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

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu May 31 20:57:49 EDT 2007

Tom Van Baak wrote:
>> 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.

The usual solution to this is to use analog circuitry to calculate 
(I1-I2)/(I1+I2) rather than just (I1-I2) although the location of the 
point of equality (of I1 and I2) is independent of I1 and I2 provided 
that the responses of the 2 sensors track. I1 is the photocurrent from 
sensor 1, I2 is the photocurrent from sensor 2.
> 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.
Suitable spheres are available from edmund optics.
> /tvb

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