Keelan,

Its not at all confusing. I briefly explored the possibility of using a
linear CCD array in this manner. I rejected it on two counts.
1. The walls are of wood & will be poor reflectors of light.
2. The walls will be slightly rough, so no guarantees that the light will
return to land on the array. Maybe something could be done using a normal
CCD sensor.

The main problem is the size of the bore (6mm at one end & 12 mm on the
other). 

Joe

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
Keelan Lightfoot
Sent: 16 December 2004 23:49
To: Microcontroller discussion list - Public.
Subject: Re: [EE]:Interesting measurement problem.


Here's my contribution:

On the scanning carriage of previously helical scanning apparatus, place a
small laser diode, or LED, that projects a narrow beam of light towards the
wall of the tube at a 45 degree angle parallel to the axis of the helical
scanning screw. Essentially, imagine standing in a giant tube facing the
opening, and shining a laser pointer in front of yourself, but towards the
'ceiling' at a 45 degree angle.

'In front' of this LED, place a line of photodiodes (probably a linear
array, where they're packed in really close together). In the standing in a
tube analogy, this would be like putting a line of photodiodes in front of
you, in your path towards the opening of the tube:

/ - Path of light
o - photodiode
'-' - carriage

---/oooooooooo---

Given that somewhat confusing description of the apparatus required, here's
how it would work: The closer the wall of the tube is 'vertically' to the
light source, the closer 'horizontally' towards the light source the spot
would appear. A lens would probably be required to focus the light into the
photodiode array. The distance from the light source to the wall of the tube
would correspond linearly to the distance between the light source and the
photodiode that 'sees' the spot of light on the wall. Sensitivity could be
increased by decreasing the angle of the light source relative to the
carriage, and adding a longer line of photodiodes, but this would cause more
problems when nearing the ends of the tube.

- Keelan
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