Research On Electromagnetic Ultrasonic Transducer Based On Halbach Principle

Electromagnetic acoustic testing technology is a non-destructive testing technology,which has just emerged in recent decades.Because of non-contact,no coupling,and high-speed on-line detection,it is widely used in industrial fields.Electromagnetic ultrasonic generation is more flexible.Ultrasonic transverse wave and longitudinal wave can be generated by setting different magnets and coils,which are suitable for different detection sites.In the process of electromagnetic ultrasonic testing,without pre-processing,it can effectively prevent the test material from being damaged.However,the disadvantage of Electromagnetic Acoustic Transducer(EMAT)is that the signal-to-noise ratio(SNR)is low,which also increases the difficulty in practical applications.In this thesis,aluminum plate and natural gas plate are taken as the research objects respectively.The mechanism of thickness measurement is studied by electromagnetic ultrasonic technology.The structure of the transducer is designed and optimized simulation,then the experimental verification and signal noise reduction are carried out.The main tasks are as follows:Firstly,in this thesis,the Lorentz force-based model and the magnetostrictive force-based model are derived in the form of mathematical modeling.On the one hand,boundary conditions of the electromagnetic ultrasonic equation are given,which theoretically verifies the correct mechanism of electromagnetic ultrasonic transduction.On the other hand,the structure of the electromagnetic ultrasonic transducer is introduced.According to the characteristics of the ultrasonic waveform,the N52 permanent magnet and spiral coil structure are selected.Then,the thickness measurement of the metal plate is performed by ultrasonic transverse wave.Next,the finite element software is used to simulate and compare the three bias magnetic fields.Under the same physical conditions,the permanent magnet based on the Halbach principle has a good"one-sided" effect,which can concentrate the magnetic line on one side of the magnet.It is advantageous for attenuating the magnetic field on the other side of electromagnetic ultrasonic transducers,thereby the structure of the transducer is determined.Then,the finite element simulation method is used to optimize the design of the selected permanent magnet structure.The simulation results show:With the structure of the armature plate increasing,the magnetic induction intensity is enhanced.The optimal armature thickness is 2mm.The fixed armature thickness is optimized for the length,width and height parameters of the permanent magnet.The optimal parameters are:length and width are both 12mm and height is 30mm.Through theoretical calculations,the optimized permanent magnet structure can increase the SNR of the transducer by 2.16 times.In addition,by comparing different signal post-processing methods,the wavelet threshold denoising method is selected to denoise the original signal.The threshold is selected as the Heursure threshold,and the original signal is processed by the soft threshold method.Finally,the SNR is calculated by the formula to increase the SNR by 19.9644 dB.Finally,based on the above work,an experimental system of electromagnetic ultrasonic is built.The thickness of the 9mm natural gas pipeline and the thickness of the 10mm aluminum plate are measured respectively.The accuracy of the experiment is verified by mathematical statistics.The experimental results show that the electromagnetic thickness measurement error is controlled within 3%.