Development Of Three-Dimensional Electromagnetic Tomography System Based On Lock-In Amplifier Technology

Electromagnetic Tomography(EMT)technology is a new type of process tomography technology.It is based on the principle of electromagnetic induction.It has non-contact and non-intrusive characteristics,and has a wide application prospect.The principle of electromagnetic tomography is:generate a plurality of magnetic fields at different angles by excitation in the object field.And the conductive magnetic material in the object space can generate an induced magnetic field.Then this induced magnetic field can change the original magnetic field.Then the detection voltage of each angle through the detection coils at different angles will be measured.Finally,according to the measured data,a specific image reconstruction algorithm is used to reconstruct the distribution of conductive and magnetic material in the measured object space.In this paper,by studying,learning and summarizing the existing electromagnetic tomography system,every key step to improve the imaging quality is analyzed,and the development of three-dimensional electromagnetic tomography system based on lock-in amplifier technology is proposed.By using a lock-in amplifier,the signal to be acquired is phase-locked and amplified,the noise signal of other frequencies is filtered out,and the signal-to-noise ratio is improved.In addition,the network control relay module is introduced to design the channel switching circuit,which greatly reduces the on-resistance of the channel switching,increases the voltage of the excitation coils,enhances the excitation magnetic field,therefore improves the amplitude of the value of the detection signal.Finally,using the 3D electromagnetic tomography system,the data acquisition experiment is carried out,and the 3D image reconstruction is performed according to the collected data.At the same time,the influence of the lock-in amplifier on the detection signal under different parameter settings is also analyzed.The specific content of this paper is as follows:(1)A 3D electromagnetic tomography system is designed and implemented.Firstly,a three-dimensional sensor array is designed.Three different sensor array design schemes are compared by three-dimensional simulation.Finally,one design scheme is determined,including the size of the object space,the size of the excitation coil and the detection coil,and the determination of electrical parameters.Then,three different kinds of lock-in amplifiers of different companies are analyzed and compared,and one lock-in amplifier is determined,and the circuit connection design and remote control programming are also carried out.The network control relay module is introduced to replace the original channel switching circuit,and the circuit design,connection and remote control programming are carried out.Finally,a 3D electromagnetic tomography system is built,and all the controlled modules in the system are jointly programmed to realize the automatic operation of the system and the data acquisition experiment is carried out.(2)Three-dimensional image reconstruction based on data collected by this system.Twenty-four groups of data collected by this system are used to reconstruct three-dimensional images.Firstly,the finite element partition of the three-dimensional object space is carried out by using Ansys,then the joint programming is performed by finite element computing software and VB scripting language,the sensitivity matrix is calculated,finally the three-dimensional image reconstruction algorithm is designed and the reconstructed image is carried out.(3)Based on this system,the quality of the detection signal is analyzed and the parameters are optimized.By setting the output voltage,the sampling frequency and the 3dB bandwidth of the lock-in amplifier,the influence of different parameters on the detection signal is analyzed.Finally,the better optimized parameters are selected,which is most suitable for the signal excitation and data acquisition of the system.Picture:34,Table:9.Reference:62.