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Dim3nsioneer

Two stepper motors on one axis

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Is there a way to run two steppers for the same axis? I'm thinking about a all-direct-drive having one stepper on each x and y rods.

Theoretically there are two possibilities:

- Connect two steppers with the same stepper driver. Most likely the driver is not designed to drive two steppers in parallel, correct?

- Connect one stepper per driver and connect two drivers in parallel to the Arduino Shield. Possible? I don't know the interface.

Is there anybody with know-how about the UM1 electronics who could tell me something about the limits of the stepper drivers (Pololu A4988)?

I'm not looking for a quick and dirty solution. It has to be safe.

 

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I'm not looking for a quick and dirty solution. It has to be safe.

 

Then you better not do this IMHO.

The Printrbot uses 2 motors on 2 different Z axiz, and that already gives some issues. They are controlled with a single driver and split motor cables.

The pololus are rated 2A max, but that's with more cooling then you have. 1.5A is about the max where they will shut down. The default is around 1.1A/1.2A for the UM. So you do not have a lot of extra power there.

 

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Besides extra torque I would like to know if there is any difference in accuracy and ringing . Theoretically, there should be an advantage, as with the standard setup the part of the belt which pulls the head in one direction is three times as long as the part that pulls in the opposite direction.

 

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@Daid: As I'm not giving up so easily :wink: : What about the second theoretical possibility? I just had a quick lock at the scheme. I guess there should be no problem with the pull-ups (maybe a separate pull-up resistor for the second MS1? Or even a separate jumper block including the 4.7K). VDD, VMot and GND are anyway common to all the drivers. RST and SLP have to be connected separately for each driver. What about EN, STEP and DIR? Would these signals break down in amplitude when connected to two drivers? And last but most important question: Is there enough power left from the Shield to run additional two motors? Most probably that is the problem...

@foehnsturm: Without any belts I would expect the backlash to go down to something very small. The torque per axis would actually be significantly smaller as one stepper would not have to drive two connected rods.

 

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If you connect the steppers in series instead of in parallel, then it may work ;)

That way you don't need double the current, but double the voltage across the motors. Afaik the motors run on a very low voltage - most of the power is actually burnt away in the drivers, so you may even get cooler stepper drivers by connecting two motors in series ;)

Owen mentioned in another thread that he connected 3 steppers in series. Haven't tried it myself but I don't see any problems with that...

Still, you'll need lots of steppers which is kind of uneconomical (what a hypocrite I am, always being the uneconomical one myself, I know I know).

 

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If you connect the steppers in series instead of in parallel, then it may work ;)

That way you don't need double the current, but double the voltage across the motors. Afaik the motors run on a very low voltage - most of the power is actually burnt away in the drivers, so you may even get cooler stepper drivers by connecting two motors in series ;)

Owen mentioned in another thread that he connected 3 steppers in series. Haven't tried it myself but I don't see any problems with that...

Still, you'll need lots of steppers which is kind of uneconomical (what a hypocrite I am, always being the uneconomical one myself, I know I know).

 

No, I didn't say I did it Jonny. I was thinking of doing it. But as for the reasons you've stated, the current is the same and double the voltage required, which the stepper drivers should be able to handle. Also an open circuit will make both steppers lose the circuit together which should be a better thing.

 

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If you connect the steppers in series instead of in parallel, then it may work ;)

That way you don't need double the current, but double the voltage across the motors. Afaik the motors run on a very low voltage - most of the power is actually burnt away in the drivers, so you may even get cooler stepper drivers by connecting two motors in series ;)

Owen mentioned in another thread that he connected 3 steppers in series. Haven't tried it myself but I don't see any problems with that...

Still, you'll need lots of steppers which is kind of uneconomical (what a hypocrite I am, always being the uneconomical one myself, I know I know).

 

That's actually the way to deal with mutliple current driven devices, yes.

It should even be safe in terms of the controller doing still the right thing (limit the current). However it is to check if the stepper driver has components with a power limit which could be exceeded. I guess the isolation of the two coils inside the steppers against ground (casing) is good enough. Otherwise one stepper would be charged up by some volts. Any idea what voltage is needed for the typical 1.1-1.2 amps? (yes, I could measure it, but if one of you knows it already... :rolleyes: )

The two additional steppers are not the only thing which is uneconomical. But nearly any modification of an Ultimaker is uneconomical... :wink:

 

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It would normally lie in the region of 1 to 3 Volts max. but usually 1.68V to 2V for each motor. So if you double that then you are well under what the stepper drivers could provide. They are supplied with 19V for the motor circuit so they could probably supply up to at least 17V if they needed to.

 

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On the topic of multiple drives, I noticed this thing which uses ball bearings clamped at an angle around a steel rod to simmulate a ballscrew arrangement. The main thing is that I am not sure if the steppers could spin fast enough. I think stock the steppers move something like 1"/turn and I this would be more like 1/4" per a turn or less.

http://www.thingiverse.com/thing:125529

 

 

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That thing is a remix from http://www.thingiverse.com/thing:112718

There are a bunch of remixes at different pitches, so you may be able to find or design one that has a pitch that will match the 1" per turn.

If you are thinking of using this to emulate a 3 ball screw z-axis arrangement, you may want to run some tests to make sure you can print well matched copies.

 

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I see what you mean, it would certainly be a problem if one side of the axes moved more per a turn than the other. In principle, you could drive each stepper independently, so you could calibrate for steps per an inch separately.

BTW, if one was going to go through all the effort to have three z-axis screws, than why not just build a Delta bot?

 

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BTW, if one was going to go through all the effort to have three z-axis screws, than why not just build a Delta bot?

 

Deltabots take up about 2X as much space vertically as they can print. They look very cool but I'm not sure if they are any more practical.

 

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Deltabots take up about 2X as much space vertically as they can print. They look very cool but I'm not sure if they are any more practical.

 

It is vertical space though, which isn't as valuable.

My main thought was that having 3 independent vertical axis is sufficient to make a delta-bot. So once you are already going that route, the delta bot might make more sense.

 

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