Encoder Counts Fine, but Homing Never Finishes? Check the Z Channel
Healthy A and B counting does not prove the encoder index channel is available. A homing method that requires an index can wait indefinitely while position appears normal.
Offline diagnostic example; index characteristics are encoder-specific.
What is happening?
Quadrature channels describe incremental movement; an index provides a reference event according to the encoder design. The controller must receive and evaluate the required reference signal through a compatible input path.
Check these five things
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Confirm whether the selected homing method needs an index.
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Read the encoder output and index specifications.
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Check the documented input assignment and polarity.
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Inspect index diagnostics separately from position counts.
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Use an approved reference test or offline signal model.
Worked example
A teaching axis receives valid A/B transitions but no simulated Z event. Position changes correctly while an index-based home sequence remains in its search state. Injecting the expected event in the model demonstrates the missing condition.
Quick diagnostic reference
| Signal | Role |
|---|---|
| A and B | Incremental movement |
| Z or index | Reference event |
| Home status | Sequence result |
The mistake to avoid
Do not disable the index requirement merely to clear the symptom. The selected reference strategy may be necessary for the required repeatability.
How to verify the fix
Verify the intended index event, reference position, and repeated homing completion. Confirm the sequence handles a missing event with an appropriate fault.
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
Beckhoff: TwinCAT homing documentation. The examples and diagnostic tables above are original teaching material; they do not replace the product manual.