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Horii, S. and Nakamura, T.Paper:An In-Pipe Mobile Robot for Use as an Industrial EndoscopeBased on an Earthworms Peristaltic CrawlingShota Horii and Taro NakamuraChuo University1-13-27 Kasuga, Bunkyou-ku, Tokyo 112-8551, JapanE-mail: nakamuramech.chuo-u.ac.jpReceived September 30, 2011; accepted October 22, 2012Many pipe accidents caused by corrosion or deterio-ration have been reported recently; hence, in-pipe in-spection is needed to prevent such problems. Fiber-scopes are currently used as industrial endoscopes toinspect defects in pipes.Because of friction, how-ever, they cannot be inserted into pipes that are morethan 15 m long or into complex pipes such as elbows.Therefore in-pipe inspection robots need to be self-propelled in order to be inserted into these environ-ments.We are developing a robot capable of pro-pelling itself through various pipes, such as long pipesand elbow pipes, specifically, a peristaltic crawlingrobot using DC brushless motors for in-pipe inspec-tion. In this study, the robot we developed was usedin straight and elbow pipes with an inner diameter of27 mm. In this paper, we derive theoretical formulasfor robot locomotion speed and propulsion force andpropose a special motion pattern, known as the mid-dle motion pattern, for the robots peristaltic crawl-ing pattern. We performed several experiments in a27-mm-diameter acrylic pipe to examine the locomo-tion speed and propulsion force. We also developed arobot that can pass through an elbow and conductedseveral experiments to confirm this.Keywords: peristaltic crawling, in-pipe inspection, in-dustrial endoscope, earthworm1. IntroductionPipelines are used in many countries as main facilitiesprimarily for transporting water, gas and oil. Many pipeaccidents caused by corrosion or deterioration have beenreported recently. Hence, in-pipe inspection is needed toprevent such problems. Fiberscopes are currently used asindustrial endoscopesto inspect defects in pipes. Becauseof friction, however, they cannot be inserted into pipesthat are greater than 15 m in length or into complex pipessuch as elbows. Therefore in-pipe inspection robots needto be self-propelled in order to be inserted into these en-vironments.Various in-pipe robots havebeen developedfor pipe in-spection. Each, however, has drawbacks. A snake-likerobot 1, for instance, needs space that is wider than it-self to operate. Vibration mechanisms2 cannotgoback-ward. The wheeled robots 35 developed thus far havebodies that are difficult to downsize because of their com-plex structure.In contrast, a peristaltic crawling robot has been de-veloped that has many advantages in in-pipe inspection.It easily goes backward and forward and it has a sim-ple mechanism. In addition, it continues to propel itselfeven if pipe diameter changes. We have therefore devel-oped several peristaltic crawling robots 69. Downsiz-ing these robots using servomotors is difficult, however,because of the nature of its mechanism. The speed ofthe robot using a pneumaticactuator decreases,moreover,with increasing distance from the compressor.We are developing a robot capable of propelling itselfthroughvariouspipes,suchaslongpipesandelbowpipes,specifically, a peristaltic crawling robot using DC brush-less motors and screws for in-pipe inspection.In this paper, the robot we developed was used instraight pipes with an inner diameter of 27 mm.Wederived theoretical formulas for the robots locomotionspeed and propulsion force and proposed a special mo-tion pattern, known as the middle motion pattern, for therobots peristaltic crawling pattern. We also performedseveral experiments in a 27-mm-diameter acrylic pipe toexamine locomotion speed and propulsion force, and de-veloped a robot that passes through an elbow pipe. Weconducted several experiments to confirm this.2. Peristaltic CrawlingAn earthworm moves by peristaltic crawling in whichextensionand contraction waves are propagatedin the an-teroposterior direction by variations in the thickness andlength of segments. Fig. 1 shows the peristaltic crawlingpatternofanearthworm. First, theearthwormcontractsitsanteriorsegments,therebyincreasingfriction betweenthesurface and segments. This occurs because thicker seg-ments are in contact with the surface being crawled uponduring locomotion, while contraction propagates back-ward. This movement pulls rear segments in the directionof movement. Anterior segmentsare extendedin the axialdirection after a contraction is completed.1054JournalofRoboticsandMechatronicsVol.24No.6,2012An In-Pipe Mobile Robot Based on Earthworms Peristaltic CrawlingFriction area Direction Contraction Fig. 1.Locomotion pattern of an earthworm in peristalticcrawling.57.3 13.5 401.1 Cigarette Fig. 2. Developed peristaltic crawling robot.Trapezoidal screw MotorGuide Belt Cylinder (a) Extension(b) ContractionFig. 3. Schematic view of unit.Table 1. Specifications of unit (normal type).Weight13.8 gExtensionLength57.3 mmDiameter13.5 mmContractionLength44.3 mmDiameter37.3 mm3. In-Pipe Mobile Robot for Straight PipesFigure 2 shows the in-pipe mobile robot that we de-veloped by implementing actual earthworm motion. Thecomplexmechanismof anearthwormwas simplified. Therobot weighs 96.6 g and is 401.1 mm long fully extendedand 310.1 mm long fully contracted. It consists of sevenunits, each equivalent to the segment of an actual earth-worm. The structure of the unit is shown in Fig. 3, and itsspecifications are listed in Table 1. A trapezoidal screw isattached to a motor shaft. When the motor starts, a cylin-der moves directly along guides as the screw turns. Thiscauses the unit to extend and contract in the axial direc-tion. As a result, the robot achieves motion similar to thatof an earthworm.Wavelength ? Direction Fig. 4. Example of basic motion pattern 4-1-1.?Direction Fig. 5. Example of middle motion pattern 5-1-1 3.4. Motion Patterns4.1. Basic Motion PatternSeveral motionpatterns are achievedby altering the ex-tension or contraction of each unit. Motion patterns con-sist of wavelength, propagation speed, number of wavesand number of units, i.e., l, s, n, and N, respectively.Wavelength is the number of units extended in the axialdirection. Propagation speed is the number of units prop-agated backward. Henceforward, we identify these basicmotion patterns as motion l-s-n, e.g., motion 4-1-1 (num-ber of units is 7, wavelength is 4, propagation speed is1, and number of waves is 1) is shown in Fig. 4. Eachparameter must satisfy the following equations:(l+s)n s, . . . . . . . . . . . . . . .(2)l s. . . . . . . . . . . . . . .(3)4.2. Middle Motion PatternConsidering stationary units, the robot achieves pat-terns of smooth motion. That is, the contraction of unitsin each basic motion pattern is divided into m parts; in themotion patterns thus generated, the time each unit takesto extend and contract shortens. Hence, the time that con-tracted units propagate backward decreases.Movement required in these motion patterns becomes1/m. Examplesare shownin Fig. 5. Here, eachparametermustsatisfy the followingequationin additionto Eqs.(1),(2), and (3) to achieve the middle motion pattern:N 2m. . . . . . . . . . . . . .(4)In addition, propagation speed s must satisfy s = 1. Weuse m in square brackets ( ) after basic motion pat-tern l-s-n to describe middle motion patterns, e.g., motionl-s-n m.JournalofRoboticsandMechatronicsVol.24No.6,20121055Horii, S. and Nakamura, T.0.671.342.011.342.692.692.014.036.042.695.373.366.170.01.02.03.04.05.06.07.01-1-11-1-21-1-32-1-12-2-12-1-23-1-13-2-13-3-14-1-14-2-15-1-15-1-13Motion patternLocomotion speed mm/s Fig. 6. Theoretical locomotion speed.5. Theoretical Model5.1. Theoretical Locomotion SpeedFirst, we derive the theoretical locomotion speed ofbasic motion patterns, several of which are considered.In each basic motion pattern, the distance the robot pro-gressesandthetimerequiredforawavetopropagatefromthe headto thetail are dBandTB, respectively. We expressdBand TBasdB= rnl, . . . . . . . . . . . . . . .(5)TB=Nts. . . . . . . . . . . . . .(6)Here, r is the amount of contraction, and t is the time thateach unit takes when from Eqs. (5) and (6), we expressthe theoretical locomotion speed of basic motion patternvBasvB=dBTB=rnslNt. . . . . . . . . . .(7)Next, we derive the theoretical locomotion speed of themiddlemotionpatterninwhichthe distancethe robotpro-gressesandthetimerequiredforawavetopropagatefromthe head to the tail are dMand TM, respectively. These areexpressed asdM= rn?l2m1i=1im?, . . . . . . . . .(8)TM=Ntm. . . . . . . . . . . . . .(9)From Eqs. (8) and (9), we express the theoretical locomo-tion speed of middle motion pattern vMasvM=dMTM=rmn?l2m1i=1im?Nt. . . . (10)Using rotational speed of the motorsand pitch of trape-zoidal screws p, t is expressed ast =rp. . . . . . . . . . . . . . (11)Substituting Eq. (11) into Eqs. (7) and (10), we expressvBand vMasvB=dBTB=nslpN,. . . . . . . . . . . (12)vM=dMTM=mn?l2m1i=1im?pN. . . (13)The theoretical values of locomotion speed fromEqs. (12) and (13) are shown in Fig. 6. Note that loco-motion speedincreaseswith increasing wavelength,prop-agation speed or number of waves, and that maximum lo-comotion speed is the value of motion 5-1-1 3. In addi-tion, from Eqs. (12) and (13), the amount of contractiondoesnotdependonlocomotionspeed. For this reason,therobot proceeds at the same locomotion speed even if theinner diameter of the pipe changes. Moreover, units canbe arranged roughly because the distance between unitsdoes not depend on locomotion speed. In addition, if oneof the units breaks down, speed of the robot can be calcu-lated by considering the distance of a broken unit as thedistance between units.5.2. Theoretical Propulsion ForceThe robot moves using trapezoidal screws. Thus, thethrustforceisequaltotheextensionforceofaunit. Wesetup the thrust force of the trapezoidal screw to determinethe extension force of a unit.First, we calculate the thrust force of a square screw.Fig. 7 shows how material is applied to a slope of thesquare screw.Fig. 8 shows a developmental view of1056JournalofRoboticsandMechatronicsVol.24No.6,2012An In-Pipe Mobile Robot Based on Earthworms Peristaltic CrawlingQDP Fig. 7. Frame format view of square screw.ps Fig. 8. Development of square screw. Fig. 9. Development view of square screw considering friction.Fig. 7. P, Q, D and psare horizontal force, thrust force,effective diameter and pitch, respectively. From Fig. 8,tanis expressed astan=psD. . . . . . . . . . . . (14)If friction is taken into account, Fig. 8 can be redrawnas Fig. 9. W is resultant force. The angle of friction is. From this figure, the thrust force of square screw Qsisexpressed asQs=Ptan(+). . . . . . . . . . (15)P is expressed as follows using torquethat turns thescrew:P =2D. . . . . . . . . . . . . . (16)Substituting Eq. (16) into Eq. (15), we express the thrustforce of square screw QsasQs=2Dtan(+). . . . . . . . . . (17)Here, we consider the apex angle of trapezoidal screw 2because we examine its thrust force. A cross-sectional di-agram of the trapezoidal screw is shown in Fig. 10. Fromthis figure, the thrust force of the trapezoidal screw Qtis?QtQs Fig. 10. Cutaway view of trapezoidal screw.Table 2. Specifications of trapezoidal screw.Pitch p2 mmEffective diameter D7.25 mmLead angle5Apex angle 230expressed asQt= Qscos=2Dtan(+)cos. . . (18)The extension force of a unit was therefore defined be-cause it is equal to thrust friction force. Table 2 showsparameters of the trapezoidal screw.is approximated as0 because trapezoidal screws are lubricated with grease.Substituting each parameter into Eq. (18), it is possible tocalculate. Here, an extension force is 45.03 N and maxi-mum motor torque is 14.3 mNm.If the friction force of the robot is greater than thisforce, propulsionforceis equalto extensionforce. In con-trast, if friction force is lower than this force, propulsionforce is equal to the friction force of the robot.6. Experimental Results and Discussion ofRobot for Straight Pipes6.1. Locomotion SpeedWe tested the robot and measured locomotion speed ineach motion pattern. These experiments were performedwith an acrylic pipe 27 mm in diameter. The relationshipbetween locomotion speed and motion pattern is shownin Fig. 11. The maximum locomotion speed is 5.24 mm/sin motion 5-1-1 3. Locomotion speed increases withincreasing wavelength, propagation speed or number ofwaves.Experimental values are, however, never less than the-oretical values. This is explained as follows: althoughtheoreticalwas determined by moving one unit, realwas different. Realdecreased because the unit wasforcedbyits anteroposteriorunits, andtheoreticalvaluedecreased with load for these types of force. In addi-tion, these errors between theory and reality accumulateas waves propagate backward. This is the reason for theJournalofRoboticsandMechatronicsVol.24No.6,20121057Horii, S. and Nakamura, T.0.460.801.441.341.781.991.484.272.565.240.01.02.03.04.05.06.07.01-1-11-1-21-1-32-1-12-2-12-1-23-1-13-2-13-3-14-1-14-2-15-1-15-1-13Motion patternLocomotion speed mm/s Fig. 11. Experimental result of the relationship between locomotion speed and motion pattern. Fig. 12. Experimental setup for measuring friction force.experimental value of locomotion speed to be less thanthe theoretical value.6.2. Friction ForceThe maximum friction force of the robot was measuredusing the experimental setup shown in Fig. 12. Here, thefriction coefficient between the robot and the acrylic pipeis 0.76.The number of contracted units is varied from one toseven. The maximum friction force is measured by a loadcell. First, units of the robot were contractedin a horizon-tal pipe and cut off from the electricity supply to main-tain contraction. We then connected the robot to the loadcell and pulled the acrylic pipe along a guide in a straightline. When the acrylic pipe slips from the robot, frictionforce is the maximum static friction force. The relation-shipbetweenthis forceandthenumberofcontractedunitsis shown in Fig. 13. The maximum static friction forceincreases in proportion to the number of contracted units.Here, the number of units contracted in the axial directionin each motion pattern NCisNC N nl. . . . . . . . . . . . (19)We therefore can observe maximum static friction forcecorresponding to each motion pattern.In addition, we measured static friction force of oneunit when its diameter changed. Fig. 14 shows this exper- 01234567891001234567The number of contracting unitsMaximum static friction force N 01234567891001234567The number of contracting unitsMaximum static friction force N Fig. 13.Experimentally observed relationship betweenmaximum static friction force and number of contractedunits. 00.511.522.533.511.051.41.45Ratio( Maximum diameter of the unit / Diameter of the acrylic pipe?Maximum static friction force N .00.511.522.533.511.051.41.45Ratio( Maximum diameter of the unit / Diameter of the acrylic pipe?Maximum static friction force N .Fig. 14.Experimentally observed relationship betweenmaximum staticfriction force andexpansion diameter of oneunit.imentally obtained relationship between maximum staticfriction force and maximum expansion diameter of oneunit. Maximum static friction force increases with maxi-mum diameter of the unit.1058JournalofRoboticsandMechatronicsVol.24No.6,2012An In-Pipe Mobile Robot Based on Earthworms Peristaltic Crawling Fig. 15.Experimental setup for measuring extensionforce of unit. Fig. 16. Experimental setup for measuring propulsion force.8.995.825.315.695.754.835.526.185.924.885.213.283.520123456789101-1-11-1-21-1-32-1-12-2-12-1-23-1-13-2-13-3-14-1-14-2-15-1-15-1-13Motion patternMaximum propulsive force N Fig. 17. Experimentally observed relationship between propulsion force and motion pattern.6.3. Extension Force of a UnitTheextensionforceofaunitwasmeasuredusingaloadcell in the experimental setup shown in Fig. 15. The max-imum extension force of a unit is 44.24 N. This experi-mental result is about the same as the theoretical value.Here, as previously mentioned, we assume that propul-sion force is equal to friction force of the robot becausefriction force is lower than propulsion force.6.4. Time Response of Head UnitPropulsion force is needed when the robot passes ofpipe. The robot was measured in each motion throughshapes such as elbows. The propulsion force pattern usesa load cell in the experimental setup shown in Fig. 16.The relationship between propulsion force and the mo-tion pattern is shown in Fig. 17. Maximum propulsionforce is 8.99 N in motion 1-1-1. Propulsion force de-creases with fewer contracted units because friction de-creases. When propulsion force of each motion pattern iscompared to the number of contracted units, propulsionforce is higher than static friction force corresponding toeach motion pattern. 48505254565860626401020304050607080Time sDistance mm48505254565860626401020304050607080Time sDistance mmFig. 18.Experimentally observed time response of headunit extension.6.5. Propulsion ForceFigures 13 and 17 show that propulsion force corre-sponding to each motion pattern exceeds maximum staticfriction force for that pattern. Here, we take motion 1-1-1,which shows the greatest difference between propulsionforce and maximum static friction force, as an example todiscuss the reason for this excess.Figure 18 shows the time response of head unit lengthin the propulsion force experiment in motion 1-1-1.JournalofRoboticsandMechatronicsVol.24No.6,20121059Horii, S. and Nakamura, T. Head part Joint 475 Fig. 19. Endoscope robot for moving in small or curved tubes.Table 3. Specifications of unit (elbow type).Weight11.5 gExtensionLength41.8 mmDiameter13.3 mmContractionLength34.5 mmDiameter29.5 mmLength was analyzed using movement analysis software.The figure shows that the head unit normally extends andcontracts for about 30 s. Shortly afterward, the head unitcollides with the load cell and cannot extend fully; it con-tracts normally and repeats this movement, however. Asa result, the head unit gradually shortens. In addition,from Fig. 14, maximum static friction force increasesas the head unit shortens. This is why propulsion forcefor each motion pattern exceeds maximum static frictionforce. Suchmovementispreventedbycontrollingthemo-tor pulse.7. In-Pipe Mobile Robot for Elbow PipesThe robot has three problems in passing through elbowpipes:1. Joints are unbending because of material and con-nection.2. The unit is too large to pass through elbow pipes.3. The head unit cannot get over uneven obstacles injoints between straight pipes and elbow pipes.Based on these problems, we developed a robot thatpasses through elbow pipes.7.1. In-Pipe Mobile Robot for Elbow PipesFigure 19 shows the robot we developed for passingthrough elbow pipes. Its specifications are listed in Ta-ble 3. It consists of six units. Each joint uses a rubbertube so that it bends passively. To avoid squashing thejoint when it bends, we set a spring in the rubber tube.Its units are also downsized to pass through elbow pipesby using motors for the next size down. We reduced unitextension by 27.1% and unit contraction by 22.1% of thatof the robot for straight pipes. We also developed a headto pass obstacles in joints between straight pipes and el-bow pipes. It is composed of a rubber tube, a spring, and3.0 9.8 Rubber tube (7 mm in inner diameter) Spring Fig. 20. Joint.Motor Rubber part Rubber tubeFig. 21. Top. 1 2 3 4 Fig. 22. Displacement of top.7 mm micromotors. A rubber part attached to the mo-tor shaft rotates when the motor starts so it passes throughelbow pipes. Figs. 20, 21, and 22 show its new joint, thehead,andoperationin motion,respectively. From this fig-ure, note that the rubber part revolves along the inner wallof the elbow in one direction. It was confirmed that it iscaught in the following pipe.7.2. Driving Experiment of Robot for Elbow PipesWe tested therobotto see if it couldpassthroughelbowpipes. This experiment was performed with two acrylicpipes and an elbow pipe having inner diameters 27 mm(25 A).Experimental results for performance driving throughan elbow pipe are shown in Fig. 23. Here, motion 2-2-1is employed as the motion pattern of the robot. Thesefigures show that the robot passes through the elbow pipe.In contrast, it was difficult for the robot to pass throughthe elbow pipe using other motion patterns. There was asignificant decrease in locomotion speed because of thelack of force pushing units forward into the elbow pipe(anterior units) and pulling back units from it (posteriorunits).1060JournalofRoboticsandMechatronicsVol.24No.6,2012An In-Pipe Mobile Robot Based on Earthworms Peristaltic Crawling? ? ? ? ? ? ? ? Fig. 23. Performance driving through elbow pipe of motorrobot.8. ConclusionsWe have developed robots used in straight and elbowpipes with an inner diameter of 27 mm. We have alsoderived theoretical formulas for robot locomotion speedand propulsion force, and performed several experimentsin 27-mm-diameter acrylic pipes to examine locomotionspeed and propulsion force. From these, we obtained thefollowing results:1. We examined the relationship between locomotionspeed and motion pattern and found that maximumlocomotion speed is 5.24 mm/s in motion 5-1-1 3on the robot for straight pipes.2. We examined the relationship between propulsionforce and motion pattern and found that maximumpropulsion force is 8.99 N in motion 1-1-1 using therobot for straight pipes.3. The robot for elbow pipes passed through elbowpipes.In future, the robot for elbow pipes will be improvedas follows: we will let pushing and pulling force increaseby increasing friction force through the use of differentfriction materials and passing cables inside the robot.References:1 A. Kuwada, K. Tsujino, K. Suzumori, and T. Kanda, “IntelligentActuators Realizing Snake -like Robot for Pipe Inspection,” Proc.IEEE Int. Symposium on Micro-nano Mechatronics and HumanScience, pp. 1-6, 2006.2 M. Konyo, K. Hatazaki, K. Isaki, and S. Tadokoro, “Developmentofan Active Scopecamera Driven byCiliary Vibration Mechanism,”Proc. of the 12th Robotics-symposia, pp. 460-465, 2007.3 P. Li, S. Ma, B. Li, and Y. Wang, “Development of an AdaptiveMobile Robot for In-pipe Inspection Task,” Proc. IEEE Int. Conf.on Mechatronics and Automation, pp. 3622-3627, 2007.4 T. Okada and T. Sanemori, “MOGER: A Vehicle Study and Real-ization for In-pipe Inspection Tasks,” IEEE J. of Robotics and Au-tomation, Vol.RA-3, No.6, December 1987.5 A. H. Heidari, M. Mehrandezh, R. Paranjape, and H. Najjaran, “Dy-namic Analysis and Human Analogous Control of a Pipe CrawlingRobot,” Proc. IEEE/RSJ Int. Conf. on Intelligent Robots and Sys-tems, pp. 733-740, 2009.6 T. Nakamura, T. Kato, T. Iwanaga, and Y. Muranaka, “Developmentof a Peristaltic Crawling Robot Using Servo Motors Based on theLocomotion Mechanism of Earthworms,” Proc. of IEEE Int. Conf.on Robotics and Automation (ICRA 2006), pp. 4342-4344, 2006.7 T.Nakamura and T. Iwanaga, “Locomotion Strategy for a PeristalticCrawling Robot in a 2-Dimensional Space,” Proc. IEEEIn. Conf. onRobotics and Automation, pp. 238-243, 2008.8 N. Saga and T. Nakamura, “Development of peristaltic crawlingrobot using magnetic fluid on the basis of locomotion mechanismof earthworm,” Proc. of SPIE, Smart structures, Devices, and Sys-tems, SPIE, pp. 369-377, 2002.9 H. Omori, T. Hayakawa, and T. Nakamura, “Locomotion and Tur