On 08/07/10 13:01, dave wrote: > On Wed, 2010-07-07 at 20:43 -0500, Chris Radek wrote: > >> On Thu, Jul 08, 2010 at 10:32:50AM +1000, Jake Anderson wrote: >> >>> We have converted our mill to CNC and EMC2 and all is fairly well. >>> http://wiki.linuxcnc.org/cgi-bin/emcinfo.pl?JakeAndRussells >>> >> ... >> >> >>> Unfortunatly when I change that cruise speed (g1 F200 vs G1 F100 say) >>> the ferror goes up, and I start either leading or lagging the set point. >>> I can tune to hover around 0 error at any given speed, but as the speed >>> changes I start getting error again. >>> >> >>> Any tips? >>> >> Yes but beware they're all half-baked. >> >> I see you're using bare H bridges which means you don't have a >> velocity loop. I think this problem with FF1 is because of that, and >> it is a fairly fundamental problem with a torque-mode setup. >> >> In hand-wavey terms, a full velocity mode servo amp has a commanded >> velocity (output of emc's pid in our case) and actual velocity (from a >> tachometer) and it uses the difference between the two to determine >> the torque/current to apply to the motor. >> >> It seems like you can fake this up by using the mesa encoder's >> velocity output as your pretend velocity feedback. Use sum2 (one gain >> negative) to calculate the difference between your pid output and your >> velocity feedback. Pick your scales carefully so everything matches >> up. Use this resulting difference to drive your pwmgen. >> >> I haven't tried this and the whole idea came out of a late-night chat >> session with Kim K. Unfortunately I can't find the archive of it. >> >> I'd be thrilled to hear whether this gives you a stable loop and >> whether it tunes up better. Please do keep notes and report back if >> you try it. >> >> One step more complicated is to have dual pid loops, a torque loop >> inside a velocity. I'm not sure what you'd use for torque/current >> feedback, though. Normally that's current sensing in hardware, and >> you have no equivalent that I can see. >> >> Simpler is to leave your pid as-is and try using I instead of FF1. If >> you can increase your I by a lot and still keep the loop stable, >> you'll get correct following at any (steady) speed. The tradeoff is >> sloppy settling at speed changes, like at the beginning and ending of >> moves. >> >> Chris >> > Hi all, > > I is not that I know anything about tuning torque mode. Just did one a > few weeks ago. I started out like usual as for a tach/velocity mode > tune. Not the best idea. > > Take a look at the parameters used on the Mazak conversion. > ~/emc2/emc2-trunk/configs/demo_mazak/demo_mazak.ini or something close > to that. > > Those values look pretty radical but it gives you an idea what is needed > for tuning torque mode. Maybe try 1/20th or less of those values and see > what happens. Pretty much you get it going with a reasonable P and then > use I and D get stability and then increase everything in steps to get > decent stiffness and response. > That's not the problem though, it works ok except for the fixed offset that varies with speed. > At least on my machine FF(n) didn't do much good and believe me I did > try them. > > Linear scales on machines that are not really low in backlash can be a > challenge. I got better results with encoders on the ballscrew. > There is very little lash in the setup, we can't measure it because its less than our dialguages can measure. We are using .001mm scales. on the Z drive there is more and we notice that at low movement rates. > The mill was a Cinci contourmaster with servos that had, and still does, > about 0.003" backlash. > > In terms of tuning, linear glass scales, encoders on the servo motors > then encoders on the ballscrews; encoders on the ballscrews being by far > the easiest to tune. SEM MT30H4s with tachs. for X and Y and a torque > mode amp for the Z. > > Hope this helps. > > Dave > > >> ------------------------------------------------------------------------------ >> This SF.net email is sponsored by Sprint >> What will you do first with EVO, the first 4G phone? >> Visit sprint.com/first -- http://p.sf.net/sfu/sprint-com-first >> _______________________________________________ >> Emc-users mailing list >> Emc-users@lists.sourceforge.net >> https://lists.sourceforge.net/lists/listinfo/emc-users >> > > ------------------------------------------------------------------------------ > This SF.net email is sponsored by Sprint > What will you do first with EVO, the first 4G phone? > Visit sprint.com/first -- http://p.sf.net/sfu/sprint-com-first > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users >
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