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Comp Table and Cross Coupled Gantry


kmonroe023
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Hopefully this is a fairly simple question. We have an x/y gantry, and one axis uses the x-coupled gantry algorithm. We will be using a laser interferometer to compare actual position to commanded. We plan to use the results to setup a comp table for both axis in the gantry, which leads to my question.

 

When setting up a comp table for a gantry axis using two motors with the x-coupled gantry algorithm, do you setup a comp table for the leader motor, or both motors?

 

Thanks,

kmonroe

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Power PMAC has two features for gantry systems, which while commonly used together are technically independent from each other.

 

The first is the leader/follower feature, in which the follower motor does not compute its own commanded trajectory, but uses the trajectory computed by the leader motor. Note that this deals only with commanded trajectories.

 

The second feature is the cross-coupled servo control. In this feature, not only does each motor close a loop based on its own following error, but adjusts its servo output to try to drive the difference in following error to zero. (Most commonly, the two motors that are cross-coupled are the leader and follower motors, but this is not necessarily the case.

 

I think the best way to look at the cross-coupled servo is as the addition of a small but quick tweak to try to keep the two motors fully aligned. The main servo effort is using each motor's own command and feedback. So measurement errors in the feedback for each motor, which in general will be different, can and should be corrected by a compensation table for that individual motor.

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