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

One of the most commonly used printing methods of today is offset printing that uses an intermediate blanket cylinder 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 plate cylinder, ink and dampening rollers, a rubber blanket, a blanket cylinder and an impression cylinder. The printing plate comprises hydrophilic, water receptive parts and hydrophobic, ink receptive parts. The water receptive parts of the plate form non-image areas whereas the ink receptive parts form image areas. The dampening rollers apply chemically modified water, so-called fountain solution, to the non-image areas of the plate, and then ink is applied to the image areas of the plate. The ink and the fountain solution are transferred to the printing substrate from the printing plate via a rotating blanket cylinder which is covered with a smooth rubber blanket.

During the printing operation impurities are accumulated on 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 adversely and therefore repeated cleaning of the blanket is

necessary. Cleaning of the blanket is usually performed after each print job, but sometimes the print quality is so badly affected by the build-up on the blanket that a print 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 response and may give rise to a number of prints with bad print quality. It has been proposed to optimize the cleaning frequency based on the amount of ink consumed, see e.g. US 5,740,030. However, it remains a need for a better, faster and more efficient way to monitor the build-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 surface to be monitored,

b) illuminating the measuring area with at least two

light sources,

c) acquiring images of the measuring area with at least one image sensor,

d) acquiring a topographic illustration of said images by means of a topographic measurement system,

e) repeating the above steps b) , c) and d) one or more times ,

f) registering an increasing amount of accumulated

impurities from the acquired topographic

illustrations, and

g) interrupting the operation of the printing press for cleaning said surface when the amount of accumulated impurities 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 1 shows an arrangement for carrying out the method according 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 printing press 1, wherein the amount of impurities is measured by means of a topographic measurement technique on a

determined measuring area 3 of the rubber blanket 2. The accumulation or build-up of impurities can be observed as a change in the height difference between printing and non-printing areas. Preferably, the topographic

measurement technique is based on photometric stereo. According to this technique, the surface is illuminated from different directions, whereby several images are acquired. These are then used to calculate the

topography .

The information can then be used to predict surface accumulation based on past, current and future states of the process. In the printing machine, the accumulation amount and suitable measurement areas can be decided based on the previous, current and next printing layouts. Figure 1 shows a preferred embodiment of an arrangement for carrying out the method according to the invention, wherein a light source 101 may comprise a plurality and even hundreds of coherent light sources in order to mix the coherence and to avoid the speckle phenomenon. The point or area 3 of the surface 2 to be imaged may be defined by a template and/or a lens 103, the form of which does not necessarily have to be limited to the one shown in figure 1. From the point being imaged light is transferred to a dichromatic film 104 that distributes the illumination wavelength-specifically along a route corresponding to the wavelength of the light source to a branch of an optical path and to an image sensor or camera 102, located at the end of it. Even though figure 1 shows such an optical path where the

wavelength-specific branch is both at the beginning and the 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 shown for illustration purposes, according to an embodiment of the invention, there may be several light sources 101. The light source 101 is monochromatic, but it is arranged as incoherent. This is implemented, for example, by utilizing a set of coherent light sources, whose

coherence is intentionally mixed in order to prevent the speckle phenomenon, but still for creating sufficient illumination efficiency for short pulses for taking a still image. The light source 101 is preferably a LED light source.

In the method for imaging the topography of a specific point or area 3 of a rotating cylinder surface 2,

illumination implemented by means of incoherent

radiation, which comprises at least a first wavelength component λ±ε and a second wavelength component λ, comprises the following phases:

- directing the incoherent illumination along an

optical path OP. la, OP.l b, OP.2, OP.3, OP.4a, OP4b to a specific point of the cylinder surface 2 to be imaged,

- pulsing said illumination with pulsing means for

taking an instantaneous still image with each camera 102 arranged at the end of an optical path,

- directing said illumination from said specific point of the measuring area 3 to a wavelength-selective λ, λ±ε film, which is on said optical path OP. la, OP.l b, OP.2, OP.3, OP.4a, OP4b, for

- creating illumination for distributing a first wavelength component λ±ε of the used radiation via a first optical event, such as reflection, to a first branch OP.4a of the optical path to a first camera 102 at the end of it, and for - creating illumination for distributing a second wavelength component λ of the used radiation via a second optical event to a second branch OP.4b of said optical path to a second camera 102 at the end of it,

- storing an image with said first camera as a first image 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 is reasonable from the point of view of directing, pulsing and other practical implementation of illumination.

According to one embodiment of the method according to the invention, an image is taken along with the frequency of the pulsing of the illumination with at least a first or a second camera. It is therefore possible to create an image 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 pixel matrix to be added onto a cumulative image, to the value before imaging of each variable of an elementary unit of which matrix is directed an arithmetic calculation with the value of the variable of the image taken of said elementary unit for forming a new cumulative image. Thus, the image can also be processed in other ways, for example for detecting aspects affecting the feedback.

Thus, an advantage is reached in that the controller can be presented with an image where deviations are shown better in which case it is easier for the observer to stop erroneous production.

The arrangement comprises two incoherent light sources 101 with different wavelengths (λ, λ±ε) . The light source 101 pair and camera pair 102 are on the same level. The arrangement may also comprise a second corresponding light source 101 pair, however without limiting the selection 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 first light source pair and a second light source pair, the use is affected by how many branches of optical path are arranged on the optical path after a dichromatic mirror to lead to a camera. Thus, the response of said mirror for penetrating wavelength of radiation has an effect on the more individual placement of cameras and light sources . When the topography of impurities is examined as defined by a camera pair for calculating the gradient of the surface being measured, it is possible to use a

photometric stereo known per se (R. Woodhamin (1980), Photometric Method for Determining surface Orientation from Multiple Images, Optical engineering vol. 19, no. 1, pp. 139-144 suitable parts) . Hanson and Johansson

(P. Hanson, P. Johansson (2000), Topography and

reflectance analysis of paper surface using a photometric stereo method, Optical engineering, vol. 39, no. 9, pp. 2555-2561) disclose formulas for a

2-light-photometric stereo:

(1)

df _ 1 k - dx tan( ) l + I ₂ wherein I is intensity to a first camera and I ₂ is intensity to a second camera and o is the angle of incidence of light in relation to the normal of the surface for both light sources. Thus, the topography of the surface can be calculated by integrating the gradient filed by the Hanson and Johansson method by using

integration 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 a printing press 1, the surface 2 is rotating about an axis 4 of said rubber blanket. According to the invention the surface 2 is illuminated from one direction at a point of time and a corresponding image is acquired with the camera system 102. At least two images with distinct illumination directions are acquired. The camera 102 head is at one position, taking images of the same spot, but the light is coming from different sides. The images may be taken on subsequent rotations of the blanket cylinder 1.

The method comprises the step of determining a measuring area 3 on said rotating surface 2 to be monitored. The measuring 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 illuminating the measuring area 3 from at least two different

directions. The number of illumination directions can be 2, 3, 4, or more without any upper limit. There is no need for pairs of illumination directions, each direction can be separated for example with 120 degrees such as in the case with three illumination directions. When using a plurality of light sources, the light sources 101 may have the same wavelength or different wavelengths such as white light. The color of lights is decided based on the reflectance properties of the surface 2 and the

accumulation. Preferably, said light sources 101 are LED-light sources. When using different wavelengths, the method further comprises a step of separating the

different wavelengths by using a wavelength selective mirror/prism 104. The separated wavelengths are directed to different image sensors 102 or directed to a camera 102 with a wavelength selective filter 103 such as a Bayer filter. The method also comprises the step of acquiring images of the area 3 with at least one image sensor 102. The images may be acquired on each round of rotation of the cylinder 1 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 a topographic illustration of said images by means of a topographic measurement system. The accumulation or build-up of impurities can be observed as a change in the height difference between printing and non-printing areas. Preferably, the topographic measurement technique is based on photometric stereo. According to this

technique, the measuring area 3 is illuminated from different directions, whereby several images are

acquired. These images are then used to calculate the topography. The amount of impurities on the measuring area 3 is measured by comparing printing and non-printing areas. 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 an increasing amount of accumulated impurities from the acquired topographic illustrations.

Finally, the method comprises the step of interrupting the operation of the printing press for cleaning said surface 2 when the amount of accumulated impurities on the 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 be attached to a linear slide in order to measure topography from the whole width of the surface 2. Topographies in the cylinder rotation direction may be measured by delaying illumination and image acquisition based on the rotation speed of the cylinder. Thus, the camera system may be connected to the speed sensor of the printing press and is continously synced with the press speed. For instance, if the press is running slower, the camera automatically waits for the correct illumination moment.