Abstract:
In an ink droplet ejecting method and apparatus, when a continuous dot printing is performed and also when a continuous dot printing is followed by a one-dot rest and again subsequent printing, it is intended to suppress the meniscus oscillation of ink, prevent the decrease in ink droplet ejecting speed of some dots and prevent the ink droplet ejecting direction from becoming unstable. A plurality of driving waveforms are provided in advance, and in accordance with whether there is ink ejection just before and just after one dot, an appropriate driving waveform for the dot is selected, whereby it becomes possible to suppress the meniscus oscillation of ink and a stable ink droplet ejection is ensured in a continuous dot printing and also when a continuous dot printing is followed by a one-dot rest and against subsequent printing.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an ink droplet ejecting method and apparatus of an ink jet printhead. 
     2. Description of Related Art 
     According to a known ink jet printer using an ink jet printhead, the volume of an ink flow path is changed by deformation of a piezoelectric ceramic material, and when the flow path volume decreases, the ink present in the ink flow path is ejected as a droplet from a nozzle, while when the flow path volume increases, ink is introduced into the ink flow path from an ink inlet. In this type of printing head, a plurality of ink chambers are formed by partition walls of a piezoelectric ceramic material, and ink supply means, such as ink cartridges, are connected to one end of each ink chamber of the plurality of ink chambers, while at the opposite end of each of the ink chambers is an ink ejecting nozzle (hereinafter referred to simply as “nozzle” or “nozzles”). The partition walls are deformed in accordance with printing data to make the ink chambers smaller in volume, whereby ink droplets are ejected onto a printing medium from the nozzles to print, for example, a character or a figure. 
     As this type of an ink jet printer, a drop-on-demand type ink jet printer which ejects ink droplets is popular because of a high ejection efficiency and a low running cost. As an example of the drop-on-demand type there is known a shear mode type using a piezoelectric material, as is disclosed in Japanese Published Unexamined Patent Application No. Sho 63-247051. 
     As shown in FIGS. 12A-13, (which are also applicable to the instant invention), this type of an ink droplet ejecting apparatus  600  comprises a bottom wall  601 , a top wall  602  and shear mode actuator walls  603  located therebetween. The actuator walls  603  each comprise a lower wall  607  bonded to the bottom wall  601  and polarized in the direction of arrow  611  and an upper wall  605  formed of a piezoelectric material, the upper wall  605  being bonded to the top wall  602  and polarized in the direction of arrow  609 . Adjacent actuator walls  603 , in a pair, define an ink chamber  613  therebetween, and next adjacent actuator walls  603 , in a pair, define a space  615  which is narrower than the ink chamber  613 . 
     A nozzle plate  617  (FIG. 12B) having nozzles  618  is fixed to one end of the ink chambers  613 , while to the opposite end of the ink chambers is connected an ink supply source (not shown). On both side faces of each actuator wall  603  are formed electrodes  619 ,  621 , respectively, as metallized layers. More specifically, the electrode  619  is formed on the actuator wall  603  on the side of the ink chamber  613 , while the electrode  621  is formed on the actuator wall  603  on the side of the space  615 . The surface of the electrode  619  is covered with an insulating layer  630  for insulation from the ink. The electrode  621  which faces the space  615  is connected to a ground  623 , and the electrode  619  provided in each ink chamber  613  is connected to a controller  625  which provides an actuator drive signal to the electrode. 
     The controller  625  applies a voltage to the electrode  619  in each ink chamber, whereby the associated actuator walls  603  undergo a piezoelectric thickness slip deformation in directions to increase the volume of the ink chamber  613 . For example, as shown in FIG. 13, when voltage E(V) is applied to an electrode  619   c  in an ink chamber  613   c,  electric fields are generated in the directions of arrows  629 ,  631  and  630 ,  632  respectively in actuator walls  603   e  and  603   f , so that the actuator walls  603   e  and  603   f  undergo a piezoelectric thickness slip deformation in directions to increase the volume of the ink chamber  613   c . At this time, the internal pressure of the ink chamber  613   c , including a nozzle  618   c  and the vicinity thereof, decreases. The applied state of the voltage E(V) is maintained for only a one-way propagation time T of a pressure wave in the ink chamber  613   c . During this period, ink is supplied from the ink supply source. 
     The one-way propagation time T is a time required for the pressure wave in the ink chamber  613  to propagate longitudinally through the ink chamber. Given that the length of the ink chamber  613  is L and the velocity of sound in the ink present in the ink chamber  613  is a, the time T is determined to be T=L/a. 
     According to the theory of pressure wave propagation, upon the lapse of time T, or an odd-multiple time thereof, after the above application of voltage, the internal pressure of the ink chamber  613   c  reverses into a positive pressure. In conformity with this timing, the voltage being applied to the electrode  619   c  in the ink chamber  613   c  is returned to 0 (V). As a result, the actuator walls  603   e  and  603   f  revert to their original state (FIG. 13) before the deformation, whereby a pressure is applied to the ink. At this time, the above positive pressure and the pressure developed by reverting of the actuator walls  603   e  and  603   f  to their original state before the deformation are added together to afford a relatively high pressure in the vicinity of the nozzle  618   c  in the ink chamber  613   c , whereby an ink droplet is ejected from the nozzle  618   c.  An ink supply passage  626  communicating with the ink chamber  613  is formed by members  627 ,  628 . 
     Heretofore, in this type of an ink droplet ejecting apparatus  600 , when jet pulses (an optimum pulse width is an odd-multiple value of T) are applied to an actuator continuously at a predetermined frequency to effect a continuous dot printing and when the continuous dot printing is followed by, for example, a one-dot rest and subsequent input of the next dot printing instruction, the ink droplet speed and the direction of droplet ejection become unstable at the portion of the printing instruction under the influence of remaining meniscus oscillation of the ink present in the nozzle concerned, thus giving rise to the problem that a printing line is curved or thinned at that portion, resulting in deterioration of the print quality. 
     In the case where an ink droplet of a small volume is to be ejected for enhancing the printing resolution, it has been proposed to add, for one dot, a non-jet pulse after application of a jet pulse and before completion of ink ejection. In this case, the remaining meniscus oscillation is suppressed and the ejection of ink becomes stable in a continuous dot printing, but there arises the problem that the energy efficiency is low because it is necessary to continue adding the non-jet pulse. In both cases noted above, the printing instruction is issued without considering whether there is ejection of ink just before and just after the dot concerned. 
     Now, with reference to FIGS. 1A,  1 B and FIGS. 2 and 3, a description will be given of results obtained by conducting two printing operations and actually measuring ink droplet ejecting speeds. FIG. 1A shows a jet pulse signal A (designated the first driving waveform) of pulse width 1 T for one dot and FIG. 1B shows the jet pulse signal A of pulse width 1 T for one dot and a non-jet additional pulse signal B (both designated the second driving waveform). In this case, a time difference between a fall timing of the jet pulse signal A and a rise timing of the additional pulse signal B is set at 2.25 T and that the pulse width of the additional pulse signal B is set at 0.5 T. Here there was used a certain waveform (the first or the second driving waveform) irrespective of whether there is ejection of ink. Table 1 below shows measurement data on the ink droplet ejecting speed (m/s) obtained by a continuous dot printing (1˜5) with use of each driving waveform, subsequent one-dot rest (6) and subsequent two-dot printing (7, 8). Printing frequency was set at 10.0 kHz. As is seen from Table 1, the ink droplet ejecting speed greatly decreases at the second dot (8) after the rest which follows the continuous printing using the first driving waveform. 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 DRIVING 
                 DOT 
               
             
          
           
               
                 WAVEFORM 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
               
               
                   
               
               
                 1 ST  DRIVING 
                 8.0 
                 9.0 
                 9.5 
                 9.5 
                 9.5 
                 — 
                 9.2 
                 6.5 
               
               
                 WAVEFORM 
               
               
                 2 ND  DRIVING 
                 8.0 
                 7.5 
                 8.1 
                 8.1 
                 8.1 
                 — 
                 8.0 
                 7.5 
               
               
                 WAVEFORM 
               
               
                   
               
             
          
         
       
     
     In the case where printing is performed at a high frequency in such a manner that a continuous dot printing is followed by a one-dot rest and subsequent printing with plural dots, there arises the problem that the second dot after the rest cannot be ejected or the ink droplet of the second dot becomes smaller in continuous dot printing. 
     SUMMARY OF THE INVENTION 
     The invention addresses and solves the above-identified problems. According to the invention, the driving waveform for printing the dot concerned is changed according to whether there is ejection of ink just before and just after the printing, whereby in a continuous dot printing and when a continuous dot printing is followed by a one-dot rest and again subsequent printing, it becomes possible to suppress the meniscus oscillation of the ink and the decrease in ink droplet ejecting speed of some dots is prevented. The instability of the droplet ejecting direction is also prevented. In addition, the driving energy efficiency is improved. It is an object of the invention to provide an ink droplet ejecting method and apparatus capable of attaining these results. 
     For achieving the above-mentioned object, the invention resides in an ink droplet ejecting method wherein a jet pulse signal is applied to an actuator which is for changing the volume of an ink chamber filled with ink, to generate a pressure wave within the ink chamber, thereby applying pressure to the ink and allowing a droplet of the ink to be ejected from a nozzle, wherein, on the basis of whether there is ejection of ink just before and just after one dot, a driving waveform which forms the one dot is deformed. 
     In this method, the state of ink meniscus in printing the dot differs according to whether there is ejection of ink just before and just after the dot. However, since the driving waveform of the dot is changed according to whether there is ejection of ink just before and just after the dot, it becomes possible to stabilize the meniscus, and when printing is started again after a continuous dot printing or after a one-dot rest in the continuous dot printing, the decrease in ink droplet ejecting speed is prevented and the ink ejecting direction is stabilized. 
     The invention resides in an ink droplet ejecting method, wherein two to four types of driving waveforms are provided in advance as jet pulse signals to be applied to the actuator at a predetermined cyclic timing in accordance with a one dot or plural continuous dots printing instruction, and any of the pre-provided driving waveforms is selected on the basis of whether there is ejection of ink just before and just after one dot. 
     According to this method, a suitable driving waveform of one dot is selected from among several pre-provided driving waveforms on the basis of whether there is ejection of ink just before and just after the dot. By so doing, an appropriate driving waveform can be selected easily and there are attained the same effects as above. 
     The invention resides in an ink droplet ejecting method, wherein if there is ejection of ink just after the dot, ink ejection is performed using a first driving waveform comprising one or plural jet pulses, while if there is no ejection of ink just after the dot, ink ejection is performed using a second driving waveform which comprises the first driving waveform and a non-jet pulse added after the first driving waveform. 
     According to this method, it is possible to stabilize the dot ejection in the case where there is no ejection of ink just after the dot. Besides, it becomes unnecessary to always add a non-jet pulse for one dot. 
     The invention resides in an ink droplet ejecting method, wherein if there is ejection of ink just before the dot and there is no ejection of ink just after the dot, the wave width of the jet pulse is shifted from an odd-multiple of time T required for one-way propagation of the pressure wave through the ink chamber, and in other cases the wave width of the jet pulse is set at an odd-multiple of the one-way propagation time T. 
     According to this method, when continuous dots are subjected to printing with a cycle of time T and when the wave width of one-dot jet pulse is set at an odd-multiple (say, 1 T or 3 T) of time T, the pressure increases in relation to propagation of the pressure wave and the ink droplet ejecting speed increases, while if the wave width is shifted from an off-multiple time, say 1.5 T, the pressure does not increase and the droplet ejecting speed decreases. Therefore, by adopting the above driving waveform for a dot not immediately followed by dot ejection, it is possible to suppress the residual meniscus oscillation and the droplet ejecting speed can be stabilized. 
     The invention resides in an ink droplet ejecting method, wherein if there is ejection of ink just before and just after the dot, ink ejection is performed at a frequency at which the ink droplet ejecting speed remains the same or increases, and in other cases ink ejection is performed at a frequency at which the ink droplet ejecting speed decreases. 
     According to this method, in continuous printing, the frequency of a driving signal for some dots is slightly increased or decreased with respect to a predetermined printing frequency, with the result that the dot ejection timing changes at that dot portion. Consequently, the influence on the residual meniscus oscillation changes and so does the droplet ejecting speed. In view of this point, a dot not followed by dot ejection before or after the dot is driven at a frequency at which the droplet ejecting speed decreases (the ejection timing becomes faster), whereby the influence of the residual meniscus oscillation can be diminished and it is possible to stabilize the droplet ejecting speed. 
     The invention resides in an ink droplet ejecting apparatus including an ink chamber filled with ink, an actuator for changing the volume of the ink chamber, a driving power source for applying an electric signal to the actuator, and a controller which makes control so that a jet pulse signal is applied to the actuator from the driving power source to increase the volume of the ink chamber and thereby generate a pressure wave in the ink chamber and so that when the time required for one-way propagation of the pressure wave through the ink chamber is assumed to be T, the volume of the ink chamber is decreased from the increased state to a normal state after the lapse of an odd-multiple of the time T, thereby applying pressure to the ink present in the ink chamber and allowing an ink droplet to be ejected, wherein the controller control is such that, in accordance with a one-dot printing instruction and on the basis of whether there is ejection of ink just before and just after the one dot, a driving waveform which forms the one dot is deformed and a jet pulse signal of the driving waveform is applied to the actuator from the driving power source. This structure affords the same effects as the first aspect of the invention. 
     The invention resides in an ink droplet ejecting apparatus, wherein two or four types of driving waveforms are provided in advance as jet pulse signals to be applied to the actuator at a predetermined cyclic timing in accordance with a one dot or plural continuous dots printing instruction, and any of the pre-provided driving waveforms is selected on the basis of whether there is ejection of ink just before and just after one dot. This structure affords the same effects as the second aspect of the invention. 
     The invention resides in an ink droplet ejecting apparatus, wherein if there is ejection of ink just after the dot, ink ejection is performed using a first driving waveform comprising one or plural jet pulses, while if there is no ejection of ink just after the dot, ink ejection is performed using a second driving waveform which comprises the first driving waveform and a non-jet pulse added after the first driving waveform. This structure affords the same effects as the third aspect of the invention. 
     The invention resides in an ink droplet ejecting apparatus wherein, if there is ejection of ink just before the dot and there is no ejection of ink just after the dot, the wave width of the jet pulse is shifted from an odd-multiple of time T required for one-way propagation of the pressure wave through the ink chamber, and in other cases the wave width of the jet pulse is set at an odd-multiple of the one-way propagation time T. This structure affords the same effects as the fourth aspect of the invention. 
     The invention resides in an ink droplet ejecting apparatus, wherein if there is ejection of ink just before and just after the dot, ink ejection is performed at a frequency at which the ink droplet ejecting speed remains the same or increases, and in other cases ink ejection is performed at a frequency at which the ink droplet ejecting speed decreases. This structure affords the same effects as the fifth aspect of the invention. 
     According to the ink droplet ejecting method and apparatus according to the invention, as set forth above, the driving waveform for printing a dot is changed in accordance with whether there is ejection of ink just before and just after the dot, whereby in a continuous printing and when a continuous dot printing is followed by a one-dot rest and again subsequent printing, it becomes possible to suppress the meniscus oscillation of ink and prevent the decrease in ink droplet ejecting speed of some dots and the destabilization of the droplet ejecting direction. Moreover, the driving energy efficiency is improved because it is not necessary to always add a non-jet pulse to one dot. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the invention will be described in detail with reference to the following figures wherein: 
     FIG. 1A is a diagram showing a jet pulse signal waveform for one dot and 
     FIG. 1B is a diagram showing both jet pulse signal waveform and non-jet additional pulse signal waveform for one dot; 
     FIG. 2 is a diagram showing the driving waveforms used in a first embodiment of the invention; 
     FIG. 3 is a diagram showing a third driving waveforms used in the second embodiment of the invention; 
     FIG. 4 is a diagram showing the driving waveforms used in the second embodiment; 
     FIGS. 5A-5D are diagrams showing driving waveforms according to a further embodiment of the invention; 
     FIGS. 6A-6D are diagrams showing driving waveforms according to a still further embodiment of the invention; 
     FIGS. 7A-7D are diagrams showing driving waveforms according to a still further embodiment of the invention; 
     FIGS. 8A-8C are diagrams showing a satisfactory state of printing in a continuous dot ejection in FIG.  8 A and FIGS. 8B and 8C are diagrams each showing an unsatisfactory state of printing in a continuous dot ejection; 
     FIG. 9 is a diagram showing a drive circuit in an ink droplet ejecting apparatus embodying the invention; 
     FIG. 10 is a diagram showing storage areas of a ROM used in a controller of the ink droplet ejecting apparatus; 
     FIG. 11 is a functional block diagram of the controller; 
     FIG. 12A is a longitudinal sectional view of an ink jet portion of a printing head and 
     FIG. 12B is a transverse sectional view thereof take along  12 B— 12 B of FIG. 12A; and 
     FIG. 13 is a longitudinal sectional view showing the operation of the ink jet portion in the printing head. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the invention will be described below with reference to the drawings. The structure of the mechanical portion in the ink droplet ejecting apparatus embodying the invention is the same as that shown in FIGS. 12A,  12 B and  13 , previously described. Therefore an explanation thereof is here omitted. 
     An example of dimensions of the ink droplet ejecting apparatus, indicated at  600 , will be described. The length L of the ink chamber  613  is 7.5 mm. As to the dimensions of the nozzle  618 , its diameter on an ink droplet ejection side is 40 μm, its diameter on the ink chamber  613  side is 72 μm, and its length is 100 μm. The viscosity, at 25° C., of ink used in an experiment is about 2 mPas and the surface tension thereof is 30 mN/m. The ratio of the above length L to a sonic velocity, a, in the ink present within the ink chamber  613 , i.e., L/a (=T), was 8 μsec. 
     The driving waveform to be applied to an electrode  619  in the ink chamber  613  used in this apparatus is outputted at a predetermined cyclic timing in accordance with a single dot or plural continuous dots printing instruction, and there is selected any of several types (2 to 4) of driving waveforms which are provided in advance on the basis of whether there is ejection of ink just before or just after one dot, i.e., the current dot for printing. 
     Table 2 below shows driving waveform conditions used in the first embodiment. In the table, the first and second driving waveforms are those shown in FIGS. 1A and 1B, respectively. The driving waveforms of FIGS. 1A and 1B are pulses for one dot printing, of which FIG. 1A comprises a jet pulse signal A (the first driving waveform) having a pulse width of an odd-multiple of 1 T, and FIG. 1B comprises the jet pulse signal A and a non-jet pulse B (the second driving waveform) which follows application of the jet pulse signal A. In the first embodiment, if there is ejection of ink immediately after one dot has been printed, ink ejection is performed using the first driving waveform, while if there is no ejection of ink immediately after the one dot, ink ejection is performed using the second driving waveform. Peak values (voltage values) of the jet pulse signal A and the additional pulse B are both assumed to be E(V), for example, say 20 (V). 
     
       
         
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 PRECEEDING 
                 FOLLOWING 
                 DRIVING WAVEFORM FOR 
               
               
                 DOT 
                 DOT 
                 CURRENT DOT 
               
               
                   
               
             
             
               
                 ON 
                 ON 
                 1 ST  DRIVING WAVEFORM 
               
               
                 ON 
                 OFF 
                 2 ND  DRIVING WAVEFORM 
               
               
                 OFF 
                 ON 
                 1 ST  DRIVING WAVEFORM 
               
               
                 OFF 
                 OFF 
                 2 ND  DRIVING WAVEFORM 
               
               
                   
               
             
          
         
       
     
     In this case, the wave width of the jet pulse signal A is set equal to an odd-multiple, a value peculiar to a head, of the ratio, L/a (=T), of the above length L to a sonic velocity, a, in the ink present within the ink chamber  613 . A time difference between a fall timing of the jet pulse signal A and a rise timing of the additional pulse B, as well as the wave width of the additional pulse B, are as noted previously. The cycle of pulses in the case of printing the next dot in a continuous manner is assumed to be approximately an even-multiple of T, which is set so that the residual oscillation based on the jet pulse signal A promotes the next ink ejection. For example, the pulse cycle is 100 μsec, assuming that the driving frequency is 10 kHz. 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 DRIVING 
                 DOT 
               
             
          
           
               
                 WAVEFORM 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
               
               
                   
               
               
                 DRIVING 
                 8.0 
                 9.0 
                 9.5 
                 9.5 
                 9.5 
                 — 
                 9.2 
                 8.7 
               
               
                 WAVEFORM 
               
               
                 DETERMINED 
               
               
                 BY TABLE 2 
               
               
                   
               
             
          
         
       
     
     Table 3 above shows measurement data on the ink droplet ejecting speed (m/s) obtained by performing printing continuously (with a one-dot rest halfway) with use of the first or the second driving waveform under the driving waveform conditions in the first embodiment shown in Table 2 above. The printing frequency was set at 10.0 kHz. In the same manner as in Table 1, printing was conducted by a continuous dot printing (1˜5), subsequent one-dot rest (6) and subsequent continuous dot printing (7, 8). FIG. 2 shows the driving waveform applied to this example. For the fifth and eighth dots, the second driving waveform was used because neither was immediately followed by dot ejection, and for the other dots there was used the first driving waveform. A comparison of the data with the data obtained by using only the first driving waveform in Table 1 shows that the eighth dot ejection not immediately followed by dot ejection does not decrease so much and that the eighth ejection is stable. Besides, in comparison with the use of only the second driving waveform with the conventional art, the second dot ejecting speed in the first embodiment does not decrease. Moreover, the energy efficiency is improved because the second driving waveform with a non-jet pulse added thereto is not normally in use. Further, an appropriate driving waveform can be selected easily from among several types of driving waveforms which are provided in advance. 
     FIG. 3 shows one jet pulse signal C (a third driving waveform, pulse width: 1.5 T) used in a second embodiment and Table 4 shows driving waveform conditions used in the second embodiment. Either the first or the third driving waveform is used according to whether there is ejection of an ink dot just before and just after the one dot. The third driving waveform is used in the case where there is ejection of ink just before the one dot to be printed and there is no ejection of ink just after. In other cases the first driving waveform is used. 
     
       
         
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 PRECEEDING 
                 FOLLOWING 
                 DRIVING WAVEFORM FOR 
               
               
                 DOT 
                 DOT 
                 CURRENT DOT 
               
               
                   
               
             
             
               
                 ON 
                 ON 
                 1 ST  DRIVING WAVEFORM 
               
               
                 ON 
                 OFF 
                 3 RD  DRIVING WAVEFORM 
               
               
                 OFF 
                 ON 
                 1 ST  DRIVING WAVEFORM 
               
               
                 OFF 
                 OFF 
                 1 ST  DRIVING WAVEFORM 
               
               
                   
               
             
          
         
       
     
     Table 5 shows measurement data on the ink droplet ejecting speed (m/s) obtained by performing printing in a continuous manner (with a one-dot rest halfway) with respect to the case where only the third driving waveform was used and the case (Example) where either the first or the third driving waveform is used according to the driving waveform conditions in the second embodiment shown in Table 4. 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 DRIVING 
                 DOT 
               
             
          
           
               
                 WAVEFORM 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
               
               
                   
               
               
                 3 RD  DRIVING 
                 6.0 
                 6.8 
                 7.1 
                 7.1 
                 7.1 
                 — 
                 6.9 
                 4.9 
               
               
                 WAVEFORM 
               
               
                 DRIVING 
                 8.0 
                 9.0 
                 9.5 
                 9.5 
                 7.9 
                 — 
                 8.0 
                 8.5 
               
               
                 WAVEFORM 
               
               
                 DETERMINED 
               
               
                 BY TABLE 4 
               
               
                   
               
             
          
         
       
     
     FIG. 4 shows the driving waveform applied to the example of Table 5, bottom row. For the fifth and eighth dots, the third driving waveform is used because there is ink ejection just before and no ink ejection just after the respective dots. For the other dots there is used the first driving waveform. A comparison of the Example with the use of only the third driving waveform shows that the ejection speed of the eighth dot not immediately followed by dot ejection exhibits no decrease, proving stable ejection. 
     In the second embodiment, the wave width of the jet pulse in the first driving waveform is set equal to an odd-multiple (say 1 T or 3 T) of time T required for one-way propagation of a pressure wave through the ink chamber, while in the third driving waveform the wave width of the jet pulse is shifted, for example, say 1.5 T, from an odd-multiple of the time T. If continuous dots are subjected to printing with a cycle of time T and if the jet pulse wave width of one dot is assumed to be an odd-multiple of time T, the pressure increases and the ejection speed also increases in relation to propagation of the pressure wave, while if the wave width is shifted from the odd-multiple, the pressure does not increase and the ejection speed decreases. Therefore, for a dot not immediately followed by dot ejection, there is adopted such a driving waveform as mentioned above, whereby it is possible to dampen the residual oscillation of the meniscus and stabilize the ejection speed. 
     
       
         
               
               
               
             
           
               
                 TABLE 6 
               
               
                   
               
               
                 PRECEEDING 
                 FOLLOWING 
                 PRINTING FREQUENCY FOR 
               
               
                 DOT 
                 DOT 
                 CURRENT DOT 
               
               
                   
               
             
             
               
                 ON 
                 ON 
                 10.0 KHZ 
               
               
                 ON 
                 OFF 
                 10.8 KHZ 
               
               
                 OFF 
                 ON 
                 10.8 KHZ 
               
               
                 OFF 
                 OFF 
                 10.8 KHZ 
               
               
                   
               
             
          
         
       
     
     Table 6 above shows driving wave conditions used in the third embodiment of the invention. If there is ejection of ink just before and just after one dot to be printed, ink ejection is performed at a frequency (say 10.0 kHz as will be described later) at which the ink droplet ejecting speed remains the same or increases, and in other cases ink ejection is performed at a frequency (say 10.8 kHz) at which the ink droplet ejecting speed decreases. The first driving waveform is used in both cases. Table 7 below shows measurement data on the ink droplet ejecting speed (m/s) obtained by performing printing continuously, with a one-dot rest halfway, with respect to the case where ink ejection is conducted at plural frequencies of 10.0 kHz or so and the case where ink ejection is conducted at frequencies according to the driving waveform conditions in the third embodiment shown in Table 6. 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 7 
               
             
             
               
                   
               
               
                 FREQUENCY 
                 DOT 
               
             
          
           
               
                 [kHz] 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
               
               
                   
               
             
          
           
               
                  9.2 
                 8.0 
                 9.5 
                 10.0 
                 10.0 
                 10.0 
                 — 
                 9.7 
                 6.8 
               
               
                  9.6 
                 8.0 
                 9.3 
                 9.8 
                 9.8 
                 9.8 
                 — 
                 9.5 
                 6.6 
               
               
                 10.0 
                 8.0 
                 9.0 
                 9.5 
                 9.5 
                 9.5 
                 — 
                 9.2 
                 6.5 
               
               
                 10.4 
                 8.0 
                 8.2 
                 9.0 
                 9.0 
                 9.0 
                 — 
                 8.6 
                 6.1 
               
               
                 10.8 
                 8.0 
                 7.0 
                 8.1 
                 8.1 
                 8.1 
                 — 
                 7.8 
                 5.6 
               
               
                 11.2 
                 8.0 
                 7.5 
                 8.7 
                 8.7 
                 8.7 
                 — 
                 8.6 
                 6.8 
               
               
                 11.6 
                 8.0 
                 8.2 
                 9.2 
                 9.2 
                 9.2 
                 — 
                 9.0 
                 6.4 
               
               
                 FREQUENCY 
                 8.0 
                 9.5 
                 9.6 
                 9.6 
                 9.5 
                 — 
                 8.8 
                 9.1 
               
               
                 DETERMINED 
               
               
                 BY TABLE 6 
               
               
                   
               
             
          
         
       
     
     As is seen from the measurement data of Table 7, when the frequency of 10.0 kHz is used, the ink droplet ejecting speed in the second dot ejection is higher than that in the first dot ejection, while at the frequency of 10.8 kHz the droplet ejecting speed in the second dot ejection is lower than that in the first dot ejection. The reason why the ejection speed varies is that the frequency of a driving signal in a certain dot ejection increases or decreases slightly in continuous printing relative to a predetermined printing frequency, resulting in the dot ejection timing being changed at the dot portion concerned, and that therefore the influence on the residual meniscus oscillation changes. Accordingly, the dots not preceded by or not followed by dot ejection, here the first and fifth dots, as well as the seventh and eighth dots, are ejected at a frequency (10.8 kHz) at which the ejection speed decreases, whereby the ejection timing is faster (by 7.4 μs) and dot ejection can be carried out at a time point where the meniscus oscillation is small, so that the ejection speed can be stabilized. The reason why the ejection timing becomes faster by 7.4 μs is because the pulse cycle is 100 μs at 10.0 kHz and is 92.6 μs at 10.8 kHz. The second dot is ejected substantially at 9.3 kHz. 
     FIGS. 5A-5D show driving waveforms (driving voltage constant) used in another embodiment of the invention. 
     
       
         
               
               
               
             
           
               
                 TABLE 8 
               
               
                   
               
               
                 PRECEEDING 
                 FOLLOWING 
                 PULSE WIDTH OF DRIVING 
               
               
                 DOT 
                 DOT 
                 WAVEFORM FOR CURRENT DOT 
               
               
                   
               
             
             
               
                 ON 
                 ON 
                 1 T 
               
               
                 ON 
                 OFF 
                 0.7 T 
               
               
                 OFF 
                 ON 
                 0.9 T 
               
               
                 OFF 
                 OFF 
                 0.8 T 
               
               
                   
               
             
          
         
       
     
     In the same figure, driving voltages of jet pulses for the dot concerned are shown under the conditions of FIGS. 5A to  5 D. If the jet pulse width T in FIG. 5A with dots present just before and just after the dot concerned is assumed to be a reference pulse width, the jet pulse width in FIG. 5B with a dot present just before and no dot present just after the dot concerned may be made shorter than that in FIG. 5A, the jet pulse width in FIG. 5C with no dot present just before and a dot present just after the dot concerned may be made longer than that in FIG.  5 B and shorter than T (FIG.  5 A), and the jet pulse width in FIG. 5D with no dot present just before and after the dot concerned may be as short as that in FIG.  5 B. The change of voltage waveform is not limited to the above examples. For example, the waveform of FIG. 5C may become equal to the waveform of FIG. 5A, or the waveforms of FIGS. 5B and 5D may be different, according to various conditions, including the shape of an ink flowing path. This is also the case with the following embodiments illustrated in FIGS. 6A-6D and  7 A- 7 D. 
     FIGS. 6A-6D show driving waveforms used in a still further embodiment of the invention, in which the voltage value of the jet pulse is changed according to whether a dot is present just before and/or just after a dot of concern. The conditions of use of the driving waveforms are shown in Table 9 below. 
     
       
         
               
               
               
             
           
               
                 TABLE 9 
               
               
                   
               
               
                 PRECEEDING 
                 FOLLOWING 
                 DRIVING VOLTAGE OF DRIVING 
               
               
                 DOT 
                 DOT 
                 WAVEFORM FOR CURRENT DOT 
               
               
                   
               
             
             
               
                 ON 
                 ON 
                 20 V 
               
               
                 ON 
                 OFF 
                 15 V 
               
               
                 OFF 
                 ON 
                 19 V 
               
               
                 OFF 
                 OFF 
                 18 V 
               
               
                   
               
             
          
         
       
     
     If a peak value of jet pulse in FIG. 6A with dot present before and just after a dot concerned is assumed to be a reference peak value, there may be adopted such peak values as illustrated in the same figure under the same conditions as above. 
     FIGS. 7A-7D shows driving waveforms used in a still further embodiment of the invention, in which inclinations at the leading and trailing edges of the jet pulse are changed according to whether a dot is present just before and/or just after a dot of concern. The conditions of use of the driving waveforms of FIGS. 7A-7D are shown in Table 10 below. 
     
       
         
               
               
               
               
             
           
               
                 TABLE 10 
               
               
                   
               
               
                   
                   
                 DELAY TIME 
                 DELAY TIME 
               
               
                   
                   
                 OF LEADING 
                 OF TRAILING 
               
               
                 PRECEEDING 
                 FOLLOWING 
                 EDGE FOR 
                 EDGE FOR 
               
               
                 DOT 
                 DOT 
                 CURRENT DOT 
                 CURRENT DOT 
               
               
                   
               
             
             
               
                 ON 
                 ON 
                 0 
                 0 
               
               
                 ON 
                 OFF 
                 0.25 T 
                 0 
               
               
                 OFF 
                 ON 
                 0.1 T 
                 0 
               
               
                 OFF 
                 OFF 
                 0 
                 0.25 T 
               
               
                   
               
             
          
         
       
     
     If such a jet pulse as in FIG. 7A with a dot present before and just after the dot of concern is made a reference pulse, there may be adopted such pulse waveforms as have the illustrated inclinations under the same conditions as above. 
     All of the above measurement data have been obtained taking note of the case where a continuous dot ejection is followed by a one-dot rest and subsequent dot ejection. FIGS. 8A-8C illustrate a continuous dot ejection, in which FIG. 8A shows a satisfactory state of a continuous dot printing and FIGS. 8B and 8C each show the state of a continuous dot printing performed at a frequency of, say, 10.8 kHz without any change of jet pulse. From FIGS. 8B and 8C it is seen that the droplet volume of the second dot is small, affording a thin print, or there occurs a drop-out of a dot, respectively. Such a problem is apt to occur when printing is performed at a high frequency. 
     In the invention, as described in the above embodiments, the driving waveform (voltage, pulse width, the number of pulse) is changed in accordance with whether a dot is present just before and/or just after the dot concerned, thereby affording the favorable printing result shown in FIG.  8 A. 
     Now, an example of a controller for implementing such various driving waveforms as discussed above will be described with reference to FIGS. 9 and 10. A controller  625  shown in FIG. 9 comprises a charging circuit  182 , a discharge circuit  184  and a pulse control circuit  186 . The piezoelectric material of the actuator wall  603  and electrodes  619 ,  621  are represented equivalently by a capacitor  191 . Numerals  191 A,  191 B denote terminals thereof. 
     Input terminals  181 ,  183  are for inputting pulse signals to adjust the voltage to be applied to the electrode  619  in each ink chamber, to E(V) or 0(V). The charging circuit  182  comprises resistors R 101 , R 102 , R 103 , R 104 , R 105  and transistors TR 101 , TR 102 . 
     When an ON signal (+5V) is applied to an input terminal  181 , the transistor TR 101  conducts through resistor R 101 , so that an electric current flows from a positive power source  187 , passes through resistor R 103 , and flows from the collector to the emitter of transistor TR 101 . Consequently, a divided voltage of the voltage applied to the resistors R 104 , R 105  which are connected to the positive power source  187  increases and so does the electric current flowing in the base of the transistor TR 102 , providing conduction between the emitter and the collector of the transistor TR 102 . A voltage of 20 (V) from the positive power source  187  is applied to the capacitor  191  and terminal  191 A via the collector and emitter of the transistor TR 102  and resistor R 120 . 
     The following description is now provided about the discharge circuit  184 . The discharge circuit  184  comprises resistors R 106 , R 107  and a transistor TR 103 . When an ON signal (+5V) is applied to an input terminal  183 , the transistor TR 103  turns conductive via resistor R 106  and the terminal  191 A on the resistor R 120  side of the capacitor  191  is grounded via resistor R 120 , so that the electric charge imposed on the actuator wall  603  of the ink chamber  613 , shown in FIGS. 12A,  12 B and  13 , is discharged. 
     Reference will now be made to the pulse control circuit  186  which generates pulse signals to be received by the input terminal  181  of the charging circuit  182  and the input terminal  183  of the discharge circuit  184 . Provided in the pulse control circuit  186  is a CPU  110  which performs various arithmetic operations. To the CPU  110  are connected a RAM  112  for the storage of printing data and various other data and a ROM  114  which stores sequence data for generating ON-OFF signals in accordance with control program and timing in the pulse control circuit  186 . In the ROM  114 , as shown in FIG. 10, there are provided an area  114 A for the storage of ink droplet ejection control program and an area  114 B for the storage of driving waveform data. Thus, sequence data of driving waveforms are stored in the area  114 B. 
     The CPU  110  is further connected to an I/O bus  116  for transmission and reception of various data, and to the I/O bus  116  are connected a printing data receiving circuit  118  and pulse generators  120 ,  122 . The output of the pulse generator  120  is connected to the input terminal  181  of the charging circuit  182 , while the output of the pulse generator  122  is connected to the input terminal  183  of the discharge circuit  184 . 
     The CPU  110  controls the pulse generators  120 ,  122  in accordance with the sequence data stored in the driving waveform data storing area  114 B of the ROM  114 . Therefore, by having various patterns of the foregoing timing stored beforehand in the driving waveform data storing area  114 B of the ROM  114 , it is possible to apply an appropriate driving pulse of an appropriate driving waveform to the actuator wall  603 . 
     The pulse generators  120 ,  122 , the charging circuit  182  and the discharge circuit  184  are provided in the same number as the number of nozzles used. Although the above description was directed to controlling one nozzle, the same control is applied also to the other nozzles. 
     FIG. 11 is a functional block diagram of the controller  625 , showing the flow of a printing instruction signal. In FIG. 11, a printing instruction is supplied from a computer, such as a personal computer (PC), or a word processor, to the pulse control circuit  186  (FIG. 9) where it is applied as a control signal to a driver circuit (the charging circuit  182  and the discharge circuit  184 ). That is, the printing instruction passes through the printing data receiving circuit  118  and is stored in RAM  112 . The CPU  110  using control routines and data stored in ROM  114  outputs signals to the pulse generators  120 ,  122  on the basis of the processed printing instruction. The output of the pulse generators  120 ,  122  controls the charging and discharge circuits  182 ,  184  to drive an actuator which is an ink channel  613  and represented by capacitor  191 . In this case, the controller  625  stores in RAM  112  beforehand where there has been ejection of ink before each dot and then changes the driving waveform in the manner described above in accordance with whether the answer is affirmative or negative and on the basis of the data read from the ROM. 
     Although the invention has been described above by way of embodiments thereof, the invention is not limited thereto. For example, a drive signal having only one jet pulse A has been shown above as a main drive signal, which signal, however, may comprise two jet pulses for example. Also the structure of the ink droplet ejecting apparatus  600  it is not limited to the structure adopted in the above embodiments. There may be adopted even one which is opposite in polarizing direction of the piezoelectric material. 
     Although in the above embodiments air chambers  615  are provided on both sides of each ink chamber  613 , ink chambers may be formed directly adjacent each other without forming an air chamber therebetween. Further, although a shear mode type actuator was used in the above embodiments, there may be adopted a structure wherein layers of a piezoelectric material may be laminated together and a pressure wave is generated by deformation in the laminated direction. No limitation is placed on the piezoelectric material. Any other material may be used insofar as it generates a pressure wave in each ink chamber.