Methods for analyzing the response characteristics of a pulse-width-modulated electro-hydraulic servomechanism are presented. Except for the modulator, the hydraulic control system is of conventional configuration. The modulator transfer characteristic is derived from a double Fourier series analysis of the pulse-width-modulated signal and this indicates the effect of the modulation parameters on the system performance. The valve flow which controls the mechanical load motion has been shown to be the dominant nonlinearity in a valve-controlled hydraulic system, and the system becomes more complicated due to the valve spool dither which is a consequence of the pulse-width-modulated signal. A dual-input describing function is developed for the equivalent valve flow gain in the determination of the system closed-loop frequency response. A numerical simulation is used to obtain the closed-loop transient response. Experimental verification of the theoretical analyses is carried out on a practical system. Good agreement is obtained between the predicted and the experimental responses.

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