Inkjet printing device

Abstract

To provide a technology capable of improving printing quality by making a switched boundary difficult to notice while ensuring that droplets are properly discharged in the case of switching a waveform of a drive signal to be supplied to a discharge part in accordance with a change in a state during discharging.SOLUTION: An inkjet printing device includes a discharge part, a detection part for detecting one of a conveyance speed of a printing medium and an ink temperature in an ink chamber, and a drive signal generation part 64. If one of the conveyance speed and the ink temperature exceeds a first threshold to change, the drive signal generation part generates a drive signal Ds by multiplying an image signal Is by a mixed waveform obtained by mixing an unchanged basic waveform being a basic waveform W corresponding to one of an unchanged conveyance speed and the ink temperature with a changed basic waveform being the basic waveform W corresponding to one of a changed conveyance speed and the ink temperature to generate the drive signal Ds, supplies the drive signal to the discharge part during a prescribed period including at that point time, thereby, properly discharges droplets and can achieve printing quality that makes a switched boundary difficult to notice.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present invention relates to an inkjet printing apparatus that performs printing on a print medium such as paper using an inkjet method.

[0002] Inkjet printing devices are known that print characters and images by ejecting ink droplets onto a print medium transported in a predetermined direction. Such printing devices include an ejection unit that ejects the ink droplets, and a control unit that generates a drive signal for driving the ejection unit.

[0003] When printing is performed using an inkjet printing device, the transport speed of the print medium may change. Also, the temperature of the ink stored in the ejection section before ejection may change. When the conditions during ejection change in this way, the waveform of the drive signal supplied to the ejection section is usually switched according to the changed conditions. Here, when the waveform of the drive signal is switched, a change occurs in the density of the ink printed on the print medium as the printing result at the boundary where the drive signal is switched, and the boundary may appear as a streak. Therefore, a technology for dealing with such an event is disclosed, for example, in Patent Document 1. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5268838 Summary of the Invention [Problem to be solved by the invention]

[0005] Patent Document 1 discloses a technique for dealing with the phenomenon in which ink droplets become difficult to eject due to the effects of ink thickening, etc., when a piezoelectric element, which is an actuator for ejecting ink in an inkjet head, does not operate for a long period of time. In particular, it points out that when ejection is resumed after a period of inactivity of the actuator, the ejection speed of the first ink droplet is slow, resulting in a large difference in the ejection speed of each subsequent ink droplet (paragraph 0003).

[0006] Therefore, in Patent Document 1, when generating drive pulse voltages for the actuators (Z1, Z2, ... Zm), the level of a predetermined number of drive pulse voltages is set higher than the level of the remaining drive pulse voltages from the beginning, and the level of the predetermined number of drive pulse voltages is gradually changed in a downward direction (paragraph 0026). This makes it possible to increase the ejection speed even when the ink is in a viscous state (paragraph 0029).

[0007] However, when the level of the drive pulse voltage is gradually changed downward for a predetermined number of times, the level of the drive pulse voltage after the change may become a value or shape that has never been experienced before. Also, in general, in the ejection section, when the drive pulse voltage (drive signal) supplied to the piezoelectric element becomes a certain value or waveform (frequency), the ink droplets may not be ejected properly or the ejection position may be shifted. In other words, the method of Patent Document 1 does not guarantee that the ink droplets are ejected appropriately.

[0008] The present invention has been developed in consideration of the above circumstances, and aims to provide a technology that can appropriately eject ink droplets when switching the waveform of the drive signal supplied to the ejection section in accordance with changes in the conditions during ejection, while making the switching boundary less noticeable and improving print quality. [Means for solving the problem]

[0009] In order to solve the above problems, the first invention of the present application is an inkjet printing device that performs printing on a printing medium by an inkjet method, and includes a transport mechanism, an encoder, an ejection unit, a drive signal generation unit, an image signal supply unit, a detection unit, a basic waveform storage unit, and a basic waveform selection unit. The transport mechanism transports the printing medium in a longitudinal direction along a transport path. The encoder outputs a pulse signal according to a transport speed of the printing medium transported by the transport mechanism. The ejection unit has a nozzle that ejects ink droplets onto the printing medium transported by the transport mechanism at an ejection position on the transport path, an ink chamber that communicates with the nozzle, and a piezoelectric element that applies pressure to the ink in the ink chamber. The drive signal generation unit generates a drive signal for driving the piezoelectric element and supplies it to the ejection unit. The image signal supply unit generates an image signal indicating an image to be printed at the ejection position based on position information of the printing medium detected from the pulse signal output from the encoder, and supplies it to the drive signal generation unit. The detection unit detects one of the transport speed of the printing medium and the temperature of the ink in the ink chamber, which are related to the pulse signal. The basic waveform storage unit stores a range of the value of the one of the transport speed of the printing medium and the temperature of the ink in the ink chamber in association with the corresponding basic waveform. The basic waveform selection unit reads out the basic waveform from the basic waveform storage unit based on the detection result of the one of the transport speed of the printing medium and the temperature of the ink in the ink chamber by the detection unit, and supplies it to the drive signal generation unit. When the one of the transport speed of the printing medium and the temperature of the ink in the ink chamber changes beyond a first threshold, the drive signal generation unit generates the drive signal by multiplying the image signal by a mixed waveform obtained by mixing a pre-change basic waveform, which is the basic waveform corresponding to the one of the transport speed of the printing medium and the temperature of the ink in the ink chamber before the change, and a post-change basic waveform, which is the basic waveform corresponding to the one of the transport speed of the printing medium and the temperature of the ink in the ink chamber after the change, during a predetermined period including that time.

[0010] The second invention of the present application is an inkjet printing device of the first invention, wherein the drive signal generating unit generates the drive signal by multiplying the image signal with a mixed waveform in which the pre-transformation fundamental waveform and the post-transformation fundamental waveform are mixed at a predetermined mixing ratio.

[0011] The third invention of the present application is an inkjet printing device of the first or second invention, wherein, when one of the transport speed of the printing medium and the temperature of the ink in the ink chamber changes beyond the first threshold and then changes beyond the second threshold, the specified period is the period less than halfway between the point at which one of the transport speed of the printing medium and the temperature of the ink in the ink chamber changes beyond the first threshold to the point at which it changes beyond the second threshold.

[0012] A fourth invention of the present application is an inkjet printing device of the second invention, wherein, during the specified period, the mixing ratio changes so that the ratio of the pre-transition basic waveform gradually decreases and the ratio of the post-transition basic waveform gradually increases.

[0013] A fifth aspect of the present invention is the inkjet printing apparatus according to any one of the first to fourth aspects, wherein the one of the two is a transport speed of the print medium.

[0014] A sixth aspect of the present invention is the inkjet printing apparatus according to any one of the first to fourth aspects, wherein the one of the two is a temperature of the ink in the ink chamber. Effect of the Invention

[0015] According to the first to sixth aspects of the present application, when switching the waveform of the drive signal supplied to the ejection section in response to changes in the ejection situation, the drive signal is generated using a mixed waveform that mixes the pre-change basic waveform and the post-change basic waveform. This makes the boundary between the switched waveform less noticeable, improving print quality. In addition, because the pre-change basic waveform and the post-change basic waveform are mixed without changing the basic waveform itself, ink droplets can be ejected more appropriately than when the basic waveform itself is changed. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 is a diagram conceptually illustrating a configuration of an inkjet printing apparatus according to a first embodiment. [Diagram 2] FIG. 2 is a view showing one discharge portion according to the first embodiment as viewed from below. [Diagram 3] FIG. 2 is a diagram conceptually showing the configuration of one ejection section and one ink supply section according to the first embodiment. [Figure 4] 2 is a block diagram showing electrical connections between a control unit and each unit in the inkjet printing apparatus according to the first embodiment. FIG. [Diagram 5] FIG. 2 is a block diagram conceptually showing some of the functions of a control unit according to the first embodiment. [Figure 6] 3A to 3C are diagrams showing examples of fundamental waveforms stored in a fundamental waveform storage unit according to the first embodiment. [Figure 7] FIG. 4 is a diagram for explaining waveform switching according to the first embodiment. [Figure 8] 4 is a diagram showing an example of the ratio between the number of pre-transformation fundamental waveforms and the number of post-transformation fundamental waveforms included in a mixed waveform, which is stored in a mixture ratio storage unit according to the first embodiment. FIG. [Figure 9] 13 is a diagram conceptually showing the configuration of one ejection section and one ink supply section according to a second embodiment. FIG. [Figure 10] FIG. 11 is a block diagram conceptually showing some of the functions of a control unit according to a second embodiment. [Figure 11] 13 is a diagram showing an example of a fundamental waveform stored in a fundamental waveform storage unit according to the second embodiment. FIG. [Figure 12] FIG. 11 is a diagram for explaining waveform switching according to the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Note that the components described in the embodiment are merely examples, and are not intended to limit the scope of the present invention. In addition, in the drawings, the dimensions and numbers of each part may be exaggerated or simplified as necessary for ease of understanding.

[0018] <1. First embodiment> <1-1. Configuration of Inkjet Printing Device> FIG. 1 is a conceptual diagram showing the configuration of an inkjet printing device 1 according to a first embodiment of the present invention. The inkjet printing device 1 is an inkjet printer that conveys a long strip of continuous paper 9 and ejects droplets of water-based ink from a plurality of ejection units 21 toward the continuous paper 9, thereby recording characters and images on the recording surface of the continuous paper 9. However, the long strip of continuous paper 9 is merely one example of a printing medium. The printing medium may be cut paper, or may be a plastic film, cardboard, metal foil, or a glass substrate. In other words, the inkjet printing device 1 may be a device that prints on a printing medium by an inkjet method. As shown in FIG. 1, the inkjet printing device 1 has a conveying mechanism 10, a printing unit 20, an encoder 30, and a control unit 40.

[0019] The transport mechanism 10 transports the continuous paper 9 in a transport direction along its longitudinal direction. That is, the transport mechanism 10 transports the continuous paper 9 in the longitudinal direction along a transport path. The transport mechanism 10 has a number of rollers including an unwinding roller 11, a number of transport rollers 12, and a take-up roller 13. The continuous paper 9 is hung around these rollers under tension. This prevents the continuous paper 9 from sagging or wrinkling during transport.

[0020] The transport mechanism 10 also has a motor (not shown) that rotates at least one roller (hereinafter referred to as the "drive roller") of the multiple transport rollers 12. By driving the motor, each transport roller 12, including the drive roller, rotates about an axis parallel to the width direction of the continuous paper 9 (a direction perpendicular and horizontal to the transport direction). As a result, the continuous paper 9 is unwound from the unwind roller 11 and guided to the downstream side of the transport path formed by the multiple transport rollers 12. After being transported, the continuous paper 9 is collected by the take-up roller 13.

[0021] The printing unit 20 has a plurality of (four in this embodiment) ejection units 21 and a plurality of (four in this embodiment) ink supply units 22. The four ejection units 21 are arranged at intervals along the transport direction of the continuous paper 9. The four ejection units 21 are arranged above each of the ejection positions P1, P2, P3, and P4 on the transport path. The four ejection units 21 have the same structure. The continuous paper 9 moves below the four ejection units 21, approximately parallel to the arrangement direction of the four ejection units 21. At this time, the recording surface of the continuous paper 9 faces the ejection units 21. In other words, the recording surface of the continuous paper 9 faces upward.

[0022] Each of the four ejection units 21 is a unit that ejects ink droplets from a nozzle 83 (see FIG. 2 and FIG. 3 described later) toward the recording surface (upper surface) of the continuous paper 9 transported by the transport mechanism 10. In this embodiment, the four ejection units 21 eject ink droplets of different colors (for example, cyan, magenta, yellow, and black) to record monochromatic images on the recording surface of the continuous paper 9. Then, a multicolor image is formed on the recording surface of the continuous paper 9 by superimposing the four monochromatic images.

[0023] FIG. 2 is a view of one ejection section 21 as viewed from below. In FIG. 2, the continuous paper 9 is indicated by a virtual line (two-dot chain line). As shown enlarged in FIG. 2, a plurality of nozzles 83 capable of ejecting ink droplets are provided on the lower surface of the ejection section 21. In this embodiment, the plurality of nozzles 83 are two-dimensionally arranged in the transport direction and the width direction on the lower surface of the ejection section 21. In addition, each nozzle 83 is arranged with its position shifted in the width direction. In this way, by arranging the plurality of nozzles 83 two-dimensionally, the positions of each nozzle 83 in the width direction can be made close to each other. However, the plurality of nozzles 83 may be arranged in a line along the width direction.

[0024] FIG. 3 is a diagram conceptually illustrating the configuration of one ejection section 21 and one ink supply section 22. In this embodiment, each ejection section 21 is composed of a plurality of (five in this embodiment) heads 80 adjacent to each other. The five heads 80 have the same structure. For this reason, in FIG. 3, only one head 80 out of the five heads 80 is illustrated in detail, and the remaining four heads 80 are illustrated in a simplified manner. As shown in FIG. 3, the nozzle 83 is a hole provided in the lower part of the head 80. The nozzle 83 faces the recording surface (upper surface) of the continuous paper 9 transported by the transport mechanism 10 in the up-down direction. In addition, the head 80 has one ink chamber 81 and one piezoelectric element 82 as an actuator for each nozzle 83. The head 80 has multiple sets of the nozzle 83, the ink chamber 81, and the piezoelectric element 82.

[0025] An internal tank 85 is provided inside a housing 84 that forms the outer frame of the head 80. The internal tank 85 is located above the multiple nozzles 83 and the multiple ink chambers 81, and is a space capable of temporarily storing ink. Each of the multiple ink chambers 81 in one head 80 communicates with the internal tank 85. Each ink chamber 81 is a cavity that communicates with the nozzle 83. The piezoelectric element 82 is provided on the wall surface of the ink chamber 81.

[0026] When ink is ejected, the ink flows down from the internal tank 85 to the ink chamber 81. Then, a drive signal Ds, which will be described later, is supplied to the ejection unit 21. Here, the drive signal Ds is a signal obtained by connecting drive voltages (pulse-shaped waveforms) applied to the piezoelectric element 82 in a time series in order to drive the piezoelectric element 82. The piezoelectric element 82 expands toward the ink chamber 81 in response to the drive signal Ds supplied. This applies pressure to the ink in the ink chamber 81. As a result, ink droplets are ejected from the nozzle 83. That is, the multiple nozzles 83 of the multiple ejection units 21 eject ink droplets onto the recording surface of the continuous paper 9 transported by the transport mechanism 10 at each of the ejection positions P1 to P4 on the transport path of the continuous paper 9.

[0027] As described above, the four ejection units 21 eject ink droplets of different colors (for example, cyan, magenta, yellow, and black) in response to the drive signal Ds from the multiple nozzles 83. Then, a multi-color image is recorded on the recording surface of the continuous paper 9 by superimposing monochromatic images formed by the ink droplets of these colors.

[0028] A drying processing unit that dries the ink ejected onto the recording surface of the continuous paper 9 may be further provided downstream in the transport direction of the four ejection units 21. The drying processing unit dries the ink by, for example, blowing heated gas toward the continuous paper 9 to vaporize the solvent in the ink adhering to the continuous paper 9. However, the drying processing unit may also harden or dry the ink by other methods such as light irradiation.

[0029] Next, a brief description will be given of the structure of the ink supply unit 22. The ink supply unit 22 is a device for supplying ink to the ejection unit 21 while circulating a portion of the ink. As described above, the inkjet printing device 1 of this embodiment has four ink supply units 22. Each of the four ink supply units 22 is connected to one ejection unit 21. The four ink supply units 22 have the same structure.

[0030] 3, each ink supply unit 22 has a storage tank 221, a supply pump 222, a reflux pump 224, a pipe 225, a filter 226, and a plurality (five in this embodiment) of on-off valves 227. The pipe 225 includes a first supply pipe 91, a plurality (five in this embodiment) of second supply pipes 92, a plurality (five in this embodiment) of first reflux pipes 93, and a second reflux pipe 94.

[0031] The storage tank 221 is a container that stores ink. The storage tank 221 is equipped with a temperature adjustment mechanism (not shown) for adjusting the temperature of the stored ink. The storage tank 221 is connected to each head 80 of one ejection unit 21 via a first supply pipe 91 and a second supply pipe 92 branching off from the first supply pipe 91. A supply pump 222 and a filter 226 are inserted in the first supply pipe 91.

[0032] The supply pump 222 generates a flow of ink inside the first supply pipe 91 from the storage tank 221 toward each head 80 in accordance with an operation signal from the control unit 40. As a result, the ink that has been stored inside the storage tank 221 and has been temperature-adjusted is supplied to the internal tank 85 of each head 80 via the first supply pipe 91 and the second supply pipe 92. In addition, the filter 226 removes solid components and foreign matter from the ink passing through the first supply pipe 91. This prevents solid components and foreign matter from being mixed into the ink supplied to each head 80.

[0033] Each head 80 is equipped with a liquid level sensor (not shown). The liquid level sensor is a sensor that detects the height of the liquid level of the ink stored in the internal tank 85. The control unit 40 detects the height of the ink level in the internal tank 85 based on a signal from the liquid level sensor, and determines whether or not to supply ink to the internal tank 85. When supplying ink from the storage tank 221 to the internal tank 85, the control unit 40 operates the supply pump 222.

[0034] The five first return pipes 93 and the second return pipes 94 are pipes that connect the five heads 80 and the storage tank 221. An on-off valve 227 is interposed in each of the first return pipes 93. When the on-off valve 227 is in a closed state, communication of the first return pipes 93 in which the on-off valve 227 is interposed is blocked. On the other hand, when the on-off valve 227 is in an open state, communication of the first return pipes 93 in which the on-off valve 227 is interposed is ensured.

[0035] In addition, a reflux pump 224 is interposed in the second reflux pipe 94. The reflux pump 224 generates a flow of ink from each of the first reflux pipes 93 toward the storage tank 221 inside the second reflux pipe 94 according to an operation signal from the control unit 40. As a result, the ink stored in the internal tank 85 of each head 80 (ink that has not been ejected for a long period of time and has remained in the internal tank 85 and whose temperature has significantly decreased) is returned to the storage tank 221 via the first reflux pipe 93 and the second reflux pipe 94. This makes it possible to prevent the temperature, viscosity, etc. of the ink stored in the internal tank 85 of each head 80 from changing significantly (for example, by about 10° C. or more). As a result, deterioration of the ink ejected from each nozzle 83 can be suppressed, and printing quality can be improved.

[0036] The encoder 30 is a sensor for detecting the transport speed of the continuous paper 9. The encoder 30 is attached to the axis of one of the multiple transport rollers 12 (the transport roller 121 in FIG. 1). The encoder 30 outputs a pulse signal En once every time the transport roller 121 rotates a predetermined angle. Therefore, the pulse signal En becomes data reflecting the change over time in the transport speed of the continuous paper 9 transported by the multiple transport rollers 12 including the transport roller 121. Therefore, when the continuous paper 9 is transported at a constant transport speed, the encoder 30 outputs successive pulse signals En at a constant cycle according to the transport speed.

[0037] The pulse signal En is transmitted from the encoder 30 to the control unit 40. That is, the encoder 30 supplies the pulse signal En corresponding to the transport speed of the continuous paper 9 transported by the transport mechanism 10 to the control unit 40. However, the encoder 30 only needs to be directly or indirectly connected to at least one of the multiple transport rollers 12, and the connection destination is not limited to the transport roller 121.

[0038] The control unit 40 is an information processing device for controlling the operation of each unit in the inkjet printing apparatus 1. Fig. 4 is a block diagram showing the electrical connection between the control unit 40 and each unit in the inkjet printing apparatus 1. As shown in Fig. 4, the control unit 40 is configured by a computer having a processor 41 such as a CPU, a memory 42 such as a RAM, and a storage unit 43 such as a hard disk drive. The storage unit 43 stores a computer program P and data D for controlling the operation of each unit and ejecting ink droplets from the ejection unit 21 onto the continuous paper 9 while transporting the continuous paper 9.

[0039] 4, the control unit 40 is communicably connected to the transport mechanism 10, the four ejection units 21 of the printing unit 20, the four ink supply units 22 of the printing unit 20, and the encoder 30, either by wire or wirelessly. The control unit 40 reads out a computer program P and data D from the storage unit 43 to the memory 42, and operates the processor 41 in accordance with the computer program P and data D. This causes the transport and printing process of the continuous paper 9 to proceed.

[0040] <1-2. Detailed configuration of the control unit functions> Next, a more detailed configuration of the functions of the control unit 40 will be described. FIG. 5 is a block diagram conceptually illustrating some of the functions of the control unit 40. As shown in FIG. 5, the control unit 40 of this embodiment has a conveying speed calculation unit 61, a basic waveform selection unit 62, a basic waveform storage unit 63, a driving signal generation unit 64, a printing position identification unit 65, an image signal supply unit 66, and a mixture ratio storage unit 67. The functions of the conveying speed calculation unit 61, the basic waveform selection unit 62, the basic waveform storage unit 63, the driving signal generation unit 64, the printing position identification unit 65, the image signal supply unit 66, and the mixture ratio storage unit 67 are realized by temporarily reading the computer program P and data D stored in the storage unit 43 into the memory 42, and having the processor 41 perform arithmetic processing based on the computer program P and data D.

[0041] As described above, the encoder 30 outputs a pulse signal En according to the conveying speed of the continuous paper 9 conveyed by the conveying mechanism 10, and supplies the signal to the control unit 40. The conveying speed calculation unit 61 calculates the measured value Ms (m / sec) of the conveying speed of the continuous paper 9 conveyed by the plurality of conveying rollers 12 including the conveying roller 121 based on the supplied pulse signal En. That is, the conveying speed calculation unit 61 detects the conveying speed of the continuous paper 9 related to the pulse signal En. The conveying speed calculation unit 61 corresponds to the "detection unit" in the first embodiment of the present invention. The conveying speed calculation unit 61 also supplies the calculated measured value Ms of the conveying speed of the continuous paper 9 to the basic waveform selection unit 62 and the drive signal generation unit 64, respectively.

[0042] When the measured value Ms of the transport speed of the continuous paper 9 is supplied from the transport speed calculation unit 61, the basic waveform selection unit 62 queries the basic waveform storage unit 63. The basic waveform storage unit 63 stores the range of values ​​of the transport speed of the continuous paper 9 and the corresponding basic waveform W, which is the minimum unit constituting the drive signal Ds, in association with each other. Here, as described above, the drive signal Ds is a signal obtained by connecting, in a time series, drive voltages (pulse-shaped waveforms) applied to the piezoelectric element 82 in order to drive the piezoelectric element 82 of the ejection unit 21. The basic waveform W is the drive voltage (pulse-shaped waveform), which is a voltage applied to the piezoelectric element 82 in order to eject one ink droplet from the nozzle 83.

[0043] 6 is a diagram showing an example of the basic waveforms W stored in the basic waveform storage unit 63. As shown in Fig. 6, for example, when the conveying speed V (m / sec) of the continuous paper 9 is "0.8≦V<1.2", the basic waveform W corresponding to this is "W1", and when "1.2≦V<1.6", the basic waveform W corresponding to this is "W2". The basic waveform selection unit 62 reads out the basic waveform W from the basic waveform storage unit 63 based on the measured value Ms of the conveying speed of the continuous paper 9 supplied from the conveying speed calculation unit 61, and supplies it to the drive signal generation unit 64.

[0044] On the other hand, the print position specifying unit 65 detects the position in the transport direction of the continuous paper 9 transported by the transport mechanism 10. Specifically, the print position specifying unit 65 specifies the position on the continuous paper 9 where an image should be printed, based on the pulse signal En supplied from the encoder 30, and supplies the specified position information Pi to the image signal supply unit 66.

[0045] The image signal supply unit 66 generates an image signal Is indicating an image to be printed by each ejection unit 21 based on the input image data Id and the position information Pi on the continuous paper 9 to be printed, which is supplied from the print position specification unit 65. Specifically, the image signal supply unit 66 converts the input image data Id into images of each color component, for example, cyan, magenta, yellow, and black. Then, based on the images of each color component, it recognizes to which position on the continuous paper 9 the ink droplets of each color of cyan, magenta, yellow, and black should be ejected. Furthermore, it determines the time when the ink droplets of these inks should be ejected. That is, the image signal Is is time-series data obtained by connecting data indicating at which time and from which ejection unit 21 the ink droplets of each color of cyan, magenta, yellow, and black should be ejected.

[0046] Further, the image signal supply unit 66 supplies the generated image signal Is to the drive signal generation unit 64. That is, the image signal supply unit 66 generates an image signal Is indicating an image to be printed at each of the ejection positions P1 to P4 based on position information Pi of the continuous paper 9 detected from the pulse signal En output from the encoder 30, and supplies the image signal Is to the drive signal generation unit 64.

[0047] The drive signal generating unit 64 generates a drive signal Ds for driving the piezoelectric element 82 and supplies it to each of the four ejection units 21. Specifically, when the measured value Ms of the conveying speed of the continuous paper 9 supplied from the conveying speed calculation unit 61 is substantially constant or when the fluctuation width of the measured value Ms of the conveying speed is within a predetermined range, the drive signal generating unit 64 generates the drive signal Ds by multiplying a basic waveform W corresponding to the measured value Ms of the conveying speed by an image signal Is supplied from the image signal supplying unit 66. More precisely, when there is no change in the basic waveform W supplied from the basic waveform selecting unit 62, the drive signal generating unit 64 generates the drive signal Ds by multiplying the basic waveform W by the image signal Is supplied from the image signal supplying unit 66.

[0048] Each of the four ejection units 21 ejects droplets of ink of each color from the nozzle 83 to an appropriate position in the transport direction on the continuous paper 9 at ejection positions P1 to P4 based on a drive signal Ds supplied from the drive signal generation unit 64. As a result, each of the four ejection units 21 records a single-color image at an appropriate location on the recording surface of the continuous paper 9. Then, a high-quality multi-color image is formed on the upper surface of the continuous paper 9 by superimposing the four single-color images.

[0049] Here, it is assumed that the conveying speed of the continuous paper 9 changes beyond the first threshold value. For example, as shown in FIG. 7, it is assumed that the conveying speed of the continuous paper 9 changes from conveying speed V=1.0 (m / sec) to conveying speed V=1.3 (m / sec) at time t1. Here, the basic waveform storage unit 63 stores that the basic waveform W corresponding to the conveying speed V of the continuous paper 9 when "0.8≦V<1.2" is "W1", and the basic waveform W corresponding to the conveying speed V of the continuous paper 9 when "1.2≦V<1.6" is "W2". In this case, the conveying speed V=1.2 (m / sec) is the first threshold value. Before time t1, the basic waveform selection unit 62 reads out the "basic waveform before change W1" as the basic waveform corresponding to the conveying speed of the continuous paper 9 before the change, and supplies it to the drive signal generation unit 64. Furthermore, after time t1, the basic waveform selection unit 62 reads out the “post-change basic waveform W2” as the basic waveform corresponding to the post-change transport speed of the continuous paper 9, and supplies it to the drive signal generation unit 64.

[0050] At this time, it is assumed that the drive signal generating unit 64 generates the drive signal Ds by multiplying the pre-change basic waveform W1 supplied from the basic waveform selecting unit 62 by the image signal Is supplied from the image signal supplying unit 66 before time t1, and each of the four ejection units 21 ejects ink droplets of each color based on the drive signal Ds. It is also assumed that the drive signal generating unit 64 generates the drive signal Ds by multiplying the post-change basic waveform W2 supplied from the basic waveform selecting unit 62 by the image signal Is supplied from the image signal supplying unit 66 after time t1, and each of the four ejection units 21 ejects ink droplets of each color based on the drive signal Ds. In this way, if the waveform of the drive signal Ds is immediately switched at time t1, a change in the density of the ink printed on the continuous paper 9 as a printing result occurs at the boundary where the switch is made, and the boundary may appear as a streak.

[0051] Therefore, the driving signal generating unit 64 generates the driving signal Ds by multiplying the mixed waveform Wm, which is a mixture of the pre-change fundamental waveform W1 supplied before time t1 and the post-change fundamental waveform W2 supplied after time t1, by the image signal Is supplied from the image signal supplying unit 66, during the period from time t1 when the driving signal Ds is switched to the subsequent time tz. The mixed waveform Wm is a waveform in which the pre-change fundamental waveform W1 and the post-change fundamental waveform W2 are connected in time series and at equal or unequal intervals. That is, when the conveying speed of the continuous paper 9 changes beyond the first threshold value at time t1, the driving signal generating unit 64 generates the driving signal Ds by multiplying the mixed waveform Wm, which is a mixture of the pre-change fundamental waveform W1 and the post-change fundamental waveform W2, by the image signal Is during a predetermined period PP including that time (time t1), and supplies the driving signal Ds to each of the four ejection units 21. By configuring the waveform of the drive signal Ds not to be switched immediately, print boundaries become less noticeable, improving print quality.

[0052] More specifically, when the drive signal generating unit 64 recognizes that the measured value Ms of the conveying speed of the continuous paper 9 supplied from the conveying speed calculating unit 61 has changed beyond the first threshold, the drive signal generating unit 64 inquires of the mixture ratio storing unit 67. Here, the mixture ratio storing unit 67 stores the range of the value of the time elapsed ratio R1 of the above-mentioned predetermined period PP and the corresponding ratio R2 of the number of pre-change fundamental waveforms W1 and the number of post-change fundamental waveforms W2 included in the mixed waveform Wm in association with each other. FIG. 8 is a diagram showing an example of the ratio R2 of the number of pre-change fundamental waveforms W1 and the number of post-change fundamental waveforms W2 included in the mixed waveform Wm stored in the mixture ratio storing unit 67.

[0053] For example, assume that the above-mentioned predetermined period PP is "1.0 seconds". In this case, as shown in FIG. 8, the driving signal generating unit 64 mixes the basic waveforms W1 and W2 included in the mixed waveform Wm so that the ratio of the number of pre-change basic waveforms W1 is 80% and the ratio of the number of post-change basic waveforms W2 is 20% in the period from 0.0 seconds to 0.1 seconds of the predetermined period PP based on the mixing ratio R2 read from the mixing ratio storage unit 67. Similarly, in each remaining period, the pre-change basic waveform W1 and the post-change basic waveform W2 are mixed at the ratio R2 shown in FIG. 8. That is, the driving signal generating unit 64 generates the driving signal Ds by multiplying the mixed waveform Wm in which the pre-change basic waveform W1 and the post-change basic waveform W2 are mixed at a predetermined mixing ratio by the image signal Is.

[0054] Also, as shown in Fig. 8, in the predetermined period PP, the mixture ratio R2 of the pre-transformation fundamental waveform W1 and the post-transformation fundamental waveform W2 changes so that the ratio of the pre-transformation fundamental waveform W1 gradually decreases and the ratio of the post-transformation fundamental waveform W2 gradually increases. This makes the print boundary less noticeable and further improves the print quality. However, in the predetermined period PP, the mixture ratio R2 of the pre-transformation fundamental waveform W1 and the post-transformation fundamental waveform W2 may be different from the state shown in Fig. 8, and may be maintained at a constant value throughout the entire predetermined period PP.

[0055] Here, the above-mentioned predetermined period PP will be described in detail. It is assumed that the transport speed of the continuous paper 9 changes beyond the first threshold at time t1, and then changes beyond the second threshold at time t2. The second threshold is the transport speed at which the basic waveform W switches from W2 to W3. In this case, the end time tz of the predetermined period PP is a time before the intermediate time between time t1 and time t2. That is, the above-mentioned predetermined period PP is a period before the halfway point between the time (time t1) when the transport speed of the continuous paper 9 changes beyond the first threshold and the time (time t2) when it changes beyond the second threshold. This makes it possible to sufficiently secure a period during which the drive signal Ds is generated based only on the changed basic waveform W2, not on the mixed waveform Wm, before time t2.

[0056] However, the predetermined period PP may be set longer than this. The predetermined period PP may also be set before time t1. In other words, if it is known in advance that the transport speed of the continuous paper 9 will change beyond the first threshold at time t1, the drive signal Ds may be generated using the mixed waveform Wm before the change.

[0057] As described above, in this embodiment, when the waveform of the drive signal Ds supplied to the discharge unit 21 is switched in accordance with the change in the transport speed of the continuous paper 9, a period is provided in which ink droplets are discharged according to the drive signal Ds generated using the mixed waveform Wm in which the pre-change fundamental waveform W1 and the post-change fundamental waveform W2 are mixed. This makes the print boundary between before and after the switching less noticeable, improving the print quality. In addition, it has already been confirmed that ink droplets are properly discharged when the discharge unit 21 is driven by the drive signal Ds generated using the pre-change fundamental waveform W1 and the post-change fundamental waveform W2 included in the mixed waveform Wm. Therefore, by using the mixed waveform Wm in which such known fundamental waveforms W1 and W2 are mixed, it is guaranteed that ink droplets can be properly discharged. In other words, by using the mixed waveform Wm in which the fundamental waveforms W1 and W2 themselves are not changed, ink droplets can be more properly discharged than when the fundamental waveforms W1 and W2 themselves are changed. As a result, the print quality can be further improved.

[0058] <2. Second embodiment> Next, the configuration of an inkjet printing apparatus 1 according to a second embodiment of the present invention will be described. In the following, differences from the first embodiment will be mainly described, and some of the overlapping descriptions of the same parts as the first embodiment will be omitted. Figure 9 is a conceptual diagram showing the configuration of one ejection unit 21 and one ink supply unit 22 according to this embodiment.

[0059] 9, in the inkjet printing apparatus 1 according to this embodiment, the ink chamber 81 of each ejection unit 21 is provided with a temperature detection unit 811B for measuring the temperature of the ink in the ink chamber 81. The temperature detection unit 811B corresponds to the "detection unit" in the second embodiment of the present invention. The temperature detection unit 811B measures the temperature of the ink in the ink chamber 81, and outputs a measurement value Dt related to the detection result. The measurement value Dt is transmitted from the temperature detection unit 811B to the control unit 40.

[0060] 10 is a block diagram conceptually showing some of the functions of the control unit 40 according to this embodiment. As shown in FIG. 10, the control unit 40 according to this embodiment has a basic waveform selection unit 62, a basic waveform storage unit 63, a drive signal generation unit 64, a print position specification unit 65, an image signal supply unit 66, and a mixture ratio storage unit 67.

[0061] When the temperature measurement value Dt of the ink in the ink chamber 81 is supplied from the temperature detection unit 811B, the basic waveform selection unit 62 inquires of the basic waveform storage unit 63. In the basic waveform storage unit 63, the range of the temperature value of the ink in the ink chamber 81 and the corresponding basic waveform W, which is the minimum unit constituting the drive signal Ds for driving the piezoelectric element 82, are stored in association with each other. FIG. 11 is a diagram showing an example of the basic waveform W stored in the basic waveform storage unit 63 according to this embodiment. As shown in FIG. 11, for example, the basic waveform W corresponding to the ink temperature C (°C) in the ink chamber 81 being "29.8≦C<30.2", is "W1", and the basic waveform W corresponding to the ink temperature C (°C) being "30.2≦C<30.6" is "W2". The basic waveform selection unit 62 reads out the basic waveform W from the basic waveform storage unit 63 based on the temperature measurement value Dt of the ink in the ink chamber 81 supplied from the temperature detection unit 811B, and supplies it to the drive signal generation unit 64.

[0062] On the other hand, the print position specifying unit 65 detects the position in the transport direction of the continuous paper 9 transported by the transport mechanism 10. Specifically, the print position specifying unit 65 specifies the position on the continuous paper 9 where an image should be printed, based on the pulse signal En supplied from the encoder 30, and supplies the specified position information Pi to the image signal supply unit 66.

[0063] The image signal supplying unit 66 generates an image signal Is indicating an image to be printed at each ejection unit 21 based on the input image data Id and position information Pi at which an image is to be printed on the continuous paper 9, which is supplied from the print position specifying unit 65. That is, the image signal supplying unit 66 generates an image signal Is indicating an image to be printed at each ejection position P1 to P4 based on the position information Pi of the continuous paper 9 detected from the pulse signal En output from the encoder 30, and supplies the image signal Is to the drive signal generating unit 64.

[0064] The drive signal generating unit 64 generates a drive signal Ds for driving the piezoelectric element 82 and supplies it to each of the four ejection units 21. Specifically, when the measured value Dt of the ink temperature in the ink chamber 81 supplied from the temperature detection unit 811B is substantially constant or when the fluctuation width of the measured value Dt of the ink temperature is within a predetermined range, the drive signal generating unit 64 generates the drive signal Ds by multiplying the basic waveform W corresponding to the measured value Dt of the ink temperature by the image signal Is supplied from the image signal supply unit 66. More precisely, when there is no change in the basic waveform W supplied from the basic waveform selection unit 62, the drive signal generating unit 64 generates the drive signal Ds by multiplying the basic waveform W by the image signal Is supplied from the image signal supply unit 66.

[0065] Here, it is assumed that the temperature of the ink in the ink chamber 81 changes beyond the first threshold value. For example, as shown in FIG. 12, it is assumed that the temperature of the ink in the ink chamber 81 changes from temperature C=30.0 (°C) to temperature C=30.3 (°C) at time t1. Here, the basic waveform storage unit 63 stores that the basic waveform W corresponding to the ink temperature C (°C) in the ink chamber 81 is "29.8≦C<30.2", is "W1", and the basic waveform W corresponding to the ink temperature C (°C) in the ink chamber 81 is "W2". In this case, the temperature C=30.2 (°C) is the first threshold value. Before time t1, the basic waveform selection unit 62 reads out the "basic waveform W1 before change" as the basic waveform W corresponding to the ink temperature in the ink chamber 81 before the change, and supplies it to the drive signal generation unit 64. Furthermore, after time t1, the basic waveform selection section 62 reads out the “post-change basic waveform W2” as the basic waveform W corresponding to the post-change temperature of the ink in the ink chamber 81, and supplies it to the drive signal generation section 64.

[0066] At this time, the drive signal generating unit 64 generates the drive signal Ds by multiplying the mixed waveform Wm, which is a mixture of the pre-change fundamental waveform W1 supplied before time t1 and the post-change fundamental waveform W2 supplied after time t1, by the image signal Is supplied from the image signal supplying unit 66, during the period from time t1 when the drive signal Ds is switched to the subsequent time tz. That is, when the transport speed of the continuous paper 9 changes beyond the first threshold value at time t1, the drive signal generating unit 64 generates the drive signal Ds by multiplying the mixed waveform Wm, which is a mixture of the pre-change fundamental waveform W1 and the post-change fundamental waveform W2, by the image signal Is during a predetermined period PP including that time (time t1), and supplies the drive signal Ds to each of the four ejection units 21. As a result, by configuring the waveform of the drive signal Ds not to be switched immediately, the boundary of the print becomes less noticeable, and the print quality is improved.

[0067] More specifically, when the driving signal generating unit 64 recognizes that the measured value Dt of the temperature of the ink in the ink chamber 81 supplied from the temperature detecting unit 811B has changed beyond the first threshold, the driving signal generating unit 64 inquires of the mixing ratio storing unit 67. Here, as in FIG. 8 of the first embodiment, the mixing ratio storing unit 67 stores the range of the value of the time elapsed ratio R1 of the above-mentioned predetermined period PP and the corresponding ratio R2 of the number of pre-change fundamental waveforms W1 and the number of post-change fundamental waveforms W2 included in the mixed waveform Wm in association with each other. The driving signal generating unit 64 generates the driving signal Ds by multiplying the mixed waveform Wm, which is obtained by mixing the pre-change fundamental waveform W1 and the post-change fundamental waveform W2 at a predetermined mixing ratio stored in the mixing ratio storing unit 67, by the image signal Is.

[0068] Also, in the same manner as in the first embodiment, in the predetermined period PP, the mixture ratio R2 of the pre-transformation fundamental waveform W1 and the post-transformation fundamental waveform W2 changes so that the ratio of the pre-transformation fundamental waveform W1 gradually decreases and the ratio of the post-transformation fundamental waveform W2 gradually increases. This makes the print boundary even less noticeable, and further improves the print quality. However, in the predetermined period PP, the mixture ratio R2 of the pre-transformation fundamental waveform W1 and the post-transformation fundamental waveform W2 may be different from the above-mentioned aspect, or may be maintained at a constant value.

[0069] Also, as in the first embodiment, a case is assumed in which the transport speed of the continuous paper 9 changes beyond the first threshold at time t1, and then changes beyond the second threshold at time t2. The second threshold is the temperature at which the basic waveform W switches from W2 to W3. In this case, the end time tz of the predetermined period PP is a time before the intermediate time between time t1 and time t2. That is, the above-mentioned predetermined period PP is a period before the halfway point between the time (time t1) when the transport speed of the continuous paper 9 changes beyond the first threshold and the time (time t2) when it changes beyond the second threshold. This makes it possible to sufficiently secure a period in which the drive signal Ds is generated based only on the changed basic waveform W2, not on the mixed waveform Wm, before time t2.

[0070] However, the predetermined period PP may be set longer than this. The predetermined period PP may also be set before time t1. In other words, if it is known in advance that the temperature of the ink in the ink chamber 81 will change to exceed the first threshold value at time t1, the drive signal Ds may be generated using the mixed waveform Wm before the change.

[0071] As described above, in this embodiment, when the waveform of the drive signal Ds supplied to the ejection unit 21 is switched in accordance with the change in the temperature of the ink in the ink chamber 81, a period is provided in which ink droplets are ejected according to the drive signal Ds generated using the mixed waveform Wm in which the pre-change fundamental waveform W1 and the post-change fundamental waveform W2 are mixed. This makes the print boundary before and after the switching less noticeable, improving the print quality. In addition, it has already been confirmed that ink droplets are appropriately ejected when the ejection unit 21 is driven by the drive signal Ds generated using the pre-change fundamental waveform W1 and the post-change fundamental waveform W2 included in the mixed waveform Wm. Therefore, by using the mixed waveform Wm in which such known fundamental waveforms W1 and W2 are mixed, it is guaranteed that ink droplets can be appropriately ejected. In other words, by using the mixed waveform Wm in which the fundamental waveforms W1 and W2 themselves are not changed, ink droplets can be ejected more appropriately than when the fundamental waveforms W1 and W2 themselves are changed. As a result, the print quality can be further improved.

[0072] <3. Modifications> Although the first and second embodiments of the present invention have been described above, the present invention is not limited to the above-mentioned embodiments.

[0073] As described above, in an inkjet printing device that performs printing on a print medium by an inkjet method, the inkjet printing device includes a transport mechanism that transports the print medium in a longitudinal direction along a transport path, an encoder that outputs a pulse signal according to a transport speed of the print medium transported by the transport mechanism, an ejection unit having a nozzle that ejects ink droplets onto the print medium transported by the transport mechanism at an ejection position on the transport path, an ink chamber that communicates with the nozzle, and a piezoelectric element that applies pressure to the ink in the ink chamber, a drive signal generation unit that generates a drive signal for driving the piezoelectric element and supplies the drive signal to the ejection unit, and a speed of the print medium detected from the pulse signal output from the encoder. The device may have an image signal supply unit that generates an image signal indicating an image to be printed at the ejection position based on medium position information and supplies the image signal to the drive signal generation unit; a detection unit that detects one of the transport speed of the printing medium and the temperature of the ink in the ink chamber related to the pulse signal; a basic waveform storage unit that associates and stores the range of values ​​of the one of the transport speed of the printing medium and the temperature of the ink in the ink chamber with a basic waveform corresponding thereto; and a basic waveform selection unit that reads out the basic waveform from the basic waveform storage unit based on the detection result of the one of the transport speed of the printing medium and the temperature of the ink in the ink chamber by the detection unit, and supplies the basic waveform to the drive signal generation unit. Then, when one of the transport speed of the printing medium and the temperature of the ink in the ink chamber changes beyond a first threshold value, during a predetermined period including that point in time, the drive signal generating unit may be configured to generate the drive signal by multiplying the image signal by a mixed waveform that is a mixture of a pre-change basic waveform, which is the basic waveform corresponding to one of the transport speed of the printing medium and the temperature of the ink in the ink chamber before the change, and a post-change basic waveform, which is the basic waveform corresponding to one of the transport speed of the printing medium and the temperature of the ink in the ink chamber after the change, and supply the drive signal to the ejection unit.

[0074] Furthermore, in the case where one of the transport speed of the printing medium and the temperature of the ink in the ink chamber changes beyond the first threshold value and then further changes beyond the second threshold value, it is more desirable for the specified period to be the period more than halfway between the point at which one of the transport speed of the printing medium and the temperature of the ink in the ink chamber changes beyond the first threshold value and the point at which it further changes beyond the second threshold value.

[0075] The "one side" may be the transport speed of the print medium, or the temperature of the ink in the ink chamber.

[0076] Furthermore, the elements appearing in the above-described embodiments and modifications may be combined as appropriate to the extent that no contradiction arises. [Explanation of symbols]

[0077] 1 Inkjet printing device 9 Continuous Paper 10. Conveyor mechanism 12 Transport roller 20 Printing Department 21 Discharge part 22 Ink supply unit 30 Encoder 40 Control section 61 Conveying speed calculation unit 62 Basic waveform selection section 63 Basic Waveform Storage Section 64 Drive signal generator 65 Print position identification section 66 Image signal supply unit 67 Mixture ratio storage section 80 Head 81 Ink chamber 82 Piezoelectric element 83 Nozzle 121 Transport roller 811B Temperature detection unit Ds drive signal Dt (Ink temperature) measurement En (encoder) pulse signal Is Image signal Ms (continuous paper transport speed) measured value PP (using mixed waveforms) for a given period Pi location information V (continuous paper) transport speed W1 Basic waveform before change W2 Basic waveform after change Wm mixed waveform

Claims

1. An inkjet printing device that performs printing on a print medium using an inkjet method, a transport mechanism that transports the print medium in a longitudinal direction along a transport path; an encoder that outputs a pulse signal corresponding to a transport speed of the print medium transported by the transport mechanism; an ejection unit having a nozzle that ejects ink droplets onto the print medium transported by the transport mechanism at an ejection position on the transport path, an ink chamber that communicates with the nozzle, and a piezoelectric element that applies pressure to the ink in the ink chamber; a drive signal generating unit that generates a drive signal for driving the piezoelectric element and supplies the drive signal to the ejection unit; an image signal supply unit that generates an image signal indicating an image to be printed at the ejection position based on position information of the printing medium detected from the pulse signal output from the encoder, and supplies the image signal to the drive signal generation unit; a detection unit that detects one of a transport speed of the print medium and a temperature of the ink in the ink chamber, the transport speed being related to the pulse signal; a basic waveform storage unit that stores a range of one of the values ​​of the transport speed of the printing medium and the temperature of the ink in the ink chamber and a basic waveform corresponding thereto in association with each other; a basic waveform selection unit that reads out the basic waveform from the basic waveform storage unit based on a detection result of one of the transport speed of the print medium and the temperature of the ink in the ink chamber by the detection unit, and supplies the basic waveform to the drive signal generation unit; having When one of the transport speed of the print medium and the temperature of the ink in the ink chamber changes beyond a first threshold value, during a predetermined period including that point in time, The drive signal generating unit a pre-change basic waveform that corresponds to one of the conveyance speed of the printing medium and the temperature of the ink in the ink chamber before the change; a changed basic waveform, which is the basic waveform corresponding to one of the changed transport speed of the printing medium and the changed temperature of the ink in the ink chamber; an inkjet printing apparatus that generates the drive signal by multiplying the mixed waveform by the image signal, and supplies the drive signal to the ejection section.

2. 2. The inkjet printing apparatus of claim 1, In an inkjet printing device, the drive signal generation unit generates the drive signal by multiplying the image signal by a mixed waveform in which the pre-change fundamental waveform and the post-change fundamental waveform are mixed at a predetermined mixing ratio.

3. 3. The inkjet printing apparatus according to claim 1, When one of the transport speed of the print medium and the temperature of the ink in the ink chamber changes beyond the first threshold value and then changes beyond a second threshold value, In an inkjet printing device, the specified period is a period that is less than halfway between the point at which one of the transport speed of the printing medium and the temperature of the ink in the ink chamber changes beyond the first threshold value and the point at which it further changes beyond the second threshold value.

4. 3. The inkjet printing apparatus of claim 2, An inkjet printing device, wherein during the specified period, the mixing ratio changes so that the ratio of the pre-change fundamental waveform gradually decreases and the ratio of the post-change fundamental waveform gradually increases.

5. 3. The inkjet printing apparatus according to claim 1, In an inkjet printing apparatus, the one of the two is a transport speed of the print medium.

6. 3. The inkjet printing apparatus according to claim 1, The inkjet printing apparatus, wherein the one of the two is a temperature of the ink in the ink chamber.

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