Research On Performance Enhancement Of Residence Time Difference Fluxgate Sensors Based On The Dynamic Hysteresis Model

Since the "13th Five-Year Plan" for the Science and Technology Innovation and Development Project of China was proposed,the explorations of deep earth,the aerospace,and the deep sea have never stopped,and various types of magnetic sensors for weak magnetic field detection have played an important role on the stage of social development.The fluxgate magnetometer is still of the interests in weak or DC magnetic field detection on account of its stability and simple structure.Due to the increasing demand for magnetic field measurement,the wave of the explorations of new fluxgate technologies has also been launched internationally,aiming to design magnetic fluxgate magnetometers with simpler structure,better performance and wider application range.The residence-time-difference(RTD)fluxgate sensor technology,a novel fluxgate technology,is a measuring method in time domain.It is effectively immune from the symmetry error of the fluxgate probe of the traditional harmonic fluxgates,and is featured with a simple structure and low power consumption.Since 2005,the researches on RTD fluxgate magnetometers have been widely carried out across the world,including the analyses of the working principle of the RTD fluxgate sensor,and the design for the structure and the peripheral circuit,which have achieved certain results.However,the hysteresis characteristic of the magnetic core is the basis of the fluxgate sensor,and the inherent fluctuation characteristic of the hysteresis loop limits the further improvement of the performance of the RTD fluxgate sensor.Therefore,focusing on the intrinsic coercivity of the magnetic core,the fluctuation of the coercivity is utilized to characterize the volatility of the hysteresis loop.This thesis analyzes and quantifies the influence of the coercivity fluctuation on the sensor output and proposes the dynamic hysteresis model of the RTD magnetic fluxgate sensor.Firstly,based on the dynamic hysteresis model,the sensitivity and the noise level of RTD fluxgates sensors have been studied deeply,and the sensitivity model near zero magnetic field and the noise transfer model of the RTD fluxgate sensors have been established.And then the physical structure and the signal conditioning circuit of RTD fluxgate probe have been developed and applied.Finally,the performance of the RTD fluxgate sensor has been tested and a single-frequency time difference method has been proposed to effectively improve the resolution of the sensor and reduce its noise level.The experimental results show that the fluctuation range of the time difference is ±0.5 ?s.And the calculation results of the Allen variance show that the resolution of the RTD-type fluxgate sensor designed in this paper is 0.064 nT,which is better than the RTD-type fluxgate sensor developed abroad.The goal of performance enhancement of the RTD fluxgate sensor has been achieved.(1)The dynamic hysteresis model of RTD fluxgate sensors has been established,which directly shows the dynamic relationship between the external magnetic field and the magnetization intensity.RTD magnetic fluxgate sensor is a device operated in the time domain to detect the time difference between positive and negative saturation states of magnetic cores,while the coercive force is the threshold for switching between positive and negative states.Hence,the Langevin equation is applied in this thesis to describe the fluctuation of coercivity,and according to this characteristic,the statistical model of dynamic hysteresis is established.On this basis,the influence of the fluctuation of coercive force on the time difference fluctuation is quantified,and it concludes that the magnetic noise of the magnetic core is the main noise source of the RTD fluxgate magnetometers.(2)The sensitivity model of RTD magnetic fluxgate sensors near zero magnetic field has been established,which could calculate the sensitivity accurately through the physical parameters of the RTD fluxgate probe(number of turns,length,and diameter of the excitation coil),the excitation conditions,and the dynamic hysteresis states of the magnetic core.And the relative errors of the calculation results were no more than 4%.The sensitivity of the RTD fluxgate sensor is simultaneously related to the excitation condition,the coercive force,and the external magnetic field,and the coupling relationship between the coercive force of the magnetic core and the excitation condition makes the optimization of the sensitivity difficult.Based on the dynamic hysteresis model,the sensitivity model utilizes the dynamic differential permeabilities instead of the coercivity in the sensitivity mathematical formula,which decouples the coercive force and excitation conditions.As a result,this model achieves a fast calculation of the sensitivity of RTD fluxgate sensors and simplifies the design process of sensor sensitivity.(3)The noise transfer model of the RTD fluxgate system has been established to quantify the influence of the fluctuation of coercivity on the time difference output,and some effective methods for reducing noise have been proposed.Firstly,the different noise sources of the RTD fluxgate sensor are analyzed,and the noise transfer model is established for the RTD fluxgate sensor according to the error transfer principle.And then the dynamic hysteresis loop measurement system is applied to measure and quantify the effect of coercive force fluctuation on time difference.Finally,some effective methods for noise reduction are proposed for different noise sources,combined with the relationship between the noise characteristics and the system performance parameters.(4)The system of the RTD fluxgate sensor has been designed,and some advices about the selection of the excitation conditions and the magnetic core material have been given.The test results showed that the time difference fluctuates within 1 ?s and the maximum sensitivity can reach 10 ?s/nT.According to the interinhibitive relationship between sensor sensitivity and noise performance,the excitation conditions and magnetic core materials are selected.The preamplifier,filter and hysteresis comparator are designed as well.Finally,the development of an RTD fluxgate sensor system is fabricated.Finally,based on the experimental test of the sensor system and the analysis of the results,a single-frequency-time-difference(SFTD)method which can suppress the high frequency electronic noise superimposed on the induced voltage while maintaining the sensitivity of the RTD probe,has been proposed for the noise characteristics of the RTD fluxgate sensor.The experimental results show that the fluctuation range of time difference of the RTD sensor output is ±0.5 ?s,which is better than ±1 ?s of the RTD sensor developed abroad and ±5 ?s of the primitive RTD sensor developed by Jilin University under the same excitation condition.The calculation results of Allen variance show that the resolution of the system can reach 0.064 nT,which is 5 times higher than the traditional time difference detection method.The purpose of improving the performance of the RTD fluxgate sensor has been achieved.