In corresponding with Randall Peters <peters_rd@Mercer.EDU> in March, 2002, he suggested making a horizontal sensor from a folded pendulum. He wrote:
"Have you thought about a 'folded pendulum' for seismometry. The Aussies (headed by Blair) have done some interesting work with them. It's even the basis (I think) for some isolation work at LIGO. I think we could make a simple unit employing three point supports and a tungsten wire to yield a decent horizontal seismometer that would be more compact than the ones the amateurs favor. Perhaps it could be more stable than the 'torsion-gravity' unit that I've used in the past."
I constructed a crude one to get a feeling for how it might work.
|Overview of folded pendulum. There are two pendulums, one hanging from the top of the aluminum box and the other inverted and standing on the metal base. A horizontal plastic arm connects the top of the inverted pendulum with the bottom of the hanging pendulum. The pivots are made from four razor blades.|
|Closer view of lower three pivots.|
|A block of aluminum is used for the mass. The period of the pendulum is adjusted by sliding the mass back and forth along the horizontal element of the pendulum.|
Mun Woo <
email@example.com > has a page
devoted to the folded pendulum.
This geometry is also known as a "Watt's Linkage." There is a good animation here.
A Peaucellier linkage is shown here.
Brady Romberg suggested an alternative geometry.