Dual motor Y-axis, single driver, has a speed limit

ppvi
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Joined: Wed Mar 23, 2016 9:46 pm

Dual motor Y-axis, single driver, has a speed limit

Post by ppvi » Wed Mar 23, 2016 10:54 pm

Hi, this is my first post. There are many threads about dual motor-single driver problems but I haven't found this particular one. :x

When I move the Y-axis above a certain speed (11000mm/min), both motors stall at the same time. They accelerate and then make a horrible sound. I can live with lower speeds but now I want to know if this is normal or why this is happening.
  • If I connect the X-axis motor to the Y driver, I can move it at F30000 easily, so there's no software/driver limitation
  • I removed the gantry to reduce load to the minimum and I loosened the wheels. The speed limit stays constant and both motors behave the same way.
  • I use beefy NEMA17s (2.5A per coil, probably a bad purchase decision!), but increasing the current of the driver (up to 2.5A total) has no effect.
My theory is that having two motors in parallel is producing some kind of counter-current and they are cancelling each other once a rpm is reached. One of the motors is connected to a longer set of wires. Before trying to match them, any other ideas?

I have a (e)ShapeOko with ramps1.4 running Sprinter (similar to Marlin) and both drv8825 and a4988 produce the same results. Tried 1/4, 1/8, 1/16 and 1/32 microstepping. 12V PSU.

Thanks!

ppvi
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Joined: Wed Mar 23, 2016 9:46 pm

Re: Dual motor Y-axis, single driver, has a speed limit

Post by ppvi » Thu Mar 24, 2016 10:33 am

Looks like my results are in line with this reddit thread (Running two stepper motors from the same drive by wiring in parallel).
you can run two steppers off single driver up to a certain RPM limit and then it will stall. I recall it to be around 300 or 400 rpm but that is dependent on motor/driver combination.
So I guess I'll hack the firmware to use the E axis as Y2. I'm worried about what will happen when one of the Y drivers stop working... :oops:
In any case I'd like to know why this happens.

Jimf
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Joined: Wed Feb 11, 2015 8:58 pm

Re: Dual motor Y-axis, single driver, has a speed limit

Post by Jimf » Thu Mar 24, 2016 1:25 pm

I wrote that reply in that Reddit thread. 300-400 rpm limit was what I recall reading on Cnczone. I never got around to actually verify/testing the rpm limit with a drv8825. I should run a speed test.

Your 12volt motor supply may be a factor in your motor speed limit. Try a higher supply voltage to see if you can get a few more rpm.

Jimf
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Re: Dual motor Y-axis, single driver, has a speed limit

Post by Jimf » Thu Mar 24, 2016 5:00 pm

Ok I did a bench test with a drv8825 driver and two nema17, 1.5amp, 76oz-in motors. Both are same make and model. Stepper wire length are the same.

I used a external Tektronix frequency generator to avoid any Arduino pulse jitter and inconsistency.

301 rpm was the max I was able to get. Power supply was variable between 12 and 20volts. It didn't matter what voltage setting was used.

At 12volts, single motor max rpm was about 625rpm.

Rpm was measured with a digital rpm meter.

Jimf
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Re: Dual motor Y-axis, single driver, has a speed limit

Post by Jimf » Thu Mar 24, 2016 6:08 pm

http://youtu.be/bjw5Z9R2o3U

Short video of the dual stepper bench test

ppvi
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Joined: Wed Mar 23, 2016 9:46 pm

Re: Dual motor Y-axis, single driver, has a speed limit

Post by ppvi » Thu Mar 24, 2016 9:23 pm

Wow, that's a very sofisticated setup. Thanks for confirming this. I wonder if more expensive drivers allow for it somehow.

Jimf
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Re: Dual motor Y-axis, single driver, has a speed limit

Post by Jimf » Thu Mar 24, 2016 9:58 pm

[quote="ppvi"]Wow, that's a very sofisticated setup. Thanks for confirming this. I wonder if more expensive drivers allow for it somehow.[/

I will test with a geckodrive when I get a chance. My guess it will be similar.

Jimf
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Joined: Wed Feb 11, 2015 8:58 pm

Re: Dual motor Y-axis, single driver, has a speed limit

Post by Jimf » Fri Mar 25, 2016 1:31 am

I ran the test again with a Gecko202 stepper driver. 290rpm was the most I can get with the same dual nema17 motors connected.

This is a older anti resonant driver however it is still a very high performance one. I was able to spin a single nema17 motor 8900rpm using only a 20volt power supply.

That's right almost 9 thousand rpm!!! Also tells you how good the automation direct motor is.

cvoinescu
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Re: Dual motor Y-axis, single driver, has a speed limit

Post by cvoinescu » Tue Mar 29, 2016 9:27 am

If you have a RAMPS, why not use one driver per motor? It's the easiest way to work around the problem.

Your motors are fairly low impedance (lower than the A4988 drivers were designed for). Connecting them in parallel makes that worse. Try series, although the 12 V supply voltage is going to be even more of a limiting factor in that case.

As for them stalling at the same time, that's because the stall of one motor induces a current that makes the other motor do the same thing. It's normal for parallel-wired motors.
Proud owner of ShapeOko #709, eShapeOko #0, and of store.amberspyglass.co.uk

ppvi
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Joined: Wed Mar 23, 2016 9:46 pm

Re: Dual motor Y-axis, single driver, has a speed limit

Post by ppvi » Tue Mar 29, 2016 9:55 am

Yes I'm working on it. I wanted to make sure the behaviour was normal. I'll check out your patch for Marlin to make sure I'm not missing anything.

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