921

Navigate to DIAGNOSIS > SERVO screen and observe diagnostic numbers D0300-D0303 (for axes 5-8 respectively) to identify which axis triggered the alarm and note the servo alarm number displayed.

Check the servo amplifier LED display on the suspected axis drive module (axes 5-8). Record the LED blink pattern or alphanumeric code shown, cross-reference with the specific amplifier alarm list (αi series shows A0-A9, B0-B9, L0-L9, etc.).

Review parameters 1825#0-#7 (servo alarm detection mask) for the affected axis to verify alarm detection is enabled. Check parameter 2022 (rotary axis position loop gain) and 2023 (velocity loop gain) for unusual values that may cause instability on rotary axes.

If sub-alarm indicates overcurrent (A0, A1) or overload (A8), verify motor cable connections are secure, check for mechanical binding on the rotary axis, measure motor resistance phase-to-phase (should be balanced), and verify motor specifications match parameter 2020 (axis type) settings.

Do not repeatedly reset and test if overcurrent alarms occur - this may damage the amplifier power modules

Access DIAGNOSIS > SERVO and compare D0500-D0503 (position deviation for axes 5-8) against parameter 1826 (in-position check width). If deviation exceeds several thousand encoder counts, investigate mechanical resistance or encoder signal integrity.

Verify encoder cable shielding and connections at both motor and amplifier ends for axes 5-8. Check DIAGNOSIS > SERIAL for axes 5-8, monitor D0020-D0023 (serial pulse coder feedback) while manually rotating the axis - values should increment smoothly without jumps.

Check parameters 1821 (position loop gain), 1822 (velocity loop gain), and 1860 (acceleration time constant) for the affected rotary axis. Compare against OEM specifications; excessive gain or short acceleration times can cause instability on high-inertia rotary tables.

If position error persists, perform encoder alignment check by navigating to SYSTEM > PMC > SIGNAL and monitoring encoder phase A/B signals while jogging the axis slowly. Verify quadrature relationship and absence of signal dropout in diagnostic D0010-D0013.

Encoder misalignment or electrical noise can cause cumulative position errors leading to alarm 921

Navigate to DIAGNOSIS > SERIAL screen and check D0900-D0903 for axes 5-8 encoder communication status. A value other than 0 indicates a specific encoder fault code (refer to encoder manufacturer documentation for BiS-C or αi encoder error definitions).

If absolute encoder battery alarm is indicated, check parameter 3111#6 and diagnostic D1390-D1393 for battery voltage status on axes 5-8. Replace encoder battery if voltage is below specification (typically <3.0V for lithium cells) and perform reference position return after replacement.

Loss of absolute position requires re-establishing reference position per OEM procedure to prevent workpiece coordinate errors

Inspect encoder cable routing for axes 5-8, ensuring separation from power cables and AC spindle motor cables (minimum 100mm separation). Check cable shield grounding at amplifier side only, verify ferrite cores are installed per Fanuc connection manual specifications.