Development of Online Electromagnetic Tomography Demodulation System for Rail Defect Inspection

Rail inspection is the most important maintenance method for the safety of rail transportation. With the advantages of noncontact inspection, electromagnetic tomography can be an ideal technique to realize online inspection. In this paper, a novel Electromagnetic Tomography rail inspection prototype developed with Field Programmable Gate Array(FPGA) as front circuit control and signal demodulation unit. The rail inspection sensor designed with 12 coils matrix formed as “L” shape to meet the wheel and track running condition. In the sensor matrix four excitation coils are excited by four individual direct digital synthesizer(DDS) implemented by FPGA. Four signals with four optimized frequencies applied simultaneously to excite four tomographic projection. Other coils pairs in the array are acting as detection coils to measure response signals at the same time. This excitation strategy makes the four projection of electromagnetic tomography applied parallel to improve the efficiency of inspection. The measurement coils' output signals demodulated by FPGA in real time and then the demodulation results are transferred to an image reconstruction computer with user diagram protocol(UDP) by Ethernet. Linear Back Projection image reconstruction algorithm used to reconstruct the rail defect to meet the construction speed requirement. Rail inspection experiments show that the prototype is feasible to be used in real time rail inspection.