[palish]

Hey, thank you for the critique. It's a rare gift to find someone who's willing to disagree + solid reasoning + references to existing work. Thank you.

In a later post, you mentioned mapping teeth with a toothbrush. That's not a trivial thing to do - without a fixed frame of reference it would not only be very complex mathematically, but also pretty unreliable.

Of course. That is why I'd incorporate a frame of reference into the design of the toothbrush. The point of the post was to quickly paint a picture of what "the future" could be like if I were given time and freedom to pursue these designs. I assumed that if anyone was interested in the details, then they'd simply ask me to provide them.

The toothbrush contains two technologies that enable it to build a map of the surfaces of the teeth you brush. #1: a three-axis gyroscope + accelerometer (e.g. same as an iPhone's). This gets me the rotation of the toothbrush relative to the direction of gravity. Now, of course, this would not be enough by itself; we also need some way of determining the position of the bristles within the mouth. This is accomplished with sensor #2: along the length of the toothbrush near the mouthpiece, there are dozens of tiny light sensors. The more the toothbrush is inserted into the mouth, the less light that reaches the light sensors. This gets you a reading of "how deeply is the toothbrush into the mouth?" which you can easily combine with "what are the current angles of rotation?" to conclude e.g. "the bristles are therefore currently brushing the front surface of your bottom-left-rearmost molar."

Light sensors are easy to work with, but probably not the most reliable way of doing this. I would look into using some kind of sonar sensor, possibly. Or you could do something as simple as "detect which part of the toothbrush that your lips are currently touching"; that might work too.

The goal is straightforward: "to determine an accurate-enough approximation of how deeply the toothbrush is currently inserted into the mouth", because that can be combined with the rotation angles to give you a solid frame of reference.

I of course don't know which technique will prove to be the most pragmatic, since I haven't tested any them yet. =) This is a solvable problem, however.

I've got to run for now, so unfortunately I ran out of time to address your other [excellent] points. But I'm not sure anyone is even reading this discussion anymore anyway. Feel free to post a reply if you'd like to continue, or shoot me an email (see profile). Thanks again!