Influence of Controller on Cable Driven Lower Limb Rehabilitation Exoskeleton (C-LREX): PD vs MPC

Cable driven exoskeletons are preferred over link-driven exoskeletons due to numerous advantages such as remote actuation, lightweight, reconfigurability, and many more. PD (proportional derivative) controllers are widely implemented in exoskeletons due to simplicity and faster response. However, in cable driven exoskeleton, the PD controller doesn't directly distribute the cable tension and thus the possibility of tracking error is higher. In this work, we formulated a non-linear model predictive controller (NMPC) for C-LREX that directly allocates cable tension and is further simplified to reduce the time delay. The simulation result indicated that simplified NMPC is faster than NMPC but slower than PD for C-LREX. NMPCs tracked trajectory with the least error, however, allocated higher cable tension. PD should be preferred when speed is prioritized while NMPC should be preferred for performance.