Saturday, June 9, 2012

Scooty Brakes (and a stint through the rain)



Using your shoe as a brake is okay for testing, but the wear gets ridiculous. So, I put a few holes in the front fork to mount a plate and the plate to mount a set of brake calipers on the front wheel.

The plate mounts right on the front fork, held on by 6 4-40 screws. The plate is 1/4" steel.
The calipers I used were inexpensive bike calipers from Amazon. While I knew braking onto a plastic rim is not such a good idea, I thought it would still work okay.

Then, parts and assembly, after a particularly nasty ride in the rain back from work.
Ghetto extra waterproofing with masking tape

Still alive!

Yes, that is water on the inside on my caps.

Attaching the brake to the brake plate. Haha what clearance.
Due to the calipers blocking the screws, I had to slowly inch the parts together, screwing down half the screws, moving the caliper around, and screwing down the other half.
Derp.
And then, I got excited and forgot to take more pictures. Long story short, after some testing, I decided they were worse than failure. Not only do the brakes not work, but the brakes and rims must be mortal enemies because they destroy each other. The pads exhibit significant wear and some chunks coming off already, and this is from a nice walking speed.
Overexposed for your dark caliper viewing pleasure.
 On the rim, it seems these bits have melted on. I'm not sure which surface made them.
Can't brush this off. Its melted on.
I can't imagine can guess what would happen slowing from full speed. Rim failure and tire blowout, or I eat through the pads so much they stop applying pressure, or both.

Well, back to the CAD board. This time, I'll use one of these.
70 mm rotor band brake

Sunday, June 3, 2012

Scooty Puff Build

*Note: somehow the styles on this post crapped out and all the headers are tiny. I can't figure it out*

My first build out of real necessity (not something like, I really want a delta robot)! Scooty Puff is a brushless motor powered wonderfully fast Razor style folding scooter.

The problem:
I needed a way to get around in Dallas, specifically from where I live to work and back.

The constraints:
  • It had to fit in one of my suitcases. This rules out bikes.
  • It has to go some reasonable range.
  • I can carry it on a train.

Nice to have things:
  • Speed.
  • Pneumatic wheels to not rattle my joints apart.
  • Brakes.
All signs point to scooter! Name: Scooty Puff, after Fry's ride in the Futurama episode "The Why of Fry."

The original Scooty Puff Jr.
When designing, I had a lot of help from Jamison and his experience making scooters, and referred to Shane's Pneu Scooter and Charles' RazEr rEVolution and Instructable on electric scooters quite a bit. 

Parts List

Design Considerations

As always, a link to the Solidworks design files are linked below in the appendix.

The Rear Wheel

One tricky part was getting the sprocket mounted on the rear wheel. The hub of the wheel is plastic with spokes, so there was not a lot of material I could remove for, say, holes in the spokes.
So, after asking some friends around the Invention Studio for advice and taking inspiration from Pneu Scooter's hub motor, I came to a solution. I would use a flat free wheel. The sprocket is mounted onto a plate, and that plate onto the wheel with screws that go through the entire wheel assembly, like so.
Wheel in the flesh!

This way there are no threads pulling on the plastic hub. Now this solution works all hunky-dory, until unforeseen material properties smacks me in the face. First, the tire does not have as firm a grip on the hub, so it tends to pull away during turns. This creates the terrifying feeling of the rear wheel slipping around corners. Second, the flat free tire wears down really, really fast.
This wheel used to be round.
Just one week of use.
How is it supposed to last the summer with these terrible characteristics? I had to switch to a pneumatic wheel, with a redesign of the sprocket mounting plate. Two plates now, one on each side of the wheel, are screwed onto the hub (yes, threading into plastic). The sprocket mounting bolts go through both plates to hold the plates in compression, minimizing the chance of the plastic threads pulling out.
There is a hole on the plate opposite of the sprocket for the valve stem. Thus, three of the sprocket bolts go through the entire assembly, the last one only goes through one plate.

The Motorpod

Heavily inspired by Razor Wind's wheel pod, it is a self contained unit for the rear wheel, motor, and power transmission.

Motor Selection


I used the EMP/Turnigy C6374 because not only is it appropriately sized, but it also has a bearing on both sides of the motor. This makes the can more resilient to shaking and vibration and other nasty real world conditions.

Controller

I use a cheap sensorless ebike controller from China. After reading Charles' Beyond Unboxing of the controllers, I thought they would do well after a bit of modification, and they do. The mods:
  • Reduced shunt resistance to 2 milliohms. This, in theory, boosts the controller's wattage from 250W to close to 1000W
  • Due to Charles' reports on the bus caps getting warm, an additional 4700uF of capacitance.
  • Swapping the power FETs with IRF3207s to more than halve the on resistance (Rdson)
This give the controller enough beef to accelerate uphill.

The rest of the scooter was designed to be made with the waterjet, manual mill, and lathe. After two weeks of machining, it looks like this. 

A scooter!

Scooterbros


With a 3D printed fender
The stats:
  • 4.25 mile range
  • 28 mph theoretical top speed
  • 25 mph observed top speed
  • 2 hour 15 minute charge time from empty to full, limited by charger power
  • 26.1 Watt-hour/mile efficiency 
Lets see how long this thing lasts with a 3.5-4 mile long round trip commute every weekday this summer.

Appendix

Complete design files on github: https://github.com/aaronbot3000/scooty-puff

Saturday, June 2, 2012

Catching Up

Wow it's been a long time since I last posted. Busyness and schoolwork and then moving to Dallas for a summer internship at TI and general laziness added up, I guess.

In chronological order:

My good friends Jamison Go and Xo Wang and I visited MIT and MITERS over spring break mid-March. We managed to bring three electric vehicles in two pieces of luggage: Safety Razor, Razor Wind, and the newest incarnation of Velociryder with breadboarded (but functional) circuitry. This involved a panicked redistribution of weight in front of the luggage check in as our suitcase ended up being 70 pounds, while the limit is 50. In the end, our flight was successful, and the TSA screeners didn't lose any parts.

While at MITERS, us Georgia Tech guys completed a 12 hour build, the Scooter-Ass-Kart, one of the most dangerous vehicles to ever have been constructed there.

Two people wiped out riding it, and one battery pack damaged. Thank god for dent tolerant A123 cells.

Upon return, I started assembling the Velociryder's actual circuit board. While the breadboard + Arduino works, its not pretty, and doesn't use the encoders.

Hot air reflow.

Everything is so nice and straight and aligned.
I ignored common sense and soldered everything on at once. Luckily, neither soldering errors nor circuit design flaws damaged the PIC microcontroller. When I was clicking around the programmer, I accidentally sent 5 volts to the chip instead of the required 3.3 volts. Now, whenever I try to program it, I get verification errors, always in the same block of memory. Ah poop. Discouraged by this setback, I put the Velociryder back on the shelf.

While at MIT, everyone else had practical vehicles. That is, they move faster than the Velociryder's slightly-above-walking speed. And they were lighter. Knowing I would be going to Dallas over the summer, most likely car-less, I decided what I was going to do. Make something useful! Thus, project Scooty Puff was born.