Comparison of Different Technologies for Soft Robotics Grippers

sensors

Review Comparison of Different Technologies for Soft Robotics Grippers

Silvia Terrile * , Miguel Argüelles and Antonio Barrientos

Centre for Automation and Robotics (CAR), Universidad Politécnica de Madrid—Consejo Superior de Investigaciones Cientíﬁcas, 28006 Madrid, Spain; [email protected] (M.A.); [email protected] (A.B.) * Correspondence: [email protected]

Abstract: Soft grippers have experienced a growing interest due to their considerable flexibility that allows them to grasp a variety of objects, in contrast to hard grippers, which are designed for a specific item. One of their most remarkable characteristics is the ability to manipulate soft objects without damaging them. This, together with their wide range of applications and the use of novels materials and technologies, renders them a very robust device. In this paper, we present a comparison of different technologies for soft robotics grippers. We fabricated and tested four grippers. Two use pneumatic actuation (the gripper with chambered fingers and the jamming gripper), while the other two employ electromechanical actuation (the tendon driver gripper and the gripper with passive structure). For the experiments, a group of twelve objects with different mechanical and geometrical properties have been selected. Furthermore, we analyzed the effect of the environmental conditions on the grippers, by testing each object in three different environments: normal, humid, and dusty. The aim of this comparative study is to show the different performances of different grippers tested under the same conditions. Our findings indicate that we can highlight that the mechanical gripper with a passive structure shows greater robustness. Citation: Terrile, S.; Argüelles, M.; Barrientos, A. Comparison of Keywords: grasping and manipulation of soft objects; soft robotics; soft gripper; pneumatic gripper; Different Technologies for Soft jamming gripper; passive gripper Robotics Grippers. Sensors 2021, 21, 3253. https://doi.org/10.3390/ s21093253 1. Introduction Academic Editor: Seokheun Choi Robotics grippers are one of the most important components in the industry because Received: 1 April 2021 of their capability to manipulate objects. They not only realize transportation tasks but Accepted: 28 April 2021 also perform quality control to optimize robotic work cells [1]. They can be used in several Published: 8 May 2021 applications, such as industry, medicine, space exploration, and so on. Each application presents different characteristics and needs specific solutions, so grippers can be divided Publisher’s Note: MDPI stays neutral into categories based on their function as described in [2]. Thanks to the advance in soft with regard to jurisdictional claims in material and soft actuators, soft grippers have gained relief in the field of soft robotics in published maps and institutional affil- the last years. iations. Traditional hard robotic grippers consist of a set of mostly rigid joints and links [3] that have to precisely adapt to the object to not damage it. Consequently, their use is limited to a specific item type. If a variety of objects needed to be manipulated, it will be very difficult, if not impossible, to grasp them with the same rigid gripper. Copyright: © 2021 by the authors. The solution is to increase the degrees of freedom of the fingers that form the gripper, Licensee MDPI, Basel, Switzerland. using, e.g., flexible materials for finger fabrication. This allows grippers to passively adapt This article is an open access article to objects. Using the principles of soft robotics, gripping tools with a greater robustness distributed under the terms and can be developed. This implies that they do not require changes in their programming or conditions of the Creative Commons construction to manipulate different objects, and the precision needed to make the grips Attribution (CC BY) license (https:// is reduced. creativecommons.org/licenses/by/ 4.0/).

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2. Related Work Although the development of soft robotics is relatively recent, a variety of soft grippers exist, as seen in [4]. Rigid gripping tools with electromechanical actuation are the first step toward soft robotics. Using a chain of rigid elements, grippers can adapt to the shape of the objects. The greater the number of elements that compose the chain, the better the finger profile approximate to that of a continuous line. This is interesting because a larger contact area between the gripping tool and the object leads to a reduced pressure exerted on that area, thus resulting in a lower risk of damaging delicate objects. A prototype based on this idea was developed in 1978. This gripper could adapt to objects of almost any shape and size and grip them with uniform pressure along with the whole finger. Moreover, it presented a relatively simple mechanism and control implemented only with the traction of a pair of wires [5]. An evolution of the previous example is a tendon-driver gripper fabricated through shape deposition manufacturing, which presents joints formed by an elastomer as well as actuators and sensors incorporated in hard rigid polymers [6]. In another case, two different types of silicone have been used to realize the fingers of the gripper: one more rigid for the base and another more flexible for the section to be in contact with objects. The actuation is realized only with a cable to allow an easier adaptation of the gripper to objects [7]. Other examples are [8–12]. The need to increase the contact surface between the finger and the object is also the idea behind the grippers that use passive structures actuated with external motors. In this case, cables are not required, because when grippers deform, they adapt to the geometry of the objects. The fourth gripper presented in this study and in [13] are based on this concept. Another large family of soft grippers is the one using pneumatic actuators. In general, they have deformable pneumatic chambers realized with different types of silicone or other elastomers. What distinguishes them is the shapes of the pneumatic chambers. Examples of this type of gripper can be seen in [14–19]. Two interesting types of pneumatic soft grippers are those that use the pneumatic system to create the vacuum and grasp objects by suction, such as [20,21], and those that exploit the principle of jamming transition such as [22–25]. It is also worth mentioning the fabric-based soft grippers; these pneumatic actuators present some advantages over others due to their high flexibility, stiffness customizability, and high force-to-weight ratio [26–28]. Apart from these two categories into which the four grippers described in this research fall, many other technologies have been used: electro-adhesion [29–31], Gecko- adhesion [32,33], dielectric elastomer actuators (DEAs) [34–37], fluidic elastomer actuators (FEAs) [38,39], magnetorheological (MR) fluid [40], and shape-memory materials [41]. In some cases, two different types of actuation are implemented in the same gripper to allow stable and reliable grasping under different working environments as in [42], which presents the combination of the suction and traditional linkage-driven grippers or in [18], which presents the integration of an electro-adhesive system and a pneumatic actuator. All these last technologies have achieved good results, although to date, they are not yet used, especially in the industrial sector, since they do not have the same performance as pneumatic and mechanical grippers. This work presents a review and comparison of different technologies for soft robotic grippers to highlight the more robust mechanical design. Usually, specific robotic gripper designs are presented and tested to demonstrate their effectiveness. Nevertheless, soft robotic grippers present the important characteristic of gripping a variety of objects, so it is useful to know what the best option is and why when robustness is the first requirement. Our results have the purpose of showing how different types of mechanical actuation systems and material selection can influence gripping capability. The mechanical design of grippers is illustrated in Section3, describing separately the first two pneumatic grippers and the last two mechanical grippers. Then, experiments and results are discussed in Section4, followed by the conclusions in Section5. Sensors 2021, 21, x FOR PEER REVIEW 3 of 17

The mechanical design of grippers is illustrated in Section 3, describing separately the first two pneumatic grippers and the last two mechanical grippers. Then, experiments and results are discussed in Section 4, followed by the conclusions in Section 5.

3. Mechanical Design of Grippers In this work, four soft robotic grippers have been recreated starting from commercial products, except for the third one. Each gripper has been designed from scratch, thus being able to modify in part their original design, but always aiming for the finest quality. To realize an effective comparison, two actuation systems (pneumatic and electromechanical) and four design solutions have been chosen. More specifically, two grippers employ a pneumatic actuation system, while the another two utilize an electromechanical actuation system. In addition, grippers present similar sizes and fit the UR3, which is the robotic arm used in the experiments. In the first part of this section, each gripper will be described in terms of its design and actuation. The two type of actuation systems are separately described, being common

Sensors 2021, 21, 3253 to every pair of grippers. 3 of 18

3.1. Pneumatic System 3. Mechanical Design of Grippers InThe this work, two four pneumatic soft robotic grippers grippers have been use recreated the starting actuation from commercial system shown in Figure 1. products, except for the third one. Each gripper has been designed from scratch, thus being able toThe modify pneumatic in part their original module design, but is always composed aiming for the of finest two quality. solenoid To valves. One of the solenoid valvesrealize an effectiveis used comparison, to connect two actuation the path systems that (pneumatic carries and electromechanical) pressurized air to the gripper with the one and four design solutions have been chosen. More specifically, two grippers employ a frompneumatic which actuation air system, is extracted while the another with two the utilize vacuum an electromechanical pump. actuation The other solenoid valve connects the system. In addition, grippers present similar sizes and fit the UR3, which is the robotic arm tubeused in from the experiments. the pressurized air reservoir with the inlet of the first solenoid valve and a blind tube.In theIt operates first part of this as section, an o eachpen/ gripperclose will valve. be described Finally, in terms a of pressure its design regulator controls the pressure and actuation. The two type of actuation systems are separately described, being common thatto every is pairadministered of grippers. to the gripping tools. For compressed air, a compressor with a small

tank3.1. Pneumatic is used, System whereas an electric vacuum pump is used for vacuum generation. The two pneumatic grippers use the actuation system shown in Figure1.

Figure 1. Pneumatic system. (a) Connection diagram. (b) Wiring diagram control. Signals 1, 2, and 3 are the cables that connect to the UR3 for control.

FigureThe pneumatic1. Pneumatic module system. is composed (a of) Connection two solenoid valves. diagram One of. the(b) solenoid Wiring diagram control. Signals 1, 2, and 3valves are the is used cables to connect that the connect path that carries to the pressurized UR3 for air cont to therol gripper. with the one from which air is extracted with the vacuum pump. The other solenoid valve connects the tube from the pressurized air reservoir with the inlet of the first solenoid valve and a blind tube. It operates as an open/close valve. Finally, a pressure regulator controls the pressure 3.1.1.that is administered Gripper to 1 the gripping tools. For compressed air, a compressor with a small tank isThe used, whereasfirst gripper, an electric vacuum shown pump in is used Figure for vacuum 2, is generation. a pneumatic gripper based on the one com- mercialized3.1.1. Gripper 1 by Soft Robotics [43]. They developed several configuration solutions de- The first gripper, shown in Figure2, is a pneumatic gripper based on the one commer- pendingcialized by Soft on Robotics the application. [43]. They developed In several this configurationwork, the solutions four depending-finger solution, with fingers symmetri- callyon the application.spaced Inaround this work, thethe four-finger center solution, of the with gripp fingers symmetricallyer, has been spaced adopted. around the center of the gripper, has been adopted.

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Figure 2. First gripper: (a) CAD model, (b) Gripper during the operation, (c) Fingers flexed, (d) and Fingers in extension.

The most important aspect of the design of fingers is its asymmetry along the length- wise plane. This is necessary to allow movements. The inner side presents a flat surface with small reliefs that stick out to improve gripping capacity. The outer side presents six chambers that inflate when pressurized air is introduced inside, generating the contrac- tion movement. Vice versa, creating a vacuum inside, they contract and generate the extension movement. At one end, the finger has a section that allows it to be held at the base of the gripper, as well as allowing air to enter and exit the chamber. On the other end, a section with Figure 2. First gripper: (a) CAD model, (b) Gripper during the operation, (c) Fingers flexed, (d) and Fingers in extension. Figure 2. First grippergreater: (a) CAD rigidity model, (andb) Gripper with duringa triangular the operation profile, (c) hasFingers been flexed added, (d) to the finger. This provides and Fingers in extension. The most important aspect of the design of fingers is its asymmetry along the length- a wiserelatively plane. This straight is necessary section to allow at movements. the end The of inner the side finger presents to aperform flat surface a better grip with the ex- The most importanttremity.with smallaspect Fingers reliefs of the that measure design stick out of to 11 improvefingers cm in grippingis lengthits asymmetry capacity. and 4.5 The along cm outer in the side width. length- presents six chambers that inflate when pressurized air is introduced inside, generating the con- wise plane. This is necessarytractionFor movement. the to creationallow Vice movements. versa, of this creating prototype, aThe vacuum inner inside, twoside they partspresents contract can anda beflat generate distinguished surface the : manufacturing sil- with small reliefs thaticoneextension stick fingers out movement. to and improve the 3D gripping printing capacity. of the restThe outerof the side pieces presents that formsix the base of the gripping chambers that inflate whenAt one pressurized end, the finger air has is a section introduced that allows inside, it to be generat held at theing base the ofthe contrac- gripper, toolas well (all as 3D allowing printed air to parts enter and are exit made the chamber. with Onthe the thermoplastic other end, a section polyester with PLA). tion movement. Vicegreater versa,To rigidity obtaincreating and a with asilicone vacuum a triangular finger, inside, profile hastheya mold been contract added has to beenand the finger. generate realized This provides the with ex- a 3D printing, as shown in tension movement.Figure relatively 3. straight It comprises section at the three end of thedifferent finger to perform parts: a bettera male grip withthat the provides extremity. the internal shape of the At one end, the Fingersfinger measurehas a section 11 cm in that length allows and 4.5 it cm to in be width. held at the base of the gripper, finger,For and the creation two females, of this prototype, which two provide parts can the be distinguished: exterior. The manufacturing silicone used is the Silastic® 3483, as well as allowing air to enter and exit the chamber. On the other end, a section with withsilicone a cure fingers time and theof 3Done printing hour. of the rest of the pieces that form the base of the greater rigidity and withgripping a triangular tool (all 3D printed profile parts has are been made withadded the thermoplasticto the finger. polyester This PLA).provides a relatively straight sectionThisTo obtain at silicone the a silicone end allowsof finger, the afinger get moldting has to beenperformthe realizeddesired a withbetter characteristics 3D grip printing, with as shownthe with ex- in regard to stiffness, while otherFigure silicones3. It comprises such three as different Ecoflex parts:® a00 male-30 that have provides proved the internal too shapesoft offor the this application. tremity. Fingers measurefinger, 11 and cm two in females, length which and provide 4.5 cm the in exterior. width. The silicone used is the Silastic® 3483, For the creationwith of thisThe a cure prototype, working time of one hour.twopressure parts canof the be distinguishedgripping tools: m anufacturingis around 2 bars.silicone fingers and the 3D printing of the rest of the pieces that form the base of the gripping tool (all 3D printed parts are made with the thermoplastic polyester PLA). To obtain a silicone finger, a mold has been realized with 3D printing, as shown in Figure 3. It comprises three different parts: a male that provides the internal shape of the finger, and two females, which provide the exterior. The silicone used is the Silastic® 3483, with a cure time of one hour. This silicone allows getting the desired characteristics with regard to stiffness, while other silicones such as Ecoflex® 00-30 have proved too soft for this application. The working pressure of the gripping tools is around 2 bars.

Figure 3. Mold for manufacturing silicone ﬁngers: (a) Mold for inner side, (b) Mold for outer side, Figure(c) Male 3. mold, Mold (d) The for assembled manufacturing mold. silicone fingers: (a) Mold for inner side, (b) Mold for outer side, (c) Male mold, (d) The assembled mold.

3.1.2. Gripper 2 The second gripper, as shown in Figure 4, is also pneumatic and is based on the Versaball, which is a gripper produced by the company Empire Robotics Inc. [44]

Figure 3. Mold for manufacturing silicone fingers: (a) Mold for inner side, (b) Mold for outer side, (c) Male mold, (d) The assembled mold.

3.1.2. Gripper 2 The second gripper, as shown in Figure 4, is also pneumatic and is based on the Versaball, which is a gripper produced by the company Empire Robotics Inc. [44]

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Its operation is based on the jamming process by which granular materials increase their viscosity when the density of particles increases. The gripper presents a spherical chamber made of an elastomeric plastic membrane filled with granular material. It is possible to get the liquid and solid pseudo-states when pressurized air and vacuum are applied, respectively. To grab an object, a small amount of air has to be introduced in the chamber so that Sensors 2021, 21, 3253 the granular material acts similar to a liquid. Then, the gripper5 of 18 has to be pressed against the object, adapting the contents of the chambers to the outer shape of the object. Finally, the vacuum is created inside the chamber, in order to solidify its interior and maintain the This silicone allows getting the desired characteristics with regard to stiffness, while shapeother silicones of the such object. as Ecoﬂex As® 00-30 long have as proved the vacuum too soft for thisis maintained, application. the object is held by the gripper. Subsequently,The working pressure it is enough of the gripping to fill tools the is around chamber 2 bars. with air again, to return to the “liquid” state and3.1.2. release Gripper 2 the object. This method of gripping requires that the object size be approxi- The second gripper, as shown in Figure4, is also pneumatic and is based on the matelyVersaball, half which of is a the gripper diameter produced by of the the company pneumatic Empire Robotics chamber. Inc. [44]

FigureFigure 4. Second4. Second gripper: g (rippera) CAD model: (a) ofCAD the gripper, model (b) Gripper of the during gripper the operation., (b) Gripper during the operation. Its operation is based on the jamming process by which granular materials increase their viscosity when the density of particles increases. TheThis gripper gripper presents is a spherical composed chamber of made the of previously an elastomeric plastic described membrane pneumatic module, a piece to joinfilled this with granularmodule material. with It isthe possible claw, to get two the liquidpieces and to solid keep pseudo-states the filter when in place, and two final pieces pressurized air and vacuum are applied, respectively. thatTo hold grab anthe object, pneumatic a small amount camera. of air has to be introduced in the chamber so that the granularAs in material the previous acts similar tocase, a liquid. the Then, pneumatic the gripper has chamber to be pressed is against the basic element of the gripper. the object, adapting the contents of the chambers to the outer shape of the object. Finally, Tothe realize vacuum is it, created a latex inside balloon the chamber, of inapproximately order to solidify its interior1-mm and thickness maintain and about 8 cm in diameter hasthe shapebeen of chosen. the object. AsThe long chosen as the vacuum thickness is maintained, provides the object the is heldchamber by the with resistance against the gripper. Subsequently, it is enough to fill the chamber with air again, to return to the abrasive“liquid” state action and release of thethe object. granular This method material of gripping and requires ensures that the that object it size is not damaged when coming intobe approximately contact with half of theobjects. diameter At ofthe the pneumatic same chamber.time, a smaller thickness adapts easier to the small This gripper is composed of the previously described pneumatic module, a piece to irregularitiesjoin this module with of thethe claw, objects, two pieces besides to keep the gaining filter in place, flexibility and two final and pieces elasticity. that holdSeveral the pneumatic options camera. of granular materials have been tested, such as ground coffee, fine As in the previous case, the pneumatic chamber is the basic element of the gripper. sand,To realize and it, a latexbreadcrumbs. balloon of approximately After testing 1-mm thickness ground and about coffee 8 cm inand diameter breadcrumbs, it has experimen- tallyhas been been chosen. observed The chosen that thickness with provides ground the chamber coffee with, the resistance chamber against the adapts much better to objects. abrasive action of the granular material and ensures that it is not damaged when coming Thisinto contact is the with same objects. material At the same used time, a in smaller the thicknessresearc adaptsh previously easier to the small mentioned. irregularitiesA filter of the is objects, necessary besides gainingto allow flexibility air andto elasticity.pass in both directions and maintain the granular Several options of granular materials have been tested, such as ground coffee, fine materialsand, and breadcrumbs. isolated After from testing the ground pneumatic coffee and breadcrumbs, system. itThe has experimentally filter is composed of a felt circle with thebeen same observed diameter that with ground as the coffee, upper the chamber section adapts of much the better base to objects. and Thisa thickness is of approximately one the same material used in the research previously mentioned. millimeter.A filter is necessary The working to allow air pressure to pass in both is directions around and 1 maintainbar. the granular material isolated from the pneumatic system. The filter is composed of a felt circle with 3.2. Electromechanical System The other two grippers are mechanical and use a 6 V DC motor with a reduction gearbox. To calculate the reduction, two parameters must be considered: the approximate closing time of the gripper and its generated torque. For the third gripper, a great reduction ratio is necessary, since the torque generated by the motor must be enough to pull the three cables of the three fingers of the gripper simultaneously, so a reduction of 1:298 is necessary at least.

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the same diameter as the upper section of the base and a thickness of approximately one millimeter. The working pressure is around 1 bar. 3.2. ElectromechanicalIndeed, Systema fourth gripper presents a spindle nut system that generates a suitable The other two grippers are mechanical and use a 6 V DC motor with a reduction gearbox.torque To for calculate this the application. reduction, two parameters Thus, mustthe be reduction considered: the is approximate chosen only based on the gripper closing closing time of the gripper and its generated torque. timeFor and the third is gripper,equal a greatto 1: reduction125. ratio is necessary, since the torque generated by the motorThe mustsame be enough structure to pull the is three used cables to of mount the three ﬁngers the ofmotors the gripper on the two grippers, as the motor simultaneously, so a reduction of 1:298 is necessary at least. sizeIndeed, is identical a fourth gripper in presentsspite aof spindle the nutdifferent system that generatesreduction a suitable ratio. torque This structure is composed of two for this application. Thus, the reduction is chosen only based on the gripper closing time andparts: is equal a tobase 1:125. and an adapter for the robotic arm UR3. The base part houses the motor insideThe same and structure is designed is used to mount to thebe motorstightened on the two to grippers, prevent as the motorits longitudinal size displacement. is identical in spite of the different reduction ratio. This structure is composed of two parts: a base andFinally, an adapter the for the simplified robotic arm UR3. electrical The base part scheme houses the motoris showed inside and in Figure 5. The LR group repre- issents designed the to beDC tightened motor to prevent, and its the longitudinal H bridge displacement. is composed of the four bipolar signals and the two Finally, the simpliﬁed electrical scheme is showed in Figure5. The LR group represents thecontrol DC motor, signals. and the H bridgeThese is composed are the of theones four bipolarthat signalsconnect and the to two the control input/output panel of the UR3. signals. These are the ones that connect to the input/output panel of the UR3.

Figure 5. Wiring diagram of the electromechanical system. Figure 5. Wiring diagram of the electromechanical system. 3.2.1. Gripper 3 The first mechanical gripper, as shown in Figure6, presents articulated fingers actuated by3.2.1. cables. Gripper When the cables3 are pulled by a motor connected to a pulley, the fingers are flexed. The first mechanical gripper, as shown in Figure 6, presents articulated fingers actuated by cables. When the cables are pulled by a motor connected to a pulley, the fingers are flexed.

Figure 6. Third gripper: (a) CAD model of the gripper, (b) Real Gripper.

The shape that fingers adopt is a function of the object to be grasped; the links of the finger will adapt to the outer surface of the object. Here, the number of fingers selected to achieve optimum grip is three, which are arranged at 120 degrees. To work properly, the three fingers must perform their movements simultaneously. To carry out this movement, the three cables (one for each finger) are wound on the same pulley that collects or releases the cables, generating thus the flexion or extension of the fingers on the basis of the rotating direction. The pulley represents a crucial part of the design, since it is responsible for the syn- chronization of the movement of fingers. It has an inner section in which a nut is inserted

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Indeed, a fourth gripper presents a spindle nut system that generates a suitable torque for this application. Thus, the reduction is chosen only based on the gripper closing time and is equal to 1:125. The same structure is used to mount the motors on the two grippers, as the motor size is identical in spite of the different reduction ratio. This structure is composed of two parts: a base and an adapter for the robotic arm UR3. The base part houses the motor inside and is designed to be tightened to prevent its longitudinal displacement. Finally, the simplified electrical scheme is showed in Figure 5. The LR group represents the DC motor, and the H bridge is composed of the four bipolar signals and the two control signals. These are the ones that connect to the input/output panel of the UR3.

Figure 5. Wiring diagram of the electromechanical system.

3.2.1. Gripper 3 The first mechanical gripper, as shown in Figure 6, presents articulated fingers actuated by cables. When the cables are pulled by a motor connected to a pulley, the fingers Sensors 2021, 21, 3253 7 of 18 are flexed.

Figure 6. Third gripper: (a) CAD model of the gripper, (b) Real Gripper. Figure 6. Third gripper: (a) CAD model of the gripper, (b) Real Gripper. The shape that fingers adopt is a function of the object to be grasped; the links of the finger will adapt to the outer surface of the object. Here,The the shape number that of fingers fingers selected adopt to achieve is a optimumfunction grip of is three,the object which are to be grasped; the links of the fingerarranged will at 120 adapt degrees. to To the work outer properly, surface the three of fingers the must object. perform their movements simultaneously. ToHere, carry out the this number movement, of the fingers three cables selected (one for each to finger) achieve are wound optimum on the grip is three, which are ar- rangedsame pulley at that120 collects degrees. or releases To the work cables, properly, generating thus the the three flexion orfingers extension must of perform their movements the fingers on the basis of the rotating direction. simultaneouThe pulley representssly. a crucial part of the design, since it is responsible for the synchronizationTo carry of out the movement this movement of fingers. It, the has an three inner sectioncables in (one which afor nut each is finger) are wound on the inserted to fasten the pulley to the flat face of the motor shaft. The pulley is designed with samea diameter pulley that is that small collects enough to generateor releases a good the torque cables for the, application generating but large thus the flexion or extension of enough to be printed in 3D and not too fragile. It also has a duct with a circular section thethat fingers connects the on outer the face basis of the pulley,of the where rotating the cables direction. are wound, with the underside of theThe pulley. pulley represents a crucial part of the design, since it is responsible for the syn- The three fingers are held on a base, and an adapter is used for the connection to the chronizationrobot. Each finger of comprises the movement three base modules, of fingers. one end, It and has a mount. an inner Modules section are in which a nut is inserted joined with screws that allow their relative rotation. Each finger measures 12 cm in length and 2.5 cm in width.

3.2.2. Gripper 4 The fourth gripper, as shown in Figure7, is based on the MultiChoice Gripper product manufactured by the FESTO® Company [45]. This design is based on the mechanics of the human hand where the thumb serves to impose an effort in the same direction but opposite orientation as the rest of the fingers. To adapt to different object typologies, the tool developed by FESTO is able to reorient the fingers and obtain different configurations. For example, for objects with round perimeter are placed with ternary symmetry. However, for objects with parallel faces, one finger is placed in opposition to the other two. Sensors 2021, 21, x FOR PEER REVIEW 7 of 17

to fasten the pulley to the flat face of the motor shaft. The pulley is designed with a diameter that is small enough to generate a good torque for the application but large enough to be printed in 3D and not too fragile. It also has a duct with a circular section that connects the outer face of the pulley, where the cables are wound, with the underside of the pulley. The three fingers are held on a base, and an adapter is used for the connection to the robot. Each finger comprises three base modules, one end, and a mount. Modules are joined with screws that allow their relative rotation. Each finger measures 12 cm in length and 2.5 cm in width.

3.2.2. Gripper 4 The fourth gripper, as shown in Figure 7, is based on the MultiChoice Gripper product manufactured by the FESTO® Company [45]. This design is based on the mechanics of the human hand where the thumb serves to impose an effort in the same direction but opposite orientation as the rest of the fingers. To adapt to different object typologies, the tool developed by FESTO is able to reorient the fingers and obtain different configurations. For example, for objects with round perimeter are placed with ternary symmetry.

Sensors 2021, 21, 3253 However, for objects with parallel faces, one finger is placed8 of in 18 opposition to the other two.

Figure 7. Fourth gripper: (a) CAD model of the gripper, (b) Gripper during the operation. Figure 7. Fourth gripper: (a) CAD model of the gripper, (b) Gripper during the operation. Nevertheless, for the reproduction of this prototype, this feature has been obviated to focus attention on the design of the fingers: passive structures capable of adapting to theNevertheless, objects that deform f them.or the This reproduction gripping tool comprises of this three prototype, fingers of a this flexible feature has been obviated tomaterial. focus Theattention fingers are on triangular the design with thin of walls the and,fingers: longitudinally, passive they structures present a capable of adapting to thenucleus objects formed that by rigid deform parts that them. prevent This the walls gripping of the finger tool from comprises approaching. three With fingers of a flexible ma- this morphology, the tip remains approximately in the same place when trying to deform terial.the finger, The but thefingers body bends are aroundtriangular the object. with This deformationthin walls of theand, finger longitudinally, allows it to they present a nu- cleusadapt toformed the shape by of the rigid surface parts of the that object withprevent which itthe will walls make contact, of the and, finger thanks from approaching. With to its flexibility, it will deform before damaging it. In this case, the choice of the number of thisfingers morphology, that form the gripper the tip could remains have been approximately two, three, or four. Thein the number same three place has when trying to deform thebeen finger, selected becausebut the the body spindle bends mechanism around makes the the movement object. perfectly This synchronized,deformation of the finger allows it and it is unnecessary to complicate the design by adding a fourth finger, although in some toscenarios, adapt it to might the be shape useful to of have the it. surface of the object with which it will make contact, and, thanksThe grippingto its flexibility, tool consists ofit threewill flexible deform fingers before and a rigiddamaging base. The fingersit. In arethis case, the choice of the numbermade from of the fingers elastomeric that thermoplastic form the produced gripper by could the company have Recreus been [46two,] called three, or four. The number FilaFlex, and they measure 11 cm in length and 1.5 cm in width. The base is formed by two threepieces: thehas main been one onselected which the because fingers are mountedthe spindle and to whichmechanism the motor ismakes coupled, the movement perfectly synchronized,and a secondary one and that servesit is unnecessary to join the gripping to tool complicate with the UR3. the Furthermore, design theby adding a fourth finger, mechanical actuation is composed of a direct current motor, a coupler flexible, a spindle althoughand a nut, and in a some handle toscen whicharios the nut,, it themight finger be connector useful as wellto have as the motorit. support are attached.The gripping tool consists of three flexible fingers and a rigid base. The fingers are To generate the grip movement, each finger can be held with rotational joints at both madeends of from its base. the By maintainingelastomeric the positionthermoplastic of the outer produced ends of the fingerby the bases company and Recreus [46] called FilaFlex,changing the and height they of the measure inner ends, 11 the cm opening in andlength closing and movement 1.5 cm of thein handwidth. is The base is formed by twoachieved. pieces This: movement,the main which one can on be which generated the in many fingers ways, isare done mount with a spindleed and to which the motor is nut mechanism. A motor rotates the spindle, which causes the nut to move up and down coupled,when its rotation and a is secondary prevented. The one inner that ends serves of the fingers to join are the connected gripping to the nuttool with the UR3. Further- more,by connectors, the mechanical which allow the actuation possibility ofis movement composed while of preventing a direct the current rotation of motor, a coupler flexible, the nut. The rigid sections that form the internal structure of the finger have been made with a wire. This wire has enough rigidity to allow the deformation of the finger but at the same time allow it to return to its initial position.

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a spindle and a nut, and a handle to which the nut, the finger connector as well as the motor support are attached. To generate the grip movement, each finger can be held with rotational joints at both ends of its base. By maintaining the position of the outer ends of the finger bases and changing the height of the inner ends, the opening and closing movement of the hand is achieved. This movement, which can be generated in many ways, is done with a spindle nut mechanism. A motor rotates the spindle, which causes the nut to move up and down when its rotation is prevented. The inner ends of the fingers are connected to the nut by connectors, which allow the possibility of movement while preventing the rotation of the Sensors 2021, 21, x FOR PEER REVIEW 8 of 17 nut. The rigid sections that form the internal structure of the finger have been made with a wire.a Thisspindle wireand a nut, has and enough a handle to rigiditywhich the nu tot, the allow finger connectorthe deformation as well as the of the finger but at the motor support are attached. same timeTo allow generate it the to grip return movement, to eachits initialfinger can position. be held with rotational joints at both ends of its base. By maintaining the position of the outer ends of the finger bases and changing the height of the inner ends, the opening and closing movement of the hand is 4. Experimentsachieved. This movement, which can be generated in many ways, is done with a spindle nut mechanism. A motor rotates the spindle, which causes the nut to move up and down Preparationwhen itsof rotat theion Experiments is prevented. The inner ends of the fingers are connected to the nut by Sensors 2021, 21, 3253 connectors, which allow the possibility of movement while preventing9 of 18 the rotation of the Thenut. objective of this work is to test the robustness of the gripping tools and their The rigid sections that form the internal structure of the finger have been made with grasp stability.a wire. This wireTo hastest enough these rigidity parameters, to allow the deformation an experiment of the finger hasbut at beenthe designed in which the 4. Experiments ability sameof the time gripperallow it to return to gripto its initial a certain position. object, to hold it while the robotic arm realizes a Preparation of the Experiments trajectoryThe objective4. Experiments, and of this worksuccessively is to test the robustness release of the grippingit, is toolsevaluated and their grasp (as illustrated in Figure 8). The test is stability. To test these parameters, an experiment has been designed in which the ability deemedof the gripperPreparation tosuccessful grip a certainof the Experiments object, if the to hold object it while thedoes robotic not arm realizesfall aeither trajectory, during the grip or the displacement and successively if the Theobject release objective it, is is evaluated notof this damaged work (as illustrated is to test after inthe Figure robustness gripping8). The testof the is action. deemed gripping tools and their successful ifgrasp the object stability. does notTo test fall eitherthese duringparameters, the grip an or experiment the displacement has been and designed if the in which the object is notability damaged of the after gripper gripping to action. grip a certain object, to hold it while the robotic arm realizes a trajectory, and successively release it, is evaluated (as illustrated in Figure 8). The test is deemed successful if the object does not fall either during the grip or the displacement and if the object is not damaged after gripping action.

Figure 8. Test execution: the gripper picks up one of the twelve objects in a predetermined posi- Figure 8. Testtion execution: and place the gripperit in the picks desired up onelocation. of the twelve objects in a predetermined position Figureand place it 8. in the Test desired execution: location. the gripper picks up one of the twelve objects in a predetermined posi- Since one of the most important aspects is the capability of grippers to manipulate tionSince and one place of the mostit in important the desired aspects is location. the capability of grippers to manipulate objects with different characteristics, i.e., different shape, rigidity, and fragility, twelve objects with different characteristics, i.e., different shape, rigidity, and fragility, twelve objects, as shownobjects, in Figureas shown9, have in beenFigure chosen 9, have for experimentation. been chosen for Each experimentation. of them presents Each of them pre- differentSince characteristicssents differentone to of represent characteristics the amost wider to range important represent of objects. a wider The aspects weight range and of dimensionsobjects.is the The capability weight and di- of grippers to manipulate objectsof objects are mensionswith limited by differentof the objects capacity are andlimitedcharacteristics, dimensions by the capacity of the grippers. and i.e., dimensions different of the grippers.shape, rigidity, and fragility, twelve objects, as shown in Figure 9, have been chosen for experimentation. Each of them presents different characteristics to represent a wider range of objects. The weight and dimensions of objects are limited by the capacity and dimensions of the grippers.

Figure 9. SelectionFigure of objects 9. Selection for experimentation of objects for experimentation with its identification with its identiﬁcation number. (a number.) Tennis (balla) Tennis #1, (b ball) Screen #1, Cleaner #2, (c) Vaseline Container(b) Screen #3, Cleaner(d) Metallic #2, (c )Container Vaseline Container #4, (e) Nut #3, #5 (d, )(f Metallic) Rock #6 Container, (g) Tomato #4, ( e#7) Nut, (h) #5,Egg (f )#8 Rock, (i) #6,Glue Stick #9, (l) Dish Sponge #10(g, )(m Tomato) Broccoli #7, ( h#11) Egg, and #8, (n (i)) Candy Glue Stick Box #9, #12. (l) Dish Sponge #10, (m) Broccoli #11, and (n) Candy Box #12. Objects are oriented in different ways to not facilitate any gripper. For example, object #9 (see Table1) is arranged vertically, whereas object #12 is arranged horizontally. In some cases, external elements have been used to ensure the correct positioning of the object, such as for the egg (#8) (as can be seen in Figure 10 or for the tennis ball (#1), which would have slipped once placed on the surface due to their physical characteristics.

Figure 9. Selection of objects for experimentation with its identification number. (a) Tennis ball #1, (b) Screen Cleaner #2, (c) Vaseline Container #3, (d) Metallic Container #4, (e) Nut #5, (f) Rock #6, (g) Tomato #7, (h) Egg #8, (i) Glue Stick #9, (l) Dish Sponge #10, (m) Broccoli #11, and (n) Candy Box #12.

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Table 1. Selection of objects for experimentation. Each object has been assigned a number to facilitate the subsequent display of results (ﬁrst column). The “Dimensions” column shows the signiﬁcant dimensions of each object, and the “Weight” column shows the weight of the objects used in the experiments. In the last column “Relevant Aspects”, the aspects that differentiate each object from the others are highlighted.

ID Object Dimensions Weight Relevant Aspects Quite rigid and with a rough 1 Tennis ball 70 mm diameter 57.6 g surface 80 mm diameter Considerably soft and 2 Screen Cleaner 12.8 g 30 mm thickness deformable Vaseline 80 mm diameter 3 147 g Very smooth surface container 50 mm height Metallic Prismatic form with some 4 85 × 50 × 30 mm 47.9 g container irregularities, smooth surface 35 mm diameter 5 Nut 14.4 g Very small and lightweight 45 mm height The smallest object of the 6 Rock 25 × 15 × 10 mm 12.9 g experiment 90 mm diameter The heaviest object of the 7 Tomato 229.6 g 70 mm height experiment 50 mm diameter 8 Egg 71 g Hard but very fragile 65 mm height 25 mm diameter Svelte with irregularities at 9 Glue stick 19.6 g 100 mm height the ends Considerably soft and 10 Dish sponge 105 × 80 × 25 mm 10.2 g deformable with a slightly stiffer surface 90 mm diameter Irregular object that should Sensors 2021, 21, x FOR PEER REVIEW 11 Broccoli 130.4 g 10 of 17 120 mm height not be deformed 12 Candy box 60 × 35 × 15 mm 7.9 g Little gripping surface

FigureFigure 10. 10.Test Test execution execution with Gripperwith Gripper 1 (a,b), Gripper 1 (a,b), 2 Gripper (c,d), Gripper 2 (c 3,d (e),,f Gripper), Gripper 3 4 (ge,,hf).), Gripper 4 (g,h).

The time used for successful gripping is different for each of the grippers. In the case of Gripper 1, this time is in the range of 1–2 s, since the fingers must be inflated to a greater or lesser extent according to the object’s size. In the case of Gripper 2, this time is about 3 s, since it is necessary that the gripper first adjusts to the shape of the object to subsequently perform the vacuum. In the case of Gripper 3 and Gripper 4, this time is less than or equal to one second.

5. Discussion The data obtained from the experiments are presented in the following graphics and Tables 2–5.

Table 2. Experiments with Gripper 1. On the left, the table with a success rate for every object, in the last column, the average for every object is calculated, and at the end of the table, the average success rate for every environment is determined. On the right, the graphic representation of Table 2 is reported.

Experiments with Gripper 1 Obj. Normal Humid Dusty Average 1 100% 100% 100% 100% 2 100% 100% 100% 100% Gripper 1 3 100% 100% 0% 67% 4 100% 95% 65% 87% 100% 80% 5 100% 100% 100% 100% 60% 40%

6 20% 30% 0% 17% 20% Rate of successRate of 7 95% 95% 5% 65% 0% 8 0% 0% 0% 0%

9 100% 100% 100% 100% Normal Humid Dusty 10 80% 100% 75% 85% 11 95% 100% 95% 97% 12 100% 100% 100% 100% Avg. 83% 85% 62% 77%

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In the following table, an identification number is assigned to each object to simplify the result tables. The experiment has been carried out in three different environments: normal, humid, and dusty. The normal environment is achieved without changing the properties of the environment. In this case, objects are not damp or dusty. The humid environment consists in moistening both the gripping tool and the object. The presence of humidity causes greater effects on some objects than on others, depending Figure 10. Test execution with Gripper 1 (a,b), Gripper 2 (c,d), Gripper 3 (e,f), Gripper 4 (g,h). on the surface of the object and how much the water can wet it. For instance, while a plastic container with smooth walls may only slightly be affected by a humid environment, The timeproperties used for of asuccessful sponge, such gripping as its weight is anddifferent surface for adhesion, each greatlyof the changegrippers. in such In anthe case of Gripper 1,environment. this time is Finally, in the the range dusty of environment 1–2 s, since is simulated the fingers by spreading must be a generousinflated amount to a greater or lesser extentof flour according both by the to tool the and object by the’s object.size. In The the last case two situationsof Gripper can occur2, this in agriculturetime is about 3 or in specific industrial processes. s, since it is necessaryFor each experiment,that the gripper 20 repetitions first haveadjusts been to performed. the shape This of means the thatobject for eachto subsequently performobject, the 60 repetitions vacuum. have In beenthe case performed of Gripper with each 3 tooland and Gripper twenty have4, this been time performed is less than or equal to onefor each second. environment, which are 240 for each gripper and object. Being twelve objects, a total of 2880 repetitions are performed. In Figure 10, it is possible to see some captions of these experiments. 5. Discussion The time used for successful gripping is different for each of the grippers. In the case The dataof Gripperobtained 1, this from time the is in experiments the range of 1–2 are s, since presented the fingers in must the befollowing inflated to graphics a greater and or lesser extent according to the object’s size. In the case of Gripper 2, this time is about 3 s, Tables 2–5. since it is necessary that the gripper first adjusts to the shape of the object to subsequently perform the vacuum. In the case of Gripper 3 and Gripper 4, this time is less than or equal Table 2. Experiments with Gripper 1. Onto onethe second.left, the table with a success rate for every object, in the last column, the average for every object is calculated, and at the end of the table, the average success rate for every environment is deter- 5. Discussion mined. On the right, the graphic representation of Table 2 is reported. The data obtained from the experiments are presented in the following graphics and Experiments with Gripper Tables1 2–5. Obj. Normal TableHumid 2. Experiments Dusty with Gripper Average 1. On the left, the table with a success rate for every object, in the last column, the average for every object is calculated, and at the end of the table, the average success rate for every environment is determined. On 1 100% the right,100% the graphic100% representation of100% Table 2 is reported.

2 100% 100% Experiments100% with Gripper100% 1 Gripper 1 3 100% Obj. 100%Normal 0% Humid Dusty67% Average 1 100% 100% 100% 100% 4 100% 95% 65% 87% 100% 2 100% 100% 100% 100% 80% 3 100% 100% 0% 67% 60% 5 100% 4100%100% 100% 95% 100% 65% 87% 5 100% 100% 100% 100% 40% 6 20% 30% 0% 17% 20%

6 20% 30% 0% 17% successRate of 7 95% 795% 95%5% 95% 65% 5% 65% 0% 8 0% 0% 0% 0% 8 0% 90% 100%0% 100% 100%0% 100% 10 80% 100% 75% 85% 9 100% 11100% 95%100% 100% 100% 95% 97% Normal Humid Dusty 12 100% 100% 100% 100% 10 80% Avg.100% 83%75% 85% 85% 62% 77% 11 95% 100% 95% 97% 12 100% 100% 100% 100% Avg. 83% 85% 62% 77%

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Table 3. Experiments with Gripper 2. On the left, the table with success rate for every object, in the last column, the average for every object is calculated, and at the end of the table, the average success rate for every environment is determined. On the right, the graphic representation of Table 3 is reported.

Experiments with Gripper 2

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1 0% 0% 0% 0% 2 0% 0% 0% 0% 3 Table 3. Experiments0% with0% Gripper 2.0% On the left, the0% table with success rate for every object, in the last column, the average for every object is calculated, and at the end of the table, the average success rate for every environment is determined. 4 On the right0%, theSensors graphic2021,0%21 representation, 3253 0% of Table 3 0%is reported. 12 of 18 5 70% 90% 60% 73% Experiments with Gripper 2 6 85% 90% 0% 58% Obj. Normal TableHumid 3. Experiments Dusty with Gripper 2.Average On the left, the table with success rate for every object, in the last column, the average 7 0% for every0% object is calculated,0% and at the0% end of the table, the average success rate for every environment is determined. On 1 0% the right,0% the graphic representation0% of0% Table 3 is reported. 8 0% 0% 0% 0% 2 0% 0% Experiments0% with Gripper0% 2 9 100% 100% 95% 98% 3 0% Obj. 0% Normal0% Humid Dusty0% Average 10 0% 10% 0%0% 0% 0%0% 0% 4 0% 20% 0%0% 0% 0%0% 0% 11 0% 0% 0% 0% 5 70% 390% 0%60% 0% 73% 0% 0% 12 95% 4100% 0%100% 0% 98% 0% 0% 6 85% 590% 70%0% 90% 60%58% 73% Avg. 29% 632% 85%21% 90% 27% 0% 58% 7 0% 70% 0%0% 0% 0%0% 0% 8 0% 0% 0% 0% 8Table 4. Experiments0% 9 with0% Gripper 100% 3. 0%On 100% the left, the 95%0% table with 98% success rate for every object, in the last column, the average 10 0% 0% 0% 0% 9for every100% object is calculated11100% , 0%and at95% the 0% end of the98% 0% table , the 0% average success rate for every environment is determined. On the right, the graphic12 representation 95% 100% of Table 4 100% is reported. 98% 10 0% Avg.0% 29%0% 32% 21%0% 27% 11 0%Experiments 0% with Gripper0% 3 0% Obj. Normal TableHumid 4. Experiments Dusty with Gripper 3.Average On the left, the table with success rate for every object, in the last column, the average 12 95% for every100% object is calculated,100% and at the98% end of the table, the average success rate for every environment is determined. On Avg.1 29%50% the right,32%55% the graphic21% representation20% of27% Table42% 4 is reported. 2 100% 30% Experiments95% with Gripper75% 3 3Table 4. Experiments0% Obj. with0% GripperNormal 3. On Humid0% the left, the Dusty0% table with Average success rate for every object, in the last column, the average for every object is calculated1, 50% and at the 55% end of the 20% table, the 42% average success rate for every environment is determined. 4On the right0%, the graphic2 0%representation 100%0% 30% of Table 4 95% is0% rep orted. 75% 5 0% 30% 0%0% 0% 0%0% 0% Experiments4 with 0% Gripper 0% 3 0% 0% 6 0% 50% 0%0% 0% 0%0% 0% Obj. Normal 6Humid 0% Dusty 0% Average 0% 0% 7 100% 780% 100% 100% 80% 100%93% 93% 1 50% 855% 0%20% 0% 0%42% 0% 8 0% 90% 30%0% 20% 10%0% 20% 2 100% 1030% 85%95% 20% 65%75% 57% 9 30% 1120% 85%10% 70% 50%20% 68% 3 0% 120% 0%0% 0% 0%0% 0% 10 85% Avg.20% 38%65% 23% 28%57% 30% 4 0% 0% 0% 0% 11 85% 70% 50% 68% 5 0% 0% 0% The ﬁrst0% three columns of data correspond to the success rate of the gripping tools 12 0% 0% 0% 0% 6 0% 0% 0%in the three0% scenarios: normal, humid, and dust. The average column of success rates for Avg. 38% 23% 28%each object30% in all scenarios is presented in the fourth column. In the last row, the average 7 100% 80% 100%success rate93% for each environment is calculated, and the global success rate is calculated as the average between the rates of each object in each scenario. Each table is accompanied by 8 0% 0% 0%a histogram0% to display the data more intuitively. 9 30% 20% 10% From20% the results (Figure 11), it can be seen that the success order of the grippers is as follows: Gripper 4, Gripper 1, Gripper 3, and Gripper 2. 10 85% 20% 65% Gripper57% 1, with 77% of the global success rate, is the best of the two pneumatic grippers 11 85% 70% 50%and ranks second68% among the four presented grippers. Table2 indicates that Gripper 1 can grab eight of the 12 objects with a success rate equal to or greater than 85% and that only in 12 0% 0% 0%two cases, with0% objects #6 and #8, it has not been able to grasp them. This is because the size of object #6 (the stone) was too small for this type of gripper, while the surface of the object Avg. 38% 23% 28%#8 (the egg)30% was the most slippery, besides the impossibility of applying high forces to avoid breaking the object. The best performance is achieved in a humid environment. This is because the silicone of the ﬁngers has increased adhesion in the presence of moisture.

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Table 5. Experiments with Gripper 4. On the left, the table with success rate for every object, in the last column, the average for every object is calculated, and at the end of the table, the average success rate for every environment is determined. On the right, the graphic representation of Table 5 is reported.

Experiments with Gripper 4 Obj. Normal Humid Dusty Average 1 100% 100% 100% 100% 2 100% 100% 100% 100% 3 100% 100% 100% 100% 4 75% 45% 55% 58% 5 100% 100% 100% 100% 6 85% 90% 75% 83% 7 100% 100% 100% 100%

Sensors 2021, 21, x FOR PEER REVIEW 12 of 17 8 100% 100% 100% 100% 9 100% 100% 100% 100% Sensors 2021, 21, 3253 13 of 18 10Table 5. Experiments95% with Gripper90% 4. On the left, the95% table with success93% rate for every object, in the last column, the average for every object is calculated, and at the end of the table, the average success rate for every environment is determined. On the right, the graphic representation of Table 5 is reported. 11 100% 100% Vice versa,100% the friction coefﬁcient100% decreases in the presence of dust, and therefore, the worst 12 Experiments10% with Gripper0% results4 are0% obtained in the third3% scenario. Obj. Normal TableHumid 5. Experiments Dusty with Gripper 4.Average On the left, the table with success rate for every object, in the last column, the average Avg. 89%for every object is85% calculated, and at the85% end of the table, the average86% success rate for every environment is determined. On 1 100% the right,100% the graphic100% representation of100% Table5 is reported.

2 100% 100% Experiments100% with Gripper100% 4 3 100% Obj. 100%Normal 100% Humid The Dusty100% first three Average columns of data correspond to the success rate of the gripping tools in 1 100%the 100% three 100% scenarios 100% : normal, humid, and dust. The average column of success rates for each 4 75% 245% 100%55% 100% 100%58% 100% 5 100% 3100% 100% 100%object 100% in 100% 100%all scenarios 100% is presented in the fourth column. In the last row, the average suc- 4 75% 45% 55% 58% 6 85% 590% 100%75%cess 100% rate 100% 83%for each 100% environment is calculated, and the global success rate is calculated as 6 85% 90% 75% 83% 7 100% 7100% 100% 100%the 100% average 100%100% between 100% the rates of each object in each scenario. Each table is accompanied 8 100% 100% 100% 100% 8 100% 9100% 100% 100%by 100% a histogram 100%100% 100%to display the data more intuitively. 10 95% 90% 95% 93% 9 100% 11100% 100% 100% 100% From 100%100% the results 100% (Figure 11), it can be seen that the success order of the grippers is as 12 10% 0% 0% 3% 10 95% Avg.90% 89%95%follows: 85% 85%Gripper93% 86% 4, Gripper 1, Gripper 3, and Gripper 2. 11 100% 100% 100% 100% 12 10% 0% 0% 3% Avg. 89% 85% 85% 86%

The first three columns of data correspond to the success rate of the gripping tools in the three scenarios: normal, humid, and dust. The average column of success rates for each object in all scenarios is presented in the fourth column. In the last row, the average success rate for each environment is calculated, and the global success rate is calculated as the average between the rates of each object in each scenario. Each table is accompanied by a histogram to display the data more intuitively. From the results (Figure 11), it can be seen that the success order of the grippers is as follows: Gripper 4, Gripper 1, Gripper 3, and Gripper 2.

Figure 11. Summary of test results for the four grippers in the three environments.

FigureGripper 11. Summary 2 presents the of lower test global results success for rate, the which four is equalgrippers to 27%. in This the was three environments. expected due to the restriction by which it cannot take objects more than 50% of its diameter. It achieved good results only with the smallest and lightest objects, but also, in this case, it didGripper not do better than1, with the other 77% grippers. of Forthe the global same reasons success as in the previousrate, is case, the the best of the two pneumatic grip- presence of humidity seems to indicate better performance. pers andGripper ranks 3, with second 30% of the among global success the rate, four is slightly presented better than grippers. Gripper 2. Table 2 indicates that Gripper 1 canHowever, grab it iseight evident of from the Table 124 that objects it is not better.with Despite a success its ability rate to grab equal many to or greater than 85% and that of the objects, it fails to complete the task in most cases. It reached 100% only in three onlycases: in with two object cases, #2 in the with normal objects environment #6 and and with object#8, it #7 has in the not normal been and dusty able to grasp them. This is because environment. These results indicate that this gripper is suitable for large objects and is not theaffected size byof weight. object Unlike #6 Gripper (the 2,stone) it is unable was to grasp too small small or rectangular for this objects, type and of gripper, while the surface of Figure 11. theSummarythe object presence of test #8 of results humidity, (the for egg) in the general, four was grippers worsens the its inmost performance the three slippery, environments. (as in the casebesides of objects the #2, impossibility of applying high #7, and #10). Gripperforces 1, with to 77%avoid of the break globaling success the object.rate, is the The best best of the performance two pneumatic grip- is achieved in a humid environ- pers and ment.ranks second This among is because the four the presented silicone grippers. of the Table fingers 2 indicates has thatincrease Gripperd adhesion in the presence of 1 can grab eight of the 12 objects with a success rate equal to or greater than 85% and that only in two cases, with objects #6 and #8, it has not been able to grasp them. This is because the size of object #6 (the stone) was too small for this type of gripper, while the surface of the object #8 (the egg) was the most slippery, besides the impossibility of applying high forces to avoid breaking the object. The best performance is achieved in a humid environment. This is because the silicone of the fingers has increased adhesion in the presence of

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Gripper 4, with 86% of the global success rate, is the best gripper of this study. As can be seen in Table5, its performances, with a success rate of 89% in the normal environment and of 85% in the other two cases, are not significantly influenced by the type of environment. It is capable of grasping both small and light objects as large and heavy objects. Fingers adapt to objects by applying uniform pressure without the need to exert much force. The gripper presents problems only with two objects, #4 and especially #12. In both cases, it is a rectangular object with smooth surfaces (in the case of #12, the thickness is least). In this case, the gripper, with its three fingers, is unable to surround the object and adapt to it. In summary, Gripper 1 can grab many different objects with similar performance. Its limitations proved to be the dimensions below those of the nut and the thin objects. In the case of delicate objects such as #7 and #11, the gripper has left them intact. Gripper 2 can grab little and lightweight objects, better if with regular surfaces, as in the case of objects #9 and #12. Almost certainly, some improvements in the realization of the gripper would allow achieving better results, but due to the limitations mentioned above, it would still not be the most robust gripper among the four proposed. Gripper 3 is suitable for grabbing sufficiently large and heavy round objects that cannot escape laterally. In general, it did not cause damage to the most delicate objects; however, it is necessary to generate more force on the surface of the object to ensure a firm grip. Finally, Gripper 4 proved to be the most robust gripper of the four proposed. It is able to grasp objects without exerting a significant force, so it is suitable for delicate objects, and it has been seen that neither the tomato nor the broccoli has been dented. In Figure 12, comparison graphs have been realized to highlight the most significant differences. Figure 12a compares the performance of Gripper 1 and Gripper 4, the two best grippers. Both exhibit acceptable behavior with most objects. This indicates that in general, they are very robust tools of grip. It is interesting to note that the two grippers compensate, as with objects #8 and #12, and in part with #4 and #6. Although the shape of approaching the object is similar, the presence of a fourth finger in Gripper 1 makes the difference in being able to grab rectangular objects, even if the dimensions of these same fingers are prejudicial for grasping small objects. Figure 12b compares the two grippers with lower performances. In this case, it is evident that the type of object that each can grasp is completely different. Figure 12c compares the pneumatic grippers. Only with object #6 is Gripper 2 better than Gripper 1. Finally, Figure 12d compares the mechanical grippers. In this case, Gripper 4 is always better than Gripper 3. Electromechanical grippers behave best in the normal environment and not in a humid environment such as pneumatic grippers. This is because with this type of gripper, the grip of the objects depends to lesser extent on the coefficients of friction. Analyzing the data, it is interesting to observe that for certain objects, results are not affected by the change of environment, while for others, they change significantly. Gripper 1, for example, exhibits an important fall in the success rate with tomato in the dusty environment. These significant variations can be attributed to the surface properties of the object. The surface of a tomato, e.g., is perfectly smooth and not porous. As a consequence, powder remains on its surface and directly affects the grip. This effect does not occur with the tennis ball due to its surface roughness. Another example of this phenomenon occurs with the stone. The second pneumatic grip tool, with success rates of 85% and 90% in normal and humid environments respectively, is unable to grasp the stone in the presence of dust. Object #9 (stick glue), with an 80% success rate globally (in all environments and with all grippers), is the object exhibiting the best results. The three objects with the worst results have been objects #4 (metallic container), #6 (the stone), and #3 (the Vaseline container), with overall results of 36%, 40%, and 42% respectively. Sensors 2021, 21, x FOR PEER REVIEW 13 of 17

moisture. Vice versa, the friction coefficient decreases in the presence of dust, and therefore, the worst results are obtained in the third scenario. Gripper 2 presents the lower global success rate, which is equal to 27%. This was expected due to the restriction by which it cannot take objects more than 50% of its diameter. It achieved good results only with the smallest and lightest objects, but also, in this case, it did not do better than the other grippers. For the same reasons as in the previous case, the presence of humidity seems to indicate better performance. Gripper 3, with 30% of the global success rate, is slightly better than Gripper 2. How- ever, it is evident from Table 4 that it is not better. Despite its ability to grab many of the objects, it fails to complete the task in most cases. It reached 100% only in three cases: with object #2 in the normal environment and with object #7 in the normal and dusty environment. These results indicate that this gripper is suitable for large objects and is not affected by weight. Unlike Gripper 2, it is unable to grasp small or rectangular objects, and the presence of humidity, in general, worsens its performance (as in the case of objects #2, #7, and #10). Gripper 4, with 86% of the global success rate, is the best gripper of this study. As can be seen in Table 5, its performances, with a success rate of 89% in the normal environment and of 85% in the other two cases, are not significantly influenced by the type of environment. It is capable of grasping both small and light objects as large and heavy objects. Fingers adapt to objects by applying uniform pressure without the need to exert much force. The gripper presents problems only with two objects, #4 and especially #12. In both cases, it is a rectangular object with smooth surfaces (in the case of #12, the thickness is least). In this case, the gripper, with its three fingers, is unable to surround the object and adapt to it. In summary, Gripper 1 can grab many different objects with similar performance. Its limitations proved to be the dimensions below those of the nut and the thin objects. In the case of delicate objects such as #7 and #11, the gripper has left them intact. Gripper 2 can grab little and lightweight objects, better if with regular surfaces, as in the case of objects #9 and #12. Almost certainly, some improvements in the realization of the gripper would allow achieving better results, but due to the limitations mentioned above, it would still not be the most robust gripper among the four proposed. Gripper 3 is suitable for grabbing sufficiently large and heavy round objects that cannot escape laterally. In general, it did not cause damage to the most delicate objects; however, it is necessary to generate more force on the surface of the object to ensure a firm grip. Finally, Gripper 4 proved to be the most robust gripper of the four proposed. It is able to grasp objects without exerting a significant force, so it is suitable for delicate objects, and it has been seen that neither the tomato nor the broccoli has been dented. Sensors 2021, 21, 3253 In Figure 12, comparison graphs have been realized to highlight the 15most of 18 significant differences.

Sensors 2021, 21, x FOR PEER REVIEW 14 of 17

Figure 12. Graphic comparison between (a) Gripper 1 and Gripper 4. As can be seen from the graph, the two grippers Figure 12. Graphic comparison between (a) Gripper 1 and Gripper 4. As can be seen from the graph, the two grippers generally have similar performances. (b) Gripper 2 and Gripper 3. These are the two grippers with the worst performances, generally haveand similar each one performances. proved to be suitable (b) only Gripper for a specific 2 and type Gripper of object. 3. (Thesec) Graphic are comparison the two grippers between Gripper with the 1 and worst Gripper performances, and each one2 (theproved two pneumatic to be suitable grippers). only As for can a be specific seen from type the graph,of object. Gripper (c) 1Graphic works significantly comparison better between than Gripper Gripper 2. (d) 1 and Grip- per 2 (the twoGraphic pneumatic comparison grippers). between Gripper As can 3 andbe Gripperseen from 4 (the the two graph, mechanic Gripper grippers). 1 As works can be seensignificantly from the graph, better Gripper than Gripper 2. (d) Graphic 3comparison works significantly between better Gripper than Gripper 3 and 4. Gripper 4 (the two mechanic grippers). As can be seen from the graph, Gripper 3 works significantly better than Gripper 4. Finally, we present some considerations with respect to the construction and operation of the grippers. The tool with the fastest and easiest manufacturing process turned out Figureto be Gripper 12a compares 4. In addition, the its performance control is very of simple Gripper and requires 1 and onlyGripper a power 4, supply,the two best grippers. Bothwhile inexhibit the case acceptable of pneumatic behavior grippers, a with compressor, most aobjects. pressure airThis tank, indicates and a vacuum that in general, they arepump very are robust needed. tools of grip. It is interesting to note that the two grippers compensate, Another factor to consider is the noise generated by the tools during use. The two as withelectromechanical objects #8 and grippers #12, and are muchin part quieter with than #4 and the pneumatic #6. Although ones due the to shape the work of of approaching the objectthe compressor is similar, and the the presence vacuum pump, of a although fourth thefinger latter in could Gripper be replaced 1 makes by a vacuum the difference in being ejector,able to which grab is muchrectangular quieter. objects, even if the dimensions of these same fingers are The work environment is another factor that affects performances. In the food industry, prejudicialthe pneumatic for grasping tools may small be a better objects. option due to the ease and speed to clean them. Another Figureadvantage 12b is compares that the compressor the two and grippers the tank maywith be lower away fromperformances. tools and out In of thethis case, it is evidentmanipulation that the type environment. of object Inthat contrast, each can in the grasp electromechanical is completely tools, different. the operating Figuremechanisms 12c compares are in direct the contact pneumatic with the work grippers. environment Only and with could object compromise #6 is Gripper the 2 better safety of the installation. than GripperIf speed 1. of action plays a paramount role in the application, as in pick and place Finally,tasks in whichFigure the 12d speeds compares of opening the and mechanical closing of the toolsgrippers. are critical In this for productivity, case, Gripper 4 is always pneumaticbetter than grippers Gripper should 3. beElectromechanical considered as the better grippers option due behave to their best higher in speed. the normal environment andHowever, not in from a thehumid perspective environment of the maintenance such as ofpneumatic gripping tools, grippers. it is not easyThis is because to determine which is the most appropriate. Although changing a component of an with thiselectromechanical type of gripper, gripper the requires grip of a longerthe objects time, pneumatic depends elements to lesser are extent more likely on the to coefficients of friction.fail than mechanic ones. Analyzing the data, it is interesting to observe that for certain objects, results are not affected by the change of environment, while for others, they change significantly. Grip- per 1, for example, exhibits an important fall in the success rate with tomato in the dusty environment. These significant variations can be attributed to the surface properties of the object. The surface of a tomato, e.g., is perfectly smooth and not porous. As a consequence, powder remains on its surface and directly affects the grip. This effect does not occur with the tennis ball due to its surface roughness. Another example of this phenomenon occurs with the stone. The second pneumatic grip tool, with success rates of 85% and 90% in normal and humid environments respectively, is unable to grasp the stone in the presence of dust. Object #9 (stick glue), with an 80% success rate globally (in all environments and with all grippers), is the object exhibiting the best results. The three objects with the worst results have been objects #4 (metallic container), #6 (the stone), and #3 (the Vaseline container), with overall results of 36%, 40%, and 42% respectively. Finally, we present some considerations with respect to the construction and operation of the grippers. The tool with the fastest and easiest manufacturing process turned out to be Gripper 4. In addition, its control is very simple and requires only a power supply, while in the case of pneumatic grippers, a compressor, a pressure air tank, and a vacuum pump are needed. Another factor to consider is the noise generated by the tools during use. The two electromechanical grippers are much quieter than the pneumatic ones due to the work of

Sensors 2021, 21, 3253 16 of 18

6. Conclusions Four prototypes of soft robotic grippers, including their design and manufacture, have been developed for a later stage of experimentation. To realize a more effective comparison, two grippers use pneumatic system actuation, while the other two employ electromechanical actuation, and each of them shows a different solution design. All grippers have been tested with twelve objects in three different work environments. These objects have carefully been selected to be a representative group of items that could be manipulated by the grippers in a real industrial work environment. This group includes objects with different geometric and mechanical characteristics, in terms relative to the dimensions of the grippers. An overview of the work and real tests is available in Supplementary Materials. From the results of our tests, it can be seen that Gripper 4 (the electromechanical gripper with passive structure) and Gripper 1 (the pneumatic gripper with chambered ﬁngers) present the highest success rate, with rates of 86% and 77% respectively. Thus, it can be concluded that Gripper 4 represents the best option, when robustness and good behavior for the manipulation of unknown objects are needed. Gripper 1 also represents an excellent option despite some limitations due to the dimensions of ﬁngers that do not allow grasping certain types of objects. It is worth noting that the improvements in the mechanical design of the other grippers may increase their percentage of success rate.

Supplementary Materials: The following are available online at: https://youtu.be/fzxXVgQEElA, Supplementary materials Overview of the work and real tests. Author Contributions: M.A. developed grippers and performed experiments, S.T. designed and oversaw the study, analyzed the data, and wrote the paper, A.B. oversaw the study and revised the paper. All authors have read and agreed to the published version of the manuscript. Funding: Work result of research activities carried out at the Centre for Automation and Robotics, CAR (UPM-CSIC), within the Robotics and Cybernetics research group (RobCib). Supported by the RoboCity2030-DIH-CM, Madrid Robotics Digital Innovation Hub, S2018/NMT-4331, funded by “Programas de Actividades I + D en la Comunidad de Madrid” and co-funded by Structural Funds of the EU, and also by the project TASAR (Team of Advanced Search And Rescue Robots), PID2019-105808RB-I00, funded by the Ministerio de Ciencia e Innovación (Government of Spain). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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