At 04:58 PM 11/11/97 -0800, you wrote:
>At 07:16 PM 11/11/97 -0500, Sean Breheny wrote:
>>I have
>>two questions, what type of maximum range do you want this to handle and
>>what will be the accoustics of the areas where you envision it being used?
>>This is tough to implement >10meters range when indoors because stray
>>reflections are detected.
>
>Sorry I forgot to actually anser your questions in my first reply... :-)
>
>As I initially envisioned this, it was a tool for model robotics.  As such, I
> imagined ranges of a few inches to about 10 feet maximum, and primarily
indoor
> usage.  Some PICLISTers have suggested other uses, such as measuring
stock in
> grain silos or water or oil tanks.  In those cases, it would be ranges from
> several feet to several dozen yards.
>
>By placing the transducers inside of pipes or cylinders lined with some
> sound-absorbing substance (like velvet or some other cloth) we should be
able
> to get good directivity, and with different resistor values in the op-amp
> circuits, we should be able to tweak the gain to accomodate the longer
> distances.  I have been planning on 8 bits of granularity in the
measurements,
> scaled over whatever time period is appropriate.  For robotics work, the
scale
> would obviously be smaller than for the grain silo application....
>
>Because PICs are so easy to reprogram, it makes sense to have several
"standard"
> Ranging Unit programs available which scale the 256 possible output
values over
> various distances.  End users could simply pick whatever range was most
> appropriate for their application.
>
>What do you think?
>
> - Rick
>---

My design was originally for use in physics demonstrations or velocity and
acceleration, so my goal was resolution of about 3 cm or so over about 7
meters, about the same as your robotics idea. I have worked on the design
of a similar project before, and we then experienced significant difficulty
in working over 15 feet reflected distances (30 feet total round trip) when
indoors, presumably due to reflections. The other guy who was the main
designer of the project (who is also on this list, Louis Marquette), has
since tried placing the transducers in cylinders and I think that he had
good success.

Previously, you said that you wanted to avoid using an AGC. Theoretically,
if we design the comparator/clipper circuit well enough and provide enough
gain, it should respond equally well to weak signals and strong ones. You
may not need to adjust the gain for longer distances. The main thing you
need to do is adjust the timing and guard against the multipath.

You might also have difficulty with getting the circuit to work with very
close ranges, like several inches, for a few reasons:
 1.) The transmitting transducer as well as the whole device will vibrate
during the TX period. They will continue to vibrate slightly at 40KHz for a
few microseconds or so after the TX time. If you start listening for an
echo  soon enough to hear one which comes from only a few inches away, you
will most likely hear the vibrations instead of the echo.
2.) The two transducers are probably separated by a few centimeters on the
device. This means that the signal path does not go straight out, but
instead must bounce off at an angle and return, forming an equilateral
triangle, not a straight line. Since the wavefront spreads out over long
distances, and the length of the path becomes long compared to the distance
between transducers, this effect is negligible at several feet. But I
imagine that it may become pronounced at very short distances.

I have also seen in several places recommendations that a specific pattern
of pulses be used rather than a striaght 40KHz burst. This allows you to
throw out noisy readings because the return pulses will not fit the
expected pattern. I imagine that the pattern must be carefully designed so
that it is still recognizable after being passed through the transducers,
which act effectively as a narrow 40KHz band pass filter.

Sean



> You can help design a Serial Sonar Unit for model robots!
>  Vist http://www.notesguy.com/notesguy for details, or
>   send mail to Rick Dickinson at rtd@notesguy.com
>
Sean Breheny,KA3YXM
Electrical Engineering Student