Proportional control of on/off solenoid operated hydraulic valve by nonlinear robust controller

The growth of electronics control systems, automation and robotics for agricultural and construction machinery demands less expensive, robust electrohydraulic devices to interface fluid power systems. Servo and proportional valves need two or three stages of amplification and delicate mechanisms or sensors in the loops, to control high oil flow by small electromechanical motors linearly. Traditional on/off solenoids are cheaper, robust, have higher power-mass ratio and could have small and large sizes, but they are not usually used for this aim by high nonlinearity. The present work investigates continuous and proportional control of an on/off solenoid driving valve spool by modern nonlinear control like VSC and using a solenoid model to estimate spool position. A magnetic model of solenoid and hydraulic flow forces of a valve are carried out without using FEM techniques. The inevitable inaccuracies of these models and the impossibility to consider and value friction forces suggested use of VSC. Due to its robustness, the architecture of the control system is based on an external VSC position loop and an internal current loop with a traditional PI controller to reduce problems due to chatter. We also investigated estimating position, in particular, when the valve is powered by a solenoid only, to avoid use of an additional sensor. For this aim the derivative of current is used in a simple way. Control and di/dt measurement systems are designed to fit a practical and not expensive approach.