| According to the requirements of detecting the inner and outside wall crack in pipelines,a method using low frequency pulsed signal as excitation based on ACFM,which is called as pulsed alternating current field measurement,acts on an in-line detecting probe and a system is developed to realize the inner and outside wall crack of pipeline identification and quantification.By theoretical analysis,and finite element numerical simulation,the electromagnetic perturbation near the cracks and the transient signal are analyzed.Then,a new feature is extracted to identify the cracks between inside and outside pipeline.Besides,based on the curve of the new feature,not only length quantification of cracks using crest and trough of feature curve,but also depth quantification using BP neutral net algorithm are present.The system of PACFM pipeline inspection including an optimazated probe structure,hardware equitements and the software program is developed.At last,an identification and quantification experiment of artificial inner and outside wall defects of pipelines is set up to testing the system capability.The main works are summarized as follows:(1)Theoretical and simulation models of PACFM detecting pipelinesAccording to the theory of classitical electromagnetism,a PACFM model based on ACFM technology using low frequency pulsed signal is set up.The finite element method(FEM)is employed to analyze the distribution of electromagnetic field under the affect of the inner and outside wall crack.The relationship between cracks and the transient induced signal is mainly studied.(2)Identification and quantification algorithm of PACFM inspecting pipelinesBased on the finite element model,the transient signal of inner and outside cracks of pipeline is studied in time domain and a feature is extracted.According to the curves of the new feature,the identication algorithm and the length quantification algorithm are defined.Besides,the depth algorithm based on the BP neural net is also defined by the charateristic curve.(3)The hardware equipment and software design,the system development and testingThe detecting probe is designed including optimaztion the detecting probe structure using the two-dimensional finite element model and assembling the probe with radial array sensors.The hardware equipment and software are designed and a system of PACFM detecting the inner and outdide wall crackss of pipeline is development.According to the tesing result,the system can detect the depth 2mm of the inner crackss and the hidden depth 6mm of the outside cracks.Besides,this system can identify the inner and outside cracks correctly as well as quantificate the cracks. |
