Development of a Velocity-Adaptable Alternating Current Field Measurement Device for Crack Inspection in Rails

The alternating current field measurement (ACFM) technique has been validated for its velocity-insensitive feature, which is a merit in the high-velocity detection of rails. However, little has been done to explain its velocity-insensitive mechanism. Moreover, as the detecting velocity varies during detection, the cutoff frequency in the key module lock-in amplifier of the ACFM device should be adaptable, and no method of determining the cutoff frequency is currently available. In the present study, using the Doppler effect, the velocity-insensitive mechanism of ACFM was investigated, and a method for determining the cutoff frequency in the lock-in amplifier was provided. A velocity-adaptable ACFM device was also developed. The device consists of a velocity sensor and a digital lock-in amplifier developed through a field-programmable gate array. Experiments were carried out to verify the significance of a proper cutoff frequency and to test the performance of the developed device. The experimental results indicate that an improper cutoff frequency will lead to missing detection or bring in noise. With the use of the developed device, the cutoff frequency can be adaptably adjusted in fixed- and variable-velocity experiments. Compared to the results at a low velocity of 0.2 m/s, the signal error at a variable velocity was below 5.0%.