Electromagnetic rail inspection using AC bridge measurements

This paper introduces an electromagnetic rail inspection method using excitation and detection coils in an AC bridge configuration. The differential output and summing output of two detection coils in the bridge can be used to evaluate rail defects and lift-off distance, respectively. When the sensor scans a non-defective rail track, the differential output will be near zero and the summing output can be used to calculate the lift-off distance. Then the lift-off distance can be used to compensate the differential output to evaluate rail defects. To improve the sensor sensitivity, a digital lock-in algorithm is used to measure the imbalance signal. Experiments on rail defect inspection using a sample with artificial cracks have verified this method.