Subject:
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Re: Ultrasonic sensors - comparison & performance?
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Newsgroups:
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lugnet.robotics
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Date:
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Tue, 6 Apr 2004 21:32:50 GMT
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Viewed:
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1557 times
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In lugnet.robotics, Brian Davis wrote:
> John Barnes wrote:
>
> > Air movement at right angles to the beam will cause
> > the beam path out and back to have to be curved, increasing
> > tha apparent distance with increasing airspeed. (yes you can
> > make air movement sensors using this kind of technology :)
>
> How sensitive can you get this way? Specificly I'm thinking of having the
> sensor looking down a long "hallway" with airflow directed toward or away from
> the sensor (this too should result in a differing time-of-flight, yes?).
Yes, this should work, but you won't be able to measure the direction of the
wind, just it's velocity. But then this isn't different from the cross-wind
situation either.
Remember velocity=distance/time (v=d/t). The way to measure wind speed is to fix
the distance (d) to a known value. Solve for t to get t=d/v. Let Vs = speed of
sound and Vw = speed of wind. Going in one direction you'll have v = Vs + Vw,
the other you'll have v = Vs - Vw. The total time equals the sum of the time in
both directions.
t = d/(Vs+Vw) + d/(Vs-Vw)
= 2*d*Vs/(Vs^2-Vw^2) // with a bit of algebra
Google says the speed of sound is 340.29 m/s in air (probably at sea level, and
some specific temperature like 20C), so you can plug that in, plus your distance
which is fixed.
Measure your time to get the signal back, and solving for Vw you get
Vw^2 = Vs^2 - 2*d*s/t
Punch in your numbers, take the square root, and you'll have your answer. This
also shows why you won't be able to tell the direction - the square root could
be plus or minus. But it should be obvious as you are measuring the return time
for a signal to travel the distance, and either way this time will be the same -
one direction it goes with the wind, the other against the wind. You just don't
know which is first.
Now the only problem you have with John's sensor is that he's made it for what
would be most useful for Lego robots -- it returns a 'distance' measurement, not
a time measurement. But with a generic ultrasound sensor you'll get the time for
a signal to return. You could probably fiddle with the sensor John built to get
the time from it. That's a bit more work -- an exercise for the reader :-)
--
David Schilling
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Message is in Reply To:
 | | Re: Ultrasonic sensors - comparison & performance?
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| (...) How sensitive can you get this way? Specificly I'm thinking of having the sensor looking down a long "hallway" with airflow directed toward or away from the sensor (this too should result in a differing time-of-flight, yes?). (21 years ago, 6-Apr-04, to lugnet.robotics)
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