FANUC spindle speed fluctuation detection: why exceeding the percentage does not always trigger an alarm
The 16i/18i/20i/21i-A rules use the greater of fixed and percentage-based speed tolerances. Reaching speed and timing out are separate detection-start events, explaining why the same deviation can produce different outcomes.
Contents
After setting a percentage speed tolerance, an observed deviation exceeds that percentage but no alarm occurs. Besides whether detection is enabled, check the fixed speed-deviation width and whether monitoring has actually started.
The greater of percentage and fixed widths applies
The FANUC 16i/18i/20i/21i-A connection manual B-63003EN-1/02, §9.6, pages 754–755 (public mirror of the original manual) defines allowable fluctuation as Sm=max(Sd,Sr). Sd is a fixed speed width; Sr is commanded speed Sc multiplied by the allowed percentage. Sc is the effective command after multiplying the S-specified speed by spindle override, not the unadjusted S value. Actual deviation from Sc must exceed Sm to meet this alarm criterion. This edition uses alarm 704 and signal SPAL; that number must not be transferred unconditionally to other editions.
The fixed width can dominate at low speed
An original threshold example uses Sd=120 r/min and allowed ratio r=5%. All Sc values below already incorporate override; these are calculation assumptions, not recommended settings:
| Sc | Sr=5%×Sc | Sm | Actual deviation 100 r/min |
|---|---|---|---|
| 1000 r/min | 50 r/min | 120 r/min | Exceeds 5%, but not Sm |
| 4000 r/min | 200 r/min | 200 r/min | Does not exceed Sm |
The widths are equal at Sc=120/0.05=2400 r/min. Below this, the fixed width dominates; above it, the percentage width dominates. Incorrectly selecting the smaller value predicts alarms the system does not require.
Detection starts on reaching speed or timing out
The same section states that after the command speed changes, detection starts when actual speed enters the target tolerance band or waiting time p expires. These are alternative conditions, not conditions that must both be met.
An original timing example sets a new target of 2000 r/min and target tolerance q=3%, giving a band of 1940–2060 r/min, with an assumed wait of 2 s. If actual speed reaches 1950 at 0.8 s, detection can start without waiting the full 2 s. If speed is still only 1700 at 2 s, timeout also starts detection. Retaining Sd=120 and r=5%, the deviation of 300 exceeds Sm=120, satisfying the fluctuation criterion.
Target-reaching tolerance is not alarm tolerance
q determines when checking begins; r and Sd determine the permitted deviation afterward. Treating q and r as the same percentage confuses entering monitoring with an alarm already being present. Normal acceleration before monitoring must also be distinguished from a speed drop after monitoring starts.
This is speed-deviation detection logic. An alarm establishes that the logic triggered; it does not alone prove that a temperature sensor has detected overheating or uniquely identify a motor, drive or mechanical seizure. Compare command-change time, effective reference speed and actual speed on one timeline. This article provides no instructions for disabling detection or widening thresholds. Further resources are in the OPTION section.