FANUC SP9021 Alarm 21: Position Sensor Polarity Error and Troubleshooting

FANUC SP9021 means that the polarity specified for the spindle position sensor does not match the machine configuration. The CNC may display SP9021 or a series-dependent serial spindle alarm, while the spindle amplifier displays 21.

FANUC CNC alarm lists instruct technicians to check and correct parameter 4000#0 and parameter 4001#4. The αi Series spindle amplifier maintenance manual also instructs technicians to check the position sensor feedback cable.

For the surrounding alarm numbers, see the FANUC Serial Spindle Alarm Codes SP9001–SP9033.

What Does FANUC SP9021 Mean?

The spindle control must interpret the position feedback in the correct rotational direction. SP9021 is generated when the polarity defined for that feedback is inconsistent with the configured relationship between the spindle, motor, and position sensor.

This alarm commonly appears after:

• Spindle parameters have been initialized, restored, copied, or edited

• A CNC, spindle amplifier, motor, position sensor, or feedback cable has been replaced

• The spindle drive or sensor wiring has been changed during maintenance

• A replacement parameter file does not match the machine's original spindle configuration

These circumstances are useful diagnostic clues, but they do not replace verification against the correct machine data. Do not change parameter bits by trial and error.

What Do Parameters 4000#0 and 4001#4 Control?

FANUC AC Spindle Motor αi/βi Series Parameter Manual B-65280EN identifies the two relevant settings as follows:

FANUC sample configurations repeatedly mark these settings as configuration-dependent. There is no universal combination that is correct for every machine.

Before changing either bit, compare the current values with:

• The machine builder's parameter list

• A verified backup from the same machine

• The FANUC spindle parameter manual for the installed sensor configuration

• Commissioning data recorded before the recent repair or replacement

Changing only 4001#4 may be the correct procedure under the αi spindle amplifier maintenance manual, while CNC alarm lists for some control series direct technicians to verify both 4000#0 and 4001#4. Follow the documentation for the exact CNC and spindle system.

1. Confirm SP9021 and Amplifier Alarm 21

Before changing parameters or wiring, confirm:

• The CNC shows SP9021, POS SENSOR POLARITY ERROR, or the equivalent series-specific spindle alarm

• The spindle amplifier status display shows 21

• The alarm belongs to the spindle axis being diagnosed

• No separate position-coder, motor-lock, or sensor-disconnection alarm is present

Record the alarm screen and the following equipment information:

• Complete CNC series and model

• Exact spindle amplifier or SPM model, including every suffix

• Spindle motor model

• Position sensor or position coder type

• Current values of 4000#0 and 4001#4

• Any maintenance, parameter restoration, cable replacement, or hardware replacement performed immediately before the alarm appeared

The CNC-side alarm number can vary among FANUC series, so the amplifier display 21 and the applicable manual are important confirmation points.

2. Back Up the Existing Parameters

Create or confirm a complete parameter backup before making any change. Record the original values of 4000#0 and 4001#4 separately so they can be restored if necessary.

Do not copy spindle parameters from a visually similar machine unless the CNC, spindle amplifier, motor, sensor, gear or belt arrangement, and machine builder specification are confirmed to be identical. A parameter combination that works on one machine may command the wrong rotational relationship on another.

Parameter access and power-cycle requirements differ by CNC series and machine builder. Use the applicable FANUC and machine documentation, and allow only qualified personnel to modify the spindle settings.

3. Verify Parameters 4000#0 and 4001#4

Compare both bits with the verified machine data:

1. Confirm whether the mechanical transmission makes the spindle rotate in the same or opposite direction relative to the motor.

2. Confirm the installed orientation of the spindle position sensor.

3. Check that 4000#0 matches the spindle-to-motor rotational relationship.

4. Check that 4001#4 matches the spindle sensor mounting direction.

5. Correct only the value proven to be inconsistent with the documented configuration.

If the alarm began immediately after parameter initialization or restoration, first determine whether the wrong parameter file or default data was loaded. Correcting the source data prevents the same fault from returning during the next restore.

Do not simply toggle both bits at once. Doing so hides which setting was wrong and can create an incorrect spindle direction or position-feedback relationship.

4. Inspect the Position Sensor Feedback Cable

If the parameters match the verified machine specification but alarm 21 remains, FANUC B-65285EN/04 directs technicians to check the position sensor feedback cable.

With machine power safely isolated and the DC link discharged according to the applicable manual, inspect:

• Whether the feedback cable is connected to the correct spindle amplifier and sensor connectors

• Loose, partially inserted, damaged, or contaminated connectors

• Bent, recessed, or damaged connector pins

• Incorrectly repaired or replaced cable assemblies

• Wiring changes made during recent sensor, motor, or amplifier work

• Cable continuity and shielding according to the exact FANUC connection diagram

The instruction to check the cable does not mean that every position sensor uses the same A/B-phase wiring. FANUC spindle systems support different sensor types and interfaces. Verify pin assignments from the correct connection manual rather than swapping conductors experimentally.

5. Retest the Spindle Safely

After restoring the correct documented parameter settings or repairing a confirmed feedback-cable problem, retest according to the machine builder's procedure.

Confirm that:

• SP9021 and amplifier alarm 21 no longer appear

• The commanded and actual spindle directions agree

• Spindle speed feedback is stable

• Orientation, threading, rigid tapping, spindle synchronization, and other installed position-control functions operate correctly

• No new position-coder or motor-lock alarm appears

If another alarm is displayed, diagnose that exact alarm separately. Do not continue treating every sensor-related alarm as SP9021.

Checks That Do Not Belong to the Standard SP9021 Procedure

Several items in broad spindle troubleshooting lists are useful for other alarms but are not FANUC's standard alarm 21 procedure:

• Motor U/V/W phase sequence: FANUC lists this under motor-lock and incorrect-rotation troubleshooting, including alarm 31. It is not a routine SP9021 step.

• Motor feedback A/B phase sequence: The αi maintenance manual mentions this under alarm 31. It should not be applied universally to every position sensor used with alarm 21.

• V-belt slippage: Slippage is relevant to certain one-rotation signal, gear-ratio, and sensor-tracking alarms. It is not listed as the standard cause of alarm 21.

• Immediate sensor replacement: Alarm 21 first requires parameter and cable verification. Replace a sensor only when signal testing or a separate applicable alarm confirms that it is faulty.

• Immediate spindle amplifier replacement: FANUC does not specify amplifier replacement as the normal first remedy for alarm 21. Confirm the parameters, cable, and exact alarm before considering hardware repair.

This distinction prevents unnecessary parts replacement and keeps the diagnosis aligned with FANUC's alarm-specific procedure.

Difference Between SP9021 and Related Spindle Sensor Alarms

An alarm involving a position sensor is not automatically a polarity alarm. Always use the number shown on the spindle amplifier.

FANUC SP9021 Diagnostic Table

Recommended Troubleshooting Order

1. Confirm SP9021 on the CNC and alarm 21 on the spindle amplifier.

2. Record the CNC, spindle amplifier, motor, sensor, and recent maintenance information.

3. Back up all parameters and record the original values of 4000#0 and 4001#4.

4. Verify the spindle-to-motor rotation relationship and spindle sensor mounting direction.

5. Compare 4000#0 and 4001#4 with the applicable machine builder and FANUC documentation.

6. Correct only a confirmed parameter mismatch.

7. If the alarm remains, safely inspect the position sensor feedback cable and connectors.

8. Retest spindle direction, speed feedback, orientation, and other installed position-control functions.

9. If another alarm appears, diagnose that alarm separately.

Technical References

• FANUC AC Servo Motor αis/αi Series, AC Spindle Motor αi Series, Servo Amplifier αi Series Maintenance Manual, B-65285EN/04, Alarm Code 21

• FANUC AC Spindle Motor αi/βi Series Parameter Manual, B-65280EN/08, spindle sensor configurations and parameters 4000#0 and 4001#4

• Applicable FANUC CNC maintenance manual alarm list for the installed control series, including the SP9021 procedure

Always use the manual edition that applies to the exact CNC, spindle amplifier, motor, sensor, and machine configuration.

Need Help With FANUC SP9021 Alarm?

REACO CNC provides independent troubleshooting, testing, repair, and replacement support for FANUC spindle amplifiers, motors, position sensors, and feedback cables. Send us the CNC alarm screen, amplifier display, complete CNC model, exact spindle amplifier and motor models, sensor type, current values of 4000#0 and 4001#4, and details of recent maintenance.

Visit the FANUC Technical Support Center for more FANUC alarm guides, or contact REACO CNC for assistance.

Reference Source: Beijing FANUC. This article is based on FANUC technical documentation. The correct polarity parameters and verification procedure depend on the exact CNC, spindle system, sensor installation, and machine builder configuration.