NL2023189B1

NL2023189B1 - Sheet stacker comprising an idle time counter for ejecting a sheet stack after a predetermined idle period

Info

Publication number: NL2023189B1
Application number: NL2023189A
Authority: NL
Inventors: J Scheppink Erik
Original assignee: Canon Production Printing Holding Bv
Priority date: 2019-05-24
Filing date: 2019-05-24
Publication date: 2020-12-02
Also published as: DE102020112223A1

Abstract

Sheet stackers generally contain completed sheet stacks inside their housing until said housing needs to be cleared for a consecutive sheet stack. |t was found, especially during the management a printer fleet with large numbers of sheet printers, that completed sheet stacks were overlooked by operators, resulting in undesired delays. This overlooking may be avoided by counting an idle time from a completion of said sheet stack, in which idle time no sheets are stacked and emitting an operator awareness signal, such as ejecting the sheet stack to an unloading position outside the housing of the sheet stacker, when the counted idle time exceeds a predetermined idle time setting.

Description

P4157NL01 1

SHEET STACKER COMPRISING AN IDLE TIME COUNTER FOR EJECTING A SHEET STACK AFTER A PREDETERMINED IDLE PERIOD

FIELD OF THE INVENTION The present invention generally pertains to a sheet stacker for receiving sheets processed by a printing system and for forming sheet stacks of said received sheets. The invention further relates to a printing system comprising such a sheet stacker as well as to a method for handling sheet stacks.

BACKGROUND ART Sheet stackers, as known from e.g. European Patent EP 2776352 B1, are positioned to receive sheets printed by a printing system. The sheet stacker consecutively receives said sheets and stacks these on top of one another to form a sheet stack. The sheet stack is preferably formed inside a housing of the sheet stacker for safety and/or environmental reasons. The sheet stacker comprises a stack ejector actuator for moving the sheet stack from inside the housing to an external unloading position. The housing may comprise a door through which the sheet stack passes when being ejected. In practice, a completed sheet stack is contained inside the housing of the sheet stacker until a predetermined maximum capacity of the sheet stacker is reached. The present sheet stack is then ejected to clear the stacking position inside the housing for the new sheet stack. It was found that when operating printer fleets comprising a large number of sheet printing systems undesired delays occurred in the retrieval of sheet stacks from their respective stackers. Therein operators continuously pass through the fleet to retrieve completed sheet stacks and bring these to their respective further processing stations, such as book binders. Several print jobs suffered undesired delays in reaching the respective customer due to the operator overlooking the completion of a sheet stack.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a simple and low costs solution to indicate the completion of a sheet stack to one or more operators managing a sheet printer fleet. In a first aspect of the present invention, a sheet stacker is provided.

P4157NL01 2 The sheet stacker is configured and positioned for stacking sheets received from a sheet printer. The sheet stacker comprises: - a stacking unit for receiving sheets and forming a sheet stack of said received sheets on a stack support; - a controller having a memory storing a predetermined idle time setting, which controller is configured to: - determine an idle time wherein the stacking unit has been idle since a completion of a sheet stack; and - emit an operator awareness signal when the controller determines that the determined idle time exceeds the predetermined idle time setting.

It is the insight of the inventor that in the known sheet stacker a completed sheet stack is effectively hidden from the operator until a new print job is sent to the respective printing system. It was found that this overlooking of completed sheet stacks resulted in the delayed arrival of the completed sheet stacks at their respective further processing stations. It is additionally the insight of the inventor that such delays are avoided if the sheet stack is placed outside of the housing of the stacker after completion. It is a further insight of the inventor that by postponing the sheet stack ejection until the sheet stack has been idle for a predetermined time, productivity is substantially unaffected by said sheet stack ejection.

The stacking unit stacks the received sheets into the sheet stacks until the stack or print job is complete. The stacking unit then enters an idle state, wherein no further sheets are stacked by the stacking unit onto the completed sheet stack. The controller counts the idle time from the completion of the sheet stack as long as the stacking unit remains idle. In case further sheets are supplied to the idle stacking unit, the stacking unit would then activate to commence further stacking, thereby exiting its idle state and terminating the idle time period and the idle time counting. The controller compares the counted idle time to the predetermined idle setting time which is stored on the controller's memory. While the counted idle time is below the predetermined idle time setting and the stacking unit remains in its idle state, the controller keeps counting the idle time. When the counted idle time reaches the predetermined idle time setting, the controller emits the operator awareness signal, such that the operator is informed of the completed sheet stack. Thereby, the operator may timely retrieve the completed sheet stack for further processing by having the sheet stacker eject the sheet stack. Since the stack ejection occurs while the stacking unit is idle, the productivity of the printing system is not affected and the integrity of the sheet stacks and/or print jobs is preserved. The

P4157NL01 3 present invention requires little modification to existing sheet stackers and printing systems resulting in a simple and low costs solution. Thereby, the object of the present invention is achieved.

In an embodiment, the idle time setting defines a non-zero idle time period. The idle time setting is preferably sufficiently large to prevent the sheet stack from being ejected in between adjoining stack jobs, as the ejection process momentarily halts the stacking process. The idle time setting thus preferably corresponds to or exceeds one minute, preferably two minutes, even more preferably 5 minutes.

In embodiment, the controller is further configured to: - receive stack job information defining a sheet stack to be formed by the stacking unit, and - count the idle time from the completion of the sheet stack for said stack job information as long as no further stack job information is received. The controller receives stack job information as defined by the print job input to the controller for example via a network or user interface. The stack job preferably defines the total number of sheets in the sheet stack, such that the number of stacked sheets may be counted and compared to the total number to verify whether the stack job has been completed. The idle time is counted from the moment when the controller determines that the stack job is complete, for example when the number of sheets in the sheet stack matches the predefined total number of the stack job. If the controller then determines that no new stack job is received, the controller continues to count the idle time. The idle time counting is interrupted when a new stack job is received before the idle time setting is reached. In the latter case, the controller preferably ejects the sheet stack to clear the stack support for the new sheet stack or allows the next stack job to be stacked on top of the present sheet stack if sufficient stacking space is available. The idle time counter is thereby preferably reset and counting starts again when the stacking unit re-enters its idle state. Should no new stack job be received when the counted idle time matches the idle time setting, then the controller emits the operator awareness signal to indicate that a sheet stack is awaiting pick-up for further processing. In an embodiment, the controller is configured to: - determine whether the stacking unit is one of an active state and an idle state, - count the idle time from a start of the idle state; and

P4157NL01 4 - interrupt said counting when the stacking unit enters the active state.

The stacking unit when stacking is in its active state. When no sheets are received by the stacking unit, the stacking unit enters its idle state, wherein it refrains from stacking sheets. The controller is configured to determine in which of said states the stacking unit resides. The controller counts the idle time only when the stacking unit is in its idle state.

Preferably, the idle time is reset whenever the stacking unit enters its active state.

Thereby, the overall productivity of the printer fleet is maintained as the idle time sheet stack ejection is performed only when a respective printer has entered an idle period.

In an embodiment, the operator awareness signal is configured to activate an indicator to indicate the completion of the sheet stack to an operator, wherein the indicator comprises one of the group of: - a user interface visualizing the completion of the sheet stack to the operator; - an indicator light or speaker for visually and/or auditively indicating the completion of the sheet stack; - an stack ejector for ejecting the sheet to an unloading position.

The user interface may be a display provided on the printer or on a mobile device. The operator awareness signal may be displayed on the user interface as a visual prompt indicating the completion of the respective sheet stack. Preferably, the visual prompt further provides information regarding the sheet stack, such as its respective position on or in its respective printer, further processing information regarding a destination of the respective sheet stack, and/or detail regarding the respective print job. The indicator light or speaker is preferably mounted at or near the stacking unit or unloading position, so as to be easily identifiable when activated. In a simple and low costs solution, the sheet stack is ejected to an unloading position on the outside of the sheet stacker so as to be visible to an operator.

In an embodiment, a sheet stacker according to the present invention further comprises a stack ejector actuator for moving a sheet stack on the stack support to an unloading position, wherein the controller sends the operator awareness signal to the stack ejector actuator, such that the sheet stack is ejected to the unloading position when the idle time exceeds the predetermined idle time setting. The operator awareness signal activates the stack ejector actuator, so that the sheet stack is moved outside of a housing of the sheet stacker to its unloading position. Thereby, the completed sheet stack is placed in view of a passing operator, who then may retrieve the sheet stack

P4157NL01 from the unloading position for further processing. In a printer fleet sheet printers are generally provided with a stack ejector actuator configured to clear the stack support for a new stack job that would result in the sheet stack exceeding the maximum capacity of the stack support. The present invention teaches triggering such sheet printers to place 5 a completed sheet stack outside of the housing of the sheet stacker also when no new print or stack job is received, specifically after expiry of the predefined idle time period or setting. This provides a low costs solution to prevent the operator from overlooking completed sheet stacks.

In an embodiment, the controller is further configured to eject the sheet stack, when the controller determines that the sheet stack on the stack support exceeds a predetermined maximum capacity.

For productivity reasons, the sheet stacker may eject a sheet stack when a maximum capacity is reached, such that printing and stacking may continue. To improve productivity different stack jobs are stacked on top of one another as long as sufficient space is available or a stack ejection command is given by the controller. As such, there is no need for the operator to separately bring each sheet stack to a further processing device. Further, the stack ejector actuator may be activated dependent on a stack ejection command from the controller predetermined by or embedded in the print job.

For the ease of processing the controller may group sheets belonging to a single print job together, for example in the case of a multi-copy print job. The copies may then be ejected as a single stack, such that these may be transported together to their respective further processing point. Likewise, the controller may group together into a single stack job sheets destined for the same further processing point. Such a group of stacked sheets may comprise separation sheets, such as tab sheets. The sheet stack of a group may also comprise jogged portions, wherein a substack of sheets in the sheet stack is shifted slightly with respect to an adjacent substack in the same sheet stack. However, when such a group exceeds the maximum capacity of the sheet stacker, the controller preferably ejects the stack jobs in subgroups, one of which fit within the maximum capacity.

In an embodiment, the stack ejector comprises a slider actuator for sliding the stack support from a stacking position to the unloading position. To preserve the integrity and/or straightness of the sheet stack, the stack support is used to move the sheet stack. This prevents the sheet stack from becoming skewed when moving.

P4157NL01 6 In an embodiment, the sheet stacker comprises a door positioned between a stacking position where the sheet stack is formed and an unloading position on an opposite side of the door with respect to the stacking position, and the controller is configured for opening said door when the sheet stack is to be ejected.

The sheet stack is then moved through the door opening to the unloading position.

The sheet stacker comprises a housing inside which the sheet stack is formed on the stack support.

The housing protects the sheet stack from the environment as well as the operator from the fast moving components of the stacking unit.

While the door is closed, a completed sheet stack is substantially hidden from the operator's view.

Providing a window in the door or housing was proven to provide insufficient visibility of the sheet stack for an operator managing a printer fleet.

Especially from a distance, a completed sheet stack inside the housing is difficult to determine for an operator.

The stack ejector actuator comprises a door actuator for opening the door when ejecting the sheet stack to the unloading position, which unloading position is positioned outside the housing.

Preferably, the door actuator is controlled to close the door after an ejection, such that stacking may continue in the closed housing.

In a further aspect, the present invention provides a sheet printer assembly comprising a sheet printer and the sheet stacker according to the present invention and as described above.

Said assembly preferably comprises a user interface which is configured for inputting and/or adjusting the idle time setting.

The idle time setting is preferably adjustable via the user interface, such that an operator may input his or her desired idle time period.

As such, each operator may optimize productivity of his or her printer fleet by choosing and setting an appropriate idle time setting.

Alternatively, the idle time setting may be determined by the controller for example based on production parameters, such as the print job planning for said day.

Self learning or adaptive algorithms may be applied to tune such a dynamic idle time setting to the specifics of each printer fleet.

For example, in one example the longest possible idle time may be applied while in another example a mean or median idle time may be calculated based on a print job planning and be used as a value for the idle time setting.

For example compatible algorithms for similar logistical situations, such as transportation, distribution, and/or automated production systems may be applied.

P4157NL01 7 In another aspect, the present invention provides a method for handling sheet stacks, comprising a sheet stacker performing the steps of: - forming a sheet stack on a stack support inside a housing of the sheet stacker; - counting an idle time from a completion of said sheet stack, in which idle time no sheets are stacked; - ejecting the sheet stack to an unloading position outside the housing, when the counted idle time exceeds a predetermined idle time setting. In its embodiments, the sheet stacker operates in the above described manners.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying schematical drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: Fig. 1Ais a schematical cross-sectional side view of an embodiment of a printing system comprising an embodiment of a sheet stacker according to the present invention; Fig. 1Bis a schematical top view of the embodiment of the printing system in Fig. 1; Fig. 2 is a schematic perspective view of a stacking unit of the sheet stacker in Fig. 1; and Fig. 3 is a schematic block diagram listing the different steps of an embodiment of the method according to the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS The present invention will now be described with reference to the accompanying drawings, wherein the same reference numerals have been used to identify the same or similar elements throughout the several views.

P4157NL01 8 FIG. 1A shows schematically an embodiment of a printing system 1 according to the present invention. The printing system 1, for purposes of explanation, is divided into an output section 5, a print engine and control section 3, a local user interface 7 and an input section 4. While a specific printing system is shown and described, the disclosed embodiments may be used with other types of printing system such as an ink jet print system, an electrographic print system, etc. The output section 5 comprises a first stack support 52 for holding printed image receiving material, for example a plurality of sheets. The output section 5 may comprise a second stack support 55. While two stack supports are illustrated in FIG. 1A, the number of stack supports may include one, two, three or more stack supports. The printed image receiving material is transported from the print engine and control section 3 via an inlet 53 to the output section 5. One or more output sections may be formed by a stacker which comprises a stacking unit 71, 72 for receiving sheets from the print engine and for forming sheet stacks of said received sheets. Details of the stacking unit 71, 72 are shown in Fig. 2. When a stack ejection command is invoked by the controller 37 for the first stack support 52, a first stack ejector actuator 54 is activated in order to eject the plurality of sheets on the first stack support 52 outwards to a first external unloading position UP outside of the housing 5A, as shown in Fig. 1B. The first external unloading position UP is formed by the stack support 52 being positioned at least partially outside the housing 5A. From the loading position UP the sheet stack may be retrieved by an operator, robot, or other suitable retrieving means. When a stack ejection command is invoked by the controller 37 for the second stack support 55, second guiding means 56 are activated in order to eject the plurality of sheets in the second stack support 55 outwards to a second external unloading position UP, for example an external stack support. The housing 5A effectively surrounds the sheet stack and thereby hides the sheet stack in the stacking position SP from an operator's view in at least a plurality of different directions, while in the unloading position UP, the sheet stack is visible to an operator from a sufficiently large distance and a sufficient number of different directions.

The output section 5 is digitally connected by means of a cable 60 to the print engine and control section 3 for bi-directional data signal transfer. Doors 73, 74 are provided to, along with the housing 5A, shield a sheet stack on the first or second stack supports 52,

P4157NL01 9 56 at a stacking position SP from the ambient atmosphere present at the unloading positions UP. The stack ejection command further controls the doors 73, 74 to be opened to allow the sheet stacks to be ejected to their respective the unloading positions UP. Thereto, the doors 73, 74 are provided with door actuators 75, 76 configured to open and close the doors 73, 74.

The print engine and control section 3 comprises a print engine and a controller 37 for controlling the printing process and scheduling the plurality of sheets in a printing order before they are separated from input holder 44, 45, 46.

The controller 37 is a computer, a server or a workstation, connected to the print engine and connected to the digital environment of the printing system, for example a network N for transmitting a submitted print job to the printing system 1. In FIG. 1 the controller 37 is positioned inside the print engine and control section 3, but the controller 37 may also be at least partially positioned outside the print engine and control section 3 in connection with the network N in a workstation N1. The controller 37 comprises a print job receiving section 371 permitting a user to submit a print job to the printing system 1, the print job comprising image data to be printed and a plurality of print job settings. The controller 37 comprises a print job queue section 372 comprising a print job queue for print jobs submitted to the printing system 1 and scheduled to be printed. The controller 37 comprises a sheet scheduling section 373 for determining for each of the plurality of sheets of the print jobs in the print job queue an entrance time in the paper path of the print engine and control section 3, especially an entrance time for the first pass and an entrance time for the second pass in the loop in the paper path according to the present invention. The sheet scheduling section 373 will also be called scheduler 373 hereinafter. The sheet scheduling section 373 takes the length of the loop into account. The length of the loop corresponds to a loop time duration of a sheet going through the loop dependent on the velocity of the sheets in the loop. The loop time duration may vary per kind of sheet, i.e. a sheet with different media properties.

Resources may be recording material located in the input section 4, marking material located in a reservoir 39 near or in the print head or print assembly 31 of the print engine, or finishing material located near the print head or print assembly 31 of the print engine or located in the output section 5 (not shown).

P4157NL01 10 The paper path comprises a plurality of paper path sections 32, 33, 34, 35 for transporting the image receiving material from an entry point 36 of the print engine and control section 3 along the print head or print assembly 31 to the inlet 53 of the output section 5. The paper path sections 32, 33, 34, 35 form a loop according to the present invention.

The loop enables the printing of a duplex print job and/or a mix-plex job, i.e. a print job comprising a mix of sheets intended to be printed partially in a simplex mode and partially in a duplex mode.

The print head or print assembly 31 is suitable for ejecting and/or fixing marking material to image receiving material.

The print head or print assembly 31 is positioned near the paper path section 34. The print head or print assembly 31 may be an inkjet print head, a direct imaging toner assembly or an indirect imaging toner assembly.

While an image receiving material is transported along the paper path section 34 in a first pass in the loop, the image receiving material receives the marking material through the print head or print assembly 31. A next paper path section 32 is a flip unit 32 for selecting a different subsequent paper path for simplex or duplex printing of the image receiving material.

The flip unit 32 may be also used to flip a sheet of image receiving material after printing in simplex mode before the sheet leaves the print engine and control section 3 via a curved section 38 of the flip unit 32 and via the inlet 53 to the output section 5. The curved section 38 of the flip unit 32 may not be present and the turning of a simplex page has to be done via another paper path section 35. In case of duplex printing on a sheet or when the curved section 38 is not present, the sheet is transported along the loop via paper path section 35A in order to turn the sheet for enabling printing on the other side of the sheet.

The sheet is transported along the paper path section 35 until it reaches a merging point 34A at which sheets entering the paper path section 34 from the entry point 36 interweave with the sheets coming from the paper path section 35. The sheets entering the paper path section 34 from the entry point 36 are starting their first pass along the print head or print assembly 31 in the loop.

The sheets coming from the paper path section 35 are starting their second pass along the print head or print assembly 31 in the loop.

When a sheet has passed the print head or print assembly 31 for the second time in the second pass, the sheet is transported to the inlet 53 of the output section 5.

P4157NL01 11 The input section 4 may comprise at least one input holder 44, 45, 46 for holding the image receiving material before transporting the sheets of image receiving material to the print engine and control section 3. Sheets of image receiving material are separated from the input holders 44, 45, 46 and guided from the input holders 44, 45, 46 by guiding means 42, 43, 47 to an outlet 36 for entrance in the print engine and control section 3. Each input holder 44, 45, 46 may be used for holding a different kind of image receiving material, i.e. sheets having different media properties. While 3 input holders are illustrated in FIG. 1, the number of input holders may include one, two, three or more input holders.

The local user interface 7 is suitable for displaying user interface windows for controlling the print job queue residing in the controller 37. In another embodiment a computer N1 in the network N has a user interface for displaying and controlling the print job queue of the printing system 1.

Figure 2 is a diagram showing a stacking unit 71 for use in the printing system 1. Such a stacking unit 71, 72 as well as alternative embodiments are known from European Patent EP 2776352 B1, the description of which stacking units 71, 72 is herein incorporated by reference. The stacking unit 71 comprises a stack support or receiving plane 52 and two rotatably arranged elements 22 and 23, which are connected to an electrically driven motor 25 and rotation shaft 24. A stacking unit 71 may comprise one or more rotatably arranged elements of this kind. Electrically driven motor 25 may, for example, be an electric servo motor or stepping motor. The rotatably arranged elements comprise slots 26 and 27 in which an incoming sheet may be accepted in whole or in part from input clamping arrangement 15. In the position shown in figure 2, rotatable elements 22 and 23 are located in the first rotation zone, in which the leading edge of a sheet may be accepted into slots 26 and 27. Mounted pivotably on each of the rotatably arranged elements 22 and 23, cam following pivot elements 50 on which a closed-loop shaped friction elements 28 and 29 have been mounted. The cam following pivot elements are biased inwards by means of a spring force, while a cam engages with these cam following pivot elements such that just before the sheet is urged against stop 30. By pivoting outward in radial direction of the rotatably arranged elements 22 and 23, the friction elements do not interfere with an incoming sheet during acceptance of that sheet in the slot 26, 27, while an urging force is applied during the urging of the sheet against the stop 30. Note that, for reasons of

P4157NL01 12 clarity of illustration the rotatably arranged elements 22 and 23 have been illustrated partially. In practice, the rotatably arranged elements 22 and 23 are formed as a full cylinder comprising two slots and two pivotable elements. By actuating motor 25, rotation shaft 24 is driven in the direction of arrow B, where rotatably arranged elements 22 and 23, and as a result, also slots 26 and 27, as well as the sheet accepted within, are conveyed in the direction of stop 30 according to arrow B. During the movement of rotation shaft 24, closed-loop shaped friction elements 28 and 29 connected to it will exercise a force with a component in the direction of the stop on the upper side of the sheet that was last deposited onto stack support 52, so that it is conveyed against stop

30. In the description below of figures 4 a-f, the movement cycle of the device will be described in more detail. It will be understood by those skilled in the art that, when stack support 52 does not contain any sheets, the closed-loop shaped friction elements 28 and 29 will brush over stack support 52. The stack support 52 in Fig. 2 is in its stacking position SP where the sheets are stacked on top of one another. When completed the stack support 52 is slid by means of a stack ejector actuator 54 through the opening formed by door 73 towards an unloading position UP outside of the housing 5A.

The embodiment shown in figure 2 furthermore comprises a tool to restrict the freedom of movement of the edge of any sheets that have already been deposited onto stack support 52. This will reduce any curling or other form deformation of the sheet edge, which will have a beneficial effect on the registration behaviour of sheets conveyed onto stack support 52. In this example of an embodiment, retention hooks 61 and 62 have been mounted, which have been rotatably attached to the frame end and are bent at the other side, in such a way that any incoming sheets conveyed from slots 26 and 27 at the level of stop 30 onto the receiving plane may easily be conveyed under here, whilst the freedom of movement of the sheet edge is restricted. The retention force of the hooks onto the sheet edge is predominantly directed downwards and may, for example, be delivered exclusively by the hooks’ own weight in a gravity field and/or by means of a torsion spring at the position of attachment to the frame or by means of magnetic pull.

Fig. 3 shows schematically a flow diagram for the method according to the present invention. The controller 37 receives a print job in step i, which comprises stack job information, specifying the number of sheets to be directed towards a stacking unit 71,

72. The print job information is processed and then executed, resulting in a number of printed sheets. The printed sheets are consecutively output from the printing system 1 to the stacking unit 71, 72, which starts forming the stack in step ii. The stacking unit 71,

P4157NL01 13 72 receives the printed sheets, forming a sheet stack on the stack support 52, 55 in step iii.

The stacking unit 71, 72 comprises a sheet counter {not shown) for counting the sheets passing said sheet counter.

Such a sheet counter may for example be an optical detector positioned opposite from an optical emitter with regard to a sheet transport path in between said detector and emitter.

The presence of a sheet can then be derived from the light intensity received by the detector, allowing the number of sheets to be tracked and/or counted in step iv.

While stacking the controller 37 constantly compares the number of sheets in the stack to a maximum capacity of the stack support 52, 55 to prevent the stack height from exceeding the maximum capacity (step v). Preferably, the controller 37 performs the capacity check in step v prior to starting a new stack job by comparing the number of sheets in the stack job to the number of sheets already present on the stack support 52, 55 and the maximum capacity {which together yield the remaining available space for stacking sheets). As such, the controller 37 may determine that insufficient space is available for an upcoming stack job and choose to eject the sheet stack currently present on the stack support 52, 55 to make room for the new stack job (step xiii). Parallel to monitoring the sheet stack height with respect to the maximum capacity, in step vi, the controller 37 compares the measured number of sheets in the sheet stack to the number of sheets defined by the stack job information.

Stacking continues while the determined number of sheets in the sheet stack is below the number prescribed by the stack job information (or until the maximum capacity of the stacking unit is reached). While the stack job is incomplete and sufficient space is available, the stacking unit 71, 72 continues stacking the sheets onto the stack support 52, 55. When the controller 37 determines that the number of sheets in the sheet stack has reached the number defined by the stack job information, the controller 37 places the stacking unit 71, 72 in an idle state and commences tracking the duration of said idle state in step vii.

In step vii, the controller 37 determines that the stack job has been completed and commences counting the idle time, wherein the stacking unit 71, 72 is inactive and wherein the sheet stack rests on the stack support 52, 55 in the stacking position SP.

Without any further input the completed sheet stack remains in said stacking position SP while the idle times counter counts towards the predetermined idle time setting (step viii). Should however a new stack job be input to the controller 37 (step ix) before the idle time counter reaches said idle time setting, then the stacking unit 71, 72 is activated to stack the sheets for the new stack job on top of the sheet stack present on the stack support 52, 55, given that the addition of the new stack job does not increase the sheet stack beyond the maximum capacity (in which case the

P4157NL01 14 present sheet stack is ejected to make room for the new stack job). Upon receipt of the stack job information, the idle time counter is reset to its starting value, as shown in step xi. With the idle time counter reset, the new stack job is then stacked by the stacking unit 71 on the present sheet stack or on the cleared stack support 52, 55 in case a stack ejection was performed to prevent exceeding the maximum capacity.

In case no new stack job is received within the time period defined by the idle time setting, the controller 37 ejects the completed sheet stack when the idle times counter reaches the idle time setting. Therein, the controller 37 controls the door actuator 75, 76 to open the door 73, 74, such that the sheet stack may pass through the door opening from the stacking position SP to the unloading position UP. The sheet stack is moved by the stack ejector actuator 54, 56. Preferably, the stack ejector actuator 54, 56 is configured to move, specifically to slide, the stack support 52, 55 with the sheet stack on it from the stacking position SP to the unloading position UP. Thereby, the stack support 52, 55 is cleared for the new stack job. The completed sheet stack is thereby visibly placed in view of a passing operator, who may then retrieve the sheet stack for further processing by a finishing device, such as a book binder. As such, the risk of the operator overlooking a completed sheet stack is reduced with a low costs and easy to implement solution. The operator may adjust the idle time setting via the user interface 7 to suit the specific workflow of the operator. Detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. In particular, features presented and described in separate dependent claims may be applied in combination and any advantageous combination of such claims are herewith disclosed. Further, it is contemplated that structural elements may be generated by application of three-dimensional (3D) printing techniques. Therefore, any reference to a structural element is intended to encompass any computer executable instructions that instruct a computer to generate such a structural element by three-dimensional printing techniques or similar computer controlled manufacturing techniques. Furthermore, such

P4157NL01 15 a reference to a structural element encompasses a computer readable medium carrying such computer executable instructions. Further, the terms and phrases used herein are not intended to be limiting; but rather, to provide an understandable description of the invention. The terms "a" or "an", as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language). The term coupled, as used herein, is defined as connected, although not necessarily directly.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

P4157NL01 16

EMBODIMENTS

1. A sheet stacker (5) for stacking sheets received from a sheet printer, the sheet stacker (5) comprising: -a stacking unit (71, 72) for receiving sheets and forming a sheet stack of said received sheets on a stack support (52, 55); - a controller (37) having a memory storing a predetermined idle time setting, which controller (37) is configured to: - determine an idle time wherein the stacking unit (71, 72) has been idle since a completion of a sheet stack; and - emit an operator awareness signal when the controller (37) determines that the determined idle time exceeds the predetermined idle time setting.

2. The sheet stacker (5) according to embodiment 1, wherein the controller (37) is further configured to: - receive stack job information defining a sheet stack to be formed by the stacking unit (71,72), and - count the idle time from the completion of the sheet stack for said stack job information as long as no further stack job information is received.

3. The sheet stacker (5) according to any of the previous embodiments, wherein the controller (37) is configured to: - determine whether the stacking unit (71, 72) is one of an active state and an idle state, - count the idle time from a start of the idle state; and - interrupt said counting when the stacking unit (71, 72) enters the active state.

4. The sheet stacker (5) according to any of the previous embodiments, wherein the operator awareness signal is configured to active an indicator (7, 54, 56) to indicate the completion of the sheet stack to an operator, wherein the indicator comprises one of the group of: - a user interface (7) visualizing the completion of the sheet stack to the operator; - an indicator light or speaker for visually and/or auditively indicating the completion of the sheet stack; - an stack ejector actuator (54, 58) for ejecting the sheet stack to an unloading position (UP).

P4157NL01 17

5. The sheet stacker (5) according to any of the previous embodiments, further comprising a stack ejector actuator (54, 56) for moving a sheet stack on the stack support (52, 55) to an unloading position (UP}, wherein the controller (37) sends the operator awareness signal to the stack ejector actuator (54, 56), such that the sheet stack is ejected to the unloading position (UP) when the idle time exceeds the predetermined idle time setting.

6. The sheet stacker (5) according to embodiment 5, wherein the controller (37) is further configured to eject the sheet stack, when: - the controller (37) receives further stack job information for a forming further sheet stack; and/or - the controller (37) determines that the sheet stack on the stack support (52, 55) exceeds a predetermined maximum capacity.

7. The sheet stacker (5) according to embodiment 5 or 6, wherein the stack ejector (54, 56) comprises a slider actuator (54, 56) for sliding the stack support (52, 55) from a stacking position to the unloading position (UP).

8. The sheet stacker (5) according to embodiment 5, 6, and/or 7, wherein the sheet stacker (5) comprises a door (73, 74) positioned between a stacking position where the sheet stack is formed and an unloading position (UP), and the controller (37) is configured for opening said door (73, 74) when the sheet stack is to be ejected from the stacking position to the unloading position (UP).

9. A sheet printing system (1) comprising a sheet printer and the sheet stacker (5) according to any of the previous embodiments, further comprising a user interface (7) which is configured for inputting and/or adjusting the idle time setting.

10. A method for handling sheet stacks, comprising a sheet stacker (5) performing the steps of: - forming a sheet stack on a stack support (52, 55) inside a housing (5A) of the sheet stacker (5); - counting an idle time from a completion of said sheet stack, in which idle time no sheets are stacked;

P4157NL01 18 - ejecting the sheet stack to an unloading position (UP) outside the housing (5A), when the counted idle time exceeds a predetermined idle time setting.

Claims

P4157EN01 19 CONCLUSIONS

A sheet stacker (5) for stacking sheets from a sheet printer, the sheet stacker (5) comprising: - a stacking unit (71, 72) for receiving sheets and forming them on a stacking support (52, 55). a sheet stack of the received sheets; - a control unit (37) with a memory on which a predetermined rest time setting is stored, which control unit (37) is arranged for: - determining a rest time in which the stacking unit (71, 72) is at rest since the completion of a sheet stack; and - transmitting a driver awareness signal when the control unit (37) determines that the determined rest time exceeds the predetermined rest time setting.

A sheet stacker (5) according to claim 1, wherein the control unit (37) is further arranged to: - receive stack order information determining a sheet stack to be formed by the stack unit (71, 72), and - count the rest time from the completing the sheet stack for the stack job information, as long as no further stack job information is received.

Sheet stacker (5) according to any of the preceding claims, wherein the control unit (37) is arranged to: - determine whether the stacking unit (71, 72) is in one of an active state and a rest state; - counting the rest time from the beginning of the rest state; and - interrupting the counting when the stacker (71, 72) enters the active state.

A sheet stacker (5) according to any one of the preceding claims, wherein the driver awareness alert signal is arranged to activate an indicator (7, 54, 56) to indicate the completion of the sheet stack to a driver, the indicator (7, 54, 56) includes one of the group of: - a user interface (7) for visualizing the completion of the sheet stack for the operator;

P4157EN01 20 - An indicator light or indicator loudspeaker for visually or audibly indicating the completion of the sheet stack to the operator; - a stack ejector (54, 56) for ejecting a completed sheet stack to an unload position (UP).

A sheet stacker (5) according to any one of the preceding claims, further comprising a stack ejector (54, 56) for moving a sheet stack on the stack support (52, 55) to an unload position (UP), wherein the control unit (37) driver awareness signal to the stack ejector (54, 56) such that the sheet stack is ejected to the unload position (UP) when the rest time exceeds the predetermined rest time setting.

A sheet stacker (5) according to claim 5, wherein the control unit (37) is further arranged to eject the sheet stack when: - the control unit (37) receives further stack order information to form a further sheet stack; and / or - the controller (37) determines that the sheet stack on the stacking support (52, 55) exceeds a predetermined maximum capacity.

A sheet stacker (5) according to claim 5 or 6, wherein the stack ejector (54, 56) comprises a slide actuator (54, 56) for sliding the stack support (52, 55) from a stack position to the unload position (UP).

A sheet stacker (5) according to claim 5, 6, and / or 7, where the sheet stacker (5) comprises a door (73, 74) located between a stack position where the sheet stack is formed and an unload position (UP), and wherein the control unit (37) is arranged to open the door (73, 74) when the sheet stack is intended to be ejected from the stack position to the unload position (UP).

Sheet printing system (1) comprising a sheet printer and the sheet stacker (5) according to any one of the preceding claims, further comprising a user interface (7) which is adapted to enter and / or adjust the rest time setting.

A method of processing sheet stacks, comprising the following steps performed by a sheet stacker:

P4157NL01 21

- forming a sheet stack on a stack support (52, 55) within a housing (5A) of the sheet stacker (5); - counting a rest time from a completion of the sheet stack, in which rest time no sheets are stacked;

- ejecting the sheet stack to an unload position (UP) outside the housing (5A), when the counted rest time exceeds a predetermined rest time setting.