Answer / carl_ellis

There's two kinds of feedback from a control valve:
a) the change of the process variable because the valve
position changes
b) position feedback directly from the positioner to the
controller/control system.

We'll assume that the question is directed at the loss of
position feedback signal from the loop's positioner.

1) Position feedback cards are options on smart
transmitters and need to be initialized after installation.
Has it been initialized?

2) DC power is typically loop power for position feedback.
Is the loop power supply still functioning?

3) Has the position feedback card or its wiring been
disabled or removed?

4) If the position feedback loop is intrinsically safe, is
the loop's I/S barrier open circuited?

5) Has the loop wiring been changed in some manner so as to
affect the position feedback signal integrity?

Carl Ellis
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