Method and device for controlling the circulation speed of an endless belt and arrangement for generation of a braking force on an endless belt

In a method for control of circulation speed of an endless belt arranged in a printer or copier, the endless belt is directed over at least two rollers where the belt is driven with a preset first circulation speed via at least one of the rollers as a driven roller. Various load states act on the endless belt in successive operating phases during a printing or copying process, and via said various load states the belt being braked with different strengths so that a slippage is generated at least between the belt and the driven roller. A braking force acting directly on the endless belt is generated. Braking force is controlled such that a substantially constant slippage is generated between the driven roller and the belt based on the operating phases so that the endless belt is braked to a second circulation speed.

BACKGROUND

The preferred embodiments concern a method and a device for controlling the circulation speed of an endless belt, in which an endless belt is guided over at least two rollers. The belt is driven by a least one of the rollers with a preset first circulation speed.

In electrophotographic printer or copiers, a print image is electrophotographically generated on a photoconductor, for example an OBC belt (organic photo conductor-photoconductor) in that a charge image is generated on the photoconductor with the aid of a character generator and subsequently developed with toner. The toner image is then transferred onto a belt-shaped intermediate carrier with defined electrical properties. The intermediate carrier can, for example, be a transfer belt.

The toner image located on the intermediate carrier is subsequently directly transferred onto a carrier material (for example a paper web) at a transfer printing station, or the toner image located on the intermediate carrier is re-supplied to the transfer printing region between photoconductor and intermediate carrier in order to print a further (in particular differently-colored) toner image over the toner image already located on the intermediate carrier. This method of printing toner images one over the other is also designated as pick-up of the toner images in a collection mode. The second toner image can, for example, have a toner color different from that of the first toner image or contain a special toner, in particular a machine-readable microtoner. A two-color print with a base color and an additional color can thereby be generated.

Furthermore, printers and copiers are known in which three or four different-colored toner images are printed over one another in order to thereby obtain a print image in full-color printing. During the pick-up of the toner images, the intermediate carrier is pivoted away from the carrier material such that no contact between the intermediate carrier and the carrier material is present during the collection. Only when all toner images are printed over one another on the intermediate carrier is a mechanical contact produced between the intermediate carrier and the carrier material in order to transfer the complete, collected toner image onto the carrier material. The mechanical contact is advantageously established at the point in time at which the leading edge of the toner image located on the intermediate carrier has reached the transfer printing location for transfer-printing of the toner image from the intermediate carrier onto the carrier material. A cleaning station is subsequently pivoted onto the carrier element when the point at which the leading edge of the transferred toner image on the intermediate carrier has been located and has reached the cleaning station.

Corresponding stress states (i.e. load states) of the intermediate carrier thereby result due to the different operating phases, due to which stress states the circulation speed of the intermediate carrier is changed. The operating phases and the load states resulting from these are subsequently explained in further detail in the Figure descriptions regardingFIGS. 1 through 14b.

A slippage occurring dependent on the load state results at the drive roller from the different load states. The circulation speed and the circulation time of the intermediate carrier change due to the different slippage at the drive roller. These changes of the circulation speed or circulation time effect a displacement relative to one another of the position of a plurality of successive toner images transferred onto the intermediate carrier as well as the compression of individual toner images or parts of the toner images in the transport direction of the intermediate image carrier.

A print and copier device for performance-adapted monochrome and color one- and two-sided printing of a recording medium is known from the international patent application WO 98/39691 and the U.S. Pat. No. 6,246,856. A plurality of different-colored toner images are thereby generated on a photoconductor belt and subsequently transferred onto a transfer belt on which the toner images are collected before they are transferred all together onto a paper web. The collection and transfer occurs in a start-stop operation of the paper web. In the continuous monochrome printing, the toner images are continuously generated in succession on the photoconductor, and transferred onto the transfer belt whereby the transfer belt in continuous operation directly further transfers a toner image onto the paper web. The contents of the international patent application WO 98/39691 and of the U.S. Pat. No. 6,246,856 are herewith incorporated by reference into the present specification.

Furthermore, in the prior art a plurality of attempts have been made to prevent the position displacement and the length variation of a toner image of the same desired length. It was thus attempted to keep the load change optimally low given the restriction of the transfer belt to a paper web via reduction of the speed difference between paper web and transfer belt. However, depending on the paper properties of the paper web a minimum speed difference is necessary, whereby given a change of the paper type of the paper web to be printed, and in particular of the paper width and the paper thickness, the paper speed, or the speed difference between transfer belt and paper web must be readjusted. An arrangement for reduction load given an activated cleaning unit is known from the German patent document DE 199 42 116 C2. The contents of the patent document DE 199 42 116 C2 as well as the patents or patent applications cited therein is herewith incorporated by reference into the present specification. Due to the arrangement known from this document, the forces acting on the transfer belt which are caused by the cleaning unit are reduced. However, a load change that leads to the disadvantages already described remains upon activation of the cleaning unit.

In the prior art there were also solution approaches to compensate the print image displacement via an adaptation of the write speed by the imaging unit, i.e. by the character generator or the laser exposure device, in that the subsequent position displacement and/or compression or stretching of the toner image is already taken into account in the generation of the latent print image.

Alternatively, solution proposals are known in which the speed-influenced pivot movements occur before or after the toner image generation or after the transfer-printing of the toner image onto the carrier material. However, the overall print speed of the printer is therewith significantly reduced.

SUMMARY

It is an object to specify a method and a device for controlling the circulation speed of an endless belt in which a substantially constant circulation speed of the belt is ensured even given a plurality of different load states.

In a method for control of circulation speed of an endless belt arranged in a printer or copier, the endless belt is directed over at least two rollers where the belt is driven with a preset first circulation speed via at least one of the rollers as a driven roller. Various load states act on the endless belt in successive operating phases during a printing or copying process, and via said various load states the belt being braked with different strengths so that a slippage is generated at least between the belt and the driven roller. A braking force acting directly on the endless belt is generated. Braking force is controlled such that a substantially constant slippage is generated between the driven roller and the belt based on the operating phases so that the endless belt is braked to a second circulation speed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Via a method for controlling the circulation speed of an endless belt, it is achieved that the endless belt can be braked to a second circulation speed, which is in particular advantageous when the belt is braked by the method of the preferred embodiment in a phase with lesser load effect and the belt is not braked or is only slightly braked during operating phases with a load effect of components of the printer or copier on the endless belt.

Due to the direct effect of the braking force on the endless belt, inaccuracies and time delays in the generation of a braking effect are prevented.

A second aspect of the preferred embodiment concerns an arrangement for controlling the circulation speed of an endless belt. This arrangement contains an endless belt that is guided over at least two rollers. A drive unit drives the belt over at least one of the rollers with a preset first circulation speed. A braking unit introduces a braking force directly into the belt, via which the belt is braked to a second circulation speed.

Via this arrangement it is achieved that the endless belt is simply brought to the second circulation speed via braking.

A third aspect of the preferred embodiment concerns an arrangement for generation of a braking force on an endless belt. An electrically-conductive surface is arranged essentially parallel to the endless belt. A voltage relative to the ground potential is provided on the surface for generation of a braking force.

Via this arrangement it is achieved that the braking force directly acts on the endless belt, and thus the belt is braked directly and without temporal delays.

A printing unit is shown inFIG. 1in which a charge image is generated on a photoconductor belt22with the aid of a character generator (not shown), which charge image is subsequently inked with colored toner material (advantageously black toner material) with the aid of a developer unit28. A toner image is thereby generated on the photoconductor belt22. The photoconductor belt22is guided over deflection rollers24and25as well as over a drive roller26. Deflection rods27a,27b,27cfor direction of the photoconductor belt22are also provided.

The drive roller26is connected with a drive motor (not shown) and drives the photoconductor belt22in the direction of the arrow23. The printing unit also contains a belt drive for guidance of a transfer belt17. The belt drive has a drive roller1as well as guidance and deflection rollers1,5a,5b,7,9,11,13,16. The rollers5a,5band16are arranged stationary in the belt drive, whereby the guidance and deflection rollers7,9,11,13are connected with one another via a lever arrangement with levers6,8,10,12,15such that a pivot movement of the transfer belt17onto a paper web19and onto a cleaning unit21occurs given a constant belt tension of the transfer belt17. Two drive units (not shown) are also provided for execution of the pivot movements. The transfer belt17is driven in the direction of the arrow18with the aid of the drive roller1that is connected with a drive unit (not shown).

A load-dependent slippage arises at the drive roller1upon driving the transfer belt17with the aid of the drive roller1. The different load states in particular occur via pivoting of the transfer belt17onto the paper web19, the pivoting of the transfer belt17onto the cleaning unit21, the activation of the cleaning corotron21cand the pivoting of a pressure roller20in the transfer printing region between transfer belt17and paper web19.

The rollers5aand5bare arranged immediately next to a transfer printing location between the photoconductor belt22and the transfer belt17and continuously press the transfer belt17against the photoconductor belt22guided to the transfer printing location by the deflection roller24.

The printing unit according toFIG. 1is shown inFIG. 2, whereby (as described in connection withFIG. 1) toner images are generated via inking of the charge images (generated by a character generator) on the photoconductor belt22with toner material via the developer unit28, which charge images are subsequently transferred onto the transfer belt17. Two toner images29c,29dare arranged on the photoconductor belt22inFIG. 2, whereby a first part of the toner image29chas already been transfer-printed onto the transfer belt17and the developer unit28subsequently further inks the latent print image (present as a charge image on the photoconductor belt22) on the toner image29d. The toner images29band29ainked beforehand by the developer unit28have already been transferred onto the transfer belt17and are transported in the direction of the arrow18with the transfer belt17on its surface up to a transfer printing location at which they are then transferred from the transfer belt17onto the paper web19. In the operating phase shown inFIG. 2, the transfer belt17is pivoted onto a cleaning unit21such that the cleaning unit21is activated. The pivoting occurs with the aid of a drive unit (not shown) for movement of the lever8, whereby the levers6and10are also moved. The transfer belt17is pivoted onto the cleaning unit21via this lever movement. The belt tension of the transfer belt17always remains constant given the pivot movement of the levers6,8,10. The levers6,810participating in the pivot movement are shown hatched inFIG. 2.

The printing unit according toFIGS. 1 and 2is shown inFIG. 3, whereby the transfer belt17is pivoted both onto the paper web19and onto the cleaning unit21such that the toner images29bthrough29flocated on the transfer belt17are transferred onto the paper web19. The paper web19is accelerated to transport speed just before the pivoting of the transfer belt17and moved in the direction of the arrow30. The pivot levers6,8,10,12and15are thereby directed with the aid of drive units such that the transfer belt17contacts the paper web19in a transfer printing region between the rollers11and20, whereby the pressure roller20is pivoted from below onto the paper web19simultaneously with the pivoting of the transfer belt17onto the paper web19. The levers6,8,10,12,15participating in the pivot movements are shown with a hatched fill inFIG. 3.

The pivot lever mechanism is moved with the aid of a second drive unit such that the transfer belt17is in particular pivoted onto the cleaning unit21via the direction of the roller9, after which at least one part of the first generated toner image29hhas been transfer-printed onto the paper web19and at least the point of the transfer belt17at which the leading edge of the toner image29hwas located arrives in the cleaning region of the cleaning unit21. The cleaning unit21contains a discharge corotron21cvia whose high voltage corotron the toner residues located on the transfer belt are discharged.

The cleaning unit21also contains a brush21bthat brushes the toner residues located on the transfer belt17off from this, whereby the rotation direction of the cleaning brush21bis provided counter to the transport direction of the transfer belt17, such that a large brush effect (and thus an efficient cleaning effect) is achieved. With the aid of a suitable device, the toner material removed with the aid of the brush21bis separated from this and re-supplied to the developer unit28. Alternatively, the brush21bcan also move in the opposite direction, for example with a circumferential speed different from the circulation speed of the transfer belt17. The removed toner material can alternatively be supplied to a residual toner reservoir.

Toner images29a,29b,29c,29d,29e,29f,29g,29are shown inFIG. 3that have been successively inked with the aid of the developer unit28, whereby toner image29awas inked first and the toner image29hwas inked last. The toner image29hhas not yet been completely generated and is subsequently further completed via inking of a charge image present on the photoconductor belt22. As already described, the toner images29athrough29hare successively inked on the photoconductor belt22with the aid of the developer unit29, subsequently transferred from this photoconductor belt22onto the transfer belt17and subsequently transferred onto the paper web19. The generation of the toner images29athrough29hoccurs continuously, whereby the photosensitive belt22, the transfer belt17and the paper web19are driven with essentially the same speed after the pivoting of the transfer belt17onto the paper web19. To tighten the paper web19, the drive speed of the transfer belt17is slightly higher than the drive speed of the paper web19. The transfer belt17is thereby essentially braked to the drive speed of the paper web19after the pivoting onto the paper web19. The pivoting of the paper web19thus effects a speed difference of the circulation speed of the transfer belt17due to the lower drive speed of the paper web. A greater slippage at the drive roller1of the transfer belt drive is generated by the braking of the transfer belt17upon contact with the paper web19and the contact pressure of the pressure roller20.

A printing unit ofFIG. 4is similar to the printing unit according toFIGS. 1 through 3, whereby a two-color toner image can be generated on the paper web19. Identical elements have identical reference characters. InFIG. 4, four toner images29athrough29dhave been generated with the aid of the developer unit28, whereby the toner images are inked with black toner material. Upon inking with toner material of the charge images generated via the character generator, the developer unit28is activated and a developer unit31for development of toner images with red toner material is deactivated. In the operating phase shown inFIG. 4, the transfer belt17is pivoted away from the paper web19and onto the cleaning unit21.

The printing unit according toFIG. 4is shown inFIG. 5, whereby the toner image29dhas been entirely generated with the aid of the developer unit28and has almost completely been transferred from the photoconductor belt22onto the transfer belt17. A further toner image32ahas subsequently been generated on the photoconductor belt22with the aid of the activated developer unit31given a deactivated developer unit28, whereby the character generator has previously generated a corresponding charge image on the photoconductor belt22. In the operating state shown inFIG. 5, only a first part of the entire toner image32ais inked with red toner material by the developer unit31. The further print image of the toner image32ais already located on the photoconductor belt22as a charge image and is thus present as a latent print image that is subsequently inked with red toner material with the aid of the developer unit31.

The printing unit according toFIGS. 4 and 5is shown inFIG. 6, whereby the toner image32ais transfer-printed on the toner image29athat is located on the transfer belt17and has been re-supplied to the transfer printing location between the photoconductor belt22and the transfer belt17. The leading edge of the toner image29acoincides with the leading edge of the toner image32asuch that the toner images29aand32aare essentially congruent. In the operating state shown inFIG. 6, a further toner image32bhas been generated with the aid of the developer unit31, whereby the separation between the toner images32aand32bessentially corresponds to the separations of the toner images29aand29b;29band29c;29cand29d. The printing unit according toFIG. 6thus has generated a second red toner image32aon the black toner image29a.

The printing unit according toFIGS. 4 through 6is shown inFIG. 7, whereby after the pivoting of the transfer belt17onto the paper web19with the aid of the levers10,12and15, a first part of the toner images29aand32aprinted over one another have been transferred onto the paper web19. The pressure roller20is pivoted onto the paper web19from below simultaneously with the pivoting of the transfer belt17onto the paper web19. The paper web19has been accelerated to transport speed before both pivoting processes, as is described in connection withFIGS. 1 through 3for the printing unit shown there.

Further toner images32a,32c,32dwere generated in a red toner color with the aid of the developer unit31and are essentially congruent in the outer dimensions with the toner images previously inked black with the aid of the developer unit28. The toner image32ais superimposed on the toner image29a, the toner image32cis superimposed on the toner image29cand the toner image32dis superimposed on the toner image29d. This superimposition of the toner images is also designated as pick-up. The generation of the toner images placed atop one another thus occurs in a collection mode. In the operating state shown inFIG. 7, the transfer belt17is not yet pivoted onto the cleaning unit21since the point at which the leading edge of the toner images29aand32ahas been located has not yet reached the cleaning region in the cleaning unit21.

InFIG. 7, a further toner image33ahas been generated with black toner material on the photoconductor belt22with the aid of the developer unit28.

The printing unit shown according toFIGS. 4 through 7is shown inFIG. 8, whereby a further part of the toner images29aand32ahas been transferred onto the paper web19. The point at which the leading edge of the toner images29aand32ahas been located has reached the cleaning region of the cleaning unit21, whereby the pivot levers6,8and10are moved (at the latest upon arrival at this point of the transfer belt17in the cleaning region of the cleaning unit21) with the aid of a drive unit (not shown) such that the transfer belt17is pivoted onto the cleaning unit21, whereby the positions of the pivot levers12and15are not altered upon pivoting of the transfer belt17onto the cleaning unit21. The belt tension of the transfer belt17is also not changed both upon pivoting of the transfer belt17onto the paper web19and upon pivoting of the transfer belt17onto the cleaning unit21.

A printer for performance-adapted monochrome and color one- and two-sided printing of a recording medium is known from WO 98/39691 and the U.S. Pat. No. 6,246,856, whereby the pivoting of the transfer belt onto and off of the recording medium is described in detail in this patent application or in this patent. The content of the patent application WO 98/39691 and the content of the U.S. Pat. No. 6,246,856 are herewith incorporated by reference into the present specification.

The printing unit according toFIGS. 4 through 8is shown inFIG. 9, whereby the entire toner images shown collected (i.e. printed over one another) inFIG. 8have been transferred to the paper web19. The trailing edge of the print images29d/32dwere transferred last onto the paper web19. After the trailing edges of these toner images29d/32dhave been transferred onto the paper web19, the transfer belt17has been pivoted away from the paper web19with the aid of the drive device (not shown) via movement of the levers10,12and15. Furthermore, both the discharge corotron21cand the cleaning brush21bare activated, whereby the transfer belt17is furthermore pivoted onto the cleaning unit21. The cleaning brush21band the cleaning corotron21cremain activated at least until the point on the transfer belt17at which the trailing edges of the toner images29d/32dlocated on the transfer belt17has completely passed through the cleaning region of the cleaning unit21. The toner image22aalready generated in the operating mode or operating state already shown inFIG. 8is at least partially transferred from the photoconductor belt21onto the transfer belt17. A further toner image33bis generated on the photoconductor belt22with the aid of the developer unit28. Both the toner image22aand the toner image33bhave been inked with the toner material in the color black.

The printing unit according toFIGS. 1 through 3is shown inFIG. 10, whereby in contrast to the operating states shown inFIGS. 1 through 3the printing unit is shown in an operating state in which print images29athrough29dare transported on the photoconductor belt22and the transfer belt17, whereby the transfer belt17is pivoted away from the cleaning unit21an the paper web19. A load state of the transfer belt17similar to the load state according to Figure is thus shown inFIG. 10, whereby in contrast to the load state according toFIG. 1toner images29athrough29dare generated or transported. No braking effect is thereby exerted on the transfer belt17due to the contact of the transfer belt17with the cleaning unit21and no braking effect is exerted on the transfer belt17due to the contact of the transfer belt17with the paper web19. The toner images29a,29band29ctransferred from the photoconductor belt22onto the transfer belt17have thus been transferred at a higher first circulation speed v1of the transfer belt17according toFIG. 10. Given the load state according toFIG. 2in which the transfer belt17is pivoted onto the cleaning unit21, at least the toner image29cis transferred from photoconductor belt22onto the transfer belt17at a second middle circulation speed v2of the transfer belt17. Given a transfer belt17pivoted onto the paper web19and onto the cleaning unit21, at least the toner image29cinFIG. 3is transferred onto the transfer belt17at a third low circulation speed v3of the transfer belt17.

The circulation speed of the photoconductor belt22is thereby constant, independent of the circulation speed of the transfer belt17. The toner images are thereby not transferred and compressed at the first circulation speed v1of the transfer belt17at the transfer printing location between photoconductor belt22and transfer belt17, meaning that the length of the toner images on the photoconductor belt22corresponds to the subsequent length of the same toner images on the transfer belt17. If the toner images are transferred from the photoconductor belt22onto the transfer belt17at middle circulation speed v2, the toner image is compressed by a first amount upon transfer and is compressed by a second amount upon transfer of a toner image at the third, lower circulation speed v3of the transfer belt17.

The toner images are thereby compressed in a range between a thousandth and multiple millimeters. This affects the length of the subsequent print image generated on the paper web19as well as its position on the paper web19. Given the load state according toFIG. 10, the circulation of the transfer printing17occurs with a first high circulation speed v1that is additionally designated inFIG. 10with the reference character34. The speed v of the photoconductor belt22is also additionally designated with the reference character35.

The printing unit according toFIG. 10is shown inFIG. 11, whereby the print images29athrough29dhave been generated in the same manner as inFIG. 10, whereby, however, the transfer belt17is pivoted onto the cleaning unit21with the aid of the levers6,8and10at least upon transfer of the toner image29cfrom the photoconductor belt onto the transfer belt17. Due to the pivoted cleaning unit21, the circulation of the transfer belt17occurs with the second, middle circulation speed v2, whereby inFIG. 11the middle circulation speed v2is additionally designated with the reference character33.

The printing unit according toFIGS. 4 through 9is shown inFIG. 12, whereby the transfer belt17is pivoted both onto the cleaning unit21and onto the paper web19. The pressure roller20is also pivoted onto the paper web19from below. The circulation of the transfer belt17thereby occurs with a lower third circulation speed v3that is additionally designated with the reference character35inFIG. 12.

A circulation time diagram40is shown inFIG. 13as a screen printout of an evaluation software for evaluation of measurement values determined with the aid of a unit on the printing unit according to one of the printing units shown inFIGS. 1 through 12. The current time is thereby plotted on the abscissa and the circulation time of a belt circulation of the transfer belt17is plotted on the ordinate. In the diagram40, the circulation speeds v1, v2and v3of the transfer belt17during the operating states shown inFIGS. 10 through 12have been determined. During the operating states41aand41b, the transfer belt17has neither mechanical contact with the cleaning unit21nor mechanical contact with the paper web19. In these operating states, the transfer belt17has a circulation time of 1788.51 ms, which corresponds to the circulation speed v1. During the operating state42, the transfer belt17has mechanical contact with the activated cleaning unit21, however no mechanical contact with the paper web19. During the operating state42, the transfer belt17has a circulation time of 1788.58 ms, which corresponds to a speed v2.

During the operating state43, the transfer belt17has both mechanical contact with the cleaning unit21and mechanical contact with the paper web19. During the operating state43, the transfer belt17has a circulation speed of 1788.67 ms and thus a speed v3. The circulation speed of the transfer belt17thereby varies between the speeds v1through v3. The circulation speed of the photoconductor belt22always remains constant during the operating phases41a,41b,42and43. The circulation time of the transfer belt17results from the quotient of the length of the transfer belt17and the circulation speed of the transfer belt17.

In the printing units according toFIGS. 1though12, the relative speed deviation is less than 2/1000 of the nominal circulation speed. However, in practice (in particular in two-color and multi-color printing) it has visible effects. With the aid of the printing units according toFIGS. 1 through 12, one page or multiple pages with a total length of up to 1650 mm can be generated in an exemplary design embodiment of these printing units. After the load-conditional reduction of the circulation speed of the transfer belt17by (relatively) 1/1000 of the circulation speed after the transfer printing of a first toner image and before the transfer printing of a second toner image, the second toner image transferred from the photoconductor belt22onto the transfer belt17is compressed by 1z,900relative to the first toner image during this transfer such that, given a congruent start of the page by both toner images, the page end of the second toner image ends earlier than the page end of the first toner image.

Given a write length of the first color separation of 1650 mm, i.e. given a toner image with a length of 1650 mm in a first toner color, and given a compression of the subsequent printing of a second toner image in a second color on this first toner image, the second toner image is shorter by 1.65 mm than the first toner image (1z,900of 1650 mm write length of the first circulation).

A second toner image transferred at a higher circulation speed (in comparison to a first circulation speed) is expanded in the same manner in relation to the first toner image. The relative speed difference results from the quotients of the speed vx1at which the first toner image is transferred and the speed vx2at which the second toner image is transferred, whereby the amount 1 is subtracted from this quotient. The absolute length error dl results from the multiplication of the write length possible on the transfer belt17and the relative speed difference.

The product from 1650 mm×0.01=1.65 mm thus results in the present example for calculation of the length error, whereby a positive algebraic sign of the length error results given a speed increase and a negative algebraic sign of the length error results given a speed reduction. The human eye very clearly detects a line offset given a plurality of print images of different colors printed over one another and feels this to be disturbing, whereby this offset is generally designated as color fringe in printing technology. 2/100 mm offset is thereby already clearly detectable and is sensed as disturbing. It results from this that, given a possible length of a print image printed over one another of 1650 mm, the speed change may maximally amount to 0.012z,900, whereby this value is calculated as follows:

The effects of the compression of the print images at the transfer printing location between photoconductor belt22and transfer belt17are shown inFIGS. 14athrough14dusing schematically shown print sides48athrough48d. Five print images of print sides that are successively generated and transferred onto the transfer belt17are shown inFIG. 14a, which print sides are subsequently transfer-printed onto an endless paper web45. In contrast to this, the second color printout that is designated with48binFIG. 14vwas transferred onto the transfer belt17with a circulation speed v2of the transfer belt17after the pivoting of the cleaning unit21, whereby the print sides shown inFIG. 14bshould at least correspond in the outer contours to the print sides according toFIG. 14a. However, the length of the print sides is different due to the different circulation speed v1, v2of the transfer belt17, i.e. due to the difference of the speeds v1and v2. The second toner image according toFIG. 14bis also generated in second toner color differing from the toner image according toFIG. 14a. The transfer belt17has an even lower circulation speed v3after the pivoting of the transfer belt17onto the paper web19,45.

The transfer belt17is subsequently pivoted away from both the paper web19and the cleaning unit21, such that the transfer belt17again has a circulation speed v1. The subsequently generated print images are then again transferred uncompressed from the photoconductor belt22onto the transfer belt17. The change of the total length of the five successively-generated print sides of 1650 mm given a change of the circulation speed of the transfer belt17from the circulation speed v1to the circulation speed v2is designated by the arrow49a; the offset given a change of the circulation speed v1to the circulation speed v3is designated with the arrow49b; and the offset given the change of the circulation speed v3to the circulation speed v1is designated with the arrow49c.

The physical length of one page on the paper web45is specified with the aid of the dimensioning, the physical length of the toner image transferred onto the transfer belt17(which toner image is transferred onto the paper web17after the collection of the toner images on the transfer belt17) is respectively specified with the dimensions47athrough47d. InFIGS. 14athrough14d, the physical page lengths are respectively specified by perpendicular dashed lines.

The offset of the toner images generated or transfer-printed at the circulation speed v1in relation to the toner images generated or transfer-printed at the speed v2is clarified by the dash-dot lines indicated between the print images ofFIGS. 14aand14b, whereby the offset between the individual print images is clarified via the increasing slope of the initially horizontal dash-dot line in successive print image starts and ends. InFIG. 14bit is likewise visible that the third print image is already transfer-printed onto the paper web45before the physical page border, whereby a part of the toner image of this print page is truncated in a subsequently cutting process. Larger parts of the print page are then truncated in the subsequently printed fourth and fifth print pages, whereby parts of the subsequent print page are respectively contained on the preceding print page according to the layout.

In the solution of the preferred embodiment to the problem, the individual influences that lead to a speed reduction of the transfer belt17from the circulation speed v1to the circulation speed v3are not prevented by elaborate measures such as in the prior art; rather, the transfer belt17is braked to the speed v3even given load states with higher circulation speeds v1and v2, or is braked to a speed lower than the speed v3during all load states.

Devices to reduce the circulation speed of the transfer belt17are subsequently specified inFIGS. 15 through 22. These devices thereby form the basis of the realization that conductive surfaces connected to a voltage source, to which conductive surfaces a belt-shaped material (in particular an endless belt) is directed, exert a braking force in this belt due to the generated electrostatics and thus effect a braking effect on the belt. In the devices according toFIGS. 15 through 22, this perception is used for realization of a braking arrangement via which the circulation speed of the transfer belt17is reduced. The braking force of the braking arrangement is thereby advantageously changed dependent on the load states, such that a constant circulation speed of the transfer belt17is generated given all load states.

Printing units similar to the printing units according toFIGS. 1 through 12are shown inFIGS. 15 through 19as well as21and22. Identical elements have the same reference characters. A metal plate rounded at the edges is arranged on the inner side of the transfer belt17, over which metal plate the transfer belt17is directed upon actuation of the transfer belt17with the aid of the drive roller1. The metal plate55is supplied with a high voltage (adjustable relative to the ground potential of the printer) with the aid of a high voltage source56with adjustable voltage. Due to the high voltage, a braking force58is generated in the transfer belt17that directly acts on the transfer belt17, whereby the transfer belt17is braked.

A diagram57is also shown inFIG. 15in which is shown a graph of the voltage curve of the high voltage (represented with the aid of a point line) and, with a graph shown with a solid line, the braking force (generated by the high voltage) with which the transfer belt17is braked. The time curve of the high voltage is controlled dependent on the different load states already described, such that an essentially constant circulation speed of the transfer belt17is effected. InFIG. 15, the transfer belt17is pivoted away from both the paper web19and the cleaning unit21such that the transfer belt17is braked with maximum required braking force to a constant low circulation speed v4.

In contrast toFIG. 15, inFIG. 16a high voltage source with constant high voltage is provided, whereby the high voltage source60according toFIG. 16supplies the high voltage to the metal plate in the form of voltage pulses of different pulse breadth or pulse width. If a greater braking force is required, the pulse widths are increased and the pauses between the individual pulses are reduced. Conversely, if a smaller braking effect is required, the pulse width of the individual pulses is reduced and the pauses between the pulses are increased. This dependency of the braking force on the pulse width is also shown in diagram61, whereby the voltage pulses are represented by hatched surfaces and the braking force resulting from this are represented with the aid of a graph shown with a solid line.

In contrast toFIGS. 15 and 17, no metal plate55is provided in the printing unit shown inFIG. 17; rather a plurality of strip-shaped metal plates65athrough65darranged next to one another and insulated from one another are arranged in the transport direction of the transfer belt17, to which metal plates65athrough65dis alternately supplied a constant high voltage (generated by a high voltage source67) via a switch66athrough66d. The surface charged with the high voltage is thereby simply changed with the aid of the switch66athrough66d, whereby the braking force is dependent on the effective surface charged with high voltage. The dependency of the braking force on the effective surface is likewise represented in the force-time diagram68, whereby the metal plate65aforms the areal segment A1, the metal plate65bforms the areal segment A2, the metal plate65cforms the areal segment A3and the metal plate65dforms the areal segment A4. The total braking force acting on the transfer belt17then changes dependent on the area of the individual elements, as shown in the diagram68. The areal segments charged with high voltage are specified with the aid of the footnotes of the areal segments. Given the areal segment A34, the metal plates65cand65dare thus charged with high voltage via closing of the switches66cand66d.

The arrangement for braking the transfer belt17shown inFIG. 18is similar to the arrangement according toFIG. 17, whereby given a state in which no high voltage of the high voltage source67is supplied to the metal plates65athrough65d, but rather ground potential is supplied via a circuit arrangement. Floating potentials of these metal plates65athrough65dare thereby prevented. The braking effect of this arrangement essentially coincides with the braking effect of the arrangement according toFIG. 17, as is also shown in diagram68.

An arrangement for generation of a braking force that acts directly on the transfer belt17is shown inFIG. 19. The arrangement of the metal plates65athrough65dessentially coincides with the arrangement according toFIGS. 17 and 18. A first high voltage generated by a high voltage source67or a second high voltage generated by a high voltage source71can selectively be supplied to the individual metal plates65athrough65dvia the switches66athrough66d(which are realized as change-over switches). A potential difference differing from the ground potential can thereby be generated between the individual metal plates65athrough65d, whereby in particular one of the high voltage sources67and71can generate a high voltage negative relative to the ground potential. A braking force is generated via feeding the high voltage source67to the individual metal plates65athrough65din the same manner as inFIGS. 17 and 18, whereby the surface-dependent braking force is shown in the diagram68that essentially coincides with the diagrams68according toFIGS. 17 and 18.

Three diagrams75,76and77are shown inFIG. 20, whereby the braking forces active due to the different load states on the transfer belt17are shown in the diagram75and the braking force generated by one of the braking according toFIGS. 15 through 19is shown dependent on the time in the diagram76. A diagram77is also shown inFIG. 17, in which the sum of the braking forces from the diagrams75and76is shown, whereby a constant resulting braking force78is generated by the braking arrangement controlled dependent on load. In the diagram75, the braking force generated by the cleaning unit21is designated with FCle, the braking force resulting due to the pivoting of the transfer belt17onto the paper web19is designated with FPaper, the braking force generated given a pivoted transfer belt17on the paper web19and simultaneous pivoting of the transfer belt17onto the cleaning unit21is designated with FCle+Pap. The resulting braking force78can thus be held constant over the entire time span (i.e. during the various operating phases with the different load states) due to the shown braking arrangements, whereby the transfer belt17has a constant circulation speed. The different lengths of the toner images are thus effectively prevented. Toner images with an exact preset length are generated. Exactly congruent toner images are even generated given multi-color printing, whereby a color image fringe is prevented.

A braking arrangement according toFIGS. 15 and 16is shown inFIG. 21, whereby (in contrast toFIGS. 15 and 16) the metal plate55is supplied with a high voltage generated at a high voltage source80. The high voltage source80can output an adjustable variable high voltage, whereby the level of the output high voltage can be adjusted with the aid of a microprocessor81connected with a control input of the high voltage source80.

The microprocessor81furthermore controls drive motors83aand83bfor execution of the pivot movements of the transfer belt17onto the cleaning unit21and onto the paper web19with the aid of the lever mechanism of the levers6,8,10,12,15. The outputs of the microprocessor81for activation of the drive motors83aand83bare connected with power converters82a,82bthat convert the control signals of the microprocessor81into motor activation signals for activation of the motors83aand83b, whereby the motors83aand83bare advantageously step motors. The motor83athereby executes a pivot movement of the lever6and the motor83bexecutes a pivot movement of the lever10. The same microprocessor81thereby controls high voltage generating the braking effect and the pivot movement of the transfer belt17. The load changes generated by the pivot movements can thus very simply be taken into account in the determination of the high voltage to be set and the braking force resulting from this, whereby a corresponding change of the braking force effected by the metal plate is be generated at the same point in time at which a load change occurs (or, in the event that it is necessary, before this point in time) in order to ensure the constant braking force78shown inFIG. 20.

The braking arrangement according toFIG. 21is shown inFIG. 22, whereby the high voltage source80is activated by a microprocessor84to which a desired value86of the circulation speed of the transfer belt17is supplied and to which a real value of the circulation speed is supplied with the aid of a sensor85to detect the circulation speed of the transfer belt17. As an alternative to the velocity sensor85, the circulation time of the transfer belt17can also be detected with the aid of a suitable sensor arrangement from which the circulation speed is then simply determined with the aid of the belt length of the endless belt. The microprocessor84implements a real value-desired value comparison and, dependent on the control deviation, generates a control signal that supplies the high voltage source80to the microprocessor84. The high voltage source80thus serves as a control element of the control loop.

The circulation times of the transfer belt17are respectively shown inFIGS. 23athrough23edependent on the set direct voltage. The effective surface of the metal plate55is thereby 545 cm2, whereby the circulation speed v1is only 992 mm/s at a high voltage of 0 kV. The average circulation time is respectively shown inFIGS. 23athrough23ewith the aid of a dash-dot line. As already mentioned, inFIG. 23ano high voltage is applied to the metal plate55; rather, ground potential or a potential corresponding to ground potential is applied. The average belt circulation time is 1790.94 ms. A diagram is shown inFIG. 23bin which the circulation time of the transfer belt17occurs given a charging of the metal plate55with a high voltage of 0.4 kV. The average circulation time of the transfer belt17is thereby likewise 1790.94 ms.

The circulation time of the transfer belt17is shown inFIG. 23cgiven a charging of the metal plate55with a high voltage of 0.80 kV. The circulation time of the transfer belt17is thereby on average 1791.09 ms. The circulation time of the transfer belt17given a charging of the metal plate55with a voltage of 1.2 kV is shown inFIG. 23d. The average circulation time is thereby 1791.21 ms. Given a charging of the metal plate55with a voltage of 1.6 kV, the average circulation time of the transfer belt17is 1791.35 ms, as shown inFIG. 23e.

A circulation time/circulation speed-voltage diagram is shown inFIG. 24, in which the change of the absolute circulation time and the change in the circulation time is represented dependent on the change of the supplied voltage. The graph represented with the aid of a dashed line thereby specifies the variation of the absolute circulation time, and the graph represented with the aid of a solid line specifies the change of the circulation time with increasing voltage. The metal plate55or the metal plates65athrough65dare arranged on the inner side of the transfer belt17in the exemplary embodiments.

The braking effect on the transfer belt17may be based on the fact that an electrical field through which the transfer belt17is directed is generated between the metal plate55or the metal plates65athrough65dand the components of the printer that have a potential differing from the potential of the metal plate55,65athrough65d. The metal plate55,65athrough65dis thus a capacitor plate. The electrical field effects a temporary displacement of charges in the transfer belt17. Due to the displacement, a concentration of charges opposite the charge of the capacitor plate occurs in the transfer belt17towards the metal plate55,65athrough65d. The charges in the transfer belt17are thereby attracted by the charge of the capacitor plate55,65athrough65dwith a force according to Coulomb's Law. Due to this force, the transfer belt17is drawn in the direction of or against the metal plate55,65athrough65d(i.e. capacitor plate), whereby, given a contact between the metal plate55,65athrough65dand the transfer belt17, depending on the size of this attractive force a friction force is generated between metal plate55,65athrough65dand transfer belt17that reduces the transport speed. A braking force independent of the rollers of the belt drive is thereby generated that acts directly on the transfer belt17. A further metal plate can also be arranged on the side of the transfer belt17opposite the metal plate55,65athrough65d, essentially in parallel with the metal plate55,65athrough65d, at a preset distance from the transfer belt. To generate the braking force, the further metal plate then has a potential (advantageously ground potential) differing from the potential of the metal plate55,65athrough65d.

In other exemplary embodiments, the metal plate55,65athrough65dcan also be arranged on the outer side of the transfer belt17at a distance from the transfer belt17, such that a toner image located on the transfer belt17is not damaged by the metal plates55,65athrough65d. As an alternative to a direct voltage, the high voltage sources56,60,67,71can also generate an alternating voltage with which the plates55,65athrough65dare charged. The braking force generated via the feed of the high voltage acts directly and without temporal delay on the transfer belt17. A very exact and time-precise control of the braking force is thereby possible. The metal plates65athrough65d,55advantageously extend over the entire width of the transfer belt17. Due to the inventive braking arrangements, the transfer belt17and the plates55,65athrough65dare subject to only very slight wear.

As an alternative to the shown embodiments, the surface generating the braking force can also be divided up into segments transverse to the transfer belt17that can be charged individually or in groups with high voltage of the same voltage level or different voltage levels. The metal plates55,65athrough65dare metal plates that contain a stainless steel alloy, copper or a copper alloy or that contain an aluminum alloy or aluminum. The plates can also be subjected to a surface treatment or be provided with a coating. Alternatively, electrically-conductive plastics can also be used as a plate55. The plates55are advantageously provided with a smooth surface or with a suitable surface structure. Further variations of the regulation (for example the detection of the real value of the circulation speed with the aid of the circulation time) of a desired value specification controlled by a further process are possible in order to realize inventive applications. The braking arrangement of the preferred embodiment was provided in the shown exemplary embodiments for braking of the transfer belt17. However, such a braking arrangement for braking of the photoconductor belt22or further belt-shaped carrier material is also possible, whereby the endless carrier material does not necessarily have to be an endless, circulating belt. Rather, the belt17to be braked can also be a paper web or single sheets with a relatively large length.

Although a preferred exemplary embodiment with various modifications been shown and described in detail in the drawings and in the preceding specification, it should be viewed as purely exemplary and not as limiting the invention. It is noted that only the preferred exemplary embodiment is shown and described, and all variations and modifications that presently and in the future lie within the protective scope of the invention should be protected.