JP2013071395A - Printing apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress deterioration in print image quality regarding a printing apparatus for performing printing on a printing medium such as paper.SOLUTION: A first to a third movement amount sensors 8A to 8C are arranged such that they are separated from one another in a paper P conveyance direction and a distance between adjacent ones of movement amount sensors is smaller than the predetermined minimum paper length. All of inkjet heads 22A to 22D are arranged between the first movement amount sensor 8A arranged in the most upstream position in the conveyance direction and the third movement amount sensor 8C arranged in the most downstream position. A control part 9 selects a movement amount sensor that detects the movement amount of the paper P according to the movement of the paper P in the conveyance direction from the first to the third movement amount sensors 8A to 8C and performs ejection control on the inkjet heads 22A to 22D using the movement amount of the paper P detected by the movement amount sensor thus selected.

Description

  The present invention relates to a printing apparatus that performs printing on a printing medium such as paper.

  2. Description of the Related Art Conventionally, printing apparatuses that perform printing by discharging ink from an inkjet head onto a sheet while conveying the sheet while holding the sheet on a conveyance belt are known.

  When printing is performed in such a printing apparatus, the conveyance belt is driven and controlled to move at a constant speed. However, the movement speed of the conveyor belt may change due to the influence of so-called uneven thickness of the conveyor belt, eccentricity of a roller around which the conveyor belt is wound, and the like. If the moving speed of the conveying belt is not constant, a so-called “landing deviation” occurs in which the landing position of the ink ejected from the ink jet head on the sheet deviates from a desired position. The landing deviation causes a decrease in print image quality.

  In order to suppress such landing deviation, profile data indicating unevenness of the thickness of the conveyor belt and eccentricity of the roller around which the conveyor belt is wound is extracted and stored in advance, and the ejection timing of the inkjet head is used using this profile data. A technique for controlling the above is known.

  However, the above technique requires a step of extracting profile data in advance. On the other hand, in Patent Document 1, the amount of movement is detected by picking up an image of a sheet or a conveyance belt, and the ejection deviation of the inkjet head is controlled according to the amount of movement, thereby suppressing landing deviation and lowering the print image quality. There has been proposed a printing apparatus that suppresses this.

JP 2009-107310 A

  In the printing apparatus of Patent Document 1, the amount of movement is detected by imaging a sheet or a conveyance belt with a single movement amount sensor disposed upstream of the plurality of inkjet heads in the sheet conveyance direction. Therefore, after the trailing edge of the paper passes the movement amount sensor, the movement amount of the paper passing under the inkjet head cannot be directly detected. For example, the ejection control of the inkjet head based on the movement amount of the transport belt. Is done.

  For this reason, for example, when the movement between the paper and the conveyor belt does not match due to slippage between the paper and the conveyor belt, the ejection control based on one motion amount sensor such as the printing apparatus of Patent Document 1 In some cases, it is impossible to control the ejection of the inkjet head in accordance with the actual movement. For this reason, there is a possibility that the print image quality is deteriorated.

  The present invention has been made in view of the above, and an object of the present invention is to provide a printing apparatus that can suppress a decrease in print image quality.

  In order to achieve the above object, a first feature of a printing apparatus according to the present invention includes a transport unit that transports a print medium, and one or more inkjet heads that eject ink onto the print medium transported by the transport unit. A plurality of motion amount detection units that detect the amount of movement of the print medium conveyed by the conveyance unit; and a control unit that performs ejection control of the inkjet head. Arranged so that the distance between the adjacent motion amount detection units spaced apart from each other in the transport direction is smaller than a predetermined minimum print medium length, and the most downstream motion amount detection unit and the most downstream in the transport direction All the inkjet heads are arranged between the movement amount detection unit and the control unit according to the movement of the print medium in the transport direction among the plurality of movement amount detection units. Of selecting the motion amount detecting unit that detects a motion amount is to perform ejection control of the ink-jet head using the motion amount of the printing medium detected by the motion amount detecting unit selected.

  A second feature of the printing apparatus according to the present invention is to sequentially switch and select one movement amount detection unit used for ejection control of the inkjet head in accordance with the movement of the print medium in the transport direction. It is in.

  A third feature of the printing apparatus according to the present invention is that the control unit is detected by the plurality of motion amount detection units during a period in which there are a plurality of the motion amount detection units that detect the motion amount of the print medium. The ejection control of the inkjet head is performed using the average of the movement amount of the printed medium.

  According to the first feature of the printing apparatus of the present invention, the plurality of motion amount detection units are separated from each other in the print medium conveyance direction, and the distance between the adjacent motion amount detection units is a predetermined minimum print medium. It arrange | positions so that it may become smaller than length. Further, all the inkjet heads are arranged between the most upstream motion amount detection unit and the most downstream motion amount detection unit in the transport direction. Then, the control unit selects a motion amount detection unit that detects the motion amount of the print medium from the plurality of motion amount detection units according to the movement of the print medium, and is detected by the selected motion amount detection unit. The ejection control of the inkjet head is performed using the amount of movement of the print medium. As a result, the control unit can perform ejection control according to the actual movement of the print medium until the print medium passes through the most downstream ink jet head, so that it is possible to suppress a decrease in print image quality.

  According to the second feature of the printing apparatus of the present invention, the control unit sequentially switches and selects one movement amount detection unit used for ejection control of the inkjet head in accordance with the movement of the print medium. Thereby, the processing burden of the control unit can be reduced.

  According to the third feature of the printing apparatus of the present invention, the control unit is detected by the plurality of motion amount detection units in a period in which there are a plurality of motion amount detection units that detect the motion amount of the print medium. The ejection control of the ink jet head is performed using the average amount of movement of the print medium. Thereby, the control part can perform more suitable discharge control with respect to the behavior of the printing medium.

1 is a schematic configuration diagram of a printing apparatus according to an embodiment. FIG. 2 is a block diagram illustrating a configuration of a control system of the printing apparatus illustrated in FIG. 1. It is a figure which shows arrangement | positioning of the 1st-3rd motion amount sensor. It is a schematic block diagram of an imaging part. It is a flowchart for demonstrating discharge control of the inkjet head using the 1st-3rd motion amount sensor. It is a figure for demonstrating the discharge control using the motion amount detected by a motion amount sensor. It is a figure which shows the timing of the paper detection in a paper sensor and a 1st-3rd motion amount sensor. FIG. 6 is a diagram illustrating a movement state of a sheet in a conveyance printing unit. It is a figure for demonstrating correction | amendment of discharge timing. FIG. 6 is a diagram illustrating a movement state of a sheet in a conveyance printing unit. FIG. 6 is a diagram illustrating a movement state of a sheet in a conveyance printing unit. FIG. 6 is a diagram illustrating a movement state of a sheet in a conveyance printing unit.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. Hereinafter, embodiments of the present invention will be described with reference to the drawings. Throughout the drawings, the same or equivalent parts and components are denoted by the same or equivalent reference numerals. However, it should be noted that the drawings are schematic and different from the actual ones. Moreover, it is a matter of course that portions having different dimensional relationships and ratios are included between the drawings.

  Further, the embodiment described below exemplifies an apparatus or the like for embodying the technical idea of the present invention, and the technical idea of the present invention is to arrange the components and the like as follows. It is not something specific. The technical idea of the present invention can be variously modified within the scope of the claims.

  FIG. 1 is a schematic configuration diagram of a printing apparatus according to an embodiment of the present invention, and FIG. 2 is a block diagram showing a configuration of a control system of the printing apparatus shown in FIG. In the following description, the vertical direction and the horizontal direction indicate the vertical direction and the horizontal direction shown in FIG.

  In FIG. 1, a path indicated by a thick line is a transport path through which the print medium is transported. Among the transport routes, a route indicated by a solid line is a normal route RC, a route indicated by a one-dot chain line is a reverse route RR, a route indicated by a broken line is a paper discharge route RD, and a route indicated by a two-dot chain line is a paper feed route RS. In the following description, upstream and downstream mean upstream and downstream in the transport path.

  As shown in FIGS. 1 and 2, the printing apparatus 1 according to the present embodiment includes a paper feeding unit 2, a transport printing unit 3, an upper surface transport unit 4, a paper discharge unit 5, a reversing unit 6, A sheet sensor 7, first to third motion amount sensors 8A to 8C, a control unit 9, and a housing 10 that stores or holds each unit are provided.

  The paper feeding unit 2 feeds paper to the transport printing unit 3. The paper feed unit 2 is provided on the most upstream side of the transport path. The paper feed unit 2 includes an external paper feed tray 11, an external paper feed roller 12, a plurality of internal paper feed stands 13, a plurality of internal paper feed rollers 14, a plurality of pairs of vertical transport rollers 15, and a registration roller 16. With.

  The external paper feed tray 11 is partly exposed to the outside of the housing 10 and is loaded with paper P that is a printing medium.

  The external paper feed roller 12 takes out the paper P one by one from the external paper feed tray 11 and conveys it toward the registration roller 16 along the paper feed path RS. The external paper feed roller 12 is disposed on the upper side of the external paper feed tray 11.

  The internal paper feed tray 13 is provided inside the housing 10 and on which the paper P is stacked.

  The internal paper feed roller 14 takes out the paper P one by one from the internal paper feed tray 13 and sends it out to the paper feed path RS. The internal paper feed roller 14 is provided on the upper side of the internal paper feed tray 13.

  The vertical conveyance roller 15 is arranged along the paper feed path RS and conveys the paper P taken out from the internal paper feed tray 13 toward the registration roller 16.

  The registration roller 16 temporarily stops the paper P conveyed from the external paper feed tray 11, the internal paper feed tray 13, and the reversing unit 6, and then sends the paper P to the transport printing unit 3. The registration roller 16 is disposed on the normal route RC in the vicinity of the junction point between the sheet feeding route RS and the reverse route RR.

  The paper feeding unit 2 includes a motor that rotationally drives the external paper feeding roller 12 and the most downstream vertical conveying roller 15, a motor that rotationally drives the internal paper feeding roller 14 and the other vertical conveying roller 15, and a registration roller. And a motor (none of which is shown in the figure) that rotationally drives 16.

  The transport printing unit 3 performs printing on the paper P while transporting the paper P. The transport printing unit 3 is disposed on the downstream side of the paper feeding unit 2. The conveyance printing unit 3 includes a belt conveyance unit 21 and inkjet heads 22A to 22D.

  The belt transport unit 21 holds and transports the paper P transported from the registration rollers 16. The belt conveyance unit 21 corresponds to a conveyance unit in claims. The belt conveyance unit 21 is disposed on the downstream side of the registration roller 16 and below the inkjet heads 22A to 22D. The belt conveyance unit 21 includes a conveyance belt 23, a driving roller 24, driven rollers 25 to 27, and a motor (not shown) that rotates the driving roller 24.

  The conveyor belt 23 is an annular belt that is stretched around the driving roller 24 and the driven rollers 25 to 27. A number of belt holes for attracting and holding the paper P are formed in the transport belt 23. The conveyance belt 23 sucks and holds the paper P by the suction force generated in the belt hole by driving a fan (not shown). The conveyance belt 23 rotates clockwise in FIG. 1 by driving the drive roller 24, and conveys the paper P sucked and held on the conveyance surface (upper surface) 23a in the right direction.

  The driving roller 24 and the driven rollers 25 to 27 are for the conveyance belt 23 to be stretched over. The driving roller 24 rotates the conveyor belt 23. The driven rollers 25 to 27 are driven by the driving roller 24 via the conveyance belt 23.

  The ink jet heads 22 </ b> A to 22 </ b> D print images by ejecting ink onto the paper P conveyed by the belt conveyance unit 21. The inkjet heads 22A to 22D eject inks of different colors (for example, black, cyan, magenta, yellow). The inkjet heads 22A to 22D are line types in which a plurality of nozzles are arranged in a direction substantially orthogonal to the conveyance direction of the paper P below the inkjet heads 22A to 22D. The ink jet heads 22 </ b> A to 22 </ b> D are disposed above the belt conveyance unit 21 and spaced apart from each other at a predetermined interval in the left-right direction.

  The upper surface transport unit 4 transports the paper P transported by the belt transport unit 21 so as to make a U-turn from the right to the left. The upper surface conveyance unit 4 includes a plurality of pairs of upper surface conveyance rollers 31 and a plurality of motors (not shown) that rotationally drive the plurality of pairs of upper surface conveyance rollers 31.

  The upper surface conveyance roller 31 nips and conveys the paper P. The plurality of pairs of upper surface transport rollers 31 are arranged along a normal path RC between the transport printing unit 3 and the paper discharge unit 5. The pair of upper surface transport rollers 31 is disposed upstream of the reverse path RR.

  The paper discharge unit 5 discharges and stacks the printed paper P. The paper discharge unit 5 includes a switching unit 41, a paper discharge roller 42, a paper discharge table 43, a solenoid (not shown) that drives the switching unit 41, and a motor (not shown) that rotates the paper discharge roller 42. ).

  The switching unit 41 is disposed at a branch point between the paper discharge path RD and the reversal path RR, and switches the transport path of the paper P between the paper discharge path RD and the reverse path RR. The paper discharge roller 42 is arranged between the switching unit 41 and the paper discharge table 43, conveys the paper P conveyed by the upper surface conveyance unit 4 along the paper discharge path RD, and discharges it to the paper discharge table 43. To do. The paper discharge tray 43 stacks the paper P conveyed by the paper discharge roller 42.

  The reversing unit 6 reverses the single-side printed paper P and conveys it to the registration roller 16 in order to re-feed the single-side printed paper P to the conveyance printing unit 3 during duplex printing. The reversing unit 6 rotates the reversing roller 51, the switchback unit 52, the refeed roller 53, the switching gate 54, a motor (not shown) that rotates the reversing roller 51, and the refeed roller 53. A motor to be driven (not shown).

  The reverse roller 51 temporarily transports the paper P transported by the upper surface transport unit 4 to the switchback unit 52 and then transports it to the refeed roller 53. The reverse roller 51 is disposed on a reverse path RR between the uppermost transport roller 31 on the most downstream side and the carry-in port of the switchback unit 52.

  The switchback portion 52 is a space for the reversing roller 51 to temporarily carry in the paper P, and is formed below the paper discharge platform 43. The switchback portion 52 is opened in the vicinity of the reversing roller 51 for carrying the paper P therein.

  The refeed roller 53 conveys the paper P conveyed by the reversing roller 51 to the registration roller 16. The refeed roller 53 is disposed on the reverse path RR between the reverse roller 51 and the registration roller 16.

  The switching gate 54 guides the paper P conveyed by the upper surface conveyance roller 31 to the reverse roller 51. Further, the switching gate 54 guides the paper P carried out from the switchback unit 52 by the reversing roller 51 to the refeed roller 53. The switching gate 54 is disposed in the vicinity of the center of gravity at three locations of the uppermost transport roller 31, the reverse roller 51, and the refeed roller 53 on the most downstream side.

  The sheet sensor 7 is disposed between the registration roller 16 and the first movement amount sensor 8A, and detects the sheet P conveyed on the normal route RC. The paper sensor 7 outputs an ON signal when the paper P exists at the detection target position, and outputs an OFF signal when the paper P does not exist.

  The first to third motion amount sensors 8 </ b> A to 8 </ b> C are arranged above the belt transport unit 21 and detect the motion amount in the transport direction of the paper P transported on the normal route RC by the belt transport unit 21. Specifically, the first to third motion amount sensors 8A to 8C irradiate the moving paper P with a laser beam to pick up an image, and a specific pattern (for example, an uneven pattern on the surface of the paper P) is obtained. ) To detect the amount of movement of the paper P. In a state where there is no paper P at the imaging target position, the first to third motion amount sensors 8A to 8C capture the transport surface 23a of the transport belt 23 and detect the motion amount. The first to third motion amount sensors 8A to 8C correspond to a motion amount detection unit in claims.

  The first to third motion amount sensors 8A to 8C are spaced apart from each other in the left-right direction and are arranged at substantially equal intervals. Here, as shown in FIG. 3, the distance between the adjacent first motion amount sensor 8A and the second motion amount sensor 8B and the distance between the second motion amount sensor 8B and the third motion amount sensor 8C. Let the distance be the inter-sensor distance Ls. The first to third motion amount sensors 8A to 8C are arranged such that the inter-sensor distance Ls is smaller than the minimum sheet length (minimum print medium length) Lmin. The minimum sheet length Lmin is the value of the sheet length Lp of the sheet P having the smallest sheet length Lp that is the length in the transport direction among the various sheets P used in the printing apparatus 1.

  Of the first to third motion amount sensors 8A to 8C, the most upstream first motion amount sensor 8A is disposed upstream of the most upstream ink jet head 22A. The most downstream third motion amount sensor 8C is disposed on the downstream side of the most downstream inkjet head 22D. That is, all the inkjet heads 22A to 22D are arranged between the first motion amount sensor 8A and the third motion amount sensor 8C in the left-right direction.

  The first motion amount sensor 8 </ b> A includes an imaging unit 61 and a motion amount calculation unit 62. The second and third motion amount sensors 8B and 8C have the same configuration as the first motion amount sensor 8A.

  The imaging unit 61 captures the moving paper P or the conveyance belt 23 and generates an image. A schematic configuration of the imaging unit 61 is shown in FIG. The imaging unit 61 includes a laser diode 65, condenser lenses 66 and 67, and an area sensor 68.

  The laser diode 65 emits laser light that irradiates the paper P to be imaged or the transport belt 23.

  The condensing lens 66 condenses the laser light emitted from the laser diode 65 on the paper P or the conveyance belt 23. The condensing lens 67 condenses the reflected light, which is the laser light reflected by the paper P or the conveyance belt 23, on the area sensor 68.

  The area sensor 68 has an image sensor such as a CCD (Charge Coupled Device), receives reflected light from the paper P or the conveyor belt 23, converts the received light into an electrical signal, and based on the electrical signal, an image is obtained. Is generated.

  The movement amount calculation unit 62 calculates the amount of movement of the paper P or the conveyance surface 23a of the conveyance belt 23 by calculating how much the specific pattern in the image obtained by the imaging unit 61 has moved by a correlation function. . The motion amount calculation unit 62 generates a pulse signal for each predetermined motion amount, and outputs this to the control unit 9. That is, the motion amount calculation unit 62 outputs a pulse signal indicating the motion amount of the paper P, such as an encoder pulse signal.

  The control unit 9 includes a CPU, a memory (both not shown) and the like, and controls each unit of the printing apparatus 1. At the time of printing, the control unit 9 selects a movement amount sensor that detects the movement amount of the paper P from the first to third movement amount sensors 8A to 8C according to the movement of the conveyed paper P. The ejection control of the ink jet heads 22A to 22D is performed using the motion amount of the paper P detected by the selected motion amount sensor.

  Next, the operation of the printing apparatus 1 will be described.

  The printing operation of the printing apparatus 1 starts when print data is input from a PC (personal computer), for example.

  First, the control unit 9 controls the paper feed unit 2 to take out the paper P from either the external paper feed tray 11 or the plurality of internal paper feed trays 13 and transport the paper P toward the registration rollers 16. Further, the control unit 9 rotationally drives the drive roller 24 of the belt conveyance unit 21. As a result, the conveyor belt 23 is driven to rotate. Further, the control unit 9 starts driving the paper sensor 7 and the first to third movement amount sensors 8A to 8C.

  When the sheet P hits the registration roller 16 and a predetermined amount of slack is formed on the sheet P, the control unit 9 stops driving the external paper feed roller 12 or the vertical conveyance roller 15 and drives the registration roller 16. To start. As a result, the registration roller 16 sends the paper P toward the belt conveyance unit 21 while eliminating the slack. The belt conveyance unit 21 conveys the paper P sent from the registration rollers 16 while holding it on the conveyance surface 23 a of the conveyance belt 23.

  The first to third motion amount sensors 8 </ b> A to 8 </ b> C image the transport surface 23 a of the transported paper P or the transport belt 23 by the imaging unit 61. Then, the motion amount calculation unit 62 calculates the motion amount of the paper P or the transport surface 23 a of the transport belt 23 and outputs a pulse signal corresponding to the motion amount to the control unit 9.

  The controller 9 causes the ink jet heads 22 </ b> A to 22 </ b> D to eject ink onto the paper P transported by the transport belt 23. At this time, the control unit 9 performs discharge control of the inkjet heads 22A to 22D according to the movement of the paper P using the first to third movement amount sensors 8A to 8C. The discharge control of the inkjet heads 22A to 22D using the first to third motion amount sensors 8A to 8C will be described later.

  When the paper length Lp of the paper P used for printing is larger than the distance between the first motion amount sensor 8A and the third motion amount sensor 8C, that is, when Lp> 2 × Ls, the inkjet heads 22A to 22A. It is not necessary to use the second motion amount sensor 8B for 22D discharge control. If Lp> 2 × Ls, when the trailing edge of the paper P passes through the first movement amount sensor 8A, the leading edge of the paper P reaches the third movement amount sensor 8C, and the control unit 9 This is because the movement amount of the paper P can be continuously acquired by the movement amount sensor 8A and the third movement amount sensor 8C. Therefore, the control unit 9 can omit the driving of the second motion amount sensor 8B when Lp> 2 × Ls. The control unit 9 can determine the paper length Lp of the paper P used for printing from the type of the paper P used for printing. The type of paper P used for printing is instructed by print data from a PC, for example.

  In the case of single-sided printing, the paper P printed by the conveyance printing unit 3 is sent from the belt conveyance unit 21 to the upper surface conveyance unit 4 and conveyed by the upper surface conveyance unit 4. Then, the paper P is guided to the paper discharge roller 42 by the switching unit 41 of the paper discharge unit 5 and discharged onto the paper discharge tray 43.

  In the case of double-sided printing, the paper P printed on one side is guided from the upper surface transport unit 4 to the reversing unit 6 by the switching unit 41 as in the case of single-sided printing. In the reversing unit 6, the paper P is guided to the reversing roller 51 by the switching gate 54, and is carried into the switchback unit 52 by the reversing roller 51. Thereafter, the paper P is unloaded from the switchback unit 52 by the reversing roller 51 and guided to the refeed roller 53 by the switching gate 54. Next, the paper P is conveyed to the registration roller 16 by the refeed roller 53 and is sent out to the belt conveyance unit 21 by the registration roller 16. Here, since the paper P is reversed by the reversing unit 6, the unprinted surface (the other surface) is directed to the inkjet heads 22A to 22D. After printing on the other surface of the paper P by the ink jet heads 22 </ b> A to 22 </ b> D, the paper P that has been printed on both sides is transported by the upper surface transport unit 4 and discharged to the paper discharge tray 43 by the paper discharge unit 5.

  Next, ejection control of the ink jet heads 22A to 22D using the first to third motion amount sensors 8A to 8C during the printing operation described above will be described. Here, a case where the paper length Lp of the paper P used for printing is equal to or smaller than the distance between the first motion amount sensor 8A and the third motion amount sensor 8C, that is, a case where Lp ≦ 2 × Ls will be described. . In this case, the control unit 9 uses all of the first to third motion amount sensors 8A to 8C for the ejection control of the inkjet heads 22A to 22D.

  FIG. 5 is a flowchart for explaining ejection control of the inkjet heads 22A to 22D using the first to third motion amount sensors 8A to 8C.

  When the printing operation is started, in step S10 in FIG. 5, the control unit 9 determines whether or not the output signal of the paper sensor 7 has become an ON signal (whether the leading edge of the paper P has been detected). When it is determined that the output signal of the paper sensor 7 is an OFF signal (step S10: NO), the control unit 9 repeats the process of step S10.

  When it is determined that the output signal of the paper sensor 7 has become an ON signal (step S10: YES), in step S20, the control unit 9 starts ejection control using the motion amount detected by the first motion amount sensor 8A. To do.

  Specifically, the control unit 9 generates an ejection pulse signal based on a pulse signal indicating the movement amount of the paper P input from the first movement amount sensor 8A. The ejection pulse signal is a clock signal for determining the ejection driving timing of the inkjet heads 22A to 22D. Specifically, when the pulse input from the first motion amount sensor 8A first after the output signal of the paper sensor 7 becomes the ON signal is a pulse Pa (1) shown in FIG. The counting by the built-in timer is started from the time when Pa (1) rises. And the control part 9 memorize | stores count value Ca (1) until the rise of pulse Pa (2) input next. The control unit 9 generates a pulse Pt (1) with a duty ratio of 50% with the time corresponding to the count value Ca (1) as the period T (1). That is, the pulse width τ (1) of the pulse Pt (1) is ½ of the period T (1). Here, since the control unit 9 generates the pulse Pt (1) after acquiring the count value Ca (1), the pulse Pt (1) is generated by the pulse Pa (1) from the corresponding first motion amount sensor 8A. ) Appears later in time. If a CPU having a high processing speed is used as the control unit 9, this delay can be shortened.

  Similarly, the control unit 9 generates pulses Pt (2), Pt (3),... From the pulses Pa (2), Pa (3),. This is an ejection pulse signal. The pulse widths τ (2) and τ (3) and the periods T (2) and T (3) of the pulses Pt (2) and Pt (3) are calculated from the count values Ca (2) and Ca (3). Value.

  Based on the number of ejection pulse signals from the time when the output signal of the paper sensor 7 becomes the ON signal and the image data to be printed, the control unit 9 applies ink from each nozzle of the inkjet heads 22A to 22D. Control the discharge. The controller 9 presets the number of pulses corresponding to the distance from the paper sensor 7 to each nozzle as the number of pulses indicating the timing at which ink is ejected from each nozzle. As shown in FIG. 7, until the leading edge of the paper P reaches the first movement amount sensor 8A, the first movement amount sensor 8A detects the movement amount of the conveyance surface 23a of the conveyance belt 23. become. Here, FIG. 7 is a diagram illustrating the timing of paper detection in the paper sensor and the first to third motion amount sensors when only one page is passed.

  Next, in step S <b> 30, the control unit 9 determines whether or not the time t <b> 1 has elapsed from the time when the output signal of the paper sensor 7 becomes the ON signal. The time t1 is a time set in advance as the time from when the leading edge of the paper P reaches the paper sensor 7 to the second movement amount sensor 8B at the conveyance speed of the belt conveyance unit 21. When determining that the time t1 has not elapsed (step S30: NO), the control unit 9 repeats the process of step S30.

  When it is determined that the time t1 has elapsed (step S30: YES), in step S40, the control unit 9 switches the motion amount sensor used for the discharge control from the first motion amount sensor 8A to the second motion amount sensor 8B. In addition, the control unit 9 starts correcting the discharge timing described later.

  As described above, since the inter-sensor distance Ls is smaller than the minimum sheet length Lmin, Ls <Lp. Therefore, as shown in FIGS. 7 and 8, when the leading edge of the paper P reaches the second motion amount sensor 8B, the first motion amount sensor 8A also detects the paper P. Here, by switching the movement amount sensor used for the discharge control from the first movement amount sensor 8A to the second movement amount sensor 8B, the control unit 9 can continuously acquire the movement amount of the paper P and perform discharge control.

  The time t1 may be a value larger than the theoretical value of the time required to transport the paper P from the paper sensor 7 to the second movement amount sensor 8B at the transport speed of the belt transport unit 21. This makes it possible to switch the movement amount sensor used for the discharge control from the first movement amount sensor 8A to the second movement amount sensor 8B after the paper P has reliably reached the second movement amount sensor 8B.

  The controller 9 performs ejection based on the motion amount of the paper P detected by the second motion amount sensor 8B during the period in which the paper P is detected by both the first motion amount sensor 8A and the second motion amount sensor 8B. The timing is corrected using the movement amount of the paper P detected by the first movement amount sensor 8A.

  The ejection timing correction will be described. As described above, the controller 9 controls the ejection pulse signal based on the pulse signal input from the first motion amount sensor 8A until the time t1 elapses from the time when the output signal of the paper sensor 7 becomes the ON signal. Is generated. That is, as shown in FIG. 9, the count values Ca (m−2) and Ca (m−1) corresponding to the pulses Pa (m−2) and Pa (m−1) input from the first motion amount sensor 8A. ), Pulses Pt (m−2) and Pa (m−1) of the ejection pulse signal are generated. The pulse widths τ (m−2) and τ (m−1) and the periods T (m−2) and T (m−1) of the pulses Pt (m−2) and Pt (m−1) are counted values. The value is calculated from Ca (m-2) and Ca (m-1).

  When the time t1 has elapsed, the control unit 9 adds a pulse input from the first motion amount sensor 8A to the count value Cb (n) corresponding to the pulse Pb (n) input from the second motion amount sensor 8B. The count value Ca (m) corresponding to Pa (m) is also used to generate the pulse Pt (m) of the ejection pulse signal. Specifically, the control unit 9 calculates an average value (Cb (n) + Ca (m)) / 2 of the count value Cb (n) and the count value Ca (m). Then, the control unit 9 sets a time corresponding to the average value (Cb (n) + Ca (m)) / 2 as a period T (m), and a pulse Pt having a pulse width τ (m) = T (m) / 2. (M) is generated. Thereafter, similarly, the control unit 9 determines the pulses Pt (m + 1) and Pt (m + 2) of the ejection pulse signal from the pulses Pb (n + 1), Pb (n + 2),... And the pulses Pa (m + 1), Pa (m + 2),. , ... are generated.

  In this way, the control unit 9 corrects the ejection timing based on the pulse signal input from the second motion amount sensor 8B using the pulse signal input from the first motion amount sensor 8A. In other words, the control unit 9 controls the discharge timing using the average of the motion amounts detected by the first motion amount sensor 8A and the second motion amount sensor 8B.

  Next, in step S50, the control unit 9 determines whether or not the time t2 has elapsed from the time when the output signal of the paper sensor 7 becomes the ON signal. The time t2 is set in advance as the time from when the leading edge of the paper P reaches the paper sensor 7 until the trailing edge of the paper P reaches the first movement amount sensor 8A at the conveyance speed of the belt conveyance unit 21. It's time. The time t2 varies depending on the paper length Lp of the paper P to be used. When determining that the time t2 has not elapsed (step S50: NO), the control unit 9 repeats the process of step S50.

  When it is determined that the time t2 has elapsed (step S50: YES), in step S60, the control unit 9 ends the above-described correction of the ejection timing. As shown in FIGS. 7 and 10, when the trailing edge of the paper P reaches the first motion amount sensor 8A, the motion amount detected by the first motion amount sensor 8A is not the paper P but the transport belt 23. This is the amount of movement of the surface 23a. Therefore, when the time t2 has elapsed, the control unit 9 performs discharge control using only the second motion amount sensor 8B. Specifically, as shown in FIG. 9, when the time t2 elapses, the control unit 9 counts Cb (corresponding to pulses Pb (n + i + 1) and Pb (n + i + 2) input from the second motion amount sensor 8B. Based on (n + i + 1) and Cb (n + i + 2), pulses Pt (m + i + 1) and Pt (m + i + 2) of the ejection pulse signal are generated.

  The time t2 is a theoretical value of the time from when the leading edge of the paper P reaches the paper sensor 7 until the trailing edge of the paper P reaches the first movement amount sensor 8A at the conveyance speed of the belt conveyance unit 21. It is good also as a larger value. Thereby, the correction of the ejection timing can be ended after the trailing edge of the sheet P has surely passed through the first movement amount sensor 8A.

  Next, in step S70, the control unit 9 determines whether or not the time t3 has elapsed since the output signal of the paper sensor 7 became an ON signal. The time t3 is a time set in advance as the time from when the leading edge of the paper P reaches the paper sensor 7 until it reaches the third motion amount sensor 8C at the transport speed of the belt transport unit 21. When determining that the time t3 has not elapsed (step S70: NO), the control unit 9 repeats the process of step S70.

  When it is determined that the time t3 has elapsed (step S70: YES), in step S80, the control unit 9 switches the motion amount sensor used for the discharge control from the second motion amount sensor 8B to the third motion amount sensor 8C. Further, the control unit 9 starts correcting the ejection timing. As shown in FIGS. 7 and 11, when the leading edge of the paper P reaches the third motion amount sensor 8C, the second motion amount sensor 8B also detects the paper P.

  Here, the correction of the discharge timing performed by the control unit 9 is to correct the discharge timing based on the pulse signal input from the third motion amount sensor 8C using the pulse signal input from the second motion amount sensor 8B. is there. Since the content of this correction process is the same as the process content described in step S40 above, the description thereof is omitted.

  The time t3 may be a value larger than the theoretical value of the time required for transporting the paper P from the paper sensor 7 to the third movement amount sensor 8C at the transport speed of the belt transport unit 21. This makes it possible to switch the movement amount sensor used for the discharge control from the second movement amount sensor 8B to the third movement amount sensor 8C after the paper P has reliably reached the third movement amount sensor 8C.

  Next, in step S90, the control unit 9 determines whether or not the time t4 has elapsed since the output signal of the paper sensor 7 became an ON signal. The time t4 is set in advance as the time from when the leading edge of the paper P reaches the paper sensor 7 until the trailing edge of the paper P reaches the second movement amount sensor 8B at the conveyance speed of the belt conveyance unit 21. It's time. The time t4 varies depending on the paper length Lp of the paper P to be used. When determining that the time t4 has not elapsed (step S90: NO), the control unit 9 repeats the process of step S90.

  When it is determined that the time t4 has elapsed (step S90: YES), in step S100, the control unit 9 ends the ejection timing correction started in step S80. As shown in FIGS. 7 and 12, when the trailing edge of the paper P reaches the second motion amount sensor 8B, the motion amount of the paper P is detected only by the third motion amount sensor 8C. Therefore, when the time t4 has elapsed, the control unit 9 performs discharge control using only the third motion amount sensor 8C. That is, the control unit 9 generates an ejection pulse signal based on the pulse signal input from the third motion amount sensor 8C.

  The time t4 is a theoretical value of the time from when the leading edge of the paper P reaches the paper sensor 7 until the trailing edge of the paper P reaches the second movement amount sensor 8B at the conveyance speed of the belt conveyance unit 21. It is good also as a larger value. Thus, the ejection timing correction can be completed after the trailing edge of the paper P has surely passed the second motion amount sensor 8B.

  In step S110, the control unit 9 determines whether or not ink ejection based on the image data to be printed has been completed. When it is determined that ink ejection has not ended (step S110: NO), the control unit 9 repeats the process of step S110. When it is determined that the ink ejection has ended (step S110: YES), the control unit 9 ends the ejection control of the inkjet heads 22A to 22D.

  Thus, printing for one page is completed. When printing a plurality of pages, the process of the flowchart of FIG. 5 described above is performed for each page.

  As described above, the printing apparatus 1 according to the present embodiment includes the first to third motion amount sensors 8A to 8C. The first to third motion amount sensors 8A to 8C are arranged such that the inter-sensor distance Ls is smaller than the minimum sheet length Lmin. All the inkjet heads 22A to 22D are disposed between the first motion amount sensor 8A and the third motion amount sensor 8C in the left-right direction. And the control part 9 selected the motion amount sensor which has detected the motion amount of the paper P from the 1st-3rd motion amount sensors 8A-8C according to the movement of the conveyed paper P, and selected The ejection control of the ink jet heads 22A to 22D is performed using the motion amount of the paper P detected by the motion amount sensor. Thereby, in the printing apparatus 1, since the discharge control according to the actual motion of the paper P can be performed until the paper P passes through the most downstream ink jet head 22D, it is possible to suppress the deterioration of the print image quality.

  Further, when the movement amount sensor used for ejection control is switched, the control unit 9 uses the movement amount of the paper P detected by the movement amount sensor before switching, and the paper detected by the motion amount sensor after switching. The ejection timing based on the amount of movement of P is corrected. That is, during the period in which the plurality of motion amount sensors detect the movement amount of the paper P, the ejection of the inkjet heads 22A to 22D using the average of the movement amounts of the paper P detected by the plurality of motion amount sensors. Take control. Thereby, more appropriate discharge control can be performed with respect to the behavior of the paper P. For example, there may be a difference in the behavior of the paper P between the front end side and the rear end side due to the influence of the rear end portion of the paper P coming off the registration roller 16. Even in such a case, by performing the above-described correction of the discharge timing, it is possible to perform appropriate discharge control and suppress a decrease in print image quality.

  Note that the above-described correction of the ejection timing may be omitted. That is, the control unit 9 may sequentially switch and select one motion amount sensor used for ejection control of the inkjet heads 22A to 22D according to the movement of the paper P. In this case, for example, in the example of FIG. 9, the control unit 9 performs ejection based only on the pulses Pb (n), Pb (n + 1),... Input from the second motion amount sensor 8B after the time t1 has elapsed. Start generating pulse signals. In this way, the processing load of the discharge timing control by the control unit 9 can be reduced. For example, when the period during which the motion amount of the paper P is detected by a plurality of motion amount sensors is short, or when the behavior of the entire paper P can be approximated from the behavior of the leading edge of the paper P, the processing of the control unit 9 The discharge control according to the actual movement of the paper P can be performed while reducing the burden.

  In the present embodiment, the first to third motion amount sensors 8A to 8C are arranged at equal intervals in the left-right direction, but may not be equal intervals. Also, the number of motion sensors need not be three. Further, the number of inkjet heads is not limited to four. It is only necessary that the distance between the sensors is smaller than the minimum sheet length Lmin, and all the inkjet heads are arranged between the most upstream motion amount sensor and the most downstream motion amount sensor.

  Moreover, it is also possible to use a detector having a laser Doppler velocimeter as the first to third motion amount sensors 8A to 8C.

  The present invention is not limited to the above-described embodiments as they are, and can be embodied by modifying the components without departing from the scope of the invention in the implementation stage. Various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiments. For example, some components may be deleted from all the components shown in the embodiment.

DESCRIPTION OF SYMBOLS 1 Printing apparatus 2 Paper feed part 3 Conveyance printing part 4 Upper surface conveyance part 5 Paper discharge part 6 Inversion part 7 Paper sensor 8A-8C 1st-3rd motion amount sensor 9 Control part 21 Belt conveyance part 22A-22D Inkjet head 61 Imaging Unit 62 Motion amount calculation unit

Claims (3)

A transport unit for transporting the print medium;
One or more inkjet heads that eject ink onto a print medium conveyed by the conveyance unit;
A plurality of movement amount detection units for detecting the movement amount of the print medium conveyed by the conveyance unit;
A control unit that performs ejection control of the inkjet head,
The plurality of motion amount detection units are arranged so as to be spaced apart from each other in the conveyance direction of the print medium so that the distance between the adjacent motion amount detection units is smaller than a predetermined minimum print medium length,
All the inkjet heads are arranged between the most upstream motion amount detection unit and the most downstream motion amount detection unit in the transport direction,
The control unit selects a movement amount detection unit that detects a movement amount of the print medium from the plurality of movement amount detection units according to the movement of the print medium in the transport direction, and the selected movement A printing apparatus that performs ejection control of the inkjet head using a movement amount of a print medium detected by an amount detection unit.   2. The control unit according to claim 1, wherein the control unit sequentially switches and selects one of the movement amount detection units used for discharge control of the inkjet head in accordance with the movement of the print medium in the transport direction. The printing apparatus as described.   The control unit uses the average of the movement amounts of the print medium detected by the plurality of movement amount detection units in a period in which there are a plurality of the movement amount detection units that detect the movement amount of the print medium. The printing apparatus according to claim 1, wherein ejection control of the inkjet head is performed.