With the recent increased concerns over global environmental problems, the water hydraulic technique using pure tap water as a pressure medium has become a new drive source comparable to electric, oil hydraulic and pneumatic drive systems. This technique is also preferred due to its power, high safety against fire hazard in production plants, and easy availability. However, the main problems for precise control in a water hydraulic servo motor system are steady state errors and overshoot due to its large friction torque in the low velocity range of a motor. This accounts for the development of a strategy to compensate for such uncertainties and to ensure a closed—loop system maintain its specified performance.
In this research, the rotational angle and velocity control of a water hydraulic servo motor were considered with the H_∞ control, the disturbance—observer—type control, the sliding mode control (SMC) and the adaptive control methods. We examined the motor control performance for load fluctuations. In this paper as the first report, controllers with H_∞ theory were designed and applied to the water hydraulic servo system. This theory can explicitly treat uncertainties in a plant based on the mathematical model and/or frequency response. Two types of controller were designed, an 1DOF(Degree Of Freedom) —H_∞ controller and a 2DOF controller. In the latter, the feedback controller was designed with H_∞ theory. The control results show the easy implementation of controllers and the superiority of the 2DOF controller for angle and velocity control in the 1DOF—H_∞ case.

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A water hydraulic system has many benefits compared with other existing drive systems. Its greatest merits with respect to application to the manufacturig plants are its environmental friendliness and non flammability. On the other hand the control problems arising in water hydraulics are serious because this system has greater friction compared to oil hydraulics which has a good control performance. In this research, several robust controller design approaches were examined and compared for the water hydraulic servo motor system.
In this paper as the second report, the control performances with a disturbance—observer—type control and a sliding mode control are discussed and examined. The former algorithm estimates the disturbance to be one of the system state based on the difference between a nominal and real system, and uses it to cancel the disturbance directly. On the other hand, the latter constrains the system states to a designed hyperplane which satisfies the desired dynamical properties. In spite of the fact that these methods are often applied to the oil hydraulics, no papers have reported on how they are effective for a water hydraulic system. Moreover a controller combining both strategies was also applied to the system. The experimental results show that the combined controller gives better performance in terms of rotational angle control than H_∞ design method.

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The pressure regulator is a substantial element in a pneumatic system. In this paper, we propose a regulator that achieves precise pressure control with high a dynamic response using a pressure differentiator (PD sensor) which we have developed. The regulator consists of an isothermal chamber, a servo valve, a pressure sensor, a laminar flow meter, and the PD sensor. Slight changes of pressure in the chamber can be detected by the PD sensor and is fed back to the servo valve to maintain the pressure at a desired value. The effectiveness of the developed regulator was confirmed experimentally in comparison with commercially available pressure regulators.

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