Torque control of a series elastic actuator using an ultrasonic motor with angular-velocity saturation

A series elastic actuator (SEA) using an ultrasonic motor (USM) and an elastic element has attracted attention as a compact actuator with high torque-weight ratio and high force control performance. However, an elastic element in a SEA often causes mechanical vibrations. To suppress these vibrations, studies implemented the state feedback of the motor- and load-side angles. However, as the controller has many integrators, the control system becomes unstable when the angular-velocity of the USM is saturated. Therefore, in this study, integrators in the controller were reduced in number. Furthermore, a limit error feedback controller was designed to suppress the windup phenomenon caused by the integrators. The frequency analysis using a describing function showed that stability is guaranteed even if the angular-velocity is saturated. In addition, the experimental results indicate that an overshoot of the torque response is suppressed.