WO2008004929A1

WO2008004929A1 - Print unit, color printer and method for improving print quality in a color printer

Info

Publication number: WO2008004929A1
Application number: PCT/SE2007/000150
Authority: WO
Inventors: Bengt Bern
Original assignee: Bengt Bern
Priority date: 2006-02-17
Filing date: 2007-02-19
Publication date: 2008-01-10

Abstract

A method for improving print quality in a color printer (1) having a printer body (2) with at least one print unit (3a-d) attached thereto, the method comprising the steps of providing (40), at a printer body reference position, a sensor (7) capable of providing an output indicative of a position of a stimulus facing the sensor, positioning (41) the print unit (3a-d) at a print unit reference position adjacent to the printer body reference position such that a target structure (1 la-d) provided on the print unit (3a-d) at least partly faces the sensor (7), the target structure (lla-d) being adapted to provide a stimulus to the sensor (7), acquiring, by means of the sensor (7), a signal indicative of a position of the target structure (lla-d) in relation to the printer body reference position, determining, based on the acquired signal, a misalignment of the print unit (3a-d), and compensating (46) for the determined misalignment, thereby im rovin rint ualit in the color rinter 1).

Description

PRINT UNIT, COLOR PRINTER AND METHOD FOR IMPROVING PRINT QUALITY IN A COLOR PRINTER

Technical Field of the Invention

The present invention relates to a print unit for use in a color printer, a color printer and a method for improving print quality in a color printer.

Technical Background

Today, color printers, especially small and cost-efficient ink-jet printers, can be found in almost every office and home in the industrialized world. These color printers are increasingly being used to produce color documents ranging from business graphics to holiday snap shots. Regarding color printers, one of the most important factors for customer satisfaction is print quality. Consequently, development efforts are continuously made in order to improve print quality.

Color printers, in general, have replaceable print units or print cartridges which need to be replaced by the user at intervals depending on the frequency of use of the printer. Such a replaceable print unit typically consists of a container holding ink or another marking substance and a print head. For a color printer, output from several of these print units typically need to interact in order to create a desired color on a printed document. In a given area, an output from a second print unit is printed on top of an output from a first print unit. Where these outputs overlap, the desired color is created. If the print units are perfectly aligned with each other, a flawless printed structure of the desired color can be created. If, as is practically always the case, there is a misalignment between the print units, the outputs from the print units will only partly overlap, which results in that the structure having the desired color is surrounded by ghost-structures having the colors of the first and second print unit outputs respectively. In order to minimize this misalignment, various high-precision mechanical structures for holding the print units in place have been developed. This, in combination with a factory alignment calibration may result in an alignment which is initially satisfactory for most uses. However, when the ink or other marking substance in a print unit runs out, the user will need to replace that print unit with a new unit. This will result in newly introduced misalignment, since no two print units have the exact same dimensions and since the insertion itself may induce additional misalignment. The traditional way of reducing misalignment is to let the user print special calibration patterns and provide the printer controller with feedback indicating a suitable misalignment compensation. This is cumbersome for the user, who wants to start printing right away. Furthermore, a satisfactory result is not guaranteed, since the average user typically is not used to assessing print quality and providing the correct feedback.

According to another method known in the art, disclosed in for example US 6 076 915, a printed pattern is evaluated by a sensor in the printer and misalignment compensation is carried out based on the output from the sensor. Once again, a special pattern, requiring user interaction in the calibration, needs to be printed. This leads to unwanted waste of paper and precious ink and is tedious to the user.

Hence, there is a need for an improved color printer for alleviating the above drawbacks of the prior art.

Objects of the Invention

In view of the above-mentioned and other drawbacks of the prior art, a general object of the present invention is to provide an improved print unit, color printer and method for improving print quality in a color printer.

Summary of the Invention

According to a first aspect of the present invention, these and other objects are achieved by a print unit for use in a color printer having a printer body which is adapted to receive the print unit, the print unit comprising a container for containing a marking substance, a print head configured to transfer the marking substance from the container to a print medium, and a target structure adapted to, following positioning of the print unit at a print unit reference position in the color printer, provide a stimulus to a sensor which is arranged on the printer body.

The print unit may be a mono-color print unit or a multi-color print unit having several containers containing marking substances of different colors, and one print head for each color in the print unit. Furthermore, the print unit is typically replaceable.

The color printer may function according to any marking technology. However, the print unit according to the present invention is especially useful in various forms of direct printing, such as ink-jet printing.

The "marking substance" may be ink based on any solvent, including water, wax, organic and inorganic solvents, or it may be a dry substance, such as toner. The "print medium" may be any kind of medium used for printing onto, such as paper, plastic film, textiles etc. The print medium may, furthermore, be an intermediate member, such as a belt or a drum, onto which the marking substance is transferred by the print head. In this case, the marking substance is subsequently transferred to the final medium, such as paper, plastic film, textiles, etc.

In the context of the present application, the "print head" should be understood as a structure which is controllable to selectively transfer an amount of the marking substance to a given location on the print medium. For direct printing technologies, the print head typically includes a plurality of nozzles which are configured to restrict or allow passage of a controlled amount of marking material in order to produce a "dot" on the print medium.

Through the provision of the target structure adapted to, following positioning of the print unit at a print unit reference position in the color printer, provide a stimulus to a sensor which is arranged on the printer body, an accurate determination of a misalignment of the print unit following insertion of the print unit in the color printer is enabled. This misalignment can then be automatically compensated for, without any user intervention. Hereby, improved print quality may be maintained without any tedious manual calibration operation. The misalignment may be determined and compensated for following an exchange of print unit, or print cartridge, and/or at predetermined intervals thereinbetween. Of course, a misalignment compensation may also be prompted by the user.

The target structure may advantageously be formed by a portion of the print head.

As stated above, the human eye is very sensitive to color variations at edges in particular. Therefore, also a very small misalignment must be detectable through a sensing of the target structure. Since the position of a printed dot is determined by the position of the print head, the positioning tolerance of the target structure in relation to the print head should preferably be kept as small as possible. By forming the target structure by a portion of the print head, including extending or modifying a conventional print head to include a target structure, the positioning tolerance of the target structure with respect to the print head will be determined by the manufacturing tolerances of the print head, which are typically in the range of 0.1 μm to 5 μm. According to one embodiment of the print unit according to the present invention, the target structure may be conductive. By forming the target structure from a conductive material, a stimulus can be provided by the target structure to a variety of sensors employing different sensing mechanisms. Such sensor types, for example, include optical sensors, capacitive sensors and conductive sensors.

This conductive target structure may, furthermore, preferably be configured to be connected to a voltage source when the print unit is positioned at the print unit reference position in the color printer. Hereby, placing the target structure at a voltage (AC or DC) determined by the sensor control circuitry is enabled and, as a consequence thereof, additional types of suitable sensors may be employed. The sensor control circuitry may be provided within or outside the sensor itself and the voltage may be applied to the target structure directly by the sensor, or indirectly by means of an intermediate driving circuit. In order to increase the signal-to-noise ratio of the sensor reading, and thereby achieve an improved accuracy in the position determination, an intermediate driving circuit capable of delivering a higher voltage and/or current than the sensor itself may advantageously be employed.

Furthermore, the print unit according to the present invention may advantageously be comprised in a color printer further comprising a printer body having print medium transportation means, a sensor capable of providing an output indicative of a position of a stimulus facing the sensor, the sensor being provided at a printer body reference position, and control circuitry for controlling operation of the printer. The above-mentioned output from the sensor may indicate a position of the stimulus in one dimension, a two-dimensional in-plane position, or a three- dimensional position in space.

The sensor may preferably comprise a plurality of pixels. Although certain sensors including one sensor element only, such as the PSD- detector manufactured by the company SiTek AB (SE), are capable of outputting a signal indicative of a position of a stimulus facing the sensor, the provision of a plurality of pixels in the sensor may simplify position readout and enables use of several sensor technologies which are already utilized in other technology fields, such as image sensors in digital cameras or fingerprint sensors. The sensor may, furthermore, be capable of, for each pixel, providing an output of a plurality of values within an output range.

Hereby, a position of a stimulus may be determined with far greater accuracy than the pixel resolution of the sensor. With a typical response to a target stimulus being different "gray scale" values from a cluster of pixels, the location of the target stimulus may determined with high accuracy through a simple center-of-mass calculation, the gray scale value here representing mass.

The output from the sensor may preferably be digital. According to one embodiment, furthermore, the sensor may be a capacitive sensor.

Through the provision of a capacitive sensor, no light is necessary to obtain a signal indicative of the position of the stimulus. Furthermore, the target structure can be designed without regard to the optical contrast, the only requirement being that there is a sufficient contrast in electric conductivity between the target structure and its surroundings.

According to a second aspect of the present invention, the above and other objectives are achieved by a color printer comprising a printer body having a print medium transportation mechanism, at least one print unit attached to the printer body, a sensor capable of providing an output indicative of a position of a stimulus facing the sensor, the sensor being provided at a printer body reference position, and control circuitry for controlling operation of the printer, wherein the print unit has a target structure configured to provide a stimulus to the sensor, and wherein the control circuitry is adapted to position the print unit at a print unit reference position adjacent to the printer body reference position such that the target structure at least partly faces the sensor, acquire, by means of the sensor, a signal indicative of a position of the target structure in relation to the printer body reference position, determine, based on the acquired signal, a misalignment of the print unit, and compensate for the determined misalignment, thereby improving print quality of the color printer.

Through this second aspect of the present invention, advantageous effects are obtained which are largely analogous to those described above in connection with the first aspect of the invention.

According to a third aspect of the present invention, the above and other objectives are achieved by a method for improving print quality in a color printer having a printer body with at least one print unit attached thereto, the method comprising the steps of providing, at a printer body reference position, a sensor capable of providing an output indicative of a position of a stimulus facing the sensor, positioning the print unit at a print unit reference position adjacent to the printer body reference position such that a target structure provided on the print unit at least partly faces the sensor, the target structure being adapted to provide a stimulus to the sensor, acquiring, by means of the sensor, a signal indicative of a position of the target structure in relation to the printer body reference position, determining, based on the acquired signal, a misalignment of the print unit, and compensating for the determined misalignment, thereby improving print quality in the color printer. Through this third aspect of the present invention, advantageous effects are obtained which are largely analogous to those described above in connection with the first aspect of the invention.

The step of acquiring may comprise the steps of activating the target structure such that a stimulus is provided to the sensor, and providing as an output from the sensor, in response to the stimulus, a signal indicative of a position of the target structure in relation to the printer body reference position.

By "activating" the target structure should here be understood influencing the target structure in such a way that the sensor becomes able to sense it. Depending on sensor technology, the target structure may be activated in a number of ways including, for example, illuminating it, directly or indirectly, heating it, and applying a voltage to it. Hence, the stimulus sensed by the sensor may be provided in the form of light generated or reflected by at least part of the target structure, heat generated or reflected by at least part of the target structure, or voltage or current emitted by at least part of the target structure.

By activating the target structure a stronger stimulus can be provided to the sensor than without the activation, which leads to a larger signal-to-noise ratio and consequently a better measurement value.

The target structure may advantageously be activated by means of an activation signal which is triggered by a sensor output.

Hereby, the activation of the target structure can be synchronized with the sensor operation and a better output signal from the sensor achieved. The sensor output referred to above may preferably be a specific trigger output from the sensor.

This activation signal may be a voltage pulse train. By providing the activation signal as a voltage pulse train which is triggered by a sensor output, the use of certain reflective type capacitive sensors having very good sensing performance is enabled.

Brief Description of the Drawings These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing a currently preferred embodiment of the invention.

Fig 1 is a schematic plane view of a portion of a color printer according to a preferred embodiment of the present invention. Figs 2a-b are schematic plane views illustrating misalignment in different directions of print units according to a preferred embodiment of the present invention. Fig 3 a schematically illustrates the sensor comprised in the color printer of fig l.

Fig 3b schematically illustrates position determination according to a preferred embodiment of the method of the present invention, by means of the sensor in fig 3a

Fig 4 is a flow chart which schematically illustrates a preferred embodiment of the method according to the present invention.

Detailed Description of a Preferred Embodiment of the Invention In the following description, a preferred embodiment of the present invention is described with reference to an ink-jet color printer having a capacitive sensor mounted on the printer body in order to sense a position of a target structure on a print unit attached to the printer body when the print unit is positioned at a print unit reference position. The ink-jet printer described herein has 4 mono-color print units. It should be understood that this by no means limits the scope of the present invention, which is equally applicable to other color printers, such as so-called solid ink printers, laser printers and other types of color printers for direct and indirect printing. The invention is, obviously, also applicable to color printers having different numbers of print units than 4. For example, the color printer may have 6 print units each intended for printing a particular different color. Additionally, as mentioned above, a single print unit may consist of several print heads and several corresponding containers each containing a different color marking substance, such as ink.

In the present description, similar features are indicated with the same numerals throughout. Furthermore, parts that are well known in the art and not of direct relevance to the present invention have been left out in order not to unnecessarily obscure the present invention.

Fig 1 schematically shows a portion of a color printer according to a preferred embodiment of the present invention. hi fig 1, an ink-jet color printer 1 is partly shown comprising a printer body 2 and four replaceable print units 3a-d. The printer body comprises a carriage 4 to which the print units 3a-d are attached, the carriage 4 being arranged to move along an axis 12 extending in the x-direction, and an optical positioning strip 5 for determining the position in the x-direction of the carriage 4, a mechanism (not shown) for transporting a print medium 6 in the y-direction past the print units 3a-d, and a sensor 7, for sensing a location of a target structure provided on each of the print units, which is mounted in a printer body 2 reference position in the color printer 1. Two of the print units 3a,d are shown to eject ink droplets 8a,d through nozzles (not shown, see fig 2b) in print heads 9a-d comprised in the print units towards the print medium 6.

Typically, the print medium 6 is held stationary while the carriage 4 moves the print units 3a-d across the surface of the print medium 6 in the x-direction. At locations determined by the image to be printed, the print units 3a-d eject ink droplets through selected print head nozzles to thereby form a strip of the image to be printed. After the carriage 4 has completed one (or several) such passages across the surface of the print medium 6, the print medium is forwarded one step in the y-direction.

However, unless some compensation of misalignment of the print units 3a-d is performed, the droplets will end up at unintended locations, leading to print defects, such as color mismatch and shadow images.

In figs 2a-b misalignment in the three dimensions (x, y, and z) is illustrated. With reference to figs 2a-b, furthermore, a preferred embodiment of a print unit according to the present invention will be described. In fig 2a, the print unit 3c is shown to be located at the print unit reference position, with its print head 9c facing the sensor 7. The distance from this print head 9c to the sensor surface is denoted Z₃. For the other print units 3a,b,d, this distance is denoted Z₁, Z₂, and Z₄, respectively. As is illustrated (with exaggerated proportions) in fig 2a, these z-direction distances are typically different from each other. As a consequence thereof, the ink droplets ejected from the respective print heads 9a-d need different amounts of time to reach the print medium surface, that is, they have different times-of -flight. Since the print units 3a-d are not stationary during printing, but rather moving across the print medium 6 with the carriage 4, ink droplets which are intended for the same location on the print medium but are ejected through print heads having different distances Z_1-4 to the print medium 6 will end up on different locations on the print medium surface.

In fig 2b, misalignment in the x- and y-directions is schematically illustrated. Here, the print units 3a-d are viewed as seen from the print medium 6 and the sensor 7, and nozzles 10 (only one of which here being indicated by a reference numeral for the sake of clarity of drawing) as well as target structures 1 la-d, here in the form of small metallic, essentially pyramid-shaped structures are shown. The target structures lla-d are connected to respective connection pads 12a-d for enabling connection of the target structure to a voltage source (not shown) which is triggered by the sensor 7 to supply a pulsed voltage to the target structure of the print unit currently being in the print unit reference position, facing the sensor 7. Note that the target structures lla-d are here provided on the print heads 9a-d. Preferably, the target structures lla-d are manufactured at the same time as the nozzles and other structures of the print heads 9a-d are formed, to thereby achieve very small positioning tolerances between target structure and nozzle (typically in the sub-micron range).

As shown in fig 2b, print units 3a-d are, in addition to the above described z- direction misalignment, typically misaligned in the x- and y-directions as well. Fig 3 a schematically illustrates the sensor comprised in the above described color printer according to a preferred embodiment of the present invention.

In fig 3 a, a position determination sensor 7 is schematically shown including a plurality of pixels, or sensor elements 20. The sensor elements 20 are arranged in a matrix with a pitch P, which is typically in the range of 50 μm, and a gray scale value for each sensor element 20 can be read from the sensor. These values can be acquired for discrete positions denoted by (X₁, V₁) - (x_n, y_n) in fig 3 a. The useful gray scale range is typically determined by the signal-to-noise ratio of the sensor. As will be described with reference to fig 3b, this ability to output gray scale values from a plurality of pixels, or sensor elements, enables position determination with a far greater accuracy than the pitch P. Furthermore, the ability to output sensor element gray scale values enables determination not only of position in the x-y-plane, but additionally of position in the z-direction, or in other words, the distance between sensor 7 and target structure lla-d.

Fig 3b schematically illustrates position determination according to a preferred embodiment of the method of the present invention, by means of the sensor in fig 3 a

In fig 3b, the output values of sensor elements 20 of the sensor 7 as a response to stimulus by a target structure is schematically shown. The position of the target structure lla-d is schematically illustrated by the hatched area 30 in fig 3b, and the output gray scale values of selected sensor elements are indicated in diagrams along the x- and y-axis, respectively, of the sensor 7. In these diagrams, it can be seen that the output values from the sensor elements are dependent on the 3D-shape of the target structure providing the sensor stimulus. In the case illustrated in fig 3b, the target structure is essentially pyramid shaped. In order to determine the position of the apex of the pyramid shaped target structure, a center-of-mass calculation is performed on the sensor output, yielding the apex position with an accuracy being far greater than would have been possible without the gray scale ability of the sensor elements. Fig 4 is a flow chart which schematically illustrates a preferred embodiment of the method according to the present invention. Referring to fig 4, a first step 41 of providing, at a printer body reference position, a sensor capable of providing an output indicative of a position of a stimulus facing the sensor is followed by a step 42 of positioning the print unit at a print unit reference position adjacent to the printer body reference position such that a target structure provided on the print unit at least partly faces the sensor, the target structure being adapted to provide a stimulus to the sensor. In a subsequent step 43, the target structure is activated such that the stimulus is provided to the sensor, and in a following step 44, a signal indicative of a position of the target structure in relation to the printer body reference position is providing as an output from the sensor, in response to the stimulus. In the subsequent step 45, the misalignment of the print unit is determined by performing a centre-of-mass calculation on the pixel values output from the sensor, and finally, in step 46, compensating for the determined misalignment.

The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, the print medium motion need not be stepped, but could be continuous. Furthermore, the print units need not be carried on a carriage, but could be provided in the form of so-called page wide arrays. In this case, several sensors, one for each print unit may be provided in the printer body.

Claims

1. A print unit (3a-d) for use in a color printer (1) having a printer body (2) which is adapted to receive said print unit (3a-d), said print unit (3a-d) comprising: a container for containing a marking substance; a print head (9a-d) configured to transfer said marking substance from said container to a print medium (6); and a target structure (lla-d) adapted to, following positioning of said print unit (3a-d) at a print unit reference position in said color printer (1), provide a stimulus to a sensor (7) which is arranged on the printer body (2).

2. A print unit (3a-d) according to claim 1, wherein said target structure (lla-d) is formed by a portion of said print head (3a-d).

3. A print unit (3a-d) according to claim 1 or 2, wherein said target structure

(lla-d) is conductive.

4. A print unit (3a-d) according to claim 3, wherein said target structure (lla-d) is configured to be connected to a voltage source when the print unit is positioned at said print unit reference position.

5. A color printer (1) comprising a printer body (2) having print medium transportation means; a sensor (7) capable of providing an output indicative of a position of a stimulus facing said sensor, said sensor (7) being provided at a printer body reference position; control circuitry for controlling operation of said printer; and a print unit (3a-d) according to claim 1.

6. A color printer (1) according to claim 5, wherein said sensor (7) comprises a plurality of pixels (20).

7. A color printer (1) according to claim 6 wherein said sensor (7) is capable of, for each pixel (20), providing an output of a plurality of values within an output range.

8. A color printer (1) according to claim 7, wherein said output is digital.

9. A color printer (1) according to any one of claim 5 to 8, wherein said sensor (7) is a capacitive sensor.

10. A color printer (1) comprising a printer body (2) having a print medium transportation mechanism; at least one print unit (3a-d) attached to the printer body (2); a sensor (7) capable of providing an output indicative of a position of a stimulus facing said sensor, said sensor (7) being provided at a printer body reference position; and control circuitry for controlling operation of said printer (1), wherein said print unit (3a-d) has a target structure (1 la-d) configured to provide a stimulus to said sensor (7), and wherein said control circuitry is adapted to: position said print unit (3a-d) at a print unit reference position adjacent to said printer body reference position such that said target structure (lla-d) at least partly faces said sensor (7); acquire, by means of said sensor (7), a signal indicative of a position of said target structure (lla-d) in relation to said printer body reference position; determine, based on said acquired signal, a misalignment of said print unit

(3a-d); and compensate for said determined misalignment, thereby improving print quality of the color printer (1).

11. A color printer (1) according to claim 10, wherein said sensor (7) comprises a plurality of pixels (20).

12. A color printer (1) according to claim 11 wherein said sensor (7) is capable of, for each pixel (20), providing an output of a plurality of values within an output range.

13. A color printer (1) according to claim 12, wherein said output is digital.

14. A color printer (1) according to any one of claim 10 to 13, wherein said sensor (7) is a capacitive sensor.

15. A color printer (1) according to any one of claim 10 to 13, wherein said sensor (7) is an optical sensor.

16. A method for improving print quality in a color printer (1) having a printer body (2) with at least one print unit (3a-d) attached thereto, the method comprising the steps of: providing (40), at a printer body reference position, a sensor (7) capable of providing an output indicative of a position of a stimulus facing said sensor; positioning (41) said print unit (3a-d) at a print unit reference position adjacent to said printer body reference position such that a target structure (1 la-d) provided on said print unit (3a-d) at least partly faces said sensor (7), said target structure (lla-d) being adapted to provide a stimulus to said sensor (7); acquiring, by means of said sensor (7), a signal indicative of a position of said target structure (lla-d) in relation to said printer body reference position; determining, based on said acquired signal, a misalignment of said print unit

(3a-d); and compensating (46) for said determined misalignment, thereby improving print quality in the color printer (1).

17. A method according to claim 16, wherein said step of acquiring comprises the steps of: activating (42) said target structure (lla-d) such that a stimulus is provided to said sensor (7); and providing (43) as an output from said sensor (7), in response to said stimulus, a signal indicative of a position of said target structure (lla-d) in relation to said printer body reference position.

18. A method according to claim 17, wherein said target structure is activated by means of an activation signal which is triggered by a sensor output.

19. A method according to claim 18, wherein said activation signal is a voltage pulse train.

20. A method according to any one of claim 16 to 19, wherein said acquired signal represents a plurality of pixel values within an output range.

21. A method according to claim 20, wherein said misalignment is determined through a center of mass calculation on said pixel values.