Image forming apparatus with predetermined target density and method

Abstract

A development bias calculation and an electrifying bias calculation are executed in this order. In the development bias calculation, a plurality of toner images are formed as first patch images while changing the development bias. An optimal development bias, which is necessary to obtain the target density, is determined based on densities of the first patch images. In the electrifying bias calculation, toner images are formed as second patch images while changing the electrifying bias with the development bias fixed to the optimal development bias. An optimal electrifying bias, which is necessary to obtain the target density, is determined based on densities of the second patch images.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus and an image forming method in which an electrifying bias applied to electrifying means electrifies a surface of a photosensitive member, an electrostatic latent image is thereafter formed on the surface of the photosensitive member, and a development bias is thereafter applied to developer means so that a toner visualizes the electrostatic latent image into a toner image.

2. Description of the Related Art

This type of an image forming apparatus often sees a change in an image density due to the following factors: fatigue, degradation with age or the like of a photosensitive member and a toner; a change in a temperature, a humidity or the like around the apparatus; and other causes. Noting this, a number of techniques have been proposed which aim at stabilizing an image density through appropriate adjustment of a density control factor such as an electrifying bias, a development bias, a light exposure dose, etc. For example, the invention described in the Japanese Patent Application Laid-Open Gazette No. 10-239924 requires to properly adjust an electrifying bias and a development bias in an effort to stabilize an image density. That is, according to this conventional technique, reference patch images are formed on a photosensitive member while changing an electrifying bias and/or a development bias and an image density of each reference patch is detected. An optimal electrifying bias and an optimal development bias are thereafter determined based on the detected image densities, and a density of a toner image is accordingly adjusted.

However, the conventional technique described above requires to identify an electrifying bias/development bias characteristic before forming reference patch images, and to set an electrifying bias and a development bias for creation of reference patch images, such that the characteristic is satisfied. In order to stabilize an image density based on a calculated optimal electrifying bias and development bias, it is necessary to identify an electrifying bias/development bias characteristic of each image forming apparatus, which is troublesome.

Further, an electrifying bias/development bias characteristic does not always stay constant but may change with time. If the characteristic changes, it is difficult to accurately calculate an optimal electrifying bias or an optimal development bias. While appropriate updating of the electrifying bias/development bias characteristic solves this problem, the updating is bothersome and disadvantageous in terms of maintainability.

Meanwhile, other technique for stabilizing an image density is the invention described in Japanese Patent Application Laid-Open Gazette No. 9-50155. According to the described invention, a reference patch image, which is a patch image obtained by outputting groups of three-dot lines for every three dots, is formed on a photosensitive drum, and a sensor reads patch images thus created, whereby a line width is detected. A laser power is controlled based on the detected line width, a light exposure dose is accordingly adjusted so that a desired line width will be obtained, and an ideal line image is obtained.

However, a line image is basically a one-dot line which is drawn with one laser beam, and therefore, simply controlling a line width of a multi-dot line as in the conventional technique can not realize a precise adjustment of a line image.

SUMMARY OF THE INVENTION

A main object of the present invention is to provide an image forming apparatus and an image forming method with which it is possible to stabilize an image density at a high accuracy in a simple manner.

Other object of the present invention is to provide an image forming apparatus and an image forming method with which it is possible to stabilize an image density of a line image.

In fulfillment of the foregoing object, an image forming apparatus and method are provided and are particularly well suited to density adjustment of a toner image based on image densities of a plurality of patch images.

According to a first aspect of the present invention, control means performs a development bias calculation and an electrifying bias calculation in this order. In the development bias calculation, after sequentially forming a plurality of toner images as first patch images while changing the development bias, densities of the first patch images are detected, and an optimal development bias, which is necessary to obtain the target density, is determined based on the densities of the first patch images. In the electrifying bias calculation, after sequentially forming a plurality of toner images as second patch images while changing the electrifying bias with the development bias fixed to the optimal development bias, densities of the second patch images are detected, and an optimal electrifying bias, which is necessary to obtain the target density, is determined based on the densities of the second patch images. Thus, it is possible to obtain an optimal electrifying bias and an optimal development bias without using an electrifying bias/development bias characteristic.

According to a second aspect of the present invention, a plurality of patch images, each of which is formed by a plurality of one-dot lines that are apart from each other, are formed on a photosensitive member or a transfer medium. Control means adjusts an image density of a toner image based on the image density of the patch images. Hence, it is possible to stabilize an image density of not only a line image which is formed by a P-dot (P≧2) line but of a line image which is formed by a one-dot line, to thereby stably form a fine image with an appropriate image density.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing showing a preferred embodiment of an image forming apparatus according to the present invention;

FIG. 2 is a block diagram showing an electric structure of the image forming apparatus of FIG. 1 ;

FIG. 3 is a flow chart showing a density adjustment operation in the image forming apparatus of FIG. 1 ;

FIG. 4 is a flow chart showing an operation of development bias calculation of FIG. 3 ;

FIG. 5 is a flow chart showing an operation of the bias calculation of FIG. 4 in a wide range;

FIGS. 6A through 6D are schematic diagrams showing an operation of the processing of FIG. 5 and an operation of the bias calculation in a narrow range;

FIG. 7 is a drawing showing a first patch image;

FIGS. 8A through 8D are drawings showing an order of forming patch images;

FIG. 9 is a flow chart showing an operation of bias calculation (1) of FIG. 4 in the narrow range;

FIG. 10 is a flow chart showing an operation of bias calculation (2) of FIG. 4 in the narrow range;

FIGS. 11A and 11B are schematic diagrams showing the operation of the processing of FIG. 10 ;

FIG. 12 is a flow chart showing an operation of the electrifying bias calculation of FIG. 3 ;

FIGS. 13A and 13B are schematic diagrams showing the operation of the processing of FIG. 12 ;

FIG. 14 is a drawing showing a second patch image;

FIGS. 15A and 15B are drawings showing a relationship between the first patch images, a surface potential and a development bias potential; and

FIGS. 16A and 16B are drawings showing a relationship between the second patch images, a surface potential and a development bias potential.

FIG. 17 is a graph showing a light intensity distribution of laser light which is irradiated onto a surface of a photosensitive member;

FIGS. 18A and 18B are schematic diagrams showing a relationship between one-dot lines and a detect area which a patch sensor detects, with a change in line intervals;

FIG. 19 is a view for describing a detect deviation which occurs as positions of the detect area of the patch sensor and one-dot lines change relative to each other;

FIG. 20 is a graph showing a change in an output from the patch sensor with a change in line intervals;

FIG. 21 is a schematic diagram of other preferred embodiment of a patch image;

FIG. 22 is a graph showing attenuation of a surface potential as photosensitive member is exposed at various exposure powers;

FIG. 23 is a drawing showing a relationship between a development bias and a contrast potential when the development bias is changed with an electrifying bias fixed;

FIG. 24 is a drawing showing a relationship between an electrifying bias and a contrast potential when the electrifying bias is changed with a development bias fixed;

FIG. 25 is a drawing showing the relationship between the development bias and the contrast potential;

FIG. 26 is a drawing showing variations in the contrast potential and the exposed area potential in accordance with a change in the electrifying bias;

FIG. 27 is a drawing showing a relationship between the development bias and the contrast potential as the electrifying bias is set according to a first variation;

FIG. 28 is a drawing showing a relationship between the electrifying bias and the development bias in the first variation;

FIG. 29 is a drawing showing a relationship between the electrifying bias and the development bias in a second variation;

FIG. 30 is a drawing showing a relationship between an exposure power and a surface potential;

FIG. 31 is a drawing showing a relationship between the development bias and the contrast potential at the exposure power shown in FIG. 30 ;

FIG. 32 is a drawing showing a relationship between the development bias and the contrast potential as the electrifying bias is set according to the second variation;

FIG. 33 is a drawing showing a relationship between the electrifying bias and the development bias in a third variation;

FIG. 34 is a drawing showing a relationship between an exposure power and a surface potential;

FIG. 35 is a drawing showing a relationship between the development bias and the contrast potential at the exposure power shown in FIG. 34 ;

FIG. 36 is a drawing showing a relationship between the development bias and the contrast potential as the electrifying bias is set according to the third variation;

FIG. 37 is a drawing showing the relationship between the development bias and the contrast potential;

FIG. 38 is a drawing showing a relationship between the electrifying bias and the development bias in a fourth variation;

FIG. 39 is a drawing showing a relationship between the development bias and the contrast potential as the electrifying bias is set according to the fourth variation;

FIG. 40 is a drawing showing the relationship between the development bias and the contrast potential;

FIG. 41 is a drawing showing a relationship between the electrifying bias and the development bias in a fifth variation;

FIG. 42 is a drawing showing a relationship between the development bias and the contrast potential as the electrifying bias is set according to the fifth variation; and

FIGS. 43A through 43D are drawings showing an order of forming patch images according to still other preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A. Overall Structure of Image Forming Apparatus

FIG. 1 is a drawing showing a preferred embodiment of an image forming apparatus according to the present invention. FIG. 2 is a block diagram showing an electric structure of the image forming apparatus of FIG. 1 . The image forming apparatus is an apparatus which overlaps toner images in four colors of yellow (Y), cyan (C), magenta (M) and black (K) to thereby form a full-color image or uses only a black (K) toner to thereby form a monochrome image. When an image signal is supplied to a main controller 11 of a control unit 1 from an external apparatus such as a host computer, an engine controller 12 controls respective portions of an engine part E in accordance with an instruction from the main controller 11 , whereby the image forming apparatus forms an image which corresponds to the image signal on a sheet S.

The engine part E is capable of forming a toner image on a photosensitive member 21 of an image carrier unit 2 . That is, the image carrier unit 2 comprises the photosensitive member 21 which is rotatable in the direction of an arrow in FIG. 1 . Disposed around the photosensitive member 21 and in the rotation direction of the photosensitive member 21 in FIG. 1 are an electrifying roller 22 which serves as electrifying means, developers 23 Y, 23 C, 23 M and 23 K which serve as developing means, and a cleaning part 24 . Applied with a high voltage from an electrifying bias generation part 121 and in contact with an outer peripheral surface of the photosensitive member 21 , the electrifying roller 22 uniformly electrifies the outer peripheral surface of the photosensitive member 21 .

An exposure unit 3 irradiates laser light L toward the outer peripheral surface of the photosensitive member 21 which is electrified by the electrifying roller 22 . The exposure unit 3 , as shown in FIG. 2 , is electrically connected with an image signal switching part 122 . In accordance with an image signal which is supplied through the image signal switching part 122 , the laser light L scans over the photosensitive member 21 and consequently exposes the photosensitive member 21 , whereby an electrostatic latent image corresponding to the image signal is formed on the photosensitive member 21 . For example, when the image signal switching part 122 is in conduction with a patch generation module 124 , based on an instruction from a CPU 123 of the engine controller 12 , a patch image signal outputted from the patch generation module 124 is fed to the exposure unit 3 so that a patch latent image is formed. On the other hand, when the image signal switching part 122 is in conduction with a CPU 111 of the main controller 11 , the laser light L scans over and consequently exposes the photosensitive member 21 in accordance with an image signal which is supplied through an interface 112 from an external apparatus such as a host computer, so that an electrostatic latent image corresponding to the image signal is formed on the photosensitive member 21 .

The electrostatic latent image which is formed in this manner is developed by a developer part 23 . In other words, according to the preferred embodiment, disposed as the developer part 23 are the developer 23 Y for yellow, the developer 23 C for cyan, the developer 23 M for magenta and the developer 23 K for black which are arranged in this order around the photosensitive member 21 . The developers 23 Y, 23 C, 23 M and 23 K are each structured so as to freely separate from and come close to the photosensitive member 21 . In accordance with an instruction given from the engine controller 12 , one of the four developers 23 Y, 23 C, 23 M and 23 K selectively contacts the photosensitive member 21 . A development bias generation part 125 thereafter applies a high voltage to the photosensitive member 21 , and the toner in the selected color moves to the surface of the photosensitive member 21 , thereby visualizing the electrostatic latent image on the photosensitive member 21 . The voltages supplied to the respective developers may be simply D.C. voltages, or alternatively, A.C. voltages superimposed over D.C. voltages.

The toner image developed by the developer part 23 is primarily transferred onto an intermediate transfer belt 41 of a transfer unit 4 in a primary transfer region R 1 which is located between the black developer 23 K and the cleaning part 24 . A structure of the transfer unit 4 will be described in detail later.

The cleaning part 24 is disposed at a position further ahead in a circumferential direction (the direction of the arrow in FIG. 1 ) from the primary transfer region R 1 , such that a toner remaining on the outer peripheral surface of the photosensitive member 21 after the primary transfer treatment is scraped off.

Next, the structure of the transfer unit 4 will be described. According to the preferred embodiment, the transfer unit 4 comprises rollers 42 through 47 , the intermediate transfer belt 41 which is spun around the rollers 42 through 47 , and a secondary transfer roller 48 which secondarily transfers an intermediate toner image transferred to the intermediate transfer belt 41 onto a sheet S. A transfer bias generation part 126 applies a primary transfer voltage upon the intermediate transfer belt 41 . Toner images in the respective colors formed on the photosensitive member 21 are laid one atop the other on the intermediate transfer belt 41 into a color image, while the sheet S is taken out from a cassette 61 , a hand-feeding tray 62 or an additional cassette (not shown) by a paper feed part 63 of a paper feed/discharge unit 6 and conveyed to a secondary transfer region R 2 . The color image is thereafter secondarily transferred onto the sheet S, thereby obtaining a full-color image. Meanwhile, when a monochrome image is to be transferred onto a sheet S, only a black toner image on the photosensitive member 21 is formed on the intermediate transfer belt 41 , and transferred onto a sheet conveyed to the secondary transfer region R 2 to thereby obtain a monochrome image, as in the case of forming a color image.

After secondary transfer treatment, a toner remaining on and sticking to an outer peripheral surface of the intermediate transfer belt 41 is removed by a belt cleaner 49 . The belt cleaner 49 is disposed opposite to the roller 46 across the intermediate transfer belt 41 , and a cleaner blade contacts the intermediate transfer belt 41 at appropriate timing and scrapes off a toner from the outer peripheral surface of the intermediate transfer belt 41 .

Further, disposed in the vicinity of the roller 43 is a patch sensor PS which detects a density of a patch image which is formed on the outer peripheral surface of the intermediate transfer belt 41 as described later, and so is a read sensor for synchronization RS which detects a reference position of the intermediate transfer belt 41 .

Referring to FIG. 1 again, the description on the structure of the engine part E will be continued. The sheet S now seating the toner image transferred by the transfer unit 4 is conveyed by the paper feed part 63 of the paper feed/discharge unit 6 to a fixing unit 5 which is disposed on the downstream side to the secondary transfer region R 2 along a predetermined paper feed path (dot-dot-dash line), and the toner image on the conveyed sheet S is fixed on the sheet S. The sheet S is thereafter conveyed to a paper discharge part 64 along the paper feed path 630 .

The paper discharge part 64 has two paper discharge paths 641 a and 641 b . The paper discharge path 641 a extends from the fixing unit 5 to a standard paper discharge tray, while the paper discharge path 641 b extends approximately parallel to the paper discharge path 641 a between a paper re-feed part 66 and a multi-bin unit. Three roller pairs 642 through 644 are disposed along the paper discharge paths 641 a and 641 b , so as to discharge the sheets S toward the standard paper discharge tray or the multi-bin unit and convey the sheets S toward the paper re-feed part 66 for the purpose of forming images on non-printing surfaces of the sheets S.

Aiming at conveying a sheet S which was inverted and fed from the paper discharge part 64 as described above to a gate roller pair 637 of the paper feed part 63 along a paper re-feed path 664 (dot-dot-dash line), the paper re-feed part 66 is formed of three paper re-feed roller pairs 661 through 663 which are disposed along the paper re-feed path 664 as shown in FIG. 1 . In this manner, the sheet S sent from the paper discharge part 64 is returned to the gate roller pair 637 along the paper re-feed path 664 and a non-printing surface of the sheet S is directed toward the intermediate transfer belt 41 within the paper feed part 63 , which makes it possible to secondarily transfer the image onto the non-printing surface.

In FIG. 2 , denoted at 113 is an image memory which is disposed in the main controller 11 such that the image memory stores image data supplied from an external apparatus such as a host computer through the interface 112 , denoted at 127 is a RAM which temporarily stores control data for controlling the engine part E, a calculation result obtained by the CPU 123 , etc., and denoted at 128 is a ROM which stores a calculation program which is executed by the CPU 123 .

B. Density Adjustment by Image Forming Apparatus

Now, a description will be given on how the image forming apparatus having such a structure as described above adjusts a density of an image.

FIG. 3 is a flow chart showing a density adjustment operation in the image forming apparatus of FIG. 1 . In the image forming apparatus, as shown in FIG. 3 , it is determined at a step S 1 whether the density adjustment operation should be executed to thereby update an electrifying bias and a development bias. For example, the image forming apparatus may start setting the biases when the image forming apparatus becomes ready to form an image after a main power source of the image forming apparatus is turned on. Alternatively, the image forming apparatus may set the biases every few hours while a timer (not shown) disposed in the image forming apparatus measures hours of continuous use.

When it is determined YES at the step S 1 and setting of the biases is accordingly started, steps S 2 and S 3 are executed to calculate an optimal development bias, and the calculated bias is set as the development bias (step S 4 ). Following this, a step S 5 is executed to calculate an optimal electrifying bias, and the calculated bias is set as the electrifying bias (step S 6 ). The electrifying bias and the development bias are optimized in this manner. In the following, a detailed description will be given on an operation of each one of the development bias calculation (step S 3 ) and the electrifying bias calculation (step S 5 ).

B-1. Development Bias Calculation

FIG. 4 is a flow chart showing an operation of the development bias calculation shown in FIG. 3 . In the development bias calculation (step S 3 ), the CPU 123 determines whether this is first calculation or the second or subsequent calculation after the main power source of the image forming apparatus is turned on (step S 301 ). When the current calculation is the first one, after setting up such that patch images will be created in all colors (which are the four colors of yellow (Y), cyan (C), magenta (M) and black (K) in this preferred embodiment) (step S 311 ), an immediately subsequent step S 312 is executed. In other words, a plurality of patch images are formed while gradually changing the development bias at relatively long intervals within a relatively wide range, thereby tentatively identifying a development bias which is necessary to obtain an optimal image density based on densities of the respective patch images. Now, an operation of this processing will be described in detail with reference to FIGS. 5 and 6A through 6 D.

FIG. 5 is a flow chart showing an operation of the bias calculation of FIG. 4 within a wide range. FIGS. 6A through 6D are schematic diagrams showing an operation of the processing of FIG. 5 and an operation of the bias calculation within narrow range which will be described later. During this calculation, a color in which patch images are to be generated is set as the first color, e.g., yellow (step S 312 a ). With the electrifying bias set to a default value which is set in advance at the step S 2 , the development bias is set to four different values which are apart at relatively long intervals (first intervals) within the wide range (step S 312 b ). For instance, in this preferred embodiment, the wide range is the entirety of a programmable range (Vb 01 -Vb 10 ) of development bias which can be supplied to the developer part 23 from the development bias generation part 125 , and four points Vb 01 , Vb 04 , Vb 07 and Vb 10 within the wide range (Vb 01 -Vb 10 ) are set as development biases. In this manner, according to this preferred embodiment, the first intervals W 1 are:

W 1 =Vb 10 -Vb 07 =Vb 07 -Vb 04 =Vb 04 -Vb 01

Four yellow solid images ( FIG. 7 ) are sequentially formed on the photosensitive member 21 with this bias setup, and the solid images are transferred onto the outer peripheral surface of the intermediate transfer belt 41 as shown in FIG. 8A to thereby form first patch images PI 1 (step S 312 c ). The first patch images PI 1 are solid images in this preferred embodiment. The reason of this will be described in detail later.

At a subsequent step S 312 d , whether patch images are formed in all of patch generation colors is determined. While a result of the judgement stays NO, the next color is set as a patch generation color (step S 312 e ) and the steps S 312 b and S 312 c are repeated. This adds further first patch images PI 1 on the outer peripheral surface of the intermediate transfer belt 41 , in the order of cyan (C), magenta (M) and black (K), as shown in FIGS. 8B through 8D .

On the contrary, when it is determined YES at the step S 312 d , image densities of the sixteen (=4 types×4 colors) patch images PI 1 are measured on the basis of a signal outputted from the patch sensor PS (step S 312 f ). While the image densities of the patch images PI 1 are measured at once after forming the patch images PI 1 in all patch generation colors in this preferred embodiment, the image densities of the patch images PI 1 may be measured sequentially color by color every time the patch images PI 1 in one patch generation color are formed. This applies to the later bias calculation ( FIGS. 9 , 10 and 12 ) as well.

Following this, a development bias corresponding to a target density is calculated at a step S 312 g , and the calculated bias is stored temporarily in the RAM 127 as an interim bias. When a measurement result (image density) matches with the target density, a development bias corresponding to this image density may be used as the interim bias. When the two density values fail to match, as shown in FIG. 6B , it is possible to calculate an interim bias through linear interpolation, averaging or other appropriate methodology in accordance with data D (Vb 04 ) and data D (Vb 07 ) which are on the both sides of the target density.

Once the interim bias is determined in this manner, the bias calculation (1) in the narrow range shown in FIG. 4 is executed. FIG. 9 is a flow chart showing an operation of the bias calculation (1) of FIG. 4 in the narrow range. During this calculation, a color in which patch images are to be generated is set as the first color, e.g., yellow (step S 313 a ), as in the earlier calculation (step S 312 ). With the electrifying bias set to the default value which is set in advance at the step S 2 , the development bias is set to four different values which are apart at narrower intervals (second intervals) than the first intervals W 1 within a narrow range which includes the interim bias (step S 313 b ). For instance, in this preferred embodiment, the narrow range is approximately ⅓ of the programmable rang (Vb 01 -Vb 10 ) of development bias. When the interim bias is between development biases Vb 05 and Vb 06 as shown in FIG. 6B , four points Vb 04 , Vb 05 , Vb 06 and Vb 07 are set as development biases (FIG. 6 C). In this manner, according to this preferred embodiment, the second intervals W 2 are:

W 2 =Vb 07 -Vb 06 =Vb 06 -Vb 05 =Vb 05 -Vb 04

Four yellow solid images ( FIG. 7 ) are sequentially formed on the photosensitive member 21 with this bias setup, and the solid images are transferred onto the outer peripheral surface of the intermediate transfer belt 41 as shown in FIG. 8A to thereby form first patch images PI 1 (step S 313 c ). As in the earlier calculation (step S 312 ), the next color is set as a patch generation color (step S 313 e ) and the steps S 313 b and S 313 c are repeated until it is determined at a step S 313 d that patch images are formed in all of patch generation colors. As a result, first patch images PI 1 are further formed on the outer peripheral surface of the intermediate transfer belt 41 , in the order of cyan (C), magenta (M) and black (K).

Once sixteen (=4 types×4 colors) patch images PI 1 are formed on the intermediate transfer belt 41 in this manner, image densities of the respective patch images PI 1 are measured on the basis of a signal outputted from the patch sensor PS (step S 313 f ). Following this, at a step S 313 g , a development bias corresponding to a target density is calculated. When a measurement result (image density) matches with the target density, a development bias corresponding to this image density may be used as an optimal development bias. When the two density values fail to match, as shown in FIG. 6D , it is possible to calculate an optimal development bias through linear interpolation, averaging or other appropriate methodology in accordance with data D (Vb 05 ) and data D (Vb 06 ) which are on the both sides of the target density.

The RAM 127 stores the optimal development bias which is calculated in this manner (step S 302 in FIG. 4 ), and reads it out as the development bias during calculation of the electrifying bias which will be described later or while an image is formed in a normal manner.

Thus, the preferred embodiment described above carries out a two-stage development bias calculation. In the first stage, patch images PI 1 are formed at the first intervals W 1 in the wide range to calculate a development bias, which is necessary to obtain an image having a target density, as an interim development bias. In the second stage, patch images PI 1 are formed at the narrower intervals (i.e., the second intervals) W 2 in the narrow range which includes the interim bias to calculate a development bias which is necessary to achieve the target density. Finally, the calculated bias is set as an optimal development bias. This realizes the following effects.

For example, upon turning on of the main power source of the image forming apparatus, it is totally impossible to predict variations in characteristics of the photosensitive member and the toners, humidity and temperatures around the apparatus, etc. Hence, it is necessary to form patch images after setting a development bias such that the programmable range (Vb 01 -Vb 10 ) of development biases is entirely covered and to determine an optimal development bias. Therefore, the optimal development bias can be obtained by the following approach: The approach requires to divide the programmable range (Vb 01 -Vb 10 ) of development biases into a plurality of narrow ranges and to execute similar processing to the bias calculation (1) described above in each one of the narrow ranges. However, this comparative approach has a problem that the number of steps to be executed increases in proportion to the number of the divided ranges and calculation of an optimal development bias therefore takes time. Conversely, if the programmable range is divided into a smaller number of narrow ranges, although the problem described earlier is solved, bias intervals within each divided range become wider than the second bias intervals W 2 . This creates another problem that an accuracy of calculating an optimal development bias drops down and an image density therefore can not be accurately adjusted to the target density.

In contrast, according to the above embodiment, a development bias is tentatively calculated through the bias calculation processing (step S 312 ) in the wide range, and the development bias is changed at the narrower intervals (i.e., the second intervals) W 2 in the narrow range in the vicinity of the interim bias, so that an optimal development bias is finally calculated. Hence, it is possible to more accurately calculate an optimal development bias in a shorter period of time than in the comparative approach above.

By the way, while an optimal electrifying bias and an optimal development bias change due to fatigue, degradation with age or the like of a photosensitive member, a toner, etc., the changes possess a continuity to a certain extent. Hence, where an image density is repeatedly adjusted, it is possible to predict an optimal development bias based on an image density which is measured immediately previously (e.g., the step S 313 f , and steps S 322 f and S 510 which will described later). Noting this, in the bias calculation (step S 3 ) according to this preferred embodiment, when the current calculation is determined to be the second or subsequent calculation after the main power source of the image forming apparatus is turned on, that is, when it is determined at the step S 301 in FIG. 4 to follow the SECOND OR SUBSEQUENT path, after setting up such that patch images will be created in all colors (which are the four colors of yellow (Y), cyan (C), magenta (M) and black (K) in this preferred embodiment) (step S 321 ), an immediately subsequent step S 322 is executed. In other words, bias calculation (2) within the narrow range is executed to thereby calculate an optimal development bias without calculating an interim bias. Now, an operation of this processing will be described in detail with reference to FIG. 10 .

FIG. 10 is a flow chart showing an operation of the bias calculation (2) of FIG. 4 within the narrow range. FIGS. 11A and 11B are schematic diagrams showing the operation of the processing shown in FIG. 10 . This calculation processing is largely different from the bias calculation (1) within the narrow range described earlier in regard to the following. During the calculation (1) shown in FIG. 9 , the electrifying bias set to the default value, and four different types of development biases are set based on an interim bias (step S 313 b ). Meanwhile, during the bias calculation (2), the electrifying bias is the optimal electrifying bias which is calculated through immediately preceding measurement and stored in the RAM 127 , and four different types of development biases are set within the narrow range based on the optimal development bias which is stored in the RAM 127 (step S 322 b ). The bias calculation (2) is structured otherwise the same as the bias calculation (1), and therefore, a redundant description will be simply omitted.

In this manner, during the second or subsequent density adjustment, the four different types of development biases are set. The four biases are apart at the second intervals within the narrow range using the development bias which is calculated immediately previously (preceding optimal development bias) without calculating an interim bias, the patch images are formed in the respective colors, and the optimal development bias is calculated. Hence, it is possible to calculate an optimal development bias in a further shorter time.

The engine controller 12 writes the optimal development bias which is calculated in this manner over the preceding optimal development bias which is already stored in the RAM 127 , thereby updating the optimal development bias (step S 302 in FIG. 4 ). The sequence thereafter returns to FIG. 3 which requires to read the optimal development bias from the RAM 127 and set the retrieved optimal development bias as the development bias. An optimal electrifying bias is thereafter calculated (step S 5 ) and set as the electrifying bias (step S 6 ).

B-2. Optimal Electrifying Bias Calculation

FIG. 12 is a flow chart showing an operation of the electrifying bias calculation of FIG. 3 . FIGS. 13A and 13B are schematic diagrams showing the operation of the processing shown in FIG. 12 . During the electrifying bias calculation (step S 5 ), after setting up such that patch images will be created in all colors (which are the four colors of yellow (Y), cyan (C), magenta (M) and black (K) in this preferred embodiment) (step S 501 ), a color in which second patch images are to be generated is set as the first color, e.g., yellow at a step S 502 .

As in the development bias calculation, the CPU 123 determines whether the current electrifying bias calculation is first such calculation or the second or subsequent calculation after the main power source of the image forming apparatus is turned on (step S 503 ). When the current calculation is determined to be the first one, a step S 504 is executed. When the current calculation is determined to be the second or subsequent calculation, a step S 505 is executed.

At the step S 504 , the electrifying bias is set to four different values. The four biases are apart at relatively narrow intervals (third intervals) within the narrow range which includes the default value. Meanwhile, at the step S 505 , the electrifying bias is set to four different values which are apart at relatively narrow intervals (third intervals) within the narrow range which includes a preceding optimal electrifying bias. In this manner, unlike the development bias calculation, the electrifying bias calculation executes only narrow-range calculation without calculating within the wide range. In this preferred embodiment, the narrow range is approximately ⅓ of a programmable range (Va 01 -Va 10 ) of electrifying bias. When the default value or an immediately preceding optimal electrifying bias is between electrifying biases Va 05 and Vb 06 as shown in FIG. 13A , four points Va 04 , Va 05 , Va 06 and Va 07 are set as electrifying biases. That is, according to this preferred embodiment, the third intervals W 3 are:

W 3 =Va 07 -Va 06 =Va 06 -Va 05 =Va 05 -Va 04

Once four types of electrifying biases are set up for the yellow color in this manner, respective yellow halftone images (See FIG. 14 ) are sequentially formed on the photosensitive member 21 and transferred onto the outer peripheral surface of the intermediate transfer belt 41 , whereby second patch images PI 2 are formed (FIG. 8 A: step S 506 ). The electrifying bias is increased stepwise because when an electrifying bias is to be changed stepwise, increasing the electrifying bias achieves a superior response of the power source as compared to decreasing the electrifying bias. In the preferred embodiment above, the second patch images PI 2 are halftone images which are defined by a plurality of one-dot lines which are arranged parallel to each other but apart from each other at the intervals of five lines (n=5). The reason of this will be described in detail later together with the reason why the first patch images are solid images.

At a subsequent step S 507 , whether the second patch images are formed in all of patch generation colors is judged. While a result of the judgement stays NO, the next color is set as a patch generation color (step S 508 ) and the steps S 503 through S 507 are repeated. This adds further second patch images PI 2 on the outer peripheral surface of the intermediate transfer belt 41 , in the order of cyan (C), magenta (M) and black (K), as shown in FIGS. 8B through 8D .

On the contrary, when it is determined YES at the step S 507 , image densities of the sixteen (=4 types×4 colors) patch images PI 2 are measured on the basis of a signal outputted from the patch sensor PS (step S 509 ). Following this, an electrifying bias corresponding to a target density is calculated (step S 510 ), and the calculated electrifying bias is stored in the RAM 127 as an optimal electrifying bias (step S 511 ). When a measurement result (image density) matches with the target density, an electrifying bias corresponding to this image density may be used as an optimal electrifying bias. When the two density values fail to match, as shown in FIG. 13B , it is possible to calculate an optimal electrifying bias through linear interpolation, averaging or other appropriate methodology in accordance with data D (Va 05 ) and data D (Va 06 ) which are on the both sides of the target density.

Once the optimal electrifying bias is determined in this manner, the optimal electrifying bias calculated as described above is read from the RAM 127 and set as the electrifying bias, in addition to the optimal development bias already set as the development bias. When an image is formed with this setup, the resultant image has the target density. In other words, the image density is stable.

As described above, according to this preferred embodiment, it is possible to calculate an optimal electrifying bias and an optimal development bias without using an electrifying bias/development bias characteristic which is essential in the conventional technique to adjust an image density. Hence, it is possible to adjust an image density to a target density and accordingly stabilize the image density in a simple manner. Further, even despite a change with time in an electrifying bias/development bias characteristic, this preferred embodiment allows to accurately calculate an optimal electrifying bias and an optimal development bias without an influence of the change.

Further, as described above, since calculation of an optimal development bias is achieved in the two stages of bias calculation in the wide range (step S 312 ) and bias calculation in the narrow range (step S 313 ), it is possible to calculate the optimal development bias at a high accuracy in a short period of time.

Further, this preferred embodiment makes it possible to calculate an optimal electrifying bias and an optimal development bias, adjust an image density to a target density, and stabilize the image density. According to this preferred embodiment, in particular, each patch image PI 2 is formed by a plurality of one-dot lines which are arranged apart from each other. Since an image density of each such patch image PI 2 is detected and an image density of a toner image is adjusted to a target density based on the detected image densities of the patch images PI 2 , it is possible to stabilize an image density of not only a line image which is formed by a P-dot (P≧2) line but of a line image which is formed by a one-dot line, and hence, to stably form a fine image with an appropriate image density.

Further, with respect to calculation of an optimal electrifying bias, since the electrifying bias calculation is executed with an optimal development bias calculated through immediately preceding calculation set as a development bias, it is possible to accurately calculate an optimal electrifying bias.

Further, during the second or following calculation of a development bias and an electrifying bias, since the biases are calculated based on immediately preceding results of image density measurements (i.e., an optimal electrifying bias and an optimal development bias), it is possible to accurately calculate an optimal electrifying bias and an optimal development bias in a short period of time.

C. Patch Images

By the way, the following is the reason why solid images are used as the first patch images for calculation of a development bias, while for calculation of an electrifying bias, used as the second patch images are halftone images in which a plurality of one-dot lines are arranged parallel to each other but apart from each other at intervals of n lines.

As an electrostatic latent image LI 1 of a solid image (first patch image) PI 1 (See FIG. 7 ) is formed on the surface of the photosensitive member 21 which is electrified uniformly at a surface potential V 0 , a surface potential corresponding to the electrostatic latent image LI 1 largely drops down to a potential (exposed area potential) Von as shown in FIGS. 15A and 15B , whereby a well potential is developed. Now, even if the electrifying bias is increased to raise the surface potential of the photosensitive member 21 from the potential V 0 up to a potential V 0 ′, the exposed area potential will not depart largely from the potential Von. Hence, a toner density is determined only in accordance with the development bias Vb despite any small change in the electrifying bias.

Meanwhile, a halftone image (second patch image) PI 2 (See FIG. 14 ) contains one-dot lines DL formed at predetermined intervals. As an electrostatic latent image LI 2 of the halftone image is formed on the surface of the photosensitive member 21 which is electrified uniformly at a surface potential V 0 , surface potentials corresponding to the positions of the lines largely drop down to the potential (exposed area potential) Von, as shown in FIGS. 16A and 16B . As a result, a comb-shaped well potential is developed. If the electrifying bias is increased in a similar manner to described above to raise the surface potential of the photosensitive member 21 from the potential V 0 up to the potential V 0 ′, the exposed area potential corresponding to each line changes greatly from the potential Von to a potential Von′. Hence, as the electrifying bias changes, a toner density corresponding to the development bias Vb changes with the change in the electrifying bias. A relationship between such bias setup (the optimal development bias and the optimal electrifying bias) and a toner density will be described in detail in “D. Setting of Electrifying Bias in Development Bias Calculation” below.

From the above, it is found that use of a solid image reduces the influence of the electrifying bias over the toner density, and therefore, it is possible to adjust an image density of the solid image by means of adjustment of the development bias. In short, when the development bias calculation is executed using solid images as the first patch images as in the preferred embodiment above, it is possible to accurately calculate an optimal development bias regardless of the value of the electrifying bias.

Further, to form an image in a stable manner, adjustment at a maximum gradation (maximum density) alone is not sufficient. Density adjustment of a line image is necessary as well. However, when halftone images of line images are used, as shown in FIGS. 16A and 16B , the set development bias and the set electrifying bias strongly influence an eventual image. To deal with this, the preferred embodiment above requires to calculate an optimal development bias first. While changing the electrifying bias with the development bias set to the optimal development bias, the second patch images of halftone images are formed. As a result, therefore, the optimal electrifying bias needed to obtain an image density, which meets the target density, is calculated.

In addition, a line image (second patch image PI 2 ) is formed by a halftone image which is obtained by arranging a plurality of one-dot lines parallel to each other but apart from each other at intervals of n lines, for the following reason. That is, although one approach to adjust an image density of a one-dot line is to form the second patch image PI 2 as a single one-dot line and detect a density of the one-dot line with the patch sensor PS, since an image density of a one-dot line is extremely low, it is difficult to detect an image density of a one-dot line with the patch sensor PS. Noting this, the present invention requires to form a patch image with a plurality of one-dot lines to solve this problem.

Where a patch image is formed by a plurality of one-dot lines, the issue is how to arrange the one-dot lines for the following reason. Laser light L irradiated toward the photosensitive member 21 from the exposure unit 3 has a light intensity distribution of a Gaussian type as that shown in FIG. 17 , for example. In a normal apparatus design, in most cases, a design spot diameter is set which is needed to attain a design resolution. An apparatus is designed such that a spot diameter approximately at 50% of a maximum light intensity matches a design resolution. However, an effective exposure spot diameter corresponding to 1/e 2 which is effective as an exposure power is larger than the design spot diameter. Hence, when a line interval between adjacent one-dot lines DL is narrow, a toner adheres between the lines. In other words, if the line interval n between the adjacent one-dot lines DL ( FIG. 16 A) is one line, adjacent effective exposure spots partially overlap with each other, a surface potential at the overlap position changes, and a toner adheres. Because of this, it is necessary that a line interval between adjacent one-dot lines DL is at least two lines or more.

Conversely, the following problem occurs if the line intervals are too wide. That is, a sensitivity of the patch sensor PS to detect an image density is closely related with the number of one-dot lines DL which are contained in a detect area of the patch sensor PS. Where a density change of each one-dot line DL is X and the number of lines covered by the detect area is m, an image density changeΔ detected by the patch sensor PS is:

Δ= m·X

Thus, the larger the number of lines contained in the detect area is, the higher the detect sensitivity is. For instance, as shown in FIG. 18A , with line intervals of n 1 , when the number of lines contained in the detect area IR of the patch sensor PS is five, an image density changeΔ a is:

Δa= 5 ·X

On the other hand, as shown in FIG. 18B , with line intervals n 2 (>n 1 ), the number of lines contained in the detect area IR of the patch sensor PS decreases to four, and therefore, an image density changeΔ b is:

Δb= 4 ·X

thereby decreasing the detect sensitivity.

While results of various experiments have identified that it is necessary to improve the detect sensitivity of the patch sensor PS approximately one digit in order to ensure sufficient density adjustment, the number of lines contained in the detect area IR must be set to ten or larger for that purpose. Now, where the size of the detect area IR is φ(mm) and the design resolution of the apparatus, namely, the number of dots contained in a unit length (1 mm) is R, if the line intervals are n, the number of lines m within the detect area IR is:

m=φ·R/ (1+ n )

For the number of lines m to be ten or larger, the following must be satisfied:

φ·R/ (1+ n )≧10

Modifying the inequality,

n≦ ( φ·R− 10)/10  (1)

Thus, if the line intervals n are set so as to satisfy the inequality (1) above, it is possible to detect image densities of the patch images PI 2 at an excellent detect sensitivity.

While where the patch sensor PS is to read image densities, repeated reading while changing a read position aims at improving the detect accuracy. If images to be detected are patch images in which one-dot lines are arranged parallel to each other but apart from each other at predetermined intervals, due to positional differences between the detect area of the patch sensor PS and the patch images relative to each other, the number of one-dot lines contained in the detect area differs maximum one line. When the detect area IR of the patch sensor PS and the patch image PI 2 are positioned relative to each other as shown in FIG. 19A , for example, the number of one-dot lines DL contained in the detect area IR is five, whereas the relative positions are as shown in FIG. 19B , the number of the lines is six. Hence, even though the patch sensor PS reads the same patch image PI 2 , the patch sensor PS detects different image densities in the two different situations, and the detect deviation between the two different situations is:

Detect deviation (%)=(1 /m )×100

where m denotes the number of the lines contained in the detect area IR. Thus, the larger the number of the lines m contained in the detect area IR becomes, the smaller the detect deviation becomes. This makes it possible to improve the accuracy of measurement.

For highly accurate control of densities, it is necessary to suppress the detect deviation to 5% or smaller, and therefore, it is desirable to set the number of the lines m to twenty or larger. In short, the inequality below must be satisfied:

φ·R /(1 +n )≧20

Modifying the inequality,

n≦ ( φ·R− 20)/20  (2)

Thus, if the line intervals n are set so as to satisfy the inequality (2) above, it is possible to suppress the detect deviation and detect image densities of the patch images PI 2 at an even better detect accuracy.

An actual example as described below was tried to verify the condition above regarding the line intervals. In the actual example, patch images were created while changing the line intervals n under the following conditions and voltages detected by the patch sensor PS were measured, thereby obtaining a graph as that shown in FIG. 20 :

Design resolution R: 23.6 lines/mm (600 DPI); and

Size of detect area IR of patch sensor PS φ: 8 mm

The result in the graph well matches with the condition described above regarding the line intervals.

That is, while it is necessary to set the line intervals n to two or larger in order to avoid a mutual influence between adjacent one-dot lines, as clearly seen in FIG. 20 , if the line intervals n are set to 1, it is not possible to distinguish from solid images.

On the contrary, it is desirable to set the line intervals n such that the inequality (1) above is satisfied in order to obtain a sufficient detect sensitivity. Therefore, in the actual example, it is desirable to set the line intervals n to seventeen or smaller, i.e., satisfy the following:

  n≦ (8×23.6−10)/10=17.88 (lines)

In this respect, as clearly seen in FIG. 20 , if the line intervals n are 18 or larger, it is not possible to distinguish from a blank image, and hence, it is difficult to accurately detect image densities.

Further, it is desirable to satisfy the inequality (2) described above for highly accurate detection with a suppressed detect deviation. Therefore, in the actual example, it is desirable to set the line intervals n to eight or smaller, i.e., satisfy the following:

n≦ (8×23.6−20)/20=8.44 (lines)

Thus, it is most desirable to set the line intervals n to five in the actual example.

In addition, although the patch images PI 2 are images which are obtained by arranging a plurality of one-dot lines DL parallel to each other but apart from each other at the predetermined intervals n in the preferred embodiment above, as shown in FIG. 21 , for instance, perpendicular lattice images PI 2 ′ may be used which are obtained by arranging a plurality of one-dot lines DL in the configuration of a lattice. In this case, the detect area IR of the patch sensor PS covers more lines, and hence, the detect sensitivity is better and a larger improvement is made to the accuracy as compared to where the patch images PI 2 are formed by one-dot lines which are arranged parallel to each other (See FIG. 14 ). Moreover, it is possible to widen the line intervals n, owing to the increased number of lines. Widening the line intervals particularly in the sub-scanning direction reduces an influence by an uneven density in the drive direction, which in turn allows to control while detecting more stable images. Of course, a lattice structure of patch images is not limited to a perpendicular lattice, but may be various types of lattices in which case as well a similar effect is obtained.

D. Setting of Electrifying Bias in Development Bias Calculation

By the way, when second patch images are formed while changing an electrifying bias, an exposed area potential (bright part potential) Von of a latent image sometimes largely changes as the electrifying bias changes.

FIG. 22 is a graph showing attenuation of a surface potential as a photosensitive member is exposed at various exposure powers, in which curves C(Va- 1 ), C(Va- 2 ), C(Va- 3 ) and C(Va- 4 ) express attenuation of a surface potential caused by electrification at electrifying biases Va- 1 through Va- 4 which are different from each other. In FIG. 22 , “EXPOSURE POWER” denotes a dose of exposure applied upon a photosensitive member 21 per unit area from the exposure unit 3 . As clearly shown in FIG. 22 , a surface potential in a surface area of the exposed photosensitive member 21 , namely, the exposed area potential changes in accordance with the electrifying bias and the exposure power supplied to the exposed photosensitive member 21 from the exposure unit 3 . The exposed area potential is approximately the same between the attenuation curves regardless of a value of the electrifying bias when the exposure power is relatively large. On the other hand, the exposed area potential is different in accordance with the electrifying bias when the exposure power is relatively small. Such a tendency is as already described with reference to FIGS. 15A , 15 B, 16 A and 16 B.

Hence, when the exposure power is set relatively high, even if the electrifying bias set during the development bias calculation is largely deviated from the optimal electrifying bias, a contrast potential (=development bias−surface potential) during the development bias calculation matches with a contrast potential after setting of the optimal electrifying bias. Therefore, it is possible to stably form an image at a target density by means of the optimal development bias and the optimal electrifying bias which are calculated according to the preferred embodiment above.

Conversely, when the exposure power is set relatively small, since the surface potential differs depending on the electrifying bias, it is sometimes impossible to stably form an image at a target density even despite setting the optimal development bias and the optimal electrifying bias which are calculated according to the preferred embodiment above. This is because when the electrifying bias set during the development bias calculation is largely deviated from the optimal electrifying bias, the contrast potential (=development bias−surface potential) during the development bias calculation becomes different from the contrast potential after setting of the optimal electrifying bias. With the contrast potential varied in such a manner, it is difficult to stabilize an image density.

Noting this, in a preferred embodiment described below, the electrifying bias is changed in accordance with a change in the development bias during the development bias calculation processing, to thereby solve the problem above which occurs when the exposure power is relatively small. First, a relationship between the development bias Vb and the contrast potential will be described before describing how the electrifying bias is specifically changed.

During the development bias calculation processing, as shown in FIG. 23 for instance, if the electrifying bias is fixed at a bias Va- 2 and latent images of first patch images are formed by exposing light at an exposure power P 1 , the exposed area potential of the latent images become a potential Von 1 . As the development bias Vb is changed in this condition, a contrast potential Vcon 1 changes in accordance with the change in the development bias Vb, thereby changing densities of the first patch images. Hence, during the development bias calculation according to the preferred embodiment described above, a plurality of first patch images is formed while changing only the development bias Vb and the optimal development bias is thereafter determined.

On the other hand, during the electrifying bias calculation processing, as shown in FIG. 24 for example, the electrifying bias is set to various levels while fixing the development bias to the optimal development bias Vb, and latent images of second patch images are formed by exposing light at an exposure power P 2 . The exposed area potential of the latent images becomes largely different between the different electrifying bias levels. Since second patch images are halftone images as those shown in FIG. 16 A. Hence, even though the latent images are formed with an exposure beam having the exposure power P 1 , an effective exposure power for exposure with an isolated beam is smaller than the exposure power P 1 . As a result, the lowest potential level of a comb-shaped well potential is not as low as the lowest potential level that is observed during solid exposure. Noting a macro surface potential of halftone latent images, this is the same as solid exposure at the exposure power P 2 that is smaller than the exposure power P 1 . Therefore, considering that the latent images of the second patch images are images solidly exposed at the exposure power P 2 , the exposed area potential of these latent images becomes largely different depending on the electrifying bias.

For instance, the exposed area potential becomes a potential Von 2 - 2 to generate the contrast potential Vcon 2 - 2 when the electrifying bias has the level Va- 2 , whereas when the electrifying bias has the level Va- 3 , the exposed area potential becomes a potential Von 2 - 3 to generate the contrast potential Vcon 2 - 3 . In this manner, the contrast potential Vcon 2 changes as the electrifying bias Va changes, and a density of the second patch image accordingly changes. For this reason, the electrifying bias calculation according to the preferred embodiment described above requires to form a plurality of second patch images while changing only the electrifying bias Va in order to determine an optimal electrifying bias.

If the optimal electrifying bias resulting from such electrifying bias calculation processing is different from the electrifying bias set during the development bias calculation (i.e., the electrifying bias Va- 2 in FIG. 23 ), the contrast potential Vcon 1 determined through the development bias calculation is changed. Hence, despite application of the optimal development bias, an image density may deviate from a target density. The possibility of this is high particularly when the exposure power drops.

FIG. 25 shows a relationship between the development bias Vb and the contrast potential that is identified based on the optimal attenuation curves C(Va-a) and C(Va-b). In FIG. 25 , the horizontal axis denotes the development bias Vb while the vertical axis denotes the contrast potential. Further, straight lines L(P 1 , Va-a), L(P 1 , Va-b), L(P 2 , Va-a) and L(P 2 , Va-b) respectively denote contrast potentials Vcon 1 -a, Vcon 1 -b, Vcon 2 -a and Vcon 2 -b which are shown in FIG. 26 .

When first patch images are formed with the electrifying bias Va-a, changing the development bias Vb causes proportional change in the contrast potential Vcon 1 -a as denoted at the straight line L(P 1 , Va-a) shown in FIG. 25 . Meanwhile, when first patch images are formed with the electrifying bias Va-b, changing the development bias Vb causes proportional change in the contrast potential Vcon 1 -b as denoted at the straight line L(P 1 , Va-b) shown in FIG. 25 . When second patch images are formed with the electrifying bias Va-a, changing the development bias Vb causes proportional change in the contrast potential Vcon 2 -a as denoted at the straight line L(P 2 , Va-a) shown in FIG. 25 . Further, when second patch images are formed with the electrifying bias Va-b, changing the development bias Vb causes proportional change in the contrast potential Vcon 2 -b as denoted at the straight line L(P 2 , Va-b) shown in FIG. 25 . A development bias/contrast potential characteristic is determined based on the optimal attenuation curves in this manner.

In FIG. 25 , a target contrast potential Vcon 01 corresponds to the target density during the development bias calculation processing and a target contrast potential Vcon 02 corresponds to the target density during the electrifying bias calculation processing. In order to even more accurately adjust a density, it is necessary to set the optimal development bias Vb and the optimal electrifying bias Va such that these two contrast potentials Vcon 01 and Vcon 02 are simultaneously satisfied.

According to this embodiment, during the development bias calculation processing, as shown in FIG. 27 , the development bias Vb is varied in its programmable range while at the same time changing the electrifying bias from the level Va-a to the level Va-b. As the electrifying biases Va-a and Va-b are set so that the two target contrast potentials Vcon 01 and Vcon 02 are simultaneously satisfied with approximately the same development bias Vb 0 , the optimal development bias Vb and the optimal electrifying bias Va are set at a high accuracy.

Now, as variations of the electrifying bias during the development bias calculation processing, five variations will be described. In each one of the five variations below, the electrifying bias increases as the development bias increases.

(1) First variation: FIG. 28

FIG. 28 is a drawing showing a first variation of the development bias and the electrifying bias during the development bias calculation processing. In the first variation, a quantity of change ΔVa (=Va-b −Va-a) in the electrifying bias is set equal to a quantity of change ΔVb in the development bias, and the electrifying bias Va is set to a value which is expressed as below:

Va=Vb+C

where C is a constant that is determined in accordance with a structure, operations and the like of an image forming apparatus.

(2) Second variation: FIG. 34

FIG. 34 is a drawing showing a second variation of the development bias and the electrifying bias during the development bias calculation processing. In the second variation, a quantity of change ΔVa (=Va-b−Va-a) in the electrifying bias is set smaller than a quantity of change ΔVb in the development bias. Such setup is suitable to a situation where, as shown in FIG. 30 , the exposure power P 1 during the development bias calculation processing is relatively high thereby accompanying a small change in the exposed area potential Von 1 with a change in the electrifying bias, whereas the exposure power P 2 during the electrifying bias calculation processing is relatively low thereby accompanying a large change in the potential Von 2 with a change in the electrifying bias. The reason of this will now be described with reference to FIGS. 30 through 32 .

Where an attenuation characteristic is as shown in FIG. 30 , the straight line L(P 2 , Va-a) and the straight line L(P 2 , Va-b) shown in FIG. 31 are apart relatively far from each other. Because of this, even when the electrifying bias is changed from the level Va-a to the level Va-b, the contrast potential Vcon 2 shows only a small change, thereby making it impossible sometimes to calculate appropriate values which are necessary to obtain the target contrast potential Vcon 02 .

To deal with this, the second variation requires to set an electrifying bias change ΔVa smaller than a quantity of change ΔVb in the development bias Vb. Hence, the straight line L(P 2 , Va-b) shifts closer to the straight line L(P 2 , Va-a) as shown in FIG. 32 , accompanying a large change in the contrast potential Vcon 2 . As a result, it is possible to reliably calculate appropriate values (the optimal development bias and the optimal electrifying bias) which are necessary to obtain the target contrast potential Vcon 02 .

(3) Third variation: FIG. 33

FIG. 33 is a drawing showing a third variation of the development bias and the electrifying bias during the development bias calculation processing. In the third variation, a quantity of change ΔVa (=Va-b−Va-a) in the electrifying bias is set larger than a quantity of change ΔVb in the development bias. Such setup is suitable to a situation where, as shown in FIG. 34 , the exposure power P 1 during the development bias calculation processing is relatively high thereby accompanying a small change in the exposed area potential Von 1 with a change in the electrifying bias, and the exposure power P 2 during the electrifying bias calculation processing is also relatively high thereby accompanying a small change in the potential Von 2 with a change in the electrifying bias. The reason of this will now be described with reference to FIGS. 34 through 36 .

Where an attenuation characteristic is as shown in FIG. 34 , the straight line L(P 2 , Va-a) and the straight line L(P 2 , Va-b) shown in FIG. 35 are apart relatively close to each other. In this condition, even when the electrifying bias is changed from the level Va-a to the level Va-b, the exposed area potentials Von 2 -a, Von 2 -b of second patch images shows only a small change, which arrives at virtually one optimal solution (the optimal electrifying bias). Because of this, as shown in FIG. 35 , the target contrast potential Vcon 01 of first patch images and the target contrast potential Vcon 02 of second patch images sometimes become inconsistent to each other. In short, a deviation ΔVb 0 is sometimes created between the optimal development bias Vb 0 of first patch images and the optimal development bias of second patch images.

To deal with this, the third variation requires to set the electrifying bias change Δ Va larger than a quantity of change Δ Vb in the development bias Vb (FIG. 33 ). Hence, the straight line L(P 2 , Va-b) is far from the straight line L(P 2 , Va-a) as shown in FIG. 36 , thereby expanding a range of an optimal solution. This ensures consistency between the target contrast potential Vcon 01 of first patch images and the target contrast potential Vcon 02 of second patch images.

(4) Fourth variation: FIG. 38

It is desirable to set the electrifying bias in accordance with a change in the development bias such that a development bias Vb 01 satisfying the target contrast potential Vcon 01 and a development bias Vb 02 satisfying the target contrast potential Vcon 02 become approximately equal to each other, as described above. However, depending on a process of forming images, as described earlier, it is difficult in some cases to match the development biases Vb 01 and Vb 02 with a linear change in the electrifying bias. For example, when the electrifying bias is changed according to the first variation (FIG. 28 ), the development bias Vb 02 sometimes becomes smaller than the development bias Vb 01 as shown in FIG. 37 to thereby create a deviation ΔVb 0 to the development bias. When this occurs, the electrifying bias may be changed logarithmically as shown in FIG. 38 , which moves the development bias Vb 02 which satisfies the target contrast potential Vcon 02 closer to the development bias Vb 01 which satisfies the target contrast potential Vcon 01 so that the two development biases Vb 01 and Vb 02 approximately match with each other (FIG. 39 ).

(5) Fifth variation: FIG. 41

When the electrifying bias is changed according to the first variation (FIG. 28 ), the development bias Vb 02 sometimes becomes larger than the development bias Vb 01 as shown in FIG. 40 , creating a deviation ΔVb 0 to the development bias. When this occurs, the electrifying bias may be changed exponentially as shown in FIG. 41 , which moves the development bias Vb 02 which satisfies the target contrast potential Vcon 02 closer to the development bias Vb 01 which satisfies the target contrast potential Vcon 01 so that the two development biases Vb 01 and Vb 02 approximately match with each other (FIG. 42 ).

E. Other

The present invention is not limited to the preferred embodiment above, but can be modified in various manners other than those described above without departing from the essence of the present invention. For example, although the foregoing requires to use the electrifying roller 22 as the electrifying means, an electrifying brush may be used. The present invention is also applicable to an image forming apparatus in which non-contact electrifying means electrifies the photosensitive member 21 , instead of an image forming apparatus utilizing such contact electrification in which a conductive member, such as an electrifying roller and an electrifying brush, touches a surface of a photosensitive member 21 for electrification.

Further, while the patch images PI 1 are formed as clusters in each color as shown in FIGS. 8A through 8D in the preferred embodiment described above, the patch images PI 1 may be formed in each color in turn as shown in FIGS. 43A through 43D . More specifically, first, yellow patch images PI 1 (Y) are formed on the intermediately transfer belt 41 at relatively wide intervals. Next, cyan patch images PI 1 (C) are formed one by one, starting at a position which is shifted by one patch image and a blank between the adjacent-patch images in the sub scanning direction (the right-hand side in FIGS. 43A through 43D ) as viewed from the yellow patch images PI 1 (Y). Following this, magenta patch images PI 1 (M) and black patch images PI 1 (K) are formed in a similar manner. Where the respective patch images are thus formed at relatively wide intervals, it is possible to ensure a stabilization time for switching of the biases, and hence, to form the respective patch images at the set biases without fail. Although the description immediately above is related to first patch images, the same directly applies to second patch images as well.

Further, while the preferred embodiment above is related to an image forming apparatus which is capable of forming a color image using toners in four colors, an application of the present invention is not limited to this. The present invention is naturally applicable to an image forming apparatus which forms only a monochrome image as well. In addition, although the image forming apparatus according to the preferred embodiment above is a printer for forming an image supplied from an external apparatus such as a host computer through the interface 112 on a sheet such as a copying paper, a transfer paper, a form and a transparent sheet for an over-head projector, the present invention is applicable to image forming apparatuses of the electrophotographic method in general such as a copier machine and a facsimile machine.

Further, in the preferred embodiment above, toner images on the photosensitive member 21 are transferred onto the intermediate transfer belt 41 , image densities of patch images formed by said toner images are detected, and an optimal development bias and an optimal electrifying bias are thereafter calculated based on the detected image densities. However, the present invention is also applicable to an image forming apparatus in which a toner image is transferred onto other transfer medium except for the intermediate transfer belt 41 , to thereby form a patch image. In this case, the patch sensor is disposed at a position PS 2 as shown in FIG. 1 , to detect a density of a patch image formed on the other transfer medium. The other transfer medium includes a transfer drum, a transfer belt, a transfer sheet, an intermediate transfer drum, an intermediate transfer sheet, a reflection-type recording sheet, a transmission memory sheet, etc. Further, instead of forming a patch image on a transfer medium, a patch sensor may be disposed at a position PS 1 as shown in FIG. 1 , so as to detect a density of a patch image which is formed on a photosensitive member. In this case, the patch sensor detects image densities of patch images on the photosensitive member and an optimal development bias and an optimal electrifying bias are calculated based on the detected image densities.

Further, in the preferred embodiment above, the RAM 127 of the engine controller 12 stores an optimal development bias and an optimal electrifying bias. Hence, when the main power source of the image forming apparatus is turned off, the contents stored in the RAM 127 disappear. When the main power source is turned on once again, the image forming apparatus recognizes the current development bias calculation and the current electrifying bias calculation as “the first”calculation and executes processing in accordance with this recognition. Instead of this, a nonvolatile memory such as an EEPROM may be used to store an optimal development bias and an optimal electrifying bias which are calculated in sequence, so that as the main power source is turned on once again, the processing for “the second or subsequent” calculation is executed during the development bias calculation and the electrifying bias calculation.

Further, although the optimal development bias is determined in the two-stage calculation during the development bias calculation processing after it is judged that it is the “FIRST TIME” in the preferred embodiment described above, the optimal development bias may be calculated only through the bias calculation processing in the wide range (step S 312 ) alone.

Further, the narrow range is defines as approximately ⅓ of the programmable range (Vb 01 -Vb 10 ) of development bias in the preferred embodiment above. Although the width of the narrow range is not limited to this, if the width of the narrow range is wide, the use of the narrow range becomes less meaningful and degrades the accuracy of calculation of an optimal development bias. For this reason, it is necessary to set the narrow range as approximately ½ of or narrower than the programmable range for development bias. This also applies to the narrow range for electrifying biases as well.

Further, although the four types of biases are set in the wide and the narrow ranges in the preferred embodiment described above, the number of bias values (the number of patch images) in the range is not limited to this but may be optional to the extent that more than one types of bias values are used. Alternatively, the number of bias values may be different between the wide range and the narrow range such that the number of patch images is different between the wide range and the narrow range.

Further, while the first patch images are each a solid image whose area ration is 100% in the preferred embodiment above, an image whose area ratio is approximately 80% or more may be used instead of using a solid image. Even when such an image is used as the first patch images, a similar effect to that promised when solid images are used is obtained. The term “area ratio” refers to a ratio of dots to the area of a patch image as a whole.

Further, although the preferred embodiment above requires to change an electrifying bias which is supplied to the electrifying roller 22 as a density controlling factor to sequentially form patch images PI 2 , PI 2 ′, other density controlling factor may be used, i.e., patch images of more than one one-dot lines may be formed while changing a development bias, an exposure dose, etc. In such a modification as well, as densities of the patch images are detected and an optimal value which is needed to achieve a target density is determined based on the detected image densities, it is possible to stabilize an image density of a line image.

Further, in the preferred embodiment above, after executing the development bias calculation (step S 3 ), the electrifying bias calculation (step S 5 ) is further executed, in order to calculate an optimal development bias and an optimal electrifying bias. However, the manner in which an optimal development bias and an optimal electrifying bias are calculated is not limited to this. For example, a plurality of patch images may be formed while changing the development bias and the electrifying bias at the same time, so that an optimal development bias and an optimal electrifying bias are calculated based on image densities of the patch images and density adjustment is executed. In this case, memory means such as a RAM and a ROM stores the development bias and the electrifying bias for every density adjustment and the memory means reads out the most recent development bias and the most recent electrifying bias in preparation for the next density adjustment. The plurality of patch images are formed while changing the development bias and the electrifying bias at the same time based on the most recent development bias and the most recent electrifying bias. This realizes a similar effect to that according to the preferred embodiment above. Still further, the present invention is applicable to where calculation of an optimal development bias is executed first and an optimal electrifying bias is thereafter calculated followed by density adjustment, in which case as well it is possible to achieve a similar effect to that described above.

Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiment, as well as other embodiments of the present invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.

Claims

1. An image forming apparatus for forming an image which has a predetermined target density, comprising:a photosensitive member; electrifying means for electrifying a surface of said photosensitive member; exposing means for forming an electrostatic latent image on the electrified surface of said photosensitive member; developing means for visualizing said electrostatic latent image with a toner to form a toner image; transferring means for transferring the toner image from said photosensitive member to a transfer medium; density detecting means for detecting an image density of a patch image on said photosensitive member; and control means for adjusting the image density of said toner image to a predetermined target density, based on a result of a detection determined by said density detecting means, wherein said patch image is formed by a plurality of one-dot lines which are spaced apart from each other by at least a two line interval.

2. An image forming method comprising:electrifying a surface of a photosensitive member; forming an electrostatic latent image on the electrified surface of said photosensitive member; developing said electrostatic latent image with a toner to form a toner image which has a predetermined target density; sequentially forming a plurality of toner images as patch images while changing a density controlling factor which influences an image density of the toner image; detecting the densities of said patch images; and determining an optimal density controlling factor necessary to control and adjust said predetermined target density based on the densities of said patch images, wherein each of said patch images is a plurality of one-dot lines spaced apart from each other by at least a two line interval.

3. An image forming apparatus for forming an image which has a predetermined target density, comprising:a photosensitive member; electrifying means for electrifying a surface of said photosensitive member; exposing means for forming an electrostatic latent image on the electrified surface of said photosensitive member; developing means for visualizing said electrostatic latent image with a toner to form a toner image; transferring means for transferring the toner image from said photosensitive member to a transfer medium; density detecting means for detecting an image density of a patch image on said transfer medium; and control means for adjusting the image density of said toner image to a predetermined target density, based on a result of a detection determined by said density detecting means, wherein said patch image is formed by a plurality of one-dot lines which are spaced apart from each other by at least a two line interval.

4. The image forming apparatus according to claim 3, wherein said transfer medium is one of a transfer drum and a transfer belt.

5. The image forming apparatus according to claim 3, wherein said transfer medium is an intermediate transfer medium.

6. The image forming apparatus according to claim 3, wherein said transfer medium is a sheet.