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The $50 Segway

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joshua_

Sun Feb 12 2006, 11:36PM

Registered Member #61 Joined: Thu Feb 09 2006, 05:50AM
Location: Mountain View, CA
Posts: 43

At my school, we are required to do a project for physics class second semester. The only (academic) requirement for the project is that it demonstrates something about physics. They're pretty open about the fact that nearly everything had to do with physics, though, so it's pretty much a "make anything" project. There are a few other requirements, the most important of which being that it can't cost more than $50 per person.

I thought long and hard about this, and was leaning this way and that - I had various other projects that I was working on that didn't feel like they'd take the requisite 3 months. What could I build? I decided on a self-balancing device. I'd seen these on the web before, and they didn't seem particularly difficult. (e: might not've been clear: it needs to carry me, too)

I'll be updating this thread with my progress as I go through it. Before you dismiss this as totally impossible, understand that I'm dumping all semblances of safety, performance, really anything to cut costs; check the project proposal for details.

2/10/06: Some samples ordered.
2/11/06: Project proposal written. To be submitted to physics teacher 2/13. Viewable at my site.
2/13/06: Proposal submitted.
2/14/06: Freescale samples arrived. Breakout board for X-only accel created: PDF available (total board size: .8x.8in)
2/15/06: Proposal accepted.
2/17/06: Breakout board for MMA7260 (X-Y-Z accelerometer) created. Total size: 25x21.5mm. SMD size: 6x6mm. To be etched and tested later today.
2/18/06: Couldn't find any copper clad, so I just dead bugged a MMA6260 (X-Y accel)
2/19/06 early morning: Got a PIC speaking I2C to my Matrix Orbital LCD -- pictures of the "hello" program later. Also will try to set up the ADC later.
2/20/06 early morning: Got the ADC going, and got periodic output to the LCD going using TMR0. Scrounged up a .1uF cap to try to filter out some of the oscillations from the accelerometer, but minimal luck. May need to decouple the whole accelerometer (like I should've done when I first hooked it up). I've been building this thing on the board that I was previously using for my UPS project, so it still has the ATX transformer on it. That'll probably be removed, and this will probably be made into the main board for this project. Pictures lower down on the page, or linked here:
full test setup, PIC test board, ADC hello world. I've been having some problems with in circuit serial programming, and I've already killed one PIC inserting and removing it from the socket (ESD maybe from the cold air? dunno) -- the next task, then, is probably a bootloader. Maybe I'll just throw the Microchip USB loader on it and be done with it...
2/21/06 early morning: Disaster strikes! I etched a beautiful breakout board for the MMA7260, but I then discovered that somehow, Vcc didn't get hooked up. No problem -- I added a wire under the IC. It was too tall to sit nicely on its own, so I just let it seat on its own in the toaster oven. I had it all set to go, but I wasn't watching carefully enough, and I made a nice crispy board. The IC probably still worked, and the board, other than being crispy, was probably fine, so I tried fixing solder bridges, but to no avail. Worse, pads started coming up. I removed the IC with some pliers, but evidently in doing so, I ripped a pad from the bottom of the IC (G-SELECT1). Anyway, bad. I'm going to use the X-Y accels for the time being. Oh well...
3/06/06: Haven't updated in a wihle. Built a H-Bridge recently, and wrote a USB bootloader for the 18F2550. Will update with pictures later today.
05/07/06: After a long hiatus, I have returned! I'm surprised that I didn't remember to post pictures. Anyway, yesterday I got some Fisher Price motors and gearboxes for free from a local Power Wheels Authorized Service Center. Thanks! I decided to disassemble an old bike of mine today to get the wheels. In my bump at the bottom, I'll post pictures of the motor and the bootloader. I also got the front wheel from an old bike attached to a motor -- I got very lucky with the spoke layout! Now to figure out how to attach the back wheel, which has a larger hub...

By the way, I will only bump from now on when hardware has been constructed or a milestone has been reached (or when I feel like some other significant update has happened), just to avoid bumping for every little thing.

Think I'm nuts? Feel free to comment. I have no idea how this will play out, but I'm sure it'll be fun no matter what. There's nothing difficult but the whole thing ...

Maz

Mon Feb 13 2006, 12:46AM

Registered Member #111 Joined: Fri Feb 10 2006, 01:04AM
Location: Menasha,Wisconsin
Posts: 65

if you want to be really cheap about it (which i think is the whole goal) rig up a mouse's scroll wheel with a weight so the platform would be balanced when the wheel hangs freely (no motor power) and when it swings either direction it goes forward or backwards.

joshua_

Mon Feb 13 2006, 02:47AM

Registered Member #61 Joined: Thu Feb 09 2006, 05:50AM
Location: Mountain View, CA
Posts: 43

Maz wrote ...

if you want to be really cheap about it (which i think is the whole goal) rig up a mouse's scroll wheel with a weight so the platform would be balanced when the wheel hangs freely (no motor power) and when it swings either direction it goes forward or backwards.

Yeah, but that causes pretty bad oscillations. The accelerometer and gyro were free, so cost on those is not a problem. The way to damp those oscillations, by the way, would be to use a gyro...

Maz

Mon Feb 13 2006, 03:29AM

Registered Member #111 Joined: Fri Feb 10 2006, 01:04AM
Location: Menasha,Wisconsin
Posts: 65

im sorry i didnt even notice your proposal link. i figured a gyro would cost mucho $$$ so i didnt suggest it.

joshua_

Mon Feb 20 2006, 09:56AM

Registered Member #61 Joined: Thu Feb 09 2006, 05:50AM
Location: Mountain View, CA
Posts: 43

Evidently last time my pictures were too big, so retry:

Full test setup

PIC test board

ADC hello world

joshua_

Sun May 07 2006, 07:24PM

Registered Member #61 Joined: Thu Feb 09 2006, 05:50AM
Location: Mountain View, CA
Posts: 43

OK, so here are pictures of the motors, as promised: (click for bigger)

Wow, how perfect can you get? They turn at quite a reasonable speed, and they're torquey as all get out.

It attaches nicely.

Close up on how it attaches. Maybe I will make notches in the bottom of the little black plastic thing so that it will stay put. Either that, or I will just use liberal amounts of epoxy....

The bootloader running.

The H-Bridge board populated only with the SMD FETs.

Front side fully populated.

And, for good measure, the schematic and the board for the H-Bridge.

Enjoy!

GimpyJoe

Mon May 08 2006, 03:05PM

Registered Member #316 Joined: Mon Mar 13 2006, 01:30PM
Location: Marietta, GA
Posts: 212

Wow! Are those powerwheels motors?

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