|
Fig. 8. Proposed high-speed MFL
inspection system with three sensor arrays.
5. Conclusion and further work
In this paper, based on numerical simulations carried out in ANSOFT
Maxwell EM, eddy currents in steel specimen due to moving probe and their
effect on MFL signals from a highspeed MFL inspection system are
investigated. According to the numerical simulations on high-speed MFL
inspection systems, it is concluded that:
† Despite that DC excitation current is also employed in highspeed MFL
inspection systems, eddy currents are generated because of the probe that
travels at high speed. The influence of eddy currents in specimen on MFL
signal involves not only the shape but also the magnitude of the signal
measured. With the probe speed rising, the signal magnitude drops, which
results in low signal-to-noise ratio in practical inspection system. As a
result, high-sensitivity sensor/sensor array is in demand. Moreover, in
high-speed MFL inspection systems, it is necessary to apply the noise
suppression techniques to cripple systematic noise and extraneous noise.
Compared to static MFL inspection system, more sophisticated techniques,
e.g. adaptive filter and algorithms, wavelet analysis [24] for signal
processing might be employed in a high-speed MFL inspection systems.
† Since the inspection system runs at high velocity, the signal
acquisition time should be shortened. Therefore, the instrumentation of
signal acquisition should have high sampling frequency and comply with
Nyquist law, and sensor/sensor array in high-speed MFL inspection systems
are required to have high bandwidth in order to ensure fast response to an
instantaneous magnetic field change.
† The features of MFL signals from high-speed MFL inspection systems used
for defect characterization will be determined by the electromagnetic
simulations for inverse models. The inverse models will rely on the probe
moving speed. Therefore, the measurement and characterisation will
integrate the speed of the probe for the defect identification and
analysis.
|