Detailed Sequence Of Positioner Operations - Flowserve Logix 3400MD User Instructions

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.
Figure 4: Standard Rotary Mounting
Logix 3400MD
Digital Positioner
Logix 3400MD Digital Positioner FCD LGENIM3404-00 – 9/09
• 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
Positioner Bolts ¼-20 (4)*
/
Bracket Bolts -18 (2, not shown)
5
16
Follower Arm
Take-off Arm, Rotary
Lock Washer (2)
10-32 Bolt
10-32 Nut
Self-tapping Screws (2)
Spline Lever Adapter
10-32 Nut
Lock Washer
* Located in appropriate
hole pattern as indicated on
bracket. (25, 50, 100/200)
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