A Novel Equivalent Continuous Metering Control With a Uniform Switching Strategy for Digital Valve System

Pulse number modulation (PNM) combined with pulse width modulation (PWM) control is an effective solution to improve the resolution of digital valve systems. However, the numerous discrete variables that use parallel on / off valves cause difficult control coordination and uneven switching. To address this issue, this article defines the equivalent spool displacement of the digital flow control unit by the number of PNM-controlled valves and the duty cycle of PWM-controlled valves to replace multiple discrete variables and develops the equivalent continuous metering control method. Furthermore, a uniform switching control strategy is proposed for the PWM-controlled valve using a uniformly distributed permutation for each on / off valve. The proposed control methods are verified by simulation on the built mathematical model of the equal-coded digital valve system. Experimental results for the displacement control of a hydraulic cylinder at 1 rad/s show that the average error of the equivalent continuous metering control is about 0.236 mm and the dispersion index reaches 20%, while the uniform switching control strategy achieves 80% with an average error of 0.215 mm. Simulated and experimental results demonstrate that the equivalent continuous metering control with a uniform switching strategy can almost evenly distribute switching numbers without compromising the accuracy of the displacement control.