From the FAQ:
Q: We’ve assembled everything and it works pretty well except the infrared trackers are tracking beacon reflections off of reflective surfaces. What do we do?
A: You think like an engineer and find a creative way to solve the problem. Didn’t you see the flawless demonstration performed at the kick-off? If so, you know it can be done.
Q: Okay, we’re four weeks into the build and we’re spazing because we’re too wired on Mountain Dew, not getting enough sleep and we really don’t need flippant answers like the one above. So what’s the deal with reflections? Don’t make us come over there and steal the plastic pink flamingos from your front yard!
A: <sigh> Okay, If the sensors rotate in a plane that includes the beacon, the tracker will work just fine if it can’t see (reflections) above or below that plane. Our original prototype looked like a side-by-side shotgun mounted to the servo. Imagine looking through a long, dark piece of tube while you slowly spin around a dark room that also has a friend holding an LED flashlight at the same height as the tube. As you spin around, what are the chances that you’ll see a reflection off of any surface? Pretty slim because the point of reflection must also reside on that idealized plane. The chances are also made even slimmer when you realize that not only does the point of origin have to reside on the plane, but the photon must also pass through one specific spot on the plane: your eye.
We found this setup to work pretty well until the servo had to try to keep up with a moving target. So we had a minor epiphany and realized that if we cut a slit down the tubes such that one of the sensors would always have a view of the beacon, we would always know which direction to move the servo to catch up.
Q: I still don’t think it’s going to work because when I fire-up the beacon in my bedroom, the tracker still sees reflections from the walls, the carpet, and my chihuahua.
A: Okay, I know it’s convenient to build and test gizmos in one’s bedroom, and instead of driving out to the desert, I would love to bring home the odd prototype Mars rover for testing in my bedroom, where I’m near an ample supply of Diet Pepsi and tools. Sure, it makes a lot of sense to design and initially test stuff in the lab (or bedroom), but before you make a final judgment on how well your gizmo works, you’ve got to test it in it’s intended environment, which, in this case, is a FIRST robotics playing field. Here are a few of the differences:
- When your bedroom was built, I’ll bet that little thought was put into the suppression of infrared beacon energy. Update: Okay, so a few of you do have infrared energy-suppressing bedrooms. Please stop e-mailing me because you’re creeping me out.
- The ambient light level is too low for the amount of infrared energy that’s being transmitted. The infrared sensor has a built-in AGC (Automatic Gain Circuit) that greatly amplifies the output of the infrared photo diode when the noise level (ambient light) is low. With the gain on the amplifier cranked-up, the sensor is going to be able to sense (and react to) the faintest reflection. Add to this an infrared beacon designed to blast enough energy to fill a full-size 48’ x 24’ playing field and it shouldn’t be any wonder the the tracker tracks your chihuahua. Either reduce the amount of transmitted energy with a filter, increasing the value of the current-limiting resistors, etc. or increase the amount of ambient light in the room. As an example of just how well these sensors can detect the infrared beacon, while testing the receiver code, I setup a beacon LED next to a very bright flashlight and had no problem tracking it from a foot away or across the room. For extra credit, at night, fire-up the beacon in your bedroom after turning-off the lights, duct-taping the cracks around the door(s), etc. Pretty dark, right? Now, turn-on a camcorder or digital camera that has a LCD viewfinder. Unless, your camera has a built-in infrared filter, you should see the room through the viewfinder as thought it were dimly lit. This is how the sensor “sees” your room.
- Your bedroom probably isn’t the size of the Georgia Dome. The chances of a reflection making its way back to the sensor with enough energy to be detected, is reduced in large spaces because of the inverse square law of light brightness, which states that the amount of light energy deposited on a fixed area is inversely proportional to the square of the distance traveled. As an example, the reflection off the bulb in your bedrooms table lamp has on the order of one-hundred times more energy than one returned from the bulb in the fixture on the ceiling of the Georgia Dome.
Q: We noticed that the sensor can sometimes detect the beacon even when the little window isn’t pointed at it. Is this a problem?
A: Yes, I can see where you might think the little hemisphere window is where the infrared light must enter to be detected. Actually, the entire epoxy package, that the sensor is embedded within, is transparent to infrared light. I’ve noticed that unless I shield everything except that window, the sensor will detect the beacon from just about any angle. This is why you’ll see the sensor in my example tracking assembly is covered by dark gray heat shrink tubing.