Servo In-Position Bit Is On but the Part Is Off? Check What Feedback Measures
A motor encoder can confirm motor position without directly measuring the final load position. Backlash, slip, compliance, and coupling problems may exist between them. Determine where feedback is measured before treating an in-position bit as proof of workpiece accuracy.
Motor-versus-load feedback framework; acceptance measurements are application-specific.
What is happening?
The control loop can only evaluate the feedback available to it. A motor-side measurement and a load-side measurement observe different points in the mechanism. Reversal and changing load can reveal mechanical error that is small or invisible during a one-direction unloaded test.
Check these five things
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Identify every feedback device and the physical point each measures.
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Compare command, motor feedback, and independent load-position evidence where available.
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Test direction reversals and representative load through the approved commissioning procedure.
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Inspect coupling, transmission, and mechanical condition using controlled isolation.
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Assess any compensation or feedback-design change with the responsible motion engineer.
Worked example
A motor-side encoder returns exactly to its target, but a worn transmission allows the load to settle at different positions depending on approach direction. The controller's in-position indication can be consistent with its feedback while the product position varies.
Quick diagnostic reference
| Observation | Investigation |
|---|---|
| Error changes with direction | Lost motion or backlash |
| Error changes with load | Compliance or slip |
| Motor and load both wrong | Reference or scaling |
The mistake to avoid
Do not increase software compensation until mechanical condition and the measurement point are understood. Compensation can hide deterioration or create new behavior.
How to verify the fix
Measure load-position accuracy and repeatability under the required directions and loads, not merely the motor's reported error.
Field-work boundary: These are educational diagnostic guides. Use the exact equipment manuals and approved site procedures. Electrical testing and machinery changes belong to qualified, authorized personnel; control hazardous energy and validate affected functions before release.
Technical reference
Rockwell Automation: motion configuration and mechanical backlash considerations. The examples and diagnostic tables above are original teaching material; they do not replace the product manual.