Servo Valve
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Steam Turbine Controls Consulting JOSEPH F. BYRD, JR., INC. PO Box 52 Dunn, NC 28335 PH: (910) 658-1291 [email protected] Servo Valve The servo valve is the key to EHC control. It accepts the low level signal from the DEHC electronics and converts EHC fluid in the very high force needed to control MWs of electrical generation. The servo valve is used to control fluid flow into and out of the actuator to position the valve. The servo valve flow is proportional to an error signal. The DEHC sums its demand signal with the demodulated LVDT position signal to derive the error signal. For example, If the demand is 50% and 1) The LVDT demodulator is at 50% there is zero error. (Assuming a gain of 20:1), 2) If the demand changes to 55% the error signal will be 20 X (demand-demodulator) or 20 X (55-50)= +100% for a “go open” signal to the servo valve. 3) If the demand changed to 45%, then the error signal would be 20 X (45-50)= -100% for a “go close” signal to the servo By being proportional, the flow rate can be precisely control to move the actuator at defined rates or hold it at precise position. The graph shows that servo full rated flow at rated current either into or out and zero flow near zero demand (null) There are several designs of servo valve. There is the 1) jet tube, 2) the flapper and 3) the proportional solenoid. The design shown below is the jet tube. It operates by a torque motor controlling the nozzle of a jet tube that can port the jet fluid to either end on the spool valve. The spool position then meters the EHC fluid into or out of the actuator. I do not know the type HTC uses.
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Transcript of Servo Valve
Steam Turbine Controls Consulting
JOSEPH F. BYRD, JR., INC. PO Box 52 Dunn, NC 28335 PH: (910) 658-1291 [email protected]
Servo Valve The servo valve is the key to EHC control. It accepts the low level signal from the DEHC electronics and converts EHC fluid in the very high force needed to control MWs of electrical generation. The servo valve is used to control fluid flow into and out of the actuator to position the valve. The servo valve flow is proportional to an error signal. The DEHC sums its demand signal with the demodulated LVDT position signal to derive the error signal. For example, If the demand is 50% and
1) The LVDT demodulator is at 50% there is zero error. (Assuming a gain of 20:1),
2) If the demand changes to 55% the error signal will be 20 X (demand-demodulator) or 20 X (55-50)= +100% for a “go open” signal to the servo valve.
3) If the demand changed to 45%, then the error signal would be 20 X (45-50)= -100% for a “go close” signal to the servo
By being proportional, the flow rate can be precisely control to move the actuator at defined rates or hold it at precise position. The graph shows that servo full rated flow at rated current either into or out and zero flow near zero demand (null)
There are several designs of servo valve. There is the
1) jet tube, 2) the flapper and 3) the proportional solenoid.
The design shown below is the jet tube. It operates by a torque motor controlling the nozzle of a jet tube that can port the jet fluid to either end on the spool valve. The spool position then meters the EHC fluid into or out of the actuator. I do not know the type HTC uses.
Steam Turbine Controls Consulting
Page 2
JOSEPH F. BYRD, JR., INC.
NULL The demand is near zero, the jet directs equal flow (purple) to each end of the spool valve to position it with the 1st land at null, so the fluid in the actuator (blue) is held constant with no EHC fluid (red) being admitted nor going to drain (green) and the piston holds its position.
This mode is represented with the servo in the center position
Given there is leakage in the piston, for DEHC to hold a precise position there will be a slight positive error (and servo flow into the actuator).
Steam Turbine Controls Consulting
Page 3
JOSEPH F. BYRD, JR., INC.
OPENING The demand is near +100%, the torque motor positions the jet to direct flow (purple) to the #1 end of the spool valve and drains fluid (green) from the #2 end to position it with the 1st land at full open, so EHC fluid is admitted into the actuator (red) and the piston opens at maximum rate (full stroke in a few seconds).
This mode is represented with the servo at the end position
Steam Turbine Controls Consulting
Page 4
JOSEPH F. BYRD, JR., INC.
CLOSING The demand is near -100%, the torque motor positions the jet to direct flow (purple) to the #2 end of the spool valve and drains fluid (green) from the #1 end to position it with the 1st land at full drain, so EHC fluid is drained from the actuator (green) and the piston closes by spring force at maximum rate (full stroke in a few seconds).
This mode is represented with the servo at the other end position
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