Processing methods of the pipeline crack detection signal by a balanced field electromagnetic technique based on phase characteristics

Abstract

The balanced field electromagnetic technique is an effective way of in-line inspection to detect cracks in pipelines. A signal demodulation method based on phase characteristics is proposed for the problem of interference signals generated by the sensor tilt shaking during the detection, which affects the judgment of the cracks. The method uses a reference signal whose phase is orthogonal to the signal generated by the sensor shaking to demodulate the detection signal to eliminate the shake interference. The generation principles of crack detection signals and interference signals generated by sensor shaking are analyzed, and the influence of sensor lift-off on detection is compared. A demodulation model is established based on the characteristic of that same frequency and different phases of crack and shake signals. The feasible conditions of the method are analyzed by simulation, and the phase value of the reference signal in the demodulation method is determined. The platform detection experiment and pulling tests at different speeds are carried out, respectively, to verify the effectiveness of the proposed method. The results show that there is a significant phase difference between the signals generated by the sensor shaking and the crack. For carbon steel pipelines, the signal phase of different shake angles is −4°. When the sensor structure and excitation frequency in this study are used, the reference signal phase is chosen to be 86°. The method preserves the detection signal characteristics before processing and enables the linear output responses to be obtained for different depths of cracks.