Magnetically soft higher order harmonic stress and temperature sensors

A stress and temperature sensor based on changes in the higher order harmonics of a magnetically soft magnetoelastic sensor is reported. When excited by a low-frequency ac magnetic field, the nonlinear permeability of the ferromagnetic sensor causes the emission of higher order magnetic induction that can be detected by a coil. The sensor can be used for temperature monitoring since temperature affects the permeability and magnetic hardness of the sensor material, causing a change in the higher order harmonic amplitudes. The higher order harmonics also respond to stress since the sensor material is magnetoelastic. A theoretical model is presented to explain the effects of temperature and stress on the higher order harmonics. Experimental results are presented illustrating application of this sensor technology to monitoring of stress and temperature from within solid structures such as wooden and concrete beams.