On Tuesday 26 May 2020 16:29:17 Chris Albertson wrote: > Gene, > > I'm also looking at using a closed-loop stepper for a z-axis drive. > On my system the z-motor's torque holds the full weight of the head. > With a standard open loop motor full current is used and the motor > gets hot because there is more current than needed. The closed loop > system puts only as much current into the motor as is required to hold > position. > > My calculation says that the force of gravity pulling the head down > will generate roughly just under 1.2 Nm of static torque on the ball > screw. That is the price one pays for using ball screws vs acme lead > screws. The ball screw is back drivable with very little friction. > But it has zero measurable backlash. (The cheap $50 Chinees screw is > perfect as far as my dial indicator can tell.) > If its loaded by head weight, and the gibs are well adjusted and wet, the weight preload cancels the backlash. Too much there will make sloppy rigid threads.
> The build-in z-motor encoder has 1000 lines per revolution, so I > assume if I use 1/4 micro-steps that the motor will always have at > most 1/1000 of a revolution position error. The z-axis lead screw is > 5mm pitch so hopefully, I get 0.005 mm resolution (Yes, this is better > than needed) Absolutely, theres more spring in the frame iron than that. > > Using a closed-loop motor should make a big improvement. Stepper > online has a 12Nm NEMA 34 close-loop 1000 line per rev kit with motor, > controller and cables for $120. > > *Back on-topic.* With the price of glass scales having dropped to > way-cheap. Is there any reason NOT to place a closed-loop control > around even open-loop steppers? Look at this: > amazon.com/TOAUTO-Digital-Readout-Milling-Machine... > <https://www.amazon.com/TOAUTO-Digital-Readout-Milling-Machine/dp/B086 >X9171J/ref=sr_1_9?dchild=1&keywords=glass%2Bdro%2Bscale&qid=1590523919& >sr=8-9&th=1> > > This is a serious question. I wonder if doing so would remove the > problem of backlash and flex? These are 5 uM scales for $65 Even if > they are 4X worse than spec'd they would offer a big improvement. > I'd say, but how to install them so they are protected, the 64 million sheckel question indeed. > On Tue, May 26, 2020 at 11:45 AM Gene Heskett <ghesk...@shentel.net> wrote: > > On Monday 25 May 2020 20:43:45 Gene Heskett wrote: > > > On Monday 25 May 2020 18:33:29 Jon Elson wrote: > > > > On 05/25/2020 02:50 PM, N wrote: > > > > > Do anyone here have any suggestion for FERROR and MIN_FERROR > > > > > values? > > > > > > > > It depends on your "user units", as well as the general > > > > accuracy and speed of the machine. > > > > FERROR is a multiplier to velocity in user units/second that > > > > is added to MIN_FERROR. > > > > MIN_FERROR is the allowable following error with the machine > > > > not moving. > > > > > > > > On a servo system, you should have Halscope graph the > > > > following error and find out what > > > > your actual errors are, and then set MIN_FERROR > > > > accordingly. Then, set FERROR to > > > > accommodate larger error at rapid traverse speed. > > > > > > > > There's no one good value for everybody. > > > > > > > > Jon > > > > > > Perhaps this might be a good time to discuss/develop a recommended > > > good practice procedure to be applied to a machine w/o PID's as > > > there's none in the pi running the sheldon, and none in the > > > D5215MW running the 6040. > > > > > > Can you recommend a procedure that Just Works, where the machine > > > is expected to follow what the TP actually outputs, with a > > > tolerance setting profile that its capable of staying inside of? > > > > > > Without PID's, where do we get a following error that we can see > > > on the halscope? Or do I need to put PID's in before this makes > > > sense? Or do I make a sub2 out of a sum2 just to develop the error > > > signal? Eventually that would use up the cpu cycles saved by not > > > using the PID's. And it all, to me, adds up to more give between > > > the TP and what the machine does, allowing more error than w/o the > > > PID. > > > > > > Seems to me there should be some "rules of thumb" to apply here. > > > > I just tried an experiment. Since there are no pid's in that > > sheldon setup, I commented out the *ERROR entries in the .ini file. > > Interesting as it made zero difference in how it ran. So, it appears > > that I'll need to synth an error signal, and the best way I can > > think of it to subtract the command to a stepgen from the feedback, > > or vice-versa, but w/o an encoder actually measuring that axis, its > > not going to tell me anything but the time lags caused by the addf > > order. Food to feed a "what if" at some point... Perhaps if I cab > > break into the encoder? > > > > Acc to fedex I should get those 3 phase closed loop motors late > > today. AIUI, they do have an error output, but its yay or nay I > > think, but the 3NM is way too long to fit behind the apron and drive > > the X. I do intend to put one of the 3NM's on the Z. Lots of room > > there. That 1600 oz/in there now is so much overkill its not even > > funny. It rattles the whole machine when it moves. That should at > > least give me an error stop if the motor doesn't follow orders. > > > > Stay well everybody. > > > > 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 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
