Receiver for a pulsed eddy current system

1 . A receiver for a Pulsed Eddy Current, PEC, system configured to detect a changing electromagnetic field generated by eddy currents induced in an object of an electrically conductive material, the receiver comprising:

an electrically conductive receiver coil;

a high-voltage receiver channel, HVRC;

a low-voltage receiver channel, LVRC; and

an over-voltage protection circuit, OVP, connected between the receiver coil and the LVRC, the OVP including:

a bias circuit;

a diode connected between the receiver coil and the bias circuit; and

a capacitor connected between the bias circuit and the LVRC;

wherein the bias circuit is configured for providing a bias threshold for forward biasing or reverse biasing the diode depending on a voltage induced in the receiver coil; and

wherein the capacitor is configured for preventing the LVRC from seeing a bias voltage or current of the bias circuit.

2 . The receiver of claim 1 , wherein the diode is a radio frequency, RF, diode.

3 . The receiver of claim 1 , wherein the diode when forward biased has an absolute value of series impedance of at most 2 Ω.

4 . The receiver of claim 1 , wherein the diode when forward biased has an absolute value of series impedance of at most one tenth of the absolute value of an input impedance of the LVRC.

5 . The receiver of claim 3 , wherein the bias circuit is configured to provide a bias threshold such that the diode has said absolute value of series impedance.

6 . A PEC system comprising:

a transmitter configured to generate a changing electromagnetic field which induces eddy currents in an object of an electrically conductive material arranged within the electromagnetic field; and

a receiver for a Pulsed Eddy Current, PEC, system configured to detect a changing electromagnetic field generated by eddy currents induced in an object of an electrically conductive material, the receiver including:

an electrically conductive receiver coil;

a high-voltage receiver channel, HVRC;

a low-voltage receiver channel, LVRC; and

an over-voltage protection circuit, OVP, connected between the receiver coil and the LVRC, the OVP including:

a bias circuit;

a diode connected between the receiver coil and the bias circuit; and

a capacitor connected between the bias circuit and the LVRC;

wherein the bias circuit is configured for providing a bias threshold for forward biasing or reverse biasing the diode depending on a voltage induced in the receiver coil; and

wherein the capacitor is configured for preventing the LVRC from seeing a bias voltage or current of the bias circuit.

7 . A method of determining a thickness of the object by means of the PEC system comprising a transmitter configured to generate a changing electromagnetic field which induces eddy currents in an object of an electrically conductive material arranged within the electromagnetic field; and

a receiver for a Pulsed Eddy Current, PEC, system configured to detect a changing electromagnetic field generated by eddy currents induced in an object of an electrically conductive material, the receiver including:

an electrically conductive receiver coil;

a high-voltage receiver channel, HVRC;

a low-voltage receiver channel, LVRC; and

an over-voltage protection circuit, OVP, connected between the receiver coil and the LVRC, the OVP including:

a bias circuit;

a diode connected between the receiver coil and the bias circuit; and

a capacitor connected between the bias circuit and the LVRC;

wherein the bias circuit is configured for providing a bias threshold for forward biasing or reverse biasing the diode depending on a voltage induced in the receiver coil; and

wherein the capacitor is configured for preventing the LVRC from seeing a bias voltage or current of the bias circuit;

the method comprising:

by means of the transmitter, inducing eddy currents in the object;

by means of the receiver measuring, as a function of time, a voltage induced in the receiver coil by the changing electromagnetic field generated by the induced eddy currents, wherein:

for a voltage induced in the receiver coil which is above a bias threshold provided by the bias circuit, the OVP prevents the induced voltage from being seen by the LVRC, and

for a voltage induced in the receiver coil which is below the bias threshold, the OVP allows the induced voltage to be seen by the LVRC; and

based on the measurement of the voltage, determining the thickness of the object.

8 . The method of claim 7 , wherein the thickness is less than 0.5 mm.

9 . The method of claim 7 , wherein the diode when forward biased has a voltage drop of at most 1 V.

10 . The method of claim 7 , wherein the bias threshold is at least 20% above the voltage drop of the diode when forward biased.

11 . The receiver of claim 2 , wherein the diode when forward biased has an absolute value of series impedance of at most 2 Ω.

12 . The receiver of claim 2 , wherein the diode when forward biased has an absolute value of series impedance of at most one tenth of the absolute value of an input impedance of the LVRC.

13 . The receiver of claim 4 , wherein the bias circuit is configured to provide a bias threshold such that the diode has said absolute value of series impedance.

14 . The method of claim 8 , wherein the diode when forward biased has a voltage drop of at most 1 V.

15 . The method of claim 8 , wherein the bias threshold is at least 20% above the voltage drop of the diode when forward biased.