Research On Technology Of Digital Tracking And Locking For Optomagnetic Resonance Frequency Of Optically Pumped Helium Magnetometer

The technology of magnetic field measurement is playing more and more significantrole in scientific research and human life. The Optical Pumped Magnetometer (OPM)has attracted extensive attention worldwide for its outstanding performance, and hasmade remarkable progress. OPM is widely used in many fields, such as space magneticmeasurement, antisubmarine, biomedical and so on. It has the characteristics of highprecision and good real-time performance, etc. Optical pumped magnetometer’s abilityof tracking frequency reflects the instrument’s ability to adapt to external magneticfield. The instrument is required to have fast tracking speed to optical magneticresonance frequency to adapt to external magnetic field and lock in it if it is used in theapplications such as aviation, or the magnetic environment is complex and changeable.This thesis introduces the fundamental detection principle, tracking method andlocking process of OPM. Main research events and achievements are listed as follows:1)Starting with the basic principle of OPM, the reasons why chooses the outputsignal as the controlled subject are sketched, combining with the working process ofclosed loop, the principle and consistence of the detection system are analyzed. Finally,it is proposes that the essence of OPM is a system to track and lock the frequency, andthe OPM should be regarded as a phase-locked loop.2)According to the frequency nature of the helium optical pumping magneticsensor, a simulator of sensor is designed, it implement the function of Helmholtz coilby modulation domain frequency measurement technology to receive a feedback signalof the closed loop, and generate a magnetic resonance signal simultaneously based onFPGA, it can simulate the specific magnetic field value by setting the frequency. 3)The model of control system is established under the minimum deviationprinciple, and the rationality of the model is also verified, the digital controller base onFPGA is designed by using proportional-integral-differential (PID) control, whoseparameters are selected according to the Nichols-Ziegler parameter tuning method.This thesis proposes the principle of selecting the parameters, and analyzes the stabilityof the control system.4)Analyzing the effect of sampling rate on tracking performance, designing amulti-rate filter under the condition of selecting sampling rate reasonably to realize thetracking of the magnetic field without aliasing or distortion.5)Making simulation for digital controller, measuring the tracking performance ofdevice and testing the simulator for sensor. Comparing the tracking and lockingperformance of different parameters. After setting a reasonable sampling rate, the LeastSignificant Bit (LSB) of noise is tested out, then analyzing the sources of the noise andresuming the limit of noise to improve the tracking speed.