What is in this article?:
Flow-control valves include simple orifices to sophisticated closed-loop electrohydraulic valves that automatically adjust to variations in pressure and temperature.
Pressure-compensated proportional flow-control valves are 2-port valves in which the main control orifice is adjusted electronically. Similar to conventional pressure-compensated flow-control valves, a pressure-compensated proportional flow-control valve maintains constant flow output by keeping the pressure drop constant across the main control orifice. The proportional valve, however, is different in that the control orifice is modified to work in conjunction with a stroke controlled solenoid.
In a 2-port, pressure-compensated proportional flow-control valve, an electrically adjustable control orifice is connected in series with a pressure reducing valve spool, known as a compensator, Figure 11. The compensator is located upstream of the main control orifice and is held open by a light spring. When there is no input signal to the solenoid, the light spring force holds the main control orifice closed. When the solenoid is energized, the solenoid pin acts directly on the control orifice, moving it downward against the spring to open the valve and allow oil to flow from port A to port B.
At the same time, the LVDT provides the necessary feedback to hold position. In this case, the LVDT provides feedback to maintain a very accurate orifice setting.
Pressure compensation is achieved by incorporating a pilot passage at the inlet of the valve that connects to one side of the compensator spool, A2. There is another pilot passage located near the outlet of the valve beyond the control orifice, and it is connected to the opposite side of the compensator spool, A3. A bias spring on this side of the spool keeps the compensator in the open position. Load-induced pressure at the outlet port - or pressure deviations at the inlet port - modulate the compensator spool to increase or decrease the pressure drop across the compensator's metering orifice. Acting as a pressure reducing valve, the compensator ensures that the main control orifice sees a constant pressure drop. When the pressure drop is constant, the flow remains constant.
The amplifier provides time controlled opening and closing of the orifice. For reverse free-flow, check valve C, built into the valve, provides a flow path from port B to A. Proportional flow-control valves are also available with either linear or progressive flow characteristics. The input signal range is the same for both. However, the progressive flow characteristic gives finer control at the beginning of orifice adjustment.
In case electrical power or feedback is lost, solenoid force drops to zero and the force exerted by the spring closes the orifice. When feedback wiring is connected incorrectly or damaged an LED indicates the malfunction on the amplifier card.
Proportional flow logic valves
Proportional flow-control logic valves are basically electrically adjustable flow controls that fit into a standard logic valve cavity. The cover and cartridge are assembled as a single unit, with the cover consisting of a proportional force solenoid and a pilot controller, Figure 12.
When an electrical signal is fed into an electronic amplifier, the solenoid and controller adjust the pilot pressure supplied from port A to change spool position. An LVDT then feeds back the position to the amplifier to maintain the desired orifice condition for flow from port A to port B. The proportional logic valve is available with either linear or progressive flow characteristics, and the valve drivers respond to voltage (0- to 10-V DC) or current (0- to 20-mA) command signals. The typical valve amplifier card requires a 24-V DC power supply.
Because the valve remains relatively unaffected by changes in system pressure, it can open and close the orifice in the same length of time. This maximum time can be changed on the amplifier card by adjusting a built-in ramp generator.
The amplifier can be used in several ways. An external potentiometer can make the orifice remotely adjustable while maximum spool acceleration is still limited by this internal ramp; or a limit switch can be added to turn the ramp on and off. In case of power failure, the element will return to its normally closed position.
|Which flow control to use for an application|
|Application||Type of flow-control valve|
|Load on the actuator and supply pressure both are constant: ±5% accuracy||Non-compensated, fixed or variable flow control, depending on application|
|Load on the actuator, supply pressure, or both undergo changes: ±3-5% accuracy||Pressure-compensated, fixed or variable flow control, depending on application|
|Load on the actuator, supply pressure, or both change, and fluid temperature varies ±30° F (±17° C): ±3-5% accuracy||Pressure- and temperature-compensated, fixed or variable flow control|
|The best type of flow control valve to use depends on the design parameters of the application. Above are general guidelines based on common application characteristics.|