All fine and good but doesn't help anyone choose a stepper motor. For example: https://www.geckodrive.com/support/step-motor-basics.html tries to explain corner speed but fakes the curves by showing that the torque of a stepper motor is constant up to a certain speed. I've yet to see any curves of real motors that look like that.
Perhaps you can find some and post those that do show flat torque up to a 'corner speed'? In either case that doesn't really help anyone choose a motor so that Gecko article and corner speed are effectively techo-babble framed in a way to help them sell their drivers which are limited to 80VDC. Similarly the stepper motor suppliers provide 1/2 step curves leaving out the resonance point so unless you stay below this 'corner frequency' and never reach that point the torque curves are somewhat obscure. Perhaps explain how _you_ choose a stepper motor for a given axis? What process do you go through to do this? That might help more. John > From: Robin Szemeti via Emc-users [mailto:[email protected]] > > "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 ste" > > Yes, that is exactly what the corne frequency is ... the step frequency at > which the current no longer reaches the desired value before the end of the > step. It's obviously dependent on inductance and maximum available drive > voltage. > > On Sat, 5 Feb 2022 at 11:50, Robin Szemeti <[email protected]> wrote: > > > 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 <[email protected]> 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:[email protected]] > >> > > >> > 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 <[email protected]> > >> 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: > >> > > [email protected]] > >> > > > 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 <[email protected]> > >> > > wrote: > >> > > > > >> > > > > > >> > > > > > >> > > > > > From: Kenneth Lerman [mailto:[email protected]] > >> > > > > > 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 < > >> > > > > [email protected]> > >> > > > > > 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 < > >> [email protected]> > >> > > > > 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 > >> > > > > > > > [email protected] > >> > > > > > > > https://lists.sourceforge.net/lists/listinfo/emc-users > >> > > > > > > > > >> > > > > > > > >> > > > > > > > >> > > > > > > -- > >> > > > > > > > >> > > > > > > Chris Albertson > >> > > > > > > Redondo Beach, California > >> > > > > > > > >> > > > > > > _______________________________________________ > >> > > > > > > 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 > >> > > > > > >> > > > > > >> > > > > > >> > > > > _______________________________________________ > >> > > > > 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 > >> > > > >> > > > >> > > > >> > > _______________________________________________ > >> > > Emc-users mailing list > >> > > [email protected] > >> > > https://lists.sourceforge.net/lists/listinfo/emc-users > >> > > > >> > > >> > > >> > -- > >> > > >> > Chris Albertson > >> > Redondo Beach, California > >> > > >> > _______________________________________________ > >> > 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 > >> > > > > _______________________________________________ > 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
