[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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