Nested compliant admittance control for robotic mechanical assembly of misaligned and tightly toleranced parts

In this paper, we propose a closed-loop force sensor based nested admittance/impedance control strategy to actively estimate and minimize the effects of geometric misalignment that naturally occur during assembly tasks with compliant robots. The method allows the robot to be used with a stiff impedance control setting, which is beneficial for free air motion performance, yet allows to adjust for large misalignment errors between parts that need be assembled. First, the stability bounds on the control parameters of this method are established through numerical simulation, after which they are compared with the experimentally determined parameters. Trial peg-in-hole insertion experiments are performed with a 6-DOF KUKA LWR-4+ robot under various degrees of rotational misalignment, where metal pegs are being inserted into metal holes under tight tolerances. The proposed method allows successful peg insertions even under large rotational misalignments of up to 20° (13 fold increase compared to 1.5° we achieved with traditional impedance control alone), without the need to adjust the position trajectories with complex models on the fly. Moreover, it provided a 5 fold reduction of the average forces exerted on the environment compared with using impedance control alone.