The Magnetic Differentiation Technique for GMI Sensor

The “magnetic differentiation” technique is an original method for magnetic-field measurement. It is used for giant magnetoimpedance (GMI) sensors. The response is very different from the classical impedance measurement. To measure the external field Hext, small increments of magnetic field, alternatively positive and negative ±ΔH, are added to H
_ext
by a small coil around the GMI wire. The measured signal is the slope of the curve of the impedance Z(H
_ext
). The “magnetic differentiation” technique exhibits many advantages. The obtained response is a relatively simple curve with an odd symmetry. The zero of the sensor exactly corresponds to the zero magnetic field. The sensor is able to measure the “absolute zero” field, without any zero-adjustment. Around the zero-field point, the response is linear for a range of several tenths of A/m, with a very high sensitivity. The sensor response around the zero stays constant when the GMI wire is submitted to variable influence parameters, such as temperature or strain. Moreover, the measurement of high magnetic field can be easily made with an additional coil supplied by a closed-loop system; the operation point remains around the zero-field. Globally, the magnetic differentiation technique is an original way of using the GMI effect. The null signal at zero magnetic field, the linearity around the zero, and the odd response are very interesting properties, allowing the development of a new generation of GMI sensors.