Flowserve Logix 3800zb User Manual

Logix™ 3800zb Digital Positioner FCD AIIOM001030_EN 02/23
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7.4 Control Loop
The Logix 3800zb uses a proportional control loop, stem-
positioning algorithm. 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 digital output control
to move the 4-Way valve in a direction, depending upon the
deviation. 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.
7.5 Detailed Sequence of Positioner
Operations
A more detailed example explains the control function.
Assume the unit is configured as follows:
• The unit is in Analog command source. (if using
HART)
• 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 signal of
50%
• Loop calibration (if using HART): 4 mA = 0% command,
20 mA = 100% command.
• Write to Final_Value to change command.
• The actuator is tubed and positioner is configured air-
to-open.
Given an input command of 50%, since custom
characterization is disabled, the command source is passed
1:1 to the final command. Since zero deviation exists, the stem
position is also at 50%. With the stem at the desired position,
the 4-Way valve will be unenergized at a middle position that
balances the pressures above and below the piston in the
actuator. This is commonly called the null position.
Assume the input signal changes from 50% to 75%. The
positioner sees this as a command source of 75%. With linear
characterization, the final command becomes 75%. Deviation
is the difference between final command and Stem Position:
Deviation = 75% - 50% = +25%, where 50% is the present
stem position. With this positive deviation, the control
algorithm sends a signal to the 4-Way valve. from its present
position. As the 4-Way valve opens, the supply air is applied
to the bottom of the actuator, and the 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%. As
the stem moves, the Deviation begins to decrease. This
process continues until the Deviation goes to zero. At this
point, the 4-Way valve will close. The desired stem position is
achieved when the stem movement stops.

Flowserve Logix 3800zb Specifications

General

Brand	Flowserve
Model	Logix 3800zb
Category	Industrial Equipment
Language	English