Balanced-Field Electromagnetic Detection Method for Pipeline Cracks With a High-Frequency Stability Signal Source
Wenxue Zheng, Jiayin Li, Z.C. Pan, Lijian Yang, Luyao He, Bin Liu
Abstract
Balanced field electromagnetic technique has been demonstrated to be an effective method for in-line crack detection in pipelines. Although this technology offers high sensitivity and omnidirectional detection capabilities, the traditional method of excitation using crystal oscillator frequency division within its application domain suffers from frequency fluctuations. This causes random drift in the amplitude of detection signals, making it difficult to identify the signals from inner wall cracks and buried cracks, which have a weak signal-to-noise ratio. To address the issue of noise interference resulting from unstable signal source frequency, this study proposes a crack detection method based on a phase-locked loop. This study establishes a theoretical correlation model between frequency stability and noise and analyses the influence of frequency stability on crack detection with numerical simulation. The gain effect of pipeline crack detection is verified through experiments. The results show that the frequency stability of the output signal is on the order of pHz when the phase-locked loop is used as the signal source, and it can achieve the crack detection of steel pipes with a burial depth of more than 4 mm. When detecting inner surface cracks of pipelines, the signal-to-noise ratio with a phase-locked loop as the signal source is nearly four times that of crystal oscillator excitation. Using a signal source with high-frequency stability enables the detection of pipeline cracks greater than skin depth by the balanced-field electromagnetic technique.