High- and low-pass filters in modern digital eddies generally have multi-channel digital high- and low-pass filtering. The eddy current signal after the detection is filtered to remove low and high frequency interference components in the useful signal, improve the test signal to noise ratio, and obtain a clearer display image (parameter). Its working principle is shown in. It indicates the UXUY signal output of the two components UX and UY of a crack signal U after high-low-pass filtering.
The results shown in the diagram of the high and low pass filters are conditional, that is, they must be satisfied: fH<1/S The A-scan and C-scan A-scan maps are an image in which the measurement data obtained by rotating the probe from the reference point for one week is described in real time. When the probe moves axially one position, its time axis coincides with the previous one. Therefore, the A-scan can only give the position of the defect on the circumference, but the axial length cannot be determined, as shown. The C-scan is a display that does not overlap the signal data collected every revolution of the probe. If the probe is scanned in a spiral rotation, all eddy current data of the detected hole will be recorded and displayed, and the defect can be positioned as shown. The A-scan of defects a, b shows the selection of the scan variable of the graph. When the eddy current signal is displayed in A or C scan, UX or UY can be selected. Usually the variable to be measured is used in the two-dimensional processing, using the phase rotation function to obtain the maximum value on the Y-axis and the main disturbance variable to the horizontal axis. Thus, when UY-(0~2P) is selected during A or C scanning, the response signal on the scanned image is only related to the measured variable, and the interference variable is eliminated (because it is projected as 0 on the Y-axis). In addition, in the A or C scan image, there is no phase information of the signal. If there is any doubt about the signal in the image, the signal (single-frequency or multi-frequency) impedance plan can be taken out and extended analysis can be performed to evaluate the defect properties. Bolt hole automatic rotation scanning system configuration example Rotary scanning probe probe can generally be used according to working speed, structural parameters (suitable hole probe can generally be according to working diameter and hole depth), coil type (absolute, differential, reflection), whether shielding, etc. Classification constitutes a variety of series. The contact type rotary probe is suitable for low speed scanning and has high sensitivity, but the working life is not long. The non-contact type refers to the probe sensitive point and the hole surface are not in contact (maintaining a small gap). It is suitable for high speed scanning, long life but sensitivity. Slightly worse. The choice of probe type is the same as that of a normal point probe. The shielded probe has a small eddy current field radius and a strong resolution for short defects. In addition, there are some flexible rotating probes for narrow space detection and a threaded rod rotating probe for axially feeding the scanner without axial feeding. These are listed as special types of rotating probes. The main technical parameters of the rotating probe are: working diameter, frequency range, effective scanning width and dynamic range. Detection method and result Connect the instrument and accessories as required, and set the parameters of the detection system. After the system is calibrated, the handheld scanner scans the three á10 holes of the tested piece according to the uniform feeding time (≥4s). In the single impedance plane and single time base display mode, the acousto-optic threshold is set with a standard damage of 4/5 amplitude. If the hole to be inspected is harmless, Y1 is a straight line with no obvious signal. There is only scanning background noise at the X1-Y1 impedance plane equilibrium point; if there is a crack in the hole, Y1-T is at the baseline and X1-Y1 The crack signal is displayed in real time on the impedance plan.
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