Abstract:
In a drive signal where three ink droplets are ejected for one printing command, the following expressions are satisfied: 0.8T≦T1≦1.2T, 0.4T≦T2≦1.2T, 0.4T≦T3≦0.8T, W1&gt;W2, W1&gt;2T, wherein T1, T2, T3 are pulse widths for drive pulses P1, P2, P3 each to eject an ink droplet and W1, W2 are pulse intervals. When the drive signal meeting the above conditions is applied to an actuator for a printing operation, ink droplets can be ejected stably over a wide range of temperatures without dispersion in density and can be prevented from coalescing into one globule along the trajectory.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of Invention  
           [0002]    The invention relates to an ink ejection apparatus that ejects ink droplets from a nozzle by driving an actuator to generate a pressure wave in an ink chamber, particularly to an ink ejection apparatus capable of ejecting three or more ink droplets for one printing command.  
           [0003]    2. Description of Related Art  
           [0004]    Non-impact type printing devices have recently taken the place of conventional impact type printing devices and are holding an ever-growing share of the market. Of these non-impact type printing devices, ink jet type printing devices have the simplest operation principle, but are still capable of effectively and easily performing multi-gradation and color printing. Of these devices, a drop-on demand type that ejects ink droplets only used for printing has rapidly gained popularity because of its excellent ejection efficiency and low running cost.  
           [0005]    A conventional ink ejection apparatus used in a drop-on demand type printing device includes a nozzle from which ink is ejected, an ink chamber that is provided on the back of the nozzle and stores ink, an actuator that changes the volume of the ink chamber, and a driving device that drives the actuator to generate pressure wave vibrations in the ink chamber causing ink to be ejected from the nozzle. This kind of ink ejection apparatus is of a design wherein the driving device drives the actuator to generate the pressure wave vibrations in the ink chamber in response to a change in the volume of the ink chamber, thereby ejecting ink from the nozzle.  
           [0006]    The actuator may be made of a piezoelectric element that deforms through the application of a drive voltage. In this case, ink is ejected by applying a pulse voltage (hereinafter referred to as a drive pulse) to the piezoelectric elements from a drive circuit. In this kind of ink ejection apparatus, it is conceivable that the drive pulse is repeatedly applied to the actuator in response to one print command, to eject multiple ink droplets from one nozzle, so that one dot is formed. As one dot is produced from large quantity of ink in this case, an image can be formed having a deep color.  
           [0007]    There is a shear mode type of piezoelectric element in an ink ejection apparatus using the piezoelectric element as the actuator, for example. An exemplary ink ejection apparatus of this kind, which also is the apparatus to which the invention is applied, is shown in FIGS. 10A and 10B. FIG. 10A is a sectional view taken along line  10 - 10  of FIG. 10B. FIG. 10B is a sectional view taken along line  11 - 11  of FIG. 10A.  
           [0008]    As shown in FIG. 10A, an ink ejection apparatus  600  includes a bottom wall  601 , a top wall  602 , and elongated shear mode actuator walls  603  sandwiched therebetween. Each actuator wall  603  includes an upper wall  605  of piezoelectric material, which is adhesively attached to the top wall  602  and polarized in a direction indicated by an arrow  609 , and a lower wall  607  of piezoelectric material, which is adhesively attached to the bottom wall  601  and polarized in a direction indicated by an arrow  611 . Alternating pairs of actuator walls  603  form in alternation between ink chambers  613  and spaces  615 , the spaces  615  narrower than the ink chambers  613 .  
           [0009]    As shown in FIG. 10B, a nozzle plate  617  having nozzles  618  is fixedly secured to one end of each ink chamber  613  and an ink supply source (not shown) is connected to the other end of each ink chamber  613  via a manifold  626 . The manifold  626  includes a front wall  627  formed with openings in positions corresponding to the ink chambers  613 , a rear wall  628  for sealing the space between the bottom wall  601  and the top wall  602 . The manifold  626  is structured to distribute the ink supplied from the ink supply source to the front wall  627  and the rear wall  628  into each of the ink chambers  613 .  
           [0010]    Electrodes  619 ,  621  are provided on both sides of each of the actuator walls  603 . Specifically, the electrode  619  is provided on the actuator wall  603  in the ink chamber  613  and the electrode  621  is provided on the actuator wall  603  in the space  615 . The electrode  621  is also provided on the outer side surface of each of the two outermost actuator walls  603 . The electrode  619  is covered by an insulating layer (not shown) to insulate it from the ink. Each electrode  621  is connected to a ground  623 . Each electrode  619  provided in the ink chamber  613  is connected to a control unit  625  and carries a voltage (drive signal) described later.  
           [0011]    When the control unit  625  applies the voltage to the electrodes  619  in the ink chambers  603 , pairs of the actuator walls  603  deform in the shear mode such that the volume of each ink chamber  613  increases. An example of this operation is shown in FIG. 11. When a voltage of E volts, which is the crest value, is applied to an electrode  619   c  of the ink chamber  613   c , an electric field develops in each of the actuator walls  603   e  and  603   f  in the directions indicated by the arrows  631  and  632 , respectively. The actuator walls  603   e  and  603   f  deform in the shear mode to increase the volume of the ink chamber  613   c . At this time, the pressure in the ink chamber  613   c  including the nozzle  618   c  decreases.  
           [0012]    The voltage of E volts is applied to the electrode  619  only for a one-way propagation time T. While the voltage is applied, ink is supplied from the ink supply source. The one-way propagation time T is a time required for a pressure wave in the ink chamber  613  to propagate once in the lengthwise direction of the ink chamber  613 . The one-way propagation time T is calculated by the following expression:  
           
         T=L/a,  
       
           [0013]    wherein L is the length of the ink chamber  613  and a is the speed of sound in the ink in the ink chamber  613 .  
           [0014]    According to the theory of pressure wave propagation, the pressure in the ink chamber  613  reverses into a positive pressure when the one-way propagation time T passes after the application of the voltage. When the pressure becomes positive, the control unit  625  returns the voltage applied to the electrode  619  of the ink chamber  613  to zero volts, so that the deformed actuator walls  603   e  and  603   f  revert to their initial shape, as shown in FIG. 10A, and pressure is applied to the ink. The pressure reverted to positive and the pressure generated when the deformed actuator walls  603   e  and  613   f  return to their initial shape are combined into a relatively high pressure that develops near the nozzle  618   c  in the ink chamber  613   c , ejecting ink from the nozzle  618   c.    
           [0015]    However, when three or more ink droplets are ejected for one printing command in a drive waveform, as shown in FIG. 8E, the drive pulses are set as follows:  
           T1=T2=T3=T,  
             W 1 =W 2=2 T,    
           [0016]    wherein T is the one-way propagation time, T1 is a pulse width of a drive pulse P1 for ejecting a first ink droplet, T2 is a pulse width of a drive pulse P2 for ejecting a second ink droplet, T3 is a pulse width of a drive pulse P3 for ejecting a third ink droplet, W1 is an interval between the drive pulses P1 and P2, and W2 is an interval between the drive pulses P2 and P3.  
           [0017]    In this case, the application of the pressure to the ink chamber and the cancellation of the pressure application are performed in synchronization with the one-way propagation time T. In other words, the pressure is applied in accordance with a rising point of the ink pressure wave and the application of the pressure is cancelled in accordance with a falling point of the ink pressure wave. Therefore, the pressure wave is gradually amplified to perform efficient ink ejection. However, the pressure applied to the ink becomes greater whenever the ink droplet is ejected, and ejecting speed becomes faster for a later ink droplet. As a result of the influence of the pressure wave, the ink may be ejected from an adjacent nozzle, ink ejection may become unstable and the interval to eject ink droplets may become short when the printing command is continuously executed on the same nozzle. As shown in FIG. 9B, ink droplets  99  may coalesce into one along the trajectory. If the ink droplets  99  coalesce or unify, during the trajectory in this manner, deviation in trajectory occurs, lowering printing quality. Further, when the temperature of the ink is changed, the one-way propagation time T is also changed, becoming out of synch with the application of the ink pressure wave and the cancellation of the application. As a result, ink droplets vary in size, printing density is changed, and ink ejection becomes unstable.  
         SUMMARY OF THE INVENTION  
         [0018]    The invention provides an ink ejection apparatus capable of ejecting three or more ink droplets for one printing command stably without dispersion in density over a wide range of temperatures and of preventing ink droplets from coalescing into a globule during the trajectory without difficulty to improve printing quality.  
           [0019]    According to one aspect of the invention, an ink ejection apparatus includes a nozzle from which ink is ejected, an ink chamber provided on a back of the nozzle where the ink is stored, an actuator that changes a volume of the ink chamber, and a drive device that drives the actuator by applying a drive signal including a plurality of pulses to the actuator to cause the actuator to generate a pressure wave vibration in the ink chamber, thereby ejecting the ink from the nozzle. The drive device generates positive and negative pressure waves in the ink chamber through application of one drive pulse to the actuator. When three or more ink droplets are ejected for one printing command, the drive signal satisfies the following expressions:  
           0.8 T≦T 1≦1.2 T,    
           0.4 T≦T 2≦1.2 T,    
           0.4 T≦T 3≦0.8 T,    
           W1&gt;W2,  
             W 1&gt;2 T,    
           [0020]    wherein T1 is an effective pulse width of a drive pulse P1 to eject a first ink droplet, T2 is an effective pulse width of a drive pulse P2 to eject a second ink droplet, T3 is an effective pulse width of a drive pulse P3 to eject a third ink droplet, W1 is an interval between the drive pulses P1 and P2, W2 is an interval between the drive pulses P2 and P3, and T is a one-way propagation speed where a pressure wave is propagated in the ink chamber once.  
           [0021]    Under these expressions, as T3 is set shorter than T and W1 is set longer than 2T, the first ink droplet ejection does not have an adverse effect upon the second and third ink droplets, thereby reducing the pressure applied to the ink during the ejection of the second and third ink droplets. This enables ink droplets to be ejected stably and separately thereby preventing the ink droplets from coalescing into one globule. The nozzle is not affected by the previous ink ejection and ink ejection by an adjacent nozzle, thereby improving printing quality. As there is no need to insert a non-ejection pulse between the drive pulses, as has been conventional, the invention can preferably correspond to high-speed printing. The ink droplets can be stably ejected over a wide range of temperatures, thereby stably obtaining a specific printing density.  
           [0022]    In the above structure, it is preferable that T2, T3, W1, and W2 further satisfy the following expressions:  
           0.4 T≦T 2= T 3≦0.8 T,    
           1.8 T≦W 2≦2.2 T,    
           and  
           2.2 T≦W 1≦2.8 T.    
           [0023]    It has been found from various experiments that printing quality can be improved further preferably and stably when T2, T3, W1 and W2 satisfy the above expressions.  
           [0024]    Further, it is preferable that T1, T2 and T3 satisfy the following expression:  
           T1≧T2&gt;T3.  
           [0025]    It has been found from various experiments that ink droplets can be further preferably ejected over a wide range of temperatures and a specific printing density can be stably obtained when T1, T2 and T3 satisfy the above expression. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    The invention will be described in greater detail with reference to preferred embodiments thereof and the accompanying drawings wherein;  
         [0027]    [0027]FIG. 1 is an exploded perspective view of an ink jet printer head as one embodiment of an ink ejection apparatus;  
         [0028]    [0028]FIG. 2 is an exploded perspective view of a cavity plate of the ink jet printer head;  
         [0029]    [0029]FIG. 3 is an enlarged perspective view of a main structure of the cavity plate;  
         [0030]    [0030]FIG. 4 is a sectional view showing the structure of the cavity plate taken along line  5 - 5  of FIG. 3;  
         [0031]    [0031]FIG. 5 is an exploded perspective view of a piezoelectric actuator of the ink jet printer head;  
         [0032]    [0032]FIG. 6 is a sectional view taken along line  5 - 5  of FIG. 3 when the piezoelectric actuator is mounted on the cavity plate;  
         [0033]    [0033]FIG. 7 is a circuit diagram of a drive circuit used in the ink jet printer head;  
         [0034]    [0034]FIG. 8A is a drive voltage waveform of a drive voltage applied to the piezoelectric actuator;  
         [0035]    [0035]FIG. 8B is a drive voltage waveform of a drive voltage applied to the piezoelectric actuator;  
         [0036]    [0036]FIG. 8C is a drive voltage waveform of a drive voltage applied to the piezoelectric actuator;  
         [0037]    [0037]FIG. 8D is a drive voltage waveform of a drive voltage applied to the piezoelectric actuator;  
         [0038]    [0038]FIG. 8E is a conventional drive voltage waveform of a drive voltage applied to the piezoelectric actuator;  
         [0039]    [0039]FIG. 9A shows ink ejection when the drive voltage waveform of the embodiment is applied;  
         [0040]    [0040]FIG. 9B shows ink ejection when the conventional drive voltage waveform is applied;  
         [0041]    [0041]FIG. 10A is a sectional view taken along line  10 - 10  of FIG. 10B, which shows a structure of the ink jet apparatus of the invention (and the related art);  
         [0042]    [0042]FIG. 10B is a sectional view taken along line  11 - 11  of FIG. 10A; and  
         [0043]    [0043]FIG. 11 shows an example of an operation of the ink ejection apparatus shown in FIGS. 10A and 10B. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0044]    As shown in FIG. 1, the ink jet printer head is formed by laminating a cavity plate  10  made of metal, a piezoelectric actuator  20  and a flexible, flat connecting cable  30 . The connecting cable  30  goes to external equipment.  
         [0045]    The cavity plate  10  (FIG. 2) is made up of five thin metal plates of substantially rectangular shape; a nozzle plate  11 , two manifold plates  12 , a spacer plate  13 , and a base plate  14 .  
         [0046]    The nozzle plate  11  has a line of nozzles  15  of minute diameter for ejecting ink droplets, which are provided lengthwise along the centerline  11   a  at a micro pitch. The manifold plates  12  each have ink passages  12   a  that extend along both sides of the line of nozzles  15 . The ink passages  12   a  are defined by sandwiching the manifold plates  12  between the nozzle plate  11  and the spacer plate  13 .  
         [0047]    The base plate  14  has a number of narrow ink chambers  16  each of which extends in a direction orthogonal to a centerline  14   a  along the length of the base plate  14 . As shown in FIGS. 3 and 4, ink outlets  16   a  of the ink chambers  16  are positioned on the centerline  14   a  in such a manner that alternate ink chambers  16  extend from the ink outlets  16   a  in direction opposite to each other. The ink outlets  16   a  of the ink chambers  16  communicate with the nozzles  15  in the nozzle plate  11  via through holes  17  provided in both the spacer plate  13  and the manifold plates  12 . The ink inlets  16   b  of the ink chambers  16  communicate with the corresponding ink passages  12   a  through holes  18  provided in the spacer plate  13 .  
         [0048]    With this structure, the ink fed from supply holes  19   a ,  19   b  provided on one side of both the spacer plate  13  and the base plate  14  flows to the ink passages  12   a , and passes through each of the through holes  18 , thereby to be directed to each of the ink chambers  16 . After that, the ink passes through each of the through holes  17  aligned with each of the ink outlets  16   a  of the ink chambers  16  and reaches an associated one of the nozzles  15 . Each of the ink chambers  16  has a narrow groove  16   c  adjacent to the ink inlet  16   b  and a beam  16   d  for reinforcement in a central portion. The narrow groove  16   c  and the beam  16   d  are partially thinned and integrally formed in the ink chamber  16 .  
         [0049]    As shown in FIG. 5, the piezoelectric actuator  20  is constructed by laminating three piezoelectric sheets  21 ,  22 ,  23 . Narrow drive electrodes  24  are formed on the upper surface of the lowermost piezoelectric sheet  21  (closest to the cavity plate  10 ) so as to be aligned with the respective ink chambers  16  in the cavity plate  10 . In addition, the drive electrodes  24  are formed in such a manner that one end  24   a  of each of the drive electrodes  24  is bare on either of right and left side surfaces  20   c  (FIG. 3) in the direction of the length of the piezoelectric actuator  20 .  
         [0050]    A common electrode  25  is formed on the upper surface of the middle piezoelectric sheet  22  so that projecting parts  25   a  of the common electrode  25  extend out to the right and left side surfaces  20   c . Further, surface electrodes  26  facing the respective drive electrodes  24  and surface electrodes  27  facing projecting parts  25   a  of the common electrode  25  are provided on the upper surface of the uppermost piezoelectric sheet  23 , so as to be arranged along the right and left side surfaces  20   c . The numerals  24 ′ and  25 ′ indicate electrodes for dummy patterns.  
         [0051]    In FIGS. 3 and 4, side-mounted electrodes  28 , that electrically connect the drive electrodes  24  and the respective surface electrodes  26 , and side-mounted electrodes  29 , that electronically connect the projecting parts  25   a  of the common electrode  25  and the surface electrodes  27 , are formed at the side surfaces  20   c . In the above description, the piezoelectric sheet  21  on which the drive electrodes  24  are formed and the piezoelectric sheet  22  on which the common electrode  25  is formed are laminated only in one pair, however, then may be laminated in a plurality of pairs.  
         [0052]    The piezoelectric actuator  20  structured in this manner is fixedly laminated to the cavity plate  10 . The lamination is made to block each ink chamber  16  on the underside of the piezoelectric sheet  21  mounting the drive electrodes  24  thereon. Further, the flexible flat cable  30  is fixedly laminated onto the piezoelectric actuator  20  so that a printed pattern (not shown) exposed at the underside of the flat cable  30  can be electrically connected to the surface electrodes  26 ,  27 .  
         [0053]    In the ink jet printer head, when a voltage is applied between one of the drive electrodes  24  and the common electrode  25  in the piezoelectric actuator  20 , the piezoelectric sheet  22  sandwiched between the drive electrode  24  and the common electrode  25  deforms by piezoelectric effect in a direction where the sheets are laminated. By this deformation, the volume of the ink chamber  16  corresponding to the drive electrode  24  is reduced, causing ink stored in the ink chamber  16  to be ejected as a droplet from the associated nozzle  15 . Alternatively, the drive voltage can be applied to all drive electrodes  24  in advance before an ejection command is input, to cause the piezoelectric sheet  22  to deform in relation to all ink chambers  16 . In this case, when the ejection command is input to one of the drive electrodes  24 , the voltage application to the drive electrode  24  is cancelled and the volume of the corresponding ink chamber  16  is increased. Then when the voltage is again applied to the drive electrode  24 , the piezoelectric sheet  22  aligned with the ink chamber  16  is returned to the deformed state and the pressure is applied to the ink chamber  16 . This causes ink stored in the ink chamber  16  to be ejected as a droplet from the associated nozzle  15 .  
         [0054]    In the ink jet printer head of this embodiment, holes  41 ,  42  are opened in the base plate  14  so as to be aligned with the side-mounted electrodes  28 ,  29 . This can preferably prevent a short circuit between the side-mounted electrodes  28 ,  29  and the base plate  14  when the piezoelectric actuator  20  is placed on the cavity plate  10 , as shown in FIG. 6.  
         [0055]    In this embodiment, to apply the drive voltage to the drive electrodes  24 , a drive circuit  100  is connected to the surface electrodes  26 ,  27  on the piezoelectric actuator  20  via the flexible flat cable  30 . FIG. 7 is a circuit diagram showing the configuration of the drive circuit  100  in the ink jet printer head.  
         [0056]    As shown in FIG. 7, the drive circuit  100  includes a charging circuit  182 , a discharge circuit  184 , and a pulse control circuit  186 . In addition, the piezoelectric sheet  22 , the drive electrodes  24 , and the common electrode  25  are equivalently represented by a capacitor  191 . Terminals  191 A,  191 B of the capacitor  191  correspond to the drive electrodes  24  and the common electrode  25 , respectively. The terminal  191 A is connected to the drive circuit  100  and the terminal  191 B is connected to a ground  623 .  
         [0057]    An input terminal  187  provided in the charging circuit  182  and an input terminal  188  provided in the discharge circuit  184  are terminals to input a signal for applying the drive voltage of E volts (e.g. 20V) or 0V to the terminal  191 A (the drive electrode  24  of the associated ink chamber  16 ) from the pulse control circuit  186 .  
         [0058]    The charging circuit  182  includes resistors R 101 , R 102 , R 103 , R 104  and R 105 , and transistors TR 101  and TR 102 . A base of the transistor TR 101  is connected to the input terminal  187  via the resistor R 101  and is grounded via the resistor R 102 . An emitter of the transistor TR 101  is directly grounded and a collector thereof is connected to a positive power supply  189  of E volts via the resistor R 103 . A base of the transistor TR 102  is connected to the positive power supply  189  via the resistor R 104  and to the collector of the transistor TR 101  via the resistor R 105 . An emitter of the transistor TR 102  is connected directly to the positive power supply  189  and a collector thereof is connected to the terminal  191 A via the resistor R 120 .  
         [0059]    When an ON signal (+5V) is applied to the input terminal  187 , the transistor TR 101  becomes conductive, allowing current from the positive power supply  189  to flow from the collector of the transistor TR 101  to the emitter. This raises the voltage dividedly applied to the resistors R 104 , R 105 , connected to the positive power supply  189 , and increases the current flowing to the base of the transistor TR 102 , making the transistor TR 102  conductive between the emitter and the collector of the transistor TR 102 . As a result, the voltage of E volts is applied from the positive power supply  189  to the terminal  191 A (that is, the drive electrodes  24 ) of the capacitor  191  via the collector and the emitter of the transistor TR 102  and the resistor R 120 .  
         [0060]    The discharge circuit  184  includes resistors R 106 , R 107  and transistor TR 103 . A base of the transistor TR 103  is connected to the input terminal  188  via the resistor R 106  and grounded via the resistor R 107 . An emitter of the transistor TR 103  is directly grounded and a collector thereof is connected to the terminal  191 A via the resistor R 120 . As a result, when an ON signal (+5V) is applied to the input terminal  188 , the transistor TR 103  becomes conductive, allowing the terminal  191 A (that is, the drive electrodes  24 ) of the capacitor  191  to ground via the resistor R 120 .  
         [0061]    When an ON signal is applied to the input terminal  187  from the pulse control circuit  186  and an OFF signal is applied to the input terminal  188 , the drive voltage of E volts can be applied to the drive electrode  24 . When an OFF signal is applied to the input terminal  187  from the pulse control circuit  186  and an ON signal is applied to the input terminal  188 , the drive electrode  24  can be maintained at 0 volts as with the common electrode  25 .  
         [0062]    The variations in voltage applied to the drive electrode  24  via the charging circuit  182  and the discharge circuit  184  actually include a delay corresponding to the capacitance of the piezoelectric sheet  22 . However, the following description will be made on the assumption that variations in signal to be input to the input terminals  187 ,  188  are synchronized with the variations in voltage to be applied to the drive electrode  24 .  
         [0063]    The pulse control circuit  186  includes a CPU  210  that performs various calculations, which is connected to a RAM  212  and a ROM  214 . The RAM  212  stores print data and other data in it. The ROM  214  stores the control programs for the pulse control circuit  186  and sequence data for generating ON and OFF signals of the waveforms shown in FIGS. 8A to  8 D.  
         [0064]    In addition, the CPU  210  is connected to an I/O bus  216  via which various data can be input and output. The I/O bus  216  is connected to a print data receiving circuit  218  and pulse generators  220 ,  222 . An output from the pulse generator  220  is input to the input terminal  187  of the charging circuit  182  and an output from the pulse generator  222  is input to the input terminal  188  of the discharge circuit  184 .  
         [0065]    In the pulse control circuit  186  configured above, the CPU  210  controls the pulse generators  220 ,  222  in accordance with the sequence data stored in the ROM  214  to apply the drive voltage to the appropriate drive electrode  24  in a timed relationship associated with the sequence data. Pulse generators  220 ,  222 , the charging circuit  182  and the discharge circuit  184  are provided for each nozzle  15  of the ink jet printer head. The CPU  210  outputs a drive signal to a drive electrode  24  associated with the print data, causing the ink to be ejected from the corresponding nozzle  15 .  
         [0066]    An exemplary waveform of the above drive signal (hereinafter referred to as a drive waveform) in the drive circuit  100  is shown in FIG. 8A. The drive circuit  100  outputs the drive signal while the ink jet printer head is moved by the carriage. The drive waveform shown in FIG. 8A represents a case where three ink droplets are ejected for one printing command. T1 is an effective pulse width of a drive pulse P1 for ejecting a first ink droplet. T2 is an effective pulse width of a drive pulse P2 for ejecting a second ink droplet. T3 is an effective pulse width of a drive pulse P3 for ejecting a third ink droplet. W1 is an interval between the drive pulses P1, P2. W2 is an interval between the drive pulses P2, P3.  
         [0067]    With this drive signal, the voltage of E volts is applied under normal circumferences, and stopped (0 volts) and applied again in timed relationship among the drive pulses P1, P2 and P3. Accordingly, the piezoelectric actuator  20  is normally deformed in a direction causing the volume of each of the ink chambers  16  to shrink. The deformation of the piezoelectric actuator  20  is stopped when the voltage application is canceled at each drive pulse, which enlarges the volume of the corresponding ink chamber  16 . Then, when the voltage is applied again, the piezoelectric actuator  20  is returned to the deformed state in the direction causing the volume of the ink chamber  16  to shrink, providing ink in the ink chamber  16  with an ejection pressure.  
         [0068]    It is found that ink droplets  99  can be prevented from coalescing into a globule (FIG. 9A) easily and stably, without a need to insert a non-ejection pulse between the drive pulses as has been conventional, when the drive signal satisfies the following expressions:  
         0.8 T≦T 1≦1.2 T,    
         0.4 T≦T 2≦1.2 T,    
         0.4 T≦T 3≦0.8 T,    
         W1&gt;W2,  
           W 1&gt;2 T,    
         [0069]    where T is a one-way propagation time where the pressure wave of the ink propagates through the ink chamber  16 .  
         [0070]    It is believed that, as T3 is set shorter than T and W1 is set longer than 2T, the first ink droplet ejection can not have any adverse effect upon the second and third ink droplets, thereby reducing the pressure applied to the ink during the ejection of the second and third ink droplets.  
         [0071]    In this embodiment, T1, T2, T3, W1, and W2 are set in ranges indicated in the above expressions according to the sequence data. Thus, the ink droplets  99  ejected from the nozzle  15  do not coalesce into a globule along the trajectory as shown in FIG. 9A, thereby preferably improving printing quality. In addition, as such coalescence can be stably prevented, the nozzle  15  is not affected by the previous ink ejection or ink ejection by an adjacent nozzle  15 , thereby preferably and stably improving printing quality. Further, it is not necessary to insert a non-ejection pulse between the drive pulses, as has been conventional. Therefore, the embodiment provides high-speed printing.  
         [0072]    On condition that T1=T, W2=2.0T, T2=T3, experiments to find optimum ranges of W1, T2 and T3 were conducted by printing various test patterns while changing values of W1, T2 and T3 while maintaining a controlled environment. The printing quality results are shown in Table 1 below. Ink used for the experiments is a water-base ink having a viscosity 3.4 mPa·s and a surface tension of 33 mN/m at a specified temperature.  
         [0073]    In Table 1, O indicates that preferable printing was obtained. × indicates that a dot was not formed at a desired position because of a deviation in the trajectory occurred or a dot was not appropriately formed because a satellite droplet (excess ink droplet subsequent to the ink droplets  99 ) was ejected.  
                                                                               TABLE 1                                       T2 = T3                0.2 T   0.4 T   0.6 T   0.8 T   1.0 T                        W1   2.0T   X   X   X   X   X           2.2 T   X   ◯   X   X   X           2.4 T   X   ◯   ◯   X   X           2.6 T   X   ◯   ◯   ◯   X           2.8 T   X   X   ◯   ◯   X           3.0 T   X   X   X   X   X                  
 
         [0074]    Similar results were obtained when 0.8T≦T1≦1.2T and 1.8T≦W2 ≦2.2T. Therefore, as shown in Table 1, preferable printing quality was obtained in the following ranges:  
         0.8 T≦T 1≦1.2 T,    
         0.4 T≦T 2 =T 3≦0.8 T,    
         2.2 T≦W 1≦2.8 T,    
         1.8 T≦W 2≦2.2 T,    
         W1&gt;W2,  
         and  
         W1&gt;2 T.    
         [0075]    In this case, it is thought that, as T2 and T3 are set shorter than the one-way propagation time T and W1 is set longer than 2T, the first ink droplet ejection can not have any adverse effect upon the second and third ink droplets, thereby reducing the pressure applied to the ink during the ejection of the second and third ink droplets. However, preferable printing quality was not obtained outside the defined ranges.  
         [0076]    Experiments to find optimum ranges of T1, T2, and T3 were conducted by changing ambient temperatures in stages. The values of W1 and W2 were left unchanged as with the above experiments of Table 1.  
         [0077]    Table 2 provides a summary of experimental results. O indicates that ink droplets were stably ejected and high density printing quality was obtained. Δ indicates that ink droplets were stably ejected but printing was of a slightly inferior quality due to density. × indicates ink ejection was unstable, i.e., a deviation in the trajectory occurred or a satellite droplet was ejected.  
                                                           TABLE 2                           PULSE   T1   1.4 T   1.2 T   1.0 T   1.0 T   1.0 T   1.0 T   0.8 T   0.8 T   0.8 T   0.6 T           T2   1.4 T   1.2 T   1.0 T   0.8 T   0.6 T   0.6 T   1.0 T   0.8 T   0.6 T   0.4 T           T3   0.8 T   0.8 T   0.6 T   0.6 T   0.6 T   0.8 T   0.6 T   0.8 T   0.4 T   0.2 T       TEMP    5° C.   Δ   Δ   Δ   X   Δ   X   X   X   X   X           10° C.   X   Δ   Δ   Δ   ◯   X   X   X   Δ   X           15° C.   X   ◯   ◯   Δ   ◯   X   X   X   Δ   X           20° C.   X   ◯   ◯   ◯   Δ   X   Δ   X   ◯   X           25° C.   X   ◯   ◯   ◯   Δ   X   ◯   X   ◯   X           30° C.   X   ◯   ◯   ◯   Δ   X   Δ   X   ◯   X           35° C.   X   ◯   ◯   ◯   X   X   X   X   Δ   X           40° C.   X   Δ   ◯   ◯   X   X   X   X   Δ   X                  
 
         [0078]    Therefore, as shown in Table 2, preferable printing quality was obtained over a wide range of temperatures in ranges indicated by the following expressions:  
         0.8 T≦T 1≦1.2 T,    
         0.4 T≦T 2≦1.2 T,    
         0.4 T≦T 3≦0.8 T,    
           T 1≦T2≦T3,  
         2.2 T≦W 1≦2.8 T,    
         1.8 T≦W 2≦2.2 T,    
         W1&gt;W2,  
         and  
           W 1&gt;2 T.    
         [0079]    It should be understood that the invention is not limited in its application to the details of structure and the arrangement of parts illustrated in the accompanying drawings. The invention is capable of other embodiments and of being practiced or performed in various ways without departing from the technical idea thereof, based on existing and well-known techniques among those skilled in the art. For example, the above embodiment uses rectangular waveforms, however, a trapezoidal waveform can be used. In this case, values of T1, T2, T3, W1 and W2 may be set with respect to a center of each of the oblique lines of the trapezoidal waveform, as shown in FIG. 8B. The invention can be applied to a case where four or more ink droplets are ejected. In addition, after the three ink droplets are ejected, a non-ejection pulse to suppress fluctuations of the pressure can be applied to reduce a cycle of the print command, thereby further facilitating high-speed printing.  
         [0080]    In the above embodiment, the ink jet printer head is constructed by lamination of the cavity plate  10  and the piezoelectric actuator  20 . However, the invention can be applied to an ink ejection apparatus wherein side walls of an ink chamber are made up of the piezoelectric element as shown in the related art of FIGS.  10 B, and  11 , for example, disclosed in U.S. application Ser. No. 09/069,777 corresponding to Japanese Laid-Open Patent Publication No. 10-296975. In this case, drive waveforms, such as shown in FIGS. 8C and 8D are used.  
         [0081]    In the above embodiment, the actuator is formed of the piezoelectric elements. However, the actuator may be formed of another medium as long as it can provide positive and negative pressure wave fluctuations within the ink in the ink chamber through the application of a drive pulse.