Structured H∞ framework for impedance minimization on robot arm with compliant actuation

In most cases where robots have a physical interaction with humans, it is needed to minimize the feeling of the robot opposition to the human movement during the free movement phase. This is usually done by minimizing the robot interaction port apparent impedance. This task is particularly challenging in presence of a flexibility between the actuator and the considered interaction port. In this paper, we propose a new approach that minimizes the robot arm apparent impedance to the absolutely minimal achievable impedance ensuring passivity on the interaction port. Impedance minimization is formulated as a particular standard two-block H_∞ problem. Passivity constraint is considered using the positive real lemma applied to the admittance of the interaction port. The resulting problem is a structured H_∞ problem for which solvers based on non-smooth optimization have been recently developed. The proposed methodology was evaluated experimentally on a testbed and allowed to obtain very satisfying results.