Some Basic Questions about servo control

[acondit]

I actually think that I know the answer to some of these questions, but I am not certain.

Assume that you are using EMC2 to close the loop on a servo system:
If one servo is failing to move at the commanded rate (i.e., fewer encoder counts have been received than are expected) this is a following error, correct? If not, what exactly is a following error?

A following error is only a show stopper if the difference between commanded motion and reported motion exceeds the ferror value set in the ".ini" file, correct?

Will Emc2 automatically allow more time for the slow axis to catch up, or will it just continue as commanded until it errors out?

Now assume that you are allowing Gecko servo drives to close the loop:
If you are using Gecko servo drives and one axis is approaching the internal limit of counts on the following error (but not exceeding it) EMC2 will not know that is happening, correct? So if the following error in the Gecko drives, is +/- 128counts it might create problems that go unreported to the system, (depending on how many encoder counts it takes to move the distance specified by ferror), right?

Alan

[skullworks]

Basically you have it right.

If the reported position varies from the commanded position more than the ferror amount EMC will error out and stop.

With servos the system often has headroom margin to apply extra power to correct for a lag before the Ferror is reached. and the Ferror limit can be a very small number of counts - like 3.

Geck 320's or 340's - are a different issue - they are stepper drives for servos. They treat a servo like a stepper with feed back to the Gecko - the following error is fixed in the gecko at +/- 128 counts - so depending on what 128 counts is in real movement you could be way off yet be within the Gecko internal following error.

Since the loop is closed between the servo and Gecko, using EMC to run this requires setting it up as a stepper since EMC is out of the loop for Ferror and may not even have actual encoder feedback unless a special breakout board like a PICO Gecko servo interface, and I think PMDX and maybe others allow a way to fork the encoder signal back to EMC.