Method of transporting sheets in a digital printing machine

Abstract

 The invention relates to a method of transporting sheets in a digital printing machine.
The invention is characterized in that an individual sheet in a transport loop can essentially be negatively accelerated and essentially be positively accelerated.
Further, the invention relates to a device for transporting a sheet, in particular after a sheet has been picked off a stack and separated.
In accordance with the invention, this object is achieved in that the sheet is continuously accelerated from a stopped state to a transport speed.

Description

[0001] The invention relates to a method of transporting sheets in a digital printing machine, wherein an individual sheet, preferably each sheet, is brought from a first transport speed to a second transport speed, preferably for transfer from one segment of the transport path to another segment of the transport path, said sheet experiencing at least one speed change (positive and/or negative acceleration).

[0002] EP-A-1 470 925 discloses an image forming apparatus with a return unit having a double-sided image forming function, more particularly to the control of the media transport speed in the apparatus.

[0003] EP-A-0 807 862 discloses an image forming apparatus with conveying sections, more particularly for forming an image on both sides or in a multiplex fashion, which is capable of properly straightening curls of a recording medium and increasing the image processing speed.

[0004] A method of the aforementioned type has basically been known from US 6,533,264 B1.
With this method, a pre-specified distance between two successive sheets, for example in a copier or printer, is to be set. As a result, a uniform distribution of the sheets as well as a uniform utilization of the printing machine capacity are to be achieved.

[0005] In principle, this is welcome. However, the method described in the cited document is not adequate for processing batches of print jobs that involve duplex-printing (recto and verso printing) jobs.

[0006] Therefore, the object of the invention is to provide a method of the aforementioned type permitting an improved processing of print jobs involving recto-printing and verso-printing, specifically also when sheets having different weights, thus exhibiting different running times in the transport path, are used.

[0007] In accordance with the invention, this object is achieved in that, in order to process sets of sheets in batches by recto-printing and verso-printing (duplex printing), the sheets are fed to a transport loop of a transport path in order to pass, before and after a side-reversing inverting for printing, at least one printing unit, that the transport loop can be loaded with a natural number (n ∈ N) of sheets, said number being a function of the format of the sheets to be processed, whereby the transport loop (virtually) is divided into a corresponding number (n) of loading fields (frames) for the sheets, and that the individual sheet in this transport loop is essentially negatively accelerated for the length of a corresponding transport path segment, preferably before passing the minimum of one printing unit, and is essentially positively accelerated for the length of a corresponding transport path segment, preferably after passing said printing unit.

[0008] Basically, there are - so to speak logistically - two procedures used in a digital printing machine in order to print selected sheets of a print job, i.e., one sheet, every sheet, a few consecutive or even non-consecutive sheets, not only in verso mode but also in recto mode, i.e., the so-called batch mode or the so-called interleaf mode.

[0009] In interleaf mode, the sheets that are to be perfected (verso-printed) are returned and turned over in a transport loop after having been recto-printed and then, like in a zipper, alternately merged or "meshed" with the sheets that are still being recto-printed. The minimum of one printing unit is alternately fed one recto printing sheet and then, again, one verso printing sheet. This process requires a rather complex coordination, because, for example, it may happen more frequently that, in the course of one print job, one or the other type of printing creates longer breaks because many successive sheets are to be printed in recto mode only or every sheet is to be printed also in verso mode, which may certainly result in a situation in which, considering individual sheets, only the rear side is to be printed, i.e., only verso-printing without recto-printing is supposed to occur. As a result of this, the print job, which potentially is also to be bound sorted, can be mixed up significantly and the minimum of one printing unit is most likely not optimally utilized, despite complex time management.

[0010] Therefore, in the rather to be preferred batch mode, the sheets of a print job are processed in batches; this means, sheets that are to be verso-printed are turned over in a greater number and again fed to the minimum of one printing unit and, in so doing, "interleaved" with batches of sheets that are still being recto-printed or "added on" to previous batches. Thus, sets (batches) of sheets alternate as they pass the printing unit. This causes specifically two problems.

[0011] The transport loop must be divided into a positive whole number, for example 18, of imaginary or virtual fields or frames in order to accommodate exactly one pre-specified batch of sheets. The field size, and thus the number of fields potentially accommodated in the transport loop, is a function of the respective format of the sheets. In so doing, less deviating formats such as, e.g., DIN A3 and DIN A3+ do not result in a change of field size but the field is measured to have a size that will still accommodate the largest sheet of this format but that, considering a sheet having a format of, e.g., DIN A4, the field size can be reduced to half and the number of fields can be doubled.

[0012] The second problem is that, in particular two successive batches must follow each other positioned correctly (registered). In so doing, it must be taken into consideration that sheets having different weights exhibit different running time behaviors in the transport path. In this case, it is not inertia that leads to a slower transport of heavier sheets, but heavier sheets are transported even faster, this probably being due to the fact that heavier sheets are also stiffer and thus are not bent as much by transport rollers and do not sag as much between transport rollers but have a more direct, abbreviated effective transport path.

[0013] Documents DE-A- 102 34 629 and DE-A- 103 38 949, to which express reference is made here, address this problem of processing sheets in a digital printing machine and the feeding of such sheets, preferably in batch mode.

[0014] The cited DE-A- 102 34 629 explains in particular that, in an electrophotographically operating printing machine - which will be described in detail for the sake of clarity, without, however, representing a restriction to such a type of digital printing machine - sheets to be printed are fed by one or more feeders to a paper path or, in more general terms, to a transport path for printing material of any type. If several feeders or feeding units are provided, printing material - specifically of different formats, weights, consistencies or the like - can be made available. This is of particular advantage especially in the case of a digital printing machine because a new image is created anyhow for each new page to be printed and thus even mixed print jobs can be processed without problems, namely those, in which, for example, such an individual print job consists of pages of a brochure which are directly fed in successive order to the printing unit and, subsequently, optionally also to a finishing step, in which case, for example, the front and rear cover sheets may consist of a heavier-weight paper and the subsequent papers may consist of a lighter-weight paper and, in between even films of plastic material with diagrams or the like may be printed. Such different printing materials would be made available in different feeding units and would be fed in a preselected, quasi intermittent, order to the transport path.

[0015] In so doing, a first transport path segment that starts at the feeding units, may consist for example of rotating driven grip belts, between which sheets are transported. Thereafter, the sheets could be transferred to and placed onto a rotating driven transport belt and adhere there due to electrostatic forces. In most cases this transport belt is a transparent web of plastic material and passes through a printing system, which, for color printing, may of course comprise several printing units. In electrophotographic printing one latent toner image per color separation is transferred to the sheet. Thereafter, the sheet is transported to a fusing unit, in which the toner image is fused to the printing material, specifically melt-deposited there, and cooled. Considering the transport into and through the fixing unit, a change of the transport member could again occur. Only sheets to be printed on one side are then continued to be transported or ejected into a tray. After the fusing step, sheets to be printed on both sides are returned to pass the printing unit and are turned over via a transport path loop for continued printing. The reverse transport and the turn may take place at the same time, for example, in that, also on this transport path segment, grip belts are used which take an approximately helical course and, in so doing, rotate the sheets about their longitudinal axes (pointing in the transport direction) by 180 degrees.

[0016] In particular the transport belt passing through the printing system and being frequently referred to as a web in electrophotography, is to be loaded with sheets to be printed, whereby the space between sheets is to be small enough to achieve the greatest possible throughput per unit of time, i.e., ensure the highest possible printing output. On the other hand, minimum distances between successive sheets must be maintained. This applies to simplex-printing of only the front side of sheets, as well as to duplex-printing when the front and rear sides of the sheets are printed and perfected.

[0017] In order to achieve an optimal or matched loading of the web, the web is divided virtually, or also by means of controllers, into areas which can be described as frames, in which respectively one sheet - taking into consideration common formats - is to be precisely placed for printing. In so doing, an area of the web is recessed, said area optionally having a transverse seam, by means of which the ends of the web are connected in order to form a closed loop. Usually, for convenience, this seam is also used as a mark that is detected by a sensor in order to allow a control of the rotary position of the web and to have a reference point. Therefore, this seam must not be covered by a sheet. Of course, other marks could also be taken into consideration, in particular those which are applied only along the edge of the web.

[0018] To ensure, even in duplex-printing mode, that these frames on the transport belt will be exactly met again after the return transport of the sheets in order for the sheets to be transferred, the ratio of the running time of the sheets rotating via the return after the first side has been printed with respect to the running time of the web must represent a whole number.

[0019] However, despite this, problems may still occur in that sheet running times inside the considered printing machine are affected by various parameters. For example, the weight of the paper and the length of the paper have been found to represent dominant paper variables. Likewise, machine-specific parameters such as, for example, exact transport path length, roller diameter and motor speeds are contributing factors.

[0020] This behavior is the reason for various problems (e.g., image quality, insufficient distance between paper sheets) that occur when the machine is running. This is particularly noticeable when mixed paper print jobs are run. For example, thick (heavy) sheets have shorter running times than thin sheets. Consequently, during their run through the machine, the distance between two successive sheets can decrease distinctly (the fast, thick one catches up), this leading to an interruption of the printing function due to too small a distance between the sheets and hence to a clear loss of machine performance. Likewise, sheets could be deposited on the web seam and thus trigger image errors.

[0021] Therefore, the cited DE-A- 102 34 629 suggests that a starting time for feeding a sheet from any, or the only, feeding unit is chosen with respect to the type of printing material of which said sheet consists.

[0022] In this reference, advantageously, sheets are started depending on their type - specifically their length and/or weight - at different times, i.e., fed by the respective feeding unit to the transport path, in order to apply a counter-error to potentially (even with respect to each other) wrong positioned sheets as expected during transport for correction at the onset, so that the desired position will be taken during transport. In order to be able to perform such a preliminary control in a quantitatively targeted manner, a modification in this case provides that information for the selection of the starting time is yielded beforehand by at least one trial run with at least one type of printing material, preferably by trial runs with different types of material, while a corresponding empirical table is created, for example, configured as a look-up table, i.e., a specified table.

[0023] However, it has been found that the generation and use of such tables is very complex and still does not always provide fully satisfactory results. In particular such a table is advantageously avoided in accordance with the inventive method in that the actual position of the individual sheets can be corrected in a special manner, thus leaving sufficient time due to the special inventive velocity profiles of the printing process, despite time and space constraints.

[0024] Modifications of the invention provide that the sheet, after being turned over, is essentially positively accelerated and/or the velocity profile is controlled.

[0025] It is particularly advantageous, in view of an optimal usability of available time and length ratios for correction of a potential wrong position of a sheet if, as provided by another modification of the invention, the acceleration is shifted with respect to time for a lengthening or shortening of one or another transport path segment of the transport path loop, as needed. In particular, in this way, a faster sheet may be retained for a longer time at the lower speed and a slower sheet may be retained for a shorter time in order to correct the position.

[0026] Additionally or alternatively, another modification of the inventive method advantageously provides that, during the essentially positive acceleration, the sheet is initially subjected to a negative acceleration from a first transport speed (v₁) to a (slightly) lower speed (v_min), then subjected to a positive acceleration to a high speed (v_max), and only then brought to the second transport speed (v₂), said second transport speed being (slightly) lower than the high transport speed (v_max) but (distinctly) higher than the first transport speed (v₁), whereby the positive acceleration from the lower speed (v_min) to the high speed (v_max) is chronologically shifted as needed, and/or that, during the essentially negative acceleration, the sheet is initially subjected to a positive acceleration from the second transport speed (v₂) to the high speed (v_max), then negatively subjected to a negative acceleration to the lower speed (v_min), and only then brought to the first transport speed (v₁), whereby the negative acceleration from the high speed (v_max) to the lower speed (v_min) is chronologically shifted as needed.

[0027] Another modification of the invention provides that, in order to detect the time of arrival of an edge of a sheet, specifically the lead edge and/or the rear edge of the sheet, in a pre-specified position, an edge sensor is provided and that, based on this detection, a comparison of this position of the sheet with a desired position of the sheet to be assumed at this point in time is to take place, and that the velocity control is based on this. For example, based on this, the exact time of arrival of the lead edge of a respective sheet at the beginning of the acceleration segment can be determined, when - considering this acceleration - said sheet's rear edge is cleared, and/or when said sheet's lead edge or rear edge reaches the end of this acceleration segment.

[0028] Preferably, a precisely controllable high-performance stepper motor is used for the accelerations.

[0029] Considering the accelerations, preferably a path segment between successive transport members, specifically pairs of rollers, of the transport path is used. An edge sensor is placed preferably at the start of said path segment.

[0030] A method not covered by the claims, but useful for understanding the invention, is a method of transporting sheets in a digital printing machine, whereby a single sheet, preferably each sheet, is brought from a first transport speed to a second transport speed, preferably for transfer from one segment of the transport path to another segment of the transport path, said sheet experiencing at least one speed change (positive and/or negative acceleration), said method independently achieving the object and being characterized in that a counted clock pulse reflecting the position of loading fields (frames), into which a transport loop for sheets is (imaginarily) divided, is generated, and that, based on this, the insofar absolute position of the sheet, i.e., the position of the sheet being a direct function of the positions of the previous and subsequent sheets, is determined with respect to at least one loading field, and that this position is used for a comparison with a desired position of this sheet, preferably with respect to the said loading field.

[0031] Thus, the inventive method permits, in an especially advantageous manner, a precise positioning of sheets for batch mode, without encountering the above-described problems.

[0032] For example, it may be additionally considered that the space potentially required for marks, specifically register marks, on a transport element (web) for sheets is available.

[0033] The invention provides in particular the advantage that sheets can be fed directly by at least one feeder to a duplex turning loop configured as a transport loop for recto-printing and verso-printing, independent of whether the respective sheet is actually to be printed in recto-printing mode and verso-printing mode or only in recto-printing mode or verso-printing mode. As a result of this, in particular a separate, extended and additional feeder transport path segment is unnecessary, the feeder or feeders - specifically configured as several drawer-like paper trays can be better integrated in the machine frame, which, among other things, can minimize the footprint, and this platform can be configured in modules that can be modified as needed.

[0034] A device not covered by the claims, but useful for understanding the invention, is a device for transporting sheets in a digital printing machine, said device being used for bringing a single sheet, preferably each sheet, from a first transport speed to a second transport speed, preferably for transfer from one segment of the transport path to a another segment of the transport path, said sheet experiencing at least one speed change (positive and/or negative acceleration), preferably for carrying out the described method, said device independently achieving the object and being characterized in that, in order to process sets (batches) of sheets in recto-printing mode and in verso-printing mode (duplex), a transport loop of a transport path is provided, in which the sheets pass at least one printing unit before and after a side-reversing turn for printing, that the transport loop can be loaded with a natural number (n ∈ N) of sheets which is a function of the format of the sheets to be processed, whereby the transport loop (virtually) is divided into a corresponding number (n) of loading fields (frames) for the sheets, and that the individual sheet in this transport loop can essentially be negatively accelerated for the length of a corresponding transport path segment and subsequently essentially be positively accelerated for the length of a corresponding transport path segment.

[0035] The advantages resulting from this device, modifications of which are disclosed have already been sufficiently described in conjunction with the method.

[0036] The invention relates to a method for transporting a sheet, in particular after a sheet has been picked off a stack and separated, preferable for the feeder-side use in a printing machine, with said sheet being held by a transport element, preferably a transport belt configured as a suction belt, and brought to a transport speed, preferably for transfer to another transport path.

[0037] Further, the invention relates to a device for transporting a sheet, in particular after a sheet has been picked off a stack and separated, preferably for the feeder-side use in a printing machine, said device comprising a transport element, preferably a transport belt configured as a suction belt, for holding the respective sheet and for bringing the sheet to a transport speed, as well as, preferably, for transferring said sheet to another transport path in order to carry out the aforementioned method.

[0038] A method and a device of the aforementioned type have been known, in principle, from document DE 196 07 826 A1, corresponding to document US 5 634 634 A. Specifically (see DE 196 07 826 A1, column 10, lines 22 through 60), a clutch is selectively actuated to couple a motor with a roller set in such a manner that the transport belts are driven such that the grasped sheet is transported off a stack of sheets and is then ready for further processing. As a result of this abrupt clutching operation, in particular when relatively heavy paper is accelerated, it could be well that such a jolt occurs that the sheet loses its hold and is transported in an uncontrolled manner.

[0039] Therefore, the object of the invention is to provide a controlled method of the aforementioned type and a device which permits the use of this method, in particular, considering sheets having a larger size and/or weight per unit area.

[0040] In accordance with the invention, this object is achieved, in so far as the method is concerned, in that the sheet is continuously accelerated from the stopped state to the transport speed.

[0041] As a result of this, potential problems that occur when a sheet is made available from a stack and when said sheet is first transported away, are significantly reduced.

[0042] A modification of the inventive method provides that the velocity profile of the transport element essentially comprises three time phases, i.e., a first phase, namely the so-called acceleration phase of continuous acceleration from the stopped state to the transport speed, a second phase, which is the essentially constant transport speed, and a third phase, which is the reduction of the transport speed back to the stopped state.

[0043] This ensures a particularly gentle and controlled transport of each sheet.

[0044] Also, the progress of acceleration or the velocity profile plays a part in a problem-free acceleration of a sheet during the acceleration phase. Particularly advantageously, another modification provides that the acceleration profile, during acceleration from the stopped state to the transport speed as a function of time (t), follows substantially a function sin^xt, where the exponent x represents a number that is greater than or equal to 1 to smaller than or equal to 4. Preferably, exponent x is approximately equal to 2.

[0045] Likewise, the speed reduction profile of the reduction during the third phase from the transport speed to the stopped state as a function of time (t) follows essentially a function sin^x t, where the exponent x represents a number that is greater than or equal to 1 to less than or equal to 4. In this case, the negative acceleration profile of speed reduction can also be steeper than during the positive acceleration of the first phase, so that, preferably, in this case the exponent x may be approximately equal to 4.

[0046] Another modification of the inventive method provides that the acceleration profile representing the acceleration from the stopped state to the transport speed is computed and stored, or otherwise recorded, before transport of the sheets, so that during the actual operation, no additional time-critical interventions are necessary during the operating cycle.

[0047] Preferably, the velocity profile is controlled electronically. To achieve this, the acceleration profile is preferably stored in electronic control means.

[0048] Furthermore, protection is claimed for an inventive device for transporting a sheet, in particular for picking a sheet off a stack and separating said sheet, preferably for the feeder-side arrangement in a printing machine, said device comprising a transport element, preferably a transport belt configured as a suction belt, for holding the respective sheet and for bringing said sheet to a transport speed, as well as, preferably, for transferring said sheet to another transport path, in order to carry out the aforementioned method, said method being characterized in that the transport element can be driven in such a manner that it can be continuously accelerated from the stopped state to the transport speed.

[0049] The advantages resulting from this have already been explained in conjunction with the inventive method.

[0050] In order to provide a sheet from a stack and to pull this sheet off the stack, the inventive device preferably comprises a (delivery) motor for driving the transport element. Preferably, this delivery motor is a high-performance stepper motor.

[0051] Embodiments of the invention, which could result in additional inventive features without, however, restricting the scope of the invention thereto, are referred to by the drawings.

[0052] They show in

Fig. 1

a schematic side elevation of a part of a digital printing machine for carrying out the inventive method, and

Figs. 2 through 9

velocity profiles, respectively showing the velocity v of a sheet as a function of time t,

Fig. 10

a side elevation of an inventive device, and

Fig. 11

a velocity and acceleration profile for carrying out the inventive method as a function of time (t).

[0053] Fig. 1 shows a schematic side elevation of a part of a printing machine for carrying out the inventive method.

[0054] Referring to the printing machine, there are indicated a feeder 1 for sheets of printing material, a feeding segment 2 for sheets of printing material from said feeder 1, in which case also more than one feeder may be provided, and a pocket 3 for sheets of printing material, in which case an output segment 14 leads to said pocket. The main part of a transport path for sheets of printing material consists of a duplex turning loop 5, which represents a segment of the transport path, the turning operation being symbolically indicated by bent arrows 16. An integral part of this duplex turning loop 5 is a closed rotating transport belt 4, which, in particular, moves the sheets of printing material past printing units 15 in order for said sheets to be printed.

[0055] Various positions of sheets of printing material are shown for example on transport path segments 4, 5, 14. Sheets 7 and 8 are already on transport belt 4. Sheets 6 and 9 are in the duplex turning loop 5 and are either just leaving feeder 1 or have already been passed by printing units 15 and been turned over in zone 16. Within a short time, they will (optionally again) move onto transport belt 4. Sheets 10 and 11 have already again left transport belt 4 and/or duplex turning loop 5 and are on their way to pocket 3.

[0056] Sheets 6, 9, at least when they, coming from feeder 1, are to move for the first time into duplex turning loop 5 and to transport belt 4, are detected, while they are being transferred to transport belt 4, by a lead edge sensor 13 as shown by the example, said sensor being connected to a controller of the printing machine which comprises, for example, a processor. The arriving sheets 6, 9 are fed in a timed manner to transport belt 4 as sheets 7, 8, for example, in such a manner that the printing machine is used in the optimal possible manner, in particular when one printing unit or several color printing units 15 are used optimally. Likewise, sheets 6, 9 are (again) fed to transport belt 4 when said sheets have passed through the duplex turning loop 5 in order to now be printed (after their front sides have been printed or not) on their rear sides by printing units 15. To achieve this, a sheet 6, 9 can again be detected by lead edge sensor 13. The circulating time of sheet 6, 9 from lead edge sensor 13 via transport belt 4 into duplex turning loop 5 and through the latter back to lead edge sensor 13 is known, so that, during the first pass of a sheet through printing units 15, the sheet's return for printing its rear side could already be planned with respect to time, because, in particular, it is already known during the first pass by lead edge sensor 13 whether the just detected sheet 6, 9 is to be printed in simplex mode or in duplex mode.

[0057] However, it must be taken into consideration that, for example, sheets having different weights exhibit different running time behavior in turning loop 5 and that sheets 6, 9, respectively, are to be batch-processed, in which case the number of sheets 6, 9, indicated here only as an example and sporadically for the sake of clarity, in a batch is such that the sheets of the batch just fill the turning loop 5, and that successive batches are to follow in registered position. Therefore, for potential skew correction in accordance with the invention, sheets 6, 7, 8, 9 experience two velocity changes in the duplex turning loop, i.e., at the sites indicated by dashed arrows 19 and 20, which is respectively downstream of turn 16 and upstream of lead edge sensor 13. Transport path segments exhibiting different speeds, for example, make sense even without skew correction in order to be able to feed sheets from feeder 1 at high speed into turning loop 5, for example, and be able to print, with printing units 15, at a relatively low process speed.

[0058] Referring to zones 19 and 20, an individual sheet 6 through 9 is accelerated by so-called "velocity ramps" 17, 18 indicated in Fig. 1 schematically by enlarged details in lined boxes, i.e., the sheet is brought in region 19 by ramp 17, essentially positively, to a higher speed and in region 20 by ramp 18, essentially negatively (decelerating), to a lower speed. Referring to boxes 17, 18, schematic side elevations of transport roller pairs are indicated as circles which may form parts of the transport path in the duplex turning loop 5. Between these transport roller pairs, velocity profiles of the individual sheets are indicated, whereby it is assumed that, in upward direction, the velocity is plotted as a function of time toward the right. For the sake of simplicity, box 17 indicates a linear velocity increase and box 18 indicates a linear velocity decrease, this resulting in a triangular shape with the coordinate axes, reminding of the side elevation of a ramp. In addition, referring to the double arrows, it is indicated in boxes 17, 18 that the starting time of the respective positive or negative acceleration may be shifted chronologically as needed, in which case the time interval available is the time during which the sheet is located in the intermediate space between the indicated transport roller pairs.

[0059] As a reminder and precaution, it should be repeated here that, in view of the optimal usability of available time and length ratios, the acceleration for an on-demand increase or decrease of one or the other transport path segment of the duplex turning loop 5, namely the segment from zone 19 to zone 20 or the segment from zone 20 and, again, to zone 19, is shifted with respect to time. In so doing, specifically a faster sheet can be maintained longer at the lower speed and a slower sheet can be maintained for a shorter time in order to achieve a timing correction in this manner.

[0060] Additionally or alternatively, the inventive method preferably provides that, during the essentially positive acceleration, the sheet initially is subjected to a negative deceleration from a first transport speed (v₁) to a (slightly) lower speed (v_min), then subjected to a positive acceleration to a high speed (v_max), and only then brought to the second transport speed (v₂), said second transport speed being (slightly) lower than the high transport speed (v_max) but (distinctly) higher than the first transport speed (v₁), whereby the positive acceleration from the lower speed (v_min) to the high speed (v_max) is chronologically shifted as needed, and/or that, during the essentially negative acceleration, the sheet is initially subjected to a positive acceleration from the second transport speed (v₂) to the high speed (v_max), then subjected to a negative acceleration to the lower speed (v_min), and only then brought to the first transport speed (v₁), whereby the negative acceleration from the high speed (v_max) to the lower speed (v_min) is chronologically shifted as needed. Referring to Figs. 2 through 9, this will be explained in detail hereinafter.

[0061] Referring to Figs. 2 through 5, the essentially positive acceleration process in zone 19 will be described and explained now.

[0062] Figs. 2 through 5, as well as Figs. 6 through 9, show detailed velocity/time profiles, respectively in boxes 17 and 18, as indicated by a simple triangular shape. In each case the velocity v is plotted as a function of time t (or a distance s). Taking into consideration velocity v or Δ v, time intervals Δ t can be converted, respectively, into distance intervals Δ s between successive transport rollers, as indicated in part in Figs. 2 through 9. The total chronologically or quasi spatially possible shift region of the "ramps" between roller pairs, said shift region being indicated by a double arrow in boxes 17, 18 in Fig. 1, can be identified, in this context with a double arrow, as distance L in Figs. 2 through 9.

[0063] Figs. 2 and 3 now show that a respective sheet could be accelerated simply linearly from a speed v₁ to a speed v₂, whereby this acceleration could begin later (Fig. 2) or sooner (Fig. 3) in order to change the length ratio between the above-addressed transport path segments for faster or slower sheets as needed. In fact, the acceleration is somewhat more complex than shown by Figs. 2 and 3, namely as shown by Figs. 4 and 5. As already mentioned above, during the essentially positive acceleration, the sheet is initially subjected to a negative deceleration from a first transport speed (v₁) to a (slightly) lower speed (v_min), then subjected to a positive acceleration to a high speed (v_max), and only then brought to the second transport speed (v₂), said second transport speed being (slightly) lower than the high transport speed (v_max) but (distinctly) higher than the first transport speed (v₁), whereby the positive acceleration from the lower speed (v_min) to the high speed (v_max) is chronologically shifted in segment Δ s₂ as needed. This procedure allows better consideration of given requirements and, at the same time, of conditions regarding space and time.

[0064] Figs. 6 through 9 show the corresponding relationships and procedures, essentially in reverse, for the essentially negative acceleration in zone 20. In so doing, Figs. 6 and 7, again show a simple linear negative acceleration, which can be shifted with respect to time, while Figs. 8 and 9 again show a more complex, preferred procedure, which provides that, during the essentially negative acceleration, the sheet is initially subjected to a positive acceleration from the second transport speed (v₂) to the high speed (v_max), then negatively subjected to a negative acceleration to the lower speed (v_min), and only then brought to the first transport speed (v₁), whereby the negative acceleration from the high speed (v_max) to the lower speed (v_min) can be chronologically shifted as needed.

[0065] Fig. 10 shows an outline of a highly schematic illustration of another device, in side elevation.

[0066] The device comprises a separating device 21 for lifting a sheet off a stack 22 and for separating said sheet, and for a first transport of the separated sheet into an (additional) transport path. The separating device 21 substantially comprises a transport belt 23, which is configured as a suction belt and is looped around driving rollers 24 and is designed for grasping a sheet lifted off stack 22 and for transporting said sheet in the direction of an arrow 25, and which comprises a suction chamber 26 for attracting a sheet to transport belt 23 and for holding said sheet during its transport by transport belt 23.

[0067] At least one of the driving rollers 24 is permanently connected with a motor 27 by means of a clutch 28, said motor also driving a first pair of transport rollers 29 of a transport path following separating device 21. Said motor 27 is a high-performance stepper motor, which is activated by electronic control means 30. With the use of said electronic control means 30, motor 27 - following a pre-specified velocity profile, can be activated and operated. This will be explained in detail in conjunction with Fig. 11.

[0068] Viewed in transport direction 25 of the sheet, the pair of transport rollers 29 is followed by another pair of transport rollers 31, to which the sheet can be transferred. Apart from that, the further progress of the transport path is not illustrated in detail. For the transfer of the sheet from transport rollers 29 to transport rollers 31, a sensor - in viewing direction of an arrow 32 - is provided for sheet detection, whereby said sensor may also be connected with electronic control means 30 in order to detect the time of arrival of the trailing edge of said sheet.

[0069] Fig. 11 shows a velocity and acceleration profile of the sheet, or motor 27, and transport belt 23 during transport through separating device 21.

[0070] A solid line shows the profile of velocity v as a function of time t. A broken line shows the profile of the associate acceleration as a function of time t for the acceleration phase of the velocity profile.

[0071] The velocity profile is divided into three phases. First, during an acceleration phase, acceleration occurs to a constant transport speed, then, during a second phase, this transport speed is maintained for a certain period of time, and finally, during a third reduction phase, the speed is again reduced to a stopped state. The illustrated acceleration during the first acceleration phase, in so doing, follows a sin² (t) function. In contrast, the reduction of speed during the third reduction phase may be significantly steeper; in this case, the (negative) acceleration could follow, for example, a sin⁴ (t) function. The reduction phase could be started, in particular, when sensor 32 detects the trailing edge of the sheet, i.e., when the sheet leaves the inventive device.

Itemized subject matter

[0072] 

1. Method of transporting sheets in a digital printing machine, wherein an individual sheet, preferably each sheet, is brought from a first transport speed to a second transport speed, preferably for transfer from one segment of a transport path to a another segment of a transport path, said sheet experiencing at least one speed change (positive and/or negative acceleration),
and in order to process sets of sheets in batches by recto-printing and verso-printing (duplex printing), the sheets are fed to a transport loop of a transport path in order to pass, before and after a side-reversing inverting for printing, at least one printing unit; the transport loop can be loaded with a natural number (n ∈ N) of sheets, said number being a function of the format of the sheets to be processed, whereby the transport loop (virtually) is divided into a corresponding number (n) of loading fields (frames) for the sheets; and
the individual sheet in this transport loop is essentially negatively accelerated for the length of a corresponding transport path segment and is later essentially positively accelerated for the length of a corresponding transport path segment and the velocity profile is controlled, characterized in that, during the essentially positive acceleration, the sheet is initially subjected to a deceleration from a first transport speed (v₁) to a (slightly) lower speed (v_min), then subjected to a positive acceleration to a high speed (v_max), and only then brought to the second transport speed (v₂), said second transport speed being (slightly) lower than the high transport speed (v_max) but (distinctly) higher than the first transport speed (v₁).

2. Method as in 1, characterized in that the positive acceleration from the lower speed (v_min) to the high speed (v_max) can be chronologically shifted as needed.

3. Method as in one of 1 or 2, characterized in that, during the essentially negative acceleration, the sheet is initially subjected to a positive acceleration from the second transport speed (v₂) to the high speed (v_max), then subjected to a negative acceleration to the lower speed (v_min), and only then brought to the second transport speed (v₂).

4. Method as in 3, characterized in that the negative acceleration from the high speed (v_max) to the lower speed (v_min) can be chronologically shifted as needed.

5. Method of transporting sheets in a digital printing machine, whereby an individual sheet, preferably each sheet, is brought from a first transport speed to a second transport speed, preferably for transfer from one segment of the transport path to a another segment of the transport path, said sheet experiencing at least one speed change (positive and/or negative acceleration), characterized in that
a counted clock pulse reflecting the position of loading fields (frames), into which a transport loop for sheets is (imaginarily) divided, is generated; and,
based on this, the insofar absolute position of the sheet, i.e., the position of the sheet being a direct function of the positions of the previous and subsequent sheets, is determined with respect to at least one loading field, and that this position is used for a comparison with a desired position of this sheet.

6. Method as in 5, characterized in that, in order to detect the time of arrival of an edge of a sheet in a pre-specified position, an edge sensor is provided and that, based on this detection, a comparison of this position of the sheet at the edge sensor with a desired position which the sheet should have assumed at this point in time takes place, and that the velocity control is based on this.

7. Method as in one of the previous 1 to 6, characterized in that the space potentially required for marks, specifically register marks, on the transport element for sheets is taken into consideration.

8. Method as in one of the previous 1 to 7, characterized in that sheets are directly fed by at least one feeder to a duplex turning loop configured as a transport loop for recto-printing and verso-printing, independent of whether the respective sheet is actually to be printed in recto-printing mode and verso-printing mode or only in recto-printing mode or verso-printing mode.

9. Device for transporting sheets in a digital printing machine, whereby an individual sheet, preferably each sheet, is brought from a first transport speed to a second transport speed, preferably for transfer from one segment of the transport path to a another segment of the transport path, said sheet experiencing at least one speed change (positive and/or negative acceleration), preferably for carrying out the method and in order to process sets of sheets in batches by recto-printing and verso-printing (duplex printing), the sheets are fed to a transport loop of a transport path in order to pass, before and after a side-reversing inverting for printing, at least one printing unit;
the transport loop can be loaded with a natural number (n ∈ N) of sheets, said number being a function of the format of the sheets to be processed, whereby the transport loop (imaginary) is divided into a corresponding number (n) of loading fields (frames) for the sheets; and
the individual sheet in this transport loop can be essentially negatively accelerated for the length of a corresponding transport path segment and can be later essentially positively accelerated for the length of a corresponding transport path segment. and the essentially positive acceleration of the sheet is provided after a inverting zone. and the velocity profile is controlled,
characterized in that the acceleration zone is chronologically shifted for a lengthening or shortening of one or another transport path segment of the transport path loop, as needed.

10. Device as in 9, characterized in that the essentially positive acceleration of the sheet is controlled in such a manner that the sheet is initially subjected to a deceleration from a first transport speed (v₁) to a (slightly) lower speed (v_min), then subjected to an acceleration to a high speed (v_max), and only then brought to the second transport speed (v₂), said second transport speed being (slightly) lower than the high transport speed (v_max) but (distinctly) higher than the first transport speed (v₁).

11. Device as in 13 and 14, characterized in that the positive acceleration from the lower speed (v_min) to the high speed (v_max) can be chronologically shifted as needed.

12. Device as in one of 10 or 11, characterized in that the essentially negative acceleration of the sheet is controlled in such a manner that the sheet is initially subjected to a positive acceleration from the second transport speed (v₂) to the high speed (v_max), then subjected to a negative acceleration to the lower speed (v_min), and only then brought to the second transport speed (v₂).

13. Device as in 12, characterized in that the negative acceleration from the high speed (v_max) to the lower speed (v_min) can be chronologically shifted as needed.

14. Device as in one of the previous 9 to 13, characterized in that, in order to detect the time of arrival of an edge of a sheet in a pre-specified position, an edge sensor is provided and that, based on this detection, a comparison of this position of the sheet at the edge sensor with a desired position which the sheet should have assumed at this point in time takes place, and that the velocity control is based on this.

15. Device for transporting sheets in a digital printing machine, whereby an individual sheet, preferably each sheet, is brought from a first transport speed to a second transport speed, preferably for transfer from one segment of the transport path to a another segment of the transport path, said sheet experiencing at least one speed change (positive and/or negative acceleration), preferably in accordance with one of the previous 9 to 14,
characterized in that
a pulse generator for generating countable clock pulses reflecting the position of loading fields (frames), into which a transport loop for sheets is (virtually) divided; and,
based on this, the insofar absolute position of the sheet, i.e., the position of the sheet being a direct function of the positions of the previous and subsequent sheets, is determined with respect to at least one loading field, and that this position is used for a comparison with a desired position of this sheet.

Claims

1. Method for transporting a sheet, in particular after a sheet has been picked off a stack and separated, preferable for the feeder-side use in a printing machine, with said sheet being held by a transport element, preferably a transport belt configured as a suction belt, and brought to a transport speed, preferably for transfer to another segment of the transport path ,
and the sheet is continuously accelerated from a stopped state to a transport speed. and the velocity profile of the transport element essentially comprises three time phases, i.e., a first phase, namely the so-called acceleration phase of continuous acceleration from the stopped state to the transport speed, a second phase, which is the essentially constant transport speed, and a third phase, which is the reduction of the transport speed back to stopped state.
characterized in that the acceleration profile, during acceleration from the stopped state to the transport speed as a function of time (t), follows substantially a function sin^x t, where the exponent x represents a number that is greater than or equal to 1 to less than or equal to 4.

2. Method as in Claim 1, characterized in that exponent x is approximately equal to 2.

3. Method as in Claim 2, characterized in that the speed reduction profile of the speed reduction during the third phase from the transport speed to the stopped state as a function of time (t) follows essentially a function sin^x t, where the exponent x represents a number that is greater than or equal to 1 to less than or equal to 4.

4. Method as in Claim 3, characterized in that the exponent x is approximately equal to 4.

Drawing