> I took apart my daughter's car. At the heart of it was a small DC motor and
> reduction gearing system that spins a gear with a tube through the middle.
> The axel of the back wheel goes though the tube.
>
> It's quite powerful. However a couple of open issues:
>
> 1) The gearing is set up to spin only forward. A "jiggle" gear slips if
> the motor is spun backward.
Presumably this is an anti-damage device which could be easily removed.
> 2) Since only one motor is driving the back wheels, steering much be
> accomplished some other way.
Two-motor differential steering still seems like the best bet to me.
It is simple and effective, needs no clever mechanics and its behaviour
can be accurately predicted by simple software. It would be much harder to
both build and model a steered-wheel system.
> Agreed. What about plastic? Metal seems a bad idea due to weight and rusting...
Depends on the metal. Aluminium is a good candidate when considering
both weight and rust.
> Well I found a place here in Atlanta that sells the biggest PowerSonic
> sealed gel cell I've ever seen (get this): 12V 30 AHr! Wow!
RS lists 12 sizes of ``Dryfit'' 12V sealed gel cells ranging from 2Ah
to 115Ah. The latter weighs 40kg (90lb) and costs 300UKP (450US$)!
A more realistic buy might be the 12V, 7Ah ``Global'' from Maplin at
26UKP (39US$). This measures 150x96x64mm (6x3.75x2.5in) and weighs 2.4kg
(5.3lb). To get this capacity with NiCds would need 14 1.5V 4Ah D cells
costing around 70UKP (105US$). At that price, I don't think NiCds are
an option.
> The last interesting and inexpensive motor I've seen is from BG Micro
> (http://www.bgmicro.com catalog is in PDF format). 12V 8A stall, 3.5A
> continous unloaded. No torque specs available. 1/2 inch long 1/8 diameter
> shaft. $4.95 each.
That is incredibly cheap. At that price, you could afford to buy some to
test, and not feel bad about not using them if htey weren't good enough!
It might be worth asking for more specs first though.
> I'm figuring that I can get the battery, base and motors for under $100.
> Any clue to guage the run time say for 3 motors (two for drive, one for
> cutting?) May be easier simply to get the setup and do empirical tests.
Assuming maximum 3A for the three motors and a 7Ah battery, that's minimum
two hours run time (since it's best not to run a lead-acid battery flat).
With better motors we could run 2 or maybe even 3 times longer.
Robin.
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