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Typical transfer functions
Passive air vibration isolation
Parallel type active air
Serial type active
“Stacked” air on serial type
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Historically, improvements to optical table vibration performance have focused on increasing the structural damping of
the top—the goal being to combine extremely high stiffness-to-
weight ratios achieved with steel honeycomb technology with
high structural damping and little amplification at resonance.
In general, this has been successful, with the best tops now
achieving critical damping at their lowest resonant frequencies.
To the extent that this has had good success, further improve-
ments now provide diminishing returns. Until recently, there
have been fewer advances in the isolation systems that support
the tops from floor vibration.
The low-frequency isolation challenge
As scientists and engineers seek to make measurements and
achieve resolution at ever-smaller scales, optical table applications are increasingly sensitive to low-frequency floor vibration.
Such vibration in the 0. 5–30 Hz range is not attenuated by even
the stiffest and best-damped tops.
Vibration in this frequency range reaching the isolated surface
simply results in rigid body motion of the top. Rather, to effectively attenuate vibration in this frequency range, it must be isolated from reaching the top. The best vibration isolation support
stands provide limited isolation in this frequency range. They typ-
ically consist of passive self-leveling air isolators with low verti-
cal and horizontal resonant frequencies that amplify floor vibration in the 1–4 Hz
range and begin to
isolate above 4 Hz.
have been applied to
optical table vibra-
tion isolation supports to improve
low-frequency isolation. The term “
active” has been used
loosely in industry
jargon to describe
various control sys-
tems that include
such simple feedback mechanisms
as the mechanical
self-leveling of air isolators by combining them with pressure
regulators and a mechanical linkage.
For clarity, when we refer to “active,” we specifically refer
to inertial feedback active systems in which the signal from
an inertial sensor such as a geophone or accelerometer (which
measure velocity or acceleration, respectively) is conditioned,
amplified, and ultimately used in closed-loop feedback to cancel unwanted vibration through an electromechanical or other type of actuator.
FIGURE 2. When vibration isolation
systems are stacked, the transfer functions
are additive; the shaded area in the model
represents improvement from passive air to
stacked air on serial-type active systems.