EP3475073B1 - Wood material panel pressing device and method for monitoring a wood material panel pressing device - Google Patents

Description

The invention relates to a wood-based panel pressing device for pressing a fiber cake to a wooden board, with a control device which is designed to output a signal in the event of a fault and / or to monitor the fiber cake in the lead-in area. The second aspect relates to the invention a method for monitoring a wood-based panel pressing device for pressing a fiber cake to a wood-based panel according to claim 9.

Wood-based panel pressing devices are used to compact fiber cakes or to compress them to wooden-based panels. The invention relates in particular to continuously operating wood-based panel pressing devices in which the fiber cake is continuously scattered and pressed and / or pressed. It is desirable to operate such a wood-based panel pressing device at the highest possible speed. However, high feed speeds at which the fiber cake is moved through the wood-based panel pressing device are more likely to lead to material defects. For example, the fiber cake has to be compressed during pressing. But this presupposes that air is pressed out of the fiber cake. If this happens too quickly, the escaping air can tear fibers of the fiber cake with it, which leads to crater-like breakouts. Outbreaks of this type, which are also called blowers, generally lead to the fact that the wood-based panel produced cannot be used in the area of the outbreak, so that rejects occur.

Another source of error is a fluctuation in the fiber cake beyond a predetermined tolerance.

From the DE 196 22 712 B4 , of the DE 10 2007 019 390 A1 and the DE 10 2005 049 880 A1 is known to have thin metal wires looped over the fiber cake, which make electrical contact with a press plate of the one pressing device when the thickness of the fiber cake exceeds a predetermined maximum thickness. The disadvantage of this system is that only binary monitoring is possible, namely whether the maximum thickness has been exceeded. Another disadvantage is that breakouts as described above are only detected if they occur directly under a wire. DE 10 2005 049880 A1 further discloses the possibility of using a camera to monitor the fiber cake to be pressed.

From the WO 2009/071738 A1 is known a press system for gluing wooden sticks to glued wood panels. In order to ensure an even bond, a thermographic image is taken by means of a thermal camera after the bond. If the temperature of the finished glued wood panels deviates too much from a specified temperature, the heating output of the press system is corrected. For a particularly accurate measurement result, the temperature of the arranged wooden sticks can also be measured using a thermal camera before gluing. In this way it can be determined where the press system has too little local heating power. Such a press system is unsuitable for pressing fiber cakes into wood-based panels, since a fiber cake requires high surface pressure and cannot withstand lateral pressure. Furthermore, with such a system, only the (local) heating output can be controlled or regulated; it is unsuitable for avoiding blowouts.

The invention has for its object to reduce rejects.

The invention solves the problem by a generic wood-based panel pressing device, in which the control device has a camera and an evaluation unit, and the camera is arranged on an inlet area of the wooden-based panel pressing device and the evaluation unit is designed to automatically carry out a method with the steps (i ) Continuous recording of images of the lead-in area, (ii) Continuous acquisition of measurement data for evaluation area pixels from images which belong to a predetermined evaluation area, which comprises at least one area which adjoins a desired horizon of the fiber cake upwards, so that Evaluation data are obtained, and (iii) output a signal when the evaluation data changes by more than a predetermined tolerance value.

According to the invention, there is also a wood-based panel pressing device for pressing a fiber cake for producing a wooden board, with (a) a control device which is designed to output a signal in the event of a fault, (b) the control device having a camera and an evaluation unit, (c ) the camera is arranged on an entry area of the wood-based panel pressing device and (d) the evaluation unit is designed to automatically carry out a method with the steps: (i) continuously taking pictures of the entry area, (ii) continuously acquiring measurement data for evaluation area pixels of images belonging to a predetermined evaluation area, which comprises at least one area which adjoins a desired horizon of the fiber cake upwards, so that evaluation data are obtained, and (iii) emitting a signal when the evaluation data is more than change a specified tolerance value. The preferred embodiments described below relate to both aspects of the invention mentioned above.

According to a second aspect, the invention solves the problem by a method for monitoring a wood-based panel pressing device for pressing a fiber cake to a wooden-based panel, with the above-mentioned steps.

An advantage of the invention is that monitoring is possible in real time. In other words, a time period between the occurrence of an error and the detection of the error is very small and can be, for example, less than one second. This enables a quick response so that, for example, process parameters can be changed so that the error is avoided in the future.

Another advantage is that this monitoring is possible with very little effort. The necessary components such as the camera and the evaluation unit in the form of evaluation electronics are standard products that can be easily obtained.

The fact that the process monitoring by means of a camera works at all was not considered realistic because such wood-based panel pressing devices can be extremely dusty. It was therefore to be feared that chips would lead to false alarms and / or to rapid contamination of the camera. In addition, spray devices generally exist on the inlet side of wood-based panel pressing devices, by means of which a liquid, for example water containing surfactant, can be sprayed onto the fiber cake. Positioning the camera so that it can serve its purpose usually requires a modification of these devices.

The camera can be an infrared camera or a camera that captures visible light.

The camera has an optical axis which forms an angle of incidence of preferably at most 70 °, in particular at most 60 °, preferably at most 50 °, very particularly preferably at most 40 °, with the material flow direction. The angle of attack is preferably at least 0 °, in particular at least 5 °, preferably at least 10 °.

In the context of the present description, a wood-based panel pressing device is understood to mean, for example, a device by means of which the fiber cake can be pressed continuously to form a wooden board. For this purpose, the wood-based panel pressing device preferably has a rotating pressing plate which is heated and pressed onto the fiber cake. The wood-based panel is preferably an HDF, an MDF or an OSB board. Alternatively, the wood-based panel pressing device is a pre-press, which is part of a plant for the production of wood-based panels, and has a pressing element in the form of a pressing blanket.

The control device is understood in particular to be a device which allows a plant operator to record the condition of the wood-based panel pressing device. It is possible, but not necessary, for the control device to be connected to a machine control so that the signal leads to a change in at least one machine parameter without human intervention, that is to say directly. A machine parameter is understood to be a parameter that determines the production conditions of the wood-based panel press device, for example press pressure, temperature or inlet angle. The entry angle is the angle at which a pressing element is oriented on the entry side relative to the horizontal immediately before the pressing element contacts and compresses the fiber cake.

Continuous recording of the images of the lead-in area means in particular that images are recorded at regular time intervals. It is possible and preferred that the images are taken at equidistant time intervals, but this is not necessary. In particular, an image is recorded at least every three seconds, preferably at least once per second. It is particularly favorable if an image is taken several times per second. Cameras usually have a frame rate of more than 10 frames per second. It is possible, but usually not necessary, that more than 20 pictures are taken per second.

The evaluation area is understood to mean an area of the wood-based panel pressing device, in particular the section of the pressing element, which at a given time is at a predetermined distance from the target horizon of the fiber cake. The target horizon of the fiber cake is the imaginary line that describes the top of the fiber cake in the field of view of the camera at the point where the top comes into contact with the press plate. Under ideal production conditions, the target horizon is a straight line, in particular a horizontal one, which does not change its position.

The measurement data are in particular the data that the camera captures for each pixel in each acquisition cycle. The measurement data are usually voltages or electrical currents encoding a color and a brightness of the light falling on the corresponding pixel.

The output of a signal is understood to mean, in particular, that a human-perceptible or imperceptible sign is emitted, for example a change in voltage or a digitally coded signal. Of course, it is possible for signals to be emitted continuously, even if the evaluation data does not change by more than a predetermined tolerance value. The only decisive factor is that if the deviation is greater than the tolerance value, a signal is emitted or an otherwise emitted signal which indicates a fault-free state is omitted.

It is possible, but not necessary, for this signal to be forwarded to a machine control which automatically changes at least one production parameter. Rather, it is also possible for the signal for a machine operator to be displayed optically and / or acoustically, so that the operator can take suitable measures.

It is possible, but not necessary, that additional pixels that are not evaluation area pixels are evaluated. For example, the control device can be designed such that it continuously detects a background brightness, so that fluctuations in the brightness, which are caused, for example, by changing light conditions, do not lead to a false alarm.

The feature that the evaluation data changes by more than the predetermined tolerance value is also understood in particular to mean that data or quantities derived from the evaluation data, for example changes over time (i.e. the first time derivative) or changes in changes (i.e. the second temporal one) Derivation) or other quantities or size curves calculated from the measurement data are recorded or compared and compared with the predetermined tolerance value.

The pressing element is understood in particular to mean a pressing plate or a pressing cloth. A press plate is in particular a circumferential plate, by means of which the fiber cake is pressed into a wood-based panel. A press cloth is understood in particular to mean an air-permeable element which is part of a pre-press and serves to push air out of the fiber cake. The press cloth can be a textile. However, it is also possible for the press cloth to be another air-permeable element, for example a wire mesh or a perforated plate.

According to a preferred embodiment, the measurement data are recorded for a large number of regions which adjoin one another and together cover a full width of the fiber cake. In other words, the fiber cake is captured over a full width by the camera and the corresponding measurement data is evaluated.

The wood-based panel pressing device preferably has a circumferential pressing element, which can also be referred to as a press plate, if the wooden-based panel pressing device is a hot press, the control device being designed to automatically carry out a method which includes the step of continuously collecting measurement data for reference area pixels comprises, wherein the reference area pixels belong to at least one predetermined reference area and each evaluation area is assigned exactly one reference area which lies in front of the evaluation area with respect to a pressing element movement direction.

The continuous acquisition of measurement data for evaluation area pixels then preferably includes calculating a deviation between the measurement data of the evaluation area pixels and the measurement data of the reference area pixels shifted by a time delay, the time delay being the time that a section of the pressing element from the reference area to Evaluation area required. This has the advantage that any inhomogeneities in the pressing element, in particular the pressing plate, do not lead to false alarms. The background is that the press plate in the event of damage, is often repaired by cutting out the damaged area. A suitable piece of sheet metal is inserted into the resulting hole. In particular, at the joints between the sheet metal used and the surroundings, color changes can occur, which in turn can trigger false alarms. This is avoided by the specified process steps. According to the invention is also a method according to the invention, which has these steps.

According to a preferred embodiment, the wood-based panel pressing device is a hot press, that is to say a press, on the outlet side of which a dimensionally stable wood-based panel runs out. A dimensionally stable wood-based panel is understood to mean a wood-based panel that bears its own weight when placed on a side edge. A hot press is understood in particular to be a press which introduces heat into the fiber cake due to heat conduction, and in particular free of microwaves.

Alternatively, the wood-based panel pressing device is a pre-press, on the outlet side of which a fiber cake that is not dimensionally stable runs out.

According to the invention is also a wood-based panel pressing system that has at least one wood-based panel pressing device. Preferably, one of these wood-based panel pressing devices is a pre-press, which has the above-mentioned control device, a second wood-based panel pressing device in the form of a hot press for pressing the fiber cake flowing out of the pre-press to a wooden board is arranged behind this pre-press.

The wood-based panel press device preferably has an illumination device for illuminating the inlet area. This has the advantage that any external fluctuations in the lighting conditions in an environment of the inlet area lead to smaller percentage fluctuations in the measurement results.

The control device is preferably designed to automatically carry out a method according to the invention. This is understood in particular to mean that the control device carries out the specified steps even without operator intervention.

The method preferably comprises the steps (i) grouping evaluation area pixels into a first area and at least a second area and (ii) for all areas detecting those pixels in which there is a property, in particular the color, the brightness and / or the Contrast with at least one neighboring pixel, changes more than a threshold value, and (iii) calculate a change parameter from the number of these pixels and compare the change parameter with the tolerance value.

It is possible, but not necessary, that the change in the property is calculated from measurement results that follow one another in time. It is also possible that averaging over two, three or more measured values that follow one another in time or that a measured value processed in another way is calculated from these measurement results.

In an ideal process, the measurement data of the pixels do not change over time. In a trouble-free operation, the measurement data fluctuate, for example due to statistical measurement errors or randomly changing environmental conditions. If a fault occurs, for example an outbreak of a gas bubble described above, the property changes more rapidly in time.

The threshold value is determined by the fact that the temporal fluctuation of the property is recorded in trouble-free operation. The threshold value is selected, for example, such that it is exceeded at most once per minute and pixel in the trouble-free operation due to random fluctuation.

The tolerance value for the change parameter is determined in that the change parameter is initially in trouble-free operation for a predetermined time of an hour, for example. Then the feed speed and / or the inlet angle α is increased until the above-mentioned breakouts or blowouts occur, which lead to rejects. The change parameter is determined at the same time. The tolerance value is chosen so that it lies below the change value at which rejects occur.

The evaluation area pixels are preferably grouped into at least 20 areas. It is favorable if these areas are adjacent to one another. This means that there are no pixels or so few pixels between two areas that blow-out can still be reliably recognized. It is particularly favorable if the areas are directly adjacent to one another, since the highest detection probability of errors is achieved in this way.

According to a preferred embodiment, the output of the signal causes a change in the entry angle, the entry angle being the angle between the horizontal plane and a straight line through the pressing element. Alternatively or additionally, the output of the signal causes the feed rate to be reduced. In order to achieve this, the control device can be connected directly to a machine control. For example, the feed rate is reduced between 3% and 5% if the tolerance value is exceeded.

According to a preferred embodiment, the method comprises the steps of calculating an actual horizon of the fiber cake and outputting a signal if the actual horizon deviates from the target horizon by more than a level error threshold value. If the actual horizon exceeds the target horizon by more than one alarm value, the wood-based panel pressing device is preferably shut down automatically in order to avoid damage to the press.

Figur 1

eine schematische Ansicht einer erfindungsgemäßen Holzwerkstoffplatten-Pressvorrichtung,

Figur 2a

eine perspektivische Detailansicht des Einlaufbereichs der Holzwerkstoffplatten-Pressvorrichtung gemäß Figur 1 und

Figur 2b

ein Bild des Einlaufbereichs, in dem die Gebiete, die zur Auswertung verwendet werden, eingezeichnet sind.

Figur 3

zeigt eine alternative Ausführungsform einer erfindungsgemäßen Holzwerkstoffplatten-Pressvorrichtung, die eine Vorpresse aufweist.

The invention is explained in more detail below with reference to the accompanying drawings. It shows

Figure 1

2 shows a schematic view of a wood-based panel pressing device according to the invention,

Figure 2a

a detailed perspective view of the inlet area of the wood-based panel pressing device according to Figure 1 and

Figure 2b

an image of the inlet area, in which the areas that are used for evaluation are drawn.

Figure 3

shows an alternative embodiment of a wood-based panel pressing device according to the invention, which has a pre-press.

Figure 1 shows a wood-based panel pressing device 10 according to the invention in the form of a hot press for pressing a fiber cake 12 into a wood-based panel 14. The wood-based panel pressing device has a spreading device 16 for spreading the fiber cake 12 onto a conveyor 18.

The wood-based panel pressing device 10 has a control device 20 which has a camera 22 and an evaluation unit 24 connected to it. An optical axis A is oriented at an axis angle α to the horizontal H, which is generally between 0 ° and 45 ° and in the present case is α = 10 °. A positive axis angle means that the camera is looking down.

The wood-based panel pressing device 10 has a pressing element 26 in the form of a pressing plate which forms a closed loop and is pressed onto the fiber cake 12 by means of rollers 28.1, 28.2, ... with a pressing force F. Via a schematically drawn heater 29, the pressing element 26 is also heated to a predetermined temperature T ₂₆ by means of hot thermal oil. In the present case, the wood-based panel pressing device 10 also has a second pressing element 26 ', which is pressed against the fiber cake 12 from below via rollers 28'.1, 28'.2, ... The second pressing element 26 'is also heated by heating elements 29'.

The pressing element 26 runs in the area in which it first comes into contact with the fiber cake 10 at an entry angle ε to the horizontal H. The entry angle ε is adjustable, as is indicated by the dashed-drawn press plate.

The camera 22 records images of an inlet area 30 in which the fiber cake 12 comes into contact with the pressing element 26 for the first time. In the interior of the fiber cake has a 12 actual horizon at a height _h, the in the in Figure 1 situation shown a desired height h _should correspond. A level error Δh lies below a level error threshold value Δh _S.

In operation, the fiber cake 12 moves in a material flow direction M at a feed rate v. A trimming system 32 and / or further processing devices, for example a digital printing device for printing on the wood-based board 14, is arranged in the material flow direction 12 behind the wood-based board pressing device.

Figure 2a shows a perspective view of the inlet area 30 of the wood-based panel pressing device. The fiber cake 12, the pressing element 26, an illumination device 34, in the present case in the form of an LED lamp, the camera 22 and a moistening system 36 can be seen. The moistening system 36 can be used to spray a liquid mist, in particular from water containing tenside, onto the fiber cake 12 be sprayed on.

Figure 2b shows an image B of the inlet area 30, which was recorded with the camera 22. An actual horizon H _{ist of} the fiber cake 12 lying at the height hist can be seen in the lower part of the figure. Is located also an evaluation portion 37, the upward _istsoll lying at the height h on the target horizon H _to adjacent the fiber cake 12, wherein in the present case _is h = h _to apply.

The evaluation area 37 is divided into N areas G _i (i = 1, 2, ..., N), each of which has a plurality of pixels P. The areas G _i could also be called clusters.

Each region G _i relates to an image of a region B _{i of} the evaluation region 37. In other words, regions denote sections of the wood-based panel pressing device, for example the pressing element 26, whereas the regions G _{i are} groups of pixels P _{n, m} , n and m are the counting indices of the pixels of the camera 22.

A measured value P _{n, m} (t _j ) is recorded for each pixel P _{n, m} at regular time intervals tj (j = 1, 2, ...), in particular the brightness b _{n, m} (t _j ) and the color f _{n, m} (t _j ). In addition, the brightness contrast, for example K _{n, m} (t _j ) = b _{n, m} (t _j ) - b _{n-1, m} (t _j ) and the color contrast k _{n, m} (t _j ) = f _{n, m} (t _j ) - f _{n-1, m} (t _j ) calculated. Alternatively, the brightness contrast can be defined differently, for example K _{n, m} ( _t j) = b _{n, m} (t _j ) - b _{n + 1, m} (t _j ) or K _{n, m} (t _j ) = b _{n , m} (t _j ) - b _{n, m-1} (t _j ) or K _{n, m} (t _j ) = b _{n, m} (t _j ) - b _{n, m + 1} (t _j ) or an average these values. The same applies to the color contrast k _{n, m} (t _j ). The measurement data of the evaluation area pixels are called evaluation data, since preferably only they participate in the evaluation.

For the evaluation data _{n b, m} (t _j), f _{n, m} (t _j), K _{n, m} (t _j) and k _{n, m} (t _j) determines the evaluation unit 24 at each time t _j and for each Area i a change parameter D _i , whether they are each greater than a predetermined threshold. It is therefore checked for all n and m for which the pixel P _{n, m} lies in the area i whether b _{n, m} (t _j )> b _max , f _{n, m} (t _j )> f _max , K _{n, m} (t _j )> K _max and k _{n, m} (t _j )> k _max applies. For each area G _i, the pixels are counted for which at least one of the conditions is met; the corresponding number is the change parameter D. If the change parameter D _{i exceeds} a tolerance value D _T for at least one area G _i , a signal is output.

Alternatively, the temporal changes b (t _j ) - b (t _j-1 ), f (t _j ) - f (t _j-1 ), K (t _j ) - K (t _j-1 ), k ( t _j ) - k (t _j-1 ) is determined by the evaluation unit 24 at every time t _j and for each area i a change parameter D _i $$D_i\left(t_j\right)={\displaystyle {\sum }_{n,m\mathit{With}{P}_{n,m}\in G_i,}\left(\mathrm{\alpha }{b}_{\mathrm{n},\mathrm{m}}\left(\mathrm{tj}\right)+{\mathit{\beta f}}_{\mathrm{n},\mathrm{m}}\left(\mathrm{tj}\right)+{\mathit{\gamma K}}_{\mathrm{n},\mathrm{m}}\left(\mathrm{tj}\right)+{\mathit{\delta k}}_{\mathrm{n},\mathrm{m}}\left(\mathrm{tj}\right)\right).}$$

The parameters α, β , y, δ are real numbers that are determined in preliminary tests. If the change parameter D _i exceeds a tolerance value D _T, ie D _i > D _T in at least one area Gi, a signal is emitted.

Figure 2b also shows that reference areas R _i are recorded in image B. The reference areas R _i are preferably as many pixels wide as the areas G _i . The pressing element 26 moves at the feed speed v. In order to keep external influences on the measurement result as small as possible, the relative brightness b̃ , which indicates the difference from the same area on the pressing element 26 when passing through the reference area R _i , is preferably used before the absolute brightness B.

Alternatively, the change parameter D is calculated as described above, but before the comparison with the tolerance value Tt, a reference change parameter D _{R is} subtracted, which is calculated from the corresponding pixels of the reference area R _i at a time when the corresponding area of the pressing element 26 has passed the reference area R _i .

Figure 3 shows a second embodiment of a wood-based panel pressing system 42, in which a pre-press 40 is arranged in front of a hot press 38, as described above, which compresses the fiber cake 12 before it enters the hot press 38. The pressing element 26 of the pre-press 40 is designed as a pressing cloth. <b> List of reference symbols </b> 10th Wood-based panel pressing device D _r Tolerance value 12th Fiber cake D Change parameters 14 Wood-based panel F Pressing force 16 Spreader f colour 18th Sponsor G _i Territory H horizontal 20th Control device h _is Actual height 22 camera h _should Target height 24th Evaluation unit Δh Level error 26 Pressing element Δh _S Level error threshold 28 roller H horizon 29 Heating elements i Running index i = 1.2, .. N j Counting index 30th Infeed area K Brightness contrast 32 Trimming system k Color contrast 34 Lighting device M Material flow direction 36 Lighting system 37 Evaluation area N Number of areas 38 Hot press P _n , _m (t _j ) Reading 40 Pre-press R Reference range t time A optical axis v Feed rate B picture b brightness α Axis angle D _R Reference change parameters β Entry angle

Claims (14)

1. A wood material panel pressing device (10) for pressing a fibrous press cake (12) for producing a wood material panel (14), comprising:

   (a) an inspection device (20), which is designed to emit a signal in the event of a disruption, wherein:

   (b) the inspection device (20) has a camera (22) and an evaluation unit (24),

   (c) the camera (22) is arranged in an intake region (30) of the wood material panel pressing device (10); and characterized by that

   (d) the evaluation unit (24) is designed to automatically carry out:

   (i) continuous recording of images (B) of the intake region (30),

   (ii) continuous detection of measurement data for evaluation region pixels of images (B), which belong to a predefined evaluation region comprising at least one region bordering a target horizontal of the fibrous press cake (12) at a top, such that evaluation data is obtained, and

   (iii) emission of a signal if the evaluation data change by more than a predetermined tolerance value (D_T).
2. The wood material panel pressing device according to claim 1, further comprising a spreading device (16) for spreading the fibrous press cake (12) onto a conveyor (18).
3. The wood material panel pressing device according to one of the previous claims, further comprising:

   - measurement data is recorded for a number of regions (B_i),

   - the regions (B_i) border one another and together cover a full width (B) of the fibrous press cake (12).
4. The wood material panel pressing device according to one of the previous claims, further comprising:

   (a) a circulating press element (26) for pressing the fibrous press cake (12),

   (b) wherein continuous recording of measurement data for reference region pixels which belong to at least one predetermined reference region (R_i), wherein precisely one reference region (R) is allocated to each evaluation region, said reference region being located in front of the evaluation region in terms of a direction of movement of the circulating press element, and

   (c) wherein the continuous recording of measurement data for evaluation region pixels, which belong to at least one predetermined evaluation region, comprises a calculation of a deviation between:

   - the measurement data of the evaluation region pixels; and

   - the measurement data of the reference region pixels that have been shifted by a time delay, and

   wherein the time delay is the time needed for a section of the circulating press element to pass from the reference region to the evaluation region.
5. The wood material panel pressing device according to one of the previous claims, characterized in that

   - the wood material panel pressing device (10) is a hot press (38) and
   the press element (26) is a press plate or

   - the wood material panel pressing device (10) is a pre-press and
   the press element (26) is a press cloth.
6. The wood material panel pressing device according to one of the previous claims, further comprising a lighting device (34) for illuminating the intake region (30).
7. The wood material panel pressing device according to one of the previous claims, wherein the inspection device (20) is designed to automatically carry out a method according to one of the Claims 9 to 14.
8. The wood material panel pressing device according to one of the previous claims, further comprising an optical axis (A) of the camera forms an axis angle (α) with a horizontal (H) that lies between 0° and 20°.
9. A method for monitoring a wood material panel pressing device for pressing a fibrous press cake (12) to produce a wood material panel (14) comprising:

   (i) continuous recording of images (B) of the intake region (30),

   (ii) continuous detection of measurement data for evaluation region pixels of images, which belong to a predefined evaluation region that borders a target horizontal of the fibrous press cake (12) at the top, such that evaluation data is obtained, and

   (iii) emission of a warning signal if the evaluation data change by more than a predetermined tolerance value (D_T).
10. The method according to claim 9, further comprising:

    (i) grouping of evaluation pixels into a first region (G₁) and at least one second region (G₂), and

    (ii) for all regions (G_i):

    - detection of the pixels which feature one property that changes more drastically than a threshold value,

    - calculation of a change parameter (D) from the number of these pixels and

    - comparison of this change parameter (D) with the tolerance value (D_T).
11. The method according to claim 9 or 10, wherein the evaluation region pixels are grouped into at least 20 regions (G).
12. The method according to one of the claims 9 to 11,

    - wherein the emission of the warning signal effects an adjustment of an intake angle, and

    - wherein the intake angle (β) is the angle between the horizontal plane and the press element (26) of the wood material panel pressing device (10).
13. The method according to one of the claims 9 to 12, wherein the emission of the warning signal effects an adjustment of a feed speed (v).
14. The method according to one of the claims 9 to 13, further comprising:

    (i) calculation of a position of the actual horizontal of the fibrous press cake, and

    (ii) emission of a signal if the actual horizontal deviates from the target horizontal by more than one level error threshold value (Δh_S).

Images