Research Of The Transfer Characteristic Of Search Coil Magnetometer With Magnetic Feedback

The magnetotellurics is a geophysical method to study the underground electricalstructure by observing the induction electromagnetic field in the interior of the earthgenerated by the electromagnetic wave high in air. It is widely used in study ofinternal structure of the earth, and exploration areas including petroleum structure,coalfield structure, groundwater, mineral resources and engineering geology, andearthquake precursory monitoring research, etc.Search coil magnetometer is currently the most widely applied magnetic sensor. Itcan directly generate induced voltage with changing magnetic field and has a widefrequency band and high sensitivity, and also has a small volume, light weight andlower production cost. The domestic magnetic sensor technology index still cannotcatch up with that of Germany, Canada, America and other countries. In currentapplications, most of the electromagnetic measurement instruments depend onimports and the applications subject to many restrictions, which seriously restrict thedevelopment of electromagnetic equipment in China.This paper studied the search coil magnetometer with magnetic feedback. Theapplication of magnetic feedback can make the transfer characteristic flat near theresonance frequency, expand the band width and improve the stability of the system.In addition, there are still a lot of factors influencing the transfer characteristic of thesystem. Only fully grasped the influences on the transfer characteristic of these factors,can we design the optimal magnetic sensor. The work of the full text is summarized asfollows:1. Studied the main parameters of the coil, including resistance, inductance andcapacitance; understood the parameters of magnetic core and chose the right core.Established the model of the coil using Maxwell, simulated influences on the coil inductance of the subsection number, paragraph spacing, coil diameter, the length andcross-sectional area of the magnetic core, so as to determine the design of coilstructure; emphatically analyzed the relation between the effective permeability andthe length-diameter ratio of the core, finally simulated and tested the eddy current lossof the magnetic core.2. Mastered the principle of the search coil magnetometer; simply analyzed thesystem model of the magnetometer without magnetic feedback; then emphaticallyanalyzed the system model of the magnetometer with magnetic feedback andsimulated the feedback loop; mastered technical indexes of the magnetic sensor,mainly including frequency range, output sensitivity, calibration sensitivity, feedbackcut-off frequency and sensor noise.3. Designed the preamplifier, which is composed of a pair of differential low noiseJFET and an operational amplifier with low power consumption; simulated thepreamplifier and the feedback loop by PSpice, grasped the influence on the transfercharacteristic by different resistance and capacitance to guide optimization of thecircuit; simulated and tested the magnetometer.4. Based on the analysis of general negative feedback loop stability, studied thestability of the magnetometer with magnetic feedback. In order to make the circuitwith feedback works steadily at predetermined cases as well as changed situations, wemust make it far from the self-excited state which is described by the gain margin andphase margin.