A Dual-Parameter Oscillation Method for Eddy Current Testing With the Aid of Impedance Nonlinearity

The ferrite core of a sensing coil enhances its magnetic field, thereby improving its detection sensitivity in eddy current testing (ECT). Due to the hysteresis caused by the ferrite core, the lumped impedance of a coil-target system is essentially nonlinear. In this article, the impedance nonlinearity is used and a dual-parameter oscillation method for ECT is proposed. By utilizing the nonlinear impedance of the coil-target system in the oscillating circuit, a self-oscillation signal can be generated. Subsequently, the dual parameters of oscillation, i.e., the amplitude and the frequency are obtained by the detection circuit. The amplitude reflects the nonlinear resistance of the coil-target system, while the frequency reflects the nonlinear inductance. A negative resistor is used to ensure the oscillation and adjust the amplitude to the appropriate range and a capacitor is used to adjust the frequency. This method is capable of real-time defect detection. The correlation of dual parameters is analyzed and used, and a synthesis parameter is proposed to suppress the lift-off effect and noise, therefore the performance of detection for small defects is enhanced. The proposed dual-parameter maps provide a graphical observation of the location and the depth of defects in real-time scanning. The proposed method is applied to detect the surface and subsurface defects of the metal sample as well as of a ferromagnetic target behind the nonferromagnetic shield.