Frequency Response of an Electro-Hydraulic Vibrator with Inertial Load

Abstract

During the past few years systematic vibration testing has played an increasing part in the evaluation of mechanical devices. The electro-hydraulic vibrator, consisting of a valve-controlled hydraulic cylinder, is capable of high thrusts over a fairly wide range of frequency. This paper examines the theoretical behaviour of such a vibrator which is itself similar to a hydraulic positioning system. The cylinder equations are extended to include end leakages and the servo valve is examined both for transfer characteristics and the interdependence of flow and load. It is shown that the classical square root relationship between pressure drop and flow does not necessarily apply for a two-stage valve with a spring-centred second stage and that a dimensionless number known as the valve load compensation coefficient describes these characteristics. For practical valves it is possible for the flow to be almost independent of the load over a wide range of operating conditions and thus a linear transfer function can be used for the valve-cylinder combination. The experimental work shows good agreement between the theoretical characteristics and those found in practice.