Patent Publication Number: US-11034159-B2

Title: Inkjet recording apparatus and method of controlling the same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a Continuation of U.S. patent application Ser. No. 16/019,137, filed Jun. 26, 2018, entitled “INKJET RECORDING APPARATUS AND METHOD OF CONTROLLING THE SAME”, the content of which is expressly incorporated by reference herein in its entirety. Further, the present application claims priority from Japanese Patent Application No. 2017-133836, filed Jul. 7, 2017, which is also hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The disclosure relates to an inkjet recording apparatus configured to record images using a recording head. 
     Description of the Related Art 
     Japanese Patent Application Laid-Open No. 2014-144611 discusses an inkjet recording apparatus including an inkjet head which discharges ink and an ink tank which stores ink to be supplied to the inkjet head. The ink tank and the inkjet head are connected to each other by a supply channel and a collection channel to circulate the ink between the ink tank and the inkjet head. Further, the ink flows also in the vicinity of a nozzle of the recording head due to a difference in pressure between the first and second channels to prevent the ink in the vicinity of the nozzle from thickening so that stable discharge performance is maintained. 
     However, the apparatus discussed in Japanese Patent Application Laid-Open No. 2014-144611 performs operations of opening and closing a supply valve provided in the first channel and a collection valve provided in the second channel to circulate the ink and also performs driving adjustment, so that it takes time to start the next recording operation in some cases. 
     SUMMARY OF THE INVENTION 
     The disclosure is directed to an inkjet recording apparatus capable of improving throughput up to a start of a recording operation in a structure of circulating ink between a recording head and an ink tank. 
     According to an aspect of the disclosure, an inkjet recording apparatus includes a recording head which discharges an ink to perform a recording operation, an ink tank which stores the ink to be supplied to the recording head, a supply channel through which the ink is supplied from the ink tank to the recording head, a supply pump provided in the supply channel, a collection channel through which the ink is collected from the recording head to the ink tank, a collection pump provided in the collection channel, and a buffer chamber provided in the recording head or the collection channel and having a volume which is changed according to a pressure. The recording head includes a discharge opening through which the ink is discharged, a common channel connected with the supply channel and the collection channel, and a pressure control unit which changes, according to the pressure, between an opened state in which the ink is passed through the common channel and a closed state in which the ink is not passed through the common channel. The inkjet recording apparatus forms a circulation state in which the ink is circulated between the recording head and the ink tank by driving the supply pump and the collection pump in a case where the recording operation is performed, whereas the inkjet recording apparatus stops the driving of the supply pump and the collection pump and forms a pause state in which the ink is not circulated in a case the recording operation is not performed. When the inkjet recording apparatus is to be changed from the circulation state to the pause state, the inkjet recording apparatus stops the driving of the supply pump and the collection pump and forms a waiting state in which the pressure in the collection channel is lowered than that in the pause state by the buffer chamber. 
     Further features and aspects of the disclosure will become apparent from the following description of numerous example embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an inkjet recording apparatus in a waiting state. 
         FIG. 2  is a block diagram illustrating a control configuration of the inkjet recording apparatus. 
         FIG. 3  illustrates the inkjet recording apparatus in a recording state. 
         FIGS. 4A to 4C  are diagrams each illustrating a conveyance route of a recording medium fed from a first cassette. 
         FIGS. 5A to 5C  are diagrams each illustrating a conveyance route of a recording medium fed from a second cassette. 
         FIGS. 6A to 6D  are diagrams each illustrating a conveyance route in a case in which a recording operation is performed on the back surface of a recording medium. 
         FIG. 7  illustrates the inkjet recording apparatus in a maintenance state. 
         FIGS. 8A and 8B  are perspective views illustrating a structure of a maintenance unit. 
         FIG. 9  is a schematic view illustrating an ink supply unit. 
         FIGS. 10A-B  illustrate a structure of an ink discharge unit of a recording head (head unit). 
         FIG. 11A-C  illustrate a structure of a first negative pressure control unit of the recording head (head unit). 
         FIG. 12  is a flowchart illustrating control for changing the inkjet recording apparatus from the recording state to the waiting state. 
         FIG. 13  is a flowchart illustrating control for changing the inkjet recording apparatus from the waiting state to the recording state. 
         FIG. 14  is a flowchart illustrating control for changing the inkjet recording apparatus from the waiting state to a pause state. 
         FIG. 15  is a flowchart illustrating control for changing the inkjet recording apparatus from the pause state to the recording state. 
         FIG. 16  is a flowchart illustrating control of a collection pump in changing the inkjet recording apparatus to the recording state. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     A first example embodiment will be described below.  FIG. 1  is an interior structure diagram illustrating an inkjet recording apparatus (hereinafter, “recording apparatus”)  1  of the present example embodiment. Hereinafter, x-, y-, and z-directions respectively refer to the horizontal direction, the direction in which discharge openings of a recording head  8  are aligned (perpendicular direction with respect to the sheet surface), and the vertical direction. 
     The recording apparatus  1  is a multi-function peripheral including a printing portion  2  and a scanner portion  3 , and the printing portion  2  and the scanner portion can execute various types of processing relating to recording and reading operations either separately or in conjunction. The scanner portion  3  includes an auto-document feeder (ADF) and a flatbed scanner (FBS) and is capable of reading documents fed automatically by the ADF and is also capable of reading (scanning) documents placed on a document plate of the FBS by the user. While the present example embodiment describes the multi-function peripheral including both the printing portion  2  and the scanner portion  3 , the scanner portion  3  can be omitted.  FIG. 1  illustrates the recording apparatus  1  in a waiting state in which neither recording operations nor reading operations are performed. 
     In the printing portion  2 , first and second cassettes  5 A and  5 B which store recording mediums (cut sheets) S are removably attached to the lower bottom part of a housing  4  in the vertical direction. The first cassette  5 A stores relatively small recording mediums up to A4 size which are laid out flat, and the second cassette  5 B stores relatively large recording mediums up to A3 size which are also laid out flat. In the vicinity of the first cassette  5 A is provided a first sheet feeding unit  6 A which separates and feeds the stored recording mediums one by one. Similarly, in the vicinity of the second cassette  5 B is provided a second sheet feeding unit  6 B. When a recording operation is performed, the recording medium S is selectively fed from one of the first and second cassettes  5 A and  5 B. 
     Conveyance rollers  7 , a sheet ejection roller  12 , pinch rollers  7   a , spurs  7   b , a guide  18 , an inner guide  19 , and a flapper  11  are a conveyance mechanism for guiding the recording medium S in a predetermined direction. The conveyance rollers  7  are driving rollers which are disposed on the upstream and downstream sides of the recording head  8  and are driven by a conveyance motor (not illustrated). The pinch rollers  7   a  are driven rollers which are disposed on the upstream side of the recording head  8  and nip and rotate the recording medium S together with the conveyance rollers  7 . The sheet ejection roller  12  is a driving roller which is disposed at the most downstream part of the conveyance route and is driven by the conveyance motor (not illustrated). The spurs  7   b  are disposed on the downstream side of the recording head  8  and guide the recording medium S in a predetermined direction. The spurs  7   b  that are disposed to face the conveyance rollers  7  or the sheet ejection roller  12  sandwich and convey the recording medium S together with the conveyance rollers  7  or the sheet ejection roller  12 . 
     The guide  18  is provided in the conveyance route for the recording medium S and guides the recording medium S in the predetermined direction. The inner guide  19  is a member which extends in the y-direction and includes a curved side surface. The inner guide  19  guides the recording medium S along the side surface. The flapper  11  is a member for changing the conveyance direction of the recording medium S in two-sided recording operations. A sheet ejection tray  13  is a tray for stacking and holding the recording medium S that undergoes a recording operation has been performed on, and has been ejected by the sheet ejection roller  12 . 
     The recording head  8  of the present example embodiment is a full-line type color inkjet recording head, and the plurality of discharge openings from which ink is discharged based on recording data is aligned in width corresponding to the width of the recording medium S in the y-direction in  FIG. 1 . When the recording head  8  is in a standby position as illustrated in  FIG. 1 , a discharge opening surface  8   a  of the recording head  8  is capped with a cap unit  10 . The position of the cap unit  10  is also referred to as “capping position”. When the recording head  8  performs a recording operation, the orientation of the recording head  8  is changed by a print controller  202  described below such that the discharge opening surface  8   a  faces a platen  9 . The platen  9  includes a flat plate extending in the y-direction and supports from the back surface the recording medium S on which the recording head  8  performs a recording operation. The movement of the recording head  8  from the standby position to a recording position will be described in detail below. 
     An ink tank unit  14  stores four color inks to be supplied to the recording head  8 . A channel connecting the ink tank unit  14  and the recording head  8  to each other is provided with an ink supply unit  15  disposed between the ink tank unit  14  and the recording head  8 , and the ink supply unit  15  adjusts the pressure and flow rate of ink in the recording head  8 . In the present example embodiment, a circulation-type ink supply system is employed, and the ink supply unit  15  adjusts within suitable ranges the pressure of ink supplied to the recording head  8  and the flow rate of ink collected from the recording head  8 . 
     A maintenance unit  16  includes the cap unit  10  and a wiping unit  17  and causes the cap unit  10  and the wiping unit  17  to operate at predetermined timings to perform a maintenance operation on the recording head  8 . Details of the maintenance operation will be described below. 
       FIG. 2  is a block diagram illustrating a control configuration in the recording apparatus  1 . The control configuration mainly includes a print engine unit  200  which comprehensively controls the printing portion  2 , a scanner engine unit  300  which comprehensively controls the scanner portion  3 , and a controller unit  100  which comprehensively controls the recording apparatus  1 . The print controller  202  controls various mechanisms of the print engine unit  200  according to instructions from a main controller  101  of the controller unit  100 . Various mechanisms of the scanner engine unit  300  are controlled by the main controller  101  of the controller unit  100 . The following describes details of the control configuration. 
     In the controller unit  100 , the main controller  101  including a central processing unit (CPU) comprehensively controls the recording apparatus  1  using a random-access memory (RAM)  106  as a work area based on a program and various parameters stored in a read-only memory (ROM)  107 . For example, if a print job is input from a host apparatus  400  via a host interface (I/F)  102  or a wireless I/F  103 , an image processing unit  108  performs predetermined image processing on received image data based on instructions from the main controller  101 . Then, the main controller  101  transmits, to the print engine unit  200  via a print engine I/F  105 , the image data having undergone the image processing. 
     The recording apparatus  1  can acquire image data from the host apparatus  400  via wireless or wired communication or from an external storage apparatus (e.g., universal serial bus (USB) memory) connected with the recording apparatus  1 . Communication methods for use in the wireless or wired communication are not limited. For example, Wi-Fi (Wireless Fidelity) (registered trademark) and Bluetooth (registered trademark) are applicable as a communication method for use in the wireless communication. Further, USB or the like is usable as a communication method for use in the wired communication. Further, for example, if a reading command is input from the host apparatus  400 , the main controller  101  transmits the reading command to the scanner portion  3  via a scanner engine I/F  109 . 
     An operation panel  104  is a mechanism for the user to perform inputting and outputting on the recording apparatus  1 . The user can give an instruction to perform an operation such as a copy or scan operation, set a printing mode, and recognize information about the recording apparatus  1  via the operation panel  104 . 
     In the print engine unit  200 , the print controller  202  including a CPU controls various mechanisms of the printing portion  2  using a RAM  204  as a work area based on a program and various parameters stored in a ROM  203 . If various commands and image data are received via a controller I/F  201 , the print controller  202  temporarily saves the various commands and image data in the RAM  204 . The print controller  202  causes an image processing controller  205  to convert the saved image data into recording data so that the recording head  8  can use the recording data in a recording operation. 
     If the recording data is generated, the print controller  202  causes the recording head  8  via a head I/F  206  to execute a recording operation based on the recording data. At this time, the print controller  202  drives via a conveyance control unit  207  the first and second sheet feeding units  6 A and  6 B, the conveyance rollers  7 , the sheet ejection roller  12 , and the flapper  11  in  FIG. 1  to convey a recording medium S. The recording head  8  executes a recording operation in conjunction with the operation of conveying the recording medium S to perform printing processing according to instructions from the print controller  202 . 
     A head carriage control unit  208  changes the orientation and position of the recording head  8  based on the operation state of the recording apparatus  1  such as a maintenance state or recording state. An ink supply control unit  209  controls the ink supply unit  15  to adjust the pressure of ink supplied to the recording head  8  within a suitable range. A maintenance control unit  210  controls the operations of the cap unit  10  and the wiping unit  17  of the maintenance unit  16  when a maintenance operation is performed on the recording head  8 . 
     In the scanner engine unit  300 , the main controller  101  controls hardware resources of a scanner controller  302  using the RAM  106  as a work area based on a program and various parameters stored in the ROM  107 . In this way, various mechanisms of the scanner portion  3  are controlled. For example, the main controller  101  controls the hardware resources in the scanner controller  302  via a controller I/F  301  to convey a document placed on the ADF by the user via a conveyance control unit  304  and reads the document with a sensor  305 . Then, the scanner controller  302  saves the read image data in a RAM  303 . The print controller  202  converts the thus acquired image data into recording data so that the recording head  8  can execute a recording operation based on the image data read by the scanner controller  302 . 
       FIG. 3  illustrates the recording apparatus  1  in the recording state. Compared to the waiting state illustrated in  FIG. 1 , the cap unit  10  is separated from the discharge opening surface  8   a  of the recording head  8 , and the discharge opening surface  8   a  faces the platen  9 . In the present example embodiment, a flat surface of the platen  9  is inclined by about 45 degrees with respect to the horizontal direction, and the discharge opening surface  8   a  of the recording head  8  in the recording position is also inclined by about 45 degrees with respect to the horizontal direction to maintain a predetermined distance from the platen  9 . 
     When the recording head  8  is moved from the standby position illustrated in  FIG. 1  to the recording position illustrated in  FIG. 3 , the print controller  202  moves the cap unit  10  downward to a retracted position illustrated in  FIG. 3  using the maintenance control unit  210 . In this way, the discharge opening surface  8   a  of the recording head  8  is separated from a cap member  10   a . Thereafter, the print controller  202  rotates the recording head  8  by 45 degrees while lowering the height of the recording head  8  in the vertical direction using the head carriage control unit  208  so that the discharge opening surface  8   a  faces the platen  9 . The foregoing steps are reversely performed by the print controller  202  to move the recording head  8  from the recording position to the standby position after the recording operation is completed. 
     Next, the conveyance route for the recording mediums S in the printing portion  2  will be described below. If a recording command is input, the print controller  202  first moves the recording head  8  to the recording position illustrated in  FIG. 3  using the maintenance control unit  210  and the head carriage control unit  208 . Thereafter, the print controller  202  drives one of the first and second sheet feeding units  6 A and  6 B using the conveyance control unit  207  according to the recording command to feed the recording medium S. 
       FIGS. 4A to 4C  illustrate a conveyance route for feeding the A4-size recording mediums S stored in the first cassette  5 A. In  FIGS. 4A to 4C  and subsequent figures, the recording medium S that is conveyed is specified by a dotted line. The uppermost recording medium S in the first cassette  5 A is separated from the second and subsequent recording mediums S by the first sheet feeding unit  6 A and is conveyed toward a recording region P between the platen  9  and the recording head  8  while being nipped by the conveyance rollers  7  and the pinch rollers  7   a .  FIG. 4A  illustrates a conveyance state immediately before the leading edge of the recording medium S reaches the recording region P. The traveling direction of the recording medium S is changed from the horizontal direction (x-direction) to a direction inclined by about 45 degrees with respect to the horizontal direction after the recording medium S is fed to the first sheet feeding unit  6 A and before the recording medium S reaches the recording region P. 
     At the recording region P, the inks are discharged from the plurality of discharge openings of the recording head  8  toward the recording medium S. The back surface of the recording medium S, in the region to which the inks are to be applied, is supported by the platen  9 , and the distance between the discharge opening surface  8   a  and the recording medium S is maintained constant. The recording medium S with the inks applied thereon is guided by the sheet ejection roller  12  and the spurs  7   b  so that the leading edge of the recording medium S is passed through the left side of the flapper  11  inclined rightward as in  FIGS. 4A to 4C  and is conveyed upward in the vertical direction of the recording apparatus  1  along the guide  18 .  FIG. 4B  illustrates the state in which the leading edge of the recording medium S is passed through the recording region P and is conveyed upward in the vertical direction. The traveling direction of the recording medium S, after the recording medium S is passed through the recording region P inclined by about 45 degrees with respect to the horizontal direction, is changed to the upward direction in the vertical direction by the conveyance rollers  7  and the spurs  7   b.    
     After conveyed upward in the vertical direction, the recording medium S is ejected onto the sheet ejection tray  13  by the sheet ejection roller  12  and the spurs  7   b .  FIG. 4C  illustrates the state in which the leading edge of the recording medium S is ejected into the sheet ejection tray  13 . The recording medium S is ejected such that the surface on which an image is recorded by the recording head  8  faces downward, and the recording medium S is held on the sheet ejection tray  13 . 
       FIGS. 5A to 5C  illustrate a conveyance route for feeding the A3-size recording mediums S stored in the second cassette  5 B. The uppermost recording medium S stacked in the second cassette  5 B is separated from the second and subsequent recording mediums by the second sheet feeding unit  6 B and is conveyed toward the recording region P between the platen  9  and the recording head  8  while being nipped by the conveyance rollers  7  and the pinch rollers  7   a.    
       FIG. 5A  illustrates the conveyance state immediately before the leading edge of the recording medium S reaches the recording region P. The plurality of conveyance rollers  7 , the pinch rollers  7   a , and the inner guide  19  are provided in the conveyance route from the point at which the recording medium S is fed by the second sheet feeding unit  6 B to the point at which the fed recording medium S reaches the recording region P so that the recording medium S is curved in an S shape and conveyed to the platen  9 . 
     The conveyance route thereafter is the same as the conveyance route for the A4-size recording mediums S in  FIGS. 4B and 4C .  FIG. 5B  illustrates the state in which the leading edge of the recording medium S is passed through the recording region P and is conveyed upward in the vertical direction.  FIG. 5C  illustrates the state in which the leading edge of the recording medium S is ejected into the sheet ejection tray  13 . 
       FIGS. 6A to 6D  illustrate the conveyance route in the case in which a recording operation (two-sided recording) is performed on the back surface (second surface) of the A4-size recording medium S. In the case in which the recording apparatus  1  performs two-sided recording, the recording apparatus  1  performs a recording operation on a first surface (front surface) and then on a second surface (back surface). The conveyance process in the recording on the first surface is similar to that in  FIGS. 4A to 4C , so that description thereof is omitted. The conveyance process following  FIG. 4C  will be described below. 
     If the recording head  8  completes the recording operation on the first surface and the tail edge of the recording medium S is passed through the flapper  11 , the print controller  202  rotates the conveyance rollers  7  backward to convey the recording medium S to the inside of the recording apparatus  1 . At this time, the leading edge of the flapper  11  is controlled to incline leftward in  FIG. 6  by an actuator (not illustrated), so that the leading edge (which is the tail edge in the recording operation on the first surface) of the recording medium S is passed through the right-hand side of the flapper  11  and conveyed downward in the vertical direction.  FIG. 6A  illustrates the state in which the leading edge (i.e., the tail edge in the recording operation on the first surface) of the recording medium S is passed through the right-hand side of the flapper  11 . 
     Thereafter, the recording medium S is conveyed along the curved outer surface of the inner guide  19  and conveyed again to the recording region P between the recording head  8  and the platen  9 . At this time, the second surface of the recording medium S faces the discharge opening surface  8   a  of the recording head  8 .  FIG. 6B  illustrates the conveyance state immediately before the leading edge of the recording medium S reaches the recording region P for the recording operation on the second surface. 
     The conveyance route thereafter is similar to that in the recording on the first surface as illustrated in  FIGS. 4B and 4C .  FIG. 6C  illustrates the state in which the leading edge of the recording medium S is passed through the recording region P and conveyed upward in the vertical direction. At this time, the flapper  11  is controlled by the actuator (not illustrated) to move to a position such that the leading edge of the flapper  11  is inclined rightward.  FIG. 6D  illustrates the state in which the leading edge of the recording medium S is passed through the sheet ejection roller  12  and ejected onto the sheet ejection tray  13 . A similar conveyance process is performed in the two-sided recording on the A3-size recording mediums S. 
     Next, a maintenance operation on the recording head  8  will be described below. As described above with reference to  FIG. 1 , the maintenance unit  16  includes the cap unit  10  and the wiping unit  17  and causes the units to perform maintenance operations at predetermined timings. 
       FIG. 7  illustrates the recording apparatus  1  in the maintenance state. To move the recording head  8  from the standby position illustrated in  FIG. 1  to the maintenance position illustrated in  FIG. 7 , the print controller  202  first moves the recording head  8  diagonally upward in the vertical direction and moves the cap unit  10  downward in the vertical direction. Then, the print controller  202  moves the wiping unit  17  from the retracted position to the right direction as illustrated in  FIG. 7 . Thereafter, the print controller  202  moves the recording head  8  downward in the vertical direction to the maintenance position to enable a maintenance operation. 
     To move the recording head  8  from the recording position illustrated in  FIG. 3  to the maintenance position illustrated in  FIG. 7 , on the other hand, the print controller  202  first moves the recording head  8  upward in the vertical direction while rotating the recording head  8  by about 45 degrees. Then, the print controller  202  moves the wiping unit  17  from the retracted position to the right direction. Thereafter, the print controller  202  moves the recording head  8  downward in the vertical direction to the maintenance position to enable a maintenance operation by the maintenance unit  16 . 
       FIG. 8A  is a perspective view illustrating the maintenance unit  16  in the standby position, and  FIG. 8B  is a perspective view illustrating the maintenance unit  16  in the maintenance position.  FIG. 8A  corresponds to the position of the maintenance unit  16  illustrated in  FIG. 1 , and  FIG. 8B  corresponds to the position of the maintenance unit  16  illustrated in  FIG. 7 . When the recording head  8  is in the standby position, the maintenance unit  16  is in the standby position illustrated in  FIG. 8A , the cap unit  10  is in the capping position, and the wiping unit  17  is stored in the maintenance unit  16 . The cap unit  10  includes the cap member  10   a , which extends in the y-direction and is box-shaped, and the cap member  10   a  is closely appressed to the discharge opening surface  8   a  of the recording head  8  to prevent evaporation of ink from the discharge openings. Further, the cap unit  10  also has the function of collecting ink discharged to the cap member  10   a  by preliminary discharge or the like and causing a suction pump (not illustrated) to suction the collected ink. 
     On the other hand, in the maintenance position illustrated in  FIG. 8B , the cap unit  10  has been moved to the retracted position located downward in the vertical direction, and the wiping unit  17  is removed from the maintenance unit  16 . The wiping unit  17  includes two wiper units, i.e., a blade wiper unit  171  and a vacuum wiper unit  172 . 
     The blade wiper unit  171  includes a blade wiper  171   a  for wiping the discharge opening surface  8   a  along the x-direction. The blade wiper  171   a  is provided in the y-direction over the length corresponding to the region where the discharge openings are aligned. When a wiping operation is performed using the blade wiper unit  171 , the wiping unit  17  moves the blade wiper unit  171  in the x-direction in a state where the recording head  8  is positioned at the height at which the recording head  8  comes into contact with the blade wiper  171   a . By this movement, the inks on the discharge opening surface  8   a  are wiped by the blade wiper  171   a.    
     The opening of the maintenance unit  16  from which the blade wiper  171   a  is stored is provided with a wet wiper cleaner  16   a  for removing the inks on the blade wiper  171   a  and applying a wet liquid to the blade wiper  171   a . In this way, each time the blade wiper  171   a  is stored in the maintenance unit  16 , adhered substances on the blade wiper  171   a  is removed by the wet wiper cleaner  16   a  and the wet liquid is applied to the blade wiper  171   a  by the wet wiper cleaner  16   a . Then, when the discharge opening surface  8   a  is wiped thereafter, the wet liquid is transferred onto the discharge opening surface  8   a  to improve the slipperiness between the discharge opening surface  8   a  and the blade wiper  171   a.    
     Further, the vacuum wiper unit  172  includes a flat plate  172   a , a carriage  172   b , and a vacuum wiper  172   c . The flat plate  172   a  includes an opening portion extending in the y-direction. The carriage  172   b  is movable in the opening portion in the y-direction. The vacuum wiper  172   c  is mounted on the carriage  172   b . The vacuum wiper  172   c  is capable of wiping the discharge opening surface  8   a  in the y-direction along with the movement of the carriage  172   b . A suction opening connected with the suction pump (not illustrated) is formed in the leading edge of the vacuum wiper  172   c . Thus, if the carriage  172   b  is moved in the y-direction while the suction pump is operated, ink and other substances adhered on the discharge opening surface  8   a  of the recording head  8  are wiped by the vacuum wiper  172   c  and suctioned into a suction opening. At this time, positioning pins  172   d  provided to the respective end portions of the flat plate  172   a  and the opening portion are used to adjust the position of the discharge opening surface  8   a  with respect to the vacuum wiper  172   c.    
       FIG. 9  illustrates the ink supply unit  15  used in the recording apparatus  1 . The ink supply unit  15  is configured to supply ink from the ink tank unit  14  to the recording head  8 . While the configuration with respect to one ink color is described, a similar configuration is provided for each ink color. The ink supply unit  15  is controlled basically by the ink supply control unit  209  illustrated in  FIG. 2 . The configurations of the respective units will be described below. 
     The ink is circulated mainly between a sub-tank (ink tank)  151  and the recording head  8  (head unit  8  in  FIGS. 9 to 16 ). The head unit  8  performs an ink discharge operation based on image data, and the ink that is not discharged is collected again by the sub-tank  151 . 
     The sub-tank  151  which stores a predetermined amount of ink is connected with a supply channel C 2  for supplying the ink to the head unit  8  and a collection channel C 4  for collecting the ink from the head unit  8 . Specifically, the sub-tank  151 , the supply channel C 2 , the head unit  8 , and the collection channel C 4  form a circulation route through which the ink is circulated. 
     The sub-tank  151  is provided with a liquid surface detection member  151   a  including a plurality of pins, and the ink supply control unit  209  detects the presence/absence of conduction current between the plurality of pins to detect the height of the ink surface, i.e., the amount of remaining ink in the sub-tank  151 . The sub-tank  151  to which the ink is supplied includes an ink chamber and an air chamber. The ink chamber is located in the lower part of the sub-tank  151  where ink is accumulated. The air chamber is located in the upper part of the sub-tank  151  where no ink is accumulated. A vacuum pump P 0  is a negative pressure generation source for reducing the pressure in the sub-tank  151 . An atmospheric air communication valve V 0  is a valve to allow or not to allow the inside of the sub-tank  151  to communicate with the atmospheric air. 
     A main tank  141  is a tank which stores the ink to be supplied to the sub-tank  151 . The main tank  141  includes a flexible member, and the sub-tank  151  is filled with the ink according to change in the volume of the flexible member. The main tank  141  is attachable to and removable from the main body of the recording apparatus  1 . A tank connection channel C 1  connecting the sub-tank  151  and the main tank  141  to each other is provided with a tank supply valve V 1  for opening/closing the connection between the sub-tank  151  and the main tank  141 , and the tank supply valve V 1  is disposed between the sub-tank  151  and the main tank  141 . 
     With the above-described configuration, the ink supply control unit  209  closes the atmospheric air communication valve V 0 , a supply valve V 2 , a collection valve V 4 , and a head exchange valve V 5  if the liquid surface detection member  151   a  detects that the ink in the sub-tank  151  is less than a predetermined amount. In this state, the ink supply control unit  209  opens the tank supply valve V 1  and operates the vacuum pump P 0 . Consequently, the pressure inside of the sub-tank  151  becomes negative, and then the ink is supplied from the main tank  141  to the sub-tank  151 . If the liquid surface detection member  151   a  detects that the ink in the sub-tank  151  exceeds the predetermined amount, the ink supply control unit  209  closes the tank supply valve V 1  and stops the vacuum pump P 0 . 
     The supply channel C 2  is a channel for supplying the ink from the sub-tank  151  to the head unit  8 , and a first check valve V 6 , a supply pump P 1 , and the supply valve V 2  are disposed in this order from the side closer to the sub-tank  151  between the sub-tank  151  to the head unit  8 . The first check valve V 6  is a one-way valve which allows ink to flow only in one direction from the sub-tank  151  to the head unit  8  and regulates flow of ink in the opposite direction. The first check valve V 6  is opened or closed according to a difference in pressure between the upstream and downstream sides. Specifically, if a predetermined difference in pressure occurs between the upstream and downstream sides of the first check valve V 6 , the first check valve V 6  is opened to allow the supply of ink from the sub-tank  151  to the head unit  8 . The first check valve V 6  is a valve for preventing the ink in the supply channel C 2  from being drawn into the sub-tank  151  when the vacuum pump P 0  is driven to reduce the pressure in the sub-tank  151 . The first check valve V 6  also serves in the role of controlling the pressure in the ink flow channel using the pressure difference for opening and closing the valve. The reduction of pressure in the sub-tank  151  is performed not only in the above-described case of filling the sub-tank  151  with the ink from the main tank  141  but also in the case of performing a deaeration operation to deaerate the ink in the channel so that formation of bubbles is prevented. 
     The supply valve V 2  is a driving-type valve and is opened during a recording operation performed by the head unit  8 . The supply valve V 2  is closed to fill the part of the supply channel C 2  that is downstream of the supply valve V 2  and the inside of the head unit  8  with the ink by suction from the discharge openings in a state where the head unit  8  is capped with the cap unit  10 . Specifically, the supply valve V 2  and a buffer chamber shut-off valve V 8  described below are closed to form a closed space between the supply valve V 2  and the head unit  8 , and a negative pressure is charged into the closed space by suction. After the negative pressure is charged for a predetermined time, the supply valve V 2  is opened so that the inside of the head unit  8  is filled with the ink due to the charged large negative pressure (choke suction). Further, a similar suction operation is performed to defoam the ink in the channel between the supply valve V 2  and the buffer chamber shut-off valve V 8 . 
     During a recording operation, the supply pump P 1  is driven in a state where the supply valve V 2  is opened, so that the ink is circulated through the circulation route while the ink is supplied to the head unit  8 . The amount of ink consumed by the head unit  8  per unit time varies depending on image data. Thus, the flow rate of the supply pump P 1  is set such that the flow rate is also able to accommodate a case of performing a discharge operation in which the amount of ink consumption per unit time in the head unit  8  reaches the maximum amount. 
     A relief channel (detour route) C 3  is a channel with one end connected with a portion between the first check valve V 6  and the supply pump P 1  and the other end connected with a portion between the supply pump P 1  and the supply valve V 2 , and a relief valve V 3  which is a differential pressure valve is provided in the relief channel C 3 . If the amount of ink supply from the supply pump P 1  per unit time is greater than the total value of the amount of discharge of the head unit  8  per unit time and the flow rate of a collection pump P 2  per unit time, a pressure is applied to the relief valve V 3  to open the relief valve V 3 . Consequently, a cyclic channel is formed by a part of the supply channel C 2  and the relief channel C 3 . The above-described structure of the relief channel C 3  is provided so that the amount of ink supply to the head unit  8  is adjusted according to the amount of ink consumption in the head unit  8  to stabilize the fluid pressure in the circulation route regardless of image data. 
     The collection channel C 4  is a channel for collecting the ink from the head unit  8  to the sub-tank  151 , and a second check valve V 7 , the collection valve V 4 , the collection pump P 2 , a buffer chamber  85 , and the buffer chamber shut-off valve V 8  are disposed in the collection channel C 4  in this order from the side closer to the sub-tank  151 . To circulate the ink in the circulation route, the collection pump P 2  becomes a negative pressure generation source and suctions the ink from the head unit  8 . The collection pump P 2  is driven so that a suitable difference in pressure is generated between an IN-channel  80   b  and an OUT-channel  80   c  in the head unit  8 , and the ink flows from the IN-channel  80   b  to the OUT-channel  80   c . Details of the channel structure in the head unit  8  will be described below. 
     The second check valve V 7  is a one-way valve that allows the ink to flow only in one direction from the head unit  8  to the sub-tank  151  and regulates the flow of the ink in the opposite direction. The second check valve V 7  is, in the similar manner to the first check valve V 6 , opened and closed according to a difference in pressure between the upstream and downstream sides of the second check valve V 7 . Specifically, if a difference in pressure occurs between the upstream and downstream sides of the second check valve V 7 , the second check valve V 7  is opened to allow the ink to be collected from the head unit  8  into the sub-tank  151 . 
     The second check valve V 7  is a valve that prevents the ink from flowing from the sub-tank  151  into the head unit  8  through the collection channel C 4  when no recording operation is performed, i.e., when no ink is circulated in the circulation route. In the circulation route of the present example embodiment, the sub-tank  151  is disposed above the head unit  8  in the vertical direction (refer to  FIG. 1 ). Thus, when the supply pump P 1  and the collection pump P 2  are not driven, the ink can flow backward from the sub-tank  151  to the head unit  8  due to a water head difference between the sub-tank  151  and the head unit  8 . To prevent such a backward flow, the second check valve V 7  is provided to the collection channel C 4  in the present example embodiment. 
     The collection valve V 4  is a driving-type valve for preventing the ink in the collection channel C 4  from being caught into the sub-tank  151  when the vacuum pump P 0  is driven to reduce the pressure in the sub-tank  151 . 
     The buffer chamber  85  includes a spring, which is a biasing member, and a flexible member, and the volume of the buffer chamber  85  is changeable. The flexible member is biased by the spring in the direction in which the volume of the buffer chamber  85  expands. When bubbles are mixed in the ink in the head unit  8  and the collection channel C 4 , the buffer chamber  85  plays the role of absorbing changes in volume due to the expansion and contraction of the bubbles to maintain the negative pressure in the channel at a constant level (maintain the pressure within a suitable range). In this way, the meniscus in the head unit  8  is maintained to prevent the ink from leaking from the head unit  8  and prevent the air from being drawn into the discharge openings. 
     The buffer chamber shut-off valve V 8  is disposed between the head unit  8  and the buffer chamber  85 . The buffer chamber shut-off valve V 8  is closed during the choke suction described above to play the role of preventing the ink to be suctioned and ejected through the cap unit  10  from flowing into the collection channel C 4 . 
     Next, a head exchange channel (first channel) C 5  is a channel that connects the supply channel C 2  and an air chamber (space storing no ink) of the sub-tank  151  to each other, and the head exchange valve V 5  is disposed in the head exchange channel C 5 . One of the ends of the head exchange channel C 5  is connected with a portion of the supply channel C 2  that is upstream of the head unit  8 , and the other one of the ends is connected with an upper portion of the sub-tank  151  to communicate with the inside air chamber. The head exchange channel C 5  is used to collect the ink from the head unit  8  in use at the time of replacing the head unit  8 , transporting the recording apparatus  1 , and other operations. 
     The head exchange valve V 5  is controlled by the ink supply control unit  209  such that head exchange valve V 5  is closed during a period other than the time of initial filling of the recording apparatus  1  with the ink and the time of collecting the ink from the head unit  8 . The head exchange channel C 5  is connected with a portion of the supply channel C 2  that is downstream of the supply valve V 2 . Specifically, the supply valve V 2  described above is provided in the supply channel C 2  between the portion connected with the head exchange channel C 5  and the portion connected with the relief channel C 3 . 
     While the structure in which the first check valve V 6  is provided to the supply channel C 2  has been described in the present example embodiment, the first check valve V 6  does not have to be provided. When the vacuum pump P 0  is driven to reduce the pressure in the sub-tank  151 , the supply valve V 2  is closed to prevent the ink in the supply channel C 2  from being drawn into the sub-tank  151 . Similarly, a structure in which the second check valve V 7  is not provided can be employed. The collection valve V 4  is closed to prevent the ink from flowing backward from the sub-tank  151  to the head unit  8 . 
     Next, the channel structure in the head unit  8  will be described below. The ink supplied from the supply channel C 2  to the head unit  8  is passed through a filter  83  and thereafter supplied to a first negative pressure control unit (first pressure control unit)  81 , which generates a small negative pressure, and a second negative pressure control unit (second pressure control unit)  82 , which generates a large negative pressure. Hereinafter, the first negative pressure control unit  81  and the second negative pressure control unit  82  are also referred to collectively as “negative pressure control unit”. The absolute value of the pressure generated by the second negative pressure control unit  82  is smaller than the absolute value of the pressure generated by the first negative pressure control unit  81 . The pressures in the first negative pressure control unit  81  and the second negative pressure control unit  82  are generated by the driving of the collection pump P 2 . 
     An ink discharge portion  80  includes a plurality of recording element substrates  80   a  in which the plurality of discharge openings is aligned to form a long discharge opening array. Each of the recording element substrates  80   a  includes a recording element  2323  (refer to  FIG. 10B ) which is a heat generation element for causing the ink to foam with the heat energy. A common supply channel  80   b  (IN-channel) for guiding the ink supplied from the first negative pressure control unit  81  and a common collection channel  80   c  (OUT-channel) for guiding the ink supplied from the second negative pressure control unit  82  also extend along the direction in which the plurality of recording element substrates  80   a  is aligned. The common supply channel  80   b  is connected with the supply channel C 2 . The common collection channel  80   c  is connected with the supply channel C 2  and the collection channel C 4 . 
       FIG. 10A  is an enlarged schematic plan view illustrating a portion of the recording element substrates  80   a , and  FIG. 10B  is a schematic cross-sectional view along the cross-sectional taken along line X-X specified in  FIG. 10A . The recording element substrate  80   a  includes a pressure chamber  2402  to be filled with the ink and the discharge openings  2311  for discharging the ink. In the pressure chamber  2402 , the recording element  2323  is provided in a position facing the discharge opening  2311 . Further, the recording element substrate  80   a  includes a plurality of separate supply channels  2321  connected with the common supply channel  80   b  and a plurality of separate collection channels  2322  connected with the common collection channel  80   c.    
     The above-described structure generates the flow of the ink from the common supply channel  80   b  having a relatively small negative pressure (high pressure) into the common collection channel  80   c  having a relatively large negative pressure (low pressure) in the recording element substrates  80   a . More specifically, the ink flows through the common supply channel  80   b , the separate supply channel  2321 , the pressure chamber  2402 , the separate collection channel  2322 , and the common collection channel  80   c  in this order. If the ink is discharged by the recording element  2323 , some of the ink that moves from the common supply channel  80   b  to the common collection channel  80   c  is discharged from the discharge openings  2311  to be ejected therefrom, whereas the ink that is not discharged moves through the common collection channel  80   c  to the collection channel C 4 . Alternatively, the recording element substrate  80   a  may be configured not to include the common collection channel  80   c , the separate supply channel  2321  or the separate collection channel  2322 . In such a configuration, the common supply channel  80   b  is directly connected with the collection channel C 4  such that the ink is collected from the common supply channel  80   b.    
       FIGS. 11A to 11C  illustrate the first negative pressure control unit  81  provided in the head unit  8 .  FIGS. 11A and 11B  are external perspective views.  FIG. 11B  illustrates the inside of the first negative pressure control unit  81  without the illustration of a flexible film  232 .  FIG. 11C  illustrates the cross section taken along line XIVC-XIVC specified in  FIG. 11A . The first negative pressure control unit  81  and the second negative pressure control unit  82  are differential pressure valves having a similar structure except for the difference in control pressure (initial load of the spring), so that description of the second negative pressure control unit  82  is omitted. 
     In the first negative pressure control unit  81 , a first pressure chamber  233  is formed by a pressure reception plate  231  illustrated in  FIG. 11B  and a flexible film  232  sealing the space around the pressure reception plate  231 . The flexible film  232  is welded to the circular edge illustrated in  FIG. 11B  and the pressure reception plate  231 . The flexible film  232  and the pressure reception plate  231  welded to the flexible film  232  are displaced upward or downward according to an increase or decrease in the ink in the first pressure chamber  233 . 
     A second pressure chamber  238  connected with the supply pump P 1 , a shaft  234  coupled to the pressure reception plate  231 , a valve  235  coupled to the shaft  234 , and an orifice  236  being in contact with the valve  235  are provided on the upstream side of the first pressure chamber  233  in the direction in which the ink is supplied. The orifice  236  in the present example embodiment is formed in the boundary of the first pressure chamber  233  and the second pressure chamber  238 . Further, the valve  235 , the shaft  234 , and the pressure reception plate  231  are biased upward in the vertical direction by a biasing member (spring)  237 . 
     If the absolute value of the pressure in the first pressure chamber  233  is equal to or greater than a first threshold value (if the negative pressure is smaller than the first threshold value), the valve  235  is brought into contact with the orifice  236  by the biasing force of the biasing member  237  to block the connection between the first pressure chamber  233  and the second pressure chamber  238 . On the other hand, if the absolute value of the pressure in the first pressure chamber  233  is less than the first threshold value, i.e., if a larger negative pressure than the first threshold value is applied to the first pressure chamber  233 , the flexible film  232  is contracted and displaced downward. In this way, the pressure reception plate  231  and the valve  235  are displaced downward against the biasing by the biasing member  237 , and the valve  235  and the orifice  236  are separated to connect the first pressure chamber  233  and the second pressure chamber  238  with each other. This connection causes the ink supplied by the supply pump P 1  to flow into the first pressure chamber  233 . 
     The first negative pressure control unit  81  has the structure of the differential pressure valve described above to control the inflow pressure and outflow pressure to maintain the inflow pressure and outflow pressure constant. To generate a larger negative pressure than that generated by the first negative pressure control unit  81 , the second negative pressure control unit  82  employs the biasing member  237  having a greater biasing force than the first negative pressure control unit  81 . Specifically, in the second negative pressure control unit  82 , the valve is opened if the pressure becomes less than a second threshold value which has a smaller absolute value of the pressure than the first threshold value. Thus, if the collection pump P 2  is driven, first, the first negative pressure control unit  81  is changed from the closed state to the opened state, and then the second negative pressure control unit  82  is changed from the closed state to the opened state. 
     When a recording operation is performed with the ink supply unit  15  and the head unit  8  having the above-described structures, the ink supply control unit  209  closes the tank supply valve V 1  and the head exchange valve V 5  and opens the atmospheric air communication valve V 0 , the supply valve V 2 , and the collection valve V 4 . In this state, the ink supply control unit  209  drives the supply pump P 1  and the collection pump P 2 . As a result of driving the supply pump P 1  and the collection pump P 2 , there arises a predetermined pressure difference in the channel, so that the first check valve V 6  and the second check valve V 7  are opened to allow the ink to flow. In this way, the circulation route of the sub-tank  151 , the supply channel C 2 , the head unit  8 , the collection channel C 4 , and the sub-tank  151  in this order is established. If the amount of ink supply from the supply pump P 1  per unit time is greater than the total value of the amount of discharge of the head unit  8  per unit time and the flow rate of the collection pump P 2  per unit time, a predetermined pressure is applied to the relief valve V 3  to open the relief valve V 3 . Consequently, the ink flows from the supply channel C 2  into the relief channel C 3 . In this way, the excessively-supplied ink flows into the relief channel C 3  to adjust the flow rate of the ink that flows from the supply channel C 2  into the head unit  8 . 
       FIG. 12  illustrates a sequence of stopping an ink circulation after an end of a recording operation and changing the recording apparatus  1  to the waiting state which is a provisional pause state. Specifically, the waiting state is a state in which a recording operation can be started immediately after a recording instruction is received, and the pause state is a state in which the recording apparatus  1  has been unoperated for a long time. If a recording operation is completed, the print controller  202  moves the head unit  8  to the standby position using the maintenance control unit  210  and the head carriage control unit  208 . In other words, the head unit  8  is capped with the cap unit  10 . In the ink supply unit  15 , as illustrated in  FIG. 12 , the ink supply control unit  209  stops the supply pump P 1  and the collection pump P 2 . In this way, the flow of the ink from the sub-tank  151  into the head unit  8  is prevented. Further, the ink supply control unit  209  closes the atmospheric air communication valve V 0  to prevent evaporation of the ink in the sub-tank  151 . In the case of a structure without the first check valve V 6  and the second check valve V 7 , the ink supply control unit  209  closes the supply valve V 2  and the collection valve V 4  to prevent the flow of the ink from the sub-tank  151  into the head unit  8  due to a water head difference. 
     At this time, between the collection pump P 2  and the second check valve V 7 , the difference in pressure is decreased due to the stop of the collection pump P 2 , and the second check valve V 7  is changed to the closed state. On the upstream side of the collection pump P 2 , the buffer chamber  8  is biased by the spring in the direction where the volume of the buffer chamber  8  expands, so that the suction force for sucking the ink into the buffer chamber  8  acts thereon. At this time, on the downstream side of the buffer chamber  8 , the second check valve V 7  is in the closed state, so that the ink is not sucked by the buffer chamber  8 . Consequently, the ink is sucked by the buffer chamber  8  from the channel between the head unit  8  disposed on the upstream of the buffer chamber  8  and the sub-tank  151 . If the pressure in the ink flow channel is balanced by the suction of the ink by the buffer chamber  8 , the pressure difference is decreased, and the first check valve V 6  is closed. As a result, the change in volume of the buffer chamber  85  due to the suction of the ink ceases. In the present example embodiment, since the pressure at which the first check valve V 6  is closed is smaller than the first threshold value, at least the first negative pressure control unit  81 , out of the first negative pressure control unit  81  and the second negative pressure control unit  82 , is maintained in the opened state. In the configuration that does not include the first check valve V 6  and the second check valve V 7 , the supply valve V 2  and the collection valve V 4  are closed, so that the ink in the channel between the supply valve V 2  and the collection valve V 4  is sucked by the buffer chamber  8 . Also in this case, at least the first negative pressure control unit  81 , out of the first negative pressure control unit  81  and the second negative pressure control unit  82 , is maintained in the opened state because the pressure after the volume change in the buffer chamber  8  is smaller than the first threshold value. The negative pressure maintained by the biasing member of the buffer chamber  8  is larger than that in the pause state described below. Further, the negative pressure maintained by the buffer chamber is smaller than the first threshold value, at least the first negative pressure control unit  81  is maintained in the opened state. 
     As described above, the recording apparatus  1  is changed to the waiting state while maintaining a part of the channel at a pressure equal to or lower than a predetermined pressure and maintaining at least the first negative pressure control unit  81 , out of the first negative pressure control unit  81  and the second negative pressure control unit  82 , in the opened state. In this way, the next time the recording apparatus  1  receives image data, the time (first print-out time (FPOT)) from the point at which the state is changed from the waiting state to the recording state (circulation state) to the point at which the recording operation on the first recording medium is ended can be reduced. 
     This point will be described below. To perform a recording operation, a predetermined negative pressure needs to be applied to the first negative pressure control unit  81  and the second negative pressure control unit  82  to change the valve  235  illustrated in  FIG. 11C  from the closed state to the opened state. To open the valve  235  of the second negative pressure control unit  82 , the pressure in the first pressure chamber  233  needs to be smaller than the second threshold value, and it takes a predetermined time for the pressure to become smaller than the second threshold value after the driving of the collection pump P 2  is started. However, use of the negative pressure due to a biasing member of the buffer chamber  85  as in the present example embodiment maintains a constant negative pressure, so that the time required to displace the valve  235  after the driving of the collection pump P 2  is started can be reduced. This reduces the time of driving the collection pump P 2  for a recording operation, so that the throughput until a start of recording improves. 
       FIG. 13  illustrates a sequence during a change from the waiting state to the recording state. If image data is received, the ink supply control unit  209  drives the supply pump P 1  and the collection pump P 2 , and the atmospheric air communication valve V 0  is opened. As a result of driving the supply pump P 1  and the collection pump P 2 , the first check valve V 6  and the second check valve V 7  are also opened such that the circulation route is re-established. Specifically, since the first negative pressure control unit  81  and the channel on the downstream side of the second negative pressure control unit  82  is maintained at a pressure equal to or lower than the predetermined pressure, if the supply pump P 1  and the collection pump P 2  are driven, the circulation route is immediately established such that a recording operation is executable. As described above, when the recording apparatus  1  is changed from the waiting state to the recording state, the head unit  8  can start a recording operation about 2 to 2.5 seconds after the start of the circulation. 
       FIG. 14  is a flowchart illustrating a process of changing the recording apparatus  1  from the waiting state to the pause state. If the recording apparatus  1  has been in the waiting state for a long time in the state where a slight amount of air bubbles is included in the head unit  8  and the channel, the air bubbles may expand and contract due to a change in ambient temperature. For example, the contraction of the buffer chamber  85  is developed when the ambient temperature decreases, and the negative pressure in the collection channel C 4  can increase. Consequently, the air is easily drawn through the discharge openings  2311  into the channel. The buffer chamber  85  serves in the role of accommodating such an expansion and contraction of air bubbles. In the present example embodiment, the control is performed to change to the pause state if a predetermined time passes after the recording apparatus  1  is changed to the waiting state, and this prevents air from entering the channel even if the recording apparatus  1  has been unoperated for a long time. Also, in a case where the recording apparatus  1  is turned off by a user or detects an error while in the waiting state, the recording apparatus  1  is changed to the pause state. Further, in a case where the recording apparatus  1  is turned off or detects an error while in the recording state (circulation state), control may be performed such that the recording apparatus  1  is immediately changed to the pause state without passing through the waiting state. The positions of the head unit  8  and the maintenance unit  16 , while the recording apparatus  1  is in the pause state, are similar to those in the waiting state illustrated in  FIG. 1 . 
     First, in step S 1401 , the ink supply control unit  209  opens the atmospheric air communication valve V 0 . In step S 1402 , the supply pump P 1  is driven for a preset time (predetermined time). As a result of driving the supply pump P 1 , the first check valve V 6  is opened, and the ink is pressure-supplied to the head unit  8 . Since at least the first negative pressure control unit  81  is opened, the ink is supplied to the downstream side of the negative pressure control unit as a result of driving the supply pump P 1 . In this way, the volume of the buffer chamber  85  becomes larger and the pressure on the downstream side of the negative pressure control unit increases to weaken the negative pressure. The predetermined time for which the supply pump P 1  is driven in step S 1402  is set to a sufficient time for the volume of the buffer chamber  85  to increase such that the buffer chamber  85  can accommodate the expansion and contraction of bubbles in the channel that are caused by a change in ambient temperature. If the predetermined time passes, then in step S 1403 , the ink supply control unit  209  stops the supply pump P 1 . 
     In step S 1404 , the ink supply control unit  209  opens the head exchange valve V 5 . This enables the pressure increased by the pressure supply by the supply pump P 1  to escape through the head exchange channel C 5  into the sub-tank  151  which is in communication with the atmospheric air. Since the head exchange channel C 5  is filled with the ink, the ink in the head exchange channel C 5  is pushed into the sub-tank  151  by the increased pressure. As a result of the foregoing pressure adjustment, the pressures in the supply channel C 2  and the head exchange channel C 5  become substantially equal to the pressure of the atmospheric air. 
     Thereafter, in step S 1405 , the ink supply control unit  209  closes the head exchange valve V 5 , and in step S 1406 , the ink supply control unit  209  closes the atmospheric air communication valve V 0 . Further, in step S 1407 , the ink supply control unit  209  closes the supply valve V 2  and the collection valve V 4 . The supply valve V 2  and the collection valve V 4  each include a diaphragm valve and are biased in the closing direction by a biasing member such as a spring. When the recording apparatus  1  is to be changed to the pause state, the supply valve V 2  and the collection valve V 4  are closed to prepare for the situation in which the recording apparatus  1  is unoperated for a long time, and this reduces the burden on the biasing member and the diaphragm. By the above-described control, the process of changing the recording apparatus  1  from the waiting state to the pause state is completed. 
     While the structure including the supply valve V 2  and the collection valve V 4  is described in the present example embodiment, the supply valve V 2  and the collection valve V 4  are not necessarily be included when the first check valve V 6  and the second check valve V 7  are provided. That is, only at least one of the first check valve V 6  and the supply valve V 2  and at least one of the second check valve V 7  and the collection valve V 4  need to be included to implement an example embodiment of the disclosure. While the example in which the predetermined time during the change from the waiting state to the pause state is an arbitrarily-set fixed value is described in the present example embodiment, the disclosure is not limited to the example. The recording apparatus  1  can include a sensor capable of measuring an environmental change (e.g., temperature change), and the predetermined time can be changed if air bubbles are likely to expand or contract according to measurement results. 
       FIG. 15  illustrates a sequence of changing the recording apparatus  1  from the pause state to the recording state. The ink supply control unit  209  drives the supply pump P 1  and the collection pump P 2  and opens the atmospheric air communication valve V 0 . Furthermore, the ink supply control unit  209  opens the supply valve V 2  and the collection valve V 4 . In the case in which the recording apparatus  1  is changed from the pause state to the recording state, it takes time for a pressure difference to occur between the first negative pressure control unit  81  and the second negative pressure control unit  82 , so that the head unit  8  starts a recording operation about 20 seconds after the start of the circulation. 
     As described above, if a recording operation is ended, the recording apparatus  1  is first changed from the recording state to the waiting state which is a provisional pause state. This reduces the time needed to start the next recording operation so that the throughput of the recording operation improves. Further, in the case in which the recording apparatus  1  has been left in the waiting state for a long time, the recording apparatus  1  is changed to the pause state after the pressure in the channel is adjusted and the volume of the buffer chamber  85  is changed. This prevents air from being drawn and the ink from leaking through the discharge openings  2311 . As described above, the recording apparatus  1  is provided which includes the waiting state where the head unit  8  is capped with the cap unit  10  as illustrated in  FIG. 1  and the pause state. The recording apparatus  1  can reduce FPOT while adjusting the pressure in the channel by changing between the waiting state and the pause state based on a certain condition such as an elapsed time. 
     A second example embodiment will be described below. In the second example embodiment, a method of controlling the driving amount of the collection pump P 2  at the time of changing from the waiting state or the pause state to the recording state, in addition to the control in the first example embodiment, will be described below. Specifically, when a circulation of the ink is started to change to the recording state, the driving amount of the collection pump P 2  is set greater than that in the recording state. The structure of the recording apparatus  1  is similar to that in the first example embodiment, so that description thereof is omitted. 
       FIG. 16  is a flowchart illustrating the control of the collection pump P 2  at the time of changing to the recording state. The control other than the control of the collection pump P 2  at the time of changing to the recording state is similar to those in  FIGS. 13 and 15 . 
     In step S 161 , the ink supply control unit  209  starts driving the collection pump P 2  in a driving amount required to realize a second flow rate which is higher than a normal flow rate (first flow rate) of ink circulation. In step S 162 , the recording apparatus  1  waits until the predetermined time elapses. If the predetermined time elapses (YES in step S 162 ), the processing proceeds to step S 163 . 
     In step S 163 , the ink supply control unit  209  changes the driving amount of the collection pump P 2  to realize the first flow rate. In step S 164 , the print controller  202  controls the head carriage control unit  208  to execute a recording operation. 
     As described above, in the present example embodiment, the control is performed for the predetermined time at the time of the start of ink circulation such that the flow rate of the collection pump P 2  is higher than that in normal ink circulation. This enables a larger negative pressure than a normal negative pressure to be applied to the negative pressure control unit, so that the time taken to generate a pressure difference between the first negative pressure control unit  81  and the second negative pressure control unit  82  can be reduced, thereby further reducing the FPOT. The control is effective especially in the case where the recording apparatus  1  is changed from the pause state to the recording state. 
     Specifically, the disclosure provides an inkjet recording apparatus capable of improving the throughput until a start of a recording operation in the structure in which ink is circulated between a recording head and an ink tank. 
     While the disclosure has been described with reference to example embodiments, it is to be understood that the invention is not limited to the disclosed example embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.