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MarkV Control System
LVDT .7 V adjustment procedure?

Can anybody suggest the correct procedure for SRV & GCV LVDT zero position .7V adjustment?

Some people putting feeler (I think 40, though not sure) gauge between the stem and plug without hydraulics supply (shut down condition) and adjusting .7V and some people starting hydraulic supply and adjusting .7V.

Which one is correct method? If some body knows please help me with correct procedures.

1 out of 2 members thought this post was helpful...

This "drawing" is NOT to scale; it's just to give you an idea of how the LVDTs that GE typically uses on their heavy duty gas turbines work.

LVDT |<-----------LVDT Linear Output Range----------->| LVDT
Non-Linear | | Non-Linear
Output | | | | Output
Range | |<----GE-Specified Output Range Limits---->| | Range
| | | |
0.58 VAC | | 3.70 VAC
0.70 VAC 3.50 VAC

The drawing (graph) shows that the linear output range of the LVDT from the manufacturer is to be from 0.58 VAC RMS to 3.70 VAC RMS. Then, GE chooses the LVDT such that when the zero stroke voltage is set to 0.70 VAC RMS, +/-0.02 VAC RMS, the output at the maximum travel of the device the LVDT will be attached to will not be greater than 3.50 VAC RMS. (It can--and often is--less than 3.50 VAC RMS, but as long as it's less that approximately 3.50 VAC RMS when the zero stroke voltage is set to 0.70 VAC RMS, +/-0.02 VAC RMS, the output will be linear.)

GE says to set the zero stroke voltage at 0.70 VAC RMS, +/-0.02 VAC RMS, so that the output of the LVDT will linear over the stroke (range of travel) of the device the LVDT is attached to. This ensures the output will be linear, because the GE-specified linear range limits are LESS THAN the LVDT's linear output range, as long as the zero stroke voltage is set correctly (usually, 0.70 VAC RMS, +/-0.02 VAC RMS).


As long as you set the zero stroke voltage at 0.70 VAC RMS, +/-0.02 VAC RMS, with the typical GE-provided LVDT, the output will be linear over the range of travel of the LVDT. And that's ALL the Mark* cares about--a linear output over the range of travel of the device the LVDT is attached to.

Now, for the gas valves GE used for decades on their heavy duty gas turbines. The valve stems of the valve WERE NOT physically connected to the valve plugs. This means the valve stems were free to move, 0.040 inches, +/-0.010 inches. This means that when the valve actuator (piston) rod is fully down (when there's NO hydraulic pressure on the system) AND the valve stem is fully up--in contact with the valve plug--there will be a gap of 0.040 inches, +/-0.010 inches, between the bottom of the valve stem and the top of the actuator (piston) rod. THIS IS THE TRUE ZERO STROKE POSITION OF THE VALVE--WHEN THE VALVE STEM IS UP (IN CONTACT WITH THE VALVE PLUG) AND THERE IS A GAP BETWEEN THE TOP OF THE ACTUATOR (PISTON) ROD AND THE BOTTOM OF THE VALVE STEM.

During calibration and verification it is necessary to put something between the top of the actuator (piston) rod and the bottom of the valve stem to ensure the valve stem stays in the TRUE ZERO STROKE POSITION during calibration and verification. That's why it's necessary to put feeler gauges to fill up the full gap between the top of the actuator (piston) rod and the bottom of the valve stem during calibration and verification. Because without the feeler gauges, the valve stem can move, and the calibration can fail. And, without the feeler gauges filling the gap between the top of the actuator (piston) rod and the bottom of the valve stem the valve stem--and the LVDT--will not be in the true zero stroke position.

But, the exact value of zero stroke voltage from the LVDT is NOT a single number; rather, it's a range: from 0.68 VAC RMS to 0.72 VAC RMS.

When the feeler gauges are removed, the valve stem may drop--indicating a negative valve stroke. THIS IS NORMAL AND TO BE EXPECTED because the valve stem is NOT connected to the valve plug, it rises to open the valve plug, and it can drop below the zero stroke position of the valve plug. It should only be a couple of percent negative, but that is normal after a proper calibration of the valve (SRV or GCV). It will certainly indicate a negative position (presuming there is a gap between the actuator (piston) rod and the valve stem!) after the first time the unit is STOPped, or tripped, and the valve spring slams the valve shut. AGAIN, this is NORMAL and TO BE EXPECTED because of the way the valve is constructed.

I don't believe you said what Frame size machine you were asking about, but this thread has another rather crude "drawing" of how the actuator rod and valve stem and valve plug and LVDTs are arranged for a combined SRV/GCV assembly such as were used for decades on GE-design Frame 5 and Frame 6B heavy duty gas turbines:

Finally, calibrating LVDTs without actually measuring the stroke and the actual device position with either the graduated scale supplied on most combined gas valve assemblies, or by using a dial indicator or some kind or calipers, is just wasting time. And, not checking the calibration using some kind of scale or measuring device is also just wasting time. What's important is: Is the scaled feedback (the indicated position after an AutoCalibrate procedure) nearly equal to the actual physical position, as physically measured ---NOT if the indicated position is nearly equal to the reference position on the operator interface display without checking the actual physical position.

And, one should always start any LVDT calibration AFTER the null bias value is set to the default value AND ensuring the servo-valve coils are receiving the proper polarity of current (in that order). It is rarely necessary to change the null bias current value from the default, and the procedure for calculating the null bias current value provided in GE Control Specification procedures is wrong; it's just not possible to calculate null bias. It has to be derived from a trial and error procedure (which is too long to go into at this writing).

So, the 0.70 VAC RMS "zero stroke" LVDT voltage value is really anything between 0.68 VAC RMS and 0.72 VAC RMS, AND it should be set with the device the LVDT is attached to at it's true zero stroke position (and for combined SRV/GCV assemblies, that is when the valve stem is up, against the valve plug. And, the feeler gauges are used during calibration and verification to ensure the valve remains in its true zero stroke position as it is moved and returned to "0".

Hope this helps!

Thanks for the prompt reply. We tried to calibrate the our valve which is using in our frame-5 machine. But still we are getting 2% error for zero stroke command after down loading new values. But when we give -25% we are getting feed back 0%. We reduced the null bias value in the IO configurator from 2.6 to 1.2, still we are getting .8% error at zero command. How can we get exact zero at zero command? Our gap is 30 thou.

Are you using a meter or the diagnostic counter page?

Try using the voltages on the diagnostic counter page to calibrate your valve.


1) When you inserted the feeler gauges you were sure that all the gap between the actuator (piston) rod and the valve stem was filled and there was no gap between the actuator (piston) rod and the valve stem. You used a piece of wood (approximately 1 metre long) to first pry down firmly but not excessively on the bar which the LVDT cores are attached to (to push the actuator rod down) then used the piece of wood to pry firmly but not excessively up on the LVDT bar to expose the gap and while holding the wood up and putting the maximum possible amount of feeler gauges in the gap. Then you removed the piece of wood and let the feeler gauges remain in the gap during the calibration and verification process.

2) You used a dial indicator or calipers to measure the distance between the base of the valve assembly and the top of the LVDT bar. Then you stroked the valve fully open to the mechanical stop and took another measurement. The difference between the two measurements would be the 100% stroke of the valve. And you used your measuring means to verify that the indicated percentage stroke was nearly equal to the actual measured stroke when verifying the accuracy of the calibration.

3) You took the AVERAGE of the 0% and 100% stroke values from the AutoCalibrste Display after the calibration and put them in the I/O Configurator, clicked on 'Verify Screen,' then exited the I/O Configurator--saving the changes--then downloaded the IOCFG partition to the three control processors.

The very act of having to average the 0% and 100% voltage values introduces error! And if the three values are significantly different from each other then the error will be larger. There's no way around this. Full stop. Period. HOWEVER, if immediately after the calibration you perform the verification and the actual physical position (using measurements) is within approximately 0.5% of the indicated position on the display, then the calibration is successful. At that time the 0% and 100% stroke voltage values in each of the three control processors are EXACTLY what they need to be. You can and should calculate the average for each of the values, put them in the I/O Configurator and download them to the three control processors--BUT YOU SHOULD NOT RE-BOOT THE PROCESSORS. Re-booting the processors replaces the exact values with the average values--which introduces an error. The unit will start and run just fine with the exact values in RAM--even better than with the average values!

Then you can remove the feeler gauges, but understand the value after a shutdown or trip will usually be a negative value, and it will not always be the same negative value--because the valve stem is free to move 0.030-0.050 inches, and nothing prevents it from stopping at any point in between the fully up and fully down positions (between 0.030- and 0.050 inches).

The ONLY time you should need to change the bill bias current value is when the actual measured stroke percentage is nearly equal to the calibrated feedback percentage (within approximately 0.5%, but it should less than that difference presuming there is no mechanical looseness in the system) BUT there is a large difference between the reference and the actual positions (more than 0.8% or so).

And then, the limit of adjustment of the bill bias current is between 1.3 and 4.0. Anything less or more than those two values to get the actual position to be within +/-0.5% of the reference value means there is a problem with the servo OR something is wrong with the hydraulic actuator.

One more thing--the default min and max position values in ACALIB.DAT are usually 0.1 and 100. This means that even if the valve stem is truly at 0.0% when AutoCalibrate will think it's at 0.1%, which introduces another small error.

If there is a 0.2% difference between the actual, measured physical position and the calibrated feedback on the HMI display AND the actual, measured physical position is very close to the reference position then the calibration was successful. Calibration is about getting the error between the actual, measured percentage position and the calibrated feedback percentage on the HMI display to be nearly equal, and a difference of as much as 0.5% is acceptable for proper operation of the machine. Adjustment of the bill bias current is only necessary if the difference between the actual, measured position percentage and the reference is greater than approximately 0.8%, and the the limits of bull bias adjustment are 1.3 to 4.0. Realize that when you change the bill bias current value, download and reboot that the average calibration 0% and 100% stroke values will overwrite the "perfect" 0% and 100% stroke voltage values, introducing error into the calibrated feedback percentage.

So, attempting to achieve perfection can cause unintended consequences and error.

Perfect 0% readings are never necessary because the Mark V will usually never be trying to control 0%.

And if this is for the SRV, the SRV regulator is a pressure control loop--not a position control loop. The valve will be moved by the Mark V to whatever position is required to make the actual P2 pressure equal to the P2 pressure reference. And that almost requires a position of 0%.

If you require additional help you will need to provide the 0% and 100% stroke voltage values from the AutoCalibrate display immediately after a calibrstion, and the average values you put into the I/O Configurator to download to the Mark V. At a bill bias current value of 2.67. And you will need to provide the the reference values and actual measured position percentage values for at least two positions other than 0% and 100% (for example, 25% reference and 75% reference).

When the feeler gauges are removed after a successful calibration the valve stem is free to move to any position between 0.030- and 0.050 inches LESS THAN 0% stroke. It will not always go to the same position. And the Mark V will usually never be trying to control 0% for any valve or device.

Hope this helps!

Hello All,

In the past we've been putting AVERAGE voltage values for 0% and 100%, as mentioned in this thread. But in several last years we are using MEDIAN values, as written in Maintenance Manual GEH-5980E.

To be honest, can't tell our calibrations are better or worse now.



We rectified the problem by replacing the servo valve with new one.



>We rectified the problem by replacing the servo valve with
>new one.

So, this is THE problem with saying "calibrate the valve" (or "calibrate the IGVs"). The implication is that AutoCalibrate does something to the servo-valve regulator in the Mark* (and it DOES NOT), and that AutoCalibrate does something to the gain or stability of the valve (or the IGVs), and it DOES NOT.

AutoCalibrate ONLY affects the scaling of the IGV feedback--NOTHING ELSE! LVDTs are inherently very stable and don't require frequent calibration/scaling.

But because people use the term "calibrate the valve" or "calibrate the IGVs" and because the IMPLICATION is that AutoCalibrate somehow affects the servo-regulator and/or the servo-valve and/or the gain of the loop people are CONSTANTLY performing AutoCalibrations trying to improve stability or hoping that it will somehow solve a problem that is perceived to be caused by the "valve" (meaning the servo-valve) or the IGVs (servo-valve).

I hope you returned the null bias current value back to 2.67 after, or during, the replacement of the servo-valve. AND, it IS NOT necessary to calibrate LVDT feedback when a servo-valve is replaced.

Finally, servo-valves do wear out over time, AND it is most often poor oil quality that causes servo-valve issues. Especially with L.O. produced after approximately the year 2000 or so (when the refiners changed the formulation of turbine lube oil to improve lubricity, which has had a negative effect on turbines that use lube oil for hydraulic systems).

Thanks for the feedback; and glad to hear you resolved your issue(s)! "Feedback is the most important contribution!"(c) here at When people take the time to provide meaningful feedback it helps everyone who reads these threads--both now and in the future.