Magnetic Excitation and the Effects on Modal Frequency and Damping

Abstract

Non-contact excitation sources are critical for exciting small structures that cannot be excited by conventional means, such as electromagnetic shakers, due to the added mass and stiffness effects or by impact testing, due to potential surface damage. Magnetic excitation is one possible method of non-contact excitation. However, if the material is non-ferrous, magnetic excitation will require that a ferrous material, potentially a small magnet, be attached at a desired excitation location. The addition of the magnet will have some small effects on the modal frequency and damping. The addition of multiple magnets, combined with phased magnetic excitation to enhance bending or torsion modes, is also possible, and can cause the effects to be greater. This paper will examine the effects that the added mass has on the natural frequencies and damping of a cantilever plate. It will examine four different plate configurations; an unloaded plate (for reference), and three different mass configurations, all of which will be added to the free tip of the plate. Experimental modal analysis will be performed using two different excitation sources, an impact hammer and magnetic excitation, for comparison purposes. The natural frequencies and damping for the first five deformation modes will be compared for the two different excitation sources and between the different mass configurations. A finite element modal analysis model will be created and calibrated to the unloaded plate experimental data. Then, mass modifications (magnets) will be added into the finite element model and used to predict the natural frequencies of the various mass configurations.