Thickness measurement using a pulsed eddy current system

A computer program product comprising a non-transitory computer readable storage medium storing computer-executable components, wherein the computer executable components when run on processing circuitry of a controller of a Pulsed Eddy Current, PEC, system causes the controller to perform a method of non-contact measurement of a thickness (d) of an object of an electrically conductive material, the method including:

• during a predetermined supply time period, supplying a constant current to a transmitter coil of the PEC system, said supplied constant current generating an electromagnetic field which penetrates the object;
• after the predetermined supply time period, after a start time point (t0), turning off the supplied constant current, resulting in induced eddy currents in the object and a decaying magnetic field;
• at a receiver coil of the PEC system, during a predetermined measurement time period starting at said start time point (t0), measuring a voltage induced by the decaying magnetic field at a first time point (t1), a second time point (t2) and at least one later time point (t3);
• calculating a total magnetic flux (Φ1) which is generated by the induced eddy currents at the first time point (t1) and picked up by the receiver coil, by comparing a measured flux (Φplate) at the first time point (t1) with a predetermined total flux (Φ0) picked up by the receiver coil when no object is present, wherein the first time point (t1) is set to the earliest time satisfying a provision that the magnetic flux (Φ(t1)) at the first time point (t1) is zero when no object is present, wherein Φ1 is calculated as Φ0-Φplate;
• normalizing a measured magnetic flux ((Φec) resulting from the induced eddy currents and picked up by the receiver coil, using the calculated total magnetic flux (Φ1) as a normalization factor such that a normalized eddy current flux ({circumflex over (Φ)}ec) is independent of a distance between the object and the transmitter and receiver coils whereby the normalized eddy current flux ({circumflex over (Φ)}ec) is zero at the first time point (t1) and 1 at a time after the induced eddy currents have died out;
• relating the normalized eddy current flux ({circumflex over (Φ)}ec) to a normalized time (τ) using a time constant (T) as a normalization factor, such that T=0 when a real time t=t1 and τ=1 at a time after the turning off of the supplied constant current when the eddy currents have diffused to just reach a surface of the object facing away from the transmitter coil, whereby the normalized eddy current flux ({circumflex over (Φ)}ec) is independent of the thickness (d) within a normalized time interval from τ=0 to τ=1 and independent of a resistivity (ρ) of the object at times above τ=1, wherein the time constant (T) is calculated as:
• T
• ρ
• ,
• d
• ≡
• μ
• Θ
• ⁢
• d
• 2
• π
• 2
• ⁢
• ρ
• wherein d is the thickness of the object, as previously estimated, ρ is the resistivity of the object, as previously estimated, and μ0 is a magnetic permeability of vacuum;
• and
• based on the measurements at the first time point, the second time point and the at least one later time point, determining the thickness (d) and the resistivity (ρ) of the object, wherein the second time point (t2) is set to within the normalized time interval from τ=0 to τ=2, where the normalized eddy current flux {circumflex over (Φ)}ec (τ2) is only dependent on the resistivity of the object, and the at least one later time point (t3) is set such that τ>2, where the normalized eddy current flux {circumflex over (Φ)}ec (τ3) is only dependent on the thickness of the object.

A PEC system for non-contact measurement of a thickness (d) of an object of an electrically conductive material, the PEC system including a transmitter coil, a receiver coil, and a controller comprising:

• processing circuitry;
• and
• storage storing instructions executable by said processing circuitry whereby said controller is operative to:
• during a predetermined supply time period, supply a constant current to the transmitter coil, said supplied constant current generating an electromagnetic field (B) which penetrates the object;
• after the predetermined supply time period, after a start time point (t0), turn off the supplied constant current, resulting in induced eddy currents in the object and a decaying magnetic field (B);
• at the receiver coil, during a predetermined measurement time period starting at said start time point (t0), measure a voltage induced by the decaying magnetic field at a first time point (t1), a second time point (t2) and at least one third time point (t3);
• calculate a total magnetic flux (Φ1) which is generated by the induced eddy currents at the first time point (t1) and picked up by the receiver coil, by comparing a measured flux (Φplate) at the first time point (t1) with a predetermined total flux (Φ0) picked up by the receiver coil when no object is present, wherein the first time point (t1) is set to the earliest time satisfying a provision that the magnetic flux (Φ(t1)) at the first time point (t1) is zero when no object is present, wherein Φ1is calculated as Φ0-Φplate;
• normalize a measured magnetic flux (Φec) resulting from the induced eddy currents and picked up by the receiver coil, using the calculated total magnetic flux (Φ1) as a normalization factor such that a normalized eddy current flux ({circumflex over (Φ)}ec) is independent of a distance (D) between the object and the transmitter and receiver coils whereby the normalized eddy current flux (Φec) is zero at the first time point (t) and 1 at a time after the induced eddy currents have died out;
• relate the normalized eddy current flux ({circumflex over (Φ)}ec) to a normalized time (τ) using a time constant (T) as a normalization factor, such that τ=0 when a real time t=t1 and τ=1 at a time after the turning off of the supplied constant current when the eddy currents have diffused to just reach a surface of the object facing away from the transmitter coil, whereby the normalized eddy current flux ({circumflex over (Φ)}ec) is independent of the thickness (d) within a normalized time interval from τ=0 to τ=1 and independent of a resistivity (ρ) of the object at times above τ=1, wherein the time constant (T) is calculated as:
• T
• ρ
• ,
• d
• ≡
• μ
• Θ
• ⁢
• d
• 2
• π
• 2
• ⁢
• ρ
• wherein d is the thickness of the object, as previously estimated, p is the resistivity of the object, as previously estimated, and μ0 is a magnetic permeability of vacuum;
• and
• based on the measurements at the first time point, the second time point and the at least one third time point, determine the thickness (d) and the resistivity (ρ) of the object, wherein the second time point (t2) is set to within the normalized time interval from τ=0 to τ=2, where the normalized eddy current flux {circumflex over (Φ)}ec (τ2) is only dependent on the resistivity of the object, and the at least one third time point (t3) is set such that τ>2, where the normalized eddy current flux {circumflex over (Φ)}ec (τ3) is only dependent on the thickness of the object.