Patent Publication Number: US-8534791-B2

Title: Liquid droplet jetting apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority from Japanese Patent Application No. 2010-078526, filed on Mar. 30, 2010, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid droplet jetting apparatus for jetting liquid droplets. 
     2. Description of the Related Art 
     An ink jet printer has been hitherto widely known, in which an ink-jet head and tanks (ink cartridges) for storing inks are connected by means of tubes, and the inks are supplied to the ink-jet head via the tubes. A flexible tube, which is formed of, for example, a synthetic resin material, is generally used for each of the tubes, for the following various reasons. That is, it is intended that the tube can be freely laid out to secure the degree of freedom of the layout. In another viewpoint, it is intended to avoid the disturbance of movement of the head as thoroughly as possible, which would be otherwise caused by the tubes when the head is moved. 
     The state of the ink contained in the tube has been hitherto detected for various reasons. For example, an ink jet printer (ink-jet recording apparatus) disclosed in Japanese Patent Application Laid-open No. 5-57905 includes a sensor (optical detector) which detects the presence or absence of the ink and which is provided at an intermediate position of a tube for connecting an ink jet head and an ink cartridge, wherein the exhaustion of the ink in the ink cartridge is detected by means of the sensor. 
     Japanese Patent Application Laid-open No. 10-157161 discloses an ink-jet printer which is configured to perform the gradational recording by using a plurality of types of inks having different concentrations. The concentration of the ink is raised on account of the evaporation or vaporization of a solvent. An ink concentration detecting section, which detects the increase in the ink concentration, is provided at an intermediate position of a tube for connecting an ink jet head and an ink cartridge. The amount of a dilution liquid (diluent) to be added to the ink is controlled to adjust the concentration of the ink to be supplied to the ink-jet head on the basis of a detection result obtained by the ink concentration detecting section. 
     On the other hand, it is feared that the air may be mixed in from a connecting portion between an ink jet head and a tube and/or a connecting portion between the tube and a tank to form any bubble. If such a bubble is fed to the head together with the ink, any harmful influence is exerted on the liquid droplet jetting performed by the head. In order to solve this problem, an ink jet printer disclosed in Japanese Patent Application Laid-open No. 2004-188647 has a transmission type photosensor which detects any bubble contained in a tube and which is provided at a connecting portion of an ink-jet head connected to the tube. 
     As disclosed in Japanese Patent Application Laid-open Nos. 10-157161 and 2004-188647, any bubble is generated in the tube during the ordinary use, and/or the ink contained in the ink is dried to increase or raise the concentration (viscosity) of the ink. In the following description, the expression that “the viscosity of the ink is increased or raised” also includes the meaning of the increase in the concentration of the ink. For example, the air enters through the connecting portion of the tank when the tank (ink cartridge) is exchanged, and the bubble is mixed into the tube in some cases. On the other hand, the flexible tube, which is formed of, for example, the synthetic resin material, generally has the gas permeability. Therefore, if the flexible tube is left to stand for long period of time, then the air is mixed into the tube to generate the bubble in some cases, and/or the solvent of the ink contained in the tube is evaporated to increase the viscosity of the ink in other cases. If the bubble and the viscosity-increased ink contained in the tube as described above are fed to the head, then the abnormality (for example, the nozzle-absence by which the ink droplets are not jetted, or the jetting-curvature by which the ink droplets are not jetted in the right direction) arises in the liquid droplet jetting performed with the nozzle by the head, and the printing quality is deteriorated. 
     However, in Japanese Patent Application Laid-open Nos. 5-57905, 10-157161, and 2004-188647, the detecting means, which detects the presence or absence of the ink in the tube, the concentration, and/or the presence or absence of the bubble, has the fixed position with respect to the tube. The detecting means can detect only the state of the ink at a certain portion of the tube. Therefore, if the bubble passes through the predetermined detection position by some chance during the period in which the state of the ink in the tube is not detected (for example, during the period in which the power source is turned OFF), it is feared that the bubble may thereafter arrive at the head without being detected. 
     Further, in Japanese Patent Application Laid-open Nos. 5-57905, 10-157161, and 2004-188647, it is impossible to recognize at what position the bubble and/or the viscosity-increased ink exists in the tube extending over a long distance from the head to the tank, and it is of course impossible to recognize the size of the bubble and the extent of the viscosity increase. Therefore, it is impossible to optimize the discharge operation depending on, for example, the size of the bubble if it is intended to discharge the bubble from the ink supply system ranging to the head by means of any discharge means until the bubble or the like arrives at the head when the detecting means detects the bubble and/or the viscosity-increased ink contained in the tube. For example, the following problems arise. That is, a large amount of the ink (normal ink not subjected to the viscosity increase) is discharged together with the bubble or the like during the discharge operation for discharging the bubble or the like. In another situation, the discharge operation is performed for an unnecessarily long period of time. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a liquid droplet jetting apparatus which is capable of detecting a state of a liquid at a plurality of positions of a tube. 
     According to the first aspect of the present invention, there is provided a liquid droplet jetting apparatus which jets liquid droplets of a liquid onto a medium, including: 
     a liquid droplet jetting head which jets the liquid droplets; 
     a storage tank which stores the liquid to be jetted by the liquid droplet-jetting head; 
     a tube which connects the liquid droplet jetting head and the storage tank; and 
     a liquid-state detecting mechanism which detects a state of the liquid contained in the tube at a plurality of positions in a longitudinal direction of the tube. 
     According to the present invention, the state of the liquid in the tube (for example, bubble and/or viscosity-increased liquid) is detected at the plurality of positions by means of the liquid-state detecting mechanism. Therefore, the opportunity is increased to detect the bubble and/or the viscosity-increased liquid existing in the tube, and the reliability of the detection is enhanced. Further, it is possible to grasp not only the presence or absence of the bubble and/or the viscosity-increased liquid in the tube but also the position and the amount of the bubble and/or the viscosity-increased liquid. Therefore, the amount of the liquid to be discharged (wasted or discarded) can be suppressed to be small, for example, by optimally controlling the discharge operation for the bubble and/or the viscosity-increased liquid on the basis of the information about the position and the amount of the bubble and/or the viscosity-increased liquid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a plan view schematically illustrating a printer according to an embodiment of the present invention. 
         FIG. 2  shows a side view illustrating a carriage shown in  FIG. 1  as viewed from the left side of  FIG. 1 . 
         FIG. 3  shows a sectional view illustrating a subtank. 
         FIG. 4  shows an enlarged view illustrating an ink state detection sensor shown in  FIG. 2 . 
         FIGS. 5A and 5B  illustrate the bubble detection in a tube, wherein  FIG. 5A  shows a state in which a bubble exists in the tube, and  FIG. 5B  shows the output change of the sensor in the state shown in  FIG. 5A . 
         FIGS. 6A and 6B  illustrate the viscosity-increased ink detection in a tube, wherein  FIG. 6A  shows a state in which a viscosity-increased ink exists in the tube, and  FIG. 6B  shows the output change of the sensor in the state shown in  FIG. 6A . 
         FIG. 7  shows a block diagram illustrating an electrical arrangement of the printer. 
         FIG. 8  shows a flow chart illustrating the bubble discharge control. 
         FIG. 9  shows a flow chart illustrating the viscosity-increased ink discharge control. 
         FIG. 10  shows a sectional view illustrating a tube according to a first modified embodiment. 
         FIGS. 11A and 11B  show a tube according to a second modified embodiment, wherein  FIG. 11A  shows a sectional view, and  FIG. 11B  shows a perspective view illustrating a tube structure in which those corresponding to four colors are integrated into one unit. 
         FIGS. 12A and 12B  show an ink state detection sensor according to a fourth modified embodiment, wherein  FIG. 12A  shows a state provided when an end position is detected, and  FIG. 12B  shows a state provided when an ink state in a tube is detected. 
         FIG. 13  shows an ink state detection sensor composed of a transmission type photosensor according to a fifth modified embodiment. 
         FIGS. 14A ,  14 B and  14 C show tube holding members according to a seventh modified embodiment. 
         FIG. 15  shows a plan view illustrating a schematic arrangement of a printer according to an eighth modified embodiment. 
         FIGS. 16A and 16B  show a tube and a sensor holding member according to a ninth modified embodiment, wherein  FIG. 16A  shows a sectional view illustrating the tube, and  FIG. 16B  shows a perspective view. 
         FIG. 17  shows a flow chart illustrating the bubble discharge control according to a thirteenth modified embodiment. 
         FIG. 18  shows a flow chart illustrating the bubble discharge control according to a fourteenth modified embodiment. 
         FIGS. 19A and 19B  show flow charts illustrating the bubble discharge control according to a fifteenth modified embodiment. 
         FIG. 20  shows a schematic arrangement of a printer to illustrate a sixteenth modified embodiment. 
         FIG. 21  shows a block diagram illustrating an electrical arrangement of the printer according to the sixteenth modified embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Next, an embodiment of the present teaching will be explained. As shown in  FIG. 1 , a printer  1  (liquid droplet jetting apparatus) includes a carriage  2  which is constructed reciprocatively movably in the scanning direction as shown in  FIG. 1 , an ink jet head  3  (liquid droplet jetting head) and subtanks  4  which are carried on the carriage  2 , a holder  6  to which ink cartridges  5  (storage tanks) for storing inks are installed, a purge mechanism  7  which discharges, for example, any bubble existing in the ink supply system of the ink-jet head  3 , and a control unit  8  which controls respective sections of the printer  1 . 
     The carriage  2  is constructed reciprocatively movably along two guide rails  12 ,  13  extending in parallel in the left-right direction (scanning direction) as shown in  FIG. 1 . An endless belt  18  is connected to the carriage  2 . When the endless belt  18  is driven to travel by a carriage driving motor  19  (moving driving mechanism), the carriage  2  is moved in the scanning direction in accordance with the travel of the endless belt  18 . The printer  1  is provided with a linear encoder  10  having a large number of light transmitting portions (slits) which are arranged while providing spacing distances in the scanning direction. On the other hand, the carriage  2  is provided with a position detection sensor  11  including a transmission type photosensor having a light-emitting element and a light-receiving element. The printer  1  is constructed so that the present position of the carriage  2  in the scanning direction can be recognized from a counted value (number of times of detection) of the light transmitting portions of the linear encoder  10  detected by the position detection sensor  11  during the movement of the carriage  2 . 
     The ink-jet head  3  and the four subtanks  4  are carried on the carriage  2 . A plurality of liquid droplet jetting nozzles  14  are formed on the lower surface of the ink-jet head  3  (surface disposed on the back side in relation to the plane of paper (the paper surface) in  FIG. 1 ). In  FIG. 1 , the nozzles  14  are shown while being enlarged as compared with actual diameters of the nozzles  14  so that the nozzles  14  can be viewed with ease. As shown in  FIG. 1 , the four subtanks  4  are arranged and aligned in the scanning direction above the ink-jet head  3 . The four subtanks  4  are connected to the holder  6  respectively by means of four tubes  15  connected to the carriage  2 . 
     As shown in  FIG. 3 , a damper chamber  20 , an ink supply channel  21 , and a gas discharge channel  22  are formed in each of the subtanks  4 . The damper chamber  20  is formed to have a shape which is spread horizontally. A damper film  23 , which is composed of a flexible film, is provided at an upper wall portion thereof. The ink, which is supplied from the tube  15 , is temporarily stored in the damper chamber  20 . When any pressure fluctuation is caused in the ink contained in the damper chamber  20 , the flexible damper film  23  functions to damp or attenuate the pressure fluctuation. 
     The damper chamber  20  is communicated with an upper end portion of the ink supply channel  21  extending in the vertical direction. A lower end portion of the ink supply channel  21  is connected to the ink-jet head  3 . The ink is supplied from the damper chamber  20  via the ink supply channel  21  to the ink-jet head  3 . The ink supply channel  21  also plays such a role that the bubble, which flows into the subtank  4  together with the ink from the tube  15 , is separated from the ink so that the bubble is stored (bubble storage member of the present invention). Specifically, a space  24  is defined on the upper side of the ink supply channel  21  as compared with the communication port with respect to the damper chamber  20 . Therefore, the bubble, which is mixed in the ink supplied from the damper chamber  20  to the ink supply channel  21 , is trapped by the space  24 , and the bubble is progressively stored. A gas discharge channel  22 , which is communicated with the space  24  and through which the bubble stored in the space  24  is discharged, is connected to the ceiling surface of the ink supply channel  21 . Further, the gas discharge channel  22  is connected to a suction pump  31  of a purge mechanism  7  as described later on. 
     As shown in  FIG. 1 , the holder  6  and the four subtanks  4  of the carriage  2  are connected to one another by means of the four tubes  15 . The four tubes  15  are composed of a light-transmissive synthetic resin material such as polyimide. Further, the four tubes  15  are bendable because they have the flexibility. As shown in  FIG. 2 , the four tubes  15  are arranged while being aligned in the up-down direction. As shown in  FIG. 1 , each of the tubes  15  is connected at one end thereof to the left side surface of the carriage  2 . Each of the tubes  15  is led out leftwardly (toward one side in the scanning direction) from the carriage  2 , and then each of the tubes  15  is bent and inverted so that each of the tubes  15  is laid out rightwardly (toward the other side in the scanning direction). The other end of each of the tubes  15  is connected to the holder  6 . An ink-state detection sensor  40 , which detects the state of the ink contained in a tube portion  15   a , is provided opposingly to the tube portion  15   a  of the tube  15  which is bent, inverted, and connected to the holder  6 , on the left side surface of the backward end portion of the carriage  2  (end portion disposed on the downstream side in the paper feeding direction). The ink-state detection sensor  40  will be described in detail later on. 
     The four ink cartridges  5 , which store the inks of four colors (black, yellow, cyan, and magenta) respectively, are removably installed to the holder  6 . The four color inks, which are stored in the four ink cartridges  5  respectively, are supplied to the four subtanks  4  via the four tubes  15 . The four color inks are temporarily stored in the subtanks  4 , and then the color inks are supplied to the ink-jet head  3 . 
     In the printer  1  as described above, the recording paper P is fed from the upper position as viewed in  FIG. 1  to the position disposed on the lower side of the ink jet head  3  (back side in relation to the plane of the paper in  FIG. 1 ) by means of a transport mechanism  9  of the recording paper having a plurality of transport rollers for the recording paper (transport mechanism, see  FIG. 7 , omitted from the illustration in  FIG. 1 ). The ink-jet head  3  jets the ink liquid droplets onto the recording paper P from the plurality of nozzles  14  provided on the lower surface of the ink jet head  3 , while performing the reciprocative movement in the scanning direction together with the carriage  2 . Accordingly, a desired image and letters are recorded on the recording paper P. In the printer  1  shown in  FIG. 1 , the transport direction, in which the recording paper P is transported by the transport mechanism  9 , is perpendicular to the scanning direction of the carriage  2 . However, the present teaching is not limited thereto. It is not necessarily indispensable that the transport direction should be perpendicular to the scanning direction provided that the transport direction intersects the scanning direction. 
     As shown in  FIG. 1 , the purge mechanism  7  is arranged at the position disposed at the outside (on the right side as shown in  FIG. 1 ) of the printing area in relation to the scanning direction, the printing area facing the recording paper P within the range of movement of the carriage  2 . A storage cap  25  is provided at the position disposed at the outside of the printing area on a side opposite to the purge mechanism  7  (on the left side as shown in  FIG. 1 ), with the printing area intervening therebetween. The storage cap  25  is installed to the ink jet head  3  so that the nozzles  14  are covered therewith in order to avoid the drying of the inks from the nozzles  14  when the ink-jet head  3  stops. 
     In the printer  1  of the embodiment of the present teaching, when any bubble is mixed into the ink to be jetted by the ink-jet head  3 , or when the solvent of the ink is evaporated to increase the viscosity, then the malfunction, which includes, for example, the nozzle-absence and the jetting-curvature, is caused in the ink-jet head  3 . Accordingly, the purge mechanism  7  is provided with a mechanism to discharge the viscosity-increased ink and the bubble mixed into the ink which cause the malfunction of the ink jet head  3  as described above. 
     Specifically, the purge mechanism  7  has a suction cap  30 , a suction pump  31 , and a switching unit (changeover unit)  32 . The suction cap  30  is constructed movably in the up-down direction (direction perpendicular to the plane of the paper in  FIG. 1 ). The plurality of nozzles  14 , which are disposed on the lower surface of the ink jet head  3 , can be covered therewith by moving the suction cap  30  upwardly in a state in which the carriage  2  is opposed to the suction cap  30 . 
     The suction pump  31  is connected to the switching unit  32 . Further, the switching unit  32  is connected to the suction cap  30  via a suction tube  33 . The switching unit  32  is also connected to the gas discharge channels  22  of the subtanks  4  (see  FIG. 3 ) via a gas discharge tube  34 . The switching unit  32  switches the connection/disconnection between the suction cap  30  and the suction pump  31 , and the connection/disconnection between the subtanks  4  and the suction pump  31  respectively. 
     When the suction pump  31  is operated in a state in which the suction cap  30  covers the nozzles  14  of the ink jet head  3  and the suction cap  30  is connected to the suction pump  31  by means of the switching unit  32 , then the inks can be sucked from the nozzles  14 , and it is possible to perform the suction purge so that the bubbles and the viscosity-increased ink contained in the ink-jet head  3  are discharged. On the other hand, when the suction pump  31  is operated in a state in which the subtanks  4  is connected to the suction pump  31  by means of the switching unit  32 , it is possible to perform the gas discharge purge so that the bubbles stored in the subtanks  4  are discharged from the gas discharge channels  22 . 
     By the way, the phenomenon of the mixing of bubbles and/or the viscosity increase of the ink, which causes the jetting malfunction in the ink-jet head  3 , is often caused by the tubes  15  for connecting the carriage  2  and the holder  6 . For example, when the ink cartridge  5  is installed or removed with respect to the holder  6 , then the air makes invasion from the connecting portion between the holder  6  and the ink cartridge  5  in some cases, and the air enters the tube  15  to form the bubble. Further, the tube  15 , which is composed of the synthetic resin material, has the gas permeability. Therefore, the invasion of the air into the tube  15  and the evaporation of the ink contained in the tube  15  proceed, although the invasion and the evaporation proceed gradually. Therefore, when the state, in which the printer  1  is not used, is continued for a long period of time, the bubble contained in the tube  15  greatly grows. In another situation, the degree of viscosity increase of the ink becomes serious in the tube  15 . 
     As shown in  FIG. 1 , the printer of the embodiment of the present invention is provided with the ink-state detection sensor  40  (liquid-state detecting mechanism) which detects the state of the ink contained in the tube  15  (presence or absence of any bubble and/or any viscosity-increased ink). The suction purge and the gas discharge purge can be performed by the purge mechanism  7  only when they are required, on the basis of the detection result of the sensor  40 . However, in order to perform the purge by the purge mechanism  7  more efficiently, it is preferable to successfully detect at what position the bubble and/or the viscosity-increased ink exists in the tube  15 , and it is preferable to successfully detect the extent of the size (amount) of the bubble and/or the viscosity-increased ink existing in the tube  15 . The ink-state detection sensor  40  of the embodiment of the present teaching is movable in the longitudinal direction of the tube  15  (an extending direction along the tube). Therefore, the state of the ink contained in the tube  15  can be detected by the sensor  40  at a plurality of positions in the longitudinal direction of the tube  15 . Further, the state of the ink (the ink-state) can be detected continuously in the longitudinal direction. Accordingly, it is possible to detect the position and the size of the bubble and/or the viscosity-increased ink. 
     An arrangement of the ink-state detection sensor  40  will be specifically explained below. As shown in  FIGS. 1 ,  2 , and  4 , the ink-state detection sensor  40  is provided on the carriage  2  which is reciprocatively driven in the scanning direction by the carriage driving motor  19  (movement-driving mechanism). As shown in  FIG. 1 , the tubes  15 , which are connected to the ink-jet head  3  carried on the carriage  2 , are led out leftwardly (toward one side in the scanning direction), and then the tubes  15  are bent and inverted so that the tubes  15  are laid out rightwardly (toward the other side in the scanning direction) and the tubes  15  are connected to the holder  6 . In other words, the tube portions  15   a , which range from the bent and inverted portions  15   c  to the holder  6 , are arranged in the scanning direction on the downstream side in the paper feeding direction as compared with the carriage  2 . On this condition, the ink-state detection sensor  40  is movable in the scanning direction together with the carriage  2  while being opposed to the tube portions  15   a.    
     The second tube portions  15   b  of the tubes  15 , which range to the bent and inverted portions  15   c  and which are directly connected to the ink-jet head  3 , are moved in the scanning direction in accordance with the movement of the carriage  2 . On the contrary, the first tube portions  15   a  of the tubes  15 , which range from the bent and inverted portions  15   c  to the holder  6 , are scarcely moved, because the position of the holder  6  is fixed. Therefore, when the ink state detection sensor  40  is moved in the scanning direction together with the carriage  2 , the ink-state detection sensor  40  is relatively moved in the longitudinal direction of the tubes  15  with respect to the first tube portions  15   a.    
     As shown in  FIG. 4 , the ink-state detection sensor  40  is provided with four photosensors  41  which individually detect the states of the respective inks of the four tubes  15 . As also shown in  FIGS. 2 and 4 , the four tubes  15  are arranged while being aligned in the up-down direction on the side of the carriage  2 . Therefore, the four photosensors  41 , which correspond to the four tubes  15 , are also arranged while being aligned in the up-down direction. Each of the photosensors  41  is a reflection type photosensor having a light-emitting element  42  and a light-receiving element  43  which are arranged closely to one another at positions opposed to the tube  15 . The light is emitted from the light-emitting element  42  toward the tube  15  as the objective, and the light reflected by the ink contained in the tube  15  is received by the light-receiving element  43 . 
     As described above, the tube  15  is formed of the light-transmissive synthetic resin material. The light, which is emitted from the light-emitting element  42 , is firstly transmitted through the wall of the tube  15  disposed on the side of the carriage  2 . If any bubble is not present in the tube  15 , then a part of the light is reflected by the ink, and the part of the light is received by the light-receiving element  43 . On the contrary, if any bubble is present, the light is hardly reflected. Therefore, the light is not reflected by the interior of the tube  15 , and a large amount of the light is transmitted through the tube  15  as compared with the case in which the bubble is absent. Therefore, if any bubble is present, the light-receiving amount of the light-receiving element  43  is decreased. On the other hand, if any viscosity-increased ink is present in the tube  15 , then the concentration of the viscosity-increased ink is higher than that of the ink having the ordinary viscosity, the viscosity-increased ink has the lower light-transmissive property, and the light reflected by the viscosity-increased ink is increased. Therefore, if any viscosity-increased ink is present, the light-receiving amount of the light-receiving element  43  is increased. 
     The ink-state detection sensor  40  can be moved in the longitudinal direction of the tube  15 . Therefore, it is possible to detect not only the presence or absence of the bubble and/or the viscosity-increased ink but also the position and the size (amount) thereof. Further, not only the light-transmissive property (reflectance) differs but the refractive index also differs between the viscosity-increased ink and the ink having the ordinary viscosity. Accordingly, the present teaching is not limited to only the utilization of the difference in the light-transmissive property between the viscosity-increased ink and the ink having the ordinary viscosity. It is also allowable to detect the position and the size of the viscosity-increased ink by utilizing the difference in the refractive index between the viscosity-increased ink and the ink having the ordinary viscosity. 
     As shown in  FIG. 5A , a situation is assumed, in which the bubble  51  exists in the ink  50  in a range La having a certain length in the tube  15 . In this situation, the ink-state detection sensor  40 , which is moved at a predetermined velocity in the scanning direction (longitudinal direction of the tube  15 ), provides the output (light amount received by the light-receiving element  43 ; light-receiving amount of the light-receiving element  43 ) which is decreased during a time Ta of the light emission from the light-emitting element  42  in an interval of the range La as shown in  FIG. 5B . Therefore, the length of the range La of the presence of the bubble  51 , i.e., the size of the bubble  51  can be detected from the velocity of movement of the sensor  40  (scanning velocity of the carriage  2 ) and the time Ta in which the output is decreased as compared with a predetermined first threshold value V 1 . The position in the scanning direction of the carriage  2  has been detected by the position detection sensor  11 . Therefore, it is also possible to detect the position of existence of the range La in the tube  15 . It is noted that the detection of the presence or absence of the bubble  51 , the position of the bubble, and the size of the bubble is performed by a bubble detecting section  66  of the control unit  8  as described later on, on the basis of the detection signal of the ink-state detection sensor  40 . 
     As shown in  FIG. 6A , a situation is assumed, in which the viscosity-increased ink  52 , which has the viscosity higher than that of the ink  50 , exists in the ink  50  having the ordinary viscosity in a range Lb having a certain length in the tube  15 . In this situation, the ink-state detection sensor  40  provides the output which is increased during a time Tb of the light emission from the light-emitting element  42  in an interval of the range Lb as shown in  FIG. 6B . Therefore, the length of the range Lb of the presence of the viscosity-increased ink  52 , i.e., the amount of the viscosity-increased ink  52  can be detected from the velocity of movement of the sensor  40  and the time Tb in which the output is increased as compared with a predetermined second threshold value V 2  (&gt;first threshold value V 1 ) in the same manner as in the bubble detection described above. Further, the degree of the increase in viscosity can be also detected from the output value (magnitude of the light-receiving amount). Further, it is also possible to detect the position of existence of the range Lb in the tube  15  from the position of the carriage  2  detected by the position detection sensor  11 . It is noted that the detection of the presence or absence of the viscosity-increased ink, the position of the viscosity-increased ink, the amount of the viscosity-increased ink, and the degree of the increase in viscosity is performed by a viscosity-increased ink detecting section  67  of the control unit  8  as described later on, on the basis of the detection signal of the ink-state detection sensor  40 . 
     Therefore, it is possible to detect not only the presence or absence of the bubble and/or the viscosity-increased ink in the tube  15  but also the position and the size (amount) thereof. Therefore, it is possible to optimize the suction purge and the gas discharge purge performed by the purge mechanism  7  so that only a necessary amount of the ink is discharged when the discharge is required, on the basis of the information in relation to the bubble and/or the viscosity-increased ink as described above. This feature will be described in detail in the explanation of the purge control described later on. 
       FIGS. 5B and 6B  are illustrative of the exemplary cases in which the light is continuously emitted by the light-emitting element  42  and the light is continuously received by the light-receiving element  43  during the period in which the sensor  40  is moved so that the state of the ink is continuously detected by the sensor  40  in the longitudinal direction of the tube  15 . However, the present teaching is not limited thereto. The light may be intermittently emitted by the light-emitting element  42  and/or the light may be intermittently received by the light-receiving element  43  at every predetermined time intervals. Accordingly, the ink-state may be detected at every predetermined intervals in the longitudinal direction of the tube  15 . However, in order to accurately detect the position and the size of the bubble  51  and/or the viscosity-increased ink  52 , it is of course preferable to perform the detection continuously in the longitudinal direction of the tube  15 . 
     The light, which is emitted by the light-emitting element  42 , is basically the white light. However, the light amount and/or the wavelength of the light emitted by the light-emitting element  42  may differ depending on the color of the ink contained in the tube  15  subjected to the detection. The first threshold value V 1  for the bubble detection and the second threshold value V 2  for the viscosity-increased ink detection described above are individually set respectively depending on the color of the ink contained in the tube  15  subjected to the detection. 
     Next, an explanation will be made in detail with reference to a block diagram shown in  FIG. 7  about the control system of the ink-jet printer  1  composed of the control unit  8  as a main component. The control unit  8  of the printer  1  shown in  FIG. 7  is provided with a microcomputer including, for example, Central Processing Unit (CPU), Read Only Memory (ROM) which stores therein, for example, various programs and data for controlling the overall operation of the printer  1 , and Random Access Memory (RAM) which temporarily stores therein, for example, data to be processed by the CPU. The programs stored in ROM are executed by CPU to thereby perform various types of control as explained below. Alternatively, the control unit  8  may be composed of a hardware in which various circuits including a computing circuit are combined. 
     The control unit  8  includes a printing control section  60 , a state detecting section  61 , and a purge control section  62 . The respective functions of the printing control section  60 , the state detecting section  61 , and the purge control section  62  explained below are actually realized by the operation of the microcomputer described above or the operation of various circuits including the computing circuit. 
     The printing control section  60  has a head control section  63  which controls the liquid droplet jetting operation of the ink jet head  3 , a carriage control section  64  which controls the carriage driving motor  19  on the basis of the output of the position detection sensor  11  to control the position of the carriage  2  in the scanning direction, and a transport control section  65  which controls the transport operation of the recording paper in the transport mechanism  9 . The printing control section  60  performs the printing on the recording paper P by controlling the ink-jet head  3 , the carriage driving motor  19 , and the transport mechanism  9  on the basis of the data (printing data) which is inputted from PC  70  and which relates, for example, to an image to be printed. 
     The state detecting section  61  has a bubble detecting section  66  and a viscosity-increased ink detecting section  67 . The bubble detecting section  66  (bubble-detecting mechanism) detects the presence or absence of the bubble, the position of the bubble, and the size of the bubble for each of the four tubes  15  on the basis of the detection signal of the ink-state detection sensor  40  (change of the light-receiving amount) and the position information of the carriage  2  detected by the position detection sensor  11 . Similarly, the viscosity-increased ink detecting section  67  (viscosity-increased liquid detecting mechanism) detects the presence or absence of the viscosity-increased ink, the position of the viscosity-increased ink, the size (amount) of the viscosity-increased ink, and the degree of the viscosity increase for each of the four tubes  15 . 
     The purge control section  62  (bubble discharge control mechanism, liquid discharge control mechanism) performs the suction purge and the gas discharge purge by controlling the suction pump  31  (bubble discharge mechanism, viscosity-increased liquid discharge mechanism) on the basis of the information about the bubble and the viscosity-increased ink detected by the state detecting section  61  so that the bubble and the viscosity-increased ink are efficiently discharged. 
     Each of the timing at which the ink-state detection sensor  40  described above detects the state of the ink (timing at which the ink-state detection sensor  40  is subjected to the scanning), the timing at which the bubble and the viscosity-increased ink are detected on the basis thereof, and the timing at which the purge is performed on the basis of the detection result, is not limited to any specified timing. The timings may be appropriately set. However, when the state, in which the printing is not performed, is continued for a long period of time, the possibility is raised for the presence of the bubble and the viscosity-increased ink in the tube  15 . Therefore, it is effective to perform the operation, for example, immediately after the power source is turned ON. Alternatively, when the degree of time elapse from the previous printing can be recognized, for example, by an internal clock or timer contained in the printer  1 , the detection of the bubble or the like and the purge may be performed at an appropriate timing determined on the basis thereof. 
     Of course, it is also allowable to appropriately perform the foregoing detection during the period in which the power source of the printer  1  is turned ON. For example, when the printing instruction (printing data) is inputted from PC  70 , the detection may be performed immediately before the printing so that any harmful influence is not exerted on the printing by the bubble and the viscosity-increased ink. In this way, the bubble and the viscosity-increased ink may be reliably discharged by means of the purge. 
     Next, an explanation will be made below as exemplified by a specified example while being classified into the case in which the bubble is discharged and the case in which the viscosity-increased ink is discharged, in relation to the purge control to be performed on the basis of the detection results of the bubble detecting section  66  and the viscosity-increased ink detecting section  67 . 
     &lt;Bubble Discharge Control&gt; 
     In the bubble discharge control, if the bubble in the tube  15  is detected by the bubble detecting section  66  on the basis of the detection signal of the ink-state detection sensor  40 , the purge control section  62  allows the purge mechanism  7  to perform the gas discharge purge for discharging the bubble from the subtank  4 . 
     When the bubble discharge control is started, then the control unit  8  firstly moves the carriage  2  at a constant velocity in the scanning direction by means of the carriage driving motor  19 , and the state in the tube  15  (tube portion  15   a  shown in  FIG. 1 ) is detected (S 10 ) while moving (scanning) the ink-state detection sensor  40  in the longitudinal direction with respect to the tube  15 . Subsequently, the bubble detecting section  66  detects whether or not the bubble exists in the tube  15  from the change of the output of the ink-state detection sensor  40  (change of the light-receiving amount of the light-receiving element  43 ) (S 11 ). 
     As explained with reference to  FIG. 5 , if the bubble exists in the tube  15 , the light-receiving amount (output value) of the light-receiving element  43  is lowered. Therefore, if the light-receiving amount is lower than the predetermined first threshold value V 1  which corresponds to the light-receiving amount obtained when the ink exists (time Ta shown in  FIG. 5B ), the bubble detecting section  66  judges that the bubble is present in the tube  15  (S 11 : Yes). On the other hand, if it is judged that the bubble is absent in the tube  15  (S 11 : No), the routine returns from the sequence shown in  FIG. 8  without executing Steps S 12  to S 14  described later on. 
     When the bubble is present, the bubble detecting section  66  detects the position of the bubble in the longitudinal direction of the tube  15  from the timing at which the output of the sensor  40  is changed and the position of the carriage  2  which is provided at the timing and which is obtained by the position detection sensor  11 . Further, the size of the bubble is detected from the time in which the output of the sensor  40  is changed (S 12 ). 
     Subsequently, the purge control section  62  determines the optimum suction amount (minimum necessary suction amount) of the suction pump  31  when the bubble is discharged by means of the gas discharge purge on the basis of the position and the size of the bubble detected by the bubble detecting section  66  (S 13 ). The position and the size of the bubble in the tube  15  are known. Therefore, it is possible to know the extent of the minimum suction by the suction pump  31  in order to successfully discharge the bubble contained in the tube  15  from the gas discharge channel  22  of the subtank  4  completely. That is, when the minimum necessary suction amount of the suction pump  31  is determined, then it is possible to shorten the gas discharge time, and it is possible to maximally suppress the amount of the ink discharged together with the bubble from the subtank  4  by the suction purge. After determining the suction amount, the purge control section  62  allows the connection target of the suction pump  31  to be connected to the gas discharge channel  22  of the subtank  4  by means of the switching unit  32 . Further, the purge control section  62  controls the suction pump  31  so that the suction is performed in the amount determined in S 13  to execute the gas discharge purge (S 14 ). 
     &lt;Viscosity-Increased Ink Discharge Control&gt; 
     In the viscosity-increased ink discharge control, when the viscosity-increased ink in the tube  15  is detected by the viscosity-increased ink detecting section  67  on the basis of the detection signal of the ink state detection sensor  40 , the purge control section  62  allows the purge mechanism  7  to perform the suction purge for discharging the ink from the nozzles  14  unlike the bubble discharge control described above. 
     When the viscosity-increased ink discharge control is started, the control unit  8  performs the detection of the state in the tube  15  (first tube portion  15   a ) (S 20 ) while moving (scanning) the ink-state detection sensor  40  in the longitudinal direction with respect to the tube  15  together with the carriage  2 . Subsequently, the viscosity-increased ink detecting section  67  detects whether or not the viscosity-increased ink exists in the tube  15  from the change of the output of the ink-state detection sensor  40  (change of the light-receiving amount of the light-receiving element  43 ) (S 21 ). 
     As explained with reference to  FIG. 6 , when the viscosity-increased ink exists in the tube  15 , the light-receiving amount (output value) of the light-receiving element  43  is increased. Therefore, when the light-receiving amount exceeds the second threshold value V 2  which corresponds to the light-receiving amount obtained when the ink, which is not subjected to the viscosity increase, exists (time Tb shown in  FIG. 6B ), the viscosity-increased ink detecting section  67  judges that the viscosity-increased ink is present in the tube  15  (S 21 : Yes). On the other hand, when it is judged that the viscosity-increased ink is absent in the tube  15  (S 21 : No), the routine returns from the sequence shown in  FIG. 9  without executing Steps S 22  to S 24  described later on. 
     When the viscosity-increased ink is present, the viscosity-increased ink detecting section  67  detects the position of the viscosity-increased ink in the longitudinal direction of the tube  15  from the timing at which the output of the sensor  40  is changed and the position of the carriage  2  which is provided at the timing and which is obtained by the position detection sensor  11 . Further, the amount of the viscosity-increased ink is detected from the time in which the output of the sensor  40  is changed. Further, the degree of the viscosity increase of the viscosity-increased ink is detected in accordance with the extent by which the output of the sensor  40  exceeds the second threshold value V 2  (S 22 ). 
     Subsequently, the purge control section  62  determines the optimum suction amount (minimum necessary suction amount) of the suction pump  31  and the suction speed (speed of rotation of the suction pump  31 ) when the viscosity-increased ink is discharged by means of the suction purge on the basis of the position and the size of the viscosity-increased ink and the degree of the viscosity increase detected by the viscosity-increased ink detecting section  67  (S 23 ). The position and the size of the viscosity-increased ink in the tube  15  are known. Therefore, it is possible to know the extent of the minimum suction by the suction pump  31  in order to successfully discharge the viscosity-increased ink contained in the tube  15  from the nozzles  14  completely. Further, the higher the degree of the viscosity increase is, the stronger the required suction is. However, the degree of the viscosity increase is known. Therefore, it is possible to reliably discharge the viscosity-increased ink by changing the suction speed depending thereon. 
     After determining the suction amount, the purge control section  62  allows the connection target of the suction pump  31  to be connected to the suction cap  30  by means of the switching unit  32 , and the suction cap  30  is brought in tight contact with the lower surface of the ink-jet head  3  on which the nozzles  14  are open. On this condition, the suction pump  31  is controlled so that the suction is performed in the amount and the suction speed determined in S 23  to execute the suction purge (S 24 ). 
     As explained above, in this embodiment, the state of the ink contained in the tube  15  (presence or absence of the bubble and/or the viscosity-increased ink) can be detected at the plurality of positions in the longitudinal direction of the tube  15 , and the state of the ink contained in the tube  15  can be detected continuously in the longitudinal direction of the tube  15  by means of the ink-state detection sensor  40  which is movable in the longitudinal direction of the tube  15 . Therefore, the opportunities to detect the bubble and/or the viscosity-increased ink existing in the tube are increased, and the reliability of the detection is enhanced. 
     The position and the size (amount) of the bubble and/or the viscosity-increased ink can be detected on the basis of the detection result of the ink-state detection sensor  40 . Therefore, it is possible to shorten the period of time required for the discharge and it is possible to suppress the discharged (wasted or discarded) ink amount to be small by optimally controlling the operation (suction amount of the suction pump  31 ) when the suction purge and the gas discharge purge are performed to discharge the bubble and the viscosity-increased ink on the basis of the information about the position and the size of the bubble and the viscosity-increased ink. 
     Further, even when the bubble and/or the viscosity-increased ink is/are detected, if the size of the bubble or the viscosity-increased ink is small to such an extent that it is judged that the influence, which is exerted on the ink jet head  3 , is almost absent, then it is also possible not to perform the gas discharge purge and the suction purge. For example, when the bubble is extremely small, the bubble storage amount is scarcely changed even when the bubble is absorbed by the bubble stored in the subtank  4 . Therefore, no influence is exerted on the head  3 . On the other hand, when the amount of the viscosity-increased ink is extremely small, no influence is exerted on the head  3  by mutually mixing the viscosity-increased ink and the ink having the ordinary viscosity disposed therearound. 
     In the embodiment of the present teaching, the carriage  2 , on which the ink-jet head  3  is carried, is provided with the ink-state detection sensor  40 . The ink-state detection sensor  40  is constructed movably in the longitudinal direction of the tube  15  integrally with the carriage  2 . Therefore, it is unnecessary to distinctly provide any exclusive arrangement for moving the ink-state detection sensor  40 . 
     The ink-state detection sensor  40  has the four photosensors  41 . The four photosensors  41  are used to individually detect the states of the respective inks contained in the four tubes for supplying the four color inks to the ink jet head  3  respectively. Therefore, it is possible to perform, for example, the optimization of the gas discharge purge and the suction purge for each of the tubes  15  (i.e., for each of the ink colors). 
     Next, modified embodiments, in which various modifications are applied to the embodiment described above, will be explained. However, those constructed in the same manner as those of the embodiment described above are designated by the same reference numerals, any explanation of which will be appropriately omitted. 
     The arrangement of the ink state detection sensor  40  can be variously changed as follows. 
     First Modified Embodiment 
     As shown in  FIG. 10 , when the surface (irradiation surface) of a portion  15   d  of a tube  15 , which is irradiated with the light emitted from the light-emitting element  42 , is a flat surface, then the refraction of the light and the irregular reflection are decreased on the irradiation surface, and the detection accuracy of the light is raised for the light-receiving element of the sensor  40 . Further, when the wall thickness of the portion  15   d  of the tube  15 , which is irradiated with the light, is thinner than those of the other portions, then the portion, which is irradiated with the light emitted from the light-emitting element  42 , is thinned, and the light is transmitted with ease. Therefore, the detection accuracy is raised as well. 
     Second Modified Embodiment 
     As shown in  FIG. 11A , a tube  15  may have a tube body  73  which is made of a synthetic resin material or an elastomer and which is formed with a groove  73   a  extending in the longitudinal direction, and a thin film  74  which is composed of a synthetic resin or the like. As for the tube  15 , the film  74  is stuck so that the groove  73   a  is covered therewith, and thus an ink channel is formed in the tube body  73 . In this arrangement, the light, which is emitted from the light-emitting element  42 , is radiated onto a flat outer surface of the thin film  74 , and thus the light is easily transmitted between the inside and the outside of the tube  15 . Alternatively, as shown in  FIG. 11B , a tube  15  is also available, wherein four grooves  73   a , which allow the four color inks to flow therethrough, are formed in a tube body  73 , and a film  74  is stuck so that the four grooves  73   a  are commonly covered therewith. Accordingly, ink channels, through which the four color inks are allowed to flow respectively, are integrated into one unit. 
     Third Modified Embodiment 
     In the embodiment described above, the ink-state detection sensor  40  is provided with the four photosensors  41  for performing the detection for the four tubes  15  respectively (see  FIG. 4 ). However, in the present teaching, it is not necessarily indispensable that the number of the tubes as the detection objectives should be coincident with the number of the ink-state detection sensors. A plurality of ink-state detection sensors may be provided for one tube. On the contrary, one ink-state detection sensor may be provided for a plurality of tubes. For example, one photosensor  41  may be constructed movably in the direction in which the plurality of tubes  15  are aligned (in the up-down direction in the example shown in  FIG. 4 ). The ink states of the plurality of tubes  15  may be detected by means of one photosensor  41 . 
     Fourth Modified Embodiment 
     When the carriage  2  is provided with a photosensor for detecting the end position in relation to the scanning direction of the recording paper P (recording medium), i.e., an end position detection sensor (end-position detecting mechanism) having a photosensor for detecting the recording paper width of the recording paper P, the end-position detection sensor can be also used as the ink-state detection sensor  40  for detecting the ink-state of the tube  15 . When the end-position detection sensor, which is provided for the carriage  2 , is used as the ink-state detection sensor as described above, it is unnecessary to provide any exclusive sensor for detecting the state of the ink contained in the tube  15 . 
     However, the end-position detection sensor emits the light toward the recording paper P when the end-position is detected. Therefore, in order to use the end-position detection sensor as the ink-state detection sensor when the tube  15  is positioned at the side position of the carriage  2  as shown in  FIG. 4  of the embodiment described above, it is necessary that the end-position detection sensor should be constructed so that the light emitting direction of the sensor can be changed. For example, as shown in  FIG. 12 , an ink-state detection sensor  40  may be attached to the carriage  2  so that the ink-state detection sensor  40  can be rotated by 90 degrees. When the end position is detected in the scanning direction of the recording paper P (direction perpendicular to the plane of the paper in  FIG. 12 ) as shown in  FIG. 12A , the light emitting direction of the ink-state detection sensor  40  is directed downwardly. As shown in  FIG. 12B , when the state of the ink contained in the tube  15  is detected, the light emitting direction of the ink-state detection sensor  40  is switched to the lateral direction. 
     Fifth Modified Embodiment 
     The ink-state detection sensor  40  may be composed of a transmission type photosensor. For example, as shown in  FIG. 13 , the following arrangement is also available. That is, the ink state detection sensor  40 , which is provided on the carriage  2 , includes a sensor body  80 , and a light-emitting element  83  and a light-receiving element  84  which are provided on the sensor body  80  so that the tubes  15  are interposed therebetween. The light, which is emitted from the light-emitting element  83  toward the tube  15  and which is transmitted through the tube  15 , is received by the light-receiving element  84  disposed on the opposite side. 
     Sixth Modified Embodiment 
     The ink-state detection sensor  40  is not limited to the photosensor described above. It is possible to use various sensors capable of detecting the liquid-state including, for example, the concentration of the ink contained in the tube, the viscosity, the presence or absence of the bubble, and the color. For example, it is also possible to use a sensor which utilizes the ultrasonic wave and a sensor which utilizes the difference in the capacitance or the electric conductivity depending on the presence or absence of the bubble contained in the tube  15 . Further, it is also allowable to use an image sensor as those adopted in image scanners. In this case, the position and the size of the bubble and/or the viscosity-increased ink can be detected by photographing the interior of the transparent tube  15  by means of the image sensor and applying the known image recognition processing to the photographed or picked up image data. 
     Seventh Modified Embodiment 
     The flexible tube  15  is easily bent and deformed to change the position when any external force is allowed to act during the movement of the carriage  2 . Therefore, in order to accurately detect the ink-state by means of the ink-state detection sensor  40 , it is preferable that the displacement of the tube portion as the detection objective is regulated to be small. Therefore, a member, which regulates the displacement of the tube, may be provided on a part or all of the flexible tube. For example, as shown in  FIG. 14A , the carriage  2  may be provided with a tube holding member  85 A, and the displacement of the tube  15  may be regulated thereby. The tube holding member  85 A is provided with an ink-state detection sensor  40  having a reflection type photosensor, and the tube holding member  85 A is formed with four grooves  86  which are opposed to the ink-state detection sensor  40 . The four tubes  15  are retained in the four grooves  86  respectively so that the four tubes  15  are slidable in the longitudinal direction. Further, the displacement of a portion of the tube, which is provided especially in the up-down direction and which is detected by the ink-state detection sensor  40 , is regulated when the carriage  2  is moved. It is not necessarily indispensable that the ink-state detection sensor  40  should be provided on the tube holding member  85 A as shown in  FIG. 14A . The ink-state detection sensor  40  may be provided on the carriage  2  as shown in  FIG. 4  of the embodiment described above. 
     Alternatively, as shown in  FIG. 14B , a tube holding member  85 B may have an elastic member  87  which is formed with an insertion hole  87   a  and which is divided into two at an upper portion of the insertion hole  87   a . Further, the tube  15  may be inserted into the insertion hole  87   a  of the elastic member  87 , the tube  15  may be tightened by the elastic member  87 , and thus the tube  15  may be retained by the tube holding member  85 B. In this arrangement, both of the displacements of the tube  15  in the two directions perpendicular to the longitudinal direction respectively (up-down direction and left-right direction in the drawing) are regulated. Further alternatively, as shown in  FIG. 14C , a tube holding member  85 C may have a pair of rollers  88  for interposing the tube  15 . In this arrangement, the tube holding member  85   c  can be smoothly moved in the longitudinal direction with respect to the tube  15  while regulating the displacement of the tube  15  in the left-right direction in the drawing by means of the pair of rollers  88 . In  FIGS. 14B and 14C , the ink-state detection sensor  40  is omitted from the illustrations. However, the ink-state detection sensor  40  is provided at any appropriate position of the carriage  2  or the tube holding members  85 B,  85 C opposed to the tube  15 . 
     As described above, when the part of the tube  15  to be detected by the ink-state detection sensor  40  is retained or held by the tube holding member  85  ( 85 A to  85 C) which is movable integrally with the ink-state detection sensor  40 , the displacement of the tube  15  is regulated in the direction perpendicular to the longitudinal direction. In this case, the position of the part of the tube  15  as the detection objective of the ink-state detection sensor  40  is hardly deviated with respect to the ink-state detection sensor  40 . Therefore, the detection accuracy of the ink-state detection sensor is improved. 
     Eighth Modified Embodiment 
     In the embodiment described above, the ink-state detection sensor  40  is constructed such that the ink-state detection sensor  40  is provided on the carriage  2  and the ink-state detection sensor  40  is movable in the longitudinal direction of the tube  15  integrally with the carriage  2 . Therefore, when the detection is performed by means of the ink-state detection sensor  40 , the ink jet head  3  is also moved together with the carriage  2 . During this process, the tube  15 , which is connected to the head  3 , is deformed or displaced by the external force allowed to act thereon in some cases. In such a situation, the detection accuracy of the sensor  40  is lowered. In this viewpoint, it is preferable that the ink-state detection sensor  40  is constructed to be relatively movable with respect to the ink-jet head  3  so that the ink-state detection sensor  40  is moved independently in the longitudinal direction with respect to the tube  15  in the stationary state to perform the detection, without allowing the ink-state detection sensor  40  to move integrally with the ink-jet head  3  (carriage  2 ) to perform the detection. 
     For example, as shown in  FIG. 15 , an ink-state detection sensor  40  may be provided distinctly from the carriage  2 . The ink-state detection sensor  40  may be movable in the longitudinal direction of the tube  15  in a state of being guided by a guide shaft  90  extending in the scanning direction (longitudinal direction of the tube  15 ). In this arrangement, the ink-state detection sensor  40  can be driven by a sensor driving motor  91  (movement-driving mechanism) even in a state in which the carriage  2  stops and the tube  15  stands still. Therefore, the ink state of the tube  15  can be detected while moving the ink-state detection sensor  40  in the longitudinal direction of the tube  15 . 
     Ninth Modified Embodiment 
     In the embodiment described above, as also appreciated from  FIG. 1 , it is difficult that the entire length of the tube  15  connected to the ink jet head  3  carried on the carriage  2  as well is designated as the detection objective by using the ink-state detection sensor  40  carried on the carriage  2 . In the embodiment described above, only the tube portion  15   a , which ranges from the bent and inverted portion  15   c  to the holder  6 , is designated as the detection objective. However, in this arrangement, it is impossible to detect the bubble and the viscosity-increased ink existing at the bent and inverted portion  15   c  and the tube portion  15   b  ranging from the bent and inverted portion  15   c  to the ink-jet head  3 . Accordingly, in view of the fact that the detection is performed over the entire length in the longitudinal direction of the tube  15 , it is preferable that the ink-state detection sensor  40  can be moved along the tube  15  while following the bending thereof as well. 
     A specified example is shown below. As shown in  FIG. 16 , a guide section  15   e , which extends in the longitudinal direction of the tube  15 , is integrally formed on the outer surface of the tube  15 , and a sensor holding member  92 , which is engageable with the guide section  15   e  of the tube  15 , is constructed movably along the guide section  15   e . Although not shown, the sensor holding member  92  is provided with an ink-state detection sensor and a driving mechanism which allows the sensor holding member  92  to travel by itself. The sensor holding member  92  is allowed to travel by itself along the tube  15  while following the bent portion thereof as well, and thus the ink-state can be detected over the entire length of the tube  15 . 
     Tenth Modified Embodiment 
     The ink-jet printer concerning the embodiment described above is the so-called serial type ink-jet printer in which the ink-jet head  3  is carried on the carriage  2  and the ink-jet head  3  is reciprocatively moved in the scanning direction parallel to the widthwise direction of the recording paper P. However, the present teaching is not limited thereto. The present teaching is also applicable, for example, to a printer having a line type ink-jet head  3  which is lengthy or is elongated in the widthwise direction of the recording paper P (scanning direction of the carriage as shown in  FIG. 1 ), wherein the liquid droplet jetting position is fixed. However, in this case, the carriage  2  for moving the ink-jet head  3  is absent. Therefore, in order to move the ink state detection sensor  40 , it is necessary to provide an arrangement for moving the ink-state detection sensor  40  singly with respect to the tube  15 , for example, as in the eighth modified embodiment ( FIG. 15 ) and the ninth modified embodiment ( FIG. 16 ). 
     Eleventh Modified Embodiment 
     It is not necessarily indispensable that one ink-state detection sensor  40  should be moved in the longitudinal direction of the tube  15 . The ink state can be also detected at a plurality of portions of the tube  15  by arranging a plurality of ink-state detection sensors  40  in the longitudinal direction of the tube  15 . 
     Twelfth Modified Embodiment 
     It is also allowable that the ink-state detection sensor  40  detects whether or not any ink of another color is mixed in the ink, without being limited to the detection to detect whether or not the viscosity-increased portion is present in the ink in the tube  15  or whether or not the bubble is mixed. It is considered that the ink of any different color is mixed into the tube, for example, if a user erroneously inserts an ink cartridge of any different color into the cartridge installing section when the ink cartridge is exchanged, or if any color mixture ink causes the counterflow from the nozzles on account of any reason. In particular, if the dark color ink such as the black ink or the like is mixed into the pale color ink such as the yellow ink or the like, then the printing quality is extremely deteriorated, if such an ink is used as it is for the printing and any color mixture ink is discharged, because the difference in the color between the color mixture ink and the original ink is extremely large. In such a situation, the ink-state detection sensor  40  may have a color mixture ink sensor  40  (color mixture liquid sensor, see  FIG. 7 ) for detecting the color mixture ink. In this case, the color mixture ink sensor  40   a  may be a color sensor for detecting the color of the ink. The color mixture ink has a concentration different from those of the other inks. Therefore, the color mixture ink sensor  40   a  may be a concentration sensor for detecting the concentration of the ink. As described above, for example, if the dark color ink such as the black ink or the like is mixed into the pale color ink such as the yellow ink or the like, the light transmittance and the light reflectance of the portion of the color mixture ink are greatly different from those of the portion of the pale color ink. Accordingly, the color mixture ink sensor  40   a  may be an optical sensor of the transmission type or the reflection type as described above. Alternatively, the color mixture sensor  40   a  may be a combination of the sensors as described above. In any case, the presence or absence of the color mixture ink, the position of the color mixture ink, and the size of the color mixture ink are detected for each of the four tubes  15  from the detection signal of the color mixture ink sensor  40   a  and the position information of the carriage  2  detected by the position detection sensor  11  by using a color mixture ink detecting section  68  (color mixture liquid-detecting mechanism, see  FIG. 7 ) provided for the state detecting section  61 . If the color mixture ink detecting section  68  detects that the color mixture ink is present in the tube  15 , the color mixture ink can be removed from the tube  15  by using the purge mechanism  7  in the same manner as the case in which the bubble and/or the viscosity-increased ink is/are removed as described above. 
     It is possible to perform the control and the detection in various forms other than those of the control and the detection referred to in the embodiment described above by using the information (position and size) in relation to the bubble contained in the tube  15  detected by the bubble detecting section  66  on the basis of the detection signal of the ink-state detection sensor  40 . An explanation will be made below about a modified embodiment in relation to the use of the detection signal of the ink-state detection sensor  40 . 
     Thirteenth Modified Embodiment 
     A certain amount of the bubble can be stored in the subtank  4 . Therefore, even when the bubble contained in the tube  15  is detected, it is unnecessary to perform the gas discharge purge every time when the bubble is detected. In other words, the following procedure is also available. That is, it is judged whether or not the total bubble amount in the subtank  4  exceeds the maximum storage amount assuming that the bubble, which is detected at the present point in time, is fed to the subtank  4 . The gas discharge purge is performed only when it is predicted that the total bubble amount in the subtank  4  exceeds the maximum storage amount. 
       FIG. 17  shows a flow chart illustrating the bubble discharge control according to a thirteenth modified embodiment. At first, in the thirteenth modified embodiment, the control unit  8  of the printer  1  retains or stores the information (detection hysteresis) about the size of the bubble contained in the tube  15  detected by the bubble detecting section  66  during a period until arrival at the present point in time after the completion of the previous gas discharge purge. 
     When the bubble discharge control is started, the scanning is performed with the ink state detection sensor  40  (S 30 ). When any bubble is detected (S 31 : Yes), the size V 1  of the bubble is calculated by the bubble detecting section  66  (S 32 ). Subsequently, the bubble detecting section  66  refers to the detection hysteresis described above, and the sizes (amounts) of bubbles detected in the past are added up to thereby estimate the bubble amount V stored in the subtank  4  at the present point in time (S 33 ). That is, the bubble detecting section  66  corresponds to the bubble storage amount-estimating mechanism according to the present teaching. 
     Subsequently, the purge control section  62  calculates the bubble amount in the subtank V 2  (=V+V 1 ) on the assumption that the bubble contained in the tube  15  is fed to the subtank  4  (S 34 ). It is judged whether or not the bubble amount in the subtank V 2  exceeds the predetermined maximum bubble storage amount Vmax capable of being stored in the subtank  4  (S 35 ). It is noted that the maximum bubble storage amount Vmax is the storage limit bubble amount which is determined in view of the design, for example, from the volume and the internal channel structure of the subtank  4 , at which it is judged that the bubble flows to the ink-jet head  3  when the suction purge or the printing operation of the ink-jet head  3  is performed, if any additional amount of the bubble flows into the subtank  4 . 
     If V 2  is not more than Vmax (S 35 : No), then it is judged that it is unnecessary to discharge the bubble at the present point in time, and the routine returns. On the other hand, if V 2  exceeds Vmax (S 35 : Yes), the purge control section  62  judges that it is necessary to discharge the bubble so that the bubble does not flow into the ink-jet head  3 . The suction amount of the suction pump  31  is determined (S 36 ) so that both of the bubble amount V contained in the subtank  4  and the bubble V 1  contained in the tube  15  can be discharged. The suction pump  31  is controlled to perform the gas discharge purge (S 37 ). All of the bubbles contained in the subtanks  4  are discharged by the gas discharge purge. Therefore, the past bubble detection hysteresis is reset or erased (S 38 ), and the routine returns from the sequence shown in  FIG. 17 . 
     In this way, the gas discharge purge is performed only when it is predicted that the bubble storage amount in the subtank  4  exceeds the maximum bubble storage amount. Accordingly, it is possible to maximally avoid any unnecessary execution of the gas discharge purge. If the suction pump  31  is driven while exceeding the bubble amount stored in the subtank  4 , then not only the bubble contained in the subtank  4  is discharged, but the ink is also discharged simultaneously. However, when the suction amount of the suction pump  31  is controlled, it is possible to decrease the ink to be discharged (wasted or discarded) during the discharge of the bubble. 
     Fourteenth Modified Embodiment 
     When the bubble contained in the tube  15  is disposed at a position separated far from the ink jet head  3 , a certain period of time is required until the bubble arrives at the ink-jet head  3 . Therefore, it is unnecessary to immediately discharge the bubble. In the case of the gas discharge purge, the bubble is discharged from the subtank  4  connected directly to the ink jet head  3 . Therefore, it is necessary that a large amount of the ink, which exists between the subtank  4  and the bubble, should be discharged previously before the bubble contained in the tube  15  is discharged. Therefore, it is preferable that the bubble is discharged when the bubble exists at a position disposed near to the ink-jet head  3  (subtank  4 ). However, it is necessary to avoid such a situation that the bubble arrives at the ink-jet head  3  when the ink contained in the tube  15  is moved toward the ink-jet head  3  in accordance with the printing operation (liquid droplet jetting operation) of the ink-jet head  3 . In view of the above, it is also allowable to determine whether the bubble is discharged immediately or the bubble is discharged after performing the printing operation by the ink-jet head  3  to some extent depending on the position of the bubble when the bubble is detected by the bubble detecting section  66 . 
       FIG. 18  shows a flow chart illustrating the bubble discharge control according to a fourteenth modified embodiment. When the bubble discharge control is started, the scanning is performed with the ink state detection sensor  40  (S 40 ). If any bubble is detected (S 41 : Yes), the position and the size of the bubble are detected by the bubble detecting section  66  (S 42 ). Further, the purge control section  62  estimates the ink amount existing in the region ranging from the position of the bubble detected by the bubble detecting section  66  to the subtank  4 , i.e., the ink amount Va to be discharged together with the bubble when the bubble is discharged from the subtank  4  by performing the gas discharge purge (S 43 ). The routine waits in this state until the printing instruction is inputted. When the printing instruction is inputted from PC  70  (S 44 : Yes), the head control section  63  estimates the ink amount to be consumed by the ink jet head  3  when the printing is performed on the basis of the inputted printing data (S 45 ). 
     Subsequently, the purge control section  62  compares the ink amount Va estimated in S 43  with the ink consumption amount Vb to be consumed by the ink-jet head  3  as estimated in S 45  (S 46 ). When Va is larger than Vb (S 46 : Yes), it is judged that the bubble contained in the tube  15  does not arrive at the subtank  4  even if the printing is performed with the ink-jet head  3 . The head control section  63  allows the ink jet head  3  to perform the printing operation. 
     On the other hand, when Va is not more than Vb (S 46 : No), it is judged that the bubble contained in the tube  15  arrives at the subtank  4  even if the printing is performed with the ink jet head  3 . The purge control section  62  determines the suction amount of the suction pump  31  (S 47 ) to perform the gas discharge purge (S 48 ). After that, the head control section  63  allows the ink jet head  3  to perform the printing operation (S 49 ). 
     When the printing operation is performed without performing the gas discharge purge (S 46  Yes→S 49 ), the bubble still remains in the tube  15 . Therefore, it is preferable to execute the series of steps from S 40  thereafter every time when the printing instruction is supplied from PC  70 . 
     In the fourteenth modified embodiment, it is possible to estimate whether or not the bubble contained in the tube  15  arrives at the ink jet head  3  (subtank  4 ) when the ink is consumed by the printing operation performed by the ink-jet head  3 . Therefore, it is possible to perform the gas discharge purge only when the gas discharge purge is really required by determining whether or not the gas discharge purge is to be performed prior to the printing operation. 
     Fifteenth Modified Embodiment 
     A fifteenth modified embodiment is an embodiment obtained by combining the thirteenth modified embodiment and the fourteenth modified embodiment described above. That is, when the bubble contained in the tube  15  is disposed at a position separated far from the ink-jet head  3 , a certain period of time is required until the bubble arrives at the ink-jet head  3 . Therefore, it is unnecessary to immediately discharge the bubble (fourteenth modified embodiment). Further, if a certain amount of the bubble can be stored in the subtank  4 , even when the bubble contained in the tube  15  is detected, then it is unnecessary to perform the gas discharge purge every time when the bubble is detected (thirteenth modified embodiment). 
       FIG. 19  shows a flow chart illustrating the bubble discharge control according to the fifteenth modified embodiment. In the fifteenth modified embodiment, the control unit  8  of the printer  1  retains the information (detection hysteresis) about the size of the bubble contained in the tube  15  detected by the bubble detecting section  66  during a period until arrival at the present point in time after the completion of the previous gas discharge purge. 
     When the bubble discharge control is started, the scanning is performed with the ink-state detection sensor  40  (S 50 ). When any bubble is detected (S 51 : Yes), the position and the size (V 1 ) of the bubble are detected by the bubble detecting section  66  (S 52 ). The purge control section  62  estimates the ink amount Va existing in the region ranging from the position of the bubble detected by the bubble detecting section  66  to the subtank  4  (S 53 ). After that, if the printing instruction is inputted from PC  70  (S 54 : Yes), the head control section  63  estimates the ink amount Vb to be consumed by the ink jet head  3  if the printing is performed based on the inputted printing data (S 55 ). 
     Subsequently, the purge control section  62  compares the estimated ink amount Va with Vb. When Va is larger than Vb (S 56 : Yes), it is judged that the bubble contained in the tube  15  does not arrive at the subtank  4  even if the printing is performed with the ink-jet head  3 . The head control section  63  allows the ink-jet head  3  to perform the printing operation (S 57 ). 
     On the other hand, when Va is not more than Vb (S 56 : No), the bubble contained in the tube  15  arrives at the subtank  4  if the printing is performed with the ink-jet head  3 . 
     At this stage, the bubble detecting section  66  refers to the detection hysteresis described above, and the sizes (amounts) of bubbles detected in the past are added up to thereby estimate the bubble amount V stored in the subtank  4  at the present point in time (S 58 ). 
     Subsequently, the purge control section  62  calculates the bubble amount in the subtank V 2  (=V+V 1 ) on the assumption that the bubble contained in the tube  15  is fed to the subtank  4  when the printing is performed with the ink-jet head  3  (S 59 ). It is judged whether or not the bubble amount in the subtank V 2  exceeds the predetermined maximum bubble storage amount Vmax capable of being stored in the subtank  4  (S 60 ). 
     When V 2  is not more than Vmax (S 60 : No), then it is judged that it is unnecessary to discharge the bubble at the present point in time, and the head control section  63  allows the ink-jet head  3  to perform the printing operation (S 57 ). On the other hand, when V 2  exceeds Vmax (S 60 : Yes), the purge control section  62  judges that it is necessary to discharge the bubble so that the bubble does not flow into the ink jet head  3 . The suction amount of the suction pump  31  is determined (S 61 ) so that both of the bubble amount V contained in the subtank  4  and the bubble V 1  contained in the tube  15  can be discharged. The suction pump  31  is controlled to perform the gas discharge purge (S 62 ). It is noted that all of the bubbles contained in the subtanks  4  are discharged by the gas discharge purge. Therefore, the past bubble detection hysteresis is reset or erased (S 63 ). After that, the ink jet head  3  is allowed to perform the printing operation (S 57 ). 
     Sixteenth Modified Embodiment 
     When the time elapses while allowing the ink jet head  3  to stop in a state in which the bubble exists in the tube  15 , the bubble grows on account of the invasion of the external air and the continuous evaporation of the ink. The growth of the bubble can be grasped from the position of the bubble detected at the present point in time by the bubble detecting section  66  and the position of the bubble detected therebefore. The growth of the bubble equals to the decrease in the volume of the ink and the progress of the viscosity increase corresponding to the amount of the growth. Therefore, it is possible to estimate the viscosity of the ink from the growth of the bubble (change of the bubble amount). 
     An explanation will be made about a specified technique for estimating the viscosity. As shown in  FIG. 20 , the subtank  4 , which is positioned above the ink-jet head  3 , is connected via the tube  15  to the ink cartridge  5  which is open to the atmospheric air by the atmospheric air communication hole  5   a . On the other hand, as shown in  FIG. 21 , a control unit  8  of the printer of a sixteenth modified embodiment has a viscosity estimating section  93  (viscosity-estimating mechanism) which estimates the viscosity based on the position of the bubble detected by the bubble detecting section  66 . 
     When the evaporation (viscosity increase) of the ink and the growth of the bubble  51   a  are caused in the tube  15 , then the income and the outgo of the ink are caused between the tube  15  and the ink cartridge  5 , and the liquid surface of the ink cartridge  5  is varied or fluctuated. However, the meniscus retaining force, which is provided on the side of the ink-jet head  3  (nozzles), is sufficiently higher than that provided on the side of the ink cartridge  5 . Therefore, the meniscus position is not varied or fluctuated, and the income and the outgo of the ink are not caused on the nozzle side. Accordingly, in this procedure, the evaporation rate of the ink is estimated by detecting the volume change of the ink on the downstream side (nozzle side) of the bubble  51   a.    
     The evaporation rate “r” of the ink is determined from the volume change of the ink on the downstream side from the bubble  51   a  during the period ranging from the last estimation to the present estimation. That is, the evaporation rate “r” of the ink can be expressed as eq. 1 by using the volume V 0  of the ink on the downstream side at the last estimation (in the last time) and the volume V of the ink on the downstream side at the present estimation (in the present time).
 
 r=V 0 −V/V 0  (eq. 1)
 
     The volume V of the ink on the downstream side in the present time can be calculated from eq. 2 by using the volume V 0  of the ink on the downstream side in the last time and the frontward movement volume ΔVt 1  of the bubble in the tube.
 
 V=V 0 −ΔVt 1  (eq. 2)
 
     In this procedure, the ink volume V 0  in the last time is the value which has been determined during the viscosity estimation in the last time and which is stored. The ink volume V 0  in the last time is updated every time when the viscosity estimation is performed. The frontward movement volume ΔVt 1  of the bubble in the tube indicates the frontward movement amount of the bubble  51   a  toward the downstream side, which is determined from the change of the position of the downstream end of the bubble  51   a  detected by the bubble detecting section  66 . 
     However, in the above described equation (eq. 2), the growth of the bubble  51   b  existing in the subtank  4  is not taken into consideration. In order to take the bubble  51   b  in the subtank  4  into consideration as well, the growth volume ΔVs of the bubble in the subtank is further determined, and the following equation (eq. 3) is used in place of the equation (eq. 2) to determine the ink volume V on the downstream side in the present time.
 
 V=V 0 −ΔVt 1−Δ Vs   (eq. 3)
 
     In this procedure, the growth volume ΔVs of the bubble in the subtank can be calculated by multiplying the growth volume ΔVt 2  of the bubble in the tube determined from the bubble position detected by the bubble detecting section  66  by a predetermined coefficient C. The coefficient C relates to the condition of the evaporation of the ink, such as the material, the structure, and the surface area, and reflects the difference in the bubble growth speed between the tube  15  and the subtank  4  having different conditions of the evaporation of the ink. The coefficient C is previously set at the designing stage, and it is stored in the control unit  8 . 
     When the ink evaporation rate is determined from the equation (eq. 1), the viscosity estimating section  93  estimates the ink viscosity on the downstream side of the bubble from a previously stored correspondence table between the ink evaporation rate and the ink viscosity. 
     When the ink viscosity can be estimated as described above, it is possible to perform the following control. For example, when the head control section  63 , which controls the ink-jet head  3 , is allowed to give the high energy to the ink when the ink viscosity is high, it is possible to realize the same jetting characteristic as that obtained when the viscosity is low. When the purge control section  62 , which controls the purge mechanism  7 , raises the suction speed of the suction pump  31  when the ink viscosity is high, it is possible to reliably discharge the ink having the high viscosity by means of the suction purge. Further, it is also possible to estimate the viscosity of the ink in the ink-jet head  3  and/or the ink cartridge  5  by estimating the evaporation rate of the ink in the ink jet head  3  and/or the ink cartridge  5  from the evaporation rate “r” of the ink described above. In this procedure, if the viscosity of the ink is high, it is feared that the jetting failure may arise in the nozzle  14 . Therefore, it is also effective to increase the amount and/or the number of times of the flashing to be performed before the printing or during the printing, and it is also effective to increase the frequency of the suction purge. 
     Seventeenth Modified Embodiment 
     The movement velocity of the ink in the tube  15  is determined from the change of the position of the bubble detected by the bubble detecting section. It is also possible to estimate the viscosity of the ink on the basis thereof. 
     For example, when the movement velocity of the ink, which is determined from the change of the position of the bubble detected by the bubble detecting section  66  when the ink is consumed by performing the liquid droplet jetting from the ink jet head  3  and/or performing the suction purge in a state in which the bubble exists in the tube  15 , is smaller than the movement velocity of the ink which is assumed from the ink not subjected to the viscosity increase, the fluidization resistance is increased. Therefore, it can be judged that the viscosity of the ink is raised. The viscosity can be also estimated by detecting the movement velocity of the bubble when the suction purge is performed by the purge mechanism  7  to discharge a predetermined amount of the ink by means of the purge. 
     Alternatively, the flow (movement velocity) of the ink caused by the inertia, which is predicted from the scanning velocity of the carriage when the carriage  2  is moved in the scanning direction at a predetermined velocity, is compared with the movement velocity which is determined from the change of the position of the bubble detected by the bubble detecting section  66 . The viscosity of the ink can be also estimated from the difference therebetween. 
     Eighteenth Modified Embodiment 
     If the leak arises at a part of the tube  15 , when a predetermined amount of the ink is consumed by the ink-jet head  3  to generate the flow of the ink in the tube  15 , then the actual movement velocity of the ink in the tube  15  is different from the movement velocity of the ink estimated from the ink amount to be consumed. Accordingly, when the movement velocity of the ink is determined from the position of the bubble brought about when the ink in the tube  15  flows, then it is also possible to detect the leak from the tube  15 . 
     The embodiment and the modified embodiments thereof described above are described merely by way of example in every sense. The present invention is not limited to the embodiment and the modified embodiments. For example, a plurality of the modified embodiments may be carried out appropriately in combination. 
     The embodiment and the modified embodiments thereof described above are the examples in which the present teaching is applied to the ink jet printer which is one of liquid droplet jetting apparatuses. However, the application objective of the present teaching is not limited thereto. That is, the present teaching is applicable to any liquid droplet jetting apparatus provided with the structure for supplying various liquids to the head via the tube or tubes irrelevant to the type of the liquid to be jetted, the way of use, and the technical field in which the present teaching is used.