This paper introduces the composition of electro-hydraulic servo control system. Especially, the triangular wave generator, PWM principle and the power amplifier stage circuit in the servo amplifier are described. Finally, the frequency response of servo valve to the PWM driving signal is analysed.

The power amplifier has five voltage inputs ( one inverting) and acurrent irput O-20mA. Adjustments for set zero point or deadband

compensation and for gain allow the amplifier to be easily tuned to theproportiona1 contro1 valve The ramp fnction generator can be switched onand off using the " ramp enab1e”contro1.