Image forming apparatus

Abstract

To provide an image forming apparatus capable of acquiring a proper driving voltage corresponding to a print sheet.SOLUTION: A density adjusting unit 83 is configured to: cause a recording element group of recording heads 1a to 1d to eject ink for a first test chart with a first driving voltage, as well as to eject ink for a second test chart with a second driving voltage that is different from the first driving voltage; (b) acquire a density of the first test chart and a density of the second test chart; (c) derive a difference between the density of the first test chart and the density of the second test chart; and (d) derive a driving voltage corresponding to a target density on the basis of the difference between the density of the first test chart and the density of the second test chart and a difference between the first driving voltage and the second driving voltage.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus. [Background technology]

[0002] In one inkjet recording device, in the first correction, a correction value for the drive voltage of the recording head corresponding to the print density (image read value) of the test chart is determined based on a predetermined characteristic curve, and in the second correction, a density correction value for the image data is determined (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-81344 Summary of the Invention [Problem to be solved by the invention]

[0004] In the inkjet recording device described above, the shape of the characteristic curve is constant, so the amount of density change corresponding to the amount of correction of the driving voltage varies depending on the type of print sheet, etc., and it may not be possible to obtain a driving voltage that can appropriately correct the density.

[0005] SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has as its object to provide an image forming apparatus that can obtain an appropriate driving voltage according to the print sheet. [Means for solving the problem]

[0006] The image forming apparatus of the present invention includes a group of recording elements that eject ink according to a drive voltage; and a density adjustment unit that (a) causes the group of recording elements to eject ink for a first test chart at a first drive voltage and eject ink for a second test chart at a second drive voltage different from the first drive voltage, (b) acquires the density of the first test chart and the density of the second test chart, (c) derives the difference between the density of the first test chart and the density of the second test chart, and (d) derives a drive voltage corresponding to a target density based on the difference between the density of the first test chart and the density of the second test chart and the difference between the first drive voltage and the second drive voltage. [Effects of the Invention]

[0007] According to the present invention, an image forming apparatus can be obtained that can obtain an appropriate driving voltage according to the print sheet.

[0008] The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a side view illustrating the mechanical internal configuration of an image forming apparatus according to an embodiment of the present invention. [Figure 2] FIG. 2 is a plan view showing an example of the recording heads 1a, 1b, 1c, and 1d in the image forming apparatus 10 shown in FIG. [Figure 3] FIG. 3 is a block diagram showing the electrical configuration of the image forming apparatus 10 according to the embodiment of the present invention. [Figure 4] FIG. 4 is a diagram showing an example of a test chart. [Figure 5] FIG. 5 is a diagram illustrating the relationship between the amount of change in driving voltage and the amount of change in density. [Figure 6] FIG. 6 is a diagram for explaining the suppression of density unevenness by a plurality of printing element groups. [Figure 7]FIG. 7 is a flowchart illustrating the operation of the image forming apparatus shown in FIGS. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0011] 1 is a side view illustrating the internal mechanical configuration of an image forming apparatus 10 according to an embodiment of the present invention. The image forming apparatus 10 according to this embodiment is a device such as a printer, a copier, a facsimile machine, or a multifunction peripheral.

[0012] 1 includes a print engine 10a and a sheet conveying unit 10b. The print engine 10a physically forms a page image to be printed on a print sheet (e.g., a print sheet). In this embodiment, the print engine 10a is a line-type inkjet print engine.

[0013] In this embodiment, print engine 10a is equipped with line-type recording heads 1a to 1d corresponding to four ink colors: cyan, magenta, yellow, and black.

[0014] FIG. 2 is a plan view showing an example of the recording heads 1a, 1b, 1c, and 1d in the image forming apparatus 10 shown in FIG. 2. For example, as shown in FIG. 2, in this embodiment, each of the recording heads 1a, 1b, 1c, and 1d has a plurality of (three in this example) head units 11. These head units 11 are arranged along the main scanning direction and are detachable from the apparatus main body. Note that each of the recording heads 1a, 1b, 1c, and 1d may have only one head unit 11.

[0015] The head section 11 of each of the print heads 1a, 1b, 1c, and 1d includes printing elements arranged in the main scanning direction. Each printing element includes a nozzle for ejecting ink, a pressure chamber connected to the nozzle and supplied with ink, and a piezoelectric actuator that is driven by a drive signal corresponding to the image data of the image to be printed to push ink from the pressure chamber to the nozzle and eject it from the nozzle. A drive signal with a drive voltage set for each printing element group consisting of multiple printing elements is applied to each printing element. In other words, the same drive voltage is applied to all printing elements in a given printing element group, and the drive signal corresponding to that printing element is applied to each printing element group. This drive voltage is the voltage amplitude of the drive signal, and the drive voltage set for each printing element group is applied to the printing element group according to the waveform of the drive signal by an electrical circuit (not shown).

[0016] The sheet transport section 10b transports the print sheet before printing to the print engine 10a along a predetermined transport path, and transports the print sheet after printing from the print engine 10a to a predetermined discharge destination (such as a discharge tray 10c).

[0017] The sheet transport unit 10b includes a main sheet transport unit 10b1 and a circulating sheet transport unit 10b2. In double-sided printing, the main sheet transport unit 10b1 transports print sheets to be used for printing the page image on the first side to the print engine 10a, and the circulating sheet transport unit 10b2 transports print sheets from the rear stage to the front stage of the print engine 10a while retaining a predetermined number of print sheets.

[0018] In this embodiment, the main sheet conveying section 10b1 includes a circular conveying belt 2 arranged opposite the print engine 10a to convey the print sheet, a drive roller 3 and a driven roller 4 on which the conveying belt 2 is suspended, an adsorption roller 5 that nips the print sheet together with the conveying belt 2, and a pair of discharge rollers 6, 6a.

[0019] A drive roller 3 and a driven roller 4 rotate the conveyor belt 2. Then, an attraction roller 5 nips a print sheet conveyed from paper feed cassettes 20-1 and 20-2 (described later), and the nipped print sheet is conveyed by the conveyor belt 2 to the printing positions of recording heads 1a to 1d in order, where images of each color are printed by the recording heads 1a to 1d. After color printing is completed, the print sheet is discharged onto a discharge tray 10c or the like by a pair of discharge rollers 6 and 6a.

[0020] Furthermore, the main sheet transport section 10b1 is equipped with a plurality of paper feed cassettes 20-1 and 20-2. The paper feed cassettes 20-1 and 20-2 store print sheets SH1 and SH2, and lift plates 21 and 24 push the print sheets SH1 and SH2 upward to contact pickup rollers 22 and 25. The print sheets SH1 and SH2 placed in the paper feed cassettes 20-1 and 20-2 are picked up one by one from above by the pickup rollers 22 and 25 onto paper feed rollers 23 and 26. The paper feed rollers 23 and 26 transport the print sheets SH1 and SH2, which have been fed from the paper feed cassettes 20-1 and 20-2 by the pickup rollers 22 and 25, onto a transport path one by one. Transport roller 27 is a transport roller on a transport path shared by the print sheets SH1 and SH2 transported from the paper feed cassettes 20-1 and 20-2.

[0021] During double-sided printing, the circulating sheet transport unit 10b2 returns the print sheet from a predetermined position downstream of the print engine 10a to a predetermined position upstream (here, a predetermined position upstream of the line sensor 31, which will be described later). The circulating sheet transport unit 10b2 includes a transport roller 41 and a switchback transport path 41a that reverses the traveling direction of the print sheet in order to switch the side of the print sheet facing the print engine 10a from the first side to the second side.

[0022] Furthermore, the image forming apparatus 10 includes a line sensor 31 and a sheet detection sensor 32 .

[0023] The line sensor 31 is an optical sensor that is arranged along a direction perpendicular to the conveyance direction of the print sheet and detects the positions of both edge portions (edges on both sides) of the print sheet. For example, the line sensor 31 is a CIS (Contact Image Sensor). In this embodiment, the line sensor 31 is arranged between the registration rollers 28 and the print engine 10a.

[0024] The sheet detection sensor 32 is an optical sensor that detects when the leading edges of the print sheets SH1 and SH2 have passed a predetermined position on the conveyance path. The line sensor 31 detects the positions of both edge portions of the print sheets SH1 and SH2 when the leading edges of the print sheets SH1 and SH2 are detected by the sheet detection sensor 32.

[0025] For example, as shown in FIG. 1, the print engine 10a is arranged above or below (here, above) the print sheet transport path, the line sensor 31 is arranged above or below (here, below) the print sheet transport path, and the circulating sheet transport section 10b2 transports the print sheet by switching back from the downstream side of the print engine 10a to the upstream side of the line sensor 31.

[0026] In addition, a line sensor 31 is provided to detect the position of the print sheet. For example, a test chart, which will be described later, is printed on the print sheet, and the print sheet is transported by the circulating sheet transport section 10b2, and the image of the printed test pattern is read by the line sensor 31.

[0027] Fig. 3 is a block diagram showing the electrical configuration of image forming apparatus 10 according to the embodiment of the present invention. As shown in Fig. 3, image forming apparatus 10 includes image output unit 71 having the mechanical configuration shown in Figs. 1 and 2, as well as operation panel 72, storage device 73, image reader 74, and controller 75.

[0028] The operation panel 72 is arranged on the surface of the housing of the image forming device 10 and is equipped with a display device 72a such as an LCD display and an input device 72b such as hard keys or a touch panel, and displays various messages to the user on the display device 72a and accepts user operations on the input device 72b.

[0029] The storage device 73 is a non-volatile storage device (such as a flash memory or a hard disk drive) that stores data, programs, etc. necessary for controlling the image forming apparatus 10. The storage device 73 stores data indicating the drive voltage set for each recording element group in each head unit 11.

[0030] The image reading device 74 is equipped with a platen glass and an automatic document feeder, and optically reads the image of a document placed on the platen glass or a document transported by the automatic document feeder, and generates image data of that image.

[0031] The controller 75 includes a computer that executes software processing according to a program, an ASIC (Application Specific Integrated Circuit) that executes predetermined hardware processing, and the like, and operates as various processing units. The computer includes a CPU (Central Processing Unit), ROM (Read Only Memory), RAM (Random Access Memory), and the like, and operates as various processing units (together with the ASIC, if necessary) by loading programs stored in the ROM, storage device 73, etc. into the RAM and executing them on the CPU. Here, the controller 75 operates as a control unit 81, an image processing unit 82, and a density adjustment unit 83.

[0032] The control unit 81 controls the image output unit 71 (print engine 10a, sheet conveying unit 10b, etc.) and executes a print job requested by a user. In this embodiment, the control unit 81 causes the image processing unit 82 to perform predetermined image processing and controls the print engine 10a (head unit 11) to eject ink and form a print image on a print sheet. Specifically, the control unit 81 supplies drive signals of drive voltages (voltage amplitudes) set for each recording element group to each piezoelectric actuator of the head unit 11, causing ink to be ejected from the nozzles. The image processing unit 82 performs predetermined image processing such as RIP (Raster Image Processing), color conversion, and halftoning on image data of an image to be printed on a print sheet.

[0033] In this way, the control unit 81 causes the print engine 10a to print the user document image based on the print image data specified by the user.

[0034] In this embodiment, the control unit 81 has an automatic centering function that (a) identifies the center position of the print sheet as the actual sheet center position based on the positions of both edge portions of the print sheet detected by the line sensor 31, and (b) adjusts the center position of the image to be printed based on the actual sheet center position, and executes the automatic centering function as hardware processing. Specifically, in the automatic centering function, the control unit 81 changes the drawing position of the image to be printed along the main scanning direction by the difference between the reference center position of the print engine 10a and the actual sheet center position. In this embodiment, the nozzles in the recording heads 1a to 1d do not move, so the nozzles corresponding to each pixel in the image to be printed are changed depending on the drawing position of the image to be printed.

[0035] In this way, the control unit 81 determines the nozzles (nozzles corresponding to each pixel) corresponding to the image to be printed according to the position on the print sheet, and causes the recording heads 1a to 1d to eject ink from the nozzles.

[0036] The density adjustment unit 83 (a) causes each recording element group to eject ink for a first test chart at a first drive voltage and eject ink for a second test chart at a second drive voltage different from the first drive voltage, (b) acquires the density of the first test chart and the density of the second test chart, (c) derives the difference between the density of the first test chart and the density of the second test chart, and (d) derives a drive voltage corresponding to the target density based on the difference between the density of the first test chart and the density of the second test chart and the difference between the first drive voltage and the second drive voltage.

[0037] FIG. 4 is a diagram showing an example of a test chart. For example, as shown in FIG. 4, test charts 101C, 101M, 101Y, and 101K are printed for each ink color by the recording heads 1a to 1d, respectively. The test chart 101C is a cyan test chart, the test chart 101M is a magenta test chart, the test chart 101Y is a yellow test chart, and the test chart 101K is a black test chart. Each of the test charts 101C, 101M, 101Y, and 101K has patches 111 arranged in the main scanning direction. Each patch 111 is printed by a single recording element group associated with that patch. However, multiple patches 111 may be printed by one recording element group. The first and second test charts described above are printed based on image data of the same test chart.

[0038] The density of each test chart 101C, 101M, 101Y, and 101K (i.e., the density of the patch 111) is determined based on image data obtained by optically reading the test chart using, for example, the line sensor 31 or the image reading device 74. This density is an ID value, RGB value, L*a*b* value, etc. When multiple patches 111 are printed using one recording element group, the average value of the densities of those patches 111 is determined to be the density of the test chart corresponding to that recording element group.

[0039] Figure 5 is a diagram that explains the relationship between the amount of change in drive voltage and the amount of change in density. For example, as shown in Figure 5, when the drive voltage of the printing element group is increased, the density of the printed image also increases. However, since ink bleeding and color development differ depending on the type of print sheet, the amount of change in density relative to the amount of change in drive voltage differs depending on the type of print sheet.

[0040] Therefore, the density adjustment unit 83 (a) uses the first test chart or the second test chart as the reference test chart, (b) derives the difference between the target density and the density of the reference test chart, (b) derives a correction amount for the drive voltage from the difference between the target density and the density of the reference test chart, the difference between the densities of the first test chart and the second test chart, and the difference between the first drive voltage and the second drive voltage, and (c) derives a drive voltage corresponding to the target density from the correction amount and the first drive voltage or the second drive voltage corresponding to the reference test chart, and updates the above-mentioned drive voltage data with the derived drive voltage. Thereafter, when printing, a drive signal for that drive voltage is applied to the printing element group.

[0041] 5, the density adjustment unit 83 (a) derives a correction coefficient k from the ratio (dD / dV) of the difference dD between the density D1 of the first test chart and the density D2 of the second test chart and the difference dV between the first drive voltage V1 and the second drive voltage V2, and (b) derives the above-mentioned correction amount by multiplying or dividing the difference between the target density and the density (D1 or D2) of the reference test chart by the correction coefficient k. At this time, division is performed when the above-mentioned ratio is dD / dV, and multiplication is performed when the above-mentioned ratio is dV / dD.

[0042] 6 is a diagram illustrating the suppression of density unevenness caused by a plurality of printing element groups. For example, as shown in FIG. 6, for each of printing element groups #1 to #12, the density adjustment unit 83 adjusts the drive voltage to adjust the ink ejection amount of the printing element group, thereby bringing the density of the image drawn by the printing element group closer to the target density (for example, the target density for the maximum solid density), thereby suppressing density unevenness caused by the plurality of printing element groups. Note that here, each printing element group #i corresponds to one or two patches 111.

[0043] After deriving the drive voltage corresponding to the target density as described above, the density adjustment unit 83 (a) causes the printing element group to eject ink for the third test chart at the derived drive voltage, (b) obtains the density of the third test chart, and (c) determines whether the error between the density of the third test chart and the target density is less than a predetermined threshold. If the error between the density of the third test chart and the target density is not less than the predetermined threshold, the density adjustment unit 83 derives a correction coefficient based on the densities of the first, second, and third test charts, as well as the first drive voltage, the second drive voltage, and the derived drive voltage, and (b) derives an additional correction amount from the correction coefficient and the error, and corrects the drive voltage corresponding to the target density with the additional correction amount.

[0044] For example, the correction coefficient is derived by the least squares method based on the density of the first test chart, the density of the second test chart, the density of the third test chart, the first drive voltage, the second drive voltage, and the derived drive voltage.

[0045] Next, the operation of the image forming apparatus will be described below. Fig. 7 is a flowchart illustrating the operation of the image forming apparatus shown in Figs.

[0046] The density adjustment unit 83 sets the drive voltage of each recording element group to drive voltage V1 (step S1), supplies a drive signal to each recording element to cause each recording element to eject ink, and prints test charts 101C, 101M, 101Y, and 101K on a predetermined type of print sheet (step S2).

[0047] Next, the density adjustment unit 83 sets the drive voltage of each recording element group to drive voltage V2 (step S3), supplies a drive signal to each recording element to cause each recording element to eject ink, and prints test charts 101C, 101M, 101Y, and 101K on a print sheet of the same type as the above-mentioned specified type (step S4).

[0048] This provides two test charts corresponding to the drive voltages V1 and V2. These test charts may be printed on a single print sheet.

[0049] The density adjustment unit 83 acquires the density measurement values ​​D1 and D2 of the two test charts (step S5), derives a correction coefficient k from the drive voltages V1 and V2 and the density measurement values ​​D1 and D2 (step S6), and adjusts the drive voltage based on the correction coefficient k so as to bring the density of the test chart closer to the target density (step S7).

[0050] Then, the density adjusting unit 83 sets the adjusted drive voltage V3, supplies a drive signal to each recording element, and causes each recording element to eject ink, thereby printing the test charts 101C, 101M, 101Y, and 101K (step S8).

[0051] This provides a test chart corresponding to the drive voltage V3.

[0052] The density adjustment unit 83 acquires the density measurement value D3 of the test chart (step S9), and determines whether the density error (the difference between the density measurement value D3 and the target density) is less than a predetermined threshold value (step S10).

[0053] If the density error is less than the predetermined threshold, the adjustment of the driving voltage is terminated. Note that the above-described adjustment of the driving voltage is performed for each ink color and for each printing element group.

[0054] On the other hand, if the density error is not less than the predetermined threshold, the process returns to step S6, where the density adjustment unit 83 derives a new correction coefficient k from the drive voltages V1-V3 and the density measurements D1-D3 using a least-squares method or the like (step S6), adjusts the drive voltage based on the correction coefficient k so that the test chart density approaches the target density (step S7), sets the adjusted drive voltage Vi (i=4, 5, . . .) and prints test charts 101C, 101M, 101Y, and 101K (step S8), obtains the density Di of the printed test charts 101C, 101M, 101Y, and 101K (step S9), and determines whether the density error (here, the difference between the density measurement value Di and the target density) is less than the predetermined threshold (step S10). Note that the density error may be the difference between the maximum and minimum densities of the test chart for all recording element groups for each ink color.

[0055] In this way, additional test charts are repeatedly printed until the density error becomes less than the predetermined threshold, and correction coefficients are derived based on the drive voltages V1 to Vi and the density measurement values ​​D1 to Di, and the drive voltages are additionally adjusted based on the correction coefficients.

[0056] As described above, according to the above embodiment, the density adjustment unit 83 causes the printing element groups of the printing heads 1a to 1d to eject ink for a first test chart at a first drive voltage and eject ink for a second test chart at a second drive voltage different from the first drive voltage, (b) acquires the density of the first test chart and the density of the second test chart, (c) derives the difference between the density of the first test chart and the density of the second test chart, and (d) derives a drive voltage corresponding to the target density based on the difference between the density of the first test chart and the density of the second test chart and the difference between the first drive voltage and the second drive voltage.

[0057] This allows the amount of change in print density corresponding to the amount of change in drive voltage to be specified for each print sheet type, so that an appropriate drive voltage can be obtained for the print sheet regardless of the characteristics of the print sheet (degree of bleeding and color development).

[0058] It should be noted that various changes and modifications to the above-described embodiments will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the spirit and scope of the subject matter and without diminishing its intended advantages. In other words, it is intended that such changes and modifications be included within the scope of the claims.

[0059] For example, in the above embodiment, the drive voltage corresponding to the target density is derived from two test charts (first and second test charts) in steps S1 to S7, but instead, the above-mentioned correction coefficient may be derived using the least squares method or the like from three or more test charts printed at mutually different drive voltages.

[0060] In the above embodiment, the reference test chart may be selected from the first test chart and the second test chart, whichever has the smaller overall error from the target density.

[0061] In addition, in the above embodiment, instead of measuring the density of the test chart with the line sensor 31 or the image reading device 74, the density of the test chart may be measured with a colorimeter and the measurement value may be input to the density adjustment unit 83.

[0062] In addition, in the above embodiment, drive voltage data associated with the print type may be stored in the memory device 73, and the controller 75 may identify the print type of the print sheet to be used for printing, read out the drive voltage for the identified print type, and apply that drive voltage to the group of recording elements to print a user image (an image specified by the user). [Industrial Applicability]

[0063] The present invention is applicable to, for example, an image forming apparatus. [Explanation of symbols]

[0064] 10 Image forming device 83 Concentration Adjustment Department

Claims

1. a group of printing elements that eject ink in response to a driving voltage; (a) a density adjustment unit that causes the recording element group to eject ink for a first test chart at a first drive voltage and eject ink for a second test chart at a second drive voltage different from the first drive voltage, (b) acquires the density of the first test chart and the density of the second test chart, (c) derives a difference between the density of the first test chart and the density of the second test chart, and (d) derives a drive voltage corresponding to a target density based on the difference between the density of the first test chart and the density of the second test chart and the difference between the first drive voltage and the second drive voltage; An image forming apparatus comprising:

2. 2. The image forming apparatus according to claim 1, wherein, when either the first test chart or the second test chart is used as a reference test chart, the density adjustment unit (a) derives the difference between the target density and the density of the reference test chart, (b) derives a correction amount for the drive voltage from the difference between the target density and the density of the reference test chart, the difference between the density of the first test chart and the density of the second test chart, and the difference between the first drive voltage and the second drive voltage, and (c) derives a drive voltage corresponding to the target density from the correction amount and the first drive voltage or the second drive voltage corresponding to the reference test chart.

3. The image forming apparatus according to claim 2, characterized in that the density adjustment unit (a) derives a correction coefficient from the ratio of the difference between the density of the first test chart and the density of the second test chart and the difference between the first driving voltage and the second driving voltage, and (b) derives the correction amount by multiplying or dividing the correction coefficient by the difference between the target density and the density of the reference test chart.

4. 2. The image forming apparatus according to claim 1, wherein the density adjustment unit (a) causes the recording element group to eject ink for a third test chart at the derived drive voltage, (b) acquires the density of the third test chart, and (c) if an error between the density of the third test chart and the target density is not less than a predetermined threshold, derives a correction coefficient based on the densities of the first test chart, the second test chart, and the third test chart, as well as the first drive voltage, the second drive voltage, and the derived drive voltage, and (b) derives an additional correction amount from the correction coefficient and the error, and corrects the drive voltage corresponding to the target density with the additional correction amount.

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