Abstract:
An image forming apparatus includes a duplex conveyance path where a recording medium can be reversed and re-fed; and an output image detector to detect an image or a test pattern formed on a surface of the recording medium. The image forming apparatus is configured to: detect the test pattern formed on the first side of the recording medium by the output image detector; adjust and correct image forming conditions based on the detection result of the detector; convey the recording medium the first side of which the test pattern is formed on, to a duplex conveyance path; re-feed the recording medium and form a test pattern for verifying correction effect on a backside of the recording medium after correcting and updating the image forming conditions; and detect the test pattern formed on the backside of the recording medium again by the output image detector, thereby verifying the correction effect.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from Japanese patent application number 2010-230915, filed on Oct. 13, 2010, the entire contents of which are incorporated by reference herein. 
     FIELD OF THE INVENTION 
     The present invention relates to a multi-function image forming apparatus having one or more capabilities as a copier, a printer, a facsimile machine, a plotter, and the like, and in particular relates to an image forming apparatus including a detector to obtain image information from an image formed on a recording medium by the image forming apparatus, and a correction effect verification method based on the obtained image information. 
     BACKGROUND OF THE INVENTION 
     It is known to provide, in an interior of an image forming apparatus, a detector to detect a final image on a recording medium or sheet and many proposals have been made so far. The image formed on the recording medium is detected by the detector and the image quality is determined based on the detection result, or used as feedback to recursively change or correct image forming condition. 
     To detect the image formed on the recording medium, in the case of using a test pattern that is not an actual image, the number of sheets used must be minimized. Accordingly, a method of using both sides of the recording medium has been proposed. 
     JP-2005-202028-A discloses a technique to automatically detect both sides of the sheet in one test printing. Specifically, when a color detector to detect a patch on the recording medium is disposed at a duplex-print conveyance path and a discharge conveyance path and the patch is detected by the color detector, a first surface and a second or backside surface are detected in one test printing. This technique has an advantage that the test patches on the first and second surfaces of the recording medium can be detected at once. However, after detection of the test patches on the first or the second surface, when the detection result is fed back to change the image forming condition and the changing or correction effect needs to be confirmed by forming test patches again, another sheet needs to be conveyed again, which does not contribute to the reduction of the recording sheet. 
     JP-2005-215340-A discloses a configuration in which each of the first surface and the second surface is provided with an individual reading means. The same also discloses a technique in which an image forming condition for the first surface is obtained from the image formed on the first surface and that for the second surface is obtained from the image formed on the second surface, respectively, so that each image forming condition is made different. However, the use of two reading means increases both the size and the cost of the apparatus. Moreover, even in this case, another sheet needs to be supplied to confirm the correction result after test patches have been formed again. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is conceived in light of the foregoing problems, and provides an inexpensive, compact image forming apparatus capable of detecting an image on a recording medium; forming a correction test pattern to securely obtain the detection result of the image on the recording medium; and forming a correction effect verifying test pattern, without uselessly consuming the recording medium. 
     As an aspect of the present invention, the image forming apparatus includes a duplex conveyance path in which a recording medium can be reversed and re-fed and an output image detector to detect an image or a test pattern formed on a surface of the recording medium. The image forming apparatus is configured to: detect the test pattern formed on the first side of the recording medium with the output image detector; adjust and correct image forming conditions based on the detection result of the detector; convey the recording medium on the first side of which the test pattern is formed to a duplex conveyance path; re-feed the recording medium and form a test pattern for verifying correction effect on a backside thereof after correcting and updating the image forming conditions; and detect the test pattern formed on the backside of the recording medium again with the output image detector, thereby verifying the correction effect. 
     These and other objects, features, and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a general configuration of an image forming apparatus according to embodiments of the present invention; 
         FIG. 2  is a general configuration of an output image detector; 
         FIG. 3  is a plan view illustrating a spatial relation between a test pattern formed on a recording sheet and the output image detector; 
         FIG. 4  is a side view illustrating a state of detection by another output image detector; 
         FIG. 5A  is a graph illustrating surface test pattern readings and  FIG. 5B  is a graph illustrating backside surface correction effect verification test pattern readings; and 
         FIG. 6  is a plan view illustrating a spatial relation between the test pattern formed on a recording sheet and the output image detector as in  FIG. 3 , in which a sequence of the test pattern in the main scanning direction is changed. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. 
     A general configuration of an image forming apparatus according to one embodiment of the present invention will now be described with reference to  FIG. 1 . As illustrated in  FIG. 1 , an image forming section of the image forming apparatus has a full-color electrophotographic system, employing a tandem-type intermediate transfer method. It should be noted, however, that the present invention is not limited to such a system but is also applicable to various apparatuses using one-drum intermediate transfer belt method, a tandem-type direct transfer method, a one-drum direct transfer method (for monochrome printing), and the like. 
     As illustrated in  FIG. 1 , photoreceptor drums  2 Y,  2 M,  2 C, and  2 K as image carriers are disposed in parallel along a developing surface of the intermediate transfer belt  1  as an intermediate transfer body. The suffix “Y” indicates the color yellow and M, C, and K, respectively, denote magenta, cyan, and black. Hereinafter, a yellow image forming station will be described as representative. Around the photoreceptor drum  2 Y, a charger  3  as a charging means, a writing unit  4 Y, a developing unit  5 Y, a primary transfer roller  6 Y, a photoreceptor cleaning unit  7 Y, and a quenching lamp  8 Y are disposed along a direction of rotation of the photoreceptor drum  2 Y. The other image forming stations are configured similarly. At an upper side of the writing unit  4 , a scanner  9  and an Automatic Document Feeder (ADF)  10  are disposed. 
     The intermediate transfer belt  1  is rotatably supported by a plurality of rollers  11 ,  12 , and  13 . An intermediate transfer belt cleaning unit  15  is disposed opposite the roller  12 . A secondary transfer roller  16  as a secondary transfer means is disposed opposite the roller  13 . In the bottom of the apparatus, there are provided a plurality of sheet feed trays  17  as a sheet feed section. Recording sheets  20  as recording media contained in these trays are fed one by one by a pickup roller  21  and a sheet feed roller  22 , are conveyed by a pair of conveyance rollers  23 , and are sent to the secondary transfer section at a predetermined timing by a pair of registration rollers  24 . A fixing unit  25  as a fixing means is disposed downstream of the secondary transfer section in the sheet conveyance direction. Reference numeral  26  in  FIG. 1  denotes a sheet discharge tray,  28  denotes a duplex conveyance path, and  27  denotes a pair of switchback rollers. The duplex conveyance path  28  includes a sheet reversing section  29  and a re-feed conveyance path  30 . An output image detector  31  as an output image detecting means is disposed in the vicinity and downstream of the fixing unit  25 . 
     An image forming operation conducted using the configuration as illustrated in  FIG. 1  will be described briefly. 
     When a print start command is input, each roller disposed around the photoreceptor drum, around the intermediate transfer belt, along the sheet feed and conveyance path, and the like, starts rotating at a predetermined timing and feeding of a recording sheet from the sheet feed tray starts. 
     At the same time, a surface of each photoreceptor drum  2  is charged to a uniform electric potential by the charger  3 , and the writing unit  4  radiates light on the surface to expose it according to image data. The electric pattern formed after exposure is called electrostatic latent image. The photoreceptor drum  2  on which the electrostatic latent image is carried is supplied with toner from the developing unit  5 , whereby the electrostatic latent image carried thereon is developed into color toner images of specific colors. 
     As illustrated in  FIG. 1 , there are four photoreceptors  2  respectively used for a toner image of yellow, magenta, cyan, and black, and therefore, a toner image of each color is formed on the corresponding photoreceptor drum  2 . (The order of the colors is different from system to system.) 
     The color toner image developed on each of the photoreceptor drums  2  is transferred onto the intermediate transfer belt  1  at a contact point between each drum  2  and the intermediate transfer belt  1  by a primary transfer bias applied to the primary transfer roller  6  disposed opposite the photoreceptor drum  2  and by pressure. This primary transfer operation is repeated for four colors with a matched timing, thereby forming a full-color toner image on the intermediate transfer belt  1 . 
     The full-color toner image formed on the intermediate transfer belt  1  is transferred at the secondary transfer roller  16  onto the recording sheet  20  which is conveyed at a matched timing by the pair of registration rollers  24 . In this case, secondary transfer is performed with the secondary transfer bias applied to the secondary transfer roller  16  and pressure. The recording sheet  20  on which a full-color toner image has been transferred is heated while passing through the fixing unit  25 , and the toner image carried thereon thus heated is fused and fixed onto the recording sheet  20 . 
     In a case of single-sided copying or printing, the recording sheet  20  is directly conveyed to the sheet discharge tray  26 . In a case of duplex copying or printing, the recording sheet  20  is reversed upside down and is conveyed to the sheet reversing section  29 . Upon the sheet recording sheet  20  arriving at the sheet reversing section  29 , the conveyance direction of the recording sheet  20  is reversed by the pair of switchback rollers  27 , and therefore, the trailing edge of the sheet first passes through the sheet reversing section  29 . This operation by which the sheet surface is reversed is called a switchback. 
     The thus-reversed recording sheet  20  does not return to the fixing unit, passes through the re-feed conveyance path  30 , and joins the normal sheet conveyance path. Thereafter, the toner image is transferred to the recording sheet  20  similarly to the case of the first-side print, and the recording sheet  20  passes through the fixing unit  25  and is discharged outside. The duplex print operation is thus terminated. 
     The photoreceptor drum  2  that has passed through the primary transfer section carries residual toner after the primary transfer on the surface thereof. This residual toner is removed by the photoreceptor cleaning unit  7 . The cleaning unit  7  includes a blade, a brush, and the like. Then, the surface of the photoreceptor drum  2  is electrically uniformly discharged by the quenching lamp  8  and is prepared for the electric charge for a next image formation. 
     The intermediate transfer belt  1  that has passed through the secondary transfer section carries residual toner thereon. This residual toner remaining on the intermediate transfer belt  1  is also removed by the intermediate transfer belt cleaning unit  15  including a blade, a brush, and the like. Then, the intermediate transfer belt  1  is prepared for a next toner image transfer. With such repeated operation, the one-side print or the duplex print is performed. 
     As described above, the output image detector  31  disposed downstream of the fixing unit  25  detects an image on the recording sheet  20 . After the image or the test pattern formed on the surface of the recording sheet  20  has been detected, the recording sheet  20  is conveyed to the sheet reversing section  29  with its direction reversed upside down by a conveyance path switching means, not shown, passes through the re-feed conveyance path  30 , and comes to be in a re-feedable state. 
     Meanwhile, image forming conditions (various parameters) are adjusted and corrected based on the detected information on the image on the surface of the recording sheet  20 . Upon above various adjustment/correction having been completed and image forming conditions having been updated, the recording sheet  20  being in a standby mode at the re-feed conveyance path  30  starts to be fed again and a test pattern for verifying effect is formed on the reverse surface of the recording sheet. 
     This effect-verifying test pattern is again fixed and read by the output image detector  31 , and the recording sheet  20  is discharged to the sheet discharge tray  26 . 
       FIG. 2  shows an example of the output image detector  31  of the present embodiment. The output image detector  31 , being a single-eyed spot detector includes a light-emitting element  58  and a light-receiving element  59  with an RGB filter. As illustrated in  FIG. 2 , the output image detector  31  further includes a dustproof glass or lens  53  and a detector case  54 . 
     When using this type of detector for a full-color apparatus, because one detector detects only one point in the main scanning direction, a plurality of detectors for each color needs to be installed. Even a single detector itself with an RGB filter can detect many colors as illustrated in  FIG. 2 , but considering the limited size of the recording sheet  20  in the sheet conveyance direction, it is very difficult to arrange patterns of all colors on the recording sheet  20  in the sheet conveyance direction. It is possible to arrange one patch for each color, but the plurality of gradation patterns can hardly be arranged. Accordingly, it is preferred that a detector head for each color be disposed along the main scanning direction so that the test pattern information of each color can be collected simultaneously 
       FIG. 3  shows an optimal arrangement of the detectors. A detector board  60  is disposed opposite and above the surface of the recording sheet  20 . Each of the output image detectors  31 A,  31 B,  31 C, and  31 D is disposed on the detector board  60  so as to be opposite a corresponding row of the test patterns M, Y, C, and Bk, respectively. M represents a row of test patterns of magenta, Y a row of test patterns of yellow, C a row of test patterns of cyan, and Bk a row of test patterns of black, respectively. Each test pattern row includes gradation levels and its gradation gradually decreases downstream in the conveyance direction (arrow F direction in  FIG. 3 ) of the recording sheet. 
       FIG. 4  shows an output image detector  31 ′. 
     An output image detector  31 ′ in another embodiment is formed of a line detection-type detector such as Charge Coupled Device (CCD) or Contact Image Detector (CIS). These linear detection detectors are basically formed of small spot detectors connected serially as illustrated in  FIG. 2 . 
     When the linear detection detector is used, patterns of each color need not be arranged in the sheet conveyance direction and the gradation patterns may be disposed also in the main scanning direction, thereby making the pattern layout freer. However, the linear detection detector is more expensive than the spot detector. 
     In the present invention, a test pattern for correction control is formed on the first surface of the recording sheet and a test pattern for verifying the effect of the correction is formed on the second surface thereof. In this case, the test pattern formed on the first surface may adversely affect the detection output of the second side of the recording sheet. 
     In such a case, the first-side pattern must not be overlaid positionally on the second-side pattern, thereby halving the useable surface of the recording medium. Accordingly, because there may be a case in which the spot detector cannot handle the necessary pattern number and patch size, the linear type detection detector may be installed alternatively. 
     The correction test pattern to be formed on the first side and the test pattern for verifying the correction result formed on the second side are basically the same. For example, when the gradation pattern as illustrated in  FIG. 3  is output on the first surface, the gradation levels as illustrated in  FIG. 5A  can be obtained. As long as the gradation levels are corrected as illustrated in  FIG. 5B , it is determined that the control effect is satisfactory and the same gradation pattern is satisfactory. 
     In addition, if a pattern with a large number of gradation levels needs to be formed on the first side as a correction pattern and the number of gradation levels to be formed on the second side needs to be reduced so that the second side pattern does not overlay the first side pattern, the second side pattern may be simplified. 
     As a result of detecting the verifying pattern formed on the second side, there may be a case in which correction effect is not properly obtained. In such a case, after the gradation levels as illustrated in  FIG. 5A  are obtained as to the first side, the gradation levels as illustrated in  FIG. 5A  are again obtained as to the second side. In such a case, the correction control needs to be performed again using another recording sheet. When performing the correction control step and the verifying step again, positions of the test patterns in the main scanning direction are changed from the previous time.  FIG. 6  shows an example of patterns. In addition, compared to the previous pattern as illustrated in  FIG. 3 , a sequence of the colors in the main scanning direction is changed. For example, in a case in which a cyan photoreceptor includes a certain defect (such as smears or filming) and as a result the pattern of the second spot detector position from the right in  FIG. 3  has a problem, the consecutive control effect may be improved by shifting the cyan pattern to the left end spot detector position. 
     If an abnormal image noticeable to a user is formed, replacement of a part is necessary. However, even when a slight defect is observed and an optimal correction control effect cannot be obtained as a result, there is a possibility that a good effect can be obtained by shifting the pattern as described above. 
     The determination of the above correction effect can be performed automatically by a controller, not shown. The controller, may be implemented as a central processing unit (CPU) provided with a random-access memory (RAM) and a read-only memory (ROM), for example, and executing programs stored in memory. If the controller determines, based on the measurement data or readings as illustrated in  FIG. 5 , that the image defect exceeds a predetermined level noticeable to the user, the controller outputs a message to a display panel, not shown, notifying the user that parts replacement is required. 
     Additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.