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 displacement measurements. This tech note will cover the effective range that Kaman's eddy current sensors can achieve, and the concerns that should be addressed when selecting a sensor for a particular application.
The sensor itself contains a precisely wound coil of wire 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's displacement sensors use the electromagnetic field to induce eddy currents in the surface of a conductive "target". As 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 - 50% 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.
Kaman's sensors are calibrated at the factory using a flat target based on the thumb rules described in the preceding paragraph. This ensures the system will perform to the published specifications. For ideal performance the same target diameter thumb rules should be followed in actual application.
Deviating From Thumb Rules
Some applications do not allow enough room for an appropriately sized target. If the ideal target diameter is not possible, measurement can still be achieved with reduced performance. Contact Kaman for application assistance.