An experimental and theoretical study on the effect of sampling time delay on the stability of a PI position controller of a hydraulic cylinder

Abstract

The stability properties of the feedback control of a hydraulic cylinder is analysed with an emphasis on the effect of the control delay due to the discrete sampling. A simple theoretical model is developed for low-load cases, i.e. if the forces acting of the piston are small compared to the pressure forces inside the cylinder. It is shown that in the case of a traditional PI controller, there is an upper and lower limit on the proportional gain corresponding to significantly different mechanisms of stability loss. It is shown that the frequency of the appearing oscillations at low proportional gains is one range smaller than that the sampling frequency. The theoretical results are confirmed by measurements.