A PASSIVE MECHANISM THAT IMPROVES ROBOTIC POSITIONING THROUGH COMPLIANCE AND CONSTRAINT

This paper presents the design of a passive robotic wrist that is capa ble of establishing and maintaining an accurate position relative to a workpart edge through compliance and constraint (force guidance). In previous work, we have shown that, through proper selection of a manip ulator's impedance, a manipulator's end-effector can be guided to its desired relative position despite errors in its commanded position. Th e selected proper impedance is attained here through the design of a p assive micromanipulator that is mounted on the end-effector of a conve ntional manipulator. The micromanipulator consists of three linkages c onnected by revolute joints and torsional springs. The outermost linka ge contacts the workpart at multiple locations providing multidirectio nal unilateral kinematic constraint. This kinematic constraint in conj unction with the compliance provided by the torsional springs causes t he linkage to be re-positioned so that any existing misaligmnent (that inevitably occurs) is eliminated and a unique planar position/orienta tion with respect to the workpart edge is attained. Here, we present t he procedure used in the parametric design of this mechanism. The desi red compliant properties identified in task space (using Cartesian var iables (x, y, and theta) for force and motion) are extended here to jo int space (using joint variables (theta(1), theta(2), and theta(3)) fo r torque and motion). The appropriate micromanipulator link lengths, i nitial linkage angles, and the appropriate torsional spring constants are selected using an optimization procedure. Computer simulation of t he constrained manipulator/workpart interaction demonstrates that the desired force guidance behavior is attained.