OPTIMAL DESIGN OF MAGNETIC (EDDY CURRENT) DAMPERS FOR TUNED DAMPING APPLICATIONS

Abstract

Damping is an essential element of any structure prone to vibration. The energy dissipation provided by dampers reduces structural vibration, enhancing serviceability of the structure and in some cases prevents damage caused by fatigue. When most of the vibration energy in a structure resides in a single mode, the go-to method of abating such vibration is tuned mass damping. Numerous applications, including pipework and structures, necessitate that the tuned mass damper be tuned to a specific natural frequency in a particular vertical or horizontal direction. When structure vibration occurs at multiples modes with their corresponding natural frequencies, then viscous dampers are the more suitable choice due to their broadband damping capacity.

The motivation for conducting this study is to explore the use of eddy current damping as an alternative to liquid-based viscous and solid-based viscoelastic damping mechanisms in tuned mass dampers as well as stand-alone/broadband viscous dampers. Eddy current damping (also known as magnetic damping) is generated by the eddy current induced in a conductive material subjected to a time-varying magnetic field (normally created by several rare earth permanent magnets) and tuned mass damper that is used eddy current damper as damper element are explored. The eddy current in the conductor generates its own magnetic field resulting in an electromagnetic force opposing the force of the original time-varying magnetic field.

The main contribution of this effort is the development of a numerical tool for synthesizing eddy current dampers and optimizing them for particular applications. The utility and efficacy of this numerical tool are demonstrated and verified by synthesizing two different designs of eddy current dampers and laboratory testing one of them.

The first design consists of a stack of eight axially magnetized NdFeB (N42) permanent magnets that are separated by seven iron pole sections moving within a tubular eddy current damper and consists of a highly conductive aluminum cylindrical shell. The interaction between the magnetic field of the permanent magnet assembly and the magnetic field of the electromagnet created by the eddy current within the aluminum tube results in energy dissipation. The tubular eddy current damper design is synthesized using the finite element based numerical tool and is built and tested in the laboratory. The result shows a good agreement with the predicted ones, verifying the fidelity of the numerical tool.

The tubular eddy current damper (ECD) exhibits viscoelastic trait at high frequencies. The viscoelastic attribute is described effectively by the Maxwell model, which represents viscous dampers as a series combination of pure viscous dampers and springs. Two methodologies for determining Maxwell model parameters for a viscous dampers (including ECDs) are introduced. Both approaches rely on damping force vs. displacement identification employing harmonic displacement loading (sinusoidal loading) across various excitation frequencies. Both methodologies have been employed in identifying the parameters of an eddy current damper at .different frequencies, yielding similar results

The second design, a plate eddy current damper, consists of a conductive copper plate and five NdFeB (N52) magnet plates two of which are axially and the remaining three radially magnetized. The magnets are placed in Halbach array arrangement, on two iron plates. In this damping scheme, the relative motion between the copper plate and magnets produces a repulsive force that opposes the changing in the magnetic flux density. The damping effectiveness is much higher in this damper configuration than the tubular eddy current damper. The plate eddy current damper is also synthesized using finite element numerical tool mentioned earlier.

Lastly, the utility of ECD as the dissipating element of a lateral tuned mass damper is explored.