phd@speech1.cs.cmu.edu.UUCP (03/15/87)
Does anybody out there know what the fundamental accuracy limitations are for Ultrasonic rangefinders? I would guess that air temperature and pressure changes would severly limit performance. A previous post mentioned the severe ringing of most ultrasonic transducers. For single transducer systems, this is especially troublesome, since one must wait for much of the ringing to die out before looking for an echo. Does anyone try to actively damp this ringing? Most of the detection circuits I've seen (i.e. National's LM1812) just use some sharp filter to look for the echo. Since the signal is rather slow, wouldn't it be better to perform some sort of cross-correlation between the transmit and receive signals. This way, a somewhat more random looking signal could be used in noisy environments. (Is this what the Polaroid system does?) Intuitively, I would expect the cross-correlation to give more accurate results, if one could get a transducer to do more than just oscillate sinusoidally at its resonant frequency. Has anyone tried any of these things? --- Paul Dietz Dept. of Electrical and Computer Engineering Carnegie-Mellon University Pittsburgh (America's MOST livable city!), PA phd@speech1.cs.cmu.edu.ARPA Disclaimer: My employer takes no responsibility since I'm unemployed.
guest@hpfcrj.UUCP (03/20/87)
Response to ultrasonic transduser accuracy limits:
The most serious limiters of accuracy in ultrasonic rangefining are WIND
(or any type of air motion of which there is a lot) and temperature. The
speed of sound in air does not depend on air pressure as it is assosicated
only with the thermal velocity of the molecules not their mean free paths.
Most ultrasonic transducers are highly resonant so that significant sound
pressure levels may be generated with low voltage low power drivers.
The result of course is that they ring a lot and the signal cannot be
modulated at frequencys higher than the resonant frequency, virtually
eliminating the possibility of signal encoding for autocorrelation.
To use auto-correlation a wide bandwith transmitter and reciever are
nessisary, the best method of "modulation" is to use matched SAW devices to
convert pulses to widband signals for transmittion and widband signals
to pulses for reception.
The physical size of the transducer is important too as the directionallity
of the signal (transmitt or recieve) is determined largly by the diffraction
limits assosiated with the size and cohearancy of the device.
The polaroid transducers are relativly wide bandwidth and require a
large voltage drive (say 100 V) for many applications a more highly tuned
transducer is better like the Projects Unlimited SQ-40's or the MASSA TR-89's
these transducers produce resonable sound pressure levels with only 5V drive.
As far as damping is concerned the transducers are inherently high impedance
and do not respond well to passive damping, I have tried active damping with
some success but it must be carfully controlled.
Hewlett-Packard
3404 E. Harmony Rd.
Ft. Collins Co.
303-339-2384ornitz@kodak.UUCP (03/24/87)
In article <3590001@hpfcrj.HP.COM> guest@hpfcrj.HP.COM (guest) writes: >Response to ultrasonic transduser accuracy limits: >The most serious limiters of accuracy in ultrasonic rangefining are WIND >(or any type of air motion of which there is a lot) and temperature. The >speed of sound in air does not depend on air pressure as it is assosicated >only with the thermal velocity of the molecules not their mean free paths. This statement is a good approximation for air at normal atmospheric conditions (i.e. temperatures and pressures normally found on the surface of the earth). It also is based on an ideal gas and the assumptions that the velocity of the medium is small (no wind) and the sound is a simple harmonic compression wave. Large disturbances such as shock waves propagate at much higher velocities [Hirschfelder, Curtiss & Bird: Molecular Theory of Gases and Liquids]. With these assumptions in mind, the additional major cause of error is the assumption that the composition of air is constant. This is not the case with changes in humidity. The Handbook of Chemistry and Physics gives equations for the velocity of low-frequency sound waves with humid air with varying temperatures. Barry