By Alla Katsnelson
How It Works: Passive vibrational isolation
Passive vibrational isolation tables offer the most vibrational noise reduction for the price. They work on the same basic principle as the suspension of a car - though the wheels move up and down rapidly as you drive over a bumpy road, the spring supporting the mass of the cab keeps passengers from feeling the vertical bounce. Unlike air tables, in which air pumped into the system acts as the spring, and active tables, which use sensors and actuators to electronically correct for positional information, passive isolators are entirely passive, as the name implies.
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Illustration: Andrew Meehan
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1. Vertical vibrations are isolated by the spring's interaction with four pairs of flexures. The weight of the instrument compresses the pre-loaded spring, floating the isolator and aligning the flexures.
2. A squeeze force from another spring, controlled by the knob, is applied to the outside of the flexures via a screw. The "squeezed" flexures constitute a "negative stiffness mechanism" (NSM) that acts like the negative of a spring, reducing the stiffness of the system.
3. The crank moves the base of the spring up and down to compensate for changes in the weight of the payload and to keep the flexures in their straight, aligned position. If you increase the weight on the spring (by swapping a lighter microscope for a heavier one, for example), its base must be raised by turning the crank clockwise.
4. Four beam-columns connecting an upper and lower column plate act as a horizontal spring to isolate the horizontal motion. The beam-columns are vertically very stiff, but bend slightly in response horizontal vibration. The weight on the deflected beam-columns reduces the
stiffness of the spring, making the system behave like a spring with an NSM.
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The idea is simple, explains David Platus, president of Minus K Technology, which manufactures such systems. "A passive isolator can be a piece of cork - as long as it provides a much lower frequency than the frequency of the vibration you want to attenuate." The lower the natural frequency of your isolation system, the lower the frequencies it will be able to cancel; the bigger the gap between your system and the noise it's combating, the better the isolation. (Minus K tables operate at about 0.5 Hz, and start isolating at 0.7 Hz.) Minus K tables combine a stiff spring with a "negative stiffness mechanism," which effectively loosens the spring while maintaining its load supporting capacity. For example, if a 10-pound load would normally deflect the top of the spring downward by an inch, that same deflection might take just a single pound.
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