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A bit of background: A want to build a motorised board for my son is 3 and has very limited mobility. He can't crawl but can use his hands to press buttons so I want to build something he can lay on and by using 2 switch buttons he can move himself back and forth.

The dimentions of the board I have are 1m by 60cm. I already have an arduino board but started looking at getting motors and motor controllers and got lost. I need a good place to start.

Here are the actual questions: Do I buy one or 2 motors? What type of motors would you recommend? Any tutorials on how to wire the motors and what motor controller to get will be great. What wheels should I buy? It will need to move about 20kg max. What other parts would I need?

Thanks

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    \$\begingroup\$ I would consider purchasing something similar and modifying it. There safety concerns including getting fingers stuck in the wheels. \$\endgroup\$ – Brian Carlton Dec 5 '12 at 21:32
  • \$\begingroup\$ Motorized skateboards come to mind, though they will not have the reverse movement you need. Modifying one for your purposes might be easier than building one, for an inexperienced maker. \$\endgroup\$ – Anindo Ghosh Dec 6 '12 at 4:24
  • \$\begingroup\$ Or hack a Roomba. Probably disable to vac motor, too. \$\endgroup\$ – Bobbi Bennett Dec 6 '12 at 4:50
  • \$\begingroup\$ Or start from a bottom unit from a discarded power wheelchair. \$\endgroup\$ – Bobbi Bennett Dec 6 '12 at 4:54
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    \$\begingroup\$ Make a tank ;) The treads will serve you better than wheels. Take a look at this thing, no idea on the cost but even if it's too much you can get an idea of the types of motors and power requirements required to move 40kg. If nothing else write them and say what you're trying to do. I'd be suprised if they didn't help: microrobo.com/… \$\endgroup\$ – Some Hardware Guy Dec 8 '12 at 13:44
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If you are thinking of building from scratch (i.e. sourcing/making and integrating the battery, controller, control logic, firmware, board, wheels, drive chain), this is probably a larger project that you first expected.

Just to back up my point, I followed the same route, and after 550 hours and NZ$3,500 later (yes, 550 man hours!) came up with this...http://cladlab.com/electronics/projects/electric-skateboard/electric-skateboard-design

electric skateboard image

This isn't an advertisement (not that I make any money from it), but hopefully you can glean some information from that project page. In summary, I used an 800W brushed DC motor (you could go for a brushless one, slightly more expensive, and more complex control but better power-to-weight and size ratios), which fitted onto a custom axle, drive chain, and wheels, all designed for an electric skateboard already. Have a look on eBay for these. Doing this yourself would add significant time and resources (e.g. requiring a lathe/milling machine).

One motor will be sufficient, as long as it is powerful enough (one-wheel drive works surprisingly well, as mine is). Two motors means two drive chains, possibly two controllers, and stuff like differential control for steering e.t.c.

Also, power electronics are notoriously difficult to design, and I definitely recommend buying a motor drive module. You can get cheap ones like the one shown in the image below off http://www.aliexpress.com/ (and here is a link to one to get you started).

motor controller

As for the control logic, this is something I recommend you do yourself, and you have the right idea by using an Arduino.

As mentioned already, probably the best way to start is to modify something that already moves (like an electric wheelchair). An interesting project, and I wish you all the best.

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I am putting this as an answer, so it can be edited and extended, but it is more of a suggestion.

Get an old power wheelchair, and strip it down.

The motors are almost always a gear box, brake and motor assembly. I had a pair on Ebay, there are quite a few other guys' still listed, look up 'battlebot' parts to see what they look like. You could bolt a pair of these to the bottom of the plywood sheet and have a platform that would be apx 8" off the ground.

You would want a wheel well around the wheels sticking out either side, to keep fingers out. In fact some crib walls would be good to ensure the boy is fully inside. Maybe even pressure switches on the walls, to stop it moving if anything is out of place.

Add fore and aft caster wheels to make it stable.

Keep all the wheelchair electronics, cables etc. Joystick up front for the boy to push. Sling the rest beneath. You can under-spec the batteries a lot, since they do not have so much work to do. Use 12 V lamps inline with the motors to keep the torque down;' the thing without limiters would be pretty dangerous for a toddler.

If you have just the motors, they take 24v max but would turn fine with 12. They usually have a brake solenoid that takes 12v. You probably do not need reverse, just forward (v-right=6, v-left=6), right (v-right=5, v-left=7) and left (v-right=7, v-left=5), so just a simple PWM FET circuit would do. Don't forget a wheeling diode! The motor load is quite inductive.

You could skip the arduino and electronics entirely, and use three medium duty switches with light-bulb limiters. Still need a snubber diode across the switches, though.

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