Flowserve Logix 3200IQ Digital Positioner - Manual (Page 4)

Flowserve Corporation 1350 N. Mountain Springs Parkway 1978 Foreman Dr.
Flow Control Division Springville, Utah 84663-3004 Cookville, TN 38501
www.flowserve.com Phone: 801 489 2233 Phone: 931 432 4021
FCD AXAIM3200-00 9/04 Page: 4 of 32
© 2004, Flowserve Corporation, Printed in USA
Automax Logix 3200IQ Digital Positioner
Installation, Operation and Maintenance Instructions
user-defined 21-point output curve is defined using a
handheld or PC software. In addition, two user-defined
features, Soft Limits and MPC (Minimum Position Cutoff),
may affect the final input signal. The actual command
being used to position the stem, after any characterization
or user limits have been evaluated, is called the Control
Command.
The Logix 3200IQ uses a two-stage, stem-positioning
algorithm. The two stages consist of an inner-loop,
spool control and an outer-loop, stem position control.
Referring again to Figure 1, a stem position sensor
provides a measurement of the stem movement. The
Control Command is compared against the Stem Position.
If any deviation exists, the control algorithm sends a
signal to the inner-loop control to move the spool up
or down, depending upon the deviation. The inner-loop
then quickly adjusts the spool position. The actuator
pressures change and the stem begins to move. The
stem movement reduces the deviation between Control
Command and Stem Position. This process continues
until the deviation goes to zero.
The inner-loop controls the position of the spool valve
by means of a driver module. The driver module consists
of a temperature-compensated hall effect sensor and a
piezo valve pressure modulator. The piezo valve pressure
modulator controls the air pressure under a diaphragm by
means of a piezo beam bender. The piezo beam deflects
in response to an applied voltage from the inner-loop
electronics. As the voltage to the piezo valve increases,
the piezo beam bends, closing off against a nozzle
causing the pressure under the diaphragm to increase. As
the pressure under the diaphragm increases or decreases,
the spool valve moves up or down respectively. The hall
effect sensor transmits the position of the spool back to
the inner-loop electronics for control purposes.
Detailed Sequence of Positioner Operations
A more detailed example explains the control function.
Assume the unit is configured as follows:
• Unit is in Analog command source.
• Custom characterization is disabled (therefore
characterization is Linear).
• No soft limits enabled. No MPC set.
• Valve has zero deviation with a present input signal
of 12 mA.
• Loop calibration: 4 mA = 0% command, 20 mA =
100% command.
• Actuator is tubed and positioner is configured
air-to-open.
Given these conditions, 12 mA represents a Command
source of 50 percent. Custom characterization is disabled
so the Command source is passed 1:1 to the Control
Command. Since zero deviation exists, the Stem Position
is also at 50 percent. With the stem at the desired
position, the spool valve will be at a middle position that
balances the pressures above and below the piston in the
actuator. This is commonly called the null or balanced
spool position.
Assume the input signal changes from 12 mA to 16 mA.
The positioner sees this as a Command source of 75
percent. With Linear characterization, the Control
Command becomes 75 percent. Deviation is the difference
between Control Command and Stem Position: Deviation
= 75% - 50% = +25%, where 50 percent is the present
stem position. With this positive deviation, the control
algorithm sends a signal to move the spool up from its
present position. As the spool moves up, the supply air is
applied to the bottom of the actuator and air is exhausted
from the top of the actuator. This new pressure differential
causes the stem to start moving towards the desired
position of 75 percent. As the stem moves, the Deviation
begins to decrease. The control algorithm begins to
reduce the spool opening. This process continues until
the Deviation goes to zero. At this point, the spool will be
back in its null or balanced position. Stem movement will
stop and the desired stem position is now achieved.
One important parameter has not been discussed to this
point: Inner loop offset. Referring to Figure 3, a number
called Inner loop offset is added to the output of the
control algorithm. In order for the spool to remain in its
null or balanced position, the control algorithm must
output a non-zero spool command. This is the purpose
of the Inner loop offset. The value of this number is
equivalent to the signal that must be sent to the spool
position control to bring it to a null position with zero
deviation. This parameter is important for proper control
and is optimized and set automatically during stroke
calibration.