Robust position control of a novel series elastic actuator via disturbance observer

This paper proposes a new robust position control method for a novel series elastic actuator (SEA). It is a wellknown fact that SEAs provide many benefits in force control, e.g., lower reflected inertia and impedance, greater shock tolerance, safety, and so on. However, current SEA designs have a common performance limitation due to the compromise on the selection of spring stiffness. The performance of an SEA can be significantly improved by changing the stiffness of the spring; however, designing a variable-stiffness SEA is a quite challenging task. In this paper, a novel variable-stiffness SEA, which can relax the fundamental performance limitation of conventional SEAs, is proposed. It consists of torsional and linear springs, which have different compliances, in series. The soft and hard springs improve the performance when low and high force control applications are performed, respectively. Although SEAs have several advantages in force control, their position control problem is more complicated than the force control one. Moreover, using extra springs increases the number of vibration mode, which may significantly deteriorate the performance, in the position control problem of the SEA. In this paper, a new position control system, which improves the performance by increasing the robustness and suppressing the vibration, is proposed for a novel SEA. Experimental results are given to validate the proposal.