Nonlinear control of a hydraulic steer-by-wire test rig using extended linearisation techniques

This paper presents a nonlinear model-based control design for a hydraulic steer-by-wire test rig, which consists of two mechanically coupled hydraulic cylinders actuated each by a separate servo-valve. For both cylinders — a steering cylinder and a servo cylinder — control-oriented nonlinear mathematical models are derived. The proposed control structure involves a combined gain-scheduled feedforward and feedback control of the cylinder position for the steering cylinder, whereas the servo cylinder uses a combined gain-scheduled feedforward and feedback control law with the coupling force as controlled variable. A reduced-order observer estimates an external disturbance force representing 1) a simulated drive force — generated by the servo cylinder within a hardware-in-the-loop concept — and 2) nonlinear friction. The design of both decentralised control structures is based on extended linearisation techniques. The efficiency of the proposed control structure is demonstrated by experimental results from a dedicated test rig.