DESIGN OF A MANIPULATOR PROTOTYPE FOR ON-ORBIT MODULE REPLACEMENT

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1 DESIGN OF A MANIPULATOR PROTOTYPE FOR ON-ORBIT MODULE REPLACEMENT Hu Bngshan, Tang Pn, Chen Meng, Zou Huawu, Han Langlang Shangha Insttute of Aerospace Sstems Engneerng, No. 3888, Yuanjang Road, Mnhang Dstrct, Shangha, Chna, Emal:hubngshan@gmal.com ABSTRACT Performance and lfespan of spacecraft can be enhanced and prolonged b space manpulator s on-orbt servce. In ths paper, a space manpulator prototpe s desgned for module on-orbt replacement, and the knematcs model of the manpulator s deduced. Knematcs, dnamcs and control algorthm co-smulaton model s set up for task smulaton. Smulaton results show that the manpulator prototpe n ths paper can complete the module replacement task. 1. INTRODUCTION In mcrogravt, hgh temperature range and hgh radaton space envronments, operatng capact and range of the astronauts are lmted, but the space manpulator could work well. So, there are mportant sgnfcances n economc and safet aspects b usng space manpulator to replace astronauts completng space operaton, and the space manpulator has attracted more and more attenton of aerospace department all over the world. Manpulators puttng nto orbt applcaton nclude the Shuttle Remote Manpulator Sstem (SRMS), the Space Staton Remote Manpulator Sstem (SSRMS) and the Japanese experment module manpulator sstem (JEMRMS). These manpulators lengths are generall more than meters, and the load capact are more than 7 tons, even up to hundreds of tons. Ther man functon s to carr large loads. Whle the small dexterous space manpulator can acheve hgh tp control accurac, so t can complete fne operaton, such as small module replacement, pushng button and so on. In ths feld, the Unted States, German and Japan had carred out n orbt valdaton, such as ETS-VII, orbtal express and so on [1][2][3]. As mentoned before, small dexterous space manpulators can be placed nsde the space staton cabn, satellte or space exposure platform, to complete fne manpulaton tasks ndependentl or assst astronauts do the tasks. In ths paper, a small dexterous space manpulator s desgned to complete the module replacement task. To verf the feasblt of the robot arm to complete the task, frstl the knematcs model of the robot arm s establshed, and the task smulaton model s set up. The knematcs, dnamcs characterstcs of the manpulator n the whole task process are smulated and analsed. 2. TASK ANALYSIS As shown n Fg.1, the dexterous manpulator prototpe desgned n ths paper s mounted on the spacecraft smulator, and the process of completng the module replacement task s as follows: frstl, the dexterous manpulator arm moves and extracts the old module from an old module box, and then the old module s put nto the module store. Secondl, a new module s extracted from the module store, and nstalled nto the old module box. Fgure 1 Flowchart of module replacement The nertal coordnate sstem orgn O 0 s one vertex of the spacecraft smulator. Axs O 0 X 0 and axs O 0 Y 0 are parallel to the spacecraft's two sdes respectvel. The axs O 0 Z 0 s perpendcular to the nstallaton surface of the module store and devates from the spacecraft smulator. The poston of the module store n the nertal coordnate sstem s: 400mm X 00mm 200mm Y 500mm (1) 0mm Z 185 mm The poston of the old module n the nertal coordnate sstem s: -185 mm X 0 mm -07 mm Y -327 mm (2) -250 mm Z 0 mm The dexterous manpulator s reachable space must obtan upon postons as shown n formula (1) and (2). 3. THE SMALL DEXTEROUS MANIPULATOR PROTOTYPE 3.1 PROTOTYPE CONFIGURATION It s feasble to complete the module replacement task 15th European Space Mechansms & Trbolog Smposum ESMATS 2013 Noordwjk, The Netherlands, September 2013

2 shown n Fg 2 usng a DOF manpulator sstem, whch s composed of shoulder aw jont, shoulder roll jont, shoulder ptch jont, elbow ptch jont, wrst ptch jont, wrst roll jont, one end effector and manpulator control sstem. To decrease the envelope sze, the elbow jont uses offset laout and other jonts use collnear tpe laout. Fgure 2 Confguraton of the manpulator 3.2 PRECISION ANALYSIS AND DESIGN To fnsh the fne operaton, control accurac of the manpulator s tp should be as hgher as better. But consderng the dffcult of mplementaton, the absolute poston accurac requrements for the manpulator can be reduced through ncreasng capture tolerance of the end effector. Consderng the arm length, module sze and other factors, the capture tolerance of the end effector s desgned to 15 mm/1.5. Moreover, a hand-ee camera and a sx axs force sensor are mounted on the end of the arm, and these sensors are used for provdng feedback nformaton to mprove the accurac of closed loop control. When the absolute poston accurac s 5 mm and the atttude accurac s 0.5, the manpulator can capture the replaceable module and complete the module replacement task. The man sources of manpulator end errors are the lnk length error lnk twst error, lnk offset error and jont angle error. Except the jont angle error, the former three errors manl come from producton and assemblng, and these errors can offset b calbraton. Accordng to manpulator s forward knematcs, the jont angle error s nfluence on the manpulator s tp precson could be obtaned. Through calculaton, when the jont s accurac s better than 0.05, the manpulator s absolute accurac can be guaranteed. In the desgn of the jont, we use a harmonc reducer whch has a small gap and hgh precson, and a hgh accurac poston sensor s used, to meet the requrement of the jont poston accurac. 4. ESTABLISHMENT OF SIMULATION MODEL 4.1 KINEMATICS MODEL Accordng to the dexterous manpulator s confguraton, a lnk coordnate sstem s establshed as shown n Fg.3. DH parameters of the manpulator can be gotten as shown n Tab.1. Fgure 3 Lnk coordnate of the manpulator Table 1 DH parameters of the manpulator θ Lnk α -1 a -1 (mm) (Intal D angle) (mm) θ1 (0 ) θ2 (90 ) θ3 (-90 ) θ4 (0 ) θ5 (-90 ) θ (0 ) The generalzed coordnate transformaton matrx between two adjacent lnk +1 and s, -1 T = Rot( Z, q ) Trans(0,0, d ) Trans( a,0,0) Rot( X, a ) È cosq -snq 0 a- 1 snq cosa cosq cosa -sna -sna d = snqsna- 1 cosqsna- 1 cosa- 1 cosa- 1d Î (3) Substtutng the DH parameters n Tab.1 nto the Eq (3) respectvel, the transformaton matrx -1 T and the forward knematcs for the manpulator s, Ènx ox ax px 0 n o a p T = nz oz az pz Î = ( q ) ( q ) ( q ) ( q ) ( q ) ( q ) T T T T T T INVERSE KINEMATICS MODEL are gotten, Usng the manpulator s nverse knematcs, the demand jont angle q can be derved from the manpulator s tp poston and posture ( xzaq,,,,, b). The transformaton matrx of the tp pont relatve to the base coordnate of the manpulator s, Ènx ox ax px 0 n o a p T = nz oz az pz (5) Î È c( b)c( q) -cbsq sb x sasbcq + casq - sasbsq + cacq -sacb = - casbcq + sasq casbsq + sacq cacb z Î In Eq (5), c refers to cos, s refers to sn. To make Eq (4) equal to Eq (5), usng separate varable method, q and 2 (4)

3 q are calculated drectl, q = atan2( ab, ) + atan2( ± a + b -d4,-d4) (8) q2 = acos( n zsn( q) + oz cos( q)) or q2 =- acos( n zsn( q) + ozcos( q)) (9) a= np x x + np + np z z -dna 7 z z -dna 7 -dna 7 x x (10) b= op x x + op + op z z -doa 7 z z -doa 7 -doa 7 x x(11) Substtutng q 2 and q to Eq (4), the other jont angles can be gotten, and the manpulator s nverse knematcs s establshed. 4.3 PATH PALNNING ALGORITHM In the process of extractng or nsertng the module, the trajector of the manpulator s tp must be perpendcular to the module slot. In the other poston, there aren t such specal requrement and the tp of the manpulator just move from one pont to another pont freel. Accordng to the above requrements, n the process of extractng or nsertng the module, the path plannng algorthm based on the tp poston n Cartesan space s used. In the other poston, the path plannng algorthm based on the poston n jont space s used to reduce the amount of calculaton. The flow of the jont space path plannng algorthm s as followng: frstl, the ntal angle q and end angle 0 q f ( =1,2 ) of each jont s obtaned b usng the nverse knematcs formulas accordng to the manpulator tp s startng pose m1 and endng pose m2. It s supposed that ever jont has the same rotatng tme t f, so n tme t, each jont s angle can be gotten accordng to the fve order polnomal nterpolaton [4][5], q () t = a + at+ a t + at + a t + at (12) Constrants of the Eq (12) s, ( ) Ï q 0 = q0 = 0 q( tf ) = q f q& (0) = 0 Ì q& ( tf ) = 0 q&& ( 0) = 0 q&& Ó ( tf ) = 0 (13) Substtutng formula (13) to formula (12), we can get jont angle q () t, whch s used for jont poston s real tme control. The flow of the Cartesan space path plannng algorthm s as followng: frstl, the manpulator s pose x(t) (t refer to tme) between the startng pose m1 and endng pose m2 s gotten usng the fve order polnomal nterpolaton, then each jont s angle q () t s obtaned b the arm s nverse knematcs formulas, and q () t s each jont s control command. 4.4 SIMULATION MODEL To carr out knematcs, dnamcs and control arthmetc co-smulaton durng the whole module replacement process, the manpulator s nverse knematcs soluton program and path plannng algorthm s realzed usng C++ software. The cosmulaton model s n Fg.4, n whch ZT s path plannng module, and FT s nverse knematcs soluton module. The dnamcs model s establshed usng ADAMS software as showng n Fg. 5. All of these modules are connected b ADMAS Mechansm. Fgure 4 Co-smulaton model Fgure 5 The manpulator s dnamcs model Some hpotheszes are gven before smulaton: each jont s maxmal speed s 0.35 o /s, the module s weght s 2 kg, the maxmal contact force between the manpulator s tp and the module s 20N. 5. SIMULATION RESULTS 5.1. REACHABLE SPACE ANALYSIS The manpulator tp s reachable space s analsed b the

4 knematcs, and ts reachable space s as showng n Fg.. The reachable space s an ellpsod, whose long axs s 1590 mm, and short axs s mm. It s obvous that the module s poston s contaned n the reachable space, so the robot arm can reach the old module and the module store. Fgure 7 Trajector relatve to spacecraft smulator Fgure The manpulator s reachable space 5.2. MOTION TRAJECTORY ANALYSIS The module replace task smulaton process s as follows: frstl, the manpulator moves from ntal poston to the old module s above place, and the manpulator s tp do a lnear moton to capture the old module, after that, the manpulator s tp do a lnear moton agan to the opposte drecton to extract the old module. Secondl, the manpulator carrng the old module moves to the above place of the module store, and nsert the old module to a slot. The manpulator tp s trajector relatve to spacecraft smulator s shown n Fg.7. In Fg.7, mb1 s the manpulator s ntal pont, mb2 s the pont above the old module, mb3 s the pont where the manpulator capture the old module, mb4 s the pont where the manpulator has extracted the old module, mb5 s the pont of the slot, mb s the pont that the manpulator has nsert the old module nto the slot. From Fg.7, we can fnd that the path plannng algorthm s feasble, and there sn t nterference between the manpulator and the spacecraft smulator, so the manpulator has the ablt of completng the module replacement task JOINT TORQUE ANALYSIS In the module replacement task, each jont torque s shown n Fgure 7, and the maxmum torque s 107.5Nm whch appears n the shoulder ptch jont. The maxmum torque s smaller than the jont s maxmum output torque. Fgure 8 Output torque of each jont durng module replacement. CONCLUSION In ths paper, a small dexterous manpulator prototpe s desgned, and ts knematcs, dnamcs and control cosmulaton model s establshed to analse the whole module replacement task. Smulaton results show that, the module s n the reachable space of the manpulator. There sn t nterference between the manpulator and the spacecraft smulator, the path plannng algorthm n ths paper s feasble. The maxmum torque n the replacement process s smaller than the jont s maxmum output torque, so the manpulator n ths paper has the ablt of completng module replacement task. 7. REFERENCES 1. James, F. A., Peter,D. S. (1993). The Development Test Flght of the Flght Telerobotc Servcer: Desgn Descrpton and Lessons Learned. IEEE Transactons on Robotcs and Automaton. 9(3), Cargnan, C. (199). A Decouplng Inverse Knematcs Algorthm for the Ranger Dexterous Manpulator, IEEE Int. Conf. on Robotcs and Automaton, Mnneapols, Unted State.

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