A NEW RESEARCH METHOD FOR CORROSION DEFECT IN METAL PIPELINE BY USING PULSED EDDY CURRENT

Pulsed eddy current (PEC) is a new technique to distinguish corrosion defeats inside and outside the metal pipeline. In comparison with other eddy current techniques, the PEC technique has the advantage of being simple and high velocity. In this article, a brand-new PEC probe based on differential conductivity is established through the combination of modules like square wave generator, eddy current coil bridge, differential current, voltage sample circuits and so on. The 50% duty cycle square wave is used as the driving signal. To measure differential conductance, a coil bridge configuration with two legs is adopted. One leg is composed of measurement eddy current coil and the in-series resistor, and the other is reference eddy current coil and the in-series resistor. Because the two legs go through defects in pipeline non-synchronously, there is a differential conductance between the two coils. A trans-impedance amplify circuit is used to detect coil eddy current. At the same time, two amplifiers are used to measure the differential voltage between the two coils. A 14 bit ADC is used to sample differential voltage, measurement and reference eddy currents which transferred to differential current by main processor Complex Programmable Logic Device (CPLD). CPLD is used to get differential conductance by differential current divide differential voltage. At last the eddy current signal sampling sequence is developed. A dynamic testing fixture with artificial defects carved on the pipeline is used to validate PEC probe's accuracy. The differential conductance signals were displayed on the oscilloscope. Results showed that the inside defect had two peaks, positive peak and negative peak, but the outside defect only had one positive peak. We can conclude that the brand-new PEC probe has high accuracy in distinguishing the inside and outside defects.