WO2005018937A1

WO2005018937A1 - Compass/register errors in a printing machine

Info

Publication number: WO2005018937A1
Application number: PCT/US2004/026169
Authority: WO
Inventors: Rolf Johannes Spilz
Original assignee: Eastman Kodak Company
Priority date: 2003-08-18
Filing date: 2004-08-12
Publication date: 2005-03-03
Also published as: DE10337861A1

Abstract

Determining compass and/or register errors in a printing machine by using an arrangement of triangles (1, 2) serving as register marks, whereby at least one first triangle (1) is printed onto a sheet (3) near its leading edge (7) and at least one second triangle (2) is printed onto a sheet (3) near its trailing edge (18), the first triangle and the second triangle are detected by a sensor (10), and a displacement of the sheet at right angles to the sheet's direction of travel and/or an angular displacement of the sheet, is determined.

Description

COMPASS/REGISTER ERRORS IN A PRINTING MACHINE

FIELD OF THE INVENTION This invention relates in general to determining register errors, and more particularly to sensing printed triangles to determining compass and/or register errors in a printing machine. BACKGROUND OF THE INVENTION In the printing industry, a number of print patterns or print images are transferred onto various printing media, usually onto various types of paper. In this process, the locational positioning of the print images is of great importance for the print quality. The locational positioning of each of the individual color applications, which combine to form a printed image in color, is also of great importance. To achieve correct locational positioning, marks are used in the printing industry, which are usually imprinted onto the printing medium and which serve for one thing, as a comparison for detecting possible deviations of the printed image on the printing medium from the desired position. In another respect, the marks serve for detecting the position of the printing medium on the conveying mechanism and for determining whether that position of the printing medium on the conveying mechanism deviates from intended position. These marks are referred to as register marks in black on white printing, and as compass marks in color printing. When these marks are not located where they should be, the resultant deviation is called a register error or a compass error. The location of the marks is measured and from the measurement a determination is made as to whether the printed image is properly positioned, whether the printing medium is properly positioned on the conveying mechanism, and/or the magnitude of any such deviation. Position deviations of the printed image, or the printing medium on the conveying mechanism that have been determined in this way can be appropriately corrected. In multi-color printing, for instance, during which several layers of color or color applications are printed one upon one other, marks are used for each individual color application. In the case of duplex printing, i.e., imprinting the printing medium on both sides, i.e., the first form side and the back side, the state-of-the-art discloses the use of marks on the first form side as well as on the back side, which are detected separately. The state-of-the-art discloses a number of documents, which propose the use of triangles as marks. For example, DE 40 14 706, describes a process for detecting compass and/or register errors on a print product that contains marks, whereby the marks are photo-electronically scanned as the print product passes through the printing machine. SUMMARY OF THE INVENTION The goal of the invention is to suitably identify compass and/or register errors during printing. The invention provides a process for determining compass and/or register errors in a printing machine that uses an arrangement of triangles as marks, whereby at least one first triangle is imprinted near the leading edge of a sheet, and at least one second triangle is imprinted near the trailing edge of the sheet, the first triangle and the second triangle are detected by a sensor, and any displacement of the sheet that is at right angles to the sheet's direction of travel and/or any side to side displacement of the sheet is identified. In one embodiment, the displacement of the sheet at right angles to the sheet's direction of travel and or the angular displacement of the sheet on the first form side thereof is/are identified, the sheet is turned over, and the displacement of the sheet at right angles to the sheet's direction of travel and/or the angular displacement of the sheet on the back side is/are identified. In a further embodiment, the first triangle and the second triangle are detected through the sheet. In this way, it becomes unnecessary to imprint the marks on the back side and also when, after being turned over, the sheet passes through the printing machine for the second time, any shifting of the sheet on the back side, when compared to the first passage of the sheet on the first form side, can be detected. Beneficially, a mechanism corrects the displacement of the sheet at right angles to the sheet's direction of travel and/or the angular displacement of the sheet. The process can be executed with particular ease if the position of the sheet is corrected such that the quotient of the distance d_lx between the sides of the first triangle detected by the sensor and the distance d_2x between the sides of the second triangle equals one. Using this criterion it is easy to determine whether the sheet is located in its error-free position on the conveyor belt. The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiment presented below. BRIEF DESCRIPTION OF THE DRAWINGS In the detailed description of the preferred embodiment of the invention presented below, reference is made to the accompanying drawings, in which: FIG. 1 shows an overhead view of the first form side of a sheet on a conveyor belt, on which a first triangle and a second triangle are shown, as they would appear as marks when the sheet is not displaced; FIG. 2 shows an overhead view of the first form side of a sheet on a conveyor belt, on which a first triangle and a second triangle are shown as they would appear as marks when the sheet is displaced; and FIG. 3 shows an overhead view of the back side of a sheet on a conveyor belt, on which a first triangle and a second triangle are shown as they would appear as marks when the sheet is displaced. DETAILED DESCRIPTION OF THE INVENTION Referring now to the accompanying drawings, FIG. 1 shows an overhead view of the first form side of a sheet 3 that is being conveyed through a printing machine on a driven conveyor belt 6 in the direction shown by the arrow. The conveyor belt 6 is usually continuous and is stretched over guide rollers; in FIGS. 1 through 3 a section of the conveyor belt 6 is shown. In the printing mechanisms or printing modules of the printing machine, the sheet is imprinted with a first register mark, which is located near the leading edge 7 of the sheet 3 and is in the form of a triangle; this is a first triangle 1. Near the trailing edge 8 of the sheet 3 a second register mark is imprinted, which is also in the form of a triangle; this is the second triangle 2, the shape of which should preferably be identical to that of the first triangle 1. In this example, the first triangle 1 and the second triangle 2 are equilateral triangles whose base lines run approximately parallel to the longitudinal side of the sheet 3 and to the direction of travel of the sheet 3. Customarily, the marks (first triangle 1 and second triangle 2) serve to assure that the imprinted image on a printing medium, in this case, sheet 3, is in compass and/or in register. The term "in register" refers to the proper positioning of an image imprinted on the printing medium, while the term "in compass" refers to the proper positioning of a color image imprinted on the printing medium, sheet 3. The imprint will not be in compass or in register if the printing medium becomes undesirably displaced on its mechanism of conveyance, in this case, the conveyor belt 6, or if the imprint is applied at the wrong time. Above the conveyor belt, a sensor 10 is mounted, which detects the first triangle and the second triangle. For detecting the triangles 1 and 2, the sensor 10 identifies the light to dark transitions between the sheet 3 and the triangles 1 and 2. The line optically traversed by the sensor 10, is shown schematically by the dashed line 100. The sensor 10 is connected to the control mechanism 20 in the printing machine, which is schematically shown. The sensor 10 identifies the light to dark transitions for each of the two sides of triangles 1 and 2. In the first triangle 1, the sensor 10 identifies the light to dark transitions of the sides 11 and 12, and in the second triangle 2, the sensor 10 identifies the light to dark transitions of the sides 21 and 22. The signals from the sensor 10 are transmitted to the control mechanism 20, in which the distances dπ and d₂₁ can be calculated from the signals. The distance dπ, is the distance between the two sides 11 and 12 of the first triangle 1, through which the dashed line 100 passes and which are detected by the sensor 10. The distance d₂₁ is the distance between the two sides 21 and 22 of the second triangle 2, through which the dashed line 100 passes and which is detected by the sensor 10. In the control mechanism 20, clock pulses are counted which are governed by signals from the sensor 10 and which begin in each case when the first side of the triangles 1 and 2 are detected and stop when the second side of these triangles 1 and 2 are detected. The clock pulses that are counted in this way are correlated with a distance dπ or d₂₁. If the triangles 1 and 2 are displaced, the distances between the sides 11 and 12, and 21 and 22 that are detected by the sensor 10 are changed. In such case, the sensor 10 detects the sides 11 and 12, and 21 and 22 of the triangles

1 and 2 at different points on these sides. In FIG. 1, the sheet 3 is located in an error-free orientation relative to the conveyor belt 6, the sheet 3 is not displaced, and the first triangle 1 and the second triangle 2 have been imprinted at the correct locations on the sheet 3. Provided that the first triangle 1 and the second triangle

2 are identical in size and have the same orientation on the sheet 3, i.e., the distances of the triangles 1 and 2 to the longitudinal side of the sheet 3 and the orientations of the angles of the triangles 1 and 2 on the sheet 3 are also identical, the distances dπ and d₂₁ which are detected by the sensor 10 will be the same. In such case, then, in which the sheet 3 is being conveyed error free on the conveyor belt 6 without any angular shift, the quotient derived from the distances dπ and d ₁, equals one. This quotient is calculated in the control mechanism 20. Thus, a simple criterion is available for determining the error free angular orientation of sheet 3. If the quotient calculated using the distances dπ and d₂₁ is equal to one, the sheet 3 is conveyed error-free relative to its angular position on the conveyor belt 6. If, however, the quotient calculated using the distances dπ and d₂₁ does not equal one, the sheet 3 is being conveyed on the conveyor belt at an incorrect angle. Angular displacements are thus determined with the aid of the aforementioned quotient. Angular displacements of the sheet 3 are specifically determined by correlating the value of the calculated quotient with values found in a correlation table contained in the control mechanism 20. An angle α, by which the triangles 1 and 2 and, thus, the sheet 3 on the conveyor belt 6 are displaced, is specifically assigned to each calculated quotient. It always is a prerequisite in this regard that the positions of the triangles 1 and 2 on the sheet 3 are perfect, i.e., the triangles 1 and 2 are imprinted error- free on precisely the correct locations on the sheet 3. This prerequisite is met when the sheet 3 is oriented on the conveyor belt 6 directly before the print 3 is imprinted with the triangles 1 and 2. In addition, a displacement of the sheet 3 that is at right angles to its direction of travel, the so-called cross-track, is determinable. If the sheet 3 is displaced at right angles to its direction of travel, the distances dπ and d₂₁ change equally, provided that the sheet 3 is not angularly displaced. For example, a shifting of the sheet 3 in FIG. 1 toward the left, at right angles to the direction of travel of the sheet 3, results in an increase of the distances dπ and d₂₁ because the triangles 1 and 2 in such case lie such that the sides of the triangles spread out in a rightward direction. A shifting of the sheet 3 in FIG. 1 toward the right, for example, and at right angles to the direction of travel of the sheet 3, results in a decrease of the distances dπ and d₂₁. In the control mechanism, the distances dπ and d₂₁ are assigned specifically to values for a displacement of the sheet 3 at right angles to the direction of travel. FIG. 2 shows an overhead view of the first form side of the sheet 3 that is similar in some respects to FIG. 1. The first triangle 1 and the second triangle 2 have sides of equal length, have equal sizes, and their base sides run at equal distances from and parallel to the longitudinal side of the sheet 3, as was also the case in FIG. 1. The first triangle 1 is located close to the leading edge 7 of the sheet 3 and the second triangle 2 is located close to the trailing edge 8. In this representation, the sheet 3 is displaced on the conveyor belt 6 by an angle _\ from its center point; there is no displacement of the sheet 3 on the conveyor belt in the plane that is at right angles to the direction of travel. A displacement of the sheet 3 in the direction of its travel is not covered by the invention. In FIG. 2, an angular displacement of the sheet 3 is present during the first passage of the sheet 3 through the printing machine, whereby the first form side is up, whereas in the preferred embodiments, the sheet 3 becomes oriented error-free on the conveyor belt 6 and is immediately thereafter imprinted with the triangles 1 and 2. In such case, the sheet 3 lies error-free on the conveyor belt 6 with its first form side up and an angular displacement such as that shown in FIG. 2 does not take place. Because of the displacement of the sheet, the sensor 10 detects the first triangle 1 at different points of intersection on the triangle 1 to the background formed by the sheet 3 than is the case in FIG. 1. As a consequence, the distance d₁₂ between the points on the sides 11 and 12 of the first triangle 1 is different from the corresponding distance dπ, i.e., when the sheet 3 has shifted angularly, the distance dπ does not equal the distance d₁₂. In the instant case as shown in FIG. 2 the distance d₁ is greater than the distance dπ. The same applies to the second triangle 2, whereby the sensor 10 measures a distance d₂₂ from the first side 21 relative to the direction of travel to the second side 22 of the second triangle 2, whereby the distance d₂₂ is greater than the distance d₂₁. With the aid of the existing distances d₁₂ and d₂₂ for the first triangle 1 and the second triangle 2, respectively, the angular displacement oi_! of the sheet 3 on the conveyor belt 6 is specifically determined in the control mechanism 20. The quotient of the distances d₁ and d₂₂ is calculated in the control mechanism 20. The quotient thus calculated is then correlated with a value for an angle in the correlation table that corresponds to the angular displacement of the sheet 3, which in the instant example is the angle αi. A displacement of the sheet 3 on the conveyor belt 6 that is at right angles to its direction of travel in the direction of the two-pointed arrow results in changes in the distances d₁₂ and d₂₂ that are detected by the sensor 10. In this case, too, the quotient of the distances d₁₂ and d₂₂ is calculated in the control mechanism, whereby this quotient is correlated with a displacement path at right angles to the direction of travel of the sheet 3. If the sheet 3 is shifted exclusively at right angles to its direction of travel, the quotient derived from the distances d₁₂ and d₂₂ remains constant. FIG. 3 shows an overhead view of the back-side of the sheet 3 on the conveyor belt 6 along with the first triangle 1 and the second triangle 2, which are used as marks as they were in FIG. 2. Here, the sheet 3 has already been run through the printing machine and has been turned over. It should be noted here, that after being turned over such that the back-side is now facing upward, the sheet 3 is not once again imprinted with marks. Instead, the sensor 10 detects the triangles 1 and 2, which have already been imprinted on the sheet 3, through the sheet 3. It is also possible for the second sensor to be mounted underneath both the sheet 3 and the conveyor belt 6, and this sensor, instead of the sensor 10, would then detect the triangles 1 and 2, whereby the two sensors would be aligned with one another and would detect the triangles 1 and 2 at the same places relative to the conveyor belt 6. In the present case, the conveyor belt 6, having only one sensor, is transparent. Here, the sheet 3 has experienced a wider angular displacement in its passage through the printing machine than was the case in FIG. 2, i.e., the angular displacement here is now α₂, which is not equal to α_1; whereby the angles from the longitudinal side of the sheet 3 are measured. In the preferred embodiment of the invention, in which the triangles 1 and 2 are imprinted directly after the sheet 3 has been correctly aligned on the conveyor belt 6, the angle α₂ is the only angular displacement of the sheet 3, and the angle oii equals zero. The distances d₁₃ and d₂₃ are measured as described above and then transmitted to the control mechanism. In the control mechanism 20, the distances d₁₃ and d₂ are compared with the distances dπ and d₁₂, which represent the error- free distances. The results of this comparison show whether the sheet 3 became displaced on the conveyor belt between the detection by the sensor 10 of the triangles 1 and 2 on the first form side and the detection of the triangles 1 and 2 after the sheet 3 had passed through the printing machine and had been turned over. If, for example, the measured distances d₁₃ and dπ and d and d₂₁ are the same, the sheet 3 will not have been displaced during its passage through the printing machine, the position of the sheet 3 on the conveyor belt will not have changed, and a correction of the position of the sheet 3 on the conveyor belt will not be necessary. If distances d₁ and dπ and/or d₂₃ and d₂₁ are unequal, then the sheet 3 will have been displaced on the conveyor belt 6 between the detection that took place when the first form side was up and the detection after passage through the printing machine, that was followed by turning the sheet 3 over. The distances dπ and d₂₁ indicate the distances of the first triangle 1 or the second triangle 1, respectively, in the error-free situation as shown in FIG. 1; these distances are stored in the control mechanism 20. The differences in values between the detected distances of the first triangle 1, i.e., dπ and d₁₃, and of the second triangle 2, i.e., d₂₁ and d₂₃ are calculated. These differential values are correlated in the correlation table of the control mechanism 20 with a displacement of the sheet 3, as described above. Thus, the angle α₂ is determined, which identifies the angular displacement of the sheet 3 on the conveyor belt 6, which occurred during the passage of the sheet 3 through the printing machine. This displacement is then corrected before the sheet 3 is conveyed to the printing mechanisms or printing modules of the printing machine and printed on the back-side.

Claims

CLAIMS:

1. A process for determining compass and/or register errors in a printing machine that uses an arrangement of triangles (1 and 2) as register marks, characterized by said fact that at least one triangle (1) is printed near said leading edge (7) of said sheet (3) and at least one additional triangle (2) is printed near said trailing edge (8) of said sheet (3), said first triangle (1) and said second triangle (2) are detected by a sensor (10) and a displacement of said sheet (3) at right angles to said direction of travel of said sheet (3) and/or an angular displacement of said sheet (3) is determined.

2. A process for determining compass and/or register errors according to Claim 1, characterized by said fact that said displacement of said sheet (3) at right angles to said direction of travel of said sheet (3) on said first form side (4) is determined, said sheet (3) is turned over, said displacement of said sheet (3) at right angles to said direction of travel of said sheet (3) and/or said angular displacement of said sheet (3) on said back side (5) is determined.

3. A process for determining compass and/or register errors according to Claim 2, characterized by said fact that said first triangle (1) and said second triangle (2) are detected through said sheet (3).

4. A process for determining compass and/or register errors according to Claim 3, characterized by said fact that displacement of said sheet (3) at right angles to said direction of travel of said sheet (3) and/or said angular displacement of said sheet (3) is or are corrected.

5. A process for determining compass and/or register errors according to Claim 4, characterized by said fact that said position of said sheet (3) continues to be corrected until said quotient, derived from said distance d_lx between said sides (11 and 12) of said first triangle (1) that have been detected by » said sensor (10) and said distance d_2x between said sides (21 and 22) of said second triangle (2) that have been detected by said sensor (10), equals one.