An automatic power transmission has some pressure control valves for wet-clutch engaging or band-brake tightening control. Recent transmissions are being changed from hydromechanical control systems to electrohydraulic systems for improvement of shifting quality and control flexibility. A conventional hydromechanical system has a large accumulator for the smooth pressure control.; however, the size is sometimes a problem during installation on vehicles.
In this paper, a two-stage pressure control valve is applied to the control system, consisting of a PWM control switching valve and a secondary pressure control valve. The secondary valve is provided with a hydraulic position limiting mechanism for reducing the pressure surge. The electrohydraulic pressure control system is applied to advanced automatic transmissions. The system reduced the torque change at gear-stage shifting by half.

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In an oil hydraulic system such as a power steering system, a disk valve is used with a constant flow hydraulic source to regulate the pressure acting on a hydraulic cylinder. In such a disk valve, cavitation occurs when the inlet pressure of the disk valve is high, that is, the clearance between the valve seat and disk is small. On the disk valve, the flow characteristic, pressure distribution and thrust force were formerly investigated, but very little could be found about its cavitation characteristic.
In this paper, the cavitation and pressure characteristic of a disk valve are investigated theoretically, and experimental investigations are also performed. The results of this study are as follows :
(1) In the theoretical flow model in this paper, the constriction at the entrance of the valve clearance is considered. And the flow near the entrance is treated like an orifice flow and the other part is treated like a radial viscous flow in parallel disks. By this model, one can continuously treat the whole flow from valve inlet to valve outlet.
(2) The pressure and cavitation characteristic predicted by this theory agree with that measured by the experiments.
(3) When the cavitation is occurring, the relationship between the inlet pressure and valve clearance is not influenced by the outlet pressure.
(4) The cavitation is difficult to occur with an increase in the radius of the disk, or with a decrease in oil temperature.

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It seems to be useful to apply the model reference adaptive control (MRAC) method for pneumatic servo systems. Since pneumatic servo systems are easily effected by the change in system parameters such as the load mass or supply pressure, etc.
The MRAC method for the pneumatic cylinder position servo system using a diaphragm-type electro-pneumatic converter is proposed in this paper. The converter acts with a very short lag time and has a simple first order transfer function. So the total transfer function of the servo system can be represented to the lower order system and this is suitable for the execution of MRAC.
The experimental tests were carried out by using a 200-mm stroke and 40-mm bore single rod-type air cylinder. The reference model was set to the second order system which has an optimal dumping factor. Also, the response of the MRAC was tested under the various input conditions such as a rectangular signal, sinusoidal signal or triangular signal was yielded, respectively.
As a result, the accommodations for the change in load mass, supply pressure or magnification of servo amplifier could be clarified.

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Measurement of unsteady flowrate through pipelines and equipment is very important to analyze dynamic characteristics of hydraulic control systems.
In this research, we propose two approaches, the remote and quasi-remote measurement method for instantaneous flowrate using hydraulic pipeline dynamics which are well-established in the form of transfer matrix relating to the pressures and flowrates between two cross sections (distance L) of the pipeline.
In particular, the remote measurement method of instantaneous flowrate for estimating flowrate through an arbitrary cross section from the measured values of pressure and flowrate at the another remote cross section are investigated in detail.
The estimated flowrate waveforms are compared with the measured flowrate waveforms directly by another cylindrical choke type instantaneous flowmeter, which is used for calibration. The excellent agreement obtained between the estimated and directly measured flowrate waveforms illustrates the validity of the proposed method.
It is shown that the remote measurement method of instantaneous flowrate proposed here is useful in estimating flowrate through the arbitrary cross section of hydraulic pipeline under unsteady laminar flow.

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