Abstract:
An ink jetting device includes a wall that constitutes at least a part of an ink channel and is formed of piezoelectric ceramic material polarized in one direction. A first electrode is formed wholly over one surface of the wall, a second electrode is formed partially on the other surface of the wall, and a third electrode is formed at a position that is spaced from the second electrode on the other surface of the wall. A controller is connected to the second and third electrodes but is not connected to the first electrode. The controller induces a potential difference between the second and third electrodes to deform the wall with a piezoelectric effect, so that the ink in the ink channel is pressurized to jet an ink droplet from the ink channel.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ink jetting device. 
     2. Description of Related Art 
     In recent years, non-impact type printing devices have superseded previously used impact type printing devices and have increasingly propagated in the market. Of these non-impact type printing devices, the ink jetting type printing device is more popular because it has the simplest printing principle and facilitates a color printing operation with high gradation. In this type of printing device, a drop-on-demand type printing device, in which only an ink droplet for printing is jetted, has rapidly increased in popularity in the market because of its high ink jetting efficiency and low running cost. 
     Examples of the drop-on-demand type printer include the Kyser type, as disclosed in Japanese Patent Publication No. 53-12138, and the thermal jet type, as disclosed in Japanese Patent Publication No. 61-59914. However, these types of printing devices have the following critical problems. With respect to the former, it is difficult to design the device with a compact size. With respect to the latter, ink is heated at a high temperature, thus requiring ink with a high heat-proof property. 
     To solve both of the above problems at the same time, a shear mode type, as disclosed in Japanese Laid-open Patent Publication No. 63-247051, is proposed as a new type of printing device. 
     FIGS. 3A and 3B show a shear mode type of ink jetting device. As shown in FIG. 3A, the shear mode type of ink jetting device  10  comprises a bottom wall  20 , a ceiling wall  22 , a rigid wall  26 , an actuator wall  60  and an ink channel  24 , which is surrounded so as to be sealed and defined by the above walls. 
     The actuator wall  60  is formed of piezoelectric ceramic material that is polarized in a Z-direction perpendicular to the ceiling wall  22  and the bottom wall  20 , and it is firmly fixed to the bottom wall  20  and the ceiling wall  22 . The wall surfaces  65  and  66  of the actuator wall  60  are provided with metal electrodes  68  and  69  at the lower side thereof and with metal electrodes  68 ′ and  69 ′ at the upper side thereof so as to be spaced from the metal electrodes  68  and  69 . The metal electrodes  68 ,  68 ′,  69  and  69 ′ are electrically connected to a controller C. 
     As shown in FIG. 3B, when ink is jetted, the controller C controls the metal electrodes  68 ′ and  69  to be grounded and applies a driving voltage V to the metal electrodes  68  and  69 ′. Through this operation, electric fields in opposite directions occur at the upper and lower portions of the actuator wall  60 . Therefore, the upper and lower portions of the actuator wall  60  are displaced by thickness shear in such directions that the volume of the ink channel  24  is reduced. This deformation of the actuator wall  60  pressurizes the ink in the ink channel  24 , so that an ink droplet is jetted from nozzles (not shown in this view) that intercommunicate with the ink channel  24 . 
     In the ink jetting device described above, the metal electrodes  68 ′ and  69  are grounded, and the metal electrodes  68  and  69 ′ are supplied with the driving voltage. Thus, the metal electrodes  68 ,  68 ′,  69  and  69 ′ must be connected to the controller C. Accordingly, this ink jetting device has a disadvantage that a large number of connections between the controller C and the metal electrodes are required, and thus the manufacturing cost of the device is high. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an ink jetting device that has a small number of connections of metal electrodes to a controller and can be manufactured at low cost. 
     To attain the above and other objects, an ink jetting device according to the present invention includes a wall that constitutes at least a part of an ink channel and is formed of piezoelectric ceramic material polarized in one direction. A first electrode is formed wholly over one surface of the wall, a second electrode is formed partially on the other surface of the wall, and a third electrode is formed at a position not connected to the second electrode on the other surface of the wall. A controller is connected to the second and third electrodes but is not connected to the first electrode. The controller induces a potential difference between the second and third electrodes to deform the wall with a piezoelectric effect, whereby ink in the ink channel is pressurized and an ink droplet is jetted from the ink channel. 
     In the ink jetting device of this invention thus constructed, the controller induces the potential difference between the second and third electrodes so that an electric field in a direction perpendicular to the polarization direction of the wall is produced between the first and second electrodes. Simultaneously, an electric field in the opposite direction to the direction of the electric field occurring between the first and second electrodes is produced between the first and third electrodes. The wall is deformed by the piezoelectric effect of the piezoelectric ceramic material, and the ink is pressurized in the ink channel so that the ink droplet is jetted from the ink channel. As noted above, the controller is connected to the second and third electrodes, but it is not connected to the first electrode. Therefore, the electrical contact (connection) between the electrodes and the controller can be performed in a simple manner. Thus, the productivity is excellent, and the manufacturing cost can be reduced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the present invention is described in detail with reference to the following figures wherein: 
     FIG. 1A is a schematic side view in cross section showing the construction of an ink jetting device according to a first embodiment of the present invention; 
     FIG. 1B is a schematic side view in cross section showing the operation of the ink jetting device of FIG. 1A according to a first embodiment of the present invention; 
     FIG. 1C is a schematic side view in cross section showing a modification of the ink jetting device of FIG. 1A according to the present invention; 
     FIG. 2A is a schematic side view in cross section showing the construction of an ink jetting device according to a second embodiment of the present invention; 
     FIG. 2B is a schematic side view in cross section showing the operation of the ink jetting device of FIG. 2A according to a second embodiment of the present invention; 
     FIG. 2C is a schematic side view in cross-section showing a modification of an ink jetting device according to a second embodiment of the invention; 
     FIG. 3A is a schematic side view in cross section showing the construction of a conventional shear mode type of ink jetting device; and 
     FIG. 3B is a schematic side view in cross section showing the operation of the conventional shear mode type of ink jetting device. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments according to the present invention are described with reference to the accompanying drawings. In the following description, the same elements as the conventional ink jetting device as described in the background section above are represented by the same reference numerals, and the description thereof is omitted. 
     FIGS. 1A and 1B schematically show an ink jetting device of a first embodiment of the invention. Like the conventional ink jetting device shown in FIG. 3A, the ink jetting device  110  of this embodiment basically comprises a rigid wall  26 , an actuator wall  60 , a bottom wall  20 , a ceiling wall  22  and an ink channel  24 , which is defined by the above walls. The wall surface  65  of the actuator wall  60  is provided with a metal electrode  68  serving as a second electrode at the lower side thereof and with a metal electrode  68 ′ serving as a third electrode at the upper side thereof spaced from the metal electrode  68 . Further, a metal electrode  70  serving as a first electrode is formed on the whole wall surface  66  of the actuator wall  60 , which corresponds to an inner surface of the ink channel  24 . 
     The metal electrodes  68  and  68 ′ are connected to a controller C, and the metal electrode  70  is not connected to the controller C. Controller C selectively applies voltage and grounds the electrodes  68  and  68 ′ depending upon the activation of the ink channel. Also, the electrode  68  can be grounded while the electrode  68 ′ has voltage applied thereto if desired. 
     Next, the operation of the ink jetting device of the first embodiment is described. When an ink droplet is jetted, the controller C grounds the metal electrode  68 ′ formed at the upper portion of the actuator wall  60  and applies a driving voltage V′ to the metal electrode  68  formed at the lower portion of the actuator wall  60 . Through this operation, an electric field  73  in the Y-direction occurs between the metal electrode  68  and the metal electrode  70  in the actuator wall  60 , and an electric field  74  in a direction opposite to the Y-direction occurs between the metal electrode  68 ′ and the metal electrode  70 . Further, a leak electric field  75  is directly formed between the metal electrode  68  and the metal electrode  68 ′. However, since the actuator wall  60  is designed so that the thickness (Y-direction) thereof is extremely small as compared with the height (Z-direction) thereof, the leak electric field  75  is still weaker than the electric field  73  and the electric field  74 , which serve to deform the actuator wall  60 . Thus, it is negligible. In FIGS. 1A and 1B, the actuator wall  60  is illustrated as being thicker than an actual size for purposes of explanation. 
     The directions of the electric fields  73  and  74  are opposite to each other and are perpendicular to the Z-direction corresponding to the polarization direction of the actuator wall  60 . So, the actuator wall  60  is deformed toward the inside of the ink channel  24  by the thickness shear effect of the piezoelectric ceramic material. Through this deformation, the ink in the ink channel  24  is pressurized, and the ink droplet is jetted from nozzles (not shown) intercommunicating with the ink channel  24 . 
     Here, the actuator wall  60  basically serves as a capacitor. Thus, the direction of current that flows at the rise-up time and the fall time of the driving voltage V′ applied to the metal electrode  68  is coincident with the direction of the electric field  73 ,  74 . Therefore, the current flows from the metal electrode  68  to the metal electrode  70 , and further flows through the metal electrode  70  to the metal electrode  68 ′. Accordingly, the upper and lower portions of the actuator wall  60  are electrically connected to each other in series. As compared with the conventional ink jetting device in which the upper and lower portions are apparently connected in parallel (see FIGS.  3 A and  3 B), a double driving voltage must be applied to induce the same deformation in the actuator wall  60  in this embodiment. However, as described above, the actuator wall  60  basically serves as a capacitor, and the capacitance of the actuator wall  60  at the in-series connection is a quarter of that at the in-parallel connection. The supplied energy (power) for the supplied driving voltage is proportional to the capacitance of the capacitor and also proportional to the square of the applied voltage. Therefore, the same energy efficiency is obtained in this embodiment and the prior art. 
     As described above, in the ink jetting device  110  of the first embodiment, the controller C is connected to the metal electrodes  68  and  68 ′, that is, it is connected to two portions only. On the other hand, the controller of the prior art is connected to four portions, totally. Therefore, the number of the connections is reduced to half. Usually, a printing operation is carried out using a printing head in which a plurality of ink channels thus constructed are provided. Thus, the manufacturing cost can be remarkably reduced if the connection number is reduced to half. 
     In the prior art (see FIGS.  3 A and  3 B), the metal electrode  69  disposed in the ink channel  24  is grounded, and the metal electrode  69 ′ is supplied with the voltage V. Therefore, an electric field occurs in the ink that is filled in the ink channel  24 , so that ink particles are charged by the electric field. The charged ink particles are electrostatically attracted to and impinge against the metal electrodes  69  and  69 ′. So, the metal electrodes  69  and  69 ′ may deteriorate. In addition, the charged ink particles adhere to the metal electrodes  69  and  69 ′. Therefore, the metal electrodes  69  and  69 ′ are liable to corrode. So, the lifetime of the ink jetting device and its reliability is reduced. 
     However, in the first embodiment as described above, the metal electrode  70  provided inside of the ink channel  24  is not directly connected to the controller C, and the ink is never charged if one ink channel is provided. Accordingly, the deterioration and corrosion of the metal electrode  70  are prevented because the ink does not cling to the electrode  70 . So, the lifetime of the ink jetting device is longer than the prior art, and its reliability is more improved than the prior art. However, when a printing head having plural ink channels  24  is used, the ink located between the metal electrode  70  of an activated ink channel  24  and the metal electrode  70  of an unactivated ink channel  24  becomes charged because a current is induced between the electrodes of each channel via the ink. However, these metal electrodes are disposed farther away from each other as compared with the prior art because they are disposed on one end of each channel. Therefore, the induced current would have to travel through the ink down one channel and up the other channel to the other electrode. Therefore, as the induced current is very weak, it takes an extremely long time for the ink particles to adhere to the metal electrode  70  in the channel. So, the metal electrode  70  hardly suffers corrosion, and the lifetime of the ink jetting device is much longer than the prior art. 
     Next, a second embodiment of the ink jetting device according to the present invention is described with reference to FIGS. 2A and 2B. In an ink jetting device  210  of the second embodiment, an actuator wall  80  is further used in place of the rigid wall  26  of FIGS. 1A and 1B. The actuator wall  80  is formed of piezoelectric ceramic material like the actuator wall  60 . The wall surface  85  of the actuator wall  80  is provided with a metal electrode  78  at the lower side thereof and with a metal electrode  78 ′ at the upper side thereof spaced from the metal electrode  78 . Further, a metal electrode  71  is formed on the whole wall surface  86  of the actuator wall  80  that corresponds to an inner surface of the ink channel  24 . The metal electrodes  78  and  78 ′ are connected to a controller C, and the metal electrode  71  is not connected to the controller C. 
     When the controller C connects the metal electrodes  68 ′ and  78 ′ to ground and applies a driving voltage V to the metal electrodes  68  and  78 , as shown in FIG. 2B, electric fields  73  and  74  occur in the actuator wall  60  while electric fields  76  and  77  occur in the actuator wall  80 . Through this operation, the actuator walls  60  and  80  are deformed so that the volume of the ink channel  24  is reduced to pressurize the ink in the ink channel  24 , thereby jetting the ink from nozzles (not shown). 
     In comparison between the second embodiment for deforming the two walls and the first embodiment for deforming only one wall, in order to obtain the same ink pressure in the ink channel  24 , it is sufficient in the second embodiment to supply each wall with a half deformation amount of the first embodiment. Accordingly, the driving voltage of the second embodiment is set to half of the driving voltage V′ of the first embodiment. Further, the actuator walls  60  and  80  are deformed in the second embodiment, and thus the capacitance of the capacitor is increased to double of the first embodiment. However, the supply energy for the supplied driving voltage is proportional to the capacitance of the capacitor, and also proportional to the square of the applied voltage. So, the supply energy of the second embodiment is half of that of the first embodiment. Therefore, in the second embodiment the ink droplet can be performed with half the supply energy of the first embodiment. Accordingly, the power consumption can be reduced, and the running cost can be lowered. Further, the driving voltage is small, and thus durability of the actuator wall can be improved. 
     The same effect could be obtained if the two-wall deforming operation as described above is used in the prior art. However, in this case the number of connections between the metal electrodes and the controller is increased twice as compared with the case where only one wall is driven (deformed). Therefore, the connections become more complicated, and the cost is also increased. On the other hand, the second embodiment can obtain the improved effect as described above with the same connection number in the case where the one-wall deforming operation is used in the prior art. 
     In the first and second embodiments as described above, only one ink channel  24  is provided. However, a plurality of ink channels may be provided. In this case, an ink droplet may be jetted from those ink channels selected from the plural ink channels. 
     Further, in the first and second embodiments, the ink in the ink channel  24  is pressurized by reducing the volume of the ink channel  24  from its usual or initial state (i.e., the volume when no voltage is applied) to thereby jet the ink droplet. However, the ink droplet may be jetted in the following manner. That is, driving voltages each having the opposite polarity are applied to the metal electrodes to increase the volume of the ink channel  24  from the usual state. Then, the application of the driving voltages to the metal electrodes is released to return the increased volume of the ink channel  24  to the usual state and pressurize the ink in the ink channel  24  after a predetermined time elapses, thereby jetting the ink from the ink channel  24 . 
     Still further, in the first and second embodiments, the actuator wall  60  and the bottom wall  20  are formed of different members. However, they may be integrally formed by processing one surface of a piezoelectric ceramic plate to form grooves on the surface of the plate. The grooves may be formed on both surfaces of the plate. 
     If the ink has proper conductivity, the metal electrodes  70  and  71  in the ink channel  24  are not necessarily required as shown in FIG.  1 C and FIG.  2 C. In this case, current flows through the conductive ink  90 , and there is no problem if no electrochemical deterioration occurs in the ink. 
     While advantageous embodiments have been chosen to illustrate the invention, it will be understood by those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention defined in the appended claims.