EP1410113B1 - Method and device for controlling a print process with high colour density - Google Patents

Description

The invention relates to a method for controlling the printing process in a printer or copier, wherein a character generator on a subcarrier generates a latent image and a latent toner mark. Furthermore, the invention relates to a device for carrying out the method.

In electrographically operating printers or copiers toner material according to the image structure on a support, such as paper, applied and fixed to generate the printed image. Examples of such printing processes are the electrophotographic processes and the magnetoelectric process. In this case, a latent printed image, for example on a photoconductor, is first produced on an intermediate carrier, this latent printed image is inked and subsequently transferred to the carrier. To achieve a high print quality, the coloring of the printed image must be kept within narrow predetermined limits. Such a print image may include solid areas, halftone halftone areas, lines, characters, and other relatively complex picture elements. For precise control of the printing process, the degree of inking for the printed image is indirectly determined by means of a toner mark, and the printing process is controlled or regulated as a function of the printing result with this toner mark. Therefore, a latent toner mark is produced on the intermediate carrier in addition to the latent image which is important for the customer. Such a toner mark is relatively small compared to the area of the printed image.

At high ink densities or degrees of coloration, a high concentration of the dye is required, ie associated toner mark is relatively dark. When scanning such a dark toner mark by an optical reflection sensor whose sensitivity is reduced, ie the characteristic of the color density on the toner mark on the toner concentration is relatively flat. As a result, setting the exact toner concentration is difficult at high color densities. This effect is further enhanced by the fact that small areas are colored darker than large areas, ie the relatively small toner marks are colored darker than the larger area elements of the printed image.

The DE-A-39 38 354 describes a method for controlling the printing process in an image forming device. In creating a latent toner mark, the energy is lowered from the energy for creating a latent print image. A reflex sensor determines the color density of the inked toner brand. Depending on the signal of this reflex sensor, either the lamp voltage of the character generator, the bias for a developing process or the grid voltage of the main charger is set. The influence of the toner concentration in the development station is not described.

The document WO 99/36834 The same applicant describes the scanning of toner marks using a reflex sensor. Depending on the measurement result of the reflex sensor, the toner concentration in the developer station is set.

The document DE-A-199 00 164 The same Applicant describes measuring the toner density of a toner mark. Depending on the measured value, the toner concentration is set.

The document US-A-4,962,407 describes an electrophotographic copying device, wherein measuring signals of two reference marks of different color density are used to determine the toner concentration. From the difference signal for both toner brands is closed to the toner concentration and in case of deviation from a target value, the toner concentration is readjusted in the developer station.

The documents DE-A-41 26 446 and DE-A-41 26 457 describe a developing unit for developing colored toner images, in which image patterns are scanned, which were generated by means of two different potentials. Based on the measured signals can then be closed to the toner concentration and tracked.

It is the object of the invention to provide a method and a device for controlling the printing process at which even at high color density, the toner concentration can be adjusted with high accuracy.

This object is achieved for a method by the features of claim 1.

According to the invention, the energy per unit area for producing the latent toner mark is lowered relative to the energy per unit area for producing the latent print image with an otherwise identical image structure. In electrophotographic printing processes e.g. this energy is in the form of radiant energy. This means that in electrophotographic processes, this radiant energy for generating the latent toner mark, which is embodied, for example, as a solid surface, is lowered relative to the radiant energy of an equally large solid surface of the latent print image. This has the consequence that the color density of the solid surface of the toner mark is lower than the color density in the corresponding printed image. Accordingly, the signal of the reflection sensor which scans the toner mark is larger; In addition, the operating point of the reflection sensor is on a characteristic of the color density on the toner concentration in a steeper area. Accordingly, a change in the color density on the toner mark causes a correspondingly larger change in the signal of the reflection sensor, whereby the toner concentration can be tracked with a higher accuracy in order to achieve the desired color density on the printed printed image.

According to a further aspect of the invention, a device having the features of claim 15 is provided. The technical effects achievable with this device have already been described in connection with the method.

Fig. 1

schematisch den Aufbau wichtiger Funktionselemente in einem elektrofotografischen Drucker,

Fig. 2

ein Blockschaltbild zur Regelung des Einfärbegrades bei einer Absenkung der Lichtleistung für die Tonermarke,

Fig. 3

die Einfärberegelung bei normaler und geringer Farbdichte, und

Fig. 4

ein Diagramm, welches die Einfärbung auf dem Papier als Druckbildträger abhängig von der Tonerkonzentration zeigt.

An embodiment of the invention will be explained below with reference to the drawing. It shows

Fig. 1

schematically the construction of important functional elements in an electrophotographic printer,

Fig. 2

a block diagram for controlling the inking degree with a decrease in the light output for the toner brand,

Fig. 3

Dyeing at normal and low color density, and

Fig. 4

a diagram showing the coloring on the paper as a print image carrier depending on the toner concentration.

Fig. 1 schematically shows the structure of important components of a device for controlling an electrophotographic printing process. A character generator 10 is in the form of an LED comb. It contains a multiplicity of LEDs whose light is emitted onto the surface of a photoconductor drum 12 in order to generate there latent image structures in the form of a charge image. In region a, the character generator 10 emits light with the light output L1. This area a is an area in which print images are generated for the customer. In an adjacent track having the area b, the LEDs of the character generator 10 emit light for generating the structure of a toner mark 14. However, this light has a lower light output L2 than the light output L1 in the region a for a same image structure, in the present case a full-tone image structure. The toner-colored mark 14 (the process step of developing with toner was described in U.S.P. Fig. 1 omitted for clarity) is scanned on the photoconductor 12 by a reflection sensor 16.

Its electrical signal is used to control or regulate the printing process, as will be explained in more detail below.

In a later process step, the toner-colored print image 18 is transferred to a paper web 20. The toner mark 14 is not included in the printed image 18 for the customer.

Schritt a:

Es wird das vom Kunden gewünschte Druckbild mit einer am Drucker einstellbaren Farbdichte gedruckt.

Schritt b:

Mit Hilfe eines Densitometers wird die Farbdichte im Druckbild des Kunden gemessen. Diese Farbdichte ist ein logarithmisches Maß des Verhältnisses aus Reflexionsgrad des Hintergrunds zum Reflexionsgrad der Bildstruktur.

Schritt c:

Falls die gewünschte Farbdichte für das Druckbild des Kunden nicht erreicht wird, so wird die Tonerkonzentration manuell oder automatisch geändert, bis die tatsächliche Farbdichte mit der gewünschten Farbdichte übereinstimmt.

Schritt d:

Die Lichtleistung L2 für die Tonermarke ist bei hohen Farbdichten, d.h. bei dunklen Bildstrukturen im Druckbild gegenüber der Lichtleistung L1 abgesenkt.

Schritt e:

Die Grundeinstellung des Druckprozesses ist abgeschlossen. Beim anschließenden Drucken einer Vielzahl von Seiten wird die Tonerkonzentration so gesteuert oder geregelt, daß die mit der verringerten Lichtleistung L2 auf der Tonermarke vorhandene Farbdichte konstant bleibt.

In order to print the print image for the customer with the desired color density or the desired degree of coloration, a basic setting and further steps are carried out:

Step a:

The print image desired by the customer is printed with a printer-adjustable color density.

Step b:

With the help of a densitometer, the color density in the printed image of the customer is measured. This color density is a logarithmic measure of the ratio of the reflectance of the background to the reflectance of the image structure.

Step c:

If the desired color density for the printed image of the customer is not reached, the toner concentration is changed manually or automatically until the actual color density matches the desired color density.

Step d:

The light output L2 for the toner mark is lowered at high color densities, ie at dark image structures in the printed image compared to the light output L1.

Steps:

The basic setting of the printing process is complete. In the subsequent printing of a plurality of pages, the toner concentration is controlled so that the color density existing on the toner mark with the reduced light power L2 remains constant.

Fig. 2 shows in a block diagram schematically the control of the coloring at high color density of the printed image for the customer. The blocks 22 and 24 describe the generation of the latent print image 18 and the latent toner mark 14, respectively. The light energy L2 of the LEDs for generating the latent toner mark is lowered by a constant factor compared to the light energy L1 for generating the latent print image 18. For example, this can be done by lowering the current that is supplied to the corresponding LEDs of the LED comb. The colored toner mark is scanned by means of the reflection sensor 16 and the measured color density is imaged in a signal 26. This signal 26 represents an actual value of the printing process. This actual value 26 is compared with a desired value 28. This setpoint was previously set in a trial operation in the above-described metering method according to step c. A setpoint-actual value comparator 30 determines the actual value deviation 32. Due to this deviation 32, the toner concentration is changed, for example, increased or decreased. For adjusting the toner concentration, it is possible, for example, to act on a conveyor coil which effects the toner transport to the developer station. With the thus-adjusted toner concentration, the print image 18 and the toner mark 14 are generated. The print image 18 is then output.

Fig. 3 shows the example with normal and low color density; the same parts are the same. In the process of applying the latent print image 18 and the latent toner mark 14, the lowering of the energy for generating the toner mark 14 is canceled, that is, the light power L2 is equal to the light power L1.

Fig. 4 shows characteristics of the coloring as a function of the toner concentration in the developer station. Plotted is the color density on the printed paper on the toner concentration in percent. If the same light energy per unit area is used for the latent toner starch 14 and the latent print image 18 with respect to otherwise identical image structures, the characteristic 36 results. It should be noted that the toner mark has a relatively small area compared to the printed image 18. Accordingly, the coloring is darker and the color density is increased. The characteristic 38 results for a relatively large area of the printed image. The characteristic 40 shows the relationship of toner concentration and color density of the printed image on the paper when the latent mark is produced with a lowered light power L2.

At the operating point with a toner concentration of 5% can be seen from the characteristic curve 36 that a relatively large color density is achieved on the paper. However, this characteristic curve 36 is very flat, so that with a change in the toner concentration, the color density on the paper barely changes, ie there is a very small control stroke for the control process. By lowering the light output, a large control stroke can be achieved with the same toner concentration, as the characteristic curve 40 shows. The slope of the characteristic or the value of the differential of the color density on the paper to the toner concentration is increased in the characteristic curve 40 at this operating point. That way, too Control accuracy improved to control the printing process. The reduction of the light output L2 for the production of the latent toner mark 14 can be greater, the greater the desired color density of the printed printed image on the paper. The reduction of the light power L2 for the toner mark 14 can also be such that the ratio or the differential quotient of toner concentration and color density at the operating point of the reflection sensor 16 exceeds a predetermined value.

As based on the Fig. 4 can be seen, the characteristics 36, 38, 40 are not linear. In the range of low toner concentrations, a sufficient slope of the characteristic curve 36 is present. This means that a reduction in the light output L2 for the toner brand 14 is no longer absolutely necessary and can be reversed. Therefore, in one embodiment of the invention, starting from a color density value ≦ a predetermined threshold value, the lowering of the light output is reversed.

Numerous variants of the described invention are possible. For example, the intermediate carrier on which the toner mark 14 is applied may also be a photoconductor belt. The invention can also be used for electromagnetic. Printing method can be applied. It is also possible to transfer the toner mark 14 to a transfer belt and to scan this toner mark 14 on the transfer belt. Furthermore, it is possible to adjust the toner concentration on the basis of predetermined characteristics representing the relationship between color density on the substrate and the toner concentration with lowered energy for the generation of the latent mark.

LIST OF REFERENCE NUMBERS

10

character generator

12

Fotoleitertrommer

14

toner mark

16

reflective sensor

18

print image

20

paper web

22

function block

24

function block

26

signal

28

setpoint

30

Setpoint actual value comparison

32

Actual value deviation

36

curve

38

curve

40

curve

L1

light output

a

Area

b

Area

L2

light output

Claims (27)

1. Method to control the print process in a printer or copier,
   in which a character generator (10) generates a latent print image (18) and a latent toner marking (14) on an intermediate carrier (12),
   the exposure energy per area unit for the generation of the latent toner marking (14) is decreased in comparison to the energy per area unit for the generation of the latent print image (18), given an otherwise identical image structure,
   the latent print image (18) and the latent toner marking (14) are inked with toner in a developer station,
   an optical reflection sensor (16) determines the colour density of the inked toner marking (14), whereby via the stated decrease of the energy the slope of the characteristic line (40) of the colour density (40) of the toner marking (14) over the toner concentration increases,
   and in that the toner concentration is adjusted in the developer station dependent on the signal of the reflection sensor (16).
2. Method according to claim 1, in that the toner concentration is adjusted in the developer station using a predetermined function of the colour density over the toner concentration.
3. Method according to claim 1 or 2, in that the toner concentration is adjusted in a calibration event such that the desired colour density is present on the printed print image (18), and in that the signal of the optical reflection sensor (16) is determined for the toner marking (14).
4. Method according to claim 3, in that the toner concentration is adjusted in a regulation process such that the signal determined by the reflection sensor (16) has a largely constant value for the toner marking (14).
5. Method according to any of the preceding claims, in that the decrease of the energy for the generation of the latent marking (14) ensues ever greater the higher the desired colour density of the printed print image (18).
6. Method according to any of the preceding claims, in that the decrease of the energy for the toner marking ensues such that the ratio or the differential quotient from toner concentration and colour density in the operating point of the reflection sensor (16) exceeds a predetermined value.
7. Method according to any of the preceding claims, in that, for a colour density value greater than or equal to a predetermined threshold, the decrease of the energy for the toner marking is cancelled.
8. Method according to any of the preceding claims, in that a densitometer is used to establish the colour density of the printed print image.
9. Method according to any of the preceding claims, in that an electrophotographic print process is used as a print process for the printer or copier.
10. Method according to claim 9, in that an LED character generator whose LEDs can be individually controlled is used as a character generator (10).
11. Method according to any of the preceding claims, in that a photoconductor drum or a photoconductor band is used as an intermediate carrier (12).
12. Method according to any of the preceding claims, in that the toner marking (14) is arranged outside of the region of the print image (18).
13. Method according to claim 11 or 12, in that the toner marking (14) inked on the intermediate carrier (12) is scanned by the optical reflection sensor (16).
14. Method according to any of the preceding claims, in that the toner marking is scanned on a transfer band.
15. Device to control the print process in a printer or copier,
    in which a character generator (10) generates a latent print image (18) and a latent toner marking (14) on an intermediate carrier (12),
    means are provided that decrease the exposure energy per area unit for the generation of the latent toner marking (14) in comparison to the energy per area unit for the generation of the latent print image (18), given an otherwise identical image structure,
    the latent print image (18) and the latent toner marking (14) are inked with toner in a developer station, an optical reflection sensor (16) determines the colour density of the inked toner marking (14), whereby via the stated decrease of the energy the slope of the characteristic line (40) of the colour density (40) of the toner marking (14) over the toner concentration increases,
    and in that the toner concentration is adjusted in the developer station dependent on the signal of the reflection sensor (16).
16. Device according to claim 15, in that the toner concentration is adjusted in the developer station using a predetermined function of the colour density over the toner concentration.
17. Device according to claim 15 or 16, in that the toner concentration is adjusted in a calibration event such that the desired colour density is present on the printed print image (18), and in that the signal of the optical reflection sensor (16) is determined for the toner marking (14).
18. Device according to claim 17, in that the toner concentration is adjusted in a regulation process such that the signal determined by the reflection sensor (16) has a largely constant value for the toner marking (14).
19. Device according to any of the preceding claims, in that the decrease of the energy for the generation of the latent marking (14) ensues ever greater the higher the desired colour density of the printed print image (18).
20. Device according to any of the preceding claims, in that the decrease of the energy for the toner marking ensues such that the ratio or the differential quotient from toner concentration and colour density in the operating point of the reflection sensor (16) exceeds a predetermined value.
21. Device according to any of the preceding claims, in that, for a colour density value greater than or equal to a predetermined threshold, the decrease of the energy for the toner marking is cancelled.
22. Device according to any of the preceding claims, in that an electrophotographic print process is used as a print process for the printer or copier.
23. Device according to claim 22, in that an LED character generator whose LEDs can be individually controlled is used as a character generator (10).
24. Device according to any of the preceding claims, in that a photoconductor drum or a photoconductor band is used as an intermediate carrier (12).
25. Device according to any of the preceding claims, in that the toner marking (14) is arranged outside of the region of the print image (18).
26. Device according to claim 24 or 25, in that the toner marking (14) inked on the intermediate carrier (12) is scanned by the optical reflection sensor (16).
27. Device according to any of the preceding claims, in that the toner marking is scanned on a transfer band.

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