Thickness gauging of single-layer conductive materials with two-point non linear calibration algorithm
AssigneeThe United States of America as represented by the Administrator of the National Aeronautics and Space Administration
InventorsJames P. Fulton, Min Namkung, John W. Simpson, Russell A. Wincheski, Shridhar C. Nath
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Abstract
A thickness gauging instrument uses a flux focusing eddy current probe and two-point nonlinear calibration algorithm. The instrument is small and portable due to the simple interpretation and operational characteristics of the probe. A nonlinear interpolation scheme incorporated into the instrument enables a user to make highly accurate thickness measurements over a fairly wide calibration range from a single side of nonferromagnetic conductive metals. The instrument is very easy to use and can be calibrated quickly.
Claims16
An instrument for single-layer thickness gauging of nonferromagnetic conductive materials, comprising:
- a flux focusing eddy current probe comprising an excitation coil, a pick-up coil, and a flux focusing lens disposed between the excitation coil and the pick-up coil;
- electronic means for exciting the excitation coil at predetermined frequency increments within a predetermined frequency range;
- recording means for recording pick-up coil voltages induced by the excitation coil;
- two-point nonlinear means for calibrating the instrument and interpolating thickness from pick-up coil output;
- data input means for inputting data during instrument set-up and calibration;
- and
- display means for prompting data input during set-up and calibration of the instrument and for presenting thickness results.
A method for measuring the thickness of a single-layer nonferromagnetic conductive material, comprising:
- calculating the summed outputs, represented by the variable x.sub.1, of a pick-up coil, the pick-up coil being contained within a flux focusing eddy current probe comprising an excitation coil, a pick-up coil, and a flux-focusing lens disposed between the excitation coil and the pick-up coil, induced by the excitation coil over predetermined frequency increments within a predetermined frequency range for a thin test coupon having a thickness represented by the variable y.sub.1 and having the same material as the sample to be measured;
- calculating the summed outputs represented by the variable x.sub.2 of the pick-up coil induced by the excitation coil over predetermined frequency increments within a predetermined frequency range for a thick test coupon having a thickness represented by the variable y.sub.2 and having the same material as the sample to be measured;
- calculating the variables a and b in the algorithm y=ax.sup.b using the data points (x.sub.1,y.sub.1) and (x.sub.2,y.sub.2), thereby calibrating the instrument;
- calculating the thickness of the sample using the algorithm, the calculated variables a and b, and the summed pick-up coil outputs for the sample over predetermined frequency increments within a predetermined frequency range.
Dependencies are inferred from the claim text.