I remembered the company's name http://www.mimio.com/ .  The technology
section describes the device:
http://www.mimio.com/meet/technology.html

And says, in part,
"mimio uses a high-resolution ultrasonic position capture system
consisting of a capture bar, color-coded marker sleeves and an
electronic eraser. The capture bar is a two-foot long ultrasonic
tracking array positioned along the upper left edge of the whiteboard or
flip chart. The capture bar connects to a personal computer - either PC
or Macintosh - through either a serial or USB interface cable.

The electronic marker sleeves transmit an ultrasonic signal to the
capture bar, which triangulates the pen's position on the board as the
user writes. The only change users must make is to be sure they use the
electronic eraser to make corrections, since mimio can't capture changes
made with a standard eraser or with one's fingers."

So I was wrong in stating that the ultrasonic waves travelled on the
writing surface, they just go through the air.

However, since the positioning is relative then I don't see how speed of
sound variations would make any significant difference - it would affect
the signal equally relative to all the transducers, and at this high
frequency I suspect that air temperature affects it even less.

I don't know how the notepad clip on works, it could be really only be
light, sound, or radio.  Out of those three, given the size of the pen
and its power supply I doubt sound would really work out well.  It would
be nice to get a close up of the pieces to see if there are any ir
windows on the pen or capture device.

I wonder if you could design a passive pen which bounces rf or em
signals from the clip back to itself.  Wacom uses this for their
tablets, but the range is only an inch or three above the writing
surface, not at one edge.

Would be neat to look up the patent for it.

-Adam

Brendan Moran wrote:

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>
>
>>Another company has made a similar product for some time geared
>>towards white boards.  They transmit an ultrasonic frequency
>>through the pen tip, which travels across the surface and a rather
>>basic circuit
>>determines the phase difference and can locate the tip.
>>
>>My first thought was a 2-axis accelerometer near the tip, but the
>>clip needing to be attached to the writing surface makes me think
>>it's
>>similar to the whiteboard solution above.
>>
>>It would be fun to make a simple system like this from scratch.
>>
>>
>
>Ultra-sound likely won't work for you, due to the speed of sound
>changing in different pressures/temperatures as well as minor
>variations due to atmospheric composition.  I think the IR system
>that was proposed is likely the best.  However, measureing position
>from the speed of light is not really a practical solution.  So
>you'll need two high-res IR recievers that don't need to move to give
>you an angle that they're recieving the signal on.
>
>Btw, I want to see schematics for that sucker when it's done :)
>
>- --Brendan
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