Patent Publication Number: US-7588308-B2

Title: Liquid ejection apparatus and air bubble determination method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid ejection apparatus and an air bubble determination method, and more particularly, to ejection determination in a liquid ejection apparatus which forms an image, or the like, on a medium by ejecting liquid from a nozzle. 
     2. Description of the Related Art 
     Inkjet recording apparatuses which comprise an inkjet head having a plurality of nozzles and record images onto a medium by ejecting ink toward the medium from the inkjet head, are known. 
     In an inkjet recording apparatus, if there is an increase in the viscosity of the ink or infiltration of air bubbles into the inkjet head, or if dirt, paper dust, or other foreign matter adheres to the ink ejection surface, then the nozzles may become blocked and it is difficult to eject ink droplets. If the nozzle blockages occur, then dot omissions occur in the image formed on the medium, and this causes degradation of the image quality. Some inkjet recording apparatuses are composed in such a manner that the nozzle blockages are determined and a maintenance operation is carried out in respect of the nozzles thus determined to have blockages. 
     An embodiment of ejection abnormality determination in nozzles relating to the related art is described with respect to  FIGS. 17A to 17D . In the ejection abnormality determination according to the related art, the pressure generated in the pressure chambers is determined by means of pressure sensors provided in the pressure chambers, and ejection abnormalities in the nozzles connected to the pressure chambers are judged on the basis of the magnitude correlation between a peak value V p  of the waveform of the generated pressure and a prescribed threshold value V th . 
       FIG. 17A  shows a sensor signal  300  obtained from a pressure sensor. As shown in  FIG. 17A , the sensor signal  300  has a voltage (waveform) that is directly proportional to the pressure of the pressure chamber (pressure waveform), and a pressure abnormality in the corresponding pressure chamber is judged on the basis of the magnitude correlation between the peak value V p  of the sensor signal  300  and the predetermined threshold voltage V th . 
     A composition is adopted in which, if the peak value V p  of the sensor signal  300  is greater than the threshold voltage V th , then the pulse signal  302  is obtained as shown in  FIG. 17B . Furthermore, if the pulse signal  302  is obtained, then it is judged that the pressure in the pressure chamber is normal and that the nozzle connected to the pressure chamber is in normal ejection state. 
     On the other hand, the sensor signal  304  shown in  FIG. 17C  has a peak value V′ p  that is smaller than the threshold voltage V th , and therefore the pulse signal (represented by numerical sign “ 302 ” in  FIG. 17B ) is not obtained as shown in  FIG. 17D . If the pulse signal is not obtained, then it is judged that the pressure abnormality has occurred in the corresponding pressure chamber, and hence it is judged that the nozzle connected to the pressure chamber is in ejection abnormality state. In other words, by setting the threshold voltage V th  to an appropriate value, it is possible to judge that the pressure abnormality in the pressure chamber occurs and therefore the nozzle connected to the pressure chamber suffering the pressure abnormality is in the ejection abnormality state. 
     Furthermore, systems have also been proposed in which maintenance processing is carried out for the nozzles at a constant time interval, without determining the ejection abnormalities in the nozzles. In the system which carries out maintenance processing for the nozzles before the occurrence of the ejection abnormality in this way, it is possible to prevent the occurrence of the ejection abnormalities, in advance, by setting the time interval for maintenance processing to a suitable time. 
     The invention disclosed in Japanese Patent Application Publication No. 10-114074 relates to an inkjet recording head comprising electrostrictive vibrating elements provided respectively in the ink flow channels of a plurality of nozzles, which ejects ink droplets by applying a drive voltage to the electrostrictive vibrating elements. The inkjet recording head further comprises an air bubble determination device which determines the presence or absence of air bubbles in the ink flow channels by determining whether the voltage occurring in the electrostrictive vibrating elements due to the volume change in the ink flow channels becomes an excess voltage (in other words, the value of the voltage occurring in the electrostrictive vibrating elements becomes the value of the drive voltage or more) or not constantly during a printing operation. 
     However, in the embodiments shown in  FIGS. 17A to 17D , the pressure abnormalities in the pressure chambers are judged on the basis of one constant threshold voltage. Therefore, even if there is a pressure abnormality that does not reach a sufficient level to affect ejection, it is judged on the basis of the pressure abnormality that the ejection abnormality has occurred, and hence the maintenance processing is carried out for the corresponding nozzle. 
     Furthermore, in a composition in which maintenance processing is carried out at a constant time interval, an expensive timer circuit is required in order to manage the interval for the maintenance processing, and since the maintenance is carried out irrespectively of the presence or absence of the ejection abnormalities, then the ink consumption increases, which is not economical. 
     In the invention disclosed in Japanese Patent Application Publication No. 10-114074, the presence or absence of air bubbles is determined by determining whether the voltage occurring in the electrostrictive vibrating elements becomes an excess voltage, which is the drive voltage or more. Therefore, if there is even an air bubble of a small size which is not sufficient to affect ink droplet ejection, it is judged that the ejection abnormality has occurred. Therefore, unnecessary and wasteful restoration processing is carried out, and the ink consumption increases, which is not economical. 
     SUMMARY OF THE INVENTION 
     The present invention has been contrived in view of the foregoing circumstances, an object thereof being to provide a liquid ejection apparatus and an air bubble judgment method whereby the presence or absence of an ejection abnormality can be determined by means of a simple composition and method. 
     In order to attain the aforementioned object, the present invention is directed to a liquid ejection apparatus, comprising: a liquid ejection head including a nozzle which ejects liquid, a pressure chamber provided to correspond to the nozzle, a pressure generating element which is disposed on a first wall surface of the pressure chamber and adjusts pressure in the pressure chamber, and a determination element which is disposed on a second wall surface of the pressure chamber and generates a first determination signal corresponding to the pressure in the pressure chamber adjusted by the pressure generating element; a threshold value setting device which sets a threshold value in accordance with a waveform of the first determination signal generated by the determination element; a comparing device which acquires a comparison result obtained by comparing the first determination signal with the threshold value which is set by the threshold value setting device; and an evaluation device which acquires an evaluation result obtained by evaluating a size of an air bubble present in the pressure chamber according to the comparison result acquired by the comparing device. 
     According to this aspect of the present invention, the threshold value is set on the basis of the determination signal which is obtained from the determination element and is directly proportional to the pressure in the pressure chamber, and the size of an air bubble present in the pressure chamber is evaluated from the result of comparing the threshold value with the waveform of the determination signal. Therefore, it is possible to determine the size of the air bubble occurring in the pressure chamber by means of such a simple composition, and furthermore, improved determination accuracy can be expected. 
     In other words, when an air bubble occurs inside the pressure chamber, the pressure loss which is directly proportional to the size of the air bubble occurs in the pressure chamber, and hence the waveform variation corresponding to the pressure loss (a voltage drop directly proportional to the pressure loss) appears in the determination signal. Therefore, by setting the threshold value in accordance with the variation in the waveform of the determination signal, the threshold value that is suitable for determining the size of the air bubble is set. 
     A fluorine resin-based piezoelectric element made of PVDF (polyvinylidene fluoride) or the like, is suitable for use as the pressure determination element. Furthermore, a ceramic-based piezoelectric element (piezoelectric actuator) made of PZT (lead zirconate titanate) or the like, is suitable for use as the pressure generating element. If a plurality of piezoelectric element parts are required for the pressure determination element(s) and the pressure generating element(s), then it is possible to adopt a composition in which a piezoelectric body is formed as a single member for all of the plurality of pressure chambers, and drive signal application electrodes are provided in the respective parts of the piezoelectric body corresponding to the pressure chambers, or a composition in which piezoelectric bodies are formed respectively for the pressure chambers and drive signal application electrodes are provided respectively for the piezoelectric bodies. 
     A configuration is also possible in which the liquid ejection apparatus comprises a signal processing device which carries out signal processing, such as amplification or noise reduction, on the determination signal obtained from the determination element. 
     The liquid ejection head may be a line type head including a row of ejection holes having a length corresponding to the full width of the recording medium (the width of the possible image formation region of the recording medium), or a serial head in which a short head including a row of ejection holes having a length that does not reach the full width of the recording medium is used, and this head is made to scan in the breadthways direction of the recording medium. 
     Such a line type of liquid ejection head may have a length corresponding to the full width of the recording medium by combining short heads having rows of ejection holes which do not reach a length corresponding to the full width of the recording medium in such a manner that the short heads are joined together in a staggered matrix fashion. 
     The state where pressure of prescribed pressure value in the pressure chamber is generated (adjusted) by means of the pressure generating element, includes a state where the pressure generated in the pressure chamber serves as an ejection force and consequently liquid of a prescribed volume is ejected from the nozzle. In other words, the pressure generating element may also function as an ejection force generating element that applies the ejection force to the liquid inside the pressure chamber. 
     The first wall surface and the second wall surface of the pressure chamber may be formed separately, or may be formed integrally. Thus, the pressure generating element and the pressure determination element may be provided on the same wall, or may be provided on the separate walls respectively. 
     Preferably, the liquid ejection apparatus further comprises: an air bubble removal device which carries out an air bubble removal processing with respect to the liquid ejection head; and a control device which controls the air bubble removal device according to the evaluation result acquired by the evaluation device. 
     According to this aspect of the present invention, the air bubble removal processing is carried out with respect to the liquid ejection head in accordance with the evaluation result of the evaluation device, and therefore desirable air bubble removal processing is carried out in accordance with the size of the air bubble present in the pressure chamber. 
     For example, there is a mode in which, if a (small) air bubble of a size that dose not affect liquid ejection is present in the pressure chamber, then the air bubble removal processing is not carried out. In such a mode where the air bubble removal processing is not carried out if the size of an air bubble present in the pressure chamber is small enough not to affect liquid ejection, it is possible to restrict ink consumption composition due to the air bubble removal processing. Furthermore, since it is possible to reduce the number of times for the air bubble removal processing, then the load of the air bubble removal processing is reduced. 
     The air bubble removal processing includes processes known as maintenance processing, such as purging and suctioning, and there is a mode in which processing such as removal of ink of increased viscosity inside a nozzle is carried out simultaneously with the removal of air bubbles inside the pressure chambers. For the air bubble removal processing, it is possible to use restoration processing which is carried out when the apparatus is initialized by switching on the power source, changing the settings, or the like. 
     Preferably, the control device controls the air bubble removal device in such a manner that, when the size of the air bubble present in the pressure chamber evaluated by the evaluation device is sufficient to cause an ejection abnormality, the air bubble removal processing is carried out with respect to the liquid ejection head. 
     According to this aspect of the present invention, since the air bubble removal processing is carried out if the air bubble that is liable to affect liquid ejection (the air bubble that is liable to lead to the ejection abnormality) is present in the pressure chamber, then it is possible to prevent the ejection abnormalities caused by the presence of an air bubble in the pressure chamber, in advance. 
     Preferably, the threshold value setting device sets the threshold value according to a maximum value of the first determination signal generated by the determination element. 
     According to this aspect of the present invention, since the threshold value is set on the basis of the maximum value of the determination signal, which is correlated with the size of the air bubble, then it is possible to evaluate the size of the air bubble by means of such a simple method. 
     Preferably, the liquid ejection apparatus further comprises a reference maximum value storage device which stores the maximum value of the first determination signal as a reference maximum value, wherein the determination element further generates a second determination signal corresponding to the pressure in the pressure chamber; and the threshold value setting device sets the threshold value to a differential value between the reference maximum value stored in the reference maximum value storage device, and a maximum value of the second determination signal generated by the determination element. 
     According to this aspect of the present invention, since the differential between a previously stored reference maximum value and a determination signal obtained from the determination device is acquired and is set as the threshold value, then it is possible to evaluate the size of the air bubble with good accuracy. 
     The maximum value storage device may also function as another storage device (storage element). 
     It is also possible to set the threshold value to a value obtained by adding the differential between the reference maximum value and the maximum value of the determination signal obtained from the determination element, to a predetermined reference threshold value. Desirably, the liquid ejection apparatus comprises a storage device (threshold value storage device) storing an initial threshold value (default value) which is used as the reference threshold value. 
     Preferably, the liquid ejection head includes a plurality of the nozzles, a plurality of the pressure chambers corresponding to the nozzles, and a plurality of the determination elements corresponding to the pressure chambers; and the threshold value setting device sets the threshold value for each of the pressure chambers. 
     According to this aspect of the present invention, since threshold values are set respectively for the plurality of nozzles (or for the plurality of pressure chambers), then desirable air bubble evaluation in which the individual differences (variations) between the pressure determination elements provided in the pressure chambers are taken account of, is carried out. 
     In order to attain the aforementioned object, the present invention is directed to an air bubble determination method for a liquid ejection head including a nozzle which ejects liquid, a pressure chamber provided to correspond to the nozzle, a pressure generating element which is disposed on a first wall surface of the pressure chamber and adjusts pressure in the pressure chamber, and a determination element which is disposed on a second wall surface of the pressure chamber and generates a determination signal corresponding to the pressure in the pressure chamber adjusted by the pressure generating element, the air bubble determination method comprising the steps of: adjusting the pressure in the pressure chamber by driving the pressure generating element; acquiring the determination signal generated by the determination element when the pressure in the pressure chamber is adjusted by the pressure generating element; setting a threshold value according to the determination signal generated by the determination element; acquiring a comparison result obtained by comparing the determination signal with the threshold value; and acquiring an evaluation result obtained by evaluating a size of an air bubble present in the pressure chamber according to the comparison result. 
     In the step of generating (adjusting) the pressure in the pressure chamber, the pressure of prescribed pressure value in the pressure chamber may be generated (adjusted) by means of the pressure generating element in such a manner that liquid of a prescribed volume is ejected from the nozzle. 
     Preferably, the air bubble determination method further comprises the step of implementing an air bubble removal processing with respect to the liquid ejection head according to the evaluation result. 
     Preferably, the threshold value is set, each time the pressure in the pressure chamber is adjusted by driving the pressure generating element; the comparison result is acquired by comparing the threshold value with the determination signal generated by the determination element, each time the pressure in the pressure chamber is adjusted by driving the pressure generating element; and the evaluation result is acquired by evaluating the size of the air bubble present in the pressure chamber, each time the pressure in the pressure chamber is adjusted by driving the pressure generating element. 
     According to this aspect of the present invention, it is possible to evaluate the size of the air bubble in real time, each time the pressure in the pressure chamber is adjusted (i.e., each time the pressure generation step is carried out). Furthermore, since the air bubble removal processing is carried out in real time, it is not necessary to perform a maintenance process at regular time intervals. 
     Preferably, the air bubble determination method further comprises the steps of: performing a restoration processing with respect to the liquid ejection head; and storing a maximum value of the determination signal as a reference maximum value, the determination signal being generated by the determination element when the pressure in the pressure chamber is adjusted by the pressure generating element immediately after the restoration processing is performed. 
     According to this aspect of the present invention, the maximum value of the determination signal obtained when the pressure in the pressure chamber is generated (adjusted) immediately after carrying out the restoration processing (i.e., the maximum value of the determination signal obtained in an ideal state where there is no air bubble), is stored as the reference maximum value. 
     The term “immediately after the restoration processing” may also include a time period from the end of the restoration processing step until the pressure generating elements is driven by means of a prescribed drive signal. 
     According to the present invention, the threshold value is set on the basis of the determination signal which is obtained from the determination element and is directly proportional to the pressure in the pressure chamber, and furthermore the size of an air bubble present in the pressure chamber is evaluated on the basis of the result of comparison between the threshold value and the waveform of the determination signal. Therefore, it is possible to determine the size of the air bubble occurring in the pressure chamber by means of a simple composition, and furthermore, improved determination accuracy is expected. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The nature of this invention, as well as other objects and benefits thereof, are explained in the following with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures and wherein: 
         FIG. 1  is a general schematic drawing of an inkjet recording apparatus relating to an embodiment of the present invention; 
         FIG. 2  is a principal plan diagram of the peripheral area of a print unit in the inkjet recording apparatus shown in  FIG. 1 ; 
         FIGS. 3A to 3C  are plan view perspective diagrams showing embodiments of the composition of a head; 
         FIG. 4  is a cross-sectional diagram showing the three-dimensional structure of a head; 
         FIG. 5  is a cross-sectional diagram showing a further embodiment of the structure of the head shown in  FIG. 4 ; 
         FIG. 6  is a block diagram showing the approximate composition of an ink supply system of the inkjet recording apparatus shown in  FIG. 1 ; 
         FIG. 7  is a principal block diagram showing the system configuration of the inkjet recording apparatus shown in  FIG. 1 ; 
         FIG. 8  is a block diagram showing the composition of the signal processing unit shown in  FIG. 7 ; 
         FIGS. 9A and 9B  are diagrams showing a sensor signal obtained from a sensor; 
         FIG. 10  is a diagram for describing the details of the sensor signal shown in  FIGS. 9A  and  9 B; 
         FIGS. 11A and 11B  are diagrams for describing a sensor signal and a pulse signal during normal ejection; 
         FIGS. 12A to 12C  are diagrams for describing a sensor signal and a pulse signal in a case where a small-size air bubble has occurred; 
         FIGS. 13A to 13C  are diagrams for describing a sensor signal and a pulse signal in a case where a medium-size air bubble has occurred; 
         FIGS. 14A to 14C  are diagrams for describing a sensor signal and a pulse signal in a case where a large-size air bubble has occurred; 
         FIG. 15  is a flowchart showing a control sequence for ejection abnormality determination according to an embodiment of the present invention; 
         FIG. 16  is a flowchart showing a control sequence for the reference peak value determination shown in  FIG. 15 ; and 
         FIGS. 17A to 17D  are diagrams for describing ejection abnormality determination according to the related art. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     General Configuration of Inkjet Recording Apparatus 
       FIG. 1  is a general configuration diagram of an inkjet recording apparatus (a liquid ejection apparatus) according to an embodiment of the present invention. As shown in  FIG. 1 , the inkjet recording apparatus  10  comprises: a printing unit  12  having a plurality of inkjet heads (hereafter, called “heads”)  12 K,  12 C,  12 M, and  12 Y provided for ink colors of black (K), cyan (C), magenta (M), and yellow (Y), respectively; an ink storing and loading unit  14  for storing inks of K, C, M and Y to be supplied to the heads  12 K,  12 C,  12 M, and  12 Y; a paper supply unit  18  for supplying recording paper  16  which is a recording medium; a decurling unit  20  removing curl in the recording paper  16 ; a suction belt conveyance unit  22  disposed facing the nozzle face (ink-droplet ejection face) of the printing unit  12 , for conveying the recording paper  16  while keeping the recording paper  16  flat; and a paper output unit  26  for outputting image-printed recording paper (printed matter) to the exterior. 
     The ink storing and loading unit  14  has ink supply tanks for storing the inks of K, C, M and Y to be supplied to the heads  12 K,  12 C,  12 M, and  12 Y, and the tanks are connected to the heads  12 K,  12 C,  12 M, and  12 Y by means of prescribed channels. The ink storing and loading unit  14  has a warning device (for example, a display device or an alarm sound generator) for warning when the remaining amount of any ink is low, and has a mechanism for preventing loading errors among the colors. 
     As shown in  FIG. 1 , a composition which has a magazine for rolled paper (continuous paper) as an embodiment of the paper supply unit  18  is adopted; however, more magazines with paper differences such as paper width and quality may be jointly provided. Moreover, papers may be supplied with cassettes that contain cut papers loaded in layers and that are used jointly or in lieu of the magazine for rolled paper. 
     In the case of a configuration in which a plurality of types of recording paper  16  can be used, it is preferable that an information recording medium such as a bar code and a wireless tag containing information about the type of paper is attached to the magazine, and by reading the information contained in the information recording medium with a predetermined reading device, the type of recording paper  16  to be used (type of medium) is automatically determined, and ink-droplet ejection is controlled so that the ink-droplets are ejected in an appropriate manner in accordance with the type of medium. 
     The recording paper  16  delivered from the paper supply unit  18  retains curl due to having been loaded in the magazine. In order to remove the curl, heat is applied to the recording paper  16  in the decurling unit  20  by a heating drum  30  in the direction opposite from the curl direction in the magazine. The heating temperature at this time is preferably controlled so that the recording paper  16  has the curl in which the surface on which the print is to be made is slightly round outward. 
     In the case of the configuration in which roll paper is used, a cutter (first cutter)  28  is provided as shown in  FIG. 1 , and the continuous paper is cut into a desired size by the cutter  28 . The cutter  28  has a stationary blade  28 A, whose length is not less than the width of the conveyor pathway of the recording paper  16 , and a round blade  28 B, which moves along the stationary blade  28 A. The stationary blade  28 A is disposed on the reverse side of the printed surface, and the round blade  28 B is disposed on the printed surface side across the conveyor pathway. When cut papers are used, the cutter  28  is not required. 
     The recording paper  16  that is decurled and cut is delivered to the suction belt conveyance unit  22 . The suction belt conveyance unit  22  has a configuration in which an endless belt  33  is set around rollers  31  and  32  so that the portion of the endless belt  33  facing at least the nozzle face of the printing unit  12  forms a horizontal plane (flat plane). 
     The belt  33  has a width that is greater than the width of the recording paper  16 , and a plurality of suction apertures (not shown) are formed on the belt surface. A suction chamber  34  is disposed in a position facing the nozzle surface of the printing unit  12  on the interior side of the belt  33 , which is set around the rollers  31  and  32 , as shown in  FIG. 1 . The suction chamber  34  provides suction with a fan  35  to generate a negative pressure, and the recording paper  16  is held on the belt  33  by suction. 
     The belt  33  is driven in the clockwise direction in  FIG. 1  by the motive force of a motor ( 88  in  FIG. 7 ) being transmitted to at least one of the rollers  31  and  32 , which the belt  33  is set around, and the recording paper  16  held on the belt  33  is conveyed from left to right in  FIG. 1 . 
     Since ink adheres to the belt  33  when a marginless print job or the like is performed, a belt-cleaning unit  36  is disposed in a predetermined position (a suitable position outside the printing area) on the exterior side of the belt  33 . Although the details of the configuration of the belt-cleaning unit  36  are not shown, embodiments thereof include a configuration of nipping a brush roller and a water absorbent roller, an air blow configuration in which clean air is blown, or a combination of these. In the case of the configuration of nipping the cleaning rollers, it is preferable to make the line velocity of the cleaning rollers different from that of the belt  33  to improve the cleaning effect. 
     The inkjet recording apparatus  10  can comprise a roller nip conveyance mechanism, instead of the suction belt conveyance unit  22 . However, there is a drawback in the roller nip conveyance mechanism that the print tends to be smeared when the printing area is conveyed by the roller nip action because the nip roller makes contact with the printed surface of the paper immediately after printing. Therefore, the suction belt conveyance in which nothing comes into contact with the image surface in the printing area is preferable. 
     A heating fan  40  is disposed on the upstream side of the printing unit  12  in the conveyance pathway formed by the suction belt conveyance unit  22 . The heating fan  40  blows heated air onto the recording paper  16  to heat the recording paper  16  immediately before printing so that the ink deposited on the recording paper  16  dries more easily. 
     The heads  12 K,  12 C,  12 M and  12 Y of the printing unit  12  are full line heads having a length corresponding to the maximum width of the recording paper  16  used with the inkjet recording apparatus  10 , and comprising a plurality of nozzles for ejecting ink arranged on a nozzle face through a length exceeding at least one edge of the maximum-size recording paper  16  (i.e., the full width of the printable range) (see  FIG. 2 ). 
     The heads  12 K,  12 C,  12 M and  12 Y are arranged in color order (black (K), cyan (C), magenta (M), yellow (Y)) from the upstream side in the feed direction (paper feed direction) of the recording paper  16 , and these heads  12 K,  12 C,  12 M and  12 Y are fixed extending in a direction substantially perpendicular to the paper conveyance direction. 
     A color image can be formed on the recording paper  16  by ejecting inks of different colors from the heads  12 K,  12 C,  12 M and  12 Y, respectively, onto the recording paper  16  while the recording paper  16  is conveyed by the suction belt conveyance unit  22 . 
     By adopting a configuration in which the full line heads  12 K,  12 C,  12 M and  12 Y having nozzle rows covering the full paper width are provided for the respective colors in this way, it is possible to record an image on the full surface of the recording paper  16  by performing just one operation (one sub-scanning operation) of relatively moving the recording paper  16  and the printing unit  12  in the paper conveyance direction (the sub-scanning direction), in other words, by means of a single sub-scanning action. Higher-speed printing is thereby made possible and productivity can be improved in comparison with a shuttle type head configuration in which a recording head reciprocates in the main scanning direction. 
     Although the configuration with the KCMY four standard colors is adopted in the present embodiment, combinations of the ink colors and the number of colors are not limited to those. Light inks, dark inks or special color inks can be added as required. For example, a configuration is possible in which inkjet heads for ejecting light-colored inks such as light cyan and light magenta are added. Furthermore, there are no particular restrictions of the sequence in which the heads of respective colors are arranged. 
     A post-drying unit  42  is disposed following the print unit  12 . The post-drying unit  42  is a device to dry the printed image surface, and includes a heating fan, for example. It is preferable to avoid contact with the printed surface until the printed ink dries, and a device that blows heated air onto the printed surface is preferable. 
     In cases in which printing is performed with dye-based ink on porous paper, blocking the pores of the paper by the application of pressure prevents the ink from coming contact with ozone and other substance that cause dye molecules to break down, and has the effect of increasing the durability of the print. 
     A heating/pressurizing unit  44  is disposed following the post-drying unit  42 . The heating/pressurizing unit  44  is a device to control the glossiness of the image surface, and the image surface is pressed with a pressure roller  45  having a predetermined uneven surface shape while the image surface is heated, and the uneven shape is transferred to the image surface. 
     The printed matter generated in this manner is outputted from the paper output unit  26 . The target print (i.e., the result of printing the target image) and the test print are preferably outputted separately. In the inkjet recording apparatus  10 , a sorting device (not shown) is provided for switching the outputting pathways in order to sort the printed matter with the target print and the printed matter with the test print, and to send them to paper output units  26 A and  26 B, respectively. When the target print and the test print are simultaneously formed in parallel on the same large sheet of paper, the test print portion is cut and separated by a cutter (second cutter)  48 . The cutter  48  is disposed directly in front of the paper output unit  26 , and is used for cutting the test print portion from the target print portion when a test print has been performed in the blank portion of the target print. The structure of the cutter  48  is the same as the first cutter  28  described above, and has a stationary blade  48 A and a round blade  48 B. 
     Although not shown in  FIG. 1 , the paper output unit  26 A for the target prints is provided with a sorter for collecting prints according to print orders. 
     Structure of the Liquid Ejection Head 
     Next, the structure of a liquid ejection head (hereinafter referred to as a head) is described. The heads  12 K,  12 C,  12 M and  12 Y of the respective ink colors have the same structure, and a reference numeral  50  is hereinafter designated to any of the heads. 
       FIG. 3A  is a plan view perspective diagram showing an embodiment of the structure of a head  50 , and  FIG. 3B  is an enlarged diagram of a portion of same. Furthermore,  FIG. 3C  is a plan view perspective diagram showing a further embodiment of the composition of a head  50 , and  FIG. 5  is a cross-sectional diagram showing a three-dimensional composition of an ink chamber unit (being a cross-sectional view along line  4 - 4  in  FIGS. 3A and 3B ). 
     The nozzle pitch in the head  50  is required to be minimized in order to maximize the density of the dots formed on the surface of the recording paper  16 . As shown in  FIGS. 3A to 3C , the head  50  according to the present embodiment has a structure in which a plurality of ink chamber units (ejection elements)  53 , each comprising a nozzle  51  forming an ink droplet ejection port, a pressure chamber  52  corresponding to the nozzle  51 , and the like, are disposed two-dimensionally in the form of a staggered matrix, and hence the effective nozzle interval (the projected nozzle pitch) as projected in the lengthwise direction of the head (the main-scanning direction perpendicular to the paper conveyance direction) is reduced and high nozzle density is achieved. 
     The composition of forming one or more nozzle rows through a length corresponding to the entire width of the recording paper  16  in a direction substantially perpendicular to the conveyance direction of the recording paper  16  is not limited to the embodiment described above. For example, instead of the configuration as described with reference to  FIG. 3A , a line head having nozzle rows of a length corresponding to the entire width of the recording paper  16  can be formed by arranging and combining, in a staggered matrix, short head blocks  50 ′ having a plurality of nozzles  51  arrayed in a two-dimensional fashion, as shown in  FIG. 3C . 
     The present embodiment describes an aspect in which the planar shape of the pressure chambers  52  is substantially a square shape, but the planar shape of the pressure chambers  52  is not limited to a substantially square shape, and it is possible to adopt various other shapes, such as a substantially circular shape, a substantially elliptical shape, a substantially parallelogram (diamond) shape, or the like. Furthermore, the arrangement of the nozzles  51  and the supply ports  54  is not limited to the arrangement shown in  FIGS. 3A to 3C , and it is also possible to arrange nozzles  51  substantially in the central region of the pressure chambers  52 , or to arrange the supply ports  54  in the side walls of the pressure chambers  52 . 
     As shown in  FIG. 3B , the high-density-nozzle head according to the present embodiment is achieved by arranging a plurality of nozzles in a lattice configuration, according to a fixed arrangement pattern following a row direction which is aligned with the main scanning direction, and an oblique column direction which forms a prescribed, non-perpendicular angle θ with respect to the main scanning direction. 
     In other words, by adopting a structure in which a plurality of ejection elements  53  are arranged at a uniform pitch d in line with a direction forming an angle of θ with respect to the main scanning direction, the pitch P of the nozzles  51  projected to an alignment in the main scanning direction is d×cos θ, and hence it is possible to treat the nozzles as if they are arranged linearly at a uniform pitch of P. By means of this composition, it is possible to achieve a nozzle composition of high density, in which the nozzle columns projected to an alignment in the main scanning direction reach a total of 2400 per inch (2400 nozzles per inch). 
     When implementing the present invention, the arrangement structure of the nozzles is not limited to the embodiment shown in  FIG. 3A , and it is also possible to apply various other types of nozzle arrangements, such as an arrangement structure having one nozzle row in the sub-scanning direction. 
       FIG. 4  is a cross-sectional diagram showing the three-dimensional composition of ejection elements  53 . As shown in  FIG. 4 , a piezoelectric actuator  58  (pressure generating device) provided with an individual electrode  57  is bonded to a pressure plate  56 . The pressure plate  56  forms the upper face of the pressure chambers  52  and also serves as a common electrode. The piezoelectric actuator  58  is deformed when a drive voltage (drive signal) is supplied to the individual electrode  57 , thereby causing ink to be ejected from the nozzle  51 . When ink is ejected, new ink is supplied to the pressure chamber  52  from a common flow passage  55 , via the supply port  54 . 
     On the other hand, if a sensor  59  (determination element) provided on the opposite side of the pressure chamber  52  from the piezoelectric actuator  58  receives the pressure due to ejection or refilling of the ink, or the like, then distortion (stress) corresponding to the pressure occurs in the sensor  59 , and a voltage corresponding to the distortion is obtained from the sensor  59  as a determination signal (sensor signal). In other words, it is possible to extract the voltage (waveform) corresponding to the pressure generated in the pressure chamber  52 , from the sensor  59 . 
     In this inkjet recording apparatus  10 , the size of an air bubble present in the pressure chamber  52  is determined on the basis of the sensor signal obtained from the sensor  59 , and it is judged whether or not the ejection abnormality has occurred in the nozzle  51  connected to the pressure chamber  52 , on the basis of the determined size of the air bubble. 
     The sensor  59  is provided with extraction electrodes  100  and  102  for the sensor signal, which are provided respectively on the surface adjacent to the pressure chamber and the surface opposite to the pressure chamber, in such a manner that the sensor signal is obtained from the extraction electrode  100  on the pressure chamber side and the extraction electrode  102  on the side opposite to the pressure chamber. 
     It is possible to adopt a floating-output type of sensor in which the extraction electrode  102  outputs an inverted signal which is equivalent to a signal obtained by inverting the sensor signal output from the extraction electrode  100 , for the sensor  59  shown in the present embodiment. In other words, the sensor signal obtained from the extraction electrode  100  and the sensor signal obtained from the extraction electrode  102  have substantially the same phase and have a mutually inverse relationship. 
     The surface of the extraction electrode  100  on the pressure chamber side and the surface of the extraction electrode  102  on the opposite side to the pressure chamber  52  are insulated. It is preferable that a cavity part is provided on the opposite side of the extraction electrode  102  of the sensor  59  from the pressure chamber  52 , in such a manner that the displacement of the sensor  59  is not obstructed. 
     Furthermore, a flexible cable  110  (a flexible printed circuit board) having a wiring pattern (not shown) for transmitting drive signals to be applied to the piezoelectric actuators  58  and sensor signals obtained from the sensors  59  is provided on the opposite side of the piezoelectric actuators  58  from the pressure plate  56 . Between the flexible cable  110  and the pressure plate  56 , a cavity part  112  between the piezoelectric actuator  58  and the flexible cable  110  is formed, and a supporting member  114  which supports the flexible cable  110  from below is provided. 
     By providing the cavity part  112  to the upper side of each piezoelectric actuator  58  (between the piezoelectric actuator  58  and the flexible cable  110 ), it is possible to suppress loss of the pressure generated by the piezoelectric actuators  58 , without restricting the displacement of the piezoelectric actuators  58  when the piezoelectric actuators  58  are driven. 
     The flexible cable  110  has a conducting layer made of copper, or the like, which is surrounded by supporting layer (insulating layer) made of a resin material, such as epoxy or polyimide. In the present embodiment, the flexible cable used has a multi-layer structure in which a three or more conducting layers and a plurality of supporting layers are bonded together alternately. 
     The individual electrodes  57  of the piezoelectric actuators  58  are connected to horizontal wires (not shown) formed on the piezoelectric actuator installation surface  56 A of the pressure plate  56  (in other words, the individual electrodes  57  are extended onto the piezoelectric actuator installation surface of the pressure plate  56  and are bonded electrically to the horizontal wires), and each of the horizontal wires is connected to a vertical wire  120  (indicated by broken lines in the diagram) penetrating through the supporting member  114 . Moreover, the vertical wires  120  are electrically connected to the wiring pattern of the flexible cable  110 . 
     In other words, the drive signals to be supplied to the piezoelectric actuators  58  are transmitted from the head driver (reference numeral “ 84 ” in  FIG. 7 ) to the individual electrodes  57  of the piezoelectric actuators  58 , through the wiring pattern of the flexible cable  110 , the vertical wires  120 , and the horizontal wires (not shown). 
     Furthermore, the sensor signals obtained from the sensors  59  are supplied to the signal processing unit  85  shown in  FIG. 7 , via the extraction electrodes  100  and  102 , horizontal wires  122  and  124  connected respectively to the extraction electrodes  100  and  102 , vertical wires  126  and  128  penetrating the flow channel structure  50 A and the supporting member  114 , the pressure plate  56 , the supporting member  114 , and the wiring pattern of the flexible cable  110 . 
     In other words, the drive signal wires, along which the drive signals are transmitted, include the wiring pattern of the flexible cable  110 , the vertical wire  120 , and the horizontal wire (not shown), and the sensor signal wires, along which the sensor signals are transmitted, include the wiring pattern of the flexible cable  110 , the vertical wires  126  and  128 , and the horizontal wires  122  and  124 . 
     For the piezoelectric actuator  58  shown in  FIG. 4 , it is suitable to adopt a piezoelectric element using ceramic material such as PZT (Pb(Zr—Ti)O 3 , lead zirconate titanate). For the sensor  59 , it is suitable to adopt a piezoelectric element using a fluoride resin material, such as a PVDF (polyvinylidene-fluoride) or PVDF-TrFE (a copolymer of polyvinylidene fluoride and trifluoride ethylene). 
     In general, for an actuator which generates the ejection force, it is desirable to use a piezoelectric element having large absolute values of the equivalent piezoelectric constants (d constant, electrical-mechanical conversion constant, piezoelectric strain constant) and excellent drive characteristics; and for a sensor which determines pressure, it is desirable to use a piezoelectric element having large values for the piezoelectric output coefficients (g constant, mechanical relectrical conversion constant, piezoelectric stress constant) and excellent determination characteristics. In other words, a ceramic material, such as PZT, is suitable for the piezoelectric element having excellent drive characteristics, whereas a fluorine-based resin material, such as PVDF or PVDF-TrFE, is suitable for the piezoelectric element having excellent determination characteristics. PZT is basically composed of lead titanate (PbTiO 3 ) ,which is a ferroelectric material, and lead zirconate (PbZrO 3 ), which is an antiferroelectric material. By changing the mixing ratio of the two components, it is possible to control various properties of the ceramic material, such as the piezoelectric, dielectric and elastic characteristics. 
     The piezoelectric actuator  58  which applies the ejection force to the ink inside the pressure chamber  52 , and the sensor  59  which determines the pressure inside the pressure chamber  52  are not restricted to being arranged in the positions shown in  FIG. 4 , and a configuration is possible in which the piezoelectric actuator  58  and the sensor  59  is provided on the same wall of the pressure chamber  52 , or on different walls of the pressure chamber  52  respectively. Furthermore, a mode is also possible in which the piezoelectric actuator  58  and the sensor  59  are provided inside the pressure chamber  52 . In a mode where the piezoelectric actuator  58  and the sensor  59  are provided inside the pressure chamber  52 , a prescribed ink resistance processing (insulation treatment) is applied to the parts of the piezoelectric actuator  58  and the sensor  59  that are exposed to the ink. 
       FIG. 5  illustrates a further embodiment of the structure of a head  50 . The head  50  shown in  FIG. 5  has a vertical wire  120  which is formed so as to rise up in a vertical direction from the individual electrode  57  of the piezoelectric actuator  58  which is provided corresponding to each pressure chamber  52 . 
     Furthermore, the vertical wires  126  and  128 , which transmit the sensor signals, are formed so as to rise up from the extraction electrodes  100  and  102  for the sensor  59  and pass through the flow channel structure  50 A, the pressure plate  56 , and the space where the vertical wires  120  are erected (i.e., formed so as to be erected in the space where the vertical wires  120  are disposed). The reference numerals  130  and  132  shown in  FIG. 5  denote an insulating layer (protecting layer) formed on the pressure chamber side of the extraction electrodes  100  for the sensors  59 , and an insulating layer formed on the opposite side of the extraction electrodes  102  from the pressure chambers, respectively. 
     In this way, the space in which the column-shaped vertical wires  120 ,  126  and  128  are erected between the pressure plate  56  and the flexible cable  110  forms a common flow channel (common liquid chamber)  55  for supplying ink to the pressure chambers  52  via supply side flow channels  54 A and supply ports (supply restrictors)  54 . 
       FIG. 5  shows just a single ejection element  53  and only a portion of the common flow channel  55  and the flexible cable  110 , but the common flow channel  55  of the present embodiment constitutes one large space formed over the whole region in which the pressure chambers  52  are formed, in order to supply ink to all of the pressure chambers  52  shown in  FIG. 3A . The structure of the common flow channel  55  is not limited to one in which it is formed as a single large space in this way, and it may also be formed as a plurality of spaces by dividing into several regions. 
     The vertical wires  120 ,  126  and  128  shown in  FIG. 5  support the flexible cable  110  from below and create a space which forms the common flow channel  55 . The vertical wires  120  which rise up as columns in this way may be referred to as “electrical columns”, and the vertical wires  126  and  128  may be referred to as “sensor columns”. In the present embodiment, each of the vertical wires  120  is formed in a one-to-one correspondence with each of the piezoelectric actuators  58 , and the vertical wires  126  and  128  are formed respectively in a one-to-one correspondence with the extraction electrodes  100  and  102  for the sensors  59 . In order to reduce the number of wires, the wires corresponding to a plurality of piezoelectric actuators  58  may be gathered together into a single vertical wire  120 , and the wires corresponding to a plurality of sensors  59  may be gathered into single vertical wires  126  and  128 . 
     Description of on an Ink Supply System 
       FIG. 6  is a schematic drawing showing the configuration of the ink supply system in the inkjet recording apparatus  10 . 
     The ink supply tank  60  is a base tank that supplies ink and is set in the ink storing and loading unit  14  described with reference to  FIG. 1 . The aspects of the ink supply tank  60  include a refillable type and a cartridge type: when the remaining amount of ink is low, the ink supply tank  60  of the refillable type is filled with ink through a filling port (not shown) and the ink tank  60  of the cartridge type is replaced with a new one. In order to change the ink type in accordance with the intended application, the cartridge type is suitable, and it is preferable to represent the ink type information with a bar code or the like on the cartridge, and to perform ejection control in accordance with the ink type. 
     A filter  62  for removing foreign matters and air bubbles is disposed between the ink supply tank  60  and the head  50  as shown in  FIG. 6 . Preferably, the filter mesh size is not greater than the diameter of the nozzle and commonly about 20 μm. 
     Although not shown in  FIG. 6 , it is preferable to provide a sub-tank integrally to the head  50  or nearby the head  50 . The sub-tank has a damper function for preventing variation in the internal pressure of the head and a function for improving refilling of the head. 
     The inkjet recording apparatus  10  is also provided with a cap  64  as a device to prevent the nozzles  51  from drying out or to prevent an increase in the ink viscosity in the vicinity of the nozzles, and a cleaning blade  66  as a device to clean the nozzle face. A maintenance unit including the cap  64  and the cleaning blade  66  can be relatively moved with respect to the head  50  by a movement mechanism (not shown), and is moved from a predetermined holding position to a maintenance position below the head  50  as required. 
     The cap  64  is displaced up and down relatively with respect to the head  50  by an elevator mechanism (not shown). When the power of the inkjet recording apparatus  10  is turned OFF or when in a print standby state, the cap  64  is raised to a predetermined elevated position so as to come into close contact with the head  50 , and the nozzle face is thereby covered with the cap  64 . 
     During printing or standby, if the use frequency of a particular nozzle  51  is low, and if a state of not ejecting ink continues for a prescribed time period or more, then the solvent of the ink in the vicinity of the nozzle evaporates and the viscosity of the ink increases. In the situation described above, it is difficult to eject ink from the nozzle  51 , even if the piezoelectric actuator  58  is operated. 
     Therefore, before a situation of this kind develops (i.e., while the ink is within a range of viscosity which allows it to be ejected by operation of the piezoelectric actuator  58 ), the piezoelectric actuator  58  is operated, and a preliminary ejection (“purge”, “blank ejection”, “liquid ejection” or “dummy ejection”) is carried out toward the cap  64  (ink receptacle), in order to expel the degraded ink (i.e., the ink in the vicinity of the nozzle which has increased viscosity). 
     Furthermore, if air bubbles enter into the ink inside the head  50  (inside the pressure chamber  52 ), then even if the piezoelectric actuator  58  is operated, it is difficult to eject ink from the nozzle. In the case described above, the cap  64  is placed on the head  50 , the ink (ink containing air bubbles) inside the pressure chamber  52  is removed by suction, by means of a suction pump  67 , and the ink removed by suction is then supplied to a recovery tank  68 . 
     This suction operation is also carried out in order to remove degraded ink having increased viscosity (hardened ink), when ink is loaded into the head for the first time, and when the head starts to be used after having been out of use for a long period of time. Since the suction operation is carried out with respect to all of the ink inside the pressure chamber  52 , the ink consumption is considerably large. Therefore, desirably, preliminary ejection is carried out when the increase in the viscosity of the ink is still minor. 
     In the inkjet recording apparatus  10  described in the present embodiment, the ejection state of a nozzle  51  connected to a pressure chamber  52  is judged (evaluated) on the basis of the size of the air bubble present in the pressure chamber  52 , and if it is judged that the air bubble greater than a prescribed size is present in the pressure chamber  52  and that the ejection abnormality is occurring in the corresponding nozzle  51 , then a maintenance process (air bubble removal process), such as the suctioning described above, is carried out. In the present embodiment, a mode is adopted in which a cap  64  is placed in close contact with the head  50  and the ink inside the head  50  is then suctioned from the nozzles as an air bubble removal process. However, it is also possible to remove the air bubbles inside the pressure chambers  52  by means of processing other than suctioning. 
     The cleaning blade  66  is composed of rubber or another elastic member, and can slide on the ink ejection surface (surface of the nozzle plate) of the head  50  by means of a blade movement mechanism (a wiper) which is not shown in drawings. When ink droplets or foreign matter has adhered to the nozzle plate, the surface of the nozzle plate is wiped and cleaned by sliding the cleaning blade  66  on the nozzle plate. Preliminary ejection is performed in order to prevent foreign matters from entering the nozzle  51  in use of the blade when the ink ejection surface is cleaned by the blade mechanism. 
     Description of Control System 
       FIG. 7  is a principal block diagram showing the system configuration of the inkjet recording apparatus  10 . The inkjet recording apparatus  10  comprises a communications interface  70 , a system controller  72 , a memory  74 , a motor driver  76 , a heater driver  78 , a print controller  80 , an image buffer memory  82 , a head driver  84 , and a signal processing unit  85 , and the like. 
     The communications interface  70  is an interface unit for receiving image data sent from a host computer  86 . A serial interface such as USB (universal serial bus), IEEE 1394, Ethernet™, wireless network, or a parallel interface such as a Centronics interface may be used as the communications interface  70 . A buffer memory (not shown) may be mounted in this portion in order to increase the communication speed. The image data sent from the host computer  86  is received by the inkjet recording apparatus  10  through the communications interface  70 , and is temporarily stored in the memory  74 . 
     The memory  74  is a storage device for temporarily storing images inputted through the communications interface  70 , and data is written and read to and from the memory  74  through the system controller  72 . The memory  74  is not limited to a memory composed of semiconductor elements, and a hard disk drive or another magnetic medium may be used. 
     The system controller  72  is constituted by a central processing unit (CPU) and peripheral circuits thereof, and the like, and it functions as a control device for controlling the whole of the inkjet recording apparatus  10  in accordance with a prescribed program, as well as a calculation device for performing various calculations. More specifically, the system controller  72  controls the various sections, such as the communications interface  70 , memory  74 , motor driver  76 , heater driver  78 , and the like, as well as controlling communications with the host computer  86  and writing and reading to and from the memory  74 , and it also generates control signals for controlling motors of the conveyance system such as the motor  88  and heaters such as the heater  89  for the post drying unit  42 . 
     The program executed by the CPU of the system controller  72  and the various types of data which are required for control procedures are stored in the memory  74 . The memory  74  may be a non-writeable storage device, or it may be a rewriteable storage device, such as an EEPROM. The memory  74  is used as a temporary storage region for the image data, and it is also used as a program development region and a calculation work region for the CPU. 
     The motor driver  76  is a driver (drive circuit) which drives the motor  88  in accordance with instructions from the system controller  72 . Furthermore, the heater driver  78  is a driver which drives the post drying unit  42 , and the heater  89  of the temperature adjustment heater in the inkjet recording apparatus  10  and in the head  50 , and the like, in accordance with instructions from the system controller  72 . 
     The print controller  80  has a signal processing function for performing various tasks, such as correction processing and other types of processing for generating print control signals from the image data stored in the memory  74  in accordance with commands from the system controller  72 , and the print controller  80  supplies the generated print data (dot data) to the head driver  84 . Prescribed signal processing is carried out in the print controller  80 , and the ejection amount and the ejection timing of the ink droplets from the respective heads  50  are controlled via the head driver  84 , on the basis of the print data. By this means, desired dot size and dot positions are achieved. 
     The print controller  80  is provided with the image buffer memory  82 ; and image data, parameters, and other data are temporarily stored in the image buffer memory  82  when image data is processed in the print controller  80 . Also possible is an aspect in which the print controller  80  and the system controller  72  are integrated to form a single processor. 
     The head driver  84  drives the piezoelectric actuators  58  of the heads of the respective colors,  12 K,  12 C,  12 M and  12 Y, on the basis of print data supplied by the print control unit  80 . In other words, in the head driver  84 , drive signals to be supplied to the piezoelectric actuators  58  are generated on the basis of the dot data obtained from the print controller  80 , and the drive signals are supplied to the respective piezoelectric actuators  58  via the prescribed circuitry and wiring. In order to maintain uniform driving conditions in each head, a feedback control system may also be incorporated into the head driver  84 . 
     The image data to be printed is externally inputted through the communications interface  70 , and is stored in the memory  74 . In this stage, the RGB image data is stored in the memory  74 . 
     The image data stored in the memory  74  is sent to the print controller  80  through the system controller  72 , and is converted to the dot data for each ink color in the print controller  80 . In other words, the print controller  80  performs processing for converting the inputted RGB image data into dot data for four colors, K, C, M and Y. The dot data generated by the print controller  80  is stored in the image buffer memory  82 . 
     The head driver  84  generates drive control signals for the head  50  on the basis of the dot data stored in the image buffer memory  82 . By supplying the drive control signals generated by the head driver  84  to each head  50 , the ink is ejected from each head  50 . By controlling the ink ejection from the heads  50  in synchronization with the conveyance velocity of the recording paper  16 , an image is formed on the recording paper  16 . 
     The signal processing unit  85  is a signal processing block which carries out prescribed signal processing on the sensor signals obtained from the sensors  59  in accordance with the pressure in the pressure chambers  52  shown in:  FIG. 4 , compares each of the sensor signals with a prescribed threshold value, and sends the comparison results to an air bubble judgment unit  92  (judgment device) inside the print controller  80 . In the inkjet recording apparatus  10 , the size of the air bubbles present in the pressure chambers  52  is judged (evaluated) on the basis of the comparison results, and if there is an air bubble that is larger than a prescribed size, then the nozzle  51  connected to the pressure chamber  52  where this air bubble is present is judged to be in the ejection abnormality state. If the ejection abnormality state is determined, then the system controller  72  activates the cap movement mechanism (not shown) in such a manner that the cap  64  shown in  FIG. 6  is placed tightly against the nozzle forming surface of the head  50 , and removes the air bubbles inside the head  50  (pressure chambers  52 ) from the nozzles  51  by operating the suction pump  67  shown in  FIG. 6 . In other words, the system controller  72  shown in  FIG. 7  functions as a device for controlling the maintenance process. 
     A detailed description of the signal processing unit  85  and the air bubble judgment control procedure is described later. 
     Various control programs are stored in the program storage unit  90  shown in  FIG. 7 , and the control program is read out and executed in accordance with commands from the system controller  72 . As the program storage unit  90 , a semiconductor memory such as a ROM or EEPROM, or a magnetic disk, or the like, may be used. The program storage unit  90  may be provided with an external interface, and a memory card or PC card may also be used as the program storage unit  90 . Furthermore, of such storage media, various types of storage media may also be provided in combination. The program storage unit  90  may also function as a storage device (not shown) for storing operational parameters, and the like. 
     In the present embodiment, the system controller  72 , the memory  74 , the print controller  80 , and the like, are depicted as separate functional blocks, but they may also be integrated to form one single processor. Furthermore, it is also possible to achieve a portion of the functions of the system controller  72  and a portion of the functions of the print controller  80 , by one processor. 
     Description of the Signal Processing Unit 
     Next, the signal processing unit  85  shown in  FIG. 7  is described.  FIG. 8  is a block diagram showing the approximate composition of the signal processing unit  85 . The signal processing unit  85  comprises: a switch array (multiplexer circuits)  202  having N switching elements  200  ( 200 - 1  to  200 -n) corresponding to the sensors  59  (the sensors  1  to N); a charge amplifier (amplification circuit)  208  which amplifies the sensor signals obtained from the sensors  59  (see  FIG. 9A ) at a prescribed gain; a peak value determination circuit  210  which determines the peak value of each sensor signal amplified by the charge amplifier  208  (see  FIG. 9B ), and a storage circuit  212  which stores the peak value determined by the peak value determination circuit  210 . 
     Switching on and switching off of the switching elements  200  of the switch array  202  is controlled on the basis of a synchronization signal  204 . In other words, the switch array  202  functions as a device for selecting the sensors that sensor signals are acquired from, from the sensors  59  (the sensors  1  to N), on the basis of the synchronization signal  204 . 
     For the peak value determination circuit  210  which determines the peak value of each sensor signal, which is an analog signal, it is suitable to use a sample and hold (S&amp;H) circuit. 
     The storage circuit  212  comprises: an A/D converter (A/D conversion circuit)  214  which converts the peak value determined by the peak value determination circuit  210  into digital data; a CPU  218  which stores the peak value (digital data) in a memory  216 , on the basis of the synchronization signal  204  supplied to the switch array  202 ; and a D/A converter (D/A conversion circuit)  220  which converts the peak value read out from the memory  216  via the CPU  218 , into an analog signal. 
     The CPU  218  used in the storage circuit  212  functions as a memory controller which controls the writing of data to the memory  216  and the read out of data from the memory  216 . The CPU  218  may also be used as a processor including the system controller  72  and the print controller  80  shown in  FIG. 7 . Furthermore, the memory  216  may also be used as another memory, such as the memory  74  or the image buffer memory  82  shown in  FIG. 7 . 
     The signal processing unit  85  comprises: a threshold value changing circuit  222  which extracts the differential between the reference peak value stored in the storage circuit  212  and the peak value of the sensor signal obtained from each sensor  59 , and changes a predetermined reference threshold value to the threshold value used for judging the air bubble size, on the basis of the differential; and a comparing circuit  224  which compares the threshold value obtained from (updated by) the threshold value changing (updating) circuit  222  with the sensor signal obtained from the charge amplifier  208 . 
     In the composition shown in  FIG. 8 , the reference threshold value is set to 0V (i.e., no reference threshold value is provided). In the composition shown in  FIG. 8 , the differential of the peak value is set directly as the threshold value. By adopting a composition in which no reference threshold value is provided in this way, it is possible to omit the composition (processing) for adding the differential to the reference threshold value, and hence the composition and processing of the signal processing unit  85  are simplified. 
     The comparing circuit  224  includes a comparator circuit which outputs a rectangular wave (pulse) when the sensor signal which has undergone the prescribed signal processing becomes greater than the prescribed threshold value. 
     Furthermore, the signal processing unit  85  includes a switch  226  which opens and closes the circuit between the peak value determination circuit  210  and the storage circuit  212 , and a switch  228  which opens and closes the circuit between the storage circuit  212  and the threshold value changing circuit  222 . 
     In other words, at each ejection operation, the predetermined reference threshold value that is previously established on the basis of the peak value of the sensor signal obtained from each of the sensors  59  is updated (or set), and the size of the air bubble present in the corresponding pressure chamber  52  is judged on the basis of the updated threshold value. 
       FIG. 9A  is a diagram showing a sensor signal  240  obtained from each sensor  59  (a sensor signal input to the input part  208 A of the charge amplifier  208 ), and  FIG. 9B  is a diagram showing a sensor signal  242  amplified by the charge amplifier  208  shown in  FIG. 8  (a signal obtained from the output part  208 B of the charge amplifier  208  in  FIG. 8 ), and the peak value V p  of the sensor signal  242  (the signal obtained from the output part  210 A of the peak value determination circuit  210  in  FIG. 8 ). If the sensors  59  shown in  FIG. 8  have good sensitivity (i.e., if a sensor signal  240  obtained from each sensor  59  has a sufficient voltage which can be identified as a signal by the subsequent circuits), then the charge amplifier  208  shown in  FIG. 8  is not necessary. 
     Description of Air Bubble Judgment Control 
     Next, a sensor signal obtained from each sensor  59  (see  FIG. 9A ) is described: with reference to  FIG. 10 . A sensor signal obtained from each sensor  59  has a voltage directly proportional to the pressure in the pressure chamber  52 . The sensor signal  250  denoted by the solid lines in  FIG. 10  is the sensor signal in a case where the pressure of the pressure chamber  52  is normal and it has a peak value of V p1 . 
     Furthermore, the sensor signal  252  denoted by the broken line in  FIG. 10  is the sensor signal in a case where an air bubble of small size (in the present embodiment, an air bubble having a diameter of approximately 10 μm to 20 μm) is present, the sensor signal  254  denoted by the single-dotted line is the sensor signal in a case where an air bubble of medium size (in the present embodiment, an air bubble having a diameter of approximately 30 μm to 120 μm) is present, and the sensor signal  256  denoted by the double-dotted line is the sensor signal in a case where an air bubble of large size (in the present embodiment, an air bubble having a diameter of 130 μm or larger) is present.  FIG. 10  shows a typical example of sensor signals corresponding to pressure chambers where a small-size, medium-size and large-size air bubbles are present. 
     As shown in  FIG. 10 , the relationship between the peak value V p1  of the sensor signal  250 , the peak value V p2  of the sensor signal  252 , the peak value V p3  of the sensor signal  254 , and the peak value V p4  of the sensor signal  256  is: V p1 &gt;V p2 &gt;V p3 &gt;V p4 , and this relationship indicates that the peak value of the sensor signal becomes smaller as the size of the air bubble present in a pressure chamber  52  becomes larger. 
     When an air bubble occurs in a pressure chamber  52 , the pressure loss proportional to the size of the air bubble occurs in the pressure of the pressure chamber  52 . When this pressure loss becomes large, then the ejection abnormality occurs and the ink cannot be ejected normally from the nozzle  51  even if the prescribed ejection force is applied to the piezoelectric actuator  58 . In other words, if the air bubble of a large size that affects ink ejection has occurred, then it is necessary to carry out a maintenance process promptly in the pressure chamber  52  and the nozzle  51  to be connected to the pressure chamber  52 . 
     In the inkjet recording apparatus  10  shown in the present embodiment, if the sensor signal  256  corresponding to the large-sized air bubble that is liable to affect ink ejection or the sensor signal  254  corresponding to the medium-sized air bubble that is liable to affect ink ejection is obtained, then the ejection is judged abnormal (it is judged that pressure abnormality occurs in the pressure chamber  52 ). On the other hand, if the sensor signal  252  corresponding to the small-sized air bubble that dose not affect ejection or the normal sensor signal in the case where no air bubble is present is obtained, then it is judged that the ejection is normal (the pressure in the pressure chamber  52  is normal). 
     In other words, since the threshold value used for judging the air bubble size is updated in synchronism with the ejection operation, in accordance with the peak value V p  of the amplified sensor signal  242  shown in  FIG. 9B , then it is possible to judge the presence or absence of the ejection abnormality in the nozzle  51  connected to a pressure chamber  52 , on the basis of the size of the air bubble in the pressure chamber  52 . 
     More specifically, the differential ΔV between the reference peak value V p0  which is the peak value of the sensor signal in the case of normal ejection and is previously stored in the memory  216  shown in  FIG. 8 , and the peak value V p  which is the peak value of a sensor signal obtained from each sensor  59  during an ejection operation, is derived. The differential ΔV is added to a predetermined reference threshold value V th0  to obtain a threshold value for judging the air bubble size in each ejection operation. The reference threshold value V th0  is stored in the memory  216  in  FIG. 8 . The reference threshold value V th0  may be stored as one of the system parameters. 
     A specific embodiment of changing (updating) the threshold value for judging the air bubbles at each ejection operation is described below with reference to  FIGS. 11A to 14C . In the present embodiment, the predetermined reference threshold value V th0  is set to 0V. In other words,  FIGS. 11A to 14C  show an embodiment where there is no reference threshold value V th0  and the derived differential ΔV is directly set as the threshold value. 
       FIG. 11A  is a diagram showing a sensor signal  250  in the state where no air bubble is present inside a pressure chamber  52  (see  FIG. 10 ; a signal obtained from the output part  208 B of the charge amplifier  208  in  FIG. 8 ). The sensor signal  250  shown in  FIG. 11A  has the peak value V p1  which is substantially the same as the reference peak value V p0  stored in the memory  216  of the  FIG. 8  (i.e., V p0 ≈V p1 ), and therefore the differential ΔV 1  between the reference peak value V p0  and the peak value V p1  of the sensor signal  250  is substantially zero. Consequently, the threshold value V th1  which is obtained by updating the reference threshold value V th0 , is substantially the same as the reference threshold value V th0  (V th0  is changed to the threshold value V th1 , where V th1 =V th0 ). Accordingly, as a result of comparing the threshold value V th1  (which is equal to the reference threshold value V th0 ) with the sensor signal  250 , a pulse signal  260  having three rectangular waves (pulses) is obtained from the output part  224 A of the comparing circuit  224  in  FIG. 8 , as shown in  FIG. 11B . 
       FIG. 12A  is a diagram showing a sensor signal  252  in the state where a small-size air bubble has occurred inside a pressure chamber  52  (see  FIG. 10 ; a signal obtained from the output part  208 B of the charge amplifier  208  in  FIG. 8 ). 
     The sensor signal  252  shown in  FIG. 12A  has the peak value V p2  which is smaller than the reference peak value V p0 , and as shown in  FIG. 12B , the threshold value V th2  (=V th0 +ΔV 2 ) which is derived by adding ΔV 2  (=V p0 −V p2 ) to the reference threshold value V th0  is obtained from the output part  222 A of the threshold value changing circuit  222  in  FIG. 8 . 
     The threshold value V th2 , which is obtained by changing the reference threshold value V th0  (i.e., is obtained by updating the reference threshold value) in this way, and the sensor signal  252  are compared by the comparing circuit  224  in  FIG. 8 , and thus a pulse signal  262  having two square waves is obtained from the output part  224 A of the comparing circuit  224 , as shown in  FIG. 12C . 
       FIG. 13A  is a diagram showing a sensor signal  254  in the state where a medium-size air bubble has occurred inside a pressure chamber  52  (see  FIG. 10 ; a signal obtained from the output part  208 B of the charge amplifier  208  in  FIG. 8 ). The sensor signal  254  shown in  FIG. 13A  has the peak value V p3  which is smaller than the reference peak value V p0  and even smaller than the peak value V p2  of the sensor signal  252  shown in  FIG. 12A , and as shown in  FIG. 12B , a new threshold value V th3  (=V th0 +ΔV 3 ) which is derived by adding ΔV 3  (=V p0 −V p3 ) to the reference threshold value V th0  is obtained from the output part  222 A of the threshold value changing circuit  222  in  FIG. 8 . 
     The threshold value V th3 , which is obtained by changing the reference threshold value V th0  (i.e., is obtained by updating the reference threshold value) in this way, and the sensor signal  254  are compared by the comparing circuit  224  in  FIG. 8 , and thus a pulse signal  264  having one square wave is obtained from the output part  224 A of the comparing circuit  224 , as shown in  FIG. 13C . 
       FIG. 14A  is a diagram showing a sensor signal  256  in the state where a large-size air bubble has occurred inside a pressure chamber  52  (see  FIG. 10 ; a signal obtained from the output part  208 B of the charge amplifier  208  in  FIG. 8 ). The sensor signal  256  shown in  FIG. 14A  has the peak value V p4  which is smaller than the reference peak value V p0  and even smaller than the peak value V p3  of the sensor signal  254  shown in  FIG. 13A , and as shown in  FIG. 14B , the new threshold value V th4  (=V th0 +ΔV 3 ) which is derived by adding ΔV 4  (=V po −V p4 ) to the reference threshold value V th0  is obtained from the output part  222 A of the threshold value changing circuit  222  in  FIG. 8 . 
     The threshold value V th4 , which is obtained by changing the reference threshold value V th0  (i.e., is obtained by updating the reference threshold value) in this way, and the sensor signal  256  are compared by the comparing circuit  224  in  FIG. 8 , and thus no pulse signal is obtained from the output part  224 A of the comparing circuit  224 , as shown in  FIG. 14C . 
     Thus, the threshold value for judging the air bubble size is changed in accordance with the peak value of each sensor signal, and pulse signals having different numbers of pulses are obtained by comparing each sensor signal with the threshold value which is changed (updated). The size of an air bubble occurring inside a pressure chamber  52  is judged on the basis of the pulse signal that is obtained in the aforementioned way. 
     The sensors  59  used in the inkjet recording apparatus  10  have a sensitivity (electrostatic capacitance) which varies with the operating environment of the head  50 , such as the temperature or humidity. In other words, since a sensor signal (reference numeral  240  in  FIG. 9A ) obtained from a sensor  59  varies with the operating conditions of the head  50 , then its peak value (the peak value V p  of the amplified sensor signal  242  in  FIG. 9B ) also changes. 
     In the inkjet recording apparatus  10  of the present embodiment, in order to cope with change in a sensor signal occurring as a result of environmental factors of this kind, a composition is adopted in which the threshold value used for judging the air bubble size is changed at each ejection operation, on the basis of the peak value V p  of a sensor signal. 
       FIGS. 15 and 16  are flowcharts showing a sequence, used in the inkjet recording apparatus  10 , for controlling the judgment of the ejection abnormality. In the ejection abnormality judgment control according to the present embodiment, the reference peak value V p0  is derived in the state where no air bubble is present immediately after carrying out the restoration processing (initialization) while the apparatus is off-line (in a non-printing state), and is stored in the memory  216  shown in  FIG. 8 . 
     When the apparatus is on-line state (in a printing state), the differential ΔV between the reference peak value V p0  and the peak value of a sensor signal obtained at each ejection operation is derived, and then the size of an air bubble is judged on the basis of a new threshold value which is obtained by adding this differential ΔV to the reference threshold value V th0 . 
     In other words, when the power supply is turned on (step S 10 ), first, the peak value (reference peak value V p0 ) of the pressure waveform in the normal ejection is determined (step S 12 ).  FIG. 16  shows a flowchart of the control sequence for calculating the reference peak value V p0  shown in step S 12 . 
     As shown in  FIG. 16 , when the reference peak value calculation control sequence starts (step S 100 ), the apparatus enters an off-line state (step S 102 ), and a restoration process is carried out (step S 104 ). 
     Subsequently, the switch  226  shown in  FIG. 8  is switched on (the peak value determination circuit  210  and the A/D converter  214  are connected), the switch  228  is switched off (the D/A converter  220  and the threshold value changing circuit  222  are disconnected), and when a piezoelectric actuator  58  (see  FIG. 4 ) is operated and a pressure waveform is output from the corresponding sensor  59  (step S 106 ), then the peak value of the sensor signal (which becomes the reference peak value V p0 ) is determined (step S 108 ). The reference peak value V p0  is stored in the memory  216  shown in  FIG. 8  (step S 110 ). 
     The steps from S 100  to S 110  are carried out for each nozzle  51 , and when the reference peak values V p0  for the respective nozzles have been stored, then the reference peak value calculation control sequence terminates (step S 112 ), and the procedure advances to step S 14  in  FIG. 15 . 
     At step S 14 , the apparatus enters an online state and a standby state where it waits print data. The switch  226  shown in  FIG. 8  is turned off (the peak value determination circuit  210  and the A/D converter  214  are disconnected), and the switch  228  is turned on (the D/A conversion circuit  220  and the threshold value changing circuit  222  are connected). 
     When a printing operation is carried out by acquiring print data (in a normal ejection state), then piezoelectric actuators  58  are operated and a pressure waveform is output from each sensor  59  (step S 16 ). When the pressure waveform is output from each sensor  59  at step S 16 , then the peak value V p  of the sensor signal for each nozzle is determined at each ejection operation. In the threshold value changing circuit  222  in  FIG. 8 , the differential ΔV between the peak value V p  and the reference peak value V p0  is calculated, and the threshold value V thi  corresponding to each ejection operation is derived by adding this differential ΔV to the predetermined reference threshold value V th0  (step S 18  in  FIG. 15 ). 
     When the threshold value V thi  is compared with the sensor signal obtained for each ejection operation in the comparing circuit  224  in  FIG. 8 , the pulse signal corresponding to the comparison result is output from the output part  224 A of the comparing circuit  224  (step S 20 ). 
     The number of the square waves (pulses) of the pulse signal obtained from the comparing circuit  224  is counted up, and it is judged whether or not the pulse signal contains two or more pulses (step S 22 ). If the pulse signal contains only one pulse or less (“NO” verdict), then the procedure advances to step S 24 , and it is judged whether or not the pulse signal contains a pulse. 
     If it is judged at step S 24  that the pulse signal contains one pulse (“NO” verdict), then the procedure advances to step S 26 . If the pulse signal contains two pulses, then it is judged that the medium-size air bubble is present, and the maintenance processing (air bubble removal processing) corresponding to the medium-size air bubble is carried out. 
     Thereupon, the procedure advances to step S 28  where it is judged whether a subsequent sensor signal has been obtained or not, and if the subsequent sensor signal has not been obtained (“NO” verdict), then the current pressure abnormality judgment control sequence is terminated. On the other hand, if the subsequent sensor signal has been obtained (“YES” verdict), then the procedure returns to step S 16 . 
     Furthermore, if it is determined that the pulse signal contains no pulse at step S 24  (“YES” verdict), then it is judged that the large-size air bubble is present and the maintenance process (air bubble removal processing) corresponding to the large-size air bubble is carried out (step S 32 ), whereupon the procedure advances to step S 28 . 
     At step S 22 , if it is determined that the pulse signal contains two or more pulses (“YES” verdict), then it is judged that the pressure in the pressure chamber  52  is in the normal state (or that it contains the small-size air bubble which is not sufficient to affect ejection), and the procedure then advances to step S 16 . 
     If the operating environment of the head  50  is changed, then the determination control sequence for the reference peak value shown in  FIG. 16  is carried out, and the memory  216  in  FIG. 8  is rewritten. Examples of such a case where operating environment of the head  50  is changed include the following cases: a case where conditions of the circumstances, such as temperature and humidity, have moved outside a prescribed range; and a case where the type of ink used has changed (a case where the ink has been refilled). 
     In the maintenance processing corresponding to the large-size air bubble and the maintenance processing corresponding to the medium-size air bubble, purging times are set in accordance with the size of the air bubble. More specifically, in a case where the air bubble of a large size is removed, the purging time is set to a longer time than in a case where the medium-size air bubble is removed. By controlling and altering the maintenance times in accordance with the size of the air bubble in this way, it is possible to shorten the required maintenance time in comparison with maintenance control carried out simply on the basis of time management or print intervals as in the related art. 
     The inkjet recording apparatus  10  having the composition described above has a composition in which the threshold value used for judging the size of an air bubble present in a pressure chamber  52  is changed from the reference threshold value V th0  in accordance with the sensor signal obtained from the sensor  59 , and the air bubble size is judged on the basis of this threshold value V thi  which has been changed (updated). Therefore, even if a small-size air bubble which is not sufficient to affect the ejection has occurred, it is not judged that the ejection abnormality has occurred. Therefore, by means of a simple circuit composition, the size of air bubbles present in the pressure chambers are determined, and it is judged whether an ejection abnormality is present or absent on the basis of the determined air bubble size. Consequently, wasteful and unnecessary maintenance processing is not carried out, the ink consumption is suppressed, and the load of maintenance processing is reduced. Since the threshold value is changed for each nozzle and each ejection operation, it is possible to deal with variations between the sensors and changes in the environment, and the air bubble size is determined in real time. 
     In the foregoing embodiments, the inkjet recording apparatus using the page-wide full line type head having a nozzle row of a length corresponding to the entire width of the recording paper  16  is described, but the scope of application of the present invention is not limited to this. The present invention may also be applied to an inkjet recording apparatus using a shuttle head which performs image recording while moving the recording head of short dimensions in a reciprocal fashion. 
     The embodiments described above relate to the inkjet recording apparatus  10  for forming images on the recording paper  16  by ejecting ink from nozzles provided in the head, but the scope of application of the present invention is not limited to those. The present invention may also be applied broadly to image forming apparatuses which each form an image (a three-dimensional shape) by means of a liquid other than ink, such as resist, and to liquid ejection apparatuses, such as a dispenser, which eject liquid chemicals, water, or the like, from a nozzle (ejection hole). 
     It should be understood that there is no intention to limit the invention to the specific forms disclosed, but on the contrary, the invention is to cover all modifications, alternate constructions and equivalents falling within the spirit and scope of the invention as expressed in the appended claims.