Fundamental Analysis of an Electrohydraulic Servomechanism Operated by PWM Mode

Abstract

When an electrohydraulic servomechanism is used as a high performance servo, the PWM mode is interesting for the reduction of required accuracy in manufacturing valves and for removing the effects such as frictions of various parts. The manufactured servomechanism is composed of a torquemotor, a flapper-nozzle, a spool valve and an actuator. The input current to the torquemotor is in the form of a pulse-width-modulated wave supplied by a multivibrator, and the displacements of the flapper and the spool are restricted by the stoppers. The forces acting on the flapper caused by jets from nozzles are analyzed, and it is shown that the forces become as small as negligible, and sometimes accelerate the the flapper movement with a proper choice of discharge coefficient of nozzle. This means that the flapper-nozzle is favorable for a preamplifier in the PWM mode operation. As the results of the analysis, the displacement of the spool is approximately in the form of a trapezoidal pulse with certain time lag for the input to the torquemotor. When the load of the actuator consists of a mass and friction, the dither amplitude of the actuator piston is obtained exactly for the trapezoidal pulse input. The pulse period is determined by the waveshape of the spool displacement and the dither amplitude of the actuator piston. The analytical results were verified by experiments.