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Design Considerations for Attachment and Detachment in RobotClimbing with Hot Melt Adhesives

Liyu Wang, Fabian Neuschaefer, Remo Bernet and Fumiya Iida

Abstract Robust climbing in unstructured environments isa long-standing challenge in robotics research. Recently therehas been an increasing interest in using adhesive materials forthat purpose. For example, a climbing robot using hot meltadhesives (HMAs) has demonstrated advantages in high attach-ment strength, reasonable operation costs, and applicability todifferent surfaces. Despite the advantages, there still remainseveral problems related to the attachment and detachmentoperations, which prevent this approach from being used ina broader range of applications. Among others, one of themain problems lies in the fact that the adhesive characteristicsof this material were not fully understood n the context of robotic climbing locomotion. As a result, the previous robotoften could not achieve expected locomotion performances andcontaminated the environment with HMAs left behind. Inorder to improve the locomotion performances, this paperfocuses on attachment and detachment operations in robotclimbing with HMAs. By systematically analyzing the adhesiveproperty and bonding strength of HMAs to different materials,we propose a novel detachment mechanism that substantiallyimproves climbing performances without HMA traces.

I. INTRODUCTION

There has been an increasing interest in using adhesivematerials for robotic climbing [1-5]. In contrast to prior tech-nical solutions such as by using vacuum suction [6], magneticattachment [7] or gripping mechanisms [8], adhesion-basedapproach does not require continuous energy supply (as withvacuum suction) or pre-dened structures (i.e. grippers orferromagnetic surfaces) to maintain a robot on a surface.However, most adhesive materials have a relatively lowattachment strength ( 103 -104 Pa), and some materials losetheir adhesiveness after being used for a limited time.

In our lab we are developing climbing robots based on hotmelt adhesives (HMAs). HMAs have a number of interestingproperties: they are adhesive uids with a low bondingstrength at a high temperature, while solid with a highbonding strength at room temperature; they can be repeatedlytransformed between uid and solid phases by controllingtheir temperature; and the material has been used in many

industrial applications and proved to be economical. Byexploiting these properties, we have previously prototypeda climbing robot that demonstrated successful steps on avertical surface [9]. The advantage of the approach lies inits high attachment strength ( 105 -106 Pa), and that it canbe applied to different surfaces. However, HMAs adhesive

This work was supported by Swiss National Science Foundation, GrantNo. PP00P2123387/1.

All authors are with Bio-Inspired Robotics Laboratory, Departmentof Mechanical and Process Engineering, ETH Zurich, 8092, Zurich,Switzerland [liyu.wang, fumiya.iida]@mavt.ethz.ch,[nfabian, rebernet]@student.ethz.ch

properties were not fully understood, and the attachmentand detachment operations during climbing were done in anaive way. For example, the robot used a large amountof HMAs to achieve a strong attachment, while detachmentwas realized by heating up the HMA to a high temperatureso that it became uidic and adhesive. There are severalnegative consequences from this. First, since the HMAbecame adhesive during detachment, a trace was left onthe climbing surfaces. Second, due to the continuous loss,new HMAs had to be supplied for subsequent steps, whichnot only increased design and control complexity, but also

required longer time to climb. Third, over-use of HMAs costmore effort for the robot to detach itself. All those problemsare critical for a broader use of the HMA-based method, andconsiderations for attachment and detachment operations aresignicant from this aspect.

The goal of the paper is to consider the design for betterattachment and detachment operations in the HMA-basedclimbing approach. By exploiting HMAs adhesive propertiesand bonding strength to various materials, we propose anovel detachment mechanism for the robot not to leavea trace on the climbing surfaces. Under this detachmentmechanism, we conduct systematic investigations on han-dling of HMAs for climbing locomotion. With a set of

feasible models that describe bonding force and bondingmoment, we show how an allowable amount range of HMAscan be determined from the climbing requirement, and howattachment can be made stronger with minimal HMAs in therange. We further implement the design considerations in anew climbing robot, which demonstrates faster climbing ona vertical surface without leaving an HMA trace.

The rest of the paper is arranged as follows. Section IIintroduces a detachment mechanism that leaves no HMAs onclimbing surfaces. Section III analyzes force and moment,and suggests how to improve the attachment performancewith HMAs. Section IV presents our newly designed climb-ing robot. Section V shows the climbing experiment result of the robot. Section VI gives conclusions and future directions.

I I . T RACE -F RE E D ETACHMENT M ECHANISM

When using HMAs for robot climbing, attachment anddetachment operations need to be carefully planned. In thissection we propose a novel detachment mechanism to avoidthe robot leaving an HMA trace on the climbing surfaces.

The problem of HMAs being left on a climbing sur-face was caused before in a detachment operation. Morespecically, the climbing robot was designed with two feetand climbed in a rotational manner similar to that in Fig.

2012 IEEE International Conference on Robotics and AutomationRiverCentre, Saint Paul, Minnesota, USAMay 14-18, 2012

978-1-4673-1405-3/12/$31.00 2012 IEEE 1181


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Fig. 1. Trace-Free Detachment Mechanism for HMA-based robot climbing.The robot has two feet with HMAs on them. Within one climbing step,(a) the robot initially stays with both feet attached to a climbing surface(parallel to the paper). (b) It then detaches one foot (lower one) away fromthe surface without heating HMAs. (c) While rotating the body about thestill-attached foot, the HMA on the detached foot is heated. (d) Once nextfoothold is reached, the foot attaches back onto the climbing surface.

1 (more detail see [9]). However, different from Fig. 1b,detachment happened after the HMA on a foot had beenheated up, so that the bonding strength could be loweredfor easy operation. However that made the HMA becomeadhesive, according to the materials properties introducedin Section I. As a result, a trace was left behind, and newHMAs had to be supplied for subsequent steps.

The detachment mechanism proposed in this paper, whichwe call the Trace-Free Detachment Mechanism (TFDM),primarily requires that a robots foot should only detachfrom a surface when HMAs are not adhesive. There is atemperature parameter called the bond formation temperatureT bf , below which HMAs are not adhesive [10]. When therobots foot detaches at a temperature T detach below T bf ,it is possible that no HMAs will be left on the surface. Inthe paper, we consider T detach at room temperature T r (25-30 C ) for control simplicity (Fig. 1b).

TFDM also requires HMAs to remain on the robots foot,so that they can be used for multiple steps. That is achievedby exploiting HMAs bonding strength to different material.When HMAs are used to bond two surfaces, they form abonding area A in between. The maximum bonding forceF max when bonding breaks is proportional to A:

F max = A (1)

The proportion is HMAs bonding strength, which is amaterial dependent constant at a given temperature for a

certain type of HMAs. In the case of HMA bonding betweentwo different types of material, like a robots foot attached toa climbing surface (Fig. 2b), bonding will break at the surfacewhere F max is smaller. By assuming the HMA bonding hasthe same area on both surfaces:

F max = min ( foot A, wall A) (2)

For a climbing robot, two types of bonding forces need to beconsidered, these being a clamping bonding force F c whichis normal to the surfaces, and a shear bonding force F s whichis tangential to the surfaces (Fig. 2b).

Fig. 2. (a) A 0.97kg robot climbing on a vertical surface with 50 mm 2

HMAs on each foot, as presented in the paper. (b) A free-body diagram of the robot on a vertical wall with a single foot attached with HMAs.

Fig. 3. HMAs clamping bonding strength between material. TFDMrequires a robots feet to be constructed with a material whose bondingstrength with HMAs is higher than that of the climbing surface.

To validate the relationship between bonding forces andbonding area, a set of experiments were carried out bymanually breaking HMA bondings between two surfaces of the same material at T r . The values of F max were recordedwith a hand-held scale (VOLTCRAFT HS-10L), and HMAbonding areas A were measured after each trial. Experimentalresult of the maximum clamping bonding force F cmax of HMAs to different material (i.e., copper and aluminium)is shown in Fig. 3. Both F cmax and the maximum shearbonding force F smax of HMAs to a certain material (i.e.,aluminium) is shown in Fig. 4 (more detail in Section III).Both gures conrm Eq. (1) that F max is propotional to A, and the proportion is material dependent (i.e., at T r , cCu =0.98 MPa,

cAl =0.69 MPa). Table I further lists values

of the clamping bonding strength c and the shear bondingstrength s for a type of HMA to various material.

The difference in the HMAs bonding strength to variousmaterials is important for choosing the right constructingmaterial for a robots feet. When the feet have a larger than the climbing surface, bonding will break at the climbingsurface, and HMAs will remain on the feet:

foot > wall (3)

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TABLE I

HMA S 1 BONDING S TRENGTH 2 WITH VARIOUS M ATERIALS

Material Clamping c (MPa) Shear s (MPa)HMA 6.2-26.1 N/APeltier element ceramic 0.1-0.2 0.1Stone 0.2-0.3 0.2Normal steel 3 0.3-0.4 0.4-0.5Anticorodal hard aluminium 3 0.6-0.7 0.9-1.0Copper ETP 3 0.9-1.0 1.3-1.5Roof batten r wood 3 1.5-2.5 4.3-5.2Window glass > 2.0 > 2.01 Pattex Hot Sticks Transparent, Henkel, Germany2 T detach = T r3 From ETH D-PHYS-Shop, https://lager.phys.ethz.ch/de/R/0200/

For example, when copper Electrolytic-Tough-Pitch (ETP) isused on the feet, a robot can climb without trace on stone,metallic and ceremic surfaces according to Table I.

It is important to note that the cohesive strength of HMAsis higher than their bonding strength with other materials(Table I), which means bonding will only break at theclimbing surface given Eq. (3), but not within HMAs.

III . H ANDLING OF HMA S FOR ATTACHMENT

This section considers how to handle the use of HMAs forattachment under TFDM. A piece of HMAs on the robotsfoot, on one hand has to provide enough force and momentfor stable climbing, and on the other, has to be kept onboardfor multiple climbing steps. With a set of force and momentanalysis, we show how a minimal amount of HMAs can bedecided for a stronger attachment.

A. Shear and Clamping Bonding Force

As Fig. 2b illustrates, the shear bonding force F s of HMAsby a single attached foot should be able to compensate the

robots weight. In this case, the maximum shear bondingforce F smax of HMAs on the attached foot should be largerthan robots weight:

F smax > mg

Considering Eq. (1), the smallest bonding area A min shall be:

Amin = mg s

From Table I, we can see s is in the order of 0.1 MPafor most materials at T r . That means, with as small as 1cm 2 bonding area on each foot, the HMA can keep a robotweighing more than 1 kg on most vertical surfaces.

With TFDM, a minimal amount of HMAs is howeverpreferred for the following reason. As Fig. 4 shows, while alarger bonding area generates larger F smax which enhancesstable climbing, it also leads to larger F cmax which requires astronger motor for the detachment operation. Given a certainmotor force F leg , F cmax is constrained by:

F cmax < F leg

Therefore the largest HMA bonding area A max should be

Amax = F leg

c

Fig. 4. Shear and clamping bonding forces determines an allowable rangeof HMA bonding area. When climbing on a surface, the lower boundarycondition is determined by the robots weight. The upper boundary conditionis determined by the motor force for foot detachment.

Fig. 4 also shows an allowable range of HMA bondingarea in the case of a robot climbing on a vertical surface:

Amin < A < A max (4)

And A should be as small as possible so that F leg can beobtained from a motor with a smaller weight and torque.

B. Shear Bonding Moment

The shear bonding moment is important for stable climb-ing when the robot is moving. For example, the maximumshear bonding moment M smax for the attached foot shouldbe larger than the moment of the robots weight, so that therobot does not fall off during rotation. Given the necessityof a minimal size of HMAs described in the previoussubsection, we try to augment its M smax . For that purpose, wehave developed a model of the shear bonding moment, whichsuggests that multiple bondings generate a larger M smax thana single bonding with the same total area.

We assume the centre of gravity (COG) is in the middleof a robot. The maximum moment of weight M wmax occurswhen the robot is in a horizontal posture on a vertical surface,when the weight has the longest moment arm of half therobots length L. M smax should be larger than M

wmax :

M smax > M wmax = mg

L2

(5)

Fig. 5a illustrates two cases of HMA bonding(s) on a

single foot of a robot. A1 is a single-bonding case wherethe area is a circle with radius R 1 , and A2 is a double-bonding case where the total area is equal to A1. The bondingforces in the two cases are the same according to Eq. (1).The maximum shear bonding moment M 1smax for A1 canbe obtained by integral in a polar coordinate system:

M 1smax = s

2

0

R 1

0r 2 drd =

23

s R 1 3 (6)

For A2, we assume the two bondings are identical and bothlocated R 0 from the centre of A1. Each can be seen as part

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Fig. 5. (a) A model of maximum shear bonding moment for strongerattachment with HMAs. (b) With the same total area, a double-bondingcase has larger moment than a single bonding. The model is validated byexperimental data (star and cross marks).

of a ring with outer and inner radius R 2 and R 0 , and a spanangle . The maximum shear bonding moment M 2smax is:

M 2smax = 2 s

0 R 2

R 0r 2 drd = 2

3 s (R 2 3 R 0 3 ) (7)

We assume each area can be approximated by a square:

R 0 = R 2 R 0 (8)

Substituting Eq. (8) into Eq. (7) yields:

M 2smax = 23R 0

s (R 2 R 0 )(R 2 3 R 0 3 ) (9)

It is worth mentioning that the multiple-bonding method canbe scaled up to any number of bondings in a similar way toEq. (7) and (8). In this paper, we use a double-bonding caseas it is sufcient to show the benet of multiple bondings.

To validate the model, we conducted another set of exper-iments, in which M smax was measured for HMA bondingsbetween aluminium and copper surfaces for both cases, withtwo bonding areas 10 cm apart. M smax was calculated asthe product of the shear force when a bonding was brokenmanually, and a moment arm length of that shear force.Results from Fig. 5b shows that the model ts experimentaldata very well. With the same total area, the double-bondingcase results in a much larger M smax , for example, 4 Nmwhen the total area is 50 mm 2 , while under 0.5 Nm with asingle HMA bonding.

Fig. 6. Hardware implementation of the climbing robot and one of its foot.

TABLE II

COMPARISON OF T WO D ESIGNS OF HMA-B ASED C LIMBING ROBOTS

Spec Previous design [9] New designMass 0.60 kg 0.97 kgDimension (L W H) 200 120 110 mm 3 260 160 120 mm 3

Leg actuator torque 0.2 Nm (foot lift-up) 3.06 NmPedestal actuator torque 3 Nm 2.90 NmActuator number 6 4Peltier number 2 4Resistor number 0 8Speed 0.16 m/min 0.45 m/minHMA supply Yes NoTrace Yes NoClimbing surface Almost any Not wood or glass

IV. H ARDWARE IMPLEMENTATION

We have designed a new climbing robot with the aboveconsiderations for attachment and detachment operations.The robot is simpler than the previous version due to the

absence of a module for supplying HMAs [9]. Each robotsfoot has been made larger to accommodate two pre-supplieddistant HMA pieces based on the analysis in Section III.

A. Overall Design

The robot is 260 mm long and weighs 0.97 kg. It has twoidentical feet, and each foot has two actuators. One actuatoris for rotating the body within the climbing surface, labelledas pedestal actuator in a blue lined circle in Fig. 6 (KondoKRS-2350 HV ICS servomotor). It has enough torque torotate the robot on a vertical surface ( M wmax = 1.30 Nm). Theother actuator is for rotation away and towards the climbingsurface, labelled as leg actuator in a green lined circle inFig. 6 (Futaba S9157 servomotor). It can generate 100 NF leg for detachment with a 30 mm lever arm. The robotis manually assembled with laser cut PMMA parts. Morespecications of the robot and a comparison to the previousversion can be found in Table II.

B. Foot Design

As the most important part in the climbing robot, the footrequires careful design with three major principles. First,its surface material should have a higher than climbingsurfaces in order to implement TFDM. Second, the foot must

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