Great explanation Peter! Happy New Year. John Dammeyer > -----Original Message----- > From: Peter Homann [mailto:[email protected]] > Sent: January-03-16 5:36 PM > To: Enhanced Machine Controller (EMC) > Subject: Re: [Emc-users] Lost fractions of a step > > > It depends on the drive that you have. > > Geckodrives morph from 10 microstepping to full stepping by the time the > motor > reaches a few revs per second, so no torque is lost. You get the best of both > worlds, very smooth slow speed movement and the torque of full stepping. > > Also, a lot of people do not understand the purpose of microstepping. It is > not there to increase the resolution, but to provide smooth movement at > low > speeds. > > On most machines, if you issue a single microstep, the axis is unlikely to > move at all as there is insufficient force to overcome the friction in the > drive system and the stepper detent force. > > When determining the resolution or accuracy of the machine, you should > use the > distance moved by a full step, and not the microstep distance. In a pinch you > could use the distance travelled by a half step as the accuracy value as it is > quite good. Any higher microstep than the half step is not accurate. > Sure you will have higher resolution, but that is not accuracy, and it is > accuracy that you are after. > > > Cheers, > > Peter > > > > On 03-Jan-16 5:51 PM, Valerio Bellizzomi wrote: > > Good year to all, > > I have a board that can do microstepping but I would not use > > microstepping because the manual says that while it rises the precision > > it also reduces the torque. > > > > > > On Sun, 2016-01-03 at 01:02 -0500, Cecil Thomas wrote: > >> My question is about what happens to the "leftovers" when the > >> precision of the g code commanded position cannot be met by the > >> hardware executing it. > >> Several years ago I wrote a program to "generate" involute gear teeth > >> by making multiple cuts of the same tooth from differing angles with > >> a rack shaped cutter. This eliminates the need for the different > >> cutters when making only one cut per tooth. I have used it many > >> times to cut relatively large gears with a relatively small number of > >> teeth with virtually no noticeable error. > >> > >> A few days ago a friend who repairs watches wanted to know if I could > >> figure out what gear (wheel to you watch guys) size, pitch or module > >> and number of teeth would be required to replace a missing one. (the > >> original was long gone). I had no problem working from the center > >> distance and the matching pinion coming up with the appropriate design. > >> > >> However, when I cut the gear I had the right number of teeth but the > >> last tooth was much too wide. > >> > >> It would appear that I had lost a bunch of steps on the rotary > >> axis. Further investigation reveals what I think is the root cause > >> but I would like someone with more knowledge than me to confirm or > >> disprove my analysis. > >> > >> The gear had 86 teeth (in the power train, not in the timing train) > >> and I made 9 cuts per tooth. That is 774 commands and about all but > >> 86 of them in the same direction. > >> > >> My rotary axis is a 200 step stepper into a 30 to 1 worm drive > >> microstepped by 10 so 1.8 degrees divided by 300 equals .006 degrees > >> per microstep or 166.6667 steps per degree. > >> > >> Unfortunately when the g code calls for a 1 degree move the motion > >> planner can only issue 166 steps since it can't issue .6667 > >> steps. That means that the actual movement of the A axis is only > >> 166/166.66667 or .996 degree. That is .004 degree lost as far as I > >> can tell. That might be close enough for one or even several > >> commands but after 688 comands in the same direction that constitutes > >> 688 x .004 or 2.7 degrees lost. > >> > >> That is a significant portion of a tooth on a high tooth number > >> wheel. Depending on the actual value of the command the actual lost > >> motion could be anything from nothing to essentially a whole step or > >> .0059999 degrees. > >> > >> I think that I can lessen the impact of the lost portion of the steps > >> by using the MOD operator to determine how much is left over after > >> dividing the commanded move by .006. Then use IF ELSE, IF the > >> remainder is Greater Than .5 steps then ADD a full step (command = > >> command PLUS .006 degrees) ELSE issue the commanded number (do > nothing). > >> > >> This should statistically reduce the error by rounding up or down and > >> redistribute it randomly among all the cuts although it will not > >> eliminate it. The greater the number of cuts the better the > >> approximation will be. > >> > >> Sorry for the long post but I couldn't condense it much and get the > >> idea across. Can anyone confirm or disprove my observation or come > >> up with a better solution? Obviously I could add another reduction > >> stage to my rotary axis but I would like to avoid that if possible. > >> > >> Cecil > >> > >> > >> ---------------------------------------------------------------------------- -- > >> _______________________________________________ > >> Emc-users mailing list > >> [email protected] > >> https://lists.sourceforge.net/lists/listinfo/emc-users > > > > > > > > > > ---------------------------------------------------------------------------- -- > > _______________________________________________ > > Emc-users mailing list > > [email protected] > > https://lists.sourceforge.net/lists/listinfo/emc-users > > > > -- > ---------------------------------------------------------------------------- - > eStore: http://www.homanndesigns.com/store > Web : http://www.homanndesigns.com > email : [email protected] > Phone : +61 421 601 665 > > --- > This email has been checked for viruses by Avast antivirus software. > https://www.avast.com/antivirus > > > ---------------------------------------------------------------------------- -- > _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users
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