Mitigation of Motions of Tall Buildings with Specific Examples of ...

6.3.5 Additional Applications of Active Control
Active Gyrostabilizers, which has been observed to perform best in tower-like structures, have
been developed commercially for application (Kazao et al. 1992). The system is composed of a
high-speed rotating flywheel called a “rotor” and a supporting frame for the rotor called a “gimbal.”
The system has two servo motors: one rotating the flywheel at high speed levels, and the
other controlling the angle of the gimbal to generate the gyroscopic moment actively. The
moment along the y-axis stabilizes the bending response of the structure on which the gyro is
placed. The sensor system measures the horizontal velocity at the top of the structure, and PD
operation is executed on the velocity response by an A/D converter. The executed signals from
the digital computer are D/A converted and sent to the servo driver as the speed instruction to
obtain the control moment giving precession to the gimbal. The absolute angle of the gimbal is
also measured and fedback to give a slight restoring force to the gimbal. A full-scale demonstration
was conducted on a 60 m tower-like structure equipped with 2 gyrostabilizers with a 408 kg
flywheel rotating at 1260 rpm. The system is supported by a gimbal driven by a servo motor with
reduction gear. The damping coefficient, found through free vibration tests, without control was
found to be 0.96% and with the addition of the device, the damping coefficient was found to
increase to 8.1%. Under actual wind loads, the peak response acceleration with control was
reduced to 30% to 80% of that without control, and the rms response acceleration with control
was reduced to 25% to 60% of the tower alone.
6.4 Hybrid Dampers
Another genre of control systems, hybrid systems, were also devised to overcome the shortcomings
of a passive system, e.g. its inability to respond to suddenly applied loads like earthquakes
and weather fronts. In the case of a TMD, the building may be equipped with a passive auxiliary
mass damper system and a tertiary small mass connected to the secondary mass with a spring,
damper, and an actuator. The secondary system is set in motion by the active tertiary mass, and it
is driven in the direction opposite to the TMD, magnifying its motion, and hence, making it more
effective (Sakamoto 1993, Sakamoto & Kobori 1996).
Hybrid Mass Dampers (HMDs), behave as either a TMD, utilizing the concept of moving masssupported
mechanisms of the same natural period as the building, or an AMD according to the
wind conditions and building and damper mass vibration characteristics (Tamura 1997). As a
result of this unique feature, the devices are often termed tuned active dampers (TAD). The active
portion of the system is only used when there is high building excitation, otherwise, it behaves
passively. In such systems, the device will typically maintain active control, and in the event of a
power failure or extreme excitations which exceed the actuator capabilities, will automatically
switch into passive mode until the system can safely resume normal operations. This combination
of passive and active systems in Japan has been found to reduce structural responses by more than
50%. While these systems are expensive to install, the reduced operation of the AMD implies low
maintenance and operation costs.
Japanese researchers have devoted numerous studies toward the application of hybrid devices in
structures. In fact, most applications of active control technologies are of the hybrid type, as Table
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