in Vibration Cancellation Applications
Figure 1: Basic configuration of a parallel type
active cancellation system,
shown in one dimension
for simplicity. Image used
with permission from TMC.
motors to provide a corrective force to the payload
based on a known and predictable payload motion.
The application of feedforward parallel style systems
to cancelling of on-board motion is well suited for
settling-time critical applications, such as cancelling the payload motion caused by a moving stage
in a semiconductor inspection tool. In stage motion
cancellation, the need for a payload to stop resonating and “settle” to its original position quickly after
a stage motion is critical in maximizing inspection throughput in semiconductor manufacturing
processes. The feedforward input is known, since
the stage position, velocity, and mass are known
and constant, and the appropriate cancellation
signal can be fed-forward to the actuators in order
to cancel out the predicted payload disturbances.
Using powerful linear motors, settling times after
stage motion can be reduced from seconds to hundreds of milliseconds or less, and throughput can
be significantly increased. The use of parallel type
feedforward active cancellation systems in this configuration, however, does nothing for cancellation of
floor vibration, and therefore must often be used in
conjunction with a separate floor vibration cancellation system. Due to the limitations of parallel type
systems in cancellation of low frequency floor vibrations, their inherent coupling to payload resonances,
and their strengths in feedforward stage motion
cancellation, the way to maximize the payback
from a parallel type system is to use it in settling-time critical applications, and rely on a different
approach to for floor vibration cancellation.
Serial type systems
A serial type vibration cancellation system places
the passive element in series with the active element, and the two are separated by an intermediate
mass that is small relative to the payload (Fig. 3).
The transfer function of a serial type system is a
sum of the transfer functions of both the active element and the passive element, and it is important
to note that the intrinsic resonance of each element
contributes to the overall isolation performance of
the system. Therefore, there must be a significant
difference in stiffness between the two components
such that their transfer functions may sum, but the
components do not couple and oscillate (Fig. 4). A
stiff elastomer mount with resonant frequency of
15-20Hz can be employed in series with a piezo-
electric transducer as the active actuator, which will
have a stiffness value over 100 times greater than
the elastomer, resulting in a sufficient resonance
separation to ensure no coupling between the pas-
sive (elastomer) and active (piezoelectric) elements.
For vibration cancellation, the most effective serial
type systems always employ a closed loop feedback
system, where the inertial vibration sensor is placed
on the intermediate mass, and the signal detected
is filtered and fed back to the piezoelectric actuator,
so it may expand or contract with a displacement
of reverse phase to the detected signal. Piezoelectric
transducers are uniquely suited to achieve very low
frequency performance because of their stiffness
and response and can achieve vibration cancella-
The main design features of a serial type system
which include placing the vibration sensor on the
intermediate mass, and the inherent stiffness of
each component result in three critical implications
Figure 2: Changes in floor
vibration measured in a
Semiconductor Fab over
time. As the Fab is populated with tools, vibration
Gendreau, M. and H.
Amick, “’Maturation’ of
the Vibration Environment
in Advanced Technology
Facilities,” J. Institute of
and Technology (2004).
Re-printed with permission from IEST.