Subject:
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Re: Polaroid Sonar Questions
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Wed, 10 Feb 1999 21:57:55 GMT
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Original-From:
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Will Bain <WILLBAIN@CS.saynotospamUMT.EDU>
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Viewed:
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3049 times
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Nick Taylor wrote:
> I'm looking for help/advice with my sonar setup. • <snip>
> 1. How to deal with "jitter" of about ±1 cm at all ranges. My
> tentative solution is to average 3 or 4 readings.
> 2. How to deal with the occasional (once out of 10 or 20 pings)
> large error. My tentative solution is to discard ranges that
> have changed a "large" amount since the previous reading ...
> faster than the 'bots max velocity ... but what about a
> rapidly approaching predator such as my cat?
I found that returning the median (middle value) of three successive
readings filters out the noisy readings quite well--especially the
occasional large error. Just a couple nested if-then statements are
needed.
> 3. How to determine when the minimum reliable range has been
> reached. Tentative: just discard anything less than 20 cm.
Sure, that sounds about right.
> 4a.Are my time of flight calculations correct? Polaroid implies
> that the speed of sound is about 1.1 feet/sec. My reference
> says 331.6 m/sec at 0°C and approx. 354 m/sec at 20°C (68°F).
> If my grade school math is correct this works out to a sonar
> inch (out and back time) equal to 143.5 usec (287 counts) and
> a sonar centimeter of 56.5 usec (113 counts).
> 4b.Using Dr. Martin's sonar.c to test the 6500 the about figures
> give reasonably accurate readings, but it is necessary to
> subtract about 1000 counts (0.5 msec) from the returned result
> prior to converting to inches or centimeters.
When I calibrated mine, the counts per cm ended up about 143 (maybe
because I live up in the thin air?)
> 5. How best to implement the software to take full advantage of
> the 6500's capabilities?
>
> I intend using Adam Cox's four transducer hookup at:
> http://el.www.media.mit.edu/groups/el/projects/handy-board/software/contrib/adam/mxducr1.gif
There's a lot to be said for simple solutions. The Seattle Robotics
Society web site had a great page about sonar (I don't know if it's
still there). It described overlapping the 10-degree fields of "view"
of a couple of sonar transducers by five degrees to provide a 15-degree
field with 5-degree effective resolution. I plan to follow their lead.
I'll let you all know how it turns out. Have a blast.
--Will
, ,
__@_/ \_@__ |/
| /__, o @_/
)\ ) ( \ (\/\\,
~~~~~~~~~ ' ` ~~~~~~~~~~~ ` ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Wendy Parson, If you steal from one author, it's plagiarism;
Will Bain, if you steal from many, it's research.
& Tatoosh --Wilson Mizner
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Message is in Reply To:
 | | Polaroid Sonar Questions
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| I'm looking for help/advice with my sonar setup. At present I've got a single sonar connected to the new Expansion Board and have been doing a calibration. My goal is to use four transducers to cover a 45° arc. Here are my problems/questions: 1. How (...) (28 years ago, 9-Feb-99, to lugnet.robotics.handyboard)
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