FANUC Maintenance & Troubleshooting of CNC Robots 2 — Questions and Answers
Question 1: A FANUC robot displays alarm code SRVO-023 (Stop Error Excess). What is the most likely cause?
- The servo amplifier has overheated
- The robot reached a position error exceeding the defined threshold during motion (Correct answer)
- The teach pendant battery is low
- The emergency stop circuit is open
Correct answer: The robot reached a position error exceeding the defined threshold during motion
SRVO-023 indicates the positional deviation between commanded and actual position exceeded the stop error limit, typically caused by mechanical binding or incorrect gain settings.
Question 2: When replacing a FANUC pulse coder (encoder), which step must be performed immediately after installation?
- Update the robot firmware
- Perform a mastering (zero-point calibration) procedure (Correct answer)
- Reset all I/O assignments
- Replace the servo amplifier fuse
Correct answer: Perform a mastering (zero-point calibration) procedure
A new pulse coder loses the stored zero-point data, so mastering must be performed to re-establish the robot's reference position.
Question 3: Which FANUC diagnostic screen shows real-time axis torque values useful for detecting mechanical overloads?
- ALARM History screen
- SYSTEM Variables screen
- Status/Servo screen (servo diagnostic monitor) (Correct answer)
- I/O Status screen
Correct answer: Status/Servo screen (servo diagnostic monitor)
The Servo diagnostic monitor (Status > Servo) displays real-time torque percentage, current draw, and positional error for each axis.
Question 4: A FANUC CNC robot exhibits excessive backlash on Joint 2. After ruling out software causes, what mechanical component should be inspected first?
- The motor encoder cable shield
- The harmonic drive reducer or gearbox (Correct answer)
- The brake resistor pack
- The servo amplifier capacitors
Correct answer: The harmonic drive reducer or gearbox
Excessive backlash on a robot joint is most commonly caused by wear or damage in the harmonic drive reducer or gearbox that transmits motor torque to the link.
Question 5: What does the FANUC controller variable $SCR_GRP[1].$SOFTLMT indicate?
- The current servo loop gain multiplier
- The software joint travel limit boundaries for group 1 (Correct answer)
- The screen refresh rate in milliseconds
- The network timeout for the controller
Correct answer: The software joint travel limit boundaries for group 1
$SCR_GRP[1].$SOFTLMT contains the software over-travel limit values that the controller checks against joint angles to prevent exceeding the defined workspace.
Question 6: During preventive maintenance, the recommended method to check FANUC robot grease condition in joint reducers is to:
- Inspect grease color and quantity at the drain port during scheduled interval (Correct answer)
- Run the joint at maximum speed and measure motor current spike
- Check the grease level via the teach pendant grease monitoring screen
- Replace grease only when a SRVO alarm appears
Correct answer: Inspect grease color and quantity at the drain port during scheduled interval
FANUC maintenance schedules require draining and inspecting grease at defined intervals (typically 3,840 operating hours) by checking the drain port for color, contamination, and correct quantity.
Question 7: A FANUC robot arm drifts out of position overnight with the controller powered off. What is the most likely cause?
- Corrupted TP program memory
- Worn or failed axis brake not holding joint position (Correct answer)
- Misconfigured DCS safe zone
- Faulty Ethernet communication card
Correct answer: Worn or failed axis brake not holding joint position
If a robot axis drifts when power is removed, the electromagnetic brake for that joint is failing to engage and hold, requiring brake inspection or replacement.
FANUC Maintenance & Troubleshooting of CNC Robots 2
A FANUC robot displays alarm code SRVO-023 (Stop Error Excess).
What is the most likely cause?