| The fluxgate is a magnetic vector sensor based on the hysteresis saturation characteristic of soft magnetic materials.As the fluxgate has the advantages of high sensitivity,small size,and low power consumption,etc,it has been widely used in geomagnetic survey,magnetic navigation,aeronautics and astronautics,ocean exploration,biomedicine,and other fields.The even harmonic amplitude fluxgate has the problems of odd harmonics amplitude noise interference and circuit complexity.The Residence Times Difference(RTD)fluxgate measures the target magnetic field,using residence time difference between positive and negative saturations,which has the relationship with the target magnetic field.Based on the detection principle,it avoids the problems of the amplitude type fluxgate.The RTD-fluxgate has the characteristics of simple detection procedure and strong anti-interference ability,etc and is the research hotspot and development trend of fluxgate.In order to reduce the noise level of RTD-fluxgate and improve the accuracy and stability of measurement,the theoretical model and low noise technology were deeply studied in this paper.The selection of excitation waveform,the modeling of magnetic core noise,the readout and processing of time difference and the design of sensor were studied.The trapezoidal wave magnetic field excitation,the Barkhausen noise model based on the arctangent hysteresis using dynamic permeability,the Negative Magnetic Saturation Time(NMST)readout technique and the variable coefficient discrimination-equal weight endpoint smoothing time difference processing algorithm were proposed.According to the above research,the low noise RTD-fluxgate prototype was completed development and its performance was tested.The main research works of this paper were as follows:(1)From the two aspects of bistable and hysteresis characteristics of the sensitive unit magnetic core,the detection principle of RTD-fluxgate was studied.The output models of RTD-fluxgate under sinusoidal and triangular waveform excitations were compared.As to the problems of the output model had poor linearity and complex expression under sinusoidal wave excitation and the instability of output time difference caused by triangular wave excitation,the RTD-fluxgate under trapezoidal wave excitation was proposed.The output model under trapezoidal wave excitation was constructed.The model not only had the characteristics of simple structure and linear output,but also could solve the problem of unstable time difference caused by the sudden change of triangular wave excitation spike.Through simulation and experiment,the stability of time difference and the noise level of sensor were compared with different waveform excitations.The results showed that the trapezoidal wave excitation could improve the stability of time difference and reduce the noise level and power consumption of the sensor.(2)When RTD-fluxgate worked,Barkhausen noise generated by the magnetic core of the sensitive unit under the excitation of periodic alternating magnetic field.The relationship between the noise and the dynamic permeability of the magnetic core was analyzed.Based on the arctangent hysteresis model using dynamic permeability,the model of Barkhausen noise was constructed and simulated.The results showed that Barkhausen noise of magnetic core was high frequency random noise.The band range of Barkhausen noise which was mainly distributed in the induction signal pulse position was 10kHz~100kHz.When the pulse reached peak,the positive and negative amplitude of the Barkhausen noise were the largest and the absolute values were approximately equal.In order to analyze the influence of noise on the stability of the time difference readout,the noise separation method of wavelet threshold was used.The time difference fluctuation decreased from 4660?s to 35?s before and after the noise separated and the time difference signal-to-noise ratio improved 133 times.The analysis showed that the noise affected the time difference readout,which was the main factor to restrict the improvement of the time difference stability.(3)The influence of the induction signal noise on readout technique about the bidirectional magnetic saturation time difference was studied.Based on the relationship between the variable state of magnetic core and the measured magnetic field,the NMST readout technique was proposed,which measured the magnetic field after getting the negative saturation time read by known reference point of excitation signal passing zero.The output model of the NMST readout technique was established and the output characteristics were studied.The stabilities of the two readout schemes were compared.When the NMST readout technique was used,the degree of the time difference affected by noise reduced to original 1/(?) in theory.The experimental results showed that the standard deviation of the time difference was reduced by 35% and the fluctuation decreased by 32%,which increased the stability of the time difference.(4)There was time difference fluctuation caused by random interference during measuring.As to above problems,the time difference processing algorithm was studied.A variable coefficient discrimination-equal weight endpoint smoothing time difference processing algorithm formed by combining the variable coefficient Pauta criterion and equal-weight endpoint smoothing was proposed.The algorithm processed time difference in sensor fast,simply and easily.As it was applied to NMST readout technique,the time difference fluctuation was further reduced.And then the time difference standard deviation was reduced from 0.26?s to 0.05?s,the fluctuation was reduced from 1.83?s to 0.17?s.The algorithm could effectively reduce the time fluctuation caused by random disturbance and was suitable for real-time measurement of dynamic magnetic field by RTD-fluxgate.(5)The development and testing of the low noise RTD-fluxgate were important research contents in the paper.The RTD-fluxgate included the sensitive unit and the detection circuit.The sensitive unit was studied from a few aspects below: magnetic core material selection and performance comparison,dimension design and winding mode,etc.The co-based amorphous ribbon which was 0.8mm in width,0.025 mm in thickness and 100 mm in length was fabricated as the core of sensitive unit with symmetrical winding.The detection circuit was low noise designed from a few aspects below: excitation signal generating module,induction signal processing module,time difference reading and processing module,respectively.The main performances of RTD-fluxgate prototype were tested.The results showed that the linear error was less than ±0.12%,the measurement accuracy was 0.013% of the full range(FVM-400 measurement accuracy was ±0.25% of the full range)and the 1Hz noise was 50pT/?Hz which was close to the noise level of RTD-fluxgate abroad.The noise level of the RTD-fluxgate was reduced,so the accuracy and the stability of the sensor were improved by adopting the low noise technology.Overall,the RTD-fluxgate and related low noise technology were studied in this paper.The types and research directions of fluxgate were expanded from the perspective of the detection principle.It had important research value in reducing the noise level of sensor and improving the accuracy and the stability of measurement. |
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