Position Error Fault

HARDWARE: This fault can occur on all drive types.

A PositionErrorFault occurs when the difference between the commanded position and the encoder position is more than the value of the PositionErrorThreshold Parameter.

A PositionErrorFault can occur only on an enabled axis. The Position Error Fault box must be selected in the FaultMask Parameter in the Configure workspace for the system to monitor for errors. If this is not selected, a fault condition will not show a PositionErrorFault.

If possible, Aerotech recommends to always enable the PositionErrorFault. This fault helps with protection against a runaway axis, which can cause damage to the equipment and the user operating the equipment.

Use the tables and charts in the sections that follow to help find the cause and solution of the PositionErrorFault on your system.

New axis that has a PositionErrorFault when it runs for the first time

The table that follows includes troubleshooting for a new axis that is running for the first time and goes into a fault state when enabled or commanded to move.

Table: New Axis running for the first time

Cause	Examine your System	Solution
Feedback device configuration error	Feedback configuration causes a change in position feedback without a change in position command. Feedback moves in the opposite direction of the command.	Make sure that the position feedback is configured to read from the correct device in Machine Setup. Make sure that the Primary Feedback Direction in the FeedbackSetup Parameter is set correctly. Make sure that PrimaryFeedbackType Parameter has the correct type of feedback device selected. If the axis has two feedback devices, make sure that the AuxiliaryFeedbackType Parameter has the correct type of feedback device selected. If the axis has one feedback device, the AuxiliaryFeedbackType parameter should be set to None.
Motor configuration error	The motor configuration causes a change in position feedback without a change in position command. The current feedback does not obey the current command. Feedback moves in the opposite direction of command.	Make sure that the motor cable is connected to the correct device as identified in Machine Setup. Use the Motor Phasing Module in the Configure workspace to find possible problems with the motor wiring. Look at the settings for the CommutationInitializationSetup Parameter. If set to Hall-Effect Switches, make sure that the Hall Commutation Direction is set to the correct polarity. If set to Auto-MSET, adjust the AutoMsetCurrent Parameter or AutoMsetTime Parameter. If set to Commutation Search, adjust the CommutationSearchCurrent Parameter or CommutationSearchTime Parameter. If applicable, make sure that the CurrentCommandCouplingFollowerDirection Parameter is set to the correct polarity. Go through Machine Setup again, and make sure that the correct electrical and mechanical devices are identified. Make sure that the bus voltage is set correctly on the electrical device. Then, on the summary page, accept the changes to only the parameters that apply to the current loop gains.
Mechanical restriction	The position feedback does not keep up with the position command. The brake does not disengage when the stage is enabled.	Make sure that the red shipping clamps are removed. Make sure that there are not external parts that are interfering with stage travel. If the stage has a brake, make sure that the BrakeOutput Parameter and BrakeSetup Parameter are set correctly. Make sure that the brake power supply is working and is wired correctly.
Bus voltage error	The current feedback does not obey the current command. The position feedback does not change when the position command changes.	Make sure that bus power is being applied to the amplifier. Make sure that the bus voltage is sufficient for the drives.
Servo tuning error	The servo loop is unstable and goes into a fault state when the stage is enabled. The position feedback does not keep up with the position command.	Run EasyTune in the Servo module of the Configure workspace to set the servo gains.

Existing axis that has a PositionErrorFault with all moves

The table that follows includes troubleshooting for an existing axis that ran successfully but now goes into a fault state when enabled or when trying to do all types of moves.

Table: Existing axis that has a fault with all moves

Cause	Examine your System	Solution
Feedback device failure	The position feedback does not change when the position command changes.	If you made changes to your system, make sure that the position feedback is configured to read from the correct device in Machine Setup. Do a check for disconnected feedback cables or for failures with the feedback cables. Contact Global Technical Support for replacements if necessary. Do a check for feedback in the Status Utility by manually moving the stage while it is disabled. If the feedback does not change, the device is probably damaged. Contact Global Technical Support to ask about repairs.
Motor or amplifier failure	The position feedback does not keep up with the position command. The current feedback does not obey the current command.	Make sure that the motor cable is connected to the correct device as identified in Machine Setup. Make sure that the motor and amplifier cables are not damaged or disconnected. Contact Global Technical Support for replacements if necessary.
Mechanical failure	The position feedback does not keep up with the position command. The brake does not disengage when the stage is enabled.	Examine system for external part interferences with stage travel. Do a check for damage to the drive train of the stage. Contact Global Technical Support for repairs. Examine the stage bearings. Make sure that they are not damaged and that they are lubricated per the product manual. Contact Global Technical Support for repairs. If the stage has a brake, make sure that the brake parameters are set correctly. Make sure that the motor/brake cable has not failed.
Bus voltage problem	The current feedback does not obey the current command. The position feedback does not keep up with the position command.	Make sure that bus power is being applied. Make sure that the bus voltage is sufficient for the demand from the drives.
Servo tuning problem	The payload changed, which caused the servo to be unstable. This causes a fault when the stage is enabled.	Use the EasyTune module in the Servo topic of the Configure workspace. Use a custom performance target to get the specified motion profile.

Existing axis that has a PositionErrorFault with only some types of moves

The table that follows includes troubleshooting for an existing axis that can move through travel but goes into a fault state when commanded to do specified types of moves.

Table: Existing axis that has a fault with some types of moves

Cause	Examine Your System	Solution
Amplifier or motor capability problems	The current feedback plateaus while the current command increases. The current feedback does not obey the current command.	Do a check to see if the current is clamped at the value set by the MaxCurrentClamp Parameter. The current threshold of the drive is possibly not sufficient for the commanded move. Make sure that the bus voltage is sufficient for the demand from the drives. Go through Machine Setup again. On the summary page, accept the changes to only the parameters that apply to the current loop gains. The amplifier power limiting protection is enabled and is limiting output to the motor. Contact Global Technical Support if you need help finding the power requirements for specified moves.
Incorrect servo gains	The position feedback does not keep up with the position command. The system becomes unstable during some motion.	Use the EasyTune module in the Servo topic of the Configure workspace. Use a custom performance target to get the specified motion profile.
Speed or feedback capabilities	The position feedback does not keep up with the position command. The position feedback does not change during high velocity moves.	Examine the maximum speed limitations for the axis or feedback device. Decrease the speed during the motion profile that is causing the position error fault. Increase the PositionErrorThreshold Parameter to a value that is within acceptable limits.