Servo Hits the Position but Shakes the Machine? Review Jerk
Correct final position does not prove that a motion profile suits the mechanics. Abrupt acceleration changes can excite vibration. Compare command shape with measured response before treating every shake as a tuning problem.
Profile behavior and units depend on the selected motion instruction.
What is happening?
Jerk describes the rate of change of acceleration. S-curve profiles control that change, but smoothness, move duration, and peak demands interact. The controller defines how its jerk parameters and profile options are interpreted.
Check these five things
-
Capture position, velocity, and relevant drive diagnostics for the move.
-
Identify whether vibration begins at acceleration transitions or elsewhere.
-
Check profile type, units, and the documented jerk convention.
-
Review the motion envelope with the responsible controls and mechanical engineers.
-
Test approved changes with representative payloads and settling requirements.
Worked example
A teaching axis reaches its destination accurately but rings after each sharp acceleration transition. A reviewed S-curve profile reduces excitation while increasing move duration. Whether it improves the actual cycle depends on the reduced settling time as well as travel time.
Quick diagnostic reference
| Measure | Reason |
|---|---|
| Travel duration | Profile cost |
| Settling time | Usable completion |
| Peak demand | Drive and mechanical limits |
The mistake to avoid
Lower jerk is not automatically better. Excessive smoothing can miss timing requirements, while mechanical looseness still needs repair rather than profile compensation.
How to verify the fix
Compare total useful cycle time, settling, tracking, and limits across intended payloads. Save the accepted profile and measured results together.
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: Trapezoidal and S-curve motion profiles. The examples and diagnostic tables above are original teaching material; they do not replace the product manual.