Correction Method
Signal correction techniques used in electromagnetic testing adjust for the unavoidable changes in signal amplitude that occur when the distance between a sensor and the test specimen varies. This technique, known as lift-off compensation, utilizes the phase difference between the impedance changes caused by air-gap variations and those caused by material properties. By isolating the material signal along a specific phase direction, the instrument remains insensitive to minor probe wobble.
Sensor Interaction
As the probe moves away from the surface, the coupling between the coil and the material decreases rapidly. This reduces the amplitude of the induced currents.
Calibration Protocol
To establish the direction of the compensation vector, the operator performs a lift-off execution by manually raising and lowering the probe on a reference block. The instrument calculates the phase angle of this trajectory and adjusts the rotation of the complex display so that lift-off compensation occurs along the horizontal axis. This process is repeated whenever a different probe or cable is connected.
Operational Limits
The correction algorithm is highly effective for small gaps but becomes less accurate as the distance exceeds the effective range of the coil. For most high-frequency probes, the maximum allowable gap is limited to approximately one millimeter. Exceeding this limit causes the signal-to-noise ratio to drop below the threshold required for reliable inspection.
