Geckobot and Waalbot: Small-Scale Wall ClimbingRobots

Ozgur Unver†oUnver@andrew.cmu.edu

Michael P. Murphy†mikemurphy@cmu.edu

†Department of Mechanical EngineeringCarnegie Mellon University

Pittsburgh, PA, USA

Metin Sitti†sitti@cmu.edu

Abstract— This paper proposes two small-scale agile wallclimbing robots able to navigate on smooth vertical surfaceswhich use adhesive materials for attachment. Geckobot is alizard-inspired climbing robot with similar kinematics to a geckoclimbing gait. Waalbot uses two actuated legs with rotary motionand two passive revolute joints at each foot. Due to their compactdesign, a high degree of miniaturization is possible. Each hasonboard power, computing, and wireless communication whichallow for semi-autonomous operation. Various aspects of func-tioning prototype design and performance are discussed in detail,including leg and feet design and gait dynamics. Geckobot andWaalbot prototypes can climb 85◦ and 90◦ slopes respectively,and steer to any angle reliably. These robots are intended forinspection and surveillance applications and, ultimately, spacemissions.

I. INTRODUCTION

Mobile robots with the ability to climb and navigate on sur-faces of any orientation without leaving residue or damagingthe surface have many potential applications. One of the mostnotable situations where such a robot could be useful, andperhaps life-saving, is for spacecraft hull inspection and repair.Terrestrial uses include surveillance or inspection in hazardousor difficult to reach areas. Many of the first wall scaling robotswere intended for cleaning in hazardous environments such asnuclear reactors [1].

Researchers have proposed a great variety of climbingrobots for various applications. In general these robots use oneof three types of adhesion mechanism; vacuum suction [1]–[7], magnetic attraction [8], or gripping with claws or grasp-ing mechanism [9], [10]. Each of these mechanisms has itsadvantages and drawbacks. For instance, magnetic adhesioncan be very strong and has good power failure mitigation,but is only applicable for ferromagnetic surfaces. Suctionadhesion requires an ambient pressure for attachment and istherefore not suitable for space applications. Recently, robotsusing micro-claws have shown good performance on brickbuldings [9], but clawed and grasping robots cannot climbsmooth surfaces smooth metal or painted structures.

This paper proposes two semi-autonomous small-scale robotprototype capable of navigating on smooth flat vertical sur-faces. The novelties of the Geckobot are gecko-inspired gait,methodology for inspired active tail assisted robust attachment,

Fig. 1. Geckobot - CAD model. A: Adhesive footpads, B: Servomotors, C:Front legs, D: Waist, E: Servomotor propeller, F: Rear legs G: Active tail, H:Onboard components.

tail based steering for high maneuverability, and optimalpeeling mechanisms for power efficient detachment. Waalbotis actuated by two motors, each controlling a set of three-footed wheel-legs. Wheel-leg designs have been successfullyimplemented in ground walking robots such as RHex [11],Whegs [12] and miniWhegs [13] robots but are not specializedfor climbing vertical surfaces. Waalbot’s specialized legs en-able vertical climbing and agile movements including steeringand plane to plane transitions.

A. Dry Adhesion

Recently, passive attachment mechanisms have been pro-posed for robust climbing. The Tokay gecko, for example,can weigh up to 300 grams and reach lengths of 35 cm yetis still able to run inverted and cling to smooth walls usingfibrillar dry adhesives in their footpads by exploiting contactforces such as van der Waal’s Force (from which Waalbotdraws its name). Synthetic versions of the biological Geckoadhesives may allow agile movements and robust climbingfor robots which do not need energy to stay on the surfaceor pressure differences to climb. Since dry adhesion does notrely heavily on the surface material or atmospheric pressure, itallows climbing on a wider variety of surfaces and is uniquelysuitable for use in the vacuum of space.

Synthetic fibrillar dry adhesive technology is not currently

mature enough to be used for climbing robots, however Geck-obot and Waalbot are designed with the intention of eventuallyutilizing the technology. While efforts to develop the syntheticfibrillar dry adhesive continue, Waalbot uses a conventionaladhesive material (foam tape) which shares many performancecharacteristics with the envisioned dry adhesive material andGeckobot utilizes dry adhesive patterned elastomers for at-tachment. Both the dry adhesive and the foam tape must bepressed to the surface with a preload force in order to providean adhesive force on detachment. This adhesive force increaseswith preload force up to a saturation point. The elastomers gaintheir adhesion performance by deforming into the microscalesurface features of any smooth surface and creating a largecontact area. Using these adhesives as substitutes until thefiber based dry adhesives have suitable performance for thisapplication allows testing and improvements to the robotdesign.

II. ROBOT DESIGNS

A. Geckobot Design

This paper aims to design a gecko-inspired robot that canwalk, climb, and steer robustly and power efficiently as shownin Fig. 2. In order to achieve efficient wall climbing, therobot should be able to change orientation and move inall directions. The abilities to climb at any direction, avoidobstacles, steer and actively preload the tail are required.Peeling is a very crucial and challenging task for climbingrobots to improve their climbing ability and to minimizepower consumption. For autonomous performance, the sourceof energy, microprocessor, actuators and sensors have to beplaced on the robot.

1) Walking on a Flat Surface: The robot body kinematicsis composed of a four-bar mechanism as shown in Fig. 3a.AB, BC, CD and ground are four linkages and the circleson the lines illustrate the joints. Geckobot sequentially pullstwo diagonally opposed feet up by using a motor on the four-bar mechanism, propelling itself forward. Then, the feet thatare aloft are attached to the ground and the opposite feet are

Fig. 2. Waalbot: Photograph of prototype climbing a 90◦ (vertical) surface.

Fig. 3. (a) Four-bar mechanism of the Geckobot (left image), (b) The safetyregion of the Geckobot (right image).

lifted for the next forward motion. The aim is to keep thecenter of gravity (CG) inside the safety region (SR) shown inFig. 3b, which is formed by the ground legs to keep the robotin balance. Firstly, the angle between waist and centerline, ϕ,has to be found to obtain the safety region

tanϕ = (a + b)/L (1)

sinϕ = R/B (2)

where a is the front, b is the rear waist width, R is the radiusof the foot, B is the vertical distance between safety regionlines, and L is the waist length. When the robot is stationary,as long as CG is in the SR robot does not fall aside when twofeet are aloft. However, when the robot moves, the CG movesa distance forward due to the rotation of the links, where θ isthe angle of the front foot rotation and s is the step size ofthe CG.

s = a sin θ (3)

Increasing the step size concludes decreasing the safety regionwith the amount of the CG shift, s. It is obvious that if θ,during walking, gets smaller and the foot diameter gets bigger,maintaining the balance becomes easier. In other words, thesafety region increases. The lengths a and b should be selectedvery carefully in order to make the CG lie intersection ofthe centerline that connects front and rear leg centers and theline that connects rear and front waist motors. The main aimbehind this idea is overlapping the CG point and SR center tomaximize the usage of the SR when the robot steps forwardand backward. The safety region is calculated for varying legwidths and waist lengths. According to the calculations, thewaist length should be as small as possible, and the front andrear widths should be around 110 mm to maximize the safetyregion.

2) Climbing Analysis: Unlike in ground walking, the pro-jection of the CG shifts backward during climbing. This shift,∆S, is related with the inclination, α, and CG height, h, ofthe robot as shown in Fig. 4.

Fig. 4. Projection of CG shift during slope-up climbing.

Fig. 5. Safety region shift during side-walking.

∆S = h tanα (4)

However, the CG shifts sideways with the side slope, β,when Geckobot is walking across a slope. The CG height anddimensions of the robot directly affect the amount of side-shiftas shown in Fig. 5. Due to the shift, safety region decreaseswith the amount of loss (∆L). Side shift (Ss) can be expressedas

Ss = h tanβ (5)

and loss of the safety region can be expressed as

Ls = Ss/tanϕ (6)

In order to find the optimum width of the legs, climbing bothup and across a slope are analyzed. First, the waist length ofthe robot is taken arbitrarily and the leg width was taken as avariable between 50 mm, and 300 mm for both side-walkingand slope-up climbing to obtain a 3-D safety region. Aftercombining these graphs into the same graph, an intersectionline is observed in Fig. 6. This line indicates the optimumwidth of the front and rear legs for the waist length chosenarbitrarily. If any other width is chosen, the performance ofthe side-walking might be increased but slope-up climbingperformance is diminished or vice versa. As a result, choosingthe intersection value gives us the optimum width duringwalking in any direction for the given waist length.

Fig. 6. Optimization of the width of the Geckobot according to the safetyregion.

A tail is one of the most important limbs for climbinganimals and robots. There are some benefits of having a tailwhile climbing, such as; holding onto supports, maintainingbalance and moving from one place to another, but the mainaim of having a tail is preloading. Pressing against the surfacewith a tail increases the normal force on the front toes leadingsecure climbing. If a maximum slope-up climbing angle isexceeded, the front toes detach from the surface due to themoment about the rear legs caused by the CG. A tail is addedto the system in order to transfer some load from the rear toesto the front toes increasing the maximum slope the robot canclimb. When Geckobot is climbing at 90 ◦, at least 50 mNadhesion force is needed to keep the front toes on the surfacewithout a tail.However, with the aid of the tail, the adhesionforce needed can be decreased down to 25mN by preloadingthe tail 50 mN as shown in Fig. 7. Fig. 8 shows the forcesthat act on Geckobot’s toes and tail. Ftx can be assumed zerosince the friction coefficient of the tail is very small and thepreload on the tail is not more than 60mN . Fty is the tailpreload and can be controlled by the rotation of the motor.Normal force under the rear toe becomes

Fry = (mg cos αL1 +mg sin αh−Fty(L1 +L2 +L3))/(L1 +L2) (7)

The normal force under the front toe turns out to be

Fry = mg cos α − Fty − Fry (8)

In Fig. 9, the force distribution during side walking is shown.Fdx and Fux are the normal forces underneath the toes duringside-walking.

Fux = (mg cos β − sinβh)/2a (9)

Fdx = mg cos β − Fux (10)

Climbing is achieved if normal force on the pad is multipliedby coefficient of friction is bigger then the total force whichpulls the robot downwards parallel to the surface. If there aretwo components connected to each other and they have the

Fig. 7. Force distribution with tail during slope-up climbing while havingequal normal forces underneath the toes.

Fig. 8. Force distributions during slope-up climbing.

same coefficient of friction, the one that has lesser amountof normal force slides first down if pulling down forces areequal, and cause the other component to slide as well. Forthis reason, rear and front normal forces has to be as close aspossible for better climbing. As a conclusion, the tail motorshould push against the surface at just the right amount thatthe front and rear toes normal forces become equal at any timeand any slope for stable climbing. This is because, although itis a four-bar mechanism, two actuators should be used to shareclimbing forces. In other words, the rear motor pushes and thefront motor pulls the robot up. Since their normal forces areequal and they are using the same pad materials, front andrear toes slip forces are going to be equal as well.

First, the tail normal force versus climbing angle versusnormal force underneath the toes is drawn in 3-D mesh graph.The intersections of the front and rear preload surfaces givethe force on the tail to make the normal force of the toesequal. A cubic equation is fitted to the intersection curve y =

Fig. 9. Force distributions during side-walking.

1.5e− 005x3 − 0.0057x2 +0.92x+25. x is slope-up angle indegrees and y is the tail normal force in mN’s. The slope canbe detected using an accelerometer, and put into the equationdetermining the required normal force needed on the tail. Foraccurate and precise preloading, the position of the tail mustbe controlled. To accomplish this, a servomotor is chosen foractuation. If a tail is machined from a stiff material, very smallrotation of the servomotor will create a large preload force onthe tail, which is undesirable. Instead, a relatively compliantmaterial should be used to get good resolution out of the servorotation. When the servomotor presses on a complaint tail, itstarts to bend, but the preload force does not reach a very highlevel abruptly; instead, it is increased gradually, which givesbetter preload resolution.

3) Steering: Steering is directly related to the system’sdegree of freedom (DOF). Since Geckobot is a four-barmechanism, it has one DOF. As explained in the previoussubsection, one more motor should be placed in the four-barmechanism for increasing climbing performance. However, notproper synchronization may result in a foot slip or inner torqueaccumulation that bends some part of the robot or increasesinstability. Synchronization is realized by using kinematicanalysis of the Geckobot as shown in Fig. 10.

From kinematics, it is found out that when the front motorrotates with a specific degree, there is just one correct positionfor the rear motor due to the single DOF as shown in Fig. 11.

beiθ3 + weiθ2 + aeiθ1 = |AD| (11)

b cos θ3 + a cos θ1 = w − w cos θ2 (12)

b sin θ3 + a sin θ1 = a + b − wsinθ2 (13)

where, θ’s are the angle between the links as shown, w is thewaist length, and |AD| is the vectorial distance between pointA and D. The rear motor rotation angle is (180 − θ3 + θ2)and, the front motor rotation angle is (θ1 − θ2) and both ofthem can be controlled.

Fig. 10. Kinematic diagram of the Geckobot.

Fig. 11. Kinematic angles of rear and front motor rotations.

Steering is realized by controlling two servomotors onthe four-bar mechanism separately when the rear feet arealoft. While the robot is walking, the two motors rotate ina synchronized manner. When steering starts, its rear feet peeloff the ground and Geckobot’s tail presses harder against theground. Then, the front motor rotates the whole body to thedesired angle while the tail is sliding. At the same time, theback motor adjusts itself for the next step to the right anglewhile rear legs are aloft. The rotation of the rear servomotordoes not directly affect steering; however, right after steering,when the hind legs touch the ground, their position has to beplaced properly in relation to the position of the front legs.

4) Peeling Mechanism: The peeling mechanism is verycrucial for climbing robots for power-efficient detachment asseen in geckos. For instance, to remove an ordinary piece oftape from an item, if pulled perpendicular to the surface fromthe center, a relatively high force would be required. However,instead of pulling the tape directly upwards, if it is peeledstarting from one side, it would come off very easily. Likethe tape example, the Geckobot has to peel during climbingin order to minimize the foot detaching force.

The working mechanism of the peeling system is as follows.When the motor is energized, due to the rotation and dis-placement of the servo arm, all fishing lines and the adhesive

Fig. 12. CAD design of the peeling mechanism. A: Servomotor, B: Adhesivepads, C: Strings, D: Revolute joint, E: Spring, F: Cylindrical rod

Polydimethyl siloxane (PDMS) elastomer surfaces are pulled.Then starting from the edge, the PDMS mold starts beingdeflected and peels off. There is a compression spring onthe shaft so when the pulling force exceeds the deflectionthreshold of the spring, foot starts to move up. Reattachmentto the ground is accomplished by rotating the servo arm toits original position and releasing the foot. Since compressionsprings are used, they push the whole mechanism back to itsoriginal position.

B. Waalbot Design

1) Waalbot Mechanical Design: In order to create thepreload force required to bring the adhesive into intimatecontact with the surface, the robot has been designed tomaximize the pressing force when the adhesive pads comeinto contact with the climbing surface. This requirement guidesthe design of the legs and feet mechanisms. A gear motor’soutput shaft is connected to a triangular shaped leg, where eachpoint of the triangle holds a foot assembly on a revolute anklejoint (Fig. 13). This ankle joint assembly is spring loaded toalways return to the forward position (the robot cannot travelin reverse).

Fig. 13. CAD model of the Tri-Foot design (left) and assembled prototypecomponents (right). Each foot pad has two degrees of freedom allowing forclimbing and turning.

On the distal end of the foot assembly is another revolutejoint connecting a foot pad which holds the adhesive material.This final revolute joint enables steering capabilities (discussed

later in Section II-C). Only the outer annulus of the foot padis covered with adhesive (Fig. 2,13). This ensures that the footpad adheres flat to the surface while not sticking so much thatthe robot cannot peel the foot away.

Fig. 14. Schematic side view of Waalbot climbing a vertical surface (top)and still photo frames from a video of the Waalbot prototype performing thesame actions (bottom).

The principle of operation is as follows. During forwardtravel the two legs are synchronized and step in unison. Asthe motors turn, the tail of the robot presses against the surfaceand the triangular legs rotate forward. The two feet which areadhered to the surface (one on each side) support the weight ofthe robot (Fig. 14a). Soon the forward feet come into contactwith the surface (Fig. 14b). At this point there are 5 contactpoints with the surface; 2 feet on each side and the tail. Themotor torque provides an internal moment which presses thefront feet onto the surface while pulling the rear feet awayfrom the surface. When the rear foot normal force FRn reachesa critical peeling value of Fcr, the adhesive peels away fromthe surface and the robot steps forward.

Fig. 15. Free body diagrams showing the forces during a forward step (left)where the front foot is pressed against the surface and the rear foot is peeled,and in the configuration in which the robot experiences the highest peelingforce while climbing (right).

Examining the free body diagram (Fig. 15a) and assuming

quasi-static dynamics and symmetric loading it is possible tofind a system of equations which describe the forces on therobot during the stepping transfer:

∑Fy = 0 = Ft + FRn + FFn − W cos θ∑Fx = 0 = FRx + FFx − W sin θ∑Mc = 0 = M + (FRn − FFn)(

dstep

2)

+ Ft(Lt) + W sin θ(Lyc − Lycg)−(FRx + FFx)(Lyc) − W cos θ(Lxcg)

(14)

where W is the weight, θ is the slope of the climbing surface,Ft, FRn, FFn are the normal forces at the tail, rear foot, andfront foot respectively. FRx and FFx are the shear forceson the rear and front feet respectively. dstep is the distancebetween the centers of the rear and front feet, Lyc is thedistance from the climbing surface to the center of the leg,and Lycg, Lxcg are the distances from the center of gravity(Cg) to the surface and center of the leg respectively. Lt isthe distance between the center of the leg and the tail-surfacecontact point.

There are five unknown forces (FFn, FRn, FFx, FRx, Ft)and only three equations, so in order to solve for the unknownssome assumptions must be made. Assuming FRn = Fcr willgive the forces just before the peel-off occurs, which is whenFFn is at a maximum. We also assume that the shear forceson the front and rear feet are equal (FFx = FRx). Lastly weassume that the torque just before peel-off is approximatelyequal to the the torque necessary to peel the rear feet andpreload the front feet with an equal force(M = Fcr·dstep).This assumption also guides the minimum motor torque re-quirements when designing the physical robot.

Using these assumptions and (14) it is possible to examinethe effect of slope angle θ on the preload force FFn. Sincethis preload force is important for creating intimate contactand thus adhesion, it is of critical importance. The results ofthis modeling are used to guide the robot design to acheivehigh preload force.

The adhesives used with the robot have very high shearresistance and therefore always detach because of a normalforce pulling the foot away from the surface. The minimumadhesion normal force required to keep the robot attached tothe wall during climbing can be found by examining the casewhen only one foot per side is attached and the Cg is at itsfurthest distance from the surface. The quasi-static equationfor the case when the Cg farthest from the wall (Fig. 15b)gives a maximum force which the adhesives must be able toprovide:

FRn =W

Lt

(Lycg(max) sin θ − (Lt − Lxcg) cos θ

)(15)

If this force exceeds the magnitude of Fcr then the robot willdetach from the surface. Therefore, this equation gives theminimum adhesive performance needed to climb. Of course,

because of the consequences of a fall, a safety factor is usedwhen choosing an adhesive and adhesive foot area.

From (15) it is clear that in order to minimize the peelingforce on the adhesive pad during climbing, the length of thetail should be increased and also the center of gravity should bemoved close to the wall. Finally the weight of the robot shouldbe minimized. This may be accomplished through fabricatingthe robot from lighter materials or through miniaturization.Miniaturization is advantageous to this robot design becausemass is proportional to L3 while the adhesion force is pro-portional to area, thus L2. As the robot shrinks in size, theadhesion force will be reduced less than the gravitational force,so we will see an increase in the performance of the robot.

Both equations suggest that the tail should be long toincrease preload force and to minimize the peeling force.However, it is important to note that the weight of the robotis also increased with the tail length, so there is a couplingbetween W and Lt. Furthermore, a longer tail increasesthe amount of room necessary for turning, as the tail maysweep out and contact obstacles causing the robot to loseadhesion and fall, so a longer tail limits the ability to climbin small areas. Therefore, for maximum performance withoutcompromising agility the tail length is chosen to be the longestlength such that the tail remains fully within the turning circle(Fig. 16).

A printed circuit board (PCB) acts as the chassis for therobot instead of using an additional body frame in order tokeep the mass low. The heaviest components of the system arethe leg assemblies (fabricated by rapid prototyping), motors,and batteries. In order to move the center of gravity as close aspossible to the surface and balanced around the motor axes, allof these parts are located around the motors, with the batteriesbeneath the PCB, almost touching the climbing surface.

2) Electronics and Sensors: The robot is controlled by aPIC microcontroller (PIC16F737) and is able to perform pre-programmed actions such as climbing and turning without ainstructions from the user. Gait is controlled with feedbackfrom foot position sensors. Limit switches are triggered whenthe legs are aligned such that only one foot is contactingthe surface. This information is used to keep the robot’s gaitsynchronized by pausing one of the motors until the opposingmotor catches up, so that (14) applies. This is also importantfor safely putting the robot into steering mode.

Infrared (IR) RC5 communication is used to teleoperatethe robot to climb straight, stop, and turn. Commands canbe sent for turning 180◦, 90◦, and in increments of 15◦ ineither direction. In the absence of user commands the robotwill perform a preprogrammed routine.

Power is provided by two lithium ion polymer batterieswhich are placed in series beneath the body of the robot for 7.4volts. Power is regulated to 5V to drive the logic and sensorswhile unregulated power goes to the motors. The two motors(Sanyo 12GA-N4s) have a torque output of approximately400 mN·m each, which is enough to peel the rear feet fromthe surface.

C. Agility

1) Steering: When only one foot on a side is contacting thesurface, that foot can be used as a pivot point for the robotto turn around. By advancing the opposite motor, the robotrotates around the passive revolute joint in the pivoting foot.If the robot attempted to turn while two feet were attached ona side, the robot would shear itself off of the surface since thecenter of rotation would not be aligned with a joint. This canbe a catastrophic failure for a climbing robot, so foot positionsensors are used to prevent this occurrence.

In steering mode, the robot takes discrete steps around thepivoting foot. The turning radius is less than the width ofthe robot so tight turns are possible (Fig. 16). The ability tomake tight turns is an important feature for climbing throughsmall passageways or for avoiding closely spaced obstacles.Path planning and odometry is simple for the Tri-Foot Waalbotbecause of the discrete nature of the movements, as well asthe non-slipping gait.

Fig. 16. Waalbot’s small turning radius (Rt) and turning circle radius (Rtc)let it climb around obstacles and operate in narrow corridors with sharp bends

The prototype Waalbot was designed to make discreteturning increments which change the heading by 15◦ per step.This angle is convenient as it allows for turns of 45◦, 90◦, 180◦

as well as smaller adjustments.2) Plane Transitions: The robot’s unique jointed legs allow

it to perform transitions between planes. For example, therobot can climb along a floor and transition to climb up avertical wall, then transition onto a wall or ceiling. The stepsof the transition process can be seen in Figure 17. As the robotapproaches a junction the forward foot makes contact with thenew plane (Fig. 17a) and the robot makes the planar transition(Fig. 17b,c).

The Tri-Foot design gives the Waalbot the capability ofmaking planar transitions of various angles including 90◦,however the transitions are not fully robust. This is due tothe non-perpendicular angle at which the forward foot maycontact the new plane. Since the foot position is optimizedfor flat surface walking, when a new plane is encountered thefoot may not come into full contact immediately. In this casethe foot may not be preloaded correctly and adhesion cannot

Fig. 17. Schematic side view showing the steps in a 90◦ plane transition(top) and still photo frames of prototype Waalbot performing the transition(bottom).

be guaranteed. In the worst case, the forward foot may landin the junction, touching both planes and making very littlecontact, causing almost certain failure of adhesion. Therefore,depending on the relative angles of the approaching foot andthe new plane, the robot may or may not successfully makethe transition.

III. PROTOTYPES

A. PROTOTYPE

1) Body Materials and Circuitry: The chassis of the Geck-obot was built from Delrin R©. The robot was equipped withseven servomotors (GWS Pico STD), four used for lifting therobotic legs, two for robot locomotion, and one for the activetail. The output torque of the servomotors is 70 Nmm when 5V is applied. PDMS is used as the dry adhesive elastomermaterial. PDMS adhesion pressure for various preloads isdisplayed in Fig. 18. On top of the PDMS layer, a very thinstainless steel is used for both fixing the fishing lines securelyand giving the spring back behavior to the whole foot. Fishinglines pass between the PDMS layer and the stainless steelsheet. Fishing lines go out of the mold from the edges and areconnected to the servomotor with the aid of superglue. In thefour-bar mechanism, there is a pushpin on top of the PDMSlayer, half buried into the PDMS, as a revolute joint. All thefishing lines are passed through a same hole, when the motoris actuated, it is thereby guaranteed that all fishing lines ordeflected PDMS move the same distance.

A compact on-board circuit is placed on the waist. This cir-cuit expands the functionalities of the robot by adding modulessuch as wireless infrared (IR) communication, obstacle avoid-ance, serial communication, and in-circuit serial programming(ICSP) compatibility. The robot can be controlled via IR overRC5 protocol or serial communication over RS232. The IRproximity sensors allow the robot to avoid obstacles and steeraway from them. The motors are driven by a microcontroller,which also runs the main program. A three-axis accelerometeris used to sense orientation and control the active tail.

Fig. 18. PDMS preload vs adhesion graph.

Fig. 19. Photo of the Geckobot as parts displayed in Fig.1

B. EXPERIMENTS

The overall weight of the robot is 100 grams includingthe electronic board. The total length is 190 mm withoutthe tail, width is 110 mm and tail length is 100 mm. Thedistance between the front legs and the CG is 100 mm asshown in Fig. 19. In these experiments, flat adhesives withoutpatterned surface are used on the feet. The speed of therobot is 5 cm/s during walking on the ground, but whenclimbing at higher angles, it decreases to 1 cm/s due tostability reasons. The robot is slowed due to attachment anddetachment vibrations during climbing caused by the PDMSpad adhesives. Geckobot can climb up to 85 ◦ stably onPlexiglas surfaces as seen in Fig. 20. However, beyond thisangle stability diminishes abruptly. Since Geckobot is mainlydesigned for slope-up climbing instead of side-walking, itslength is longer than its width, which diminishes the robot’sside-walking performances. Geckobot cannot walk sidewayson a slope of more than 50 ◦. Steering can be done effectivelyand very stably until 45 ◦, as displayed in Fig. 21. The powerconsumption of the robot is around 1.4 Watts for 85 ◦ climbingmainly due to the servomotors.

Experimental results of the tail preload force versus servo-motor rotation angle, as demonstrated in Fig. 22, is exactlymatched with the theoretical calculations.

Fig. 20. Photo snapshots of the Geckobot climbing on an acrylic flat surfacewith 85 degrees slope.

Fig. 21. Photo snapshots of the Geckobot steering on a horizontal surface.

C. DISCUSSIONS

The performance of the robot depends on the chosen adhe-sive and peeling mechanism. The presented research opens anew avenue in the design of high performance miniature wall-climbing robots using active tail and peeling mechanism withdry adhesives. Here, flat PDMS elastomer is chosen as the dryadhesive. Although PDMS is a stable material, it is degradedand contaminated by the surface and the air within the time.That is, after sometime it looses its adhesive characteristicsand some properties.

Since an open-loop control system is used on the tail

Fig. 22. Experimental tail normal force for varying servomotor rotationangle.

mechanism, the robot does not control its tail through feedbackfrom the system, potentially causing some problems duringclimbing. In addition, the Geckobot cannot peel very effec-tively due to the lack of molding techniques.

If the force calculations are realized again considering thesame preload under the front and rear toes

Ffy = (mg cos α(L3 + b) − sinαh)/(L1 + L2 + 2L3) (16)

Fux = mg(cos β − sinβh)/(a + b) (17)

are derived for slope-up climbing and side-walking. It is seenthat, for climbing steeper angles the lengths of the robot shouldbe increased. However, tail length is much more important thanL1 or L2. For side-walking, the robot should be as wide asit can be. For both cases, Geckobot should be designed verylight and very close to the ground.

D. ExperimentsA prototype Waalbot was built according to the design

aspects previously mentioned. This prototype has the followingspecifications:

Prototype SpecificationsMass 100 gLength 13 cmHeight 5 cmWidth (total) 12.3 cmTurning Radius (Rt) 10 cmTurning Circle Radius (Rtc) 11.15 cmStep Length (dstep) 2.6 cmAdhesive Area (per foot) 1.5 cm2

Speed 6 cm/sTurning Speed 37.5◦/s

Videos of the prototype performing various agility tasks canbe viewed at [14].

The current Waalbot prototype was tested on a smooth clearacrylic climbing surface. It is capable of climbing any directionon a planes of various orientations. This includes climbing up,down, or across a vertical (90◦) wall, all at the speed of 6 cm/s.The maximum angle which the prototype was able to climbreliably was found to be 110◦ (20◦ past vertical).

Power consumption was measured to be an average of2.42 watts with a maximum instantaneous power draw of2.66 watts while climbing vertically. This is very high for arobot of such small size. The pausing power consumption isapproximately 240 mW, and the average power consumptionrunning with the robot lifted off the ground measured as1.99 watts and a peak draw was 2.27 watts. This indicates thatthe work done to preload and peel the feet and to overcomegravity is around 400 mW and that the remainder of the powerconsumption is likely caused by frictional losses within themotors and drive train. Improving the drivetrain should reducepower consumption significantly.

The prototype Waalbot is able to make left and right handturns without falling from the climbing surface. Both small and

Fig. 23. Still frames from a video showing a Waalbot prototype turning whileclimbing on a vertical surface. The prototype turns 90◦ (line overlayed) insix steps (bottom a–f).

large turning angles are routinely made. The prototype can beteleoperated to navigate around obstacles while climbing.

The current prototype is capable of making transitions ofvarious angles, including 90◦. We had not anticipated therobot successfully transitioning to a surface which was notexactly perpendicularly aligned with the robot’s path, but inmany tests, the prototype was indeed able to transfer when thesurface was misaligned by up to 10◦.

IV. CONCLUSION

Two semi-autonomous tetherless robot prototypes were de-signed and fabricated which are able to climb on smoothsurfaces of various orientations. The robots can steer and turnwith a small turning radii to travel around tight corners. TheWaalbot prototype is able to successfully perform plane toplane transitions of a wide variety of angles including 90◦

junctions. IR communication is used to control the robots.Geckobot demonstrates effective climbing behavior on in-

clined surfaces up to 85 ◦ at a speed of 1 cm/s. The robotdesign is demonstrated to be efficient, reliable, and robust.Future versions will address many unresolved issues with thecurrent prototype including 90 ◦ surface climbing, obstacleavoidance, and autonomous navigation.

Waalbot demonstrates climbing at slopes up to 110 ◦ ata speed of 6 cm/s. One of the major disadvantages of theWaalbot robot design is that there is very little redundancy incase of adhesion failure. At some times during operation thereare only two feet attached to the surface. Inverted walking hasnot been possible with the prototype due to the mass of therobot. Furthermore, the possibility of adhesion failure duringa transition if improper foot placement occurs is a dangerousflaw for a climbing robot. The adhesives used on the feet ofthe robot gather dust and other contaminants their performancedegrades quickly. Therefore, these adhesives are not suitablefor dirty outdoor environments, walking across indoor floors,or for long term tasks.

Future work includes implementing the synthetic dry ad-hesives in place of the conventional adhesives when thetechnology is mature. Further miniaturization of the robots

is required to improve performance due to increased area-to-mass ratio. Most of the electronic components are available insmaller, lighter surface mount-packages and along with supe-rior fabrication methods their use will lead to a reduction inoverall weight, allowing smaller, lighter actuators. Decreasingthe robot mass will result in higher payload capacity, allowingfor other sensors or tools and lower power consumption forlonger operation time.

Acknowledgments

The authors acknowledge Ali Uneri and Alper Aydemir fortheir contributions to robot electronics and software, Will Tsofor his work on the Waalbot prototype design, fabrication, andtesting, and all of the NanoRobotics Lab members for theirsupport.

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[14] [Online]. Available: http://www.me.cmu.edu/faculty1/sitti/nano/projects/waalbots/tri-leg.html