Model-free based adaptive nonsingular fast terminal sliding mode control with time-delay estimation for a 12 DOF multi-functional lower limb exoskeleton

The design of a 12 DOF lower limb multi-functional exoskeleton is realized in SolidWorks with the analysis of kinematics. For controller design, this paper presents the model-free based adaptive nonsingular fast terminal sliding mode control which comprises three parts: the intelligent PI controller, time-delay estimation, and adaptive sliding mode compensator. The proposed control strategy drives tracking errors to a nonsingular fast terminal sliding surface and realizes fast convergence in finite time. The designed adaptive law approximates the upper bound of estimation error in time-delay estimation and thus it reduces the fundamental chattering on the switching manifold. The stability of controller is verified by using Lyapunov theory. To validate the proposed method, the virtual prototype is transferred to MATLAB/SimMechanics as a platform for co-simulation. The proposed method shows improved tracking performance compared with the existing model free controller in co-simulations.