JP4951993B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to image quality control in an electrophotographic image forming apparatus.

  The image quality of an electrophotographic image forming apparatus is determined by various factors. These elements (hereinafter referred to as “image quality factor”) include not only the density of the image or toner image itself (hereinafter referred to as “image density”), but also the sensitivity of the photoreceptor, charging potential, exposure amount, development bias potential, toner. There are remaining amount and toner concentration (mixing ratio of toner and carrier). In order to appropriately adjust such image quality factors and maintain the image quality in an appropriate state, various detection means (sensors) have been conventionally used in image forming apparatuses (see, for example, Patent Documents 1 to 5). .

On the other hand, due to the demand for downsizing and cost reduction of the image forming apparatus, it is required to realize high image quality with the simplest possible structure. For this reason, the number of sensors as described above tends to be reduced. Since it is mainly the image density that finally determines the image quality, in such a case, only the image density sensor for detecting the image density is generally left. When only the image density sensor is left in this way, the image quality factors other than the image density are only estimated experimentally or empirically.
Japanese Patent No. 2884526 Japanese Patent No. 3172170 Japanese Patent Laid-Open No. 5-273828 JP 2002-148927 A JP 2003-270874 A

  However, in an electrophotographic image forming apparatus, there is a possibility that the image quality factor may change greatly depending on the installation environment and usage conditions of the apparatus. When the image quality factor changes greatly, if only relying on the prediction as described above, there is a possibility that the deterioration of the image quality caused by a change in the image quality factor is corrected by adjusting another image quality factor. As a result, any image quality factor deviates from an appropriate state, and as a result, there is a possibility that the image quality cannot be maintained in a good state.

  The present invention has been made in view of such circumstances, and its purpose is to detect image quality factor states other than image density using an image density sensor, and to correct and maintain them in a good state, thereby improving image quality. It is an object of the present invention to provide a technique that makes it possible to maintain a good state.

In order to achieve the above object, a photoconductive photoconductor, a charging unit for charging the photoconductor to a predetermined charging potential, and a photoconductor charged by the charging unit are exposed to the surface of the photoconductor. Formed by an exposure means for forming an electrostatic latent image, a development means for generating a developing bias potential and forming a toner image corresponding to the electrostatic latent image formed on the photosensitive member by the exposure means, and the development means A density detection unit that detects a density of the toner image; and a control unit that controls operations of the exposure unit and the development unit. The control unit causes the exposure unit to form a predetermined electrostatic latent image, and the development unit And generating a plurality of first toner images by generating different developing bias potentials, and the toner is detected based on the first toner image in which the density detected by the density detecting means is equal to or higher than a predetermined density. Guess developing bias potential development is initiated, have rows potential setting that determines the potential difference between the inferred developing bias potential becomes a predetermined value as the exposure unit potential, determined by the potential setting The state detection second potential is detected by changing at least one of the charging potential and the developing bias potential in the charging means so that the developing contrast value which is the difference between the exposure portion potential and the developing bias potential becomes a predetermined value. When a density of the second toner image detected by the density detection unit exceeds a predetermined density, the toner density in the developing unit is decreased while the second toner image is formed. When the density of the toner image is lower than the predetermined density, toner density control is performed to increase the toner density in the developing unit. After the toner density control, a third toner image for detecting the density of the toner image is formed. After that, there is a difference between the density of the third toner image detected by the density detecting means and a predetermined density. In some cases, an image forming apparatus that performs image density control to change image forming conditions in a direction to eliminate the difference is provided.

  Note that the present invention is not limited to an image forming apparatus, but includes an image quality control method in an image forming apparatus including the above-described charging unit, exposure unit, developing unit, density detection unit, and control unit, It is also possible to provide a program for executing toner density control and image density control.

  Embodiments of the present invention will be described below with reference to the drawings. As examples of the present invention, two embodiments of an electrophotographic image forming apparatus including a rotary developing device and an intermediate transfer belt will be described below. In the following description, unless otherwise specified, the reference potential is 0 V, and various potentials are shown as differences from the reference potential.

[First Embodiment]
FIG. 1 is a block diagram schematically showing the overall configuration of an image forming apparatus 100 according to the first embodiment of the present invention. As shown in the figure, the configuration of the image forming apparatus 100 is roughly divided into a control unit 10, a storage unit 20, a communication unit 30, an operation unit 40, and an image forming unit 50. The control unit 10 is an arithmetic unit including a CPU (Central Processing Unit), an ASIC (Application Specific Integrated Circuit), a RAM (Random Access Memory), a ROM (Read Only Memory), and the like, and executes a program stored in the ROM. Thus, the operation of each part of the image forming apparatus 100 is controlled. The control unit 10 also has a function of executing image processing for obtaining image data suitable for image formation in the image forming unit 50 by ASIC or the like. The storage unit 20 is a storage device such as an HDD (Hard Disk Drive), and stores various data used for image formation. The communication unit 30 is an interface device for exchanging image data with an external device such as a personal computer or a scanner. The operation unit 40 is an input device including a touch panel, displays various types of information related to image formation, and receives instructions from the user. The image forming unit 50 forms a toner image corresponding to the image data supplied from the control unit 10 and fixes the toner image on the recording paper.

  FIG. 2 is a diagram showing the configuration of the image forming unit 50 in more detail. In the figure, the arrows indicated by broken lines indicate the conveyance path of the recording paper. Here, the recording paper is a sheet-like medium for forming an image, and is not particularly limited to paper. The image forming unit 50 includes a photosensitive drum 501, a charger 502, an exposure device 503, a developing device 504, a primary transfer roll 505, a photosensitive cleaner 506, an erase lamp 507, an intermediate transfer belt 511, A plurality of conveying rolls 512, a secondary transfer roll 513, a backup roll 514, an image density sensor 515, a belt cleaner 516, and a fixing device 517 are provided.

  The photosensitive drum 501 is an image carrier having a photosensitive layer on the surface, and is rotated in the direction of arrow A in the drawing by a driving unit (not shown). The charger 502 generates a predetermined potential difference with the photosensitive drum 501 by a high voltage power source, and uniformly charges the surface thereof. The potential generated on the photosensitive drum 501 at this time is hereinafter referred to as “charging potential”. The image forming apparatus 100 of the present embodiment applies a DC voltage or a voltage obtained by superimposing a DC component and an AC component during charging, and the charging potential (effective value) is about −500V. Since the charging potential is stably maintained by the performance of the high-voltage power supply itself, changes due to the state of the photosensitive drum 501 and the like are relatively small.

  The exposure device 503 exposes the photosensitive drum 501 by irradiating light beams according to the image data supplied from the control unit 10. Since the photosensitive drum 501 loses the charge corresponding to the exposure amount, the potential of the photosensitive drum 501 is changed in the portion irradiated with the beam light (hereinafter referred to as “exposure portion”). Hereinafter, the potential of the photosensitive drum 501 in the exposure unit is referred to as “exposure unit potential”. An electrostatic latent image is formed on the exposed portion due to the difference between the charged potential and the exposed portion potential.

  The developing device 504 is a so-called rotary developing device that includes developing units for yellow (Y), magenta (M), cyan (C), and black (K). Each developing unit includes a toner supply unit including a developing roll and an auger, and a toner cartridge for supplying developer to the toner supply unit. Note that the developer in the present embodiment is a two-component developer and includes toner and a carrier. The developing device 504 generates a predetermined potential difference between the developing roll and the photosensitive drum 501 and transfers toner to the photosensitive drum 501 by this potential difference. The potential generated at the developing roll at this time is hereinafter referred to as “developing bias potential”.

  The primary transfer roll 505 generates a predetermined potential difference at a position where the intermediate transfer belt 511 faces the photosensitive drum 501, and transfers the toner image to the intermediate transfer belt 511 by this potential difference. The potential generated in the primary transfer roll 505 at this time is hereinafter referred to as “primary transfer potential”. The photoreceptor cleaner 506 removes untransferred toner remaining on the surface of the photoreceptor drum 5041 after the toner image is transferred. The erase lamp 507 removes static electricity by strongly exposing the surface of the photosensitive drum 5041.

  The intermediate transfer belt 511 is an endless belt member, and the conveyance roll 512 stretches the intermediate transfer belt 511. At least one of the transport rolls 512 has a drive unit, and moves the intermediate transfer belt 511 in the direction of arrow B in the drawing. As the intermediate transfer belt 511 rotates, the toner image transferred by the primary transfer roll 505 is moved to a nip region formed by the secondary transfer roll 513 and the backup roll 514.

  The image density sensor 515 is an optical sensor that detects the density of the toner image before the secondary transfer (hereinafter referred to as “image density”) on the upstream side of the nip region. Specifically, the image density sensor 515 detects the image density based on the intensity of the reflected light (regular reflected light or diffuse reflected light) when the toner image is irradiated with light. The image density sensor 515 of the present embodiment outputs a voltage corresponding to the amount of reflected light, and compares and standardizes this value with the output value of a reflector having a known reflectivity, so that “0” to “1023”. Is converted to the value of "" and output. In this embodiment, the lower the output value, the higher the image density (darker), and the higher the output value, the lower the image density (lighter).

  The secondary transfer roll 513 and the backup roll 514 generate a predetermined potential difference at a position where the intermediate transfer belt 511 faces the recording sheet, and the toner image is transferred to the recording sheet by the potential difference. Note that the secondary transfer roll 513 is electrically grounded and has a reference potential. Hereinafter, the potential generated in the backup roll 514 is referred to as “secondary transfer potential”. A belt cleaner 516 removes untransferred toner remaining on the intermediate transfer belt 511 without being transferred by the secondary transfer roll 513. The fixing device 517 includes a heating roll and a pressure roll, and the toner image transferred onto the recording paper is fixed by heating and pressing the recording paper with these roll members.

  With the above configuration, the image forming apparatus 100 forms a toner image corresponding to the image data input by the user, and fixes the toner image on the recording paper. In addition to such normal image formation, the image forming apparatus 100 forms a toner image (hereinafter referred to as “state detection pattern”) for detecting the state of the image forming unit 50 at a predetermined timing. Image density is controlled by detecting this density with the image density sensor 515. The timing at which the image forming apparatus 100 performs the image quality control is, for example, every predetermined number of prints, when the image forming apparatus 100 is idle, or when the power is turned on. This image quality control is desirably executed when there is a sufficient margin in the remaining amount of toner.

Image quality control performed by the image forming apparatus 100 is as follows:
(1) Potential setting for forming a state detection pattern (2) Toner density control based on the detection result of the state detection pattern (3) Image density control based on the detection result of the state detection pattern In any stage, the image forming apparatus 100 forms a state detection pattern, performs an operation of detecting the pattern by the image density sensor 515, and corrects the image quality factor using the detection result. That is, the image forming apparatus 100 forms the state detection pattern three times in a series of image quality control. Hereinafter, the image quality control executed by the image forming apparatus 100 will be described in detail for each of the above-described stages.

(1) Potential Setting When the image forming apparatus 100 executes toner density control, the image forming apparatus 100 determines in advance the exposure portion potential for performing toner density control. This is to eliminate density fluctuations caused by image quality factors other than toner density from the state detection pattern formed during toner density control. In other words, the processing in (1) is performed in order to set the potential condition (reciprocal relationship between the charging potential, the exposure portion potential, the developing bias potential, etc.) when performing toner density control to a constant level.

  The reason for performing such a process is that a residual potential that varies depending on the environment, usage conditions, and the like is generated on the photosensitive drum 501. Here, the residual potential is a potential generated by electric charges remaining on the surface of the photosensitive drum 501 without being completely discharged even by strong exposure. In general, the residual potential varies depending on the degree of use of the photosensitive drum 501 and is used for a long time, or a strong physical load (contact pressure of the photosensitive cleaner 506, etc.) or an electrical load (charging potential) is applied to the photosensitive drum 501. Or a primary transfer potential) or an environmental load (temperature / humidity, etc.) may be applied, or may be temporarily increased suddenly by applying these loads in combination. The exposed portion potential changes depending on this residual potential.

In normal image formation, the influence of the exposure portion potential on the image quality may be corrected by other image quality factors, so that any value of the exposure portion potential is not a big problem. Therefore, in the image forming apparatus 100, it is assumed that the residual potential RP changes as shown in FIG. 3, and the exposure portion potential V _low is predicted based on the residual potential RP. However, in toner density control, if the potential condition is not kept constant, the factor of density fluctuation, that is, the image quality factor causing density fluctuation cannot be specified. turn into. Therefore, when the toner density control is executed, the exposure portion potential is determined in order to realize a condition that the development contrast value described later is constant.

  In the graph shown in FIG. 3, the vertical axis represents the potential (V), and the horizontal axis represents the thickness (film thickness) of the photosensitive layer of the photosensitive drum 501. In this embodiment, the film thickness (T) is a value predicted based on the total number of rotations of the photosensitive drum 501 and decreases as the number of rotations increases. In the image forming apparatus 100 of this embodiment, the control unit 10 calculates the film thickness by measuring the number of rotations of the photosensitive drum 501, and the time when the film thickness reaches 160 μm Approximate lifetime (replacement time). The film thickness is a value obtained by multiplying the number of rotations of the photosensitive drum 501 by a predetermined coefficient, but the coefficient value varies depending on the charging conditions. The charging conditions here include, for example, a state where charging is not performed, a state where charging is performed using only a DC component, a state where charging is performed by superimposing a DC component and an AC component, and a state where charging is performed with different voltages. This shows the conditions such as the potential of each part. Instead of the total number of rotations of the photosensitive drum 501, prediction based on the driving time of the photosensitive drum 501 may be performed, or these may be combined.

Specifically, the exposure part potential is determined as follows. First, FIG. 4 shows a graph showing the relationship between the image density (D) and the development contrast value (V _deve ) indicated by the state detection pattern. Here, the development contrast value is a difference between the exposure portion potential and the development bias potential, that is, a potential difference. In the present embodiment, an exposure portion potential that satisfies V _deve = “0” is obtained and used for toner density control. That is, the exposed portion potential to be obtained at this time is equal to the developing bias potential, and the developing bias potential at this time is such a potential that D = “d ₁ ”.

In order to obtain such a development bias potential, the control unit 10 of the image forming apparatus 100 forms a plurality of state detection patterns with different development bias potentials, and causes the image density sensor 515 to read each state detection pattern. Process. At this time, the control unit 10 predicts a rough value of the exposed portion potential in advance (hereinafter, this predicted value is referred to as “predicted potential”), and the difference from the predicted potential is about ± 100 V or less. The state detection pattern is formed at the developing bias potential. Then, the control unit 10 specifies a state detection pattern showing the image density closest to the above-described image density d ₁ from these, and determines the developing bias potential when the state detection pattern is formed as the exposure unit potential.

FIG. 5 is a schematic diagram illustrating an example of this determination process. The relationship between the potential (V) of the photosensitive drum 501 and the developing bias potential (V _bias ) and the output value (image density sensor 515 corresponding to this) S). Here, the charging potential V _{h is set} to −500 V, and the predicted potential V _p is set to −240 V. Further, in order to easily understand the contents of the processing, the exposure portion potential determined here is matched with the predicted potential. As shown in the figure, in the state detection pattern (P ₁ and P ₂ ) where the development bias potential is high, the output value of the image density sensor 515 remains constant at “1023”. This indicates that the toner does not transfer at such a developing bias potential, and a state detection pattern is not formed. However, after that, as the developing bias potential becomes lower, the output value of the image density sensor 515 decreases almost linearly. That is, this indicates that the image density of the state detection pattern is gradually increasing.

Here, the control unit 10 indicates the state detection pattern indicating the output value closest to the predetermined threshold Th (“773” in the example of FIG. 5) (that is, the image density is closest to “d ₁ ”). (State detection pattern) is specified, and the developing bias potential when this state detection pattern is formed is specified. In the example of FIG. 5, the state detection pattern P ₅ is specified at this time. Then, the control unit 10 stores the developing bias potential at this time as it is as the exposure unit potential. In this way, the exposed portion potential is determined.

In the image forming apparatus 100 of the present embodiment, the development bias potential (V _bias ) and the output value (S) of the image density sensor 515 have the relationship shown in FIG. As indicated by the solid line in the figure, the output value of the image density sensor 515 decreases almost linearly at the point where development starts. However, the change in the output value is not necessarily constant, and may vary within a range as indicated by a broken line in the drawing according to the film thickness of the photosensitive drum 501 and the toner density difference. Therefore, the threshold value Th is provided in the vicinity of the output value when development is started. In this way, the developing bias potential (that is, the vicinity of the exposed portion potential) can be specified at a position where the fluctuation range is small, so that the error in the exposed portion potential can be reduced.

  In the example of FIG. 5, the case where the predicted potential and the exposure portion potential match is shown. However, the predicted potential and the exposure portion potential do not always match. Therefore, in such a case, a state detection pattern showing an output value closest to the predetermined threshold Th is specified, and the difference between the threshold Th and the output value (with respect to the developing bias potential when the state detection pattern is formed ( That is, a value obtained by performing correction according to the density difference may be used as the exposure portion potential. As described above, the output value of the image density sensor 515 changes substantially linearly. Therefore, when the output value and the threshold value Th match to some extent, the value corresponding to the slope of the straight line is obtained in advance, and the actual exposure portion potential can be accurately predicted therefrom. At this time, if the density difference is a minute value within a negligible range, the value of the developing bias potential may be used as it is without correcting the value of the developing bias potential. If the prediction as described above is used, it is not necessary to form a large number of state detection patterns, and it may be about one or two.

(1a) Modification of potential setting There are other methods for obtaining the exposure portion potential such that the development contrast value is “0”. In this method, the development contrast value at the development start point is specified, and the exposure portion potential is predicted therefrom. Here, the development start point is a point where the output value of the image density sensor 515 starts to deviate from a value when a region where no toner is developed is detected. Specifically, In FIG. 4, the development contrast value (V _deve ) is “V ₀ ”. Since the difference ΔV ₁ between the development start point and the point at which the development contrast value is “0” is almost constant in each apparatus (although it differs for each apparatus), this difference ΔV ₁ is experimentally determined beforehand. It is possible to obtain the exposure part potential by obtaining the above. The processing for obtaining the development start point is performed as follows.

The control unit 10 of the image forming apparatus 100 forms a plurality of state detection patterns by changing the developing bias potential in the same manner as described in the above (1), and each state detection pattern is formed by the image density sensor. A process of causing 515 to read is performed. The relationship between the output value (S) of the image density sensor 515 and the developing bias potential (V _bias ) at this time is as shown in the graph of FIG. 7, as in the case of the schematic diagram of FIG. Of these output values, the controller 10 calculates a straight line L using two or more output values excluding an output value at which the image density is equal to or higher than the image density at the development start point. In the example shown in FIG. 7, straight line approximation is performed by the least square method using output values S _{4 to} S ₉ to calculate straight line L. Incidentally, there it is also possible to use the output value S ₃ in the calculation of the straight line L, but the output value of the developing start point vicinity because it contains much noise, is excluded here.

  When the straight line L is calculated in this way, the intersection point X between the straight line of S = 1023 where the image density is “0” and the approximate straight line L is obtained. Since this intersection X substantially coincides with the development start point described above, the development bias potential (here, −200 V) at the development start point can be obtained by estimation. By adding the above-described difference ΔV (here, 40 V) to this development bias potential, the development bias potential when the development contrast value is “0”, that is, when the development contrast value is “0”. The exposed portion potential can be obtained.

(2) Toner Density Control Next, the control unit 10 forms a toner image under such a potential condition that the exposed portion potential obtained by the above-described potential setting is obtained, and causes the image density sensor 515 to read the toner image. The toner image at this time may be the same as or different from the state detection pattern described above. The control unit 10 stores a target value of the output value of the toner image in advance. This target value coincides with an output value detected when a toner image is formed under the above potential condition when the toner density is an appropriate density. The control unit 10 compares the output value of the image density sensor 515 with this target value to determine whether or not the state detection pattern is formed with an appropriate image density.

  At this time, if the output value of the image density sensor 515 exceeds the target value (that is, the image density is thinner than the target), the control unit 10 supplies more developer from the toner cartridge of the developing device 504 to the toner supply unit. Replenish and increase toner density. As the toner density increases, the charge amount of the toner decreases, and as a result, more toner is transferred to the photosensitive drum 501. On the other hand, when the output value of the image density sensor 515 is lower than the target value (that is, the image density is higher than the target), the control unit 10 stops the replenishment from the toner cartridge for a predetermined time or forcibly supplies the toner. To lower the toner density. As the toner density decreases, the charge amount of the toner increases, and as a result, the toner transferred to the photosensitive drum 501 decreases. Note that the charge amount of the toner may be controlled by increasing or decreasing the stirring speed of the toner by an auger or the like.

  By performing such processing, the image forming apparatus 100 can always perform toner density control under a constant potential condition. Therefore, the image forming apparatus 100 can accurately measure and correct the toner density without being affected by density fluctuations caused by other image quality factors.

(3) Image Density Control After the toner density control described above is executed and the toner density in the developing device 504 is set to an appropriate state, the control unit 10 performs image density control. Image density control is to control image forming conditions so that a toner image is formed at a target image density. Since the toner density has already been corrected and is in an appropriate state, the potential condition at this time may not match the potential condition at the time of toner density control. The image forming condition to be controlled at this time is at least one of a charging potential, an exposure amount, a developing bias potential, a primary transfer potential, and a secondary transfer potential, and the combination of the control is arbitrary. is there.

  By executing the stepwise image quality control as described above, the image forming apparatus 100 according to the present embodiment detects the exposure portion potential without using the surface potential sensor, and the toner concentration without using the toner concentration sensor. It becomes possible to detect. In addition, the image forming apparatus 100 can perform toner density control without being affected by other image quality factors, and can perform image density control with an appropriate toner density. That is, according to the image forming apparatus 100 of the present embodiment, it is possible to satisfactorily detect image quality factors other than image density using the image density sensor and maintain the image quality in a good state.

[Second Embodiment]
Subsequently, a second embodiment of the present invention will be described. This embodiment eliminates the inconvenience that may occur in the first embodiment described above. That is, also in this embodiment, the configuration and operation of the image forming apparatus are basically the same as those in the first embodiment. Therefore, in the following, the description will be focused on the parts different from the first embodiment. In the following description, the image forming apparatus of the present embodiment is referred to as “image forming apparatus 200” and is distinguished from the image forming apparatus 100 of the first embodiment, but the configuration (see FIGS. 1 and 2) is an image. Similar to the forming apparatus 100.

As in (1) of the first embodiment, when the state detection pattern is formed by a plurality of different development bias potentials, the potential difference between the charging potential and the development bias potential (hereinafter referred to as “ΔV ₂ ”) is very large. There is a problem that variations occur. For example, in the example shown in FIG. 5, the potential difference ΔV ₂ when forming the state detection pattern P ₁ is 380V, while the potential difference ΔV ₂ when forming the state detection pattern P ₉ is 140V. In general, the appropriate range of the potential difference ΔV ₂ is about 50 to 250 V. When the voltage difference is less than 50 V, so-called fog is likely to occur. : Beads Carry Over). Therefore, when the state detection pattern is formed by a plurality of different development bias potentials, it is necessary to keep the potential difference ΔV ₂ within an appropriate range for all the state detection patterns.

Therefore, in the present embodiment, the image forming apparatus 200 uses a charging potential different from that during normal image formation when setting the potential so that the potential difference ΔV ₂ is within an appropriate range for all the state detection patterns. Specifically, as shown in FIG. 8, the charging potential V _h is changed from −500 V to −300 V, and the difference in the development bias potential (V _bias ) of each state detection pattern is changed from 30 V to 20 V. By doing so, it is possible to form each state detection pattern in an appropriate state without switching the charging potential. Note that, as shown in FIG. 8, the threshold value Th also changes because the potential condition changes. In the present embodiment, the threshold value Th is “873”, and the development bias potential and the exposure portion potential are determined to be −170V.

Subsequently, when executing the toner density control, the image forming apparatus 200 returns the charging potential to −500V. Since the charging potential is changed, it is necessary to correct the development bias potential and the exposure portion potential according to the change in the charging potential. This correction is performed by using a conversion table as shown in FIG. Using this conversion table, the image forming apparatus 200 corrects the exposure portion potential (V _low ) determined as described above to a value corresponding to the actual charging potential (−500 V). The conversion table is experimentally obtained in advance and stored in the storage unit 20. The image forming apparatus 200 stores a plurality of conversion tables corresponding to the state of the photosensitive drum 501 in the storage unit 20, and switches the conversion table to be used according to the state or the like.

[Modification]
Although the present invention has been described with the two embodiments, the present invention is not limited to these embodiments, and can be implemented in various other modes. In the present invention, for example, the following modifications can be applied to the above-described embodiment. These modifications can be combined as appropriate.

  The above-described embodiment has been described using an image forming apparatus including a so-called rotary type developing device and an intermediate transfer belt, but it is needless to say that an image forming apparatus having another configuration can also be used. In other words, a so-called tandem system in which the number of photosensitive drums is the same as the number of colors of toner may be adopted, or a paper transport belt is provided instead of the intermediate transfer belt, and the image is directly transferred to the recording paper without performing secondary transfer. You may employ | adopt the structure which performs. Further, the image density sensor is not limited to the one that detects the toner image on the intermediate transfer belt, but may be one that detects the toner image on the photosensitive drum or the recording paper.

  Further, in the above-described embodiment, it has been described that the state detection pattern is formed by a plurality of different development bias potentials. However, these may be separated from each other or may be in contact with each other. Good. The state detection patterns in contact with each other can be formed, for example, by forming a single belt-like electrostatic latent image and changing the developing bias potential stepwise within the range of the electrostatic latent image.

  Further, as described above, the image forming apparatus 100 executes image quality control at a time other than normal image formation. In other words, the image forming apparatus 100 has a mode for performing normal image formation (hereinafter referred to as “first mode”) and a mode for performing image quality control (hereinafter referred to as “second mode”). The control unit 10 operates the entire image forming apparatus 100 while appropriately switching to the second mode between the first modes. The switching timing is predetermined in the image forming apparatus 100, but may be configured so that the user can select it. For example, a button for instructing execution of the above-described second mode may be provided on the operation unit so that the user can execute the second mode at an arbitrary timing.

  Further, the photoconductive drum 501 of the image forming apparatus 100 has high stability when it is new, but the stability decreases due to an increase in residual potential with use. For this reason, it is desirable to vary the execution frequency of image quality control depending on the degree of use of the photosensitive drum 501. Specifically, for example, the control unit 10 of the image forming apparatus 100 calculates the film thickness of the photosensitive drum 501 using the graph shown in FIG. 3, and increases the frequency of executing image quality control as the film thickness decreases. It is good to do so.

  Further, since the residual potential may temporarily increase in accordance with the load applied to the photosensitive drum 501, it is desirable to perform image quality control also in such a case. The load applied to the photosensitive drum 501 increases when the charging time per unit time (for example, several hours) increases, when the charging potential itself increases, or when the primary transfer potential increases (see FIG. The charging potential and the primary transfer potential may be changed by image density control, or may be changed by the environment and the type of recording paper). Therefore, even in such a case, the image forming apparatus 100 may increase the frequency of executing image quality control.

  Further, as described above, the residual potential increases as the temperature and humidity increase. Therefore, the image forming apparatus 100 may be provided with a sensor for measuring the temperature or humidity inside the apparatus, and the frequency of executing image quality control may be increased as the temperature and humidity increase.

1 is a block diagram schematically showing an overall configuration of an image forming apparatus according to a first embodiment of the present invention. FIG. 2 is a diagram illustrating a configuration of an image forming unit of the image forming apparatus in more detail. It is the figure which showed the prediction of the change of residual potential RP and exposure part potential _Vlow . FIG. 6 is a diagram illustrating a relationship between an image density (D) and a development contrast value (V _deve ) indicated by a state detection pattern. FIG. 3 is a schematic diagram illustrating an example of determination processing performed by the image forming apparatus. FIG. 6 is a diagram showing a relationship between a developing bias potential (V _bias ) and an output value (S) of an image density sensor. FIG. 6 is a diagram showing a relationship between a developing bias potential (V _bias ) and an output value (S) of an image density sensor. FIG. 3 is a schematic diagram illustrating an example of determination processing performed by the image forming apparatus. It is the figure which illustrated the conversion table for correct | amending exposure part electric potential.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100, 200 ... Image forming apparatus, 10 ... Control part, 20 ... Memory | storage part, 30 ... Communication part, 40 ... Operation part, 50 ... Image forming part, 501 ... Photosensitive drum, 502 ... Charger, 503 ... Exposure apparatus, 504 ... Developing device, 505 ... Primary transfer roll, 506 ... Photoconductor cleaner, 507 ... Erase lamp, 511 ... Intermediate transfer belt, 512 ... Conveying roll, 513 ... Secondary transfer roll, 514 ... Backup roll, 515 ... Image density sensor 516 ... Belt cleaner, 517 ... Fixing device

Claims (1)

A photoconductive photoconductor;
Charging means for charging the photosensitive member to a predetermined charging potential;
Exposing the photosensitive member charged by the charging unit to form an electrostatic latent image on the photosensitive member; and
Developing means for generating a developing bias potential and forming a toner image corresponding to the electrostatic latent image formed on the photosensitive member by the exposure means;
Density detecting means for detecting the density of the toner image formed by the developing means;
Control means for controlling operations of the exposure means and the developing means,
The control means includes
A predetermined electrostatic latent image is formed on the exposure means;
A plurality of first toner images are formed by causing the developing means to generate different development bias potentials,
A development bias potential at which development of a toner image is started is estimated based on a first toner image whose density detected by the density detection unit is equal to or higher than a predetermined density, and a difference from the estimated development bias potential is estimated. There have line potential setting that determines the potential to be a predetermined value as the exposure unit potential,
At least one of the charging potential or the developing bias potential in the charging unit is changed so that the developing contrast value, which is the difference between the exposure portion potential determined by the potential setting and the developing bias potential, becomes a predetermined value. A second toner image for state detection is formed, and after that, when the density of the second toner image detected by the density detection means exceeds a predetermined density, the toner density in the developing means is lowered. On the other hand, when the density of the second toner image is lower than the determined density, toner density control for increasing the toner density in the developing unit is performed,
After the toner density control, a third toner image for detecting the density of the toner image is formed, and then the difference between the density of the third toner image detected by the density detecting means and a predetermined density is determined. An image forming apparatus that performs image density control to change an image forming condition in a direction in which the difference is eliminated .

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