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Thread: Ultrasonic Sensors
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face BY : Matt Bennett email (remove spam text)



Fred Bailey wrote:
>
> I have found that it is not the capaictance of the sensor but the physical
> diameter of the sensor that determines how long it rings. It's surface flexes
> when the voltage spike is applied. The ringing is electromechanical and not just
> due to capacitance. The echo is the reverse and it will have a similar ring down
> time.
>
> Best Regards, Fred

I'd have to agree with Fred- the ringing of an ultrasonic sensor is a
mechanical result of the excitation voltage.  The large polaroid sensors
rely on this mechanical resonance for a *very* powerful pulse.
Unfortunately, a pulse that rings for a long time will make your
distance resolution poorer than what it could be.  If your transmitter
rings for a long time, so will any response from your target, which
makes it that much harder to distinguish an object.

You can reduce the ringing by applying a load to the transmitting sensor
immediately after you send the pulse out.  In my experiments I tried a
bunch of different resistors, but then found it was most effective to
just short it out.  Since I've used separate transmit and recieve
transducers, I just used a transistor to short out the recieve
transducer so that I could resolve close-in targets.

The polaroid and other ultrasonic sensors use the high initial pules and
measure off the leading edge of the response- which is fine if you just
want a single measurement, but you lose S/N that could be used to
resolve quieter targets.  I built an ultrasonic sonar
<http://web.hazmat.com/~mjb/projects/picsonar/> that gives a continuous
time response.  If you are familar with radar terminology, I can get
what is called an "A-scope" which the response is measured in distance
on the horizontal axis, and amplitude on the vertical axis.  You can
wrap this into a circle by rotating your sensor, and you can get what is
called a "PPI" display (Plan Position Indicator) which is essentially
the traditional radar scope that most people expect.

The biggest problem with ultrasonic sensors is that the angular
resolution is poor.  The small sensors I've been using have a beam width
of roughly 45 degrees between 3dB points.  The Polaroid sensors, with
their large size with respect to the frequency, can get a pretty good
angular resolution, but you lose the ability to resolve small objects
behind a big reflector.  I would like to build a phased array radar, but
all the transducers available are just too big- you need to get the
spacing between the sensors to about .7 of the wavelength for it to be
effective, otherwise, you get *huge* sidelobes which will obscure your
target.  The other method I have rolling around for increasing the
angular resolution is a "synthetic aperture" sonar, which may be
possible with my sonar, since the transducers move in a semi-circle
about a central point.  But this project is on hold, since usually once
I get to a place where I'm pretty sure I could succeed, I lose interest,
and start tilting at the next windmill (a pic-controlled walking robot
at this point).

Mechanically focusing the sonar is a possibility too (with a parabolic
reflector), but that has its own mechanical and time response problems.
I've never gotten to that part of my project.

Matt

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