Subject:
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Sonar questions (technical ones!)
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Wed, 25 Feb 1998 21:28:05 GMT
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Original-From:
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Skye Legon <slegon@SAYNOTOSPAMwatfast.uwaterloo.ca>
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Viewed:
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1405 times
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Hello everyone,
A few sonar questions (other than where to buy them):
1: The speed of sound has been widely quoted to be 0.9 ft/ms (274m/s),
but my Physics text lists it as 343 m/s (at 20C/70F), which is about
1.13 ft/ms, or 25% faster. So why the 0.9 value (or just cuz
the Polaroid guys say so)?
2: The sonar routines (e.g. sonar_sample()) return the distance as an
int, meaning the greatest round-trip distance possible is 32768
half-microsecs, or 8192 microsecs one-way. By my math, 8.2ms is
about 3m (10 ft). Is this our max distance? Any way to improve
this?
3: My sonar works well, but about 10% of the time it returns a spurious
low value (around 2200 counts/8 inches), even if nothing has moved
or changed in the environment. I have written a kludge to simply
re-pulse if it receives such a result, which works fine, but it's
ugly and I'm just treating the symptom, not the cause. Any ideas
why this is? (I'm using sonar_closeup(), BTW)
4: I'm confused by the guts of the sonar routines. The start_time is
read from the system TCNT register as a 16-bit integer (max 32768
counts). If TCNT increments 2,000,000 counts/sec, does this mean
TCNT rolls over every 16ms? If so, it seems quite likely that the
counter will roll over while we're waiting for a pulse to return
depending on when we peek it for the start_time. However, the
routines work fine, so can someone please clear up my
misunderstanding?
Cheers! Skye.
+-----------------------------------------+----------------------------+
| Skye Legon | University of Waterloo |
| Systems Design Engineering | __/ __/ __/ __/ |
| Pattern Analysis & Machine Intelligence | __/ __/ __/ __/ __/ |
| 143 Columbia St. West, Unit E-4 | __/ __/ __/ ____/ __/ |
| Waterloo Ontario CANADA N2L 3L2 | __/__/__/ __/__/\__/__/ |
| +1(519)888-9249 | ______/ ___/ \___/ |
| slegon@uwaterloo.ca | DC 2620, 888-4567 x5192 |
+-----------------------------------------+----------------------------+
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Message has 2 Replies: | | Re: Sonar questions (technical ones!)
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| My sonar seems to be pretty accurate. I would notice a 25% difference. ??? Check your calculations. Maybe you missed something. ---...--- ericson mar Master of Engineering Candidate Project: Mobile Robotics mar@cooper.edu (212)353-4356 Department of (...) (27 years ago, 26-Feb-98, to lugnet.robotics.handyboard)
| | | Re: Sonar questions (technical ones!)
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| Good questions!! (...) The polaroid number/widely quoted number is 0.9 MILLISECONDS PER FOOT -- or 1.11 ft/ms. hence the conversion matches! I think also I have this wrong on my HB sonar page! This confused me for a while too. (...) Yes, this is (...) (27 years ago, 26-Feb-98, to lugnet.robotics.handyboard)
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