Or use the much cheaper 2-axis Analog devices accelerometer
ADXL202. PWM output so it's really easy to interface to a PIC.
There is also a Microchip app note on using the device, complete
with source code. You really don't need to know if the head is moving
up and down, just where it's tilted so a 2 axis device is sufficient.

You might also want to look
at piezo gyros (as used in the Gyropoint mouse) to collect
relative rotation information (head spin).

And you might also look at 4 quadrant photodiodes. With a lens
in front of the 2cm square device, you get an output that is
proportional
to imaged beam position. Use an IR laser to point (filtering out
white light at the sensor), and a visible one to show the aim point.

The problem with using a video camera is cost. You need the camera,
and some pretty hefty compute power on the back end to find and
track the moving target.

And how about turning the problem around? Have modulated LED's
at your target points, and a pinhole/telescope photodetector
on the head. Each target can have a different modulation (frequency,
data, whatever), and since LED's are cheaper than photodiodes...

If you projected a TV raster onto the work surface (old B&W with
a cheap fresnel lens), you could use
the photodetector much like the old light pens. Measure the time
between vertical and horizontal sync, and the 'flash' of the
beam imaged by the detector, to get position.
You could probably use an old Nintendo 'shoot 'em up' game
to test the concept easily.

KISS. And do let us know what you finally use.

Robert

Douglas Butler wrote:
>
> Might I suggest an accellerometer package such as a 3 axis Crossbow.  It
> is small and light and costs about US$300.  The user could swirl his/her
> head to turn pages foreware and backward.  Various nods could also be
> decoded.  I think it could be very intuitive.
> Check out www.xbow.com product CXL04M3.
>
> Just my $.02, Sherpa Doug
>
> > -----Original Message-----
> > From: Alan B. Pearce [mailto:A.B.Pearce@RL.AC.UK]
> > Sent: Friday, August 10, 2001 8:59 AM
> > To: PICLIST@MITVMA.MIT.EDU
> > Subject: Re: [EE]: Video Camera as head position indicator - ideas
> > sought
> >
> >
> > >It occurred to me that the availability of low cost video cameras in
> > various
> > >forms (serial port, parallel, USB, separate video ...) should allow a
> > >reasonably low cost solution where the camera monitored a target and
> > >detected head orientation. This could still include a head
> > mounted target
> > or
> > >source but this could be more discrete and if active it
> > could be infrared
> > >and pulsed. While this sounds eminently achievable there would almost
> > >certainly prove to be "more to it" than meets the eye at
> > this stage. Such a
> > >system is notionally more expensive in material cost than a
> > headpointer /
> > >beam system but would be more flexible, more easily adapted
> > to related uses
> > >and less user intrusive.
> >
> > Using a webcam type device looking over the top of the book
> > at the person,
> > how would the following work out?
> >
> > If the person is dark haired this may be easier, but with using IR
> > sensitivity could possibly work with anyone - sense the area
> > and position of
> > the face e.g. look towards ceiling (or straight at camera) as
> > you suggested
> > to "start command" and then rock head to one side or the
> > other to turn page
> > in same direction as head rock. Sense head position by light
> > area of face.
> >
> > If the person has reasonable limb mobility it may be possible
> > to provide
> > additional commands by moving left or right arm after using
> > head movement
> > "start command". Sense it is an arm by long thin object
> > instead of round
> > blob object you would use for face. By seeing if there is a
> > line on the left
> > or right side of the image you may get more commands. If you
> > can distinguish
> > between low lifted arm and high lifted arm you could have two
> > commands per
> > arm.
> >
> > Doing it this way the person would not need to have any
> > headgear at all, let
> > alone be connected to anything.
> >
> > --
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> >
> >
> >
>
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