10 May 2012

Crüscooter Megaupdate 1: Final Design

Alright, the semester's nearly over and I now have a good excuse to update my blog: my grade depends on it. In order to not spam one semester's worth of blogging into a single post, I'll split it up into a few. Without further ado, Crüscooter's final design! 

I want a scooter that's portable (lightweight and foldable), has room for both my feet to be at rest, is reasonably weatherproof, can brake similarly to Razor scooter, can drive me around MIT's campus reasonably fast (20MPH), has decent acceleration and can go uphill, is robust against bumps and railroad tracks and such, and can generally put up with day-to-day use. Given these criteria, here's the design I came up with. 

Approach and Initial Decisions

My approach to designing the scooter was to stand on the shoulders of giants and learn from the successes and failures(read: lessons learned) of the many others who've built electric vehicles before me. 

I chose to use a timing belt-drive instead of chain-and-sproket or other transmissions because belts are smooth and silent. I also chose to go with a sensored motor controller because it can get me going from a standstill. Also, I need 16 Inches of free space on top to fit both my feet. Also, because the back wheel pulley had 72 teeth, I chose a 14-tooth motor pulley for a gear ratio of Kg = 72 / 14 = 5.153. 

Brake Design

The only real innovation for this entire project, my brake design combines the simple and intuitive Razor step brake with a band brake that doesn't wear the tire down. The fender also serves to prevent water from being kicked up all over my back when riding on wet surfaces. 

The result is a brake you step down on that pulls on a short brake cable, which is attached to the brake. The cable actuates a pulley on the brake which tightens the band brake to slow down the wheel. When tuned properly, the brake maintains full effectiveness while preventing the fender from ever contacting the wheel. To quote my instructor, "this mechanism is legit."

Choosing a Motor and Drivetrain

I need a motor and drivetrain that can get me to 20MPH as quick as possible. Hobbyking.com seems to have the best selection of Cheap-ass Motors, or "ICBM"s (Iridescent Canadian Banana Monsters?) so that's where I started looking for the perfect outrunner motor for me. 

First of all, I had the option of choosing 1 to 3 batteries. The batteries provided to us are A123 12V 4.5Ah batteries with 40-amp fuses. Because of the space each battery took up and the fact that I didn't want a 4-foot long scooter, I decided to run on 24 Volts (2 batteries). 

Motors are characterized by a constant that is manipulated in various ways to show off a different type of spec. On Hobbyking, that value is its Kv which has units of RPM/Volt. I converted these to Newton*meters/Amp with a little but of unit magic. I narrowed down my motor selection to the SK3 class, which has a 63mm can diameter (59 mm stator) and comes in different sizes. After doing some math, most of which is detailed in the Instructables link above, I picked a motor and determined my scooter would have the following theoretical specifications: 

Turnigy Aerodrive SK3 - 6364-190kv Brushless Outrunner Motor
Voltage: 24V nominal, 26.4V charged
Current limit: 40Amps

SK3 6364
Kv = 190 RPM/V 
Kt = 0.05026 Nm/A

Gear Ratio: = 72 / 14 = 5.153
Top Speed = 23.21 MPH
Max Acceleration = 1.1199 m/s^2

Additional Design Features: 


The chassis is 1/4" Aluminum plate because it is light, strong, and easily waterjettable. Using T-slots and tabs, the chassis can be assembled easily I chose 6-32 button cap screws and square nuts because they would sit flush against the chassis's surface. The front fork is also designed to be waterjetted and t-slot fastened, essentially copying Charles Guan's design (see above "failures" link, which is Charles's answer to a broken plastic front fork). 


In order to tension the belt, I chose to put slots that will allow the motor mounts to rotate about a pivot point, giving the belt a few centimeters of play. 


There's a hole in the side that mayOrMayNot be covered, but it allows me to access some wires on the internals, and the serial port. Yes, my scooter will have a serial port. Also, there's a hole on top that was originally for a Hella Switch, but the CAD model that website provided was like half as large as an actual switch or something. I ended up putting the status lights, precharge switch (A main motor power switch with a resistor, so you don't run infinite current into the controller when you turn it on) and a "Dean's key" for main power switching (That I learned from these guys) slot there instead. 



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