Abstract:
A jetting head is provided with a nozzle orifice, a pressure chamber communicated with the nozzle orifice, and a piezoelectric element which is deformable to cause pressure fluctuation to liquid contained in the pressure chamber. A drive signal generator simultaneously generates a plurality of drive signals, each provided with waveform elements including at least one drive pulse in every unit jetting cycle. The drive pulse deforms the piezoelectric element to cause such pressure fluctuation as to eject a liquid droplet from the nozzle orifice. A switcher selectively supplies at least one of the waveform elements included in one of the drive signals to the piezoelectric element. A switch controller controls a selective supply operation of the switcher in accordance with amount data which indicates an amount of the liquid droplet to be ejected. A time period in which the drive pulse is generated in one of the drive signal and that in another one of the drive signals overlap at least partly.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a liquid jetting apparatus such as an ink jet recording apparatus, a display manufacturing apparatus, an electrode forming apparatus, a biochip manufacturing apparatus or the like, which can control ejection of liquid droplets from nozzle orifices by controlling supply of drive pulses to pressure generating elements in accordance with a jetting amount, as well as to a method for driving such an apparatus. 
     Various kinds of the liquid jetting apparatus have hitherto been known. For example, there have been known an image forming apparatus which records information on recording paper by jetting ink droplets, an electrode forming apparatus which forms an electrode on a board by jetting liquid-state electrode material, a biochip manufacturing apparatus which manufactures a biochip by jetting biological specimen, and a micropipette for jetting a predetermined amount of sample into a vessel. 
     A liquid jetting apparatus capable of changing the amount of liquid to be ejected from nozzle orifices with a view toward pursuing both higher-speed jetting operation and higher jetting amount accuracy has hitherto been known. 
     For example, an ink jet recording apparatus which is one kind of the liquid jetting apparatus has, for example, a recording head which has nozzle orifices communicating with a pressure chamber, and pressure generating elements capable of causing a change in the pressure of the ink stored in the pressure chamber; and a drive signal generator capable of producing a drive signal to be supplied to the pressure generating elements. The drive signal is a single signal formed by connecting a plurality of drive pulses into a string of pulses within one recording cycle. A required portion of the drive signal is supplied to the pressure generating element in accordance with recording data (i.e., gradation data), thereby changing the amount of ink to be ejected from a nozzle orifice. Such a configuration is disclosed in Japanese Patent Publication No. 10-81012A (see Page 9 and FIG. 9). 
     However, a related-art configuration in which a required portion of a single drive signal is supplied to pressure generating elements encounters difficulty in causing a jetting head (recording head) to sufficiently offer original performance thereof. More specifically, since a plurality of drive pulses are included in one jetting (recording cycle), there is no alternative but to actuate a jetting head (i.e., a pressure generating element) at a frequency lower than the maximum frequency at which the jetting head can be actuated. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a liquid jetting apparatus which can be constructed so as to be able to actuate a jetting head at a higher frequency, along with a method for driving such an apparatus. 
     In order to achieve the above object, according to the invention, there is provided a liquid jetting apparatus, comprising: 
     a jetting head, provided with a nozzle orifice, a pressure chamber communicated with the nozzle orifice, and a piezoelectric element which is deformable to cause pressure fluctuation to liquid contained in the pressure chamber; 
     a drive signal generator, which simultaneously generates a plurality of drive signals, each provided with waveform elements including at least one drive pulse in every unit recording cycle, the drive pulse deforming the piezoelectric element to cause such pressure fluctuation as to eject a liquid droplet from the nozzle orifice; 
     a switcher, which selectively supplies at least one of the waveform elements included in one of the drive signals to the piezoelectric element; and 
     a switch controller, which controls a selective supply operation of the switcher in accordance with amount data which indicates an amount of the liquid droplet to be ejected, 
     wherein a time period in which the drive pulse is generated in one of the drive signal and that in another one of the drive signals overlap at least partly. 
     In such a configuration, the recording cycle can be shortened as compared with that achieved when a plurality of drive pulses are included in one drive signal in the form of a single pulse train. As a result, a jetting head can be actuated at a higher frequency. 
     Preferably, the waveform elements in each drive signal include a drive waveform element which constitutes the drive pulse, and a constant-potential waveform element which maintains a potential of the drive signal at a leading-end potential and a trailing-end potential thereof. 
     Here, the switch controller may control the switcher such that the drive waveform element in one of the drive signals and the drive waveform element in another one of the drive signals are supplied to the piezoelectric element in the unit jetting cycle. 
     Alternatively, the switch controller may control the switcher such that the drive waveform element in one of the drive signals and the constant-potential waveform element in another one of the drive signals are supplied to the piezoelectric element in the unit jetting cycle. 
     Alternatively, the switch controller controls the switcher such that the constant-potential waveform element in at least one of the drive signals is supplied to the piezoelectric element in the unit jetting cycle. 
     Since the waveform elements of respective drive signals are supplied in combination to the pressure generating element within a jetting cycle by switch controller, a jetting head can be actuated in a new pattern which is not originally contained in respective drive signals. As a result, complicated control can be realized while the drive frequency of the jetting head is enhanced. 
     When the constant-potential waveform element is used, the piezoelectric element can be maintained at a constant potential. As a result, there can be prevented a drop in the potential of the piezoelectric element, which would otherwise be caused by an electric discharge. Thus, there can be prevented occurrence of failures, such as erroneous ejection of a liquid droplet. 
     Preferably, the switcher includes a plurality of switches interposed between the drive signal generator and the piezoelectric element such that each of the switches is associated with one of the drive signals. 
     Here, it is preferable that the switch controller selectively activates one of the switches such that one of the drive signals associated with an activated switch is supplied to the piezoelectric element. 
     Preferably, the switcher includes a plurality of input contacts each associated with one of the drive signals and an output contact electrically connected to the piezoelectric element. Here, the switch controller selectively connects one of the input contacts and the output contact such that one of the drive signals associated with a selected input contact is supplied to the piezoelectric element. In this case, the switching control can be simplified. 
     Preferably, the drive signals include: a first drive signal, in which at least two first drive pulses each for ejecting a first amount of liquid droplet are arranged at a predetermined interval; and a second drive signal, in which at least one second drive pulse for ejecting a second amount of liquid droplet is generated at a timing between timings at which the first drive pulses are generated. Here, the predetermined interval is determined such that the first drive pulses are still arranged at the predetermined interval even when the first drive signal is successively selected in adjacent unit jetting cycles. 
     In such a configuration, there can be prevented occurrence of an offset, which would otherwise arise in an interval between ejection of liquid droplets, thereby enabling an improvement in jetting amount accuracy. 
     Here, it is preferable that the first drive pulse including: an expanding element, in which a potential of the first drive signal is varied from a reference potential to a first potential at a constant gradient, so that a volume of the pressure chamber is expanded from a reference volume to a first volume; and first holding element, which maintains the volume of the pressure chamber at the first volume. On the other hand, the second drive pulse including: a second holding element, in which a potential of the second drive signal is maintained at the first potential to maintain the volume of the pressure chamber at the first volume; and a contracting element, in which the potential of the second drive signal is varied from the first potential to the reference potential at a constant gradient, so that the volume of the pressure chamber is contracted from the first volume to the reference volume. Here, the switch controller controls the switcher so as to supply the expanding element, the first holding element, the second holding element and the contracting element, to cause pressure fluctuation such an extent that no liquid droplet is ejected, when the amount data indicates no jetting is to be performed. 
     Further, it is preferable that; each of the first drive pulses is interposed between first constant-potential waveform elements which maintain a potential of the first drive signal at a reference potential so that an initial end and a termination end of each first drive pulse are set to the reference potential; the second drive pulse is interposed between second constant-potential waveform elements which maintain a potential of the second drive signal at the reference potential so that an initial end and a termination end of the second drive pulse are set to the reference potential; and the switch controller controls the switcher so as to supply one of the first drive pulses and one of the second constant-potential waveform element, so that a potential of the piezoelectric vibrator is set to the reference potential while the first drive pulse is not supplied, when the amount data indicates the first amount of liquid droplet to be ejected. 
     According to the invention, there is also provided a method of driving a liquid jetting apparatus which comprises a jetting head, provided with a nozzle orifice, a pressure chamber communicated with the nozzle orifice, and a piezoelectric element which is deformable to cause pressure fluctuation to liquid contained in the pressure chamber, the method comprising the steps of: 
     generating simultaneously a plurality of drive signals, each provided with waveform elements including at least one drive pulse in every unit jetting cycle, the drive pulse deforming the piezoelectric element to cause such pressure fluctuation as to eject a liquid droplet from the nozzle orifice; 
     providing a switcher which selectively supplies at least one of the waveform elements included in one of the drive signals to the piezoelectric element; and 
     controlling a selective supply operation of the switcher in accordance with amount data which indicates an amount of the liquid droplet to be ejected, 
     wherein a time period in which the drive pulse is generated in one of the drive signal and that in another one of the drive signals overlap at least partly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and advantages of the present invention will become more apparent by describing in detail preferred exemplary embodiments thereof with reference to the accompanying drawings, wherein: 
     FIG. 1 is a functional block diagram showing an ink jet recording apparatus according to a first embodiment of the invention; 
     FIG. 2 is a cross-sectional view showing the configuration of a recording head of longitudinal vibration mode; 
     FIG. 3 is a diagram for describing a drive signal to be generated by a drive signal generator and supply control of the drive signal; 
     FIG. 4 is a diagram for describing control of supply of the drive signal during non-recording operation; 
     FIG. 5 is a diagram for describing control of supply of a drive signal at the time of small dot recording operation; 
     FIG. 6 is a diagram for describing control of supply of a drive signal at the time of middle dot recording operation; 
     FIG. 7 is a diagram for describing control of supply of a drive signal at the time of large dot recording operation; and 
     FIG. 8 is a block diagram showing a switcher an ink jet recording apparatus according to a second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the invention will be described hereinbelow by reference to the accompanying drawings. The following explanations are for an ink jet recording apparatus which one kind of a liquid jetting apparatus. The ink jet recording apparatus jets ink droplets which is one kind of liquid droplets of the invention. 
     FIG. 1 shows a printer serving as an ink jet recording apparatus according to a first embodiment of the invention. The printer comprises a printer controller  1  and a print engine  2 . The printer controller  1  has an external interface  3  for receiving print data or the like from an unillustrated host computer or the like; a RAM  4  for storing a variety of data sets; a ROM  5  for storing routines for use in processing a variety of data sets; a controller  6  provided as a CPU or the like; an oscillator  7  for generating a clock (CK) signal; a drive signal generator  9  for generating drive signals (COM 1 , COM 2 ) to be supplied to a recording head; and an internal interface  10  for transmitting recording data, drive signals, or the like to the print engine  2 . 
     The external interface  3  receives, from a host computer, print data consisting of one type of data or a plurality of types of data selected from, e.g., a character code, a graphic function, and image data. The external interface  3  outputs a busy (BUSY) signal and an acknowledgement (ACK) signal to the host computer. 
     The RAM  4  is utilized as a receiving buffer, an intermediate buffer, an output buffer, work memory (not shown), and the like. Print data that have been output from the host computer and received by the external interface  3  are temporarily stored in a receiving buffer. Intermediate code data that have been converted into an intermediate code by the controller  6  are stored in an intermediate buffer. Data to be recorded (hereinafter called “recording data”) are expanded into an output buffer. The ROM 5  stores various control routines, font data, graphics functions, and various procedures. 
     The drive signal generator  9  comprises a first drive signal generating section  9 A capable of generating a first drive signal COM 1  and a second drive signal generating section  9 B capable of generating a second drive signal COM 2 . As shown in FIG. 3, the first drive signal COM 1  is a signal train which includes two middle dot drive pulses DP 1 , DP 2  within one recording cycle T and is generated at every recording cycle T. The second drive signal COM 2  is a signal train which includes a small-dot drive pulse DP 3  within one recording cycle T and is generated at every recording cycle T. The second rive signal COM 2  is repeatedly generated at every recording cycle T. The drive signals COM 1 , COM 2  will be described in detail later. 
     The controller  6  controls generation of a signal to be sent to the drive signal generator  9  and converts the print data output from the host computer into recording data. At the time of conversion of print data into recording data, the controller  6  reads print data from the inside of the receiving buffer, converts the thus-read print data into an intermediate code, and stores intermediate code data into an intermediate buffer. Next, the controller  6  analyzes the intermediate code data read from the intermediate buffer and converts the intermediate code data into recording data on a per-dot basis by reference to the font data and the graphics functions stored in the ROM 5 . 
     The recording data of the embodiment is constituted such that one bit is formed from two-bit gradation data. The gradation data comprise gradation data [ 00 ] indicating a non-recording state (meniscus vibrating operation); gradation data [ 01 ] indicating recording to be performed through use of small dots; gradation data [ 10 ] indicating recording to be performed through use of middle dots; and gradation data [ 11 ] indicating recording to be performed through use of large dots. Accordingly, such a data structure enables recording of each dot in four levels of tone. 
     The controller  6  constitutes a part of a timing signal generator and supplies a latch (LAT) signal and channel (CH-A, CH-B) signals to the recording head  8  by way of the internal interface  10 . Latch pulses included in the latch signal and channel pulses included in the channel signals define start timings of supply of a plurality of waveform elements constituting the drive signals COM 1 , COM 2  and supply of adjustment elements (PS 1  to PS 6 , and P 0 , P 20 ). 
     Specifically, as shown in FIG. 3, a latch pulse LAT 1  defines a start timing of supply of an adjustment element P 0  to be generated during a charging period t 10  and a start timing of supply of an adjustment element P 20  to be generated during a charging period t 20 . 
     A first channel pulse CH 11  appearing in a first channel signal CH-A defines a start timing of supply of a first waveform section PS 1  to be generated during a period t 11  of the first drive signal COM 1 . A second channel pulse CH 12  defines a start timing of supply of a second waveform section PS 2  to be generated during a period t 12 . A third channel pulse CH 13  defines a start timing of supply of a third waveform section PS 3  to be generated during a period t 13 . 
     Similarly, a first channel pulse CH 21  appearing in a second channel signal CH-B defines a start timing of supply of a fourth waveform section PS 4  to be generated during a period t 21  of the second drive signal COM 2 . A second channel pulse CH 22  defines a start timing of supply of a fifth waveform section PS 5  to be generated during a period t 22 . A third channel pulse CH 23  defines a start timing of supply of a sixth waveform section PS 6  to be generated during a period t 23 . 
     The print engine  2  will now be described. As shown in FIG. 1, the print engine  2  has a recording head  8 , a carriage mechanism  11 , and a paper feeding mechanism  12 . 
     The carriage mechanism  11  is constituted of a carriage having the recording head  8  mounted thereon, and a drive motor (e.g., a DC motor) which causes to the carriage to travel by way of a timing belt or the like. The carriage mechanism  11  moves the recording head  8  in the primary scanning direction. The paper feeding mechanism  12  is constituted of a paper feeding motor and a paper feeding roller and like rollers. The paper feeding mechanism  12  performs secondary scanning by sequentially feeding recording paper (i.e., a kind of print recording medium). 
     The recording head  8  will now be described in detail. First, the structure of the recording head  8  will be described by reference to FIG.  2 . The illustrated recording head  8  has a vibrator unit  24  into which a plurality of piezoelectric vibrators  21  a fixation plate  22 , and a flexible cable  23  are assembled as a unit; a case  25  capable of housing the vibrator unit  24 ; and a channel unit  26  joined to a leading end face of the case  25 . 
     The case  25  is a block-shaped member which is formed from synthetic resin and defines a housing space  27  whose front and rear ends are open. The vibrator unit  24  is housed and fixed in the housing space  27 . 
     The piezoelectric vibrator  21  is a kind of pressure generating element and formed into a longitudinally-elongated comb shape. The piezoelectric vibrator  21  is a piezoelectric vibrator of lamination type formed by laminating piezoelectric material layers and internal electrodes one on top of the other. The piezoelectric vibrator  21  is of longitudinal vibration mode, in which the vibrator can swell and shrink in a longitudinal direction orthogonal to the direction in which the piezoelectric material layers are laminated. Leading-end faces of the respective piezoelectric vibrators  21  are joined to an island portion  28  of the channel unit  26 . 
     The piezoelectric vibrator unit  21  acts in the same way as does a capacitor. Specifically, when supply of a signal is stopped, the potential of the piezoelectric vibrator  21  (i.e., the potential of the vibrator) is held at a potential attained immediately before supply of the signal is stopped. 
     The channel unit  26  is constituted by sandwiching a channel forming substrate  29  between a nozzle plate  30  and an elastic plate  31 , which oppose each other. 
     The nozzle plate  30  is formed from a thin metal plate material (e.g., a stainless steel plate) having a plurality of nozzle orifices  32  (e.g.,  96  nozzle orifices) provided in a secondary scanning direction. The channel forming substrate  29  is a plate-shaped member in which an ink flow passage is defined by a common ink reservoir  33 , an ink supply port  34 , a pressure chamber  35 , and a communication port  36 . In the embodiment, the, channel forming substrate  29  is made of a silicon wafer by etching. The elastic plate  31  is a composite plate material of a dual structure and formed by laminating a stainless steel support plate  37  with a resin film  38 . The island portion  28  is formed by annually removing a portion of the support plate  37  opposing the pressure chamber  35 . 
     In the recording head  8 , a string of ink flow passages are defined for each nozzle orifice  32  so as to extend from the common ink reservoir  33  to a corresponding nozzle orifice  32  by way of the pressure chamber  35 . The piezoelectric vibrator  21  is deformed as a result of being subjected to discharging and charging. Specifically, the piezoelectric vibrator  21  of longitudinal vibration mode contracts in a longitudinal direction thereof when subjected to recharge and extends in the same direction when subjected to discharge. When the potential of the vibrator is increased as a result of a charging operation, the island portion  28  is pulled toward the piezoelectric vibrator, whereby the resin film  38  located around the island portion  28  is deformed and the pressure chamber  35  expands. In contrast, when the potential of the vibrator is lowered as a result of a discharging operation, the pressure chamber  35  contracts. 
     In this way, the volume of the pressure chamber  35  can be controlled in accordance with the potential of the vibrator, and hence the pressure of the ink stored in the pressure chamber  35  can be changed, thereby ejecting an ink droplet from the nozzle orifice  32 . For instance, the pressure chamber  35  having a reference volume is caused to abruptly shrink after having been expanded, thereby enabling ejection of an ink droplet. 
     An electrical configuration of the recording head  8  will now be described. 
     As shown in FIG. 1, the recording head  8  has a shift register circuit constituted of a first shift register  41  and a second shift register  42 ; a latch circuit constituted of a first latch  43  and a second latch  44 ; a level shifter circuit constituted of a decoder  45 , a control logic  46 , a first level shifter  47 , and a second level shifter  48 ; a switching circuit constituted of a first switcher  49  and a second switcher  50 ; and the piezoelectric vibrators  21 . 
     A plurality of shift registers  41 ,  42 ; a plurality of latches  43 ,  44 ; a plurality of level shifters  47 ,  48 ; a plurality of switchers  49 ,  50 ; and a plurality of piezoelectric vibrators  21  are provided so as to correspond to the respective nozzle orifices  32 . 
     In accordance with the recording data (SI) output from the printer controller  1 , the recording head  8  ejects ink droplets. In the embodiment, a group of higher order bits of recording data and a group of lower order bits of recording data are sent to the recording head  8 , in this sequence. Hence, the group of higher order bits of recording data are first set in the second shift register  42 . When the group of higher order bits of recording data have been set in the second shift register  42  with regard to all the nozzle orifices  32 , the group of lower order bits of recording data are subsequently set in the second shift register  42 . In association with setting of the group of lower order bits of recording data, the group of higher order bits of recording data are shifted and set to the first shift register  41 . 
     The first latch  43  is electrically connected to the first shift register  41 . The second latch  44  is electrically connected to the second shift register  42 . When a latch pulse (LAT 1 ) output from the printer controller  1  is input to the respective latch circuits  43 ,  44 , the first latch  43  latches the group of higher order bits of recording data, and the second latch  44  latches the group of lower order bits of recording data. 
     The recording data (i.e., the group of higher order bits and the group of lower order bits) latched by the latch circuits  43 ,  44  are respectively input to the decoder  45  The decoder  45  performs translating operation on the basis of the higher order bits and lower order bits of recording data, thereby producing waveform selection data to be used for selecting the waveform elements PS 1  to PS 6  and the adjustment elements P 0 , P 20 , which constitute the drive signals COM 1 , COM 2 . 
     In the embodiment, the waveform selection data are generated for each of the drive signals COM 1 , COM 2 . Specifically, first waveform selection data corresponding to the first drive signal COM 1  are formed from a total of four bits of data; that is, the bits being assigned respectively to a first adjustment element P 0  (a period t 10 ), a first waveform section PS 1  (a period t 11 ), a second waveform section PS 2  (a period t 12 ), and a third waveform section PS 3  (a period t 13 ). Second waveform selection data corresponding to the second drive signal COM 2  are formed from a total of four bits of data; that is, the bits being assigned respectively to a second adjustment element P 20  (a period t 20 ), a fourth waveform section P 54  (a period t 21 ), a fifth waveform section PS 5  (a period t 22 ), and a sixth waveform section PS 6  (a period t 23 ). 
     The decoder  45  serves as a waveform selection data generator and generates a plurality of sets of waveform selection data from the recording data (i.e., gradation data), the data being equal in number to drive signals. 
     A timing signal output from the control logic  46  is also input to the decoder  45 . The control logic  46  serves as the timing signal generator along with the controller  6 . In synchronism with input of a latch signal (LAT) and channel signals (CH-A, CH-B), timing signals (TYM-A, TYM-B) are generated. 
     The timing signal is also generated for each of the drive signals COM 1 , COM 2 . Specifically, the control logic  46  generates the first timing signal (TYM-A) from the latch pulse (LAT 1 ) and channel pulses (CH 11  to CH 13 ) for the first drive signal COM 1 . Further, the control logic  46  generates the second timing signal (TYM-B) from the latch pulse and channel pulses (CH 21  to CH 23 ) for the second drive signal COM 2 . 
     The four bits of waveform selection data generated by the decoder  45  are input to the respective level shifters  47 ,  48  in descending order from the high order bits at a timing specified by the timing signal. In accordance with timings at which respective timing pulses included in the first timing signal TYM-A are to be generated, the first waveform selection data are input to the first level shifter  47 . Moreover, in accordance with timings at which respective timing pulses included in the second timing signal TYM-B are to be generated, the second waveform selection data are input to the second level shifter  48 . 
     The level shifters  47 ,  48  serves as voltage amplifiers. In a case where the waveform selection data assume a value of [1], the level shifters  47 ,  48  output an electric signal which has been boosted up to a voltage at which corresponding switchers  49 ,  50  can be activated; for example, approximately tens of volts. More specifically, when the first waveform selection data assume a value of [1], an electric signal is output to the first switcher  49 . When the second waveform selection data assume a value of [1], an electric signal is output to the second switcher  50 . 
     The first drive signal COM 1  is supplied to an input side of the first switcher  49  from the drive signal generator  9 . The second drive signal COM 2  is supplied to an input side of the second switcher  50  from the same. Further, the piezoelectric vibrator  21  is electrically connected to output sides of the switchers  49 ,  50 . The switchers  49 ,  50  are provided in accordance with the type of a drive signal to be generated. The switchers  49 ,  50  are interposed between the drive signal generator  9  and the piezoelectric vibrator  21  and selectively supply the drive signals COM 1 , COM 2  to the piezoelectric vibrator  21 . 
     The waveform selection data are used to control operation of the switcher  49  and that of the switcher  50 . During a period in which the waveform selection data input to the first switcher  49  assumes a value of [1], the first switcher  49  is brought into conduction, and the first drive signal COM 1  is supplied to the piezoelectric vibrator  21 . Similarly, during a period in which the waveform selection data input to the second switcher  50  assumes a value of [1], the second switcher  50  is brought into conduction, and the first drive signal COM 1  is supplied to the piezoelectric vibrator  21 . In response to the thus-supplied drive signals COM 1 , COM 2 , a potential of the piezoelectric vibrator  21  is changed. During a period in which the waveform selection data input to the switcher  49  and those input to the switcher  50  assume a value of [ 0  ], an electric signal to be used for activating the switchers  49 ,  50  is output from neither the level shifter  47  nor the level shifter  48 . Hence, a drive signal is not supplied to the piezoelectric vibrator  21 . In other words, the adjustment elements P 0 , P 20  and the waveform elements (i.e., the first waveform section PS 1  through the sixth waveform section PS 6 ), which have arisen during a period in which a value of [1] is set as waveform selection data, are selectively supplied to the piezoelectric vibrator  21 . 
     In the embodiment, the decoder  45 , the control logic  46 , and the level shifters  47 ,  48  serve as a switch controller. The switchers  49 ,  50  are controlled in accordance with recording data (i.e., gradation data). 
     The drive signals COM 1 , COM 2  generated by the drive signal generator  9  will now be described, along with control of supply of the drive signals COM 1 , COM 2  to the piezoelectric vibrator  21 . 
     As mentioned above, the drive signals shown in FIG. 3 are embodied as the first drive signal COM 1  and the second drive signal COM 2 . The first drive signal COM 1  comprises a first adjustment element P 0  generated during the period t 10 ; a first waveform section PS 1  generated during the period t 11 ; a second waveform section PS 2  generated during the period t 12 ; and a third waveform section PS 3  generated during the period t 13 . The second drive signal COM 2  comprises a second adjustment element P 20  generated during the period t 20 ; a fourth waveform section PS 4  generated during the period t 21 ; a fifth waveform section PS 5  generated during the period t 22 ; and a sixth waveform section PS 6  generated during the period t 23 . 
     The first drive signal COM 1  will first be described. 
     The first adjustment element P 0  is formed from a waveform element which is uniform at an intermediate potential Vhm. As will be described later, the first adjustment element P 0  is supplied to the piezoelectric vibrator  21  so as to adjust the potential of the vibrator to the intermediate potential Vhm at the beginning of the recording cycle T. 
     Here, the intermediate potential Vhm is a kind of reference potential and also serves as leading-edge and trailing-edge potentials of the respective drive pulses DP 1  through PD 3 . 
     The first waveform section PS 1  is formed from a first constant potential element P 1 , a first expanding element P 2 , and a first expansion holding element P 3 . The first constant potential element P 1  is a waveform element which is constant at an intermediate potential Vhm. The first expanding element P 2  is a waveform element for causing a potential to increase from the intermediate potential Vhm to a first expansion potential Vh 1  at such a relatively gentle-constant gradient that no ink droplets are ejected. The first expansion holding element P 3  is a waveform element which is constant at the first expansion potential Vh 1 . 
     The second waveform section PS 2  is formed from a second expansion holding element P 4 , a first ejection element P 6 , a contraction holding element P 6 , a damping element P 7 , and a second constant potential element P 8 . The second expansion holding element P 4  is a waveform element which is constant at the first expansion potential Vh 1 . The first ejection element P 5  is a waveform element for causing a potential to drop from the first expansion potential Vh 1  to a contraction potential VL at a relatively steep gradient. The contraction holding element P 6  is a waveform element which is constant at the contraction potential VL. The damping element P 7  is a waveform element for causing a potential to increase from the contraction potential VL to the intermediate potential Vhm at such a relatively gentle constant gradient that no ink droplets are ejected. Moreover, the second constant potential element P 8  is a waveform element which is constant at the intermediate potential Vhm. 
     The third waveform section PS 3  is formed from a third constant potential element P 9 , a first expanding element P 10 , an expansion holding element P 11 , a first ejection element P 12 , a contraction holding element P 13 , and a damping element P 14 . 
     The third constant potential element P 9  is a waveform element which is constant at the intermediate potential Vhm. The expansion holding element P 11  is a waveform element which is constant at the first expansion potential Vh 1 . A period of time during which the expansion holding element P 11  is generated is set to a value equal to the sum of the duration of the first expansion holding element P 3  and the duration of the second expansion holding element P 4 . 
     The remaining waveform elements; that is, the first expanding element P 10 , the first ejection element P 12 , the contraction holding element P 13 , and the damping element P 14 , are identical with the first expanding element P 2 , the first ejection element P 5 , the contraction holding element P 6 , and the damping element P 7 , all belonging to the first and second waveform elements PS 1 , PS 2 , and hence their repeated explanations are omitted. 
     In relation to the first drive signal COM 1 , the first expanding element P 2 , the first expansion holding element P 3 , the second expansion holding element P 4 , the first ejection element P 5 , the contraction holding element P 6 , and the damping element P 7 , all belonging to the first and second waveform elements PS 1 , PS 2 , constitute the first middle dot drive pulse DP 1 . Moreover, the first expanding element P 10 , the expansion holding element P 11 , the first ejection element P 12 , the contraction holding element P 13 , and the damping element P 14 , all belonging to the third waveform section PS 3 , constitute the second middle dot drive pulse DP 2 . The middle dot drive pulses DP 1 , DP 2  assume identical waveform patterns. When the middle dot drive pulses DP 1 , DP 2  are supplied to the piezoelectric vibrator  21 , the amount of ink corresponding to a middle dot is ejected from a corresponding nozzle orifice  32 . 
     Descriptions are now given by taking the first middle dot drive pulse DP 1  as an example. As a result of supply of the first expanding element P 2 , the piezoelectric vibrator  21  contracts in a longitudinal direction thereof. In contract, a corresponding pressure chamber  35  expands from a reference volume corresponding to the intermediate potential Vhm (reference potential) to an expanded volume corresponding to a first expansion potential Vh 1 . By the expanding action of the pressure chamber  35 , ink is supplied from the common ink reservoir  33  to the inside of the pressure chamber  35 . The expanded state of the pressure chamber  35  is maintained during a period in which the first and second expansion holding elements P 3  and P 4  are supplied. 
     Subsequently, the first ejection element P 5  is supplied to the piezoelectric vibrator  21 , whereby the piezoelectric vibrator  21  is extended. In association with extension of the piezoelectric vibrator  21 , the pressure chamber  35  is abruptly contracted from the expanded volume to a contracted volume corresponding to the contraction potential VL. The ink stored in the pressure chamber  35  is compressed as a result of abrupt contraction of the pressure chamber  35 , whereby a predetermined quantity of ink is ejected from a corresponding nozzle orifice  32 . 
     The contracted state of the pressure chamber  35  is maintained over a period during which the contraction holding element P 6  is supplied. During this period, the pressure of the ink stored in the pressure chamber  35 , the pressure having dropped by ejection of an ink droplet, is again increased by the natural vibration of ink. The damping element P 7  is supplied in step with the timing at which the pressure increases. As a result of supply of the damping element P 7 , the pressure chamber  35  expands and is restored to the reference volume, thereby absorbing changes in the pressure of the ink stored in the pressure chamber  35 . 
     In relation to the first drive signal COM 1 , the first middle dot drive pulse DP 1  and the second middle dot drive pulse DP 2  are connected together at the leading edge and trailing-edge potentials thereof (i.e., the intermediate potential Vhm), by the first adjustment element P 0 , the first constant potential element P 1 , the second constant potential element P 8 , and the third constant potential element P 9 . Thus, the middle dot drive pulses DP 1 , DP 2  are generated at given intervals over adjacent recording cycles T. Specifically, the sum of a period of time during which the first adjustment element P 0  is generated and a period of time during which the first constant potential element P 1  is generated is set to a value identical with that of the sum of a period of time during which the second constant potential element P 8  is generated and a period of time during which the third constant potential element P 9  is generated. 
     Given that the middle dot drive pulses DP 1 , DP 2  are generated at given intervals over adjacent recording cycles T, when the medium drive pulses DP 1 , DP 2  are continuously supplied to the piezoelectric vibrator  21 , the status of a meniscus achieved at the beginning of supply of the drive pulses can be maintained constant. As a result, the flight of an ink droplet can be stabilized, thereby realizing an attempt to improve image quality. 
     In relation to the first drive signal COM 1  having the foregoing configuration, the first expanding elements P 2 , P 10 , the first expansion holding element P 3 , the second expansion holding element P 4 , the expansion holding element P 11 , the first ejection elements P 5 , P 12 , the contraction holding elements P 6 , P 13 , and the damping elements P 7 , P 14 , serve as drive waveform elements. 
     On the other hand, the first adjustment element P 0 , the first constant potential element P 1 , the second constant potential element P 8 , and the third constant potential element P 9 , serve as constant-potential waveform elements. 
     The second drive signal COM 2  will now be described. 
     The second adjustment element P 20  is formed from a waveform element which is constant at the intermediate voltage Vhm, in the same manner as is the first adjustment element P 0 . In order to adjust the potential of the vibrator to the intermediate potential Vhm at the beginning of the recording cycle T, the second adjustment element P 20  is also supplied to the piezoelectric vibrator  21 . 
     In the embodiment, either the second adjustment element P 20  or the first adjustment element P 0  is supplied to the piezoelectric vibrator  21  at the beginning of the recording cycle T. Hence, a period t 20  of time during which the second adjustment element P 20  is generated is set to become identical in duration with a period t 10  of time during which the first adjustment element P 0  is generated. 
     The fourth waveform section PS 4  is formed from a fourth constant potential element P 21 . The fourth potential element P 21  is a waveform element which is constant at the intermediate potential Vhm and is generated at a point in time between the period t 11  and the period t 12  of the first drive signal COM 1 . Specifically, generation of the waveform element is commenced at the start of the period t 11  and terminated at an intermediate point during a period of time in which the contraction holding element P 6  of the second waveform section PS 2  is generated. 
     The fifth waveform section PS 5  is formed from a fifth constant potential element P 22 , a second expanding element P 23 , an expansion holding element P 24 , a second ejection element P 25 , and a first contraction holding element P 26 . The fifth potential element P 22  is a waveform element which is constant at the intermediate potential Vhm and is generated over an extremely short period of time. The second expanding element P 23  is a waveform element which causes a potential to abruptly increase from the intermediate potential Vhm to a second expansion potential Vh 2 . The expansion holding element P 24  is a waveform element which is constant at the second expansion potential Vh 2 . The second ejection element P 25  is a waveform element which causes a potential to abruptly drop from the second expansion potential Vh 2  to an ejection potential Vh 3 . The first contraction holding element P 26  is a waveform element which is constant at the ejection potential Vh 3 . 
     The ejection potential Vh 3  of the embodiment is made equal to the first expansion potential Vh 1  of the first drive signal COM 1 . 
     The sixth waveform section PS 6  is formed from a second contraction holding element P 27 , a damping element P 28 , and a sixth constant potential element P 29 . The second contraction holding element P 27  is a waveform element which is constant at the ejection potential Vh 3  and is generated over an extremely short period of time. The damping element P 28  is a waveform element for causing a potential to drop from the ejection potential Vh 3  to the intermediate potential Vhm at a relatively gentle, constant gradient. The sixth constant potential element P 29  is a waveform element which is constant at the intermediate potential Vhm and is generated from the trailing edge of the damping element P 28  to the trailing edge of the recording cycle T. 
     In relation to the second drive signal COM 2 , the second expanding element P 23 , the expansion holding element P 24 , the second ejection element P 25 , the contraction holding elements P 26 , P 27 , and the damping element P 28 , all belonging to the fifth and sixth waveform elements PS 5 , PS 6 , constitute the small dot drive pulse DP 3  When the small dot drive pulse DP 3  is supplied to the piezoelectric vibrator  21 , a nominal amount of ink corresponding to a small dot is ejected from the nozzle orifice  32 . 
     Specifically, as a result of supply of the second expanding element P 23 , the piezoelectric vibrator  21  rapidly contracts in the longitudinal direction thereof. The pressure chamber  35  rapidly expands from the reference volume corresponding to the intermediate potential Vhm to an expanded volume corresponding to the second expansion potential Vh 2 . As a result of expansion, relatively high negative pressure develops in the pressure chamber  35 , thereby strongly drawing a meniscus (i.e., an exposed free surface of ink in the nozzle orifice  32 ) toward the pressure chamber  35 . The expanded state of the pressure chamber  35  is held over a period during which the expansion holding element P 24  is supplied. During this period, the moving direction of a center portion of the meniscus is reversed to the direction in which ink is to be ejected. The center portion becomes raised in the form of a pillar. 
     Subsequently, the second ejection element P 25  is supplied to the piezoelectric vibrator  21 , whereupon the vibrator extends. As a result of extension of the piezoelectric vibrator  21 , the pressure chamber  35  is abruptly contracted from the expanded volume to an ejection volume corresponding to the second expansion potential Vh 3 . By abrupt contraction of the pressure chamber  35 , the ink stored in the pressure chamber  35  is compressed, thereby promoting growth of the pillar portion. The pillar portion is broken at an intermediate position thereof, whereby ink is ejected in the form of an ink droplet. 
     The second ejection element P 25  is followed by supply of the first contraction holding element P 26  and supply of the second contraction holding element P 27  Subsequently, the damping element P 28  is supplied. The damping element P 28  contracts the pressure chamber  35  so as to compensate for the drop in pressure of the ink stored in the pressure chamber  35  resulting from ejection of an ink droplet. Specifically, the pressure chamber  35  is contracted to a reference volume by supply of the damping element P 28 , thereby absorbing a change in the pressure of the ink stored in the pressure chamber  35 . 
     A period of time during which the respective waveform elements (P 23  through P 28 ) constituting the small dot drive pulse DP 3  are to be generated partially overlaps periods of time during which the respective waveform elements (P 2  to P 7 , P 10  to P 14 ) constituting the middle dot drive pulse DP 1 , DP 2  are to be generated. Specifically, a period of time during which the second expanding element P 23  of the small dot drive pulse DP 3  is generated partially overlaps a period of time during which the damping element P 7  of the first middle dot drive pulse DP 1  is to be generated. Further, a period of time during which the damping element P 28  of the small dot drive pulse DP 3  is to be generated overlaps, at the trailing edge, a period of time during which the first expanding element P 10  of the second middle dot drive pulse DP 2  is to be generated. 
     In this way, the drive pulses DP 1  to DP 3  are divided into the drive signals COM 1 , COM 2  and generated so as to be superimposed on each other with respect to time. In this case, the drive pulses DP 1  through DP 3  and the first vibrating pulse VP 1  can be efficiently arranged in even a recording cycle T of limited length. Consequently, high-frequency driving of the recording head  8  can be realized. 
     In relation to the second drive signal COM 2 , the small dot drive pulses DP 3  are connected together at the leading-edge and trailing-edge potentials thereof (i.e., the intermediate potential Vhm), by the second adjustment element P 20 , the fourth constant potential element P 21 , the fifth constant potential element P 22 , and the sixth constant potential element P 29 . 
     A timing at which the small dot drive pulse DP 3  is to be generated is set to an intermediate point in time between the first middle dot drive pulse DP 1  and the second middle dot drive pulse DP 2 . In detail, a timing at which the second ejection element P 25  of the small middle dot drive pulse DP 3  is to be generated is set to an exactly intermediate point in time between a timing at which the first ejection element P 5  of the first middle dot drive pulse DP 1  is to be generated and a timing at which the first ejection element P 12  of the second middle dot drive pulse DP 2  is to be generated, in an attempt to improve image quality. 
     As will be described later, in the embodiment, the first middle dot drive pulse DP 1  and the second middle dot drive pulse DP 2  are supplied to the piezoelectric vibrator  21  at the time of recording of a large dot, and the second middle dot drive pulse DP 2  is supplied to the piezoelectric vibrator  21  at the time of recording of a middle dot. Further, at the time of recording of a small dot, the small dot drive pulse DP 3  is supplied to the piezoelectric vibrator  21 . 
     Here, if the small dot drive pulse DP 3  is generated at an intermediate point in time between the first middle dot drive pulse DP 1  and the second middle dot drive pulse DP 2 , an interval between ejection of an ink droplet and ejection of the next ink droplet can be made uniform even when a recording gradation is changed between a preceding recording cycle T and a current recording cycle T. For instance, an interval between ejection of ink for producing a small dot during a preceding recording cycle T and ejection of ink for producing a large dot during a current recording cycle T can be made equal to that existing between ejection of ink for producing a large dot during a preceding recording cycle T and ejection of ink for producing a small dot during the current recording cycle T. 
     As a result, the status of a meniscus generated during a current recording cycle T becomes uniform, and ejection of an ink droplet can stabilized, and by extension image quality can be improved. 
     In relation to the second drive signal COM 2  having the foregoing configuration, the second expanding element P 23 , the expansion holding element P 24 , the second ejection element P 25 , the first contraction holding element P 26 , the second contraction holding element P 27 , and the shrinking damping element P 28 , serve as drive waveform elements. On the other hand, the second adjustment element P 20 , the fourth constant potential element P 21 , the fifth constant, potential element P 22 , and the sixth constant potential element P 29 , serve as constant-potential waveform elements. 
     Control of multiple gradations to be performed in the embodiment will now be described by reference to FIGS. 3 through 7. During control of multiple gradations, the switchers  49 ,  50  are controlled by the switch controller (embodied by a combination of the decoder  45 , the control logic  46 , and the level shifters  47 ,  48 ; the same also applies to any counterparts in the following descriptions). The respective switchers  49 ,  50  supply the selected drive signals COM 1 , COM 2  to the piezoelectric vibrator  21 . Specifically, the first drive signal COM 1  and the second drive signal COM 2  are not simultaneously supplied to the piezoelectric vibrator  21 , in order to stabilize the potential of the vibrator  21 . 
     An explanation will first be given of the case of non-recording operation (meniscus vibration). In this case, the decoder  45  generates the first waveform selection data [ 1100 ] and the second waveform selection data [ 0001 ] by translation of gradation data [ 00 ] for non-recording operation. The switch controller controls operation of the first switcher  49  and that of the second switcher  50  on the basis of the thus-generated waveform selection data, which in turn controls supply of the first drive signal COM 1  and the second drive signal COM 2  to the piezoelectric vibrator  21 . 
     During the period t 10  (t 20 ), the first adjustment element P 10  is supplied to the piezoelectric vibrator  21 . As a result, the potential of the vibrator is adjusted to the intermediate potential Vhm. Here, one is selected from the first adjustment element P 0  and the second adjustment element P 20  in accordance with the next waveform element (i.e., waveform element) to be sent, and the selected element is supplied to the piezoelectric vibrator  21 . Specifically, if the next waveform element to be supplied is a waveform element of the first drive signal COM 1 , the fist adjustment element P 0  is selected. If the next waveform element to be supplied is a waveform element of the second drive signal COM 2 , the second adjustment element P 20  is selected. Such a selecting operation is performed in order to reduce the number of times the switchers  49 ,  50  operate. More specifically, if the number of times the switchers  49 ,  50  operate is reduced, a drive signal supplied to the piezoelectric vibrator  21  is stabilized, in turn stabilizing operation of the piezoelectric vibrator  21 . 
     During the period t 11 , the first switcher  49  is brought into a connected state. During the period t 21 , the second switcher  50  is brought into a disconnected state. Specifically, as indicated by a bold line shown in FIG. 4, the first waveform section PS 1  of the first drive signal COM 1  is supplied to the piezoelectric vibrator  21 . The pressure chamber  35  is expanded to an expanded volume by the first expanding element P 2 . In association with swelling of the pressure chamber  35 , the ink stored in the pressure chamber  35  is slightly decompressed. 
     During subsequent periods t 12  and t 13 , the first switcher  49  is controlled and brought into a disconnected state, and the second switcher  50  is controlled and brought into a disconnected state during a period t 22 . As a result, neither the first drive signal COM 1  nor the second drive signal COM 2  is supplied to the piezoelectric vibrator  21  from the beginning of the period t 12  to the end of the period t 22 . Consequently, as indicated by a semi-bold line shown in FIG. 4, the potential of the vibrator is maintained at the first expansion potential Vh 1  which appears immediately before disconnection of the first switch, and the expanded volume of the pressure chamber  35  is maintained. During the period, pressure fluctuations in the ink stored in the pressure chamber  35  are induced by the depressurization that has arisen during the period t 11 . 
     During a period t 23 , the second switcher  50  is controlled and brought into a connected state. As a result, as indicated by a bold line shown in FIG. 4, a sixth waveform section PS 6  of the second drive signal COM 2  is supplied to the piezoelectric vibrator  21 , whereby the pressure chamber  35  is contracted to the reference volume by the damping element P 28 . In association with contraction of the pressure chamber  35 , the ink stored in the pressure chamber  35  is slightly compressed. 
     By pressure fluctuations imparted to ink, a meniscus is minutely vibrated toward the pressure chamber  35  as well as in a direction in which an ink droplet is to be ejected. By the minute vibration of the meniscus, the ink that is located in the vicinity of the nozzle orifice  32  and whose viscosity is increased is dispersed, thereby preventing an increase in the viscosity of ink. 
     In the embodiment, the first expansion potential Vh 1  of the first drive signal COM 1  and the ejection potential Vh 2  of the second drive signal COM 2  are set so as to assume the same potential. Hence, when the sixth waveform section PS 6  (i.e., a second contraction holding element P 27 ) is supplied to the piezoelectric vibrator  21  during the period t 23 , the potential of the vibrator and the leading-edge potential of the sixth waveform section PS 6  are made equal to each other. Hence, the sixth waveform section PS 6  can be smoothly supplied to the piezoelectric vibrator  21 . 
     In the embodiment, in the case of a recording gradation for non-recording, portions of the waveform elements constituting the first drive signal COM 1  (i.e., the first expanding element P 2  and the first expansion holding element P 3 ) and a portion of the waveform element constituting the second drive signal COM 2  (i.e., the second contraction holding element P 27  and the damping element P 28 ) are supplied, in combination, to the piezoelectric vibrator  21 , thereby minutely vibrating a meniscus. As a result, the meniscus can be vibrated minutely without provision in the respective drive signals COM 1 , COM 2  of the waveform elements specifically designed for minute vibration, thereby preventing an increase in the viscosity of the ink located in the vicinity of the nozzle orifice  32 . 
     There, will now be described a case where recording is performed through use of small dots. In this case, the decoder  45  generates first waveform selection data [ 0000 ] and second waveform selection data [ 1111 ] by translation of gradation data [ 01 ] pertaining to small dots. The switch controller controls supply of the first and second drive signals COM 1 , COM 2  to the piezoelectric vibrator  21  on the basis of the thus-generated waveform selection data. 
     Specifically, during the period t 10  (t 20 ), the second adjustment element P 20  is supplied to the piezoelectric vibrator  21 , whereby the potential of the vibrator is adjusted to the intermediate potential Vhm. During the periods t 11  to t 13 , the first switcher  49  is controlled and brought into a disconnected state. During periods t 21  to t 23 , the second switcher  50  is controlled and brought into a connected state. 
     As a result, the fourth waveform section PS 4  is supplied to the piezoelectric vibrator  21  during the period t 21 ; the fifth waveform section PS 5  is supplied to the same during the period t 22 ; and the sixth waveform section PS 6  is supplied to the same during the period t 23 . More specifically, the small dot drive pulse DP 3  is supplied to the piezoelectric vibrator  21 . 
     Consequently, as indicated by a bold line shown in FIG. 5, the potential of the vibrator is changed in accordance with the second drive signal COM 2 , and a nominal amount of ink is ejected from the nozzle orifice  32  by the small dot drive pulse DP 3 . 
     There will now be described the case of recording of middle dots. In this case, the decoder  45  generates first waveform selection data [ 0001 ] and second waveform selection data [ 1100 ] by translation of gradation data [ 10 ] pertaining to middle dots. The switch controller controls supply of the first and second drive signals COM 1 , COM 2  to the piezoelectric vibrator  21  on the basis of the thus-generated waveform selection data. 
     During the period t 10  (t 20 ), the first adjustment element P 0  and the second adjustment element P 20  are supplied to the piezoelectric vibrator  21 , and the potential of the piezoelectric vibrator  21  is adjusted to the intermediate potential Vhm. During the periods t 11  and t 12 , the first switcher  49  is brought into a disconnected state. During the period t 21 , the second switcher  50  is brought into a connected state. As indicated by a bold line shown in FIG. 6, the second waveform section PS 4  of the second drive signal COM 2  is supplied to the piezoelectric vibrator  21 , and the potential of the vibrator is maintained at the intermediate potential Vhm by the fourth constant potential element P 21 . 
     During the subsequent period t 22 , the second switcher  50  is controlled and brought into a disconnected state. During a period from the beginning of the period t 22  to the end of the period. t 13 , neither the first drive signal COM 1  nor the second drive signal COM 2  is supplied to the piezoelectric vibrator  21 . Consequently, as indicated by a semi-bold line shown in FIG. 6, the potential of the vibrator is maintained at the intermediate potential Vhm which arises before disconnection of the switchers. Since the fourth constant potential element P 21  has already been supplied to the piezoelectric vibrator  21  during the preceding period t 21 , the period of time during which the drive signals are not supplied becomes relatively short. 
     During the period t 13 , the first switcher  49  is controlled and brought into a connected state. During the period t 23 , the second switcher  50  is controlled and brought into a disconnected state. As indicated by the bold line shown in FIG. 6, the third waveform section PS 3  of the first drive signal COM 1  is supplied to the piezoelectric vibrator  21 . As a result, the second middle dot drive pulse DP 2  is supplied to the piezoelectric vibrator  21 , whereby a small amount of ink corresponding to a middle dot is ejected. 
     In the embodiment, even in the case of a medium-dot recording gradation, portions of the waveform elements constituting the first drive signal COM 1  (i.e., the third constant potential element P 9 , the first expanding element P 10 , the expansion holding element P 11 , the first election element P 12 , the damping hold element P 13 , and the damping element P 14 ) and a portion of the waveform element constituting the second drive signal COM 2  (i.e., the fourth constant potential element P 21 ) are supplied, in combination, to the piezoelectric vibrator  21 . During a period of time during which the first drive signal COM 1  cannot be supplied to the piezoelectric vibrator  21  (the periods t 11 , t 12 ), the fourth constant potential P 21  of the second drive signal COM 2  is supplied, thereby maintaining the potential of the vibrator at the intermediate potential Vhm. 
     This is intended for shortening, to the greatest possible extent, the period of time during which the drive signals COM 1 , COM 2  are not supplied to the piezoelectric vibrator  21 . More specifically, when a printer is used at high humidity or the insulation resistance of the piezoelectric element has dropped as a result of long-term use of the piezoelectric vibrator  21 , an electric-charge retaining capability of the piezoelectric vibrator  21  may drop. When a drop has arisen in the electric-charge retaining capability of the piezoelectric vibrator  21 , the potential of the piezoelectric vibrator  21  is gradually lowered by electric discharge which arises during a period of time in which the drive signals are not supplied to the vibrator. Therefore, when the period of time during which the drive signals are not supplied to the vibrator is long, the extent to which the potential of the vibrator is decreased becomes larger. When the next drive signals are supplied to the vibrator, a difference between the potential of the drive signal and the potential of the vibrator becomes greater. In this case, abrupt deformation arises in the piezoelectric vibrator  21 , thereby causing erroneous ejection of an ink droplet. 
     As in the case of this embodiment, so long as the period of time during which the drive signals COM 1 , COM 2  are not supplied to the vibrator is shortened to the greatest possible extent, the extent to which the potential of the vibrator drops can be made smaller even when a drop has arisen in the electric-charge retaining capability of the vibrator. Hence, the drive signals COM 1 , COM 2  can be supplied without any trouble. 
     There will now be described the case of recording of large dots. In this case, the decoder  45  generates first waveform selection data [ 1111 ] and second waveform selection data [ 0000 ] by translating gradation data [ 11  ] pertaining to large dots. In accordance with the thus-generated waveform selection data, the switch controller controls supply of the first drive signal COM 1  and the second drive signal COM 2  to the piezoelectric vibrator  21 . 
     Specifically, during the period t 10  (t 20 ), the first adjustment element P 0  is supplied to the piezoelectric vibrator  21 , and the potential of the vibrator is adjusted to the intermediate potential Vhm. During the periods t 11  to t 13 , the first switcher  49  is controlled and brought into a connected state. During the periods t 21  to t 23 , the second switcher  50  is controlled and brought into a disconnected state. As a result, during the period t 11 , the first waveform section PS 1  is supplied to the piezoelectric vibrator  21 . During the period t 12 , the second waveform section PS 2  is supplied to the piezoelectric vibrator  21 . Further, during the period t 13 , the third waveform section PS 3  is supplied to the same. More specifically, the first middle dot drive pulse DP 1  and the second middle dot drive pulse DP 2  are supplied to the piezoelectric vibrator  21 . 
     Consequently, as indicated by a bold line shown in FIG. 7, the potential of the vibrator is changed in accordance with the first drive signal COM 1 , and a small amount of ink is continuously ejected from the nozzle orifice  32  twice in response to the middle dot drive pulse. Large dots are recorded by these ink droplets. 
     As has been described, in the embodiment, two middle dot drive pulses DP 1 , DP 2  are included in the first drive signal COM 1 . One small dot drive pulse DP 3  is included in the second drive signal COM 2 . A period of time during which the middle dot drive pulses DP 1 , DP 2  are generated and a period of time during which the small dot drive pulse DP 2  is generated partially overlap each other, thereby shortening the recording cycle T. As a result, the piezoelectric vibrator  21  can be driven at a higher frequency, thereby enabling the recording head  8  to provide sufficient performance. 
     Since a portion of the waveform elements constituting the first drive signal COM 1  and a portion of the waveform elements constituting the second drive signal COM 2  are supplied, in combination, to the piezoelectric vibrator  21 , the recording head can be driven in accordance with a new pattern which is not explicitly specified by the drive signals. For example, a meniscus can be minutely vibrated without use of a dedicated vibrating pulse. Moreover, periods during which no drive signals are supplied to the piezoelectric vibrator  21  can be shortened to the shortest possible extent. As a result, a complicated control operation can be achieved while the recording head  8  is actuated at a higher frequency. 
     In this embodiment, the drive signals COM 1 , COM 2  are selectively supplied to the piezoelectric vibrator  21  by the first and second switchers  49 ,  50  that are provided in accordance with the types of drive signals to be generated. However, the invention is not limited to such a switcher. For instance, the drive signals COM 1 , COM 2  may be selectively supplied to the piezoelectric vibrator  21  by a changeover switch shown in FIG. 8 as a second embodiment of the invention. 
     The changeover switch  61  is provided for each of the piezoelectric vibrators  21 . The changeover switch  61  has a first input contact point  61   a , a second input contact point  61   b , an off-contact point  61   c , all being provided in accordance with the types of drive signals to be generated, and an output terminal  61   d  to be electrically connected to the piezoelectric vibrator  21 . One of the contact points  61   a  through  61   c  is selectively, electrically connected to the output terminal  61   d . The first input contact point  61   a  is electrically connected to a line for feeding a first drive signal COM 1 ; the second input contact point  61   b  is electrically connected to a line for feeding a second drive signal COM 2 ; and the off-contact point  61   c  has no electrical connection. 
     The drive signals COM 1 , COM 2  can be selectively supplied to the piezoelectric vibrator  21  by switching the contact points  61   a  through  61   c , all being electrically connected to the output terminal  61   d . Specifically, the first drive signal COM 1  can be supplied by electrically connecting the first input contact point  61   a  to the output terminal  61   d . The second drive signal COM 2  can be supplied by electrically connecting the second input drive signal COM 2  to the output terminal  61   d . Neither the first drive signal COM 1  nor the second drive signal COM 2  is supplied when the off-contact point  61   c  is electrically connected to the output terminal  61   d.    
     The operation of the changeover switch  61  is controlled by the decoder  62  and the switch controller  63 . The decoder  62  serves as a switching data generator and generates switching data representing any one of the first input contact point  61   a  ([ 1 ]), the second input contact point  61   b  ([ 2 ]), and the off-contact point  61   c  ([ 0 ]) by translation of recording data (gradation data). The switching data are output to the switch controller  63  in synchronism with a timing output from the control logic  46 ′. 
     An explanation will be given by reference to a drive signal shown in FIG.  3 . In the case of gradation data [ 00 ] the decoder  62  generates switching data [ 110002 ]. The switching data are output to the switch controller  63  at a start timing of period t 10  (t 20 ), a start timing of the period t 11  (t 21 ), a start timing of the period t 12 , a start timing of a period t 22 , a start timing of a period t 13 , and a start timing of a period t 23 . 
     During the periods t 10  and t 11 , the changeover switch  61  is electrically connected to the first input contact point  61   a , whereby the first adjustment element P 0  and the first waveform section PS 1  of the first drive signal COM 1  are supplied to the piezoelectric vibrator  21 . Subsequently, the changeover switch  61  is switched to the off-contact point  61   c  immediately before the period t 23 , whereby supply of a drive signal is interrupted. During the period t 23 , the changeover switch  61  is switched to the second input contact point  61   b , whereby the sixth waveform section PS 6  of the second drive signal COM 2  is supplied to the piezoelectric vibrator  21 . 
     Consequently, as in the case of the embodiment, the meniscus vibrating operation can be effected. 
     In the case of the gradation data [ 01 ], the decoder  62  generates switching data [ 42222 ]. As a result, the changeover switch  61  is electrically connected to the second input contact point  61   b  over the entire period of the recording cycle T. The second adjustment element P 20 , the fourth waveform section PS 4 , the fifth waveform section PS 5 , and the sixth waveform section PS 6  are supplied to the piezoelectric vibrator  21 . 
     Consequently, as in the case of the embodiment, an amount of ink corresponding to a small dot can be ejected. 
     In the case of the gradation data [ 10 ], the decoder  62  generates switching data [ 222011 ]. As a result, the changeover switch  61  is electrically connected to the second input contact point  61   b  immediately before start of the period t 22 , whereupon the second adjustment element P 20  and the fourth waveform section PS 4 , both belonging to the second drive signal COM 2 , are supplied to the piezoelectric vibrator  21 . The changeover switch  61  is switched to the off-contact point  61  from a start point of the period t 22  to a point immediately before start of the period t 13 , thereby interrupting supply of a drive signal. Subsequently, the changeover switch  61  is switched to the first input contact point  61   a  during the period t 13 , whereupon the third waveform section PS 3  of the first drive signal COM 1  is supplied to the piezoelectric vibrator  21 . 
     Consequently, as in the case of the embodiment, an ink droplet corresponding to a middle dot can be ejected. 
     In the case of the gradation data [ 11 ], the decoder  62  generates switching data [ 111111 ]. As a result the changeover switch  61  is electrically connected to the first input contact point  61   a  over the entire period of the recording cycle T. The first adjust element P 0 , the first waveform section PS 1 , the second waveform section PS 2 , and the third waveform section PS 3 , all belonging to the first drive signal COM 1 , are supplied to the piezoelectric vibrator  21 . 
     Consequently, as in the case of the embodiment, an ink droplet corresponding to a large dot can be ejected. 
     By such a configuration, control of one changeover switch  61  with regard to one piezoelectric vibrator  21  is sufficient, and hence simplification of control of the switcher can be attempted. 
     Here, the invention is not limited to the above-described embodiment and is susceptible to various modifications within the scope of the invention defined by the appended claims. 
     In connection with the pressure generating element, the embodiment has described a case where the piezoelectric vibrator  21  of so-called longitudinal vibration mode is used. However, the invention can be carried out in the same manner, through use of a piezoelectric vibrator of so-called deflection vibration mode. Alternatively, an electrostatic actuator may be used in addition to a piezoelectric vibrator. 
     The embodiment has described the two types of drive signals COM 1 , COM 2 . However, even when three or more types of drive signals are generated, the invention can be carried out in the same manner. 
     The invention can be applied to plotters, facsimiles, copiers, or various types of ink jet recording apparatuses, as well as to printers. 
     The invention can be also applied to display manufacturing apparatuses, electrode forming apparatuses, biochip manufacturing apparatuses, or various types of liquid jetting apparatuses, as well as ink jet recording apparatuses. In such cases, one ordinary skilled in the art can easily realize that the words “ink”, “recording”, “small dot”, “medium dot”, “large dot” and “recording gradation” used in the foregoing explanations may be respectively replaced with “liquid”, “jetting”, “small droplet”, “medium droplet”, “large droplet” and “jetting amount”.