A New Adjustable Stiffness Rotary Series Elastic Actuator Based on Lockable Spring Elements

Abstract In this work, we present a new adjustable stiffness rotary series elastic actuator where the stiffness is adjusted by changing the length of spring elements using lockable pins. The design of spring elements is optimized for undergoing maximum deformation based on compliance maximization. To ensure symmetry, a pair of spiral spring elements that are equally phased out are used. On each spiral spring element, a guided movable pin that can be locked to a rigid back plate is mounted. By locking the pins to the back plate, the effective length of the spiral spring elements can be adjusted, resulting in adjustable stiffness of the series elastic actuator. The stiffness is determined using finite element analysis and experimentally validated. An assembly is designed with an adjustable stiffness elastic member and a rotational encoder to determine the torque transmitted by the series elastic actuator. This apparatus can be used for partially assisted therapy operation where the load torque can be calculated from the deformation of the elastic member and the input torque from the motor can be adjusted according to user effort. Such a partial assisted therapy mode of operation enables simpler control of prime movers in assistive applications, thus increasing safety in human–robot interactions. Future work will focus on improving the partially assisted therapy operation control using data from a series elastic actuator to train machine-learning methods.