Many studies on the control of an oil hydraulic servo system have been carried out. The adaptive or robust control schemes are useful improving the performance of the controlled system. In this study, we have proposed such a robust servo system based on a combination of a disturbance observer and sliding mode control. Also, to eliminate chattering phenomena, we used a sliding mode control with an accelerative reaching rule.
From the viewpoint of climinating the chattering phenomena and a reduction in the disturbance, a sliding mode contol with an accelerative reaching rule working in combination with a disturbance observer was employed to control the positioning of an oil hydraulic cylinder. The experiments were carried out under several conditions, and the results obtained were compared with those of the conventional PID controller. As a result, the proposed method has shown good control performance in that the robust and stable positioning of the oil hydraulic cylinder can be achieved accurately under undesirable chattering.

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This study concerns with a semi-active damper, in which a pressure regulating mechanism similar to a balanced piston-type relief valve is built-in. In the previous paper, a mathematical model for the damper was derived and it was pointed out by numerical simulation that the system is less stable in the extended stroke than in the compressed stroke. The present paper focuses on the stability of the extended stroke. A modified equivalent model has been proposed to obtain the steady operating point. The effects of system parameters on the stability are investigated on the basis of this model. Furthermore, the effect of the damping coefficient of the pilot valve is investigated by a reduced order model, and it is clarified that the increase in the damping of the pilot valve deteriorates the stability of the system.

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A hydraulic holding valve of the poppet type has been modified and improved in order to prevent the occurrence of cavitation and reduce cavitation noise near the orifice. The ways in which cavitation occur are investigated by visually observing the flow using half-cut models of the valves in case of converging flow. Test facilities were designed to control the flow rate, upstream pressure, downstream pressure, and valve lift. Moreover, good results regarding noise reduction could be obtained by noise measurements of the half-cut models of the improved valve. Especially, with respect to the relation between the forms of occurrence of the cavitation and noise level, the detailed studies were undertaken by changing Reynolds number and cavitation number.

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Structural vibrations of an oil-hydraulic pump working under real operating conditins have been investigated using a FEM analysis so as to obtain fundamental design data to reduce the pump's audible noise level through the use of a vibration control technique. A variable-displacement, swash-plate type axial piston pump was used as the test pump. Particular attention was paid to the FEM analysis by substituting the moving excitiing force and moments due to the transient pressure fluctuation in the cylinder near the bottom and top dead centers and due to the rotary motion of the cylinder block with some equivalent forces capable of handling the FEM analysis, as well as assigning appropriate coupling conditions for the contacting surface between the bolted flanges and the L-type bracket, for carrying out accurate modeling for the forced vibration of a three-dimensional solid structure with a moving load being applied. Especially for the former, a new method capable of replacing the real moving exciting force and moments accurately with the stationary virtual forces applied on the three node points of the FEM model has been devised.
The harmonic amplitudes of casing surface vibration simulated by the FEM analysis agree relatively well with measured results for its complex structure for almost all harmonics up to around 3 kHz except for the frequency ranges around some resonances and anti-resonances. It has also been shown that the pump casing vibrations in the direction of trunnion axis was mainly excited by the pressure ripple in the delivery line.

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