Research Of Sensors Based On Giant Magnetoimpedance Effect Of Co-rich Amorphous Materials

With the rapid development of science of material and modern communication technology, in the magnetic measurement field, some traditional sensors can not meet demands of people in certain aspects, such as size, sensitivity, full scale, thermodynamic stability, power expend, cost and so on. For example, Hell sensor, flux gate sensor, librated or rotated coils, there are some flaws, for instance, the circuits are too complicated and cost is high. Since 1992 giant magnetioimpedance (GMI) effect had been discovered, whose sensitivity could reach 20%/Oe, it could produce a new non-contact sensor with well stability, high sensitivity, high speed respond and low cost. The GMI effect is the phenomenon that exhibits a huge variation of the electrical impedance of the magnetic materials as a function of the applied magnetic field. It has been interpreted in terms of the classical skin effect as a consequence of the change in the penetration depth of the alternating current. The penetration depth depends on the magnetic permeability which varies with the applied magnetic field. In the recent years, great interest has been paid to the applications of the GMI materials for their huge potential application value.There are same classical designs of circuits in the magnetic field sensor application research. Most of the magnetic field sensors are designed by Japanese research groups, such as Mohri and Panina group in Nagoya University. Most of the previous GMI magnetic sensors adopt that complicated and elaborate coils winding around the probes to supply a bias magnetic field to fix the working point of the probes. When the bias coils are carrying current, they bring excess power consumption, and production of heat also influence the sensor performance. Therefore, in my thesis research, we present a novel design of magnetic sensor based on the GMI effect of commercial Co-rich amorphous ribbons (AT&M 2714) and self-made Co72Zr8B20 amorphous ribbons in laboratory, it can measure the low intensity quasi-static magnetic field. The magnetic sensor probe consists of GMI probe, Colpitts oscillator, per-amplifier and rectifier. In the Colpitts oscillator, Co-rich amorphous ribbon becomes a part of emitter load of transistor. Because the frequency of the Colpitts oscillator only depends on the intrinsic frequency of the crystal oscillator (3.579 MHz), the Colpitts oscillator provides GMI probe a stable ac driving current. The pre-amplifier is used to enlarge the ac voltage of the amorphous ribbon. Then the enlarged signal of pre-amplifier is transformed to dc voltage of the probe by rectifier. The gain bandwidth of pre-amplifier is only 15 MHz, so the high frequency noise can be restrained. In this thesis, we introduce two processing output method of the magnetic sensor: linear output and nonlinear output, which are based on the two above amorphous ribbons respectively.The magnetic sensor using AT&M 2714 adopt linear output. First, the as-cast AT&M 2714 are cut into three samples with sizes of 1×60 mm,2×60 mm and 3×60 mm. Then the as-cast ribbons are annealed in a high vacuum condition at the temperature range 200℃for 30 min. Both of the as-cast and the annealed ribbons are jointed in the probe circuit. All the output characteristics with different ribbons are measured. It is noted that both MIR curves of the as-cast and the annealed ribbons are double peaks and asymmetry. The ribbons show the good transversal anisotropy. Compared with the as-cast ribbons, the annealed ribbon GMI effect is not improved obviously after releasing internal stress. In view of the output characteristics and the actual conditions, the as-cast ribbon with the size of 2×60 mm is adopted. In the design of the magnetic sensor, differing from the formerly GMI magnetic sensor, two NdFeB permanent magnets are used to supply a bias magnetic field for the amorphous ribbon working point setting, which are avoiding the problems of the complicated bias coils. Furthermore, a negative feedback-coil is adopted in order to linearize the sensor output. Finally, this sensor can detect the magnetic field in the range of±1 Oe with the sensitivity 1041 mV/Oe, the measurement precision is less than 0.11%. All the technical parameters are better than that of the amorphous metal alloy double core multivibrator bridge magnetometer which is widely used in the low intensity magnetic field measurement.The magnetic sensor using Co72Zr8B20 amorphous ribbon is nonlinear output. The as-cast and the annealed ribbons are jointed in the probe circuit, the annealed one is obtained in a high vacuum condition at the temperature range 495℃for 10 min, both probe output characteristics are measured. We find that both MIR curves of the as-cast and the annealed ribbons are single peaks. It is indicated that both ribbons have longitudinal anisotropy. And the curves of the probe output for the positive and negative magnetic field are superposition, but nonlinear. The annealed Co72Zr8B20 ribbon is used in actual sensor design. The probe output is enlarged by amplifier with zero-adjust properly, then the enlarged signal is converted into digital signal by A/D conversion. According to the fitting relation of the digital signal and applied magnetic field, the micro processor can process the digital data and control the LED display to show the value of the magnetic field. This GMI magnetic sensor resolution factor reach up 0.01 Oe, the measuring range is 0～50 Oe, and the measurement precision is less than 0.068%.To sum up, the new GMI magnetic sensor is design in this thesis. We can use the permanent magnets to supply a bias magnetic field for the amorphous ribbon working point setting. The sensor exhibits stable frequency, simple structure and low cost. It can be used widely the low intensity magnetic field measurement of industry and education.In other application fields of the GMI sensor, the researches are focused on the current sensor. Most of the GMI current sensor probes adopt ring cores structure which are composed of amorphous materials, and the conductor wire flowing current is coaxial with the axis of the ring cores. Complicated and elaborate coils are winding around the ring cores to supply a bias magnetic field for the amorphous materials. When the bias coils are carrying current, they bring excess power consumption, and production of heat also influence the sensor probes working capability. For the weak signal of some above GMI current sensors, even some sophisticated equipments, such as lock-in amplifiers (LIA), are employed in the last port of signal detection. In this thesis, we design and fabricate two non-contact type current sensors with the novel array structure and the spiral structure double-probe, respectively. The permanent magnets are used to supply the bias magnetic field for fitting the probe working point, which are avoiding the problems of the complicated bias coils on the traditional ring cores. Meanwhile, we introduce two sensor design ideals of using GMI materials: GMI probe to be a load or inductance of the oscillator circuit.In the design of GMI current sensor with array structure double-probe, the GMI probes are fabricated using some AT&M 2714 pieces, which are parallel to array each other in equidistance and welded with copper in series form. The identical array structure probes (probe 1 and probe 2) are set on both sides of the conducting wire. A permanent magnet supplies a dc bias magnetic field Hb to set the working point of the probes, the Hb can be obtained by adjusting the distance between the probes and the permanent magnet. When the conducting wire carries measuring current, a circular induced magnetic field Hi is applied on the longitudinal direction of the amorphous ribbon pieces. Due to the existence of bias magnetic field Hb, the total magnetization decreases in probe 1 (Hb-Hi) and increases in probe 2 (Hb+Hi), resulting in that the impedance of both probes change differently with the current. The vibration and rectifier circuit of the current sensor is same as the above probe circuit of the magnetic sensor, which is composed of Colpitts oscillator, pre-amplifier and rectifier. The GMI probe becomes a part of emitter load of transistor. The signals on the probes are transformed into dc output voltages U1, U2 by pre-amplifier and rectifier. U1, U2 are in proportion to the impedances of the probe 1 and probe 2. both the single-probe output U1 and the double-probe output U (U2 -U1) are measured at a dc bias magnetic field Hb ranging from 1.08 to 18.38 Oe. The differential output U shows excellent sensitivity, linearity and stability at Hb of 7.40 Oe. The sensitivity of the sensor reaches up to 1 V/A by adjusting the reference voltage and the gain of the amplifier with zero-adjust. the measuring range is±3 A, and the measurement precision is less than 0.15%. The technical parameters are better than that of the Hall sensor which is widely used. The best advantage of this sensor is open-loop probe structure. When the current is measuring, you only need to put the wire in the spacing between the double probes. The distance of the double probes can be adjusted. So the installation and take-down of this sensor have no influence on the working circuit with the measured current.In the design of GMI current sensor with spiral structure double-probe, the GMI probes are made of AT&M 2714 ribbons which are wound around rubber sleeving in spiral form. A couple of permanent magnets are used to supply bias magnetic field for the probe 1. The amorphous ribbon of the probe 1 is divided into two magnetization regions (the top part and bottom part) by the permanent magnets. The magnetization directions of two regions are reversed, clockwise and counter-clockwise. When the distance between the probe 1 and the permanent magnets is adjusted properly, the magnetization of top part is much higher than that of bottom part. So the total magnetization of the probe 1 is clockwise and not amounts to 0. The identical spiral-structure probe 2 and a couple of permanent magnets are set in the same way except that the magnetization direction of the permanent magnets is reversed. So the total magnetization of the probe 2 is equal to probe 1, but counter-clockwise. A circular induced magnetic field is applied nearly on the longitudinal direction of the spiral amorphous ribbons of the probes when the conducting wire carries the measured current. On account of the reversed direction of double-probe magnetization, the total magnetization decreases in probe 1 and increases probe 2. The impedance of both probes change differently with the measured current. The vibration and rectifier circuit is composed of Clapp oscillator and rectifier. The GMI probe becomes an inductance of the Clapp oscillator. When the impedance and inductance of the probes are changing with the measured current, the frequency of the oscillator signal and the rectified voltages U1, U2 are varying too. According to the actual conditions, we detect the current by measuring the voltages of the circuit. The as-cast AT&M 2714 are cut into three samples with sizes of 1 mm,2 mm and 3 mm in width. Three as-cast ribbons are curled to spiral tubes. The spiral tubes are annealed in a high vacuum condition at 200℃for 1 hour. Both of the as-cast and the annealed ribbons are jointed in circuit. All the output characteristics with different ribbons are measured. Finally, it is found that the annealed spiral probe of 3 mm in width shows lower hysteresis, and it is easy to make two identical probes. When the distance between the probe and permanent magnets is fixed at 2.4cm, the single-probe output U1 changes with current monotonically and shows low hysteresis. The differential double-probe output U (U2 -U1) exhibits excellent linearity and stability. The sensitivity of this GMI current sensor reaches up to 1 V/A, the measuring range is±1.5 A, and the measurement precision is less than 0.16%. The circuit of this GMI current sensor is more simple, economical, practical and convenient.In conclusion, we design two current sensors based on GMI effect of commercial Co-rich amorphous ribbons, the sensors with the array structure and the spiral structure double-probe, respectively. The permanent magnets are used to supply the bias magnetic field for fitting the probe working point. These low-cost sensors have simple structure and exhibits excellent stability, and reliability. So we find two non-contact methods of quasi-static current measurement.