Sliding mode control for a system of two coupled hydraulic cylinders

This paper presents a model-based control design for a hydraulic system that consists of two rigidly coupled hydraulic cylinders actuated each by a separate servo valve. Based on a sixth-order nonlinear mathematical model of the test rig, a subsequent model simplification results in a fully controllable fourth-order design model. In this contribution, a decentralised control approach is proposed using Lyapunov techniques: for the position of the coupled cylinders a sliding mode control is designed, whereas the second one is force-controlled. The control structure is extended with a sliding mode disturbance observer that estimates an overall disturbance force, which represents the force generated by the second cylinder as well as further force contributions due to nonlinear friction and parameter uncertainties. The efficiency of the proposed control structures is demonstrated by experimental results from a dedicated test rig.