Control.com - Nerds in Control

I have two questions about these regulators:

1) While doing the calibration of these servo valves, what is the difference between the Manual Calibration & the Auto Calibration?

2) In the configuration of these regulators in the TCQA Card, for example the regulator 43, we need to enter the following parameters:

Current Bias: (0 to 100% rated [10,20,40])
Current Gain: (0 to 200% rated_cur./%pos.)

Zero Stroke (0 to 6.667 Vrms):- LVDT 1: LVDT 2:
100% Stroke (0 to 6.667 Vrms):- LVDT 1: LVDT 2:

Pos limits (-128% to 128%):- Low High:
Integrator convergence gain (0 to 16 psi/%):
Position reference Gain (0 to 2 %/psi):

The Current Bias, Zero Stroke and 100%Stroke parameters are well documented in GE Manual on how can be calculated. However other parameters, Current Gain, Pos. limits, Integrator convergence gain and Position reference Gain, I couldn’t find a procedure that calculates these parameters. Are these parameters are calculated and updated automatically during execution the AUTOCALIB or there is exists a procedure to calculate them?

Good questions.

"Calibration" of a servo-valve operated device (fuel valves, IGVs, steam valves, etc.) is *only* necessary if the device is equipped with LVDTs. One is not calibrating the device or the servo-valve, one is calibrating the LVDT feedback from the device. If a device, such as a Liquid Fuel Bypass Valve, is not equipped with LVDTs then no calibration is required. LVDTs should only require calibration if they are replaced, removed and reinstalled during maintenance activities, or the device they are installed on is disassembled and reassembled for repair or maintenance.

One should periodically check the calibration of LVDT feedback, and, if necessary re-calibrate the feedback. But many people make the mistake of attempting to calibrate LVDT feedback during annual maintenance outages without really understanding how to calibrate LVDT feedback with a Mark V and end up causing *lots* of problems. When most I&C technicians are "calibrating" pressure switches or -transmitters or temperature switches or transmitters, they don't start the process by calibrating them "from scratch." They begin by documenting the as-found condition, and only if the as-found condition is outside the acceptable deadband for the device's calibration do they change the as-found calibration. It just makes me cringe when people who also calibrate pressure switches and temperature transmitters don't apply the same procedure to calibrating LVDT feedback. Only if the LVDT feedback is outside acceptable limits should the LVDT feedback be re-calibrated using AutoCalibrate or by some manual method; or, if something is done to possibly change the mechanical stroke of the device or if the LVDT(s) are replaced should LVDT feedback require calibration.

1) AutoCalibration will "stroke" devices equipped with LVDTs from mechanical stop to mechanical stop and then back to the original mechanical stop (to check to ensure the device returns to the original starting position), and will calculate the 0% and 100% stroke voltages for each processor. NOTE: Mechanical stops may or may not be equal to 0- and 100% stroke; in the case of the IGVs, the minimum mechanical stop is less than the closed operating condition and the maximum mechanical stop is more than the full open operating condition. Such conditions must be determined and manually entered in ACALIB.DAT *before* performing an AutoCalibrate.

Manual Calibration isn't really "calibration." It's used to move a servo-operated device with LVDTs to a setpoint entered via the Manual feature of AutoCalibrate.

2) Current Bias should almost always be 2.67% per processor for a TMR control panel (4.0% for a SIMPLEX panel) if the polarity of the servo-valve current has been verified to be correct. Current Gain, Integrator Convergence Gain, Position Limits, and Position Reference Gains are all calculated by the packager of the turbine and *should not* be changed from the values supplied with the turbine and control system. They are determined by knowing such details as hydraulic supply pressure, servo-valve flow-rate, actuator volume, desired slew-rate, etc. These values should not ever change as long as OEM-compatible devices (servo-valves and actuators) are used on the turbine.

I couldn't agree more about checking the LVDTs by manually stroking the valves or IGVs PRIOR to any calibration of their respective LVDTs. I would also add that if I find one or more LVDTs on an actuator shift their zero or span, I'm definitely going to look at the actuator to try and find out why the shift has occurred.

For example, we had a main steam stop valve LVDT on an Alstom steam turbine shift its zero position by 12%. (Fortunately the Turbotrol measures actual stop valve travel rather than just using limit switches.) By measuring the stem travel and comparing it to the original stem travel record, we knew that the valve had lost travel. Disassembly during an outage revealed that the seat had dislodged and cocked slightly. I could have just "calibrated" the LVDT to compensate for the zero shift, but this would only have masked the mechanical problem.

Autocalibration is a great tool, and it's tempting to sit in a nice PECC or DCS room rather than going out and looking at the machinery, but it's just a bad idea to compensate for mechanical problems through software. Minor sensor drift is one thing, but significant change in measurements should be investigated.