Monday, July 22, 2013

√−1 Scooter

As part of my last free this summer, I've taken up the task of building a personal scooter to transport me around campus and to MITERS.

Design parameters:
  • 5-8 mile range
  • cruise speed of 20mph
  • reliability
I used Charles Guan's instructable as a guide in how to succeed not to destroy everything.

Among the most useful "steps" was the motor page as, having no previous EV experience, sizing up a reasonable motor alone would lead to either utter disappointment or too much scooter for any sane person to handle.

To safely fulfill the 20 mph (8.9 m/s) cruising speed, I'd need a motor with the following output wattage:

P = Force(drag)*Distance/Time

Substituting in air's density at STP (1.1839 kg*m^-3), and my drag coefficient at 1 into the drag equation gets me 38.0N, which translates to 338W. It's recommended that this number be less than 15% of the max motor input as ideally, only half of the input wattage goes into mechanical output. 

This would mean I'd need a motor rated for ~2.3kW. 

Hot damn.

With waterjetting out of the question, I ran into the idea of constructing the entire chassis out of u-channel to avoid racking up costs (the actual theoretical budget for the project is around ~$400). 

Going for $47.06, three feet of 4"x2.25" McMaster u-channel seemed to fit the job. 

At this point, several issues arise with trying to build a suitable motor into the frame of this form factor.
  • Outrunners of the correct wattage tend to be either too big for the frame, or run at voltages that would cause battery costs to be too damn high.
  • Having an outrunner too long/large would mean having to machine out the appropriately sized  hole in the other side of the chassis while mounting the motor to the frame. 
  • Batteries are expensive.
  • Ideally, the motor should completely inside the chassis to avoid gunk acquisition that would otherwise limit its lifespan.
This meant downsizing the motor from the previously melon-esque Turnigy SK3 63mm motor to the following:

Running the system at 36V also means an overall cheaper system and a happier wallet.

This did mean, however, my safe cruising speed would take a fall to 18 mph (hardly slow).

After settling on the motor, I could finally go shopping for the other assortment of parts I'd need to make this scooter scoot. The BOM can be viewed here along with the drag equation conveniently available in table form.

More to come...

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