Development Of Broadband Inductive Magnetic Sensor

The inductive magnetic sensor is used widely as magnetic field receiver in FEM methodof MT,and it has some superiorities such as wide bandwidth,wide range of detection depth,small size,light weight,simple operation,low cost and so on.The inductive magnetic sensorswere researched in the end of the1970s in China by some research institutions and made greatprogress in the the early21st century.However, there is large distance compared with similarforeign instruments.High performance magnetic sensors are imported from Europe and theUnited States for a long time with expensive price of one hundred thousand yuan.There aresome disadvantages such as long cycle of repair, impediment to development domesticinstruments,high price of the instrument in a monopoly position.The magnetic sensors havebecome an important factor to restrict the development of the electromagnetic instrument.Weanalysis and optimize the schemes which affect the performance of inductive magneticsensor.Finally we achieved a product with wide band width,high sensitivity,low noise.Thefrequency range of the sensors is0.001Hz-10kHz,and the sensitivity is0.3V/(nT*Hz)@0.01Hz,0.92V/nT@1kHz.The result of the tests in field shows that the noise of sensors is0.1nT@0.001Hz,1*10-6nT@1kHz.The stability and other parameters of the sensor are closeto the foreign product.And lay the foundation for electromagnetic instrument.This research issupported by the research of the wide band magnetic sensors based on magnetic feedback ofthe National Natural Science Foundation and it is also funded by the development of theelectromagnetic instrument detection of large depth of underground resources of industrialtechnology projects Jilin Province.The main contents and results are as follows:(1) The research of the principle measuring magnetic. According to Faraday's law, weobtain SBL=2π*f*N*μa*S*G at low frequency, SBLis the dynamic sensitivity of thesensor.We analysis the parameters affecting the sensitivity of the sensor including the number of turns of the coil, effective area of the core, apparent permeability and magnification of thecircuit. And we propose the key topics which can improve the performance of the inductivemagnatic sensor.(2) The methods of increasing the effective permeability and the effective area isresearched. The permalloy is used as core of the sensor according to the magnetic relativepermeability, conductivity, temperature stability,frequency stability and the productionprocess.We advance the method of measuring the apparent permeability and simulating thedistributing with finite element software. We determine that the apparent permeabilitydecreasing from the center of the core to the sides. The core is added magnetic fluxconcentrators on both sides of it, the apparent permeability is twice when the diameter are7cm,and the thick4cm.The magnetic metal sheets(30layers) increases the effective area to17times and reduces the eddy current loss to3％.(3) According to Basic model of the coil,we propose the tandem model which canexplain amplitude-frequency characteristics of the coil and summe up the formula by whichcan calculate parameters of the coil accurately.By using different methods to test theamplitude-frequency characteristic and phase-frequency characteristics indicate that thetraditional model of the coil can not explain its ture features.We obtainthe DC resistance,dwis the diameter d0icucu Rcuis internaldiameter of the skeleton. We obtain the formula of distributed capacitance,the error is lowerthan7％the error of the formula of inductance is lower than6％.Finally we can estimate theresonant frequency of the coil and provide a basis for the design of sensor.(4) We designed the chopper circuit which has lower1/f noise applicating at the lowfrequency10-3Hz~30Hz and high precison amplifier which applicating at high frequency30Hz~10kHz. The noise of the amplifier circuit has1/f noise at the low frequency. Theinduced voltage is small and easily drowned by1/f noise departuring from the principle ofmodulation and demodulation the chopper amplifier circuit is design by discretecomponents. The circuit structure is optimized. The chopper amplifier circuit is design bydiscrete components.The circuit structure is optimized. The chopping frequency is determined.The results show that the sensor noise power is below2*10-5V/Hz1/2. The corner frequency of1/f noise is lower than0.001Hz.The induced voltage is high at high frequency,we selecthigh-performance and low-noise amplifier to design high-frequency circuit. The amplitude ofthe noise is below4*10-5V according to the results of experiment in field.(5) The method of magnetic flux feedback is proposed.Because that the electriccompensation can not solve the mutation of the phase at the resonance frequency of the coil.The flux feedback method is brought forward. The principle of magnetic flux feedback is enlarging the induction coil's output voltage signal and transforming it to the current throughthe feedback resistance. Then forms the feedback magnetic field through the feedback coilwhich is twist on the introduction coil, so the feedback magnetic field's direction is oppositeto the measured magnetic field. And the feedback magnetic field is formed to the negativefeedback way.We calculate the transfer function of the flux feedback system by using theprinciple of mutual inductance. Analyzing the influence of the number of coil turns N1,theinductance Lp,the capacitance C,the apparent permeability μapp,, length of core l,magnification, feedback resistor Rfb,turn of the feedback coil N2to sensor bandwidth andpassband gain and optimizing the sensor. The sensitivity curve is flat at100Hz-2kHz andphase-frequency characteristics is linear approximately at resonant frequency(750Hz).(6) The calibration device is maked for calibrating the sensor in the laboratory. The noiseof the sensor has been tested with correlation coefficient method in the field environment.Because the inductive magnetic sensor is sensitive magnetic instrument, it is necessary tocalibrate in no interference environment. The electrical magnetic shielding cylinder in theelectrical shield room could reduce the interference effectively, the result shows that thesensitivity of sensorⅡ is0.3V/(nT*Hz)@0.01Hz,0.92V/nT@1kHz.Three sensors Ⅱ areburied underground with4m apart, what made them measure the same magnetic fieldcomponent and interference. The method also avoids the trouble of impact. Using the samechannel in the acquisition system to sample the data. Because there are both useful signal andinterference signal in the sampled data, a acquisition-related processing solution had beendone in order to get the power spectrum of the noise. The results showe that the sensor noiseis0.1nT/Hz1/2@0.001Hz,1*10-6nT/Hz1/2which can meet the expected target.(7) The contrast experiments of sensor Ⅰin field are carried out. In the CSAMT,homemade sensor and Germany MFS06sensor are placed in parallel with4m apart and thenthe magnetic field value in the same direction will be measured for many times in the samefrequency. Through the comparison of the results, homemade sensor and MFS06almost hasthe same stability. Besides, the spectrum amplitude in other frequencies shows that the outputamplitude of the two sensors is consistent with the calibrated sensitivity and the noise index isalmost same. Earth resistivity measured by the two sensors is similar besides the relative largedifference at8kHz.The frequency band of sensor is0.001Hz~10kHz; the sensitivity is0.3V/(nT*Hz)@0.01Hz,0.92V/nT@1kHz; the noise level is0.1nT/Hz1/2@0.001Hz,1*10-6nT/Hz1/2@1kHz.The frequency band is narrower slightly,higher sensitivity,lower noise compared toMFS06.The sensor Ⅱcan meet requirement in MT and CSAMT.This paper innovation points: (1) The tandem model and a new calculation formula are put forward. Because the coilamplitude frequency characteristics and phase frequency characteristics appear many peaksand phase point mutations by different measuring methods.The tandem model is put forwardaccording to the conventional equivalent model.It is can be seen that the tandem model can beused to explain coil properties. The tandem model also can show that its frequency of thesecond peak point decreases with the decreasing of the resonance of the frequency, because inpeak point, the phase of the coil still jumps and all compensation methods cannot solve thisproblem. The frequency of the second peak point is10.1kHz. The sensor's frequency bandextended.A new calculation formula is put forward by modifying the constant in thetraditional inductance coil calculation formula, its Error is lower than6％according to finiteelement method and the experiment test method. The actual test results of coil in magneticcore show that the traditional magnetic core inductance coil calculation formula has largererror, and the parameters (such as the effective permeability) and coil parameters in theformula of magnetic core is correct. The calculation error is caused by the constant type.Getting the fit formula of the traditional formula through the measured value of multiplemagnetic core inductance coils. And then correct the constant in formula to get a newcalculation formula.(2) The core is designed with flux collectors which can greatly improve the apparentpermeability. The geometry of the core is changed by flux collectors, which can gathermagnetic. The apparent permeability of the core is significantly improved compared with byincreasing length according simulating by finite element software. The apparent permeabilityis twice when the diameter is7cm,and the thick4cm. The outside diameter of induction coil is7cm. So the flux collectors don't increase the volume of the sensor.(3) The scheme of frequency compensation is proposed that the the chopper amplifier isapplied at low-frequency and magnetic flux feedback is used at high-frequency. The inducedvoltage is small and easily drowned by1/f noise at low-frequency. The amplifier is built bydiscrete components. It reduces the1/f noise (lower0.001Hz) and the temperature drifteffectively compared to integrated chopper amplifier. The results show that the sensor's noisepower is below2*10-5V/Hz1/2. Because that the electric compensation can not solve themutation of the phase at the resonance frequency of the coil, the flux feedback method isbrought forward. It can solve the mutation of the phase at the resonance frequency andreduce the dynamic range of the coil according the Experimental and Simulative results. Thesensitivity curve is flat at a wide frequency range. The frequency range is0.001Hz~10kHz. Itmeets the design requirements.(4) The correlation coefficient method is proposed to test the noise of the sensor.Thetesting of the noise should be in space without any magnetic interference,otherwise it isimpossible to distinguish the interference signal or noise in output signal.There is only a zero-magnetism space laboratory in our country so that to test noise is very difficult. Based onthe correlation between the interference and noise we test the noise of three sensors in theenvironment with interference. We extract the noise from time signal acquired withcorrelation coefficient method.Experimental results show that the noise is0.1nT/Hz1/2@0.001Hz,1*10-4nT/Hz1/2@1Hz,1*10-6nT/Hz1/2@1kHz,which is lower thannatural field signal.It can Meet the requirements of design...