Nonlinear Control Design for a Gravity Compensation Mechanism for Human Lower Limb Rehabilitation

Dynamics of a two degree-of-freedom suspension mechanism is incorporated into nonlinear control design to facilitate its potential use as a rehabilitation device to aid people with lower-limb injuries. The proposed mechanism is a variation of the standard four-bar linkage with an extra link and two springs. The system dynamic model is first extracted based on the Lagrange’s equations in conservative form. The performance deviations due to the link inertia is demonstrated in open-loop numerical simulations under an impulsive force scenario. Finally, the dynamic model of the suspension mechanism is incorporated into feedback control design based on nonlinear, sliding mode control strategy that can add robustness against modeling uncertainties and external disturbances. The tracking performance of the proposed nonlinear controller is validated in closed-loop numerical simulations to demonstrate possible performance improvements under feedback control.