Abstract—IBSS_Gecko_6 is a bio-inspired legged robot thatcan climb on smooth vertical surface. In this paper, we design anew footpad which equipped with polyurethane adhesive.Different from pervious study, we directly apply the adhesive inthe footpad and investigate optimal attaching and detachingtrajectory for wall climbing. The optimal adhesion forcedemonstrated that the leg can perform strong attaching forceand modest detaching force, which can supply a reliable forcefor wall climbing. The optimal trajectory can apply in the wallclimbing robot in future.

I. INTRODUCTION

The extraordinary locomotory abilities of the gecko havebeen well-known for many centuries, which are associatedwith compliant micro/nano-scale high aspect ratio beta-keratinstructures at their feet [1]. Because of this, the gecko hasbecome a source of inspiration for the design of dry adhesives[2, 3, 4, 5]. Despite much effort and the significant progressmade on the design of fabrication of the dry adhesives, only afew of them have applied to climbing robots. For example, alegged robot which uses directional polymer adhesives wasconstructed to climb up smooth surface [6]. And agecko-inspired robot called stickybot that was previouslyrestricted to climbing smooth surfaces is able to climb verticalsurfaces such as wood panels, painted metals, and plastics byusing hierarchical adhesives [7].

Figure 1. IBSS_Gecko_6: a gecko-inspired robot capable of climbing verticalsurface, body mass: 500g. Insert: mechanical structure of the leg.

A safe and efficient climbing gait is essential for leggedclimbing robots. The literatures on gait planning of leggedclimbing robots which based on theoretically calculation and

This work was supported by the National Natural Science Foundation ofChina (grant nos. 51105201, 61161120323) and Doctoral Fund of Ministryof Education of China (grant no. 20123218110031).

Zhongyuan Wang, Zhendong Dai, Zhiwei Yu and Danni Shen are with theInstitute of Bio-inspired Structure and Surface Engineering, NanjingUniversity of Aeronautics and Astronautics, Nanjing, PR China. (e-mail:zhongyuanwang@nuaa.edu.cn; zddai@nuaa.edu.cn; yuzhiwei@nuaa.edu.cn;taoyannisdn@163.com).

*Corresponding author: zddai@nuaa.edu.cn

software simulation were mainly focus on gaining stability byplacing the center of gravity inside the supporting polygon andminimizing the required torques of actuators [8, 9]. A Leggedrobot equipped with dry adhesives for vertical climbingrelying on the locomotion behavior of its leg. It requirespreloading their footpads against the wall to adapt to thesubstrate and maximize the contact area for high adhesion.Furthermore, it needs to peel off and separates from thesubstrate, and minimize the detachment force. Consequently,how to apply the adhesives to the climbing robot’s foot toachieve strong adhesive properties and easy-removalcapabilities has become a major concern that may restrict thewide applications of the adhesives on the climbing robots.

Here, we have developed a gecko-inspired robot capableof climbing a wide range of surfaces: IBSS_Gecko_6 (Fig. 1)[10]. The robot’s leg is composed of two linearly moving armsand a footpad. The leg and the footpad were connected with aball hinge to improve passive degree of freedoms of the legand reduce the impact of the leg with the contact surface.Previously, the robot was equipped with a specialized footpadthat allows it to climb smooth vertical surface using stickingtap. The improvements presented in this work build upon thebasic design principles of the robot. One major change was theutilization of polyurethane adhesive (From a local company).Since the adhesion ability increased due to the betterperformance of the adhesive, an elastic polyvinyl chloridepolymer (PVC) plate with a certain radian was introduced tomatch with the adhesive (Fig. 2).

Figure 2. Photograph of the new footpad with polyurethane adhesive

In this work, we combined locomotion gait planning of therobot’s leg with the adhesion performance testing of the dryadhesion. Using a force plate, different attaching anddetaching trajectories of the dry adhesive which directly applyto the footpad were investigated. We aims to find out theoptimal attaching and detaching trajectory of the footpad,which can be satisfied the required high attaching force andsmall detaching force for wall climbing.

Optimal attaching and detaching trajectory for bio-inspired climbingrobot using dry adhesive

Zhongyuan Wang, Zhendong Dai*, Zhiwei Yu and Danni Shen

2014 IEEE/ASME International Conference onAdvanced Intelligent Mechatronics (AIM)Besançon, France, July 8-11, 2014

978-1-4799-5736-1/14/$31.00 ©2014 IEEE 990

II. EXPERIMENTAL DESIGN

Mechanical setupThe mechanical setup of the experimental apparatus

includes force-measuring setup and three dimensional motionsetup (Fig. 3). The force-measuring setup consists of a twodimensional force sensor and a signal processing device (IBSS,custom-built). The force sensor was made up of H-shapedaluminum alloy, and was used to measure the tangential andnormal forces. A load carrier (100×100 mm) was fixed on thetop of the sensor to mimic the substrate. Forces were measuredfrom the deflection of the cantilever beams by foil straingauges glued to the beams. The resolution of the sensor foreach direction was 2 mN; the natural frequency for the x- , y-and z-axis is 1397 Hz, 4023 Hz and 8256 Hz, respectively.Signals from the force plate channels were amplified andcollected by the signal processing device on a computer(Lenovo, Beijing, China) at a sampling frequency of 500 Hz.The three dimensional setup follows the mechanical structureof the robot’s single leg. A groove in the vertical plate wasused to fix the leg and adjust the vertical distance between theleg and the substrate. Another groove in the horizontal platewas used to fix the force sensor and adjust the horizontaldistance between the leg and the substrate. Three motors (The1st motor and 2nd motor, Hitec HS-65MG, South Korea; The3rd motor, Hitec HS-85MG, South Korea) are able to executeprescribed motion paths in three dimensions.

Figure 3. Schematic diagram of mechanical setup.

Experimental methodsThe basic concept used was to separate each step into the

two phases that were previously introduced: attaching phaseand detaching phase. Each phase can be seen as a distinctoptimization problem. All the motion is based on the gait offootpad’s heel.

Figure. 4. Attaching and detaching phase of the optimal trajectory design.

In the attaching phase, the friction/adhesion of theadhesive was studied. For an adhesive interface, thestatic-friction force can be given as [11]

ALf (1)

where is the static-friction coefficient, L is the compressivenormal load, A is the real contact area, and is the elastomershear strength. So the adhesive friction, the normal load andthe real contact area determine the friction force.

Because of the structure of the leg, the force here is notloading normally first. An extrusion period is introduced tomaximize the contact area. So we define the displacement in Ydirection as the extrusion magnitude. Because the normal loadis also an essential component for the friction, after extrusion,the footpad preloads on the substrate under a certain depths forhigh adhesion. We define the depth as preload depth. Then itdrags until it slips on the substrate. Various extrusionmagnitudes and preload depths were performed to obtain themaximum attaching force.

In the detaching phase, the footpad peels with a continuousshift from the back of the foot towards the distal end of the toe.Then, it separates from the substrate after a critical angle.

The peeling mechanism is crucial for power-efficientdetachment and the Kendall model of peeling was applied toestimate the peeling force [12]:

)]cos1(2)cos1([ 2 dERbdEF (2)

where F is the peeling force, d is the thickness of theadhesive, b is width of the adhesive, E is the elastic modulusof the adhesive, is the peeling angle, and R is the energyrequired per unit of the interface. For a certain adhesive, thepeeling force is relative to the peeling angle. Here, variouspeeling angles were performed to obtain the minimumdetaching force.

Theoretically, minimum backing layer thickness leading tothe best attachment and easy-removal properties under thecondition that fracture of the backing layer does not occur [13].Actually, a thin backing layer does not distribute forces well[14]. In this work, the adhesive attached a PVC backing layerwith 2mm thickness was chosen.

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III. RESULT

A. Optimal attaching trajectoryThe attaching phase explored the effect of extrusion

magnitude and preload depth on the tangential adhesion force.The extrusion magnitude was added between 0mm and 5mmin 1mm increments, the preload depth was varied between0mm and 5mm. Fig. 5A shows tangential adhesion forcesunder different extrusion magnitudes. Because of the sametrend is seen across different preload depths, only one preloaddepth (3mm) is shown. The data clearly show the adhesionforce is maximized around an extrusion magnitude of 3mm.For the optimal extrusion magnitude, the tangential adhesionforce increases as preload depth increases up until a saturationpoint (3mm) is reached, as shown in Fig. 5B. All the tangentialadhesion forces were calculated by averaging the maximumpreload recorded for the experiment which repeated five timeseach time.

Figure 5. (A) Relationship between extrusion magnitude and averagetangential adhesion force at a preload depth of 3mm. (B) Relationship betweenpreload depth and average tangential adhesion force at an extrusion magnitudeof 3mm.

B. Optimal detaching trajectoryThe detaching phase explored the effect of peeling angle

on the normal and tangential force. Different peeling angleswill lead to different peeling heights. In order to facilitate thecalculation, peeling heights were introduced (the heightbetween the footpad’s heel and substrate when peels off).

As shown in Fig.6, normal force increases with the peelingheight, and tangential force decreases with the peeling height.At a peeling height of 33mm, both the normal and tangentialforces are at relative low values. The height was recognized asthe optimal peeling height.

Fig. 6. Relationship between peeling height and average adhesion force.

IV. DISCUSSION

The optimal attaching and detaching trajectory is used inplanning force trajectories for the foot during attaching anddetaching cycle. First the frontend of the footpad approachesthe substrate and then it extrudes on the substrate to maximizethe contact area and then it loads on the substrate under acertain depths for high adhesion. Next, the footpad is pulledtoward to transfer the shear load. Once the foot no longerneeds to carry a shear load, it peels off the substrate, thenswing to reset (Fig. 7).

Figure 7. The programmed optimal gait of the leg during one cycle.

Fig. 8 shows the tangential and normal contact forcesexerted by the leg which imitate the motion of robot’s frontleft foot. The test is planned to apply in wall climbing, thelateral force is small. Here, only the relationship betweentangential and normal force is shown. The leg uses a relativesmall preload force to obtain a relative large adhesiontangential force.

The adhesion cycle starts with a relative small preload(1.2N) (A) to bring the footpad into full and well contact withthe substrate. The normal force is modest so that the robotwon’t be pushed off when climbing on the wall. Then thetangential force is increased to an adequate value (3.2N, A-B)which can satisfy the required adhesion. The adhesion alsoincreases during the stance phase (B-C) to main equilibrium ofthe robot. During detaching phase (C-D), the normal andtangential forces are reduced gradually to obtain an effortlessand smooth peeling.

Figure 8. Tangential and normal force of the footpad during one climbing step.

Adhesion of the footpad follows the frictional-adhesionmodel [15]. The tangential and normal force limits can bemodeled as:

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TN

TN

FF

FF

tan

1

(3)

where is the critical angle of detachment, is thecoefficient of friction, NF is tangential force, and TF isnormal force.

TABLE I. MAXIMUM ADHESION FORCES DURING ATTACHING ANDDEACHING PHASE IN ONE CYCLE

Attaching phase Detaching phase

Tangential Tangential Normal

Maximum adhesionforces (N) 3.2 1.8 0.8

During wall climbing, the required maximum tangentialadhesion force was at least 2.5N under diagonal gait. In ourexperiment, the force (3.2N) which generated by one singleleg was obviously satisfied for wall climbing. Although thedetaching force (TableⅠ) of the polyurethane adhesive is notas low as directional adhesive, the maximum adhesion forceof the leg can apply is larger than the directional adhesive.Furthermore, the modest detaching force and the strongattaching force, which are much better than the previous leg’sforce using sticking tape [10], can obtain a reliable climbing.

V. CONCLUSION AND FUTURE WORK

In this paper, a polyurethane flat elastomer is used as anadhesive material for climbing robot. We combine the motionof the robot’s leg with the adhesion performance of theadhesive. Various motion trajectories were applied to explorethe attaching and detaching characteristic of the adhesive byusing a force plate. During attaching phase, the effects of bothextrusion and normal load magnitude were investigated. Theadhesion force is maximized around an extrusion magnitudeof 3mm. For the optimal extrusion magnitude, the tangentialadhesion force increases as preload depth increases up until asaturation point (3mm) is reached. During detaching phase,both the normal and tangential forces are at relative values ata peeling height of 33mm.We analyze tangential and normal force of the footpad

during one climbing step under optimal gait. The force wascompared with the frictional-adhesion model. The resultshows the adhesive can supply strong attaching force andmodest detaching force for wall climbing.In previous study, the robot can climb on vertical smooth

surface by using sticking tap. The adhesive used in this papercan ensure the robot climb on the smooth surface morereliable in future. The method and analysis can also beextended to explore other adhesive apply in the robot to climbon even overhanging surface.

ACKNOWLEDGMENT

We thank all the members in the Institute of Bio-inspiredStructure and Surface Engineering for their useful discussion.

This work was supported by the National Natural ScienceFoundation of China (grant nos. 51105201, 61161120323)and Doctoral Fund of Ministry of Education of China (grantno. 20123218110031).

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