Our negative pressure pneumatic servo system operates in the region ranging between atmospheric pressure and negative pressure produced using a vacuum pump. A labyrinth sealed power cylinder is used as a very small friction actuator because the working force on the piston of ordinary size is comparatively small. The air is regulated by a couple of 3 ports solenoid valves with on-off actions. Accordingly, limit cycles appear naturally. However, the limit cycles can be suppressed by proper feedback compensation, and around the stationary states, the control will be nearly equal to the PWM mode. The negative supply pressure is set at-0.052MPa. The bandwidth is estimated to be about 1.8Hz from the frequency response. Stic slips hardly occur against slow velocity inputs.

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This study deals with hydrostatic transmission (HST) which consists of a variable displacement axial piston pump connected in a closed circuit to a load cylinder. The cylinder adopted in this HST-system is an actuator instead of a hydraulic motor in conventional HST-systems in order to realize a translational motion control. For the variation of the swashplate angle of the pump, a hydraulic rotary actuator is used. This actuator is driven by two on-off solenoid valves based on the differential PWM method, for the purpose of making it easier for the HST-system to introduce a digital control algorithm. The system is designed by using an optimal control scheme. It is verified by experiment and digital simulation that the system has good control performance.

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A hydraulic pressure control system with an accumulator and a servovalve has nonlinear and complicated dynamics which is induced by the gas dynamics of the accumulator and the nonlinear flow characteristics of the servovalve. Because of this, the effect of pressure control using the ordinary linear control method is insufficient. Therefore we apply the time delay control (TDC) method to this system. Using the state signal information that is observed before one sampling time step, the TDC method enables estimation of the influences of unknown dynamics and unexpected disturbances, and generation of a control signal to cancel their effects.
However, if the control element has a delay characteristic, the ordinary TDC method may render the system unstable. In this paper, an extended TDC pressure control method is proposed. This proposed control method shows that by shifting the sampling instant of the state signal from the output instant of the control input, it is possible to avoid the influence of the delay characteristics of the control element. The effect of this control method is confirmed by simulation and experiments.
Moreover, as an example of its application, we apply the extended TDC method to pressure control system of a hydraulic suspension mechanism which consists of an accumulator and a cylinder with a mass and a spring load.

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