Component Oriented Modelling of a Valve Controlled Asymmetric Cylinder Drive Using a Generalized Formulation for Model Linking

Abstract

A computer aided design facility for the analysis and simulation of hydraulic servo-systems is being developed, and this work has highlighted the lack of a suitable component oriented model for the asymmetric cylinder drive. Current literature characterizes the linking between the valve and the asymmetric cylinder by the service line pressures and flows which together represent up to four variables. This paper outlines a simplified approach by defining and utilizing generalized definitions of load flow and load pressure so that adequate linking is made possible by the two simple product variables of power. This enables a model to be produced in a way that is directly analogous to that of a symmetric cylinder drive. Particular difficulties associated with spool operation in the underlap region and with laminar leakage modelling are addressed. The model thus provides an intuitive insight into the particular control problems associated with asymmetric cylinder drives when controlled by a symmetric four-way valve, a combination found in many applications. Computer generated results are compared with observed results from a test rig. A steady state model solution is shown to emulate the steady state characteristics of the drive. A dynamic model solution, via simulation, is shown to correspond well with observed response. Both open- and closed-loop responses are tested. The characteristic of an asymmetric cylinder to demonstrate a higher gain and lower damping when extending (compared to retracting) is clearly evident in the results. The generalized definitions demonstrate potential for application in a wide range of analysis techniques.