Hi Philip,
For low rate measurements like a pendulum clock the timestamping method
works well. A number of us do it that way. The same method works for
GPS/1PPS-like signals and also mains (raw 50/60, or divided down to 1) Hz.
Correct, if your measurements are precise enough, you should see rate
modulation due to uneven mechanical effects like slightly unbalanced
hands or gears with uneven wear. You might also see the effects of
winding. The more complicated the clock the more rich the spectral peaks.
You might want to fix the seconds hand so that it's balanced. Most clock
hands are long and thin on one side and short and fat on the other so
that rotational balance is maintained.
Note that both TimeLab and Stable32 work with pendulum data. In
Stable32, you'll find both PSD (Power Spectral Density) and ACF
(Autocorrelation function) very useful when analyzing data from
mechanical clocks.
You asked if anyone else is interested in this level of detail. Yes,
you're not alone. There are active discussions like this right now on
the Synchronome list:
https://groups.io/g/synchronome1/
Also, check out 30 years of wonderful HSN (Horological Science
Newsletter) articles:
http://www.hsn161.com/
Bryan's Microset website is a great resource for clock data and analysis:
https://www.bmumford.com/microset.html
Some nice examples of my own analyzing pendulum data:
http://leapsecond.com/pend/shortt/
http://leapsecond.com/pend/clockb/
http://leapsecond.com/hsn2006/
We tend not to discuss the subject of mechanical clocks and watches here
on time-nuts; mostly because those topics are already well represented
with magazines, journals, books, and multiple web forums or email lists.
But if you have specific questions about performance or measurement
technique that's probably ok. See also the picPET timer chip, which does
sub-microsecond timestamping [1]. And the TAPR TICC, which does
sub-nanosecond timestamping [2].
/tvb
[1] http://leapsecond.com/pic/ and http://leapsecond.com/pic/pp06.htm
[2] https://www.tapr.org/kits_ticc.html
On 11/19/2019 5:10 PM, Philip Gladstone wrote:
I've started to monitor the individual ticks on a grandfather clock from
the 1790s. Essentially I timestamp whenever the pendulum breaks/restores a
light beam.
The data that I get is surprising in that the pendulum swing varies
according to the position of the hands on the clock. It appears that the
amplitude of the swing depends on the driving force imparted by the
escapement. Since the second hand is not counterweighted, there is slightly
more energy available to drive the escapement during the first half of each
minute and slightly less in the second half. There is much bigger effect at
the end of each hour when the mechanism has to move a lever to trigger the
strike mechanism. This 'end of hour' effect changes the pendulum swing
enough so that the period is noticeably affected (maybe by 300ppm)
Anyway, my google-fu did not reveal anybody else interested in this
stuff... Anybody here interested?
Philip
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