Optimal design of spiral coil electromagnetic acoustic transducers considering lift-off sensitivity operating on non-ferromagnetic media

Abstract The generation efficiency of electromagnetic acoustic transducers (EMATs) dramatically reduces with increasing lift-off, thus restricting their applications. This paper aims to provide methods to decrease lift-off sensitivity. The process of generating a shear wave in a spiral coil EMAT is established based on a 2-D axisymmetric model using the finite element method. An equivalent excitation circuit, including an impedance-matching network and a limited power source, is presented for calculating the excitation current. Such an equivalent circuit has not been given adequate attention in previous publications in this field, but it has proven to be of great importance in determining the efficiency of ultrasonic generation. The predictions of static magnetic field, eddy current density and excitation current are validated by experimental results. The conversion efficiency and lift-off sensitivity of the EMAT are analysed with respect to impedance matching parameters, magnet dimensions, the presence of a copper backplate between the magnet and the coil, and backplate-to-coil distance. The results indicate that impedance matching parameters, magnet diameter and plate-to-coil distance have obvious effects on lift-off sensitivity. The backplate can be used to decrease lift-off sensitivity, and it can rearrange the source current density across the cross-sectional area of the coil conductor. Furthermore, compared with the original EMAT, the lift-off sensitivity of the optimised EMAT can be decreased by 7.9 dB at 2 mm lift-off.

[1]  Steve Dixon,et al.  A new electromagnetic acoustic transducer (EMAT) design for operation on rail , 2014 .

[2]  Anthony N. Sinclair,et al.  Optimal design of EMAT transmitters , 2004 .

[3]  W. E. Deeds,et al.  ANALYTICAL SOLUTIONS TO EDDY--CURRENT PROBE COIL PROBLEMS. , 1967 .

[4]  R. Edwards,et al.  Depth gauging of defects using low frequency wideband Rayleigh waves. , 2006, Ultrasonics.

[5]  R. Dewhurst,et al.  Modelling of magnetic fields to enhance the performance of an in-plane EMAT for laser-generated ultrasound. , 2006, Ultrasonics.

[6]  S. Boonsang,et al.  A new magnetic configuration for a small in-plane electromagnetic acoustic transducer applied to laser-ultrasound measurements: Modelling and validation , 2006 .

[7]  A. M. Hussein.  Current distribution and input impedance of a finite electromagnetic acoustic transducer , 1991 .

[8]  Krishnan Balasubramaniam,et al.  Improving the signal amplitude of meandering coil EMATs by using ribbon soft magnetic flux concentrators (MFC). , 2011, Ultrasonics.

[9]  S Dixon,et al.  Coupling mechanism of an EMAT. , 2006, Ultrasonics.

[10]  Wei Zhao,et al.  Study on the Lift-off Effect of EMAT , 2009 .

[11]  Ivo Cap,et al.  Special problems of electromagnetic-acoustic transducer for material non-destructive testing , 2002 .

[12]  K. Grattan,et al.  A model for pulsed Rayleigh wave and optimal EMAT design , 2006 .

[13]  Lei Kang,et al.  Modeling of Lorentz forces and radiated wave fields for bulk wave electromagnetic acoustic transducers , 2013 .

[14]  Rachel S. Edwards,et al.  Effect on ultrasonic generation of a backplate in electromagnetic acoustic transducers , 2007 .

[15]  A. Konrad.  Integrodifferential finite element formulation of two-dimensional steady-state skin effect problems , 1982 .

[16]  Xinjun Wu,et al.  Enhancement of the Excitation Efficiency of a Torsional Wave PPM EMAT Array for Pipe Inspection by Optimizing the Element Number of the Array Based on 3-D FEM , 2015, Sensors.

[17]  R. Ludwig,et al.  Numerical simulations of an electromagnetic acoustic transducer-receiver system for NDT applications , 1993 .

[18]  Rachel S. Edwards,et al.  Optimising ultrasonic wideband Rayleigh wave generation by pulsed electromagnetic coils , 2005 .

[19]  Daniel Massicotte,et al.  Wavelet-transform-based method of analysis for Lamb-wave ultrasonic NDE signals , 2000, IEEE Trans. Instrum. Meas..

[20]  Steve Dixon,et al.  Lift-off performance of ferrite enhanced generation EMATs , 2008 .

[21]  S. Palmer,et al.  Wideband electromagnetic accoustic transducers utilising neodymium iron boron permanent magnets , 1990, International Conference on Magnetics.

[22]  Tino Morgenstern,et al.  Ultrasound generation with high power and coil only EMAT concepts. , 2014, Ultrasonics.

[23]  Krishnan Balasubramaniam,et al.  A two-stage finite element model of a meander coil electromagnetic acoustic transducer transmitter , 2011 .

[24]  Lei Kang,et al.  Enhancement of signal amplitude of surface wave EMATs based on 3-D simulation analysis and orthogonal test method , 2013 .

[25]  Anthony N. Sinclair,et al.  Comparison of three formulations for eddy-current and skin effect problems , 2002 .

[26]  Reinhold Ludwig,et al.  Numerical simulation of electromagnetic acoustic transducer in the time domain , 1991 .

[27]  Steve Dixon,et al.  Eddy current generation enhancement using ferrite for electromagnetic acoustic transduction , 2006 .

[28]  Dale Ensminger,et al.  Ultrasonics : data, equations, and their practical uses , 2008 .

[29]  Y. Berlinsky,et al.  Effect of liftoff on accuracy of phase velocity measurements made with electromagnetic-acoustic transducers , 1992 .

[30]  H. J. Salzburger,et al.  Non-destructive testing of laser welds in tailored blanks using electromagnetic transducers , 2007 .

[31]  S Dixon,et al.  Lift-off compensation for improved accuracy in ultrasonic lamb wave velocity measurements using electromagnetic acoustic transducers (EMATs). , 2006, Ultrasonics.

[32]  Rachel S. Edwards,et al.  Coupling mechanism of electromagnetic acoustical transducers for ultrasonic generation , 2006 .

[33]  Riichi Murayama,et al.  Development of an on-line evaluation system of formability in cold-rolled steel sheets using electromagnetic acoustic transducers (EMATs) , 1996 .

[34]  Huang Songling,et al.  Circuit-field coupled finite element analysis method for an electromagnetic acoustic transducer under pulsed voltage excitation , 2011 .

[35]  Z. Filus.  THE EFFECT OF TUNING OF BULK WAVE EMATS ON THEIR LIFT-OFF , 1994 .

[36]  Hirotsugu Ogi,et al.  EMATS for science and industry , 2003 .

[37]  C. B. Scruby,et al.  Non-contact ultrasonic measurements on steel at elevated temperatures , 1993 .