Differential-Type GMI Magnetic Sensor Based on Longitudinal Excitation

A novel design of a linear differential-type giant magneto-impedance magnetic sensor found on the Fe-based amorphous ribbon with the longitudinal excitation is presented and the performances of the GMI sensor are carefully studied. A double-sensing element structure is used to construct the sensing head. Each of the sensing elements separately uses an 18-mm-long, 2-mm-wide and 20-mu m-thick amorphous ribbon wrapped by a 6-mm-long coil, exhibiting a giant magneto-impedance ratio. The asymmetrical behavior of the GMI effect is required because the sensitivity and linearity for the magnetic field are the most important parameters in the practical application of the GMI effect to magnetic sensors, and this has been realized by providing a bias current through the coil in the sensing element to offer a bias magnetic field. By providing two bias magnetic fields to the double-sensing elements in the opposite direction, the characteristic slopes of the first and the second sensing elements in the working section are decreased and increased, respectively. Accordingly, a differential-type amplifying circuit is designed for subtracting the voltage drops over the two sensing elements to attain the output of the sensor. The sensitivity is found to be 200 mV/Oe before the amplification which is twice that of the single-element structure in the dynamic range of +/- 2.5 Oe. The linearity error is 0.48% and the bandwidth of the sensor is more than 2 kHz at -3 dB. Besides, the stability is improved and the offset drift is reduced with the differential-type sensing head.