SEMI-ACTIVE AIR-SUSPENDED TUNED MASS DAMPER
Passive tune mass dampers (TMDs) target (add damping to) a particular structural mode and thus are effective in a narrow frequency range. They tend to become less than optimally tuned and thus less effective, as the dynamic characteristics of their target structures vary with time rendering the TMDs de-tuned. This is especially true for TMDs with small mass ratios.
Semi-active TMDs, also called adjustable passive TMDs, have the same configuration as passive TMDs but with the ability to have their stiffness and/or damping adjusted, in-site, in real-time. Semi-active TMD can readily be fine-tuned and re-tuned, awithout changing any hardware, and thus remain always optimally tuned to their target mode.
Figure 1 Horizontal pendulum configuration
DEICON has extended the use of its ‘Computer Controlled Air Isolation Technology’ developed originally for vibration isolation, to realize semi-active (adjustable) tuned mass dampers. That is, air springs equipped with damping and stiffness adjustability are used as both the resilient element and energy dissipating (damping) element in DEICON’s Semi-active Air-Suspended Tuned Mass Dampers.
In addition to its utility as a tuned damper, DEICON’s air suspended tuned device can be used as a dynamic absorber.
Figure 2 Air-suspended tuned mass damper (black) installed on a structure (red)
The schematic of DEICON’s patented ‘Semi-active Air-Suspended Tuned Mass Damper appended to a vibrating structure (depicted as a one degree of freedom spring-mass-dashpot system M1-K1-C1) is presented schematically in Figure 1.
The sensors installed on the air suspended tuned device are used to measure the deflection of the air spring. The dashed lines in Figure 2 show the sensory information feeding the controller and the control signal feeding the servo-valve which in turn manipulates the flow of air in and out of the air spring. Depending on the makeup of the control scheme, the controller residing in a small computer (micro-controller) regulates the pressure or height, realizes damping and adjusts the stiffness (and thus the tuning frequency) of the tuned device.
DEICON’s Semi-active Air-Suspended Tuned Mass Damper is different form of active tuned mass dampers which use full authority actuators (electromagnetic or hydraulic) in parallel to, or in place of, the suspension of the tuned mass dampers. In DEICON’s semi-active tuned damping technology, no full-authority actuator is used. We still use the air spring as the spring and only take advantage of its adjustability to adjust its parameters (stiffness and damping).
A notable feature of DEICON’s semi-active TMD is the absence of a physical damping element in its make-up. The required viscous damping is introduced into the TMD using a semi-active control scheme residing in a micro-controller regulating the flow of air in and out of the air springs. In addition to introducing damping into the TMD, the semi-active control scheme adjusts the stiffness of the TMD.
Figure 3 Inverted pendulum configuration
The DEICON’s semi-active TMD can be configured either as horizontal pendulum (also known as hammer configuration), vertical pendulum (both inverted shown in Figure 3 and non-inverted/regular), as well as the translational spring mass system, depending on the application; the horizontal pendulum and inverted pendulum configurations are shown in Figures 1 and 3.
The ability of DEICON’s Semi-active Air-Suspended Tuned Mass Damper in adjusting its tuning frequency and its internal damping is demonstrated by four sets of frequency response functions (FRFs) presented in Figure 4. Each set is comprised of 3 FRFs of the semi-active TMD, with different levels of internal damping, tuned to tuned to four different frequencies of 2.8, 3.9, 4.8, and 5.9 Hz. The damping and tuning frequency adjustments are done thru software with no change in hardware.
Figure 4 Frequency response functions of the semi-active TMD, with different levels of internal damping, tuned to four different frequencies
TUNING, RE-TUNING, & SELF-TUNING
As stated earlier, initial fine-tuning and occasional re-tuning of traditionally tuned mass dampers involves modifying the hardware and could be quite challenging, especially in large tuned mass dampers. On the other hand, tuning, fine-tuning and re-tuning of Semi-active Air-Suspended Tuned Mass Dampers can readily be done thru software without physically modifying the hardware. Moreover, self-tuning capabilities can be incorporated into the controller of air suspended tuned mass dampers so that they continuously and automatically fine-tune themselves to the target frequency.
DEICON’s semi-actively controlled tuned mass damper uses air springs as both the resilient and damping elements; the TMD uses no passive dampers. The control strategy a) adjusts the stiffness of and b) induces viscous damping into the air springs. The semi-active TMD can readily be fine-tuned and re-tuned, automatically, without changing any hardware and thus remain always optimally tuned to their target mode.
DEFCON’s Semi-active Air-Suspended Tuned Mass Damper can be used in a variety of structural damping applications including, but not limited to, floor systems, pedestrian bridges, and towers.