Re: [CAD_CAM_EDM_DRO] Re: Make your own linear scales

On Friday 01 March 2002 09:34 pm, Spehro wrote:

> At 12:52 AM 3/2/02 +0000, you wrote:
> >Pick up the received signal from the microphone and compare it
> >against the transmitted signal. At 40kHz, the wave-lenght is
> >about .025".
>
> Hi, Mariss,
>
> I've been working on some (unrelated) sonar stuff recently. You're
> off by about 10:1 there, wavelength is more like 0.025 *foot*
> (about 1/3").

OK, the speed of sound in air at 70 degrees F is 1129 feet per second. Doing
the math gives a wavelength at 40kHz of 0.3387". BUT! If we fill the tube
with water, the speed of sound goes up to 4856 feet/second which makes our
wavelength 1.4568"... Oops, wrong way... (Brief thoughts of using a partial
vacuum are discarded at this point.)

Well, there are some ultrasound transducers in the Digikey catalog at 215kHz,
but they cost $56 (and we need 4/axis - *) and that only gets us down to
1/16" wavelength. If we can discriminate 1.5 degree phase shifts the basic
resolution would be 0.000262558139534884", not bad...

If cheap high frequency transducers could be found, this may very well work!

Matt

* - Actually we only need one fixed transducer in a single tube, plus one
movable and one fixed.