On Wednesday 09 December 2020 13:58:14 Todd Zuercher wrote: > -----Original Message----- > From: dave engvall <dengv...@charter.net> > Sent: Wednesday, December 09, 2020 1:10 PM > To: emc-users@lists.sourceforge.net > Subject: Re: [Emc-users] Tuning a servo > > On 12/9/20 8:21 AM, Jon Elson wrote: > > On 12/08/2020 10:53 PM, Gene Heskett wrote: > >> What does that 22mv mean in folllowing error since I have that set > >> for about half an inch at the moment? > > > > It is not milli VOLTS, it is milli-units. Generally inches or mm, > > depending on your preference. > > so 22m means 0.022" assuming that's what you use. Or, if this is a > > rotary, then it could be > > 0.022 degrees. > > > > My method of tuning is to first increase MIN_FERROR to prevent > > following error trips, then reduce P to a small value, just enough > > that error doesn't creep up. Adjust > > FF1 until error is small at moderate speed, less than 50 encoder > > counts, perhaps. Then, assuming a real axis with encoder feedback, > > add a LITTLE FF2 to reduce the error spikes on accel/decel. (Don't > > bother on stepper-simulated axes.) Then, increase P to drive the > > error down to a small fraction, add a little D to improve stability. > > It takes practice and experience to know what to adjust based on the > > Halscope trace. > > > > Jon > > Tuning seems to be a highly individualized endeavor. Hopefully a > damped (in following error) search. :-) I start with a classic ZN ... > increase P until you get un-dampened oscillation. Make starting P .6 > of that value. Set FF1 equal to one and then adjust as necessary to > minimize following error. Adjust FF2 to minimize spikes on accel and > decel. Adjust FF0 if needed for zero crossing. Try small amounts of D > and I although I usually leave them a zero. Following error is < 1e-3 > and can get down as good as 5e-4. The values in PID, etc do seem to > interact so going thru the loop a few times can't hurt. As always > YMMV. The major error on my machine is backlash. Adding a glass scale > to X and Y might help. (future project). > > I suspect that having tach feedback makes tuning much easier. I've > never had much success with torque mode. > > If I had the skill in linuxcnc internals I'd would try a well > constrained Monte Carlo. No chance of my doing that. ;-( > > > That’s all well and good if the command input of the amp is velocity, > but I don't think that is the case for this one. In torque/current > mode the FF1 and FF2 have very different affects than with a velocity > drive. (and I never really completely figured it out, before I gave > up and set up two nested PID loops.) For a single loop you'll need as > much P and D as you can make stable, and lots of I. I've also found > that setting the max error I limit pin to something that prevents > windup can allow for very quick response from the I term without the > negative side effects. (I've had good luck with setting the pid > maxerrorI = 10/I). I've also noticed some strangeness, where adding I > actually seemed to made the steady state following error worse up > until it was large, then the error started getting better until it > finally came back in line and more I started to help a lot. I did > this searching for the point where more I caused instability, which > I'm not sure if I ever did find. (This was done before I tried > limiting the max error for I.) > > One of the main reasons I gave up on the single loop, was that the > tune seemed to be very changeable and touchy, every few weeks I was > tweaking on it again trying to get rid of following errors, > instability, or overheating the drives. I've not had to mess with it > since switching to the double loop.
When I get it all assembled, which is slowly getting closer, I may do this, and pick your brain via PM for help. But as I just posted, I have major problems understanding the SCALE setting because its obviously different for steppers than for servo's, and until that is sorted, and its assembled, I think I'm barking at an empty tree. Thanks Todd. Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) If we desire respect for the law, we must first make the law respectable. - Louis D. Brandeis Genes Web page <http://geneslinuxbox.net:6309/gene> _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
