Patent Publication Number: US-6702418-B2

Title: Ink jet recording device capable of detecting defective nozzle with high signal-to-noise ratio

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ink jet recording device having a monitor function for monitoring ink droplet generating conditions. 
     2. Related Art 
     There has been proposed a line scanning type ink jet printer, capable of printing images on an elongated uncut recording sheet at a high printing speed. This type of printer includes a head having a plurality of nozzles and an elongated width covering over the entire width of the recording sheet. When printing images, ink droplets are ejected from the nozzles, charged, and deflected, and then impact on the recording sheet that is being fed at a high speed in its longitudinal direction. The impact positions of the ejected ink droplets on the recording sheet are controlled based on a recording signal. By controlling the impact positions of the ink droplets and the feed of the recording sheet, a desired image is formed on the recording sheet. 
     There are two types of line scanning type ink jet printer. One includes a continuous ink jet head, and the other includes an on-demand ink jet head. 
     Although, the printer with the on-demand ink jet head is slow in printing speed compared with the printer with the continuous ink jet head, the on-demand ink jet head requires a simple ink system, and so is well suited for a general-purpose high-speed printer. 
     When a nozzle of ink jet printers becomes defective, a part of an image corresponding to the defective nozzle may be left out or may have an unevenness in ink density, resulting in degradation of image quality. Therefore, in order to maintain a high quality of images, it is necessary to monitor the ink ejection condition of each nozzle. 
     Japanese Patent-Application Publication No. SHO-61-53053 discloses an ink jet printer having a monitor function for monitoring ink droplet generation. After an ink-droplet-charging signal is generated to charge ink droplets for a certain period of time, a charged-amount-detection signal is detected for a certain period of time so as to detect charging condition of the ink droplets. A changeable amplifying means amplifies the charged-amount-detection signal at an amplification rate. An amplification-rate-control-signal generation circuit generates and outputs an amplification-rate-control signal to control the changeable amplifying means to change the amplification rate. Specifically, the amplification-rate-control signal controls the changeable amplifying means to set to a lower amplification rate when the ink-droplet charging signal is being generated, and to a higher amplification rate when the charged-amount-detection signal is being detected. In this way, the charged amount, i.e., charging condition of ink droplet, is detected while preventing a detection error, because electrical noise is not amplified other than when the charged amount-detection signal is being detected. 
     SUMMARY OF THE INVENTION 
     However, in the above printer, because a pulse-shaped high voltage signal is used as the ink-droplet charging signal, its influence is reflected in the charged-amount detection signal, which is a weak signal, so the signal-to-noise ratio (SNR) becomes small. 
     It is an object of the present invention to overcome the above problems, and also to provide an ink jet recording device capable of detecting the ink droplet generation condition with high SNR. 
     In order to achieve the above and other objective, there is provided an ink jet recording device including a head formed with a nozzle and selectively ejecting an ink droplet from the nozzle, a deflecting means for deflecting a flying direction of the ink droplet ejected from the nozzle, the deflecting means including a first electrode and a second electrode, a mode selecting means for selecting one of a first mode and a second mode, an applying means for applying a direct voltage to the first electrode and another direct voltage to the second electrode throughout the first mode and the second mode, the direct voltage differing from the another direct voltage, and a detection means for detecting a quantity of electricity relating to an electric discharge flowing through the first electrode in the second mode. 
     There is further comprising a control method for controlling an ink jet recording device. The control method comprises the steps of a) selecting a first mode, b) applying a direct voltage to a first electrode and another direct voltage to a second electrode throughout the first mode and a second mode, the direct voltage differing from the another direct voltage, c) ejecting an ink droplet from a nozzle of an ink jet head in the first mode, d) switching from the first mode to the second mode, and e) detecting a quantity of electricity relating to an electric discharge flowing through the first electrode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a plan view showing a configuration of an ink jet printer according to an embodiment of the present invention; 
     FIG.  2 ( a ) is a time chart of a print-mode/detect-mode switching signal; 
     FIG.  2 ( b ) is a time chart of ejection signal; 
     FIG.  2 ( c ) is a time chart of voltage applied to a first deflector electrode; 
     FIG.  2 ( d ) us a time chart of voltage applied to a second deflector electrode; 
     FIG.  2 ( e ) is a time chart of a detection signal; 
     FIG.  2 ( f ) is a time chart of charging-mode/detection-mode switching signal; 
     FIG.  2 ( g ) is a time chart of a condition of a switch; 
     FIG.  2 ( h ) is a time chart of a condition of a photo-coupler; 
     FIG.  2 ( i ) is a time chart of a condition of a photo-coupler; 
     FIG. 3 is a plan view of components, partially indicated in a block diagram, of the ink jet printer; 
     FIG. 4 is a magnified view of component of FIG. 3; 
     FIG.  5 ( a ) is an explanatory view showing charging-deflection control signals applied to the charger electrodes of the ink jet printer; 
     FIG.  5 ( b ) is an explanatory view showing PZT driving signals applied to nozzles and corresponding deflection amounts of ink droplets; and 
     FIG. 6 is an explanatory view showing dots formed on a recording sheet. 
    
    
     PREFERRED EMBODIMENT OF THE PRESENT INVENTION 
     Next, an ink jet printer  1  according to an embodiment of the present invention will be described while referring to the attached drawings. 
     The ink jet printer  1  shown in FIG. 1 has a print mode and a detect mode. The print mode is for printing operation for forming images on a recording medium. The detect mode is for detecting any nozzle that has became defective. The detect mode is automatically set when a main power of the ink jet printer  1  is turned ON, or once every hour or once every 1,000 pages printing, for example. Needless to say, the detect mode can be manually set as desired or can be set both manually and automatically. 
     The detect mode includes a charging mode for charging operation and a detection mode for detection operation. Typically, the charging operation and the detection operation together require 1 ms. Performing these two operations twice (2 ms) improves detection precision. 
     First, the printing operation in the print mode will be described while also explaining a configuration of the ink jet printer  1 . 
     The ink jet printer  1  of the present embodiment forms an image on an elongated uncut recording sheet  100  of FIG.  3 . Specifically, the elongated uncut recording sheet  100  has a width in a first direction A and a length in a second direction B perpendicular to the first direction A, and is transported in the second direction B at a predetermined speed. The ink jet printer  1  forms dots on scanning lines  110  (FIG. 4) on the recording sheet  100  at a dot density of DS so as to form a dot image on the recording sheet  100  at a high speed. 
     As shown in FIGS. 3 and 4, the ink jet printer  1  includes a recording head  200 , which includes a plurality of head modules  210  arranged in the first direction A and a frame  220  for supporting the head modules  210 . Each head module  210  has the same configuration, and is formed with a nozzle line  211  extending in a third direction C. The nozzle line  211  includes N nozzles  230  aligned in the third direction C at a pitch of Pn, and each nozzle  230  has a nozzle hole  231  opened to a nozzle surface of the head module  210 . The recording head  200  is positioned so that the nozzle surface faces a recording surface of the recording sheet  100  while keeping the distance of 1 mm through 2 mm therebetween. 
     As shown in FIG. 4, each nozzle  230  has the same configuration and has an ink chamber  232  with the nozzle hole  231 , an ink supply port  233  for introducing ink into the ink chamber  232 . The head module  210  is formed also with a manifold  234  for distributing ink to the ink supply port  233  of each nozzle  230 . The ink chamber  232  is provided with a piezoelectric element  235 , such as PZT, serving as an actuator. The piezoelectric element  235  changes a volume of the ink chamber  232  when applied with recording signals. 
     In the present example, scanning lines  110  extend in the second direction B and have a line density DS of 300 dpi in the first direction A. The angle θ of the third direction C with respect to the second direction B is approximately 11.3 degrees (=tan −1  (⅕)). The nozzle-hole pitch Pn is {fraction (2/300)} (sin(⅕)) −1  inches, i.e., approximately 0.034 inches. The number N of nozzles  230  is 96. 13 head modules  210  are used, which is sufficient for covering over the entire width of recording head  200 . 
     The ink jet printer  1  also includes a plurality of pairs of deflector electrodes  310 ,  320 , an electrode substrate  330 , a deflection-control-signal generating unit  400 , and an ink-ejection control-signal generating unit  500 . Each pair of electrodes  310 ,  320  are provided between the recording sheet  100  and the recording head  200  and sandwich a corresponding one of the nozzle lines  211  therebetween. The electrode  310  serves as a positive-polarity deflector electrode, and the electrode  320  serves as a negative-polarity deflector electrode. The electrodes  310 ,  320  are connected to a positive-polarity deflector-electrode terminal  341  and a negative-polarity deflector-electrode terminal  342 , respectively, which are provided on the electrode substrate  330 . 
     The deflection-control-signal generating unit  400  is for applying deflection control signals to the deflector electrodes  310 ,  320 , and includes a charging-signal generating unit  410 , a positive-polarity deflector voltage supply  421 , a negative-polarity deflector voltage supply  422 , a positive-polarity biasing circuit  431 , and a negative-polarity biasing circuit  432 . 
     The charging-signal generating unit  410  generates charging signal voltage for charging ink droplets. The positive-polarity deflector voltage supply  421  and the negative-polarity deflector voltage supply  422  generate and output deflector voltages. The positive-polarity biasing circuit  431  and the negative-polarity biasing circuit  432  superimpose the charging signal voltage onto the deflector voltage, thereby generating charging-deflecting control signals S 1 , S 2 , which are applied to the electrodes  310 ,  320 , respectively. 
     The ink-ejection control-signal generating unit  500  includes a recording-signal generating circuit  510 , a timing-signal generating circuit  520 , a PZT-driving-pulse generating circuit  530 , and a PZT driver circuit  540 . The recording-signal generating circuit  510  generates pixel data of images based on input data or test pattern data. The timing-signal generating circuit  520  generates a timing signal for determining operation timings of the ink jet printer  1 . The PZT-driving-pulse generating circuit  530  generates a PZT driving pulse for each nozzle  230  based on the pixel data and the timing signal. The PZT driving pulse is for controlling the proper ink ejecting timing. The PZT-driver circuit  540  amplifies the PZT driving pulse to a signal level sufficient for driving the piezoelectric element  235 , and outputs the amplified PZT driving pulse to the piezoelectric element  235  of each nozzle  230 , so that an ink droplet is ejected from the nozzle  230  at a proper timing. The timing-signal generating circuit  520  also generates print-mode/detect-mode switching signals  605 , charging-mode/detection-mode switching signals  606 , and ejection signals  607  as described later. 
     FIG.  5 ( a ) shows the charging-deflecting control signals S 1  and S 2  applied to the electrodes  310  and  320 , respectively. FIG.  5 ( b ) shows PZT driving pulses Sa through Sd for each nozzle  230  and also corresponding ink-droplet deflection amounts Ca through Cd. FIG. 6 shows dots recorded on the recording sheet  100 . Details will be described next. 
     When the electrode  310  for a positive polarity is applied with the charging-deflecting control signals S 1 , a deflector voltage of +H and a charger voltage are applied to the electrode  310 . Similarly, when the electrode  320  for a negative polarity is applied with the charging-deflecting control signals S 2 , a deflector voltage of −H and the charger voltage are applied to the electrode  320 . The magnitude of the charger voltage changes every time period T in a stepped manner among 0 V and ± Vc. As a result, a charger electric field for charging ink droplets and a deflector electrostatic field for deflecting the charged ink droplets are generated. 
     The ink held in the recording head  200  is electrically connected to the ground, i.e., has OV. Therefore, at the time when the ink droplet  130  is about to be ejected from the nozzle hole  231 , the charger voltage is applied between the ink droplet  130  and the electrodes  310 ,  320 . Because the ink has an excellent conductivity of lower than several hundreds Ω cm, at the time of when the ink droplet  130  separates from the rest of the ink, the ink droplet  130  is charged by an amount corresponding to the charger voltage applied at that moment. Then, the charged ink droplet  130  flies toward the recording sheet  100 . Before impacting on the recording sheet  100 , the ink droplet  130  is deflected by the deflector electrostatic field by a deflection amount in proportion to the charged amount toward a fourth direction D perpendicular to the third direction C (FIG.  4 ). 
     Referring to FIG. 4, an ink droplet  130 A ejected from a nozzle hole  231 A can impact on any scanning lines  110   n+1  to  110   n+5  depending on its deflection amount, and therefore can form any dot  140   AN+1  to  140   AN+5 . Similarly, an ink droplet  130 B ejected from a nozzle hole  231 B can impact on any scanning lines  110   n+3  to  110   n+7  by deflection, and an ink droplet  130 C from a nozzle hole  231 C is deflected to impact on any scanning lines  110   n+5  to  110   n+9 . That is, the ink droplets  130 A,  130 B,  130 C from three different nozzle holes  231 A,  231 B, and  231 C can impact on the single scanning line  110   n+5 . Also, two ink droplets from different nozzle holes can impact on the scanning line  110   n+4 . The same is true for the scanning line  110   n+6 . 
     The recording operations will be described further in more detail. It should be noted that as described above the PZT driving pulses Sa through Sd of FIG.  5 ( b ) are applied to the piezoelectric elements  235  for ejecting ink droplets  130 . FIG. 6 shows dots formed on the recording sheet  100  and projections  231 A′,  231 B′,  231 C′ of the nozzle holes  231 A,  231 B,  231 C of FIG.  4 . The line segments extending perpendicular to the direction C are time division/deflection reference lines L. The interval of the reference lines L indicates the time interval T, the direction of the reference lines L indicate a direction of the deflection, and the length of the reference lines L indicates the deflection amount. 
     As shown in FIGS.  5 ( a ) and  5 ( b ), at the time T 1 , the charger voltage is ±0. Accordingly, the ink droplet  130 A ejected from the nozzle hole  213 A at the time T 1  is not charged. Accordingly, the ink droplet  130 A is not deflected but flies straight, and then impacts on, for example, a pixel  120 A T1  on the scanning line  110   n+3  of FIG. 6, forming a dot thereon. At a subsequent time T 2 , because the PZE driving signal pulse is not applied to the piezoelectric element  235  of the nozzle  230 A, no ink droplet is ejected at the time T 2 , and so not dot is formed. At the time T 3 , the charger voltage is −Vc, so an ink droplet ejected at the time T 3  is deflected by an amount of − 2 . The ink droplet impacts on a pixel  120 A T3  on the scanning line  110   n+5 , and forms a dot thereon. At the time T 4 , no dot is formed by an ink droplet from the nozzle hole  231 A. At the time T 5 , the charger voltage is +½ Vc, so an ink droplet ejected at the time T 5  is deflected by an amount of +1. The ink droplet impacts on a pixel  120 A T5  on the scanning line  110   n+2 , and forms a dot thereon. The same operation is performed with respect to the nozzle-holes  231 B,  231 C,  231 D, and on, so that dots are formed on other pixels also as shown in FIG.  6 . 
     In this manner, ink droplets  130 A ejected from the nozzle hole  231 A are selectively deflected and able to impact on every pixel on the five scanning lines  110   n+1  through  110   n+5 . 
     Next, the operation in the detect mode will be described while referring to a monitoring mechanism of the ink jet printer  1 . 
     It is assumed in this example that the nozzle  230  shown in FIG. 1 is defective, and an ink droplet  608  that is smaller in size than a proper ink droplet is ejected from the nozzle  230 . The nozzle  230  becomes defective for different reasons, for example, when the nozzle  230  is clogged, when air bubbles are trapped in the nozzle  230 , or when a portion around the nozzle hole  231  is unevenly wet with ink. In this condition, the defective nozzle is incapable of ejecting ink, or ink droplet is ejected at an angle. Sometimes, an ink droplet is ejected along with additional minute ink droplets. 
     As shown in FIG. 1, the ink jet, printer  1  further includes a monitoring mechanism  10  provided to each nozzle  230 . The monitoring mechanism  10  includes switches  600 ,  601 , and  602 , which together determine the operation mode of the ink jet printer  1 . For example, the connection conditions shown in FIG. 1 of the switches  600 ,  601 ,  602  indicate that the ink jet printer  1  is in the charging mode of the detect mode. 
     The switches  600  and  601  are connected to the deflector electrodes  310  and  320 , respectively, and change their connection condition in response to the print-mode/detect-mode switching signal  605 . The switch  602  is turned ON and OFF in response to the charging-mode/detection-mode switching signal  606 . Each of the switching signals  605  and  606  are output from the timing-signal generating circuit  520  and takes the value of either “0” or “1”. 
     When the switching signal  605  of “1” is output to the switches  600  and  601 , this means that the print mode is selected, the switches  600  and  601  connect the electrodes  310 ,  320  to the deflection-control-signal generating unit  400 . 
     When setting to the detect mode, the switching signal  605  is switched from “1” to “0”, so that the switches  600  and  601  are switched into the connection condition shown in FIG. 1, and the operation mode is switched from the print mode to the detect mode. 
     When the switching signal  605  is switched to “0” in this manner as shown in FIG.  2 ( a ), the switching signal  606  is initially set to “0” as shown in FIG.  2 ( f ). As a result, the switch  602  is turned ON as shown in FIGS.  1  and  2 ( g ), and the operation mode is set to the charging mode. 
     In this charging mode, that is, in the condition shown in FIG. 1, the deflector electrode  310  is connected to the ground, that is, set to 0 V (FIG.  2 ( c )). On the other hand, the deflector electrode  320  is connected to a charger voltage supply (battery)  603  via a resister  604  and the switch  602 . The charger voltage source  603  supplies a DC voltage of −V1 to the deflector electrode  320  (FIG.  2 ( d ). At the same time, a condenser  609  is also charged with −V1 from the charger voltage supply  603  via the resister  604 . 
     The ejection signal  607  shown in FIG.  2 ( b ) is output to the piezoelectric element  235 . Because the nozzle  230  of FIG. 1 is defective as mentioned above, the minute ink droplets  608  are ejected from the nozzle  230 . At this time, the minute ink droplets  608  are positively charged by a charger electric field generated by the deflector electrode  301  with 0 V and the deflector electrode  320  with −V1. 
     Next, as shown in FIG.  2 ( f ), the switching signal  606  is switched to the value of “1”, and the operation mode is switched from the charging mode to the detection mode. As a result, the ejection signal  607  is stopped (FIG.  2 ( b )), and the switch  602  is turned OFF (FIG.  2 ( g )). Because no ejection signal  607  is output, no ink droplet is ejected from the nozzle  230 . Also, because the switch  602  is turned OFF, the charged voltage of the condenser  609 , which is negatively charged during the charging mode, is applied to the deflector electrode  320 , so that the second deflector electrode  320  is maintained at −V1 (V) (FIG.  2 ( d )). 
     Accordingly, the positively charged ink droplets  608  are pulled toward the negatively charged deflector electrode  320  and impact thereon. It should be noted that because an ink droplet in a proper size flies at a higher speed, the positively-charged ink droplet having a proper size does not impact on the deflector electrode  320  but reaches the recording sheet  100 . However, because the minute ink droplets  608  are slow in their flying speed, the droplets  608  are pulled toward the deflector electrode  320  during both the charging mode and the detection mode and impact thereon eventually. 
     When the positively charged ink droplets  608  impacts and cling on the deflector electrode  320 , the negative charge of the condenser  609  is canceled out by the positive charge of the ink droplets  608 . As a result, the positive charge at a side of the condenser  609  opposite to the side connected to the deflector electrode  320  flows to the ground via a field effect transistor (FET)  618  of a photo-coupler  610 . That is, the electric discharge occurs by the amount equivalent to the charging amount of the minute ink droplets  608  clinging on the deflector electrode  320 . 
     The photo-couplers  610  and  612  control the electric current flowing through light-emitting diodes (LEDs)  617  and  619  (input side) so as to control the ON resistance of the FETs  618  and  620  (output side), respectively. The ON resistance can change from tens Ω to hundreds MΩ. 
     In the detection mode, the switching signal  606  is “1” as mentioned above. Therefore, no electric current flows to the LED  617  of the photo-coupler  610 , so that the ON resistance of the photo-coupler  610  is large (FIG.  2 ( h )). Also, because an inverter  616  outputs a signal of “0” to the photo-coupler  612 , an electric current flows to the LED  619  of the photo-coupler  612 , so that the ON resistance of the FET  620  of the photo-coupler  612  is small (FIG.  2 ( i )). 
     Accordingly, the discharge due to the charged minute ink droplets  608  is detected as a large detection voltage at the both sides of the FET portion  618 , impedance-converted at the operation amplifier  611 , amplified at an operation amplifier  613  at an amplification rate, which is determined by the resistance of the resister  614  and the ON resistance of the FET portion  620 , and so producing a detection output  615  (FIG.  2 ( e )). That is, the detection output  615  is amplified at a high rate in the detection mode. Because the charger voltage of the condenser  609  is static and has no noise, even when the detection output  615  is amplified at the high rate, the noise during the detection is greatly suppressed. 
     On the other hand, when the switching signal  606  is “0” in the charging mode, the ON resistance of the photo-coupler  610  is small, and the ON resistance of the FET  620  is large, so that the amplification rate is small. 
     In this way, because stable and low noised deflector DC voltage is used over the charging period in the charging mode to the detection period in the detection mode, and also because the voltage is controlled to the lower amplification rate at the charging period and to the higher amplification rate in the detection period, the detection output  615  with a high SNR can be obtained. 
     By performing the above charging operation and detection operations twice, the detection precision is enhanced as mentioned above. In other words, in the detect mode, the ejection signal  607  are intermittently output, and a defective nozzle is detected based on the discharge due to the ink droplets impacted on the electrodes  320  at the time of when the ejection signal  607  is not output. The electrode  320  is applied with a negative voltage from the battery  603  and the condenser  609  in the detect mode. 
     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.