Calibration of two cooperative manipulators via pseudo-closed-loop method

Calibration of two or more cooperative robots is a basic issue and very important for coordination control. Considering fast, low-cost and even autonomous calibration, the closed-loop calibration method for a single robot is extended to two cooperative robots for establishing the relation between robot base coordinates. For collecting physical date the closed-chain formed by two robot arms must be driven to different points in the space. This is impossible, however, when the relation between base coordinates of the two robot arms have not yet been established. To overcome this difficulty, at every selected point the two robot arms perform an accurate peg-in-hole assembly with the use of a contact force control technique for a single robot system. A rigid peg with a shaft key and a rigid hole with a keyway are grasped tightly by two robots respectively, the clearance between the peg and the hole is very small. So the closed-chain formed after any one peg-in-hole assembly at any point in the space holds the same. In this sense, this method is called the pseudo-closed-loop method. Two identification algorithms are developed, one takes for granted that the closed-chain is very rigid while another takes into account the deformation of the object connecting the endpoints of the two robots. Experiments are conducted with two PUMA industrial robots. Experimental results validate the proposed method.