Research On The Key Technology Of Laser Ultrasonic Pipeline Inspection System

The main form of natural gas transmission is pipeline. Internal corrosion is found inevitably in urban pipeline because of the increasing service age,which leads to wall thickness reduction and pipeline cracking, consequently causing gas leak. Therefore, the gas pipeline system must be regularly tested.Laser ultrasonic technology is a new type of nondestructive testing technology,which has high temporal and spatial resolutions. It has great potential in the detection of micro cracks in gas pipeline, but the laser ultrasonic signal has the disadvantages of low SNR, low frequency interference and so on. The key technologies of the laser ultrasonic technique in the detection of pipeline defects is studied in this paper, mainly includes the following aspects:(1) The SNR of laser ultrasonic signal was low. We introduced the de-noising principle of average method and wavelet de-noising, analyzed the influence of different digital average times and different wavelet parameters on the de-noising effect, compared wavelet denoising simulation analysis results and the experimental results. The advantages and disadvantages of the time domain average method and the wavelet denoising method are analyzed.Finally, combining the two methods to denoise the signal.(2)In order to deal with the laser ultrasonic signal with noise of trend, two different methods to process those signals were put forward, which is wavelet analysis and differential method. Then we introduced the principle of wavelet analysis and differential method to extract the mutation point in the signal, and proposed a fast algorithm of differential. Finaly, the advantages and disadvantages of wavelet method and differential method of feature extraction were compared to select the most suitable algorithm for laser ultrasonic signal feature extraction.(3)we developed and designed the laser-generated ultrasonic pipeline inspection system software based on Labview. By using the modular designmethod, the data acquisition, motion control and data processing are completed by each sub module. The data processing module has the functions of noise reduction, filtering, differential feature extraction and so on, which could meet the demands of laser ultrasonic signal processing.(4)The external surface and inner surface crack of the pipeline are tested by suing the laser-generated ultrasonic pipeline inspection system. The results show that this system can efficiently detect the crack defects, and the thickness measurement experiment shows that the system can detect small changes in pipe wall thickness.