Study On The Noise Reduction Of TMR Magnetic Sensor Based On Axially Energized Magnetic Shielding Modulation

The tunnel magnetoresistance(TMR)sensor has the advantages of high sensitivity,small size and low consumed power.It has broad application prospects in medical diagnosis,nondestructive test,micro satellite and other fields.However,its low frequency noise is large,which limits the improvement of resolution.Therefore,a variety of noise reduction technologies have been studied around the TMR magnetic sensor at home and abroad,including:increasing the number of magnetoresistance units of the sensor bridge arm,using Sigma-Delta modulation circuit to reduce the output noise of the sensor,chopping to modulate the magnetic field around the sensor and then demodulating the chopping signal.Among them,magnetic shielding chopper which is to reduce noise does not need to change the sensor structure,and has the advantages of simple signal conditioning circuit.Current magnetic shielding chopper modulation mainly uses the second harmonic method to extract magnetic signals.Through the research,it is found that after applying DC bias magnetic field,the sensitivity of the sensing signal based on fundamental detection is higher than the second harmonic,and the noise reduction ability is better,which has guiding significance for the design of high-resolution TMR sensor..Firstly,the modulation-demodulation measurement system for Fundamental-Mode TMR sensor is designed:the sensor is placed in a micro magnetic shielding cylinder made of soft magnetic material,and the alternating current with DC bias is applied along the cylinder axis to generate a unidirectional loop magnetic field in the wall.By controlling the current,the soft magnetic material is supersaturated once per cycle.As a result,the shielding coefficient changes periodically and the low-frequency magnetic field outside the cylinder is modulated into the high-frequency magnetic field inside the cylinder.In this way,the measurement signal of the TMR magnetic sensor is separated from the low-frequency noise.Then the low frequency magnetic field signal can be restored and the low frequency noise of the sensor can be suppressed by using the demodulation circuit.In order to further clarify the working mechanism of the system,the signal output model of TMR sensor under magnetic shielding chopper is established by using electromagnetics method.Then,the modulation-demodulation circuit of the magnetic shielding modulation system is designed,which is based on the principle of lock in amplifier.It includes five circuit modules:signal excitation,preamplification,band-pass filter,phase sensitive detection and low-pass filter.In order to verify the noise reduction ability of the fundamental-mode magnetic shielding chopper measurement system,this paper establishes the noise equivalent models for the TMR sensor and modulation-demodulation circuit.By analyzing the thermal noise,shot noise and 1/f noise of the TMR sensor,the background noise of the sensor is calculated.By analyzing the noise of components and amplifying factor,the modulation-demodulation circuit noise is calculated.The simulation results show that the noise of the TMR sensor with magnetic shielding chopper is lower than the background noise of the TMR sensor without chopper,and the noise reduction ability of the chopper measurement system based on the fundamental wave detection is better than that of the second harmonic,which shows that the DC bias can improve the noise reduction performance of the magnetic shielding chopper measurement system.In this paper,the micro magnetic shielding cylinder and the modulating-demodulating circuit board with TMR sensor are made and packed in the processed casing.Using the magnetic sensor test platform in the laboratory,the magnetic shielding chopper system is tested and studied.Firstly,the influence of the remanence of the magnetic shielding cylinder on the measurement system is tested,and the remanence is restrained by the degaussing method.Secondly,the influence of the bias current amplitude,the alternating current amplitude,the alternating current frequency and the thickness of the shielding cylinder on the measurement system is analyzed theoretically,and verified by the experimental method Finally,the sensitivity,linearity and noise characteristics of the magnetic shielding chopper system are measured for the uniaxial sensor TMR2102 and triaxial sensor TMR2305M.The results show that:(1)for TMR2102,the signal sensitivity based on the fundamental wave detection is 31.5V/T,the linearity is better than 1%,and the noise characteristic is 4.07nT/(?)@1Hz,which is 80%lower than that without chopper.The signal sensitivity based on the second harmonic detection is 5.71V/T,linearity is better than 1%,and the noise characteristic is 12.56nT/(?)@Hz,which is 53%lower than that without chopper.(2)For TMR2305m,based on the fundamental wave detection,the noise reduction efficiency of Xaxis is 56%,Y-axis is 77%,and Z-axis is 78%.The above test results confirm that the magnetic shielding chopper measurement system has realized the noise reduction function,and the noise reduction effect based on the fundamental wave detection is better than that of the second harmonic,and the noise reduction effect of the radial chopper is better than the axial chopper.