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IEEE TRANSACTIONS ON ROBOTICS, VOL. 29, NO. 1, FEBRUARY 2013 251

A Compact and Compliant ExternalPipe-Crawling Robot

Puneet Singh and G. K. Ananthasuresh

Abstract—The focus of this paper is on the practical aspects ofdesign, prototyping, and testing of a compact, compliant externalpipe-crawling robot that can inspect a closely spaced bundle ofpipes in hazardous environments and areas that are inaccessible tohumans. The robot consists of two radially deployable compliantring actuators that are attached to each other along the longitu-dinal axis of the pipe by a bidirectional linear actuator. The robotimitates the motion of an inchworm. The novel aspect of the com-pliant ring actuator is a spring-steel compliant mechanism thatconverts circumferential motion to radial motion of its multiplegripping pads. Circumferential motion to ring actuators is pro-vided by two shape memory alloy (SMA) wires that are guided byinsulating rollers. The design of the compliant mechanism is de-rived from a radially deployable mechanism. A unique feature ofthe design is that the compliant mechanism provides the necessarykinematic function within the limited annular space around thepipe and serves as the bias spring for the SMA wires. The robothas a control circuit that sequentially activates the SMA wires andthe linear actuator; it also controls the crawling speed. The robothas been fabricated, tested, and automated. Its crawling speed isabout 45 mm/min, and the weight is about 150 g. It fits within anannular space of a radial span of 15 mm to crawl on a pipe of60-mm outer diameter.

Index Terms—Compliant mechanism, inchworm motion, pipecrawler, pipe inspection, shape memory alloy (SMA).

I. INTRODUCTION

P IPE crawlers can be classified as internal and externalcrawlers: Internal ones crawl inside the pipe, while ex-

ternal ones traverse on the outside. They can also be classi-fied, based on locomotion methods, as wheeled crawler, walk-ing crawler, sliding crawler, and inchworm crawler [1]–[5].Wheeled crawlers work well only on even surfaces. Walkingcrawlers require sensors and sophisticated control to have thenecessary gait and to avoid obstacles. Sliding crawlers needtracks or treads. In comparison with other crawlers, inchwormcrawlers are less cumbersome and are less affected by irregular-ities. Different variations of inchworm crawlers are proposed inthe literature (see, e.g., [6]–[11]).

Manuscript received March 10, 2012; revised June 29, 2012; acceptedAugust 18, 2012. Date of publication September 7, 2012; date of current versionFebruary 1, 2013. This paper was recommended for publication by AssociateEditor Y. Choi and Editor B. J. Nelson upon evaluation of the reviewers’ com-ments. This work was supported in part by the Government of India under aBoard of Research in Nuclear Sciences Grant. This paper is an enhanced andcomprehensive version of three papers that presented different aspects of thework in national conferences held in India, as cited in this paper.

The authors are with the Department of Mechanical Engineering,Indian Institute of Science, Bangalore 560012, India (e-mail: [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TRO.2012.2214560

Fig. 1. Space constraints of an external pipe crawler. The side view of a bundleof three pipes is shown at the top, while the cross-sectional view of the spaceavailable for one pipe is shown at the bottom.

Many of the crawlers reported in the literature are internalcrawlers. However, the focus of this paper is not on internalcrawlers because of the need to inspect the thickness of a pipecarrying corrosive fluids. External inchworm-type pipe crawlershave received modest attention in the literature. Schempfet al. [8] reported an external inchworm crawler using a four-bar linkage to generate clamping and releasing actions. Thebulky rigid-body mechanisms used in it made it occupy a lotof space outside the pipe because its function was to removeand bag asbestos. Fukuda et al. [9] also designed external pipecrawler using a winch mechanism and used sophisticated controlsystem. Choi et al. [10], [11] reported pneumatically actuatedexternal crawlers that used hinged clamps to hold the pipe. Thepipe-inspection robot of Chatzakos et al. [12] used omnidirec-tional wheels and linkages. Except the crawlers reported by Choiet al. [10], [11], other external crawlers are not compact to meetthe requirements of the application considered in this paper.

The application that motivated our work is an environmentin which there is a bundle of pipes with limited space betweenthem. Typically, only 10–20-mm radial space is available arounda pipe of 60-mm outer diameter. This means that with a radialspan of 15 mm, there is an annular space between circles of 90and 60 mm diameters (see Fig. 1). Thus, the main challengeaddressed in this study is an external crawler that is compactin terms of mechanism and actuation. The crawlers reported by

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252 IEEE TRANSACTIONS ON ROBOTICS, VOL. 29, NO. 1, FEBRUARY 2013

Choi et al. in [10], [11] seem to be compact but use pneumaticactuation, which needs flexible pipes to crawl along. We useshape memory alloy (SMA) wires for actuation and joint-freecompliant design for the mechanism. Using SMA wire has anadvantage over pneumatic actuation in that the crawler can beuntethered if it is equipped with a battery. The compliant mech-anism concept is amenable for further reduction in size withoutputting undue demand on manufacturability.

Fig. 1 shows the main features of the crawler: It has tworings that hold the pipe tightly when there is no actuation; itcrawls like an inchworm by alternately releasing the grip ofone of its rings with actuation, and moving to and fro with alinear actuator that connects the rings. The clamping mechanismand linear actuators of the inchworm crawler must fit withinthe narrow space indicated in Fig. 1. Hence, it necessitateda compact mechanism and actuator. Since multiple dedicatedcrawlers are needed for all the pipes, it is also imperative thatthe cost is kept low.

This implies that the design should be simple, use as few partsas possible, and minimize assembly. This challenge is met bya unibody compliant ring actuator actuated by SMA wires. Thering actuator has a compliant mechanism that converts circum-ferential motion to radial motion of its multiple gripping pads.Unlike the hinged clamps used in [11], the ring actuator canapply equal pressure on all its pads to have uniform gripping onthe pipe.

The ring actuator and SMA wires operate synergistically:SMA provides the compact actuation to the mechanism, whilethe compliant mechanism serves as the bias spring for SMAin cyclical operation. As stated earlier, the inchworm motion isachieved by a pair of compliant ring actuators connected by bidi-rectional linear actuators. One of the rings translates forward,while the other is clamped and the linear actuator is extended.Then, the translated ring is clamped and the first ring advances asthe linear actuator reduces the length between the rings. A sim-ple electronic circuit controls the cyclical operation. Apart fromthe design of the joint-free compliant ring actuator, the otherchallenges faced in this study include electrical isolation of theSMA wires from the metal used in the ring actuator, speed ofcrawling based on heating/cooling times of SMA wires, a linearactuator that is compact and adequately powerful, and designfor manufacturability of the entire crawler robot.

The remainder of this paper is organized as follows. Thedesign of the ring actuator is explained in Section II. Section IIIdeals with the characterization of SMA wires used in this study.Sections IV and V describe the clamp-and-push “inchworm”motion and integration of the control electronics. Section VIpresents the details of prototyping and the results of testing.Concluding remarks are in Section VII.

II. COMPLIANT RING ACTUATOR

In recent years, radially deployable mechanisms havegained popularity in various forms: retractable structures(see, e.g., [13]), solar arrays (see, e.g., [14]), and toys(www.hoberman.com). These mechanisms are overconstrained(i.e., they have negative degrees of freedom as per the Grubler

Fig. 2. (a) Geometric model of the radially deployable mechanism. (b) Ra-dially deployable mechanism: (i) clamping and (ii) releasing. It is made of16 angulated members connected with 24 revolute joints and arranged in twolayers. One angulated element is shown in Fig. 2(b-i).

formula [15]) but have special geometry that permits themto move radially in or out, as shown in Fig. 2(a) and (b).Such a mechanism, which is known as the Hoberman linkage[www.hoberman.com], can be used to clamp around a pipe andrelease the grip upon actuation. However, there is a problemwith it because all of its points move radially outward and needsliding guidance or need to be pulled away using anchors lo-cated outside. This issue was resolved with a general theory ofplanar radially deployable mechanisms presented by Patel andAnanthasuresh [16]. A new type of mechanism that has fewpoints that move along a circle was identified in that work. Itenables circumferential actuation to achieve the radial deploy-ment and avoids the need for radial guidance.

The circumferentially actuated mechanism is illustrated inFig. 3(a) and (b). However, this multibody jointed mechanismis also not suitable to fit into the narrow annular space such asthe one shown in Fig. 1. Rather, it will be either difficult or noteconomical to manufacture hinged parts of small size. There-fore, we developed a radially deployable compliant mechanismwith the same kinematic behaviors the one in Fig. 3(a) and (b)and satisfying the space constraint. This is done by using elas-tic deformation instead of joint-based motion as in compliantmechanisms [17].

A. Radially Deployable Compliant Mechanism

A radially deployable compliant mechanism is shown inFig. 4(a) and (b). When its compliant rings are rotated relativeto each other, its interior points move outward. This mechanismwas derived—although it may not be apparent at first sight—from the rigid-body mechanism shown in Fig. 3(a) and (b). Itconsists of two layers of identical planar compliant mechanisms

SINGH AND ANANTHASURESH: COMPACT AND COMPLIANT EXTERNAL PIPE-CRAWLING ROBOT 253

Fig. 3. (a) Geometric model of the circumferentially actuated radially deploy-able mechanism. (b) Circumferentially actuated radially deployable mechanism:(i) clamping and (ii) releasing. It consists of two identical layers each containingfour identical triangles and one ring. The triangles are attached to each otherand the ring with 16 revolute joints. Each triangle is attached to one triangle inthe same layer, one ring, and one triangle in the other layer.

Fig. 4. (a) Geometric model of the circumferentially actuated radially deploy-able compliant mechanism. (b) Circumferentially actuated radially deployablecompliant mechanism prototype: (i) undeformed and (ii) deformed. This is madeof two identical layers of compliant segments connected to each other with eightrevolute joints.

of certain shape attached to each other at eight interior points.The two layers are arranged one over the other wherein one is amirror image of the other about an in-plane axis. When the ringportions of each layer are rotated circumferentially relative toeach other, eight attachment points move radially.

To understand the radially deployable compliant mechanismshown in Fig. 4(a) and (b), consider a single compliant segment

Fig. 5. (a) Deformation of a single compliant segment. (b) Deformation of asingle compliant ring.

Fig. 6. Geometric model of the SMA-actuated radially deployable compliantmechanism.

shown in Fig. 5(a). It consists of two relatively rigid portionsconnected by a slender curved beam of a certain shape. Imaginethat the rigid portion attached to the ring is constrained to movealong the circumference of a circle whose center coincides withthat of the arc of the ring. Imagine also that the pad is con-strained to move along a radial line passing through the samecenter. As shown in Fig. 5(a), the elastic deformation showsthat when this element is actuated circumferentially at one end,amplified radial motion ensues at the other end. The shape of theslender curved beam is the crucial component for this behavior.The shape of the compliant segment was intuitively chosen atfirst by observing the motion of the rigid links of the mechanismof Fig. 3. A few shapes were then explored using elastic defor-mation analysis until satisfactory behavior was obtained. Theconstraints assumed in Fig. 5(a) are achieved by using an arrayof four of these elements arranged symmetrically around thecircumference of a ring, as shown in Fig. 5(b). The constrainton the pad to move along a radial line is achieved by placing onesuch ring flipped over another and attaching both at the pads.This gives rise to the ring actuator shown in Fig. 6.


Fig. 7. SMA-actuated radially deployable compliant mechanism (a) with asingle wire and (b) with two SMA wires.

B. Shape-Memory-Alloy-Actuated Radially DeployableCompliant Mechanism

Using the principle of elastic deformation depicted in Fig. 5(a)and (b), the pads of the ring actuator move radially when its tworings are rotated circumferentially relative to each other. Thisdifferential circumferential motion to the two rings is providedby two SMA wires. The ends of each of the two SMA wiresare first elongated, wrapped around half of the rings, cappedwith plastic clips, and then crimped firmly to two rings so thatthey can be pulled together when the SMA wire contracts uponheating. We use spring-steel to make the compliant rings becausethe restoring force to bring the SMA wires had to be sufficientlylarge (which is not possible with plastic materials unless the sizeis large) and to have good elastic behavior for repetitive action.

Since spring-steel in an electrical conductor, it warrantsproper electrical insulation between the SMA wires and therings. For this, several insulating roller guides are used. Rollingguides solve two problems: electrical insulation and guiding thewire with reduced friction. It can be noticed in Fig. 6 that thering actuator is compact in the radial direction around a circularobject on which it is to be mounted. Its annular span in the radialdirection is only 15 mm, while its overall radius is 90 mm. Thiscompact design was arrived at after a few trials reported earlierby us in conference presentations [18], [19].

In one of our earlier designs [18], [19], which is shown inFig. 7(a), we had used a single SMA wire that was put along thecircumference of the rings. This caused nonuniform motion ofthe gripping pads. This is because the mechanism, as well as theradially deployable rigid-body shown in Fig. 3(a) and (b), hasan extra degree of freedom that makes the two rings translaterelative to each other. This will prevail unless the two rings

Fig. 8. Previous prototypes of pipe crawler. (a) SMA strips for linear actuation.(b) SMA helical springs for linear actuation.

are made to only rotate relative to each other. When a singleSMA wire is wrapped around the ring, it pulls unevenly on thefour compliant mechanism segments. Hence, the four grippingpads move unequally to the extent that one pad moves in theinward direction while the rest of the three move in the outwarddirection. This is confirmed by finite-element simulation donein ABAQUS [www.simulia.com].

To solve this problem, we used two SMA wires instead of one.Two SMA wires were put in such a way that both SMA wiresapply the force uniformly on the ring and avoid the undesirableextra degree of freedom. Its simulation is shown in Fig. 7(b),in which it can be seen that all gripping pads move equally inthe outward direction by 1.5 mm. In ABAQUS simulation, weassumed that the lower ring is fixed and the forces were appliedon the upper ring, as shown in Fig. 7(a) and (b). Tetrahedral finiteelements were used in ABAQUS simulation, and the mesh wasmade sufficiently fine to ensure convergence. We noticed thatusing four SMA springs is even better than using two, but itleads to structural complexity in terms of capping plastic clipsand electrical connections.

In our previous attempts [18], [19] at prototyping the pipecrawler with the ring actuator, we attempted to use SMA stripscustom-made in U-shape or as helical springs. These are shownin Fig. 8(a) and (b). SMA strips needed a lot of current to heatup and actuate, and worse yet, they needed a lot of time to coolback to room temperature. SMA helical springs suffered fromexcessive lateral flexibility. Therefore, SMA actuation is limitedonly to the ring actuator in the final design and prototype [20].

III. SHAPE MEMORY ALLOY WIRE AS AN ACTUATOR

SMA material can be used in two modes: 1) super- orpseudoelastic and 2) shape memory [21]. In this study, it is used


Fig. 9. Phase diagram of SMA with compliant mechanism.

primarily in the latter mode although the first is needed to pre-stress the SMA wires. When an SMA wire is electrothermallyheated by passage of electric current, it heats up and shrinks asit transforms to its martensite state from the austenite state, asshown in Fig. 9. The shortening of the wire causes the relativecircumferential motion of the rings. Compliant segments trans-form this to amplified radial motion at the gripping pads, as perthe principle illustrated in Fig. 5(a) and (b). This releases thegrip on the circular object, i.e., pipe. Upon turning off the elec-tric current, the wires cool and reach the martensite state again.Then, the restoring force of the compliant segments pulls thewires back to their stressed states (see atomic level schematicshown in Fig. 9). This cycle repeats as many times as neededprovided that the original state is restored in terms of stress inthe SMA wires and temperature.

A. Characterization of Shape Memory Alloy

The principle of using SMA material as an actuator is shownin Fig. 10. It shows the SMA wire as a spring stretched fromits “memory shape” at room temperature and connected to aload or a bias spring (here, the compliant mechanism). SinceSMA exhibits pseudoplasticity in this condition, it stretches andstays that way. After this, the SMA wire is heated so that it canreturn to its memory shape and pull the load or the bias springin this process. When the SMA wire cools down to the roomtemperature, it is the load or the bias spring that pulls the SMAback to the stressed state. Otherwise, reheating does not makeany further movement. Thus, the load or bias spring plays acrucial role here. If the bias spring is not stiff enough, the SMAwire does not return to the sufficiently stressed state, and hence,the stroke of actuation will be reduced. If the load/bias spring istoo stiff, the SMA will give a very small stroke. Therefore, thestiffness value is important to make repeated cyclical motionpossible. The stiffness calculations are explained in the nextsection.

B. Stiffness Characterization of the Shape Memory Alloy Wire

In order to match the stiffness of the complaint mechanismwith the SMA wire, we performed an experiment with the SMAwire to characterize its stiffness. The SMA wire that we used

Fig. 10. Actuation of SMA wire with load/bias spring.

needs to be stretched up to 2% strain before assembling onto thedevice because it was trained to recover a maximum strain of 2%for unlimited number of cycles. We used 1-mm diameter Ni–Ti–Cu SMA wires (National Aerospace Laboratories, Bangalore,India). From the manufacturers, it is known that the wire canexert a stress of about 100 MPa, which for a 1-mm diameterwire translates to about 78.54-N force

F = σ × Across section = 78.54 N.

The length of the wire and the prestrain in it had to be chosencarefully to exert adequate force and displacement that matchthose of the compliant segments. Only then, repeatable operationis possible. In other words, the stiffness of the SMA wire andthe compliant segments should be matched. Therefore, it wasimperative that the SMA wires be properly characterized.

The experimental setup to characterize the SMA wires isshown in Fig. 11. The wire was prestretched by about 1.75%.As the wire was heated, there was deformation in the SMAwire. To see the deformation in the wire against different appliedweights, we applied weights from 1 to 8 kg. With 8-kg weight,the deformation was 4 mm, indicating that the wire is able tocontract against a force of as much as 78 N. To calculate thestiffness in the martensite phase, we took the 226-mm-long SMAwire and fixed it at one end, while the other end was attachedto 8-kg weight. The SMA wire deformed with the weight by1 mm. Therefore, the stiffness of SMA wire in the martensitephase is calculated as 78.48 kN/m

KMartensite Phase =L

δ=

8 × 9.811

= 78.48 kN/m.

To calculate the stiffness in the “cooling phase” (during whichmaterial transitions from the austenite to the martensite phase),we took a 230-mm (prestretched by 4 mm) long wire and fixedits one end in a bench-vice and applied 8-kg weight at theother end. With a 2-V power supply, the ensuing current heatsup the wire and transforms it to austenite phase changing itslength to 226 mm. Then, we turned the power supply OFF. As aresult, austenite phase starts to transform into martensite phase.Due to weight, the SMA wire again deformed in cooling phaseup to 229 mm. Therefore, the SMA wire deformed by 3 mm


Fig. 11. Schematic of experimental setup for SMA wire characterization.

in cooling phase. We repeated this experiment five times andfound stiffness in the cooling phase to be 26.16 kN/m

KCooling Phase =L

δ=

8 × 9.813

= 26.16 kN/m

KSMA CoolingPhase < KCompliant Mechanism .

Now, to make the repetitive motion possible, the stiffness ofcompliant ring should be greater than the stiffness of SMA wirein the cooling phase. Only then, the compliant ring will stretchthe SMA wire back to the stressed state. The stiffness of the com-pliant ring was calculated to be 69.11 kN/m by finite-elementsimulation [see Fig. 7(b)]. This stiffness value is more than thestiffness of the SMA wire in the cooling phase. Therefore, itensures repetitive cyclic motion.

C. Thermal Characterization of the Shape Memory Alloy Wire

It is important to understand the electrothermal behavior ofthe SMA wire and the entire ring actuator. Two issues arisehere: 1) ensuring that the ring cools to room temperature withinreasonable time after one cycle of operation and 2) ensuring thatall SMA wires heat to 40 ◦C transition temperature with 2 V.For 1), we performed an experiment on the ring actuator andthe SMA wire and measured the temperatures. We used K-typethermocouples to measure the temperature shown in Fig. 12(a).The dotted line in the graph shows the temperature of the SMAwire, while the other line shows the temperature of the spring-steel portion. It can be noticed that it takes only about 1 s to heat,while it takes 9 s to cool. Thus, it takes about 10 s to completea cycle. It is worth noting that it is the cooling time that decidesthe overall cycle time. To make it faster than 10 s, one wouldneed to use a cooling mechanism such as a fan or Peltier cooler,which comes at the expeese of more complexity.

To address the second issue, we performed 2-D transientelectrothermal modeling of SMA wire using CMOSOL Multi-physics [www.comsol.com] to calculate the temperature profileby applying 2-V electric potential. The temperature profile isshown in Fig. 12(b) at time 2 s. It can be seen that the en-tire wire reaches 40 ◦C. This distribution continues to be sowhen the voltage is sustained for longer time. Hence, all of theSMA wire uniformly undergoes phase transition. The materialproperties used in thermal modeling were thermal conductiv-ity = 18 W/(m·K), density = 6800 Kg/m3 , and resistivity =

Fig. 12. (a) Temperature versus time in the experiment. (b) Temperature dis-tribution of the SMA wire in its electrothermal modeling at time 2 s.

7.6e-7 Ω·m for the SMA wire. The two ends of the wire wereheld at room temperature in this modeling, which is justifiablebecause the spring-steel portion does not heat much, as can beseen in Fig. 12(a).

IV. CLAMP-AND-PUSH MOTION

As demonstrated in Fig. 13, the robot consists of two SMA-actuated radially deployable compliant ring actuators, which arelabeled A and B.

In order to crawl to the left, ring B grips the pipe tightly andstays unactuated, while ring A is actuated to release its grip.The translation actuator pushes ring A closer to ring B. At thisstage, actuation to ring A is stopped, thus making it grip thepipe again while ring B is released and translation actuators areactivated in reverse direction. Now, ring B is pushed to the left.This constitutes one cycle and leads to a finite movement to theright. By repeating this cycle, the device crawls over the pipe.By reversing the roles of rings A and B in a cycle, the crawlingdirection can be changed. This concept works for an internalcrawler too, but here we focus on only the external crawlerwithin limited space.

V. ELECTRONICS INTEGRATION

An electronic circuit is necessary to automate the robot. Elec-tric current has to be supplied to actuate the two pairs of SMAwires that help the gripping pads release their grip and thetranslation actuators that connect the two ring actuators. Thisrequires a driver circuit to effect these actuations cyclically in a


Fig. 13. Schematic illustration of a clamp-and-push concept of a pipe crawlerrobot.

sequence. As stated before, we had used SMA strips and springsfor translation actuation [see Fig. 8(a) and (b)] which took a lotof time to heat and cool. Therefore, we decided to change thetranslation actuator to a geared DC motor with lead screw.

The current-driving circuit on a printed circuit board is usedto effect the cyclical operation of triggering the passage andstoppage of current in the two pairs of SMA wires and oneset of translation actuators (shown in Fig. 14). The circuit hasATMEL-made AVR microcontroller with three current sourcesin parallel, as shown in Fig. 15.

A switched mode power supply (SMPS) of 400-W capac-ity was taken from a personal computer to supply the currentneeded for the SMA wires and translation actuators. The currentcan be switched by the microcontroller and the value of currentcan be set by the three potentiometers. High-power MOSFETs(metal–oxide field-effect transistors) are used to get the currentloads needed for the actuators. The microcontroller is interfacedby universal serial bus converter. A command-driven programis used to set the various actuator timings to control the speed.

Fig. 14. Cyclic chart for the pipe crawler robot.

The sequence of the actuation is programmed into the micro-controller so that switching of the actuators is automatic.

VI. PROTOTYPING AND ASSEMBLY

Compliant radially deployable mechanism was assembledusing two compliant rings, as shown in Fig. 16. The com-pliant rings, as stated earlier, convert the circumferentialmotion into radial motion. First, we designed a geometricmodel of compliant rings in SolidWorks [www.solidworks.com]and generated the machining code—the G-code—for wire-cutelectrodischarge-machining (EDM) using customized ELAPTsoftware. The compliant rings were made out of spring steel(AISI 1080; EN 42 J) cut on a wire-cut EDM. Since the tworings are in two different parallel planes, they were soft-solderedat the gripping pads. Two Ni–Ti–Cu SMA wires were connectedin between the compliant rings with the help of a plastic clipand a brass crimpable tube. The SMA wires were connected inopposite directions. The insulating rollers on which the SMAwire sits can be seen in Fig. 17 along with the other parts. In-sulating acrylic rollers were made on the CO2 laser machine.To increase the friction coefficient between the pipe and grip-ping pads, a butyl layer of 1-mm thickness was affixed onto thegripping pads. A full assembly of SMA-actuated ring actuatoris shown in Fig. 17.

The electronics unit was packaged into a custom-designedbox, as shown in Fig. 18. The complete prototype of the crawl-ing robot is shown in Fig. 17. Two SMA-actuated radially de-ployable compliant mechanism rings are attached to each otherby translating DC actuator. The translation actuator consists ofthree parts: DC motor, gear box, and lead screw. When the po-tential is applied, the motor rotates, and the gear box reduces thespeed of the motor by 1:100 and the lead screw with a nut thattransmits the force on the compliant mechanism ring to moverelative to each other.


Fig. 15. Electronic schematic diagram for cyclic operation of the robot.

VII. TESTING AND RESULTS

Fig. 19 shows the test setup of SMA-actuated radial deploy-able compliant ring actuator. The setup shows the ring actuatorput on a pipe that is set vertically. Since the outer diameterof the pipe is larger than the diameter of the circle passingthrough the four clamping points, there is a gripping force of1.8 N (per clamping pad) applied on the pipe, as calculated bythe finite-element simulation in ABAQUS. With 2-V potential,the ring loosens the grip on the pipe because of the radiallyoutward movement of the gripping pads. This experiment wasdone 20 times to calculate the deformation in the clamping padswithout mounting it on the pipe. The maximum deformationon one clamping pad was 1.2 mm with a standard deviation of0.02 mm. When we look at the movement of all four pads, thereis about 2.4-mm change in the diameter of the circle passingthrough the outer edges of the gripping pads in the unactuatedand actuated configurations.

Fig. 16. SMA-actuated radial deployable compliant ring actuator.

Fig. 17. Complete prototype of the crawling robot.

Fig. 18. Electronic control system for pipe crawler.

Fig. 19. Test setup for the actuator (a) at 0 V. The pipe is tightly grippedagainst gravity (b) at 1.35 V; the ring got released and fell down.


Fig. 20. Demonstration of pipe crawler robot in horizontal pipe.

Fig. 21. Demonstration of pipe crawler robot on a pipe set at 30◦ incline.

Figs. 20 and 21 show the test setup of the pipe crawler robot.The setup shows the robot put on a pipe that is set horizontaland at 30◦. Initially, the gripping pads of both the compliant ringactuators tightly hold the pipe. When the SMA wires of the firstring are activated, the compliant mechanism releases the gripon the pipe. At this time, the translation actuator is activated tobring the ring actuators closer together, as shown in Steps 2 and3 in Figs. 20 and 21. After this, the first ring is deactivated to gripthe pipe. This is followed by release of the grip of the secondring actuator. After this, the translation actuator is activated inthe reverse direction, as shown in Steps 4 and 5 in Figs. 20 and21. That leads the robot to its initial configuration but moved by30 mm in 40 s as well. Thus, it was observed that the measuredcrawling speed in both configurations is 45 mm/min.

There is room for improvement in the crawling robot in termsof increasing its speed and reducing the size. This is largely de-pendent on the linear actuator used to effect the relative motionbetween the pair of ring actuators. It should be noted that theSMA wire actuation is improved to the extent possible to takeas little time as possible to heat and cool. It takes about 1 s toheat up and then 9 s to cool down an SMA wire. This is thelimiting step in this device. Hence, a suitable motor that is notmuch more expensive than the one used now would help in-crease the crawling speed. The robot is not yet tested on curvedpipes, but it could be done by including flexible joints at theconnection points of the linear actuator. The lead screw and DCmotor, if made smaller, can further reduce the overall size ofthe crawling robot and make it the curvature of the pipe sharper

than is currently possible. Friction and sliding may also need tobe modeled.

The main contribution of the work presented here is the com-pactness, simplicity, and cost effectiveness of the SMA-actuatedring actuator. It may be useful for other applications as well.Another contribution is the concept of designing the stiffness ofthe bias spring (here, the compliant ring actuator) to match thatof the SMA wire. This too is important in many applicationsbeyond the one presented here.

VIII. CLOSURE

In this paper, we have presented a compact external pipecrawler robot that follows clamp-and-push motion and imitatesinchworm motion. Its compactness is a result of radially deploy-able compliant mechanism and SMA actuation. The compliantmechanism used here was designed on the basis of a kinematictheory developed for radially deployable linkages. A novel fea-ture of the design is that the compliant mechanism provides thenecessary kinematic function within limited space and servesas the bias spring for the SMA wires. The stiffness of the SMAwire and that of the compliant mechanism were matched so thatthey work synergistically. A description of its working principle,design details, and practical aspects were provided. A controlcircuit is built to effect the cyclical triggering in a sequence ofsix steps. The total cycle time is 40 s, and the distance crawledis 30 mm per cycle. Thus, a crawling speed of 45 mm/min wasachieved in the prototype. Other sizes and speeds are also pos-sible by changing the design of the compliant mechanism in thering actuator.

ACKNOWLEDGMENT

This work is a result of synergistic effort of several studentsof the Multidisciplinary Multi-scale Devices and Design groupat the Indian Institute of Science, Bangalore, India. The authorswould like to thank G. Balaji and P. Biradar for initial designsand prototypes; B. M. V. Kumar and A. R. Kumar for their helpin fabrication; and D. K. Badige and S. Yamadagni for theirhelp in building the electronics unit. The SMA wires were madeby C. N. S. Krishna and Ramaiah in Dr. S. Bhaumik’s groupat the National Aerospace Laboratories, Bangalore. They alsothank A. Haruray, who brought the application to their attentionand helped with the specifications for the crawler. This help isgratefully acknowledged.

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[2] S. G. Roh and H. R. Choi, “Differential-drive in-pipe robot for movinginside urban gas pipelines,” IEEE Trans. Robot., vol. 21, no. 1, pp. 1–17,Feb. 2005.

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[4] A. Zagler and F. Pfeiffer, “MORTIZ, a pipe crawler for tube junctions,” inProc. IEEE Int. Conf. Robot. Autom., Taipei, Taiwan, Sep. 13–14, 2003,pp. 2954–2959.


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[8] H. Schempf, E. Mutschler, B. Chemel, and S. Boehmke, “BOA II & PipeTaz: Robotic pipe-asbestos insulation abatement systems,” in Proc. IEEEInt. Conf. Robot. Autom., Albuquerque, NM, Apr. 1997, pp. 52–59.

[9] T. Fukuda, H. Hosokai, and M. Otsuka, “Autonomous pipeline inspectionand maintenance robot with inch worm mobile mechanism,” in Proc. IEEEInt. Conf. Robot. Autom., Mar. 1987, pp. 539–544.

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[12] P. Chatzakos, Y. P. Markopoulos, K. Hrissagis, and A. Khalid, “On thedevelopment of a modular external-pipe crawling Omni-directional mobilerobot,” in Proc. 8th Int. Conf. Climbing Walking Robot. Support Technol.Mobile Mach. Part 11, 2006, pp. 693–700.

[13] T. Buhl, F. V. Jensen, and S. Pellegrino, “Shape optimization of coverplates for retractable roof structures,” Comput. Struct., vol. 88, pp. 1227–1236, 2004.

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[15] G. Erdman, G. N. Sandor, and S. Kota, Mechanism Design: Analysis andSynthesis. Englewood Cliffs, NJ: Prentice–Hall, 2001.

[16] J. Patel and G. K. Ananthasuresh, “A kinematic theory for radially foldableplanar linkages,” Int. J. Solids Struct., vol. 44, pp. 6279–6298, 2007.

[17] L. L. Howell, Compliant Mechanisms. New York: Wiley, 2001.[18] G. Balaji, P. Biradar, C. N. Saikrishna, K. Venkata Ramaiah, S. K.

Bhaumik, A. Haruray, and G. K. Ananthasuresh, “An SMA-actuated,compliant mechanism-based pipe-crawler,” presented at the Int. Conf.Smart Mater., Struct., Syst., Bangalore, India, Jul. 2008, Paper no. 96.

[19] B. M. Vinod Kumar, D. K. Badige, S. Hegde, and G. K.Ananthasuresh, “An improved compact compliant mechanism for an ex-ternal pipe-crawler,” presented at the 14th Nat. Conf. Mach. Mechanisms,Durgapur, India, Dec. 2009.

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[21] M. G. Faulkner, J. J. Amalra, and A. Bhattacharyya, “Experimental in-vestigation and thermal and electrical properties Ni-Ti-Cu shape memorywires,” Int. J. Smart Mater. Struct., vol. 9, pp. 632–639, 2009.

Puneet Singh received the B.Tech. degree in mechan-ical and automation engineering from Guru GobindSingh Indraprastha University, Delhi, India, in 2010.

He is currently a Research Assistant with theMultidisciplinary Multiscale Devices and DesignLaboratory, Department of Mechanical Engineering,Indian Institute of Science, Bangalore, India. His re-search interests include robotics, biorobotics, brain–machine interfaces, microsystems, compliant mech-anisms, and biomechanics.

G. K. Ananthasuresh received the B.Tech. degreefrom the Indian Institute of Technology Madras,Chennai, India, in 1989 and the Ph.D. degree fromthe University of Michigan, Ann Arbor, in 1994.

He is currently a Professor of mechanical en-gineering with the Indian Institute of Science,Bangalore, India. He worked as a Postdoctoral Re-search Associate with the Massachusetts Institute ofTechnology, Cambridge (during 1995–1996), taughtat the University of Pennsylvania, Philadelphia (dur-ing 1996–2004), and served as a Visiting Scientist

with the University of Cambridge, Cambridge, U.K., and the Katholike Uni-vesiteit, Leuven, Belgium. He is on the Editorial Boards of eight journals. Heis a coauthor of more than 180 papers in journals and conferences, as wellas a textbook, two edited books, and ten book chapters. His current researchinterests include compliant mechanisms, kinematics, multidisciplinary designoptimization, microsystems technology, micro- and mesoscale manufacturing,protein design, and cellular biomechanics.

Dr. Ananthasuresh received the National Science Foundation CAREERAward (1998–2002) in the U.S. and the Swarnajayanthi Fellowship (2007–2012) and Shanti Swarup Bhatnagar Prize (2010) in India, as well as seven bestpaper awards at international and national conferences.

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