In this study, the force control of hydraulic cylinders was carried out using a system consisting of a single proportional-type valve with a built-in overlap-type 4-way valve, two pressure sensors and personal computer. For the force control by using On/Off operation to the valve, we adopted a learning control and a PWM (Pulse Width Modulation) control with a constant off-time which is relatively short.
For the purpose of obtaining a high response, the PWM control with a constant off-time which is modified a general PWM control by an off-time being constant was introduced to the force control. The system was good in response with a stable off-time of 2ms. In this case, the time lag from the electric current off to the valve closure was about 1ms. In the first place, the on-time table of the PWM has been prepared for the desired force and the error in the learning control. Also, the on-time table is corrected by the control which is in progress; therefore, this control response becomes higher than the case before learning. The thrust fluctuation of the hydraulic cylinder was able to be made smaller by controlling the spool position in the off-time of the valve.

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Among high-speed On/Off valves used for PWM control of oil hydraulic pressure, a 3-way ball valve has the advantage of structural simplicity. To improve the performance of a solenoid-driven ball valve, one of the requirements is to reduce so-called 'fluid force' working on the ball. The present paper aims to clarify how such fluid force is produced in a 3-way ball valve with a steady-state oil flow. Measurements of pressure losses, flow rates, ball thrusts and displacements, in addition to computational studies on the flow around the ball, have revealed the following : the pressure falls along the two constriction gaps formed between the ball and valve seats on the supply and drain sides substantially dominate the fluid force. The gap on the drain side works to increase the thrust on the ball, while the one on the supply side decreases it. Furthermore, the structural modification of the valve seat based on the present findings has proved to effectively reduce the fluid force on the ball.

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This paper presents a method linearizing the dynamic characteristics of a valve controlled hydraulic motor-inertia load system, and an application of model reference adaptive control (MRAC) for the plant.
At first, it is shown by the experimental results and theoretical analysis on the assumption of the small perturbation from equilibrium state that a formula expressing the time constant of the load angular velocity response is approximated to a simple equation of the input. By using the approximated formula, the analog compensation circuit to eliminate the dependence on the input is composed and the fruitions of the analog compensation circuit are experimentally confirmed.
Next, the applications of the MRAC for the plant with an integral element to eliminate steady error are conducted, and it is experimentally shown that the more excellent MRAC is obtained by using the analog compensation circuit to linearize the dynamic characteristics of the plant.

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In this paper, the silting characteristics of capillary passages, whose diameters are much larger than those of the contaminants in fluids, are experimentally studied.
The parameters are : the properties of the hydraulic fluids, the concentration of additives (Amine type, Sulfonate type), the ratio of the average particle size (d*) of classified Air Cleaner Coarse Test Dust to the inner diameter of the capillary passage (D) and the inlet pressure of capillary passage. Furthermore, to clarify the effect of inlet pressure of capillary passage, the release pressure, that is, the pressure required to remove the silted particles is conformed.
The results are as follows :
1) Properties of the hydraulic fluids have a great influence on the silting frequency.
2) Silting frequency decreases with the concentration of additives (detergent dispersant) and the silting frequency of Sulfonate type is lower than that of Amine type.
3) Silting frequency increases with the average particle size. The silting may not occur below d*/D=3×10^-2. Moreover, particle size distributions have a great influence on the silting time as well as the silting frequency.
4) Even if the pressure of inlet capillary passage increases, the change in the silting frequency is not so large.
5) If the inlet pressure of capillary passage increases, the release pressure and unremoval frequency is large. Moreover, the release pressure of outer silting is larger than that of inner silting.

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