calculating which Stepper to use

[jamiec]

Hi There

I was wondering if someone can help I am try to work out what Stepper to buy, I am trying to build a vertical scanner but i am building out of wood so it has to left quite a heavyish and there quite a bit of friction. so i was wondering if there's any calculation i can use try and work whats best, also is there a tool out there i could use that would how much force was needed as i guess wood friction is quite hard to calculate

Here my plan
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Any help on this would be greatly appreciated

James

[Soeren]

Hi,

[...] i am building out of wood so [...] there quite a bit of friction.

Any reason to not use either metal or plastic, or even ball bearings for the frictional parts?

[...] is there a tool out there i could use that would how much force was needed

Yes, it's called a weight 
Geet one of those used for weighing your travel luggage, or one for weighing your catch when fishing, or even a parcel/paper weight... Whatever have a range that seems to cover your needs.
It has to be one of the types with a hook in one end and a kind of handle in the other.

Put a nail in your drum, hook the weight to that and draw the weight out perpendicular to the radii described by the nail to the center of the drum.
Pull in a steady manner, building up the pull strength until it moves - the number on the weight should be read as the highest reached right when it starts to move.

Now say your drum is 20cm in diameter and the "high score" on the weight was 200g you can calculate the minimum needed force to just move it.
radius = 20cm/2 = 10cm
Force = 200g/10cm
Convert to whatever measure you need, like: = 2kg/cm = 20g/m = 0.196Nm (= 27.8 Oz/Inch)

Remember... This is just to counter inertia and friction. To move it at any speed, you need more torque.

Don't use a stepper for this... You'll be glad you didn't

What's it for?

[vinito]

Add a counterweight and you can negate most of all the force required to do the lifting. Why put the expense in a large motor when you can just add some simple ballast?

[Soeren]

Hi,

Add a counterweight and you can negate most of all the force required to do the lifting.

The force to overcome is friction (apparently wood to wood), so how would a counterweight do any good?
(It's not lifting but rotation).

[vinito]

Add a counterweight and you can negate most of all the force required to do the lifting.

No, overcoming the friction is only part of the force to overcome. He said "it has to left quite a heavyish" and also that it is vertical rather than horizontal, so I was suggesting the counterweight simply to counter the weight (duh). Yes friction still has to be overcome, and personally I would eliminate as much of that as possible mechanically too, but that's just me and since the solution for this may or may not be a bunch of parts and design work I left that alone. A counterweight is relatively simple.

Note emphasis added in the quoted section above. Maybe I was incomplete saying this was to negate the lifting force only while friction issues remain.

[photomark]

From your diagram the object you wish to lift is 0.6Kgs , normal force = mg ( mass x gravity ) = 0.6 x 9.8 = 5.88N . this is the force needed to move the object.  The torque needed from the motor will depend on the radius of your motor pulley, for example if you have a pulley that is 30mm in diameter you simply multiply the force needed (5.88N) by the radius of the pulley in meters (0.015m)   5.88 x 0.025 =0.0882 N.M  .

The friction force created by the wood is hard to calculate from the little information you have given

 the true friction coefficient could be very high so  I would recommend bearings and ceramic bearings really are not that heavy .

This is just a simple explanation but it should help        

[Soeren]

Hi,

No, overcoming the friction is only part of the force to overcome.

Sorry, my bad - I somehow got into my head that the "drum" (or whatever) should rotate in place. Well, scanners can be do many different things, now I see it's more like a flatbed scanner raised on an edge.
Sorry.

And yes... Plus one to the counterweight, now that I've finally got the idea 

[jamiec]

Thanks for all you help guys it's really appreciated

The reason I am making it in Wood is more of a aesthetic thing more art project that engineering, I had tried a to counterweight but it seem to add more friction (it's a bit badly made at the moment) I think i start looking into integrate some ball bearing

Soeren: What would you suggest instead of a stepper ?

Many Thanks

James

[Soeren]

Hi,

Soeren: What would you suggest instead of a stepper ?

A simple DC motor. It will be much easier for you to control, they're cheaper and they're more powerfull for any given size.

It's quite often I see art students making something that takes a little engineering and that's cool, but only when things compose into a higher unity.
A problem with friction is so easy to solve and, at least in my mind, would detract a lot from the appearance of the finished "product". 4 ball bearings would cure it and they can be mounted hidden under/behind the wood parts.

[abuse.bot]

I think a stepper *is* the best idea.  "Loot" a big one from an old printer or something, and you can make it go in circles with just 3 transistors and an old PC printer port (or anything more fancy if you're wanting to make it more complicated than necessary).

The benefit of the stepper is that you'll know exactly how far you've moved your frame - if youre stepper does 1-degree steps, and you've sent 360 of them - you know it's done a full rotation... that's really easy to program into whatever scanner software you're coding.  If you use anything other than a stepper - you're going to have to build some extra stuff to measure how far it's moved after - loads more effort...

[Soeren]

Hi,

I think a stepper *is* the best idea. 

You're allowed your opinions of course, but steppers are much weaker, for their size, than DC motors (and that's an easily verifiable fact, not a personal opinion).

Steppers are imprecise as well.
When you run it for say 100 revolutions (assuming you could guarantee that it wouldn't skip), it will be fairly precise, but their step to step precision depends on how well it was produced, as it is a question on the precision of the pole pieces - and, over the years, I have had to toss a good many steppers that didn't had the precision I needed.

"Loot" a big one from an old printer or something,

The "big" ones I have seen in printers have all been what I would call very small and weak (And I have stripped printers as old as from before 1975 - Which didn't use steppers at all, but a few slow synchros).
I do have a single large stepper lying around, but it's still very weak compared to its DC size equivalent.

and you can make it go in circles with just 3 transistors and an old PC printer port

A DC motor just needs DC to turn 

(or anything more fancy if you're wanting to make it more complicated than necessary).

Oh, you can get it to run in squares then?   

The benefit of the stepper is that you'll know exactly how far you've moved your frame

In theory yes, but in real life there's the problem with skipping, so you don't know the exact position with 100% accuracy after all.

- if youre stepper does 1-degree steps, and you've sent 360 of them - you know it's done a full rotation... that's really easy to program into whatever scanner software you're coding.  If you use anything other than a stepper - you're going to have to build some extra stuff to measure how far it's moved after - loads more effort...

A little more effort, but that will be precise, as it is a low mass being moved by the axle.
With steppers you need to find the optimum acceleration and decelleration ramps and even then you cannot guarantee against skipping., Don't believe me? Just check a scanner or printer using a stepper - most of them have positional wheel or linear encoders and they all add a zero position sensor to keep skips from adding up to disastrous amounts - and they don't add as much as a single screw that isn't absolutely needed.

Steppers have their place, but they take circuitry to run (which the DC motor doesn't and while the DC motor needs a positional decoder, the stepper needs a periodic positional reset or skipping might soon accumulate to something serious.
Further, the output of steppers are way lower than DC's, when you compare their size and weight.

All said, the final decision will often hang on what's already available and how complicated it is allowed to be.