The Development Of Eddy Current Testing System And Study On The Forward Problem

Eddy current testing, as one method of the nondestructive testing, is a low cost, fast detection and non-contact technique, and has been being widely used in the aerospace, manufacture and nuclear industries. Supported by the National Nature Science Foundation of China, the paper mainly studied on the design of critical subsystems of a new type of eddy current testing experimental system and researched on the forward problems of eddy current testing. The main research work includes:(1) The development techniques of several critical subsystems of the eddy current testing experimental system were illuminated. A power source whose ripple is limited in 1mV was designed. A balance circuit of probes was used to take full advantage of the amplifier. Different methods of program controlled amplifier were compared and experimental results show that the method based on OP and multi-channel chips is the best one and thus it has been adopted in the circuit. The phase sensitive detection was designed with small errors. Auto-zero circuit was used to take full advantage of ADC. And the software of microcontroller was designed to improve the degree of automation of the system.(2) The upper computer software of the eddy current testing experimental system was developed based on the LabView platform. Signal collection, display and storage modules were developed, in which a panel displays four parameters. The median filter was used to eliminate the high frequency interference. Data can be transformed into TXT files for the sake of sharing. The digital phase sensitive detection was used to simplify the hardware. When scan speed is low, the precision of the digital phase sensitive detection is high. The protocol of serial communication was designed and has been verified by experiments.(3) The necessity of introducing GMR sensors to eddy current testing in conductive multi-layered structures was analyzed. A probe with three GMR sensors has been designed. The output of coil sensors and GMR sensors were compared by experiments. The result shows that the sensitivity of coil sensors has relationship with the incentive frequency while GMR sensors are in contrast.(4) The liftoff elimination technique of eddy current testing on conductive multi-layered structures has been studied. A method to eliminate interference of liftoff by phase testing of signal was presented. Theory analyses, simulation and experiments were used to support this method.