Force distortion in a coil/magnet vibration exciter for shallow structures subjected to large displacements

Abstract

An electromagnetic noncontact vibration exciter has been designed for investigating nonlinear vibrations of shallow structures such as gongs, cymbals, and spherical shells. It is composed of two devices: a coil driven by a current and a small magnet glued on the structure. One interest of this system is that the mass of the magnet is small compared to the mass of the structure, so that its dynamic properties are only slightly modified. The magnitude of the force applied to the structure via the magnet depends on the relative position of magnet and coil. As a consequence, an harmonic distortion appears in the force wave form as the magnitude of the magnet displacement increases. A theoretical study is first carried out in order to find an optimal geometry for the coil that minimizes the force distortion. In a second step, a simple degree-of-freedom model is used to evaluate the distortion rate. It is found that this rate not only depends on the exciter properties, but also on the dynamics of the system under testing. A multiple-scale technique is used for the resolution of the nonlinear model that governs the distortion. Experimental results will be presented which confirm the theoretical predictions.