Development and testing of a minimal model for a pressure compensated hydraulic cylinder control system

This paper proposes a minimal model of a pressure compensated hydraulic system driven by a constant displacement pump developed from sets of equations describing hydraulic fluid flows and pressures. The transfer characteristics of these equations are used to develop a 2nd order Ordinary Differential Equation (ODE), which may be considered linear time invariant or linear time variant depending on type of model being used. The results show the minimal model parameters extracted from the observation of a low frequency square wave response and two state variables is capable of accurately modelling the time domain response to both the square wave and low frequency sine waves. This is despite the fact neither the internal pressure of the hydraulic system nor the exact operating point of the electro-hydraulic valves as a function of pressure and actuation current is known. While the minimal model captures the overall dynamics of the hydraulic system, lack of observation of the internal hydraulic pressure means that effects such as stick slip stiction are not captured. However, these areas where the linearised model is not accurate have been determined in terms of observable characteristics of the system. Future research includes extension of the algorithm to real time measurement of load forces.