2.4.2 ACTIVE BRACING SYSTEM
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SEMI-ACTIVE PROTECTION SYSTEM
A semi-active control system typically requires a small external power
source for operation (e.g. a battery) and utilizes the motion of the
structure to develop the control forces, the magnitude of which can be
adjusted by the external power source. Control forces are developed
based on feedback from sensors that measure the excitation and/or the
response of the structure. A semi-active control system generally
originates from a passive control system which has been subsequently
modified to allow for the adjustment of mechanical properties. For
example, supplemental energy dissipation devices which dissipate energy
through shearing of viscous fluid, movement of fluid through orifice,
and sliding friction have been modified to behave in a semi-active
manner. The mechanical properties of these systems may be adjusted based
on feedback from the excitation and/or from the measured response. As in
an active control system, a controller monitors the feedback
measurements and generates an appropriate command signal for the
semi-active devices. As in a passive control system, however, the
control forces are developed as a result of the motion of the structure
itself. The control forces are developed through appropriate (based on a
pre-determined control algorithm) adjustment of the mechanical
properties of the semi-active control system. Furthermore, the control
forces in many semi-active control systems primarily act to oppose the
motion of the structural system and therefore promote the global
stability of the structure. Semi-active control systems generally
require a small amount of external power for operation (on the order of
tens of watts)[12].
Semi-active control systems are a class of active control systems for
which the external energy requirements are orders of magnitude smaller
than typical active control systems. Typically, semi-active control
devices do not add mechanical energy to the structural system (including
the structure and the control actuators), therefore bounded-input
bounded-output stability is guaranteed. Semi-active control devices are
often viewed as controllable passive devices.