Let's be serious here. Radar is the wrong part of the E-M
spectrum. Use light. It is cheap, easy to detect, and
there are plenty of reflections to go around.
Or better still, listen to the tic. Then you don't even
have to open the case.... Which brings this full circle.
-Chuck Harris
Jim Lux wrote:
On 12/14/15 9:12 PM, ed breya wrote:
This may be totally ridiculous, but maybe there's another way to get a
balance wheel signal. The X-band Doppler type microwave motion detectors
can pick up various object signals in free air from quite a distance, so
maybe up close there would be enough resolution and penetration of the
metal parts of a timepiece to get a usable signal in and out. It would
tend to accentuate the fastest part of any motion - the balance wheel in
this case. I can picture setting one up with the horn pointed at the
thinnest part, likely the watch face, from maybe a few inches away - or
whatever it takes to not overload the detector. The audio detector
signal (if sufficient) could then be processed in the same way as with a
microphone sound signal.
As it happens, I have a fair amount of recent experience detecting small (<1mm)
motions using radar.
Yes, remarkably tiny holes will let enough signal in and out, but, it's going
to be
very, very position dependent. You have a lot of multipath in this kind of
testing,
and it's easy to wind up in a null zone.
You might want to look for K-band (24GHz) units: the shorter the wavelength,
the more
phase shift you get from the tiny motion. To put some numbers on it: at 3 GHz,
a 1mm
displacement gives you about 6-7 degrees; at 24 GHz, you're going to be getting
50-60
degrees.
You'll be wanting some form of homodyne detector (which has the nice property
that
the phase noise of the source cancels out, so you can have a pretty grungy
quality
oscillator). The signal you're looking for, though, is phase shifts occurring
at a
1Hz kind of rate. Most of the cheap "motion detectors" have a high pass filter
(1
m/sec at 3 cm wavelength is 66 Hz) and the amplifier chain is AC coupled.
You'll need a good low noise amplifier with a low 1/f knee.
For reference, a receiver gain of about 60 dB gives you a millivolt kind of
signal
from a 1mm displacement with 1mW at 3GHz from a 0.1 square meter target at 10
meter
distance. You can scale to your situation.
You'll probably want some way to subtract out the static baseline, so your high
gain
amplifier stages don't need enormous dynamic range. In my radars, I do this
with an
adjustable "leakage" path from Tx to Rx. You could probably do it with a
movable
metal target next to your clock/watch and you adjust it for a null.
You probably also want a I/Q output: if you think about the signal you're
receiving,
it's a slowly moving vector that spans a fairly small phase angle (because it
combines a very large static response from stuff that's moving plus a little
tiny
moving component). If that vector happens to point at 90 degrees to your I axis
only,
then you're great: the variation shows up in the I axis. But if the vector
happens to
point parallel to the I axis, the motion is very small.
With I/Q, you can either do a arctan demodulation, or you can rotate the
signal to
make the variation largest (basically using the sin x=x approximation for small
x)
Ed
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