Control of four wheel steering using independent actuators

In four wheel steering (4WS) system the steering angles in each wheel must follow Ackermann steering geometry in order to eliminate the lateral slippage of the wheels. In agricultural vehicles, the steering actuators are typically hydraulic cylinders. In pure four wheel steering with two degrees of freedom and without additional constraints, a straight forward way is to install one actuator with a position sensor per wheel to realize Ackermann principle. In agricultural robot studies four wheel steering is a common kinematic structure. In this paper, modeling and control of steering in a 4WS remote controlled tractor is studied. The tractor is equipped with four asymmetric cylinders that steer each wheel, without any additional mechanical rods. The hydraulic system consists of one variable displacement pump and four proportional directional valves with load sensing function. The system model is split into subsystems and the overall model contains both dead-time delay, first order dynamics, static nonlinearities, temperature dependency and combined saturation. Besides modeling and analysis, the paper presents a control design that compensates nonlinearities but also takes care of saturation while compensating the dynamics. The objective in control is synchronous motion in the position control of the steering angles of all wheels. The results show controller performance in single wheel motion without saturation and overall controller performance under saturated pump flow.