Switching control of a water hydraulic cylinder with slow parallel-connected on/off valves

Digital hydraulics is traditionally divided into parallel-connected systems and switching control systems. Parallel-connected systems require more valves but can produce number of discrete flow values. In contrast, switching control normally relies on one or few fast on/off valves and continuous spectrum of values is created by e.g. pulse width modulation. Recently, a new control method has been introduced, which combines the idea of switching control and parallel connection. Only few mutually similar valves are needed and the effective switching frequency is much higher than the switching frequency of the single valve. Accurate path tracking control has been demonstrated with 3 ms prototype valves. This paper studies the suitability of the approach for a water hydraulic cylinder actuator when slower (about 8 ms) commercial on/off valves are used. Experimental results show that the approach works also with slower valves and gives good controllability especially at higher velocities. However, the significant pressure and velocity ripple occurs at lower velocities. It is shown by simulations that damping volumes are one potential way to reduce fluctuations. Results show that switching control is possible with the new approach without ultra-fast valves.