Subject:
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Re: sonar (fwd)
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Sat, 23 Nov 1996 10:05:55 GMT
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Original-From:
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John Whitten <brat@naxs.com/stopspammers/>
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Reply-To:
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brat@naxs%nospam%.com
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Viewed:
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1419 times
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Richard Vannoy wrote:
>
> Just to add a basic newbie note on receiver blanking that will have to be designed in
> if you build it yourself.
>
> A transmit pulse is a very powerful (relatively speaking) pulse. For example, the
> Polaroid transducer, about the size of a silver dollar, puts out 400 volts at two
> amps for several milliseconds.
This is true of the POLAROID Ultrasonic Sensor module, but not
necessarily true of all Ultrasonic Sensor modules. It depends on the
type of Ultrasonic Transducer the circuit uses. The one I built uses a
Piezo-Electric transducer available through a number of surplus places,
and outputs a 9VDC pulse. Of course it also needs two transducers, one
to send, the other to receive. The Polaroid unit is based around a
single Capacitive transducer. It uses such a high voltage to bias the
transducer. This allows the unit to send a pulse at full power in its
"on" state, and still detect the return pulse using the leftover "half"
voltage that is biasing the transducer in its "off" state. The
transducer is coupled to the sending and receiving circuitry by means of
a small transformer which allows the two circuits to have relatively
independent access to the transducer. I'm basing this assertion on the
information I have on the now-defunct LM1812 Ultrasonic Chip, and the
Texas Instrauments Sonar Module, which appear to use pretty much the
same setup as the Polaroid unit. I don't have a Polaroid unit and have
never actually had one in person, so I might be wrong.
> If the receiver is left on, the shock of the transmit
> pulse will rattle around the mechanics and the receiver causing all kinds of false
> signals. On transmit, the receiver must be turned off for a short period. Just
> enough time to let the transmitted pulse "get away" from the transmitter, and for any
> mechanical "ringing" in the mechanics to fall off.
This is true. However, there are several ways to deal with this problem.
One is mechanical damping, meaning adding foam or some other "sound
absorbing" material around the receiver. This helps cut down on induced
ringing. Of course, in the case of the Polaroid unit, this would be
difficult since the transmitter and receiver are the same. Also, it is
not totally effective, at best it can only reduce the ringing. Another
way to handle this is if you're using a microprocessor to detect the
return pulse is you can simply "ignore" the receiver for a short period
of time while the ringing dies down. In an analog detection scheme, you
would have to add some sort of delay-timing to the detector, or to turn
it off momentarily as you suggested.
> The blanking signal turns the
> reciever back on a few milliseconds later, so the first echo (hopefully) will be of
> an external target, and not residue of the transmit pulse. Polaroid has receiver
> blanking built in, and steps up the gain of the receiver every few feet to compensate
> for the rapid attenuation of the returned signal. As an ex-sonar technician, I STILL
> don't want to design all that from scratch, therefore $50 per unit for Polaroid seems
> like a great bargain to me unless the hobbyist really gets off on building from
> scratch.
Well, I'm NOT a sonar technician, and I suspect I could learn a thing or
two from you, but I do have complete schematics and a nice write-up on
how it all works available on my page:
http://www.vnet.net/wizorg/wizorg.html. Its not as sophisticated as the
Polaroid unit, nor has quite as much range, but it is reliable and best
of all CHEAP! Feel free to give it a look and let me know if you see
anything that could be improved, I'd like to know about it. I do know
that I could add gain-ramping, but I didn't in this case to keep the
circuit simple and affordable. I plan to experiment with that in my next
go-round.
> > Basic idea behind Sonar is this: Send a pulse with the transducer. The
> > sound pulse travels across the room, hits an object, and reflects back (and
> > everywhere else). The time that it between when you sent the pulse and
> > when you recieved a return pulse is twice the time that it takes sound to
> > travel to the nearest object in the line of the pulse.--
To include some numbers for anyone else who might be reading this and
wondering, sound travels approximately 1065 feet or so in air at
sea-level. This will be affected some by the altitude, temperature,
humidity and barometric pressure. But that's a good enough number to use
for hobbiest work. This means that a pulse sent from an Ultrasonic
Transducer travels at about 12780 inches per second from the transmitter
out to an object and back. Remember to DIVIDE the total time by 2 to get
the one-way distance. So that means if you use a range-calculation
clocking frequency of 12780 Hz, you'll get 1 inch per clock tick. You
can skip the math if you like by dividing the clock in half, or 6390 Hz,
to get the ONE-WAY distance in terms of 1 inch per tick.
> Richard T. Vannoy II richardv@abac.com
> PO Box 103 Computer Programmer
> La Mesa, CA 91944-0103 College Electronics Instructor\
Ok Richard, now I have a question for you... in my setup I'm currently
using two small Murata transducers. They're a matched pair, one to
transmit, the other to receive. I'm currently getting somewhere around
10 feet or so but was wondering if I could reliably get more. Here's my
question: would it be helpful if I "ganged" multiple transmitters and/or
receivers together to make a "larger" array? What would the diffulties
in doing that be (in terms of synchronization and so forth) be and would
it be worth trying out? Also, do you know of any good online sources for
more in-depth information, like what the roll-off is in distance versus
db for the receiver?
And now a general question for anyone, does anyone know of a source for
the "capacitive tranducers" that Polaroid uses in its module?
John Whitten
brat@naxs.com
--
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Message has 1 Reply:
 | | Re: sonar (fwd)
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| (...) My 20 years of navy sonar help me somewhat with active/passive transducers and things in the kilowatt range... :-) but I'm a rank newbie to most of the sonar talked about here. Sounds like Piezo would be very cheap. True? I bought two (...) (28 years ago, 23-Nov-96, to lugnet.robotics.handyboard)
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Message is in Reply To:
| | Re: sonar (fwd)
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| Just to add a basic newbie note on receiver blanking that will have to be designed in if you build it yourself. A transmit pulse is a very powerful (relatively speaking) pulse. For example, the Polaroid transducer, about the size of a silver dollar, (...) (28 years ago, 18-Nov-96, to lugnet.robotics.handyboard)
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