A method for measuring the thickness of a workpiece in a rolling mill

Abstract

The present invention relates to determining the current thickness of a workpiece during machining in a rolling mill (100), the method comprising: acquiring (S102) a data signal reflecting the time-dependent nature of eddy current decay in the workpiece caused by an applied pulsed magnetic field; determining (S104) a thickness parameter value based on the acquired signal, the thickness parameter value being determined from a sample in the data signal, the thickness parameter value depending on the ratio between the thickness of the workpiece and the resistivity of the workpiece; calculating (S106) the ratio between a reference thickness value of the workpiece and the thickness parameter value, thereby providing an instantaneous resistivity value; determining (S108) an average resistivity value based on the instantaneous resistivity value; and providing (S110) an output signal based on the average resistivity of the workpiece and the thickness parameter value, the output signal indicating the determined current thickness of the workpiece.

Description

Technical Field

[0001] The present invention relates to a method, a control unit, and a rolling mill for determining the thickness of a workpiece during machining in a rolling mill. Background Technology

[0002] Metal rolling typically involves producing metal workpieces with reduced and uniform thickness by rolling the metal workpiece between two rotating work rolls.

[0003] To ensure high product quality, the thickness of the workpiece is precisely monitored and controlled. It is especially important to monitor rapid thickness changes within the workpiece (e.g., a sheet of metal), even for very thin metal sheets. Mill operation can be controlled based on measured thickness. In particular, the last stand mill, i.e., the mill at the end of the milling line, can be controlled based on thickness measured upstream of the last stand to ensure final product quality.

[0004] Traditional pulsed eddy current measurement techniques are based on measuring the eddy currents induced in a metal plate by a rapidly changing magnetic field applied to it. The resistivity and thickness of the metal plate are then extracted based on the measured eddy currents.

[0005] However, for sufficiently thin plates, traditional methods are not accurate enough and are often affected by noise measurements, resulting in limited applicability below the lower limit of metal plate thickness.

[0006] Therefore, it is necessary to improve the accuracy of thickness measurement in rolling mills, especially for relatively thin metal sheets. Summary of the Invention

[0007] In view of the above and other disadvantages of the prior art, one object of the present invention is to provide a method for determining the thickness of a workpiece with improved accuracy when machining a workpiece in a rolling mill.

[0008] According to a first aspect of the invention, a method is provided for determining the current thickness of a workpiece when machining it in a rolling mill, the method comprising acquiring a data signal reflecting the time correlation of eddy current decay in the workpiece caused by an applied pulsed magnetic field.

[0009] Furthermore, the method includes determining thickness parameter values ​​based on the acquired signal. The thickness parameter values ​​are determined from samples in the data signal. Additionally, the thickness parameter values ​​depend on the ratio between the workpiece thickness and the workpiece resistivity.

[0010] In addition, the method includes calculating the ratio between a reference thickness value and a thickness parameter value of the workpiece, thereby providing an instantaneous resistivity value.

[0011] In addition, the average resistivity value is determined based on the instantaneous resistivity value, and an output signal is provided based on the average resistivity and thickness parameter values ​​of the plate, which indicates the current thickness of the workpiece.

[0012] This invention is at least partly based on the implementation of processing a thickness parameter value that reflects the ratio between the workpiece's thickness and its resistivity to determine the workpiece's thickness. Therefore, instead of attempting to derive the thickness directly from the thickness parameter value and an uncertain resistivity value (e.g., temperature-dependent), the thickness parameter value is used in conjunction with a reference thickness value to obtain an instantaneous resistivity value, which may depend on the thickness variation of the workpiece (e.g., a metal sheet) depending on how the reference thickness value was obtained. Based on the instantaneous resistivity value, the true average resistivity of the sheet can be determined, thus eliminating the need to use the uncertain resistivity value contained in the correlation of the thickness parameter value.

[0013] Using the proposed method, thickness measurements can be obtained with greater accuracy, at least for thin workpieces. Furthermore, this allows for improved control of the thickness of the rolled workpiece within the rolling mill, thereby enabling higher production speeds and better quality of the final processed workpiece.

[0014] The thickness parameter value depends on the ratio between the workpiece's thickness and resistivity, meaning the thickness parameter value reflects this ratio. The thickness parameter value depends almost entirely on this ratio, although other minor correlations may exist. This ratio is thickness divided by resistivity.

[0015] Preferably, the thickness parameter value can be determined from a sample of the data signal after a certain time delay. This time delay is long enough to avoid any initial high signal transients caused by the pulsed magnetic field in the sampled data signal. The time delay is long enough that the time correlation of eddy current attenuation in the workpiece depends primarily on the ratio between thickness and resistivity, and also on the distance between the measuring devices used to detect the magnetic field generated by the eddy currents to determine the attenuation; these measuring devices are typically provided as receiver coils. Therefore, the thickness parameter value can be calculated based on the time correlation of eddy current attenuation.

[0016] More precisely, the thickness parameter value is preferably determined by the time derivative of the magnetic flux generated by the eddy currents in the workpiece after a predetermined delay, and the distance between the workpiece and the measuring device. For example, if the distance remains constant, the thickness parameter value can be determined by detecting the time correlation of eddy current decay, and then using a model that correlates the time correlation of eddy current decay with the thickness parameter value (i.e., the ratio between workpiece thickness and resistivity). This model can be established theoretically, but it can also be based on extensive prior measurements.

[0017] As described above, eddy currents can be detected by a receiving coil arranged at a certain distance from the workpiece. In this case, the time derivative of the magnetic field generated by the eddy currents in the workpiece induces a voltage signal in the receiving coil. The voltage signal is preferably amplified and integrated to generate the acquired signal.

[0018] The thickness parameter value can be determined based on the theoretical model.

[0019] However, in one embodiment, the thickness parameter value is determined based on an empirically determined model that correlates the time dependence of eddy current decay with the ratio between the workpiece thickness and the workpiece resistivity. By repeatedly measuring or observing the relationship between the eddy current decay time dependence and the thickness parameter value for different workpieces with different thicknesses and resistivities, an empirical model that correlates the thickness parameter with the eddy current decay derivative can be formed. This empirical model is advantageously used to establish accurate models.

[0020] In this embodiment, the time correlation of eddy current decay can be measured by a magnetic field measuring device positioned at a distance from the workpiece, determined from samples of the data signal during the initial stage of eddy current decay, where the thickness parameter value is further determined based on the determined distance. Therefore, the distance between the measuring device and the workpiece can advantageously be determined from the data signal itself. It is well known that magnetic field strength decreases with distance from the source. This knowledge can be used to calculate the distance from the measuring device to the workpiece, rather than measuring the distance using a separate measuring device (e.g., an optical or capacitive measuring device).

[0021] In this embodiment, an empirically determined model can correlate the time correlation of eddy current decay with the ratio between workpiece thickness and workpiece resistivity for different distances between the workpiece and the magnetic field measuring device. Therefore, this model can advantageously take into account the distance between the workpiece and the magnetic field measuring device, thus providing a more accurate model, especially in measurement cases where the distance between the measuring device and the workpiece changes rapidly.

[0022] In a preferred embodiment, the average resistivity value can be determined by filtering the instantaneous resistivity value. The filtering can be a low-pass time-domain filter. Therefore, the instantaneous resistivity value calculated from the measured thickness parameter value is passed through a low-pass time-domain filter, and the output of the time-domain filter is the average resistivity. The instantaneous resistivity obtained in this way depends primarily on the thickness variation of the workpiece, since it can be assumed that the resistivity of the workpiece changes slowly. Therefore, by filtering the instantaneous resistivity using a time-domain filter, the true average resistivity of the workpiece can be advantageously obtained.

[0023] The length and properties of the time-domain filter (which can be a digital filter) depend on the specific implementation available.

[0024] Depending on how the reference thickness value is obtained, the concept of the present invention applies to at least three different main embodiments. Therefore, the reference thickness value can be obtained in various conceivable ways.

[0025] First, if the reference thickness value reflects the instantaneous thickness of the workpiece—that is, when the workpiece is being processed in the rolling mill, the instantaneous thickness is measurable. This is often the case for sufficiently thick workpieces, where the thickness measuring device measures the workpiece thickness with a certain acceptable accuracy. In this case, a filter is used to reduce measurement noise from the instantaneous thickness measurement; the filter should be as short as possible while providing satisfactory noise reduction.

[0026] Secondly, reference thickness parameter values ​​can be provided as an estimated or predetermined average thickness of the workpiece, i.e., as the nominal thickness. For example, the predetermined average thickness value can be provided or measured before the workpiece is machined in the rolling mill.

[0027] In this case, the ratio between the reference thickness value provided as the nominal thickness value and the thickness parameter value (i.e., the ratio that forms the instantaneous resistivity value) depends on the reciprocal of the workpiece thickness. However, by passing the instantaneous resistivity value through a time-domain filter, the output signal is a high-pass filtered value of the workpiece thickness, meaning the signal reflects the thickness variation with respect to the average or nominal thickness. In an embodiment, the high-pass filtered value of the thickness variation can be used for feedforward control, and in this case, the length of the filter and its time-domain characteristics must be selected based on the parameters and speed of the entire control loop.

[0028] Third, if the reference thickness value is provided by an inherently slow measurement, or by a slow measuring device or method. Here, "slow" means that the time constant of the reference thickness value measurement is slower than the time constant of the thickness parameter value measurement. The time constant refers to, for example, the window length of the filter in the case of a windowed filter. It can also refer to the full frequency response of the measurement or filter. For example, one way to obtain a reference thickness value is to measure the thickness after the next work roll downstream in the mill and use the speed of the workpiece measured before and after that work roll to estimate the thickness reduction of the workpiece. These devices are typically inherently slow in producing results or require long filtering times. In this third case, the thickness value measured by the thickness measuring device can be filtered using a time-domain filter to provide the reference thickness value. This helps improve the accuracy of the determined workpiece thickness parameters.

[0029] Preferably, the time constant obtained from the reference thickness value filtered by the time-domain filter should be matched with the time constant of the time-domain filter used to filter the thickness parameter value. In other words, the thickness parameter value can be filtered before forming a ratio with the filtered reference thickness value. By matching the time constants, the changes in the signal (i.e., the reference thickness value and the thickness parameter value) will be equal, and the ratio between them will remain unchanged. In other words, if the reference thickness value and the thickness parameter value are filtered by filters with equal time constants or frequency responses, the instantaneous resistivity value is equal to the average resistivity of the workpiece. This is particularly advantageous when the measuring device used to determine the reference thickness value is slow, and it also helps to speed up the determination of the signal indicating the current thickness of the workpiece. Therefore, embodiments of the present invention can help improve the measurement speed of such a slow device, and when the device is used for measurements in a rolling mill, it will improve the resolution of thickness information along the workpiece. The method provided by the embodiments here helps to obtain updated current thicknesses of the workpiece at a higher frequency, i.e., the time between updates can be improved even when using inherently slow thickness measuring devices.

[0030] In one embodiment, the method may include providing the output signal as a feedforward control signal to the rolling mill. In other words, the output signal can advantageously be provided to control the operation of a set of work rolls downstream of the rolling mill before the workpiece reaches the work rolls, i.e., in a feedforward manner. This advantageously contributes to providing higher quality final products, such as better thickness control.

[0031] Therefore, in an embodiment, the method may include controlling the operation of a mill stand supporting a set of work rolls based on a feedforward control signal, thereby changing the thickness of the workpiece.

[0032] In an embodiment, the output signal can be based on a determined thickness calculated by multiplying the workpiece's average resistivity and thickness parameter values.

[0033] In this embodiment, the reference thickness value is a thickness value measured by a thickness measuring device of the rolling mill. For example, such a thickness measuring device may be an X-ray thickness measuring device, an isotope thickness measuring device, an optically based thickness measuring device, etc.

[0034] The embodiments described herein are advantageously applied to workpieces that are metal sheets.

[0035] According to a second aspect of the invention, a control unit is provided, configured to generate an output signal indicating the current thickness of a workpiece during processing in a rolling mill, the control unit being configured to: acquire a data signal reflecting the time-dependent nature of eddy current decay in the workpiece caused by an applied pulsed magnetic field; determine a thickness parameter value based on the acquired signal, wherein the thickness parameter value is determined according to a sample in the data signal, the thickness parameter value depending on the ratio between the thickness of the workpiece and the resistivity of the workpiece; calculate a ratio between a reference thickness value of the workpiece and the thickness parameter value, thereby providing an instantaneous resistivity value; determine an average resistivity value based on the instantaneous resistivity value; and provide an output signal based on the average resistivity of the plate and the thickness parameter value, the output signal indicating the determined current thickness of the workpiece.

[0036] The further effects and features of the second aspect of the present invention are very similar to those described above in conjunction with the first aspect of the present invention.

[0037] According to a third aspect of the invention, a rolling mill is provided, comprising: a control unit according to an embodiment of the invention; a set of work rolls configured to process a workpiece between the work rolls to a predetermined workpiece thickness; and a mill stand supporting the work rolls, the mill stand being controllable based on an output signal as a feedforward signal to change the distance between the work rolls, thereby changing the thickness of the workpiece processed in the rolling mill.

[0038] The further effects and features of the third aspect of the present invention are very similar to those described above in conjunction with the first and second aspects of the present invention.

[0039] Further features and advantages of the invention will become apparent when examined in light of the appended claims and the following description. Those skilled in the art will recognize that different features of the invention can be combined to create embodiments other than those described below without departing from the scope of the invention. Attached Figure Description

[0040] These and other aspects of the invention will now be described in more detail with reference to the accompanying drawings, which illustrate exemplary embodiments of the invention, wherein:

[0041] Figure 1 A workpiece processed in a rolling mill according to an embodiment of the present invention is conceptually illustrated;

[0042] Figure 2 This is a block diagram illustrating the inventive concept according to an embodiment of the present invention;

[0043] Figure 3 This is a flowchart of the method steps according to an embodiment of the present invention; and

[0044] Figure 4 This is a flowchart of the method steps according to an embodiment of the present invention. Detailed Implementation

[0045] In this detailed description, various embodiments of the invention are described with reference to specific implementations. Specific terminology is used for clarity in the description of the embodiments. However, the invention is not limited to the specific terminology chosen. While specific exemplary embodiments have been discussed, it should be understood that this is for illustrative purposes only. Those skilled in the art will recognize that other components and configurations can be used without departing from the scope of the invention.

[0046] Figure 1 A rolling mill 100 is conceptually illustrated, comprising a set of work rolls 102a and 102b adapted to process a workpiece 104. The work rolls 102a-b rotate while the workpiece 104 (e.g., a metal sheet) is fed between the work rolls 102a-b. As will be understood by those skilled in the art, the work rolls 102a-b reduce the thickness of the workpiece.

[0047] Precise control of the thickness of the workpiece 104 output downstream of the work rolls 102a-b is desired. For this purpose, a pulsed eddy current technology device 106 based on applying a pulsed magnetic field to the workpiece 104 is typically employed. The pulsed eddy current technology device 106 detects the eddy currents induced in the workpiece 104 to estimate the thickness of that portion of the workpiece before it reaches the work rolls 102a-b. This invention addresses improving thickness estimation. For example, for thin workpieces, such as metal sheets less than 1 mm thick, pulsed eddy current technology generates noise and has unsatisfactory accuracy.

[0048] A control unit 108 is conceptually shown here, which is configured to generate an output signal indicating the thickness of the workpiece 104 when the workpiece is being processed in a rolling mill.

[0049] Control unit 108 is configured to acquire a data signal S reflecting the time-dependent nature of eddy current decay in a workpiece caused by an applied pulsed magnetic field. In other words, control unit 108 is wirelessly or hardwired connected to pulsed eddy current technology device 106, allowing control unit 108 to receive data signals from pulsed eddy current technology device 106. The time-dependent nature of eddy current decay reflects the derivative of eddy current decay in workpiece 102.

[0050] The pulsed eddy current technology device 106 includes a receiving coil 106a in which a magnetic field-induced voltage signal generated by eddy currents in the workpiece 104 is received. The pulsed eddy current technology device 106 includes electronic equipment for amplifying and integrating the voltage signal and providing the resulting signal S to a control unit 108.

[0051] Based on the acquired signal, the control unit 108 can determine the thickness parameter value (E). The thickness parameter value (E) is determined by samples from the acquired signal. Importantly, the thickness parameter value depends on the ratio between the workpiece thickness (t) and the workpiece resistivity (r). In other words, E ~ t / r.

[0052] For thin workpieces, such as those with a thickness of less than 1 mm, the thickness parameter value can be regarded as the reciprocal of the thin-film resistance, i.e., 1 / the thin-film resistance of the workpiece, and can be referred to as the thin-film conductivity.

[0053] Although the thickness parameter value reflects the ratio between the workpiece's thickness and resistivity, the thickness is not directly extracted from the thickness parameter value, because it requires knowledge of the workpiece's resistivity, and the resistivity depends on temperature.

[0054] Instead, the control unit is configured to calculate the reference thickness value (t) of the workpiece. r The ratio between the resistivity value (r) and the thickness parameter value E provides the instantaneous resistivity value (r). i In other words, the instantaneous resistivity value is given by the following formula: Therefore, the instantaneous resistivity value r i It mainly depends on the thickness variation in workpiece 104, which is determined by the reference thickness t. r The ratio between and thickness t This reflects that thickness t is a part of the thickness parameter value. It can be assumed that the resistivity r changes very slowly, therefore not affecting the instantaneous resistivity value r. i The changes.

[0055] Furthermore, the control unit 108 is configured to base its operation on the instantaneous resistivity value r. i Determine the average resistivity value r f Average resistivity value r f Advantageously, the instantaneous resistivity value r is filtered by using a time-domain filter (preferably a low-pass filter). i This allows for the determination of the accurate average resistivity of the workpiece. If the reference thickness value comes from a noisy thickness measurement calculated at the same rate as the thickness parameter value, the instantaneous resistivity value remains almost unchanged and depends only on the noise in the reference thickness value. In this case, a time-domain filter is advantageously used to reduce measurement noise.

[0056] Control unit 108 is configured to provide an output signal based on the workpiece's average resistivity and thickness parameter values. The output signal indicates the current thickness of the workpiece. The current thickness (T) is preferably indicated by the workpiece's average resistivity r. f The product of the thickness parameter value (E) is calculated, i.e.

[0057] When the reference thickness value is a predetermined average thickness value, the output signal is the high-pass filtered thickness of the workpiece. For example, in response to a sudden thickness change of, say, 10%, the thickness parameter value (E) also increases by about 10%. If the time-domain filter is a low-pass filter, the average resistivity does not change immediately. Therefore, the output signal is the product of the average resistivity and the thickness parameter value (E), and since the average resistivity has not yet changed due to the low-pass filtering, the output signal will initially increase by about 10%. After a period of time, the average resistivity will decrease by about 10%, and the output signal will again equal the reference thickness value, just as before the sudden thickness change. In other words, the embodiment described here is advantageously used for monitoring changes in the thickness of a workpiece.

[0058] The cutoff frequency of a high-pass filter thickness depends on the characteristics of the time-domain filter. For example, for a simple moving average with a 10-second window, the cutoff frequency is approximately 0.024 Hz.

[0059] The time-domain filter can be a running window average, such as a running or moving average window, with a window length of approximately 10 seconds. Running window averaging is well known in the art and can be performed in various forms, such as simple moving average, cumulative moving average, center moving average, weighted moving average, Gaussian window, etc. Other example filters can be first-order exponential filters or binomial filters.

[0060] It should be understood that the process described above for determining the current thickness of the workpiece is performed simultaneously with the workpiece 104 being processed in the rolling mill. Even with an increase in the processing speed in the rolling mill (i.e., the feed rate of the workpiece 104), accurate determination of the current thickness helps to improve control over the workpiece thickness. Therefore, the control unit operates to determine the current thickness online while the workpiece 104 is fed through the rolling mill.

[0061] Figure 2 This is a block diagram illustrating the inventive concept according to an embodiment of the present invention. Figure 3 This is a flowchart of the method steps according to an embodiment of the present invention, and will be combined with Figure 2 Describe it.

[0062] First, in step S102, a data signal reflecting the time correlation of eddy current attenuation in the workpiece caused by the applied pulsed magnetic field is acquired. The acquired data signal S includes a set of data points, wherein initial data points S0 are provided to module 204 from a data sampling module 202 including suitable data acquisition electronics. Module 204 can calculate the time correlation of eddy current attenuation in the workpiece caused by the applied pulsed magnetic field. Figure 1The distance d from the workpiece 104 is calculated. Furthermore, at least one subgroup of data points S' is provided to the thickness calculation module 206. The entire acquired data signal S can be provided to the thickness calculation module 206, although only the selected data points are sufficient. Data point S' should reflect the time correlation of eddy current decay in the workpiece 104.

[0063] In step S104, the thickness parameter value E is determined based on the acquired signal. The thickness parameter value is determined by samples from the data signal. Furthermore, the thickness parameter value depends on the ratio between the workpiece thickness and the workpiece resistivity.

[0064] The thickness parameter value can be determined by model 208, which processes the determined eddy current time decay, such as the time derivative of eddy current decay, and calculates the thickness parameter value. Model 208 can be an empirically determined model that correlates the time correlation of eddy current decay with the ratio between the workpiece thickness and the workpiece resistivity. In other words, based on the currently determined eddy current time correlation, model 208 can correlate it with previous measurements and find the thickness parameter value that best matches the currently determined eddy current time correlation.

[0065] Furthermore, the thickness parameter value can also be determined based on a defined distance d. Therefore, distance d can be input as a parameter into model 208. The distance between the receiving coil 106a and the workpiece affects the intensity of the detected magnetic flux. Therefore, this distance is a parameter that can be included in the determination of the thickness parameter value E. In other words, the empirically determined model 208 can correlate the time correlation of eddy current decay with the ratio between the workpiece thickness and the workpiece resistivity for different distances between the workpiece and the magnetic field measuring device 106a.

[0066] Furthermore, in step S106, the reference thickness value t of the workpiece 104 is calculated. r The ratio between the thickness parameter value E and the instantaneous resistivity value r provides the instantaneous resistivity value. i .

[0067] Reference thickness values ​​can be obtained in several ways. For example, the reference thickness value t r The thickness value can be measured by the thickness measuring device 212 of the rolling mill.

[0068] The thickness value measured by the thickness measuring device can be filtered using a time-domain filter to provide a reference thickness value. Therefore, the thickness measurement performed by the thickness measuring device 212 is filtered using a time-domain filter, and the filtered thickness measurement is the reference thickness value t. rThe time constant of the time-domain filter used to provide the reference thickness value is preferably the same as the time constant of the time-domain filter used to filter the thickness parameter value E. This provides a constant ratio between the thickness parameter value and the filtered reference thickness value, resulting in an instantaneous resistivity equal to the average resistivity of the workpiece, and providing a more accurate thickness value in the output signal, even if the thickness measuring device is inherently slow. However, if the reference thickness value comes from a thickness measurement calculated at the same rate as the thickness parameter value, it is advantageous to calculate the average resistivity value directly from the quotient between the reference thickness value and the thickness parameter value without filtering.

[0069] The time constant refers to, for example, the window length of a filter in the case of running a windowed filter. It can also refer to a measurement or the complete frequency response of the filter.

[0070] In other embodiments, the reference thickness value is a predetermined average thickness value of the workpiece. This predetermined average thickness value can be a manual measurement of the workpiece performed before processing in the rolling mill. In this case, the ratio between the reference thickness value (e.g., provided as a nominal thickness value) and the thickness parameter value E depends on the reciprocal of the workpiece thickness. However, by passing the ratio, which is a value of instantaneous resistivity, through a time-domain filter, the output signal is a high-pass filtered value of the workpiece thickness, i.e., the signal reflects the thickness variation with respect to the average or nominal thickness.

[0071] Box 212 can also represent a processing module that calculates reference thickness values ​​based on certain algorithms.

[0072] In step S108, the average resistivity value r is determined based on the instantaneous resistivity value. f The average resistivity value is preferably determined by filtering the instantaneous resistivity value in the time-domain filter 210.

[0073] In step S110, an output signal C is provided based on the workpiece's average resistivity and thickness parameter values, indicating the determined current thickness of the workpiece. The determined thickness is calculated using arithmetic operation A, which is the product of the workpiece's average resistivity and thickness parameter values.

[0074] like Figure 1 As shown, the output signal can be provided to the rolling mill as a feedforward control signal.

[0075] The method may include, for example Figure 4 As shown in the flowchart, Figure 4 This also includes Figure 3 In step S110, the operation of the mill stand supporting a set of work rolls is controlled based on the feedforward control signal C, thereby changing the thickness of the workpiece 104.

[0076] The control unit may include a microprocessor, microcontroller, programmable digital signal processor, or other programmable device. The control unit may also, or alternatively, include an application-specific integrated circuit (ASIC), a programmable gate array (FPGA) or programmable array logic, a programmable logic device, or a digital signal processor. When the control unit includes a programmable device, such as the microprocessor, microcontroller, or programmable digital signal processor described above, the processor may also include computer-executable code that controls the operation of the programmable device.

[0077] Communication between the devices, control units or other modules described herein may be wireless or hardwired (as the case may be), and appropriate protocols shall be implemented for the specific circumstances.

[0078] Although the invention has been described with reference to specific exemplary embodiments thereof, many different changes, modifications, etc. will become apparent to those skilled in the art.

[0079] 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 invention. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite articles "a" or "an" do not exclude a plural. The fact that certain measures are listed in mutually different dependent claims does not in itself imply that combinations of these measures cannot be used advantageously.

Claims

1. A method for determining the current thickness of a workpiece during machining in a rolling mill (100), the method comprising: (S102) Acquire (a) a data signal reflecting the time correlation of eddy current decay in the workpiece caused by the applied pulsed magnetic field. Based on the acquired signal, a thickness parameter value is determined (S104) from samples in the data signal. This thickness parameter value depends on the ratio between the workpiece's thickness and its resistivity. (S106) Calculate the ratio between the reference thickness value and the thickness parameter value of the workpiece to provide an instantaneous resistivity value. Based on the instantaneous resistivity value, determine (S108) the average resistivity value, and Provide (S110) an output signal based on the average resistivity and the thickness parameter value of the workpiece, the output signal indicating the current thickness of the workpiece as determined.

2. The method of claim 1, wherein the thickness parameter value is determined based on an empirically determined model that correlates the time correlation of eddy current decay with the ratio between the thickness of the workpiece and the resistivity of the workpiece.

3. The method according to any one of claims 1 and 2, wherein the time correlation of the eddy current attenuation is measured by a magnetic field measuring device arranged at a certain distance from the workpiece. The distance is determined from samples of the acquired data signal during the initial phase of the eddy current decay, and the thickness parameter value is also determined based on the determined distance.

4. The method of claim 2, wherein the empirically determined model correlates the time correlation of eddy current decay with the ratio between the thickness of the workpiece and the resistivity of the workpiece for different distances between the workpiece and the magnetic field measuring device.

5. The method according to any one of the preceding claims, wherein the average resistivity value is determined by filtering the instantaneous resistivity value.

6. The method according to any one of the preceding claims, comprising: The output signal is provided to the rolling mill as a feedforward control signal.

7. The method of claim 6, wherein the output signal is based on the current thickness calculated by the product of the average resistivity of the workpiece and the thickness parameter value.

8. The method according to any one of claims 6 and 7, comprising: The operation of the mill stand supporting a set of work rolls is controlled based on the feedforward control signal, thereby changing the thickness of the workpiece.

9. The method according to any one of the preceding claims, wherein the reference thickness value is a thickness value measured by the thickness measuring device (106) of the rolling mill.

10. The method of claim 9, wherein the thickness value measured by the thickness measuring device is filtered using a time-domain filter to provide the reference thickness value.

11. The method of claim 10, wherein the frequency response of the time-domain filter used to provide the reference thickness value is matched with the frequency response of the time-domain filter used to filter the thickness parameter value.

12. The method according to any one of claims 1 to 8, wherein the reference thickness value is a predetermined average thickness value of the workpiece.

13. The method according to any one of the preceding claims, wherein the workpiece is a metal sheet.

14. A control unit (108) configured to generate an output signal indicating the current thickness of a workpiece during machining in a rolling mill, the control unit being configured to: Acquire data signals reflecting the time-dependent decay of eddy currents in the workpiece (104) caused by the applied pulsed magnetic field. A thickness parameter value is determined based on the acquired signal, wherein the thickness parameter value is determined from samples in the data signal, and the thickness parameter value depends on the ratio between the thickness of the workpiece and the resistivity of the workpiece. The ratio between the reference thickness value and the thickness parameter value of the workpiece is calculated to provide an instantaneous resistivity value. The average resistivity value is determined based on the instantaneous resistivity value, and An output signal is provided based on the average resistivity and thickness parameter values ​​of the workpiece, the output signal indicating the determined thickness of the workpiece.

15. A rolling mill (100), comprising: The control unit according to claim 14, A set of work rolls (102a-b) is configured to process a workpiece between the work rolls to a predetermined workpiece thickness; and a mill stand supporting the work rolls, the mill stand being controllable based on the output signal as a feedforward signal to change the distance between the work rolls, thereby changing the thickness of the workpiece being processed in the mill.

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