COMPLIANT END-EFFECTOR COUPLING FOR ROBOTIC ASSEMBLY

Abstract

Many manipulation tasks require compliance, i.e., the robot's ability to comply with the environment and accomplish force as well as position control. Examples of such tasks are constrained motion, and tasks associated with touch or feel in fine assembly. Few compliance related tasks have been automated and is usually by means of active compliance control. The need for passive compliance offered by the manipulator itself has been recognised and has led to the development of compliant end-effectors and/or wrists. This thesis presents a novel passively compliant coupling, the compliant end-effector coupling (CEEC), that aids automated assembly. It serves as a mechanical interface between the end of the robot arm and the end-effector. The coupling has 6 degrees of freedom. The design of the coupling is based on a "Lock" & "Free" assembly idea. The coupling is locked and behaves like a stiff member during robot motion and is free (compliant) during constrained motion. It features an air bearing, a variable stiffness air spring and a centre-locking mechanism. The end-effector assembly being centrally unlocked floats within the designed compliance limits assisted by the air bearing. These frictionless and free conditions facilitate a fast correction of any initial lateral and angular misalignments. In a peg insertion assembly, such accommodation is possible provided that the tip of the peg is contained within the chamfer of the hole. An "air spring" has been incorporated in the design to allow for variable stiffness in the vertical direction. The CEEC is designed to be mounted vertically at the end of the robot arm. It should remain always vertical during operation and robot motion because of the air bearing. Thus the CEEC is well suited for Cartesian and SCARA robots. The motion interaction of CEEC while performing chamfer crossing and insertion has been analysed using a motion simulator. The results have shown that the CEEC is a sound lateral error accommodator. The assembly operation started with a small assembly force provided by the weight of the floatable end-tool. This is one of the unique features of the CEEC where the assembly force is delivered in stages.

The CEEC has been used in experiments that measured its functional performance and as part of an automated disk assembly demonstration project. Equipped with the CEEC, the robot succeeded in accomplishing a series of peg-hole insertion tests that includes lateral error of up to 1 mm and 2° angular offset. The average clearances between the parts were as small as 0.0235 mm. A mathematical study and practical examination ascertained that the designed air bearing was able to support a mass of 6 kg comfortably.