Research On TMR Magnetic Sensing Technology With AC Excitation

Tunnel magnetoresistance(TMR)has many obvious advantages,such as high sensitivity,low power consumption and easy miniaturization,and has great potential to be developed into high-performance small magnetic sensors.However,in the aspect of high-performance linear magnetic measurement,TMR still has many practical problems,such as large 1/f noise,low resolution of low-frequency magnetic field,and insufficient output stability.To solve the above problems,this thesis proposes a design scheme of TMR magnetic sensing system with AC excitation.By combining theoretical analysis and experimental research,the dynamic noise characteristics,output response characteristics and output stability of TMR under AC excitation are studied,and the characteristic change rule of TMR under AC excitation is mastered.A TMR magnetic sensing system with AC excitation is developed.The test and evaluation show that the noise performance of the TMR magnetic sensing system with AC excitation is improved by about 4.3 times,the system zero bias is reduced by about 96.8%,and the output drift with time is reduced by 83.3% compared with the DC supply condition..The main research work is as follows:1.The dynamic noise characteristics and output response characteristics of AC excited TMR are studied by combining theoretical analysis and experimental exploration.The study shows that the noise of AC excited TMR is reduced by about 30% and the sensitivity of TMR is not lost.Firstly,the variation mechanism of TMR noise characteristics and output response characteristics is analyzed theoretically.Then the dynamic noise characteristics and output response characteristics of TMR under AC excitation are studied by using the noise testing system and output response characteristics testing system.The experiment shows that the TMR noise under AC excitation is separated,and about 30% of the noise is left in the low frequency band.In terms of output response,the sensitivity of TMR under AC excitation is not lost,and its core characteristics such as output response curve,sensitivity and hysteresis are consistent with those under DC power supply.2.Theoretical analysis and experimental research onTMR output stability under AC excitation are carried out.The research shows that AC excitation can significantly reduce TMR output fluctuation and output drift,and effectively improve the repeatability of TMR output drift under variable temperature.In this thesis,the internal and external factors affecting the output stability of Wheatstone bridge TMR structure are analyzed,and the influence mechanism of AC excitation on the output stability of TMR is clarified.The output stability of TMR under different excitation conditions,different magnetic field conditions and different temperature conditions was studied by constructing a test system.Experimental results show that AC excitation can reduce TMR output fluctuation by 62.5% under constant temperature,and the maximum output drift can be reduced to 1/9 of that under DC power supply.Under the condition of variable temperature,AC excitation reduces TMR output drift to 1/6 of that under DC excitation,and significantly improves the repeatability of TMR output drift,which lays a foundation for the establishment of high precision temperature drift compensation method.3.The design scheme of "AC excitationTMR magnetic sensing system" is proposed,and an AC excitationTMR magnetic sensing system is established,which integrates AC excitation generation and regulation,AC signal acquisition and processing,magnetic field real-time compensation and calculation.In this thesis,AC excitation and magnetic field compensation are integrated into the TMR magnetic sensing system,and the design scheme of "AC excitationTMR Magnetic Sensing System" is proposed.The overall architecture of the magnetic sensing system is designed,the probe module of TMR magnetic sensing is designed,and the software and hardware of the measurement and control system are designed.In the aspect of circuit system,the power supply module,AC excitation(chopper)circuit module,compensation coil constant current driver module and serial port sending module are designed and prepared.In software,the main program of measurement and control based on digital signal processor and output display platform based on virtual instrument Lab VIEW are established.Finally,a TMR magnetic sensor system is realized,which integrates AC excitation generation and regulation,AC signal acquisition and processing,magnetic field compensation and calculation.4.A performance evaluation experimental system is constructed to test and evaluate the performance of the designed TMR magnetic sensor system.The evaluation results show that the noise performance of the AC excitationTMR magnetic sensor system is improved by about 4.3 times,the system zero bias is reduced by about 96.8%,and the drift of system output with time is reduced by83.3%.The experimental system is constructed to test and evaluate the software and hardware performance level of the measurement and control system and the key performance indexes of the AC excitationTMR magnetic sensor system.The evaluation results show that the noise of the AC excitationTMR magnetic sensor system is about 2.1nT,which is 4.3 times lower than the 9.2nT noise performance under the DC condition.The system zero deviation is about 0.35 nt,which is96.8% lower than the 10.8nt of DC power supply.The drift of system output over time is about0.015%/h at most,compared with DC power supply conditions,it is reduced by 83.3%.The experimental results further verify the feasibility of the proposed scheme,and also accumulate experience for the development of TMR magnetic sensor.