Equal coded digital hydraulic valve system-improving tracking control with pulse frequency modulation

摘要

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.