SV0411 Excess Error (Moving) fault in Fanuc drive

SV0411 Excess Error (Moving) fault in Fanuc drive

Description

While SV0410 refers to an error while stopped, SV0411 specifically indicates an "Excess Following Error" occurring while the axis is in motion. In servo theory, a "following error" is necessary for movement; the difference between commanded and actual position is what generates the torque command. However, this error must remain proportional to the velocity. If the error widens beyond the calculated limit set by the parameters (specifically the "Error Limit" parameter based on feed rate), the CNC assumes it has lost control of the axis trajectory. This alarm is a safety feature to prevent the tool from deviating from the programmed path, which could result in a scrapped part or a collision with the workpiece.

Cause

This fault usually indicates that the motor is physically unable to keep up with the commanded speed.

• Voltage Starvation: If the input voltage to the machine dips (brownout) or the DC Link voltage sags under load, the motor loses torque at high RPMs.
• Excessive Cutting Forces: Using a dull tool or taking a depth of cut that exceeds the machine's horsepower rating. The motor applies maximum current but still drags behind the command.
• Parameter Tuning: If the Servo Loop Gain is set too low, the axis becomes "lazy" and lags behind. Conversely, if the acceleration time constant is set to zero (step command) for a heavy load, physics dictates the motor cannot react instantly.
• Torque Limit Setting: The torque limit parameter (usually 200%+) might be set too low artificially, capping the motor's ability to correct errors.

Solution

Troubleshooting involves distinguishing between electrical limitation and mechanical overload:

1. Analyze Operation: Does this happen only during Rapid (G00) moves or cutting (G01) moves? If it's during rapids, friction is likely the culprit. If during cutting, check tool load.
2. Check Input Voltage: Measure the 3-phase input while the machine is accelerating. A significant voltage drop indicates weak shop power or undersized wiring.
3. Tuning Adjustment: For older machines, you may need to slightly increase the "Error Limit" parameter (Parameter 1828) temporarily to see if it clears, though this masks the root cause.
4. Mechanical Friction: Lubricate the ways. A dry slideway increases the "break-away" torque required, causing the axis to lag immediately upon starting a move.