I just picked up a new rotary table that has a Fanuc AC a2 motor with pulse coder.
I've been running AMC drives on all my brushed servo axes for years now and it always seemed intuitive to me that one would connect the tachometer feedback to the drive to close the +/- 10V analog velocity control, and to feed the encoder signals back to LinuxCNC to control the overall position loop. For the longest time, I've been searching specifically for tables with motors with tachometer feedback because I have a pile of amps that take tach feedback, and probably because... well, I just didn't know any better.
But now that I'm faced with the Fanuc motor with no tachometer, it suddenly dawns on me - must I close the loop on the amplifier? It strikes me as odd that I didn't see this before, but if I leave the amplifier in open-loop mode (i.e. No feedback), then can I not use a motor with an encoder and feed the encoder right back to LinuxCNC which will close the loop anyway? I've seen threads describing position vs velocity, but I thought the PID causes feedback to the motor command (analog signal for direction and velocity) that would be sufficient to close the loop...
Anyway, I'm looking for an expert opinion here. I've been running my machine for a few years now and can hold tolerances of +/- 0.0005, so I'm not going to screw with a good thing, but now when faced with the prospect of interfacing with this new AC drive (and given that I happen to have an AC brushless motor that does NOT have a tach feedback), I'm open to suggestions on why closing the loop between the motor and the amp independent from the loop between encoder and LinuxCNC may or may not be a good idea.
Any takers?
Torin...
--
www.walker-tech.com
I've been running AMC drives on all my brushed servo axes for years now and it always seemed intuitive to me that one would connect the tachometer feedback to the drive to close the +/- 10V analog velocity control, and to feed the encoder signals back to LinuxCNC to control the overall position loop. For the longest time, I've been searching specifically for tables with motors with tachometer feedback because I have a pile of amps that take tach feedback, and probably because... well, I just didn't know any better.
But now that I'm faced with the Fanuc motor with no tachometer, it suddenly dawns on me - must I close the loop on the amplifier? It strikes me as odd that I didn't see this before, but if I leave the amplifier in open-loop mode (i.e. No feedback), then can I not use a motor with an encoder and feed the encoder right back to LinuxCNC which will close the loop anyway? I've seen threads describing position vs velocity, but I thought the PID causes feedback to the motor command (analog signal for direction and velocity) that would be sufficient to close the loop...
Anyway, I'm looking for an expert opinion here. I've been running my machine for a few years now and can hold tolerances of +/- 0.0005, so I'm not going to screw with a good thing, but now when faced with the prospect of interfacing with this new AC drive (and given that I happen to have an AC brushless motor that does NOT have a tach feedback), I'm open to suggestions on why closing the loop between the motor and the amp independent from the loop between encoder and LinuxCNC may or may not be a good idea.
Any takers?
Torin...
--
www.walker-tech.com