The basic characteristics of hydraulic cylinder drives are fairly high natural frequency and low damping. In order to achieve good positioning accuracy a quite high position loop gain is required due to the nonlinearities of a drive. MOst severe nonlinearities in a hydraulic position servo cylinder drive are friction force, load force and the nonlinearities of a servo/proportional valve. The nonlinearities of a valve have the biggest influence on the positioning accuracy of a hydraulic servo cylinder drive in many applications. When the cost of servo/proportional valve is tried to keep low, the valve should be without electrical feedback. This means that the remarkably big null point drift of a valve might occur. The null point drift is a function of the variations of temperature and pressure. The null point drift of a so called industrial quality servo/proportional valve might be several percent of the maximum control signal of a valve. In this paper different factors influencing the accuracy of hydraulic cylinder drive position servos are presented. Their relative influences are discussed. Especially the influence of the null point drift of a valve on the position accuracy of a position servo is studied and a compensating principle is presented. Its usefulness is studied experimentally.
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