r - The Hong Kong Polytechnic University

al. 2005; Nagarajaiah and Sonmez 2007; Nagarajaiah 2009; Chey et al. 2010), is suggested to improve the
performance of the PF-TMD. By regulating the slip force applied to the friction interface in real time, the
SAF-TMD can adjust its slip force in response to structural motion and/or external excitation; thus, the
SAF-TMD can respond to earthquakes with different intensities. Moreover, the SAF-TMD has the following
features that differentiate it from active mass dampers (AMDs). (1) It does not require a significant amount of
control energy as controlling the clamping force does not generally require a large control stroke. (2) It does not
pump energy into the controlled structures; thus, the effects of control spill-over and control instability are
eliminated. (3) It has a control force restraint, i.e., it can only provide a resistance (passive) force to the
controlled structure. In other words, the friction force produced by a SAF-TMD is always in a direction opposite
to that of the device in motion.
A SAF-TMD consists of a mass block and a variable friction device (VFD) (Lin et al. 2010). The many
innovative VFDs described in the structural control literature include Kannan et al. 1995, who applied hydraulic
and electromagnetic driven forces to regulate the clamping force of a semi-active friction device. Other
researchers (Xu et al. 2008; Chen and Chen 2004; Lu et al. 2010) controlled the clamping force of a semi-active
friction device via an embedded piezoelectric stack actuator for generating high stress with very low electrical
current when the piezoelectric actuator is properly confined. All these studies agree that combining a
semi-active friction device with TMD systems may be a feasible technology.
To provide this adaptability, an SAF-TMD implementation must include a controller with a control law for
on-line determination of the clamping forces in the VFD. A major advance in friction controller technology is
the use of bang-bang control laws for VFD, which was first proposed by Kannan et al. 1995. Their methods are
simple and merely require the measurement of the velocity directions of the controlled structure. However,
repetitive on and off damper actions can amplify structural acceleration by causing discontinuous control forces
that exert a high-frequency structural response. Inaudi et al. 1997 proposed a modulated homogenous friction
control strategy for producing a slip force proportional to the prior local peak of the damper deformation.
Similarly, this algorithm may also cause discontinuous control forces. In a subsequent study, Yang and Agrawal
2002 modified the Inaudi’s method for controlling isolated structures subject to near-fault ground motion. Lin et
al. 2010b applied a non-sticking friction (NSF) controller that is able to keep the SAF-TMD in its slip state
constantly for an earthquake with any intensity.
For feasibility tests of the proposed SAF-TMD, a prototype SAF-TMD was fabricated and tested dynamically
via a shaking table test. Firstly, the configuration and the constituent elements of the SAF-TMD are explained,
and the mathematic model and the numerical analysis method for structure with an SAF-TMD are discussed.
The test setup for the shaking table test of the prototype SAF-TMD, and the test results are explained. The
experimental performance of the SAF-TMD is also compared to those of its uncontrolled and passive
counterparts. Finally, conclusions are presented.
SEMI-ACTIVE FRICTION TUNED MASS DAMPER (SAF-TMD)
Configuration of SAF-TMD
In order to evaluate the performance of the proposed SAF-TMD, the seismic response of a prototype SAF-TMD
was tested via an experiment using a shaking table. Figure 1 is a schematic diagram of the prototype SAF-TMD,
which mainly consists of a sliding platform and a piezoelectric friction damper (PFD) (Lu et al. 2010). The main
components of the sliding platform are the guide rails, sliding blocks and springs. The springs provide not only
stiffness and resilience, but also a tuning frequency.
Mass block
Sliding platform
Piezoelectric friction damper (PFD)
Load cell Friction pad Friction bar
System
dynamic response
Friction bar
Spring
Controller
(PC+ A/D card)
Control voltage
(DC 0-10 V)
Sliding block
Guide rail
Figure 1 Schematic diagram of the SAF-TMD.
Pre-compression screw
Driving voltage
(DC 0-1000V)
Voltage Amplifier
Figure 2 Control diagram of the PFD in the SAF-TMD.
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