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

Hi Mariss -

Wouldn't the speed of sound also change depending on its
composition? Would you need a nitrogen/oxygen/etc ratio detector?

Carlos Guillermo
VERVE Engineering & Design

-----Original Message-----
From: mariss92705 [mailto:mariss92705@...]
Sent: Friday, March 01, 2002 7:53 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Re: Make your own linear scales

Hi,

Here's a crazy idea I've thought about but never pursued beyond a
few
experiments to see if it is feasable:

Take two long cylinders (like a double-barreled shotgun). In one
cylinder place fixed ultrasonic transmitter (speaker) at one end,
have a movable ultrasonic receiver (microphone) that can move the
lenght of the tube, linked to an axis.

Drive the transmitter with a 40 kHz signal; this will launch sound
waves down the tube. The tube acts as waveguide so there is no
attenuation of the sound down its lenght.

Pick up the received signal from the microphone and compare it
against the transmitted signal. At 40kHz, the wave-lenght is
about .025". As you move the microphone, a phase detector
(receiver
vs. transmitter) would show a 360 degrees phase shifht for every
.025"
you move. Resolve the phase detector output to about 1.5 degrees
(not
hard to do) and you can read a 0.0001" movement.

Now for the other tube. Have the speaker and microphone fixed at
the
ends a known distance apart. Do the same with the signals as for
the
first tube. This is the reference channel.

The speed of sound changes with temperature, humidity and
barometric
pressure; probably other things as well. These changes will
produce a
phase shift in the reference channel. Servo the transmitter
frequency
(up or down) to keep the reference phase equal to zero (give or
take
a degree). This cancels the change in the speed of sound.