Vision / force control of parallel robots 2

30 Recently, parallel robots have drawn a lot of interest in the robotic community due to their theoretical superiority over the 31 classical serial structures in terms of stiffness, accuracy, high speed and payload in spite of their more complex kinematics and 32 smaller workspace compared to serial manipulators. These specific benefits allowed the parallel kinematic machines to perform 33 better some industrial tasks requiring accurate and fast motion like the pick and place of light objects. Moreover, being stiff, 34 parallel robots have potential abilities to perform bettermost ofmachining operations (like deburring, polishing,...) than the serial 35 ones because these lasters are subject to link deflections under external load when exerting force on a rigid environment. Such 36 deflections have significant impact on robot performances when dealing with tasks involving both Cartesian position and contact 37 forces control. For such tasks, the interaction force must be controlled properly, since otherwise the arising contact forces may 38 damage the object or the robot tip. To this end, different force control approaches have been proposed in the literature and applied 39 for serial machines. The case of parallel machines has rarely been addressed in view of the complexity of their mechanical 40 architecture, which leads to difficulty to obtain the relation determining the pose of the end effector from the joint coordinates 41 (Forward KinematicModel). Indeed, solving the Forward KinematicModel (FKM) of parallel machines remains a difficult problem. 42 The Forward Kinematic Model is indispensable to achieve robot position control in Cartesian space (using joint sensors) which is 43 more convenient when the interaction forces between the robot end effector and the environment must be controlled as well. 44 Also, force control involves the dynamics of the mechanical structure which is easily described in Cartesian space for a parallel 45 machine. An alternative to obtain the end effector Cartesian pose without calculating the fastidious Forward Kinematic Model of a 46 parallel robot is the use of an exteroceptive measure, specially, a camera since vision systems have shown good efficiency to guide 47 robot using image information (visual servoing). The present work focuses on coupling force feedback and visual servoing to 48 control both contact forces and the end effector Cartesian pose of a parallel robot. The two controlled variables (contact forces and 49 Cartesian pose of the end effector) are directly measured by exteroceptive sensors (force sensor and camera) within parallel Mechanism and Machine Theory xxx (2011) xxx–xxx ⁎ Corresponding author. Tel.: +33 473407766; fax: +33 473407262. Q1 E-mail address: Saliha.BELLAKEHAL@lasmea.univ-bpclermont.fr (S. Bellakehal). MAMT-01832; No of Pages 20 0094-114X/$ – see front matter © 2011 Published by Elsevier Ltd. doi:10.1016/j.mechmachtheory.2011.05.010