Secondary guiding system for a print medium on a graphics printer

Abstract: The secondary guiding system comprises a fan arrangement (16) for generating an air flux (50) and directing said flux to a part (20a) of a print medium that hangs freely from the graphics printer, so as to maintain the hanging part (20a) away from the graphics printer or any object supported by it. Preferably, in the case of a pen plotter equipped with devices for creating a partial vacuum beneath the print medium in an active zone of the writing instrument, the air flux is at least produced by these suction devices.

Description: BACKGROUND OF THE INVENTION Field of the invention The present invention relates to a secondary guiding system for a print medium on a graphics printer. The present invention more particularly relates to a vector mode graphics printer (pen plotter) in which a drawing is obtained by a relative movement along two orthogonal directions between a print medium, such as a sheet of paper, and a writing head. However, the present invention is also applicable to graphics printers operating in a scanning mode (raster printer) in which the drawing is composed of points obtained by a line-by-line scanning while the print medium is driven past one or a plurality of writing heads. More generally, the present invention applies to any graphics printer in which a part of the print medium hangs loosely from the printer during printing. In the case of a pen plotter, for example, a drawing is generally produced by a to-and-fro movement of the print medium along a first direction (X), and a to-and-fro movement of the writing head along a second direction (Y), perpendicular to the X direction. The plotter can be adapted to use either a pre-cut sheet or a continuous roll. In the former case, the print plotter is supplied with pre-cut sheets of the desired formats. In the latter case, the print medium is drawn from a supply reel to obtain a length corresponding to the format of the drawing to be made, and the drawing is then reproduced by appropriately controlling the displacements of the print medium and the writing head. During printing, the print medium is mainly guided by drive means acting along the X direction, e.g. a drum against which the print medium is applied by means of pressure rollers. In the vicinity of the writing zone the print medium passes over a platform that may be extended towards the front and/or the rear by a skirting. The platform may be provided with openings that communicate with suction means in order to create a partial vacuum beneath the print medium in the zone where the printing instrument is active. These guide means ensure that the print medium is properly supported at the level of the writing zone, in order to prevent it from lifting from the platform or to start vibrating as a result of the movements imparted to it. However, once the print medium is no longer in contact with these guiding means, it hangs freely from the graphics printer. In the case of a continuous print medium drawn from a supply reel, the remanence of the rolled state gives a tendency for the free end of the medium, hanging at the front of the graphics printer, to roll up on itself, so forming a loop. Accordingly, as the printed face of the print medium curls towards the rear, it can come into contact with a part of the printer or any object supported by the printer, causing the drawing to be smeared. This risk of smearing is particularly high if the printer is equiped with an automatic storage device for the drawings. Such a device generally comprises a horizontal bar (which can be part of the printer stand) located at a lower level and slightly recessed with respect to the print medium outlet, which is at the front of the printer. The rubbing action between the loop formed at the end of the print medium on which a drawing is being made and a stored sheet on which a drawing has already been made can cause a smearing of both drawings. This is all the more likely to occur if the print medium is made of a non-absorbing material, as is the case of a sheet of polymer, on which the ink takes a long time to dry. The contact between a loose end of the print medium and a part of the printer or an object supported by the printer can create a non-negligible sound disturbance in the immediate environment of the printer, since the medium is generally displaced by rapid to-and-fro movements with very high accelerations (up to 6 g in ultra-high-speed machines). These movements are transmitted to the hanging portion of the medium and tend to cause that portion to oscillate, thereby generating a noise by contacts. This is all the more of a nuisance if it is the free edge of the medium that establishes the contact. The prior art approaches to solving this problem rely solely on a mechanical secon dary guiding means to steer away the hanging portion of the print medium. These means are, e.g. bars, rollers or other guides arranged in various ways to maintain the sheet in a non-contacting position, by acting on its unprinted surface. Accordingly, all these known solutions turn out to be relatively complex from a mechanical point of view. Moreover, they add to the printer's overall dimensions, at least in its operating configuration. Further, these mechanisms prove to be noisy owing to the print medium's displacement speeds and the snatching movements produced by the high accelarations. SUMMARY OF THE INVENTION WITH OBJECTS It is an object of the present invention to provide a secondary guiding system that is free of the drawbacks of the prior art solutions. This object is achieved by a system that comprises means for generating an air flux and means for controlling a flow path of said air flux in a direction and to a location so as to prevent a freely hanging part of said print medium from contracting the printer, or any object supported by it. Advantageously, when the pen plotter is equipped with a suction means intended t create a partial vacuum beneath the print medium in an active zone of the graphic printer's writing instrument, the air flux is at least partially produced by the suction means. In the latter case, there can also be provided a dedicated air flux generator capable of co-operating with the suction means to provide the air flux. The dedicated generator is then activated as a function of the air flow that can be derived from the suction means, or as a function of the characteristics of the print medium material. The secondary guiding system may also comprise a deflector, possibly defining a variable cross-section air outlet, so as to orient the air flux towards the print medium. BRIEF DESCRIPTION OF THE DRAWINGS The present invention shall be more clearly understood from reading the following description of the preferred embodiments given as a non-limiting example, with reference to the appended drawings in which: figure 1 is a schematic perspective view of a pen plotter to which the present invention is applicable, figure 2 is a highly schematic cross-sectional view partially showing the upper part of a pen plotter equipped with a secondary guiding system according to the present invention, figure 3 is a highly schematic longitudinal cross-sectional view partially showing the upper part of a pen plotter equipped with a secondary guiding system according to a second embodiment of the present invention, and figures 4A to 4E are schematic views showing the operation of a pen plotter equipped with a secondary guiding system according to the invention and an automatic storage device for drawings. DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the secondary guiding system according to the invention shall be described in the context of a pen plotter that uses a continuous print medium and is equipped with an automatic storage device for drawings. As can be seen from figures 1 and 2, such a plotter 10 comprises a platform 11 over which is guided a print medium 20 drawn from a supply reel 21. The print medium 20 is displaced along an X direction (longitudinal direction of the print medium) by means of a drum 12 having a horizontal axis perpendicular to the X direction. The print medium 20 is pressed against the drum 12 at its edge portions by means of pressure rollers 13, via a longitudinal groove formed in the platform 11. A write head 30 is movable in a Y direction, perpendicular to the X direction, along a writing zone 31 located at the surface of the platform 11. The write head carries a writing instrument 32. Several different writing instruments can be stored in a turret 33 at the terminal point of the travel of the write head 30 to allow an automatic change-over of the writing instrument carried by the latter. As is already well known in the art, a drawing is produced on the print medium 20 by a combination of a to-and-fro movement thereof in the X direction, and a to-and-fro movement of the write head 30 in the Y direction. Before starting a drawing, a length of print medium 20 corresponding to the format of the drawing is drawn from the supply reel 21 so as to define a loop 20b, thus limiting the print medium's inertia during its displacements for the execution of the drawing. During execution of a drawing, the print medium 20 is held in contact with the platform 11 by means of a partial vacuum created beneath the print medium. To this end, the platform 11 comprises openings 14, at least in the active zone of the write head 30, which communicate with a suction chamber 15 containing the drum 12. A partial vacuum is created in the chamber 15 by means of a fan 16. When the drawing is finished, the drawn format is separated from the rest of the print medium by a cutting action produced by a cutting device 35 fixed to the write head 30. The finished drawings 20' can be automatically stored by being suspended over a horizontal bar 40. This bar can form a part of the printer stand that serves to maintain the platform 11 raised from the floor. The drawings are transferred to the storage bar 40 e.g. by means of mobile horizontal bars 41, 42 supported at their end by arms 43, 44 that are articulated on the printer around a horizontal axis. The operation of the storage device shall be described in more detail further on. It will be noted that an automatic storage device for cut sheets for a pen plotter or raster plotter operating from a continuous print medium is described in French Patent Application No. 89 00920. As is shown more clearly in figure 2, the free edge of the print medium 20 has a tendency to curl round itself to form a loop 22, owing to the remanence of its rolled state in the supply reel 21. Accordingly, the print medium 20 exposes a part of its printed face towards the storage bar 40, which is located beneath and at a retracted position from the print medium outlet, at the front of the graphics printer. It is thus necessary to maintain the part 20a of the print medium 20 which hangs beyond the printer away from the storage bar 40 (or from any other part of the printer) inorder to prevent smearing of the drawing formed on the print medium, and possibly smearing of the drawing stored on the bar 40. Indeed, the movements imparted to the print medium during execution of a drawing, combined with the aerodynamic loads at the end 22, induce displacements of that end 22, which could cause it to contact and rub violently with the bar 40 or the drawings stored thereon. According to the present invention, there is provided a secondary guiding of a print medium by means of an air flux 50 which is generated and directed so as to maintain the part 20a of the print medium 20 hanging from the printer away from contact with any parts of the latter or from an object supported by it, during execution of a drawing. Advantageously, the air flux 50 is at least partially produced by the fan 16 used for generating a partial vacuum beneath the print medium 20 at the level of the platform 11. As shown in figure 2, the air is driven away from suction chamber 15 via openings 51 formed in the lower wall of the chamber, and is directed by means of a deflector 52 along a substantially horizontal direction, towards the front of the printer, immediately beneath the outlet for the print medium 20 at the front of the printer. In the illustrated embodiment, the deflector 52 forms, with the lower wall of the chamber 15, a duct 53 that defines a chamber opening at the front of the printer. The outlet 54 of the duct 53 is rectangular and its width is e.g. equal to the maximum width of the print medium usable with the printer. As shown in figure 2, the cross-section of the duct 53 can be variable, e.g. by narrowing towards the outlet 54. Air is admitted into the chamber 15 by means of openings 14 that are not covered by the print medium. The openings 14 are normally provided over a distance along the Y direction corresponding to the maximum width of the print medium 20 usable with the printer. Thus, to ensure that there is a minimum of air flux 50, in the case where the openings 14 are all, or practically all, covered by the print medium, there is provided at least one additional air inlet 55 in the chamber 15. This air inlet 55 is e.g. formed in an end wall of the chamber 15 or in the lower wall of the latter, outside the duct 53 (figure 3). This solution may prove to be inadequate in some cases. Thus, according to a variant of the embodiment according to the present invention, there is provided dedicated means for complementing the air flux produced by the fan 16. This dedicated means consists of at least one additional fan 57 (figure 3). The latter is e.g. lodged laterally in the chamber 15 and has an outlet giving on to the duct 53. The fan 57 can then be switched on controllably as a function of the width of the print medium used, in other words as a function of the air flow that can be supplied by the fan 16, and/or as a function of the nature of its material. Indeed, in the case of a material having a large surface density or rigidity, a relatively strong air flux will be necessary to ensure effectively the required secondary guiding. The fan 57 may be controlled manually or automatically. In the latter case, the fan can be controlled by a microprocessor receiving data identifying the format and the characteristics of the print medium. According to another embodiment, there may be provided an additional deflector, parallel to deflector 52, to direct the air flux produced by the additional fan. According to another variant, the additional fan can be used as a complement to a fan 16 and may serve not only for the production of the air flux 50 but also for the suction through the openings 14. Of course, the required number of fans working in parallel in the chamber 15 is not limited to two. The foregoing examples concern a plotter using suction means to maintain the print medium onto the platform 11 in the vicinity of the active zone of the write head. The present invention of course also applies to plotters that do not use such suction means. The air flux necessary for the secondary guiding is then produced by at least one dedicated fan, such as fan 57 of figure 3. Reference will now be made to figures 4A to 4E, which illustrate the successive stages of the execution and storage of a drawing with a plotter of the type shown in figure 1. During execution of the drawing, the bars 41, 42 are in a rear position and the air flux 50 holds the hanging portion 20a of the print medium away from the storage bar 40, and towards the front of the machine (figure 4A). The air flux 50 also has the effect of making this portion 20a of the print medium less sensitive to ambiant draughts (e.g. from open windows, air conditioning etc.) which could force it against the printer. After execution of the drawing, the print medium is driven rearwards, the air flux 50 is stopped, and the bars 41, 42 are raised towards the front. The print medium is then placed towards the front so that its end 22 engages between bars 41, 42 and the storage bar 40, while at the same time secondary guiding is provided by means of the air flux 50 (figure 4B). When the middle of the drawing reaches the level of the bars 41, 42, the latter are brought to a rear position to press the print medium onto the top of the storage bar 40 substantially at the level of the center of the drawing (figure 4C). It should be noted that this pressure is provided by bar 41 which comes into abutment against the top of bar 40, whereas bar 42 can pass over bar 40, the arms 43 descending lower than arms 44. Next, the advance of the print medium is stopped when the rear edge of the drawing reaches the level of the cutting device 35. The latter is then activated (figure 4D). Finally, the air flux 50 is stopped and bar 42 is displaced rearwardly to allow the second part of the cut drawing to pass behind the storage bar 40, in order to suspend the cut drawing over the latter (figure 4E). Bar 42 is then brought to its initial position. Although the present examples concern a pen plotter working with a continuous print medium, it will be understood that the invention also applies to pen plotters working with cut sheets. In the latter case, the secondary guiding of the print medium will advantageously be carried out not only at the front, but also at the rear of the printer, by means of two separate air fluxes directed in opposite directions towards the parts of the print medium that hang freely from the front and rear of the printer. Moreover, it will be understood that the present invention also applies to raster plotters in which the drawing is made from successive lines of dots.

Claims: Secondary guiding system for a print medium (20) on a graphics printer, characterized in that it comprises means (16; 57) for generating an air flux (50) and means for controlling a flow path of said air flux (50) in a direction and to a location so as to prevent a freely hanging part (20a) of said print medium (20) from contacting the printer, or any object supported by it. System according to claim 1, for secondary guiding of a print medium (20) on a graphics printer having suction means (16) intended to create a partial vacuum beneath said print medium in an active zone of a writing instrument (32) of said graphics printer, characterized in that said air flux (50) is at least partially produced by said suction means (16). System according to claim 2, for secondary guiding of a print medium (20) on a graphics printer having a platform (11) supporting the print medium in an operating zone of said writing instrument (32) and provided with apertures (14) cooperating with said suction means (16), characterized in that there is provided at least one air inlet (55) that is not susceptible of being occluded by said print medium, in order to ensure a minimum air flux when all said apertures in said platform are covered by said print medium. System according to any one of claims 1 to 3, characterized in that said air flux (50) is at least partially produced by a dedicated generator (57). System according to any one of claims 2 and 3, characterized in that it further comprises at least one additional air flux generator (57) capable of cooperation with said suction means (16) to produce said air flux (50). System according to claim 5, characterized in that said additional air flux generator (57) is controllably activable as a function of the air flow produced by said suction means. System according to any one of claims 5 and 6, characterized in that said additional air flux generator (57) is controllably activable as a function of the characteristics and/or the width of the material constituting said print medium. System according to any one of claims 5 to 7, characterized in that said suction means and said air flux generating means (57) are comprised of a plurality of fans. System according to any one of claims 1 to 8, characterized in that it further comprises a deflector (52) to direct said air flux (50). System according to claim 9, characterized in that said deflector (52) defines an air duct (53) of variable cross-section.

Inventors: ARMIROLI JEAN, COINTRE JEAN-PIERRE, ARMIROLI, JEAN, COINTRE, JEAN-PIERRE
Applicants: OCE GRAPHICS FRANCE, OCE GRAPHICS FRANCE S.A.
Application year: 1990
Main classification: B41J15
IPC codes: B41J15, G01D15