Research On GMR Sensing Technology Of High Dynamic Direct Current

With the development of smart grid,and the ever-increasing load of direct current,the power system requires current measurement technology with wider measurement range.In order to monitor the status of transmission and distribution lines better,the power system needs to collect information of multiple power nodes.Due to the long distance of the transmission and distribution lines and wide range of dynamic current,many important power nodes have not achieved current monitoring yet.At the other hand,electric vehicle is an important load accessing to the smart grid in the future.Accurate measurement of electric vehicles load under the fast charge and slow charge mode is the vital interest of power grid and users.This thesis designs a new type of current sensor which can realize high dynamic DC measurement based on GMR material.This thesis compares principle and characteristics,and analyzes advantages and disadvantages of different DC current sensors.The research status of Giant Magnetoresistance(GMR)current sensors is introduced emphatically.By establishing a GMR static experimental platform,the sensitivity characteristic,hysteresis characteristic,temperature characteristic and magnetic sensitive shaft of the GMR were investigated.The test results show that the GMR chip hasissues of hysteresis,temperature drift and location sensitivity due to its magnetic sensitive properties.For the shortcomings of low sensitivity and poor linearity of GMR chip,this thesis presents a structure of open-loop current sensor using GMR chip as magnetic probe.The open-loop GMR current sensor consists of a GMR chip,a magneto-magnetic ring and a differential operational amplifier,which improves the sensitivity of sensor effectively.Based on the open-loop GMR current sensor structure,a closed-loop GMR current sensor design is proposed.This design compensates the magnetic field through the feedback circuit and improves the accuracy and dynamic measurement range of GMR current sensor.The electromagnetic relationship of GMR current sensor is studied by finite element software,and the closed-loop control principle is verified.By setting up GMR current sensor test platform,the static characteristics of GMR current sensors with different structures is tested.The experimental results show that the accuracy of the open-loop GMR current sensor is 2.5%,and the dynamic measurement range is 100mA~7.65 A.The accuracy of the closed-loop GMR current sensor is 0.77%,and the dynamic measurement range is 10mA~25A.It should be noticed that closed-loop GMR current sensor can measure the maximum current of 430 A or more with the increasing of the number of feedback coil turns,in actual industrial production.A hysteresis model based on hyperbolic tangent function is presented in this thesis.The experimental results proved that the hysteresis model is helpful to improve the accuracy of GMR current sensor,which provides valuable experience for the integrated development of GMR current sensor.The GMR current sensor can be used in detection of transient electromagnetic current,with the range of transmitter current and receiver current from several milliamperes to hundreds of amperes.According to the specific test,GMR current sensor can accurately reflect the waveform of transmitter current and receiver current,and has the capability of high dynamic DC measurement.