Patent Publication Number: US-8523339-B2

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C §119 to Japanese Patent Application No. 2010-271174 filed Dec. 6, 2010, the entire contents of which are hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus having a recording head and a head tank, the recording head discharging liquid droplets, the head tank supplying a fluid to the recording head. 
     2. Description of the Related Art 
     As an image forming apparatus such as a printer, a facsimile machine, a copier, a plotter, a multi function peripheral thereof and the like, there has been known an inkjet recording apparatus and the like employing a liquid discharging recording method using a recording head discharging ink droplets or the like. 
     In the image forming apparatus employing the liquid discharging recording method, an image is formed by discharging ink droplets from a recording head onto a fed sheet. Herein, the term “forming” is a synonym of the terms recording, typing, imaging, and printing. 
     Further, herein, the term “sheet” is not limited to paper, and refers to any appropriate medium (e.g., the OHP) to which ink droplets, other liquid and the like may be adhered. The image forming apparatus employing the liquid discharging recording method may be classified into two types: a serial-type image forming apparatus and a line-type image forming apparatus. 
     In the serial-type image forming apparatus, an image is formed by discharging ink droplets from the recording head while the recording head moves in the main scanning direction. On the other hand, in the line-type image forming apparatus, an image is formed by discharging ink droplets from the line-type recording head while the recording head does not change its position. 
     Further, in an embodiment of the present invention, the term “image forming apparatus” employing the liquid discharging recording method refers to an apparatus forming an image by discharging a liquid onto a medium including paper, thread, fiber, textile, leather, metal, plastic, glass, wood, ceramics and the like. 
     Further, the term “image forming” refers to not only forming a meaningful image such as characters, figures, and the like on a medium but also forming a meaningless image such as a pattern and the like on a medium (including simply discharging droplets onto a medium). 
     Further, the term “ink” is collectively used and herein refers to not only any material called “ink” but also any liquid for forming an image which may be called recording liquid, fixing processing liquid, liquid, a DNA sample, a patterning material, a resin and the like. 
     Further, the “image” is not limited to a planar image. For example, the “image” includes an image formed on a material that is three-dimensionally formed and an image three-dimensionally formed made of three-dimensional figures. 
     Among such image forming apparatuses there is a known image forming apparatus employing an ink supply method in which ink is supplied from a main tank to a head tank, the main tank (a.k.a. an ink cartridge) being detachably mounted on a main body of the apparatus, the head tank (also called a sub tank or a buffer tank) supplying ink to the recording head. 
     When the ink cartridge is in an ink-end condition (an end state) (i.e., an ink-empty condition (an empty state)), including an ink-near-end condition (i.e., an ink-near-end condition), exchange of the ink cartridge may be suggested. However, when there is no stock of the ink cartridge, the ink cartridge may not be exchanged immediately. As a result, printing may have to be interrupted. 
     To overcome the inconvenience, there is a know technique in which emergency printing using ink remaining in the head tank is performed to continue printing (see, Japanese Laid-open Patent Publication No. 2011-183729). 
     To that end, in a case where ink is supplied from the main tank to the head tank, when the pressure in the head tank is less than a predetermined pressure, an air opening mechanism of the head tank is opened to increase the pressure in the head tank. Then, the liquid is pumped from the head tank to the main tank by a reversible-type fluid supply unit to control reducing the negative pressure in the head tank. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, an image forming apparatus includes a recording head discharging liquid droplets; a head tank including a liquid container containing liquid to be supplied to the recording head and an air opening mechanism opening and closing the liquid container to air; a main tank containing liquid to be supplied to the head tank; a reversible-type fluid supply unit disposed between the head tank and the main tank and supplying liquid; a pressure detector detecting a pressure in the head tank; a first control unit, when the pressure in the head tank is not restored to a predetermined pressure while liquid is supplied from the main tank to the head tank, determining that the state of the main tank is an empty state, causing the reversible-type fluid supply unit to reversely rotate to reversely supply a predetermined amount of liquid from the head tank to the main tank, and stopping printing; and a second control unit, when the printing is stopped by the first control unit and emergency printing is instructed using liquid remaining in the image forming apparatus, causing the air opening mechanism of the head tank to open, causing the reversible-type fluid supply unit to normally rotate to the supply liquid to the head tank, the liquid having been reversely supplied to the main tank, causing the air opening mechanism to close, and causing the reversible-type fluid supply unit to reversely rotate to reversely supply the liquid from the head tank to the main tank to generate a predetermined negative pressure in the head tank. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features, and advantages of the present invention will become more apparent from the following description when read in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic side view of a mechanical part of an image forming apparatus according to an embodiment of the present invention; 
         FIG. 2  is a top view of a main part of the mechanical part; 
         FIG. 3  is a schematic top view illustrating an example of a head tank; 
         FIG. 4  is a schematic front view of the head tank of  FIG. 3 ; 
         FIG. 5  is a schematic view illustrating an ink supply exhaust system; 
         FIG. 6  is a view of a main part of the carriage and a full-tank detection sensor illustrating a pressure detection of the head tank; 
         FIGS. 7A and 7B  are schematic views illustrating the pressure detection of the head tank; 
         FIGS. 8A and 8B  are schematic views illustrating an ink supply operation to the head tank and end detection of the ink cartridge; 
         FIG. 9  is a schematic block diagram illustrating a control section; 
         FIG. 10  is a flowchart illustrating the control according to a first embodiment of the present invention; 
         FIG. 11  is a flowchart illustrating the emergency printing in  FIG. 10 ; 
         FIG. 12  is a flowchart illustrating the control according to a second embodiment of the present invention; 
         FIG. 13  is a flowchart illustrating the emergency printing in  FIG. 12 ; 
         FIG. 14  is a flowchart illustrating the emergency printing in the control according to a third embodiment of the present invention; 
         FIG. 15  is a flowchart illustrating the control according to a fourth embodiment of the present invention; 
         FIG. 16  is a flowchart illustrating the control according to a fifth embodiment of the present invention; 
         FIG. 17  is a schematic view of an ink supply system according to a sixth embodiment of the present invention; 
         FIG. 18  is a flowchart illustrating the control according to the sixth embodiment of the present invention; and 
         FIG. 19  a flowchart illustrating the control according to a seventh embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In an image forming apparatus in the related art, when an emergency printing using ink remaining in the head tank can be performed, it may be desired to continue printing as long as possible by reducing ink to be wastefully consumed. 
     The present invention is made in light of the above requirement, and may provide an image forming apparatus that reduces ink to be wastefully consumed in the emergency printing. 
     In the following, embodiments of the present invention are described with reference to the accompanying drawings. First, one example of an image forming apparatus according to an embodiment of the present invention is described with reference to  FIGS. 1 and 2 .  FIG. 1  is a side view illustrating the entire configuration of the image forming apparatus.  FIG. 2  is a top view of a main part of the apparatus of  FIG. 1 . 
     The image forming apparatus is a serial-type image forming apparatus. Namely, as illustrated in  FIG. 2 , a carriage  33  is slidably supported in the main scanning direction by a main guide rod  31  and a sub guide rod  32  which are guide members bridged between side plates  21 A and  21 B on the left and right sides, respectively, of an apparatus main body  1 . As a result, the carriage  33  can move and scan in the carriage main scanning direction driven by a main-scanning motor and a timing belt (transfer belt) described below. 
     The carriage  33  includes recording heads  34   a  and  34   b  (which may be collectively referred to as “recording head  34 ”) discharging ink droplets of yellow (Y), cyan (C), magenta (M), and black (K) colors and having a nozzle line that includes plural nozzles and that is arranged in the sub scanning direction orthogonal to the main scanning direction in a manner such that the ink discharging direction is directed downward. 
     The recording heads  34  have two nozzle lines, so that one nozzle line of the recording head  34   a  discharges black (K) liquid droplets and the other nozzle line of the recording head  34   a  discharges cyan (C) liquid droplets, and one nozzle line of the recording head  34   b  discharges magenta (M) liquid droplets and the other nozzle line of the recording head  34   b  discharges yellow (Y) liquid droplets. 
     Further, in the carriage  33 , head tanks  35   a  and  35   b  (which may be collectively referred to as a “head tank  35 ”) are mounted for supplying color inks corresponding to the nozzle lines of the recording heads  34 . Color recording liquids are supplied from ink cartridges  10   y ,  10   m ,  10   c ,  10   k  (which may be collectively referred to as an “ink cartridge  10 ”) to the respective head tanks  35  by a supply unit  24  via respective ink supply tubes  36 . The ink cartridges  10  which are main tanks detachably mounted on a cartridge mounting section  4 . 
     On the other hand, as a sheet feeding section for feeding a sheet  42  piled on a sheet piling section (platen plate)  41  of a sheet feeding tray  2 , there are a half moon roller (feed roller)  43  and a separation pad  44 . The half moon roller  43  separates and feeds the sheets  42  from the sheet piling section  41  one by one. The separation pad  44  faces the half moon roller  43  and is made of a material having a high friction coefficient. Further, the separation pad  44  is biased toward the half moon roller  43  side. 
     Further, in order to further feed the sheet  42  fed from the sheet feeding section to the lower side of the recording head  34 , there are provided a guide member  45  guiding the sheet  42 , a counter roller  46 , a feed guide member  47 , a pressing member  48  having a head pressing roller  49 , and a transfer belt  51  which is a transmission unit that electrostatically attracts and feeds the sheet  42  to the side facing the recording head  34 . 
     The transfer belt  51  is an endless belt bridged between a feed roller  52  and a tension roller  53 , and rotates in the belt feeding direction (sub scanning direction). Further, there is provided a charging roller  56  serving as a charging unit charging a surface of the transfer belt  51 . The charging roller  56  is in contact with a surface layer of the transfer belt  51 , so that the charging roller  56  rotates in accordance with the rotation of the transfer belt  51 . 
     The transfer belt  51  is rotated and moved in the belt feeding direction by the rotation of the feed roller  52  driven by a sub scanning motor described below via a timing belt. 
     Further, as a sheet discharging section for discharging the sheet  42  printed by the recording head  43 , there are provided a separation pawl  61  to separate the sheet  42  from the transfer belt  51 , a sheet discharging roller  62 , a spur  63  which is another discharging roller, and a discharge tray  3  below the discharging roller  62 . 
     Further, a two-sided unit  71  is detachably mounted on the rear part of the apparatus main body  1 . The two-sided unit  71  takes in the sheet  42  returned by the rotation in the reverse direction of the transfer belt  51 , reverses the sheet  42 , and feeds the sheet  42  between the counter roller  46  and the transfer belt  51  again. Further, there is a manual tray  72  on the upper surface of the two-sided unit  71 . 
     Further, in one non-printing area on one side of the scanning direction of the carriage  33 , there is a maintenance and recovery mechanism  81  to maintain and recover the status of the nozzles on the recording head  34 . The maintenance and recovery mechanism  81  includes cap members (hereinafter a “cap” or a “suction cap”)  82   a  and  82   b  (which may be collectively referred to as “cap  82 ”) to cap the nozzle surfaces of the recording head  34 , a wiper member (wiper blade)  83  to wipe the nozzle surfaces, a preliminary discharge tray  84 , and a carriage lock  87  to lock the carriage  33 . 
     The preliminary discharge tray  84  receives droplets upon a preliminary discharge that preliminarily discharges thicker recording fluid and that makes no contribution to printing. Further, under the maintenance and recovery mechanism  81 , a waste fluid tank  100  is exchangeably provided to the apparatus main body  1  to contain waste fluid produced by the maintenance and recovery operation. 
     Further, in another non-printing area on the other side of the scanning direction of the carriage  33 , there is provided a preliminary discharge tray  88  to receive droplets upon the preliminary discharge that preliminarily discharges recording fluid having been made thicker during printing or the like and that makes no contribution to printing. The preliminary discharge tray  88  includes an opening  89  which is open along the nozzle line direction of the recording head  34 . 
     In an image forming apparatus having the configuration described above, the sheets  42  are separated and fed from the feeding tray  2  one by one. Then, the sheet  42  fed substantially in the vertical upward direction is guided by the guide member  45 , and further fed between the transfer belt  51  and the counter roller  46 . Then, the header of the sheet  42  is further guided by a transfer guide  37  and pressed toward the transfer belt  51  by the head pressing roller  49  so that the transfer direction of the sheet  42  is changed by approximately 90 degrees. 
     Then, an alternating voltage alternately repeating a plus output and a minus output is applied to the charging roller  56 . As a result the transfer belt  51  is alternately charged to have an alternating band voltage pattern in the sub scanning direction (i.e., the rotating direction) in a manner such that plus charges and minus charges are alternately charged and have a predetermined width. 
     When the sheet  42  is fed on the transfer belt  51  where plus charges and minus charges are alternately charged, the sheet  42  is attracted to the transfer belt  51  and fed in the sub scanning direction by the rotating movement of the transfer belt  51 . 
     Then, by moving the carriage  33  and driving the recording head  34  in accordance with an image signal, ink droplets are discharged onto the halted sheet  42 , so as to record one line of data. Then, the sheet  42  is fed by a predetermined distance and the data of the next line is recorded. 
     Upon receipt of a record end signal or a signal indicating the arrival of the tail of the sheet  42  at the recording area, the recording operation ends and the sheet  42  is discharged to the discharge tray  3 . 
     On the other hand, to maintain and recover the nozzles of the recording head  34 , the carriage  33  is moved to a home position so as to face the maintenance and recovery mechanism  81  and the capping by the cap member  82  is performed. Then, the maintenance and recovery operations such as the nozzle suction operation to suction through the nozzles and the preliminary discharge operation discharging liquid droplets making no contribution to image forming are performed. By doing in this way, it may become possible to stably form an image by discharging liquid droplets. 
     Next, an example of the head tank  35  is described with reference to  FIGS. 3 and 4 .  FIGS. 3 and 4  are schematic top and front views, respectively, of the head tank  35  corresponding to one nozzle line. 
     The head tank  35  includes a tank case  201  forming (defining) (a part of) an ink container storing ink and having an opening on one side of the ink container. The opening of the tank case  201  is sealed by a flexible film  203 . Further, a spring  204  as an elastic member is disposed in the tank case  201  so as to always bias the flexible film  203  toward outside. Due to the biasing force applied to the flexible film  203  of the tank case  201  toward outside by the spring  204 , a negative pressure may be generated (increased) in response to a decrease of a remaining ink amount in the tank case  201 . 
     Further, outside of the tank case  201 , there is a displacement member  205  (hereinafter may be referred to as a “full-tank detection filler”) that has one end side swingably supported by a supporting axle  202  and that is biased toward the tank case  201  side. The displacement member  205  is fixed onto the flexible film  203  with glue or the like, so that the displacement member  205  is displaced in accordance with the movement of the flexible film  203 . By detecting the displacement member  205  using a full-tank detection sensor  301 , it may become possible to detect the remaining ink amount in the head tank  35  and the like. 
     Further, on the upper side of the tank case  201 , a supply opening  209  to supply ink from the ink cartridge  10  is formed and connected to the ink supply tube  36 . Further, on one side part of the tank case  201 , an air opening mechanism  207  to open the inside of the head tank  35  to the atmosphere is formed. The air opening mechanism  207  includes a valve body  207   b  to open and close an air opening path  207   a  in communication with the inside of the head tank  35  and a spring  207   c  biasing the valve body  207   b  so as to close the air opening path  207   a . The air opening path  207   a  is open when an air opening solenoid  302  is used to press the valve body  207   b , so that the inside of the head tank  35  is in communication with the atmosphere (air opened state). 
     Further, there are provided electrode pins  208   a  and  208   b  to detect an ink fluid surface height in the head tank  35 . Ink is electrically conductive. Therefore, when ink is in contact with the electrode pins  208   a  and  208   b , a current flows between the electrode pins  208   a  and  208   b , thereby changing the resistance value between the electrode pins  208   a  and  208   b . By using the characteristics, it may become possible to detect where the ink fluid surface height is equal to or lower than a predetermined height. Namely, it may become possible to determine whether an amount of air in the head tank  35  is equal to or greater than a predetermined amount. 
     Next, a negative pressure generating operation in the image forming apparatus is described with reference to  FIG. 5  schematically illustrating the ink supply system of the image forming apparatus. First, there is a fluid supply pump  241  between the ink cartridge  10  and the head tank  35 . The fluid supply pump  241  herein is a reversible-type pump (i.e., a pump capable of supplying and suctioning fluid) such as a tube pump. 
     While the air opening mechanism  207  of the head tank  35  is closed, the fluid supply pump  241  is reversely driven to supply ink from the head tank  35  to the ink cartridge  10  (hereinafter this operation may be called a “ink reverse supply” or “ink suction”). By doing this, due to the biasing force applied to the flexible film  203  toward outside by the spring (elastic member)  204 , the negative pressure may be generated in the head tank  35 . 
     Further, while the air opening mechanism  207  of the head tank  35  is closed, the nozzle surface of the recording head  34  is capped (sealed) by the cap members  82   a  of the maintenance and recovery mechanism  81  and a suction pump  812  of the maintenance and recovery mechanism  81  is driven. 
     Then, it may become possible to suction ink in the head tank  35  by suctioning ink from the nozzle of the recording head  34  via a suction tube  812 . By doing this, due to the biasing force applied to the flexible film  203  toward outside by the elastic member  204 , the negative pressure may be generated in the head tank  35 . The suctioned waste ink is exhausted to a waste fluid tank  813 . 
     Further, while the air opening mechanism  207  of the head tank  35  is closed, liquid droplets not contributing to image forming are discharged from the recording head  34  to the preliminary discharge tray  84  so as to consume ink in the head tank  35 . Similar to the above, by ding this, due to the biasing force applied to the flexible film  203  toward outside by the elastic member  204 , the negative pressure may be generated in the head tank  35 . 
     Next, the detection of the negative pressure (pressure) in the head tank  35  is described with reference to  FIG. 6  and  FIGS. 7A and 7B . 
     As illustrated in  FIG. 6 , on the apparatus main body  1  side, the full-tank detection sensor  301  as a transmission-type optical sensor is disposed at the position where a distal end  205   a  of the displacement member  205  (full-tank detection filler) of the head tank  35  passes when the carriage  33  is moved in the main scanning direction. Herein, the position of the carriage  33  in the main scanning direction is detected by reading an encoder scale  332  arranged along the carriage main scanning direction using an encoder sensor  331  ( FIG. 6 ). 
     Therefore, as illustrated in  FIG. 7A , when the negative pressure (pressure) value in the head tank  35  is normal, by moving the carriage  33  in the arrow direction from a predetermined position illustrated in solid lines so that the head tank  35  is moved by a distance L 1 , the full-tank detection sensor  301  may detect the distal end  205   a  of the displacement member  205 . 
     On the other hand, as illustrated in  FIG. 7A , when the negative pressure (pressure) value in the head tank  35  is not normal (i.e., abnormal) because the negative pressure in the head tank  35  is lower (i.e., the pressure in the head tank  35  is higher), the displacement member  205  that should be more shifted to the head tank  35  side is more separated from the head tank  35  (i.e., the displacement member  205  is more biased outward by the restoring force of the elastic member  20 ). 
     Because of this feature, when the carriage is similarly moved in the arrow direction from the predetermined position illustrated in the solid lines, the full-tank detection sensor  301  may detect the distal end  205   a  of the displacement member  205  when the head tank  35  is moved by a distance L 2  which is less than the distance L 1 , the distance when the negative pressure in the head tank  35  is normal. 
     Similarly, when the negative pressure (pressure) value in the head tank  35  is not normal (i.e., abnormal) because the negative pressure in the head tank  35  is higher (i.e., the pressure in the head tank  35  is lower), the displacement member  205  that should be more separated from the head tank  35  side is more shifted to the head tank  35  (i.e., the displacement member  205  is pressed inward against the restoring force of the elastic member  20 ). 
     Because of this feature, when the carriage is similarly moved in the arrow direction from the predetermined position (i.e. the position in a normal value) illustrated in the solid lines, the full-tank detection sensor  301  may detect the distal end  205   a  of the displacement member  205  when the head tank  35  is moved by a distance greater than the distance L 1 , the distance when the negative pressure in the head tank  35  is normal. 
     As described above, by detecting the position (moving distance) of the head tank  35  when the distal end  205   a  of the displacement member  205  is detected, it may become possible to detect a displacement amount of the displacement member  205  (the displacement mount corresponding to the displacement amount of the elastic member  204  in the head tank  35 ), the pressure (negative pressure) in the head tank  35 , and the ink remaining amount in the head tank  35  because the displacement member  205  is displaced in accordance with the ink remaining amount. 
     Next, an ink supply operation of supplying ink to the head tank  35  and an end detection of the ink cartridge  10  are described with reference to  FIGS. 8A and 8B . 
     As described above, the displacement amount (open amount) of the displacement member  205  may be prescribed as the relative positional relationship of the carriage  33  relative to the full-tank detection sensor  301 . 
     Namely, as illustrated in  FIG. 8A , by supplying ink to the head tank  35  while the carriage  33  is disposed at a predetermined position relative to the full-tank detection sensor  301 , the displacement member  205  is displaced with the increase of the ink amount in the head tank  35 , and the full-tank detection sensor  301  may detect the displacement member  205 . 
     In this case, it may be determined that the head tank  35  is full (full tank) at a timing when the open amount of the displacement member  205  becomes L 11  by supplying a predetermined amount of ink to the head tank  35 , and then the ink supply operation may stop. 
     Next, ink in the head tank  35  is consumed by discharging ink from the recording head  34 , suctioning ink via the suction cap  82   a  and the like. As illustrated in  FIG. 8B , the open amount of the displacement member  205  may become smaller to L 12 , for example. In this case, it may be determined (detected) that the head tank  35  is full at a timing when a counted value determined based on the counted discharge amount and the counted suction amount via the suction cap  82   a  becomes a predetermined amount. 
     Otherwise, the ink supply operation may be performed by detecting the open amount L 12  of the displacement member  205 . Namely, by the control so that the open amount of the displacement member  205  is in a range between L 11  and L 12 , an appropriate negative pressure for discharging a liquid (ink) may be maintained. 
     As described above, in a normal use condition, the ink (liquid) supply operation is intermittently repeated so that the open amount of the displacement member  205  is in the range between L 11  and L 12 . 
     On the other hand, when the full-tank detection sensor  301  does not detect the displacement member  205  even when the ink supply operation described with reference to  FIG. 8A  is performed for a predetermined time period, it may be determined that there is no ink in the ink cartridge  10 . It may also be possible to determine the cartridge end (i.e., no ink remaining in the cartridge) by calculating the ink remaining amount in the ink cartridge  10  by counting the discharge amount of ink from the recording head  34 . 
     When this method is used, however, an error between the amount based on the counted value and the actual remaining amount may occur. Due to the error, it may be necessary to determine the cartridge end in advance even when it is detected that a predetermined amount of ink still remains. On the other hand, by using the method that the cartridge end is determined when the displacement member  205  is not detected within the predetermined time period, it may become possible to more certainly use ink until the cartridge end of the cartridge  10 , thereby enabling using ink more economically. 
     Next, the outline of the control section  500  of the image forming apparatus is described with reference to  FIG. 9 .  FIG. 9  is a schematic block diagram of the control section  500 . 
     As illustrated in  FIG. 9 , the control section  500  includes a CPU (Central Processing Unit)  501 , a ROM (Read Only Memory)  502 , a RAM (Random Access Memory)  503 , a rewritable non-volatile memory  504 , and an ASIC (Application Specific Integrated Circuit)  505 . The CPU  501  performs control of the entire apparatus. The ROM  502  stores programs including programs and other fixed data, the programs including a program performing control (processes) according to the present invention and executed by the CPU  501 . 
     The RAM  503  temporarily stores image data and the like. The non-volatile memory  504  is provided for storing data even when the power of the apparatus is turned OFF. The ASIC  505  performs various signal processing operations on image data, image processing such as a process of changing the order of data, and processing on input and output signals to control the entire apparatus and the like. 
     The control section  500  further includes a print control section  508 , a head driver (driver IC)  509 , a motor drive section  510 , an AC bias supply section  511 , a solenoid drive section  512 , a pump drive section  516  and the like. The print control section  508  includes a data transfer unit and a drive signal generation unit to drive and control the recording head  34 . 
     The motor drive section  510  drives a head driver (driver IC)  509  to drive the recording head  34  provided on the carriage  33  side, a main scanning motor  554  to move and scan the carriage  33 , a sub scanning motor  555  to rotate and move the transfer belt  51 , and a maintenance and recovery motor  556  of the maintenance and recovery mechanism  81 . 
     The AC bias supply section  511  supplies an AC bias to the charging roller  56 . The solenoid drive section  512  drives the air opening solenoid  302  to open and close the air opening mechanism  207  of the head tank  35 . The pump drive section  516  drives the fluid supply pump  241 . 
     Further, the control section  500  is connected to an operation panel  514  displaying information necessary for the apparatus and accepting the input of the information. 
     The control section  500  further includes a host interface (“I/F”)  506  for transmitting and receiving data and signals from and to a host side, so that the I/F  506  of the control section  500  receives the data and the signals from a host  600  including an image processing apparatus such as a personal computer, an image reading apparatus such as an image scanner, an image acquisition apparatus such as a digital camera and the like via a cable or a network. 
     Further, the CPU  501  of the control section  500  reads and analyzes the print data in a receiving buffer of the I/F  506 . The ASIC  505  performs a process of changing the order of the data and the like, and the image data are transferred from the print control section  508  to the head driver  509 . Further, the dot pattern data for the image output are generated by a printer drive  601  on the host  600  side. 
     The print control section  508  transfers the image data as serial data, and outputs the transfer clock signal, the latch signal, the control signal and the like necessary for the transfer of the image data and the confirmation of the transfer. Further, the print control section  508  includes a D/A converter to perform D/A conversion on the pattern data of driving pulses stored in the ROM  502 , a voltage amplifier, and a drive signal generator including a current amplifier, and outputs a drive signal including one or more drive pulses to the head driver  509 . 
     The head driver  509  drives the recording head  34  by applying a drive pulse of the drive signal to a drive element (e.g., piezoelectric device), the drive signal being given from the print control section  508  based on the image data corresponding to one line of the recording head  34  and being input in series, the drive element generating energy for selectively discharging liquid droplets from the recording head  34 . 
     In this case, by selecting the drive pulse of the drive signal, it becomes possible to select different sizes of dots such as large droplets, middle-sized droplets, and small droplets to be discharged. 
     An I/O section  513  (of the control section  500 ) acquires information from a sensor group  515  mounted in the apparatus and extracts information necessary for controlling the printer, so that the extracted information is used in, for example, the print control section  508 , the motor drive section  510 , and the control of the AC bias supply section  511 . 
     The sensor group  515  includes, for example, an optical sensor to detect the position of the sheet, a thermistor to monitor the temperature and the humidity in the apparatus, a sensor monitoring a voltage of a charged belt, and an interlock switch detecting the opening/closing the cover. The I/O section  513  can process various sensor information. The sensor group  515  includes the full-tank detection sensor  301  detecting the displacement member  205  of the head tank  35 , and the electrode pins  208   a  and  208   b.    
     The CPU  501  of the control section  500  serves as (constitutes) a unit to detect the pressure in the head tank  35  by detecting the displacement member  205  based on the detection signal from the full-tank detection sensor  301 . Further, the CPU  501  constitutes a unit (herein may be referred to as first or third control unit and the like) that controls ink liquid flow between the ink cartridge  10  and the head tank  35 . 
     Next, a first embodiment of the present invention is described. 
     First, as described above, when the ink-end condition (i.e., the ink-empty condition) of the ink cartridge is detected (determined), the exchange of the empty ink cartridge  10  with a new ink cartridge  10  is suggested to the user. 
     In this case, even after the ink cartridge  10  is empty (end), if the fluid supply pump  241  is driven to supply ink from the ink cartridge  10  to the head tank  35  side, the fluid supply path between the fluid supply pump  241  and the ink cartridge  10  may become a vacuum state. Even after the fluid supply pump  241  is stopped, due to the valve mechanism of the fluid supply pump  241 , the vacuum state may be maintained. 
     Under the state, if the ink cartridge  10  is exchanged, air may be introduced into the fluid supply path in the vacuum state right after the ink cartridge  10  is removed. After that, if a new ink cartridge  10  is attached and ink is supplied to the head tank  35 , the introduced air may also be transferred to the head tank  35 . 
     As a result, air in the head tank  35  may be formed as air bubbles, which may cause a malfunction in the detection using the electrode pins  208 . Further, due to the increase of the air amount, the ink amount may be relatively reduced. 
     To respond to the problem, when the ink cartridge is in the ink-end (ink-empty) condition, before suggesting the exchange of the ink cartridge  10  to the user, the negative pressure of the fluid supply path in the vacuum state between the ink cartridge  10  and the fluid supply pump  241  is released. 
     To that end, when the full-tank detection sensor  301  does not detect the displacement member  205  of the head tank  35  within a predetermined time period since the fluid supply pump  241  is driven, the fluid supply pump  241  is stopped. Then, the fluid supply pump  241  is reversely driven to return (supply) a small amount of ink from the head tank  35  to the ink cartridge  10  side to release the negative pressure of the fluid supply path in the vacuum state. Then, the fluid supply pump  241  is stopped. 
     Then, after the vacuum state is released, the exchange of the empty ink cartridge  10  with a new ink cartridge  10  may be suggested to the user on the display of the operation panel  514  or the like. 
     Normally, the user may exchange the ink cartridge  10  under this state. However, there may be case where there are no new ink cartridges  10  or the user wishes to continue printing a little bit more. In such a case, ink remaining in the head tank  35  is not sufficient, so that the head tank  35  is in a negative pressure state. 
     Namely, it may be possible to continue printing for a while but as the printing is continued, the volume of the head tank  35  may be reduced and the negative pressure in the head tank  35  may be increased. When the negative pressure becomes higher, air may be suctioned through the nozzles of the recording head  34 , which may cause an unstable discharging condition. 
     Specifically, when it is determined that the ink cartridge  10  is in the ink-end condition, the open amount of the displacement member  205  of the head tank is an arbitrary amount in the range between L 11  and L 12  which is described above with reference to  FIGS. 8A and 8B . Under this state, when the printing is continued, the open amount of the displacement member  205  may be less than the open amount L 12 . 
     As the result, the negative pressure may be increased too much and air may be more likely to be suctioned through the nozzles of the recording head  34 . In other words, in a case where the open amount of the displacement member  205  is close to the open amount L 12  at the timing when it is determined that the ink cartridge is in the ink-end condition, if the printing is continued, the quality of the output image may be immediately degraded. 
     In respond to the problem, in this embodiment, in a case where, even when the fluid supply pump  241  (which may be herein referred to as a “reversible-type fluid supply unit”) is used to perform the fluid supply for a predetermined time period, if the pressure in the head tank  35  is not restored to a predetermined pressure, it is determined that the ink cartridge  10  (which is the main tank) is in the ink-end condition. 
     As a result, printing is stopped and the exchange of the ink cartridge  10  is suggested to the user. In this case, however, if the user wishes to continue printing, a printing mode is changed to an emergency printing mode, namely emergency printing is performed. In the emergency printing, the air opening mechanism  207  of the head tank  35  is opened to increase the pressure which is temporarily increased. 
     After that, the fluid supply pump  241  is reversely driven (controlled) to return (reversely supply) ink from the head tank  35  to the ink cartridge (main tank)  10  to reduce the pressure in the head tank  35  (the negative pressure is generated) to a desired pressure. 
     Next, the control in this embodiment is described with reference to the flowcharts of  FIGS. 11 and 12 . 
     First, during printing, it is determined whether a predetermined amount of ink in the head tank  35  is consumed (step S 1 , hereinafter step numbers are simplified as “S 1 ” for instance). When determining that the predetermined amount of ink is consumed, the ink supply operation supplying a predetermined amount of ink is started (S 2 ). 
     Then, the open amount of the displacement member  205  (hereinafter may be simplified as a “lever”) in response to the predetermined amount is confirmed (checked) (S 3 ). Then, the data of the open amount of the lever is referred to, the data having been stored in the ROM  502 , and the open amount of the lever in response to the predetermined amount (i.e., a target supply amount) is determined (S 4 ). 
     Then, the, the fluid supply pump  241  is normally driven to supply ink from the ink cartridge (main tank)  10  to the head tank  35  (S 5 ). In this case, it is determined whether the target supply amount of ink has been supplied (i.e., as described above, it is determined whether the full-tank detection sensor  301  detects the lever) within the predetermined time period (S 6 , S 9 ). 
     When determining that the full-tank detection sensor  301  detects the lever, since it means that the supply of the target supply amount of ink has been completed, the fluid supply pump  241  is stopped to stop the ink supply (S 7 ), the ink supply operation of the predetermined amount of ink is stopped (S 8 ), and printing is continues. 
     On the other hand, it is determined that, even when the fluid supply pump  241  is normally driven to supply ink from the ink cartridge (main tank)  10  to the head tank  35 , if the full-tank detection sensor  301  does not detect the lever within the predetermined time period, it is determined that the ink cartridge  10  is in the ink-end condition, and the fluid supply pump  241  is stopped to stop the ink supply (S 10 ). 
     Then, the fluid supply pump  241  is reversely driven to return (reversely supply) a predetermined amount of ink from the head tank  35  to the ink cartridge (main tank)  10  (S 11 ). The “predetermined amount” may not be accurately determined as long as the “predetermined amount” at least corresponds to the capacity defined between the pressure cutting portion of the fluid supply pump  241  and the ink supply port (needle) of the ink cartridge  10 . 
     In this case, the reversely supplied amount may be detected (determined) based on, for example, the time period of driving the fluid supply pump  241  or the number of rotations of the fluid supply pump  241  to supply ink. 
     After that, the state (e.g., a message) indicating that the ink cartridge  10  is in the ink-end condition (the state of the cartridge end) is displayed, and the operation of the apparatus is stopped (S 12 ). In this case, the instructions to select either “cartridge exchange” or “emergency printing” are displayed on the operation panel or on the display device of the host  600  side for the user (S 13 ). 
     When the “cartridge exchange” is selected (instructed), after the old ink cartridge is exchanged (replaced) by a new ink cartridge  10 , the ink supply operation (not shown) of supplying ink from the new ink cartridge  10  to the head tank  35  is performed (S 4 ). 
     On the other hand, when the “emergency printing” is selected (instructed), the process goes to step S 15  to perform the emergency printing operation illustrated in  FIG. 10  (S 15 ). 
     Next, the emergency printing operation is described with reference to  FIG. 11 . First, by normally driving the fluid supply pump  241 , the ink having been reversely supplied from the head tank  35  to the ink cartridge  10  as described above is supplied from the ink cartridge  10  to the head tank  35  (S 21 ). 
     In this case as well, the ink supply operation is stopped (i.e., the ink supply operation is stopped) by stopping the fluid supply pump  241  when, for example, the driving (supplying) time period or the number of the rotations of the fluid supply pump  241  is equal to the predetermined driving (supplying) time period or the number of the rotations, respectively (S 22 ). 
     At this point, the ink amount in the head tank  35  is substantially equal to the ink amount before ink is reversely supplied from the head tank  35  to the ink cartridge  10 , and the open amount of the lever (displacement member  205 ) is in a range between L 11  and L 12  of  FIG. 8 . 
     After that, the air opening mechanism  207  of the head tank  35  is opened (S 23 ). By doing this, the air is introduced into the head tank  35  and the negative pressure is released and the open amount of the lever is greater than the open amount L 11  of  FIG. 8 . After that, the air opening mechanism  207  of the head tank  35  is closed (S 24 ). 
     Then, the fluid supply pump  241  is reversely driven to return (reversely supply) a predetermined amount of ink from the head tank  35  to the ink cartridge (main tank)  10  (S 25 ). The “predetermined amount” refers to the amount so that appropriate negative pressure is generated in the head tank  35  and preferably the open amount of the lever (displacement member  205 ) is close to the open amount L 11  of  FIG. 8 . From that point of view, the operating (driving) time period of the fluid supply pump  241  may be set by detecting the lever or may be determined in advance. 
     By doing this, an appropriate negative pressure may be generated in the head tank  35  and the vacuum state in the path from the fluid supply pump  241  to the ink cartridge  10  may be released. 
     After that, the ink consumption counter value is reset (S 26 ). This is performed in order to perform the process, described with reference to  FIG. 10 , in which ink is supplied from the ink cartridge  10  to the head tank  35  when the estimated ink consumption amount (e.g., accumulated amount of the number of droplets or drops) in the head tank  35  is equal to the corresponding threshold value in normal printing. Namely, the counter is reset because the negative pressure in the head tank  35  is set to the initial value. 
     Then, the emergency printing is executed (S 27 ). In this case, a normal printing operation is performed. 
     In this case, based on the counter value of the ink consumption counter, it is determined whether the open amount of the lever (displacement member  205 ) is equal to the open amount L 12  of  FIG. 8  (S 28 ). When the counter value of the ink consumption counter is a predetermined value, the printing is stopped and the state of the cartridge end is displayed (S 29 ). 
     AS described above, an image forming apparatus according to the embodiment includes a first control unit, when the pressure in the head tank is not restored to a predetermined pressure while liquid is supplied from the main tank to the head tank, determining that the state of the main tank is an empty state, cause the reversible-type fluid supply unit to reversely rotate to reversely supply a predetermined amount of liquid from the head tank to the main tank, and stop printing; and a second control unit, when the printing is stopped by the first control unit and emergency printing is instructed using liquid remaining in the image forming apparatus, causing the air opening mechanism of the head tank to open, causing the reversible-type fluid supply unit to normally rotate to supply the liquid to the head tank, the liquid having been reversely supplied to the main tank, causing the air opening mechanism to close, and causing the reversible-type fluid supply unit to reversely rotate to reversely supply the liquid from the head tank to the main tank to generate a predetermined negative pressure in the head tank. 
     By having this configuration, it may become possible to reduce ink to be wastefully consumed in the printing operation using the liquid (ink) remaining in the head tank. 
     In this embodiment, a case is described where the second control unit, when the printing is stopped by the first control unit and emergency printing is instructed using liquid remaining in the image forming apparatus, causes the air opening mechanism of the head tank to open, causes the reversible-type fluid supply unit to normally rotate to the supply liquid to the head tank, the liquid having been reversely supplied to the main tank, causes the air opening mechanism to close, and causes the reversible-type fluid supply unit to reversely rotate to reversely supply the liquid from the head tank to the main tank to generate a predetermined negative pressure in the head tank. 
     However, when the printing is stopped by the first control unit and emergency printing is instructed using liquid remaining in the image forming apparatus, the second control unit may cause the reversible-type fluid supply unit to normally rotate to supply the liquid to the head tank, the liquid having been reversely supplied to the main tank, cause the air opening mechanism of the head tank to open, cause the air opening mechanism to close, and cause the reversible-type fluid supply unit to reversely rotate to reversely supply the liquid from the head tank to the main tank to generate a predetermined negative pressure in the head tank. 
     Next, the control according a second embodiment of the present invention is described with reference to flowcharts of  FIGS. 12 and 13 . 
     In the above first embodiment, when the cartridge end is determined (i.e., when the full-tank detection sensor  301  does not detect the lever within the predetermined time period,) the fluid supply pump  241  is reversely driven to return (reversely supply) the predetermined amount of ink from the head tank  35  to the ink cartridge  10  to release the vacuum state of the supply path between the fluid supply pump  241  and the ink cartridge (S 11 ). 
     Then, the state of the cartridge end is displayed and operating of the device is stopped. In this second embodiment, however, the state of the cartridge end is displayed and operating of the device is stopped without reversely supplying ink from the head tank  35  to the ink cartridge  10 . 
     After that, when the user selects the “cartridge exchange”, after the fluid supply pump  241  is reversely driven to reversely supply ink from the head tank  35  to the ink cartridge  10  to release the vacuum state of the supply path between the fluid supply pump  241  and the ink cartridge  10  (S 16 ), the cartridge is exchanged (S 14 ). 
     When the user selects the “emergency printing”, as illustrated in  FIG. 13 , the process starts with the process to open the air opening mechanism  207  of the head tank  35  (i.e., the process starts with the process in step S 23  of  FIG. 11 ). 
     Unlike the first embodiment, when the “emergency printing” is instructed (directed) by the user, it may become possible to immediately start the negative pressure generation process by opening the air opening mechanism  207  of the head tank  35  because of skipping the process of reversely supply ink from the head tank  35  to the ink cartridge  10 . 
     Next, the control according a third embodiment of the present invention is described with reference to a flowchart of  FIG. 14 .  FIG. 14  illustrates another example of the emergency printing when the “emergency printing” is selected in S 13  of the first embodiment. 
     In this embodiment, as described in the first embodiment, after the fluid supply pump  241  is reversely driven to return (reversely supply) the predetermined amount of ink from the head tank  35  to the ink cartridge  10  to release the vacuum state of the supply path between the fluid supply pump  241  and the ink cartridge (S 11  of  FIG. 10 ), the air opening mechanism  207  of the head tank  35  is opened to introduce air into the head tank  35  to release the negative pressure (S 31 ). 
     Then, the air opening mechanism  207  of the head tank  35  is closed (S 32 ). Then, to generate negative pressure in the head tank  35 , the fluid supply pump  241  is reversely driven to return (reversely supply) a predetermined amount of ink from the head tank  35  to the ink cartridge  10  side (S 33 ). By doing this, the ink cartridge  10  side includes ink that has been reversely supplied twice. 
     Then, after the ink consumption counter value is reset (S 34 ), the emergency printing is performed (S 35 ). When it is determined that the ink consumption counter value is a predetermined counter value and the negative pressure in the head tank  35  approaches a limit value (S 36 ), ink having been returned to the ink cartridge  10  side is supplied to the head tank  35  again (S 37 , S 38 ). 
     Then, after the air opening mechanism  207  of the head tank  35  is temporarily opened to release the negative pressure (S 39 ), the air opening mechanism  207  of the head tank  35  is closed again (S 40 ) to generate negative pressure. Then, a predetermined amount of ink is reversely supplied to release the vacuum state in the supply path from the fluid supply pump  241  to the ink cartridge  10  side (S 41 ). 
     By doing this, it may become possible to set the negative pressure of the head tank  35  to be the initial value again, so that the emergency printing can be performed again. Namely, it may become possible to multiply (increase) the number of printed sheets after the cartridge end is detected (displayed). 
     Then, the ink consumption counter value is reset (S 42 ), and it is determined that the ink consumption counter value is the predetermined counter value and the negative pressure in the head tank  35  approaches the limit value (S 43 ). When determined so, the state of the cartridge end is displayed and the operation of the apparatus is stopped (S 44 ). 
     Next, the control according a fourth embodiment of the present invention is described with reference to a flowchart of  FIG. 15 . 
       FIG. 15  of this embodiment illustrates an example of the emergency printing where cartridge end is detected while the air is introduced into the head tank  35 . In this example, the air amount in the head tank  35  is detected with the electrode pins  208   a  and  208   b.    
     Namely, when the height of the liquid surface of ink in the head tank  35  is greater than the heights of the electrode pins  208   a  and  208   b , a current may flow between the electrode pins  208   a  and  208   b . Therefore, it may be determined that there is (sufficient) ink in the head tank  35 . On the other hand, when the height of the liquid surface of ink in the head tank  35  is less than the height of the electrode pin  208   b  which is less than the height of the electrode pins  208   a , no current may flow between the electrode pins  208   a  and  208   b.    
     Therefore, it may be determined that there is no (insufficient) ink in the head tank  35 . When air is introduced into the head tank  35 , the height of the liquid surface is lowered. Due to this, when determined that the there is no (insufficient) ink in the head tank  35 , the air opening mechanism  207  is opened and ink is supplied until current flows between the electrode pins  208   a  and  208   b  so that air amount in the head tank  35  is reduced. This operation may be herein called a “air open filling”. Under this state, however, the cartridge end may occur. 
     First, as illustrated in  FIG. 15 , when the printing is started or during the printing, it is determined whether there is (sufficient) ink in the head tank  35  using the electrode pins  208   a  and  208   b  which are provided for detecting the ink amount in the head tank (S 51 ). When determining that there is no (insufficient) ink (i.e, when the air amount is equal to or greater than a predetermined amount), the above “air open filling” operation is started (S 52 ). 
     In this “air open filling” operation, first, the air opening mechanism  207  of the header tank  35  is opened (S 53 ), and the ink supply of supplying ink from the ink cartridge  10  to the header tank  35  is started (S 54 ). By doing this, extra air may be discharged though the air opening mechanism  207 . 
     In this case, it is determined whether the state where there is (sufficient) ink is detected by using the electrode pins  208   a  and  208   b  within a predetermined time period (S 55 , S 61 ). When determining that the state where there is (sufficient) ink is detected within the predetermined time period, the ink supply operation is stopped (S 56 ), and the air opening mechanism  207  is closed (S 57 ). 
     Then, the recording head  34  is capped by the suction cap  82   a  and ink is suctioned (head suction) to generate negative pressure in the head tank  35 . The negative pressure may alternatively be generated by reversely driving the fluid supply pump  241  to reversely supply ink. After that, a wiping operation is performed on the recording head  34  (S 59 ), and the “air open filling” operation is terminated (S 60 ), so that the printing may be started. 
     On the other hand, when determining that the state where there is (sufficient) ink is not detected within the predetermined time period, it is determined that it is in the state of the cartridge end, and the ink supply operation is stopped (S 62 ). Then, after the air opening mechanism  207  is closed (S 63 ), a predetermined amount of ink is (reversely) supplied from the head tank  35  to the ink cartridge  10  to generate negative pressure in the head tank  35  (S 64 ). Then the operation of the apparatus is stopped and an ink end display suggesting the exchange of the cartridge exchange is displayed (S 65 ). 
     Then, when the user instructs (selects) the exchange of the ink cartridge  10 , the cartridge exchange process is performed (S 67 ). On the other hand, when the user instructs (selects) the emergency printing, the emergency printing similar to that described with reference to  FIG. 11  is performed (S 68 ). 
     Next, the control according a fifth embodiment of the present invention is described with reference to a flowchart of  FIG. 16 . 
     Herein, the relationship of this fifth embodiment to the fourth embodiment corresponds to the relationship of the second embodiment to the first embodiment. Namely the fifth embodiment is similar to the fourth embodiment except that, similar to the second embodiment, the reverse supply of ink from the head tank  35  to the ink cartridge  10  side is not performed before the operation of the apparatus is stopped to exchange the ink cartridge  10 . When the ink cartridge is exchanged, the ink is reversely supplied (S 69 ). When the emergency printing is instructed, a similar process as that in second embodiment is performed. 
     Next, a sixth embodiment of the present invention is described with reference to  FIG. 17 .  FIG. 17  schematically illustrates an ink supply system of this embodiment. 
     In this embodiment, as illustrated in  FIG. 17 , the ink supply system includes two head tanks  35 A and  35 B connected to the same ink cartridge  10  via respective supply paths. The two supply paths include respective fluid supply pumps  241 A and  241 B and supply tubes  36 A and  36 B. Namely, the ink supply system in this embodiment includes more than one head tank for each color ink. 
     In this embodiment, ink is supplied to the head tanks  35 A and  35 B using the fluid supply pumps  241 A and  241 B, respectively, using the two supply paths separated from the single ink cartridge  10 . Therefore, the state of the cartridge end of the ink cartridge  10  may be detected during the ink supply operation to either the head tank  35 A or the head tank  35 B. 
     Next, the control according the sixth embodiment of the present invention is described with reference to a flowchart of  FIG. 18 . 
     During printing, when it is determined that a predetermined amount of ink is consumed in the head tank  35 A (S 71 ), a predetermined amount of ink supply to the head tank  35 A is started (S 72 ). Then, it is determined whether the ink supply is completed within a predetermined time period (S 73 ). When determining that the ink supply is not completed within a predetermined time period, the operation of the apparatus is stopped and the ink end display suggesting the exchange of the ink cartridge  10  is displayed (S 75 ). Then, depending on the user&#39;s instructions (S 76 ), the exchange of the ink cartridge  10  (S 77 ) or the emergency printing (S 78 ) is performed. 
     On the other hand, during printing, when it is determined that a predetermined amount of ink is consumed in the head tank  35 B (S 81 ), a predetermined amount of ink supply to the head tank  35 B is started (S 82 ). Then, it is determined whether the ink amount in the other head tank  35 A is zero (not sufficient) (S 83 ). 
     When determining that the ink amount in the head tank  35 A is zero, the process goes to S 75 , where, as described above, the operation of the apparatus is stopped and the ink end display suggesting the exchange of the ink cartridge  10  is displayed (S 75 ). Otherwise, the process goes to S 71 . 
     Namely, the ink consumption count values are separately counted for each of the head tanks  35 A and  35 B. Further, when the ink consumption count values exceed the threshold values, ink is supplied timely to the corresponding head tanks  35 A and  35 B. 
     In this case, when the ink supply is not completed within a predetermined time period, it is determined that the ink cartridge  10  is in the state of the cartridge end. Then, the printing is stopped and the exchange of the ink cartridge  10  is suggested to the user. 
     Further, in the emergency printing, when the state of the ink cartridge  10  is determined as the cartridge end, the ink amount of one head tank  35  to which ink is being supplied from the ink cartridge  10  may not be sufficient for printing. 
     In this case, however, the other (another) head tank  35  may be more likely to have sufficient ink for printing. From this point of view, the emergency printing may be performed using only the head tank  35  having sufficient ink, and when it is determined that it is necessary to supply ink to the head tank  35 , the printing may be completely stopped. 
     Next, the control according a seventh embodiment of the present invention is described with reference to a flowchart of  FIG. 19 . 
     In this embodiment, for example, in a case where the state of the ink cartridge  10  is determined as the cartridge end while ink is supplied to a first head tank as described in the sixth embodiment, when the emergency printing is selected, ink is reversely supplied from a second head tank having sufficient ink to the ink cartridge side by reversely driving the fluid supply pump for the second head tank. 
     Then ink having reversely supplied from the second head tank is supplied from the ink cartridge to the first head tank not having sufficient ink by normally driving the fluid supply pump for the first head tank. By doing this, it may become possible to mutually supply ink between (among) head tanks to make it possible to execute the emergency printing. 
     To that end, for example, first, an ink usable amount of the head tank other than the head tank which requires ink supply is calculated (S 91 ). Then, a total usable amount of ink in the head tanks corresponding to the color is calculated, and the calculated total usable amount of ink is divided by the number of the total head tanks corresponding to the color (in this example, the number of the total head tanks is “2”) to acquire the amount of ink to be reversely supplied to the ink cartridge  10  (S 92 ). 
     Then, the acquired amount of ink having been reversely supplied to the ink cartridge  10  is supplied to the head tank needing ink to be supplied (S 93 , S 94 ). In this case, a longer operating time may be set for the ink supply operation so that all mount of ink having been returned to ink cartridge  10  is supplied to the head tank needing ink to be supplied. When all the amount of ink is supplied to the head tank, the ink supply operation is completed. 
     After that, the emergency printing is continued (S 95 ). Then, the ink consumption amount is counted for each of the head tanks. When determining that any of the ink consumption amounts of the head tanks is equal to a predetermined threshold value (i.e., when determining that ink remaining amount of any one of the head tanks is none (not sufficient)) (S 96 ), the printing is stopped (S 97 ). 
     The control relevant to the ink supply (normal and reverse supplies) and the various processes such as determination processes and detection processes according to the embodiments of the present invention as described above may be executed by a computer using a program stored in the ROM  502 . The program may be downloaded to an image processing apparatus (host  600 ) and installed in the image forming apparatus. 
     Further, the above processes may be configured to be executed by a printer driver on the image processing apparatus (host  600 ) side. Further, by combining an image forming apparatus of the present invention and an image processing apparatus, or by combining an image forming apparatus and an image processing apparatus of the present invention, an image forming system of the present invention may be constituted. 
     According to an embodiment of the present invention, an image forming apparatus includes a recording head discharging liquid droplets; a head tank including a liquid container containing liquid to be supplied to the recording head and an air opening mechanism opening and closing the liquid container to air; a main tank containing liquid to be supplied to the head tank; a reversible-type fluid supply unit being disposed between the head tank and the main tank and supplying liquid; a pressure detector detecting a pressure in the head tank; a first control unit, when the pressure in the head tank is not restored to a predetermined pressure while liquid is supplied from the main tank to the head tank, determining that the state of the main tank is an empty state, causing the reversible-type fluid supply unit to reversely rotate to reversely supply a predetermined amount of liquid from the head tank to the main tank, and stopping printing; and a second control unit, when the printing is stopped by the first control unit and emergency printing is instructed using liquid remaining in the image forming apparatus, causing the air opening mechanism of the head tank to open, causing the reversible-type fluid supply unit to normally rotate to the supply liquid to the head tank, the liquid having been reversely supplied to the main tank, causing the air opening mechanism to close, and causing the reversible-type fluid supply unit to reversely rotate to reversely supply the liquid from the head tank to the main tank to generate a predetermined negative pressure in the head tank. 
     According to an embodiment of the present invention, an image forming apparatus includes a recording head discharging liquid droplets; a head tank including a liquid container containing liquid to be supplied to the recording head and an air opening mechanism opening and closing the liquid container to air; a main tank containing liquid to be supplied to the head tank; a reversible-type fluid supply unit being disposed between the head tank and the main tank and supplying liquid; a pressure detector detecting a pressure in the head tank; a first control unit, when the pressure in the head tank is not restored to a predetermined pressure while liquid is supplied from the main tank to the head tank, determining that the state of the main tank is an empty state, causing the reversible-type fluid supply unit to reversely rotate to reversely supply a predetermined amount of liquid from the head tank to the main tank, and stopping printing; and a second control unit, when the printing is stopped by the first control unit and emergency printing is instructed using liquid remaining in the image forming apparatus, causing the reversible-type fluid supply unit to normally rotate to supply the liquid to the head tank, the liquid having been reversely supplied to the main tank, causing the air opening mechanism of the head tank to open, causing the air opening mechanism to close, and causing the reversible-type fluid supply unit to reversely rotate to reversely supply the liquid from the head tank to the main tank to generate a predetermined negative pressure in the head tank. 
     According to an embodiment of the present invention, an image forming apparatus includes a recording head discharging liquid droplets; a head tank including a liquid container containing liquid to be supplied to the recording head and an air opening mechanism opening and closing the liquid container to air; a main tank containing liquid to be supplied to the head tank; a reversible-type fluid supply unit being disposed between the head tank and the main tank and supplying liquid; a pressure detector detecting a pressure in the head tank; a first control unit, when the pressure in the head tank is not restored to a predetermined pressure while liquid is supplied from the main tank to the head tank, determining that the state of the main tank is an empty state and stopping printing; and a third control unit, when the printing is stopped by the first control unit and exchange of the main tank is instructed, causing the reversible-type fluid supply unit to normally rotate to the supply liquid from the main tank to the head tank, and when the printing is stopped by the first control unit and emergency printing is instructed using liquid remaining in the image forming apparatus, causing the air opening mechanism of the head tank to open, causing the reversible-type fluid supply unit to normally rotate to supply the liquid to the head tank, the liquid having been reversely supplied to the main tank, causing the air opening mechanism to close, and causing the reversible-type fluid supply unit to reversely rotate to reversely supply the liquid from the head tank to the main tank to generate a predetermined negative pressure in the head tank. 
     According to an embodiment of the present invention, an image forming apparatus may further include a control unit, when the negative pressure in the head tank is increased in the emergency printing and it becomes necessary to supply liquid from the main tank, causing the air opening mechanism of the head tank to open, causing the reversible-type fluid supply unit to normally rotate to the supply liquid to the head tank, the liquid having been reversely supplied to the main tank, causing the air opening mechanism to close, and causing the reversible-type fluid supply unit to reversely rotate to reversely supply the liquid from the head tank to the main tank to generate a predetermined negative pressure in the head tank. 
     According to embodiments of the present invention, an image forming apparatus includes a first control unit, when the pressure in the head tank is not restored to a predetermined pressure while liquid is supplied from the main tank to the head tank, to determine that the state of the main tank is an empty state, causing the reversible-type fluid supply unit to reversely rotate to reversely supply a predetermined amount of liquid from the head tank to the main tank, and stop printing; and a second control unit, when the printing is stopped by the first control unit and emergency printing is instructed using liquid remaining in the image forming apparatus, either causing the air opening mechanism of the head tank to open and then cause the reversible-type fluid supply unit to normally rotate to supply the liquid to the head tank, the liquid having been reversely supplied to the main tank, or cause the reversible-type fluid supply unit to normally rotate to supply the liquid to the head tank, the liquid having been reversely supplied to the main tank and then causing the air opening mechanism of the head tank to open, and then causing the air opening mechanism to close and causing the reversible-type fluid supply unit to reversely rotate to reversely supply the liquid from the head tank to the main tank to generate a predetermined negative pressure in the head tank. By having this configuration, it may become possible to reduce ink to be wastefully consumed in the printing operation using the liquid (ink) remaining in the head tank. 
     Further, according to an embodiment of the present invention, an image forming apparatus includes a first control unit, when the pressure in the head tank is not restored to a predetermined pressure while liquid is supplied from the main tank to the head tank, determining that the state of the main tank is an empty state and stopping printing; and a third control unit, when the printing is stopped by the first control unit and exchange of the main tank is instructed, causing the reversible-type fluid supply unit to normally rotate to supply the liquid from the main tank to the head tank, and when the printing is stopped by the first control unit and emergency printing is instructed using liquid remaining in the image forming apparatus, causing the air opening mechanism of the head tank to open, causing the reversible-type fluid supply unit to normally rotate to supply the liquid to the head tank, the liquid having been reversely supplied to the main tank, causing the air opening mechanism to close, and causing the reversible-type fluid supply unit to reversely rotate to reversely supply the liquid from the head tank to the main tank to generate a predetermined negative pressure in the head tank. By having this configuration, it may become possible to reduce ink to be wastefully consumed in the printing operation using the liquid (ink) remaining in the head tank. 
     Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teachings herein set forth.