JP6690088B2 - Image forming device - Google Patents

Description

  The present invention relates to an image forming apparatus, and more particularly to an electrophotographic image forming apparatus that performs image stabilization control.

  Generally, in an image forming apparatus such as a copying machine or a printer using an electrophotographic method, there is a problem that the density of an output image is not stable due to changes in the photosensitive characteristics of the photosensitive member or the charge amount of the developer. In order to prevent such deterioration of image quality, in each image forming apparatus, a test pattern with toner is formed on the image carrier under a predetermined image forming condition before or during the printing operation, The density of the test pattern is optically detected, and the result is fed back to image forming conditions such as charging voltage, developing bias voltage, exposure light amount, input / output image data characteristics (gamma correction data), etc. 1). This type of feedback control is called image stabilization control, and mainly maintains maximum density, halftone density, and line width (dot diameter).

  When performing image stabilization control, when printing is not being sent to the image forming unit before printing operation or during continuous printing operation in order to avoid print productivity from decreasing as much as possible due to interruption of printing operation. The state detection control for detecting the state of the image density is executed. In the state detection control, a test pattern shorter than the test pattern formed during image stabilization control (hereinafter referred to as a control pattern to distinguish from the test pattern for image stabilization control) is formed on the image carrier, and the optical sensor is used. There is known a control that determines whether or not the image density to be printed is within a predetermined range based on the density detection result, and determines whether or not to execute image stabilization control. In the state detection control, control patterns suitable for confirming the qualities such as maximum density, halftone density, and line reproducibility are formed.

JP, 2009-217163, A

  By the way, the means (optical sensor) for detecting the density of the toner image has a problem that it cannot detect in the high density area. The density is detected by the optical sensor by utilizing the phenomenon that the light is almost totally reflected in the area where the toner is not adhered and part of the light is absorbed by the toner in the area where the toner is adhered and the reflected light is reduced. ing. In the toner adhesion amount detection characteristic of the optical sensor, as shown in FIG. 8, the sensor output value decreases as the adhesion amount increases. In other words, the optical sensor does not measure the color of the test pattern or the absolute value of the adhesion amount, but detects the change in the hiding ratio that changes with the increase in the adhesion amount of the toner as a substitute characteristic of the adhesion amount. . Therefore, in the high-concentration region X shown in FIG. 8, the detection sensitivity decreases, and the concentration cannot be detected accurately.

  In the maximum density control in the image stabilization control, as shown in FIG. 9, the characteristic that the toner adhesion amount changes linearly with respect to the developing bias voltage is used to control the maximum density which is not directly detected by the optical sensor. .

  In the maximum density state detection control, the method of forming the test pattern with the maximum density and detecting the maximum density state is the simplest and easiest. However, since the detection sensitivity of the optical sensor is low and the maximum density cannot be detected, it is necessary to predict the state of maximum density by forming a pattern with a gradation that is lower than the maximum density. In order to predict the state of maximum density, from the viewpoint of control accuracy, it is necessary to form a test pattern with a density as high as possible within the range detectable by the optical sensor and detect the density. That is, as shown in FIG. 10, the change in the halftone density accompanying the change in the maximum density is larger in the high density portion (high gradation). It is desirable to set the gradation so that changes in density can be detected.

  However, the detectable range of the optical sensor differs depending on the individual difference of the sensor itself, the variation in mounting accuracy, and also changes with time due to dirt from toner dust or paper dust. Therefore, even if a test pattern is formed with a density as high as possible and it is attempted to detect the density, there remains a problem that the maximum density state cannot be accurately detected due to variations among image forming apparatuses and changes over time. That is, as shown in FIG. 11, the maximum density fluctuation cannot be detected in the region Y due to the maximum density fluctuation.

  An object of the present invention is to provide an image forming apparatus capable of accurately determining the most preferable gradation of a test pattern used in maximum density state detection control.

The image forming apparatus according to one aspect of the present invention is
Image forming means for forming a toner image on the image carrier,
Detection means for optically detecting the density of the toner image formed on the image carrier,
Control means for controlling the operation of the image forming means,
Equipped with
The control means forms a control pattern consisting of a plurality of density levels of toner on the image carrier, estimates a maximum density fluctuation according to a density detection limit of the control pattern by the detection means, and determines the test pattern. Execute the state detection control to determine the gradation of
The control means forms a test pattern with the determined gradation on the image carrier, and performs maximum density state detection control for determining whether the density detection result of the test pattern by the detection means is within a predetermined range. Doing,
Is characterized by.

  In the image forming apparatus, the control means forms a control pattern having a plurality of density levels by toner on the image carrier, and a gradation is obtained when the density detection result of the control pattern by the optical detection means is the maximum density. The state detection control for determining the gradation of the test pattern in the maximum density state detection control is executed. As a result, the most preferable gradation of the test pattern used in the maximum density state detection control, which is a prerequisite for the image stabilization control, is accurately determined regardless of the individual difference of the device or the change over time.

  According to the present invention, it is possible to accurately determine the most preferable gradation of the test pattern formed on the image carrier for the maximum density state detection control.

FIG. 1 is a schematic configuration diagram of an image forming apparatus that is an embodiment. It is a flowchart figure which shows image stabilization control. It is a schematic diagram which shows a gamma correction control pattern. It is a flowchart figure which shows state detection control. It is a chart figure which shows the detected density of a gamma correction control pattern. It is a chart figure which shows the setting density of a test pattern. It is a flowchart figure which shows maximum density state detection control. 6 is a graph showing toner adhesion amount detection characteristics by an optical sensor. 6 is a graph showing a toner adhesion amount with respect to a developing bias voltage. 6 is a graph showing a change in halftone density accompanying a change in maximum density. It is a graph which shows the area | region where the fluctuation of the maximum density cannot be detected.

  Embodiments of an image forming apparatus according to the present invention will be described below with reference to the accompanying drawings. In each figure, the same members and parts are designated by the same reference numerals, and duplicate description will be omitted.

  As shown in FIG. 1, the image forming apparatus 1 which is an embodiment is configured to form a color image by a tandem system. That is, four process units 10 (a unit for yellow image 10 y, a unit for magenta image 10 m, a unit for cyan image 10 c, and a unit for black image 10 k) are juxtaposed and formed on each photoconductor 11. The toner images of the respective colors are transferred / combined (primary transfer) onto the intermediate transfer belt 31 which is rotationally driven in the direction of arrow A by an electric field applied from each transfer roller 32. After that, the synthetic toner image is secondarily transferred onto a recording medium (paper) by an electric field applied from the transfer roller 35.

  A charging roller 12, a developing device 13, a cleaning blade 14 for residual toner, and the like are arranged around each photoconductor 11, and a laser scanning optical unit 20 for forming an electrostatic latent image on the photoconductor 11 is further provided. It is arranged. A process of forming a toner image on the photoconductor 11 by the electrophotographic method by the process unit 10 of this type, primary-transferring the toner image onto the intermediate transfer belt 31, and then secondary-transferring onto the recording medium is well known. is there.

  The recording media (paper) are stacked and housed in a paper feed cassette 50, and are fed one by one by a paper feed roller 51. The fed paper is conveyed to the secondary transfer portion via the registration roller pair 52, and the toner image is secondarily transferred from the intermediate transfer belt 31. After that, the sheet is heated and fixed with the toner in the fixing unit 55, and is ejected from the ejection roller pair 56 to the ejection portion 57.

  In the image forming apparatus 1, first, a pattern is formed with toner on the photoconductor 11 with image forming conditions such as the amount of charge, the amount of exposure light, and the developing bias voltage as predetermined values, and the pattern is transferred onto the intermediate transfer belt 31. Then, the density of the toner pattern transferred onto the intermediate transfer belt 31 is optically detected by the sensor SE1, and based on the detected values, image stabilization control, state detection control, and maximum density state detection described below. Take control. These controls are executed by the CPU forming the control unit 60 together with the overall control of the image forming apparatus 1.

  As shown in FIG. 2, the image stabilization control is calibrated so that the output voltage of the optical sensor SE1 is appropriate (step S1), and the amount of toner adhered to the photoconductor 11 (developing efficiency) becomes maximum density. Thus, the developing bias voltage is controlled (step S2), the LD light amount (exposure light amount) of the optical unit 20 is appropriately controlled (step S3), and gamma correction control is executed (step S4).

  FIG. 3 shows control patterns 36Y (yellow), 36M (magenta), 36C (cyan), and 36K (black) used for gamma correction control. In FIG. 3, the control patterns 36Y, 36M, 36C, and 36K are shown in a state in which they are formed on the respective photoconductors 11 and transferred onto the intermediate transfer belt 31, and a plurality of control patterns from low density to maximum density are shown. It is formed with gradation. Gamma correction is a well-known control that is performed using this control pattern, and this control pattern is also used in the state detection control in this embodiment.

  FIG. 4 is a procedure for calculating the detection range of the optical sensor SE1 using the control patterns 36Y, 36M, 36C and 36K used for gamma correction control, and determining the gradation of the test pattern used for maximum density state detection control. Indicates. Specifically, the gamma correction control pattern is formed on the intermediate transfer belt 31 (step S11), the density thereof is detected by the optical sensor SE1 (step S12), and the detected value is converted into the density (step S12). S13). The density with respect to the detection value of the optical sensor SE1 is measured in advance and stored in the CPU 60 in a look-up table or the like.

  Next, the detection range of the optical sensor SE1 is calculated (step S14), and the gradation of the test pattern is determined (step S15). As shown in FIG. 5, the output voltage is saturated at a concentration higher than the detection range of the optical sensor SE1, and the concentration converted from the detected value is also saturated. That is, the density exceeding the detection range of the optical sensor SE1 is saturated. In this way, the saturated detection density can be calculated as the detection range of the optical sensor SE1. The detection density of 255 gradations may be treated as the saturation density. If the gradation (density) of the test pattern for maximum density state detection control is formed at the detected saturation density, or if the maximum density changes in the direction of increasing density, the density change cannot be detected, so it is formed at a density lower than the detected saturation density. It is necessary to calculate from the allowable fluctuation range of the maximum concentration.

  Specifically, as shown in FIG. 6, when the allowable fluctuation range of the maximum density is set to ΔM%, the detection upper limit density of the optical sensor SE1 is L, and the density of the state detection control pattern is T, T = L It is calculated by / (1 + M / 100). It is assumed that the high density part also fluctuates at the same rate as the fluctuation range of the maximum density.

  The maximum density state detection control is executed at a timing such as during the continuous printing operation, when the paper is not fed to the image forming unit, after the initial operation when the power of the device 1 is turned on, and immediately before the printing operation. To do. As shown in FIG. 7, a test pattern is formed on the intermediate transfer belt 31 with the gradation determined in step S15 (step S21), and its density is detected by the optical sensor SE1 (step S22). The value is converted into a density (step S23). If the density detection result is within the fluctuation allowable range (YES in step S24), the image forming condition is finely adjusted (step S25). The image forming conditions include a developing bias voltage, an LD light amount (exposure light amount), and the like. On the other hand, if the density detection result fluctuates beyond the allowable range (NO in step S24), the printing operation is interrupted, the image stabilization control is activated, and the maximum density control is executed (step S26).

  If the density fluctuates in the dark direction, the test pattern for maximum density control is set to a light density, and if the density fluctuates in the light direction, the test pattern for maximum density control is set to a dark density.

  By the above control, the most preferable gradation of the test pattern used in the maximum density state detection control is accurately determined regardless of the individual difference of the device or the change over time.

  The image forming apparatus according to the present invention is not limited to the above-described embodiment, but can be variously modified within the scope of the gist.

  In the above-described embodiment, the example in which the detection range of the optical sensor is calculated by using the gamma correction control has been described. However, the density of the control pattern is changed by changing the LD light amount and the developing bias voltage to detect the detection range of the optical sensor. May be calculated. Further, the density T of the state detection control pattern may be set in advance as a look-up table.

  The image forming apparatus may have any basic structure or detailed structure, and may be a multifunction peripheral having a communication function, a facsimile function, or the like, in addition to a print function alone.

  As described above, the present invention is useful for an image forming apparatus, and is particularly excellent in that the most preferable gradation of a test pattern used for maximum density state detection control can be accurately determined.

DESCRIPTION OF SYMBOLS 10 ... Process unit 11 ... Photosensitive member 13 ... Developing device 31 ... Intermediate transfer belt 36 ... Control pattern 60 ... Control part SE1 ... Density detection optical sensor

Claims (8)

Image forming means for forming a toner image on the image carrier,
Detection means for optically detecting the density of the toner image formed on the image carrier,
Control means for controlling the operation of the image forming means,
Equipped with
The control means forms a control pattern composed of a plurality of density levels of toner on the image carrier, estimates the maximum density fluctuation according to the density detection limit of the control pattern by the detection means, and Executes state detection control that determines gradation,
The control means forms a test pattern with the determined gradation on the image carrier, and performs maximum density state detection control for determining whether the density detection result of the test pattern by the detection means is within a predetermined range. Doing,
An image forming apparatus characterized by.   In the state detection control, the control unit changes the exposure light amount, the developing bias voltage, or the density gradation to form a control pattern having a plurality of density levels, and the detection unit detects the density of the control pattern. The image forming apparatus according to claim 1, wherein a density detection limit of the detection unit is calculated from the result.   The image forming apparatus according to claim 1, wherein the control unit sets a gradation of the test pattern to a gradation having a density lower than a density detection limit of the detection unit.   The image according to any one of claims 1 to 3, wherein the control unit determines the gradation of the test pattern based on a variation allowable width of the maximum density and a density detection limit of the detection unit. Forming equipment.   The control means performs the maximum density state detection control during continuous printing operation when the image forming apparatus is not driven for a predetermined time, when a predetermined number of sheets are printed, when temperature and humidity change outside a predetermined range. The image forming apparatus according to any one of claims 1 to 4, characterized in that the image forming apparatus is executed when any one of the sheets is not fed to the image forming unit.   The control means changes either the developing bias voltage or the exposure light amount when the density detection result of the test pattern by the detecting means is within a predetermined range. The image forming apparatus according to any one of claims. When the density detection result of the test pattern by the detection means is out of a predetermined range, the control means suspends the printing operation and executes image stabilization control for maintaining the maximum density. The image forming apparatus according to claim 1.   The image forming apparatus according to any one of claims 1 to 4, wherein the state detection control is executed and a gradation of the test pattern is determined in a manufacturing process of the image forming apparatus.