Sunday, July 11, 2010
Sometimes consumer technologies march forward and open possibilities in completely unrelated fields. Such a situation seems to exist with the problem of position and distance sensing as it might apply to PRT. Traditional technologies include SONAR, LADAR, (similar to sonar but with lasers instead of sound) as well as magnetic position sensors and RFID tags. Meanwhile, digital photography has taken off, and computing speed has made interpreting camera information essentially instantaneous. The following is a brief exploration of the possibility of using ordinary “webcams” for this purpose. This is made possible by the relatively controlled lighting conditions in an enclosed track. The following assumes a light on the front and a hollow square reflector on the back of each bogie. My sample webcam has been simplified to the point of having almost no resolution, with only 100 pixels. In real practice the light sources and cameras would be in pairs, offering redundancy.
Here you can see the reflected square as captured by the camera. I have made it off-center to simulate an approaching curve in the track downward and to the left. Because of curves in the track the camera will not always be aimed directly at the leading vehicle. There can even be blind turns, an issue I’ll avoid for now.
In the example above, the pixels 52-55, 62, 65, 72, 75, and 82-85 are activated. It would be a simple matter for the computer to recognize the patterns, since the horizontal lines are characterized by consecutive numbers and the vertical lines are characterized by incrementing by tens. In either case, (up & down or across) the count is four.
In the second picture the longest string of numbers (consecutive or by tens) is three digits, not four. The box is smaller, as it would appear if the lead vehicle were further away.
Here is a nearly blind turn. In this case the only information that the computer can use is that the pixels 30, 40, 50, 60, and 70 are red. The computer would rightly interpret this as a five. Now it is apparent why a chose a hollow square reflector. As long as there is at least one straight line with a beginning and an end, the distance to the lead vehicle can be determined.
The example above is a very primitive, I know, but it would work about the same with a higher resolution system. For example, the lowest resolution “webcam” that I found online was 640 by 480 pixels, about a third of a megapixel. I found a “two-pack” of 1-megapixel cameras for $33. (U.S.)
Consider how such resolution would apply to distance determination. Assuming a bit of edge-blur from the optics of, say, 3 pixels, that would still differentiate over two hundred different sizes/distances. Again, this is with the very least available resolution.
Video is commonly captured at 30 frames per second, so this gives you an idea of the sampling speed. Were a vehicle to be stalled, for example, such sampling would establish a possible problem by the second frame, confirm it by the third, and double check the results by the fourth, activating a braking routine. I would assume that such cameras would also be able to receive track location information as well, perhaps by simple shape or pattern recognition, like a bar code.
But these handy little cameras do even more. They can also serve as a WDM style demultiplexer. WDM stands for “Wavelength Division Multiplexing” and is a method of cramming multiple simultaneous data streams into a single fiber optic cable. As long as each stream has it’s own frequency, (color) the streams can be unscrambled at their destination. In our case different colors could simply mean different things, just like the way traffic lights communicate with red, yellow and green. Since the computer is already equipped to recognize color information from the camera, there can be many input “channels” that can be utilized, all with their own color. Of course this is all still very primitive, especially because 30 frames per second is too slow for meaningful serial communication. Yet between simple shape recognition, color differentiation, and the (painfully slow) serial transmission, this gives an autonomous redundancy/backup capability to vehicles otherwise directed by more complex and capable wireless communication methods. And it’s hard to beat the price!