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10.00 FLUID POWER SYSTEMS Task 10.60: Describe the operation and application of electric flow control devices.
Flow control valves influence actuator speeds in a fluid power system. Increasing or decreasing the opening to
Outcome 10.60.1: Given the describes the and appliflow at a throttling point adjusts thefluid flow topower optimizeapplication; the system performance. This flowoperation at a throttling point may cation of hydraulic flow control devices. be calculated dependentelectric on the type of orifice, pressure drop, and the type of fluid. The electronic proportional flow
control is similar to standard two-port flow controls except the main control orifice is adjusted electronically. The Flow control valves influence actuator speeds in a fluidwith power system. Increasing or decreasing theof opening to control orifice has been modified to work in conjunction a force-controlled solenoid. If the position the spool flowisatmonitored a throttling adjusts the flow the system performance. This flow at athen throttling point withpoint a feedback device suchtoasoptimize a linear variable displacement transformer (LVDT), a strokemay be calculated dependent on the solenoid type of is orifice, drop and the type of fluid. The electronic controlled used topressure control the valve opening. proportional flow control is similar to standard two port flow controls except the main control orifice is adjusted electronically. The control orifice has been modified to work in conjunction with a force controlled solenoid. If the position of the spool is monitored with a feedback Specifically, two-port flow condevice asuch as proportional an LVDT pressure (linear compensated variable displacement trol valve has an electrically control orifice connected in to transformer) then aadjustable stroke controlled solenoid is used A2 the valve opening. series control with a hydrostat (essentially a pressure-reducing valve), as shown
in the illustration. Specifically, a two port proportional pressure compensated P1 Theflow hydrostat is located the main control orifice and control valveupstream has an of electrically adjustable control A is heldorifice open byconnected a light spring. input signal to the(essentially solenoid is a in When seriesthe with a hydrostat P2 P3 B zero, the light spring force holds the main orifice closed. When pressure reducing valve). See control Fig. 10.24. the solenoid is energized, the control orifice opens. When the solenoid The hydrostat is located upstream of the main control is open, the control orifice opens in proportion to the control signal and orifice and is held open by a light spring. When the input allows the oil to flow from port A to port B. Hydrostat signal to the solenoid is zero, the light spring force holds At the same time, the LVDT, if so equipped, provides the necessary 10.00 FLUID POWER SYSTEM the main control orifice closed. When the solenoid is feedback to hold position. In thisorifice case, the LVDT provides feedback to is energized, the control opens. When the solenoid Outcome 10.60.2: Given the fluid A3power application; describes the operation and appl maintain a very setting. open theaccurate controlorifice orifice opens in proportion to the control cation of pneumatic electric flow control devices. C Thesignal controland signal provides opening and of allows thetime-controlled oil to flow from port A toclosing B. An the electropneumatic proportional valve is an open loop flow control valve that controls the flow rate the orifice. For reverse free flow, a check valve may be built into accordance At the same time, the LVDT, if so equipped, provides thewith current through the coil. However, the system can incorporate a feedback loop as shown valve. Proportional flow control valves are available with either the linear example rpm Proportional of an air motor, Fig. 10.25. Fig. the 10.24 Pressure necessary feedback to hold position. In this case, the of a circuit controlling Proportional pressure-compensated flow control Compensated Flow Control or progressive flow characteristics. The input signal range is the same LVDT provides feedback to maintain a very accurate for both. However, the progressive flow characteristic gives finer control orifice setting. at the beginning of orifice adjustment. In case of electrical power or Power amplifier The control signal provides time-controlled opening and closing of the orifice. For reverse free flow, a check control feedback failure, the solenoid force drops to zero, and the force exerted (feedback circuit number of valve may be built into the valve. Proportional flow control valves are available with either linear or progressive circuit by the spring closes the orifice. Typically, when feedback wiring is conbuilt-in type) rotations flow characteristics. The input signal range is the same for both. However, the progressive flow characteristic nected incorrectly or damaged, the system uses an LED to indicate the gives finer control at the beginning of orifice adjustment. In case of electrical feedback power or feedback failure, the malfunction on the amplifier solenoid force dropscard. to zero and the force exerted by the spring closes the orifice. Typically, when feedback Some flow control valves consist of a proportional-force solenoid wiring is connected incorrectly or damaged, the and system has an LED (light emitting diode) indicating the tachometer a pilotmalfunction controller. Because thisamplifier valve is relatively on the card. unaffected by changes in system pressure, it can open and close the orifice in the same length Some flow control valves consist of a proportional force solenoid and a pilot controller. Because this valve is of time. This maximum time can be changed on the amplifier card by relatively unaffected by changes in system pressure, it can open and close the orifice in the same length of electro-pneumatic electropneumatic airadjusting motor a built-in ramp adjusting a built-in ramp generator. time. This maximum time can be changed on the amplifier card by generator. proportionalvalve valve proportional An electropneumatic proportional valve is an open loop flow control type, (flow two port) valve that controls the flow rate in accordance with current through the coil. However, the system can incorporate a feedback loop as shown in Controlling rotation of air motor the illustration of a circuit controlling the rpm of an air motor. Fig. 10.25 Controlling Rotation of Air Motor Under normal conditions, what directly controls the output rpm of the air motor? a. Supply pressure to the electropneumatic c. Dither level to the solenoid. proportional valve. d. Restrictions in the muffler. b. Flow rate through the electropneumatic e. None of the above. proportional valve. See page 31 for the solution. WWW.IFPS.ORG • WWW.FLUIDPOWERJOURNAL.COM
NOVEMBER 2020
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