[time-nuts] Sensing pendulum position, speed, or height

Tom Van Baak tvb at LeapSecond.com
Thu May 31 20:02:48 EDT 2007


>>     Lunar/Solar Tides and Pendulum Clocks (part 1)
>>     http://www.leapsecond.com/hsn2006/ch1.htm 
> 
> Nice.  Thanks.  Did you ever write chapter 2?

In progress. I'll let you know when it's done.

>...
> Or a digital camera: take a sequence of pictures and interpolate as to when 
> it crosses the center.
> 
> The hardware used for bar code scanners might be a useful starting place.  I 
> assume you would have to hack the firmware/whatever to output time/position 
> info rather than bar code data.

You have lots of good ideas; I'm sure some been tried but I
don't have references for you. From what I remember many
modern attempts at super pendulum clocks end up using
optical sensors.

But you can save weeks of prototype work with a few minutes
of calculation. For example, given the length of the pendulum
and the half-angle amplitude you can calculate the velocity at
center swing. Then knowing the geometry of the optical gate
gives you the rise-time. If the IR emitter/transistor pair have a
circular aperture instead of slit then you're getting a slower
rise time as the pendulum crosses the circular beam, etc.

One could do a series of experiments to obtain the most
precise signal. Or the most stable swing-to-swing samples.
But that's only part of the problem...

I would also worry about long-term stability of any optical
emitter and sensor pair. Any decline in light output over
time might appear in the data as a phase shift which could
affect amplitude or period stability. Then there's any tempco
or voltco issues to consider with optoelectronic elements.

By the way, a really nice technical paper on a precision
pendulum clock is this 1996 paper by De Marchi:

A Measurement of the Period Stability of a Free Pendulum
http://www.leapsecond.com/history/1996-DeMarchi-Pendulum-Stability.pdf

Clever solution. His optical gap is something like 5 microns.

/tvb




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