Passive and accurate torque control of series elastic actuators

The principle of series elastic actuation offers considerable advantages for haptic displays compared to stiff actuators. The interaction force between motor and load is directly proportional to their relative position, which corresponds to the elongation of the elastic element. This way, the torque control task is transformed to a position control task, which comes natural to traditional DC motors. In this paper, several existing control strategies are analyzed and compared with respect to passivity concerns. Cascaded control with a fast inner velocity loop results to be the best option. Based on the analysis, boundaries for the parameters are presented, such that the force controller may contain integral action without jeopardizing passivity.