Dual z-axis thoughts

[Russ Taber]

I always wondered why the dual z-axis motors were used.  In some ways it simplifies things, but in other ways, it adds complexity. Seems to make more of a challenge for leveling since I've easily got my lead screws out of sync when my the motors are powered off. 

I've seen a design mod for an i3 where you go over the top with a belt design: http://www.thingiverse.com/thing:1205037

but wondered why not use a dual shaft and go under the bed? This is over simplified:

I would think a dual shaft motor would be a bit cheaper than two single shafts and balance the cost of a belt and pulleys.

Is there any inherent issues with using dual shaft motors that I'm simply overlooking?

[Ryan Carlyle]

I think when you price it out and include the additional fab/alignment of the closed-loop belt and tensioner and screw support bearings, it'll be a little cheaper to use two motors. It's easier to run more wire to the second motor than figure out a sync belt. 

So yeah, it would perform better, but that doesn't mean people will use it. Mendel / Prusa style printers these days are all about low cost and parts count. There are LOTS of design/performance compromises. 

[Ryan Carlyle]

For what it's worth, I recently designed a two-screw Z stage mod for my (cantilevered) CoreXY, and I decided to go with dual motors (although with integral screws to avoid coupling wobble issues, and serial wiring instead of the usual parallel wiring, to help the motors stay in sync when depowered). Why? Pure laziness. Bolting another NEMA 17 motor mount bracket to the frame takes a tenth as much design and fab effort as coming up with a way to support and belt-sync the second screw. 

[Russ Taber]

Makes great sense. Doing often beats theory. How does serial keep the motors in sync? Any links to serial stepper info. A book would be nice--yours--hint hint.

[Russ Taber]

Sorry. Should have googled before opening my yap:

http://www.instructables.com/id/Wiring-Your-Z-Stepper-Motors-in-Series/

[Ryan Carlyle]

1) Half the current requirement
2) Both motors must get the same current, which may help if the coils are slightly mismatched
3) When you spin one depowered motor, the induced current must flow through the other motor in the same direction, making them slightly tend to move in sync (as opposed to the opposite effect when wired in parallel)

[Whosawhatsis]

Re: 3, are you implying that with parallel wiring, turning one motor while the driver isn't powered will tend to make the other motor turn the opposite direction? This is definitely not the case, both empirically and from my understanding of the physics. When you turn one stepper with another wired in parallel it acts as a generator producing a voltage that produces the same sequence of positive/negative voltages to make that motor turn the same direction. I can maybe see how it would cause the other motor to move one step out of phase, but it definitely turns in the same direction.

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[Ryan Carlyle]

Hmm, I had to think about it for a minute, but you're right, they'll want to turn together if wired EITHER series or parallel. I was thinking having them in parallel would be like flipping the plug (which does reverse the direction) but no.

Paired motors with one driving the other:
Series: each coil must see the same current direction and the coils have the same relative phase
Parallel: each coil will see the opposite current, but because both coils are reversed, the relative phase and direction of motion is the same

Single motor rewiring:
Flipping one coil: the flipped coil has opposite polarity so it reverses the coils' relative phase
Flipping a plug: each coil sees the opposite direction current AND the relative phase flips, so three reversals makes one net reversal

What I'm not sure about is how the freewheeling diodes in the de-energized driver will affect the generator action induced current. When you run one stepper, it has to have enough speed to generate enough back-emf voltage to push current through the H-bridge body diodes before current will flow. When you have the two motors in parallel, small loop currents will flow at lower speeds, perhaps?

[whosawhatsis]

Yeah, I've been wondering all this time, as you've been referring to series motors turning together, why you were making a point of it when the same happens when they're wired in parallel.

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[Ryan Carlyle]

Just fuzzy thinking on my part. Thanks for the correction.

Only significant difference I can think of is that the parallel motor being driven will be two full steps out of phase from its normal energized position.

I need to do some more thinking about back-emf phase and load angles. The driven motor is also producing back-emf that opposes the driving motor. But one of the motors should be braking (negative load angle) and the other should be driving (positive load angle) and that phase difference is what allows there to be a net driving force.