Research Of The Method And System Of The Nuclear Magnetic Resonance Magnetometer Based On Echo Induction

Nuclear magnetic resonance(NMR)magnetometer is a precision measuring instrument with the accuracy of 1nT based on the strict linear correlation between the frequency of nuclear resonance and the strength of magnetic field.However,due to the limitation of bad SNR in weak field,its measurement range generally not less than 20 mT.Some new methods,such as atom magnetometer and quantum interferometer,can achieve the ultra-high measurement accuracy of 1fT.However,it only applies to the measurement of near-zero magnetic field under 0.1mT.To achieve the high precision measuremen of the 0.1mT ~ 20 mT magnetic field,a fluid-type NMR magnetometer was studed in this paper.The capability for weak field measurement of the NMR magnetometer was promoted by pre-polarization technology.An improved signal detection method based on echo induction principle was proposed.The main research contents of this paper include the following parts.(1)The measurement principle and the mathematical physical equations involved in the process of the fluid-type NMR magnetometer was described in detail.(2)By analyzing the working principle of the autodyne detector and inductive detector,with the application of CPMG sequence,the method of longitudinal magnetization vector detection by using induction detector instead of the autodyne detector was proposed.(3)By constructing the mathematical model of absorption peak acuity,the influence of system parameters on the measurement results was analyzed.(4)A fluid-type NMR magnetometer containing a 1.3T Halbach polarized magnet,a 0.182 T homocentric squares detection magnet,saddle excitation coil,solenoid detection coil,control circuit and PC software was completed.The control circuit included two parts: the radio frequency(RF)signal transceiver based on field programmable gata array(FPGA)and the system management circuit based on microcontrollers(STM32).The former realizes the digital design of induction-receiver by means of digital demodulation of echo signal and digital modulation of RF signal.Then the anti-jamming ability was improved.In addition,a more versatile list-type sequence controller was implemented by extracting the repeatable sequence model.The latter achieves efficient coordination of RF excitation,flow rate control,data transmission and other functions through by means of the voltage control current source(VCCS)and SDRAM used for data cacheExperimental results show that,the inductive detector has better reliability and stability for the detection of longitudinal magnetized vector.The prepolarization treatment increased the intensity of the echo signal by 5.78 times.Considering the influence of the uniformity of the measured magnetic field,the fitting equation with a correction coefficient was highly consistent with the measurement data,because of that,the measurement speed and measurement accuracy were improved.The intensity of excitation and velocity of flow-water have significant influence on the measurement curve.Optimization of system structure and measuring parameters can obtain more sharp absorption peaks.Finally,the measurement of magnetic induction intensity of 0.136mT~1.215 mT was realized by this method,and the measurement accuracy is better than 0.1?T,which proves that this method is capable of accurate measurement of weak magnetic field.