METHOD AND APPARATUS FOR MONITORING THE PASSAGE OF MARKS ON A WEB

Abstract: A method and apparatus for monitoring the passage of a mark (13-16) on a web (5). The web (5) is caused to move through a detection station (7) at which at least that portion of the web carrying the mark is illuminated. An image of the mark (13-16) is projected onto a quadrant array (20) of photodetector elements at the detection station (7). The axes (25,26) defined between the elements (21-24) are tranverse to the direction of movement of the web. The time t₁, t₂ at which substantially equal areas of a mark are projected on either side of the axes is determined which provide an indication of registration of the web.

Description: The invention relates to a method and apparatus for monitoring the passage of marks on a web. In the printing industry, it is common, particularly in the case of colour printing, to cause the web onto which a picture is to be printed to pass in series through a number of printing stations. At each printing station, one of the colour components of the picture is printed onto the web. It is important that each colour component is registered with each of the other colour components. Registration is achieved by closely controlling the position of the web at each printing station. In order to monitor or control registration, it is conventional to print onto the web in a track extending along the length of the web a number of register marks, at least one for each colour component. Typically, these marks have an accurate, predetermined shape and are then detected at a detection station. The time interval between successive marks corresponding to different colour components then provides an indication of whether the colour components are registered in the direction of web travel while the relative lateral positions between the marks indicates the degree of registration of the colour components in the lateral direction. In the case of gravure printing, a detection station is provided after each printing station and the output of the detection apparatus at each detection station is used to control the position of the web as it passes through the printing station subsequently so as to provide real time correction. In offset web printing, it is more common to print all the colour components of a picture and then monitor the register marks downstream of all the printing stations. As a result of this monitoring stage, subsequent web movement is adjusted to compensate for any registration errors. An example of a method of web register control is described in GB-A-1253426. US-A-4485982 describes a number of different web tracking systems utilising detector assemblies formed by arrays of detectors. These are relatively complex to operate requiring the measurement of voltages and the like to detect the passage of register marks. US-A-4518856 also discloses a web tracking system using detector assemblies which, in this case, comprise two detectors and a source juxtaposed relative to the position of register marks in use. In this case, there are limitations on the types of register marks which can be tracked. A further problem with the known register control systems is the need for each register mark to have the same closely defined shape. This is particularly important where the mark is to be used for providing lateral offset (or sidelay) information in which the lateral position is encoded into the shape of the registration mark. In accordance with one aspect of the present invention, a method of monitoring the passage of a mark on a web comprises causing a web to move through a detection station and illuminating at least that portion of the web carrying the mark; causing an image of the mark to be projected onto an array of detector elements at the detection station, the array being symmetrical about at least two axes transverse to the direction of movement of the web; and determining the times (t₁,t₂) at which substantially equal areas of a mark are projected on either side of the axes. This invention deals with the problem described above by transferring the problem of mark shape from the mark to the detector. An example of a suitable array is a quadrant array which is well known for use in monitoring the form of a radiation beam such as a laser beam and an example of such use is described in EP-A-0057339. However in all these uses the "image" of the beam and the array are stationary. Typically, the array will have just four detector elements, each having substantially the same shape and size. However, variations in the form of the array are possible and for example other numbers of elements such as six or eight could be used. Typically the elements of the array are arranged symmetrically about a first line transverse, preferably orthogonal, to the web direction, the method further comprising determining a time representative of when the mark crosses said line. Preferably, the elements are also arranged symmetrically about a second line orthogonal to the first line. Conveniently, the method further comprises determining a time (t₃), at which the mark crosses a line orthogonal to the direction of movement of the web, wherein t₃ = (t₁ + t₂)/2 The invention is particularly applicable to a method of monitoring the registration of a web carrying a number of register marks normally spaced apart in the direction of the web movement in which a method according to the first aspect of the invention is carried out on each mark and in which the elements of the array are symmetrically arranged about a line orthogonal to the direction of movement of the web, the method comprising determining a time representative of when each mark crosses said line. The difference between each determined time then indicates the degree of misregistration in the direction of web movement. The method according to the first aspect of the invention is also applicable to a method of monitoring the registration of a web carrying at least one register mark, the method comprising determining the difference between the times t₁, t₂ to generate a lateral offset factor. This lateral offset factor provides an indication of the lateral position of the web (or sidelay) relative to some predetermined, normal position. In practice, the lateral offset factor is determined from the difference between the times t₁, t₂ and the velocity of the web which could be determined, for example, by monitoring the time between register marks for the same colour component passing through the detection station. In accordance with a second aspect of the present invention, a method of web register control comprises printing on a web two or more register marks one behind the other in a track extending in the direction of travel of the web, monitoring the passage of the marks through a detection station by carrying out a method in accordance with the first aspect of the invention, and controlling subsequent movement of the web in response to the determined times such that the determined times satisfy the registration condition. For example, the speed of the web could be controlled to maintain the time intervals between marks passing through the detection station substantially constant (which may be determined in the manner explained above); and/or adjusting the lateral position of the web in response to a lateral offset factor related to the difference between the two times t₁, t₂. In accordance with a third aspect of the present invention, apparatus for monitoring the passage of a mark on a web comprises feed means for feeding the web through a detection station; a light source for illuminating that portion of the web carrying the mark at the detection station; an array of detector elements onto which an image of the mark is projected at the detection station, the array being symmetrical about at least two axes transverse to the direction of movement of the web; and processing means for determining the times t₁, t₂ at which substantially equal areas of a mark are projected on either side of the axes. As explained above, such monitoring apparatus could be incorporated into conventional printing apparatus such as gravure or offset printing apparatus to achieve register control. Typically, the detector elements comprise photodetectors. An example of a web offset printing system incorporating web register control apparatus according to the invention will now be described with reference to the accompanying drawings, in which:- Figure 1 is a schematic view of the printing apparatus; Figure 2 is a schematic plan of part of the detection station shown in Figure 1; and, Figure 3 is a circuit diagram of the logic circuitry and the quadrant array contained in the detector head of the detection station shown in Figure 2. The printing apparatus shown in Figure 1 has a conventional form comprising in series four offset web printing units 1-4 which print onto a web 5 respective colour components of an image. Typically, these colour components will be cyan, magenta, yellow,and black. The position and speed of the web 5 as it passes through each of the printing units 1-4 is controlled by each of those printing units in response to signals from a controller 6 incorporating a microcomputer. Downstream of the printing units 1-4 the web 5 is fed via driven rollers 7A-7C through a register mark detection station 7. The detection station 7 includes a light source 8 which illuminates that section 9 of the web within the detection station 7. Light reflected from a side portion of the web 5 passes to a detector head 10. As will be explained below, the detector head 10 includes logic circuitry 40 which generates two signals P, Q which are passed along respective lines 11, 12 to the controller 6. In a track alongside the image printed on the web, each printing unit 1-4 prints a register mark 13-16 (Figure 2). In normal operation, these register marks 13-16 will be spaced apart from one another by a predetermined distance (normally the same distance between each pair of marks) and their lateral positions relative to the feed system will also be substantially the same. For reasons to be explained below, the leading printing unit 1 also prints a code mark 17 upstream of the register marks 13-16. The detector head 10 includes a linear charged coupled device (CCD) array 18 at its upstream end coupled via a microprocessor 19 with a quadrant array 20. The quadrant array has a conventional form and comprises four photodetectors 21-24 positioned within respective quadrants which define two orthogonal axes 25, 26. As can be seen in Figure 2, the axes 25,26 are transverse to the direction of web travel indicated by an arrow 38. Each of the photodetectors 21-24 is connected via a switch 39 controlled by the microprocessor 19 to respective conductors 27-30 shown in more detail in Figure 3. The photodetectors 21-24 generate signals, labelled A, B, C, D respectively. The conductors 27-30 are connected to logic circuitry 40 (Figure 3) comprising four adder circuits 31-34 and two subtraction circuits 35, 36. The output signals from the subtraction circuits 35, 36 are fed to the conductors 11, 12 respectively. The signal A is fed to adder circuits 31, 33; the signal B is fed to the adder circuits 31, 34; the signal C is fed to the adder circuits 32, 34; and the signal D is fed to the adder circuits 32, 33. The output signals from the adder circuits 31, 33 representing A + B and C + D respectively are fed to the subtraction circuits 35 while the output signals from adder circuits 33, 34 representing A + D and B + C respectively are fed to the subtraction circuit 36. In operation, the microprocessor 19 monitors the output signals from the CCD array 18 onto which light reflected from the section of the web 9 in the detection station 7 impinges. Initially, the switch 39 is open so that no signals are passed to the logic circuitry 40 which will thus not respond to erroneous marks appearing on the web. As soon as the code mark 17 is detected by the CCD array 18, the microprocessor 19 responds to this detection by closing the switch 39. At this stage, each photodetector 21-24 in the quadrant array should receive substantially the same intensity of light and thus the signals A-D will be the same. Thus, the output signals P,Q will both be zero. When the first register mark 13 reaches the detector station 7, an image of that mark will be reflected onto the quadrant array 20. The image will traverse the array 20 as the web moves and thus the intensity of light received by each of the photodetectors 21-24 will vary during that movement. As will be seen from Figure 3, the output signal P is representative of the effect of the image of the register mark on photodetectors 21, 22 as compared with the image on photodetectors 23, 24. Thus, this signal P will start from a null value rise to a first peak, pass through a null position and rise to a second peak (or trough) and then return to the null position. The intermediate null position will occur when the mark is centred about the axis 26 indicating the time at which equal areas of the mark are positioned on either side of the axis 26. Similarly, the signal Q will pass through an intermediate null position when the mark is centred about the axis 25. The controller 6 monitors the output signals P, Q and measures the times t₁, t₂ at which the signals P, Q respectively pass through intermediate null values. From these times t₁, t₂, a time t₃ can be calculated corresponding to a time at which the register mark crosses an imaginary line 37 orthogonal to the web direction 38. This time t₃ is given by the equation: t₃ = (t₁ + t₂)/2 This calculation is repeated by the controller 6 for each of the register marks 13-16 and the difference between the times t₃ is calculated. These differences should, if registration is correct, be the same for corresponding pairs of register marks and if there are any variations in these differences, these variations can than be used in a conventional manner to determine the corrections needed to minimise the variations by subsequent web speed control. It is also necessary to monitor the sidelay (s) of the web - that is the lateral offset of the web from some predetermined position. The sidelay is measured by determining the difference between the times for the register mark to cross the axes 25, 26 ie. t₁-t₂. After compensation for the speed of the web, this difference between the times t₁, t₂ provides an indication of the degree of side lay. That is: s = k(t₁-t₂)v where k is a constant and v is the velocity of the web. For example, if the mark passes exactly through the middle of the quadrant array 20, t₁ = t₂ and s = 0. The greater the sidelay towards quadrant 24, for example, then the smaller s and the greater the sidelay towards quadrant 22, the larger s. Once again, the controller 6 responds to the value s to adjust the feed associated with the appropriate printing unit 1-4 so as to correct for the lateral offset in a subsequent print run. It should be noted that in order to obtain the web speed, one of the printing units 1-4 can lay down two register marks a predetermined distance apart, the controller 6 responding to the times t₃ calculated for each of these marks in the way specified above to determine from the difference between those times and the distance between the marks the speed of the web.

Claims: A method of monitoring the passage of a mark (13-16) on a web, the method comprising causing a web (5) to move through a detection station (7) and illuminating at least that portion of the web carrying the mark; causing an image of the mark to be projected onto an array (20) of detector elements at the detection station (7), the array being symmetrical about at least two axes (25,26) transverse to the direction of movement of the web; and determining the times (t₁,t₂) at which substantially equal areas of a mark are projected on either side of the axes. A method according to claim 1, wherein the array (20) of detector elements comprises a quadrant array. A method according to claim 1 or claim 2, wherein the elements (21-24) of the array are arranged symmetrically about a first line (37) orthogonal to the web direction, the method further comprising determining a time representative of when the mark crosses the first line (37). A method according to claim 3, wherein the elements are also arranged symmetrically about a second line orthogonal to the first line (37). A method according to any of the preceding claims, further comprising determining a time (t₃), at which the mark crosses a line orthogonal to the direction of movement of the web, wherein t₃ = (t₁ + t₂)/2 A method of monitoring the registration of a web (5)carrying a number of register marks (13-16) normally spaced apart in the direction of the web movement in which a method according to at least claim 3 is carried out on each mark, the method comprising determining a time representative of when each mark crosses the first line (37). A method of monitoring the registration of a web (5)carrying at least one register mark (13-16), the method comprising performing a method according to any of claims 1 to 5; and determining the difference between the times t₁, t₂ to generate a lateral offset factor. A method according to claim 7, wherein the lateral offset factor (S) is defined as: S = k(t₁-t₂)V where k is a constant and V is the velocity of the web. A method of web register control comprising printing on a web two or more register marks (13-16) one behind the other in a track extending in the direction of travel of the web, monitoring the passage of the marks through a detection station (7) by carrying out a method in accordance with any of claims 1 to 5, and controlling subsequent movement of the web in response to the determined times such that the determined times satisfy the registration condition. Apparatus for monitoring the passage of a mark (13-16) on a web (5), the apparatus comprising feed means (7A-7C) for feeding the web (5) through a detection station (7); a light source (8) for illuminating that portion of the web carrying the mark at the detection station (7); an array (20) of detector elements onto which an image of the mark is projected at the detection station, the array being symmetrical about at least two axes (25,26) transverse to the direction of movement of the web; and processing means (40,6) for determining the times t₁, t₂ at which substantially equal areas of a mark are projected on either side of the axes. A method or apparatus according to any of the preceding claims, wherein the detector elements (21-24) comprise photodetectors.

Inventors: GOUCH, MARTIN PHILIP
Applicants: HARLAND CROSFIELD LIMITED
Application year: 1988
Main classification: B41F33
IPC codes: B41F33