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ADVANCES IN OPTICAL TABLE DESIGN continued
This approach is particularly effective
at low frequencies, achieving up to two
orders-of-magnitude improvement over
passive air isolators in the 1–3 Hz range.
In addition, the hard-mount support offers the fringe benefit of maintaining the
payload’s positional stability with respect
to off-board beam sources, which is impractical with a soft-air support.
Though the serial-type approach provides dramatic improvements at very low
frequencies, there is little benefit at higher frequencies. Since the passive support
spring is much stiffer than a conventional
passive air spring, there is less isolation at
higher frequencies. In the 10–30 Hz range,
serial-type active systems do not provide
better vibration attenuation than passive,
self-leveling air isolation supports.
Stacked systems for
> 10 Hz isolation
Always seeking more innovative solutions,
scientists have experimented with stacking
systems by placing a serial-type active sys-
tem beneath a passive air isolator. Such
systems provide the combined isolation of
each sub-system since the transfer func-
tions are additive.
In effect, this approach provides two
stages of isolation: one active and one
passive. For example, with 30 dB of at-
tenuation at 10 Hz actively and 30 dB
of attenuation at 10 Hz passively, 60 d B
of total attenuation at 10 Hz is achieved
(see Fig. 2). And since the serial-type
architecture is an active hard mount, it
can be stacked with no risk of instabili-
ty or crosstalk between the isolation sys-
tem. The 15–20 Hz hard-mount spring
is sufficiently stiffer than the 2 Hz pas-
sive air spring to achieve adequate im-
pedance mismatch, ensuring stability.
Unfortunately, this do-it-yourself ap-
proach of stacking systems is inconvenient
and cumbersome. A better approach is an
integrated, two-stage, passive-over-active
isolation system (see Fig. 3). Such systems
are now commercially available with six-
DOF performance and provide the best
of both worlds: aggressive low-frequen-
cy vibration cancellation from an inher-
ently stable architecture with two stages
of vibration isolation since the attenua-
tion of the air isolators and piezo isola-
tors are additive.
Passive-over-active systems are being
adopted for some of the most sensitive
applications, including single-molecule
biophysics, multiphoton imaging, atom-
ic force microscopy, confocal microsco-
py, and interferometric studies of large
samples where sub-nanometer and even
sub-angstrom resolution is desired. As res-
olution is sought at ever-smaller scales,
rapid advances in vibration isolation tech-
nology ensure that floor vibration need
not be the critical limiting factor.
Steve Ryan is divisional vice president of TMC,
a unit of AME TEK Ultra Precision Technologies,
Peabody, MA; e-mail: steve.ryan@ametek.com;
www.techmfg.com.
