| 摘要 |
A hydraulically actuated control valve normally includes a hydraulic cylinder which has an internal piston connected through a shaft to the actuating mechanism in the control valve. Ports are provided on each side of the piston through which fluid flows to move the piston axially inside the cylinder. The piston can be controlled electrically by an electro-hydraulic servo-controlled valve which includes an electrical input, a mechanical feedback input, and first and second outputs. The outputs are connected to the two ports in the cylinder, a mechanical feedback is coupled to the shaft and the mechanical feedback input of the control valve, and an electrical circuit is coupled to the electrical input of the servo-controlled valve to signal the hydraulic portion of the valve to either move the shaft in a manner to close the valve or move the shaft in a manner to open the valve. An improved control apparatus is provided which serially connects a solenoid valve in one of the lines of the hydraulic output of the servo-controlled valve with one of the ports in the cylinder. The solenoid valve can be bypassed with a needle valve or a controllable valve. A mechanical actuator on the shaft is coupled through an electrical circuit to the solenoid valve. Thus, if the valve is signaled to close at a high rate of speed at a predetermined time, the mechanical circuit will trigger the solenoid valve, causing the speed to diminish to a preselected rate in order to prevent water hammer. As the valve reaches a position where it is nearly closed, an electrically controlled valve can be substituted for the solenoid valve so that the rate of closure can be other than linear if desired. Furthermore, the circuit can be modified to change the opening of the valve in any desired manner.
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