When no target material is specified, nonmagnetic systems will typically be set up and calibrated using a 6000 series aluminum target. For magnetic systems, 4130 steel is typically used.
Sensor Diameter vs. Target Diameter Relationships
Calibrated range-sensor diameter
The calibrated range is typically the first specification to be considered when selecting an eddy current sensor for position/displacement measurements.
The sensor itself contains a precision-wound wire coil that is excited by a high-frequency AC signal. This produces a toroidal-shaped oscillating electromagnetic field with the coil itself at the center of the toroid. As with all electromagnetic coils, the strength of the field produced is proportional to the distance from the coil.
Kaman displacement sensors use the oscillating electromagnetic field to induce eddy currents in the surface of a conductive “target.” Since the magnitude of the eddy current is proportional to the field strength, the amount of eddy current induced in the target decreases with increasing distance between the sensor and the target. The effective measuring range of a given coil is typically equal to 30 percent to 50 percent of the coil diameter.
Sensor diameter-target diameter
For this discussion, the coil diameter and sensor diameter are interchangeable. The electromagnetic field produced by unshielded sensors extends radially from the sensor to a diameter equal to 2.5 to 3 sensor diameters. For shielded sensors, the field diameter extends 1.5 to 2 sensor diameters. Any electrically conductive material in this field will have eddy currents induced into its surface, thereby affecting sensor output.
Sensors are typically calibrated by the manufacturer using a flat target based on the thumb rules described above. To ensure the
sensor will perform to the manufacturer’s published specifications, the same target diameter thumb rules should be followed in actual application.