Research Of Proton Magnetic Field Sensor And Signal Conditioning Circuit

Proton magnetometer is a magnetic field measuring instrument based on the principle of Larmor precession effect of protons in magnetic field.It is widely used in weak magnetic measurement fields such as geological survey,mineral survey and military exploration because of its high accuracy and low cost.As the absolute measurement accuracy of commercial proton magnetometers is about ±1n T at present,but in applications such as magnetic field measurement,the measuring accuracy of magnetic field measuring instruments is better than 0.1n T are needed,so the accuracy of the instrument can't meet the application requirements at present.Therefore,in order to further improve the measurement accuracy of proton magnetometer,this design designs and optimizes the proton magnetometer from three aspects: sensor probe,signal conditioning circuit and frequency measurement method.Specifically,the main contents of this paper are as follows:Firstly,the proton magnetic field sensor is studied from four aspects: structure,working substance,electromagnetic shielding structure and inductance coil.In order to improve the gradient tolerance,the size and structure of the sensor are optimized.In order to improve the measurement accuracy,the lateral and longitudinal relaxation time of different working substances are compared and analyzed.The aviation kerosene with the lateral relaxation time of 1 s and the longitudinal relaxation time of 0.5 s is selected as the working substance of the sensor.In order to improve the anti-jamming ability,three shielding modes are studied and simulated,the double shielding structure of copper foil and conductive coatings is selected.The simulation results show that the shielding effect can reach 137 d B.In the aspect of coil design,in order to obtain a larger value of Larmor induction signal,a quasi-square coil is proposed to replace the conventional circular coil.Under the restriction of determining the inner diameter of the sensor shell,aiming at maximizing the amplitude of Larmor induction signal,the coil structure parameters are optimized.The experimental results show that the induction signal amplitude of quasi-square coil is 1.43 times that of circular coil,and the SNR of induction signal is increased by 3.1d B.Secondly,in order to test the developed sensor,the signal conditioning circuit is designed in this paper.The circuit consists of five parts: polarization,tuning,amplification,filtering and shaping.Based on the analysis of the coil current and voltage changes during the switching-on and switching-off of polarization circuit,the corresponding switching and protection circuits are designed to ensure the safety of subsequent circuits and the reception of signals.Then,combining the parameters of inductance coil and the frequency range of Larmor signal,a capacitance tuning scheme is designed to select and amplify the output signal of coil.After selective frequency amplification,the signal is still a low SNR signal of micro volt level.In order to amplify the signal to volt level,a multi-stage amplifier circuit with high gain and low noise is designed in this paper.In order to meet the needs of frequency measurement,a waveform conversion circuit is designed to convert sinusoidal signal into square wave signal.Through the analysis of the conditioned output signal,the SNR is over 36 d B.Finally,on the basis of the multi-cycle frequency measurement method,aiming at the insufficient use of signal information,this paper improves it and proposes a multi-average frequency measurement method.The theoretical calculation shows that the relative error of this method is more than ten times lower than that of the multi-cycle frequency measurement method.The comparative experiment of frequency measurement shows that the accuracy of the ten-average frequency measurement method increases by more than 3.13 times under different signal frequencies,SNR and measurement time.