FC 150 - Hydraulic Servo Slave

This function code interfaces a hydraulic servo slave (HSS) to a Harmony controller.  The hydraulic servo slave provides the necessary inputs and outputs to position a hydraulic actuator.  The value of the linear variable differential transformer (LVDT) differential voltages determined during the I/O module calibration cycle are stored permanently in S8 and S9 replacing the initial values or previous calibration values. On HSS module or controller restart, the calibration data from S8 and S9 is downloaded from the module to the HSS module. If necessary, new S8 and S9 values can be established via an interface device such as a HSI.

 

 

Outputs:

Blk

Type

Description

N

R

Percent actuator position with quality

N+1

B

LVDT at null point:

0 = no

1 = yes

N+2

B

Actuator positioning status:

0 = good

1 = bad

N+3

B

A/D or D/A status:

0 = good

1 = bad

N+4

B

LVDT primary status:

0 = good

1 = bad

N+5

B

LVDT secondary status:

0 = good

1 = bad

N+6

B

Output 1 status:

0 = good

1 = bad

N+7

B

Output 2 status:

0 = good

1 = bad

N+8

B

I/O module mode:

0 = normal

1 = E-STOP manual

N+9

B

Calibrate status:

0 = in progress

1 = complete

N+10

B

I/O module hardware status:

0 = good

1 = bad

N+11

B

I/O module communication and watchdog timer status:

0 = good

1 = bad

 

 

 

 

 

Specifications:

Spec

Tune

Default

Type

Range

Description

S1

N

0

I

0 - 63

Expander bus address of I/O module

S2

N

5

I

Note 1

Block address of the percent position demand

S3

N

0

I

Note 1

Block address of calibrate mode select:

1 = calibrate

S4

N

0

I

Note 1

Block address of the calibration go/hold select:

0 = hold

1 = go

S5

N

0

I

Note 1

Block address of the LVDT null check mode select:

0 = no

1 = yes

S6

N

6

I

Note 1

Block address of the calibration stroke time selection:

1.0 = 30 secs

2.0 = 60 secs

3.0 = 35 min

4.0 = 70 min

S7

N

6

I

Note 1

Block address of the calibration cycles count (1.0 to

8.0 cycles)

S8

Y

-10.000

R

Full

LVDT differential voltage at 0% actuator position

S9

Y

+10.000

R

Full

LVDT differential voltage at 100% actuator position

S10

Y

0.000

R

Full

Spare real parameter

S11

N

0

I

Note 1

Spare boolean block input

 

NOTES:

1. Maximum values are:9,998 for the BRC-100, IMMFP11/12 and 31,998 for the HAC

 

 

150.1   Explanation

 

The position of the actuator is measured using a linear variable differential transformer (LVDT). The HSS module provides an adjustable AC frequency excitation signal for the LVDT and demodulates the two LVDT outputs. The difference between the LVDT outputs is a measure of the actuator position. The actuator is positioned by a servo valve which moves hydraulic fluid into and out of the actuator drive cylinder. The HSS module compares the actuator position, from the LVDT, to the position demand from the module and modulates the current flow in the servo valve coils to move the actuator to the desired position. The positioning function is performed by the analog controller on the HSS module. Other hydraulic servo slave functions are performed by its on-board microprocessor. In the normal operating mode, the actuator is positioned to match the position demand from the module.

 

For maximum LVDT linearity, the LVDT null point should occur at the actuators mid-travel position. To check and adjust the LVDT null point, the calibrate and null check modes are selected. When the go mode is selected, the actuator ramps to and holds at the LVDT null point (LVDT secondaries have equal voltages). While the actuator is held at the LVDT null point, the mechanical zero on the LVDT can be adjusted until the actuator is at its mid-travel position. After LVDT zeroing, turn the null check mode off, select hold and proceed with calibration.

 

In the calibrate mode, the actuator is held at its last position until the go mode is selected. When in the calibrate go mode, the actuator moves to the 100 percent position at the selected stroke time <S6>. The HSS module drives the actuator to the 100 percent end of travel stop as verified by no further position movement and saturated servo valve coil current. The LVDT differential voltage is recorded in (S9) while the actuator is held against the 100 percent end of travel stop.

 

After recording the 100 percent reading, the actuator moves at the selected stroke time to the zero percent actuator position. The HSS module drives the actuator to the zero percent end of travel stop as verified by no further position movement and saturated servo valve coil current. The LVDT differential voltage is recorded in S8 while the actuator is held against the zero percent end of travel stop. After recording the zero percent reading, the actuator moves at the selected stroke time to the position demand from the module. If the number of calibration cycles (S7) to be performed is greater than one, the 100 percent to zero percent cycle will be repeated the specified number of times before ramping to the position demand <S2>.

 

The HSS module provides a number of status outputs. The actuator positioning status will be bad if the measured actuator position deviates beyond an established deadband from the position set point established in the HSS module. For steam turbine valve control applications, this bad status is called a valve contingency. The A/D or D/A status is bad when the A/D reading of the voltage reference is outside specifications or the D/A output as read by the A/D is outside specifications.

 

The LVDT primary status is bad if both LVDT secondaries read approximately zero volts. LVDT secondary status is bad if one secondary reads approximately zero volts, while the other is reading a nonzero value.

 

NOTE: These alarms are enabled only when the actuator position status is bad. For more information, refer to the IMHSS03 Hydraulic Servo product instruction.

 

The output one or two status is bad if its servo valve coil voltage indicates the coil is open or shorted. The HSS module can continue to control the actuator even if one coil is open or shorted.

The current output to the servo valve coils is set to zero if A/D or D/A status is bad, or the I/O module hardware watchdog timer times out. The output is also set to zero until the HSS module restart is complete. The servo valve will hold the actuator approximately in place if the coil current is zero. The servo valve will drift based on its mechanical biasing when the coil is zero.

 

If the watchdog timer (time-out condition) determines that communication with the module is lost, the HSS module reverts to and indicates E-STOP (executed stop) manual mode. In E-STOP mode, the actuator position can be changed by activating the module's raise and lower contact inputs. When communications are re-established with the module, the HSS module will not transfer to position control from the module until the module's position demand equals the present actuator position. This is to prevent an undesired actuator position change.

 

The HSS module is typically used in the positioning of steam turbine, throttle and control valves, gas turbine fuel valves, inlet guide vanes and nozzle angle.