Servo Moves the Wrong Distance but Repeats Perfectly? Check Scaling
Repeatability does not prove dimensional accuracy. An axis can make the same wrong move every time when its gear ratio, screw pitch, feedback resolution, or engineering-unit conversion is incorrect. Compare physical travel with commanded units before editing every target position.
Original dimensional example; scaling changes require motion-system review.
What is happening?
Axis scaling converts feedback and motion quantities into application units. The relevant ratio direction and reference shaft must match the documentation. A gearbox ratio entered backward or a millimeter/inch mismatch can create systematic errors while the controller's internal tracking looks excellent.
Check these five things
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Write the conversion chain from motor or feedback revolution to load travel in physical units.
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Compare gear ratio, screw pitch or circumference, and feedback configuration with the installed mechanics.
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Check the software's numerator and denominator definitions rather than relying on a remembered ratio convention.
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Measure an approved low-risk test move using suitable independent measurement.
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Validate several distances and directions after the accepted correction.
Worked example
In a simplified model, one motor revolution produces 5 millimeters of travel, but the project assumes 10. The controller can track its configured units closely while the load moves the wrong physical distance. The correction belongs in the verified conversion chain.
Quick diagnostic reference
| Conversion | Evidence |
|---|---|
| Motor to load rotation | Gear ratio and direction |
| Load rotation to travel | Pitch or circumference |
| Software units | Configured scaling definition |
The mistake to avoid
Changing scale can also affect velocity, acceleration, limits, and stored positions. Review all dependent quantities before applying it.
How to verify the fix
Record independent travel measurements and confirm dependent motion settings and limits remain consistent with the machine requirements.
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: axis scaling and ratio definitions. The examples and diagnostic tables above are original teaching material; they do not replace the product manual.