Contactless Excitation and Readout of Passive Sensing Elements Made by Miniaturized Mechanical Resonators

Mechanical resonance has been contactless excited and detected in conductive nonmagnetic structures to be used as sensors without the need for external magnets, or electrical connections to the structure to supply current lines. An external coil generates a magnetic field at frequency f that induces eddy currents in the structure. The interaction between the eddy currents and the magnetic field itself causes Lorentz forces at frequency 2f that can set the structure into resonance. An additional dual-coil arrangement applies and senses a probing magnetic field at higher frequency and exploits it to measure the resonator vibrations. The principle was tested on millimeter-size metallic beams, obtaining operation distances in the order of 1 cm and values of the resonant frequencies in agreement with measurements taken by an optical system. The resulting resonators can be used as passive sensing elements for a variety of quantities, being especially attractive for harsh and inaccessible environments.