What is in this article?:
- Engineering Essentials: Flow-Control Valves
- Other flow controls
Flow-control valves include simple orifices to sophisticated closed-loop electrohydraulic valves that automatically adjust to variations in pressure and temperature.
Other flow controls
Flow dividers — A flow-divider valve is a form of pressure-compensated flow-control valve that receives one input flow and splits it into two output flows. The valve can deliver equal flows in each stream or, if necessary, a predetermined ratio of flows. The circuit in Figure 9 shows how a flow divider could be used to roughly synchronize two cylinders in a meter-in configuration.
Like all pressure- and flow-control devices, flow dividers operate over a narrow bandwidth rather than at one set point. Thus, flow variations in the secondary branches are likely. Therefore, precise actuator synchronization cannot be achieved with a flow-divider valve alone. Flow dividers can also be used in meter-out circuit configurations or cascaded - connected in series to control multiple actuator circuits, Figure 10.
Rotary flow dividers — Another technique for dividing one input flow into proportional, multiple-branch output flows is with a rotary flow divider. It consists of several hydraulic motors connected together mechanically by a common shaft. One input fluid stream is split into as many output streams as there are motor sections in the flow divider. Because all motor sections turn at the same speed, output stream flow rates are proportional and equal to the sum of displacements of all the motor sections. Rotary flow dividers can usually handle larger flows than flow divider valves.
The pressure drop across each motor section is relatively small because no energy is delivered to an external load, as is the usual case with a hydraulic motor. However, designers should be aware of pressure intensification generated by a rotary flow divider. If, for any reason, load pressure in one or more branches drops to some lower level or to zero, full differential pressure will be applied across the motor section in each particular branch. The sections thus pressurized will act as hydraulic motors and drive the remaining section(s) as pump(s). This results in higher (intensified) pressure in these circuits branches. When specifying rotary flow dividers, system designers must be careful to minimize the potential for pressure intensification. A pressure relief valve should be placed in any actuator fluid line where this condition may occur. Rotary flow dividers can also integrate multiple branch return flows into a single return flow.
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Proportional flow-control valves
Proportional flow-control valves combine state-of-the-art hydraulic valve actuation with modern, sophisticated electronic control. These valves help simplify hydraulic circuitry by reducing the number of components a system may require while, at the same time, substantially increasing system accuracy and efficiency.
An electronically controlled, proportional flow-control valve modulates fluid flow in proportion to the input current it receives. The valves can easily control cylinders or smaller hydraulic motors in applications that require precise speed control or controlled acceleration or deceleration. Most proportional flow-control valves are pressure-compensated to minimize flow variations caused by changes in inlet or outlet pressure.
An electrohydraulic proportional valve consists of three main elements:
- a pilot or proportional solenoid
- a metering area (where the valve spool is located), and
- an electronic position-feedback device, often an LVDT (linear variable differential transformer).
Valve operation begins when it receives a signal from an outside controlling device such as a computer, programmable logic controller (PLC), traditional logic relay, or potentiometer. The control device delivers analog electrical signals to the valve driver card, which, in turn, sends a current signal to the solenoid on the valve.
The electromechanical force on the spool causes it to shift, gradually opening a flow path from the pump to the actuator port. The greater the command input signal, the greater the current to the valve solenoid, and, thus, the higher the flow from the valve. The important feature of this proportional valve is that all elements are proportional; thus, any change in input current changes force signals proportionately as well as the distance the valve spool will shift, the size of the flow path, the amount of fluid flowing through the valve, and finally the speed at which the actuator moves.
As the spool shifts, its motion is detected and monitored very accurately by an LVDT or other type of position-feedback transducer. This signal is fed back to the driver card where it is continuously compared with the input signals from the controller. If the two differ, the driver adjusts spool position until the two signals match.
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 Vdc) or current (0 to 20 mA) command signals. The typical valve amplifier card requires a 24-Vdc 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 electronic control can make the orifice remotely adjustable while maximum spool acceleration is still limited by this internal ramp; or a 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.
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|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.|