5-7 SERVO-VALVE AUTOMATIC CALIBRATION

The Mark V Automatic Calibrate (AUTOCAL) function establishes the relationship between the position of the servo-valve and feedback voltage. AUTOCAL calibrates the feedback signals in the position control sections of the various Servo Valve Outputs (SVO)s. Calibration means to calculate the values of I/O Configuration Constants used to scale the input voltage signals from the position of the device to the proportional voltage from the feedback devices, Linear Variable Differential Transfer/Reactor (LCDT/R). Prior to performing an automatic calibration, the actuator or device must be mechanically adjusted, the mechanical end stops set correctly, and the LVDT/R must be adjusted to the correct minimum voltage position. AUTOCAL is designed to be used:

• during initial unit start-up and commissioning activities • after an LVDT/R is replaced or repaired

• after reassembly of the actuator or driven device that was disassembled for maintenance or repair

• to verify Under Position Control as a maintenance tool to check mechanical devices, such as the servo-valve, actuator, or driven device for mechanical binding or sticking

• to verify Under Current Control the linearities of the LVDT/R(s) feedback signal • to manually position the device to a user-defined position for testing or maintenance

It is not necessary to use AUTOCAL to recalibrate the feedback signal(s) of the LVDT/R(s) whose faulty servo-valve has been replaced, unless the servo-valve itself has a LVDT/R(s) attached (as on some medium steam turbine applications).

5-7.1. AUTOCAL Display

This section describes the format of the data file, ACALIB.DAT. The program ACALIB.EXE executes the AUTOCAL function. ACALIB.DAT defines the calibration parameters and the format of the screen display for each SVO. A typical AUTOCAL display is shown in Figure 5-1.ACALIB.DAT must be located in the unit configuration directory, F:\UNITn. (For further information on directories, see Chapter 3.)The three main sections to this data file are Status Codes, Trace

Information, and Display Definitions.

Status Codes converts I/O card status codes to text. These are used to decipher the hexadecimal I/O card state information to text for ease of understanding.

Trace Information defines how many position traces are to be shown for each regulator type when displaying a Verify Under Current Control or Verify Under Position Control plot.

Display Definitions section defines all the different AUTOCAL displays for each regulator. Each definition includes items such as the processor, I/O card, I/O processor number, regulator number, positions at current saturation, logic permissive for sending commands, and the data to be displayed. When AUTOCAL is run, it displays a list of the regulators. The user selects a regulator and the definition of the regulator is used to control the AUTOCAL functions and provide data to display. The AUTOCAL function changes only the TCQA RAM values of the I/O Configuration Constants; it does not modify the I/O Configuration Constants in EEPROM or the <I>. This means that if the processor(s) is re-booted after AUTOCAL is

Configuration Constant values must be written down or printed from the AUTOCAL display, entered into the I/O Configurator, and downloaded to EEPROM in the processor(s) in the Mark V control panel. This is the only method of making the EEPROM and RAM values equal.

5-7.2. Operation

The AUTOCAL Display starts by reading ACALIB.DAT for the current unit. (The current unit is the unit selected from the main menu. The data file defines all the SVOs that require calibration and presents a menu of the defined SVO. Once the desired SVO is selected, information is displayed on the screen. However, no commands can be sent to the unit until the permissive logic signal is met. If the Mark V does not respond or the AUTOCAL function is being used by another <I>, the data fields remain blank.

The data file can define a logic PERMISSIVE signal that must be in a required state before calibration commands can be sent to the Mark V. If the data file defines a signal name that does not exist for this unit, the display issues an error message about an invalid PERMISSIVE signal name and refuses to enable commands. To enable commands the data file must add the PERMISSIVE signal and the logic that drives it to the Mark V. If the user has the required privilege and the permissive signal is found to be valid, a warning message is displayed.

5-7.2.1. AUTOCAL TARGETS. When AUTOCAL is enabled the following seven targets appear.

START CALIBRATE sends the command to the Mark V to start AUTOCAL. This executes the position calibrate function and reports the resulting information on the display. When all required permissives are met and hydraulic operation of the valve is possible, AUTOCAL controls the servo-valve output current(s) to the servo-valve device to position the actuator or device. The Mark V then measures the feedback voltages and calculates I/O Configuration Constants. The currents

determined during the AUTOCAL procedure are stored in TCQA RAM.

VERIFY POSITION starts the "verify under position" control function. This function ramps the actuator from actuator mechanical minimum travel to mechanical maximum travel then ramps back again at a constant rate (for example, constant inches per minute). While verifying under position control, AUTOCAL causes the servo-valve output current to increase or decrease as necessary to maintain the fixed rate of travel as indicated by the LVDT/R feedback signal(s). Servo current data is collected at 128 Hz rate and is placed in the Mark V’s buffer. If the servo-valve, the actuator, or device is not mechanically binding or sticking, the amount of current required to maintain the fixed rate of travel should be constant. Such mechanical binding could be caused by such things as valve packing, a scored hydraulic actuator cylinder, or a damaged valve stem. The results of verifying under position control can be plotted on the <I> and printed for further analysis.

VERIFY CURRENT starts the "verify under current" control function. This function causes AUTOCAL to output a fixed servo-valve output current that causes the device to move at a constant rate of approximately 30% travel-per-second from actuator minimum mechanical to maximum mechanical travel and back. If the processor is re-booted or another valve calibrated, the current values from the most recent AUTOCAL procedure are lost. If no values exist in TCQA RAM for moving the device at the 30% per second rate, a message is displayed indicating that the device must be calibrated before the operation can occur. The current for each direction is different because of the null bias current required to overcome the fail- safe spring bias in the servo-valve

MANUAL SETPOINT defines the position reference if manual control is enabled. Manual control is used to check the accuracy of calibration or to hold the device in some position for mechanical inspection or maintenance. The desired position is entered, ENABLE MANUAL is selected, then the valve is driven to the setpoint position. Changing the MANUAL

SETPOINT when manual control is enabled ramps the device at a fixed 30% stroke-per-second rate to the new setpoint. IDLE halts any calibration, verify, or manual control and clears any status or error condition from a previous command. VIEW VERIFY is used to collect and plot data in the capture buffer.

5-7.3. Precautions/Preliminary Steps

Personnel should keep clear of the area during an automatic calibration sequence or when one of the automatic calibration options is executed. The hydraulic actuator is moving the device from mechanical stop to mechanical stop at rates as quick as approximately 4 seconds using full hydraulic pressure.

63. Safety concerns and precautions. To enable AUTOCAL, high-pressure shaft turbine speed must be below 28%, meaning the unit must be off-line, shut down, or on crank in certain applications.

Insure LVDT is in the linear region and that the minimum travel is set (see Figure 5-2). LVDT/Rs are usually specified so that the linear range of output voltage is slightly greater than the mechanical range of travel of the actuator or device. 64. Since the output voltages of LVDT/Rs are non-linear at the extreme ends of travel, it is important to mechanically

adjust the LVDT/Rs so that at its minimum travel, such as at one of the mechanical stops of the actuator or devicer, the output voltage is in the linear range. See documentation for minimum travel voltage.

65. Verify pretravel. Pretravel dimensions are defined and explained in the turbine manual.

66. Understand difference between LVDT range of travel, Actuator range of travel, and actual mechanical range of travel. The following information must be known before AUTOCAL can calibrate LVDT/R position feedback signal(s) from a device: (see Figure 5-3)

LVDT/R Output Voltage Range of Travel LVDT/R Range of Travel Actuator/ Device Range of Travel Effective Stroke

The graph represents a plot of position versus voltage. The LVDT/R is usually adjusted such that its output is linear over the mechanical range of travel of the actuator/device. In some cases, the effective stroke of the device may be less (or more) than the actuator/ device range of travel.

Figure 5-2. Voltage vs Range of Travel Curve

1 LVDT Mechanical Minimum Travel

2 LVDT Minimum Voltage

3 Actuator Mechanical Minimum Travel (distance between 3 and 4 is called "pretravel") 4 Device Mechanical Minimum Travel (distance between 4 and 5 is "effective travel or stroke")

5 Device Mechanical Maximum Travel or Normal Operating Maximum Position (distance between 3 and 5

is actual mechanical range of travel)

6 Actuator Mechanical Maximum Travel

7 LVDT Maximum Voltage

8 LVDT Mechanical Maximum Travel

9 actual mechanical range of travel of the device/actuator, such as closed-end mechanical stop to open-end mechanical stop, in engineering units (inches, centimeters, degrees of angle, etc.)

--- Position of Actuator or LVDT ---

| | | | | | | | 1 2 3 4 Operating Range 5 6 7 8

Figure 5-3. Linear Representation of Travel Terms (see definitions above)

The effective travel of the device whose LVDT/R(s) is being calibrated is very important when it is not equal to the actual mechanical range of travel. For many steam turbine applications, the effective travel of a steam valve is equal to the actual mechanical range of travel. However, many heavy duty gas turbines employ an actuator for the Gas Control Valve which has a maximum mechanical range of travel of approximately 2.40 inches, but the effective range of travel is only 1.75 inches. At 1.75 the flow characteristics of the valve’s internal plug and seat might cause the maximum expected fuel flow for that unit. In this case, the control system is calibrated so that the 100% travel for the valve (effective travel) is when the valve is at 1.75 inches. The "effective travel" of the device being calibrated is usually defined in either the unit’s Control Specifications, Lineup Instructions, or unit

instruction manuals.

If the device’s effective travel is different than its mechanical range of travel, the actual mechanical range of travel must be known and entered into the unit-specific ACALIB.DAT file on the <I>. While the mechanical range of travel is usually listed in the unit’s documentation, it is best to actually measure the range of travel using a dial indicator, a machinist’s rule, a machinist’s protractor, or some appropriate measuring instrument. Once the actual mechanical range of travel is known for a particular actuator or device, it can be entered into the unit- specific file and does not need to be changed unless the actuator or device is replaced or has been disassembled for repair or maintenance.

The values for POSITION-POS-SAT and POSITION-NEG-SAT can be entered in inches, millimeters, or

other convenient units of measure. Often they are expressed as a percentage of effective travel or device mechanical maximum travel. This is probably the easiest to understand, but does require the user to know on what the value is based.

67. Enable Maintenance level Password or above

68. Gather the necessary data from Figure 5-3 and from ACALIB.DAT file: 68.1 Saturation position values

68.2 CDB logic signal for AUTOCAL permissive

69. Establish Hydraulic pressure. In order to use AUTOCAL, the unit must be shut down and the hydraulic system of the unit must be placed in operation by the user. Any dump valves or trip solenoids must be in the correct position to allow the operation of the hydraulic system. AUTOCAL does not enable the hydraulic system automatically. For example, in some heavy-duty gas turbines without motor-driven hydraulic pumps, the unit has to be cranked to establish hydraulic pressure. Consult the documentation to determine how to enable the hydraulics.

70. Forcing points to enable AUTOCAL. In most cases, the AUTOCAL function requires that a particular CDB logic point be in a specific state in order to enable AUTOCAL. The CDB logic signal pointname is displayed on the AUTOCAL screen for the device being calibrated. If there is any question about whether a logic permissive has been specified for

the device, the CDB pointname and the required state of the logic signal can be viewed in the unit-specific

ACALIB.DAT file. Most applications use the CDB logic signal L3ADJ for the AUTOCAL permissive and require it to be a logic " 1 ". By reviewing the unit’s Control Sequence Program, the conditions which make L3ADJ (or the

particular specified CDB logic signal point) a logic " 1 " can be determined.

5-7.4. Executing AUTOCAL

The following procedures can be used to calibrate the Servo Valve Outputs: