When To Consider An Extended Range Calibration
Kaman's measuring systems have specified ranges that allow them to meet published performance specifications. There are times when physical constraints prevent the use of a larger sensor with the range capability required of the application. In such applications, and when the performance at the far end of the range is not as important as close in, an extended range calibration may be the answer!
Eddy current sensors work because there is a change in the sensor coil's inductance with changes in distance between the coil and the target. This diagram shows a typical inductance curve that shows the change in inductance with respect to distance.
The standard calibrated range of our sensors is also noted on the inductance curve. Above the upper end of the calibrated range the ratio of change in inductance/change in displacement, decreases dramatically. Although it is still a usable change, you can see that the linearity over the entire calibrated span would be sacrificed.
In order to repeatably produce a quality, high precision system, we limit the standard calibrated range to a value that will produce a stable output with good linearity and usable resolution. If your application cannot support the use of a standard sensor calibrated over a standard range, we can offer extended range calibrations. A thumb rule for extended range calibration is to not exceed approximately 1.5 times the standard range.
What You Can Expect
Performance expectations for an extended range calibration equal to 1.5 times the published range include:
- Non-linearity will increase to about twice the published specification.
- Thermal sensitivity will increase at the far end of the range due to small temperature changes causing a greater inductance change than that caused by a change in displacement.
- Resolution at the far end of the range will be significantly less due to the decrease in the ratio of inductance/displacement.
One important note: The performance over the standard range portion of the extended range calibration will still be per published specification.
Contact Kaman for application assistance.
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