JP7842616B2 - Image forming apparatus and method for adjusting image density in an image forming apparatus - Google Patents

Description

This invention relates to an image forming apparatus and a method for adjusting image density in an image forming apparatus, and more particularly to an image forming apparatus and a method for adjusting image density in an image forming apparatus that include an image forming means for performing image forming processing by an electrophotographic method.

In this type of image forming apparatus, the state (result) of the image forming process may change due to the deterioration of each element constituting the image forming means or the developer over time, or due to changes in the surrounding environment (temperature, humidity, etc.). To compensate for this, process control is performed to adjust the density of the image (output image) formed by the image forming process. An example of this process control technology is disclosed in Patent Document 1.

According to the technology disclosed in Patent Document 1, a first relational expression representing the relationship between the amount of toner deposited on the photoreceptor drum, which constitutes the image forming means, and the optical reflectance density measured by an optical reflectance density measuring means, which is an image density detection means for detecting the density of the toner image formed on the photoreceptor drum, is stored in advance. In addition, a plurality of second relational expressions representing the relationship between the amount of toner deposited on the photoreceptor drum and the development bias voltage, which change depending on the state of the photoreceptor drum and developer, are also stored in advance. Then, a toner patch is created on the photoreceptor drum as a test toner image using a predetermined development bias voltage, and the density of this toner patch is measured by the optical reflectance density measuring means. Based on the first relational expression, the density of the toner patch is converted to the amount of toner deposited. Furthermore, a relational expression that is close to the relationship between the converted toner deposited amount and the development bias voltage at the time of toner patch creation is extracted from the plurality of second relational expressions. Based on the extracted relational expression, an appropriate development bias voltage is determined.

Japanese Patent Publication No. 2010-139553

The technology disclosed in Patent Document 1 assumes that the relationship between the amount of toner deposited and the optical reflectance measured by the optical reflectance measuring means is constant. However, this relationship can change due to the degradation of the developer containing the toner. Therefore, the amount of toner deposited, derived (converted) from the optical reflectance measured by the optical reflectance measuring means, may differ from the actual amount of toner deposited. In this case, accurate process control cannot be achieved.

Therefore, the present invention aims to provide a novel image forming apparatus and a method for adjusting image density in an image forming apparatus that can achieve precise process control even when the developer deteriorates.

To achieve this objective, the present invention includes a first invention relating to an image forming apparatus and a second invention relating to a method for adjusting image density in an image forming apparatus.

The first invention, relating to an image forming apparatus, comprises an image forming means that performs image forming processing using an electrophotographic method, and further comprises an image forming control means, an image density detection means, an approximation means, a correction means, and a derivation means. The image forming control means controls the image forming means to form a test toner image on an image carrier constituting the image forming means under conditions where the development bias voltage differs. The image density detection means detects the density of the test toner image. The approximation means approximates the relationship between the density detection value of the test toner image by the image density detection means and the development bias voltage using a linear approximation formula. The correction means corrects the density detection value by the image density detection means based on the slope and intercept of the linear approximation formula approximated by the approximation means. Finally, the derivation means derives an appropriate development bias voltage based on the density detection value corrected by the correction means.

Furthermore, in this first invention, a reference curve storage means may be provided. This reference curve storage means stores data related to the reference curve in advance. The reference curve is a curve that approximates the relationship between the slope and intercept of the aforementioned linear approximation formula for test toner images formed using a reference developer and under different environmental conditions—in other words, formed with developers with different developing properties. In this case, the correction means corrects the density detection value based on the amount of deviation of the slope and intercept of the linear approximation formula related to the density detection value to be corrected by the correction means from the reference curve.

The standard developer referred to here is one whose degree of deterioration is below a specified level; in other words, it is essentially a brand-new developer that has not deteriorated.

Furthermore, the deviation from the reference curve refers to the deviation of the intercept of the linear approximation equation, which is the slope of the linear approximation equation related to the concentration detection value subject to correction by the correction means, from the reference curve.

The image forming means in this first invention may include an intermediate transfer belt. In this case, the image density detection means detects the density of the test toner image transferred from the image carrier to the intermediate transfer belt.

The second invention of this invention, relating to an image density adjustment method in an image forming apparatus, includes an image formation control step, an image density detection step, an approximation step, a correction step, and a derivation step. Here, the image forming apparatus comprises image forming means that perform image forming processing using an electrophotographic method. Furthermore, in the image formation control step, the image forming means is controlled to form a test toner image on an image carrier constituting the image forming means under conditions of different development bias voltages. In the image density detection step, the density of the test toner image is detected. In the approximation step, the relationship between the density detected value of the test toner image obtained in the image density detection step and the development bias voltage is approximated by a linear approximation formula. In the correction step, the density detected value is corrected based on the slope and intercept of the linear approximation formula approximated in the approximation step. Finally, in the derivation step, an appropriate development bias voltage is derived based on the density detected value after correction in the correction step.

According to this invention, precise process control can be achieved even if the developer deteriorates.

Figure 1 is a schematic diagram showing the internal configuration of a multifunction device according to one embodiment of the present invention. Figure 2 is a magnified view of the image forming section of a multifunction printer according to one embodiment of the present invention. Figure 3 is a block diagram showing the electrical configuration of a multifunction device according to one embodiment of the present invention. Figure 4 is a view from below of the intermediate transfer belt in which a test toner image has been formed according to one embodiment of the present invention. Figure 5 shows the relationship between the normalized sensor output value and the amount of toner deposited in one embodiment of the present invention. Figure 6 shows the relationship between the development bias voltage and the amount of toner deposited based on the normalized sensor output value in one embodiment of the present invention. Figure 7 is a schematic diagram showing the state of toner adhesion on the intermediate transfer belt in one embodiment of the present invention. Figure 8 is a diagram that hypothetically shows how the relationship between the normalized sensor output value and the amount of toner deposited changes due to the deterioration of the developer in one embodiment of the present invention. Figure 9 shows the state in which the relationship between the development bias voltage and the amount of toner deposited based on the normalized sensor output value changes due to the deterioration of the developer in one embodiment of the present invention. Figure 10 shows a reference curve in one embodiment of the present invention. Figure 11 is a diagram conceptually showing the configuration of a correction coefficient table in one embodiment of the present invention. Figure 12 is a conceptual diagram showing the configuration of a reference correction amount table in one embodiment of the present invention.

One embodiment of the present invention will be described using the multifunction printer 10 shown in Figure 1 as an example.

The multifunction device 10 according to this first embodiment is a type of image forming apparatus and has multiple functions such as copying, printing, image scanning, and faxing. Figure 1 is a view of the internal configuration of the multifunction device 10 as seen from the front of the device when it is set up for use. That is, the vertical direction in Figure 1 corresponds to the vertical direction of the multifunction device 10. The horizontal direction in Figure 1 corresponds to the horizontal direction of the multifunction device 10. Furthermore, the front of the page in Figure 1 corresponds to the front of the multifunction device 10. And the back of the page in Figure 1 corresponds to the rear of the multifunction device 10.

An image reading unit 12, an example of an image reading means, is provided at the top of this multifunction printer 10. The image reading unit 12 is responsible for image reading processing, which reads an image of a document (not shown) and outputs two-dimensional read image data corresponding to the image of the document. For this reason, the image reading unit 12 has a document tray 14 on which the document is placed. The document tray 14 is formed of a transparent material such as glass in a roughly rectangular flat plate shape, and is provided so that both of its main surfaces are aligned horizontally. Below the document tray 14, an image reading unit 16 is provided. Although a detailed explanation is omitted, the image reading unit 16 has a light source, mirror, lens, line sensor, etc., and forms a linear image reading position Pr on the upper surface of the document tray 14 that extends along the front-to-back direction of the multifunction printer 10. Furthermore, below the document tray 14, a drive mechanism (not shown) is provided for moving (scanning) the image reading position Pr of the image reading unit 16 along the left-to-right direction of the multifunction printer 10. In other words, with the document placed on the document tray 14, the image reading position Pr of the image reading unit 16 is moved by the drive mechanism, thereby reading the image of the document using a so-called fixed-reading method. The front-to-back direction of the multifunction device 10 is called the main scanning direction, and the left-to-right direction is called the sub-scanning direction.

Furthermore, an automatic document feeder (ADF) 18, which also serves as a document holder cover for holding down documents placed on the document tray 14, is provided above the document tray 14. The ADF 18 is designed to transition between a state where the upper surface of the document tray 14 is exposed to the outside and a state where it covers the upper surface of the document tray 14. Therefore, the ADF 18 is connected to the main body (housing) of the multifunction printer 10 via a suitable movable support member (not shown), such as a hinge. Figure 1 shows the ADF 18 covering the upper surface of the document tray 14. The ADF 18 performs its intended function when it is in the state shown in Figure 1, covering the upper surface of the document tray 14.

The automatic document feeder 18 has a document tray 20. This document tray 20 can hold documents, specifically sheet-like documents, and in particular, multiple documents can be stacked on it. While a detailed explanation is omitted, the automatic document feeder 18 takes in documents placed on the document tray 20 one at a time and transports them along the document transport path 22 within the automatic document feeder 18. Along the way, the documents pass through an image reading position Pr, specifically a fixed image reading position Pr. This allows the image of the document to be read using a so-called "swiping" method. Afterward, the documents are discharged into the document output tray 24.

Below the image reading unit 12, an image forming unit 28 is provided as an example of a color image forming means. This image forming unit 28 performs a printing process, i.e., an image forming process, to form an image on a sheet-like image recording medium (not shown) based on appropriate image data, such as the aforementioned read image data. This image forming process is performed using a known electrophotographic method. Furthermore, the image forming unit 28 employs a tandem system to perform color image forming.

Specifically, the image forming unit 28 has four image forming stations (sometimes called "process units") 32, 32, ... for individually forming monochrome toner images of four different colors, such as yellow, magenta, cyan, and black. In addition, the image forming unit 28 includes an exposure device 34 as an exposure means for performing the exposure necessary for forming the monochrome toner images by each image forming station 32, 32, ..., a transfer unit 30 as a transfer means for transferring a color toner image, which is a composite image of the monochrome toner images formed by each image forming station 32, 32, ..., onto paper, and a fixing device 36 as a fixing means for fixing the transferred toner image onto the paper. Furthermore, the image forming unit 28 includes an image sensor 38 as an example of an image density detection means for detecting the density of the test toner images 200, 200, ... formed by each image forming station 32, 32, ... (described later).

The image forming unit 28 will be described in more detail with reference to Figure 2. First, the transfer unit 30 includes an intermediate transfer belt (sometimes called a "primary transfer belt") 40, a drive roller 42 for rotating the intermediate transfer belt 40, a driven roller 44 for tensioning the intermediate transfer belt 40 together with the drive roller 42, four intermediate transfer rollers (sometimes called "primary transfer rollers") 46, 46, ... located inside the intermediate transfer belt 40 at positions corresponding to each image forming station 32, 32, ..., and a secondary transfer roller 48.

The intermediate transfer belt 40 is stretched by a drive roller 42 and a driven roller 44. The drive roller 42 rotates under the driving force of a motor (not shown) acting as a belt drive means, for example, in counterclockwise direction in Figure 2. As a result, the intermediate transfer belt 40 moves circumferentially, and the driven roller 44 rotates. The lower region of the intermediate transfer belt 40 between the drive roller 42 and the driven roller 44 is stretched horizontally, and each image forming station 32, 32… is positioned opposite this horizontally stretched region. This region of the intermediate transfer belt 40 opposite each image forming station 32, 32… is referred to as the primary transfer region 40a. Within the primary transfer region 40a, the intermediate transfer belt 40 moves (travels) from left to right of the multifunction device 10, as indicated by the arrow 100 in Figure 2, that is, along the sub-scanning direction.

The intermediate transfer belt 40 is a flexible, endless strip-shaped body made of synthetic resin or rubber with an appropriate amount of conductive material such as carbon black. Although a detailed explanation is omitted here, the driven roller 44 also has the function of preventing slack in the intermediate transfer belt 40 by applying appropriate tension to it. The intermediate transfer rollers 46, 46, ... and the secondary transfer roller 48 will be explained in detail later.

Next, each image forming station 32, 32, ... will be described. Each image forming station 32, 32, ... is located below the primary transfer region 40a of the intermediate transfer belt 40, and is positioned at a constant interval L along the direction of movement 100 of the intermediate transfer belt 40 in the primary transfer region 40a, that is, along the sub-scanning direction. Each of these image forming stations 32, 32, ... individually forms single-color toner images of four colors—yellow (Y), magenta (M), cyan (C), and black (K)—on the intermediate transfer belt 40. Here, each image forming station 32, 32, ... is positioned in the order of yellow, magenta, cyan, and black, from upstream to downstream in the direction of movement 100 of the intermediate transfer belt 40 in the primary transfer region 40a. Note that each image forming station 32, 32, ... has the same structure as the others, except that they form single-color toner images of different colors on the intermediate transfer belt 40.

Each image forming station 32 includes a photoreceptor drum 50 as an image carrier, a charging device 52 as a charging means, a developing device 54 as a developing means, a cleaning device 56 as a cleaning means, and an anti-static device (not shown). The photoreceptor drum 50 has a cylindrical conductive substrate made of a conductive material such as aluminum, and a photosensitive layer is formed on the surface (outer surface) of this substrate, which exhibits insulating properties in areas not irradiated with light and conductive properties in areas irradiated with light. The photoreceptor drum 50 is positioned so that its surface (substrate) is in contact with the outer surface of the intermediate transfer belt 40. The photoreceptor drum 50 then rotates by receiving a driving force from a motor (not shown) which is a drum driving means, and rotates clockwise, for example, in Figure 2. The photoreceptor drum 50 rotates at a speed matching the peripheral speed of the intermediate transfer belt 40. More precisely, its peripheral speed is slightly lower than the intermediate transfer belt 40's speed, for example, by 0.1% to 0.3%. This is to facilitate the transfer of the single-color toner image formed (supported) on the surface of the photoreceptor drum 50 to the outer surface of the intermediate transfer belt 40. In other words, it prevents the phenomenon known as "gap transfer," where the single-color toner image is not transferred from the surface of the photoreceptor drum 50 to the outer surface of the intermediate transfer belt 40.

The charging device 52 applies static electricity to the surface of the photoreceptor drum 50, charging the surface of the photoreceptor drum 50 to a predetermined potential, for example, a negative potential. When the surface of the photoreceptor drum 50, thus charged, is exposed by the exposure device 34, the illuminated area becomes conductive, and the surface potential of that area becomes zero (0). As a result, an electrostatic latent image (not shown) corresponding to the image data to be used for image formation processing is formed on the surface of the photoreceptor drum 50. The exposure device 34 is a laser scanning unit having a laser diode (not shown) as a light-emitting means, a polygon mirror 60 as a deflection means, etc., and is installed below the row of image forming stations 32, 32, ... The exposure device 34 performs exposure by irradiating the surface of the photoreceptor drum 50 of each image forming station 32 with laser light, for example, from below. Instead of a laser scanning unit, an LED unit with an LED array arranged in a row of LEDs may be used as the exposure device 34.

Developing is performed on the surface of the photoreceptor drum 50, where an electrostatic latent image has been formed in this manner, by the developing device 54. Although detailed illustrations are omitted, the developing device 54 employs a two-component developer. The carriers contained in this two-component developer, together with the toner contained in the two-component developer, are attracted to the surface (outer surface) of the developing roller (magnetic roller) 54a of the developing device 54, forming a magnetic brush. When this magnetic brush approaches the surface of the photoreceptor drum 50, toner flies towards the electrostatic latent image formed on the surface of the photoreceptor drum 50. The toner then adheres to the electrostatic latent image, causing it to become visible. At this time, a negative developing bias voltage DVB is applied to the developing roller 54a. The magnitude (absolute value) of this developing bias voltage DVB affects the flight of toner from the surface (magnetic brush) of the developing roller 54a to the surface (electrostatic latent image) of the photoreceptor drum 50, and thus affects the so-called developing performance. For example, the higher the development bias voltage (DVB), the greater the developability, and consequently, the greater the amount of toner adhering to the surface of the photoreceptor drum 50. Through this development process, a monochrome toner image is formed on the surface of the photoreceptor drum 50.

The monochrome toner image formed on the surface of the photoreceptor drum 50 is transferred (primary transfer) from the surface of the photoreceptor drum 50 to the outer surface of the intermediate transfer belt 40 at the contact point between the surface of the photoreceptor drum 50 and the outer surface of the intermediate transfer belt 40. At this time, static electricity is applied to the intermediate transfer belt 40 from each of the intermediate transfer rollers 46.

In other words, each intermediate transfer roller 46, 46, ... is provided in correspondence with each image forming station 32, 32, ... More specifically, each intermediate transfer roller 46 is provided in the primary transfer region 40a so as to face the photoreceptor drum 50 of the corresponding image forming station 32 across the intermediate transfer belt 40. Furthermore, each intermediate transfer roller 46 is provided so as to contact the inner surface of the intermediate transfer belt 40 with its own surface (outer surface). Each intermediate transfer roller 46 rotates by receiving the driving force from the circumferential movement of the intermediate transfer belt 40, for example, in Figure 2, it rotates counterclockwise. Then, a predetermined voltage (primary transfer voltage) is applied to each intermediate transfer roller 46. As a result, static electricity is applied from the intermediate transfer roller 46 to the intermediate transfer belt 40, and a transfer electric field is formed between the surface of the photoreceptor drum 50 and the outer surface of the intermediate transfer belt 40. Due to the action of this transfer electric field, a monochrome toner image is transferred from the surface of the photoreceptor drum 50 to the outer surface of the intermediate transfer belt 40.

As a result, individual, or sequential, monochrome toner images of four colors—yellow, magenta, cyan, and black—are formed on the intermediate transfer belt 40. These four monochrome toner images then overlap, forming a color toner image on the intermediate transfer belt 40.

In each image forming station 32, after the monochrome toner image is transferred from the surface of the photoreceptor drum 50 to the outer surface of the intermediate transfer belt 40 as described above, the toner remaining on the surface of the photoreceptor drum 50 is removed by the cleaning device 56. Subsequently, static electricity is removed from the surface of the photoreceptor drum by a static elimination device (not shown), and the subsequent steps, including charging by the charging device 52, are repeated.

The toner image formed on the intermediate transfer belt 40 is transferred to a sheet of paper (not shown) at the transfer nip 62 between the intermediate transfer belt 40 and the secondary transfer roller 48. Specifically, the secondary transfer roller 48 is positioned opposite the drive roller 42, straddling the intermediate transfer belt 40, and is positioned to press against the intermediate transfer belt 40 between itself and the drive roller 42. The secondary transfer roller 48 rotates due to the driving force generated by the circumferential movement of the intermediate transfer belt 40, for example, rotating clockwise in Figure 2. A predetermined voltage (secondary transfer voltage) with the opposite polarity to the toner image is then applied to the secondary transfer roller 48. This creates a transfer electric field between the intermediate transfer belt 40 and the secondary transfer roller 48. In this state, when the sheet of paper passes through the transfer nip 62 between the intermediate transfer belt 40 and the secondary transfer roller 48, the toner image formed on the intermediate transfer belt 40 is transferred to the sheet of paper (secondary transfer).

The image sensor 38 is a sensor for detecting a single-color toner image formed on the intermediate transfer belt 40, and more precisely, it is a sensor for detecting the density of the test toner images 200, 200, ... which will be described later. The image sensor 38 will be explained in detail later, but it is, for example, a reflective photoelectric sensor. The image sensor 38 is located below the primary transfer region 40a and downstream of the arrangement of the image forming stations 32, 32, ... in the direction of movement of the intermediate transfer belt 40. Furthermore, the image sensor 38 is positioned with its detection unit (light-emitting unit and light-receiving unit) facing upward (directly upward), that is, facing the primary transfer region 40a of the intermediate transfer belt 40. While at least one image sensor 38 is sufficient, multiple sensors may be provided at appropriate intervals from each other in the direction of extension of the rotation axis of the drive roller 42, that is, in the main scanning direction. In this embodiment (though not visible in Figures 1 and 2), two sensors are provided. Furthermore, the image sensor 38 is also used as a sensor for resist adjustment; however, since resist adjustment is not directly related to the essence of this invention, its explanation is omitted here.

Looking again at Figure 1, the multifunction printer 10 has a paper transport path 66 inside, which runs from the paper feeding section 64 (described later) through the transfer nip section 62 to the output tray 26 (more precisely, the paper output opening to the output tray 26). Multiple transport rollers (more precisely, pairs of rollers) 68, 68, ... are provided at appropriate positions along the paper transport path 66 to transport the paper from the paper feeding section 64 to the output tray 26. In other words, the paper to be processed by the image forming section 28 is transported along the paper transport path 66, passing through the transfer nip section 62 along the way.

Furthermore, the transport roller 68 located upstream of the transfer nip section 62 in the paper transport path 66, and closest to the transfer nip section 62, is a register roller (sometimes called a "paper stop roller") 70. This register roller 70 timed the passage of the paper through the transfer nip section 62 and temporarily stopped the paper transport for that purpose. Then, the register roller 70 started transporting the paper in synchronization with the transfer unit 30. At this time, the register roller 70 rotates at the same peripheral speed as the intermediate transfer belt 40.

A fixing device 36 is provided in the paper transport path 66, between the transfer nip section 62 and the output tray 26; in other words, downstream of the transfer nip section 62 in the paper transport direction of the paper. The fixing device 36 includes a heat roller 72 and a pressure roller 74. These heat roller 72 and pressure roller 74 are positioned so that their surfaces (outer surfaces) are in close contact with each other. The heat roller 72 is heated to a predetermined temperature (fixing temperature). Simultaneously, the heat roller 72 is driven by a motor (not shown) acting as a fixing roller driving means, and rotates counterclockwise, for example, in Figure 1. The pressure roller 74 also rotates accordingly, for example, clockwise, in Figure 1. The paper that has passed through the transfer nip section 62 then passes through the fixing nip section 76, which is sandwiched between the heat roller 72 and the pressure roller 74. This fixes the toner image on the paper. With this, the series of image forming processes by the image forming unit 28 is completed.

The paper, or printed material, after this image forming process is performed is discharged into the output tray 26. The output tray 26 is located between the image forming unit 28 and the image reading unit 12, within the internal space of the multifunction printer 10. Alternatively, the output tray 26 may be located on the outside of the multifunction printer 10.

Furthermore, a paper feeding unit 64 is provided at the bottom of the multifunction printer 10. The paper feeding unit 64 has one or more paper cassettes 78. Each paper cassette 78 can accommodate multiple sheets of paper stacked on top of each other. In addition, the paper feeding unit 64 has a manual feed tray 80. The manual feed tray 80 is, for example, located on the right side of the multifunction printer 10. Multiple sheets of paper can be stacked on this manual feed tray 80. The paper feeding unit 64 uses either the paper cassette 78 or the manual feed tray 80 as a paper source and supplies paper one sheet at a time to the paper transport path 66.

In addition, a transport path 82 for double-sided printing is provided inside the multifunction printer 10. This transport path 82 is designed to take in paper that has already passed through the fuser nip section 76—that is, paper that has undergone image formation processing—and to re-process it for image formation. Specifically, paper taken into the transport path 82 for double-sided printing is supplied again to the paper transport path 66, more precisely to the upstream side of the registration roller 70. At this point, the front and back sides of the paper are reversed. This allows image formation processing to be applied to the back side of the paper, achieving double-sided printing. Transport rollers 84 are also provided at appropriate positions along the transport path 82 for double-sided printing.

Figure 3 is a block diagram showing the electrical configuration of the multifunction printer 10. As shown in Figure 3, the multifunction printer 10 includes an image reading unit 12, an automatic document feeder 18, an image forming unit 28, and a paper feeding unit 64, as well as a control unit 86, an auxiliary storage unit 88, and the like. These are connected to each other via a common bus 94. The multifunction printer 10 also includes various other elements, such as an operation unit (not shown), but elements not directly related to the essence of the present invention, including the operation unit, are omitted from the illustration and description here. Furthermore, the image reading unit 12, automatic document feeder 18, image forming unit 28, and paper feeding unit 64 are as described above.

The control unit 86 is an example of a control means that oversees the overall control of the multifunction printer 10. Therefore, the control unit 86 includes a computer, such as a CPU 86a, as a control execution means. In addition, the control unit 86 has a main memory unit 86b as a main memory means directly accessible by the CPU 86a. The main memory unit 86b includes ROM and RAM (not shown). The ROM stores a control program, so-called firmware, for controlling the operation of the CPU 86a. The RAM constitutes a work area and buffer area when the CPU 86a executes processing based on the control program.

The auxiliary storage unit 88 is an example of an auxiliary storage means. That is, various data, such as the aforementioned read image data, are appropriately stored in the auxiliary storage unit 88. This auxiliary storage unit 88 may, for example, include a hard disk drive (not shown). Additionally, the auxiliary storage unit 88 may include rewritable non-volatile memory such as flash memory.

Now, in the multifunction printer 10 according to this embodiment, process control is performed, or more precisely, high-density process control is performed. This high-density process control is performed, for example, when the power of the multifunction printer 10 is turned on (at startup), or each time a predetermined number of image forming processes are executed.

To explain this high-density process control in detail, in this high-density process control, four high-density, so-called solid-color test toner images (patches) 200, 200, ... for yellow, magenta, cyan, and black, as shown in Figure 4, are formed on the intermediate transfer belt 40. Figure 4 shows the intermediate transfer belt 40 with the test toner images 200, 200, ... formed on it, viewed from below; in other words, it shows a part of the image forming unit 28 viewed from below. Furthermore, each test toner image 200, 200, ... is formed individually by separate image forming stations 32, 32, ... for yellow, magenta, black, and cyan.

As shown in Figure 4, the yellow and black test toner images 200 and 200 of each test toner image 200, 200, ... are formed at a position closer to one side edge (downward in Figure 4) in the main scanning direction (up and down in Figure 4) of the intermediate transfer belt 40. The magenta and cyan test toner images 200 and 200 are formed at a position closer to the other side edge (upward in Figure 4) in the main scanning direction of the intermediate transfer belt 40. Furthermore, the yellow and black test toner images 200 and 200 are formed in this order from upstream to downstream in the movement direction 100 of the intermediate transfer belt 40. Similarly, the magenta and cyan test toner images 200 and 200 are formed in this order from upstream to downstream in the movement direction 100 of the intermediate transfer belt 40. Note that the formation positions of each test toner image 200, 200, ... are not limited to those shown in Figure 4. Furthermore, while the shape of each test toner image 200, 200, ... is rectangular in Figure 4, it is not limited to this shape.

Two image sensors 38 are provided to detect the density of each test toner image 200, 200, ... Specifically, one image sensor 38 is positioned in the sub-scanning direction to detect the yellow and black test toner images 200 and 200. The other image sensor 38 is positioned in the sub-scanning direction to detect the magenta and cyan test toner images 200 and 200.

Each image sensor 38 displays an output value Vs corresponding to the density of the test toner image 200 being detected; for example, a higher density results in a smaller output value Vs. In other words, the output value Vs of each image sensor 38 corresponds to the amount of toner Cr deposited on the intermediate transfer belt 40, and is correlated with the amount of toner Cr. Figure 5 shows the relationship between the output value Vs of the image sensor 38 and the amount of toner Cr deposited on the intermediate transfer belt 40, and more precisely, the relationship between the normalized sensor output value Vn and the amount of toner Cr deposited on the intermediate transfer belt 40. The normalized sensor output value Vn is the value obtained by dividing the output value Vs1 of the image sensor 38 for the test toner image 200 by the output value Vs0 of the image sensor 38 for the substrate portion on the intermediate transfer belt 40 where the test toner image 200 is not formed, and is expressed by the following equation 1.

《Formula 1》
Vn=Vs1/Vs0
As shown in Figure 5, the normalized sensor output value Vn, that is, the output value Vs (Vs1) of the image sensor 38, and the amount of toner deposited on the intermediate transfer belt 40, Cr, are related by a quadratic curve X. Using this relationship, the amount of toner deposited on the intermediate transfer belt 40, Cr, is derived (estimated) based on the output value Vs of the image sensor 38, or more precisely, based on the normalized sensor output value Vn. For this purpose, data representing the relationship shown in Figure 5 is acquired in advance for each image sensor 38, for example, through experiments, and stored in the auxiliary storage unit 88.

Then, for each image forming station 32, as shown in Figure 6, a test toner image 200 is formed under different conditions for the development bias voltage DVB, for example, under conditions where the development bias voltage DVB is DVB1, DVB2, DVB3, and DVB4 (DVB1 < DVB2 < DVB3 < DVB4). The density of the test toner image 200 is detected by the image sensor 38, and based on the output value Vs of the image sensor 38, more precisely based on the normalized sensor output value Vn, the amount of toner deposited Cr on the intermediate transfer belt 40 is derived. In other words, the amount of toner deposited Cs, which is an estimated value of the amount of toner deposited Cr, is determined based on the normalized sensor output value Vn. Furthermore, based on the amount of toner deposited Cs at each development bias voltage DVB (DVB1, DVB2, DVB3, and DVB4), the relationship between the development bias voltage DVB and the amount of toner deposited Cs is approximated by the following linear approximation formula, which is expressed by equation 2. This approximation can be performed, for example, by the least squares method.

《Formula 2》
Cs=a・DVB+b
In addition, a target toner deposition amount Ct is set in advance. The intersection point of this target toner deposition amount Ct and the straight line Y, which follows the linear approximation equation expressed by Equation 2, is determined, and the development bias voltage DVB at this intersection point is determined as the appropriate development bias voltage DVBf. By adjusting the development bias voltage DVB to be equivalent to this appropriate development bias voltage DVBf, high-density process control is completed.

Incidentally, even if the same amount of toner adheres to the intermediate transfer belt 40, the output value Vs of the image sensor 38 may differ depending on the degree of deterioration of the developer containing the toner. For example, if the developer is relatively new and its degree of deterioration is relatively small, the toner adheres uniformly to the intermediate transfer belt 40, as shown in Figure 7A. In contrast, if the developer is relatively old and its degree of deterioration is relatively large, the toner aggregates, as shown in Figure 7B, creating gaps in the toner on the intermediate transfer belt 40. As a result, when the degree of developer deterioration is large, as shown in Figure 7B, the amount of reflected light to the image sensor 38 is greater than when the degree of developer deterioration is small, as shown in Figure 7A.

Consequently, as shown in Figure 8, the relationship between the normalized sensor output value Vn and the actual toner deposition amount Cr becomes a relationship represented by the dashed curve X', deviating from the original relationship represented by the solid curve X. This makes it impossible to accurately derive the actual toner deposition amount Cr from the normalized sensor output value Vn, and consequently, to accurately control the high-concentration process.

Figure 9A shows an example of a situation where, due to developer degradation, an error occurs in the toner deposition amount Cs based on the normalized sensor output value Vn, making accurate high-density process control impossible. Specifically, the relationship between the development bias voltage DVB and the toner deposition amount Cs based on the normalized sensor output value Vn becomes as shown by the thick dashed line Y', deviating from the original (ideal) relationship shown by the thick solid line Y. As a result, an incorrect voltage DVBf' is obtained as the appropriate development bias voltage DVBf.

Furthermore, the toner's developability, as mentioned above, changes depending on environmental factors such as ambient temperature and humidity. When this toner developability changes, the relationship between the development bias voltage DVB and the toner deposition amount Cs based on the normalization sensor output value Vn also changes. Figure 9B shows an example of the relationship between the development bias voltage DVB and the toner deposition amount Cs based on the normalization sensor output value Vn when the toner's developability decreases.

In other words, even when the developer is not significantly degraded, as can be seen from the comparison between the thick solid line Y in Figure 9B and the thick solid line Y in Figure 9A, the relationship between the development bias voltage DVB and the toner deposition amount Cs based on the normalized sensor output value Vn changes due to the change in the toner's developability. Furthermore, when the developer degrades, as shown by the thick dashed line Y' in Figure 9B, the relationship between the development bias voltage DVB and the toner deposition amount Cs based on the normalized sensor output value Vn changes even further, resulting in the determination of an even more significantly incorrect voltage DVBf' as the appropriate development bias voltage DVBf.

To resolve these inconveniences, this embodiment focuses on the slope a and intercept b of the first-order approximation equation (Equation 2) that represents the relationship between the development bias voltage DVB and the toner deposition amount Cs based on the normalized sensor output value Vn. Based on the slope a and intercept b of this first-order approximation equation, the output value Vs of the image sensor 38 is corrected, and more precisely, the normalized sensor output value Vn is corrected.

Specifically, a developer with a degree of degradation below a predetermined level, i.e., one with a relatively low degree of degradation, is prepared as the reference developer. For example, a developer that is less than one month old from the time of production, essentially a new product, is used as the reference developer. Then, using the reference developer, a linear approximation formula is determined according to the procedure described above. Furthermore, by intentionally altering the surrounding environment, various changes are introduced to the developability of the toner contained in the reference developer. Then, using reference developers with varying toner developability, a linear approximation formula is determined using the same procedure; in other words, multiple linear approximation formulas are obtained.

The slope a and intercept b of each linear approximation formula obtained in this way are plotted on a two-dimensional Cartesian coordinate system (graph), as shown in Figure 10, with slope a on the horizontal axis and intercept b on the vertical axis. The white circles (○) in Figure 10 represent the slope a and intercept b for each reference developer, or in other words, the relationship between the slope a and intercept b.

As can be seen from Figure 10, the slope a and intercept b for each reference developer lie roughly on a quadratic curve Z. In other words, the relationship between the slope a and intercept b for each reference developer is approximated by the quadratic curve Z. This approximation can be performed, for example, by the least squares method.

Furthermore, a linear approximation formula can be obtained using a developer with a relatively high degree of degradation, following the same procedure. In addition, even with a developer with a relatively high degree of degradation, various changes in the developability of the toner contained in the reference developer can be introduced by intentionally altering the surrounding environment. Then, using this developer, a linear approximation formula can be obtained using the same procedure as described above, and the slope a and intercept b of this linear approximation formula are plotted on the orthogonal coordinate system shown in Figure 10. The black circles (●) in Figure 10 indicate the relationship between the slope a and intercept b for a developer with a high degree of degradation.

As is clearly visible in Figure 10, which includes the black circle, the relationship between the slope a and intercept b for the developer with a high degree of degradation deviates significantly from curve Z. In other words, by focusing on the slope a and intercept b of the linear approximation equation, the degree of developer degradation can be determined relatively easily and accurately. It is difficult to easily and accurately determine the degree of developer degradation from the straight line Y (and Y') representing the linear approximation equation itself, as shown in Figure 9 (Figures 9A and 9B).

Therefore, in this embodiment, data representing the curve Z shown in Figure 10, in other words, data representing the reference curve Z that serves as a standard for determining the degree of deterioration of the developer, is acquired in advance, for example, through experimentation, and stored in the auxiliary storage unit 88. Then, when the slope a and intercept b for a certain developer are obtained, the amount of deviation Δb from the reference curve Z of the intercept b at the slope a is measured. If this deviation amount Δb is greater than or equal to a predetermined threshold Th, it is determined that the degree of deterioration of the developer is relatively large, or that the degree of deterioration of the developer is so large that it is necessary to correct the normalization sensor output value Vn in the manner described in detail later. On the other hand, if the deviation amount Δb is less than the predetermined threshold Th, it is determined that the degree of deterioration of the developer is relatively small, or that the degree of deterioration of the developer is so small that it is not necessary to correct the normalization sensor output value Vn. Note that Δb in Figure 10 represents the amount of deviation for a developer with a slope a of approximately 2.17 and an intercept b of approximately -60. Furthermore, the unit of the intercept b in Figure 10 is arbitrary and is merely an example. The threshold Th is determined appropriately, taking into account the impact of developer degradation on the output image. Additionally, the threshold Th may be changed as appropriate according to the slope a.

As mentioned above, when the deviation amount Δb is greater than or equal to the threshold Th, the normalized sensor output value Vn is corrected, and the corrected sensor output value Vn' is calculated based on the following equation 3.

《Formula 3》
Vn'=Vn-α・Vz
In Equation 3, α is the correction coefficient, and Vz is the reference correction amount. Equation 3, which includes these correction coefficients α and reference correction amount Vz, is derived experimentally beforehand. In particular, for the correction coefficient α, a value corresponding to the deviation amount Δb is applied, and specifically, a value based on the correction coefficient table 300 shown in Figure 11 is applied. For the reference correction amount Vz, a value corresponding to the normalized sensor output value Vn is applied, and specifically, a value based on the reference correction amount table 400 shown in Figure 12 is set.

Based on Equation 3, after the normalized sensor output value Vn is corrected, that is, after the corrected sensor output value Vn' is determined, the toner deposition amount Cr is derived based on the corrected sensor output value Vn', that is, the toner deposition amount Cs based on the corrected sensor output value Vn' is determined. From this point onward, as described above, the relationship between the development bias voltage DVB and the toner deposition amount Cs is approximated by a first-order approximation formula, and consequently, the appropriate development bias voltage DVBf is determined. Then, by adjusting the development bias voltage DVB to be equivalent to the appropriate development bias voltage DVBf, high-density process control is completed. As mentioned above, if the deviation amount Δb is less than a predetermined threshold Th, the normalized sensor output value Vn is not corrected.

As described above, according to this embodiment, if the degree of developer degradation is relatively large and the normalized sensor output value Vn deviates from its original value to the extent that the toner deposition amount Cr cannot be accurately derived, the normalized sensor output value Vn is corrected. Then, based on the corrected normalized sensor output value Vn', the toner deposition amount Cr is derived, and consequently, the appropriate development bias voltage DVBf is determined. This allows for accurate high-density process control even under conditions of developer degradation.

Furthermore, by focusing on the slope a and intercept b of the linear approximation equation representing the relationship between the development bias voltage DVB and the toner deposition amount Cs based on the normalized sensor output value Vn, the degree of developer degradation is determined based on this slope a and intercept b, and consequently, the normalized sensor output value Vn is corrected. Therefore, compared to, for example, attempting to determine the degree of developer degradation from the linear line Y (and Y') representing the linear approximation equation itself, as shown in Figure 9 (Figures 9A and 9B), the degree of developer degradation can be determined more accurately, and consequently, the normalized sensor output value Vn can be corrected more accurately.

As mentioned above, the test toner images 200, 200, ... are formed individually by each image forming station 32, 32, ..., but the image forming unit 28, including each of these image forming stations 32, 32, ... is controlled by the control unit 86. This control unit 86, which is responsible for controlling the image forming unit 28, is an example of the image forming control means according to the present invention.

Furthermore, the control unit 86 is responsible for approximating the relationship between the development bias voltage DVB, expressed by the first-order approximation formula in Equation 2 above, and the toner deposition amount Cs based on the normalized sensor output value Vn. This control unit 86, which is responsible for this approximation, is an example of the approximation means according to the present invention.

Furthermore, the control unit 86 is also responsible for correcting the normalized sensor output value Vn based on the aforementioned Equation 3. This control unit 86, which handles the correction, is an example of the correction means according to the present invention.

Then, based on the corrected sensor output value Vn', which is the corrected normalized sensor output value Vn, the toner deposition amount Cr is derived, and consequently, the appropriate development bias voltage DVBf is determined. The control unit 86 also handles the calculation to determine this appropriate development bias voltage DVBf. This control unit 86, which handles this calculation, is an example of the derivation means according to the present invention.

In addition, the data representing the aforementioned reference curve Z is stored in the auxiliary storage unit 88. The auxiliary storage unit 88 that stores this data representing the reference curve Z is an example of the reference curve storage means according to the present invention.

This embodiment is one specific example of the present invention and does not limit the technical scope of the invention. The present invention can also be applied to situations other than those described in this embodiment.

For example, as mentioned above, it is possible to determine the degree of developer degradation based on the slope a and intercept b of a first-order approximation equation representing the relationship between the development bias voltage DVB and the toner deposition amount Cs based on the normalization sensor output value Vn. Therefore, the relationship between the normalization sensor output value Vn and the actual toner deposition amount Cr may be obtained in advance through experiments for each relationship between slope a and intercept b, and data representing this relationship may be stored. Then, the system may be configured so that the actual toner deposition amount Cr is determined by applying the stored data—that is, the actually obtained relationship between slope a and intercept b and the normalization sensor output value Vn—to the stored data.

Furthermore, although the image forming unit 28 employs a tandem system in this embodiment, the present invention can also be applied to configurations employing other systems, such as a rotary system. In particular, in configurations employing a rotary system, a test toner image is formed on the surface of the photoreceptor drum.

Furthermore, the present invention can be applied not only to configurations employing a color image forming unit 28, but also to configurations employing a monochrome image forming unit.

Furthermore, although this embodiment describes a configuration using a two-component developer, the present invention can also be applied to a configuration using a one-component developer.

Furthermore, while this embodiment describes a configuration in which high-density process control is performed as the process control, the present invention is not limited to this. The present invention can also be applied to configurations in which mid-tone process control is performed using mid-tone test toner images.

Furthermore, although this embodiment uses a multifunction printer 10 as an example, the present invention can also be applied to other image forming devices such as dedicated copiers, dedicated printers, and dedicated fax machines.

Furthermore, the present invention is not limited to the form of an image forming apparatus, but can also be provided in the form of a method for adjusting image density in an image forming apparatus.

10... Multifunction printer 28... Image forming unit 30... Transfer unit 32... Image forming station 38... Image sensor 40... Transfer belt 50... Photoreceptor drum 54... Developing device 54a... Developing roller 86... Control unit 86a... CPU
86b... Main memory unit 88... Auxiliary memory unit 200... Test toner image a... Slope b... Intercept Z... Reference curve

Claims (6)

An image forming apparatus comprising image forming means for performing image forming processing by an electrophotographic method,
Image forming control means for controlling the image forming means to form a test toner image on an image carrier constituting the image forming means under conditions of different development bias voltages,
Image density detection means for detecting the density of the test toner image,
Approximation means for approximating the relationship between the density detection value of the test toner image by the image density detection means and the development bias voltage using a first-order approximation formula,
Correction means for correcting the concentration detection value based on the slope and intercept of the aforementioned first-order approximation formula, and
An image forming apparatus further comprising a derivation means for deriving an appropriate development bias voltage based on the density detection value after correction by the correction means. The system further includes a reference curve storage means that stores data relating to a reference curve in which the relationship between the slope and intercept of the first-order approximation formula is approximated for the test toner images formed under different environmental conditions using a standard developer,
The image forming apparatus according to claim 1, wherein the correction means corrects the concentration detection value based on the amount of deviation of the slope and intercept of the first-order approximation formula related to the concentration detection value to be corrected by the correction means from the reference curve. The image forming apparatus according to claim 2, wherein the standard developer is a developer whose degree of degradation is below a predetermined degree. The image forming apparatus according to claim 2 or 3, wherein the amount of deviation is the amount of deviation of the intercept of the first-order approximation formula from the reference curve in the slope of the first-order approximation formula relating to the density detection value subject to correction by the correction means. The image forming means includes an intermediate transfer belt,
The image forming apparatus according to claim 1, wherein the image density detection means detects the density of the test toner image transferred from the image carrier to the intermediate transfer belt. A method for adjusting image density in an image forming apparatus equipped with an image forming means that performs image forming processing by an electrophotographic method,
Image forming control step: Controls the image forming means to form a test toner image on an image carrier constituting the image forming means under conditions where the development bias voltage is different.
Image density detection step for detecting the density of the test toner image,
An approximation step in which the relationship between the density detection value of the test toner image obtained in the image density detection step and the development bias voltage is approximated by a first-order approximation formula,
A correction step in which the concentration detection value is corrected based on the slope and intercept of the first-order approximation formula, and
An image density adjustment method comprising a derivation step of deriving an appropriate development bias voltage based on the density detection value after correction by the correction step.