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Calibration of eddy current profilometry

  • US 4,942,545 A
  • Filed: 06/06/1988
  • Issued: 07/17/1990
  • Est. Priority Date: 06/06/1988
  • Status: Expired due to Fees
First Claim
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1. A method of calibration an eddy current profilometry probe in a system for inspecting a substantially cylindrical field tube, the inspection system including a probe having a plurality n of sensor coils, a step tube standard having nominal and step diameters, a data acquisition subsystem connected to the probe, a data storage subsystem connected to the data acquisition system, and a data analysis subsystem including means for accessing the data in the storage subsystem and means for inferring from the accessed data, the radius of the tube at a plurality of axial locations in the tube, and a user interface for communicating the inferred radii to the user, wherein the method comprises the steps of:

  • passing the probe through the standard to acquire and record in the data acquisition subsystem and data storage subsystem, respectively, eddy current test (ECT) calibration values for each of the n coils at the nominal and the step diameters of the standard;

    passing the probe through the tube to acquire and record in the data acquisition subsystem and the data storage subsystem respectively, ECT inspection values for each of the n coils at each of said axial tube locations;

    with the data analysis subsystem, selecting from the data storage subsystem the ECT calibration values acquired from the nominal diameter portion of the standard with each of the n coils and assigning to each of the n coils a first set of paired values consisting of the nominal radius and the ECT calibration value from the nominal diameter portion of the standard;

    with the data analysis subsystem, selecting from the data storage subsystem the ECT calibration values acquired from the step diameter portion of the standard with each of the n coils and assigning to each of the n coils a second set of paired values consisting of the step radius and the ECT calibration value from the step diameter portion of the standard;

    forming n systems of equations, each system having two equations in two unknowns, each equation defining a quantitative relationship equivalent to
    
    
    space="preserve" listing-type="equation">X.sub.i =-A.sub.i /(Y.sub.i +B.sub.i)+C whereXi is the tube radiusYi is the probe ECTAi is a scaling factorBi is an adjacent constantC is an adjacent constant radiusi is each of the 1 through n coils;

    for each of the n system of equations, using said first and second sets of paired radii and ECT calibration values, for the quantities X and Y, respectively, to determine the values of A and B for each of the n coils;

    selecting an ECT inspection value from the data storage subsystem for at least one of the n coils and determining a corresponding value of field tube radius using said quantitative relationship including said determined values of A and B for said selected coil; and

    communicating the determined value of the field tube radius to said user interface.

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