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How to Fix FANUC Alarm SRVO-021 – Excess Error (Position Deviation)
Quick Fix for FANUC SRVO-021
If SRVO-021 (Excess Error) appears, try these immediate actions:
- Reset the alarm and restart motion at low speed
- Check for mechanical obstruction or collision
- Reduce override speed and acceleration
- Verify payload and tool settings
- Inspect servo motor and encoder cables
These steps may quickly restore operation if the deviation is temporary.
What is FANUC SRVO-021 Excess Error?
FANUC SRVO-021 is a position deviation alarm, triggered when the actual motor position deviates too far from the commanded position.
This is a protection mechanism to prevent:
- Mechanical damage
- Loss of motion control
- Servo instability
The controller continuously compares commanded vs actual position. If the difference exceeds the limit, the alarm is generated.
What Does SRVO-021 Mean?
SRVO-021 indicates that the servo system cannot follow the commanded motion accurately.
Typical impact:
- Robot stops immediately
- Affected axis is disabled
- Motion becomes unstable or inaccurate
This usually points to either mechanical resistance or servo performance issues.
Common Causes of FANUC SRVO-021
Mechanical Obstruction or Collision
The most common cause.
- Robot hits an object
- External interference blocks movement
- Tight or damaged joints
Excessive Load or Incorrect Payload
Improper payload settings affect motion accuracy.
- Payload too heavy
- Incorrect center of gravity
- Poor tool configuration
Servo Motor or Amplifier Weakness
The servo system may not produce enough torque.
- Aging motor
- Amplifier performance degradation
- Overheating
Encoder or Feedback Issues
Position feedback errors can cause deviation.
- Encoder signal instability
- Loose feedback connections
- Data mismatch
High Speed or Aggressive Motion Settings
Demanding motion profiles increase deviation risk.
- High acceleration or deceleration
- Sudden direction changes
- High override speeds
Cable or Connection Problems
Signal interruption leads to control errors.
- Damaged encoder cables
- Loose motor connectors
- Electrical noise interference
How to Troubleshoot FANUC SRVO-021
Step 1: Check for Mechanical Issues
- Inspect robot path for obstruction
- Manually jog the robot slowly
- Listen for abnormal resistance or noise
Step 2: Reduce Speed and Load
- Lower override speed (e.g., 10–25%)
- Test motion under reduced load
- Adjust acceleration settings
Step 3: Verify Payload Configuration
- Check payload weight and center of gravity
- Update tool data in controller
- Ensure correct TCP settings
Step 4: Inspect Servo Motor and Amplifier
- Check for overheating
- Observe servo amplifier status
- Replace faulty components if needed
Step 5: Check Encoder and Feedback System
- Inspect encoder signals
- Reseat connectors
- Replace encoder cable if damaged
Step 6: Test and Monitor
- Reset alarm
- Run robot in controlled motion
- Observe if error returns under load
Pro Diagnostic Tip (Quick Isolation Method)
1. Test Without Payload (Load Isolation) Remove the tool or payload and run the affected axis at a low speed. If the alarm clears, the issue is likely rooted in the payload weight or an incorrect tool configuration.
2. Evaluate Motor/Servo Performance If the alarm persists even without a load, inspect the servo motor for signs of overheating, mechanical wear, or inconsistent torque output.
3. Inspect Encoders and Cables Check for stable encoder signals. Since loose or damaged connections are a common culprit, reseat or replace the encoder cables as needed.
4. Field Isolation Strategy This systematic approach quickly distinguishes between mechanical load issues and feedback system faults. It allows you to pinpoint whether the problem lies within the servo motors, amplifiers, or encoder connections, significantly reducing downtime.
Preventing SRVO-021 Excess Error
- Always configure accurate payload data
- Avoid aggressive acceleration profiles
- Perform routine mechanical inspections
- Maintain servo motors and amplifiers
- Replace worn cables and connectors early
Related FANUC Alarm Codes
These alarms are often related to motion deviation and servo instability.
Conclusion
FANUC SRVO-021 is a critical motion deviation alarm, usually caused by mechanical resistance, incorrect payload, or servo system limitations.
Most issues can be resolved by reducing load, checking mechanics, and verifying servo performance.
A structured diagnostic approach ensures fast recovery and prevents repeated downtime.
FAQ
Q1: What causes FANUC SRVO-021?
It is caused by excessive position deviation due to mechanical blockage, overload, servo weakness, or feedback issues.
Q2: Is SRVO-021 serious?
Yes, it stops robot motion to prevent damage.
Q3: Can I reset and continue operation?
Only if the cause is temporary. Repeated alarms require troubleshooting.
Q4: What should I check first?
Start with mechanical obstruction and payload settings.
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