First Stepper Motor Project - guidance requested

Hi all, I've used Arduino in the past to do some fun solid state stuff, but I've never made anything MOVE. I went out and bought parts that ended up not working together (got an H-bridge for a unipolar motor), so I'm coming to you all on advice before I spend more money. I'm also open to any other general advice on motor sizing, power supplies, etc. as I'm being very honest when I say I know little. I'm open to buying different equipment if really necessary, I'll always have other projects.

Project: Motorize a hand-crank apple peeler (this one has a potato... weirdos)

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(You can see something like this in action at the following YouTube video, if it helps you visualize.)

This specific motor assembly is intended to drive the central screw-shaft in pic above. My working theory is to mate the motor directly to the back of the screw (where the handle connects in the above picture) and put the motor assembly on a sliding rail so it cannot rotate but travels back and forth with the screw as it moves. I've used these before, and not too much force is required to push an apple through the peeler, especially when the cutting blade assembly is kept well.

Motor: A stepper motor was desired for low cost and high holding torque so that an operator can load/unload apples at the start and end of each peeling cycle. I currently have the following motor:

Type : 6-wire unipolar
Rated Voltage : DC 12-40V
Rated Current : 1.2A
Rated Speed : 1000RPM
Rated Torque : 3.5Kg.cm

(Amazon page: http://www.amazon.com/gp/product/B008ICMY2E/ref=oh_details_o00_s00_i00 )

Power supply: Just a standard wall-outlet transformer.

Rated voltage: 12V DC
Rated amperage: 1500mA

I've done my research and realized that I need something like a Darlington transistor array to drive the unipolar motor. The chip that comes up constantly in my searches is the U2003, but my reading of the spec sheet says that it only supports 500mA per channel, and according to the motor I have above I believe I need something that supports at least 1200mA per channel. However, I'm unable to find any array that supports that kind of current.

(U2003 specs: http://rocky.digikey.com/WebLib/Texas%20Instruments/Web%20data/ULN2001A-4A,ULQ2003A-4A.pdf )

How does the project sound so far? Can anyone point me to a driver that handles the current I'm looking for? Will I have a problem driving the motor at 12V when the operating range is 12-40V? Have I said anything really stupid?

Thanks for your help!

"I've never made anything MOVE"
For a first project to make things move I would recommend a DC gear motor and use your H-bridge to drive it.
I would not recommend a stepper motor for this project.

Thanks LarryD,

Why do you suggest a DC motor project? Is it just the complexity of driving a stepper motor vs a regular continuous DC motor?

I don't know if this project would work well with a continuous DC motor, because I need some idea of how far I've spun the corkscrew, and then I'm either looking at some kind of time-based driver, which would get out of sync over time, or some kind of sensor to detect how far to go, which seems unnecessarily complex. But maybe you're seeing something I'm not.

I'm also more than happy to rise to the challenge. The whole reason I do things like this is to learn.

I was concerned your stepping motor would not have enough drive power. However, there is a gain since you will be using a screw...
I often use surplus windshield motors for high torque applications (easy to get locally).

because I need some idea of how far I've spun the corkscrew

You could have a limit switch to detect the stop/rev/crash position

unnecessarily complex

I don't think so.
Note: you can use PWM for speed control on the gear motor.

If you are just starting to use motor drive circuits the gear motor is a good choice.
A DC motor is usually smaller when compared to a stepping motor considering drive power.
They also use a fraction of drive current.
Using a stepping motor as a first project is OK if you insist on using one, rather if you want to learn about them.

The whole reason I do things like this is to learn.

If we aren't learning, are we dead? To learn is to be young.

dsf900:
Hi all, I've used Arduino in the past to do some fun solid state stuff, but I've never made anything MOVE. I went out and bought parts that ended up not working together (got an H-bridge for a unipolar motor),

If the unipolar motor is 8 or 6 wire then it can be configured as bipolar and driven from two H-bridges. If its 5-wire
then I think its also possible (the winding centre taps are commoned, so you have to be a bit more careful of how
you drive the H-bridges (PWM for instance is problematic).

Stepper motor is just fine for this project. Stpeer motors produce their torque at low speeds and require less of a gear train to perform well at low speed.

Post the part numbers and any other info on your stepper motor and driver so we can give you some better advice as to how to connect things up. If you ran the stepper in unipolar mode you could make a somple driver from 4 MOSFETS and use 4 outputs from your Arduino to control the outputs in proper sequence. I have done this in developing a stepper controller for my own use - Here's the url - http://arduino.cc/forum/index.php/topic,84809.0.html - It is for a freestanding stepper driver, but it started out life as fully Arduino Based.

You will need a LOT of power to run those devices. That will be an expensive Stepper & PSU. Try fitting a pully where the handle goes, wrap a cord around it and hang a weight off of it - it needs to handle worse case.

Your arm is very strong. Making robot arms to move 5kg loads about ain't cheap! You do it without thinking!

I would tend to use a cheap 12 DC motor with a reduction grear-box, and a couple of relays, or employ a small child to peel my spuds.

Check out ebay for say 12V 30 RPM - which is probably too fast

Seteppers are quite expensive. I think you only need a geared dc motor. You can accurately control the angle with a stepper. The geared dc motor also has a lot of torque.