Capacitive and eddy-current sensors respond very differently to differences in target material. The magnetic field of an eddy-current sensor penetrates the target and induces an electric current in the material which creates a magnetic field that opposes the field from the probe. The strength of the induced sensor current and the resulting magnetic field depend on the permeability and resistivity of the material. These properties vary between different materials. They can also be changed by different processing techniques such as heat treating or annealing. For example, two otherwise identical pieces of aluminum that were processed differently may have different magnetic properties. Between different nonmagnetic materials such as aluminum and titanium the variance of permeability and resistivity can be small, but a high performance eddy-current sensor calibrated for one nonmagnetic material will still produce errors when used with a different nonmagnetic material.
The differences between nonmagnetic materials like aluminum and titanium and magnetic materials such as iron or steel are enormous. While the relative permeability of aluminum and titanium are approximately one, the relative permeability of iron can be as high as 10,000.
Eddy-current sensors calibrated for nonmagnetic materials are not likely to function at all when used with magnetic materials. When using eddy-current sensors for precise measurements, it is critical that the sensor be calibrated for the specific material used in the application.
The high permeability of magnetic materials such as iron and steel can also cause small eddy-current sensor errors within the same piece of material. Within any imperfect material, there are microscopic cracks and material variations. The material’s permeability changes slightly around these areas. While the changes are relatively small, the extremely high permeability of magnetic materials enables high-resolution eddy-current sensors to detect these changes. This problem is most evident in rotating targets of magnetic materials.
The electric field of a capacitive sensor uses the target as a conductive path to ground. All conductive materials offer this equally well, so capacitive sensors measure all conductive materials the same. Once a capacitive sensor is calibrated, it can be used with any conductive target with no degradation in performance.An eddy-current sensor can be mounted to measure the runout of a rotating shaft. But even if the shaft is ideal, with absolutely no runout, a high-resolution eddy-current sensor will detect a repeatable pattern of changes as the shaft rotates. These changes are a result of small variations in the material. This phenomenon is well-known and is called electrical runout. These errors can be very small, often in the micron range. Many shaft runout applications, especially those in hostile environments where eddy-current sensors are the norm, are looking for much larger errors and can therefore tolerate these errors. Other more precise applications will need to use techniques to address these errors or use a different sensing technology such as capacitive sensors.
Because the electric field of a capacitive sensor does not penetrate the material, variations within the material do not affect the measurement. Capacitive sensors do not exhibit the electrical runout phenomenon of eddy-current sensors and can be used with rotating targets of any conductive material without additional error.
Eddy-current sensors should be calibrated to the same material as the target in the application and should not be used with rotating magnetic material targets unless the electrical runout errors are acceptable in the application.