Research On Mechanism Of Electromagnetic Acoustic Transducer For Solidification End And Point Focusing Transducer Design

Continuous casting billet is the main source of machinery and construction industry,and its quality and material properties are closely related to the national economy and people’s lives.Accurately detecting and predicting the position information of the solidification end not only provides a reference for the machine design and process control of continuous casting,but also the premise and key to realize the soft reduction and electromagnetic stirring technique,and has a special significance for improving the internal quality and productivity of slabs.Electromagnetic acoustic transducer(EMAT)has many advantages such as non-contact,no coupling agent,high temperature resistance(> 900℃)and waterproof fog,as well as the development of high-power ultra-short pulse excitation power and signal processing technique.The electromagnetic acoustic technique has become a new way to detect the solidification end of continuous casting billet,and has wide application value and prospect in the future industrial field.However,as the core component of electromagnetic acoustic testing,EMAT not only has a low conversion efficiency,but also has many problems such as multi-mode,poor directivity,and resonant frequency that is susceptible to external environment,which greatly restricts its further promotion and development.To solve the above problems,this paper carries out in-depth research on the optimal design and resonant frequency tracking control of EMAT for solidification end detection of continuous casting billets.For the EMAT applied to the detection of solidification end,the mechanism of electromagnetic acoustic detection is analyzed by the established theoretical model of the electromagnetic acoustic transmission line.Then,based on the equations of electric field,magnetic field,elastic-dynamics and sound field,a three-dimensional finite element model of EMATs considering both static and dynamic Lorentz force is established.The model covers all the physics fields involved in the transduction process and structural parameters of EMATs,which effectively compensates for the lack of accuracy and completeness of the EMAT model,and ladies foundation for optimizing design and analysis of the point-focusing EMAT.Aiming at the shortcomings of meander-line coil EMAT,such as weak receiving signal and poor directivity,based on the simulation calculation of three-dimensional finite element model of EMATs,the signal intensity,focus offset and beam focusing mode under different aperture angles are studied.The influence of design depth of focus is also considered.Then,the optimal design method of point-focusing SV wave meander-line coil EMAT based on orthogonal test method is proposed.Through this method,the influence level between the optimized design parameters and target is obtained,the optimal parameter combination of the transducer’s design factors are determined,and the basic design principle of the point-focusing SV wave meander-line coil EMAT are summarized.Aiming at the problem of resonance frequency drift,the admittance model of EMAT’s equivalent circuit near the resonant frequency is established,and the impedance matching characteristics of EMAT are analyzed from the theoretical perspective,then the basis and law of the impedance matching method are summarized.Based on the admittance model of equivalent circuit,a low-pass filter is introduced to convert the non-affine system implicitly in the nonlinear uncertain function of the transducer system into an affine system that is easy to study.Then,a neural network adaptive resonant frequency tracking algorithm is proposed,and the control algorithm of external disturbance factors considering ambient temperature and lift-off distance is simulated.The results show that the neural network adaptive resonant frequency tracking algorithm can realize automatically tracking control of the natural resonant frequency of the EMAT,and the controller has good dynamic characteristics and stability.Finally,to verify the ability of the designed point-focusing SV wave EMAT for detecting the solidification end of the continuous casting billet,the finite element model of the continuous casting billet including liquid core is established,and the effect of the offset distance,depth and width of liquid core on the detection capability are analyzed.Aiming at the actual engineering background of the solidification end detection of continuous casting billet,an electromagnetic acoustic testing system is designed and established,and the end position of the liquid core is detected by the designed point-focusing SV wave meander-line coil EMAT.The design technique of EMATs for the solidification end detection of continuous casting billet in this paper will lay a good foundation for the online automatic detection technique of continuous casting billet.