Folded Pendulum

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.

fp_over.jpg (419216 bytes)

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.


fp_pivot.jpg (266517 bytes)

Closer view of lower three pivots.


fp_mass.jpg (208784 bytes)

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 < > 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.