A fractional-order impedance and momentum observer-based compliant interaction strategy for upper-limb exoskeletons

This study proposes a composite control framework for upper-limb rehabilitation exoskeletons integrating fractional-order impedance control, computed-torque compensation, and a momentum-based nonlinear disturbance observer. By combining Jacobian-transpose force mapping, fractional-order impedance shaping, and observer-based compensation, the framework enhances compliant yielding and tracking robustness under non-ideal dynamics. Simulation results show that, under severe spasticity-like impact, interaction force remains within 45 N. Under 30% mass mismatch and strong friction, tracking accuracy improves without reaching the 60 N·m torque limit. Under tremor-like disturbance, 4-8 Hz oscillations are attenuated while low-frequency voluntary motion is preserved, improving compliance, disturbance rejection, and torque smoothness.