JP2007101821A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of simultaneously executing a plurality of requests of the same level severe in timing requirement without using an expensive CPU high in processing speed and reducing color smear. <P>SOLUTION: The image forming apparatus is provided with; a plurality of belt position detecting means detecting a belt position in the orthogonal direction to running direction of an intermediate transfer belt, a main scanning position control means controlling a main scanning start position for a writing means based on a detected result of each belt position detecting means, a sub-scanning position control means controlling a sub-scanning start position for a writing means according to the difference in time between a detection signal of a belt mark which is to be image forming start reference provided on the intermediate transfer belt and a synchronizing signal generated in the writing means, and a developing switching control means switching the developing means so that a selected developing means is made effective. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus using an intermediate transfer body, a change (shift) in a direction orthogonal to a running direction during running of the intermediate transfer body and a phase between a belt rotation position and a writing system scanning position ( The present invention relates to a technique for reducing misalignment of superimposed images caused by time differences.

In recent years, the development of local networks (hereinafter abbreviated as “LAN”) has progressed, and a plurality of PCs and printers that print out image data have been connected to this LAN. Under the control of network operations, each PC can access the printer. It has become. In such a network system, the frequency of access to the printer becomes very high, and efficient printer output is desired. Several techniques have been proposed for this purpose. As one of them, in Patent Document 1, interrupt processing is performed first as much as necessary, and the remaining processing is performed after the current processing is completed, and the current printing processing is performed while ensuring the necessary number of interrupt printing copies. The delay of the end time can be reduced. Further, Patent Document 2 compares the priority order of instructions, interrupts a low order instruction, and executes it from a high order instruction, so that an urgently required document can be output immediately. Further, Patent Document 3 changes the predetermined order as required to suppress timing deviation to a minimum.
JP 2001-320528 A JP 2003-246122 A Japanese Patent Laying-Open No. 2005-080267

  However, in any of the above Patent Documents 1 to 3, when a plurality of interrupt processes occur simultaneously with respect to the interrupt process control in which the time from the control request to the control end is questioned, the process is performed according to the priority order. One is sacrificed even if is at the same level. For example, even when an instruction for controlling the main scanning position and the sub-scanning position of an image is generated at the same time, one of them cannot be controlled immediately, so that color misregistration cannot be reduced and the image quality of a color (superimposed) image deteriorates. Further, one CPU cannot process the plurality of interrupt requests simultaneously.

  The present invention is intended to solve these problems. An image that can simultaneously execute a plurality of demands on the same level and has a severe timing requirement without using an expensive CPU capable of high-speed processing, and reduces color misregistration. An object is to provide a forming apparatus.

  The image forming apparatus of the present invention is an image forming apparatus that forms a superimposed image by transferring an image formed on an image carrier onto an intermediate transfer belt. Furthermore, the image forming apparatus of the present invention has a plurality of image forming means arranged to face the moving surface of the intermediate transfer belt, and each image forming means has an image carrier, a writing means, and a writing means on the image carrier. A developing unit including a plurality of developing means for developing the electrostatic latent image formed by the developing unit, and a developing switching means for selecting one of the developing means of the developing unit. The image forming apparatus of the present invention includes a plurality of belt position detecting means for detecting a belt position in a direction orthogonal to the traveling direction of the intermediate transfer belt, and a writing means based on the detection result of each belt position detecting means. The main scanning position control means for controlling the scanning start position, and the writing means according to the time difference between the belt mark detection signal provided on the intermediate transfer belt and the belt mark detection signal and the synchronization signal generated by the writing means. It is characterized in that it has sub-scanning position control means for controlling the sub-scanning start position and development switching control means for switching the developing means so that the selected developing means becomes effective.

  Arithmetic processing means are provided in each of the main scanning position control means, the sub-scanning position control means, and the development switching control means, and each control processing means provided in each control means starts an instruction required for each control means. Execute the instructions. Therefore, even when a control instruction with a strict time request, that is, when a plurality of interrupt requests are generated at the same time, the control operation can be immediately started.

  Further, two belt position detection means are provided in the vicinity of the transfer position between each image carrier and the intermediate transfer belt, and the main scanning position control means determines the main scanning start position of the writing means based on the difference value of each belt position detection means. Control. Therefore, the color shift in the main scanning direction due to the meandering of the belt can be reduced.

  Further, the sub-scanning position control means starts sub-scan image formation for each color as it is based on the time difference between the belt mark detection signal and the synchronization signal generated by the writing means, or forms a one-scan delay or an image head line. Writing is performed by selecting a beam from a plurality of subsequent beams. Therefore, the color shift in the sub-scanning direction due to the phase shift of the scanning beam of the writing system when the belt mark is detected can be reduced.

  Further, the development switching control means swings the development unit to a predetermined position within a predetermined time so that the development means selected by the development switching means becomes effective, so that the color mixing is performed in the development unit having a plurality of development colors. The color can be switched accurately and at high speed.

  In addition, by constructing each arithmetic processing means in a programmable device, a flexible and flexible control system can be constructed.

  According to the image forming apparatus of the present invention, even when request instructions for main scanning position control, sub-scanning position control, and development switching control are generated at the same time, high-quality images with reduced color misregistration can be processed simultaneously. 2 station type image forming apparatus can be constructed.

  FIG. 1 is a schematic diagram showing the configuration of the image forming apparatus. The image forming apparatus shown in the figure is a so-called two-station type image forming apparatus, which generates a toner image by a developing unit from a latent image formed on an image carrier by a scanning type writing unit, and transfers the toner image on an intermediate transfer member. The image forming apparatus forms a color image by repeating the process of transferring to a plurality of times for each color and sequentially superimposing toner images for each color. In the figure, a charging unit 2a, a developing unit 4a, a transfer unit 5a, and a cleaning unit 6a are disposed around an image carrier 1a composed of a photosensitive drum. The developing unit 4a selects an effective developing color from among a plurality (two) of developing units 4a1 and 4a2 for developing colors as necessary, and switches them with a developing switching unit (not shown). These are collectively referred to as an image station, and those including the image carrier 1a are referred to as an image station a. Further, each constituent means is arranged around the image carrier 1b similarly to the image station a, and the one including the image carrier 1b is referred to as an image station b. That is, the two image stations a and b are arranged to face the intermediate transfer belt 7 that is an intermediate transfer member. Further, an intermediate transfer belt 7 in the form of a belt or the like is disposed above the image carriers 1a and 1b. Further, the intermediate transfer belt 7 has a mark (not shown) indicating the reference position of the belt.

  Next, the operation of the image forming apparatus having such a configuration will be outlined. First, the surfaces of the image carriers 1a and 1b rotating in the direction of the arrow A are charged by the charging means 2a and 2b. On the other hand, when the intermediate transfer belt 7 moves in the direction of the arrow B and the mark detection means 10 detects the mark, exposure based on the image data is started by the exposure beams 3a and 3b from the writing means 20, and the image carriers 1a and 1b. To form a latent image. The latent image is visualized as a toner image by the developing means 4 a and 4 b and transferred to the intermediate transfer body 7 by the transfer means 5 a and 5 b at the contact point with the intermediate transfer belt 7. After the transfer, the image carrier 1a, 1b is cleaned of residual toner by cleaning means 6a, 6b.

  When a color (multiple color) image is formed, the above-described steps of image formation with different colors are repeated by switching the developing means 4a, 4b to the other color by the switching means, and the intermediate transfer belt 7 is repeated. Overlay the images of each color. When the four color images are superimposed on the intermediate transfer belt 7, the transfer roller 8 comes into contact with the intermediate transfer body 7, and the image is transferred to a recording medium 30 such as paper conveyed to the contact / separation position of the transfer roller 8. To do. When the transfer is completed, the transfer roller 8 is separated from the intermediate transfer belt 7. The paper onto which the image has been transferred is fixed by a fixing device (not shown) and discharged outside the device. Further, the cleaning blade 9 comes into contact with the intermediate transfer belt 7 over an area where an image is formed in order to clean the transfer residual toner on the intermediate transfer belt 7, and is separated when the cleaning is completed.

  As described above, due to the contact and separation of the transfer roller and the cleaning blade, the belt meanders or the position of the belt shifts due to a change in the tension of the intermediate transfer belt, and the superimposed images are displaced, resulting in deterioration of image quality. In addition, since the scanning position of the exposure beam of the writing means with respect to the reference position (belt mark) of the intermediate transfer belt also changes depending on each color, the sub-scanning also shifts and the image quality deteriorates.

  In order to prevent these degradations, the present invention obtains the amount of belt position fluctuation (meandering, offset) perpendicular to the running direction of the intermediate transfer belt at the image transfer position, and based on this, determines the image formation timing of the main scanning of the writing means. Simultaneously with the control, the image forming position of the sub-scan is controlled according to the time difference between the belt mark detection signal and the synchronization detection signal of the writing means. Further, in order to form an image having no color mixture at high speed, the developing means is accurately switched at a predetermined time.

  FIG. 2 is a schematic diagram showing the configuration of an image forming apparatus according to an embodiment of the present invention. In the figure, the same reference numerals as those in FIG. 1 denote the same components. As different constituent elements, belt position detecting means 11a and 11b for detecting the displacement of the belt position of the intermediate transfer belt 7 are provided in the vicinity of the arrangement position of the transfer means 5a and 5b, and a reference parallel to the belt running direction is provided. The belt end face is detected at a different position on the face. The belt position detection means 11a, 11b detects any mark or pattern on the belt in addition to the belt end face as long as it can detect the amount of variation (displacement) in the direction perpendicular to the belt running direction. It does not matter if it is configured.

  FIG. 3 is a block diagram showing the configuration of the image forming apparatus according to the embodiment of the present invention. An image forming apparatus 100 according to this embodiment shown in the figure is a CPU 101, a ROM 102 for storing a control program executed by the CPU 101 and other data, and a working memory for developing image data to be processed, an execution program, and the like. The RAM 103, the interface unit 104 for exchanging with an external device via Ethernet (registered trademark), telephone line, radio, etc., the writing means 105 corresponding to the writing means 20 in FIG. 2, the developing means 4a, FIG. The developing means 106 corresponding to 4b, the development switching control means 107 for switching the developing means 4a and 4b in FIG. 2, the intermediate transfer belt driving means 108 for controlling the driving of the intermediate transfer belt 7 in FIG. 2, and the belt in FIG. Belt position detecting means for detecting the belt position of the intermediate transfer belt based on the detection signals of the position detecting means 11a and 11b. 109, a main scanning position control unit 110 that controls the main scanning position of writing by the writing unit 105 based on the position of the intermediate transfer belt detected by the belt position detection unit 109, and an intermediate transfer belt detected by the belt position detection unit 109. And sub-scanning position control means 111 for controlling the sub-scanning position of writing by the writing means 105 based on the positions of the two, and each is connected via an internal bus 112.

Next, the operation of the image forming apparatus according to the present embodiment having such a configuration will be described with reference to FIGS.
When there is a print request from the host, the image forming apparatus starts driving the rotary polygon mirror (polygon) by the intermediate transfer belt 7 in FIG. 2 and the writing means 105 in FIG. 3 by the intermediate transfer belt driving means 108 in FIG. When the intermediate transfer belt 7 is driven, the two belt position detecting means 11a and 11b provided in the vicinity of the transfer positions of the image carriers 1a and 1b in FIG. 2 detect marks provided in advance on the belt edge or belt. A belt position in a direction orthogonal to the moving direction of the belt is detected. When the belt mark which is the image formation start reference is detected by the mark detection unit 10 in FIG. 1 after the intermediate transfer belt starts to be driven, the two belt position detection units 11a and 11b immediately obtained at an arbitrary predetermined period are used. The main scanning position control unit 110 in FIG. 3 instructs the writing unit 105 to control the start position of main scanning writing so as to reduce the color misregistration at the time of superimposed image formation based on the difference value. .

  Similarly, when the belt mark is detected by the mark detection means 10 in FIG. 1, the time difference between the belt mark signal and the synchronization detection signal generated by the writing means is obtained, and writing is started from the optimum (low color misalignment) sub-scanning position. 3 is instructed to the writing means 105 by the sub-scanning position control means of FIG. If the above control instructions overlap, processing is performed according to priority. When priority is given to the control of the main scanning position control means 110, the meandering component of the belt is obtained from the difference between the two belt position detection means 11a and 11b, and the main scanning start position is controlled based thereon. Thereafter, control processing of the sub-scanning position control unit 111 is performed.

  Next, sub-scanning position control processing by the sub-scanning position control means in the present embodiment will be described with reference to FIG. 4 which is an operation time chart. From the synchronization detection signal (LSync) immediately after the belt mark detection, image formation in main scanning is performed. It is necessary to control the process for the start position of the sub-scan at time tc until the effective width signal (LGate) is generated. Conversely, when the control of the sub-scanning position control means is prioritized, the time difference between the belt mark detection signal and the writing system synchronization signal is obtained and compared with a certain reference value to start image formation as it is. Whether scanning is delayed or writing is started from a succeeding beam of two (two) beams is determined and controlled. After such control is finished, the main scanning start position is controlled based on the meandering component obtained by the main scanning position control process, that is, the belt position detecting means.

  In particular, since the sub-scanning position control is performed by obtaining the relationship with the image formation start position of the color already formed after the second color, the processing time increases. Therefore, it is difficult to control the main scanning start position without a large delay from the time when the instruction is issued. Therefore, an arithmetic processing unit corresponding to each control unit is provided individually, so that even when an instruction required for each control unit is in a situation of multiple interrupt processing, the instructions can be processed immediately. The same applies to the development switching control means 107 in FIG. The development switching control means 107 drives the stepping motor, rotates the displacement means, and changes the effective development color by swinging the whole developing means (unit) around a certain point. The development switching control unit instructs the generation of the stepping motor drive signal so as to gradually increase (slow up) the frequency of the stepping motor drive signal so that the motor does not step out when switching is started. The stepping motor drive signal is generated by inverting “1” and “0” by this instruction signal. Immediately after displacement to a predetermined position, a stop instruction is output to the motor, and the displacement of the developing unit is terminated. If priority is given to the main scanning position control or sub-scanning position control instruction, the motor drive signal generation instruction is put into a standby state for the processing, and when the switching starts, the slow-up waveform is not generated correctly and step out occurs. There is a risk that when the switching is stopped, the developing unit cannot be stopped immediately at a predetermined position, and it becomes difficult to accurately maintain the gap between the image carrier and the effective developing roller.

  Accordingly, the development switching control means 107 in FIG. 3 can also be immediately controlled when a switching instruction is issued. Here, an example of a basic processing flow of the various control means is shown in FIGS. It is assumed that there is one belt mark. When there is a print request, the polygon of the writing means starts rotating at the same time as the intermediate transfer belt starts to rotate. Here, each system (program) centering on each control starts. Sub-scanning position control is included as part of the overall sequence control, such as motor start / stop that is not time-critical and lighting / power control of the light source of the writing means. Further, it is assumed that the operation flow of FIGS. 6 and 7 starts as a system (program) in which main scanning position control and development switching control are separate systems. In FIG. 5, when the belt rotates to detect the belt mark (sig1) (step S101; YES), the sub-scanning position control is started, the time difference from the writing system synchronization signal is obtained, and the sub-scanning position control is performed. By detecting the belt mark (sig1), it is determined whether or not the synchronization signal (sig2) is generated (step S102). When the synchronization signal (sig2) is generated (step S102; YES), a time difference Δt from the belt mark (sig1) is immediately obtained (step S103). Next, it is determined what color this image is (step S104), and if it is the first color (step S104; YES), a preset value is compared with the obtained value Δt (step S105) Δt. Is larger (step S106; YES), the sub-scanning image formation start signal (FGate) generated by the writing means is controlled to start writing as it is (step S107), and when it is smaller than Δt (step S106; NO) Control is performed to start the image form with a delay of one line (step S108). In the case of the second and subsequent colors (step S104; NO), the relationship with the already formed image start time is obtained to control the sub-scan image formation start position (step S109). If the start of image formation for the fourth color is confirmed (step S110; YES), the sub-scanning position control process is terminated, and the subsequent sequence control (paper feed conveyance, transfer, fixing, paper discharge) is performed (step S111). ).

  Next, in the main scanning position control flow shown in FIG. 6, when a belt mark (sig1) is detected, a timer is started (step S201; YES, step S202), and a difference between the two belt position detection means is obtained (step S203). . In accordance with the difference value, a shift signal for controlling the main scanning start position is output to the writing means (step S204). Then, the writing means controls the main scanning start position by shifting the synchronization signal along the time axis in accordance with the shift signal (step S205). While the image forming start signal (FGate) of the writing means is being output (step S206; NO, step S207; NO), the difference (meandering component) between the two belt position detecting means is obtained at a preset cycle, and the above control is performed. (Steps S201 to S205). After the Fgate is completed, the next mark signal detection is awaited, and the same control is performed after the detection (step S208; YES, steps S203 to S205, step S206; NO, step S207; YES).

  Next, in the development switching control flow shown in FIG. 7, each developing means (unit) is oscillated and switched by default to a position where the first and second development colors become valid when the power is turned on (step S301). ; NO, step S302, step S303; YES, step S304). When image formation (FGate signal) for the first color (second color) is completed (step S305; YES, step S306; NO), the development color is changed to another color (the image station a is the third color and the image station b is the fourth color). Switching control is performed (steps S302 to S304). The switching operation is terminated when the predetermined amount is displaced. While the printing operation continues, the development switching operation is performed.

  Next, control performed by an instruction required for main scanning position control will be described. Here, FIG. 8 shows a view of an intermediate transfer belt having a basic form as seen obliquely from the lower side (image carrier side). In the figure, belt position detecting means 11a and 11b for detecting belt end faces are provided below the flat portion of the intermediate transfer belt 7, that is, on the image transfer side, at different positions on the reference plane parallel to the belt running direction. . The intermediate transfer belt moves in the direction of arrow B, that is, from the belt position detecting means 11a toward the belt position detecting means 11b. From the experiment, a substantially same waveform as the fluctuation waveform (amount) of the belt position obtained at the position of the belt position detecting means 11a is obtained by the belt position detecting means 11b with a delay of time td. This time td is the time required for one point on the belt to move from the belt position detecting means 11a to the belt position detecting means 11b.

  Here, if a ′ obtained by delaying the displacement a of the belt position detecting means 11a by the time td is obtained and the difference ba−a ′ with respect to the displacement b of the belt position detecting means 11b is obtained, the unevenness of the belt end surface is canceled and the belt The belt fluctuation amount (meandering component) can be obtained when the arbitrary point is moved from the belt position detecting means 11a to the belt position detecting means 11b. By calculating the difference at any point on the belt as described above and adding / subtracting to / from the previously obtained difference value of the adjacent point, the meandering component from which the influence of the unevenness of the belt end surface is removed can be obtained.

  Therefore, the two detection means are arranged so that the distance between the detection positions of the belt position detection means 11a and the belt position detection means 11b satisfies the detection point pitch on the belt × n (integer).

  First, when n = 1, that is, when the distance between the belt position detection means and the pitch of the detection points on the belt are equal, the belt position (displacement) at each detection position when the belt end is viewed from the upper surface is schematically illustrated. As can be seen from FIG. 9, when p1 on the belt reaches the detection position of the belt position detection means 11b at time t1, p2 at the detection position adjacent to p1 is detected at the detection position of the belt position detection means 11a. . An arrow C indicates the direction of belt displacement that occurs as the belt moves. Further, the position where p1 on the belt is detected at the detection position of the belt position detecting means 11a at time t0 is set as a reference position d0. In FIG. 9, as the belt moves in the direction of arrow B, it is displaced upward, and at time t1, p1 is detected at the detection position of belt position detection means 11b, and p2 is detected at the detection position of belt position detection means 11a. The By subtracting (difference) the value of the belt position detecting means 11a at the time t0 from the belt position detecting means 11b at the time t1, the displacement (meandering) amount Δ1 accompanying the belt movement at p1 on the belt is obtained. In other words, since the distance between the belt position detection means is as short as several centimeters, the belt position detection means between the belt end faces (the belt end face regarded as having no irregularities) is substantially parallel to the reference plane set in the belt moving direction. Therefore, it can be said that p2 detected by the belt position detector 11a at the time t1 is also displaced by Δ1 with respect to the reference d0.

  Further, when the belt is displaced upward in the drawing as the belt moves, and p2 is detected at the detection position of the belt position detection means 11b and p3 is detected at the detection position of the belt position detection means 11a at time t2, the belt at time t2 is detected. By subtracting the value of the belt position detecting means 11a at the time t1 from the value of the position detecting means 11b, a displacement (meandering) amount Δ2 accompanying the belt movement at p2 on the belt is obtained. Here, since p2 is displaced by Δ1 with respect to the reference d0 at the detection position of the belt position detecting means 11a, the displacement d2 with respect to the reference d0 is d2 = Δ1 + Δ2. At this time, at the detection position of the belt position detecting means 11a, p3 is similarly displaced by d2 with respect to the reference d0.

  Next, when the belt is displaced downward as it moves, when p3 is detected at the detection position of the belt position detection unit 11b and p4 is detected at the detection position of the belt position detection unit 11a at time t3, belt position detection at time t3 is detected. By subtracting the value of the belt position detecting means 11a at time t2 from the value of the means 11b, the displacement (meandering) amount Δ3 accompanying the belt movement at p3 on the belt is obtained. Here, since p3 is displaced by d2 with respect to the reference d0 at the detection position of the belt position detecting means 11a, the displacement d3 with respect to the reference d0 is d3 = d2 + (− Δ3) = Δ1 + Δ2-Δ3. At this time, at the detection position of the belt position detection means 11a, p4 is similarly displaced by d3 with respect to the reference d0. Thereafter, similarly, the displacement from the reference position can be obtained.

  Next, FIG. 10 schematically shows a case where the distance between the belt position detecting means is the pitch of detection points on the belt × 2 (n = 2). The position where p1 on the belt is detected at the detection position of the belt position detecting means 11a at time t0 is defined as a reference d0. In FIG. 10, p2 is detected at the detection position of the belt position detection means 11a at time t1. At this time, the belt is displaced upward, and the displacement from the reference d0 is Δ10. Subsequently, when p1 reaches the detection position of the belt position detection means 11b at time t2, p3 is detected at the detection position of the belt position detection means 11a. Here, by subtracting the value of the belt position detecting means 11a at the time t0 from the value of the belt position detecting means 11b at the time t2 (difference), the displacement (meandering) amount Δ11 accompanying the belt movement at p1 on the belt is obtained. At this time, at the detection position of the belt position detecting means 11a, p3 is displaced by Δ11 with respect to the reference d0. When the belt further moves and p2 reaches the detection position of the belt position detection means 11b at time t3, p4 is detected at the detection position of the belt position detection means 11a. Accordingly, the value of the belt position detecting means 11a at the time t1 is subtracted from the value of the belt position detecting means 11b at the time t3 to obtain the displacement (meandering) amount Δ12 accompanying the belt movement at p2 on the belt. Here, since p2 is displaced by Δ10 with respect to the reference d0 at the detection position of the belt position detecting means 11a, the displacement d2 with respect to the reference d0 is d2 = Δ10 + Δ12. At this time, at the detection position of the belt position detecting means 11a, p4 is similarly displaced by d2 with respect to the reference d0.

  Further, when the belt is displaced downward as shown in FIG. 10 and p3 reaches the detection position of the belt position detection means 11b at time t4, p5 is detected at the detection position of the belt position detection means 11a. Therefore, by subtracting the value of the belt position detecting means 11a at the time t2 from the value of the belt position detecting means 11b at the time t4, the displacement (meandering) amount Δ13 accompanying the belt movement at p3 on the belt is obtained. Since p3 is displaced by Δ11 with respect to the reference d0 at the detection position of the belt position detecting means 11a, the displacement d3 with respect to the reference d0 is d3 = Δ11 + (− Δ13) = Δ11−Δ13. Thereafter, the displacement can be obtained in the same manner for all the detection points.

  Based on the belt fluctuation amount thus obtained, a shift signal is generated and the image formation start position in the main scanning direction is controlled as shown in FIG. This figure shows an example of the synchronizing signal of the writing means. The scanning beam detection output is binarized with a threshold value to generate a synchronization signal. The main scanning write start signal (LGate) is generated after a certain time from the rising edge of the synchronization signal. For example, when the amount of change in the location (p1) detected when the belt mark is detected, the shift amount is set to zero, and the synchronization signal is generated with the default threshold value th0. As shown in FIG. 12, with respect to the main scanning write start signal (LGate) when the synchronization signal is used as the reference position, a portion (variation amount + d) that has changed from the reference position to the belt center side is a scanning time corresponding to + d. The threshold is changed as shown by th1 in FIG. 11 so as to delay the Lgate signal generation timing by t. Further, as shown in FIG. 12, the position (fluctuation amount -d) that fluctuates from the reference position to the belt outer side is set to a threshold value by a shift signal so that the Lgate signal generation timing is advanced by a scanning time t corresponding to -d as shown in FIG. Change like th2.

  Note that image formation start position control in the main scanning direction is not controlled by the synchronization signal as described above, but there is also a method of controlling the generation start time of the LGate signal according to the shift signal with reference to the synchronization signal.

  Next, sub-scanning position control performed according to an instruction required for sub-scanning position control will be described. Image formation for each color is started with reference to the mark on the intermediate transfer member 7 in FIG. 2, but when the writing means 20 in FIG. 2 is a scanning type using a laser scanning optical system, the mark detection means 10 in FIG. Since the mark detection signal of the intermediate transfer body 7 and the synchronization signal that serves as the writing reference of the writing means 20 are asynchronous, even if image formation is started based on the mark reference of the intermediate transfer body 7, the superimposed images of the respective colors are displaced. More specifically, an example of the relationship between the image forming start signal a) in the sub-scanning direction generated by detecting the mark on the intermediate transfer body 7 by the mark detecting unit 10 in FIG. As shown in FIG. 13, the time difference between the image formation start signal a) and the synchronization signal is shifted by the synchronization signal period T at the maximum as shown in the signals b) and c). When the reference (first) image formation is performed with the synchronization signal p1 at the timing b) in FIG. 3, the image formation start timing outside the reference image (after the second color) can no longer be corrected, and the synchronization signal at the timing of the signal c). Is generated by the synchronization signal p2, and a deviation of a maximum of one scan (Δt: hatched portion) occurs.

  Therefore, in the present invention, first image formation is performed from a synchronization signal after a certain time has elapsed after detection of an image formation start signal. Here, FIG. 14 shows an example of dot formation positions under the control of the present invention when the plurality of beams are two beams (gray beam, white beam). Note that the gray beam is the preceding (starting) side beam of the sub-scanning. When the first color (reference) image formation is detected after the image formation start (belt mark) signal in the sub-scanning direction is detected and 3T / 4 or more has elapsed, the first color (reference) image formation is started with the synchronization signal. In the case of FIG. 14, since t1> 3T / 4, the operation starts in synchronization with the first synchronization signal. In the second color, when the synchronization signal p1 is generated at the time t2 after the mark detection, the synchronization signal p2 is delayed by one scan so that the difference Δt (absolute value) from the writing start position of the first color is minimized. Start image formation. The image formation start time from the belt mark signal at this time is t2 + T. T is the period of the synchronization signal. Here, the average tc1 (centroid synchronization 1) of the image formation start times of the first and second colors is tc1 = (t1 + t2 + T) / 2. For the third color, image formation is started by selecting a beam (white beam: L1) so that the time difference Δt between the average start time tc1 of the first and second colors and the start time of the third color is minimized. Assuming a pseudo synchronization signal (a dotted line between the gray beam (Du) and the white beam (L1) in FIG. 14) for the white beam L1 selected at this time, it is set as an image formation start synchronization signal for the third color. Therefore, the image formation start time with respect to the mark reference for the third color is t3 + T / 2. When the average tc2 (centroid synchronization 2) between tc1 and the image formation time of the third color is obtained, tc2 = (tc1 + t3 + T / 2) / 2. For the fourth color, since the time difference Δt between the time t4 from the belt mark signal to the synchronization signal and the average image formation start time tc3 up to the third color is smaller than the time difference in the case of delay or beam selection, image formation is started as it is. To do.

In the example of FIG. 14, the criterion for determining the writing start position is one scan delay when the time difference Δt is Δt ≧ 3T / 4, and beam (subsequent) selection when T / 4 ≦ Δt <3T / 4. However, if Δt <T / 4, it remains as it is.
In addition, the first and second colors of the first and second colors that have already been formed are determined so that the time difference between the average image formation start time tc1 of the first color and the second color is minimized so that the image formation start time of the third color obtained by delaying the write timing or beam selection control If the deviation from the fluctuation (variation) of the image formation start time with respect to the mark standard of the image is deviated from the deviation from the minimum or maximum value of the fluctuation range and the time determined by the write timing delay or beam selection control, it is determined in advance. Assume a start time that is advanced or delayed in the beam sub-scanning direction. Then, the deviation from the minimum value or the maximum value of the fluctuation range at that time is compared, image formation is started at the image formation start time at which the deviation becomes small, and the image formation start time with respect to the third color mark reference is obtained first. An average image formation start time tc2 with tc1 is obtained. Then, the image formation start time of the fourth color obtained by the delay of the write timing or the beam selection control so that the time difference from the average image formation start time tc2 is minimized, with respect to the mark reference of the already formed images of the first to third colors. When deviating from the fluctuation range of the image formation start time, it is advanced or delayed in the sub-scanning direction by a predetermined beam amount from the deviation from the minimum value or maximum value of the fluctuation range and the time obtained by delaying the write timing or beam selection control. Assumes a bad start time. Control may be performed so that the deviation from the minimum value or the maximum value of the fluctuation range at that time is compared, and image formation is started at the image formation start time at which the deviation becomes small.

  Next, an example of development switching control will be described with reference to FIG. A cam shaft 42 that rotates about an axis parallel to the rotation center axis 41 of the developing unit is provided in the developing unit frame 44, and an eccentric cam 43 is fixed to the cam shaft 42. The eccentric cam 43 is in contact with a roller member that is rotatable with respect to the developing roller shaft. The eccentric cam 43 is rotated by a motor so that the developing unit is displaced to a predetermined position, thereby swinging the developing unit itself and switching the effective developing color (roller).

  Further, as a configuration in which a plurality of CPUs are built in a programmable device (FPGA) and a plurality of interrupt processing requests can be controlled simultaneously, main scanning position control, sub-scanning position control, and development switching control of the image forming apparatus are performed.

  The present invention is not limited to the above-described embodiments, and it goes without saying that various modifications and substitutions are possible as long as they are described within the scope of the claims.

1 is a schematic diagram illustrating a configuration of an image forming apparatus. 1 is a schematic diagram illustrating a configuration of an image forming apparatus according to an exemplary embodiment of the present invention. 1 is a block diagram illustrating a configuration of an image forming apparatus according to an exemplary embodiment of the present invention. It is a time chart which shows the sub-scanning position control process by the sub-scanning position control means in the present embodiment. It is a flowchart which shows the basic process of a development switch control means. It is a flowchart which shows the process of main scanning position control. It is a flowchart which shows the process of development switching control. FIG. 2 is a view of an intermediate transfer belt having a basic form as viewed obliquely from below. It is a figure which shows typically the belt position in each detection position which looked at the belt edge part from the upper surface when the distance between belt position detection means and the pitch of the detection location on a belt are equal. It is a figure which shows typically the belt position in each detection position which looked at the belt edge part from the upper surface in case the distance between belt position detection means is pitch * 2 of the detection location on a belt. It is a figure which shows an example of the synchronizing signal of a writing means. 5 is a time chart showing a main scanning write start signal, a main scanning write start signal which is delayed by a scanning time t and a scanning time t is advanced. 6 is a time chart illustrating an example of a relationship between an image formation start signal in a sub-scanning direction and a synchronization signal of a writing unit. It is a figure which shows an example of the dot formation position by control of this invention at the time of using two beams as a plurality of beams. It is a schematic diagram showing an example of development switching control.

Explanation of symbols

11a, 11b; belt position detecting means;
106; developing means 107; development switching control means;
108; intermediate transfer belt driving means; 109; belt position detecting means;
110; main scanning position control means; 111; sub-scanning position control means.

Claims (6)

An image forming apparatus for forming an overlapped image by transferring an image formed on an image carrier onto an intermediate transfer belt, the image forming apparatus having a plurality of image forming means arranged to face the moving surface of the intermediate transfer belt, and each image The forming means selects an image carrier, a writing means, a developing unit including a plurality of developing means for developing an electrostatic latent image formed on the image carrier by the writing means, and one of the developing units. In an image forming apparatus having development switching means for
A plurality of belt position detecting means for detecting a belt position perpendicular to the traveling direction of the intermediate transfer belt;
Main scanning position control means for controlling the main scanning start position of the writing means based on the detection result of each belt position detection means;
A sub-scanning position control unit that controls a sub-scanning start position of the writing unit according to a time difference between a belt mark detection signal provided on the intermediate transfer belt and serving as a reference for image formation and a synchronization signal generated by the writing unit; ,
An image forming apparatus comprising: a development switching control unit that switches the developing unit so that the selected developing unit is effective.   Arithmetic processing means are provided in each of the main scanning position control means, the sub-scanning position control means, and the development switching control means, and instructions required for the respective control means are given to the respective arithmetic processing means provided in the respective control means. The image forming apparatus according to claim 1, wherein each control start instruction is executed by.   Two belt position detection means are provided in the vicinity of the transfer position between each image carrier and the intermediate transfer belt, and the main scanning position control means determines the main scanning start position of the writing means based on the difference value of each belt position detection means. The image forming apparatus according to claim 1, wherein the image forming apparatus is controlled.   The sub-scanning position control means starts the sub-scan image formation for each color as it is based on the time difference between the belt mark detection signal and the synchronization signal generated by the writing means, or a beam for forming one scanning delay or image head line. The image forming apparatus according to claim 1, wherein writing is performed by selecting from a plurality of subsequent beams.   The image forming apparatus according to claim 1, wherein the development switching control unit swings the development unit to a predetermined position within a predetermined time so that the developing unit selected by the development switching unit is effective.   The image forming apparatus according to claim 1, wherein each arithmetic processing unit is built in a programmable device.

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