On Tue, 2013-08-06 at 09:13 -0400, Stephen Dubovsky wrote: > Something to remember w/ steppers is that they produce no torque at rest. > The maximum pullout torque is at 1/4 full step of error. W/ a typ 200 step > motor thats 1/800th of a revolution. W/ a 0.2"(5tpi) leadscrew like on a > bridgeport that is 0.2/800= +-.00025" error depending on moving/cutting > force direction. Bigger motors w/ torque reserve help but the torque curve > vs displacement is sinusoidal so there is always a 'null' around the > commanded location even if you oversize the motors by a factor of 100. > That error can be more than acceptable in many applications (esp for ones > geared down), but for high precision some other form of feedback may be > necessary. > > Stephen Too early in the morning and still on first cup of coffee but: if you are within the 1/4 step (null) then increased feedback of some kind won't fix it. Gearing eg. gears or timing belt with zero (?) backlash or slack will certainly help. BTW - there was an implication in an earlier email that implied that a tach would help positioning; well only in better control of velocity which makes tuning easier. The tach is a velocity loop inside the position loop. It REALLY makes it easier to tune although I've seen good tuning without it.
Also alluding to an earlier email cradek tried a synthetic tach off the encoder counts and it worked rather well at speed. Of course when you get down to creeping is when the analog tach really shines. Now off to work on the coffee. ;-) Dave > > On Tue, Aug 6, 2013 at 4:54 AM, Viesturs Lācis > <viesturs.la...@gmail.com>wrote: > > > 2013/8/6 Sven Wesley <svenne.d...@gmail.com> > > > > > 2013/8/6 Sven Wesley <svenne.d...@gmail.com> > > > > > > > > > > > > > > > > > > > 2013/8/6 Viesturs Lācis <viesturs.la...@gmail.com> > > > > > > > >> 2013/8/6 Sven Wesley <svenne.d...@gmail.com> > > > >> > > > >> > > > > >> > The palette I'm going to use as a table is pretty heavy, and put a > > > mould > > > >> > half on top of that. That's at least 100 kg moving parts, maybe > > more. > > > >> And I > > > >> > want at least 5 m/min transport. > > > >> > > > > >> > > > >> Max acceleration is what matters most for the motor. In my router I > > have > > > >> 7500 mm/min maxvel and 300 mm/s^2 maxaccel (it reaches max velocity in > > > 0,4 > > > >> seconds, which is fine for me). > > > >> It requires 30N force to move 100 kg at 0,3 m/s^2. With a 5 mm pitch > > > >> ballscrews it would require 0.023 Nm of torque applied to the screw. > > > >> This calculation assumes no friction and other losses, so add 30-40% > > for > > > >> safety and then another 50% to make sure that stepper motor is not > > > running > > > >> out of torque. It still is less than 0.1 Nm to move the machine itself > > > >> around. > > > >> IMHO milling forces have to be calculated to take them into account as > > > >> well > > > >> as that might much bigger load than these 100 kg. > > > >> > > > >> I definitely do like those largest Nema23 motors - SY60STH86-3008B: > > > >> http://www.zappautomation.co.uk/en/attachment.php?id_attachment=140 > > > >> > > > >> At 60V/4.3 A and halfstepping they have something like 1.3 Nm at 1500 > > > RPM. > > > >> And 1500 RPM with 5 mm ballscrew pitch will give 7500 mm/min velocity. > > > >> > > > >> -- > > > >> Viesturs > > > >> > > > >> > > > > Good writing Viesturs. My small Nidec servo's can keep up with the > > > > acceleration I have now and it makes the machine shake at short runs. I > > > > don't remember the acceleration config but it's just a fraction to max > > > > speed and the portal is around 80-90 kg. > > > > > > > > > > When checking Zapp's webshop they have hybrid closed loop steppers, it's > > > seems to be quite easy to beef up with strong steppers at low cost... > > > > > > > Thanks! I build these machines for living so I should be doing some basic > > calculations :)) > > > > I just checked and discovered that they have also 2 phase steppers with 4 > > wires for the closed loop (I somehow thought that they are using _only_ > > 3-phase stepper motors). > > Here is one of their 2-phase 4-wire closedloop motors: > > > > http://www.zappautomation.co.uk/en/easy-servo-stepper-motors-closed-loop-stepper-system/882-es-m22430-3nm-holding-torque.html > > Based on datasheet it seems that it is Nema23, so I was wondering, if > > "regular" Nema23 stepper motor could be used instead, because there is > > significant price difference: > > > > http://www.zappautomation.co.uk/en/nema-23-stepper-motors/380-sy60sth88-3008bf-nema-24-stepper-motor.html > > > > -- > > Viesturs > > > > If you can't fix it, you don't own it. > > http://www.ifixit.com/Manifesto > > > > ------------------------------------------------------------------------------ > > Get your SQL database under version control now! > > Version control is standard for application code, but databases havent > > caught up. So what steps can you take to put your SQL databases under > > version control? Why should you start doing it? Read more to find out. > > http://pubads.g.doubleclick.net/gampad/clk?id=48897031&iu=/4140/ostg.clktrk > > _______________________________________________ > > Emc-users mailing list > > Emc-users@lists.sourceforge.net > > https://lists.sourceforge.net/lists/listinfo/emc-users > > > ------------------------------------------------------------------------------ > Get your SQL database under version control now! > Version control is standard for application code, but databases havent > caught up. So what steps can you take to put your SQL databases under > version control? Why should you start doing it? Read more to find out. > http://pubads.g.doubleclick.net/gampad/clk?id=48897031&iu=/4140/ostg.clktrk > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users ------------------------------------------------------------------------------ Get your SQL database under version control now! Version control is standard for application code, but databases havent caught up. So what steps can you take to put your SQL databases under version control? Why should you start doing it? Read more to find out. http://pubads.g.doubleclick.net/gampad/clk?id=48897031&iu=/4140/ostg.clktrk _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
