Abstract
This paper describes a new method of eddy current impedance extraction and hardness classification for valves, which is used in online classification. A finite element simulation is conducted to estimate the base magnetic response in specimens with various initial permeabilities, and a scatter diagram between the hardness calibration and initial permeability is plotted experimentally; thus, the hardness classification model is obtained by using the initial permeability as the intermediate variable. A lock-in amplifier is commonly used to measure the amplitudes of impedance components in conventional eddy current testing. This study replaces the lock-in amplifier with a cross-correlation algorithm with the objective of integrating the impedance processing with the subsequent classification. Moreover, an incremental and decremental support vector machine is used to classify the valve specimens based on the extracted impedance due to its demonstrated excellent performance for both linearly separable and nonlinearly indivisible problems. The cross-correlation algorithm and the incremental and decremental support vector machine are implemented on a field-programmable gate array, which has the advantage of parallel processing and can satisfy the online requirements. A comparison with the traditional impedance comparison method shows that the proposed method not only is suitable for online hardness classification but also achieves good performance in correlating impedance amplitude changes caused by a temperature increase of the coil and various heat treatment conditions with the hardness values.
Highlights
A valve is a component that is responsible for inputting air into an engine or exhaust gas after combustion
A lock-in amplifier (LIA) is commonly used to measure the amplitudes of impedance components and the phases of impedance signals in conventional eddy current testing, which has the advantage of simultaneously obtaining the magnitudes of the resistance and inductance components [9], [10]; LIAs are uncontrollable for the online extraction of features in manufacturing processes, and difficulties are encountered in the integration of impedance processing with subsequent classification [11]
When a hardness-calibrated valve is placed into the reference coil and no valve is placed into the testing coil, as shown in Fig. 11(a), channel 1 shows the waveform of the impedance signal that is acquired by the AD, and channel 2 shows the waveform of the reference signal that is acquired by the fieldprogrammable gate array (FPGA)
Summary
A valve is a component that is responsible for inputting air into an engine or exhaust gas after combustion. A lock-in amplifier (LIA) is commonly used to measure the amplitudes of impedance components and the phases of impedance signals in conventional eddy current testing, which has the advantage of simultaneously obtaining the magnitudes of the resistance and inductance components [9], [10]; LIAs are uncontrollable for the online extraction of features in manufacturing processes, and difficulties are encountered in the integration of impedance processing with subsequent classification [11]. This study uses a finite element model (FEM) to simulate the eddy current field distribution for valve samples to obtain a theoretical basis for establishing a hardness testing model that is based on an impedance feature. This study replaces the LIA with a cross-correlation algorithm with the aim of extracting the magnitude of the impedance This algorithm is a classic algorithm, its application in hardness feature extraction in the field of eddy current detection gives rise to new academic and technical problems.
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