Patent Publication Number: US-9885991-B2

Title: Image forming apparatus and control method for an image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an image forming apparatus, e.g., a copying machine or a printer. 
     Description of the Related Art 
     An electrophotographic image forming apparatus includes a photosensitive member, a charging device configured to charge the photosensitive member, an exposure device configured to expose the charged photosensitive member to form an electrostatic latent image, and a developing device configured to develop the electrostatic latent image with a developer. The image forming apparatus is configured to transfer an image onto a sheet, and to then cause a fixing device to fix the image to the sheet by heat and pressure. 
     In this case, for example, a streak-like image (hereinafter referred to as “vertical streak image”) may appear in the image formed on the sheet due to a stain of the exposure device, coating unevenness of the developer caused by the developing device, a scratch on a surface of the photosensitive member, or the like. For example, an image forming apparatus described in U.S. Pat. No. 7,583,408 is configured to form a measurement image on an image bearing member, and to determine whether or not an abnormality has occurred in the image formed by the image forming apparatus based on a result of detecting the measurement image by a sensor. 
     However, the image forming apparatus described in U.S. Pat. No. 7,583,408 is unable to determine that an abnormality has occurred when, for example, the abnormality has occurred in a transfer portion configured to transfer the image onto the sheet. Therefore, the present invention has an object to appropriately determine that an abnormality has occurred in a component of the image forming apparatus. 
     SUMMARY OF THE INVENTION 
     An image forming apparatus according to the present disclosure includes a first image forming unit configured to form an image in a first chromatic color; a second image forming unit configured to form an image in a second chromatic color different from the first chromatic color; and a controller configured to: control the first image forming unit and the second image forming unit to form, on a sheet, a superimposed measurement image in which a first measurement image in the first chromatic color is superimposed on a second measurement image in the second chromatic color; acquire read data relating to the superimposed measurement image formed on the sheet; analyze the read data; and detect a streak image within the first measurement image and a streak image within the second measurement image based on a result of the analysis, wherein: the read data is output from a reading device; the first image forming unit comprises: a first photosensitive member; a first charging unit configured to charge the first photosensitive member; a first exposure unit configured to expose the first photosensitive member to form an electrostatic latent image on the first photosensitive member; and a first developing unit configured to develop the electrostatic latent image on the first photosensitive member using a developer of the first chromatic color on a first sleeve; the second image forming unit comprises: a second photosensitive member; a second charging unit configured to charge the second photosensitive member; a second exposure unit configured to expose the second photosensitive member to form an electrostatic latent image on the second photosensitive member; and a second developing unit configured to develop the electrostatic latent image on the second photosensitive member using a developer of the second chromatic color on a second sleeve; the controller is configured to control the first image forming unit so that an absolute value of a first developing bias applied to the first sleeve becomes larger than an absolute value of a surface potential of the first photosensitive member charged by the first charging unit in order to form the first measurement image; and the controller is configured to control the second image forming unit so that an absolute value of a second developing bias applied to the second sleeve becomes larger than an absolute value of a surface potential of the second photosensitive member charged by the second charging unit in order to form the second measurement image. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a configuration diagram of an image forming apparatus. 
         FIG. 2  is a configuration diagram of a controller. 
         FIG. 3  is an explanatory diagram of relationships between kinds of vertical streaks and components to be replaced. 
         FIG. 4A  and  FIG. 4B  are explanatory diagrams of a state in which a developing sleeve is coated with a developer. 
         FIG. 5  is a diagram for exemplifying how the developer adheres to the developing sleeve and a photosensitive drum. 
         FIG. 6  is a diagram for exemplifying a state in which an intermediate transfer belt has been plastically deformed into a convex shape. 
         FIG. 7  is an explanatory diagram of a state in which exposure with laser light is blocked. 
         FIG. 8  is a diagram for exemplifying a measurement chart. 
         FIG. 9A  and  FIG. 9B  are graphs for showing relationships between a potential of the photosensitive drum and a developing bias. 
         FIG. 10  is a flowchart for illustrating image diagnostic processing. 
         FIG. 11  is an exemplary diagram of a notification image displayed when there is a problem in image quality. 
         FIG. 12  is an explanatory diagram of processing for determining a unit that has caused appearance a vertical streak. 
         FIG. 13A  and  FIG. 13B  are diagrams for exemplifying the measurement chart. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Referring to the accompanying drawings, an embodiment of the present invention is described below. 
     Image Forming Apparatus 
       FIG. 1  is a configuration diagram of an image forming apparatus  1  according to this embodiment. The image forming apparatus  1  includes an image reading section  200  and a printer section  4 . 
     The image reading section  200  includes an original table  202 , a light source  203 , an optical system  204 , a light receiver  205 , a reference white board  206 , a baffle member  207 , and a read image processing portion  208 . The original table  202  is made of glass, and an original  201  to be read is placed on the original table  202 . The light source  203  is configured to irradiate the original  201  placed on the original table  202  with light. The optical system  204  is configured to image the light emitted from the light source  203  and reflected by the original  201  onto the light receiver  205 . The light receiver  205  is, for example, a group of charge-coupled-device (CCD) line sensors for red, green, and blue that are arrayed in three rows, and is configured to receive the light reflected by the original  201 . The light receiver  205  is configured to transmit a color component signal being an electric signal of the corresponding color to the read image processing portion  208  for each line based on the received light. The read image processing portion  208  is configured to conduct image processing for the color component signal acquired from the light receiver  205 , to thereby transmit a generated image signal to the printer section  4 . 
     The printer section  4  includes an image forming portion  10 , an intermediate transfer belt  31 , a fixing device  40 , a conveyance system configured to convey a sheet P, and a printer control portion  209 . The printer section  4  is configured to cause the printer control portion  209  to acquire the image signal transmitted from the read image processing portion  208  of the image reading section  200 . The printer control portion  209  is configured to control image forming processing conducted by the printer section  4 . The image forming portion  10  is configured to generate toner images in respective colors of yellow, magenta, cyan, and black. The image forming portion  10  is configured to transfer the toner images in the respective colors onto the intermediate transfer belt  31  being an intermediate transfer member. The intermediate transfer belt  31  is configured to transfer the transferred toner images onto the sheet P conveyed by the conveyance system. The fixing device  40  is configured to fix the toner images to the sheet P onto which the toner images have been transferred. The sheet P to which the toner images have been fixed is delivered onto a delivery tray  64  provided to the image forming apparatus  1 . 
     Four image forming portions  10  are provided so as to correspond to the respective colors of yellow, magenta, cyan, and black, and each includes a photosensitive drum  11 , a charging device  12 , an exposure device  13 , a developing device  14 , and a drum cleaner  15 . In  FIG. 1 , the image forming portion  10  configured to generate the toner image in yellow, the image forming portion  10  configured to generate the toner image in magenta, the image forming portion  10  configured to generate the toner image in cyan, and the image forming portion  10  configured to generate the toner image in black are arrayed in a row in the stated order from the left of the drawing. 
     In this embodiment, the photosensitive drum  11  is a drum-shaped photosensitive member having an outer diameter of 30 mm and a circumference of approximately 94.2 mm. The photosensitive drum  11  is configured to be rotated by a drive source (not shown) counterclockwise as viewed in the drawing. The charging device  12 , the exposure device  13 , the developing device  14 , and the drum cleaner  15  are arranged around the photosensitive drum  11 . A primary transfer blade  17  is arranged in a position opposed to the photosensitive drum  11  across the intermediate transfer belt  31 . The toner image is formed on the photosensitive drum  11 . 
     In this embodiment, in the developing device  14 , a two-component developer formed of a non-magnetic toner and a low-magnetization high-resistance carrier is used as a developer. The non-magnetic toner is formed by using appropriate amounts of: a binder resin, e.g., a styrene-based resin or a polyester resin; a colorant, e.g., a carbon black, a dye, or a pigment; a release agent, e.g., wax; a charge control agent; and the like. Such a non-magnetic toner is manufactured by a method, for example, a pulverization method or a polymerization method. Examples of a magnetic carrier to be used include a resin carrier formed by dispersing magnetite in a resin as a magnetic material and dispersing a carbon black in order to achieve conductivity and adjust resistance. Another example of the magnetic carrier to be used is a carrier formed by oxidizing or reducing a surface of a single magnetite material, e.g., ferrite, for resistance adjustment. Further, another example of the magnetic carrier to be used is a carrier formed by coating the surface of a single magnetite material, e.g., ferrite, with a resin for the resistance adjustment. 
     The intermediate transfer belt  31  is looped around a secondary transfer inner roller  34  and rollers  36  and  37 , and is configured to rotate clockwise as viewed in the drawing. The secondary transfer inner roller  34  and a secondary transfer outer roller  35  sandwich the intermediate transfer belt  31 , and form a secondary transfer portion. The secondary transfer portion causes toner images to be transferred from the intermediate transfer belt  31  onto the sheet P. The fixing device  40  fixes the toner images to the sheet P onto which the toner images have been transferred. 
     The sheet P is received in a sheet feeding cassette  20  or placed on a sheet feeding tray  25 . The conveyance system controls the driving of the respective conveyance rollers after the sheet P is fed from the sheet feeding cassette  20  or the sheet feeding tray  25  until the sheet P is delivered onto the delivery tray  64  under control of the printer control portion  209 . The conveyance system includes a registration roller pair  23  on upstream of the secondary transfer portion. The registration roller pair  23  conducts skew feed correction for the sheet P. The registration roller pair  23  conveys the sheet P to the intermediate transfer belt  31  at a timing suitable for a timing at which the intermediate transfer belt  31  is to carry the toner images to the secondary transfer portion. 
     The printer section  4  having the above-mentioned configuration forms an image on the sheet P in the following manner. First, the photosensitive drum  11  of the image forming portion  10  has the surface charged uniformly by the charging device  12 . The exposure device  13  forms an electrostatic latent image in the color corresponding to the photosensitive drum  11  by irradiating the charged photosensitive drum  11  with laser light modulated by the image signal of the corresponding color under the control of the printer control portion  209 . The developing device  14  develops the electrostatic latent image with a developer including a toner of the corresponding color, to thereby form a toner image on the photosensitive drum  11 . 
     The toner image formed by each photosensitive drum  11  is primarily transferred onto the intermediate transfer belt  31  by the primary transfer blade  17 . The toner remaining on the photosensitive drum  11  after the transfer is removed by the drum cleaner  15 . The photosensitive drum  11  from which the toner has been removed by the drum cleaner  15  becomes ready to form a subsequent toner image. The intermediate transfer belt  31  has the toner images transferred from the respective photosensitive drums  11  so as to be superimposed on each other. With this configuration, a full-color toner image is formed on the intermediate transfer belt  31 . 
     The sheets P are conveyed from the sheet feeding cassette  20  or the sheet feeding tray  25  to the registration roller pair  23  one by one by the conveyance system. The registration roller pair  23  conducts the skew feed correction for the conveyed sheet P, and stands by until a timing to convey the sheet P to the secondary transfer portion. The registration roller pair  23  conveys the sheet P to the secondary transfer portion at the timing suitable for the timing at which the toner images formed on the intermediate transfer belt  31  are to be carried to the secondary transfer portion. The secondary transfer portion secondarily transfers the toner images formed on the intermediate transfer belt  31  onto the sheet P. While conducting the secondary transfer, the secondary transfer portion conveys the sheet P to the fixing device  40  by the rotation of the secondary transfer inner roller  34  and the secondary transfer outer roller  35 . The toner remaining on the intermediate transfer belt  31  after the secondary transfer is removed by a transfer cleaner  38 , and the intermediate transfer belt  31  becomes ready for subsequent image formation. The fixing device  40  heats and pressurizes the sheet P, to thereby cause the toner image to adhere to the sheet P. The fixing device  40  delivers the sheet P to which the image has been fixed to the delivery tray  64 . 
     Such an image forming apparatus  1  employs a replaceable replacement unit for a part of components in order to facilitate maintenance. As a typical replacement unit, there is a process unit obtained by integrating the photosensitive drum  11  with at least one of a toner cartridge configured to receive a toner, the charging device  12 , the developing device  14 , and the drum cleaner  15 . 
     In this embodiment, the photosensitive drum  11 , the charging device  12 , and the drum cleaner  15  are integrally formed as one process cartridge  50 . The process cartridge  50  is a component (unit) removably attached to a main body of the image forming apparatus  1 . The process cartridge  50  allows the photosensitive drum  11 , the charging device  12 , and the drum cleaner  15  to be replaced at a time. The exposure device  13  is a component (unit) removably attached to the main body of the image forming apparatus  1 . The developing device  14  is a component (unit) removably attached to the main body of the image forming apparatus  1 . Each of the image forming portions  10  for the respective colors may be provided as one component (unit) removably attached to the main body of the image forming apparatus  1 . The primary transfer blade  17  and the intermediate transfer belt  31  are integrally formed as a transfer unit. The transfer unit is a component (unit) removably attached to the main body of the image forming apparatus  1 . The transfer unit allows the primary transfer blade  17  and the intermediate transfer belt  31  to be replaced at a time. The transfer cleaner  38  is a component (unit) removably attached to the main body of the image forming apparatus  1 . Those components (units) removably attached to the main body of the image forming apparatus  1  can be replaced with ease. Therefore, complication in the maintenance of the image forming apparatus  1  is resolved, and a maintenance time period is shortened, which improves convenience. 
     Controller 
       FIG. 2  is a configuration diagram of a controller configured to control an operation of the image forming apparatus  1 . A controller  102  is built into the image forming apparatus  1 . The image forming apparatus  1  is capable of communicating with an information processing apparatus  124  and a server  128 , which are external apparatus, through networks  123  and  130 . The information processing apparatus  124  includes a printer driver  125 , and is configured to transmit print data including information relating to image formation to the image forming apparatus  1  through the network  123 . The server  128  is configured to transmit print data including information relating to image formation to the image forming apparatus  1  through the networks  123  and  130 . The network  130  and the network  123  are separate network environments connected to each other and located apart from each other at, for example, external buildings. There may be provided a plurality of information processing apparatus  124  and a plurality of servers  128 . 
     In order to control operations of the printer section  4  and the image reading section  200  that are described above, the image forming apparatus  1  includes the controller  102 , a network I/F  122 , a display portion  118 , an input portion  120 , and a storage portion  121 . 
     The network I/F  122  is a communication interface between the information processing apparatus  124  and the server  128 . The network I/F  122  is configured to receive the print data transmitted from the information processing apparatus  124  and the server  128 . The network I/F  122  is configured to transmit the received print data to the controller  102 . 
     The controller  102  includes an interpreter  104 , a central processing unit (CPU)  103 , a renderer  112 , a raster image storage portion  113 , an image processing portion  114 , and an image diagnostic portion  126 . In order to analyze the print data, the controller  102  includes a source profile storage portion  107 , a destination profile storage portion  108 , and a device link profile storage portion  110 . The controller  102  is not only formed of a discrete product but also achieved by, for example, a one-chip semiconductor product. Examples of the one-chip semiconductor product include a micro-processing unit (MPU), an application specific integrated circuit (ASIC), and a system-on-a-chip (SOC). 
     The interpreter  104  is configured to acquire the print data from the network I/F  122 . The interpreter  104  is configured to input the acquired print data to the CPU  103 . The CPU  103  is configured to analyze a page description language (PDL) part of the acquired print data from the interpreter  104 , and to generate intermediate language data. The generated intermediate language data is stored in a first intermediate language data storage portion  105 . The CPU  103  includes two CMSs  106  and  109 , and color conversion of the intermediate language data is conducted using the CMSs  106  and  109  to generate the intermediate language data (after CMS). The intermediate language data (after CMS) is stored in a second intermediate language data storage portion  111 . Which of the CMS  106  and the CMS  109  is to be used to conduct the color conversion is set by, for example, the printer driver  125 . 
     The CMS  106  is configured to conduct the color conversion of the intermediate language data using a source profile stored in the source profile storage portion  107  and a destination profile stored in the destination profile storage portion  108 . The “source profile” is a profile for converting a device-dependent color space, e.g., RGB or CMYK, into a device-independent color space, e.g., L*a*b or XYZ defined by the International Commission on Illumination (CIE). In XYZ, colors are expressed by tristimulus values. The “destination profile” is a profile for converting the device-independent color space into a CMYK color space dependent on the image forming apparatus  1 . 
     The CMS  109  is configured to conduct the color conversion of the intermediate language data using a device link profile stored in the device link profile storage portion  110 . The “device link profile” is a profile for directly converting the device-dependent color space, e.g., RGB or CMYK, into the CMYK color space dependent on the image forming apparatus  1 . 
     The renderer  112  is configured to acquire the intermediate language data (after CMS) from the second intermediate language data storage portion  111  to generate a raster image. The renderer  112  is configured to store the generated raster image in the raster image storage portion  113 . The image processing portion  114  is configured to acquire the raster image from the raster image storage portion  113 , and to subject the raster image to image processing for use in the image forming processing to be conducted by the printer section  4 . The image processing portion  114  is also configured to subject the image signal acquired from the image reading section  200  to the image processing for use in the image forming processing to be conducted by the printer section  4 . The image diagnostic portion  126  is configured to form a measurement chart (measurement image) to be used for problem analysis on the sheet P in a case where a problem occurs in image quality of the formed image. The measurement chart formed on the sheet P is used for image diagnostic processing described later. 
     The display portion  118  is an output interface configured to display images indicating an instruction for a user and a status of the image forming apparatus  1 . The input portion  120  is an input interface configured to receive input through the user&#39;s operation. The input portion  120  includes various operation buttons and a touch panel integrated with the display portion  118 . The storage portion  121  is configured to store data processed by the controller  102  and various kinds of data acquired by the controller  102 . 
     Vertical Streak Image 
     As one of abnormalities that occur in the image formed on the sheet P, there is a vertical streak image that appears along a conveyance direction of the sheet P. Examples of a cause of appearance of the vertical streak image include a stain of the exposure device  13 , coating unevenness of the developer caused by the developing device  14 , a scratch on the surface of the photosensitive drum  11 , damage done to the photosensitive drum  11  by the drum cleaner  15 , and a deformation of the intermediate transfer belt  31 . There is also a case where an abnormality occurs in the secondary transfer portion to cause an abnormality in the image formed on the sheet P. 
     In this embodiment, an image is formed on a sheet, and it is determined which component has caused an abnormality based on a result of reading the image. Hitherto, a large number of sheets are required to be output in order to form images in the respective colors on the sheet independently and determine which component has caused an abnormality, and work of reading the sheet becomes complicated. The increase in number of sheets to be used is not preferable from the viewpoint of an environmental load. 
     A description is made of a relationship between each replaceable component of the image forming apparatus  1  and a vertical streak image that appears in the formed image.  FIG. 3  is an explanatory diagram of relationships between kinds of vertical streak image and components to be replaced (replacement units). The kinds of the vertical streak image include a “development white streak”, a “photosensitive drum cleaning defect streak”, an “intermediate transfer belt cleaning defect streak”, a “photosensitive drum scratch streak”, an “intermediate transfer belt deformation white streak”, and an “exposure defect white streak”. The image forming apparatus  1  stores a table for showing such relationships between the vertical streak images and the replacement units in a predetermined memory area in advance. 
     The “development white streak” is a streak image that appears depending on a state in which a developing sleeve of the developing device  14  is coated with a developer.  FIG. 4A  and  FIG. 4B  are explanatory diagrams of the state in which the developing sleeve is coated with the developer. As illustrated in  FIG. 4A , the developing device  14  includes a developing sleeve  142  which contains a magnet  141  as a carrying member for the developer, and which is supported by a developing container  143  so as to be free to rotate. The developing sleeve  142  is configured to rotate counterclockwise as viewed in the drawing. The developing sleeve  142  has the surface coated with a developer, and supplies the developer to the photosensitive drum  11  at a closest part  145  to the photosensitive drum  11 . An amount (thickness) of the developer caused to adhere to the developing sleeve  142  is regulated by a regulating blade  146 . This guarantees the amount of the developer to be supplied to the photosensitive drum  11  at the closest part  145 . 
     As illustrated in  FIG. 4B , in a case where foreign matter  148 , e.g., hair, is stuck at a part between the developing sleeve  142  and the regulating blade  146 , the developing sleeve  142  fails to be coated in the above-mentioned part with the developer.  FIG. 5  is a diagram for exemplifying how the developer adheres to the developing sleeve  142  and the photosensitive drum  11  in the above-mentioned state. The developing sleeve  142  exhibits an area  51  in which the developer is not borne due to the influence of the foreign matter  148 . This causes the photosensitive drum  11  to exhibit an area  52  of not being developed. In the thus-formed image, a continuous white streak appears in a straight line due to the area  52 . The “development white streak” is caused in this manner. The cause lies in the foreign matter  148  of the developing device  14 , and hence the unit to be replaced in order to resolve the development white streak is the developing device  14 . 
     The “photosensitive drum cleaning defect streak” is a streak image that appears due to a cleaning defect of the photosensitive drum  11  caused by the drum cleaner  15 . The photosensitive drum cleaning defect streak appears because a part of the drum cleaner  15  in abutment with the photosensitive drum  11  is chipped to inhibit the toner remaining on the photosensitive drum  11  from being cleared after the transfer. The color of the streak image that has appeared is a color of the image formed on the photosensitive drum  11 . A yellow streak image appears in the case of the cleaning defect of the photosensitive drum  11  on which the image in yellow is formed. A magenta streak image appears in the case of the cleaning defect of the photosensitive drum  11  on which the image in magenta is formed. A cyan streak image appears in the case of the cleaning defect of the photosensitive drum  11  on which the image in cyan is formed. A black streak image (black streak) appears in the case of the cleaning defect of the photosensitive drum  11  on which the image in black is formed. The streak image due to the cleaning defect of the photosensitive drum  11  continuously appears on the image, in particular, on the sheet P in a white ground part, substantially in a straight line. The cause lies in the cleaning defect of the photosensitive drum  11 , and hence the unit to be replaced in this case is the process cartridge  50  corresponding to the color of the streak image that has appeared. 
     The “intermediate transfer belt cleaning defect streak” is a streak image that appears due to a cleaning defect of the intermediate transfer belt  31  caused by the transfer cleaner  38 . The intermediate transfer belt cleaning defect streak appears because a part of the transfer cleaner  38  in abutment with the intermediate transfer belt  31  is chipped to inhibit the toner remaining on the intermediate transfer belt  31  in the chipped part from being cleared after the secondary transfer. The streak image that has appeared exhibits a mixed color of the respective colors of yellow, magenta, cyan, and black. The cause lies in the cleaning defect of the intermediate transfer belt  31 , and hence the unit to be replaced in this case is the transfer cleaner  38 . 
     The “photosensitive drum scratch streak” is a streak image that appears due to a scratch on the photosensitive drum  11 . The photosensitive drum scratch streak appears because, for example, a concave scratch appears on the surface of the photosensitive drum  11  when the photosensitive drum  11  is rotated with high-hardness foreign matter being sandwiched between the drum cleaner  15  and the photosensitive drum  11 . The photosensitive drum  11  is not sufficiently charged in the part of the concave scratch by the charging device  12 , and exhibits a lower potential of the electrostatic latent image than in other parts. Hence, at a time of development, the electrostatic latent image is developed with a larger amount of toner than in other parts, which increases a density of the image on the sheet P. When a scratch appears on the photosensitive drum  11 , the streak image on the sheet P appears with a cycle of an outer circumferential length (in this case, approximately 94.2 mm) of the photosensitive drum  11 . Therefore, in order to positively detect the streak image, it is necessary to form an image having a length equal to or longer than the outer circumferential length of the photosensitive drum  11  in the conveyance direction of the sheet P. The cause lies in the scratch on the photosensitive drum  11 , and hence the unit to be replaced in this case is the process cartridge for the color of the streak image that has appeared. 
     The “intermediate transfer belt deformation white streak” is a streak image that appears due to the plastic deformation of the intermediate transfer belt  31 . The intermediate transfer belt  31  may become deformed into a convex shape in a part in which an inner scraped powder adheres to the rollers  36  and  37  due to long-term use.  FIG. 6  is a diagram for exemplifying a state in which the intermediate transfer belt  31  has been plastically deformed into a convex shape. When a deformed part  61  having a convex shape appears on the intermediate transfer belt  31 , parts on both sides of the deformed part  61  become difficult to be brought into contact with the photosensitive drum  11  and the sheet P at a time of the transfer. For that reason, the above-mentioned parts of the image formed on the sheet P become white vertical streak images. The white vertical streak images appear in all the colors of yellow, magenta, cyan, and black. The cause lies in the plastic deformation of the intermediate transfer belt  31 , and hence the unit to be replaced in this case is the transfer unit. 
     The “exposure defect white streak” is a streak image that appears due to blocking of the exposure of the photosensitive drum  11  with the laser light emitted from the exposure device  13 .  FIG. 7  is an explanatory diagram of a state in which the exposure with the laser light is blocked.  FIG. 7  is an illustration of a state in which foreign matter  135  (dust, hair, toner, or the like) adheres to a dust-proof window  132  of the exposure device  13 . A part of the laser light emitted from the exposure device  13  is blocked by the foreign matter  135 , which inhibits the electrostatic latent image from being formed on the photosensitive drum  11 . This causes a white vertical streak image. The cause lies in an exposure defect caused by the exposure device  13 , and hence the unit to be replaced in this case is the exposure device  13 . The white streak image appears due to the blocking of the laser light, and therefore does not appear in such an image formed without using the laser light described later. 
     Measurement Chart 
     In order to specify, from the vertical streak image, the unit (component) which is the cause of the streak, the image forming apparatus  1  forms a measurement chart (measurement image) on the sheet P. A description is made below of an example of forming the measurement chart on the sheet P having an A3 portrait size (297 mm in width direction length and 420 mm in conveyance direction length). 
       FIG. 8  is a diagram for exemplifying the measurement chart. The measurement chart includes white ground parts W 1  and W 2 , an analog image A 1  and a digital image D 1  that are formed using toners of chromatic colors (yellow, magenta, and cyan), and an analog image A 2  and a digital image D 2  that are formed using a toner of black. The analog image A 1  and the digital image D 1  are formed by transferring an image in yellow, an image in magenta, and an image in cyan onto the sheet P with those images being superimposed on each other. The digital images are each an image formed using the irradiation with the laser light emitted from the exposure device  13 . The analog images are each an image formed without using irradiation with the laser light emitted from the exposure device  13 . 
     A description is made of formation methods for the digital images D 1  and D 2  and the analog images A 1  and A 2 .  FIG. 9A  and  FIG. 9B  are graphs for showing relationships between a potential of the photosensitive drum  11  and a developing bias applied to the developing sleeve  142  which are exhibited when the digital images D 1  and D 2  and the analog images A 1  and A 2  are respectively formed. In  FIG. 9A  and  FIG. 9B , the Y-axis indicates a surface potential of the photosensitive drum  11 . In the image forming apparatus  1 , the developing device  14  agitates the toner, to thereby charge the toner to a negative polarity. 
     When the digital images D 1  and D 2  are formed, the surface potential of the photosensitive drum  11  is controlled as shown in  FIG. 9A . The charging device  12  charges the photosensitive drum  11  so that the surface potential of the photosensitive drum  11  becomes a dark part potential Vd. The exposure device  13  emits the laser light to expose the photosensitive drum  11 . The potential of the photosensitive drum  11  in the area irradiated with the laser light corresponds to a bright part potential Vl, and the potential of the photosensitive drum  11  in the area that is not irradiated with the laser light corresponds to the dark part potential Vd. A direct-current developing bias Vdc is applied to the developing sleeve  142 . A surface potential of the developing sleeve  142  corresponds to the developing bias Vdc. When the digital images D 1  and D 2  are formed, the developing bias Vdc is controlled to a value higher than the dark part potential Vd and lower than the bright part potential Vl. The dark part potential Vd is, for example, −400 V, the developing bias Vdc is, for example, −300 V, and the bright part potential Vl is, for example, −100 V. With this configuration, the toner (negative polarity) borne on the developing sleeve  142  adheres to the photosensitive drum  11  in the area (bright part potential Vl) irradiated with the laser light, but does not adhere to the photosensitive drum  11  in the area (dark part potential Vd) that is not irradiated with the laser light. 
     When the analog images A 1  and A 2  are formed, the surface potential of the photosensitive drum  11  is controlled as shown in  FIG. 9B . The charging device  12  charges the photosensitive drum  11  so that the surface potential of the photosensitive drum  11  becomes the dark part potential Vd. The exposure device  13  does not expose the photosensitive drum  11 . Then, a direct-current developing bias Vdc 2  is applied to the developing sleeve  142 . A surface potential of the developing sleeve  142  corresponds to the developing bias Vdc 2 . When the analog images A 1  and A 2  are formed, the developing bias Vdc 2  is controlled to a value lower than the dark part potential Vd. The dark part potential Vd is, for example, −400 V, and the developing bias Vdc 2  is, for example, −550 V. With this configuration, the toner (negative polarity) borne on the developing sleeve  142  adheres to the photosensitive drum  11  in an area opposed to the developing sleeve  142 . 
     A difference between the bright part potential Vl and the developing bias Vdc is referred to as “developing potential difference Vc”. A difference between the dark part potential Vd and the developing bias Vdc is referred to as “developing potential difference Vb”. A difference between the dark part potential Vd and a developing bias Vdc 2  is referred to as “developing potential difference Vc 2 ”. The developing potential difference Vc exhibited when the digital images D 1  and D 2  are formed is different from the developing potential difference Vc 2  exhibited when the analog images A 1  and A 2  are formed. The image forming apparatus  1  switches the developing bias when using the developing bias, to thereby form the analog images A 1  and A 2  and the digital images D 1  and D 2 . 
     The analog images A 1  and A 2  of the measurement chart are formed to have a conveyance direction length of, for example, approximately 30 mm, which allows detection of whether or not an exposure white streak has appeared. The digital images D 1  and D 2  have a conveyance direction length of, for example, 100 mm, which allows detection of the white streak image ascribable to the scratch that has appeared on the photosensitive drum  11 . The above-mentioned length is a length that allows the white streak image ascribable to the scratch that has appeared on the photosensitive drum  11  to be positively detected on the photosensitive drum  11  having a circumferential length of approximately 94.2 mm. 
     The analog image A 1  and the digital image D 1  of the measurement chart are both formed with the images in the chromatic colors of yellow, magenta, and cyan being superimposed on each other. The image reading section  200  according to this embodiment is configured to cause the light receiver  205  to receive the light reflected from the original  201  after passing the reflected light through filters of red, blue, and green. The light receiver  205  receives a cyan component bypassing the reflected light through the filter of red, a yellow component bypassing the reflected light through the filter of blue, and a magenta component by passing the reflected light through the filter of green. When the image in black is superimposed on the images in other colors, it is difficult for the light receiver  205  to receive the other colors with high precision. Therefore, in the measurement chart according to this embodiment, the image in black and the images in the other chromatic colors are formed separately from each other. With this configuration, according to this embodiment, it is possible to positively detect the vertical streak image. 
     Determination of Replacement Unit 
       FIG. 10  is a flowchart for illustrating the image diagnostic processing conducted by the image forming apparatus  1  configured as described above. The controller  102  of the image forming apparatus  1  executes the image diagnostic processing in response to an instruction to diagnose the image, which is issued by the user or a service person through the input portion  120 . 
     When acquiring the instruction to diagnose the image, the controller  102  of the image forming apparatus  1  causes the printer section  4  to form the measurement chart exemplified in  FIG. 8  on the sheet P and to output the sheet P (Step S 301 ). The output sheet P is placed on the original table  202  of the image reading section  200  by the user or the service person. The controller  102  causes the image reading section  200  to execute an operation for reading the measurement chart. The image reading section  200  reads the measurement chart, and outputs read data (image signal) corresponding to a result of reading the measurement chart to the controller  102 . The controller  102  acquires the read data (image signal) on the measurement chart output from the image reading section  200  (Step S 303 ). 
     The controller  102  executes analysis processing for the acquired read data (image signal) on the measurement chart to acquire an image feature amount (Step S 305 ). In this embodiment, the controller  102  analyzes the vertical streak image based on the measurement chart. The controller  102  analyzes the image signal of the measurement chart, and stores a raster image of the measurement chart in the raster image storage portion  113 . The image diagnostic portion  126  of the controller  102  uses the raster image of the measurement chart to detect a line that meets conditions from the uniform image parts within a surface of the measurement chart. For example, the image diagnostic portion  126  extracts straight lines extending in the conveyance direction of the sheet P and a direction perpendicular to the conveyance direction. The controller  102  measures a width and a length of the extracted line, and acquires a result of the measurement as the image feature amount. 
     The image diagnostic portion  126  of the controller  102  compares the image feature amount with a predetermined threshold value to determine whether or not a problem has occurred in image quality (Step S 307 ). In this case, in a case where the width of the line being the image feature amount is wider than a threshold value Wth, or in a case where the length of the line is longer than a threshold value Lth, the image diagnostic portion  126  determines that the vertical streak image has appeared in the measurement chart. In short, the controller  102  determines that a problem has occurred in the image quality. 
     When a problem has occurred in the image quality (Y in Step S 307 ), the controller  102  causes the display portion  118  to notify that there is a problem in the image quality (Step S 309 ).  FIG. 11  is a diagram for exemplifying a notification image displayed on the display portion  118  when there is a problem in the image quality. In this example, such a specific message as to be easily understood by the user or the service person and information converted into a code are displayed. The notification image of  FIG. 11  includes details of the problem in the image quality and a name of the unit that has caused the problem. When no problem has occurred in the image quality (N in Step S 307 ), the controller  102  causes the display portion  118  to notify that there is no problem in the image quality (Step S 310 ). The controller  102  stores the image feature amount in the storage portion  121  after the notification of a result of diagnosing the image quality (Step S 311 ). In this manner, the occurrence of the problem in the image quality and the replacement unit are notified by displaying a specific message, which allows the user or the service person to easily determine the unit that needs to be replaced. 
       FIG. 12  is an explanatory diagram of processing for determining the unit that has caused the appearance of a vertical streak. This processing corresponds to the processing of Step S 305  and Step S 307  of  FIG. 10 . 
     The image diagnostic portion  126  of the controller  102  uses the raster image of the measurement chart to determine whether or not the white ground parts W 1  and W 2  are free from the appearance of the streak image having a vertical streak shape and a chromatic color (Step S 401 ). When the streak image has appeared (N in Step S 401 ), the image diagnostic portion  126  determines whether or not the streak image exhibits a color mixture (Step S 402 ). When the identified color is a single color of yellow, magenta, cyan, or black (N in Step S 402 ), the image diagnostic portion  126  determines that the streak image is ascribable to the cleaning defect of the photosensitive drum. In this case, the controller  102  determines that the process cartridge  50  for the corresponding color is the replacement unit, and displays to that effect on the display portion  118 . When the identified color is a color mixture (Y in Step S 402 ), the image diagnostic portion  126  determines that the appearance of the streak image is ascribable to the cleaning defect of the intermediate transfer belt  31 . In this case, the controller  102  determines that the transfer cleaner  38  is the replacement unit, and displays to that effect on the display portion  118 . 
     In a case where no streak image has appeared in the white ground part W 1  or W 2  (Y in Step S 401 ), the image diagnostic portion  126  uses the raster image of the measurement chart to determine whether or not the streak image has appeared in the digital images D 1  and D 2  (Step S 403 ). In a case where no streak image has appeared (N in Step S 403 ), the controller  102  determines that no vertical streak has appeared, and displays to that effect on the display portion  118 . 
     In a case where the streak image has appeared in the digital images D 1  and D 2  (Y in Step S 403 ), the image diagnostic portion  126  determines whether or not the white streak image having a vertical streak shape has appeared in the analog images A 1  and A 2  (Step S 404 ). In a case where no streak image has appeared (N in Step S 404 ), the image diagnostic portion  126  determines that the appearance of the streak image within the digital images D 1  and D 2  is ascribable to the exposure defect. In this case, the controller  102  displays an image for instructing to clean or replace the exposure device  13  on the display portion  118 . 
     In a case where the white streak image has appeared in the analog images A 1  and A 2  (Y in Step S 404 ), the image diagnostic portion  126  determines whether or not the color of the streak image that has appeared in the digital images D 1  and D 2  is only a specific color (Step S 405 ). In a case where the color of the streak image includes all the colors of yellow, magenta, cyan, and black (N in Step S 405 ), the image diagnostic portion  126  determines that the appearance of the streak image is ascribable to the plastic deformation of the intermediate transfer belt  31 . In this case, the controller  102  determines that the transfer unit is the replacement unit, and displays to that effect on the display portion  118 . 
     In a case where the color of the streak image that has appeared in the digital images D 1  and D 2  is a specific color among yellow, magenta, cyan, and black (Y in Step S 405 ), the image diagnostic portion  126  examines appearance cycle of the streak image along the conveyance direction. The image diagnostic portion  126  determines whether or not the streak image is ascribable to the scratch on the photosensitive drum  11  based on whether or not the appearance cycle of the streak image corresponds to a rotation cycle of the photosensitive drum  11  (Step S 406 ). 
     In a case where the appearance cycle of the streak image is the same as the circumferential length of the photosensitive drum  11 , which is 94.2 mm (Y in Step S 406 ), the image diagnostic portion  126  determines that the appearance of the streak image is ascribable to the scratch on the photosensitive drum  11 . In this case, the controller  102  determines that the process cartridge  50  for the color identified in the processing of Step S 405  is the replacement unit, and displays to that effect on the display portion  118 . In a case where the appearance cycle of the streak image is different from the circumferential length of the photosensitive drum  11  (N in Step S 406 ), the image diagnostic portion  126  determines that the appearance of the streak image is ascribable to a coating defect of the developing device  14 . In this case, the controller  102  determines that the developing device  14  for the color identified in the processing of Step S 405  is the replacement unit, and displays to that effect on the display portion  118 . 
     The above-mentioned processing using the measurement chart allows the image forming apparatus  1  to determine the unit that has caused the streak image and needs to be replaced. The image forming apparatus  1  displays the unit that needs to be replaced on the display portion  118 , to thereby greatly reduce a time period necessary for the user or the service person to identify the cause of the streak image. This greatly reduces downtime of the image forming apparatus  1  that occurs due to maintenance. It is also possible to positively identify the unit that has caused the appearance of the streak image, which prevents a unit irrelevant to the appearance of the streak image from being replaced, and suppresses a wasteful maintenance cost. 
     In this embodiment, the measurement chart includes the analog image A 1  and the digital image D 1  that are obtained by superimposing yellow, magenta, and cyan on each other, and hence the image diagnostic processing can be conducted with one sheet P having the measurement chart formed thereon. Hitherto, at least two A3-size sheets P are required when analog images and digital images are respectively formed in yellow, magenta, and cyan. Hence, in this embodiment, inconvenience felt by the user or the service person causing the image forming apparatus  1  to read the sheet P having the measurement chart printed thereon through the image reading section  200  can be suppressed to a minimum level. It is also possible to minimize the number of sheets P required for identifying the unit that has caused the streak image. 
     The measurement chart of  FIG. 8  is merely an example, and the layout of the white ground parts W 1  and W 2 , the analog images A 1  and A 2 , and the digital images D 1  and D 2  is not limited thereto. A measurement chart obtained by arranging those images in another order can produce the same effect by the above-mentioned processing. In this embodiment, as conditions for forming the analog images A 1  and A 2  and the digital images D 1  and D 2 , the dark part potential of the photosensitive drum  11  is fixed, while the developing bias is changed. The controller  102  controls a charging voltage applied to the charging device  12  so that the surface potential of the photosensitive drum  11  charged by the charging device  12  becomes the dark part potential Vd. Then, the controller  102  controls the direct-current developing bias applied to the developing sleeve  142  to a first value Vdc and a second value Vdc 2 . However, the dark part potential of the photosensitive drum  11  may be changed while the developing bias is fixed. The controller  102  controls the direct-current developing bias applied to the developing sleeve  142  to a predetermined value Vdc. Then, the controller  102  controls the dark part potential of the photosensitive drum  11  to a first value Vd 1  and a second value Vd 2 . For example, the first value Vd 1  is set to −400 V, and the second value Vd 2  is set to −200 V. In short, the controller  102  may change at least one of the dark part potential and the developing bias to form the analog images A 1  and A 2  and the digital images D 1  and D 2 . 
     The measurement chart includes the analog image A 1  and the digital image D 1  that are obtained by superimposing yellow, magenta, and cyan on each other, but the present invention is not limited thereto. For example, the measurement chart may independently include an analog image and a digital image, which are obtained by superimposing any two colors among yellow, magenta, and cyan on each other, and an analog image and a digital image, which are formed in the remaining one color. Even such a measurement chart allows the unit that has caused the streak image to be identified. The measurement chart may independently include analog images and digital images that are formed in the respective colors of yellow, magenta, cyan, and black. Such a measurement chart allows the unit that has caused the streak image to be identified, but the number of sheets P to be used becomes larger. 
     The image forming apparatus  1  according to this embodiment notifies the replacement unit by displaying a notification to that effect on the display portion  118 , but the notification may be conducted by another method. For example, the image forming apparatus  1  may notify the replacement unit to an information processing apparatus possessed by the service person through the network  123 . In this case, the service person can prepare the replacement unit in advance, and can efficiently conduct the maintenance. 
     The images of the measurement chart illustrated in  FIG. 8  are formed on the A3-size sheet P, but may be separately formed on two sheets P having an A4 size (297 mm in width direction length and 210 mm in conveyance direction length). In general, an image forming apparatus is more likely to use the A4 size than the A3 size as the size of the sheet P. For that reason, it is useful to provide measurement charts for the A4 size capable of producing the same effect as the measurement chart for the A3 size.  FIG. 13A  and  FIG. 13B  are diagrams for exemplifying the measurement charts provided in such a case. 
       FIG. 13A  is a first measurement chart, and includes the white ground parts W 1  and W 2 , the analog image A 1  obtained by superimposing the chromatic colors of yellow, magenta, and cyan on each other, and the digital image D 1  obtained by superimposing the chromatic colors of yellow, magenta, and cyan on each other.  FIG. 13B  is a second measurement chart, and includes white ground parts W 3  and W 4 , the analog image A 2  in black, and the digital image D 2  in black. The image forming apparatus  1  can cause the image reading section  200  to read two sheets P having such measurement charts formed thereon, and can identify the unit that has caused the appearance of the streak image by such processing as described with reference to  FIG. 10  and  FIG. 12 . 
     As described above, the image forming apparatus  1  according to this embodiment detects the streak image ascribable to a replaceable component based on as small a number of sheets P as possible, and positively identifies the component, to thereby suppress the downtime. It is also possible to reduce wasteful work, e.g., the replacement of the component that does not need to be replaced, and to reduce costs in terms of time and price. Accordingly, the image forming apparatus  1  can appropriately determine that an abnormality has occurred in the component. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2016-018647, filed Feb. 3, 2016 which is hereby incorporated by reference herein in its entirety.