> From: TONY NIXON 54964 <Anthony.nixon@eng.monash.edu.au>
>
> I need to measure a distance from a 'sensor' to a wall which will
> have a rough surface. The measuring distance should not be any more
> than 5 inches. The PIC will be used to control the 'sensors' movement
> across the wall and to gather the distance data, and thus I should be
> able to map the surface contour.
>
> It would seem a reasonable problem except that the wall will be under
> murky water. Also the wall may have mud sticking to it at places so this
> means I could not have a sensor which actually touched the wall. If
> this happened then mud may attach itself to the sensor giving me
> inaccurate data.
>
> Any leads on a type of sensor would be appreciated.

Hey - I need one of these to clear the roots from my blocked drains!
Are you going to attach a saw to it?

Depending on the particle size, IR might be able to pierce the gloom.
How about some sort of triangulation arrangement such as situating an
IR source (laser) and photodiode at different points on the Z axis of
the device.  The detector has a small aperture so that it only 'sees'
the laser spot at one point:


   Z--->    det           laser
             |              |
           --|--aperture  scanner
             |             /
             |            /
             |           /
             |          /
             |         /
             |        /
             |       /  <------>
             |      /
             |     /
             |    /
             |   /
             |  /
             | /
             |/
        =============================== wall


The scanner could be a galvo mirror or rotating prism.  If the PIC knows
the angle of the beam from the Z axis (theta) and can pick up the 'blips'
from the detector, then it can calculate the distance of the wall as
distance(laser-detector) * tan(theta).

Regards,
SJH
Canberra, Australia