IMPROVED CONTACTLESS DETECTION OF VIBRATIONS IN METAL BELTS

Abstract

A measuring assembly with a mechanical excitation device that excites the metal belt of a transport device at an excitation frequency (fA) to produce mechanical vibrations. The measuring assembly has a metal plate that faces the metal belt. The metal plate is equipped with sensor elements which are offset relative to one another when viewed in the belt width direction with which measurement signals (MA) that characterize the amplitude (A) of the excited mechanical vibrations are detected for corresponding regions of the metal belt. The sensor elements protrude beyond the upper face of the metal plate and up to the metal belt. A cover for the measuring assembly that is made of an electrically insulating material covers the sensor elements on the upper face thereof, and laterally seals the sensor elements.

Claims

1. A measuring arrangement in a transport device for a metal strip, wherein the measuring arrangement is arranged between a front device and a rear device of the transport device, said rear device being arranged downstream of the front device, wherein the measuring arrangement has a mechanical excitation device by means of which the metal strip can be excited so as to vibrate mechanically in its thickness direction at an excitation frequency (fA), wherein the measuring arrangement has a metal plate, the upper side of which faces the metal strip, wherein a plurality of sensor elements is arranged in the metal plate, wherein the sensor elements are arranged offset relative to one another as viewed in a width direction of the metal strip, wherein it is possible by means of the sensor elements to acquire for each of the plurality areas of the metal strip which are offset relative to one another in the width direction a measurement signal (MA) that is characteristic of the amplitude (A) of the excited mechanical vibration of the respective area of the metal strip, wherein: the sensor elements project towards the metal strip beyond the upper side of the metal plate and the measuring arrangement has a cover which consists of an electrically insulating material and covers the sensor elements on their upper side and seals them on their sides.

2. The measuring arrangement as claimed in claim 1, wherein the cover has recesses on its underside that faces the sensor elements, so that a number of flow channels for a cooling medium is formed between the metal plate and the cover, wherein the sensor elements can be cooled by means of said cooling medium.

3. The measuring arrangement as claimed in claim 2, wherein the flow channels are designed in such a way that, with respect to one of the flow channels, the sensor elements are arranged sequentially one behind the other as viewed in the direction of flow of the cooling medium.

4. The measuring arrangement as claimed in claim 3, wherein: the cover has receptacles for the sensor elements on its underside, insofar as these project beyond the upper side of the metal plate, the receptacles each have an inlet for the cooling medium and an outlet for the cooling medium, the outlet of a respective receptacle is communicatively connected via a respective connecting section of the respective flow channel to the inlet of the respective next receptacle as viewed in the direction of flow of the cooling medium, and the inlet and the outlet of a respective receptacle are arranged opposite one another as viewed from the respective sensor element.

5. The measuring arrangement as claimed in claim 1, wherein the mechanical excitation device has a flat boundary surface that faces the metal strip.

6. The measuring arrangement as claimed in claim 5, wherein an upper side of the cover lies in the plane that is formed by the plane boundary surface.

7. The measuring arrangement as claimed in claim 1, wherein the cover consists of a ceramic or a plastic.

8. The measuring arrangement as claimed in claim 1, wherein channels for a cooling liquid are arranged in the metal plate, wherein the metal plate and thus indirectly also the sensor elements can be cooled by means of said cooling liquid.

9. The measuring arrangement as claimed in claim 1, wherein: the sensor elements each have a sleeve with an external thread, in the respective sleeve a respective sensor is arranged, by means of which in each case one of the measurement signals (MA) can be acquired, and a respective fixing element is applied to the respective sleeve, said fixing element having a collar that projects radially outwards over the respective sleeve in the radial direction.

10. The measuring arrangement as claimed in claim 9, wherein the metal plate has receptacles for the sensor elements, which in turn each have a radially inwardly projecting support ring for the respective collar of the respective sensor element.

11. The measuring arrangement as claimed in claim 9, wherein a plastic hood is applied to the respective sensor on its side that faces the metal strip, so that the respective sensor, insofar as it projects beyond the metal plate, is sealed in an airtight and watertight manner.

12. The measuring arrangement as claimed in claim 1, wherein the sensor elements comprise eddy current sensors and in that the eddy current sensors of sensor elements that are arranged directly adjacent in the metal plate are operated at mutually different operating frequencies (f1, f2, f3).

13. The measuring arrangement as claimed in claim 12, wherein the sensor elements have a coding which is characteristic of the operating frequency (f1, f2, f3) of the respective sensor element.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The characteristics, features and advantages of this invention described above and the manner in which they are achieved will become clearer and more comprehensible in connection with the following description of the embodiments, which will be explained in more detail in connection with the drawings. Hereby in a schematic illustration:

[0045] FIG. 1 shows a side view of a rolling facility,

[0046] FIG. 2 shows a perspective side view of a metal strip and a measuring arrangement,

[0047] FIG. 3 shows a perspective view from above of a part of the measuring arrangement of FIG. 2, wherein a cover is partially removed,

[0048] FIG. 4 shows a part of the measuring arrangement of FIGS. 2 and 3,

[0049] FIG. 5 shows an individual sensor,

[0050] FIG. 6 shows a perspective view of an underside of a cover,

[0051] FIG. 7 shows a section through a metal plate and a cover,

[0052] FIG. 8 shows an individual sensor element,

[0053] FIG. 9 shows a section of the metal plate and an individual sensor element,

[0054] FIG. 10 shows sensor elements and digitization devices,

[0055] FIG. 11 shows an installation unit comprising a metal plate with sensor elements and digitization devices,

[0056] FIG. 12 shows the installation unit of FIG. 11 with a removed cover plate,

[0057] FIG. 13 shows a method of signal transmission,

[0058] FIG. 14 shows an assignment of operating frequencies to sensor elements and

[0059] FIG. 15 shows a possible evaluation of transmitted signals.

DESCRIPTION OF THE EMBODIMENTS

[0060] According to FIG. 1, a transport device for a metal strip 1 has a front device 2. The front device is usually a roll stand. Only the working rollers of the roll stand are shown in FIG. 1. However, the roller stand often has additional rollers, for example in the case of a four-high stand, supporting rollers in addition to the working rollers and in the case of a six-high stand, intermediate rollers and supporting rollers in addition to the working rollers. Other configurations are also possible, for example as a 20-roller roll stand or as a 12-roller roll stand.

[0061] A further roll stand can be arranged upstream of the roll stand on the in-feed side. Multiple further roll stands can also be arranged upstream of the roll stand on the in-feed side. It is also possible, for example, that a decoiler for uncoiling the metal strip 1 is arranged directly upstream of the roll stand. The front device 2 itself can also be a device other than a roll stand. Examples of such devices are a drive roller set and a decoiler. Which of these embodiments is given is of secondary importance in the context of the present invention. For this reason, the configuration of the rolling facility on the inlet side of the front device 2 is also not shown in the FIGS and is also not explained in more detail.

[0062] The transport device also has a rear device 3. The rear device 3 is arranged downstream of the front device 2. The rear device 3 can, for example, comprise a reel 4 and a deflection roller 5 upstream of the reel 4 as shown in FIG. 1. The specific configuration of the downstream device 3 is of secondary importance. The decisive factor is that a pass line for the metal strip 1 is defined by the front device 2 and the rear device 3, along which the metal strip 1 is conveyed in a conveying direction x from the front device 2 to the rear device 3. The conveying direction x is generally horizontal or at least almost horizontal. A transport speed at which the metal strip 1 runs out of the front device 2 can be up to 400 m/min in the case of the configuration of the front device 2 as a roll stand, sometimes even slightly higher.

[0063] Various devices can be arranged between the front device 2 and the rear device 3, said various devices being of secondary importance in the context of the present invention, for example a thickness measuring device. The decisive factor in the present case is that a measuring arrangement 6 is arranged between the front device 2 and the rear device 3.

[0064] The measuring arrangement 6 has a mechanical excitation device 7. By means of the mechanical excitation device 7, the metal strip 1 can be excited so as to vibrate mechanically in its thickness direction. Specifically, the metal strip 1 is shown in FIG. 1 in a solid line in a center position. The deflection from the center position is indicated in FIG. 1 by a double arrow 8. The excitation to the mechanical vibration takes place at an excitation frequency fA.

[0065] The mechanical excitation device 7 can, for example, be designed as a suction device as shown in FIG. 1. This configuration is established, robust and reliable.

[0066] For example, a suction fan 9 can extract air from the area between the metal strip 1 and the measuring arrangement 6 via suction openings 10 (see in particular FIGS. 2 and 3; only a few of the suction openings 10 are provided there with their reference character) and a suction channel 11 and thus periodically apply a negative pressure to one side of the metal strip 1. The extent to which air is extracted can be varied by directly activating the suction fan 9 and/or by activating a modulator element 12. If the modulator element 12 is activated, the modulator element 12 periodically varies the prevailing cross-section and thus the flow resistance of the suction duct 11. The modulator element 12 can, for example, be designed as an oval or elliptical element that is rotated in the suction channel 11.

[0067] As already mentioned, this configuration is well established. Detailed explanations are therefore not necessary.

[0068] In order to be able to cause the metal strip 1 to vibrate effectively, the mechanical excitation device 7 usually has a flat boundary surface 13. The flat boundary surface 13 faces the metal strip 1 and runs at a small distance (usually in the single-digit millimeter range) from the pass line. The suction openings 10 are arranged in the boundary surface 13.

[0069] The measuring arrangement 6 also has a metal plate 14 as shown in FIGS. 3 and 4. The metal plate 14 is arranged next to the boundary surface 13. The metal plate 14 has an upper side 15 that faces the metal strip 1. The metal plate 14 is offset from the boundary surface 13 so that the upper side 15 is at a greater distance from the pass line than the boundary surface 13.

[0070] A plurality of sensor elements 16 is arranged in the metal plate 14 as shown in FIGS. 1, 3 and 4. The measuring arrangement 6 thus has this plurality of sensor elements 16. For the sake of clarity, only one of the sensor elements 16 is shown in FIG. 1. In FIGS. 3 and 4, only some of the sensor elements 16 are labeled with their reference characters. It is evident that the sensor elements 16 are not flush with the upper side 15 of the metal plate 14, but protrude beyond the upper side 15 towards the metal strip 1. The measuring arrangement 6 therefore has a cover 17. The cover 17 covers the sensor elements 16 on their upper side (i.e. towards the metal strip 1). The cover 17 seals the sensor elements 16 towards the sides of the sensor elements 16. An upper side 18 of the cover 17 lies in the plane formed by the flat boundary surface 13 as shown in FIG. 2.

[0071] The cover 17 consists of an electrically insulating material. For example, the cover 17 can be made of a ceramic or a plastic. Suitable ceramics and suitable plastics, for example polyimides and polyester ester ketones (PEEK), are known to those skilled in the art.

[0072] The sensor elements 16 are arranged offset from one another as viewed in the width direction of the metal strip 1. In the specific embodiment of the present invention, the sensor elements 16 form two rows, with the corresponding sensor elements 16 being arranged next to one another within the respective row as viewed in the width direction, and the sensor elements 16 of the rows being arranged offset relative to the sensor elements 16 of the other rows as viewed in the width direction in the overall view of the rows. This configuration, i.e. with multiple rows of sensor elements 16 and rows offset from one another, is currently preferred, but is of secondary importance as a result.

[0073] It is possible by means of the sensor elements 16 to acquire for a respective area of the metal strip 1 a respective measurement signal MA that is characteristic of the amplitude A of the excited mechanical vibration of the respective area of the metal strip 1. The areas of the metal strip 1 are also offset from one another in the width direction of the metal strip 1 in accordance with the arrangement of the sensor elements 16.

[0074] The acquisition of the respective measurement signal MA is contactless. Possible configurations for this are generally known to persons skilled in the art. It is preferable that the sensor elements 16 (=assembly unit) comprise eddy current sensors as actual sensors 19 which acquire the respective measurement signal MA. In an eddy current sensorsee FIG. 5 for an individual eddy current sensoran eddy current is induced in the respective area of the metal strip 1 by means of an excitation current IA of an excitation coil 20. The extent to which the eddy current is induced can be detected. The respective (analogue) measurement signal MA is derived from this extent.

[0075] The excitation current IA has an excitation frequency f, hereinafter referred to as the operating frequency so as to distinguish it from the excitation frequency fA. The operating frequency f is usually in the range of multiple kHz, sometimes even in the single-digit MHz range. The measurement signal MA also has the operating frequency f. The respective prevailing distance of the respective area of the metal strip 1 from the measuring arrangement 6 can thus be determined from the measuring signal MA in a manner known per se. The development of this distance over time provides the amplitude A of the mechanical vibration of the corresponding area of the metal strip 1.

[0076] This procedure is generally known and familiar to persons skilled in the art. It therefore does not need to be explained in detail.

[0077] According to FIG. 6, the cover 17 has receptacles 21 on its underside, i.e. the side that faces the sensor elements 16.

[0078] When the cover 17 is mounted on the metal plate 14, the sensor elements 16 are immersed in the receptacles 21 (of course only insofar as they protrude beyond the upper side 15 of the metal plate 14).

[0079] The cover 17 also has recesses 22 on its underside. In FIG. 6, only some of the recesses 22 are provided with their reference character. The recesses 22 in their entirety form a number of flow channels for a cooling medium 23 between the metal plate 14 and the cover 17. The sensor elements 16 can thus be actively cooled by means of the cooling medium 23.

[0080] In the illustration according to FIG. 6, a single flow channel is formed. However, multiple flow channels could also be formed. The following explanations regarding the sequence of sensor elements 16 refer to the respective flow channel. In the case of multiple flow channels, the flow channels are separated from one another. The sensor elements 16 are therefore only integrated into a single flow channel in each case. In the case of multiple flow channels, the following statements are valid for each flow channel individually.

[0081] According to FIG. 6, the flow channel is designed in such a way that the sensor elements 16 are arranged sequentially one behind the other as viewed in the direction of flow of the cooling medium 23. The cooling medium 23 therefore first cools one of the sensor elements 16, then the next sensor element 16, then the next sensor element 16 and so on until all sensor elements 16 of the respective flow channel are cooled.

[0082] It can also be seen from FIG. 6 that the receptacles 21 each have an inlet 24 for the cooling medium 23 and each have an outlet 25 for the cooling medium 23. In FIG. 6, the inlets 24 and the outlets 25 are only provided with their reference characters for some of the receptacles 21 for the sake of clarity. On the one hand, it can be seen that the outlet 25 of a respective receptacle 21 is communicatively connected to the inlet 24 of the respective next receptacle 21 via a respective connecting section of the respective flow channel. The term next receptacle in this context refers to the next receptacle 21 as viewed in the direction of flow of the cooling medium 23. On the other hand, the inlet 24 and the outlet 25 of a respective receptacle 21 are arranged opposite each other as viewed from the respective sensor element 16. As a result, the respective sensor element 16 is completely surrounded by the cooling medium 23 and thus cooled.

[0083] The cooling medium 23 can be (purified) compressed air, for example. This configuration offers the additional advantage that minor leakages are not critical. This is because the compressed air has a higher pressure than the ambient air. Despite the leakage no foreign bodies can therefore penetrate into the space that is covered by the cover 17. Nevertheless, the cooling of the sensor elements 16 can be maintained if the leakage is small enough.

[0084] In some cases, it may be sufficient to cool the sensor elements 16 solely with the cooling medium 23. In other cases, it is necessary to arrange channels 26 for a cooling liquid in the metal plate 14 as shown in the schematic diagram in FIG. 7. In this case, the metal plate 14 is cooled directly by the cooling liquid. Indirectly, the sensor elements 16 are also cooled as a result.

[0085] The structure of an individual sensor element 16 is explained in more detail below in conjunction with FIGS. 8 and 9.

[0086] According to FIGS. 8 and 9, the sensor elements 16 are pre-assembled units consisting of multiple components. These components arein addition to the sensor 19, by means of which the respective measuring signal MA is acquired-a sleeve 27 and a fixing element 28. In addition, a plastic hood 29 can be applied to the sensor 19 itself on its side that later faces the metal strip 1. By means of the plastic hood 29 (if present), the sensor 16 (insofar as it protrudes beyond the metal plate 14 in the mounted state) is sealed airtight and watertight.

[0087] According to FIG. 8, the sensor 19 (possibly including the plastic hood 29) is arranged in the sleeve 27. The sleeve 27 in turn has an external thread 30. The fixing element 28 is attached to the sleeve 27. The fixing element 28 thus has a corresponding internal thread (not shown). The fixing element 28 has a collar 31 which projects radially outwards over the sleeve 27 in the radial direction.

[0088] The components, i.e. the sensor 19, the sleeve 27 and the fixing element 28, are fixed relative to each other. For example, the sensor 19 can be glued into the sleeve 27 and the fixing element 28 can be fixed to the sleeve 27 via soldering points or spot welds. When mounting the sensor element 16, the distance between the lower edge or the upper edge of the collar 31 and the upper side of the sensor 19 (or, if present, the upper side of the plastic hood 29) can be set in a defined manner. For example, the sensor 19 can be fixed in the sleeve 27 first. Before or after this, the plastic hood 29 can be placed on the sensor 19 if necessary. The distance between the lower edge or the upper edge of the collar 31 is then set. Only finally is the fixing element 28 fixed to the sleeve 27.

[0089] The metal plate 14 hassee in particular FIG. 9receptacles 32 for the sensor elements 16. The receptacles 32 in turn each have a radially inwardly projecting support ring 33. When the corresponding sensor element 16 is mounted in the corresponding receptacle 32, the collar 31 thus rests on the support ring 33.

[0090] As already explained above, the sensor elements 16 can be used to acquire a respective measurement signal MA for the areas of the metal strip 1. The acquisition is contactless, usually via eddy current sensors. For this purpose, the eddy current sensors have excitation coils 20 to which excitation currents IA of multiple kHz, sometimes even in the single-digit MHz range, are applied. The acquired measurement signals MA are initially analogue.

[0091] In the prior art, the measurement signals MA are transmitted via corresponding cables to an evaluation device 34 (see FIGS. 1 and 10) which is arranged outside the measuring arrangement 6, usually in a control cabinet. In the context of the present invention, however, digitization devices 35 are arranged inside the measuring arrangement 6. FIG. 1 shows an individual one of the digitization devices 35, FIG. 10 shows a few of the digitization devices 35. The acquired analogue measurement signals MA are digitized by means of the digitization devices 35.

[0092] As in the prior art, the evaluation device 34 is arranged outside the measuring arrangement 6, for example in a control cabinet. In the simplest case, the digitization devices 35 transmit the digitized measurement signals themselves to the evaluation device 34 as transmitted signals MA. Alternatively, the digitization devices 35 can transmit signals derived from the digitized measurement signals to the evaluation device 34 as transmitted signals MA.

[0093] The arrangement of the digitization devices 35 within the measuring arrangement 6 can be as required. For example, the digitization devices 35 can be designed as independent elements separate from the sensor elements 16 as shown in FIGS. 1, 10, 11 and 12. In this case, the digitization devices 35 are connected to the sensor elements 16 within the measuring arrangement 6 via cables 36. With regard to their arrangement, the digitization devices 35 can be arranged below the metal plate 14 in particular. The cables 36 are generally very short (usually a maximum of 100 cm, often only 50 cm or less).

[0094] The digitization devices 35 are shown in FIGS. 11 and 12see also FIG. 4as cigar-like elements. The word cigar-like in this context refers both to the basic shape (an elongated cylinder) and to the absolute dimensions (length approx. 15 cm to 30 cm, diameter approx. 1.0 cm to 3.0 cm). This form of the digitization devices 35 is currently preferred, but in no way mandatory.

[0095] The cables 36 are generally detachably connected to the digitization devices 35, for example via a screw connection or a bayonet-type connection. At the transition to the sensor elements 16, the cables 36 are preferably hermetically sealed (i.e. airtight and watertight). The seal can be made, for example, as is known for the spark plug connectors of motor vehicle engines, by means of rubber-elastic sleeves which are slidably arranged on the corresponding cable 36. Alternativelyand this is currently preferredthe cables 36 can be non-detachably connected to their respective sensor element 16.

[0096] With regard to the connection of the digitization devices 35 to the evaluation device 34, it is possible in principle to establish the connection via individual corresponding connection cables, as is done for the transmission of the analogue measurement signals MA in the prior art. However, it is preferable if the transmitted signals MA are transmitted to the evaluation device 34 via a common armored cable 37 with pre-assembled connections 38 as shown in FIG. 13. This is explained in more detail below.

[0097] According to FIG. 13, individual lines or thin cables 39 run from the digitization devices 35 to a pre-assembled plug connection 40. One of the pre-assembled connections 38 of the armored cable 37, which is arranged at one end of the armored cable 37, can be plugged onto the pre-assembled plug connection 40. Within the armored cable 37, lines 41 that correspond to the individual lines or thin cables 39 are routed to the other pre-assembled connection 38, which is arranged at the other end of the armored cable 37. This pre-assembled connection 38 is connected to a further plug connection 42, from which lines are routed to the evaluation device 34.

[0098] The armoring 43 of the armored cable 37 can, for example, correspond to that which is usual for hydraulic lines whose hydraulic fluid is under a pressure in the range from 100 bar to 500 bar.

[0099] The dashed line L in FIG. 13 is intended to indicate the boundary of the measuring arrangement 6. As shown in FIG. 13, the armored cable 37 can therefore be connected from outside the measuring arrangement 6 without having to open the measuring arrangement 6. Alternatively, it is of course also possible to arrange the pre-assembled plug connection 40 inside the measuring arrangement 6.

[0100] FIG. 13 also shows another preferred embodiment. In this embodiment, the armored cable 37 comprises in each case dedicated lines 41 for the transmitted signals MA of groups of multiple sensor elements 16 in each case. Specifically, the digitized measurement signals of three sensor elements 16 are combined in the digitization devices 35 in each case. The number three is of secondary importance in this context.

[0101] FIG. 14 shows a schematic plan view of the sensor elements 16, i.e. as viewed from the metal strip 1. Reference characters f1, f2 and f3 are drawn in the sensor elements 16. f1, f2 and f3 are also operating frequenciesanalogous to the operating frequency f. However, they differ from one another in pairs. For example, the operating frequency f1 can have the value 280 kHz, while the operating frequency f2 has the value 300 kHz and the operating frequency f3 has the value 320 kHz. The values mentioned are, of course, purely exemplary.

[0102] It is evident that the eddy current sensors of sensor elements 16, which are arranged directly adjacent to each other in the metal plate 14, are operated at mutually different operating frequencies f1, f2, f3. This allows any crosstalk behavior to be significantly reduced.

[0103] In the case of mutually different operating frequencies f1, f2, f3, it is also possible, as shown in FIGS. 13 and 14, that the number of sensor elements 16 whose digitized measurement signals are transmitted via an individual one of the lines 41 is equal to the number of operating frequencies f1, f2, f3. In this case, the sensor elements 16 can in particular be grouped in such a way that each operating frequency f1, f2, f3 is represented once in the groups. In FIG. 14, the corresponding groups are framed in dashed lines.

[0104] In the case of mutually different operating frequencies f1, f2, 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 is characteristic of the operating frequency f1, f2, f3 of the respective sensor element 16. The coding 44 can be of an optical nature. For example, different colors can be used for the different operating frequencies f1, f2, f3, for example red, green and blue or yellow, red and blue. The coding 44 can also be of a haptic nature. For example, the operating frequency f1 can be coded by a circumferential ring on the cable 36, while the operating frequencies f2 and f3 can be coded by two circumferential rings on the cable 36, wherein the distinction between the operating frequency f2 and the operating frequency f3 can be made by the distance between the two rings. A further operating frequency f could, for example, be coded by three such rings. Mechanical coding is also possible in a similar way.

[0105] Preferably, the digitization devices 15 also have a corresponding coding 45, so that the correct assignment is also readily apparent.

[0106] As shown in FIG. 15, the evaluation device 34 receives the transmitted (digital) signals MA from the digitization devices 35. As part of an evaluation of the transmitted signals MA, it determines the amplitude A of the mechanical vibration of the metal strip 1 for the respective area of the latter.

[0107] In order to determine the amplitudes A, the evaluation device 34 first performs a linearization of the transmitted signals MA in a linearization block 46. The linearization block 46 thus outputs modified signals MA, the respective value of which is proportional to the corresponding deflection of the respective area of the metal strip 1 at the time at which the corresponding (analogue) measurement signal MA was acquired. As part of the linearization process, the evaluation device 34 evaluates a characteristic curve K. The characteristic curve K is determined by the evaluation device 34 specifically for the metal strip 1. It can be determined, for example, as a function of geometric properties G and/or chemical properties C and/or thermodynamic properties T (for example temperature) and/or the history H of the metal strip 1. If necessary, an operating temperature T of the sensor elements 16 can also be taken into account when determining the characteristic curve K.

[0108] In order to determine the characteristic curve K, for example, the associated characteristic curves K can be stored in a determination device 47 for specific values of the geometric properties G, the chemical properties C, etc., so that the characteristic curve K actually utilized can be determined by selection and/or interpolation.

[0109] The modified signals MA are fed to a determination block 48 within the evaluation device 34. In the determination block 48, the evaluation device 34 determines the respective amplitude A of the excited mechanical vibration of the metal strip 1 for the areas of the metal strip 1. In the context of determining the amplitudes A, the evaluation device 34 preferably uses a Goertzel algorithm as shown in FIG. 15. It is particularly preferred that the evaluation device 34 takes into account the excitation frequency fA within the Goertzel algorithm.

[0110] The determined amplitudes A can be fed to a further determination block 49. In the determination block 49, the evaluation device 34 uses the amplitudes A to determine a flatness error PF for each of the areas of the metal strip 1. The determination of the flatness errors PF as such is no longer the object of the present invention. The evaluation device 34 can, for example, output the determined flatness errors PF to a control device (not shown) for the front device 2, so that the control device can activate flatness actuators of the front device 2 in such a way that the flatness errors PF are eliminated as far as possible.

[0111] The present invention has many advantages. The use of the cover 17 improves the sensitivity of the sensors 19. The replacement of the sensor elements 16 is significantly simplified. By configuring the sensor elements 16 as pre-assembled units, the positioning of the sensor elements 16 in the metal plate 14 can also be ensured reliably and precisely. This also applies to the subsequent replacement of a defective sensor element 16 with a new sensor element 16. The configuration of the sensor elements 16 as pre-assembled units also reduces the protection of the sensors 19 against moisture, dirt and, within limits, against high heat input. This improves the durability of the sensor elements 16. The cooling of the sensor elements 16 by means of the cooling medium 23 also provides improved protection against dirt and moisture. Crosstalk can be largely eliminated by using multiple operating frequencies f1, f2, f3. Thanks to the very early digitization of the measurement signals MA within the measuring arrangement 6, the measurement signals MA can be converted very quickly into a form that is immune to interference. This enables, among other things, an increase in the analyzable measuring range. Evaluation in conjunction with characteristic curves K, which are specific to the metal strip 1, also enables improved evaluation and evaluation in an enlarged measuring range. By using the characteristic curve K, an evaluation of the measurement signals MA is possible that is optimized for the respective metal strip 1. The evaluation using a Goertzel algorithm provides superior results with reduced computational effort.

[0112] Although the invention has been illustrated and described in detail by the preferred exemplary embodiment, the invention is not limited by the disclosed examples and other variants can be derived by the person skilled in the art without departing from the scope of protection of the invention.

TABLE-US-00001 List of reference characters 1 Metal strip 2 Front device 3 Rear device 4 Reel 5 Deflection pulley 6 Measuring arrangement 7 Mechanical excitation device 8 Double arrow 9 Suction fan 10 Suction openings 11 Suction channel 12 Modulator element 13 Boundary surface 14 Metal plate 15, 18 Upper side 16 Sensor elements 17 Cover 19 Sensors 20 Excitation coil 21, 32 Receptacles 22 Recesses 23 Cooling medium 24 Inlets 25 Outlets 26 Channels 27 Sleeve 28 Fixing element 29 Plastic hood 30 External thread 31 Collar 33 Support ring 34 Evaluation device 35 Digitization devices 36 Cables 37 Armored cable 38 Pre-assembled connections 39 Wires or thin cables 40, 42 Plug connections 41 Lines of the armored cable 43 Armoring 44, 45 Coding 46 Linearization block 47 Determination device 48, 49 Determination blocks A Amplitudes C Chemical properties f, f1 to f3 Operating frequencies fA Excitation frequency G Geometric properties H History IA Excitation current K Characteristic curves L Line MA Analogue measurement signal MA Transmitted signals MA Modified signals PE Flatness error T Thermodynamic properties T Operating temperature x Conveying direction