Electromagnetic Field Penetration and Eddy Current Skin Depth Mechanics
Standard skin depth defines the exponential field decay boundary, dictating coil drive frequencies and target thickness minimums for displacement sensing.
Liftoff Trajectory
Impedance Representation
Impedance plane displays track the changes in coil impedance as a sensor moves away from a metallic surface. This movement forms a curved path on the impedance display known as the liftoff trajectory, which represents the pure effect of distance on the coil. Because the path is highly repeatable, it establishes a reference line for sensor calibration.
This line is used to define the zero-point of the measurement system.
Signal Discrimination
Phase analysis techniques separate unwanted spacing variations from actual material defects. The liftoff trajectory is oriented at a specific angle on the impedance plane, while defect signals are oriented at a different angle. By rotating the phase of the signal, the effect of varying distance can be minimized, allowing the system to detect surface cracks even when the sensor vibrates.
This technique is standard in automated eddy current testing systems.
Sensor Alignment
Mechanical guide systems keep the probe aligned to minimize variations in spacing during automated scans. If the sensor is allowed to wobble, the signal moves along the liftoff trajectory, which can obscure small defects. Specialized springs or air bearings are used to maintain a constant pressure and keep the probe perpendicular to the surface.
This mechanical control is essential for achieving high repeatability in high-speed manufacturing environments.
Measurement Limitation
Sensor sensitivity decreases rapidly as the physical separation from the material increases. When the sensor is too far from the surface, the signal along the liftoff trajectory becomes too weak to distinguish from background electronic noise, establishing a hard limit for the maximum allowable air gap. To maintain accurate readings, the system must operate within a tightly controlled range close to the material.
Beyond this distance, the eddy currents cannot couple effectively with the specimen, rendering the measurements unreliable and prone to failure.