Patent Publication Number: US-8534785-B2

Title: Inkjet printer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2011-093233, filed Apr. 19, 2011, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an inkjet printer comprising an ink circulation path. 
     2. Description of the Related Art 
     An inkjet printer which records images by ejecting inks to recording media from an inkjet head is widely used. As an example of such an inkjet printer, there is a known printer comprising an ink circulation path which circulates an ink between an ink tank storing an ink and an inkjet head. 
     For example, Jpn. Pat. Appln. KOKAI Publication No. 2001-219580 Publication discloses an inkjet printer comprising an ink circulation path which comprises: a recording head (inkjet head); a first ink tank provided above the recording head; and a second ink tank provided below the recording head; and a pump which pumps the ink from the second ink tank to the first ink tank. On this ink circulation path, image recording is performed while the ink is circulated through the ink circulation path by supplying an ink from the first ink tank to the second ink tank through the recording head utilizing height differences between positions of the first ink tank, the recording head, and the second ink tank and by drawing up the ink collected into the second ink tank further into the first ink tank by the pump. 
     Further, the inkjet printer according to Jpn. Pat. Appln. KOKAI Publication No. 2001-219580 is provided with a liquid surface detector which detects an ink liquid surface inside each of the first and second ink tanks. These liquid surface detectors monitor ink surfaces in the first and second ink tanks, and output signals from the liquid surface detectors are transferred to a control unit. If the ink in the first ink tank is determined to be insufficient by the output signal from the liquid surface detector for the first ink tank, the ink is suctioned from the second ink tank to the first ink tank to supply the first ink tank with the ink. Further, if an ink in the second ink tank is determined to be insufficient by the output signal from the liquid surface detector for the second ink tank, the second ink tank is supplied with the ink from a supply ink tank. Thus, even when the ink is ejected from the recording head during circulation of the ink, an amount of the ink in the ink circulation path is maintained properly. 
     Meanwhile, the inkjet printer described in the publication No. 2001-219580 is based on a prerequisite that the liquid surface detectors provided respectively for the first and second ink tanks operate properly. However, the liquid surface detectors can cause a malfunction or operation error due to external disturbance factors. In this case, a proper ink circulation operation and ink charging operation as described above cannot be achieved through the ink circulation path. For example, even when sufficient ink remains in an ink tank, the ink can possibly be determined to be short. Consequently, an excessive portion of the ink over a capacity flows in and overflows to outside the tank, thereby staining inside of the printer. 
     Therefore, there is a need to detect whether the liquid surface detectors properly operate or not. However, the publication No. 2001-219580 discloses nothing about detection of whether the liquid surface detectors properly operate or not. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to providing an inkjet printer capable of achieving a proper ink circulation operation by detecting malfunctions and operation errors in a liquid surface detector which detects an ink liquid surface in an ink tank. 
     According to a embodiment of the invention, an inkjet printer is provided, the inkjet printer comprising: an ink circulation path comprising an ink head comprising a nozzle surface where a plurality of nozzles which eject an ink are formed, a first tank which is positioned above the nozzle surface along a gravitational direction and stores the ink to be supplied for the ink head, a second tank which is provided below the nozzle surface in the gravitational direction and stores the ink collected from the ink head, a pump which pumps the ink from the second tank to the first tank; first and second liquid surface detectors which respectively detect ink liquid surfaces in the first and second tanks; an ink supply unit which supplies the ink circulation path with the ink; a supplementary feed valve which makes the ink supply unit and the ink circulation path communicate with or shut from each other; a control unit which controls driving and a stop operation of the pump, an open/close operation of the supplementary feed valve, and the whole inkjet printer; a storage unit which stores information detected by the first and second liquid surface detectors; and a determination unit which determines whether or not an abnormality occurs in the first and second liquid surface detectors, based on the information, wherein when the ink circulates through the ink circulation path, the second liquid surface detector detects whether or not the ink liquid surface in the second tank is higher than a predetermined level, the detected information is accumulatively stored into the storage unit, the determination unit determines whether or not an abnormality occurs in the second liquid surface detector, based on the accumulatively stored information, the first liquid detector thereafter detects whether or not an ink liquid surface in the first tank is higher than a predetermined level, the detected information is accumulatively stored into the storage unit, and the determination unit determines whether or not an abnormality occurs in the first liquid surface detector, based on the accumulatively stored information. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
         FIG. 1  schematically shows a configuration of an ink path in an inkjet printer; 
         FIG. 2  schematically shows the configuration of the ink circulation unit; 
         FIG. 3A  is a transition table showing a first mode of operations of a pump and a supplementary feed valve; 
         FIG. 3B  is a transition table showing a second mode of operations of the pump and supplementary feed valve; 
         FIG. 3C  is a transition table showing a third mode of operations of the pump and supplementary feed valve; 
         FIG. 4A  is a flowchart showing a whole ink circulation operation according to the embodiment; 
         FIG. 4B  is a flowchart showing a subroutine of sensor on/off for charging an ink in  FIG. 4A ; 
         FIG. 4C  is a flowchart showing a subroutine of sensor on/off for detecting an operation of a second sensor in  FIG. 4A ; 
         FIG. 4D  is a flowchart showing a subroutine of pump stopping/driving for detecting an operation of the pump in  FIG. 4A ; 
         FIG. 4E  is a flowchart showing a subroutine of sensor on/off for detecting an operation of a first sensor in  FIG. 4A ; and 
         FIG. 4F  is a flowchart showing a subroutine for a standby processing in  FIG. 4A  and  FIGS. 4C to 4E . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, an embodiment of the invention will be described. 
       FIG. 1  schematically shows a configuration of an ink path  1  in an inkjet printer according to an embodiment of the invention. Although  FIG. 1  typically shows only a configuration of an ink path concerning an ink of a color, the inkjet printer comprises ink paths for four colors of inks, such as, cyan (C), black (K), magenta (M), and yellow (Y). 
     In addition to an ink path  1  shown in  FIG. 1 , the inkjet printer has the same configuration as a conventional inkjet printer, such a supply unit which supplies recording media, a conveyor unit which conveys the supplied recording media, a discharge unit which feeds out the recording media with an image recorded on and a maintenance unit which performs a maintenance operation for an ink head. 
     Where roughly divided, the ink path  1  comprises: an image recording unit  3  comprising a plurality of ink heads  2 ; an ink circulation unit  6  which comprises at least a first tank  4  and a second tank  5  and circulates an ink for the image recording unit  3 ; a supply unit  7  which supplies the ink circulation unit  6  with the ink; a drainage unit  8  to contain an unrequired ink and an overflowing ink; a first-tank-shared air chamber  9  and a second-tank-shared air chamber  10 ; and a pressure adjuster  11 . For example, when the inkjet printer comprises ink paths for four colors of inks, the inkjet printer comprises four independent systems of ink paths (ink circulation units) and image recording units. However, the drainage unit  8 , first-tank-shared air chamber  9 , second-tank-shared air chamber  10  and pressure adjuster  11  are common to all of these colors. 
     The inkjet printer also comprises: a control unit  60  which performs control of a whole printer, such as an image recording operation of the image recording unit  3 ; a storage unit  70  such as a program memory in form of a RPM which stores required control programs or a data memory in form of a RAM which stores information output from individuals of components forming the inkjet printer; and a determination unit  80  which makes a determination for controlling the individual components of the inkjet printer, based on information stored in the storage unit  70 . 
     The image recording unit  3  will be described first. 
     The image recording unit  3  comprises a plurality of ink heads  2 , an ink distributor  12  to distribute an ink to the plurality of ink heads  2 , and an ink collector  13  to collect the ink from the plurality of ink heads  2 . The ink distributer  12  is connected to communicate with the plurality of ink heads  2  and the first tank  4 . The ink collector  13  is connected to communicate with the plurality of ink heads  2  and the second tank  5 . 
     In the present embodiment, the ink distributer  12  is provided in an upstream side of the ink heads  2 , and the ink collector  13  is provided in a downstream side thereof. However, the first tank  4  and second tank  5  are not limited to this configuration but may be connected directly to the ink heads  2 . 
     In the present embodiment, the plurality of short ink heads  2  which are shorter in width than a width (recording width) of a recording medium are arrayed, for example, in a zigzag layout along a width direction of the recording medium (perpendicular to a conveying direction of the recording medium), thereby to form a line head wider than the recording width. The embodiment is not limited to these ink heads but may employ a long ink head. 
     In a nozzle surface  50  of each ink head  2 , a plurality of nozzles are formed in columns. The ink heads  2  each are internally maintained at a negative pressure (approximately −1 kPa at a gauge pressure in the present embodiment) during ink circulation, which creates a spherically concave meniscus inside each nozzle, to enable a proper recording operation. During recording, the ink heads  2  each eject the ink from the nozzles to a recording medium which is conveyed by a conveyor unit not shown, based on an image signal input from outside. In this manner, a desired image is recorded on the recording medium. 
     Next, the supply unit  7  will be described. 
     The supply unit  7  comprises: an ink cartridge  14  filled with a refill ink; an ink cartridge tray  15  on which the ink cartridge  14  is arranged; a joint unit  16  detachably connected to a supply port of the ink cartridge  14 ; an ink cartridge determination unit  17  for preventing the ink cartridge  14  from being erroneously attached and for detecting a residual ink amount; and a supplementary feed valve  19  which is interposed in a tube  18  connected from the joint, unit  16  to the first tank  4  and makes the supply unit  7  and ink circulation unit  6  to communicate with or shut from each other. 
     In the present embodiment, the supply unit  7  is connected to the first tank  4  through a tube  18 . However, the supply unit  7  may alternatively be connected to the second tank  5 . In this case, the second tank  5  is supplied with the ink from the supply unit  7 . 
     Next, the drainage unit  8  and the first- and second-tank-shared air chambers  9  and  10  will be described. 
     The drainage unit  8  comprises: an overflow tank  20  which collects an (overflow) ink overflowing from the ink circulation unit  6 ; a drainage tank  22  which is detachably coupled to the overflow tank  20  through a tube  21 ; a drainage tank tray  23  on which the drainage tank  22  is arranged; a drainage amount detector  24  which detects an amount of ink contained in the drainage tank  22 , from a difference in weight or liquid levels; and a drainage-tank attachment detector  25  which detects presence or absence of mounting of the drainage tank  22  by optical detection. 
     The overflow tank  20  is provided below the pump  30  so as to receive all of an overflowing ink even when the pump  30  described later is damaged and an ink overflows from the ink circulation unit  6 . Further, the overflow tank  20  is connected to the first-tank-shared air chamber  9  through a tube  27  with an air release valve  26  interposed therebetween, as well as connected to the second-tank-shared air chamber  10  through a tube  29  with an air release valve  28  interposed therebetween. Further, the first-tank-shared air chamber  9  is connected to the first tank  4  through a tube  37 , and the second-tank-shared air chamber  10  is connected to the second tank  5  by a tube  38 . 
     The configuration as described above allows an overflowing ink to be collected into the overflow tank  20  through the tube  37 , the first-tank-shared air chamber  9  and tube  27  from the first tank  4 , as well as through the tube  38 , the second-tank-shared air chamber  10  and tube  29  from the second tank  5 , even when an ink overflows from the first tank  4  or second tank  5 . The ink thus contained in the overflow tank  20  is further collected into the drainage tank  22  through the tube  21 . 
     Since the tube  27  extended from the first-tank-shared air chamber  9  is inserted into the overflow tank  20  through the air release valve  26 , the first-tank-shared air chamber  9  can be opened/closed to the air by opening/closing (releasing/shutting) the air release valve  26 . Similarly, the tube  29  extended from the second-tank-shared air chamber  10  is inserted into the overflow tank  20  through the air release valve  28 , the second-tank-shared air chamber  10  can be opened/closed to the air by opening/closing (releasing/shutting) the air release valve  28 . That is, the air release valve  26  allows inside of the first tank  4  to communicate with or shut from the air. The air release valve  28  allows inside of the second tank  5  to communicate with or shut from the air. 
     Next, the ink circulation unit  6  will be described. 
     The ink circulation unit  6  comprises the first tank  4 , second tank  5 , pump  30 , a one-way valve  31 , a heat exchanger  32  and a filter  33 . As for these components, nozzle surfaces  50  of the ink head  2 , an ink liquid surface  51  inside the first tank  4  and an ink liquid surface  52  inside the second tank  5  are arranged so as to satisfy a positional relationship that the ink liquid surface  51 , nozzle surfaces  50 , and ink liquid surface  52  are positioned in this order from the highest position to the lowest position along a perpendicular direction (gravitational direction). The ink circulation unit  6  comprises an ink circulation path which allows the ink to flow from the first tank  4  and return to the first tank  4 , orderly through the ink distributer  12 , ink heads  2 , ink collector  13 , second tank  5 , pump  30 , one-way valve  31 , heat exchanger  32 , and filter  33 , connected to one another by tubes  34  to  36 . 
       FIG. 2  schematically shows an enlarged view of the configuration of the ink circulation unit  6  in  FIG. 1 . Arrows added to the tubes  34  to  36  in  FIG. 2  indicate directions in which the ink flows when the ink circulates through the ink circulation path. The heat exchanger  32  and filter  33  are omitted from  FIG. 2 . 
     The ink circulation unit  6  according to the present embodiment is divided into a first path  40  and a second path  41 , as shown in  FIG. 2 . The first path  40  is to flow the ink from the first tank  4  to the second tank  5  through the ink heads  2 . The second path  41  is to flow the ink up from the second tank  5  to the first tank  4  through the one-way valve  31 , heat exchanger  32  and filter  33  by the pump  30 . 
     At first, components of the first path  40  will be described in details. 
     The first tank  4  stores an ink to be supplied for the ink heads  2 . The first tank  4  is provided with an ink inlet port  4   a , an ink outlet port  4   b , an air port  4   c  and an ink supply port  4   d.    
     The ink inlet, port  4   a  is connected to the filter  33  through the tube  36 , and makes the ink, which flows out of the filter  33 , flow into the first tank  4 . An opening of the ink inlet port  4   a  in the first tank  4  is provided to be lower (see  FIG. 1  and at a bottom of the first tank  4  in  FIG. 2 ) than the ink liquid surface in the first tank  4  along the perpendicular direction (gravitational direction) so that air bubbles may not be mixed into the ink flowing in. 
     The ink outlet port  4   b  is connected to the ink distributer  12  through the tube  34 , and makes the ink flow from the first tank  4  to the ink distributer  12 . The ink which has flowed into the ink distributer  12  is distributed evenly to the ink heads  2  (K 1  to K 6 ). The ink which has flowed into the ink heads  2  is ejected through nozzles formed in the nozzle surfaces  50  of the ink heads  2 . In this manner, an image is recorded on a recording medium conveyed by the conveyor unit not shown. 
     An amount of ink which flows into the ink heads  2  is set to be greater than an amount of the ink ejected from the nozzles. Therefore, a portion of the ink which is not ejected from the nozzles is once collected by the ink collector  13 , and then flows out to the second tank  5  through the tube  35 . 
     The air port  4   c  is connected to the first-tank-shared air chamber  9  through the tube  37 . The first-tank-shared air chamber  9  is connected also to air ports of first tanks  4  for the other colors. 
     The ink supply port  4   d  is connected to the ink cartridge  14  through the tube  18 . In the present embodiment, the ink is supplied to the first tank  4  from the ink cartridge  14  due to differences in height therebetween by opening the supplementary feed valve  19 . However, the embodiment is not limited hitherto but the ink needs only to be fed from the ink cartridge  14  to the first tank  4 . For example, the ink may be fed by a pump in place of the supplementary feed valve  19 . 
     Further, in order to maintain an ink liquid surface at a predetermined height, that is, to maintain an ink in the first tank  4  at a predetermined amount, the first tank  4  is provided with a first liquid surface detector  42  which detects whether or not the ink liquid surface in the first tank  4  is at a predetermined level or higher. 
     The first liquid surface detector  42  comprises: a float member  42   a  which is floated on the ink liquid surface; a liquid surface position sensor  42   b  attached to a wall surface of the first tank  4 ; and a magnet  42   c  which is attached to the float member  42   a.    
     The float member  42   a  is pivotally supported by a support shaft  42   d  so as to be pivotal depending on the height of the ink liquid surface in the first tank  4 . The liquid surface position sensor  42   b  is made of, for example, a magnetic sensor, and detects the position of the float member  42   a , that is, the height of the ink liquid surface in the first tank  4  by detecting magnetic force of the magnet  42   c  attached to the float member  42   a . Further, whether an ink amount in the first tank  4  is proper or not is determined, based on a liquid surface detection signal detected by the first liquid surface detector  42 . Further, the control unit  60  controls drive and stop operations of the pump  30  and an open/close operation of the supplementary feed valve  19 . Thus, the ink amount in the first tank  4  is maintained at a predetermined amount. 
     The second tank  5  stores an ink collected from the ink heads  2 . The second tank  5  is provided with: an ink inlet port  5   a  which is connected to the ink collector  13  through the tube  35  and allows the ink to flow in from the ink collector  13 ; an ink outlet port  5   b  which is connected to the pump  30  through the tube  36  and feeds the ink to the pump  30 ; and an air port  5   c  which is connected to the second-tank-shared air chamber  10  through the tube  38 . 
     Further, in order to maintain an ink liquid surface at a predetermined height, the second tank  5  is provided with a second liquid surface detector  43  which detects whether or not the ink liquid surface in the second tank  5  is lower than a predetermined level. Similarly to the first liquid surface detector  42 , the second liquid surface detector  43  comprises a float member  43   a , a liquid surface position sensor  43   b , and a magnet  43   c  attached to the float member  43   a . These components have the same configuration as the first liquid surface detector  42 , and therefore, descriptions thereof will be omitted herefrom. 
     In the present embodiment, the first and second liquid surface detectors  42  and  43  are magnetic sensors. However, the embodiment is not limited hitherto but may employ optical sensors each comprising a light emitting element and a light receiving element. 
     Next, constitutive components of the second path  41  will be described in details. 
     For example, an electromagnetic piston pump may be used as the pump  30 . The pump  30  is driven/stopped to feed the ink from the second tank to the first tank by the control unit  60  in accordance with detection results of the first liquid surface detector  42  and second liquid surface detector  43 , in a manner that heights of the ink liquid surface  51  and ink liquid surface  52  are respectively maintained within desired ranges in the first, tank  4  and second tank  5 . 
     Also in the present embodiment, liquid feed capability of the pump  30  is designed to be able to feed into the first tank  4  an amount of ink which is greater than that flowing into the second tank  5 . This design is to prevent overflow from the second tank  5 . That is, in ordinary use, overflow from the second tank  5  is prevented by increasing a flow rate of the ink, at which the pump  30  can feed up when the pump  30  is driven, to be greater than a flow rate of the ink which flows into the second tank  5 . 
     Although the present embodiment employs an electromagnetic piston pump as the pump  30 , the pump  30  needs ability only to be able to feed a greater amount of ink than an amount of ink flowing into the second tank  5 . For example, a diaphragm pump, gear pump, tube pump, rotary pump, or centrifugal pump which has such a feeding ability may be used. 
     The one-way valve  31  is interposed in the tube  36  in an ink ejection side (a path in a liquid feed side toward the first tank  4 ) of the pump  30 . The one-way valve  31  prevents reverse flow of ink or, specifically, flow of ink toward the second tank  5  from the first tank  4 , which is caused by a difference in height between the ink liquid surface  51  in the first tank  4  and the ink liquid surface  52  in the second tank  5 . 
     That is, as described above, the liquid feed capability of the pump  30  is set to be able to feed up a greater amount of ink than an amount of ink which flows form the first tank  4  to the second tank  5  through the ink heads  2 . Therefore, when an ink circulation operation is performed, the pump  30  performs an intermittent operation. When the pump  30  stops, the ink reversely flows from the first tank  4  to the second tank  5 . Therefore, the one-way valve  31  prevents this reverse flow. 
     The heat exchanger  32  comprises a heat sink unit, a cooling fan, a heater unit and an ink flow path. The configuration as described except the ink flow path is common to the ink paths for all colors. The heat exchanger  32  functions to control the temperature of the ink which flows inside the ink circulation unit  6  to a desired temperature. In order to control the heat exchanger  32  by detecting the temperature of the ink, each of the ink heads  2  or the ink flow path near the ink heads  2  is provided with a temperature sensor  44  as shown in  FIG. 1 . 
     The filter  33  is provided to remove foreign materials contained in the ink supplied for the ink heads  2  and to eliminate recording errors caused by clogging of nozzle holes. The filter  33  comprises meshed member to allow the ink to penetrate. In order to allow the ink to pass a diameter of each nozzle hole of the ink heads  2 , a mesh having a mesh size is so selected as to remove sufficiently small foreign materials. 
     Next, the pressure adjuster  11  will be described with reference to  FIG. 1 . 
     The pressure adjuster  11  comprises a bellows unit  45  which generates a negative pressure, a weight unit  46 , and a bellows elevation mechanism  47 . 
     The bellows unit  45  is connected to the second-tank-shared air chamber  10  by the tube  39 . As the bellows unit  45  extends and drops due to a load of the weight unit  46  with the air release valve  28  shut off from the air, inside of the second-tank-shared air chamber  10  is put in a state of a negative pressure. That is, when the air release valve  28  is closed to drop the bellows elevation mechanism  47  to a position as shown in  FIG. 1 , the bellows unit  45  which is apart from the bellows elevation mechanism  47  is pulled down by a weight of the weight unit  46 . Accordingly, a negative pressure is generated in the second-tank-shared air chamber  10  in balance with the gravity acting on the weight unit  46 . 
     Further, the same negative pressure as generated in the second-tank-shared air chamber  10  is generated in the second tank  5  connected to the second-tank-shared air chamber  10  through the tube  38 . The negative pressure in the second tank  5  is arranged so as to apply a pressure adequate for recording (for example, a nozzle pressure of approximately −1 kPa during ink circulation) to inside of the ink heads  2  which communicates through the tube  35  during ink circulation, or more specifically, to inside of the ink heads  2  near the nozzles. By this pressure, a meniscus is formed in each nozzle. 
     A standby position of the bellows elevation mechanism  47  is arranged such that the bellows unit  45  shrinks and a negative pressure can be generated in a short time when ink circulation is performed. In addition, a partition wall (not shown), only an upper part of which communicates, is provided inside the second-tank-shared air chamber  10  so that the ink in the second tank  5  may not enter into the pressure adjuster  11  even when the ink overflows. In this manner, only transfer of a gas is performed inside the tube  39  which communicates from the second tank  5  to the pressure adjuster  11 . 
     Next, with reference to  FIGS. 3A to 3C , operations of the pump  30  and supplementary feed valve  19  will be described under each state (on or off) detected by the first liquid surface detector  42  of the first tank  4  and the second liquid surface detector  43  of the second tank  5 . 
     In transition tables shown in  FIGS. 3A , to  3 C, an on-state of the liquid surface detector  42  indicates a state in which the ink liquid surface in the first tank  4  has reached a desired level (height) (i.e., a desired amount of ink is stored in the first tank  4 ) and the first liquid surface detector  42  outputs on. Similarly, an on-state of the liquid surface detector  43  indicates a state in which the ink liquid surface in the second tank  5  has reached a desired level (height) (i.e., a desired amount of ink is stored in the second tank  5 ) and the second liquid surface detector  43  outputs on. 
     The off-state of the first liquid surface detector  42  indicates a state in which the ink liquid surface in the first tank  4  has not vet reached the desired level (i.e., the desired amount of ink is not stored in the first tank  4 ) and the first liquid surface detector  42  outputs off. Similarly, an off-state of the liquid surface detector  43  indicates a state in which the ink liquid surface in the second tank  5  has not yet reached the desired level (i.e., the desired amount of ink is stored in the second tank  5 ) and the second liquid surface detector  43  outputs off. 
     In descriptions below, a state of outputting on from the first liquid surface detector  42  or the second liquid surface detector  43  will be simply referred to as “on”, and a state of outputting off will be simply referred to as “off”. 
     Further, an off-state of the pump  30  indicates a state in which the pump  30  stops, and an on-state of the pump  30  indicates a state in which the pump  30  drives. Further, an off-state of the supplementary feed valve  19  indicates a state in which the supplementary feed valve  19  is closed and no ink is supplied into the first tank  4 . Inversely, an on-state of the supplementary feed valve  19  indicates a state in which the supplementary feed valve  19  is opened and an ink is supplied from the ink cartridge  14  into the first tank  4 . 
       FIG. 3A  is a transition table showing a first mode of operations of the pump  30  and supplementary feed valve  19  during ink circulation. In the first mode, the pump  30  and supplementary feed valve  19  are controlled to both be off regardless of whether the second liquid surface detector  43  is on (first state  61 ) or off (second state  62 ), when the first liquid surface detector  42  is on. That is, when the first liquid surface detector  42  detects the first tank  4  as containing sufficient ink, any ink is neither fed upward to the first tank  4  from the second tank  5  nor supplied from the ink cartridge  14  to the first tank  4  regardless of the amount of the ink in the second tank  5 . 
     When the first liquid surface detector  42  is off and the second liquid surface detector  43  is on (third state  63 ), the pump  30  and supplementary feed valve  19  are respectively controlled to be on and off. That is, when the first liquid surface detector  42  detects the first tank  4  as containing sufficient ink and the second liquid surface detector  43  also detects the second tank  5  as containing sufficient ink, an ink is fed upward to the first tank  4  from the second tank  5  while no ink is supplied from the ink cartridge  14  to the first tank  4 . 
     Further, when the first liquid surface detector  42  is off and the second liquid surface detector  43  is also off (fourth state  64 ), the pump  30  and supplementary feed valve  19  are respectively controlled to be off and on. That is, when the first liquid surface detector  42  detects the first tank  4  as not containing sufficient ink and the second liquid surface detector  43  also detects the second tank  5  as not containing sufficient ink, an ink is not fed upward to the first tank  4  from the second tank  5  while an ink is supplied from the ink cartridge  14  to the first tank  4 . 
     Accordingly, the pump  30  is on only when the first liquid surface detector  42  is off and the second liquid surface detector  43  is on (third state  63 ). The supplementary feed valve  19  is on only when the first liquid surface detector  42  is off and the second liquid surface detector  43  is also off (fourth state  64 ). 
       FIG. 3B  is a transition table showing a second mode of operations of the pump  30  and supplementary feed valve  19  during ink circulation. The second mode differs from  FIG. 3A  when the liquid surface detector  42  is on and the second liquid surface detector  43  is also on. That is, in  FIG. 3B , when the first liquid surface detector  42  is on and the second liquid surface detector  43  is also on, control is performed so as to transit to a fifth state  65  in which the supplementary feed valve  19  is off and the pump  30  is maintained to drive or stop (state maintenance control). 
     “State maintenance” means that an operation of the pump  30  before the transition to the fifth state  65  is continued. For example, when the pump  30  is on and the liquid surface detectors are both on, the supplementary feed valve  19  is off as shown in  FIG. 3A . However, the pump  30  is continuously controlled to be kept on. When the pump  30  is off and the liquid surface detectors are both on, the supplementary feed valve  19  and pump  30  are controlled to be off and continuously off, respectively. That is, the second mode implies that the pump  30  is stopped only when the second liquid surface detector  43  is off. 
       FIG. 3C  is a transition table showing a third mode of operations of the pump  30  and supplementary feed valve  19  during ink circulation. The third mode differs from  FIG. 3A  when the first liquid surface detector  42  is off and the second liquid surface detector  43  is also off. That is, in  FIG. 3C , when the first liquid surface detector  42  is off and the second liquid surface detector  43  is also off, control is performed so as to transit to a sixth state  66  in which the supplementary feed valve  19  is on and pump  30  is maintained to drive or stop (state maintenance control). 
     For example, when the operation of the pump  30  is on and the liquid surface detectors are both off, the supplementary feed valve  19  is on as in  FIG. 3A  and the pump  30  is controlled to be continuously kept on. When the pump  30  is off and the liquid surface detectors are both off, the supplementary feed valve  19  is also on while the pump  30  is controlled to be continuously kept off. That is, the third mode implies that the pump  30  is stopped only when the first liquid surface detector  42  is on. 
     In the present embodiment, the pump  30  and supplementary feed valve  19  are operated in any of the first to third modes as described above. 
     Next, the ink circulation operation of the ink path  1  according to the present embodiment will be described with reference to  FIGS. 4A to 4F . 
       FIG. 4A , is a flowchart showing the whole ink circulation operation. 
     In the present embodiment, in an initial state (standby state) in which no ink is circulated, the supplementary feed valve  19  and the air release valve  26  for the first tank  4  are closed while the air release valve  28  for the second tank  5  is opened. In addition, the pump  30  stops. 
     At first, to start an ink circulation operation, six parameters a to f for the storage unit  70  are set a=0, b=0, c=0, d=0, and f=0 (step S 1 ). Next, the air release valve  28  for the second tank  5  is closed (step S 2 ). Next, the bellows elevation mechanism  47  of the pressure adjuster  11  is operated to move down the bellows unit  45  by the weight of the weight unit  46  (step S 3 ). Further, the air release valve  26  of the first tank  4  is opened (step S 4 ). Thus, a pressured state is created in the first tank  4 , and a negative state is created in the second tank  5 . From step S 5 , the ink is circulated through the ink circulation path until the ink circulation is determined to have ended, in step S 9 . 
     After step S 4 , the processing transits to a subroutine to turn sensors on/off for charging an ink (step S 5 ).  FIG. 4B  is a flowchart showing the subroutine to turn sensors on/off for charging an ink (steps S 11  to S 16 ). 
     At first, whether or not the first liquid surface detector  42  (hereinafter referred to as first sensor  42 ) which detects an ink liquid surface in the first tank  4  is off and the second liquid surface detector  43  which detects an ink liquid surface in the second tank  5  (hereinafter referred to as a first sensor  43 ) is on is determined (step S 11 ). If not (NO) in step S 11 , the pump  30  is stopped (kept stopped) as in the first, second, and fourth to sixth states  61 ,  62 , and  64  to  66  in  FIGS. 3A to 3C  (step S 12 ). If so (Yes) in step S 11 , the pump  30  is driven (maintains driving) as in the third state  63  in  FIGS. 3A to 3C  (step S 13 ). Thus, when only the ink in the first tank  4  is determined to run short of the ink, the ink in the second tank  5  is fed upward to the first tank  4  by the pump  30 . 
     Next, whether or not the first sensor  42  and the second sensor  43  are both off is determined (step S 14 ). If not (NO) in step S 14 , the supplementary valve  19  is closed (maintains closing) as shown in the first to third and fifth states  61  to  63  and in  FIGS. 3A to 3C  (step S 15 ). If so (Yes) in step S 14 , the supplementary valve  19  is opened (maintains opening) as shown in the fourth or six state  64  or  66  in  FIGS. 3A to 3C  (step S 16 ). Thus, when both of the first and second tanks  4  and  5  are determined to run short of the ink, the ink is supplied from the ink cartridge  14  through the tube  18  to the first tank  4 . 
     Referring back to  FIG. 4A , following step S 5 , the processing transits to a subroutine of sensor on/off (step S 6 ) for detecting an operation of the second sensor  43 .  FIG. 4C  is a flowchart showing a subroutine of sensor on/off (steps S 17  to S 24 ) for detecting an operation of the second sensor  43 . 
     At first, whether or not the second sensor  43  is off is determined (step S 17 ). If not off, i.e., if on (NO), “a” is incremented a=a+1) (step S 18 ) and “b” is reset (b=0) (step S 19 ). That is, in repetition of steps S 5  to S 9 , if the processing sequentially goes to NO in step S 17 , “a” is incremented in step S 18  and accumulatively stored in the storage unit  70 . Then, whether N&lt;a or not is determined (step S 20 ). In this step, N is an arbitrarily set value. For example, where a looping duration from step S 5  to S 9  is supposed to be 100 milliseconds, N=30 is set if abnormality is determined to be occurring when an output waveform stays unchanged continuously for three seconds. 
     More specifically, when steps S 5  to S 9  are repeated, an ink which is not ejected from the nozzles of the ink heads  2  usually flows into the second tank  5  in a downstream side, or an intermittent operation of the pump  30  causes the first tank  4  to be drawn upward to the first tank  4  from the second tank  5 . Therefore, the height of the liquid surface in the second tank  5  is supposed to change, and an output waveform of the second sensor  43  is also supposed to repeatedly turn on and off. Accordingly, if the output waveform of the second sensor  43  stays unchanged for the number of times N which is set by the second sensor  43  or, in other words, for a predetermined time (three seconds according to the setting described above) according to setting, an abnormality may have occurred in the second sensor  43  or may have hindered proper ink circulation operation by the abnormality. Therefore, an abnormality (operation error) is determined to be occurring. 
     Unless N&lt;a (NO) in step S 20 , this subroutine is terminated and the processing goes to step S 7 . If N&lt;a (YES), the second sensor  43  is kept on and stays unchanged for a predetermined time. Therefore, an abnormality is determined to be occurring, and the processing goes to a subroutine for a standby processing (see  FIG. 4F ) (step S 10 ). Further, a service call is made to notify a user or service person of the abnormality (step S 21 ). 
     The service call may be a unit for visually or aurally informing a user or service person by displaying a position where the abnormality occurs in a display not shown or by outputting an audible alert or a sound. Of course, the unit described above may be combined together. Thus, if an abnormality is determined to be occurring, the processing transits to the standby processing. Thereafter, the circulation operation can be stopped early by notifying the user of the abnormality occurring. 
     If the second sensor  43  is off in step S 17  (YES), “b” is incremented (b=b+1) (step S 22 ), and “a” reset (a=0) (step S 23 ). That is, in repetition of steps S 5  to S 9 , if the processing sequentially goes to YES in step S 17 , “b” is incremented in step S 22  and accumulatively stored in the storage unit  70 . Then, whether N&lt;b or not is determined (step S 24 ). 
     Unless N&lt;b (NO) in step S 24 , this subroutine is terminated and the processing goes to step S 7 . If N&lt;b (YES), the second sensor  43  is kept off and stays unchanged for a predetermined time (three seconds according to the setting described above). Therefore, an abnormality is determined to be occurring, and the processing goes to the subroutine described later for the standby processing (step S 10 ). Further, a service call is made (step S 21 ). 
     Referring back to  FIG. 4A , following step S 6 , the processing goes to a subroutine of pump stopping/driving (step S 7 ) for detecting an operation of the pump  30 .  FIG. 4D  is a flowchart showing a subroutine of pump stopping/driving (steps S 25  to S 31 ) for detecting an operation of the pump  30 . 
     At first, whether or not the pump  30  stops is determined (step S 25 ). If not stopped, i.e., if driving (NO), “c” is incremented (c=c+1) (step S 26 ) and d is reset (d=0) (step S 27 ). That is, in repetition of steps S 5  to S 9 , if the processing sequentially goes to NO in step S 25 , “c” is incremented in step S 26  and is accumulatively stored in the storage unit  70 . Then, whether N&lt;c or not is determined (step S 28 ). 
     As described above, if the ink circulation operates properly, the pump  30  operates intermittently. Unless N&lt;c (NO) in step S 28 , this subroutine is terminated and the processing goes to step S 8 . If N&lt;c (YES), an abnormality is determined to be occurring, and the processing goes to the subroutine for the standby processing described later (step S 10 ). Further, a service call is made (step S 21 ). 
     If the pump  30  is stopped in step S 25  (YES), “d” is incremented (d=d+1) (step S 29 ), and “c” is reset (c=0) (step S 30 ). That is, in repetition of steps S 5  to S 9 , if the processing sequentially goes to YES in step S 25 , “d” is incremented in step S 29  and accumulatively stored in the storage unit  70 . Then, whether N&lt;d or not is determined (step S 31 ). 
     Unless N&lt;d (NO) in step S 31 , this subroutine is terminated and the processing goes to step S 9 . If N&lt;d (YES), an abnormality is determined to be occurring, and the processing goes to the subroutine for the standby processing described later (step S 10 ). Further, a service call is made (step S 21 ). 
     Referring back to  FIG. 4A , following step S 7 , the processing transits to a subroutine of sensor on/off (step S 8 ) for detecting an operation of the first sensor  42 .  FIG. 4E  is a flowchart showing the subroutine of sensor on/off (steps S 32  to S 44 ) for detecting an operation of the first sensor  42 . 
     At first, whether or not the first sensor  42  is off is determined (step S 32 ). If not off, i.e., if on (NO), “e” is incremented (e=e+1) (step S 33 ), and “f” is reset (f=0) (step S 34 ). That is, in repetition of steps S 5  to S 9 , if the processing sequentially goes to NO in step S 32 , “c” is incremented in step S 33  and is accumulatively stored in the storage unit  70 . Then, whether e&gt;T5 or not is determined (step S 35 ). “T5” is a value which is set based on the same concept for N as described above. 
     Unless e&gt;T5 (NO) in step S 35 , this subroutine is terminated and the processing goes to step S 9 . If e&gt;T5 (YES), the first sensor  42  is kept on and stays unchanged for a predetermined time. Therefore, the processing goes to the subroutine for the standby processing described later (step S 10 ). Further, a service call is made (step S 21 ). 
     If the first sensor  42  is off in step S 32  (YES), “f” is incremented (f=f+1) (step S 36 ), and “e” is reset (a=0) (step S 37 ). That is, in repetition of steps S 5  to S 9 , if the processing sequentially goes to YES in step S 32 , “f” is incremented in step S 36  and is accumulatively stored in the storage unit  70 . Then, whether f&lt;T6 or not is determined (step S 38 ). This “T6” is a value which is also set based on the same concept for N as described above. 
     Unless f&lt;T6 (NO) in step S 38 , this subroutine is terminated and the processing goes to step S 9 . If f&gt;T6 (YES), the first sensor  42  is considered to be continuously kept off in the repetition of steps S 5  to S 9 . With respect to it, two reasons can be considered, i.e., an abnormality occurs in the first sensor  42  itself or sufficient ink does not remain (empty) in the ink cartridge  14  as an ink supply and the first tank  4  is therefore not supplied with an ink while the first sensor  42  operates properly. Therefore, if f&gt;T6 (YES), the processing goes to step S 39  and later steps to specify which of the two reasons described above causes the first sensor  42  to be off for a predetermined time. 
     At first, the pump  30  is stopped (kept stopped) (step S 39 ). Next, the supplementary feed valve  19  is closed (kept closed), and the ink supply operation is stopped (step S 40 ). Further, whether the second sensor  43  is on or not is determined (step S 41 ). If on (YES), the processing goes to step S 43 . If not on (NO), on is waited for a predetermined time (step S 42 ). Since the pump  30  is stopped in step S 39 , an ink flows into the second tank  5  through the ink heads  2  from the first tank  4  after waiting for a while in step S 42  and the second sensor  43  is off, even if the second sensor  43  is not on in step S 41 . The second sensor  43  is confirmed to be on in steps S 41  and S 42 , in order to secure that the second tank  5  still contains an ink a predetermined amount or more. 
     If on (YES) step S 41  or after step S 42 , the pump  30  is driven again (step S 43 ). Then, the ink in the second tank  5  is fed upward to the tank  4 . Further, whither or not the first sensor  42  is turned on within a time T 1  is determined (step S 44 ). This time T 1  is set to a time when the first sensor  42  is always turned on if only the pump  30  is continuously driven for time T 1 . 
     If the first sensor  42  is turned on within time T 1  (YES) in step S 44 , the first sensor  42  is determined to turn on although the first sensor  42  has been continuously determined to be off in step S 32  in the repetition of steps S 5  to S 9 . Therefore, on/off operations of the first sensor  42  are excellent. Thus, a reason for the first sensor  42  continuously determined to be off attributes to an insufficient ink remaining in the ink cartridge  14 . 
     Further, the processing goes to the standby processing described later (step S 10 ) and notifies that the ink cartridge  14  is empty (i.e., only insufficient ink remains in the ink cartridge  14 ) (step S 45 ). This notification can be performed by displaying on a display, generating an alert, and a combination thereof, like a service call (step S 21 ) described above. 
     Unless the first sensor  42  turns on within time T 1  in step S 44  (NO), the first sensor  42  is determined to be occurring in the first sensor  42  itself, and the processing goes to the standby processing described later (step S 10 ). Further, a service call is made (step S 21 ). 
     Referring back to  FIG. 4A , following step S 8 , whether or not the ink circulation operation is terminated is determined (step S 9 ). If the ink circulation operation is continued (NO), the processing returns to step S 5  and repeats steps S 5  to S 9 . If the ink circulation operation is to be terminated (YES), the processing goes to the standby processing (step S 10 ). 
       FIG. 4F  is a flowchart showing the subroutine (steps S 46  to S 49 ) of the standby processing. 
     At first, the supplementary feed valve  19  is closed (kept closed) (step S 46 ). Further, the pump  30  is stopped (kept stopped) (step S 47 ). Next, the air release valve  28  for the second tank  5  is opened (step S 48 ), and the air release valve  26  for the first tank  4  is closed (step S 49 ). Thus, a standby state is created (the nozzles of the ink heads  2  are applied with a slight negative pressure). 
     When step S 10  is terminated, the ink circulation operation then ends. 
     As has been described above, according to the present embodiment, an on/off output from the second liquid surface detector  43  is detected, i.e., whether or not the ink liquid surface in the second tank  5  is lower than a predetermined level is detected. Detected information thereof is accumulatively stored in the storage unit  70 . Further, the determination unit  80  determines whether or not an abnormality occurs in the second liquid surface detector  43 . Whether or not the on/off output of the first liquid surface detector for the first tank  4  is lower than a predetermined level, detected information thereof is accumulatively stored into the storage unit  70 . Further, the determination unit  80  determines whether or riot an abnormality is occurring in the first liquid surface detector  42 , in accordance with the accumulatively stored information. Thus, proper operations of the liquid surface detectors can be confirmed by monitoring liquid surface detectors which detect ink liquid surfaces in two ink tanks having a difference in height between each other, with use of a principle of a so-called watch dog timer. Accordingly, a proper ink circulation operation can be maintained. 
     Also in the present embodiment, an abnormality is determined to be occurring if a detection signal from the second liquid surface detector  43  is continuously kept in a state (on or off) for a predetermined time during the ink circulation operation, i.e., if the ink liquid surface in the second tank  5  continues to be not lower or to be lower than a predetermined level for a predetermined time. Further, if a detection signal from the first liquid surface detector  42  continues to be on for a predetermined time during the ink circulation operation, an abnormality is determined to be occurring. Alternatively, if the detection signal continues to be off for the predetermined time, occurrence of an abnormality is not immediately notified but the determination unit  80  determines whether the abnormality is caused by a liquid surface detector or shortage of ink in an ink cartridge. Therefore, whichever liquid surface detector causes an abnormality, occurrence of the abnormality can be detected accurately. Further, a factor which causes the abnormality can be specified, and a user or service person can easily cope with the abnormality during a maintenance service. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.