Two-stage adaptive impedance control applied to a legged robot

In this paper we propose an adaptive impedance control scheme consisting of two stages. The first stage performs an online estimation of the robot parameters imposing the desired mechanical impedance. It constitutes the kernel of the control system and remains active during the complete, free or constrained, motion of the robot. A simple algorithm for the numerical computation of the defined impedance error is presented where only the available feedback information is used. The imprecision in the parameters of the environment (stiffness, positioning) is compensated by the second stage of adaptation. This one constitutes an external force control loop closed around the internal nonlinear impedance controller. Simulation results obtained for a single leg of a pneumatic driven, quadruped robot show the effectiveness of the proposed control scheme in case of considerable uncertainty both in the robot and ground parameters.