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'[PIC]:ultrasonic range finder'
2001\04\23@132537 by sam woolf

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I'm a PIC (and electronics) newbie and have just completed my first project.. a sonar rage-finder based on the design at http://www.mindspring.com/~sholmes/robotics/ultrasnd.htm

It's working nicely, but I could do with better range. Farnells sell three sizes of ultrasonic transducers/ receivers, and I'm currently using the smallest. Would using the larger size significantly increase the range? Other than the size difference, the specs for all the transducers look very similar.
Do any other companies in U.K supply better transducers (though still reasonably cheap ones)? (the ones at Maplins look to me like the small Farnell ones, and RS don't seem to do any)
Is there anything else I can try to increase range?

Any advice much appreciated!
Sam Woolf
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2001\04\23@151806 by Lawrence Lile

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Sam, I'm not sure of the answer to getting more range, other than "try the
biggest sensor" which is just a guess.  I think I understood that the old
panasonic sensors had high range, if you can find one surplus, that's a
possible idea.

I've got a related question about ultrasonic rangefinders.  Maybe you can do
a quick experiment with yours, and find out the answer.  Would an ultrasonic
transducer return a different signal (signature) when the pulse was bounced
off concrete, versus grass?  You mowbot freaks will know where I am going
with this.  I'd love to have a "concrete sensor" on my 'bot, so the thing
won't wander out of my yard out into the street.

Sam, if you could simply point your rangefinder at a concrete sidewalk, and
then at some grass (preferably from a vertical angle, close range)  I'd love
to see a scope output (or a sketch) and try to figure out if the differences
are stark enough that a PIC could figure out the difference.


-- Lawrence Lile

P.S.  Been working hard on a Mowbot chassis, got it moving last weekend,
working on the mower head drive this week!


{Original Message removed}

2001\04\24@112545 by rottosen

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Sam:
How to get additional range? Some quick thoughts...

1. Increase the transmitted power.

This can be done with a larger transducer that focusses the ultrasonic
beam better where it is needed.

Or choose a transducer that has a lower impedance (more capacitance).

Driving the transducer with a larger voltage may also work. Some
transducers may be damaged by more than 15 volts while others can accept
a couple of hundred volts.

2. Increase the receiver sensitivity.

Again, a larger transducer with a better focus will help. Use a low
noise preamp on the receiver. This preamp will have to be protected
against the transmitter voltage. If the transmitter and the receiver
transducers are separate then this will not be as much of a problem.

3. A more efficient transducer.

Some transducers may simply have better power conversion efficiencies
when transmitting or receiving.

4. Bias the transducer.

I don't know why, but the Polaroid transducers seem to need a DC bias
voltage to give their best performance.

5. Tune the transmitter or receiver.

A tuned circuit gives better transfer of power to and from the
transducer. This is sometimes done by putting a coil in parallel with
the transducer. The transducers mechanics/acoustics and capacitance
determine the value of the inductor.



Lyle:
Some quick thoughts about the differences between grass and concrete...

The concrete will give a very large reflection because of the large
difference in impedance between it and air.

The grass will give less reflection because its density is closer to
air. The grass will also absorb more of the sound I think.

The concrete will give a very sharp echo because it is a smooth surface.

The grass will give a "fuzzy" echo because of the many surfaces at
different angles and distances from/to the transducer.

I think the concrete will give large echoes when the ultrasound strikes
it at 90 degrees. It will give small echoes at other angles. This is
because it looks like a mirror.

The grass will give about the same size echoes from a wider range of
angles because the blades are at many angles.


-- Rich



Lawrence Lile wrote:
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> {Original Message removed}

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