| Based on Underwater Alternating Current Field Measurement(ACFM)technology,this paper is dedicated to develop a set of inspection instrument which is able to detect cracks underwater.The main research object to be detected is underwater structures.Through simulations which are aimed to simulate the influence of the underwater environment to electromagnetic field generated by the cracks and therefore to the variations of the characteristic signals,the underwater electromagnetic simulation models were fully developed and especially studied of the liftoff effect on the characteristic signals.The underwater ACFM probe using 3-axis TMR and the underwater cabin were designed and fabricated while an Ethernet-based signal transmission path and its signal processing hardware circuit were designed accordingly.Cracks identification algorithm was developed by using LabVIEW,whose function includes cracks identification,display,Ethernet communication,data storage and re-use and et cetera.Software development,circuit design,signal processing system and related hardware design and optimization were completed,which finally developed a set of portable,intelligent underwater ACFM inspection instrument.The result from experiments shows that,it's competent to detect the cracks and alarm as well as show the cracks in a graphic pattern,therefore quantify the cracks quickly and sufficiently at a precision around 95%.The main work of this paper is as follows:(1)Theoretical model establishment of the underwater ACFM inspection systemBased on the ACFM model,the influence of the simulated marine environment on the distribution of the induced electromagnetic fields and the characteristic detection signals are studied,which is for the study of distribution of the ACFM'S characteristic signals and the defect recognition method in the defect area.The characteristic signals under different liftoffs are especially studied,with which the underwater ACFM model was built.(2)Design and optimization of the ACFM probe and design of the cabinThe design of the underwater probe includes the excitation design,sensor design and structural seal design in order to receive the best characteristic signal.Signal processing module of weak electromagnetic signals were completed to transfer better signals to the ADC moduel.Finnaly,the assembly of the probe was accomplished.The underwater ACFM cabin's structure is designed and the force analysis is completed as well as the safty analysis.All the modules inside of the cabin are designed and distributed reasonably,which is compact and effective.The excitation signal generation module,Ethernet acquisition and signal transmission module,signal conditioning module,power supply module,voltage conversion and voltage regulation module are fully developed and therefore an portable instrument module is built.(3)Inspection system soft-development by LabVIEWThe relationship between characteristic signals and the dimensions of the cracks in underwater environment are studied and therefore the algorithm of display and quantification are developed.The communication between the probe,acquisition board and the MCU are fulfilled under consideration of signal communication's specialty.Programming including the design of identification algorithm,data storage and re-use are complete to enhance the intelligence of the instrument.(4)Establishment of the whole system and fabrication of the intelligent instrumentThe whole ACFM inspection system is established which fulfill the communication among the probe,the cabin and the data process module meanwhile the functionality and reliability were experimented and tested.By studying the characteristic signal under different liftoffs,the experimental tests are verified.Finally,portable intelligent detection instrument are fully complete and underwater cracks are tested to verify the reliability of the system.In the embedded system,a program is built based on Ethernet communication which is designed for data transition and storage to achieve a better portality and intelligence. |
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