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.
Ferromagnetic Targets
Electromagnetic Interaction
Metallic test specimens with high magnetic permeability alter the inductance of nearby sensing coils by attracting flux lines. When these ferromagnetic targets are placed in the path of a sensor’s magnetic field, they cause a substantial increase in coil inductance. This effect is much stronger than the eddy current effect, which would otherwise decrease the inductance.
Sensor systems must be designed to handle this high-amplitude signal response.
Material Permeability
Internal magnetic domains within the material respond strongly to external magnetic fields. This high permeability means that small changes in distance produce large output variations. For ferromagnetic targets, the relationship between distance and impedance is non-linear.
Signal Calibration
Calibration curves must be adjusted when switching from one alloy composition to another. Variations in carbon content or heat treatment can change the magnetic properties of ferromagnetic targets, leading to measurement errors if the sensor is not recalibrated. Testing processes use reference blocks made of the same specific alloy to establish a baseline.
This calibration process ensures that the sensor output reflects the actual physical position rather than material variations.
Temperature Sensitivity
Thermal exposure can drastically alter the magnetic characteristics of the material under test. As the temperature of ferromagnetic targets increases, their magnetic permeability decreases, which can cause appreciable measurement drift. If the temperature reaches the Curie point, the material loses its ferromagnetic properties entirely and becomes paramagnetic.
For this reason, high-temperature applications require sensors with built-in thermal compensation or specialized targets that maintain stable magnetic properties across the operating range.