A Pneumatically Driven, Disposable, Soft Robotic GripperEquipped with Retractable, Telescopic Fingers

Lucas Gerez and Minas Liarokapis

Abstract— Robots use their end-effectors to grasp and manip-ulate objects in unstructured and dynamic environments. Robothands and grippers can vary from rigid and complex designsto soft, inflatable, and lightweight structures. In this paper,we focus on the development of a soft, retractable, telescopicgripper that exhibits an adaptive behaviour that allows efficientand stable grasps with a wide range of objects. The efficiency ofthe proposed device is experimentally validated through threedifferent types of experiments: i) grasping experiments thatinvolve different everyday objects, ii) force exertion experimentsthat capture the maximum forces that can be exerted by theproposed device, and iii) grasp resistance experiments thatfocus on the effect of the inflatable structure on resistingenvironmental uncertainties and disturbances. The proposedgripper is able to grasp a plethora of everyday life objects andit can exert more than 8 N of grasping force. The design is solow-cost that the soft fingers can be used in a disposable manner,facilitating the execution of specialized tasks (e.g., grasping insterilized environments, handling of medical waste, etc).

I. INTRODUCTION

Robotic end-effectors have evolved over the past fewdecades from simple, parallel jaw grippers to complex robothands with multiple degrees of freedom (DoF) that requirecomplex control laws and sophisticated sensing. Dexterousrobot hands can efficiently grasp objects applying differenttypes of actuation mechanisms, such as pulley and geartransmissions [1] and cable-driven systems [2]. In mostcases, dexterous hands are rigid, difficult to develop, heavy,and expensive [3]. For that reason, Dollar et al. [4] presenteda method for manufacturing intrinsically compliant robothands combining elastomer materials and rigid parts. Theserobot hands are highly adaptable during grasping, enhancinggrasp stability by design.

More recently, following the path of intrinsical compli-ance in robotic devices, soft grippers based on completelysoft pneumatic actuators (SPAs) have received an increasedinterest from the research community due to their flexibility,high customizability, and environmental adaptability [5].Rigid manipulators require a combination of joints and rigidlinks to execute grasps, while the motion of soft actuatorsis promoted by the intrinsic mechanical properties of softmaterials. Soft robotic hands are suitable for grasping andmanipulating delicate and/or fragile objects and complexshapes by conforming to the object’s geometry and distribut-ing contact forces among the surface [6]. This type of robothands and grippers do not require specific control inputs,

Lucas Gerez and Minas Liarokapis are with the New Dexterity re-search group, Department of Mechanical Engineering, The Universityof Auckland, New Zealand. E-mails: lger871@aucklanduni.ac.nz, mi-nas.liarokapis@auckland.ac.nz

Fig. 1. Soft robot gripper equipped with retractable, telescopic, inflatablefingers in two different scenarios: deflated (left) and inflated (right). Thegripper can achieve stable grasps and can be used in unknown environmentsdue to its high conformability and compactness.

reducing the complexity and cost of the device withoutcompromising the grasping efficiency and safety [7]. Thehigh conformability of soft actuators can be an advantagewhen using soft grippers inside non-traditional environments(e.g., cavities and narrow spaces) or to grasp fragile objects.In unstructured and unknown environments, the shape of theobjects to be grasped is not predictable, so robot hands withhigh adaptability are required [8].

Soft robotic grippers have been proven to be a goodalternative when grasping different types of objects [9]–[12].Although the size of soft robotic grippers is a concern forrobot gripper and hand designers, most of these devices havesimilar dimensions with the traditionally used bulky, rigidrobot grippers. One of the advantages of soft actuators is thatthey can be easily molded to shapes that reduce the amount ofspace taken when they are not inflated (e.g., accordion-styleparts, origami structures, and foldable actuators) [13]–[16].

In this paper, we focus on the development of a soft roboticgripper equipped with pneumatically driven, retractable, tele-scopic fingers that can be employed in environments thatrequire a disposable device with minimal size (e.g., a gripperfor handling of medical waste that can be easily stored).The performance of the proposed gripper is experimentallyvalidated using three types of experiments: i) grasping teststhat involve different everyday objects, ii) force exertionexperiments that capture the maximum forces that can beexerted, and iii) grasp resistance experiments that focus onthe effect of the inflatable structure on resisting environmen-tal uncertainties.

Fig. 2. When the soft telescopic actuator is inflated, each inner ring inflates separately, starting with the most exterior ring and finishing with the mostinterior one. Such a configuration allows object grasping with ease and stability. At the rest position, the soft actuator is 20 mm thick. However, wheninflated, it can reach a height of 107 mm. The actuator can be inflated at different speeds, grasping objects quickly or slowly. During the deflation process,the finger naturally returns to its original flat position without requiring any external forces or negative pressure inside the actuator (vacuum). Thus,retraction is completely passive. The finger bending is caused by an internal angle between the base of the actuator and the telescopic structure (sectionview). Each telescopic ring has a different thickness, making the finger bend when inflated.

The rest of the paper is organized as follows: Section IIpresents the design of the soft, robotic gripper, Section IIIdetails the experimental setup used and presents the results,while Section IV concludes the paper.

II. SOFT GRIPPER DESIGN

In this section, we present the design of the soft roboticgripper, also discussing its manufacturing process, function-alities, and operation. The device is composed of two mainparts: a robot mount and two soft, inflatable, telescopic fin-gers. The robot mount was designed to connect the gripper tothe robot arm (UR10, Universal Robots, Odense, Denmark).A control box is used to inflate and deflate the fingers. Thecontrol box contains a 12V mini air pump and a solenoidvalve. The air pump is in charge of inflating the fingerswhile the solenoid valve releases the pressure of the systemso the finger can retract to its original and flat position. Thesoft robotic fingers developed for this gripper are retractableand telescopic. More precisely, the fingers are flat when notinflated (20 mm) and elongate to more than 100mm longwhen inflated due to their foldable structure. The fingers arecalled retractable because they naturally / passively returnto their original flat position when the internal pressure isreleased, without requiring any external forces or negativepressure inside the actuator (vacuum). The soft robotic fingercan be inflated at different speeds (varying the flow rate in theinlet), being able to grasp objects either quickly and firmlyor slowly, carefully, and delicately.

A. Soft Telescopic Actuator Design

The soft telescopic actuator is based on a urethane rubber(Smooth-On Vytaflex 40) structure designed for graspingassistance during the execution of activities of daily living.The foldable structure was designed in such a way thatit does not influence the interaction with the environmentwhen the gripper is not inflated due to its small thickness

Fig. 3. The manufacturing process of the inflatable, telescopic finger(made out of Vytaflex 40) involves the following three molding steps: i)the foldable part of the actuator is fabricated using two separate molds, ii)the base layer is fabricated using a third mold, with the base layer being1.5mm thick and 2 mm smaller than the foldable part in all directions sothat they can be molded together, iii) after both parts are cured, a forthmold is used to combine the upper part and the base layer part, filling theremaining gaps between the two parts and bonding them together. The finalinflatable structure is 20 mm thick and 90 mm wide.

and telescopic behaviour. The finger bending is caused byan internal angle between the base of the actuator and thetelescopic structure (see the section view in Fig. 2). Eachtelescopic ring has a different thickness, making the fingerto move in different proportions when inflated, thus bendingit. The rounded shape of the actuator was chosen so as tomaximize the size of the objects that could be grasped byemploying the actuator. The actuator operates at a pressure of30 kPa, weighs 50 g, is 20 mm thick, and 90 mm wide. Themanufacturing process of the telescopic actuator involvesthree different molding steps. Fig. 3 illustrates the steps to

Fig. 4. The first experiment was executed to evaluate the grasping performance of the gripper for different everyday life objects. Each object was placedon a flat surface and the gripper was attached on a Universal Robots UR10 robot arm. Upon grasp execution, the robot arm lifts the object and holdsit steady for 10 seconds. The robot gripper is then moved in the horizontal direction and finally placed back on the surface. A successful grasp is alsoconsidered stable if there is no visible object reorientation (motion in any direction) or slippage during the task execution. A total of 10 out of 12 objectsexamined, were successfully and stably grasped.

TABLE ICOMPARISON BETWEEN ELONGATION RATIO IN MULTIPLE SOFT

ACTUATORS (HEIGHT OF THE ACTUATOR IN THE INFLATED STATE, Li ,DIVIDED BY THE HEIGHT OF THE ACTUATOR IN THE REST STATE, Lr ).

Soft Actuator Li/LrConnolly et al. [17] 1.2

Bryant et al. [18] 1.4Deshpande et al. [19] 2.1

Li et al. [20] 1.9Martinez et al. [14] 3.6

Rafsanjani et al. [21] 1.25Soft Telescopic Actuator (this study) 5.3

manufacture the actuator. Initially, the foldable part and thebase layer of the actuator are fabricated. The base layer is2 mm smaller than the upper part in all directions so thatthey can be molded together. After both parts are cured, athird mold is used to combine the upper part and the baselayer part, filling the remaining gaps between the two partsand bonding them together. This technique avoids leakagesand deformations in the actuator. Although having a thickelastomer base, a fabric layer can be added to the base torestrict the extension of the actuator along the base axis.

The actuator behaviour during the inflation process can beseen in Fig. 2. When the soft telescopic actuator is inflated,each inner ring inflates separately, starting with the mostexterior ring and finishing in the most interior one. Suchconfiguration allows object grasping with ease and stability.During its rest position, the soft actuator is 20 mm thick.However, when inflated, it can reach a height of 107 mm,having an elongation ratio bigger than five (height of theactuator in the inflated state divided by the height of theactuator in the rest state). If compared to the current soft

TABLE IIOBJECTS USED DURING THE GRASPING EXPERIMENT.

Object Mass (g) Dimension (mm) SuccessBanana 66 36x190 Yes

Blue Ball 55.3 41 YesHammer 665 24x32x135 NoSoft Ball 191 96 Yes

Die 5.2 16.2 YesCredit Card 5.2 54x85x1 NoWine Glass 133 89x137 Yes

Mustard Bottle 603 58x95x190 YesCan 349 66x101 Yes

Gelatine Box 187 35x110x89 YesEgg 49 53x40 Yes

Muffin 72 57x45 Yes

actuators found in the literature, the proposed telescopic ac-tuator has the highest elongation ratio among all the analyzedsolutions. Table I shows the elongation ratio of different softactuator designs found in the literature. The simplicity andthe compactness of the proposed design lead to the develop-ment of a cost-effective robotic gripper. Each inflatable fingercosts less than 1 USD of material to be manufactured. Thus,the proposed soft robotic gripper is a good alternative forsterilized environments where the use of disposable productsguarantees a high level of cleanness (e.g., dust-free factoriesand hospitals). In particular, in hospitals, such a gripper canbe used to handle dangerous medical waste in a disposablemanner to reduce the transmission of bacteria and viruses(e.g., COVID-19).

III. EXPERIMENTS AND RESULTS

The experiments that were conducted to assess the per-formance of the robotic gripper were divided into threeparts. The first part focused on evaluating the use of the

Fig. 5. Twelve objects were used in the experiments: a can, a soft ball, amustard bottle, a credit card, a blue ball, a die, a jello box, a cup, a hammer,a plastic banana, an egg, and a muffin.

soft actuators for grasping multiple everyday life objects.The second experiment assessed the forces that the deviceis capable of exerting. The third experiment focused onassessing the efficiency of the gripper on resisting externalforces that try to pull the objects away from the gripper.

A. Daily Life Objects Grasping Experiment

The first experiment was executed to evaluate the graspingperformance for different everyday life objects (Fig. 5).The goal of these tests is to verify if the gripper wascapable of executing different grasping tasks. A total of 12objects were tested. 10 objects used in this experiment wereselected from the YCB object set [22], an object set designedfor facilitating benchmarking in robotic manipulation andgrasping. The other two objects (a raw egg and a muffin),tested the gripper’s capability on grasping fragile objects.The objects ranged from a tiny die to a heavy hammer.Each object was placed on a flat surface, and the gripperwas connected to the robot arm. Five grasping attemptswere executed for each object. After executing the grasp, therobot arm lifts the object and holds it for 10 seconds. Therobot gripper was then moved repeatedly in the horizontaldirection and finally placed back on the surface. Assessmentof grasp stability was based on a successful grasp withno visible object reorientation (motion in any direction) orslippage during the task execution. A total of 10 out of 12objects were successfully grasped (see Fig. 4), while thecredit card and hammer could not be grasped (see Table II).

Fig. 6. The second experiment focused on assessing the forces that thedevice can apply to grasp objects. In this experiment, a dynamometer wasused to measure the forces exerted by the soft gripper while grasping thedevice. The dynamometer was placed in multiple positions between thefingers, and the maximum force obtained was 8.1 N.

During the experiments, it was noticed that grasps with highcontact area between the object and the gripper were easilyexecuted. Meanwhile, flat and thin objects like the credit cardrequire a combination of fine motions and the provision offingernails to be successfully grasped from a table surface.Such objects can be slid to the edge of the table surfaceand be successfully grasped there with a simple grasp. Thehammer was the heaviest object among all tested objects.The contact area between the hammer and the gripper wassignificantly small, hindering a successful and stable grasp.

B. Grasp Force Experiment

The second experiment focused on assessing the forcesthat the device can apply while grasping objects. In thisexperiment, a Biopac MP36 data acquisition unit (BiopacSystems, Inc., USA) was used with the SS25LA dynamome-ter to measure the forces exerted while grasping the device,as shown in Fig. 6. During the experiment, the system wasactuated until the maximum pressure of the actuator wasreached (30 kPa). The dynamometer was placed in differentpositions between the fingers. The maximum force obtainedduring the experiments was 8.1 N. The maximum forcewas reached close to the base of the actuator (stiffer regioncompared to the tip of the soft finger). According to [23],the required force to grasp objects during activities of dailyliving does not exceed 10 to 15 N. Although the amountof force applied by the fingers is not considered high whencompared to rigid grippers [3], [24], the large contact regionand high friction between the object and the soft materialof finger compensate the lower forces, and the gripper isable to grasp a large range of objects. Also, the amount offorce applied by the proposed soft robotic gripper is withinthe range of forces that the particular class of grippers canusually achieve [5], [10], [12]. The gripper can be improvedto exert higher forces by increasing the thickness of the wallsor changing the finger material. Stiffer walls of the actuatorwould increase the maximum pressure that the actuator canwithstand and, consequently, increase the amount of forcethat the soft robotic gripper can apply.

Fig. 7. The third experiment focused on measuring the amount of forcerequired to pull the objects out of the grasp of the robot gripper. Threedifferent types of objects were used: a sphere, a cube, and a cylinder. Theforce required to pull the ball away from the gripper reached more than 10N, while for the cylinder and the cube these forces were mostly between 5N and 6 N.

C. Pulling Force Experiment

The third experiment focused on measuring the amount offorce required to pull objects away from the robot gripper. Inthis experiment, three different types of objects were used:a rubber sphere (41 mm), a PLA plastic cube (70x70x70mm), and a PLA plastic cylinder (50x50 mm). The telescopicgripper was mounted to the robot arm equipped with aforce/torque sensor (FT300, Robotiq, Canada), and the objectwas grasped by the gripper until reaching a predefinedchamber pressure. The object was then pulled off the gripper,and the maximum force in each trial was recorded. Theexperiment was repeated ten times for each object at apressure of 30 kPa. The object was pulled at differentspeeds to simulate different environmental conditions. Theexperimental results are reported in Fig. 7. The force requiredto pull the ball away from the gripper reached more than 10N, while for the cylinder and the cube, the forces were mostlybetween 5 N and 6 N. The ball is made out of rubber, whichincreases the friction between the object and the gripper. Theresults demonstrate that the gripper can resist the same rangeof pulling forces of the soft grippers found in the literature[9], [10]. It was noticed that the resistance force between theobject and the gripper could be increased by having a largerbending angle of the fingers. Such change would cage theobject, creating a higher resistance against external forces.The bending angle of the finger can be changed by increasingthe internal angle of the actuator (described in the sectionview of Fig. 2).

D. Video Demonstration

A video presenting the soft robotic gripper and demon-strating some of its uses can be found at the following URL:

www.newdexterity.org/telescopicsoftgripper

IV. CONCLUSIONS AND FUTURE DIRECTIONS

In this paper, we presented a soft, robotic gripper equippedwith retractable, telescopic fingers that increase grasp sta-bility and can be used in multiple environments. The per-formance of the proposed soft, telescopic gripper was ex-perimentally validated using three types of experiments,involving grasping everyday life objects and force exertionmeasurements. The experiments demonstrated that the grip-per can grasp efficiently most of the examined daily lifeobjects exerting a significant amount of force despite itsnon-traditional behaviour. Also, the telescopic and retractablecharacteristics allow the gripper to be used in unstructuredand dynamic environments and grasp fragile objects.

Regarding future directions, we plan to design a gripperwith three fingers to facilitate grasping of a bigger range ofobjects. We also plan to integrate force sensors into the softstructure to control the forces being applied by the actuator.

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