This study deals with a hydraulic actuator system which is operated by two 2-way solenoid valves in the PWM-Digital mode. The solenoid valves act as converters of electronic-pulse signals to hydraulic-ones.
It is considered that the dynamic characteristics of the system depend largely on the be h avior of on/off action of the solenoid valves. The response performance of the system, for example, might be inevitably affected by the time lag existing between the input and output signals of the valves. Details of such effects of the solenoid valves, however, have yet to be satisfactorily clarified, mainly because an adequate mathematical model for the system has not been developed.
The object of this study is, therefore, to prop o se a mathematical model for the PWM-actuator system. The dynamic charateristics of the system are investigated by the method of digital simulation based on the proposed model. The simulated time responses are compared with the measured ones to confirm the validity of the mathematical model.

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In oil hydraulic systems, various restrictors are used. When cavitation occurs, it produce, a violent noise. Although the multi-stage restrictor is proposed to prevent the cavitation, very little is known about its cavitation performance.
This paper deals with the cavitation in a two-stage restrictor consisting of an upstream orifice and a downstream round edged choke, in which the upstream oriffice does not affect the pressure flow characteristic of the downstream choke. The results obtained from the tests in such restrictors are as follows:
(1) When two restrictors are in series with the distance of more than 100mm, the flow characteristics of up-and downstream restrictors are the same as that of individual restrictors.
(2) The critical conditions of cavitation inception on an upstream restrictor are not affected by the downstream restrictor and they are identical with those of individual restrictors.
(3) When the cavitation does not occur on an upstream restrictor, the critical conditions of cavitation inception on a downstream restrictor are the same as those of individual ones without an upstream restrictor.
(4) However, when the cavitation is occurring on an upstream restrictor, the critical outlet pressure for cavitation inception on a downstream one is higher than that observed on an individual restrictor under the same inlet p ressure.
(5) In the case of a two-stage restrictor, the cavitation noise is much less than that of a single-stage round edged choke.

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