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 ------------------------------------------------------------------------------ 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
