Patent Publication Number: US-2023140394-A1

Title: Liquid ejecting device

Description:
The present application is based on, and claims priority from JP Application Serial Number 2021-177281, filed on Oct. 29, 2021, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a liquid ejecting device such as a printer. 
     2. Related Art 
     For example, as in JP-A-2020-82475, there is a liquid ejecting device that ejects ink serving as one example of a liquid from a liquid ejecting head to perform printing. The liquid ejecting head causes a vibrating plate to be displaced to eject the ink from a nozzle. The displaced vibrating plate performs damped oscillation. This damped oscillation is also called residual vibration. In a case of the residual vibration, a damping ratio of frequency and amplitude varies when an abnormality of ejection occurs. The liquid ejecting device includes an ejection-abnormality detecting circuit serving as one example of an ejection-failure detecting unit configured to identify a cause of an ejection failure on the basis of the residual vibration. 
     In a case of JP-A-2020-82475, identification is made on the basis of residual vibration as to whether the cause of the ejection failure is the presence of air bubbles, or an increase in the viscosity of a liquid, or attachment of foreign substance. However, the cause may be difficult to be identified in some cases such as when foreign substances are attached, for example. When the cause of the ejection failure cannot be identified, it is difficult to deal with the failure in an appropriate manner. 
     SUMMARY 
     The liquid ejecting device that solves the problem described above includes a liquid ejecting head configured to eject a liquid from a plurality of nozzles to perform printing, a camera configured to image a nozzle surface at which the plurality of nozzles are provided, an ejection-failure detecting unit configured to detect whether the plurality of nozzles have an ejection failure, a maintenance unit configured to perform maintenance of the liquid ejecting head, a notification unit configured to perform notification, and a control unit, in which, when the ejection-failure detecting unit detects the ejection failure, the control unit causes the camera to image a nozzle in which the ejection failure is detected, infers a cause of the ejection failure based on a result of the imaging, and causes at least one of the maintenance and the notification to be performed based on the inferred cause. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic view illustrating an embodiment of a liquid ejecting device. 
         FIG.  2    is a schematic cross-sectional view illustrating a liquid ejecting head. 
         FIG.  3    is a block diagram illustrating an electrical configuration of the liquid ejecting device. 
         FIG.  4    is a diagram illustrating a calculation model for simple harmonic motion representing residual vibration of a vibrating plate. 
         FIG.  5    is an explanatory diagram used to explain a relationship between an increase in the viscosity of a liquid and waveforms of the residual vibration. 
         FIG.  6    is an explanatory diagram used to explain a relationship between the existence of air bubbles and a waveform of the residual vibration. 
         FIG.  7    is a flowchart showing a nozzle inspection routine. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Embodiments 
     Liquid Ejecting Device 
     Below, an embodiment of a liquid ejecting device will be described with reference to the drawings. For example, the liquid ejecting device is an ink jet-type printer configured to eject ink serving as one example of a liquid onto a medium such as a sheet, fiber, vinyl, a plastic component, a metal component, to perform printing. 
     In the drawings, on the assumption that a liquid ejecting device  11  is placed on a horizontal surface, the direction of gravity is illustrated as the Z-axis, and directions along the horizontal surface are illustrated by the X-axis and the Y-axis. The X-axis, the Y-axis, and the Z-axis are perpendicular to each other. In the following description, a direction parallel to the X-axis is also referred to as a scanning direction X. 
     As illustrated in  FIG.  1   , the liquid ejecting device  11  may include a supporting leg portion  12 , an outer case  13 , a support  14 , a guide shaft  15 , a supporting mechanism  16 , a driving mechanism  17 , and a carriage  18 . The liquid ejecting device  11  includes a liquid ejecting head  20 , a camera  21 , a maintenance unit  22 , a notification unit  23 , and a control unit  24 . 
     The supporting leg portion  12  supports the outer case  13 . The liquid ejecting device  11  may include one or more supporting leg portions  12 . 
     The outer case  13  may include an opening/closing section  26  configured to enable access to the inside thereof. By opening the opening/closing section  26 , a user can perform maintenance within the liquid ejecting device  11 , which includes cleaning of the liquid ejecting head  20  or replacement of the liquid ejecting head  20 . 
     The support  14  supports a medium  27 . The medium  27  is transported on the support  14  in a transport direction by a transport unit that is not illustrated. In the present embodiment, the transport direction is a direction parallel to the Y-axis. After printing is performed on the support  14 , the medium  27  is discharged outside of the outer case  13 . 
     The liquid ejecting device  11  may include one or more guide shafts  15 . In the present embodiment, the guide shaft  15  extends along the X-axis. The guide shaft  15  guides movement of the carriage  18 . 
     The supporting mechanism  16  supports the guide shaft  15 . The supporting mechanism  16  may support the guide shaft  15  so as to be able to ascend and descend. 
     The carriage  18  holds the liquid ejecting head  20 . The carriage  18  reciprocates along the guide shaft  15  with drive of the driving mechanism  17 . That is, the carriage  18  moves the liquid ejecting head  20  along the guide shaft  15  toward the scanning direction X and a direction opposite to the scanning direction X. In other words, the liquid ejecting head  20  is able to move in the scanning direction X. 
     The liquid ejecting head  20  ejects a liquid from a plurality of nozzles  28  to perform printing. The liquid ejecting head  20  includes a nozzle surface  29  at which the plurality of nozzles  28  are provided. In the present embodiment, the liquid ejecting head  20  discharges a liquid while moving, to perform printing on the medium  27  supported at the support  14 . 
     The carriage  18  holds one or more first liquid accommodating bodies  31 . A plurality of first liquid accommodating bodies  31  may individually accommodate different types of liquids. The different types of liquids are, for example, inks having different colors. The first liquid accommodating bodies  31  may be configured to be supplied with liquids from another second liquid accommodating body  32  through a supply tube that is not illustrated. The number of second liquid accommodating bodies  32  provided may be the same number of first liquid accommodating bodies  31 , for example. 
     Each of the first liquid accommodating bodies  31  and the second liquid accommodating body  32  is, for example, a tank, a cartridge, or a pack configured to accommodate the liquid. The first liquid accommodating bodies  31  and the second liquid accommodating body  32  may be the same item or may be different items. The first liquid accommodating bodies  31  may be configured to be attached to the carriage  18  in a detachable manner. The second liquid accommodating body  32  is disposed inside of or outside of the outer case  13 . The second liquid accommodating body  32  may be attached in a detachable manner. 
     The liquid ejecting device  11  may include one or more cameras  21 . In the present embodiment, the liquid ejecting device  11  includes two cameras  21 . In the present embodiment, one of the cameras  21  is also referred to as a first camera  21   f , and the other one of the cameras  21  is referred to as a second camera  21   s . The first camera  21   f  and the second camera  21   s  are provided with the support  14  being interposed between them in the scanning direction X. 
     Each of the cameras  21  is able to image the nozzle surface  29 . That is, the first camera  21   f  is able to image the nozzle surface  29  of the liquid ejecting head  20  located at a first image-capturing position P 1  indicated by the solid line in  FIG.  1   . The second camera  21   s  is able to image the nozzle surface  29  of the liquid ejecting head  20  located at a second image-capturing position P 2  indicated by the long dashed double-short dashed line in  FIG.  1   . 
     The liquid ejecting device  11  may include a movement unit  34  configured to move the camera  21 . The movement unit  34  may move at least one camera  21  of the plurality of cameras  21 . In the present embodiment, the movement unit  34  moves the first camera  21   f . The movement unit  34  is able to move the first camera  21   f  in a movement direction differing from the scanning direction X. The movement direction is, for example, a direction parallel to the Y-axis. The movement unit  34  moves the first camera  21   f  located at a standby position, which is not illustrated, in the movement direction so as to be located at an imaging position Pi. The imaging position Pi is a position where the nozzle  28  of the liquid ejecting head  20  located at the first image-capturing position P 1  serving as one example of an image-capturing position is able to be imaged. 
     The liquid ejecting device  11  may include one or more covers  35  configured to protect the camera  21 . In the present embodiment, the liquid ejecting device  11  includes two covers  35 . The covers  35  may be of a fixed type or may be of a movable type. 
     In the present embodiment, the cover  35  configured to protect the first camera  21   f  is of a fixed type. Of the first camera  21   f  located at the standby position, the cover  35  for the first camera  21   f  covers at least a lens that the first camera  21   f  includes. As the first camera  21   f  is moved to the imaging position Pi, the lens is deviated from the cover  35 . 
     In the present embodiment, the cover  35  configured to protect the second camera  21   s  is of a movable type. The cover  35  for the second camera  21   s  is able to move to a covering position where at least a lens that the second camera  21   s  includes is covered and an exposing position where the lens is exposed. The cover  35  for the second camera  21   s  may be located at the covering position during the time when the second camera  21   s  does not perform imaging and may move to the exposing position when the second camera  21   s  performs imaging. 
     The first camera  21   f  and the second camera  21   s  may have different imaging ranges in terms of imaging. For example, the first camera  21   f  may perform imaging in a manner such that a portion of the nozzle surface  29  is enlarged. For example, the second camera  21   s  may perform imaging the entire nozzle surface  29 . 
     When the imaging range in the scanning direction X is a portion of the nozzle surface  29 , the control unit  24  may adjust the first image-capturing position P 1  and the second image-capturing position P 2  such that the nozzle  28  to be imaged exists in the imaging range. That is, the first image-capturing position P 1  and the second image-capturing position P 2  are not limited to positions where the center, in the scanning direction X, of the liquid ejecting head  20  is immediately above the center of the camera  21 , and may be positions where these centers are not aligned in the scanning direction X. 
     Specifically, the first image-capturing position P 1  may be a position at which a given nozzle  28  can be imaged with the first camera  21   f  located at the imaging position Pi. The second image-capturing position P 2  may be a position at which a given nozzle  28  can be imaged with the second camera  21   s . The given nozzle  28  may be, for example, a nozzle  28  for which an ejection failure of the liquid ejecting head  20  is detected. 
     The maintenance unit  22  performs maintenance of the liquid ejecting head  20 . The maintenance unit  22  may include a wiper  37 , a cleaning unit  38 , and a cleaning member  39 . The cleaning unit  38  may include a cap  40  and a suction pump  41 . In the present embodiment, the wiper  37 , the cap  40 , and the cleaning member  39  are individually able to move to a position where they can be in contact with the liquid ejecting head  20  and a position where they are not in contact with the liquid ejecting head  20 . 
     The wiper  37  includes, for example, a sheet-like elastic body. The wiper  37  causes the elastic body to be brought into contact with the nozzle surface  29  of the liquid ejecting head  20  that is passing through above the wiper  37  to be able to wipe the nozzle surface  29 . The maintenance in which the wiper  37  wipes the liquid ejecting head  20  is also referred to as wiping. 
     The cap  40  is opposed to the liquid ejecting head  20  located at a cleaning position that is not illustrated. In the present embodiment, the cleaning position is disposed immediately above the cap  40 . When the cap  40  ascends in a case where the liquid ejecting head  20  is at the cleaning position, the cap  40  is brought into contact with the nozzle surface  29  so as to surround the nozzle  28 . As the cap  40  is brought into contact with the liquid ejecting head  20 , a closed space where the nozzle  28  is opened is formed between the cap  40  and the liquid ejecting head  20 . The maintenance in which the cap  40  is brought into contact with the nozzle surface  29  so as to surround the nozzle  28  is also referred to as capping. The capping is performed to prevent the nozzle  28  from drying when the liquid ejecting device  11  is at rest or stopped. 
     The suction pump  41  reduces the pressure within the closed space formed by the cap  40  with the liquid ejecting head  20 . This causes the liquid within the liquid ejecting head  20  to be forcibly discharged through the nozzle  28 . The maintenance in which the suction pump  41  causes the liquid to be discharged is also referred to as suction cleaning. The cleaning unit  38  is able to perform the suction cleaning serving as one example of cleaning in which a liquid is forcibly discharged from the liquid ejecting head  20 . 
     The cap  40  may accommodate the liquid ejected from the liquid ejecting head  20  located at the cleaning position. The maintenance in which the liquid is ejected from the liquid ejecting head  20  is also referred to as flushing. 
     The cleaning member  39  is able to clean a side surface of the liquid ejecting head  20 . The cleaning member  39  may include, for example, an elastic body such as rubber. The cleaning member  39  may include an absorption body that is able to absorb a liquid. 
     The notification unit  23  performs notification. The notification unit  23  may include a display unit  43  that can display various types of information. The display unit  43  performs display as the notification. The display unit  43  may be a monitor or may be a touch panel at which operation can be performed. The notification unit  23  may include a speaker  44  that can produce sounds. 
     As illustrated in  FIG.  2   , the liquid ejecting device  11  may include a supply flow path  46 . The supply flow path  46  couples the first liquid accommodating body  31  and the liquid ejecting head  20 . 
     The liquid ejecting head  20  may include a shared liquid chamber  48 , a plurality of pressure chambers  49 , a plurality of communication paths  50 , a plurality of actuators  51 , a vibrating plate  52 , and an accommodation chamber  53 . 
     The supply flow path  46  is coupled to the shared liquid chamber  48 . A liquid is supplied from the first liquid accommodating body  31  through the supply flow path  46  to the shared liquid chamber  48 . 
     Each of the pressure chambers  49  communicates with the shared liquid chamber  48  through the corresponding communication path  50 , and also communicates with the corresponding nozzle  28 . A portion of a wall surface of the pressure chamber  49  is formed of the vibrating plate  52 . 
     The accommodation chamber  53  is disposed at a position differing from that of the shared liquid chamber  48 . The accommodation chamber  53  accommodates the actuator  51 . The actuator  51  is provided at a surface of the vibrating plate  52  that is disposed at an opposite side from a portion that faces the pressure chamber  49 . 
     In the present embodiment, the actuator  51  is formed of a piezoelectric element that contracts when a driving voltage is applied. After the vibrating plate  52  is caused to deform in association with contraction of the actuator  51  due to application of the driving voltage, the application of the driving voltage to the actuator  51  is stopped. This changes the volume of the pressure chamber  49  to eject the liquid within the pressure chamber  49  from the nozzle  28  as liquid droplets. 
     As illustrated in  FIG.  3   , the control unit  24  comprehensively controls constituent elements of the liquid ejecting device  11 . The control unit  24  includes an interface unit  55 , a CPU  56 , a memory  57 , a control circuit  58 , and a driving circuit  59 . The interface unit  55  transmits and receives data between a computer  60  serving as an external device and the liquid ejecting device  11 . The driving circuit  59  generates a drive signal that causes the actuator  51  to drive. 
     The CPU  56  is an arithmetic processing device. The memory  57  is a storage device used to secure a region where a program for the CPU  56  is stored, a working region, or the like, and includes a storage element such as a RAM, an EEPROM, or the like. The CPU  56  controls the driving mechanism  17 , the camera  21 , the maintenance unit  22 , the notification unit  23 , the liquid ejecting head  20 , and the like through the control circuit  58  in accordance with programs stored in the memory  57 . 
     The liquid ejecting device  11  includes a detection unit group  62 . The detection unit group  62  is controlled by the control unit  24 . The detection unit group  62  monitors situations in the liquid ejecting device  11 . The detection unit group  62  outputs results of detection to the control unit  24 . The detection unit group  62  may include, for example, a linear encoder configured to detect a state of movement of the carriage  18 , and a medium detection sensor configured to detect the medium  27 . 
     The detection unit group  62  includes an ejection-failure detecting unit  63 . The ejection-failure detecting unit  63  detects the presence or absence of an ejection failure at the plurality of nozzles  28 . In the present embodiment, the ejection-failure detecting unit  63  is a circuit configured to detect a vibration waveform of the pressure chamber  49  to detect a state within the pressure chamber  49 . The ejection-failure detecting unit  63  detects the presence or absence of an ejection failure at the plurality of nozzles  28  on the basis of the residual vibration of the vibrating plate  52  that is displaced. The ejection-failure detecting unit  63  may include a piezoelectric element that constitutes the actuator  51 . 
     The plurality of actuators  51  are individually driven by the driving circuit  59  to partially displace the vibrating plate  52 . The displaced vibrating plate  52  causes a liquid to be ejected from a nozzle  28  corresponding to the driven actuator  51 . 
     That is, once a voltage is applied to the actuator  51  in response to a signal from the driving circuit  59 , the vibrating plate  52  bends and is deformed. This varies a pressure within the pressure chamber  49 . This variation causes the vibrating plate  52  to vibrate for a while. This vibration is called residual vibration. Detecting a state of the pressure chamber  49  and a state of the nozzle  28  communicating with the pressure chamber  49  on the basis of the state of the residual vibration is called nozzle inspection. 
       FIG.  4    is a diagram illustrating a calculation model for simple harmonic motion representing the residual vibration of the vibrating plate  52 . 
     As the driving circuit  59  applies a drive signal to the actuator  51 , the actuator  51  expands and contracts in accordance with the voltage of the drive signal. The vibrating plate  52  is flexed in response to expansion and contraction of the actuator  51 . This causes the volume of the pressure chamber  49  to expand and then contract. At this time, due to the pressure occurring within the pressure chamber  49 , a portion of the liquid that exists in the pressure chamber  49  is ejected from the nozzle  28  as liquid droplets. 
     During the series of operations of the vibrating plate  52  described above, the vibrating plate  52  performs free oscillation at a natural oscillation frequency determined by: a flow path resistance r determined by the shape of a flow path in which a liquid flows, the viscosity of the liquid, and the like; an inertance m due to the weight of the liquid in the flow path; and a compliance C of the vibrating plate  52 . This free oscillation of the vibrating plate  52  is the residual vibration. 
     The calculation model for the residual vibration of the vibrating plate  52  illustrated in  FIG.  4    can be expressed by using a pressure P, the inertance m, the compliance C, and the flow path resistance r described above. The step response at the time of applying the pressure P to the circuit in  FIG.  4    is calculated in terms of the volume velocity u. This makes it possible to obtain the following equation. 
     
       
         
           
             
               
                 
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       FIG.  5    is an explanatory diagram illustrating a relationship between an increase in the viscosity of the liquid and the waveform of the residual vibration. In  FIG.  5   , the horizontal axis indicates time, and the vertical axis indicates the magnitude of the residual vibration. For example, when the liquid at and around the nozzle  28  dries, the viscosity of the liquid increases, that is, the increased viscosity. When the viscosity of the liquid increases, the flow path resistance r increases, which leads to an increase in the oscillation cycle and the damping of the residual vibration. 
       FIG.  6    is an explanatory diagram illustrating a relationship between the existence of air bubbles and a waveform of the residual vibration. In  FIG.  6   , the horizontal axis indicates time, and the vertical axis indicates the magnitude of the residual vibration. For example, when air bubbles enter the flow path for the liquid or the tip of the nozzle  28 , the inertance m that is the weight of the liquid reduces by the amount of the existence of air bubbles, as compared with a case when the state of the nozzle  28  is normal. From Equation (2), as the “m” reduces, the angular velocity co increases, which results in a reduction in the oscillation cycle. That is, the oscillation frequency increases. 
     In addition, when foreign substances such as paper dust adhere at and around the opening of the nozzle  28 , the amount of the liquid in the pressure chamber  49  and the seeping liquid as viewed from the vibrating plate  52  increases as compared with the normal state. It is considered that this increase causes an increase in the inertance m. It is considered that fiber of paper dust attached at and around the outlet of the nozzle  28  causes an increase in the flow path resistance r. Thus, when paper dust is attached at and around the opening of the nozzle  28 , the frequency reduces as compared with the normal ejection, and the frequency of the residual vibration increases as compared with the frequency when the viscosity of the liquid increases. 
     When the viscosity of the liquid increases or an air bubble exists or foreign substances adhere or the like, the state of the nozzle  28  and the inside of the pressure chamber  49  becomes abnormal. This typically results in a fact that a liquid is not ejected from the nozzle  28 . This causes a missing dot to occur in an image recorded on the medium  27 . Even when liquid droplets are ejected from the nozzle  28 , the amount of the liquid droplets may be small, or the direction of fly of the liquid droplets may be deviated and the liquid droplets may not land on the targeted location. A nozzle  28  having such an ejection failure is also referred to as an abnormal nozzle. 
     In this manner, the residual vibration of the pressure chamber  49  communicating with the abnormal nozzle differs from the residual vibration of the pressure chamber  49  communicating with a normal nozzle  28 . Thus, the ejection-failure detecting unit  63  detects a vibration waveform of the pressure chamber  49  to detect the state of the inside of the pressure chamber  49 . 
     The ejection-failure detecting unit  63  may infer whether the state within the pressure chamber  49  is normal or abnormal, on the basis of the vibration waveform of the vibrating plate  52 . When the state within the pressure chamber  49  is abnormal, it is inferred that a nozzle  28  communicating with this pressure chamber  49  is an abnormal nozzle that suffers an ejection failure. On the basis of the vibration waveform of the pressure chamber  49 , the control unit  24  may be configured to infer whether the state within the pressure chamber  49  is abnormal due to the existence of air bubbles or the state within the pressure chamber  49  is abnormal due to an increase in the viscosity of the liquid. On the basis of the vibration waveform of the pressure chamber  49 , the control unit  24  may be configured to infer the total volume of air bubbles existing at the pressure chamber  49  and the nozzle  28  communicating with the pressure chamber  49  and the degree of the increase in the viscosity of the liquid at the pressure chamber  49  and the nozzle  28  communicating with the pressure chamber  49 . 
     The frequency of the vibration waveform detected in a state where air bubbles exist in the pressure chamber  49  and the nozzle  28  that are filled with the liquid is higher than the frequency of the vibration waveform detected in a state where no air bubble exists in the pressure chamber  49  and the nozzle  28  that are filled with the liquid. The frequency of the vibration waveform detected in a state where the pressure chamber  49  and the nozzle  28  are filled with air is higher than the frequency of the vibration waveform detected in a state where air bubbles exist in the pressure chamber  49  and the nozzle  28  that are filled with the liquid. As the size of air bubbles existing in the pressure chamber  49  and the nozzle  28  that are filled with the liquid increases, the frequency of the vibration waveform increases. 
     The frequency of the vibration waveform detected in a state where the viscosity of the liquid increases is lower than the frequency of the vibration waveform detected in a state where the viscosity of the liquid does not increase. As the degree of the increase in the viscosity of the liquid increases, the frequency of the vibration waveform reduces. That is, the driving waveform in a case where the degree of the increase in the viscosity is small is similar to the vibration waveform in a case where foreign substances adhere at and around the opening of the nozzle  28 . Thus, when a cause of an ejection failure is foreign substance at the nozzle surface  29  and when the degree of the increase in the viscosity is small, it is difficult to identify the cause of the ejection failure on the basis of the vibration waveform. 
     When an ejection failure is detected by the ejection-failure detecting unit  63 , the control unit  24  causes the camera  21  to image a nozzle  28  for which the ejection failure is detected. Specifically, when an ejection failure is detected by the ejection-failure detecting unit  63 , the control unit  24  causes the liquid ejecting head  20  to move to the first image-capturing position P 1  or the second image-capturing position P 2 . 
     The control unit  24  may select which one of the first image-capturing position P 1  and the second image-capturing position P 2  the liquid ejecting head  20  is moved to. For example, the control unit  24  may move the liquid ejecting head  20  to an image-capturing position located closer to the position of the liquid ejecting head  20  when the ejection failure is detected. For example, the control unit  24  may move the liquid ejecting head  20  in accordance with the range of the nozzle surface  29  to be imaged. That is, for example, when the entire nozzle surface  29  is imaged, the liquid ejecting head  20  is moved to the second image-capturing position P 2  to cause the second camera  21   s  to perform imaging. For example, when a portion of the nozzle surface  29  is imaged, the liquid ejecting head  20  is moved to the first image-capturing position P 1  to cause the first camera  21   f  to perform imaging. 
     The camera  21  may image the liquid ejecting head  20  that is stopped at the image-capturing position or may image the liquid ejecting head  20  that is passing through the image-capturing position. The first camera  21   f  may image the liquid ejecting head  20  while moving. 
     When the liquid ejecting head  20  is caused to move to the first image-capturing position P 1 , the control unit  24  causes the first camera  21   f  located at the standby position to the imaging position Pi. The control unit  24  may start movement of the first camera  21   f  to the imaging position Pi while causing the liquid ejecting head  20  to move to the first image-capturing position P 1 . 
     The control unit  24  infers the cause of an ejection failure on the basis of a result of imaging by the camera  21 . For example, when no issue exists at the nozzle surface  29 , the control unit  24  may infer that the cause of the ejection failure is an increase in the viscosity of the liquid. When foreign substances attached at the nozzle surface  29  cover the nozzle  28 , the control unit  24  may infer that the nozzle  28  is blocked with the foreign substance. When the end of foreign substances attached to the nozzle surface  29  is located at the nozzle  28 , the control unit  24  may infer that the foreign substances are stuck in the nozzle  28 . 
     The control unit  24  may infer the state of the liquid ejecting head  20  on the basis of a result of imaging. For example, a mist that is a liquid in a form of fine spray may be attached on the nozzle surface  29 , or the liquid may spatter at and around the nozzle  28  due to the existence of air bubbles in the nozzle  28  to cause the nozzle surface  29  to get soiled. The control unit  24  may infer the presence or absence of dirt on the nozzle surface  29  on the basis of a result of imaging. The control unit  24  may infer the presence or absence of an indentation or damage at the nozzle surface  29  or the presence or absence of a mark made due to the cap  40  being brought into contact, on the basis of a result of imaging. 
     Next, a nozzle inspection routine will be described with reference to a flowchart illustrated in  FIG.  7   . The nozzle inspection routine is performed at the timing when the plurality of actuators  51  are driven. 
     As illustrated in  FIG.  7   , in step S 101 , the control unit  24  causes the ejection-failure detecting unit  63  to perform nozzle inspection. That is, the ejection-failure detecting unit  63  detects the presence or absence of an ejection failure at the plurality of nozzles  28 , on the basis of the residual vibration of the vibrating plate  52 . 
     In step S 102 , the control unit  24  determines whether or not an ejection failure exists in the nozzle  28 . When no ejection failure exists in the nozzle  28 , step S 102  results in NO, and the control unit  24  ends the process. When an ejection failure exists in the nozzle  28 , step S 102  results in YES, and the control unit  24  cause the process to proceed to step S 103 . 
     In step S 103 , the control unit  24  determines whether or not the number of times of detection in which an ejection failure is detected after the nozzle inspection routine starts is more than a threshold value for the number of times. The threshold value for the number of times is one or more and is set in advance. When the number of times of detection is more than the threshold value for the number of times, step S 103  results in YES, and the control unit  24  causes the process to proceed to step S 104 . In step S 104 , the control unit  24  causes the notification unit  23  to perform notification, and ends the process. 
     When the number of times of detection is equal to or less than the threshold value for the number of times, step S 103  results in NO, and the control unit  24  causes the process to proceed to step S 105 . In step S 105 , the control unit  24  determines whether or not the cause of the ejection failure is an increase in the viscosity of the liquid or the existence of air bubbles. 
     When the cause of the ejection failure is identified as the increase in the viscosity of the liquid or the existence of air bubbles, step S 105  results in YES, and the control unit  24  causes the process to proceed to step S 106 . In step S 106 , the control unit  24  causes the cleaning unit  38  to perform suction cleaning. In the step S 107 , the control unit  24  causes the wiper  37  to perform wiping. Then, the control unit  24  causes the process to proceed to step S 101 . In step S 101 , the control unit  24  may cause the liquid ejecting head  20  to perform flushing, and at the same time, perform a nozzle inspection on the basis of residual vibration associated with the flushing. 
     When, in step S 105 , the cause of the ejection failure cannot be identified as the increase in the viscosity of the liquid or the existence of air bubbles, step S 105  results in NO, and the control unit  24  causes the process to proceed to step S 108 . In step S 108 , the control unit  24  causes the camera  21  to image a nozzle  28  for which the ejection failure is detected. 
     In step S 109 , the control unit  24  determines whether or not foreign substances are stuck in the nozzle  28 . When foreign substances are stuck in the nozzle  28 , step S 109  results in YES, and the control unit  24  causes the process to proceed to step S 110 . In step S 110 , the control unit  24  causes the wiper  37  to perform wiping. Then, the control unit  24  causes the process to proceed to step S 106 . In step S 109 , for example, when foreign substances cover the nozzle  28 , step S 109  results in NO, and the control unit  24  causes the process to proceed to step S 106 . 
     Operation of Embodiment 
     Operation according to the present embodiment will be described. 
     The ejection-failure detecting unit  63  may detect an ejection failure on the basis of the residual vibration when a liquid is ejected for the purpose of performing printing or performing flushing. The ejection-failure detecting unit  63  may detect an ejection failure on the basis of the residual vibration when a liquid is ejected regardless of whether the ejection is performed to perform printing or perform flushing, or may detect an ejection failure on the basis of the residual vibration when the vibrating plate  52  vibrates such that a liquid is not ejected. 
     When an ejection failure is detected by the ejection-failure detecting unit  63 , the control unit  24  causes the camera  21  to image a nozzle  28  for which the ejection failure is detected. When an ejection failure is detected during printing, the control unit  24  may cause the camera  21  to image a nozzle  28  for which the ejection failure is detected, after the printing ends. For example, the control unit  24  may cause the camera  21  to perform imaging after the end of one instruction that causes printing to be performed. The control unit  24  may cause the camera  21  to perform imaging after the end of printing to one medium  27 . After the end of printing performed by the liquid ejecting head  20  with movement in one direction, the control unit  24  may cause the camera  21  to perform imaging at the time of turning around the liquid ejecting head  20 . The control unit  24  may change the timing of imaging by the camera  21  according to whether or not it is possible to use another nozzle  28  to perform printing as an alternative to the nozzle  28  for which the ejection failure is detected. That is, when it is not possible to use other nozzles  28  as an alternative to perform printing, it may be possible to cause the camera  21  to perform imaging when the liquid ejecting head  20  is turned around. When it is possible to use another nozzle  28  to perform printing, it may be possible to cause the camera  21  to perform imaging after the end of one instruction. 
     The control unit  24  infers the cause of the ejection failure on the basis of a result of imaging, and causes at least one of maintenance and notification to be performed on the basis of the inferred cause. 
     When the cause of the ejection failure inferred on the basis of the result of imaging is inferred to be foreign substances that cover the nozzle  28 , the control unit  24  may perform suction cleaning, wiping, and flushing as the maintenance. The control unit  24  may perform a nozzle inspection on the basis of the residual vibration of the vibrating plate  52  associated with flushing. 
     When the cause of the ejection failure inferred on the basis of a result of imaging is foreign substances stuck in the nozzle  28 , the control unit  24  may cause wiping to be performed as maintenance and then cause suction cleaning, wiping, and flushing to be performed. The control unit  24  may perform the nozzle inspection on the basis of the residual vibration of the vibrating plate  52  associated with flushing. 
     When the cause cannot be eliminated even by performing the maintenance a predetermined number of times, the control unit  24  may cause the notification unit  23  to perform notification. The control unit  24  may cause the display unit  43  to perform display as the notification. The control unit  24  may cause the notification unit  23  to make notification as to at least one of the cause of the ejection failure, options concerning the maintenance, and recommended operations for eliminating the cause. The control unit  24  may make notification as to the number of nozzles  28  and the location of the nozzles  28  that have the ejection failure. 
     The control unit  24  may make notification of, for example, an increase in the viscosity of the liquid, the existence of air bubbles, attachment of foreign substances, damage of the nozzle  28 , or the like, as the cause of the ejection failure. 
     The control unit  24  may make notification of at least one of suction cleaning, wiping, flushing, cleaning of the liquid ejecting head  20  by a user, cleaning of the nozzle  28  by a user, and replacement of the liquid ejecting head  20 , as an option concerning the maintenance and recommended operation. 
     Effects of Embodiment 
     Effects of the present embodiment will be described. 
     (1) When an ejection failure is detected by the ejection-failure detecting unit  63 , the control unit  24  causes the camera  21  to image a nozzle  28  for which the ejection failure is detected. Thus, when the ejection failure is caused such that the accuracy of detection by the ejection-failure detecting unit  63  is low, it is possible to infer the cause on the basis of a result of imaging by the camera  21 . This makes it possible to improve the accuracy in identifying the cause, which makes it possible to take a measure suitable for the cause of the ejection failure. 
     (2) The ejection-failure detecting unit  63  detects the presence or absence of the ejection failure on the basis of the residual vibration of the vibrating plate  52 . Thus, it is possible to simplify the configuration as compared, for example, with a case where a sensor configured to detect an ejected liquid is provided. 
     (3) The maintenance unit  22  includes the cleaning unit  38 . The cleaning unit  38  performs cleaning in which a liquid is forcibly discharged from the liquid ejecting head  20 . Thus, when the cause of the ejection failure can be removed by discharging the liquid, it is possible to cause the state of the nozzle  28  to recover by performing cleaning. 
     (4) The notification unit  23  includes the display unit  43 . The display unit  43  displays information based on the cause of the ejection failure. Thus, it is possible to notify a user of occurrence of the ejection failure. 
     (5) The control unit  24  causes the notification unit  23  to make notification of the cause of the ejection failure. Thus, it is possible to notify a user of the cause of the ejection failure. 
     (6) The control unit  24  causes the notification unit  23  to make notification of an option concerning the maintenance. Thus, it is possible to enable a user to select the maintenance performed when an ejection failure occurs. 
     (7) The control unit  24  causes the notification unit  23  to make notification of a recommended operation. Thus, when an ejection failure occurs, it is possible to enable a user to easily select an operation suitable for the ejection failure. 
     (8) When the ejection-failure detecting unit  63  detects an ejection failure during printing, the control unit  24  causes the camera  21  to perform imaging after the end of printing. Thus, it is possible to suppress a reduction in the throughput, as compared with a case where the camera  21  is caused to perform imaging during printing. 
     (9) The movement unit  34  moves the camera  21 . Thus, it is possible to move the camera  21  to a position where the camera can image the nozzle  28  in which the ejection failure occurs. This makes it possible to easily capture an image necessary to infer the cause of the ejection failure. 
     (10) The control unit  24  starts to move the first camera  21   f  during a period of time when the liquid ejecting head  20  is moved to the first image-capturing position P 1 . Thus, it is possible to reduce the period of time from a time when the ejection failure is detected to a time when the first camera  21   f  performs imaging, as compared with a case where the liquid ejecting head  20  is moved to the first image-capturing position P 1  and then, movement of the first camera  21   f  starts. 
     (11) When the cause of the ejection failure cannot be removed by the maintenance, the control unit  24  causes the notification unit  23  to make notification. This makes it possible to ask a user to deal with it. 
     Modification Examples 
     The present embodiment described above may be modified in the following manner and be implemented. The present embodiment and the modification examples described below may be implemented in combination within a range in which a technical contradiction does not arise.
         The liquid ejecting device  11  may include an accommodating portion separately from the cap  40 , the accommodating portion being configured to accommodate a liquid discharged through flushing.   The liquid ejecting device  11  may separately include a moisture-retentive cap configured to perform capping of the liquid ejecting head  20  when the liquid ejecting device  11  is at rest or is stopped, and also include a suction cap configured to perform suction cleaning.   The liquid ejecting device  11  may include a cleaning unit configured to pressurize a liquid within the liquid ejecting head  20 . The cleaning unit may perform pressurized cleaning serving as one example of cleaning in which a liquid is forcibly discharged from the liquid ejecting head  20 . The cap  40  may accommodate a liquid discharged from the nozzle  28  in association with the pressurized cleaning.   The cleaning unit  38  may include a valve configured to restrict flow of a liquid in the supply flow path  46  or the liquid ejecting head  20 . The cleaning unit  38  may perform choke cleaning serving as one example of cleaning in which a pressure within the closed space formed by the cap  40  is reduced in a state in which the flow of a liquid is restricted, and then, the restriction of the flow of the liquid is removed to discharge the liquid in a catapult manner.   The control unit  24  may combine different cleanings to perform it. For example, the control unit  24  may be configured such that the suction cleaning is caused to be performed as the first cleaning, and the choke cleaning is caused to be performed as the second cleaning in a case where the ejection failure is not eliminated.   The control unit  24  may cause the maintenance unit  22  to perform wiping as the maintenance performed on the basis of the inferred cause. The control unit  24  may perform wiping when the cause of the ejection failure is inferred to be foreign substances on the basis of a result of imaging by the camera  21 .   After the cause of the ejection failure is inferred on the basis a result of imaging by the camera  21 , the control unit  24  may cause notification to be made without performing the maintenance. For example, when the liquid ejecting head  20  is in a state in which the liquid ejecting head  20  cannot recover through maintenance, the control unit  24  may ask a user to deal with it. When the liquid ejecting head  20  has an indentation, damage, or the like and is in a state of being unable to be addressed through cleaning, the control unit  24  may make notification to recommend replacing the liquid ejecting head  20 .   The control unit  24  may cause the maintenance and the notification to be performed together. The control unit  24  may be configured to perform the maintenance, and make the notification that an ejection failure occurs and the notification that the maintenance is being performed.   The control unit  24  may cause the maintenance to be performed after causing the notification to be made. The control unit  24  may cause the notification of occurrence of an ejection failure to be made, and after receiving confirmation from a user, cause the maintenance to be performed.   The liquid ejecting device  11  may include a plurality of movement units  34 . The movement units  34  may individually move the corresponding camera  21 .   The control unit  24  may cause the liquid ejecting head  20  to be moved to the first image-capturing position P 1 , and then, starts to move the first camera  21   f.      After starting to move the first camera  21   f  to the imaging position Pi, the control unit  24  may start to move the liquid ejecting head  20  to the first image-capturing position P 1 .   The movement direction of the camera  21  may be a direction parallel to the Z-axis. For example, when foreign substances are attached so as to droop from the nozzle surface  29 , imaging is performed while moving along the Z-axis. This makes it easy to detect the foreign substances.   The control unit  24  may infer the cause of the ejection failure on the basis of a plurality of images captured by the camera  21  or may infer the cause on the basis of a portion of an image.   The ejection-failure detecting unit  63  may be a sensor configured to detect a liquid ejected from the nozzle  28 . The sensor may be an electrode sensor including an electrode configured to detect that the ejected liquid is brought into contact, or may be an optical sensor using light to detect the liquid.   The liquid ejecting device  11  may be a liquid ejecting device configured to jet or eject a liquid other than ink. The state of the liquid ejected from the liquid ejecting device as a very small amount of liquid droplet includes a particle shape, a teardrop shape, and a fiber shape having a tail. For the liquid as used herein, it may be possible to use any material as long as the liquid ejecting device can eject. For example, the liquid may be any liquid phase substance, and includes a liquid body having high viscosity or low viscosity, sol, gel water, and a fluid body such as inorganic solvent, organic solvent, solution, liquid-like resin, liquid-like metal, or melted metal. The liquid not only includes a liquid as one state of a substance but also includes a substance obtained by dissolving, dispersing, or mixing, in a solvent, particles made of a functional material formed of a solid substance such as pigment or metal particle. A typical example of the liquid includes ink as described in the embodiment above, liquid crystal, or the like. Here, the ink includes various liquid compositions such as general water-based ink, oil-based ink, gel ink, or hot melt ink. A specific example of the liquid ejecting device includes a device configured to eject a liquid containing a material such as an electrode material or a color material in a state of being dispersed or dissolved, which is used in manufacturing or the like of a liquid crystal display, an electroluminescence display, a field emission display, or a color filter. The liquid ejecting device may be a device configured to eject a bioorganic material used in manufacturing a bio chip, a device used as an accurate pipette and configured to eject a liquid serving as a sample, a printing machine, a microdispenser, or the like. The liquid ejecting device may be a device configured to eject lubricant oil by pinpoint to a precision machine such as a watch or a camera, or a device configured to eject a transparent resin solution such as an ultraviolet curing resin onto a substrate so as to form a micro-hemispherical lens, an optical lens or the like used in an optical communication element. The liquid ejecting device may be a device configured to eject an etching solution such as acid or alkali so as to etch a substrate or the like.       

     Below, description will be made of technical ideas or operation and effect thereof obtained on the basis of the embodiment and the modification examples described above. 
     (A) A liquid ejecting device includes: a liquid ejecting head configured to eject a liquid from a plurality of nozzles to perform printing; a camera configured to image a nozzle surface at which the plurality of nozzles are provided; an ejection-failure detecting unit configured to detect presence or absence of an ejection failure at the plurality of nozzles; a maintenance unit configured to perform maintenance of the liquid ejecting head; a notification unit configured to perform notification; and a control unit, in which, when the ejection failure is detected by the ejection-failure detecting unit, the control unit causes the camera to image a nozzle for which the ejection failure is detected to infer a cause of the ejection failure on a basis of a result of imaging, and causes at least one of the maintenance and the notification to be performed on a basis of the inferred cause. 
     With this configuration, when an ejection failure is detected by the ejection-failure detecting unit, the control unit causes the camera to image a nozzle for which the ejection failure is detected. Thus, even when the ejection failure occurs such that the accuracy of detection of the cause by the ejection-failure detecting unit is low, it is possible to infer the cause on the basis of a result of imaging by the camera. This makes it possible to improve the accuracy in identifying the cause, which makes it possible to take a measure suitable for the cause of the ejection failure. 
     (B) The liquid ejecting device may be configured such that the liquid ejecting head includes a plurality of actuators and a vibrating plate, in which the plurality of actuators are individually driven by a driving circuit to partially displace the vibrating plate, the displaced vibrating plate causes the liquid to be ejected from a nozzle corresponding to the driven actuator, and the ejection-failure detecting unit detects presence or absence of the ejection failure at the plurality of nozzles on the basis of residual vibration of the displaced vibrating plate. 
     With this configuration, the ejection-failure detecting unit detects the presence or absence of an ejection failure on the basis of the residual vibration of the vibrating plate. Thus, it is possible to simplify the configuration as compared, for example, with a case where a sensor configured to detect the ejected liquid is provided. 
     (C) The liquid ejecting device may be configured such that the maintenance unit includes a cleaning unit configured to be able to perform cleaning in which the liquid is forcibly discharged from the liquid ejecting head, and the control unit causes the cleaning to be performed as the maintenance. 
     With this configuration, the maintenance unit includes the cleaning unit. The cleaning unit performs cleaning in which a liquid is forcibly discharged from the liquid ejecting head. Thus, when the cause of the ejection failure can be eliminated by discharging the liquid, it is possible to recover the state of the nozzle by performing the cleaning. 
     (D) The liquid ejecting device may be configured such that the notification unit includes a display unit configured to be able to display various types of information, and the control unit causes the display unit to perform display as the notification. 
     With this configuration, the notification unit includes the display unit. The display unit displays information based on the cause of an ejection failure. Thus, it is possible to notify a user of occurrence of the ejection failure. 
     (E) The liquid ejecting device may be configured such that the control unit causes the notification unit to make notification of the cause. 
     With this configuration, the control unit causes the notification unit to make notification of the cause of the ejection failure. Thus, it is possible to notify a user of the cause of the ejection failure. 
     (F) The liquid ejecting device may be configured such that the control unit causes the notification unit to make notification of an option for the maintenance. 
     With this configuration, the control unit causes the notification unit to make notification of an option for the maintenance. Thus, it is possible to enable a user to select the maintenance performed when an ejection failure occurs. 
     (G) The liquid ejecting device may be configured such that the control unit causes the notification unit to make notification of a recommended operation used to eliminate the cause. 
     With this configuration, the control unit causes the notification unit to make notification of a recommended operation. Thus, when an ejection failure occurs, it is possible to enable a user to easily select an operation suitable for the ejection failure. 
     (H) The liquid ejecting device may be configured such that, when the ejection failure is detected during printing, the control unit causes the camera to image the nozzle for which the ejection failure is detected, after the end of the printing. 
     With this configuration, when the ejection-failure detecting unit detects an ejection failure during printing, the control unit causes the camera to perform imaging after the end of printing. Thus, it is possible to suppress a reduction in throughput, as compared with a case where the camera is caused to perform imaging during printing. 
     (I) The liquid ejecting device may further include a movement unit configured to be able to move the camera. 
     With this configuration, the movement unit moves the camera. Thus, the camera can be moved to a position where the camera can image the nozzle in which the ejection failure occurs. This makes it possible to easily capture an image necessary to infer the cause of the ejection failure. 
     (J) The liquid ejecting device may be configured such that the liquid ejecting head is configured to be able to move in a scanning direction, the movement unit is configured to be able to move the camera in a movement direction differing from the scanning direction, the camera located at an imaging position is able to image the nozzle, in the liquid ejecting head, for which the ejection failure is detected, the liquid ejecting head being located at an image-capturing position, and when the ejection failure is detected by the ejection-failure detecting unit, the control unit starts to move the camera to the imaging position during a period of time when the liquid ejecting head is moved to the image-capturing position. 
     With this configuration, the control unit starts to move the camera during a period of time when the liquid ejecting head is moved to the image-capturing position. Thus, it is possible to reduce the period of time from a time when the ejection failure is detected to a time when the camera performs imaging, as compared with a case where the liquid ejecting head is moved to the image-capturing position and then, movement of the camera starts. 
     (K) The liquid ejecting device may be configured such that, when the cause cannot be eliminated even by performing the maintenance a predetermined number of times, the control unit causes the notification unit to perform the notification. 
     With this configuration, when the cause of the ejection failure cannot be eliminated by the maintenance, the control unit causes the notification unit to perform notification. Thus, it is possible to ask a user to deal with it.