Discharge failure detection device, image forming apparatus, and discharge failure detection method

Abstract

To provide a discharge failure detection device capable of efficiently detecting a discharge failure without requiring labor and time.SOLUTION: A discharge failure detection device 24 includes: a plate 30 (translucent) that receives a plurality of ink droplets D discharged from a plurality of nozzles of a recording head 14; a capping device 22 in which a surface 22C facing the plate 30 is colored in white and which covers a nozzle surface 14A; a reading unit 31 that reads the plurality of ink droplets D attached onto the plate 30 while the nozzle surface 14A is kept covered with the capping device 22; and a control unit that calculates a difference between light reception signals from light receiving elements adjacent to each other in a main scanning direction. In a case where the number of the light receiving elements 36 having a difference larger than a difference threshold value is larger than a number threshold value, the control unit determines that a nozzle corresponding to a light receiving element 36 having a difference equal to or smaller than the difference threshold value has a discharge failure. In a case where the number is equal to or smaller than the number threshold value, the control unit moves the reading unit 31 in the main scanning direction by a predetermined distance which is less than an interval between the adjacent light receiving elements.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to a discharge defect detection device, an image forming apparatus, and a discharge defect detection method. [Background technology]

[0002] A printing device is known that prints an image on a printing medium using a print head in which multiple nozzles that eject ink are arranged in a predetermined direction (see Patent Document 1). A print position adjustment pattern and a non-discharge detection pattern are printed on the printing medium by ejecting ink from some nozzles and not ejecting ink from other nozzles. The printing medium on which each pattern is printed is set on the glass surface of a reading unit, and the reading unit reads the print position adjustment pattern on the printing medium. The number of defective ejection nozzles is obtained based on the reading results of the non-discharge detection pattern, and if the number of defective ejection nozzles is smaller than a predetermined threshold, a value based on the reading results of the print position adjustment pattern is determined as an adjustment value for adjusting the misalignment of the printing head. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 5473435 Summary of the Invention [Problem to be solved by the invention]

[0004] However, with the above-mentioned technology, if there is a slight misalignment between the position of the ink droplets on the recording medium and the position of the optical sensor (light-receiving element) in the reading unit, it may not be possible to accurately read the recording position adjustment pattern (ink droplets). In this case, nozzles that are normally ejecting ink are erroneously detected as not ejecting ink, resulting in the problem of unnecessary maintenance operations (wiping, etc.) being performed on the recording head. Furthermore, with the above-mentioned technology, since each pattern is recorded on the recording medium by ejecting ink from only a portion of the multiple nozzles, it is not possible to detect ejection defects for all nozzles. If ejection defects for all nozzles were to be detected, each pattern would have to be recorded on multiple recording media while switching between nozzles that eject ink, and the ejection defect detection process would have had to be performed multiple times, which was time-consuming and labor-intensive.

[0005] In consideration of the above circumstances, the present invention provides a discharge defect detection device, an image forming apparatus, and a discharge defect detection method that can detect discharge defects efficiently without requiring much time and effort. [Means for solving the problem]

[0006] The discharge defect detection device includes: a recording head that discharges droplets from a plurality of nozzles that are aligned along a main scanning direction on a nozzle surface; a plate that is formed into a flat plate shape from a light-transmitting material and is disposed opposite the nozzle surface and that receives the plurality of droplets discharged from the plurality of nozzles; a cap portion that covers the nozzle surface and has a surface that faces the plate colored white; and a reading portion that faces the nozzle surface across the plate, is provided so as to be movable along the main scanning direction, and includes a plurality of light-receiving elements that are aligned along the main scanning direction, and that reads the plurality of droplets that have adhered to the plate using the plurality of light-receiving elements when the nozzle surface is covered by the cap portion. and a control unit that receives light receiving signals from the plurality of light receiving elements and calculates the difference (V) of the light receiving signals from adjacent light receiving elements in the main scanning direction across all the light receiving elements, and when the number (P) of the light receiving elements that results in the difference (V) greater than the difference threshold (Vth) is greater than a number threshold (Pth), the control unit performs a detection operation to determine that the nozzle corresponding to the light receiving element whose difference (V) is equal to or less than the difference threshold (Vth) is defective in ejection, and when the number (P) is equal to or less than the number threshold (Pth), the control unit performs a main scanning adjustment operation to move the reading unit in the main scanning direction by a predetermined distance that is less than the spacing between adjacent light receiving elements.

[0007] In this case, the control unit can repeat the main scanning adjustment operation a predetermined number of times (N), and the predetermined distance is preferably half (0.5G) of the spacing (G) between adjacent light receiving elements divided by the predetermined number of times (N).

[0008] In this case, the recording heads are arranged in a sub-scanning direction perpendicular to the main scanning direction, the plurality of nozzles are aligned in a line in the main scanning direction to form a nozzle row, the plurality of nozzle rows are aligned along the sub-scanning direction on the nozzle surface, the reading unit is provided to be movable along the sub-scanning direction, the droplets are ejected toward the plate from the plurality of nozzles constituting one of the nozzle rows located at one end in the sub-scanning direction, the droplets are not ejected from the plurality of nozzles constituting the other of the nozzle rows, and the reading unit reads the main scanning direction from the nozzles on the plate. The plurality of droplets arranged in a line in the scanning direction are read, and the control unit calculates the difference (V) based on the reading result by the reading unit, and when the number (P) is greater than the number threshold (Pth), the droplets are not ejected from the plurality of nozzles that make up one nozzle row, and the droplets are ejected toward the plate from the plurality of nozzles that make up the other nozzle row, and the reading unit reads the plurality of droplets that have adhered to the plate while moving in the sub-scanning direction, and the control unit performs the detection operation based on the reading result by the reading unit.

[0009] In this case, it is preferable that the image forming apparatus further includes a wiper that is movable in the sub-scanning direction in conjunction with the reading unit and that wipes off the droplets remaining on the plate.

[0010] The image forming apparatus includes any one of the ejection defect detection devices described above.

[0011] The ejection defect detection method includes a discharge step of discharging a plurality of droplets from a plurality of nozzles arranged along a main scanning direction on a nozzle surface of a recording head toward a plate formed in a flat plate shape from a light-transmitting material; a reading step of covering the nozzle surface with a cap portion whose surface facing the plate is colored white, and reading the plurality of droplets deposited on the plate by a reading unit that faces the nozzle surface across the plate and includes a plurality of light-receiving elements arranged along the main scanning direction; and a reading step of reading the plurality of droplets deposited on the plate by a reading unit that faces the nozzle surface across the plate and includes a plurality of light-receiving elements arranged along the main scanning direction. The method includes a difference calculation step of calculating the signal difference (V) across all of the light receiving elements, a detection step of determining that the nozzle corresponding to the light receiving element whose difference (V) is equal to or less than the difference threshold (Vth) is defective in ejection when the number (P) of the light receiving elements whose difference (V) is greater than the difference threshold (Vth) is greater than a number threshold (Pth), and a main scanning adjustment step of moving the reading unit in the main scanning direction by a predetermined distance that is less than the interval between adjacent light receiving elements when the number (P) is equal to or less than the number threshold (Pth).

[0012] In this case, the main scanning adjustment process can be repeated a predetermined number of times (N), and the predetermined distance is preferably half (0.5G) of the spacing (G) between adjacent light receiving elements divided by the predetermined number of times (N).

[0013] In this case, the recording head is arranged in a sub-scanning direction perpendicular to the main scanning direction, the plurality of nozzles are arranged in a line in the main scanning direction to form a nozzle row, the plurality of nozzle rows are arranged in the sub-scanning direction on the nozzle surface, and in the ejection step, the droplets are ejected toward the plate from the plurality of nozzles constituting one of the nozzle rows located at one end in the sub-scanning direction, and the droplets are not ejected from the plurality of nozzles constituting the other nozzle rows, and in the reading step, the reading unit reads the plurality of nozzles arranged in a line in the main scanning direction on the plate. In the difference calculation process, the difference (V) is calculated based on the reading results by the reading unit, and in the detection process, if the number (P) is greater than the number threshold (Pth), the droplets are not ejected from the multiple nozzles that make up one nozzle row, and the droplets are ejected toward the plate from the multiple nozzles that make up the other nozzle row, and the reading unit reads the multiple droplets that have adhered to the plate while moving in the sub-scanning direction, and determines the nozzle that is experiencing ejection problems based on the reading results by the reading unit.

[0014] In this case, it is preferable that the method further includes a cleaning step in which a wiper provided to be movable in the sub-scanning direction in conjunction with the reading unit wipes off the droplets remaining on the plate prior to the ejection step. [Effects of the Invention]

[0015] According to the present invention, ejection defects can be detected efficiently without requiring much time and effort. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a schematic diagram (front view) showing the internal structure of an image forming apparatus according to one embodiment of the present invention. [Figure 2] FIG. 2 is a plan view showing a line head of the image forming apparatus according to the embodiment of the present invention. [Figure 3] 1 is a block diagram showing a maintenance unit and the like of an image forming apparatus according to an embodiment of the present invention; [Figure 4] 1 is a front view showing a head unit, a conveying unit, and a maintenance unit of an image forming apparatus according to an embodiment of the present invention. [Figure 5] 1 is a perspective view showing a discharge defect detection device according to an embodiment of the present invention; [Figure 6] 1 is a front view showing a discharge defect detection device according to an embodiment of the present invention; [Figure 7] 1 is a flowchart illustrating a discharge defect detection method according to an embodiment of the present invention. [Figure 8] 10A to 10C are explanatory views (plan views, cross-sectional views, etc.) for explaining a main scanning adjustment step (with misalignment) of a discharge defect detection method according to one embodiment of the present invention. [Figure 9] 10A to 10C are explanatory views (plan views, cross-sectional views, etc.) for explaining a main scanning adjustment process (without misalignment) of a discharge defect detection method according to one embodiment of the present invention. [Figure 10] 1 is a front view showing a discharge defect detection device (detection step) according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. Note that Fr, Rr, L, R, U, and Lo shown in the drawings represent front, rear, left, right, top, and bottom. The front-to-back direction (main scanning direction), left-to-right direction (sub-scanning direction), and top-to-bottom direction are perpendicular to one another. Terms indicating directions and positions are used in this specification, but these terms are used for convenience of explanation and do not limit the technical scope of the present invention.

[0018] [Image forming device] An image forming apparatus 1 according to one embodiment will be described with reference to Figures 1 and 2. Figure 1 is a schematic diagram (front view) showing the internal structure of the image forming apparatus 1. Figure 2 is a plan view showing a line head 13.

[0019] The image forming apparatus 1 is an inkjet printer that ejects ink droplets D to form an image on paper M (medium). As shown in FIG. 1, the image forming apparatus 1 includes a box-shaped housing 3 that houses various devices. A paper feed cassette 4 in which paper M is set is housed in the lower part of the housing 3, and a paper output tray 5 on which printed paper M is stacked is provided on the upper left side of the housing 3. A first transport path 6 is formed on the right side of the housing 3 for transporting paper M from the paper feed cassette 4 toward the head unit 12. A paper feed unit 10 is provided upstream of the first transport path 6, and a registration roller 11 is provided downstream of the first transport path 6. In this specification, the direction in which paper M is transported is referred to as the "transport direction."

[0020] The head unit 12 has four line heads 13 corresponding to four colors of ink: black, cyan, magenta, and yellow. As shown in FIG. 2, each line head 13 has a head base 13A formed like a plate that is long in the front-to-rear direction (main scanning direction). A plurality of (e.g., three) recording heads 14 are arranged in a staggered pattern on the head base 13A of each line head 13. The plurality of recording heads 14 are arranged so as to cover the front-to-rear width of the paper M (the width perpendicular to the conveying direction). Since the four line heads 13 are arranged in a row in the left-to-right direction (sub-scanning direction) (see FIG. 1), the plurality of recording heads 14 mounted on the four line heads 13 are also arranged in the left-to-right direction (sub-scanning direction). More specifically, the recording heads 14 are arranged in two rows in the left-to-right direction per head base 13A, so that the head unit 12 as a whole has eight rows of recording heads 14 in the left-to-right direction. Each recording head 14 is connected to an ink pack (not shown) of the corresponding color via a tube (not shown), and receives a supply of ink (liquid) from the ink pack. Since the four line heads 13 have the same structure, the following description will mainly focus on one line head 13. Similarly, since the multiple recording heads 14 also have the same structure, the following description will mainly focus on one recording head 14.

[0021] As shown in FIGS. 1 and 2, the recording head 14 is supported by the head base 13A with the nozzle surface 14A facing downward (toward the transport unit 15, described later). A plurality of nozzles N are aligned in a line on the nozzle surface 14A along the front-to-rear direction (main scanning direction) to form a nozzle row NL. A plurality of (e.g., two) nozzle rows NL are aligned along the left-to-right direction (sub-scanning direction) on the nozzle surface 14A. Specifically, each recording head 14 has two nozzle rows NL in the left-to-right direction, so the head unit 12 as a whole has 16 nozzle rows NL in the left-to-right direction. The recording head 14 has a plurality of ink chambers formed corresponding to the plurality of nozzles N and a plurality of piezoelectric elements provided corresponding to the plurality of ink chambers (neither of which is shown). Each piezoelectric element is electrically connected to a high-voltage power supply (not shown), which applies a drive signal (high voltage) to each piezoelectric element under the control of the control unit 19. The recording head 14 is a so-called piezo-type inkjet head that ejects ink (liquid) from nozzles N by applying a drive signal to a piezoelectric element to deform an ink storage chamber.

[0022] As shown in Fig. 1, a transport section 15 for transporting paper M is provided below the head unit 12. The transport section 15 includes a transport belt 16 that is wound around a plurality of tension rollers 16A. A large number of through holes (not shown) are formed in the transport belt 16, and a suction section 16B is provided on the inside of the transport belt 16.

[0023] A decurling device 17 that corrects curls in the paper M is installed on the left side of the head unit 12. A second transport path 7 that transports the paper M from the decurling device 17 toward the paper output tray 5 is formed on the left side of the housing 3, and a paper output unit 18 is provided downstream of the second transport path 7. A third transport path 8 that transports the paper M from the middle of the second transport path 7 to the registration rollers 11 is formed in the upper part of the housing 3.

[0024] A control unit 19 for appropriately controlling various control target devices is provided inside the housing 3. The control unit 19 includes a processor or the like that executes various arithmetic processes in accordance with programs and parameters stored in memory. Note that the control unit 19 may be realized by a logic circuit (hardware) formed in an integrated circuit or the like, instead of a processor or the like that executes programs or the like.

[0025] [Printing Processing] Here, the printing process (image forming process) will be described. The control unit 19 controls various control target devices as appropriate, and executes the printing process as follows.

[0026] The paper feed unit 10 feeds the paper M taken out of the paper feed cassette 4 to the first transport path 6. The registration rollers 11 temporarily block the paper M to correct skew, and then feed the paper M onto the transport belt 16 in time with the ejection of ink droplets D (liquid droplets) from the recording head 14. The transport unit 15 transports the paper M at a position opposite the head unit 12 (recording head 14). The paper M is transported while being adsorbed onto the transport belt 16, and the recording head 14 ejects ink droplets D (liquid droplets) from multiple nozzles N toward the paper M. As a result, a full-color image is formed (printed) on the paper M. After passing below the head unit 12, the paper M is sent to a decurling device 17 where curls are corrected.

[0027] The paper M printed on one side passes through the second transport path 7 and is discharged to the paper output tray 5. When performing double-sided printing, the paper M printed on one side enters the third transport path 8, is turned over, and is transported again toward the registration rollers 11. An image is then formed on the back side of the paper M in the same order as in the single-sided printing described above, and the double-sided printed paper M is straightened and discharged to the paper output tray 5.

[0028] [Maintenance Department] The image forming apparatus 1 is equipped with a maintenance unit 20 that maintains the recording head 14. The maintenance unit 20 will be described below with reference to Figs. 3 to 6. Fig. 3 is a block diagram showing the maintenance unit 20 and other components. Fig. 4 is a front view showing the head unit 12, the transport unit 15, and the maintenance unit 20. Fig. 5 is a perspective view showing the discharge defect detection device 24. Fig. 6 is a front view showing the discharge defect detection device 24.

[0029] The maintenance unit 20 is disposed inside the housing 3 near the left side of the head unit 12 (see FIG. 1). The maintenance unit 20 performs a maintenance operation to prevent and unclog the nozzles N of the recording head 14, and a discharge defect detection operation to detect clogged nozzles N. As shown in FIG. 4, the maintenance unit 20 includes a head moving device 21, a capping device 22, a wiping device 23, and a discharge defect detection device 24. The maintenance unit 20 also includes a box-shaped maintenance housing 25 with an open right side. The capping device 22, the wiping device 23, and the discharge defect detection device 24 are each supported inside the maintenance housing 25 via a sliding device 26. The sliding device 26 is driven and controlled by the control unit 19 (see FIG. 3), and moves a selected one of the capping device 22, the wiping device 23, and the discharge defect detection device 24 back and forth in the left-right direction.

[0030] <Head moving device> The head moving device 21 reciprocates the head unit 12 in the vertical direction (the direction in which the head unit 12 moves away from or toward the transport unit 15) (see FIG. 4). The head moving device 21 includes an electric motor (not shown) connected to the head unit 12 via a drive transmission mechanism (not shown). The head moving device 21 is electrically connected to the control unit 19 and moves the head unit 12 up and down under the control of the control unit 19. As shown in FIG. 4, the head unit 12 moves (lifts and lowers) between a printing position P1 where the head unit 12 is close to the transport unit 15 (transport belt 16) and can perform a printing process, and a maintenance position P2 that is set at a position above and spaced apart from the printing position P1 (see FIG. 4). By disposing the head unit 12 in the maintenance position P2, the maintenance operations and ejection defect detection operations described above can be performed.

[0031] <Capping device> As shown in FIG. 4, the capping device 22 (cap section) includes a plurality of individual caps 22B mounted on a cap carriage 22A. The cap carriage 22A is formed in a flat plate shape, and the individual caps 22B are formed in a tray shape with edges protruding upward from the bottom surface. The plurality of individual caps 22B are arranged on the cap carriage 22A so as to correspond to the plurality of recording heads 14. The capping device 22 (cap carriage 22A) is provided so as to be movable in the left-right direction between a storage position (see FIG. 4) in which it is stored in a maintenance housing 25 and a capping position (see FIG. 6) set directly below the head unit 12 disposed at a maintenance position P2. When capping, which is an example of a maintenance operation, is performed, the head moving device 21 moves the head unit 12 from the print position P1 to the maintenance position P2, and the slide device 26 moves the capping device 22 from the storage position to the capping position. Thereafter, the head moving device 21 lowers the head unit 12 at the maintenance position P2, causing the edges of the individual caps 22B to come into close contact with the nozzle faces 14A of the recording heads 14 and seal the areas where the nozzles N are formed (see FIG. 6). As described above, the capping device 22 (cap portion) covers the nozzle faces 14A of the multiple recording heads 14, thereby suppressing the drying (thickening) of ink in the nozzles N and preventing clogging of the nozzles N. The lower surface (surface 22C facing the plate 30, which will be described later) of the capping device 22 (cap carriage 22A) is colored white.

[0032] Furthermore, the individual cap 22B is connected to a suction pump 27 (see FIG. 3) via a tube (not shown). When purging (discharge of waste) is performed as an example of a maintenance operation, the suction pump 27 is driven and controlled by the control unit 19 with the edge of the individual cap 22B in close contact with the nozzle surface 14A, and a negative pressure is created in the space surrounded by the nozzle surface 14A and the individual cap 22B. This forcibly sucks out ink that has thickened inside the nozzles N, making it possible to prevent (or eliminate) clogging of the nozzles N. The sucked ink is stored in a waste tank (not shown).

[0033] When capping or purging is completed, the head moving device 21 raises the head unit 12 to the maintenance position P2, and the slide device 26 returns the capping device 22 to the storage position.

[0034] <Wiping device> As shown in FIG. 4, the wiping device 23 includes four wiper carriages 23A, a plurality of head wipers 23B, and a wiper moving device 23C. The wiping device 23 is provided so as to be movable in the left-right direction between a storage position (see FIG. 4) housed in a maintenance housing 25 and a wiping position (not shown) set directly below the head unit 12 disposed at maintenance position P2. The four wiper carriages 23A are provided so as to correspond to the four line heads 13. The wiper carriage 23A is provided so as to be movable in the front-rear direction when disposed at the wiping position. Three head wipers 23B corresponding to the three recording heads 14 are provided in an upright position on each wiper carriage 23A. The head wipers 23B are formed in a plate shape using an elastic material such as synthetic rubber. The wiper moving device 23C reciprocates the wiper carriage 23A (head wipers 23B) in the front-rear direction.

[0035] When wiping, an example of a maintenance operation, is performed, the head moving device 21 moves the head unit 12 from the print position P1 to the maintenance position P2, and the sliding device 26 moves the wiping device 23 from the storage position to the wiping position. The head moving device 21 then lowers the head unit 12 from the maintenance position P2, causing the tip (upper end) of the head wiper 23B to contact the nozzle surface 14A. The wiper moving device 23C is then driven and controlled by the control unit 19 to move the wiper carriage 23A from front to rear, causing the head wiper 23B to wipe away ink adhering to the nozzle surface 14A. When wiping is completed, the head moving device 21 raises the head unit 12 to the maintenance position P2, the wiper moving device 23C returns the wiper carriage 23A to the wiping start position, and the sliding device 26 returns the wiping device 23 to the storage position. Wiping may be performed, for example, after purging ink.

[0036] <Discharge defect detection device> The discharge defect detection device 24 includes a plurality of recording heads 14, a control unit 19, a capping device 22, and a detection main body 28. The recording heads 14 are elements that make up the head unit 12, the capping device 22 is an element that makes up the maintenance unit 20, and the control unit 19 is an element that controls the image forming apparatus 1, but they are also elements that make up the discharge defect detection device 24.

[0037] 4 to 6, the detection main body 28 includes a plate 30, a reading unit 31, and a plate wiper 32. The detection main body 28 is provided so as to be movable in the left-right direction between a storage position (see FIG. 4) in which it is stored in the maintenance housing 25 and a detection position (see FIGS. 5 and 6) set directly below the head unit 12 disposed at the maintenance position P2.

[0038] (plate) The plate 30 is formed in a flat plate shape from a light-transmitting material such as glass. The plate 30 is formed to be longer than the overall length of the head unit 12 in the left-right direction (transport direction). When the detection main body 28 is placed at the detection position, the plate 30 is placed opposite the nozzle surface 14A with a gap therebetween and receives the multiple ink droplets D ejected from the multiple nozzles N. A white reference portion 30A is provided at the most upstream portion of the upper surface of the plate 30 to be read when calibrating a light-receiving element 36 of the reading unit 31, which will be described later.

[0039] (Reading unit) The reading unit 31 is disposed below the plate 30 and faces the nozzle surface 14A across the plate 30. The reading unit 31 is movable in the front-to-rear direction (main scanning direction) and the left-to-right direction (sub-scanning direction). The reading unit 31 has a reading main body 33 and a reading movement unit 34.

[0040] The reading main body 33 is a so-called CIS (Contact Image Sensor) that includes a light source, a lens, and an image sensor 35. The light source (not shown) is, for example, a light-emitting diode (LED) and is electrically connected to the control unit 19. The light source emits light toward the ink droplets D on the plate 30 while its blinking is controlled by the control unit 19. A lens (not shown) causes light reflected by the ink droplets D on the plate 30 to enter the image sensor 35 (a plurality of light-receiving elements 36). The image sensor 35 includes a plurality of light-receiving elements 36 (see FIG. 8, described later) arranged along the front-rear direction (main scanning direction). The length of the row formed by the plurality of light-receiving elements 36 is longer than the length of the nozzle row NL. Each light-receiving element 36 is electrically connected to the control unit 19 (see FIG. 3) and converts the incident light into a light-receiving signal SL (e.g., an electrical signal such as a voltage (see FIG. 8, described later)) and transmits it to the control unit 19. That is, the reading unit 31 optically reads the ink droplets D adhering to the plate 30 using the light receiving elements 36 .

[0041] As shown in FIGS. 5 and 6, the reading movement unit 34 includes a reading carriage 40, a main scanning motor 41, and a sub-scanning motor 42. The reading carriage 40 is formed in a box shape that is elongated in the front-rear direction, and the reading main body unit 33 described above is mounted on the reading carriage 40. The reading carriage 40 is provided so as to be movable in the front-rear direction while being guided by a main scanning guide (not shown) that extends in the front-rear direction. The reading carriage 40 is provided so as to be movable in the left-right direction while being guided by a sub-scanning guide 43 that extends in the left-right direction. The main scanning motor 41 and the sub-scanning motor 42 are, for example, stepping motors with reducers, and are electrically connected to the control unit 19 (see FIG. 3). The control unit 19 controls the rotation angles of rotor shafts 41A and 42A of the main scanning motor 41 and the sub-scanning motor 42.

[0042] As shown in FIG. 5, the main scanning motor 41 is disposed, for example, below and in front of the reading carriage 40. A pinion gear (not shown) is fixed to a rotor shaft 41A of the main scanning motor 41, and the pinion gear meshes with a rack gear (not shown) fixed to the underside of the reading carriage 40. The main scanning motor 41 moves the reading carriage 40 in the front-to-rear direction while the control unit 19 controls the rotation angle of the rotor shaft 41A. As shown in FIGS. 5 and 6, the sub-scanning motor 42 is disposed, for example, below the right rear corner of the plate 30. A drive belt 45 is stretched between the rotor shaft 42A of the sub-scanning motor 42 and a driven pulley 44 disposed below the left rear corner of the plate 30, and the reading carriage 40 is fixed to the drive belt 45. The sub-scanning motor 42 moves the reading carriage 40 in the left-to-right direction while the control unit 19 controls the rotation angle of the rotor shaft 42A.

[0043] (plate wiper) As shown in FIG. 6, the plate wiper 32 is disposed above the plate 30 and is located slightly to the right of the reading main body 33 (upstream in the transport direction) across the plate 30. The plate wiper 32 is formed into a plate shape from an elastic material such as synthetic rubber. The plate wiper 32 is connected to the reading carriage 40 (or drive belt 45) of the reading unit 31 via a connecting arm (not shown) and is provided so as to be movable in the left-right direction (sub-scanning direction) in conjunction with the reading unit 31. The plate wiper 32 wipes off ink droplets D remaining on the plate 30 by bringing its lower end into contact with the upper surface of the plate 30 and moving in the left-right direction (sub-scanning direction) together with the reading unit 31.

[0044] [Method for detecting defective discharge] Next, with reference to Figures 6 to 10, a discharge defect detection method (discharge defect detection operation) for detecting clogged nozzles N using the discharge defect detection device 24 will be described. Figure 7 is a flowchart showing the discharge defect detection method. Figures 8 and 9 are explanatory diagrams (plan view, cross-sectional view, etc.) for explaining the main scanning adjustment step S6 of the discharge defect detection method. Figure 10 is a front view showing the discharge defect detection device 24 (detection step S5).

[0045] For example, when a user operates a touch panel (not shown) of the image forming apparatus 1, the control unit 19 appropriately controls various devices to be controlled and executes the discharge defect detection method (discharge defect detection operation) as follows. Note that, for ease of explanation, in this specification, the recording head 14 and nozzle row NL located at the most upstream (right) in the transport direction may also be referred to as the "first row" recording head 14 and the "first row" nozzle row NL.

[0046] First, as a preliminary step to the ejection defect detection method (ejection defect detection operation), the head moving device 21 moves the head unit 12 from the print position P1 to the maintenance position P2, and the slide device 26 moves the detection main body 28 from the storage position to the detection position. Thereafter, the head moving device 21 slightly lowers the head unit 12 at the maintenance position P2, bringing the nozzle surface 14A of each recording head 14 close to the upper surface of the plate 30.

[0047] The ejection defect detection method includes a cleaning step S1, an ejection step S2, a reading step S3, a difference calculation step S4, a detection step S5, and a main scanning adjustment step S6 (see FIG. 7).

[0048] <Cleaning process> In the cleaning step S1, the sub-scanning motor 42 of the reading movement unit 34 moves the reading main body unit 33 (reading carriage 40) from left to right (from downstream to upstream in the transport direction) until the reading main body unit 33 (reading carriage 40) is positioned directly below the first row of recording heads 14 (first row of nozzle rows NL) (see FIG. 6). The plate wiper 32, which is integrated with the reading unit 31, wipes off ink droplets D, dust, etc. remaining on the plate 30 while moving from left to right in conjunction with the reading unit 31.

[0049] <Discharge process> In the ejection step S2, a plurality of ink droplets D are ejected toward the plate 30 from the plurality of nozzles N of the recording head 14. More specifically, in the ejection step S2, ink droplets D are ejected toward the plate 30 from the plurality of nozzles N constituting the first nozzle row NL (one nozzle row NL located at one end in the sub-scanning direction), and ink droplets D are not ejected from the plurality of nozzles N constituting the other nozzle rows NL. In other words, ink droplets D are not ejected from all the nozzles N of all the recording heads 14, but ink droplets D are ejected only from the nozzles N constituting the first nozzle row NL. On the plate 30, a plurality of ink droplets D corresponding to the first nozzle row NL adhere (land) in a line in the front-to-back direction (main scanning direction), forming only one line (see the plan views at the top of FIGS. 6 and 8).

[0050] <Reading process> In the reading step S3, the capping device 22 covers the nozzle surface 14A, and the reading unit 31, which includes multiple light-receiving elements 36, reads multiple ink droplets D adhering to the plate 30 (see the AA cross-sectional views in FIGS. 6 and 8 ). Specifically, similar to capping as an example of a maintenance operation, the head moving device 21 raises the head unit 12 from a position close to the plate 30 to the maintenance position P2, and the sliding device 26 moves the capping device 22 from the storage position to the capping position. The head moving device 21 then lowers the head unit 12, which is in the maintenance position P2, causing the individual caps 22B to seal the nozzle surface 14A of the recording head 14. With the capping device 22 covering the nozzle surface 14A, the reading unit 31 (reading main body 33), stopped directly below the first nozzle row NL, uses the multiple light-receiving elements 36 to read multiple ink droplets D adhering to the plate 30 in a line in the front-to-rear direction (main scanning direction). Each light receiving element 36 transmits to the control unit 19 a light receiving signal SL (see the lower part of FIG. 8) corresponding to the intensity of the reflected light.

[0051] Because the underside (opposing surface 22C) of the cap carriage 22A is colored white, the light-receiving element 36 reads the ink droplets D against the white opposing surface 22C as a background. Therefore, the light-receiving signal SL indicating the white background has a higher value than the light-receiving signal SL indicating the ink droplets D. However, if there is a slight misalignment between the ink droplets D on the plate 30 and the light-receiving element 36 of the reading unit 31 in the front-to-rear direction (main scanning direction) (see the AA cross-sectional view in FIG. 8 ), the ink droplets D may not be accurately read. In this case, for example, a nozzle N that is normally ejecting ink may be erroneously detected as not ejecting ink, resulting in unnecessary maintenance operations being performed on the recording head 14. Therefore, the ejection defect detection method (ejection defect detection device 24) according to this embodiment is configured to adjust for slight misalignment between the ink droplets D and the light-receiving element 36.

[0052] <Difference calculation process> In the difference calculation step S4, the control unit 19 receives light-receiving signals SL from the multiple light-receiving elements 36 and calculates the difference (V) between the light-receiving signals SL from adjacent light-receiving elements 36 in the front-to-back direction (main scanning direction) for all of the light-receiving elements 36. For example, as shown in the AA cross-sectional view in the middle of FIG. 8, if one ink droplet D on the plate 30 is positioned so as to straddle two adjacent light-receiving elements 36 (if the ink droplet D and the light-receiving elements 36 are slightly misaligned in the front-to-back direction (main scanning direction)), each light-receiving element 36 receives a mixture of reflected light reflected by both the opposing surface 22C, which is the background, and the ink droplet D. Therefore, as shown in the bottom of FIG. 8, the difference (V) between the light-receiving signals SL becomes small. In contrast, for example, as shown in the BB cross-sectional view in the middle of FIG. 9, when one ink droplet D on the plate 30 is positioned on one light receiving element 36 (there is no (or little) misalignment between the ink droplet D and the light receiving element 36), each light receiving element 36 receives light reflected by the opposing surface 22C in the background or the ink droplet D. Therefore, as shown in the bottom of FIG. 9, the difference (V) in the light receiving signals SL becomes large. From the above, the control unit 19 calculates the difference (V) in the light receiving signals SL based on the reading result by the reading unit 31, and can determine the misalignment between the ink droplet D on the plate 30 and the light receiving element 36 based on the magnitude of the difference (V).

[0053] <Detection process> In the detection step S5 (detection operation), if the number (P) of light-receiving elements 36 with a difference (V) greater than the difference threshold (Vth) is greater than the number threshold (Pth), the control unit 19 determines that the nozzle N corresponding to the light-receiving element 36 with a difference (V) equal to or less than the difference threshold (Vth) is defective in ejection. The difference threshold (Vth) is set based on, for example, the linearity of the image sensor 35 (light-receiving element 36) and the reflectivity of the ink droplets D, and is stored in advance in the memory of the control unit 19. The number threshold (Pth) is set based on, for example, the number of nozzles N determined to be defective as a result of printing (forming an image) on paper M, and is stored in advance in the memory of the control unit 19.

[0054] The detection step S5 (detection operation) will be described in detail. The control unit 19 calculates the number (P) of light receiving elements 36 for which the difference (V) exceeds the difference threshold (Vth), in other words, the number of light receiving elements 36 that can be determined to be in general agreement with the position of the ink droplets D (step S51). The control unit 19 also determines whether the number (P) is greater than the number threshold (Pth) (step S51). If the number (P) is greater than the number threshold (Pth) (YES in step S51), the positions of the ink droplets D and the light receiving elements 36 are in general agreement, and therefore it is determined that the reading unit 31 can properly read the ink droplets D, and the following steps (operations) are executed.

[0055] The head moving device 21 raises the head unit 12 from the position where it contacts the individual cap 22B to the maintenance position P2, and the slide device 26 moves the capping device 22 from the capping position to the storage position. Thereafter, the head moving device 21 lowers the head unit 12 at the maintenance position P2 to a position close to the plate 30, and ink droplets D are not ejected from the plurality of nozzles N constituting the first nozzle row NL, but ink droplets D are ejected from the plurality of nozzles N constituting the other nozzle rows NL toward the plate 30 (step S52). That is, in step S52, ink droplets D are ejected from the nozzles N that did not eject ink droplets D in the above-described ejection step S2. The ink droplets D ejected using all the nozzles N adhere (land) on the plate 30 (see FIG. 10).

[0056] Next, the head moving device 21 raises the head unit 12 from a position close to the plate 30 to the maintenance position P2, and the sliding device 26 moves the capping device 22 from the storage position to the capping position. The head moving device 21 then lowers the head unit 12 from the maintenance position P2, causing the individual caps 22B to seal the nozzle surface 14A of the recording head 14 (see FIG. 10 ). As shown in FIG. 10 , with the capping device 22 covering the nozzle surface 14A, the reading unit 31 moves left and right (the sub-scanning direction) to read the ink droplets D adhering to the plate 30 (step S53). Specifically, the sub-scanning motor 42 of the reading moving unit 34 moves the reading main body 33 from right to left (from upstream to downstream in the transport direction), and the reading main body 33 (light receiving element 36) sequentially reads the ink droplets D on the plate 30 as it moves. Each light receiving element 36 transmits a light receiving signal SL corresponding to the intensity of the reflected light to the control unit 19. The plate wiper 32 moves from right to left in conjunction with the reading unit 31, wiping away the ink droplets D after they have been read by the reading main body unit 33.

[0057] Next, the control unit 19 determines which nozzle N has a discharge defect based on the reading result by the reading unit 31. Specifically, the control unit 19 determines whether or not there is a light-receiving element 36 for which the difference (V) calculated in the difference calculation step S4 is equal to or less than the difference threshold (Vth) (step S54). If there is a light-receiving element 36 for which the difference (V) is equal to or less than the difference threshold (Vth) (YES in step S54), the control unit 19 determines that the nozzle N corresponding to that light-receiving element 36 has a discharge defect (non-discharge, insufficient discharge amount, misaligned deposition position, etc.), and displays the determination result on the touch panel (step S55). On the other hand, if there is no light-receiving element 36 for which the difference (V) is equal to or less than the difference threshold (Vth) (NO in step S54), the control unit 19 determines that there is no nozzle N with a discharge defect, and displays the determination result on the touch panel (step S56).

[0058] This completes the discharge defect detection method. If it is determined that there is a discharge-defective nozzle N (step S55), the above-mentioned maintenance operations such as purging and wiping are carried out.

[0059] <Main scanning adjustment process> Next, if the number (P) is equal to or less than the number threshold (Pth) (NO in step S51), it is determined that the reading unit 31 cannot properly read the ink droplets D because the positions of the ink droplets D and the light receiving element 36 are misaligned in the front-to-back direction, and a main scanning adjustment step S6 (main scanning adjustment operation) is executed as follows. Note that step S51 is a step included in the detection step S5, but is also a step included in the main scanning adjustment step S6.

[0060] In the main scanning adjustment step S6, the reading unit 31 (reading main body unit 33) is moved in the front-to-rear direction (main scanning direction) by a predetermined distance that is less than the interval (G) between adjacent light receiving elements 36. The control unit 19 repeatedly executes the main scanning adjustment step S6 (main scanning adjustment operation) a predetermined number of times (N). As an example, the predetermined number of times (N) is set to an initial value of "0" and an upper limit value of "2." The predetermined distance is half (0.5G) of the interval (G) between adjacent light receiving elements 36 divided by the predetermined number of times (N) (=0.5G / N). The interval (G) between the light receiving elements 36 is the distance between the centers of adjacent light receiving elements 36 (see Figures 8 and 9).

[0061] The main scanning adjustment step S6 (main scanning adjustment operation) will be described in detail. If the number (P) is equal to or less than the number threshold (Pth) (NO in step S51), the control unit 19 determines whether the predetermined number (N) is 2 (step S61). If the predetermined number (N) is not 2 (NO in step S61), the control unit 19 adds 1 to the predetermined number (N), and the main scanning motor 41 of the reading movement unit 34, under the control of the control unit 19, moves the reading unit 31 (reading main body unit 33) backward (or forward) by a predetermined distance (0.5 G / N), for example (step S62). Thereafter, the reading unit 31 and the control unit 19 again execute the reading step S3, the difference calculation step S4, and step S51. Until the number (P) becomes greater than the number threshold (Pth) (until YES is returned in step S51), the control unit 19 repeats the main scanning adjustment step S6 (main scanning adjustment operation) a maximum of two times.

[0062] If the number (P) does not exceed the number threshold (Pth) even after repeating the main scanning adjustment step S6 twice, that is, if the predetermined number of times (N) is "2" (YES in step S61), the control unit 19 displays a detection error on the touch panel (step S63) and ends the discharge defect detection operation. Note that if a detection error is determined (step S63), the above-mentioned maintenance operations such as purging and wiping are performed.

[0063] In the ejection defect detection device 24 (ejection defect detection method) according to the present embodiment described above, a light-transmitting plate 30 receives a plurality of ink droplets D (liquid droplets) ejected from a plurality of nozzles N, and a reading unit 31 (a plurality of light-receiving elements 36) reads the plurality of ink droplets D adhering to the plate 30 with the nozzle surface 14A covered by the capping device 22. The light-receiving elements 36 read the ink droplets D on the plate 30 with the opposing surface 22C (white) of the capping device 22 as the background, so that the light-receiving signals SL at positions where ink droplets D are present have lower values than the light-receiving signals SL at positions where ink droplets D are not present. With this configuration, differences in the light-receiving signals SL tend to occur between adjacent light-receiving elements 36 in the main scanning direction, making it easy to calculate the difference (V) between the light-receiving signals SL of adjacent light-receiving elements 36. Furthermore, if the number (P) of light-receiving elements 36 that result in a difference (V) greater than the difference threshold (Vth) is greater than the number threshold (Pth), it can be estimated that the misalignment between the ink droplets D on the plate 30 and the light-receiving elements 36 in the front-to-back direction (main scanning direction) is small. By identifying the light-receiving elements 36 whose difference (V) is equal to or less than the difference threshold (Vth), it is possible to identify nozzles N that are experiencing ejection defects (such as non-ejection). On the other hand, if the number (P) is equal to or less than the number threshold (Pth), it is estimated that the misalignment between the ink droplets D and the light-receiving elements 36 in the front-to-back direction (main scanning direction) is large, which may result in the ink droplets D not being accurately read by the light-receiving elements 36. Therefore, by moving the reading unit 31 a predetermined distance (0.5 G / N) in the main scanning direction, it is possible to eliminate (reduce) the misalignment between the ink droplets D on the plate 30 and the light-receiving elements 36. This makes it easier to identify the light receiving element 36 (nozzle N with ejection defects) for which the difference (V) is equal to or less than the difference threshold (Vth).

[0064] Furthermore, with the discharge defect detection device 24 according to this embodiment, for example, ink droplets D discharged from all nozzles N can be received by one plate 30, and discharge defects can be detected while adjusting the positional deviation in the main scanning direction of the reading unit 31. This makes it possible to detect discharge defects efficiently without requiring much time and effort, compared to the case where discharge defect detection is repeated using multiple sheets of paper onto which ink droplets D have been discharged.

[0065] Furthermore, in the ejection defect detection device 24 according to this embodiment, the main scanning adjustment step S6 is repeated a predetermined number of times (N), and the movement distance (predetermined distance (0.5G / N)) of the reading unit 31 per main scanning adjustment step S6 is set to 0.5G / N (G: the distance between adjacent light receiving elements 36). This configuration allows the reading unit 31 to gradually move in the main scanning direction until the number (P) of light receiving elements 36 at which the difference (V) is greater than the difference threshold (Vth) exceeds the number threshold (Pth). This allows for accurate detection of nozzles N with ejection defects. While the upper limit of the predetermined number of times (N) was set to "2," this is not a limitation and may be set to "1" or greater. However, if the upper limit of the predetermined number of times (N) is large, the number of times the main scanning adjustment step S6 is repeated increases, and the ejection defect detection operation takes time. Therefore, it is preferable to set the upper limit of the predetermined number of times (N) to approximately "2" to "5."

[0066] Furthermore, in the discharge defect detection device 24 according to this embodiment, in order for the control unit 19 to calculate the difference (V), only the nozzle row NL (plurality of nozzles N) located in the first row is configured to discharge ink droplets D toward the plate 30. Also, only when the detection step S5 is executed is the other nozzle rows NL (plurality of nozzles N) configured to discharge ink droplets D toward the plate 30. With this configuration, even if the positional deviation between the ink droplets D and the light receiving element 36 is not resolved in the main scanning adjustment step S6 and the detection step S5 is not executed, it is possible to prevent unnecessary discharge of ink droplets D toward the plate 30.

[0067] Furthermore, in the ejection defect detection device 24 according to this embodiment, the plate wiper 32 is arranged to be movable in the sub-scanning direction in conjunction with the reading unit 31, and is configured to wipe off ink droplets D remaining on the plate 30. This configuration allows the light transmittance of the plate 30 to be maintained, and the reading unit 31 (light receiving elements 36) can properly read the ink droplets D on the plate 30. This makes it possible to properly calculate the difference (V) between the light receiving signals SL of adjacent light receiving elements 36.

[0068] In the discharge defect detection device 24 (discharge defect detection method) according to this embodiment, the difference (V) in the light-receiving signal SL is expressed as the difference between the light-receiving signals SL from adjacent light-receiving elements 36 in the front-to-back direction (main scanning direction). However, the present invention is not limited to this. Because each light-receiving element 36 constitutes a pixel of the scanned image, the control unit 19 may calculate the difference in brightness (luminance) between adjacent pixels in the front-to-back direction (main scanning direction) as the "difference (V)." In the detection step S5, if the number (P) of pixels with a difference (V) greater than the difference threshold (Vth) is greater than the number threshold (Pth), the nozzle N corresponding to the pixel with the difference (V) equal to or less than the difference threshold (Vth) may be determined to have a discharge defect. In the main scanning adjustment step S6, if the number (P) is equal to or less than the number threshold (Pth), the reading unit 31 may be moved in the front-to-back direction (main scanning direction) by a predetermined distance (0.5 / N pixels) that is less than the spacing between adjacent pixels.

[0069] Furthermore, in the ejection defect detection device 24 according to this embodiment, one light receiving element 36 corresponds to one pixel, but the present invention is not limited to this. For example, a plurality of adjacent light receiving elements 36 may be grouped together and treated as one pseudo light receiving element. In this case, the pseudo light receiving element corresponds to one pixel, and the control unit 19 calculates the difference (V) between the light receiving signals SL from the pseudo light receiving elements adjacent in the main scanning direction.

[0070] Furthermore, in the ejection step S2 of the ejection defect detection method according to this embodiment, the nozzles N constituting the first nozzle row NL eject ink droplets D, and the other nozzles N do not eject ink droplets D, but the present invention is not limited to this. For example, in the ejection step S2, ink droplets D may be ejected toward the plate 30 from all the nozzles N of all the recording heads 14. In this case, steps S52 and S53 of the detection step S5 can be omitted, further shortening the time required for ejection defect detection.

[0071] Furthermore, in the ejection defect detection method according to this embodiment, the cleaning step S1 is performed prior to the ejection step S2, but the present invention is not limited to this. The cleaning step S1 may be performed after the ejection step S2. For example, in the ejection step S2, only the nozzles N constituting one nozzle row NL located at the most downstream (16th row) in the transport direction eject ink droplets D, and in the reading step S3, the ink droplets D located at the most downstream in the transport direction are read. Then, in step S53 of the detection step S5, the reading unit 31 reads the ink droplets D on the plate 30 while moving from left to right (from downstream to upstream in the transport direction), and the plate wiper 32 moves integrally with the reading unit 31 to wipe off the ink droplets D after reading. In other words, step S53 of the detection step S5 may also serve as the cleaning step S1.

[0072] Furthermore, in the ejection defect detection device 24 according to this embodiment, the reading main body 33 of the reading unit 31 employs the so-called CIS type image sensor 35, but this is not limiting, and for example, a CCD (Charge Coupled Device) type image sensor (not shown) may also be employed.

[0073] In addition, in the image forming apparatus 1 according to the present embodiment, the maintenance unit 20 and the like are controlled by the control unit 19 that performs overall control, but the present invention is not limited to this. For example, a dedicated control unit (not shown) that controls the maintenance unit 20 and the like may be provided.

[0074] Furthermore, in the image forming apparatus 1 according to the present embodiment, the head unit 12 includes four line heads 13. However, this is not a limitation, and the head unit 12 may include one or more line heads 13. Furthermore, while three recording heads 14 are mounted on one line head 13, this is not a limitation, and the head unit 12 may include one or more recording heads 14. Furthermore, while the recording heads 14 are arranged in a staggered pattern on the head base 13A, this is not a limitation, and the arrangement may be freely changed, for example, to a single line. Furthermore, while the recording head 14 ejects ink (ink droplets D) from the nozzles N, the present invention is not limited to this. For example, the recording head may eject droplets of water, molten synthetic resin, or the like from the nozzles N (not shown).

[0075] Furthermore, in the image forming apparatus 1 according to the present embodiment, the maintenance unit 20 is provided downstream of the head unit 12, but the present invention is not limited to this. The maintenance unit 20 may be located near the head unit 12 regardless of whether it is upstream or downstream in the transport direction of the paper M.

[0076] Furthermore, the image forming apparatus 1 according to the present embodiment is a color printer, but is not limited to this and may be a monochrome printer, a copier, a facsimile, or the like.

[0077] The above-described embodiment shows one aspect of the image forming apparatus according to the present invention, and the technical scope of the present invention is not limited to the above-described embodiment. The present invention may be variously changed, substituted, or modified without departing from the spirit of the technical concept, and the claims include all embodiments that may fall within the scope of the technical concept. [Explanation of symbols]

[0078] 1. Image forming device 14 Recording head 14A Nozzle surface 19 Control Unit 22 Capping device (cap part) 22C Opposite surface 24. Discharge defect detection device 30 plates 31 Reading unit 32 Plate wiper (wiper) 36 Photodetector S1 Cleaning process S2 Discharge process S3 Reading process S4 Difference calculation process S5 Detection process S6 Main scanning adjustment process D. Ink droplets (droplets) N nozzle NL nozzle row

Claims

1. a recording head that ejects droplets from a plurality of nozzles arranged along the main scanning direction on a nozzle surface; a plate formed in a flat plate shape from a light-transmitting material, disposed opposite the nozzle surface, and configured to receive the droplets ejected from the nozzles; a cap portion having a surface facing the plate colored white and covering the nozzle surface; a reading unit that faces the nozzle surface across the plate, is provided to be movable along the main scanning direction, and includes a plurality of light receiving elements that are aligned along the main scanning direction, and that reads the plurality of droplets that have adhered to the plate using the plurality of light receiving elements when the nozzle surface is covered with the cap unit; a control unit that receives light receiving signals from a plurality of the light receiving elements and calculates a difference (V) between the light receiving signals from the light receiving elements adjacent to each other in the main scanning direction across all the light receiving elements, The control unit When the number (P) of the light receiving elements for which the difference (V) is greater than the difference threshold (Vth) is greater than a number threshold (Pth), a detection operation is performed to determine that the nozzle corresponding to the light receiving element for which the difference (V) is equal to or less than the difference threshold (Vth) is defective in ejection; A defective discharge detection device characterized in that, when the number (P) is less than the number threshold (Pth), a main scanning adjustment operation is performed to move the reading unit in the main scanning direction by a predetermined distance that is less than the spacing between adjacent light receiving elements.

2. the control unit is capable of repeatedly executing the main scanning adjustment operation a predetermined number of times (N), 2. The ejection defect detection device according to claim 1, wherein the predetermined distance is a value obtained by dividing half (0.5G) of the interval (G) between adjacent light receiving elements by the predetermined number (N).

3. The recording heads are arranged in a sub-scanning direction perpendicular to the main scanning direction, the plurality of nozzles are aligned in a line in the main scanning direction to form a nozzle row, the nozzle rows are arranged in plurality along the sub-scanning direction on the nozzle surface, the reading unit is provided so as to be movable along the sub-scanning direction, the droplets are ejected toward the plate from a plurality of nozzles constituting one of the nozzle rows located at one end in the sub-scanning direction, and the droplets are not ejected from a plurality of nozzles constituting another of the nozzle rows, the reading unit reads the plurality of droplets lined up in a line in the main scanning direction on the plate, and the control unit calculates the difference (V) based on the reading result by the reading unit; 3. The ejection defect detection device according to claim 1, wherein when the number (P) is greater than the number threshold (Pth), the droplets are not ejected from the plurality of nozzles constituting one of the nozzle rows, and the droplets are ejected toward the plate from the plurality of nozzles constituting the other of the nozzle rows, the reading unit reads the plurality of droplets adhered to the plate while moving in the sub-scanning direction, and the control unit performs the detection operation based on the reading results by the reading unit.

4. 4. The ejection defect detection device according to claim 3, further comprising a wiper that is movable in the sub-scanning direction in conjunction with the reading unit and wipes off the droplets remaining on the plate.

5. 5. An image forming apparatus comprising the ejection defect detection device according to claim 1.

6. a discharge step of discharging a plurality of droplets from a plurality of nozzles arranged along a main scanning direction on a nozzle surface of a recording head toward a plate formed in a flat shape from a light-transmitting material; a reading step of covering the nozzle surface with a cap portion whose surface facing the plate is colored white, and reading the plurality of droplets adhered to the plate by a reading unit that faces the nozzle surface across the plate and includes a plurality of light receiving elements that are arranged along the main scanning direction; a difference calculation step of calculating a difference (V) between light receiving signals from the light receiving elements adjacent to each other in the main scanning direction across all the light receiving elements; a detection step of determining that the nozzle corresponding to the light-receiving element whose difference (V) is equal to or smaller than the difference threshold (Vth) has an ejection defect when the number (P) of the light-receiving elements whose difference (V) is greater than the difference threshold (Vth) is greater than a number threshold (Pth); and a main scanning adjustment process for moving the reading unit in the main scanning direction by a predetermined distance that is less than the spacing between adjacent light receiving elements when the number (P) is equal to or less than the number threshold (Pth).

7. The main scanning adjustment process can be repeated a predetermined number of times (N), 7. The method for detecting defective discharge according to claim 6, wherein the predetermined distance is a value obtained by dividing half (0.5G) of the interval (G) between adjacent light receiving elements by the predetermined number (N).

8. The recording heads are arranged in a sub-scanning direction perpendicular to the main scanning direction, the plurality of nozzles are aligned in a line in the main scanning direction to form a nozzle row, the nozzle rows are arranged in plurality along the sub-scanning direction on the nozzle surface, in the ejection step, the droplets are ejected toward the plate from a plurality of nozzles constituting one of the nozzle rows located at one end in the sub-scanning direction, and the droplets are not ejected from a plurality of nozzles constituting another of the nozzle rows; In the reading step, the reading unit reads the plurality of droplets arranged in a line on the plate in the main scanning direction, In the difference calculation step, the difference (V) is calculated based on the reading result by the reading unit, The method for detecting defective ejection described in claim 6 or 7, characterized in that in the detection process, when the number (P) is greater than the number threshold (Pth), the droplets are not ejected from the multiple nozzles that make up one nozzle row, and the droplets are ejected toward the plate from the multiple nozzles that make up the other nozzle row, the reading unit reads the multiple droplets that have adhered to the plate while moving in the sub-scanning direction, and the nozzle that has defective ejection is determined based on the reading result by the reading unit.

9. The ejection defect detection method according to claim 8, further comprising a cleaning step in which a wiper movable in the sub-scanning direction in conjunction with the reading unit wipes off the droplets remaining on the plate prior to the ejection step.

Images