ISAAC technology: how do active mass dampers work?
All the structures, from the terraced houses of the city suburbs to the ever-taller skyscrapers of the financial districts from the motorway viaducts to the longest suspension bridges in the world, are subject to vibrations of various nature and extent. The first thing that comes to mind is how much we think about oscillation phenomena affecting the structures are undoubtedly i earthquakes.
The action of an earthquake, however, is not the only phenomenon that can induce significant phenomena of vibrations. Think for example of the wind which has an extremely important effect on tall and slender structures, or the traffic induced vibrations of heavy transport such as trucks or trains, a problem well known to those who live near a subway line in a large city.
The reasons, therefore, to reduce the oscillations affecting a structure can be different and range from comfort inside the rooms, to the protection of vital plants up to protection of structural elements and seismic prevention in general.
Active Mass Damper (AMD)
An AMD (from the English acronym Active Mass Damper) is an active vibration damper, a device that arises precisely from the need to eliminate, or reduce, the oscillations to which the structures are subject. It is an oscillating mass, free to move in one or more directions, driven by a mechanical actuator capable of generating high forces, installed on a structure in order to reduce its oscillations in the presence of an oscillation induced, for example, by 'seismic action.
The force generated by the AMD depends on the real-time reading of the accelerometric sensors installed in significant points of the structure and follows a control law defined in the design phase, which allows the achievement of the performance of vibration reduction You want.
To better understand the principle of operation from the comparison between the AMD and its "passive" cousin, the TMD.
The “passive” cousin: the TMD
TMD (from English Tuned Mass Damper) is an oscillating mass capable of discharging forces on the structural elements on which it is anchored by means of a spring-damping connection.
The TMD allows to "capture" the movement energy of the structure around its resonant frequency. To work efficiently, the TMD needs to be “calibrated” in terms of mass and stiffness in such a way as to suitably “couple” with the resonance frequency of the structure on which it is installed.
Certainly the most famous example of existing TMD concerns the Taipei101 skyscraper in Taiwan, where a 660-ton pendulum was installed, equipped with a total of eight enormous hydraulic dampers.
Figure 1 - TMD installed on Taipei101, Taiwan
The advantages of the Active Mass Damper
In this sense, the main advantage of using a Active Mass Damper compared to a Tuned Mass Damper is that the former does not require an ad-hoc design for each structure on which it is to be applied. Not to mention that in the event of damage to the structure itself, the natural frequency can change over time, making the action of the TMD less effective, almost totally canceling itself out due to very significant changes in the dynamics of the building.
In addition to the great flexibility with which a Active Mass Damper is able to adapt to dynamic changes in structures, another great advantage in using this technology lies in the forces that can be generated for the same installed mass. The installation of large masses on top of the structures involves considerable problems of a design and structural nature. The forces that can be generated with an Active Mass Damper do not depend on the total mass but only on the nature of the actuator with which it is moved.
The first Active Mass Damper developed and tested by ISAAC anti-seismic, I-Pro 1, is equipped with a total mass of 4 tons (of which 2.2 tons of mobile mass and 1.8 tons of fixed mass) and, thanks to a hydraulic actuator that works with pressures up to 280 bar it is able to generate horizontal thrusts exceeding 20 tons of force.
Figure 2 - I-Pro 1 installed at the Politecnico di Milano for functional tests
A second AMD, always developed within the ISAAC anti-seismic laboratories, Electro-Pro20x, responds to the need for extreme simplicity of installation in the field. With a fixed mass anchored to the structure that varies between 100 and 200 kg (depending on the overall length) and a mobile mass that can be adjusted between a minimum of 250 and a maximum of 1000 kg, it can be entirely assembled directly in the final place. installation. The actuator is a linear electric motor capable of developing horizontal forces greater than 20 kN (2 tons of thrust).
Figure 3 - Electro Pro 20x installed in the ISAAC Lab for performance tests
The compactness of the solution allowed the first commercial applications of the technology which, until now, due to the high costs and technical difficulties, had been possible are purely theoretical.
Author: Stefano Cii