Flowserve 3400IQ Digital Positioner - Manual (Page 12)

Logix 3400IQ Digital Positioner FCD LGENIM3401-00 – 06/06
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position the stem, after any characterization or user limits have been evaluated, is called the Control
Command.
The Logix 3400IQ 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.
4.3 Detailed Sequence of Positioner Operations
A more detailed example explains the control function. Assume the unit is configured as follows:
• Unit is in OOS.
• Custom characterization is disabled (therefore characterization is Linear).
• No soft limits enabled. No Final Value Cutoff set.
• Valve has zero deviation with a present input command of 50.
• Write to Final_Value to change command.
• Actuator is tubed and positioner is configured air-to-open.
Given these conditions, 50 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 50 to 75. 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