Primary Principle
Electromagnetic induction generates circular flows of electrons within conductive targets to measure position or material integrity. Eddy current transduction relies upon an excitation coil driven by an alternating current to produce a fluctuating primary magnetic field. When this field interacts with a metallic substrate, secondary loops of current develop in opposition to the primary source.
The sensor head detects the resulting change in impedance to determine the proximity of a conductor or the presence of a flaw.
Signal Processing
Precision depends upon the selection of an excitation frequency that penetrates the target to a depth determined by electrical conductivity and magnetic permeability. Lower frequencies allow deeper penetration for subsurface inspection of thick plates. High frequencies focus the sensitivity on surface defects and thin wall thicknesses where the skin effect dominates the interaction.
Demodulation circuits isolate the resistive and inductive components of the sensor impedance to separate signals from noise or thermal drift.
Calibration Procedure
Standardisation occurs by measuring against known geometric references or material samples with calibrated defect sizes. A reference block matching the metallurgy of the actual part ensures that gain settings compensate for variances in conductivity. Ambient temperature shifts alter the resistivity of both the coil wire and the target material, which requires active compensation through a bridge circuit or reference channel.
Verification of the accuracy limits involves repeated scans of a notched reference piece to confirm that the amplitude variation stays within defined tolerances.
Operational Constraint
Proximity to sharp edges or geometry changes creates false signals through the edge effect. Field distortion near corners complicates the interpretation of defect data because the current paths crowd together at the boundary. Signal suppression techniques allow the filtering of these geometric artifacts provided the operator defines a clear separation between the edge response and the signal of interest.
Consistent signal quality rests on the maintenance of a stable lift off distance between the probe and the material surface.
