Patent Publication Number: US-2023165151-A1

Title: Piezoelectric actuator

Description:
REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation application of U.S. patent application Ser. No. 17/388,197 filed on Jul. 29, 2021, which is a continuation application of U.S. patent application Ser. No. 16/856,658 filed on Apr. 23, 2020, now U.S. Pat. No. 11,081,636, which is a continuation application of U.S. patent application Ser. No. 16/433,508 filed on Jun. 6, 2019, now U.S. Pat. No. 10,669,372, which is a continuation application of U.S. patent application Ser. No. 15/375,917 filed on Dec. 12, 2016, now U.S. Pat. No. 10,316,138, which is a continuation application of U.S. patent application Ser. No. 14/997,036 filed on Jan. 15, 2016, now U.S. Pat. No. 9,527,284, which is a continuation application of U.S. patent application Ser. No. 14/458,870 filed on Aug. 13, 2014, now U.S. Pat. No. 9,238,368, which is a continuation application of U.S. patent application Ser. No. 12/872,410 filed on Aug. 31, 2010, now U.S. Pat. No. 8,833,910, which claims priority from Japanese Patent Application No. 2009-200744, filed on Aug. 31, 2009, the disclosures of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND ART 
     A piezoelectric actuator has been hitherto known, including a channel unit, a top plate (vibration plate) which is arranged on an upper end of the channel unit, and a piezoelectric layer which is stacked on the vibration plate, wherein individual electrodes facing pressure chambers are arranged on one surface of the piezoelectric layer, and a first constant electric potential electrode which faces substantially central portions of the individual electrodes and a second constant electric potential electrode which faces both end portions in the transverse direction of the individual electrodes are arranged on the other surface of the piezoelectric layer. In the piezoelectric actuator constructed as described above, the portion, of the piezoelectric layer, facing the individual electrode and the first constant electric potential electrode and another portion, of the piezoelectric layer, facing the individual electrode and the second constant electric potential electrode are simultaneously deformed. Accordingly, the portions, of the piezoelectric layer and the vibration plate, which face the pressure chambers are deformed. Accordingly, it is possible to avoid the so-called crosstalk which would be otherwise caused such that the deformation of any portion of the piezoelectric layer facing a certain pressure chamber is transmitted to another portion facing another pressure chamber. 
     Another piezoelectric actuator has been known, including a vibration plate, a piezoelectric layer which is stacked on an upper surface of the vibration plate, pressure chambers which are aligned in two arrays in one direction on a surface, of the piezoelectric layer, facing the vibration plate, individual electrodes which are arranged at portions of the vibration plate facing the pressure chambers, a common electrode which is arranged to extend continuously while ranging over the plurality of pressure chambers on the other surface, of the piezoelectric layer, not facing the vibration plate, a driver IC which is arranged on the upper surface of the vibration plate, and wiring lines which are led out to the upper surface of the vibration plate and which connect the driver IC and the electrodes to one another. In the piezoelectric actuator constructed as described above, the wiring lines, which are connected to the individual electrodes, are laid out on the same surface, of the vibration plate, as the surface on which the individual electrodes are arranged, and the wiring lines are connected to the driver IC. Therefore, the reliability is enhanced for the connection between the individual electrodes and the driver IC. In other words, the various electrodes such as the individual electrodes and the like are connected to the driver IC without using any wiring member such as FPC or the like. Therefore, the reliability is enhanced for the electrical connection and the physical connection between the electrodes and the driver IC. 
     DESCRIPTION 
     The present inventors have contrived a piezoelectric actuator which has two types of common electrodes, wherein wiring lines, which are connected to various electrodes, are led out to an upper surface of a vibration plate, and the wiring lines are connected to a driver IC. In the case of the piezoelectric actuator constructed as described above, it is also possible to avoid the crosstalk. Further, the electrodes such as individual electrodes and the like can be connected to the driver IC without using any wiring member such as FPC or the like. Therefore, the reliability is enhance for the physical and electrical connection between the driver IC and the electrodes such as the individual electrodes and the like. 
     Further, the present inventors have found out such a possibility that the electric potential distribution of a common electrode may be nonuniform on account of the following reason in relation to a piezoelectric actuator contrived by the present inventors. Thus, the present disclosure has been achieved. 
     According to the knowledge of the present inventors, it is preferable that the electric potential of the common electrode is uniform at all portions in order to uniformize the driving characteristics of the portions corresponding to respective pressure chambers in the piezoelectric actuator. If the wiring line for applying the electric potential is connected to the common electrode at only one position, it is feared that the electric potential (first electric potential) of the portion (first portion) of the common electrode connected to the wiring line is not identical with the electric potential (second electric potential) of the portion (second portion) separated from the terminal portion. According to the knowledge of the present inventors, it is feared that the second electric potential may be deviated from the first electric potential as the distance of electrical connection is more prolonged between the first portion and the second portion. In view of the above, the present inventors have found out the fact that it is necessary to connect the wiring lines at a plurality of mutually separated positions on the common electrode in order to suppress the nonuniform electric potential. 
     The two types of common electrodes described above are provided corresponding to the pressure chambers (individual electrodes) which are arranged in two arrays in one direction. Therefore, at least one of the two types of common electrodes has two portions which correspond to the individual electrodes included in the respective arrays and which are arranged while being separated from each other with the other common electrode intervening therebetween in relation to the direction perpendicular to the one direction. Therefore, in order to uniformize the electric potential between the mutually separated two portions, it is necessary that the portions should be in conduction with each other. 
     In this case, assume that the common electrode has the portions which are separated from each other as described above and the common electrode has no portion which allows the portions to be in conduction. Then it is possible to connect the wiring lines to the mutually separated portions respectively for the two types of common electrodes on the surface of the piezoelectric layer on which the common electrode is arranged. However, the present inventors have found out the following fact. That is, in this case, it is feared that any dispersion (fluctuation) may arise in the electric potential between the mutually separated portions. 
     An object of the present disclosure is to provide a piezoelectric actuator which is constructed such that two types of common electrodes retained at mutually different electric potentials are arranged on an identical surface of a piezoelectric layer and which makes it possible to uniformize the electric potentials of the two types of common electrodes at all portions respectively, and a liquid discharge apparatus provided with the same. 
     According to an aspect of the present disclosure, there is provided a piezoelectric actuator including: a vibration plate; a piezoelectric layer which is stacked on the vibration plate; a plurality of individual electrodes which are arranged on one surface of the piezoelectric layer and which form individual electrode arrays arranged in two arrays in one direction; a first common electrode which is arranged on the other surface of the piezoelectric layer and which has a plurality of first facing portions each facing a part of one of the individual electrodes and a first connecting portion connecting the first facing portions; a second common electrode which is arranged on the other surface of the piezoelectric layer, which is electrically insulated from the first common electrode, and which has a plurality of second facing portions each facing another part of one of the individual electrodes and a second connecting portion connecting the second facing portions; a plurality of first connecting wirings which are connected to the first common electrode, in the first connecting portion at a plurality of mutually separated positions; a plurality of first wiring sections which are arranged on the vibration plate and which are connected to the first common electrode via the first connecting wirings; a plurality of second connecting wirings which are connected to the second common electrode, in the second connecting portion at a plurality of mutually separated positions; and a plurality of second wiring sections which are arranged on the vibration plate and which are connected to the second common electrode via the second connecting wirings. One of the first connecting wirings connects the first connecting portion and one of the first wiring sections while striding over the second common electrode. 
     Accordingly, the first connecting wiring, which connects the first connecting portion and the first wiring portion to one another while striding over (stepping over or crossing over) the second connecting portion of the second common electrode, is provided, and thus the first and second connecting wirings can be connected to the plurality of mutually separated positions of the first and second common electrodes arranged on the same surface of the piezoelectric layer. It is possible to uniformize the electric potentials of the first and second common electrodes, respectively. The description is made, for example, in the following embodiments for the purpose of convenience assuming that the common electrode, which has the facing portions facing the substantially central portions of the individual electrodes, is regarded as the first common electrode, and the common electrode, which has the facing portions facing the portions except for the substantially central portions of the individual electrodes, is regarded as the second common electrode. However, no problem arises even if it is assumed that the common electrode, which has the facing portions facing the substantially central portions of the individual electrodes, is regarded as the second common electrode, and the common electrode, which has the facing portions facing the portions except for the substantially central portions of the individual electrodes, is regarded as the first common electrode. 
     According to the present disclosure, it is possible to uniformize the electric potentials of the first common electrode and the second common electrode arranged on the same surface of the piezoelectric layer respectively. 
    
    
     
       BRIEF EXPLANATION OF DRAWINGS 
         FIG.  1    shows a schematic arrangement of a printer according to a first embodiment. 
         FIG.  2    shows a plan view illustrating an ink-jet head shown in  FIG.  1   . 
         FIG.  3    shows a plan view illustrating an upper surface of a vibration plate shown in  FIG.  2   . 
         FIG.  4    shows a plan view illustrating an upper surface of a piezoelectric layer shown in  FIG.  2   . 
         FIG.  5    shows a sectional view taken along a line V-V shown in  FIG.  2   . 
         FIG.  6    shows a sectional view taken along a line VI-VI shown in  FIG.  2   . 
         FIG.  7    shows a view according to a first modified embodiment corresponding to  FIG.  4   . 
         FIG.  8    shows a view according to a second modified embodiment corresponding to  FIG.  6   . 
         FIG.  9    shows a view according to a third modified embodiment corresponding to  FIG.  6   . 
         FIG.  10    shows a view according to a fourth modified embodiment corresponding to  FIG.  4   . 
         FIG.  11    shows a view according to a second embodiment corresponding to  FIG.  4   . 
         FIG.  12    shows a view according to a fifth modified embodiment corresponding to  FIG.  4   . 
     
    
    
     FIRST EMBODIMENT 
     A first embodiment of the present teaching will be explained below. 
     As shown in  FIG.  1   , a printer  1  includes, for example, a carriage  2 , an ink-jet head  3 , and a transport roller  4 . The carriage  2  is reciprocatively movable along a guide shaft  5  which extends in the scanning direction (left-right direction in  FIG.  1   ). The ink-jet head  3  is arranged on a lower surface of the carriage  2 , and the ink-jet head  3  discharges inks from nozzles  15  (see  FIG.  2   ) arranged on the lower surface thereof. The transport roller  4  (transport mechanism) transports the recording paper P in the paper feeding direction (frontward direction in  FIG.  1   ) in cooperation with an unillustrated motor. 
     In the printer  1 , the inks are discharged to the recording paper P, which is transported in the paper feeding direction by the transport roller  4 , by the ink-jet head  3  which is reciprocatively moved in the scanning direction together with the carriage  2 , and thus the printing is performed on the recording paper P. The recording paper P, for which the printing has been completed, is transported in the paper feeding direction by the transport roller  4 , and thus the recording paper P is discharged from the printer  1 . 
     Next, the ink-jet head  3  will be explained. In order to recognize the positional relationship more comprehensively, a piezoelectric layer  42  described later on is depicted by two-dot chain lines in  FIG.  3   , and a through-hole  46   a  of an insulating layer  46  described later on and a portion of a wiring line  47   e  arranged on the upper surface of the insulating layer  46  are depicted by two-dot chain lines respectively in  FIG.  4   . 
     The ink-jet head  3  includes a channel unit  31  which is formed with ink channels including, for example, pressure chambers  10  and the nozzles  15 , and a piezoelectric actuator  32  which is arranged on the upper surface of the channel unit  31  and which applies the pressure to the inks contained in the pressure chambers  10 . 
     As shown in  FIGS.  5  and  6   , the channel unit  31  is formed by mutually stacking four plates of a cavity plate  21 , a base plate  22 , a manifold plate  23 , and a nozzle plate  24 . The three plates  21  to  23  except for the nozzle plate  24 , which are included in the four plates  21  to  24 , are formed of a metal material such as stainless steel. The nozzle plate  24  is formed of a synthetic resin material such as polyimide. Alternatively, the nozzle plate  24  may be also formed of a metal material in the same manner as the other three plates  21  to  23 . 
     A plurality of pressure chambers  10  and an ink supply port  9  (see  FIG.  2   ) are arranged in the cavity plate  21 . The pressure chambers  10  have substantially elliptical shapes in a plan view in which the scanning direction (left-right direction in  FIG.  2   ) is the longitudinal direction of the elliptical pressure chamber  10 , and the pressure chambers  10  are arranged in two arrays in the paper feeding direction (one direction). Substantially circular through-holes  12 ,  13  are arranged respectively at portions of the base plate  22  opposed to the both end portions of the plurality of pressure chambers  10  in the scanning direction. The ink supply port  9  is formed at a position communicated with a manifold channel  11  as described later on. 
     The manifold plate  23  is formed with the manifold channel  11  which extends in the paper feeding direction (up-down direction in  FIG.  2   ) so that the manifold channel  11  is overlapped with substantially half portions of the pressure chambers  10  in each of the arrays disposed on the side of the through-holes  12  in the scanning direction. In this arrangement, the ink is supplied from the ink supply port  9  to the manifold channel  11 . Further, the manifold plate  23  is formed with substantially circular through-holes  14  at portions overlapped with the through-holes  13 . The nozzle plate  24  is formed with the nozzles  15  at portions overlapped with the through-holes  14 . 
     In the channel unit  31 , the manifold channel  11  is communicated with the pressure chambers  10  via the through-holes  12 , and the pressure chambers  10  are communicated with the nozzles  15  via the through-holes  13 ,  14 . In this way, a plurality of individual ink channels, which range from the outlets of the manifold channel  11  via the pressure chambers  10  to arrive at the nozzles  15 , are formed in the channel unit  31 . 
     The piezoelectric actuator  32  is provided with a vibration plate  41 , a piezoelectric layer  42 , a plurality of individual electrodes  43 , common electrodes  44 ,  45 , an insulating layer  46 , and a plurality of wiring lines  47 . 
     The vibration plate  41  is arranged on the upper surface of the cavity plate  21  so that the plurality of pressure chambers  10  are covered therewith. The vibration plate  41  is formed of, for example, a ceramics material such as alumina, zirconia, PZT described later on or the like or a metal material such as stainless steel or the like. However, when the vibration plate  41  is formed of the metal material, in order to avoid the conduction between the conductive vibration plate  41  and the individual electrodes  43  and/or the wiring lines  47  described later on, it is necessary that an insulating layer (not shown), which is formed of, for example, a ceramics material, should be arranged on the upper surface of the vibration plate  41  and that the individual electrodes  43  and the wiring lines  47  should be arranged thereon. 
     The piezoelectric layer  42  is formed of a piezoelectric material (PZT) containing a main component of lead titanate zirconate as the mixed crystal of lead titanate and lead zirconate. The piezoelectric layer  42  is arranged on the upper surface of the vibration plate. The piezoelectric layer  42  extends continuously while ranging over the portions facing the plurality of pressure chambers  10 . 
     The individual electrodes  43  are arranged on the upper surface of the vibration plate  41  (lower surface (one surface) of the piezoelectric layer  42 ), corresponding to the pressure chambers  10 . The individual electrodes  43  have substantially elliptical shapes which are slightly larger than those of the pressure chambers  10  in a plan view, and the individual electrodes  43  are arranged so that the pressure chambers  10  are entirely covered therewith. Accordingly, the individual electrodes  43  are arranged in two arrays in the paper feeding direction in the same manner as the pressure chambers  10 . The individual electrodes  43  are connected to a driver IC  50  as described later on. Any one of the electric potentials of the ground electric potential and the predetermined positive electric potential (for example, about 20 V) is selectively applied by the driver IC  50 . 
     The common electrode  44  (first common electrode) is arranged on the upper surface of the piezoelectric layer  42 . Further, the common electrode  44  is connected to the driver IC  50  as described later on. The common electrode  44  is always retained at the ground electric potential by the driver IC  50 . The common electrode  44  has a plurality of facing portions  44   a  (first opposing portions, first facing portion), two connecting portions  44   b , and a joining portion  44   c . In this embodiment, a combination of the two connecting portions  44   b  and the joining portion  44   c  corresponds to the first connecting portion according to the present teaching. 
     The facing portions  44   a  have substantially elliptical shapes which are one size smaller than those of the pressure chambers  10  as viewed in a plan view, and the facing portions  44   a  are arranged so that the facing portions  44   a  face substantially central portions (certain portions) of the pressure chambers  10  (individual electrodes  43 ). Accordingly, the facing portions  44   a  are arranged in two arrays in the paper feeding direction in the same manner as the individual electrodes  43 . The two connecting portions  44   b  extend in the paper feeding direction (one direction) respectively. The two connecting portions  44   b  mutually connect left end portions of the facing portions  44   a  constituting the left array and right end portions of the facing portions  44   a  constituting the right array. The joining portion  44   c  extends in the scanning direction. The joining portion  44   c  mutually connects upper end portions of the two connecting portions  44   b.    
     The common electrode  45  (second common electrode) is arranged in an area on the same upper surface of the piezoelectric layer  42  as the surface on which the common electrode  44  is arranged, the area being surrounded by the two connecting portions  44   b  and the joining portion  44   c . The common electrode  45  is provided with a plurality of facing portions  45   a  (second opposing portions, second facing portions) and a connecting portion  45   b  (second connecting portion). The facing portions  45   a  are arranged to face outer portions, of the pressure chambers  10 , disposed outside the facing portions of the pressure chambers  10  facing the common electrode  44 . The connecting portion  45   b  is the portion, of the common electrode  45 , as provided by excluding the facing portions  45   a  from the common electrode  45 . The connecting portion  45   b  mutually connects the facing portions  45   a.    
     The common electrode  45  is connected to the driver IC  50  as described later on. The common electrode  45  is always retained at the constant positive electric potential by the driver IC  50 . 
     In this arrangement, the portions of the piezoelectric layer  42  described above which are interposed between the individual electrodes  43  and the common electrode  44  (facing portions  44   a ) and the portions interposed between the individual electrodes  43  and the common electrode  45  (facing portions  45   a ) are polarized in the thickness direction of the piezoelectric layer  42 . 
     The insulating layer  46  is formed of an insulative material including, for example, a ceramics material such as alumina, zirconia, PZT or the like or a synthetic resin material. The insulating layer  46  is arranged over the substantially entire region of the upper surface of the piezoelectric layer  42  so that the common electrodes  44 ,  45  are covered therewith. The common electrode  44  and the common electrode  45  are arranged closely to one another on the same upper surface of the piezoelectric layer  42 . Therefore, it is feared that the common electrode  44  and the common electrode  45  may be in conduction on account of the migration caused by the material for forming the electrodes. However, the insulating layer  46  is arranged to cover the common electrode  44  and the common electrode  45 . Therefore, the occurrence of the migration as described above is avoided. Further, when a thin-layered insulating layer  46  is used, it is possible to thin the piezoelectric actuator  32 . A substantially circular through-hole  46   a , which is filled with a conductive material, is formed through the insulating layer  46  at a portion opposed to a right-upward end portion of the common electrode  45  as viewed in  FIG.  2   . 
     The plurality of wiring lines  47  include a plurality of wiring lines  47   a  which are individually connected to the individual electrodes  43 , two wiring lines  47   b ,  47   c  which are connected to the common electrode  44 , and two wiring lines  47   d ,  47   e  which are connected to the common electrode  45 . First ends of the wiring lines  47  are connected to the respective electrodes  43 ,  44 ,  45 , and second ends of the wiring lines  47  are connected to the driver IC  50  arranged on the vibration plate  41 .  FIGS.  2  to  4    show portions, of the wiring lines  47 , disposed in the vicinity of the connecting portions with respect to the electrodes  43  to  45  and other portions disposed in the vicinity of the connecting portions with respect to the driver IC  50 . The other portions of the wiring lines  47  are omitted from the illustration. 
     The wiring lines  47   a  to  47   e  will be explained in further detail below. The first end of the wiring line  47   a  is connected to the end portion of the individual electrode  43  disposed on the side opposite to the nozzle  15  in the scanning direction. Further, the wiring line  47   a  is laid out from the concerning portion on the upper surface of the vibration plate  41  which is positioned on the same surface as the surface on which the individual electrode  43  is arranged. The second end of the wiring line  47   a  is connected to the driver IC  50 . 
     In this arrangement, the wiring lines  47   a , which are provided individually for the individual electrodes  43 , have the largest number among the wiring lines  47 . However, the wiring lines  47   a  are laid out on the same surface as the surface on which the individual electrodes  43  are arranged, and the wiring lines  47   a  are connected to the driver IC  50 . Therefore, for example, the disconnection or breaking of the wire is hardly caused, and the reliability of the connection is enhanced. 
     As shown in  FIG.  2   , the first ends of the two wiring lines  47   b ,  47   c  are connected to the upper end portions of the left connecting portion  44   b  and the right connecting portion  44   b , respectively, on the upper surface of the piezoelectric layer  42 . Further, the wiring lines  47   b ,  47   c  are led out, from the connecting portions, to the upper surface of the vibration plate  41  via the left and right side surfaces of the piezoelectric layer  42  as viewed in  FIG.  2   , respectively. Further, the wiring lines  47   b ,  47   c  are laid out on the vibration plate  41 , and the second ends of the wiring lines  47   b ,  47   c  are connected to the driver IC  50 . In this embodiment, the portions of the two wiring lines  47   b ,  47   c , which are arranged on the upper surface of the vibration plate  41 , correspond to the first wiring portions according to the present teaching, and the portions, which connect the first wiring line portions and the connecting portions  44   b , correspond to the first connecting wirings according to the present teaching. 
     The first end of the wiring line  47   d  is connected to the lower-left end portion (connecting portion  45   b ) of the common electrode  45  as viewed in  FIG.  2    on the upper surface of the piezoelectric layer  42 . The wiring line  47   d  is led out from the connecting portion to the upper surface of the vibration plate  41  via the lower side surface of the piezoelectric layer  42  as viewed in  FIG.  2   . Further, the wiring line  47   d  is laid out on the upper surface of the vibration plate  41 , and the second end of the wiring line  47   d  is connected to the driver IC  50 . 
     The first end of the wiring line  47   e  is positioned at the portion of the upper surface of the insulating layer  46  (surface disposed on the side opposite to the piezoelectric layer  42 ) facing the through-hole  46   a , and the first end of the wiring line  47   e  is connected to the upper-right end portion (connecting portion  45   b ) of the common electrode  45 , as viewed in  FIG.  2   , via the conductive material charged into the through-hole  46   a . The wiring line  47   e  extends from the connecting portion on the upper surface of the insulating layer  46  to the upper end of the insulating layer  46  in the upward direction as viewed in  FIG.  2   . In this arrangement, the wiring line  47   e  is arranged on the upper surface of the insulating layer  46  so that the wiring line  47   e  strides over (steps over) the joining portion  44   c  of the common electrode  44 . 
     As shown in  FIG.  6   , the wiring line  47   e  is further led out to the upper surface of the vibration plate  41  via the insulating layer  46  and the side surface of the piezoelectric layer  42 . The wiring line  47   e  is laid out on the vibration plate  41 , and the second end of the wiring line  47   e  is connected to the driver IC  50 . 
     In this embodiment, the portions of the wiring lines  47   d ,  47   e , which are arranged on the upper surface of the vibration plate  41 , correspond to the second wiring portions according to the present teaching. The portions of the wiring lines  47   d ,  47   e , which connect the second wiring portions and the common electrode  45  (connecting portion  45   b ), correspond to the second connecting wirings according to the present teaching. In the first embodiment, as described above, the second connecting wiring line extends while striding over the first connecting portion. 
     In this arrangement, unlike the wiring lines  47   a  connected to the individual electrodes  43 , the wiring lines  47   b  to  47   e , which are connected to the common electrodes  44 ,  45 , are led out from the connecting portions with respect to the respective electrodes to the upper surface of the vibration plate  41  which is positioned at the height different from that of the connecting portions. That is, the first and second connecting wirings, which are connected to the first and second wiring line portions, are arranged on the surface having the height (position in relation to the stacking direction of the vibration plate  41  and the piezoelectric layer  42 ) different from that of the surface on which the first and second wiring portions are arranged. Therefore, it is feared that the reliability of the connection may be deteriorated or lowered as compared with the wiring lines  47   a . However, as described above, the two wiring lines  47   b ,  47   c  are connected to the common electrode  44 , and the two wiring lines  47   d ,  47   e  are connected to the common electrode  45 . Therefore, even if any one of the two wiring lines is discontinued or broken, any fear, in which the electric potential of the electrode may be greatly fluctuated, is not caused. Any fear, in which the piezoelectric actuator  32  cannot be driven, is not caused as well. 
     When the numbers of the wiring lines connected to the common electrodes  44 ,  45  are further increased, it is also possible to improve the reliability of the connection. Even in this case, the numbers can be increased with ease as compared with the case in which the number of the wiring lines connected to the respective individual electrodes  43  having the large number is further increased. 
     The method for driving the piezoelectric actuator  32  will now be explained. In the piezoelectric actuator  32 , the plurality of individual electrodes  43  are previously retained at the ground electric potential. As described above, the constant positive electric potential is applied to the common electrode  45 . Therefore, the electric potential difference arises between the individual electrodes  43  and the common electrode  45  (facing portions  45   a ). The electric field, which is in the direction parallel to the polarization direction, is generated at the portion (first active portion) of the piezoelectric layer  42  interposed between the electrodes. Accordingly, the concerning portion of the piezoelectric layer  42  is shrunk in the horizontal direction perpendicular to the direction of the electric field. As a result, the portions of the vibration plate  41 , the piezoelectric layer  42 , and the insulating layer  46 , which face the pressure chambers  10 , are deformed as a whole to protrude toward the opposite side of the pressure chambers  10 . The volume of the pressure chambers  10  are increased as compared with the state in which the deformation is not caused. 
     When the piezoelectric actuator  32  is driven, the electric potential of any desired individual electrode  43  is switched from the ground electric potential to the positive electric potential. In this situation, the individual electrode  43 , which has been switched to have the positive electric potential, has the electric potential equal to that of the common electrode  45 . Therefore, the portion of the piezoelectric layer  42 , which is interposed between the individual electrode  43  and the common electrode  45 , is returned to the state having been provided before the shrinkage. On the other hand, as described above, the electric potential difference arises between the individual electrode  43  and the common electrode  44  (facing portion  44   a ), because the ground electric potential is applied to the common electrode  44 . Accordingly, the electric field, which is directed in the direction parallel to the polarization direction, is generated at the portion (second active portion) of the piezoelectric layer  42  interposed between the individual electrode  43  and the common electrode  44  (facing portion  44   a ). The concerning portion of the piezoelectric layer  42  is shrunk in the horizontal direction perpendicular to the direction of the electric field. Accordingly, the portions of the vibration plate  41 , the piezoelectric layer  42 , and the insulating layer  46 , which are opposed to the corresponding pressure chamber  10 , are deformed as a whole to protrude toward the corresponding pressure chamber  10 , and the volume of the pressure chamber  10  is decreased. As a result, the pressure of the ink contained in the pressure chamber  10  is raised, and the ink is discharged from the nozzle  15  communicated with the corresponding pressure chamber  10 . After the ink discharge, the electric potential of the individual electrode  43  is switched from the positive electric potential to the ground electric potential, and the piezoelectric actuator  32  is returned to the initial state. 
     In this situation, in the piezoelectric actuator  32 , when the electric potential of the individual electrode  43  is switched from the ground electric potential to the positive electric potential, the portion of the piezoelectric layer  42 , which is interposed between the individual electrode  43  and the facing portion  44   a , is shrunk in the horizontal direction. Further, simultaneously therewith, the portion of the piezoelectric layer  42 , which is interposed between the individual electrode  43  and the facing portion  45   a , is elongated from the shrunk state to the state having been provided before the shrinkage. Accordingly, the influence exerted by the shrinkage of the piezoelectric layer  42  and the influence exerted by the elongation thereof are counteracted with each other. As a result, it is possible to avoid the so-called crosstalk which would be otherwise caused such that the deformation of any portion of the piezoelectric layer  42  facing a certain pressure chamber  10  is transmitted to any other portion facing another pressure chamber  10 . 
     In order to uniformize, for example, the discharge speed and the volumes of the ink droplets discharged from the respective nozzles  15  when the piezoelectric actuator  32  is driven, it is necessary that the electric field, which is generated in the piezoelectric layer  42 , should be uniformized. For this purpose, it is necessary that the electric potentials of the common electrodes  44 ,  45  should be uniformized at all portions respectively. It is hypothetically assumed that one contact is provided between each of the wiring lines  47  and each of the common electrodes  44 ,  45 . In this case, it is difficult to uniformize the electric potentials at all portions of the common electrodes  44 ,  45  respectively, because the common electrodes  44 ,  45  are the relatively large electrodes which are provided to range over the plurality of pressure chambers  10  (individual electrodes  43 ). That is, the electric potentials of the common electrodes  44 ,  45  are not uniform in all areas. As the portions of the common electrodes  44 ,  45  are separated farther from the portions of connection with respect to the wiring lines  47 , the electric potentials of the concerning portions are more deviated from the electric potentials provided at the contact portions with respect to the wiring lines  47 . Therefore, in order to uniformize the electric potential, as described above, it is necessary that the wiring lines  47  (first and second connecting wirings) should be connected at a plurality of positions of the common electrodes  44 ,  45  separated from each other. 
     As for the common electrode  44 , the two connecting portions  44   b  are arranged while being separated from each other in the scanning direction. Therefore, in order to uniformize the electric potential of the common electrode  44 , it is necessary that the two connecting portions  44   b  should be connected to one another. 
     However, it is difficult that, on the upper surface of the piezoelectric layer  42 , all of the wiring lines  47 , which are connected to the plurality of mutually separated portions of the common electrodes  44 ,  45 , are connected to the common electrodes  44 ,  45 , and that, on the upper surface of the piezoelectric layer  42 , the two connecting portions  44   b  are connected to one another. 
     This situation will be explained specifically below. When the common electrode  44  has the joining portion  44   c , and the two connecting portions  44   b  are connected to one another by the joining portion  44   c  on the upper surface of the piezoelectric layer  42  as in this embodiment, then the electric potential is uniformized for the two connecting portions  44   b . However, in this case, the common electrode  45  is surrounded on the three sides by the two connecting portions  44   b  and the joining portion  44   c . Therefore, if the situation holds as it is, it is inevitable that the wiring line  47   d , which is connected to the common electrode  45 , should be connected to only the lower end portion of the common electrode  45 . In this case, it is feared that the electric potential provided at the upper end portion of the common electrode  45  separated from the connecting portion with respect to the wiring line  47  and the electric potential provided in the area near to the connecting portion with respect to the wiring line  47   d  may be nonuniform. 
     On the other hand, if the joining portion  44   c  is removed from the common electrode  44 , then the wiring lines  47  can be connected to the upper end portion and the lower end portion of the common electrode  45  (connecting portion  45   b ) on the upper surface of the piezoelectric layer  42 , and hence it is possible to uniformize the electric potential of the common electrode  45 . However, in this case, the two connecting portions  44   b  are not connected to one another. Therefore, it is feared that any dispersion may arise in the electric potential between the two connecting portions  44   b.    
     On the contrary, in this embodiment, as described above, the two connecting portions  44   b  of the common electrode  44  are connected by the joining portion  44   c , and the wiring line  47   d  is connected to the lower end portion of the connecting portion  45   b  of the common electrode  45 . Further, the wiring line  47   e  (second connecting wiring), which extends on the upper surface of the insulating layer  46  while striding over the joining portion  44   c , is connected to the upper end portion of the connecting portion  45   b . Therefore, it is possible to uniformize the electric potentials of the common electrodes  44 ,  45  respectively. 
     Further, the wiring line  47   e , which is connected to the common electrode  45  while striding over the joining portion  44   c , is arranged on the upper surface of the insulating layer  46  which covers the common electrodes  44 ,  45  therewith. Therefore, it is possible to prevent the wiring line  47   c  from any conduction with the joining portion  44   c . The portion of the common electrode  45  to which the wiring line  47   e  is connected and the portion to which the wiring line  47   d  is connected are formed at the positions (positions substantially in point symmetry with respect to the center of the common electrode  45 ) separated from each other in the diagonal line direction of the substantially rectangular common electrode  45 , in order to secure the distance between these portions. In this way, in order to uniformize the electric potential distribution of the common electrode as uniformly as possible, it is desirable that the wiring lines, which supply the electric potential to the common electrode, are connected to the common electrode at the positions which are separated from each other as far as possible. 
     Next, modified embodiments, in which various modifications are applied to the first embodiment, will be explained. However, the components or parts, which are constructed in the same manner as those of the embodiment of the present teaching, are designated by the same reference numerals, any explanation of which will be appropriately omitted. 
     In a first modified embodiment, as shown in  FIG.  7   , the common electrode  44  further includes a joining portion  44   d  which connects the lower end portions of the two connecting portions  44   b . Accordingly, the common electrode  45  is surrounded on the four sides by the two connecting portions  44   b  and the two joining portions  44   c ,  44   d.    
     A wiring line  47   f  is provided in place of the wiring line  47   d . The first end of the wiring line  47   f  is connected to the lower-left end portion of the common electrode  45  via the conductive material charged into the through-hole  46   a  formed at the lower-left end portion of the insulating layer  46  opposed to the common electrode  45 . The wiring line  47   f  extends downwardly from the connecting portion on the upper surface of the insulating layer  46  (see  FIG.  5   ) while striding over (stepping over) the joining portion  44   d.    
     When the common electrode  45  is surrounded on the four sides by the two connecting portions  44   b  and the two joining portions  44   c ,  44   d , the wiring lines cannot be connected to the common electrode  45  on the upper surface of the piezoelectric layer  42 . Even in such a situation, the wiring lines  47   e ,  47   f , which extend while striding over the joining portions  44   c ,  44   d , are provided as the wiring lines which are connected to the common electrode  45 . Accordingly, it is possible to connect the wiring lines to the mutually separated portions of the common electrode  45 , and it is possible to uniformize the electric potential of the common electrode  45 . 
     In this case, the lower end portions of the two connecting portions  44   b  are connected to one another by the joining portion  44   d  in addition to the fact that the upper end portions of the two connecting portions  44   b  are connected to one another by the joining portion  44   c . Therefore, the electric potential of the common electrode  44  is further uniformized. 
     In the first embodiment, the portion of the wiring line  47   e  (second connecting wiring), which extends while striding over the joining portion  44   c , is arranged on the upper surface of the insulating layer  46  which is arranged on the upper surface of the piezoelectric layer  42  so that the entire common electrodes  44 ,  45  are covered therewith. However, the present teaching is not limited thereto. It is not necessarily indispensable that the insulating layer is formed to cover the entire common electrode. 
     In a second modified embodiment, as shown in  FIG.  8   , an insulating layer  81  is arranged on the upper surface of the piezoelectric layer  42  so that only the joining portion  44   c  is covered therewith. The portion of the wiring line  47   e , which extends while striding over the joining portion  44   c , is arranged on the upper surface of the insulating layer  81 . Further, another insulating layer  82  is arranged on the upper surface of the piezoelectric layer  42  on which the insulating layer  81  and the abovementioned portion of the wiring line  47   e  are arranged so that the common electrodes  44 ,  45  are covered therewith. The common electrode  44  and the common electrode  45  are prevented from any conduction on the upper surface of the piezoelectric layer  42  owing to the insulating layer  82 . 
     Also in this case, it is possible to avoid any conduction between the wiring line  47   e  and the joining portion  44   c  by means of the insulating layer  81 . 
     In a third modified embodiment, the conductive material is not charged into the through-hole  46   a  of the insulating layer  46 . As shown in  FIG.  9   , a first end of a wire bonding line  91  is connected to a portion (connecting portion  45   b ) of the common electrode  45  exposed from the through-hole  46   a , and a second end of the wire bonding line  91  is connected to a wiring line  47   g  arranged on the upper surface of the vibration plate  41 . Accordingly, the common electrode  45  and the wiring line  47   g  are connected to one another by the wire bonding line  91 . Further, in order to avoid any exfoliation of the wire bonding line  91 , an insulating material  92 , which is composed of, for example, a synthetic resin material and which is formed by the potting, is arranged to cover not only the wire bonding line  91 , but also the connecting portions with respect to the common electrode  45  and the wiring line  47   g . In this case, the wiring line  47   g  corresponds to the second wiring portion according to the present teaching, and the wire bonding line  91  corresponds to the second connecting wiring according to the present teaching. 
     In this arrangement, the wire bonding line  91  and the common electrode  45  are connected to one another and the wire bonding line  91  and the wiring line  47   g  are connected to one another, for example, by means of the ultrasonic welding. In this case, the first end and the second end of the wire bonding line  91  are pressed against the foregoing portion of the common electrode  45  and the wiring line  47   g  respectively (by applying the pressure) to effect the abutment. In this state, the ultrasonic wave is applied to the abutment portions. Accordingly, the portions of the wire bonding line  91  and the common electrode  45  (the portions of the wire bonding line  91  and the wiring line  47   g ), at which they mutually make the abutment, are converted into the alloy, and the wire bonding line  91  and the common electrode  45  (the wire bonding line  91  and the wiring line  47   g ) are connected to one another. 
     Also in this case, the portion of the common electrode  45 , which is surrounded by the connecting portions  44   b  (see  FIG.  4   ) and the joining portion  44   c , can be connected to the wiring line  47   g  by means of the wire bonding line  91 . Therefore, it is possible to uniformize the electric potential of the common electrode  45  in the same manner as in the first embodiment. 
     When the insulating material  92  is formed by means of the potting, the insulating material  92  also flows into the portion disposed on the lower side of the wire bonding line  91 . Therefore, the portion of the insulating layer  46 , which covers the joining portion  44   c , may be omitted. Further, the wire bonding line  91  is arranged in the state of being separated from the joining portion  44   c . Therefore, it is not necessarily indispensable to form the insulating material  92 . 
     In the first embodiment and the first to third modified embodiments, the wiring lines  47   e ,  47   f ,  47   g  and the wire bonding line  91  are allowed to stride over the joining portions  44   c ,  44   d . However, they may be allowed to stride over any one of the two connecting portions  44   b.    
     In the first embodiment, the common electrode  44  (first common electrode) has the two connecting portions  44   b  which are provided corresponding to the individual electrodes  43  arranged in the two arrays and the joining portion  44   c  which connects the two connecting portions  44   b , and the common electrode  45  (second common electrode) is surrounded by the two connecting portions  44   b  and the joining portion  44   c . However, the present teaching is not limited thereto. 
     In a fourth modified embodiment, as shown in  FIG.  10   , common electrodes  74 ,  75  are arranged on the upper surface of the piezoelectric layer  42  in place of the common electrodes  44 ,  45 . The common electrode  74  (first common electrode) has a plurality of facing portions  74   a  (first facing portions) which face the plurality of pressure chambers  10  respectively and a connecting portion  74   b  (first connecting portion) which mutually connects the plurality of facing portions  74   a.    
     The common electrode  75  (second common electrode) is arranged to surround the common electrode  74 . The common electrode  75  has a plurality of facing portions  75   a  (second facing portions) which face the plurality of pressure chambers  10  respectively and a connecting portion  75   b  (second connecting portion) which mutually connects the plurality of facing portions  75   a.    
     Wiring lines  47   h ,  47   i  are connected to an upper end portion and a lower end portion of the connecting portion  74   b  of the common electrode  74  respectively. Wiring lines  47   j ,  47   k  are connected to an upper-right end portion and a lower-left end portion (connecting portions  75   b ) of the common electrode  75  respectively as viewed in  FIG.  10   . 
     The first ends of the wiring lines  47   h ,  47   i  are connected to the upper end portion and the lower end portion of the connecting portion  74   b , respectively, via the conductive material charged into through-holes  46   c ,  46   d  which are formed at portions of the insulating layer  46  (see  FIG.  5   ) facing the upper end portion and the lower end portion of the connecting portion  74   b . The wiring line  47   h  extends, from the connecting portion in the upward direction as viewed in  FIG.  10   , on the upper surface of the insulating layer  46  to stride over the common electrode  75  (connecting portion). Further, the wiring line  47   h  is led out to the upper surface of the vibration plate  41  via the upper side surface of the insulating layer  46 . Similarly, the wiring line  47   i  also extends, from the connecting portion in the leftward direction, on the upper surface of the insulating layer  46  to stride over the common electrode  75  (connecting portion). Further, the wiring line  47   i  is led out to the upper surface of the vibration plate  41  via the left side surface of the insulating layer  46 . 
     As described above, the first ends of the wiring lines  47   j ,  47   k  are connected to the portions of the common electrode  75 , respectively, on the upper surface of the piezoelectric layer  42 . Further, the wiring lines  47   j ,  47   k  are led out to the upper surface of the vibration plate  41  via the right and lower side surfaces of the piezoelectric layer  42  as viewed in  FIG.  10   . 
     The wiring lines  47   h  to  47   k , which are led out to the upper surface of the vibration plate  41 , are laid out on the upper surface of the vibration plate  41 , and the second ends of the wiring lines  47   h  to  47   k  are connected to the driver IC  50 , in the same manner as described in the first embodiment. 
     In the fourth modified embodiment, the portions of the wiring lines  47   h ,  47   i , which are arranged on the upper surface of the vibration plate  41 , correspond to the first wiring portions according to the present teaching. The portions of the wiring lines  47   h ,  47   i , which are arranged to stride over the common electrode  75  and which connect the first wiring portions and the upper/lower end portion of the connecting portion  74   b , correspond to the first connecting wirings according to the present teaching. The portions of the wiring lines  47   j ,  47   k , which are arranged on the upper surface of the vibration plate  41 , correspond to the second wiring portions according to the present teaching. The portions of the wiring lines  47   j ,  47   k , which are allowed to pass along the side surfaces of the piezoelectric layer  42  to connect the second wiring portions and the common electrode  45 , correspond to the second connecting wirings according to the present teaching. In the fourth modified embodiment, as described above, the first connecting wirings extend while striding over the second connecting portion. 
     Also in this case, the wiring lines  47   h  to  47   k  are connected to the mutually separated portions of the common electrodes  74 ,  75 . Therefore, it is possible to uniformize the electric potentials of the common electrodes  74 ,  75  respectively. 
     Second Embodiment 
     Next, a second embodiment of the present disclosure will be explained. However, in the second embodiment, only a part of the arrangement of the piezoelectric actuator is different from that of the first embodiment. Therefore, only portions, which are different from those of the first embodiment, will be explained below. 
     As shown in  FIG.  11   , in the second embodiment, the common electrode  44  does not have the connecting portion  44   c  (see  FIG.  4   ) on the piezoelectric layer  42 . The two connecting portions  44   b  are arranged while being separated from each other with the common electrode  45  intervening therebetween in the scanning direction (left-right direction as shown in  FIG.  11   ). A wiring line  47   l  is connected to the common electrode  45  in place of the wiring line  47   e  (see  FIG.  4   ). 
     As shown in  FIG.  11   , the first end of the wiring line  47   l  is connected to an upper-right end portion (connecting portion  45   b ) of the common electrode  45  on the upper surface of the piezoelectric layer  42 . Further, the wiring line  47   l  is led out to the upper surface of the vibration plate  41  (surface disposed on the side opposite to the channel unit) via the upper side surface of the piezoelectric layer  42 . Further, the wiring line  47   l  is laid out on the upper surface of the vibration plate  41 , and the wiring line  47   l  is connected to the driver IC  50 . The portion of the wiring line  47   l , which is arranged on the upper surface of the vibration plate  41 , corresponds to the first wiring portion according to the present teaching. The portion of the wiring line  47   l , which is allowed to pass along the side surface of the piezoelectric layer  42  to connect the first wiring portion and the common electrode  45 , corresponds to the first connecting wiring according to the present teaching. 
     A bypass wiring  101  is arranged on the upper surface of the insulating layer  46  (see  FIG.  5   ). The bypass wiring  101  extends in the scanning direction while striding over the common electrode  45 . The both ends thereof are connected to upper end portions of the two connecting portions  44   b  respectively via the conductive material charged into through-holes  46   e ,  46   f  formed at portions of the insulating layer  46  facing the upper end portions of the two connecting portions  44   b . Accordingly, the two connecting portions  44   b  are connected to one another by the bypass wiring  101 . 
     Also in this case, the wiring lines  47   d ,  471  (second connecting wiring) are connected respectively to the mutually separated lower-left and upper-right end portions (a plurality of portions) of the common electrode  45  on the upper surface of the piezoelectric layer  42 , in the same manner as in the first embodiment. Therefore, it is possible to improve the uniformity of the electric potential of the common electrode  45 . 
     Further, the two connecting portions  44   b , which are arranged while being separated from each other with the common electrode  45  intervening therebetween in the scanning direction, are connected to one another by the bypass wiring  101  which extends while striding over the common electrode  45 . Therefore, the electric potential of the common electrode  44  is uniformized. 
     Next, a modified embodiment, in which various modifications are applied to the second embodiment, will be explained. However, the components or parts, which are constructed in the same manner as those of the second embodiment, are designated by the same reference numerals, any explanation of which will be appropriately omitted. 
     In the second embodiment, the two connecting portions  44   b , which are arranged and separated from each other, are connected to one another by the bypass wiring  101 . However, the present teaching is not limited thereto. 
     In a fifth modified embodiment, as shown in  FIG.  12   , the common electrode  44  further includes a joining portion  44   f  which connects substantially central portions of the two connecting portions  44  to one another. Therefore, the connecting portion  45   b  of the common electrode  45  is divided into two portions which are separated from each other while interposing the joining portion  44   f  in relation to the up-down direction as viewed in  FIG.  12   . 
     In this arrangement, conversely to the second embodiment, the common electrode  45  corresponds to the first common electrode according to the present teaching, and the common electrode  44  corresponds to the second common electrode according to the present teaching. The respective portions of the connecting portion  45   b , which are divided into two, correspond to the first connecting portions according to the present teaching respectively. That is, the common electrode  45  has the two first connecting portions. 
     In the fifth modified embodiment, the portion corresponding to the first wiring portion and the portion corresponding to the second wiring portion are reversed to one another, and the portion corresponding to the first connecting wiring and the portion corresponding to the second connecting wiring are reversed to one another, as compared with the second embodiment. 
     Further, a bypass wiring  102  is arranged on the upper surface of the insulating layer  46  (see  FIG.  5   ). The bypass wiring  102  extends in a direction slightly inclined with respect to the paper feeding direction (left-right direction as viewed in  FIG.  12   ) while striding over the joining portion  44   f  The both ends of the bypass wiring  102  are connected respectively to the abovementioned two portions of the connecting portion  45   b  via the conductive material charged into two through-holes  46   g ,  46   h  formed respectively at portions of the insulating layer  46  facing the two portions of the common electrode  45 . Accordingly, the two portions of the connecting portion  45   b  are connected to one another by the bypass wiring  102 . 
     Also in this case, it is possible to uniformize the electric potentials of the common electrodes  44 ,  45  respectively at all portions. 
     Also in the second embodiment, the two connecting portions  44   b  may be connected to one another by means of a wire bonding provided in the same manner as in the third modified embodiment described above, in place of the bypass wiring  101  arranged on the upper surface of the insulating layer  46 . 
     In the second embodiment, the bypass wiring  101  mutually connects the upper end portions of the two connecting portions  44   b . However, the bypass wiring  101  may connect other portions of the two connecting portions  44   b . Further, a plurality of bypass wirings may be provided. 
     In the second embodiment, the common electrode  44  (first common electrode) is provided with the two connecting portions  44   b  (first connecting portions) corresponding to the arrays of the individual electrodes  43 . However, the present teaching is not limited thereto. It is not necessarily indispensable that the first connecting portion should be provided corresponding to the array of the individual electrode  43 , provided that the first connecting portion connects at least two first facing portions to one another. The number of the first connecting portions is not limited to two as well. The first common electrode may have three or more first connecting portions. When the three or more first connecting portions are provided, for example, a plurality of bypass wirings are provided to connect every mutually different two of the three or more first connecting portions. In this way, it is possible to allow all of the first connecting portions to be in conduction. 
     While the invention has been described in conjunction with various example structures outlined above and illustrated in the figures, various alternatives, modifications, variations, improvements, and/or substantial equivalents, whether known or that may be presently unforeseen, may become apparent to those having at least ordinary skill in the art. Accordingly, the example embodiments of the disclosure, as set forth above, are intended to be illustrative of the invention, and not limiting the invention. Various changes may be made without departing from the spirit and scope of the disclosure. Therefore, the disclosure is intended to embrace all known or later developed alternatives, modifications, variations, improvements, and/or substantial equivalents. Some specific examples of potential alternatives, modifications, or variations in the described invention are provided below: 
     In the first and second embodiments described above, the plurality of individual electrodes  43  are arranged on the lower surface of the piezoelectric layer  42 , and the common electrodes  44 ,  45  are arranged on the upper surface of the piezoelectric layer  42 . However, conversely to the above, the common electrodes  44 ,  45  may be arranged on the lower surface of the piezoelectric layer  42 , and the plurality of individual electrodes  43  may be arranged on the upper surface of the piezoelectric layer  42 . 
     In this case, for example, an insulating layer is arranged between the piezoelectric layer  42  and the vibration plate  41 . The portions of the wiring lines  47   e ,  47   f ,  47   h ,  47   i  which extend while striding over the joining portions  44   c ,  44   d  and the common electrode  75  (first and second connecting wirings) and the bypass wiring  101  are arranged on the lower surface of the insulating layer. 
     In the first and second embodiments, the common electrode  44  (facing portion  44   a ) faces the substantially central portion of the pressure chamber  10 , and the common electrode  45  (facing portion  45   a ) is opposed to the portion other than the substantially central portion of the pressure chamber  10 . However, there is no limitation thereto. For example, conversely to the above, the facing portions of the first and second common electrodes may face portions of the pressure chamber  10  different from those in the first embodiment. That is, the second facing portion may face the substantially central portion of the pressure chamber  10  and the first facing portion may face the portion other than the substantially central portion of the pressure chamber  10 . In this case, for example, the positions and the shapes of the connecting portions of the respective common electrodes may be appropriately changed in conformity with, for example, the positions and the shapes of the facing portions of the first and second common electrodes. 
     In the foregoing embodiments, the piezoelectric actuator is arranged with only any one of the wiring line which is connected to one of the two types of the common electrodes while striding over the other common electrode and which is exemplified by the wiring lines  47   e ,  47   f ,  47   h ,  47   i  and the wire bonding line  91 . Alternatively, the piezoelectric actuator is arranged with the bypass wiring which mutually connects the two or more connecting portions of one of the two types of the common electrodes while striding over the other common electrode and which is exemplified by the bypass wirings  101 ,  102 . However, the piezoelectric actuator may be arranged with both of the two types of the wiring lines depending on the arrangement of the two types of the common electrodes. 
     In the foregoing embodiments, the wiring lines  47   e ,  47   f ,  47   h ,  47   i  are arranged on the insulating layer, or the insulating material is arranged around the wire bonding line  91  by means of the potting. However, there is no limitation thereto. The wiring lines  47 , the wire bonding line  91 , and the bypass wiring lines  101 ,  102  may be formed with coated electric wires or electric cables. 
     The foregoing embodiments are illustrative of the exemplary case in which the present teaching is applied to the piezoelectric actuator to be used for the ink-jet head of the so-called serial type printer for discharging the inks from the nozzles while being reciprocatively moved in the scanning direction together with the carriage. However, there is no limitation thereto. It is also allowable to apply the present teaching to a piezoelectric actuator to be used for a so-called line head which is fixed to the printer and which extends over the entire length in the widthwise direction of the recording paper. Further, it is also possible to apply the present teaching to a piezoelectric actuator to be used for any apparatus or device other than the ink-jet head. The piezoelectric actuator according to the present teaching is also preferably applicable to any liquid discharge head to be carried on a liquid discharge apparatus usable, for example, for an apparatus for producing a color filter of a liquid display apparatus by discharging any liquid other than the ink including, for example, a coloring liquid and an apparatus for forming electrical wiring lines by discharging a conductive liquid.