Research On Technologies Of GMI Sensor Based On Self-balancing Bias Magnetic Field

Giant magneto-impedance(GMI)effect includes a large and sensitive change in the complex impedance(ac voltage)of a soft magnetic specimen subjected to an ac current with a high frequency upon the application of an external magnetic field.Due to the advantages such as high resolution,fast response,small size,low power consumption and so on,the magnetic sensor based on GMI effect(GMI sensor)has a broad application prospect in the field of long-distance ferromagnetic object detection and magnetoencephalogram(MEG)detection.However,the GMI sensor is facing many new challenges.The thesis focused on the GMI sensor in which the Co-based amorphous wire was used as the sensing element,and investigated the GMI characteristics of the Co-based amorphous wire,the structure of the magnetic detector and the sensor circuit,the self-balancing technology based on the bias magnetic field and the actual circuit of the GMI sensor.The main research work is summarized as follows.Based on the action mechanism and theoretical model of GMI effect,the simple measurement system of GMI effect was presented and the parameters having an effect on the GMI characteristics such as the length of the Co-based amorphous wire,the amplitude and frequency of the ac excitation current along with the connection type between the amorphous wire and the PCB pads were optimized in terms of the experiment results.According to the action mechanism and theoretical model of GMI effect,influence of length,amplitude and frequency on the GMI effect were analyzed.At the same time,it was pointed out that the actual GMI effect was also related to the factors including composition,prepared methods and annealing treatments.Therefore,the parameters including the length of the Co-based amorphous wire,the amplitude and frequency of the ac excitation current were determined in terms of the experiments results.In addition,it was found that the connection style had an important effect on the GMI characteristics.The connection style using conductive silver paint is more suitable for the connection between the amorphous wire and PCB pads.The thesis proposed the differential type GMI sensor circuit and its working principle.Based on the differential type GMI sensor,an integrated gradient type amorphous wire detector based on a pair of amorphous wires with the series configuration and the function elements of bias magnetic field self-balancing technology was presented.Since the GMI profile is unipolar and nonlinear near zero magnetic field point,the bias magnetic field was required to determine the static operating point.The changes in sensing direction or position of the GMI sensor have a remarkable effect on the bias magnetic field due to the uniform geomagnetic field without any magnetic shield.The magnetic anomaly signal from long-distance ferromagnetic object and MEG are week enough to be submerged in noise such as geomagnetic noise,electromagnetic noise and so on.The surrounding temperature has an effect on the GMI characteristics and further results in the output instability of GMI sensors.Therefore,an integrated gradient type amorphous wire detector and the differential type GMI sensor circuit were proposed.The thesis proposed self-balancing technology based on the bias magnetic field in order to provide a constant bias magnetic field whether the sensing direction or position of the GMI sensor changes.As we all known,the geomagnetic field is an anisotropic vector field.The vector components of uniform geomagnetic field in different direction or position on the surface of the earth are different.Thus,the variation in sensing direction or position of amorphous wire detector will cause a change of the vector component along the sensing direction.The vector component in combination with the external magnetic field induced the change of GMI output and the fluctuation of the output voltage signal of the GMI sensor.Moreover,it was difficult to determine whether the output voltage signal is caused by the variations in sensing direction or position of amorphous wire detector or the change in the detected magnetic anomaly signal.Therefore,the self-balancing technology based on the bias magnetic field was proposed.The variations of the vector components of uniform geomagnetic field were considered as the disturbance signal superimposed on the bias magnetic field instead of the magnetic anomaly signal.The self-balancing technology based on the bias magnetic field using the principle of the feedback control can self-balance the bias magnetic field so as to keep it in the initial value.The thesis proposed the optimum design methods in order to improve the noise level of the GMI sensor circuit,and designed and fabricated the actual sensor circuit.The development of high-performance GMI sensor depended not only on the selection of soft magnetic materials with good softness,the optimum of the geometry and measurement parameters and so on but also on the optimum design of the GMI sensor circuit.Combined with the overall design scheme of the GMI sensor circuit,all the functional modules of sensor circuit were investigated and optimized by comparing the different structures of the circuits.In the design process of the sensor circuit,the impedance matching and electromagnetic compatibility were taken into account.The simulation and experiment results demonstrated the theory and technology proposed were feasible and valid.They will provide important reference and guidance for the further study of GMI sensors with higher performance.