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BrickAC Gantry mode
#1
I have a Power Brick AC controlling a gantry system. I am new to PPMAC and have some questions regarding the gantry mode:
1. Do the two axis have to have the same position before enabling gantry mode?
2. Is gantry mode disabled if a drive faults? If yes, how is this done and can it be disabled?
3. Do the drives have to be disabled to enable gantry mode?
4. Is there a proper method to enable gantry mode or preparation before enabling gantry mode?

Thanks,

Adrian
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#2
There are two separate features for "gantry mode", which are technically independent of each other, but often used together.

The first is "leader/follower" mode, that provides the same trajectory to each motor. It does this by telling the "follower" motor not to use its own trajectory (by setting Motor[x].ServoCtrl to 8, not 1) but instead to use the trajectory from the "leader" motor (by setting the follower motor's Motor[x].CmdMotor to the number of the leader motor).

The two motors do not have to be in the same position when enabling leader/follower mode, and in general will not be. We call the difference "skew". When you do a homing search move for a leader/follower motor pair, this skew will automatically be removed after the home trigger for both motors has been found. This is explained in detail in the User's Manual.

Most users will have leader/follower mode enabled automatically on power-on/reset, and the first thing done with this motor pair is to do the joint homing search move. If the servo loops are ever opened (e.g. for a fault), any skew will automatically be removed when the loops are closed again.

You have choice as to how faults are shared. Most users define the follower motor as a "null axis" (#x->0) in the same coordinate system as the leader motor so that faults can be "shared". If Motor[x].FaultMode = 1 for both motors, if one drive faults, PMAC automatically "kills" the other motor.

When first enabling leader/follower mode in the lab, I would definitely recommend having the drives disabled so there are no transition issues. As noted above, in the actual application, most users have the motors in this configuration immediately on power-on/reset (from the saved configuration), so they are automatically in this mode before they are enabled.

Virtually all gantry users employ this leader/follower mode, but most use separate servo algorithms to close the loops on each motor. Generally, only if the resulting accuracy is not good enough do people employ the second feature for gantries: the cross-coupled feedback (GantryXctrl).
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