Patent Publication Number: US-2009225113-A1

Title: Method of driving a liquid jet head and a liquid jet apparatus

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present application claims priority to Japanese Patent Application No. 2008-48751 filed in the Japanese Patent Office on Feb. 28, 2008 and Japanese Patent Application No. 2009-2956 filed in the Japanese Patent Office on Jan. 8, 2009, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of driving a liquid jet head and a liquid jet apparatus. 
     2. Description of the Related Art 
     If the arrangement density of nozzles increases, the section area of each pressure generation chamber decreases. The decrease in the section area of each pressure generation chamber causes deterioration of the ink supply characteristic of the pressure generation chamber. That is, in order to secure a necessary volume of the pressure generation chamber, the length of the pressure generation chamber increases as the section area decreases. The increase in the length of the pressure generation chamber causes deterioration of the ink supply characteristic of the pressure generation chamber. When this happens, for example, the Helmholtz vibration cycle Tc of the pressure generation chamber is lengthened or the return of a meniscus toward a nozzle opening is delayed. As a result, it may be impossible to increase a driving frequency. 
     In order to solve such a problem, for example, JP-A-2003-276199 suggests an ink jet type recording head that has a plurality of communicating grooves (sub pressure chambers) communicating with the pressure generation chambers (main pressure chamber). In such an ink jet type recording head, auxiliary piezoelectric elements are individually provided in regions corresponding to the communicating grooves. 
     In the ink jet type recording head described in Patent Document 1, ink droplets are ejected by displacement of the piezoelectric elements. In this case, the auxiliary piezoelectric elements provided in the regions opposite the communicating grooves are selectively displaced, and accordingly ink droplets are jet from a predetermined nozzle. That is, when ink droplets are jet, an auxiliary piezoelectric element corresponding to a nozzle which jets ink droplets is displaced, and compliance of a vibrating plate on the corresponding communicating groove decreases. Meanwhile, an auxiliary piezoelectric element corresponding to a nozzle which does not jet ink droplets is not displaced, and compliance of a vibrating plate on the corresponding communicating groove is maintained so as to be comparatively high. 
     With such a driving method, while the driving frequency increases, ink droplets may be selectively jet from a predetermined nozzle. However, only by changing compliance of the vibrating plate, it is impossible to control a pressure given to ink when a piezoelectric element is driven. As a result, ink droplets may be simultaneously jet from the nozzles. 
     SUMMARY OF THE INVENTION 
     The invention has been finalized in order to solve at least some of the above-described problems, and it may be realized by the following aspects or examples. 
     An aspect of the invention provides a method of driving a liquid jet head. The liquid jet head includes a plurality of nozzles jetting liquid droplets, pressure generation chambers individually communicating with the nozzles and having a predetermined width, a reservoir common to a plurality of pressure generation chambers, and piezoelectric elements, each causing a change in pressure of a corresponding one of the pressure generation chambers. Each of the pressure generation chambers includes a main pressure chamber communicating with the reservoir through a liquid supply channel, and a plurality of sub pressure chambers arranged in parallel in a width direction of the main pressure chamber, the sub pressure chambers independently communicating with an end surface of the main pressure chamber opposite to the liquid supply channel, and each of the sub pressure chambers communicating with the corresponding nozzle. Each of the piezoelectric elements includes a main piezoelectric element provided to correspond to the main pressure chamber, and sub piezoelectric elements provided to correspond to the sub pressure chambers and to have a width smaller than that of the main piezoelectric element. When the main piezoelectric element is driven so as to jet liquid droplets from a predetermined nozzle, a driving signal which is in opposite phase to a driving signal to be applied to the main piezoelectric element is applied to a sub piezoelectric element corresponding to a sub pressure chamber communicating with a nozzle, which does not jet liquid droplets. 
     The above and other features and objects of the invention will become apparent from the following description with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For better understanding of the invention and its advantages, reference will be made in the following description and the accompanying drawings. 
         FIG. 1  is a schematic perspective view of a recording apparatus according to an embodiment of the invention. 
         FIG. 2  is an exploded perspective view of a recording head according to an embodiment of the invention. 
         FIG. 3  is a plan view and a sectional view of a recording head according to an embodiment of the invention. 
         FIG. 4  is a block diagram showing the configuration of a recording apparatus according to an embodiment of the invention. 
         FIG. 5  is a diagram showing an example of basic driving signals. 
         FIG. 6  is a diagram showing patterns of driving signals. 
         FIG. 7  is a diagram showing patterns of driving signals. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     At least the following will become apparent from the description herein and the accompanying drawings. 
     An aspect of the invention provides a method of driving a liquid jet head. The liquid jet head includes a plurality of nozzles jetting liquid droplets, pressure generation chambers individually communicating with the nozzles and having a predetermined width, a reservoir common to a plurality of pressure generation chambers, and piezoelectric elements, each causing a change in pressure of a corresponding one of the pressure generation chambers. Each of the pressure generation chamber includes a main pressure chamber communicating with the reservoir through a liquid supply channel, and a plurality of sub pressure chambers arranged in parallel in a width direction of the main pressure chamber, the sub pressure chambers independently communicating with an end surface of the main pressure chamber opposite to the liquid supply channel, and each of the sub pressure chamber communicating with a corresponding one of the nozzles. Each of the piezoelectric elements includes a main piezoelectric element provided to correspond to the main pressure chamber, and sub piezoelectric elements provided to correspond to the sub pressure chambers and to have a width smaller than that of the main piezoelectric element. When the main piezoelectric element is driven so as to jet liquid droplets from a predetermined nozzle, a driving signal which is in opposite phase to a driving signal to be applied to the main piezoelectric element is applied to a sub piezoelectric element corresponding to a sub pressure chamber communicating with a nozzle, which does not jet liquid droplets. 
     In a method of driving a liquid jet head according to another aspect of the invention, a driving signal in opposite phase to a driving signal to be applied to the main piezoelectric element is applied to a sub piezoelectric element. Therefore, no liquid droplets are jet from a nozzle corresponding to the sub piezoelectric element, and liquid droplets are selectively jet from another nozzle. 
     The sub piezoelectric elements may be separately driven so as to jet liquid droplets of a first size, the main piezoelectric element may be separately driven so as to jet liquid droplets of a second size larger than the first size, and the main piezoelectric element and the sub piezoelectric elements may be simultaneously driven so as to jet liquid droplets of a third size larger than the second size. 
     In this way, if liquid droplets having a plurality of sizes are jet, a liquid jet head is used for various purposes. 
     The sub piezoelectric elements may be driven by a voltage higher than a voltage to be applied to the main piezoelectric element. 
     That is, the main piezoelectric element and the sub piezoelectric elements are driven by voltages depending on the sizes (widths) of the main piezoelectric element and the sub piezoelectric elements, respectively. 
     Therefore, an adequate pressure depending on the size of the pressure chamber is given to the liquid, and thus liquid droplets are satisfactorily jet from a predetermined nozzle. 
     Another aspect of the invention provides a liquid jet apparatus. The liquid jet apparatus includes a liquid jet head and a driving control unit. The liquid jet head includes a plurality of nozzles jetting liquid droplets, pressure generation chambers individually communicating with the nozzles and having a predetermined width, a reservoir common to a plurality of pressure generation chambers, and piezoelectric elements, each causing a change in pressure of a corresponding one of the pressure generation chambers. Each of the pressure generation chambers includes a main pressure chamber communicating with the reservoir through a liquid supply channel, and a plurality of sub pressure chambers arranged in parallel in a width direction of the main pressure chamber, the sub pressure chambers independently communicating with an end surface of the main pressure chamber opposite to the liquid supply channel, and each of the sub pressure chambers communicating with a corresponding one of the nozzles. Each of the piezoelectric elements includes a main piezoelectric element provided to correspond to the main pressure chamber, and sub piezoelectric elements provided to correspond to the sub pressure chambers and have a width smaller than that of the main piezoelectric element. When the main piezoelectric element is driven so as to jet liquid droplets from a predetermined nozzle, the driving control unit applies a driving signal, which is in opposite phase to a driving signal to be applied to the main piezoelectric element, to a sub piezoelectric element corresponding to a sub pressure chamber communicating with a nozzle, which does not jet liquid droplets. 
     In the liquid jet apparatus according to the invention, the driving control unit applies a driving signal, which is in opposite phase to a driving signal to be applied to the main piezoelectric element, to a sub piezoelectric element. Therefore, no liquid droplets are jet from a nozzle corresponding to the sub piezoelectric element, and liquid droplets are selectively jet from another nozzle. 
     Hereinafter, a preferred embodiment of the invention will be described with reference to the drawings. The following embodiment is described as an example of the invention, and all the parts to be described below are not always essential parts. 
     Best Embodiment 
     Hereinafter, an embodiment will be described with reference to the drawings. 
     First Embodiment 
     Hereinafter, the invention will be described in detail with reference to an embodiment. 
       FIG. 1  is a perspective view showing the schematic configuration of an ink jet type recording apparatus, which is an example of a liquid jet apparatus. 
     As shown in  FIG. 1 , an ink jet type recording apparatus I is configured such that cartridges  2 A and  2 B constituting an ink supply unit are detachably mounted on recording head units  1 A and  1 B each having an ink jet type recording head (hereinafter, simply referred to as a recording head), which will be described below, respectively. A carriage  3  having mounted thereon the recording head units  1 A and  1 B is provided so as to freely move in an axial direction with respect to a carriage shaft  5  attached to an apparatus main body  4 . The recording head units  1 A and  1 B eject a black ink composition and a color ink composition, respectively. 
     If a driving force of a driving motor  6  is transmitted to the carriage  3  through a plurality of gears (not shown) and a timing belt  7 , the carriage  3  having mounted thereon the recording head units  1 A and  1 B moves along the carriage shaft  5 . A platen  8  is provided in the apparatus main body  4  along the carriage shaft  3 . A sheet discharge roller  9  is provided near the platen  8  so as to be rotatable by a driving force of a sheet feed motor (not shown). A recording sheet S that is a recording medium, such as paper or the like, fed by the sheet feed roller and the like, is transported. 
     In such an ink jet type recording apparatus I, the carriage  3  moves along the carriage shaft  5 , and ink is ejected by the recording head units (recording head)  1 A and  1 B and printed on the recording sheet S. 
     Next, the configuration of a recording head to be mounted on the above-described ink jet type recording apparatus I will be described.  FIG. 2  is an exploded perspective view showing the schematic configuration of a recording head according to this embodiment.  FIG. 3  is a plan view of  FIG. 2  and a sectional view taken along the line A-A′ of  FIG. 2 . 
     As showing in  FIG. 2  and  FIG. 3 , in this embodiment, a flow channel forming plate  10  constituting a recording head II flow channel forming plate  10  is made of a silicon monocrystal plate having a surface direction ( 110 ). An elastic film  50  made of silicon dioxide is formed on one surface of the flow channel forming plate  10  by thermal oxidization. Pressure generation chambers  12  which are partitioned by a plurality of partition walls  11  are provided in the flow channel forming plate  10  so as to be arranged in parallel in a width direction (lateral direction) of the flow channel forming plate  10 . Ink supply channels  13  and communicating channels  14  are partitioned by the partition walls  11  at an end portion of the flow channel forming plate  10  in a longitudinal direction of each of the pressure generation chamber  12 . A communicating portion  15  that constitutes a part of a reservoir  100  serving as a common ink chamber (liquid chamber) of the pressure generation chambers  12  is formed at one end of each of the communicating channels  14 . 
     Each of the pressure generation chambers  12  includes a main pressure chamber  12   a  communicating with the corresponding ink supply channel  13 , and two sub pressure chambers (first sub pressure chamber  12   b  and second sub pressure chamber  12   c ) arranged in parallel in a width direction of the main pressure chamber  12   a  to independently communicate with an end surface of the main pressure chamber  12   a  opposite to the ink supply channel  13 . The sub pressure chambers  12   b  and  12   c  have a width smaller than that of the main pressure chamber  12   a , and extend in a longitudinal direction of the main pressure chamber  12   a.    
     A nozzle plate  20  is fixed onto an opening surface side of the flow channel forming plate  10  by an adhesive, a thermally welding film, or the like. The nozzle plate  20  is provided with nozzles  21  individually communicating with the sub pressure chambers  12   b  and  12   c . The nozzle plate  20  is made of, for example, glass ceramics, a silicon monocrystal plate, stainless steel, or the like. 
     On a side opposite to the opening surface of the flow channel forming plate  10 , as described above, the elastic film  50  is formed. An insulator film  55  is formed on the elastic film  50 . Piezoelectric elements  300  each having a lower electrode film  60 , a piezoelectric material layer  70 , and an upper electrode film  80  are formed on the insulator film  55 . Each of the piezoelectric elements  300  includes a main piezoelectric element  301  that is provided to be opposite the main pressure chamber  12   a  constituting the corresponding pressure generation chamber  12 , and sub piezoelectric elements (first sub piezoelectric element  302  and second sub piezoelectric element  303 ) that are provided to correspond to the sub pressure chambers  12   b  and  12   c . The main piezoelectric element  301  and the sub piezoelectric elements  302  and  303  are formed to correspond to the main pressure chamber  12   a  and the sub pressure chambers  12   b  and  12   c  so as to have widths smaller than those of the main pressure chamber  12   a  and the sub pressure chambers  12   b  and  12   c , respectively. That is, the sub piezoelectric elements  302  and  303  that are provided to correspond to the sub pressure chambers  12   b  and  12   c  having a width smaller than that of the main pressure chamber  12   a  are formed so as to have a width smaller than that of the main piezoelectric element  301 . 
     In this embodiment, the lower electrode film  60  forms a common electrode of the piezoelectric elements  300 , and the upper electrode film  80  forms individual electrodes of the piezoelectric elements  300 , but this configuration may be reversed depending on the specific arrangement of the driving circuit or wiring. Each piezoelectric element  300  and a vibrating plate where displacement occurs when the piezoelectric element  300  is driven are collectively called an actuator. The vibrating plate refers to a portion which forms one surface of the corresponding pressure generation chamber  12  and at which deformation occurs when the piezoelectric element  300  is driven. In this embodiment, the elastic film  50 , the insulator film  55 , and the lower electrode film  60  serve as the vibrating plate, but the invention is not limited thereto. For example, only the lower electrode film  60  may serve as a vibrating plate, while the elastic film  50  and the insulator film  55  may not be provided. Alternatively, the piezoelectric element  300  itself may serve as a vibrating plate. 
     A lead electrode  90  is connected to the upper electrode film  80  serving as the individual electrode of the main piezoelectric element  301 . The lead electrode  90  is led from near an end portion in the longitudinal direction of the main piezoelectric element  301  to a region outside the pressure generation chamber  12 . The lead electrode  90  is made of, for example, gold (Au) or the like. In this embodiment, the lead electrode  90  extends from near the end portion in the longitudinal direction of the piezoelectric element  300  to a region opposite a through portion  33  of a protective plate  30 , and is connected to a driving IC or the like by a connection wire (not shown) which extends through the through portion  33 . 
     Lead electrodes  91  are connected to the upper electrode films  80  serving as the individual electrodes of the sub piezoelectric elements  302  and  303 , respectively. The lead electrodes  91  are led from near end portions in the longitudinal direction of the sub piezoelectric elements  302  and  303  to regions outside the sub pressure chambers  12   b  and  12   c , respectively. The lead electrodes  91  extend to near the end portion of the flow channel forming plate  10 , and are connected to a driving IC or the like by connection wires (not shown), similarly to the lead electrode  90 . 
     A protective plate  30  is bonded onto the flow channel forming plate  10 . The protective plate  30  is used to protect the piezoelectric elements  300  and has a piezoelectric element holding portion  31 . The piezoelectric elements  300  are formed within the piezoelectric element holding portion  31 , and thus they are protected without being almost influenced by an external environment. The piezoelectric element holding portion  31  may be sealed or unsealed. A reservoir portion  32  is provided in the protective plate  30  so as to constitute at least a part of the reservoir  100 . In this embodiment, the reservoir portion  32  passes through the protective plate  30  in its thickness direction and extends along the width direction of the respective pressure generation chambers  12 . The reservoir portion  32  communicates with the communicating portion  15  of the flow channel forming plate  10  and constitutes the reservoir  100  serving as the ink chamber common to the pressure generation chamber  12 . The through portion  33  is provided in a region of the protective plate  30  between the piezoelectric element holding portion  31  and the reservoir portion  32  to pass through the protective plate  30  in its thickness direction. As described above, near the end portions of the lead electrodes  90  led from the respective piezoelectric elements  300  are exposed through the through portion  33 . The protective plate  30  is made of, for example, glass, a ceramics material, a metal, resin, or the like. Preferably, the protective plate  30  is made of a material having the substantially same thermal expansion coefficient as the flow channel forming plate  10 . 
     A compliance plate  40  having a seal film  41  and a fixed plate  42  is bonded to a region of the protective plate  30  corresponding to the reservoir portion  32 . The seal film  41  is made of a flexible material having low rigidity. The seal film  41  seals one surface of the reservoir portion  32 . The fixed plate  42  is made of a hard material, such as a metal or the like. A region of the fixed plate  42  opposite the reservoir  100  is completely removed in its thickness direction to form an opening  43 , and thus one surface of the reservoir  100  is sealed only by the flexible seal film  41  which has the flexibility. 
     In such a recording head II, ink is supplied from an external ink supply unit (not shown), and filled of ink from the reservoir  100  to the nozzles  21 . A predetermined driving signal is selectively applied to a predetermined piezoelectric element  300  (the main piezoelectric element  301  and the sub piezoelectric elements  302  and  303 ) in accordance with a recording signal from a driving circuit (not shown). When this happens, the piezoelectric element  300  is deformed in a deflection manner. Accordingly, pressure in the corresponding pressure generation chamber  12  increases, and thus ink droplets are jet from a predetermined nozzle  21 . 
     In the recording head II having the above-described configuration, each of the pressure generation chambers  12  includes the main pressure chamber  12   a  and the first and second sub pressure chambers  12   b  and  12   c . For this reason, the nozzles  21  can be comparatively easily arranged with high density. Therefore, a stable ink ejection characteristic can be obtained and print quality can be improved. In addition, the driving frequency can be increased and thus high-speed printing can be realized. 
     As shown in a block diagram of  FIG. 4 , the ink jet type recording apparatus I has a driving control unit  200  for controlling driving of the recording head II, in particular, driving of the piezoelectric elements  300  constituting the recording head II. In the invention, as described above, each of the piezoelectric elements  300  has the main piezoelectric element  301  and the sub piezoelectric elements  302  and  303 . Accordingly, the driving control unit  200  selectively applies the driving signal to the main piezoelectric element  301  and the sub piezoelectric elements  302  and  303  in accordance with input of a print signal such that liquid droplets are jet from a predetermined nozzle communicating with each pressure generation chamber. 
     A driving method of the recording head II by the driving control unit  200  will be described.  FIG. 5  is a diagram showing an example of basic driving signals which are output to the main piezoelectric element  301  and the sub piezoelectric elements  302  and  303 .  FIG. 6  and  FIG. 7  are diagrams showing examples of patterns of driving signals which are applied to the main piezoelectric element  301  and the sub piezoelectric elements  302  and  303 . 
     As shown in  FIG. 5 , the basic driving signals that are applied to the piezoelectric elements  300  include a basic driving signal (COM 1 ) for the main piezoelectric element  301  and a basic driving signal (COM 2 ) for the sub piezoelectric elements  302  and  303 . 
     The basic driving signal (COM 1 ) for the main piezoelectric element  301  has two jet driving signals  400 A and  400 B for jetting ink droplets from the nozzle  21  as one cycle. The basic driving signal (COM 2 ) for the sub piezoelectric elements  302  and  303  has a jet driving signal  500  that is used to jet ink droplets from the nozzle  21 , and a nonjet driving signal  501  that is in opposite phase to the jet driving signal  500  and used not to jet ink droplets from the nozzle  21 . The basic driving signal (COM 2 ) has the successive jet driving signal  500  and nonjet driving signal  501  as one cycle. 
     The basic driving signals (COM 1  and COM 2 ) have voltage depending on the size (width) of each piezoelectric element  300 . Specifically, the basic driving signal (COM 1 ) for the main piezoelectric element  301  has a peak voltage set so as to be lower than that of the basic driving signal (COM 2 ) for the sub piezoelectric elements  302  and  303 . This is because the main piezoelectric element  301  and the sub piezoelectric elements  302  and  303  generate adequate pressure depending on the size of each pressure generation chamber  12 . 
     One jet driving signal in one cycle of the basic driving signal (COM 1 ) is selectively applied to the main piezoelectric element  301 . The jet driving signal  500  and the nonjet driving signal  501  in one cycle of the basic driving signal (COM 2 ) are selectively applied to the first sub piezoelectric element  302  and the second sub piezoelectric element  303 , respectively. In this way, ink droplets are jet from a predetermined nozzle  21 . In this embodiment, the main piezoelectric element  301  and the sub piezoelectric elements  302  and  303  are driven in combination so as to jet ink droplets having different sizes (an ink droplet of a first size (small dot), an ink droplet of a second size larger than the first size (medium dot), and an ink droplet of a third size larger than the second size (large dot)). 
     Hereinafter, the patterns of the driving signals which are applied to the main piezoelectric element  301  and the sub piezoelectric elements  302  and  303  will be described. 
     A pattern  1  of driving signals shown in  FIG. 6(   a ), as shown in Table 1 is an example of driving signals which are used to jet ink droplets of a large dot from a nozzle  21 B communicating with the first sub pressure chamber  12   b  and a nozzle  21 C communicating with the second sub pressure chamber  12   c . Specifically, one jet driving signal  400 A in one cycle selected from the basic driving signal (COM 1 ) is applied to the main piezoelectric element  301 , and the jet driving signal  500  in one cycle selected from the basic driving signal (COM 2 ) is applied to the first and second sub piezoelectric elements  302  and  303 . Accordingly, a change in pressure given to ink in the first and second sub pressure chambers  12   b  and  12   c  is maximized, and as a result, ink droplets of a large dot are jet from the nozzles  21 B and  21 C communicating with the first and second sub pressure chambers  12   b  and  12   c . 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Pattern 
                 Nozzle 
                 Ink Droplet 
               
               
                   
               
             
            
               
                 1 
                 Nozzle 21B 
                 Large 
               
               
                   
                 Nozzle 21C 
                 Large 
               
               
                 2 
                 Nozzle 21B 
                 Large 
               
               
                   
                 Nozzle 21C 
                 OFF 
               
               
                 3 
                 Nozzle 21B 
                 Medium 
               
               
                   
                 Nozzle 21C 
                 Medium 
               
               
                 4 
                 Nozzle 21B 
                 Medium 
               
               
                   
                 Nozzle 21C 
                 OFF 
               
               
                 5 
                 Nozzle 21B 
                 Small 
               
               
                   
                 Nozzle 21C 
                 Small 
               
               
                 6 
                 Nozzle 21B 
                 Small 
               
               
                   
                 Nozzle 21C 
                 OFF 
               
               
                   
               
            
           
         
       
     
     A pattern  2  of driving signals shown in  FIG. 6(   b ) is an example of driving signals which are used to jet ink droplets of a large dot only from the nozzle  21 B, as shown in Table 1. That is, the jet driving signal  400 B selected from the basic driving signal (COM 1 ) is applied to the main piezoelectric element  301 . In addition, the jet driving signal  500  selected from the basic driving signal (COM 2 ) is applied to the first sub piezoelectric element  302 , and the nonjet driving signal  501  is applied to the second sub piezoelectric element  303 . Accordingly, a change in pressure given to ink in the first sub pressure chamber  12   b  is maximized, and an ink droplet of a large dot is jet from the nozzle  21 B communicating with the first sub pressure chamber  12   b . Meanwhile, a change in pressure of the second sub pressure chamber  12   c  due to displacement of the main piezoelectric element  301  is substantially cancelled by a change in pressure due to displacement of the second sub piezoelectric element  303 , and thus no ink droplets are jet from the nozzle  21 C. 
     A pattern  3  of driving signals shown in  FIG. 6(   c ) is an example of driving signals which are used to jet ink droplets of a medium dot from the nozzles  21 B and  21 C, as shown in Table 1. That is, the jet driving signal  400 A selected from the basic driving signal (COM 1 ) is applied to the main piezoelectric element  301 . In addition, neither the jet driving signal  500  nor the nonjet driving signal  501  is applied to the first and second sub piezoelectric elements  302  and  303 . That is, the first and second sub piezoelectric elements  302  and  303  are not displaced. Accordingly, ink droplets of a medium dot are jet from the nozzles  21 B and  21 C due to a change in pressure of ink caused only by displacement of the main piezoelectric element  301 . 
     A pattern  4  of driving signals shown in  FIG. 7(   a ) is an example of driving signals which are used to jet an ink droplet of a medium dot only from the nozzle  21 B, as shown in Table 1. That is, the jet driving signal  400 B selected from the basic driving signal (COM 1 ) is applied to the main piezoelectric element  301 . In addition, neither the jet driving signal  500  nor the nonjet driving signal  501  is applied to the first sub piezoelectric element  302 , and the nonjet driving signal  501  selected from the basic driving signal (COM 2 ) is applied to the second sub piezoelectric element  303 . Accordingly, an ink droplet of a medium dot is jet from the nozzle  21 B communicating with the first sub pressure chamber  12   b  due to a change in pressure caused by displacement of the main piezoelectric element  301 . Meanwhile, a change in pressure of the second sub pressure chamber  12   c  due to displacement of the main piezoelectric element  301  is substantially cancelled by a change in pressure due to displacement of the second sub piezoelectric element  303 , and thus no ink droplets are jet from the nozzle  21 C. 
     A pattern  5  of driving signals shown in  FIG. 7(   b ) is an example of driving signals which are used to jet ink droplets of a small dot from the nozzles  21 B and  21 C, as shown in Table 1. Specifically, the jet driving signal  400  is not applied to the main piezoelectric element  301 , and the jet driving signal  500  selected from the basic driving signal (COM 2 ) is applied to the first and second sub piezoelectric elements  302  and  303 . Accordingly, ink droplets of a small dot are jet from the nozzles  21 B and  21 C due to a change in pressure caused only by displacement of the first and second sub piezoelectric elements  302  and  303 . 
     A pattern  6  of driving signals shown in  FIG. 7(   c ) is an example of driving signals which are used to jet an ink droplet only from the nozzle  21 B, as shown in Table 1. Specifically, the jet driving signal  500  selected from the basic driving signal (COM 2 ) is applied only to the first sub piezoelectric element  302 . That is, while the main piezoelectric element  301  and the second sub piezoelectric element  303  are not displaced, only the first sub piezoelectric element  302  is displaced by the jet driving signal  500 . Accordingly, an ink droplet of a small dot is jet from the nozzle  21 B due to a change in pressure caused by displacement of the first sub piezoelectric element  302 , and no ink droplets are ejected from the nozzle  21 C. 
     As various patterns of driving signals are illustrated, according to the driving method of the invention, ink droplets having different sizes can be jet from the nozzle  21 , and thus fine printing becomes possible. Therefore, it is possible to improve print quality, and also to cope with various kinds of printing. In addition, with respect to a sub pressure chamber communicating with a nozzle which does not jet ink droplets, a nonjet driving signal in opposite phase to a jet driving signal is applied to the corresponding sub piezoelectric element, such that a change in pressure due to displacement of the main piezoelectric element  301  is cancelled by a change in pressure due to displacement of the sub piezoelectric element. As a result, ink droplets can be reliably jet from a predetermined nozzle. 
     The above-described patterns of driving signals are just examples, but the driving method of the invention is not limited to the examples. For example, in any cases, the jet driving signal  400 B with the same timing as the jet driving signal of the basic driving signal (COM 2 ) or the jet driving signal  400 B with the same timing as the nonjet driving signal may be applied to the main piezoelectric element  301 . 
     In this embodiment, a thin film ink jet type recording head that is manufactured by using film deposition and lithography processes has been described, but the invention is not limited thereto. For example, the driving method of the invention may be adopted for a thick film ink jet type recording head that is formed by, for example, patching a green sheet or the like. 
     In this embodiment, the driving method of the invention has been described with reference to an ink jet type recording head which includes pressure generation chambers each having two sub pressure chambers. However, the driving method of the invention may also be applied to an ink jet type recording head which includes pressure generation chambers each having three or more sub pressure chambers. 
     In this embodiment, an ink jet type recording apparatus in which an ink jet type recording head is mounted on a carriage and moves in a main scanning direction has been described, but the invention may also be applied to other types of ink jet type recording apparatuses. For example, the invention may also be applied to a so-called line-type ink jet type recording apparatus that has a plurality of fixed ink jet type recording heads and performs printing only by moving the recording sheet S, such as paper or the like, in a sub scanning direction. 
     In this embodiment, a case in which an ink jet type recording head is used as an example of a liquid jet head has been described. However, the invention is intended for all kinds of liquid jet heads, and of course, it may be applied to a liquid jet head that jets a liquid other than ink. Other examples of the liquid jet head include, for example, various recording heads used in an image recording apparatus, such as a printer or the like, a color material jet head used in manufacturing a color filter for a liquid crystal display or the like, an electrode material jet head used in forming electrodes for an organic EL display, an FED (Field Emission Display), or the like, a bio-organic material jet head used in manufacturing a biochip.