how big do these motors need to be?

As a software engineer with almost no hardware experience, I'm thinking that a decent way for me to get started in robotics would be to make a
simple wheeled platform and strap my laptop on top of it. I could control the motors via something like Pololu's serial motor controllers. Then I can make use of a USB or Firewire camera, add some sensors (e.g. using a Phidgets interface), and so on, and do all the programming in an environment with which I'm already comfortable.
The big unknown for me is: what size motors do I need to carry around a laptop, batteries, and a handful of other stuff? Totalling maybe 10 or 15 pounds?
Thanks for any recommendations. - Joe
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Sorry to follow up my own post, but for example: would this motor from Jameco be a good one?
<http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1 &storeId001&catalogId001&productId0641&pa0641PS>
3500 g-cm sounds like a lot of torque to me, but it seems odd that it's only drawing 105 mA... that's less than a standard 9V LEGO motor. What am I missing?
Thanks, - Joe
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wrote:

Joe, you might want to check out:
https://www.zagrosrobotics.com /
You may find a prebuilt base that will work for you.
Hopes this helps,
// Jim
|| Horror Novel: http://staydead.jamesdunavant.com || Sci-Fi Novel: http://keepers.jamesdunavant.com
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Zagan wrote:

This here one would do the trick;
https://www.zagrosrobotics.com/shop/item.asp?itemidc9&catid 
we see from the stall current that it is a 72W motor, the reduction is pretty big...flat out with 100mm wheels you are going to move at 100mm/s...still a fair clip...expensive though!
depends too on what it is meant for, tooling across the floor inside the force needed is much smaller;
if V is 100mm/s and it takes 2s to get there (seems reasonable)
V = at, a = 50mm/s^2
from F = M*a, F = 10kg*0.05 = 0.5N
the torque required depends on the gear ratio, so using V = 100mm/s and D = 50mm (inside use, does not require big wheels) then wheel speed is 0.6rps, say loaded motor speed is 2000rpm, reduction is 55:1, moment at wheel is T, T = F*r = 0.5N*0.025m = 0.0125Nm, through gearbox 0.0125/55 = 0.00023Nm @ motor.
So...P = T*w
w = 2pi*f, f = 2000rpm/60 = 33rps, w = ~200rad/s
P = 0.045W, Stall current @ 12V = ~0.4mA
So if my maths is right, an itty bitty teeny tiny motor will make the 10kg unit move across a flat frictionless surface at 100mm/s, and it will take 2s to reach this speed
From previous post, a 50W motor will haul the unit up any hill limited by the traction.
How big does the motor have to be?
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It would, if I had $300 to spend on it -- I'm hoping to put something together for under $100. It's possible that I'm being unrealistic, but I have to try.
Thanks, - Joe
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Joe Strout wrote:

these motors are cheap & cheerful, there would be something like this wherever you live;
http://www.oatleyelectronics.com/motors.html
if two wheels are driven and steering is effected by differences in velocity then velocity encoders are needed...these can be either an itty bitty DC motor used to generate a voltage proportional to speed, fed in by an AD converter to printer port, or a pulse generating 'thingy' (optical, hall effect, reed switch...whatever takes your fancy) that triggers an interrupt to count pulses...i think most people use this form
but one wheel can be driven and turned using a servo to give steering, then velocity is of less interest
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$100 is on the low end... at that price you'd have to machine the base yourself. For $150 I'll bet you could put together a base with a laser cut lexan platform, two gear motors, wheels, caster and shaft encoders.
I've not used them, but I've heard good things about Pololu and I'm keeping them in mind for future projects.
http://www.pololu.com/laser_cutting.html
If you're talking about including motor controller, laptop interface, microcontroller card, etc. in that $150, then you might be a tad optimistic. But that's the fun thing about this stuff, most of the cool tech is DIY anyway.
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Yes, I just found that site last night too. It looks like a great service -- a fairly simple platform looks like it would cost about $10 (price depends on how many cuts I need, but I'd be happy to drill mounting holes myself).
I also found the motors I need; Zagros sells the ones used in its platforms as individual units for $35. So I'm looking at two $35 motors, a $10 platform, and a $20 Pololu serial dual motor controller... a total of exactly $100, plus a few more bucks for misc. hardware (mounting brackets etc.), and maybe another $20 (?) for a battery (but it seems that the Zagros mobile platforms don't include that either, so it's fair to leave it out in comparison).
Still a bit of a stretch for my budget, but if I can talk my brother-in-law into joining the effort, we could be up and running pretty soon!
Thanks to all for your help.
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wrote:

Unless you really want to get into the building of a base, then you should look for something cheap and ready made. The below child's electric ATV for ~$40 should hold the 15lb with out problems. Once you get the programming and control issues worked out, then you can spend time on building a custom platform. You might check the big discount stores for similar electric toys at a reasonable price.
http://www.homier.com/detail.asp?dpt=7&catD&sku5984
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Joe Strout wrote: > The big unknown for me is: what size motors do I need to carry around a

well, call it 10kg = 22lb = ~100N
P=F*v = 100N @ say 0.5m/s (1.5ft/s)
PPW = V*I, @ 12V = 4A current
Speed of DC motor is ~2500rpm = 41rps
If you use 100mm diameter wheels;
C = pi*D = 314mm @ 0.5m/s = 1.6rps
41/1.6 = ~25:1 reduction
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I'd just go with a fist-sized DC gearmotor with around 100 rpm.
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"Everything should be made as simple as possible, but not simpler." AE
:-)
DOC

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Joe Strout wrote:

I would suggest that you look at the LEAF project: http://www.leafproject.org/ -- D. Jay Newman
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Joe Strout wrote:

Windscreen-wiper motors? Just make sure you buy a motor controller that can switch enough current while protecting your laptop. Two such controllers will probably blow your $100 budget.
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    --For low budget robotic platforms it's hard to beat a reinforced cardboard box. A pal of mine did this to test a theory he was working on. As for motors: go to Harbor Freight and pick up variable speed reversing hand drills. Strip them down to their essence and you've got a nifty motor-and-gearbox, plus you've got the motor controller. When there's a sale on (most of the time) you can get these for maybe $15.- apiece.
--
"Steamboat Ed" Haas : Bummed to be living in the
Hacking the Trailing Edge! : Golden Age of Bullshit...
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I like that idea also.
I did a 30 pound spinner a while back using a couple of B&D 9 volt cordless drill motors and the planetary gears.
I had to do some machining to make them a little more robust, but they're a lot better than the windshield wiper jobs.
Bit of a comical aside...
When I was done with the cordless drills, I put what was left of them back in the nice plastic carrying cases.
Didn't quite know what to do with the things, so they hung around in my car for several months.
Then I got inspired and figured humm...
I could just leave them in the parking lot of my local Canadian Tire. (Car parts and crappy tool store.)
They were gone within 30 seconds!
:-)
DOC

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