Hysteresis reduction in hydraulic proportional valve control : Tuning parameters in dither compensation method

Abstract

The purpose of this thesis was to examine the phenomenon of hysteresis occurring in hydraulic proportional valve control and to find out a parameter tuning method in dither compensation to achieve an optimal reduction of hysteresis.

The existence of hysteresis causes nonlinearity in proportional valve resulting the controlled system cannot achieve the desired control effect. The main reason that causes hysteresis is static friction or stiction between the spool and the valve body. To eliminate the negative effects of stiction in a valve, the dither compensation method has been found in practice. A dither signal is superimposed on the PWM command signal with a specific amplitude and frequency to always keep the spool valve in micro-movement. Further, it has been discovered that a low frequency PWM can efficiently reduce hysteresis.

To examine the valve dynamic and the dither effect on it, a model of a proportional valve and several dynamic friction models were executed in this thesis project. Based on the relationship between dither parameters, stiction compensation and valve performance, a parameter tuning method was proposed to obtain optimal hysteresis without causing any other problems. A series of simulations and practical experiments were conducted to verify the effectiveness of dithering and the correctness of the parameters tuning method. The simulation and the measurement results showed that optimal hysteresis could be achieved with a proper set of PWM frequency, dither frequency and dither amplitude.