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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