Inkjet printing device

Abstract

This inkjet printing device comprises a discharge unit (21), a detection unit, and a drive signal generation unit. The discharge unit (21) discharges ink droplets onto a printing medium conveyed by a conveyance mechanism. The detection unit detects the conveyance speed of the printing medium. If the conveyance speed of the printing medium changes to exceed a first threshold value, during a prescribed period of time that includes the point of time of said change, the drive signal generation unit (64) multiplies an image signal (Is) by a mixed waveform produced by mixing a pre-change basic waveform that is a basic waveform (W) corresponding to the conveyance speed of the printing medium before the change and a post-change basic waveform that is a basic waveform (W) corresponding to the conveyance speed of the printing medium after the change, so as to generate a drive signal (Ds) for driving a piezoelectric element of the discharge unit (21) and supplies same to the discharge unit (21).

Description

Inkjet printing device

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

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

[0003] When printing using an inkjet printing device, the transport speed of the print medium may change. Furthermore, the temperature of the ink stored in the ejection unit before ejection may also change. When the conditions during ejection change in this way, the waveform of the drive signal supplied to the ejection unit is typically switched to match the changed conditions. Switching the waveform of the drive signal may cause a change in the density of the ink printed on the print medium as a printing result at the boundary where the switch is made, and this boundary may appear as a streak. Therefore, a technology to address such issues is disclosed, for example, in Patent Document 1.

[0004] Patent No. 5268838

[0005] Patent Document 1 discloses a technique for dealing with the phenomenon in which ink droplets become difficult to eject due to the influence of ink viscosity increase, etc., when a piezoelectric element, which is an actuator for ejecting ink in an inkjet head, is not in operation 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, and a large difference occurs 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 initially set higher than the level of the remaining drive pulse voltages. The level of the predetermined number of drive pulse voltages is then gradually decreased (paragraph 0026). This allows the ejection speed to be increased 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 take on a value or shape never experienced before. Furthermore, in general, when the drive pulse voltage (drive signal) supplied to the piezoelectric element has a specific 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 will be ejected properly.

[0008] The present invention has been made in consideration of the above circumstances, and aims to provide a technology that can eject ink droplets appropriately 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 boundary between the switching states less noticeable and improving print quality.

[0009] To solve the above problems, a first invention of the present application provides an inkjet printing device that performs inkjet printing on a print medium, 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 print medium in the longitudinal direction along a transport path. The encoder outputs a pulse signal corresponding to the transport speed of the print medium transported by the transport mechanism. The ejection unit includes, at an ejection position on the transport path, a nozzle that ejects ink droplets onto the print medium transported by the transport mechanism, an ink chamber communicating 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 print medium detected from the pulse signal output from the encoder, and supplies the image signal to the drive signal generation unit. The detection unit detects one of the print medium transport speed and the ink temperature in the ink chamber associated with the pulse signal. The basic waveform storage unit stores a range of variation of the one of the print medium transport speed and the ink temperature in the ink chamber in association with a corresponding basic waveform. The basic waveform selection unit reads the basic waveform from the basic waveform storage unit based on the detection result of the one of the print medium transport speed and the ink temperature in the ink chamber by the detection unit, and supplies the basic waveform to the drive signal generation unit. When one of the print medium transport speed and the ink temperature 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 that mixes a pre-change basic waveform, which is the basic waveform corresponding to the one of the print medium transport speed and the ink temperature in the ink chamber before the change, and a post-change basic waveform, which is the basic waveform corresponding to the one of the print medium transport speed and the ink temperature in the ink chamber after the change, for a predetermined period including that point in time.

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

[0011] The third invention of the present application is an inkjet printing device according to 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 time when the transport speed of the printing medium and the temperature of the ink in the ink chamber changes beyond the first threshold and the time when the temperature changes beyond the second threshold.

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

[0013] A fifth aspect of the present invention is the inkjet printing apparatus of 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 ink jet printing apparatus of any one of the first to fourth aspects, wherein the one of the two is the temperature of the ink in the ink chamber.

[0015] According to the first to sixth aspects of the present invention, when switching the waveform of the drive signal supplied to the ejection unit in accordance with changes in the ejection situation, the drive signal is generated using a mixed waveform that mixes a pre-modification fundamental waveform and a post-modification fundamental waveform. This makes the boundary between the switching points less noticeable, improving print quality. Furthermore, because the pre-modification fundamental waveform and the post-modification fundamental waveform are mixed without changing the fundamental waveform itself, ink droplets can be ejected more appropriately than when the fundamental waveform itself is changed.

[0016] 1 is a diagram conceptually illustrating the configuration of an inkjet printing apparatus according to a first embodiment; FIG. 2 is a diagram of one ejection unit according to the first embodiment as viewed from below; FIG. 3 is a diagram conceptually illustrating the configuration of one ejection unit and one ink supply unit according to the first embodiment; FIG. 4 is a block diagram showing the electrical connection between a control unit according to the first embodiment and each unit in an inkjet printing apparatus; FIG. 5 is a block diagram conceptually illustrating some of the functions of a control unit according to the first embodiment; FIG. 6 is a diagram showing an example of basic waveforms stored in a basic waveform storage unit according to the first embodiment; FIG. 7 is a diagram for explaining waveform switching according to the first embodiment; FIG. 8 is a diagram showing an example of the ratio between the number of pre-transition basic waveforms and the number of post-transition basic waveforms included in a mixed waveform, which is stored in a mixing ratio storage unit according to the first embodiment; FIG. 9 is a diagram conceptually illustrating the configuration of one ejection unit and one ink supply unit according to a second embodiment; FIG. 10 is a block diagram conceptually illustrating some of the functions of a control unit according to the second embodiment; FIG. 11 is a diagram showing an example of basic waveforms stored in a basic waveform storage unit according to the second embodiment;

[0017] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the components described in these embodiments are merely examples and are not intended to limit the scope of the present invention. Furthermore, in the drawings, the dimensions and number of each part may be exaggerated or simplified as necessary to facilitate understanding.

[0018] 1. First Embodiment 1-1. Configuration of Inkjet Printing Apparatus FIG. 1 is a conceptual diagram illustrating the configuration of an inkjet printing apparatus 1 according to a first embodiment of the present invention. The inkjet printing apparatus 1 is an inkjet printer that transports a long strip of continuous paper 9 while ejecting droplets of aqueous ink from multiple 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 print medium. The print medium may be cut paper, plastic film, cardboard, metal foil, or a glass substrate. In other words, the inkjet printing apparatus 1 may be any device that prints on a print medium using an inkjet method. As shown in FIG. 1, the inkjet printing apparatus 1 includes a transport 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 that follows 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 multiple rollers, including an unwind roller 11, multiple transport rollers 12, and a take-up roller 13. The continuous paper 9 is stretched over these multiple 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 of the multiple transport rollers 12. Hereinafter, this roller will be referred to as a "drive roller." By driving this motor, each transport roller 12, including the drive roller, rotates around an axis that is parallel to the width direction of the continuous paper 9. Here, the width direction is a direction that is 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 transport, the continuous paper 9 is collected by the take-up roller 13.

[0021] The printing unit 20 has multiple ejection units 21 and multiple ink supply units 22. In this embodiment, the printing unit 20 has four ejection units 21 and four ink supply units 22. The four ejection units 21 are arranged at intervals along the transport direction of the continuous paper 9. Each of the four ejection units 21 is arranged above 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 FIGS. 2 and 3 described below) 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 droplets of ink of different colors, thereby recording a monochromatic image on the recording surface of the continuous paper 9. In this embodiment, the four ejection units 21 eject droplets of ink of each color, for example, cyan, magenta, yellow, and black. 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 unit 21 as seen from below. In FIG. 2, the continuous paper 9 is indicated by an imaginary line (two-dot chain line). As shown enlarged in FIG. 2, the lower surface of the ejection unit 21 is provided with a plurality of nozzles 83 capable of ejecting ink droplets. In this embodiment, the plurality of nozzles 83 are arranged two-dimensionally in the transport direction and width direction on the lower surface of the ejection unit 21. Furthermore, the nozzles 83 are arranged with their positions shifted in the width direction. By arranging the plurality of nozzles 83 two-dimensionally in this way, the positions of the nozzles 83 in the width direction can be made closer to each other. However, the plurality of nozzles 83 may also be arranged in a line along the width direction.

[0024] FIG. 3 is a conceptual diagram illustrating the configuration of one ejection unit 21 and one ink supply unit 22. In this embodiment, each ejection unit 21 is composed of multiple adjacent heads 80. In this embodiment, each ejection unit 21 is composed of five heads 80. The five heads 80 have identical structures. For this reason, FIG. 3 illustrates only one of the five heads 80 in detail, while the remaining four heads 80 are illustrated in a simplified manner. As shown in FIG. 3, the nozzles 83 are holes provided in the lower part of the head 80. The nozzles 83 face the recording surface (top surface) of the continuous paper 9 transported by the transport mechanism 10 in the vertical direction. The head 80 also has one ink chamber 81 and one piezoelectric element 82 serving as an actuator for each nozzle 83. The head 80 has multiple sets of nozzles 83, ink chambers 81, and piezoelectric elements 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 that can temporarily store 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 downward from the internal tank 85 to the ink chamber 81. A drive signal Ds, which will be described later, is then supplied to the ejection unit 21. Here, the drive signal Ds is a signal formed by connecting, in time series, values ​​indicating a drive voltage that is applied to the piezoelectric element 82 to drive the piezoelectric element 82. The drive voltage has a pulse waveform. The piezoelectric element 82 expands toward the ink chamber 81 in response to the supplied drive signal Ds. This applies pressure to the ink in the ink chamber 81. As a result, ink droplets are ejected from the nozzles 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 droplets of ink of different colors from the multiple nozzles 83 in response to the drive signal Ds. The four ejection units 21 eject droplets of ink of each color, for example, cyan, magenta, yellow, and black. A multicolor image is then recorded on the recording surface of the continuous paper 9 by superimposing the monochromatic images formed by the droplets of these colors of ink.

[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 that supplies ink to the ejection unit 21 while circulating a portion of the ink. As described above, the inkjet printing apparatus 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, piping 225, a filter 226, and multiple on-off valves 227. In this embodiment, each ink supply unit 22 has five on-off valves 227. The piping 225 includes a first supply piping 91, multiple second supply piping 92, multiple first reflux piping 93, and a second reflux piping 94. In this embodiment, the piping 225 includes five second supply piping 92 and five first reflux piping 93.

[0031] The storage tank 221 is a container for storing 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 into 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, ink that has been stored inside the storage tank 221 and whose temperature has been 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 detects the height of the ink level 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 ink is to be supplied 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 inserted in each of the first return pipes 93. When the on-off valve 227 is in a closed state, communication is blocked for the first return pipes 93 in which the on-off valve 227 is inserted. On the other hand, when the on-off valve 227 is in an open state, communication is ensured for the first return pipes 93 in which the on-off valve 227 is inserted.

[0035] A reflux pump 224 is also inserted into the second reflux pipe 94. In response to an operation signal from the control unit 40, the reflux pump 224 generates a flow of ink inside the second reflux pipe 94 from each first reflux pipe 93 toward the storage tank 221. This causes the ink stored in the internal tank 85 of each head 80 to return to the storage tank 221 via the first reflux pipe 93 and the second reflux pipe 94. Here, the ink stored in the internal tank 85 of each head 80 refers to ink that has not been ejected for a long period of time and has remained in the internal tank 85, resulting in a significant drop in temperature. This prevents the temperature, viscosity, and other properties of the ink stored in the internal tank 85 of each head 80 from changing significantly (for example, by approximately 10°C or more). For example, the temperature of the ink stored in the internal tank 85 of each head 80 can be prevented from changing by approximately 10°C or more. As a result, deterioration of the ink ejected from each nozzle 83 is prevented, improving print quality.

[0036] The encoder 30 is a sensor for detecting the transport speed of the continuous paper 9. The encoder 30 is attached to the shaft core of one of the multiple transport rollers 12. In this embodiment, the encoder 30 is attached to the shaft core of the transport roller 121 in FIG. 1. The encoder 30 outputs a pulse signal En once each time the transport roller 121 rotates a predetermined angle. Therefore, the pulse signal En is data that reflects changes 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 a continuous pulse signal En at a constant cycle that corresponds 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 control unit 40 with a pulse signal En that corresponds to the transport speed of the continuous paper 9 transported by the transport mechanism 10. However, the encoder 30 only needs to be directly or indirectly connected to at least one of the multiple transport rollers 12, and its 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 within the inkjet printing apparatus 1. Figure 4 is a block diagram showing the electrical connections between the control unit 40 and each unit within the inkjet printing apparatus 1. As shown in Figure 4, the control unit 40 is made up of a computer having a processor 41 such as a CPU, a memory 42 such as 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 for 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 a computer program P and data D from the storage unit 43 into the memory 42, and operates the processor 41 in accordance with the computer program P and data D. This allows the transport of the continuous paper 9 and the printing process to proceed.

[0040] <1-2. Detailed Configuration of Functions of Control Unit> 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 drive signal generation unit 64, a print 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 drive signal generation unit 64, the print position identification unit 65, the image signal supply unit 66, and the mixture ratio storage unit 67 are realized by the processor 41 temporarily reading out a computer program P and data D stored in the storage unit 43 into the memory 42 and performing arithmetic processing based on the computer program P and data D.

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

[0042] When the basic waveform selection unit 62 receives the measured value Ms of the conveyance speed of the continuous paper 9 from the conveyance speed calculation unit 61, it queries the basic waveform storage unit 63. The basic waveform storage unit 63 stores the range of values ​​of the conveyance speed of the continuous paper 9 and the corresponding basic waveform W in association with each other. Here, the basic waveform W is the smallest unit constituting the drive signal Ds. As described above, the drive signal Ds is a signal formed by connecting, in time series, drive voltages (pulse-like waveforms) that are applied to the piezoelectric elements 82 of the ejection unit 21 to drive the piezoelectric elements 82. The basic waveform W is the drive voltage (pulse-like waveform) that is applied to the piezoelectric elements 82 to eject one ink droplet from the nozzle 83.

[0043] 6 is a diagram showing examples of the basic waveforms W stored in the basic waveform storage unit 63. As shown in Fig. 6, for example, when the transport speed V (m / sec) of the continuous paper 9 is "0.8≦V<1.2", the corresponding basic waveform W is "W1", and when "1.2≦V<1.6", the corresponding basic waveform W 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 transport speed of the continuous paper 9 supplied from the transport speed calculation unit 61, and supplies the basic waveform W to the drive signal generation unit 64.

[0044] Meanwhile, 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. The print position specifying unit 65 then 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 position information Pi on the continuous paper 9 where the image is to be printed, which is supplied from the print position identification 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, the image signal supply unit 66 recognizes the positions on the continuous paper 9 at which cyan, magenta, yellow, and black ink droplets should be ejected. Furthermore, the image signal supply unit 66 determines the times at which these ink droplets should be ejected. In other words, the image signal Is is time-series data that connects together data indicating the times at which cyan, magenta, yellow, and black ink droplets should be ejected from which ejection unit 21.

[0046] The image signal supply unit 66 also 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 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 generation unit 64.

[0047] The drive signal generation 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 range of the measured value Ms of the conveying speed is within a predetermined range, the drive signal generation unit 64 generates the drive signal Ds by multiplying the basic waveform W corresponding to the measured value Ms of the conveying speed 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 generation 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.

[0048] Based on the drive signal Ds supplied from the drive signal generating unit 64, each of the four ejection units 21 ejects droplets of ink of each color from the nozzle 83 onto an appropriate position in the transport direction on the continuous paper 9 at ejection positions P1 to P4. 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 top surface of the continuous paper 9 by superimposing the four single-color images.

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

[0050] In this case, it is assumed that the drive signal generation unit 64 generates the drive signal Ds before time t1 by multiplying the pre-change fundamental waveform W1 supplied from the fundamental waveform selection unit 62 by the image signal Is supplied from the image signal supply unit 66. It is also assumed that 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 generation unit 64 generates the drive signal Ds after time t1 by multiplying the post-change fundamental waveform W2 supplied from the fundamental waveform selection unit 62 by the image signal Is supplied from the image signal supply unit 66. It is also assumed that 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 occurs at the boundary where the switch occurred, and streaks may appear at the boundary as a printing result.

[0051] Therefore, the drive signal generation unit 64 generates the drive signal Ds by multiplying a mixed waveform Wm of the pre-transformation fundamental waveform W1 and the post-transformation fundamental waveform W2 by the image signal Is supplied from the image signal supply unit 66 from time t1, when the drive signal Ds is switched, to the subsequent time tz. Here, the mixed waveform Wm is a waveform in which the pre-transformation fundamental waveform W1 supplied before time t1 and the post-transformation fundamental waveform W2 supplied after time t1 are mixed. The mixed waveform Wm is a waveform in which the pre-transformation fundamental waveform W1 and the post-transformation fundamental waveform W2 are connected in time series at equal or unequal intervals. That is, when the transport speed of the continuous paper 9 changes beyond the first threshold value at time t1, the drive signal generation unit 64 generates a drive signal Ds by multiplying a mixed waveform Wm, which is a mixture of a pre-change fundamental waveform W1 and a post-change fundamental waveform W2, by the image signal Is during a predetermined period PP that includes that point (time t1). The drive signal generation unit 64 then supplies the generated drive signal Ds to each of the four ejection units 21. This configuration prevents the waveform of the drive signal Ds from being immediately switched, making print boundaries less noticeable and improving print quality.

[0052] More specifically, when the drive signal generation unit 64 recognizes that the measured value Ms of the conveyance speed of the continuous paper 9 supplied from the conveyance speed calculation unit 61 has changed beyond the first threshold, it queries the mixture ratio storage unit 67. The mixture ratio storage unit 67 stores the domain of the value of the time elapsed ratio R1 of the predetermined period PP in association with the corresponding ratio R2. The ratio R2 is the ratio between the number of pre-transformation fundamental waveforms W1 and the number of post-transformation fundamental waveforms W2 included in the mixed waveform Wm. FIG. 8 shows an example of the ratio R2 between the number of pre-transformation fundamental waveforms W1 and the number of post-transformation fundamental waveforms W2 included in the mixed waveform Wm, which is stored in the mixture ratio storage unit 67.

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

[0054] 8, during the predetermined period PP, the mixing ratio R2 of the pre-modified fundamental waveform W1 and the post-modified fundamental waveform W2 changes so that the ratio of the pre-modified fundamental waveform W1 gradually decreases and the ratio of the post-modified fundamental waveform W2 gradually increases. This makes the print boundary less noticeable and further improves print quality. However, during the predetermined period PP, the mixing ratio R2 of the pre-modified fundamental waveform W1 and the post-modified fundamental waveform W2 may differ from the aspect shown in FIG. 8. The mixing ratio R2 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. As shown in FIG. 7 , assume that the transport speed of the continuous paper 9 exceeds the first threshold at time t1, and then exceeds the second threshold at time t2. The second threshold is the transport speed at which the fundamental waveform W switches from W2 to W3. In this case, the end time tz of the predetermined period PP is a time before the midpoint between time t1 and time t2. In other words, the above-mentioned predetermined period PP is a period that is more than halfway between the time point at which the transport speed of the continuous paper 9 exceeds the first threshold (time t1) and the time point at which it further exceeds the second threshold (time t2). This ensures a sufficient period before time t2 during which the drive signal Ds is generated based only on the changed fundamental waveform W2, not the mixed waveform Wm.

[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 and exceed the first threshold value at time t1, the drive signal Ds may be generated using the mixed waveform Wm before the change occurs.

[0057] 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 changes in the transport speed of the continuous paper 9, a period is provided in which ink droplets are ejected in accordance with the drive signal Ds generated using the mixed waveform Wm, which is a mixture of the pre-transformation fundamental waveform W1 and the post-transformation fundamental waveform W2. This makes the boundary between the print before and after the switching less noticeable, thereby improving print quality. Furthermore, it has already been confirmed that ink droplets are ejected appropriately when the ejection unit 21 is driven by the drive signal Ds generated using the pre-transformation fundamental waveform W1 and the post-transformation fundamental waveform W2 included in the mixed waveform Wm. Therefore, using the mixed waveform Wm, which is a mixture of such known fundamental waveforms W1 and W2, ensures that ink droplets can be ejected appropriately. In other words, by using the mixed waveform Wm, which is a mixture of the fundamental waveforms W1 and W2 without changing them, ink droplets can be ejected more appropriately than when the fundamental waveforms W1 and W2 themselves are changed. As a result, 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. The following description will focus on differences from the first embodiment, and some overlapping descriptions of portions that are the same as those in 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 sent from the temperature detection unit 811B to the control unit 40.

[0060] 10 is a block diagram conceptually illustrating 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 includes a basic waveform selection unit 62, a basic waveform storage unit 63, a drive signal generation unit 64, a print position identification unit 65, an image signal supply unit 66, and a mixture ratio storage unit 67.

[0061] When the temperature detection unit 811B supplies the basic waveform selection unit 62 with the measured temperature value Dt of the ink in the ink chamber 81, the basic waveform selection unit 62 queries the basic waveform storage unit 63. The basic waveform storage unit 63 stores the temperature range of the ink in the ink chamber 81 in association with the corresponding basic waveform W. Here, the basic waveform W is the smallest unit constituting the drive signal Ds for driving the piezoelectric element 82. FIG. 11 shows 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, when the temperature C (°C) of the ink in the ink chamber 81 is "29.8≦C<30.2", the corresponding basic waveform W is "W1", and when "30.2≦C<30.6", the corresponding basic waveform W 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 Dt of the temperature of the ink in the ink chamber 81 supplied from the temperature detection unit 811 B, and supplies it to the drive signal generation unit 64 .

[0062] Meanwhile, 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. The print position specifying unit 65 then supplies the specified position information Pi to the image signal supply unit 66.

[0063] The image signal supply unit 66 generates an image signal Is indicating an image to be printed at each discharge unit 21 based on the input image data Id and position information Pi at which the image is to be printed on the continuous paper 9, which is supplied from the print position identification unit 65. That is, the image signal supply unit 66 generates an image signal Is indicating an image to be printed at each discharge 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 generation unit 64.

[0064] The drive signal generation 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 range of the measured value Dt of the ink temperature is within a predetermined range, the drive signal generation 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 generation 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] Now, assume that the temperature of the ink in the ink chamber 81 changes beyond the first threshold. For example, as shown in FIG. 12 , assume 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 the basic waveform W corresponding to the ink temperature C (°C) in the ink chamber 81 being "29.8 ≦ C < 30.2", which is "W1", and the basic waveform W corresponding to the ink temperature C (°C) being "30.2 ≦ C < 30.6", which is "W2". In this case, temperature C = 30.2 (°C) becomes the first threshold. Before time t1, the basic waveform selection unit 62 reads out "pre-change basic waveform W1" as the basic waveform W corresponding to the ink temperature in the ink chamber 81 before the change, and supplies this to the drive signal generation unit 64. Furthermore, after time t1, the basic waveform selection unit 62 reads out the "changed basic waveform W2" as the basic waveform W corresponding to the changed temperature of the ink in the ink chamber 81 and supplies it to the drive signal generation unit 64.

[0066] At this time, the drive signal generation unit 64 generates the drive signal Ds by multiplying a mixed waveform Wm of the pre-transformation fundamental waveform W1 and the post-transformation fundamental waveform W2 by the image signal Is supplied from the image signal supply unit 66 from time t1, when the drive signal Ds is switched, until the subsequent time tz. Here, the mixed waveform Wm is a waveform that mixes the pre-transformation fundamental waveform W1 supplied before time t1 and the post-transformation fundamental waveform W2 supplied after time t1. In other words, when the transport speed of the continuous paper 9 changes beyond the first threshold at time t1, the drive signal generation unit 64 generates the drive signal Ds by multiplying the mixed waveform Wm, which is a mixture of the pre-transformation fundamental waveform W1 and the post-transformation fundamental waveform W2, by the image signal Is during a predetermined period PP that includes that time (time t1). The drive signal generation unit 64 then supplies the generated drive signals Ds to each of the four ejection units 21. By configuring the waveform of the drive signal Ds not to be switched immediately, the print boundary becomes less noticeable, improving print quality.

[0067] More specifically, when the drive signal generation unit 64 recognizes that the measured temperature Dt of the ink in the ink chamber 81 supplied from the temperature detection unit 811B has changed beyond the first threshold, it queries the mixture ratio storage unit 67. Here, similar to FIG. 8 of the first embodiment, the mixture ratio storage unit 67 stores the domain of the value of the time elapsed ratio R1 of the predetermined period PP in association with the corresponding ratio R2. The ratio R2 is the ratio between the number of pre-transformation fundamental waveforms W1 and the number of post-transformation fundamental waveforms W2 included in the mixture waveform Wm. The drive signal generation unit 64 generates the drive signal Ds by multiplying the mixture waveform Wm, which is a mixture of the pre-transformation fundamental waveform W1 and the post-transformation fundamental waveform W2 at a predetermined mixture ratio stored in the mixture ratio storage unit 67, by the image signal Is.

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

[0069] As in the first embodiment, as shown in FIG. 12 , assume that the temperature of the ink in the ink chamber 81 exceeds the first threshold at time t1, and then exceeds the second threshold at time t2. The second threshold is the temperature at which the fundamental waveform W switches from W2 to W3. In this case, the end time tz of the predetermined period PP is a time before the midpoint between time t1 and time t2. In other words, the predetermined period PP is the period more than halfway between the time t1 at which the temperature of the ink in the ink chamber 81 exceeds the first threshold and the time t2 at which the temperature exceeds the second threshold. This ensures a sufficient period before time t2 during which the drive signal Ds is generated based only on the changed fundamental waveform W2, not the mixed waveform Wm.

[0070] However, the predetermined period PP may be set longer than this. Also, the predetermined period PP may be set to start 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 and exceed the first threshold value at time t1, the drive signal Ds may be generated using the mixed waveform Wm before the change occurs.

[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 changes in the temperature of the ink in the ink chamber 81, a period is provided in which ink droplets are ejected in response to the drive signal Ds generated using the mixed waveform Wm, which is a mixture of the pre-transformation fundamental waveform W1 and the post-transformation fundamental waveform W2. This makes the boundary between the print before and after the switching less noticeable, thereby improving print quality. Furthermore, it has already been confirmed that ink droplets are ejected appropriately when the ejection unit 21 is driven by the drive signal Ds generated using the pre-transformation fundamental waveform W1 and the post-transformation fundamental waveform W2 included in the mixed waveform Wm. Therefore, using the mixed waveform Wm, which is a mixture of such known fundamental waveforms W1 and W2, ensures that ink droplets can be ejected appropriately. In other words, by using the mixed waveform Wm, which is a mixture of the fundamental waveforms W1 and W2 without changing them, ink droplets can be ejected more appropriately than when the fundamental waveforms W1 and W2 themselves are changed. As a result, 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-described embodiments.

[0073] As described above, in an inkjet printing device that performs inkjet printing on a print medium, 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 the transport speed of the print medium transported by the transport mechanism, a discharge unit that has, at a discharge position on the transport path, a nozzle that discharges ink droplets onto the print medium transported by the transport mechanism, 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 it to the discharge 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 the 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 stores the range of values ​​of 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; and a basic waveform selection unit that reads out the basic waveform from the basic waveform storage unit based on the detection result of 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, during a predetermined period including that point, the drive signal generation unit may be configured to generate the drive signal and supply it to the ejection unit 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.

[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 and then changes beyond the second threshold, it is more desirable that the specified period be the period more than halfway between the time 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 the time when it changes beyond the second threshold.

[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 within the scope of not causing any contradiction.

[0077] 1 Inkjet printing device 9 Continuous paper 10 Transport mechanism 12 Transport roller 20 Printing unit 21 Discharge unit 22 Ink supply unit 30 Encoder 40 Control unit 61 Transport speed calculation unit 62 Basic waveform selection unit 63 Basic waveform storage unit 64 Drive signal generation unit 65 Print position identification unit 66 Image signal supply unit 67 Mixed ratio storage unit 80 Head 81 Ink chamber 82 Piezoelectric element 83 Nozzle 121 Transport roller 811B Temperature detection unit Ds Drive signal Dt Measured value (of ink temperature) En Pulse signal (of encoder) Is Image signal Ms Measured value (of transport speed of continuous paper) PP Predetermined period (using mixed waveform) Pi Position information V Transport speed (of continuous paper) W1 Basic waveform before change W2 Basic waveform after change Wm Mixed waveform

Claims

1. An inkjet printing device for printing on a printing medium by an inkjet method, comprising: a transport mechanism for transporting the printing medium in a longitudinal direction along a transport path; an encoder for outputting a pulse signal according to the transport speed of the printing medium transported by the transport mechanism; an ejection section having a nozzle for ejecting ink droplets onto the printing medium transported by the transport mechanism at an ejection position on the transport path, an ink chamber communicating with the nozzle, and a piezoelectric element for applying pressure to the ink in the ink chamber; a drive signal generation section for generating a drive signal for driving the piezoelectric element and supplying the drive signal to the ejection section; an image signal supply section for generating 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 supplying the image signal to the drive signal generation section; a detection section for detecting one of the transport speed of the printing medium and the temperature of the ink in the ink chamber related to the pulse signal; and a basic waveform storage section for storing a range of values ​​of one of the transport speed of the printing medium and the temperature of the ink in the ink chamber in association with a basic waveform corresponding thereto. an ink jet printing device having a basic waveform selection unit that reads out the basic waveform from the basic waveform storage unit based on the detection result of 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, wherein when one of the transport speed of the printing medium and the temperature of the ink in the ink chamber changes by exceeding a first threshold, during a predetermined period including that point in time, the drive signal generation unit generates the drive signal by multiplying the image signal by a mixed waveform that mixes 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.

2. An inkjet printing device as described in claim 1, wherein the drive signal generating 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. An inkjet printing device as described in claim 1 or claim 2, 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 value and then changes beyond a second threshold value, 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. An inkjet printing device as described in claim 2, wherein, during the specified period, the mixing ratio changes so that the ratio of the pre-change basic waveform gradually decreases and the ratio of the post-change basic waveform gradually increases.

5. An inkjet printing device according to any one of claims 1 to 4, wherein the one of the two is a transport speed of the printing medium.

6. An inkjet printing device according to any one of claims 1 to 4, wherein the one of the two is a temperature of the ink in the ink chamber.

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