ADVANCES IN OPTICAL TABLE DESIGNcontinued
Inertial feedback active systems
The first embodiments of inertial feedback active systems were
parallel-type configurations in which the inertial sensor is mounted to the isolated surface, and the cancellation actuator is mount-
ed in parallel with springs (air isolator supports) that support the
isolated surface. This approach can effectively suppress the resonant amplification of the support air isolators in the 1–4 Hz range.
However, attempts to isolate over wide bandwidths with this
approach meet with difficulty, as the sensors cannot differentiate between rigid body motion of the top and resonances of
structures on the isolated payload. The control system will attempt to cancel both, leading to system instability. While the
compromise has been to limit the bandwidth of such systems
to < 8 Hz, effectively suppressing resonant amplification of the
air isolators and improving stability, it does little to improve vibration isolation over the broad 0. 5–30 Hz range.
An alternative approach has been developed using a seri-
al-type configuration. Here, the support spring is placed in se-
ries with the cancellation actuator. The sensor is mounted to
an ultrastiff inner mass that supports the payload through a
stiff, 15–20 Hz spring. The actuator supports the inner mass
to ground (see Fig. 1).
With this approach, linear motors and other conventional ac-
tuators are not feasible because the actuator in a serial-type con-
figuration must support the static weight of the top. But, devel-
opments in piezoelectric actuator technology make piezos the
ideal choice for serial-type configurations, as they can now be
designed to support a large static mass and have excellent re-
sponse characteristics to very low displacements.
In this embodiment, floor vibration is sensed at the inner
mass as it is transmitted through the stiff actuator. The feed-
back loop is closed at the inner mass as the actuators “filter”
floor noise from reaching the inner mass. That is, as the floor
moves upward, the actuators contract; as the floor moves down-
ward, they expand.
A three-axis design expands this control behavior to all six
DOFs. Such systems are inherently robust, as the payload res-
onances are filtered from reaching the inner mass by the stiff
spring and the sensor is mounted to an inner mass that can be
designed to achieve the very high stiffness required—>1000 Hz.
Therefore, gain settings can be aggressive with bandwidths of
up to 150 Hz frequently achieved, leading to very high levels of
vibration attenuation with little risk of instability.
FIGURE 3. Researchers attempt to improve performance by stacking
passive optical table isolators on an active, hard-mount support (a);
however, commercially available two-stage, passive-on-active systems
(b) can provide 20–30 dB more isolation across a broad frequency range.