GantryMisalignmentThreshold Parameter

The GantryMisalignmentThreshold parameter defines Decoupling Control behavior of a gantry. It is set on the Yaw (Theta) axis and it specifies absolute value of the maximum allowable Yaw (Theta) position. If the absolute value of the Yaw (Theta) position command or position feedback is more than this threshold, the controller generates a GantryMisalignmentFault. This threshold prevents the Yaw (Theta) axis of a gantry from excessive yaw motion that would damage the mechanical device.

For Rigid gantries, the Yaw (Theta) is a null axis, and this parameter is similar to the PositionErrorThreshold with one notable exception. Position error faults are triggered only by position errors that exceed the PositionErrorThreshold, while gantry misalignment faults are additionally triggered by commands that exceed the GantryMisalignmentThreshold. This includes commands from things like calibration correction and home offsets. For Rigid gantries, set the GantryMisalignmentThreshold parameter high enough to allow for normal positional errors but low enough to prevent damage from too much relative motion between the spars. Use the procedure that follows to set an initial value for this threshold in the case of a Rigid H-bridge gantry:

1. Configure the gantry using Device Catalog and Machine Setup. Make sure that the Yaw (Theta) units are set up as desired.
2. Move the gantry axis to one end of travel by hand.
3. Reset the controller with the gantry at the end of travel.
4. While you apply pressure at the approximate midpoint of the bridge, gently push the gantry through travel. Make note of the maximum absolute value of the position feedback of the Yaw (Theta) axis.
5. Set the initial value of the GantryMisalignmentThreshold parameter to twice the value noted in step 4.

For Flexible gantries, this parameter must be set to the maximum allowable travel of the Yaw (Theta) axis. Commands to the Yaw (Theta) axis that exceed this threshold will be prevented by the controller to protect the stage from damaging itself.

If this parameter is set to 0 on the Yaw (Theta) axis, no GantryMisalignmentFault will occur.

This parameter must be set to 0 on the Linear (R) axis.