Accurate Modeling and Identification of Servo-Hydraulic Cylinder Systems in Multi-Axial Test Applications

Servo-hydraulic control is regarded as a key technology in most mechanical testing systems for the investigation of strength and fatigue properties of materials, mechanical components and structures. High performance but robust control strategies are necessary as part of the test control system to ensure high test quality and reproducible test results which are of special importance when considering multi-axial component tests using high channel count test rigs with several active and synchronously operating servo-hydraulic test cylinders. Such high performance multidimensional control algorithms demand sufficient information about the dynamic characteristics of the used test cylinders, the test rig and – at least as a good estimate – of the device under test. In this paper we introduce a special kind of identification strategy for deriving accurate mathematical models for a class of commonly used servo-hydraulic test cylinders: low-cost symmetric test cylinders with significant friction. In order to accurately describe the dynamics of the servo-hydraulic cylinder system, we first build a mathematical model of the system which includes some still unknown parameters. These unknown parameters are determined by utilizing a special identification scheme based on the black box identification method to estimate initial values of the unknown parameters, followed by the grey box identification method for accurately estimating the complete cylinder system model. The developed black-grey box identification method delivers accurate information on the cylinder dynamics. The comparison between simulation results and measured data from real world experiments demonstrates the high accuracy of the derived servo-hydraulic cylinder system model, and thus validates the effectiveness of the developed system identification method.