Inkjet head and inkjet printer

An inkjet head in which cross-talk between adjacent pressure chambers is suppressed. The inkjet head has a channel unit having pressure chambers and nozzles communication with the pressure chambers, and an actuator unit fixed to one surface of the channel unit for changing the volume of the pressure chamber. The actuator unit includes individual electrodes provided opposed to the plurality of pressure chambers for receiving a drive signal to change the volume of the pressure chamber; a common electrode provided over the plurality of pressure chambers; a piezoelectric sheet provided between the individual electrode and the common electrode; and an independent electrode provided between adjacent individual electrodes and electrically isolated from the common electrode and the individual electrodes. An inductor is electrically connected between the independent electrode and a portion whose electric potential is substantially the same as that of the common electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an inkjet head for ejecting ink onto a recording medium and an inkjet printer for performing printing operations with the inkjet head.

2. Description of the Related Art

An inkjet head distributes ink supplied from an ink tank to a plurality of pressure chambers to eject ink through nozzles that are in fluid communication with the pressure chambers by selectively applying pressure in pulses to the pressure chambers one of methods for selectively applying pressure to the pressure chambers is to use an actuator unit formed of a plurality of ceramic piezoelectric sheets laminated together.

One inkjet head having this type of actuator unit is disclosed in Japanese unexamined patent application publication HEI-4-341852 (FIG. 1). The inkjet head has a plurality of individual electrodes disposed opposite a plurality of pressure chambers for changing the volume of the pressure chambers in response to drive signals; a common electrode disposed over the plurality of pressure chambers and maintained at ground potential; and a piezoelectric sheet interposed between the individual electrodes and the common electrode. When the individual electrodes are set at a potential different from that of the common electrode to cause an electric field across the polarizing direction of the piezoelectric sheet, the piezoelectric sheet interposed between the individual electrodes and the common electrode and polarized in the laminating direction of the sheets deform in the laminating direction according to a longitudinal piezoelectric effect. If the directions of the electric field and polarization are the same, then the piezoelectric sheet expands in the laminating direction. This deformation of the piezoelectric sheet changes the volume in the pressure chamber, causing ink to eject through a nozzle in communication with the pressure chamber toward a recording medium.

As the density of pressure chambers continues to increase in this type of inkjet head in recent years in order to meet high resolution and high-speed printing needs, a problem called structural cross-talk has arisen. The structural cross-talk means that deformation in the piezoelectric sheet facing a certain pressure chamber accidentally results in deforming another portion of the sheet facing adjacent pressure chambers. As a result, ink may be ejected from nozzles where such ejection is not intended. The amount of ink intended to be ejected from the target nozzle may be changed.

Due to the structural cross-talk, when ink is ejected from that pressure chamber, the amount of deformation in the piezoelectric sheet facing a given pressure chamber may be changed depending on whether ink is simultaneously ejected from neighboring pressure chambers. Accordingly, the amount of ink ejected from this pressure chamber is not stable.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention to provide an inkjet head that suppress structural cross-talk occurring when the volume of pressure chambers adjacent to the target pressure chamber changes accidentally, by suppressing deformation in the piezoelectric sheet at not-opposing areas which does not face the pressure chambers.

The present invention provides an inkjet head having a channel unit and an actuator unit. The channel unit has a flat shape. The channel unit has a plurality of pressure chambers arranged adjacent to one another in a plane perpendicular to a thickness direction of the channel unit, and a plurality of nozzles provided on a first surface of the channel unit and being in communication with the plurality of pressure chambers. Each of the plurality of pressure chambers has a volume. The actuator unit is fixed to a second surface of the channel unit for changing the volume of each of the plurality of the pressure chambers. The actuator unit has a plurality of individual electrodes, a common electrode, a piezoelectric sheet, and an independent electrode. The plurality of individual electrodes is provided opposed to the plurality of pressure chambers, respectively. Each of the plurality of individual electrodes receives a drive signal to change the volume of corresponding one of the pressure chambers. The common electrode is provided over the plurality of pressure chambers. The piezoelectric sheet is provided between the plurality of the individual electrodes and the common electrode. The independent electrode is provided between adjacent individual electrodes on a non-opposing portion of the piezoelectric sheet that is not opposed to the pressure chambers. The independent electrode is electrically isolated from the common electrode and the plurality of individual electrodes. The inkjet head further has an inductor electrically connected between the independent electrode and a portion whose electric potential is substantially the same as that of the common electrode.

The present invention provides an inkjet printer having an inkjet head. The inkjet head has a channel unit and an actuator unit. The channel unit has a flat shape. The channel unit has a plurality of pressure chambers arranged adjacent to one another in a plane perpendicular to a thickness direction of the channel unit, and a plurality of nozzles provided on a first surface of the channel unit and being in communication with the plurality of pressure chambers. Each of the plurality of pressure chambers has a volume. The actuator unit is fixed to a second surface of the channel unit for changing the volume of each of the plurality of the pressure chambers. The actuator unit has a plurality of individual electrodes, a common electrode, a piezoelectric sheet, and an independent electrode. The plurality of individual electrodes is provided opposed to the plurality of pressure chambers, respectively. Each of the plurality of individual electrodes receives a drive signal to change the volume of corresponding one of the pressure chambers. The common electrode is provided over the plurality of pressure chambers. The piezoelectric sheet is provided between the plurality of the individual electrodes and the common electrode. The independent electrode is provided between adjacent individual electrodes on a non-opposing portion of the piezoelectric sheet that is not opposed to the pressure chambers. The independent electrode is electrically isolated from the common electrode and the plurality of individual electrodes. The inkjet head further has an inductor electrically connected between the independent electrode and a portion whose electric potential is substantially the same as that of the common electrode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An inkjet head according to a first embodiment of the present invention will be described next. It should be noted that the direction expressions such as “front”, “rear”, “above”, “below”, “top”, and “bottom” are used throughout the description to define the various parts when a printer is disposed in an orientation in which it is intended to be used. The inkjet head according to a first embodiment of the present invention is provided in an inkjet printer (not shown) for ejecting ink onto a paper conveyed in the inkjet printer in order to record images on the paper.

FIGS. 1 and 2show the inkjet head1having a main head member70for ejecting ink to paper and a base block71. The main head member70has a flat rectangular shape extending in a main scanning direction. The base block71has two ink reservoirs3for supplying ink to the main head member70. The two ink reservoirs3are positioned above the main head member70.

The main head member70includes: a channel unit4in which ink channels are formed; and a plurality of actuator units21bonded to a top surface of the channel unit4. The channel unit4and the actuator units21have a laminated structure in which a plurality of thin plates are stacked and bonded together. Flexible printed circuits (FPCs)50are bonded to the top surfaces of the actuator units21for supplying electric power to the same. The FPCs50are led out from the actuator units21on the both sides thereof. The base block71is formed of a metal material such as stainless steel. The ink reservoir3is provided inside the base block71and includes a hollow portion having a substantially rectangular parallelepiped shape extending in a longitudinal direction of the base block71.

The bottom surface73of the base block71protrudes downward from openings3bof the ink reservoir3. The base block71contacts the channel unit4only in regions73aaround the openings3bon the bottom surface73. Accordingly, regions other than the openings3bof the bottom surface73of the base block71are separated from the main head member70, forming spaces therebetween. The actuator units21are disposed in respective these spaces.

The inkjet head1includes a holder72. The holder72includes a retaining part72a,and a pair of plate-shaped protruding parts72bprotruding perpendicularly to the top surface of the retaining part72aand forming a prescribed gap therebetween. The base block71is bonded and fixed in a recess formed in the bottom surface of the retaining part72a.The FPCs50bonded to the actuator units21are arranged along the surfaces of the protruding parts72bthrough an elastic material83such as a sponge material. A driver IC80is provided on each FPC50disposed on the surface of the protruding part72bof the holder72. The FPC50is electrically connected by soldering to both the driver IC80and the actuator unit21for transferring drive signals from the driver IC80to the actuator unit21.

A heat sink82substantially shaped like a rectangular parallelepiped is disposed in close contact with the outer surface of the driver IC80for efficiently dissipating heat generated by the driver IC80. A circuit board81is disposed on the outer side of each FPC50above the driver IC80and heat sink82. Seal members84are affixed between the top surface of the heat sink82and the circuit board81and between the bottom surface of the heat sink82and the FPC50.

FIG. 3is a plan view of the main head70shown inFIG. 1. The ink reservoirs3formed in the base block71are depicted in phantom by dotted lines inFIG. 3. Two ink reservoirs3extend in the longitudinal direction of the main head70parallel to one another and separated by a prescribed distance. Each of the two ink reservoirs3has an opening3aat one end of the base block71. The ink reservoirs3are in fluid communication with an ink tank (not shown) via the openings3a,enabling the ink reservoirs3to be full of ink at all times. A plurality of the openings3bare provided in each of the ink reservoirs3along the longitudinal direction of the main head70for connecting the ink reservoirs3to the channel unit4, as described above. Pairs of openings3bpositioned close to one another are disposed in the longitudinal direction of the main head70. Pairs of openings3bcommunicating with one ink reservoir3are disposed in a staggered relationship with pairs of openings3bcommunicating with the other ink reservoir3.

A plurality of the actuator units21having a planar trapezoidal shape are arranged in a staggered pattern opposing each pair of openings3bin regions not occupied by the openings3b.The parallel sides (top and bottom sides) of each actuator unit21are aligned with the longitudinal direction of the main head70, while the slanted sides of neighboring actuator units21overlap in the widthwise direction of the main head70.

FIG. 4is an enlarged view showing a region inFIG. 3delineated by a broken line with alternating long and short dashes. As shown inFIG. 4, the openings3bprovided in the ink reservoirs3are in fluid communication with manifolds5serving as common ink chambers. The end of each manifold5is split into two sub-manifolds5a.In a plan view, two sub-manifolds5abranching from an adjacent opening3bextend through the actuator units21from both slanted sides thereof. Hence, a total of four sub-manifolds5aseparated from one another extend along the bottom of the actuator unit21in the direction of the parallel sides of the actuator unit21.

Ink ejection regions are formed on the bottom surface of the channel unit4facing regions on which the actuator units21are bonded. A plurality of nozzles8are arranged in a matrix on the surface of the ink ejection regions as described later. For simplification, only a few of the nozzles8have been depicted inFIG. 4, while in actuality the nozzles8are disposed along the entire ink ejection area of the actuator units21.

FIG. 5is an enlarged view of an area inFIG. 4delineated by a broken line with alternating long and short dashes.FIGS. 4 and 5are views in a direction perpendicular to the ink ejection surface and show a plurality of pressure chambers10that are arranged in the channel unit4in a matrix configuration. Each pressure chamber10has a planar and substantially diamond-shape having round corners. A longer diagonal line between opposing corners is parallel to the width direction of the channel unit4. One end of the pressure chamber10is in fluid communication with a nozzle8, while the other end is in fluid communication with the sub-manifold5avia an aperture12(seeFIG. 6). In a plan view, individual electrodes35having a planar shape similar to but slightly smaller than that of the pressure chambers10are formed on top of the actuator unit21at positions overlapping each pressure chamber10. For the sake of description, only a few of the plurality of individual electrode35have been depicted inFIG. 5. The pressure chambers10and apertures12have been depicted with solid lines inFIGS. 4 and 5, although they are beneath the actuator units21and should be depicted with dotted lines.

As shown inFIG. 5, a plurality of virtual diamond-shaped regions10xaccommodating each of the pressure chambers10is arranged in a matrix formation so that rows are formed in the direction A and a direction B with adjacent diamond-shaped regions10xsharing the same sides, but not overlapping. The direction A is the longitudinal direction of the inkjet head1, that is, the direction in which the sub-manifolds5aextend, and is parallel to the shorter diagonal lines between opposing angles in the diamond shaped regions10x.The direction B is aligned with slanted sides of the diamond shaped regions10xand forms an obtuse angle θ with the direction A. The pressure chambers10share the same center point with the opposing diamond shaped regions10x,but the contours of both are spaced apart when seen in a plan view.

Neighboring pressure chambers10in this matrix configuration are spaced in the direction A at intervals corresponding to 37.5 dpi. Further, eighteen of the pressure chambers10are aligned in the direction B within a single ink ejection area. However, the pressure chambers on both ends in the direction B are dummy chambers and do not contribute to ink ejection.

The plurality of pressure chambers10arranged in the matrix configuration form a plurality of pressure chamber rows along the direction A as shown inFIG. 5. When viewed in a direction perpendicular to the surface of the drawing inFIG. 5(third direction), the pressure chamber rows are divided into first pressure chamber rows11a,second pressure chamber rows11b,third pressure chamber-rows11c,and fourth pressure chamber rows11din accordance with the relative position to the sub-manifold5a.From the top side of the actuator unit21to the bottom side, the four pressure chamber rows11a–11dare arranged cyclically in the order11c→11d→11a→11b→11c→11d→ . . . →11b.

In a pressure chamber10aconfiguring part of the first pressure chamber row11aand the pressure chamber10bconfiguring part of the second pressure chamber row11b,the nozzles8are densely distributed at the bottom of the pressure chambers with respect to a direction orthogonal to the direction A (fourth direction) when viewed in the third direction. The nozzle8is positioned on the bottom end of the corresponding diamond-shaped region10x.

However, in a pressure chamber10cconfiguring part of the third pressure chamber row11cand a pressure chamber10dconfiguring part of the fourth pressure chamber row11d,the nozzles8are densely distributed on the top of the pressure chambers with respect to the fourth direction. The nozzle8is positioned on the top end of the corresponding diamond shaped region10x.When viewed in the third direction, a region greater than half the pressure chambers10aand10doverlaps the sub-manifolds5ain the first pressure chamber rows11aand11d.Also when viewed from the third direction, the entire regions of the pressure chambers10band10cdo not overlap the sub-manifolds5ain the second pressure chamber rows11band11c.For this reason, the nozzles8in fluid communication with the pressure chambers10belonging to all pressure chamber rows do not overlap the sub-manifolds5a,while the width of the sub-manifolds5ais set as large as possible to smoothly supply ink to the pressure chambers10.

Next, the cross-sectional structure of the main head member70will be described referring toFIGS. 6 and 7.FIG. 6shows the pressure chambers10ain the first pressure chamber rows11a.Referring toFIG. 6, the nozzle8is in fluid communication with a submanifold5avia the pressure chamber10(10a) and an aperture12. Accordingly, an individual ink channel32is formed in the main head member70for each pressure chamber10and extends from the outlet of the submanifold5ato the nozzle8via the aperture12and the pressure chamber10.

As shown inFIG. 7, the main head member70has a laminated structure that includes a total of ten stacked sheets. From top to bottom these sheets include the actuator unit21, a cavity plate22, a base plate23, an aperture plate24, a supply plate25, manifold plates26,27, and28, a cover plate29, and a nozzle plate30. The channel unit4is configured of nine of the above metal plates, excluding the actuator unit21.

As shown inFIG. 9, the actuator unit21includes four laminated piezoelectric sheets41–44. The topmost sheet of the sheets41–44has active layer portions (hereinafter referred to as the “active layer”) when a voltage is applied from electrodes, while the remaining three sheets remains inactive layers. The piezoelectric sheets41–44are made from a dielectric material and have piezoelectric effect.

Referring toFIG. 6, the cavity plate22is made from metal and provided with a plurality of substantially diamond-shaped openings facing the pressure chambers10. The base plate23is made of metal and provided with a communication hole connecting the pressure chamber10and aperture12, and another communication hole connecting the pressure chamber10to the ink nozzle8for each pressure chamber10in the cavity plate22. The aperture plate24is a metal plate provided with the aperture12communicating the pressure chamber and the submanifold5a,and a communication hole connecting the pressure chamber10and the ink nozzle8. The holes in the aperture plate24is made by etching. The supply plate25is a metal plate provided with a communication hole connecting the aperture12and the submanifold5a,and a communication hole connecting the pressure chamber10and the ink nozzle8.

The manifold plates26,27, and28are each provided with a hole for configuring the submanifold5awhen the plates are laminated together, and a communication hole connecting the pressure chamber10to the nozzle8. The cover plate29is a metal plate provided with a communication hole connecting the pressure chamber10to the nozzle8. The nozzle plate30is a metal plate provided with a nozzle8for each pressure chamber10in the cavity plate22.

These nine metal plates are aligned and stacked together to form the ink channel32shown inFIG. 6. The ink channel32begins from the submanifold5aproceeding upward, extends horizontally in the aperture12before again proceeding upward, again extends horizontally in the pressure chamber10, and then proceeds downward to the nozzle8, first at a slant away from the aperture12and then straight downward.

Next, the structure of the actuator unit21will be described referring toFIGS. 8 and 9. As shown inFIG. 9, the actuator unit21includes the four piezoelectric sheets41–44, each having the same thickness of approximately 15 μm. These piezoelectric sheets41–44are continuous laminated plates (continuous planar layers) that span the plurality of pressure chambers10formed in a single ink ejection region of the main head member70. By disposing the piezoelectric sheets41–44as continuous planar layers over the plurality of pressure chambers10, the electrodes35can be densely arranged on the piezoelectric sheet41using a screen printing technique. Therefore, the pressure chambers10can also be densely arranged at positions corresponding to the electrodes35, enabling the printing of high-resolution images. The piezoelectric sheets41–44are formed of ferroelectric ceramics such as lead zirconate titanate (PZT).

The individual electrodes35are formed on top of the piezoelectric sheet41, the topmost layer. A common electrode34formed as a sheet with a uniform thickness of approximately 2 μm is interposed between the piezoelectric sheets41and42. Both the electrodes35and the common electrode34are formed of a metal material such as Ag—Pd.

As shown inFIG. 8, the individual electrodes35are arranged in a matrix (see alsoFIG. 5). Each individual electrode35has a main electrode part35aand a land part35b(connection terminal). The main electrode part35ais substantially diamond-shaped (having four sides with opposing sides parallel to one another) similar to the shape of the pressure chamber10, and has a thickness of approximately 1 μm. The land part35bextends from the main electrode part35ato a region not opposing the pressure chambers10, and is connected to a signal wire through which drive signals are supplied. As shown inFIG. 8, one acute angle portion of each diamond-shaped main electrode part35aextends in the same direction (downward inFIG. 8). The land part35bextends from this acute angle portion. The land part35bhas a circular shape with a diameter of approximately 160 μm, and is electrically connected to the main electrode part35a.The land part35bis formed of gold including glass frit, for example, and is electrically bonded to the surface of the extended part extending from the individual electrode35. Further, the land part35bis electrically bonded to a contact provided on the FPC50. Drive signals from the driver ICs80(seeFIG. 2) are inputted into the main electrode part35avia the land part35bto change the volume of the pressure chamber10. Further, as shown inFIG. 9, the land part35bis disposed in a region not opposing the pressure chambers10.

The common electrode34is grounded so that all of the common electrodes34are maintained equally at a ground potential for all areas corresponding to the pressure chambers10. Further, the individual electrodes35are connected to the driver ICs80via the lands36and the FPCs50, which include a plurality of independent lead wires for each individual electrode35in order to independently control the potential corresponding to each pressure chamber10.

Further, an independent electrode60is disposed between pairs of neighboring individual electrodes35on regions of the piezoelectric sheet41that do not oppose the pressure chambers10. The independent electrode60is electrically insulated from the individual electrodes35and is grounded to maintain the same potential as that of the common electrode34. The independent electrode60will be described in greater detail below.

Next, a method of driving the actuator unit21will be described. The polarizing direction of the piezoelectric sheet41is equal to the direction of its thickness. Specifically, the actuator unit21has a unimorph structure in which the single piezoelectric sheet41on the top side (separated from the pressure chamber10) has an active layer, while the three piezoelectric sheets42–44on the bottom side (near the pressure chamber10) are inactive layers. Accordingly, when a prescribed positive or negative voltage is applied to the electrode35, and the directions of the electric field and polarization are the same, areas in the piezoelectric sheet41interposed between the electrodes34and36and over which a voltage is applied function as active layers to compress in a direction orthogonal to the polarizing direction due to the transverse piezoelectric effect.

However, since the piezoelectric sheets42–44are not affected by the electric field and therefore do not spontaneously compress, a difference in strain between the piezoelectric sheet41and the piezoelectric sheets42–44is produced in the direction orthogonal to the polarizing direction, causing all of the piezoelectric sheets41–44to deform in a convex shape on the inactive side (unimorph deformation). As shown inFIG. 9, since the bottom surface of the piezoelectric sheets41–44is fixed to the top surface of the cavity plate22, which serves to partition the pressure chambers, the piezoelectric sheets41–44effectively deform in a convex shape toward the pressure chamber side. As a result, the capacity of the pressure chamber10decreases, increasing the pressure of the ink and causing ink to eject from the nozzle8. When the individual electrodes35are subsequently returned to the same potential as that of the common electrode34, the piezoelectric sheets41–44return to their original shape and the pressure chamber10returns to its original capacity, drawing ink in from the manifold5.

In an alternative method, the individual electrode35may be maintained at a different potential from that of the common electrode34initially. And, in response to request for ejecting ink, the individual electrode35may be temporarily changed to the same potential as that of the common electrode34, and then subsequently returned to the potential different from that of the common electrode34at a prescribed timing. When the individual electrode35is changed to the same potential as that of the common electrode34in this case, the piezoelectric sheets41–44return to their original shape, causing the capacity of the pressure chamber10to increase from its initial state in which the potential applied to the individual electrode35was different from that of the common electrode34. As a result, ink from the manifolds5is drawn into the pressure chamber10. Subsequently, when the potential of the individual electrode35becomes different from that of the common electrode34, the piezoelectric sheets41–44deform in a convex shape toward the pressure chamber10, decreasing the volume of the pressure chamber10and increasing the pressure on ink therein, causing the ejection of ink.

If the direction of the electric field applied to the piezoelectric sheet41is opposite the polarizing direction, the active layer in the piezoelectric sheet41interposed between the individual electrode35and common electrode34will attempt to expand in a direction orthogonal to the polarizing direction by the transverse piezoelectric effect. Accordingly, the piezoelectric sheets41–44will deform in a concave shape on the side of the pressure chamber10, thereby increasing the volume of the pressure chamber10and drawing ink in from the manifold5. Subsequently, when the potential of the individual electrode35is returned to normal, the piezoelectric sheets41–44return to their original flat shape, which returns the pressure chamber10to its original volume and causes ink to eject from the nozzle8.

Generally, when a drive signal is applied to the individual electrode35corresponding to a given pressure chamber10, a part of the piezoelectric sheet41corresponding to the given pressure chamber10is deformed in response to the drive signal. However, the deformation of the part of the piezoelectric sheet41may simultaneously cause deformation of another part of the piezoelectric sheet41corresponding to a neighboring pressure chamber10. As a result, ink may be ejected from a nozzle not intended for ink ejection, or the resultant amount of ejected ink may be changed. The so-called structural cross-talk happens. In the inkjet head1of the first embodiment, the pressure chambers10are arranged adjacent to one another in a matrix formation when seen in a plan view. The space between two adjacent pressure chambers10is small, so that the structural cross-talk is inevitable.

Referring toFIGS. 8 and 9, the actuator unit21further includes an independent electrode60and a coil61in order to suppress the above cross-talk. The independent electrode60is formed on the piezoelectric sheet41between neighboring individual electrodes35. The independent electrode60is positioned on non-opposing chamber regions, and extends in a continuous linear manner in an arranging direction B of the pressure chambers10and a direction C forming an angle φ with the direction B. Thus, each individual electrode35is surrounded by the independent electrode60extending in both the directions B and C. In this embodiment, it should be noted that “a non-opposing chamber region” or “a non-opposing portion” is a portion of the piezoelectric sheets41–44which do not oppose the pressure chamber10.

Further, the independent electrode60is electrically insulated from the individual electrodes35. The independent electrode60is connected to a ground through the coil61. In other words, the coil61is electrically connected between the independent electrode60and the ground point. In this embodiment, the coil61is provided in the driver IC80(seeFIG. 2), and is connected to the independent electrode60via a lead wire.

FIG. 10shows a circuit diagram configured with the common electrode34, independent electrode60, and coil61. As shown inFIG. 10, a capacitor62is formed by the electrodes34and60and a portion of the piezoelectric sheet41interposed therebetween. The capacitor62forms a close circuit with the coil61. The close circuit shown inFIG. 10is a parallel resonance circuit formed by the capacitor62and the coil61.

In the circuit, the parallel resonance caused by the capacitor62and coil61prevents charge transfer, thereby restricting electrostatic induction in the capacitor62. Therefore, generation of an electric field in the piezoelectric sheet41between the independent electrode60and the common electrode34is suppressed. This leads to an increase in the mechanical impedance in the non-opposing chamber region. Accordingly, deformation of the piezoelectric sheet41between the independent electrode60and common electrode34is suppressed.

In this embodiment, the pressure chambers10are arranged in the matrix configuration. Further each pressure chamber10is substantially surrounded by the independent electrode60from different directions. Thus, the deformation and/or stress of the pressure chamber10can be effectively prevented from acting on neighboring chambers beyond the independent electrode60which is close to the pressure chamber10.

As described above, the deformation of the piezoelectric sheet41is suppressed in non-opposing chamber regions. Accordingly, the deformation of the piezoelectric sheet41in response to a drive signal to the individual electrode35associated with a pressure chamber10is prevented from transferring to portions of the piezoelectric sheet41opposing another pressure chamber10.

Suppose that the LC parallel circuit ofFIG. 10formed of the capacitor62and coil61resonates at a frequency f0. It is preferable that the coil61has an inductance L defined by the frequency f0and the capacitor62. It should be noted that the frequency f0is equal to a frequency of the drive signal applied to the individual electrode35to excite the corresponding pressure chamber10. In this case, oscillation having the frequency f0excites the pressure chamber10. Here, if we assume ω0is the angular frequency of the oscillation exciting the pressure chamber10corresponding to the individual electrode35(ω0=2Πf0) and C is the capacitance of the capacitor62, then an ideal inductance of the coil61: L0=1/(ω02·C).

On the other hand, an inductance Z of the LC parallel circuit inFIG. 10is Z=jωL/(1−ω2LC). Hence, as L approaches L0, the inductance Z increases and the current flowing in the LC parallel circuit decreases. In other words, since the amount of charge flowing in the piezoelectric sheet41interposed between the independent electrode60and common electrode34, i.e., the capacitor62is decreasing, this portions of the piezoelectric sheet41are less likely to deform.

More specifically, it is preferable for L to be set within a range ⅓L0<L<3 L0, and more preferable for L to be set nearly equal to L0, so that most charges do not flow in the piezoelectric sheet41. Since the value of the capacitance C differs depending on the type of the inkjet head1, a variable coil for adjusting an inductance L can be used as the coil61.

As described above, the independent electrode60extends in continuous linear way in the arranging direction B of the pressure chamber10and the direction C that forms the obtuse angle φ with the direction B. And each of the plurality of individual electrodes35is surrounded by the independent electrode60extending in these directions B and C. Hence, when a drive signal is applied to a given individual electrode35and the piezoelectric sheet41corresponding to this individual electrode35deforms, deformation of the piezoelectric sheet41in the non-opposing chamber region under the independent electrode60is suppressed by the increased mechanical inductance due to the coil61and capacitor62. In other words, even when the drive signal is supplied to one individual electrode to deform a corresponding area of the piezoelectric sheet, this deformation is not transferred to the neighboring pressure chambers, thereby reducing structural cross-talk. Accordingly, the piezoelectric sheet41under adjacent individual electrodes35deforms very little, thereby reliably reducing structural cross-talk. In the preferred embodiment, each of the regions surrounded by the independent electrode60including the individual electrode35has the same area. Accordingly, even if deformation of the piezoelectric sheet41in the non-opposing chamber region close to the individual electrode35which a drive signal is applied is not completely suppressed, and this deformation acts on neighboring portions of the piezoelectric sheet41corresponding to neighboring individual electrodes35, the effects of deformation that propagates to the neighboring individual electrodes35is substantially equal, thereby reducing irregularity in the amount of ink ejected from a plurality of nozzles in fluid communication with the plurality of pressure chambers10. Further, since the independent electrode60extends in a continuous linear way in the directions B and C ofFIG. 8, the independent electrode60can be more easily formed on the piezoelectric sheet41than when discrete independent electrodes60are provided.

In the above embodiment, the common electrode34is grounded. However, the common electrode34may be connected to a reference potential other than a ground.

Next, modifications of the first embodiment will be described, wherein like parts and components are designated by the same reference numerals to avoid duplicating description.

While the independent electrode60in the actuator units21of the first embodiment surrounds each individual electrode35, an independent electrode may be partially provided at a position on a minimum distance between two adjacent individual electrodes35.

As in the first embodiment described above, by arranging the plurality of individual electrodes35adjacent to one another in a matrix configuration, the distance between adjacent individual electrodes35is shortest at the land parts35b.Therefore, in an actuator unit21A shown inFIG. 11, independent electrodes60A extending in the directions B and C may be discreetly provided at positions near the land parts35boverlapping virtual lines65that connect two adjacent individual electrodes35where they are in closest proximity.

As described above, the independent electrode60A is formed at a position between two adjacent individual electrodes35a35a,so that deformation of the pressure chamber caused by one of the two individual electrodes is effectively prevented from being transferred to the other pressure chamber corresponding to the other of the two adjacent individual electrodes. Therefore, the structural cross-talk can be reliably reduced.

Alternatively, in actuator units21B shown inFIG. 12, independent electrodes60B shaped like the letter “C” may be provided around the periphery of each land part35bextending from the individual electrode35to a position on the non-opposing chamber region.

In either case, a dummy land part (not shown) having substantially the same shape and size as those of the land part35bmay be provided on the opposite side of the individual electrode35from the land part35b.The dummy land part is electrically connected to the independent electrode60, and electrically insulated from the individual electrode35. The independent electrode60is electrically connecting to the coil61in the FPC50through the dummy land part. Accordingly, this structure increases and enhances the joint strength between the FPC50and the actuator unit. Further, the above structure will facilitate an electrical connection between the dummy land part and the discretely disposed independent electrodes.

FIG. 13shows another modification of the actuator unit21C. Referring toFIG. 13, a land part35bis provided in a facing chamber region in which the piezoelectric sheets41–44facing the pressure chamber10. In this embodiment, independent electrodes60C are provided at positions between main electrode parts35aof neighboring individual electrodes35where they are in closest proximity. Alternatively, the independent electrodes60C may be provided around the main electrode parts35aas in the first embodiment described above.

FIG. 14shows further modification of the actuator unit21D. In the actuator unit21D, an independent electrode60D of a conductive layer may be provided over most of the entire surface of the piezoelectric sheet41except the individual electrodes35. One method of forming the independent electrode60D is to form a conductive layer over the piezoelectric sheet41by PVD or electroplating, and subsequently form loop-shaped grooves66in the conductive layer by photolithography or laser machining. In this way, the conductive layers on the inner and outer sides of the grooves66form the individual electrodes35and the independent electrode60D that are insulated from each other by the grooves66. By forming the grooves66in an area opposing the pressure chamber10, the individual electrodes35are positioned in an area substantially opposing the pressure chambers10, while the independent electrode60D may extend from an area not-facing chamber region to an area opposing the pressure chambers10but not overlapping the individual electrodes35. By spreading the independent electrode60D over the not-facing chamber regions and part of the facing chamber regions opposing the pressure chambers10in this way, deformation the piezoelectric sheet41not opposing the pressure chambers10can be reliably suppressed in order to reduce structural cross-talk.

The inkjet head of the present invention is not limited to the inkjet head described in the first embodiment, wherein the pressure chambers10are arranged in a planar matrix structure. For example, the present invention may also be applied to an inkjet head in which pressure chambers are arranged adjacent to one another in lateral rows.

The location of the coil61is not limited in the driver IC80as described in the first embodiment. The coil61may be provided on the outer side of the circuit board81and may be connected to the independent electrode60by a signal wire separate from the lead wire of the FPC50. Instead of the coil61, any component having an inductance in the LC parallel circuit such as a transformer may be used.

Next, an inkjet printer according to a second embodiment of the present invention will be described.FIG. 15shows the inkjet printer101of the second embodiment. The inkjet printer101is a color inkjet printer having four inkjet heads1A. Each of the inkjet heads1A has the same structure as that of the inkjet head1of the first embodiment except the position of the coil61. The inkjet printer101has a paper supply unit111and a discharge unit112.

The inkjet printer101has a paper conveying path formed inside for conveying paper from the paper supply unit111to the discharge unit112. A pair of conveying rollers105aand105bfor pinching and conveying paper loaded in the paper supply unit111is disposed on the downstream side of the paper supply unit111. Paper is conveyed from the left side of the drawing toward the right by the conveying rollers105aand105b.Two belt rollers106and107and an endless conveying belt108looped around the belt rollers106and107are disposed in the central area of the paper conveying path. The outer surface of the conveying belt108, that is, the paper conveying surface, is subjected to a silicon treatment to generate a tackiness on the conveying surface. Paper supplied by the conveying rollers105aand105bis gripped by the tacky conveying surface and conveyed downstream (toward the right) by the clockwise rotation of the belt roller106(indicated by the arrow inFIG. 15).

Each of the four inkjet heads1A is provided with the main head70described in the first embodiment on the bottom end thereof and positioned adjacent to one another. The bottom surface of each main head70faces the paper conveying path. The nozzles8having apertures with a diameter on the micron order described in the first embodiment (seeFIG. 7) are provided in the bottom surfaces of the main heads70for ejecting ink from the respective main head70in the colors magenta, yellow, cyan, and black.

The main heads70are disposed such that a small gap is formed between the bottom surfaces of the main heads70and the conveying surface of the conveying belt108. In the small gap, the paper conveying path is formed. With this construction, ink of each color is ejected from the nozzles8toward the top surface of the paper, that is, the printing surface, as the paper conveyed on the conveying belt108passes directly under each of the main heads70in sequence, thereby forming a desired color image on the paper.

The inkjet printer101is also provided with a maintenance unit117for automatically performing maintenance on the inkjet heads1A. Further, the belt rollers106and107and the conveying belt108are supported in a casing113. When the maintenance unit117performs a maintenance operation, a shaft114eccentrically positioned in a cylindrical member115is rotated to change the height of the cylindrical member115in order to raise and lower the chassis113.

The inkjet printer101is also provided with a controller120for controlling various operations of the inkjet printer101, such as the ejection of ink from the four inkjet heads1A and the conveying of paper by the belt rollers106and107. The controller120is provided with a coil61A connected to the independent electrode60in the actuator unit21described above (seeFIGS. 8 and 9). The independent electrode60is grounded via the coil61A.

Hence, as in the first embodiment described above, a circuit is formed of the coil61and capacitor62, which is formed of the piezoelectric sheet41interposed between the independent electrode60and common electrode34(see FIG.10). Mechanical inductance increases through interaction between the capacitor62and coil61, thereby suppressing deformation of the piezoelectric sheet41interposed between the independent electrode60and the common electrode34. A description of the other operations and effects of the inkjet printer will be omitted as they are similar to the first embodiment described above.