Patent Publication Number: US-RE38941-E

Title: Ink droplet ejecting method and apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an ink droplet ejecting method and apparatus of an ink jet type. 
     2. Description of Related Art 
     According to a known ink jet printer of an ink jet type, the volume of an ink flow path is changed by deformation of a piezoelectric ceramic material. When the ink flow path volume decreases, the ink present in the ink flow path is ejected as a droplet from a nozzle. However, when the ink flow path volume increases, the ink is introduced into the ink flow path from an ink inlet. In this type of printing head, multiple ink chambers are formed by partition walls of a piezoelectric ceramic material. An ink supply device, such as ink categories, are connected to one end of each of the multiple ink chambers. The opposite end of each of the ink chambers is provided with an ink ejecting nozzle (hereinafter referred to simply as “nozzles”). The partition walls are deformed in accordance with printing data to make the ink chambers smaller in volume, whereby ink droplets are ejected onto a printing medium from the nozzles to print, for example, a character of a figure. 
     An example of this type of ink jet printer is a drop-on-demand type ink jet printer that ejects ink droplets, which is popular because of a high ejection efficiency and a low running cost. An example of a drop-on-demand type ink jet printer is a shear mode type that uses a piezoelectric material, which is disclosed in Japanese Published Unexamined Patent Application No. Sho 63-247051. 
     As shown in FIGS.  7 (a) and  7 (b), this type of ink droplet ejecting apparatus  600  includes a bottom wall  601 , a top wall  602  and shear mode actuator walls  603  (shown in  FIG. 8  as  603 a-g) located therebetween. The actuator walls  603  each include a lower wall  607  bonded to the bottom wall  601  and polarized in the direction of arrow  611 , and an upper wall  605  formed of a piezoelectric material, the upper wall  605  being bonded to the top wall  602  and polarized in the direction of arrow  609 . Adjacent actuator walls  603 , as a pair, define ink chamber  613  (shown in  FIG. 8  as  613 a-d) therebetween. The actuator walls  603  that are adjacent to the ink chamber, in a pair, define a space  615  which is narrower than the ink chamber  613 . 
     A nozzle plate  617  having nozzles  618  (shown in  FIG. 8  as  618 a-d) is fixed to one end of each of the ink chambers  613 , while the opposite end of each of the ink chambers is connected to an ink supply source (not shown). Electrodes  619  (shown in  FIG. 8  as  619 a-d) and  621  are respectively formed on both side faces of each actuator wall  603 , as metallized layers. More specifically, electrode  619  is formed on the actuator wall  602  on the side of the ink chamber  613 , while electrode  621  is formed on the actuator wall  603  on the side of the space  615 . The surface of electrode  619  is covered with an insulating layer  630  for insulation from ink. Electrode  621 , which faces the space  615 , is connected to a ground  623 , and electrode  619 , which is provided in each ink chamber  613 , is connected to a controller  625 , which provides an actuator drive signal to the electrode. 
     The one-way propagation time T is a time required for the pressure wave in the ink chamber  613  to propagate longitudinally through the same chamber. Given that the length of the ink chamber  613  is L and the velocity of sound in the ink present in the ink chamber  613  is a, the time T is determined to be T=L/a. 
     According to the theory of pressure wave propagation, upon lapse of time T, or an odd-multiple time thereof, after the above application of voltage, the internal pressure of the ink chamber  613  reverses into a positive pressure. In conformity with this timing, the voltage being applied to the electrode in the ink chamber  613 c is returned to 0(V). As a result, the actuator walls  603 e and  603 f revert to their original state (FIGS.  7 (a) and  7 (b) before the deformation, whereby a pressure is applied to the ink. At this time, the above positive pressure, and the pressure developed by the reverting of the actuator walls  603 e and  603 f to their original state before the deformation, are added together to provide a relatively high pressure in the vicinity of the nozzle  618 c in the ink chamber  613 c, whereby an ink droplet is ejected from the nozzle  618 c. An ink supply passage  626 , shown in FIG.  7 (b), that communicates with each of the ink chambers  613 , is formed by members  627  and  628 . 
     Conventionally, in this type of ink droplet ejecting apparatus  600 , when an ink droplet of a small volume is to be ejected for enhancing the printing resolution, a control has been provided to decrease the driving voltage in multiple steps, for example. However, such a method of controlling the voltage in multiple steps leads to an increase in cost of a driver IC, etc., and attempting to reduce the volume of an ink droplet gives rise to the problem that even the speed of the ink droplet decreases. In order to obtain an ink droplet of a small volume without decreasing the ink droplet speed, it has been proposed to use in additional pulse of a low voltage level, after application of a jet pulse and before completion of ink ejection. However, this proposal also leads to an increase in cost of a driver IC, etc. because multiple voltages are needed as driving pulses. 
     SUMMARY OF THE INVENTION 
     The invention solves the above-mentioned problems, and it is an object of the invention to provide an ink droplet ejecting method and apparatus, wherein, after a driving waveform for a primary ejection of ink, only one additional pulse is added, thereby making it possible to obtain an ink droplet of a desired volume and also possible to minimize the decrease of the ink droplet speed. 
     In order to achieve this object, an ink droplet ejecting method is provided, wherein a jet pulse signal is applied to an actuator, for changing the volume of an ink chamber filed with ink, to generate a pressure wave within the ink chamber, thereby applying pressure to the ink and allowing a droplet of the ink to be ejected from a nozzle. Both the jet pulse signal and an additional pulse signal are applied to the actuator in accordance with a one-dot printing instruction. The jet pulse signal has a pulse width which allows the volume of the ink chamber to increase upon application of a voltage of the actuator, thereby causing a pressure wave to be generated within the ink chamber, and which, after the lapse of time T required for an approximately one-way propagation of the pressure wave through the ink chamber or after the lapse of an odd-multiple time of the time T, allows the volume of the ink chamber to decrease from the increased state to a normal state. The additional pulse signal has a pulse width of approximately 0.2T to 0.6T relative to the jet pulse signal, and a time difference between a fall timing of the jet pulse signal and a rise timing of the additional pulse signal is 0.3T to 0.7T. 
     According to the above method, the ink present in the ink chamber is about to rush out from the nozzle at the leading edge and the trailing edge of the jet pulse signal, and with the additional pulse signal which is subsequently applied halfway at the above timing, a part of the ink droplet which is rushing out from the nozzle is pulled back. Consequently, it is possible to reduce the size of the flying ink droplet after ejection, and hence possible to attain a high printing resolution easily. Further, since it is not necessary to change the driving voltage to reducing the size of the ink droplet, the cost is reduced and the ink droplet speed is only minimally decreased. 
     In accordance with another aspect of the ink droplet ejecting method, the jet pulse signal and the additional pulse signal have the same peak value. According to this method, a single drive voltage source is sufficient to obtain a small-sized ink droplet, and therefore the cost can be reduced. 
     An ink droplet ejecting apparatus is also provided that includes an ink chamber filled with ink, an actuator for changing the volume of the ink chamber, a driving power source for applying an electric signal to the actuator, and a controller which provides controls so that, in accordance with a one-dot printing instruction, a jet pulse signal and an additional pulse signal are applied to the actuator from the driving power source, thereby causing the ink present in the ink chamber to be ejected. The controller provides control so that the jet pulse signal has a pulse width which allows the volume of the ink chamber to increase upon application of a voltage to the actuator, thereby causing a pressure wave to be generated within the ink chamber, and which, after the lapse of time T required for an approximately one-way propagation of the pressure wave through the ink chamber or after the laps of an odd-multiple time of the time T, allows the volume of the ink chamber to decrease from the increased state to a normal state. The controller also provides control so that the additional pulse signal has a pulse width of approximately 0.2T to 0.6T relative to the jet pulse signal, and a time difference between a fall timing of the jet pulse signal and a rise timing of the additional pulse signal is 0.3T to 0.7T. 
     This structure provides the same advantages as the corresponding method in accordance with the invention discussed above. 
     In accordance with another aspect of this ink droplet ejecting apparatus, the jet pulse signal and the additional pulse signal have the same peak value. This structure provides the same advantages as the corresponding aspect of the method in accordance with the invention discussed above. 
     An ink droplet ejecting apparatus is also provided that includes an ink chamber filled with ink, an actuator for changing the volume of the ink chamber, a driving power source for applying an electric signal to the actuator, and a controller which provides control so that in accordance with a one-dot printing instruction, a jet pulse signal for ejecting the ink present in the ink chamber and an additional pulse signal for withdrawing a part of an ink droplet which has rushed out from a nozzle in accordance with the jet pulse signal, are applied from the driving power source to the actuator. The controller determines whether or not the additional pulse signal is to be used. According to the apparatus, the volume of ink droplet can be adjusted by either applying, or not applying, the additional pulse signal in accordance with a preset resolution. 
     An ink droplet ejecting apparatus is also provided that includes an ink chamber filled with ink, an actuator for changing the volume of the ink chamber, a driving power source for applying an electric signal to the actuator, and a controller which provides control so that, in accordance with a one-dot printing instruction, a jet pulse signal for ejecting the ink present in the ink chamber and an additional pulse signal for withdrawing a part of an ink droplet which has rushed out from a nozzle in accordance with the jet pulse signal, are applied to the actuator from the driving power source. The controller provides controls so that a time difference from the application of the jet pulse signal up to the application of the additional pulse signal, and the pulse width of the additional pulse signal, can be adjusted. According to this apparatus, since a control is provided so that the time difference from the application of the jet pulse signal up to the application of the additional pulse signal can be adjusted in accordance with a preset resolution, it is possible to adjust the volume of an ink droplet. 
     In accordance with another aspect of this ink droplet ejecting apparatus, the jet pulse signal has a pulse width which allows the volume of the ink chamber to increase upon application of a voltage to the actuator thereby causing a pressure wave to be generated within the ink chamber, and which, after the lapse of time T required for an approximately one-way propagation of the pressure wave through the ink chamber or after the lapse of an odd-multiple time of the time T, allows the volume of the ink chamber to decrease from the increased state to a normal state. The pulse width of the additional pulse signal is controlled so as to be adjustable in the range of approximately 0.2T to 0.6T relative to the jet pulse signal, and the time difference is controlled so as to be adjustable in the range of approximately 0.3T to 0.7T from a trailing edge of the jet pulse signal up to a leading edge of the additional pulse signal. According to this apparatus, the decrease of the ink catalyst speed is minimized, so that the same advantages can be attained as the ink droplet ejecting apparatus discussed above. 
     According to the invention, as set forth above, by adding a predetermined additional pulse signal to a jet pulse signal for a one-dot printing instruction, a small volume ink droplet can be provided at high speed, without decreasing the ink droplet speed. Further, since the volume of an ink droplet can be adjusted as desired, it is possible to obtain a desired printing resolution easily. 
     Further, unlike the conventional art, multiple driving voltages are not required to reduce the size of an ink droplet. One driving voltage source is sufficient, and it is not necessary to change the driving voltage, thus reducing the cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A preferred embodiment of the invention will be described in detail with reference to the following figures wherein: 
         FIG. 1  is a diagram showing a driving waveform used in an ink droplet ejecting apparatus according to an embodiment of the invention; 
         FIG. 2  is a diagram showing a drive circuit used in the ink droplet ejecting apparatus; 
         FIG. 3  is a diagram showing storage areas of a ROM used in the ink droplet ejecting apparatus; 
       FIG.  4 (a) is a diagram showing how the ink droplet speed changes upon application of various driving waveform signals and 
       FIG.  4 (b) is a diagram showing how the ink droplet volume changes upon application of various driving waveform signals; 
       FIGS.  5 (a)- 5 (e) are drawings showing how an ink droplet is ejected from a nozzle upon application of a conventional driving waveform signal; 
       FIGS.  6 (a)- 6 (e) are diagrams showing how an ink droplet is ejected from a nozzle upon application of a driving waveform signal according to the invention; 
       FIG.  7 (a) is a longitudinal sectional view of an ink jet portion of a printing head, and 
       FIG.  7 (b) is a transverse sectional view thereof; 
         FIG. 8  is a longitudinal sectional view showing how the ink jet portion of the printing head operates; 
         FIG. 9  is a diagram showing dots ejected at resolutions of 360 dpi, 720 dpi and 1440 dpi; and 
         FIG. 10  is a flowchart explaining control contents of the ROM in the controller of the ink droplet ejecting apparatus according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the invention will be described hereinunder with reference to the drawings. The structure of a mechanical portion of an ink droplet ejecting apparatus of this embodiment is the same as that shown in FIGS.  7 (a) and  7 (b), referred to above, and therefore an explanation thereof is omitted. 
     An example of specific dimensions of this ink droplet ejecting apparatus, indicated as  600 , will now be described. The length L of an ink chamber  613  is 9 mm. As to the dimensions of a nozzle  618 , its diameter on an ink droplet ejection side is 40 μm, its diameter on the ink chamber  613  side is 72 μm, and its length is 100 μm. In an experiment, the viscosity of 25° C. of ink used was about 2 mPas and the surface tension thereof was 30 mN/m. The ratio of the above length L to a sonic velocity, a, in the ink present within the ink chamber  613 , i.e., L/a (=T) was 10 μsec. 
       FIG. 1  shows a driving waveform to be applied to an electrode  619  disposed in the ink chamber  613  in the embodiment of the invention. This driving waveform, indicated at  10 , is a pulse signal including a jet pulse signal A for the ejection of an ink droplet, and an additional pulse signal B subsequent to the jet pulse signal A. The additional pulse signal B, which reduces the sizes of a flying ink droplet, has a pulse width smaller than that of the jet pulse signal A, and is applied at a timing at which a part of the ink droplet rushed out from the nozzle in accordance with the jet pulse signal A can be withdrawn. The jet pulse signal A and the additional pulse signal B have the same peak value (voltage value) of E(V), for example 20(V). 
     The wave width Wa of the jet pulse signal A is set equal to the ratio, L/a (=T), of the foregoing length L to a sonic velocity, a, in the ink present in the ink chamber  613 , or corresponds to an odd-multiple time thereof (a value peculiar to a head), for example, 10 μsec, that a time difference, d, between a fall timing of the jet pulse signal A and a rise timing of the additional pulse signal B is 0.3T to 0.7T, that is, approximately 3 to 7 μsec, and that the wave width Wb of the additional pulse signal B is 0.2T to 0.6T, that is, approximately 2 to 6 μsec. The total time of the time difference, d, and the wave width, Wb, is approximately 5 to 13 μsec. At the wave width Wb, the additional pulse signal B does not cause ejection of an ink droplet. The pulse cycle in the case of printing the next dot in a continuous manner is 100 μsec, assuming that the driving frequency is 10 kHz. 
     Next, an example of a controller for implementing the driving waveform  10  will be described with reference to  FIGS. 2 and 3 . A controller  625  shown in  FIG. 2  includes a charging circuit  182 , a discharge circuit  184  and a pulse control circuit  186 . A piezoelectric material of an actuator wall  603  and electrodes  619 ,  621  are represented equivalently by a capacitor  191 . Numerals  191 A and  191 B denote terminals thereof. 
     Input terminals  181  and  183  are for inputting pulse signals to adjust the voltage to be applied to the electrode  619  in each ink chamber  613 , to E(V) or 0(V). The charging circuit  182  includes resistors R 101 , R 102 , R 103 , R 104 , R 105  and transistors TR 101 , TR 102 . 
     When an ON signal (+5V) is applied to the input terminal  181 , the transistor TR 101  conducts through resistor  101 , so that an electric current flows from a positive power source  187 , passes through resistor R 103 , and flows from the collector to the emitter of transistor TR 101 . Consequently, a divided voltage of the voltages applied to the resistor R 104  and R 105 , which are connected to the positive power source  187 , increases and so does the electric current flowing in the base of the transistor TR 102 , providing conduction between the emitter and the collector of the transistor TR 102 . A voltage of 20(V) from the positive power source  187  is applied to the terminal  191 A of the capacitor  191  via the collector and emitter of the transistor TR 102  and resistor R 120 . 
     The following description is now provided regarding the discharge circuit  184 . The discharge circuit  184  includes resistors R 106 , R 107  and a transistor TR 103 . When an ON signal (+5V) is applied to the input terminal  183 , the transistor TR 103  turns conductive via resistor R 106  and the terminal  191 A on the resistor R 120  side of the capacitor  191  is grounded via resonator R 120 , so that the electric charge imposed on the actuator wall  603  of the ink chamber  613  shown in FIGS.  7 (a),  7 (b) and  8  is discharged. 
     Reference will now be made to the pulse control circuit  186  which generates pulse signals to be received by the input terminal  181  of the charging circuit  182  and the input terminal  183  of the discharge circuit  184 . A CPU  110  is provided in the pulse control circuit  186  which performs various arithmetic operations. To the CPU  110  are connected, a RAM  112  for the storage of printing data and various other data, and a ROM  114  which stores sequence data for generating ON-OF signals in accordance with control program and timing in the pulse control circuit  186 . In the ROM  114 , as shown in  FIG. 3 , an area  114 A for the storage of an ink droplet ejection control program, and an area  114 B for the storage of driving waveform are provided. Thus sequence data of the driving waveform  10  is stored in the driving waveform data storage area  114 B. 
     In the control program storage area  114 A, as shown in  FIG. 10 , there is stored a program according to which the CPU  110  judges whether a setting made by a user is for enhancing the resolution (that is, reducing the volume of a one-dot ink droplet ejected) (S 1 ), and on the basis of the result of the this judgment, it is determined whether the additional pulse B is to be added to the jet pulse signal stored in the waveform data storage area  114 B (S 2 , S 3 ). A program is also stored in the control program storage area  114 A for controlling the time difference, d, and the wave width, Wb, in an adjustable manner. 
     The CPU  110  is further connected to an I/O bus  116  for transmission and reception of various data. A printing data receiving circuit  118  and pulse generations  120  and  122  are also connected to the I/O bus  116 . The output of the pulse generator  120  is connected to the input terminal  181  of the charging circuit  182 , while the output of the pulse generator  122  is connected to the input terminal  183  of the discharge circuit  184 . 
     The CPU  110  controls the pulse generators  120  and  122  in accordance with the sequence data stored in the driving waveform data storage area  114 B of the ROM  114 . Therefore, by having various patterns of the foregoing timing stored beforehand in the driving waveform data storing area  114 B of the ROM  114 , it is possible to apply a driving pulse of the driving waveform  10  shown in  FIG. 1  to the actuator wall  603 . 
     The same number of pulse generators  120 ,  122 , charging circuit  182  and discharge circuit  184  are provided as the number of nozzles used. Although the above description is directed to controlling one nozzle, the same control can also be applied to the other nozzles. 
     Reference is now made to the results of ink droplet ejection tests conducted in accordance with the driving method of this embodiment. FIGS.  4 (a) and (b) are characteristic diagrams showing changes in ink droplet speed and one-dot link droplet volume in various combinations of the time difference, d, and the wave width, Wb, in connection with the driving waveform  10  shown in FIG.  1 . In both figures, broken lines represent values of ink droplet speed (8 m/s) and ink droplet volume, 45 pl (picoliter), obtained by using only the jet pulse A for the ejection of ink. The values of ink droplet volume obtained by using both jet pulse signal A and additional pulse signal B are all smaller than the values when only the jet pulse signal A is used. Particularly, in any of the combinations 0.3T to 0.5T as time difference, d, and 0.2T to 0.67T as wave width, Wb, of the additional pulse signal B a considerably reduction in the size of ink droplet is attained. In comparison with the use of only the jet pulse signal A, the ink droplet speed decreases partially (at the time difference, d, of 0.3T), but in many of the other cases (0.5T to 0.7T in the time difference, d), the ink droplet speed does not decrease very much. By using the additional pulse signal B in such combination ranges of time difference and wave width as mentioned above, the ink droplet speed does not decrease very much, and a small ink droplet is obtained as compared with the use of only the jet pulse signal A. 
     FIGS.  5 (a)- 5 (e) show the manner in which an ink droplet is ejected from a nozzle by applying only the jet pulse signal A to the actuator with respect to one dot, and FIGS.  6 (a)- 6 (e) show the manner in which an ink droplet is ejected from the nozzle by using both jet pulse signal A and additional pulse signal B as in the embodiment of the invention shown in FIG.  1 . In  FIG. 5 , at the leading edge of the jet pulse signal A, the volume of the ink chamber  11  increases and an ink meniscus  13  temporarily retracts inwardly of the nozzle  12  temporarily, as shown in FIG.  5 (b). Then, at the trailing edge of the jet pulse signal A after the lapse of time required for one-way propagation of the pressure wave through the ink chamber  11 , the volume of the ink chamber  11  decreases from the increased state to a normal state, whereby the ink is ejected from the nozzle while forming the ink droplet  14 . 
     On the other hand, as shown in  FIG. 6  according to this embodiment, the additional pulse signal B is applied after the fall of the jet pulse signal A, whereby a part of the ink droplet being ejected from the nozzle  12  is pulled back, resulting in a meniscus  15  as shown in FIG.  6 (d), whereby an ink droplet  16  ejected from the nozzle  12  is made smaller in size than ink droplet  14 . In this way, without changing the driving voltage and hence without increase of cost, the ejection of an ink droplet of a small volume can be attained by merely adding one pulse after the main driving waveform. Also, the ink droplet speed is only minimally decreased. 
       FIG. 9  shows printed states of continuous dot printings performed at resolution of 360 dpi, 720 dpi and 1440 dpi, respectively. As shown in  FIG. 1  referred to previously, if the additional pulse signal B is annexed to the jet pulse signal A as a one-dot printing instruction and if, for example, the time difference, d, is set at 0.7T and the wave width, Wb, at 0.6T the ink droplet volume becomes 40 pl or so, which is suitable for printing at a resolution of 360 dpi. If the time difference, d, is set at 0.3T and the wave width, Wb, is set at 0.6T, the ink droplet volume becomes 25 pl or so, which is suitable for printing at a resolution of 720 dpi. Further, at a time difference, d, of 0.3T and a wave width, Wb, of 0.2T, the ink droplet volume is about 15 pl, which is suitable for printing at a resolution of 1440 dpi. 
     Although an embodiment of the invention has been described above, the invention is not limited therein. For example, although the main driving signal used in the above embodiment has only one jet pulse A, it may be a driving signal that includes two jet pulses. Also, regarding the ink droplet ejecting apparatus  600 , no limitation is placed on the structure described in the above embodiment. Further, a similar apparatus can be used that is opposite in polarizing direction of the piezoelectric material. 
     Although in the above embodiment, air chambers  615  are formed on both sides of each ink chamber  613 , ink chambers may be formed in a directly adjacent manner with no air chamber  615  therebetween. Further, although the actuator used in the above embodiment is a shear mode type, a structure may be used wherein layers of a piezoelectric material are laminated together and a pressure wave is generated by a deformation in the laminated direction. No limitation is imposed on the piezoelectric material. Further, any other material can be used insofar as a pressure wave is generated in each ink chamber.