A multi-frequency impedance analysing instrument for eddy current testing

Abstract

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.

An experimental comparison of multi-frequency and chirp excitations for eddy current testing on thin defects

“…The former has important representatives in pulsed (PEC) [2][3][4][5] or pseudo-noise excitations [6,7], while the latter proposes signals as the Multi-Frequency (MF-ECT) [8][9][10] and chirp one [11,12]. Pulsed signals are typically analyzed in the time-domain signal processing, while MF-ECT and chirp signals are instead analyzed by the use of transformed domains as the Fourier and the Chirplet.…”

Noncontact Thickness Measurement of Metal Films Using Eddy-Current Sensors Immune to Distance Variation

“…The response of MEC sensors can be modeled using complex phase notation [13], or a few other features [14], and solutions of different sensor structures [15] have been developed to determine and reduce the lift-off effect to some extent. However, such methods require a multifrequency impedance analyzing instrument, which is technically complex and expensive [16]. A simple lock-in instrument was developed for thickness measurement by abstracting the phase signature by phase comparators, which was affected by the lift-off variations.…”

A Recursive Demodulator for Real-Time Measurement of Multiple Sinusoids

“…Because the response sensor signal is also composed of multiple sinusoids with the same frequencies and the frequencies are pre-known, only the amplitudes and phases of the sinusoids need to be obtained. The active measurement is required in many industrial and biomedical applications, such as industrial process tomography [1]- [3], bio-impedance measurement [4]- [6], eddy current testing [7], [8] and built-in self-test [9], etc. Because the characteristic parameters of the target object are often rapidly changing, real-time and fast measurement of the amplitudes and phases of multiple sinusoids in the response sensor signal is highly desired in sensor data processing.…”