Abstract:
When ink droplets are ejected, angled or splashed where a plurality of minute ink droplets are generated, angled or splashed ink clings on an electrode 401, 402 and increases the amount of electric current conducted therethrough. Hence, the defectiveness of ink ejection can be detected by monitoring the amount of the electric current. When the defectiveness of ink ejection is detected, ejection data D is retrieved and updates the ejection data D based on a condition register S, and set to a defect register E. When the defect register E has only one element that takes a condition value of 1 indicating defectiveness, the corresponding nozzle is identified as defective. The restoring means reallocates dots, which have been originally allocated to the defective nozzle, to neighboring nozzle.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a multi-nozzle ink jet recording device, and more specifically to a highly reliable multi-nozzle ink jet recording device capable of automatically detecting defective nozzles and restoring normal printing without performing a test-pattern printing. 
     2. Related Art 
     Japanese Patent Publication No. SHO-47-7847 discloses a conventional ink jet recording device formed with a plurality of nozzles aligned in a line in a widthwise direction of a recording sheet. Ink droplets ejected from the nozzles impact and form dots on the recording sheet while the recording sheet is moved in a sheet feed direction perpendicular to the widthwise direction, thereby forming dot images on the recording sheet. The ejected ink droplets are uniform in their size and separated one from the other. 
     The recording device also includes electrodes that generate a charging electric field and a deflector electric field for respective nozzles. The charging electric field charges the ejected ink droplets based on a recording signal, and the deflector electric field having a uniform magnitude changes a flying direction of the charged ink droplets as needed, thereby controlling the impact positions of the ink droplets with respect to the widthwise directions so as to form the dots on exact target positions. 
     There has been also proposed a nozzle array where a plurality of nozzles are formed in an arrayed manner, which improves recording speed. However, increase in the number of nozzles sacrifices the reliability of the device. 
     Air bubbles and any foreign substances existing within a nozzle will result in ink droplets ejected at an angle and also in a splash where unintended minute ink droplets are generated. In worse case, no ejection is performed. 
     Moreover, when the ejection direction is angled or when the splash is caused in this manner, ink droplets may impact and cling on the electrodes. Especially, the splashed minute ink droplets have a low flying speed and a greater deflection amount because of their small diameter, so a large number of minute ink droplets cling on the electrodes. Because the ink has been charged by the charging electric field, the ink clinging on the electrodes increases an electric current conducting through the electrodes. Therefore, a nozzle corresponding to the electrodes with increased electric current can be easily detected defective. 
     However, the above method for detecting defective nozzles is not useful in a recording device including common electrodes used common to a plurality of nozzles. Specifically, when there is any change in electric current, it can be known that there is a defective nozzle(s). However, because the amount of change in the electric current due to a single defective nozzle is unknown and fluctuates, it is impossible to detect the number of defective nozzle(s) or to identify the defective nozzle(s). For example, even if defective ejection is detected when droplets are ejected from two nozzles at one time, it cannot detect which one of the two nozzles is defective or whether both of the two nozzles are defective. 
     It is conceivable to perform a test-pattern printing where an ink droplet is ejected from each one of the nozzles one at a time. Then, it is determined whether or not each nozzle is defective or normal by using a laser beam or a CCD sensor. However, this method is time consuming and wastes ink, and it is impossible to perform such a time-consuming test-pattern printing during actual image forming operations. Moreover, there is hardly a case when an ink droplet is ejected from only a single nozzle during the actual printing, not the test-pattern printing, so it is unrealistic to wait such a single-nozzle printing during the actual printing to detect defectiveness of each nozzle. That is, even when a nozzle becomes defective during printing, there has been no conventional means for restoring the normal printing during the printing, so that there has been no choice but to continue the defective printing with the defective nozzle. 
     There is a choice to temporarily stop the printing to detect a defective nozzle. However, this method wastes time required for the detection, wastes ink contributed for the detection, and requires disposing the ink contributed for the detection. Accordingly, it is preferable to avoid such an operation as much as possible. 
     SUMMARY OF THE INVENTION 
     It is therefore an objective of the present invention to overcome the above problems, and also to provide a highly reliable multi-nozzle ink jet recording device capable of automatically detecting defective nozzles and restoring normal printing without performing a test-pattern printing. 
     In order to overcome the above and other objectives, there is provided an ink jet recording device including a head, electrodes, first detecting means, and identifying means. The head is formed with a plurality of nozzles through which ink droplets are selectively ejected based on ejection data during printing. The electrodes are provided common to the plurality of nozzles. The first detecting means detects whether all of selected nozzles through which ink droplets are ejected are normal or at least one of the selected nozzles is defective. The identifying means automatically identifies a defective nozzle while the head is continuously performing the printing when the first detecting means detects that the ejection is performed defective. 
     There is also provided a detecting method of detecting a defective nozzle among a plurality of nozzles formed to a head of an ink jet recording device that includes the head and electrodes for generating a deflection electric field common to the plurality of nozzles. The detecting method includes the steps of a) detecting whether all of selected nozzle through which ink droplets are ejected are normal or at least one of the selected nozzles is defective, and b) identifying a defective nozzle among the plurality of nozzles while the head continuously performing the printing when the ejection is detected defective in step a). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a block diagram of components of an ink jet recording device according to an embodiment of the present invention; 
     FIG. 2 is a cross-sectional view of a nozzle formed to a recording head of the ink jet recording device; 
     FIG.  3 ( a ) is a plan view partially showing an ejection surface of the recording head; 
     FIG.  3 ( b ) is a plan view showing the ejection surface of the recording head; 
     FIG. 4 is an explanatory plan view showing the ejection surface and common electrodes; 
     FIG. 5 is an explanatory cross-sectional view showing ink droplet deflection; 
     FIG. 6 is a table indicating deflection results; 
     FIG. 7 is an explanatory view showing a partial configuration of engine portion including the recording head  107 ; 
     FIG.  8 ( a ) is an explanatory view showing a dot frequency and a deflected-dot frequency; 
     FIG.  8 ( b ) is an explanatory view showing change in magnitude of a deflector electric field; 
     FIG.  8 ( c ) is an explanatory view showing ejection data; 
     FIG.  8 ( d ) is an explanatory view showing a positional relationship between an orifice and an impact position of a deflected ink droplet; 
     FIG.  8 ( e ) is an explanatory view showing a positional relationship between an orifice and an impact position of a deflected ink droplet; 
     FIG.  8 ( f ) is an explanatory view showing a positional relationship between an orifice and an impact position of a deflected ink droplet; 
     FIG.  8 ( g ) is an explanatory view showing a positional relationship between an orifice and an impact position of a deflected ink droplet; 
     FIG. 9 is an explanatory view showing an example of ink ejection and deflection; 
     FIG. 10 is an explanatory view of adjusted ink ejection and deflection for when a nozzle N 2  becomes defective; 
     FIG. 11 is a flowchart representing a detecting process according to a first embodiment of the present invention; 
     FIG. 12 is a flowchart representing a restoring process executed in S 1110  and S 1115  of FIG. 11; 
     FIG. 13 is a flowchart representing a detecting process according to a second embodiment of the present invention; 
     FIG. 14 is a cross-sectional view showing a configuration of an ink jet head according to a third embodiment of the present invention; and 
     FIG. 15 is a plan view sowing a laser beam generator and a laser beam receptor according to the third embodiment of the present invention. 
    
    
     PREFERRED EMBODIMENTS OF THE PRESENT INVENTION 
     Next, a line-scanning-type multi-nozzle ink jet recording device and a recording method according to an embodiment of the present invention will be described while referring to the accompanying drawings. 
     First, overall configuration of the line-scanning-type multi-nozzle ink jet recording device  1  will be described while referring to FIGS. 1 to  8 . 
     As shown in FIG. 1, the ink jet recording device  1  includes a signal processing portion  101 , a data memory  103 , an engine portion  102 , and a detection-restoring unit  111 . The engine portion  102  includes a control unit  105 , a piezoelectric driver  106 , a recording head  107 , a common electrode power source  104 , a sheet feed unit  108 , and a detection unit  110 . The recording head  107  is formed with a plurality of nozzles  107   a  (FIG.  2 ). Because the piezoelectric driver  106  has a well-known configuration, detailed description thereof will be omitted. 
     When the ink jet recording device  1  is a full-color recording device, a plurality of recording heads  107  are provided for a plurality of different colored ink. However, in the present embodiment, it is assumed that the ink jet recording device  1  is a monochromatic recording device, and that only one recording head  107  is provided. 
     The signal processing portion  101  is a well-known microcomputer, and receives a bitmap data  109 , which is binary data, from an external computer and the like (not shown). When the ink jet recording device  1  is the full-color recording device, a plurality of sets of the bitmap data  109  are usually provided for the recording heads  107 . 
     Upon receipt of the bitmap data  109 , the signal processing portion  101  generates ejection data  112  for each of the nozzles  107   a  of the recording head  107  based on the bitmap data  109  and a prestored program. The ejection data  112  is arranged, based on position information of each nozzle  107   a  and deflection information of ink droplets, in an order in which ink droplets are ejected. The signal processing portion  101  temporarily stores one-scanning-worth or one-page-worth of the ejection data  112  into the data memory  103 . 
     The control unit  105  of the engine portion  102  controls the sheet feed unit  108  and the common electrode power source  104 . When printing is started, the sheet feed unit  108  starts feeding a recording sheet. At the same time, the common electrode power source  104  applies an electric voltage to common electrodes  401 ,  402  (FIGS. 4 and 5) to be described later, thereby generating a charging electric field and a deflector electric field. When a recording position of the recording sheet reaches the recording head  107 , the control unit  105  outputs a request command to the data memory  103 , the request command requesting the signal data memory  103  to output the ejection data  112 . The ejection data  112  is input to the piezoelectric driver  106 , and the piezoelectric driver  106  outputs a print signal  113  to each nozzle  107   a  of the recording head  107 . As a result, an image  114  is formed on the recording sheet. 
     In the ink jet recording device  1  of the present embodiment, printing is performed by the recording head  107  that is held still while the recording sheet is transported. 
     As shown in FIG. 2, each nozzle  107   a  of the recording head  107  includes a diaphragm  203 , a piezoelectric element  204 , a signal input terminal  205 , a piezoelectric element supporting substrate  206 , a restrictor plate  210 , a pressure-chamber plate  211 , an orifice plate  212 , and a supporting plate  213 . The diaphragm  203  and the piezoelectric element  204  are attached to each other by a resilient member  209 , such as a silicon adhesive. The restrictor plate  210  defines a restrictor  207 . The pressure-chamber plate  211  and the orifice plate  212  define a pressure chamber  202  and an orifice  201 , respectively. The orifice plate  212  has an ejection surface  301 . A common ink supply path  208  is formed above the pressure chamber  202  and is fluidly connected to the pressure chamber  202  via the restrictor  207 . Ink flows from above to below through the common ink supply channel  208 , the restrictor  207 , the pressure chamber  202 , and the orifice  201 . The restrictor  207  regulates an ink amount supplied into the pressure chamber  202 . The supporting plate  213  supports the diaphragm  203 . The piezoelectric element  204  deforms when a voltage is applied to the signal input terminal  205 , and maintains its initial shape when no voltage is applied. 
     The diaphragm  203 , the restrictor plate  210 , the pressure-chamber plate  211 , and the supporting plate  213  are formed from stainless steel, for example. The orifice plate  212  is formed from nickel material. The piezoelectric element supporting substrate  206  is formed from an insulating material, such as ceramics and polyimide. 
     The print signal  113  output from the piezoelectric driver  106  is input to the signal input terminal  205 . In accordance with the print signal  113 , uniform ink droplets separated from each other are ejected, ideally outwardly with respect to a normal line of the orifice plate  212 , from the orifice  201 . 
     As shown in FIG.  3 ( b ), a plurality of orifice lines  107   b  are formed to the recording head  107 . Details will be described below. 
     As shown in FIG.  3 ( b ), the ejection surface  301  is formed with a plurality of the orifice lines  107   b  arranged side by side in an x direction and each extending in an orifice-line direction  302 , which is inclined by θ with respect to a y direction perpendicular to the x direction. As shown in FIG.  3 ( a ), each orifice line  107   b  includes  128  orifices  201  arranged at a pitch of 75 orifices/inch in the orifice-line direction  302 . Although not indicated in the drawings, adjacent orifice lines  107   b  usually overlap each other in the x direction by several-dot-worth amount. 
     In actual assembly, a plurality of head portions of FIG.  3 ( a ) are assembled into the single head  107  of FIG.  3 ( b ). The above arrangement prevents unevenness in color density of recorded image, which appears in a black or white band, due to erroneous attachment of the head portions and uneven nozzle characteristics, and also enables assembly of the recording head  107  elongated in the x direction. 
     As shown in FIGS. 4 and 5, the common electrodes  401 ,  402  are provided for each orifice line  107   b , at positions between the ejection surface  301  and a recording sheet  502 . The common electrodes  401 ,  402  extend parallel to and sandwich the corresponding orifice line  107   b  in a plan view. In the present embodiment, a distance D1 from the orifice plate  212  to the recording sheet  502  is 1.6 mm. A distance D2 from the orifice plate  212  to the common electrode  401  ( 402 ) is 0.3 mm. Each common electrode  401 ,  402  has a thickness T1 of 0.3 mm in the y direction. The common electrodes  401  and  402  are separated from each other by a distance of 1 mm. 
     As shown in FIG. 4 there are provided an alternate current (AC) power source  403  and a pair of direct current (DC) power sources  404 . The AC power source  403  outputs an electric voltage Vchg. As will be described later, the value of the electric voltage Vchg is changed among several different values in a predetermined frequency. Each of the DC power sources  404  outputs an electric voltage Vdef/2. With this configuration, an electric voltage of Vchg+VdefY/ 2  and Vchg−Vdef/ 2  are applied to the common electrodes  401  and  402 , respectively. The orifice plate  212  having the ejection surface  301  is connected to the ground. 
     As shown in FIG. 5, the common electrodes  401 ,  402  and the orifice plate  212  together generate a charging electric field E 1  in a region near the orifice  201 . Because the orifice plate  212  is conductive and connected to the ground, the direction of the charging electric field E 1  is parallel to the normal line of the orifice plate  212  as indicated by an arrow A 1 . The common electrodes  401  and  402  also generate a deflector electric field E 2  having a direction from the common electrode  401  to the common electrode  402  as indicated by an arrow A 2 . That is, the deflector electric field E 2  has the direction A 2  perpendicular to the orifice-line direction  302 . The magnitude of the deflector electric field E 2  is in proportion to the electric voltage Vdef. The electric voltage Vdef is maintained at 400V in this embodiment. 
     Because the orifice  201  is separated from both the electrodes  401  and  402  by the same distance, the electric voltage applied to an ink droplet  501 , which is about to be ejected, is in proportion to the electric voltage Vchg. Accordingly, at the time of ejection from the orifice  201 , the ink droplet  501  is charged with a voltage of Q which has a magnitude in proportion to the electric voltage Vchg and a polarity opposite to the electric voltage Vchg. In this way, the electric field E 1  charges the ink droplet  501 . 
     After ejection, the flying speed of the ink droplet  501  is accelerated by the charging electric field E 1 . When the ink droplet  501  reaches between the common electrodes  401  and  402 , the deflector electric field E 2  deflects the ink droplet  501  toward the direction A 2  of the electric field E 2  and changes its flying direction to a direction indicated by an arrow A 3 . Then, the ink droplet  501  impacts on the recording sheet  502  at a position  502   b  shifted in the direction A 2  by a distance C from an original position  502   a  where the ink droplet  501  would have impacted if not deflected at all. The distance C between the actual impact position  502   b  and the original position  502   a  is referred to as deflection amount C hereinafter. 
     FIG. 6 shows a table indicating the relationships among the deflection amounts C (μm) and average flying speeds Vav (m/sec) obtained when the AC voltage Vchg are 200V, 100V, 0V, −100V, and −200V. The average flying speed Vav indicates an average flying speed of the ink droplet  501  from when the ink droplet  501  is ejected from the orifice  201  until impacts on the recording sheet  502 . 
     It should be noted that a flying time T from when the ink droplet  501  is ejected until when the ink droplet impacts on the recording sheet  502  is ignored in the explanation. This is because fluctuation in the deflection amount C within values that the deflection amount C takes during actual printing hardly varies the flying time T. A possible explanation for this is that when the deflection amount C is relatively large, a flying distance of the ink droplet  501  increases. However, in this case, the charging amount Q also increases, and this in turn increases acceleration rate cased caused by the charging electric field E 1  and the deflecting electric field E 2 , thereby increasing the average speed Vav of the ink droplet  501 . Accordingly, the flying time T stays unchanged regardless of the deflection amount C. 
     Next, an x-y coordinate system used in this embodiment will be described while referring to FIG.  7 . The x-y coordinate system is defined on the recording sheet  502 , and includes a plurality of x-scanning lines  701  and a plurality of y-scanning lines  702 . The x-scanning lines  701  extend in the x direction and align at a uniform interval of dy in the y direction, which is referred to as “resolution interval dy”. On the other hand, the y-scanning lines  702  extend in the y direction and align at a uniform interval of dx in the x direction, which is referred to as “resolution interval dx”. These x-scanning lines  701  and y-scanning  702  lines intersect one another and define a plurality of grids  704  having grid corners  704   a . The ink droplets  501  are controlled to impact on one of grid corners  704   a , which is defined by a coordinate value (dx, dy). It should be noted that in the present embodiment, the recording sheet  502  is moved in the y direction during printing. 
     In the present embodiment, the recording head  107  is positioned above the recording sheet  502  while its ejection surface  301  faces and extends parallel to the recording sheet  502 . The distance between the recording sheet  502  and the ejection surface  301  is between 1 mm and 2 mm. 
     Next, a specific example of the present embodiment will be described while referring to FIG.  7 . In this example, tan θ is set to ¼. Also, the charging electric field E 1  takes four different magnitudes, i.e., a deflection number n is  4 , so an ink droplet  501  ejected from a single orifice  201  is deflected by one of four deflection amounts C, and impacts on one of four impact positions  703 . Because it is desirable to decrease the deflection amount C, the four impact positions  703  are symmetrically arranged to the left and right sides of the orifice  201 . 
     Also, in the present example, two adjacent orifices  201  are separated in the x direction by two grids  704  (2dx). Accordingly, the nozzle interval in the y direction is 8dx (=2dx/tan θ). 
     Because the orifice pitch in the orifice-line direction  302  is set to  75  orifices/inch as described above, the resolution interval dx is 41 μm, so the resolutions of the printed image  114  in the x and y directions are both  619  dpi (1/dx and 1/dy, respectively). 
     Although the adjacent orifices  201  are separated by 2dx in the x direction, because ink droplets  501  ejected from a single orifice  201  hit on four different x-scanning lines  701 , a dot on every grid corners is formed by two ink droplets  501  ejected from two orifices  201 . 
     FIGS.  8 ( a ) to  8 ( c ) show relationships between the charging electric field E 1 , the ejection data  112 , and the impact positions  703 . In FIG.  8 ( a ), a sheet-feed time t 0 , t 1 , t 2 , . . . is a time duration required to move the recording sheet  502  by a single grid in the y direction (1dy), which is referred to as “dot frequency”. The sheet-feed time is further divided into n dot-forming time segments t 00 , t 01 , t 02 , t 03 , t 10 , t 11 , t 12 , t 13 , t 20 , . . . , which is referred to as “deflected-dot frequency”. In each dot-forming time segment, a single dot is formed by a single nozzle  107   a . Because the deflection number n is 4 in this example, the dot-forming time segment is ¼ of the sheet-feed time. 
     As shown in FIGS.  8 ( a ) and  8 ( c ), the ejection data  112  is output for a dot (x 3 , y 0 ) at the dot-forming time t 00 . As a result, as shown in FIG.  8 ( d ), an ink droplet  501  ejected from the orifice  201  is deflected rightward perpendicular to the orifice-line direction  302 , and impacts on a y-scanning line x 3  on the recording sheet  502 . At this time, the impact position  703  is on the grid corner (x 3 , y 0 ). 
     At the subsequent dot-forming time t 01 , the magnitude of the charging electric field E 1  has been changed as shown in FIG.  8 ( b ), and the ejection data  112  for (x 2 , y 0 ) is output. Accordingly, the ejected ink droplet  501  is deflected rightward and impacts on the y-scanning line x 2  as shown in FIG.  8 ( e ). Because the recording sheet  502  has been transported by a distance of 1dy/4 by this moment, the impact position  703  is on the grid corner (x 2 , y 0 ). Then, at the dot-forming time of t 02 , the magnitude of the charging electric field E 1  has been changed as shown in FIG.  8 ( b ), and the recording sheet  502  has been moved by a distance of another 1dy/4. The ejection data  112  for (x 1 , y 0 ) is output, and as shown in FIG.  8 ( f ), the ejected ink droplet  501  is deflected leftward perpendicular to the orifice-line direction  302  and impacts on the grid corner (x 1 , y 0 ) on the y-scanning line x 1 . At the dot-forming time t 03 , the magnitude of the charging electric field E 1  has been changed as shown in FIG.  8 ( b ), and the ejection data  112  for (x 2 , y 0 ) is output. Accordingly, as shown in FIG.  8 ( g ), the ejected ink droplet  501  is deflected leftward and impacts on the y-scanning line x 0 . 
     During the sheet-moving time t 1  and on, the same processes are performed, so dots are formed on every grid corner. 
     It should be noted that because the flying time T is constant regardless of the deflection amount C as described above, it is unnecessary to take the flying time T (sheet transporting speed) into consideration when determining the ink ejection timing. In actual printing, the recording sheet  502  is moved by a predetermined distance in the y direction while the flying time T. Therefore, it would be only necessary to be aware that all the actual impact positions  703  would shift by a predetermined distance in the y direction. Also, the timing of changing the magnitude of the charging electric field E 1  is set to the exact time of when the ink droplet  501  is generated, that is, when the ink droplet  501  is separated from remaining ink in the nozzle  107   a . This can be achieved by setting the actual timing to a time a predetermined time duration after the ejection data  112  is output, that is, after the piezoelectric element is driven. This timing can be obtained through experiments. 
     Next, an example of ejection-deflection operation will be described while referring to FIG.  9 . 
     When printing is performed using all the nozzles  201 , two ink droplets  501  from different nozzles  201  forms one dot on respective grid corners  704   a . Accordingly, it is possible to select which one of two nozzles  201  to use for forming a dot on a corresponding grid corner  704   a . In an example of FIG. 9, a dot (x 0 ,y 0 ) is formed by a nozzle N 1 . A dot (x 0 , y 1 ) is formed by a nozzle N 2 . A dot (x 0 , y 2 ) is formed by the nozzle N 1 , and a dot (x 0 , y 3 ) is formed by the nozzle N 2 . By forming dots on a single y scanning line  702  by using two nozzles  201  in alternation in this manner, it is possible to prevent uneven color density appearing on an image in a form of banner extending in the y direction, which is due to uneven nozzle characteristics. 
     Next, a restored printing for when a nozzle  201  becomes defective will be described while referring to FIG.  10 . In this example, it is assumed that the nozzle N 2  becomes defective. When the nozzle N 2  becomes defective, dots (x 1 , y 0 ), (x 0 , y 1 ), (x 1 , y 2 ), (x 0 , y 3 ), (x 1 , y 4 ), and on, which are originally allocated to the nozzle N 2 , are formed by the nozzle N 1 , and dots (x 3 , y 0 ), (x 2 , y 1 ), (x 3 , y 2 ), (x 2 , y 3 ), (x 3 , y 4 ) and on, which are also originally allocated to the nozzle N 2 , are formed by the nozzle N 3 . 
     In this restored printing, dots can be formed on all grid corners  704   a  without using the defective nozzle N 2 . Although this operation is not useful for when two adjacent nozzles become defective, there is only a slight possibility that any one nozzle  201  becomes defective during printing, there is hardly a possibility that two adjacent nozzles  201  become defective, so that there is no need to take that possibility into consideration. Therefore, it can say that the above operation enables forming of dots on all grid corners  704   a without using any defective nozzles  201 . In this case, the ejection data  112  is generated by the signal processing portion  101  in accordance with the current restored printing. 
     Next, an explanation will be provided for the switching of the printing while referring to FIGS. 1,  11 , and  12  and also a table shown later. The ejection-deflection operations are when at least one nozzle is detected defective in a detection operation. 
     The detection unit  110  shown in FIG. 1 detects whether the printing is normal or defective, and outputs a detection signal to the detection-restoring unit  111 . Specifically, the detection unit  110  detects whether all the nozzles having performed ejection are normal or at least one of the nozzles having performed ejection is defective. If all the nozzles are normal, a detection signal of 1 is output. On the other hand, if at least one of the nozzles is defective, then a detection signal of 0 is output. 
     Upon reception of the detection signal of 0, the detection-restoring unit  111  outputs a restore signal to the signal processing portion  101 , commanding to restore the printing. The signal processing portion  101  changes a generation method of the ejection data  112  so as to generate the ejection data  112  that is adjusted for a restoring printing. It should be noted that the actual detection-restoring unit  111  is realized as one of the processes of the signal processing portion  101 . However, in the present example, the detection-restoring unit  111  is described as a component separated from the signal processing portion  101  so as to facilitate the explanation. 
     Next, the detection unit  110  is described. The detection unit  110  detects change in electric current conducted through a power source that generates the deflecting voltage Vdef of the charging voltage Vchg. As described above, a charged ink droplet from a normal nozzle  201  reaches the recording sheet without impacting on the electrode  401  nor  402 . Therefore, no electric current is conducted through the electrodes  401 ,  402 . However, an ink droplet ejected at an angle or a splashed minute ink droplet from a defective nozzle  201  impacts on the electrode  401  or  402 . Because these ink droplets are charged, electric current is conducted through the electrode  401 ,  402 . The detection unit  110  outputs the detection signal of 1 when detecting no current and the detection signal of 0 when detecting the current. 
     It should be noted that this detection method cannot detect ejection failure although it can detect angled ejection and splash. Usually, ejection failure is detected by a laser beam light or CCD sensor after test printing. However, the angled ejection or splash usually occurs before the nozzles become completely clogged. That is, the detection method of the present invention can detect presence of defective nozzles before these nozzles become incapable of ink ejection. 
     In the present detection, a condition register S is used. The register S is a memory region with a specific function secured within the signal processing portion  101 . The condition register S includes a plurality of elements for respective nozzles  201 . In this example, it is assumed that n nozzles  201  and accordingly n elements are provided. Each of the n elements takes three condition values 0, 1, 2, wherein the condition value of 0 represents that a corresponding nozzle is defective, the condition value of 2 represents that a corresponding nozzle is normal, and the condition value of 1 represents that a condition of a corresponding nozzle is unknown. Usually, all the elements of the register S initially take the condition value of 1, indicating unknown. Needless to say, if there is any nozzle whose condition is known, the corresponding value takes either 0 or 2, instead. 
     Ejection data D is detected by the detection-restoring unit  111  before ejection. The ejection data D includes n bits for the respective n nozzles. Each bit takes an ejection value of 1 for ejection or a non-ejection value of 0 for non-ejection. When the signal processing portion  101  generates one-page-worth of the ejection data  112 , the one-page worth of or a portion of the one-page-worth of the ejection data D is stored in the detection-restoring unit  111 , and the detection-restoring unit  111  refers to thus stored ejection data D at the time of detection. 
     The detection-restoring unit  111  does not perform the detection of the ejection data D every time when the ejection is performed because it is time consuming. In the present embodiment, the detection is performed every time the ejection is performed 1,024 times, or about 5 Hz. That is, the detection-restoring unit  111  stores the ejection data D once every 1,024 times the signal processing portion  101  generates the ejection data  112 . 
     At the time of the selective ink ejection, when the detection unit  110  outputs the detection signal of 1 indicating normal ejection, this means that all the nozzles having performed the ink ejection, that is, the nozzles corresponding to the ejection value of 1, are normal. On the other hand, when the detection unit  110  outputs the detection signal of 0 indicating defective ejection, this means that at least one of the nozzles having performed the ejection is defective. However, as described above, a defective nozzle cannot be identified by simply detecting the conducted electric current. 
     In the present embodiment, the defective nozzle is identified in a detection process represented by a flowchart of FIG.  11 . Details will be described next. 
     It should be noted that in the present example it is assumed that the condition of all the n nozzles are unknown, and also that there is no defect register E (Hereinafter referred to as ‘defective register’) at the beginning. 
     When the routine is started in S 1101 , first in S 1102  all the condition values of the condition register S are initialized to 1 that represents the unknown condition. 
     Next, in S 1103 , it is determined whether or not there is no nozzle with unknown condition, i.e., whether or not the condition register S has any element with the condition value of 1. If not (S 1103 :NO), the present process is brought to a normal end in S 1104 . 
     On the other hand, if so (S 1103 :YES), then the process proceeds to S 1105 . In S 1105 , detection restoring unit  111  gets the ejection data D from the signal processing portion  101 , and gets the detection signal from detection unit  110 . Then, in S 1106 , it is judged whether or not the printing is normal based on the signal from the detection unit  110 . 
     If the printing is defective (S 1106 :NO), this means that there is at least one defective nozzle among the nozzles with the ejection value of 1, then the process proceeds to S 1107 . In S 1107 , the ejection data D is updated based on the condition register S, where the values in the ejection data D are set to 0 for the normal nozzles, that is the nozzles with the condition value of 2, and the values for the others are maintained the same. Next in S 1108 , the updated ejection data D is compared with a set of defective registers E to judge whether or not there is any defective register E that matches the updated ejection data D. If so (S 1108 :YES), the process returns to S 1103 . On the other hand, if S 1108  results in a negative determination (S 1108 :NO), and then in S 1109  the updated ejection data D is set to a defective register E and added to the set of the defective registers E. The defective register E is generated in this manner and increases its number. 
     Next, in S 1110 , the newly added defective register E is set as an argument, and a restoring process is executed for the defective register E. FIG. 12 shows a flowchart representing the restoring process. 
     When the restore process is started, first in S 1201  it is detected whether or not the number of the element in the defective register E that has the value of 1 is only one. If not (S 1201 :NO), the present routine is ended. On the other hand, if so (S 1201 :YES), this means that the nozzle that corresponds to the element with the value of 1 is the one that is defective. Then in S 1202 , the condition register S is updated such that the condition value for the defective nozzle is set to 0 indicating the defective condition. 
     Next in S 1203 , it is determined whether or not the detected defective nozzle is adjacent to an existent defective nozzle which has been detected earlier. If so (S 1203 : YES), then in S 1204  the present routine is brought to an end. That is, because the above-described restored printing is not useful in this case as described above, the printing is stopped, and then any restoring operation, such as cleaning operation, is performed. 
     As described above, there is hardly a possibility that two adjacent nozzles become defective during the printing. Also, even if a plurality of nozzles becomes defective, printing can be properly performed as long as the plurality nozzles are not adjacent to one another. 
     If S 1203  results in a negative determination (S 1203 :NO), then in S 1205 , restored printing is performed without using the defective nozzle. In S 1206 , all the defective registers E so that the condition value for the detected defective nozzle is  1  is deleted from the set of the defective registers E. Then, the process returns. 
     On the other hand, if S 1106  results in a positive determination (S 1106 :YES), this means that all the nozzles with the ejection value of 1 are normal. Then, in S 1111 , the condition register S is updated so that the condition values for these normal nozzles are set to 2. Next, in S 1112 , it is determined whether or not any unprocessed defective register E exists in the set of the defective registers E. If not (S 1112 :NO), the process returns to S 1103 . If so (S 1112 :YES), then in S 1113  one unprocessed defective register E is retrieved, and in S 1114  the defective register E I updated so that the value for the normal nozzle is set to 0. Then, in S 1115 , the adjustment process shown in FIG. 12 is executed, and the process returns to S 1112 . The same process is repeated for any unprocessed defective register E. 
     A specific example of the above-described detecting process will be described while referring to a following table T. Explanation will be provided while referring to line numbers (No.) in the left most column of the table 1. In the present example, it is assumed that the ink jet recording device is formed with eight nozzles in order to simplify the explanation. Also, it is assumed that second and seventh nozzles among the eight nozzles are defective as indicated at No.  1  of the table 1. Needless to say, the defectiveness of the second and seventh nozzles is unknown before the detecting process. 
     
       
         
               
               
               
               
               
               
               
               
             
           
               
                 TABLE T 
               
               
                   
               
               
                   
                   
                   
                   
                 VALUES 
                 NUMBER 
                 DETECTION 
                   
               
               
                 No 
                 STEP 
                 ITEM 
                 REGISTER 
                 12345678 
                 OF 1 
                 RESULT 
                 DESCRIPTION 
               
               
                   
               
             
             
               
                  1 
                   
                 nozzle conditions 
                   
                 20222202 
                 8 
                   
                 0: Defective 2: Normal 
               
               
                   
                   
                 (initially unknown) 
                   
                   
                   
                   
                 2: Normal 1: Unknown 0: Defective 
               
               
                  2 
                 1102 
                 condition register S (initial value) 
                 S 
                 11111111 
               
               
                  3 
                 1105 
                 ejection data D 
                 D 
                 01110001 
                   
                   
                 0: Nonejection 1: Ejection 
               
               
                   
                   
                 detection result 
                   
                   
                   
                 1 
                 sensor result (1: Defective 0: Normal) 
               
               
                  4 
                 1106 
                 defective 
               
               
                  5 
                 1107 
                 update ejection data D 
                 D 
                 01110001 
                   
                   
                 normal nozzle (Sn = 2) → 0 
               
               
                  6 
                 1109 
                 defective register E1/number of 1 
                 E1 
                 01110001 
                 4 
                   
                 number is not 1 → skip 
               
               
                  7 
                 1105 
                 ejection data D 
                 D 
                 01011101 
                   
                   
                 0: Nonejection 1: Ejection 
               
               
                   
                   
                 detection result 
                   
                   
                   
                 1 
                 sensor result (1: Defective 0: Normal) 
               
               
                  8 
                 1106 
                 defective 
               
               
                  9 
                 1107 
                 update ejection data D 
                 D 
                 01011101 
                   
                   
                 normal nozzle (Sn = 2) → 0 
               
               
                 10 
                 1108 
                 no D in defective registers 
               
               
                 11 
                 1109 
                 defective register E2/number of 1 
                 E2 
                 01011101 
                 5 
                   
                 not 1 → skip 
               
               
                 12 
                 1105 
                 ejection data D 
                 D 
                 00010101 
                   
                   
                 0: Nonejection 1: Ejection 
               
               
                   
                   
                 detection result 
                   
                   
                   
                 0 
                 sensor result (1: Defective 0: Normal) 
               
               
                 13 
                 1106 
                 normal 
               
               
                 14 
                 1111 
                 update condition register S 
                 S 
                 11121212 
                   
                   
                 normal nozzle → 2 
               
               
                 15 
                 1114 
                 defective register E1/number of 1 
                 E1 
                 01100000 
                 2 
                   
                 normal nozzle → 0, not 1 → skip 
               
               
                 16 
                 1114 
                 defective register E2/number of 1 
                 E2 
                 01001000 
                 2 
                   
                 normal nozzle → 0, not 1 → skip 
               
               
                 17 
                 1105 
                 ejection data D 
                 D 
                 01010101 
                   
                   
                 0: Nonejection 1: Ejection 
               
               
                   
                   
                 detection result 
                   
                   
                   
                 1 
                 sensor result (1: Defective 0: Normal) 
               
               
                 18 
                 1106 
                 defective 
               
               
                 19 
                 1107 
                 update ejection data D 
                 D 
                 01000000 
                   
                   
                 normal nozzle (Sn = 2) → 0 
               
               
                 20 
                 1108 
                 no D in defective registers 
               
               
                 21 
                 1109 
                 defective register E3/number of 1 
                 E3 
                 01000000 
                   
                   
                 is 1 → restore 
               
               
                 22 
                 1201 
                 E3/number = 1 → restore 
                   
                   
                 1 
               
               
                 23 
                 1202 
                 update condition register S 
                 S 
                 10121212 
                   
                   
                 defective nozzle → 0, restore 
               
               
                 24 
                 1206 
                 delete defective register E 
                   
                   
                   
                   
                 delete E including defective nozzle 
               
               
                 25 
                 1105 
                 ejection data D 
                 D 
                 00001110 
                   
                   
                 0: Nonejection 1: Ejection 
               
               
                   
                   
                 detection result 
                   
                   
                   
                 1 
                 sensor result (1: Defective 0: Normal) 
               
               
                 26 
                 1106 
                 defective 
               
               
                 27 
                 1107 
                 update ejection data D 
                 D 
                 00001010 
                   
                   
                 normal nozzle (Sn = 2) → 0 
               
               
                 28 
                 1108 
                 no D in defective registers 
               
               
                 29 
                 1109 
                 defective register E1/number of 1 
                 E1 
                 00001010 
                 2 
                   
                 normal nozzle → 0, not 1 → skip 
               
               
                 30 
                 1105 
                 ejection data D 
                 D 
                 00101100 
                   
                   
                 0: Nonejection 1: Ejection 
               
               
                   
                   
                 detection result 
                   
                   
                   
                 0 
                 sensor result (1: Defective 0: Normal) 
               
               
                 31 
                 1106 
                 normal 
               
               
                 32 
                 1111 
                 condition register S 
                 S 
                 10222212 
                   
                   
                 normal nozzle → 2 
               
               
                 33 
                 1114 
                 defective register E1/number of 1 
                 E1 
                 00000010 
                 1 
                   
                 normal nozzle → 0, is 1 → restore 
               
               
                 34 
                 1201 
                 E1/number = 1 → restore 
               
               
                 35 
                 1202 
                 condition register S 
                 S 
                 10222202 
                   
                   
                 defective nozzle → 0, restore 
               
               
                 36 
                 1206 
                 delete defective register E 
                   
                   
                   
                   
                 delete E including defective nozzle 
               
               
                 37 
                 1105 
                 ejection data D 
                 D 
                 10010000 
                   
                   
                 0: Nonejection 1: Ejection 
               
               
                   
                   
                 detection result 
                   
                   
                   
                 0 
                 sensor result (1: Defective 0: Normal) 
               
               
                 38 
                 1106 
                 normal 
               
               
                 39 
                 1111 
                 update condition register S 
                 S 
                 20222202 
                   
                   
                 normal nozzle → 2 
               
               
                 40 
                 1112 
                   
                   
                   
                   
                   
                 no set of defective registers E 
               
               
                 41 
                 1103 
                 number of 1 
                 S 
                 20222202 
                 0 
               
               
                   
                   
                 within condition register S 
               
               
                 42 
                 1104 
                   
                   
                   
                   
                   
                 normal end 
               
               
                   
               
             
          
         
       
     
     First, at No.  2 , the process of S 1102  in FIG. 11 is executed, and all the elements of the condition register S are initialized to the condition value of 1, the condition value of 1 indicating unknown condition. Accordingly, S 1103  results in a negative determination (S 1103 :NO). Next, at No.  3 , the process of S 1105  is executed to perform selective ejection. The ejection data D at this time is “01110001”, for example. Also in S 1105 , detection signal is received from the detection unit  110 . In this example, the detection signal of  1  is received, and so S 1106  results in a negative determination (S 1106 :NO), i.e., defective, at No.  4 . At No.  5 , the ejection data D is updated based on the condition register S in S 1107 . In this example, the ejection data D is unchanged at this time. 
     Because there is no defective register E identical to the updated ejection data D (S 1108 :NO), the updated ejection data D is set as a defective register E 1  and added to a set of defective registers E (S 1109 ) at No.  6 . Because the defective register E 1  includes four values of 1 at this time, S 1201  results in a negative determination (S 1201 :NO), and the process returns to S 1103 . The same processes are repeated at No.  7  through No.  11 . 
     At No.  12  in S 1105 , the detection signal of 0 is received, so it is judged the normal ejection in S 1106  at No.  13  (S 1106 :YES). Because the ejection data D at No.  12  has the ejection value of 1 for the fourth, sixth, and eighth nozzles, these nozzles are determined to be normal, and the condition values of the condition register S for these normal nozzles are set to 2 in S 1111  at No.  14 . In S 1114  at No.  15 , the defective register E 1  shown at No.  6  is updated as shown at No.  15 , where the condition values for the normal nozzles are changed from 1 to 0. The resultant values are “01100000” as shown at No.  15 . Because there are two condition values of 1, S 1201  results in a negative determination (S 1201 :NO), and so the process returns to S 1112 . Then, the same process is executed to the subsequent defective register E 2  in S 1113  and S 1114 . No.  16  shows the defective register E 2  updated from the defective register E 2  of No.  11  by replacing the value of 1 to 0 for the normal nozzles. In this case also, the number of the condition values of 1 is not one, but two, so S 1201  results in a negative determination, and the process returns to S 1112 . Because there is no more unprocessed defective register E (S 1112 :NO), the process returns to S 1103 . 
     At No.  17  through No.  21 , the processes of S 1105  through S 1109  are performed in the same manner. At No.  22 , because the defective register E 3  has only one value of 1, it is determined that the second nozzle, in this example, is defective. S 1201  results in an affirmative determination at No.  22  (S 1201 :YES), and then in S 1202 , the condition register S is updated so that the condition value for the defective nozzle (second nozzle) is changed to 0. In this case, the updated register S has the values of “10121212” as shown at No.  23 . Because there is no adjacent defective nozzle in this example (S 1203 :NO), it is stopped using the defective nozzle, and normal nozzles next to the defective nozzle cover up the defective nozzle and form dots, which are originally allocated to the defective nozzle, instead of the defective nozzle. Then, all the defective registers E having the condition value of 1 for the defective nozzle (second nozzle) are deleted in S 1206  at No.  24 . In this example, the defective registers E 1 , E 2 , and E 3  are all deleted. 
     When the same process is repeatedly executed, the remaining seventh nozzle is detected to be defective in S 1114  at No.  15 , and eventually at No.  41  it is determined in S 1103  that the condition register S includes no condition value of 1. Then, the process is brought to the normal end in S 1104 . 
     Although not described in the above example, it may be determined in S 1203  that the defective nozzle is in adjacent to another defective nozzle, and then the defective end may result. In this case, the printing is stopped, and the restoring process is executed as described above. 
     According to the present embodiment, when a nozzle becomes defective during the printing, the defective nozzle is automatically detected and proper printing can be restored without a need to stop the printing. 
     Next, a detecting process according to a second embodiment of the present invention will be described while referring to a flowchart shown in FIG.  13 . In the above-described first embodiment, the restoring operation is performed only after the number of condition values of 1 within the defective register E becomes one. However, when two or more nozzles become defective and when these defective nozzles are those that highly likely perform ink ejection at the same time, the number of condition values of 1 will not easily reach one. In this case, it takes relatively a long period of time before the restoring operation starts. Moreover, the accumulated number of the defective registers E becomes so large that the data value may exceed the capacity of the memory, resulting in memory overflow. 
     The restoring process of the second embodiment overcomes such a problem. Specifically, when the number of the defective registers E reaches a predetermined number, the following process is executed. Also, there is provided a defective additional memory ES including a plurality of elements for the respective nozzles. Each of the elements includes a plurality of bits, and functions as a memory for storing an element value. Details will be described below. 
     In the flowchart of FIG. 13, when the process starts in S 1301 , all element values of the defective additional memory ES are initialized to 0 in S 1302 . Next in S 1303 , it is detected whether or not there is any unprocessed defective register E. If so (S 1303 :YES), then in S 1304 , one unprocessed defective register E is retrieved. Then, in S 1305 , the condition values of the retrieved defective register E are added to the corresponding elements of the defective additional memory ES, and the process returns to S 1303 . The same processes of S 1304  and S 1305  are executed to all unprocessed defective registers E. When S 1303  results in a negative determination (S 1303 :NO), then in S 1306 , the ejection data D is received. When the received ejection data D have only the values of 0, indicating no ejection, (S 1307 :YES), then the process proceeds to S 1308 . In S 1308 , a nozzle corresponding to an element of the defective additional memory ES with the largest value is identified, and the value of the ejection data D for the detected nozzle is changed from 0 to 1 so that only the detected nozzle performs the ejection. In this way, the ejection data D is updated. Next, in S 1309  the ejection is performed based on the updated ejection data D, and the detection signal is received from the detection unit  110 . At this time, one dot is formed (test printed) on a recording sheet although no dot is supposed to be formed. However, degradation in the printed result due to the one unnecessary dot is far less than that caused by defective ink ejection from a defective nozzle, and such degradation is small enough to ignore. Then, in S 1310 , it is determined whether or not the ejection is normal. If defective (S 1310 :NO), the same operations as that of S 1107  through S 1110  in FIG. 11 are executed in S 1312 , and the process is ended in S 1313 . On the other hand, if normal (S 1310 :YES), the same processes of S 1111  through S 1115  in FIG. 11 are executed in S 1311 , and the process is ended in S 1313 . 
     As described above, according to the second embodiment, the test printing is performed where a single dot is formed by a single nozzle. Because the single nozzle is highly likely the defective nozzle, the defective nozzle can be promptly detected in an effective manner. Accordingly, the defective nozzle is stopped from being used at an earlier stage, so that degradation of an image quality can be reduced. Further, the number of the defective registers E is greatly reduced regardless of whether the tested nozzle is normal or defective, so that the memory overflow can be prevented. 
     Next, a third embodiment of the present invention will be described while referring to FIG.  14 . In this embodiment, the electric-current detection is performed by using a laser beam. 
     The ink jet head  107  of the third embodiment includes a laser-beam generator  1501  and a laser-beam receptor  1504  shown in FIG. 15 at the ends of corresponding nozzle line for generating a laser beam  1401  or  1402  shown in FIG. 14 between the laser-beam generator  1501  and the laser-beam receptor  1504 . The laser-beam generator  1501  includes a well-known semiconductor laser  1502  and a collimate lens  1503 . The laser-beam receptor  1504  includes a well-known photodiode  1504  and a signal detection circuit (not shown). The axis of the laser beam  1401 ,  1402 , is parallel to the nozzle line direction  302 . A plurality of concentric circles of the laser beam  1401 ,  1402  indicates its strength distribution. A charged splash from a nozzle  201  flies to the electrode  401  as indicated by an arrow  1403  and impacts thereon. Because the laser beam  1401  intersects the path  1403 , the splash flying along the path  1403  blocks the laser beam  1401 , so that the amount of the laser beam  1401  received be the laser-beam receptor  1504  reduces. Accordingly, the occurrence of splash can be detected by detecting change in the amount of the laser beam  1401 ,  1402  reaching the laser-beam receptor  1504 . Because the splash flies to the electrode  402  in the same manner, only one of the laser beams  1401  and  1402  is necessary for the detection. 
     In this method also, a defective nozzle cannot be identified by merely detecting the change in the laser beam amount, although the occurrence of defective ejection can be detected. However, the same process as that of the first or second embodiment can be executed in the third embodiment also in order to identify the defective nozzle. 
     According to the third embodiment, ink droplets ejected at angle and splashed minute droplets can be detected even when these do not reach and impact on the electrodes  401 ,  402 , so a nozzle, which is not completely defective but incapable of proper ejection, can also be detected. Accordingly, the above restoring operation can be performed at an earlier stage, and so the degradation of the image quality can be minimized. 
     As described above, according to the present invention, the electrodes for generating a charging electric field and deflector electric field can be provided common to a plurality of nozzles. This provides a highly reliable multi-nozzle head. Also, because ink droplet ejections are performed at constant intervals, a maximum ejection rate available for the nozzles can be used. Further, it is possible to perform a multiple ejection, where a single dot is formed by a plurality of ink droplets from different nozzles, and so the reliability can be increased as needed. Moreover, the ink droplet ejection in a non-rectangular coordinate system with honeycomb shape is also possible. In this case, the amount of overlapping regions among adjacent dots can be minimized, so the ink consumption can be reduced. 
     While some exemplary embodiments of this invention have been described in detail, those skilled in the art will recognize that there are many possible modifications and variations which may be made in these exemplary embodiments while yet retaining many of the novel features and advantages of the invention. 
     Although in the above-described embodiment, the orifices  201  are aligned in the pitch of  75  orifices/inch, the nozzles  107   a  can be aligned in the pitch of  150  orifices/inch. In this case, a resolution will be twice the above-described resolution. Also, the number of nozzles  107   a  (orifices  201 ) is not limited to  128 . 
     Also, the present invention can be also applied to an ink jet recording device where printing is performed while a recording head is moved and a recording sheet stays still rather than where the printing is performed while the recording sheet is moved and the recording sheet stays still. 
     Further, the present invention can also be applied to bubble jet recording device where an air bubble is generated by applying head, and ejecting ink by utilizing the pressure of the generated air bubble.