Study On Trans-impedance Measurement Technique Of Planar Electromagnetic Arrays

Array eddy current technique is a new electromagnetic nondestructive testing(NDT) technique developed in recent years, which has wide application foreground in damage quantitative detection and evaluation for metal structures. Planar electromagnetic array has nice coherence, fast respond speed and high sensitivity, which can be commonly used for micro-damage detection of kernel parts in equipments, and the key point of improving the detection precision is to achieve accurate measurement of multi-channel trans-impedances. Currently the impedance analyzers are effective for single channel impedance measurement, but it is difficult to achieve synchronized multi-channel impedance measurement. Therefore, the multi-channel trans-impedance measurement technique for planar electromagnetic array has been studied in the paper.At the beginning of this paper,the research status and the development trend home and abroad of the array eddy current technique and impedance measurement technique are illustrated.The basic principle is introduced. Then, the basic working principle of planar electromagnetic array and the characteristics of trans-impedance are analyzed. The theory, model and test methods of the planar electromagnetic sensors are discussed. The system of impedance detection which based on FPGA is designed and established. In this study, The weak signal detection circuit and high-speed synchronous data acquisition circuit are designed and the digital lock-in detection technique is studied. The digital lock-in amplifier is designed with FPGA and the accurate measurement of multi-channel impedance is accomplished. Noise can be eliminated effectively by this method, and the amplitude and phase as well as the real part and imaginary part of the trans-impedance without data acquisition of the reference signal. The impedance measure system is established and utilized to inspect the flaw in the workpiece, and the test data is analyzed.The experimental results indicate that the repeated measurement precision of the amplitude and phase of trans-impedance achieved 0.04% and 0.02% respectively, which establish a foundation for the quantitative detection of micro-damage.In the end,the conclusions are given and some suggestions for the further research are made.