Multistable series elastic actuators: Design and control

In this paper we propose a novel actuation concept, consisting of a conventional DC motor in series with a compliant element having multiple configurations of equilibrium. The proposed device works similarly to a traditional series elastic actuator, where the elasticity increases safety and force control accuracy, but presents the possibility of achieving higher efficiency and releasing energy at a higher bandwidth. An introduction on the mechanical properties of the multistable element explains its working principle and provides simple model-based guidelines to its design. We characterize the actuator and propose a robust algorithm to control both storage and rate of release of its elastic energy. Using only an incremental encoder on the motor's axis, we show that we can reliably control the position of the actuator and its convergence towards a state of stable equilibrium. The proposed robust control architecture sensibly improves the tracking accuracy with respect to conventional PID controllers. Once reconfigured, no additional energy from the motor is required to hold the position, making the actuator appealing for energy-efficient systems. We conclude with a discussion on the limitations and advantages of such technology, suggesting avenues for its application in the field of assistive robotics.