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Logix
®
520MD+ and 510+ Digital Positioners FCD LGENIM0105-15-AQ – 05/16
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1.12 Command Input and Final Command
The Command Input signal (in percent) passes through a characteri-
zation/limits modifier block. This function is done in software, which
allows for in-the-field customer adjustment. The characterization
block can apply no adjustment (Linear), one of several pre-defined
characterization curve 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 algo-
rithm 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.
1.13 Outer Loop
The Logix 500+ 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.
1.14 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-compen-
sated hall-effect sensor and a Piezo valve pressure modulator. The
Piezo valve pressure modulator controls the air pressure under a dia-
phragm 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 decreas-
es, the spool or poppet valve moves up or down respectively. The Hall
Effect sensor transmits the position of the spool or poppet back to the
inner-loop electronics for control purposes.
Figure 1: Principles of Operation of Logix 500+