Analysis of Compressed Air Flow Through a Spool Valve

A pneumatic servo-mechanism with a sophisticated control algorithm based on variable closed-loop gains has been developed recently on specially designed pneumatic drives. Unfortunately, due to the compressibility of air, models derived using traditional linearization methods could not describe the behaviour of the system over its full stroke and hence prevented the generalization of the control algorithm on a commercial platform. The problem is tackled by the component-oriented approach which segregates the pneumatic servo-mechanism into individual components for more precise analysis. This paper describes the analysis of one of the components—a five-port spool valve from the concept of a flow node in the valve land. The new valve model, which incorporates an extension to include an upstream flow head in the one-dimensional adiabatic flow equation, can predict the mass flowrates at all spool displacements satisfactorily. The flow node model can also be used to derive the theoretical pressure variations due to leakage which cannot be deduced in the conventional approach.