Newsgroups: comp.robotics
Path: brunix!news.Brown.EDU!noc.near.net!howland.reston.ans.net!agate!ames!decwrl!netcomsv!netcom.com!nagle
From: nagle@netcom.com (John Nagle)
Subject: Re: Ultrasonic Rangefinders - My personal experience
Message-ID: <nagleC55FJu.M4s@netcom.com>
Organization: NETCOM On-line Communication Services (408 241-9760 guest)
References: <msf.734205713@skaro>
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Date: Thu, 8 Apr 1993 05:07:05 GMT
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msf@skaro.as.arizona.edu (Michael Fulbright) writes:
> I have built a working ultrasonic rangefinder for a few bucks and
>I thought I'd share my experiences. I also have some questions which
>hopefully someone can answer.

      That's nice.  Most people use the Polaroid units, which use the
same transducer for sending and receiving.  That has some advantages
and some disadvantages, the most notable disadvantage being that the
thing has a minimum range of about a foot, because it takes a while
for the transducer to quiet down after sending so it can be used for
receiving.  You don't have that problem.

      Those false signals at 6"-1' are probably due to the fact that
air itself reflects sound.  The usual solution to this problem is to have
a circuit which increases the gain of the receiver as time (and range)
increase, so that for short ranges, the gain is low, and as range
increases, the gain increases too.  TI has a chipset for doing this job,
(TL852/TL853), and that's what Polaroid uses.  I don't know an analog
solution for that problem offhand.  Some kind of scheme with a variable
gain amplifier with gain controlled by an integrator is indicated.
It's possible to use a FET as a voltage-controlled variable resistor,
and that's how remove volume controls usually work.
If you're happy with five feet of range, you might just have two gains, and
something that increases the gain at a range of 2' or so.  If
you go with something as complex as the TI chipset, you can get 40' range.

     There's nothing wrong with using two amplification stages to get a
big gain.  That's how it's usually done.

     Congratulations on getting this to work without an oscilloscope.
That's hard to do.

					John Nagle
