Serial vs. Parallel Type Active Systems
TMC The need to provide adequate vibration isola- tion presents an increasingly important and complicated challenge, particularly at very low frequencies. Many users have begun to
favor active vibration cancellation systems, which
incorporate the use of sensors, actuators, and con-
trol algorithms to detect and mitigate vibration.
The basic architecture of an active system can be
characterized as one of two types: a serial type
active system or a parallel type active system.
Most active vibration cancellation systems
incorporate the use of three key elements — a
passive isolator, an electro-mechanical actuator,
and a control system. An active system is, in fact,
a damped spring which functions as a passive isolator. It can be mechanical, such as an elastomer
mount or a metal die spring, or it can be pneumatic. In addition to the passive isolator, every active
system also includes an electro-mechanical actuator, which is often a linear motor or piezoelectric
transducer. Finally, the control system provides the
necessary feedforward and/or feedback information
to the system’s actuators, enabling them to provide
the corrective force or displacement required for
active vibration cancellation.
Parallel type systems
The first type of active vibration cancellation system developed was the
parallel type system, which is still used
widely today. Starting with an existing
simple, mass-spring-damper passive
vibration isolation system, a parallel
active system is created by adding a
vibration sensor to the payload and
applying a cancellation force through
an electro-mechanical actuator to
damp any residual motion of the payload that has not been isolated by the
passive isolation system.
The basic system architecture uses
a passive isolator, typically a damped
spring, in parallel with an electro-
mechanical actuator, each of which
acts simultaneously on the payload (Fig. 1). The
passive isolator in most parallel type commercial
systems is typically an elastomer mount, a metal
die spring, or a pneumatic isolator, and the electro-
mechanical actuator is typically a linear motor (or
voice coil motor). The location of the inertial vibra-
tion sensor in a parallel type system is important.
Some early parallel type systems utilized a ground
motion sensor to sense ground position, and deliv-
ered a feedback signal to the linear motor to apply
an out of phase countering force to the payload in
an attempt to cancel the detected vibration. This
helped to track response to earth motion, but the
dual-sensor method has two inherent limitations.
The first limitation is that the system cannot
track information from two sensors at the same
time at low frequencies. If there’s a need to cancel
out a disturbance at 2Hz on the payload, the gain
of the payload sensor must be raised, but the gain
of the sensor on the earth must be lowered. Also,
any resonances from the passive isolators (which,
in the case of most pneumatic isolators, is around
2Hz) are added to the gain of the whole system.
The result is a very high sensor gain at the payload
at 2Hz, which requires a de-tuning of the ground
sensor to the point where it is ineffective at sensing
The second limitation: using feedback sensing of
the payload results in all payload resonances being
fed into the control loop of the cancellation system.
The actuators, in response to the signal from the
payload sensor, will incorrectly try to counteract
these payload resonances, leading to an unstable
system. The alternative is to “de-tune” the system to
ensure stability but often to an extent that no significant vibration cancellation is achieved.
At frequencies above 3Hz, bandwidth limitations on sensors play less of a role in the performance of parallel type active cancellation systems.
Using modern digital controllers, parallel type systems can be carefully tuned to counteract specific
resonances on the floor above 3Hz, but it has been
found that tuning to specific frequencies is a high-maintenance activity. As facility conditions change
(Fig. 2), frequent re-tuning of a parallel type system
is a reality of system ownership.
Given the limitations of parallel type systems at
low frequencies, designers sought a more effective
use of parallel type systems, which takes advantage
of their ability to be tuned to very accurately counteract motion on the payload (but not the floor).
This use was found in the application of parallel
type systems in feedforward control configurations,
which apply a pre-programmed signal to the linear
SEM image of nano-lithography pattern
created with severe low
frequency floor vibration
present. Photo courtesy
SEM image of the same
lithography tool, when
supported by a serial-type active vibration cancellation system. Photo
courtesy of SEMATECH.