Comparison of Nonlinear Flatness-Based Control of Two Coupled Hydraulic Servo Cylinders

This paper presents two nonlinear model-based control designs for a hydraulic system that consists of two mechanically coupled hydraulic cylinders actuated each by a separate servo-valve. Based on a physically-oriented nonlinear mathematical model of the test rig, a further model simplification results in a completely controllable MIMO system. Two different control structures are discussed and compared to each other in this paper: First, a cascaded flatness-based control is designed, where fast inner control loops determine the difference pressure in each hydraulic cylinder, while the position as well as the generated force is controlled in the outer loop. In the second approach, a centralised flatness-based control, with the same outputs as in the outer loop of the cascaded approach, is developed for the MIMO system. Model parameter uncertainties are estimated by a reduced-order disturbance observer and compensated by the control algorithm. The efficiency of the proposed control structures is demonstrated by experimental results from a dedicated test rig.