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
