Improved non-contact vibration detection in metal strips.

Abstract

Between the front device (2) and the rear device (3) of the conveying device for the metal strip (1) a measuring device (6) is arranged. A mechanical excitation device (7) of the measuring device (6) excites the metal strip (1) in the thickness direction to mechanical vibrations at an excitation frequency (fA). The measuring device (6) has sensor elements (16) which are arranged offset relative to one another in the width direction. By means of the sensor elements (16), analog measurement signals (MA) are detected which are characteristic of the amplitude (A) of the excited mechanical vibrations for the corresponding areas of the metal strip (1). A digitizing device (35) is arranged inside the measuring device (6) by means of which the detected analog measurement signals (MA) are digitized and the digitized measurement signals or signals extracted from the digitized measurement signals are transmitted from the digitizing device as transmission signals (MA') to an evaluation device (34) arranged outside the measuring device (6). The sensor elements 16 include eddy current sensors, and the eddy current sensors of the sensor elements 16 arranged immediately adjacent to each other in the width direction operate at different operating frequencies (f1, f2, f3). However, when viewed across the entire sensor element 16, each of the sensor elements 16 operates at the same operating frequency (f1, f2, f3).

Description

[Technical Field]

[0001] The invention is based on a measuring device in a conveying device for metal strips, the measuring device is arranged between a front device of the transport device and a rear device arranged downstream of the front device, the measuring device comprises a mechanical excitation device by means of which the metal strip can be excited to mechanically oscillate in its thickness direction at an excitation frequency; the measuring device comprises a plurality of sensor elements, the sensor elements are arranged offset from one another in the width direction of the metal strip, - Using the sensor elements, analog measurement signals can be detected for each of a number of regions of the metal strip that are offset from one another in the width direction, the analog measurement signals being characteristic of the amplitude of the excited mechanical vibrations of each region of the metal strip, a digitizing device is arranged inside the measuring device, the detected analog measurement signals are digitized using the digitizing device, and the digitized measurement signals or signals extracted from the measurement signals are transmitted from the digitizing device as transmission signals to an evaluation device that is arranged outside the measuring device. [Background technology]

[0002] The invention is further based on a combination of the measuring device with an evaluation device, which within the scope of the evaluation determines from the signals transmitted to it the respective amplitudes of vibration of corresponding areas of the metal strip for areas of the metal strip that are offset from one another in the width direction of the metal strip.

[0003] Such measuring devices are known. Purely by way of example, reference may be made to DE 1 02 04 11 522. From the detected amplitude of the mechanical vibrations in the area of ​​the metal strip, the flatness of the metal strip can be determined. This is also explained in detail in DE 1 02 04 11 522.

[0004] Non-Patent Document 1 discloses a measuring device for a metal strip conveying device, which is arranged between a front device of the conveying device and a rear device arranged downstream of the front device. The measuring device has a mechanical excitation device, by means of which the metal strip can be excited to mechanically oscillate in its thickness direction at an excitation frequency. The measuring device also has a plurality of sensor elements, which are arranged offset from one another in the width direction of the metal strip. The sensor elements are used to detect analog measurement signals for a plurality of regions of the metal strip offset from one another in the width direction, which analog measurement signals are characteristic of the amplitude of the excited mechanical vibrations of each region of the metal strip. The detected signals are supplied in analog form via a coaxial cable to a digitizing device arranged near the rolling line. The data is then transmitted from the digitizing device to an evaluation device. The coaxial cable connecting the sensor to its electronics can have a length of up to 9 m. Similar disclosures can be found in Patent Documents 2 and 3.

[0005] Patent Document 4 discloses a measuring device for a metal strip conveying device, which is arranged between a front device of the conveying device and a rear device arranged downstream of the front device. The measuring device has a plurality of sensor elements, which are arranged offset from one another in the width direction of the metal strip. The sensor elements are capable of detecting analog measurement signals for a plurality of regions of the metal strip that are offset from one another in the width direction. The measuring device has a coil, which can be used to deflect the metal strip in its thickness direction. A constant voltage is applied to the coil to adjust the distance of the metal strip from the measuring device.

[0006] Patent Document 5 discloses a measuring device for a metal strip conveying device, which is arranged between a front device of the conveying device and a rear device arranged downstream of the front device. The measuring device has a plurality of sensor elements, which are arranged offset from one another in the width direction of the metal strip. The sensor elements are used to detect analog measurement signals for a plurality of regions of the metal strip offset from one another in the width direction, which measurement signals are characteristic of the distance of the metal strip from the respective sensor elements in each region. The signals appear to be preprocessed in analog form and supplied in analog form to a microprocessor. The signals appear to be first digitized in the microprocessor. There is no mention of the arrangement of the microprocessor. Each sensor element operates at its own operating frequency.

[0007] When rolling metal strip, the flatness of the rolled metal strip is an important quality feature, and in particular, corrugation of the rolled metal strip after rolling should be avoided.

[0008] To detect the corresponding measured values, measuring devices of the type described above can be used, which have the advantage over conventional measuring devices, in particular, that the signal detection is carried out contactlessly and therefore there is no risk of damaging the metal strip. However, such measuring devices also have some disadvantages.

[0009] In practice, for example, the sensor element is attached to the metal plate flush with the upper surface of the metal plate. This means that the sensor of the sensor element, i.e., the element that detects the measurement signal, is laterally surrounded by the material of the metal plate. This results in signal attenuation. This means that the detected measurement signal has a relatively low level and therefore a relatively small signal-to-noise ratio (SNR). Furthermore, in practice, the detected measurement signal is transmitted as an analog signal via a cable to the evaluation device. Due to the length of the cable, distortions due to temperature effects, crosstalk, and other interferences can occur. This makes the evaluation of the detected measurement signal difficult. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] DE 19706691 A1 [Patent Document 2] International Publication No. 98 / 38482 Brochure [Patent Document 3] DE 19839286 [Patent Document 4] U.S. Patent No. 3,502,968 [Patent Document 5] U.S. Patent No. 4,677,578 [Non-patent literature]

[0011] [Non-Patent Document 1] “Non-contact measurement of strip flatness”, Steel Times International, July / August 2003, p.16-17 Summary of the Invention [Problem to be solved by the invention]

[0012] The object of the present invention is to create the possibility of avoiding the drawbacks of the prior art. [Means for solving the problem]

[0013] This problem is solved by a measuring device having the features of claim 1. Advantageous embodiments of the measuring device according to the invention are the subject of dependent claims 2 to 7.

[0014] According to the invention, a measuring device of the type mentioned at the beginning is configured in such a way that the sensor elements include eddy current sensors, the eddy current sensors of sensor elements arranged immediately adjacent to one another in the width direction each operate at one of two or three different operating frequencies, and the number of sensor elements is greater than the number of operating frequencies, so that, when viewed across the entire sensor element, a number of sensor elements each operate at the same operating frequency.

[0015] This allows the number of operating frequencies to be kept small. In particular, the number of operating frequencies is less than the number of sensor elements. By using (only) two or three operating frequencies, the number of components is minimized. Also, within the scope of the present invention, although several sensor elements operate at the same operating frequency, the sensor elements operating at the same operating frequency are nevertheless separated from each other by other sensor elements operating at a different operating frequency or at two different operating frequencies.

[0016] Within the scope of the present invention, the measurement signal is digitized as early as possible and is therefore very robust against any subsequent interference.

[0017] The analog-to-digital conversion can also take place within each sensor element. However, it is currently preferred that the digitizing device be configured as a separate element from the sensor element and be connected to the sensor element via a cable within the measuring device. However, the cable is relatively short. The length of the cable is usually significantly less than 1 meter, for example, about 20 cm to about 50 cm.

[0018] It is also possible to carry out additional processing of the analog and / or digitized measured values ​​immediately before or after digitization, i.e., inside the measuring device. Although this is possible, it is not necessary and in many cases not reasonable.

[0019] Preferably, the cable is sealed at the transition to each sensor element. The term "sealed" refers to an airtight and watertight seal. In particular, the cable may be permanently connected to each sensor element to provide such a seal. This ensures that the transition from the sensor element to the cable is well protected from environmental influences, especially from the ingress of water. The permanent connection of the cable to the sensor element is particularly practical since the cable is relatively short. This contrasts with the prior art, in which the cable is led to a control cabinet outside the measuring device and therefore has a length of several meters.

[0020] Preferably, the digitizing device is arranged below the metal plate on which the sensor elements are arranged, which allows the digitizing device to be arranged in a space-saving yet well-protected manner.

[0021] Preferably, the transmitted signals are transmitted to the evaluation device via a common armored cable with pre-assembled connections. This is particularly advantageous in the harsh environment of a rolling mill. Suitable armored cables are modeled after hydraulic hoses. Such armored cables (with pre-assembled connections) are offered, for example, by Harting GmbH, Vienna (Austria).

[0022] The armored cable may include dedicated conductors for transmitting signals of individual sensor elements, but it is particularly preferred that the armored cable includes dedicated conductors for transmitting signals of groups of several sensor elements, which allows the number of conductors and the overall size and complexity of the armored cable to be minimized.

[0023] Preferably, the sensor elements have a coding that indicates the operating frequency characteristic of each sensor element. The coding is such that it can be directly perceived by humans with their sensory organs. This allows the sensor elements to be quickly and easily distinguished by an operator and ensures that the sensor elements are attached to the "right" place on the metal plate. The coding may be mechanical and / or tactile and / or optical, as required. In the case of mechanical coding, it may even be possible, in some circumstances, to make it impossible to attach a sensor element to the "wrong" position on the metal plate.

[0024] Alternatively or additionally, it is possible, for example, for the evaluation device to check the operating frequency at which the sensor elements are each operated, in which case, if the evaluation device stores which sensors should be operated at which operating frequency, the evaluation device can carry out the check and output an error message in the event of an error.

[0025] The problem is further solved by a combination of the measuring device according to the invention with an evaluation device having the features of claim 8. Advantageous embodiments of this combination are the subject of dependent claims 9 to 11.

[0026] According to the invention, a combination of a measuring device according to the invention and an evaluation device is created, in which the evaluation device: receiving a transmission signal from a digitizing device, determining the amplitude of the excited mechanical vibrations of each area of ​​the metal strip based on the transmitted signal for each area of ​​the metal strip; - Within the amplitude determination range, at least a characteristic curve specific to the metal strip is utilized.

[0027] By taking into account the characteristic curve, the transmitted signal can be linearized and the amplitude of vibration of the area of ​​the metal strip can be determined based on the linearized signal. Since the characteristic curve is determined specifically for the metal strip (e.g. depending on the width, thickness, temperature and / or material), on the one hand, an improved linearization and on the other hand a linearization over a larger measurement range are possible compared to the prior art.

[0028] It is also possible for the characteristic curve to additionally depend on the operating temperature of the sensor element, which allows for a further improvement in the linearization of the transmitted signal.

[0029] Preferably, the evaluation device uses the Goertzel algorithm within the scope of the amplitude determination, which allows for a faster and better evaluation of the transmitted signal, especially if the evaluation device takes the excitation frequency into account within the Goertzel algorithm.

[0030] The above-mentioned characteristics, features and advantages of the present invention, as well as the manner in which they are obtained, will become more apparent and easier to understand in connection with the following detailed description of an embodiment thereof, taken in conjunction with the drawings, in which: [Brief explanation of the drawings]

[0031] [Figure 1] FIG. [Figure 2] FIG. 2 is a side perspective view of a metal strip and a measuring device. [Figure 3] 3 is a perspective view of part of the measuring device according to FIG. 2, seen from above, with the cover partly removed; FIG. [Figure 4] FIG. 4 shows a part of the measuring device according to FIGS. 2 and 3; [Figure 5] FIG. 2 is a diagram showing the sensors individually. [Figure 6] FIG. [Figure 7] FIG. 4 is a cross-sectional view of a metal plate and a cover. [Figure 8]FIG. 2 is a diagram showing the sensor elements individually. [Figure 9] FIG. 2 is a diagram showing a metal plate and each sensor element. [Figure 10] FIG. 1 shows a sensor element and a digitizing device. [Figure 11] FIG. 1 shows a mounting unit including a metal plate with a sensor element and a digitizing device. [Figure 12] 12 shows the mounting unit according to FIG. 11 with the cover plate removed. [Figure 13] FIG. 1 is a diagram illustrating a signal transmission method. [Figure 14] FIG. 10 is a diagram illustrating allocation of operating frequencies to sensor elements. [Figure 15] FIG. 1 illustrates a possible evaluation of a transmitted signal. DETAILED DESCRIPTION OF THE INVENTION

[0032] According to FIG. 1, the conveying device for the metal strip 1 comprises a front device 2, which is typically a rolling mill stand. In FIG. 1, only the work rolls of the rolling mill stand are shown. However, rolling mill stands often comprise additional rolls, for example, in the case of a four-high stand, support rolls in addition to the work rolls, and in the case of a six-high stand, intermediate rolls and support rolls in addition to the work rolls. Other configurations are also possible, for example, 20-roll or 12-roll rolling mill stands.

[0033] On the infeed side, a further rolling mill stand may be arranged upstream of the rolling mill stand. On the infeed side, several further rolling mill stands may be arranged upstream of the rolling mill stand. Likewise, it is possible, for example, to arrange a reel for unwinding the metal strip 1 directly upstream of the rolling mill stand. The front device 2 itself may also be a device other than a rolling mill stand. Examples of such devices include a drive roll set and a reel. Which of these embodiments is provided is of secondary importance within the scope of the present invention. For this reason, the configuration of the rolling device on the infeed side of the front device 2 is not shown in the figures or described in detail.

[0034] The conveying device further comprises a rear device 3, which is arranged downstream of the front device 2. The rear device 3 includes, for example, a reel 4 and a pulley 5 arranged upstream of the reel 4, as shown in FIG. 1 . The specific configuration of the rear device 3 is of secondary importance. What is crucial is that the front device 2 and the rear device 3 define a pass line for the metal strip 1, along which the metal strip 1 is conveyed from the front device 2 to the rear device 3 in a conveying direction x. The conveying direction x is generally horizontal, or at least approximately horizontal. The conveying speed of the metal strip 1 as it leaves the front device 2 can be up to 400 m / min, or even slightly higher, if the front device 2 is configured as a rolling mill stand.

[0035] Between the front device 2 and the rear device 3, various devices of secondary importance within the scope of the present invention, such as thickness measuring devices, may be arranged, where the arrangement of the measuring device 6 between the front device 2 and the rear device 3 is crucial.

[0036] The measuring device 6 includes a mechanical excitation device 7. The mechanical excitation device 7 can be used to mechanically vibrate the metal strip 1 in its thickness direction. Specifically, the metal strip 1 is shown in FIG. 1 as a solid line at its center position. The deflection from the center position is indicated by a double arrow 8 in FIG. 1. The excitation to mechanical vibration is performed at an excitation frequency fA.

[0037] The mechanical excitation device 7 may be configured as a suction device, for example, as shown in Figure 1. Such a configuration is well established, robust and reliable.

[0038] For example, the intake fan 9 can suck air out of the area between the metal strip 1 and the measuring device 6 through the suction openings 10 (see in particular Figures 2 and 3; only some of the suction openings 10 are numbered) and the suction channels 11, thus periodically applying a negative pressure to one side of the metal strip 1. The degree to which the air is sucked out can be varied by directly activating the suction fan 9 and / or by activating the modulator element 12. When activated, the modulator element 12 periodically changes the current cross-section of the suction channel 11 and thus the flow resistance of the suction channel 11. The modulator element 12 can be configured, for example, as an elliptical or oval element rotating within the suction channel 11.

[0039] As already mentioned, the configuration is well established and therefore no further explanation is necessary.

[0040] In order to effectively vibrate the metal strip 1, the mechanical exciter 7 has a flat boundary surface 13. The flat boundary surface 13 faces the metal strip 1 and extends a short distance (usually in the single-digit millimeter range) from the pass line. Suction openings 10 are arranged in the boundary surface 13.

[0041] 3 and 4, the measuring device 6 further includes a metal plate 14. The metal plate 14 is disposed next to the interface 13. The metal plate 14 has an upper surface 15 facing the metal strip 1. The metal plate 14 is offset with respect to the interface 13, so that the upper surface 15 is at a greater distance from the pass line than the interface 13.

[0042] As shown in Figures 1, 3, and 4, multiple sensor elements 16 are arranged on the metal plate 14. This means that the measuring device 6 has multiple sensor elements 16. For clarity, only one of the sensor elements 16 is shown in Figure 1. In Figures 3 and 4, only some of the sensor elements 16 are labeled with reference numerals. It is clear that the sensor elements 16 are not flush with the upper surface 15 of the metal plate 14, but rather protrude beyond the upper surface 15 toward the metal strip 1. Therefore, the measuring device 6 has a cover 17. The cover 17 covers the sensor element 16 with its upper surface (i.e., toward the metal strip 1). The cover 17 seals the sensor element 16 toward its side. The upper surface 18 of the cover 17 is in the plane formed by the flat boundary surface 13, as shown in Figure 2.

[0043] The cover 17 is made of an electrically insulating material. For example, the cover 17 may be made of a ceramic or a plastic. Suitable ceramics and suitable plastics, such as polyimide and polyester ester ketone (PEEK), are known to those skilled in the art.

[0044] The sensor elements 16 are arranged offset relative to one another in the width direction of the metal strip 1. In a specific embodiment of the invention, the sensor elements 16 form two rows, with corresponding sensor elements 16 arranged adjacent to one another in each row in the width direction, and with the sensor elements 16 of one row offset relative to the sensor elements 16 of the other rows when viewed across the rows. This embodiment, i.e., with multiple rows of sensor elements 16 offset from one another, is currently preferred but is of secondary importance in the end.

[0045] By means of the sensor elements 16, a respective measurement signal MA can be detected for each region of the metal strip 1, each measurement signal MA being characteristic of the amplitude A of the excited mechanical vibrations of each region of the metal strip 1. The regions of the metal strip 1 are likewise offset from one another in the width direction of the metal strip 1 according to the arrangement of the sensor elements 16.

[0046] The respective measurement signals MA are detected contactlessly. Possible embodiments for this are generally known to those skilled in the art. In particular, the sensor element 16 (= assembly) comprises an eddy current sensor as the actual sensor 19 for detecting the respective measurement signals MA. In the eddy current sensor (see FIG. 5 for the individual eddy current sensor), eddy currents are induced in the respective regions of the metal strip 1 by means of an excitation current IA in an excitation coil 20. The extent to which the eddy currents are induced is detectable. The respective (analog) measurement signal MA is derived from this extent.

[0047] The excitation current IA has an excitation frequency f, which will be referred to below as the operating frequency to distinguish it from the excitation frequency fA. The operating frequency f is usually in the range of a few kHz, and can even be in the single-digit MHz range. The measurement signal MA likewise has an operating frequency f. From the measurement signal MA, the current distance of each area of ​​the metal strip 1 from the measuring device 6 can therefore be determined in a manner known per se. From the change in this distance over time, the amplitude A of the mechanical vibrations of the corresponding area of ​​the metal strip 1 is obtained.

[0048] This procedure is commonly and well known to those skilled in the art and therefore need not be described in detail.

[0049] 6, the cover 17 has a receiving portion 21 on its underside, i.e. the side facing the sensor element 16. When the cover 17 is attached to the metal plate 14, the sensor element 16 enters the receiving portion 21 (obviously only insofar as it protrudes beyond the upper surface 15 of the metal plate 14).

[0050] The cover 17 further has a recess 22 on its underside, only a portion of which is marked with a reference number in Figure 6. The recess 22 as a whole forms a number of flow paths for the cooling medium 23 between the metal plate 14 and the cover 17. The sensor element 16 can therefore be actively cooled using the cooling medium 23.

[0051] According to Fig. 6, one flow path is formed. However, several flow paths may also be formed. The following explanation regarding the order of the sensor elements 16 relates to each flow path. In the case of several flow paths, the flow paths are separated from one another, i.e., a sensor element 16 is incorporated in only one flow path in each case. If several flow paths are present, the following explanation is valid for each flow path individually.

[0052] 6, the flow paths are configured so that the sensor elements 16 are arranged one behind the other in the flow direction of the cooling medium 23. That is, the cooling medium 23 first cools one of the sensor elements 16, then the next sensor element 16, and so on until all the sensor elements 16 in each flow path have been cooled.

[0053] 6 further shows that the receptacles 21 each have an inlet 24 for the cooling medium 23 and an outlet 25 for the cooling medium 23. In FIG. 6, the inlets 24 and outlets 25 are labeled with reference numerals only for some of the receptacles 21 for clarity. On the one hand, it is clear that the outlet 25 of each receptacle 21 is connected to the inlet 24 of the next receptacle 21 via the respective connections of the respective flow paths. In this context, the term "next receptacle" refers to the next receptacle 21 in the direction of flow of the cooling medium 23. On the other hand, the inlets 24 and outlets 25 of each receptacle 21 are arranged opposite each other when viewed from each sensor element 16. As a result, each sensor element 16 is completely surrounded by the cooling medium 23 and cooled.

[0054] The cooling medium 23 may be, for example, (purified) compressed air. This embodiment offers the additional advantage that even a small leak is not fatal, since compressed air has a higher pressure than ambient air. Therefore, despite the leak, foreign matter cannot enter the space covered by the cover 17. Nevertheless, cooling of the sensor element 16 can be maintained as long as the leak is sufficiently small.

[0055] In some cases, it may be sufficient to cool the sensor element 16 exclusively with the cooling medium 23. In other cases, it is necessary to arrange a conduit 26 for the coolant in the metal plate 14, as shown diagrammatically in Figure 7. In this case, the metal plate 14 is cooled directly by the coolant, which also indirectly cools the sensor element 16.

[0056] The structure of one sensor element 16 will be described in detail below with reference to FIGS.

[0057] 8 and 9, the sensor element 16 is a pre-assembled unit consisting of several components: a sleeve 27 and a fixing element 28, in addition to the sensor 19 used for detecting the respective measurement signal MA. Furthermore, a plastic cover 29 may be attached to the sensor 19 on the side that will later face the metal strip 1. By means of the plastic cover 29 (if present), the sensor 16 (insofar as it protrudes from the metal plate 14 in the attached state) is sealed airtight and watertight.

[0058] 8, the sensor 19 (which may include a plastic cover 29) is disposed within a sleeve 27. The sleeve 27 has an external thread 30. A fixing element 28 is attached to the sleeve 27. The fixing element 28 therefore has a corresponding internal thread (not shown). The fixing element 28 has a collar 31 that projects radially outward from the sleeve 27.

[0059] The components, i.e., sensor 19, sleeve 27, and fixing element 28, are fixed relative to one another. For example, sensor 19 may be glued to sleeve 27, and fixing element 28 may be fixed to sleeve 27 via soldering or welding points. This allows the distance between the lower or upper edge of collar 31 and the upper surface of sensor 19 (or the upper surface of plastic cover 29, if present) to be set as desired when mounting sensor element 16. For example, sensor 19 may first be fixed to sleeve 27. Before or after this, plastic cover 29 can be placed over sensor 19, if necessary. The distance between the lower or upper edge of collar 31 is then set. Finally, fixing element 28 is fixed to sleeve 27.

[0060] 9, the metal plate 14 has receptacles 32 for the sensor elements 16. Each receptacle 32 has a radially inwardly projecting support ring 33. The collar 31 is supported by the support ring 33 when the corresponding sensor element 16 is mounted in the corresponding receptacle 32.

[0061] As already explained, the sensor elements 16 can be used to detect a measurement signal MA for each area of ​​the metal strip 1. This detection is carried out contactlessly by means of an eddy current sensor. For this purpose, the eddy current sensor has an excitation coil 20 to which an excitation current IA in the range of a few kHz, possibly even in the single-digit MHz range, is applied. The detected measurement signal MA is initially analog.

[0062] In the prior art, the measurement signal MA is transmitted via a corresponding cable to an evaluation device 34 (see FIGS. 1 and 10) which is arranged outside the measuring device 6, typically in a control cabinet. However, within the scope of the present invention, digitizing devices 35 are arranged inside the measuring device 6. One of the digitizing devices 35 is shown in FIG. 1, and several of them are shown in FIG. 10. Using the digitizing devices 35, the detected analog measurement signal MA is digitized.

[0063] As in the prior art, the evaluation device 34 is arranged outside the measuring device 6, for example in a control cabinet. In the simplest case, the digitizing device 35 transmits the digitized measurement signal itself as a transmission signal MA' to the evaluation device 34. Alternatively, the digitizing device 35 can transmit a signal extracted from the digitized measurement signal to the evaluation device 34 as a transmission signal MA'.

[0064] The arrangement of the digitizing device 35 within the measuring device 6 may be as required. For example, the digitizing device 35 may be configured as an independent element separate from the sensor element 16, as shown in Figures 1, 10, 11 and 12. In this case, the digitizing device 35 is connected to the sensor element 16 within the measuring device 6 via a cable 36. In terms of its arrangement, the digitizing device 35 may in particular be arranged below the metal plate 14. The cable 36 is generally very short (usually at most 100 cm, often less than 50 cm).

[0065] Digitizing device 35 is shown in Figures 11 and 12 (see also Figure 4) as a cigar-shaped element. The term "cigar-shaped" in this context relates to both the basic shape (an elongated cylinder) and the absolute dimensions (length about 15 cm to about 30 cm, diameter about 1.0 cm to about 3.0 cm). This shape of digitizing device 35 is presently preferred, but by no means required.

[0066] The cables 36 are typically releasably connected to the digitizing device 35, for example via a screw or bayonet connection. At the transition to the sensor element 16, the cables 36 are preferably sealed (i.e., air-tight and water-tight). Sealing can be achieved by means of an elastic rubber sleeve slidably arranged on the corresponding cable 36, as is known, for example, from spark plug connectors in automobile engines. Alternatively, it is also possible, and currently preferred, for the cables 36 to be non-releasably connected to the respective sensor element 16.

[0067] With regard to the connection of the digitizing device 35 to the evaluation device 34, it is in principle possible to establish the connection via individual corresponding connecting cables, as is done in the prior art for the transmission of analog measurement signals MA. However, it is preferred that the transmission signals MA' are transmitted to the evaluation device 34 via a common armored cable 37 with a pre-assembled connection 38, as shown in Figure 13. This will be explained in more detail below.

[0068] 13, individual conductors or thin cables 39 extend from the digitizing device 35 to a pre-assembled plug connection 40. One of the pre-assembled connections 38 of the armored cable 37, arranged at one end of the armored cable 37, can be plugged into the pre-assembled plug connection 40. Inside the armored cable 37, conductors 41 corresponding to the individual conductors or thin cables 39 are guided to the other pre-assembled connection 38 arranged at the other end of the armored cable 37. The pre-assembled connection 38 is connected to a further plug connection 42, from which conductors are guided to the evaluation device 34.

[0069] The sheath 43 of the sheathed cable 37 may correspond, for example, to the sheath that is typical for hydraulic wiring in which the hydraulic fluid is under pressure in the range of 100 bar to 500 bar.

[0070] The dashed line L in Figure 13 is intended to indicate the boundaries of the measuring device 6. This means that, as shown in Figure 13, the armored cable 37 can be connected from outside the measuring device 6 without opening the measuring device 6. Alternatively, it is obviously also possible to arrange a pre-assembled plug connection 40 inside the measuring device 6.

[0071] 13 also shows a further preferred embodiment, in which the armored cable 37 includes a dedicated conductor 41 for the transmission signal MA' of each group of sensor elements 16. In particular, the digitized measurement signals of three sensor elements 16 are combined in the digitizing device 35. In this context, the number "3" has secondary importance.

[0072] 14 is a schematic top view of the sensor element 16, i.e., as seen from the metal strip 1. Within the sensor element 16, reference characters f1, f2, and f3 are marked. f1, f2, and f3 are operating frequencies, similar to the operating frequency f. However, they differ from each other in pairs. For example, the operating frequency f1 can have a value of 280 kHz, while the operating frequency f2 has a value of 300 kHz and the operating frequency f3 has a value of 320 kHz. Obviously, the values ​​given are purely exemplary.

[0073] It can be seen that the eddy current sensors of the sensor elements 16 arranged directly adjacent to one another in the metal plate 14 operate at different operating frequencies f1, f2, f3, which makes it possible to significantly reduce possible crosstalk behavior.

[0074] In the case of different operating frequencies f1, f2, f3, the number of sensor elements 16 whose digitized measurement signals are transmitted over the individual conductors 41 may be equal to the number of operating frequencies f1, f2, f3, as shown in Figures 13 and 14. In this case, the sensor elements 16 may be grouped in particular such that each operating frequency f1, f2, f3 is represented once in a group. In Figure 14, the corresponding groups are surrounded by dashed lines.

[0075] For different operating frequencies f1, f2, and f3, the sensor elements 16 preferably have a coding 44, as shown in FIGS. 11 and 12 (see also FIG. 9 for an individual sensor element 16). The coding 44 indicates the characteristics of the operating frequencies f1, f2, and f3 of each sensor element 16. The coding 44 may be optical. For example, different colors can be used for the different operating frequencies f1, f2, and f3, such as red, green, and blue, or yellow, red, and blue. The coding 44 may also be tactile. For example, the operating frequency f1 may be coded by a circumferential ring on the cable 36, while the operating frequencies f2 and f3 may be coded by two circumferential rings on the cable 36, with the distinction between the operating frequencies f2 and f3 being made by the distance between the two rings. A further operating frequency f may be coded by, for example, three such rings. Mechanical coding is also possible in a similar manner.

[0076] Preferably, the digitizing device 15 also has a corresponding coding 45, so that the correct allocation is also easily evident.

[0077] As shown in Figure 15, the evaluation device 34 receives the transmission (digital) signal MA' from the digitizing device 35. Within the scope of the evaluation of the transmission signal MA', the evaluation device 34 determines, for each area of ​​the metal strip 1, the amplitude A of the mechanical vibrations of the metal strip 1.

[0078] To determine the amplitude A, the evaluation device 34 first linearizes the transmission signal MA' in a linearization block 46. This means that the linearization block 46 outputs a corrected signal MA", the value of which is proportional to the corresponding deflection of the respective region of the metal strip 1 at the time the corresponding (analog) measurement signal MA was detected. Within the linearization, the evaluation device 34 utilizes a characteristic curve K, which is determined by the evaluation device 34 specifically for the metal strip 1. This determination can be made, for example, depending on the geometrical properties G and / or chemical properties C and / or thermodynamic properties T (e.g. temperature) and / or history H of the metal strip 1. If necessary, the operating temperature T' of the sensor element 16 can also be taken into account within the determination of the characteristic curve K.

[0079] To determine the characteristic curve K, for example, the determination device 47 may store associated characteristic curves K for specific values ​​of the geometrical property G, the chemical property C, etc., and the specifically utilized characteristic curve K can be determined by selection and / or interpolation.

[0080] The correction signal MA" is supplied in the evaluation device 34 to a decision block 48. In the decision block 48, the evaluation device 34 determines the respective amplitude A of the excited mechanical vibrations of the metal strip 1 for the area of ​​the metal strip 1. In determining the amplitude A, the evaluation device 34 preferably uses the Goertzel algorithm, as shown in Figure 15. It is particularly preferable for the evaluation device 34 to take the excitation frequency fA into account in the Goertzel algorithm.

[0081] The determined amplitude A can be supplied to a further decision block 49. In decision block 49, the evaluation device 34 determines the respective flatness error PF for the region of the metal strip 1 on the basis of the amplitude A. The determination of the flatness error PF per se is no longer the subject of the present invention.

[0082] The evaluation device 34 can output the determined flatness error PF to, for example, a control device (not shown) for the front device 2, so that the control device can control the flatness actuators of the front device 2 so that the flatness error PF is eliminated as much as possible.

[0083] The present invention has many advantages. The use of the cover 17 improves the sensitivity of the sensor 19. Replacement of the sensor element 16 is significantly easier. The design of the sensor element 16 as a pre-assembled assembly also ensures reliable and accurate positioning of the sensor element 16 within the metal plate 14. This also applies to the subsequent replacement of a defective sensor element 16 with a new one. The design of the sensor element 16 as a pre-assembled assembly reduces the sensor 19's protection from moisture, dirt, and large heat input within its boundaries. This improves the durability of the sensor element 16. Cooling the sensor element 16 with the cooling medium 23 also improves protection from dirt and moisture. The use of multiple operating frequencies f1, f2, and f3 virtually eliminates crosstalk. Very early digitization of the measurement signal MA within the measuring device 6 allows the measurement signal MA to be converted very quickly into a form that is resistant to interference. This allows, among other things, an extension of the assessable measurement range. Evaluation in combination with a characteristic curve K specific to the metal strip 1 allows for an improved evaluation and also for an evaluation over an extended measuring range. The use of the characteristic curve K allows for an optimized evaluation of the measurement signal MA for each metal strip 1. Evaluation using the Goertzel algorithm provides excellent results with a reduced amount of calculation.

[0084] Although the present invention has been shown and described in detail by way of preferred embodiments, the present invention is not limited to the disclosed examples, and those skilled in the art may derive other variations without departing from the scope of protection of the present invention. [Explanation of symbols]

[0085] 1 metal strip 2 Front device 3 Rear device 4 reels 5 pulleys 6. Measuring equipment 7 Mechanical exciter 8 Double Arrow 9 Intake fan 10 Suction opening 11 suction channel 12 Modulator Elements 13 Boundary 14 Metal plate 15, 18 Top 16 Sensor element 17 Cover 19 Sensors 20 Excitation coil 21, 32 Receptor 22 recess 23 Cooling medium 24 Entrance 25 Exit 26 Conduit 27 Sleeve 28 Fixed Elements 29 Plastic Cover 30 male thread 31 Color 33 Support ring 34 Evaluation equipment 35 Digitization equipment 36 Cable 37 Armored Cable 38 Pre-assembled connections 39 Conductor or thin cable 40, 42 Plug connection 41 Armored cable conductor 43 Exterior 44, 45 Coding 46 Linearization Blocks 47 Determination device 48, 49 Decision Block A amplitude C Chemical properties f, f1~f3 operating frequency fA Excitation frequency G Geometric properties H History IA excitation current K characteristic curve L linear MA Analog measurement signal MA' transmission signal MA” correction signal PF flatness error T Thermodynamic properties T' Operating temperature x Transport direction

Claims

1. A measuring device in a conveying device for metal strips (1), comprising: the measuring device is arranged between a front device (2) of the conveying device and a rear device (3) arranged downstream of the front device (2), the measuring device comprises a mechanical excitation device (7) by means of which the metal strip (1) can be excited to mechanically oscillate in the thickness direction of the metal strip at an excitation frequency (fA); the measuring device comprises a plurality of sensor elements (16), the sensor elements (16) are arranged offset from one another in the width direction of the metal strip (1); - by means of said sensor elements (16), analog measurement signals (MA) can be detected for a number of areas of said metal strip (1) offset from one another in the width direction, said analog measurement signals being characteristic of the amplitude (A) of the excited mechanical vibrations of each area of ​​said metal strip (1); a measuring device in which a digitizing device (35) is arranged inside the measuring device, by means of which the detected analog measuring signal (MA) is digitized, and the digitized measuring signal or a signal extracted from the digitized measuring signal is transmitted as a transmission signal (MA') from the digitizing device to an evaluation device (34) arranged outside the measuring device, The measuring device is characterized in that the sensor elements (16) include eddy current sensors, the eddy current sensors of the sensor elements (16) arranged immediately adjacent to each other in the width direction each operate at one of two or three operating frequencies (f1, f2, f3) that are different from each other, and the number of the sensor elements (16) is greater than the number of the operating frequencies (f1, f2, f3), so that when viewed across the entire sensor elements (16), each of the sensor elements (16) operates at the same operating frequency (f1, f2, f3).

2. 2. The measuring device according to claim 1, wherein the digitizing device (35) is configured as a separate element from the sensor element (16) and is connected to the sensor element (16) within the measuring device via a cable (36) having a length of at most 1 m, in particular at most 50 cm.

3. 3. The measuring device according to claim 2, wherein the cable (36) is sealed at the transition to each of the sensor elements (16) and is in particular non-releasably connected to each of the sensor elements (16).

4. 4. Measuring device according to claim 2 or 3, characterized in that a digitizing device (35) is arranged on the underside of the metal plate (14) on which the sensor elements (16) are arranged.

5. 2. The measuring device according to claim 1, wherein the transmission signal (MA') is transmitted to the evaluation device (34) via a common armored cable (37) with a pre-assembled connection (38).

6. 6. The measuring device according to claim 5, characterized in that the armored cable (37) comprises dedicated conductors (41) for the transmission signals (MA') of individual sensor elements (16) or for the transmission signals (MA') of groups of several sensor elements (16), respectively.

7. 2. The measuring device according to claim 1, characterized in that the sensor elements (16) have a coding (44) characteristic of the operating frequency (f1, f2, f3) of each of said sensor elements (16).

8. 2. A combination of a measuring device (6) and an evaluation device (34) according to claim 1, wherein the evaluation device (34) comprises: receiving the transmission signal (MA') from the digitizing device (35); - determining the amplitude (A) of the excited mechanical vibrations of each area of ​​the metal strip (1) based on the transmitted signal (MA') for said area, - a combination that utilizes, within the determined range of said amplitude (A), at least a characteristic curve (K) specific to said metal strip (1).

9. 9. The combination according to claim 8, characterized in that the characteristic curve (K) additionally depends on the operating temperature (T') of the sensor element (16).

10. 10. The combination according to claim 8 or 9, characterized in that the evaluation device (34) uses the Goertzel algorithm within the determination of the amplitude (A).

11. 11. The combination according to claim 10, characterized in that the evaluation device (34) takes into account the excitation frequency (fA) within the Goertzel algorithm.

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