Design Of Three Axis Weak Magnetic Field Detection System Based On TMR Sensor

With the development of electronic technology and magnetoelectrics,magnetic field detection technology plays a more and more important role in power system.With the development trend of intelligent and informational power system,higher requirements for the precision of magnetic field detection are put forward.At present,there are many measurement techniques for magnetic field,such as superconducting quantum interferometer,optical pump magnetometer,fluxgate magnetometer and magnetoresistance magnetometer.The magnetoresistance sensor has the most research prospects because of its high sensitivity,small package size,cheap price and high reliability.Tunneling magnetoresistance sensor,as the latest generation of magnetoresistance sensor,has attracted much attention in recent years because its sensitivity is much larger than that of AMR sensor and GMR sensor.But the tunnel magnetoresistance sensor insufficient background noise is too large,according to the disadvantages,through the lock-in technology to design and develop the detection system of tunnel magnetoresistance sensor based on three axis magnetic,further improve the sensitivity of the tunnel magnetoresistance sensor,and reduce its noise as well.First of all,through the chip investigation,this paper selects the three axis linear sensor TMR2305 of Multidimensional Technology Co.,Ltd.Jiangsu as the front end sensitive element of the detection system.In order to reduce the interference,the TM2305 is soldering on a PCB plate as the probe of the detection system,and the probe circuit board and signal are adjusted through the connector.The circuit board is connected to complete the transmission of the signal.Then,according to the characteristics of TMR2305 output signal,a signal conditioning circuit based on phase-locked amplification is designed,which includes modulation circuit,amplifying circuit,AC coupling,phase sensitive detection and low pass filtering.The DC voltage signal output by the sensor is first converted to the AC signal of 2k Hz by the modulation circuit and then amplified by the low noise amplifying circuit.Therefore the noise of the system is preliminarily suppressed by the AC coupling.The output signal is multiplicative by the phase sensitive detection circuit and the 2k Hz reference signal,and a difference frequency,a sum frequency component and some high frequency noises are obtained.Finally,a sum frequency component and large amounts of noise are filtered after low pass filter circuit,an amplified DC signal is obtained.The minimum system of FPGA is designed to control the A/D conversion of analog signals of three AD7710 complete signal conditioning circuits.After the analog digital conversion,the data is uploaded to the upper computer software of the detection system by RS232.The software part is based on the idea of modularization and hierarchy of Lab VIEW,and adopts the design method from top to bottom.The main function is to collect and process magnetic field data and display and store it in the front panel of Lab VIEW.Finally,the performance indexes of the weak magnetic detection system are calibrated and tested.The standard three-dimensional magnetic field produced by the magnetic field generator custom-made by the East Changing company is adopted,and the five-layer shield cylinder custom-made by the China Iron and Steel Research Institute is used to shield the geomagnetic field and the stray magnetic field to provide a stable calibration environment.The three-axial performance of the system are equal,the sensitivity is about38 u V/n T,the linearity is 0.9999,the noise is about 0.9n T.The sensitivity of the detection system is not only raised about 150 times,but also the oversize of the background noise of the TMR sensor is also improved.The experimental results show that the weak magnetic detection system designed in this paper meets the high accuracy requirement of magnetic field measurement in power system.