Patent Publication Number: US-2010110127-A1

Title: Liquid Ejecting Apparatus And Control Method Of Liquid Ejecting Apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid ejecting apparatus including a liquid ejecting head such as an ink jet type recording head which ejects liquid from a nozzle opening, and a control method of the liquid ejecting apparatus. 
     2. Description of the Related Art 
     Liquid ejecting heads which discharge (eject) liquid as a liquid droplet from a nozzle opening by generating pressure change in the liquid in a pressure generating chamber include, for example, an ink jet type recording head (hereinafter simply referred to as “recording head”) used in an image recording apparatus such as an ink jet type recording apparatus (hereinafter simply referred to as “printer”), a color material ejecting head used to manufacture a color filter of a liquid crystal display or the like, an electrode material ejecting head used to form an electrode of an organic EL (Electro Luminescence) display, an FED (Field Emission Display), and the like, a bioorganic material ejecting head used to manufacture a biochip (biochemical element), and the like. 
     For example, in the recording head mentioned above, there is a risk that the recording head causes a trouble such as ink discharge failure by thickening and fixing of ink due to evaporation, pressure loss caused by a bubble mixed into ink which absorb the pressure change, and the like. 
     Various maintenance processings are performed to prevent such ink discharge failure. For example, a recording head is proposed in which thickened ink and a bubble mixed into ink are forcibly eliminated by driving a pressure generating element to provide pressure change in the pressure generating chamber and perform empty discharge of liquid droplets from the nozzle (hereinafter referred to as “flushing”) while the nozzle is capped and negative pressure is generated by a pump (for example, refer to Patent Document 1). 
     [Prior Art Document] 
     [Patent Document] 
     [Patent Document 1] JP-A-2007-136989 
     SUMMARY OF THE INVENTION 
     However, in the recording head described above, it is difficult to sufficiently eliminate the bubble because the pressure change is not sufficiently provided to a bubble having a minute diameter (for example, diameter of several tens of μm). On the other hand, depending on a waveform of a drive signal for driving the pressure generating element, a minute ink droplet called “satellite ink droplet” is generated following a main ink droplet, and the satellite ink droplet does not reach ink absorbing material and becomes a mist. There is a problem that the ink droplet which becomes the mist is scattered while flying in the air, and not only smears the inside of the printer, but also adheres to electronic components such as a circuit board to cause a failure such as a short circuit. 
     The present invention has been made in view of the above situation, and an object of the present invention is to provide a liquid ejecting apparatus which can eliminate a bubble mixed into liquid and discharge the liquid from a nozzle more stably, and a control method of the liquid ejecting apparatus. 
     MEANS FOR SOLVING THE PROBLEM 
     To achieve the object, the liquid ejecting apparatus of the present invention is a liquid ejecting apparatus for ejecting liquid, and characterized by comprising: 
     a pressure generating chamber in which the liquid is filled; 
     a pressure generating element for changing a volume in the pressure generating chamber; 
     a nozzle connecting to the pressure generating chamber; and 
     a control section for generating a drive signal for controlling the pressure generating element, 
     wherein 
     the control section can generate a maintenance drive pulse group for discharging a bubble in the liquid from the pressure generating chamber, 
     the maintenance drive pulse group includes a plurality of drive pulses including a first pulse element for changing the volume of the pressure generating chamber to a first state by driving the pressure generating element, a second pulse element for maintaining the first state for a predetermined time period, and a third pulse element for changing the volume of the pressure generating chamber from the first state to a second state having a volume different from the volume of the first state, and a first time period is set between a trailing edge of the third pulse element and a leading edge of the first pulse element of a next drive pulse, 
     a voltage value of the drive pulses is set to 30 V or more, 
     the first time period is set to a time period between 0.55 Tc and 0.75 Tc when a Helmholtz resonance period of the liquid filled in the pressure generating chamber is assumed to be Tc, 
     a second time period is set between a maintenance drive pulse group and another maintenance drive pulse group following the former maintenance drive pulse group, and 
     the second time period is set to 20 times the first time period or more. 
     According to the above configuration, since the time period between the trailing edge of the third pulse element and the leading edge of the first pulse element of the next drive pulse is set as the first time period, and the first time period is set between 0.55 Tc and 0.75 Tc, the next drive pulse is applied to the pressure generating element at a timing when a free surface (meniscus) of the liquid in the nozzle is displaced toward the opposite direction to the fluid discharge direction by residual vibration after the fluid is discharged from the pressure generating chamber, so that, when the bubble mixed into the liquid are eliminated, a discharged liquid by a former drive pulse and a discharged liquid by a latter drive pulse are discharged in a state in which the former discharged liquid and the latter discharged liquid are linked to each other. Therefore, it is possible to prevent the discharged liquid from becoming a mist, and disadvantage that the misty liquid adheres to devices around the nozzle to cause a failure of the recording head, or the like can be reduced. As a result, the liquid can be stably discharged from the nozzle opening of the recording head, and the failure due to the mist does not occur. Here, the first state indicates a state in which the volume of the pressure generating chamber is different from a volume when the voltage is not applied to the pressure generating element, and means a state in which the voltage value is changed from an initial voltage value to a certain direction (for example, electricity charging direction). The second state indicates a state having a different volume of the pressure generating chamber from the volume in the first state, and means a state in which the voltage value is changed to the opposite direction to the certain direction (for example, electricity discharging direction). 
     Since the second time period is set between a maintenance drive pulse group and another maintenance drive pulse group following the former maintenance drive pulse group, and the second time period is set to 20 times the first time period or more, even when the drive pulse in the maintenance drive pulse group is set to a voltage higher than that of a normal discharge drive pulse, residual vibration generated by the maintenance drive pulse group can be sufficiently attenuated before the pressure generating element is driven by the next maintenance drive pulse group. As a result, the pressure generating element can be driven by the maintenance drive pulse group in a state in which the residual vibration is sufficiently attenuated, so that the bubble mixed into the liquid can be efficiently eliminated. 
     Further, in addition to the above, the control section may change the second time period in accordance with the number of the drive pulses included in the maintenance drive pulse group. According to the above configuration, even when the number of repetition times of the drive pulse included in the maintenance pulse group is changed and the residual vibration increases or decreases, the residual vibration is sufficiently attenuated independently of the number of repetition times of the drive pulse. 
     The control method of the liquid ejecting apparatus of the present invention is proposed to achieve the object describe above, and is a control method characterized by controlling drive of a pressure generating element, 
     in a liquid ejecting apparatus including: 
     a pressure generating chamber in which the liquid is filled; 
     a pressure generating element for changing a volume in the pressure generating chamber; and; 
     a nozzle connecting to the pressure generating chamber, 
     by providing a maintenance drive pulse group having a plurality of drive pulses including a first pulse element for changing the volume of the pressure generating chamber to a first state by driving the pressure generating element, a second pulse element for maintaining the first state for a predetermined time period, a third pulse element for changing the volume of the pressure generating chamber from the first state to a second state, and a first time period set between a trailing edge of the third pulse element and a leading edge of the first pulse element of a next drive pulse, 
     wherein 
     a voltage value of the drive pulses is set to 30 V or more, 
     the first time period is set to a time period between 0.55 Tc and 0.75 Tc when a Helmholtz resonance period of the liquid filled in the pressure generating chamber is assumed to be Tc, 
     a second time period is set between a maintenance drive pulse group and another maintenance drive pulse group following the former maintenance drive pulse group, and 
     the second time period is set to 20 times the first time period or more. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a printer; 
         FIG. 2  is an exploded perspective view of a recording head; 
         FIG. 3  is a plan view and a cross-sectional view of the recording head; 
         FIG. 4  is a block diagram illustrating an electrical configuration of the printer; 
         FIG. 5  is a waveform diagram illustrating a maintenance drive pulse group; 
         FIG. 6  is a schematic view illustrating behavior of a bubble in a pressure generating chamber; 
         FIG. 7  is an illustration explaining a relationship between a Helmholtz resonance period and a drive pulse; 
         FIG. 8  is a view showing an experimental result of discharge condition with respect to change of the drive pulse; 
         FIG. 9  is a view showing a relationship between a time width between the drive pulses and an applied voltage; and 
         FIG. 10  is a view showing a relationship between the time width between the drive pulses and the number of repetition times. 
     
    
    
       1  . . . printer,  12  . . . nozzle opening,  19  . . . piezoelectric element,  21  . . . pressure generating chamber,  46  . . . control section, DP 1 , DP 2  . . . drive pulse, FL . . . maintenance drive pulse group, P 11 , P 21  . . . first pulse element, P 12 , P 22  . . . second pulse element, P 13 , P 23  . . . third pulse element, Pdis 1 , Pdis 2  . . . time width 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. Although, in the embodiments described below, there are various limitations as preferred embodiments of the present invention, the scope of the present invention is not limited to these embodiments unless there is a specific description that limits the present invention in the description below. Hereinafter, a case in which the liquid ejecting apparatus of the present invention is applied to an ink jet type recording apparatus (hereinafter abbreviate as printer) shown in  FIG. 1  will be illustrated. 
     A printer  1  is schematically configured to include a recording head  2  which is a kind of a liquid ejecting head, the recording head being attached to the printer  1 , a carriage  4  to which an ink cartridge  3  is attachably and detachably attached, a platen  5  arranged under the recording head  2 , a carriage movement mechanism  7  for moving the carriage  4  on which the recording head  2  is mounted in a paper width direction of recording paper  6  (a kind of a discharge target or an ejection target), a paper transport mechanism  8  for transporting the recording paper  6  in a paper transport direction perpendicular to a head moving direction, and the like. Here, the paper width direction is a main scanning direction, and the paper transport direction is a sub-scanning direction. The ink cartridge  3  may be a type which is mounted on the carriage  4 , or may be a type which is mounted on a housing of the printer  1  and from which an ink is supplied to the recording head  2  via an ink supply tube. 
     The carriage  4  is mounted on a guide rod  9  while being axially supported by the guide rod  9  installed in the main scanning direction, and configured to move in the main scanning direction along the guide rod  9  by the action of the carriage move mechanism  7 . The position of the carriage  4  in the main scanning direction is detected by a linear encoder  10 , and a detection signal is transmitted to a printer controller  40  (refer to  FIG. 4 ) as position information. In this way, the printer controller  40  can control a recording operation (ink ejection operation) or the like of the recording head  2  while recognizing the scanning position of the carriage  4  on the basis of the position information from the linear encoder  10 . 
     A home position which is a scanning start position of the recording head  2  is set in a moving range of the recording head  2  and outside of the platen  5 . A capping mechanism  11  is provided at the home position. The capping mechanism  11  seals a nozzle surface of the recording head  2  by a cap member  11 ′, and prevents ink solvent from evaporating from a nozzle opening  12  (corresponding to the nozzle of the present invention, refer to  FIG. 2 ). The capping mechanism  11  is used for a cleaning operation for eliminating a bubble mixed into the ink and a flushing operation described below for eliminating a thickened ink by providing negative pressure to the sealed nozzle surface to forcibly suction and remove the ink from the nozzle opening  12 . 
       FIG. 2  is an exploded perspective view showing a configuration of the recording head  2 ,  FIG. 3(   a ) is a plan view of the recording head  2 , and  FIG. 3(   b ) is a cross-sectional view taken along the line A-A′ in (a). The recording head  2  of the embodiment is configured by laminating a flow path forming substrate  15 , a nozzle plate  16 , an elastic body film  17 , an insulating body film  18 , a piezoelectric element  19  (corresponding to the pressure generating element of the present invention), a protective substrate  20 , and the like. 
     The flow path forming substrate  15  is constituted by a silicon single crystal substrate in the embodiment, and a plurality of pressure generating chambers are aligned in a width direction of the flow path forming substrate  15 . A connection section  22  is formed in an area outside in the longitudinal direction of the pressure generating chamber  21  of the flow path forming substrate  15 , and the connection section  22  and each pressure generating chamber  21  are connected via an ink supply path  23  provided for each pressure generating chamber  21 . The connection section  22  is connected to a reserver section  30  of the protective substrate  20  described below, and constitutes a part of the reserver  31  which is a shared ink chamber for each pressure generating chamber  21 . The ink supply path  23  is formed to have a width narrower than that of the pressure generating chamber  21 , and maintains a flow path resistance of the ink flowing into the pressure generating chamber  21  from the connection section  22  at a constant level. 
     A nozzle plate  16  in which nozzle openings  12  connected to an end portion in the opposite side of the ink flow path  23  of each pressure generating chamber  21  are provided in an open manner is fixed to an opening surface of the flow path forming substrate  15  via an adhesive material, a heat adhesive film, or the like, and each nozzle openings  12  connects to one of the pressure generating chambers  21  in an end portion in the opposite side of the ink supply path  23 . The nozzle plate  16  includes a plurality of lines of the nozzle openings  12 , and one nozzle line is constituted by, for example, 360 nozzle openings  12 . The recording head  2  of the present invention is configured to be able to accommodate 4 ink cartridges each of which stores an ink (a kind of the liquid of the present invention) having a color different from one another, specifically stores inks of a total of 4 colors of cyan (C), magenta (M), yellow (Y), and black (K), and a total of 4 nozzle lines are formed in the nozzle plate  15  in accordance with these colors. 
     On the other hand, on the surface opposite to the opening surface of the flow path forming substrate  15 , the elastic film  17  constituted by silicon dioxide (SiO 2 ) having a thickness of, for example, about 1.0 μm is formed, and the insulating body film  18  constituted by zirconium oxide (ZrO 2 ) having a thickness of, for example, about 0.4 μm is formed on the elastic film  17 . On the insulating body film  18 , a lower electrode film  25  having a thickness of, for example, about 0.2 μm, an piezoelectric body layer  26  (piezoelectric body film) having a thickness of, for example, about 1.0 μm, and an upper electrode film  27  having a thickness of, for example, about 0.05 μm are formed, so that, as a total, the piezoelectric element  19  (thin film piezoelectric element) having a thickness of 1.25 μm is constituted. Therefore, the piezoelectric element  19  is constituted by including the lower electrode film  25 , the piezoelectric body layer  26 , and the upper electrode film  27 , using either one of the upper electrode and the lower electrode as a shared electrode, and performing patterning on the other electrode and the piezoelectric body layer  26  for each pressure generating chamber  21 . A portion which is constituted by one of the electrodes and the piezoelectric body layer  26  on which the patterning is performed, and in which piezoelectric strain is generated by applying voltages to both electrodes is called a piezoelectric body active portion. Although, in the embodiment, the lower electrode film  25  is used as the shared electrode of the piezoelectric element  19 , and the upper electrode film is used as individual electrodes of the piezoelectric element  19 , for convenience of drive circuit and wiring, an opposite configuration of the above can be employed. In both cases, the piezoelectric body active portion is formed for each pressure generating chamber  21 . A lead electrode  28  made of, for example, gold (Au) is connected to the upper electrode film  27  of each piezoelectric element  19  respectively, and a voltage is selectively applied to each piezoelectric element  19  via the lead electrode  28 . 
     The protective film  20  including a piezoelectric element holding section  29  having a space, the size of which is enough so that the piezoelectric element holding section  29  does not block the displacement of the piezoelectric element  19  is connected to an area facing the piezoelectric element  19  on the surface of the side of the piezoelectric element  19  on the flow path forming substrate  15 . Since the piezoelectric element  19  is accommodated in the piezoelectric element holding section  29 , the piezoelectric element  19  is protected in a state in which the piezoelectric element  19  is hardly affected by an external environment. In addition, in the protective substrate  20 , the reserver section  30  is provided in an area corresponding to the connection section  22  in the flow path forming substrate  15 . The reserver section  30  is arranged along the alignment direction of the pressure generating chambers  21  while penetrating the protective film  20  in the thickness direction, and constitutes a reserver  31  to be a shared ink chamber for each pressure generating chamber  21  by being connected to the connection section  22  of the flow path forming substrate  15  as described above. 
     In an area between the piezoelectric element holding section  29  in the protective film  20  and the reserver section  30 , a through-hole  32  penetrating the protective film  20  in the thickness direction is provided, a part of the lower electrode film  25  and a top portion of the lead electrode  28  are exposed in the through-hole  32 , and an end portion of a wiring line from a drive IC not shown in the figures is electrically connected to the lower electrode film  25  and the lead electrode  28 . A compliance substrate  35  constituted by a sealing film  33  and a fixing board  34  is bonded on the protective substrate  20 . The sealing film  33  is made of a material having a low rigidity and a flexibility (for example, polyphenylene sulfide film having a thickness of 6 μm), and a surface of the reserver section  30  is sealed by the sealing film  33 . The fixing board  34  is formed by a hard material such as a metal (for example, stainless steel having a thickness of 30 μm). Since an area facing the reserver  31  in the fixing board  34  is an opening portion  36  in which the fixing board  34  is completely removed in the thickness direction, a surface of the reserver is sealed by only the sealing film  33  having a flexibility. 
     In the recording head  1  of the above configuration, ink is taken from an ink supply means such as the ink cartridge, the inside from the reserver  31  to the nozzle opening  12  is filled with the ink, and thereafter, by providing a drive signal from the printer controller  40  of the printer main body, a voltage is applied between the lower electrode film  25  and the upper electrode film  27  corresponding to each pressure generating chamber  21  to bend the elastic film  17 , the insulating body film  18 , the lower electrode film  25 , and the piezoelectric body layer  26 , so that the pressure in the pressure generating chamber  21  is increased, and by controlling the pressure change, an ink droplet is ejected (discharged) from the nozzle opening  12  provided in an open manner in the nozzle plate  16 . 
       FIG. 4  is a block diagram illustrating an electrical configuration of the printer  1 . The printer  1  of the embodiment is schematically constituted by the printer controller  40  and a print engine  41 . The printer controller  40  includes an external interface (external I/F)  42  from which print data or the like from an external apparatus such as a host computer is inputted, a RAM  43  for storing various data or the like, a ROM  44  storing a control program or the like for various controls, a non-volatile memory element  45  constituted by an EEPROM, a flash ROM, and the like, a control section  46  for performing an overall control of each section in accordance with the control program stored in the ROM  44 , an oscillation circuit  47  for generating a clock signal, a drive signal generation circuit  48  for generating a drive signal provided to the recording head  2 , and an internal interface (internal I/F)  49  for outputting dot pattern data obtained by developing the print data into each dot, the drive signal, and the like to the recording head  2 . 
     The print engine  41  is constituted by the recording head  2 , the carriage movement mechanism  7 , the paper transport mechanism  8 , and the linear encoder  10 . The recording head  2  includes a shift register  50  in which the dot pattern data is set, a latch circuit  50  for latching the dot pattern data set in the shift register  50 , a decoder  52  for translating the dot pattern data from the latch circuit and generating pulse selection data, a level shifter  53  functioning as a voltage amplifier, a switch circuit  54  for controlling supply of the drive signal to the piezoelectric element  19 , and the piezoelectric element  19 . 
       FIG. 5  is a waveform diagram illustrating a maintenance drive pulse group FL which is one of the drive signals generated by the drive signal generation circuit  48  in the above configuration. The control section  46  described above can generate a drive signal COM for controlling the piezoelectric element  19 . The maintenance drive pulse group (flushing waveform group) FL illustrated in  FIG. 5  is a drive pulse for discharging fluid including a bubble  57  (refer to  FIG. 6 ) mixed into an ink  56  in the pressure generating chamber  21  when the flushing operation for discharging the thickened ink  56  (refer to  FIG. 6 ) from the nozzle opening  12  is performed. The maintenance drive pulse group includes sets of a drive pulse DP 1  and a drive pulse DP 2 , and the repetition of the drive pulse DP 1  and the drive pulse DP 2  constitutes the maintenance drive pulse group. 
     The drive pulse DP 1  is a pulse signal having a generally trapezoidal shape as shown in  FIG. 5 , and constituted by a first pulse element P 11  which raises the voltage from a base voltage VB to a highest voltage VH at a constant gradient in a time width Pwc, a second pulse element P 12  which maintains the highest voltage VH that is a trailing edge voltage of the first pulse element P 11  in a certain time period (time width Pwh), and a third pulse element P 13  which lowers the voltage from the highest voltage VH at a constant gradient in a time width Pwd. 
     The drive pulse DP 2  is constituted by generally the same waveform elements as those of the drive pulse DP 1  described above, and generated later than the drive pulse DP 1 . The drive pulse DP 2  is constituted by a first pulse element P 21  which raises the voltage from the base voltage VB to the highest voltage VH at a constant gradient in a time width Pwc, a second pulse element P 22  which maintains the highest voltage VH that is a trailing edge voltage of the first pulse element P 21  in a certain time period in a time width Pwh, and a third pulse element P 23  which lowers the voltage from the highest voltage VH at a constant gradient in a time width Pwd. A voltage difference between the lowest voltage VL and the highest voltage VH is a drive voltage (applied voltage) Vd[V]. 
       FIG. 6  is a schematic view illustrating behavior of the bubble  57  in the pressure generating chamber  21 . When the drive pulse DP 1  or DP 2  is provided to the piezoelectric element  19 , the action below is performed. First, as shown in  FIG. 6(   a ), in a state in which the bubble  57  is mixed into the ink  56  in the pressure generating chamber  21 , when the first pulse element P 11  or P 21  is provided to the piezoelectric element  19 , the piezoelectric element  19  convexly bends to the opposite side of the pressure generating chamber  21 , and accordingly the pressure generating chamber  21  expands from a base volume corresponding to the base voltage VB to a maximum volume corresponding to the highest voltage VH (refer to  FIG. 6(   b )). Because of this first state, a negative pressure is generated in the ink  56  in the pressure generating chamber  21 , and the meniscus exposed to the nozzle opening  12  is pulled toward the pressure generating chamber  21 . The expanded state of the pressure generating chamber  21  is constantly maintained while the second pulse element P 12  or P 22  is provided. At this time, as the pressure in the pressure generating chamber  21  decreases, the diameter of the bubble  57  increases. 
     Following the second pulse element P 12  or P 22 , when the third pulse element P 13  or P 23  is provided to the piezoelectric element  19 , the piezoelectric element  19  is restored to a flat shape, so that the pressure generating chamber  21  contracts from the maximum volume to the base volume corresponding to the base voltage VB (refer to  FIG. 6(   c )). Because of this second state, the ink  56  in the pressure generating chamber  21  is given pressure from the elastic body film  17  and discharged from the nozzle opening  12 . At this time, the bubble  57  whose diameter has been increased gradually approaches the nozzle opening  12  by an ink flow generated by the discharge of the ink  56 , and finally discharged outside from the nozzle opening  12 . 
     By the way, depending on the waveform of the maintenance drive pulse group FL constituting the drive pulses DP 1  and DP 2 , by the volume change of the pressure generating chamber  21 , a minute ink droplet called “satellite ink droplet” may be generated from the nozzle opening  12  following a main ink droplet, and there is a possibility that the satellite ink droplet becomes a mist. Therefore, in the maintenance drive pulse group FL of the present invention, the time width Pdis between the drive pulse DP 1  and the drive pulse DP 2  is appropriately set in accordance with the Helmholtz resonance period Tc which is a natural vibration period of the ink  56  (fluid) including the bubble  57  in the pressure generating chamber  21 , so that the free surface (meniscus) of the ink  56  in the nozzle opening  12  is controlled. The Helmholtz resonance period Tc is a natural vibration period when a vibrational wave generated by the volume change of the pressure generating chamber  21  is transmitted to the ink  56  in the pressure generating chamber  21 , and is a value determined by the shapes or the like of the nozzle opening  12 , the pressure generating chamber  21 , the ink supply path  23 , and the like. 
     The Helmholtz resonance period (natural vibration period) Tc is a value determined by the shapes or the like of the nozzle opening  12  and the pressure generating chamber  21 , and a vibration period Tc of the ink in the pressure generating chamber  21  is represented by the following formula. 
         Tc= 2π√[(( Mn×Ms )/( Mn+Ms ))× Cc]   (1) 
     In the formula (1), Mn is an inertance in the nozzle opening  12 , Ms is an inertance in the ink supply path  23  connected to the pressure generating chamber  21 , Cc is a compliance (volume change per pressure change, shows a degree of flexibility) of the pressure generating chamber  21 . In the formula (1), the inertance M shows a mobility of the ink in the ink flow path, and is an ink mass per unit area of cross section. When the ink density is ρ, the cross-sectional area of the flow path perpendicular to the ink flow direction is S, and the length of the flow path is L, the inertance M can be approximately represented by the following formula (2). 
       Inertance  M =(density σ×length  L )/cross-sectional area  S   (2) 
     Tc is not limited to the above formula (1), and Tc may be a vibration period included in the pressure generating chamber  21 . 
       FIG. 7  is an illustration explaining a relationship between the Helmholtz resonance period Tc (upper part) and the drive pulses DP 1  and DP 2  (lower part).  FIG. 8  is a view showing an experimental result of discharge condition with respect to the change of the drive pulse. Here, the interval between the trailing edge of the third pulse element P 13  of the drive pulse DP 1  of the maintenance drive pulse group FL and the leading edge of the first pulse element P 21  of the drive pulse DP 2  is defined as a time width Pdis 1  (corresponding to the first time period of the present invention), and the trailing edge of the third pulse element P 23  of the drive pulse DP 2  and the leading edge of the first pulse element P 11  of the drive pulse DP 1  is defined as a time width Pdis 2  (corresponding to the second time period of the present invention). The Helmholtz resonance period Tc which is the natural vibration period of the ink  56  (fluid) including the bubble  57  in the pressure generating chamber  21  is defined as Tc (for example, 6.4 [μm]). In the coordinate axes in  FIG. 7 , the horizontal axis shows a time progress and the vertical axis shows the position of the free surface (meniscus) of the ink  56  in the nozzle opening  12  so that the direction facing the back side (inside) of the nozzle opening  12  is the positive direction. 
     Here, the coordinate origin is a time point when the liquid is discharged, which is a first peak of the free surface (meniscus) and corresponds to the proximity of the third pulse element P 13  of the drive pulse DP 1 . Although,  FIG. 7  shows that the free surface moves in the same amplitude range even when the time passes, the amplitude decreases as the time passes because the amplitude practically attenuates. 
       FIG. 8(   a ) is the experimental result when the time width Pdis 1  is changed in a state in which the time width Pdis 2 /Pdis 1  is fixed to 100, and it is preferred that the time width Pdis 1  is within a range between 0.55 Tc and 0.75 Tc as shown in the experimental result.  FIG. 8(   b ) is the experimental result when the time width Pdis 2  is changed in a state in which the time width Pdis 1 /Tc is fixed to 0.65, and it is preferred that the time width Pdis 2  is 20 times the time width Pdis 1  or more as shown in the experimental result. In  FIG. 8 , “◯” shows that approximately all the nozzle openings  12  have discharged successfully, “Δ” shows that the discharge of at least one nozzle opening  12  has failed at a probability of 30% or less, and “x” shows that the discharge of the nozzle opening  12  has failed at a probability of 50% or less. 
     Here, the fact that the time width Pdis 1  is set within the range between 0.55 Tc and 0.75 Tc means that the next drive pulse DP 2  is provided within a range depicted by dashed lines in  FIG. 7 , and the range corresponds to the time point when the free surface starts to move toward the back side. 
       FIG. 9  is a view showing a relationship between the time width Pdis 2 /Pdis 1  between the drive pulses DP and the applied voltage Vd[V] to the piezoelectric element  19 . As shown in  FIG. 9 , it is preferred that the applied voltage (voltage value) of the drive pulse DP to the piezoelectric element  19  is set to 30 V or more, in addition to that the time width Pdis 2  is set to 20 times the time width Pdis 1  or more. Therefore, as shown in  FIG. 9 , it is known that, by providing the maintenance drive pulse group FL in which the time width of the drive pulse DP and the applied voltage are set within the above mentioned ranges to the piezoelectric element  19 , the occurrence rate of the discharge failure can be reduced. In  FIG. 9 , “xx” shows that the discharge failure has occurred in 50% or more of all the nozzle openings  12 , “x” shows that the discharge failure has occurred in more than or equal to 30% and less than 50% of the nozzle openings, “Δ” shows that the discharge failure has occurred in less than 30% of the nozzle openings, and “◯” shows that all the nozzle openings  12  have discharged successfully. 
     In summary, it is preferred that time width Pdis 1  is within the range between 0.55 Tc and 0.75 Tc, and the time width Pdis 2  is 20 times the Pdis 1  or more. The time width Pdis 1  and the time width Pdis 2  may not be out of the above ranges. 
     When the applied voltage is low, the pressure change in the pressure generating chamber  21  decreases and the bubble discharge ability in the pressure generating chamber  21  decreases, so that the above described discharge failure of the nozzle opening  12  occurs. Therefore, as the applied voltage of the drive pulse DP to the piezoelectric element  19  increases, the pressure change in the pressure generating chamber  21  increases, so that the bubble discharge ability in the pressure generating chamber  21  can be improved. However, when the applied voltage increases, the residual vibration in the pressure generating chamber  21  also increases. Therefore, the time width Pdis 2  of the drive pulse DP is set to 20 times the time width Pdis 1  or more so that, in the maintenance drive pulse group FL of the present invention, after the residual vibration generated by applying the maintenance drive pulse group FL 1  (indicated by a reference symbol FL 1  in  FIG. 5 ) to the piezoelectric element  19  attenuates, the timing when the next maintenance drive pulse group FL 2  (indicated by a reference symbol FL 2  in  FIG. 5 ) is applied to the piezoelectric element  19  is set. In this way, the meniscus becomes a stable state, so that the discharge failure can be suppressed. 
     The time width Pdis 1  and the time width Pdis 2  are alternately set between the drive pulse DP 1  and DP 2 , so that, at a timing when a free surface (meniscus) of the ink  56  in the nozzle opening  12  is displaced toward the opposite direction to the fluid discharge direction by the residual vibration after the fluid is discharged from the pressure generating chamber  21  by the drive pulse DP 1 , if the drive pulse DP 2  is applied to the piezoelectric element  19  to contract the pressure generating chamber  21  again, the ink droplet discharged by the drive pulse DP 1  and the ink droplet discharged by the drive pulse DP 2  are discharged in a state in which the two ink droplets are linked to each other. This is considered that the ink droplets are attracted to each other by surface tension or the like. Because of this, so-called “ink trailing” in which an ink droplet (satellite ink droplet) generated following the discharged droplet flies is decreased, and the mist generation is suppressed. Therefore, it is possible to reduce failure of the recording head  2  or the like caused by the misty ink which adheres to devices around the nozzle opening  12 . As a result, the ink  56  can be stably discharged from the nozzle opening  12  of the recording head  2 , and the failure due to the mist does not occur. The time width Pdis 2  is properly set to a value of 20 times the time width Pdis 1  or more, so that the frequency of the drive signal COM can be changed. 
     Since the time width Pdis 2  is set between the maintenance drive pulse group FL 1  and the maintenance drive pulse group FL 2  following the maintenance drive pulse group FL 1 , and the time width Pdis 2  is set to 20 times the time width Pdis 1  or more, even when the drive pulse DP in the maintenance drive pulse group FL is set to a voltage higher than that of a normal discharge drive pulse, the residual vibration generated by the maintenance drive pulse group FL 1  can be sufficiently attenuated before the piezoelectric element  19  is driven by the next maintenance drive pulse group FL 2 . As a result, the piezoelectric element  19  can be driven by the maintenance drive pulse group FL 2  in a state in which the residual vibration is sufficiently attenuated, so that the bubble mixed into the ink can be efficiently eliminated. 
     By the way, the present invention is not limited to the above embodiment, and various modifications are possible on the basis of the description of claims. 
     Although, in the above embodiment, an example in which the maintenance drive pulse group FL is configured to include sets of one drive pulse DP 1  and one drive pulse DP 2  is shown, the present invention is not limited to this, and the maintenance drive pulse group FL may be configured to include sets of two or more drive pulses DP. 
       FIG. 10  is a view showing a relationship between the time widths Pdis 2 /Pdis 1  between the drive pulses DP 1  and DP 2  and the number of repetition times of the drive pulse DP included in the maintenance drive pulse group FL. 
     Further, in the present invention, the time width Pdis 2  may be changed in accordance with the number of the drive pulses DP included in the maintenance drive pulse group FL. For example, although, when the number of the drive pulses DP included in the maintenance drive pulse group FL is increased from 2 to 3, the residual vibration increases because the number of applied pressure changes increases, the residual vibration can be sufficiently attenuated by prolonging the time width Pdis 2 . In this way, by setting the time width Pdis 2  in accordance with the number of the drive pulses DP included in the maintenance drive pulse group FL, the residual vibration can be sufficiently attenuated regardless of the number of the drive pulses DP. 
     Although, in the above embodiment, an example is shown in which the thin film piezoelectric element constituted by the lower electrode film  25 , the piezoelectric body layer  26  (piezoelectric body film), and the upper electrode film  27  is used as the pressure generating element, the piezoelectric element of the present invention is not limited to this, and for example, a so-called flexural vibration mode piezoelectric element individually provided for each pressure generating chamber  21 , a longitudinal vibration type piezoelectric element, or the like can be employed. The pressure generating element may be a magnetostrictor or the like, and also may be a heater element when using an ink generating a bubble. 
     Further, material and structure of each member are not limited to the above embodiment, and various configurations can be employed. Even when a different structure is employed, the maintenance drive pulse group FL may be determined on the basis of Tc of the structure. 
     Although, in the above embodiment, an ink jet type recording head mounted on an ink jet printer is illustrated, of course, the present invention can be applied to an apparatus which ejects liquid other than ink. Other liquid ejecting apparatuses include, for example, various recording heads used in an image recording apparatus such as a printer, a color material ejecting head used in a manufacturing apparatus of a color filter of a liquid crystal display or the like, an electrode material ejecting head used in an electrode forming apparatus for an organic EL display, an FED (Field Emission Display), and the like, a bioorganic material ejecting head used to manufacture a biochip, and the like.