Patent Description:
In a sheet printer an endless belt conveyor may be positioned over a suction chamber and facing a print assembly to provide a moveable sheet support surface during printing. The conveyor may comprise detectors for sensing the speed, position, and/or orientation of the belt and actuators which actively control the belt to maintain its position constant with respect to the print head assembly. The sheet is generally transferred to the belt in a predetermined position suitable for printing, which position was determined by means of a sheet registration device upstream of the belt conveyor. When transferring the sheet to the belt, undetected displacement may occur between the sheet and the belt, such that the position of the sheet on the belt is uncertain or unknown. The sheet may for example slip over the belt and trail behind its intended position. This could result in misalignment of the image printed on the sheet, as accurate knowledge of the sheet's position is required to timely start the jetting of ink droplets onto the sheet. It is known to eliminate such uncertainty by providing the print head assembly with means for directly sensing the position of the sheet irrespective of the belt. This may reduce productivity as a correction of the sheet's and/or belt's position has to be performed prior to printing. Direct sensing of the sheet may further be difficult to achieve in cases wherein the sheet is transparent or has a similar color and/or material as that of the belt. Additionally, such sensors require additional components, resulting in greater costs.

<CIT> and <CIT> B1dislcose printers wherein a sheet is transferred between pairs of rollers. <CIT> disclose a printer, wherein a sheet is transferred from a pair of rollers to a vacuum belt.

It is an object of the invention to provide an alternative sheet conveyor assembly, which allows for accurate transfer of the sheet between conveyors.

In accordance with the present invention, a sheet conveyor for a printer according to claim <NUM>, a sheet printer according to claim <NUM>, a method according to claim <NUM>, a computer-readable storage medium storing instructions according to claim <NUM>, and a memory for storing data according to claim <NUM> are provided.

The sheet conveyor assembly comprises a first conveyor with a first holding device configured for exerting a holding force on a sheet proportional to a coverage of the sheet on the first conveyor and a second conveyor upstream of the first conveyor comprising a second holding device configured for holding and releasing the sheet, wherein the first conveyor comprises a moveable sheet support surface onto which the sheet is at least partially held while on the first conveyor in the form of an endless, air permeable belt positioned over a suction chamber in connection to a suction source for applying an underpressure to at least a portion of sheet on the first conveyor, such that a holding force applied by the underpressure increases as the sheet's coverage over the suction chamber increases, and a controller storing on its memory a sheet media catalogue and configured to control a release timing of the second holding device for releasing the sheet partially engaged by the first conveyor based on a media type parameter selected from the media catalogue, wherein the release timing determines a coverage of the sheet over the first conveyor.

It is the insight of the inventors that undetected displacement of the sheet with respect to the first conveyor occurs due to an insufficient holding force as a consequence of too little coverage of the sheet on the first conveyor at the moment that the sheet is released by the second conveyor. It is the further insight of the inventors that reliable transfer may be achieved by releasing the sheet from the second conveyor after a sufficient amount of coverage on the first conveyor has been achieved. Said amount of coverage varies for different media types, so the inventors propose providing release timing determining means to determine a suitable moment of releasing a sheet in accordance with its media type. This may be achieved by determining a suitable release timing for releasing the sheet from the second conveyor, for example by storing a media type characteristic corresponding to a minimum coverage over the first conveyor which should be achieved by a sheet for any relevant media types, or by means of an algorithm which determines a suitable release timing from other pre-stored characteristics of the media types. This ensures that the second conveyor releases the sheet only after sufficient coverage of and holding by the first conveyor is present.

Displacement due to e.g. slippage is prevented and the sheet is transferred to the first conveyor in the same position and orientation as on the second conveyor. The sheet is transferred synchronously with the movement of the first conveyor, such that the position of the sheet may be accurately derived from the first conveyor's transport information (such as its transport velocity). This reduces the need for direct sensing of the sheet on the first conveyor. A reliable and low costs sheet conveyor assembly is thus formed, which allows the print head assembly to be controlled based on transport information from the first conveyor without the need for directly sensing the sheet. Thereby the object of the present invention has been achieved.

More specific optional features of the invention are indicated in the dependent claims.

In an embodiment, the sheet conveyor assembly further comprises a registration device for adjusting a lateral position and/or orientation of the sheet, such that the sheet is transferred from the second conveyor to the first conveyor in the lateral position and/or orientation determined by the registration device. The registration device is configured to position the sheet in a predetermined position with respect to the belt and/or a print head assembly for printing an image on the sheet. The registration device is configured to laterally shift and/or rotate the sheet. To avoid misalignment of the printed image on the sheet, the lateral position and/or orientation of the sheet should be maintained as the sheet is transferred onto the first conveyor. The longitudinal position of the sheet in the transport direction is further accurately tracked to time the arrival of the leading edge of the sheet with the start of the printing of the image, as the sheet moves synchronously with the first conveyor after release.

In an embodiment, the controller is configured to control the release of the sheet from the second conveyor after the holding force of the first conveyor is sufficient to prevent displacement of the sheet with respect to the first conveyor, preferably during and/or after transfer. Displacement of the sheet with respect to the first conveyor is avoided ensuring accurate tracking of the sheet's position by the controller. The sheet's lateral position and orientation have been determined by the registration device and the sheet is transferred to the first conveyor while maintaining said lateral position and orientation. Further, the longitudinal position of the sheet, i.e. its position in the transport direction, was determined and/or set by the registration device. In order to allow the controller to track further progress of the sheet without direct sensing, the sheet should be transported further in a controlled manner. While held by the second conveyor, the longitudinal position of the sheet may be tracked by and/or derived from transport information from the second conveyor. The controller may for example compare a speed of the second conveyor to an elapsed time since last determining the sheet's longitudinal position, for example in the registration device to determine the sheet's current longitudinal position. Similarly, the controller may track progress of the sheet on the first conveyor in a similar manner. This requires the sheet to be controllably handed over from the second to the first conveyor, without the sheet slipping or otherwise moving over the first conveyor after the second conveyor has released the sheet. Preferably the controller controls the first and second conveyors, such that the sheet moves synchronously with the first conveyor. This achieved by suitably timing the release of the second conveyor on or after the moment at which a secure holding of the first conveyor on the sheet is known to have been achieved. This moment may vary for different media types, dependent on their dimensions and materials. The controller's memory thereto stores suitable means for determining this release time upon selection of the applied media type. These means may include a look-up table and/or algorithm which yields a release timing upon selecting and/or inputting a certain media type for a sheet.

In an embodiment, the sheet is positioned on both the first and second conveyors when the controller controls the second conveyor to release the sheet. The release is performed while the sheet is still partially on or over the second conveyor. The second conveyor comprises preferably a holding device for actively holding the sheet, for example a transport pinch formed of opposing rollers, which can be moved apart from one another to release the sheet. At the moment of the release timing, the controller controls the holding device to release the sheet, which at that time is securely held by the first conveyor, such that the sheet is further transported synchronously with the first conveyor. It will be appreciated that dependent on the distance between the holding device of the second conveyor and the first conveyor certain shorter sheets may have passed or exited the holding device of the second conveyor before the determined release timing. Therefore, the holding device of the second conveyor is preferably adjacent or near the first conveyor.

In an embodiment, the controller comprises release timing determining means to derive the release timing from the selected media type parameter. The release timing determining means yield a release timing when a media type parameter is input or selected. The media type parameter has been determined when the sheet arrives the second conveyor. The media type parameter may be input via the user interface or as part of a print job prescribing the use of a certain media type. The release timing determining means apply the media type parameter to determine or derive the release timing belonging to a sheet of the respective media type. In another embodiment, the release timing determining means comprise a predetermined release timing or length parameter for each of media type in the media catalogue and/or an algorithm configured to derive release timing parameter from other predetermined, prestored characteristics for each of media type in the media catalogue, preferably a sheet dimension characteristic and/or a sheet air permeability characteristic. The release timing determining means may comprise a look-up table or list storing predetermined release timing properties per media type, for example as part of the media catalogue. Selection of the media type then automatically includes a selection of the respective release timing properties via the look-up table. The releases timing properties may be expressed as a point or position on the first conveyor, a time or distance measured from e.g. the arrival of the sheet at the first conveyor or any other suitably point, etc. The release timing determining means may further comprise an algorithm which determines or calculates a value for the release timing parameter for a media type based on one or more media characteristics in the media catalogue.

In an embodiment, the coverage is proportional to a sheet dimension characteristic and/or a sheet air permeability characteristic. The holding force of the first conveyor is proportional to its coverage by the sheet. A sheet with a greater air permeability but similar dimensions will in most cases require a larger coverage to achieve the same secure holding on the first conveyor as a sheet with lower or zero air permeability. Sheets of the same material but with different dimensions may require different release timings to achieve sufficient coverage for a secure holding on the first conveyor.

In an embodiment, the sheet conveyor assembly further comprises a detector assembly for sensing the transport of the first conveyor, preferably a belt tracking sensor assembly. The progress of the sheet, specifically its arrival at the print head assembly, is determined by the controller from transport information generated by the detector assembly, specifically by the belt tracking sensor assembly. Since the handover of the sheet between the first and second conveyor was performed with accurate knowledge of the sheet's position, it is possible to determine the sheet's arrival at the print head assembly by detecting the movement of the belt. This allows the print head assembly to be controlled with the correct timing to position the image on the intended position on the sheet. The detector assembly may be formed of an optical detector configured to detect markers on or in the belt at different positions and/or comprise one or more encoders positioned at the belt, at one or more of its rollers, and/or at its drive.

In another embodiment, the first conveyor comprises a moveable sheet support surface onto which the sheet is at least partially held while on the first conveyor, in the form of an endless, air permeable belt positioned over a suction chamber in connection to a suction source for applying an underpressure to at least a portion of sheet on the first conveyor. The belt forms a sheet support surface and the holding device is provided for holding the sheet against the belt. Such holdings means may be pressing means or electro-static attractors, or preferably a suction chamber connected to a suction source for drawing in air through through-holes in the belt.

In an embodiment, the second sheet conveyor comprises a releasable holding device having a first state, wherein the sheet is actively held against a support surface of the second conveyor and a second state wherein the sheet is freely supported on the support surface of the second conveyor. The second conveyor comprises a holding device such as a pressing means, a suction source, electrostatic attractors, etc. which can controlled to whether or not actively hold the sheet against the support surface. Preferably, the second conveyor comprises a transport pinch comprising at least one roller moveable in a direction perpendicular to a plane of the sheet away from the sheet on the second conveyor for releasing the sheet from the second conveyor. The roller can be moved to a remote position away from the support force to bring the holding device in the second state. In the first state the roller presses the sheet onto the support surface, which may be formed by one or more rollers, a support plate, etc. Preferably, the roller is part of the registration device, wherein the roller is opposite one or more independently driveable wheels or rollers for adjusting a lateral position and/or orientation of the sheet.

The present invention further relates to a sheet printer comprising the sheet conveyor assembly according to the present invention, wherein a print head assembly is positioned over the first conveyor and further comprising a detector assembly for sensing the transport of the first conveyor and a controlled configured to control the print head assembly to commence printing the sheet based on transport information from the detector assembly. The printer is preferably an inkjet printer with a controller which is able to accurately track further progress of the sheet beyond a point upstream of the first conveyor, at which point the sheet's position was determined or known. No additional direct sensing of the sheet beyond that point is required, since the sheet is controllably transferred to the first conveyor, such that the current position of the sheet can be accurately derived from the transport information from the detector assembly.

The present invention further relates to a method for transferring sheets having a predetermined position and/or orientation from a second conveyor to first conveyor, wherein the first conveyor comprises a moveable sheet support surface onto which the sheet is at least partially held while on the first conveyor in the form of an endless, air permeable belt positioned over a suction chamber in connection to a suction source for applying an underpressure to at least a portion of sheet on the first conveyor, such that a holding force applied by the underpressure increases as the sheet's coverage over the suction chamber increases, the method comprising the steps of:.

When a sheet is transferred between two conveyors, the holding force of the first conveyor should be sufficiently large to prevent slippage between the sheet and the first conveyor before the second conveyor releases the sheet. On the second conveyor progress of the sheet is determined by the first conveyor, which allows the sheet's position to be determined based on transport information from the second conveyor. Further progress of the sheet on the first conveyor can only be accurately tracked from transport information of the first conveyor if the sheet is controllable transferred between the conveyors, i.e. without displacement of the sheet with respect to the first conveyor after the second conveyor releases the sheet. This is ensured by determining a suitable release timing for each media type applied. This determination is preferably performed by the printer's controller based on the input or selection of a certain media type. The sheet is released from the second conveyor on or after the determined release timing, for example after passing a predetermined length from the upstream end of the first conveyor. The release timing for each media type is selected, such that the holding force of the first conveyor is sufficiently great to prevent displacement or slippage between the sheet and the first conveyor after release. This allows the progress of the sheet towards the print head assembly to be accurately derived from the transport information of the first and second conveyors, such that the print head assembly can be controlled based thereon to position the image on the sheet at the intended position.

In an embodiment, the method further comprises the step of determining the release timing comprises determining a parameter corresponding to a minimal coverage of the sheet over the first conveyor, and wherein the second conveyor is controlled to release its holding on the sheet when said minimal coverage has been exceeded and/or reached. As indicated above, the release timing is a measure for the minimum amount of coverage a sheet of a certain media has to attain on the first conveyor to achieve secure holding.

The present invention further relates to a computer-readable storage medium storing instructions that when executed by a sheet printer as described above cause the sheet printer to perform a method for for transferring sheets having a predetermined position and/or orientation from a second conveyor to first conveyor to print an image on a sheet , the method comprising:.

The instructions instruct the computer to determine a release timing for a sheet of a certain media type based on said media type. The media type parameter is selected in correspondence to the media type of the applied sheets for a print job. The sheet is transferred from the second conveyor to the first conveyor with a predetermined position. By determining an appropriate release timing for releasing the sheet from the second conveyor, it is prevented that the position of the sheet shifts with respect to the first conveyor. The further progress of the sheet can then be derived from position transport information of the first conveyor in combination with the determined position of the sheet at the moment of release. This allows the sheet's progress towards the print head assembly to be tracked without directly sensing the sheet itself. Based on said progress, the print head assembly is controlled to timely commence printing on the sheet in order to correctly position the image on the sheet.

<FIG> shows schematically an embodiment of a printer <NUM> according to the present invention. The printer <NUM>, for purposes of explanation, is divided into an output section <NUM>, a print engine and control section <NUM>, a local user interface <NUM> and an input section <NUM>. While a specific printer is shown and described, the disclosed embodiments may be used with other types of printer such as an ink jet print system, an electrographic print system, etc..

The output section <NUM> comprises a first output holder <NUM> for holding printed image receiving material, for example a plurality of sheets. The output section <NUM> may comprise a second output holder <NUM>. While <NUM> output holders are illustrated in <FIG>, the number of output holders may include one, two, three or more output holders. The printed image receiving material is transported from the print engine and control section <NUM> via an inlet <NUM> to the output section <NUM>. When a stack ejection command is invoked by the controller <NUM> for the first output holder <NUM>, first guiding means <NUM> are activated in order to eject the plurality of sheets in the first output holder <NUM> outwards to a first external output holder <NUM>. When a stack ejection command is invoked by the controller <NUM> for the second output holder <NUM>, second guiding means <NUM> are activated in order to eject the plurality of sheets in the second output holder <NUM> outwards to a second external output holder <NUM>.

The output section <NUM> is digitally connected by means of a cable <NUM> to the print engine and control section <NUM> for bi-directional data signal transfer.

The print engine and control section <NUM> comprises a print engine and a controller <NUM> for controlling the printing process and scheduling the plurality of sheets in a printing order before they are separated from input holder <NUM>, <NUM>, <NUM>.

The controller <NUM> is a computer, a server or a workstation, connected to the print engine and connected to the digital environment of the printer, for example a network N for transmitting a submitted print job to the printer <NUM>. In <FIG> the controller <NUM> is positioned inside the print engine and control section <NUM>, but the controller <NUM> may also be at least partially positioned outside the print engine and control section <NUM> in connection with the network N in a workstation N1.

The controller <NUM> comprises a print job receiving section <NUM> permitting a user to submit a print job to the printer <NUM>, the print job comprising image data to be printed and a plurality of print job settings. The controller <NUM> comprises a print job queue section <NUM> comprising a print job queue for print jobs submitted to the printer <NUM> and scheduled to be printed. The controller <NUM> comprises a sheet scheduling section <NUM> 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 <NUM>, 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 <NUM> will also be called scheduler <NUM> hereinafter.

The sheet scheduling section <NUM> 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 <NUM>, marking material located in a reservoir <NUM> near or in the print head or print assembly <NUM> of the print engine, or finishing material located near the print head or print assembly <NUM> of the print engine or located in the output section <NUM> (not shown).

The paper path comprises a plurality of paper path sections <NUM>, <NUM>, <NUM>, <NUM> for transporting the image receiving material from an entry point <NUM> of the print engine and control section <NUM> along the print head or print assembly <NUM> to the inlet <NUM> of the output section <NUM>. The paper path sections <NUM>, <NUM>, <NUM>, <NUM> 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 <NUM> is suitable for ejecting and/or fixing marking material to image receiving material. The print head or print assembly <NUM> is positioned near the paper path section <NUM>. The print head or print assembly <NUM> 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 <NUM> in a first pass in the loop, the image receiving material receives the marking material through the print head or print assembly <NUM>. A next paper path section <NUM> is a flip unit <NUM> for selecting a different subsequent paper path for simplex or duplex printing of the image receiving material. The flip unit <NUM> 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 <NUM> via a curved section <NUM> of the flip unit <NUM> and via the inlet <NUM> to the output section <NUM>. The curved section <NUM> of the flip unit <NUM> may not be present and the turning of a simplex page has to be done via another paper path section <NUM>.

In case of duplex printing on a sheet or when the curved section <NUM> 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 <NUM> until it reaches a merging point 34A at which sheets entering the paper path section <NUM> from the entry point <NUM> interweave with the sheets coming from the paper path section <NUM>. The sheets entering the paper path section <NUM> from the entry point <NUM> are starting their first pass along the print head or print assembly <NUM> in the loop. The sheets coming from the paper path section <NUM> are starting their second pass along the print head or print assembly <NUM> in the loop. When a sheet has passed the print head or print assembly <NUM> for the second time in the second pass, the sheet is transported to the inlet <NUM> of the output section <NUM>.

The input section <NUM> may comprise at least one input holder <NUM>, <NUM>, <NUM> for holding the image receiving material before transporting the sheets of image receiving material to the print engine and control section <NUM>. Sheets of image receiving material are separated from the input holders <NUM>, <NUM>, <NUM> and guided from the input holders <NUM>, <NUM>, <NUM> by guiding means <NUM>, <NUM>, <NUM> to an outlet <NUM> for entrance in the print engine and control section <NUM>. Each input holder <NUM>, <NUM>, <NUM> may be used for holding a different kind of image receiving material, i.e. sheets having different media properties. While <NUM> input holders are illustrated in <FIG>, the number of input holders may include one, two, three or more input holders.

The local user interface <NUM> is suitable for displaying user interface windows for controlling the print job queue residing in the controller <NUM>. In another embodiment a computer N1 in the network N has a user interface for displaying and controlling the print job queue of the printer <NUM>.

<FIG> shows a schematic cross-sectional view of a first conveyor <NUM> positioned at the paper path section <NUM>. The first conveyor <NUM> comprises a plurality of rollers <NUM> which support and drive an endless conveyor belt <NUM>. At least one of the rollers <NUM> is provided with a drive or motor for driving the belt <NUM>. The belt <NUM> is permeable to gas, specifically to air, to apply an underpressure to a sheet <NUM> of an image receiving member positioned on the belt <NUM>. The sheet <NUM> is thereby held in position against the belt <NUM>. Generally, the belt <NUM> is or has been aligned with respect to the print head assembly <NUM>. A belt detector <NUM> is provided to detect movement of the belt <NUM>, specifically its velocity, lateral position, and/or orientation with respect to the print head assembly <NUM>. In <FIG>, the belt detector <NUM> is formed as a plurality of optical detectors <NUM> positioned at different longitudinal positions along the belt <NUM> in the transport direction D1. By detecting markers, such as perforations or indicators at two or more different points, the velocity, lateral position, and/or orientation of the belt <NUM> may be derived. Other types of detectors, such as encoders may also be applied.

The belt <NUM> is provided with a matrix of through-holes to draw in air through the belt <NUM>. The belt <NUM> is positioned above a suction chamber <NUM> which is connected to a suction source <NUM>, such as a pump or fan, via line <NUM>. It will appreciated that the suction source <NUM> may be positioned remote from the suction chamber <NUM> by extending the line <NUM>. To achieve good image quality the sheet <NUM> should be flatly positioned below the print head assembly <NUM>. This prevents any irregularities in the sheet <NUM> from resulting in print artifacts. It further allows for a narrow print gap between the print head assembly <NUM> and the sheet <NUM>, which allows for more accurate ink droplet positioning. Means for holding the sheet <NUM> onto the belt <NUM> other than suction may be applied for holding the sheet <NUM> onto the belt <NUM>, such as mechanical pressing means, electrostatic charges, etc..

Upstream of the first conveyor <NUM>, the second conveyor <NUM> is positioned. The second conveyor <NUM> is formed of a transport pinch <NUM> in combination with a sheet guide, which in <FIG> is formed as a guide plate which supports the sheet <NUM>. The transport pinch <NUM> comprises a holding device formed of opposing rollers <NUM>, <NUM>. As shown in <FIG>, the bottom rollers <NUM> are formed as laterally spaced apart wheels <NUM>, while the upper roller <NUM> is formed of a longer and/or larger cylinder. One of the rollers <NUM>, <NUM> is drivable by means of a drive or motor (not shown) to transport the sheet <NUM> further along in the transport direction D1. To drive the sheet <NUM>, the rollers <NUM>, <NUM> are positioned in an adjacent position A, which ensures sufficient engagement of the sheet <NUM> by the driven roller <NUM>. This position is generally referred as the "closed" position A. The rollers <NUM>, <NUM> are further moveable with respect to one another into a remote position B, which is commonly referred as the "open" position. In <FIG>, the upper roller <NUM> is moveably mounted on a support and provided with an actuator for moving the roller <NUM> between position A and position B. Different means for actuating the rollers <NUM>, <NUM> between the positions A, B may be applied, such as pivots, hinges, linear actuators, etc. In the open position B, the sheet <NUM> is substantially released from any holding force exerted on it by the second conveyor <NUM>. In practice, some minor frictional forces may remain where the sheet <NUM> is supported by second conveyor <NUM>, though these forces are significantly less than the holding force of the second conveyor <NUM> with the rollers <NUM>, <NUM> in the closed position A.

The second conveyor <NUM> further comprises a registration <NUM> device for laterally shifting and/or rotating the sheet <NUM>. In <FIG> the registration device <NUM> is formed by a plurality of individually drivable wheels or rollers <NUM>. The rollers <NUM> are spaced in the lateral direction D2 and positioned sufficiently adjacent in the transport direction D1 to simultaneously engage the sheet <NUM>. The orientation and/or position of a sheet <NUM> is determined by means of a sheet detector <NUM>, which may be configured to detect passage of the sheet <NUM> along a predetermined point or points in the transport path. The sheet detector <NUM> in <FIG> preferably comprises a pair of stationary optical sensors for detecting passage of the sheet <NUM> along the sensors at at least two different points. This allows the controller <NUM> to derive the sheet's position in the transport direction D1, its rotational orientation, and/or its lateral position. By controlling one roller <NUM> to rotate at a different velocity than the other roller <NUM>, the sheet <NUM> may be rotated. Similarly, the sheet <NUM> may laterally shifted, for example by consecutive rotations which involve a lateral shift, but which cancel each other out in an angular direction measured around an axis in the perpendicular direction D3. The rollers <NUM> may further be independently rotatable around axes in the perpendicular direction D3 for improved control over the sheet registration. It will be appreciated that the sheet sensors <NUM> may be positioned to allow the controller <NUM> to track the repositioning and/or re-orientation of the sheet <NUM> during its registration, for example by positioning optical sensors downstream of the rollers <NUM>, <NUM>. Different registration devices may be applied, such as a longitudinal guide block positioned at a lateral side of the second conveyor at the desired lateral position of the sheet <NUM>, wherein the sheet <NUM> is rotated into the desired orientation via contact with the guide block. It will further be appreciated that the registration device <NUM> may be provided separate from the sheet conveyor <NUM>, such that the registration is performed at a different position than where the holding force of the second conveyor <NUM> is applied.

<FIG> illustrate the steps of transferring a sheet between conveyors <NUM>, <NUM> according to an embodiment of the present invention. The steps are further schematically indicate in the diagram in <FIG>. In step i, shown in <FIG>, the sheet <NUM> of a certain media type is received by the second conveyor <NUM>, which determines the position and orientation of the sheet <NUM> using the sensors <NUM>. In <FIG> the sheet <NUM> is skewed with respect to the print head assembly <NUM> and should be repositioned into the desired alignment with respect to the print head assembly <NUM>. Step i further includes registering the sheet <NUM>. The sheet <NUM> in <FIG> is engaged and held by the second conveyor <NUM>, which allows the second conveyor <NUM> to drive the sheet <NUM> forward, and to adjust the position and/or orientation of the sheet <NUM> to achieve the registered state shown in <FIG>. In the registered state the sheet <NUM> is suitably positioned with respect to the print head assembly in the lateral direction D2. The second conveyor <NUM> in its "closed" position A transports the sheet <NUM> in its registered state onto the first conveyor <NUM> at a predetermined speed, such that the controller <NUM> is able to accurately track the position of the sheet <NUM> in the transport direction D1. It will be appreciated that while the sheet <NUM> is held by the rollers <NUM>, <NUM> further progress of the sheet <NUM> on the second conveyor <NUM> beyond the point of the sensors <NUM> may be derived from transport information sent from the second conveyor <NUM> to the controller <NUM>. For example, by sensing a position of a lateral and/or longitudinal edge of the sheet <NUM> may be sensed by the sensors <NUM>. The position information is sent to the controller, which is able to determine further progress of the sheet by receiving transport information, such as velocity and/or direction, from the rollers <NUM>, <NUM>. Thereby, after registration, the position of the leading edge of the sheet can be derived from the transport information from the second conveyor <NUM> up to the moment the second conveyor <NUM> releases its hold on the sheet <NUM> by moving the roller <NUM> to position B. Before said release, the sheet <NUM> is partially transferred to onto the first conveyor <NUM>.

As shown in <FIG>, the sheet <NUM> is in step ii transferred onto the first conveyor <NUM> while still being held and transported by the second conveyor <NUM>. The controller <NUM> controls the first and second conveyors <NUM>, <NUM> at substantially identical transport velocities. The second <NUM> drives the sheet <NUM> onto the first conveyor <NUM>, such that the leading edge of the sheet <NUM> moves synchronously with the belt <NUM>. Initially, the first conveyor <NUM> will not exert any holding force on the sheet <NUM>, specifically while the leading edge of the sheet <NUM> is still over the upstream roller <NUM>. As the leading edge of the sheet <NUM> is pushed by the second conveyor <NUM> beyond the upstream roller <NUM>, the sheet is partially positioned over the suction chamber <NUM>, which exerts an underpressure on the sheet <NUM>. The holding force applied by the underpressure increases as the sheet's coverage over the suction box <NUM> increases. This is in part to the increasing area to which the underpressure is applied and/or to a reduction of air leaking into the suction chamber <NUM> through uncovered portions of the belt <NUM>. The position of the sheet <NUM> with respect to the print head assembly <NUM> was determined by the controller <NUM> initially from transport information from the second conveyor <NUM> and the sensed position information from the sensors <NUM>. The controller <NUM> is further able to derive further progress of the sheet <NUM> after transfer to the first conveyor <NUM>, from transport information from the first conveyor <NUM>, for example from the belt detector <NUM>. This requires however that the sheet <NUM> moves synchronously with the belt <NUM> after the sheet <NUM> is released from the rollers <NUM>, <NUM>, as otherwise the actual position of the sheet <NUM> trails behind the sheet position determined by the controller <NUM> from the transport information of the first conveyor <NUM>. An early release of the sheet <NUM> could however result in an insufficient holding force of the first conveyor at the initial stage of transfer, causing the belt <NUM> to slip with respect to the sheet <NUM>, thereby causing a discrepancy between the sheet position as derived from the belt transport information and the actual sheet position. This is avoided by the controller <NUM> determining a release timing for the second conveyor <NUM>. To prevent displacement of the sheet <NUM> after release from the second conveyor <NUM>, the release is timed to occur only after a sufficiently large holding force of the first conveyor <NUM> on the sheet <NUM> has been achieved.

The controller <NUM> has determined a media type parameter (90A in <FIG>), corresponding to a media type of the respective sheet <NUM>. The media type parameter 90A is selected from the media catalogue <NUM>, for example from information in the print job, via the user interface <NUM> when re-supplying the input section <NUM>, or via automatic media type detection means provided e.g. at the input section <NUM>. The media catalogue <NUM> defines for each media type 90A a plurality of characteristics 90B-90E, such as a factors indicative of sheet length 90B, sheet width 90C, sheet thickness, air permeability 90D, sheet weight, material density, and/or grammage. Additional characteristics may be included and/or above described factors may be expressed through different properties. In the example in <FIG>, the media catalogue further comprises a pre-stored release time or length characteristic 90E for each media type 90A. The pre-stored release time or length characteristic 90E defines a release point on the first conveyor <NUM> in the longitudinal direction beyond which the holding force from the suction chamber <NUM> on the sheet <NUM> should be sufficient to avoid slippage between said sheet <NUM> and the belt <NUM>, for the respective media type of the sheet <NUM>. When a media type 90A has been selected, the controller <NUM> in step iii retrieves the respective release time or length characteristic 90E from the media catalogue <NUM>. In the example, of <FIG>, the controller <NUM> determines in this manner a release length L measured from the upstream end of the suction box <NUM>. By tests or calculations, it was determined that when the leading edge of the sheet <NUM> has passed beyond said release length L, the coverage of the sheet <NUM> on the suction chamber <NUM> or belt <NUM> is sufficient to achieve a secure holding force of the first conveyor <NUM> on the sheet <NUM>. Slipping of the sheet <NUM> with respect to the sheet support surface of the belt <NUM> is thereby reduced or even prevented. The step iv of the roller <NUM> releasing the sheet <NUM> is illustrated in <FIG>. The controlled handover allows the controller <NUM> in step v to accurately derive the sheet position from the transport information of the first conveyor <NUM> after release. Consequently, the controller <NUM> in step vi is able to suitably control the jetting timing of the print head assembly <NUM> in accordance with the arrival of the leading edge of the sheet <NUM> at the print head assembly <NUM>. This allows for an accurate positioning of the printed image on the sheet <NUM> without additional sheet position sensors at the first conveyor <NUM>. It will be appreciated that the controller <NUM> may in another embodiment comprise an algorithm, which determines the release length or time characteristic from other properties of each media type stored in the media catalogue <NUM>, for example the sheet length 90B, sheet width 90C, and sheet air permeability 90D.

Claim 1:
A sheet conveyor assembly for a printer (<NUM>), comprising a first conveyor (<NUM>) with a first holding device configured for exerting a holding force on a sheet (<NUM>) proportional to a coverage of the sheet (<NUM>) on the first conveyor (<NUM>) and a second conveyor (<NUM>) upstream of the first conveyor (<NUM>) comprising a second holding device configured for holding and releasing the sheet (<NUM>), wherein the first conveyor (<NUM>) comprises a moveable sheet support surface onto which the sheet (<NUM>) is at least partially held while on the first conveyor (<NUM>) in the form of an endless, air permeable belt (<NUM>) positioned over a suction chamber (<NUM>) in connection to a suction source (<NUM>) for applying an underpressure to at least a portion of sheet (<NUM>) on the first conveyor (<NUM>), such that a holding force applied by the underpressure increases as the sheet's coverage over the suction chamber (<NUM>) increases, characterized by a controller (<NUM>) storing on its memory a sheet media catalogue (<NUM>) and configured to control a release timing of the second holding device (<NUM>) for releasing the sheet (<NUM>) partially engaged by the first conveyor (<NUM>) based on a media type parameter (90A) selected from the media catalogue (<NUM>), wherein the release timing determines a coverage of the sheet (<NUM>) over the first conveyor (<NUM>).