An Eddy Current-Capacitive Crack Detection Probe with High Insensitivity to Lift-Off

This paper proposes a novel probe for reliable detection of crack/defect in a conductive target. This probe is realized by combining the eddy current and capacitive sensing techniques. Lift-off is a main concern for the eddy current based defect detection systems as the output of the sensor is very sensitive to the air gap between the target and the probe. The effect is significant when the probe is kept close to the target. In the proposed probe, like in some of the conventional ones, a planar coil is used as the eddy current sensing element. The same planar coil is used as a capacitive sensing element to sense the value of the capacitance between the coil and the target. This value is a function of the air gap between the probe and the target. The measured value is fed back to a magnetic actuator on which the entire probe is fitted. The magnetic actuator uses the capacitance value information and automatically adjusts the gap between the probe and the target to a preset value. Thus, the liftoff is corrected automatically. The functionality of the new probe has been analyzed and optimized using finite element analysis before building a prototype probe. Tests conducted on the probe developed confirms the effectiveness of the probe.