Image forming apparatus

An image forming apparatus includes a density detector, a determination unit, and a change unit. The density detector detects a density of a patch image formed on an image carrier over plural places. The determination unit determines whether plural detection results of the density detector are within a threshold. When the number of determination results, by the determination unit, that the density of the patch image is not within the threshold exceeds a predetermined number, the change unit changes (i) a detection timing at which the patch image is detected by the density detector or (ii) a formation position of the patch image on the image carrier.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2019-181464 filed Oct. 1, 2019.

BACKGROUND

1. Technical Field

2. Related Art

In the related art, an image forming apparatus is configured to form a patch image on an image carrier and control an image density by detecting a density of the patch image in order to appropriately maintain the image density (JP-B-4820067 and the like). JP-B-4820067 discloses a technique in which only a patch pattern detection result of a central portion area of the patch pattern is selected from among patch pattern detection results stored in a storage unit from a positional deviation amount of the patch pattern and an image density of the patch pattern is calculated by a density calculation unit.

SUMMARY

Aspects of non-limiting embodiments of the present disclosure relate to securing the number of patch images effective for density detection even when a margin set before and after a detection area of the patch image is narrowed, as compared with a case where only a detection result of a central portion area is selected from among detection results of the patch image stored in a storage unit based on a positional deviation amount of the patch image to calculate an image density of the patch image.

According to an aspect of the present disclosure, there is provided an image forming apparatus including a density detector, a determination unit, and a change unit. The density detector detects a density of a patch image formed on an image carrier over plural places. The determination unit determines whether plural detection results of the density detector are within a threshold. When the number of determination results, by the determination unit, that the density of the patch image is not within the threshold exceeds a predetermined number, the change unit changes (i) a detection timing at which the patch image is detected by the density detector or (ii) a formation position of the patch image on the image carrier.

DETAILED DESCRIPTION

First Exemplary Embodiment

FIG. 1illustrates an image forming apparatus according to a first exemplary embodiment.

Overall Configuration of Image Forming Apparatus

An image forming apparatus1is a full-color printer that finally forms an image with toner on recording paper9based on image information composed of characters, photographs, figures, and the like by adopting an electrophotographic process. The recording paper9is an example of a recording medium. In the image forming apparatus1, as illustrated inFIG. 1, an image forming device20as an example of an image forming unit that forms a toner image with toner as a developer, an intermediate transfer device30that holds the toner image formed by the image forming device20by being subjected to primary transfer, and then transports the toner image to a secondary transfer position where the toner image is secondarily transferred onto the recording paper9, a paper feeding device40that accommodates and supplies the recording paper9to be supplied to the secondary transfer position of the intermediate transfer device30, and a fixing device50that fixes the toner image secondarily transferred by the intermediate transfer device30to the recording paper9are disposed. A thick solid line illustrated inFIG. 1is a transport path of the recording paper9.

The image forming device20includes four image forming devices20Y,20M,20C and20K that individually form developer (toner) images of four colors of yellow (Y), magenta (M), cyan (C), and black (K). As illustrated inFIG. 1, each of the image forming devices20(Y, M, C, and K) include a photoconductor drum21that is driven to rotate in a direction indicated by an arrow A. The photoconductor drum21is an example of an image carrier. Around each photoconductor drum21, a charging device22, an exposure device23, a developing device24, a primary transfer device25, a drum cleaning device26, and the like are disposed. The reference numerals of the photoconductor drum21and the members disposed around the photoconductor drum21are assigned only to image forming device20K of black (K), and are omitted in the other image forming devices20(Y, M, and C).

The photoconductor drum21is, for example, a drum-shaped photoconductor in which an image-forming surface having a photo-dielectric layer (photoconductive layer) made of a photosensitive material such as OPC is formed on a peripheral surface of a cylindrical or columnar conductive base material to be grounded. The photoconductor drum21is rotatably driven in a direction indicated by the arrow A by receiving power from a driving device (not illustrated).

As the charging device22, for example, a non-contact type charging device such as a scorotron, which is disposed in a non-contact state on an image-forming surface of the photoconductor drum21and is supplied with required charging bias of a negative polarity, is used. The charging device22is not limited to a non-contact type charging device such as a scorotron, but may use a contact type charging device provided with a charging roller.

The exposure device23is, for example, a non-scanning type exposure device configured using a light emitting diode and an optical component, or a scanning type exposure device configured using an optical component such as a semiconductor laser and a polygon mirror. The exposure device23receives each image information, which is input from the outside through a communication unit, an image reading device, or the like, or image information stored in an internal storage, as an image signal separated into respective color components (Y, M, C, and K) of yellow (Y), magenta (M), cyan (C), and black (K) after being subjected to required processing by an image controller110. The exposure device23performs exposure according to the input image signal.

The developing device24is, for example, a developing device24(Y, M, C, and K) that uses a two-component developer containing a toner of one of the four colors (Y, M, C, and K) and a magnetic carrier. The developing device24(Y, M, C, and K) is used, for example, to charge the toner to a negative polarity to perform reversal development. The developing device24(Y, M, C, and K) includes a developing roller241that is stored in a casing, holds the two-component developer, and rotates so as to transport the two-component developer to a developing area facing the photoconductor drum21. The developing roller241is an example of a developer holding unit. The developing roller241is supplied with the developing bias in which, for example, a DC component is superimposed on an AC component between the developing roller241and the photoconductor drum21.

Each of the developing devices24(Y, M, C, and K) of yellow (Y), magenta (M), cyan (C), and black (K) is supplied with a toner of the corresponding color from the toner cartridge242containing the corresponding color toner by rotating a supply motor243as a developer supply driving unit. The drive timing and the drive time of each supply motor243are controlled by a drive controller120. In each of the developing device24(Y, M, C, and K), appropriate supply of the corresponding color toner from the toner cartridge242changes the toner concentration in the developing device24(Y, M, C, and K). The image controller110and the drive controller120are implemented by, for example, a controller101of a control device100described later.

The primary transfer device25is, for example, disposed so as to be in contact with an image-forming surface portion (in a state via an intermediate transfer belt31described later), which is a primary transfer position of the photoconductor drum21and to be driven to rotate, and is a contact type transfer device including a primary transfer roller to which a required primary transfer bias is supplied.

The drum cleaning device26is disposed so as to be in contact with at least the image-forming surface portion of the photoconductor drum21after the primary transfer at an opening for clean work of the casing, and includes a cleaning member such as an elastic plate that scrapes and removes unnecessary matters such as residual toner on the image-forming surface.

The intermediate transfer device30is disposed at a position below the four image forming devices20(Y, M, C, and K). The intermediate transfer device30includes an intermediate transfer belt31disposed that rotates in a direction indicated by an arrow B while passing through primary transfer positions facing the primary transfer device25of the photoconductor drum21respectively in the image forming device20(Y, M, C, and K). The intermediate transfer belt31is an example of an intermediate transfer unit (image carrier). The intermediate transfer belt31is an image carrier that holds a patch image200described later formed by each of the image forming devices20(Y, M, C, and K) of yellow (Y), magenta (M), cyan (C), and black (K).

The intermediate transfer belt31is formed into an endless belt shape having a required thickness and electric resistance value by using a material obtained by dispersing a resistance adjusting agent such as carbon black on a base material such as polyimide resin or polyamide-imide resin.

The intermediate transfer belt31is stretched around plural support rollers32ato32cand is rotatably supported. The support roller32aserves as a driving roller. The support roller32bis configured as a sensor roller that supports the intermediate transfer belt31wound around an outer peripheral surface thereof and detects a patch image as a density control image formed on the intermediate transfer belt31by a density sensor60. The density sensor60is an example of a density detector. The support roller32cis configured as a secondary transfer backup roller. The density sensor60is disposed at a position facing the surface of the intermediate transfer belt31, and is disposed downstream of the downstreammost image forming device20K of black (K) in a moving direction of the intermediate transfer belt31among the four image forming devices20(Y, M, C, and K) of yellow (Y), magenta (M), cyan (C), and black (K).

FIG. 2is a configuration diagram illustrating an optical density sensor.

As illustrated inFIG. 2, the density sensor60includes a light emitting element61made of an LED or the like, a light shielding member62that irradiates an exposure position on the intermediate transfer belt31with the light flux, which is emitted from the light emitting element61, to be focused in a substantially circular shape, a first condensing lens63that collects specularly reflected light from the patch image200formed on the intermediate transfer belt31, a first ultraviolet light cut filter64that blocks ultraviolet light, a first light receiving element65implemented by a photodiode, a phototransistor, and the like that receives the specularly reflected light from the patch image200formed on the intermediate transfer belt31through the first condensing lens63and the first ultraviolet light cut filter64, a second condensing lens66that collects diffuse reflection light from the patch image200formed on the intermediate transfer belt31, a second ultraviolet light cut filter67that blocks ultraviolet light, and a second light receiving element68implemented by a photodiode, a phototransistor, and the like that receives the diffuse reflection light from the patch image200formed on the intermediate transfer belt31through the second condensing lens66and the second ultraviolet light cut filter67. An incident angle of the light emitting element61is set to about 80 degrees with respect to the patch image200on the intermediate transfer belt31, for example. Further, a light receiving angle of the first light receiving element65is about 80 degrees in order to receive specularly reflected light from the patch image200on the intermediate transfer belt31. On the other hand, the light receiving angle of the second light receiving element68is set to about 30 degrees, for example, to receive diffuse reflection light from the patch image200on the intermediate transfer belt31. The detection signal from the density sensor60is input to the image controller110as illustrated inFIG. 1. The detection signal (sensor value) of the density sensor60is obtained by subjecting output signals from the first and second light receiving elements65and68to required arithmetic processing, and decreases as the toner amount of the patch image200increases.

The intermediate transfer device30includes a secondary transfer device33that secondarily transfers the toner image, which is transferred on the intermediate transfer belt31, to the recording paper9, and a belt cleaning device34that removes and cleans unnecessary matters such as the toner remaining on and attached to the image carrying surface on the outer peripheral surface of the intermediate transfer belt31, and the like. The secondary transfer device33is an example of a transfer unit. The belt cleaning device34is an example of a cleaner for the intermediate transfer device30.

As the secondary transfer device33, as illustrated inFIG. 1, for example, a contact type transfer device including a secondary transfer roller331, which is disposed to rotate in contact with an image carrying surface portion supported by the support roller32cof the intermediate transfer belt31during normal image-forming, is employed. A secondary transfer voltage having the same polarity or opposite polarity to a charged polarity of the toner is applied to the support roller32cor the secondary transfer roller331that supports the intermediate transfer belt31from the back surface by a power supply (not illustrated). The support roller32cor the secondary transfer roller331that supports the intermediate transfer belt31from the back surface is configured so that the secondary transfer roller331can be brought into contact with and separated from the intermediate transfer belt31by a contact and separation unit35. When a patch image is formed on the intermediate transfer belt31, the secondary transfer roller331is separated from the intermediate transfer belt31by the contact and separation unit35.

The belt cleaning device34is disposed so as to be in contact with at least the image carrying surface portion of the intermediate transfer belt31after the secondary transfer at the opening for cleaning work of the casing, and includes a cleaning member such as an elastic plate that scrapes and cleans unnecessary matters such as residual toner on the image carrying surface.

The paper feeding device40is disposed at a position below the intermediate transfer device30. The paper feeding device40includes a storage body41in which the recording paper9of a required size, type, and the like is stored in a stacked state on a loading plate (not illustrated), and a delivery device42that delivers the recording paper9one by one from the storage body41toward the paper feeding transport path. The number of the storage bodies41and the delivery devices42is increased or decreased as needed.

The recording paper9can be applied to any recording medium which can be transported by a transport path and onto which a toner image can be transferred and fixed. Examples of the recording paper9include thin paper such as plain paper and tracing paper used in an electrophotographic copier and a printer, or an OHP sheet. In order to further improve smoothness of the image surface after fixing, the surface of the recording paper9is desirably as smooth as possible, and, for example, so-called thick paper having a relatively large basis weight, such as coated paper in which the surface of plain paper is coated with a resin or the like, art paper for printing, and the like can be suitably used.

The fixing device50is disposed below the intermediate transfer device30in the transport direction of the recording paper9near the secondary transfer position. The fixing device50is installed with a heating rotating body52having a shape of a roller or a belt that rotates inside the casing51in a direction indicated by an arrow and is heated by a heating unit so that the surface temperature is maintained at a predetermined temperature, and a pressurizing rotating body53in the form of a roller or a belt that is driven and rotated in contact with a predetermined pressure in a state of running substantially along the axial direction of the heating rotating body52. In the fixing device50, a portion where the heating rotating body52and the pressurizing rotating body53are in contact is configured as a fixing processing unit where the recording paper9holding the toner image is introduced and is subjected to fixing processing (heating and pressing).

The image forming apparatus1is provided with a supply transport path RT1connecting between the paper feeding device40and the intermediate transfer device30, a relay transport path RT2connecting between the secondary transfer position of the intermediate transfer device30and the fixing device50, and an output transport path RT3connecting between the fixing device50and a paper output port (not illustrated) as paper transport paths for the recording paper9.

In the supply transport path RT1, one or plural paper transport roller pairs43for transporting the recording paper9supplied from the paper feeding device40to the secondary transfer position, a transporting guide (not illustrated), and the like are disposed. The paper transport roller pair43disposed adjacent to the intermediate transfer belt31upstream of the secondary transfer position is configured as a registration roller that transports the recording paper9in synchronization with an image on the intermediate transfer belt31.

Basic Image Forming Operation by Image Forming Apparatus

In the image forming apparatus1, a basic image forming operation described below is performed. Here, an operation in the case of forming a full-color image formed by combining toner images of four colors (Y, M, C, K) will be described as an example.

First, in the image forming apparatus1, as illustrated inFIG. 1, when the control device100(seeFIG. 5) receives a request command for an image forming operation from outside or the like, each photoconductor drum21is driven to rotate in the direction indicated by arrow A and each charging device22receives a supply of a charging current and generates a corona discharge, in the four image forming devices20(Y, M, C, and K). With this configuration, the image-forming surface of each photoconductor drum21is charged to a required polarity (for example, a negative polarity) and a potential.

Subsequently, each exposure device23performs exposure on the image-forming surface of each photoconductor drum21after charging according to the image signal decomposed into each of the color components (Y, M, C, and K). With this configuration, an electrostatic latent image of each color component having a predetermined potential is formed on the image-forming surface of each photoconductor drum21.

Subsequently, each of the developing devices24(Y, M, C, and K) supplies the toner of each of colors (Y, M, C, and K) charged to a predetermined polarity (negative polarity) from a developing roller241, and electrostatically adheres the toner to an electrostatic latent image portion of each color component on the image-forming surface of the photoconductor drum21by a developing electric field formed between the developing roller241and the photoconductor drum21by receiving the supply of a developing bias. With this configuration, a toner image of a corresponding color among the four colors (Y, M, C, and K) is individually formed on the image-forming surface of each photoconductor drum21.

Subsequently, each primary transfer device25receives the supply of the primary transfer current and forms a primary transfer electric field between the primary transfer device25and the photoconductor drum21, so that the toner image on each photoconductor drum21is primarily transferred to the image carrying surface of the intermediate transfer belt31in the intermediate transfer device30in order (in the order of Y, M, C, and K). The drum cleaning device26cleans the image-forming surface of each photoconductor drum21after the primary transfer or the like, and prepares for the next image creating operation in each photoconductor drum21.

Next, in the intermediate transfer device30, an unfixed toner image primarily transferred and held on the image carrying surface is transported to a secondary transfer position facing the secondary transfer device33by rotating the intermediate transfer belt31in the direction indicated by the arrow B. On the other hand, in the paper feeding device40, after the delivery device42delivers the recording paper9from the storage body41to the supply transport path RT1, the paper transport roller pair43supplies the recording paper9to the secondary transfer position of the intermediate transfer device30. Then, at the secondary transfer position, the secondary transfer device33receives the supply of the secondary transfer bias and forms a secondary transfer electric field between the secondary transfer device33and the intermediate transfer belt31, so that the toner images of four colors on the intermediate transfer belt31are secondarily transferred to one side of the recording paper9.

Next, the recording paper9on which the unfixed toner image is secondarily transferred is transported so as to be delivered to the fixing device50via the relay transport path RT2after being separated from the intermediate transfer belt31. In the fixing device50, the recording paper9is heated and pressurized when the recording paper9is introduced into and passed through the fixing processing unit which is a contact portion between the heating rotating body52and the pressurizing rotating body53. With this configuration, the toner configuring the toner image is melted under pressure, and the toner image is fixed onto the recording paper9.

Subsequently, the recording paper9after the toner image is fixed is output from the inside of a casing51of the fixing device50, is transported via the output transport path RT3, and finally is output from a paper output port (not illustrated) to the outside.

By performing the operations described above, one recording paper9on which a full-color image is formed is output. When a request command for plural image forming operations is received, the image forming operation is repeatedly performed by the number of sheets in the same manner.

In addition, the image forming apparatus1can form a monochromatic image by operating any one of the four image forming devices20(Y, M, C, and K) or a color image other than a full-color image can be formed by operating two or three of the four image forming devices20(Y, M, C, and K) in combination, in the image forming operation.

Configuration of Feature Portions of Image Forming Apparatus

In the image forming apparatus1configured as described above, as illustrated inFIG. 3, in order to maintain color stability and the like of an image printed on the recording paper9constant, it is necessary to form the patch image200on the intermediate transfer belt31, detect the patch image200by the density sensor60, and accurately calculate the density of the patch image200.

By the way, in order to detect the density of the patch image200formed on the intermediate transfer belt31by the density sensor60, it is necessary to primarily transfer the patch images200Y,200M,200C, and200K of corresponding colors formed by the image forming devices20(Y, M, C, and K) of yellow (Y), magenta (M), cyan (C), and black (K) onto the intermediate transfer belt31, and to detect the density of the patch image200by the density sensor60at the timing when the patch image200of each color of yellow (Y), magenta (M), cyan (C), and black (K) is moved to the position of the density sensor60.

In order to improve productivity which is the number of recording paper9on which an image can be printed per unit time in the image forming apparatus1, as illustrated inFIG. 3, a configuration in which, while forming an image in an image area300set in advance on the intermediate transfer belt31, the patch images200Y,200M,200C,200K of respective colors of yellow (Y), magenta (M), cyan (C), and black (K) are sequentially formed one by one in a non-image area301provided between two adjacent image areas300and the patch images200Y,200M,200C,200K are detected by the density sensor60, is adopted.

The detection results of the patch images200Y,200M,200C, and200K by the density sensor60are used immediately for controlling the image density. For that reason, it is desirable to set the density of the patch image200to alternately change between the high density side and the low density side such that, as the patch images200Y,200M,200C, and200K of colors of yellow (Y), magenta (M), cyan (C), and black (K), for example, the first set of patch images200Y,200M,200C, and200K has an image density of Cin=80%, the second set of patch images200Y,200M,200C, and200K has an image density of Cin=40%, the third set of patch images200Y,200M,200C, and200K has an image density of Cin=60%, and the fourth set of patch images200Y,200M,200C, and200K has an image density of Cin=20%.

As illustrated inFIG. 4A, when detecting the density of the patch image200, the density sensor60starts density detection from a detection start position positioned a predetermined distance L2inward from a tip that is a downstream end of the patch image200in the moving direction B of the intermediate transfer belt31, and detects the density of the patch image200at plural places (for example, about 20 to 30 points) at predetermined time intervals ΔT (corresponding to the distance ΔL).

In the first exemplary embodiment, as illustrated inFIG. 4A, the distance L2from the downstream end of the patch image200in the moving direction B of the intermediate transfer belt31to the detection start position is shorter than in the related art and an upstream margin in the moving direction B of the intermediate transfer belt31of the patch image200is set relatively narrow. In the related art, the distance L2from the downstream end of the patch image200in the moving direction B of the intermediate transfer belt31to the detection start position is set to be substantially the same as the detection area of the patch image200. Incidentally, in the related art, the distance L1from the upstream end of the patch image200in the moving direction B of the intermediate transfer belt31to the detection end position is also set to be substantially the same as the detection area of the patch image200.

At this time, a shift may occur in the detection timing of the patch image200by the density sensor60, for example, (i) there is a mounting error of which magnitude is a tolerance or greater at a mounting position of the density sensor60disposed on downstream of the image forming device20K of black (K) in the moving direction B of the intermediate transfer belt31in the image forming apparatus1, (ii) the control device100(seeFIG. 5) that controls the detection timing at which the density sensor60detects the patch image200on the intermediate transfer belt31is affected by another program processing or noise, and the like.

As described above, when the shift in the detection timing of the patch image200by the density sensor60occurs, as illustrated inFIG. 4B, in the case where the detection timing of the patch image200by the density sensor60deviates to the downstream in the moving direction B of the intermediate transfer belt31of the patch image200, there is a possibility that the density of the patch image200cannot be accurately detected over plural predetermined detection places, such as erroneously detecting the surface of the intermediate transfer belt31other than the patch image200as the patch image200.

The image forming apparatus1according to the first exemplary embodiment includes a determination unit that determines whether or not plural detection results of the density sensor60are within a predetermined threshold and a change unit. When the number of determination results, by the determination unit, that the density of the patch image is not within the threshold exceeds a predetermined number, the change unit changes (i) a detection timing at which the patch image200is detected by the density sensor60or (ii) a formation position of the patch image200on the intermediate transfer belt31.

Configuration of Control Device

FIG. 5is a block diagram illustrating a control device of the image forming apparatus according to the first exemplary embodiment.

A control device100includes a controller101that functions as an example of a determination unit and a change unit. The controller101comprehensively controls the operation of the image forming apparatus1including a control operation of an image density based on a detected density of the patch image200by executing a program stored in a storage (not illustrated). The controller101executes a forming operation of a color image and the like and a control operation of an image density by controlling a developer supply driving unit243, the charging device22, the exposure device23, the developing device24, and the like in each of the image forming devices20(Y, M, C, and K) of yellow (Y), magenta (M), cyan (C), and black (K).

The controller101is connected to an image acquisition unit102that acquires image information from outside and an image processor103that performs required image processing on the image information acquired by the image acquisition unit102.

Density detection data of the patch image200is input to the controller101from the density sensor60as an image density detector.

When it is determined that it is the timing to execute the image density control operation, the controller101generates a timing signal for image density control. The timing at which the image density control operation is to be performed is, for example, when the power of the image forming apparatus1is turned on, when an image is printed on a predetermined number of recording paper9, or when a jam occurs on the recording paper9.

When the image density control timing signal is generated, the controller101outputs a patch generation signal for generating the patch image200, and forms the patch image200of a corresponding color in each of the image forming devices20(Y, M, C, and K) of yellow (Y), magenta (M), cyan (C), and black (K). Although the patch image200of each color of yellow (Y), magenta (M), cyan (C) and black (K) is formed over plural densities, here, a case where the patch image200is formed with only one density will be described.

The controller101is configured the following manner. When the density sensor60detects the density of the patch image200of each color of yellow (Y), magenta (M), cyan (C), and black (K), the controller101determines whether or not plural detection results of the density sensor60are within a predetermined threshold. When it is determined that the number of the determination results that the density of the patch image is not within the threshold exceeds a predetermined number, the controller101changes (i) the detection timing at which the patch image200is detected by the density sensor60or (ii) the formation position of the patch image200on the intermediate transfer belt31.

Operation of Feature Portions of Image Forming Apparatus

In the following manner, the image forming apparatus1according to the first exemplary embodiment secures the number of patch images effective for density detection even when the margin set before and after the detection area of the patch image is narrowed, as compared with a case where only the detection result of the central portion area is selected from among the detection results of the patch image stored in the storage unit based on the positional deviation amount of the patch image to calculate the image density of the patch image.

That is, in the image forming apparatus1according to the first exemplary embodiment, as illustrated inFIG. 6, when the controller101of the control device100determines that the timing is the control timing of the image density, the image density control operation is executed. In this image density control operation, an operation of creating a patch image200of each color of yellow (Y), magenta (M), cyan (C) and black (K), and an operation of detecting the patch image200of each color by the density sensor60and calculating the image density of the patch image200of each color are executed.

The controller101of the control device100creates the patch images200Y,200M,200C, and200K composed of toner images of the corresponding colors in the image forming devices20(Y, M, C, and K) of yellow (Y), magenta (M), cyan (C), and black (K) (step S101).

Next, the controller101detects densities of the patch images200Y,200M,200C, and200K of respective colors formed by the image forming devices20(Y, M, C, and K) of yellow (Y), magenta (M), cyan (C), and black (K) by the density sensor60and collect density data of the patch images200Y,200M,200C, and200K of respective colors (step S102).

Thereafter, the controller101determines whether or not there are one or more measurement points in which the density is a predetermined threshold or greater, based on the density data of the patch images200Y,200M,200C, and200K of respective colors (step S103). Here, determining whether or not there is a measurement point in which the density is a predetermined threshold or greater is made, as illustrated inFIG. 2, by detecting the densities of the patch images200Y,200M,200C, and200K by the density sensor60by detecting the specularly reflected light and the diffuse reflection light from the intermediate transfer belt31on which the patch images200Y,200M,200C, and200K are formed by the first and second light receiving elements65and68. For that reason, this is because, when the density sensor60detects the surface of the intermediate transfer belt31without detecting the patch images200Y,200M,200C, and200K, the component of the specularly reflected light from the surface of the intermediate transfer belt31increases, and the output of the density sensor60deviates in the increasing direction. Here, instead of determining whether there is a measurement point in which the density is a predetermined threshold or greater, a configuration in which whether or not there is a measurement point in which the density is a predetermined first threshold or greater and in which the density is a predetermined second threshold or less is determined may also be adopted.

As illustrated inFIG. 7A, when it is determined that there is no measurement point in which the density is the predetermined threshold or greater among the density data of the patch images200Y,200M,200C, and200K of respective colors and all of the measurement points are less than the predetermined threshold, the controller101calculates the image densities of the patch images200Y,200M,200C, and200K (step S104), and ends density calculation processing of the patch images.

The calculation of the image densities of the patch images200Y,200M,200C, and200K is performed by calculating an average value of the density data excluding the maximum value and the minimum value among the density data of the patch images200Y,200M,200C, and200K of respective colors.

On the other hand, when it is determined that there is at least one measurement point in which the density is a predetermined threshold or greater among the density data of the patch images200Y,200M,200C, and200K of respective colors, as illustrated inFIG. 7B, the controller101determines whether or not the patch image in which the number of measurement points in which the density is the threshold or greater is five points or greater and the threshold is exceeded at five points or more is continuous for all colors of yellow (Y), magenta (M), cyan (C), and black (K) (step S105).

Then, when it is determined that the condition that the patch image in which the number of measurement points in which the density is the threshold or greater is five points or greater and the threshold is exceeded at five points or more is continuous for all colors of yellow (Y), magenta (M), cyan (C), and black (K) is not satisfied, the controller101deletes the density data of the measurement points in which the density is the threshold or greater (step S106), and calculates the image densities of the patch images200Y,200M,200C, and200K (step S104).

Here, the calculation of the image densities of the patch images200Y,200M,200C, and200K is performed by obtaining the average value of the other density data excluding the maximum value and the minimum value, as described above, after deleting the density data of the measurement points in which the density is the threshold or greater from among the density data of the patch images of respective colors. Here, the average value of the other density data may be immediately obtained without excluding the maximum value and the minimum value, after deleting the density data of the measurement points in which the density is the threshold or greater from among the density data of the patch images of respective colors.

When it is determined that the patch image in which the number of measurement points in which the density is the threshold or greater is five points and the threshold is exceeded at five points or more is continuous for all colors of yellow (Y), magenta (M), cyan (C), and black (K), the controller101executes processing for changing the detection timing of the patch images200Y,200M,200C, and200K (step S107).

In the processing for changing the detection timing of the patch images200Y,200M,200C, and200K, as illustrated inFIG. 8, the controller101determines whether the detection timing of the patch images200Y,200M,200C, and200K deviates downstream or the upstream in the moving direction of the intermediate transfer belt31, and detects a deviation amount of the detection timing of the patch images200Y,200M,200C, and200K as a time or a distance with respect to the moving speed of the intermediate transfer belt31.

The direction in which the detection timings of the patch images200Y,200M,200C, and200K deviate and the deviation amount, as illustrated inFIG. 7B, are detected by the controller101, for example, by analyzing whether the density data of the measurement point in which the density is the threshold or greater exists upstream or downstream in the moving direction B of the intermediate transfer belt31, or analyzing the number of density data at the measurement point in which the density is the threshold or greater.

As illustrated inFIG. 7B, the controller101calculates a deviation amount L3of the detection timing of the patch image200based on the analysis result of the position where the density data of the measurement points in which the density is the threshold or greater exists and the number of pieces of the density data of the measurement points in which the density is the threshold or greater, calculates a correction amount (L3+L2) of the detection timing of the patch image200, and the like in consideration of the deviation amount L3of the detection timing and margins L1and L2set upstream and downstream of the patch image200in the moving direction B of the intermediate transfer belt31such that the detection timing of the patch images200Y,200M,200C, and200K by the density sensor60falls within a required range of the patch image, and executes the processing for changing the detection timing of the patch images200Y,200M,200C, and200K based on the correction amount (L3+L2) of the detection timing (step S107).

In this case, as illustrated inFIG. 8, the controller101executes processing for changing the detection timing of the patch images200Y,200M,200C, and200K so that the detection timing is delayed by a time corresponding to the deviation amount (L3+L2).

The controller101may be configured to change the formation position of the patch images200Y,200M,200C, and200K on the intermediate transfer belt31instead of changing the detection timing of the patch images200Y,200M,200C, and200K by the density sensor60.

In this case, the controller101may change the formation positions of the patch images200Y,200M,200C, and200K in the image forming devices20(Y, M, C, and K) of yellow (Y), magenta (M), cyan (C), and black (K) to the downstream in the moving direction of the intermediate transfer belt31.

After that, the controller101returns to step S101, executes an operation of creating the patch images200Y,200M,200C, and200K from the toner images of the corresponding colors in the image forming devices20(Y, M, C, and K) of yellow (Y), magenta (M), cyan (C), and black (K), detects the patch images200Y,200M,200C, and200K at the changed detection timing by the density sensor60and collects density data (step S102), and executes processing for calculating the image densities of patch images200Y,200M,200C, and200K (step S104).

After that, based on the calculated image density of the patch image, the controller101executes an operation for controlling the image density to be in an appropriate range in the image forming devices20(Y, M, C, and K) of yellow (Y), magenta (M), cyan (C), and black (K).

As described above, with the image forming apparatus1according to the first exemplary embodiment, even when a margin set before and after a detection area of the patch image200is narrowed, the number of patch images effective for density detection can be secured by changing (i) the detection timing at which the patch image200is detected by the density sensor60or (ii) the formation position of the patch image200on the image forming device when the number of the determination results, by the determination unit, that the density of the patch image is not within the predetermined threshold exceeds the predetermined number, as compared with the case where only the detection result of the central portion area is selected from among the detection results of the patch image stored in the storage unit from the positional deviation amount of the patch image and the image density of the patch image is calculated.

Second Exemplary Embodiment

FIG. 9illustrates an image forming apparatus according to a second exemplary embodiment. The image forming apparatus according to the second exemplary embodiment is configured such that the density detector detects the density of the patch image before the start of the image forming operation.

That is, in the image forming apparatus1according to the second exemplary embodiment, as illustrated inFIG. 9, a configuration in which the patch images200Y,200M,200C, and200K of yellow (Y), magenta (M), cyan (C) and black (K) of the corresponding colors are continuously formed in the image forming devices20(Y, M, C, and K) prior to the image forming operation and the density of the patch images200Y,200M,200C, and200K is detected by the density sensor60is adopted.

As described above, by continuously forming the patch images200Y,200M,200C, and200K prior to the normal image forming operation and detecting the density of the patch images200Y,200M,200C, and200K by the density sensor60, image quality of the initially formed image can be improved as compared with the case where the density detection of the patch images200Y,200M,200C, and200K by the density sensor60is performed between plural image areas300formed by the image forming operation.

Other configurations and operations are the same as those in the first exemplary embodiment, and thus description thereof will be omitted.

Third Exemplary Embodiment

FIG. 10illustrates an image forming apparatus according to a third exemplary embodiment. The image forming apparatus1according to the third exemplary embodiment is configured such that the operation after the controller101determines whether there are one or more measurement points in which the density is a predetermined threshold or greater based on the density data of the patch images200Y,200M,200C, and200K of respective colors is different from that of the first exemplary embodiment described above.

That is, in the image forming apparatus1according to the third exemplary embodiment, as illustrated inFIG. 10, after the controller101determines whether there is one or more measurement points in which the density is a predetermined threshold or greater based on the density data of the patch images200Y,200M,200C, and200K of respective colors (step S103), when it is determined that among the density data of the patch images200Y,200M,200C, and200K of each color, at least one measurement point in which the density is the predetermined threshold or greater is determined, the controller101determines whether or not the patch image in which the number of measurement points in which the density is the threshold or greater is five points or greater and the threshold is exceeded at three points or more is continuous for all colors of yellow (Y), magenta (M), cyan (C), and black (K) (step S108).

Accordingly, the controller101is configured to proceed to step S107and change the detection timing when five or more measurement points in which the density is the threshold or greater exist among the density data of the patch images200Y,200M,200C, and200K of respective colors, or when the patch image in which the threshold is exceeded at three points or more is continuous for all colors of yellow (Y), magenta (M), cyan (C), and black (K).

As such, in the third exemplary embodiment, the controller101changes the detection timing when five or more measurement points in which the density is the threshold or greater exist among the density data of the patch images200Y,200M,200C, and200K of respective colors, or when the patch image in which the threshold is exceeded at three points or more is continuous for all colors of yellow (Y), magenta (M), cyan (C), and black (K).

For that reason, in the third exemplary embodiment, under severe condition when five or more measurement points in which the density is the threshold or greater exist even in the density data of the patch images200Y,200M,200C, and200K of one color, or when the patch image in which the threshold is exceeded at three points or more is continuous for all colors of yellow (Y), magenta (M), cyan (C), and black (K), it is possible to maintain the detection accuracy of the patch images200Y,200M,200C, and200K while allowing the margin of the patch images200Y,200M,200C, and200K to be reduced, by changing the detection timing of the patch images200Y,200M,200C, and200K.

Other configurations and operations are the same as those in the first exemplary embodiment, and thus description thereof will be omitted.

In the exemplary embodiments described above, although a full-color image forming apparatus including the image forming devices20(Y, M, C, and K) of yellow (Y), magenta (M), cyan (C), and black (K) has been described as an image forming apparatus, it is needless to say that image forming apparatus can be similarly applied to a monochrome image forming apparatus.

In the exemplary embodiments described above, the electrophotographic image forming apparatuses have been described. The present disclosure is applicable to image forming apparatuses other than electrophotographic ones. For example, the present disclosure may be applied to an inkjet image forming apparatus. More specifically, the present disclosure may be applied to an image forming apparatus that draws an ink image on an intermediate transfer body with an ink ejection head and transfers the ink image from the intermediate transfer body to paper.