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Fault 15
Pro

Fault 15 — Analog Valve Feedback Drift

What you'll learn

**Reported symptom:** The valve local display is at 50%, but PLC feedback is consistently 7.5% high across the range.

Lab time: ~15 minutes.

Lesson briefing

Analog Valve Feedback Drift

Reported symptom: The valve local display is at 50%, but PLC feedback is consistently 7.5% high across the range.

The circuit was commissioned from a known-good drawing. Treat every terminal as suspect, but test from source to load instead of guessing. Use voltage mode on an energised control circuit and continuity only after the simulated circuit is isolated.

Your task

Compare the loop signal at the valve, marshalling terminals and PLC input, then separate wiring loss from calibration offset.

Choose the fault category only when the measurements support it.

Hints

Hint 1

A fixed offset across several test points suggests zero/calibration drift rather than changing wire resistance.

Hint 2

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Hint 3

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This lesson uses 7 placed components on the lab canvas. Components are vendor-neutral (no proprietary trademarks); the wiring rules apply to any equivalent industrial part.

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Competency and practice field guide

Valve feedback drift fault lesson: practice plan

Direct answer

The learner can trend command, position and process response, distinguish calibration drift from stiction or scaling and prove recovery at several operating points.

Written for instrumentation and maintenance learners diagnosing a valve whose position feedback gradually disagrees with its command or process effect.

an instrumentation calibration bench connecting pressure, temperature, load, level and smart sensors to PLC input channels and reference measurements while studying control-valve command, position feedback drift and proving tests
System map / 02

NODE 01observable

Scope

Command signal, fail action, actuator, positioner, travel range, feedback transmitter, scaling, tolerance, deadband, hysteresis, stiction, process response and maintenance history.

NODE 02observable

Signal path

PLC command through output scaling, positioner and actuator to valve travel, feedback measurement, input scaling, deviation alarm and process result.

NODE 03observable

Baseline practice

Low, midpoint and high commands produce repeatable travel and process response within the declared tolerance.

NODE 04observable

Edge cases

Slow ramp, reversal, small step, loss of air, sticky travel, feedback offset, span drift, saturated output, restart and fail position.

NODE 05observable

Fault practice

A command, output, positioner, actuator, mechanics, feedback, scaling, alarm or process mismatch.

NODE 06observable

Transfer to the job

The isolated valve inspected, calibrated and stroked under approved procedure with process checks.

Answer surface / 07

How do you detect valve position-feedback drift?

Trend command, independent position feedback and process response across known points and directions, then compare deviation with the declared tolerance.

Can valve command equal valve position?

No. Command is intent; position feedback and process response are separate evidence that the final element moved and had the expected effect.

Motor, wiring and VFD path

Connect the control command to physical motor behavior

Move from safe control wiring and contactors into VFD parameters, measurements, faults and PLC command paths.