Patent Publication Number: US-11378908-B2

Title: Image forming apparatus and program

Description:
The entire disclosure of Japanese patent Application No. 2020-140290, filed on Aug. 21, 2020, is incorporated herein by reference in its entirety. 
     BACKGROUND 
     Technological Field 
     The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile apparatus, or a multifunction peripheral having functions of the plurality of apparatuses, and a program. 
     Description of the Related Art 
     As the image forming apparatus as described above, particularly, as an image forming apparatus employing an electrophotographic method as a printing method, one that applies a lubricant to a surface of a photoreceptor as an image carrier is known. The lubricant plays a role of reducing a friction coefficient of the surface of the photoreceptor and reducing loads on the photoreceptor and a cleaning device. The lubricant also plays a role of improving transfer efficiency of toner and protecting the photoreceptor from a discharge product. 
     Such a lubricant is applied to the photoreceptor by a lubricant application device. However, when the application performance is deteriorated due to temporal degradation or the like of the lubricant application device, an application amount of the lubricant changes and an application unevenness is likely to occur, and the effect of the lubricant on the photoreceptor cannot be sufficiently exhibited. Therefore, it is necessary to detect the application unevenness of the lubricant at an early stage. 
     JP 6010852 B2 discloses an image forming apparatus that calculates a life or a total consumption of a solid lubricant from a total travel distance or a total drive time of a lubricant supply roller. 
     JP 2007-164105 A discloses an image forming apparatus that measures the number of rotations of a lubricant application brush, holds correspondence data between the number of rotations of the lubricant application brush and a life of a solid lubricant, and determines the life of the solid lubricant from the measured number of rotations and the correspondence data. 
     Furthermore, JP 5790962 B2, JP 6120130 B2, JP 5235846 B2, and JP 3406099 B2 disclose techniques for detecting or determining a remaining amount of a lubricant. 
     However, JP 6010852 B2, JP 2007-164105 A, JP 5790962 B2, JP 6120130 B2, JP 5235846 B2, and JP 3406099 B2 disclose that the life of the lubricant application device is estimated by the travel distance, the drive time, or the number of rotations of the lubricant supply roller, or the remaining amount of the lubricant is directly detected, but do not sufficiently disclose a technique for accurately detecting the application unevenness of the lubricant at an early stage. 
     SUMMARY 
     An object of the present invention is to provide an image forming apparatus and a program capable of accurately detecting an application unevenness of a lubricant at an early stage. 
     Another object of the present invention is to provide an image forming apparatus and a program capable of calculating a replacement time or a next detection time of a lubricant applicator, controlling an application amount of the lubricant, controlling an image forming condition, and the like based on a detection result of the application unevenness of the lubricant. 
     To achieve at least one of the abovementioned objects, according to an aspect of the present invention, there is provided an image forming apparatus having an application unevenness detection mode, and the image forming apparatus reflecting one aspect of the present invention comprises: an image carrier; a lubricant applicator that applies a lubricant to the image carrier; a hardware processor that is capable of changing an application amount of the lubricant applied to the image carrier by the lubricant applicator, and detects an application unevenness of the lubricant applied by the lubricant applicator; an image former that forms an image on the image carrier to which the lubricant has been applied by the lubricant applicator, and forms, in the application unevenness detection mode, a density unevenness measurement image on the image carrier in a state where the application amount of the lubricant applied to the image carrier is changed by the hardware processor in association with a temporal change in the application amount; and a density unevenness measurer that is provided in the image forming apparatus or outside the image forming apparatus, and measures a first density unevenness for at least one of the density unevenness measurement image formed by the image former in the application unevenness detection mode or a density unevenness measurement image obtained by primary or subsequent transfer of the density unevenness measurement image, wherein the hardware processor detects the application unevenness of the lubricant applied by the lubricant applicator based on the first density unevenness measured by the density unevenness measurer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages and features provided by one or more embodiments of the invention will become more fully understood from the detailed description given hereinbelow and the appended drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention: 
         FIG. 1  is a cross-sectional view of a main part of an electrophotographic image forming apparatus according to an embodiment of the present invention; 
         FIG. 2  is a diagram illustrating a configuration of a lubricant application device; 
         FIG. 3A  is a diagram illustrating a relationship between the number of printed sheets and an application amount of a lubricant; 
         FIG. 3B  is a diagram illustrating a relationship between the number of printed sheets and a density unevenness; 
         FIG. 4  is a flowchart for describing an operation in an application unevenness detection mode of the image forming apparatus; 
         FIG. 5  is a plan view illustrating an intermediate transfer belt on which a density unevenness measurement image is formed; 
         FIG. 6  is a flowchart for describing another operation in the application unevenness detection mode of the image forming apparatus; and 
         FIG. 7  is a flowchart for describing still another operation in the application unevenness detection mode of the image forming apparatus. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, one or more embodiments of the present invention will be described with reference to the drawings. However, the scope of the invention is not limited to the disclosed embodiments. 
       FIG. 1  is a cross-sectional view of a main part of an electrophotographic image forming apparatus according to an embodiment of the present invention. 
     In  FIG. 1 , a reference numeral  1  denotes a photoreceptor drum as an image carrier to which a lubricant is applied. The photoreceptor drum  1  rotates in a direction of an arrow a, and a transfer unit including a charging unit  2 , an exposure unit  3 , a developing unit  4 , a primary transfer member  5 , an intermediate transfer belt  6 , and the like, and a cleaning unit  7  are disposed around the photoreceptor drum  1  along a rotation direction. 
     The intermediate transfer belt  6  travels and moves downward from the top of the page of  FIG. 1 . A density sensor  12  that measures a density of an image is disposed at a position facing a surface (transfer surface) of the intermediate transfer belt  6  on a front side of the primary transfer member  5  in a moving direction of the intermediate transfer belt  6 . 
     The cleaning unit  7  mainly includes a cleaning blade  71  that abuts on the photoreceptor drum  1 , a lubricant application device  72 , a conveying screw  73  that conveys collected toner to the outside of the cleaning unit  7 , and the like. 
     In the image forming apparatus illustrated in  FIG. 1 , the exposure unit  3  exposes, according to image data, a surface of the photoreceptor drum  1  charged by the charging unit  2  to form an electrostatic latent image, and then the developing unit  4  develops the electrostatic latent image to form an image (toner image) on the surface of the photoreceptor drum  1 . 
     In this embodiment, the image forming apparatus has a normal image forming mode and an application unevenness detection mode for image formation. In the image forming mode, the image formed on the photoreceptor drum  1  is primarily transferred to the intermediate transfer belt  6  by the primary transfer member  5 . The image primarily transferred to the intermediate transfer belt  6  is conveyed to a secondary transfer position (not illustrated) by the intermediate transfer belt  6 , and is transferred to a sheet by a secondary transfer member at the secondary transfer position. The image transferred to the sheet is fixed by a fixing device and discharged to the outside of the apparatus. 
     On the other hand, in the application unevenness detection mode, a density unevenness measurement image including a predetermined pattern image is formed on the photoreceptor drum  1 , and the density unevenness measurement image is primarily transferred to the intermediate transfer belt  6  by the primary transfer member  5 . A density of the density unevenness measurement image primarily transferred to the intermediate transfer belt  6  is measured by the density sensor  12  when the image passes through the density sensor  12 , which accompanies the conveyance of the intermediate transfer belt  6 , and a controller  100  described later calculates a density unevenness based on the measured density. 
     Note that the density unevenness measurement image formed on the intermediate transfer belt  6  may be erased without being secondarily transferred to a sheet after the density measurement. Furthermore, in this embodiment, the density unevenness measurement image is formed on the intermediate transfer belt  6 , but the density of the density unevenness measurement image formed on the photoreceptor drum  1  may be measured, so that the density unevenness may be calculated. 
     Alternatively, the density unevenness measurement image on the intermediate transfer belt  6  may be secondarily transferred to a sheet and then fixed by the fixing device (not illustrated), and the density of the density unevenness measurement image fixed on the sheet may be measured, so that the density unevenness may be calculated. In this case, instead of the image forming apparatus measuring the density of the density unevenness measurement image and calculating the density unevenness, an external device such as an automatic quality optimization unit (IQ-501 manufactured by KONICA MINOLTA, INC.) or a scanner device connected to the image forming apparatus may read the density unevenness measurement image transferred to and fixed on the sheet and measure the density and/or calculate the density unevenness, and the image forming apparatus may receive the result to detect the application unevenness. 
     In addition, in an image forming apparatus not including the intermediate transfer belt  6 , the density unevenness of the density unevenness measurement image formed on the photoreceptor drum  1  or the density unevenness of the density unevenness measurement image transferred to and fixed on a sheet can be measured. 
     Note that, in a color image forming apparatus including a plurality of photoreceptor drums  1 , a density unevenness measurement image including a color image may be formed, or one of the plurality of photoreceptor drums  1 , for example, a black photoreceptor drum  1  may be used to form a density unevenness measurement image. 
     On the photoreceptor drum  1  having the image transferred to the intermediate transfer belt  6 , residual toner is removed and collected by the cleaning blade  71  of the cleaning unit  7 . The collected toner is conveyed to the outside of the cleaning unit  7  by the conveying screw  73 . The lubricant application device  72  applies the lubricant to the surface of the photoreceptor drum  1  from which the residual toner has been removed. 
     As illustrated in  FIG. 2 , the lubricant application device  72  includes a lubricant application brush  8 , a solid lubricant  9 , a pressing spring  10 , and a uniformizing member  11  (illustrated in  FIG. 1 ). The lubricant  9  is located below the lubricant application brush  8 , is provided in pressure contact with a lower region of the lubricant application brush  8  with an upward biasing force applied by the pressing spring  10  from below, and supplies the lubricant to the lubricant application brush  8 . The lubricant application brush  8  has a configuration in which a brush is disposed on a surface of a roller rotationally driven by a driver such as a motor (not illustrated), and is in contact with the surface of the photoreceptor drum  1 . The roller of the lubricant application brush  8  is rotated when the photoreceptor drum  1  rotates, so that the lubricant supplied from the lubricant  9  is applied to the surface of the photoreceptor drum  1 . 
     The lubricant plays a role of reducing a friction coefficient of the surface of the photoreceptor drum  1 , reducing loads on the photoreceptor drum  1  and the cleaning unit  7 , improving transfer efficiency of the toner, protecting the photoreceptor drum  1  from a discharge product, and the like. 
     The uniformizing member  11  plays a role of uniformly leveling the lubricant applied to the surface of the photoreceptor drum  1 . Note that a lower end of the pressing spring  10  is fixed to a cover  13  attached to a housing  14 . 
     The controller  100  includes a CPU  101 , a ROM  102  in which an operation program of the CPU  101  and other data are stored, and a RAM  103  that serves as a work area when the CPU  101  operates, and comprehensively controls the entire image forming apparatus by the CPU  101  operating according to the operation program stored in the ROM  102  or the like. 
     Specifically, the controller  100  controls an image forming operation by an image former. The image former includes members relating to image formation, such as the photoreceptor drum  1 , the charging unit  2 , the exposure unit  3 , the developing unit  4 , the primary transfer member  5 , the intermediate transfer belt  6 , the cleaning unit  7 , and the secondary transfer member (not illustrated). 
     Furthermore, the controller  100  changes an application amount of the lubricant applied to the photoreceptor drum  1  by the lubricant application device  72 , calculates the density unevenness of the image from the density of the image measured by the density sensor  12 , detects the application unevenness of the lubricant applied to the photoreceptor drum  1  based on the density unevenness, and performs various determinations and controls based on the detection result of the application unevenness. Details of these operations will be described later. 
     Incidentally, in the image forming apparatus including the lubricant application device  72 , when the amount of lubricant on the surface of the photoreceptor drum  1  decreases to a predetermined amount or less, a predetermined amount or more of difference is likely to occur in the amount of lubricant between adjacent positions. That is, the application unevenness of the lubricant is likely to occur. When the application unevenness occurs, the density unevenness of the image occurs. This is because a surface potential of the photoreceptor drum  1  depends on the application amount of the lubricant. 
     Specifically, as indicated by a solid line in  FIG. 3A , the application amount of the lubricant changes in a decreasing direction due to deterioration or the like of the lubricant application device  72  as the number of printed sheets increases (the lubricant application device  72  endures the increase in the number of printed sheets), and the application unevenness occurs eventually. Furthermore, as indicated by a solid line in  FIG. 3B , the density unevenness of the image increases as the number of printed sheets increases (the lubricant application device  72  endures the increase in the number of printed sheets), and eventually reaches the limit. As described above, there is a correlation between the occurrence of the application unevenness and the density unevenness. Therefore, if the density unevenness is measured, it is possible to know the application unevenness. 
     Even if the density unevenness does not occur under a normal lubricant application condition in which the application amount of the lubricant is not intentionally suppressed, temporarily suppressing the application amount of the lubricant greatly reduces the amount of lubricant on the photoreceptor drum  1  as compared with a case where the application amount of the lubricant is not suppressed, as indicated by a broken line in  FIG. 3A , and also increases the difference in the amount of lubricant between portions, thereby increasing the density unevenness, as indicated by a broken line in  FIG. 3B . 
     Thus, in the application unevenness detection mode, the application amount of the lubricant is temporarily suppressed to intentionally create a state where the application amount of the lubricant has changed (decreased) due to performance deterioration of the lubricant application device  72 . If a density unevenness of an image formed in this state is measured, it is possible to accurately detect and predict at an early stage whether an occurrence status of the application unevenness has reached the limit and a limit time of the application performance of the lubricant application device  72 . As will be described later, the limit time of the application performance of the lubricant application device  72  is calculated based on a difference from a reference value (threshold for determining the limit). 
     First Embodiment 
     An operation in the application unevenness detection mode of the image forming apparatus illustrated in  FIG. 1  will be described with reference to a flowchart of  FIG. 4 . Processing illustrated in  FIG. 4  and subsequent flowcharts is executed by the CPU  101  of the controller  100  operating according to the operation program stored in the ROM  102  or the like. 
     In this embodiment, the application unevenness of the lubricant is detected, and a life of the lubricant application device  72  is predicted based on the detection result. 
     First, in step S 01 , the density unevenness measurement image is formed under the normal condition, in other words, in a current state where the application amount of the lubricant is not suppressed. 
     Specifically, in step S 011 , a rotation speed of the lubricant application brush is set to a rotation speed at the time of normal image formation, and the lubricant is applied to the photoreceptor drum  1 . In this state, the predetermined pattern image is formed as a halftone image on the photoreceptor drum  1 . Next, the halftone image formed on the photoreceptor drum  1  is primarily transferred to the intermediate transfer belt  6 , and as illustrated in  FIG. 5 , a density unevenness measurement image  200  is formed on the intermediate transfer belt  6 . It is desirable, from the viewpoint of being capable of accurately measuring the density unevenness, to set the density of the density unevenness measurement image  200  to a density corresponding to a tone value of “64” when a solid image has a tone value of “256”. 
     Next, density unevenness analysis processing is performed on the formed density unevenness measurement image  200  in step S 02 . Specifically, the density sensor  12  measures image densities at a plurality of positions in a direction (CD direction) orthogonal to a sheet passing direction (moving direction of the intermediate transfer belt  6 ), and transmits the measurement result to the controller  100 . The image density at each measurement position is calculated by use of data measured a plurality of times in the sheet passing direction (FD direction). The controller  100  calculates a density unevenness (corresponding to a second density unevenness) based on the densities received from the density sensor  12 . The density unevenness is defined by a maximum value of density differences at all the measurement positions when a density difference from an adjacent measurement position is calculated at each measurement position in the CD direction. Note that, in the density unevenness measurement image  200  in  FIG. 5 , a plurality of linear noises  201  along the sheet passing direction (FD direction) is generated due to the application unevenness of the lubricant applied to the photoreceptor drum  1 . 
     Next, in step S 03 , the density unevenness measurement image is formed in a state where the application amount of the lubricant applied to the photoreceptor drum  1  is suppressed. Specifically, in step S 031 , the rotation speed of the lubricant application brush  8  is set to 80% of the rotation speed at the time of normal image formation, and the lubricant is applied to the photoreceptor drum  1 . In this state, a halftone image is formed on the photoreceptor drum  1 , and then the halftone image is primarily transferred to the intermediate transfer belt  6  to form the density unevenness measurement image  200 . The density of the density unevenness measurement image  200  is preferably set to the same tone value as that of the normal image formation described above. Note that the change of the rotation speed of the lubricant application brush  8  for reducing the application amount may include a rotation stop. 
     Next, in step S 04 , the density unevenness analysis processing is performed on the formed density unevenness measurement image  200 , and a density unevenness (corresponding to a first density unevenness) is calculated. This processing is performed in the same procedure as the density unevenness analysis processing at the time of normal image formation in step S 02 . 
     In step S 05 , the density unevenness (second density unevenness) at the time of normal image formation, which is calculated in step S 02 , and the density unevenness (first density unevenness) at the time of suppressing the application amount of the lubricant, which is calculated in step S 04 , are compared with the preset reference value (threshold for determining the limit) to detect the application unevenness and calculate a replacement time of the lubricant application device  72 . 
     Specifically, in step S 05 , in a case where both the density unevenness at the time of normal image formation and the density unevenness at the time of suppressing the application amount of the lubricant exceed the reference value, it is determined in step S 06  that the application unevenness has reached the limit and the lubricant application device  72  exceeds a margin of the life. In this case, for example, a message prompting replacement of the lubricant application device  72  is displayed on an operation panel (not illustrated) or the like. 
     Note that, in the replacement of the lubricant application device  72 , only the lubricant  9  may be replaced, or a cartridge in which the lubricant  9  and the lubricant application brush  8  are integrated may be replaced. Furthermore, in a case where the lubricant application device  72  is integrated with a process cartridge such as the photoreceptor drum  1 , the process cartridge may be replaced. 
     In step S 05 , in a case where the density unevenness at the time of normal image formation does not exceed the reference value, and only the density unevenness at the time of suppressing the application amount of the lubricant exceeds the reference value, it is determined in step S 07  that the application unevenness has not reached the limit, but a timing at which the limit is reached is close, and the replacement time is calculated. The replacement time is calculated by the following equation based on a ratio between the differences between the density unevennesses and the reference value.
 
Replacement time=Replacement coefficient×( Z−X )÷( Y−Z )
 
     The density unevenness at the time of normal image formation is X, the density unevenness at the time of suppressing the application amount of the lubricant is Y, and the reference value is Z. 
     In step S 05 , in a case where the density unevenness at the time of suppressing the application amount of the lubricant is equal to or less than the reference value (in this case, the density unevenness at the time of normal image formation is also equal to or less than the reference value), it is determined in step S 08  that the application unevenness has not reached the limit, there is a margin in the life, and it is not necessary to replace the lubricant application device  72  for the time being. 
     As described above, in this embodiment, in the application unevenness detection mode, a future state where the application amount of the lubricant is reduced due to the performance deterioration of the lubricant application device  72  is intentionally created in a state where the application amount of the lubricant applied to the photoreceptor drum  1  is suppressed. In this state, the density unevenness measurement image is formed to calculate the density unevenness and compare the density unevenness with the reference value, so that the application unevenness of the lubricant applied by the lubricant application device  72  is detected. Therefore, even if the application unevenness has not reached the limit and the lubricant application device  72  has not reached the end of the life at the time of detecting the application unevenness, it is possible to detect at an early stage and in advance whether the lubricant application device  72  reaches the end of the life and the application unevenness reaches the limit in the near future. Moreover, since the density unevenness larger than the actual density unevenness can be obtained, the application unevenness can be accurately detected. In addition, the timing at which the lubricant application device  72  reaches the end of the life can be accurately calculated based on the detection result of the application unevenness. 
     Note that the detection of the application unevenness in the application unevenness detection mode can be performed every predetermined number of printed sheets (for example, every 50,000 sheets). In addition, a detection interval may be lengthened or shortened. If the detection interval is shortened, the detection accuracy is enhanced. Furthermore, a detection time may be changed according to an endurance number of sheets. For example, the detection interval may be lengthened (for example, every 100,000 sheets) when the density unevenness at the time of suppressing the application amount of the lubricant is equal to or less than the reference value, and the detection interval may be shortened (for example, every 10,000 sheets) when the density unevenness at the time of suppressing the application amount of the lubricant exceeds the reference value. Note that the detection time may be changed according to an installation environment or a printing rate, such as shortening the detection interval when the temperature is low or the printing rate is high. 
     Second Embodiment 
     Another operation in the application unevenness detection mode of the image forming apparatus illustrated in  FIG. 1  will be described with reference to a flowchart of  FIG. 6 . 
     In this embodiment, the application unevenness of the lubricant is detected, and the application amount at the time of image formation by the lubricant application device  72  is controlled based on the detection result. 
     Since the processing of forming the density unevenness measurement image under the normal condition in step S 01 , the processing of setting the rotation speed of the lubricant application brush to the rotation speed at the time of normal image formation and applying the lubricant to the photoreceptor drum  1  in step S 011 , the density unevenness analysis processing in step S 02 , the processing of forming the density unevenness measurement image in a state where the application amount of the lubricant applied to the photoreceptor drum  1  is suppressed in step S 03 , the processing of setting the rotation speed of the lubricant application brush to 80% of the rotation speed at the time of normal image formation and applying the lubricant to the photoreceptor drum  1  in step S 031 , the density unevenness analysis processing in step S 04 , and the comparison processing in step S 05  are the same as the processing in each step in the first embodiment illustrated in  FIG. 4 , the same step numbers are given, and detailed description thereof will be omitted. 
     In step S 05 , the density unevenness at the time of normal image formation, which is calculated in step S 02 , and the density unevenness at the time of suppressing the application amount of the lubricant, which is calculated in step S 04 , are compared with the preset reference value, and an application condition (application amount) of the lubricant application device  72  is controlled according to the detection result and the comparison result of the application unevenness. 
     Specifically, in a case where both the density unevenness at the time of normal image formation and the density unevenness at the time of suppressing the application amount of the lubricant exceed the reference value, it is determined in step S 11  that the application unevenness has reached the limit, and the rotation speed of the lubricant application brush  8  is changed by a predetermined amount (for example, 1.1 times the linear speed) to increase the application amount of the lubricant. 
     To change the application amount of the lubricant, the rotation speed of the lubricant application brush  8  can be changed, which includes the rotation stop. Alternatively, to change the application amount of the lubricant, an electromagnetic actuator or the like may press or pull a position of the cover  13  to which the lower end of the pressing spring  10  is attached with respect to the pressing spring  10 , so that a pressing force of the pressing spring  10  that presses the lubricant  9  against the lubricant application brush  8  may be changed. Alternatively, the rotation speed of the lubricant application brush  8  and the pressing force of the pressing spring  10  may be changed at the same time. 
     Next, in step S 12 , it is determined whether an increase in the application amount of the lubricant has reached the upper limit, and if the increase amount has not reached the upper limit (NO in step S 12 ), the processing returns to step S 01  and steps S 01  to S 05  are repeated. In step S 05 , the change of the rotation speed of the lubricant application brush and the measurement of the density unevenness are repeated until the density unevenness at the time of normal image formation is equal to or less than the reference value and the density unevenness at the time of suppressing the application amount of the lubricant exceeds the reference value. Note that, when the increase in the application amount due to the change of the rotation speed of the lubricant application brush has reached the limit (YES in step S 12 ), it is determined in step S 13  that the replacement time of the lubricant application device  72  has come. 
     In step S 05 , when the density unevenness at the time of suppressing the application amount of the lubricant is equal to or less than the reference value (the density unevenness at the time of normal image formation is also equal to or less than the reference value), it is determined that the application unevenness has not reached the limit but the application amount of the lubricant is excessive, and, in step S 011 , the rotation speed of the lubricant application brush  8  is changed by a predetermined amount (for example, 0.9 times the linear speed). After the change, the density unevenness is measured again, and the change of the rotation speed of the lubricant application brush  8  and the measurement of the density unevenness are repeated until the density unevenness at the time of normal image formation is equal to or less than the reference value and the density unevenness at the time of suppressing the application amount of the lubricant exceeds the reference value. 
     In step S 05 , when the density unevenness at the time of normal image formation is equal to or less than the reference value and the density unevenness at the time of suppressing the application amount of the lubricant exceeds the reference value, it is determined that the timing at which the application unevenness reaches the limit is close but has not yet come and the application amount of the lubricant is also appropriate. In step S 14 , the next application unevenness detection time in the application unevenness detection mode is calculated, and then the processing is ended. 
     The next detection time in step S 14  is calculated by the following equation.
 
Next detection time=Detection time coefficient×( Z−X )÷( Y−Z )
 
     The density unevenness at the time of normal image formation is X, the density unevenness at the time of suppressing the application amount of the lubricant is Y, and the reference value is Z. 
     Note that, in a case where the lubricant application condition is changed in step S 11 , an image is formed in the image forming mode under the changed condition. 
     As described above, in this embodiment, in the case where both the density unevenness at the time of normal image formation and the density unevenness at the time of suppressing the application amount of the lubricant exceed the reference value, it is determined that the application unevenness has reached the limit, and the rotation speed of the lubricant application brush  8  is changed to increase the application amount of the lubricant. Therefore, the lubricant application device  72  can be used to the limit at which the lubricant application device  72  reaches the end of the life, so that it is possible to perform good image formation. 
     On the other hand, in the case where the density unevenness at the time of suppressing the application amount of the lubricant is equal to or less than the reference value, it is determined that the application unevenness has not reached the limit, but the lubricant is excessive, and the rotation speed of the lubricant application brush  8  is changed to reduce the application amount of the lubricant. Therefore, setting the application amount of the lubricant to an appropriate value makes it possible to perform good image formation, and in addition, to suppress wasteful consumption of the lubricant. 
     In the above example, the change control of the application amount of the lubricant applied to the photoreceptor drum  1  is performed in both the case where both the density unevenness at the time of normal image formation and the density unevenness at the time of increasing the detection sensitivity exceed the reference value and the case where the density unevenness at the time of increasing the detection sensitivity is equal to or less than the reference value. However, the change control of the application amount of the lubricant applied to the photoreceptor drum  1  may be performed only in one of the cases. 
     Third Embodiment 
     Still another operation in the application unevenness detection mode of the image forming apparatus illustrated in  FIG. 1  will be described with reference to a flowchart of  FIG. 7 . 
     In this embodiment, the application unevenness of the lubricant is detected, and an image forming condition in the normal image forming mode is controlled based on the detection result. 
     Since the processing of forming the density unevenness measurement image under the normal condition in step S 01 , the processing of setting the rotation speed of the lubricant application brush  8  to the rotation speed at the time of normal image formation and applying the lubricant to the photoreceptor drum  1  in step S 011 , the density unevenness analysis processing in step S 02 , the processing of forming the density unevenness measurement image in a state where the application amount of the lubricant applied to the photoreceptor drum  1  is suppressed in step S 03 , the processing of setting the rotation speed of the lubricant application brush  8  to 80% of the rotation speed at the time of normal image formation and applying the lubricant to the photoreceptor drum  1  in step S 031 , the density unevenness analysis processing in step S 04 , and the comparison processing in step S 05  are the same as the processing in each step in the first embodiment illustrated in  FIG. 4 , the same step numbers are given, and detailed description thereof will be omitted. 
     In step S 05 , the density unevenness at the time of normal image formation, which is calculated in step S 02 , and the density unevenness at the time of suppressing the application amount of the lubricant, which is calculated in step S 04 , are compared with the preset reference value, and the image forming condition is controlled according to the comparison result. 
     Specifically, in a case where both the density unevenness at the time of normal image formation and the density unevenness at the time of suppressing the application amount of the lubricant exceed the reference value, it is determined that the application unevenness has reached the limit, and the image forming condition is changed in step S 21 . 
     Next, in step S 22 , it is determined whether the change of the image forming condition has reached the limit, and if the change has not reached the limit (NO in step S 22 ), the processing returns to step S 01  and steps S 01  to S 05  are repeated. 
     The image forming condition to be changed is preferably at least one of a charging potential of the photoreceptor drum  1 , a voltage (developing bias) to be applied to a developer carrier, an alternating current (AC) voltage of the developing bias, or a frequency of the AC voltage of the developing bias. For example, both the charging potential and the developing bias are changed by −50V. In a case where the AC voltage of the developing bias is changed, the AC voltage is increased by, for example, 100 V, and in a case where the frequency of the AC voltage is changed, the frequency is increased by, for example, 1.1 times. 
     After the change, the density unevenness measurement image is formed again to calculate the density unevenness, and the change of the image forming condition and the measurement of the density unevenness are repeated until the density unevenness at the time of normal image formation is equal to or less than the reference value and the density unevenness at the time of increasing the detection sensitivity exceeds the reference value. Note that, when the change of the image forming condition reaches the limit (YES in step S 22 ), it is determined in step S 23  that the replacement time of the lubricant application device  72  has come. 
     In step S 05 , when the density unevenness at the time of increasing the detection sensitivity is equal to or less than the reference value (the density unevenness at the time of normal image formation is also equal to or less than the reference value), it is determined that the image forming condition does not need to be changed, and the processing is ended without doing anything. Similarly, when the density unevenness at the time of normal image formation is equal to or less than the reference value and the density unevenness at the time of increasing the detection sensitivity exceeds the reference value, it is determined that the image forming condition does not need to be changed, and the processing is ended without doing anything. 
     Note that, in a case where the image forming condition is changed in step S 21 , an image is formed in the image forming mode under the changed condition. 
     As described above, in this embodiment, in the case where both the density unevenness at the time of normal image formation and the density unevenness at the time of increasing the detection sensitivity exceed the reference value, the image forming condition is changed, and an image is formed in the image forming mode under the changed condition. Therefore, the lubricant application device  72  can be used to the limit at which the lubricant application device  72  reaches the end of the life, so that it is possible to perform good image formation. 
     According to an embodiment of the present invention, a lubricant is applied to an image carrier by a lubricant applicator, and an image former forms, in an application unevenness detection mode, a density unevenness measurement image on the image carrier in a state where an application amount of the lubricant applied to the image carrier is changed in association with a temporal change in the application amount. 
     A first density unevenness is measured for at least one of the density unevenness measurement image formed in the application unevenness detection mode or a density unevenness measurement image obtained by primary or subsequent transfer of the density unevenness measurement image, and an application unevenness of the lubricant applied to the image carrier is detected based on the measured first density unevenness. 
     According to an embodiment of the present invention, the density unevenness measurement image is formed in a state where the application amount is decreased by an application amount changing unit, and the density unevenness is measured. Thus, it is possible to more accurately and reliably detect the application unevenness. 
     According to an embodiment of the present invention, the first density unevenness is compared with a reference value, and it is determined that the application unevenness has not reached a limit in a case where the first density unevenness is equal to or less than the reference value, and it is determined that the application unevenness has already reached the occurrence limit or a timing at which the application unevenness reaches the limit is close in a case where the first density unevenness exceeds the reference value. Thus, it is possible to detect at an early stage that the application unevenness has already reached the limit or the timing at which the application unevenness reaches the limit is close. 
     According to an embodiment of the present invention, a second density unevenness is measured for at least one of density unevenness measurement images formed without changing the application amount of the lubricant, and in a case where the first density unevenness exceeds the reference value, it is determined that the application unevenness has already reached the limit when the second density unevenness exceeds the reference value, and it is determined that the timing at which the application unevenness reaches the limit is close when the second density unevenness is equal to or less than the reference value. Thus, it is possible to reliably detect that the application unevenness has already reached the limit and that the timing at which the application unevenness reaches the limit is close. 
     According to an embodiment of the present invention, it is possible to reliably change the application amount of the lubricant in the application unevenness detection mode by at least one of changing a rotation speed of a lubricant application brush, which includes a rotation stop, or changing a lubricant pressing force applied to the lubricant application brush. 
     According to an embodiment of the present invention, a density unevenness of a halftone image as the density unevenness measurement image is measured in the application unevenness detection mode. Thus, it is possible to measure the density unevenness with high accuracy, and furthermore, to detect the application unevenness of the lubricant with high accuracy. 
     According to an embodiment of the present invention, it is possible to calculate a replacement time of the lubricant applicator based on a detection result of the application unevenness. 
     According to an embodiment of the present invention, the second density unevenness is measured for at least one of the density unevenness measurement images formed without changing the application amount of the lubricant, and the replacement time of the lubricant applicator is calculated in a case where the first density unevenness exceeds the reference value and the second density unevenness is equal to or less than the reference value. 
     According to an embodiment of the present invention, it is possible to control the application amount of the lubricant applied to the image carrier by the lubricant applicator based on the detection result of the application unevenness. 
     According to an embodiment of the present invention, the second density unevenness is measured for at least one of the density unevenness measurement images formed without changing the application amount of the lubricant, and the application amount of the lubricant applied to the image carrier by the lubricant applicator is controlled in a case where the first density unevenness is equal to or less than the reference value and/or in a case where both the first density unevenness and the second density unevenness exceed the reference value. 
     According to an embodiment of the present invention, it is possible to calculate a next application unevenness detection time based on the detection result of the application unevenness. 
     According to an embodiment of the present invention, the second density unevenness is measured for at least one of the density unevenness measurement images formed without changing the application amount of the lubricant, and the next application unevenness detection time is calculated in a case where the first density unevenness exceeds the reference value and the second density unevenness is equal to or less than the reference value. 
     According to an embodiment of the present invention, it is possible to control an image forming condition of the image former based on the detection result of the application unevenness. 
     According to an embodiment of the present invention, the second density unevenness is measured for at least one of the density unevenness measurement images formed without changing the application amount of the lubricant, and the image forming condition is controlled in a case where both the first density unevenness and the second density unevenness exceed the reference value. 
     Although embodiments of the present invention have been described and illustrated in detail, the disclosed embodiments are made for purposes of illustration and example only and not limitation. The scope of the present invention should be interpreted by terms of the appended claims.