Patent Publication Number: US-6908166-B2

Title: Inkjet recording device with ink refresh function

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a drop-on-demand inkjet recording device and particularly to a high-speed line-scan inkjet recording device with an ink refresh function. 
     2. Description of Related Art 
     There are continuous type and drop-on-demand type inkjet recording devices. Although continuous type inkjet recording devices constantly eject ink from all nozzles, drop-on-demand inkjet recording devices eject ink droplets only as needed. Sometimes nozzles of drop-on-demand inkjet recording devices will not be fired for long periods during printing. Because inkjet recording devices mainly use water-based ink, whose main component is water, the water-based ink near the opening of non-firing nozzles can evaporate and cohere during such long non-firing periods. Once ink is ejected, the poor condition of the ink in the nozzle can adversely affect ejection performance. In bad situations, the nozzle can be completely clogged by the evaporated or cohered ink so that ejection becomes impossible. 
     Japanese Patent-Application Publication No. SHO-57-61576 discloses a method of vibrating ink to prevent clogging. During periods of non-ejection, the piezoelectric elements for ejecting ink are applied with a smaller energy than required for actually ejecting an ink droplet. This vibrates the ink near the opening of nozzles so that the ink is less likely to cohere. Therefore, vibrating ink can prevent nozzle clogs without increasing consumption of ink. However, merely vibrating the ink does not prevent the water component of the ink from evaporating. When the ink near the nozzle opening evaporates, the viscosity of the ink increases so that ejection performance can be poor. For example, ejected ink droplets may follow a curved trajectory instead of a desirable straight trajectory. Nozzles can also clog up so that ink ejection is impossible. 
     Japanese Patent-Application Publication No. HEI-9-29996 discloses performing an ink refresh operation in addition to ink vibration. During the ink refresh operation, recording operations are temporarily stopped, the recording head is moved to a predetermined position that is outside the printing range, and then ink is ejected from all of the nozzles in the head. Overly viscous or partially cohered ink near the opening of the nozzles is discharged with the ink ejection and replenished with fresh ink. This method is superior to vibrating the ink in terms of effectively maintaining ejection performance. 
     Line scan inkjet recording devices are also known in the art. Conventional line scan inkjet recording devices include a print head with an array of nozzles that extend across the entire width of a recording sheet. Line scan inkjet recording devices can record images at high speed because there is no need to transport the print head across the surface of the recording sheet In its widthwise direction. That is, the recording sheet needs to be merely transported continuously in front of the nozzles. However, whenever a refresh operation is performed, recording operations must be temporarily stopped and the print head is moved to a non-printing region. This reduces the recording speed. Further, a complicated mechanism is required for temporarily stopping sheet transport in this way. 
     Japanese Patent-Application Publication No. 2002-36566 discloses a deflection-type drop-on-demand inkjet recording device that is capable of performing refresh operations without the need to temporarily stop recording operations and move the print head out of the printing range. The nozzles of the print head are divided into groups of 128 to 1,024 nozzles. When there is a period when none of the nozzles in one of the groups is required for image recording, then all of the nozzles in the group are fired together in a refresh operation. The refresh droplets are charged by an electric field and then deflected by a deflection field away from the recording sheet toward an ink collection unit, where the refresh ink droplets are collected. 
     However, a refresh operation cannot be performed on any group of nozzles as long as even a single nozzle of the group is being used for image recording. When printing a vertical straight line or other image that is elongated in the transport direction of the recording sheet, then refresh operations cannot be performed for long periods of time on nozzle groups with nozzles used in the elongated image. Nozzles of such groups that are not used to record the image will have problems described above such as ink cohering so that ink ejection is defective or impossible. 
     To prevent such problems, it is conceivable to provide an ink refresh ejection period in addition to recording ejection periods. The ink refresh ejection period is used solely for ink refresh operations. In general, a time-sharing method is used wherein an ink refresh ejection period is interposed between two consecutive ink recording ejection periods. In order to reduce ink consumption, the fewer times that ink refresh is performed the better. It has been determined by tests that, under normal environmental conditions of temperature and humidity, sufficient effects are achieved by performing refresh operations at a frequency of only 10 Hz-20 Hz. 
     This type of refresh operation is well suited for low-speed recording devices, but not very well suited for high-speed recording devices, such as line scan inkjet recording devices. Normally recording at high speeds is achieved by electing droplets at a high ink ejection frequency f. However, in order to eject an ink droplet, each voltage drive signal that is applied to a piezoelectric element to eject an ink droplet needs to be applied for a certain time duration, for example, 80 micro seconds as shown in FIG.  1 ( a ). This time requirement for duration of the drive signal limits the frequency that signals can be applied. For example, when the drive signal must be a minimum of 80 micro seconds long, then the drive signals cannot be applied at a frequency of greater than 10 kHz, so the maximum ejection frequency fm (Hz) is 10 kHz. 
     At this time, the speed at which a recording sheet can be transported, that is, a sheet transport speed Vp, can be represented using the following formula:
 
 Vp=f/R   (1)
 
     wherein f is the ejection frequency; and 
     R is the resolution (in dots/inch) in the sheet transport direction. 
     For example, the maximum sheet transport speed Vpm is 33.3 inches/second for printing an image with a resolution of 300 dpi (dots/inch) at the maximum ejection frequency fm of 10 kHz. 
     However, when recording is performed at a high speed near or at the maximum ejection frequency fm of 10 kHz, only a short interval separates successive drive signals as shown in FIG.  1 ( a ). In this case, there is insufficient time for also outputting an ink refresh drive signal. A longer interval between successive drive signals is required if the time-sharing method is to be used. 
     However, normally both the recording resolution and sheet transport speed are maintained constant to facilitate synchronization of ink ejection and sheet transport operations. Therefore, the duration of each drive signal is also constantly the same. Accordingly, the interval between successive drive signals cannot be temporarily lengthened only at certain times. Therefore, even if ink refresh operations are performed only very infrequently, the interval between successive drive signals must be increased for all drive signals as shown in FIG.  1 ( b ). As a result, in order to enable refresh operations during printing operations, the actual ejection frequency f must be met to half the maximum ejection frequency fm of 10 kHz or less, that is, to 5 kHz or less. 
     Naturally, the recording speed Vp also decreases. That is, from formula (1) it can be understood that:
 
 Vp=f/R= 16.7 inches/second  (2)
 
     The sheet-transport speed also drops by half or less. This creates a big problem when attempting to produce a high-speed recording device. 
     SUMMARY OF THE INVENTION 
     It is an objective of the present invention to provide an inkjet recording device capable of performing an optional ink refresh operation without sacrificing recording speed. 
     In order to attain the above and other objects, the present invention provides an inkjet recording device including a plurality of nozzles for ejecting ink droplets, a first signal generator that generates a recording ejection signal, in response to which the nozzles selectively eject recording ink droplet, a changing unit that, during a frequency changing period, temporarily changes an ejection frequency that is common to all of the nozzles, a second signal generator that generates, during the frequency changing period, a refresh ejection signal in response to which the nozzles selectively eject refresh ink droplet, an electric field generator that generates an electric field for deflecting the refresh ink droplet, and an ink collector that collects the deflected refresh ink droplet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
       FIG.  1 ( a ) is timing chart showing a drive signal output at a maximum ejection frequency; 
       FIG.  1 ( b ) is timing chart showing a drive signal output at a maximum ejection frequency possible when an ink refresh operation is also performed; 
         FIG. 2  is a block diagram showing electrical components of an inkjet recording device according to an embodiment of the present invention; 
         FIG. 3  is a side view showing a recording head and a sheet transport system of the inkjet recording device of the embodiment; 
         FIG. 4  is a block diagram showing a head module and a piezoelectric element driver of the recording head; 
         FIG. 5  is a timing chart showing basic timings of the piezoelectric element driver; 
         FIG. 6  is a cross-sectional view showing the head module; 
         FIG. 7  is a plan view showing an array of head modules; 
         FIG. 8  is a perspective view showing the head module; 
         FIG. 9  is a side view showing an ejected refresh ink droplet being deflected and collected; 
         FIG. 10  is a timing chart showing various signals for driving the piezoelectric element driver; and 
         FIG. 11  is a schematic view showing trajectory of ink droplets ejected during a normal ejection mode and an ink refresh ejection made. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Next, an inkjet recording device  1  according to an embodiment of the present invention will be described with reference to the accompanying drawings. 
     As shown in  FIG. 2 , the inkjet recording device  1  includes a recording head  501  and a sheet-transport system  601 . The recording head  501  is mounted an the sheet-transport system  601 . The recording head  501  includes a plurality of nozzle modules  401  and a plurality of piezoelectric element drivers  402 . The piezoelectric element drivers  402  ere provided in a number corresponding to the number of nozzle modules  401  and are each connected to a corresponding one of the nozzle modules  401 . In order to achieve color printing, a plurality of different recording heads  501 , one for each different color, is provided. However, in order to facilitate explanation, the following explanation will be provided assuming that only a single recording head  501  is provided. 
     As shown in  FIG. 3 , the sheet-transport system  601  includes a guide  603 , a transport drive roller  604 , and a rotary encoder  605 . Although not shown in the drawings, the sheet-transport system  601  also includes a transport mechanism. Under operation of the transport mechanism, a continuous recording sheet  602  is transported in a sheet transport direction Y to the guide  603 , follows the guide  603  to a position directly below the recording head  501 , and is then discharged out via the transport drive roller  604 . The rotary encoder  605  is attached to the transport drive roller  604  and outputs a sheet position pulse  108  indicated in  FIG. 2  in accordance with the position of the continuous recording sheet  602  in the sheet transport direction Y. Although not shown in the drawings, a drive motor is attached to the transport drive roller  604 . 
     As shown in  FIG. 2 , the inkjet recording device  1  further includes a buffer memory  102 , a data processing unit  103  such as a CPU, an ejection data memory  105 , a sheet control unit  106 , an analog drive signal generator  110 , and a digital ejection signal generator  111 . Although not shown in the drawings, a computer system is connected to the inkjet recording device  1 . The user uses the computer system to prepare text and the like to be recorded using the inkjet recording device  1 . The text may be written at the computer system in any of a variety of different page-description languages. However, before sending the text to the inkjet recording device  1 , the computer system develops the text into bitmap data  101  to match the specifications (resolution and the like) of the inkjet recording device  1 . According to the present embodiment, the bitmap data  101  is monochrome bitmap data wherein a logical value of 1 indicates “record” and a logical value of 0 indicates “non-record.” It should be noted that even if the computer system supplies color or multi-bit bitmap data, the bitmap data can be easily used in the inkjet recording device  1  by converting it into monochrome bitmap data. Since such conversion operations are well known in the art, their detailed description will be omitted here. 
     When recording is started, one job&#39;s worth (i.e., a plurality of pages&#39; worth) of bitmap data  101  is input serially into the buffer memory  102 . The buffer memory  102  temporarily stores the bitmap data  101 . During or after operations for storing the bitmap data  101  into the buffer memory  102 , the data processing unit  103  serially converts the bitmap data  101 , that is temporarily stored in the buffer memory  102 , into ejection data  104  that meets the ejection specifications of the inkjet recording device  1 . The ejection data  104  is stored into the ejection data memory  105 . When storage of the ejection data  104  into the ejection data memory  105  is completed, then the sheet control unit  106  outputs an operation command  107  to the sheet-transport system  601  to command start of sheet transport. As sheet transport starts, the sheet control unit  106  starts receiving the sheet position pulses  108  from the rotary encoder  605 . According to the present embodiment, the sheet position pulses  108  are outputted at a rate of 500 pulses/inch, or once about every 17 micro millimeter. When the continuous recording sheet  602  reaches a suitable recording position, the sheet control unit  106  generates a sheet-position synchronization signal  109  that matches the resolution of the inkjet recording device  1 . The resolution of the inkjet recording device  1  according to the present embodiment is 300 dpi. Accordingly, the sheet-position synchronization signal  109  is generated once each time the sheet position pulse  108  is output five times, that is, each time the continuous recording sheet  602  moves {fraction (1/300)} th  of an inch. The sheet-position synchronization signal  109  is sent to the analog drive signal generator  110  and the digital ejection signal generator  111 . The sheet-position synchronization signal  109  is also sent to the piezoelectric element drivers  402  as a latch clock L-CLK shown in FIG.  4 . 
     As will be described later, the maximum ejection frequency fm of the inkjet recording device  1  is 10 kHz. However, the normal ejection frequency f is set to 8 kHz. The sheet transport speed Vp can be determined by substitution using formula (1):
 
 Vp=f/R= 26.7 inch/s  (3)
 
     However, because the ink ejection timing is determined based on the sheet position pulses  108  from the rotary encoder  605 , the ejection frequency f will vary a bit if the sheet transport speed Vp fluctuates. 
     The analog drive signal generator  110  prepares an analog drive signal  406  that corresponds to each of the nozzle modules  401  and supplies the analog drive signal  406  to the piezoelectric element drivers  402  in synchronization with the sheet-position synchronization signal  109 . The digital ejection signal generator  111  sends a shift clock S-CLK shown in  FIG. 4  to the ejection data memory  105  and the piezoelectric element drivers  402  in synchronization with the sheet-position synchronization signal  109 . The digital ejection signal generator  111  retrieves and amplifies the ejection data  104  from the ejection data memory  105 . The digital ejection signal generator  111  uses the ejection data  104  to prepare recording ejection data  407  and supplies the recording ejection data  407  to the piezoelectric element drivers  402 . 
     Next, the nozzle modules  402  of the recording head  501  will be described while referring to FIG.  6 .  FIG. 6  is a cross-sectional view showing an example nozzle module  401 . Each nozzle module  401  has substantially the same configuration, so the following explanation will be provided using the nozzle module  401  shown in  FIG. 6  as an example. The nozzle module  401  includes an orifice plate  312 , a pressure chamber plate  311 , a restrictor plate  310 , and a piezoelectric element fixing plate  306 . The orifice plate  312  is formed with 128 nozzles  300 , only one of which is shown in  FIG. 6. A  common ink channel  308  is formed in the nozzle nodule  401 . The common ink channel  308  is for supplying ink to all of the nozzles  300 . Each of the nozzles  300  includes a nozzle orifice  301  formed in the orifice plate  312 , a pressure chamber  302  formed in the pressure chamber plate  311 , and a restrictor  307  formed in the restrictor plate  310 . The restrictor  307  connects the common ink channel  308  with the pressure chamber  302  and controls the amount of ink that flows from the common ink channel  308  to the pressure chamber  302 . 
     The nozzles  300  each include a diaphragm  303 , a piezoelectric element  304 , and a support plate  313 . The diaphragm  303  and the piezoelectric element  304  are connected together by a resilient material  309  such as silicon adhesive. Each piezoelectric element  304  has a pair of signal input terminals  305 . The piezoelectric elements  304  are configured to contract when a voltage is applied between the corresponding signal input terminals  305  and remain unchanged in shape when no voltage is applied. The support plate  313  is for reinforcing the diaphragm  303 . 
     The diaphragm  303 , the restrictor plate  310 , the pressure chamber plate  311 , and the support plate  313  are made from stainless steel, for example. The orifice plate  312  is made from nickel, for example. The piezoelectric element fixing plate  306  is made from an insulating material, such as ceramic or polyimide. 
     Ink supplied from an ink tank (not shown) is distributed through the common ink channel  308  to the restrictors  307  and through the restrictors  307  to the pressure chambers  302  and the nozzle orifices  301 . When voltage is applied between the signal input terminals  305 , the corresponding piezoelectric element  304  deforms so that a portion of the ink in the pressure chamber  302  is ejected from the corresponding nozzle orifice  301 . It should be noted that the inkjet recording device  1  according to the embodiment uses ink with electrically conductive properties. 
     As shown in  FIG. 4 , the 128 nozzles  300  are juxtaposed in a line. Adjacent nozzles  300  are separated from each other by the same distance. The pitch of the nozzles  300  centered on the nozzle orifices  301  is 75 nozzles/inch (npi). The pitch of the nozzles  300  is also referred to as the nozzle density. As shown in  FIG. 7 , the nozzle modules  401  are juxtaposed in groups of four in the direction of sheet transport Y. By distributing the nozzle modules  401  in this way, even though each nozzle module  401  has a low nozzle pitch of 75 dpi in the width direction X of the recording sheet, the nozzle pitch of the recording head  501  overall is 300 dpi so that the recording head  501  can record images having a resolution of 300 dpi. 
     Next, the piezoelectric element drivers  402  will be explained. Each of the piezoelectric element drivers  402  is a well-known piezoelectric element driver and as shown in  FIG. 4  includes 128 analog switches  403 , a 128-bit latch  404 , and a 128-bit shift resistor  405 . The shift resistor  405  is input with the shift clock S-CLK from the sheet control unit  106  and recording ejection data  407  from the digital ejection signal generator  111 . The recording ejection data  407  is 128 bit serial data that corresponds to the 128 nozzles  300 . Each logical value of 1 in the recording ejection data  407  indicates that an ejection is to be performed and each logical value of 0 indicates that no ejection is to be performed. The latch  404  is input with 128-bit parallel data from the shift resistor  405  and the latch clock L-CLK from the sheet control unit  106 . 
     Each analog switch  403  includes a switch terminal  403   a , an input terminal  403   b , and an output terminal  403   c . Each switch terminal  403   a  is input with corresponding output from the latch  404  and each input terminal  403   b  is input with the analog drive signal  406 . The analog switch  403  outputs the analog drive signal  406  being applied to the input terminal  403   b  to the output terminal  403   c  when the switch terminal  403   a  is applied with a logical value of 1. On the other hand, the analog switch  403  opens the output terminal  403   c  when the switch terminal  403   a  is applied with a logical value of 0 so the analog drive signal  406  is not output to the output terminal  403   c . The output terminal  403   c  of the analog switch  403  is connected to one of the signal input terminals  305  of the corresponding nozzles  300 . The other signal input terminal  305  is connected to ground. That is, the analog drive signal  406  is a signal used commonly for all of the 125 nozzles  300  of the same nozzle module  401  and is for driving the 128 piezoelectric elements  304 . A variety of drive waveforms can be used as the analog drive signal  406 . According to the present embodiment, the trapezoidal waveform shown In  FIG. 5  is used. The trapezoidal waveform is produced by application of 24V for a duration (time width) Tw of about 80 microseconds. 
     Next, basic operations of the piezoelectric element drivers  402  will be described with reference to the timing chart of FIG.  5 . The latch clock L-CLK is generated when the sheet-position synchronization signal  109  is generated. When the latch clock L-CLK is input to the piezoelectric element drivers  402 , all the recording ejection data  407  that was stored in the shift resistor  405  during the preceding cycle is stored in the latch  404  and outputted to the switch terminal  403   a  of the analog switch  403 . At the same time, the analog drive signal  406  is input to the input terminal  403   b  of the analog switch  403  simultaneously with output of the recording ejection data  407  to the switch terminal  403   a . At this time, an ink droplet is ejected from nozzles  300  where the recording ejection data  407  is a logical value of 1. No ejection is performed where the recording ejection data  407  is a logical value of 0. Next, the recording ejection data  407  is serially stored in the shift resistor  405  in synchronization with the shift clock S-CLK. Once a full complement of 128 bits is stored in the shift resistor  405 , then generation of the next sheet-position synchronization signal  109  is awaited. That is, the content of the recording ejection data  407  represents which nozzles  300  will be fired during the next cycle. 
     In order to record at high speeds, normally the ink ejection frequency is raised and recording is performed at a high frequency. However, the latch clock L-CLK must have an interval between successive pulses that is long enough for the time width Tw of the analog drive signal  406 . According to the present embodiment, the time width Tw of the analog drive signal  406  is about 80 microseconds so it is impossible to drive the recording head  501  faster than 10 kHz. Therefore, the maximum ejection frequency fm is 10 kHz. 
     The inkjet recording device  1  further includes an electric field developing unit and an ink collection unit. The electric field developing unit develops an electric field for charging ink droplets and deviating the trajectory of the charged ink droplets. The same electric field developing unit is used for all of the nozzles  300  and includes, as shown in  FIG. 2 , a common electric field developing unit  112 , a common-electric-field high-voltage source  114 , and a sheet back electrode  805 . The common electric field developing unit  112  supplies a common electric field signal  113  to the common-electric-field high-voltage source  114  in synchronization with the sheet-position synchronization signal  109 . The common-electric-field high-voltage source  114  sets voltage developed at the sheet back electrode  805  in accordance with voltage of the input common electric field signal  113 . Normally, the common electric field signal  113  is not supplied to the common-electric-field high-voltage source  114 , so the common-electric-field high-voltage source  114  maintains the electric potential of the sheet back electrode  805  at 0V. 
     The ink collection unit collects ink droplets that return to the recording head  501  and, as shown in  FIGS. 8 and 9 , includes an ink collection electrode  801 , a metal mesh  802 , and plastic tubes  803 . As shown in  FIG. 8 , the ink collection electrode  801  is a single plate-shaped electrode and is attached to a nozzle surface  301 A of the orifice plate  312  in parallel with the nozzle row. The ink collection electrode  801  is separated from the nozzle orifices  301  of the nozzle rows by a distance D 1  of about 0.3 mm. The ink collection electrode  801  has the same positional relationship with all 128 of the nozzles  300 . The metal mesh  802  is adhered to a surface  801 A of the ink collection electrode  801 . The ends  802 A of the metal mesh  802  protrude from the ink collection electrode  801 . The plastic tubes  803  are attached to the ends  802 A of the metal mesh  802  that protrude to the outside of the ink collection electrode  801 . Although not shown in the drawings, a suction pump is attached to the plastic tubes  803 . The ink collection electrode  801  and the orifice plate  312  are electrically grounded. 
     As shown in  FIG. 9 , the sheet back electrode  805  is provided to the rear of the continuous recording sheet  602 . The sheet back electrode  805  is electrically insulated. The sheet back electrode  805  is a single plate-shaped electrode that extends in the direction in which the nozzle row extends. The sheet back electrode  805  has the same positional relationship with all of the 128 nozzles  300 . According to the present embodiment, the nozzle surface  301 A (nozzle orifices  301 ) and the continuous recording sheet  602  are separated by a distance D 2  of 1.5 mm and the ink collection electrode  801  is formed with a thickness D 3  of 0.4 mm. 
     As shown in  FIG. 2 , the inkjet recording device  1  further includes a refresh signal generator  120 . The refresh signal generator  120  judges whether or not a refresh operation is required. When a refresh operation is required, then the refresh signal generator  120  outputs a refresh signal  121  that switches the inkjet recording device  1  from ejection mode to a refresh ejection mode. The refresh signal generator  120  also stores refresh ejection data  901  to be described later. 
     The inkjet recording device  1  can be switched to the refresh ejection mode at any optional timing that need not be synchronized with the print signal. The refresh signal generator  120  refers to the following conditions when judging whether to switch the inkjet recording device  1  to the refresh ejection mode: 
     1) Elapse of a fixed period: a refresh operation is performed at a fixed time interval of about 10-20 Hz in the conventional manner. 
     2) Recording history: the fewer ejections shown in the past record for the nozzles  300 , the more the refresh signal generator  120  shortens the cycle at which the inkjet recording device  1  is switched to the refresh ejection mode. 
     3) Environmental conditions: the refresh signal generator  120  shortens the cycle at which the inkjet recording device  1  is switched to the refresh ejection mode under cool (low temperature) and dry (low humidity) conditions because the ink in the nozzles  300  will become viscous at low temperature and will dry more quickly at low humidity. 
     4) Passage of time: the older the nozzles  300  are, the more the refresh signal generator  120  shortens the cycle at which the inkjet recording device  1  is switched to the refresh ejection mode. 
     5) Ink conditions: the refresh signal generator  120  shortens the cycle at which the inkjet recording device  1  is switched to the refresh ejection mode when the type of ink used in the nozzles  300  is an easily drying type. 
     When the refresh signal generator  120  judges that a refresh operation is required, the refresh signal generator  120  prepares the refresh signal  121  and outputs the refresh signal  121  to the sheet control unit  106 , the analog drive signal generator  110 , the digital ejection signal generator  111 , and the common electric field developing unit  112 . Upon receiving refresh signal  121 , the sheet control unit  106 , the analog drive signal generator  110 , the digital ejection signal generator  111 , and the common electric field developing unit  112  perform operations as indicated in FIG.  10 . 
     That is, the sheet control unit  106  temporarily changes the frequency of the sheet-position synchronization signal  109 . More particularly, during the normal ejection mode, the sheet-position synchronization signal  109  is generated once each time five sheet position pulses  108  are generated. However, during the refresh ejection mode, the sheet-position synchronization signal  109  is generated once each time four sheet position pulses  108  are generated. According to the present embodiment, the refresh ejection mode continues during a time period required to transport the continuous recording sheet  602  by four dots&#39; distance at a resolution of 300 dpi. Said differently, during the normal ejection mode, the sheet-position synchronization signal  109  is generated once each time the continuous recording sheet  602  is transported one dot&#39;s distance at a resolution of 300 dpi. Therefore, the sheet-position synchronization signal  109  is generated four times during the time required to transport the continuous recording sheet  602  four dots&#39; distance at a resolution of 300 dpi. In contrast to this, during the refresh ejection mode, the sheet-position synchronization signal  109  is generated five times during the time required for the continuous recording sheet  602  by a distance equivalent to four dots at a resolution or 300 dpi. That is, the sheet-position synchronization signal  109  is generated once each time the continuous recording sheet  602  is transported by a distance equivalent to one dot at a resolution of 375 dpi. 
     These operations will be described in more detail with reference to the timing chart of FIG.  10 . When the refresh signal  121  is generated, then at the next sheet-position synchronization signal  109  the inkjet recording device  1  switches from the normal ejection mode to the refresh ejection mode. As a result, the interval of the sheet-position synchronization signal  109  is reduced, that is, the sheet-position synchronization signal  109  is generated every {fraction (1/375)} inch that the continuous recording sheet  602  is transported instead of only every {fraction (1/300)} inch. During the refresh ejection mode, the sheet-position synchronization signal  109  is generated five times at the {fraction (1/375)}-inch interval as indicated by  109 - 1 ,  109 - 2 ,  109 - 3 ,  109 - 4 ,  109 - 5  in FIG.  10 . The inkjet recording device  1  reverts to the normal ejection mode after the sheet-position synchronization signal  109  is generated for the five times  109 - 1 ,  109 - 2 ,  109 - 3 ,  109 - 4 ,  109 - 5 . Once the inkjet recording device  1  switches back to the normal ejection mode, the interval of the sheet-position synchronization signal  109  returns to {fraction (1/300)} inch. 
     On the other hand, the digital ejection signal is generator  111  retrieves the refresh ejection data  901  from the refresh signal generator  120  in synchronization with the sheet-position synchronization signal  109 - 1  and sends the refresh ejection data  901  to the piezoelectric element drivers  402 . Next, the digital ejection signal generator  111  sends the recording ejection data  407  retrieved from the ejection data memory  105  and sends the recording ejection data  407  to the piezoelectric element drivers  402  in synchronization with the sheet position synchronization signals  109 - 2  to  109 - 5 . Next, the inkjet recording device  1  is reverted back to the normal ejection mode, wherein only recording ejection data  407  retrieved from the ejection data memory  105  is sent to the piezoelectric element drivers  402  in synchronization with the 300 dpi sheet-position synchronization signal  109 . 
     The analog drive signal generator  110  prepares and outputs the analog drive signal  406  in synchronization with the sheet-position synchronization signal  109 - 1 . Then, the analog drive signal generator  110  temporarily changes the waveform of the analog drive signal  406  to produce a refresh drive signal  904  and outputs the refresh drive signal  904  in synchronization with the sheet-position synchronization signal  109 - 2 . Afterward, the analog drive signal generator  110  prepares and outputs the analog drive signal  406  in synchronization with the sheet position synchronization signals  109 - 3  to  109 - 5 . Afterward, the inkjet recording device  1  reverts to the normal ejection mode. According to the present embodiment, the analog drive signal generator  110  produces the refresh drive signal  904  by reducing the voltage value of the analog drive signal  406  compared to the voltage used for ejecting a normal ink droplet. 
     The common electric field developing unit  112  maintains the common electric field signal  113  at 0V during the normal ejection mode. However, as shown in  FIG. 10  the common electric field developing unit  112  controls the common electric field signal  113  to a negative voltage for a short time period centered on a timing T 1 . The timing T 1  is after a duration of time ts 1  of 50 to 80 microseconds elapses after the rising edge of the refresh drive signal  904 , which was generated based on the refresh signal  121 . The common electric field signal  113  is maintained at the negative voltage for a period of about 10 microseconds that centers on the timing T 1 . Then, the common electric field signal  113  is switched to a positive voltage of fixed value until a timing T 2 , which is a duration of time ts 2  after timing T 1 . Starting after the timing T 2 , the voltage value of the common electric field signal  113  is gradually decreased until it reaches a voltage value of 0V at timing T 3 . The timing T 3  is a duration of time ts 3  after the rising edge of the analog drive signal  406  that is synchronized with the sheet-position synchronization signal  109 - 5 . As a result, the voltage at the sheet back electrode  805  is maintained at a negative voltage Vcm of −15 kV for the first 10 microseconds after the common electric field signal  113  is switched to a negative voltage, is then maintained at a positive voltage Vcp of 1.5 kV until the timing T 2 , and then gradually reduced to a voltage value of 0V at timing T 3 . It should be noted that the negative voltage Vcm is not limited to −1.5 kV, but could be any value from −1.0 kV to −1.5 kV. Similarly, the positive voltage Vcp is not limited to 1.5 kV, but could be any value from 1.0 kv to 1.5 kV. 
     As described above, the orifice plate  312  and the ink collection electrode  801  are electrically grounded. Therefore, when a voltage is applied at the sheet back electrode  805 , an electric field that corresponds to the applied voltage develops between the sheet back electrode  805  and the orifice plate  312 /ink collection electrode  801 , 
     Next, the trajectory of the refresh droplet ejected during the refresh ejection mode will be described with reference to FIG.  9 . The refresh drive signal  904  that is generated in synchronization with the sheet-position synchronization signal  109 - 2  is applied to the piezoelectric element  304  through the piezoelectric element drivers  402 . As a result, a refresh ink droplet  806  is ejected from the nozzle orifice  301 . Although not shown in the drawing, at first the refresh ink droplet  806  is still connected with the meniscus of ink in the nozzle orifice  301 . However, once the refresh ink droplet  806  extends to a certain length, it breaks away from the ink of the meniscus as shown in FIG.  9 . The refresh ink droplet  806  breaks away from the ink of the meniscus at the timing T 1 , that is, after the time ts 1  elapses from the rising edge of the refresh drive signal  904 . The ink droplet break away timing T 1  is known to be consistent (i.e., not to fluctuate much) regardless of the ink droplet speed and environmental conditions. 
     An electric field E 1  shown in  FIG. 9  is generated while the negative voltage Vcm of −1.5 kV is applied to the sheet back electrode  805  for the 10 microsecond period centered on the ink droplet break away time T 1 . The electric field E 1  instantly polarizes the charge in the refresh ink droplet  806 . The electric field E 1  faces downward for the most part, although it slants slightly to the left as viewed in  FIG. 9  under influence from the side surface of the ink collection electrode  801 . Therefore, positive charge will accumulate in the lower portion of the refresh ink droplet  806  while the refresh ink droplet  806  is still connected to the meniscus. The refresh ink droplet  806  will therefore have a positive charge after breaking away from the meniscus. Next, the sheet back electrode  805  is applied with the positive voltage Vcp of 1.5 kV to generate an electric field E 2 . The electric field E 2  faces substantially upward. Therefore, the speed of the positively charged refresh ink droplet  806  toward the continuous recording sheet  602  drops dramatically until the speed and direction of the refresh ink droplet  806  reverses and the refresh ink droplet  806  starts moving hack toward the recording head  501 . Because the electric field E 2  slants slightly to the right under influence from the side surface of the ink collection electrode  801 , the refresh ink droplet  806  does not return to the nozzle orifice  301 , but instead catches in the metal mesh  802  on the ink collection electrode  801 . The ink seeps toward the plastic tubes  803  under force of capillary action. The plastic tubes  803  suck up and discharge the ink. The position where the refresh ink droplet  806  changes direction and starts to return back to the orifice plate  312  can be approximated using the following formula:
 
λ= m×v   0   2 /(2 ×q×E )  (4)
 
     wherein 1 is the maximum distance in the vertical direction V from the nozzle orifice  301  to the point where the ink droplet U-turns; 
     m is the specific gravity of the ink droplet; 
     v 0  is the ejection speed of the ink droplet; 
     q is the charge amount of the ink droplet; and 
     E is the vertical direction V component of the electric field E 2 . 
     As can be understood from equation (4), the flight speed v 0  needs to be a small value in order to prevent the refresh ink droplet  806  from impinging on the continuous recording sheet  602 . According to the present embodiment, the ejection speed V 0  of recording ink droplets is 7 m/s to 8 m/s, but the ejection speed V 0  of the refresh ink droplet  806  is set to 4.0 m/s. The ejection speed V 0  is set slower for the refresh ink droplet  806  by reducing the voltage value of the analog drive signal  406  to a lower value for the refresh drive signal  904  than when ejecting recording ink droplets. By setting the ejection speed V 0  of the refresh ink droplet  806  to 4.0 m/s, the maximum distance 1=1.0 mm, which is shorter than the distance D 1  from the nozzle orifice  301  to the continuous recording sheet  602 . Therefore, the refresh ink droplet  806  U-turns before reaching the continuous recording sheet  602  and will not impinge on the continuous recording sheet  602 . The entire process from when the refresh ink droplet  806  being ejected, to when the refresh ink droplet  806  U-turns, and further to when the ink is collected by the metal mesh  802  takes about 100 microseconds to 1 millisecond. Therefore, the positive voltage Vcp needs to be maintained at the common electric field signal  113  during this period. The common electric field signal  113  is maintained at a fixed negative voltage during the period ts 2  for this reason. 
     Next, recording ink droplets are ejected one after the other when the analog drive signal  406  is generated in synchronization with the sheet position synchronization signals  109 - 3 ,  109 - 4 ,  109 - 5 . The ink droplets ejected as a result of the sheet position synchronization signals  109 - 3 ,  109 - 4 ,  109 - 5  will be referred to as recording ink droplets  806 - 3 ,  806 - 4 , and  806 - 5 , respectively. The recording ink droplets  806 - 3 ,  806 - 4 , and  806 - 5  will be explained with reference to FIG.  11 . 
     In the same manner as for the refresh ink droplet  806 , the recording ink droplet  806 - 3  breaks away from the meniscus after extending to a certain length. The separation occurs at timing T 2  indicated in FIG.  10 . Because the positive voltage Vcp is applied to the sheet back electrode  805  at the break away timing T 2 , the recording ink droplet  806 - 3  is charged to a negative charge by the electric field E 2 . The negatively charged recording ink droplet  806 - 3  is accelerated by the electric field E 2 . At this time, the recording ink droplet  806 - 3  is deflected to the left as shown in  FIG. 9  because the electric field E 2  slants slightly to the right. Therefore, the recording ink droplet  806 - 3  impinges on the continuous recording sheet  602  at a position that is to the left of a line C that is normal from the nozzle orifice  301 . In other words, the recording ink droplet  806 - 3  impinges on the continuous recording sheet  602  at a position that is shifted upstream with respect to the sheet transport direction Y. 
     The recording ink droplet  806 - 4  is charged, accelerated, and deflected in the same manner as the recording ink droplet  806 - 3  and also impinges on the continuous recording sheet  602  at a position b that is shifted to the left from the normal line C. However, starting from the timing T 2 , the positive voltage Vcp of the common electric field signal  113  gradually drops so that the acceleration and deflection amount of the recording ink droplet  806 - 4  is less than for the recording ink droplet  806 - 3 . Therefore, the impingement position b is shifted from the normal line C to a smaller extent than the impingement position a. The acceleration and deflection amount is even smaller for the recording ink droplet  806 - 5  so the recording ink droplet  806 - 5  impinges at a position c at the timing T 3 . It should be noted that there is no need for the positive voltage Vcp to decrease in a continuous manner. The positive voltage Vcp may be reduced in a stepwise manner each time a recording ink droplet is ejected. 
     Next, a series of operations performed by the inkjet recording device  1  during printing will be explained with reference to FIG.  11 .  FIG. 11  shows the condition of ink droplets ejected from a single nozzle orifice  301  during both the normal ejection mode and the refresh ejection mode.  FIG. 11  shows the recording head  501  relatively moving from left to right across the continuous recording sheet  602 . That is,  FIG. 11  shows the relative position of the single nozzle orifice  301  at different consecutive times. It should be noted that the speed component in the movement direction of the recording head  501  is not taken into consideration. 
     In the example shown in  FIG. 11 , at first printing is performed in the normal ejection mode. During this time, recording ink droplets are ejected at timings corresponding to sheet position synchronization signals  109  for the resolution of 300 dpi. At this time, the common electric field signal  113  is not generated so the recording ink droplets move straight downward toward the continuous recording sheet  602  without any deflection. Next, the inkjet recording device  1  switches to the refresh ejection mode when the refresh signal  121  is generated. After the refresh signal  121  is generated, the five sheet position synchronization signals  109 - 1  to  109 - 5  are generated at timings that correspond to 375 dpi. 
     The recording ink droplet  806 - 1  that is ejected at the timing of the sheet-position synchronization signal  109 - 1  is not charged so flies in a straight line toward the continuous recording sheet  602  and will not be deflected even if the electric field E 1  is developed directly after the recording ink droplet  806 - 1  is ejected. The recording ink droplet  806 - 2  that is ejected at the timing of the sheet-position synchronization signal  109 - 2  is charged to a positive charge by the electric field E 1 . Therefore, the recording ink droplet  806 - 2  U-turns under influence from the positive polarity deflection electric field E 2  and is caught on the ink collection electrode  801 . A period of about 100 microseconds to 1 millisecond elapses from when the recording ink droplet  806 - 2  is ejected until it is collected. The positive polarity deflection electric field E 2  is maintained during this entire period. The three recording ink droplets  806 - 3 ,  806 - 4 , and  806 - 5  are ejected while the recording ink droplet  506 - 2  is in flight, that is, while positive polarity deflection electric field E 2  is being maintained, so are deflected in the manner described above before impinging on the continuous recording sheet  602 . 
     The recording ink droplets  806 - 3 ,  806 - 4 , and  806 - 5  ejected at the timings of the sheet position synchronization signals  109 - 3  to  109 - 5  impinge on the continuous recording sheet  602  at positions a, b, and c, respectively. The impinging positions of the ink droplets are separated by a uniform distance whether ejected during the normal ejection mode or during the refresh ejection mode. Therefore, even though the ink refresh operation is performed during recording, recording can be performed at the same resolution of 300 dpi as when no ink refresh operation is performed. When ink ejection during the refresh ejection mode is completed, the inkjet recording device  1  automatically returns to the normal ejection mode. 
     According to the present embodiment, an ink refresh operation can be performed at any optional timing while recording is being performed at a frequency of 8 kHz, which is 80% of the maximum ejection frequency fm of 10 kHz. 
     As described above, according to the present invention, the refresh ejection period can be secured by temporarily changing the ejection frequency. Refresh operations can be performed using the resultant time-sharing refresh method with a loss in ejection speed of only a few percentages compared to the maximum ejection speed. Because the refresh ink droplets are deflected and collected, there is no need to provide a complicated mechanism for retracting the recording head or stopping recording operations each time a refresh operation is performed. 
     Because a recording ink droplet ejected during the refresh ejection mode impinges on a position that is shifted from an imaginary normal line that extends from the corresponding nozzle orifice, normal recording can be performed at a predetermined interval with no dots missing from the recorded image because of the refresh ejection. 
     Because ink droplets ejected in the refresh ejection mode impinge at positions that are shifted in accordance with the deflection amount by gradually smaller distances, normal recording can be performed at a predetermined interval with no dots missing from the recorded image because of the refresh ejection. 
     Because the ejection frequency is temporarily changed at an optional timing, the ink refresh operations need not be performed in synchronization with the recording signal. Instead, whether or not a refresh operation is to be performed can be judged based on a variety of conditions, such as elapse of a fixed period, recording history, environmental conditions, passage of time, or ink conditions. 
     While the invention has been described in detail with reference to the specific embodiments thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention. 
     For example, the embodiment describes generating five sheet position synchronization signals  109  at 375 dpi during the refresh ejection mode while the continuous recording sheet  602  is transported a distance equivalent to 4 dots at 300 dpi of the normal ejection mode. However, 10 sheet position synchronization signals  109  could be generated at 333 dpi while transporting the continuous recording sheet  602  a distance equivalent to 9 dots at 300 dpi. With this configuration, recording can be performed at 9 kHz, which is 90% of the maximum ejection frequency fm of 10 kHz. 
     The embodiment describes ejecting refresh droplets from all of the nozzles during the refresh ejection mode. However, refresh droplets need only be ejected from optional nozzles that require an ink refresh operation. That is, the need for an ink refresh operation differs for each nozzle depending on the conditions that recording ink droplets were ejected. If refresh droplets are ejected only from nozzles that require an ink refresh operation, then a great deal of ink can be saved, especially in the case of inkjet recording devices with a large number of nozzles. In this case, the refresh signal generator is controlled to generate refresh signals that eject ink droplets only from those nozzles that need an ink refresh operation.