JP2012150413A - Image formation system and method for correcting image density in image formation system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress variation in image density between a first stage apparatus and a second stage apparatus in a continuous printing operation performed in a serial tandem type image formation system while suppressing degradation of productivity.SOLUTION: An image density sensor 351 is provided in a sheet conveyance path disposed from a portion following fixation in a first stage apparatus 200 to a portion before transfer in a second stage apparatus 400 so as to detect a density of a test pattern (a pattern image) formed on a sheet P by the first stage apparatus 200. A second stage apparatus control part corrects, on the basis of density data, namely, the density of the pattern image detected by the image density sensor 351, an image density condition for regulating a density of a toner image to be formed by the second stage apparatus 400.

Description

  The present invention relates to an image forming system and an image density correction method thereof.

  In recent years, a serial tandem type image forming system in which a plurality of image forming apparatuses are connected in series to perform double-sided printing or the like has been put into practical use. For example, in a series tandem type image forming system in which two image forming apparatuses are connected, an image is formed on the surface of a sheet by a preceding image forming apparatus (previous machine), and the sheet is formed by a subsequent image forming apparatus (following machine). An image is formed on the back surface of. Thereby, productivity can be improved as compared with the case where double-sided printing is performed with one image forming apparatus.

  For example, Patent Document 1 discloses an image forming system that performs double-sided printing using a plurality of image forming apparatuses as a tandem configuration. In this image forming system, the following process is executed in order to improve the color reproducibility of the front and back sides. Specifically, the paper on which the test pattern is formed in the former machine, for example, the front surface of the paper, and the paper on which the test pattern is formed in the latter stage machine, for example, the back surface of the paper, are passed through the fixing unit of the latter machine to the sensor. Each is measured, and feedback control is performed to keep the color reproducibility of one or both of the front and rear machines optimally based on the measurement results.

JP 2009-300703 A

  However, when the normal operation of forming an image with respect to input image data is continuously performed, there is a problem that productivity of double-sided printing is reduced by performing the above-described feedback control. In other words, when the measurement of the test pattern on the front and back of the paper is completed and the result is fed back to the front or rear machine, the normal operation is temporarily stopped or several sheets are left blank for control. It is necessary to secure the time lag required.

  The present invention has been made in view of such circumstances, and the object of the present invention is to reduce the image density of the preceding and succeeding machines while suppressing a decrease in productivity during continuous printing in an in-line tandem type image forming system. It is to suppress variation.

  In order to solve such a problem, the first aspect of the present invention is to provide an image forming apparatus for forming an image on a sheet of paper, and a pre-stage machine that is connected to a downstream side of the pre-stage machine and outputs an image to the paper discharged from the pre-stage machine. There is provided an image forming system having a post-stage machine that is an image forming apparatus to be formed. In this image forming system, by controlling the former machine, a former machine control unit that forms a normal image corresponding to the input image data or a pattern image corresponding to previously stored pattern data on a sheet, and a latter machine By controlling, it is provided in the post-machine control unit that forms a normal image corresponding to the input image data on the paper, and in the paper conveyance path after the image formation in the pre-stage machine and before the image formation in the post-stage machine. And a density detecting unit for detecting the density of the pattern image formed on the paper by the preceding machine. Here, the post-stage machine control unit corrects the image density condition that defines the density of the image formed by the post-stage machine based on the density of the pattern image detected by the density detection unit.

  Here, in the first invention, the front-stage machine control unit forms a pattern image on n (n: a natural number of 1 or more) sheets, and the rear-stage machine control unit is detected by the density detection unit. It is preferable to correct the image density condition regarding the n + 1th and subsequent sheets based on the density of the pattern image regarding the nth sheet. Alternatively, in the first invention, the sheet is preferably a sheet set to a predetermined size by cutting a predetermined area as a cutting area. In this case, the former machine control unit forms a pattern image in the cutting area of the n (n: natural number of 1 or more) sheet, and forms a normal image in the non-cutting area of the nth sheet, It is desirable that the post-stage machine control unit corrects the image density condition regarding the nth sheet based on the density of the pattern image regarding the nth sheet detected by the density detection unit.

  In the first invention, the pattern image is a multi-tone image in which the image density changes stepwise, and the post-machine control unit is based on the detection result of the multi-tone pattern image by the image detection unit. It is preferable to correct the image density condition so as to correspond to the image density condition obtained by the preceding machine.

  A second invention is an image forming apparatus that forms an image on a sheet, and an image forming apparatus that forms an image on a sheet that is connected to the downstream side of the preceding apparatus and is discharged from the preceding apparatus. An image density correction method for an image forming system is provided. In this image density correction method, a first step of forming a pattern image corresponding to pattern data stored in advance on a sheet by controlling the former stage machine, and after image formation in the former stage machine and before image formation in the latter stage machine. Until the second step of detecting the density of the pattern image formed on the paper by the former stage machine and the density of the pattern image detected by the second step of the image formed by the latter stage machine. And a third step of correcting an image density condition that defines the density.

  According to the present invention, it is possible to acquire density data obtained by detecting the density of an image formed by the former stage machine before performing image formation by the latter stage machine. As a result, the detected density data can be immediately provided as data for correcting the image forming conditions of the subsequent machine. As a result, even during continuous printing, it is possible to suppress variations in image density between the preceding machine and the succeeding machine while suppressing a decrease in productivity.

1 is a configuration diagram schematically showing an image forming system according to a first embodiment. Explanatory drawing of the test pattern formed on the image density sensor 351 and the paper P Explanatory drawing which shows the structure of the color sensor 351Y typically. 1 is a block diagram schematically showing a control system of an image forming system according to a first embodiment. Explanatory drawing which shows the image density correction process concerning 1st Embodiment. Flowchart showing printing procedure of image forming system including image density correction process Flowchart showing printing procedure of image forming system including image density correction process Explanatory drawing about correction of image density condition of rear stage machine 400 Explanatory drawing which shows the image density correction process concerning 2nd Embodiment.

(First embodiment)
FIG. 1 is a configuration diagram schematically illustrating an image forming system according to the first embodiment. The image forming system includes a large-capacity paper feeding device 100, a pre-stage machine 200, an intermediate transport apparatus 300, and a rear-stage machine 400. Each of the front-stage machine 200 and the rear-stage machine 400 is an electrophotographic image forming apparatus such as a copying machine. This image forming system is configured by connecting two image forming apparatuses in series. This is a tandem type image forming system. In the serial tandem type image forming system, a toner image is formed on the surface of the paper P by one image forming apparatus (for example, the front stage machine 200), and a toner is formed on the back surface of the paper P by the other image forming apparatus (for example, the rear stage machine 400). By forming an image, it is possible to increase the printing speed.

  The large-capacity paper feeding device 100 is a device for accumulating and storing a large amount of paper P and feeding the paper P to the pre-stage machine 200 and the post-stage machine 400, and is serially connected to the front stage side of the pre-stage machine 200 in the paper transport direction. It is connected. The large-capacity paper feeding device 100 is mainly configured by a paper feeding unit 110. The paper feed unit 110 includes a plurality of paper feed trays, for example, three paper feed trays 111 to 113, and each of the paper feed trays 111 to 113 accommodates paper P of various sizes and types. The paper feeding unit 110 feeds the paper P selected by the user one by one from the corresponding paper feeding trays 111 to 113 and conveys the paper P to the pre-stage machine 200.

  The pre-stage machine 200 forms a toner image on the paper P by transferring and fixing the toner image to the paper P supplied from the large-capacity paper feeder 100 or the paper P held by itself. The pre-stage machine 200 mainly includes a document reading device 210, an image forming unit 230, a fixing unit 240, and a paper feeding unit 250.

  The document reader 210 has an automatic document feeder (not shown) provided at the top of the device, and subscans the document by the automatic document feeder while reading an image of the document line by line in the main scanning direction. By conveying in the direction, an image signal is obtained. Specifically, the document reading device 210 irradiates an image of the document with a lamp and forms an image of the reflected light on the light receiving surface of the image sensor. The image sensor photoelectrically converts the incident light and outputs a predetermined image signal to the image reading control unit 220. The image reading control unit 220 performs processing such as A / D conversion, shading correction, and compression on the image signal, and outputs the image signal to the upstream machine control unit 270 (see FIG. 4) as image data. Note that the image data input to the control unit 70 is not limited to the data read by the document reading device 210, and is, for example, data received from a personal computer connected to the image forming device or another image forming device. Also good.

  The image forming unit 230 transfers a full-color image onto the paper P by arranging a plurality of photoconductors facing one intermediate transfer belt in the vertical direction. Specifically, the image forming unit 230 includes four image forming units 230Y, 230M, 230C, and 230K corresponding to yellow, magenta, cyan, and black, and an intermediate transfer unit 235. Hereinafter, the configuration of the image forming unit 230Y corresponding to yellow will be described, but the configurations of the image forming units 230M to 230K corresponding to other colors are also the same.

  The image forming unit 230Y includes a photosensitive drum 231 and a developing unit 232. The developing unit 232 is mainly composed of, for example, a charging unit, an exposure unit, and a developing unit. The charging unit uniformly charges the peripheral surface of the photosensitive drum 231. The exposure unit irradiates the photosensitive drum 231 with laser light to form an electrostatic latent image on the surface thereof. The developing unit reveals the latent image formed on the surface of the photosensitive drum 231 as a toner image. As a result, a toner image is formed on the photosensitive drum 231. The toner image formed on the photosensitive drum 231 is sequentially transferred to a predetermined position on the intermediate transfer belt 236 that constitutes the intermediate transfer unit 235.

  Each color toner image transferred onto the intermediate transfer belt 236 is transferred onto the paper P as the paper P passes through the nip portion between the transfer roller 237 and the intermediate transfer belt 236 at a predetermined timing, as will be described later. The

  The paper feed unit 250 includes one or more paper feed trays, and the paper P is stored in the paper feed tray. The paper feeding unit 250 feeds the paper P selected by the user one by one as necessary.

  The paper P selected by the user is transported through the apparatus after being introduced from the large-capacity paper feeder 100 into the pre-stage machine 200 or supplied from the paper feeder 250. The sheet P is conveyed to the transfer position of the toner image in a state where it is synchronized with the toner image by the registration roller. As a result, the toner image is transferred onto the surface of the paper P as described above. The sheet P on which the toner image is transferred is conveyed to the fixing unit 240.

  The fixing unit 240 includes a heating roller and a pressure roller. A heat source (not shown) is built in the heating roller, and the heating roller is controlled to a constant temperature by this heat source. When the paper P passes through the nip portion, which is a pressure contact portion between the heating roller and the pressure roller, in the process of transporting the paper P, the toner image transferred to the surface of the paper P is heated and pressurized, and thereby the toner The image is fixed on the paper P. The paper P on which the fixing process has been performed by the fixing unit 240 is discharged to the adjacent intermediate conveyance device 300 by a paper discharge roller.

  The intermediate conveyance device 300 is disposed between the front-stage machine 200 and the rear-stage machine 400, and conveys the paper P discharged from the front-stage machine 200 to the rear-stage machine 400. In the process of transporting the paper P, the intermediate transport device 300 corrects the curl generated on the paper P, or reverses the front and back of the paper P as necessary. The intermediate conveyance device 300 is mainly configured by a pair of curl correction units 310 and 320 and a reverse conveyance unit 330, and an internal conveyance path is configured by a large number of conveyance rollers and guide members.

  The first curl correction unit 310 corrects the upper curl, that is, the curl of the paper P having a crest at the center, and the second curl correction unit 320 has the lower curl, that is, a trough at the center. Correct the curl of the paper P. Each of the curl correction units 310 and 320 is mainly configured by a small-diameter roller and a large-diameter roller having a larger diameter than the small-diameter roller. The curling of the paper P is corrected by the paper P receiving a bending force that is applied by the small diameter roller and the large diameter roller due to the paper P passing through the curl correction units 310 and 320.

  The reverse conveyance unit 330 includes a plurality of conveyance rollers and a paper guide. The reverse conveyance unit 330 reverses the front and back of the paper P by switchback, and then discharges the paper P to the subsequent stage machine 400. When reversing the front and back of the sheet P, the reversing conveyance unit 330 causes the sheet P to enter the reversing conveyance path extending downward by the sheet guide. When the reverse conveyance unit 330 determines that the paper P has reached a predetermined position in the reverse conveyance path, the reverse conveyance unit 330 reverses the paper P by stopping the conveyance of the paper P and then reversely feeding it. Then, the reverse conveyance unit 330 discharges the paper P to the adjacent succeeding machine 400 by the paper discharge roller. When it is not necessary to reverse the front and back of the paper P, the reversing conveyance unit 330 guides the paper P to the paper discharge roller by the paper guide and discharges the paper P as it is to the subsequent stage machine 400.

  Further, as one of the features of the present embodiment, the intermediate conveyance device 300 has a function of detecting the image density of the test pattern formed on the paper P by the pre-stage machine 200. For this reason, a leading edge detection sensor 350 and an image density sensor 351 are arranged in the conveyance path of the paper P in the intermediate conveyance device 300.

  The leading edge detection sensor 350 detects that the conveyed paper P (specifically, the leading edge of the paper P) has reached a detection position (for example, a position directly below the sensor) on the conveyance path. For example, a photo sensor or the like can be used as the tip detection sensor 350. The leading edge detection sensor 350 normally outputs an off signal, but switches to an on signal when the leading edge of the sheet reaches the detection position on the conveyance path. The leading edge detection sensor 350 is disposed on the upstream side of the image density sensor 351 in the conveyance path of the paper P.

  FIG. 2 is an explanatory diagram of the test pattern formed on the image density sensor 351 and the paper P. FIG. The image density sensor 351 detects the image density of the test pattern formed on the paper P. The image density sensor 351 is attached to a support member 352 that extends in the paper width direction (direction perpendicular to the paper conveyance direction FD) above the conveyance path, and corresponds to yellow, magenta, cyan, and black. It is composed of four color sensors 351Y, 351M, 351C, and 351K.

  Here, the test pattern is formed on the paper P by the pre-stage machine 200, and is composed of four pattern images Ty, Tm, Tc, and Tk corresponding to yellow, magenta, cyan, and black. More specifically, the tone of the test pattern increases stepwise in the paper conveyance direction FD as the image forming units 230Y to 230K constituting the pre-stage machine 200 form one pattern image at different positions. The pattern image is composed of four pattern images Ty to Tk having different colors in the paper width direction.

  FIG. 3 is an explanatory diagram schematically showing the configuration of the color sensor 351Y. The color sensor 351Y that detects the pattern image Ty corresponding to yellow has a paper width direction so that the detection position and the pattern image Ty corresponding to yellow correspond to each other when the paper P passes through the sensor 351Y. The layout position in is preset. The color sensor 351Y includes a light emitting element 351Ya such as an LED, a light receiving element 351Yb, and an optical element (lens) 351Yc that focuses light from the light emitting element 351Ya reflected by the pattern image Ty on the light receiving surface of the light receiving element 351Yb. It is configured. The color sensor 351Y is controlled to be switched on and off in accordance with a drive signal from an intermediate control unit 340 (see FIG. 4) described later. The color sensor 351Y detects the image density of the pattern image Ty when passing the detection position by the light emitting element 351Ya emitting light in the ON state and the light receiving element 351Yb outputting a voltage value corresponding to the amount of received light. Note that the color sensors 351M to 351K that detect the pattern images Tm to Tk corresponding to magenta, cyan, and black also have substantially the same configuration as the color sensor 351Y, although there are differences in the emission colors of the light emitting elements.

  The rear-stage machine 400 forms a toner image on the paper P by transferring the toner image to the paper P supplied from the intermediate conveyance device 300 or the paper P held by itself and performing a fixing process. The paper P on which the toner image is formed by the rear stage machine 400 is discharged to a discharge tray (not shown). The subsequent stage machine 400 is mainly configured by a document reading device 410, an image forming unit 430, a fixing unit 440, and a paper feeding unit 450. In addition, each element 410-450 which comprises the latter stage machine 400 respond | corresponds with each element 210-250 which comprises the former stage machine 200, and replaces the hundred-digit number in a code | symbol with "2" to "4" By doing so, description can be substituted.

  FIG. 4 is a block diagram schematically showing a control system of the image forming system according to the present embodiment. The control system of this image forming system is mainly configured by a paper feed control unit 120, a front-stage machine control section 270, an intermediate control section 340, and a rear-stage machine control section 470. These control units 120, 270, 340, and 470 are configured to be able to communicate with each other.

  The sheet feeding control unit 120 has a function of controlling the large capacity sheet feeding device 100 in an integrated manner. As the paper feed control unit 120, a microcomputer mainly composed of a CPU 121, a ROM 122, a RAM 123, and an I / O interface can be used. The paper feed control unit 120 controls the paper feed unit 110 to convey the paper P according to the user's selection to the pre-stage machine 200 one by one.

  Communication units 280, 360, and 480, which will be described later, are connected to the communication unit 130. The communication unit 130 functions as an input / output unit that establishes communication with the control units 270, 340, and 470. Accordingly, the paper feed control unit 120 can communicate with each of the control units 270, 340, and 470 via the communication unit 130.

  The pre-stage machine control unit 270 has a function of controlling the image forming apparatus that is the pre-stage machine 200 in an integrated manner, and controls the other control units 120, 340, and 470 to integrate the entire image forming system. It has a function to control. As the upstream machine control unit 270, a microcomputer mainly composed of a CPU 271, a ROM 272, a RAM 273, and an I / O interface can be used.

The pre-stage machine control unit 270 acquires the image data input from the image reading control unit 220 and the like, and based on the input image data, each part of the pre-stage machine 200 (image forming unit 230, fixing unit 240, By controlling the paper feeding unit 250), the following series of processes is executed. As a result, a toner image corresponding to the input image data (hereinafter referred to as “normal image” if necessary) is formed on the surface of the paper P.
(1) The photosensitive drum is charged by the charging section (2) The electrostatic latent image is formed on the photosensitive drum by the exposure section (3) The toner is attached to the electrostatic latent image formed by the developing section (4 ) Transfer the toner image on the photosensitive drum onto the paper P. (5) Fix the paper P on which the toner image is transferred. (6) Discharge the paper P.

  Also, in relation to the present embodiment, the pre-stage machine control unit 270 has the image density of the post-stage machine 400 when it has execution conditions set by the user or the like during continuous printing in which normal images are continuously formed. The image density correction process for correcting the image is interrupted. Specifically, as shown in FIG. 5, when the pre-machine control unit 270 determines that the execution condition is satisfied after the normal image is formed on the (n−1) th sheet P (Pn−1), The formation is temporarily interrupted, and a test pattern is formed on the nth sheet P (Pn). That is, the pre-stage machine control unit 270 controls each part of the pre-stage machine 200 based on the image data corresponding to the test pattern (hereinafter referred to as “pattern data”), and performs a process according to the above (1) to (6). Execute. By storing the pattern data in the ROM 272 in advance, the pre-stage machine control unit 270 can read out from the ROM 272. When the test pattern formation is completed, the pre-machine control unit 270 resumes the normal image formation on the (n + 1) th sheet P (Pn + 1).

  The communication units 280, 360, and 480 are connected to the communication unit 280, and the communication unit 280 functions as an input / output unit that establishes communication with the control units 120, 340, and 470. Thereby, the front machine control part 270 can communicate with each control part 120,340,470 via the communication part 280 mutually. In relation to the present embodiment, the pre-stage machine control unit 120 informs that when executing the image density correction process, or sends a print start command and printing conditions to be described later via the communication unit 280 to each control unit. 120, 340, and 470.

  The intermediate control unit 340 has a function of controlling the intermediate transfer apparatus 300 in an integrated manner. As the intermediate control unit 340, a microcomputer mainly composed of a CPU 341, a ROM 342, a RAM 343, and an I / O interface can be used. The intermediate control unit 340 controls the reverse conveyance unit 330 during double-sided printing on the paper P to invert the front and back of the paper P for the paper P conveyed from the front stage machine 200 and then to the rear stage machine 400. Eject paper. Further, when executing the image density correction process, the intermediate control unit 340 monitors the detection signal of the image density sensor 351 using the passage timing of the paper P detected by the leading edge detection sensor 350 as a trigger, and thereby the test pattern related to the test pattern. Obtain concentration data.

  Communication units 130, 280, and 480 are connected to the communication unit 360, respectively, and the communication unit 360 functions as an input / output unit that establishes communication with the control units 120, 270, and 470. Accordingly, the intermediate control unit 340 can communicate with each of the control units 120, 270, and 470 via the communication unit 360. In relation to the present embodiment, the intermediate control unit 340 outputs the density data to the post-stage machine control unit 470.

  The post-stage machine control unit 470 has a function of integrally controlling the image forming apparatus that is the post-stage machine 400. As the post-stage machine control unit 470, a microcomputer mainly composed of a CPU 471, a ROM 472, a RAM 473, and an I / O interface can be used. The post-stage machine control unit 470 controls each part of the post-stage machine 400 based on the image data output from the pre-stage machine control unit 270 (for example, image data to be printed on the back side of the input image data). Thus, the series of processes (1) to (6) described above is executed. As a result, a toner image (normal image) is formed on the back surface of the paper P.

  In relation to this embodiment, the post-stage machine control unit 470 defines the density of the toner image formed by the post-stage machine based on the density data output from the intermediate control unit 340 when the image density correction process is executed. Correct the image density condition.

  The communication unit 480 is connected to communication units 130, 280, and 360, respectively, and the communication unit 480 functions as an input / output unit that establishes communication with each of the control units 120, 270, and 340. As a result, the post-stage machine control unit 470 can communicate with each of the control units 120, 270, and 340 via the communication unit 480.

  6 and 7 are flowcharts showing the printing procedure of the image forming system including the image density correction process. The process shown in this flowchart is executed with a print start command instructed by the user as a trigger.

  Here, the print start command is input to the front-stage machine control section 270 by starting an operation section (not shown) provided in the upper part of the front-stage machine 200, for example. A touch panel capable of performing an input operation can be used in accordance with displayed information. The user can instruct the pre-stage machine control unit 270 through the operation unit 5 in addition to a print start command, image forming conditions and paper conditions as printing conditions. Here, the image forming conditions are, for example, single-sided / double-sided printing type, image density and magnification, the number of copies, and the like, and the paper conditions are the type of paper such as size, plain paper / thick paper, and the amount of paper. Note that an input of a print start command or the like to the pre-stage machine control unit 270 may be performed not only using the operation unit but also using a personal computer connected to the image forming apparatus.

  In this embodiment, it is assumed that double-sided printing and a plurality of print copies are designated as printing conditions, and the description will be made assuming a continuous printing scene. When a print start command is input, the pre-machine control unit 270 instructs the control units 120, 340, and 470 to start printing, reads the input image data, and prints an image to be printed on the back side. The data is output to the subsequent machine control unit 470.

  First, in step 10 (S10), the paper feed control unit 120 feeds the paper P from any of the paper feed trays 111 to 113 selected according to the printing conditions. In step 11 (S11), the paper feed control unit 120 discharges the paper P from the large-capacity paper feeder 100.

  In step 12 (S12), the pre-stage machine control unit 270 determines whether or not to form a test pattern for the paper P introduced into the pre-stage machine 200, that is, whether or not to execute image density correction processing. . This image density correction process is executed when predetermined execution conditions are satisfied, for example, when the cumulative number of printed sheets reaches a predetermined number (1000 × n (n: natural number of 1 or more)) set by the user. If an affirmative determination is made in step 12, that is, if the image density correction process is to be executed, this is notified to the other control units 120, 340, and 470, and the process in step 13 (S13) and subsequent steps is performed. move on. On the other hand, when a negative determination is made in step 12, that is, when a normal image (a toner image corresponding to input image data) is formed without forming a test pattern, the process proceeds to step 22 (S22) described later. move on.

  In step 13, the pre-machine controller 270 reads the pattern data stored in the ROM 272, and transfers the test pattern to the paper P based on this pattern data. In step 14 (S14), the pre-stage machine control unit 270 performs a test pattern fixing process on the paper P on which the toner image (test pattern) is transferred onto the paper surface. In step 15 (S15), the pre-stage machine control unit 270 discharges the paper P from the pre-stage machine 200.

  In step 16 (S16), the intermediate control unit 340 determines whether or not the output of the tip detection sensor 350 is on. Based on the determination in step 16, it is confirmed whether or not the sheet P transported in the intermediate transport apparatus 300 has reached the lower position of the leading edge detection sensor 350, that is, whether it has reached the previous stage of the image density sensor 351. The If an affirmative determination is made in step 16, that is, if the output of the tip detection sensor 350 is on, the process proceeds to step 17 (S17). On the other hand, if a negative determination is made in step 16, that is, if the output of the tip detection sensor 350 is off, the determination in step 16 is performed again.

  In step 17, the intermediate control unit 340 switches the color sensors 351 </ b> Y to 351 </ b> K constituting the image density sensor 351 to the on state, and starts detecting the image densities of the pattern images Ty to Tk by the color sensors 351 </ b> Y to 351 </ b> K. . In step 18 (S18), the intermediate control unit 340 switches the color sensors 351Y to 351K to the off state, and ends the detection of the image densities of the pattern images Ty to Tk. The switching of each of the color sensors 351Y to 351K to the off state is determined based on the elapsed time starting from the timing when the determination in step 16 is affirmative that the trailing edge of the sheet P has passed the detection position of the image density sensor 351. As a result, it is executed.

  In step 19, the intermediate control unit 340 outputs the density data, that is, the image density detection results corresponding to the test patterns Ty to Tk output from the color sensors 351 </ b> Y to 351 </ b> K, to the subsequent-stage machine control unit 470.

  In step 20 (S20), the post-stage control unit 470 corrects the image density condition that defines the density of the toner image formed by the image forming unit 430 for each of yellow, magenta, cyan, and black based on the density data. (So-called γ correction). The concept of correcting the image density condition will be described later.

  In step 21 (S21), the rear-stage machine control unit 470 causes the paper P on which the test pattern is formed to be different from the paper discharge tray (not shown) on which the paper P on which the normal image is formed is discharged. To discharge.

  On the other hand, if a negative determination is made in step 12, as shown in FIG. 7, the pre-stage machine control unit 270 transfers the normal image onto the surface of the paper P based on the image data in step 22 (S22). In step 23 (S23), the pre-stage machine control unit 270 performs normal image fixing processing on the paper P on which the toner image (normal image) is transferred onto the paper surface. In step 24 (S24), the pre-stage machine control unit 270 discharges the paper P from the pre-stage machine 200.

  In step 25 (S25), the intermediate control unit 340 conveys the paper P introduced into the intermediate conveyance device 300, and the reverse conveyance unit 330 reverses the front and back of the paper P. In step 26 (S 26), the intermediate control unit 340 discharges the paper P from the intermediate conveyance device 300.

  In step 27 (S27), the post-stage machine control unit 470 transfers the normal image to the back surface of the paper P based on the image data. In step 28 (S28), the post-stage machine control unit 470 performs a normal image fixing process on the paper P on which the toner image (normal image) is transferred on the back side of the paper. In step 24 (S24), the rear stage machine control unit 470 discharges the paper P from the rear stage machine 400.

  Hereinafter, with reference to FIG. 8, the correction of the image density condition of the rear stage machine 400 will be described in detail. Normally, the image density conditions in the image forming units 230 and 430 are set to be the same so that the toner images output from the front-stage machine 200 and the rear-stage machine 400 correspond to each other. Specifically, in the pre-stage machine 200 and the post-stage machine 400, the target density Trg of the toner image to be output corresponding to the gradation of the image data is set in advance for each gradation as the image density condition. Therefore, if the input image data is the same in the pre-stage machine 200 and the post-stage machine 400, the density of each toner image is also the same.

  However, the output density of the toner image for the same gradation may differ between the pre-stage machine 200 and the post-stage machine 400 due to the difference in image formation frequency. In this case, when front side printing is performed by the front stage machine 200 and back side printing is performed by the rear stage machine 400, even if the same toner image is formed on the front and back of the paper P, the image density of the toner image differs between the front and back sides. As a result, the color reproducibility is degraded.

  Therefore, in the present embodiment, the image density condition of the rear stage machine 400 is corrected to match the actual output of the front stage machine 200 based on the density data corresponding to the output of the front stage machine 200. Here, in FIG. 8, “Dac” is an explanatory diagram showing density data, that is, the actual image density of the pre-stage device 200 for each gradation. The actual image density of the pre-stage machine 200 corresponding to the gradation “150” is lower than the target density Trg and coincides with the target density Trg corresponding to the gradation “123”. Therefore, when the gradation “150” is input as the image data, the post-stage machine 400 sets the image density condition (γ correction curve of the post-stage machine 400 so that the target density Trg corresponding to the gradation “123” is obtained. ) Is adjusted, it is possible to achieve a concentration balance between the upstream machine 200 and the downstream machine 400. Therefore, the image density condition of the post-stage machine 400 can be corrected by performing this operation for all gradations and further performing the same processing for all colors (yellow, magenta, cyan, black). This corrected image density condition is applied in image formation on the (n + 1) th sheet P (Pn + 1) conveyed after the sheet P (Pn) on which the test pattern is formed.

  As described above, according to the present embodiment, the image density sensor 351 is provided in the paper transport path from after the fixing in the pre-stage machine 200 to before the transfer in the post-stage machine 400. The density of the formed test pattern (pattern image) is detected. Then, the post-machine controller 470 corrects the image density condition that defines the density of the toner image formed by the post-machine 400 based on the density data, that is, the density of the pattern image detected by the image density sensor 351. .

  According to this configuration, it is possible to acquire density data relating to an image formed by the pre-stage machine 200 before performing image formation by the post-stage machine 400. As a result, the detected density data can be immediately provided as data for correcting the image forming conditions of the post-stage machine 400. As a result, even during continuous printing, variations in image density between the pre-stage machine 200 and the post-stage machine 400 can be suppressed while suppressing a decrease in productivity.

  Further, according to the present embodiment, the pre-stage machine control unit 270 forms only the test pattern (pattern image) on the nth (n: natural number of 1 or more) sheet P. Then, the post-stage machine control unit 270, based on the density of the test pattern related to the nth sheet P (Pn) detected by the image density sensor 351, the n + 1th and subsequent sheets P (Pn + 1, Pn + 2, Pn + 3,... -Correct the image density condition for).

  According to such a configuration, it is possible to suppress variations in image density between the pre-stage machine 200 and the post-stage machine 400 only by interposing at least one sheet P during continuous printing. Thereby, the fall of productivity can be controlled.

  In the present embodiment, the pre-stage machine 200 and the post-stage machine 400 have been described on the premise of an image forming apparatus that forms a color image. However, the pre-stage machine 200 and the post-stage machine 400 are image forming apparatuses that form a monochrome image. May be. In this case, the test pattern formed on the paper P by the pre-stage machine 200 only needs to be composed of one pattern image Tk corresponding to black, and the image density sensor 351 is one color sensor 351K corresponding to black. It only has to be configured.

  In the present embodiment, the image density of the pattern image formed on the nth sheet P (Pn) is detected, and the image density condition for the n + 1th sheet and subsequent sheets P (Pn + 1, Pn + 2, Pn + 3...). Was corrected. However, if there is a sufficient distance in the transport path from the installation position of the image density sensor 351 to the transfer position of the succeeding machine 400, the image density of the pattern image formed on the nth sheet P (Pn) is detected. It is also possible to correct the image density condition for the n-1th sheet P (Pn-1, Pn-2, Pn-3,...).

(Second Embodiment)
An image density correction process of the image forming system according to the second embodiment will be described. Note that the description of the same configuration as that of the first embodiment will be omitted, and the following description will be focused on the differences. In the present embodiment, the front-stage machine 200 and the rear-stage machine 400 are described as image forming apparatuses that form monochrome images, not image forming apparatuses that form full-color images.

  FIG. 9 is an explanatory diagram of a test pattern according to the second embodiment. As shown in the figure, the paper P in this embodiment is set to a predetermined size by cutting one or both predetermined areas (hereinafter referred to as “cutting areas”) at both ends parallel to the paper transport direction FD. Paper. In this paper P, a normal image is formed in the non-cut area except the cut area, and a test pattern is formed in each of the cut areas. Each test pattern is composed of a pattern image Tk corresponding to black, and the gradation changes stepwise in the paper conveyance direction.

  Corresponding to such a test pattern form, the image density sensor 351 includes two color sensors 351K corresponding to black. Further, each color sensor 351k has a layout position in the sheet width direction so that when the sheet P passes through the color sensor 351Y, the detected position and the pattern image Ty formed in the cutting area correspond to each other. Is preset.

  When such a sheet P for cutting is used, unlike the first embodiment, the pre-stage machine control unit 270 performs image density correction processing of the post-stage machine 400 simultaneously with the formation of the normal image. Specifically, as shown in FIG. 9, the pre-stage machine control unit 270 forms a test pattern simultaneously with the formation of the normal image on the nth sheet P (Pn). That is, the pre-stage machine control unit 270 controls each part of the pre-stage machine 200 based on the input image data and pattern data, and executes processes according to the above-described (1) to (6). Thereby, on the paper P (Pn), the pattern image Tk is formed in each of the cutting areas, and the normal image is formed in the non-cutting area. The pre-stage machine control unit 270 forms a normal image and a test pattern for each sheet fed, such as the (n + 1) th sheet P (Pn + 1).

  On the other hand, the intermediate control unit 340 monitors the detection signal of the image density sensor 351 with the passage timing of the paper P detected by the leading edge detection sensor 350 as a trigger, thereby obtaining density data regarding the test pattern.

  Further, the post-stage machine control unit 470 forms a toner image (normal image) on the back surface of the paper P, and the density of the toner image formed by the post-stage machine 400 based on the density data output from the intermediate control unit 340. The image density condition that regulates is corrected. This corrected image density condition is applied in image formation on the paper P from which density data has been read. For example, the density data for the nth sheet P (Pn) is applied as an image density condition for image formation for the nth sheet P (Pn) in the rear stage machine 400.

  As described above, according to the present embodiment, the detected density data can be used as data for correcting the image forming conditions of the succeeding apparatus 400. Thereby, in the serial tandem type image forming system, it is possible to suppress variations in image density between the pre-stage machine 200 and the post-stage machine 400 while suppressing a decrease in productivity during continuous printing.

  Further, according to the present embodiment, the paper P is set to a predetermined size by cutting a predetermined area (for example, one or both areas at both ends parallel to the paper conveyance direction) as a cutting area. Paper can be used. In this case, the pre-stage machine control unit 270 forms a pattern image in the cutting area of the nth sheet P (Pn) and forms a normal image in the non-cutting area of the nth sheet P (Pn). . The subsequent-stage machine control unit 470 then performs an image relating to the nth sheet P (Pn) based on the density of the test pattern (pattern image) relating to the nth sheet P (Pn) detected by the image density sensor 351. Correct the density condition.

  According to such a configuration, a test pattern can be printed together with a normal image. Therefore, when the rear stage machine 400 forms an image on the paper P on which density detection has been performed, the image density condition of the rear stage machine 400 is corrected. Can work with. Thereby, it is possible to suppress variations in image density between the pre-stage machine 200 and the post-stage machine 400 simultaneously with the continuous printing. Thereby, the fall of productivity can be controlled.

  Note that it is not necessary to form the pattern image Ty in each of the cutting regions, and the pattern image Ty may be formed on either one end side. If the cutting area is provided only at one end, it is sufficient to form the pattern image Ty in that area. In the present embodiment, the execution condition of the image density correction is set for each sheet of paper, but may be an appropriate number of sheets or user designation such as every 10 sheets. In this case, the subsequent-stage machine control unit 470 determines the nth and subsequent sheets of paper P (Pn) based on the density of the test pattern (pattern image) relating to the nth sheet of paper P (Pn) detected by the image density sensor 351. The image density condition regarding is corrected.

  In the above-described embodiment, the pre-stage machine 200 and the post-stage machine 400 have been described on the premise of an image forming apparatus that forms a monochrome image. However, the pre-stage machine 200 and the post-stage machine 400 are similar to the first embodiment in color images. It may be an image forming apparatus that forms the image. In this case, in the image density correction process, a yellow pattern image Ty is formed in one cutting area of the first sheet P and a magenta pattern image Tm is formed in the other cutting area. A cyan pattern image Tc is formed in one cutting area and a black pattern image Tk is formed in the other cutting area.

  The image forming system according to the embodiment of the present invention has been described above, but the present invention is not limited to the above-described embodiment, and it is needless to say that various modifications are possible within the scope of the invention. . For example, the serial tandem type image forming system of the present invention may have a configuration in which two image forming apparatuses are connected in series without interposing a relay device between the preceding and succeeding machines. In the above-described embodiment, the configuration in which the front-side machine performs surface printing and the back-side machine performs back-side printing as the image forming system has been described. However, the front-side machine and the rear-stage machine perform surface printing in a superimposed manner. May be. Further, the image density detection is performed in the conveyance path of the intermediate conveyance device, but after the image formation of the preceding machine (after fixing in this embodiment) to before the image formation of the preceding machine (before transfer in this embodiment). It may be performed between. In addition, the image forming apparatuses as the front-stage machine and the rear-stage machine may be image forming apparatuses that employ a method other than the electrophotographic method.

DESCRIPTION OF SYMBOLS 100 ... Large-capacity paper feeder 110 ... Paper feed part 120 ... Paper feed control part 130 ... Communication part 200 ... Pre-stage machine 210 ... Document reading device 220 ... Image reading control part 230 ... Image forming part 235 ... Intermediate transfer part 240 ... Fixing Unit 250 ... paper feeding unit 270 ... pre-stage machine control unit 280 ... communication unit 300 ... intermediate conveyance device 330 ... reverse conveyance unit 340 ... intermediate control unit 350 ... leading edge detection sensor 351 ... image density sensor 400 ... back-stage machine 410 ... document reading device 420 ... Image reading control unit 430 ... Image forming unit 440 ... Fixing unit 450 ... Paper feeding unit 470 ... Subsequent machine control unit

Claims (5)

A pre-stage machine that is an image forming apparatus that forms an image on paper, and a post-stage machine that is connected to the downstream side of the pre-stage machine and forms an image on paper discharged from the pre-stage machine. In an image forming system,
A pre-machine controller that controls the pre-machine to form a normal image corresponding to input image data or a pattern image corresponding to pre-stored pattern data on a sheet;
A post-machine control unit that controls the post-machine to form a normal image corresponding to the input image data on a sheet;
A density detection unit that is provided in a paper conveyance path between after the image formation in the preceding machine and before the image formation in the succeeding machine, and that detects the density of the pattern image formed on the paper by the preceding machine; Have
The post-stage machine control unit corrects an image density condition that defines the density of the image formed by the post-stage machine based on the density of the pattern image detected by the density detection unit. system. The preceding machine control unit forms the pattern image on n (n: a natural number of 1 or more) sheets,
The post-stage machine control unit corrects an image density condition regarding n + 1 and subsequent sheets based on the density of the pattern image regarding the nth sheet detected by the density detection unit. 1. The image forming system described in 1. The paper is a paper set to a predetermined size by cutting a predetermined area as a cutting area,
The pre-stage machine control unit forms the pattern image in a cutting area of n (n: a natural number of 1 or more) sheets and forms the normal image in a non-cutting area of the nth sheet,
2. The post-stage machine control unit corrects an image density condition for the nth sheet based on the density of the pattern image for the nth sheet detected by the density detection unit. The image forming system described in 1. The pattern image is a multi-tone image in which the image density changes stepwise,
The post-stage machine control unit corrects the image density condition so as to correspond to the image density condition by the pre-stage machine based on the detection result of the multi-tone pattern image by the image detection unit. The image forming apparatus according to claim 1. A pre-stage machine that is an image forming apparatus that forms an image on paper, and a post-stage machine that is connected to the downstream side of the pre-stage machine and forms an image on paper discharged from the pre-stage machine. In the image density correction method of the image forming system,
A first step of forming a pattern image corresponding to pre-stored pattern data on a sheet by controlling the preceding machine;
A second step of detecting the density of the pattern image formed on the paper by the preceding machine between the time after image formation in the preceding machine and before image formation in the succeeding machine;
A third step of correcting an image density condition that defines a density of an image formed by the latter-stage machine based on the density of the pattern image detected in the second step;
An image density correction method for an image forming system, comprising:

Images