Robotic surface assembly via contact state transitions

Abstract

In order for a robotic manipulator to accomplish an assembly task of mating surfaces of two parts, compliant motion planning and control is desired to overcome position and motion uncertainty. However, compliant motion planners in the literature have focused on planning a compliant path in the task space in terms of contact configurations of one part with respect to the other. Little addressed is the issue of ensuring that such a compliant path not only satisfies contact constraints but also satisfies the manipulator constraints when converted automatically to a joint-space path for the manipulator to execute without singularities and unintended collision. Moreover, compliant control strategies are often based on force sensing in task space. In this paper, we describe a method to convert automatically a complex compliant path in the task space for assembly into a singularity-free joint space trajectory for a redundant manipulator to execute. A 7-DOF Barrett WAM and hand are used for this study. We further introduce a control scheme based on limiting computed joint torques without the presence of a force/torque sensor. We have validated our approach in an assembly experiment and tested alternative compliant paths of different sequences of contact state transitions by successful execution. This demonstrates that the surface assembly can be accomplished robustly through offline planned sequences of contact state transitions, even without on-line force/torque sensing. Estimated end-effector force data also indicate the effectiveness of our control scheme.