Hybrid Active Force Control of Super-Twisting PID Sliding Mode Technique of a Lower Limb Exoskeleton

Reference trajectory tracking and guarding against system disturbances and uncertainties are the important factors in the realm of lower limb exoskeleton robots control. Sliding mode PID is one of the robust controllers, which has a sliding manifold in the form of the PID controller. Chattering is the substantial predicament of the PIDSMC so that boundary layer around the sliding manifold is applied to eliminate the phenomenon. In this step, not only the chattering phenomenon is not eliminated but the robustness of the controller is also mitigated. In this study, supper twisting PID sliding mode controller (STPIDSMC) was used to eradicate the chattering phenomenon and enhancing controller robustness. The STPIDSMC robustness is protected indigenously and without defining the boundary layer, and the chattering phenomenon is reduced. Furthermore, to meet the external disturbances and uncertainties with unlimited amplitude, adaptive active force control method is combined by STPIDSMC as a modifying input control loop. In the active force control approach, the control input is online modified based on the estimation of moment inertia of the robot links. In order to accomplish maximum performance, control parameters were optimized using harmony search algorithm. In the optimal state, the performance of the proposed controller has been compared with PIDSMC and STPIDSMC that revealed the priority of the proposed controller rather than other controllers. The results indicate that the three error criteria, ITAE, ITASE, and IASE experience significant reduction about 39, 48, and 66 percent respectively compared to STPIDSM.