What kind of stepper for Mendel?

Hello,

I read the steppers of Mendel should have at least 0.14 Nm of holding torque (1400 g.cm) and that the size of about Nema 17 or 14.

But what about the other parameters? What about "related voltage", "current/phase", "resistance", ..? Are they important at all?

For instance, I found the following stepper:
Nema 17, bipolar, 0.21 Nm, 0.4 A, 32 mH, 30 ohm

Would it be suitable for Mendel?

Edited 1 time(s). Last edit at 01/07/2010 04:14PM by Ecir Hana.

not enough torque for the extruder, might be ok for the axes. Also the resistance is far too high. As far as I know, you want a ~0.4Nm motor that you can put 2A through, so 6 ohms or less. The stepper drivers have current choppers, so they automagically reduce the effective voltage if the motor doesn't need the full 12v. If the motor needs more than 12v there's not much you can do.

I bought 0.44Nm, 1.68 ohm, 1.68A, 2.77v 3mH motors, and from everything I read they're perfect, but I haven't actually built enough of my mendel to see how well they work yet.

I'm sorry I'm not sure if I quite follow...

The needed torque for the extruder is a function of material being extruded and radius of the pinch wheel. That said 0.21 Nm might be just ok but what bothers me is something else:

If I understand it correctly voltage, current and resistance are all related (Ohm's law). So if the motor driver outputs 12 V that is, for instance, 0.4 A * 30 ohm. Shouldn't be the motor at the top ok? But you are saying that I probably need a 2 A motor - why is it so?

Also, the motor you mention has much lower values - in this case, does the chopper cuts 12 V down to 2.77 V? What does it matter then, what voltage the motor needs, if it stays bellow 12 V?

In other words, I'm sorry I'm a bit confused on this one - the only value a motor has fixed is its resistance, right? How do I know which voltage and current I need, since R is just a ratio between V and I?

when the motor is stalled, the current is indeed defined entirely by ohm's law.

However, when the motor is moving, it produces a back-emf which your drive voltage must overcome, and the inductance of the windings also requires a higher voltage for the same current to flow within a usably short period of time.

The end result of this is that a 2.77v motor needs only 2.77v when it is holding, but may need as much as 8 volts when moving quickly to maintain torque and not lose steps.

Torque is proportional to current (and heat to the product of current and voltage), so all good stepper controllers control current tightly, and allow the voltage to be whatever it must for the necessary current to flow - the opposite of what voltage regulators such as the 7805 do.

For this reason, it is advantageous to have a low voltage motor, since then it is trivial to supply enough current even at high step rates from a 12v supply.

Thanks for the explanation!

If your motor needs a higher voltage, you can still use it, but will need to get a different power supply as a computer power supply doesn't do anything higher than 12 volts.

Also, you'll have to change the controlling electronics to handle the higher voltage, which may mean getting a different driver board for them (like pololu.com's [www.pololu.com] A4983 stepper driver breakout board).

That may or may not be something you're willing to try - knowledgeable friends help.

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I'm building it with Baling Wire

What about resolution? Is 200 steps per revolution enough, or does the Mendel require 400 steps per revolution?

J-dubs: 200 steps/rev is fine. dunno about the official stepper drivers but mine have 16x microstepping so I get up to 3200 steps/rev from a 200 steps/rev motor

Triffid_Hunter Wrote:
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> J-dubs: 200 steps/rev is fine. dunno about the
> official stepper drivers but mine have 16x
> microstepping so I get up to 3200 steps/rev from a
> 200 steps/rev motor

But a loss of position accuracy that way.

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FiveD Firmware fork (supports Arduino Mega): [github.com]
reprapsource.com

Ji joaz,

> But a loss of position accuracy that way.

... why?

I have some 1.7Nm-BergerLahr/Positec-steppers with 3 phases and jumper-selectable 200/400/500/1000 steps (or 1.8° to 0.036°) per rev. in 'normal' mode with a specified inaccuracy around +/-6 arcmin (or 0.1°) over all ranges - but this isn't a real positioning error, but more an 'elastic range' around the actual step-position, dependant of the applied external forces ...

With activated microstepping all values went ten times finer, only the angular inaccuracy stays at 6 arc mins

So with the atached 5:1-planetary-gearboxes i have resolutions from 1000 to 50000 steps per rev. @8.5Nm torque with an overall mechanical error around +/-1.2 arcmin (or 0.02°) + eventual inaccuracies of the gearbox, what can be compensated with a constant pre-load ...

Viktor

Thanks,

I managed to find 12 steppers local 400 steps per revolution and 61 oz. / in. holding torque. I bought the whole lot just in case.

-J

Hmmm you mentioning 400 step stepper motors jogged my memory do you remember the old 80 track 5.25 floppy disk drives. They have a 400 step stepper stepper motor that drives the head usually on a rack and pinion.

I think they might be ideal for the X and Y Axis of Mendel as the torque required to drive the X and Y axis is very small seems to be around 0.25kg measured using my electronic luggage scales.

That would effectively double the resolution

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Bodge It [reprap.org]
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BIQ Sanguinololu SD LCD board	BIQ Stepcon	BIQ Opto Endstop
BIQ Heater Block PCB	BIQ Extruder Peek clamp replacement	BIQ Huxley Seedling
BIQ Sanguinololu mounting	BIQ standalone Sanguinololu or Ramps mounting	Print It Stick It Cut it

My rep strap: [repstrapbertha.blogspot.com]

Buy the bits from B&Q pipestrap [diyrepstrap.blogspot.com]
How to Build a Darwin without any Rep Rap Parts [repstrapdarwin.blogspot.com]
Web Site [www.takeaway3dtech.com]

i mandaged to find some sanyo denki steppers .9 deg/step but i am still trying to find more info on them they only list there ohms and ded/step on them but there nice i got them out of the plotter,

i have some pics of them on my blog, they are nima 17 ones i do know that much at least.

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[mike-mack.blogspot.com]

Cool Blog, I like your concept. It is coming together nicely.

-J

i found this stepper . was wondring if it would work

2A 75oz-in, 0.53N-m, 1.2Ohm/Phase, 2.6mH/Phase

it looks like it should have the right Torque
but i just dont know

thanks for the help

yep that's plenty of torque. is it nema17 size? I'd expect a motor with those ratings to be nema23 at least. where have you found it?

looks like the ones from Alltronics i purchased. ~8USD per stepper.

assuming they are the same ones. they have a notched shaft so they need a gear-head if you want to use them in a pinch wheel extruder.

[www.alltronics.com]

thats the one i was looking at. its a nema 17. the price is right. but i think its 24v. so finding a something that can do 24v is going to cost a bit more.

all the photos show a solid shaft. looks like i need to send them an email on the subject.

now i just need to buy it before there all sold out

It isn't a 24V motor it is actually a 1.7V motor. The spec sheet shows the performance when driven with a 24V constant current supply. You can run it from 12V, the top speed will be a bit lower that's all. Speed is not an issue unless you use a threaded rod drive.

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[www.hydraraptor.blogspot.com]

how would these steppers due for a rep rap? I can can a bunch of them.

Minebea stepper

Type 23LM-C304-43

5.1V
1.0A
5.1 ohm

Thanks

Datasheet is here, the second link is still live:

[www.cnczone.com]

Unfortunately, while it gives you the wiring sequence, the torque is left blank. Unless its possible to calculate the torque from the information given, you'd have to experiment to see if one of these can drive a reprap.

Thanks for the link, but I got that exact data sheet with the steppers. Was hoping someone was familiar enough with them to know if they will work.