Thickness Measurement Using Pulsed Eddy Current Systems

Abstract

To provide a method of non-contact measurement of a thickness of an object of an electrically conductive material by means of a PEC system including a transmitter coil and a receiver coil.SOLUTION: The method includes, after having turned off a current in the transmitter coil, measuring at the receiver coil a voltage induced by the decaying magnetic field at a first time point, a second time point and a third time point. The method also includes calculating a total magnetic flux which is generated by the eddy currents in the object at the first time point and picked up by the receiver coil, by comparing the measured flux at the first time point with a predetermined total flux when no object is present. The method also includes normalizing the magnetic flux picked up by the receiver coil using the calculated total magnetic flux as a normalization factor, so that the normalized eddy current flux is independent of a distance between the object 1 and the transmitter and receiver coils. The method also includes, based on the measurements at the first, second and third time points, determining the thickness d and the resistivity of the object.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001]

[0001] The present disclosure relates to non-contact measurement of the thickness of an object made of a conductive material by a pulsed eddy current (PEC) system comprising a transmitter configured to generate a change in an electromagnetic field that induces eddy currents in an object placed within the electromagnetic field, and a receiver configured to detect the change in the electromagnetic field generated by the eddy currents. [Background technology]

[0002] PEC, for example as described in US Pat. No. 5,059,902, has been successfully applied to the measurement of mechanical quantities such as electrical resistivity, thickness, and edge position of non-ferrous metal sheets.

[0003]

[0003] This method works by using a DC current in the transmitter coil to create a static magnetic field in the plate under measurement. The field is then abruptly removed by turning off the current, depositing magnetic energy in a suitable load resistor. The first pulse resulting from the current cut 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, it is possible to begin measuring the eddy currents induced in the plate by the sudden change in the applied magnetic field. The change in the magnetic field due to the sudden 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]

[0005] The initial portion of the eddy current decay is independent of thickness and can be used to obtain a measure of the resistivity of the plate. The subsequent portion depends on the sheet resistance, which in turn depends on the resistivity divided by the thickness. After calculating the resistivity and sheet resistance, the thickness of the plate can be inferred, for example, as described in U.S. Pat. No. 6,661,224.

[0006]

[0006] US Patent No. 7,701,205 discloses a PEC method of measuring plate thickness, in which a transmitter coil is placed on one side of the plate and a receiver coil is placed on the other side of the plate.

[0007] A potential drawback of the techniques described in the above-referenced documents is that they require coil systems on both sides of an object, e.g., a plate. The distance between the coil systems and the object in a PEC-based measurement system must be very small (less than 20 mm) for fundamental reasons, and the need to physically fit the object between the coil systems limits its general applicability. One important example where the above technique cannot be used is the measurement of a tube wall, where it is usually not possible to have one of the coil systems inside the tube. Another important example is the measurement of the thickness in the center of a wide (more than 1.5 m) metal strip during rolling, where it is practically impossible to keep two coil systems in a constant position relative to each other with sufficient precision to reach the desired accuracy of the thickness measurement. Until now, the inability to use the above-described techniques for these measurement applications has affected the general applicability of PEC techniques to thickness and resistivity measurements in the metals industry.

[0008]

[0008] U.S. Patent No. 5,059,902 describes a general technique for determining the resistivity and thickness of an object. This technique can be used to determine the thickness of an object either with relatively less accuracy for general situations with various types of materials and thicknesses, or with greater accuracy for a more limited range of materials and thicknesses. The reason for this is that the method of U.S. Patent No. 5,059,902 is difficult to implement, as three measured values ​​are given for each set of location, resistivity, and thickness, and all relationships between the measured values ​​and physical parameters are nonlinear and unknown. To use the measured values ​​to determine the three variables of location, resistivity, and thickness, test measurements must be performed using various test plates to establish the relationships, describe them, and then use them to determine the physical parameters. As an example, if the thickness of an object can vary by a factor of 10, e.g., from 0.5 mm to 5 mm, then the resistivity can vary by a factor of 10, e.g., from 20 nΩm to 200 nΩm. If the desired accuracy of the physical parameter measurement is 0.1%, which is currently the usual level of accuracy in the metallurgical industry, the relationship between the physical parameter and the measurement is so nonlinear and unknown that test measurements to establish that relationship must be performed every time the physical parameter value changes by at least 2%. As a result, in this example, approximately 14,000 tests using different values ​​of the parameters resistivity and thickness must be performed, which means that the same number of test objects, plates, must be manufactured and tested to provide sufficient information. For practical and economic reasons, it may be impossible to handle such a large number of test objects. To create a functioning measurement method based on U.S. Patent No. 5,059,902, it is necessary to either accept a lower accuracy or use a method within a more restricted parameter range. Summary of the Invention

[0009] It is an object of the present invention to provide an improved method for determining the thickness of an object o of conductive material by pulsed eddy current measurements.

[0010] According to an aspect of the present invention, a method for non-contact measurement of the thickness of an object made of a conductive material using a PEC system including a transmitter coil and a receiver coil is provided. The method includes supplying a constant current to the transmitter coil for a predetermined supply time period, the supplied current generating an electromagnetic field that penetrates the object. The method also includes turning off the supplied current after a supply time period after a start time point t0, resulting in the induction of eddy currents in the object and a decay of the magnetic field. The method also includes measuring a voltage induced by the decay of the magnetic field in the receiver coil for a predetermined measurement time period beginning at the start time point t0. The voltage is converted to magnetic flux by integration and measured at a first time point t1, a second time point t2, and at least a subsequent time point t3. The method also includes determining the measured magnetic flux Φ at the first time point t1. plate and calculating a total magnetic flux Φ(t1) produced by the eddy currents and picked up by the receiver coil at a first time point t1 by comparing Φ(t1) to a predetermined total magnetic flux Φ(t1) picked up by the receiver coil when no object is present, where the first time point t1 is set to the first time that satisfies the condition that the magnetic flux Φ(t1) at the first time point t1 is zero when no object is present. The method also includes using the calculated total magnetic flux Φ(t1) as a normalization factor to calculate the measured magnetic flux Φ(t1) resulting from the eddy currents and picked up by the receiver coil. ec , whereby the normalized eddy current flux

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[0011]

[0011] According to another aspect of the present invention, a computer program product is provided having computer-executable components that, when executed on processing circuitry included in a controller, cause a controller of an embodiment of a PEC system to perform the method disclosed herein.

[0012] According to another aspect of the present invention, a PEC system for non-contact measurement of the thickness of an object of conductive material is provided, the PEC system comprising a transmitter coil, a receiver coil, and a controller, the controller comprising a processing circuit and a memory device storing instructions executable by the processing circuit, whereby the controller operatively performs an embodiment of the method of the present disclosure.

[0013] According to the present disclosure, by normalizing the magnetic flux and relating it to normalized time, the thickness and resistivity of an object can be unambiguously determined by PEC measurements at different points in time.

[0014]

[0014] It should be noted that, where appropriate, any feature of any aspect may be applied to any other aspect. Similarly, any advantage of any aspect may be applied to any of the other aspects. Other objects, features, and advantages of the disclosed embodiments will become apparent from the following detailed disclosure, from the appended dependent claims, and from the drawings.

[0015]

[0015] In general, all terms used in the claims shall be interpreted according to their ordinary meaning in the art unless expressly defined otherwise herein. All references to "one / the elements, apparatus, components, means, steps, etc." shall be openly interpreted as referring to at least one instance of the element, apparatus, component, means, step, etc., unless expressly stated otherwise. Unless expressly stated, the steps of any method disclosed in this disclosure need not be performed in the order disclosed. The use of "first," "second," etc., for different features / components of this disclosure is intended only to distinguish that feature / component from other similar features / components, and is not intended to impose any order or hierarchy on the features / components.

[0016]

[0016] Embodiments will now be described, by way of example, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a schematic block diagram of a PEC system according to some embodiments of the present invention. [Figure 2] 1A and 1B illustrate the magnetic field generated by a constant transmitter current passing through the transmitter coil of a PEC system according to some embodiments of the present invention after the transmitter current has completely penetrated the plate, i.e., when the effects of eddy currents have disappeared and the magnetic field distribution is independent of the presence of the plate. [Figure 3]2A and 2B illustrate magnetic fields generated by eddy currents in the plates in the PEC system of FIG. 1 immediately after the transmitter current is turned off, according to some embodiments of the present invention. [Figure 4] Normalized eddy current flux as a function of real time (t) for plates with different resistivities (ρ) and different thicknesses (d) according to some embodiments of the present invention.

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[0018]

[0017] Embodiments will now be described more fully hereinafter with reference to the accompanying figures, in which specific embodiments are shown. However, other embodiments in many different forms are possible within the scope of this disclosure. Rather, the following embodiments are provided as examples so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. Like numbers refer to like elements throughout the description.

[0019] 1 shows an embodiment of a PEC system 10 arranged to measure the thickness of an object 1, typically a plate of conductive material, typically a non-ferrous conductive material such as Al (which may also be called sheet metal or metal strip), the object having a first (here lower) side 4a and a second (here upper) side 4b. In the figure, object 1, in the form of a plate, has a longitudinal axis that is perpendicular to the plane of the figure. Although object 1 is illustrated herein as a plate, the object may have other shapes in some embodiments.

[0020] The PEC system 10 includes a transmitter coil 2 of the transmitter and a receiver coil 3 of the receiver. In the figure, both the transmitter coil 2 and the receiver coil 3 are located on the same side of the object 1, which is preferred in some embodiments. However, in some other embodiments, the receiver coil 3 may be located on the opposite side of the object 1 relative to the transmitter coil 2.

[0021] The transmitter coil 2 is configured to generate a sudden change in the supplied electromagnetic field, which causes eddy currents to be induced in an object 1 placed within the electromagnetic field. The receiver coil 3 is configured to allow a voltage to be induced therein by the change in the electromagnetic field, the change in the electromagnetic field being generated, for example, first by the collapse of the magnetic field when the current through the transmitter coil is switched off, and then by eddy currents.

[0022] The PEC system 10 may include a controller 6 for controlling the transmitter and receiver, e.g., via control signals as indicated by the dotted lines in the figure. The controller 6 may also be configured with circuitry for analyzing voltages induced in the receiver coil 3 by eddy currents in the object 1 to determine the object's thickness d, as described further below. The controller may be formed as a separate device or may be partially or fully integrated with the transmitter and / or receiver. The controller 6 may, for example, include a central controller device that is located separately from the transmitters and receivers and from distributed controller devices integrated with the transmitters and / or receivers. Eddy Current Normalization

[0023] It is desirable to know the magnitude of these currents so that the thickness d and resistivity ρ can be calculated from the eddy current measurements. The transmitter current I Tr The magnitude of the eddy current generated in plate 1 when is turned off is a function of the dimensions of transmitter coil 2, the transmitter current I Tr and the distance D between the coil 2 and the plate 1. To measure the magnitude of the eddy currents according to the present invention, the transmitter current I, picked up by the receiver coil 3, Tr The total magnetic flux Φ generated by is measured as described below.

[0024] FIG. 2 shows the transmitter current I flowing through the transmitter coil 2. Tr2 and 3 show a cross-sectional view of the magnetic field B (B denotes the magnetic flux / field density, also commonly referred to as the magnetic field) generated by the transmitter coil 2. The cross-section of the transmitter coil is shown as two inner (unfilled) dots, symbolizing where the coil turns pass perpendicularly through the plane of the diagram. The cross-section of the receiver coil 3 is seen as two outer (filled / black) dots. In the examples of FIGS. 2 and 3, the transmitter coil 2 is thus positioned within the receiver coil 3, e.g., concentrically, although in other embodiments of the invention, the transmitter and receiver coils may be positioned relative to each other in other ways, e.g., side-by-side along an axis parallel to the plate 1. Preferably, both the transmitter and receiver coils are positioned on the same side of the plate when in use. The magnetic field lines follow a circular transmitter coil 2 having a single turn, although either the transmitter or receiver coil may have any suitable shape and / or any number of turns, depending on the application.

[0025] 2, the magnetic field distribution is shown after a long enough time has passed since the application of a constant (DC) transmitter current, when possible eddy current effects have disappeared, so that the distribution of the magnetic field B is independent of the presence of the plate 1.

[0026] The total magnetic flux Φ picked up by each turn of the receiver coil 3 is, by definition, equal to the surface integral of the magnetic field density B on the surface S encompassed by the turns of the receiver coil 3:

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[0027] The magnitude of the voltage V induced in each turn of the receiver coil 3 is given by Faraday's law as the time derivative of the magnetic flux:

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[0028]

[0027] When plate 1 is absent, the magnetic field disappears as soon as the transmitter current is cut off. By integrating the voltage induced in receiver coil 3 from time t0 just before the current is cut off until the current is successfully turned off at time t1 (in this case Φ(t1)=0 when plate 1 is absent), the total magnetic flux Φ0 picked up by receiver coil 3, as defined by the surface integral (above), is given by:

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[0029] In practice, the transmitter current cannot be turned off immediately. The required time t1 depends on the values ​​of the inductance, capacitance, and any damping resistors of the PEC system 10. Time t1 is therefore a constant that depends on the design parameters of the system 10.

[0030] When plate 1 is present, eddy currents induced in the plate cause the transmitter current I Tr The magnetic field B that existed just before the wire was cut off is maintained. In this case, the magnetic field at time t1 is as shown in FIG.

[0031] The magnetic field B in Figure 3 above plate 1 is the same as in Figure 2. This field is mirrored below plate 1 due to the symmetry of the eddy currents in the plate. The magnetic field B in Figure 3 is generated solely by the eddy currents and is proportional to their magnitude.

[0032] The total magnetic flux Φ(t1) produced by the eddy currents at time t1 picked up by receiver coil 3 when plate 1 is present is denoted as Φ1. Now by again integrating the voltage V induced in receiver coil 3 from time t0 to t1 in the presence of plate 1, we obtain:

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[0033] From this relationship, Φ 1 can be obtained, which serves as a normalization factor to remove the dependence of the measurement on the distance D between the plate 1 and the coils 2 and 3 of the system 10.

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[0034] To measure the time dependence of eddy currents (ec), the integration begins at time t1 and is integrated for an additional time t:

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[0035] The quantity measured in this way is equal to zero between t and t1 and increases with time as the eddy currents and magnetic flux are diffused by the resistivity of plate 1. To eliminate the dependence on the distance D, it is normalized to the total magnetic flux Φ1 at the start of the measurement:

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[0036]

[0035] This produces a normalized eddy current flux that starts at zero (when t=t1) and reaches unity (i.e., 1) after all the current has dissipated.

[0037] Next, the time dependence of the PEC measurement is calculated by this normalized eddy current flux

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[0038] At time t1, the eddy currents are concentrated on the surface of plate 1 closest to the transmitter coil 2 (bottom surface 4a, see Figures 1, 2 and 3). They are then diffused and spread out by the influence of the plate's electrical resistivity ρ. Initially, there is no influence from the plate's thickness d, so the time dependence depends only on the resistivity. Thereafter, the time dependence becomes more complex and depends on both the resistivity and the thickness.

[0039]

[0038] By way of example, Figure 4 shows the normalized eddy current flux for six plates 1 of three different thicknesses d 1.5, 3 and 4.5 mm, each having two different resistivities ρ, 28 and 56 nΩm respectively.

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[0040] As can be seen in Figure 4, if the origin is set at t1 for times below 10 μs, all curves corresponding to plates of the same resistivity coincide. During this time range, the measurement depends only on the resistivity. Then, as time goes on, the curves separate from each other. Time Normalization

[0041]

[0040] When solving problems mathematically and physically, it is generally standard procedure to introduce normalized variables as a means of simplifying the dependence on the different parameters involved. In this regard, such a procedure applied to PEC techniques significantly reduces the number of test objects required to provide the information necessary to calibrate the models used to calculate thickness and resistivity from measured samples.

[0042] In this case, the transmitter current I Tr The diffusion of eddy currents generated by cutting is described by a three-dimensional diffusion equation, where the time can be normalized to the characteristic time scale of the problem: i.e.,

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[0043] The characteristic time scale describes the time it takes for the eddy currents to spread and reach the surface opposite the transmitter coil 2 (i.e., top surface 4b, see Figures 1, 2 and 3). This also describes the normalized eddy current flux

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[0044] Normalized eddy current flux

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[0045]

[0044] In Figure 5, for the same six plates 1 as in Figure 4, the dependence on resistivity ρ is eliminated and the measurement depends only on thickness d. In this way, the derivation of resistivity and thickness from measurements of normalized eddy current flux is greatly simplified. This shows that thickness d can be determined unambiguously, for example with reference to a pre-prepared standard or calibration curve. For example, previous measurements on several objects (usually plates) with known thicknesses and different resistivities can be input into a suitable black-box model commonly known in the art.

[0046] Expressed in terms of normalized time, a first measurement is performed at an early time τ 2 , at which time the normalized eddy current flux

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[0047] A second measurement is then performed at a subsequent normalized time τ3, at which time the normalized eddy current flux

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[0048]

[0047] To facilitate the calculation of thickness and resistivity from the measured signal or to improve the signal-to-noise ratio, it may be appropriate to perform multiple measurements at each subsequent time point within such a range of normalized time relative to the third time point, rather than just one measurement at the third time point, t3. Thus, in some embodiments of the present invention, the measurement of the voltage induced by the decay of the magnetic field at the (here at least one) third time point, t3, comprises at least two measurements at each subsequent (third) time point, for example, these measurements comprise or consist of two measurements at subsequent time points, such as a first and a second subsequent time point (the second subsequent time point being different from the first subsequent time point), both typically within a normalized time interval of τ=3 to τ=10, preferably τ=4 to τ=7.

[0049]

[0048] The thickness measurements of the object 1 can be repeated as often as possible to provide a dense sample of the object, the thickness of which varies over time. This is the case, for example, when measuring the thickness of material produced in a rolling mill. The intervals between thickness measurements must be long enough so that the effects of eddy currents induced in the object in relation to a previous thickness measurement do not interfere with subsequent thickness measurements.

[0050]

[0049] Experience has shown that the supply time period is proportional to the normalized eddy current flux when the current is turned off.

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[0051]

[0050] The time scale for this process can be determined by the so-called sheet resistivity of the object 1, typically a plate. Sheet resistivity is defined as the resistivity of the object divided by its thickness. The appropriate delivery time in microseconds can then be calculated as the plate thickness, typically the nominal thickness of object 1 in μm, divided by the resistivity in nΩm, multiplied by 17.

[0052] To determine the characteristic time scale, knowledge of the resistivity ρ and thickness d of the plate being measured can be convenient. Usually, the nominal thickness of the plate is known and its resistivity can be obtained from measurements, so that the correct time scale can be obtained iteratively. Even if the thickness is not known in advance, thickness measurements are still possible by iteration.

[0053]

[0052] Figure 6 is a flowchart illustrating some embodiments of the method of the present invention. The method is for contactless measurement of a thickness d of an object 1 made of a conductive material by a PEC system 10 comprising a transmitter coil 2 and a receiver coil 3. The method includes supplying a constant current S1 to the transmitter coil 2 for a predetermined supply time period, the supplied current generating an electromagnetic field B that penetrates the object 1. The method also includes turning off the supplied current S2 after a supply time period after a start time point t0, resulting in the induction of eddy currents in the object and the decay of the magnetic field B. The method also includes measuring S3 in the receiver coil 3 a voltage induced by the decay of the magnetic field at a first time point t1, a second time point t2, and at least one subsequent or third time point t3 during a predetermined measurement time period starting at the start time point t0. The method also includes measuring a measured magnetic flux Φ at the first time point t1. plateand calculating S4 the total magnetic flux Φ(t1) produced by the eddy currents and picked up by the receiver coil 3 at a first time point t1 by comparing it to a predetermined total magnetic flux Φ(t1) picked up by the receiver coil 3 when the object 1 is not present, where the first time point t1 is set to the first time at which the magnetic flux Φ(t1) at the first time point t1 satisfies the condition that the magnetic flux Φ(t1) is zero when the object is not present. The method also normalizes the measured magnetic flux Φ(t1) resulting from the eddy currents and picked up by the receiver coil 3 using the calculated S4 total magnetic flux Φ(t1) as a normalization factor. ec Normalizing S5 the normalized eddy current flux

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[0054] FIG. 7 schematically illustrates an embodiment of the controller 6 of the present disclosure. The controller 6 includes a processing circuit 61, e.g., a central processing unit (CPU). The processing circuit 61 may include one or more processing units in the form of a microprocessor. However, other suitable devices having computing capabilities, such as an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or a complex programmable logic device (CPLD), may be included within the processing circuit 61. The processing circuit 61 is configured to execute one or several computer programs or software (SW) 63 stored in a storage device 62, e.g., a memory, of one or several storage units. The storage unit is considered a computer-readable means, as discussed herein, that together with the SW 63 stored on the storage unit as a computer-executable component forms a computer program product 62, and may be, for example, in the form of a random access memory (RAM), a flash memory or other solid-state memory, a hard disk, or a combination thereof. The processing circuit 61 may also be configured to store data in the storage device 62, as needed. The controller 6 may be configured to execute methods of the present disclosure.

[0055]

[0054] The present disclosure has been described above primarily with reference to certain embodiments. However, as will be readily apparent to those skilled in the art, other embodiments besides those disclosed above are equally possible within the scope of the present disclosure as defined by the appended claims. The following is a summary of the claims as originally filed: [1] Transmitter coil (2) and receiver coil (3) 1. A method for non-contact measurement of a thickness (d) of an object (1) made of a conductive material by a pulsed eddy current (PEC) system (10), comprising: supplying (S1) a constant current to the transmitter coil (2) for a predetermined supply time period, wherein the supplied current generates an electromagnetic field (B) penetrating the object (1); starting time point t 0 After the subsequent supply time period, turning off the supplied current (S2), resulting in the induction of eddy currents in the object and the decay of the magnetic field (B); In the receiver coil (3), the start time point t 0 During a predetermined measurement time period starting at a first time point t 1 , the second time point t 2 , and at least one subsequent time point t 3 (S3) measuring at The first time point t 1 The measured magnetic flux Φ at plate and a given total magnetic flux Φ picked up by the receiver coil in the absence of an object (1). 0 and the first time point t 1 The total magnetic flux Φ generated by the eddy currents and picked up by the receiver coil (3) 1 (S4) calculating the first time point t 1 is the time when the object is not present. 1 The magnetic flux Φ(t 1 ) is set to zero for the first time, and Φ 1 is Φ 0 -Φ plate It is calculated as (S4), The calculated (S4) total magnetic flux Φ 1 as a normalization factor to calculate the measured magnetic flux Φ resulting from the eddy currents and picked up by the receiver coil (3). ec (S5) normalizing the normalized eddy current flux [Number 27] JPEG0007822801000027.jpg6170 becomes independent of the distance (D) between the object (1) and the transmitter coil (2) and the receiver coil (3), thereby the normalized eddy current flux [Number 28] JPEG0007822801000028.jpg6170 is the first time point t 1 and is zero at a time after the eddy current has dissipated, and is one at a time after the eddy current has dissipated. The normalized eddy current flux is calculated by using the time constant T as a normalization factor. [Number 29] JPEG0007822801000029.jpg6170 to normalized time τ (S6), so that real time t=t 1 and τ=1 at the time when the eddy currents have just diffused and arrived at the surface of the object (1) opposite the transmitter coil (2) after the applied current is turned off, thereby the normalized eddy current flux [Number 30] JPEG0007822801000030.jpg6170 is independent of the thickness (d) within the normalized time interval from τ=0 to τ=1, and is independent of the resistivity (ρ) of the object (1) at times after τ=1, where the time constant T is [Number 31] JPEG0007822801000031.jpg18170 where d is the thickness of the object as previously estimated, ρ is the resistivity of the object as previously estimated, and μ 0 is the magnetic permeability of a vacuum, determining (S7) a thickness (d) and a resistivity (ρ) of the object (1) based on the measurements at the first, second, and subsequent time points, wherein at the second time point t2 is set within the normalized time interval from τ=0 to τ=2, where the normalized eddy current flux [Number 32] JPEG0007822801000032.jpg7170 depends only on the resistivity of the plate and at least one subsequent time point t 3 is set to be τ>2, in which case the normalized eddy current flux [Number 33] JPEG0007822801000033.jpg7170 depends only on the thickness of the plate, A method comprising: [2] The second time point t 2 The method described in [1], where τ is set in the range from τ=0.8 to τ=1. [3] at least the subsequent time point t 3 The method according to [1] or [2], wherein is set within the normalized time interval from τ=3 to τ=10, for example from τ=4 to τ=7. [4] The method according to any one of [1] to [3], wherein the object (1) is a plate. [5] at least one subsequent time point t 3

[0023] The method of any one of [1] to [4], wherein the subsequent time point includes a first subsequent time point and a second subsequent time point. [6] A computer program product (62) comprising computer-executable components (63), which, when executed on processing circuitry (61) contained within a controller (6), cause the controller of a PEC system (10) to perform the method of any one of [1] to [5]. [7] A PEC system (10) for non-contact measurement of the thickness (d) of an object (1) made of a conductive material, comprising a transmitter coil (2), a receiver coil (3), and a controller (6), a processing circuit (61); a memory device (62) that stores instructions (63) executable by the processing circuitry (61); Thereby, the controller supplying a constant current to the transmitter coil (2) for a predetermined supply time period, wherein the supplied current generates an electromagnetic field (B) penetrating the object (1), at a start time point t 0 after the later supply time period, turning off the supplied current, resulting in the induction of eddy currents in the object and the decay of the magnetic field (B); In the receiver coil (3), the start time point t 0 During a predetermined measurement time period starting at a first time point t 1 , the second time point t 2 , and at least one third time point t 3 and measuring at The first time point t 1 The measured magnetic flux Φ at plate and a given total magnetic flux Φ picked up by the receiver coil in the absence of an object (1). 0 and the first time point t 1 The total magnetic flux Φ generated by the eddy currents and picked up by the receiver coil (3) 1 where the first time point t 1 is the time when the object is not present. 1 The magnetic flux Φ(t 1 ) is set to zero for the first time, and Φ 1 is Φ 0 -Φ plate It is calculated as (S4), The calculated total magnetic flux Φ 1 as a normalization factor to calculate the measured magnetic flux Φ resulting from the eddy currents and picked up by the receiver coil (3). ec Normalizing the normalized eddy current flux [Number 34] JPEG0007822801000034.jpg6170 becomes independent of the distance (D) between the object (1) and the transmitter and receiver coils (2, 3), thereby the normalized eddy current flux [Number 35] JPEG0007822801000035.jpg6170 is the first time point t 1 and is zero at a time after the eddy current has dissipated, and is one at a time after the eddy current has dissipated. The normalized eddy current flux is calculated by using the time constant T as a normalization factor. [Number 36] JPEG0007822801000036.jpg6170 to normalized time τ, and thus real time t=t 1 and τ=1 at a time after the applied current is turned off when the eddy currents have just diffused and reached the surface of the object (1) opposite the transmitter coil (2), thereby the normalized eddy current flux [Number 37] JPEG0007822801000037.jpg6170 is independent of the thickness (d) within the normalized time interval from τ=0 to τ=1, and is independent of the resistivity ρ of the object (1) at times after τ=1, where the time constant T is [Number 38] JPEG0007822801000038.jpg18170 where d is the thickness of the object estimated previously, ρ is the resistivity of the object estimated previously, and μ 0 is the magnetic permeability of a vacuum, determining a thickness (d) and a resistivity (ρ) of the object (1) based on the measurements at the first, second, and third time points, wherein at the second time point t 2 is set within the normalized time interval from τ=0 to τ=2, where the normalized eddy current flux [Number 39] JPEG0007822801000039.jpg7170 depends only on the resistivity of the plate, and at the third time point t 3 is set to be τ>2, in which case the normalized eddy current flux [Number 40] JPEG0007822801000040.jpg7170 depends only on the thickness of the plate, A PEC system (10) operable to:

Claims

1. Transmitter coil (2) and receiver coil (3) 1. A method for non-contact measurement of a thickness (d) of an object (1) made of a conductive material by a pulsed eddy current (PEC) system (10), comprising: supplying (S1) a constant current to the transmitter coil (2) for a predetermined supply time period, wherein the supplied current generates an electromagnetic field (B) penetrating the object (1); Starting point t ０ After the later supply time period, turning off the supplied current (S2), resulting in the induction of eddy currents in the object and the decay of the magnetic field (B); In the receiver coil (3), the start time t ０ During a predetermined measurement time period starting at a first time t １ , the second time point t ２ , and at least one subsequent time point t ３ (S3) measuring at The first time point t １ The measured magnetic flux Φ at ｐｌａｔｅ and a given total magnetic flux Φ picked up by said receiver coil in the absence of an object (1). ０ By comparing the first time point t １ The total magnetic flux Φ generated by the eddy currents and picked up by the receiver coil (3) at １ (S4), where the first time point t １ is the time when the object is not present and the first time t １ The magnetic flux Φ(t １ ) is set to zero for the first time, and Φ １ is Φ ０ -Φ ｐｌａｔｅ is calculated as (S4), The calculated (S4) total magnetic flux Φ １ as a normalization factor to the measured magnetic flux Φ resulting from the eddy currents and picked up by the receiver coil (3). ｅｃ Normalizing (S5), thereby normalizing the normalized eddy current flux [Equation 1] becomes independent of the distance (D) between the object (1) and the transmitter coil (2) and the receiver coil (3), thereby the normalized eddy current flux [Equation 2] is the first time point t １ and is zero at times after the eddy currents have dissipated, and is one at times after the eddy currents have dissipated, The normalized eddy current flux is calculated by using the time constant T as a normalization factor. [Equation 3] to the normalized time τ (S6), so that the real time t=t １ and τ=1 at a time after the applied current is turned off when the eddy currents have just diffused and arrived at the surface of the object (1) opposite the transmitter coil (2), thereby the normalized eddy current flux [Equation 4] is independent of the thickness (d) within the normalized time interval from τ=0 to τ=1, and is independent of the resistivity (ρ) of the object (1) at times after τ=1, where the time constant T is [Equation 5] where d is the thickness of the object as previously estimated, ρ is the resistivity of the object as previously estimated, and μ ０ is the magnetic permeability of a vacuum, determining (S7) a thickness (d) and a resistivity (ρ) of the object (1) based on the measurements at the first, second, and subsequent times, wherein at the second time t ２ is set within the normalized time interval from τ=0 to τ=2, where the normalized eddy current flux [Equation 6] depends only on the resistivity of the object (1) and at least one subsequent time t ３ is set so that τ>2, in which case the normalized eddy current flux [Equation 7] depends only on the thickness of the object (1), A method comprising:

2. The second time point t ２ The method of claim 1 , wherein is set in the range of τ=0.8 to τ=1.

3. said at least one subsequent time point t ３ 3. The method of claim 1, wherein τ is set within the normalized time interval from τ=3 to τ=10, for example from τ=4 to τ=7.

4. 4. The method according to any one of claims 1 to 3, wherein the object (1) is a plate.

5. said at least one subsequent time point t ３ 5. The method of claim 1, wherein comprises a first subsequent time point and a second subsequent time point.

6. A computer program product (62) comprising computer-executable components (63) that, when executed on processing circuitry (61) contained within a controller (6), cause the controller of a PEC system (10) to perform the method of any one of claims 1 to 5.

7. A PEC system (10) for non-contact measurement of the thickness (d) of an object (1) made of a conductive material, comprising a transmitter coil (2), a receiver coil (3) and a controller (6), wherein the controller (6) A processing circuit (61); a storage device (62) for storing instructions (63) executable by said processing circuitry (61); Thereby, the controller supplying a constant current to the transmitter coil (2) for a predetermined supply time period, wherein the supplied current generates an electromagnetic field (B) penetrating the object (1); Starting point t ０ after the later supply time period, turning off the supplied current, resulting in the induction of eddy currents in the object and a decay of the magnetic field (B); In the receiver coil (3), the start time t ０ During a predetermined measurement time period starting at a first time t １ , the second time point t ２ , and at least one third time point t ３ and measuring at The first time point t １ The measured magnetic flux Φ at ｐｌａｔｅ and a given total magnetic flux Φ picked up by said receiver coil in the absence of an object (1). ０ By comparing the first time point t １ The total magnetic flux Φ generated by the eddy currents and picked up by the receiver coil (3) at １ where the first time point t １ is the time when the object is not present and the first time t １ The magnetic flux Φ(t １ ) is set to zero for the first time, and Φ １ is Φ ０ -Φ ｐｌａｔｅ is calculated as (S4), The calculated total magnetic flux Φ １ is used as a normalization factor to normalize the measured magnetic flux Φ resulting from the eddy currents and picked up by the receiver coil (3). ｅｃ Normalizing the normalized eddy current flux [Equation 8] becomes independent of the distance (D) between the object (1) and the transmitter coil (2) and receiver coil (3), thereby the normalized eddy current flux [Equation 9] is the first time point t １ and is zero at times after the eddy currents have dissipated, and is one at times after the eddy currents have dissipated, The normalized eddy current flux is calculated by using the time constant T as a normalization factor. [Equation 10] to normalized time τ, and thereby real time t=t １ and τ=1 at a time after the applied current is turned off when the eddy currents have just diffused and reached the surface of the object (1) opposite the transmitter coil (2), thereby reducing the normalized eddy current flux [0011] is independent of the thickness (d) within the normalized time interval from τ=0 to τ=1 and is independent of the resistivity ρ of the object (1) at times after τ=1, where the time constant T is [0012] where d is the thickness of the object estimated previously, ρ is the resistivity of the object estimated previously, and μ ０ is the magnetic permeability of a vacuum, determining a thickness (d) and a resistivity (ρ) of the object (1) based on the measurements at the first, second, and third time points, wherein at the second time point t ２ is set within the normalized time interval from τ=0 to τ=2, where the normalized eddy current flux [0013] depends only on the resistivity of the object (1) and at the third time t ３ is set so that τ>2, in which case the normalized eddy current flux [0014] depends only on the thickness of the object (1), A PEC system (10) operable to:

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