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From: guest@hpfcrj.UUCP
Newsgroups: sci.electronics
Subject: Re: Higher accuracy Ultrasonic Rangefinding
Message-ID: <3590001@hpfcrj.HP.COM>
Date: Fri, 20-Mar-87 02:22:31 EST
Article-I.D.: hpfcrj.3590001
Posted: Fri Mar 20 02:22:31 1987
Date-Received: Sun, 22-Mar-87 17:14:11 EST
References: <1001@speech1.cs.cmu.edu>
Organization: HP Ft. Collins
Lines: 35

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.


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