Research On Lock-in Amplification Circuit For Mems Fluxgate Sensor

The geomagnetic field is everywhere in space and plays a very important role in all aspects of our life and work.With the progress of human science and electronic technology,magnetic sensors used to detect magnetic fields have been widely used in national defense,Marine survey,satellite navigation,transportation,modern medicine and other fields.Fluxgate sensor uses the nonlinear relationship between excitation magnetic field and induced electromotive force to realize the measurement of environmental magnetic field,which has the benefits of small size,low power consumption and high sensitivity.Traditional fluxgate sensors work in low frequency band,and there are few studies on MEMS fluxgate sensors working in 100 k Hz～1MHz frequency band.This paper proposes a hybrid modulo/digital lock-in amplification technology for fluxgate sensors in this frequency band.The hybrid lock-in amplifier has the advantages of miniaturization,simple structure and low cost of the traditional analog lock-in amplifier,and has the advantage of the all-digital circuit that can adjust and generate signals in real time according to the needs.It can effectively realize the extraction and demodulation of the induced signals of the three-axis fluxgate sensor,and realize the measurement of the environmental magnetic field.In this paper,the basic working principle and detection method of fluxgate sensor are studied,and the lock-in amplification technology using the second harmonic method is determined.The basic structural characteristics of the MEMS fluxgate sensor are introduced,and the test and calibration are carried out.The results show that the comprehensive performance of the probe is the best under the excitation frequency of 400 k Hz.Based on the above theories and experiments,the basic structure of the three-phase hybrid lock-in amplifier circuit is designed and determined,which mainly includes two modules: digital circuit and analog circuit.The digital circuit uses 32 Microcontroller Unit to control the DDS chip to generate four sinusoidal waves,which are used as the excitation signal of fluxgate sensor and the reference signal required for the demodulation of lock-in circuit.The frequency,amplitude and phase parameters of sinusoidal waves can be modified and adjusted in real time by the master computer according to the needs.Compared with the full analog circuit,it has the advantages of more flexible,stable and controllable signal generation.The analog circuit mainly includes phase-sensitive detector(PSD),zero-setting circuit,voltage amplifier,low-pass filter(LPF),etc.,which is used to extract and demodulate the second harmonic in the fluxgate output signal,and finally output the DC signal carrying the magnetic field information to be measured.In order to increase the observability of the system,a high precision data collection circuit is designed.The digital mining circuit uses STM32 to control the high precision ADC chip AD7175 to realize the analog-digital conversion of the three-axis signal,which is stored and presented in PC through the upper computer in real time.Simulation verification,functional testing and calibration of the above software and hardware design modules are carried out,which proves the feasibility of the design scheme.Then combined with the three-axis MEMS fluxgate probe,the whole sensor was powered on and calibrated under laboratory conditions,including disturbance test,sensitivity,linearity test,noise test,resolution test,etc.,and the performance of the three-axis MEMS fluxgate sensor was analyzed from many aspects.The results show that the resolution of X,Y and Z axis is basically the same,all of them are 6n T,the noise level is about 500 p T/√Hz@1Hz,and all of them can reach the sensitivity of 1.0V/Oe,the best X axis can reach 1.24V/Oe.In the range of ±0.8Oe,the linearity of the triaxial varies from 1.7% to 2.4%.In the range of ±0.5Oe,the linearity of the triaxial varies from 0.2% to 0.8%.The experimental results show that the sensor excitation frequency of the three-axis mems magnetic pass sensor is100 k Hz～1MHz,which is the effective exploration and breakthrough of the low bandwidth of the traditional magnetic gate.At the same time,there is a simple and flexible advantage of small,structural and flexible,and can also measure the space vector magnetic field.