Receivers for Pulsed Eddy Current Systems

Abstract

The present disclosure relates to a receiver for a pulsed eddy current (PEC) system, which is configured to detect a changing electromagnetic field generated by eddy currents induced in an object of conductive material. The receiver includes a conductive receiver coil 20, a high voltage receiver channel (HVRC), a low voltage receiver channel (LVRC) 22, and an overvoltage protection device (OVP) 21 connected between the receiver coil and the LVRC. The OVP includes a bias circuit B, a diode D connected between the receiver coil and the bias circuit, and a capacitor C connected between the bias circuit and the LVRC.

Description

Technical Field

[0001] The present disclosure relates to a receiver for a pulsed eddy current (PEC) system, the system comprising a transmitter configured to generate a varying electromagnetic field that induces eddy currents in an object of conductive material disposed within the electromagnetic field, and a receiver configured to detect the varying electromagnetic field generated by the eddy currents. Background Art

[0002] PEC, as described, for example, in US 5,059,902, has been successfully applied to the measurement of mechanical metrics such as resistivity, thickness and edge location of non-ferrous metal sheets.

[0003] The method works by using a DC current in a transmitter coil to generate a static magnetic field in the plate being measured. The magnetic field is then suddenly eliminated by switching off the current, depositing the magnetic energy in a suitable load resistor. The first pulse generated by the current cut-off is measured, and its integral can be used to determine the distance between the plate and the coil.

[0004] After the current in the transmitter coil has decayed, the measurement of the eddy currents induced in the plate by the sudden change in the applied magnetic field can begin. The change in the magnetic field due to the rapid decay of the eddy currents in the plate can induce a small signal that can be measured and analyzed to infer the resistivity and thickness of the plate.

[0005] The measurement of eddy current signals is usually done using a separate receiver coil and a separate measurement receiver channel (RC), which is specifically designed to measure the mV signal from the eddy current decay. Since the initial pulse generated by the transmitter current cut-off can be as high as hundreds of volts, this is why the eddy current measurement channel must also include some type of overvoltage protection device (OVP, Over Voltage Protection).

[0006] The earliest part of the eddy current decay is independent of thickness and can be used to obtain a measure of the resistivity of the plate. The later part depends on the sheet resistivity divided by the thickness. After calculating the resistivity and the sheet resistance, the thickness of the plate can be deduced, for example as described in US 6,661,224. Summary of the invention

[0007] Since the initial pulse generated by the transmitter current cut-off can be several hundred volts, while the later part of the eddy current decay is in the mV range, resulting in a low voltage (LV) signal, two receive channels are used: a high voltage receive channel (HVRC) with low gain and a low voltage receive channel (LVRC) with high gain, where the LVRC is protected from the initial pulse by the OVP. When measuring the pulse from the receiver coil, at the right moment the OVP circuit switches from the blocking state to the conducting state so that the LV signal is present at the input of the LVRC and (usually) amplified with sufficient gain.

[0008] Implementing an OVP circuit using a simple solid-state series switch works well enough for measuring the thickness of relatively thick objects (such as plates), for example at least 0.5 mm thick. Unfortunately, as the thickness of the object decreases, it becomes increasingly difficult to perform the measurement. In particular:

[0009] 1) The signal-to-noise ratio decreases, requiring a low-noise, high-resolution receiver;

[0010] 2) The LV signal decay becomes shorter, making it difficult to acquire and extract resistivity information. This requires a coil system and associated front end with greater bandwidth;

[0011] 3) The behavior of the OVP circuit needs to be as close to an ideal switch as possible, with negligible switching transients. Long switching transients may lead to masking of the LV signal, making it impossible to extract resistivity and thickness information from it.

[0012] It is an object of the present invention to provide an improved PEC receiver, including an OVP, which facilitates thickness measurement of thinner objects (eg, having a thickness less than 0.5 mm).

[0013] According to one aspect of the present invention, a receiver for a pulsed eddy current (PEC) system is provided, which is configured to detect a changing electromagnetic field generated by eddy currents induced in an object of conductive material. The receiver includes a conductive receiver coil, a high voltage receiver channel (HVRC), a low voltage receiver channel (LVRC), and an overvoltage protection device (OVP) connected between the receiver coil and the LVRC. The OVP includes a bias circuit B, a diode D connected between the receiver coil and the bias circuit, and a capacitor C connected between the bias circuit and the LVRC.

[0014] According to another aspect of the present invention, there is provided a PEC system comprising a transmitter configured to generate a varying electromagnetic field that induces eddy currents in an object of conductive material disposed within the electromagnetic field and an embodiment of the receiver of the present disclosure.

[0015] According to another aspect of the present invention, a method for determining the thickness of an object by means of an embodiment of the PEC system of the present disclosure is provided. The method includes inducing eddy currents in the object by means of a transmitter. The method also includes measuring, by means of a receiver, a voltage induced in a receiver coil by a changing electromagnetic field generated by the induced eddy currents as a function of time, wherein for a voltage induced in the receiver coil above a bias threshold provided by a bias circuit, the OVP prevents the LVRC from seeing the induced voltage, and for a voltage induced in the receiver coil below the bias threshold, the OVP allows the LVRC to see the induced voltage. The method also includes determining the thickness of the object based on the measurement of the voltage.

[0016] According to another aspect of the present invention, there is provided a computer program product comprising computer executable components for causing a controller of a PEC system to perform an embodiment of the method of the present disclosure when the computer executable components are executed on a processing circuit included in the controller.

[0017] OVP is used to protect the LVRC from the initial voltage spike, so that the LVRC can stably measure the small voltages (mV range) induced by the eddy currents. With the help of the bias circuit, for low voltages induced in the receiver coil (below the bias threshold, such as the bias voltage), the diode will be forward biased, thereby turning on the LVRC and enabling the LVRC to detect and measure the signal from the receiver coil, while for higher voltages (above the bias threshold), the diode will be reverse biased and thus block the induced voltage. By properly selecting the bias threshold provided by the bias circuit, it can be ensured that the diode is turned on only when the signal from the receiver coil is below the appropriate threshold, thereby protecting the LVRC from the higher signal. Therefore, OVP operates without switching transients or only very low switching transients, resulting in no transients or at least much lower transients than when using standard semiconductor switches. The low series impedance of the diode when forward biased and the high series impedance when reverse biased correspond to an OVP circuit that behaves as a nearly ideal open circuit in the blocking state and a nearly ideal short circuit in the conducting state for the requirements of this application.

[0018] The function of a direct current (DC) blocking capacitor is to prevent the LVDC from seeing a bias, such as a bias voltage or bias current, which is DC and is therefore blocked by the capacitor.

[0019] It should be noted that any feature of any aspect may be applied to any other aspect as appropriate. Likewise, any advantage of any aspect may be applicable to any other aspect. Other objectives, features and advantages of the attached embodiments will become apparent from the following detailed disclosure, the attached dependent claims and the accompanying drawings.

[0020] In general, unless otherwise explicitly defined herein, all terms used in the claims should be interpreted according to their ordinary meaning in the technical field. Unless otherwise explicitly stated, all references to "one / an / the element, device, component, part, step, etc." should be publicly interpreted as referring to at least one instance of an element, device, component, part, step, etc. Unless explicitly stated, the steps of any method disclosed herein do not have to be performed in the exact order disclosed. The use of "first", "second", etc. for different features / components of the present disclosure is intended only to distinguish these features / components from other similar features / components, rather than to give these features / components any order or hierarchy. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Embodiments will be described by way of example with reference to the accompanying drawings, in which:

[0022] Figure 1 is a schematic block diagram of an embodiment of a PEC system of the present invention.

[0023] Figure 2 is a schematic circuit diagram of an embodiment of a receiver of the present invention.

[0024] Figure 3 is a schematic circuit diagram of an embodiment of a LV receiver chain according to the present invention.

[0025] Figure 4 is a schematic circuit diagram of another embodiment of a LV receiver chain according to the present invention.

[0026] Figure 5 is a schematic block diagram of a controller according to some embodiments of the present invention.

[0027] Figure 6 is a schematic flow chart of methods according to some embodiments of the present invention. DETAILED DESCRIPTION

[0028] Embodiments will now be described more fully below with reference to the accompanying drawings, in which certain embodiments are shown. However, within the scope of the present disclosure, many other embodiments in different forms are also possible. On the contrary, the following embodiments are provided by way of example so that the present disclosure will be thorough and complete, and the scope of the present disclosure will be fully conveyed to those skilled in the art. Throughout the description, like numbers refer to like elements.

[0029] Figure 1An embodiment of a PEC system 10 is illustrated which is arranged for measuring the thickness d of an object 1, which is typically a plate (which may also be referred to as a metal sheet or strip) of a colored conductive material such as Al, having a first (here lower) side 4a and a second (here upper) side 4b. In the figure, the object 1 in the form of a plate has a longitudinal axis perpendicular to the plane of the figure.

[0030] The PEC system 10 comprises a transmitter 2 and a receiver 3. In the figure, both the transmitter 2 and the receiver 3, in particular the respective transmitter coil and the receiver coil, are arranged on the same side of the object 1, which is preferred in some embodiments. However, in some other embodiments, the receiver coil may be arranged on the opposite side of the object 1 relative to the transmitter coil.

[0031] The PEC system 10 may include a controller 6 for controlling the transmitter 2 and the receiver 3 via control signaling (e.g., as illustrated by dashed lines in the figure). The controller may be formed as a separate device, or partially or completely integrated with the transmitter and / or the receiver. The controller 6 may, for example, include a central controller device arranged separately from the transmitter and the receiver, and a distributed controller device integrated with the transmitter and / or the receiver.

[0032] Figure 2 An embodiment of a receiver 3 is illustrated. The receiver 3 includes a receiver coil 20 in which a receiver current may be formed, which is induced by the changing electromagnetic field generated by the cut-off of the transmitter current and then by the decay of the eddy currents in the object 1. The receiver current is output as a signal to an RC, which includes an HVRC 23, which generally always receives the signal (regardless of the voltage), and an LVRC 22, which is protected from the signal by an OVP 21 when the signal has a voltage above a predetermined threshold. The OVP 21 is connected between the receiver coil 20 and the LVRC 22. Based on the signal, the HVRC 23 and the LVRC 22 may make voltage measurements to determine the properties of the object 1. An amplifier may be connected to the LVRC to facilitate the measurement of relatively low (mV range) voltages therein.

[0033] Figure 3 An embodiment of the LV receiver chain of the receiver 3 is shown. The OVP 21 is connected between the receiver coil 20 and the LVRC 22. The OVP includes at least one bias circuit B, at least one diode D connected between the receiver coil 20 and the LVRC 22, and at least one capacitor C connected between the bias circuit B and the LVRC 22. Figure 3In the embodiment of the present invention, the OVP 21 includes a positive side circuit connected to the positive terminal of the receiver coil 20 and a negative side circuit connected to the negative terminal of the receiver coil. Each of the positive side circuit and the negative side circuit includes a bias circuit B, a diode D connected between the receiver coil 20 and the bias circuit B, and a capacitor C connected between the bias circuit B and the LVRC 22. It is worth noting that the diode D of the negative side circuit is connected in reverse parallel with the diode D of the positive side circuit.

[0034] The bias circuit B is configured to provide a bias threshold, for example by means of a bias voltage V( Figure 3 The bias threshold may be, for example, the absolute value |V| of the bias voltage V+ or V- plus the absolute value of any voltage drop across the diode D. When the absolute value of the voltage induced in the receiver coil 20 is above the bias threshold, the diode D is reverse biased and the OVP 21 prevents the LVRC 22 from seeing the induced voltage, and when the absolute value of the voltage induced in the receiver coil 20 is below the bias threshold, the diode D is forward biased and the OVP 21 allows the LVRC 22 to see the induced voltage. Typically, the bias threshold provided by the bias circuit B of the negative side circuit is the same as the bias threshold provided by the bias circuit B of the positive side circuit, for example, the absolute value of the bias voltage V (possibly adjusted for any voltage drop across the diode D).

[0035] Figure 4 Another embodiment of the LV receiver chain of the receiver 3 is shown. In this embodiment, the OVP 21 is connected to only one terminal (positive or negative) of the receiver coil 20. Figure 3 Discussions related to embodiments of Figure 4 The present invention is related to the embodiment of the present invention.

[0036] Regardless of the embodiment of OVP 21 (according to Figure 3 or Figure 4 Regardless of whether it is the embodiment of the present invention or any other embodiment, at least one diode D preferably switches very quickly between its conducting (forward biased) and blocking (reverse biased) states, and vice versa. The diode D may be, for example, a radio frequency (RF) diode.

[0037] In some embodiments of the present invention, the diode D has a low series impedance or resistance when forward biased, operating in a manner close to an ideal short circuit, for example, the absolute value of the series impedance is at most 2Ω, for example in the range of 0.5-1.5Ω, such as 1Ω. For the same reason, additionally or alternatively, in some embodiments of the present invention, the diode D has an absolute value of a series impedance when forward biased that is at most one-tenth of the absolute value of the input impedance of the LVRC 22. Similarly, in order to operate in a manner close to an ideal open circuit when reverse biased, the diode D can mainly behave as a small series capacitor, for example 500fF. In some embodiments, the bias circuit B is configured to provide a bias threshold so that the diode D has the absolute value of the series impedance. Additionally or alternatively, the bias circuit B is configured to provide a bias threshold by means of a bias voltage V (e.g., the absolute value of the bias voltage), in the range of 1-3V.

[0038] Preferably, in order to reduce the required absolute value of the bias voltage V or the bias current to provide the desired bias threshold, the voltage drop across the diode D when forward biased should be relatively low, such as at most 1 V, for example in the range of 0.5-0.9 V. In some embodiments of the present invention, the bias threshold is at least 20% (i.e., 1.2 times) higher than the voltage drop of the diode D when forward biased.

[0039] Figure 5 Schematically illustrates an embodiment of the controller 6 of the present disclosure. The controller 6 includes a processing circuit 61, such as a central processing unit (CPU). The processing circuit 61 may include one or more processing units in the form of (multiple) microprocessors. However, other suitable devices with computing capabilities may be included in the processing circuit 61, such as an application-specific integrated circuit (ASIC), a field programmable gate array (FPGA) or a complex programmable logic device (CPLD). The processing circuit 61 is configured to run one or more computer programs or software (SW) 63 stored in a data storage device 62 of one or more storage units (e.g., memory). The storage unit is considered as a computer-readable component, and together with the SW 63 stored thereon as a computer executable component, a computer program product 62 is formed, as discussed herein, and may be, for example, in the form of a random access memory (RAM), a flash memory or other solid-state memory, or a hard disk, or a combination thereof. As required, the processing circuit 61 may also be configured to store data in the storage device 62. The controller 6 may be configured to perform an embodiment of the method of the present disclosure.

[0040] Figure 6Some embodiments of the method of the present disclosure are illustrated. The method is used to determine the thickness d of an object 1 by means of an embodiment of the PEC system 10 of the present disclosure. The method comprises inducing S1 eddy currents in the object 1 by means of a transmitter 2. Then, by means of a receiver 3, the method comprises measuring S2 the voltage induced in the receiver coil 20 by the changing electromagnetic field generated by the eddy currents induced S1 as a function of time. For the voltage induced in the receiver coil 20 (e.g., its absolute value) being above a bias threshold (e.g., represented by a bias voltage V) provided by the bias circuit B, the OVP 21 prevents the LVRC 22 from seeing the induced voltage (i.e., the diode D is blocked). For the voltage induced in the receiver coil 20 (e.g., its absolute value) being below the bias threshold (e.g., represented by the bias voltage V), the OVP 21 allows the LVRC to see the induced voltage (i.e., the diode D is turned on). Then, the thickness dS3 of the object 1 is determined based on the measurement of the voltage S2.

[0041] In some embodiments of the invention, the thickness d is less than 0.5 mm, for example in the range of 0.5 mm to 0.1 mm, such as from 0.4 mm to 0.2 mm. Embodiments of the invention may also be used for thicker objects, but the advantages of the invention over the prior art are more evident for thin objects 1. Embodiments of the invention may be conveniently used in cases where the thickness d is at least as small as 0.1 mm, and sometimes even less.

[0042] Above the present disclosure has mainly been described with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the present disclosure, as defined by the appended claims.

Claims

1. A receiver (3) for a pulsed eddy current (PEC) system (10), configured to detect a changing electromagnetic field generated by eddy currents induced in an object (1) of conductive material, the receiver comprising: a conductive receiver coil (20); High voltage receiver channel HVRC (23); Low voltage receiver channel LVRC (22); as well as An overvoltage protection device OVP (21) connected between the receiver coil (20) and the LVRC (22), the OVP comprising: Bias circuit (B); a diode (D) connected between the receiver coil (20) and the bias circuit (B); and a capacitor (C) connected between the bias circuit (B) and the LVRC (22); wherein the bias circuit (B) is configured to provide a bias threshold for forward biasing or reverse biasing the diode (D) according to a voltage induced in the receiver coil (20); and Wherein the capacitor (C) is configured to prevent the LVRC from seeing a bias voltage or current of the bias circuit (B).

2. The receiver according to claim 1, wherein the diode (D) is a radio frequency (RF) diode.

3. A receiver according to claim 1 or 2, wherein the diode (D) has a series impedance when forward biased with an absolute value of at most 2Ω.

4. The receiver according to claim 3, wherein the diode (D) has a series impedance having an absolute value in the range of 0.5-1.5Ω when forward biased.

5. The receiver according to claim 4, wherein the diode (D) has a series impedance having an absolute value of 1Ω when forward biased.

6. The receiver according to claim 1 or 2, wherein the diode (D) has a series impedance when forward biased whose absolute value is at most one tenth of the absolute value of the input impedance of the LVRC (22).

7. The receiver of claim 3, wherein the bias circuit (B) is configured to provide a bias threshold such that the diode (D) has an absolute value of the series impedance.

8. A pulsed eddy current (PEC) system (10), comprising: A transmitter (2) configured to generate a changing electromagnetic field, the changing electromagnetic field inducing eddy currents in an object (1) of conductive material arranged in the electromagnetic field; as well as The receiver (3) as claimed in any one of claims 1 to 7.

9. A method for determining the thickness (d) of the object (1) by means of the PEC system (10) according to claim 8, the method comprising: Inducing (S1) eddy currents in the object (1) by means of the transmitter (2); By means of the receiver (3), the voltage induced in the receiver coil (20) by the changing electromagnetic field generated by the eddy current induced (S1) is measured (S2) as a function of time, wherein: For a voltage induced in the receiver coil (20) that is above a bias threshold (V) provided by the bias circuit (B), the OVP (21) prevents the LVRC (22) from seeing the induced voltage, and For a voltage induced in the receiver coil (20) that is below the bias threshold (V), the OVP (21) allows the LVRC to see the induced voltage; as well as Based on the measurement (S2) of the voltage, the thickness (d) of the object (1) is determined (S3).

10. The method according to claim 9, wherein the thickness (d) is less than 0.5 mm. The method according to claim 10 , wherein the thickness (d) is in the range of 0.5 mm to 0.1 mm.

12. The method according to claim 11, wherein the thickness (d) is in the range of 0.4 mm to 0.2 mm.

13. The method according to any one of claims 9 to 12, wherein the diode (D) has a voltage drop of at most 1 V when forward biased.

14. The method according to claim 13, wherein the diode (D) has a voltage drop in the range of 0.5-0.9 V when forward biased.

15. The method according to any one of claims 9-12, wherein the bias threshold (V) is at least 20% higher than the voltage drop of the diode (D) when forward biased.

Images