Sigh ... " Perhaps you can find some and post those that do show flat torque up to a 'corner speed'?" ... all ... every single one, when connected to a current limited driver.
On Sun, 6 Feb 2022 at 00:45, John Dammeyer <[email protected]> wrote: > No. The motors are designed to handle N amperes although they get quite > warm that should be a 24/7 rating. Because they get so warm many drivers > back the current off when the motors have been idle for a period of time. > > Let's take a step backwards with a long explanation. When I first sized > the motor for the knee mill I tested how much torque was required to > overcome 'static' friction by attaching a bar and adding weight until it > started turning. The 'kinetic' friction is always less so as a rule of > thumb using twice that experimentally determined torque value was a good > starting point. > > But you can also use math if you know the weight of the table you are > lifting. I didn't but if I guess at 325 lbs with a 1" lead 4 TPI ACME > leadscrew with Bronze nut > https://daycounter.com/Calculators/Lead-Screw-Force-Torque-Calculator.phtml > I get pretty close to the value I determined experimentally which is 600 > oz-in > > So choosing a 600 oz-in motor for roughly 600 oz-in torque determined > experimentally looked like it would work. Then looking at the torque curve > I determined that to get that 600 oz-in torque at the desired speed would > require a 3:1 reduction belt drive to get the 600 oz-in at target speed. > I used a Gecko Drive with a 60VDC power supply and a motor with 6.5A, > 2.2mH rating. The calculator here > https://daycounter.com/Calculators/Stepper-Motor-Calculator.phtml > shows max 10.5 RPS or 630 RPM and therefore with 3:1 reduction I'd be > looking at 210 RPM into the lead screw drive. > > With a 4 TPI drive that means 52 ipm of table motion. In reality the best > I got was 15 ipm before the motor locked up. Click on the torque curve > and at 630RPM it's about 1.5NM which is 33% of the 4.5NM rating (640 oz-in). > > https://www.automationtechnologiesinc.com/products-page/nema-34/nema-34-640-oz-in-stepper-motor-kl34h280-45-8b-dual-shaft > > Long story short is that I updated to a 1200 oz-in motor and had reliable > motion with a Gecko Drive and 60VDC at 25 ipm from LCNC or MACH3. This > moved the torque up but not really the top speed since now this motor has > 6mH rather than 2.2mH with a 6A drive rating. I might have been better to > have changed the reduction belt drive to 2:1 or less. > > Back to that pesky power rating. At 60V the motor calculator suggests > it's drawing 380W. Change the voltage to 120VDC and with the same current > we get twice the RPM and twice the power 750W. So then theoretically I > should be able to get 50 ipm reliably if I had a driver that could handle > 120VDC and if the motor winding insulation was rated for that voltage. > > Instead I ball-parked an AC Servo motor, 750W, 3000 RPM, 3.5NM (495 oz-in) > and max 2000 RPM as the solution. This drive runs off 220VAC and is rated > at 3A with a peak torque of 10.5NM. With 3:1 and that 495 oz-in torque > value all the way up to 2000 RPM (667 shaft RPM or 167 ipm) I can > comfortably run this motor at 150 ipm without it ever faulting. That's 6x > the stepper motor speed. > > What that suggests is what everyone already knows is that stepper torque > falls off badly at higher RPM even with a higher voltage but they are > really good at low speeds. Change things to get more torque and if the > inductance goes up the torque at low RPM is there but not at the high RPM. > Unless your driver and motor can handle a correspondingly higher voltage. > > But look around and most drivers for steppers are in the 24V to 80V range > so with steppers, motor power in watts means nothing if you can't get the > voltage up. Now if you go 1:1 then 0.25"/200 results in a resolution of > 0.00125". Torque falls off to 50% with half stepping and micro-stepping > results in a 70.7% reduction in max torque. And Micro-stepping also can > require as many as 4 steps before static torque is overcome and the motor > shaft turning. That's just due to the nature of the current waveform so > for accuracy you must design for full step resolution. > > John > > > > -----Original Message----- > > From: Thaddeus Waldner [mailto:[email protected]] > > Sent: February-05-22 12:40 PM > > To: Enhanced Machine Controller (EMC) > > Subject: Re: [Emc-users] What Would You Suggest? > > > > I believe the flat part of the curve is defined by the motor thermal > limits. Is this correct? > > > > > On Feb 5, 2022, at 11:44 AM, John Dammeyer <[email protected]> > wrote: > > > > > > ?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 > > > > > > _______________________________________________ > > 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
