NEMA 34 Stepper motor RPM Issues

Posted by: @clark77494

@will Do you think running the Raspberry Pi Pico will solve the issue? 

I don't know, I've never used the model of NEMA nor the driver that you're using. The Pico will definitely give you a faster clock but I don't know if the driver and stepper are capable of running at the required speed (especially under load).

 

Posted by: @zander

@clark77494 I am not expert at this to be sure, but doesn't this say the pico can do much more?

https://www.codrey.com/raspberry-pi/raspberry-pi-pico-pwm-primer/

Ron, the Stepper library doesn't use PWM. It sends pulses out and only uses digital pins.

I believe that DBWS has a video about using PWM to drive steppers, but I haven't explored the topic.

Clark, it may be worth taking a look to see if you think it might provide an alternative approach.

@will Ok, I thought that was what PWM was. I will go back in my corner now.

Posted by: @zander

I will go back in my corner now.

No, stay out here 🙂 Never hurts to have a second opinion.

@will One of the reasons for my confusion is the scope trace looks similar to the one posted by @clark77494

Posted by: @zander

@will One of the reasons for my confusion is the scope trace looks similar to the one posted by @clark77494

Yes, he's the one who posted it in this thread back at 12:28.

@will I was thinking a square wave is a square wave that's all.

@zander 

Yep, that's why PWM is another alternative for driving a stepper. Gets a bit more complicated varying the speed.

@will Isn't the pulse width what controls that, I think they call it duty cycle.

Posted by: @zander

@will Isn't the pulse width what controls that, I think they call it duty cycle.

Maybe, but I'd expect the frequency of the pulses, not their duration, to be the main factor in speed. As the speed increases for some drivers (and maybe the stepper motor too) you have to vary the mark/step ratio from 50/50 (for low speeds) to something like 25/75 (for very high speeds).

I tend not to push things to the bleeding edge of capability, so I'm not too knowledgeable at this end of the performance scale.

@will I thought it was the area under the curve that dictated the amount of power. It can be varied by modulating the amplitude or the width (duration). Unless there is something else at work here I am unaware of and that is a very real possibility.

@zander The square wave that I am showing is when the motor is off and not under load. This is what the wave form looks like under load with the motor on.

@clark77494 That doesn't look healthy at all to me, but I am certainly no expert. That looks like it's stalled perhaps.

@zander 

It's not just a question of power. The rotation is caused by energizing electromagnets which push and pull permanent magnets, same as in a normal motor. The difference is that the stepper motor stops after every step (theoretically the field collapses), so a continuous stream of pulses is required. The faster the impulses are sent, the faster the stepper motor moves to the next step position.

So, raw power is important because the motor has to overcome static inertia to start with and fights against the change in rotational inertia as it speed up (or slows down). That's why acceleration is important, it allows you to start with slower (and therefore potentially longer lasting) pulses to overcome the initial resistance and then build up speed (by reducing the inter-pulse delay) while only having to overcome the gradual change in interim (as opposed to trying to jam it all at once).

Thats why you see weight lifters pulling on a train to move it instead of running directly into it at high speed 🙂

Posted by: @clark77494

@zander The square wave that I am showing is when the motor is off and not under load. This is what the wave form looks like under load with the motor on. 

Thanks, that picture is quite informative. It illustrates the stress and force induced by the driver's pulses and the counteracting force.