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Application of eddy current loop RF technology in the inspection of bolt holes and internal veins
In modern eddy current systems, high- and low-pass filters typically employ multi-channel digital filtering. After the eddy current signal is detected, it undergoes filtering to eliminate unwanted low and high-frequency noise, thereby improving the signal-to-noise ratio and producing a clearer display of the data. This process helps in identifying defects more accurately. The working principle is illustrated in the diagram, which shows the UX and UY components of a crack signal after high- and low-pass filtering.
The results shown in the filter diagram are conditional. Specifically, if the frequency of the signal falls outside the range (fH < 1/S < fL), the signal will be completely blocked. This ensures that only the desired frequency band passes through, enhancing the clarity of the measurement.
A-scan and C-scan images represent real-time data collected as the probe rotates around a reference point for one full cycle. When the probe moves axially by one position, its time axis aligns with the previous one. As a result, an A-scan provides information about the location of a defect along the circumference but does not indicate its axial length. In contrast, a C-scan displays data without overlapping, capturing all signals from each revolution. If the probe follows a spiral path, all eddy current data from the hole is recorded and displayed, allowing precise positioning of the defect.
The A-scan of defects a and b illustrates how the scan variable is selected. When displaying eddy current signals in A or C scan mode, either UX or UY can be chosen. Typically, the measured variable is used in two-dimensional processing, and the phase rotation function is applied to maximize the Y-axis value while minimizing the horizontal disturbance. By selecting UY-(0~2Ï€), the image reflects only the measured variable, effectively eliminating interference. Additionally, A and C scan images do not include phase information. If there is uncertainty about a signal, a single-frequency or multi-frequency impedance plan can be retrieved for further analysis and evaluation of the defect.
Rotary scanning probes come in various configurations depending on working speed, structural parameters (such as suitable hole diameter and depth), coil type (absolute, differential, or reflection), and whether shielding is included. These factors define different series of probes. Contact-type rotary probes are ideal for low-speed scanning and offer high sensitivity, though their lifespan is shorter. Non-contact probes maintain a small gap between the sensitive area and the surface, making them suitable for high-speed scanning with longer durability, albeit slightly reduced sensitivity. The selection of probe type follows similar criteria as standard point probes. Shielded probes have a smaller eddy current field radius and better resolution for short defects. Specialized rotating probes include flexible ones for narrow spaces and threaded rod probes for axial movement without additional feeding. These are considered special types of rotating probes.
Key technical parameters of a rotating probe include working diameter, frequency range, effective scanning width, and dynamic range. During detection, the system and accessories are connected and configured according to specifications. After calibration, the handheld scanner performs a uniform feed over three 10 mm holes in the test piece for at least 4 seconds. In single impedance plane and single time base display modes, the acousto-optic threshold is set at 4/5 amplitude using a standard defect. If the hole is free of damage, Y1 appears as a straight line with no significant signal, showing only background noise at the X1-Y1 equilibrium point. However, if a crack exists, the crack signal appears in real time on the impedance plane, visible at the baseline of Y1-T.