Method and apparatus for crack detection at edges of metallic materials

Abstract

The invention relates to a method of determining a crack (C) at an edge (E) of a metallic material (M), the edge having a curvature of a radius (R). The method comprises: feeding (S10) an electric current to a transmitter coil (3) and controlling (S20) the electric current for generating a magnetic field in the metallic material (M), detecting (S30) the magnetic field by a receiver coil (5), the detected magnetic field thereby generating a signal (S(t)) in the receiver coil, determining (S40) a first signal value (St1), a second signal value (St2) and a third signal value (St3) of the signal at a first time (t1), a second time (t2) and a third time (t3), respectively, determining (S70) a possible presence of the crack and a crack depth thereof based on the first signal value (St1), the second signal value (St2) and the third signal value (St3) by determining a characteristic relation between the signal values.

Description

Technical Field

[0001] This disclosure generally relates to the quality inspection of metallic materials, and specifically to the detection of cracks at the edges of metallic materials using electromagnetic induction. Background Technology

[0002] Known methods for non-contact crack measurement in metallic materials utilize optical devices. The metallic material can be illuminated, and cracks can be detected by optical sensors such as cameras. A drawback of optical methods is that they cannot detect cracks invisible to the surface of the metallic material, and color changes within the metallic material may be interpreted as cracks by the optical sensors. Optical methods have proven difficult to use in applications other than inspecting perfectly clean and smooth metallic surfaces.

[0003] Inductive techniques have been used to inspect metallic materials, such as those used in steel production. When using inductive techniques, a current is induced in the metallic material (e.g., a plate or sheet) by means of a time-varying magnetic field generated by a transmitter coil fed with a time-varying current. When the induced current encounters a crack in the metallic material, the crack acts as an obstacle to the induced current. As a result, the induced current at the crack changes compared to metallic material without cracks. The changed current provides a change in the magnetic field around the current. By measuring the change in the magnetic field through a receiver coil, the presence of a crack in the inspected surface portion of the metallic material can be determined. This inspection of metallic materials is disclosed, for example, in EP 2574911.

[0004] Current inductive techniques for crack detection in metallic materials have several drawbacks. For example, near the edges or corners of the metal being measured, the curvature of the edge or a sudden change in the edge's position relative to the coil may be mistaken for a crack. Because it is difficult to keep these influencing parameters constant, inductive techniques are not suitable for inspecting cracks on irregular surfaces such as cast metal surfaces.

[0005] Therefore, there is an industrial need for an improved method and apparatus for detecting cracks at the edges of metallic materials. Summary of the Invention

[0006] One object of the present invention is to overcome at least some of the aforementioned problems and to provide a solution for detecting cracks at the edges of metallic materials, which is at least partially improved compared to prior art solutions. These and other objects, which will become apparent hereinafter, are achieved by the method for determining cracks at the edges of metallic materials and the arrangement for determining cracks at the edges of metallic materials.

[0007] According to a first aspect of the present invention, a method is provided for determining cracks at the edge of a metallic material, said edge having a curvature with a radius. The method includes the steps of:

[0008] -s10: Feeds a current with a first amplitude to the transmitter coil to generate a magnetic field in the metallic material.

[0009] -s20: Controls the current so that it acquires a second amplitude when the magnetic field is estimated to have penetrated deeper than the deepest crack depth expected to be measured in the metallic material.

[0010] -s30: The magnetic field is detected by the receiver coil, and the detected magnetic field thus generates a signal in the receiver coil.

[0011] -s40: Determine the first signal value at a first time point, at which it has been estimated that any interference caused by controlling the current to obtain the second amplitude has ceased, and

[0012] -s50: Determine the second signal value of the signal at a second time after the first time, and

[0013] -s60: Determine the third signal value of the signal at a third time point after the second time point, and

[0014] -s70: The possible presence and depth of a crack are determined based on a first signal value, a second signal value, and a third signal value by determining a characteristic relationship between at least two of the following combinations of signal values: the first signal value and the second signal value; the second signal value and the third signal value; and the first signal value and the third signal value, wherein the characteristic relationship between at least two combinations of signal values ​​is independent of the location of the edge and the radius of curvature of the edge.

[0015] By determining the characteristic relationship between at least two combinations of signal values ​​based on the specified time, the crack depth can be determined independently of the edge location and the radius of curvature of the edge, without other process parameters affecting the determined crack depth value. Therefore, it should be understood that the characteristic relationship between at least two combinations of signal values ​​is independent of the edge location and the radius of curvature of the edge. Thus, reliable crack depth measurements can be provided at the edges of metallic materials.

[0016] For example, small cracks (typically <3 mm in depth) that are particularly difficult to distinguish from changes in edge radius can be detected and measured by applying the methods described above.

[0017] It should be understood that the method can be performed by feeding a first current with a first amplitude to the transmitter coil during a first time interval, and repeatedly controlling the current to obtain a second amplitude during a second time interval. The first signal value, the second signal value, and the third signal value are all measured during the second time interval when the current is controlled to obtain the second amplitude.

[0018] According to at least one example embodiment, the detection of the magnetic field is achieved by means of a receiver coil by detecting changes in the magnetic field (i.e., induced voltage or dB / dt). Furthermore, it should be noted that "at the first moment when any disturbance caused by the control current to obtain the second amplitude has ceased" is at least partially related to the voltage peak (dB / dt) caused by the portion of the magnetic field that has not penetrated into the metallic material. In other words, this voltage peak (dB / dt) should cease at the first moment.

[0019] According to at least one example embodiment, a first time is further determined in which the current induced in the metal material due to the control current to obtain a second amplitude has penetrated deeper into the metal material than the depth corresponding to the surface irregularities of the metal material and the undesirable crack depth to be measured.

[0020] Therefore, this method is independent of surface irregularities.

[0021] According to at least one example embodiment, the method further includes the step of: disposing the transmitter coil at least partially outside the edge of the metal material.

[0022] Therefore, the induced current can be concentrated at the edge of the metallic material, making it possible to pinpoint cracks in that region. According to at least one example embodiment, the receiver coil can be arranged such that its magnetic center is located inside the edge of the metallic material.

[0023] According to at least one example embodiment, the step includes placing the transmitter coil inside at least a portion of the edge of the metallic material.

[0024] Therefore, the transmitter coil can be arranged to overlap with the edge of the metallic material, thereby facilitating crack detection. According to at least one example embodiment, the ratio of the portion of the transmitter coil arranged outside the edge to the portion arranged inside the edge is between 0.1 and 0.4. It should be understood that any overlap of edges refers to overlap in a plane parallel to the surface of the metallic material.

[0025] According to at least one example embodiment, the third time is the time when any effect of the radius change of the already estimated curvature of the edge has ceased.

[0026] Therefore, the characteristic relationship between the two signal values ​​will be independent of the radius of the edge curvature, where at least one signal value is based on a third time.

[0027] According to at least one example embodiment, the method further includes the step of:

[0028] - Establish edge position parameters based on the edge's position relative to a reference position.

[0029] - Establish the radius parameter based on the radius of curvature of the edge.

[0030] The feature relationship is independent of the edge position parameter and the radius parameter.

[0031] By specifying these parameters, the independence of edge features, such as edge location and edge radius of curvature, is simplified and easily integrated into algorithms used to determine the presence and depth of cracks. The reference location can, for example, be the magnetic axis of the receiver coil. The edge location can include both the horizontal and vertical distances from the reference location to the edge, alternatively including only the horizontal distance. Thus, and according to at least one example embodiment, the edge location parameter represents or corresponds to the position of the edge relative to the reference location, and the radius parameter represents or corresponds to the radius of curvature of the edge.

[0032] According to at least one example embodiment, the step of determining the possible presence of a crack and its crack depth includes comparing the characteristic relationship with a corresponding reference signal.

[0033] Therefore, comparisons are made with metallic materials possessing known characteristics (edge ​​location, edge radius of curvature, and absence of cracks). More specifically, the characteristic relationships formed independently of edge location and edge radius of curvature are compared with the corresponding characteristic relationships of crack-free metallic materials.

[0034] According to at least one example embodiment, the method further includes the step of:

[0035] - Establish a first reference signal for a metallic material with cracks, and the first reference signal has a first radius reference value for the radius parameter and a first edge reference value for the edge position parameter;

[0036] - Establish a second reference signal for metallic materials without cracks, and the second reference signal has a first radius reference value for the radius parameter and a first edge reference value for the edge position parameter;

[0037] - Establish a third reference signal for a non-cracked metallic material, and the third reference signal has the first radius reference value and a predetermined variation of the edge position parameter relative to the first edge reference value;

[0038] - Establish a fourth reference signal for a metallic material without cracks, and the fourth reference signal has the first edge reference value and a predetermined variation of the radius parameter relative to the first radius reference value.

[0039] Therefore, the changing characteristics of the edge, at least the edge location and the radius of curvature of the edge, can be determined, and at least a first time, a second time, and / or a third time can be selected based on the determined changing characteristics. For example, the third time can be determined as the time when the effect of the change in the radius of curvature of the edge has stopped.

[0040] Furthermore, one or more measurement signals determined at least in steps S10, S20, S30, S40, S50 and S60, or step S70, can be compared with a reference signal corresponding to metallic material without cracks at the edge.

[0041] Therefore, according to at least one example embodiment, the step of determining the possible presence of a crack and its crack depth includes using characteristic relationships with corresponding signal values ​​of a first reference signal, a second reference signal, a third reference signal, and / or a fourth reference signal.

[0042] According to at least one example embodiment, the first time, the second time, and the third time are points that are separated in time.

[0043] The separate time points are based on the time elapsed since the initialization of the control current, thus giving it a second amplitude. Alternatively, the first, second, and third times are corresponding short time ranges spanning -30% to +30% of the respective time points. For example, the first, second, and third times or time ranges do not overlap and / or do not share a common start or end time.

[0044] According to at least one example embodiment, the current is substantially constant during the feeding step.

[0045] According to at least one example embodiment, the estimation of a magnetic field that penetrates deeper into a metallic material than the deepest crack depth that is expected to be measured is based on when the current is fed to the transmitter coil, the deepest crack depth that is expected to be measured, and the relative permeability and resistivity of the metallic material.

[0046] According to at least one example embodiment, a first time is estimated based on the time when the controlled current begins to obtain its second amplitude, and based on the relationship between the relative permeability and resistivity of the metallic material and / or based on the time when the current obtains its second amplitude, the deepest crack depth to be measured, and the relative permeability and resistivity of the metallic material. The first time may be, for example, 100 μs or between 100 μs and 700 μs, such as between 100 μs and 500 μs.

[0047] According to at least one example embodiment, the first signal value, the second signal value, and / or the third signal value are respectively individual signal values, average signal values, or integrals of the signal within the short time range at the first time, the second time, and the third time.

[0048] According to at least one example embodiment, the receiver coil is a first receiver coil, and the method further includes the following steps:

[0049] - The magnetic field is detected by means of a second receiver coil, and the detected magnetic field thereby generates a signal in the second receiver coil, and

[0050] - Determine the position of the edge and its distance from the surface of the metal material relative to the first receiver coil and the second receiver coil, respectively.

[0051] Therefore, the coil arrangement (i.e., the transmitter coil, and the first and second receiver coils) and their distances relative to the edges and surfaces of the metallic material can be determined. Such positioning information can be used, for example, in conjunction with a positioning device configured to reposition the coil assembly horizontally and vertically to maintain, for example, constant distances between the coil assembly and the edges, and between the coil assembly and the surfaces of the metallic material, respectively.

[0052] According to a second aspect of the invention, an apparatus is provided for determining cracks at the edge of a metallic material, said edge having a radius of curvature. The apparatus comprises:

[0053] - A transmitter coil, which is arranged to generate a magnetic field in the metallic material.

[0054] - A receiver coil, which is arranged to detect the magnetic field.

[0055] - A signal generator, which is arranged to feed a current with a first amplitude to the transmitter coil to generate the magnetic field in the metallic material.

[0056] - A control unit, arranged to control the signal generator such that when the magnetic field is estimated to have penetrated deeper than the deepest crack depth expected to be measured in the metallic material, the current acquires a second amplitude, and

[0057] - A computing device arranged to receive a signal generated by a magnetic field detected by the receiver coil, and to determine a first signal value of the signal at a first time, at which time it has been estimated that any interference caused by controlling the current to obtain the second amplitude has ceased.

[0058] The computing device is also configured to determine a second signal value of the signal at a second time after the first time, and to determine a third signal value of the signal at a third time after the second time.

[0059] The computing device is further configured to determine the possible presence and depth of a crack based on the first signal value, the second signal value, and the third signal value by determining a characteristic relationship between at least two of the following combinations of the signal values: the first signal value and the second signal value; the second signal value and the third signal value; and the first signal value and the third signal value.

[0060] The effects and features of the second aspect of the invention are largely similar to those described above in conjunction with the first aspect of the invention. The embodiments mentioned with respect to the first aspect of the invention are largely compatible with the second aspect of the invention, some of which are illustrated below.

[0061] According to at least one example embodiment, the transmitter coil is configured in operation to be arranged at least partially outside the edge of the metallic material.

[0062] Therefore, the induced current can be concentrated at the edge of the metallic material, making it possible to pinpoint cracks in that region. According to at least one example embodiment, the receiver coil can be configured in operation such that its magnetic center is located inside the edge of the metallic material.

[0063] According to at least one example embodiment, the transmitter coil is configured in operation to be arranged at least partially inside the edge of a metallic material.

[0064] Therefore, in operation, the transmitter coil can be configured to overlap with the edge of the metallic material, thereby facilitating the identification of cracks. According to at least one example embodiment, the ratio of the portion of the transmitter coil configured to be disposed outside the edge to the portion of the transmitter coil configured to be disposed inside the edge is between 0.1 and 0.4.

[0065] According to at least one example embodiment, the computing device is arranged to determine a third signal value at a third time, which is the time when any effect of the radius change of the curvature of the edge, which has been estimated, has ceased.

[0066] According to at least one example embodiment, the computer device is arranged to determine, in addition to the aforementioned criteria, the first time in which the current induced in the metal material by controlling the current to obtain the second amplitude has penetrated deeper into the metal material than the depth corresponding to the surface irregularities of the metal material and the undesirable crack depth to be measured.

[0067] According to at least one example embodiment, the computing device is arranged to determine the first signal value, the second signal value, and the third signal value at the first time, the second time, and the third time, respectively, wherein the first time, the second time, and the third time are separate time points.

[0068] As mentioned with respect to the first aspect of the invention, the separate time points are based on the time elapsed from the initialization of the control unit to the control signal generator causing the current to acquire a second amplitude. Alternatively, the first time, the second time, and the third time are respective short time ranges spanning -30% to +30% of the respective time points. For example, the first time, the second time, and the third time or time range do not overlap and / or do not share a common start or end time.

[0069] According to at least one example embodiment, the receiver coil is a first receiver coil, and the device further includes a second receiver coil arranged to detect the magnetic field, wherein the computing device is further configured to receive a signal generated by the magnetic field detected by the second receiver coil, and is configured to determine the position of the edge and the distance from the surface of the metallic material relative to the first receiver coil and the second receiver coil, respectively.

[0070] According to at least one example embodiment, the device includes a positioning device configured to adjust the position of the transmitter coil, the first receiver coil, and / or the second receiver coil. The determined edge position and distance from the metallic surface, relative to the first receiver coil and the second receiver coil, respectively, can be used as inputs to the positioning device. The positioning device can, for example, be configured to maintain such position and distance constant.

[0071] It should be understood that, for both the first and second aspects of the invention, the distance from the edge to the first receiving coil (or any reference position within the coil arrangement or crack detection arrangement) is typically a horizontal distance, i.e., a distance parallel to a horizontal surface (e.g., the surface of a metallic material). Accordingly, the distance from the surface of the metallic material relative to the second receiving coil is a vertical distance perpendicular to the horizontal distance.

[0072] Generally, all terms used in the claims will be interpreted according to their ordinary meaning in the art, unless otherwise expressly defined herein. All references to “a / an / the element, device, component, part, step, etc.” will be openly interpreted as relating to at least one instance of an element, device, component, part, step, etc., unless otherwise expressly stated. Unless expressly stated, the steps of any method disclosed herein need not be performed in the exact order disclosed. Attached Figure Description

[0073] These and other aspects of the inventive concept will now be described in more detail with reference to the accompanying drawings, which illustrate exemplary embodiments of the inventive concept, wherein:

[0074] Figure 1 This is a schematic diagram of an example device for detecting cracks in metallic materials;

[0075] Figure 2a and Figure 2b This is a schematic diagram illustrating an example of an arrangement for crack detection in metallic materials;

[0076] Figure 3a and Figure 3b It shows the method for determining, for example, by Figure 1 or Figure 2a and Figure 2b The first, second, and third time plots of the characteristic relationships of the signals detected by the device in the middle; and

[0077] Figure 4 This is a flowchart of a method for determining the crack depth at the edge of a metallic material. Detailed Implementation

[0078] In the following description, specific details such as particular components, interfaces, and techniques are set forth for purposes of explanation and not limitation, in order to provide a thorough understanding of the invention. However, it will be apparent to those skilled in the art that the invention may be practiced in other embodiments departing from these specific details. In other instances, detailed descriptions of well-known devices, circuits, and methods have been omitted so as not to obscure the description of the invention with unnecessary detail.

[0079] The apparatus presented herein is suitable for detecting cracks at the edges of metallic materials by determining the crack depth. Advantageously, the apparatus can be used under extreme conditions, such as in metal manufacturing processes, for example, in casting or rolling. The apparatus is particularly useful for the precise measurement of crack depth on rough metal surfaces.

[0080] Any metallic material with sufficiently high electrical conductivity to allow for the induction of current in the metallic material can be examined by the methods and apparatus presented herein, such as steel.

[0081] Figure 1 A schematic diagram of an example of a device 1 for detecting cracks in surface 19 at the edge E of a metallic material M is shown. The edge E extends along the longitudinal direction of the metallic material M or the direction of movement of the metallic material M, such as... Figure 2a As shown. Edge E has a curvature of radius R, which can be as follows: Figure 1The shape is curved or pointed. Device 1 includes a signal generator 7 for generating an output signal, a control unit 9 for controlling the output signal of the signal generator 7, a transmitter coil 3 for receiving the output signal from the signal generator 7 to generate a magnetic field in the metal material M to be inspected for cracks, a first resistor R1, a receiver coil 5 for detecting the magnetic field and generating a signal based on the detected magnetic field, a second resistor R2, an amplifier 11 for amplifying the signal from the receiver coil 5, and a computing device 12 configured to process the signal from the amplifier 11 to determine whether a crack C exists in the metal material M at edge E by determining the possible crack depth CD. The computing device 12 may include various sub-units, such as a first unit 13, a second unit 15, and a third unit 16.

[0082] Generally, this disclosure relates to generating a magnetic field in a metallic material M at an edge E, detecting the magnetic field, and determining the characteristic relationship of a signal related to the detected magnetic field at certain predetermined times, thereby enabling the determination of the crack depth, which will be described in detail below.

[0083] Figure 2a and Figure 2b A schematic diagram of another example of a device 1' for detecting cracks in surface 19 at the edge E of metallic material M is shown. Figure 2a It is a top view, and Figure 2b It is a side view. Figure 2a and Figure 2b The device 1' is in principle similar to Figure 1 The device 1 is the same as the one described below; the differences between the devices will be mainly described below. For example, the signal generator 7, control unit 9, amplifier 11, and computing device 12 are collectively included. Figure 2a and Figure 2b In component 17, but will be referred to below as Figure 1 The visualization is shown in the image. Device 1' includes a transmitter coil 3 and a first receiver coil 5, which have a design similar to a reference coil. Figure 1 The same function is described above. Furthermore, device 1' includes a second receiver coil 6 arranged remotely from the first receiver coil 5. The transmitter coil 3, the first receiver coil, and the second receiver coils 5 and 6 are generally referred to herein as coil assembly 18. Further, device 1' includes a platform 20 to which coil assembly 18 is connected and movable. Platform 20 is coupled to a positioning device 21, which is configured to adjust the position of coil assembly 18 in the vertical direction V and the horizontal direction H, such as... Figure 2b As shown. It should be noted that platform 20 does not need to be able to reposition coil device 18; rather, each of transmitter coil 3, first receiver coil 5, and second receiver coil 6 can be directly coupled to positioning device 21. It should be noted that, for better understanding, in Figure 2aand Figure 2b The middle part is omitted Figure 1 The first and second resistors R1 and R2 are shown, but they can be considered to be integrated into the transmitter coil 3 and the first receiver coil 5, respectively, or integrated into the platform 20. Accordingly, it can be assumed that the second receiver coil 6 is connected in parallel to the third resistor R3 in the same way that the first receiver coil 5 is connected to the second resistor R2, and this third resistor can be integrated into the second receiver coil 6 or the platform 20.

[0084] Now refer to Figure 2- Figure 4 An example of the operation of device 1' is described in more detail. The metallic material M (e.g., a plate or sheet of metal) to be inspected for crack C is placed near transmitter coil 3 and first receiver coil and second receiver coils 5, 6. More specifically, device 1' is arranged such that transmitter coil 3 is at least partially located outside edge E, denoted by distance DO, and at least partially located inside edge E, denoted by distance DI. The ratio of DO / DI is preferably between 0.1 and 0.4. Figure 2a and Figure 2b In this configuration, the first receiver coil 5 is arranged to partially overlap with edge E, such that its magnetic center MC is located inside edge E. Figure 2a and Figure 2b In this configuration, the second receiver coil 6 is entirely disposed within the edge E and on the metallic material M. Typically, the magnetic axes of the transmitter coil 3 and the corresponding magnetic axes of the first and second receiver coils 5, 6 are perpendicular to the surface 19 of the metallic material M. Preferably, each of the transmitter coil 3 and each of the first and second receiver coils 5, 6 is a flat coil with a smaller propagation along its magnetic axis compared to its perpendicularity to the magnetic axis.

[0085] During crack inspection, the metal material M typically moves relative to the coil assembly 18, allowing inspection to be performed along the surface 19 of the metal material M, particularly at the edge E of the metal material M. As described above, the transmitter coil 3 can be arranged at least partially outside the edge E. Therefore, in step S5 prior to the first step S10 below, the transmitter coil 3 is at least partially disposed outside the edge E of the metal material M. In step S5, the transmitter coil 3 can also be at least partially disposed inside the edge E of the metal material M.

[0086] Control unit 9 (only in) Figure 1 As shown in the figure, combined with Figure 2a and Figure 2b Component 17) is configured to direct signal to signal generator 7 (only in Figure 1 As shown in the figure, combined with Figure 2a and Figure 2bThe component 17 provides a control signal to control the output signal, such as current, provided to the signal generator 7 supplied to the transmitter coil 3. The signal generator 7 may include, for example, a transistor that can be controlled by the control unit 9 to be in an off state, thereby providing current to the transmitter coil 3, or in a closed state, in which it does not provide current to the transmitter coil 3.

[0087] In one embodiment, the control unit 9 is arranged to control the signal generator 7 such that the signal generator generates a substantially constant current with a first amplitude I1 during a first time span t00–t0, such as Figure 3a As shown.

[0088] In the first step S10, a current with a first amplitude I1 is fed to the transmitter coil 3, thereby generating a magnetic field in the metal material M at the edge E. During crack inspection, the surface 19 of the metal material M is arranged sufficiently close to the transmitter coil 3 so that the magnetic field around the transmitter coil 3 can penetrate into the metal material M, thereby generating a magnetic field in the metal material M. This distance can be, for example, 10mm-25mm, 10mm-15mm, or 15mm-25mm.

[0089] At time point t0, when the estimated magnetic field has penetrated deeper into the metal material M than the deepest crack depth expected to be measured, the current fed by the signal generator 7 is controlled by the control unit 9 in the second step S20, such that a substantially constant current acquires a second amplitude I2. The second amplitude I2 can, for example, be substantially zero or zero. Therefore, the second step S20 may include setting the transistor to its closed state. The change in the feed current from the first amplitude I1 to the second amplitude I2 results in an induced current in the metal material M.

[0090] The current fed by signal generator 7 is preferably as follows: Figure 3a The topmost diagram shows the form of pulse train 22a. Magnetic field measurements are typically performed between subsequent pulses, which will be explained in more detail below.

[0091] The estimate of when the magnetic field penetrates to a depth in the metal material M that is greater than the deepest crack depth that is expected to be measured in the metal material M can be based on theoretical estimation. The estimated time is programmed into the software in the control unit 9 so that it can control the current output by the signal generator 7 accordingly.

[0092] This estimate can be based on when current begins to be fed to the transmitter coil 3, the deepest crack depth to be measured, and the relative permeability μ and resistivity ρ of the metallic material M. This estimate can be provided, for example, by the following relationships:

[0093] t0-t00>1.5×μ×(CDmax)2 / ρ,

[0094] Where t0 is the time in milliseconds when the current acquires its second amplitude I2, such as Figure 3a and Figure 3b As shown, t00 is the time when the current acquires its first amplitude I1, CDmax is the maximum crack depth to be measured in millimeters, μ is the relative permeability of the metal material M, and ρ is the resistivity of the metal material M in nano-ohms (nΩm).

[0095] After the second step S20, the first resistor R1 can be used (only when...) Figure 1 As shown, combined with, for example Figure 2a and Figure 2b The energy in the transmitter coil 3 is rapidly released in the platform 20. Therefore, the first resistor R1 is configured to release energy from the transmitter coil 3 when the current reaches its second amplitude I2. In one embodiment, the first resistor R1 may be arranged in parallel with the transmitter coil 3.

[0096] In the third step S30, when the current reaches its second amplitude I2, the magnetic field generated by the induced current is detected at least by the first receiver coil 5. The magnetic field detected by the first receiver coil 5 induces a signal S(t), such as a voltage, in the first receiver coil 5, which can be amplified by the amplifier 11 (only when...). Figure 1 It is clearly shown that, combined with Figure 2a and 2b (Enlarged from component 17). According to Figure 2a and Figure 2b In an example embodiment, in optional step S35, the magnetic field generated by the induced current is also detected by the second receiver coil 6. The magnetic field detected by the second receiver coil 6 also induces a signal Sr(t), such as a voltage, in the second receiver coil 6, which can be amplified by an amplifier.

[0097] Amplifier 11 provides the amplified signal from at least the first receiver coil 5 to computing device 12 (only when...). Figure 1 It is clearly shown that, combined with Figure 2a and Figure 2b (in component 17). In one embodiment, the computing device 12 is configured to determine a first signal value St1, a second signal value St2, and a third signal value St3 of the signal in the fourth step S40, the fifth step S50, and the sixth step S60, respectively. In one embodiment, the control unit 9 is configured to provide a control signal to the first unit 13, such that the first unit 13 can determine the first signal value St1 at a first time t1, the second signal value St2 at a second time t2, and the third signal value St3 at a third time t3, as shown. Figure 3a and Figure 3b As shown.

[0098] In the third step S30, before or simultaneously with detecting the magnetic field of the first receiver coil 5, and optionally detecting the magnetic field of the second receiver coil 6, the energy generated by the magnetic field in the first receiver coil 5 is released through the second resistor R2, and the energy generated by the magnetic field in the second receiver coil 6 is released through the third resistor R3. Therefore, the second resistor R2 and the third resistor R3 are arranged to release energy from the first receiver coil 5 and the second receiver coil 6, respectively, when the current has reached its second amplitude I2. In one embodiment, the second resistor R2 is arranged in parallel with the first receiver coil 5, and / or the third resistor R3 is arranged in parallel with the second receiver coil 6.

[0099] By appropriately selecting the resistances of the first resistor R1, the second resistor R2, and the third resistor R3, and by rapidly switching between the first amplitude I1 and the second amplitude I2 of the current, the energy in the transmitter coil 3 and the first receiver coil and the second receiver coils 5 and 6 can be rapidly released, thereby allowing for a short time interval t1-t0 before the magnetic field measurement begins through the first receiver coil 5 and the second receiver coil 6.

[0100] In one embodiment, the first time t1 is a time (starting from t0) at which it is estimated that any disturbance caused by the control current to obtain the second amplitude I2 has ceased, and optionally, the induced current in the metallic material M caused by the control current to obtain the second amplitude I2 has penetrated deeper into the metallic material M than the depth corresponding to the surface irregularities of the metallic material M and the undesirable shallow crack depth. In the case where the crack depth and surface irregularities having a depth less than or equal to 1 mm are undesirable for measurement, an estimate of the time when the current has penetrated to a depth deeper than the surface irregularities of the metallic material M and the undesirable shallow crack depth can be provided by the following relationship:

[0101] t1≈800*μ / ρ,

[0102] Where t1 is time in microseconds, μ is the relative permeability of the metallic material M, and ρ is resistivity in nΩm. A similar equation can be derived based on the minimum crack depth to be measured. For example, when measuring on high-resistivity materials such as hot steel (e.g., steel at 1000°C), the decay time should be less than about 1 microsecond, and time t1 is therefore chosen to be 1 microsecond, or between 0.5 and 1 microsecond (after t0). For low-resistivity materials, a significantly longer setting for the initial time t1 can be used, for example, according to a simplified equation:

[0103] t1=30 / (ρ 1 / 2 )

[0104] Where ρ is the resistivity of the metallic material M in nanometer ohms (nΩm), and t1 is in microseconds.

[0105] In one embodiment, the third time t3 is the time after t0 when any effect of the change in the radius R of the curvature of the already estimated edge E has ceased. For example, the third time t3 used for measurement on high resistivity materials such as hot steel (e.g., steel at 1000°C) could be approximately 12 microseconds (e.g., radius R of edge E with a 2 mm radius). For another material, the third time t3 could be set as follows:

[0106] t3 = 12 × (1000 / ρ) 1 / 2

[0107] Where ρ is the resistivity of the metallic material M in nanometer ohms (nΩm), and t3 is in microseconds.

[0108] Choose a second time t2 sometime between the first time t1 and the third time t3. For example, the second time t2 could be chosen at a time (after t0):

[0109] t2=(((t1) 1 / 2 +(t3) 1 / 2 ) / 2) 2

[0110] Each of the first, second, and third times described herein is typically programmed in the software of the control unit 9, which can provide control signals to a computing device, such as the first unit 13, to determine the first signal value St1, the second signal value St2, and the third signal value St3.

[0111] For a first time t1, a first signal value St1 is determined by the computing device 12, for example, by the first unit 13. The first signal value St1 is typically a single signal value of the signal acquired at the first time t1, but it can also be the average value of the signal within a first time range extending from -30% to +30% of t1, or the integral of the signal within the first time range. Correspondingly, for a second time t2, a second signal value St2 is determined by the computing device 12, for example, by the first unit 13. The second signal value St2 is typically a single signal value of the signal acquired at the second time t2, but it can also be the average value of the signal within a second time range extending from -30% to +30% of t2, or the integral of the signal within the second time range. Finally, for a third time t3, a third signal value St3 is determined by the computing device 12, for example, by the first unit 13. The third signal value St3 is typically a single signal value of the signal acquired at the third time t3, but it can also be the average value of the signal within a third time range extending from -30% to +30% of t3, or the integral of the signal within the third time range.

[0112] The first signal value, the second signal value, and the third signal values ​​St1, St2, and St3 are provided to the second unit 15 (only when...). Figure 1 As shown in the figure, combined with Figure 2a and Figure 2b (In component 17). The first signal value, the second signal value, and the third signal values ​​St1, St2, and St3 can be provided as analog signals as voltages through a sample-and-hold circuit provided in the second unit 15, or alternatively, as digital signals through an A / D converter provided in the second unit 15.

[0113] The first, second, and third signal values, St1, St2, and St3, can be provided by the second unit 15 to the third unit 16 at a fourth time t4, which is after the third time t3 but before time t20. Measurements are repeated at time t20, and a new current pulse is generated by the signal generator 7. The first unit 13 can be reset at a fifth time t5 after the fourth time t4 for subsequent measurements, i.e., to determine the signal value of the subsequent current pulse. This is in... Figure 3b As shown in the bottom diagram. Therefore, a current pulse can be fed to the transmitter coil 3 by the signal generator 7 at time t20, wherein the above steps S10-S60 are repeated.

[0114] In step S70, the presence of a crack is determined by determining the crack depth based on a first signal value St1, a second signal value St2, and a third signal value St3. The determination of the crack depth can be performed in the third unit 16 by determining a characteristic relationship between at least two of the following combinations of signal values: the first signal value St1 and the second signal value St2; the second signal value St2 and the third signal value St3; and the first signal value St1 and the third signal value St3. The inventors have recognized that combinations of characteristic relationships, such as St1 / St2 with St1 / St3, or St2 / St3 with St1 / St3 or St1 / St2, are independent of the position of the edge and the radius of curvature of the edge, as will be explained further below. It is noteworthy that during crack detection, the resistivity of the material is substantially constant, and irregularities on the surface of the metallic material do not affect the measurement of characteristics with respect to the first time t1.

[0115] Go to Figure 3b The crack and its depth CD are determined by comparing the characteristic relationship with the corresponding characteristic relationship of the reference signal. Figure 3b In this context, a first reference signal Sa(t) has been established (theoretically or through measurements as described in reference steps S10-S70 above, but more times than t1, t2, and t3 to obtain a continuous curve), where crack C appears at the edge E of the metallic material M. The position of edge E relative to the reference position (here, the magnetic center MC of the first receiver coil 5) and the radius of curvature R of edge E can be determined accordingly by the edge position parameter and the radius parameter, respectively. The edge position parameter and the radius parameter are used to represent the first reference signal Sa(t) in the normal state, corresponding to the first edge reference value of the edge position parameter and the first reference value of the radius parameter. Furthermore, in Figure 3bThe following reference signals are established and presented: In step S64, a second reference signal Sb(t) is established (theoretically or through measurements as described in steps S10-S60 above, but more times than t1, t2, and t3 to obtain a continuous curve), for the same metallic material without cracks, and having the same radius parameter and edge position parameter (i.e., the first radius reference value and the first edge reference value, respectively); in step S65, a third reference signal Sc(t) is established (theoretically or through measurements as described in steps S10-S60 above, but more times than t1, t2, and t3). To obtain a continuous curve, a fourth reference signal Sd(t) is established in step S66 (theoretically or by measurement as described in steps S10-S60 above, but more times than t1, t2, and t3, to obtain a continuous curve), for the same metal material without cracks, using the first edge reference value as the edge position parameter and the predetermined variation of the radius parameter relative to the first radius reference value. Therefore, all reference signals can be based at least on steps S10, S20, S30, S40, S50, and S60 as described above, but at more points in time than t1, t2, and t3 to obtain a continuous curve.

[0116] The independence of the edge position can therefore be performed in step 70, and preferably in the third unit 16, by means of the following characteristic relationship between the first signal value St1, the second signal value St2 and the third signal value St3, for example by performing the following process and calculation.

[0117] First, the following characteristic relationship between the second reference signal Sb(t) and the third reference signal Sc(t) holds for the first time, the second time, and the third time t1-t3:

[0118] Sb(t1) / Sb(t2)=Sc(t1) / Sc(t2), and

[0119] Sb(t2) / Sb(t3)=Sc(t2) / Sc(t3).

[0120] Based on the aforementioned characteristic relationships, the position of the edge will therefore not interfere with the measurement.

[0121] Sb(t1) / Sb(t2) and Sc(t1) / Sc(t2) can then be multiplied by the constant factor N12 such that the product equals one (1) when there are no cracks in the metallic material, and multiplied by the first radius reference value of the radius parameter. Correspondingly, Sb(t2) / Sb(t3) and Sc(t2) / Sc(t3) can be multiplied by the constant factor N23 such that the product equals one (1) when there are no cracks in the metallic material, and multiplied by the first radius reference value of the radius parameter. Therefore, the following relationship can be established:

[0122] N12*Sb(t1) / Sb(t2)-1=N12*Sc(t1) / Sc(t2)-1=0;

[0123] N23*Sb(t2) / Sb(t3)-1=N23*Sc(t2) / Sc(t3)-1=0.

[0124] Therefore, during the measurement of S(t) as described above, the following equation is established:

[0125] R12=N12*S(t1) / S(t2)-1

[0126] R23=N23*S(t2) / S(t3)-1

[0127] Therefore, the deviations of R12 and R23 from zero (0) represent changes in the radius of curvature of the edge and / or the presence and depth of the crack, regardless of the location of the edge.

[0128] N12 and N23 can be determined by measuring a reference metallic material that has the same or similar electrical and magnetic properties as the metallic material being measured.

[0129] As from Figure 3b It is further clear that, at least compared to the difference between the same reference signals Sa(t) and Sd(t) between the second time t2 and the third time t3, there is a considerable difference between the first reference signal Sa(t) and the fourth reference signal Sd(t) between the first time t1 and the second time t2. Therefore, the R12 relative to R23 for Sa(t) will produce a relatively large difference compared to the R12 relative to R23 used for Sd(t). Based on this fact, the characteristic number CR can be determined, which is independent of the edge position and the edge radius of curvature:

[0130] CR = R12 - Const1 * R23

[0131] Const1 can be determined, for example, based on measurements of the reference material as described above, which include a predetermined change in the radius parameter relative to a first radius reference value, and wherein for such a radius change CR = 0. Alternatively, Const1 can be determined on metallic material in portions subjected to crack measurements but without cracks, and the value of Const1 gives the minimum measurement change during the measurement period. That is, the value of Const1 is chosen to give the minimum change in CR when the radius R of the edge changes. For example, if the second time t2 is given by the above equation t2 = (((t1)) 1 / 2 +(t3) 1 / 2 ) / 2) 2 If determined, then Const1 is equal to or approximately equal to one (1).

[0132] For example, by performing the method described above, steps S10-S60, and confirming that for Sd(t), CR = 0 (i.e., Const1 = R12 / R23), Const1 = 0.91, and Sb(t1) = 1, a model based on... Figure 3b The following results table:

[0133] Table 1

[0134] Sb(t) 1 0，5 0，25 0，5 0，5 0 0 0 Sc(t) 1，5 0，75 0，375 0，5 0，5 0 0 0，0000 Sd(t) 1，2 0，55 0，25 0，5 0，5 0，090909 0，1 -0，0001 Sa(t) 1，15 0，5 0，23 0，5 0，5 0，15 0，086957 0，0709

[0135] Therefore, the deviation of CR from zero (0) indicates the presence of a crack (the confirmed value of -0.0001 is approximately zero).

[0136] The crack depth CD can then be determined by the following relationship:

[0137] CD = Const2 * CR + Const3

[0138] Const2 and Const3 can be determined, for example, by measurements on a reference material containing cracks of different depths as described above, or theoretically. For example, for hot steel (steel at 1000°C) and for the time t1-t3 as described above, Const2 would be approximately 100 and Const3 would be approximately 1 mm.

[0139] It should be noted that the method described in the detailed description of patent application number EP2574911A1 utilizes a first characteristic value and a second characteristic value CV1, CV2 and their relationship to make the measurement independent of the distance of the transmitter coil 3 and the receiver coil 5 from the surface 19 of the metal material M, the resistivity ρ of the metal material M, and the possible irregularities on the surface 19 of the metal material M, and a third characteristic value CV3 to determine the crack length, referred to herein as reference, and can be used in parallel with the disclosure presented herein to determine the presence of a crack and its crack depth away from the edge.

[0140] According to at least one example embodiment, in step S37 as described below, the first receiver coil and the second receiver coils 5 and 6 are used to determine the horizontal position of edge E relative to device 1' (i.e., along or parallel to the surface 19 of the metal material), and the vertical distance between the surface 19 of the metal material M and device 1'. The two (amplified) time-dependent signals S(t) and Sr(t), as described above, i.e., S(t) for the first receiver coil 5 and Sr(t) for the second receiver coil 6, are fed to component 17, such as computing device 12. Here, signals S(t) and Sr(t) are integrated from time t0 to time t1, respectively, and these two times are controlled by control unit 9, as previously described. S(t1-t0) and Sr(t1-t0) (i.e. and The two integral values ​​of Sr(t1-t0) and their relationship are used to determine the horizontal position of edge E relative to device 1' and the vertical distance between the surface 19 of the metal material M and device 1'. More specifically, when the first receiver coil 5 is arranged relatively close to edge E, the integral value of S(t1-t0) will depend on the vertical distance between the first receiver coil 5 and the surface 19 of the metal material, and the horizontal distance between the first receiver coil 5 (e.g., the magnetic center MC) and edge E, while the integral value of Sr(t1-t0) will depend only on the vertical distance between the second receiver coil 6 and the surface 19 of the metal material (because the second receiver coil 6 is arranged away from edge E). Therefore, for example in computing device 12, the position or horizontal distance PHor of edge E and the vertical distance PVer of the surface 19 of the metal material M relative to device 1' or coil device 18 can be determined by the ratio Sr(t1-t0) / S(t1-t0). For example, by measuring the changes in PHor and PVer on a reference material (e.g., in multiple steps of 1 mm), the ratio is:

[0141]

[0142] The relationship between PHor and PVer can be determined, for example, based on the magnetic center of the transmitter coil.

[0143] For the above measurements, the first receiver coil 5 is arranged, for example, such that the horizontal distance Dr from the edge E to the magnetic center MC is approximately half the vertical distance between the surface 19 of the metallic material and the first receiver coil 5. The second receiver coil 6 is arranged, for example, such that the horizontal distance Ds from the edge E to the magnetic center of the second receiver coil 6 is greater than the vertical distance between the surface 19 of the metallic material and the second receiver coil 6.

[0144] Therefore, the positioning device 21 can move the platform 20 in response to the measurement of PHor and PVer, and is configured to keep PHor and PVer constant during measurement. For large variations in PHor and PVer (e.g., + / - 3 mm to 5 mm), the measurement independence of the edge position and the radius of curvature of the edge may not be achievable. Therefore, according to at least one example embodiment, the measurement independence of the edge position and the radius of curvature of the edge is effective for variations in PHor and PVer < + / - 3 mm.

[0145] It should be understood that, for embodiments with only the first receiver coil 5, the edge position parameter can still be established in step S62 based on the position of the edge relative to the reference position (i.e., the horizontal distance from the edge E and, for example, the magnetic center MC of the first receiver coil 5), and the radius parameter can still be established in step S63 based on the radius of curvature R of the edge E. The aforementioned characteristic relationship is suitable for use independently of the edge position parameter and the radius parameter. However, for the absolute values ​​of PHor and PVer, both the first receiver coil and the second receiver coils 5 and 6 are required.

[0146] Cracks at the edges of metallic materials should be interpreted as cracks existing near the edges, such as cracks originating from the edge and intersecting the geometric diagonal axis at 45° into the metallic material, or cracks extending from the horizontal surface of the metallic material to its transverse or vertical surfaces. According to an example embodiment, the crack depth CD can be the distance from the surface of the metallic material to the point where the crack intersects the geometric diagonal axis at 45°. Here, the measurement of the crack at edge E is achieved by at least partially positioning the transmitter coil 3 outside the edge E of the metallic material M, and preferably overlapping such that DO / DI is between 0.1 and 0.4. Furthermore, it should be noted that the curvature of the edge does not necessarily have to be circular, but can also be curved. In this case, a hydraulic radius or equivalent radius can be used instead of the radius given herein.

[0147] The concept of the present invention has been described above primarily with reference to several embodiments. However, as will be readily understood by those skilled in the art, other embodiments besides those disclosed above are also possible within the scope of the invention as defined by the appended claims. Therefore, although the invention has been described in conjunction with embodiments now considered to be the most practical and preferred, it should be understood that the invention is not limited to the disclosed embodiments; rather, the invention is intended to cover various modifications and equivalent arrangements. The order of the method steps described in this disclosure is not limited to... Figure 4 The order described herein. Without departing from the scope of the invention, one or more steps may be switched in position or occur in a different order. However, according to at least one example embodiment, the method steps are... Figure 4 The sequential execution described in the text.

[0148] Furthermore, by studying the accompanying drawings, the disclosure, and the appended claims, those skilled in the art can understand and implement variations of the disclosed embodiments when practicing the claimed inventive concept. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite articles "a" or "an" do not exclude multiple. The fact that certain measures are recited in mutually different dependent claims does not imply that combinations of these measures cannot be advantageously used.

Claims

1. A method for determining a crack (C) at an edge (E) of a metallic material (M), said edge (E) having a curvature with a radius (R), said method comprising: - A current with a first amplitude (I1) is fed (S10) to the transmitter coil (3) to generate a magnetic field in the metal material (M). - Control the current (S20) such that when the magnetic field is estimated to have penetrated deeper than the deepest crack depth expected to be measured in the metallic material (M), the current acquires a second amplitude (I2). - The magnetic field is detected (S30) by the receiver coil (5), and the detected magnetic field thereby generates a signal (S(t)) in the receiver coil (5). - Determine (S40) the first signal value (St1) of the signal at a first time (t1), at which it has been estimated that any interference caused by controlling the current to obtain the second amplitude has stopped, and - establish (S62) edge position parameters based on the position of the edge relative to the reference position. - Based on the radius of the curvature of the edge, establish the radius parameter (S63). - Determine (S50) the second signal value (St2) of the signal at a second time (t2) after the first time (t1), and - Determine (S60) the third signal value (St3) of the signal at a third time (t3) after the second time (t2), and - Based on the first signal value (St1), the second signal value (St2), and the third signal value (St3), the possible existence of a crack and its depth are determined by determining the characteristic relationship between at least two of the following combinations of signal values: the first signal value (St1) and the second signal value (St2); the second signal value (St2) and the third signal value (St3); and the first signal value (St1) and the third signal value (St3), wherein the method further includes the step (S5): disposing the transmitter coil (3) at least partially outside the edge (E) of the metal material (M), and wherein the characteristic relationship is independent of the edge position parameter and the radius parameter.

2. The method of claim 1, wherein the third time (t3) is the time at which any effect of the radius change of the curvature of the edge, which has been estimated, has ceased.

3. The method according to any one of claims 1-2, wherein the step of determining the possible existence of (S70) a crack and its crack depth comprises: The characteristic relationship is compared with the corresponding reference signal.

4. The method according to claim 3, further comprising the step of: - Establish a first reference signal (Sa(t)) for a metallic material with cracks, and the first reference signal has a first radius reference value for the radius parameter and a first edge reference value for the edge position parameter; - Establish (S64) a second reference signal (Sb(t)) for a metallic material without cracks, and the second reference signal has a first radius reference value of the radius parameter and a first edge reference value of the edge position parameter; - Establish (S65) a third reference signal (Sc(t)) for a non-cracked metallic material, and the third reference signal has a first radius reference value and a predetermined variation of the edge position parameter relative to the first edge reference value; - Establish (S66) a fourth reference signal (Sd(t)) for a non-cracked metallic material, wherein the fourth reference signal has the first edge reference value and a predetermined variation of the radius parameter relative to the first radius reference value.

5. The method of claim 4, wherein the step of determining the possible presence of a crack (S70) and its crack depth comprises: Use the aforementioned characteristic relationship with the corresponding signal values ​​of the first reference signal (Sa(t)), the second reference signal (Sb(t)), the third reference signal (Sc(t)) and / or the fourth reference signal (Sd(t)).

6. The method of claim 4, wherein the edge position parameter represents or corresponds to the position of the edge relative to a reference position, and the radius parameter represents or corresponds to the radius of the curvature of the edge.

7. The method according to any one of claims 1-2 and 5-6, wherein the first time, the second time, and the third time are separate time points.

8. The method according to any one of claims 1-2 and 5-6, wherein the receiver coil (5) is a first receiver coil (5), and the method further comprises the following steps: - The magnetic field is detected (S35) by the second receiver coil (6), and the detected magnetic field thereby generates a signal (Sr(t)) in the second receiver coil (6), and - Determine (S37) the position of the edge (E) and the distance from the surface (19) of the metal material (M) relative to the first receiver coil and the second receiver coil (5, 6), respectively.

9. An apparatus (1, 1') for determining a crack (C) at an edge (E) of a metallic material (M), said edge having a curvature with a radius (R), said apparatus comprising: - The transmitter coil (3) is arranged to generate a magnetic field in the metal material. - Receiver coil (5) is arranged to detect the magnetic field. - A signal generator (7) is arranged to feed a current having a first amplitude (I1) to the transmitter coil for generating the magnetic field in the metallic material. - A control unit (9) is arranged to control the signal generator such that when the magnetic field is estimated to have penetrated deeper than the deepest crack depth expected to be measured in the metallic material, the current acquires a second amplitude (I2), and A computing device (12) is arranged to receive a signal (S(t)) generated by the magnetic field detected by the receiver coil (5), and to determine a first signal value (St1) of the signal at a first time (t1), at which time it has been estimated that any interference caused by controlling the current to obtain the second amplitude has ceased. The computing device is further configured to determine a second signal value (St2) of the signal at a second time (t2) after the first time, and to determine a third signal value (St3) of the signal at a third time (t3) after the second time, and is configured to establish an edge position parameter based on the position of the edge relative to a reference position and a radius parameter based on the radius of the curvature of the edge. The computing device is further arranged to determine the possible presence of a crack (C) and its crack depth (CD) based on the first signal value (St1), the second signal value (St2), and the third signal value (St3) by determining a characteristic relationship between at least two of the following combinations of signal values: the first signal value (St1) and the second signal value (St2); the second signal value (St2) and the third signal value (St3); and the first signal value (St1) and the third signal value (St3), wherein the transmitter coil is configured in operation to be at least partially disposed outside the edge of the metallic material, wherein the characteristic relationship is independent of the edge position parameter and the radius parameter.

10. The apparatus of claim 9, wherein the transmitter coil is configured in operation to be arranged at least partially inside the edge of the metallic material.

11. The apparatus according to any one of claims 9-10, wherein the computing device is arranged to determine the third signal value (St3) at the third time (t3), the third time (t3) being the time at which any effect of the radius change of the curvature of the edge, which has been estimated, has ceased.

12. The apparatus according to any one of claims 9-10, wherein the computing device is arranged to determine the first signal value (St1), the second signal value (St2), and the third signal value (St3) at the first time (t1), the second time (t2), and the third time (t3), respectively, wherein the first time (t1), the second time (t2), and the third time (t3) are separate time points.

13. The apparatus according to any one of claims 9-10, wherein the receiver coil (5) is a first receiver coil (5), and the apparatus further comprises a second receiver coil (6), wherein the computing device is further configured to receive a signal (Sr(t)) generated by the magnetic field detected by the second receiver coil (6), and is configured to determine the position of the edge and the distance from the surface (19) of the metal material relative to the first receiver coil (5) or the second receiver coil (6).

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