Development And Test Of The AC Field Detection Instruments

ACFM is a new type of electromagnetic non-destructive testing technology. Itsadvantages are as follows: It has small lift-off effects, the material properties and edgeeffect have little impact on the test results, and so on. These advantages enable it tocreate precise mathematical models in order to achieve quantitative detection of themetal surface crack size.The theoretical model of ACFM is established based on Maxwell’s equations. Thesystems of ACFM can be divided into three parts: the probe, the hardware and thesoftware. The separation of the structure of the excitation coil and the detection coil isadopted, so that when the excitation coil is larger, the smaller detection coil can be used,which improves measurement accuracy. With differential design of the detecting coil,sensitivity of the detection signal gets stronger. Through the mathematical derivation,the optimal design parameters of the excitation coil and receiving coil is found.Moreover, the integrated package probe of the excitation coil and the detection coil isdesigned. The adoption of encoder for system positioning can assist in the calculation ofthe length of the defect. The specific excitation signal current source is used toprovide excitation current with high-precision and high-current. The signal conditioningcircuit is designed to amplify and filter the original signals that the detection coils detect.The designation of the high-speed A/D sampling circuit realizes the transformation ofthe analog signal to digital signal. With the accuracy of the sample12bits, the signal istransferred to the PC software without distortion is ensured effectively. Specialized PCsoftware is used to further process the digital signal. On the basis of crack identification,the accurate inversion calculation of the length of the crack can be made by theencoder’s encoding the scanning path along with the calculation of computer. Based onthe relationship between the size of the crack and the sensitivity of the detective signal,after getting the information of the length of the crack, that of the depth of the crack canbe attained by using algorithm to make inversion calculation on the input value thecomputer gets. The new method of comprehensive judgment to determine the beginningand end of the defect signal is put forward. Conclusion that while the coating thicknessincreases, the detection sensitivity decreases is achieved by the practical testing of thesteel plate with cracks and the analysis of testing data. Meanwhile, the optimal testingfrequency with different coating thickness is got. The back holes of the multilayeraluminum detection of simulated aircraft skin can be effectively detected on the circularhole-like defects of the fourth layer of aluminum; detects the surface of the aluminumplate having a thickness of10mm, can be detected in2.5mm depth from the surface ofthe defect. To detect the type of welded joint structure can be achieved, not only ofsurface cracks can be detected, but also can be detected in shallow subcutaneous defects,and ultrasonic testing to verify the accuracy of the detection result.