An intermittent homing fault on a three-axis machining center caused the affected axis to travel about 50 cm after reversing, instead of the remembered normal 3–5 cm. A later event produced about 10 cm of travel. The machine used a SELCA 3045 control, Siemens 611 drives, and apparently external motor encoders. The evidence points to a missed encoder zero mark or its signal path, but the home switch must be verified before replacing hardware.
Interpret the homing sequence
The observed sequence is: the axis approaches the home switch, the switch changes state, the axis reverses, the switch clears, and motion continues until the encoder zero mark is detected. During testing, the monitored switch input changed from 1 to 0, then returned from 0 to 1 shortly after reversal, yet the axis did not stop. This confirms that the switch changed state at the control input; it does not by itself prove that the encoder zero mark reached the counting hardware.
If the zero mark is missed, the encoder can rotate one or more additional revolutions before the next detectable mark. The resulting linear overtravel should therefore be compared with the axis travel corresponding to one encoder revolution.
Separate switch and encoder-path faults
| Observation | Engineering interpretation | Check |
|---|---|---|
| Home-switch input does not clear after reversal | The switch, actuator, wiring, or input path may be holding the homing sequence active. | Monitor the input through approach, reversal, and switch release. |
| Input clears but the axis continues | The encoder zero mark may not be detected reliably. | Inspect the encoder connector, zero channel, cable, and counting input. |
| No encoder alarm appears | Absence of an alarm does not eliminate a zero-channel fault; position signals may remain usable while the separate zero mark is missed. | Evaluate the zero-mark path independently from normal position feedback. |
| Fault occurs only during homing | This is consistent with a signal used specifically by the homing sequence. | Compare repeated homing results rather than relying only on machining-position behavior. |
Check the encoder revolution distance
Use the configured encoder counts per revolution and counts per millimetre to calculate the expected spacing between zero marks. Keep the count convention consistent: both values must use the same interpretation of encoder pulses or quadrature counts.
travel_per_encoder_revolution_mm = encoder_counts_per_revolution / counts_per_mm
missed_revolutions = abnormal_extra_travel_mm / travel_per_encoder_revolution_mmCompare the abnormal extra travel with whole multiples of the calculated revolution distance. Agreement supports the missed-zero-mark hypothesis; disagreement redirects attention to the switch sequence, scaling data, coupling, or counting path. The supplied evidence does not provide the encoder count or axis scaling values, so no numerical result can be established.
Inspect, correct, and verify
- Monitor the home-switch input and confirm that it changes state on approach and clears after reversal. Do not infer switch operation only from physical movement.
- Open and inspect the encoder connector. Emulsion was found inside the connector in this case, making contamination of the zero-mark circuit a confirmed condition.
- Dry and clean the connector using an appropriate electrical-contact treatment, then inspect the zero-channel conductors, cable, connector contacts, and counting input for persistent degradation.
- Calculate the axis travel per encoder revolution and compare it with the excess homing travel.
- Repeat the homing cycle several times while recording switch transitions and final machine position. Cleaning initially produced 3–4 successful cycles, but that temporary result alone did not prove a permanent repair.
Do not replace the encoder solely because homing failed. Replace or substitute components only after the switch sequence is confirmed and the connector, cable, zero channel, and counting input have been isolated. An incorrect reference position can also shift the controller's coordinate basis, explaining why the software limit did not stop later manual travel before the physical overtravel switch operated.
FAQ
Why does the axis pass the home position without an encoder alarm?
The normal position channels may continue counting while the separate encoder zero mark is missed. Therefore, no alarm does not rule out contamination, wiring degradation, or a zero-channel detection fault.
How can I tell whether the home switch or encoder causes the fault?
Monitor the switch input during homing. If it changes from 1 to 0 on approach and returns from 0 to 1 after reversal while motion continues, investigate the encoder zero-mark path next.
How do I confirm that the encoder missed one or more zero marks?
Calculate encoder counts per revolution divided by counts per millimetre, then compare that distance with the abnormal extra travel. Excess travel close to whole multiples of that result supports missed zero-mark detection.