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Subject: Re: Ultrasonics basics
Date: Fri, 22 Nov 2002 10:26:24 -0000
NNTP-Posting-Date: 22 Nov 2002 10:24:18 GMT
X-Newsreader: Microsoft Outlook Express 5.50.4133.2400
"Kevin Aylward" wrote in message
> Dave wrote:
> > "Kevin Aylward" wrote in message
> > news:S1nD9.144$Me5.email@example.com...
> >> Dave wrote:
> >>> "Dave" wrote in message
> >>> news:firstname.lastname@example.org...
> >>>> Hello all,
> >>>> I want to measure short distances (1 to 2 inches) using reflected
> >>>> ultrasonics (i.e. a Tx bouncing back to an Rx). Using 40KHz
> >>>> transducers the minimum achievable seems to be about 6 inches.
> >>>> Looking at existing circuits the transmitter usually sends a burst
> >>>> of 6 to 10 pulses? What is the reason for this i.e. why not one
> >>>> pulse? The reflected pulse the receiver looks for is surely the
> >>>> reflection of the last pulse transmitted?
> >>>> Is the reason for the 6 inch minimum distance because the
> >>>> transducers "ring" and so swamp any reflected pulses? If so would a
> >>>> higher frequency be the answer?
> >>>> If this is the case are any standard transducers available other
> >>>> than 40KHz? So many questions! Does anyone know the answers please?
> >>>> Thanks in anticipation, Dave.
> >>> Thanks for all the replies.
> >>> I must have my decimal point in the wrong place somewhere.
> >>> Wavelength = velocity / frequency = 330 / 40000 = 8.25mm ?
> >> Seems ok to me.
> >>> Also how would you go about measuring the phase difference? Can
> >>> someone point me towards some reading material please!
> >> I would not bother if I were you. Its just an added complication.
> >> Extracting phase info from a few cycles in a burst is going to get
> >> you into all that digital processing sort of stuff.
> > Kevin, you said previously that the minimum distance achievable is
> > the order of one wavelength. 8.25mm doesn't equate to my 150mm.
> > Dave
> Ahmmm. try posting at the bottom, that way the flow of the conversation
> can be followed!
> The one wavelength is the approximate minimum assuming that one is
> detecting only one cycle. It could be worse for a variety of other
> reasons. As already pointed out, if the transducer has a high Q, it will
> ring for quite a lot of cycles.
> Im wondering now if a typical pizeo (motorola?) speaker used in
> loudspeaker systems will go up to 40khz. I think some might. If so,
> these will be pretty flat in response, so you should be able to drive
> them with only a few pulses.
> Kevin Aylward
> SuperSpice, a very affordable Mixed-Mode
> Windows Simulator with Schematic Capture,
> Waveform Display, FFT's and Filter Design.
Thanks for your help Kevin
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