Compensation function observer based model-free adaptive boundary layer sliding mode control for hydraulic manipulators with dead-zone compensation

In this paper, a compensation function observer based model-free adaptive boundary layer sliding mode control (CFO-DIBLSMC) is proposed for hydraulic manipulators with dead-zone compensation. The proposed CFO-DIBLSMC adopts a dual-loop structure, comprising a force sub-control loop for precise force tracking using an adaptive admittance controller and a position sub-control loop where the complex hydraulic manipulator dynamics are approximated by an ultra-local model (ULM) to establish a model-free control framework. Then, considering the unknown dead-zone in the directional valve, a smooth dead-zone inverse method with online parameter adaptation is employed to compensate for dead-zone uncertainties. Besides, a compensation function observer is designed based on ULM to estimate lumped uncertainties of the system, achieving zero error estimation and robustness against high-frequency disturbances. Moreover, a nonsingular fast terminal sliding mode (NFTSM) sub-control law is constructed to accelerate error convergence, incorporating an adaptive switching sliding gain law within a boundary layer framework. The effectiveness of CFO-DIBLSMC is verified through comparative co-simulations against conventional control methods under dead-zone and joint friction disturbances.