| Unexploded ordnance,mines and other explosives that have penetrated into the ground are of various types,complex distribution and high risk.They hinder the development and use of land and threaten the safety of people's lives and property,especially in warfare and finalization tests of weapon system,various test sites,airport runways,etc.urgently need fast and accurate detection methods to find unexploded weapon systems that have penetrated into the ground.Therefore,the detection,location and elimination of unexploded ordnance have important practical significance.This article aims to study the frequency-domain detection principles and methods of underground unexploded ordnance,which is different from the widely used timedomain detection system.The main work of this paper is as follows:(1)The second-order vector potential theory(SOVP)based on the conductive half-space model and the Dodd and Deeds method in the frequency-domain electromagnetic method are introduced,and the field calculation formula of the basic coil sensor in the empty field and the object field is given,the distribution law and influencing factors of eddy current in the conductive half-space are studied.By comparing and analyzing the configuration methods and induction characteristics of coil sensors in different application backgrounds,it is proposed that rectangular planar coils with better magnetic field directionality and uniformity can be applied to the detection of underground unexploded ordnance.(2)From the perspective of the weak-coupling of the transmit-receive coil,based on the second-order vector potential theory and Maxwell finite element simulation,the effects of the coil configuration,size,angle between the shafts,and the position of the central axis on the coupling characteristics of the coil are analyzed,and then a separate vertical-coupling rectangular coil sensor with high sensitivity is designed.At the same time,the appropriate separation distance will also improve the detection depth and anti-interference of the sensor,so that it has the ability to identify small metal debris.(3)Based on the finite element method,a simplified model of the ordnance was constructed,and its response field was qualitatively analyzed.The frequency response of the simplified model of different materials and sizes was studied.The results show that different unexploded ordnance can be identified by frequency sweep analysis.And the vertical positioning and horizontal positioning schemes of unexploded ordnances were formulated.(4)The frequency-domain detection system based on FPGA+ARM architecture was introduced,and the power amplifier circuit based on LM1875 was researched and designed,and give the background of high-power application.The implementation scheme of the sinusoidal transmission circuit based on the Selective-Harmonic-Elimination-PWM spectrum control method and H-bridge control is given under the background of high-power application.The three methods of program-controlled amplifier circuit are compared and analyzed.A highprecision preprogrammed amplifier circuit based on THS7001 is designed.The digital quadrature demodulation technology based on half-period sampling and its implementation method are studied,and the host computer software based on Labview platform of the laboratory is introduced.(5)An experimental platform for the detection of unexploded ordnance was built,and the horizontal spacing between the transmitter coil and the receiver coil of the sensor,the vertical position of the central axis of the receiver coil,and the angle between the coil axes were optimized and analyzed.The frequency response of three unexploded ordnance at different depths was determined,and the available detection frequency band of the system was determined.The sensitive areas at different depths in the sensor's sensitive field are calibrated,and the anti-interference performance of the parallel sensor and the positioning scheme of unexploded ordnance are tested and verified. |
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