Liquid droplet discharge device

Abstract

To provide a liquid droplet discharge device capable of controlling a light emission amount of a light source while detecting variation in the light amount when liquid droplets pass through light.SOLUTION: A liquid droplet discharge device comprises: a discharge head having a nozzle; a light source which emits light toward a flight space through which liquid droplets discharged from the nozzle fly; a light receiving unit which detects a received light amount associated with light emitted from the light source after passing through the flight space and outputs a detection signal having a first frequency; and a control device. The control device executes feedback processing that operates at a predetermined response frequency so as to adjust the light emission amount of the light source based on the detection signal output by the light receiving unit. In the feedback processing, in a first setting, the response frequency is set to a second frequency lower than the first frequency, or in a second setting, the response frequency is set to a third frequency approximating the first frequency and an attenuation constant ξ relating to attenuation of the detection signal is set to 0<ξ<1.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present disclosure relates to a droplet ejection device that is provided in a printing device such as an inkjet printer. [Background technology]

[0002] In a conventional inkjet printer, ink droplets are ejected from a nozzle so that they pass through light emitted from a light source and received by a light-receiving element. The light-receiving element detects the amount of light received by the ink droplets as they pass through the light. The presence or absence of nozzle ejection defects is then determined based on fluctuations in the amount of light resulting from the passage of the ink droplets. However, the amount of light received by the light-receiving element can decrease due to factors such as aging of the light source, contamination of the light source and light-receiving element due to ink droplet mist, or obstacles in the optical path. This reduces the signal strength of the detection signal from the light-receiving element, making it difficult to distinguish it from noise and resulting in reduced detection accuracy. Therefore, to prevent this reduction in the signal strength of the detection signal, a correction process is known that increases the light-emitting intensity of the light source to maintain a constant level (see Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2023-152269 Summary of the Invention [Problem to be solved by the invention]

[0004] However, the function of detecting fluctuations in the amount of light emitted by the light source when ink droplets pass through the light conflicts with the function of suppressing fluctuations in the amount of light emitted by the light source and maintaining the amount of light emitted at a constant level. Therefore, it has been a challenge to control the amount of light emitted by the light source while detecting fluctuations in the amount of light.

[0005] Therefore, an object of the present disclosure is to provide a droplet ejection device that can control the light emission amount of a light source while detecting fluctuations in the light amount when droplets pass through the light. [Means for solving the problem]

[0006] The droplet ejection device disclosed herein comprises an ejection head having a plurality of nozzles that eject droplets onto a printing medium; a light source that emits light toward a flight space into which the droplets ejected from the nozzles fly; a light receiving unit that detects the amount of light received after the light emitted from the light source has passed through the flight space and outputs a detection signal of a first frequency; and a control device, wherein the control device executes a defective ejection detection process that detects defective ejection of the nozzle based on the detection signal output by the light receiving unit; and a feedback process that operates at a predetermined response frequency to adjust the amount of light emitted by the light source based on the detection signal output by the light receiving unit, wherein the feedback process has a first setting in which the response frequency is set to a second frequency that is lower than the first frequency, or a second setting in which the response frequency is set to a third frequency that is approximate to the first frequency and an attenuation constant ξ related to the attenuation of the detection signal is set to 0<ξ<1.

[0007] According to the present disclosure, in the first setting, the response frequency of the feedback process is set to a second frequency lower than the first frequency of the detection signal, thereby delaying the responsiveness of adjusting the light emission intensity of the light source relative to the fluctuation rate of the detection signal. This reduces the impact of fluctuations in the waveform (detection waveform) of the detection signal as droplets pass through the light on the feedback process that adjusts the light emission intensity. This makes it possible to control the light emission intensity of the light source while detecting fluctuations in the light intensity as droplets pass through the light. Furthermore, in the second setting, the response frequency of the feedback process is set to a third frequency that is close to the first frequency, so the detection waveform is included in the feedback waveform. However, by setting the attenuation constant ξ to 0<ξ<1, only the specific frequency portion of the detection signal can be amplified. This increases the signal-to-noise ratio of the detection signal, making it possible to control the light emission intensity while detecting fluctuations in the light intensity. [Effects of the Invention]

[0008] According to the present disclosure, it is possible to provide a droplet ejection device that is capable of controlling the light emission amount of a light source while detecting fluctuations in the light amount when droplets pass through the light. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a perspective view illustrating a printing device provided with a droplet ejection device according to an embodiment of the present disclosure. [Figure 2] 1 is a plan view illustrating a droplet ejection device according to an embodiment of the present disclosure. [Figure 3] FIG. 2 is a cross-sectional view showing the configuration of the ejection head of FIG. [Figure 4] FIG. 2 is a block diagram showing components of the printing device of FIG. 1. [Figure 5] FIG. 2 is a block diagram showing components of an automatic light intensity adjustment circuit. [Figure 6] 10A and 10B are diagrams illustrating how ink droplets ejected from an ejection head and flying are irradiated with laser light. [Figure 7] FIG. 2 is a diagram showing an optical axis and a detection axis in a cross section of a laser beam. [Figure 8] FIG. 8A is a diagram showing the waveform of a typical detection signal, FIG. 8B is a diagram showing the waveform of a feedback signal in a first setting by a feedback circuit, and FIG. 8C is a diagram showing a waveform according to a modified example of FIG. 8B. [Figure 9] FIG. 10 is a diagram showing the waveform of a feedback signal in a second setting by the feedback circuit. [Figure 10] FIG. 10 is a diagram showing a change in gain when the damping constant related to damping property is different in the second setting by the feedback circuit. [Figure 11] FIG. 11A is a diagram showing the waveform of a typical detection signal, and FIG. 11B is a diagram showing the waveform of a feedback signal when the attenuation constant in the second setting by the feedback circuit is a predetermined value. [Figure 12] FIG. 2 is a diagram showing a beam waist portion of a laser beam. [Figure 13]FIG. 10 is a diagram showing a coordinate system used when deriving a calculation formula for sensitivity K. [Figure 14] 10 is a graph showing the detection width at which the sensitivity K is maximized for each beam waist radius in relation to the radial distance from the optical axis. [Figure 15] 10 is a graph showing the detection width at which the sensitivity K becomes a predetermined value in relation to the radial distance from the optical axis for each beam waist radius. [Figure 16] FIG. 16A is a diagram showing an example of the positional relationship between the nozzle surface of the ejection head and the optical axis, and FIG. 16B is a diagram showing another example of the positional relationship between the nozzle surface of the ejection head and the optical axis. [Figure 17] 3A and 3B are diagrams illustrating the positional relationship between the ejection head, the light source, and the detection element. DETAILED DESCRIPTION OF THE INVENTION

[0010] A droplet ejection device according to an embodiment of the present disclosure will be described below with reference to the drawings. The droplet ejection device described below is merely one embodiment of the present disclosure. Therefore, the present disclosure is not limited to the following embodiment, and additions, deletions, and modifications are possible within the scope of the present disclosure.

[0011] (First embodiment) 1 is a perspective view showing a printing device 1 provided with a droplet ejection device 1a according to an embodiment of the present disclosure. In the following, an example is disclosed in which an inkjet printer capable of printing on a print medium W, which is a three-dimensional object, is used as the image forming position 1, but the printing device 1 also includes inkjet printers that can print only on paper. In other words, the embodiment of the printing device 1 in FIG. 1 is an example and is not limited to this embodiment.

[0012] 1, directions that are orthogonal to each other are referred to as a first direction Ds, a second direction Df, and a third direction Dz. In this embodiment, for example, the first direction Ds is the movement direction of a carriage 3 (described later), the second direction Df is the transport direction of a print medium W (described later), and the third direction Dz is the up-down direction. In the following description, Ds will be referred to as the movement direction, Df will be referred to as the transport direction, and Dz will be referred to as the up-down direction.

[0013] As shown in Fig. 1, the printing device 1 includes a housing 2, operation keys 4, a display unit 5, a platen 6 on which a print medium W is placed, and an upper cover 7. The printing device 1 also includes a droplet ejection device 1a (Fig. 2) that includes an ejection head 10 and a controller unit 19 including a control device 20 (Fig. 4). The ejection head 10 is an inkjet head that ejects droplets, for example, ultraviolet-curable ink droplets Id (Fig. 6).

[0014] The housing 2 is formed in a box shape. The housing 2 has an opening 2a. Operation keys 4 are provided on the housing 2. A display unit 5 is also provided near the operation keys 4. The operation keys 4 accept operation inputs from the user. The display unit 5 is configured, for example, as a touch panel, and displays predetermined information. A part of the display unit 5 also functions as an operation key. The controller unit 19 realizes a printing function and controls the display of the display unit 5 based on inputs from the operation keys 4 or external inputs via a communication interface (not shown).

[0015] The platen 6 is configured so that the print medium W can be placed on it. The platen 6 has a predetermined thickness and is made of, for example, a rectangular plate material with the transport direction Df as its longitudinal direction. The platen 6 is removably supported by a platen support base (not shown). The platen support base is configured so that it can be moved in the transport direction Df between a printing position where printing is performed on the print medium W and a detachment position where the print medium W is detached from the platen 6 by driving a transport motor 33 (FIG. 4). As a result, the platen 6 moves the print medium W in the transport direction Df relative to the ejection head 10. During printing, the platen 6 moves in the transport direction Df, so that the print medium W placed on the platen 6 is transported along the transport direction Df. In addition, the upper cover 7 is configured so that it rotates upward when its end is lifted, exposing the interior of the housing 2.

[0016] 2, the droplet ejection device 1a includes a storage tank 62, a carriage 3 on which are mounted, for example, two ejection heads 10 (10A, 10B) and two ultraviolet irradiation devices 40 (40A, 40B), and a pair of guide rails 67. Although two ejection heads 10 and two ultraviolet irradiation devices 40 are provided, this is not limiting, and one ejection head 10 and one ultraviolet irradiation device 40 may also be provided. Furthermore, if the ejection head 10 ejects normal ink droplets that are not ultraviolet-curable ink droplets, the ultraviolet irradiation device 40 is not necessary.

[0017] The carriage 3 is supported by a pair of guide rails 67 extending in the movement direction Ds, and moves back and forth in the movement direction Ds along the guide rails 67. This allows the two ejection heads 10 (10A, 10B) and the two ultraviolet irradiation devices 40 (40A, 40B) to move back and forth in the movement direction Ds. The ejection head 10 is also connected to a storage tank 62 via a tube 62a.

[0018] In this embodiment, for example, the ejection head 10A ejects ink droplets Id of each of the colors yellow (Y), magenta (M), cyan (C), and black (K), which are sometimes collectively referred to as color inks. The ink droplets Id of these four colors are ejected onto the print medium W, thereby printing a color image on the print medium W. Meanwhile, the ejection head 10B ejects white (W) ink droplets Id and clear (Cr) ink droplets Id. When printing a color image on, for example, fabric as the print medium W, white ink droplets Id are ejected first as a base ink to reduce the effect on the color and material of the fabric, and then color ink droplets Id are ejected on top of the white ink droplets Id. Clear ink droplets Id are ejected to impart gloss or protect the printed area.

[0019] Ink is stored in the storage tanks 62. A storage tank 62 is provided for each type of ink. For example, six storage tanks 62 are provided, each storing black, yellow, cyan, magenta, white, and clear ink.

[0020] The droplet discharge device 1a further includes a purge unit 50 and a receiving unit 54. The receiving unit 54 is disposed on one side of the pair of guide rails 67 in the movement direction Ds so as to overlap with the movement area of ​​the carriage 3. The purge unit 50 is disposed on the other side of the pair of guide rails 67 in the movement direction Ds so as to overlap with the movement area of ​​the carriage 3.

[0021] The purge unit 50 has a cap 51, a suction pump 52, and an unillustrated lifting mechanism that lifts and lowers the cap 51 between a suction position and a standby position. The suction pump 52 is connected to the cap 51. At the standby position, the nozzle surface NM (FIG. 3) is separated from the cap 51. At the suction position, the nozzle surface NM is covered by the cap 51, forming a sealed space. When the suction pump 52 is driven while the cap 51 is in the suction position, a purge process is performed in which the sealed space is suctioned and ink is discharged from the nozzle holes 121a (FIG. 3). In addition, the receiver 54 receives ink droplets Id ejected from the ejection head 10 during the flushing process.

[0022] Next, the structure of the ejection head 10 will be described. As shown in FIG. 3, the ejection head 10 has a plurality of nozzles 121 that eject ink droplets Id using ink from a storage tank 62. The ejection head 10 has a laminated body of a flow path forming body and a volume changing unit. An ink flow path is formed inside the flow path forming body, and a plurality of nozzle holes 121a open in the nozzle surface NM, which is the lower surface of the flow path forming body. The volume changing unit is driven to change the volume of the ink flow path. At this time, the meniscus vibrates in the nozzle holes 121a, causing ink to be ejected.

[0023] The flow path forming body of the ejection head 10 is a laminate of a plurality of plates, and the volume changing section includes a vibration plate 155 and an actuator (piezoelectric element) 160. A common electrode 161, which will be described later, is connected to the top of the vibration plate 155.

[0024] The multiple plates are stacked, including, from bottom to top, a nozzle plate 146, a spacer plate 147, a first flow path plate 148, a second flow path plate 149, a third flow path plate 150, a fourth flow path plate 151, a fifth flow path plate 152, a sixth flow path plate 153, and a seventh flow path plate 154.

[0025] Each plate has holes and grooves of various sizes formed therein. Inside the flow path forming body where the plates are stacked, the holes and grooves are combined to form a plurality of nozzles 121, a plurality of individual flow paths 164, and a manifold 122 as ink flow paths.

[0026] The nozzles 121 are formed to penetrate the nozzle plate 146 in the stacking direction. In the nozzle surface NM of the nozzle plate 146, a plurality of nozzle holes 121a, which are the tips of the nozzles 121, are aligned in the transport direction Df to form a nozzle row.

[0027] The manifold 122 supplies ink to the pressure chambers 128 to which an ejection pressure is applied. The manifold 122 extends in the transport direction Df, and is connected to one end of each of the individual flow paths 164. That is, the manifold 122 functions as a common flow path for the ink. The manifold 122 is formed by through-holes that penetrate the first flow path plate 148 to the fourth flow path plate 151 in the stacking direction and recesses that are recessed from the lower surface of the fifth flow path plate 152, which are overlapped in the stacking direction.

[0028] The nozzle plate 146 is disposed below the spacer plate 147. The spacer plate 147 is formed of, for example, stainless steel. The spacer plate 147 has a recess 145 formed by, for example, half-etching, recessing the surface on the nozzle plate 146 side in the thickness direction of the spacer plate 147, whereby a thin portion constituting the damper portion 147a and a damper space 147b are formed. As a result, the damper space 147b is formed as a buffer space between the manifold 122 and the nozzle plate 146.

[0029] A supply port 122a communicates with the manifold 122. The supply port 122a is formed, for example, in a cylindrical shape and is provided at one end in the transfer direction Df. The manifold 122 and the supply port 122a are connected by a flow path (not shown).

[0030] Each individual flow path 164 is connected to the manifold 122. The upstream end of each individual flow path 164 is connected to the manifold 122, and the downstream end is connected to the base end of the nozzle 121. Each individual flow path 164 is composed of a first communication hole 125, a supply throttle path 126 which is an individual throttle path, a second communication hole 127, a pressure chamber 128, and a descender 129, and these components are arranged in this order.

[0031] The first communication hole 125 has a lower end connected to the upper end of the manifold 122, extends upward in the stacking direction from the manifold 122, and penetrates through an upper portion of the fifth flow path plate 152 in the stacking direction.

[0032] The upstream end of supply throttle path 126 is connected to the upper end of first communication hole 125. Supply throttle path 126 is formed by half etching, for example, and is configured as a groove recessed from the lower surface of sixth flow path plate 153. Furthermore, second communication hole 127 has its upstream end connected to the downstream end of supply throttle path 126, extends upward in the stacking direction from supply throttle path 126, and is formed to penetrate sixth flow path plate 153 in the stacking direction.

[0033] The upstream end of the pressure chamber 128 is connected to the downstream end of the second communication hole 127. The pressure chamber 128 is formed to penetrate the seventh flow path plate 154 in the stacking direction.

[0034] The descender 129 is formed by penetrating the spacer plate 147, the first flow path plate 148, the second flow path plate 149, the third flow path plate 150, the fourth flow path plate 151, the fifth flow path plate 152, and the sixth flow path plate 153 in the stacking direction. The descender 129 has an upstream end connected to the downstream end of the pressure chamber 128 and a downstream end connected to the base end of the nozzle 121. The nozzle 121 overlaps the descender 129 in the stacking direction, for example, and is disposed at the center of the descender 129 in the width direction.

[0035] The vibration plate 155 is laminated on the seventh flow path plate 154 and covers the upper openings of the pressure chambers 128 .

[0036] The actuator 160 includes a common electrode 161, a piezoelectric layer 162, and an individual electrode 163, which are arranged in this order. The common electrode 161 covers the entire surface of the vibration plate 155. The piezoelectric layer 162 covers the entire surface of the common electrode 161. The individual electrode 163 is provided for each pressure chamber 128 and is arranged on the piezoelectric layer 162. One individual electrode 163, the common electrode 161, and the portion of the piezoelectric layer 162 sandwiched between the two electrodes constitute one actuator 160.

[0037] The individual electrodes 163 are electrically connected to a head driver IC. The head driver IC receives a control signal from the control device 20, generates a drive signal (voltage signal), and applies it to the individual electrodes 163. In contrast, the common electrode 161 is always maintained at ground potential. In this configuration, the active portion of the piezoelectric layer 162 expands and contracts in the planar direction together with the common electrode 161 and the individual electrodes 163 in response to the drive signal. In response, the vibration plate 155 deforms in cooperation with the drive signal, and changes in the direction of increasing or decreasing the volume of the pressure chamber 128. As a result, an ejection pressure is applied to the pressure chamber 128, causing the ink droplet Id to be ejected from the nozzle 121.

[0038] In the ejection head 10, ink flows into the manifold 122 via the supply port 122a, then flows from the manifold 122 into the supply throttle passage 126 via the first communication hole 125, and then flows from the supply throttle passage 126 into the pressure chamber 128 via the second communication hole 127. The ink then flows through the descender 129 and into the nozzle 121. When an ejection pressure is applied to the pressure chamber 128 by the actuator 160, an ink droplet Id is ejected from the nozzle hole 121a.

[0039] 4, in addition to the above-mentioned components, the printing device 1 includes a controller unit 19, a reading device 26, motor driver ICs 30 and 31, a head driver IC 32, a transport motor 33, a carriage motor 34, an irradiation device driver IC 35, a purge driver IC 36, a light source driver IC 37, a detection driver IC 38, and a motor driver IC 39. In addition to the above-mentioned components, the droplet ejection device 1a also includes a light source 65, a detection element 67, and a rotation motor 69. The detection element 67 corresponds to a light receiving unit.

[0040] The controller unit 19 has a control device 20 configured by a CPU, storage units (ROM 21, RAM 22, EEPROM 23, HDD 24), and an ASIC 25. The control device 20 is connected to the storage units and controls the driver ICs 30 to 32, 35 to 39 and the display unit 5.

[0041] The control device 20 performs various functions by executing predetermined processing programs stored in the ROM 21. The control device 20 may be implemented as a single processor in the controller unit 19, or may be implemented as multiple processors cooperating with each other. The processing programs are read by the reading device 26 from a computer-readable recording medium KB such as a magneto-optical disk or a USB flash memory, and stored in the ROM 21. The RAM 22 stores image data received from the outside and calculation results of the control device 20. The EEPROM 23 stores various initial setting information input by the user. The HDD 24 stores specific information, etc.

[0042] The ASIC 25 is connected to motor driver ICs 30, 31, and 39, a head driver IC 32, an irradiation device driver IC 35, a purge driver IC 36, a light source driver IC 37, and a detection driver IC 38. When the control device 20 receives a print job from a user, it outputs an image recording command to the ASIC 25 based on a processing program. The ASIC 25 controls the driver ICs 30-32 and 35-39 based on the image recording command. The control device 20 moves the platen 6 in the transport direction Df by driving the conveyance motor 33 using the motor driver IC 30. The control device 20 moves the carriage 3 in the movement direction Ds by driving the carriage motor 34 using the motor driver IC 31. The control device 20 also controls a rotation motor 69 using the motor driver IC 39. By controlling the rotation motor 69, the ejection head 10 can be rotated around an axis perpendicular to the nozzle surface NM.

[0043] The control device 20 converts image data acquired from an external device or the like into ejection data for ejecting ink droplets Id onto the print medium W. The control device 20 causes the head driver IC 32 to eject ink droplets Id from the ejection head 10 based on the converted ejection data. The control device 20 also causes the irradiation device driver IC 35 to irradiate ultraviolet light from each light-emitting diode chip of the ultraviolet irradiation device 40. The control device 20 drives the purge unit 50 with the purge driver IC 36. The control device 20 controls the light source 65 with the light source driver IC 37 and the detection element 67 with the detection driver IC 38.

[0044] 5 is a block diagram showing the components of the automatic light intensity adjustment circuit 20e. The automatic light intensity adjustment circuit 20e is provided in the control device 20. The automatic light intensity adjustment circuit (auto power controller (APC) circuit) 20e controls the light intensity of the light source 65 to a constant level through feedback processing, and includes a current-voltage conversion circuit 20a, an amplifier circuit 20b, and a feedback circuit 20c. The feedback circuit 20c includes a time constant adjustment unit 20d.

[0045] The light source 65 emits laser light Lz (see FIG. 6, described below) in a specific wavelength region. Examples of the light source 65 include a light-emitting diode (LED) and a semiconductor laser (LD). The detection element 67 generates a current based on the received laser light Lz and outputs the current signal to the current-voltage conversion circuit 20a. The current-voltage conversion circuit 20a converts the current signal from the detection element 67 into a voltage signal and outputs the voltage signal to the amplifier circuit 20b. The amplifier circuit 20b amplifies the voltage signal from the current-voltage conversion circuit 20a. The amplified voltage signal is used in a discharge defect detection process, which detects discharge defects in the nozzle 121.

[0046] Here, the detection element 67 outputs the current signal as a detection signal to the feedback circuit 20c. In this case, the detection element 67 outputs a detection signal of a first frequency. An example of the first frequency is 50 kHz, but it is not limited to this. The feedback circuit 20c converts the current signal from the detection element 67 into a voltage signal and performs feedback processing. The feedback circuit 20c then outputs a signal resulting from the feedback processing to the light source 65. Based on the signal from the feedback circuit 20c, the light source 65 changes the amount of light emitted so that the amount of light received by the detection element 67 reaches a predetermined value. The feedback processing will be described in detail later.

[0047] 6 is a diagram showing how a laser beam Lz is irradiated onto an ink droplet Id in flight after being ejected from the ejection head 10. The laser beam Lz corresponds to light.

[0048] As shown in FIG. 6, the light source 65 is disposed on one side of the ejection head 10 in the optical axis direction DL of the laser beam Lz emitted from the light source 65. Note that z, which will be described later, is a coordinate axis in the optical axis direction DL from a beam waist portion BW, which will be described later. The detection element 67 is disposed on the other side of the optical axis direction DL in the optical axis direction DL in relation to the position of the ejection head 10. The light source 65 irradiates the laser beam Lz toward a flight space Sh into which ink droplets Id ejected from the nozzles 121 fly. The light source 65 is disposed in a box-shaped light source housing 65a. The light source housing 65a has a slit 65b on the side in the emission direction of the laser beam Lz emitted from the light source 65. A lens 65c is disposed in the light source housing 65a so as to cover the slit 65b from the inside of the light source housing 65a. In addition to the lens 65c, one or more lenses may be provided. The light source 65 and the detection element 67 are supported by a frame 71 that extends in the optical axis direction DL of the laser light Lz.

[0049] In this configuration, the laser light Lz emitted from the light source 65 passes through the lens 65c and is then emitted from the ejection head 10, where it is irradiated onto the ink droplets Id as they fly through the flight space Sh. The detection element 67 detects the amount of light received from the laser light Lz emitted from the light source 65 after it passes through the flight space Sh. The control device 20 executes an ejection defect detection process based on a comparison between the signal output from the detection element 67 and a reference signal to detect ejection defects such as speed abnormalities, volume abnormalities, and deviations of the ink droplets Id. Deviations refer to the ink droplets Id flying in a direction different from their normal flight direction.

[0050] FIG. 7 is a diagram showing the optical axis La and detection axis Da in the cross section of the laser beam Lz. As shown in FIG. 7, the laser beam Lz has the optical axis La at the center of the cross section of the laser beam Lz. In this embodiment, the detection axis Da is an axis that is parallel to the optical axis La and is spaced a certain distance from the position of the optical axis La in the radial direction of the laser beam Lz, and that includes the center position of the vertical line segment in the cross section of the laser beam Lz where the sensitivity K is at its maximum value. The sensitivity K is based on the product of the illuminance of the laser beam Lz emitted from the light source 65 at the position of the ink droplet Id and the illuminance gradient in the optical axis direction DL of the laser beam Lz. The sensitivity K will be described in detail later.

[0051] The feedback processing by the feedback circuit 20c of the control device 20 will be described below with reference to the drawings.

[0052] FIG. 8A shows the waveform of a typical detection signal, FIG. 8B shows the waveform of a feedback signal in a first setting by the feedback circuit 20c, and FIG. 8C shows a waveform according to a modification of FIG. 8B. FIG. 9 shows the waveform of a feedback signal in a second setting by the feedback circuit 20c. FIG. 10 shows changes in gain when the attenuation constant related to attenuation is different in the second setting by the feedback circuit 20c. FIG. 11A shows the waveform of a typical detection signal, and FIG. 11B shows the waveform of a feedback signal in a second setting by the feedback circuit 20c when the attenuation constant is a predetermined value. Note that in this embodiment, the attenuation constant is a constant for adjusting the signal-to-noise ratio of a feedback signal, which will be described later, and is a constant for adjusting the amount of fluctuation in a portion Wp of the feedback signal, which will be described later (the portion from the falling edge of the feedback signal to the rising edge of the feedback signal).

[0053] The feedback circuit 20c executes a feedback process that operates at a predetermined response frequency to adjust the amount of light emitted by the light source 65 based on the signal output by the detection element 67. In this case, the feedback circuit 20c sets the response frequency of the feedback process to a second frequency that is lower than the first frequency as a first setting. The second frequency is equal to or less than 1 / 10 of the first frequency. An example of the second frequency is 100 Hz, but is not limited to this.

[0054] As shown in FIG. 8A, in a detection signal detected and amplified by a typical detection element, a convex portion indicating the detection of an ink droplet appears, and the voltage value at the concave portion increases over time to keep the light emission amount of the light source constant.

[0055] In contrast, in this embodiment, the feedback circuit 20c sets the response frequency of the feedback process in the first setting to a second frequency lower than the first frequency. As a specific example, in the first setting, the second frequency is set to at least twice the execution frequency of the ejection defect detection process for all nozzles 121. For example, if the time required for the ejection defect detection process for all nozzles 121 (i.e., the time required for one cycle) is 100 seconds, in other words, if the execution frequency of the ejection defect detection process is 0.01 Hz, the second frequency is set to at least 0.02 Hz. As a result, as shown in FIG. 8B, a convex portion indicating the detection of ink droplets does not appear in the waveform of the feedback signal. In other words, unlike the case of FIG. 8A, the detection signal component can be cut out of the feedback signal.

[0056] As another specific example, in the first setting, the second frequency is set to at least twice the ejection frequency of the nozzle 121. For example, if the detection cycle for the ejection defect detection process for the nozzle 121 is 1000 microseconds (μs), in other words, if the execution frequency of the ejection defect detection process is 1 kHz, the second frequency is set to at least 2 kHz. This also makes it possible to cut the detection signal component in the feedback signal, as shown in FIG. 8C.

[0057] Alternatively, the feedback circuit 20c may set the response frequency of the feedback process to a third frequency that is approximate to the first frequency in the second setting, and set the attenuation constant ξ related to the attenuation of the detection signal to 0<ξ<1. The attenuation constant ξ is set by the time constant adjustment unit 20d based on the third frequency. Note that the approximation refers to a setting range of the third frequency relative to the first frequency that satisfies the condition that the gain is greater than 1 when the attenuation constant is set.

[0058] When the response frequency is set to a third frequency that is close to the first frequency, as shown in FIG. 9, a convex portion indicating the detection of an ink droplet appears in the waveform of the feedback signal at the second setting by the feedback circuit 20c, and the voltage value increases due to the feedback circuit 20c detecting a decrease in the amount of light emitted by the light source 65 and correcting the decrease.

[0059] Furthermore, when the attenuation constant ξ is changed in the range of 0<ξ<1, the gain of the feedback signal changes as shown in Figure 10. In this case, the attenuation constant ξ can be set to 0.01, at which point the gain becomes the largest. In Figure 10, fp is the first frequency, fn is the second frequency, and f is the ejection frequency of the nozzle 121.

[0060] 11A, which shows a typical detection signal, the feedback signal, as shown in FIG. 11B, shows a portion Wp where the voltage value rises due to the constant light emission of the light source 65 after a drop due to attenuation of the light intensity caused by ink droplets. By setting the attenuation constant ξ to 0.01 as described above, the amount of fluctuation in the portion Wp of the feedback signal can be maximized. This improves the signal-to-noise ratio of the feedback signal.

[0061] Here, the energy density of the laser light Lz emitted from the light source 65 toward the flight space Sh varies depending on the position in the optical axis direction DL. Therefore, the first frequency of the detection signal varies depending on the flight position of the ink droplets in the optical axis direction DL. Therefore, in this embodiment, the feedback circuit 20c may change the third frequency depending on the ejection position of the ink droplets in the feedback process performed with the second setting. In this case, the feedback circuit 20c can change the third frequency depending on the position of the nozzle 121.

[0062] Furthermore, when environmental noise having a frequency close to the first frequency of the detection signal is present, the feedback circuit 20c may change the third frequency according to the magnitude of the environmental noise in the feedback process executed in the second setting.

[0063] As described above, according to the droplet ejection device 1a, in the first setting, the response frequency of the feedback process is set to a second frequency lower than the first frequency of the detection signal, thereby delaying the responsiveness of the light source 65 when adjusting the light emission amount relative to the fluctuation rate of the detection signal. This reduces the impact of fluctuations in the waveform (detection waveform) of the detection signal when the ink droplet Id passes through the laser light Lz on the feedback process that adjusts the light emission amount. This makes it possible to control the light emission amount of the light source 65 while detecting fluctuations in the light emission amount when the ink droplet Id passes through the laser light Lz. Furthermore, in the second setting, the response frequency of the feedback process is set to a third frequency that is close to the first frequency, so the detection waveform is included in the feedback waveform. However, by setting the attenuation constant ξ to 0<ξ<1, only the portion of the detection signal with a specific frequency can be amplified. This increases the signal-to-noise ratio of the detection signal, making it possible to control the light emission amount while detecting fluctuations in the light emission amount.

[0064] Furthermore, in this embodiment, the control device 20 may cause the light source 65 to emit light before starting the discharge defect detection process. In this case, the amount of light emitted by the light source 65 can be acquired without causing the ink droplets Id to be ejected, and the amount of light emitted in the discharge defect detection process can be determined based on the acquired result.

[0065] In this embodiment, the control device 20 includes an automatic light intensity control circuit (APC circuit) 20e, which allows the control device 20 to be implemented using an existing circuit.

[0066] Furthermore, in this embodiment, in the first setting, the second frequency is set to at least twice the execution frequency of the discharge defect detection process for all the nozzles 121. This makes it possible to perform light intensity adjustment by feedback processing at least once during one cycle of the discharge defect detection process for all the nozzles 121.

[0067] Furthermore, in this embodiment, in the first setting, the second frequency is set to at least twice the ejection frequency of the nozzle 121. As a result, even if a fluctuation in the amount of light emitted occurs due to foreign matter adhering to the light source 65 during execution of the ejection defect detection process, the amount of light emitted can be adjusted before the next detection signal is output.

[0068] In this embodiment, the feedback circuit 20c may change the third frequency depending on the ink droplet ejection position in the feedback process executed at the second setting. By changing the third frequency depending on the ink droplet ejection position in this way, the signal-to-noise ratio of the detection signal can be increased.

[0069] Furthermore, in this embodiment, when environmental noise having a frequency close to the first frequency of the detection signal is present, the feedback circuit 20c may change the third frequency in accordance with the magnitude of the environmental noise in the feedback process executed at the second setting. In this regard, even when environmental noise having a frequency close to the first frequency of the detection signal is present, changing the third frequency in accordance with the magnitude of the environmental noise can ensure appropriate responsiveness of the feedback process.

[0070] (Second embodiment) Next, a method for determining the radius of the beam waist BW (beam waist radius) ω0 will be described. Fig. 12 is a diagram showing the beam waist BW of the laser light Lz. Fig. 13 is a diagram showing a coordinate system used when deriving the calculation formula for the sensitivity K.

[0071] 12, the laser beam Lz emitted from the light source 65 has a beam waist portion BW where the beam diameter is smallest in the optical axis direction DL. The radius of the cross section at this beam waist portion BW is defined as the beam waist radius ω0.

[0072] In this embodiment, as will be described below, a formula for calculating the sensitivity K when detecting discharge defects is derived, and the beam waist radius ω0 is appropriately determined based on the sensitivity K obtained by this formula.

[0073] First, considering that the laser light Lz is a beam whose intensity distribution in a plane perpendicular to its optical axis direction DL is close to a Gaussian distribution, the illuminance E of the Gaussian beam at an arbitrary position away from the focusing position can be calculated using the following formula 1. In the following formula 1, E0 is the central illuminance, z is the distance in the optical axis direction DL from the beam waist BW (FIG. 12 described below), ω(z) is the laser light radius at an arbitrary coordinate in the optical axis direction DL, and ω0 is the radius of the beam waist BW (beam waist radius). Also, in the following formula 1, x is the coordinate axis in the movement direction Ds in FIG. 13 (in other words, the coordinate in the radial direction of the laser light cross section centered on the optical axis La), y is the coordinate axis perpendicular to the movement direction Ds in FIG. 13, and z is the coordinate axis in the optical axis direction DL from the beam waist BW.

[0074]

number

[0075] Furthermore, the central illuminance E0 can be calculated using the following formula 2. In the following formula 2, P0 is the amount of light (beam power).

[0076]

number

[0077] In addition, the laser beam radius of the Gaussian beam at an arbitrary position away from the focusing position can be calculated by the following formula 3. In the following formula 3, λ is the wavelength of the laser beam Lz, and M 2 is a factor representing the quality of the laser light Lz (the quality of the light source 65).

[0078]

number

[0079] Furthermore, the coordinates {x(t), y(t), z(t)} of an ink droplet ejected from the nozzle 121 at any coordinate (Xn, Yn, Zn) after t seconds can be calculated using the following formula 4. In the following formula 4, φ is the angle from the z-axis, which serves as the reference axis in the xz plane in Fig. 13, to the x-axis direction, θ is the angle formed between the ejection direction of the ink droplet in Fig. 13 and the vertical direction, v is the ink droplet speed, and Vcr is the relative speed between the ejection head 10 and the optical axis La of the laser light Lz in the carriage movement direction.

[0080]

number

[0081] Furthermore, the detection signal intensity of an ink droplet ejected from a nozzle 121 at any coordinate (Xn, Yn, Zn) can be calculated using the following formula 5. In the formula 5, E(x, y, z) is the light illuminance at the ink droplet position, S is the cross-sectional area of ​​the ink droplet, AEI is the light-to-current conversion sensitivity, AIV is the current-to-voltage conversion sensitivity, and A is the detection circuit amplification.

[0082]

number

[0083] Here, the detection signal strength is greatest when the ink droplet reaches a plane (ZX plane) that passes through an optical axis parallel to the nozzle surface, i.e., when y = 0. Therefore, if E(x, 0, z) at this time is expressed as E(x, z), the peak value Vpeak of the detection signal strength can be calculated using the following equation 6.

[0084]

number

[0085] Furthermore, the difference in peak detection signal strength (Vpeak''-Vpeak') between normal ejection and defective ejection (ejection with distortion or the like) can be found using the following formula 7. In the following formula 7, Vpeak'' is the peak value of the detection signal strength during defective ejection, Vpeak' is the peak value of the detection signal strength during normal ejection, E'' is the light illuminance at the position of the defectively ejected ink droplet, and E' is the light illuminance at the position of the normally ejected ink droplet.

[0086]

number

[0087] From Equation 7, the peak detection signal intensity difference (Vpeak''-Vpeak') is proportional to the light illuminance difference (E''-E'). In other words, the sensitivity K to defective ejection differs depending on the position of the ink droplet ejected relative to the laser light Lz. Note that K=(E''-E').

[0088] In this case, considering the landing accuracy required for inkjet technology, the difference (dx, dz) between the landing position of a normal ejection and that of a defective ejection is considered to be a very small value compared to the light diameter. Therefore, the sensitivity K at a certain position can be considered to be the product of the illuminance E(x, z) at the ink droplet coordinates (x, z) and the illuminance gradient, as shown in the following equation 8.

[0089]

number

[0090] Here, for the ink droplet velocity v, angle θ, and angle φ in Figure 13, the parameters related to ink droplets during normal ejection are v', θ', and φ', the parameters related to ink droplets during defective ejection that we wish to detect are v'', θ'', and φ'', the flight coordinates of normally ejected ink droplets are (x', z'), and the flight coordinates of defectively ejected ink droplets are (x'', z''). In this case, the sensitivity K(x', z') to defective ejection can be calculated using the following equation 9.

[0091]

number

[0092] Here, the detection signal strength peaks when the y coordinate of the ink droplet becomes 0. The time t at this time is the time when y(t) = 0, so if the distance from the nozzle to the XZ plane is taken as h(=-Yn), then the following formulas 10, 11, and 12 hold.

[0093]

number

[0094]

number

[0095]

number

[0096] However, because the calculations for each of the above equations are complicated, it can be considered realistically as follows: That is, since there is a relationship that the illuminance gradient ∂E / ∂z in the optical axis direction DL<< the illuminance gradient ∂E / ∂x in the light cross section direction, the above ∂E / ∂z can be ignored, and therefore the following equation 13 holds. Therefore, the following equation 14, which is a formula for calculating the sensitivity K, can be derived.

[0097]

number

[0098]

number

[0099] For the reasons described above, when distinguishing between normal and defective ejection from differences in signal waveforms due to differences in the passing coordinates of ink droplets, such as in the case of distortion, the sensitivity K to defective ejection can be considered to be the product of the illuminance E(x, z) and the illuminance gradient, as described above.

[0100] In this embodiment, the control device 20 determines the beam waist radius ω0 based on the sensitivity K in the discharge defect detection process. A specific description will be given below. Fig. 14 is a graph showing the detection width DW1 that becomes the maximum value in the relationship between the sensitivity K and the radial distance from the optical axis La for each beam waist radius ω0.

[0101] As shown in Fig. 14, for each beam waist radius ω0, the sensitivity K changes in a wave-like manner according to the radial distance of the beam waist portion BW from the radial center position of the laser beam Lz (i.e., the optical axis La). The maximum value of the sensitivity K differs for each beam waist radius ω0. The sensitivity K and the radial distance corresponding to the sensitivity K for each beam waist radius ω0 are stored in advance in the storage unit. The sensitivity K for each beam waist radius ω0 and the radial distance corresponding to the sensitivity K in Fig. 15, which will be described later, are the same as those in Fig. 14.

[0102] The control device 20 causes the light source 65 to emit laser light Lz having a beam waist radius ω0 at which the detection width, which is the radial distance at which the sensitivity K satisfies the requirement that the sensitivity K is equal to or greater than the SN satisfactory sensitivity, is maximized, in the relationship between the sensitivity K and the radial distance from the optical axis La in FIG. 14. The SN satisfactory sensitivity is the sensitivity K that satisfies the SN ratio of a predetermined value or greater, and is based on the amount of light emitted by the light source 65, the light receiving sensitivity of the detecting element 67, and the amount of noise in the detecting element 67. In the example of FIG. 14, the SN satisfactory sensitivity is set to 0.10000 (μW / μm 2 In this case, the control device 20 sets the sensitivity K to 0.10000 (μW / μm 2 ) is obtained as the maximum detection width DW1 among the detection widths that are the radial distances that satisfy the condition that the detection width DW1 is equal to or greater than the detection width DW1. Then, the control device 20 causes the light source 65 to emit a laser beam Lz having a beam waist radius ω0 of 110 μm, which corresponds to the detection width DW1.

[0103] Alternatively, the control device 20 may control the light source 65 as follows: Fig. 15 is a graph showing the detection width DW2 that becomes a predetermined value in the relationship between the sensitivity K and the radial distance from the optical axis La for each beam waist radius ω0.

[0104] The control device 20 causes the light source 65 to emit laser light Lz having a beam waist radius ω0 at which the detection width, which is a distance that satisfies the requirement that the sensitivity K is equal to or greater than the SN satisfactory sensitivity, is a predetermined value, in the relationship between the sensitivity K and the radial distance from the optical axis La in FIG. 15. In the example of FIG. 15, the SN satisfactory sensitivity is set to 0.10000 (μW / μm 2 In this case, the control device 20 is set to a sensitivity K of 0.10000 (μW / μm 2 ) or more, a detection width DW2 is acquired that is a predetermined value (e.g., 20 μm). The predetermined value may have a certain range. The control device 20 causes the light source 65 to emit a laser beam Lz having a beam waist radius ω0 of 150 μm, which corresponds to the detection width DW2.

[0105] Next, the beam waist radius ω0 to be determined under predetermined conditions will be explained using a specific example.

[0106] For example, if the detection length is 36 mm and the factor M relating to the quality of the laser light Lz is 2 Under the given conditions that the wavelength of the laser light Lz is 450 nm and the light intensity P0 is 5000 μW, the SN satisfactory sensitivity is P0 × 0.00002 (μW / μm 2 ), it is preferable that the beam waist radius is set to 20 μm to 160 μm. The detection length is the length of the nozzle surface NM in the optical axis direction DL. More specifically, the detection length is the length in the optical axis direction DL between the nozzle 121 located at one end in the optical axis direction DL and the nozzle 121 located at the other end.

[0107] FIG. 16A is a diagram showing an example of the positional relationship between the nozzle surface NM of the ejection head 10 and the optical axis La, and FIG. 16B is a diagram showing another example of the positional relationship between the nozzle surface NM of the ejection head 10 and the optical axis La.

[0108] The nozzle surface NM of the ejection head 10 is formed, for example, in a rectangular shape. As shown in FIGS. 16A and 16B, the ejection head 10 has a nozzle surface NM having a long side Ls and a short side Ss. In FIG. 16A, the laser light Lz is emitted from the light source 65 so that the optical axis La of the laser light Lz is parallel to the short side Ss. In this embodiment, for example, when the detection length is 100 mm and the factor M relating to the quality of the laser light Lz is 2 Under the given conditions that the value of SN satisfies P0 × 0.000008 (μW / μm 2 ), the beam waist radius is preferably set to 30 μm to 260 μm.

[0109] 16B, the laser beam Lz is emitted from the light source 65 so that the optical axis La of the laser beam Lz is parallel to the long side Ls. In this embodiment, for example, when the detection length is 1000 mm and the factor M 2 Under the given conditions that the wavelength of the laser light Lz is 450 nm and the light intensity P0 is 5000 μW, the SN satisfactory sensitivity is P0 × 0.0000016 (μW / μm2 ), the beam waist radius is preferably set to 70 μm to 600 μm.

[0110] As described above, in this embodiment, it is possible to appropriately determine the beam waist radius ω0 that can obtain a certain level of discrimination accuracy in the discharge defect detection process.

[0111] In this embodiment, the SN sufficiency sensitivity is based on the amount of light emitted by the light source 65, the light receiving sensitivity of the detection element 67, and the amount of noise in the detection element 67. In this case, the SN sufficiency sensitivity can be obtained by adjusting the amount of light emitted by the light source 65, the light receiving sensitivity of the detection element 67, and the amount of noise in the detection element 67.

[0112] Furthermore, in this embodiment, the control device 20 causes the light source 65 to emit a laser beam Lz having a beam waist radius ω0 that maximizes the detection width, which is the radial distance that satisfies the requirement that the sensitivity K is equal to or greater than the SN satisfactory sensitivity. This widens the detection width, allowing ink droplets to be ejected so that they pass within the detection width even if the relative positional accuracy between the ejection head 10 and the optical axis La is poor. This improves robustness against the relative positional accuracy between the ejection head 10 and the optical axis La. In other words, even if the positional accuracy is poor, the detection width can be maximized, so the accuracy of detecting ejection defects does not decrease. Furthermore, since there is no need to emit light with a beam waist radius ω0 larger than necessary, the optical system can be simplified in configuration. This allows for low cost and space savings.

[0113] In this embodiment, the control device 20 may also cause the light source 65 to emit a laser beam Lz having a beam waist radius ω0 such that the detection width, which is a distance that satisfies the requirement that the sensitivity K is equal to or greater than the SN satisfactory sensitivity, is a predetermined value. In this case, since it is not necessary to emit a laser beam Lz having a beam waist radius ω0 larger than necessary, the optical system can be configured simply, thereby achieving low cost and space savings.

[0114] In this embodiment, it is preferable that the beam waist radius ω0 is set to 20 μm to 160 μm. In this case, the discharge defect detection process can be performed while ensuring the desired sensitivity K and the necessary detection length.

[0115] Furthermore, in this embodiment, the laser light Lz may be emitted from the light source 65 so that the optical axis La of the laser light Lz is parallel to the short side Ss. In this case, in a configuration in which the optical axis La of the laser light Lz is parallel to the short side Ss of the nozzle surface NM, the ejection defect detection process can be executed while ensuring the desired sensitivity K and the necessary detection length.

[0116] Furthermore, in this embodiment, the laser light Lz may be emitted from the light source 65 so that the optical axis La of the laser light Lz is parallel to the long side Ls. In this case, in a configuration in which the optical axis La of the laser light Lz is parallel to the long side Ls of the nozzle surface NM, the ejection defect detection process can be executed while ensuring the desired sensitivity K and the necessary detection length.

[0117] (Variation) The present disclosure is not limited to the above-described embodiments, and various modifications are possible without departing from the spirit of the present disclosure. For example, the following modifications are possible.

[0118] FIG. 17 is a diagram showing the positional relationship between the discharge head 10, the light source 65, and the detection element 67.

[0119] 17, the light source 65 and the detection element 67 may be disposed below the discharge head 10. In this case, the light source 65 is disposed inside one end of the discharge head 10 in the optical axis direction DL of the laser light Lz, and the detection element 67 is disposed inside the other end of the discharge head 10 in the optical axis direction DL of the laser light Lz. In other words, "the light source 65 and the detection element 67 are disposed below the discharge head 10" does not mean that the light source 65 and the detection element 67 are disposed below the discharge head 10.

[0120] During high-gap printing, the droplet ejection pressure is relatively high, increasing the reliability of ink droplets being ejected straight from the nozzles 121. According to the above configuration, when performing high-gap printing, by arranging the light source 65 and the detection element 67 below the ejection head 10, ink droplets ejected from the nozzles 121 can pass through the laser light Lz even if the optical path length (the distance from the light source 65 to the detection element 67) is short, and the optical path length can be shortened. This allows for space saving in the optical axis direction DL. Furthermore, if a certain distance or more is maintained between the ejection head 10 and the light source 65 and the detection element 67, the effect of distortion on the detection signal becomes significant, resulting in improved detection accuracy. [Explanation of symbols]

[0121] 1a Droplet discharge device 10 Discharge head 20 Control device 20e Automatic light intensity adjustment circuit 65 Light source 67 Detector element 121 nozzle BW Beam waist DL optical axis direction ID ink drops La optical axis Ls Long side of nozzle surface Lz laser light NM nozzle surface Sh flight space Ss Short side of nozzle surface W Printing medium ω0 beam waist radius

Claims

1. an ejection head having a plurality of nozzles for ejecting droplets onto a print medium; a light source that emits light toward a flight space in which the droplets ejected from the nozzle fly; a light receiving unit that detects a received light amount of the light emitted from the light source after the light has passed through the flight space and outputs a detection signal of a first frequency; a control device; The control device a discharge defect detection process that detects discharge defects of the nozzles based on the detection signal output by the light receiving unit; a feedback process that operates at a predetermined response frequency to adjust the amount of light emitted by the light source based on the detection signal output by the light receiving unit; In the feedback process, as a first setting, the response frequency is set to a second frequency lower than the first frequency, or as a second setting, the response frequency is set to a third frequency approximate to the first frequency and an attenuation constant ξ related to the attenuation of the detection signal is set to 0<ξ<1, the droplet ejection device.

2. The droplet ejection device according to claim 1 , wherein the control device causes the light source to emit light before the ejection defect detection process starts.

3. The droplet ejection device according to claim 1 , wherein the control device includes an automatic power control circuit (APC circuit) that executes the feedback process.

4. The droplet ejection device according to claim 1 , wherein in the first setting, the second frequency is at least twice the frequency at which the ejection defect detection process is executed for all the nozzles.

5. The droplet ejection device according to claim 1 , wherein in the first setting, the second frequency is at least twice the ejection frequency of the nozzle.

6. the energy density of the light emitted from the light source toward the flight space varies depending on the position in the optical axis direction of the light, and therefore the first frequency of the detection signal varies depending on the flight position of the droplet in the optical axis direction; The droplet ejection device according to claim 1 , wherein the control device changes the third frequency in accordance with the ejection position of the droplet in the feedback process executed under the second setting.

7. The droplet ejection device according to claim 1 , wherein the control device changes the third frequency in the feedback process executed under the second setting in accordance with the magnitude of environmental noise.

8. the light emitted from the light source has a beam waist portion, which is a portion where the light diameter is smallest in the optical axis direction; The droplet ejection device described in claim 1, wherein the control device, in the ejection defect detection process, determines a beam waist radius, which is the radius of the light in the beam waist portion, based on sensitivity based on the product of the illuminance at the position of the droplet by the light emitted from the light source and the illuminance gradient in the optical axis direction of the light.

9. The droplet ejection device according to claim 8 , wherein the SN sufficiency sensitivity, which is the sensitivity that satisfies an SN ratio of a predetermined value or more, is based on the amount of light emitted by the light source, the light receiving sensitivity of the light receiving unit, and the amount of noise in the light receiving unit.

10. The droplet ejection device described in claim 9, wherein the control device emits light to the light source having a beam waist radius at which the detection width, which is the distance at which the sensitivity is greater than or equal to the SN satisfying sensitivity, is maximized in relation to the sensitivity and the distance from the radial center position of the light in the beam waist portion, depending on the beam waist radius.

11. The droplet ejection device described in claim 9, wherein the control device emits light to the light source having a beam waist radius such that, in a relationship between the sensitivity and the distance from the radial center position of the light in the beam waist portion according to the beam waist radius, the detection width, which is the distance that satisfies that the sensitivity is equal to or greater than the SN sufficiency sensitivity, is a predetermined value.

12. 9. The droplet ejection device according to claim 8, wherein the beam waist radius is 20 μm to 160 μm.

13. the ejection head has a nozzle surface having long sides and short sides; the control device causes the light source to emit the light such that the optical axis of the light is parallel to a short side of the nozzle surface; 9. The droplet ejection device according to claim 8, wherein the beam waist radius is 30 μm to 260 μm.

14. the ejection head has a nozzle surface having long sides and short sides; the control device causes the light source to emit the light so that the optical axis of the light is parallel to a long side of the nozzle surface; 9. The droplet ejection device according to claim 8, wherein the beam waist radius is 70 μm to 600 μm.

15. The droplet ejection device according to claim 8 , wherein the light source and the light receiving unit are disposed below the ejection head.

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