The FANUC SV0421 alarm indicates an excess error in semi-closed loop control. This alarm occurs when the difference between the full-closed loop feedback data and the semi-closed loop calculated data exceeds the value set in parameter P2118.
In a servo system using both semi-closed loop and full-closed loop feedback, the CNC system compares the motor-side calculated position with the machine-side feedback position. If the difference between these two values becomes larger than the allowable range, the system may generate the SV0421 alarm.
This alarm is usually related to incorrect parameter settings, wrong linear scale detection direction, incorrect flexible gear ratio, incorrect dual position feedback conversion coefficient, poor hardware installation, wiring problems, or damaged feedback hardware.
Quick Summary of FANUC SV0421 Alarm
| Alarm Code | SV0421 |
|---|---|
| Alarm Meaning | Excess Error in Semi-Closed Loop Control |
| Main Cause | The difference between full-closed loop feedback data and semi-closed loop calculated data exceeds the set value. |
| Key Parameter | P2118 |
| Related Parameter | P2018#0 |
| Key Check Items | Flexible gear ratio, dual position feedback conversion coefficient, linear scale direction, hardware installation, feedback hardware condition |
What Does FANUC SV0421 Alarm Mean?
SV0421: Excess Error in Semi-Closed Loop Control
The FANUC SV0421 alarm means that the difference between the semi-closed loop calculated position and the full-closed loop feedback position has exceeded the system setting.
In semi-closed loop control, the CNC system calculates the axis position mainly from the servo motor or motor-side feedback. In full-closed loop control, the system uses machine-side feedback, such as a linear scale, to detect the actual machine position. When both feedback systems are used for comparison, the difference between them must remain within the allowable range.
If the difference exceeds the value set in parameter P2118, the CNC system detects an abnormal feedback difference and generates SV0421.
Common Causes of FANUC SV0421 Alarm
The most common causes of FANUC SV0421 excess error semi-closed loop alarm include:
1. P2118 Setting Is Not Suitable
Parameter P2118 is related to the allowable difference between semi-closed loop and full-closed loop feedback data.
If this value is too small or not suitable for the actual machine condition, the CNC system may detect the feedback difference as excessive and generate SV0421.
However, P2118 should not be changed before confirming whether the feedback direction, gear ratio, conversion coefficient, and hardware installation are correct.
2. Incorrect Flexible Gear Ratio Setting
An incorrect flexible gear ratio can cause the semi-closed loop calculated data and the full-closed loop feedback data to become inconsistent.
If the gear ratio does not match the actual mechanical transmission relationship, the calculated motor-side position may not correspond correctly to the machine-side feedback position.
3. Incorrect Dual Position Feedback Conversion Coefficient
The dual position feedback conversion coefficient is important when comparing semi-closed loop and full-closed loop feedback data.
If the conversion coefficient is set incorrectly, the CNC system may compare two position values that are not scaled correctly. This can cause the difference to exceed the allowable value and trigger SV0421.
4. Incorrect Linear Scale Detection Direction
If the linear scale detection direction is incorrect, the full-closed loop feedback may change in the opposite direction from the semi-closed loop calculated data.
In this case, the feedback difference may increase quickly during axis movement. The linear scale detection direction should be checked through parameter P2018#0.
5. Poor Linear Scale Installation or Connection
SV0421 may also be caused by poor installation or incorrect connection of the linear scale or related feedback hardware.
If the scale is not installed properly, if the connection is loose, or if the feedback signal is unstable, the full-closed loop feedback data may become abnormal.
6. Damaged Feedback Hardware
Damaged hardware may also cause SV0421. Possible hardware-related causes include a damaged linear scale, faulty feedback cable, poor connector contact, or abnormal feedback interface hardware.
If parameter settings are correct but the alarm remains, the feedback hardware and installation condition should be checked carefully.
How to Troubleshoot FANUC SV0421 Alarm
When troubleshooting the FANUC SV0421 alarm, the key point is to identify why the full-closed loop feedback data and semi-closed loop calculated data are not consistent.
You may need to check:
- Whether the machine uses full-closed loop feedback or dual position feedback
- Whether the difference between semi-closed loop and full-closed loop feedback data exceeds the value set in P2118
- Whether parameter P2118 is set to a reasonable value
- Whether the flexible gear ratio is set correctly
- Whether the dual position feedback conversion coefficient is correct
- Whether the linear scale detection direction is correct
- Whether parameter P2018#0 is correctly set
- Whether the linear scale is installed correctly
- Whether the linear scale cable and connectors are secure
- Whether the feedback hardware is damaged
- Whether the alarm appeared after parameter restoration, linear scale replacement, cable replacement, or mechanical maintenance
Because SV0421 is closely related to feedback consistency, it is important to compare parameter settings with the actual mechanical and feedback configuration. Simply increasing P2118 may hide an actual feedback direction, scale conversion, or hardware installation problem.
Recommended Solution
The correct solution for FANUC SV0421 alarm depends on whether the problem is caused by parameter settings, feedback direction, conversion coefficient, hardware installation, or damaged feedback hardware.
1. Check Parameter P2118
First, check whether the semi-closed loop and full-closed loop error parameter P2118 is set to a reasonable value.
This parameter defines the allowable difference between the full-closed loop feedback data and the semi-closed loop calculated data. If the value is not suitable for the machine condition, it may need to be adjusted according to the actual configuration.
2. Check the Flexible Gear Ratio
Confirm whether the flexible gear ratio is set correctly.
If the gear ratio does not match the actual machine transmission ratio, the semi-closed loop calculated data may not correspond correctly to the full-closed loop feedback data.
3. Check the Dual Position Feedback Conversion Coefficient
Confirm whether the dual position feedback conversion coefficient is correct.
This setting affects how the CNC system converts and compares the motor-side calculated position and machine-side feedback position. If the coefficient is incorrect, SV0421 may occur even when the hardware is normal.
4. Check Linear Scale Detection Direction P2018#0
Check whether the linear scale detection direction is correct through parameter P2018#0.
If the linear scale feedback direction is reversed, the feedback difference between the semi-closed loop and full-closed loop data may increase instead of being corrected. In this case, the parameter setting should be corrected according to the actual feedback direction.
5. Check Linear Scale Installation and Connection
Check whether the linear scale is installed correctly and whether the related feedback wiring is connected properly.
Important inspection items include:
- Linear scale mounting condition
- Scale alignment and installation accuracy
- Feedback cable connection
- Connector contact condition
- Possible cable damage
- Signal stability during movement
6. Check Whether the Feedback Hardware Is Damaged
If the parameter settings and installation are correct but the alarm continues, check whether the linear scale or related feedback hardware is damaged.
Damaged feedback hardware can cause incorrect full-closed loop data, resulting in a large difference from the semi-closed loop calculated data.
If your machine shows FANUC SV0421 excess error semi-closed loop alarm, please contact REACO CNC directly. Our team can help check P2118, P2018#0, flexible gear ratio, dual position feedback settings, linear scale installation, feedback cable condition, and related hardware issues.
REACO CNC Support for FANUC SV0421 Alarm
REACO CNC provides support for FANUC CNC systems, servo feedback systems, full-closed loop control, semi-closed loop control, dual position feedback, linear scale installation, and related CNC hardware issues.
We can help with:
- FANUC CNC alarm analysis
- FANUC SV0421 alarm troubleshooting
- Semi-closed loop and full-closed loop feedback comparison
- Parameter P2118 checking guidance
- Parameter P2018#0 checking support
- Flexible gear ratio checking
- Dual position feedback conversion coefficient checking
- Linear scale direction issue analysis
- Linear scale installation and connection troubleshooting
- Replacement FANUC CNC parts supply
If you are not sure whether the SV0421 alarm is caused by P2118, flexible gear ratio, dual position feedback conversion coefficient, linear scale direction, scale installation, cable connection, or feedback hardware damage, you can send us the CNC system model, alarm screen, axis information, feedback configuration, related parameter values, and linear scale details. Our team will help check the possible cause.
FAQ About FANUC SV0421 Alarm
What is FANUC SV0421 alarm?
FANUC SV0421 alarm means excess error in semi-closed loop control. It occurs when the difference between full-closed loop feedback data and semi-closed loop calculated data exceeds the value set in parameter P2118.
What causes FANUC SV0421 alarm?
The alarm may be caused by an unsuitable P2118 setting, incorrect flexible gear ratio, incorrect dual position feedback conversion coefficient, wrong linear scale detection direction, poor hardware installation, wiring problems, or damaged feedback hardware.
Which parameter is related to the SV0421 alarm threshold?
Parameter P2118 is related to the allowable error between semi-closed loop and full-closed loop feedback data.
Which parameter controls the linear scale detection direction?
Parameter P2018#0 is related to the linear scale detection direction. If the detection direction is incorrect, the feedback data may not match the semi-closed loop calculated data.
Can incorrect flexible gear ratio cause SV0421?
Yes. If the flexible gear ratio is not set correctly, the calculated semi-closed loop position may not correspond correctly to the full-closed loop feedback position.
Can linear scale installation problems cause SV0421?
Yes. Poor linear scale installation, loose connectors, damaged cables, or faulty feedback hardware can cause abnormal full-closed loop feedback data and trigger SV0421.
Should I change P2118 directly when SV0421 appears?
No. Before changing P2118, first check the flexible gear ratio, dual position feedback conversion coefficient, linear scale detection direction P2018#0, and the hardware installation and connection condition.
How can I fix FANUC SV0421 alarm?
Check whether P2118 is reasonable, confirm the flexible gear ratio and dual position feedback conversion coefficient, verify the linear scale detection direction through P2018#0, and inspect the linear scale installation, connection, and hardware condition. If hardware is damaged, repair or replace the faulty part.
Need Help with FANUC SV0421 Alarm?
If your CNC machine has a FANUC SV0421 excess error semi-closed loop alarm, please contact REACO CNC. We can help analyze the alarm cause and provide suitable FANUC CNC repair, feedback system troubleshooting, linear scale checking, parameter guidance, or related technical support.
Reference Source: Beijing FANUC. This article is based on technical documentation provided by Beijing FANUC. For More Fanuc CNC repair Cases and technical articles, please back to Fanuc Technical Support Center.
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