Air Suspended Tuned Mass Damper
In addition to designing and building a) tuned mass dampers (TMDs) using metal springs and viscous dampers as their suspension, and b) Viscoelastic Tuned Mass Dampers, DEICON has extended the use of its patent-pending ‘Computer Controlled Air Isolation Technology’ developed originally for vibration isolation, to the realization of 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 air suspended tuned mass dampers. It should be noted that in addition to its utility as a tuned damper, DEICON’s air suspended tuned device can be used as a dynamic absorber.
The schematic of DEICON’s patent-pending ‘Air Suspended Tuned Mass Damper’ is shown in Figure 1(a). Such a device 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(b).
Figure 1 Air-suspended tuned mass damper (a) and air-suspended tuned mass damper installed on a structure (b)
The sensors installed on the air suspended tuned device are used to measure the net motion of the air spring. The dashed lines in Figure 1 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 of the tuned device.
It should be noted that DEICON’s computer controlled air suspended tuned mass damper is quite different form active tuned mass dampers discussed in the literature. Active tuned mass dampers use a full authority actuator (electromagnetic or hydraulic) in parallel to, or in place of, the spring in a traditional tuned device. The actuator which puts out most or all of the vibration control force, mainly uses the mass as an inertia to push/pull against, dynamically. In DEICON’s technology, no full-authority actuator is used. We still use the air spring as the spring and only take advantage its adjustability to adjust its parameters (stiffness and damping).
DEICON designs and fabricates tuned mass dampers for a variety of applications. DEICON TMDs use different suspension mechanisms including viscously damped coil spring, air, and viscoelastic, depending on the application.
Vibration treatment systems that use controls for adjusting their parameters are normally categorized as semi-active systems. Since semi-actively controlled systems only use a small amount of energy to modify their parameters, their needed energy is by far less than systems under fully active control which constantly use energy to impart forcing/actuation, via an actuator, to the structure.
Figure 2 depicts two different designs, amongst many possible ones, for computer controlled air suspended tuned mass dampers. In the design shown in Figure 2(a), in addition to the 4 small air springs supporting the mass of the tuned damper 4 other air springs are used to allow for adjusting the stiffness of the tuned device without over-extending the air springs. Also shown in Figure 2(a) is a part of the underdamped, all-steel (I-beam) structure the tuned damper is used for adding damping to. Figure 2(b) shows a large, computer controlled air suspended tuned mass damper, designed for damping the vibration of a massive floor system. The computer controlled air suspended tuned mass damper is appended to the vibrating structure, i.e., the floor, via its 4 posts.
Figure 2 Two air-suspended tuned mass damper designs
Another commonly used configuration for tuned dampers is ‘pendulum type”. The image of a 1 ton pendulum type tuned mass damper undergoing testing/evaluation is presented in Figure 3(b). Figure 3(a) shows the schematic of such damper.
If need be the lateral motion of the mass of tuned mass dampers, in designs similar to the ones shown in Figure 2, can be constrained using a set of laterally mounted air springs. In addition to constraining the lateral motion of the tuned damper, these lateral air springs assist in adjusting the stiffness in the main vibration direction. In pendulum type designs, shown in Figure 3, lateral motion is automatically constrained through the mechanics of the design.
Figure 3 An image (a) and the schematic (b) of a pendulum type tuned mass damper
Fine-tuning, Re-tuning, and Self-tuning
As stated earlier, initial fine-tuning and occasional re-tuning of traditional tuned mass dampers involves modifying the hardware and could be quite challenging, especially for the large ones. On the other hand, fine-tuning and re-tuning of air suspended tuned mass dampers can readily be accomplished thru the software, using their active and semi-active stiffness control features, 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-tuned themselves to the target frequency.
DEICON’s ‘Air Suspended Tuned Mass Damper’ can be used in a variety of structural damping applications including, but not limited to, floor systems, pedestrian bridges, towers, and steel stacks.
- Contrary to traditional TMDs that use two individual elements, i.e., spring and a damper, to make up their suspension, DEICON’s air suspended TMD uses only an air spring (acting both as the resilient and dissipative elements) in its suspension. This results in fewer part counts, more reliability, and lower cost.
- Friction-free, immediate response of its air suspension is another advantages of DEICON’s computer controlled air suspended tuned damper. Since air springs have no sliding seals, there is no breakaway friction (as in mechanical viscous dampers) making these TMDs respond to the slightest of perturbation on the vibration structure.
- In DEICON’s air suspended TMDs, vibration energy of the structure is dissipated by releasing some compressed air from the air suspension, not via heat dissipation (as is in viscous dampers) to the surrounding. Thus DEICON’s air suspended TMDs
- are smaller in size than the equivalent tuned mass dampers using spring+viscous damper as their suspension, and
- have temperature-independent damping attributes.