A Review of Design and Fabrication of the Bionic Flapping Wing Micro Air Vehicles

micromachines

Review A Review of Design and Fabrication of the Bionic Flapping Wing Micro Air Vehicles

Chen Chen and Tianyu Zhang * College of Instrumentation & Electrical Engineering, Key Laboratory of Geophysical Exploration Equipment, Ministry of Education of China, Jilin University, Changchun 130026, China; [email protected] * Correspondence: [email protected]; Tel.: +86-155-6776-6673

Received: 16 January 2019; Accepted: 14 February 2019; Published: 21 February 2019

Abstract: Bionic flapping-wing micro air vehicles (FWMAVs) are promising for a variety of applications because of their flexibility and high mobility. This study reviews the state-of-the-art FWMAVs of various research institutes driven by electrical motor, mechanical transmission structure and “artificial muscle” material and then elaborates on the aerodynamic mechanism of micro-winged birds and insects. Owing to their low mass budget, FWMAVs require actuators with high power density from micrometer to centimeter scales. The selection and design of the mechanical transmission should be considered in parallel with the design of the power electronic interface required to drive it. Finally, power electronic topologies suitable for driving “artificial muscle” materials used in FWMAVs are stated.

Keywords: bionic ﬂapping-wing micro air vehicle; aerodynamic mechanism; mechanical transmission; actuator; power electronic interface

1. Introduction At present, there are many complex and cluttered environments that humans cannot survive in for a long time, such as glaciers, deserts, dense forests and caves. To explore these rigorous environments, flapping-wing micro air vehicles (FWMAVs) have been included in research by many scientific institutions as one of the feasible solutions. The advantages of FWMAVs are their more flexible maneuverability and more efficient aerodynamics compared with those of fixed or rotary wing air vehicles. Remarkable achievements have been accomplished with regard to designing and optimizing the constituent subsystems of FWMAVs, including aerodynamic mechanism [1], mechanical transmission [2], actuator [3] and power electronic interface [4]. Nevertheless, several critical challenges have to be resolved urgently to increase the practical ability of FWMAVs. This manuscript presents the (a) current research progress of existing FWMAVs investigated by scientific institutions; (b) aerodynamic mechanism of FWMAVs, including birds and insects; (c) actuators composed of new material; and (d) related power electronic interface. The aforementioned literature reviews can be used as a reference by researchers. The remainder of this paper is organized as follows: Section2 introduces the latest achievements in FWMAVs conducted by several research institutes; Section3 presents the aerodynamic mechanism of FWMAVs, mechanical transmissions and various types of actuators and related power electronic interfaces; and finally, Section4 discusses the conclusions and the future research prospects for FWMAVs.

2. FWMAVs with Different Actuation Mechanisms Research on FWMAVs can be divided into three types according to the driving methods, including electrical motor, mechanical transmission and “artiﬁcial muscle” material.

Micromachines 2019, 10, 144; doi:10.3390/mi10020144 www.mdpi.com/journal/micromachines Micromachines 2019, 10, 144 2 of 20

2.1. Electrical Motor-Driven Method Many institutions investigate electrical motor-driven FWMAVs. Although the electrical motor- driven method does not deliver excellent performance in terms of energy efficiency, it remains the first choice for most FWMAVs because of its mature application scheme and low manufacturing cost. “Microbat” [5] was developed by the California Institute of Technology in the US in 2002. It was the first FWMAV driven by electrical motor. This aerial vehicle achieves self-contained autonomous flight by mimicking the morphological properties of flexible bat wings with a mass of 12.5 g, wingspan of 25 cm and more than 42 s flight time. The aircraft flew under radio control, where the pilot could control left and right turns, pitching angle and motor on/off. During this flight right and left turns were commanded and aircraft responded appropriately. The flight duration was mainly limited by the power system and vehicle’s weight. “Hummingbird” [6] is an FWMAV with a camera and autonomous control support, launched by AeroVironment, USA, in 2011. The aerial vehicle has a mass of 19 g, a wingspan of 16.5 cm and a flight time of 4 min. As it does not completely draw on the design of bionics, it still has a certain discrepancy with actual hummingbirds, leading to its poor performance in propulsion efficiency and motion sensitivity aspects. The Hummingbird is equipped to carry its own energy source with it while flying. The wings attached to the NAV will help to rotate and turn towards any angle and position as directed by the controlling crew. The flapping wings allow the NAV to control its attitude during flight. The vehicle can be controlled remotely from a distance of up to one kilometer. Nevertheless, “Hummingbird” remains one of the FWMAVs that have already been put into practical use at high level. “Phoenix” [7] was manufactured by the Massachusetts Institute of Technology, USA, in 2011. A huge lift is generated by utilizing the huge wings to support an airframe weighing 1200 g. “Phoenix” combines a mechanical, pressure-independent regulator with a high-speed position/airflow controller to meet the unique requirements of airflow control. However, because of its large extra energy requirement, the vehicle should be started manually and can fly only for a short distance. “H2bird” [8], with a mass of 13.6 g and a wingspan of 26.5 cm, was investigated by the University of California, Berkeley, USA, in 2014. Linear piece-wise affine modeling of segments of flight conditions within a maneuver was used as an effective method for determining transition points between hybrid controllers. The controllers was stored on-board to enable autonomous navigation or obstacle avoidance by picking maneuvers applicable to an observed situation. Since the models are linear, the computational overhead for onboard look-ahead simulation or computation of feedback controllers is reduced relative to complex nonlinear models. An experimental FWMAV with an integrated autopilot was fabricated by the University of Arizona, USA, in 2009 [9]. It weighs 248 g, has a wingspan of 74 cm and maintains 7 min flight endurance. The autonomous ornithopter is a test bench to investigate the aerodynamic parameters of a flapping aerial vehicle, such as aerodynamic forces, kinematics and automatic controls. The ornithopter competition involves building the smallest radio-controlled ornithopter that can fly the most laps around a pylon course in two minutes. It is believed that the overall quality of flight plan tracking can be improved by further optimization of the flight control system. Eagle flight simulator model was designed by the University of Maryland, USA, in 2008 [10]; it has a weight of 425 g and a wingspan of 107 cm. Researchers used the aircraft to obtain and verify relevant aerodynamic parameters during flight. In 2013, Gerdes et al. fabricated the pioneering flight of “Robo Raven” which is a major breakthrough for micro air vehicles [11]. “Robo Raven” has a wingspan of 150 cm and a minimum weight of 690 g. It was the first demonstration of a bird-inspired platform doing outdoor aerobatics using independently actuated and controlled wings. Independent wing control has the potential to provide a greater flight envelope. “Smart Bird” [12] is a bionic FWMAV developed by Festo Company in Germany in 2015. It is an example of bio-mimicking seagull flight. The latest smart bird has a mass of 450 g, a wingspan of 50 cm and can achieve up to 80% aerodynamic efficiency when flying on a circular trajectory/bound orbit. A powerful microcontroller calculates the optimal setting of two servo motors, which adjust the Micromachines 2019, 10, 144 3 of 20 torsion of each wing. The flapping movement and the torsion are synchronized by three Hall sensors, which determine the absolute position of the motor for the flapping movement. However, precision control cannot be easily reached. “DelFly” [13–16] was developed by Delft University of Technology in the Netherlands in 2005. A coaxial four-wing FWMAV is achieved by mimicking the flight mode of beetles. The first generation “DelFly” has a mass of 21 g and a wingspan of 50 cm and its follow-up generation achieves a low mass of 3.07 g and a short wingspan of 10 cm. In addition, “DelFly” has single degree of freedom motor-driven flapping wings for generating thrust. The control moments are generated by actuated control surfaces on the tail. The tail actuation typically consists of a rudder and an elevator and can be used for changing the MAV’s direction, height or velocity. Since the tail is relatively large, it dampens the body dynamics sufficiently to make this type of FWMAV passively stable. The motorized insect-mimicking FWMAV [17], designed by Korean Konkuk University in 2015, has a wingspan of 12.5 cm and weighs 7.36 g with batteries and power control installed. This work has successfully demonstrated a micro aerial vehicle that can stably takeoff with initial stability. An uncontrolled takeoff test successfully demonstrated that the FW-MAV possesses initial pitching stability for takeoff. “Bat Bot” [18] was fabricated by the California Institute of Technology and the University of Illinois at Urbana-Champaign in 2017. It weighs 93 g and is shaped like a bat with a wingspan of approximately 1 ft. To mimic the morphological properties of bats, researchers used custom-made silicon skin and articulated morphing wings. The flight mechanism involves several different types of joints that interlock the bones and muscles to one another, creating a musculoskeletal system that is capable of movement in more than 40 rotational directions. A flapping twin-wing robot of the size of hummingbird (Colibri in French), actively stabilized in pitch and roll and capable of hovering, was constructed by Roshanbin, A. et al. at the Universite’ Libre de Bruxelles in 2017 [19]. The prototype has a total mass of 22 g and a wing span of 21 cm. The controller was tuned for the linearized, cycle-averaged model and can stalely control the flight in 4 DOF by altering 4 parameters: the flapping frequency, the difference between the left and right wing flapping amplitude and, independently, the left and right wing offset. The robot has demonstrated successful hovering flights with a 15–20 s onboard battery for flight autonomy. A better control of lateral direction flight can be searched in the future. The FWMAV, the Golden Snitch, developed by Hsiao et al. at Tamkang University in 2012 [20]. It is an 8 g weight and 20 cm wingspan aircraft, including the fuselage, flapping wings, tail wings, battery, motor and gear system. The flapping wings are driven by a motor with a four bar linkage system. By adjusting the lengths of the four bars, various stroke angles can be achieved. Due to the limited payload-carrying capability, the P control architecture was modified so that automatic control of flight altitude of a flapping-wing MAV fewer than 10 g. Wasp AE, developed by the Aerovironment company, U.S., is the all-environment version of FWMAV [21]. With special design considerations for maritime, Wasp AE delivers exceptional features of superior imagery, increased endurance and ease of use. The commercial vehicle has a total mass of 1300 g and wing span of 108 cm. It can be operated manually or programmed for autonomous operation, utilizing the system’s advanced avionics and precise GPS navigation. Electrical motor-driven FWMAVs have the advantages of low visual and low acoustic signatures but have only short endurance flight. To enhance their flight endurance, an increase in the mass of the onboard battery is required. However, this increase adds to the overall weight of the FWMAV; in turn, more energy is consumed, especially during the ascent and hover stages. A prominent progress in increasing the FWMAV’s endurance can be achieved using a micro combustion engine as power supply but it will increase noise, resulting in poor concealment. Hence, the problem of flight endurance versus weight is a particular challenge because the benefits of flapping-wing flight are most significant at small scales or low masses. Micromachines 2019, 10, 144 4 of 20 Micromachines 2019, 10, x FOR PEER REVIEW 5 of 22 Micromachines 2019, 10, x FOR PEER REVIEW 5 of 22 Micromachines 2019, 10, x FOR PEER REVIEW 5 of 22 2.2.2.2. Mechanical Mechanical Transmission-Driven Transmission-Driven Method Method 2.2.2.2. MechanicalMechanical TransmissionTransmission--DrivenDriven MethodMethod AnotherAnother means means of of realizing realizing ultra-light ultra-light FWMAV FWMAV is is by by using mechanical transmission transmission-driven-driven Another means of realizing ultra-light FWMAV is by using mechanical transmission-driven method.Another The “art meansificial of butterfly” realizing [2 ultra2] shown-light in FWMAV Figure 1 is was by constructed using mechanical by the University transmission of -Tokyo,driven method.method. The The “artificial “artificial butterfly” butterfly”[ [2222]] shownshown in Figure 1 was constructed constructed by by the the University University of ofTokyo, Tokyo, method.Japan, in The2010 “art usingificial a butterfly butterfly”-like [2 2crank] shown-driven in Figure wing 1body was withconstructed a flight bytime the of University only a few of seconds. Tokyo, Japan,Japan, in in 2010 2010 using using a a butterfly-like butterfly-like crank-driven crank-driven wing body body with with a aflight flight time time of of only only a few a few seconds. seconds. Japan,The flight in 2010 of “artificial using a butterfly butterfly”-like is realizedcrank-driven with simplewing body flapping, with arequiring flight time little of onlyfeedback a few control seconds. of TheThe flight flight of of “artificial “artificial butterfly” butterfly” is is realized realized withwith simple flapping, flapping, requiring requiring little little feedback feedback control control of of Thethe featheringflight of “artificial angle. Even, butterfly” the stable is realized forward with flight simple could flapping, be realized requiring without little active feedback feathering control orof thethe feathering feathering angle. angle. Even, Even, the the stablestable forwardforward flight flight could could be be realized realized without without active active feathering feathering or or thefeedback feathering control angle. of the Even, wing the motion stable forward flight could be realized without active feathering or feedback control of the wing motion feedbackfeedback controlcontrol ofof thethe wingwing motionmotion

Figure 1. Artificial butterfly driven by machinery. Figure 1. Artificial butterfly driven by machinery. FigureFigure 1. 1.Artificial Artificial butterflybutterfly driven by by machinery machinery.. Additionally, Sahai et al. [23] attempted to integrate flexural hinges into a four-bar compliant Additionally,Additionally, Sahai Sahai et et al. al. [ 23[23]] attempted attempted toto integrateintegrate flexural flexural hinges hinges into into a afour four-bar-bar compliant compliant flappingAdditionally, transmission Sahai for et FWMAV al. [23] attemptedwith approximately to integrate 3 g flexural of weight, hinges as shown into a in four Figure-bar 2compliant. flappingflappingflapping transmission transmissiontransmission for forfor FWMAV FWMAVFWMAV with withwith approximately approximatelyapproximately 33 gg ofof weight, weight, asas shownshown shown inin in FF Figureigureigure 222.. .

Figure 2. The prototype of the Harvard University’s FWMAV and its four-bar transmission Figure 2. The prototype of the Harvard University’s FWMAV and its four-bar transmission FigureFiguremechanism 2. The2. The prototype. prototype of the of Harvard the Harvard University’s University’s FWMAV FWMAV and its four-bar and its transmissionfour-bar transmission mechanism. mechanism. mechanism. A distinguishingA distinguishing feature feature of of the the mechanismmechanism isis using rubber rubber-based-based flexures flexures in in two two of of its its joints joints A distinguishing feature of the mechanism is using rubber-based flexures in two of its joints (joints(joints 3A and 3distinguishing and 4). 4). According According feature to theto theof experiment, theexperiment, mechanism not not only is only using did did compliant rubber compliant-based mechanism mechanism flexures savein save two up up of to toits 20% 20% joints of of the (joints 3 and 4). According to the experiment, not only did compliant mechanism save up to 20% of input(jointsthe powerinput 3 and power and 4). 1% According and of 1% the of weight tothe the weight butexperiment, also but producedalso not produced only more did more thrust.compliant thrust. mechanism save up to 20% of the input power and 1% of the weight but also produced more thrust. theCoil inputCoil Springs Springspower have and have 1% also also of been thebeen weight directly directly but coupled coupled also produced withwith DC more motors motors thrust. for for directly directly driving driving flapping flapping wings wings Coil Springs have also been directly coupled with DC motors for directly driving flapping wings towardtowardCoil resonance. resonance. Springs Campolo have Campolo also etbeen al.et [directly24 al.] [24 presented] coupled presented a proof-of-conceptwith a proofDC motors-of-concept for flapping-wing directly flapping driving-wing micro flapping micro aerial, wings aerial shown, toward resonance. Campolo et al. [24] presented a proof-of-concept flapping-wing micro aerial, in Figuretowardshown3 in. resonance. Figure 3. Campolo et al. [24] presented a proof-of-concept flapping-wing micro aerial, shownshown inin FFigureigure 33..

Figure 3. The prototype of a motor-driven ﬂapping-wing FWMAV. Micromachines 2019, 10, x FOR PEER REVIEW 6 of 22

Figure 3. The prototype of a motor-driven flapping-wing FWMAV. Micromachines 2019, 10, 144 5 of 20 The prototype consists of two brushed DC motors, two 7 cm length wings, two helical springs and two shaft-spring-wing couplers. The pair of small helical springs is treated as the compliant structuresThe prototype for energy consists storage of and two recovery. brushed The DC motors,two separated two 7 cmDC length motors wings, can drive two helicalthe individual springs wingand two to resonate, shaft-spring-wing respectively. couplers. Experiments The pairdemonstrated of small helical the prot springsotype iscan treated successfully as the lift compliant off and thestructures maximum for energylift-to-weight storage ratio and can recovery. be achieved The two at the separated flapping DC frequency motors can10 Hz drive using the controlling individual torques.wing toresonate, respectively. Experiments demonstrated the prototype can successfully lift off and the maximumThe traditional lift-to-weight electrical ratio canmotor be- achieved and mechanical at the flapping transmission frequency-driven 10 Hz methods using controlling of FWMAVs torques. have problemsThe traditionalof low dri electricalving efficiency motor- and and large mechanical power transmission-drivenlosses at low Reynolds methods number of FWMAVs during flight. have Thus,problems initiative of low ideas driving using efficiency novel driving and large methods power lossesare proposed at low Reynolds to resolve number the described during flight.challenges. Thus, initiative ideas using novel driving methods are proposed to resolve the described challenges. 2.3. “Artificial Muscle” Material-Driven Method 2.3. “Artificial Muscle” Material-Driven Method To improve flapping frequency and aerodynamic efficiency, some scientific research institutions suggestedTo improve using “artificial flapping frequencymuscles” as and a new aerodynamic driving actuator efficiency, for someFWMAV scientific instead research of electrical institutions motor andsuggested machinery. using These “artificial new muscles” materials as have a new also driving been actuatorapplied forto some FWMAV present instead robotic of electrical applica motortions, providingand machinery. novel ideas These for new FWMAVs. materials have also been applied to some present robotic applications, providingIn 2010, novel the ideasKorean for Academy FWMAVs. of Science and Technology developed a bionic crawler robot with thermalIn 2010, material the Korean based Academy on the thermoelectric of Science and stretching Technology effect developed [25]. In a bionic 2014, the crawler Massachusetts robot with Institutethermal material of Technology based on in the the thermoelectric US developed stretching a circular effect closed [25]. In-chain 2014, robot the Massachusetts with shape-memory Institute materialof Technology that depends in the USon developedthe temperature a circular memory closed-chain effect [26]. robot The exploration with shape-memory of these new material materials that providesdepends onalternative the temperature options and memory avenues effect for [26 the]. Theactuator exploration of FWMAVs. of these Although new materials the mechanica providesl deformationalternative options and energy and avenues conversion for theefficiency actuator of ofnew FWMAVs. materials Although are excellent, the mechanical the maximum deformation response frequencyand energy is conversion less than 5 efficiencyHz, which of means new materialsit fails to meet are excellent, the requirements the maximum of folding response ratio frequency of flapping is wings.less than In 5 2013, Hz, which the University means it fails of Maryland to meet the in requirements the US developed of folding the ratio electrostatic of flapping material wings. bounce In 2013, robotthe University [27] and in of 2007, Maryland Sungkyunkwan in the US developed University the in electrostatic Korea fabricated material an bounce insect animal robot [27 robot] and [2 in8] 2007, that hasSungkyunkwan better performance University in frequency in Korea fabricatedresponse. anHowever, insect animal due to robot the limitations [28] that has of betterinherent performance electrical characteristics,in frequency response. mechanical However, deformation due to the was limitations too little of inherentto meet electrical the large characteristics, physical transformation mechanical requirementdeformation from wastoo FWMAVs. little to meet the large physical transformation requirement from FWMAVs. In addition, the UniversiUniversityty of of California, Berkeley, Berkeley, developed a a 25 mm (wingtip (wingtip-to-wingtip)-to-wingtip) FWMAV capable of realizing sustained autonomous flightflight inin 2007 [[2299].]. Figure 4 4 showsshows thethe mentionedmentioned micromechanical flying flying insect, with with four degreesdegrees-of-freedom,-of-freedom, weighing weighing approximately approximately 100 100 mg, mg, excluding battery or electronic devices. The The main main mechanical mechanical transmission transmission component, component, namely, namely, thorax, consists consists of of two four four-bar-bar mechanisms that amplify and convert the mechanical motion into wing flappingflapping and rotation. The The biologically inspired inspired system system archi architecturetecture results results in a hierarchical structure ofof differentdifferent control control methodologies, methodologies, which which give give the the possibility possibility to plan to plan complex complex missions missions from froma sequence a sequence of simple of simple flight flight modes modes and maneuvers. and maneuvers.

FigureFigure 4 4.. FWMAVFWMAV with four degrees degrees-of-freedom.-of-freedom.

In 2013, 2013, Harvard Harvard Univ Universityersity proposed proposed the the use use of of a apiezoelectric piezoelectric bimorph bimorph material material as asactuator actuator to generateto generate mechanical mechanical deformation deformation using using inverse inverse piezoelectric piezoelectric effect effect [30 [30–3–232] ] (Figure (Figure 5). This lift- lift- enhancing design of mimicking the flapping flapping mechanism of a fly’s fly’s 2 cm wingspan enabled the 80 mg FWMAV to fly autonomously. The Robobee was fitted with various individual sensors for onboard MicromachinesMicromachines 2019,, 10,, x 144 FOR PEER REVIEW 7 6of of 22 20

FWMAV to fly autonomously. The Robobee was fitted with various individual sensors for onboard feedback.feedback. Pitch Pitch and and yaw yaw control control of of the the RoboBee RoboBee using using an an onboard onboard magnetometer magnetometer was was presented presented with with thethe robot robot constrained to to rotate only ab aboutout its principal axes. T Thehe integration of a MEMS gyroscope ontoonto the the RoboBee RoboBee to to provide provide attitude attitude feedback feedback in in flight. ﬂight. However, However, it it only only worked worked with with the the connection connection fromfrom an an external external battery power supply.

FigureFigure 5 5.. “Robobee,”“Robobee,” an FWMAV an FWMAV driven driven by a new by a “artificial new “artiﬁcial muscle” muscle” material material developed developed by Harvard by UniversityHarvard University..

TheThe characteristics characteristics of the above FWMAVs in terms of driving methodmethod areare shownshown inin TableTable1 .1.

TableTable 1. CharacteristicsCharacteristics of of the the above above FWMAVs FWMAVs..

Name/ManufacturerName/Manufacturer Mass Mass (g) (g) Wingspan Wingspan (cm) (cm) Flight Flight DurationDuration (min) (min) MicrobatMicrobat 12.5 12.5 25 25 0.70.7 HummingbirdHummingbird 19 1916.5 16.5 44 PhoenixPhoenix 1200 1200 - - -- H2bird 13.6 26.5 - H2bird 13.6 26.5 - University of Arizona 248 74 7 UniversityUniversity of of Arizona Maryland 248 425 74 107 -7 UniversityRobo of Maryland Raven 425 690 107 150 15- RoboSmart Raven Bird 690 450 150 50 -15 SmartDelFly Bird 450 21 50 50 -- DelFly Micro 3.07 10 - KonkukDelFly University 21 7.36 50 12.5 -- DelFlyBat Micro Bot 3.07 93 10 30 -- Universite’Konkuk University Libre de Bruxelles 7.36 22 12.5 21 0.3- GoldenBat Bot Snitch 93 830 20 5- Wasp AE 1300 108 50 Universite´ Libre de Bruxelles 22 21 0.3 Artificial Butterfly - - A few seconds RobobeeGolden (Harvard Snitch University) 8 0.08203 -5 Wasp AE 1300 108 50 Artificial Butterfly - - A few seconds In conclusion, the FWMAV driven by electrical motor method is the most successful and Robobee widely used because of its high maturity,0.08 low cost and wide3 application in the- field. Currently, (Harvard University) the mechanical transmission-driven FWMAV is only utilized for the experimental verification of the aerodynamic model, which is not practical. Although “artificial muscle” material-driven FWMAV is in In conclusion, the FWMAV driven by electrical motor method is the most successful and widely the preliminary stage, it has wide application prospects and important research significance. used because of its high maturity, low cost and wide application in the field. Currently, the In the near future, FWMAVs will evolve to become ultra-compact in size, super light and will have mechanical transmission-driven FWMAV is only utilized for the experimental verification of the longer flight duration. Thus, the challenges of investigating aerodynamic mechanism, transmission aerodynamic model, which is not practical. Although “artificial muscle” material-driven FWMAV is mechanism and power electronic interface should be resolved. in the preliminary stage, it has wide application prospects and important research significance. 3. AerodynamicIn the near future Mechanism, FWMAVs Bases will evolve to become ultra-compact in size, super light and will have longer flight duration. Thus, the challenges of investigating aerodynamic mechanism, transmissionUnlike themechanism fixed-wing and andpower rotary-wing electronic interface aerial vehicles, should thebe resolved. body of an FWMAV is mainly constructed based on bionics inspired by birds and insects. A flutter cycle can be divided into two stages: lower flap and upper flap. The wings are twisted quickly during the transition between Micromachines 2019, 10, x FOR PEER REVIEW 8 of 22

3. Aerodynamic Mechanism Bases MicromachinesUnlike 2019 the, 10 fixed, 144 -wing and rotary-wing aerial vehicles, the body of an FWMAV is mainly7 of 20 constructed based on bionics inspired by birds and insects. A flutter cycle can be divided into two stages:the lower lower and flap upper and flaps upper and flap. start The to flipwings over are at twisted the end quickly of each during stroke. the The transition aerodynamic between basis the of lowerinsect and flight upper can be flaps divided and start into to four flip types. over at the end of each stroke. The aerodynamic basis of insect flight can be divided into four types. 1. Delayed stall mechanism. For an in-depth study on the aerodynamic mechanism of flapping 1. Delayed stall mechanism. For an in-depth study on the aerodynamic mechanism of flapping wings of insects, see that conducted by biologists C. P. Ellington and C. van den Ber et al. on insect wings of insects, see that conducted by biologists C. P. Ellington and C. van den Ber et al. on behavior [33]. They used scaled-up model of hawkmoth wings for experiments. The front edge insect behavior [33]. They used scaled-up model of hawkmoth wings for experiments. The front of the hawkmoth wing was equipped with a smoke-releasing device and a high-speed camera to edge of the hawkmoth wing was equipped with a smoke-releasing device and a high-speed record the changing formation of the air flow of its wings during flapping. The study indicated camera to record the changing formation of the air flow of its wings during flapping. The study that the large lift produced by the hawkmoth’s wings during flapping is due to the presence indicated that the large lift produced by the hawkmoth’s wings during flapping is due to the of delayed stalls. The angle of attack is much larger than the conventionally critical angle of presence of delayed stalls. The angle of attack is much larger than the conventionally critical attack, a difference that cannot be explained by classically aerodynamic principles. However, angle of attack, a difference that cannot be explained by classically aerodynamic principles. the experiment revealed that the formation of a vortex of circulating air flow at the leading edge However, the experiment revealed that the formation of a vortex of circulating air flow at the is caused by the rapid movement of the wings. A low-pressure area will be generated because the leading edge is caused by the rapid movement of the wings. A low-pressure area will be vortex is located above the wings. Thus, generating a large lift force is beneficial. The observed generated because the vortex is located above the wings. Thus, generating a large lift force is phenomenon is consistent with the basic theoretical calculation, which is in line with the study of beneficial. The observed phenomenon is consistent with the basic theoretical calculation, which Liu H. [34]. is in line with the study of Liu H. [34]. ClapClap-and-fling-and-fling mechanism. Weis Weis-Fogh-Fogh discovered discovered a a mechanism mechanism of of lift lift generation when he observed wasp flight, flight, that is, the wings are folded back (clap) and then quickly opened (fling) (fling) befo beforere the next incitement [[3355] (Figure (Figure6 6).). TheThe aerodynamic aerodynamic mechanism mechanism creates creates a a discrete discrete vortex vortex at at the the wingtip andand resultsresults inin a a big big lift. lift. Weis-Fogh Weis-Fogh named named it theit the clap-and-fling clap-and-fling mechanism, mechanism, which which explains explains the thegeneration generation of large of large lift coefficientslift coefficients by insects by insects when when hovering. hovering.

(a) (b) (c)

(d) (e) (f)

Figure 6.6. ClapClap-and-ﬂing-and-fling mechanism mechanism..

As shown in FigureFigure 44,, thethe clapclap andand flingfling mechanism mechanism consistsconsists ofof twotwo phases: phases: thethe firstfirst one,one, thethe leading edges of both wings are clapped together at the end of the upstroke (from (a) to (c)) and the second one, the wings rotaterotate around their trailing edges, thus flingingflinging apart (from (d) to (f)).(f)). During the firstfirst “fling”“fling” phase, the flingfling motion is produced by a rotationrotation of the wings about the common trailing edge, a pair of large leadingleading edgeedge vorticesvortices are formed.formed. During the secondsecond “fling”“fling” phase, air flowsflows around the leading edge of each wing which creates a bound vortex on each wing acting as the starting vortex for the opposite wing. This allows a rapid buildup of circulation as well as an incr increaseease in total lift production.

2. RotationalRotational circulation circulation mechanism. mechanism. Dickinson Dickinson M. M. H. H. et et al. al. completed completed the the experiment experiment using using a mechanicala mechanical device device to obtain to obtain the equations the equations of the of wings’ the wings’ flapping ﬂapping motion motion captured captured by the camera by the [3camera6–38]. They [36–38 simulated]. They simulated the movement the movement of insect wings of insect by driving wings the by model driving wings the model placed wingsin the cylinderplaced in and the utilized cylinder a sensor and utilized to measure a sensor the lift to measureand drag the acting lift andon the drag airfoil. acting As ona result, the airfoil. they As a result, they found that the translational force generated by the wing attack was not sufﬁcient, whereas they discovered rotational circulation mechanism generated more lift, usually two to Micromachines 2019, 10, x FOR PEER REVIEW 9 of 22

Micromachinesfound 2019that, 10the, 144 translational force generated by the wing attack was not sufficient, whereas8 ofthey 20 discovered rotational circulation mechanism generated more lift, usually two to three times the threechord times length. the The chord theory length. of Therotational theory circulation of rotational mechanism circulation is mechanism that the wing is that of the wingfruit fly of thegenerates fruit fly a generates reverse vortex a reverse when vortex the whenwings the are wings flapping are flappingforward forwardat the end. at the So end. the airflow So the airflowvelocity velocityabove the above fly is the faster fly than is faster at the than bottom, at the forming bottom, aforming pressure a difference pressure differenceand producing and producingenough lift. enough lift.

3.3. AddedAdded mass mass effecteffect mechanism.mechanism. This This is is known to play a substantial substantial role role in in defining defining the the hydrodynamichydrodynamic forcesforces actingacting onon movingmoving bodiesbodies becausebecause thethe movementmovement ofof thethe surroundingsurrounding fluidfluid requiresrequires anan additionaladditional forceforce overover andand aboveabove whatwhat isis necessarynecessary toto accelerateaccelerate thethe bodybody itselfitself [[3399].]. Moreover,Moreover, thethe mechanismmechanism waswas laterlater devoteddevoted mostlymostly toto fastfast oscillatingoscillating motionsmotions inin viewview ofof flutterflutter andand stability studies. Andro Andro J. J. Y. Y. and and Jacqin Jacqin L. L. recently recently analyzed analyzed the the added added mass mass effect effect on on a aharmonically harmonically heaving heaving airfoil airfoil by by using using 2 2-D-D direct direct numerical numerical simulations [[4040].]. BasingBasing onon previousprevious studies,studies, GiesingGiesing J.J. P.P. developed developed an an unsteady unsteady panel panel method method for for calculating calculating the the forces forces acting acting on anon airfoilan airfoil executing executing arbitrary arbitrary motions motions and calculatedand calculated the added the added mass mass coefficients coefficients [41]. A [4 fairly1]. A goodfairly agreementgood agreement was found was between found between the numerical the numerical and analytical and analytical values of thevalues coefficients. of the coefficients. Although manyAlthough researchers many re madesearchers some made achievements, some achievements, the theoretical the modeltheoretical to explain model a to variety explain of complexa variety parametersof complex stillparameters requires still further requires improvement. further improvement.

4.4. TransmissionTransmission Mechanism Mechanism Policies Policies MechanicalMechanical transmissions,transmissions, suchsuch asas electricalelectrical motorsmotors andand smartsmart materials,materials, areare investigatedinvestigated andand designeddesigned based based on on the the former former discovered discovered aerodynamic aerodynamic mechanism mechanism bases. Electricalbases. Electrical motors are motors reliable, are versatile,reliable, versatile, low cost and low easily cost and purchasable easily purchasable in the commercial in the commercial market. Most market. of the FWMAVsMost of the described FWMAVs in Sectiondescribed2 are in driven Section by electrical 2 are driven motors; by the electrical ﬁrst one motors; is Microbat the first in 2001, one whose is Microbat transmission in 2001, structure whose istransmission shown in Figure structure7[ 5]. is The shown rotation in Figure of the 7 electrical [5]. The motorrotation drives of the the electrical gear to actuatemotor drives FWMAVs the gear and isto theactuate common FWMAVs transmission and is the principle common in transm electricalission motor-driven principle in microelectrical aircraft. motor However,-driven micro mechanical aircraft. transmissionsHowever, mechanical have individualized transmissions designs have individualized based on their characteristics.designs based on their characteristics.

FigureFigure 7.7. InternalInternal transmissiontransmission structurestructure ofof Microbat.Microbat. New smart materials have also emerged that have attracted widespread attention. If the size of New smart materials have also emerged that have attracted widespread attention. If the size of the flying robot is reduced to millimeter levels, then the efficiency of the conventional electrical motor the flying robot is reduced to millimeter levels, then the efficiency of the conventional electrical motor will be reduced dramatically. Therefore, various smart actuators are an optimal alternative choice will be reduced dramatically. Therefore, various smart actuators are an optimal alternative choice for for FWMAVs. Smart actuators are micro-mechanical devices that use artificial materials to generate FWMAVs. Smart actuators are micro-mechanical devices that use artificial materials to generate deformation [42]. Table2 shows the overview characteristics of smart actuators, such as strain, stress, deformation [42]. Table 2 shows the overview characteristics of smart actuators, such as strain, stress, elastic energy density, efficiency and response speed [43]. elastic energy density, efficiency and response speed [43]. As illustrated in the table, shape memory alloy (SMA), shape memory polymer (SMP), electro-chemo-mechanical conducting polymer (EMCP), thermal polymer and mechanochemical polymer (MCP) are capable of large free strain and high resistance but have slow response and limited efficiency, which make them unsuitable for driving FWMAV. By contrast, piezoelectric actuators exhibit relatively low free strain. They have the ability to produce very high blocking forces and more efficient sensitivity. Owing to speed requirements, piezoelectric, dielectric elastomers (DEAs), electrostatics and electromagnetic actuators are effective alternatives to micro bionic flapping wing aerial vehicles. 1. Piezoelectric actuators: Piezoelectric actuators are devices that use inverse piezoelectric effects [44] (Figure8). The drive voltage of a piezoelectric actuator is typically in the range of a few tens to Micromachines 2019, 10, 144 9 of 20

several hundreds of volts. The operating voltage of piezoceramic stack actuators is realized by stacking monolithic multilayer elements in the range of 60–200 V and a higher required voltage of approximately 1000 V for discrete stack actuators. When in conjunction with a mechanical transmission, the actuator is capable of enhanced stroke amplitude and reciprocating motion for flapping flight [45,46]. In addition, piezoelectric actuators have high displacement, fast response [47,48] and high efficiency at high deformation frequency [49]. Therefore, piezoelectric materials are an optimal choice for use as an actuator in FWMAVs.

Table 2. Overview characteristics of smart actuators [43].

Maximum Maximum Speciﬁc Elastic Maximum Relative Actuator Type Strain (%) Stress (MPa) Energy Density (J/g) Efﬁciency (%) Speed Dielectric elastomer (acrylic) 380 7.2 3.4 60–80 Medium Dielectric elastomer (silicone) 63 3.0 0.75 90 Fast Electrostatic 50 0.03 0.0015 >90 Fast Electromagnetic 50 0.10 0.003 >90 Fast Piezoelectric (ceramic) 0.2 110 0.013 90 Fast Piezoelectric (single crystal) 1.7 131 0.13 90 Fast Piezoelectric (polymer) 0.1 4.8 0.0013 80 est. Fast Shape memory alloy >5 >200 >15 <10 Slow Shape memory polymer 100 4 2 <10 Slow Thermal polymer 15 78 0.15 <10 Slow Electro-chemo-mechanical 10 450 23 <5% est. Slow Conducting polymer MicromachinesMechanochemical 2019, 10, x FOR polymer PEER REVIEW >40 0.3 0.06 30 Slow11 of 22

Deforms when Voltage Applied

Piezoelectric Material V

Piezoelectric Actuator

Figure 8. Typical example of piezoelectric actuators. Figure 8. Typical example of piezoelectric actuators. At Harvard University, Wood et al. [30,45,48–53] conducted an in-depth study on an insect-scale At Harvard University, Wood et al. [30,45,48–53] conducted an in-depth study on an insect-scale flutter robot called RoboBee that uses a piezoelectric actuator. RoboBee was the first insect-sized robot flutter robot called RoboBee that uses a piezoelectric actuator. RoboBee was the first insect-sized robot with the ability to fly. with the ability to fly. 2. Dielectric elastomers: DEA is polymer material with flexible electrodes that have a large 2. Dielectric elastomers: DEA is polymer material with flexible electrodes that have a large electromechanical response to the applied electric field (Figure9)[ 53,54]. DEA typically operates electromechanical response to the applied electric field (Figure 9) [53,54]. DEA typically operates at very high voltages (about 1–10 kV) with an electric field of approximately 100 MV/m and at very high voltages (about 1–10 kV) with an electric field of approximately 100 MV/m and produces large strain at high working density [55,56]. In reference [57], DEA was used to drive produces large strain at high working density [55,56]. In reference [57], DEA was used to drive approximately 15 g of FWMAV that extends the limitation of the artificial muscle to the level approximately 15 g of FWMAV that extends the limitation of the artificial muscle to the level of of energy required for a heavyweight aerial vehicle. However, the application is limited by the energy required for a heavyweight aerial vehicle. However, the application is limited by the challenge of a high electric field requirement in the development of DEA. challenge of a high electric field requirement in the development of DEA. 3. Electrostatic elastomer: Electrostatic and piezoelectric actuators both offer efficient compliant actuation and are capable of providing high working densities [48]. Piezoelectric bimorph actuators have been successfullyPolymer Film implemented for centimeter-scale robots [47] but the performanceV of thin film required by millimeter-scale robots deteriorates [58]. To make up for this disadvantage,

Compliant Electrodes z x (on top and bottom Voltage Off Voltage On surfaces) y

Figure 9. Working principle of DEA.

3. Electrostatic elastomer: Electrostatic and piezoelectric actuators both offer efficient compliant actuation and are capable of providing high working densities [48]. Piezoelectric bimorph actuators have been successfully implemented for centimeter-scale robots [47] but the performance of thin film required by millimeter-scale robots deteriorates [58]. To make up for this disadvantage, electrostatic actuators are generally fabricated in chip level with Microelectromechanical Systems (MEMS) technique, which provide an excellent choice for mobile microrobots (Figure 10) [59]. Micromachines 2019, 10, x FOR PEER REVIEW 11 of 22

Deforms when Voltage Applied

Piezoelectric Material V

Piezoelectric Actuator

Figure 8. Typical example of piezoelectric actuators. Micromachines 2019, 10, 144 10 of 20 At Harvard University, Wood et al. [30,45,48–53] conducted an in-depth study on an insect-scale flutterelectrostatic robot called actuators RoboBee are that generally uses a piezoelectric fabricated in actuator. chip level RoboBee with Microelectromechanical was the first insect-sized Systems robot with (MEMS)the ability technique, to fly. which provide an excellent choice for mobile microrobots (Figure 10) [59]. 24.. DielectricElectromagnetic elastomers: actuators: DEA Electromagnetic is polymer material actuators with convert flexible electrical electrodes energy that to have mechanical a large electromechanicalenergy and vice versa response by using to the electromagnetic applied electric mechanical field (Figure principles. 9) [53,54 Electromagnetic]. DEA typically actuatorsoperates atexhibit very high good voltages performance (about owing 1–10 tokV) their with quick an electric response, field simple of approximately structure, easy 100 controlMV/m and produceslow voltage large requirement strain at high from working 0 to 24 Vdensity [60,61 ].[5 Electromagnetic5,56]. In reference actuators [57], DEA mainly was used consist to drive of an approximatelyelectromagnetic 15 coil, g of a permanentFWMAV that magnet extends rotor the and limitation a “virtual of spring” the artificial magnet muscle pair. Deng to the et level al. [62 of] energyrecently required used a 2.6 for g a electromagnetic heavyweight aerial actuator vehicle. to drive However, a FWMAV the application with wing-beat is limited frequency, by the as challengeshown in of Figure a high 11 electric. field requirement in the development of DEA.

Micromachines 2019, 10, x FOR PEER REVIEW 12 of 22

Polymer Film V

o o θ＝0 θ＝180

Compliant Electrodes z x (on top and bottom Voltage Off Voltage On surfaces) y Micromachines 2019, 10, x FOR PEER REVIEW 12 of 22 Figure 9 9.. Working principle of DEA.DEA.

o o 3. Electrostatic elastomer: Electrostaticθ＝0 and piezoelectric actuatorsθ＝180 both offer efficient compliant actuation and are capable of providing high working densities [48]. Piezoelectric bimorph Figure 10. Electrostatic inchworm motor. actuators have been successfully implemented for centimeter-scale robots [47] but the 4. Electromagneticperformance of thin actuators: film required Electromagnetic by millimeter actuators-scale robots convert deteriorates electrical energy [58]. To to make mechanical up for energythis disadvantage, and vice versa electrostatic by using actuators electromagnetic are generally mechanical fabricated principles. in chip Electromagnetic level with actuatorsMicroelectromechanical exhibit good performance Systems (MEM owingS) to technique, their quick which response, provide simple an structure, excellent easy choice control for andmobile low microrobots voltage requirement (Figure 10 from) [59 ].0 to 24 V [60,61]. Electromagnetic actuators mainly consist of an electromagnetic coil, a permanent magnet rotor and a “virtual spring” magnet pair. Deng et al. [62] recently used a 2.6 g electromagnetic actuator to drive a FWMAV with wing-beat frequency, as shown in FigureFigureFigure 11.10. 10.Electrostatic Electrostatic inchworminchworm motor.motor.

4. Electromagnetic actuators: Electromagnetic actuators convert electrical energy to mechanical energy and vice versa by using electromagnetic mechanical principles. Electromagnetic actuators exhibit good performance owing to their quick response, simple structure, easy control and low voltage requirement from 0 to 24 V [60,61]. Electromagnetic actuators mainly consist of an electromagnetic coil, a permanent magnet rotor and a “virtual spring” magnet pair. Deng et al. [62] recently used a 2.6 g electromagnetic actuator to drive a FWMAV with wing-beat frequency, as shown in Figure 11.

FiguFigurere 1 11.1. AA FWMAV FWMAV using electromagnetic actuator.actuator.

At present, present, most most insect-scale insect-scale FWMAVs FWMAVs are driven are driven by piezoelectric by piezoelectric actuators actuators [3,4,31,47 ]. [3,4, Although31,47]. Althoughattempts wereattempts made were to use made electromagnetic to use electromagnetic actuators andactuators insulative and elastomerinsulative actuatorselastomer (dielectricactuators (dielectricand electrostatic and electrosta actuators)tic for actuators) driving FWMAVs,for driving no FWMAVs, report indicates no report that aerialindicates vehicle that prototypes aerial vehicle can prototypesbe lifted successfully. can be lifted successfully.

5. Power Electronic Interfaces Most compact energyFigu sourcesre 11. A FWMAV potentially using suitableelectromagnetic for FWMAV actuator. applications, such as supercapacitors [63], solar cells [64] and fuel cells [65], generate output lower than 5V. At present, conventionalAt present, batteries most are insect the- scale only FWMAVs commercially are available driven by technology piezoelectric that actuators is appropriate [3,4,31 ,4 for7]. FWMAV.Although The attempts actuators were mentioned made to use earlier electromagnetic are classified actuators into two and actuation insulative modes. elastomer The first actuators is the current(dielectric mode, and which electrosta requirestic actuators) high current for anddriving relatively FWMAVs, low voltages no report and indicates corresponds that toaerial SMA, vehicle SMP, EMCP,prototypes thermal can beactuator lifted successfully.and MCP. The second is voltage mode, which requires high voltages and relatively low currents and corresponds to piezoelectric, DEA, electrostatic and electromagnetic 5. Power Electronic Interfaces actuators. TheMost use compactof the above energy actuators sources requires potentially a power suitableelectronic for interface FWMAV with applications,high power efficiency such as andsupercapacitors density to transfer[63], solar energy cells [6 from4] and power fuel cells source [65 ], to generate actuator. output The powerlower than electronic 5V. At interface present, generallyconventional consists batteries of a power are thestage, only which commercially regulates the available voltage of technology the energy thatsources is appropriateto the requir ed for levelFWMAV. and a Thedrive actuators stage, which mentioned uses the earlier output are voltage classified to generateinto two a actuation time-varying modes. signal The applied first is onthe current mode, which requires high current and relatively low voltages and corresponds to SMA, SMP, EMCP, thermal actuator and MCP. The second is voltage mode, which requires high voltages and relatively low currents and corresponds to piezoelectric, DEA, electrostatic and electromagnetic actuators. The use of the above actuators requires a power electronic interface with high power efficiency and density to transfer energy from power source to actuator. The power electronic interface generally consists of a power stage, which regulates the voltage of the energy sources to the required level and a drive stage, which uses the output voltage to generate a time-varying signal applied on Micromachines 2019, 10, 144 11 of 20

5. Power Electronic Interfaces Most compact energy sources potentially suitable for FWMAV applications, such as supercapacitors [63], solar cells [64] and fuel cells [65], generate output lower than 5V. At present, conventional batteries are the only commercially available technology that is appropriate for FWMAV. The actuators mentioned earlier are classified into two actuation modes. The first is the current mode, which requires high current and relatively low voltages and corresponds to SMA, SMP, EMCP, thermal actuator and MCP. The second is voltage mode, which requires high voltages and relatively low currents and corresponds to piezoelectric, DEA, electrostatic and electromagnetic actuators. The use of the above actuators requires a power electronic interface with high power efficiency Micromachinesand density 2019 to transfer, 10, x FOR energy PEER REVIEW from power source to actuator. The power electronic interface generally13 of 22 consistsMicromachines of a 2019 power, 10, x stage, FOR PEER which REVIEW regulates the voltage of the energy sources to the required level13 of and 22 the smart actuator. This section illustrates a potential solution (not currently used) for the power a drive stage, which uses the output voltage to generate a time-varying signal applied on the smart electronic interface of both current- and voltage-mode actuators. actuator.the smart This actuator. section This illustrates section illustrat a potentiales a solutionpotential (not solution currently (not used)currently for theused) power for the electronic power interfaceelectronic of interface both current- of both and current voltage-mode- and voltage actuators.-mode actuators. 5.1 Power Electronic Interfaces for Current-Mode Actuators 5.1 Power Electronic Interfaces for Current-Mode Actuators 5.1. PowerCurrent Electronic-mode actuators Interfaces for rely Current-Mode on high current Actuators to raise the temperature of the active material throughCurrent-modeCurrent resistive-mode heating. actuators Generally, rely on the high voltage current delivered to raise to the the temperature actuator is lower of the than active the material energy sourcethrough voltage. resistive heating. Generally, Generally, the voltage delivered to the actuator is lower than the energy sourceOne voltage. of the simplest ways to convert the energy source voltage to the required low level is to use the conventionalOne of the simplest buck converter. ways to convertTwo alternative the energy schemes source are voltage described to the to required realize lowlow levelvoltage is to with use compactthe conventional package: buc buckn-stagek converter. cascade Two buck alternative converter [6 schemes6] and tapped are described inductor to buck realize converter low voltage [67]. with compactThe package:scheme consisting n n-stage-stage cascade of an n buck-stage converter cascade [[6combination666] and tapped of buck inductor converter buckbuck converterconverter with single [[6677 ].active]. switchThe is shown scheme in consisting Figure 12 of. an n-stagen-stage cascade combination of buck converter with singlesingle active switch is shown in Figure 1212.. L1 L2 Ln-1 Ln L1 L2 Ln-1 Ln D C D C D Q RL 1 1 3 2 2n-3 C Cn n-1Q RL D1 C1 D3 C2 D2n-3 D2n-1 Cn VO Vin D D D Cn-1 2 4 2n-2 D2n-1 VO Vin D2 D4 D2n-2

FigureFigure 1 12.2. NN-stage-stage cascade cascade buck buck converter converter.. Figure 12. N-stage cascade buck converter. This kind of converter requires an active power MOSFET and 2n 2n-1-1 passive diodes and can be utilizedThis only kind when of converter the required requires number an activeof stages power is no not MOSFETt very large. and Otherwise, Otherwise, 2n-1 passive the the diodeswhole conversionand can be efficiencyefﬁciencyutilized only will when deteriorate the required due to number the parasitic of stages losses is ofno components.t very large. Otherwise, the whole conversion efficiencyAnother will feasible deteriorate topology due to named the parasitic tapped lossesinductor of components. buck converter is presented in Figure 1313.. Another feasible topology named tapped inductor buck converter is presented in Figure 13.

FigureFigure 1 13.3. TappedTapped inductor buck converter converter.. Figure 13. Tapped inductor buck converter. This circuit utilizes a tapped tapped inductor operated by one active switch switch to to achieve achieve high high s step-uptep-up ratio ratio with Thisthe square circuit ofutilizes turns a ratio tapped between inductor the primaryoperated and by one secondary active switch windings. to achieve high step-up ratio withThe the square circuit of architecture turns ratio ofbetween the two the topologies primary and reveals secondary that the windings. latter topology requires fewer componentsThe circuit than architecture the former but of thesince two no topologies tapped inductor reveals is t hatcommercially the latter topologyavailable on requires the market, fewer thecomponents circuit manufacturing than the former technology but since is nothe tapped main challenge inductor for is commerciallythe latter topology. available For theon formerthe market, one, thethe circuitlarger themanufacturing output power, technology the higher is the the main efficiency challenge is subject for the tolatter the topology. exponential For distribution.the former o ne,In addition,the larger the the upper output effic power,iency isthe limited higher by the the efficiency number ofis stages,subject whileto the the exponential latter is more distribution. efficient. In addition, the upper efficiency is limited by the number of stages, while the latter is more efficient. 5.2. Power Electronic Interfaces for Voltage-Mode Actuators 5.2. Power Electronic Interfaces for Voltage-Mode Actuators The power electronic interfaces should be able to convert the low input voltage of a lithium batteryThe to powera high voltageelectronic signal interfaces that drives should the bepiezoelectric able to convert or dielectric the low actuator input voltage[68]. Unlike of a current lithium- modebattery actuators, to a high voltage voltage -signalmode that actuators drives requirethe piezoelectric up to several or dielectric hundred actuator volts. [6 Recovering8]. Unlike current unused- energymode actuators,from the actuators voltage-mode is also actuators another requirechallenge up for to severalpower electronic hundred volts.interfaces Recovering because unusedonly a portionenergy fromof the the input actuators electrical is also energy another is con challengeverted into for apower mechanical electronic deformation interfaces of because the actuators. only a Owingportion to of losses the input in the electrical passive inductor energy is and con activeverted switch into ,a as mechanical well as a very deformation high switching of the frequency, actuators. Owing to losses in the passive inductor and active switch, as well as a very high switching frequency, Micromachines 2019, 10, 144 12 of 20

The circuit architecture of the two topologies reveals that the latter topology requires fewer components than the former but since no tapped inductor is commercially available on the market, the circuit manufacturing technology is the main challenge for the latter topology. For the former one, the larger the output power, the higher the efficiency is subject to the exponential distribution. In addition, the upper efficiency is limited by the number of stages, while the latter is more efficient.

5.2. Power Electronic Interfaces for Voltage-Mode Actuators The power electronic interfaces should be able to convert the low input voltage of a lithium battery to a high voltage signal that drives the piezoelectric or dielectric actuator [68]. Unlike current-mode actuators, voltage-mode actuators require up to several hundred volts. Recovering unused energy from the actuators is also another challenge for power electronic interfaces because only a portion of the input Micromachineselectrical energy 2019, 10 is, x converted FOR PEER REVIEW into a mechanical deformation of the actuators. Owing to losses14 in of the 22 passive inductor and active switch, as well as a very high switching frequency, the conventional boost theconverter conventional becomes boostimpractical converter to resolve becomes the above impractical challenges. to Five resolve alternative the above electrical challenges. interfaces Five are alternapresentedtive toelectrical achieve interfaces high voltages are presented in a compact to achieve package: high hybrid voltages voltage in a multiplier compact package: boost converter, hybrid voltagetapped inductormultiplier boost boost converter, converter, cascade tapped boost inductor converter, boost highconverter, conversion cascade ratio boost boost converter, converter high and conversionpower ampliﬁer ratio boost using converter a piezoelectric and power transformer amplifier (PT) using [69]. a piezoelectric transformer (PT) [69]. A hybrid topology consisting of of a a conventional boost boost converter converter cascaded cascaded with with a a switched switched-- capacitor charge charge pump pump circuit, circuit, as as shown shown in in Figure 1144,, has has been considered previously to to drive piezoelectric actuators [ [7070]] and elect electrostaticrostatic MEMS devices [ 7711]. It It is is an n n-level-level DC DC–DC–DC converter using one switch, 2n + 1 1 diodes diodes and and 2n 2n capacitors. capacitors. Operating Operating in in a a regime regime of of high high efficiency, efﬁciency, the boost converter stage stage provides provides a a moderate boost boost to to the input voltage, while its pulsed output natu naturallyrally charges up the capacitor ladder through the diodes. The The charge charge pump pump circuit circuit multiplies the boost converter’s output output voltage, ideally by a factor factor equal equal to the the number number of of charge charge pump pump stages. The The outpu outputt power is limited by the size of the charge pump cap capacitorsacitors and the maximum output power of the boost converter.

STAGE N VC

Voltage Multiplier

RL Vo STAGE II VC

STAGE I VC

Vin Q

Boost Converter Stage

FigureFigure 1 14.4. HybridHybrid voltage voltage multiplier multiplier boost boost converter converter..

As shown shown in in Figure Figure 15 1,5 replacing, replacing the the inductor inductor in a classical in a classical boost converter boost converter with a tapped with a inductor tapped inductorresults in results a combination in a combination of boost and of ﬂyback boost andconverter flyback topology, converter named topology, the tapped named inductor the tapped boost inductorconverter boost [72]. Theconverter voltage [7 gain2]. The of thisvoltage converter gain of is this greatly converter improved, is greatly which improved, depends on which the switching depends onduty the cycle switching and the duty transformer cycle and turns the transformer ratio. turns ratio.

Figure 15. Tapped inductor boost convertor.

Super voltage boost technology is widely used in electronic converter design to increase the voltage transmission gain. Despite the high complexity of the converter, the super voltage boost converter can generate output voltages that are related to the geometric progression of the cascaded circuit [73–75]. Two-stage cascade boost converter is cascaded by conventional boost converters, as shown in Figure 16. It can sufficiently meet the high driving voltage requirement of piezoelectric actuators because of its high gain performance. Micromachines 2019, 10, x FOR PEER REVIEW 14 of 22 the conventional boost converter becomes impractical to resolve the above challenges. Five alternative electrical interfaces are presented to achieve high voltages in a compact package: hybrid voltage multiplier boost converter, tapped inductor boost converter, cascade boost converter, high conversion ratio boost converter and power amplifier using a piezoelectric transformer (PT) [69]. A hybrid topology consisting of a conventional boost converter cascaded with a switched- capacitor charge pump circuit, as shown in Figure 14, has been considered previously to drive piezoelectric actuators [70] and electrostatic MEMS devices [71]. It is an n-level DC–DC converter using one switch, 2n + 1 diodes and 2n capacitors. Operating in a regime of high efficiency, the boost converter stage provides a moderate boost to the input voltage, while its pulsed output naturally charges up the capacitor ladder through the diodes. The charge pump circuit multiplies the boost converter’s output voltage, ideally by a factor equal to the number of charge pump stages. The output power is limited by the size of the charge pump capacitors and the maximum output power of the boost converter.

STAGE N VC

Voltage Multiplier

RL Vo STAGE II VC

STAGE I VC

Vin Q

Boost Converter Stage

Figure 14. Hybrid voltage multiplier boost converter.

As shown in Figure 15, replacing the inductor in a classical boost converter with a tapped inductor results in a combination of boost and flyback converter topology, named the tapped inductor boost converter [72]. The voltage gain of this converter is greatly improved, which depends Micromachines 2019, 10, 144 13 of 20 on the switching duty cycle and the transformer turns ratio.

FigureFigure 1 15.5. TappedTapped inductor inductor boost boost convertor convertor..

Super voltage boboostost technology is is widely used in electronic converter design design to to increase the voltage transmission gain. Despite Despite the the high high complexity complexity of of the the converter, converter, the the super voltage boost converter can can generate generate output output voltages voltages that that are are related related to the geometric progressi progressionon of the cascaded circuitMicromachines [ [7733––775 20195].]. Two, Two-stage10, x- stageFOR PEER cascade cascade REVIEW boost boost converter converter is iscascaded cascaded by byconventional conventional boost boost converters, converters,15 of as22 shownasMicromachines shown in inFigure 2019 Figure, 10 1,6 x16. FORIt. Itcan PEER can sufficiently sufﬁcientlyREVIEW meet meet the the high high driving driving voltage voltage requirement requirement of of piezoelectric 15 of 22 actuators because of its high gain performance.

Figure 16. Cascade boost converter. Figure 16.16. Cascade boost converter.converter. The cascade boost converter is suitable for driving piezoelectric actuators in hundred volt level. Nevertheless,TheThe cascadecascad DEAse boost are converter electrically is suitableactuated for material driving devices piezoelectric that produce actuators large in hundred deformation volt whenlevel. Nevertheless,aNevertheless, high driving DEAs voltage areare in electricallyelectrically a few thousand actuatedactuated volts materialmaterial is applied devicesdevices to the thatthat elec produceproducetrode. largeThelarge conversion deformationdeformation between whenwhen alowa highhigh voltage drivingdriving coming voltagevoltage from inin alithiuma fewfew thousandthousand battery and voltsvolts high isis appliedapplied exciting toto voltage, thethe electrode.elec whichtrode. canTheThe drive conversionconversion DEA actuators, betweenbetween lowislow not voltagevoltage enough. comingcoming Therefore, fromfrom lithiumlithiumthe conventional batterybattery andand cascade highhigh excitingexciting boost converter voltage,voltage, whichwhichbecomes can impractical. drive DEA A actuators, feasible iscircuitis notnot enough.enough. that can Therefore,Therefore, achieve a thefewthe conventional conventionalthousand volts cascadecascade with boostcompactboost converterconverter package becomesbecomes [76] is presented impractical.impractical. in AFigureA feasiblefeasible 17. circuitThecircuit two thatthat-stage cancan cascade achieveachieve boost a few converter thousand is volts derived with from compact the twopackage -stage [7 [ 76boost6] is presentedconverter in by Figure adding 117 7a. Thedouble/enhancedThe two-stagetwo-stage cascade circuit boost in each converter converter conversion is is derived derived stage. from from the the two two-stage-stage boost boost converter converter by by adding adding a adouble/enhanced double/enhanced circuit circuit in ineach each conversion conversion stage. stage.

Figure 17.17. TwoTwo-stage-stage cascade boost converter.converter. Figure 17. Two-stage cascade boost converter. FigureFigure 1818 showsshows thethe circuitcircuit conﬁgurationconfiguration ofof highhigh conversionconversion ratioratio boostboost converter.converter. ThisThis boostboost converter is used to transfer energy from the DC source V in the low-voltage side to the DC output converterFigure is 1used8 shows to transfer the circuit energy configuration from the DC of source high conversion Vinin in the low ratio-voltage boost sideconverter. to the DCThis output boost Vo in the high-voltage side. When the proposed converter is operating in the boost mode, the circuit Vconvertero in the high is used-voltage to transfer side. When energy the from proposed the DC converter source V inis in operating the low- voltagein the boost side mode,to the DC the outputcircuit characteristic is cascaded by the boost converter and ﬂyback converter with the voltage doubled [77,78]. characteristicVo in the high -isvoltage cascaded side. by When the boostthe proposed converter converter and flyback is operating converter in thewith boost the mode,voltage the doubled circuit [7characteristic7,78]. is cascaded by the boost converter and flyback converter with the voltage doubled [77,78].

Figure 18. High conversion ratio boost converter. Figure 18. High conversion ratio boost converter. PTs have high voltage gain ratio and high power density (up to 40 W/cm3) [79] and have been widelyPTs used have inhigh actuators voltage andgain sensors.ratio and Generally, high power PTs density have (up to operateto 40 W/cm close3) [7to9 ] the and mechanical have been resonancewidely used frequency in actuators to obtain and highsensors. voltage Generally, gain and PTs power have toefficiency. operate The close equivalent to the mechanical electrical resonance frequency to obtain high voltage gain and power efficiency. The equivalent electrical Micromachines 2019, 10, x FOR PEER REVIEW 15 of 22

Figure 16. Cascade boost converter.

The cascade boost converter is suitable for driving piezoelectric actuators in hundred volt level. Nevertheless, DEAs are electrically actuated material devices that produce large deformation when a high driving voltage in a few thousand volts is applied to the electrode. The conversion between low voltage coming from lithium battery and high exciting voltage, which can drive DEA actuators, is not enough. Therefore, the conventional cascade boost converter becomes impractical. A feasible circuit that can achieve a few thousand volts with compact package [76] is presented in Figure 17. The two-stage cascade boost converter is derived from the two-stage boost converter by adding a double/enhanced circuit in each conversion stage.

Figure 17. Two-stage cascade boost converter.

Figure 18 shows the circuit configuration of high conversion ratio boost converter. This boost converter is used to transfer energy from the DC source Vin in the low-voltage side to the DC output Vo in the high-voltage side. When the proposed converter is operating in the boost mode, the circuit characteristic is cascaded by the boost converter and flyback converter with the voltage doubled [7Micromachines7,78]. 2019, 10, 144 14 of 20

Micromachines 2019, 10, x FOR PEER REVIEW 16 of 22 circuit of a PT is shown in Figure 19a. The gain of a PT is high at low loads, making it a good candidate FigureFigure 1 18.8. HighHigh conver conversionsion ratio ratio boost boost converter converter.. for the high-voltage, low-current requirements of voltage-mode actuators. FigurePTsPTs have 19b high shows voltage the class gain “E” ratio resonant and highhigh topology powerpower has densitydensity a low (up(up number toto 4040W/cm W/cmof additional33))[ [7799]] and and components. have have been been Thewidely inductor used used inis in actuatorsselec actuatorsted to and resonate and sensors. sensors. with Generally, the Generally, input PTs capacitance have PTs to have operate C toin operateof close the toPT closethe at mechanicala frequencyto the mechanical resonance close to theresonancefrequency mechanical frequencyto obtain resonance high to obtainfrequency voltage high gain [80 voltage and]. The power resonance gain efﬁciency. and transfers power The efficiency. energy equivalent to the The electrical PT equivalent from circuit the inductor electrical of a PT whenis shown the switch in Figure is off.19a. The The switch gain of is a turned PT is high on again at low as loads, soon making as the voltag it a goode across candidate Cin is for zero. the Regulationhigh-voltage, of the low-current output voltage requirements is achieved of voltage-mode by varying the actuators. switching frequency.

1:N L C R

Cin Co

( a )

L D

PT RL Vin Q D C VO

( b )

FigureFigure 19 19.. (a)(a Piezoelectric) Piezoelectric transformer transformer equivalent equivalent circuit. circuit. (b) (b Class) Class “E” “E” power power amplifier ampliﬁer topology topology..

TableFigure 3 19 showsb shows some the valuable class “E” parameters resonant topology for comparing has a low and number quantifying of additional the five components. types of topologyThe inductor performance is selected parameters. to resonate with the input capacitance Cin of the PT at a frequency close to the mechanical resonance frequency [80]. The resonance transfers energy to the PT from the inductor when the switch is off.Table The 3. switch Comparison is turned of the on main again parameters as soon asof thethe five voltage described across topologies Cin is zero.. Regulation of the output voltage is achieved by varying the switching frequency. Table3 showsHybrid some valuable parametersTapped for comparingN-Stage and quantifyingHigh Conversion the ﬁve typesClass of topology “E” Components Voltage Inductor Boost Cascade Boost Ratio Boost Power performance parameters. Multiplier Convertor Converter Converter Amplifier Inductor Table1 3. Comparison of the1 main parametersn of the ﬁve described1 topologies. 1

Capacitor Hybrid2n Voltage Tapped1 Inductor N-Stagen Cascade High Conversion3 Class “E”1 Power Components Multiplier Boost Convertor Boost Converter Ratio Boost Converter Ampliﬁer DiodeInductor 2n + 1 1 1n n+ 1 13 12 Capacitor 2n 1 n 3 1 SwitchDiode 1 2n + 11 1 n +1 1 31 21 Switch 1 1 1 1 1 n DN 1 1 n N 1 Gain n DN+1 ( 1 ) n N+1 Fixed (≥100) Gain 1−D 1−D ()1−D 1−D Fixed (≥100) Note: n is the number1D of cascaded stages.1 D N is the turns1 ratioD between secondary1 D side and primary side of Note:the transformer. n is the number of cascaded stages. N is the turns ratio between secondary side and primary side of the transformer.

According to Table3, a hybrid voltage multiplier boost converter has n times gain in contrast to a conventional boost converter because it uses n times voltage multiplier. The disadvantage of this topology is its large size, high weight and low efficiency (caused by the multiplier). However, it is commonly used in hundreds of voltage outputs because it is easily fabricated, with a tapped inductor boost converter capable of achieving the boosting capability without a high duty cycle. To achieve high voltage gains, this method has a considerably lesser number of parts than the hybrid voltage multiplier boost converter. However, the rectifier diode and output capacitor must be rated for the output voltage. Additionally, a custom transformer should be demanded to meet the low mass requirement in microroboticMicromachines 2019 applications, 10, x FOR PEER because REVIEW no commercial parts below 10 g can be purchased. As indicated17 of 22 in Table3, an n-stage cascade boost converter has a higher gain than the traditional boost converter. Theboost gain converter. also depends The gain on also the switchingdepends on duty the cycle.switching With duty this cycle. simple With converter, this simple a high converter, boosting a functionhigh boosting can be functionobtained. can As illustrated be obtained. in Table As illustrated3, a high conversion in Table 3, ratio a high boost conversion converter can ratio obtain boost highconverter voltage can gain obtai byn increasinghigh voltage the gain turns by ratio increasing of the coupled the turns inductor. ratio of This the convertercoupled inductor. has the same This disadvantageconverter has as the a tapped same inductor disadvantage boost as converter. a tapped The inductor customized boost transformer converter. is The a critical customized factor andtransformer difficult tois fabricate.a critical fact APTor is and better difficult than ato magnetic fabricate. transformer APT is better because than ofa itsmagne simpletic transformer geometries, givingbecause it potentialof its simple in milligram-scale geometries, giving power it potential actuator in design. milligram-scale power actuator design.

5.3.5.3. DriveDrive StageStage AsAs mentionedmentioned earlier,earlier, thesethese convertersconverters areare DC–DCDC–DC convertersconverters whosewhose outputsoutputs areare highhigh enoughenough toto drivedrive thethe piezoelectricpiezoelectric actuator.actuator. IfIf wewe wantwant thethe wingswings ofof roboticrobotic insectsinsects toto startstart vibrating,vibrating, thenthen anan arbitraryarbitrary unipolarunipolar drivedrive voltagevoltage shouldshould bebe provided.provided. UsingUsing anan inductor,inductor, twotwo additionaladditional switchesswitches andand twotwo self-timedself-timed shutdownshutdown diodesdiodes withwith capacitivecapacitive loads,loads, [[8181]] proposedproposed aa highlyhighly remarkableremarkable energyenergy recoveryrecovery from from the the wing wing vibration. vibration. However, However, this proposed this proposed design design only focuses only focuses on the charge on the recovery charge ofrecovery piezoelectric of piezoelectric actuators withactuators quasi-square with quasi waves.-square Another waves. feasible Another method feasible is tomethod use an is LC to resonanceuse an LC toresonance obtain an to arbitraryobtain an drivingarbitrary wave driving [4]. wave This [4]. topology This topology is called is the called switching the switching ampliﬁer amplifier driver (Figuredriver (Figure20). 20).

Q1 D1 C CH L 1

Vin Vs Va

Q2 D2 C2 CL

FigureFigure 20.20. SwitchingSwitching ampliﬁeramplifier driverdriver withwith LC.LC.

After a series of charge and discharge pulses is applied to Q and Q at the appropriate time, an After a series of charge and discharge pulses is applied to Q11 and Q22 at the appropriate time, an arbitrary waveform can be generated at Va. Differently, only a small amount of energy is processed in arbitrary waveform can be generated at Va. Differently, only a small amount of energy is processed eachin each switching switching cycle, cycle, which which can can be usedbe used to minimizeto minimize the the size size ofthe of the inductor. inductor. 5.4. Control of Proposed Power Electronic Interfaces After this the design and implementation of the proposed power electronic interfaces, a control system for the diagnostic of the actuator is needed and selected to be evaluated and implemented. The control system using estimation of the feedback parameters is shown in Figure 21. The problem with the implementation of the control system is that it does not use the feedback of actuator displacement directly, instead it comes from the driving voltage/current estimation. The indirect feedback takes some time before the controller reduces/increase the control parameters when an overshoot/undershot occurs, if the estimated feedback parameters is too high or too low when Micromachines 2019, 10, x FOR PEER REVIEW 18 of 22

5.4.Micromachines Control of2019 Proposed, 10, 144 Power Electronic Interfaces 16 of 20 After this the design and implementation of the proposed power electronic interfaces, a control systema change for is the requested. diagnostic To improveof the actuator the controllability, is needed and one selected of the following to be evaluated techniques and can implemented. be used (but Theis not control limited system to these). using estimation of the feedback parameters is shown in Figure 21.

Target Control Parameters Controller

Actuator Feedback Parameters FigureFigure 21 21.. BlockBlock diagram diagram of of the the control control system system..

TheFirstly, problem the Gain with Scheduling the implementation Controller usesof the different control controlsystem parametersis that it does depending not use the either feedback on the oferror, actuator the size displacement of the step or directly, the region inst ofead the it requested comes from feedback the driving parameters voltage/current [82]. This should estimation. reduce The the indirectovershoot feedback that some takes time some when time big before steps the are controller taken. One reduces/increase advantage of this the is control that the parameters gain scheduling when ancontroller overshoot/undershot can handle the occurs, different if the regions estimated of the feedback actuator betterparameters than a is conventional too high or controllertoo low wh (staticen a changePI or PID is requested. controller, etTo al). improve But, the the disadvantage controllability, is thatone isof stillthe needsfollowing to be techniques tuned properly. can be used (but is notIn limited addition, to these). the LQ Controller uses a state space model of the actuator and an observer that is usedFirstly, to create the the Gain control Scheduling signal to Controller the system uses [83 ].different This requires control both parameters an accurate depending description either of theon thesystem error, and the observer. size of the For step the or observer the region a Kalman of the filterrequested is often feedback used. Oneparameters advantage [82]. is This that should the LQ reducecontroller the bothovershoot can handle that some disturbances time when and big follow steps are the taken. reference One signal advantage equally of goodthis is or that better the gain than schedulingconventional controller controller. can The handle disadvantage the different is that regions the LQ controller of the actuato is morer better complex than to a implement conventional and controllertune than classic(static controller.PI or PID controller, However, ifet a al). good But, model the disadvantage of the actuator is is that available is still the needs implementation to be tuned properly.of the LQ controller becomes less complex. InThe addition, Self Tuning the ControllerLQ Controller is based uses on a state a black space box model ofof thethe actuatoractuator [ 84and]. Toan getobserver an estimation that is usedof the to black create box the model, control the signal control to the system system estimation [83]. This needs requires tobe both done an online.accurate Then description the controller of the systemuses parameters and observer. from For the the control observer system a Kalman estimation filter to is calculate often used. the newOne controladvantage strategy. is that This the willLQ controllerallow the controlboth can system handle to disturbances adapt to changes and follow that occur the duereferen to differencesce signal equally in the loadgood force or better and otherthan conventionalexternal changes. controller. One advantage The disadvantage of this is thatis that it isthe capable LQ controller of handle is changesmore complex to the controlto implement system andwithout tune losing than the classic simplicity controller. of the However, PID controller. if a goodNevertheless, model of it needs the actuator to have ais control available system the implementationestimator that makes of the it LQ hard cont to guaranteeroller becomes the stability less complex. of thecontrol system because of the dynamics of the estimation.The Self Tuning Controller is based on a black box model of the actuator [84]. To get an estimationThe general of the solutionsblack box of model, the three the described control system controllers estimation can be usedneeds to to control be done the online. described Then power the controllerelectronic uses interfaces parameters for actuators from the and con eventrol cansystem be used estimation in other to applications. calculate the Self-Tuning new control Controller strategy. Thiswas chosenwill allow to bethe used control more system generally. to adapt One to other changes reason that was occur that due it uses to differences the PID structure in the load which force is andmore other understandable external changes. than theOne Gain advantage Scheduling of this Controller is that it is and capable LQ controller. of handle changes to the control system without losing the simplicity of the PID controller. Nevertheless, it needs to have a control system6. Conclusions estimator that makes it hard to guarantee the stability of the control system because of the dynamicsThis paperof the summarizesestimation. and discusses the system level of FWMAVs with a focus on state-of-the-art FWMAVs,The general aerodynamic solution mechanisms,s of the three transmission described mechanismscontrollers can and be power used electronicto control interfaces. the described First, powervarious electronic FWMAVs interfacesdriven by electrical for actuators motor, and mechanical even can transmission be used in otherstructure applications. and “artificial Self muscles”-Tuning Controllermaterial and was investigated chosen to be by used research more generally. institutes One are presentedother reason in detail.was that The it uses unique the PID aerodynamic structure whichmodes is of more bird-mimetic understandable flapping than wing the andGain insect-mimetic Scheduling Controller flapping and wing LQ aerial controller. vehicles, which are unlike those of fixed-wing and rotary-wing aerial vehicles, are likewise elaborated. The selection 6.and Conclusion design ofs the mechanical transmission are considered based on the stringent requirement of physicalThis andpaper electrical summarizes performance and discusses in micrometer- the system to centimeter-scalelevel of FWMAVs level. with Finally, a focus power on state electronic-of-the- arttopologies FWMAVs suitable, aerodynamic for driving mechanisms, “artificial muscle” transmission materials mechanisms used in FWMAVs and power are electronic stated. These interfaces. results First,present various some possibleFWMAVs solutions driven by for electrical the creation motor, of insect-sized mechanical FWMAVs transmission and astructure substantial and step “artificial toward the realization of flying microrobots. Micromachines 2019, 10, 144 17 of 20

Further size and weight reductions of FWMAVs are important issues for the future. MEMS technologies can be used to provide devices, such as lighter, smaller and less power consuming components than the current state of-the-art ones. Nanotechnology could play an important role also in aerodynamic improvements. FWMAVs will most likely be equipped with GPS and radar systems. Infrared and/or high-definition cameras could be included. Furthermore, trends could also include the development of sophisticated software that will enable the operation of future ultrasmall FWMAVs. Finally, with the improvements of artificial intelligence, some of them will have decision-making capabilities, opening the way to completely new mission profiles.

Author Contributions: C.C. and T.Y. conceived the research and wrote the paper. Funding: This research was supported by the National Key R&D Program of China (No. 2018YFC1503802, No. 2016YFC0303902), the National Natural Science Foundation of China (No. 61871199), the Science and Technology Department of Jilin Province of China (No. 20180201022GX) and the Education Department of Jilin Province of China (No. JJKH20180154KJ). Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

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