SEA force/torque servo control with model-based robust motion control and link-side motion feedback

A series elastic actuator (SEA) consists of a system with an intrinsically compliant mechanism, and introduces the possibility of external force/torque sensing and control. However, this mechanism is also known to reduce torque servo-control bandwidth and to separate link-side dynamics from actuator-side dynamics. The conventional solution to overcoming reduced actuation bandwidth and separated link motion problems is to increase the control gain, which is limited by stability problems and actuator saturation. Here, we demonstrate that treating the actuator side as a near-ideal position source gives the compliance-independent actuation bandwidth of an SEA, so that separated link-side motion can be simply but effectively compensated. Based on these observations, we developed an SEA force/torque servo controller that consists of actuator-side robust motion control and link-side motion feedback. We performed comparison experiments with conventional cascaded two-loop SEA torque servo control using a rotational flexible-joint test-bed, and the results demonstrated that the proposed control strategy yields a wider bandwidth and more robust link motion.