SE537335C2

SE537335C2 - Procedure for monitoring the accumulation of contaminants on a cloth

Info

Publication number: SE537335C2
Application number: SE1250321A
Authority: SE
Inventors: Toni Kuparinen; Lauri Eino William Faloon; Juha Kalevi Parviainen; Petri Väätäjä; Peter Petermann
Original assignee: Lauri Eino William Faloon; Toni Kuparinen; Oy Labvision Technologies Ltd; Juha Kalevi Parviainen; Peter Petermann; Stora Enso Oyj; Vaeaetaejae Petri
Priority date: 2012-03-30
Filing date: 2012-03-30
Publication date: 2015-04-07
Also published as: WO2013144078A1; SE1250321A1

Abstract

lO l2 A B S T R A C T A method for monitoring the accumulation of impurities ona surface (2) of a rubber blanket of a printing press(l), said surface rotating about an axis (4) of saidrubber blanket, characterized by the steps of a)determining a measuring area (3) on said rotating surfaceto be monitored, b) illuminating the measuring area withat least two light sources (lOl), c) acquiring images ofthe measuring area with at least one image sensor (102),d) acquiring a topographic illustration of said images bymeans of a topographic measurement system, e) repeatingthe above steps b), c) and d) one or more times, f)registering an increasing amount of accumulatedimpurities from the acquired topographic illustrations,and g) interrupting the operation of the printing pressfor cleaning said surface when the amount of accumulatedimpurities on the measuring area has reached a predetermined level.

Description

lO A method for monitoring accumulation of impurities on arubber blanket The present invention relates to a method for monitoringthe accumulation of impurities on a surface of a rubberblanket of a printing press, said surface rotating about an axis of said rubber blanket.

One of the most commonly used printing methods of todayis offset printing that uses an intermediate blanketcylinder to transfer an image from an image carrier,hereinafter mentioned as the plate, to a substrate, e.g.paper. An offset printing unit usually comprises a platecylinder, ink and dampening rollers, a rubber blanket, ablanket cylinder and an impression cylinder. The printingplate comprises hydrophilic, water receptive parts andhydrophobic, ink receptive parts. The water receptiveparts of the plate form non-image areas whereas the inkreceptive parts form image areas. The dampening rollersapply chemically modified water, so-called fountainsolution, to the non-image areas of the plate, and thenink is applied to the image areas of the plate. The inkand the fountain solution are transferred to the printingsubstrate from the printing plate via a rotating blanket cylinder which is covered with a smooth rubber blanket.

During the printing operation impurities are accumulatedon the rubber blanket. The impurities may comprise, e.g.,ink- and fountain solution residues, fibers and/or dust.These impurities may affect the print quality adverselyand therefore repeated cleaning of the blanket isnecessary. Cleaning of the blanket is usually performedafter each print job, but sometimes the print quality isso badly affected by the build-up on the blanket that aprint job has to be interrupted for cleaning operation.The cleaning frequency may, e.g., be determined by monitoring the print quality. However, this gives a late lO response and may give rise to a number of prints with badprint quality. It has been proposed to optimize thecleaning frequency based on the amount of ink consumed,see e.g. US 5,740,030. However, it remains a need for abetter, faster and more efficient way to monitor thebuild-up of impurities on the blanket and to optimize the printing process.

The method according to the invention is characterized by the steps of a) determining a measuring area on said rotating surfaceto be monitored, b) illuminating the measuring area with at least twolight sources, c) acquiring images of the measuring area with at leastone image sensor, d) acquiring a topographic illustration of said images bymeans of a topographic measurement system, e) repeating the above steps b), c) and d) one or moretimes, f) registering an increasing amount of accumulatedimpurities from the acquired topographicillustrations, and g) interrupting the operation of the printing press forcleaning said surface when the amount of accumulatedimpurities on the measuring area has reached a predetermined level.

In the following, the invention will be described further with reference to the drawings, wherein: Figure l shows an arrangement for carrying out the methodaccording to the invention for monitoring a measuring area on a rubber blanket of a printing press.

The invention relates to a method for monitoring the accumulation of impurities on a surface of a rubber blanket 2 of a rotating blanket cylinder of a printingpress 1, wherein the amount of impurities is measured bymeans of a topographic measurement technique on adetermined measuring area 3 of the rubber blanket 2. Theaccumulation or build-up of impurities can be observed asa change in the height difference between printing andnon-printing areas. Preferably, the topographicmeasurement technique is based on photometric stereo.According to this technique, the surface is illuminatedfrom different directions, whereby several images areacquired. These are then used to calculate the topography.

The information can then be used to predict surfaceaccumulation based on past, current and future states ofthe process. In the printing machine, the accumulationamount and suitable measurement areas can be decided based on the previous, current and next printing layouts.

Figure 1 shows a preferred embodiment of an arrangementfor carrying out the method according to the invention,wherein a light source 101 may comprise a plurality andeven hundreds of coherent light sources in order to mixthe coherence and to avoid the speckle phenomenon. Thepoint or area 3 of the surface 2 to be imaged may bedefined by a template and/or a lens 103, the form ofwhich does not necessarily have to be limited to the oneshown in figure 1. From the point being imaged light istransferred to a dichromatic film 104 that distributesthe illumination wavelength-specifically along a routecorresponding to the wavelength of the light source to abranch of an optical path and to an image sensor orcamera 102, located at the end of it. Even though figure1 shows such an optical path where thewavelength-specific branch is both at the beginning andthe end of the path, the light sources 101 may be located in a mixed manner. In addition, it is noted that even though only one pair of light sources 101 has been shownfor illustration purposes, according to an embodiment ofthe invention, there may be several light sources 101.The light source 101 is monochromatic, but it is arrangedas incoherent. This is implemented, for example, byutilizing a set of coherent light sources, whosecoherence is intentionally mixed in order to prevent thespeckle phenomenon, but still for creating sufficientillumination efficiency for short pulses for taking astill image. The light source 101 is preferably a LED light source.

In the method for imaging the topography of a specificpoint or area 3 of a rotating cylinder surface 2,illumination implemented by means of incoherentradiation, which comprises at least a first wavelengthcomponent Åfs and a second wavelength component Å,comprises the following phases: - directing the incoherent illumination along anoptical path OP.1a, OP.1 b, OP.2, OP.3, OP.4a, OP4bto a specific point of the cylinder surface 2 to beimaged, - pulsing said illumination with pulsing means fortaking an instantaneous still image with each camera102 arranged at the end of an optical path, - directing said illumination from said specific pointof the measuring area 3 to a wavelength-selective Å,Afa film, which is on said optical path OP.1a, OP.1b, OP.2, OP.3, OP.4a, OP4b, for - creating illumination for distributing a firstwavelength component Åfs of the used radiationvia a first optical event, such as reflection,to a first branch OP.4a of the optical path toa first camera 102 at the end of it, and for - creating illumination for distributing a secondwavelength component Å of the used radiation via a second optical event to a second branch OP.4b of said optical path to a second camera102 at the end of it,- storing an image with said first camera as a firstimage on a first storage medium, and- storing an image with said second camera as another image on another storage medium.

Said phases can be partly overlapping in a manner that isreasonable from the point of view of directing, pulsingand other practical implementation of illumination.According to one embodiment of the method according tothe invention, an image is taken along with the frequencyof the pulsing of the illumination with at least a firstor a second camera. It is therefore possible to create animage that has a resolution on the microscopic level with an arrangement of one or more camera pairs 102.

The image taken with a camera is handled as a pixelmatrix to be added onto a cumulative image, to the valuebefore imaging of each variable of an elementary unit ofwhich matrix is directed an arithmetic calculation withthe value of the variable of the image taken of saidelementary unit for forming a new cumulative image. Thus,the image can also be processed in other ways, forexample for detecting aspects affecting the feedback.Thus, an advantage is reached in that the controller canbe presented with an image where deviations are shownbetter in which case it is easier for the observer to stop erroneous production.

The arrangement comprises two incoherent light sources101 with different wavelengths (Å, Afa). The light source101 pair and camera pair 102 are on the same level. Thearrangement may also comprise a second correspondinglight source 101 pair, however without limiting theselection of the wavelength of the light sources 101 of the pair itself as such, nor the number of the light source pairs. In an embodiment, which comprises a firstlight source pair and a second light source pair, the useis affected by how many branches of optical path arearranged on the optical path after a dichromatic mirrorto lead to a camera. Thus, the response of said mirrorfor penetrating wavelength of radiation has an effect onthe more individual placement of cameras and light SOUICGS .

When the topography of impurities is examined as definedby a camera pair for calculating the gradient of thesurface being measured, it is possible to use aphotometric stereo known per se (R. Woodhamin (1980),Photometric Method for Determining surface Orientationfrom Multiple Images, Optical engineering vol. 19, no. 1,pp. 139-144 suitable parts). Hanson and Johansson(P. Hanson, P. Johansson (2000), Topography andreflectance analysis of paper surface using a photometricstereo method, Optical engineering, vol. 39, no. 9, pp. 2555-2561) disclose formulas for a 2-light-photometric stereo: (l)âf_ 1 I1*I2 âx _ tan(0)I1-klz wherein Q is intensity to a first camera and Q isintensity to a second camera and o is the angle ofincidence of light in relation to the normal of thesurface for both light sources. Thus, the topography ofthe surface can be calculated by integrating the gradientfiled by the Hanson and Johansson method by usingintegration on Fourier level combined with a Wiener filter.

Preferred embodiments of the invention will now be described. In a method for monitoring the accumulation of impurities on a surface 2 of a rubber blanket of aprinting press 1, the surface 2 is rotating about an axis4 of said rubber blanket. According to the invention thesurface 2 is illuminated from one direction at a point oftime and a corresponding image is acquired with thecamera system 102. At least two images with distinctillumination directions are acquired. The camera 102 headis at one position, taking images of the same spot, butthe light is coming from different sides. The images maybe taken on subsequent rotations of the blanket cylinder1.

The method comprises the step of determining a measuringarea 3 on said rotating surface 2 to be monitored. Themeasuring area 3 to be monitored can be selected on the past, current and future states of the process.

Moreover, the method comprises the step of illuminatingthe measuring area 3 from at least two differentdirections. The number of illumination directions can be2, 3, 4, or more without any upper limit. There is noneed for pairs of illumination directions, each directioncan be separated for example with 120 degrees such as inthe case with three illumination directions. When using aplurality of light sources, the light sources 101 mayhave the same wavelength or different wavelengths such aswhite light. The color of lights is decided based on thereflectance properties of the surface 2 and theaccumulation. Preferably, said light sources 101 areLED-light sources. When using different wavelengths, themethod further comprises a step of separating thedifferent wavelengths by using a wavelength selectivemirror/prism 104. The separated wavelengths are directedto different image sensors 102 or directed to a camera102 with a wavelength selective filter 103 such as a Bayer filter.

The method also comprises the step of acquiring images ofthe area 3 with at least one image sensor 102. The imagesmay be acquired on each round of rotation of the cylinder1 using lights of different wavelengths and/or images may be acquired from subsequent rotations of the cylinder 1.

The method further comprises the step of acquiring atopographic illustration of said images by means of atopographic measurement system. The accumulation orbuild-up of impurities can be observed as a change in theheight difference between printing and non-printingareas. Preferably, the topographic measurement techniqueis based on photometric stereo. According to thistechnique, the measuring area 3 is illuminated fromdifferent directions, whereby several images areacquired. These images are then used to calculate thetopography. The amount of impurities on the measuringarea 3 is measured by comparing printing and non-printingareas. Moreover, the amount of impurities can be measured by analyzing the shapes and sizes of raster points.

The method further comprises the step of registering anincreasing amount of accumulated impurities from the acquired topographic illustrations.

Finally, the method comprises the step of interruptingthe operation of the printing press for cleaning saidsurface 2 when the amount of accumulated impurities onthe measuring area 3 has reached a predetermined level,i.e. a predetermined threshold. Hence, the method allows an optimization of the washing intervals.

Moreover, the image acquisition device system can beattached to a linear slide in order to measure topographyfrom the whole width of the surface 2. Topographies inthe cylinder rotation direction may be measured by delaying illumination and image acquisition based on the rotation speed of the cylinder. Thus, the camera systemmay be connected to the speed sensor of the printingpress and is continuously synced with the press speed.For instance, if the press is running slower, the camera automatically waits for the correct illumination moment.

Claims

C L A I M S

1. A method for monitoring the accumulation ofimpurities on a surface (2) of a rubber blanket of aprinting press (1), said surface rotating about an axis(4) of said rubber blanket, characterized by the steps ofa) determining a measuring area (3) on said rotatingsurface (2) to be monitored, b) illuminating the measuring area (3) with at least two(101), c) acquiring images of the measuring area (3) with at light sources least one image sensor (102), d) acquiring a topographic illustration of said images bymeans of a topographic measurement system, e) repeating the above steps b), c) and d) one or moretimes, f) registering an increasing amount of accumulatedimpurities from the acquired topographicillustrations, and g) interrupting the operation of the printing press forcleaning said surface (2) when the amount ofaccumulated impurities on the measuring area (3) has reached a predetermined level.

2. The method according to claim 1, characterized bythe step of illuminating the measuring area (3) with aplurality of light sources (101) of one and the samewavelength or different wavelengths, said light sources (101) being LED-light sources.

3. The method according to claim 2, characterized bythe step of separating the different wavelengths by usinga wavelength selective mirror/prism (104), and directingthe separated wavelengths to different image sensors(102). lO ll

4. The method according to any one of claims l-3,characterized in that said topographic measuring system includes a photometric stereo principle.

5. The method according to any one of claims l-4,characterized in that the measuring area (3) to bemonitored is selected based on the past, current and future states of the process.

6. The method according to claim 5, characterized bycomparing printing and non-printing areas to measure the accumulation within the measuring area (3).

7. The method according to claim 5, characterized byanalyzing the shapes and sizes of raster points to measure the accumulation within the measuring area (3).

8. The method according to any one of claims 5-7,characterized by optimizing blanket washing intervalsbased on the threshold for the amount of accumulated impurities.

9. The method according to any one of the precedingclaims, characterized by comprising an image acquisitionsystem to a linear slide for measuring topography over the whole width of the cylindrical surface (2).

10. The method according to claim 9, characterized bymeasuring topographies in the cylinder rotation directionby delaying illumination and image acquisition based on the rotation speed of the cylinder.