Method and apparatus for measuring the thickness of a metal layer provided on a metal object

1. A method for measuring the thickness of a metal layer provided on a metal object, wherein the metal layer has a resistivity (ρ 1 ) that differs from the resistivity (ρ 2 ) of the metal object, and the method comprises:

generating a constant magnetic field in close vicinity of the metal layer,

generating a sudden variation of the magnetic field so that a current is induced in the surface of the metal layer,

measuring the changes of the magnetic field outside the metal layer due to the induced current during a time period that is longer than the time it takes for the current to propagate through the metal layer,

detecting when the measured values of the changes of the magnetic field deviate from expected values of changes of a magnetic field for a homogeneous object with the same resistivity as the layer, and, on the basis thereof, estimating the time it takes for the induced current to propagate through the layer and reach the boundary between the metal layer and the metal object, and

determining the thickness (d) of the layer based on a mathematical relation between the thickness of the layer and the time it takes for the induced current to reach the boundary between the metal layer and the metal object.

2. The method according to claim 1 , wherein said variation is generated as a step function in which the magnetic field is suddenly changed to a value that considerably differs from its previous value.

3. The method according to claim 1 , wherein said variation is generated by suddenly allowing the magnetic field to drop to zero.

4. The method according to claim 1 , wherein the changes of the magnetic field are measured by measuring the voltage across a coil positioned close to the layer.

5. The method according to claim 1 , wherein the method comprises integrating the measured changes of the magnetic field, and determining the thickness (d) of the layer based on a mathematical relation between the thickness of the layer and the integral of the measured values of the changes of the magnetic field.

6. The method according to claim 5 , wherein the method comprises determining the integral value (Iu(tδ)) when the relation between the integral of the measured change of the magnetic field and the square root of time deviates from linearity and based thereon estimating the thickness of the layer as proportional to the determined integral value.

7. The method according to claim 5 , wherein the method comprises determining the point in time (tδ) when the relation between the integral of the measured change of the magnetic field and the square root of the time deviates from linearity, which is the point in time when the current passes the boundary, and based thereon estimating the time it takes for the induced current to reach the boundary between the metal layer and the metal object.

8. The method according to claim 1 , wherein the method comprises determining the resistivity of the metal layer based on the measured changes of the magnetic field before the variation of the magnetic field has propagated through the metal layer.

9. An apparatus for measuring the thickness of a metal layer provided on a metal object, wherein the metal layer has a resistivity (ρ 1 ) that differs from the resistivity (ρ 2 ) of the metal object, characterized in that the apparatus comprises:

a first device arranged to generate a constant magnetic field in close vicinity of the metal layer, and to generate a sudden variation of the magnetic field so that a current is induced in the surface of the metal layer,

a second device arranged to measure the changes of the magnetic field outside the metal layer due to the induced current during a time period that is longer than the time it takes for the current to propagate through the metal layer, and

a computing unit configured to receive the measured changes of the magnetic field and to detect when the measured values of the changes of the magnetic field deviate from expected values of changes of a magnetic field for a homogeneous object with the same resistivity as the layer, and, on the basis thereof, to estimate the time it takes for the induced current to propagate through the layer and reach the boundary between the metal layer and the metal object, and to determine the thickness (d) of the layer based on a mathematical relation between the thickness of the layer and the time it takes for the induced current to reach the boundary between the metal layer and the metal object.

10. The apparatus according to claim 9 , wherein said first device comprises a coil supplied with a time variable current and the first device is arranged to generate said variation by suddenly changing the current supplied to the coil.

11. The apparatus according to claim 9 , wherein said second device comprises a coil and the second device is arranged to measure the changes of the magnetic field across the coil by measuring the voltage across the coil.

12. The apparatus according to claim 9 , wherein the apparatus comprises an integrator arranged to integrate the measured changes of the magnetic field, and the computing unit is configured to receive the integrated measured changes of the magnetic field, and to determine the thickness (d) of the layer based on a mathematical relation between the thickness of the layer and the integral of the measured values of the changes of the magnetic field.

13. The apparatus according to claim 9 , wherein the computing unit configured to determine the resistivity of the metal layer based on the measured changes of the magnetic field before the variation of the magnetic field has propagated through the metal layer.