Russell,

Prepare a series of sensors, one for each desired choice.
Each sensor consists of:

  1) a container or isolation shell that is at least 1" deep.
     the purpose of the isolation shell is to isolate each
     sensor so that it only "sees" the laser beam when it
     is aimed directly at that target.

  2) The front of each sensor shell consists of a translucent
     window. Something like translucent drafting material
     works well. When light from the laser pointer hits
     the target window, the translucent target will diffuse
     the light to some extent. The target window should be
     large enough to allow the user to easily keep the laser beam
     on the target for about a second.

  3) a convex lens positioned such that it gathers light from
     the entire window area and focuses it onto a photo detector
     sensitive to the laser beam.

  4) an optional narrow band optical filter that passes only
     the laser beam. This enhances the sensitivity of the device
     to the laser beam and prevents false detections. The filter
     can be quite small if it is placed just in front of
     the sensor.

  5) an optional red LED positioned such that light from the LED
     falls on the sensor, but does not obscure the light from the
     target window. A means must be provided for adjusting the
     intensity of the LED. This LED light is used to "bias" the
     sensor into its active range. It improves sensitivity.

  6) a sensor that is good at detecting the laser beam. You may
     have to experiment around to determine which sensors work
     best.

There are several ways of using these detector modules.

METHOD 1
  Have the sensor incorporated in a circuit that produces an
  output voltage that produces two states. One state indicates
  that the light level is significantly above ambient, and the
  other state indicates that the light level is at or below
  ambient. An op-amp or comparator and a potentiometer are
  all that you really need.

  A selection is made by keeping the beam on the target
  for a short period of time.

METHOD 2
  Using an electrolytic capacitor and a resistor, monitor the
  CHANGES in output voltage. Note that there will be changes
  when lights are turned on/off, or when sunlight through a
  window changes. Avoid having direct sunlight hit the sensor
  array.

  To make a selection with this setup, move the beam onto and
  off of the desired sensor a couple of times. Software looks
  at the changes and declares a hit when there are several
  rapid changes.

Method 1 is more intuitive for the user. Visual feedback of beam
detection and choice acceptance can be indicated with two LEDs
at each sensor. Red indicates beam is detected, and green indicates
that the choice has been accepted.

Fr. Tom McGahee


----- Original Message -----
From: Russell McMahon <apptech@CLEAR.NET.NZ>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Friday, August 10, 2001 8:31 AM
Subject: [EE]: Video Camera as head position indicator - ideas sought


> I have been asked to investigate providing a disabled user with the means
to
> operate an existing book-page turner.
> This device usually uses a suck / blow switch with two levels (hard /
soft)
> of suck and blow providing 4 commands.
> The user has Multiple Sclerosis and is unable to reliably use the
suck/blow
> system
> Other users would also have similar needs.
> A complication is that minimal funding is available. This is not
necessarily
> a limitation but a lower cost solution would be nice.
>
> The user has reasonable head position control and it was suggested that I
> could provide a light beam pointing system where the user pointed a head
> mounted light beam at 4 optical receptors mounted above the page turner.
> This should be a reasonably straight forward solution:
> - Use a laser pointer
> - Possibly modified to provide beam modulation.although this should not
bee
> necessary at such short range.
> - Optical saturation from ambient light should be easy to avoid in this
> application.
> - Provision of a timed "beam on" period with turn on enabled by eg looking
> upwards at a greater than usual angle would remove the otherwise annoying
> red spot on the reading material :-)
>
> HOWEVER
>
> 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.
> A rudimentary camera based defined-object location system could have a
range
> of other uses in disability aid applications.
>
> On reflection (no pun intended) it might be easier to use the camera to
> detect where a beam falls on a target than to look at the user and attempt
> to determine where the beam is pointing. The "target" could be a wall or
> screen above the page turner. The user need not necessarily be able to see
> the beam. While this is effectively just replacing optical detectors with
a
> single wide area detector the above points re flexibility etc still apply.
>
> Has anyone had experience of similar systems and / or can offer useful
> suggestions that may help reduce design time.
>
>
>
> regards
>
>         Russell McMahon
>
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