JP5286722B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus.

  Some image forming apparatuses include a sensor for detecting whether a defect has occurred in a formed image. When a recording medium on which an image is formed is transported by such a sensor for detecting image defects, the image formed on the recording medium is read, and image defects such as streaks and unevenness are detected and formed. The position of the image to be detected is detected.

  As this sensor, for example, a line image sensor having a head width or more is used to read an image printed by all nozzles of a head for forming an image.

  In relation to this technique, Patent Document 1 discloses a technique that includes a paper width reading sensor, reads a print result by the sensor, compares the result with image data, and determines whether there is an image quality defect. Japanese Patent Application Laid-Open No. 2005-228561 discloses a technique that includes a paper width reading sensor, reads a test pattern print by the sensor, determines a non-ejection nozzle, and compares it with image data to determine whether there is an image quality defect. Has been.

Further, Patent Document 3 discloses a technique for generating a reading timing for each document conveyance clock and correcting the detection output by using the voltage of the capacitor for the variation. Japanese Patent Application Laid-Open No. 2004-228561 discloses a technique for detecting unevenness in the document conveyance speed and correcting the reading timing in accordance with the actual conveyance speed.
Japanese Patent Laid-Open No. 2006-199048 JP 2004-9474 A JP 2000-278482 A JP 2002-152465 A

  The line image sensor irradiates light for a predetermined period, photoelectrically converts the reflected light, and detects the density of the print surface. If the size of the image read within the predetermined period, that is, the distance moved by the recording medium being conveyed within the predetermined period is always constant, the detection result can be compared and the defect of the image can be detected accurately. When the conveyance speed of the recording medium fluctuates, the moving distance fluctuates, and the image range indicated by the image obtained by reading only for a predetermined period is different, so that accurate detection cannot be performed.

  Specifically, for example, in density detection, if the moving distance varies, the number of detected gray dots will differ, and accurate density determination cannot be performed. Also, when detecting color registration misalignment, the detection position varies due to fluctuations in the movement distance, and therefore the position cannot be determined accurately. Further, when pattern matching is performed, the pattern expands and contracts due to fluctuations in the movement distance, and accurate comparison cannot be performed.

  As described above, since the read range of the read image becomes an image range different from the range that should be read because the movement distance fluctuates, there is a problem in that the image formed on the recording medium cannot be detected accurately.

  In view of the above problems, an object of the present invention is to provide an image forming apparatus capable of accurately detecting an image formed on a recording medium even when the moving distance varies.

In order to achieve the above object, an invention according to claim 1 is provided with a roller and an endless conveyance belt wound around the roller, and the conveyance belt driven to rotate according to the rotation of the roller causes an image to be transferred. A conveying unit configured to convey a recording medium to be formed; an image forming unit configured to form the image on the recording medium conveyed by the conveying unit in accordance with a clock signal indicating timing for forming the image; and the recording In order to detect an image formed on the medium, when the recording medium on which the image is formed by the image forming unit is being conveyed by the conveying unit, an image obtained by reading the image for a predetermined period is obtained. Based on the image acquisition means and the conveyance speed at which the conveyance belt conveys the recording medium, the clock signal is output to the image forming means, The image forming unit controls the timing for forming an image, and the maximum speed of the transporting speed is set so that the moving distance is constant when the recording medium is transported by the transporting unit during the predetermined period. Obtain the speed fluctuation width from the frequency of the clock signal corresponding to the maximum speed, and control the length of the predetermined period by calculating the number of clocks of the signal multiplied by the clock signal based on the speed fluctuation width Control means.

According to a second aspect of the present invention, in the first aspect of the invention, the image acquisition unit acquires the image by reflected light from light irradiated on the recording medium, and the image acquisition unit includes the image acquisition unit. When starting the reading, the irradiation of the light to the recording medium is started, and when the image acquisition unit finishes reading the image, the irradiation unit further ends the irradiation, the control unit, After the irradiation means starts the irradiation, the length of the predetermined period is controlled so that the light quantity by the irradiation becomes a predetermined light quantity or more.

According to a third aspect of the present invention, in the first or second aspect of the present invention, the image forming unit moves a distance that the recording medium moves by being conveyed by the conveying unit within the predetermined period. A plurality of rectangular images having a distance in the transport direction and a length in a direction crossing the transport direction as a predetermined length are recorded on the recording medium at predetermined time intervals based on the multiplied clock signal. A rectangular image group in which the rectangular images to be formed do not overlap with each other is formed with a first color for a first period, and a second color different from the first color is formed with the first color. The rectangular image group including one rectangular image in the image group and a rectangular image separated by the moving distance is formed, and the first reading image having the highest reading density among the rectangular images read by the image acquisition unit. Said formed with color A position in the recording medium in the form image, by reading density is compared with the position in the recording medium of the highest second of the rectangular image formed by the color, the first color image is formed And detecting means for detecting a shift between the position where the second color image is formed and the position where the image of the second color is formed.

  According to the first aspect of the present invention, it is possible to provide an image forming apparatus capable of accurately detecting an image formed on a recording medium even when the moving distance varies.

According to the first aspect of the present invention, the predetermined period can be controlled more strictly than in the case based on the clock signal that is not multiplied.

According to the second aspect of the present invention, it is possible to increase the sensitivity with which the image acquisition means reads an image, as compared with the case where the control is not performed so that the period becomes a predetermined light amount or more.

According to the invention of claim 3, since the reading range can be made constant even if the moving distance fluctuates, the position where the first color image is formed and the image of the second color are formed. Can be accurately detected.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Hereinafter, a case where the image forming apparatus according to the present invention is applied to an ink jet printer will be described. In this embodiment, a recording medium such as paper on which an image is formed is expressed as recording paper. Furthermore, forming an image on a recording sheet is expressed as recording an image.

  FIG. 1 shows the overall configuration of an ink jet printer (hereinafter referred to as “printer”) 10 according to the present embodiment.

  The printer 10 includes a recording head array 12 that discharges droplets to form (record) an image. The recording head array 12 includes four recording heads 14C, 14M, 14Y, and 14K corresponding to the colors C (cyan), M (magenta), Y (yellow), and K (black). The recording head array 12 corresponds to an image forming unit. In the following description, an initial letter corresponding to each color is added to the code when distinguishing each color, and an initial letter corresponding to each color is omitted unless otherwise distinguished.

  Further, each recording head 14 is a long head having a recording area equal to or larger than the width of the recording paper P, and a large number of nozzles are arranged along the width direction of the recording paper P. The recording head 14 can collectively record the entire width of the recording paper P by discharging droplets of ink liquid from each nozzle.

  The printer 10 is loaded with ink cartridges 16C, 16M, 16Y, and 16K that store CMYK ink liquids, and the ink liquids stored in the ink cartridges 16 correspond to the respective colors through pipes (not shown). It is supplied to the head 14.

  The printer 10 also includes a paper feed tray 18 that stores the recording paper P. The recording paper P supplied from the paper supply tray 18 is conveyed by a plurality of roller pairs 20 and supplied to the recording head array 12.

  An endless transport belt 24 wound around rollers 22A and 22B and a tension roller 23 is provided at a position facing the recording head array 12. The conveying belt 24 has a surface wider than the width in the direction orthogonal to the conveying direction of the recording paper P, and is given a certain tension by being pulled downward by the tension roller 23. The conveyor belt 24 is driven to rotate by the rotation of the rollers 22A and 22B and the tension roller 23. These conveyor belt 24, rollers 22A and 22B, and tension roller 23 correspond to a conveyor.

  The recording head array 12 records an image on the recording paper P conveyed by the conveying means in accordance with a clock signal indicating the timing for recording the image. Further, the roller 22A has a circumferential length (110 mm in this embodiment) that is twice the width of the recording head 14, and the length of the conveyor belt 24 is such that three A4 lateral feeds can be made in this embodiment. (660mm).

  A charging roll 26 is provided at a position facing the roller 22B, and a predetermined voltage is applied to the charging roll 26. The recording paper P transported by the plurality of roller pairs 20 is charged by being sandwiched between the charging roll 26 and the transport belt 24, is adsorbed on the surface of the transport belt 24, and is driven by the rotation of the transport belt 24. It passes through an image recording area facing the recording head array 12.

  When the recording paper P passes through the image recording area, the printer 10 discharges droplets from the nozzles of the recording heads 14 based on the image data, thereby recording a full color image on the recording paper P.

  A plurality of discharge rollers 30 are provided on the downstream side of the conveyance belt 24 in the conveyance direction of the recording paper P. The recording sheet P on which an image is recorded by the recording head array 12 is conveyed by the plurality of discharge rollers 30 and discharged to the discharge tray 32.

  The printer 10 also has a reverse path 33 for double-sided recording. The reversing path 33 is composed of a plurality of roller pairs 35, and the recording paper P on which an image is recorded on one side by the recording head array 12 is reversed by the reversing path 33 and conveyed to the image recording area again. As a result, it is possible to record images on both sides of the recording paper P.

  Furthermore, the printer 10 according to the present embodiment is provided with a sensor for detecting whether a defect such as a streak or unevenness has occurred in a recorded image.

  Therefore, the printer 10 includes a line image sensor (hereinafter referred to as “line sensor”) 42, an encoder sensor 44, and a Drive Roll Home sensor (hereinafter referred to as “home sensor”) 46 as the sensors. Details of these will be described later.

  The control unit 40 of the printer 10 controls the overall operation of the printer 10. In particular, in the present embodiment, the line sensor 42 generates an image based on information obtained from the encoder sensor 44 and the home sensor 46. A predetermined period, which is a reading period, and a recording timing of the recording head array 12 are controlled.

  Details of the line sensor 42, the encoder sensor 44, and the home sensor 46 will be described with reference to FIG. In the figure, the line sensor 42, the encoder sensor 44, the home sensor 46, the recording head array 12, and the conveying means are shown in the configuration shown in FIG.

  Among these, the line sensor 42 detects the image recorded on the recording paper P, when the recording paper P on which the image is recorded by the recording head array 12 is being transported by the transporting means, It is an image acquisition means for acquiring an image obtained by reading only.

  Further, the line sensor 42 acquires an image by reflected light by light irradiated on the recording paper P by irradiation means (not shown). This irradiating means starts irradiating the recording paper P when the line sensor 42 starts reading an image, and ends the irradiation when the line sensor 42 finishes reading the image.

  The home sensor 46 is a sensor that detects that the roller 22A has made one revolution and outputs it to the control unit 40 as a home signal.

  The encoder sensor 44 is provided with a printing timing mark coaxially with the roller 22 </ b> A as shown in the figure, reads this printing timing mark, and outputs an encoder signal to the control unit 40. The print timing mark is marked at 600 lpi, which is the same as the print resolution, and the encoder signal from the encoder sensor 44 is output as a 600 dpi clock.

  Based on the encoder signal and home signal described above, the control unit 40 outputs a clock signal indicating the timing for recording an image to the recording head array 12 and generates a period control signal indicating a period during which the line sensor 42 reads the image. The ASIC provided in FIG. 3 will be described with reference to FIG.

  In the figure, an encoder signal and a home signal input to the ASIC 50, a clock signal output from the ASIC 50, and a period control signal are shown. The ASIC 50 is equipped with a flash memory, and a table to be described later is stored in the flash memory.

  The ASIC 50 controls the timing at which the recording head array 12 forms an image by outputting a clock signal to the recording head array 12 based on the conveying speed at which the conveying means conveys the recording paper P, and at a predetermined period, A period control signal indicating the length of a predetermined period is generated based on the clock signal so that the distance traveled by P being transported by the transport means becomes a constant distance, and this period control signal is sent to the line sensor 42. By outputting as a light source lighting control signal, the length of a predetermined period during which the line sensor 42 reads an image is controlled.

  Each of the above signals will be specifically described with reference to the timing chart of FIG. FIG. 4 shows a home signal, an encoder signal, a clock signal, and a period control signal. First, the home signal rises, and correction processing described later is performed from the encoder signal that has risen for the first time in the rising state, and for example, a clock signal subjected to correction processing such as T1 to T1 'is output. Based on this clock signal, the period control signal rises. The period in which the period control signal is ON is a period corresponding to the number of clocks determined according to the conveyance speed, as will be described later.

  Specifically, a method for generating a clock signal from an encoder signal will be described. Normally, printing is performed using an encoder signal as a print clock. However, the roller 22A usually includes an eccentric error due to a manufacturing error, an assembly error, or the like. For this reason, the speed of the conveying belt 24 varies depending on the eccentricity of the roller 22A. However, since the frequency of the encoder signal from the encoder sensor 44 is not affected by the eccentricity of the roller 22A, the frequency fluctuation due to the eccentricity does not occur.

  Accordingly, the frequency of the encoder signal deviates from the actual conveyance speed of the conveyance belt 24. When the recording head array 12 is driven using the encoder signal as it is, the recording position varies due to this shift, and the image is deteriorated. In order to correct this deviation, the ASIC 50 corrects the encoder signal.

  Here, two correction methods are listed. The first is a method in which the surface speed measuring means measures the surface speed of the conveyor belt, obtains a difference from the encoder signal, and feeds back the difference to the encoder signal. The second is a method in which test printing is performed, the amount of deviation of the landing position is measured, and the result is fed back to the printing clock. Also in the second case, since the deviation amount of the landing position can be converted into a speed, the ASIC 50 outputs a clock signal based on the transport speed in any method.

  In the first method using the surface velocity measuring means described above, a Doppler surface measuring device is used. First, the conveyance speed is adjusted by synchronizing the encoder signal and the output of the Doppler surface measuring device with the home reference detected by the home sensor 46. measure.

  Then, a low-pass filter is applied to extract the eccentricity error from each measured data. In this embodiment, since the printing frequency is 16 KHz and the circumferential length of the roller 22A is 110 mm, the eccentric period is about 6 Hz. For this reason, when the sampling period is 200 μsec, a noise component of 25 Hz or more can be removed by performing 200-segment moving average.

  The difference between the data from which the noise component has been removed in this way becomes the data for the eccentricity error. The eccentricity error data is fed back to the encoder signal, and a clock signal indicating the timing for recording an image is output to the recording head array 12.

  Next, a method for generating the period control signal will be described. Here, it demonstrates concretely using the following number.

  First, the resolution recorded by the recording head array 12 is set to 600 dpi, and the normal recording timing (frequency of the clock signal) is set to 16 KHz. In this case, the recording timing interval is 0.0625 msec, and the conveyance speed is 677 mm / sec. Further, the resolution read by the line sensor 42 is 600 spi, and a predetermined period of time for reading an image is 1 msec per line.

  Accordingly, the relationship between the read timing by the line sensor 42 and the clock signal is such that the frequency divided by 16 of the clock signal becomes the read cycle of the line sensor 42. Therefore, normally, the line sensor 42 can read once every 16 clocks.

  However, since the clock signal is obtained by correcting the encoder signal due to the speed fluctuation of the conveyor belt as described above, when reading is performed once every 16 clocks, the reading may be performed with a period of less than 1 msec. There is.

  On the other hand, when the predetermined period is fixed to 1 msec without using the clock signal, the moving distance of the recording paper P varies, which affects the image reading range.

  Therefore, first, the maximum speed of the conveyor belt 24 is obtained, the speed fluctuation range is calculated, and a predetermined period for reading by the line sensor 42 in anticipation of the speed fluctuation is set, so that the reading speed fluctuation is within the range. In addition, it achieves reading with minimal reduction in reading speed. By setting the predetermined time in this way, an image is acquired only during a period corresponding to the moving distance, so that the image formed on the recording medium can be accurately detected even if the moving distance varies.

  When the frequency of the clock signal corresponding to the maximum speed of the conveyor belt 24 is 16650 Hz, the variation rate is 16650/16000 = 4%. In the case of ± 4%, the conveying speed of the conveying belt 24 varies in the range of 650 to 704 mm / sec.

  Based on the case where the conveying speed of the conveying belt 24 operates at the maximum speed of 704 mm / sec, the number of clocks that can be secured with a predetermined period of reading by the line sensor 42 as 1 msec is calculated, and the number of clocks is set as the predetermined period.

  Since the minimum clock time is 1/16650 = 60.1 μsec in this example, as shown in the timing chart of FIG. 5, the period during which the period control signal is turned on is 16 for 1 msec / 60.1 μs. Since it is 7 clocks, it is 17 clocks.

  As described above, by reading what was read at 16 clock cycles to 17 clock cycles (1.06 msec), a predetermined period of 1 msec or more can always be secured even if the speed of the conveyor belt 24 fluctuates. And the reading range can be made uniform.

  The relationship between the clock signal and the encoder signal described above may be stored in advance in the flash memory as a correction data table as shown in FIG. The correction data table shown in the figure shows a clock number and a correction time corresponding to the clock number. The ASIC 50 uses the correction data table to perform the following process from the encoder signal immediately after the home signal is input.

  First, the period of the detected encoder signal is measured. Then, the correction times in the correction data table are read in order from clock No. 1, and the correction time is added to the measured cycle. Thereafter, the process of reading and adding the clock No. is incremented by 1 is repeated. The clock signal obtained by addition may be output, for example, 100 μsec after the home signal is turned on.

  By doing so, the ASIC 50 outputs a clock signal and a period control signal.

  In the example described above, the period control signal is generated based on the clock signal. However, as shown in FIG. 7, a multiplied clock signal obtained by multiplying the clock signal is generated, and the period control signal is generated using the multiplied clock signal. It is good also as a signal for doing. By doing so, the predetermined period can be controlled more strictly.

  For example, when the conveyance speed fluctuation is ± 3%, the conveyance speed fluctuates in the range of 657 to 697 mm / sec. The required number of clocks at that time is 1 msec / 60.7 μs = 16.4 clocks since the shortest clock time is 25.4 / 600/697 = 60.7 μsec. When the double clock signal is used, the predetermined period is 33 clock cycles, so that the loss of reading time can be reduced, and the reading range during high-speed conveyance can be shortened compared with the case where no multiplication is performed. Can do.

  Next, a description will be given of a process for generating a period control signal so that the predetermined period is a period in which the amount of light by the irradiation is equal to or greater than the predetermined amount after the irradiation unit described above starts irradiation.

  If the amount of light reflected from the reading surface is low overall (for example, the paper is not white or the brightness around the detection unit is bright), the light amount of the light source may be increased to increase the reading sensitivity. . However, if the amount of light cannot be increased, the amount of received light can be increased by adjusting the lighting time. However, even when adjusting the lighting time, it is necessary to control the reading range to be uniform.

  In this case, it is necessary to obtain an appropriate lighting time first. As an appropriate measuring method of the lighting time, for example, in the recording paper P, the roller 22A is used in a portion where the image is not recorded (a portion without printing) and a portion where the recording is performed as if it is solidly printed (a portion with printing). The output time from the image acquisition means is measured by changing the lighting time each time the lens rotates once. The amount of light irradiated by the lighting time that maximizes the difference in output from the image acquisition means between the non-printing portion and the printing portion is set as a predetermined light amount, and a period longer than that is set as the optimal lighting time.

  The optimum lighting time when the optimum lighting time is 1.6 msec and the fluctuation of the conveyance speed is ± 4% (the maximum speed is 704 mm / sec (677 mm / s + 4%)) is obtained as follows.

  When the lighting time is 1.6 msec, the moving distance is from 1.6 msec × 704 mm / s to 1.1264 mm, which is 26.6 clocks. Therefore, as shown in FIG. 8, the period control signal may be turned on for 26.6 clocks from Tn ′.

  Specifically, correction values for 27 clocks from the clock No corresponding to the reading position are read from the correction data table immediately before the reading position.

  At this time, the lighting time is set to nominal printing clock (62.5 μs) × 26 + correction value (for 26 clocks) + (nominal printing clock + 27th clock correction value) * 0.6.

  The lighting time is calculated according to this equation, and the time calculated using the timer is controlled to be a predetermined period.

  As described above, in the present embodiment, the read range can be made constant even if the movement distance varies. A method for detecting a color registration shift (hereinafter referred to as “registration shift”) using this will be described with reference to FIG. When the conveyance speed is 677 mm / sec ± 4%, the line sensor 42 is controlled by 17 clocks as described above. A detection pattern having the 17 clocks and an equal width in the transport direction is created to detect a registration error. The width corresponding to 17 clocks is a movement distance that is a distance that the recording paper P moves by being conveyed by the conveying means within a predetermined period.

  FIGS. 9A and 9B show rectangular images (hereinafter referred to as “blocks”) in which the width of 17 clocks is defined as the width in the transport direction and the length in the direction intersecting the transport direction is a predetermined length. Multiple). In this block group, as shown in the figure, the blocks do not overlap each other at a predetermined time interval (one clock in the figure) with reference to the clock signal, and the block color is different (Cyan (cyan), Bk (black)). In this case, Cyan and Bk are used, but it goes without saying that other colors may be used.

  Since each block is recorded with a shift of one clock, it is shifted by one dot as shown in FIG. Furthermore, as in the blocks indicated by dotted lines in FIGS. 4A and 4B, Bk (Cyan) different from Cyan (Bk), and only for a period of 17 clocks (first period), In addition, it includes a block recorded at Bk (Cyan) that is 17 dots (moving distance) away from one block of the block group recorded at Cyan (Bk). In the following description, a block indicated by a dotted line is expressed as a reference block.

  The Cyan color block group shown in FIG. 5A is referred to as Cyan pattern 1, and the Bk color block group is referred to as Bk pattern 1. Similarly, the Cyan color block group shown in FIG. 5B is referred to as Cyan pattern 2, and the Bk color block group is referred to as Bk pattern 2.

  First, an example of detecting a Cyan shift amount with reference to the Bk pattern 1 will be described with reference to FIG. In this detection pattern, as shown in the figure, Cyan pattern 1 has 11 blocks and is shifted by one dot. Further, the reference block of the Cyan pattern 1 is 17 dots away from the reference block of the Bk pattern 1.

  The Bk pattern 1 image is read by the line sensor 42, and the Cyan pattern 1 image is read by the line sensor 42 after an interval of 17 clocks after reading.

  Then, among the read images of Cyan pattern 1, the position on the recording paper P of the block recorded in the Cyan (Cyan having the highest reading density) in which the widest range is read, and the position on the recording paper P of Bk pattern 1 Is detected to detect a deviation between the position where the Bk color image is formed and the position where the Cyan color image is formed.

  Next, FIG. 1B will be described. In FIG. 6B, the reading timing of the reference Bk pattern image is slightly shifted due to fluctuations in the conveyance speed and expansion / contraction of the paper in FIG. This is a pattern to deal with the case. As shown in FIG. 5B, the Bk pattern 2 is shifted by 1 dot in the same manner as the Cyan pattern 1. Also in this case, in the Cyan pattern 2, the reference block is 17 dots away from the reference block of the Bk pattern 2. The Cyan pattern 2 has 21 blocks, and the blocks are formed by shifting the reference block in the vertical direction by one dot at a time.

  The line sensor 42 reads the Bk pattern 2 image with reference to the reference block of the Bk pattern 2 and reads the Cyan pattern 2 image after an interval of 17 clocks after reading. Then, in the read image, the position on the recording paper P of the block of Bk (Bk having the highest reading density) in which the widest range has been read and the Cyan (Cyan having the highest reading density) in which the widest range has been read. The color registration misalignment is corrected by calculating the discrete distance between Bk and Cyan colors by comparing the position of the block) on the recording paper P and correcting the printing timing of the Cyan color based on the calculation result. To do.

  It should be noted that the number of blocks of the Cyan pattern 2 in FIG. By doing in this way, the detection method in the same figure (A) mentioned above is applicable, without adjustment of detection timing.

  In the Bk pattern and the Cyan pattern shown in the figure, it is possible to detect a deviation of the design value ± 5 dots. If there is a possibility that a further shift will occur, it can be dealt with by increasing the number of blocks to a number corresponding thereto. In this example, since the conveyance speed fluctuation is ± 4%, the detection pattern width and the reference block interval are 17 dots. However, when the conveyance speed fluctuation is different, the detection pattern width is adjusted accordingly. Further, it is possible to cope with this by making the reference block interval the same as the clock number of the period control signal of the reading means.

It is a figure which shows the inkjet printer which concerns on this Embodiment. It is a figure which shows the detail of a line sensor, an encoder sensor, and a home sensor. It is a figure which shows the signal which ASIC and the signal which ASIC outputs, and the signal input. It is a timing chart which shows a home signal, an encoder signal, a clock signal, and a period control signal. It is a timing chart which shows the corrected period control signal. It is a figure which shows a correction data table. It is a timing chart which shows the period control signal produced | generated with the multiplication clock signal. It is a timing chart which shows the period control signal for making it more than predetermined light quantity. It is a figure which shows the detection pattern in the case of detecting a registration gap.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Printer 12 Recording head array 14, 14C Recording head 16, 16C Ink cartridge 22A, 22B Roller 23 Tension roller 24 Conveying belt 40 Control part 42 Line sensor 44 Encoder sensor 46 Home sensor

Claims (3)

A conveying unit that includes a roller and an endless conveying belt wound around the roller, and conveys a recording medium on which an image is formed by the conveying belt that is driven to rotate according to the rotation of the roller;
Image forming means for forming the image on the recording medium transported by the transport means in response to a clock signal indicating a timing for forming the image;
In order to detect an image formed on the recording medium, an image obtained by reading the image for a predetermined period when the recording medium on which the image is formed by the image forming unit is conveyed by the conveying unit. Image acquisition means for acquiring;
The timing at which the image forming unit forms an image is controlled by outputting the clock signal to the image forming unit based on the conveyance speed at which the conveyance belt conveys the recording medium, and the recording is performed in the predetermined period. The maximum speed of the transport speed is determined so that the distance traveled by the medium being transported by the transport means is a constant distance, the speed fluctuation range is determined from the frequency of the clock signal corresponding to the maximum speed, Control means for controlling the length of the predetermined period by calculating the number of clocks of a signal obtained by multiplying the clock signal based on a speed fluctuation range;
An image forming apparatus. The image acquisition means acquires the image by reflected light from the light irradiated on the recording medium,
When the image acquisition unit starts reading the image, the image acquisition unit further includes irradiation unit that starts irradiation of the light to the recording medium, and ends the irradiation when the image acquisition unit finishes reading the image. And
The image forming apparatus according to claim 1, wherein the control unit controls the length of the predetermined period so that a light amount by the irradiation is equal to or greater than a predetermined light amount after the irradiation unit starts the irradiation. . The image forming means sets a moving distance, which is a distance moved by the recording medium being conveyed by the conveying means within the predetermined period, as a length in the conveying direction, and a length in a direction intersecting the conveying direction as a predetermined length. A rectangular image group in which the rectangular images formed on the recording medium do not overlap each other at a predetermined time interval based on the multiplied clock signal is a first color in a first color. The second color different from the first color, and a rectangular image separated from one rectangular image of the rectangular image group formed by the first color by the moving distance. Forming a rectangular image group,
Among the rectangular images read by the image acquisition means, the position of the rectangular image formed with the first color having the highest reading density on the recording medium and the second color having the highest reading density. By comparing the position of the formed rectangular image on the recording medium, a shift between the position where the first color image is formed and the position where the second color image is formed is detected. the image forming apparatus according to claim 1 or claim 2 further comprising a detection means for.

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