Design of a Lightweight Inflatable Sensing Sleeve for IncreasedAdaptability and Safety of Legged Robots

Taekyoung Kim1, Jaejun Park2, Sohee John Yoon1, Do Hun Kong2, Hae-Won Park2 and Yong-Lae Park1

Abstract— We propose a lightweight inflatable sensing sleevefor increased adaptability and safety of legged robots. Thesleeve contains an air chamber that can be actively inflatedfor protecting the host robot from external impacts usingthe cushioning effect. In addition, capacitive touch-sensitivepads are integrated on the surface of the sleeve for detectingcontacts made to the sleeve. Therefore, robots can acquire tactileinformation from their surroundings for increased adaptabilityand maneuverability. This paper presents the design andthe fabrication of the inflatable sleeve and capacitive touchpads. Characterization tests were carried out to evaluate thesensitivity and durability of the touch pads and the safety effectof the inflated sleeve. The proposed sleeve was also integratedwith a robotic leg to demonstrate how it could be implementedto a legged robot for controlling motions.

I. INTRODUCTION

Various types of mobile robots have been developed fordifficult tasks in disaster sites, such as search and rescuemissions in unstructured and constrained environments, asreplacement of human workers. Typical examples are con-ventional land-based wheeled robots [1], [2] and trackedrobots [3], [4]. Recently, legged robots [5]–[7] have beenactively investigated, emphasizing enhanced mobility withvarious motions, such as walking, running and crawling. Toaccomplish missions in real environments, it is importantfor the robots not only to be agile using their high mobility,but also to be adaptive to the surroundings by understandingthe external world more accurately and quickly. Therefore,different sensors can be used to collect information onenvironments. One of the most dependable information of therobots is the visual data acquired by using optical sensors,such as laser [8], LiDAR [9] and camera [10]. However,since it is difficult to function properly due to unexpectedsituations in darkness and smoke-filled fire sites, robotsneed additional means of monitoring the external world,such as tactile sensing. In addition, when robots move ina complex and narrow terrain, they are likely to collidewith surroundings often. In this case, without tactile sensing,

This work was supported in part by the Technology Innovation Program(No. 2017-10069072) funded by the Ministry of Trade, Industry & Energy,Korea, and in part by Air Force of Scientific Research (No. FA2386-17-1-4665), Unites States. (Taekyoung Kim and Jaejun Park contributed equallyto this work.) (Corresponding authors: Hae-Won Park and Yong-Lae Park)

1Taekyoung Kim, Sohee John Yoon and Yong-Lae Park are with Depart-ment of Mechanical and Aerospace Engineering, Soft Robotics ResearchCenter (SRRC) and Institute of Advanced Machines and Design (IAMD),Seoul National University, Seoul 08826, Korea. ylpark@snu.ac.kr

2Jaejun Park, Do Hun Kong and Hae-Won Park are withDepartment of Mechanical Science and Engineering, Universityof Illinois at Urbana-Champaign, Urbana IL 61801, United Stateshaewon@illinois.edu

Fig. 1. (a) Concept of inflatable sensing sleeves integrated with a leggedrobot (b) The prototype of lightweight inflatable sensing sleeve integratedwith a single robotic leg.

it is difficult for the robots to distinguish errors of thesystem caused by the external contacts, so the robot canlose their adaptability to the surroundings [11]. Furthermore,it would be extremely useful if the robots have capabilityof protecting themselves from unpredictable environmentalrisks. Therefore, we propose a lightweight inflatable sensingsleeve that can not only enhance the adaptability of mobilerobots to environments utilizing a compact and lightweighttactile sensing mechanism, but also protect the host robotswith its cushioning effect. Fig. 1 shows a conceptual designof a quadruped legged robot integrated with the protectivesensing sleeves and an actual prototype of the proposedsleeve system.

2019 2nd IEEE International Conference on Soft Robotics (RoboSoft)COEX, Seoul, Korea, April 14-18, 2019

978-1-5386-9259-2/19/$31.00 ©2019 IEEE 257

Fig. 2. Design of an inflatable sleeve and a capacitive touch pad (a) The three-layer structure of inflatable sleeve, (b) The configuration of capacitivetouch pads with a ground line on the middle layer, (c) A prototype of inflatable sleeve integrated with the pads, (d) The structure of touch pad, (e) Anactual prototype of the capacitive touch pad.

Inflatable robotic structures have been proposed based onsoft robotics technologies, such as soft actuators [12]–[14]and soft sensors [15], [16]. For example, Qi et al. havedeveloped an interactive robot arm [17] with an inflatablestructure, and Ohta et al. have also proposed inflatablesleeves made of heat-sealable polymer materials to cover arigid robotic arm [18]. However, these arms did not have anyfunctions of directly collecting tactile information throughthe inflatable skins. To overcome this weakness, Kim et al.have developed a soft inflatable sensing module made ofstretchable silicone elastomer embedded with microfluidicsensors filled with a liquid conductor [19]. Although itwas able to directly detect the locations and magnitudes ofcontacts through the sensing skin, it had drawbacks of acomplex fabrication process and a relatively heavy weightof the silicone skin material.

Since the proposed inflatable sleeve is integrated withtouch-sensitive pads, which can detect contacts as binarystates based on capacitive sensing, it can acquire the tactileinformation from surroundings and help the host robots tounderstand the unknown external world more quickly andreliably. In addition, since the touch pad and the inflatablestructure can be simply fabricated by laminating only lightmaterials, such as a conductive fabric and a thin thermo-plastic sheet, so that the weight of each sleeve can be aslow as 16 g, which does not compromise the mobility of therobot.

The rest of the paper is organized as follows. We first

describe the main design features and the fabrication pro-cesses in Sections II and III, respectively. We experimentallycharacterize the proposed system in Section IV. Finally, wediscuss an application scenarios using the sleeves integratedwith a robotic leg in Section V followed by conclusion andfuture work in Section VI.

II. DESIGN

A. Inflatable Sleeve

The proposed inflatable sleeve has three layers com-posed of thermo-plastic sheets with double-sided adhesion(Fig. 2(a)). The space between the inner layer and the middlelayer, which are bonded by a impulse heat sealer, serves asan air chamber that can be actively inflated when compressedair is supplied. On the middle layer, capacitive touch padsand the common ground line are attached (Fig. 2(b)) andcovered with an outer layer which prevents the touch padsand the ground line from being damaged by external impacts.When the sleeve is inflated, the inner layer expands inwardcompressing the structure of the host robot, and the sleeveis tightly fixed on the robot structure. Additionally, fabricrubber bands were inserted into both ends of the sleeve closeto the joint to reduce their radii not to interrupt the motionof the robot (Fig. 2(c)). The sleeve is connected with anair pressure sensor (PSE543-M5, SMC) to measure changesof internal pressure by supplied compressed air or appliedcontact forces. The weight of each sleeve is only 16 g, whichis light enough that it does not deteriorate the mobility of

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Fig. 3. Fabrication process: (a) Cutting fabrics, tapes and thermo-plastic sheets by using a laser cutter, (b) Fabricating capacitive touch padsby laminating prepared materials (conductive fabric and non-conductivedouble-side tape) using conductive double-side tapes and connecting a signalwire to the top electrode layer, (c) Integrating the completed capacitive touchpads and a ground line made of conductive fabrics on the middle layerusing conductive double-side tapes, (d) Heat-sealing the middle layer withthe outer and inner layer. (e) Inserting fabric rubber bands into the ends ofthe flat inflatable sleeve, (f) Rolling up the flat inflatable sleeve using heatsealer to make cylindrical shape.

the host mobile robot.

B. Capacitive Touch Pad

In this study, we used an off-the-shelf capacitive touchsensor breakout board (MPR121, Adafruit) with 12 chan-nels to detect external contacts. When a conductive objecttouches the electrode of a channel, the charges stored in theelectrode are discharged through the contact object. Then,the sensor detects the contact by measuring the change ofcapacitance. However, since non-conductive matters cannottransfer the charges, the sensor cannot detect contacts fromnon-conductive objects. Therefore, a capacitive touch padwith a switch structure was designed for detecting contactsfrom both conductive and non-conductive objects.

The touch pad consists of three main layers (Fig. 2(d)): atop and a bottom electrodes and a middle layer for an air gap.The top and the bottom electrodes were made of a conductivefabric (Woven conductive fabric, Adafruit), and the middlelayer was made of a ring-shaped double-sided tape (VHB-4910, 3M). The empty space in the middle layer made anair gap between the two electrodes maintaining a non-contactspace when the load is not applied. A layer of the conductivetape was also attached at the bottom of the bottom electrode,providing a means of easy attachment to the skin structure

Fig. 4. The result about contact sensitivity of the capacitive touch pad (a)The test result in the case that a conductive indenter applied a force to thepad (b) The test result in the case that a non-conductive indenter applied aforce to the pad

with conductivity to the common ground line which plays arole to maximize amount of escaping charges when the topelectrode touches the bottom one. An actual prototype of thecapacitive touch pad is shown in Fig. 2(e).

III. FABRICATION

The fabrication process of the inflatable sleeve with ca-pacitive touch pads is mainly composed of two simplesteps; cutting materials using a laser cutter and laminatingthe cut materials. The thermo-plastic sheets for inflatablestructures and all the layers for the touch pad are first cutby laser machining (Speedy 300 flexx, Trotec) (Fig. 3(a)).The prepared touch pad layers are then combined togetherand a signal wire is connected to the top layer (Fig. 3(b)).The completed touch pads and a ground line made of aconductive fabric are attached to one of the thermo-plasticlayers corresponding to the middle layer (Fig. 3(c)). Thesensor integrated middle layer is then sandwiched by theother two thermo-plastic layers using an impulse heat sealer(SK-510, Sambotech) (Fig. 3(d)). After completion of theflat sleeve sheet with fabric elastic bands inserted at bothends (Fig. 3(e)), the sheet is rolled up, and its edges arefinally bonded using the heat sealer to form a cylindricalshape (Fig. 3(f)).

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Fig. 5. The result of durability test for 3000 cycles: The test data at thebeginning of cyclic test (top zoom-in graph), The test data at the end ofcyclic test (bottom zoom-in graph)

IV. CHARACTERIZATION

A. Capacitive Touch Pad

To characterize the performance of the capacitive touchpad, two experiments were carried out. The first experimentwas for evaluating the sensitivities of contacts from bothconductive and non-conductive materials.

Since the signal wire connects the top electrode layer tothe sensor readout board (MPR121, Adafruit), if a conductiveobject touches the pad, the charges at the top electrode areremoved through the contact. The capacitive sensor at thereadout board then detects the contact by measuring thechange of charges. Therefore, after the contact force startsto be loaded by an conductive indenter, the capacitive sensorimmediately detects the contact, as shown in Fig. 4(a). Thedetected contact is also maintained until the contact force iscompletely unloaded.

On the other hand, when a non-conductive object touches

Fig. 6. Force responses of the inflatable sleeve integrated with pressuresensor for ten cycles

the pad, the charges accumulated at the top electrode cannotdirectly escape through the contact. Instead when the topelectrode layer begins to touch the bottom electrode con-nected to the ground line, the charges escape through thebottom electrode to the ground line. For this reason, contactcannot be detected until a certain level of contact force isapplied to the pad, and the force should be high enough forthe top electrode to deform and to touch the bottom one.While the sensor did not start to detect the contact untilthe applied force reached approximately 2 N, the detectedcontact was maintained until the force was decreased backto around 1 N, as shown in Fig. 4(b). This difference is dueto the hysteresis of the sensor caused by the air gap and theviscoelasticity of the middle layer. Therefore, it is possible tocontrol the detection sensitivity by controlling the thicknessand the material of the middle layer.

In addition to the sensitivity test in a static loading condi-tion, a cyclic test was performed to evaluate the durability ofthe capacitive touch pad. The contact forces were randomlyapplied between 4 N to 6 N using a commercial robotic arm(UR3, Universal Robot) equipped with conductive indenterduring a total of 3,000 cycles. The result showed that thetouch pad was operational during the entire cyclic test,confirming sensor reliability and physical robustness, asshown in Fig. 5.

B. Inflatable Sleeve

To characterize the performance of the inflatable sleeve, aloading and unloading test was carried out while measuringthe internal pressure of the sleeve. Since the capacitive touchpads can only detects the binary states of contacts, an airpressure sensor (PSE543-M5, SMC) was installed to measurethe internal pressure of the sleeve. During the loading andunloading test, contact forces up to 16 N were applied tothe sleeve using a commercial robotic arm (UR3, universalRobot) and the data from the pressure sensor was acquired.The initial pressure inside the sleeve was 4 kPa and thechange of the internal pressure was up to about 3 kPa at each

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Fig. 7. The comparison of collision test results on two different cases;the collision without sleeve and the collision with inflated sleeve, eachcase. (Inset: the mean value results of the first peak impact forces withthe standard deviation bars of three trials on each collision case.)

cycle, as shown in Fig. 6. With calibration, the magnitudeof contact force was estimated by measuring the internalpressure of the sleeve.

The safety effect of the inflated sleeve was also demon-strated via collision tests (Fig. 7). Controlled collisionswere made by pulling and releasing a revolute joint linkageconnected with a latex band and the collision tests werecarried out both without and with the sleeve. The internalpressure of the inflated sleeve was 4 kPa.

The inset of Fig. 7 shows the mean values of the first peakimpact forces with the standard deviation bars of three trialson each collision case. When the sleeve was not worn, thepeak impact force was measured up to about 83 N. Withthe sleeve, the peak impact force was about 50 N, 40%smaller than the maximum impact force without the sleeve.In this case, the impulse was more uniformly distributedby a cushioning effect of the inflated sleeve. These resultsconfirms that the cushioning effect of the inflated sleeve isuseful, and the sleeve can effectively protect robots fromimpacts.

Additionally, the capacitive touch pad located on the sur-face of the collision area and the pressure sensor connectedto the sleeve was able to detect the impact forces during thecollision, as shown in Fig. 8. Therefore, robots can respondappropriately by detecting collisions while the inflated sleevemitigates the impact forces.

V. APPLICATION

A. Integration of Robotic Leg

As an application of the proposed inflatable sensing sleeve,we integrated a robotic leg module with two sensing sleeves.The two sleeves covered the thigh and shank links of theleg, as illustrated in Fig. 9, so that the leg was capable ofdetecting unexpected external forces applied to each link.Each sleeve contained ten touch pads and connected to apressure sensor (PSE543-M5, SMC) to measure the internal

Fig. 8. The result of collision tests with inflated sleeve (a) The impactforce by a collision with inflated sleeve (b) The internal pressure and contactdata by the collision with inflated sleeve

pressure. The robotic leg was equipped with two custom-builthigh-fidelity/high-torque force control actuators [20] whichcan provide agile motions. Both motors were placed at thehip joint and used to operate the hip and the knee motionsindividually. The base of the robotic leg was fixed to theend of a passive boom system with a radius of Rboom =1.25 m. An encoder was mounted on the rotational axis ofeach motor to measure the joint angles. The leg and thesleeves were tethered to an off-board power source, an airpump, and controllers including motor drivers and the sensormodule to keep the leg lightweight. The total weight of theintegrated leg was 654 g including the on-board sensors andthe electronics while the weight of the robotic leg was 585 g.

B. Sensor Data Processing

Each sleeve has ten capacitive touch pads, summing up toa total of twenty individual touch pads for the two sleeves onthe quadruped leg. The touch pads were connected to the twocapacitive touch sensor breakout boards (MPR121, AdafruitIndustries), as shown in Fig. 9. In this system, analog voltagesignals from the pressure sensor are converted to digital sig-nals by a 16-bit ADC (Ads1115, Adafruit Industries). Thesesignals are then transferred to the slave microcontrollers(Arduino Uno & Redboard, SparkFun Electronics). Datacompiled within the slave microcontrollers are sent to themain controller (ADLQM67PC-3517UE, ADL EmbeddedSolutions). The pressure and touch signals are collected bythe slave microcontrollers at a sampling rate of 100 Hz,

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Fig. 9. System block diagrams describing the control flow of the inflatable sensing sleeve integrated robotic leg

Standby

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𝐹𝑒𝑥𝑡@𝑠ℎ𝑎𝑛𝑘 > 0

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𝐹𝑒𝑥𝑡@𝑡ℎ𝑖𝑔ℎ > 0Jumping

Changing Hip Position𝑡 > 𝑇𝑠𝑡

𝑡 > 𝑇𝑠𝑤

Reset

Fig. 10. Finite-state machine for managing transitions between the avoidingmotions

and the main controller sends the processed commands andreceives the processed data at 4000 Hz. Using Simulink, therobotic leg was programmed to read the binary touch signalsand choose actions to respond to the touch information.

C. Control Strategy and Experiment Results

In this experiment, the robot was programmed to performavoiding motions depending on the location of external

contacts on the two sleeves. A finite-state machine wasdesigned to manage the transitions between the controllersfor the avoiding motions. The external contact is recognizedas a contact on one of the four areas of the leg equippedwith sleeves: front-thigh, front-shank, rear-thigh, or rear-shank. According to the detected contact area, the controlstrategy decides the robotic leg to make one of the followingavoiding motions: a) modification of the leg configurationwithout changing the current foothold, b) jumping of the legto move the entire position. The transition of the finite-statemachine begins by distinguishing the location of the externalcontact on the link using the inflatable sensing sleeves, asshown in Fig. 10. If the contact happens on the thigh link,Fext@thigh > 0, the thigh link avoids the contact by changingthe hip position while maintaining the initial foot contactpoint. In this scenario, because the thigh is distal to thefoothold, execution of this action can make the robotic legavoid external contacts. When the external force is applied tothe shank link, Fext@shank > 0, which is close to the foothold,the leg has to jump in the opposite direction of the contact,so the position of the entire leg is moved away from thecontacted location.

When the jumping motion is processed, a decision totransition from one state to another will be made based ont, elapsed time of a current state. When the robot controllerenters Jumping, the leg pushes the ground during Tst , theduration of Stance 1 state chosen as 0.15 sec. During Swingstate, the leg is airborne, and it re-positions the foot forlanding. Landing state follows if the measured time exceeds

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Fig. 11. Displacement of the hip motor in response to unexpected external contacts detected by the inflatable sensing sleeve. (a) A contact on the shankcausing Jumping as the avoidance motion. (B) A contact on the thigh causing changing Hip Position as the avoidance motion.

the swing time Tsw = 0.25 sec where the position of the footis held to make the leg ready for landing.

Experiments were conducted under an assumption that thesleeve integrated robotic leg had a contact on one of the touchpads, and the goal was to check whether the leg could avoidthe unexpected external contacts by following the controlstrategy of the finite-state machine. In each experimentalresult, drop of the pressure and toggle of the touch pad signalfrom non-contact to contact indicates a successful evasion ofthe leg. In the case of a contact on the shank, the touch padon the lower sleeve detected the external force so that theleg evaded further away with Jumping motion (Fig. 11(a)).As the controller entered Swing state, the increase of thehorizontal displacements, as well as the drop of the pressureand the toggle of touch pad signal from non-contact tocontact, indicates that the leg successfully moved away fromthe contact point. A small peak of the pressure change atLanding state was observed due to the interference betweenthe two sleeves caused by excessive folding of the legwhile absorbing an impact from the ground. At the moment,however, no touch pads were activated, so the leg recognized

the pressure peak as the internal interference. The contacton the thigh made the leg follow ChangingHipPosition asthe avoiding motion (Fig. 11(b)). As the touch pad on thethigh was activated, Standby state changed to Stance 2 state.The drop of the pressure from the maximum value to theoriginal level accompanied with the increase of the horizontaldisplacement showed the leg evaded the external contact.

VI. CONCLUSION AND FUTURE WORK

We propose a lightweight inflatable sensing sleeve to en-hance adaptability and safety for legged robots. The inflatablesleeve has an air chamber capable of easily being inflatedso that it can protect the host robots from physical riskfactors in environments through its cushioning effect. Thesleeve is also integrated with capacitive touch pads which candetect external contact status in a binary state, and a pressuresensor which can measure the magnitudes of contact forces.This enables the robot to obtain tactile information from itssurroundings and consequently makes itself more adaptive tovarious situations.

In this paper, we carried out several tests to characterize

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the sensing and safety functions of the sleeve. The capacitivetouch pad was robust enough to generate reliable signalsduring a cyclic test. Also, in the collision tests, the sleeve wasdemonstrated the ability of protecting the robot by mitigatingimpacts. As an example of applications of the proposedsleeve, the sleeve was integrated with a robotic leg and theleg was controlled to make evasive motions by designing theavoiding strategy of the finite-state machine corresponding tothe tactile information obtained by the proposed sleeve.

The immediate area of our future work is to improvethe sensitivity of the touch pads and to increase the spatialresolution of the sleeve by adding more touch pads to thesleeve. We will also explore new designs of the sleeve andoptimize their form factors not to interfere with the dynamicmovements of the host robot, such as crawling, running, andjumping. Furthermore, the sleeve will be designed to coverthe entire body of the robot. The sensing ability and safetyfunctions of the proposed sleeve will improve the adaptabilityof the robot to the environment and enable it to performvarious tasks by implementing proper control strategies uponunexpected external disturbances. As the number of thesensor increases, an efficient way of processing sensor datawith a restricted on-board processor and sensing strategieswill be also explored.

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