Classification of Biological and Bioinspired Aquatic Systems

Home , Aquatic locomotion, Common torpedo, Diodon, Dorsal fin, Electrophorus electricus, Undulatory locomotion

Ocean Engineering 148 (2018) 75–114

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Ocean Engineering

journal homepage: www.elsevier.com/locate/oceaneng

Review Classiﬁcation of biological and bioinspired aquatic systems: A review

R. Salazar, V. Fuentes, A. Abdelkeﬁ *

Department of Mechanical and Aerospace Engineering, New Mexico State University, Las Cruces, NM, 88003, USA

ARTICLE INFO ABSTRACT

Keywords: Robotic systems capable of aquatic movement has increased exploitation in recent years due to the diverse range Aquatic systems of missions that can be performed in otherwise hostile environments. These aquatic unmanned vehicles (AUVs) Bioinspiration have begun to transition to systems that replicate biological animals as they are already extremely efficient at Fin oscillation moving in aqueous environments. The result is the abandonment of inefficient propeller based locomotion for a Fin undulation biological locomotion type suitable for the specific mission. There is a diverse range of biological locomotion's Jet propulsion available with animals that give a range of criteria to follow. In this review, existing aquatic animals and found AUVs are classified. How the bioinspired systems compare to the animals in their locomotion is investigated and discussed. Then, it is discussed what makes these systems bioinspired and biomimetic, and the AUVs that fall into these distinctive categories. Limitations and future recommendations on possible improvements for these systems are offered.

1. Introduction missions include environmental surveying, oil spill monitoring, internal pipe inspection, erosion monitoring, observation of animal species, Currently, there is a growing need for the adaptation of robotic sys- beach safety, espionage, anti-espionage, and border patrol (Blindberg, tems which can autonomously perform routine tasks in aquatic envi- 2001; Najem et al., 2012). The mission of an AUV is dependent on its ronments (Blindberg, 2001; Habib, 2013; Scaradozzi et al., 2017; Raj and design. However, a major component of the AUV is the autonomy of the Thakur, 2016; Yen and Azwadi, 2015). This need derives from the large system. An autonomous system is necessary for the completion of these scale unobservable volume of the oceans and other bodies of water which missions because signal transmission through water is difficult, espe- all present a challenge for hominoid observation. This challenge persists cially at great depths or distance. The craft needs to carry out pre- due to human fragility in aqueous environments and the underdeveloped planned mission solely on its own and should respond to changing robotic systems that are deployed. To solve the lack of mission capabil- environment. In addition, these systems need to have long endurance ities of these systems, inspiration must be taken from the diverse selec- and have the capability to move in close quarter environments with tion of species which inhabit the oceans, rivers, and lakes. Deriving constantly changing fluid flow. A close quarter environment would not inspiration from nature would mean the system is bioinspired. If the condone a system tethered with a control cable or that of those using system mimics a biological system, it is dubbed biomimicry. There have propellers. Propeller thrust degrades when fluid flow is not uniform. already been studies where researchers used bioinspiration and bio- Propellers are also inefficient in comparison with fishes where pro- mimicry to make an autonomous vehicle. These new robots are charac- pellers are estimated efficiency of 40–50 percent (Habib, 2013). Fishes terized as an aquatic unmanned vehicle (AUV). These AUVs are modeled have the ability of changing direction at a complete stop which is highly after certain fish species which the investigators deemed to be the best beneficial in close quarter environments. For example, systems com- form to replicate. This review offers a thorough classification list for parable with the Eel would be useful in pipe inspection where large biological and AUV systems. In this effort, more current and dated sys- body contortion is necessary to navigate complex bends. Furthermore, tems are compiled into one location to allow for future investigators Tuna are extremely efficient swimmers. Such, a robot that has a high searching AUV system reference information. More available information level of biomimicry could also display high speed required for specific allow the opportunity to create optimized AUVs. (see Table 26) missions. Depending on mission requirements, a biological system AUVs would have application in a wide range of civilian and mili- should be the center point for the design because of their swimming tary missions for exploitation in marine or aquatic environments. These capabilities.

* Corresponding author. E-mail address: [email protected] (A. Abdelkeﬁ). https://doi.org/10.1016/j.oceaneng.2017.11.012 Received 14 September 2017; Accepted 5 November 2017

The current unmanned submersible systems being used in civilian et al., 1999; Colgate and Lynch, 2004; Lauder, 2015; Neveln et al., 2013) and military missions are all relatively similar where by systems use allow for a starting point where ideas can be gathered and amassed into propellers for thrust. These unmanned submersibles have been adapted one place. The reviews of AUVs lack certain topics or overview of sys- from their larger manned relatives when the utility and safety of these tems. This review fills voids in this research and offers it in a clearer systems was realized (Blindberg, 2001). Autonomous aquatic robotic format. The review is organized as follows: in Section 2 the biological systems have been used with rapid growth of use since the 1970s locomotion and species are outlined. In Section 3, the found AUVs are (Blindberg, 2001). However, the most substantial exploitation increase is described and organized. In Section 4, there is an in-depth discussion from 1990 to 2010 with the first commercial products coming about in about the best AUVs, compare them to the biological, and the most the 2000–2010 timeframe (Blindberg, 2001). These systems are often critique for biomimicry systems. In Section 5, summary conclusions outfitted with observation equipment like cameras, temperature gauges, are presented. salinity instruments, and acoustic mapping (Blindberg, 2001; Cruz et al., 1999). These systems can be separated into two classifications. There are 2. Biological locomotion and species the torpedo-shaped autonomous systems which have been widely used on more long endurance and faster speed missions. These types of sys- There are several methodologies that can be used to define the tems have been extensively used because of their long range and depth locomotion characteristics. This review proposes a more encompassing capabilities (Blindberg, 2001). Systems like those used to track migratory and straightforward classification for the biological systems that can be shark species by acoustic tag tracking have been implemented with applied to the found AUVs. Locomotion is divided into three main clas- mixed results (lost connection with signal beacon) (Lin et al., 2013). sifications, namely, Fin Oscillation, Fin Undulation, and Jet Propulsion. Scientist tag a shark with an acoustic tag and then a torpedo-shaped AUV This is the first review that combines these locomotions. There are many follows this beacon at a predetermined distance (Lin et al., 2013). This categories within these main classes, along with overlap between them. allows for the observation of depth, temperature of water, salinity, along The classification and categories are presented in Fig. 1. The Fin Oscil- with GPS location of the animal (Lin et al., 2013). There are lation has one of the largest categories, the well-known caudal fin torpedo-shaped robots capable of long range missions, known as gliders swimmers. While the largest class in Fin Undulation is pectoral fin were swept up by the military (Guizzo, 2008). These glider submersibles swimmers. The Jet Propulsion class is amongst the smallest defined, there work on an efficient and clever system that uses buoyancy and winged is not as much variety in this class. The Dorsal & Anal Fin Oscillation is flight to accomplish forward movement (no propellers) (Guizzo, 2008). described in the Fin Oscillation section. The Oscillation & Undulation is By filling and emptying ballast tanks, the craft is capable of changing detailed in the Fin Undulation. Moreover, the combination Jet & Fin depth. During depth change large control surfaces give the craft forward Undulation is described in Jet Propulsion. direction. Other types of submersible robots are the slower and less hy- The terminology for the Fin Oscillation was taken from Sfakiotakis drodynamic robots that are comparable with the Kambara (Wettergreen et al. (1999). where a detailed review is given for different types of fish et al., 1998). These systems are used often because of their appendage locomotion. Some of the same terminology is needed for the explanation attachments and their adequate close-proximity control. These systems of the AUV systems. Thus, this terminology is given to allow a better are often simple where an open frame external cage contains all the understanding of types of fins and what nomenclature are used in the equipment, and not much focus is placed on their efficiency (Wettergreen various categories of locomotion. Shown in Fig. 2(a) and (c), respec- et al., 1998). Similar submersible systems to the Kambara have been tively, are for a caudal finned fish and ray species. These classes are vastly utilized for reef exploration, wreckage surveying, and deep sea different in their swimming capabilities. However, there are similarities exploration. in the fin structures as ribs are used in the fins to give the flexible There have been extensive reviews of AUV and biological systems, membrane rigidity. These rib structures are shown in Fig. 2(b) and (d) for these reviews are used and adapted to make a comprehensive review. Use the caudal fin fishes and the ray, respectively. The most important fins to of works (Blindberg, 2001; Habib, 2013; Scaradozzi et al., 2017; Raj and take note of are caudal, pectoral, dorsal, and anal. However, the ray has Thakur, 2016; Yen and Azwadi, 2015; Laschi et al., 2013; Sfakiotakis enlarged pectoral fins in comparison with the caudal finned fish

Fig. 1. Biological locomotion classiﬁcation.

76 R. Salazar et al. Ocean Engineering 148 (2018) 75–114

Fig. 2. Terminology for (a) caudal fin fishes, (b) skeletal structure for caudal fin fishes, (c) a ray, and (d) cartilage skeletal structure for the ray (Sfakiotakis et al., 1999). Ray skeletal structure picture modified from (Hamlett, 1999).

(Sfakiotakis et al., 1999). propulsion (Gemballa et al., 2006). The skin surface also has a large effect Biological species have developed in a manner which condones the on the efficiency on the system; sharks are known for their drag reducing most efficient propulsion for their mission (Habib, 2013; Sfakiotakis skin (Lauder et al., 2016). Studying individual classes that utilize these et al., 1999; Colgate and Lynch, 2004; Webb and Weihs, 2015). Most various fins are explained in their respective sections, along with corre- fishes developed an actuation that produces a useable thrust and a body sponding species in that class. that minimizes resistance (drag). Buoyancy of fishes is maintained through use of a swim bladder that fills with gases to counteract the 2.1. Fin oscillation locomotion animals weight and to maintain depth control. Pitch and roll are controlled with auxiliary fins or main propulsion fins. Yaw is controlled The Fin Oscillation classification is the largest of all those defined. through use of the body flexion in most animals. The various terminology These biological systems offer a great variety to choose from size, speed, for directional change can be seen in Fig. 3(b) and (c). It has been found endurance, and turning capabilities range. Propulsion actuation also that fish species swim within a Strouhal number range between varies depending on the sub-classification. The chart presented in Fig. 4 0.2 < St < 0.4 (Yen and Azwadi, 2015). Depending on the fluid flow, the gives representation of the vast diversity of species found using Fin fish must compensate with either body movement or fin stabilizing Oscillation. As shown in Fig. 4, on the left there are the Caudal Fin (Webb and Weihs, 2015). High endurance or migratory swimming would swimmers, which are the first classification discussed. Then, the Pectoral require more powerful swimming muscle and efficient forward thrust Fin species. After that, Dorsal & Anal Fin and the Combination Oscillation

Fig. 3. Terminology for (a) the free body diagram a ﬁsh, and (b) the directional change with respect to pitch and roll, (c) and directional change respect to yaw.

77 R. Salazar et al. Ocean Engineering 148 (2018) 75–114

Fig. 4. Fin Oscillation ﬂowchart displaying the various species within the sub-classiﬁcations.

Fin categories are consolidated. As seen in Fig. 4, the Caudal Fin category has the largest representative group of animals. The Labriform ﬁshes are the second largest grouping while the Tetraodontiform and Ostraciiform are the two smallest.

2.1.1. Caudal fin It is clear from the flowchart in Fig. 4 that the largest category is the caudal fin. This is due to the large variety of animals that use a caudal fin for major propulsion. Caudal finned fishes are differentiated by the thrust generated by a tail realized through a body undulation. These fishes are usually labeled as body caudal fin (BCF) in other reports (Scaradozzi et al., 2017; Sfakiotakis et al., 1999). Fishes that utilize a caudal fin tend to be faster swimmers or have a high endurance and a large majority of these fish species tend to be predatory animals (Syme and Shadwick, 2011). There are long distance swimmers found in this class as some are migratory animal (van Ginneken et al., 2005). The caudal fin allows for more thrust to be generated, but thrust magnitude is dependent on the body actuation (Gillis, 1996). The different degrees of body undulation are shown in Fig. 5. The greatest degree of body undulation can be seen Fig. 5. Visual diagrams to display the differing head and body motion to realize a tail expressed in the Anguilliforms, Fig. 5(a), which the first caudal fin oscillation. (a) Anguilliform, (b) Subcarangiform, (c) Carangiform, and (d) Thunniform category discussed. The Subcarangiform, Carangiform, and Thunniform (Sfakiotakis et al., 1999). will then follow in this order respectively as the body undulation de- creases in this manner. swimming is extremely high (van Ginneken et al., 2005). Using a comparison set from two experiments, they determined the endurance of 2.1.1.1. Anguilliform. The Anguilliform caudal fin category represents these animals. One experiment lasting 173 days and the other 7 days to animals denoted with the largest body undulation to realize a caudal fin make sure, that the speeds were similar for each result. In their experi- oscillation (Sfakiotakis et al., 1999), as shown in Fig. 5(a). These animals ments, they used a water channel and subjected the animals to constant are extremely flexible and have small turning radii (Gillis, 1996). Eels flow of oxygenated rich water. During these two trials, the Eels swam and Sea Snakes are considered in this class (Gillis, 1996; Graham et al., with an average velocity of 0.22 m/s. The European Eels sustained this 1987). Both are found in reefs or protective covering where these animals pace for the full 173-day experiment, 5500 km (van Ginneken et al., display a great degree of body flexibility. This flexibility comes from the 2005). Studies have also been done on the Eel migration using pop-up large number of vertebrae. For an Eel, this number can average 104 satellite tags (PSATs). In this investigation, a PSAT of dimensions 2 whereas Sea Snakes can average 186 vertebrae (Gillis, 1996). Eels are (2 Â 13 cm ) were attached to Eels to study the increased drag and the one of the most efficient migratory swimmers. In a study conducted by impact on the Eels swimming performance (Burgerhout et al., 2011). It van Ginneken et al. (van Ginneken et al., 2005), controlled tests found was found that individuals struggled swimming and their efficiency was that the efficiency of Eel (Anguilla anguilla, aka. European Eel) migratory impacted significantly (Burgerhout et al., 2011). This shows that these

78 R. Salazar et al. Ocean Engineering 148 (2018) 75–114 animals have extremely efficient propulsion and they use energy spar- fishes use a caudal fin occasionally when their pectoral muscles are at ingly to accomplish migratory swimming. maximum endurance or when burst swimming (Korsmeyer et al., 2002). These fishes have low endurance when solely utilizing their pectoral fins 2.1.1.2. Subcarangiform & Carangiform. The Subcarangiform and Car- (Korsmeyer et al., 2002). In Fig. 6, the top and side views for the rowing angiform classifications are extremely similar. They can be distinguished and flapping swimming modes are shown for a Labriform swimmer. with a slightly varying initiation point along the body. Subcarangiform The pectoral fins of the Labriforms have rigid fin rays between the fin fishes utilize slightly more back-and-forth head movement, as presented membrane which give the fin rigidity (Westneat, 1996). The swimming in Fig. 5(b). Undulation is therefore initiated closer to the head and a strokes of the Labriform fishes can be classified into two categories, longer undulation wave realizes caudal fin oscillation which requires the namely, flapping or rowing motions, as shown in Fig. 6. The flapping activation of more muscle along the body (Altringham et al., 1993). A motion is when the fish only performs upstroke and downstroke with the common agreement is that Carangiform fishes utilize one-third of their leading edge of the fin, while the rest of the fin membrane remains posterior body for undulation (Scaradozzi et al., 2017; Sfakiotakis et al., passive. This means that both the upstroke and downstroke are both 1999). Subcarangiform fishes include animals like Salmon and Carp power strokes for flapping (Sfakiotakis et al., 1999). However, the rela- while examples of Carangiforms include Trevally, Shad, and Piranha. tive area for drag is increased when using flapping fin strokes so speed is These fishes are some of the most studied somewhat because they are the low during this actuation. Therefore, flapping is good for close quarter most easily caught and are considered efficient power thrust fish maneuvering. The rowing motion is used for faster speeds. This motion (Altringham et al., 1993). These fishes inhabit bodies of water that satisfy can be described by the leading edge making a vertical downstroke, but their survival needs. These two classes have a large variety of sizes, on the upstroke, the fin is pulled back at an angle to the vertical axis weights, and speeds. The flexibility of these two sub-categories is much (Sfakiotakis et al., 1999; Sitorus et al., 2009). less than the Anguilliform sub-category. 2.1.3. Dorsal & anal fin: Tetraodontiform 2.1.1.3. Thunniform. Thunniform fishes are most often found to be This unique class is very sparse in the literature covering species like predators or a higher order of the food chain. Noted by a very limited the Ocean Sunfish, Sharptail Mola, and Slender Sunfish. These three body undulation to the last quarter of the body (Sfakiotakis et al., 1999), represented animals are all in the same family so it is easiest to just refer as shown in Fig. 5(d). This body undulation and compact muscle to the category by the largest member which is Ocean Sunfish. The Ocean grouping force a rigid tail with powerful oscillations (Syme and Shad- Sunfish are extremely large and are the biggest bony-fish on the planet. wick, 2011; Shadwick and Syme, 2008). Bodies are usually very Fig. 7 is a representative drawing of the Ocean Sunfish. These fish have streamline and can be considered as extremely efficient fishes where they long lifespans which allows them to reach huge sizes. However, these sustain top speed for long duration to either pursue prey or avoid even fishes have the least efficient design considered out of all the ones larger Thunniform predators (Guinet et al., 2007). This sub-category mentioned. These fishes utilize two large paddle fins, one dorsal and one includes Tunas, Sharks, and Dolphins. It can be mentioned that some of anal for locomotion (Sfakiotakis et al., 1999). These fins have rigid ribs these species in this group are warm blooded which gives these predators and the fish uses the most unorthodox oscillation motions to control di- the upper-hand in terms of endurance and power (Syme and Shadwick, rection and depth (Sfakiotakis et al., 1999). Yaw and pitch control for this 2011). Warm muscle means more efficient muscle performance and animal are low. Moreover, this fish has issues navigating in close quarter hence more thrust (Syme and Shadwick, 2011). environments. These fishes are open ocean migrators who feed on jellyfish. Their body is considered flat with a slight taper from their head to 2.1.2. Pectoral fin: Labriform the underdeveloped caudal fin. The Pectoral Fin is a category of fishes which primarily uses pectoral fin locomotion to perform slower speed agile swimming (Davison, 1988). 2.1.4. Pectoral, dorsal/anal, and caudal fin: Ostraciiform Fishes like Wrasses, Parrotfish, and Sheephead utilize this form of pro- The Ostraciiform is a unique class because it uses pectoral and dorsal/ pulsion for the efficiency of close quarter swimming. The species that use anal fin oscillations to control movement (Sfakiotakis et al., 1999; Hove Labriform swimming tend to be found in reefs and areas of cover. These et al., 2001; Gordon et al., 2000). These fishes can be found in reefs and

Fig. 6. (a) Top view of rowing, (b) side view of rowing, (c) top view of ﬂapping, and (d) side view of ﬂapping (Sitorus et al., 2009).

79 R. Salazar et al. Ocean Engineering 148 (2018) 75–114

few of the most commonly found animals. For the Anguilliform, the European Eel is the midrange size but it has the highest average speed and the highest endurance. The Giant Moray is the largest Eel selected. It has an extremely flexible body but this animal does not have extreme endurance. The Giant Moray is an ambush hunter that sometimes moves along the reef to find new hunting grounds. The Sea Snake makes shallow water dives to catch prey off the coast. These animals are capable of dives lasting longer than 20 min. During these dives, they search crevasses in the ocean sea floor. These animals are not migratory swimmers. In the Subcarangiform category, two similar animals have been selected, both being Salmon having similar shape. The Chum and Chi- nook Salmon display burst swimming and migratory swimming from oceans to rivers, as all Salmon species do as part of their life cycle. These animals swim long distances to reach their hunting grounds off the Pa- cific coast of Asia and North America (Friedland et al., 2001). These Fig. 7. Side view of Ocean Sunfish and labeled main propulsion fins. Picture modified animals then return to the rivers and stream estuaries to mate. Where from (Ocean Sunfish, 2017a). depending on the estuary, Salmon must perform jumps over waterfalls. This is done by burst swimming and leaping out of the water above the close quarter environments. What makes this class stand out is the unique rapid or water fall. Burst swimming is for short periods of time as Salmon body shape for the species, namely, Boxfish and Cowfish. These two cannot sustain this high energy jumping. The Carangiforms have some relatives have a body shape that is akin to rigid box, as shown in Fig. 8. good shaped fish with slender tapered bodies that they utilize to be swift These fishes can maintain finetuned Fin Oscillations, giving this fish predators. The most dominant predator of these is the Swordfish, an good maneuverability in the small crevasses it calls home. The speed of animal capable of extremely large sizes. Even more impressively, this these animals is low because of their body shape and their inefficient fin animal is capable of extreme burst swimming of 27 m/s (Sagong et al., actuation for fast speeds (Hove et al., 2001). Osctraciiform locomotion 2013). The Swordfish body is very streamline with the enlarged sword on can be described in a few different ways. Like Labriform and Ostraciiform its nose, also known as an enlarged bill. This bill gives it predatory which use pectoral fin flapping motions, however, Osctraciiform not only advantage when attacking other fishes and helps this fish pierce the do vertical flapping, they also perform horizontal flapping too (Hove water when swimming allowing it to swim faster. The Bonefish also has a et al., 2001). The dorsal and anal fin are used to give more forward thrust streamline body, a powerful peduncle that propels this fish through the and stability and both fins flap synchronously in phase (Hove et al., water. The Trevally is a moderately fast Carangiform that has a more 2001). Occasionally, they use their extremely small caudal fin for some slender body that tapers heavily into the peduncle. This animal has burst propulsion, but is rare (Hove et al., 2001). prominent dorsal and anal fins. The Thunniform animals are on the upper tier of the food chain, apex predators of the oceans. These animals are capable of large sizes and 2.2. Consolidation of characteristics of biological fin oscillation species express extremely streamline bodies. The Tuna, Dolphin, and Shark can leisurely swim at speeds greater than 1 m/s and burst swimming greater The Fin Oscillation classification has probably the greatest degree of than 10 m/s. The Yellowfin is one of the largest Tuna with average variation of animal size, shape, speed, and swimming technique between weight of 50 kg for caught fish, but these animals can get much larger the categories. These variations allow the animals in this class to perform (Dagorn et al., 2006). However, a mature Yellowfin is larger than the a wide range of missions. This includes close quarter maneuvering, burst Skipjack Tuna. It was found that the Yellowfin Tuna, even though larger, swimming, and endurance. It should be stated that the values in Table 1 has great high speed endurance as it can maintain 3 m/s for more than should be a range. These values should rather be used as a point of 30 min (Guinet et al., 2007). The Tunas have a slender lunate tail that is reference where animal growth and speed are averaged unless otherwise semi-rigid and good for thrust. The aspect ratio of the Yellowfin can be stated. This allows for values from zero and some greater than those observed to be larger than the Skipjack Tuna. The elongated second specified. It should also be realized that these animals serve as a gener- dorsal fin and anal fin of the Yellowfin Tuna has not been found to play alization for these classes. There is a wide variety of species that use a any part in thrust generation. All fins of the Tuna are semi-rigid struc- defined locomotion, but attempts in creating a range of values through tures. Fin Oscillation and Undulation animals often have scales that cover the animals selected gives a batch of mimicable animals. Table 2 offers their bodies. These structures on their skin gives the animal a protective details about the animals’ endurance and turning capabilities as well as a layering. The skin of the predatory animals gives them an advantage picture for shape. during high speed swimming. For predators, scales can be observed as For Fin Oscillation, the most abundant animals use a caudal fin. smaller, and therefore create a smoother surface like those of the Tuna. However, since the sizes vary so drastically for these animals, each class Other animals have adapted to create scales that induce a boundary layer should be described independently. This review regularly only displays a

Fig. 8. Diagrams of a Boxﬁsh: (a) top view, (b) front view, and (c) side view (Gordon et al., 2000).

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Table 1 The Fin Oscillation animals’ characteristics including mass, length, average speed, and top speed.

Fin Oscillation Fish Mass (kg) Length (m) Average Speed (m/s) Top Speed (m/s)

Anguilliform European Eel 0.8 (van Ginneken 0.75 (van Ginneken et al., 0.4 (Burgerhout et al., 0.8 (van Ginneken et al., et al., 2005) 2005) 2011) 2005) Giant Moray 30 (Torres, 2017)3(Torres, 2017) N/A <2(Giant Moray Eel Swimming, 2017) Sea Snake (Graham et al., 1987) 0.075 0.63 0.3 0.8 Subcarangiform Chum Salmon (Tanaka and Naito, 3.25 0.65 0.5 >5 2001) Chinook Salmon (Chinook Salmon, 35 1.0 0.6 >6 2017) Lake Sturgeon Max20 (Lake Strugeon, 2.0 (Lake Strugeon, 2017) N/A Juvenile (.5 m) 1(Peake, 2017) 2008) Carangiform Swordfish Max650 (Gardieff, Max4.55 (Gardieff, 2017) 2(Sagong et al., 27 (Sagong et al., 2013) 2017) 2013) Bonefish Max9.1 (Bonefish, Max0.77 (Bonefish, 2017a) N/A 17 (Top 10 Fastest fishes, 2017a) 2017) Giant Trevally Max80 (Meyer et al., Max1.7 (Meyer et al., 2017) N/A 13 (Leis et al., 2006) 2017) Thunniform Yellowfin Tuna 50 (Dagorn et al., 2006) 1.3 (Dagorn et al., 2006) 1.2 (Dagorn et al., >16 (Guinet et al., 2007) 2006) Skipjack Tuna 2(Tuna Species Guide, 0.5 (Tuna Species Guide, .57 (Gooding et al., N/A 2017) 2017) 1981) Mako Shark (Martin, 2017a) 270 2 1.5 >13 Bottlenose Dolphin (Bottlenose 450 3 1.3 >11 Dolphin, 2017) Labriform Banded Wrasse (Davison, 1988) 0.22 0.271 .4 0.84 Spotted Wrasse (Davison, 1988) 0.058 0.158 .24 0.58 Schelegel's Parrotfish (Korsmeyer 0.2 0.22 .45 0.85 et al., 2002) Tetraodontiform Ocean Sunfish 998 (Ocean Sunfish, Wingspan2.4 (Ocean N/A 0.83 (Ocean Sunfish, 2017c) 2017b) Sunfish, 2017b) Ostraciiform Box Fish (Gordon et al., 2000) 0.04 0.13 0.42 0.88 like that found in sharks. Sharks regularly have scales microstructures 2.3. Fin undulation locomotion that trap water in vortexes along the skin surface. This greatly reduces drag and allows sharks like the Mako to achieve extremely high speed. Animals in this class use individual rib excitation in their fins to create The Mako Shark is capable of burst swimming greater than 13 m/s for undulatory waves along the fin(Habib, 2013; Sfakiotakis et al., 1999; short periods of time. The body shape of the Mako Shark, with a more Albert, 2001; Boileau et al., 2002). The animals in Fin Undulation use this elongated and tapered body compared to the Tuna. The Shark has wave propagation for thrust and stability control. Some of these animals semi-rigid fins, but these fins can still passively flex. The Dolphin is also a are able to control the direction of the wave, giving them forward and fast predator with a body comparable to the shark. The Bottlenose Dol- backward swimming capabilities or null speed turning. There are many phin is a mid-sized Dolphin that is capable of power endurance much like types of fins utilized by these animals, namely, pectoral, dorsal, anal, and the other predators in this category. Bottlenose Dolphins swim at greater dorsal/anal. There is a sub-category which uses undulatory and oscilla- than 11 m/s. tory fins. High endurance swimmers can be found in this class, but more Labriform swimmers do not have the most impressive of speeds, as often these animals are ambush predators that use structures as cover. their pectoral fins are used to swim in short range missions. These ani- For classes, such as the dorsal, anal, and combination fin, their undula- mals do use their caudal fin, but this is only when they need a boost of tory fin allows forward and backward movement which is ideal in close speed. Their shape is somewhat flat and elongated, comparable to the quarter environments (Albert, 2001). The four different types of undu- Trevally body type. Their pectoral fins give them decent turning capa- latory fin and the one combination of undulation and oscillation fin are bilities, as their bodies are flexible like the caudal fin swimmers. The presented in Fig. 9. It follows from the latter figure that this class is much Schelegel's Parrotfish is the best representation for the Labriform. It has smaller than the Fin Oscillation, but the animals are more diverse in this the highest average speed and top speed, but it is the mid-size of the three class. The largest Fin Undulation category is the Rajiforms which is dis- displayed at 0.2 kg and 0.2 m. These fishes are found in the coral reef cussed first. Then, the comparable Amiiform and Gymnotiform are with Ostraciiform species. The Ostraciiform is detailed with only one fish described. After that, the Balistiform is shown as the dorsal and anal fin as the Boxfish and Cowfish are comparable. These fishes are small with combination category. Last, the Diodontiform is presented to be another an average weight being less than 0.1 kg and length less than 0.2 m. strange locomotion as it uses both oscillation and undulation fins. These fishes hide out in alcoves along coral reefs so their multiple oscillating fins give it great control and stability with a moderate average 2.3.1. Pectoral fin: Rajiform speed relative to its length. This class includes species like Rays and Skates. The body individuals The largest animal described is the Ocean Sunfish, a Tetraodontiform. are comprised of cartilage which gives their whole body great flexibility. The maximum weight of this animal is nearly 1000 kg and a wingspan of Fin ribs extend from the body into the pectoral fin, these ribs are excited an impressive 2.4 m. This extremely large animal stays in deeper water by muscles which run through the skeleton. The skeletal structures of and has a maximum speed of less than 2 m/s. However, due to its large these animals are shown in Fig. 10. Rajiform individuals like the Com- size it is still able to cover 26 km/day (Ocean Sunfish, 2017c). The Ocean mon Stingray or Skate can control these ribs individually to create an Sunfish only has four active fins where the pectoral fins help stabilize the undulatory wave along the fin length. The rib structures for the Common colossal body. Despite these control surfaces, yaw, pitch, and roll are very Stingray are more free, compared to the Manta Ray whose ribs are difficult to control for these fishes. The body is flat with a slight disk aligned in a cross weave matrix and unify at the fin edge. The Common shape seen from a side view with a taper from the head to the tail. Stingray ribs do not have a secondary terminus connect when compared

81 R. Salazar et al. Ocean Engineering 148 (2018) 75–114

Table 2 Continuation of Fin Oscillation animal description with picture for reference. Deﬁned turning capabilities are based off our observation describing: Extremely Tight (ET), Tight (T), Moderate (Mo), Medium (Me), Low (L), and Poor (P). All pictures are from open domain websites.

Fin Oscillation Fish Endurance Turning Picture (ET, T, Mo, Me, L, P)

Anguilliform European Eel Migratory-(average speed for 173 days) (van Ginneken ET et al., 2005) (European Eel, 2017)

Giant Moray Very low endurance-ambush hunter-N/A ET

(Moray Eels, 2017)

Sea Snake Surface dives-(20–30 min) (Graham et al., 1987)ET

(Nordqvist, 2017)

Subcarangiform Chum Salmon (Friedland Migratory- 100 days T, Mo et al., 2001) -Up to 8,500 km (Quinn and Groot, 1984) (Chum Salmon,

2017) Chinook Salmon Migratory- same characteristics as Chum Salmon, T, Mo perform similar migratory patterns (Fishing and

Shellﬁshing, 2017) Lake Sturgeon Cruise swimmer- N/A Mo, Me (Lake

Sturgeon, 2017) Carangiform Swordﬁsh Extreme burst swimmer- Mo, Me 27 m/s-(<10 min) (Sagong et al., 2013) (Swordﬁsh,

2017) Boneﬁsh Burst swimmer to evade predators- N/A T, Mo (Boneﬁsh, 2017b)

Giant Trevally Burst swimmer- Max Speed-(15–28 min) (Leis et al., T, Mo 2006) (Giant Trevally, 2017)

Thunniform Yellowﬁn Tuna Endurance-3 m/s-(30–60 min) (Guinet et al., 2007) Mo, Me

(Hawaii Seafood, 2017)

Skipjack Tuna Endurance-0.54 m/s-(24 h) (Gooding et al., 1981) Mo, Me

(Skipjack tuna, 2017)

Mako Shark Burst swimming/max speed-(<20 min) (Martin, 2017a)Mo

(Mako Shark,

2017) Bottlenose Dolphin Capable of endurance predatory chases- N/A Mo (Watkins Marine

Mammal Sound Database, 2017) Labriform Banded Wrasse Incremented endurance till max speed test- T, Mo (2h)(Davison, 1988) (Five Banded Wrasse,

2017) Spotted Wrasse Incremented endurance till maximum speed test- T, Mo (2.5 h) (Davison, 1988) (Reef Frontiers, 2017)

Schelegel's Parrotﬁsh Sustained Endurance-0.66 m/s-(30 min) (Korsmeyer T, Mo et al., 2002) (continued on next page)

82 R. Salazar et al. Ocean Engineering 148 (2018) 75–114

Table 2 (continued )

Fin Oscillation Fish Endurance Turning Picture (ET, T, Mo, Me, L, P)

(Fish Identiﬁcation, 2017)

Tetraodontiform Ocean Sunﬁsh Migratory-(26 km/day) (Ocean Sunﬁsh, 2017c)P

(Ocean sunﬁsh, 2017)

Ostraciiform Boxﬁsh Endurance-0.42 m/s- (1 h) (Gordon et al., 2000)T

(Yellow Boxﬁsh, 2017)

Fig. 9. Fin Undulation ﬂowchart with species in that classiﬁcation.

Fig. 10. Skeletal structure for (a) Common Stingray and (b) Manta Ray. to the Manta Ray rib structure. Manta Rays have cross ribs to the span- edge is excited to cause the movement that forces the undulation along ning ribs which gives the pectoral fins more rigidity. The Common the fin(Rosenberger, 2001). For animals like the Common Stingray or Stingray does have some cross connection, but these are much closer to Skates, fin ribs are distributed along the entire fin surface area and the body as the rib tips must still be free to move. The difference in partake as dominant sections as the ribs are free to move independently structure is shown in Fig. 10(a) and (b). However, this difference in of its neighbor (Rosenberger, 2001). structure for the Manta Ray means that a large portion of the leading- As shown in Fig. 11, the various shapes for all the animals that partake

83 R. Salazar et al. Ocean Engineering 148 (2018) 75–114 in Rajiform swimming are presented. There are clear differences in shapes, some are more circular like the Common Stingray while the Cownose Ray has triangle shaped pectoral fins. There are two outliers in this category, a caudal fin is utilized for thrust by the animals expressed in Fig. 11(e and f). Shovelnose Guitarfish shape is shown by Fig. 11(c) while the shape of Fig. 11(f) are the Common Torpedo or Electric Ray. As seen in Fig. 9, these Rajiform members are combined with the Fin Oscillation category to represent their caudal fin. These animals use a Fig. 12. Schematic of Amiiform and the fin rays along a dorsal undulatory fin(Hu caudal fin for most of their swimming. These caudal fin Rajiforms are et al., 2009). more closely related to the shark. However, these animals have enlarged flattened pectoral fins and utilize these in swimming too, but primarily use the caudal fin. Rajiform shapes and fin structures determine what kind of swimming capabilities the animal will exhibit. The Manta and Cownose Rays can cruise more because their fins behave as more rigid structures allow these animals to glide. The Common Stingray and Electric Rays have better ability of navigating close quarter environments as their flexible fins give them extreme turning capabilities. These Rajiforms offer a diverse range of swimming capabilities.

2.3.2. Elongated undulating dorsal fin: Amiiform Amiiform animals are known by their undulating dorsal fin(Sfakio- takis et al., 1999; Jagnandan and Sanford, 2013). There are a limited number of animals that use this locomotion. Examples of species that use this locomotion are comparable to the African Aba Aba and the Bowfin. These fishes are not extremely fast but they are decently agile, as they can Fig. 13. Drawing of Gymnotiform with the undulating anal fin (Ghost Knifefish) (Al- move forward and backward by switching the direction of the wave bert, 2001). motion in the fin. Including increased directional control by rolling their fin axis off vertical, they do this by retracting muscles along their sides to as they are small and are vulnerable to larger predators outside their achieve spinal curvature (Aba Aba Knifefish, 2017; Aba Aba Knife Fish hunting zones. Feeding, 2017). Their dorsal fin runs the length of the spin and is The forward and backwards movements of the Gymnotiform and composed of a flexible membrane around closely compacted fin ribs or Amiiform allow them to perform complex maneuvering. The bend of the rays (Jagnandan and Sanford, 2013). The complex undulations in the body allows the fin to be angled to the vertical axis permits these fishes to dorsal fins are caused by the contraction of extensor and flexor muscles move with higher degree of freedom. The main undulatory fin also allows along either side of the dorsal fin, as shown in Fig. 12. These animals can these fishes to move forward, backward, upward, and downward as realize tight turning and multi-directional capabilities (Jagnandan and described in the detailed study by Youngerman et al. (2014). The Ghost Sanford, 2013). These fishes prefer cover as they are not very fast and use Knifefish can vary the undulation waves and angle of attack of the finto cover to ambush their prey and protect themselves. achieve various directional changes (Youngerman et al., 2014). The pectoral fins of the Ghost Knifefish are used as control surfaces for pitch 2.3.3. Elongated undulating anal fin: Gymnotiform and roll control (Black Ghost Knife Fish, 2017). The Gymnotiform class is comparable to the Amiiform animals. However, the undulating fin is on the posterior of the body as an elon- 2.3.4. Combination of dorsal and anal fin undulation: Balistiform gated anal fin. These fins also have many individual ribs. Albert The Balistiform fishes are classified by the use of both the dorsal and (Youngerman et al., 2014) states that these undulating fins are composed anal fins, also known as medial pair fin (MPF) undulation locomotion of 100–300 bony ribs. This allows the fish to have complete control of the (Sfakiotakis et al., 1999). The most commonly found species in this class fin for the entire length because each rib has its own set of agonist and is the Triggerfish. Fishes that utilize this propulsion type utilize MPF antagonist muscles. The membrane between the ribs are extremely combination until the muscles controlling the fin ribs are exhausted, then flexible and can move with the rib with ease (Low and Willy, 2005). The the caudal fin is used until muscle recuperation occurs (Korsmeyer et al., length and location of the anal fin along with the fins flexibility can be 2002). Therefore, even though these species primarily use their MPFs, noted in Fig. 13. These fishes are ambush predators like the Amiiforms occasionally the caudal fin is utilized to increase endurance. In experi- and prefer obstructions in the water column where they can hunt small ments performed by Korsmeyer et al. (2002), Picasso Triggerfish were prey with greater ease. These fishes do not make large migratory swims subjected to an endurance test. In this test, fluid flow was increased

Fig. 11. Different shapes for the different Rajiform animals. (a) Would be the Common Stingray, Skates could be considered in the (b–d), and (h) would be the Manta Ray and Cownose Ray (Rosenberger, 2001).

84 R. Salazar et al. Ocean Engineering 148 (2018) 75–114 intermittently till the fish was at its maximum speed where it could hunters that prefer cover. The elongated fins of the animals in these barely maintain its position in the channel. The average time lapsed till classes allow these animals to nimbly move through extremely close the fish faltered was 4 h, however, these fishes were relying heavily on quarter environments. The shape of the body of the Amiiform and their caudal fin for most of the maximum speed (Korsmeyer et al., 2002). Gymnotiform also condone this close quarter navigation as they can fit These types of fishes do not get very large and this locomotion is pri- through narrow gaps in the cover that they inhabit. The shape of these marily utilized by Triggerfish. The various types of Triggerfish are all animals is either an elongated tube shape or they have a more flattened similar where the majority are comparable to just one of the larger spe- body. The bodies of these animals are flexible allowing for the change of cies that can be found, the Picasso Triggerfish. These fishes do not yaw, pitch, and roll. The elongated fins of these animals have ribs that are perform large migrations and can be most easily studied in shallow reefs. of similar length and are distributed along the whole length of the fin. The Electric Eel was found to be the largest Gymnotiform at 20 kg and 2.3.5. Combination of pectoral fin undulation & dorsal/anal oscillation: 2 m. In the Amiiform category, the African Aba Aba is found with a Diodontiform maximum size of 18.5 kg and 1.67 m. These two animals have similar This is a unique classification of fish where the use of large undulating tubular shapes. The Black Knifefish is smaller but very nimble pectoral fins aid in slow speed motion, while oscillating dorsal and anal Gymnotiform. fins generate more thrust for higher speeds (Chan, 2010). Fishes found in The Manta Ray is the largest Fin Undulation animal with individuals this class are similar to the Porcupine Fish or Pufferfish. This fish is not averaging around 100 kg while Cownose Rays weigh about 15 kg. The considered as migratory swimmers and inhabit shallow water reefs. wingspan of Manta Ray is about 3.5 m compared to the Cownose Ray of These Porcupine Fish are not burst swimmers and rely on their body wingspan of 1.2 m. There is a large size difference between these two spines and unique attribute of swelling up with water to enlarge their size Rays. However, these animals perform migratory swimming using their to ward off predators. Fishes that also use the pectoral fin undulation are triangular pectoral fins to glide through the water. The pectoral fin rib Bluegill. These fishes do this by individual ribs within the fin(Lauder, matrix does not allow form multiple waves at a time. Although the Manta 2015). The Bluegill use these fins for low speed swimming, but use their Ray uses more of a flapping motion, it has a top speed of 9.8 m/s and has caudal fin for higher speeds. an average speed slightly over 1 m/s. The Common Stingray is capable of impressive burst swimming greater than 10 m/s. However, the Common Stingray has the circular disc shape that has the free pectoral ribs. The fi 2.4. Consolidation of characteristics of biological n undulation species free moving ribs allows this animal to have multiple undulation waves at a time. The shape of Rajiform animals range from circular disc, oblong fi The animals in the Fin Undulation classi cation vary greatly. How- disc, and triangular fins. The rib composition of the fin determines what fi ever, it should be noted that the ns which have ribs and are utilized to wave propagation that the Ray will perform. These ribs have changing fi perform undulation, vary depending on the type of n and also the type thickness depending on the length of the fin. The body of these animals of animal. These ribs allow these animals to have great control of their are flexible because their skeletal structure is comprised of cartilage. fi ns in sections. Rib length can vary depending on the animal's body Even though cartilage is not as strong as bone, these animals bodies are fl shape. Shapes vary from at body to tubular elongated bodies. The body still strong. fi types vary which allow for different n length and wave propagations. The Balistiform selected is the Picasso Triggerfish. This animal ex- fi The body of an Amiiform allows for the n to not have long ribs since the presses a good size and shape assumption for these animals based off fi n is so long, while the main ribs in the Manta Ray are long to control the observations for the Triggerfish. The Picasso Triggerfish has a size of fi leading edge of the pectoral n. These animals can be considered in 0.14 kg and 0.17 m. The Balistiform are short range swimmers when having good mobility, but are not burst swimmers. In Tables 3 and 4, the using primarily just their dorsal/anal fins. Once these muscles are characteristics of the some of the biological Fin Undulation species exhausted, the caudal fin is utilized. The caudal fin of the Picasso Trig- are presented. gerfish has a flexible lunate caudal fin. These fish have a slender body and fi The Fin Undulation category is lled with animals that include its pectoral fins are used for control surfaces for stability. These animals ambush predators capable of multi-directional motion. The animals swim close to cover to forage for food in coral reefs. The Diodontiform capable of this motion are Amiiform, Gymnotiform, and Diodontiform. animals also stay close to cover and can navigate using their undulating The Rajiform animals comparable to the Common Stingray are capable of pectoral fins and then perform fast swimming with their oscillating MPF. null speed turns. These capabilities allow these animals to navigate in The Porcupine Fish has a tubular body that tapers to the caudal fin that extremely close quarter environments. The utility of this directional this fish periodically uses. The length of these animals is greater than the control is the ability to navigate in close quarter environments. Triggerfish with a length of 0.5 m. The weight of these animals could not The Amiiform and Gymnotiform have limited information about be determined but the size can be assumed to be greater than the average speed and top speed. However, these animals are ambushed

Table 3 Fin Undulation animals and their characteristics including mass, length, average speed, and top speed.

Fin Animal Mass (kg) Length (m) Average Speed (m/ Top Speed (m/s) Undulation s)

Rajiform Manta Ray (Armstrong et al., 2016) 100 3.5 (Diameter) 1.02 9.8 (Deep Sea News, 2017) Cownose Ray (Grusha, 2005) 15 1.2 (Wingspan) (Rainer, 0.27 2.57 2017) Common Torpedo N/A 0.6 (Luna, 2017) N/A N/A Common Stingray (Rosenberger, 2001) 60 1.5 (Diameter) 0.81 (Stingray, 13 2017) Amiiform Bowfin 1(Amia calva, 0.5 (Amia calva, 2017) N/A 1 (Jagnandan and Sanford, 2017) 2013) African aba (Aba Knife fish, 2017) Max18.5 Max1.67 N/A N/A Gymnotiform Black Knifefish (Apteronotus albifrons, <10 Max0.5 N/A N/A 2017) Electric Eel (Electric Eel, 2017a) 20 2 N/A <2(Electric Eel, 2017b) Balistiform Picasso Triggerfish (Korsmeyer et al., 2002) 0.14 0.17 0.35 0.92 Diodontiform Common Pufferfish (Chan, 2010) N/A 0.5 (Pufferfish, 2017)1 9

85 R. Salazar et al. Ocean Engineering 148 (2018) 75–114

Table 4 Continuation of Fin Undulation animal description with picture for reference. Deﬁned turning capabilities are based off our observation describing Extremely Tight (ET), Tight (T), Moderate (Mo), Medium (Me), Low (L), and Poor (P). All pictures are from open domain websites.

Fin Animals Endurance Turning Picture Undulation (ET, T, Mo, Me, L, P)

Rajiform Manta Ray Constant swimming- These animals do not rest as they need to Mo constantly ﬂow water over gills to breathe (Martin, 2017b) (Ningaloo

Reef Dive, 2017) Cownose Ray Endurance-0.27 m/s-(24 h) (Grusha, 2005)T,Mo

(Georgia

Aquarium, 2017) Common Torpedo Endurance most comparable with Common Stingray Mo, Me

Common Stingray Short endurance Mo (at speed) (<1h) ET (at null speed) (Shark Trust,

2017) Amiiform Bowﬁn N/A Mo, Me

(Bowﬁn Family) African Aba Aba Slow speed visible in videos. Endurance cannot be extreme. Me (Aba Aba Knife (Aba Aba Knife Fish Feeding, 2017) Fish Feeding, 2017)

(The Aba Knife Fish, 2017) Gymnotiform Black Knifeﬁsh Slow speed visible in videos. Endurance cannot be extreme. Me (Black Ghost (Black Ghost Knife Fish, 2017) Knife Fish, 2017)

(Lidsky, 2017) Electric Eel Slow speed visible in videos. Endurance cannot be extreme. Mo (Electric Eel, (Electric Eel, 2017b) 2017b)

(Lamb, 2013) Balistiform Picasso Triggerfish Incremented endurance test- (4 h of maximum speed) Uses caudal T, Mo (Lagoon Triggerfish) fin at this maximum speed. (Korsmeyer et al., 2002) (Rhinecanthus

aculeatus, 2017) Diodontiform Common Pufferﬁsh N/A T, Mo

(The

puffer ﬁsh, 2017)

Balistiform. including the Squid or Cuttleﬁsh.

2.5.1. Bell constriction 2.5. Jet propulsion Bell constriction locomotion is defined by the movement of a flexible bell in certain biological systems. This propulsion category is applied Jet Propulsion is the smallest classification presented in this review. specifically to Jellyfish (Yeom and Oh, 2009). However, these biological This locomotion is broken down into three main categories, namely, bell systems that utilize bell constriction for locomotion have their movement constriction, mantel constriction, shell compression, and a combination strongly influenced by the ocean current because their mass and thrust of Jet and Fin Undulation. Fig. 14 shows these four categories with the are not large enough to resist it. Bell constriction is produced by the biological systems that fall in each. The bell constriction category focuses contraction and relaxation phases of the subumbrellar muscles in the bell, on the movement of Jellyfish. Mantel constriction is divided into ceph- causing movement by ejecting water due to the constriction of the vol- alopods, such as Octopus. Shell compression is specifically for mollusks, ume of the bell (Villanueva et al., 2011). Fig. 14 displays a schematic such as Scallops. Lastly, the chart presents systems that use a combination

86 R. Salazar et al. Ocean Engineering 148 (2018) 75–114

Fig. 14. Jet Propulsion flowchart with their biological classifications. drawing for the anatomy of oblate-shape and prolate-shape Jellyfish, which causes a larger amount of water to propel from their bells. This respectively. Some of the main muscles, such as the subumbrellar, rho- type of movement is often not entire circumferential constrictions. Oblate palium, tentacles, mouth, and arms are depicted in Fig. 14. The rhopa- Jellyfish impulse their bell margin as a paddle to swim by only con- lium muscle controls the pace of swimming muscle contraction, setting stricting a portion of the bell (McHenry and Jed, 2003). However, the the basic swim frequency (Katsuki and Greenspan, 2013). Because of the oblate Jellyfish can still perform a full bell deformation to displace water. shape of their bells, not all Jellyfish create thrust in the same manner. For Oblate-shape Jellyfish use less energy for movement because as their this reason, Jellyfish's propulsion movement is classified into two frequency is lower than prolate-shape Jellyfish (Peng and Alben, 2012). different swimming patterns which are jet and rowing propulsion. Spe- Shown in Fig. 13, more found biological Jellyfish fall into the rowing cies in these categories have structural similarities, such as muscles. propulsion category. This could be due to the use of currents for long However, the subumbrellar muscle is noticed to be of larger area for distance travel and the need for high energy propulsion. systems using rowing propulsion (oblate-shape) in contrast to jet propulsion (prolate-shape), as shown in Fig. 15 (a) and (b), respectively. 2.5.2. Mantel constriction The prolate-shaped Jellyfish are also recognized as bullet-shaped Octopus fall into the mantel constriction propulsion category. The Jellyfish due to their long bell appearance, as presented in Fig. 15 (a) system of propulsion by mantel constriction is generated by filling the (Dabiri et al., 2005). The Jellyfish in this category contract their bells mantle cavity with water and expelling it out through the siphon. with a higher frequency to generate the necessary thrust that they need However, locomotion is also achieved by using their limbs (arms) to walk for swimming (McHenry and Jed, 2003). However, prolate-shaped Jel- on the ocean floor and in coral reefs. Octopus mantel constriction is lyfish use more energy than oblate-shape Jellyfish for locomotion generated by using recovery and power strokes, also known as sculling. because of the higher frequency of contraction. Rowing propulsion, on Fig. 16 shows the main parts of the Octopus considered in this review. the other hand, is generated by oblate-shaped Jellyfish, also known as the The siphon not only expels water, but also specifies the directional flat-shaped Jellyfish which is shown in Fig. 15 (b) (Dabiri et al., 2005). movement and speed of the Octopus. Mantel constriction in these species Oblate-shape Jellyfish contract their bells slower using a larger volume is principally for hunting, defense, and fast swimming (Sfakiotakis et al.,

Fig. 15. Schematic for the (a) oblate-shape Jellyfish modified from (Yeom and Oh, 2009) and (b) prolate-shaped Jellyfish (Costello et al., 2008).

87 R. Salazar et al. Ocean Engineering 148 (2018) 75–114

Fig. 18. Schematic for the cuttleﬁsh.

cephalopods are equipped with 8 arms and 2 tentacles. The two tentacles are not used by Octopus. The muscular mantle, in these systems, fills and expels water to generate mantel constriction propulsion. The Jet Pro- pulsion method of Cuttlefish and Squids, comparable to the Octopus, Fig. 16. Schematic of the Octopus. forces a fast backward-swimming direction (Wang et al., 2011). Undu- lating mantel fin locomotion is given by continuous contractions of 2015). As mentioned, Octopus use this propulsion technique to escape muscles along the fin length. This undulating fin is primarily used when from predators by burst swimming away. A defensive ink is released as a swimming slow in forward or backward direction, including control of distraction technique during this defensive burst swimming. yaw, pitch, and roll. A schematic for the anatomy of Cuttlefish is shown in Fig. 18. These animals are highly efficient swimmers at slow and fast 2.5.3. Shell compression speeds (Wang et al., 2011). Shell compression locomotion is used by biological systems from the mollusk classification. Shell compression is the type of locomotion derived by the opening and closing of the two shell halves. Water is 2.6. Consolidation of characteristics of biological jet propulsion species absorbed into the abductor muscle on scallops and propelled out between the shells producing the clapping-action movement. Fig. 17 displays an Some of the aquatic species based on Jet Propulsion locomotion differ fi illustrated description for the anatomy of the Atlantic Bay Scallop. Body in many ways, such as Jelly sh from cephalopods. Even so, these species fi parts including the mantle and abductor muscle as well as the union of relate by all having little swimming endurance. Jelly sh swimming the two shells by the shell joint are shown. The ribs illustrated on the endurance is extremely low since their movement mainly depends on the fi shell are from shell growth. These ribs are not the same support ribs used ocean's current instead of self-propelling. Jelly sh turning abilities is by the Fin Oscillation and Undulation species. Scallops use shell highly characterized as minimal. Octopus cover short distances using Jet compression propulsion primarily to escape from predators (Region, Propulsion, which depends on its arms and mantle cavity for movement. fi 1995). However, this propulsion mechanism is not endurance based Cuttle sh and Squids have higher turning capabilities in contrast to oc- fi since it takes a lot of energy for scallops to use shell compression. topuses through use of their undulation n. The bell constriction category presents some Jellyfish species, such as fi 2.5.4. Mantel constriction and undulating mantel fin the Aequorea Victoria, Aurelia Aurita, and Nomura's Jelly sh. From them fi Biological systems, such as Cuttlefish and Squids use a combination all, the Aurelia Aurita, also known as Moon Jelly sh, is the most common fi method of mantel constriction and undulating mantel fin. These types of type of Jelly sh studied and observed at aquariums. These animals weigh propulsion are utilized in various modes of swimming with different approximately 0.175 kg and a length about 0.014 m. One of largest fi fi proportion of use between Undulation Fin and Jet Propulsion. This re- known Jelly sh weighing about 204 kg is Nomura's Jelly sh with a sults in slower speed control but these animals possess great agility and diameter length of about 1.8 m. There is no information presented about speed. These animals can swim forward and backward and easily change Aequorea Victoria's weight, but present a maximum length of about fi swimming directions by using both undulating fin and mantel constric- 0.25 m. Nomura's Jelly sh is capable of swimming at an average speed of tion (Wang et al., 2011). Some species of Squid produce a propulsion 0.43 m/s, reaching a top speed of 0.529 m/s. In contrast, Aequorea strong enough to shoot them out of the water. Unlike the Octopus, these Victoria propels at an average speed of 0.20 m/s. As previously stated, none of the systems present endurance since their movement mainly

Fig. 17. Schematic of the Atlantic Bay Scallop, modiﬁed from (Scallop).

88 R. Salazar et al. Ocean Engineering 148 (2018) 75–114 depends on ocean current. Jellyfish are not efficient at maneuverability oval fins on the sides of the mantle that are only a portion of the mantel because of their body shape and structure. The Nomura's Jellyfish has the length. The Sepia Officinalis Cuttlefish has a fin that starts at the top and lowest directional control. runs the length of the mantel. The shape of this fin is a crescent that Mantel constriction is represented by the Blue-Ringed Octopus, almost parallels the mantel. There are many other Squids which have Octopus Vulgaris, and Enteroctopus Dofleini. These animals were chosen varying aspect ratios to those defined, but these should be considered the to create a range of sizes for this category. The Blue-ringed Octopus is three main shapes. among the smallest octopi with a length of 0.15 m and weight of 0.028 kg. The Octopus Vulgaris is the most common of its species since it 3. AUV systems and locomotion classification is found in most of the major oceans. This animal is also differentiable by its pimple-like structures covering its mantle. These structures help this Bioinspired systems follow the same classifications defined for the animal blend into its surroundings by breaking up its silhouette to look biological systems. Investigators try to mimic the mechanical movement more like coral structure. The Octopus Vulgaris weights 10 kg reaching a using materials they deem fit. Generally, there are two different types of maximum size of 1.50 m. The Enteroctopus Dofleini, best known as the materials used to accomplish movement. These materials are either rigid Giant Pacific Octopus, is the largest identified Octopus. It weighs or soft, where rigid means the components do not deform or change approximately 50 kg with a size of around 5 m. This animal was found to shape. Rigid materials have the benefit of using actuation mechanisms have an average speed of 0.305 m/s, reaching as high as 11.2 m/s during commonly found in robotics. Soft materials can flex and deform during burst swimming. Turning capabilities for these animals is classified as movement. Soft materials have the benefit using newer and more tight since they can easily change directions with their arms, but during experimental actuation mechanisms. Some mechanical systems have Jet Propulsion, these animals have lower directional control. Swimming skins which are applied over the actuation mechanism to improve endurance for these animals cannot be extreme when using Jet hydrodynamics. Propulsion. Shell compression locomotion is denoted by the Atlantic Bay Scallop fi which is one of the most common types of Scallop. The Atlantic Bay 3.1. AUV n oscillation Scallop was selected as the sole animal for this class as other Scallops fi have high similarities in size and structure. The Atlantic Bay Scallop Robots in the Fin Oscillation classi cation follow the sub-categories fi measures 0.089 m in length, with no information about its weight. These de ned for the biological systems. While there are a lot of robots in fi animals only use shell compression to escape from predators at a speed this category, it is important to de ne them in chronological order in around 0.8 m/s. It was denoted that the system's endurance was each category based off their actuation mechanism type. The mechanical extremely low because they only swim to a safer place a few meters away layout of the systems are described for each robot in Table 7. Then, the from their starting position. The turning of the Atlantic Bay Scallop is characteristics of each robot are reviewed and compared in the consoli- described to be extremely low. dation section. Lastly, the combination of mantel constriction and undulatory fin fi locomotion is represented by the Sepia Officinalis Cuttlefish, Watasenia 3.1.1. AUV caudal n fi fi Scintillans Squid, and Vampire Squid. The Sepia Officinalis is the largest The caudal n category has robots de ned in Anguilliform, Sub- of these biological systems with a length of approximate 0.325 m and carangiform, Carangiform, and Thunniform locomotion. The large fi mass of about 6.75 kg. The Vampire Squid weights approximately number of robots in this class is due to the assumption that the caudal n fi 0.450 kg and consists of a size reaching 0.30 m which is comparable to produces the best thrust for fast speed, and body types of shes found in the Sepia Officinalis' length. The Watasenia Scintillans is among the this category condone improved hydrodynamics. smallest identified squids with a mass of 0.0089 kg and 0.06 m in size. The Sepia Officinalis is able of reaching a top speed of 0.8 m/s, compared 3.1.1.1. Anguilliform. Early efforts in this category should be considered to the Vampire Squid's top speed of 0.9 m/s. Both systems consist of short in those by Ayers et al. (2000). creating a Lamprey like robot and Crespi swimming endurances where the Sepia Officinalis lasts less than a min- et al. (2004, 2005). who created the Amphibot. The Amphibot is a chain ute. Most swimming done by these animals is through the use of the module robot where each module is of rigid box design with an actuator undulation fin where Jet Propulsion is used to burst swim or aid in connection with its neighbor. These actuators only allow for one degree directional control. The fins of these animals vary where shape and of freedom. This allows for the body undulation to only occur in one length of the fin along the mantel depend on the species. The Watasenia plane. Stefanini et al. (2012). created the LAMPETRA which is a robot Scintillans Squid consists of a triangle-shaped undulatory fin covering that uses many segments to partake in the locomotion. This system has a most of the muscular mantle, compared to the Vampire Squid having two unique muscle-like contraction actuators between each thin section. The LAMPETRA has a more flexible body due to the smaller sections and

Table 5 Jet Propulsion, combination animal, and their characteristics including mass, length, average speed, and top speed.

Jet Propulsion Fish Mass (kg) Length (m) Average Speed (m/s) Top Speed (m/s)

Bell Constriction Aequorea Victoria N/A Max0.25 (Crystal jelly) 0.020 (Najem et al., N/A 2012) Aurelia Aurita 0.175 (Pyeon et al., 2015) 0.014 (Pyeon et al., 2015) N/A N/A Nomura's Jellyfish 204 (Simpson, 2009) 1.80 (Diameter) (Simpson, 0.430 (Lee et al., 2010) 0.529 (Lee et al., 2010) 2009) Mantel Constriction Blue-Ringed Octopus 0.028 (Blue-ringed 0.15 (Blue-ringed Octopuses) N/A N/A Octopuses) Octopus Vulgaris 10 (dsantos, 2012) Max1.50 (dsantos, 2012) N/A N/A Enteroctopus Dofleini 50 (Giant Pacific Octopus) 5(Giant Pacific Octopus) 0.305 (Win, 2012) 11.2 (Win, 2012) Shell Compression Atlantic Bay Scallop N/A 0.089 (Atlantic Bay Scallop) N/A 0.8 (Hill, 2005) Jet and Undulation Sepia Officinalis 6.75 (Cuttlefish) 0.325 (Cuttlefish) N/A 0.8 (Trueman and Packard, Fin Cuttlefish 1968) Watasenia Scintillans 0.0089 (Tsuji, 2002) 0.06 (Tsuji, 2002) N/A N/A Squid Vampire Squid 0.450 (Gannon, 2015) Max0.30 (Vampire Squid) N/A 0.9 (Vampire Squid)

89 R. Salazar et al. Ocean Engineering 148 (2018) 75–114 actuators. In more recent work by Crespi et al. (2013), the Amphibot was actuation mechanisms have an alternating pattern so the next one in line updated into the Salamandra Robotica II. This system modules or has a planar motion at a ninety-degree angle relative to the previous connections were not updated, but the system was given a caudal connection. Liljeback et al. (2014). managed to compact the actuation fin appendage and multiple pectoral fins to aid in control and mechanisms and connections to make a slimmer design using rigid locomotion. components, where each module has two degrees of freedom. This robot Considering robots with multi-degree of freedom, there are multiple is waterproof and is capable of appendage attachments to help with cases. Yu et al. (2009). created a large robot that had full range of alternative missions. movement with rigid tubular modules with actuation mechanism connections that have two degrees of freedom. The connection actuation 3.1.1.2. Subcarangiform & Carangiform. This class has a great variety of mechanism is covered in a flexible skin to waterproof the system. The AUVs to choose from. The reason the Subcarangiform and Carangiform modules were equipped with wheels mounted on rigid ribs along the category are combined in the bioinspired description is because there is module surface. These wheels give this robot the capability to move on no great way to discern the robot's variation in body undulation initia- land and also give the robot more area for thrust generation in water. tion. Therefore, the robots in this class are grouped together based off Rollinson et al. (2014). created a robot that had all rigid components and their actuation mechanism. First, the three-link systems are considered, actuation mechanism. This chain of modules has actuation mechanisms second are four-linked systems, then the multi-linked systems are where each coupling has only one degree of freedom. However, the consolidated, and lastly the outliers are described.

Table 6 Continuation of Jet Propulsion animal description with picture for reference. Deﬁned turning capabilities are based off our observation describing Extremely Tight (ET), Tight (T), Moderate (Mo), Medium (Me), Low (L), and Poor (P). All pictures are from open domain websites.

Jet Propulsion Animals Endurance Turning Picture (ET, T, Mo, Me, L, P)

Bell Constriction Aequorea Victoria N/A Mo, Me (Aequorea victoria, 2016) (Talpalariu,

2008) Aurelia Aurita N/A Mo, Me (Aurelia Aurita, 2011) (Moon

Jellyﬁsh, 2012) Nomura's Jellyﬁsh N/A P(Nemopilema nomurai, 2009) (Gadd, 2014)

Mantel Constriction Blue-Ringed Moderate speed visible in videos. T(Youtube, 2015a) Octopus Endurance cannot be extreme (Youtube, 2015a) (Blue-ringed

octopus) Octopus Vulgaris Fast speed visible in videos. Endurance cannot be T(Hanlon, 2012) extreme (Hanlon, 2012) (papicy, 2013)

Enteroctopus N/A T(Youtube, 2011) Doﬂeini (Poke, 2015)

Shell Compression Atlantic Bay Low speed visible in videos. P(Youtube, 2015b) Scallop Endurance is not extreme (Youtube, 2015b) (Scientist Lears

Populatio, 2016) Mantel Constriction and Sepia Officinalis Short endurance (<1 min) (Sepia officinalis, 2014)T,Mo(Sepia officinalis, Undulatory Fin Cuttlefish 2014) (Nordsieck)

Watasenia Slow speed visible in videos. Mo (DSCN0749, 2012) Scintillans Squid Endurance cannot be extreme (DSCN0749, 2012) (CBC)

Vampire Squid Short endurance (Vampire Squid)Mo(Youtube, 2012)

(National

Geographic, 2012)

90 R. Salazar et al. Ocean Engineering 148 (2018) 75–114

Table 7 AUVs with oscillating ﬁn description and picture for reference.

Fin Oscillation Robotic Systems Description Picture

Anguilliform Lamprey Robot Rigid head unit contains electronic components Five actuators in series gives a large body undulation Actuators encased in a ﬂexible skin for reduced drag and (Ayers et al., protection Tail equipped with buoyancy foam 2000) Amphibot Small rigid cubic links which are in series One plane actuation. Performs body undulation well in singular plane (Crespi et al., 2004) Wheels for land locomotion

LAMPETRA This is a multi-segment muscle-like actuation Small head unit with sensors Caudal ﬁn after ﬂexible body (Stefanini et al., Neuro-inspired control 2012) Salamandra Robotica II Continuation of Amphibot work Tail and arm appendages were added to increase mission (Crespi et al., possibilities Still only single planar motion 2013) Series Elastic Actuated (SEA) Links in series. Snake Two types of links; one for horizontal and one for vertical. Each segment does not move in all three directions. System does have 3D motion Links are large (Rollinson et al.,

2014) Amphibious Snake-like Robot Larger tubular segment modules Each segment has two actuators to realize 3D motion for each segment This system can make complex motions in all directions Wheels oriented around circumference for land locomotion (Yu et al., 2009)

Mamba Waterproof Snake Robot Small overlapping modules, one type allows overlapping chain Each segment has on actuator, but compaction of modules to realize 3D motion Has different capabilities for appendages to be added (Liljeback et al., Modules are waterproof

2014) Subcarangiform & G9 Fish (Three-link actuation) A staple Carangiform design; heavily referenced Carangiform Three-link-motor actuation mechanism On board sensors Large scales covering over tail section attached to body (Hu et al., 2006; Liu Fins are ﬂexible

and Hu, 2010) Four-joint Robotic Fish Four-joint actuation mechanism and a lunate tail Peduncle section covered by a ﬂexible membrane Body is a rigid encasement for electronics System performed object avoidance missions in a pool using (Yu et al., 2004) overhead camera and computer solutions signal

Four-link Robotic Fish Large On board sensors and control surfaces Pectoral Fin Control Surfaces Pectoral ﬁns (square surface area) can give backwards locomotion (Yu et al., 2014) First half of body is rigid Four-link peduncle has a covering that is streamline with the body Four-link Carangiform Fish Robot Four-linked actuation mechanism attached to small head unit Small spinal skeleton for actuation Tight skin over peduncle unit not streamline to body Should be noted that this system did have to perform swimming (Koca et al., 2016) missions

(continued on next page)

91 R. Salazar et al. Ocean Engineering 148 (2018) 75–114

Table 7 (continued )

Fin Oscillation Robotic Systems Description Picture

AmphiRobot-II (Four- link) Exhibits large body undulation >60% Flexibility is low and modules are large Uses a multilink caudal peduncle actuation (Yu et al., Uses control surfaces Wheels for land locomotion

2012) Essex MT1 Robotic Fish Prototype of Liu et al. (Yu et al., 2012) after previous Essex work Has functioning control surfaces for depth control Multi-linked peduncle (Liu et al., 2005)

Essex C-turn Robot Predecessor to MT1 Slightly larger peduncle unit than MT1 Head unit is smaller (Liu and Hu, 2005) Can perform sharp C-turning behavior

Carp Robot Component encasement that is covered in an entire body shell which is flexible Mimics the Carp shape Onboard sensors housed in a component acrylic encasement Flexible tail and pectoral fins Pectoral fins can rotate 180 deg

(Ichikizaki and

Yamamoto, 2007) iSplash-I Whole body incorporates multiple links Allows for head side to side movement This is one of the ﬁrst prototypes of this design Components are made of rigid materials (Clapham and

Hu, 2014) Wire-driven Shark Robot Body undulation too large to be considered Thunniform Simple cable-actuation mechanism The control surfaces are semi-ﬁxed (Lau et al., 2015) Peduncle does not have streamline skin

Hydraulic Soft Robotic Fish Hydraulic peduncle actuation, soft peduncle and tail design Functioning control surfaces Rigid body (Katzshmann et al., 2016)

Thunniform MIT RoboTuna A test platform in which a multi-part pulley gives actuation for tuna movement The RoboTuna was ﬁxed within a water channel One of the ﬁrst to test this kind of movement to study (Tolkoff, 1999) hydrodynamic effects

Mackerel Robot Four-linked design used to study hydrodynamics Belt translation of motion down the body and skeleton Transmission shafts excite belts through vertical strut Body is mimetic of the Mackerel Flexible streamline skin

(Wen et al., 2013)

IPMC Tail Tuna IPMC peduncle attached to a rigid body The pectoral ﬁns are also rigidly ﬁxed This was tested using different aspect ratios, lengths, and thicknesses for the caudal peduncle and tail (Chen et al.,

2010) Miniature Robotic Fish Very small but functional design Rigid body and peduncle/tail sections with single actuation mechanism (Marras and Was outfitted with different caudal fin attachments to study the effects on swimming Decent turning ability and no pitch control surfaces Porfiri, 2012; Kopman and Porfiri, 2013) (continued on next page)

92 R. Salazar et al. Ocean Engineering 148 (2018) 75–114

Table 7 (continued )

Fin Oscillation Robotic Systems Description Picture

Single-Motor-Actuated Robotic This a simple actuation mechanism, two joints and a crank arm Fish Joint is at 2/3 the length of their body Skin is tight around body for waterproof seal (Yu et al., 2016a)

Multi-link Robotic Dolphin Multi-link design Movement restricted to last quarter of the body ﬁ Pectoral n control surfaces have 3 DOF (Shen et al., Rigid main body Flexible body shell

2011) Slider-crank Robotic Dolphin Novel slider crank design Body is modeled after the dolphin Pitch controlled by inner mechanism, pectoral ﬁns are rigid A single yaw directional control unit at base of peduncle that has a ﬂexible covering Head unit is rigid and houses components (Yu and Wei,

2013) Gliding Robotic Dolphin Three peduncle joints Working pectoral ﬁn control surfaces Gliding implemented for increased endurance Has rigid body compartment that holds the electronic (Wu et al., 2015) components

Dolphin Robot Capable of Leaping This design incorporates dolphin head movement to realize greater tail thrust Control surface pectoral ﬁns and dorsal ﬁn is rigid Has sensors for pitch, yaw, and roll Powerful DC motors contained in the main body and peduncle unit

(Yu et al., 2016b)

Vorticity Control Unmanned A hydraulic robotic tuna Undersea Vehicle (VCUUV) Main body is rigid Peduncle section is comprised of different rigid sections to give (Anderson and flexibility Heavy, multiple batteries, and large hydraulic actuator Pectoral fins are rigid and only rotate for pitch control Chhabra, 2002) Labriform Wrasse Robot This design has a large prismatic body Pectoral fins are large, and are capable of rowing and flapping Sensory equipment inside body (Sitorus et al., 2009)

Flexible Pectoral Fin Joint Emphasis put on the motion of the pectoral fins Labriform Robot Flexible pectoral fins used Caudal fin still performs some thrust motion Rigid body (Behbahani and Tan, 2016)

Pectoral Fin and Dual Caudal Fin This design uses pectoral ﬁns for stability and thrust Robot Dual caudal ﬁns increase yaw stability (mimics Jet Propulsion) Waterproof rigid body (Zhang et al., 2016)

Ostraciiform BoxyBot Body has two compartments for the different actuators for pectoral and caudal ﬁns All solid plastic components (Lachat et al., 2006)

Microautonomous Robotic Pectoral fins have 3 DOF Ostraciiform (MARCO) Caudal fin has one DOF Body is highly inspired from the Boxfish (continued on next page)

93 R. Salazar et al. Ocean Engineering 148 (2018) 75–114

Table 7 (continued )

Fin Oscillation Robotic Systems Description Picture

Fins are ﬂexible

(Kodati et al., 2008)

Ostraciiform Fish Robot A larger version of the Boxﬁsh-like Robot Same rigid body Earlier prototype of the Boxﬁsh-like Robot (Wang et al., 2013)

Boxfish-like Robot Rigid body for housing components Pectoral fins produce stabilizing and thrust flapping Caudal fin for thrust (Wang and Xie, Fins are rigid

2014) Boxfish Robot Completely rigid component design Found the optimal shape of pectoral fins for this design Pectoral fins flap for stability and thrust (Mainong et al., 2017) Body is multi-compartmental, for equipment

Considering the three-link actuation robots leads us to one of the most undulation mimicry. The body to peduncle was constructed using joints heavily referenced units created by Hu et al. (2006). This robot has a along the length allowing for head movement in the swimming. This rigid body unit that houses components. The robot was given a produced better flexion and thrust generated. All components for this semi-neutral buoyancy with the addition of foam inserts along the body design are rigid, but the actuation mechanism allows for components to and caudal peduncle. The body and peduncle are covered in a skin slide past each other allowing for compaction of the design. The iSplash composed of rigid scales that overlap each other, scales move freely past has a semi-constant width of body along its length, with a slight taper each other. Fins and tail are composed of flexible but strong material that near the nose and tail. allows for passive flexion of the fins. There are a few examples for the outliers in the Carangiform class. A Yu et al. (2004). robot considered the four-link caudal peduncle as- wire driven shark was constructed by Lau et al. (2015). where a sembly and a rigid body. The peduncle assembly was covered with a skin multi-segmented tail allows for a good peduncle flexion. This wire driven to waterproof the assembly. The tail is lunate to mimic those of biological peduncle used four wires and two servo motors (two wires per servo systems, but the tail is a rigid piece of composite. Yu et al. (2012). made a motor/upper and lower pair in the peduncle section). The rigid body larger AUV that was primarily composed of metal components. The housed the electrical components and the servomotors. Due to the use of different sections are mostly cubic in shape. This system was also meant two servo motors, the tail can have an angle of attack relative to the to transition from water to land by using wheels attached to the body. neutral vertical axis, which Lau et al. (2015). described as a sway Large rectangular pectoral fins are used as control surfaces. Yu et al. swimming motion. A hydraulic actuated peduncle was created by (2014). AUV had rigid body and a caudal peduncle mechanism that is Katzshmann et al. (2016). where the peduncle is made of a soft material. covered with a skin. The pectoral fins were rigid with a square area, but The hydraulic actuation mechanism that makes the peduncle flex is these fins are capable of 360-degree rotation. This allows for the pectoral through filling of different voids in the peduncle. Body and fins are fins to be altered to give this AUV backward movement. Koca et al. constructed of rigid materials, but the pectoral fins are functioning (2016). created a robot that had a small rigid body where the caudal control surfaces. peduncle actuation unit is a majority of the body length. This robot has fixed pectoral fins and a large rigid tail. The skin over the peduncle was 3.1.1.3. Thunniform. Comparable with the Subcarangiform and Car- tight were the structure of the actuation mechanism is visible. angiform class, the Thunniform robots also use a peduncle actuation unit. In 2005, multiple robots were created through the work of Lui et al However, to achieve a more concentrated tail actuation, different actu- (Liu et al., 2005; Liu and Hu, 2005). where focus was put on the complex ation mechanisms are exploited. To organize the robots in this category, turning capabilities of the robot. They created the MT1 (Liu et al., 2005) the style of the actuation mechanism is used, where single link, multi- which was a predecessor to the C-turn robot (Liu and Hu, 2005). The linked, and other various robots are described. actuation mechanism for these are multi-linked peduncle units where Firstly, one of the initial robots that was created to analyze the effects rigid components were used. Information on the construction of these of hydrodynamics of a moving body mimicking a Tuna was the RoboT- robots is limited, but the C-turn robot had a small head unit and large una created by MIT (Tolkoff, 1999). Similarly, a robot mimicking a peduncle section where the peduncle section was covered by a Mackerel was created by Wen et al. (2013). Although this system is a multi-sectioned skin. Ichikizaki et al (Ichikizaki and Yamamoto, 2007). four-linked actuation, the testing was similar to the RoboTuna. These two created a Carp inspired robot where the robot structure was contained robots were equipped with a flexible streamline skin. These robots were within a mimetic body shell. Electrical components are contained in a fixed to a strut that translates motions into belts housed in the mechanical rigid acrylic tube while the peduncle is also a multi-segmented unit. The skeleton which then force a body motion. Single actuators are the peduncle unit has a tail servo motor that has two bending joints to cause a simplest of all the considered designs. These designs were constructed flexible tail fin to bend. The fins of this robot are flexible elastomers but using both soft and rigid materials. are still semi-rigid. Clapham et al (Clapham and Hu, 2014). created a Considering the single actuators, Chen et al. (2010). created an IPMC unique prototype design (iSplash) that showed promise in body peduncle driven robot where the body and pectoral fins were rigid

94 R. Salazar et al. Ocean Engineering 148 (2018) 75–114 structures with no complacent movement. The main objective of Chen relatively rigid, but can perform both the rowing and flapping motions et al. (2010). was to study the effects of thrust using different tail aspect reminiscent of the Wrasse. In efforts by Behbahani et al. (Behbahani and ratios considering the same IPMC actuation mechanism. A miniature Tan, 2016), a Labriform swimming robot was created that had flexible robotic fish was created by Marras et al (Marras and Porfiri, 2012). where pectoral fins which could perform both rowing and flapping motions. The the body and peduncle could be considered as two separate units caudal fin is made from the same material as the pectoral fins and could controlled by a single motor and joint and the tail was a flexible caudal be activated when more forward thrust is needed. The body is rigid. fin. Similarly, this robot was used to investigate the impacts of varying Zhang et al. (2016). created a cross over robot where the robot uses the tail aspect ratios and shapes. A relatively simple pectoral fins and a dual caudal fin for swimming. The pectoral fins have 3 single-motor-actuated robotic fish was created by Yu et al. (2016a).In DOF while the dual caudal fin have 1 DOF where they compress the water their design, the motor gives motion to an eccentric wheel that drives a when their stroke comes together. Stability is highly maintained when connecting rod, giving motion to the peduncle unit which is composed of using the dual caudal fin. The body is a rigid encasement for components rigid components. The body is of uniform width with a tapered head unit. and the fins are also rigid. The final design has a protective skin covering body unit. This design has no control surfaces but was capable of performing C-turns and S-turns. 3.1.3. AUV oscillating dorsal and anal fin: Tetraodontiform A multi-linked robotic dolphin was created by Shen et al. (2011). This To the authors’ knowledge, no robotic systems have been validated to dolphin robot has a peduncle unit comprised of three links that allow for the point where they can be considered in this review. The oscillation of vertical flexion and a fourth for a smaller horizontal flexion so the tail has dorsal and anal fin would require a unique understanding of the swim- three directional movement. The pectoral fin control surfaces also had ming characteristics of the Ocean Sunfish. This would not be an three degree of freedom movement. Peduncle is encased in a rubber extremely swift design if it was to be exploited. tubing to protect it and make it waterproof. The entire body was covered in flexible mimetic body shell. In efforts by Yu et al. (Yu and Wei, 2013), a 3.1.4. AUV pectoral, dorsal/anal, and caudal fin: Ostraciiform slider-crank robotic dolphin was created that gave actuation to two The robots that fill in this category try to mimic the Boxfish or similar vertical pitch units and one yaw unit. This realizes a tail flexion with fishes. There is not much variance in the designs found where these de- three degrees of freedom and the pectoral fins are fixed surfaces. Pitch signs consider pectoral and caudal fin in the locomotion. It may be controlled using ballast sliders in the rigid body. Peduncle unit is covered recalled that the true Boxfish uses its dorsal and anal fin during its in a flexible shell to make it streamline with the body. Through efforts to swimming modes for stability and added thrust. The Ostraciiform robots increase the endurance, a gliding mode was conceived for a robotic are presented in a chronological order. First, the BoxyBot created by design by Wu et al. (2015). This design incorporated a single joint for Lachat et al. (2006). is a rigid component based robot where the body movement of the peduncle with another joint for movement of the caudal was separated into two sections, one for pectoral fin and the other for fin. Yaw and pitch movements are controlled through use of the pectoral caudal fin. The caudal fin section gives actuation to the fin but only in a fins and a buoyancy bladder which can be emptied and filled with water. one degree of freedom. The pectoral fins are capable of 360 rotation but Body and dorsal fin are rigid. The body houses all the equipment primarily performs flapping for thrust. Kodati et al. (2008). created a including the buoyancy bladder. Yu et al. (2016b). created an impressive robot in which they dubbed as Microautonomous Robotic Ostraciiform robot that was capable of fast speed and leaping out of the water. Yu et al. (MARCO). MARCO's body was a 3D printed replica of a boxfish where the (2016b). design is comparable with the one by Wu et al. (2015), but some materials used were rigid. Pectoral fins are capable of 2 DOF motion. alterations make this an intriguing design. The robot has a rigid body unit Wang and his consultants (Wang et al., 2013; Wang and Xie, 2014) that houses components, however, the head is separate unit capable of considered the same design. The newer design in 2014 was slightly vertical movement by using a neck joint. There is a joint connection for smaller, but had the same capabilities. The body is a rigid unit which is the body-peduncle and peduncle-tail. These two separate connections are not mimetic like the MARCO. The pectoral and caudal fin are rigid. The each given their own DC motor, respectively. Pectoral fins are capable of caudal fin has a single degree of freedom and the pectoral fins can rotate pitch control. Skeletal structure of the robot that holds components is 360 but also primarily used for flapping. Mainong et al. (2017). used rigid while this is encased in a flexible shell. It should be stated that this their design to invest different aspect ratios and shapes for the pectoral robotic dolphin is capable gaining enough speed to break the water's fins. This design is all rigid components, caudal fin has 1 DOF, pectoral surface with its whole-body length. fins have 360 rotation, and the body is mimetic of the Boxfish. The last Thunniform AUV that is defined is the large Vorticity Control Unmanned Undersea Vehicle (VCUUV) created by Anderson et al 3.2. Consolidation of fin oscillation AUV characteristics (Anderson and Chhabra, 2002). at the Draper Laboratory. This design was inspired from the RoboTuna. This robot was capable of extremely The AUVs that utilize Fin Oscillation make up the largest classifica- fast swimming; however, this is an extremely heavy hydraulic design. tion much like the biological animals investigated. The size, shapes, and The body is a rigid housing for the multiple batteries and other compo- capabilities do vary greatly for the AUVs in this class. Earlier AUV Fin nents while the peduncle design has a driven link chain which is encased Oscillation sections displayed shape and structure using the actuation in a rigid exostructure that folds over itself during flexion. This mechanism to organize the systems within each category. However, there exoskeleton allows for the permeation of water between each section. are systems in each class which should be recognized more so by their However, the body is waterproof to protect components. This design has defining size and performance characteristics. a main ballast to ensure the design does not roll. Pectoral fins are func- The Anguilliform AUVs have a smaller range of sizes in comparison tioning control surfaces for depth and directional control. with the Carangiform or Thunniform categories. For the found total mass of these systems, Liljeback et al. (2014). is the lightest design at 0.31 kg 3.1.2. AUV pectoral fin: Labriform while Rollinson et al. (2014). and Yu et al. (2009). are 3.657 kg and There are few found robots in this class as it is difficult to create a 6.75 kg, respectively. Yu et al. (2009). is by far the heaviest design stable robot that solely uses Fin Oscillation. The robots in this category investigated. This is due to its large module design. Anguilliform AUVs are organized as follows: pure pectoral swimmer, pectoral fin flexible fin generally follow a trend of length sizes of around 1 m with Stefanini et al. joint with caudal fin assist, and a pectoral fin/dual caudal fin hybrid. The (2012). coming closest to this at 0.99 m. Yu et al. (2009), Rollinson et al. first robot considered is a Wrasse robot created by Sitorus et al. (2009). (2014), and Liljeback et al. (2014). are all systems between 1 and 1.2 m. This design has a body with dimensions closer to a rigid cubic structure The fastest forward speed is held by Yu et al. (2009). at 0.07 m/s, but than those of the slender Wrasse. However, this body was used to house Liljeback et al. (2014). has a high joint speed which would suggest that it the considered components relatively well. The pectoral fins are possesses moderate speed capabilities. Crespi et al. (2005). has a speed

95 R. Salazar et al. Ocean Engineering 148 (2018) 75–114 which was half that of Yu et al. (2009). Yu et al. (2009). and Liljeback The Thunniform category has the largest AUVs considered where et al. (2014). are the only two systems which have depth control as they Anderson et al (Anderson and Chhabra, 2002). is the heaviest out of all have three-directional control with their head unit. Stefanini et al. AUV categories at 173.1 kg. The second largest being Shen et al. (2011). (2012). has the best endurance at 5 h while Crespi et al. (2005). has a at 23.5 kg with Wu et al. (2015). following closer behind at 18.2 kg. The defined endurance time of 2 h. lengths of these AUVs fall in the same order where Anderson et al In the Subcarangiform and Carangiform category, there is a great (Anderson and Chhabra, 2002). has the longest length of 2.4 m. The variety of systems to choose from. As for the size, the lightest design is Yu smallest Thunniform AUV is Marras et al (Marras and Porfiri, 2012). at et al. (2004). at 0.5 kg and the largest is Ichikizaki et al (Ichikizaki and 0.07 kg and 0.117 m. All other systems fall between the ranges defined by Yamamoto, 2007). at 12 kg. Yu et al. (2014). at 5.2 kg and Yu et al. these systems. The fastest AUV is smaller at 4.7 kg and 0.72 m, but Yu (2012). at 5 kg, round out the mid-range weights with all other systems et al. (2016b). design is capable of 2.1 m/s swimming which allows it to being below these values in a range (1.65–3.67 kg). The system with the leap out of the water. Even though Anderson et al (Anderson and smallest length is Clapham et al (Clapham and Hu, 2014). at 0.251 m and Chhabra, 2002). design is large, it can swim at 1.2 m/s. The turning the largest being Ichikizaki et al (Ichikizaki and Yamamoto, 2007). at capabilities of these systems are still decent. However, compared to the 0.9 m. The rest of these systems range from lengths of 0.4–0.8 m. Clap- Subcrangiform and Carangiform AUVs, these systems have lower body ham et al (Clapham and Hu, 2014). has the fastest speed of 0.85 m/s and flexibility, and hence lower turning capabilities. Depth control was also Yu et al. (2014). is the second fastest in this category at 0.7 m/s. All other not found for most of these systems, but the best performing systems systems’ speeds are higher than 0.1 m/s but are below these maximum often do have good depth control. Found endurance for these systems values. Due to the use of control surfaces, these systems have good also ranges from 1 to 3.5 h. The size of these systems allows for large directional and depth control. Endurance ranges from Katzshmann et al. battery packs equipped to power larger motors. (2016). at 35 min to Lui et al (Liu et al., 2005). at 4.5 h. The Labriform AUVs have a range of mass of 0.33–2.5 m with the two

Table 8 Characteristics of AUVs with oscillating ﬁn including mass, length, and speed.

Fin Oscillation Robotic Systems Mass (kg) Length (m) Speed (m/s)

Anguilliform Lamprey Robot N/A N/A N/A Amphibot (Crespi et al., 2005) N/A One Section (L*W*H) ¼ 0.035 (0.07*0.055*0.033) (Crespi et al., 2004) LAMPETRA (Stefanini et al., 2012) N/A 0.99 0.3 Salamandra Robotica II (Crespi et al., 2013) N/A One Section (L*W*H) ¼ N/A (0.07*0.055*0.033) Series Elastic Actuated (SEA) Snake (Rollinson et al., 2014) Per Per Module ¼ 0.064 N/A Module ¼ 0.205 Total Module Chain ¼ 1.174 Total 16 Module ¼ 3.657 Amphibious Snake-like Robot (Yu et al., 2009) 6.75 (9 Modules) ¼ 1.17 0.07 Mamba Waterproof Snake Robot (Liljeback et al., 2014) 0.31 One Section ¼ 0.089 Joint Speed¼(429 deg/sec) Total1.1 Subcarangifrom & G9 Fish (Three-link actuation) (Liu and Hu, 2010) N/A 0.52 0.6 (Hu et al., 2006; Liu and Carangifrom Hu, 2010) Four-link Robotic Fish (Yu et al., 2004) 0.5 0.4 0.32 Four-link Robotic Fish Large Pectoral Fin Control Surfaces (Yu 5.2 0.68 0.71 et al., 2014) Four-link Carangiform Fish Robot (Koca et al., 2016) N/A 0.52 0.2 AmphiRobot-II (Four-link) (Yu et al., 2012) 5 0.7 0.45 Essex MT1 Robotic Fish (Liu et al., 2005) 3.55 0.48 0.4 Essex C-turn Robot (Liu and Hu, 2005) N/A 0.8 N/A Carp Robot (Ichikizaki and Yamamoto, 2007) 12 0.9 0.41 iSplash-I (Clapham and Hu, 2014) 3.67 0.251 0.85 Wire-driven Shark Robot (Lau et al., 2015) 1.79 0.6 0.39 Hydraulic Soft Robotic Fish (Katzshmann et al., 2016) 1.65 0.45 0.1 Thunniform MIT RoboTuna N/A N/A N/A Mackerel Robot (Wen et al., 2013) N/A 0.588 0.3 IPMC Tail Tuna (Chen et al., 2010) 0.29 0.223 0.02 Miniature Robotic Fish (Kopman and Porfiri, 2013; Marras and 0.07 0.117 (without caudal fin) 0.01 Porfiri, 2012) Single-Motor-Actuated Robotic Fish (Yu et al., 2016a) N/A 0.37 1.14 Multi-link Robotic Dolphin (Shen et al., 2011) 23.5 1.2 N/A Slider-crank Robotic Dolphin (Yu and Wei, 2013) 5.2 0.75 0.78 Gliding Robotic Dolphin (Wu et al., 2015) 18.2 1.125 Gliding ¼ 0.155 Dolphin Robot Capable of Leaping (Yu et al., 2016b) 4.7 0.72 2.1 Vorticity Control Unmanned Undersea Vehicle (VCUUV) 173.1 2.4 1.2 (Anderson and Chhabra, 2002) Labriform Wrasse Robot (Sitorus et al., 2009) 2.5 0.375 0.0351 Flexible Pectoral Fin Joint Labriform Robot (Behbahani and Tan, 0.33 N/A 0.53 2016) Pectoral Fin and Dual Caudal Fin Robot (Zhang et al., 2016) 1.3 0.44 0.53 Ostraciiform BoxyBot (Lachat et al., 2006) N/A 0.25 0.37 Microautonomous Robotic Ostraciiform (MARCO) (Kodati et al., 0.49 0.15 0.0411 2008) Ostraciiform Fish Robot (Wang et al., 2013) N/A 0.35 0.38 Boxfish-like Robot (Wang and Xie, 2014) 1.4 0.33 0.347 Boxfish Robot (Mainong et al., 2017) N/A 0.165 0.106

96 R. Salazar et al. Ocean Engineering 148 (2018) 75–114 found lengths having less than 0.1 m of variance. The speed of Behbahani (passive fin) and multi-ribbed (active excitation). With the leading-edge et al (Behbahani and Tan, 2016). and Zhang et al. (2016). are the same at rib, an upstroke and downstroke are performed by means of an actuation 0.53 m/s, regardless of their designs being completely different. The unit. The stroke of the rib causes a translational wave along the mem- turning and depth control are the best for Zhang et al. (2016). brane surface, thereby causing thrust. The multi-ribbed actuation causes The variance of characteristics in the Osctraciiform AUV category is active excitation along the fin length through all the ribs engrained in the small so just an overview of the systems is stated while Tables 8 and 9 can fin. Each rib supersedes the motion of the rib before causing the trans- be referenced for specific numbers. These AUVs have few descriptions for lational wave across the fin. mass but the smallest design is Kodati et al. (2008). and the largest one is The first robot to be considered is a robotic manta ray. Gao et al. Wang et al. (Wang and Xie, 2014). In respect to length, the smallest is also (2007). created a design where a leading-edge fin rib caused passive Kodati et al. (2008). and the largest is Wang et al. (2013). Kodati et al. undulation to a membrane supported by a second rib running parallel to (2008). is the slowest Ostraciiform AUV investigated while Wang et al. the body. The body is a rigid unit with fixed control surfaces as horizontal (2013). is the fastest. Generally, these AUVs have moderate turning ca- and vertical tails. This robot was then upgraded into the Robo-Ray III pabilities. However, Wang et al (Wang and Xie, 2014). was described to created by Niu et al. (2012). where the fixed control surfaces were have depth control which makes it the only one in this category to have replaced with functioning ones that increased stability and depth control. this capability while this same AUV has the only defined endurance of A smaller leading-edge design was developed by Wang et al. (2009). around 1.5 h. except the leading edge used shape memory alloy (SMA) wires that overlaid an elastic substrate on two sides and are integrated into a polymer matrix. Having two SMA wires gives the capability for the finto 3.3. AUV fin undulation have upstroke and downstroke by alternative heating of the SMA wires through electrical resistance of the wire. This design floated on the sur- For the Fin Undulation class, it is more common to have soft materials face of the water with a buoyant rigid body. Turns can be achieved by which are more flexible due to the flexion which are required for thrust. selecting which fin to excite. An encased rigid skeleton design is a The categories for AUVs follow those defined previously in the review pneumatic fin flexion ray created by Cai et al. (2009). This design was and their description and pictures are given in Table 10. control-tethered in order to supply power to actuators that pull on the cables that cause the rigid skeleton to morph for an upstroke. The skel- 3.3.1. AUV undulatory pectoral fin: Rajiform eton was encased in a mimetic body resembling the Manta Ray. A soft The pectoral fin class is the second largest category to the caudal fin material leading-edge design is an IPMC actuated robot created by Chen fishes. Lots of different shapes, actuation mechanisms, and materials et al. (2012). The IPMC comprised most of the first half of the elastomer have been utilized in this class. For most of the robots considered, efforts membrane fin. The body is a rigid box for components. A similar design were made to mimic the same rib-membrane excitation found in bio- to the leading edge is a soft body robot created by Alvarado et al. (2013). logical systems. The order of the mentioned systems are leading edge rib

Table 9 Characteristics of AUVs of oscillating ﬁn continued. *System speciﬁes that it has depth control but does not disclose to what degree this system can perform.

Fin Oscillation Robotic Systems Turning Description (ET, T, Mo, Me, L, P) Depth Control (G, Me, P) Endurance

Anguilliform Lamprey Robot N/A N/A N/A Amphibot (Crespi et al., 2005) Me, L N/A 2 h LAMPETRA (Stefanini et al., 2012) T, Mo N/A 5 h Salamandra Robotica II Me, L N/A N/A Series Elastic Actuated (SEA) Snake Me, L N/A N/A Amphibious Snake-like Robot ET G N/A Mamba Waterproof Snake Robot ET G N/A Subcarangiform & Carangiform G9 Fish (Three-link actuation) T, Mo N/A N/A Four-link Robotic Fish T N/A N/A Four-link Robotic Fish Large Pectoral Fin Control Surfaces T N/A* N/A Four-link Carangiform Fish Robot T, Mo G N/A AmphiRobot-II (Four-link) (Yu et al., 2012) Me, L N/A 1.5 h Essex MT1 Robotic Fish (Liu et al., 2005) N/A N/A 4.5 h Essex C-turn Robot T, Mo G N/A Carp Robot (Ichikizaki and Yamamoto, 2007) Mo, Me G 1 h iSplash-I N/A N/A N/A Wire-driven Shark Robot N/A Me, P N/A Hydraulic Soft Robotic Fish (Katzshmann et al., 2016) Mo G 35 min Thunniform MIT RoboTuna N/A N/A N/A Mackerel Robot N/A N/A N/A IPMC Tail Tuna N/A N/A N/A Miniature Robotic Fish (Yu et al., 2016a) T, Mo N/A 1 h Single-Motor-Actuated Robotic Fish Mo N/A N/A Multi-link Robotic Dolphin (Shen et al., 2011) Mo N/A >2h Slider-crank Robotic Dolphin Mo N/A N/A Gliding Robotic Dolphin Mo G N/A Dolphin Robot Capable of Leaping (Yu et al., 2016b) T, Mo G 3.5 h Vorticity Control Unmanned Undersea Vehicle (VCUUV) ET Me, P N/A Labriform Wrasse Robot N/A N/A N/A Flexible Pectoral Fin Joint Labriform Robot Me, L N/A N/A Pectoral Fin and Dual Caudal Fin Robot ET G N/A Ostraciiform BoxyBot T, Mo N/A N/A Microautonomous Robotic Ostraciiform (MARCO) P N/A N/A Ostraciiform Fish Robot Mo N/A N/A Boxﬁsh-like Robot (Wang and Xie, 2014) T, Mo Me 1.5 h Boxﬁsh Robot Me, P N/A N/A

97 R. Salazar et al. Ocean Engineering 148 (2018) 75–114

Table 10 AUVs with Undulatory ﬁn description and picture for reference.

Fin Robotic Systems Description Picture Undulation

Rajiform Manta Ray Robot A predecessor to the Robo-Ray III Larger body Does not have control alternative control surfaces Has the same rigid leading edge and support rib for (Gao et al., 2007) the ﬂexible ﬁn membrane

Robo-Ray III Third generation system design Thinner body equipped with control flaps for stability and depth control Rigid leading edge of fin, along with a single support rib along length of body Membrane of the finisflexible silicon rubber (Niu et al., 2012) Leading edge has one degree of freedom actuation

Micro Manta Ray (SMA) Leading edge is comprised of two SMA wires separated by an elastic substrate and covered in a flexible skin Fin is flexible for passive membrane response (Wang et al., 2009) Body is rigid and the system is given a neutral buoyancy to float

IPMC Manta Ray The leading edge of the ﬂexible elastomer ﬁns are constructed of 40% IPMC Rigid component housing (Chen et al., 2012)

Soft Body Single-Dual Actuator Ray Soft body with a single actuator in the nose portion of the ﬁn Two prototypes with various polymer composite layout (Alvarado et al., 2013) Rigid body encased in the soft body polymer Three buoyancy tanks in body

Flexible Pectoral Foil Cownose Ray A soft body design which has a flexible exterior shell containing pneumatic muscles and skeleton Still has a control tether Fin flexion is a half upstroke and downstroke and does (Cai et al., 2009) not pass the fin neutral axis Tested in a tank in a fixed position to test hydrodynamics Bionic Fin Manta Ray Rigid leading edge of the fin leads to a more flexible free moving trailing edge Servo actuation given to the leading edge Body is rigid and houses the equipment (Chew et al., 2015)

Cownose Ray-I Ribs in a ﬂexible membrane are given actuation, timing must be precise to perform an optimal wave Body is semi-ﬂat and has multiple compartments for components (Yang et al., 2009) This system can perform a null speed pivot turn

RoMan-II Multiple ﬁn ribs give actuation to a ﬂexible membrane Body is rigid with the action motors along the side of the body (Zhou and Low, 2012)

RoMan-III Another generation of the RoMan that uses a very similar system design except length and weight are decreased System can perform null speed pivot turning (Low et al., 2011)

(continued on next page)

98 R. Salazar et al. Ocean Engineering 148 (2018) 75–114

Table 10 (continued )

Fin Robotic Systems Description Picture Undulation

IPMC Chain Ribbed Ray Chain of IPMC which excite a membrane Each arm must be given actuation at the correct time to produce a wave suitable for efﬁcient thrust This robot was tested as a surface ﬂoat system (Punning et al., 2004)

Multi-Ribbed IPMC This is a small robot with thin IPMC fin ribs with a flexible fin membrane The body is a rigid encasement Efforts made to get the IPMC ribs to perform an (Takagi et al., 2006) optimized wave

RayBot (Electric Ray Caudal Fin Propulsion Even though this robot uses a caudal fin for Robot) propulsion, it was made with the enlarged pectoral (Krishnamurthy fins of a Rajiform The body is made of soft material, covers a rigid components case and peduncle actuation unit et al., 2010) Peduncle actuation unit is fairly compact Body is flat and caudal fin is not extremely large Soft-Robotic Ray þ Tissue-engineered A micro robotic ray (1/10 scale) Soft elastomer matrix encasing a golden skeleton gives excitation to a real muscle layer through optical excitation (Park et al., 2016) Ribs of the skeleton are differing in length to give the pectoral fins a dissipating wave amplitude along the length of the fin Amiiform RoboGrilos A thin rigid body houses a simple chain of servomotors giving actuation to ribs in a flexible membrane (Hu et al., 2009)

Dorsal Undulation Fin Robot Multi-ribbed actuation dorsal ﬁn Torpedo body Rigid criss-cross tail has actuation for directional control (Xie et al., 2016)

Gymnotiform Knifeﬁsh Robot A multi-ribbed crank actuation Multiple servo motors to change angle of ﬁn Buoyancy tank (Siahmansouri et al., 2011)

Robotic Knifefish Multi-ribbed (>20) actuation with flexible fin Body is rigid and no control surfaces (Curet et al., 2011)

Anal Undulating Fin with Assisted Caudal A slider crank design gives actuation to a passive ﬁn Fin membrane wave translation Uses a caudal ﬁn for propulsion Rigid tubular body (Liu et al., 2012)

This design has a body composed of soft materials which is greater than passive undulation waves from this motion. The body is a rigid encase- 70% of all materials used. Each side of the disc body/fin has a single ment for components but has a semi-tapered design. actuator at an angle where this actuator gives the fin vertical excitation The actively excited multi-ribbed Rajiform category has a few func- for a passive translational wave. There are solid body components tioning robots. Often, the benefit of this type of actuation is that the robot encased within the soft body which act as buoyancy tanks to help control can perform a null speed turn where the robot can rotate in a stationary depth. The last leading edge design is a bionic fin manta ray created by position. The first robot to be considered using this actuation is the Chew et al. (2015). This design gives flapping actuation to a rigid leading Cownose Ray-I by Yang et al. (2009). In this design, an actuation skeleton edge in the pectoral fin. However, each fin is given an angle of attack excites multiple ribs in a flexible membrane. This skeleton is not encased during upstroke and downstroke. The trailing edge for the fin receives in a body. RoMan-II described by Zhou et al (Zhou and Low, 2012). was a

99 R. Salazar et al. Ocean Engineering 148 (2018) 75–114

Table 11 Characteristics of AUVs with undulation ﬁn including mass, length, and speed.

Fin Undulation Robotic Systems Mass (kg) Length (m) Speed (m/s)

Rajiform Manta Ray Robot (Gao et al., 2007) 3.4 (L*Wing span)¼(0.5*0.6) 0.7 Robo-Ray III (Niu et al., 2012) 3.4 (L*H*W)¼(0.56*0.12*0.643) 0.32 Micro Manta Ray (SMA) (Wang et al., 2009) .354 (L*W_span)¼(0.133*0.22) 0.057 IPMC Manta Ray (Chen et al., 2012) 0.055 0.11 0.007 Soft Body Single-Dual Actuator Ray (Alvarado et al., 2013) 0.486 0.225 0.08 Flexible Pectoral Foil Cownose Ray (Cai et al., 2009) N/A (L*W_span)¼(0.33*0.55) 0.2 Bionic Fin Manta Ray (Chew et al., 2015) 0.7677 (L*W_span)¼(0.28*0.58) 0.45 Cownose Ray-I (Yang et al., 2009) 1 (L*W_span)¼(0.3*0.5) 0.15 RoMan-II (Zhou and Low, 2012) 7.3 1 0.4 RoMan-III (Low et al., 2011) 5 0.88 0.3 IPMC Chain Ribbed Ray (Punning et al., 2004) 0.066 N/A 0.000005 Multi-Ribbed IPMC (Takagi et al., 2006) 0.315 N/A 0.015 RayBot (Electric Ray Caudal Fin Propulsion Robot) (Krishnamurthy et al., 2010) 5.5 0.75 0.1 Soft-Robotic Ray þ Tissue-engineered N/A Diameter ¼ 0.01 0.001 (1/10 scale) (Park et al., 2016) Amiiform RoboGrilos (Hu et al., 2009) N/A N/A 0.4 Dorsal Undulation Fin Robot (Xie et al., 2016) N/A Fin Length ¼ 0.6 0.424 Gymnotiform Knifefish Robot (Siahmansouri et al., 2011) N/A N/A 0.24 Robotic Knifefish (Curet et al., 2011) 2.3 Fin Length ¼ 0.326 0.18 Anal Undulating Fin with Assisted Caudal Fin (Liu et al., 2012) 11 1.2 0.25 (aided by caudal fin) larger version of RoMan-III which is described by Low et al. (2011).In multi-ribbed propulsion fin. This multi-ribbed designs are similar to these two designs, a rigid body has actuation units aligned on the sides to those found in the Amiiform bioinspired category. All components be- allow for the attachment and excitation of the pectoral fins. The body is sides the fin membrane can be considered rigid for this design. Curet tapered and the only control surfaces are the pectoral fins. Two surface et al. (2011). created a robotic Knifefish that has an actuation mechanism float systems found both use IPMC multi-ribbed actuators for swimming. encased in a rigid tubular shell where fin rib length is relatively short in Punning et al. (2004). and Takagi et al. (2006). designs are small and comparison with those described in other systems. However, ribs have a relatively similar so they are grouped together. The IPMC ribs are each small radius and the number of ribs that are powered in the actuation is individually excited to create the undulation wave. The RayBot created greater than twenty which is the most out of any considered undulatory by Krishnamurthy et al. (2010). is a Rajiform robot that uses a caudal fin fin designs. Liu et al. (2012). created a robot that used a passive fin design for propulsion which is what the Electric Ray commonly uses for thrust. where a rib on the nose and tail of the robot gives excitation to the However, even though this robot is equipped with enlarged pectoral fins flexible fin membrane stretched between them. This robot has a similar made of soft material, the pectoral fins do not perform undulations. This tubular body to the Curet et al. (2011). robotic Knifefish. This design is robot is classified in this category due to the biological systems including also equipped with a propulsive caudal fin to aid with forward thrust. Electric Rays in the Rajiform category. Similar to the other robots in this category, all components besides the The smallest robot considered is a soft-robotic ray combined with undulatory fin can be considered rigid. tissue engineering, which was created by Park et al. (2016). This design is unique because a metallic skeleton transports electrical excitation to 3.3.4. AUV undulatory pectoral fin and oscillating dorsal/anal fin: multiple ribs, which then locally excite a layer of muscle tissue to flex. Diodontiform The ribs have a descending size from front to back to cause larger un- This class is similar to the Tetraodontiform, where there are no found dulations at the forefront of the pectoral fin through larger electrical robotic systems that utilize undulatory pectoral fins and oscillating dorsal impulse delivered. The skeleton is electrically excited through laser and anal fin. Likewise, there are no found systems that use solely the flashes to two leading prongs on the skeleton. Depending on which prong undulatory pectoral fins like those found in Bluegill. This category along receives a more direct impulse determines which half of the body is with the Tetraodontiform are voids where new systems could supplied a larger excitation, causing a turning behavior. If the prongs be conceived. receive the same amount of impulse, then a straight forward thrust is generated. 3.4. Consolidation of fin undulation AUV characteristics 3.3.2. AUV undulating dorsal fin: Amiiform fi There are only two Amiiform robots which were found to be classified The AUVs that utilize an undulation n are diverse, but these systems in this category. These two designs were created by Hu et al. (2009). and are fewer in number compared to the Fin Oscillation class. Generally, the Xie et al. (2016). However, these two designs considered active size of the Rajiform AUVs is comparable with the Subcarangiform and multi-ribbed actuation. The RoboGrilos described in Hu et al. (2009). has Carangiform AUV category as masses and lengths do not exceed 10 kg a very slender rigid body that contains the necessary actuation mecha- and 1 m, respectively. The AUV with the largest mass in the Rajiform nisms to carry the translational undulation wave. Information on the category is Krishnamurthy et al. (2010). at 5.5 kg. The smallest being RoboGrilos is limited, but the fin size can be noted as large in aspect ratio Park et al. (2016). even though the mass is unknown. The relative size of in comparison with the body size. In Xie et al. (2016), an extremely this system is a fraction of the Chen et al. (2012). design which has the similar dorsal undulation fin was implemented into a rigid shell smallest found mass of 0.055 kg. Therefore, it can be assumed that Park encasement akin to the torpedo. A rigid criss-cross tail maintains stability et al. (2016). is the lightest design. The longest length is Zhou et al (Zhou and gives this robot directional control. and Low, 2012). at 1 m while Park et al. (2016). is again the smallest design, however, this design had a small tail so its length is slightly 3.3.3. AUV undulating anal fin: Gymnotiform greater than 0.01 m. The majority of these systems are in a length range – Similarly, the Gymnotiform class has few robot systems to be of 0.1 m 0.56 m. More concern is placed on wingspan of these systems as fi described. Siahmansouri et al. (2011). design incorporated a ballast/- pectoral ns are the main thrust generators. By comparing the few fi floatation tank with pitch and yaw actuation joints which connect to the de ned systems, a range of ratios of length to wingspan can be established as 0.48–0.87. The Rajiform AUV with the highest speed is Gao

100 R. Salazar et al. Ocean Engineering 148 (2018) 75–114

Table 12 Characteristics of AUVs with undulation ﬁn continued. *System speciﬁes that it has depth control but does not disclose to what degree this system can perform. **System has turning capabilities but does not disclose the degree of control.

Fin Undulation Robotic Systems Turning Description (ET, T, Mo, Me, L, P) Depth Control (G, Me, P) Endurance

Rajiform Manta Ray Robot N/A N/A N/A Robo-Ray III Mo G N/A Micro Manta Ray (SMA) T, Mo None (Surface Float) N/A IPMC Manta Ray L P N/A Soft Body Single-Dual Actuator Ray N/A N/A N/A Flexible Pectoral Foil Cownose Ray N/A N/A N/A Bionic Fin Manta Ray N/A N/A N/A Cownose Ray-I ET N/A N/A RoMan-II T, Mo G N/A RoMan-III T, Mo G N/A IPMC Chain Ribbed Ray N/A None (Surface Float) N/A Multi-Ribbed IPMC N/A None (Surface Float) N/A RayBot (Electric Ray Caudal Fin Propulsion Robot) N/A N/A N/A Soft-Robotic Ray þ Tissue-engineered P N/A N/A (1/10 scale) Amiiform RoboGrilos N/A N/A N/A Dorsal Undulation Fin Robot N/A** N/A* N/A Gymnotiform Knifeﬁsh Robot N/A** N/A N/A Robotic Knifeﬁsh N/A N/A N/A Anal Undulating Fin with Assisted Caudal Fin N/A N/A* N/A

et al. (2007). at 0.7 m/s. The next best performing system is Chew et al. species because of its similar properties to IPMC actuators in regards to (2015). at 0.45 m/s. After these two systems, there is a gradual decrease bell deformation. The bell was fabricated using heat-shrinkable polymer of performance with most systems not being able to swim faster than to mimic weight and shape of biological system. 0.3 m/s. The systems with the best turning capabilities are Niu et al. The last system introduced is AUV robotic jellyfish designed and (2012), Wang et al. (2009), Yang et al. (2009), Zhou et al. (Zhou and produced by Dooley et al. (2016). Although this robot does not meet the Low, 2012), and Low et al. (2011). Most of these systems do not have a criteria for bioinspiration, its physical aspects were influenced by jelly- defined turning capability, depth control, and none have a fish. This system uses a pulley system to actuate its tentacles which re- defined endurance. sults in an ineffective actuation under water. The bell of the AUV robot In the Amiiform and Gymnotiform AUV category, there is limited was made from transparent acrylic. The system does not propel using bell information on size and length characteristics. The largest is Liu et al. constriction, however, since it was inspired by jellyfish, it can be (2012).analfin AUV at 11 kg and 1.2 m. However, the two Amiiform compared to the last bioinspired robotic jellyfish. systems look to be of large size in Table 10 pictures. The undulation fin runsthelengthoftheseAUVswhichisgoodcharacteristic.Thespeedof 3.5.2. Mantel constriction theseAUVsisdecentastherangeofspeedis0.18–0.424 m/s consid- Octopuses' locomotion and manipulation characteristics have been an ering both categories. The capabilities of turning, depth control, and inspiration to the development of AUVs with the same capabilites. Pos- endurance are not found for these systems. In Tables 11 and 12,all eiDRONE and another bioinspired robotic octopus are covered in relation characteristics of AUV with undulation fin including mass, length, and to their skin and motion mimicry to biological species. Designed and speed are shown. manufactured by Arienti et al. (2013), PoseiDRONE is a soft bioinspired robot consisting of manipulation and locomotion capabilities including 3.5. AUV jet propulsion crawling and swimming abilities. PoseiDRONE is recognized as the first soft-bodied ROV. The system's skin is composed of rubber-like materials. The smallest classification for bioinspired aquatic robots use jet pro- The second bioinspired robot was done by Sfakiotakis et al. (2015). Re- pulsion mechanisms for movement. For this classification, it is important searchers focused on propulsion capabilities of the system where they to divide the systems by chronological order since they follow some achieved forward and backward propulsion and turning, as well as ability similarities between them. Bioinspired robots are designed using the to grasp objects. The arms are mode from polyurethane material. Inspired same materials to mimic movement of biological systems. In Table 13, by the Octopus vulgaris, the robotic octopus depends entirely on arms for AUVs that utilize jet propulsion description and picture for reference swimming. Its manipulation capabilities could bring attention to several are presented. underwater applications, such as monitoring, inspections, and rescue operations. 3.5.1. Bell constriction Robotic systems falling in this category were inspired by common 3.5.3. Mantel constriction and undulation fin jellyfish, such as Aurelia Aurita and Aequorea Victoria. Because they use Robotic systems inspired by cuttlefish and squid fall into the mantel similar aspects in their designs, robotic jellyfish are presented in chro- constriction and undulation fin locomotion classification. AUVs in this nological order. Robojelly, designed by Villanueva et al. (2011), was section are classified in chronological order, providing a short descrip- inspired by Aurelia Aurita in regards to its morphology and kinematics. tion of each robot. The cuttlefish dual undulation fin robot was designed SMA and BISMAC actuators were utilized to achieve a similar bell and fabricated by Low and Willy (2005). Although this system mimics deformation to its biological system which would allow the motion of the cuttlefish movement, it was unable to mimic the complex musculature of robot. The body is made from soft silicone poured into a mold to mimic its fins. The system does not meet the requirements for bioinspiration in the morphology of Aurelia Aurita. The second bioinspired aquatic robot regards to physical perspective. The second robotic system also designed presented was designed and fabricated by Najem et al. (2012). The Jel- by Low and Willy (2005), depends on its fins for locomotion, which lyfish robot mimics shape and swimming style of the Aequorea Victoria, mimics the majority for movement of cuttlefish. The two undulating fins known for its high swimming efficiency. Najem et al. (2012). chose this have their own actuation chain to create the motion. To conclude, the

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Table 13 AUVs that utilize jet propulsion description and picture for reference.

Jet Propulsion Robotic Systems Description Picture Systems

Bell Constriction Robojelly Robojelly uses BISMAC actuators for deformation as well as SMA actuators Depends on a rapid heating controller for contraction of the bell (Villanueva et al.,

2011) Jellyﬁsh Robot This system uses SMA actuators for propulsion Ionic Polymer Metal Composites are used for motion of the robot The system depends on ﬂoating controller to swim (Najem et al., 2012)

AUV Robotic Jellyﬁsh The movement of the system's arms is produced by a rotating pulley and cable system. Rigid body housing Because of its locomotion, the system has low maneuverability (Dooley et al.,

2016) Mantel PoseiDRONE This soft robot has a jet propulsion module arms can push the craft along Constriction The arms have great ﬂexion The system is capable of gasping objects with its arms (Arienti et al., 2013)

Robotic Octopus The system uses forward/backward propulsion and has turning capabilities the robotic octopus has manipulation capabilities allowing it to gasp objects with arms (Sfakiotakis Swimming propulsion through arm movement

et al., 2015) Jet and Undulation Cuttleﬁsh Dual Undulation Fin The system uses multiple ﬁns with multi-ribbed actuation of the Fin Robot membrane Tested in ocean (Low and Willy, No jet propulsion

2005) Multi-ribbed Dual Undulation Servomotors give actuation to multiple ribs in a ﬂexible membrane Fin Body constructed of two rows of servomotors with a buoyancy tank between them No jet propulsion (Low and

Willy, 2005) Cuttleﬁsh Robot Robot actuated by SMA wires Uses jet propulsion through mantel constriction ﬁ Fixed n, no undulation (Wang et al.,

2011) Dual Undulation Fin Robot Dual undulation fin, three ribs per fin, flexible membrane No jet propulsion Rigid body (Gilva Servomotors along body

et al., 2015)

robotic fish does not meet the criteria for bioinspiration due to its lack of locomotion of cuttlefish and rays. The system propels by lateral undu- appearance to biological systems. latory fins on the sides of the body. Because of its size, it is believed that The best bioinspired system was designed by Wang et al. (2011). This the robot may function as an educational platform for underwater robots. system imitates the Cuttlefish actuation. This design is actuated by SMA wires for the mantel constriction and membrane undulation. The Fin 3.5.4. Consolidation of jet propulsion AUV characteristics Undulation propulsion is used during low speed swimming, and mantel The AUVs based on Jet Propulsion locomotion form the smallest constriction for high speed swimming. The last underwater robot, classification of robots in the same way as their biological inspirations. designed and fabricated by Gilva et al. (2015), was inspired by the The systems presented for each category differ from one another

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Table 14 Characteristics of AUVs that utilize Jet Propulsion including mass, length, and speed.

Jet Propulsion Systems Robotic Systems Mass (kg) Length (m) Speed (m/s)

Bell Constriction Robojelly (Villanueva et al., 2011) 0.242 Diameter ¼ 0.164 0.19 Jellyfish Robot (Najem et al., 2012) 0.02 Diameter ¼ 0.15 0.0015 Height ¼ 0.05 AUV Robotic Jellyfish (Dooley et al., 2016) N/A 0.2 N/A Mantel Constriction PoseiDRONE (Arienti et al., 2013) 0.75 0.2 0.62 Robotic Octopus (Sfakiotakis et al., 2015) 2.68 N/A 0.0986 Jet and Undulation Fin Cuttlefish Dual Undulation Fin Robot (Low and Willy, 2005) N/A Two lateral fins length ¼ 0 .81 N/A Multi-ribbed Dual Undulation Fin (Low and Willy, 2005) 9 0.81 N/A Cuttlefish Robot (Wang et al., 2011) N/A N/A 0.6 Dual Undulation Fin Robot (Gilva et al., 2015) N/A 0.24 N/A

Table 15 Characteristics of AUVs that utilize Jet Propulsion continued.

Jet Propulsion Robotic Systems Turning Description (ET, T, Mo, Me, L, P) Depth Control (G, Me, P) Endurance

Bell Constriction Robojelly L N/A N/A Jellyfish Robot (Najem et al., 2012) L N/A N/A AUV Robotic Jellyfish (Dooley et al., 2016) N/A Me >20 min Mantel Constriction PoseiDRONE N/A N/A N/A Robotic Octopus (Sfakiotakis et al., 2015) Me, L N/A N/A Jet and Undulation Fin Cuttlefish Dual Undulation Fin Robot N/A N/A N/A Multi-ribbed Dual Undulation Fin N/A N/A N/A Cuttlefish Robot N/A N/A N/A Dual Undulation Fin Robot (Gilva et al., 2015)L N/A N/A regarding length, mass, and speed. However, they display similar char- AUVs like the Cuttlefish Dual Undulation Fin Robot by Low and Willy acteristics for turning capabilities, depth control, and endurance. The (2005), Multi-ribbed Dual Undulation Fin by Low and Willy (2005), AUVs from Tables 14 and 15 are organized in chronological order to give Cuttlefish Robot by Wang et al. (2011), and Dual Undulation Fin Robot a perspective of the changes made from earliest robots presented. Due to by Gilva et al. (2015). None of the AUVs named give information the lack of locomotion, there are no AUVs that use the shell constriction regarding mass except for Multi-Ribbed Dual Undulation Fin with a 9 kg propulsion. mass. Both Cuttlefish Dual Undulation Fin Robot and Multi-Ribbed Dual Bell constriction locomotion is denoted by the AUVs created by Vil- Undulation Fin consist of a length of 0.81 m. The Dual Undulation Fin lanueva et al. (2011), Najem et al. (2012), and Dooley et al. (2016).. The Robot has length of 0.24 m. From all the systems, the Cuttlefish Robot is AUV by Villanueva et al. (2011). has a mass of 0.242 kg and a diameter of the only AUV to show speed, being of 0.6 m/s. Lastly, for turning 0.164 m. Najem et al. (2012). has a mass of 0.02 kg, and diameter of description category, the Dual Undulation Fin Robot is classified as low. .15 m. The last Jellyfish robot by Dooley et al. (2016). has a length of The best AUV to imitate a biological aquatic system from Table 5 is the 0.2 m. From the systems presented, Villanueva et al. (2011). and Najem Multi-Ribbed Dual Undulation Fin for its similar size and weight to the et al. (2012). show a close resemblance to biological systems, such as biological cuttlefish. However, the Cuttlefish Robot by Wang et al. Aequorea Victoria and Aurelia Aurita. Speed for the Villanueva et al. (2011). is the most similar to the biological Cuttlefish because this AUV (2011). is 0.19 m/s, while Najem et al. (2012). is slower with a speed has an active Undulating Fin and Jet Propulsion. This actuation mecha- of 0.0015 m/s. nism closely imitates the speed of the Sepia Officinalis Cuttlefish, being Turning description was defined as low for Villanueva et al. (2011). of 0.8 m/s. The Dual Undulation Fin Robot is not a good system according and Najem et al. (2012).while the Dooley et al. (2016). did not give any to the requirements for bioinspiration for missing information about explanation for this category. However, the AUV Robotic Jellyfish was mass, speed, depth, and endurance. the only system to describe depth control, being in the medium range, and endurance description of less than 20 min. From the systems pre- 4. Constraints, limitations, and future recommendations sented, Villanueva et al. (2011). and Najem et al. (2012), showed a close resemblance to the biological jellyfish classified in Table 5. However, the Through the organization of these systems, AUVs can be expressed in Dooley et al. (2016). failed at imitating the biological systems by their respective biological locomotion category. Often AUVs derive their exceeding dimensions. size, shape, speed, materials, and endurance from nature. These deriva- The second classification for locomotion is the mantel constriction, as tions are expressed as five criteria where AUVs are compared to the show in Tables 14 and 15. Arienti et al. (2013). and Sfakiotakis et al. biological systems in their respective categories. By expressing the best (2015). are classified into this category for their locomotion description AUVs for each criterion a better understanding can be gained on indi- and inspiration on Octopus. Arienti et al. (2013). is the first Octopus AUV vidual levels of bioinspiration and biomimicry. By falling into at least one presented in the tables above, consists of a mass of 0.75 kg and length of of the criteria described the AUV is deemed bioinspired. If a system is 0.2 m. None of the AUVs presented have similar mass to biological spe- expressed in all criteria, then it is biomimicry. Assumptions will be made cies. However, the design by Sfakiotakis et al. (2015). has the closest so more systems can be categorized as biomimicry because it is extremely approach to the Blue-Ringed Octopus regarding its length of 0.15 m. difficult for an AUV to fulfill all criteria. Suggestions will be expressed Speed for these AUVs varies greatly. Arienti et al. (2013). has speed of that will help AUVs works toward biomimicry. 0.62 m/s and Sfakiotakis et al. (2015). of 0.0986 m/s. Lastly, the Sfa- kiotakis et al. (2015). is the only AUV in this category that has described turning capabilities. None of the systems present information about 4.1. Size comparison depth control and endurance. Lastly, the jet and undulation fin propulsion locomotion is denoted by AUVS can be found within the size ranges that are obtained from biological Tables 1, 3 and 5. Mass and length ranges extracted from these

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Table 16 Table 18 Anguilliform AUVs in the range of biological size. Thunniform AUVs in the biological size range.

AUV Mass Length AUV Mass Length (0.075–30 kg) (0.63–3m) (2–450 kg) (0.5–3m)

Amphibious Snake-like Robot 6.75 (9 Modules) ¼ 1.17 Vorticity Control Unmanned Undersea Vehicle (VCUUV) 173.1 2.4 Yu et al. (Yu et al., 2009) Anderson et al. (Anderson and Chhabra, 2002) LAMPETRA 0.99 Multi-link Robotic Dolphin 23.5 1.2 Stephani et al. (Stefanini et al., Shen et al. (Shen et al., 2011) 2012) Slider-crank Robotic Dolphin 5.2 0.75 SEA Snake Total 16 Total Module Yu et al. (Yu and Wei, 2013) Rollinson et al. (Rollinson et al., Module ¼ 3.657 Chain ¼ 1.174 Mackerel Robot 0.588 2014) Wen et al. (Wen et al., 2013) Mamba Waterproof Snake 0.31 One Section ¼ 0.089 Gliding Robotic Dolphin 18.2 1.125 Robot Total1.1 Wu et al. (Wu et al., 2015) Liljeback et al. (Liljeback et al., Dolphin Robot Capable of Leaping 4.7 0.72 2014) Yu et al. (Yu et al., 2016b) biological tables give parameters that distinguishes a reduced group size Table 19 of AUVs. These AUVs that fall within these ranges are defined in Ostraciiform AUVs in the biological range. Tables 16–21. In the caudal fin category, there is a good variety of robots AUV Mass Length that fall in the ranges for mass and length. Anguilliform ranges for mass (0.04 kg) (0.13 m) (0.075–30 kg) and length (0.63–3 m) reduces the number of AUVs to four Microautonomous Robotic Ostraciiform (MARCO) 0.15 out of seven as shown in Table 16. Kodati et al. (Kodati et al., 2008) In the Subcarangiform and Carangiform category has a range of mass is (3.25–650 kg) and length is (0.65–4.55 m). These large ranges allow Labriform systems exceed the biological size range. There is only one for a greater variance in AUV size. These categories are combined Ostraciiform AUV that has a size that can be compared to the Boxfish because it is hard to differentiate initiation of the body undulation for the while the other Ostraciiform AUVs exceed this size as shown in Table 19. AUVs. However, efforts should be made to differentiate AUVs between The MARCO by Kodati et al. (2008). has the smallest length of all of those these two categories in the future so they follow biological standards. The defined, but is still longer than the typical Boxfish. These systems have largest AUV is the Carp Robot by Ichikizaki et al. (Ichikizaki and difficulty to imitating the Boxfish because of their small size. Yamamoto, 2007). The rest of the AUVs not shown in Table 17 fall below The Rajiform class has a biological mass range (15–100 kg) and the mass and length ranges. A large amount of the mass and length range length range (1.2–3.5 m). The length range is defined for the wingspan is still available for the Subcarangiform and Carangiform AUVs to exploit because this is considered a more important dimension for this category. so larger AUVs can still be created. The length range is selected in this manner as the pectoral fins are the From biological Thunniform animals, the mass range (2–450 kg) and main propulsion. However, when the range is defined in this manner, length range (0.5–3 m) are extracted. These ranges are wide and AUVs of none of the AUVs fall within this range. It has been deemed by these bigger sizes can fit within this category, as shown in Table 18. The AUV in researchers that the size of their AUVs can fall more in the mass range created by Anderson et al (Anderson and Chhabra, 2002). is the largest (0.055–7.3 kg) and length range (0.01–0.6 m). The AUV range is vastly AUV found. The next two largest AUVs created by Shen et al. (2011). and lower than the Rajiform animals. Sizes do range due to growth of the Wu et al. (2015). are more than 100 kg and 1 m smaller than Anderson animal, but these AUVs are being constructed in a manner that condones et al. (Anderson and Chhabra, 2002). While lengths in this category are lower cost and ease of control. The smaller size of these crafts could also the largest described, there is still a large amount of the size range that be due to the small pools and water channels where these systems are can be exploited in this category. These longer lengths accommodate tested. However, once designs are validated, there is no reason as to why larger actuation mechanisms and components for greater thrust capa- larger versions could not be constructed. bilities. Often structures are made of rigid components that cause an The Amiiform and Gymnotiform categories are grouped together in increase in the mass, but the range defined allows for these larger sys- Table 20 as these systems are similar except for the fin location. Amii- tems. The Thunniform category can accommodate these larger sizes and forms have a mass range (1–18.5 kg) and length range (0.5–1.67 m). this allows for even more powerful systems to be developed. Gymnotiforms have a mass range (2–10 kg) and length range (0.5–2 m). The Labriform class does not have any AUVs that fall into the ranges There is only one robot in each category that falls within the biological for mass (0.058–0.22 kg) and length (0.158–0.271 m). All the found range established. These systems therefore do not follow the biological criteria for size. Table 17 The AUVs that are classified in the Jet Propulsion category have the Subcarangiform and Carangiform AUVs in biological size range. difficult task of replicating animals that have bodies void of rigid AUV Mass Length (3.25–650 kg) (0.65–4.55 m)

Essex MT1 Robotic Fish 3.55 Table 20 Liu et al. (Liu et al., 2005) Amiiform and Gymnotiform AUVs that fall in biological range for size. Essex C-turn Robot 0.8 Locomotion AUV Mass Length Liu and Hu (Liu and Hu, 2005) (1–18.5 kg) (0.5–1.67 m) AmphiRobot-II (Four-link) 5 0.7 Yu et al. (Yu et al., 2012) Amiiform Dorsal Undulation Fin Robot Fin Carp Robot 12 0.9 Xie et al. (Xie et al., 2016) Length ¼ 0.6 Ichikizaki et al. (Ichikizaki and Yamamoto, 2007) Mass Length iSplash-I 3.67 (10–20 kg) (0.5–2m) Clapham et al. (Clapham and Hu, 2014) Four-link Robotic Fish Large Pectoral Fin Control 5.2 0.68 Gymnotiform Anal Undulating Fin with Assisted 11 1.2 Surfaces Caudal Fin Yu et al. (Yu et al., 2014) Liu et al. (Liu et al., 2012)

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Table 21 Jet Propulsion AUVs that fall in biological size range.

Locomotion AUV Mass Diameter (0.175–204 kg) (0.014–1.80 m)

Bell Constriction Robojelly 0.242 0.164 Villanueva et al. (Villanueva et al., 2011) Jellyﬁsh Robot 0.15 Najem et al. (Najem et al., 2012) AUV Robotic Jellyﬁsh 0.2 Dooley et al. (Dooley et al., 2016)

Mass Length (0.028–50 kg) (0.15–5m)

Mantel Constriction PoseiDRONE 0.75 0.2 Arienti et al. (Arienti et al., 2013) Robotic Octopus 2.68 Sfakiotakis et al. (Sfakiotakis et al., 2015)

Mass Length (0.0089–6.75 kg) (0.06-.325 m)

Jet and Undulation Fin Cuttlefish Dual Undulation Fin Robot 0.81 Low and Willy (Low and Willy, 2005) Multi-ribbed Dual Undulation Fin 0.81 Low and Willy (Low and Willy, 2005) Dual Undulation Fin Robot 0.24 Gilva et al. (Gilva et al., 2015) structures. Using rigid materials in this category can make a system too Yu et al. (Yu and Wei, 2013), Wu et al. (2015), and Yu et al. (2016a). large. Bell constriction classification has a mass range (0.175–205 kg) which all have extremely good shapes. The robot dolphins all tend to and a diameter range (0.014–1.80 m). Villanueva et al. (2011). is the only have a shell applied to the mechanical structure which makes the final system that agrees with the mass and length ranges in the Jet Propulsion shape like that found in nature. In the Labriform category, Behbahani et category. Whereas the other two AUVs only fall into the length range of al (Behbahani and Tan, 2016). has the best shape considered for the the biological system, as shown in Table 21. AUVs corresponding to Labriform AUVs, as presented in Table 23. mantel constriction locomotion are based on mass range of (0.028–50 kg) The Ostraciiform AUVs have shapes similar to the Boxfish, as pre- and length range of (0.15–5 m). Arienti et al. (2013). and Sfakiotakis sented in Table 24. The AUVs also shown in Table 24 have designs where et al. (2015). fall within the mass range obtained from the Octopus. the shape was created in a CAD software. Mainong et al. (2017).3D Lastly, jet and undulation fin locomotion species have a range for mass of printed the rigid body of their Boxfish Robot. Distinguishing the AUVs (0.0089–6.75 kg) and length of (0.06–0.325 m). These ranges limit the that have shapes similar to the Boxfish limits this category down to two. number of AUVs down to three. Unfortunately, the size of the system The Rajiform AUVs do tend to have a similar shape that consider the created by Wang et al. (2011). could not be determined. However, this outline shape of the animals in Table 25 which are common shapes from system should be noted as it is constructed with proportionally more soft Fig. 11. However, the thickness of the body should follow the dimensions materials and has opportunity to fulfill the size requirements. of the Rajiform as well which Tmeans that the body should be flattened. There are a couple of soft material AUVs that have shapes with a high 4.2. Shape comparison level of biomimicry, as shown in Table 26.Cai et al. (2009), Krishna- murthy et al. (2010), Alvarado et al. (2013), and Park et al. (2016). have The shape of the AUVs derive from the animals found in that category. shapes created out of soft materials which promotes a likeness to those The more similar the shape is to an animal, the higher the level of bio- found in nature. On the other hand, Gao et al. (2007), Niu et al. (2012), inspiration. The shape helps to recreate the swimming capabilities of the and Chew et al. (2015). designs utilized a mixture of rigid and soft ma- animal. There are notable AUVs in each category which should be terials to create their shapes. These shapes have a likeness to the bio- distinguished as their shape is the most similar. In the Anguilliform logical, but not to such a high degree as those constructed with primarily category, a long slender body is consid115ed for all designs. Therefore, in soft materials. this category, all AUVs fit this criterion, as shown in Table 9. There are no similar shapes found in the Amiiform and Gymnotiform The Subcarangiform, Carangiform, Thunniform, and Labriform cate- categories. There were multiple AUVs said to be derived from animals gories can be distinguished by a long body length where the mid-body or that have long slender bodies with gradual taper, but these AUVs did not behind the head has the greatest girth. The peduncle is tapered till it mimic this shape. Curet et al. (2011). and Liu et al. (2012). considered meets the tail where the tail has a lunate shape. These shapes are more of the shape of an Electric Eel which has a long tubular body. determined by the pictures in Tables 2, 4 and 6. Those presented in However, these designs will not be considered because they also do not Table 22 are determined to be the best biological representation for each consider the taper of the body. category. In the Subcarangiform/Carangiform categories, AUV by Hu Representative examples of shape for the Jet Propulsion categories et al. (Liljeback et al., 2014), Ichikizaki et al. (Ichikizaki and Yamamoto, are shown in Table 27. The AUVs in this category that have good shapes 2007), and Katzshmann et al. (2016). have the better shapes out of the are made of soft materials and are presentedin Table 28. The soft material ones displayed in Table 22. These designs have a body shape that has allows for the curve shapes that are commonly found in this category. As varying girth along their length and height. Hu et al. (2006). and Ichi- mentioned, the Robojelly by Villanueva et al. (2011). uses bioinspired kizaki et al (Ichikizaki and Yamamoto, 2007). shapes are good because a shape memory alloy composite actuators and fabricated from RTV sili- body shell covers their mechanical structure. Ichikizaki et al (Ichikizaki cone to imitate the skin of the Aurelia Aurita jellyfish (Villanueva et al., and Yamamoto, 2007). has an entire body encasement that does not show 2011). The Jellyfish Robot by Najem et al. (2012). uses IPMC which are any evidence of its structure. Thunniform category has AUVs like those actuators matching the morphology and kinematic characteristics to the created by Tolkoff et al. (Tolkoff, 1999), Anderson et al. (Anderson and Aequorea Victoria jellyfish. The bells of these two designs are flexible and Chhabra, 2002), Chen et al. (2010), Shen et al. (2011), Wen et al. (2013), shapes are very similar to the Jellyfish. The design of the Cuttlefish Robot

105 R. Salazar et al. Ocean Engineering 148 (2018) 75–114

Table 22 emphasized for fins, as these surfaces are flexible for animals. These fins Biological Fin Oscillation defined shapes that are characteristic for their category. have varying rigidity depending on the fin type, number of ribs, thickness Biological Category Shapes of ribs, rib length, and fin size. There are several examples of AUVs which fl fi Subcarangiform & utilize exible ns. In the Anguilliform category, the LAMPETRA Carangiform developed by Stephani et al (Stefanini et al., 2012). uses a flexible caudal fin along with a large number of body sections that gives this robot better body and fin flexibility. In the Subcarangiform and Carangiform category, the designs by Hu et al. (2006), Liu et al. (Liu and Hu, 2010), Ichikizaki et al. (Ichikizaki and Yamamoto, 2007), and Katzshmann et al. (2016). utilize flexible fins. Hu et al. (2006). is the only one that uses ribs in their flexible fins. None of the studied Thunniform designs were found to actively use flexible fins. However, when considering a biological Thunniform, the assumption of rigid fins is more acceptable as fin structure is often more rigid in this category. However, these fins should still have some complacent movement or slight flexibility. The Labriform category has one robot that utilizes a flexible fin joint with a semi-rigid fin which was created by Behbahani et al. (Behbahani and Tan, 2016). This Labriform design is considered in this group because the flexible joint allows the fin surface to have some passive movement during actuation. The undulatory fin robots use a flexible fin membrane, how- Thunniform ever, there are some differences between the types of soft material used. Alvarado et al. (2013), Cai et al. (2009), and Chew et al. (2015). used a thicker membrane fin while the rest of the systems use a thin membrane. This thin membrane observed to be applied to Amiiform and Gymnoti- form systems (see Table 27). The robotic systems that use soft materials as majority of their body exhibit a large body resemblance to the biological animal. For example, all the Jet Propulsion AUVs in Table 28 utilize soft materials and actuators. Villanueva et al. (2011). and Najem et al. (2012). created systems that closely resemble jellyfish while Wang et al. (2011). and Arienti et al. (2013). manipulated a soft mantel to eject water out of a nozzle and have a likeness to their biological counterpart. These AUVs and others use soft material actuators to create thrust. However, soft actuators often have a trade-off where actuators do not produce sufficient thrust. The robotic systems that use soft actuators should be considered as special cases, as these actuators often yield a natural motion. IMPC and SMA actuators flex like real muscle, but when using these materials as a single actuator where a single wire or beam is difficult to generate thrust. There are several AUVs that use soft materials in their actuation and it can be noted that these systems exhibit a lower speed (less than 1 m/s). This low speed Labriform is due to the actuators being small relative to the AUV size. However, these actuators create a large force relative to their size. A single beam or wire cannot produce the thrust required for moving a large robot at high speed. The material properties for various soft material actuators are given in Table 29. The SMA wire is in the conducting polymer category. It should be noted that the IPMC and conducting polymer are the only two materials which have a low electrical excitation. However, the specific power of the conducting polymer is much higher. The actuation types of these materials are tensile or bending. The construction of these flexible actuators need to be considered to optimize these material characteristics. This means that there is still a possibility of improvement in the soft actuators, as you could stack these materials to create a larger force. by Wang et al. (2011). consists of SMA wires actuating the pectoral fin However, due to the electrical excitation needed, components should be and mantle, which as mentioned, presents the best practical and insulted from each other to avoid a short circuit or any other mechanical comprehensive performance. Lastly, PoseiDRONE is a soft-robotic system disturbance. If IPMC and SMA are insulated from each other a packaging primarily composed of soft materials to imitate the skin of biolog- of synthetic muscle can be created which increases potential area and ical octopuses. thus creates larger force. There are also systems that use rigid material actuators which are 4.3. Materials encased within a soft material shell. This makes the body comparable to biological systems as the shape replicates the animal hydrodynamics. Material selection has a large impact on a systems level of bio- This encasement over the mechanical structure can yield a similar motion inspiration and biomimicry. Biological systems exhibit a diverse range of to the biological cases where animals have a rigid skeletal structure with body types that bioinspired AUVs should replicate. The body structure of muscle attached to create the motion. Systems that use this construction animals can give blueprints needed for the replication of swimming method were referenced in the shape section including Ichikizaki et al. characteristics. One material suggestion is that soft materials are (Ichikizaki and Yamamoto, 2007), Shen et al. (2011), Yu et al. (Yu and

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Table 23 Subcarangiform, Carangiform, Thunnifrom, and Labriform AUVs of similar shape.

AUV Shapes AUV Shapes

G9 Fish MIT RoboTuna Hu et al. (Liljeback et al., 2014) Tolkoff et al. (Tolkoff, 1999) (Carangiform) (Thunniform)

Four-joint Robotic Fish VCUUV Yu et al. (Liu and Hu, 2010) Anderson et al. (Anderson and Chhabra, (Carangiform) 2002) (Thunniform)

Carp Robot IPMC Tail Tuna Ichikizaki et al. (Ichikizaki and Yamamoto, 2007) Chen et al. (Chen et al., 2010) (Carangiform) (Thunniform)

Four-link Robotic Fish Large Pectoral Fin Control Mackerel Robot Surfaces Wen et al. (Wen et al., 2013) Yu et al., (2014) (Thunniform) (Carangiform)

iSplash-I Multi-link Robotic Dolphin Clapham et al. (Clapham and Hu, 2014) Shen et al. (Shen et al., 2011) (Carangiform) (Thunniform)

Wire-driven Shark Robot Slider-crank Robotic Dolphin Lau et al. (Lau et al., 2015) Yu et al. (Yu and Wei, 2013) (Carangiform) (Thunniform)

Hydraulic Soft Robotic Fish Gliding Robotic Dolphin Katzshmann et al. (Lau et al., 2015) Wu et al. (Yu and Wei, 2013) (Carangiform) (Thunniform)

Flexible Pectoral Fin Joint Labriform Robot Dolphin Robot Capable of Leaping Behbahani et al. (Behbahani and Tan, 2016) Yu et al. (Yu et al., 2016b) (Labriform) (Thunniform)

Wei, 2013), Alvarado et al. (Chen et al., 2012), and Yu et al. (2016a).. All by a pneumatic rope muscles while RoboTuna and Mackerel robot are these works utilized rigid actuators bound in a soft materials encasement. actuated through a pully and belt system. The RoboTuna and Mackerel The rigid components give these systems better thrust and the soft body robot can complete a full range of motion of their caudal fin. However, gives them better efficiency. the Rajiform AUV has a limited motion range in comparison with bio- Fixed system tests have been performed where mechanical systems logical systems. are given actuation in a water channel. These robots are given actuation through either a tether cable or the support strut. The body of these systems exhibit good flexion though the internal components which are 4.4. Speed rigid like a skeleton. This rigid skeleton is encased in a soft body skin or shell that makes the body more streamline. These systems are the MIT Rigid material systems have a large amount of systems capable of fast RoboTuna designed by Tolkolf et al (Tolkoff, 1999). and Mackerel Robot speeds. One of the most impressive systems is the robotic dolphin created designed by Wen et al. (2013). Both systems utilized a rigid tail at the end by Yu et al. (2016a). which is capable of leaping. Another swift design is of the active caudal peduncle. A Rajiform AUV used in a similar test was the VCUUV by Anderson et al. (Anderson and Chhabra, 2002). These the flexible pectoral foil Cownose Ray developed by Cai et al. (2009). systems utilize different actuation mechanisms but they exhibit good fl This test was performed on this system as proof of concept prior to letting body exion and powerful thrust. These two designs try to mimic their this AUV swim in a pool without the rigid support for the water channel respective biological animal with rigid components to the best of their fi test. This Rajiform AUV also had an internal skeleton which is actuated abilities. Yu et al (Kopman and Por ri, 2013). robotic dolphin includes a head motion in their design to increase thrust through a more realistic

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Table 24 than those found in nature and can result in increased drag. On the other Osctraciiform biological animal and AUVs with similar shape. hand, Yu et al. (2014). used a smooth encasement to cover the peduncle. Biological Osctraciiform Representation Shape All the rest of the rigid component Carangiform robots exhibit similar

Boxfish speed and actuation type. While some use a peduncle encasement like Yu et al. (2014), a few do not and the system performs swimming routines with the links exposed. For the Thunniform category, the previously stated VCUUV and dolphin capable of leaping are the fastest, but there are still reputable designs left in this category. A simple design that is capable of fast speed AUV is the Single-Motor-Actuated Robotic Fish by Yu et al. (2016a). This MARCO design has continuous growth of speed depending on power input, as Kodati et al. (Kodati et al., 2008) power increases so does tail frequency. Yu et al. (2016a). did not fully max out the designs capable speed. The last notable design that was capable of decent speed was the Slider-crank Robotic Dolphin by Yu et al. (Yu and Wei, 2013). This design also had a rigid head and body attached to the large slider-crank link actuators. These rigid actuators provide powerful thrust to this design. Boxfish Robot Mainong et al. (Mainong et al., 2017) The Labriform and Ostraciiform AUVS display similar speed capabilities. However, these speeds are not as extreme as the Thunniforms and they are not very reputable. These systems are capable of slower speeds which is similar to their biological categories. The Labriform category has the swifter designs of these two categories. The rigid component pectoral and dual caudal fin robot by Zhang et al. (2016). matched speed with the flexible fin joint design of Behbahani et al. Table 25 (Behbahani and Tan, 2016). The Ostraciiforms have three designs with Biological Rajiform shape representation that is characteristic for this category. all similar speed as the BoxyBot by Lachat et al. (2006), Wang et al. Biological Shape (2013), and Wang et al (Wang and Xie, 2014). where Wang et al. (2013). Representation is a continuation of the Wang et al. (2013). design. Rajiform The undulatory fin AUVs do not display extreme speed, but there are some still worth discussing. These designs should be stated as reference for the greatest speed in their respective classification. For the Rajiform category, Gao et al. (2007), Chew et al. (2015), and Zhou et al (Zhou and Low, 2012). have a descending value of swimming speed where Gao et al. (2007). is the swiftest. The two Amiiform robots designed by Hu et al. (2009). and Xie et al. (2016). have relatively similar speeds and are faster than the Gymnotiform AUVs which do not have speed greater than 0.3 m/s. The Jet Propulsion AUVs are also not notable for fast speeds. How- ever, some systems do display similar speeds to their respective biological categories. Although none of the systems using bell constriction present similar speeds to biological jellyfish. It should be mentioned that Robojelly designed by Villanueva et al. (2011). is the fastest system due to the shape testing for the bell with the optimal thrust. Systems in the mantel constriction category present some similar speed capabilities to their biological inspirations. PoseiDRONE by Arienti et al. (2013).is between the average and maximum speed for the Enteroctopus Dofleini. The Robotic Octopus by Sfakiotakis et al. (2015). has speeds at a lower range due to its method of propulsion by using its arms to compress water away from the body. The majority of the jet and undulation fin AUVs in Tables 14 and 15 did not present information about speed except the Cuttlefish Robot designed by Low and Willy (2005). The systems show similar speed ranges as the biological Sepia Officinalis Cuttlefish. The most intriguing aspect of Low and Willy (2005) is that the design incorporates a function undulation fin along with the mantel constriction to body undulation of the Dolphin. have control at low speeds. In the Subcarangiform and Carangiform category, the iSplash created by Clapham et al (Clapham and Hu, 2014). has a complete body flexion 4.5. Endurance using rigid components. Through the incorporation of joints along the length of the body, the iSplash has a side-to-side head movement which When evaluating the endurance of the AUVs, it is hard to compare the gives this design the highest speed which can be noted in Table 7. There similarities to biological systems. As biological animals that live in reefs are other Carangiform designs that compete with the multi-jointed or some amount of structure do not have extreme endurance, and iSplash, namely, the three-link G9 Fish designed by Hu et al. (2006). migratory animals have extreme endurance. What should be noted from and four-link Yu et al. (2014). design. These two designs incorporate a the biological animals is that certain locomotion condones endurance, rigid head unit that encases sensitive components, and both use a link burst/high speed swimming, and close quarter maneuvering. The best system to make their peduncle flexible. The G9 uses scales to try and bioinspired category that follows these three biological standards are make head-peduncle-tail more streamline, the scales are much larger AUVs found in the caudal fin. However, the caudal fin category has more

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Table 26 Rajiform AUV with similar shape.

AUV Shape AUV Shape

Manta Ray Robot Soft Body Single-Dual Actuator Ray Gao et al. (Gao et al., 2007) Alvarado et al. (Alvarado et al., 2013)

Flexible Pectoral Foil Cownose Ray Bionic Fin Manta Ray Cai et al. (Cai et al., 2009) Chew et al. (Chew et al., 2015)

RayBot (Electric Ray Caudal Fin Propulsion Robot) Soft-Robotic Ray þ Tissue-engineered Krishnamurthy et al. (Krishnamurthy et al., 2010) (1/10 scale) Park et al. (Park et al., 2016)

Robo-Ray III Niu et al. (Niu et al., 2012)

Table 27 Table 28 Biological Jet Propulsion shape representations that are characteristic for each category. Jet Propulsion AUVs with similar shape.

Biological Shape AUV Shape Representation Robojelly Bell Constriction Villanueva et al. (Villanueva et al., 2011) (Bell Constriction)

Jellyﬁsh Robot Najem et al. (Najem et al., 2012) (Bell Constriction)

PoseiDRONE Arienti et al. (Arienti et al., 2013) Mantel Constriction (Mantel Constriction)

Cuttleﬁsh Robot Wang et al. (Wang et al., 2011) (Jet and Fin Undulation)

Jet and Undulation Fin

exhibit good swimming ability with moderate speed. The value of this category is that some actively excited multi-ribbed Rajiforms can perform null speed rotational turns. The value in this is that the AUV can move in extremely close quarter environments. There have been a few robots classified in the jet and undulation fin category which exhibit either just jet or undulation fin. There was no found actual combination AUV similar to the Cuttlefish. The value of having a combination of Cuttlefish AUV is the null speed turning like the Rajiforms and fast speed because of the Jet fi dif culty swimming in close quarter environments. Rajiforms are the Propulsion. While the Labriforms, Ostraciiform, Amiiform, and Gymno- next largest category which has some very functioning designs which tiform AUV categories have functioning robots capable of lower speeds or

109 R. Salazar et al. Ocean Engineering 148 (2018) 75–114

Table 29 The performance characteristics and material properties of various soft material actuators relative to natural muscle. Table acquired from (Naﬁcy et al., 2016).

Natural Muscle Dielectric Elastomer Polymer coil muscle Conducting polymer IPMC Electrostrictive polymer

Stress (MPa) 0.1–0.35 0.3–7.7 20–30 5–35 0.2–53 Strain (%) 20–60 20–350 20–60 1–10 0.5–3.3 7 Strain rate (%/s) 50–500 450–3.4 Â 104 200 0.1–10 <3.5 – Work density (kJ/m3)8–40 Up to 3.5 Â 103 Up to 2.9 Â 103 100 <5.5 1.5 Specific power (W/kg) 50–320 Up to 5 Â 103 Up to 5 Â 104 150 <2.5 – Efficiency (%) 20–68 Up to 90 1–2 1 1.5–3 – Life cycle >109 106 >106 <105 –– Operational condition – >1kV <100 V 1-10 V 1-10 V >1kV Actuation type Tensile Tensile Tensile Tensile bending Bending Bending have possibilities in close quarter maneuvering. However, much is to be In the Jet Propulsion, there are several systems that are recognized by desired in swimming performance for these categories. their mimetic design. The AUVs described in Table 28 have extremely good traits. The jellyfish AUVs by Villanueva et al. (2011). and Najem et al. (2012). are the most mimetic out of the four. However, the designs 4.6. Bioinspired, biomimetic AUVs, and future recommendations of Wang et al. (2011). and Arienti et al. (2013). should be recognized by their soft mantel constriction propulsion designs. The design of Arienti The AUVs which are considered bioinspired fall into at least one of et al. (2013). has arms that flex extremely similar to the Octopus and are the categories mentioned. The most important categories being shape able to push the AUV when touching the sea floor. and materials. To be biomimetic, the AUV should either: (1) be in all Considering all the various robot systems described in this review, the categories or (2) have an extremely high degree of mimicry materials best performing tend to align with the locomotion category that they are fi utilized to replicate biological animals. For (1), it is very dif cult for an placed in. Rigid and soft materials can be used to create the various lo- AUV to fall within all categories, namely, size, shape, material, and comotions and shapes. However, the AUVs with the fastest speeds are the speed. Endurance is considered as a low priority for this review as it is ones with rigid components. It could be further expressed that the use of a fi dif cult to match biological systems where some animals can swim for mixture of rigid and soft materials would make these robots more like extremely long periods of time. However, these endurance characteristics biological systems. A skeletal structure is the easiest way to describe how are given as a reference point that the AUVs should be working toward. the rigid materials could be used. Soft materials can cover this skeleton, fl For (2), materials are recommended to be exible with structures com- much like the RoboTuna and Mackerel and flexible fin Cownose Ray fi parable with those de ned for the biological. There can be a component designed by Cai et al. (2009). The flexible skins make these systems structure comparable to a skeleton that transfers mechanical motion, but highly mimetic and gives them more biological characteristics. fl there should be an exterior shell to have a mimetic shape and exibility. As stated, recommendations are suggested during the material se- fl This rigid skeleton should have good exion that allows for this shell to lection process. The rigid skeletal structure described allows for more have smooth movement. realistic performance for Fin Oscillation AUVs. While the soft body shells fl fi There are a few systems in Fin Oscillation that use exible ns but would help for these AUVs to mimic animal characteristics. Considering majority of the body is often made of rigid components. There are a few Fin Undulation AUVs, more soft materials are needed as large portions of Fin Oscillation AUVs that have a body shell over the skeleton. Ichikizaki the fins and body need to be flexible. Various construction techniques et al. (Ichikizaki and Yamamoto, 2007), Shen et al. (2011), Wu et al. (Yu need to be applied depending on the selection of a fin actuated by active and Wei, 2013), and Yu et al. (2016b). systems should be considered or passive wave propagation. The Fin Undulation AUVs need to have mimetic in the Fin Oscillation class. The Anguilliform AUVs have system body flexibility as this trait can be found for the biological animals which utilizes a muscle like actuation. LAMPETRA by Stephani et al described. The Jet Propulsion AUVs still need to determine the best (Stefanini et al., 2012). should be considered mimetic. Although this construction methods to recreate the thrust necessary for efficient fi system has visible section, this can be easily xed by a body sleeve to movement. The Amiiform, Gymnotiform, and Jet Propulsion AUVs make this system more hydrodynamic. A rigid component Thunniform struggle with directional control and more work needs to be done to design which are considered as mimetic is the VCUUV by Anderson et al. improve their capabilities. The Jet Propulsion AUVs have the greatest (Anderson and Chhabra, 2002). This large Tuna design is capable of possibilities for improvements out of the systems described. complex swimming missions, but the structure is made completely out of rigid components and hydraulic actuator where the peduncle has a clever 5. Conclusions flexion design that gives this body similar movement to the Tuna. This would suggest that more systems be considered mimetic, but the decision The utility of the aquatic unmanned vehicles offers a solution for a fi to include this design is based off the ful llment of criteria for (1). This is wide range of missions. In efforts to create more optimized systems, the considered the only AUV to span all these categories successfully. adaptation of biological animal traits was proposed as these offer func- Considering the Fin Undulation class, the Rajiform category is the tional design alternatives. This review compiled works by organizing only one that has potential for mimetic AUVs. The rigid structures of the AUVs into a biological locomotion based classification. The AUVs fi Amiiform and Gymnotiforms do not allow for the ful llment of the ma- described serve as a good literature reviews for each category. The bio- terials selected. However, the Rajiform category has several systems logical animals defined give criteria that can be used to create more which could be deemed mimetic. The soft structured systems by Alvarado optimized AUVs. The more intriguing AUVs found were ones that aligned et al. (2013), Cai et al. (2009), Krishnamurthy et al. (2010), and Park with the biological animals in their respective categories. Criteria et al. (2016). have extreme resemblance to their biological counterparts. extracted from each biological category creates a smaller batch of AUVs The sizes of these systems do not fall within the biological criteria. found to be more inspired from nature. From this batch of AUVs, certain However, these AUVs are taking steps in the right direction to replicate observations are made that will allow for more realistic bioinspired the biological animals in this category. The most intriguing design that is AUVs. The selection of materials is assumed to be a top priority to create revolutionary is Park et al. (2016). This design uses actual muscle tissue biomimetic AUVs. The use of rigid and soft materials was proposed as outlaid in a polymer matrix that is excited through optical impulse to a biological animals have rigid and soft structures. Body actuation and fin fi metallic skeleton to actuate the pectoral ns. Unfortunately, this design is motion are deemed a significant part of the bioinspired process. Many very difficult to scale-up.

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