Geology Reference
In-Depth Information
seismic forces that act on the structures. These
mechanisms are added to the structures to modify
their dynamic properties and, in some cases, to
increase their energy dissipation capacity.
In the last decades, the number of control
devices produced in the world has considerably
grown. At the same time, research institutes have
devoted much more attention to the analytical and
experimental study of structures protected with
these devices (Soong and Spencer, 2002).
Control systems can be classified into passive,
active, semi-active and hybrid.
These systems operate automatically as a closed
loop where the measured response (by the sensors)
is used by the controller to calculate (following
some strategy -control algorithm-) the forces to be
applied (by the actuators) to reduce the vibrations.
An example of an active system is described in
Figure 1.The figure shows a mass damper where
the connection with the main frame incorporates
a powered actuator able to accelerate the auxiliary
mass. Horizontal displacements and accelerations
in the main structure are measured by sensors
(dashed box) and this information goes (dashed
arrows) to the actuator, so governing its operation
by means of a controller. Such systems are usually
known as active mass dampers.
The feasibility and reliability of active systems
are controversial because: (i) to accelerate the
massive civil engineering constructions large
forces are required (consequently, a huge energy
source is needed) and (ii) energy supply can be
interrupted during a strong input (earthquake,
wind, etc.). Therefore, active systems might be
more appropriate for reducing the response under
minor (or even, frequent) earthquakes. In building
structures the feasibility of active control is
higher for wind input than for strong seismic
excitations because the required control forces
are smaller (roughly hundreds of tons compared
to thousands).
Passive Systems
They are the most utilized control systems because
many theoretical and experimental studies have
been carried out.
They consist of some “inert” mechanical de-
vices (here “inert” does not mean inactive, but
that they are not powered and that their behaviour
cannot be modified on-line) which are inserted in
the structure to dissipate and/or to deflect energy.
In general, passive systems are not able to adapt
to the unexpected characteristics of the excita-
tion and in some cases their effect might even be
damaging (for instance, as the natural frequencies
of the structure are shifted new resonance can oc-
cur). However, if the main features of input are
known, passive systems are extremely efficient.
Active Systems
These systems behave similarly to passive ones
but, instead of inert devices, there are highly
powered mechanisms (actuators) that are able to
push the structure to counteract the input effect.
Hydraulic cylinders driven by servo-valves are
examples of actuators. An active control system
is composed of a set of sensors to measure on-line
the response of the structure (mostly displace-
ments and accelerations), the actuators, a source
or reservoir of energy to power the actuators and a
controller (typically a computer) that decides the
amounts of forces to be exerted by the actuators.
Figure 1. Example of active (or semi-active)
control system (active mass damper)
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