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International Journal of Advanced Robotic Systems

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Research on a New Bilateral Self-locking Mechanism for an Inchworm Micro In-pipe Robot with Large Traction Regular Paper

Junhong Yang1,*, Yong Xue1, Jiangzhong Shang1 and Zirong Luo1 1 National University of Defense Technology, Changsha, China * Corresponding author E-mail: yangjunhong@nudt.edu.cn Received 31 Oct 2013; Accepted 18 Sep 2014 DOI: 10.5772/59309 © 2014 The Author(s). Licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract In this paper, we present an innovative bilaterally-controllable self-locking mechanism that can be applied to the micro in-pipe robot. The background and state of the art of the inchworm micro in-pipe robot is briefly described in the very beginning of the paper， where the main factors that influence the traction ability are also discussed. Afterwards, the micro in-pipe robots’ propulsion principle based on a unidirectional selflocking mechanism is discussed. Then, several kinds of self-locking mechanisms are compared, and a new bilaterally-controllable self-locking mechanism is proposed. By implementing the self-locking mechanism, the robot’s tractive force is no longer restricted by the friction force, and both two-way motion and position locking for the robot can be achieved. Finally, the traction experiment is conducted using a prototype robot with the new bilaterally-controllable self-locking mechanism. Test results show that this new self-locking mechanism can adapt itself to a diameter of Φ17~Φ20 mm and has a blocking force up to 25N, and the maximum tractive force of the in-pipe robot based on such a locking mechanism is 12N under the maximum velocity of 10mm/s.

Keywords Micro In-Pipe Robot, Traction Ability, SelfLocking Mechanism, Two-Way Motion, Position Locking

1. Introduction With the development of modern industrial technology, plenty of complex micro-pipes are used in the petrochemical industry, refrigerating industry and nuclear power plants. After some time, fatigue fracture, corrosion and mechanical damage may occur in these micro-pipes, which can lead to severe leakage accidents without detection and maintenance. Usually, the inner space of these pipes is narrow and the layout is complex. Furthermore, there is always a protection shield outside the pipe. These features bring many challenges to leakage detection and the maintenance of these micro-pipes. At present, the commonly used and effective detection method is to use the pipeline robot to carry the detecting elements. There are many studies concerning micro in-pipe robots that show that the maximum tractive force of most robots is less than 2N when the adapting pipe diameter ranges

Int J Adv Robot 2014, | doi: Junhong Yang, Yong Xue, Jiangzhong Shang andSyst, Zirong Luo:11:174 Research on10.5772/59309 a New Bilateral Self-locking Mechanism for an Inchworm Micro In-pipe Robot with Large Traction

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from 20mm and a 30mm [1--8]. For examp ple, the maxim mum tractive forcee is 0.22N for the t inchworm m in-pipe robott and 9.5N for the spiral wheeleed in-pipe rob bot, both with h the pe’s diameter of 20mm, dev veloped by To adapting pip okyo Institute of Technology T [33-5]. The wheeeled in-pipe robot r developed by b Toshiba Corp C had a maximum m tractive force of 1N, while the ad dapting pipelline diameter was Ø25.4mm [6]]. Shanghai University U dev veloped an ineertiadriven pipe robot r with 0.55N max-tractio on and a spiraal inpipe robot with w 1.85N, both b for Ø200mm pipes [77, 8]. Moreover, many m research hes about miccro in-pipe ro obots can be foun nd in the paapers and paatents from other o institutions, such s as Tokyo o University of o Agriculturee and Technology [9], Nippon Institute of Technology [10], nd in China the Korea Aerosspace Univerrsity [11], an Shanghai Jiao otong Universsity [12-15], N National Univeersity of Defence Teechnology [166-19], etc. Besides, otheer micro in-p pipe robots were w developeed to improve thee performancee of the colo onoscopy [20~23]. These robotss usually incclude two paarts: a locomo otion mechanism and a a steerablee robotic tip with w high dextterity [20]. Many lo ocomotion meechanisms hav ve been propo osed, such as a clamping mech hanism with a maximum pullback force off up to 0.6N [221], a magnet--based mechan nism with a drivin ng force up to 0.15N when the current den nsity is between 0.0541 0 and 46..5586A/mm2 [222], and an SMAS based mechaanism with a blocking b force of up to 0.3N [23]. The popular locomotion n mechanism ms are based d on inchworm locomotion [24]. Inchw worm locomo otion includes thee telescopic driving mecchanism and the supporting mechanism. m T The driving mechanism m iss the m is power outpu ut unit, and the supportiing mechanism used to generate a contact between the robot r and the pipe wall, forming a closed loo op of force an nd shape [25,, 26]. The supportting mechanissm has many y different fo orms, such as wheeel, elastic leg g, shape mem mory alloy, airrbag, and some oth her structuress [26, 27]. Thee challenge rellated to inchworm m locomotion is that robo ots need adeq quate adhesion forcce to propel th hemselves insiide the colon [28]. [ i robot in industry should have high A practical in-pipe tractive forcce. The larg gest tractive force is usu ually restricted by y the maximum static frictiion force betw ween the robot and d the pipe waall. Hence, normal pressuree has to be reinforcced to increase the tractive force. Howev ver, it is difficult to increase no ormal pressurre because off the pipe space. Furthermore,, enhancing the narrow in-p friction forcee will influen nce the robott’s moving sp peed. Therefore, to o improve thee robot’s tracction capacity, the contradiction n among tractive force, normal pressure,, and friction forcee should be ad ddressed. Thiss paper focusees on the supporting mechan nism and presents p a new supporting structure, which makees the tracction on and only determined d by y the independent of the frictio ng mechanism m. driving abilitty of the drivin

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Int J Adv Robot Syst, 2014, 11:174 | doi: 10.5772/59309

hworm locom motion mecha anism of thee The in-pipe inch ot based on th he unidirection nal self-lockin ng supporting g robo mecchanism is brriefly describeed in Section 2, and threee sortts of self-lockin ng supporting g mechanism are compared d n 4 presents a and analysed in detail in Secttion 3. Section nov vel direction n-controllablee bilateral self-locking g porting mechanism includiing the related d analysis and d supp mecchanical desig gn. Finally, w we conduct ex xperiments in n Secttion 5 with an n inchworm m micro-pipe rob bot prototypee baseed on this new w supporting g mechanism. This paper iss imp proved and su upplemented from the confference paperr prev viously publisshed by the au uthors [29]. 2. Prropulsion prin nciple of in-pip pe robots baseed on unid directional sellf-locking mecchanism ure 1 illustrattes the propu ulsion princip ple of in-pipee Figu robo ots based on a unidirection nal self-locking g mechanism.. The robot is com mposed of two unidirectiona al self-locking g d a telescop pic mechanism m. The coree mecchanisms and funcction of the unidirectional u self-locking mechanism m iss to lo ock the motio on at one dirrection and keep it free to o mov ve at the oth her. For exam mple, when the t telescopicc mecchanism ex xtends, un nidirectional self-locking g mecchanism 1 recceives a push force pointin ng to the left,, and its locking leegs receive fricction forces pointing to thee righ ht from the pipe p wall. Theen the locking g leg has thee tren nd to rotate clo ockwise aroun nd the joint beetween the leg g and the robot bo ody, causing iincreasingly larger l normall presssure and fricction force bettween the leg and the pipee walll. Therefore, it is difficu ult for unidirrectional self-lock king mechanissm 1 to mov ve to the left and so self-lock king can be realized. r Conttrarily, unidirrectional self-lock king mechanissm 2 receives force to the right, and itss lock king legs havee the trend to o rotate countter clockwise,, caussing the norm mal pressure and friction between thee lock king legs and d the pipe waall to becomee increasingly y sma aller. Therefore, unidirection nal self-lockin ng mechanism m 2 ca an move to th he right. On the other han nd, when thee telesscopic mechan ps take place,, nism shortenss, similar step caussing self-lockiing mechanism m 2 to be lock ked to the leftt and mechanism 1 mobile to tthe right. In this way, thee ot moves forw ward to the rig ght. The unidiirectional self-robo lock king mechanissm realizes seelf-locking by mechanisms,, and the maximum tractive force is only determined by y the telescopic mechanism’s driiving ability, not n limited by y b the ro obot and the pipe p wall. the friction force between

Figu ure 1. In-pipe robot’s propulsion n principle baseed on unid directional self-llocking mechan nism

3. Structure analysis a of threee types of un nidirectional locking supp porting mechan nism

king mechaniism is simplee The structure of this self-lock and easily realizzed. In additiion, the wholle size of thee ot can be very y small. Howev ver, two shorttcomings existt robo in such a robot. The T first is th hat, because th he supporting g press into the p pipe wall, a reelatively largee legss always pre-p fricttion is generated against the robot’ss movement,, resu ulting in loss of energy as well as severe wear of thee porting legs, therefore supp t shorrtening the lifee of the robot.. The second one iss that the motiion of the robo ot that adoptss this locking meechanism (as shown in Figure 1) iss unid directional.

3.1 Locking Meethod Using Suppporting Legs

(a) (b) (c) Figure 2. Illusttration of lockin ng method using g supporting leg gs. Pipe wall (1), elastic e supportin ng legs (2), supp porting body (3)), connecting stru ucture (4).

L Methodd Using Inclinedd Plane 3.2 Locking

This method d realizes self-llocking by prressing legs on n the a shown in n Figure 2. This structurre is pipe wall, as composed of o supporting g legs, suppo orting body and connecting mechanism. m T The supportin ng legs connect to the supportin ng body throu ugh revolute jo oints, about which w torsional sprrings are used d to overcome the weight of o the p the leegs on the piipe wall. The prelegs and to preload pressing forcce is very smaall and the leg g’s weight is very light, so they y are neglecteed in the anaalysis. N and f are assumed as the t sum of alll normal presssures, the sum m of all friction forces f between all supportting legs and d the pipe wall, respectively, r and P is th he sum of fo orces received along the directtion of the leegs, as show wn in Figure 2 (b). F is the tracttive force, and μ is the fricction coefficient beetween the leg g and the pip pe wall. To reealize self-locking, the following g condition sh hould be satissfied, i.e., f ≥F

(1)

f = Nμ

(2)

where

when the suppo orting bar stop ps For the supporting legs, w rotating, the direction of th he resultant fo orce of N and f points along the bar throug gh the centre of o the revolutee joint. If all forrces balance, the be t following equation can b obtained: (3) N = P sin(θ )

(a) (b) Figu ure 3. Illustratio on of locking meethod using incclined plane. (a)) Incliined plane’s loccking method o one: pipe wall (1), connecting g bar (2), conical bod dy (3), locking wheel (4), conn necting bar (5),, supp porting shaft (6 6), spring (7), caarriage (8). (b) Inclined I plane’ss lockiing method two o: pipe wall (1),, hold-down nu ut (2), screw rod d (3), spring s (4), wedg ge (5), conical bo ody (6), carriagee (7).

There are two kinds of lockingg method usingg inclined planee [17, 19], which arre shown in F Figure 3. It ma ainly uses thee nciple of an inclined plan ne’s self-lockin ng to realizee prin unid directional loccking. As sho own in Figuree 3 (a), if thee supp porting shaft receives push hing force to the left, thiss mecchanism has th he trend to m move to the lefft, causing thee lock king wheels to o roll up alon ng the inclined d plane of thee coniical body and d increasing th he normal preessure exerted d on the t inner wall by the locking g wheels. So th he mechanism m is lo ocked to the leeft. If the supporting shaft reeceives a push h force to the right,, the conical b body will mov ve to the right,, and the locking wheels w separatee with the pipee wall and thee m to the right. The lo ocking theory y mecchanism can move show wn in Figure 3 (b) is similarr to Figure 3 (a). ( Springs in n both h Figure 3 (a)) and (b) are used to keep p the locking g wheeels or wedgess contacting thee pipe’s inner wall.

pporting mecchanism, alon ng the telesccopic For the sup direction, thee following eq quation can be obtained:

F = P cos(θ )

(4)

ng formulas (1) to (4), the condition By combinin n of mechanism’s self-locking is: (a)

μ taan(θ ) ≥ 1

(5)

(b))

Figu ure 4. Mechanical principle of llocking method d using inclined d plan ne: (a) backward d direction and ((b) forward direection

Junhong Yang, Yong Xue, Jiangzhong Shang and Zirong Luo: Research on a New Bilateral Self-locking Mechanism for an Inchworm Micro In-pipe Robot with Large Traction

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The principle of the incllined plane lo ocking metho od is d f2 are the friction diagrammed in Figure 4, in which f1 and d N2 are the normal n pressures exerted on n the forces, N1 and wheels by thee conical body y and the pipe wall, respectiively, and F is the trractive force accted on the con nical body. onical body reeceives F poin nting to the lefft, as When the co shown in Fig gure 4 (a), with h the resultan nt action of fricction forces f1 and d f2, the lockin ng wheel recceives a clock kwise moment. Meeanwhile, the normal n pressu ures N1 and N2 are along the cen ntre of the loccking wheel, hence h they do o not produce any moment to th he wheel. As a result, the wheel w p along the incclined plane under u the actio on of will rotate up this momentt, increasing th he normal preessures N1 and d N2, e f1 and d f2. Finally, f2 is larger thaan F, which will enhance realizing selff-locking. Wh hen the conicaal body receiv ves F pointing to th he right, as sh hown in Figuree 4 (b), similarrly to above, the wheel w will rotaate down alon ng the inclinee and the locking will w be released d, so the coniccal body can move m to the right. From the above a analysis, the tractiive force of the telescopic in n-pipe robot based b on this method can n be increased as the rise of loaads, resolving g the contradicction between larg ge tractive force and friction n force. Howeever, two shortcom mings exist in such a kind off robot. The first is that the lock king mechaniism can realiize self-lockin ng in only one dirrection, and the t second is that the adaapted pipe diameteer is limited by b the length h and taper off the conical body y. If the robot is required to o cross bent pipes, p the conical bo ody cannot bee too long, and d thus the adaapted diameter ran nge is usuallly not large.. Therefore, it is significant to o make an in-p pipe robot wiith the capabilities of adapting to a wide raange of pipe diameters an nd of moving bilaterally. ms 3.3 Locking Meethod Using Cam d is based on the self-lockiing principle of a This method cam, as show wn in Figure 5. The moto or drives the lead screw to rotaate, so the mo oving nut mov ves along the axis of the lead sccrew. Meanwh hile, the nut drives d the pusshing bar, which pushes p the parallel four bars b to transfform. Thus, the cam ms mounted on o the horizontal bars presss on or separate from the innerr pipe wall, reealizing lockin ng or unlocking.

To simplify the analysis of the locking principle, p wee ume each bar’s stiffness is llarge enough, and the four-assu bar mechanism iss considered aas a rigid body y, as shown in n ure 6, f is the friction force exerted on th he cam by thee Figu pipee wall, N is thee normal presssure exerted on o the cam by y the wall and F is the tracctive force accting on thee mecchanism.

(a)

(b)

Figu ure 6. Illustratio on of a cam’s loccking mechaniccal principle: (a)) back kward direction n and (b) forward d direction

W the mecchanism receiv ves a forward tractive forcee (1) When as shown s in Figu ure 6 (a), the cam rotates anti-clockwise a e due to the friction n force f. Acco ordingly, N iss increased, ass he friction forrce f. In the eend friction fo orce f will bee it th larg ger than F and self-locking ccan be realized d. ward tractivee (2) When the mechanism recceives a backw n Figure 6 (b)), the cam rota ates clockwisee forcce, as shown in and N is decreeased, reduccing the friction force f. m is u unlocked in the t backward d Therefore, the mechanism direection. ough the abov ve analysis, a robot with su uch a locking g Thro mecchanism has two t advantag ges. First, the friction forcee exerrted on cams will increase with the tracttive force duee to the t cam’s selff-locking, so the robot has a relatively y larg ge tractive forrce. Second, b because the trransformation n scale of the paralllel four-bar meechanism is la arge, the robott can adapt a larg ge change of the pipe’s diiameter scale.. wever, such a robot has tw wo shortcomin ngs: the cam’ss How self--lock is unidirrectional, mak king the robot able to movee only y in one direcction; and therre is a large lo oad acting on n the four bars. If th he bar is thin and small, thee threshold off y the bars is aalso small. Mo oreover, if thee the load taken by bar’s dimension is i small, it is n not easy to insstall the cams.. nfiguration off in-pipe robo ots that utilizee Therefore, the con ually large. this method is usu ble Locking Mechanism M 4. A New Bilateraally-Controllab T Principle of Bilaterally-Controllable Locking g Mechanism 4.1 The

Figure 5. Illusttration of lockin ng method using g cams: pipe waall (1), carriage (2), parallel p four baars (3), cam (44), pushing bar (5), moving nut (6)), lead screw (7)), motor (8)

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In this paper, a new bilateerally-controlllable locking g mecchanism is pro oposed. The iidea is to ma ake use of thee cam m’s self-locking g theory in orrder to provid de a relatively y larg ge tractive forrce for the in n-pipe robot. As shown in n

Figure 7, thee motor drivees the screw rod, along whose w shaft the nut moves. Thus the cams on one o side are drrawn back while th hose on the oth her side contaact the pipe

To ensure the raationality of tthe following analysis, thee cam m’s stiffness iss assumed to be large enou ugh so it willl not be deformed under the acction of F. Meanwhile, thee nt between thee cam and thee pipe wall iss fricttion coefficien ng forces at po oint Q are as fo ollows: μ . Thus the actin

 N = F1y = F1 cosθ   f = N μ

(6))

To avoid a the relaative slip betw ween the cam and the pipee walll at point Q, th he following ccondition musst be satisfied: Figure 7. Illusstration of a con ntrollable lockin ng mechanism: pipe wall (1), cam (2), ( moving nut (3), connectin ng bar (4), screw w rod (5), motor (6), carriage c (7)

c fold and a unfold position can n be wall. The cams’ controlled by y changing th he position off the nut, thuss the switching of locking direcction and mov ving direction n can be achieved. This mechan ures: nism has the following f featu ve force is nott limited by th he maximum static s (1) the tractiv friction forcee; (2) by contro olling the lock king direction n, the robot can move m in two directions; d (3)) by changing g the contact poin nts on the cam surfacee, different pipe diameters caan be adapted d; (4) by contrrolling the loccking mechanism, both moving directions can n be locked at a the c be fixed at a same time, which meanss the robot can p and thiis provides some s certain posittion in the pipe, advantage in n fixed-point operations. o 4.2 Analysis off the Self-Lockingg Condition ows the cam m’s self-lockin ng mechanism m, in Figure 8 sho which xoy is the fixed recttangular coord dinate system,, F is f exerted by b the telesco opic mechanism to the tractive force the unidireectional lock king mechan nism, F1 is the component of o F along the direction of OQ, O point O is the centre of rotaating shaft of the cam, poin nt Q is the con ntact point betweeen the cam and a the pipe wall, N and f are normal presssure and frictiion force exerrted on the caam at point Q by the pipe waall, respectivelly, θ is the angle a between OQ and vertical direction, d and d F1x and F1y aree the projections of F1 on axis x and a y, respectiively.

F1x = F1 sinn θ ≤ f

(7))

ume the tang gent angle corrresponding to t μ is ϕ f and d Assu meeets: tan ϕ f = μ

(8))

From m equation (66), (7) and (8),, the further condition c thatt the cam can self-lock can be ach hieved: 0 α , where w e α is the anglle of the cam m profile as sh hown in Figu ure 9, the two o sing gular position ns will not inffluence the ex xpanding and d lock king of the cam m. L Parametter Analysis 4.5 Linkage The kinematic sccheme of the ccam mechanissm is built ass show wn in Figure 11, 1 where xoy is the Cartesian coordinatee systtem, α is the angle a of the caam profile, po oint O, G, M, K are the four join nts, β is the aangle between n the line OG G and ox axis, and ϕ is the anglee between the line OM and d ox axis. a

Figu ure 11. The kineematic scheme of the cam’s fo our bar linkagee mech hanism

According to the above sing gular position n analysis, in n ordeer to avoid the t singular p position when n the highestt poin nt of the cam contacts c the w wall of the pipee ( β = 90° ), ass show wn in Figure 10 (a), the fo ollowing condition must bee satissfied: H0
0))

(11))

o rotate to ad dapt the chang ging diameterr The cam needs to he pipe. The constraint con nditions of th he joint M can n of th be described d as: (a)

( (b)

(c)

Figure 10. Sing gular position of o the cam mecchanism: (a) sin ngular position 1, (b) singular positio on 2, and (c) sing gular position 3

In this design, the rob bot can adap pt different pipe diameters by y rotating thee cam. In ord der to enlargee the adapting ran nge of the pip pe diameter, the cam musst be able to rotate freely. Thuss it is necessaary to analysee the m. There are three t singular posiition of the caam mechanism possible sing gular position ns, as shown in Figure 100. In Figure 10 (a)), the two rod ds locate in lline with poin nt G, point M and d point K colllinearly, wherre the cam caannot rotate anymore. Figure 10 1 (b) shows that the revo olute oint M canno ot go joint M is on the line off OG. Since jo bids the cam ffrom through the ccam, this conffiguration forb rotating clock kwise. When joint M is on the line of OK K, as shown in Fig gure 10 (c), th he cam is not allowed to ro otate clockwise ag gain, since joiint M cannot pass through h the

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( x − ρ 0 cos θ ) 2 + ( y − ρ 0 sin θ ) 2 − l12 = 0  2 2 2  ( x − H 0 ) + y − l2 = 0

(12))

uations can bee derived from m Eq. (12). The following equ [(l12 − l22 + H 02 − ρ 2 ) 2 − 4((l22 − H 02 ) ρ 2 ×  2 sin 2 β ] y + [4 H (l 2 + l 2 − H 2 − ρ 2 ) − 8ρ × 0 1 2 0  x2  (l 2 − H 2 ) cos β ] y ρ sin β + 4( ρ cos β − H ) × 0 0  2 x  2 2 2 2 ( H 0l1 − H 0 ρ − l2 ρ cos β + H 0 ρ cos β ) = 0

(13))

uation (13) can n be further expressed as: Equ At 2 + Bt + C = 0

(14))

where y  t = x  2 2 2 2 2 2 2 2 2  A = (l1 − l2 + H 0 − ρ ) − 4(l2 − H 0 ) ρ sin β  2 2 2 2 2 2 [ H 0 (l1 + l2 − H 0 − ρ ) − 8 ρ (l2 − H 0 ) ×  B = [4  c β ] ρ sin β cos  C = 4( ρ cos β − H 0 )( ) H 0 l12 − H 0 ρ 2 − l22 ρ cos β +  2 H 0 ρ cos β ) + (l12 − l22 + H 02 − ρ 2 ) 2 

(15)

The following result can bee obtained fro om Eq. (14). t = f ( l1 , l 2 , H 0 , β )

(16)

Defining β ∈ [90o-α, 90o], according to the geometry y, the n the angle beetween the lin ne OG and thee line line OM is in OK, and the following f equ uation should be b satisfied: 0 ≤ tann(ϕ ) ≤ tan( β )

(17)

o the continuiity of the link kage mechaniism’s According to movement, if the boundarry condition meets m Eq. (17)), Eq. (17) can be fu urther expresssed as follows:: 0 ≤ tan(ϕ ) ≤ tan(90  − α )

(18)

owing equatio on can be derrived Given β = 90 − α , the follo from Eq. (14)) and (15). y  φ = arcttan( ) = arctan(t ) x  t = f (l1 , l2 , H 0 )

unid directional lo ocking mechanism, which h is mainly y com mposed of cam m 1, torsional spring 2, shafft 3 and fixed d mou unt 4. Three cams are located at the fixed mountt circu umferentially 120 apartt from each h other and d conn nected by the shafts. The to orsional spring g is assembled d on shaft s 3, and th he cam can alw ways contact the pipe walll due to the tension n of the torsion nal spring. Figu ure 13 shows the t structure o of the direction n-controllablee self--locking supp porting mech hanism, whicch is mainly y com mposed of the small-power D DC motor 1, flexible shaft 2, 2 univ versal joint 3, coupler 4 and the new locking g mecchanism. The new locking m mechanism is composed off two o unidirection nal self-locking g mechanism ms. These two o directional seelf-locking m mechanisms are connected d unid togeether by thee lead screw w 9 and the nut 8. Thee conn necting rod 7 combines thee nut 8 and the t cams, and d the motor mou unt 1 and u unidirectionall self-locking g mecchanisms are connected by y the universa al joint 3. Thee mottor shaft and the t lead screw w are connecteed together by y the flexible shaft 2 and the mottor coupler 4. In the design n proccess of th his direction n-controllable self-locking g mecchanism, if the motor aand this meechanism aree conn nected directlly, the robot will have a longer singlee segm ment, which requires a larrger turning radius of thee pipee. Consequenttly, the univerrsal joint and flexible shaftt are used in our design d to decrrease the turn ning radius off the robot.

(19)

priate values of o l1, l2 and H0 may When β = 90 − α , inapprop lead to the reesult that B2 − 4AC < 0 , resultting in no solu ution to Eq. (14). To T solve this problem, p the values v of l1, l2 and H0 should sattisfy the follow wing condition n: (l1 − l2 ) 2 ≤ ρ 2 sin 2 β + ( ρ cos β − H 0 ) 2 ≤ (l1 + l2 ) 2

(20)

Figu ure 12. Unidirecttional self-lockiing mechanism: cam (1), torsiional spring (2),, shaft (3), moun nt (4)

In order to get a large adapting raange of the pipe ndition of Eq. (20), diameter for a certain α, besides the con the parametters l1, l2 and d H0 should d also satisfy y the following con ndition:  0 ≤ arctan( f (l1 , l2 , H 0 )) ≤ 900 − α  2 2 0 < H 0 < (l1 + l2 ) − ρ0

(21) Figu ure 13. Direction-controllab ble self-lockin ng supporting g mech hanism: motor and its mount (1), flexible shaft (2), universall jointt (3), coupler (4), ( mount (5), cam (6), conn necting rod (7),, mov ving nut (8), lead d screw (9)

5. Developmeent and Test of o the New DirrectionControllable Locking Mech hanism Protottype 5.1 The Prototyype Developmen nt The overall design d of the locking l mechaanism is show wn in Figures 12 an nd 13. Figuree 12 shows th he structure of the

he lead screw w 9 by the fleexible shaft 2,, Mottor 1 drives th then n the lead scrrew 9 drives the moving nut n 8 moving g alon ng the lead sccrew 9. When n the locking mechanism m iss requ uired to lock moving m to thee left, the nut moves to thee


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right end, pu ulling the left connecting c rod d to draw back k the left cam; meanwhile, m t the right co onnecting rod d is slackened, so o that the righ ht cam can con ntact the pipe wall under the acction of the torsional t sprin ng. The stepss are similar for th he mechanism locking to thee right. r based on o the new selfThe inchworrm in-pipe robot locking supp porting mechaanism is show wn in Figure 14. 1 It includes two o supporting mechanisms m a two ends and at a a telescopic driver d in th he middle. The supporrting mechanism is the structu ure shown in n Figure 13. The g screw 10 and d nut telescopic driiver is composed of leading 2, leading screw’s mount 3, flexible shaaft 5, motor 6 and motor’s mou unt 7. The lead ding screw’s mount m is conneected with the mo otor’s mount by universal joints 4, and d the leading screew is connected with thee motor shafft by flexible shaftt 5. The moving nut 2 is co onnected with h the supporting mechanism m 1 by the univeersal joint 11. The motor’s mou unt is connecteed with supporting mechan nism 8 by anotherr universal joiint 9. The mottor that drivess the telescopic mechanism is a DC motor and a its velociity is determined by b the input voltage.

Figure 14. Stru ucture of the in nchworm in-pip pe robot: self-loccking supporting mechanism m (1), moving nut (2), ( leading scrrew’s mount (3), uniiversal joint (4), flexible shaft (55), driving moto or (6), driving motorr’s mount (7), self-locking s sup pporting mechaanism (8), universal jo oint-2 (9), leadin ng screw (10), universal u joint (111)

ype of the direction-con ntrollable loccking The prototy supporting mechanism m is shown in Fiigure 15. The two working con nditions, i.e., the locking mechanism locks l moving to th he left and the right, are show wn in Figure 16. 1

(a) The lock king mechanism m locks moving to the left

(b) The lock king mechanism m locks moving to the right Figu ure 16. Two wo orking condition ns of the directiion-controllablee lockiing mechanism

5.2 Experimental E Teests 5.2.1 1 Self-Locking Test T and Directtion-Controllabble Lockking Test In order o to verify the locking m mechanism’s seelf-locking and d direection-controllaable locking aabilities, experimental testss were conducted. The T test metho od is shown in n Figure 17 (a)) and the physical teesting prototyp pe is shown in n Figure 17 (b).. The test process was w as follows. F First, a line wa as connected to o each h end of the lo ocking mechanism, and thee other end off the line was conn nected to a spriing balance. The T lead screw w wass adjusted to cause the lo ocking mecha anism to lock k mov ving to the lefft. Then the m mechanism wa as placed in a pipee with an innerr diameter of Ф Ф18mm. After that, a pulling g forcee to the right was w exerted on n the mechaniism at its rightt end. As a result, th he locking mecchanism easily y moved to thee righ ht. The right sp pring balance sshowed a very y small pulling g force, which con nquered the ffriction force between thee mecchanism and th he pipe wall. Then a pullin ng force to thee left was applied aat the left end o of the mechanism. The forcee grad dually increased up to 2.5k kg and the meechanism wass still locked at its position. p A larrger force was not tested forr safety reasons. Sim milarly, the ab bove test was repeated afterr the mechanism m was switched to o the other lock king condition,, and the mechanissm was locked d moving to the right. Thee sam me result was ob btained for thee pipes with in nner diameterss of Φ17 Φ mm and Φ220 mm.

(a) Locking L mechan nism test method d

Figure 15. Th he prototype off the direction--controllable loccking supporting mechanism: locking g mechanism (1)), cam (2), conneecting bar (3)

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ototype tests (b) Physical pro Figu ure 17. Locking mechanism m testts

5.2.2 Tractive Force Test of In nchworm In-Piipe Robot Basedd L Mechan nism on the above Locking on-controllablle locking mechanism was The directio applied to develop d a miccro inchworm m in-pipe robo ot, as shown in Fig gure 18. The tractive forcee of the robot was tested with the t test metho od shown in Figure 19 and d the correspondin ng test bed sh hown in Figu ure 20. A tracction cable is conn nected to the end of the ro obot, and a loaad is applied on th he traction cab ble by a load aadjusting devicce. A tension senso or is placed between b the ro obot and the lload. The load ad djusting devicce is shown in i Figure 21. The screw is useed to increase or decreasee the pressurre to adjust the friction betweeen the wind ding reel and d the d to the screw by a friction pad. The contactorr is connected spherical hin nge. Through h this device,, continuous and variable load ds can be prod duced to test the t traction ab bility of the robott. The robot’ss velocity is measured by y the velocity deteecting device shown in Fig gure 22. When n the robot movess and pulls th he traction caable, the win nding wheel rotatess and the rotational speed is measured by y the coaxially loccated photoeelectric encod der. The rotaation speed of thee winding wh heel can be cconverted into o the robot’s veloccity by simplee calculations. The tractive fforce is measured by the tensiion detecting device show wn in T cable twin ned on the winding w reel goes Figure 23. The through the tension t dynam mometer in a zigzag z way. When W the robot moves, m the cab ble will exertt pressure on n the middle pulleey and comp pressive stresss will occur. The tractive forcee of the robot can be obtain ned by the ten nsion dynamometeer measuring the t compressive stress.

Figure 18. Inchworm I in-p pipe robot baased on direcctioncontrollable lo ocking mechan nism: motor ffor driving loccking mechanism 1 (1), ( locking mecchanism 1 (2), leead screw for drriving robot (3), mottor for driving robot (4), lock king mechanism m (5), motor for driviing locking mecchanism (6)

During the teests, all the motors are giveen maximum rated r voltages to en nsure maximu um output pow wers. In the teest of moving to th he right, at firsst, the locking g direction is set s to the left by controlling the t motors of o the supporrting mechanisms,, in which the screw nut is driven d to the right r end. As the screw nut haas a self-lockin ng feature, it will stay.

Figu ure 19. The test method m of tracttive force of the robot based on n direcction-controllab ble locking mech hanism

Figu ure 20. The test bed b of tractive fforce of the robot based on thee direcction-controllab ble locking mech hanism: in-pipee robot (1), pipee (2), the t slope adjustting device (3), p protractor (4), wire w (5), tension n sensor (6), traction cable c (7), wheels (8), encoder fo or speed (9), thee d adjusting devicce (10) load

Figu ure 21. The load adjusting devicce

Figu ure 22. The veloccity detecting device

ure 23. The tensiion detecting deevice Figu


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At its position after the motor is power off. Then, the telescopic driving mechanism starts to work. Since the robot is locked moving to the left, the telescopic driving mechanism extends, to push the right supporting mechanism to the right, and shortens to pull the left supporting mechanism to the right. In this way, the robot can move to the right step-by-step using the input voltage shown in Figure 24.

Figure 24. Input voltage of the motor when testing the traction

The robot motions are recorded under different loads. Figure 25 shows the maximum tractive force that the robot achieves when the velocity is 10mm/s, and Figure 26 plots the robot velocity under different loads. From Figure 26, the following results can be achieved: the maximum velocity of the robot is 10mm/s while the corresponding maximum tractive force is 12N; when the traction load is larger than 12N and further increases, the velocity decreases gradually. Besides, for different gradients of the climbing pipe, the results of the tractive force have slight differences because of the lightweight design of the robot. In the experiments, the pipe is filled with atmosphere and other filling materials such as water, oil, are not considered. In some cases, the pressure and the viscosity of the material could influence the performance of the robot.

6. Conclusion (1) The unidirectional locking mechanism is utilized in the telescopic in-pipe robot, which is not limited by the maximum static friction force between the robot and the pipe, thus making it possible to enlarge the tractive force. This design is very meaningful because the maximum static friction force is relatively small for the micro in-pipe robot. (2) Three types of unidirectional locking mechanism were analysed: the locking mechanisms that use supporting legs, inclined planes and cams. Based on the analysis, a new type of locking mechanism, which has a larger adapting range of pipe diameter and controllable locking direction, was proposed. It can address the problem of the traditional unidirectional locking mechanisms, which cannot move both forward and backward. Moreover, it enables the robot to fully lock at a certain place, making fixed-point operation in the pipe possible. (3) The locking conditions of the cam locking mechanism were presented, and a prototype of the new direction-controllable locking mechanism was developed. Tests showed this mechanism fitting a pipe with diameter of Φ18 mm had a tractive force up to 25N, and the adapting range of pipe diameter was Φ17~Φ20 mm. Furthermore, it was able to lock in both directions. These advantages can significantly improve the performance of the micro in-pipe robot in practical applications. (4) Large tractive force can be achieved by applying this kind of self-locking mechanism in the Inchworm micro in-pipe robot. Experimental results showed that our robot had a maximum tractive force of 12N when the maximum velocity of the robot was 10mm/s. The reason limiting the further increase of the tractive force lies only in the driving capability of the motor, instead of the friction force between the robot and the pipe wall. 7. Acknowledgments

Velocity (v /(mm/s) )

Figure 25. Maximum traction when velocity is at its maximum

10 8 6 4 0

5 10 15 20 Tractive force of the in-pipe robot (F/N)

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Figure 26. The tractive force versus velocity of the in-pipe robot based on the direction-controllable locking mechanism 10 Int J Adv Robot Syst, 2014, 11:174 | doi: 10.5772/59309

This work was funded by a grant from the National High Technology Research and Development Program of China (863 Program) (No. 2007AA04Z256) and partly funded by the National Natural Science Foundation of China (51205400). This article is a revised and expanded version of the paper entitled "Development of Controllable Two-way Self-locking Mechanism for Micro In-Pipe Robot" presented at Intelligent Robotics and Applications, Third International Conference, ICIRA 2010, Shanghai, China, November 10-12, 2010.

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