A unified passivity based control framework for position, torque and impedance control of flexible joint robots

In this paper we describe a general passivity based framework for the control of flexible joint robots. Herein the recent DLR results on torque-, position- as well as impedance control of flexible joint robots are summarized, and the relations between the individual contributions are highlighted. It is shown that an inner torque feedback loop can be incorporated into a passivity based analysis by interpreting torque feedback in terms of shaping of the motor inertia. This result, which implicitly was already included in our earlier works on torque- and position control, can also be seized for the design of impedance controllers. For impedance control, furthermore, potential shaping is of special interest. It is shown how, based only on the motor angles, a potential function can be designed which simultaneously incorporates gravity compensation and a desired Cartesian stiffness relation for the link angles. All the presented controllers were experimentally evaluated on the DLR light-weight robots and proved their performance and robustness with respect to uncertain model parameters. Herein, an impact experiment is presented briefly, and an overview of several applications is given in which the controllers have been applied.