When applying eddy current displacement sensors, consideration must be given to target size, target shape, the physical environment, cable length, and the nature of target motion.
The target -- the object being sensed -- should be electrically conductive but does not need to be grounded. The recommended minimum target diameter is 1.5 to two times the probe diameter for shielded probes and 2.5 to three times the probe tip diameter for unshielded probes. This ensures the sensor will meet published performance specifications with maximum sensitivity.
The eddy current induced in the target is a skin effect, defined as the tendency of high-frequency alternating currents to flow near the surface of a conductor. The term "skin depth" is used to define the depth at which the current is ~37 percent of the current at the surface. The recommended minimum target thickness for eddy current displacement sensors is three skin depths, and maintaining this ensures the sensor will meet published performance specifications.
With most nonferrous targets, a thickness of 0.018 inches is sufficient for optimum performance. For ferrous targets, this can be as thin as 0.002 inches.
In an ideal application, the probe would be sensing a flat target, with the surface parallel to the sensor face, with a clear area at least three times the diameter of the probe, and moving normal to the face of the sensor.
This scenario, however, represents less than one-third of all the applications that successfully use eddy current displacement sensors. More often, the target is smaller or thinner than recommended, or the target has an irregular shape and may be a rotating shaft or disc or a lever arm presenting compound motion of displacement and tilt. If your target isn't ideal, or the target motion isn't typical, contact the sensor company and ask to speak with an applications engineer. Chances are, the company will be able to come up with a solution, and your application may be one it has solved in the past.
Dan Spohn is regional sales manager for Kaman Precision Products.