Sunday, February 8, 2009

Bubbling to the Surface

It is time to confess a new obsession. Like most obsessions it begins by contagion. I caught this one from my son Alan. We are going to build a robot. Well, not exactly. We want to build a remote-controlled self-balancing two-wheeled vehicle. Think Segway, only very small. And hyphenated. It's not an original idea, of course. Besides Segway, lots of amateurs have built such things. To the extent that we have a new twist on the concept, I am interested in seeing how cheaply we can do this. I think we might be able to do it for under $100. Most such projects seem to be at least $200-300. Cheap also means small (smaller motors cost less) so ours will be smaller than the others.

I am not a hardware guy. I'm like the punchline of the old programmer joke: How many programmers does it take to change a light bulb? None. "Hey, man, that's hardware!" Fortunately for our collaboration, Alan is much more of a hands-on kind of guy. In fact, I think we make a great team. I can't wait to take a crack at the control algorithm and he's itching to do all the soldering and wiring and assembly.

Here's how we got into this. For several months Alan has been surfing websites for DIY electronics projects. For Christmas he asked for an Arduino, an inexpensive microcontroller board based on an open source hardware design. Until Alan asked for this I never knew such things existed. Then I started doing a little investigation, and the obsession began. It is such a great time to get involved in DIY electronics. I had no idea there were so many sophisticated components available so cheaply, like three axis accelerometers in an IC chip that costs only $10 or $20 dollars. And the programming reminds me of the old days programming for my first home computer: an Apple II. Low level coding on an 8 bit micro and direct manipulation of the hardware. Wonderful! (That's geek nostalgia, friend.)

I see this effort as a sequence of sub-projects:

1. Do a preliminary hardware design
2. Develop a physics model for simulating the vehicle
3. Use the simulation to develop and test the control algorithm
4. Develop an IR remote control decoder to control the vehicle
5. Build rev 1 of the vehicle as a three-wheeled scooter
6. Debug and tune the balancing on two wheels

I've already roughed out the hardware design with an eye toward selecting and pricing components online. The only piece I haven't figured out yet is how to cheaply measure distance traveled. The obvious answer is an optical wheel encoder, which you can buy as a kit for DIY robotics. But if we're going to keep the hardware budget under $100, we'll probably need to do something cheap and homebrew.

Most folks who have built one of these things uses both a gyro and an accelerometer. The gyro gives a stable rate signal that you can integrate to get angular position, but it is subject to a lot of drift. The accelerometer gives a very noisy signal, but it can be filtered and used to correct the gyro drift. Again, to save money I'd like to try to make our vehicle work with just an accelerometer. I want to use the simulation to see how much noise I can tolerate, and get a sense for the bandwidth and resolution I need on the accelerometer. This past week I've been working on the physics model. Boy, my freshman physics is rusty! I suppose since it's been nearly 35 years that's not too surprising. I've been beating my head against it for days.

At the same time I've been reading up on IR remote controls. We want to use a Sony remote from our DVD player to control the vehicle. You can buy an IR receiver for $1 or $2 but I'll need to study up the SIRC protocol and program the Arduino to decode the signal. Last night I ordered the part and started looking into the programming.

Well, that's where we are so far. I have in mind to post sporadic progress reports here as we move forward. If we ever get it working I'll post a few video clips, too. Now that I've posted this entry the pressure is on to actually do something!


Rob said...

You know, we're just too much alike. I can loan you a couple of robotics books from my last flirtation with this obsession (~10 years ago). Also, ask Dean K about the inverted pendulum project.

Have fun, and post pictures!

Bill Hensley said...

Always following in your footsteps, Rob.