Droplet ejection device and image forming apparatus

Abstract

To provide a droplet discharge device which can land droplets to a target position by following variations in medium conveyance speed.SOLUTION: A droplet discharge device includes a conveyance part for conveying a medium, a droplet discharge part for discharging droplets to a medium conveyed by the conveyance part, a conveyance speed sensor for detecting medium conveyance speed, and a controller for controlling a droplet discharge timing by the droplet discharge part on the basis of the conveyance speed detected by the conveyance speed sensor, wherein the controller has a memory for storing a look-up table for holding a discharge frequency having a small landing error of the droplets to a target position from a discharge frequency obtained by substituting the conveyance speed for F=V÷(25.4×d) associated with each of the plurality of conveyance speed, reads out the discharge frequency corresponding to the conveyance speed detected by the conveyance speed sensor from the look-up table, and allows the droplet discharge part to discharge droplets at the read-out discharge frequency.SELECTED DRAWING: Figure 7

Description

[Technical field]

[0001] The present invention relates to a droplet ejection device and an image forming apparatus. [Background technology]

[0002] Conventionally, there has been known an image forming apparatus that includes a transport unit that transports a medium and an ink ejection unit that ejects ink toward the medium transported by the transport unit. In such an image forming apparatus, if the transport speed of the medium by the transport unit varies, the ink ejected from the ink ejection unit may not land at the target position on the medium.

[0003] Therefore, in order to cause the ink ejected from the ink ejection unit to land at a target position on the medium, there is a technique for correcting the ejection timing of the ink ejection unit according to the medium transport speed detected by a sensor (see, for example, Patent Document 1). Summary of the Invention [Problem to be solved by the invention]

[0004] One method of correcting the ejection timing according to the transport speed is, for example, to calculate the calculated ejection frequency F (Hz) as the ejection timing by substituting the medium transport speed V (mm / sec) and resolution d (dpi) into the following equation 1.

[0005] F=V ×d÷25.4 ...(Formula 1)

[0006] The above-mentioned correction method assumes that the ink ejection speed is constant, and changes the calculated ejection frequency according to the transport speed. However, in reality, when the calculated ejection frequency is changed, the ejection speed also fluctuates. Therefore, the above-mentioned correction method cannot be said to sufficiently reduce the error between the target position and the actual landing position.

[0007] The present invention has been made to solve such problems, and aims to provide a droplet ejection device that can land droplets at target positions by tracking variations in the medium transport speed. [Means for solving the problem]

[0008] In order to solve the above problem, one aspect of the present invention is a droplet ejection device including a transport unit that transports a medium, a droplet ejection unit that ejects droplets onto the medium transported by the transport unit, a transport speed sensor that detects the transport speed of the medium, and a controller that controls the ejection timing of droplets by the droplet ejection unit based on the transport speed detected by the transport speed sensor. The controller calculates a ratio F=V, where V is the transport speed, d is a resolution that is a density of droplets that land on the medium, and F is an ejection frequency that is the ejection timing, in correspondence with each of a plurality of transport speeds. ×d÷25.4 The ink jet head is provided with a memory that stores a lookup table that holds the ejection frequency that has a smaller landing error of droplets with respect to a target position than the ejection frequency obtained by substituting the transport speed into the memory, and the ejection frequency that corresponds to the transport speed detected by the transport speed sensor is read from the lookup table, and droplets are ejected from the droplet ejection section at the read ejection frequency. Effect of the Invention

[0009] According to the present invention, it is possible to land droplets at target positions while following up variations in the transport speed of the medium. [Brief description of the drawings]

[0010] [Figure 1] 1 is a schematic diagram of an inkjet recording apparatus according to an embodiment of the present invention. [Diagram 2] FIG. [Diagram 3] FIG. [Figure 4] FIG. 2 is a diagram showing the hardware configuration of the inkjet recording apparatus. [Diagram 5]FIG. 4 is an explanatory diagram of a lookup table stored in a memory. [Figure 6] FIG. 2 is a functional block diagram of the controller. [Figure 7] 4 is a flowchart of an image forming process. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram of an inkjet recording apparatus (image forming apparatus, droplet ejection apparatus) 1 according to an embodiment of the present invention. As shown in FIG. 1, the inkjet recording apparatus 1 includes a paper feed section 10, an image forming section 20, a drying section 30, and a paper ejection section 40. The inkjet recording apparatus 1 forms an image by causing the image forming section 20 to eject ink onto paper P (medium) fed from the paper feed section 10. Then, the inkjet recording apparatus 1 dries the ink attached on the paper P in the drying section 30, and then ejects the paper P to the paper ejection section 40.

[0012] The paper feed section 10 includes a paper feed tray 11 on which a plurality of sheets P are stacked, a feed device 12 that separates and sends out the sheets P one by one from the paper feed tray 11, and a pair of registration rollers 13 that sends the sheets P to the image forming section 20. The feed device 12 can be any known feed device having a feeding function, such as a device using rollers or a device using air suction.

[0013] After the leading edge of the paper P sent out from the paper feed tray 11 by the feeding device 12 reaches the pair of registration rollers 13, the pair of registration rollers 13 is driven at a predetermined timing, whereby the paper is fed to the image forming unit 20. In this embodiment, the configuration of the paper feed unit 10 is not particularly limited as long as the paper feed unit 10 has a function of sending out the paper P to the image forming unit 20.

[0014] The image forming section 20 includes a paper transport drum 21 (a rotating member), an ink ejection section 22 (a droplet ejection section), a receiving drum 24, and a transfer drum 25. The paper transport drum 21, the receiving drum 24, and the transfer drum 25 are an example of a transport section. The receiving drum 24 receives the paper P from the paper feed section 10. The paper transport drum 21 is a cylindrical rotating member that carries and transports the paper P received by the receiving drum 24 on its outer circumferential surface. The ink ejection section 22 ejects ink toward the paper P transported by the paper transport drum 21. The transfer drum 25 transfers the paper P transported by the paper transport drum 21 to the drying section 30.

[0015] The paper P transported from the paper supply unit 10 to the image forming unit 20 has its leading edge gripped by a paper gripper provided on the surface of the receiving drum 24, and is transported as the surface of the receiving drum 24 moves. The paper P transported by the receiving drum 24 is delivered to the paper transport drum 21 at a position opposite to the paper transport drum 21.

[0016] A paper gripper is also provided on the surface of the paper transport drum 21, and the leading edge of the paper P is gripped by the paper gripper. In addition, a plurality of suction holes are formed and dispersed on the surface of the paper transport drum 21. A suction device 26 generates a suction air current toward the inside of the paper transport drum 21 at each suction hole. The paper P is adsorbed to the surface of the paper transport drum 21 by this suction air current. The leading edge of the paper P transferred from the receiving cylinder 24 to the paper transport drum 21 is gripped by the paper gripper, and the paper P is adsorbed to the surface of the paper transport drum 21 by the suction air current, and is transported as the surface of the paper transport drum 21 moves.

[0017] The ink ejection section 22 ejects ink of a plurality of colors, such as C (cyan), M (magenta), Y (yellow), K (black), white, and gold, to form an image. The ink ejection section 22 includes ejection heads 23A, 23B, 23C, 23D, 23E, and 23F, each for a different ink color (type of droplets). The ejection heads 23A to 23F eject ink supplied from ink cartridges of the respective colors mounted on the inkjet recording device 1. There are no limitations on the configuration of the ejection heads 23A to 23F, and any configuration may be adopted as long as they eject liquid.

[0018] The ejection operation of the ejection heads 23A to 23F is controlled by a drive signal corresponding to image data. When the paper P transported by the paper transport drum 21 passes through an area facing the ink ejection unit 22, each color ink is ejected from nozzles provided on the lower surface (nozzle surface) of each of the ejection heads 23A to 23F. As a result, an image corresponding to the image data is formed on the paper P. The ejection heads 23A to 23F may be of a known configuration.

[0019] In this embodiment, the image forming unit 20 only needs to have a function of depositing liquid onto the paper P to form an image, and the configuration for ejecting or depositing the liquid is not particularly limited.

[0020] The drying section 30 includes a drying mechanism section 31 and a suction transport mechanism section 32. The drying mechanism section 31 dries the ink attached to the paper P in the image forming section 20. The suction transport mechanism section 32 transports the paper P transported from the image forming section 20 to the paper discharge section 40 side. That is, the paper P transported from the image forming section 20 is received by the suction transport mechanism section 32, and then transported by the suction transport mechanism section 32 to pass through the drying mechanism section 31 and delivered to the paper discharge section 40. When passing through the drying mechanism section 31, the ink on the paper P is subjected to a drying process, and liquid components such as water in the ink evaporate, the ink is fixed on the paper P, and curling of the paper P is suppressed.

[0021] The paper discharge section 40 includes a paper discharge tray 41 on which a plurality of sheets of paper P are stacked. The sheets of paper P transported from the drying section 30 by the suction transport mechanism 32 are sequentially stacked and held on the paper discharge tray 41. In this embodiment, the paper discharge section 40 only needs to have a function for discharging the sheets of paper P, and the configuration thereof is not particularly limited.

[0022] The inkjet recording apparatus 1 of the present embodiment is composed of a paper feed unit 10, an image forming unit 20, a drying unit 30, and a paper discharge unit 40, but other functional units may be added as appropriate. For example, the inkjet recording apparatus 1 may include a pre-processing unit between the paper feed unit 10 and the image forming unit 20 that performs pre-processing of the image formation, and may include a post-processing unit between the drying unit 30 and the paper discharge unit 40 that performs post-processing of the image formation.

[0023] An example of the pre-processing section is a section that performs a processing liquid application process in which processing liquid that reacts with the ink to suppress bleeding is applied to the paper P. However, the content of the pre-processing is not limited to the above-mentioned example. An example of the post-processing section is a paper inversion and transport process in which the paper P on which an image has been formed in the image forming section 20 is inverted and sent back to the image forming section 20 to form images on both sides of the paper P, or a process of binding multiple sheets of paper P on which images have been formed. However, the content of the post-processing is not limited to the above-mentioned example.

[0024] In this embodiment, the inkjet recording device 1 is described as an example of an image forming device. However, the image forming device is not limited to one that visualizes meaningful images such as characters and figures by the discharged droplets as long as it is equipped with a droplet discharge unit that discharges liquid toward the surface of the sheet material to be dried. As another example, the image forming device may be one that forms a pattern or the like that has no meaning in itself.

[0025] The medium is not limited in terms of material, and may be any material to which the liquid can be attached even temporarily, such as paper, thread, fiber, cloth, leather, metal, plastic, glass, wood, or ceramics. The medium may be, for example, a film product, a cloth product for clothing, a building material such as wallpaper or flooring, or a leather product. In this embodiment, anything to which the liquid can be attached even temporarily is referred to as a "medium." In addition, a specific example of the "droplet" is not limited to the ink ejected from the ink ejection unit 22, and may be a processing liquid used in a pre-processing unit. In other words, the present invention can be widely applied to a droplet ejection device that causes droplets to land at a target position.

[0026] Moreover, the term "ejection head" refers to a functional component that ejects or sprays liquid from an ejection hole (nozzle). As an energy generation source for ejecting liquid, ejection energy generation means such as a piezoelectric actuator, a thermal actuator using an electrothermal conversion element such as a heating resistor, and an electrostatic actuator consisting of a vibration plate and a counter electrode can be used. However, the ejection energy generation means used is not limited to the above examples.

[0027] Next, various sensors provided in the image forming unit 20 will be described with reference to Figs. 2 and 3. Fig. 2 is a side view of the main parts of the image forming unit 20. Fig. 3 is a plan view of the main parts of the image forming unit 20. Note that the plan views of the ejection heads 23A, 23B, 23C, 23D, 23E, and 23F are the same, so in Fig. 3, they are represented by the "ejection head 23". The image forming unit 20 includes a first encoder 201, a second encoder 211, and a tip position sensor 220.

[0028] The first encoder 201 is a rotary encoder composed of an encoder wheel 202 and an encoder sensor 203. The encoder wheel 202 is attached to the shaft 21a of the paper transport drum 21. Furthermore, N poles and S poles are arranged alternately in the circumferential direction on the outer circumferential surface of the encoder wheel 202. The encoder sensor 203 is disposed at a position facing the outer circumferential surface of the encoder wheel 202.

[0029] The encoder wheel 202 rotates integrally with the paper transport drum 21. The encoder sensor 203 reads the N pole and S pole arranged on the outer circumferential surface of the encoder wheel 202 and outputs a pulse signal to the controller 100 (see FIG. 4). The first encoder 201 is an example of a sensor that detects the amount of rotation per unit time (i.e., the rotational speed) of the paper transport drum 21.

[0030] The second encoder 211 is a linear encoder composed of an encoder scale 212 and a plurality of encoder sensors 213 corresponding to the plurality of ejection heads 23A to 23F, respectively. The encoder scale 212 is disposed at an end of the paper transport drum 21 in the axial direction (outside the area facing the ejection heads 23). The encoder scale 212 is also provided extending in the circumferential direction along the outer circumferential surface of the paper transport drum 21. The encoder sensor 213 is disposed at a position facing the encoder scale 212. The encoder sensor 213 is also disposed at the same position as the corresponding ejection head 23 in the circumferential direction of the paper transport drum 21. The encoder sensor 213 is also disposed adjacent to the corresponding ejection head 23 in the axial direction of the paper transport drum 21.

[0031] The encoder scale 212 rotates integrally with the paper transport drum 21. Furthermore, the encoder scale 212 has N poles and S poles arranged alternately in the extension direction. The encoder sensor 213 reads the N poles and S poles arranged on the encoder scale 212 and outputs a pulse signal to the controller 100. The second encoder 211 is another example of a sensor that detects the amount of rotation per unit time (i.e., the rotational speed) of the paper transport drum 21.

[0032] The amount of rotation of the paper transport drum 21 per unit time correlates with the amount of transport per unit time of the paper P supported and transported on the paper transport drum 21 (i.e., the transport speed). That is, the first encoder 201 and the second encoder 211 are an example of a transport speed sensor that detects the transport speed of the paper P. In the following image formation process, a process of detecting the transport speed based on the pulse signal of the second encoder 211 will be described, but the transport speed may also be detected based on the pulse signal of the first encoder 201.

[0033] The leading edge position sensor 220 is disposed upstream of the ejection heads 23A to 23F in the transport direction of the paper P, facing the paper transport drum 21. The leading edge position sensor 220 detects the position of the leading edge of the paper P carried and transported on the paper transport drum 21, and outputs the detection result to the controller 100.

[0034] Fig. 4 is a hardware configuration diagram of the inkjet recording apparatus 1. As shown in Fig. 4, the inkjet recording apparatus 1 includes a central processing unit (CPU) 101, a random access memory (RAM) 102, a read only memory (ROM) 103, a hard disk drive (HDD) 104, and an interface (I / F) 105, which are connected via a common bus 109.

[0035] The CPU 101 is a calculation means and controls the overall operation of the inkjet recording apparatus 1. The RAM 102 is a volatile storage medium capable of reading and writing information at high speed, and is used as a working area when the CPU 101 processes information. The ROM 103 is a read-only non-volatile storage medium in which programs such as firmware are stored. The HDD 104 is a non-volatile storage medium with a large storage capacity that is capable of reading and writing information, and in which an OS (Operating System), various control programs, application programs, etc. are stored.

[0036] Inkjet recording apparatus 1 processes a control program stored in ROM 103, an information processing program (application program) loaded from a storage medium such as HDD 104 to RAM 102, and the like, using a calculation function provided in CPU 101. This processing constitutes a software control unit including various functional modules of inkjet recording apparatus 1. A combination of the software control unit thus constituted and hardware resources mounted on inkjet recording apparatus 1 constitutes a functional block that realizes the functions of inkjet recording apparatus 1. That is, CPU 101, RAM 102, ROM 103, and HDD 104 constitute a controller 100 that controls the operation of inkjet recording apparatus 1. Furthermore, RAM 102, ROM 103, and HDD 104 are examples of memories.

[0037] The I / F 105 is an interface that connects the paper feed unit 10, the image forming unit 20, and the drying unit 30 to a common bus 109. That is, the controller 100 controls the paper feed unit 10, the image forming unit 20, and the drying unit 30 through the I / F 105 to execute an image forming process that forms an image indicated by image data on paper P. The image data may be received, for example, from an external device (e.g., a PC) through a communication interface, or may be generated by a scanner mounted on the inkjet recording apparatus 1 reading a document.

[0038] 5 is a diagram for explaining the lookup table stored in the HDD 104. A plurality of lookup tables 500A, 500B, 500C, etc. are stored in the HDD 104. Each of the plurality of lookup tables 500A, 500B, 500C, etc. (hereinafter, these may be collectively referred to as "lookup table 500") corresponds to one of a plurality of resolutions (e.g., 1200 dpi, 800 dpi, 400 dpi) of image data recorded on the paper P. The resolution is an example of the density of droplets that land on the medium.

[0039] The lookup table 500 includes a plurality of transport speeds (the first column from the left in FIG. 5) and a plurality of corrected ejection frequencies (the second column from the right in FIG. 5) associated with each transport speed. The "transport speed" in FIG. 5 indicates the transport speed of the paper P detected by the second encoder 211. The "corrected ejection frequency" in FIG. 5 indicates the ejection timing of ink ejected onto the paper P transported at the corresponding transport speed. Meanwhile, the calculated ejection frequency, droplet speed, landing error 1, and landing error 2 in FIG. 5 are items for explaining the lookup table 500, and do not necessarily need to be included in the lookup table 500.

[0040] The ejection frequency indicates the number of ink droplets ejected from a nozzle per unit time. That is, if the ejection frequency is high, the ink ejection interval is short (i.e., the ejection timing is early), and if the ejection frequency is low, the ink ejection interval is long (i.e., the ejection timing is late). Below, the lookup table 500A associated with 1200 dpi will be described in detail.

[0041] The calculated ejection frequency (second column from the left in FIG. 5) is the result F (Hz) obtained by substituting the transport speed V (mm / sec) and resolution d (dpi) into the following formula 1. The calculated ejection frequency F is the ejection timing for causing ink to land at the target position on the paper P transported at the corresponding transport speed when the droplet speed is fixed (for example, 10.00 m / s). The droplet speed refers to the initial speed of the ink ejected from the nozzle.

[0042] F=V ×d÷25.4 ...(Formula 1)

[0043] However, in reality, the droplet speed (third column from the left in FIG. 5) also fluctuates with the change in the calculated ejection frequency. As a result, when ink is ejected at the corresponding calculated ejection frequency onto paper P transported at the transport speed, a deviation occurs between the target position and the actual landing position, as shown in landing error 1 (third column from the right in FIG. 5).

[0044] Therefore, the developer uses a dedicated device to observe the behavior (e.g., ejection speed, landing error) of ink ejected from the ejection head 23 while changing the transport speed and ejection frequency during the manufacturing process of the inkjet recording device 1. Then, for each of a plurality of combinations of transport speeds and a plurality of resolutions, the developer determines a corrected ejection frequency that reduces the landing error (typically, minimizes the landing error) when ink is ejected at the corresponding calculated ejection frequency.

[0045] As a result, when ink is ejected at the corresponding corrected ejection frequency onto paper P transported at each transport speed, the deviation between the target position and the actual landing position becomes smaller than landing error 1, as shown in landing error 2 (the first column from the right in Figure 5).

[0046] Fig. 6 is a functional block diagram of the controller 100. As shown in Fig. 6, the controller 100 includes a transport speed specifying unit 601, a lookup table selecting unit 602, a discharge frequency determining unit 603, and a discharge control unit 604.

[0047] The transport speed specifying means 601 determines the actual transport speed of the paper P based on the pulse signal output from the second encoder 211. More specifically, the transport speed specifying means 601 specifies the transport speed by dividing the length of the N pole (or S pole) of the encoder scale 212 by the output interval (sec) between two pulse signals that are adjacent in time.

[0048] The lookup table selection means 602 selects a lookup table 500 corresponding to the resolution of an image to be formed on a paper P from among a plurality of lookup tables 500A, 500B, and 500C stored in the HDD 104. The lookup table selection means 602 may acquire the resolution through an external device (e.g., a PC) or an operation panel, or may analyze image data to specify the resolution. Then, the lookup table selection means 602 reads out the selected lookup table 500 from the HDD 104 and stores it in the RAM 102.

[0049] The discharge frequency determination means 603 determines, as the discharge frequency, the corrected discharge frequency corresponding to the conveying speed specified by the conveying speed specification means 601, from among the multiple corrected discharge frequencies in the lookup table 500 stored in the RAM 102. That is, the discharge frequency determination means 603 reads out the corrected discharge frequency corresponding to the current conveying speed from the lookup table 500 selected by the lookup table selection means 602.

[0050] The ejection control means 604 causes the ejection head 23 to eject ink droplets at the corrected ejection frequency (ejection timing) determined by the ejection frequency determination means 603. The process of the ejection control means 604 is already well known, so a detailed description thereof will be omitted.

[0051] The transport speed specifying means 601, the lookup table selecting means 602, the ejection frequency determining means 603, and the ejection control means 604 execute the above-mentioned processes in parallel for each of the ejection heads 23A to 23F. That is, the controller 100 controls the ejection timing of each of the ejection heads 23 based on the transport speed detected by the encoder sensor 213 associated with the ejection head 23.

[0052] Next, the image forming process according to this embodiment will be described with reference to Fig. 7. Fig. 7 is a flowchart of the image forming process. The controller 100 starts the image forming process, for example, when an image forming instruction for a sheet P is obtained from an operator via an operation panel, or when the instruction is received from an external device (for example, a PC) via a communication network. The image forming instruction includes, for example, image data indicating an image to be formed on the sheet P and the resolution of the image.

[0053] First, the controller 100 starts conveying the paper P received from the paper feed unit 10 via the receiving cylinder 24 by rotating the paper conveying drum 21 (S701). Then, the controller 100 waits to execute subsequent processes until the leading edge of the paper P faces the ejection head 23 (S702: No). The controller 100 determines that the leading edge of the paper P faces the ejection head 23, for example, when the leading edge of the paper P is detected by the leading edge position sensor 220 and the number of pulse signals output from the first encoder 201 reaches a predetermined threshold value.

[0054] Next, after the leading edge of the paper P faces the ejection head 23 (S702: Yes), the controller 100 executes each process of steps S703 to S706. That is, the transport speed specification means 601 specifies the transport speed (S703), and then the lookup table selection means 602 selects the lookup table 500 (S704). Next, the ejection frequency determination means 603 determines the ejection frequency (S705). Next, the ejection control means 604 causes the ejection head 23 to eject ink droplets (S706).

[0055] The controller 100 repeatedly executes the processes of steps S703 to S706 each time the second encoder 211 outputs a pulse signal until the formation of the image on the paper P is completed (S707: No). However, the process of step S706 is omitted when there are no ink droplets to be ejected (for example, when the pixel at the target position is white).

[0056] According to the above embodiment, the ejection frequency suitable for each transport speed is stored in advance as a lookup table, and the ejection frequency is determined by referring to the lookup table during image formation. This allows ink droplets to land at the target position while following the variation in the transport speed of the paper P.

[0057] Furthermore, according to the above embodiment, a lookup table is prepared for each image resolution, which makes it possible to follow variations in the transport speed of the paper P and to make ink droplets land on target positions with higher accuracy.

[0058] In addition, in FIG. 5, an example is shown in which the transport speed is set at intervals of 0.1 (m / min), but by reducing the intervals of the transport speed, it is possible to make the ink droplets land at the target positions with even higher accuracy.

[0059] The present invention is not limited to the above-described embodiment, and various modifications are possible without departing from the technical gist of the present invention. The present invention covers all technical matters included in the technical ideas described in the claims. The above-described embodiment shows a preferred example, but a person skilled in the art can realize various modifications from the disclosed contents. Such modifications are also included in the technical scope described in the claims. [Explanation of symbols]

[0060] 1: Inkjet recording device 10:Paper feed section 11: Paper feed tray 12:Feeding device 13: Registration roller pair 20: Image forming section 21: Paper transport drum 21a: Axis 22: Ink ejection section 23: Discharge head 24: Receiving torso 25: Delivery torso 26:Suction device 30:Drying section 31:Drying mechanism section 32: Suction transport mechanism 40: Paper ejection section 41: Paper output tray 100: Controller 101: CPU 102: RAM 103: ROM 104: HDD 105: Interface 109: Common bus 201: First encoder 202: Encoder wheel 203,213: Encoder sensor 211: Second encoder 212: Encoder scale 220: Tip position sensor 500: Lookup table 601: conveying speed determination means 602: Lookup table selection means 603: Discharge frequency determining means 604: Discharge control means [Prior art documents] [Patent documents]

[0061] [Patent Document 1] JP 2007-237402 A

Claims

1. A transport unit that transports the medium; a droplet ejection unit that ejects droplets onto the medium transported by the transport unit; A conveying speed sensor that detects a conveying speed of the medium; a controller that controls a timing of droplet ejection by the droplet ejection unit based on the transport speed detected by the transport speed sensor, The controller: a memory for storing a lookup table for holding an ejection frequency that has a smaller landing error of droplets with respect to a target position than the ejection frequency obtained by substituting the transport speed into F=V×d÷25.4, where V is the transport speed, d is a resolution that is a density of droplets that land on a medium, and F is an ejection frequency that is the ejection timing, and the lookup table is associated with each of a plurality of transport speeds, The ejection frequency corresponding to the conveying speed detected by the conveying speed sensor is read from the look-up table; The droplet ejection device is characterized in that the droplet ejection section is caused to eject droplets at the read ejection frequency.

2. the memory stores a plurality of the lookup tables each corresponding to a plurality of the resolutions; 2. The droplet ejection device according to claim 1, wherein the controller reads out the ejection frequency corresponding to the transport speed detected by the transport speed sensor from the lookup table associated with the set resolution.

3. the transport unit includes a rotating member that rotates while supporting the medium on an outer circumferential surface, the droplet ejection unit is disposed facing an outer circumferential surface of the rotating member, The conveying speed sensor is an encoder scale that rotates integrally with the rotating member; an encoder sensor that reads the encoder scale and outputs a pulse signal; 3. The droplet ejection device according to claim 1, wherein the encoder sensor is disposed at the same position as the droplet ejection portion in the circumferential direction of the rotating member.

4. the droplet ejection unit includes a plurality of ejection heads that are disposed at positions spaced apart from each other in a circumferential direction of the rotating member and each ejects a different type of droplet; the conveying speed sensor includes a plurality of encoder sensors that are disposed at the same positions as the plurality of ejection heads in a circumferential direction of the rotating member, 4. The droplet ejection device according to claim 3, wherein the controller controls the ejection timing of each of the ejection heads based on the pulse signal output from the corresponding encoder sensor.

5. A liquid droplet ejection device according to any one of claims 1 to 4, The image forming apparatus is characterized in that the droplet ejection section forms an image on a medium by ejecting ink droplets.

Images