Appointed-time fault-tolerant prescribed performance tracking for a rehabilitation exoskeleton with flexible joints...

This article investigates the prescribed performance angular position tracking control of a rehabilitation exoskeleton with uncertain dynamics, unknown disturbances, actuator faults, and flexible joints. First, a novel appointed-time prescribed performance function (APPF) is proposed, wherein the convergence time can be accurately pre-specified. Then, the APPF is integrated with an improved sliding mode reaching law to develop an appointed-time fault-tolerant prescribed performance control (AFPPC) strategy. The attractive features of the proposed AFPPC include: 1) guaranteeing the appointed-time convergence with faster transient response and higher steady-state performance, despite the presence of uncertain dynamics, unknown disturbances, and actuator faults; 2) enhancing the robustness of the system and suppressing the vibration of the flexible joints using an improved reaching-law-based sliding mode prescribed performance framework. These features are capable of improving the safety of the rehabilitation therapy. Lyapunov-based theoretical analysis and comparative simulations demonstrate the benefits of the proposed method.