JP4591765B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus, and in particular, an image for correcting density unevenness (streaks unevenness) caused by variations such as ejection amount errors and landing position errors of recording elements (nozzles) of a line head that covers the entire recording paper width. The present invention relates to a forming apparatus.

  2. Description of the Related Art Conventionally, as an image forming apparatus, for example, an ink discharge apparatus having an inkjet head (recording head) in which a large number of nozzles (recording elements) are arranged is provided, and recording is performed while relatively moving the inkjet head and a recording medium. 2. Related Art There is known an ink jet recording apparatus (ink jet printer) that forms an image on a recording medium by ejecting ink from a nozzle toward the medium.

  In such an ink jet recording apparatus, in particular, an image can be recorded on the entire surface of the recording sheet only by performing a single operation (single pass) that covers the entire width of the recording sheet and relatively moves the recording sheet and the head. In the case of a full line head, there has been a problem that streaks in the conveyance direction of the recording paper are likely to occur due to nozzle variations.

  For example, as shown in FIG. 11A, with respect to a line type recording head 900 in which nozzles 902 are arranged in a line, recording paper 904 is placed in a direction substantially perpendicular to the nozzle arrangement direction (direction indicated by an arrow in the figure). It is assumed that ink droplets 906 are ejected from each nozzle 902 while transporting, and ink dots 908 are formed on the recording paper 904 to record an image.

  At this time, if ink droplets 906a ejected from the nozzles 902 have ink droplets 906a whose landing positions are shifted, ink droplets 906b whose ejection amount is larger than usual, etc., the white streak 910 or the density of the recording paper 904 becomes dark. As a result, dark stripes 912 and the like are formed, and density unevenness occurs. As a result, as shown in FIG. 11B, it is visually recognized as streak unevenness on the recording paper 904, which causes a problem in image quality.

  Therefore, conventionally, various techniques for correcting such density unevenness are known.

  For example, the actual density unevenness and color unevenness change depending on the use environment of the recording head, changes with time, etc., so that correction data for correcting image data is changed according to the error characteristics of each nozzle. It is known (see, for example, Patent Document 1).

Also, for example, a test pattern for detecting density unevenness is recorded on a recording medium, density unevenness of the recorded test pattern is read, density characteristics of each nozzle are calculated, and density is determined for each nozzle based on this. It is known to correct density unevenness due to errors in the ejection amount and ejection direction of each nozzle by correcting the signal (gamma correction) (see, for example, Patent Document 2).
JP-A-2-212166 JP-A-5-69545

  However, the error characteristics of the nozzles that cause density unevenness vary depending on, for example, a slight difference in the position of the meniscus surface, or a slight wetting of the nozzle plate or a nozzle state such as dust. Such a nozzle state is changed by performing head cleaning such as so-called suction or wiping processing, and as a result, the state of density unevenness is also changed.

  Accordingly, if the density unevenness correction data is changed in accordance with the density unevenness state, it is necessary to remeasure the nozzle error characteristic data every time the head cleaning is performed. However, since the head cleaning needs to be frequently performed in some cases, there is a problem that it is not practical to remeasure the error characteristic data every time the head cleaning is performed.

  In addition, according to experiments, the error characteristics of the nozzles are a combination of “reproducible errors” caused by manufacturing errors and “non-reproducible errors” that depend on the nozzle state that changes with each cleaning. I know. Therefore, it is desirable to correct the density unevenness by appropriately considering these two factors, but none of those described in the above-mentioned patent documents consider these factors, and the density unevenness is highly accurate. There is a problem that it cannot be corrected.

  The present invention has been made in view of such circumstances, and an object thereof is to provide an image forming apparatus capable of correcting density unevenness with high accuracy using effective correction data.

In order to achieve the object, the invention described in claim 1 includes a recording head in which a plurality of recording elements are arranged, a moving means for moving a recording medium relative to the recording head, and the recording head. In order to record from the recording medium to the recording medium, a driving means for driving the recording element, a cleaning means for cleaning the recording element, and a density distribution of the recording medium recorded immediately after cleaning the recording element are measured. Measurement means for performing measurement a plurality of times, characteristic data calculation means for calculating characteristic data of the recording element from density distribution data obtained by the measurement, and averaging processing of the plurality of characteristic data An image shape comprising: an averaging processing means for correcting the driving signal of the driving means based on a result of the averaging processing To provide a device.

As a result, it is possible to isolate the fluctuation factors of the characteristics of the recording element and to extract the fixed component, and to perform correction using correction data that is effective within a certain period of time. Correction is possible. In addition, it is possible to cancel a factor that randomly changes the characteristics of the recording element (error characteristics having no reproducibility), and high-precision correction is possible. Further, the non-reproducible error characteristic depending on the state of the liquid discharge port that changes every time the liquid discharge port is cleaned can be canceled, and high-accuracy correction is possible.

According to a second aspect of the present invention, when the averaging process is performed, the averaging processing unit sets a predetermined threshold and excludes the characteristic data exceeding the threshold to perform the averaging process. It is characterized by.

  Thus, by performing the averaging process by removing the abnormal value, appropriate correction data can be obtained, and appropriate correction can be performed.

According to a third aspect of the present invention, the recording element is a liquid discharge port that discharges a liquid, and the characteristic data includes a landing position or a discharge liquid of a liquid discharged from the liquid discharge port to the recording medium. Including quantity.

  This is particularly effective for correcting density unevenness in an ink jet recording apparatus having variations in recording elements.

According to a fourth aspect of the present invention, the recording head is an LED head.

According to a fifth aspect of the present invention, the recording head is a thermal head.

  As described above, according to the image forming apparatus of the present invention, it is possible to isolate the variation factors of the characteristics of the recording element, to extract the fixed component, and to perform the correction using the correction data effective within a certain period. Can suppress the occurrence of reverse correction, etc., enabling high-precision correction

  Hereinafter, an image forming apparatus according to the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is an overall configuration diagram showing an outline of an embodiment of an ink jet recording apparatus as an image forming apparatus according to the present invention.

  As shown in FIG. 1, the inkjet recording apparatus 10 includes a printing unit 12 having a plurality of printing heads (liquid ejection heads) 12K, 12C, 12M, and 12Y provided for each ink color, and each printing head 12K, 12C, 12M, and 12Y, an ink storage / loading unit 14 that stores ink to be supplied, a paper feeding unit 18 that supplies recording paper 16, a decurling unit 20 that removes curling of the recording paper 16, and the printing A belt conveyance unit 22 that is arranged to face the nozzle surface (ink ejection surface) of the unit 12 and conveys the recording paper 16 while maintaining the flatness of the recording paper 16, and a print detection unit that reads a printing result by the printing unit 12 24 and a paper discharge unit 26 that discharges printed recording paper (printed matter) to the outside.

  In FIG. 1, a magazine for rolled paper (continuous paper) is shown as an example of the paper supply unit 18, but a plurality of magazines having different paper widths, paper quality, and the like may be provided side by side. Further, instead of the roll paper magazine or in combination therewith, the paper may be supplied by a cassette in which cut papers are stacked and loaded.

  In the case of an apparatus configuration using roll paper, a cutter 28 is provided as shown in FIG. 1, and the roll paper is cut into a desired size by the cutter 28. The cutter 28 includes a fixed blade 28A having a length equal to or greater than the conveyance path width of the recording paper 16, and a round blade 28B that moves along the fixed blade 28A. The fixed blade 28A is provided on the back side of the print. The round blade 28B is arranged on the print surface side with the conveyance path interposed therebetween. Note that the cutter 28 is not necessary when cut paper is used.

  When multiple types of recording paper are used, an information recording body such as a barcode or wireless tag that records paper type information is attached to the magazine, and the information on the information recording body is read by a predetermined reader. Therefore, it is preferable to automatically determine the type of paper to be used and perform ink ejection control so as to realize appropriate ink ejection according to the type of paper.

  The recording paper 16 delivered from the paper supply unit 18 retains curl due to having been loaded in the magazine. In order to remove this curl, heat is applied to the recording paper 16 by the heating drum 30 in the direction opposite to the curl direction of the magazine in the decurling unit 20. At this time, it is more preferable to control the heating temperature so that the printed surface is slightly curled outward.

  After the decurling process, the cut recording paper 16 is sent to the belt conveyance unit 22. The belt conveyance unit 22 has a structure in which an endless belt 33 is wound between rollers 31 and 32, and at least portions facing the nozzle surface of the printing unit 12 and the sensor surface of the printing detection unit 24 are flat (flat). Surface).

  The belt 33 has a width that is wider than the width of the recording paper 16, and a plurality of suction holes (not shown) are formed on the belt surface. As shown in FIG. 1, an adsorption chamber 34 is provided at a position facing the nozzle surface of the print unit 12 and the sensor surface of the print detection unit 24 inside the belt 33 spanned between the rollers 31 and 32. Then, the suction chamber 34 is sucked by the fan 35 to be a negative pressure, whereby the recording paper 16 on the belt 33 is sucked and held.

  The power of a motor (not shown) is transmitted to at least one of the rollers 31 and 32 around which the belt 33 is wound, so that the belt 33 is driven in the clockwise direction in FIG. The recording paper 16 is conveyed from left to right in FIG.

  Since ink adheres to the belt 33 when a borderless print or the like is printed, the belt cleaning unit 36 is provided at a predetermined position outside the belt 33 (an appropriate position other than the print area). Although details of the configuration of the belt cleaning unit 36 are not shown, for example, there are a method of niping a brush roll, a water absorbing roll, etc., an air blowing method of spraying clean air, or a combination thereof. In the case where the cleaning roll is nipped, the cleaning effect is great if the belt linear velocity and the roller linear velocity are changed.

  An embodiment using a roller / nip transport mechanism instead of the belt transport unit 22 is also conceivable. However, when the roller / nip transport is performed in the print area, the roller comes into contact with the print surface of the paper immediately after printing, so that the image is likely to bleed. There is a problem. Therefore, as in this example, suction belt conveyance that does not contact the image surface in the printing region is preferable.

  A heating fan 40 is provided on the upstream side of the printing unit 12 on the paper conveyance path formed by the belt conveyance unit 22. The heating fan 40 heats the recording paper 16 by blowing heated air onto the recording paper 16 before printing. Heating the recording paper 16 immediately before printing makes it easier for the ink to dry after landing.

  FIG. 2 is a main part plan view showing the periphery of the printing unit 12 of the inkjet recording apparatus 10.

  As shown in FIG. 2, the printing unit 12 is a so-called full-line type in which a line-type head having a length corresponding to the maximum paper width is arranged in a direction (main scanning direction) perpendicular to the paper transport direction (sub-scanning direction). It has become the head of.

  Each of the print heads 12K, 12C, 12M, and 12Y is a full-line type head in which a plurality of ink discharge ports (nozzles) are arranged over a length that exceeds at least one side of the maximum size recording paper 16 targeted by the inkjet recording apparatus 10. It consists of

  Printing corresponding to each color ink in the order of black (K), cyan (C), magenta (M), and yellow (Y) from the upstream side (left side in FIG. 1) along the conveyance direction (paper conveyance direction) of the recording paper 16 Heads 12K, 12C, 12M, and 12Y are arranged. A color image can be formed on the recording paper 16 by discharging the color inks from the print heads 12K, 12C, 12M, and 12Y while the recording paper 16 is conveyed.

  Thus, according to the printing unit 12 in which the full line head that covers the entire width of the paper is provided for each ink color, the recording paper 16 and the printing unit 12 are relatively moved in the paper transport direction (sub-scanning direction). An image can be recorded on the entire surface of the recording paper 16 by a single pass in which the operation is performed only once (that is, by one sub-scan). Accordingly, high-speed printing is possible as compared with a shuttle type head in which the print head reciprocates in a direction (main scanning direction) orthogonal to the paper transport direction, and productivity can be improved.

  Here, the main scanning direction and the sub-scanning direction are used in the following meaning. That is, when driving the nozzles with a full line head having a nozzle row corresponding to the full width of the recording paper, (1) whether all the nozzles are driven simultaneously or (2) whether the nozzles are driven sequentially from one side to the other (3) The nozzles are divided into blocks, and each nozzle is driven sequentially from one side to the other for each block, and the width direction of the paper (perpendicular to the conveyance direction of the recording paper) Nozzle driving that prints one line (a line made up of a single row of dots or a line made up of a plurality of rows of dots) in the direction of scanning is defined as main scanning. A direction indicated by one line (longitudinal direction of the belt-like region) recorded by the main scanning is called a main scanning direction.

  On the other hand, by relatively moving the above-described full line head and the recording paper, printing of one line (a line formed by one line of dots or a line composed of a plurality of lines) formed by the above-described main scanning is repeatedly performed. Is defined as sub-scanning. A direction in which sub-scanning is performed is referred to as a sub-scanning direction. After all, the conveyance direction of the recording paper is the sub-scanning direction, and the direction orthogonal to it is the main scanning direction.

  Further, in this example, the configuration of KCMY standard colors (four colors) is illustrated, but the combination of ink colors and the number of colors is not limited to this embodiment, and light ink and dark ink are added as necessary. May be. For example, it is possible to add a print head that discharges light ink such as light cyan and light magenta.

  As shown in FIG. 1, the ink storage / loading unit 14 has tanks that store inks of colors corresponding to the print heads 12K, 12C, 12M, and 12Y, and each tank has a pipeline that is not shown. The print heads 12K, 12C, 12M, and 12Y communicate with each other. Further, the ink storage / loading unit 14 includes notifying means (display means, warning sound generating means, etc.) for notifying when the ink remaining amount is low, and has a mechanism for preventing erroneous loading between colors. is doing.

  The print detection unit 24 includes an image sensor (line sensor or the like) for imaging the droplet ejection result of the print unit 12, and means for checking nozzle clogging and other ejection defects from the droplet ejection image read by the image sensor. Function as. The print detection unit 24 is also used to measure a test sample and detect its density distribution in order to acquire effective correction data for correcting density unevenness described later.

  The print detection unit 24 of this example is composed of a line sensor having a light receiving element array that is wider than at least the ink ejection width (image recording width) by the print heads 12K, 12C, 12M, and 12Y. The line sensor includes an R sensor row in which photoelectric conversion elements (pixels) provided with red (R) color filters are arranged in a line, a G sensor row provided with green (G) color filters, The color separation line CCD sensor includes a B sensor array provided with a blue (B) color filter. Instead of the line sensor, an area sensor in which the light receiving elements are two-dimensionally arranged can be used.

  The print detection unit 24 reads the test patterns printed by the print heads 12K, 12C, 12M, and 12Y for each color, and detects the ejection of each head. The ejection determination includes the presence / absence of ejection, measurement of dot size, measurement of dot landing position, and the like.

  A post-drying unit 42 is provided following the print detection unit 24. The post-drying unit 42 is means for drying the printed image surface, and for example, a heating fan is used. Since it is preferable to avoid contact with the printing surface until the ink after printing is dried, a method of blowing hot air is preferred.

  When printing on porous paper with dye-based ink, the weather resistance of the image is improved by preventing contact with ozone or other things that cause dye molecules to break by blocking the paper holes by pressurization. There is an effect to.

  A heating / pressurizing unit 44 is provided following the post-drying unit 42. The heating / pressurizing unit 44 is a means for controlling the glossiness of the image surface, and pressurizes with a pressure roller 45 having a predetermined uneven surface shape while heating the image surface to transfer the uneven shape to the image surface. To do.

  The printed matter generated in this manner is outputted from the paper output unit 26. It is preferable that the original image to be printed (printed target image) and the test print are discharged separately. The ink jet recording apparatus 10 is provided with a selecting means (not shown) for switching the paper discharge path in order to select the printed matter of the main image and the printed matter of the test print and send them to the respective discharge portions 26A and 26B. ing. Note that when the main image and the test print are simultaneously formed in parallel on a large sheet, the test print portion is separated by a cutter (second cutter) 48. The cutter 48 is provided immediately before the paper discharge unit 26, and cuts the main image and the test print unit when the test print is performed on the image margin. The structure of the cutter 48 is the same as that of the first cutter 28 described above, and includes a fixed blade 48A and a round blade 48B.

  Although not shown, the paper output unit 26A for the target prints is provided with a sorter for collecting prints according to print orders.

  Next, the arrangement of the nozzles (liquid ejection ports) of the print head (liquid ejection head) will be described. Since the structures of the print heads 12K, 12C, 12M, and 12Y provided for each ink color are common, the print head is represented by the reference numeral 50 in the following, and the print head 50 is shown in FIG. The plane perspective view of is shown.

  As shown in FIG. 3, the print head 50 of this embodiment includes a nozzle 51 that ejects ink as droplets, a pressure chamber 52 that applies pressure to ink when ejecting ink, and a common flow that is not shown in FIG. The pressure chamber units 54 each including an ink supply port 53 for supplying ink from the passage to the pressure chamber 52 are arranged in a staggered two-dimensional matrix so as to increase the density of the nozzles 51.

  In the example shown in FIG. 3, when each pressure chamber 52 is viewed from above, the planar shape thereof is substantially square, but the planar shape of the pressure chamber 52 is not limited to such a square. Absent. As shown in FIG. 3, the pressure chamber 52 is provided with a nozzle 51 at one end of the diagonal and an ink supply port 53 at the other end.

  FIG. 4 is a perspective plan view showing another structural example of the print head. As shown in FIG. 4, a plurality of short heads 50 'are arranged and connected in a two-dimensional staggered pattern so that the entire length of the plurality of short heads 50' corresponds to the entire width of the print medium. One long full line head may be configured.

  FIG. 5 is a cross-sectional view taken along line 5-5 in FIG.

  As shown in FIG. 5, the pressure chamber unit 54 is formed by a pressure chamber 52 that communicates with a nozzle 51 that ejects ink. The pressure chamber 52 includes a common liquid chamber 55 that supplies ink through a supply port 53. While communicating, one surface (the top surface in the figure) of the pressure chamber 52 is constituted by a diaphragm 56, and a piezoelectric element 58 for applying pressure to the diaphragm 56 to deform the diaphragm 56 is joined to the upper portion thereof. An individual electrode 57 is formed on the upper surface of the piezoelectric element 58. The diaphragm 56 also serves as a common electrode.

  The piezoelectric element 58 is sandwiched between a common electrode (diaphragm 56) and an individual electrode 57, and is deformed by applying a driving voltage to these two electrodes 56 and 57. The diaphragm 56 is pushed by the deformation of the piezoelectric element 58, the volume of the pressure chamber 52 is reduced, and ink is ejected from the nozzle 51. When the voltage application between the two electrodes 56 and 57 is released, the piezoelectric element 58 returns to its original state, the volume of the pressure chamber 52 is restored to the original size, and passes through the supply port 53 from the common liquid chamber 55. New ink is supplied to the pressure chamber 52.

  FIG. 6 is a schematic diagram showing the configuration of the ink supply system in the inkjet recording apparatus 10. The ink tank 60 is a base tank for supplying ink to the print head 50, and is installed in the ink storage / loading unit 14 described with reference to FIG. There are two types of the ink tank 60: a method of replenishing ink from a replenishing port (not shown) and a cartridge method of replacing the entire tank when the remaining amount of ink is low. When the ink type is changed according to the usage, the cartridge method is suitable. In this case, it is preferable that the ink type information is identified by a barcode or the like, and ejection control is performed according to the ink type. The ink tank 60 in FIG. 6 is equivalent to the ink storage / loading unit 14 in FIG. 1 described above.

  As shown in FIG. 6, a filter 62 is provided in the middle of the conduit connecting the ink tank 60 and the print head 50 in order to remove foreign matter and bubbles. The filter mesh size is preferably equal to or smaller than the nozzle diameter of the print head 50 (generally, about 20 μm).

  Although not shown in FIG. 6, a configuration in which a sub tank is provided in the vicinity of the print head 50 or integrally with the print head 50 is also preferable. The sub-tank has a function of improving a damper effect and refill that prevents fluctuations in the internal pressure of the head.

  Further, the inkjet recording apparatus 10 has a nozzle surface (ink ejection surface) 50A on which a cap 64 as a means for preventing nozzle drying or preventing ink viscosity increase near the nozzle and a nozzle 51 of the print head 50 are formed. And a cleaning blade 66 as a cleaning means.

  A maintenance unit (maintenance unit) including the cap 64 and the cleaning blade 66 can be moved relative to the print head 50 by a moving mechanism (not shown), and can be moved from a predetermined retraction position to a position below the print head 50 as required. Moved to the maintenance position.

  The cap 64 is displaced up and down relatively with respect to the print head 50 by an elevator mechanism (not shown). The lifting mechanism is configured to cover the nozzle region of the nozzle surface 50 </ b> A with the cap 64 by raising the cap 64 to a predetermined raised position when the power is turned off or waiting for printing, and bringing the cap 64 into close contact with the print head 50.

  The cleaning blade 66 is made of an elastic member such as rubber, and can slide on the nozzle surface 50A of the print head 50 by means of a blade moving mechanism (not shown). When ink droplets or foreign matters adhere to the nozzle surface 50A, the cleaning blade 66 is brought into contact with the nozzle surface 50A and is slid to wipe the nozzle surface 50A and clean the nozzle surface 50A.

  During printing or standby, when a specific nozzle 51 is used less frequently and the ink viscosity in the vicinity of the nozzle 51 is increased, preliminary ejection toward the cap 64 is performed to discharge the ink that has deteriorated due to the increased viscosity. Is done.

  In addition, when bubbles are mixed in the ink in the print head 50 (ink in the pressure chamber 52), the cap 64 is applied to the print head 50, and the ink in the pressure chamber 52 (ink in which bubbles are mixed) is applied by the suction pump 67. The ink removed by suction is sent to the collection tank 68. This suction operation is also performed when the initial ink is loaded into the head or when the ink is used after being stopped for a long time, and the deteriorated ink solidified by increasing the viscosity is sucked and removed.

  That is, if the print head 50 does not discharge for a certain period of time, the ink solvent in the vicinity of the nozzles evaporates and the viscosity of the ink in the vicinity of the nozzles increases, and the discharge driving piezoelectric element 58 (see FIG. 5). Ink does not discharge from the nozzle 51 even when the operation is performed. Therefore, before this state is reached (within the viscosity range in which ink can be ejected by the operation of the piezoelectric element 58), the piezoelectric element 58 is operated toward the ink receiver, and the ink in the vicinity of the nozzle whose viscosity has increased is removed. “Preliminary discharge” is performed. Further, after the dirt on the nozzle surface 50A is cleaned by a wiper such as a cleaning blade 66 provided as a cleaning means for the nozzle surface 50A, the foreign matter is prevented from being mixed into the nozzle 51 by this wiper rubbing operation. Also, preliminary discharge is performed. Note that the preliminary discharge may be referred to as “empty discharge”, “purge”, “spitting”, or the like.

  In addition, if bubbles are mixed in the nozzle 51 or the pressure chamber 52 or if the viscosity of the ink in the nozzle 51 exceeds a certain level, ink cannot be ejected by the preliminary ejection. Do.

  That is, when bubbles are mixed in the ink in the nozzle 51 or the pressure chamber 52, or when the ink viscosity in the nozzle 51 rises to a certain level or more, ink is ejected from the nozzle 51 even if the piezoelectric element 58 is operated. become unable. In such a case, an operation in which the cap 67 is applied to the nozzle surface 50 </ b> A of the print head 50 and the ink or the thickened ink in which bubbles in the pressure chamber 52 are mixed is sucked by the pump 67.

  However, since the above suction operation is performed on the entire ink in the pressure chamber 52, the ink consumption is large. Therefore, when the increase in viscosity is small, it is preferable to perform preliminary discharge as much as possible. The cap 64 described with reference to FIG. 6 functions as a suction unit and can also function as a preliminary discharge ink receiver.

  Preferably, the inside of the cap 64 is divided into a plurality of areas corresponding to the nozzle rows by a partition wall, and each of the partitioned areas can be selectively sucked by a selector or the like.

  FIG. 7 is a principal block diagram showing the system configuration of the inkjet recording apparatus 10. The inkjet recording apparatus 10 includes a communication interface 70, a system controller 72, an image memory (memory) 74, a motor driver 76, a heater driver 78, a print control unit 80, an image buffer memory 82, a head driver 84, and the like.

  The communication interface 70 is an interface unit that receives image data sent from the host computer 86. As the communication interface 70, a serial interface such as USB, IEEE 1394, Ethernet, and wireless network, or a parallel interface such as Centronics can be applied. In this part, a buffer memory (not shown) for speeding up communication may be mounted. Image data sent from the host computer 86 is taken into the inkjet recording apparatus 10 via the communication interface 70 and temporarily stored in the image memory 74. The image memory 74 is a storage unit that temporarily stores an image input via the communication interface 70, and data is read and written through the system controller 72. The image memory 74 is not limited to a memory composed of semiconductor elements, and a magnetic medium such as a hard disk may be used.

  The system controller 72 is a control unit that controls each unit such as the communication interface 70, the image memory 74, the motor driver 76, and the heater driver 78. The system controller 72 includes a central processing unit (CPU) and its peripheral circuits, and performs communication control with the host computer 86, read / write control of the image memory 74, and the like, as well as a transport system motor 88 and heater 89. A control signal for controlling is generated. A software program executed by the system controller 72 is stored in the program storage unit 90.

  The motor driver 76 is a driver (drive circuit) that drives the motor 88 in accordance with an instruction from the system controller 72. The heater driver 78 is a driver that drives the heater 89 such as the post-drying unit 42 (see FIG. 1) in accordance with an instruction from the system controller 72.

  The print control unit 80 has a signal processing function for performing various processing and correction processing for generating a print control signal from the image data in the image memory 74 according to the control of the system controller 72, and the generated print A control unit that supplies a control signal (print data) to the head driver 84. Necessary signal processing is performed in the print controller 80, and the ejection amount and ejection timing of the ink droplets of the print head 50 are controlled via the head driver 84 based on the image data.

  The print control unit 80 includes an image buffer memory 82, and image data, parameters, and other data are temporarily stored in the image buffer memory 82 when image data is processed in the print control unit 80. In FIG. 7, the image buffer memory 82 is shown in a mode associated with the print control unit 80, but it can also be used as the image memory 74. Also possible is an aspect in which the print control unit 80 and the system controller 72 are integrated to form a single processor.

  In addition, the print control unit 80 includes a preliminary discharge control unit 94, and the preliminary discharge control unit 94 drives the maintenance unit 96 and the head driver 84 having the cap 64 and the cleaning blade 66 described above to discharge the print control unit 94. Preliminary discharge is performed to prevent defects. At this time, the preliminary ejection control unit 94 refers to the temperature / humidity of the print head 50 detected by the temperature / humidity detection unit 98, and sets the state of each nozzle 51 of the print head 50 stored in the nozzle management memory 92. Based on this, the execution of the preliminary discharge is controlled.

  Further, the print control unit 80 controls the maintenance unit 96 to execute ink suction and cleaning of the print head 50 such as a blade in addition to the preliminary ejection.

  The print control unit 80 includes a nozzle characteristic data calculation unit 100, an averaging processing unit 102, and a correction data acquisition unit 104 for density unevenness correction.

  The nozzle characteristic data calculation unit 100 calculates error characteristic data of each nozzle 51 based on density distribution data read by the print detection unit 24 from a test sample output to the recording paper 16 at a predetermined timing. As described with reference to FIG. 1, the print detection unit 24 is a block including a line sensor (not shown), reads an image (test sample) printed on the recording paper 16, performs necessary signal processing, and prints. The situation (the presence or absence of ejection, variation in droplet ejection, etc.) is detected, and the detection result is provided to the print controller 80.

  The averaging processing unit 102 performs averaging processing on a plurality of density distribution data composed of the density distribution data for each nozzle 51 obtained in the nozzle characteristic data calculation unit 100, and calculates an average value. is there.

  The correction data acquisition unit 104 acquires effective correction data for correcting density unevenness from the average value of density distribution data for each nozzle 51. That is, the correction data acquisition unit 104 creates a correction table that indicates the correspondence (conversion) between the input image signal and the output image signal for each nozzle.

  During normal printing, the print controller 80 corrects the input image signal for each nozzle by using this correction table, and outputs the converted output image signal to the head driver 84, thereby correcting the density unevenness. Do. Further, the print control unit 80 performs various corrections for the print head 50 based on information obtained from the print detection unit 24 as necessary, in addition to such correction of density unevenness.

  Hereinafter, a method for acquiring effective correction data and correcting density unevenness will be described.

  First, changes with time in error characteristics such as ejection amount error and landing position error for each nozzle will be described. FIG. 8 shows the change over time in the discharge amount error for one nozzle. Here, the discharge amount error data is shown, but the landing position error data has the same tendency.

  In FIG. 8, the horizontal axis represents time, and the vertical axis represents the deviation of the discharge amount from the ideal value as a percentage. Note that the horizontal axis merely indicates the flow of time, and is not necessarily graduated in a fixed time unit. Also, the discharge amount error on the vertical axis represents the ratio of the discharge amount deviation as a percentage, assuming that the ideal value of the ink discharge amount discharged from the nozzle is 0%.

  In FIG. 8, small black rhombuses on the solid line graph represent actual measurement values of errors. Each error data is obtained by measurement every time the head cleaning is combined with processing such as suction and wiping.

  At this time, in each of the periods T1, T2, and T3, small error fluctuations occur due to the head cleaning. In addition, during the period T1 to T2 or between the period T2 to T3, a large variation with time occurs. This large variation with time is an irreversible change caused by, for example, a minute chipping of the liquid repellent agent or a minute chipping of the nozzle on the nozzle surface.

  In order to suppress the density unevenness in consideration of such a large variation with time, correction data may be acquired again as conventionally known (see, for example, Patent Document 1 above). However, when trying to correct the image signal to be printed next time with the correction data acquired from the error data at a certain time, the state may be deteriorated.

  For example, in FIG. 8, the correction data acquired from the error data at point A in the graph is such that the discharge amount error (%) at point A is 0% of the ideal value. Therefore, if correction is attempted at the next point B using this correction data, the correction is excessive and reverse correction is performed, and density unevenness is worsened by the correction.

  Therefore, in the present invention, by taking the average value of the discharge amount errors of a plurality of times, the non-reproducible error characteristic depending on the state of the nozzle is canceled, and effective correction data is obtained to obtain this effective correction. Density unevenness is corrected by data.

  Hereinafter, a method for acquiring effective correction data will be described.

  In this method, a test sample is printed a plurality of times, and a nozzle characteristic average value within a certain period (independent of the nozzle state) is estimated from data of a plurality of measurement results obtained by measuring each test sample. At this time, it is preferable to perform the measurement a plurality of times by changing the nozzle state. As a method for changing the nozzle state, a head cleaning operation such as suction or wiping is preferably exemplified. In the example described below, the measurement is repeated a plurality of times with the head cleaning in between, and the average of the measurement results of the plurality of times is taken, but the head cleaning is merely an example, and the factor that varies randomly is canceled. Is the purpose.

  FIG. 9 is a flowchart showing a method for acquiring effective correction data for one nozzle.

  In step S100 of FIG. 9, first, an index i for counting the number of times of measurement is initialized to 1. Next, in step S110, the measurement sample (test pattern) is printed on a predetermined recording medium.

  In step S120, the printed measurement sample is measured to obtain characteristic data (error characteristic). Specifically, this is performed, for example, by optically reading the density distribution of the measurement sample with a CCD (line sensor) of the print detection unit 24 and calculating the nozzle characteristic value from the read density distribution. Note that the method for measuring the variation characteristic of the nozzle 51 is not limited to this, and other methods may be used.

  In step S130, head cleaning such as suction and wiping is performed. The nozzle state is changed by performing head cleaning. In the next step S140, the index i is incremented by one.

  In step S150, the index i is compared with the preset number of times N. If the index i still does not exceed N, the process returns to step S110 and the measurement sample is printed and measured again. . In this way, the measurement is performed by changing the nozzle state by sandwiching the head cleaning between each measurement.

  At this time, since the error characteristics change depending on various environmental conditions, it is desirable to measure the variation characteristics by varying the conditions for a plurality of parameters as follows. For example, since the error characteristics of the nozzle differ depending on the density or the printing duty, it is preferable to measure the characteristics at a plurality of levels of density or the printing duty and switch the correction data according to the density. Further, since the characteristics vary depending on the temperature and humidity, the characteristics may be measured by changing the temperature and humidity in a plurality of stages. Here, the temperature includes not only the temperature in the printer installation chamber but also a temperature change due to heat generation of the printer elements.

  Note that the relationship between the printing conditions and the error characteristics to be obtained here is “reproducible” caused by manufacturing errors, and is essentially non-reproducible within a certain period caused by head cleaning or the like. Is different. Therefore, by performing the averaging process, a non-reproducible variation factor depending on the nozzle state is canceled.

  Further, when the error characteristics are measured a plurality of times by changing the nozzle state with the head cleaning interposed therebetween, a process in the averaging processing unit 102 is preliminarily performed in order to avoid sudden errors. The threshold may be set, and a process for rejecting the abnormal value from the measured value of the characteristic error may be added.

  For example, when the nozzle is clogged instantaneously, no ejection occurs and the ejection amount is measured as 0, but it cannot be said to be a characteristic value in normal operation. If such an abnormal value is included in the measured value, the result of the averaging process performed thereafter will be affected, and effective correction data cannot be obtained and appropriate correction cannot be performed. .

  In step S150, if the index i exceeds the predetermined number of measurements N, in the next step S160, averaging processing is performed on the N pieces of measurement data obtained so far, and nozzles within a certain period of time are averaged. The characteristic average value is calculated. In step S170, effective correction data for the nozzle within a predetermined period is acquired from the average value.

  Using the effective correction data acquired in this way, the image signal is corrected within the period to correct density unevenness.

  Next, correction data acquisition timing in the entire process of the inkjet recording apparatus 10 will be described.

  FIG. 10 shows correction data acquisition timing in the overall processing flow.

  As shown in FIG. 10, when an image quality abnormality such as streak unevenness is found while the printer is operating normally, normal printing is interrupted and the process proceeds to the correction data acquisition mode.

  In the correction data acquisition mode, as described above with reference to FIG. 9, the error characteristic data is measured a plurality of times with the head cleaning interposed therebetween, and the correction data is acquired by averaging these measurement data.

  The judgment at the time of shifting to the correction data acquisition mode may be made by the operator visually judging the output print, or by measuring the density distribution by the sensor of the print detection unit 24 or the like and automatically judging the stripe unevenness of the print. May be. At this time, a test print for determining image quality may be output.

  Alternatively, the timing for periodic inspection may be set to automatically shift to the correction data acquisition mode. In this case, the inspection timing may be, for example, once a day, immediately after system startup of the day, or once a week, for example, immediately after startup at the beginning of the week.

  When valid correction data is acquired in the correction data acquisition mode, the correction data is updated with the correction data, and the printer is restarted.

  Note that the method for correcting the density unevenness using the correction data is not particularly limited. The correction data indicating what conversion is to be performed on the image signal corresponding to each nozzle is associated, and the effective correction data acquired above is tabulated (correction table). Density unevenness is corrected using the correction table.

  This correction table indicates the correspondence between the input image signal and the output image signal, and is represented, for example, as a straight line (correction curve) having a correction coefficient α multiplied by the input image signal as a slope. At this time, the correction coefficient α is switched every time new correction data is acquired in the correction data acquisition mode described above.

  During normal printing, density unevenness correction is performed by multiplying the input image signal by the slope of the correction curve (correction coefficient α) set in this way for each nozzle and converting it to an output image signal.

  As described above, according to the present embodiment, when the nozzle characteristics are measured, the head cleaning is performed to cancel the variation factor caused by the nozzle state, thereby suppressing the reverse correction and the high-precision density. Correction was possible.

  In addition, the effective correction data is extracted by repeating printing and measurement a plurality of times with the same print data with the head cleaning in between, and averaging the characteristic values obtained in each measurement. Occurrence was suppressed, and highly accurate density correction became possible.

  In the above embodiment, an inkjet recording apparatus has been described as an example of the image forming apparatus, but the present invention is not limited to inkjet. For example, the present invention can be suitably applied to an LED printer, a thermal printer, and the like.

  In the case of the ink jet recording apparatus as in the above embodiment, the correction data is obtained by performing printing and measurement a plurality of times by changing the state of the nozzle that is the recording element by head cleaning. Also in the case of the apparatus, the correction data is acquired by changing the state of the printing element. For example, in the case of an LED printer, the state of the recording element can be changed by removing dust and the like adhering to the glass surface of the LED head with a brush or an adhesive roller and cleaning it.

  In the above-described embodiment, the nozzle characteristics are measured by changing the nozzle state by performing the head cleaning, and the measurement results of a plurality of times are averaged. Continually print without changing the nozzle state by means, measure the nozzle characteristics, obtain the correction data by taking the average of the measurement results of multiple times in a certain period, and perform density unevenness correction Anyway.

  Although the image forming apparatus of the present invention has been described in detail above, the present invention is not limited to the above examples, and various improvements and modifications may be made without departing from the spirit of the present invention. Of course.

1 is an overall configuration diagram showing an outline of an embodiment of an ink jet recording apparatus as an image forming apparatus according to the present invention. FIG. 2 is a plan view of a main part around a printing unit of the ink jet recording apparatus shown in FIG. 1. FIG. 3 is a plan perspective view illustrating a structural example of a print head. It is a top view which shows the other example of a print head. FIG. 5 is a cross-sectional view taken along line 5-5 in FIG. It is the schematic which showed the structure of the ink supply system in the inkjet recording device of this embodiment. It is a principal part block diagram which shows the system configuration | structure of the inkjet recording device of this embodiment. It is a diagram which shows the error characteristic of discharge amount. It is a flowchart which shows the correction data acquisition method in this embodiment. It is explanatory drawing which shows correction data acquisition timing. (A) is explanatory drawing which shows the cause of generation | occurrence | production of a stripe unevenness, (b) is explanatory drawing which shows the example of a stripe unevenness.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Inkjet recording device, 12 ... Printing part, 14 ... Ink storage / loading part, 16 ... Recording paper, 18 ... Paper feeding part, 20 ... Decal processing part, 22 ... Adsorption belt conveyance part, 24 ... Print detection part, 26 DESCRIPTION OF REFERENCE SYMBOLS: Paper discharge part, 28 ... Cutter, 30 ... Heating drum, 31, 32 ... Roller, 33 ... Belt, 34 ... Adsorption chamber, 35 ... Fan, 36 ... Belt cleaning part, 40 ... Heating fan, 42 ... Post drying part, 44 ... heating / pressurizing unit, 45 ... pressure roller, 48 ... cutter, 50 ... print head, 50A ... nozzle surface, 51 ... nozzle, 51a ... nozzle flow path, 52 ... pressure chamber, 53 ... ink supply port, 54 ... pressure chamber unit, 55 ... common liquid chamber, 56 ... diaphragm (common electrode), 57 ... individual electrode, 58a ... piezoelectric body, 58 ... piezoelectric element, 59 ... electrode pad, 60 ... ink tank, 62 ... filter, 64 Cap, 66 ... Blade, 67 ... Suction pump, 68 ... Collection tank, 70 ... Communication interface, 72 ... System controller, 74 ... Image memory, 76 ... Motor driver, 78 ... Heater driver, 80 ... Print controller, 82 ... Image Buffer memory, 84 ... head driver, 86 ... host computer, 88 ... motor, 89 ... heater, 90 ... program storage unit, 92 ... nozzle management memory, 94 ... preliminary ejection control unit, 96 ... maintenance unit, 98 ... temperature / humidity Detection unit, 100 ... Nozzle characteristic data calculation unit, 102 ... Averaging processing unit, 104 ... Correction data acquisition unit

Claims (5)

A recording head in which a plurality of recording elements are arranged;
Moving means for moving the recording medium relative to the recording head;
Driving means for driving the recording element to record on the recording medium from the recording head;
Cleaning means for cleaning the recording element;
Measuring means for measuring a plurality of times by repeatedly measuring the density distribution of the recording medium recorded immediately after cleaning the recording element ;
Characteristic data calculating means for calculating characteristic data of the recording element from density distribution data obtained by the measurement;
Averaging processing means for averaging a plurality of the characteristic data;
Correction means for correcting the drive signal of the drive means based on the result of the averaging process;
An image forming apparatus comprising: Said averaging means is making the averaging process, by setting the predetermined threshold value, according to claim 1, characterized in that the exclusion to averaging processing the characteristic data exceeding the threshold value Image forming apparatus. The recording element is a liquid discharge port for discharging a liquid, and the characteristic data includes a landing position or a discharge liquid amount of a liquid discharged from the liquid discharge port to the recording medium. The image forming apparatus according to 1 or 2 . The image forming apparatus according to claim 1, wherein the recording head is an LED head. The image forming apparatus according to claim 1, wherein the recording head is a thermal head.

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