John, You are fundamentally incorrect when you state " the torque of the motor to drop off the faster it goes" .. although the back EMF is correct, with a modern current limited drive, the torque is flat until the corner frequency, then drops off ... up to the corner frequency the torque is constant with a good current-limited drive, above the corner frequency the torque drops off, power is constant. You are perhaps confusing the raw torque/speed curve of a motor fed from a constant voltage source, which is useful but is not how they are typically used in practice.
https://res.cloudinary.com/engineering-com/image/upload/w_640,h_640,c_limit,q_auto,f_auto/image002_bezhrr.jpg On Sat, 5 Feb 2022 at 09:00, John Dammeyer <jo...@autoartisans.com> wrote: > Hi Chris, > My issue is that a comment about corner frequency with stepper motors > _might_ well be valid as long as the maximum current for each step is > reached before or at the end of the step. But the motor is turning pretty > slowly there compared to how they are used in real life. > > However the comment about corner frequency with respect to steppers > perhaps is only backed by alternative facts? > > I must admit I've not investigated in detail the closed loop steppers. > The price of an industrial version I worked with was more than the price of > an AC servo and at higher speeds I could stop the pulley with my fingers. > Yes. It faulted. But that isn't really the point. The DC and AC servos at > higher speeds just work better. > > Stepper motors work great at low speeds usually directly coupled. > Contrary to popular belief the micro-stepping doesn't improve resolution > but gets rid of resonance and gives the appearance of better resolution. > But it doesn't change the fact that the current still has to reverse every > full step. I believe that in fact Gecko drives improve high speed torque > by switching back to full step mode above the resonance velocity. > > Now instead of 0.707 x max current in both windings (at the most) we're > back to 100% in both with an increase in torque. Absolutely nothing to do > with corner frequencies whatever they might be or how they are determined. > > John > > > > > > > From: Chris Albertson [mailto:albertson.ch...@gmail.com] > > > > John, > > > > You described it correctly. But I think what Robin meant by "Corner > > Frequency" might be the peak of the power vs. RPM graph. Basically, the > > frequency where power output starts to fall with RPM. > > > > But now it can get worse, or really better but more complex. We have > > these so-called "closed loop stepper drivers and also a few people are > > running the steppers as if they were many-pole BLDC analog (continuous, > > non-stepping) mortors > > > > On Fri, Feb 4, 2022 at 2:39 PM John Dammeyer <jo...@autoartisans.com> > wrote: > > > > > I disagree. The physics of the motor, which include inductance along > with > > > the generated back emf from the motor spinning in the magnetic field, > is > > > what cause the torque of the motor to drop off the faster it goes. > > > > > > The problem is to spin a stepper motor you have to not just change to a > > > new winding like a DC motor does but completely reverse the direction > of > > > the current through the winding. In order to do that you have to deal > with > > > the collapsing magnetic field and counter the resulting generated > voltage > > > which is based on the inductance of the windings. > > > > > > That's why the winding voltage of a stepper motor might be only 2V to > get > > > the rated 3A but you need 48V to make it turn quickly. And because > of the > > > inductance and collapsing field, time is required to change the > direction > > > of the current through the winding. If that time is longer than the > next > > > direction change then you never reach max current through the windings > and > > > you don't develop full torque. That's why a stepper motor with a 24V > power > > > supply has the same holding torque as one with a 48V power supply. The > > > current limiting of the drive holds the winding current at 3A. But > run it > > > at 24V or at 48V you get a totally different torque curve. > > > > > > If you are going to mention something called the corner frequency of a > > > stepper motor+drive please show us the graphs and specifications. I > > > haven't been able to find that rating on any stepper motor. > > > > > > Perhaps you can point it for this one? > > > http://www.automationtechnologiesinc.com/download/9259/ > > > > > > And explain how you determined that corner frequency? > > > John > > > > > > > > > > > > > > > > > > > > > > > > > -----Original Message----- > > > > From: Robin Szemeti via Emc-users [mailto: > > > emc-users@lists.sourceforge.net] > > > > Sent: February-04-22 2:01 PM > > > > To: Enhanced Machine Controller (EMC) > > > > Cc: Robin Szemeti > > > > Subject: Re: [Emc-users] What Would You Suggest? > > > > > > > > What people continually get totally wrong with steppers is failing to > > > > understand that the maximum power is delivered at the corner > frequency, > > > and > > > > power output is constant above that. > > > > > > > > If you have an application that needs to move at say 2m a minute and > your > > > > stepper stalls, there seems to be some crazy logic that says to > people > > > "Oh, > > > > the stepper stalled because it was going too fast, I need to change > the > > > > gearing so the motor spins more slowly" .. which is of course ass > > > backwards. > > > > > > > > The stepper stalled because the power output of the motor was less > than > > > the > > > > power requirement of the machine ... to increase the power output of > the > > > > motor, you need to spin it faster, not slower. Steppers motors are > > > capable > > > > of excellent performance but they do need to be used correctly ... > sadly, > > > > in most amateur applications they are not. > > > > > > > > If the corner frequency with your drive and voltage is at around 2000 > > > steps > > > > per second and you are only ever delivering 1000 steps per second, > you > > > can > > > > never got more than half the mechanical power out that the motor is > > > capable > > > > of. > > > > > > > > On Fri, 4 Feb 2022 at 17:13, John Dammeyer <jo...@autoartisans.com> > > > wrote: > > > > > > > > > > > > > > > > > > > > From: Kenneth Lerman [mailto:ler...@se-ltd.com] > > > > > > The longitudinal travel is just over a foot, and it takes about > 3-1/2 > > > > > turns > > > > > > of the crank to go that distance. I'm thinking around a second > per > > > turn > > > > > > would be about the maximum. So, that's 60 RPM. I'm thinking of a > 1:6 > > > > > ratio > > > > > > on the timing belt pulleys, so that's 360 RPM at the stepper > which is > > > > > > pretty slow. A full stepping rate would be 200 * 360/60 => 200 * > 6 > > > which > > > > > is > > > > > > only 1200 steps per second. > > > > > > > > > > You won't want to run full step. A minimum should be 8 > > > micro-steps/step > > > > > to avoid resonance and loss of position or lockup. I'd measure > the > > > torque > > > > > required to move the table by attaching a lever to the hand wheel > that > > > is > > > > > say 1' long. Set it horizontal and start hanging weight onto the > end > > > to > > > > > get ft-lbs or ft-in until it turns. That's the torque required to > > > overcome > > > > > static friction. Double that to choose your motor. > > > > > > > > > > Say that is 1 ft-lb or 192 oz-in. If you choose 3:1 for your > > > reduction > > > > > ratio you get 600 oz-in. Look at the motor torque curve (they are > all > > > > > different and if the supplier can't give you that buy one somewhere > > > else) > > > > > and see where the torque drops below 400 oz-in. Say that's 180 > RPM. > > > > > That's 3 RPS which multiplied by 2000 steps per rev for > micro-stepping > > > is > > > > > 6000 steps/second which achieves your 1 RPS on the handle. > > > > > > > > > > Or if you find it's 2 ft-lb or 400 oz-in choose a much larger motor > > > like > > > > > 1200 oz-in > > > > > http://www.automationtechnologiesinc.com/download/9259/ > > > > > Notice the curve at 3000 half steps per second is about 3.2NM. > That's > > > > > 12,000 steps per second (7.5RPS) with 8 micro-steps per step well > > > within > > > > > the reach of even a parallel port controller and 450 oz-in. That's > > > well > > > > > above the 1 RPS you need and even just 3:1 still gives you 1600 > oz-in. > > > > > > > > > > My two cents... > > > > > John Dammeyer > > > > > > > > > > > > An alternative would be to provide more gearing, but I don't > think > > > it's > > > > > > practical to get more than about a six to one ratio in a single > belt > > > > > > reduction and I'd like to avoid mechanical complexity if I can. > > > > > > > > > > > > Thoughts? > > > > > > > > > > > > Ken > > > > > > > > > > > > Kenneth Lerman > > > > > > 55 Main Street > > > > > > Newtown, CT 06470 > > > > > > > > > > > > > > > > > > > > > > > > On Fri, Feb 4, 2022 at 7:13 AM Chris Albertson < > > > > > albertson.ch...@gmail.com> > > > > > > wrote: > > > > > > > > > > > > > If looking for lowest cost solution you can us the old "Atom" > > > computer > > > > > to > > > > > > > control the grinder as long as you do not need to run the > mill and > > > > > > > grider at the same time. Get an Eiternet interface Mesa card > for > > > the > > > > > new > > > > > > > machine, You need two config files, just load the one for the > > > mill or > > > > > the > > > > > > > one for the grinder. > > > > > > > > > > > > > > Then someday you buy a second computer you only have to move > the > > > > > Ethernet > > > > > > > cable over. The best option is a newer version of the Atom. > They > > > > > seem to > > > > > > > sell for just under $200. Finally Newegg.com always has many > > > used oe > > > > > > > refurb PCs Used PCs sourced locally can be a cheap as "free" > > > > > > > > > > > > > > But 9ld PCs tend to burn up a lot of power. I am trying to get > > > mone > > > > > to do > > > > > > > "wake on LAN" so it can not use power until I need to log onto > it > > > > > > > > > > > > > > On Thu, Feb 3, 2022 at 6:52 PM Kenneth Lerman < > ler...@se-ltd.com> > > > > > wrote: > > > > > > > > > > > > > > > I'm considering converting a surface grinder to CNC. To > start, > > > I'll > > > > > > > > probably just convert the longitudinal and transverse axes. > > > > > > > > > > > > > > > > I'll go with steppers for this -- I'm thinking NEMA-42 > motors. > > > > > > > > > > > > > > > > My current Bridgeport clone uses servos and Jon Elson's > hardware > > > on a > > > > > > > > little Intel Atom Box. I'm thinking of using a Rpi for this. > It > > > will > > > > > > > need a > > > > > > > > minimal display/control panel when completed, but initially > will > > > > > need a > > > > > > > > display with touchscreen or mouse and possibly a keyboard. > In the > > > > > long > > > > > > > run, > > > > > > > > some buttons. and perhaps an mpg might be useful. > > > > > > > > > > > > > > > > I'd like to use a raw Rpi without adding special hardware > > > directly. > > > > > That > > > > > > > > probably means using a USB or ethernet interface to control > the > > > > > steppers. > > > > > > > > I'm thinking of using Mesa hardware. > > > > > > > > > > > > > > > > Can someone suggest the most cost effective way to do this? > > > > > (Although I > > > > > > > > have to admit, that after buying the timing belts and > pulleys, > > > the > > > > > > > > steppers, power supply, stepper drivers, ..., it's too late > to be > > > > > really > > > > > > > > cost effective.). And the surface grinder only cost me $300. > > > > > > > > > > > > > > > > Thanks, > > > > > > > > Ken > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Kenneth Lerman > > > > > > > > 55 Main Street > > > > > > > > Newtown, CT 06470 > > > > > > > > > > > > > > > > _______________________________________________ > > > > > > > > Emc-users mailing list > > > > > > > > Emc-users@lists.sourceforge.net > > > > > > > > https://lists.sourceforge.net/lists/listinfo/emc-users > > > > > > > > > > > > > > > > > > > > > > > > > > > > > -- > > > > > > > > > > > > > > Chris Albertson > > > > > > > Redondo Beach, California > > > > > > > > > > > > > > _______________________________________________ > > > > > > > Emc-users mailing list > > > > > > > Emc-users@lists.sourceforge.net > > > > > > > https://lists.sourceforge.net/lists/listinfo/emc-users > > > > > > > > > > > > > > > > > > > _______________________________________________ > > > > > > Emc-users mailing list > > > > > > Emc-users@lists.sourceforge.net > > > > > > https://lists.sourceforge.net/lists/listinfo/emc-users > > > > > > > > > > > > > > > > > > > > _______________________________________________ > > > > > Emc-users mailing list > > > > > Emc-users@lists.sourceforge.net > > > > > https://lists.sourceforge.net/lists/listinfo/emc-users > > > > > > > > > > > > > _______________________________________________ > > > > Emc-users mailing list > > > > Emc-users@lists.sourceforge.net > > > > https://lists.sourceforge.net/lists/listinfo/emc-users > > > > > > > > > > > > _______________________________________________ > > > Emc-users mailing list > > > Emc-users@lists.sourceforge.net > > > https://lists.sourceforge.net/lists/listinfo/emc-users > > > > > > > > > -- > > > > Chris Albertson > > Redondo Beach, California > > > > _______________________________________________ > > Emc-users mailing list > > Emc-users@lists.sourceforge.net > > https://lists.sourceforge.net/lists/listinfo/emc-users > > > > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users > _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
