Equal Coded Digital Hydraulic Valve System – Improving Tracking Control with Pulse Frequency Modulation

Abstract

Previous research on digital fluid power indicates down-scaling to be beneficial to the characteristics of digital valves. This theoretical aspect yields to use equal size on/off valves in digital hydraulic valve systems. A simple miniaturized on/off valve, having orifice diameter of 0.7 mm, have been developed to study the feasibility of this approach. A manifold manufacturing technology also has been developed to facilitate a compact framework for the valve system containing dozens of the miniature valves. The first equal coded valve system prototype contains 16 miniature valve prototypes. A natural modulation method for an equal coded valve system is Pulse Number Modulation (PNM), where the output depends on the number of activated valves. With this modulation method, resolution of the output is equivalent to the number of the valves in the control edge. Limited resolution hampers the tracking control performance especially at low velocity where small steps in flow rate are required. The main purpose of this paper is to study the feasibility of Pulse Frequency Modulation (PFM) to improve the controllability of the equal coded valve system at the velocities below the minimum velocity of PNM control. Poor controllability at low velocities is one of the main challenges of the digital hydraulic valve systems. Characteristics of the PFM method are analysed and an experimental study is carried out to measure the improvement of the PFM method on the tracking control performance. The study indicates that the PFM method is an effective way to improve tracking control performance at low velocity.

High-linear digital hydraulic valve control by an equal coded valve system and novel switching schemes

“…This trajectory shape, with the movement time of 2 s, is similar than those used in previous studies. 7,16 However, the length of the longest stroke differs from the references. The stroke lengths for this study are 15, 50, and 100 mm.…”

“…Another flaw of that solution is that a fixed PWM valve causes the uneven wearing of the valves. Paloniitty et al 16 combined an equal coded DFCU and switching control in a way that switching duty distributes for all valves. However, they used only pulse frequency modulation (PFM), which can improve the flow resolution only at small control values.…”

Investigation on the transient impact characteristics of fast switching valve during excitation stage

“…Therefore, to improve the switching frequency, an intelligent PWM control strategy is usually used in which a boost voltage is optimized to get a faster opening speed of the FSV. 12,13 However, the moment when the valve quickly opens or closes is essentially the transient impact contact process between the ball valve and the valve seat. In this process, various forms of energy conversion occur, including mechanical energy, elastic-plastic energy, friction, and sound wave dissipation.…”

Digital Hydraulic Technology for Linear Actuation: A State of the Art Review

“…Paloniity et al propose a similar scheme in [27] where pulse frequency modulation is used on one of the bits of the DFCU in order to achieve even smaller opening areas. Paloniity et al report that the method is effective at improving controllability as the average error for a slow trajectory has been reduced by a factor of approximately 4 compared with just the Pulse Number Modulation (PNM) technique.…”