Patent Publication Number: US-9421780-B2

Title: Ink circulation type inkjet printer

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2014-198713, filed on Sep. 29, 2014, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     The disclosure relates to an ink circulation type inkjet printer. 
     2. Related Art 
     Japanese Unexamined Patent Application Publication No. 2008-162262 describes an ink circulation type inkjet printer that performs printing by ejecting ink from an inkjet head while circulating the ink. 
     In some ink circulation type inkjet printers, ink is circulated by an air pump applying a positive pressure and a negative pressure to a positive pressure tank and a negative pressure tank, respectively, the positive pressure tank being disposed upstream of the inkjet head, the negative pressure tank being disposed downstream of the inkjet head. 
     When such an inkjet printer performs printing, the air pump applies a positive pressure and a negative pressure respectively to the positive pressure tank and the negative pressure tank, both of which are set in a sealed state Thus, the ink flows from the positive pressure tank to the inkjet head. The ink, which is not consumed by the inkjet head, is collected by the negative pressure tank. Also, the ink is delivered from the negative pressure tank to the positive pressure tank by an ink pump according to the liquid level heights in the positive pressure tank and the negative pressure tank. In this manner, the ink is circulated. 
     During the circulation of ink, the pressures in the ink tanks (the positive pressure tank, the negative pressure tank) are regulated in order to ensure a circulation flow rate (flow volume) and to maintain a nozzle pressure of the inkjet head at an appropriate value. The pressures are regulated, for instance, by driving the air pump and opening/closing a pressure regulation valve. 
     SUMMARY 
     In the above-described inkjet printer, variations in the pressures of the positive pressure tank and the negative pressure tank may increase due to an concurrence of a pressure regulation operation by the air pump or the like and any pressure variation factor other than the pressure regulation operation. The pressure variation factors other than the pressure regulation operation include, for instance, an ink delivery operation by an ink pump. When ink is delivered from the negative pressure tank to the positive pressure tank by an ink delivery operation, the liquid levels of the positive pressure tank and the negative pressure tank vary, and the pressure varies accordingly. 
     When a variation in the liquid levels of the positive pressure tank and the negative pressure tank increases, a variation in the nozzle pressure increases. Consequently, ejection of ink becomes unstable and printed image quality may be degraded. 
     The disclosure aims to provide an inkjet printer that can reduce degradation of printed image quality. 
     An inkjet printer in accordance with some embodiments includes: an inkjet head including a nozzle configured to eject ink; a positive pressure tank configured to store the ink to be supplied to the inkjet head; a negative pressure tank configured to receive the ink not consumed by the inkjet head; an ink deliverer configured to deliver the ink from the negative pressure tank to the positive pressure tank; a pressure regulator configured to regulate pressures of the positive pressure tank and the negative pressure tank; and a controller configured to control the ink deliverer and the pressure regulator such that an ink delivery operation by the ink deliverer and a pressure regulation operation by the pressure regulator are performed separately in time. 
     With the configuration described above, the ink delivery operation and the pressure regulation operation are performed separately in time, and thus increase in the nozzle pressure variation can be suppressed. Consequently, degradation of printed image quality can be reduced. 
     The inkjet printer may further include an ink supplier configured to supply the ink to the negative pressure tank. The controller may be configured to control the ink supplier and the pressure regulator such that an ink supply operation by the ink supplier and the pressure regulation operation by the pressure regulator are performed separately in time. 
     With the configuration described above, the ink supply operation and the pressure regulation operation are performed separately in time, and thus increase in the nozzle pressure variation can be further suppressed. Consequently, degradation of printed image quality can be further reduced. 
     An inkjet printer in accordance with some embodiments includes: an inkjet head including a nozzle configured to eject ink; a positive pressure tank configured to store the ink to be supplied to the inkjet head; a negative pressure tank configured to receive the ink not consumed by the inkjet head; an ink deliverer configured to deliver the ink from the negative pressure tank to the positive pressure tank; an ink supplier configured to supply the ink to the negative pressure tank; a pressure regulator configured to regulate pressures of the positive pressure tank and the negative pressure tank; and a controller configured to control the ink supplier and the pressure regulator such that an ink supply operation by the ink supplier and a pressure regulation operation by the pressure regulator are performed separately in time. 
     With the configuration described above, the ink supply operation and the pressure regulation operation are performed separately in time, and thus increase in the nozzle pressure variation can be suppressed. Consequently, degradation of printed image quality can be reduced. 
     The controller may be configured to control the ink supplier and the ink deliverer such that the ink supply operation by the ink supplier and the ink delivery operation by the ink deliverer are performed separately in time. 
     With the configuration described above, the ink delivery operation and the ink supply operation are performed separately in time, and thus increase in the nozzle pressure variation can be further suppressed. Consequently, degradation of printed image quality can be further reduced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic configuration diagram of an inkjet printer according to an embodiment. 
         FIG. 2  is a flow chart for explaining the operation of the inkjet printer illustrated in  FIG. 1 . 
         FIG. 3  is an explanatory diagram of pressure control. 
         FIG. 4  is an explanatory diagram of liquid level control. 
         FIG. 5  is an explanatory diagram of pressure liquid level control. 
         FIG. 6  is an explanatory diagram of the pressure liquid level control. 
         FIG. 7  is a flow chart for explaining a first separative operation. 
         FIG. 8  is a flow chart for explaining a second separative operation. 
         FIG. 9  is a flow chart for explaining a third separative operation. 
         FIG. 10  is a flow chart for explaining a fourth separative operation. 
         FIG. 11  is a flow chart for explaining a fifth separative operation. 
         FIG. 12  is a flow chart for explaining a sixth separative operation. 
         FIG. 13  is a flow chart for explaining a seventh separative operation. 
         FIG. 14  is a flow chart for explaining an eighth separative operation. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawing. 
     Description will be hereinbelow provided for an embodiment of the present invention by referring to the drawings. It should be noted that the same or similar parts and components throughout the drawings will be denoted by the same or similar reference signs, and that descriptions for such parts and components will be omitted or simplified. In addition, it should be noted that the drawings are schematic and therefore different from the actual ones. 
       FIG. 1  is a schematic configuration diagram of an inkjet printer according to an embodiment of the present disclosure. It is to be noted that the upward and downward directions in the following description indicate the vertical direction and in  FIG. 1 , UP indicates the upward direction and DN indicates the downward direction. 
     As illustrated in  FIG. 1 , an inkjet printer  1  according to the present embodiment includes an inkjet head  2 , an ink circulation unit  3 , an ink supply unit  4 , and a controller  5 . 
     The inkjet head  2  ejects ink which is supplied by the ink circulation unit  3 . The inkjet head  2  includes plural head modules  7 . 
     The head modules  7  each have an ink chamber (not illustrated) that stores ink and plural nozzles (not illustrated) through which ink is ejected. A piezoelectric element (not illustrated) is disposed in the ink chamber. Ink is ejected through a nozzle by driving the piezoelectric element. 
     The ink circulation unit  3  supplies ink to the inkjet head  2  while circulating ink. The ink circulation unit  3  includes a positive pressure tank  11 , a positive pressure tank air opening valve  12 , a positive pressure tank air opening pipe  13 , a positive pressure tank pressure regulation valve  14 , a positive pressure tank pressure regulation pipe  15 , a positive-pressure-side pressure sensor  16 , an ink distributor  17 , an ink collector  18 , a negative pressure tank  19 , a negative pressure tank air opening valve  20 , a negative pressure tank air opening pipe  21 , a negative pressure tank pressure regulation valve  22 , a negative pressure tank pressure regulation pipe  23 , a negative-pressure-side pressure sensor  24 , an ink pump  25 , an air pump  26 , a pipe  27  for air pump, and ink circulation pipes  28  to  30 . 
     The positive pressure tank  11  stores ink to be supplied to the inkjet head  2 . The ink in the positive pressure tank  11  is supplied to the inkjet head  2  through the ink circulation pipe  28  and the ink distributor  17 . An air layer is formed on the liquid level of the ink in the positive pressure tank  11 . The positive pressure tank  11  is disposed at a position lower than (under) the inkjet head  2 . 
     The positive pressure tank  11  is provided with a positive pressure tank liquid level sensor  36 . The positive pressure tank liquid level sensor  36  is for determining whether or not the liquid level height of the ink in the positive pressure tank  11  has reached a reference height. When the liquid level height of the ink in the positive pressure tank  11  is greater than or equal to a reference height, the positive pressure tank liquid level sensor  36  outputs a signal that indicates “ON” and when the liquid level height of the ink is less than the reference height, the positive pressure tank liquid level sensor  36  outputs a signal that indicates “OFF”. 
     The positive pressure tank air opening valve  12  opens and closes a flow path of the air in the positive pressure tank air opening pipe  13  for switching between a sealed state (sealed state from the atmosphere) and an air open state (open state to the atmosphere) of the positive pressure tank  11 . The positive pressure tank air opening valve  12  is disposed midway along the positive pressure tank air opening pipe  13 . 
     The positive pressure tank air opening pipe  13  forms a flow path of air for opening the positive pressure tank  11  to the atmosphere. The positive pressure tank air opening pipe  13  has one end connected to the air layer of the positive pressure tank  11  and the other end communicating with the atmosphere. 
     The positive pressure tank pressure regulation valve  14  (part of the pressure regulator) opens and closes the flow path of the air in the positive pressure tank pressure regulation pipe  15  in order to regulate the pressure in the positive pressure tank  11 . The positive pressure tank pressure regulation valve  14  is provided midway along the positive pressure tank pressure regulation pipe  15 . 
     The positive pressure tank pressure regulation pipe  15  forms a flow path of air for regulating the pressure in the positive pressure tank  11 . The positive pressure tank pressure regulation pipe  15  is formed of a pipe having a higher flow path resistance than that of the positive pressure tank air opening pipe  13 . Specifically, the positive pressure tank pressure regulation pipe  15  is formed of a pipe narrower than the positive pressure tank air opening pipe  13 . The positive pressure tank pressure regulation pipe  15  has one end connected to the air layer of the positive pressure tank  11  and the other end communicating with the atmosphere. 
     The positive-pressure-side pressure sensor  16  detects the pressure in the positive pressure tank  11 . 
     The ink distributor  17  distributes the ink supplied from the pressure tank  11  to each head module  7  of the inkjet head  2  through the ink circulation pipe  28 . 
     The ink collector  18  collects from each head module  7  the ink that has not been consumed by the inkjet head  2 . The ink collected by the ink collector  18  flows to the negative pressure tank  19  through the ink circulation pipe  29 . 
     The negative pressure tank  19  receives and stores the ink that has not been consumed by the inkjet head  2 , from the ink collector  18 . In addition, the negative pressure tank  19  stores the ink that is supplied from an ink cartridge  41  of the later-described ink supply unit  4 . An air layer is formed on the liquid level of the ink in the negative pressure tank  19 . The negative pressure tank  19  is disposed at the same height as the positive pressure tank  11 . 
     The negative pressure tank  19  is provided with a negative pressure tank liquid level sensor  37 . The negative pressure tank liquid level sensor  37  is for determining whether or not the liquid level height of the ink in the negative pressure tank  19  has reached a reference height. When the liquid level height of the ink in the negative pressure tank  19  is greater than or equal to a reference height, the negative pressure tank liquid level sensor  37  outputs a signal that indicates “ON” and when the liquid level height of the ink is less than the reference height, the negative pressure tank liquid level sensor  37  outputs a signal that indicates “OFF”. 
     The negative pressure tank air opening valve  20  opens and closes a flow path of the air in the negative pressure tank air opening pipe  21  for switching between a sealed state and an air open state of the negative pressure tank  19 . The negative pressure tank air opening valve  20  is provided midway along the negative pressure tank air opening pipe  21 . 
     The negative pressure tank air opening pipe  21  forms a flow path of air for opening the negative pressure tank  19  to the atmosphere. The negative pressure tank air opening pipe  21  has one end connected to the air layer of the negative pressure tank  19  and the other end communicating with the atmosphere. 
     The negative pressure tank pressure regulation valve  22  (part of the pressure regulator) opens and closes the flow path of the air in the negative pressure tank pressure regulation pipe  23  in order to regulate the pressure in the negative pressure tank  19 . The negative pressure tank pressure regulation valve  22  is provided midway along the negative pressure tank pressure regulation pipe  23 . 
     The negative pressure tank pressure regulation pipe  23  forms a flow path of air for regulating the pressure in the negative pressure tank  19 . The negative pressure tank pressure regulation pipe  23  is formed of a pipe having a higher flow path resistance than that of the negative pressure tank air opening pipe  21 . Specifically, the negative pressure tank pressure regulation pipe  23  is formed of a pipe which is narrower than the negative pressure tank air opening pipe  21  and has approximately the same thickness as the positive pressure tank pressure regulation pipe  15 . The negative pressure tank pressure regulation pipe  23  has one end connected to the air layer of the negative pressure tank  19  and the other end communicating with the atmosphere. 
     The negative-pressure-side pressure sensor  24  detects the pressure in the negative pressure tank  19 . 
     The ink pump  25  (ink deliverer) delivers ink from the negative pressure tank  19  to the positive pressure tank  11 . The ink pump  25  is provided midway along the ink circulation pipe  30 . 
     The air pump  26  (part of the pressure regulator) delivers air from the negative pressure tank  19  to the positive pressure tank  11 , thereby generating pressures for ink circulation in the positive pressure tank  11  and the negative pressure tank  19 . In addition, the air pump  26  is used for pressure regulation to maintain the pressures of the positive pressure tank  11  and the negative pressure tank  19  at nearly a constant level during ink circulation. The air pump  26  is disposed midway along the pipe  27  for air pump. 
     The pipe  27  for air pump forms a flow path of the air that is delivered from the negative pressure tank  19  to the positive pressure tank  11  by the air pump  26 . The pipe  27  for air pump has one end connected to the air layer of the negative pressure tank  19  and the other end connected to the air layer of the positive pressure tank  11 . 
     The ink circulation pipe  28  connects the positive pressure tank  11  and the ink distributor  17 . In the ink circulation pipe  28 , ink flows from the positive pressure tank  11  to the ink distributor  17 . The ink circulation pipe  29  connects the ink collector  18  and the negative pressure tank  19 . In the ink circulation pipe  29 , ink flows from the ink collector  18  to the negative pressure tank  19 . The ink circulation pipe  30  connects the negative pressure tank  19  and the positive pressure tank  11 . In the ink circulation pipe  30 , ink flows from the negative pressure tank  19  to the positive pressure tank  11 . The ink circulation pipes  28  to  30 , the ink distributor  17 , and the ink collector  18  constitute a circulation path for circulating ink between the positive pressure tank  11 , the inkjet head  2 , and the negative pressure tank  19 . 
     The ink supply unit  4  supplies ink to the negative pressure tank  19  of the ink circulation unit  3 . The ink supply unit  4  includes an ink cartridge  41 , an ink supply valve  42 , and an ink supply pipe  43 . 
     The ink cartridge  41  stores ink to be used in printing by the inkjet head  2 . The ink in the ink cartridge  41  is supplied to the negative pressure tank  19  of the ink circulation unit  3  through the ink supply pipe  43 . 
     The ink supply valve  42  opens and closes the flow path of ink in the ink supply pipe  43 . When ink is supplied to the negative pressure tank  19 , the ink supply valve  42  is opened. 
     The ink supply pipe  43  connects the ink cartridge  41  and the negative pressure tank  19 . In the ink supply pipe  43 , ink flows from the ink cartridge  41  to the negative pressure tank  19 . 
     The controller  5  controls the operation of each component of the inkjet printer  1 . The controller  5  includes a storage unit such as a CPU, a RAM, a ROM, and a hard disk. The controller  5  achieves the control (function) described below by executing a desirable program that is stored in the storage unit to be used in the present device. 
     The controller  5  causes the inkjet head  2  to eject ink and performs printing while circulating ink in the ink circulation unit  3 . During the circulation of ink, the controller  5  performs control such that the ink delivery operation by the ink pump  25 , the ink supply operation by the ink supply unit  4 , and the pressure regulation operation by the air pump  26 , the positive pressure tank pressure regulation valve  14 , and the negative pressure tank pressure regulation valve  22  are performed separately in time. 
     Next, the operation of the inkjet printer  1  will be described. 
       FIG. 2  is a flow chart for explaining the operation of the inkjet printer  1 . The processing of the flow chart of  FIG. 2  starts when a print job is inputted to the inkjet printer  1 . 
     In step S 1  of  FIG. 2 , the controller  5  closes the positive pressure tank air opening valve  12  and the negative pressure tank air opening valve  20 . Thus, the positive pressure tank  11  and the negative pressure tank  19  each assumes a sealed state. It is to be noted that during standby in which the inkjet printer  1  is not in operation, the positive pressure tank air opening valve  12  and the negative pressure tank air opening valve  20  are open. The positive pressure tank pressure regulation valve  14  and the negative pressure tank pressure regulation valve  22  are closed during standby. 
     Subsequently, in step S 2 , the controller  5  starts pressure liquid level control. The pressure liquid level control is for generating respective setting pressures Pks, Pfs for the positive pressure tank  11  and the negative pressure tank  19  and circulating ink while maintaining the setting pressures. The details of the pressure liquid level control will be described later. 
     The setting pressures Pks, Pfs are predetermined values of the pressure that sets the nozzle pressure of the inkjet head  2  to an appropriate value while causing circulation of ink. The setting pressure Pks for the positive pressure tank  11  is a positive pressure and the setting pressure Pfs for the negative pressure tank  19  is a negative pressure. 
     Subsequently, in step S 3 , the controller  5  determines whether or not the setting pressures Pks, Pfs have been generated in the positive pressure tank  11  and the negative pressure tank  19 . When it is determined that the setting pressures Pks, Pfs have not been generated in the positive pressure tank  11  and the negative pressure tank  19  (NO in step S 3 ), the controller  5  repeats step S 3 , 
     When it is determined that the setting pressures Pks, Pfs have been generated in the positive pressure tank  11  and the negative pressure tank  19  (YES in step S 3 ), the controller  5  starts to execute a print job. Specifically, the controller  5  ejects ink from the inkjet head  2  and prints an image on a sheet of paper based on the print job. 
     Subsequently, in step S 5 , the controller  5  determines whether or not the print job is completed. When it is determined that the print job is not completed (NO in step S 5 ), the controller  5  repeats step S 5 . 
     When it is determined that the print job is completed (YES in step S 5 ), the controller  5  terminates the pressure liquid level control in step S 6 . 
     Subsequently, in step S 7 , after the print job is completed, the controller  5  opens the positive pressure tank air opening valve  12  and the negative pressure tank air opening valve  20 . Thus, a series of operations is completed and the inkjet printer  1  is set in a standby state. 
     Next, the pressure liquid level control started in the above-described step S 2  of  FIG. 2  will be described. The pressure liquid level control combines pressure control and liquid level control. First, the pressure control and the liquid level control will be separately described. 
     The pressure control is control that causes the air pump  26 , the positive pressure tank pressure regulation valve  14 , and the negative pressure tank pressure regulation valve  22  to perform a pressure regulation operation according to a pressure condition. The pressure condition includes a combination of a magnitude relationship between value Pk detected by the positive-pressure-side pressure sensor  16  and the setting pressure Pks of the positive pressure tank  11 , and a magnitude relationship between value Pf detected by the negative-pressure-side pressure sensor  24  and the setting pressure Pfs of the negative pressure tank  19 . 
     Specifically, as illustrated in  FIG. 3 , when Pk&lt;Pks and lPfl&lt;lPfsl, the positive pressure tank pressure regulation valve  14  and the negative pressure tank pressure regulation valve  22  are closed and the air pump  26  is driven. Thus, air is delivered by the air pump  26  from the negative pressure tank  19  to the positive pressure tank  11  both in a sealed state, and thereby the pressure of the negative pressure tank  19  is decreased and the pressure of the positive pressure tank  11  is increased. 
     When Pk≧Pks and lPfl&lt;lPfsl, the positive pressure tank pressure regulation valve  14  is opened and the negative pressure tank pressure regulation valve  22  is closed. The air pump  26  is driven. Thus, air flows out from the positive pressure tank  11  through the positive pressure tank pressure regulation pipe  15 , and thereby the pressure of the positive pressure tank  11  is decreased. Also, air is sucked by the air pump  26  from the negative pressure tank  19  in a sealed state, and thereby the pressure of the negative pressure tank  19  is decreased. 
     When Pk&lt;Pks and lPfl≧lPfsl, the positive pressure tank pressure regulation valve  14  is closed and the negative pressure tank pressure regulation valve  22  is opened. The air pump  26  is driven. Thus, air is delivered by the air pump  26  to the positive pressure tank  11  in a sealed state, and thereby the pressure of the positive pressure tank  11  is increased. Also, air flows into the negative pressure tank  19  through the negative pressure tank pressure regulation pipe  23 , and thereby the pressure of the negative pressure tank  19  is increased. 
     When Pk≧Pks and lPfl≧lPfsl, the positive pressure tank pressure regulation valve  14  and the negative pressure tank pressure regulation valve  22  are opened, and the air pump  26  is stopped. Thus, air flows out from the positive pressure tank  11  through the positive pressure tank pressure regulation pipe  15 , and thereby the pressure of the positive pressure tank  11  is decreased. Also, air flows into the negative pressure tank  19  through the negative pressure tank pressure regulation pipe  23 , and thereby the pressure of the negative pressure tank  19  is increased. 
     Next, the liquid level control will be described. The liquid level control is control that causes the ink supply unit  4  and the ink pump  25  to perform an ink supply operation and liquid delivery operation, respectively according to a liquid level condition in order to maintain the liquid level heights of the positive pressure tank  11  and the negative pressure tank  19 . The liquid level condition includes a combination of ON/OFF state indicated by the positive pressure tank liquid level sensor  36  and ON/OFF state indicated by the negative pressure tank liquid level sensor  37 . 
     Specifically, as illustrated in  FIG. 4 , when the positive pressure tank liquid level sensor  36  indicates OFF and the negative pressure tank liquid level sensor  37  indicates OFF, the ink supply valve  42  of the ink supply unit  4  is opened. Thus, ink is supplied to the negative pressure tank  19 . The ink pump  25  is stopped and ink is not delivered from the negative pressure tank  19  to the positive pressure tank  11 . 
     When the positive pressure tank liquid level sensor  36  indicates ON and the negative pressure tank liquid level sensor  37  indicates OFF, the ink supply valve  42  is closed and the ink pump  25  is stopped. The same operation is also performed when the positive pressure tank liquid level sensor  36  indicates ON and the negative pressure tank liquid level sensor  37  indicates ON. In these cases, ink is not supplied to the negative pressure tank  19  and ink is not delivered to the positive pressure tank  11 . 
     When the positive pressure tank liquid level sensor  36  indicates OFF and the negative pressure tank liquid level sensor  37  indicates ON, the ink pump  25  is driven. Thus, ink is delivered from the negative pressure tank  19  to the positive pressure tank  11 . The ink supply valve  42  is closed and the ink is not supplied to the negative pressure tank  19 . 
     By such liquid level control, the ink delivery operation by the ink pump  25  and the ink supply operation by the ink supply unit  4  are performed separately in time. 
     Next, the pressure liquid level control will be described. As described above, the pressure liquid level control is formed of a combination of the pressure control and the liquid level control. That is, the pressure liquid level control is for controlling the operation of control elements, that is, the air pump  26 , the positive pressure tank pressure regulation valve  14 , the negative pressure tank pressure regulation valve  22 , the ink supply valve  42 , and the ink pump  25 , according to a combination of the pressure condition and the liquid level condition. 
     There are provided 16 combinations of the pressure condition and the liquid level condition, which are denoted by respective combinatorial condition numbers K- 1  to K- 16  in  FIGS. 5 and 6 . As illustrated in  FIGS. 5 and 6 , in the pressure liquid level control, the control elements according to the pressure condition and the liquid level condition in one of the combinations are controlled. 
     However, when ink is supplied by the ink supply unit  4 , target control elements are controlled such that an ink supply operation and a pressure regulation operation are performed separately in time. 
     Specifically, in the case of combinatorial condition number K- 1 , the ink supply valve  42  and the air pump  26  are controlled to perform later-described first separative operation. In the case of combinatorial condition number K- 5 , the ink supply valve  42 , the air pump  26 , and the positive pressure tank pressure regulation valve  14  are controlled to perform later-described second separative operation. In the case of combinatorial condition number K- 9 , the ink supply valve  42 , the air pump  26 , and the negative pressure tank pressure regulation valve  22  are controlled to perform later-described third separative operation. In the case of combinatorial condition number K- 13 , the ink supply valve  42 , the positive pressure tank pressure regulation valve  14 , and the negative pressure tank pressure regulation valve  22  are controlled to perform later-described fourth separative operation. 
     Also, when ink delivery operation is performed by the ink pump  25 , target control elements are controlled such that an ink delivery operation and a pressure regulation operation are performed separately in time. 
     Specifically, in the case of combinatorial condition number K- 3 , the ink pump  25  and the air pump  26  are controlled to perform later-described fifth separative operation. In the case of combinatorial condition number K- 7 , the ink pump  25 , the air pump  26 , and the positive pressure tank pressure regulation valve  14  are controlled to perform later-described sixth separative operation. In the case of combinatorial condition number K- 11 , the ink pump  25 , the air pump  26 , and the negative pressure tank pressure regulation valve  22  are controlled to perform later-described seventh separative operation. In the case of combinatorial condition number K- 15 , the ink pump  25 , the positive pressure tank pressure regulation valve  14 , and the negative pressure tank pressure regulation valve  22  are controlled to perform later-described eighth separative operation. 
     When the pressure liquid level control is started in step S 2  of  FIG. 2 , the controller  5  determines condition every predetermined time T. Specifically, the controller  5  determines pressure condition and liquid level condition based on the detected value Pk from the positive-pressure-side pressure sensor  16 , the detected value Pf from the negative-pressure-side pressure sensor  24 , an output signal of the positive pressure tank liquid level sensor  36 , and an output signal of the negative pressure tank liquid level sensor  37 . The controller  5  controls the control elements illustrated in  FIGS. 5 and 6  based on the result of the determination. Here, as seen from the description of later-described separative operations (the first to eighth separative operations), each separative operation takes twice as long as the predetermined time T. Therefore, when a separative operation is performed, the interval until the subsequent condition determination is 2T. 
     When a print job is completed and the pressure liquid level control is terminated in step S 6  of  FIG. 2 , the controller  5  stops the ink pump  25  and the air pump  26  when the pumps are being driven. When the positive pressure tank pressure regulation valve  14 , the negative pressure tank pressure regulation valve  22 , and the ink supply valve  42  are open, the controller  5  closes the valves. 
     Next, the first separative operation will be described. 
       FIG. 7  is a flow chart for explaining the first separative operation. The processing of the flow chart of  FIG. 7  starts when the controller  5  determines that the current condition corresponds to the combinatorial condition number K- 1 , by making condition determination. 
     In step S 11  of  FIG. 7 , the controller  5  stops the air pump  26 . In addition, the controller  5  opens the ink supply valve  42 . The opening of the ink supply valve  42  causes ink to be supplied to the negative pressure tank  19 . 
     Here, as illustrated in  FIG. 5 , in the case of combinatorial condition number K- 1 , the controller  5  closes the positive pressure tank pressure regulation valve  14  and the negative pressure tank pressure regulation valve  22 . Therefore, pressure regulation is not performed in a state where the air pump  26  is stopped. 
     Subsequently, in step S 12 , the controller  5  determines whether or not the predetermined time T has elapsed since the last condition determination was made. When it is determined that the predetermined time T has not elapsed (NO in step S 12 ), the controller  5  repeats step S 12 . 
     When it is determined that the predetermined time T has elapsed (YES in step S 12 ), the controller  5  starts to drive the air pump  26  in step S 13 . In addition, the controller  5  closes the ink supply valve  42 . The pressure regulation is performed by driving the air pump  26  to decrease the pressure of the negative pressure tank  19  and to increase the pressure of the positive pressure tank  11 . Also, the closing of the ink supply valve  42  causes ink supply to the negative pressure tank  19  to be stopped. 
     Subsequently, in step S 14 , the controller  5  determines whether or not the predetermined time T has elapsed since the processing in step S 13 . When it is determined that the predetermined time T has not elapsed (NO in step S 14 ), the controller  5  repeats step S 14 . 
     When it is determined that the predetermined time T has elapsed (YES in step S 14 ), the controller  5  terminates the first separative operation. The controller  5  makes the following condition determination. 
     Next, the second separative operation will be described. 
       FIG. 8  is a flow chart for explaining the second separative operation. The processing of the flow chart of  FIG. 8  starts when the controller  5  determines that the current condition corresponds to the combinatorial condition number K- 5 , by making condition determination. 
     In step S 21  of  FIG. 8 , the controller  5  closes the positive pressure tank pressure regulation valve  14 . The controller  5  stops the air pump  26 . The controller  5  opens the ink supply valve  42 . The opening of the ink supply valve  42  causes ink to be supplied to the negative pressure tank  19 . 
     Here, as illustrated in  FIG. 5 , in the case of combinatorial condition number K- 5 , the controller  5  closes the negative pressure tank pressure regulation valve  22 . Therefore, the pressure regulation is not performed in a state where the positive pressure tank pressure regulation valve  14  is also closed and the air pump  26  is stopped. 
     Subsequently, in step S 22 , the controller  5  determines whether or not the predetermined time T has elapsed since the last condition determination was made. When it is determined that the predetermined time T has not elapsed (NO in step S 22 ), the controller  5  repeats step S 22 . 
     When it is determined that the predetermined time T has elapsed (YES in step S 22 ), the controller  5  opens the positive pressure tank pressure regulation valve  14  and starts to drive the air pump  26  in step S 23 . In addition, the controller  5  closes the ink supply valve  42 . The opening of the positive pressure tank pressure regulation valve  14  causes the pressure of the positive pressure tank  11  to decrease, and the driving of the air pump  26  causes the pressure of the negative pressure tank  19  to decrease. In this manner, the pressure regulation is performed. Also, the closing of the ink supply valve  42  causes ink supply to the negative pressure tank  19  to be stopped. 
     Subsequently, in step S 24 , the controller  5  determines whether or not the predetermined time T has elapsed since the processing in step S 23 . When it is determined that the predetermined time T has not elapsed (NO in step S 24 ), the controller  5  repeats step S 24 . 
     When it is determined that the predetermined time T has elapsed (YES in step S 24 ), the controller  5  terminates the second separative operation. The controller  5  then makes the following condition determination. 
     Next, the third separative operation will be described. 
       FIG. 9  is a flow chart for explaining the third separative operation. The processing of the flow chart of  FIG. 9  starts when the controller  5  determines that the current condition corresponds to the combinatorial condition number K- 9 , by making condition determination. 
     In step S 31  of  FIG. 9 , the controller  5  closes the negative pressure tank pressure regulation valve  22 . The controller  5  stops the air pump  26 . The controller  5  opens the ink supply valve  42 . The opening of the ink supply valve  42  causes ink to be supplied to the negative pressure tank  19 . 
     Here, as illustrated in  FIG. 6 , in the case of combinatorial condition number K- 9 , the controller  5  closes the positive pressure tank pressure regulation valve  14 . Therefore, the pressure regulation is not performed in a state where the negative pressure tank pressure regulation valve  22  is also closed and the air pump  26  is stopped. 
     Subsequently, in step S 32 , the controller  5  determines whether or not the predetermined time T has elapsed since the last condition determination was made. When it is determined that the predetermined time T has not elapsed (NO in step S 32 ), the controller  5  repeats step S 32 . 
     When it is determined that the predetermined time T has elapsed (YES in step S 32 ), the controller  5  opens the negative pressure tank pressure regulation valve  22  and starts to drive the air pump  26  in step S 33 . In addition, the controller  5  closes the ink supply valve  42 . The opening of the negative pressure tank pressure regulation valve  22  causes the pressure of the negative pressure tank  19  to increase, and the driving of the air pump  26  causes the pressure of the positive pressure tank  11  to increase. In this manner, the pressure regulation is performed. Also, the closing of the ink supply valve  42  causes ink supply to the negative pressure tank  19  to be stopped. 
     Subsequently, in step S 34 , the controller  5  determines whether or not the predetermined time T has elapsed since the processing in step S 33 . When it is determined that the predetermined time T has not elapsed (NO in step S 34 ), the controller  5  repeats step S 34 . 
     When it is determined that the predetermined time T has elapsed (YES in step S 34 ), the controller  5  terminates the third separative operation. The controller  5  then makes the following condition determination. 
     Next, the fourth separative operation will be described. 
       FIG. 10  is a flow chart for explaining the fourth separative operation. The processing of the flow chart of  FIG. 10  starts when the controller  5  determines that the current condition corresponds to the combinatorial condition number K- 13 , by making condition determination. 
     In step S 41  of  FIG. 10 , the controller  5  closes the positive pressure tank pressure regulation valve  14  and the negative pressure tank pressure regulation valve  22 . In addition, the controller  5  opens the ink supply valve  42 . The opening of the ink supply valve  42  causes ink to be supplied to the negative pressure tank  19 . 
     Here, as illustrated in  FIG. 6 , in the case of combinatorial condition number K- 13 , the controller  5  stops the air pump  26 . Therefore, the pressure regulation is not performed in a state where the positive pressure tank pressure regulation valve  14  and the negative pressure tank pressure regulation valve  22  are closed. 
     Subsequently, in step S 42 , the controller  5  determines whether or not the predetermined time T has elapsed since the last condition determination was made. When it is determined that the predetermined time T has not elapsed (NO in step S 42 ), the controller  5  repeats step S 42 . 
     When it is determined that the predetermined time T has elapsed (YES in step S 42 ), the controller  5  opens the positive pressure tank pressure regulation valve  14  and the negative pressure tank pressure regulation valve  22  in step S 43 . In addition, the controller  5  closes the ink supply valve  42 . The opening of the positive pressure tank pressure regulation valve  14  and the negative pressure tank pressure regulation valve  22  causes the pressure of the positive pressure tank  11  to decrease and the pressure of the negative pressure tank  19  to increase. In this manner, the pressure regulation is performed. The closing of the ink supply valve  42  causes ink supply to the negative pressure tank  19  to be stopped. 
     Subsequently, in step S 44 , the controller  5  determines whether or not the predetermined time T has elapsed since the processing in step S 43 . When it is determined that the predetermined time T has not elapsed (NO in step S 44 ), the controller  5  repeats step S 44 . 
     When it is determined that the predetermined time T has elapsed (YES in step S 44 ), the controller  5  terminates the fourth separative operation. The controller  5  then makes the following condition determination. 
     By the above-described first to fourth separative operations, the ink supply operation and the pressure regulation operation are performed separately in time when the conditions for the ink supply unit  4  to perform the ink supply operation are satisfied. 
     Next, the fifth separative operation will be described. 
       FIG. 11  is a flow chart for explaining the fifth separative operation. The processing of the flow chart of  FIG. 11  starts when the controller  5  determines that the current condition corresponds to the combinatorial condition number K- 3 , by making condition determination. 
     In step S 51  of  FIG. 11 , the controller  5  stops the air pump  26 . In addition, the controller  5  causes the ink pump  25  to be driven. The driving of the ink pump  25  causes ink to be delivered from the negative pressure tank  19  to the positive pressure tank  11 . 
     Here, as illustrated in  FIG. 5 , in the case of combinatorial condition number K- 3 , the controller  5  closes the positive pressure tank pressure regulation valve  14  and the negative pressure tank pressure regulation valve  22 . Therefore, the pressure regulation is not performed in a state where the air pump  26  is stopped. 
     Subsequently, in step S 52 , the controller  5  determines whether or not the predetermined time T has elapsed since the last condition determination was made. When it is determined that the predetermined time T has not elapsed (NO in step S 52 ) and the controller  5  repeat step S 52 . 
     When it is determined that the predetermined time T has elapsed (YES in step S 62 ), the controller  5  starts to drive the air pump  26  in step S 53 . In addition, the controller  5  stops the ink pump  25 . The driving of the air pump  26  causes the pressure of the negative pressure tank  19  to decrease and the pressure of the positive pressure tank  11  to increase, thereby performing the pressure regulation. The stopping of the ink pump  25  causes ink delivery from the negative pressure tank  19  to the positive pressure tank  11  to be stopped. 
     Subsequently, in step S 54 , the controller  5  determines whether or not the predetermined time T has elapsed since the processing in step S 53 . When it is determined that the predetermined time T has not elapsed (NO in step S 54 ), the controller  5  repeats step S 54 . 
     When it is determined that the predetermined time T has elapsed (YES in step S 54 ), the controller  5  terminates the fifth separative operation. The controller  5  then makes the following condition determination, 
     Next, the sixth separative operation will be described. 
       FIG. 12  is a flow chart for explaining the sixth separative operation. The processing of the flow chart of  FIG. 12  starts when the controller  5  determines that the current condition corresponds to the combinatorial condition number K- 7 , by making condition determination. 
     In step S 61  of  FIG. 12 , the controller  5  closes the positive pressure tank pressure regulation valve  14 . In addition, the controller  5  stops the air pump  26 . The controller  5  also causes the ink pump  25  to be driven. The driving of the ink pump  25  causes ink to be delivered from the negative pressure tank  19  to the positive pressure tank  11 . 
     Here, as illustrated in  FIG. 5 , in the case of combinatorial condition number K- 7 , the controller  5  closes the negative pressure tank pressure regulation valve  22 . Therefore, the pressure regulation is not performed in a state where the positive pressure tank pressure regulation valve  14  is also closed and the air pump  26  is stopped. 
     Subsequently, in step S 62 , the controller  5  determines whether or not the predetermined time T has elapsed since the last condition determination was made. When it is determined that the predetermined time T has not elapsed (NO in step S 62 ), the controller  5  repeats step S 62 . 
     When it is determined that the predetermined time T has elapsed (YES in step S 62 ), the controller  5  opens the positive pressure tank pressure regulation valve  14  and starts to drive the air pump  26  in step S 63 . In addition, the controller  5  stops the ink pump  25 . The opening of the positive pressure tank pressure regulation valve  14  causes the pressure of the positive pressure tank  11  to decrease, and the driving of the air pump  26  causes the pressure of the negative pressure tank  19  to decrease. In this manner, the pressure regulation is performed. The stopping of the ink pump  25  causes ink delivery from the negative pressure tank  19  to the positive pressure tank  11  to be stopped. 
     Subsequently, in step S 64 , the controller  5  determines whether or not the predetermined time T has elapsed since the processing in step S 63 . When it is determined that the predetermined time T has not elapsed (NO in step S 64 ), the controller  5  repeats step S 64 . 
     When it is determined that the predetermined time T has elapsed (YES in step S 64 ), controller  5  terminates the sixth separative operation. The controller  5  then makes the following condition determination. 
     Next, the seventh separative operation will be described. 
       FIG. 13  is a flow chart for explaining the seventh separative operation. The processing of the flow chart of  FIG. 13  starts when the controller  5  determines that the current condition corresponds to the combinatorial condition number K- 11 , by making condition determination. 
     In step S 71  of  FIG. 13 , the controller  5  closes the negative pressure tank pressure regulation valve  22 . In addition, the controller  5  stops the air pump  26 . The controller  5  also causes the ink pump  25  to be driven. The driving of the ink pump  25  causes ink to be delivered from the negative pressure tank  19  to the positive pressure tank  11 . 
     Here, as illustrated in  FIG. 6 , in the case of combinatorial condition number K- 11 , the controller  5  closes the positive pressure tank pressure regulation valve  14 . Therefore, the pressure regulation is not performed in a state where the negative pressure tank pressure regulation valve  22  is also closed and the air pump  26  is stopped. 
     Subsequently, in step S 72 , the controller  5  determines whether or not the predetermined time T has elapsed since the last condition determination was made. When it is determined that the predetermined time T has not elapsed (NO in step S 72 ), the controller  5  repeats step S 72 . 
     When it is determined that the predetermined time T has elapsed (YES in step S 72 ), the controller  5  opens the negative pressure tank pressure regulation valve  22  and starts to drive the air pump  26  in step S 73 . In addition, the controller  5  stops the ink pump  25 . The opening of the negative pressure tank pressure regulation valve  22  causes the pressure of the negative pressure tank  19  to increase, and the driving of the air pump  26  causes the pressure of the positive pressure tank  11  to increase. In this manner, the pressure regulation is performed. The stopping of the ink pump  25  causes ink delivery from the negative pressure tank  19  to the positive pressure tank  11  to be stopped. 
     Subsequently, in step S 74 , the controller  5  determines whether or not the predetermined time T has elapsed since the processing in step S 73 . When it is determined that the predetermined time T has not elapsed (NO in step S 74 ), the controller  5  repeats step S 74 . 
     When it is determined that the predetermined time T has elapsed (YES in step S 74 ), controller  5  terminates the seventh separative operation. The controller  5  then makes the following condition determination. 
     Next, the eighth separative operation will be described. 
       FIG. 14  is a flow chart for explaining the eighth separative operation. The processing of the flow chart of  FIG. 14  starts when the controller  5  determines that the current condition corresponds to the combinatorial condition number K- 15 , by making condition determination. 
     In step S 81  of  FIG. 14 , the controller  5  closes the positive pressure tank pressure regulation valve  14  and the negative pressure tank pressure regulation valve  22 . In addition, the controller  5  causes the ink pump  25  to be driven. The driving of the ink pump  25  causes ink to be delivered from the negative pressure tank  19  to the positive pressure tank  11 . 
     Here, as illustrated in  FIG. 6 , in the case of combinatorial condition number K- 15 , the controller  5  stops the air pump  26 . Therefore, the pressure regulation is not performed in a state where the positive pressure tank pressure regulation valve  14  and the negative pressure tank pressure regulation valve  22  are closed. 
     Subsequently, in step S 82 , the controller  5  determines whether or not the predetermined time T has elapsed since the last condition determination was made. When it is determined that the predetermined time T has not elapsed (NO in step S 82 ), the controller  5  repeats step S 82 . 
     When it is determined that the predetermined time T has elapsed (YES in step S 82 ), the controller  5  opens the positive pressure tank pressure regulation valve  14  and the negative pressure tank pressure regulation valve  22  in step S 83 . In addition, the controller  5  stops the ink pump  25 . The opening of the positive pressure tank pressure regulation valve  14  and the negative pressure tank pressure regulation valve  22  causes the pressure of the positive pressure tank  11  to decrease and the pressure of the negative pressure tank  19  to increase. In this manner, the pressure regulation is performed. The stopping of the ink pump  25  causes ink delivery from the negative pressure tank  19  to the positive pressure tank  11  to be stopped. 
     Subsequently, in step S 84 , the controller  5  determines whether or not the predetermined time T has elapsed since the processing in step S 83 . When it is determined that the predetermined time T has not elapsed (NO in step S 84 ), the controller  5  repeats step S 84 . 
     When it is determined that the predetermined time T has elapsed (YES in step S 84 ), the controller  5  terminates the eighth separative operation. The controller  5  then makes the following condition determination. 
     By the above-described fifth to eighth separative operations, the ink delivery operation and the pressure regulation operation are performed separately in time when the conditions for the ink pump  25  to perform the ink delivery operation are satisfied. 
     As described above, in the inkjet printer  1 , the controller  5  performs control such that the ink delivery operation by the ink pump  25 , the ink supply operation by the ink supply unit  4 , and the pressure regulation operation by the air pump  26 , the positive pressure tank pressure regulation valve  14 , and the negative pressure tank pressure regulation valve  22  are performed separately in time. Thus, it is possible to suppress an increase in the nozzle pressure variation due to repeated factors of variation in the pressures of the positive pressure tank  11  and the negative pressure tank  19 . Consequently, degradation of printed image quality can be reduced. 
     It is to be noted that the operations belonging to each of only one or two combinations out of the combinations (three ways) of two operations taken from the ink delivery operation, the ink supply operation, and the pressure regulation operation may be performed separately in time. For instance, only between the ink delivery operation and the pressure regulation operation, both operations may be performed separately in time. Even in this case, the effect is obtained that degradation of printed image quality due to a nozzle pressure variation can be reduced. As might be expected, as the number of combinations of operations separated in time increases, degradation of printed image quality can be further reduced. 
     Embodiments of the present invention have been described above. However, the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. 
     Moreover, the effects described in the embodiments of the present invention are only a list of optimum effects achieved by the present invention. Hence, the effects of the present invention are not limited to those described in the embodiment of the present invention.