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Kentucky
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I'm building a small tank.  Each side of the treads will be operating independently from one another, powered by two separate huge electric dc motors ( I harvested from electric shopping carts).

I'd love for the tank to be arduino controlled as far as motion goes with 3 separate modes.  One is a photovore mode.  One is a random "roomba" mode, and the third is a manual control mode with an Xbee to cell phone control.

The problem is, I have no idea how to control these motors with an arduino.  Never done it!  Can it even be done?

The amount of power required to MOVE these beasts would fry my arduino.  I was thinking...could I use a transistor in one of the digital outputs that would allow power to flow to the motors? 

Each motor is 12 volts 1/10 HP at 12 amps.  The motor ratio is 14.3:1 and clocks at 2900 rpm
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Poole, Dorset, UK
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Sure you can do this. But you need a H-bridge thats up to dealing with 12A.

Mark
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I don't think you connected the grounds, Dave.
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Quote
Each motor is 12 volts 1/10 HP at 12 amps.
144 Watts is more like 1/5th HP
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Seattle, WA USA
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and the third is a manual control mode with an Xbee to cell phone control.
Which XBee can talk to a cell phone? Or, which cell phone can talk to an XBee?

That tree, over there, that's the one you need to be barking at.
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You may want to consider using wheels instead of tracks to start with (unless you already have the tracks). Tracks are somewhat involved to make. As others have said, an H-bridge is the usual bidirectional/speed motor control. They cost probably more than you think.
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Kentucky
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Update:

I've been tinkering and made some adjustments.  My initial goals were a little ambitious, so I backed off a little.

The tank treads are very easy to make!  Kevlar Bicycle tires work perfectly.  About $20 a tread at Walmart.  The frame of the robot can carry about 200 lb payload.  A person, other electronics, whatever.

I'm keeping it under manual joystick (from a hoveround) for now.  Just as a platform to work from.

The motor stats are totally different now, but it is irrelevant because it is all wired for me.  I was able to dumpster dive and find an old powered wheelchair.  It needed some tinkering, but I fixed it up.

Now my problem is this.  Eventually, I want to put my Xbee on my home network and control it using TouchOSC.  So I can drive my tank (about the size of a go-cart) around my property and scare children, from the comfort of my couch.

I'm not sure if anyone has done this before, but there are five wires leading from the wheelchair joystick into the voltage controller for the motors.  Five pesky wires.

I don't need a motorshield because the controller handles all that voltage for me.  But what about these wires?  I know I need to hack them, but I don't know which does what.

So my question is...how do I find out what each one does individually?  Also, once I find out, how do I control them using AnalogOut on the Arduino?
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Update:

I'm not sure if anyone has done this before, but there are five wires leading from the wheelchair joystick into the voltage controller for the motors.  Five pesky wires.

I don't need a motorshield because the controller handles all that voltage for me.  But what about these wires?  I know I need to hack them, but I don't know which does what.


Just a guess, but your 5 joystick wires will probably be:
1) Power in
2) Right motor forward
3) Right motor reverse
4) Left motor forward
5) Left motor reverse

Sorting which is which will require some trial and error.
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Ontario
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you need a H-bridge thats up to dealing with 12A.

I've been using these, and I think they're great, if a little spendy.
http://www.pololu.com/catalog/product/708

14A with no heat sink, and able to handle 30A during startup and the like.  Nice units.

The ST VNH2SP30 is a nice chip, and not too expensive at quantity.  It's too bad there's not more inexpensive boards around using these.
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The below h-bridge might be a little more affordable.

http://www.ebay.com/itm/30A-H-bridge-Coreless-Motor-Driver-Forward-Reversion-Brake-For-Smart-Car-Arduino-/180983568079?pt=LH_DefaultDomain_0&hash=item2a237616cf
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