JP7353856B2 - Image forming device - Google Patents

Description

The present invention relates to an image forming apparatus such as a copying machine, a printer, or a facsimile machine using an electrophotographic method or an electrostatic recording method.

2. Description of the Related Art Conventionally, in an image forming apparatus using an electrophotographic method, a toner image is electrostatically transferred from an image bearing member such as a photoreceptor or an intermediate transfer member to a recording material such as paper.

This transfer is often performed by applying a transfer voltage to a transfer member such as a transfer roller that comes into contact with the image carrier to form a transfer portion. If the transfer voltage is too low, "image density is low (transfer missing)" may occur, where the desired image density cannot be obtained due to insufficient transfer. In addition, if the transfer voltage is too high, discharge occurs in the transfer section, and the polarity of the toner charge in the toner image is reversed due to the influence of the discharge, resulting in "white spots" where the toner image is not partially transferred. This may occur. Therefore, in order to form a high quality image, it is required to apply an appropriate transfer voltage to the transfer member.

The amount of charge required for transfer varies depending on the size of the recording material and the area ratio of the toner image. Therefore, the transfer voltage is often applied using constant voltage control that applies a constant voltage corresponding to a predetermined current density. When the transfer voltage is applied under constant voltage control, the transfer is performed in accordance with the predetermined voltage to the area where the target toner image is located, regardless of the current flowing outside the recording material or in the areas where there is no toner image on the recording material. This is because it is easy to secure electric current. However, the electrical resistance of the transfer member that makes up the transfer section changes depending on product variations, member temperature, cumulative usage time, etc., and the electrical resistance of the recording material passing through the transfer section also changes depending on the type of recording material and the surrounding environment. (temperature, humidity), etc. Therefore, when controlling the transfer voltage at a constant voltage, it is necessary to adjust the transfer voltage in response to fluctuations in the electrical resistance of the transfer member and recording material.

Patent Document 1 discloses the following transfer voltage control method in a configuration in which the transfer voltage is controlled at a constant voltage. Immediately before the start of continuous image formation, a predetermined voltage is applied to the transfer section in a state where there is no recording material, the current value is detected, and the voltage value at which a predetermined target current is obtained is determined. Then, a recording material shared voltage corresponding to the type of recording material is added to this voltage value to set a transfer voltage value to be applied under constant voltage control during transfer. With such control, it is possible to apply a transfer voltage according to a desired target current by constant voltage control, regardless of fluctuations in the electrical resistance value of a transfer unit such as a transfer member or fluctuations in electrical resistance value of a recording material. .

Here, the types of recording materials include, for example, types that differ in the smoothness of the surface of the recording material, such as high-quality paper and coated paper, and types that differ in the thickness of the recording material, such as thin paper and thick paper. . The recording material shared voltage can be determined in advance according to the type of recording material, for example. However, there are many types of recording materials in circulation. Also, the electrical resistance of a recording material varies depending on the wet state of the recording material (the amount of moisture contained in the recording material), but even if the environment (temperature and humidity) is the same, the amount of moisture contained in a recording material varies depending on the amount of time it is left in the environment. It varies depending on. Therefore, it is often difficult to accurately determine the recording material shared voltage in advance. If the transfer voltage is not at an appropriate value, including fluctuations in the electrical resistance of the recording material, image defects such as low image density (transfer missing) and white spots may occur as described above.

To address this problem, Patent Documents 2 and 3 disclose a configuration in which the transfer voltage is controlled at a constant voltage, and the current (transfer current) supplied to the transfer unit when the recording material passes through the transfer unit is controlled. It has been proposed to provide upper and lower limits. Note that passing the recording material through the transfer section is also referred to as "paper passing." With such control, the transfer current supplied to the transfer unit during paper passing can be set within a predetermined range, so that it is possible to suppress the occurrence of image defects due to insufficient or excessive transfer current. In Patent Document 2, the upper limit value is determined based on environmental information. In Patent Document 3, the upper limit value and the lower limit value are determined based on the front and back sides of the recording material, the type of recording material, and the size of the recording material in addition to the environment.

Note that in a configuration in which the transfer voltage is controlled at a constant voltage, when the recording material passes through the transfer section, if the current flowing through the transfer member deviates from a predetermined range, the transfer is performed so that the current falls within the predetermined range. Control that changes the target voltage of constant voltage control is also called "limiter control." Further, here, the magnitude (high/low) of voltage and current is compared in absolute value.

Japanese Patent Application Publication No. 2004-117920 Patent No. 4161005 Japanese Patent Application Publication No. 2008-275946

As described above, there is a limiter control that detects the transfer current during paper passing and controls the transfer voltage so that the transfer current is within a predetermined range (below the upper limit value and above the lower limit value). In limiter control, after it is detected that the transfer current is out of a predetermined range, the transfer voltage is changed so that the transfer current falls within the predetermined range. Therefore, in the area of the recording material that passes through the transfer unit between the time the transfer current is detected and the change in transfer voltage is completed, the transfer current is out of the appropriate range, so the density due to excess or deficiency of the transfer current is Image defects such as deterioration may occur.

For example, in a region where the transfer current is less than the lower limit value in a low humidity environment, a low image density (transfer omission) occurs due to insufficient transfer current.

Incidentally, as the apparatus is used, the image density on paper may deviate from the target density. Therefore, an adjustment mode may be executed in which a test chart for density adjustment is output and the image density on paper is automatically adjusted in accordance with a user's instruction from the operation unit. In this adjustment mode, for example, a test chart in which a test image for maximum density adjustment (so-called Dmax control) is transferred onto paper may be output. In addition, a test chart may be output in which a test image is transferred to adjust the gradation from the maximum density to halftones and highlights on paper. In this way, the adjustment mode outputs a test chart in which a plurality of test images for density adjustment are formed based on a user's instruction, and a plurality of test charts may be output.

In an image forming apparatus that performs such automatic image adjustment, when the limiter control described above is performed, the following problems occur. That is, when limiter control is performed, if there is a region where the transfer current is below the lower limit value as described above, the transferability is not stable in that region. Therefore, if a toner image for density adjustment is formed in an area where the transfer current is below the lower limit value, the accuracy of the image adjustment will be reduced. For this reason, when limiter control is executed on the first sheet when outputting multiple test charts, the area where transferability deteriorates that may occur at the leading edge of the first sheet will occur repeatedly on the second and subsequent sheets. There is a possibility that it will happen.

The present invention relates to an image forming apparatus capable of executing an adjustment mode in which a plurality of test charts are output to adjust the density on paper. It is an object of the present invention to output the first test chart in the adjustment mode even if there is a region where the transferability deteriorates due to excess or deficiency of the transfer current, the second or subsequent test charts can be output. The goal is to prevent similar problems from occurring repeatedly.

The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides an image forming section that forms a toner image, an intermediate transfer belt to which the toner image formed by the image forming section is transferred, an inner roller that contacts the inner surface of the intermediate transfer belt, and an inner roller that contacts the inner surface of the intermediate transfer belt. Detects information regarding a transfer member that cooperates with the inner roller to form a transfer portion that transfers a toner image from an intermediate transfer belt to a recording material, a power source that applies voltage to the transfer portion, and a current that flows through the transfer portion. and a control section that controls the power supply, and the control section is configured to detect a detection result detected by the current detection section while the recording material is passing through the transfer section. If the voltage is within a predetermined range defined based on the type of recording material, constant voltage control is performed so that the voltage applied from the power source becomes the target voltage, and the recording material passes through the transfer section. When the detection result of the current detection unit is out of the predetermined range, the target voltage is adjusted so that the detection result is within the predetermined range, and the adjusted target voltage is used as the predetermined voltage. In the image forming apparatus that performs voltage control, the control unit includes a first test chart for adjusting image density, and a second test chart that is output after the first test chart. It is possible to execute a series of adjustment modes for adjusting image density by outputting a plurality of test charts, and when executing the adjustment mode, the control unit controls the control unit so that the first test chart passes through the transfer unit. If the detection result is out of the predetermined range and the target voltage is adjusted during the transfer process, if the tip of the second test chart that passes through the transfer section after the first test chart The target voltage applied when the first test chart passes through the transfer section is determined based on the target voltage adjusted when the first test chart passes through the transfer section , and the first test chart , a plurality of first test images for adjusting the maximum density are formed, a plurality of second test images are formed for adjusting the gradation of the image, and the control The part is configured to detect a predetermined value at the leading end of the first test chart in the conveyance direction when the detection result of the current detection part is out of the predetermined range while the first test chart is passing through the transfer part. The target voltage is changed within the area, the first test image is formed avoiding the predetermined area, and the second test image is formed in an area including the predetermined area. do.

According to the present invention, even if an area where the transferability deteriorates due to excess or deficiency of transfer current occurs when outputting the first test chart in the adjustment mode, the same problem will occur in the second and subsequent test charts. It is possible to suppress the repeated occurrence of defects.

FIG. 1 is a schematic cross-sectional view of an image forming apparatus. FIG. 3 is a schematic diagram of a configuration related to secondary transfer. FIG. 2 is a schematic block diagram showing a control mode of main parts of the image forming apparatus. FIG. 3 is a flowchart diagram for explaining an overview of secondary transfer voltage control. FIG. 3 is a schematic diagram showing an example of table data of recording material shared voltages. FIG. 3 is a schematic diagram showing an example of table data of a predetermined current range. FIG. 3 is a flowchart diagram for explaining automatic image adjustment according to the present invention. FIG. 3 is a chart diagram for explaining a voltage changing method for limiter control. They are a chart diagram and a schematic diagram of an image for explaining the problem. FIG. 2 is a chart diagram and a schematic diagram of an image for explaining the effects of the example. FIG. 3 is a schematic diagram of a test chart for maximum concentration control. FIG. 2 is a schematic diagram showing an example of the relationship between exposure intensity and image density. FIG. 3 is a schematic diagram of a test chart for automatic gradation control.

Hereinafter, the image forming apparatus according to the present invention will be explained in more detail with reference to the drawings.

[Example 1]
1. Overall Configuration and Operation of Image Forming Apparatus FIG. 1 is a schematic configuration diagram of an image forming apparatus 100 of this embodiment. The image forming apparatus 100 of this embodiment is a tandem multifunction device (having the functions of a copying machine, a printer, and a facsimile machine) that employs an intermediate transfer method and is capable of forming a full-color image using an electrophotographic method. ).

The image forming apparatus 100 includes a plurality of image forming sections (stations) including first, second, third, and fourth image forming sections SY, SM, which form images of yellow, magenta, cyan, and black, respectively. Has SC and SK. For elements with the same or corresponding functions or configurations in each image forming unit SY, SM, SC, SK, the suffix Y, M, C, K is omitted to indicate that the element is for one of the colors. This may be explained comprehensively. In this embodiment, the image forming section S includes a photosensitive drum 1, a charging roller 2, an exposure device 3, a developing device 4, a primary transfer roller 5, and a drum cleaning device 6, which will be described later.

A photosensitive drum 1, which is a rotatable drum-shaped (cylindrical) photosensitive member (electrophotographic photosensitive member), serves as a first image carrier that carries a toner image (toner image). clockwise). The surface of the rotating photosensitive drum 1 is uniformly charged to a predetermined potential of a predetermined polarity (negative polarity in this embodiment) by a charging roller 2, which is a roller-type charging member serving as a charging means. The charged surface of the photosensitive drum 1 is scanned and exposed by an exposure device (laser scanner device) 3 as an exposure means based on image information, and an electrostatic image (electrostatic latent image) is formed on the photosensitive drum 1. Ru.

The electrostatic image formed on the photosensitive drum 1 is developed (visualized) by being supplied with toner as a developer by a developing device 4 serving as a developing means, and a toner image is formed on the photosensitive drum 1. In this embodiment, the exposed area (image area) on the photosensitive drum 1, whose absolute value has decreased by being exposed to light after being uniformly charged, is charged to the same polarity as the charging polarity of the photosensitive drum 1. Toner adheres (reverse development method). In this embodiment, the normal charging polarity of the toner, which is the charging polarity of the toner during development, is negative polarity. The electrostatic image formed by the exposure device 3 is a collection of small dot images, and by changing the density of the dot images, the density of the toner image formed on the photosensitive drum 1 can be changed. In this embodiment, the maximum density of each color toner image is about 1.5 to 1.7, and the amount of toner applied at the maximum density is about 0.4 to 0.6 mg/ ^cm2. It has become.

An intermediate transfer belt 7, which is an intermediate transfer body formed of an endless belt and serves as a second image carrier carrying a toner image, is arranged so as to be able to come into contact with the surfaces of the four photosensitive drums 1. ing. The intermediate transfer belt 7 is an example of an intermediate transfer body that transports a toner image that has been primarily transferred from another image carrier to a recording material for secondary transfer. The intermediate transfer belt 7 is stretched around a drive roller 71, a tension roller 72, and a secondary transfer opposing roller 73 as a plurality of tension rollers. Drive roller 71 transmits driving force to intermediate transfer belt 7 . Tension roller 72 controls the tension of intermediate transfer belt 7 to be constant. The secondary transfer opposing roller 73 functions as an opposing member (counter electrode) of the secondary transfer roller 8, which will be described later. The intermediate transfer belt 7 rotates (circumferentially moves) in the direction of arrow R2 (clockwise) in the drawing at a conveyance speed (circumferential speed) of about 300 to 500 mm/sec by rotationally driving the drive roller 71. The tension roller 72 is applied with a force that pushes the intermediate transfer belt 7 from the inner peripheral surface side to the outer peripheral surface side by the force of a spring serving as a biasing means, and this force pushes the intermediate transfer belt 7 in the conveyance direction. A tension of about 2 to 5 kg is applied. On the inner peripheral surface side of the intermediate transfer belt 7, a primary transfer roller 5, which is a roller-type primary transfer member serving as a primary transfer means, is arranged corresponding to each photosensitive drum 1. The primary transfer roller 5 is pressed toward the photosensitive drum 1 via the intermediate transfer belt 7 to form a primary transfer portion (primary transfer nip) N1 where the photosensitive drum 1 and the intermediate transfer belt 7 are in contact with each other. . The toner image formed on the photosensitive drum 1 is electrostatically transferred (primary transfer) onto the rotating intermediate transfer belt 7 by the action of the primary transfer roller 5 in the primary transfer portion N1. . During the primary transfer process, a primary transfer voltage (primary transfer bias) is applied to the primary transfer roller 5 from a primary transfer power source (not shown), which is a DC voltage with a polarity opposite to the normal charging polarity of the toner. is applied. For example, when forming a full-color image, yellow, magenta, cyan, and black toner images formed on each photosensitive drum 1 are sequentially transferred onto the intermediate transfer belt 7 so as to be superimposed.

On the outer peripheral surface side of the intermediate transfer belt 7, a secondary transfer roller 8, which is a roller-type secondary transfer member serving as a secondary transfer means, is arranged at a position facing the secondary transfer opposing roller 73. The secondary transfer roller 8 is pressed toward the secondary transfer opposing roller 73 via the intermediate transfer belt 7 to form a secondary transfer portion (secondary transfer nip) where the intermediate transfer belt 7 and the secondary transfer roller 8 contact each other. ) form N2. The toner image formed on the intermediate transfer belt 7 is transferred to a recording material that is being conveyed between the intermediate transfer belt 7 and the secondary transfer roller 8 by the action of the secondary transfer roller 8 in the secondary transfer portion N2. (Sheet, transfer material) Electrostatically transferred to P (secondary transfer). The recording material P is typically paper, but is not limited to this, and may include synthetic paper made of resin such as waterproof paper, plastic sheets such as OHP sheets, cloth, etc. Sometimes it happens. During the secondary transfer process, a secondary transfer voltage (secondary transfer bias) is applied to the secondary transfer roller 8 from a secondary transfer power source (high voltage power supply circuit) 20, which is a DC voltage with a polarity opposite to the normal charging polarity of the toner. ) is applied. The recording material P is stored in a cassette (recording material set) 11 or the like as a feeding section (paper feeding section, storage section), and a feeding roller 12 is driven based on a feeding start signal to feed the cassette 11. The sheets of paper are fed (feeded) one by one from the paper and sent to the registration rollers 9. After the recording material P is temporarily stopped by the registration rollers 9, the recording material P is supplied to the secondary transfer portion N2 in synchronization with the toner image on the intermediate transfer belt 7.

The recording material P onto which the toner image has been transferred is conveyed to a fixing device 10 as a fixing means by a conveying member or the like. The fixing device 10 fixes (melts, fixes) the toner image on the recording material P by heating and pressurizing the recording material P carrying the unfixed toner image. Thereafter, the recording material P is discharged (output) to the outside of the main body of the image forming apparatus 100.

Further, toner remaining on the surface of the photosensitive drum 1 after the primary transfer process (primary transfer residual toner) is removed from the surface of the photosensitive drum 1 and collected by a drum cleaning device 6 serving as a photosensitive member cleaning means. Further, toner remaining on the surface of the intermediate transfer belt 7 after the secondary transfer process (secondary transfer residual toner) and deposits such as paper dust are removed from the intermediate transfer belt 7 by a belt cleaning device 74 serving as intermediate transfer body cleaning means. removed from the surface and collected.

In this embodiment, the intermediate transfer belt 7 is an endless belt having a three-layer structure from the inner peripheral surface to the outer peripheral surface: a resin layer, an elastic layer, and a surface layer. As the resin material constituting the resin layer, polyimide, polycarbonate, etc. can be used. The thickness of the resin layer is preferably 70 to 100 μm. Further, as the elastic material constituting the elastic layer, urethane rubber, chloroprene rubber, etc. can be used. The thickness of the elastic layer is preferably 200 to 300 μm. Further, as the material for the surface layer, it is desirable to use a material that reduces the adhesion force of the toner to the surface of the intermediate transfer belt 7 and makes it easier to transfer the toner to the recording material P in the secondary transfer portion N2. For example, one or more resin materials such as polyurethane, polyester, and epoxy resin can be used. Alternatively, one or more types of elastic materials such as elastic materials (elastic material rubber, elastomer) and butyl rubber can be used. In addition, one or more types of powders or particles such as fluororesin, or one or more types of these powders or particles can be added to these materials to reduce surface energy and increase lubricity. can be used by dispersing them with different particle sizes. Note that the thickness of the surface layer is preferably 5 to 10 μm. The intermediate transfer belt 7 has its electrical resistance adjusted by adding a conductive agent such as carbon black for adjusting electrical resistance, and preferably has a volume resistivity of 1×10 ⁹ to 1×10 ¹⁴ Ω·cm.

Further, in this embodiment, the secondary transfer roller 8 includes a core metal (base material) and an elastic layer formed of ion conductive foam rubber (NBR rubber) around the core metal. Ru. In this embodiment, the outer diameter of the secondary transfer roller 8 is 24 mm, and the surface roughness Rz of the secondary transfer roller 8 is 6.0 to 12.0 (μm). Further, in this embodiment, the electrical resistance value of the secondary transfer roller 8 is 1×10 ⁵ to 1×10 ⁷ Ω when measured by applying 2 kV at N/N (23° C., 50% RH), and the electric resistance value of the elastic layer The hardness is about 30 to 40° in terms of Asker-C hardness. Further, in this embodiment, the width of the secondary transfer roller 8 in the longitudinal direction (rotation axis direction) (length in the direction substantially perpendicular to the conveyance direction of the recording material P) is approximately 310 to 340 mm. In this embodiment, the width in the longitudinal direction of the secondary transfer roller 8 is the maximum width (the length in the direction substantially orthogonal to the conveyance direction) of the recording material P whose conveyance is guaranteed by the image forming apparatus 100. (maximum width). In this embodiment, since the recording material P is conveyed with the center in the longitudinal direction of the secondary transfer roller 8 as a reference, all the recording materials P whose conveyance is guaranteed by the image forming apparatus 100 are in the longitudinal direction of the secondary transfer roller 8. Pass through the length range. This makes it possible to stably transport recording materials P of various sizes and to stably transfer toner images onto recording materials P of various sizes.

Furthermore, an automatic document feeder 91 and an image reading unit (image reading device) 90 as a reading means are arranged at the top of the main body of the image forming apparatus 100. The automatic document feeder 91 automatically feeds the recording material P on which an image is formed to the image reading section 90 . The image reading unit 90 reads an image on a recording material P that is transported by an automatic document transport device 91 or placed on a platen glass 92 . The image reading unit 90 illuminates the recording material P transported by the automatic document transport device 91 or placed on the platen glass 92 with light from a light source (not shown). The image reading element (not shown) is configured to read the image formed on the recording material P at a predetermined dot density. That is, the image reading section 90 optically reads the image on the recording material P and converts it into an electrical signal.

FIG. 2 is a schematic diagram of a configuration related to secondary transfer. The secondary transfer roller 8 comes into contact with the secondary transfer opposing roller 73 via the intermediate transfer belt 7, thereby forming a secondary transfer portion N2. A secondary transfer power source 20 whose output voltage value is variable is connected to the secondary transfer roller 8 . The secondary transfer opposing roller 73 is electrically grounded (connected to the ground). While the recording material P is passing through the secondary transfer portion N2, a secondary transfer voltage, which is a DC voltage with a polarity opposite to the normal charging polarity of the toner, is applied to the secondary transfer roller 8, and the secondary transfer portion By supplying the secondary transfer current to N2, the toner image on the intermediate transfer belt 7 is transferred onto the recording material P. In this embodiment, a secondary transfer current of, for example, +20 to +80 μA is applied to the secondary transfer portion N2 during the secondary transfer. Note that a roller corresponding to the secondary transfer opposing roller 73 of this embodiment is used as a transfer member, and a secondary transfer voltage having the same polarity as the normal charging polarity of the toner is applied to the roller, and the secondary transfer roller of this embodiment is A roller corresponding to 8 may be used as a counter electrode and electrically grounded.

In this embodiment, based on information regarding the electrical resistance of the secondary transfer portion N2 (mainly the secondary transfer roller 8 in this embodiment) obtained when there is no toner image or recording material P in the secondary transfer portion N2, The secondary transfer voltage applied to the secondary transfer roller 8 is set by constant voltage control during secondary transfer. That is, constant voltage control is performed so that the voltage applied to the secondary transfer roller 8 becomes a predetermined voltage. Further, in this embodiment, the secondary transfer current flowing through the secondary transfer portion N2 is detected during paper feeding. Then, the secondary transfer power source 20 sets a value such that the secondary transfer current is below a predetermined upper limit value and above a lower limit value (herein also simply referred to as a "predetermined current range (predetermined range)"). The secondary transfer voltage to be output is controlled by voltage control (limiter control). This predetermined current range can be set based on various types of information. This various information may include, for example, the following information. First, conditions (recording This is information regarding the type of material P, etc.). It is also information regarding the detection results of the environmental sensor 32 (FIG. 3). Further, it is information regarding the electrical resistance of the secondary transfer portion N2 (mainly the secondary transfer roller 8 in this embodiment) obtained when there is no toner image or recording material P in the secondary transfer portion N2. For example, this predetermined current range can be changed based on information regarding the thickness and width of the recording material P used for image formation. Note that information regarding the thickness of the recording material P and the width of the recording material P can be acquired based on information input from the operation unit 31 or the external device 200. Alternatively, it is also possible to provide a detection means for detecting the thickness and width of the recording material P in the image forming apparatus 100 and perform control based on information acquired by this detection means.

In this embodiment, in order to perform such control, the secondary transfer power source 20 has a current (secondary transfer current) flowing through the secondary transfer portion N2 (that is, the secondary transfer roller 8 or the secondary transfer power source 20). ) is connected to a current detection circuit 21 as a current detection means (current detection section) for detecting the current. Further, a voltage detection circuit 22 is connected to the secondary transfer power supply 20 as a voltage detection means (voltage detection section) that detects the voltage (secondary transfer voltage) outputted by the secondary transfer power supply 20. Note that the control unit 50 may function as a voltage detection unit and detect the voltage output by the secondary transfer power source 20 from the instruction value of the voltage output from the secondary transfer power source 20. In this embodiment, the secondary transfer power source 20, the current detection circuit 21, and the voltage detection circuit 22 are provided within the same high voltage substrate.

2. Control Mode FIG. 3 is a schematic block diagram showing a control mode of main parts of the image forming apparatus 100 of this embodiment. The control unit (control circuit) 50 as a control means includes a CPU 51 as an arithmetic control means which is a central element that performs arithmetic processing, and memories (storage media) such as RAM 52 and ROM 53 as storage means. Ru. The RAM 52, which is a rewritable memory, stores information input to the control unit 50, detected information, calculation results, etc., and the ROM 53 stores control programs, predetermined data tables, etc. The CPU 51 and memories such as the RAM 52 and ROM 53 can transfer and read data between each other.

The control unit 50 is connected to an image reading unit 90 provided in the image forming apparatus 100 and an external device 200 such as a personal computer. Further, an operation section (operation panel) 31 provided in the image forming apparatus 100 is connected to the control section 50 . The operation unit 31 includes a display unit that displays various information to operators such as users and service personnel under the control of the control unit 50, and an input unit that allows the operator to input various settings related to image formation to the control unit 50. It is composed of the following. The operation unit 31 may be configured with a touch panel or the like having the functions of a display unit and an input unit. Job information including control commands related to image formation, such as the type of recording material P, is input to the control unit 50 from the operation unit 31 and the external device 200. The type of recording material P can be distinguished by attributes based on general characteristics such as plain paper, thick paper, thin paper, glossy paper, coated paper, manufacturer, brand, product number, basis weight, thickness, etc. It includes any information. Note that the control unit 50 can obtain information on the type of recording material P by directly inputting the information, and also, for example, when the cassette 11 that stores the recording material P is selected, the control unit 50 can acquire information on the type of recording material P in advance. It can also be acquired from information set in association with 11. Further, a secondary transfer power source 20 , a current detection circuit 21 , and a voltage detection circuit 22 are connected to the control unit 50 . In this embodiment, the secondary transfer power supply 20 applies a secondary transfer voltage, which is a DC voltage controlled at a constant voltage, to the secondary transfer roller 8 . Note that constant voltage control is control such that the voltage applied to the transfer section (that is, the transfer member) has a substantially constant voltage value. Furthermore, an environmental sensor 32 is connected to the control unit 50 . In this embodiment, the environment sensor 32 detects the temperature and humidity of the atmosphere inside the casing of the image forming apparatus 100. Information on temperature and humidity detected by the environmental sensor 32 is input to the control unit 50. The control unit 50 can determine the moisture content (moisture content, absolute moisture content) of the atmosphere inside the casing of the image forming apparatus 100 based on the temperature and humidity detected by the environment sensor 32. The environment sensor 32 is an example of an environment detection unit that detects at least one of temperature and humidity inside and outside of the image forming apparatus 100 . The control section 50 controls each section of the image forming apparatus 100 in an integrated manner based on image information from the image reading section 90 and the external device 200 and control commands from the operation section 31 and the external device 200 to perform image forming operations. Let it run.

Here, the image forming apparatus 100 executes a job (print operation), which is a series of operations to form and output an image on a single or multiple recording materials P, which is started by one start instruction (print instruction). do. A job generally includes an image forming process, a pre-rotation process, a paper spacing process when forming images on a plurality of recording materials P, and a post-rotation process. The image forming process is a period in which the electrostatic image of the image to be actually formed on the recording material P and output, the formation of the toner image, the primary transfer, and the secondary transfer of the toner image are performed. Formation period) refers to this period. More specifically, the timing of image formation differs depending on the position where each of the steps of electrostatic image formation, toner image formation, toner image primary transfer, and secondary transfer is performed. The pre-rotation process is a period from when a start instruction is input until actually starting to form an image, during which preparatory operations are performed before the image forming process. The inter-sheet process is a period corresponding to a period between recording materials P when images are formed on a plurality of recording materials P continuously (continuous image formation). The post-rotation process is a period in which organizing operations (preparatory operations) are performed after the image forming process. The non-image forming period (non-image forming period) is a period other than the image forming period, including the above-mentioned pre-rotation process, paper interval process, post-rotation process, and even when the image forming apparatus 100 is turned on or from the sleep state. This includes a pre-multi-rotation step, which is a preparatory operation for the return. In this embodiment, control for setting the initial value of the secondary transfer voltage, control for determining the upper limit value and lower limit value (predetermined current range) of the secondary transfer current during sheet feeding, etc. are executed during non-image formation. Ru. Note that the sleep state is a state in which power is not supplied to elements of the image forming apparatus 100 other than some elements such as a part of the control unit 50, for example when a predetermined time period has passed since the output of the last image. is in a stopped state. Note that, as will be described later, an adjustment mode in which a test image for density adjustment is output is sometimes called a job like a normal image forming job.

3. Overview of Secondary Transfer Voltage Control Next, an overview of secondary transfer voltage control will be described. FIG. 4 is a flowchart showing an outline of the procedure for secondary transfer voltage control. FIG. 4 shows a simplified procedure for secondary transfer voltage control among the controls executed by the control unit 50 when executing a job, and many other controls when executing a job are omitted. has been done. The same applies to the flowchart of FIG. 7, which will be described later. Further, FIG. 4 shows, as an example, a case where a job of forming an image on one sheet of recording material P is executed.

First, upon acquiring job information from the operation unit 31 or the external device 200, the control unit 50 starts the operation of the job (S1). In this embodiment, the job information includes the following information. That is, the information includes image information specified by the operator and information regarding the recording material P on which the image is formed. Information regarding the recording material P includes the size (width, length) of the recording material P, information related to the thickness of the recording material P (thickness or basis weight), records such as whether the recording material P is coated paper or not. Information related to the surface properties of the material P (paper type category) is included. The control unit 50 writes information about this job into the RAM 52.

Next, the control unit 50 determines a base voltage Vb, which is a voltage output from the secondary transfer power source 20 in order to flow a predetermined target current Itarget when there is no recording material P in the secondary transfer unit N2, and stores it in the RAM 52. It is stored (S2). This base voltage Vb corresponds to a secondary transfer partial voltage, which is a transfer voltage corresponding to the electric resistance of the secondary transfer portion N2 (mainly the secondary transfer roller 8 in this embodiment). The ROM 53 stores information indicating the correlation between environmental information and a target current Itarget for transferring the toner image on the intermediate transfer belt 7 onto the recording material P. In this embodiment, this information is set as table data indicating target current Itarget for each category of atmospheric moisture content. This table data is obtained in advance through experiments and the like. The control unit 50 acquires environmental information (temperature and humidity) detected by the environmental sensor 32. Further, the control unit 50 determines the amount of moisture in the atmosphere based on the environmental information (temperature and humidity) detected by the environmental sensor 32. Then, the control unit 50 determines a target current Itarget corresponding to the environment from information indicating the relationship between the environmental information and the target current Itarget. In addition, the control unit 50 controls the secondary transfer portion N2 (in this embodiment, mainly Obtain information regarding the electrical resistance of the secondary transfer roller 8). Then, based on the information, the base voltage Vb corresponding to the target current Itarget is determined. In this embodiment, the base voltage Vb is determined by the following ATVC control (Active Transfer Voltage Control). With the secondary transfer roller 8 and the intermediate transfer belt 7 in contact with each other, a predetermined voltage (test voltage) or current (test current) is supplied from the secondary transfer power source 20 to the secondary transfer roller 8 . Then, a current value when a predetermined voltage is being supplied or a voltage value when a predetermined current is being supplied is detected. For example, a plurality of levels of test voltages or test currents are supplied to obtain a voltage-current characteristic that is the relationship between voltage and current, and a base voltage Vb corresponding to the target current Itarget is determined based on the voltage-current characteristic. Alternatively, for example, a target current Itarget may be supplied as the test current, and the output voltage value of the secondary transfer power supply 20 at that time may be determined as the base voltage Vb.

Next, the control unit 50 calculates a recording material shared voltage Vp, which is a voltage output from the secondary transfer power source 20 by adding the electrical resistance of the recording material P, and stores it in the RAM 52 (S3). The ROM 53 stores information for determining the recording material shared voltage Vp as shown in FIG. In this embodiment, this information is set as table data indicating the relationship between the moisture content of the atmosphere and the recording material shared voltage Vp for each category of basis weight of the recording material P. The table data for determining the recording material shared voltage Vp is determined in advance through experiments or the like. The control unit 50 determines the amount of moisture in the atmosphere based on environmental information (temperature and humidity) detected by the environmental sensor 32. Then, the control unit 50 calculates the recording material shared voltage Vp from the table data based on the basis weight information of the recording material P included in the job information acquired in S1 and the environment information. . Note that the recording material shared voltage (transfer voltage corresponding to the electrical resistance of the recording material P) Vp may vary depending on the surface properties of the recording material P in addition to information related to the thickness of the recording material P (basis weight). be. Therefore, the table data may be set so that the recording material shared voltage Vp also changes depending on information related to the surface properties of the recording material P. Further, in this embodiment, information related to the thickness of the recording material P (further information related to the surface quality of the recording material P) is included in the job information acquired in S1. However, the image forming apparatus 100 may be provided with measuring means for detecting the thickness of the recording material P and the surface properties of the recording material P, and the recording material shared voltage Vp may be determined based on the information obtained by this measuring means. .

Next, the control unit 50 determines the initial value of the target value (target voltage) of the secondary transfer voltage Vtr to be applied from the secondary transfer power source 20 to the secondary transfer roller 8 during paper feeding, and stores it in the RAM 52 ( S4). That is, the control unit 50 calculates Vb+Vp, which is the sum of the base voltage Vb and the recording material shared voltage Vp, as the initial value of the secondary transfer voltage Vtr before the recording material P reaches the secondary transfer portion N2. and store it in the RAM 52. Then, preparation is made for the timing when the recording material P reaches the secondary transfer portion N2.

Furthermore, the control unit 50 determines the upper limit and lower limit (predetermined current range) of the secondary transfer current during sheet feeding (S5). The ROM 53 stores information, as shown in FIG. 6, for determining the range of current that may be allowed to flow through the secondary transfer portion N2 during sheet feeding from the viewpoint of suppressing image defects. In this embodiment, this information is set as table data indicating the relationship between the moisture content of the atmosphere and the upper and lower limits of the current that may be passed through the secondary transfer portion N2 during sheet feeding. This table data is obtained in advance through experiments and the like. The control unit 50 determines the amount of moisture in the atmosphere based on environmental information (temperature and humidity) detected by the environmental sensor 32. Then, the control unit 50 determines a predetermined current range of the secondary transfer current during sheet passing from the table data based on the environmental information.

Note that the range of current that may be applied to the secondary transfer portion N2 during paper passing changes depending on the width of the recording material P. FIG. 6 shows, as an example, table data set assuming a recording material P having a width equivalent to A4 size (297 mm). A plurality of table data may be set depending on the width of the recording material P. Alternatively, if the width of the recording material P is different from the width equivalent to A4 size, the width equivalent to A4 size can be adjusted by proportional calculation using the ratio of the width of the recording material P actually passed to the width equivalent to A4 size. The values of the table data may be corrected and used. Here, the current flowing through the transfer section when the recording material P passes through the secondary transfer section N2 includes a paper passing section current and a non-paper passing section current. The paper passing portion current is a current that flows through a region (“paper passing portion”) of the secondary transfer portion N2 through which the recording material P passes in a direction substantially perpendicular to the conveyance direction of the recording material P. Further, the non-sheet passing portion current is a current that flows in a region of the secondary transfer portion N2 through which the recording material P does not pass (“non-sheet passing portion”) in a direction substantially perpendicular to the conveyance direction of the recording material P. The current that can be detected during paper passing is the sum of the paper passing section current and the non-paper passing section current. Therefore, the range of current that can be passed through the paper passing section is set in advance in the same way as the table data above, and the current flowing through the non-paper passing section is determined. The predetermined current range may be determined by adding up the range of current that may be allowed to flow. The current flowing through the non-sheet passing portion can be determined, for example, as follows. Using the information (voltage-current characteristics) regarding the electrical resistance of the secondary transfer portion N2 acquired in S2, the current that flows when the secondary transfer voltage Vtr is applied is determined. Then, by proportional calculation using the ratio of the width of the non-paper passing portion to the width of the paper passing portion (difference between the width of the secondary transfer roller 8 and the width of the recording material P), the non-paper passing portion is calculated from the current obtained above. The current flowing through can be found. Further, the predetermined current range for suppressing image defects may vary depending on not only environmental information but also the thickness and surface properties of the recording material P. Therefore, the table data may be set such that the current range changes depending on information related to the thickness of the recording material P (basis weight) and information related to the surface properties of the recording material P. Information on the predetermined current range may be set as a calculation formula. Further, the information on the predetermined current range may be set as a plurality of table data or calculation formulas for each size of the recording material P.

Next, the control unit 50 detects the secondary transfer current by the current detection circuit 21 during sheet feeding, and if the detected secondary transfer current is outside the predetermined current range determined in S5, the control unit 50 detects the secondary transfer current by using the current detection circuit 21. The voltage Vtr is changed (limiter control) (S6). That is, the transfer voltage is changed so that the secondary transfer current is within a predetermined range. At this time, the control unit 50 changes the secondary transfer voltage Vtr by adding an offset voltage ΔVp, which will be described later, to Vb+Vp. In other words, this process corresponds to changing the secondary transfer voltage Vtr by changing Vp of Vb+Vp. To perform this operation, the high voltage substrate that supplies the secondary transfer voltage can repeat the operation of detecting the current at a certain predetermined detection time and switching the high voltage at a certain predetermined response time based on the result. It has become.

To explain further, in limiter control (current limiter control), a detection time (first period) for detecting the transfer current and a signal for changing the transfer voltage based on the detection result of the transfer current at the detection time are output. The response time (second period) in which the response is waited for is repeated. FIG. 8 schematically shows an example of changes in transfer current and transfer voltage under limiter control. This operation is performed by the control unit 50 outputting a signal for changing the voltage output to the secondary transfer power source 20 based on a signal indicating the current detection result inputted from the current detection circuit 21 during the detection period. It will be done. FIG. 8 shows an example in which the secondary transfer voltage is changed when the secondary transfer current detected during sheet feeding is below the lower limit value. As shown in FIG. 8, when the predetermined secondary transfer voltage is applied for 8 ms (response time + detection time), if the secondary transfer current is still below the lower limit value, the secondary transfer voltage is changed as follows. is changed. That is, the secondary transfer voltage is changed to a secondary transfer voltage obtained by adding a predetermined voltage fluctuation range (ΔV in the figure) to the above-mentioned predetermined secondary transfer voltage. Further, this change in the secondary transfer voltage is repeatedly executed until the secondary transfer current detected during sheet feeding reaches the lower limit value. The same applies when the secondary transfer current detected during paper feeding exceeds the upper limit value. When the predetermined secondary transfer voltage is applied for 8 ms (response time+detection time), if the secondary transfer current still exceeds the upper limit value, the secondary transfer voltage is changed as follows. In other words, the secondary transfer voltage is changed to a secondary transfer voltage obtained by subtracting the predetermined voltage variation range (ΔV in the figure) from the above-mentioned predetermined secondary transfer voltage. Further, this change in the secondary transfer voltage is repeatedly executed until the secondary transfer current detected during sheet feeding reaches the upper limit value. Note that the detection time and response time are each about 10 msec, depending on the performance of the high-voltage board. In this example, the detection time and response time are each 8 msec.

Here, the voltage fluctuation width per time in the limiter control is referred to as "voltage fluctuation width ΔVps." In addition, the cumulative value of this voltage fluctuation range ΔVps (when increasing the voltage, add ΔVps, which is a positive value, and when decreasing the voltage, add ΔVps, which is a negative value), is used in limiter control. The amount of voltage change is defined as "offset voltage ΔVp". This offset voltage ΔVp corresponds to the difference between the initial value of the secondary transfer voltage Vtr obtained by adding the base voltage Vb and the recording material shared voltage Vp, and the secondary transfer voltage Vtr after being changed by limiter control. do.

Then, the control unit 50 repeatedly performs the limiter control being notified until the output of the desired image of the job is finished, and ends the job when the output of the desired image of the job is finished.

4. Overview of automatic image adjustment Next, an overview of "automatic image adjustment" will be explained. In this embodiment, based on input from the operation unit 31, automatic image adjustment (also referred to as (automatic image) adjustment mode) can be executed to automatically output a test chart on which a test image for adjusting image density is transferred. It is. In this example, "automatic image adjustment" includes "maximum density control (so-called Dmax control)" that adjusts the maximum density on paper, and adjusts the gradation from the maximum density on paper to halftones and highlights. "Automatic gradation control" is performed.

Automatic image adjustment is an adjustment mode that can be automatically performed at any timing by the user pressing a button on the screen on the UI.

As shown in FIG. 3, when the user presses the automatic image adjustment button on the operation unit 31, the control unit 50 executes the automatic image adjustment mode. The control unit 50 first executes "maximum density control". The control unit 50 acquires the charging potential of the photosensitive drum 1 that is determined according to the charging potential of the photosensitive drum 1 that was set during the previous maximum density adjustment or the temperature and humidity detected by the environmental sensor 32. Then, the control unit 50 controls the charging device so that the charging potential of the photoreceptor drum 1 becomes the acquired charging potential. Thereafter, the control unit 50 changes the exposure intensity to 10 levels and forms a test image (a plurality of patch images) for density adjustment on the photosensitive drum 1. The formed patch image is then transferred onto a recording material, and a test chart for maximum density adjustment as shown in FIG. 11 is output. The density of the plurality of patch images formed on the test chart for maximum density adjustment is measured by the image reading section 90, and density results as shown in FIG. 12 are obtained. Based on the obtained density results, the control unit 50 determines the exposure intensity at which each color density reaches the target maximum density.

Next, after the maximum density adjustment is executed, the control unit 50 subsequently executes "automatic gradation control". The control unit 50 sets the charging potential and exposure intensity of the photosensitive drum determined in the "maximum density control" to the charging potential and exposure intensity of the photosensitive drum 1. Then, the control unit 50 controls the exposure time of the exposure device 3 to form a plurality of patch images for gradation control on the photosensitive drum 1. Then, the patch image formed on the photosensitive drum 1 is transferred to a recording material, and a test chart for automatic gradation control as shown in FIG. 13 is output. The test chart for automatic gradation control is one in which a total of 20 patches are formed for each color in intervals of 12 to 240 in 12 increments for image signals 0 to 255. The patch length in the transport direction for each signal was 19.5 to 20.5 mm. A3 size paper was used. A test chart for automatic gradation control is measured by the image reading section 90. Based on this measurement result, if there is a deviation from the target density for each image signal, the control unit 50 corrects the exposure time by a predetermined amount for the amount of deviation, and adjusts the target gradation for each color. Automatically adjust to.

This automatic gradation control is performed on multiple types of screens such as copy screens, text screens, and image screens. Therefore, the test chart for automatic gradation control shown in FIG. 18 is for one type of screen, and the other screen types are subsequently output and the image reading section 90 reads the density measurement and corrects the exposure time of the corresponding screen. This process is repeated.

When performing automatic gradation control, the greater the number of gradation patches, the higher the accuracy in obtaining the target gradation. In this example, 20 pieces were used for each color and each screen.

As described above, the maximum density and gradation density of the image output on paper can be adjusted (calibrated) by the user instructing automatic image adjustment at any timing from the operation unit 31.

5. Inheritance of Offset Voltage As described above, when limiter control is performed, a predetermined current range for the secondary transfer current during sheet feeding is determined in advance in which image defects can be suppressed. If the detected secondary transfer current is outside the predetermined current range, the transfer voltage is changed so that it falls within the predetermined current range.

However, as can be seen from the limiter control method described above, in limiter control, a time lag occurs from when it is detected that the transfer current is out of a predetermined range until the change of the transfer voltage is completed. Therefore, as mentioned above, in areas where the transfer current that passes through the transfer section until the transfer voltage change is completed is out of the appropriate range, image defects may occur due to excess or deficiency of the transfer current. There is. As described above, if such an image defect occurs on the first sheet, there is a high possibility that a similar image defect will occur on the second and subsequent sheets. This is because it is highly likely that the recording materials used for density adjustment are of the same type, and the conditions in which they are left are also considered to be the same.

FIG. 9 shows the transition of the secondary transfer voltage and secondary transfer current and the occurrence of image defects when a test chart for automatic image adjustment is output without the control of this embodiment described later. Shown schematically. The first sheet is a test chart for maximum density adjustment, and the second and subsequent sheets show the case where test charts for tone control are output. Figure 9 shows the case where image adjustment control is performed using A3 size paper with a basis weight of 90 g/ ^m2 as the recording material P in an environment of 23°C and 5% RH (moisture content 0.9 g/kg or less). It shows. Further, FIG. 9 shows an example where the secondary transfer current detected during the passing of the first sheet falls below the lower limit current. Note that with respect to the recording material P or an image formed on the recording material P, the leading edge and trailing edge refer to the conveying direction of the recording material P.

In the example of FIG. 9, the lower limit value of the predetermined current range is 50 μA, the upper limit value is 70 μA, the target current Itarget of the secondary transfer current is 60 μA, and the initial value of the secondary transfer voltage Vtr determined according to the target current Itarget is Assume that the voltage is 2500V. This secondary transfer voltage Vtr is the total value of the base voltage Vb (=1500V) and the recording material shared voltage Vp (=1000V). The target current Itarget is determined according to environmental information. The base voltage Vb is set to a target current based on information regarding the electrical resistance of the secondary transfer portion N2 (mainly the secondary transfer roller 8 in this embodiment), which is obtained when there is no recording material in the secondary transfer portion N2. Determined accordingly. Further, the recording material shared voltage Vp is determined according to information on the basis weight of the recording material P. For the recording material shared voltage Vp, a value related to the standard recording material P is set in advance as table data indicating the relationship with the environment.

The secondary transfer current detected when the secondary transfer voltage Vtr is applied to the leading edge of the first recording material P is 40 μA, which is less than the lower limit of 50 μA. This is a case where the passed recording material P has the same basis weight as the standard recording material P when the table value of the recording material shared voltage Vp was determined, but the electrical resistance is extremely high due to drying. etc. occur.

Since the secondary transfer current detected at the leading edge of the first recording material P is below the lower limit of 50 μA, the secondary transfer voltage Vtr is changed to 2600V (2500V + voltage fluctuation range ΔVps (=100V)). Then, the secondary transfer current is detected again. Thereafter, the secondary transfer voltage Vtr is changed to increase every voltage fluctuation width ΔVps (=100V) until the secondary transfer current reaches the lower limit value. Then, when the secondary transfer voltage reaches 3200V, the secondary transfer current reaches the lower limit value of 50 μA. Therefore, the secondary transfer voltage Vtr is changed seven times. After the secondary transfer current reaches the lower limit value, the change in the secondary transfer voltage Vtr is stopped, the secondary transfer voltage is maintained at 3200V, and the toner image is transferred toward the trailing edge of the first recording material P. Secondary transfer is performed.

That is, in the example shown in FIG. 9, in the section A from the leading edge of the first recording material P where the secondary transfer current is 40 μA until the secondary transfer current reaches the lower limit current of 50 μA, an image due to insufficient transfer current is generated. Image defects such as low density (transfer missing) occur. In the example shown in FIG. 9, the same secondary transfer voltage control as that for the first sheet is performed for the second sheet, so image defects such as low image density (transfer omissions) due to insufficient transfer current are performed in the same way as for the first sheet. will occur. This is because the recording material P used for the first sheet and the recording material P used for the second sheet are of the same type, and the storage conditions of these recording materials P are also the same. be. In FIG. 9, an image defect caused by an insufficient transfer current has been described as an example, but a similar problem may occur with respect to an image defect caused by an excessive transfer current.

Therefore, in this embodiment, when performing automatic image control, the offset voltage ΔVp in the limiter control of the first test chart is taken over to set the secondary transfer voltage Vtr of the second and subsequent test charts. As a result, it is possible to prevent an area where transferability is degraded due to excess or deficiency of transfer current in the first sheet under automatic image control from repeatedly occurring on the second sheet or later under automatic image control.

In this embodiment, the secondary transfer voltage Vtr for the second and subsequent sheets of automatic image control is set using an offset voltage ΔVp that is substantially the same as the offset voltage ΔVp for the limiter control of the first sheet of automatic image control. In particular, in this embodiment, settings are made as follows. The value of the secondary transfer voltage Vtr applied to the tip of the test chart for the second sheet of automatic image control is set to the voltage value obtained by adding the base voltage Vb and the recording material shared voltage Vp. The voltage value is the sum of the offset voltage ΔVp in the second limiter control. However, setting the secondary transfer voltage Vtr for the second sheet under automatic image control by taking over the offset voltage ΔVp under limiter control for the first sheet under automatic image control is not limited to the following configuration. That is, the setting is not limited to using the same offset voltage ΔVp as the offset voltage ΔVp of the limiter control for the first frame of automatic image control. In other words, if the secondary transfer voltage Vtr of the first sheet of automatic image control is changed by limiter control, the secondary transfer voltage Vtr of the second sheet of automatic image control is changed to the limiter control of the first sheet of automatic image control. It can be determined based on the amount of change in the secondary transfer voltage Vtr. Here, for the sake of simplicity, determining the secondary transfer voltage Vtr for the second sheet of automatic image control based on the amount of change in the secondary transfer voltage Vtr due to limiter control for the first sheet of automatic image control will be simply referred to as " "The offset voltage ΔVp is taken over."

FIG. 7 is a flowchart showing an outline of the procedure of secondary transfer voltage control in this embodiment, including a process of taking over the offset voltage ΔVp of the first sheet of automatic image adjustment. Further, descriptions of procedures similar to those in FIG. 4 will be omitted as appropriate.

The processes in S101 to S105 in FIG. 7 are similar to the processes in S1 to S5 in FIG. 4, respectively.

When the automatic image adjustment JOB is started, the control unit 50 determines the secondary transfer bias and transfer current range according to the flow shown in FIG. Thereafter, test charts for automatic image adjustment are sequentially output as described above. First, when outputting the first sheet (n=1) of the test chart for automatic image adjustment, a transfer voltage Vtr (=Vn=Vb+Vp) is applied (S106). In this embodiment, the first test chart for automatic image adjustment is a test chart for maximum density adjustment. The transfer current In is acquired while the test chart for maximum density adjustment is passing through the transfer section (S107). If the transfer current In when the test chart for maximum density adjustment is passing through the transfer section is outside the predetermined range (No in S108), the transfer voltage is corrected (S109). If the transfer current In does not fall outside the predetermined range (Yes in S108), a test chart for maximum density adjustment is output without correcting the transfer voltage. In this way, a test chart for maximum density adjustment is output, and the output test chart is read by the image reading section 90, and the maximum density is adjusted. When the maximum density adjustment is executed, the control unit 50 outputs a test chart for tone control as the second test chart for automatic image adjustment (S110). Here, regarding the transfer voltage applied from the leading edge of the second sheet for automatic image adjustment, if limiter control was performed during the feeding of the first sheet (No in S108), the corrected transfer voltage Vn' is adopted (S111). That is, the offset voltage ΔVp is taken over to set the secondary transfer voltage Vtr (=Vn') for the second and subsequent test charts. The same applies to the case where an automatic gradation adjustment test chart for another screen is output as the third test chart for automatic image adjustment. The same applies to subsequent test charts.

FIG. 10 shows the setting of the secondary transfer voltage Vtr to be applied to the leading edge of the second test chart by inheriting the offset voltage ΔVp of the first sheet when performing automatic image adjustment under the same conditions as in FIG. 9. FIG. 9 is a diagram similar to FIG. 9 in the case where In the example of FIG. 10, the secondary transfer voltage Vtr applied to the leading edge of the second test chart is approximately the same value as the secondary transfer voltage Vtr after being changed by the limiter control of the first sheet. . In this case, in section A of the first sheet, as in the case of FIG. 9, a region occurs where the transferability is degraded due to insufficient transfer current. However, in the second sheet, the secondary transfer voltage Vtr = 3200V, which is the voltage value obtained by adding the base voltage Vb and the recording material shared voltage Vp (table value), plus the offset voltage ΔVp stored in the first sheet. is applied from the leading edge of the recording material P. Therefore, image defects do not occur in the entire area from the leading edge to the trailing edge of the recording material P.

In this example, the number of patches in the test chart for maximum density adjustment is smaller than the number of patches in the test chart for automatic gradation adjustment, and in the case of A3 size, the length of the patches in the transport direction is 20 mm x 10 so as to avoid section A. May be placed. Here, section A refers to a region where limiter control is performed. In this embodiment, it is within a predetermined area located at the leading end of the test chart (an area sandwiched between a position a predetermined distance from the leading end of the test chart toward the rear end). In this example, when the transfer current In was out of a predetermined range, the transfer voltage was changed by 100V. In order to complete changing the transfer voltage within section A, the amount of change in transfer voltage per step may be changed as appropriate based on the transfer current In. On the other hand, in the second and subsequent gradation adjustment test charts, the gradation adjustment test image may be set to be formed in an area including the section A.

In this embodiment, the initial value of the secondary transfer voltage Vtr when outputting the second test chart is set to Vb+Vp+ΔVp (the coefficient of ΔVp is 1). That is, although the offset voltage ΔVp itself used when outputting the first test chart is inherited, the present invention is not limited to this. For example, it may be set to Vb+Vp+ΔVp×first coefficient (the predetermined coefficient is a value other than 1).

7 Intermediate transfer belt 8 Secondary transfer roller 20 Secondary transfer power source 21 Current detection circuit 22 Voltage detection circuit 50 Control section

Claims (1)

an image forming section that forms a toner image;
an intermediate transfer belt to which the toner image formed by the image forming section is transferred;
an inner roller that contacts the inner surface of the intermediate transfer belt;
a transfer member that cooperates with the inner roller to form a transfer portion that transfers the toner image from the intermediate transfer belt to the recording material;
a power source that applies voltage to the transfer section;
a current detection unit that detects information regarding the current flowing through the transfer unit;
a control unit that controls the power supply;
The control unit controls the power source when the detection result detected by the current detection unit is within a predetermined range defined based on the type of recording material while the recording material is passing through the transfer unit. constant voltage control is executed so that the voltage applied from the current detector becomes the target voltage, and when the recording material is passing through the transfer section, the detection result of the current detection section deviates from the predetermined range. In the image forming apparatus, the image forming apparatus adjusts the target voltage so that the detection result falls within the predetermined range, and executes the constant voltage control using the adjusted target voltage.
The control unit outputs a plurality of test charts including a first test chart for adjusting image density and a second test chart output after the first test chart to adjust the image density. is capable of executing a series of adjustment modes for adjusting the
When the control unit executes the adjustment mode, when the detection result deviates from the predetermined range and the target voltage is adjusted while the first test chart is passing through the transfer unit, The target voltage applied when the leading end of a second test chart that passes through the transfer unit after the first test chart passes through the transfer unit is controlled by the first test chart . determined based on the target voltage adjusted while passing;
The first test chart is formed with a plurality of first test images for adjusting the maximum density, and the second test chart is formed with a plurality of second tests for adjusting the gradation of the image. An image is formed;
The control unit is configured to control the leading end of the first test chart in the conveyance direction when the detection result of the current detection unit is out of the predetermined range while the first test chart is passing through the transfer unit. The target voltage is changed within a predetermined area, and the first test image is formed avoiding the predetermined area, and the second test image is formed in an area including the predetermined area. Features of the image forming device.

Images