Patent Application
20130220158
The present invention relates to a system and a method for detecting the water content of a paper web in a printing unit.
Different systems and methods are provided for measuring and regulating parameters of paper based materials, such as paper webs used in printing units. In a printing press, water or water based solution is added to the paper web for increasing the printing quality of the unit. The printing performance is dependent on the water content of the printing web, and the water content should thus be optimized during the printing process. The water content of the paper web is also critical for other purposes; since the paper web may become unstable if the water content is too high it is necessary to monitor the water content of the paper web in order to avoid physical damage to the paper web.
A printing unit typically provides water based solution to the paper web at positions adjacent to the ink supplying units. For example, a printing unit may have four different ink supplies for providing each one of C, M, Y, and K colors, respectively. Consequently, four different stations for regulating the water content of the paper web may be used. Traditionally, such stations are controlled manually and the resulting printing quality is thus highly dependent on the skills of the individual operator.
However, systems have been proposed in which a detector may be used for each regulating station in order to measure the water content of the moving paper web. Such detectors are based on either microwave radiation or IR radiation.
A microwave method based on a transmitter and a receiver operating at microwave frequencies (couple of GHz) is known in the art. However the water absorbance is rapidly decreasing for frequencies below 50 GHz. In addition typical paper thickness of 80 um is very thin compared to the wavelength (100 mm at 3 GHz) making it impossible to achieve acceptable accuracy in the determination of the of water content.
Another known microwave method is based on a resonator cavity where the paper web is running through the cavity. The resonant frequency shift and the quality factor of the resonance are used to estimate the water content. This method offers no spatial resolution, and the sensor dimensions are relatively large and therefore difficult to integrate in a printing press.
In the IR part of the spectrum a known method based on reflected radiation at a wavelength of a couple of microns may be used. At this wavelengths the optical depth is very low (couple of microns). However, this technique cannot be used in transmission mode, and effects of scattering and vibrations in the paper web make the measurement more difficult. Moreover, this method measures water content only at the surface of the web.
For printing presses and their operation, the water content in the paper web before applying the ink is an important parameter. Too much water may result in unnecessary hydro expansion and compromised quality as a result of color misalignments. Therefore, there is a need for a compact and fast regulating system including an improved detecting unit that can be installed in existing printing unit lines to monitor and regulate the water content.
Accordingly, the present invention preferably seeks to mitigate, alleviate or eliminate one or more of the above-identified deficiencies in the art and disadvantages singly or in any combination and solves at least the above mentioned problems by providing a system according to the appended claims.
An idea of the invention is to provide a system that enables measuring of the water content of a paper web running in a printing press.
A further idea of the invention is to provide a water content measuring system that is less expensive and less bulky than prior art systems.
A yet further idea is to provide a regulating system that may change the water content of the paper web after performing a measuring sequence on the moving paper web.
A still further idea is to provide an automatic system for optimizing the water content of a paper web in a printing press, thus optimizing the process and decreasing the required maintenance time.
As a result of considering the above mentioned drawbacks the inventors have come to the surprising conclusion that an improved measurement is achieved at frequencies where the paper thickness is a sizeable fraction of the wavelength. This part of the spectrum is often referred as mm (30-300 GHz), or sub-mm (300 GHz-3 THz). With a typical paper thickness between 50 μm and 100 μm the paper electrical thickness is about 1/9 of the wavelength at 300 GHz this results in attenuation/delay that are sensitive to the water content in the paper and at the same time convenient for measurement in transmission and/or reflection mode where the transmitter and the receiver are on the opposite and/or the same side of the paper. Another advantage of using the mm and sub-mm wavelengths is the possibility to fabricate compact optical components, resulting in compact sensor allowing positioning of the sensor almost at any point in the line.
The fact that minor differences of water in the paper produce measurable change in both amplitude and phase of the transmitted mm-wave signal can be used to provide two independent measurements of the same parameter (assuming the paper has a constant thickness) and further improve the accuracy of the measurement.
In summary, the advantage of using mm and sub-mm part of the spectra (100 GHz-3 THz) is that signals are significantly attenuated/delayed in proportion to the water content when transmitted through the thin paper layers (50-100 μm). At the microwave part of the spectra (<30 GHz) such thin layers become “invisible” since they represent a very small fraction of the wavelength whereas in the IR part of the spectra the paper thickness is optically “thick” and signals can not be transmitted through. As result effects of scattering and vibrations can affect the measurement.
According to a first aspect of the invention, a detecting unit for measuring properties of a paper web moving in a printing unit is provided comprising a transmitter arranged to emit electromagnetic radiation having a single wavelength in the range of 0.1 to 3 THz towards a paper web moving in a printing unit, a receiver arranged to receive electromagnetic radiation being transmitted through or reflected by said paper web and to create a signal proportional to the intensity and/or the delay of the received radiation, and a controller having an input channel for receiving the signal, and a calculating unit for determining a measure relating to the properties of the paper web from the signal.
According to a second aspect of the present invention, a method for detecting the water content of a moving paper web in a printing press is provided. The method comprises the steps of emitting electromagnetic radiation having a single wavelength in the range of 0.1 to 3 THz towards a paper web moving in a printing unit, receiving electromagnetic radiation being transmitted through or reflected by said paper web, creating a signal representing the received radiation, and determining a measure relating to the properties of the paper web from the signal.
These and other aspects, features and advantages of which the invention is capable of will be apparent and elucidated from the following description of embodiments of the present invention, reference being made to the accompanying drawings, in which
a and 3b are diagrams showing the attenuation and phase delay for 80 um thick paper for three different degrees of water content as a function of frequency;
c is a diagram showing the expected detection accuracy as a function of frequency of a detecting unit according to an embodiment;
a to 4c are schematic side views of a detection system of a regulating system according to three embodiments;
With reference to
The printing press may e.g. be an offset printer, and the plate cylinder 18 may thus carry a lithographic printing plate. The shown part 10 may e.g. be one of several stations of a printing press, of which each station is used and configured to provide a single color to the paper web 12.
The part 10 of the printing press has a regulating system 40 for adjusting the water content of the moving paper web 12. The regulating system 40 includes the supply 30 of the water based solution, a controller (not shown) for controlling the amount of water based solution provided to the paper web 12 via the cylinders 18, 14, and a detecting unit 50 configured to measure the water content of the moving paper web 12 and transmit the measured value of the water content to the controller. The controller then calculates if the supply 30 should increase the amount of water based solution that is provided to the paper web and consequently sends a command to the supply 30 if such action should be performed. The desired water content of the paper web, which normally is lower than 20%, is dependent on various parameters such as paper thickness and quality, press speed, desired color coverage etc.
In a further embodiment, the controller may also be connected to the ink supplying unit 20, for sending commands whether to increase or decrease the amount of ink due to the measured water content.
The detecting unit 50 comprises a transmitter 52 and a receiver 54. The transmitter 52 emits mm or sub-mm radiation, i.e. radiation in the range between 0.1 and 10 THz and preferably between 0.1 and 3 THz and the emitted radiation is collected by the receiver 54 after the radiation has interacted with the moving paper web 12.
In an embodiment, the emitted radiation has a single wavelength. However, the transmitter 52 may be capable of switching between radiation frequencies within the mm or sub-mm wavelength interval for improving accuracy of measurements.
In one embodiment, as will be further described below with reference to
b, the transmitter 52 and the receiver 54 are arranged on opposite sides of the paper web 12, such that the receiver 54 detects radiation that has been propagating through the paper web 12. Hence, the detecting system may be configured to operate in transmission mode.
In another embodiment, as will be further described below with reference to
The presented embodiments take advantage of frequencies between 100 GHz and 10 THz, preferably between 0.1 and 3 THz, and even more preferably between 0.1 and 1 THz. Frequencies in this range are strongly attenuated by water, 52 dB/mm at 200 GHz and 75 dB/mm at 500 GHz, which means that a particular good resolution may be obtained using these wavelengths. As a comparison, the paper itself is almost transparent to said electromagnetic radiation. The water attenuation is shown in
With further reference to
At the same time, emitters and detectors/receivers can be produced and manufactured at a reasonable price and with compact dimensions.
The THz transmitter 52 may be implemented by using a commercially available and cost effective oscillator, such as a voltage controlled oscillator or a dielectric resonance oscillator, and multiply the output frequency by a predetermined number of times. Using the suggested frequencies, focusing is convenient resulting in compact sensor pixels. For example, focusing of the radiation in the described setup may be done by a pair of Teflon lenses or mirrors, each having a diameter of 3 to 8 cm. The irradiated surface of the paper may thus be a couple of millimeters up to a decimeter in diameter.
For the proposed frequencies, i.e. 0.1 to 3 THz, the corresponding wavelength is larger than the surface irregularities as well as the paper thickness which makes it possible to transmit radiation through the paper web while scattering effects and vibrations have negligible effect on the accuracy of the measurement. When the detecting unit operates in transmission mode, measuring of the total water content in the paper web is thus possible.
According to an embodiment, a detecting unit for determining properties of a paper web moving in a printing press is thus provided comprising a transmitter, a receiver, and a controller.
An embodiment of a detecting unit 150 is shown in
Another embodiment of a detecting unit 250 is shown in
The embodiments shown in
In a yet further embodiment (not shown), the transmitter and the receiver may be arranged to operate in reflection mode. Such kind of setup, requiring that the transmitter and receiver are arranged on the same side of the paper web such that the receiver collects radiation being reflected by the paper web, is preferably utilized when the water content of optically thick paper webs is to be measured.
The detecting unit is able to perform remote and contact free measurements of the water content and the thickness of a moving paper web by emitting coherent THz radiation and focus it on one side of a paper web 12 running in a printing press. The emerging radiation on the other side of the paper web 12 may be focused on a receiver 54 which measures the magnitude and phase of the transmitted signal being attenuated and delayed in proportion to the water content and the thickness of the paper web 12. Hence, both the water content and the thickness of the paper web 12 may be extracted from the measurement and used as an input for the water base solution supply.
Since the system provides transmission through the paper web 12 the measured magnitude and phase is not affected by the angle at which the radiation is incident on the paper web 12. This results in an even more robust system where vibrations do not contaminate the measurement. A further advantage of a transmission based system is that the wavelength is larger than the surface irregularities, leading to no decrease of the inaccuracy of the measurement due to scattering effects.
A further embodiment of a detecting unit 350 is shown in
With reference to
The accuracy of the measurement is thus improved, since the signal transmitted through the paper web is compared to another signal detected by the reference receiver 60, which signal is fed by the same source 52 without being transmitted through the paper web. Drift in the source power and/or frequency may thus be calibrated out from the measurement. For such measurement, the receivers 54, 60 are connected to a controller (not shown) configured to convert the measured values to actual properties of the paper web. Hence, the control may have a memory in which reference values are stored corresponding to such actual properties.
The distance between the transmitter 52 and the receiver 54 is preferably in the order of 10 cm. Hence, effects such as atmospheric humidity will affect the signal only to a very little extent. There are a number of gas lines in the atmosphere where the absorptions peaks, as for example the 183 GHz water line. In the table below, examples of absorption vs. frequency and humidity are given. Even if the transmitter operates at 183 GHz (which is considered as a worst case) the attenuation will change from 15 dB/km to 48 dB/km for humidity rise from 30% to 100%. For a 15 cm path this will introduce 0.005 dB extra loss. On the other hand, if the transmitter is operating at 220 GHz the extra loss will only be 0.0005 dB.
In
For example, the beam splitting may be done by having a semi-transparent film, i.e. having optical coupling between the transmitter 52 and the reference sensor 60. In this case, the distance between the transmitter 52 and the reference sensor 60 should be held constant. However, the transmitter 52 may also be coupled mechanically to the reference sensor 60 by means of a waveguide directional coupler.
In a yet further embodiment, a number of detecting units 50 may be stationary positioned along the width of the paper web 12, thus reducing the need for a translation stage moving the components of the unit(s). In some embodiments, it may be desired to have a number of fixed detecting units disposed laterally across the paper web width. An increased number of units may thus provide increased lateral resolution, although the overall complexity of the system is increased. By using the described THz radiation, i.e. electromagnetic radiation in the range of 0.1 to 3 THz, each unit may be relatively small, e.g. having a width and depth of approximately 10 cm. The height of each unit is somewhat larger in order to provide enough space for the optical components. Hence it is possible to arrange up to ten detection units adjacent to each other for covering one meter of paper web width.
With reference to
The calculator 74 is further programmed to create a command which may be sent to either the ink supply 20, the water based solution supply 30, or both. Hence, the command will correspond to a request for increased or decreased supply of either ink or water based solution, e.g. in order to maintain minimum ink flow and improve the printing quality of the press.
The presented embodiments fill a gap in the existing methods for measurement of water content in thin paper layers. The existing systems for measuring water content are either too bulky, and thus not well-matched for installation on existing printing units, or not suited for thin paper layers.
Although the present invention has been described above with reference to specific embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the invention is limited only by the accompanying claims and, other embodiments than the specific above are equally possible within the scope of these appended claims.
In the claims, the term “comprises/comprising” does not exclude the presence of other elements or steps. Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly advantageously be combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. In addition, singular references do not exclude a plurality. The terms “a”, “an”, “first”, “second” etc do not preclude a plurality. Reference signs in the claims are provided merely as a clarifying example and shall not be construed as limiting the scope of the claims in any way. Further, any reference to “upper”, “lower”, “right”, or “left” are made only as relative determinations. It should thus be realized that such references do not limit the scope of the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1051127-7 | Oct 2010 | SE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/SE2011/051291 | 10/28/2011 | WO | 00 | 5/16/2013 |
