Neil Baylis wrote: > I made a short video of progress in tuning my Linear Motor axis. This is > going to be the Y axis of a laser machine. In the video, it's doing a series > of random rapids at 30 inches per second, or 1800 IPM. During these rapids, > the FE is less than 0.05 inches. I'm not done with the tuning yet, but these > results are encouraging. > > Linear Motor tests <http://www.youtube.com/watch?v=wxwgzj4KmCE> > I don't think it ever reached 30 IPS, maybe about half that, due to acceleration. Still, QUITE impressive.
At these sorts of speeds, you may need to up the servo sampling rate quite a bit. You can easily go to 2 KHz, and with a good PCI parallel port card, you should be able to go to 4 or 5 KHz. > The motor is driven by a Pico systems 3 phase PWM amplifier, and the Pico > systems UPC controller. This amp uses 6-step commutation, and initially I > was concerned that there would be too much force ripple. Seems I didn't have > to worry. > > You can hear two main sounds in the video. The higher pitched scraping sound > is the sound of the linear bearings. There's also a lower pitched rumble, > which is coming from the motor. This sound seems to be caused by the > Derivative term in the control loop. If I set D to zero, the rumble goes > away, but the FE becomes too great. > Yeah, I definitely hear similar things in round-motor servos, too. If you are not using FF1 and FF2, you may be able to use those with less D and get even better performance. Since you have no cutting loads to respond to, you should be able to reduce error to about 2-3 encoder counts at all speeds with the feed-forward. It only takes a little FF1 and very small FF2 to make a huge difference. It is easy to overdo the FF and make things worse. FF1 of 1-5 and FF2 of .01 are numbers I often see. That rumble comes from the sampling of the encoder count, plus your low encoder resolution, causing the velocity measured each servo cycle to jump around a lot. For instance, at 15 IPS, you are getting 15 encoder counts/sample (assuming a 1 KHz servo rate). But, the real samples are likely to jump between 14, 15 and 16 each sample. That is perceived by the PID algorithm as a 6.7% jump in velocity. This is then amplified by the D term and added to the PWM output. The higher the D, the more you amplify these apparent velocity fluctuations, even though the real velocity is not actually fluctuating. This is another of the reasons low encoder resolution causes problems. Jon ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
