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

Bill Hawkins bill at iaxs.net
Thu Mar 29 23:51:11 EDT 2007


IMHO, gluing magnets to moving parts makes them susceptible to magnetic 
fields, such as variations in the Earth's field at 10E-7, as TVB pointed out.

Had I the means to work with fused quartz and a suitable pivot, I would 
mount two photocell / LED pairs about 20 to 30% of the bob's swing apart. 
I'd break the beams with a fine wire, so the cell did not go completely dark.

Then I'd work out a program to locate the center of the optical signal and 
time the interval between those centers. Using an incredibly precise 
physical adjustment, I'd align it so the outside intervals are equal.

Call the sensors A and B. The interval between A going out and A coming back 
is proportioal to the distance from A to the end of the swing. And so it is 
for B. Now physically adjust the sensors so that A to A is precisely equal 
to the interval B to B. If the distances to the ends of the swing are equal, 
then the center is located at 1/2 the interval A to B. The interval B to A 
will be exactly the same as A to B if gravity is the only thing accelerating 
the bob, even if A and B are not centered on the swing.

I'd like to try electrostatic impulsing, if I can keep charge from building 
up on the rod. There'd be no problem timing the impulse. The impulse would 
be tiny for a high Q system, or the pendulum could run free for many swings 
before being impulsed.

I'd like to reduce the bob mass to 10 grams or so, if I could find the right 
suspension. Hmm, perhaps an electrostatic suspension? Simple enough to 
shield the vacuum tank. With precision timing electronics, I don't need a 
one second swing period.

Interesting?

Bill Hawkins






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