Design News, February 2013

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S e n S o r S/ m ac h i n e v i S i o n Position With sub micron resolution, eddy current sensors are used in stage positioning in semiconductor process tools. This same precision has been applied to insert location in plastic molding processes. Displacement Being a robust technology that can handle high temperatures and pressures, they have been used in plastic injection molding applications monitoring things like core shift and mold separation during injection. Go/No-Go In some instances the resolution and hysteresis of an inductive proximity sensor typically used in go/no-go applications is just not going to provide reliable process control. Eddy current sensors have been used in a wide variety of go/no-go applications. A common application is thread detection in automotive parts. Another example is part location prior to automated welding or staking operations. Application Concerns 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 inch is sufficient for optimum performance. For ferrous targets this can be as thin as 0.002 inch. 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, they will be able to come up with a solution, and your application may be one they have solved in the past. Dan Spohn is regional sales manager for Kaman Precision Products. For more information, go to What do you need to know when your selecting cable solution? ©2013 TURCK

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