Abstract:
A drive signal generator generates a drive signal including a first waveform component for jetting an ink drop to be a micro dot and a second waveform component for jetting an ink drop to be a middle dot. A pressure generating element varies the volume of a pressure generating chamber in accordance with the drive signal to jet an ink drop from a nozzle orifice. An ink drop to be a large dot is jetted from the nozzle orifice when the first waveform component and the second waveform component are consecutively applied to the pressure generating element. A time interval Tμm between an end point of the first waveform component and a start point of the second waveform component is set so as to substantially satisfy the following equation:          T                 μ                 m     =       n   2        Tc                             
     here, Tc denotes a natural period of the pressure generating chamber.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an ink jet recording apparatus comprising a recording head for jetting ink drops through nozzle orifices for recording and in particular to an ink jet recording apparatus intended for preventing a record image failure such as missing dots. 
     Some ink jet recording apparatuses such as ink jet printers, which will be hereinafter referred to as recording apparatuses, jet plural types of ink drops different in weight like micro dots, middle dots, and large dots through the same nozzle orifice to improve the image quality. 
     A recording head used with the recording apparatus comprises, for example, piezoelectric vibrator deformed by an applied waveform signal, pressure generating chambers expanded and contracted as the piezoelectric vibrator becomes deformed, and nozzle orifices communicating with the pressure generating chambers. 
     With such a recording apparatus, it is necessary to make a reasonable interval between waveforms. The time interval between the waveforms affects the image quality of a record image. 
     That is, if the time interval shifts from the optimum value, a record failure such as a heavy dot (a dot of a larger dot diameter than the original dot diameter) or a missing dot occurs. 
     To prevent the record failure, record patterns for causing the record failure to easily occur, for example, a one-line jetting pattern for jetting an ink drop of a large dot every eight nozzle orifices, a three-line jetting pattern for selecting three adjacent nozzle orifices every eight nozzle orifices and jetting an ink drop of a large dot through the selected nozzle orifices, and an alternate jetting pattern for jetting an ink drop of a large dot through the odd&#39;th and even&#39;th nozzle orifices alternately are actually recorded and evaluated with a recording head comprising 96 nozzle orifices per row, and the time interval between the micro dot and middle dot waveforms is determined based on the evaluation result. 
     However, although the time interval determined by evaluation with the record patterns is adopted, a record failure occurs in some cases. 
     For example, when the time interval between the micro dot and middle dot waveforms is determined based on the record patterns, if a pattern where the percentage of the nozzle orifices used for recording (recording density) is high, such as a one-line omission pattern, namely, a record pattern for setting a nozzle orifice for jetting no ink drop every eight nozzle orifices and jetting an ink drop of a large dot through the nozzle orifices except the setup nozzle orifice is recorded, the above-mentioned record failure occurs in some cases. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide an ink jet recording apparatus for making it possible to lessen record failures if a record pattern having a high recording density such as a one-line omission pattern is recorded, thereby improving recording stability. 
     In order to achieve the above object, there is provided an ink jet recording apparatus comprising: 
     a drive signal generator for generating a drive signal including a first waveform component for jetting an ink drop to be a micro dot and a second waveform component for jetting an ink drop to be a middle dot; 
     a pressure generating chamber communicated with a nozzle orifice; and 
     a pressure generating element for varying the volume of the pressure generating chamber in accordance with the drive signal to jet an ink drop from the nozzle orifice. 
     An ink drop to be a large dot is jetted from the nozzle orifice when the first waveform component and the second waveform component are consecutively applied to the pressure generating element. 
     A time interval Tμm between an end point of the first waveform component and a start point of the second waveform component is set so as to substantially satisfy the following equation:          T                 μ                 m     =       n   2        Tc                            
     here, Tc denotes a natural period of the pressure generating chamber, and n is an odd number of “3” or more. 
     Accordingly, propagation of vibration from the adjacent pressure generation chamber or the effect in the chamber caused by crosstalk of the piezoelectric element can be suppressed. 
     Thus, if a record pattern having a high recording density is recorded, recording failures can be lessened, thereby-improving the recording stability. 
     Preferably, n is set to 3. 
     Accordingly, the time interval Tμm becomes 1.5 times the characteristic vibration cycle of the pressure generating chamber Tc, so that the first waveform component and the second waveform component can be placed at a comparatively short time interval. Thus, to record a large dot, the remaining vibration after the first waveform component is applied can be used for jetting an ink drop of a middle dot, and the weight of the middle dot when a large dot is recorded can be made larger than the weight when the middle dot is recorded solely. Therefore, the ink drop weight range can be widened and recording can be executed in a wide dot diameter. 
     In order to attain the same effect, there is also provided an ink jet recording apparatus comprising: 
     a drive signal generator for generating a drive signal including a first waveform component for jetting an ink drop to be a micro dot and a second waveform component for jetting an ink drop to be a middle dot; 
     a pressure generating chamber communicated with a nozzle orifice; and 
     a pressure generating element for varying the volume of the pressure generating chamber in accordance with the drive signal to jet an ink drop from the nozzle orifice. 
     An ink drop to be a large dot is jetted from the nozzle orifice when the first waveform component and the second waveform component are consecutively applied to the pressure generating element. 
     When a frequency distribution of occurring a recording failure increases periodically as a function of a time interval Tμm between an end point of the first waveform component and a start point of the second waveform component, an actual Tμm is so as to avoid a value of which the frequency distribution increases. 
     In these apparatuses, the drive signal includes a third waveform component applied to the pressure generating element prior to the application of the first waveform component in order to finely vibrate a meniscus of the ink in the nozzle orifice. 
     Preferably, a time interval between an end point of the third waveform element and a start point of the first waveform element is set as a period in which the vibration caused by the third waveform element attenuates sufficiently. 
     Accordingly, jetting an ink drop by the first waveform component can be stabilized. 
     In these apparatuses, the pressure generating element is a piezoelectric vibrator formed into a combteeth shape in which electrodes and a piezoelectric body are laminated in a direction orthogonal to a deforming direction thereof. 
     Here, the time interval between the end point of the first waveform component and the start point of the second waveform component is not less than 3 Tc. 
     Accordingly, recording failures can be prevented more effectively. 
     In order to attain the same effect, there is also provided an ink jet recording apparatus comprising: 
     a drive signal generator for generating a drive signal including a first waveform component for jetting an ink drop to be a micro dot and a second waveform component for jetting an ink drop to be a middle dot; 
     a pressure generating chamber communicated with a nozzle orifice; and 
     a pressure generating element for varying the volume of the pressure generating chamber in accordance with the drive signal to jet an ink drop from the nozzle orifice, 
     wherein an ink drop to be a large dot is jetted from the nozzle orifice when the first waveform component and the second waveform component are consecutively applied to the pressure generating element, and 
     wherein a time interval between an end point of the first waveform component and a start point of the second waveform component is set so as to avoid ink ejection error. 
     Here, the time interval is represented as a function of a natural period of the pressure generating chamber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIG. 1 is a block diagram showing the configuration of an ink jet printer; 
     FIG. 2 is a perspective view showing the internal mechanism of the ink jet printer; 
     FIG. 3 is a sectional view showing the structure of a recording head; 
     FIG. 4 is a diagram showing an equivalent circuit for explaining characteristic vibration of ink in a cavity; 
     FIG. 5 is a block diagram showing the electric configuration in the recording head; 
     FIG. 6 is a diagram showing the relationship between drive signals and recording dots; 
     FIG. 7 is a diagram showing the drive signals; 
     FIG. 8 is a table showing evaluation of the record results when patterns are recorded while the time interval between the micro dot waveform and the middle dot waveform, Tμm, is changed; and 
     FIGS. 9A to  9 D are diagrams showing record patterns for evaluation, namely, a one-line jetting pattern, a three-line jetting pattern, an alternate jetting pattern, and a one-line omission pattern, respectively. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the accompanying drawings, there is shown an embodiment of the invention by taking an ink jet printer (simply, printer) of a representative ink jet recording apparatus as an example. As shown in FIG. 1, the printer is roughly made up of a printer controller  1  and a print engine  2 . 
     The micro dot, middle dot, and large dot throughout the specification represent dots having ink weight increased for forming the dots in the order of the micro dot, middle dot, and large dot. 
     The printer controller  1  comprises an external interface  3  (external I/F  3 ), RAM (random access memory)  4  for temporarily storing various pieces of data, ROM (read-only memory)  5  for storing a control program, etc., a controller  6  containing a CPU (central processing unit), etc., an oscillator  7  for generating a clock signal, a drive signal generator  9  for generating a drive signal (COM) supplied to a recording head  8 , and an internal interface  10  (internal I/F  10 ) for transmitting the drive signal and dot pattern data (bit map data) expanded based on print data and the like to the print engine  2 . 
     The external I/F  3  receives print data made up of character code, a graphic function, image data, etc., for example, from a host computer (not shown), etc. A busy signal (BUSY) and an acknowledge signal (ACK) are output through the external I/F  3  to the host computer, etc. 
     The RAM  4  functions as a reception buffer  4 A, an intermediate buffer  4 B, an output buffer  4 C, and work memory (not shown). The reception buffer  4 A temporarily stores the print data received through the external I/F  3 , the intermediate buffer  4 B stores intermediate code data provided by the controller  6 , and the output buffer  4 C stores dot pattern data. The dot pattern data is print data provided by decoding (translating) gradation data. 
     The ROM  5  stores font data, graphic functions, etc., in addition to the control program (control routine) for performing various types of data processing. 
     The controller  6  performs various types of control. In addition, it reads the print data in the reception buffer  4 A and stores the intermediate code data provided by converting the print data in the intermediate buffer  4 B. Also, the controller  6  analyzes the intermediate code data read from the intermediate buffer  4 B, references the font data, graphic function, etc., stored in the ROM  5 , and expands the intermediate code data into dot pattern data. After performing necessary decoration processing, the controller  6  stores the dot pattern data in the output buffer  4 C. 
     If one line of the dot pattern data that can be recorded by one main scanning of the recording head  8  is provided, it is output from the output buffer  4 C through the internal I/F  10  to the recording head  8  in sequence. When one line of the dot pattern data is output from the output buffer, the already expanded intermediate code data is erased from the intermediate buffer and the next intermediate code data is expanded. 
     The drive signal generator  9  generates a drive signal as mentioned above. In the embodiment, as shown in FIG. 6, the drive signal generator  9  generates a signal sequence containing a fine-vibration waveform  13 , a micro dot waveform  14 , and a middle dot waveform  15  with the micro dot waveform  14  followed by the middle dot waveform  15  and preceded by the fine-vibration waveform  13  as the drive signal. 
     The fine-vibration waveform  13  is a waveform for agitating ink in a nozzle orifice  16  (see FIG.  3 ), the micro dot waveform  14  is a waveform for jetting an ink drop of a micro dot (for example, an ink drop of about 3.3 ng) through the nozzle orifice  16 , and the middle dot waveform  15  is a waveform for jetting an ink drop of a middle dot (for example, an ink drop of about 10 ng) through the nozzle orifice  16 . 
     In the embodiment, as described later, the micro dot waveform  14  and the middle dot waveform  15  are applied to the recording head  8  (namely, a piezoelectric vibrator  35  described later) consecutively, whereby the weight of the ink drop jetted according to the middle dot waveform  15  is made larger than the weight of the ink drop jetted if the middle dot waveform  15  is applied solely, thereby jetting an ink drop of a large dot (ink drop of about 20 ng in total of ink drops of micro dot and middle dot). 
     The drive signal will be discussed later in detail. 
     The print engine  2  comprises a paper feed mechanism  19 , a carriage mechanism  20 , and the above-mentioned recording head  8 . 
     As shown in FIG. 2, the paper feed mechanism  19  is made up of a paper feed motor  21 , a paper feed roller  22 , etc., and feeds recording paper (a kind of print recording medium)  23  in sequence in association with the record operation of the recording head  8 . That is, the paper feed mechanism  19  moves the recording paper  23  in the recording paper feed direction, which is a subscanning direction. 
     The carriage mechanism  20  comprises a carriage  26  on which the recording head  8  and an ink cartridge  24  can be mounted, the carriage  26  being attached to a guide member  25  movably, a timing belt  29  placed on a drive pulley  27  and a driven pulley  28  and connected to the carriage  26 , and a pulse motor  30  for rotating the drive pulley  27 . 
     In the carriage mechanism  20 , the carriage  26  is reciprocated along the width direction of the recording paper  23  by the operation of the pulse motor  30 . That is, the recording head  8  mounted on the carriage  26  is moved along the main scanning direction. 
     Next, the recording head  8  will be discussed. As shown in FIG. 3, to form the recording head  8 , the piezoelectric vibrator  35  shaped like comb teeth is inserted into a chamber  34  of a case  33  shaped like a plastic box, for example, through one opening, a tip  35   a  shaped like comb teeth is made to face an opposite opening, a channel unit  36  is joined to the surface (bottom face) of the case  33  on the opening side, and the tip  35   a  is abutted against and fixed to a predetermined part of the channel unit  36 . 
     The piezoelectric vibrator  35  comprises a plate-like vibration plate comprising an alternating pattern of common internal electrodes  38  and discrete internal electrodes  39  deposited on each other with a piezoelectric body  37  in between, the vibration plate being cut like comb teeth corresponding to the dot formation density. A potential difference is given between the common internal electrode  38  and the discrete internal electrode  39 , whereby each piezoelectric vibrator  35  is expanded or contracted in the longitudinal direction of the vibrator orthogonal to the deposition direction. 
     The channel formation plate  42  is a plate member formed with a plurality of cavities (pressure generating chambers)  45  communicating with a plurality of the nozzle orifices  16  formed in the nozzle plate  43  and partitioned by a pressure generating chamber diaphragm and an elongated common ink reservoir  47  with which a plurality of ink supply ports  46  each communicating with at least one end of each cavity  45  communicate. In the embodiment, the common ink reservoir  47  is formed by etching a silicon wafer, the cavities  45  are formed matching the pitches of the nozzle orifices  16  along the longitudinal direction of the common ink reservoir  47 , and the groove-like ink supply ports  46  are formed between the cavities  45  and the common ink reservoir  47 . The ink supply port  46  is connected to one end of the cavity  45  and the nozzle orifice  16  is positioned in the proximity of the end part on the opposite side to the ink supply port  46 . The common ink reservoir  47  is a chamber for supplying ink stored in the ink cartridge  24  to the cavities  45 , and an ink supply tube  48  communicates almost at the center in the longitudinal direction. 
     The elastic plate  44  is deposited on an opposite face of the channel formation plate  44  positioned on the opposite side to the nozzle plate  43  and is of a double structure comprising a polymer film of PPS, etc., laminated as an elastic film  50  on a stainless plate  49 . The stainless plate  49  of the portion corresponding to the cavity  45  is etched to form an island portion  51  for abutting and fixing the piezoelectric vibrator  35 . 
     In the described recording head  8 , the piezoelectric vibrator  35  is expanded in the longitudinal direction of the vibrator, whereby the island portion  51  is pressed against the nozzle plate  43 , the elastic film  50  surrounding the island portion  51  becomes deformed, and the cavity  45  is contracted. If the piezoelectric vibrator  35  is contracted in the longitudinal direction of the vibrator, the volume of the cavity  45  is expanded due to elasticity of the elastic film  50 . Expansion and contraction of the volume of the cavity  45  are controlled, whereby an ink drop is jetted through the nozzle orifice  16 . 
     The characteristic vibration of ink in the cavity  45  in the described recording head  8  can be represented by an equivalent circuit shown in FIG.  4 . It is known that characteristic vibration cycle Tc of ink in the cavity  45  can be calculated according to the following expression:        Tc   =     2      π              Mn   ·   Ms       Mn   +   Ms          C                                
     where symbol M denotes inertance of the mass of a medium per unit length [Kg/m 4 ], symbol Mn denotes inertance in the nozzle orifice  16 , symbol Ms denotes inertance in the ink supply port  46 , and symbol C denotes compliance of the cavity  45  (pressure generating chamber) [m 5 /N]. 
     The characteristic vibration cycle Tc of ink in the cavity  45  calculated based on the expression is about 8 μsec in the embodiment. 
     Next, the electric configuration of the recording head  8  and control for jetting ink drops will be discussed. 
     As shown in FIG. 1, the recording head  8  comprises a shift register  54 , a latching circuit  55 , a level shifter  56 , a switching circuit  57 , the above-described piezoelectric vibrator  35 , etc. Further, as shown in FIG. 5, the shift register  54 , the latching circuit  55 , the level shifter  56 , the switching circuit  57 , and the above-described piezoelectric vibrator  35  consist of shift register elements  54 A to  54 N, latch elements  55 A to  55 N, level shifter elements  56 A to  56 N, switch elements  57 A to  57 N, and piezoelectric vibrators  35 A to  35 N, respectively, provided in a one-to-one correspondence with the nozzle orifices  16  of the recording head  8 . 
     To jet ink drops through the recording head  8 , first the controller  6  transmits print data (SI) in series starting at the most significant bit from the output buffer  4 C and sets the data in the shift register elements  54 A to  54 N in sequence in synchronization with a clock signal (CK) from the oscillator  7 . If the print data as much as all nozzle orifices  16  is set in the shift register elements  54 A to  54 N, the controller  6  outputs a latch signal (LAT) to the latching circuit  55 , namely, the latch elements  55 A to  55 N at a predetermined timing. According to the latch signal, the latch elements  55 A to  55 N latch the print data set in the shift register elements  54 A to  54 N. The latched print data is supplied to the level shifter  56 , a voltage amplifier, namely, the level shifter elements  56 A to  56 N. 
     For example, if the print data is “1,” each level shifter element  56 A- 56 N boosts the print data to a voltage value at which the switching circuit  57  can be driven, for example, several ten volts. The boosted print data is applied to the switching circuit  57 , namely, the switch element  57 A- 57 N, which then enters a connection state as the print data is applied. For example, the print data is “0,” the corresponding level shifter element  56 A- 56 N does not boost the print data. A drive signal (COM) from the drive signal generator  9  is applied to each switch element  57 A- 57 N and when the switch element  57 A- 57 N enters a connection state, the drive signal is supplied to the piezoelectric vibrator  35 A- 35 N connected to the switch element  57 A- 57 N. 
     If the drive signal is applied based on the most significant bit data, subsequently the controller  6  transmits the second most significant bit data in series and sets the data in the shift register element  54 A- 54 N. If the data is set in the shift register element  54 A- 54 N, the controller  6  applies a latch signal, thereby latching the set data, and supplies a drive signal to the piezoelectric vibrator  35 A- 35 N. After this, the same operation is repeated to the least significant bit while the print data is shifted to the low-order bit one bit at a time. 
     Thus, in the described printer, whether or not the drive signal is to be applied to the piezoelectric vibrator  35  can be controlled based on the print data. That is, the print data is set to “1,” whereby the drive signal can be applied to the piezoelectric vibrator  35 ; the print data is set to “0,” whereby applying the drive signal to the piezoelectric vibrator  35  can be stopped. 
     Therefore, the drive signal is divided in a time axis direction corresponding to an fine-vibration waveform  13 , a micro dot waveform  14 , and a middle dot waveform  15  and the bits of the print data are set corresponding to the waveform signals  13 ,  14 , and  15 , whereby the waveform signals  13 ,  14 , and  15  can be selectively applied to the piezoelectric vibrator  35 . 
     In the example shown in FIG. 6, the print data consists of three data bits D 1 , D 2 , and D 3 ; the print data bit D 1  is related to the fine-vibration waveform  13 , the print data bit D 2  is related to the micro dot waveform  14 , and the print data bit D 3  is related to the middle dot waveform  15 . The data bits D 1 , D 2 , and D 3  are changed appropriately, whereby plural types of ink drops different in weight can be jetted through the nozzle orifice  16 . 
     For example, if the print data is set as D 1 =1, D 2 =1, D 3 =0, the fine-vibration waveform  13  and the micro dot waveform  14  are applied to the piezoelectric vibrator  35  and an ink drop of a micro dot is jetted through the nozzle orifice  16 . If the print data is set as D 1 =1, D 2 =0, D 3 =1, the fine-vibration waveform  13  and the middle dot waveform  15  are applied to the piezoelectric vibrator  35  and an ink drop of a middle dot is jetted through the nozzle orifice  16 . Likewise, the print data is set as D 1 =1, D 2 =1, D 3 =1, whereby the fine-vibration waveform  13 , the micro dot waveform  14 , and the middle dot waveform  15  are applied to the piezoelectric vibrator  35  and an ink drop based on the micro dot waveform  14  and an ink drop based on the middle dot waveform  15  are jetted, forming a large dot. The print data is set as D 1 =1, D 2 =0, D 3 =0, whereby the fine-vibration waveform  13  is applied to the piezoelectric vibrator  35  and a meniscus, namely, a free surface of ink exposed at the nozzle orifice  16  is finely vibrated at the nozzle orifice  16 , whereby the ink at the nozzle orifice  16  is agitated and is prevented from increasing viscosity. 
     Next, the waveforms will be discussed in detail with reference to FIG.  7 . First, the fine-vibration waveform  13  will be discussed. 
     The fine-vibration waveform  13  is formed of a trapezoidal signal consisting of a first charge element  60  for increasing voltage at a constant gradient from GND level V 0  of reference voltage to fine-vibration drive potential V 1 , a first hold element  61  for holding the fine-vibration drive potential V 1  for a given time, and a first discharge element  62  for decreasing voltage at a constant gradient from the fine-vibration drive potential V 1  to the GND level V 0 . 
     In the embodiment, potential difference Vkp between the fine-vibration drive potential V 1  and the GND level V 0  is set based on potential difference VHμ in the micro dot waveform  14 , namely, is set to 40% of the potential difference VHμ. The application time of the first charge element  60  (charging time) is set to 7 μsec, the application time of the first hold element  61  (holding time) is set to 2 μsec, and the application time of the first discharge element  62  (discharging time) is set to 7 μsec. 
     The fine-vibration waveform  13  is applied to the piezoelectric vibrator  35 , which then is slightly contracted and expanded, and the cavity  45  is expanded and contracted only a little. The meniscus is finely vibrated with the expansion and contraction. 
     The micro dot waveform  14  is formed of a roughly trapezoidal signal consisting of a second charge element  64  for increasing voltage at a constant gradient from GND level V 0  to micro drive potential V 2 , a second hold element  65  for holding the micro drive potential V 2  for a given time, a second discharge element  66  for decreasing voltage at a constant gradient from the micro drive potential V 2  to first intermediate potential V 3 , a third hold element  67  for holding the first intermediate potential V 3  for a given time, and a third discharge element  68  for decreasing voltage at a constant gradient from the first intermediate potential V 3  to the GND level V 0 . 
     In the embodiment, separate voltage setting is executed for each recording head so that the jetted ink weight becomes 3.3 ng with respect to potential difference VHμ between the micro drive potential V 2  and the GND level V 0 . Potential difference Vcμ between the first intermediate potential V 3  and the GND level V 0  is set based on the potential difference VHμ, specifically, is set to 65% of the potential difference VHμ. The application time of the second charge element  64  is set to 8 μsec, the application time of the second hold element  65  is set to 1 μsec, and the application time of the second discharge element  66  is set to 1.5 μsec. The application time of the third hold element  67  is set to 1 μsec and the application time of the third discharge element  68  is set to 5.4 μsec. 
     When the micro dot waveform  14  is applied to the piezoelectric vibrator  35 , the piezoelectric vibrator  35  is contracted by application of the second charge element  64 , and the cavity  45  is expanded. The meniscus is pull into the inside of the cavity  45  with the expansion. An ink drop of an extremely minute amount (3.3 ng) is jetted by the force of the pulled-in meniscus attempting to return to the jet direction. 
     The middle dot waveform  15  is formed of a signal comprising an arrangement of large and small trapezoidal waveforms consisting of a third charge element  70  for increasing voltage at a constant gradient from GND level V 0  to middle drive potential V 4 , a fourth hold element  71  for holding the middle drive potential V 4  for a given time, a fourth discharge element  72  for decreasing voltage at a constant gradient from the middle drive potential V 4  to the GND level V 0 , a fifth hold element  73  for holding the GND level V 0  for a given time, a fourth charge element  74  for increasing voltage at a constant gradient from the GND level V 0  to second intermediate potential V 5 , a sixth hold element  75  for holding the second intermediate potential V 5  for a given time, and a fifth discharge element  76  for decreasing voltage at a constant gradient from the second intermediate potential VS to the GND level V 0 . 
     In the embodiment, setting is executed for each head so that the ink weight of a large dot jetted by applying the micro dot waveform  14  and the middle dot waveform  15  consecutively becomes 20 ng with respect to potential difference VHM between the middle drive potential V 4  and the GND level V 0 . Potential difference Vsp between the second intermediate potential V 5  and the GND level V 0  is set based on the potential difference VHM, specifically, is set to 20% of the potential difference VHM. 
     The application time of the third charge element  70  is set to 7.5 μsec, the application time of the fourth hold element  71  is set to 2 μsec, and the application time of the fourth discharge element  72  is set to 4 μsec. The application time of the fifth hold element  73  is set to 4 μsec, the application time of the fourth discharge element  74  is set to 4 μsec, the application time of the sixth hold element  75  is set to 2 μsec, and the application time of the fifth discharge element  76  is set to 4 μsec 
     When the middle dot waveform  15  is applied to the piezoelectric vibrator  35 , the piezoelectric vibrator  35  is contracted by application of the third charge element  70 , and the cavity  45  is expanded. The cavity  45  expanded as the piezoelectric vibrator  35  is expanded by application of the fourth discharge element  72  is contracted and an ink drop is jetted with the contraction of the cavity  45 . The fourth charge element  74 , the sixth hold element  75 , and the fifth discharge element  76  are applied, whereby opposite-phase vibration is given to the meniscus for suppressing vibration of the meniscus. 
     Next, the placement intervals between the fine-vibration waveform  13  and the micro dot waveform  14  and between the micro dot waveform  14  and the middle dot waveform  15  will be discussed. 
     First, the time interval between the micro dot waveform  14  and the middle dot waveform  15  will be discussed. In the embodiment, the time interval between the micro dot waveform  14  and the middle dot waveform  15 , namely, the time interval between the instant at which application of the third discharge element  68  ends and the instant at which application of the third charge element  70  starts is set to 11.5 μsec, about 1.5 times of the characteristic vibration cycle Tc of the cavity (in the embodiment, 8 μsec). 
     The reason why the interval between both the waveforms is thus determined is as follows: 
     FIG. 8 is a table to show evaluation of the record results when a one-line jetting pattern, a three-line jetting pattern, an alternate jetting pattern, and a one-line omission pattern are recorded with large dots while the time interval between the micro dot waveform  14  and the middle dot waveform  15 , Tμm, is changed. 
     The one-line jetting pattern is a record pattern for jetting an ink drop of a large dot every eight nozzle orifices  16  as shown in FIG. 9A; the nozzle orifice  16  for jetting an ink drop is changed every given time. The three-line jetting pattern is a record pattern for selecting three adjacent nozzle orifices  16  every eight nozzle orifices and jetting an ink drop of a large dot through the selected nozzle orifices  16  as shown in FIG. 9B; the nozzle orifices  16  for jetting an ink drop are changed every given time. The alternate jetting pattern is a record pattern for jetting an ink drop of a large dot through the odd&#39;th nozzle orifice  16  and the even&#39;th nozzle orifice  16  alternately every given time as shown in FIG.  9 C. The one-line omission pattern is a record pattern for setting a nozzle orifice  16  for jetting no ink drop every eight nozzle orifices  16  and jetting an ink drop of a large dot through the nozzle orifices  16  except the setup nozzle orifice  16  as shown in FIG.  9 D. Also in the one-line omission pattern, the nozzle orifice  16  for jetting no ink drop is changed every given time. 
     In other words, the one-line jetting pattern is a record pattern having the lowest percentage of the nozzle orifices  16  for recording (the percentage will be hereinafter referred to as recording density) among the four patterns, and the three-line jetting pattern is a record pattern having the second lowest recording density. The alternate jetting pattern is a record pattern having the second highest recording density among the four patterns, and the one-line omission pattern is a record pattern having the highest recording density. 
     In FIG. 8, the evaluation results are represented by four symbols of “∘,”“Δ,”“X,” and “XX.” The symbol “∘” means that good recording can be performed with no failure, the symbol “Δ” means that some record failure, for example, a thick dot with wide jet pattern as compared with the normal pattern or a missing dot with jet pattern unprinted occurs, the symbol “X” means that a comparatively large number of record failures occur, and the symbol “XX” means that an extremely large number of record failures occur. Only the pattern whose evaluation result is “∘” can be put to practical use. 
     In FIG. 8, MPBF denotes the average number of missing dot pages at the image printing time. The average number of missing dot pages means the average value from one page where a missing dot occurs to another page where another missing dot occurs when an evaluation image is recorded. For example, if the MPBF is “100,” it means that a missing dot occurs about every 100 pages. 
     As shown in FIG. 8, in the one-line jetting pattern described above, when the time interval Tμm is 6.5 μsec, some record failure occurs, but good recording can be performed generally with no record failure. 
     In the three-line jetting pattern, when the time interval Tμm is in the range of 6.5 to 10.5 μsec, some record failure occurs, but when the time interval Tμm becomes 11.5 μsec or more, good recording can be performed with no record failure. 
     In the alternate jetting pattern, when the time interval Tμm is 6.5 μsec and 8.5 μsec, a comparatively large number of record failures occur and when the time interval Tμm is 9.5 μsec, 10.5 μsec, and 18.5 μsec, some record failure occurs. When the time interval Tμm is in the range of 11.5 to 16.5 μsec, good recording can be performed with no record failure. 
     In the one-line mission pattern, when the time interval Tμm is in the range of 10.5 to 12.5 μsec and 18.5 μsec and 20.5 μsec, good recording can be performed with no record failure. When the time interval Tμm is 6.5 μsec and 16.5 μsec, a comparatively large number of record failures occur and when the time interval Tμm is set to 14.5 μsec, an extremely large number of record failures occur. When the time interval Tμm is 8.5 μsec, 9.5 μsec, 13.5 μsec, and 22.5 μsec, some record failure occurs. 
     Thus, as the recording density becomes higher, the time intervals that can be used for good recording are limited. That is, in the one-line jetting pattern providing the lowest recording density, good recording can be performed if the time interval Tμm is 8.5 μsec or more; however, in the one-line omission pattern providing the highest recording density, good recording can be performed only in the limited ranges of the time interval Tμm from 10.5 μsec to 12.5 μsec and from 18.5 μsec to 20.5 μsec. 
     When the recording density becomes high, periodicity appears in the time interval Tμm where a record failure occurs. That is, in the one-line omission pattern, when the time interval Tμm is 6.5 μsec, 14.5 μsec, 16.5 μsec, 22.5 μsec, a record failure occurs; it appears every 8 μsec where the time interval Tμm where a record failure occurs is almost equal to the cavity vibration cycle Tc. When the time interval Tμm is 22.5 μsec, the evaluation is “Δ” and only some record failure occurs, but considering that the elapsed time since the micro dot waveform application time is comparatively long and that meanwhile the effect of vibration caused by the micro dot waveform is lessened, it is considered that a record failure occurs. 
     The record failure in the one-line omission pattern occurs when recording is executed through the nozzle orifice  16  just after the nozzle orifice  16  passes through a no-recording state. From this fact, it is considered that the record failure in the one-line omission pattern occurs receiving the effect of ink jetting through the adjacent nozzle orifices  16 . 
     That is, vibration from the neighboring cavities propagates through the common ink reservoir  47  to the cavity (pressure generating chamber)  45  communicating with the nozzle orifice  16  not involved in recording or the piezoelectric vibrator  35  joined to the cavity  45  is vibrated because of crosstalk from the neighboring piezoelectric vibrators  35 , so that needless vibration is applied. 
     Vibration of the meniscus slightly vibrated by the fine-vibration waveform  13  is amplified depending on the vibration application timing and resultantly the amplitude of the meniscus becomes excessively large and a bubble is taken in through the nozzle orifice  16 . As the bubble is taken in, the compliance in the cavity  45  rises and first an ink drop having a large dot diameter is jetted, producing a heavy dot. As a large number of bubbles are further taken in, a missing dot occurs. 
     The timing at which such a record failure occurs (failure time) appears every 8 μsec almost equal to the cavity vibration cycle Tc, namely, when the time interval Tμm is 6.5 μsec, 14.5 μsec, or 22.5 μsec. Thus, a record failure can be prevented by setting the time interval Tμm to an almost intermediate point of the failure times. The possible reason why a record failure can be prevented is that if the time interval Tμm is set to an almost intermediate point of the failure times, propagation of vibration from the adjacent cavities or the effect of vibration in the cavity  45  caused by crosstalk of the piezoelectric vibrator  35  can be suppressed efficiently. Setting the time interval Tμm can also be represented as in the following expression:          T                 μ                 m     =       n   2        Tc                            
     where Tμm [μsec] is the time interval between the micro dot waveform  14  and the middle dot waveform  15 , Tc [μsec] is the characteristic vibration cycle of the cavity, and n is an odd number of “3” or more. 
     In the embodiment, n is set to “3” and the time interval Tμm is set to about 1.5 times the cavity vibration cycle Tc. Since the cavity vibration cycle Tc is about 8 μsec, 1.5 times the cycle becomes 12 μsec, and 11.5 μsec close to 12 μsec is set as the time interval Tμm. If the time interval Tμm is set to 11.5 μsec, MPBF (average number of missing dot pages) becomes 150 (pages); it can be confirmed that record failures can be lessened remarkably as compared with the case where the time interval Tμm is 14.5 μsec at which the MPBF value is the lowest (MPBF=10). 
     The reason why n is set to “3” is that the micro dot waveform  14  and the middle dot waveform  15  should be applied consecutively to the piezoelectric vibrator  35  in order to jet a large ink drop. 
     That is, when a large dot is jetted, the remaining vibration after the micro dot waveform  14  is applied is used, namely, the weight of an ink drop of a middle dot is made larger than the original weight 10 ng using the remaining vibration. If the time interval Tμm is too long, the effect of the remaining vibration is lessened and if the micro dot waveform  14  and the middle dot waveform  15  are applied, an ink drop of weight 13.3 ng of-the total value of the original weights results and it becomes hard to widen the weight range of an ink drop that can be jetted through the nozzle orifice  16 . Since the record cycle is also prolonged, the record speed of the printer is also lowered. 
     Thus, if the time interval Tμm is set to about 1.5 times the cavity vibration cycle Tc, record failures can be prevented, the ink drop weight range can be widened, and the record speed of the printer can be increased. 
     Next, the time interval between the fine-vibration waveform  13  and the micro dot waveform  14  will be discussed. In the embodiment, the time interval between the fine-vibration waveform  13  and the micro dot waveform  14  and the middle dot waveform  15 , namely, the time interval between the instant at which application of the first discharge element  62  ends and the instant at which application of the second charge element  64  starts is set to about 50 μsec. 
     It is considered that similar relation to that of the time interval Tμm between the micro dot waveform  14  and the middle dot waveform  15  also occurs for the time interval between the fine-vibration waveform  13  and the micro dot waveform  14 . That is, it is considered that the timing at which a record failure occurs (failure time) appears every 8 μsec almost equal to the cavity vibration cycle Tc. 
     Then, in the embodiment, the time interval between the fine-vibration waveform  13  and the micro dot waveform  14  is set long to such an extent that the effect of vibration caused by the fine-vibration waveform  13  is not received. Specifically, the time interval is set to about 50 μsec, as shown in FIG.  7 . 
     Concerning the time interval, as mentioned above, it is considered that the timing at which a record failure occurs appears every 8 μsec almost equal to the cavity vibration cycle Tc, specifically at the time interval 6.5 μsec, 14.5 μsec, 16.5 μsec, 22.5 μsec as in the case in FIG.  8 . In the case in FIG. 8, the evaluation result at the time interval 22.5 μsec is “Δ” indicating such an extent that some record failure occurs; the possible reason is that vibration is attenuated as the time interval is prolonged. 
     Thus, if the time interval is set to 30.5 μsec (22.5 μsec plus characteristic vibration cycle of the cavity Tc), it is considered that the effect of vibration caused by the fine-vibration waveform  13  can be almost ignored. Therefore, it can be considered that the time interval between the fine-vibration waveform  13  and the micro dot waveform  14  at which vibration caused by the fine-vibration waveform  13  does not affect the micro dot waveform  14  (micro dot) is approximately 3 Tc or more. In other words, the time interval between the fine-vibration waveform  13  and the micro dot waveform  14  is set to approximately 3 Tc or more, whereby the effect of vibration caused by the fine-vibration waveform  13  can be eliminated and jetting an ink drop by the micro dot waveform  14  can be stabilized. 
     In the embodiment, the pressure generation element is formed of the comb-teeth-like vibrator  35  in so-called d 31  vertical vibration mode comprising the piezoelectric body  37  and the internal electrodes  38  and  39  deposited in the direction orthogonal to the cavity pressing direction as an example; however, the invention can also be applied to a piezoelectric vibrator in so-called d 33  vertical vibration mode comprising the piezoelectric body  37  and the internal electrodes  38  and  39  deposited in the cavity pressing direction and a pressure generation element using a deflection vibration mode. 
     The comb-teeth-like vibrator  35  in the vertical vibration mode, which is joined to the adjacent vibrator in the part on the base end side in one piece, is easily affected by crosstalk from the adjacent vibrator and a record failure easily occurs. Thus, the invention is applied to the comb-teeth-like vibrator  35  in the vertical vibration mode, whereby a record failure can be prevented more effectively. 
     The words “microdot”, “middle dot” and “large dot” are used for describing the dot diameter in the above discussion, however, the size of ink drops ejected from the ink recording head according to the present invention is relative and thus the size of resultant ink dot is not limited to a specific size. Namely, the present invention is not limited to the three-size dot modulation, i.e., the microdot, the middle dot and the large dot, and may be applied to any ink jet recording apparatus wherein a drive signal waveform is controlled to modulate a dot size. Especially, remarkable effect can be obtained in cases where the present invention is applied to an ink jet recording apparatus wherein the dot-size modulation is realized by utilizing residual vibration of meniscus of ink in nozzle orifices.