Efficiency and multisimultaneous-frequency (MSF) output capability are two major criteria characterizing the performance of a power amplifier in the application of multifrequency eddy current testing (MECT). Switch-mode power amplifiers are known to have a very high efficiency, yet they have rarely been adopted in the instrumental development of MECT. In addition, switch-mode power amplifiers themselves are lacking in the research literature for MSF capability. In this article, a Class D power amplifier is designed so as to address the two issues. An MSF selective harmonic elimination pulsewidth modulation method is proposed to generate alternating magnetic fields, which are rich in selected harmonics. A field-programmable-gate-array-based experimental system has been developed to verify the design. Results show that the proposed methodology is capable of generating high MSF currents in the transmitting coil with a low distortion of signal.
This paper presents the design of a high-performance multi-frequency impedance analysing instrument (MFIA) for eddy current testing which has been developed primarily for monitoring a steel production process using an inductive sensor. The system consists of a flexible multi-frequency waveform generator and a voltage/current measurement unit. The impedance of the sensor is obtained by cross-spectral analysis of the current and voltage signals. The system contains high-speed digital-to-analogue, analogue-to-digital converters and dual DSPs with one for control and interface and one dedicated to frequency-spectra analysis using fast Fourier transformation (FFT). The frequency span of the signal that can be analysed ranges from 1 kHz to 8 MHz. The system also employs a high-speed serial port interface (USB) to communicate with a personal computer (PC) and to allow for fast transmission of data and control commands. Overall, the system is capable of delivering over 250 impedance spectra per second. Although the instrument has been developed mainly for use with an inductive sensor, the system is not restricted to inductive measurement. The flexibility of the design architecture is demonstrated with capacitive and resistive measurements by using appropriate input circuitry. Issues relating to optimizing the phase of the spectra components in the excitation waveform are also discussed.
This paper presents the modelling of an H - shaped ferrite core electromagnetic sensor for phase transformation detection in steel rolling application. It has been found that the response of the H - shaped sensor after normalisation can be described by a simple analytical model. Further, it has been found that the zero-crossing frequency for the real part of the inductance spectra and the peak frequency of the imaginary part of the inductance spectra for both air-cored and ferrite-cored sensors are linearly proportional to the magnetic permeability of the steel strip under test. These findings are helpful in understanding the response of the EM sensor and possibly lead to more accurate determination of steel magnetic property in an industrial setup and a convenient method for calibrating the device.
Detecting underground metal targets (e.g. unexploded ordnance) using simple devices and seeking to improve the detection accuracy and depth is a hot issue in electromagnetic detection in recent years. Electromagnetic detection based on the frequency-domain method has a simple excitation method, a wide excitation range, and high detection resolution and frequency flexibility. However, the primary field generated by the excitation coil and the secondary field generated by the unexploded ordnance exist at the same time, which has a large impact on the effective signal; and the current detection accuracy is not high, and the effective eigenvalues are less. To address these situations, in this paper, we use the analytical solution of the rectangular coil and finite element simulation to design a new type of electromagnetic sensor with differential multi-component reception, and the influence of the primary field can be eliminated with the help of its coupling characteristics. Based on the unique sensor structure, we propose a four-quadrant fast horizontal localisation as well as a method for target identification and a method for depth fixing of the target by SNR eigenvalue fitting. In a laboratory environment with 10 V low-power excitation, the sensor has a farthest detection depth of about 1.2 m and a positioning error of about 14.43 mm within a depth of 0.9 m, and it can be detected as far as 0.6 m in an outdoor environment.
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