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Mark 5 SRV Control
Rebuilt SRV is swinging

We have installed a rebuilt Young & Franklin SRV on one of our 2000 vintage GE 7EA machines with DLN 1.0. It has an ~.035" swing in manual mode and it won't give .7-3.5 vrms range on the new style RVDT that is wired per the instruction. The best we can get is 1.3-3.5 vrms. We took a good working valve out of this unit while it was on outage and put it in a unit that was swinging and available to fix that unit.

Has anyone swapped from the old style RVDT (4 wire) to the new style RVDT(5 wire) using a Mark 5 "B" panel? Does reducing the feedback span from 2.8 vrms to 2.1 vrms cause a swing due to reduced resolution?

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Many of the RVDTs used with Fisher Cam Vee-Balll SRVs had a different linear range than the LVDTs used on older SRVs and other devices. In some cases it was necessary to reduce the zero-stroke voltage to something around 0.5 VAC RMS, and the 100% stroke voltage was sometimes around 2.1 VAC RMS, or even less. (As wit many things, that was never well documented...) The Mark* doesn't care what the zero- and 100% stroke voltages are, as long as the output between them is linear. If the instability is only at mid-to upper stroke it may be that the zero-stroke voltage is set too high which is causing the output voltage to start to "roll over" at larger strokes.

Certainly, it doesn't help if the linear range is small, but it should be possible to get relatively stable operation.

Some of the other possible causes of instability could be: one servo coil is receiving the wrong polarity; the servo is the wrong one for the application (happens all too frequently, unfortunately), and a problem with the hydraulic actuator ( which also seems to happen more than it should), and incorrect servo null bias current. You didn't say what the servo null bias current value was in the I-O Configurator. And, you didn't say if the servo current gain values were the same for both units. There were a lot of commissioning people who would change the gain value to achieve what they perceived to be stable operation but caused problems when the valve was replaced. What are the current gain values for the two units? And the other I/O Configuration Constants for the two SRVs--are they all the same? (Some are on a different screen in the I/O Configurator.)

There have been reports of incorrect servo null bias spring adjustment on new, out-of-the-box servos. And, many refurbished servos have had spring adjustment problems, also. (It's virtually impossible to duplicate test bench conditions in the field to properly check or set null bias springs.)

I have had repeated experience with new, out-of-the-box servos with incorrect coil lead colors, so verifying polarity is very important.

Finally, some of the RVDTs were just never right and had to be replaced. In one instance only one output was good, the other output had a linear range--but the two couldn't be adjusted independently to achieve a linear range for both that was linear from valve closed to valve open.

Hope this helps! Please write back to let us know what you find.

Thanks for replying.

The servo polarities have been confirmed as well as the servo part number. The two RVDT's are linear throughout.

We swapped out the hydraulic cylinder after noticing some leak by and the amplitude of cycles dropped (this made us question the integrity of the "rebuild"). I dropped the Current Gain from 3.9 to 2.5 slowing the swing even further. The Current Bias was a 3.5 with the old servo, it was changed to 2.8 to match the average servo currents at 50%. In desperation I moved the Position Reference gain from .1 to 1.5 to get the swing as close to tolerable as possible.

We haven't run with this set up yet, as we are a peaking facility. I'll let you know what happens. Please let me know if you see any errors in my as left.


Thanks for the feedback!

You are correct--if you have to deviate from the as-found values using a rebuilt device, you should be questioning the quality of the work done on the device.

The really great thing about working on GE-design heavy duty gas turbines is that one doesn't have to tune regulators for servo-operated devices. That's because the servo gains were calculated by the design engineers knowing the hydraulic system pressure, the desired slew rate of the hydraulic actuator/device, the flow-rate of the servo-valve and the volume of the hydraulic actuator. That made things really simple (and easy!)--just make sure the right gains and settings are in there (the ones from the turbine factory--not the control system factory!) and you're practically guaranteed to have trouble-free operation.

Anyway, I'm not saying that in some cases--when the unit HAS to run--that sometimes changes aren't appropriate. What I am saying is that deviating too far from the as-found value (which may or may not have been correct, either...) is cause for concern.

As for changing the current (null) bias value, the starting point for any new servo should always be 2.67. This equates to one-third of the total current (for a TMR panel) specification for the servos GE buys and sells. And, the allowable range of adjustment for particular applications is +/-1.33. So the total allowable range of adjustment for a good servo valve is 1.33 to 4.0; any current (null) bias value outside that range is suspect--VERY suspect.

I don't subscribe to the method of calculating null bias values in GE Control Specifications--it just doesn't work. It's an ivory tower calculation, and in the real world ivory tower calculations don't always work. And in this particular case, can lead to some VERY questionable null bias values which are VERY outside the allowable range. Again, any time a new servo is installed, the current bias value should be reset to 2.67, if it's not found at that value. And any adjustments made from there--if necessary, and only up to the allowable limits (1.33 & 4.0).

I'm curious about why there was a difference in the servo currents at 50% stroke (if I understood correctly). When the SRV is in calibration mode (Auto or manual), it is in position mode. So, the reference is position (it's pressure in normal, running mode) and the feedback is provided by the RVDTs (in this case). So, if the servo current's are unbalanced at any position, and the three regulators have the same reference (which they will when in Calibration mode), then there's something amiss with the feedback.

Which brings me to my question: What did you do with the LVDT adjustment? The 0.7-3.5 VAC RMS specification is what GE tells their LVDT suppliers to guarantee the output is linear between. So, when the want an LVDT with a stroke of 3.0 inches, the LVDT output must be linear over a three-inch stroke when the zero stroke voltage is set to 0.7 VAC. But, they often used 3.0-inch stroke LVDTs on devices with 2.0 or 2.25 inches of travel--which means that the maximum output at the full stroke would be LESS than 3.5 VAC RMS. It would go to 3.5 VAC RMS--and be linear--if the device had a full three-inch stroke, but it if doesn't travel three inches on the device it's installed on, the output will never go to 3.5 VAC RMS.

It's rarely possible to get an LVDT that actually gets all the way to 3.5 VAC RMS at full stroke when the zero stroke is set to 0.7 VAC RMS. I could count on one hand the number of times I saw that (in thirty-plus years).

Early versions of the rotary SRV had the RVDTs; later versions have LVDTs. One of the reasons was because no RVDT could be sourced that had a linear output for 90 degrees of travel of 0.7-3.5 VAC RMS. And, it was felt that a lot of the instability of the SRV was because of the low range of RVDT output over the 90 degrees of travel. And, it was subsequently learned that most of the RVDTs had a very different linear range, from 0.5-1.2 VAC RMS in some cases. So, sometimes when the zero-stroke voltage was set to 0.7 VAC RMS, it was found that the RVDT output would decrease as the SRV rotated to 90 degrees--which drives the Mark V (any Mark*!) nuts. If the RVDT output increases to some point as the device is opening, and then starts to decrease as the device opens further the Mark* just doesn't really know what to do. It can lead to some very interesting "calibrations."

Please do write back with your results!