A Multi-Axis Force Measurement System for a Space Docking Mechanism

Transcription

1 3rd Internatonal Conference on Materal, Mechancal and Manufacturng Engneerng (IC3ME 215) A Mult-Axs orce Measurement System for a Space Dockng Mechansm Gangfeng Lu a*, Changle L b and Zenghu Xe c Buldng C1, State Key Laboratory of Robotcs and Systems, Harbn Insttute of Technology, Harbn, 151, Chna a lugangfeng@ht.edu.cn, b lchangle@ht.edu.cn, c @qq.com Keywords: mult-axs force measurement system, 6-UPS parallel mechansm, mpedance control system. Abstract. In order to ensure the securty buffer of the 6-UPS space dockng mechansm, real-tme control based on contact force s needed. In ths paper, a hghly effectve mult-axs force measurement system s bult to acqure and analye the real tme force from sx separate force sensors. Accordng to the data analyss of the measurement system, a compound contact force s used as the feedback sgnal to control the movement of the space dockng mechansm. Ground experment shows the good effect of the measurement system, and that the force control algorthm mproves the buffer effect at the dockng stage. Introducton The 6-UPS space dockng mechansm s workng process can be roughly dvded nto fve stages, ncludng dockng preparaton, dockng wth buffer, capturng wth electromagnet lock, pullng back of the dockng rng and lockng wth the dockng lock. If passve complance control method s adopted at the second stage whle the envronmental stuaton s not completely known, the performance of the dockng system can be unstable and hgh mpact may occur because of the system s poor robustness. Therefore, usng actve complance control method by measurng the force sgnal of the dockng mechansm s of great practcal sgnfcance. Impedance controldoes not control the desre poston or force drectly. Instead, t acheves the purpose of complance control by establshng and regulatng the dynamc relatonshp between robot s contact force and poston revson. By detectng the actual poston of the actuator, we can get the contact force wth the help of the envronment rgdty. After comparng t wth expected contact force, we can obtan the revsed poston by usng the target mpedance. Then, wth the help of robot nverse knematcs and trajectory plannng, the poston of robot s end actuator s changed to reduce the contact force. Ths control method has great robustness under dsturbed and uncertan envronment, whch can be qute useful. In ths paper, a hghly effectve mult-axs force measurement system s bult to acqure and analye the real tme force from sx separate force sensors. Accordng to the data analyss of the measurement system, a compound contact force s used as the feedback sgnal to control the movement of the space dockng mechansm. At last, after a bref ntroducton to the ground-bult dockng test system, data analyss of contact force s presented based on the practcal experments. Confguraton of orce Sensor n the Dockng Mechansm The actve part of the 6-UPS space dockng mechansm s a 6-DO parallel manpulator. Sx Hooke hnges on the moble platform are lnked correspondngly to sx equvalent sphercal hnges on the fxed platform wth sx lead screws, and there are sx separate sngle axs force sensor n the whole drvng chan, as shown n g. 1. In the meantme, there are gudng petals and electromagnets mounted unformly on the moble platform, and the passve dockng rng s just somorphc The authors - Publshed by Atlants Press 1416

2 g.1: Confguraton of orce Sensor n 6-UPS Space Dockng Mechansm Each prsmatc par of the 6-UPS dockng mechansm s drven by a brush DC motor va a reducer and a set of lead screw, whch provdes a large reducton rato. And the rotary moton of motor turns nto lnear moton after ths transformaton. Meanwhle, sx force sensors are mounted between the Hooke hnges and screws n order to measure the axal force. Load Calculatng from Sx orce Sensors In order to get the sx-dmensonal contact force ( x, y,, Mx, My, M) Τ to be used n moton controller, load calculatng must be carred on to translate the sx separate forces from sx lead screw to an ntegral sx-dmensonal contact force. The calculatng process can be dvded nto two steps, n the coordnate systemoxy. rst, couplng the tenson or pressure from sx separate axs to the pont O ; Secondly, transform the compound sx-dmensonal contact force from pont O to pont O Suppose the sngle-dmensonal contact force n the branched chan No s, and t can be v l v Τ expressed as v. Record l ( x, y,,), and decompose n drecton x y l, and under the coordnate system Oxy, as shown n g.2. g.2: orce Calculatng of the Space Dockng Mechansm n drecton x y, and can be expressed as: 1417

3 x x x x v l x + y + l + l y y y y v l l x + y + + l v l l x + y + + l Translate the three component ( x, y, ) to pont O,and then we sum the force and torque n all drectons, a sx-dmentonal force n pont O can be descrbed as: x x x x 1 1 x y 1 l + l + + y y y y 1 1 x 1 + y l + + l 1 1 x + y + 1 l + l M r r r x 2snϕ sn ϕ 1 1 x l + y l M r r r y 2cosϕ cos ϕ cosϕ 1 1 x 1 l + y + + l snϕ ycosϕ xsnϕ ycosϕ xsnϕ M ( yr2cosϕ xr2sn ϕ) r r 1 1 x 1 l + y + + l In order to get the real force to be used n moton controller, the above deduced contact force should be transformed to pont O on moble platform from pont O as follows: x ox y oy o Mx oyp py + Mox M y opx xp + Moy M oxpy ypx + Mo Besdes, the plane of pont O s the wnch pont, and t s dfferent from the real contact surface wth an offset h n drecton,so correctons can be expressed as: pnewx h(snαsnγ + cosαsnβ cos γ) + px pnewy h( snαcosγ + cosαsnβsn γ) + py pnew h cosαcos β + px The fnal transformaton after correcton can be expressed as matrx: 1418

4 x 1 x 1 y y 1 M pnew pnewy 1 x Mx M pnew pnewx 1 y M y M pnewy pnewx 1 M Structure of Measurement System The control system of the dockng mechansm s made up of one remote control computer, one master computer, one moton controller, one DAQ card, sx force sensors, sx drvers and sx DC brush motors, as shown n g. 3. g.3: Structure of Measurement System n the whole Control System The DAQ card sends force sgnal measured by sx force sensors to the master computer va the moton controller. By usng the force sgnal wth mpedance control method, the poston revson of the actve dockng rng can be calculated. After ths, motors rotatonal turns can be calculated va nverse knematcs, and then be sent to sx drvers va the moton controller, drvng the DC brush motors to the rght poston va PID poston control. Experment Results Expermental Equpment. Space dockng experment s conducted wth the ground-bult dockng test system whch s made up of 6-UPS dockng system and ground-bult ar floataton system, as shown n g. 4. The actve part and passve part of the 6-UPS dockng mechansm are mounted respectvely n the correspondng parts of ground-bult ar floataton devce. The ground-bult ar floataton system can provde buoyancy to offset the gravty of the whole devce n order to smulate weghtless envronment n space. Expermental Process and Result Analyss. After programmng the Cartesan mpedance control program wth the parameter combnaton mentoned above, dockng experment s conducted on the ground-bult dockng test platform. The changng of tme-varyng contact force and poston error n drecton Z durng the dockng process s shown n g. 5. Accordng to the expermental fgures, the contact force durng the dockng process can be controlled under 2N wth the help of mpedance control method, and t can be stable at N n the end. 1419

5 And the poston change n Z drecton s qute stable. In short, ths method can greatly ensure the securty buffer of the 6-UPS space dockng mechansm, whch s sgnfcant n practce. g.4: Ground-bult Dockng Test System g.5: Contact orce and Poston Error of Dockng Experment Concluson In ths paper, a hghly effectve mult-axs force measurement system based on the feedback sgnal from 6 separate force sensors s presented. Usng the force parameters calculated n the measurement system, ths moton controller can tremendously mprove the buffer effect at the dockng stage durng the experment. It s recommendable for future researchers to mprove the robustness of the control system n case of the envronmental mutaton condton [5], whch can expand the applcaton feld of ths method. Acknowledgement Ths work s supported by Natonal Hgh-tech R&D Program of Chna (No.214AA4161) and Chna Postdoctoral Scence oundaton (No.214M561338). References [1] Salsbury, J. Kenneth. "Actve stffness control of a manpulator n Cartesan coordnates." Decson and Control ncludng the Symposum on Adaptve Processes, th IEEE Conference on. Vol. 19. IEEE, 198. [2] Maples, J., and Joseph Becker. "Experments n force control of robotc manpulators." Robotcs and Automaton. Proceedngs IEEE Internatonal Conference on. Vol. 3. IEEE, [3] Blauer, Mchael, and P. I. E. R. R. E. R. Belanger. "State and parameter estmaton for robotc manpulators usng force measurements." Automatc Control, IEEE Transactons on (1987): [4] Kaandes, Peter, N. Scott Bradley, and W. Wolovch. "Dual-drve force/velocty control: mplementaton and expermental results." Robotcs and Automaton, Proceedngs., 1989 IEEE Internatonal Conference on. IEEE, [5] Merlet, J. P. "C-surface appled to the desgn of an hybrd force-poston robot controller." Robotcs and Automaton. Proceedngs IEEE Internatonal Conference on. Vol. 4. IEEE,