A folded pendulum isolator for evaluating accelerometer performance

Abstract

This paper describes how a requirement to evaluate a new accelerometer led to the construction of a folded pendulum isolator as part of an undergraduate civil engineering student project. The exercise has some interesting lessons about the performance of accelerometers in low-vibration environments and the importance of paying attention to the detail of their mounting. It also demonstrates a fascinating mechanical device. A folded pendulum is a compact mechanism comprising a positive pendulum a negative (or inverted) pendulum whose combined horizontal natural frequency can be made very small. We decided to build one after searching for methods to estimate noise floors of the high-grade accelerometers used for civil structure dynamic assessment by isolating them from all mechanical excitation at their supports on the ground. As with other isolation devices, the folded pendulum can be built with a natural frequency low enough to provide significant attenuation with respect to ground motion in the operational range, but compared to other isolation devices it is relatively straightforward to construct. With careful tuning the folded pendulum as constructed achieved a minimum natural frequency of 0.078Hz and proved capable of isolating the test accelerometer well enough to identify the instrument noise floor.