IRREGULAR SHAPED TARGETS
The ideal target shape for eddy current position sensors is one that is flat, parallel with the sensor face, and provides a circular area ≥ 2.5 to 3 times the diameter of the sensor. Many applications with a less than ideal target have been successful using eddy current sensors. The key to their success has been to understand the impact of the error sources, and minimizing, or eliminating them.
At times it is impossible to find a clear area on a target that meets the thumb rule. A raised edge, threaded stud, bolt head, or counter bore may engage the sensor's electromagnetic field.
An irregular feature will show up primarily as a zero shift in the output, as long as the only relative motion between the target and the sensor is normal to the face of the sensor. Any cross axis motion of this type of target can result in an output change indistinguishable from normal movement. The location of the feature can affect the magnitude of cross axis error. The majority of the eddy current produced in a target is located at a diameter equal to ±16% of the coil diameter, or for practical purposes the sensor tip diameter. Therefore it is prudent to locate the sensor such that any irregularly shaped feature is outside the ±16% zone.
Insufficient Target Area
Another category is a target that is undersized. When an eddy current sensor is calibrated at the factory, a round target ≥ three times the sensor diameter is used. A target < 2.5 times the sensor diameter presents two issues. The first is cross axis error described earlier. The second has to do with the amount of eddy current being induced. As eddy currents are induced across the entire target area, material that is not present to engage the sensor field reduces the amount eddy current produced. This results in an increase in the system output, identical to an output change from an ideal target placed farther away from the sensor.
Some applications have targets that move normal to the face of the sensor, but their surface is not parallel to the face of the sensor. Referred to as cosine error, the output of a system with a standard calibration can change when a target is tilted. Even though the tilt is such that equal halves of the target move towards and away from the sensor, minor differences in linearity and sensitivity along the calibrated span can cause the output to change.
An extreme case would be using an eddy current sensor to measure the position of a pivoting arm. In this instance the angle between the sensor and the target changes with arm movement.
Minimizing - Eliminating Errors
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