Flowserve 420 IOM - Manual (Page 5)

User Instructions - Logix® 420 Series Digital Positioners FCD LGENIM0106-06 12/13
flowserve.com
5
adjustments (including several Equal Percent), or a 21-point
Custom Characterization curve adjustment. In Linear mode,
the input signal is passed straight through to the control
algorithm in a 1:1 transfer. In Equal Percent (=%) mode, the
input signal is mapped to a standard rangeability equal
percent curve. If Custom Characterization is enabled, the
input signal is mapped to a custom, user-defined 21-point
output curve. The custom user-defined 21-point output curve
is defined using a handheld or ValveSight software. In
addition, two user-defined features, Soft Limits and Tight
Shutoff may affect the position. The actual command being
used to position the stem after the evaluation of
characterization curves and user limits, is called the Final
Command.
1.12 Outer Loop
The Logix 420 uses a two-stage, stem-positioning algorithm.
The two stages consist of an inner-loop (pilot relay 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 Final 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
relay in a direction, 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 Final Command
and Stem Position. This process continues until the deviation
goes to zero.
1.13 Inner Loop
The inner-loop controls the position of the relay 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 poppet
valve moves up or down respectively. The Hall effect sensor
transmits the position of the poppet back to the inner-loop
electronics for control purposes.
1.14 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 tight shutoff (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 Final Command.
Since zero deviation exists, the stem position is also at 50
percent. With the stem at the desired position, the poppet
valve will be at a middle position that balances the pressures
and spring force in the actuator. This is commonly called the
null or balanced poppet 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 Final Command becomes
75 percent. Deviation is the difference between Final
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 poppet up from its present position. As the poppet
moves, the supply air is applied to the bottom 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 poppet opening. This process
continues until the Deviation goes to zero. At this point, the
poppet will be back in its null or balanced position. Stem
movement will stop and the desired stem position is now
achieved.
1.15 Inner Loop Offset
The position of the poppet at which the pressure and springs
are balanced, holding the valve position in a steady state, is
called the Inner Loop Offset. The controlling algorithm uses
this value as a reference in determining the Piezo voltage.
This parameter is important for proper control and is
optimized and set automatically during stroke calibration.
Vent