The invention relates to a sheet processing machine with a device for monitoring sheets.

Sheet processing machines, also known as converting machines, are used in the packaging industry for processing raw materials, e.g. cardboard, paper or foils, into intermediate or finished products, typically in the form of sheets. Converting operations can e.g. include printing, cutting, creasing, blanking, stamping and/or folding-gluing. Typically, the individual operations are done in subsequent processing stations of the sheet processing machine with the sheets being conveyed from one processing station to the subsequent one by a transfer mechanism. The sheets can be collected in vertical stacks after converting in a designated piling area of the sheet processing machine.

Modern sheet processing machines enable a high throughput of sheets. However, not only the processing speed but also the quality of the processing operations are relevant parameters in the packaging industry. E.g., blanking operations involve breaking nicks in sheets by blanking tools, wherein the nicks have been formed in a previous processing step. If the alignment of the blanking tools is not good enough and/or the nicks have not been formed in a sufficiently precise manner, blanks pushed out of the sheets can show distorted shapes. In addition, it is possible that the blank is not pushed out of the sheet at all. Therefore, there is a need for a means for checking the state of operation of a sheet processing machine.

WO 2018/175644, discloses a method of measuring a characteristic of a sheet, e.g. its dimension, position or orientation, by means of a plurality of sheet sensors.

EP 2886498 A2 shows a sheet processing apparatus comprising an inspection section to inspect a sheet and a cutting section to cut the sheet according to a result of an inspection of the sheet in the inspection section. The apparatus can comprise a multitude of optical sensors and an array-shaped optical sensor. If the number of sheets detected in a detection section does not coincide with a number of cut sheets, any obtained count value is assumed to be “abnormal” and the processing is to be stopped.

US 2019/0144224 A1, a converting machine for processing sheets, is configured to measure a front position of register marks printed on the sheets which are used to detect the alignment of a front transverse edge. Further, an additional “pre-correction sensor”, is used to detect the passage of the front transverse edge. The alignment information is then used to control an actuator module for moving gripping elements of a gripper bar to find an optimized gripping position such that placement errors are compensated for.

The object of the invention is to provide a means for monitoring sheets in a sheet processing machine.

The object of the invention is solved by a sheet processing machine with a device for monitoring sheets and a transfer mechanism for moving sheets along a handling direction within the sheet processing machine. The device comprises a light emitting element forming a light barrier in a sheet passage between the light emitting element and the light receiving element. A control unit is connected to the light receiving element and is adapted to register each sheet passing the sheet passage.

The invention is based on the idea that a sheet passing the sheet passage will block or at least attenuate the light emitted by the light emitting element, i.e. the sheet interrupts or attenuates the light barrier. This interruption or attenuation can be detected by the light receiving element such that the control unit can register each sheet passing the sheet passage, allowing to draw conclusions about the operation of the sheet processing machine.

Preferably, the light barrier formed in the sheet passage by the light emitting element and the light receiving element is a light curtain. E.g., the light emitting element can comprise a multitude of individual light emitting sources and the light receiving element can comprise a multitude of light receiving sensors, wherein each light receiving sensor is associated to one of the light emitting sources.

Depending on the number of light emitting sources and the number of light receiving sensors, the size of the sheet passage and the spacing between the associated pairs of light emitting sources and light receiving sensors, the light curtain can be an essentially continuous light curtain.

To enhance the reliability of the device for monitoring sheets, the light curtain can extend over at least 80% of the width of the sheet passage, especially over at least 90% of the width of the sheet passage. Preferably, the light curtain extends over essentially the full width of the sheet passage to ensure that no sheet can pass the sheet passage without being registered by the control unit. Further, these variants allow using the same device for monitoring sheets for a wider size range of sheets to be processed.

In one variant, the light receiving element can be configured to measure the light intensity.

With other words, the light receiving element can detect any attenuation of the light emitted by the light emitting element, i.e. the light receiving element can provide a more nuanced information than just a Yes/No-detection.

This variant is especially useful in case the sheet is at least partially transparent, e.g. based on the thickness and/or kind of material of the sheet, such that the light can pass the sheet but is attenuated based on the transparency of the sheet.

In one variant, the light receiving element and the light emitting element are mounted on a first rail and a second rail, respectively. The first and second rail can limit the sheet passage. Further, the first rail and the second rail can have at least one mounting point for retrofitting the device for monitoring sheets in existing sheet processing machines.

To provide a simple arrangement for aligning the light emitting element and the light receiving element, the first rail and the second rail can be parallel to each other. Such an arrangement is especially advantageous in case a multitude of light emitting sources and light receiving sensors have to be carefully aligned.

The control unit especially comprises a storage module for storing information about the sheets and/or the sheet processing machine.

E.g., an expected number of sheets can be stored in the storage module of the control unit, the expected number being determined based on the number of sheets provided in a loading station of the sheet processing machine and/or the number of sheets taken from the loading station. In this variant, the control unit is adapted to compare a current number of sheets having passed the sheet passage with the expected number of sheets.

With other words, the control unit can be adapted to verify that all sheets provided in the loading station and being already taken from the loading station, respectively, have passed the device for monitoring sheets.

The expected number of sheets can be an absolute number of sheets and/or a number of sheets per time unit, e.g. per second. Accordingly, the value of the expected number of sheets can be chosen such to be the most suitable one for the kind of sheet processing job currently done by the sheet processing machine.

Additionally, the transparency of the sheet can be stored as information about the sheet in the storage module of the control unit.

In case the light receiving element can measure the light intensity, the transparency of the sheet can be used to determine whether the light barrier is blocked or not based on a threshold value. With other words, if the light intensity detected by the light receiving element is below the threshold but non-zero, the control unit can still count this measurement as a blocked light transmission

The threshold value can also be stored in the storage module of the control unit.

Further, a target shape of the sheets can be stored in the storage module of the control unit.

The target shape can be determined based on a reference sheet being passed through the sheet passage. The reference sheet can be passed through the sheet passage by an operator of the sheet processing machine before the actual sheets are processed. Accordingly, the signal sequence received by the control unit corresponds to the shape of the reference sheet from which the target shape can be calculated by the control unit and saved in the storage module.

Also, the reference sheet can be automatically determined to be the last sheet scanned during the set-up of the sheet processing machine before launching the respective processing job. Accordingly, in this version it is not necessary that the reference sheet is manually passed by an operator through the sheet passage.

In a further variant, the information on the reference sheet can be provided in form of a file to be stored in the storage module of the control device, for example a PDF file. The file may be sent to the machine through a remote computer connected to the machine, for example through a cloud computer. To perform an analysis from the shape contained in said file, the control unit may determine for each reading of the light receiving elements whether the light is totally blocked, partially blocked, or not blocked by a sheet having the expected shape. By blocked, we mean that the sheet completely covers the sensor when the reading is performed, independent from the transparency of the sheet.

With other words, it is not necessary to actually pass a reference sheet through the sheet passage and make another measurement to determine the target shape as long as the necessary information for comparing sheets with the target shape is available to the control unit. This version also allows to virtually design the target shape beforehand without the need of actually producing a reference sheet.

The target shape especially comprises the target size of the sheet and/or a target contour of the sheet. The target size especially comprises the overall length, width and height of the sheet while the target contour especially describes the course of edges of the sheet. Further, the contour can comprise information on the surface of the sheet, e.g. cutouts, nicks, perforations and/or creases.

Further, the target shape can comprise the transparency of the reference sheet. This allows setting the threshold value based on the transparency of the reference sheet.

To provide an operator of the sheet processing machine with information about the current state of the sheet processing machine, the sheet processing machine can comprise a human-machine-interface connected to the control unit, e.g. a touch-sensitive display.

The control unit can be adapted to transmit a warning message to the human- machine-interface of the sheet processing machine if the current number of sheets is lower than the expected number of sheets. Such a case might occur if a sheet has been trapped in any of the processing stations of the sheet processing machine and/or if the sheet processing machine does not operate with the intended processing speed. Accordingly, the operator can be informed by displaying the warning message on the human-machine-interface so that the operator can intervene, if necessary.

Especially, if there are no more sheets in the loading station of the sheet processing machine, the warning message is sent to the human-machine- interface. In this case, the warning message can also be an error message, wherein the error message is more prominently displayed on the human-machine- interface than the warning message. E.g., the error message can block further operation of the sheet processing machine until being explicitly acknowledged by the operator of the sheet processing machine, e.g. by a confirmation of the operator.

Analogously, the control unit can be adapted to transmit a warning message to the human-machine-interface if the target shape of at least one sheet is not matching the target shape. Accordingly, the operator can be informed by the warning message displayed on the human-machine-interface that at least one of the sheets has not been obtained in the target shape. E.g., the sheet could be damaged and/or at least one blank has not been properly pushed out from the sheet.

Further, the control unit can save a count number of the at least one sheet not matching the target shape and can send this number to the human-machine- interface to allow the operator to identify the possibly erroneous at least one sheet.

Further, if a pre-determined number of sheets are found to not match the target shape, an error message can be sent to the human-machine-interface. The pre determined number can be set such that it is indicative for a faulty processing station of the sheet processing machine. Accordingly, an operator receiving the respective error message can decide to stop the operation of the sheet processing machine to minimize the waste obtained from extended use of a not properly operating sheet processing machine.

It is also possible that the operation of the sheet processing machine is automatically stopped if a stop criterion is detected by the control unit. The stop criterion can be a pre-defined strong deviation from the target shape, e.g. a deviation which indicates that a blank who should not have removed at the position of the device for monitoring sheets is not present anymore.

In such a case, there is a high probability that the missing part is trapped within the sheet processing machine, increasing the risk of damaging the sheet processing machine resulting in undesired downtimes. By stopping the machine based on the stop criterion, the risk of damages to the sheet processing machine is at least mitigated.

With other words, the device for monitoring sheets can serve as a security device and not only as a quality control device.

The device for monitoring sheets can be arranged between a waste removal station and a blank separation station of the processing machine. This allows to check if blanks pre-formed in the sheet have been properly prepared to obtain the blanks in the blank separation station, i.e. if any waste parts have been properly removed and the blanks are in the correct shape and/or at the correct position in the sheet.

The sheets are preferably made of paper, cardboard, foil or a composite material thereof.

Further advantages and features will become apparent from the following description of the invention and from the appended figures which show a non limiting exemplary embodiment of the invention and in which:

Fig. 1 schematically shows a sheet processing machine according to the invention;

Fig. 2 schematically shows top views of a sheet being processed by the sheet processing machine of Fig. 1; and

Fig. 3 shows a perspective view of a device for monitoring sheets of the sheet processing machine of Fig. 1.

Fig. 1 schematically shows a sheet processing machine 10 making it possible to cut blanks 11 (see Fig. 2) from a succession of sheets 12. These blanks are usually intended to be subsequently folded and bonded to form packaging boxes. However, the sheets 12 might generally be made of e.g. paper, cardboard, foil, a composite material thereof or any other material routinely used in the packaging industry.

The sheet processing machine 10 comprises a series of processing stations that are juxtaposed but interdependent one another in order to form a unitary assembly. The processing machine 10 includes a loading station 14 followed by a cutting station 16 (also usually named punching station) comprising for example a die or platen press 18 where the sheets 12 are transformed by cutting, a waste removal station 20 wherein most of the waste parts are stripped, a blank separation station 22 (also usually named reception station) for separation of the blanks 11 (or blanking operation) by means of a blanking tool 23 and an evacuation station 24 for removing the residual waste sheets 25 (see Fig. 2) of the punched sheets 12.

The number and nature of the processing stations may vary depending on the nature and the complexity of the converting operations to be carried out on the sheets 12.

The sheet processing machine 10 also has a transfer mechanism 26, which in the shown embodiment is a conveyor, to make it possible to individually move each sheet 12 from an outlet of the loading station 14 to the evacuation station 24.

The conveyor uses a series of gripper bars 28 that are mounted so as to be moveable by means of two loops of chains 30 one placed laterally on each side of the sheet processing machine 10. Each loop of chains 30 travels around a loop which allows the gripper bars 28 to follow a trajectory passing successively by the cutting station 16, the waste removal station 20, the blank separation station 22 and the evacuation station 24.

Each gripper bar 28 travels on an outward path in a substantially horizontal plane of passage between a driven wheel 32 and an idler wheel 34, and then a return path in the top portion of the sheet processing machine 10. Once returned to the driven wheel 32, each gripper bar 28 is then able to grip a new sheet 12 at a front edge of the sheet 12.

In Fig. 1 , each processing station is illustrated in the form of two rectangles symbolizing respectively its top portion and its bottom portion that are positioned on each side of the plane of movement of the sheets 12. In Fig. 1, a transverse (or lateral), longitudinal and vertical direction are indicated by the orthogonal spatial system (T, L, V).

The terms "upstream" and "downstream" are defined with reference to the direction of movement of sheets 12 in a handling direction as illustrated by the arrow D in Fig. 1.

In Fig. 2, the current state and shape of the sheet 12 being processed by the sheet processing machine 10 (see Fig. 1) is shown, wherein the top views of the sheets 12 are arranged according to the sequence of the stations of the sheet processing machine 10 of Fig. 1.

In the loading station 14, the sheet 12 is provided as flat sheet with a rectangular or quadratic surface.

In the cutting station 16, a contour of the blanks 11 is prepared by forming weakened edges 38 in the sheet 12, e.g. by a sequence of nicks and/or perforations.

In the waste removal station 20, selected parts of the sheet 12 are cut out and are ejected from the sheet 12, as indicated with shaded areas 40 in Fig. 2.

Finally, the blanks 11 are separated from the sheet 12 in the blank separation station 22 such that two cross-shaped blanks 11 with a central opening 42 are obtained from the sheet 12 in the shown embodiment.

The rest of the sheet 12, i.e. the residual waste sheet 25, is removed in the evacuation station 24.

Of course, the sequence shown in Fig. 2 is illustrative in nature, only. Different shapes and sizes of the sheet 12 and/or of the blank 11 can be used and formed by the sheet processing machine 10, respectively, depending on the used type of sheets 12 and on the processing stations provided by the sheet processing machine 10.

Coming back to Fig. 1, the sheet processing machine 10 further comprises a device 44 for monitoring sheets 12 being moved downstream from the waste removal station 20 to the blank separation station 22 by the transport mechanism 26. In principle, the device 44 could be placed between any two of the processing stations of the sheet processing machine 10, depending on which current state of the sheet 12 is the most important and/or the most suited one for monitoring the correct operation of the sheet processing machine 10.

The device 44 is connected to a control unit 46, e.g. by an Ethernet connection. However, the device 44 could also be connected to the control unit 46 by any means which provides a sufficiently fast exchange of signals between the device 44 and the control unit 46. E.g., the connection can also be established wireless, e.g. by Wi-Fi.

The control unit 46 comprises a storage module 48 in which the number of sheets 12 provided in the loading station 14 and the processing speed, i.e. the number of sheets 12 per second to be processed by the sheet processing machine 10, is stored.

The control unit 46 is further connected to a human-machine-interface 50 which in the shown embodiment is a touch-sensitive display. By the human-machine- interface 50, a (not shown) operator of the sheet processing machine 10 can be informed on the current status of the sheet processing machine 10 and can control the operation of the sheet processing machine 10, too.

In Fig. 3, the device 44 for monitoring sheets 12 is shown in a perspective view.

The device 44 comprises a first rail 52 and a second rail 54, wherein the first rail 52 comprises an integrated (i.e., not explicitly shown) light emitting element and the second rail 54 comprises an integrated (i.e., not explicitly shown) light receiving element. The light emitting element has a multitude of light emitting sources, especially a multitude of laser emitting spots, while the light receiving element has a multitude of light receiving sensors, wherein each of the light receiving sensors is associated to one light emitting source. This arrangement results in an essentially continuous light curtain 56 between the first rail 52 and the second rail 54.

The first rail 52 is attached to a frame 58 of the blank separation station 22 by means of several mounting points 60 - of which only one is shown in Fig. 3 - such that the light curtain 56 is located in a sheet passage 62. As can be seen from Fig. 3, the light curtain 56 extends over most of the width of the sheet passage 62 along the transverse direction T. The light curtain 56 especially extends over at least 80% or at least 90% of the width of the sheet passage 62. In principle, the light curtain 56 could also extend over the full width of the sheet passage 62.

In the following, the mode of action of the sheet processing machine 10 in regard to the device 44 will be described in more detail.

As described before, the sheet processing machine 10 is used for converting a pile of sheets 12. More specifically, the sheet processing machine 10 is used to cut out blanks 11 from the sheets 12.

The device 44 for monitoring the sheets 12 is used to provide feedback for determining if the sheet processing machine 10 operates correctly, i.e. if all sheets 12 provided in the loading station 14 are processed and if all produced blanks 11 are of the correct shape.

For this purpose, the loading station 14 is connected to the control unit 46 such that the loading station 14 sends to the control unit 46 the total number of sheets 12 provided in the loading station 14. The total number of sheets 12 can also be provided by the operator of the sheet processing machine 10 via the human- machine-interface 50. The control unit 46 stores the total number of sheets 12 in the storage module 48.

Further, before processing the sheets 12, the operator provides a reference sheet which is passed through the sheet passage 62. The reference sheet corresponds to the desired form of the sheets 12 after leaving the waste removal station 20 and before entering the blank separation station 22 (see Fig. 3). For providing the reference sheet, the sheet processing machine 10 can have an additional (not shown) operator access in the processing station upstream of the device 44 for monitoring sheets, i.e. in the waste removal station 20 in the shown embodiment.

The control unit 46 records the signal sequence received from the light receiving element of the device 44 when the reference sheet is passed through the sheet passage 62 and stores said signal sequence in the storage module 48 for reference. Based on this signal sequence, the control unit 46 calculates a target size and/or a target contour of the sheets 12.

Additionally, the control unit 46 can store machine information about the sheet processing machine 10 in the storage module 48, e.g. a target processing speed of sheet processing machine 10 in sheets per second. The machine information can also be set by an operator via the human-machine-interface 50.

As soon as the processing job for converting the sheets 12 has started, the control unit 46 registers each sheet 12 passing the device 44, more specifically passing the sheet passage 62. Registering includes counting the sheets 12 passing the device 44 for obtaining a current number of sheets 12 as well as comparing the signal sequence provided by the light receiving element of each passing sheet 12, corresponding to the shape of each sheet 12, with the signal sequence stored in the storage module 48, i.e. the target shape.

As an alternative, the control unit 46 may use a reference file to compute a reference signal sequence received from the light receiving element of the device 44 when a sheet 12 is passed through the sheet passage 62. To do so, based on the target shape of the sheet 12, the control unit determines for each reading of the light receiving elements whether the light is totally blocked, partially blocked, or not blocked at all by a sheet 12 having the correct shape. The transparency level, i.e., the light intensity detected by the light receiving element when the light is totally blocked by the sheet, may be computed from the mean, median, or any suitable statistics of the set of totally blocked readings from the first sheet or from the first few sheets of a processing job.

Preferably, during the analysis using the reference file, the control unit may ignore the readings which may be partially blocked by the sheet and only use the ones that are either totally blocked or not blocked at all. As an alternative, the control unit my decide to assign target value for each individual light receiving element reading that depends on the proportion of the light blocked by the sheet at that location and time. Then, it may compare this target value to the real reading during the assessment. This last alternative may preferably be used when the field of view of the light receiving elements is larger than the waste (i.e., the parts of the sheet to be ejected by the machine), thereby allowing to control the quality despite a coarse array of light receiving elements. The control unit 46 is adapted to calculate an expected number of sheets 12 based on the number of sheets 12 originally provided in the loading station 14 and the number of sheets 12 already being taken from the loading station 14. For this purpose, the loading station 14 sends the number of sheets 12 already taken from the loading station 14 to the control unit 46.

The control unit 46 essentially continuously compares the expected number of sheets 12 with the current number of sheets 12. If the current number of sheets 12 is lower than the expected number of sheets 12, the control unit 46 transmits a warning message to the human-machine-interface 50 to inform the operator of the sheet processing machine 10 that there might be a problem with the operation of the sheet processing machine 10. E.g., one of the sheets 12 could be trapped inside the waste removal station 20 instead of being delivered to the blank separation station 22.

If no more sheets 12 are present in the loading station 14 and the current number of sheets 12 is still lower than the expected number of sheets 12, the control unit 46 can send an error message to the human-machine-interface 50.

The error message can be visually more prominent than the warning message to ensure that the operator cannot ignore that there is a discrepancy in the number of sheets 12.

Further, if the control unit 46 asserts that the shape of at least one sheet 12 is not matching the target shape stored in the storage module 48, the control unit 46 transmits a warning message to the human-machine-interface 50 to inform the operator that the blanks 11 coming from this sheet 12 might not have the desired shape or have not being successfully pushed out of the sheet 12 at all.

Also, the control unit 46 can transmit the current number of sheets 12 in this case such that the operator can easily identify said blanks 11 in the resulting pile of blanks 11 produced by the sheet processing machine 10 and being collected in the blank separation station 22.

Therefore, the sheet processing machine 10 according to the invention provides a simple and reliable device for monitoring the correct processing of sheets 12 in the sheet processing machine 10. In this way, any unexpected situations during production of the blanks 11 , e.g. damaged blanks 11 or missing sheets 12, can be identified to ensure high quality of the blanks 11 and minimize waste produced by and downtimes of the sheet processing machine 10.