Patent Publication Number: US-8540329-B2

Title: Liquid ejection apparatus

Description:
The present application claims priority from Japanese Patent Application No. 2009-270529, which was filed on Nov. 27, 2009, the disclosure of which is herein incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid ejection apparatus having a recording liquid ejection head which ejects recording liquid from ejection openings. 
     2. Description of the Related Art 
     The performances regarding ink supply and ink ejection in an inkjet recording apparatus significantly depend on the viscosity of ink. There is a known recording apparatus which directly measures the ink viscosity by a viscosity meter such as capillary viscosity meter, falling-ball viscosity meter, and rotational viscosity meter is provided on an ink supply passage. 
     SUMMARY OF THE INVENTION 
     However, the apparatus must be large in size when a capillary viscosity meter, a falling-ball viscosity meter, or a rotational viscosity meter is provided on the ink passage of the product. 
     An object of the present invention is to provide a liquid ejection apparatus which can precisely calculate the viscosity of liquid without requiring a viscosity meter therein. 
     A liquid ejection apparatus of the present invention includes: a tank which stores a recording liquid; a recording liquid ejection head which includes an internal passage having an inflow opening and an outflow opening and a plurality of individual recording liquid passages each connecting outlets of the internal passage with ejection openings from which the recording liquid is ejected; a supply passage which connects the inflow opening with the tank; a return passage which connects the outflow opening with the tank; a valve which adjusts a flow rate of the recording liquid on the return passage; a pump which operates to supply the recording liquid from the tank to the internal passage via the supply passage; a pump control unit which controls the pump; a valve control unit which controls the valve; a detecting unit which detects an operating state of the pump; and a calculating unit which calculates the viscosity of the recording liquid; wherein, while the valve control unit controls the valve and the pump control unit controls the pump so that the recording liquid in the tank circulates by serially passing through the supply passage, the internal passage, and the return passage, the calculating unit calculates the viscosity of the recording liquid based on the operating state of the pump detected by the detecting unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other and further objects, features and advantages of the invention will appear more fully from the following description taken in connection with the accompanying drawings in which: 
         FIG. 1  is a schematic plan view of an inkjet printer according to an embodiment of the present invention. 
         FIG. 2  is a longitudinal section of the inkjet head and the ink supply unit shown in  FIG. 1 . 
         FIG. 3  is a partial cross section of the inkjet head shown in  FIG. 2 . 
         FIG. 4  is a functional block diagram of the control unit shown in  FIG. 1 . 
         FIG. 5A  shows the flow of ink when first ink circulation is carried out for the inkjet head and the supply unit shown in  FIG. 2 . 
         FIG. 5B  shows the flow of ink when second ink circulation is carried out for the inkjet head and the supply unit shown in  FIG. 2 . 
         FIG. 6  is a flowchart showing the sequence of the maintenance operation by the control unit shown in  FIG. 4 . 
         FIG. 7  is a functional block diagram of a control unit according to a variation. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An inkjet printer  101  according to an embodiment of the present invention includes, as shown in  FIG. 1 , a conveying unit  20  which conveys sheets P from upward to downward in  FIG. 1 , four inkjet heads  1  ejecting magenta, cyan, yellow, and black ink droplets respectively onto a sheet P conveyed by the conveying unit  20 , four ink supply units  10  supplying ink to the inkjet heads  1 , and a control unit  16 . In the present embodiment, a sub-scanning direction is in parallel to the conveyance direction of the sheets P on the conveying unit  20 , whereas a main scanning direction is orthogonal to the sub-scanning direction and along the horizontal plane. 
     The conveying unit  20  has two belt rollers  6  and  7  and an endless conveyor belt  8  stretched between the rollers  6  and  7 . The belt roller  7  is a driving roller which is rotated by a driving force from an unillustrated conveyor motor. The belt roller  6  is a driven roller which rotates as the conveyor belt  8  is moved by the rotation of the belt roller  7 . A sheet P placed on the outer circumferential surface of the conveyor belt  8  is conveyed downward in  FIG. 1 . 
     The four inkjet heads  1  are elongated in the main scanning direction to be in parallel to one another. Each inkjet head  1  has a plurality of ejection openings  108  (see  FIG. 3 ) which eject ink droplets and are aligned in the main scanning direction. The inkjet printer  101  is a line-type color inkjet printer in which four inkjet heads  1  are fixedly provided to oppose the conveying passage on which sheets P are conveyed by the conveyor belt  8 . 
     When a sheet P conveyed by the conveyor belt  8  is passing through the portions immediately below the four inkjet heads  1 , ink droplets of the respective colors are serially ejected from the inkjet heads  1  to the upper surface of the sheet P, with the result that a desired color image is formed on the sheet P. 
     Now, the inkjet head  1  will be described with reference to  FIG. 2 . As shown in  FIG. 2 , the inkjet head  1  includes a reservoir unit  71  and a head main body  2 . 
     The reservoir unit  71  is a passage forming component which is fixed to the upper surface of the head main body  2  and supplies ink to the head main body  2 . Inside the reservoir unit  71  formed are an ink inflow passage  72 , an ink outflow passage  75 , a first exhaust passage  73 , and a second exhaust passage  74 . 
     The ink inflow passage  72  has an inflow opening  72   a  at the lower surface of the reservoir unit  71 . Ink supplied from the ink supply unit  10  flows into the ink inflow passage  72  via the inflow opening  72   a . The ink inflow passage  72  functions as an ink reservoir where the inflow ink is temporarily stored. At the inner wall of the ink inflow passage  72 , a hole  72   b  is formed to penetrate the upper outer wall surface of the reservoir unit  71 . The hole  72   b  is sealed from the outside of the reservoir unit  71  by a flexible resin film  76 . The resin film  78  functions a part of the inner wall surface of the ink inflow passage  72 . As the resin film  76  deforms in accordance with a change in the ink pressure in the ink inflow passage  72 , the film functions as a damper for restraining changes in the ink pressure. The resin film  76  makes it possible to realize the damper function at low cost. When printing is carried out, the resin film  76  is slightly curved toward the inside of the ink inflow passage  72 . On the outer wall surface of the reservoir unit  71 , a plate-shaped regulating member  77  is fixed to cover the resin film  76 , in order to prevent the resin film  76  from being curved toward the outside of the reservoir unit  71 . This arrangement makes it possible to prevent the resin film  76  from being excessively deformed and broken when the ink pressure in the ink inflow passage  72  becomes abnormally high. The regulating member  77  has a atmosphere introducing hole  77   a , and hence the pressure at the space between the regulating member  77  and the resin film  76  is always kept at the atmospheric pressure. This allows the resin film  76  to easily deform. 
     The ink outflow passage  75  is connected to the ink inflow passage  72  via a filter  75   a  and is also connected to the head main body  2 . When printing is carried out, ink from the ink supply unit  10  passes through the ink inflow passage  72  and the ink outflow passage  75  and is then supplied to the head main body  2 . 
     The first exhaust passage  73  is connected with the ink inflow passage  72  at an upstream of the filter  75   a , and contacts the ink supply unit  10  via a first outflow opening  73   a  formed on the lower surface of the reservoir unit  71 . At the lower inner wall surface of the first exhaust passage  73 , a hole  73   b  is formed to penetrate the lower outer wall of the reservoir unit  71 . The hole  73   b  is sealed from the outside of the reservoir unit  71  by a flexible resin film  78 . The resin film  78  functions as a part of the inner wall surface of the first exhaust passage  73 . As the resin film  78  deforms in accordance with a change in the ink pressure in the first exhaust passage  73 , the film has a damper function to restrain changes in the ink pressure. This resin film  78  makes it possible to realize the damper function at low cost. When printing is carried out, the resin film  78  is slightly curved toward the inside of the first exhaust passage  73 . On the lower outer wall surface of the reservoir unit  71 , a plate-shaped regulating member  79  is fixed to cover the resin film  78 , in order to prevent the resin film  78  from being curved toward the outside of the reservoir unit  71 . This prevents the resin film  78  from excessively deformed and broken when the ink pressure of the first exhaust passage  73  becomes excessively high. The regulating member  79  has an atmosphere introducing hole  79   a , and hence the pressure at the space between the regulating member  79  and the resin film  78  is always kept at the atmospheric pressure. This allows the resin film  78  to easily deform. During later-described first ink circulation, ink from the ink supply unit  10  passes through the ink inflow passage  72  and the first exhaust passage  73  and then returns to the ink supply unit  10  via the outflow opening  73   a  (see  FIG. 5A ). 
     The second exhaust passage  74  is connected to the head main body  2  and is also connected to the ink supply unit  10  via a second outflow opening  74   a  formed at the lower surface of the reservoir unit  71 . During the later-described second ink circulation, ink from the ink supply unit  10  passes through the ink inflow passage  72 , the ink outflow passage  75 , and the head main body  2 , and then returns to the ink supply unit  10  after passing through the second exhaust passage  74  (see  FIG. 5B ). 
     As shown in  FIG. 3 , the head main body  2  includes a passage unit  9  and an actuator unit  21 . The passage unit  9  is a laminated body in which a plurality of metal plates made of stainless steel are aligned with each other and laminated, and has a common ink chamber  105   a  connected to the ink outflow passage  75  and the second exhaust passage  74  of the reservoir unit  71  and a plurality of individual ink passages  132  connected to the common ink chamber  105   a . In the present embodiment, passages which are in the inkjet head  1  but are not the individual ink passages  132  may be termed internal passages. On the lower surface of the passage unit  9  formed is an ejection surface  2   a , where ejection openings  108  aligned in the main scanning direction are formed. Each individual ink passage  132  extends from the ejection opening of the common ink chamber  105   a  to the ejection opening  108  via the aperture  112  and the pressure chamber  110 . 
     The actuator unit  21  includes a plurality of actuators which correspond to pressure chambers  110 , respectively, and are unimorph piezoelectric elements each capable of operating independently. The actuator unit  21  has a function to selectively apply an ejection energy to the ink in the pressure chamber  110 . Each actuator is an example of an ejection energy applying element. The actuator unit  21  is constituted by unillustrated three piezoelectric sheets made of a lead zirconate titanate (PZT) ceramic material having ferroelectricity. The topmost piezoelectric sheet is polarized in the thickness directions, and the upper surface of this piezoelectric sheet is provided with a plurality of individual electrodes. Between the polarized topmost piezoelectric sheet and the piezoelectric sheet immediately below the topmost sheet, a common electrode is provided to cover the entirety of the sheets. As such, the individual electrodes corresponding to the respective pressure chambers  110  and the common electrode sandwich the polarized piezoelectric sheet. When the electric potential of an individual electrode is arranged to be different from that of the common electrode so that an electric field is applied to the topmost piezoelectric sheet in the polarization direction, the part of the piezoelectric sheet where the electric field is applied functions as a driving active portion which is warped on account of the piezoelectric effect. As a result, the ink in the pressure chamber  110  receives an ejection energy with which an ink droplet is discharged through an ejection opening  108 . 
     When printing is carried out, the ink supplied from the ink outflow passage  75  of the reservoir unit  71  flows from the ejection opening of the common ink chamber  105   a  into each individual ink passage  132 , and then reaches the ejection openings  108  as an ejection energy is applied to the ink by the actuator. During the later-described second ink circulation, the ink supplied from the ink outflow passage  75  of the reservoir unit  71  flows from the common ink chamber  105   a  into the second exhaust passage  74  of the reservoir unit  71  via an unillustrated discharging slot (see  FIG. 5B ). Alternatively, a bypass passage which connects the ink outflow passage  75  with the second exhaust passage  74  is provided between the reservoir unit  71  and the common ink chamber  105   a  and passages are provided at a plurality of parts of the bypass passage to be connected to the common ink chamber  105   a , with the result that ink supply to the common ink chamber  105   a  is stabilized and ink is circulated to pass through the bypass passage during the later-described second ink circulation. 
     The ink supply units  10  are connected to parts around the left edge of the lower surface of the inkjet head  1  in  FIG. 1 , and supply ink to the inkjet heads  1 . These ink supply units  10  will be detailed. As shown in  FIG. 2 , each ink supply unit  10  includes a sub-tank  80 , an ink supply tube  81 , an ink feeding tube  82 , a first ink return tube  83 , and a second ink return tube  84  which are connected to the sub-tank  80 , a pump  86  provided on the ink feeding tube  82 , an encoder  89  attached to the pump  86 , a first valve  87  provided on the first ink return tube  83 , and a second valve  88  provided on the second ink return tube  84 . 
     The sub-tank  80  has a storage chamber for storing ink supplied to the inkjet head  1 , and is provided with a supply opening  81   a , an outflow opening  82   a , and inflow openings  83   a  and  84   a . The supply opening  81   a  is connected to the ink supply tube  81 , the outflow opening  82   a  is connected to the ink feeding tube  82 , the inflow opening  83   a  is connected to the first ink return tube  83 , and the inflow opening  84   a  is connected to the second ink return tube  84 . 
     The ink feeding tube  82  is connected to the sub-tank  80  at one end via the outflow opening  82   a  and is connected to the ink inflow passage  72  of the reservoir unit  71  at the other end via the inflow opening  72   a . The ink stored in the sub-tank  80  is supplied to the ink inflow passage  72  of the reservoir unit  71  via the ink feeding tube  82 . The pump  86  functions as supplying means for forcibly supplying the ink stored in the sub-tank  80  to the reservoir unit  71  via the ink feeding tube  82 . In the present embodiment, the pump  86  is a diaphragm pump (displacement pump). The pump  86  also functions as a check valve which prevents ink from flowing from the ink feeding tube  82  into the sub-tank  80 . The encoder  89  is a revolution counter which detects the revolution of the pump  86  as an operating state of the pump  86  and outputs the detection result to the control unit  16  (see  FIG. 4 ). 
     The first ink return tube  83  is connected to the sub-tank  80  at one end via the inflow opening  83   a  and is connected to the first exhaust passage  73  of the reservoir unit  71  at the other end via the first outflow opening  73   a . The first valve  87  is an adjusting valve by which the flow rate of ink in the first ink return tube  83  is adjusted. 
     The second ink return tube  84  is connected to the sub-tank  80  at one end via the inflow opening  84   a  and is connected to the second exhaust passage  74  of the reservoir unit  71  at the other end via the second outflow opening  74   a . The second valve  88  is an adjusting valve by which the flow rate of ink in the second ink return tube  84  is adjusted. 
     The control unit  16  controls the entirety of the inkjet printer  101 , and includes, as shown in  FIG. 4 , a head controller  61 , a valve control unit  62 , a pump controller  63 , a conveyance controller  67 , a viscosity calculating unit  64 , a viscosity memory unit  65 , and a purge control unit  66 . The head controller  61  as driving means controls discharging of ink droplets from the ejection openings  108  by driving the actuator unit  21  of each inkjet head. In so doing, the head controller  61  drives the actuator unit  21  such that, as the ink viscosity (described later) of the ink in each inkjet head  1  which data is stored in the viscosity memory unit  65  increases, an ejection energy applied to the pressure chamber  110  increases, in other words, a drive voltage supplied to the individual electrode of the actuator unit  21  increases. 
     The valve control unit  62  controls the first valve  87  and the second valve  88 . The pump controller  63  controls the pump  86  by outputting a PWM (Pulse Width Modulation) signal. The conveyance controller  67  controls the conveying unit  20 . When printing is carried out, the pump controller  63  stops the pump  86  and the valve control unit  62  closes the first valve  87  and the second valve  88 . In this state, the conveyance controller  67  controls the conveying unit  20  so that a sheet P is conveyed at a predetermined speed, whereas the head controller  61  controls each inkjet head  1  so that ink droplets are ejected from the ejection openings  108  at timings with which a desired image is formed on the sheet P, when the sheet P passes through the positions immediately below the respective inkjet heads  1 . It is noted that the ink in the sub-tank  80  is allowed to flow into the reservoir unit  71  via the ink feeding tube  82 , even if the pump  86  has been stopped. 
     The viscosity calculating unit  64  performs a viscosity calculation process to calculate the ink viscosity of the ink in each inkjet head  1 . In the present embodiment, the viscosity calculation process is a process included in a later-detailed maintenance operation. The maintenance operation is an operation to remove foreign matters and/or bubbles from the inkjet heads  1 , and is started when the inkjet printer  101  is activated, when the standby time exceeds a predetermined time, or when an instruction is made by the user. 
     When the viscosity calculation process starts, the viscosity calculating unit  64  generates a signal with which the pump  86  drives at a predetermined instructed revolution under the control of the pump controller  63 , after the first valve  87  is opened whereas the second valve  88  is closed under the control of the valve control unit  62  (see  FIG. 5A ). The pump  86  receives a predetermined electric power corresponding to the predetermined instructed revolution. As the pump  86  drives at a revolution corresponding to the ink viscosity, the ink stored in the sub-tank  80  is forcibly supplied to the ink inflow passage  72  via the ink feeding tube  82  at the flow rate corresponding to the ink viscosity. Since the first valve  87  is open at this moment, the passage from the ink inflow passage  72  to the sub-tank  80  via the first exhaust passage  73  and the first ink return tube  83  has a lower resistance than the passages from the ink inflow passage  72  to the ejection openings  108  via the ink outflow passage  75  and the common ink chamber  105   a . For this reason, the first ink circulation is carried out so that the ink supplied to the ink inflow passage  72  hardly flows into the ink outflow passage  75  and passes through the first exhaust passage  73  and the first ink return tube  83  in this order and returns to the sub-tank  80 . However, when an amount of ink supplied by the pump  86  is excessively large, the ink pressure of the ink inflow passage  72  becomes excessively high and hence the ink supplied to the ink inflow passage  72  flows into the ink outflow passage  75 , with the result that the ink leaks out from the ejection openings  108 . It is therefore necessary to adjust an amount of ink supply in this case, as described later. 
     As the ink viscosity increases, the output (actual revolution) of the pump  86  decreases with respect to the electric power (voltage required for low-speed revolution) input from the pump controller  63  to the pump  86 . The power input to the pump  86  is represented by an average duty regarding the PWM signal. After the ink flow becomes stable after a predetermined time elapses from the start of the first ink circulation, the encoder  89  detects the revolution of the pump  86  during the first ink circulation, and then calculates the ink viscosity based on the revolution detected by the viscosity calculating unit  64  and the power input to the pump  86 . For example, the viscosity calculating unit  64  may calculate the ink viscosity by substituting for the revolution of the pump  86  and the power input to the pump  86  into a predetermined relational expression (which takes into account of the flow resistance of the passage system in the case of the first ink circulation and represents a relation among the ink viscosity, the revolution of the pump  86 , and the power input to the pump  86 ). In another example, the viscosity calculating unit  64  may calculate the ink viscosity with reference to a table which is stored in advance and shows a relation among the ink viscosity, the revolution of the pump  86 , and the power input to the pump  86 . The viscosity memory unit  65  stores the ink viscosity calculated by the viscosity calculating unit  64 . The viscosity calculating unit  64  determines that abnormalities have occurred in the pump  86  when the revolution of the pump  86  detected by the encoder  89  goes beyond a predetermined range, and the unit  64  terminates the viscosity calculation process and generates a signal to notify the user of the occurrence of abnormalities. In addition to this, the viscosity calculating unit  64  generates a signal representing information to prompt the user to discard the ink, when the calculated ink viscosity goes beyond a predetermined range. Based on these signals, notifications to the user are displayed on an unillustrated display. 
     The purge control unit  66  generates, subsequent to the viscosity calculation process, a signal which instructs to perform circulation purging for removing bubbles and/or foreign matters in the inkjet head  1 , as the pump  86 , the first valve  87 , and the second valve  88  of each ink supply unit  10  are controlled by the pump controller  63  and the valve control unit  62 . Referring to  FIG. 5A  and  FIG. 5B , the circulation purging will be described. This circulation purging is an operation to circulate ink inside the inkjet head  1  so that foreign matters and/or bubbles accumulated in the internal passages of the inkjet head  1  are removed. Ideally, the circulation purging is speedily performed so as to prevent ink from leaking out through the ejection openings. As the circulation purging starts, a first circulation operation and a second circulation operation are serially carried out. 
     In response to the signal generated by the purge control unit  66 , as shown in  FIG. 5A , the pump  86  drives under the control of the pump controller  63 , after the first valve  87  is opened whereas the second valve  88  is closed under the control of the valve control unit  62 . As a result, the first circulation operation starts, and the ink stored in the sub-tank  80  is forcibly supplied to the ink inflow passage  72  via the ink feeding tube  82 , by an amount corresponding to the ink viscosity. The ink supplied to the ink inflow passage  72  does not flow into the ink outflow passage  75 , and the first ink circulation is carried out again so that the ink passes through the first exhaust passage  73  and the first ink return tube  83  in this order and returns to the sub-tank  80 . In this case, the instructed revolution of the pump  86  (which may be represented by an input power or a flow rate of ink supplied by the pump  86  per unit of time) is determined, based on the passage resistance regarding the passages of the first ink circulation and the ink viscosity stored in the viscosity memory unit  65 , to be the highest revolution (an input power or a flow rate of ink per unit of time) within a range which is higher than the instructed revolution (power input of flow rate of ink per unit of time) of the pump  86  in the aforesaid viscosity calculation process by the viscosity calculating unit  64  but with which the ink meniscus formed at the ejection openings  108  is not broken, i.e. ink does not leak out through the ejection openings  108 . Because the withstanding pressure of the ink meniscus formed at the ejection openings  108  increases in accordance with the ink viscosity, the revolution of the pump  86  in the first ink circulation is arranged to increase as the ink viscosity stored in the viscosity memory unit  65  increases. 
     As a result of the first ink circulation, the bubbles and/or foreign matters in the ink inflow passage  72 , in particular the bubbles and/or foreign matters on the filter  75   a  serially pass through the first exhaust passage  73  and the first ink return tube  83  along with the ink, and are in the end trapped in the sub-tank  80 . 
     During the first ink circulation, the ink pressures in the ink inflow passage  72  and the first exhaust passage  73  are higher than those during printing. For this reason, the resin film  76  of the ink inflow passage  72  closely contacts the regulating member  77  and the resin film  78  of the first exhaust passage  73  closely contacts the regulating member  79 . After the first ink circulation is carried out for a predetermined time, the purge control unit  66  stops the pump  86  by the pump controller  63 , and then closes the first valve  87  by the valve control unit  62 . As a result the first circulation operation is completed. The time to execute the first ink circulation is arranged to increase as the ink viscosity stored in the viscosity memory unit  65  increases. 
     Thereafter, in response to the signal generated by the valve control unit  62 , as shown in  FIG. 5B , the pump  86  drives under the control of the pump controller  63  after the first valve  87  is closed and the second valve  88  is opened under the control of the valve control unit  62 . As a result, the second circulation operation starts so that the ink stored in the sub-tank  80  is forcibly supplied to the common ink chamber  105   a  of the head main body  2  via the ink feeding tube  82 , the ink inflow passage  72 , and the ink outflow passage  75 , by an amount corresponding to the ink viscosity. Since the second valve  88  is open at this moment, the passage from the common ink chamber  105   a  to the sub-tank  80  via the second exhaust passage  74  and the second ink return tube  84  has a lower resistance than the passages from the common ink chamber  105   a  to the ejection openings  108  via the individual ink passages  132 . For this reason, the second ink circulation is carried out so that the ink supplied to the common ink chamber  105   a  does not flow into any individual ink passages  132  and passes through the second exhaust passage  74  and the second ink return tube  84  in this order and returns to the sub-tank  80 . In this case, the instructed revolution of the pump  86  regarding the second ink circulation (which revolution may be represented by an input power or a flow rate of ink supplied by the pump  86  per unit of time) is determined, based on the passage resistance regarding the passages of the second ink circulation and the ink viscosity stored in the viscosity memory unit  65 , to be the highest revolution (an input power or a flow rate of ink per unit of time) within a range with which the ink meniscus formed at the ejection openings  108  is not broken, i.e. ink does not leak out from the ejection openings  108 . Because the withstanding pressure of the ink meniscus formed at the ejection openings  108  increases in accordance with the ink viscosity, the revolution of the pump  86  in the second ink circulation is arranged to increase as the ink viscosity stored in the viscosity memory unit  65  increases. 
     As a result of the second ink circulation, the bubbles and/or foreign matters in the ink outflow passage  75  and the common ink chamber  105   a  serially pass through the second exhaust passage  74  and the second ink return tube  84  along with the ink and are in the end trapped in the sub-tank  80 . 
     During the second ink circulation, the ink pressure in the ink inflow passage  72  is higher than the pressure during printing. For this reason, the resin film  76  of the ink inflow passage  72  closely contacts the regulating member  77 . After the second ink circulation is carried out for a predetermined time, the purge control unit  66  stops the pump  86  by the pump controller  63 , and then closes the second valve  88  by the valve control unit  62 . This is the completion of the second circulation operation. In the same manner as the first ink circulation, the time to execute the second ink circulation is arranged to increase as the ink viscosity stored in the viscosity memory unit  65  increases. This is the end of the circulation purging. 
     Now, the maintenance operation will be detailed with reference to  FIG. 6 . In the maintenance operation, the viscosity calculation process and the circulation purging are successively carried out. As the viscosity calculation process starts, the viscosity calculating unit  64  prepares for the first ink circulation to open the first valve  87  and close the second valve  88  under the control of the valve control unit  62  (S 101 ). Subsequently, the viscosity calculating unit  64  instructs, via the pump controller  63 , the pump  86  to drive at a predetermined low revolution (S 102 ). As a result the first ink circulation starts. The viscosity calculating unit  64  continues the first ink circulation until the ink flow regarding the first ink circulation becomes stable, by means of the valve control unit  62  and the pump controller  63 . More specifically, the first ink circulation is continued until a predetermined time which is determined in advance and at the end of which the ink flow regarding the first ink circulation is stable elapses from the start of the low-speed revolution of the pump  86  (S 103 ). The low-speed predetermined revolution of the pump  86  is preferably arranged to be a relatively high revolution with which the ink meniscus is not broken, i.e. ink does not leak out through the ejection openings  108  in all conceivable ink viscosity ranges. After the ink flow becomes stable, the encoder  89  detects, as an operating state of the pump  86 , an actual revolution of the pump  86  during the first ink circulation. In so doing, the encoder  89  may perform the detection under the control of the viscosity calculating unit  64 . Based on the detected revolution and the power input to the pump  86 , the viscosity calculating unit  64  calculates the ink viscosity by the above-described relational expression (S 104 ). The calculated ink viscosity is stored in the viscosity memory unit  65 . 
     Subsequent to the viscosity calculation, the purge control unit  66  determines, for the circulation purging, the revolutions (first high-speed revolution and second high-speed revolution) of the pump  86  concerning the first ink circulation and the second ink circulation, respectively, based on the ink viscosity stored in the viscosity memory unit  65 , and instructs via the pump controller  63  the pump  86  to drive at the determined first high-speed revolution. As a result the first ink circulation starts (S 106 ). The first high-speed revolution of the pump  86  is preferably arranged, based on the ink viscosity stored in the viscosity memory unit  65  and the passage resistance regarding the passages in the first ink circulation, to be a relatively high revolution which is higher than the predetermined low-speed revolution in the viscosity calculation process but with which the ink meniscus is not broken, i.e. ink does not leak out thorough the ejection openings  108 . More preferably, the first high-speed revolution is arranged to be at the upper limit in the aforesaid range. The purge control unit  66  continues the first ink circulation until a predetermined time elapses which is arranged in advance such that the removal of bubbles and/or foreign matters regarding the first ink circulation is suitably carried out within the time (S 107 ). The purge control unit  66  stops the pump  86  after the predetermined time elapses (S  108 ). 
     Thereafter, the purge control unit  66  prepares the second ink circulation to open the second valve  88  and close the first valve  87  under the control of the valve control unit  62  (S  109 ). Under the control of the pump controller  63 , the purge control unit  66  instructs the pump  86  to drive at the second high-speed revolution which has been determined As a result the start of the second ink circulation starts (S 110 ). The second high-speed revolution of the pump  86  is preferably arranged, based on the ink viscosity stored in the viscosity memory unit  65  and the passage resistance regarding the passages in the second ink circulation, to be a relatively high revolution with which the ink meniscus is not broken, i.e. ink does not leak out through the ejection openings  108 . More preferably, the second high-speed revolution is arranged to be at the upper limit in the aforesaid range. The purge control unit  66  continues the second ink circulation until a predetermined time elapses which is arranged in advance such that the removal of bubbles and/or foreign matters regarding the second ink circulation is suitably carried out within the time (S 111 ). After the predetermined time elapses, the purge control unit  66  stops the pump  86  (S 112 ). As such, the maintenance operation is completed. 
     As described above, the inkjet printer  101  of the present embodiment is advantageous in that the ink viscosity is accurately calculated without requiring an additional viscosity meter or the like because the viscosity calculating unit  64  calculates in the viscosity calculation process the ink viscosity from a revolution, as an operating state, of the pump  86  during the first ink circulation. This makes it possible to downsize the inkjet printer  101 . 
     Furthermore, the pump  86  is a displacement pump, the encoder  89  detects the revolution of the pump  86  during the first ink circulation, and the viscosity calculating unit  64  calculates the ink viscosity from the detected revolution and the power input to the pump  86 . This makes it possible to easily and accurately measure the ink viscosity. 
     In addition to the above, the instructed revolution of the pump  86  in the first ink circulation regarding the circulation purging (i.e. input power or flow rate of ink per unit of time) is arranged to be higher than the instructed revolution of the pump  86  in the viscosity calculation process (i.e. input power or flow rate of ink per unit of time) and within the range in which the meniscus of ink formed at the ejection openings  108  is not broken. It is therefore possible to efficiently remove foreign matters and/or bubbles in the inkjet head  1  without wasting ink. Moreover, since the circulation purging is carried out subsequent to the viscosity calculation process, the circulation purging is carried out based on the calculated ink viscosity before the viscosity greatly changes. The circulation purging is therefore efficiently carried out. 
     In addition to the above, since the circulation purging is carried out in each of the first ink circulation and the second ink circulation, it is possible to carry out the circulation purging at a revolution of the pump  86  suitable for each of the first ink circulation and the second ink circulation. This makes it possible to efficiently remove bubbles and/or foreign matters in the inkjet head  1 . 
     In addition to the above, when printing is carried out, the head controller  61  controls a drive voltage input to each individual electrode of the actuator unit  21  such that an ejection energy applied to the ink in the pressure chamber  110  increases as the ink viscosity in each inkjet head  1 , which is stored in the viscosity memory unit  65 , is increased. As such, the ejection energy is adjusted in accordance with the ink viscosity, and hence the ink discharging characteristics are stable and printing of high-quality images is possible. 
     In addition to the above, after the ink flow becomes stable after a predetermined time elapses from the start of the first ink circulation, the viscosity calculating unit  64  calculates the ink viscosity based on the detected revolution of the pump  86  and the power input to the pump  86 . the ink viscosity may not be accurately calculated immediately after the start of the first ink circulation because the ink flow is unstable on account of foreign matters and/or turbulence. In this regard, such a calculation error of the ink viscosity is restrained because the ink viscosity is calculated in the state of stable ink flow. 
     &lt;Variations&gt; 
     In the embodiment above, the encoder  89  detects the revolution of the pump  86  during the first ink circulation as an operating state of the pump  86  and the viscosity calculating unit  64  calculates the ink viscosity from the detected revolution and the power input to the pump  86 . Alternatively, an operating state of the pump  86  is detected by another type of means and the ink viscosity is calculated based on the detection result. For example, as shown in  FIG. 7 , an electric power detection unit  189  which detects a power supplied to the pump  86  when ink is supplied at a predetermined flow rate per unit of time, i.e. detects a power consumed by the pump  86  is provided in place of the encoder  89 , a viscosity calculating unit  164  calculates the torque of the pump  86  from the power detected by the electric power detection unit  189 , and the ink viscosity is calculated based on the torque. In this case, the viscosity calculating unit  164  may calculate the ink viscosity from an electric power detected by the electric power detection unit  189  in a state in which the revolution of the pump  86  is stable, or may calculate the ink viscosity from the rising curve of the electric power detected by the electric power detection unit  189  at the start of the rotation of the pump  86 . The pump  86  may be a non-displacement pump, e.g. an impeller pump. The displacement pump may be a tube pump. 
     According to the above, since the electric power detection unit  189  which does not perform mechanical operations is used, it is possible to downsize the apparatus. 
     According to the embodiment above, the electric power input to the pump  86  in the second ink circulation of the circulation purging is arranged to be the highest revolution within a range in which the ink meniscus at the ejection openings  108  is not broken, i.e. within a range in which ink does not leak out through the ejection openings  108 . Alternatively, the instructed revolution of the pump  86  in the second ink circulation of the circulation purging may be arranged at any revolutions as long as ink does not leak out through the ejection openings  108 . The power input to the pump  86  in the second ink circulation of the circulation purging may be constant or variable, regardless of the ink viscosity. 
     In another variation, in the circulation purging, the first valve  87  and the second valve  88  are both opened and the first ink circulation and the second ink circulation are simultaneously done. Also, the internal passages of the inkjet head may be arranged to form only one circulation passage. In this case, the circulation purging is carried out in this one circulation passage. Alternatively, the internal passages of the inkjet head may be arranged to form three or more circulation passages. In this case, the circulation purging may be carried out by selectively switching these circulation passages. 
     In addition to the above, the head controller  61  may change, in accordance with the ink viscosity stored in the viscosity memory unit  65 , the output timing and/or waveform of the drive signal with which the actuator unit  21  is driven. The head controller may not change how to control the actuator unit  21 , even if the ink viscosity is changed. 
     In the embodiment above, the viscosity calculating unit  64  generates a signal indicating information to prompt the user to discard the ink when a calculated ink viscosity is out of a predetermined range. In this regard, fresh ink may be supplied to the sub-tank  80  when a calculated ink viscosity is out of a predetermined range. In an alternative arrangement, when the printer of the embodiment above has an apparatus to control a temperature of ink in the sub-tank  80 , the temperature of the ink in the sub-tank  80  is increased when a calculated ink viscosity is higher than an allowable range whereas the temperature of the ink in the sub-tank  80  is decreased when a calculated ink viscosity is lower than the allowable range. Alternatively, a diluent may be injected into the ink in the sub-tank  80  when a calculated ink viscosity is higher than the allowable range. 
     In addition to the above, the embodiment is arranged so that the revolution of the pump  86  is detected after the ink flow becomes stable after a predetermine time elapses from the start of the first ink circulation, and the viscosity calculating unit  64  calculates the ink viscosity from the difference between the detected revolution and the instructed revolution of the pump  86 . In this regard, the timing to calculate the ink viscosity may be different from the above and may be carried out at an arbitrary timing. For example, the ink viscosity may be calculated immediately after the start of the first ink circulation. 
     In addition to the above, according to the embodiment, the operation to open the first valve  87  or the second valve  88  to switch between the first ink circulation and the second ink circulation indicates that the valve is fully opened. Alternatively, the ink flow rate may be adjusted by opening each of the first valve  87  and the second valve  88  by half or by an arbitrary degree. 
     In the embodiment above, the ejection energy applying element is an unimorph piezoelectric actuator. Alternatively, the ejection energy applying element may be a bimorph piezoelectric actuator or a heating element which generates bubbles in ink by heating the ink. 
     The present invention may be used for other liquid ejection apparatuses which eject droplets other than ink. 
     While this invention has been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention as set forth above are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention as defined in the following claims.