US 11,892,285 B2
Thickness measurement using a pulsed eddy current system
Sten Linder, Trosa (SE); Jarl Sobel, Bagarmossen (SE); Anders Eidenvall, Västerfärnebo (SE); Martin Haldin, Västerås (SE); and Aleksander Dzieciol, Uppsala (SE)
Assigned to ABB Schweiz AG, Baden (CH)
Filed by ABB Schweiz AG, Baden (CH)
Filed on Feb. 24, 2022, as Appl. No. 17/652,296.
Claims priority of application No. 21160295 (EP), filed on Mar. 2, 2021.
Prior Publication US 2022/0282964 A1, Sep. 8, 2022
Int. Cl. G01B 7/06 (2006.01)
CPC G01B 7/105 (2013.01) 7 Claims
OG exemplary drawing
 
1. A method of non-contact measurement of a thickness (d) of an object of an electrically conductive material by means of a Pulsed Eddy Current, PEC, system comprising a transmitter coil and a receiver coil, the method comprising:
during a predetermined supply time period, supplying 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), turning 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), measuring a voltage induced by the decaying magnetic field at a first time point (t), 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 (Φec) 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 (t1) and 1 at a time after the induced eddy currents have died out;
relating the normalized eddy current flux (Φec) to a normalized time (T) 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 (Φec) 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:

OG Complex Work Unit Math
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 Φec (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 Φec (τ3) is only dependent on the thickness of the object.