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acoustic interferometry

I did some simple analysis to determine the feasibility of using acoustics for position sensing.

Essentially d = s*t where s is the speed of sound in air and t is the time it takes for the sound wave to travel.

Now given that there are both errors in s and t we have dt*s + t*ds as our error term.

|s| < 1000 ft/s(actually a bit larger but simplifies the math), |t| < 1 ms(corresponds to ~ 1 ft in air)

dt is the precision of the clock which if ran at 1Mhz gives dt = 1us. ds is the precision of the the speed of sound which I'll suppose to be ~1ft/s.

With these estimates dt*s and t*ds ~ 10 mil.

It is easy enough to incease the clock frequency to marginalize dt*s. The real issue is t*ds which involves calculating the speed of sound more accurately(or reducing the distances < 1ft).

But this assumes that the method of transducing is perfect! I have no idea how piezo electrics will hold up. The main issue is one of repeatability as I believe the others can be calibrated out.

I'll worry about the problem of measuring the speed of sound after. (I think by having multiple sensers I might be able to improve the result down to <

1in if I'm lucky. 1/10 in/s will get me sub mill accuracy)

So the real issue is mainly the transducers and I have no idea if they can be precise enough.

My idea is to send a pulse of sound at some freq(higher the better I guess) and start the clock. When that pulse is recieved on the other side(by simple a threshold monitor) it will stop the clock. The problem is that start and stop isn't well defined and it depends mainly on the piezo elements I will be using.

Any ideas?

Thanks, Jon

Reply to
Jon Slaughter
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The threshold scheme will be problematic. You would need a filter to narrow the bandwidth. Too high a Q and the ringing will trip the threshold. Too wide and the scheme will not be very sensitive.

You way want to consider sending a chirp rather than a sine wave packet. Sample the signal and convolve it with the chirp you sent.

Reply to
miso

Use sine waves instead of pulses, and measure phase. The old Loran and Raydist systems did this at low RF frequencies, but it should work with sound, too.

Actually, tone bursts would be interesting... time the envelope for coarse range and phase for fine. Loran sort of works that way.

John

Reply to
John Larkin

I've done this with standard 40KHz, one sender, two receivers 50mm either side, for stereo. It's possible to detect sub-wavelength timings (perhaps 2mm out of a wavelength of 7mm), but you get +-7mm jitter depending on whether the first pulse to return was just too weak, or strong enough, to get the receiver ringing. Eight cycles transmitted with a 20Vp-p swing (use a MAX232) is enough for 6m range, if you have a timed sensitivity curve on your receiver.

With a 100mm receiver spacing, if I could have got reliable 2mm res, that's an angular resolution of a couple of degrees, nice.

As the others have suggested, you need to actually correlate the wave to detect the actual time of return, a simple threshold isn't enough. Though it's possible that having two thresholds would give you enough hysteresis to kill the jitter for the first pulse... Hmmm, didn't think of that.. I might need to revisit that project.

If you want to detect multiple returns, it's hard - it's possible that a weak return arrives out of phase after a strong one, and cancels it, so you need to actually detect unexpected changes in receiver amplitude. I guess if you had a filter that rang the same way (so you could difference it to detect a change), then lock the filter using a CMOS switch somehow to make it track the input for another pulse... probably easier these days to do it digitally however.

The other thing about accurate ultrasound is you need to compensate for temperature (v = 20*sqrt(k) for air, almost exactly). That works out to

342m/s for 20C.

Clifford Heath.

Reply to
Clifford Heath

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