Contactless harmonic detection of magnetic temperature sensor

Two new modifications of remotely interrogated passive temperature sensors using thermo-reversible permanent magnets (1) and temperature-sensitive Ni-Fe soft magnetic alloys (2) are presented in this paper. The operation of both sensors is based on the analysis of the harmonic amplitude variations generated by the sensor under the excitation of an external AC magnetic field. A Fast Fourier Transform (FFT) is used to obtain the harmonic decomposition of the sensor output voltage. In (1) a magnetically soft thin strip (core) is polarized by the bias permanent magnet made of a new type of thermo-reversible alloy (Gd-Co-Cu alloy). As a result of this biasing even harmonics appear in the magnetic induction spectrum of the sensor. The amplitude of these harmonics varies with the reversible and single-valued change of the permanent magnet biasing field, which depends strongly on the temperature. In this way, the amplitude variations of even harmonics are considered as an indicator of the temperature. In (2), a single strip of specially prepared temperature sensitive Ni-Fe alloys is used as a sensor core. It was found that under external AC excitation such strips exhibit varying harmonic signature versus temperature. In this case only odd harmonics are present in the output spectrum and they are taken as a measure of the temperature. Some experimental results are presented and analyzed in both cases.