This report deals with a theoretical and experimental analysis of the system dynamics of a solenoid-operated free piston type of compressor. The test compressor is designed to operate normally at resonant frequencies determined by a spring-mass system consisting of a piston mass, coil spring and gas spring due to the trapped air in the cylinders. Two kinds of mathematical models have been considered in the theoretical analysis : one is “Model A” in which the thermodynamical change in the state of gas in a cylinder is exactly treated with consideration given to the effects of the gas-wall and wall-environmental heat transfer and wall energy storage; the other is “Model B” in which the thermodynamics of gas compression processes in a cylinder is assumed to follow a polytropic change. It is known from a wide range of tests that the heat transfer associated with confining cylinder walls has a great effect on the system dynamics and that the simulated results of the piston displacement, gas pressure in cylinder, gas temperature in cylinder and cylinder wall temperature from “Model A” are in good agreement with the experimental results within an error around 5%. Furthermore, it has been pointed out that in the case of using “Model B”, an appropriate value for the polytropic index should be selected according to the operating conditions.

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The flow characteristics of valves and restrictions are often represented by the effective area. The effective area is often measured using a method defined in the ISO standards. The method is to measure the flow rate and differential pressure between the tested valve. Therefore, the accuracy of the method depends to a great extent on the accuracy of the flow meter.
On the other hand, a more simplified method to measure the effective area is defined in the JIS standards. This method discharges air from the chamber through the tested valve, and the effective area is measured indirectly from the pressure response of the air in th chamber. The method calculates the effective area, assuming the adiabatic conditions during discharge. Though, even in-a sudden discharge, the heat is transferred into the chamber. Therefore, this causes a measurement error.
In this study, the relation between the temperature change of the air and the measurement error of the JIS method owing to the chamber volume and the length of the pipeline are examined. Then, it became clear that by stuffing the steel wool into the chamber, the transfer area and heat transfer coefficient could be made larger and an isothermal condition could almost be realized. This chamber is called an “Isothermal chamber”. The method to measure the effective area has been proposed using an isothermal chamber. The proposed method can lessen the measurement error as a result of the chamber volume and length of the pipeline compared with the JIS method.

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As regards fluid power systems, cavitation under unsteady flow conditions plays an important role regarding their dynamic characteristics. To clarify the unsteady cavitation mechanisms, as the first step, cavitation of long orifices in unsteady flows was studied. Sinusoidal and stepwise flows were obtained from a variable displacement piston pump. Long orifices made of steel were 2.0mm and 2.5mm in diameter, and the length/diameter ratios were 5.0 and 4.0 respectively. The fluid tested was ISO VG 32.
Gavitation was detected mainly in the pressure ripples obtained by the pressure sensor positioned on the outlet side of the orifice. Also, a strobo-light and TV camera was used to observe the cavitation clouds.
The test results, related to inception and desinence of cavitation choking, were mainly shown as the relationship between cavitation number and the amplitude and frequency (0.0-1.0Hz) in case of the sinusoidal flow, and the step height of the stepwise flow. In conclusion, it could be shown that the cavitation in an unsteady flow occurs much easier than that in a steady flow and discontinuous cavitation choking with stepwise flow input could be explained by the numerical results for the unsteady pipe flows.

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To apply a pneumatic actuator more effectively for various types of automated systems, pneumatic servo technology needs to be developed. However, in using a pneumatic actuator, the load weight frequently changes. Therefore, the robust control of pneumatic servo system needs to be studied.
Pole placement has now become an accepted technique for controller design. However, when the plant parameters are uncertain the controller obtained using the conventional method may not be able to stabilize the closed-loop system. Therefore we have chosen the robust pole assignment to solve this problem. In this paper, based on the theory of robust pole assignment control, the robust speed and positioning controls of a pneumatic cylinder are studied, and the control performances are compared with the PID control ones.

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In pneumatic systems, the condensation of water due to variations in air temperature is becoming a serious matter of concern. With the growing need to improve the characteristics and reliability of pneumatic systems, this problem is recently being given a great deal of attention. In this paper, the condensation in a pneumatic system is regarded as homogeneous condensation in which the condensation does not begin until the condensation nuclei are formed spontaneously from pure vapor molecules at a sufficiently high degree of supersaturation. The features of the condensation, the conditions of generation and the influencing factors are examined using a numerical simulation with a homogeneous condensation theory, for cases in which the compressed air is being exhausted from a container at a constant volume. Moreover, the results from using the homogeneous condensation theory are compared with that from the equilibrium condensation theory based on the assumption that a large number of condensation nuclei are in the air and the interface between liquid and gas is sufficiently large and flat. The temperature and pressure variations of air, which have a great influence on condensation, are predicted by the extended BGSP.

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