Thursday, March 21, 2013

Pinewood Derby and single-track Gray codes

It's time for the anuual Pinewood Derby at the cub scout pack I serve. Last year I was out of town during the Derby so I didn't participate. This year I am here, so I am making a car. I am not competing, of course, so I am free of certain constraints. Also, I never do any project like this in some ordinary way. As Emeril would say, I always try to kick it up a notch.

The design of the car is a pretty simple one, inspired by my first car back when I was in Cub Scouts. That year, we (me and my parents) thought aerodynamics was the most important thing, and so we designed the car accordingly. And finished dead last. Anyway, I still have fond memories of that car, like trying to microwave the car to dry the paint and charring the center of the block, nailing AA batteries to the back, putting my Battlestar Galactica 2" pilot action figure as a pilot. The general car design had a duckbill front and a turtle back. It was also solid red (what color do we hate?).

This time I have nicely polished the block to the point that it shines. I am going to leave the block completely unmarked.

But, that's not kicking it up a notch. Kicking it up a notch is installing a rocket engine in it. It's installing a camera. It's installing an electric motor. It's putting in a sensor for the car to time itself. I'm going to do the latter.

I have painted the back right wheel half white, including the tread, but with the hub very carefully masked off so as to not interfere with spinning. Over that wheel, I have installed two QRD1114 line-follower sensors. Each includes a near-infrared LED and a phototransistor. The sensor is close enough to visible light that things like white paint are still white and black is still black. The two sensors are placed roughly 90deg apart around the wheel, and about 1-2mm away from the tread. The idea is that the sensors use the half-white wheel as a single-track Gray code encoder wheel. Thinking about it this way: Suppose the car is rolling, the back sensor is well over the white part, but the front sensor just transitioned from black to white. Which way did the wheel turn? It must have been forward. If the back sensor were white but the front sensor changed from white to black, that means the wheel turned backward. Using this, we can construct a directional odometer, and measure the distance the car has moved. By timing the transitions, we can measure the speed of the car as well. By properly using both sensors, we can always know the direction of wheel spin and the orientation of the wheel to within a quarter turn.
I was disappointed to see that Bele and Lokai wear gray gloves. Bele doesn't wear gloves here!

So, the car will measure its own speed. We start the car with the wheel just past ticking over, so it has to turn one full time to count a rotation. The wheels are 15mm in radius, so we can measure the distance traveled. The microcontroller reading these sensors has a clock with microsecond precision and 4-microsecond resolution, so plenty enough to measure the exact time of each wheel rotation. Putting these together gives the car's speed, which it will print on an LCD display.

The gem this week is Gray code encoders, in particular single-track encoders. These have one track of code, and several sensors over that track at different angular positions. You need one sensor for every bit of code (two in my case for a 2-bit, 4 position code), but with careful design of the code track, you can use the same track for different bits, by shifting the sensors around the wheel a bit. This can be continued until you use the same track for all the sensors.

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