On Handling Variable Stiffness Parameters in Compliance Control via MPC

In variable impedance control the desired impedance parameters of a robot manipulator are adjusted in real-time. It has been observed that time varying impedance parameters can lead to a non-passive interaction and even destabilize the system. In this paper we consider a compliance controller, where the desired impedance is specified via a time-varying stiffness and damping. We propose to compute the controller gains from an additional online optimization instead of applying the desired time-varying stiffness and damping references directly. In this way the effect of the time-varying impedance parameters is evaluated over a finite time horizon. Together with a terminal constraint on the state this controller formulation aims to avoid a destabilization of the system due to the impedance variation. The proposed method is validated for free movement in two different simulations as well as for physical interaction on a Franka Research 3.