Please educate me on the behavior of servo current and how it interacts with position feedback on a Woodward SRV based on auto calibration data contained in the attached zipped csv file.
1. At the start of the auto calibration sequence, servo current is -2.2% and position feedback is 0.0%.
2. At 13:31.52, servo current has ramped to 100% and position feedback remains at 0.0%
3. At 13:32:02, servo current goes negative, ramps down to -16% at 13:32:04 where feedback position has moved to 99.2%
4. At 13:32:12, servo current reaches -100% and position feedback is maintained at 99.2%.
5. At 13:32:29, servo current returns to -2.2% while position feedback remains at 100%.
6. At 13:32:43, servo current remains at -2.2% while position feedback indicates 0%
7. AT 13:34:01, servo current quickly ramps to -100%, then immediately steps back down to -2 and settles out at -2.7%, position feedback responds along the way and settles out at 49.8%.
8. Next at 13:34:15, servo current again rapidly shoots to -100% and steps to -14% while position feedback settles out at 100%
9. The next gyration sees servo current shoot to 100% at 13:34:26, and then steps down to 9.7% where it settles out and position feedback returns to 0%.

Questions:
1. Is the lag time between servo current change and position feedback typical?
2. In Step 8, why is servo current -14% and position feedback 100% when previously feedback position was 100% when servo current was -16%, is moving from -14% to -16% the result of the calibration process?
3. In Step 9, is the servo current move from -2.2%/position feedback 0% in Step 1 to 9.7%/position feedback position 0% again the result of the calibration process.
4. Why does servo current start positive, then go negative, and what do the positive/negative values correspond to in terms of the position being commanded?
5. At position feedback 0%, servo current as-found was -2.2% and was left at 9.7%. Again is this the result of the calibration process?
6. What's the point of the rapid servo current gyrations where servo current quickly jumps to -100% at 13:34:01 and 13:34:15 and to 100% at 13:34:26.

Thanks in advance for taking the time and the education. I could really use a handbook to more fully understand the working of servos and position feedbacks.

 

@kuhn,

I only have my phone at the moment, so I’m not able to look at the data. But the topics of Autocalibration and servo current action have been covered MANY TIMES in the past couple of decades. The Search feature of Control.com works very well.

IF I understand the basic premise of this thread you appear to be asking what is happening and why it’s happening during an Autocalibration.

Autocalibration does ONE THING AND ONE THING ONLY: It scales LVDT feedback based on two points, full-closed mechanical position and full open mechanical position. To do this the Mark* drives the device into the full closed mechanical position using positive servo current and after the LVDT feedback stops changing for a short period of time it records LVDT feedback voltage. Then it goes to negative servo current to drive the device to the full open mechanical position and waits for the LVDT feedback voltage to stop changing for a few seconds and records the voltage. It uses the voltages at the two positions (usually 0% and 100% stroke/position) to calculate offset and gain (essentially) to convert LVDT feedback voltage into indicated position.

Negative servo current increases flow to the machine (in the case of the SRV it’s gas fuel flow) and positive servo current decreases flow to the machine. In other words, negative servo current to the SRV increases the P2 pressure which means more gas fuel can flow to the machine, and positive servo current decreases the P2 pressure which means less fuel can flow to the machine. Negative servo current OPENS the SRV; positive servo current CLOSES the SRV.

When Autocalibration is moving (stroking) the device it ramps the current at a programmed rate. When the device stops moving at either end of travel the servo current keeps increasing in magnitude to drive the device to the maximum travel at each end of travel and waiting for the LVDT voltage to stop changing (remaining constant).

So, it may look like the servo current is not functioning as it would/does during normal operation—because it does operate differently during Autocalibration and normal operation. Autocalibration is not the same as normal operation.

It’s not rocket science, but neither is it documented well in GE documentation. BUT it has been covered numerous times over the last couple of decades on Control.com and those posts can be found using the Search feature of Control.com.

Hopefully you can use this information to understand what is in the .csv file. Recognize that Autocalibration operation differs from normal operation. Negative servo current opens the SRV and positive servo current closes the SRV. Servo current on a GE-design heavy duty gas turbine is not anything like 4-20mA positioner current. AND GE servos use null bias current which most other manufacturers don’t use (also covered extensively in archived posts on Control.com).

Interesting you’re a Service Engineer….

Why does the SRV LVDT feedback need calibrating? Because Autocalibration ONLY calibrates (scales) LVDT feedback vintage—NOTHING ELSE. Not the Servo; not the valve (or IGVs), only the LVDT feedback. Nothing more, and nothing less.