In this paper, the frequency characteristics of the PWM mode electrohydraulic servo are analyzed. A two-stage electrohydraulic servo was manufactured for experiments. A multivibrator was used for generating a rectangular pulse train and it was modified to obtain a high modulation gain of the pulse width. A spool in the servo valve moves in the waveshape of a trapezoidal pulse train. From the analysis of the spool movement, the equivalent transfer function of the PWM mode was obtained, and the frequency characteristics of the servo loop was analyzed. The various problems accompanying the PWM operation are also shown. The analytical results were verified by experiments
