Interaction Control of a Non-Backdriveable MR- Compatible Actuator Through Series Elasticity

This research aims at developing a magnetic resonance (MR)-compatible equivalent of an exoskeleton used for wrist movement rehabilitation therapy of neurological patients. As a crucial step towards the accomplishment of this goal, this paper investigates the development of a novel actuation architecture suitable for interaction control in MR environments, the MR-SEA (SEA stands for Series Elastic Actuator). MR-SEA consists of the combination of a non-backdriveable MR-compatible actuator and of a compliant force-sensing element. The preliminary design of a 1 DOF actuator is presented, in addition to nonlinear dynamical model of the system featuring the most relevant actuator non linearities. A switching controller is proposed, and the asymptotic stability of the resulting discontinuous dynamical system is demonstrated for force control in blocked output conditions. Simulation results show that the proposed system is adequate for the implementation of force control for wrist movement protocols in fMRI, demonstrating a bandwidth higher than 8 Hz for force control. For stiffness control, simulation results demonstrate that the system is passive for all values of desired virtual stiffness lower than the stiffness of the physical spring, and isolated stability is obtained for the entire range of stiffness values considered.