It has been widely expected that better control performance could be obtained by means of more effective utilization of future information. One of the useful methods for improving designed control system performance is to utilize the future desired signal, if at all possible. Recently, a synthesis method for digital optimal control system with a preview feed-forward compensation has been developed by T. Tsuchiya, H. Kawamura and T. Egami.
The object of this study is to find out whether their method is applicable to an electro-hydraulic servo system. A system with a preview feed-forward compensation was designed and the performance of the system was investigated by both experiments including computer simulation. It was verified that the performance of the electro-hydraulic servo system is improved considerably by employing the synthesis method.

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The refrigerated dryer mentioned in this paper is one of types of air purifiers, and is one of the popular types of compressed air dryers. The refrigerated dryer consists of a precool-reheater, main cooler, and refrigerator. In the precool-reheater, the cold humid air delivered from the main cooler is heated by the hot air delivered from the after-cooler, and changes into dry air, the relative humidity of which is fairly low. In the main cooler, the slightly hot humid air delivered from the cooling process in the precool-reheater is cooled further by the cooling medium in the refrigerator to the minimum temperature, which means a dew point of compressed air, and changes into cold and saturated humid air. The main cooler requires external energy to cool the compressed air, but the precool-reheater does not. For this reason, to miniaturize a refrigerated dryer, it is very important to reduce the thermal load in the main cooler. In this paper, the reduction in the thermal load in the main cooler is estimated with the power-saving factor that means the ratio of heating rate in the precool-reheater to one in the dryer, which is investigated theoretically and experimentally. As a result, the effect of the thermal conductance of the precool-reheater on the power-saving factor is clarified, considering compressed air temperatures at the main locations in the dryer. In addition, a proper guide for the design of a refrigerated dryer is derived, paying attention to the relationship not only between thermal conductances of an after-cooler and a refrigerated dryer but also between ones of a precool-reheater and main cooler.

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For more stable electric power service, constructional maintenance of high-voltage electric power distributed lines should be done without any breakdown and bilateral manipulators to handle active electric power lines have been developed. For such manipulators, insulated electro-hydraulic actuators are suitable from the viewpoint of performance. However, for remote control with visual sensors, and moreover, the self-organized control of the manipulator, an accurate control method for the hydraulic bilateral manipulator has yet to be sufficiently investigated. Especially, due to the oil compressibility and flexibility of the insulated actuators, the system loop gain is much lower, and the stiffness of the system cannot be maintained higher. In this study, we have examined the accurate positioning control method for electro-hydraulic manipulators with insulated actuators regarding sturdiness against any system parameter change. As a result, it was confirmed through experiments that the robust control method using a disturbance observer has a sizable effect on lessening the null point shift of servo valves, and against the case of a decrease in the system loop gain, and also the case of an increase in the inertia of the arm and the load.

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This paper is concerned with the lumped parameter approximation for pipeline dynamics in hydraulic servomechanisms in a numerical simulation, so as to reduce the huge CPU time of the distributed parameter system. Lossless line is treated here. The pipeline length being valid for a single lumped parameter approximation is found to be 1/20 or less of the propagation wavelength if the stability limit is adopted as the criterion for accuracy. This is much stricter but gives more correct results for stability and nonlinearity than the criterion based on the response waveform conventionally used. The allowable length for approximation can be greatly extended by an appropriate distribution for the pipeline capacitance of the “π type” circuit. The optimum distribution ratio is almost constant over a wide range of operating conditions.

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