Preparatory object rotation as a human-inspired grasping strategy

Transcription

1 th IEEE-RAS Interntionl Conference on Humnoid Roots Decemer ~ 3, / Dejeon, Kore Preprtory oject rottion s humn-inspired grsping strtegy Lillin Y. Chng, Grth J. Zeglin, nd Nncy S. Pollrd The Rootics Institute, Crnegie Mellon University, Pittsurgh, Pennsylvni 3, USA {lillinc,grthz,nsp}@cs.cmu.edu Astrct Humns exhiit rich set of mnipultion strtegies tht my e desirle to mimic in humnoid roots. This study investigtes preprtory oject rottion s mnipultion strtegy for grsping ojects from different presented orienttions. First, we exmine how humns use preprtory rottion s grsping strtegy for lifting hevy ojects with hndles. We used motion cpture to record humn mnipultion exmples of prticipnts grsping ojects under different tsk constrints. When sliding contct of the oject on the surfce ws permitted, prticipnts used preprtory rottion to first djust the oject hndle to desired orienttion efore grsping to lift the oject from the surfce. Anlysis of the humn exmples suggests tht humns my use preprtory oject rottion in order to reuse prticulr type of grsp in specific cpture region or to decrese the joint torques required to mintin the lifting pose. Second, we designed preprtory rottion strtegy for n nthropomorphic root mnipultor s method of extending the cpture region of specific grsp prototype. The strtegy ws implemented s sequence of two open-loop ctions mimicking the humn motion: preprtory rottion ction followed y grsping ction. The grsping ction lone cn only successfully lift the oject from -degree region of initil orienttions ( of tested conditions). Our empiricl evlution of the root preprtory rottion shows tht even using simple open-loop rottion ction enles the reuse of the grsping ction for 3-degree cpture region of initil oject orienttions ( of tested conditions). I. INTRODUCTION Rootic systems hve yet to mtch humns in dexterity for generl tool cquisition nd mnipultion. While rootic mnipultors cn e progrmmed to grsp ojects under structured conditions, humns cn esily dpt their mnipultion strtegies to novel tsk conditions. Humns typiclly use few prototypicl reching nd grsping ctions to pick up ojects. In dily life, humns must grsp ojects from vriety of initil configurtions, including mny which re not well-mtched to fmilir grsps. In our oservtion of humn grsping, we notice tht humns seldom grsp n oject directly from its exct presented configurtion. Insted, humns often mnipulte the oject to djust its configurtion prior to grsping. For exmple, person might drg mug on tle closer to the ody y pulling on the hndle with non-prehensile contct. In nother scenrio, to grsp pen from tle surfce, the fingertips my e used to quickly pivot the hndle to orient the tip for writing. These re exmples of wht we refer to s preprtory mnipultion. Preprtory mnipultion occurs whenever the interction first djusts the oject configurtion on the supporting surfce prior to the finl grsp (Fig. ). This pproch tkes dvntge of the oject s movility on the surfce to effectively chnge the intermedite tsk prmeters. In such Fig.. Preprtory mnipultion djusts the oject configurtion in the workspce prior to grsping. One exmple of preprtory mnipultion is preprtory oject rottion, shown here for hndled pn, where the oject orienttion is djusted efore grsping. cses, the ction in nticiption of grsping tsk includes chnges in the oject configurtion in the environment prior to grsping, in ddition to the mnipultor reching movement nd hnd pre-shping. In this pper, we exmine one type of preprtory motion s mnipultion strtegy used y humns: rotting hndled oject using pushing contct prior to lifting grsp. We show tht the preprtory rottion of n oject llows for single grsp ction to e reused for much wider rnge of oject configurtions. We first study the preprtory rottion strtegy used y humns. Then we implement the strtegy on n nthropomorphic root mnipultor to investigte how preprtory rottion cn extend the effective workspce of well-tuned grsping ction. II. RELATED WORK This work utilizes simple pushing strtegy to rotte n oject prior to grsping it. The enefits of utilizing the support surfce s pssive mnipultor to increse workspce nd lod limits re fmilr from the push-plnning literture []. Much of tht work is concerned with the utomtic plnning of push mnipultions to reorient ojects on supporting surfce with either known or uncertin contct conditions. For exmple, Lynch nd Mson [] explore the conditions for complete control of rigid sliding oject while pushing to n ritrry pose on plne. In contrst, our emphsis is on the identifiction of specific heuristic ction sequences which humns choose in response to prototypicl situtions. Thus, rther thn modeling the generl push dynmics, we hve deliertely chosen n exmple with simple contct conditions nd high error tolernce which dmits trivil d-hoc solution to the push-plnning prolem. Plnning methods for pushing mnipultion hve een demonstrted on other humnoid pltforms in recent work [3 ]. Our focus is on the enefits of using pushing to reuse grsping strtegies for wide rnge of oject orienttions, rther thn on the plnning of the pushing ction lone //$. IEEE

2 The strtegy is lso simple exmple of sensorless mnipultion. It does not mesure oject stte, ut ssumes the oject lies within ounded set of initil configurtions. It lso uses contct strtegy chosen to reduce the uncertinty of the oject stte until it lies within the much smller set of grsple sttes. Agin, this type of strtegy hs een wellstudied in the motion plnning literture [], ut our emphsis is on identifying smll sets of primitives which cn e used to quickly construct dequte solutions to typicl grsping prolems rther thn generl optiml plnning. Relted literture lso include studies of humn mnipultion from the psychology nd motor control community. Rosenum nd collegues hve investigted the selection of hnd grips for vriety of hndle trnsport nd hndle rottion tsks where cylinder is grsped nd then plced t different gol configurtions [7, ]. This hs led to severl inquiries testing how perceived end-stte comfort of tsk ffects the choice of initil hnd grsps in oject trnsport tsks [9, ]. In recent work, Rosenum nd Gydos [] present reltive cost pproch for evluting the movement costs for tsks such s oject-positioning nd oject-rottion. Our experiments re the first, to our knowledge, to explore wht we refer to s preprtory mnipultion strtegies in humn sujects. Deprting from the previous studies [, ] on mnipultion of lightweight ojects, we chose to investigte humn lifting of hevy ojects since we elieve preprtory mnipultion is most relevnt to more demnding tsks. III. PREPARATORY ROTATION IN HUMANS The ims of the humn study re to quntify the consistency of oject rottion s preprtory strtegy nd to exmine possile criteri tht the strtegy my optimize. In this section we review the sic experimentl protocol nd mjor findings. A complete, detiled description cn e found in Chng nd Pollrd [3]. A. Experimentl procedure Ten right-hnded dults ( mle, femle) volunteered for the study (ge =.7 ± 3.yers [men ± stndrd devition], height =.7 ±.9m, weight =.7 ±.9kg). All prticipnts signed informed consent forms pproved y the Institutionl Review Bord. Prticipnts performed the oject lifting tsks in kitchen counter top setting (Fig. ). The oject strt position ws locted on the right side counter re. The oject gol re ws locted.m to the left of the strt position nd ws mrked y circulr cover over the ottom left stove urner. At the strt position, the oject ws presented in one of eight possile orienttions, indicted y the direction of the oject hndle. In orienttion, the hndle directly fces the prticipnt (Fig. ). The two hndled ojects tested for ll ten prticipnts were plstic wter jug nd cooking pn, oth without lids (Fig. 3). The ojects were filled with wter for totl mss of 3.kg for the jug nd.kg for the pn. Kinemtic dt for the prticipnt nd ojects were recorded t Hz using Vicon cmer system (Vicon Motion. m oject gol oject strt prticipnt strt. m 3 oject Fig.. Generl lyout of the experimentl setting. () Prticipnts strted in stnding position fcing the countertop setting. Prticipnts trnsported the hndled ojects from the strt position to the gol position with their right hnd. () In ech tril, the hndled oject strted in one of eight orienttions defined y the hndle direction. In the figure, the hndled oject is in orienttion, where the hndle is fcing the prticipnt. z y cm cm z y cm Fig. 3. Prticipnts trnsported two different hndled ojects filled with wter, () n uncpped wter jug nd () cooking pn. The dshed line indictes the pproximte wter level. The coordinte systems locted t the oject hndles provide reference frme for mesuring the (c) hnd dorsum configurtion during the oject grsp. Systems, Los Angeles, Cliforni, USA). Motion of the full ody, including hnds nd fingers, ws trcked y reflective mrkers ttched to the prticipnt [3]. In ech lifting tsk tril, the prticipnt strted fcing the counter t distnce of.m from the counter edge, such tht the oject ws outside of rm s rech. For ll trils, the tsk ws to move the oject to the gol position without spilling ny wter. No specific oject orienttion ws required t the gol. Prticipnts were instructed to perform the trnsport tsk t self-selected speed with no time constrints. The experiment consisted of three phses to exmine three types of tsk scenrios (Fig. ). The first phse served s prctice trils to fmilirize the prticipnts with the tsk setting. Prticipnts were instructed to complete the trnsport tsk with no restrictions, using either or oth hnds s desired. The second phse (Fig. ) investigted unimnul lifting performnce in response to the different oject hndle orienttions. The purpose of this phse, s the min portion of the experiment, ws to oserve to wht extent oject preprtory djustment would e used s strtegy to compenste for chnges in oject orienttion. Prticipnts were instructed to complete the trnsport tsk using only their right hnd to contct the oject. Besides this unimnul constrint, there were no restrictions on the tsk performnce. The verl 9 cm y c x 7

3 jug lifts with unimnul constrint pn lifts with unimnul constrint orienttion orienttion tle surfce c Fig.. Prticipnts lifted ojects in three tsk scenrios. () Unconstrined prctice trils where imnul mnipultion ws permitted. () Unimnul constrint trils where only the right hnd ws permitted to contct the oject. (c) Oject motion constrint trils where sliding contct of the oject on the surfce ws not permitted. instructions did not suggest preprtory rottion or sliding motion s strtegy, s it ws our intent to oserve wht strtegies the prticipnts would nturlly select. The finl phse (Fig. c) tested how prticipnts would respond to different oject hndle orienttions in the sence of preprtory oject djustment on the counter surfce. The tsk performnces from this third phse provide reference mesurement for nlyzing the oject djustment motion in the second phse. Prticipnts were instructed to trnsport the oject using only the right hnd nd without sliding motion on the surfce prior to lifting the oject. B. Motion dt nlysis The key time point of oject lift-off from the surfce is the focus of our dt nlysis. This time point ws estimted utomticlly from the tril dt s the time frme when the oject s verticl motion exceeded cm. Four metrics (Tle I) were computed from the prticipnt s ody pose t the lift-off time frme: oject rottion, joint torque lod, grsp orienttion, nd grsp loction (descried in detil in Chng nd Pollrd [3]). Oject rottion ws mesured s the difference etween the initil oject orienttion nd the oject orienttion in the liftoff frme. We computed the solute mount of rottion so tht there ws no distinction etween clockwise or counterclockwise rottion. Upper ody joint torques were estimted from the lift-off ody pose using segment mss rtios nd center of mss loctions from nthropometry []. Given the fitted joint center loctions for the lower ck, shoulder, elow, nd wrist, joint torques were clculted from the lods due to distl lim segment weight nd the oject weight. The four joint torques were comined into single metric s the sum of squred joint torques. The configurtion of the hnd dorsum coordinte system ws then computed with respect to the reference coordinte system ttched to the oject hndle (Fig. 3). The grsp orienttion ws mesured s the ngle mgnitude of the single xis-ngle rottion which would lign the hnd dorsum coordinte system to the oject hndle coordinte system. The grsp loction ws mesured s the distnce etween the hnd jug lifts with oject motion constrint pn lifts with oject motion constrint orienttion orienttion tle surfce Fig.. Visuliztion of the ody postures t oject lift-off for smple suject. Poses re shown for the trils with initil oject orienttion nd, where the hndle fced wy from the prticipnt. () Poses for the trils with the unimnul constrint. () Poses for the trils with the dditionl oject motion constrint. The lift-off poses for the unimnul constrint trils were more similr to ech other ecuse the oject motion djusted the hndle direction towrd the prticipnt. When oject motion ws not permitted, the lift-off poses re more vried. In the most extreme cses for initil oject orienttion nd, the torso is tilted over the countertop nd the elow is held wy from the side of the ody. coordinte system origin t the proximl end of the third metcrpl nd the oject coordinte system origin t the se of the hndle (Fig. 3). The overll set of dependent vriles exmined in this study (Tle I) were the metric differences etween the unimnul constrint trils (second phse) nd reference oject motion constrint trils (third phse). The differences in metrics were computed etween mtched pirs of trils performed y the sme prticipnt on the sme oject for the sme initil hndle configurtion, with only difference in the tsk constrint. The differences were computed for the two sets (repetitions) of trils per oject in the unimnul constrint phse with respect to the single set of trils per oject in the motion constrint phse. We nlyzed the difference metrics with liner mixed-effect (LME) models [] (see [3] for detils). C. Humn study results When prticipnts were only restricted y the unimnul constrint, they often rotted the oject on the countertop surfce to new orienttion efore lifting nd trnsporting the oject to the gol. The resulting ody poses t oject lift-off (Fig. ) were similr in terms of the upright torso orienttion nd oject hndle directed towrd the prticipnt. In contrst, when the oject rottion strtegy ws precluded y the oject motion constrint, the resulting ody poses t

4 TABLE I RESPONSE METRICS OBSERVED FOR EACH TRIAL WHERE THE PARTICIPANT LIFTED AND TRANSPORTED A HANDLED OBJECT FROM THE PRESENTED OBJECT START ORIENTATION. THE DATA ANALYSIS MODELED THE DIFFERENCES IN EACH OF THE METRICS BETWEEN THE UNIMANUAL CONSTRAINT TRIALS AND OBJECT MOTION CONSTRAINT TRIALS. Oject lift-off posture metric Oject rottion Joint torque lod Grsp orienttion Grsp loction Computtion notes Asolute difference etween lift-off hndle ngle nd initil hndle ngle Sum of squred torques over torso, shoulder, elow, nd wrist Angle of single rottion etween hnd frme nd oject frme Distnce etween origins of hnd frme nd oject frme oject lift-off were more vried in the torso orienttion nd rm configurtion. For trils where the oject hndle fced wy from the prticipnt, the torso ws often tilted over the countertop surfce with the elow extended wy from ody to chieve the grsp of the oject hndle. One prticipnt chose to ort one lifting tril in the oject motion constrint phse fter grsping nd ttempting to lift the pn from hndle orienttion without oject motion long the surfce. Under the unimnul constrint (second phse), the mount nd direction of oject rottion vried depending on the initil hndle orienttion, s did the oject orienttion t the lift-off time frme (Fig. ). In generl, the selected lift-off orienttions were clustered in cpture region centered round hndle orienttions nd, on the prticipnt s right side. Thus, the scle for the hndle orienttion vrile is centered t the midpoint ngle etween orienttions nd for the sttisticl nlysis [3]. The differences in oject rottion (Fig. 7) etween the unimnul constrint trils nd oject motion constrint trils were the lrgest for s nd, which re opposite the cpture region t orienttions nd. The LME nlysis found the qudrtic trend of rottion mount vs. initil ngle to e sttisticlly significnt (p <.) [3]. The difference in the sum of squred joint torques lso exhiited significnt qudrtic trend with initil hndle orienttion (p <.) (Fig. ). The joint torque metric for the oject motion constrint trils ws greter thn those for the unimnul constrint trils, s seen from the primrily nonnegtive differences (Fig. ). We tested the liner correltion etween the difference in oject rottion nd the difference in the joint torque metric using the Person correltion coefficient. The correltion ws sttisticlly significnt (α =.) for of the prticipnts for the jug lifts nd for ll prticipnts for the pn lifts. Hnd grsp configurtion differences (Fig. 9) lso incresed when the initil hndle directions fced wy from the prticipnt. There ws significnt qudrtic trend for the grsp orienttion difference (p =.). For the grsp orienttion, there is lso significnt liner orienttion effect (p <.) which is seen in the symmetry of the men regression curve (Fig. 9). The symmetry is due to the unimnul constrint of right-hnd lifting. The grsp differences re smller for hndle orienttions to, where the grsps were reched on the right hnd side. Hndle orienttions to required the right hnd to cross the ody to rech the left jug rottion 3 7 jug hndle ngles t lift off initil ngle pn rottion 3 7 pn hndle ngles t lift off initil ngle pn initil rottion ngle ngle 3 7 t liftoff Fig.. Visuliztion of the oject rottion prior to lift-off for the different s. () Initil nd lift-off orienttions for the unimnul constrint trils for one prticipnt. () Oject lift-off ngles for the unimnul constrint phse for ll trils for ll prticipnts. oject rottion difference [degree] jug rottion efore lift off oject rottion difference [degree] pn rottion efore lift off oject rottion difference [degree] S S S3 S S S S7 S S9 S oject rottion efore lift off jug pn Fig. 7. Difference in oject rottion versus. () Individul prticipnt results. () Men regression curve determined from the LME regression model. The differences re the oject rottion mounts in the two unimnul constrint trils minus the oject rottion in the oject motion constrint tril. The mount of oject rottion prior to lift-off increses s the hndle orienttion moves further from the seline orienttion nturlly preferred for the lifting tsk. side for the oject motion constrint trils, which led to lrge

5 difference in squred joint torques [(N m) ] 3 torque metric for jug lifts difference in squred joint torques [(N m) ] 3 torque metric for pn lifts difference in squred joint torques [(N m) ] torque metric for oject lifts 3 jug pn Fig.. Difference in the sum of squred joint torques versus initil hndle orienttion. () Individul prticipnt results. () Men regression curve determined from the LME regression model. The differences re the torque metrics in the two unimnul constrint trils sutrcted from the torque metric in the oject motion constrint tril. The torque metric lso follows qudrtic trend, similr to tht for the oject rottion metric. difference in orienttion [degree] grsp orienttion for jug lifts difference in loction [cm] grsp loction for jug lifts difference in orienttion [degree] c grsp orienttion for pn lifts difference in loction [cm] grsp loction for pn lifts 3 3 difference in orienttion [degree] difference in loction [cm] grsp orienttion for oject lifts jug pn grsp loction for oject lifts jug pn Fig. 9. Difference in grsp s represented y the hnd dorsum orienttion nd loction with respect to the oject frme. The differences re the grsp metrics in the two unimnul constrint trils sutrcted from the grsp metric in the unimnul nd oject motion constrint tril. () Individul prticipnt results for grsp orienttion difference. () Men regression curve determined from the LME regression model for the grsp orienttion difference. (c) Individul prticipnt results for the grsp loction difference. (d) Men regression curve determined from the LME regression model for grsp loction difference. differences in grsp orienttion. Similrly, there ws significnt qudrtic trend for the grsp loction difference (p <.). The qudrtic curvture ws higher for the pn lifts thn for the jug lifts. This ws due to the length of the pn hndle, which llowed prticipnts to grsp the oject t severl different positions. For some prticipnts, the grsp loction for lifting the pn chnged drmticlly in the oject motion constrint phse where no sliding llowed. Insted of grsping close to the hndle end s they did in the unimnul constrint phse, they lifted the pn with grsp closer to the center of the pn when the d hndle ws further from rech. The use of preprtory rottion strtegy when it ws permitted in the unimnul constrint phse might e due to the preference to grsp the oject hndle t position requiring less rm rech, even though other grsps were fesile when oject motion on the surfce ws not permitted. D. Oservtions on humn strtegy Overll, we hve found tht the preprtory rottion of hevy ojects increses with the chnge in hndle orienttion wy from the preferred cpture region. When prticipnts re instructed not to pre-rotte the oject prior to lift-off, they re still le to successfully complete the oject trnsport tsk. However, without djusting the oject orienttion prior to lifting, prticipnts performed the lifting tsk with different ody poses with tilted torsos nd extended elow positions in order to rech the oject hndle. Our results suggest tht the preprtory oject djustment my e desirle ecuse it llows the oject lift to e performed with lower joint torque lod in the upper ody nd/or with preferred grsp of the oject hndle. Our experiments investigted the preprtory oject djustment in the specific context of right-hnded lifting nd lterl trnsport cross the ody. We focused on the effect of the initil oject orienttion on the selected ody posture t oject lift-off, ut severl other fctors my ffect the preprtory mnipultion. We would expect similr djustment strtegies in other tsks with different constrints. For exmple, chnging the loction of the gol my result in shifted cpture region for the lift-off hndle orienttions. Other fctors include whether the right or left hnd mnipultes the oject, timing restrictions for the tsk completion, oject weight, nd oject hndle geometry. Our nlysis focused on the difference in performnce in terms of metrics computed from lift-off time frme, which is ssumed to e representtive, qusi-sttic snpshot of the performnce. Future work investigting preprtory mnipultion could nlyze the dynmics of the motor ehvior over the entire tril durtion. IV. IMPLEMENTATION ON AN ANTHROPOMORPHIC ROBOT MANIPULATOR Inspired y the humn use of preprtory rottion in lifting hndled ojects, we implemented the strtegy on n nthropomorphic root mnipultor. The preceding results from the humn suject experiments suggest tht the preprtory rottion strtegy my e preferred to direct grsping for multiple resons, such s grsp reuse or decresed joint torque. Our gol for the root implementtion ws to focus on how preprtory rottion cn enle grsp reuse in order to extend the effective workspce of grsping prototype. A single welltuned grsping sequence my only successfully lift specific oject from smll set of initil orienttions. Preprtory rottion would reconfigure the oject such tht the susequent execution of the single grsping sequence is reusle for wider rnge of initil orienttions.

6 . gol strt pn oject..9. gol strt..9. gol strt..... PA rm nd Shdow Hnd... strt hndle ngles trnsport.. pproch Fig.. Schemtic of the two open loop ctions implemented on the root mnipultor. () In the preprtory rottion ction, the hnd trces circulr rc round the strting position to rotte the pn using single index finger contct with the edge of the hndle. () In the grsp ction, the hnd moves in reltively stright pth cross the tle to first pproch the oject from the right, grsp the hndle, nd then trnsport the oject left to the gol. grsp. Fig.. Lyout of the mnipultor experimentl setting. () The right-hnded nthropomorphic root rm trnsported hndled pn from the strt position to the gol position on tle setting. () The pivot point of pn oject is plced t the strt position. The pn strted in one of orienttions defined y the hndle direction. The orienttions re nominlly spced y degrees. Our nthropomorphic mnipultor system consists of Mitsuishi PA- 7-DOF mnipultor with -DOF Shdow Hnd C3 end-effector (Shdow Root Compny, London, UK). Fom pdding is ttched to the Shdow Hnd to modify the plm geometry. The tsk specified for the root implementtion mimicked the tsk conditions in the humn studies (Fig. ). The oject strting position on the tle is locted.9m in front of the mnipultor se. The oject gol position is locted on the tle.3m to the left of the strt position. As in the humn studies, the oject could strt in one of severl configurtions. Twenty-four hndle directions were selected to smple the full 3-degree orienttion spce t intervls of degrees. The oject in the root experiments ws smll cooking pn with hndle. The pn ws empty nd hd totl mss of.kg. Opticl mrkers were ttched to the pn in order to trck the oject configurtion using the cmer system descried in Section III-A. A. Open loop ction sequences The grsping strtegy using preprtory rottion ws implemented s two mnully-progrmmed open-loop ctions (Fig. ). One ction is the grsping ction for lifting nd trnsporting the pn y its hndle (Fig. ). The other ction is the preprtory rottion ction for reconfiguring the hndle orienttion prior to grsping (Fig. ). The grsping ction ws intended to mimic the underhnd grsp of the pn oserved in the humn study trils with the right-hnd unimnul constrint. The intended hndle orienttion for the grsping ction ws the direction fcing towrd the mnipultor nd slightly towrd the right, s oserved in the humn exmples (Fig. ). The grsping ction is sequence of three motion components: n pproch motion, the grsp motion, nd the trnsport motion (Fig. ). In the pproch motion, the Shdow Hnd mintins relxed open-hnd pose. During the grsp motion, the PA- rm configurtion remins fixed while the hnd s finger joints close round the hndle. The hnd then mintins tightly- closed pose during the trnsport motion while the PA- rm moves to lift nd trnsport left to the gol position. All three motion components were mnully progrmmed for the specific oject, intended hndle orienttion, strt position, nd gol position. Initil hnd contct with the oject often occurred during the pproch motion, when the plmr side of the fingers contct the right edge of the hndle. The rottion ction ws implemented s pushing motion using single-finger contct with the oject to rotte the cooking pn round its nturl pivot point (Fig. ). The index finger ws flexed 9 degrees such tht it pointed norml to the plm. The thum nd other three fingers were extended in the plne of the plm, which remined prllel to the tle during the rottion ction. The index fingertip first pproches the oject strt position long n initil stright segment. Then the index fingertip trces circulr rc of 3 degrees in clockwise direction round the oject strt position nd ends within the intended grsp cpture region. Trcing the full 3 degree rc pth llows the open-loop rottion ction to e executed identiclly regrdless of the initil pn hndle ngle. The degree gp in the circulr pth ws deliertely designed to void contcting the oject if the hndle strts within the originl cpture region of the grsping ction lone. B. Empiricl evlution To mesure how well the preprtory rottion enles reuse of the grsping ction, we compred the grsping ction lone to the sequence of the preprtory rottion followed y grsping ction. The two methods were ech tested on the different s in set of consecutive trils. The mnipultion ws considered successful if the grsp lifted the pn off the tle surfce nd trnsported the pn to the gol position. In the consecutive trils using the grsping ction lone, the mnipultor successfully grsped nd trnsported the pn to the gol position for of the initil hndle ngles (Fig..) The empiricl cpture region of the grsping ction lone ws degrees. The grsp component works est for the two hndle ngles in the center of the cpture region. For the two outer ngles of the region, the hndle ws rotted towrd the center of the region y either clockwise push from the pproch component or counterclockwise push from the

7 grsp ction only. successful grsp filed grsp preprtory rottion nd grsp Fig.. Empiricl test results for two open-loop mnipultion strtegies for trnsporting the pn. The plotted hndle directions re coded ccording to whether the tril resulted in successful trnsport of the pn to the gol position. () Initil oject pose for the trils using only the grsp ction. The grsp sequence cn grsp the pn from of tested ngles for degree cpture region. () Initil oject pose for trils using the preprtory rottion ction efore grsping. The mnipultor ws le to grsp the pn from ll tested ngles using rottion pn pose fter mnipultion fter trnsport gol. pre grsp, fter rottion strt. Fig. 3. The oject hndle pose for the trils using preprtory rottion strtegy. After the rottion ction (right), the preprtory ction hs significntly reduced the uncertinty in oject pose y pushing the hndle into the grsping cpture region ( degree rnge in orienttion). After the grsping ction (left), the oject hs een trnsported consistently to the finl gol position ( degree rnge in orienttion). outstretched fingers in the grsp component. In contrst, when the preprtory rottion ction is used prior to the grsping ction, the mnipultor completed the trnsport tsk successfully for ll of the consecutive trils (Fig..) In ll ut one of the trils, the index finger mde contct with the hndle t some point during the rottion rc pth nd pushed the hndle clockwise. The one exception ws the second leftmost hndle ngle of the four orienttions lredy within the cpture region of the grsping ction lone. Becuse the hndle orienttion ws lredy centered in the originl grsping cpture region, the oject remined sttionry during the rottion ction, s intended y the design of the circulr push pth. The preprtory rottion ction consistently rotted the pn into the grsping ction s cpture region (Fig. 3). The hndle ngles fter rottion nd efore the grsp were ll within -degree rnge. The grsping ction further reduced the uncertinty in oject pose t the gol position. The hndle ngles fter the grsping nd trnsport ction were ll within -degree rnge. C. Kinemtic nlysis of lterntive grsp reuse strtegy In our implementtion we hve focused on the ide of reusing n entire grsp ction consisting of the pproch motion, grsping motion, nd trnsport motion components for oth the rm nd the hnd. Ech motion component ws mnully progrmmed for the specific tsk tested in our experiments. The grsping motion is the most criticl component tht is mnully-progrmmed, ecuse the hnd pose must e crefully tuned in order to securely grsp the thin hndle of the pn. During the grsping motion component, the mnipultor rm configurtion ws sttionry while the finger joints closed round the pn hndle. An lterntive scheme to reuse well-tuned grsp would e to re-pln the rm configurtion nd rm motion components without chnging the hnd motion during the grsp. In this wy, s long s the sme reltive configurtion is mintined etween the plm of the hnd nd the oject, the sme tuned finger motion for grsping cn e used with new rm configurtions. We investigted this lterntive scheme of reusing the hnd motion while re-plnning the rm motion using kinemtic nlysis. For ech possile hndle orienttion of the pn given the sme center position, we computed the required plm trnsform in the workspce required to mintin the sme reltive configurtion to the oject hndle. Given the desired plm trnsform, we serched for n inverse kinemtics solution of n rm configurtion which would chieve the desired end effector (plm) configurtion. The inverse kinemtics solution ws computed itertively using pseudo-inverse Jcoin method []. Becuse the inverse kinemtics solutions re highly-dependent on the initil guess for the itertive serch, multiple initil configurtions were tested for ech desired plm pose. The guesses were selected from dtse of precomputed rm configurtions discretized in joint spce. Any pre-computed configurtion whose end-effector position ws in the neighorhood of the desired plm position ws evluted s n initil guess. The results of the kinemtic nlysis (Fig. ) show tht the sme reltive trnsform etween the oject nd hnd is rechle for wide rnge of hndle orienttions much lrger thn the empiricl cpture region of the single grsping ction. However, out one-third of the possile hndle orienttions re still unrechle y the mnipultor. Thus, even under the considered lterntive grsping scheme, preprtory rottion could still e used to chieve successful grsps for unrechle hndle directions. V. DISCUSSION Our implementtion of specific open-loop rottion ws intended to test the merit of the preprtory rottion strtegy in terms of extending the effective grsp cpture region. Under this scheme, the pn my e rotted in lmost full circle for some initil configurtions. The length of the rottion for some

8 rechle trnsformed grsps rechle grsp unrechle grsp Fig.. Rechle oject hndle orienttions computed for scheme where the reching motion is re-plnned ut the reltive configurtion etween hnd nd oject during grsping is the sme. If novel reching motion is plnned for ech possile hndle orienttion, the mnipultor my e le grsp from wider cpture region then the single open-loop grsping ction. However, out one-third of the orienttions re still unrechle. of the hndle directions fcing towrd the mnipultor mkes the strtegy sensitive to the initil position of the pn. It is possile for the hnd to lose contct with the hndle during the rottion if the pivot point is not plced properly. The strtegy we implemented cn e extended y dding dditionl openloop ctions tuned for different sets of initil conditions, which might e determined y only few its of sensor dt. A simple exmple would e to minimize the overll pn rottion y using counterclockwise preprtory motion for hndle orienttions on the left. This would void the need to mintin contct for long durtion long the circulr push-pth nd my improve the roustness of the rottion ction in chieving the desired hndle configurtion prior to the grsp. In ddition to extending the cpture region of welltuned grsping ction, the preprtory rottion strtegy my e desirle in humnoid roots y mking the mnipultor motion pper more humn-like. Fetures such s idirectionl rottion nd more relxed hnd pose for multi-finger pushing contct could improve the mnipultor s nturl ppernce. Other similr preprtory mnipultion strtegies include sliding, rolling, or tumling mneuvers which re-configure the oject prior to grsping. In the humn suject experiments, trnsltionl sliding of the oject ws not constrined in ny of the tsk scenrios. The humn motion cpture dt does revel tht some of the unimnul mnipultion resulted in oth plnr rottion nd trnsltion prior to lifting. In the root experiments, we found tht simple pivoting motion ws sufficient to increse the cpture region without specificlly progrmming trnsltionl displcement ction. Directions for future work in rootics include exmining which fetures of these preprtory mnipultion strtegies should e imitted in humnoid roots. Some fetures, such s the rm configurtion, might e essentil for root to pper humn-like. Other fetures, such s optimiztion of the joint torques or grsp qulity, might e importnt heuristics for chieving roust performnce in difficult tsk conditions. Further studies of humn motor control my lso uncover new concepts tht suggest how to improve the roustness of rootic mnipultion nd nthropomorphism in humnoid roots. ACKNOWLEDGMENT This work ws supported y the Ntionl Science Foundtion (IIS-33, ECS-333, nd CCF-73). L. Y. Chng received support from Ntionl Science Foundtion Grdute Reserch Fellowship nd NASA Hrriet G. Jenkins Pre-Doctorl Fellowship. 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