Abstract:
In an ink drop let jetting method and an apparatus therefor, by setting a printing frequency used when continuous dots are printed to a predetermined value, a stable jetting becomes possible, and jetting speeds and volumes of second ink droplets and subsequent droplets may be prevented from being fluctuated. A frequency of a jet pulse signal applied to an actuator in accordance with a printing command of a plurality of consecutive dots is set to be a reciprocal of the product of a sum (integer +0.5) and the time T in which a pressure wave propagates within an ink chamber in one propagation direction. Thus, it is possible to prevent speeds and volumes of the second ink droplets and subsequent ink droplets from being fluctuated.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ink jet ink droplet ejecting method and apparatus. 
     2. Description of Related Art 
     In a known ink jet printer, the volume of an ink flow path is changed by deformation of a piezoelectric ceramic material, and when the flow path volume decreases, the ink present in the ink flow path is ejected as a droplet from a nozzle. However, when the flow path volume increases, the ink is introduced into the ink flow path from an ink inlet. In this type of printing head, a plurality of ink chambers is formed by partition walls made of a piezoelectric ceramic material. Ink supply means, such as ink cartridges, are connected to first ends of the ink chambers, while at the opposite, second ends, ink ejecting nozzles (hereinafter referred to as “nozzles”) are provided. 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 or a figure. 
     For example, as this type of ink jet printer, a drop-on-demand type ink jet printer, which ejects ink droplets, is popular because of a high ejection efficiency and a low running cost. As an example of the drop-on-demand type there is known a shear-mode type that uses a piezoelectric material, as is disclosed in Japanese Published Unexamined Patent Application No. Sho 63-247051. 
     FIGS. 8A and 8B illustrate this shear-mode type of ink droplet ejecting apparatus  600  comprising a bottom wall  601 , a top wall  602  and shear mode actuator walls  603  located therebetween. Each actuator wall  603  comprises 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 , in a pair, define an ink chamber  613  therebetween, and next adjacent actuator walls  603 , in a pair, define a space  615  that is narrower than the ink chamber  613 . 
     A nozzle plate  617  having nozzles  618  is fixed to first ends of the ink chambers  613 . An ink supply source (not shown) is connected to the opposite ends of the ink chambers. As illustrated in FIG. 8B, on both side faces of each actuator wall  603  are formed electrodes  619  and  621  respectively as metallized layers. More specifically, the electrode  619  is formed on the actuator wall  603  on the side of the ink chamber  613 , while the electrode  621  is formed on the actuator wall  603  on the side of the space  615 . The surface of the electrode  619  is covered with an insulating layer  630  for insulation from ink. The electrode  621  that faces the space  615  is connected to a ground  623 , and the electrode  619  provided in each ink chamber  613  is connected to a controller  625  that provides an actuator drive signal to the electrode. 
     The controller  625  applies a voltage to the electrode  619  in each ink chamber, whereby the associated actuator walls  603  undergo a piezoelectric thickness slip deformation in different directions to increase the volume of the ink chamber  613 . For example, as shown in FIG. 9, when a voltage E(v) is applied to an electrode  619   c  in an ink chamber  613   c , electric fields are generated in directions of arrows  631  and  632  respectively in actuator walls  603   e  and  603   f , so that the actuator walls  603   e  and  603   f  undergo a piezoelectric thickness slip deformation in different directions to increase the volume of the ink chamber  613   c . At this time, the internal pressure of the ink chamber  613   c , including a nozzle  618   c  and the vicinity thereof, decreases. The applied state of the voltage E(v) is maintained for only a one-way propagation time T of a pressure wave in the ink chamber  613 . During this period, ink is supplied from the ink supply source. 
     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 pressure wave propagation theory, 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  619   c  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 (FIG. 8A) before the deformation, whereby a pressure is applied to the ink. At this time, the above positive pressure and the pressure developed by reverting of the actuator walls  603   e  and  603   f  to their original state before the deformation are added together to afford 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  communicating with the ink chamber  613  is formed by members  627  and  628 . 
     Conventionally, in this kind of apparatus  600  for jetting droplets of ink, when a printing frequency requires an increase of when droplets of ink of consecutive dots are jetted then, within a certain frequency range, the ink-jet tends to become unstable due to a meniscus vibration of ink within the nozzle. As a consequence, during continuous ink-jetting, jet speeds of second and third ink droplets and volumes of ink droplets are fluctuated and become uneven, thereby resulting in decreased printing quality. 
     Conventionally, as shown in Japanese Published Unexamined Patent Application No. Hei 6-84073, to compensate for the influence of the meniscus vibration of ink-jetting and to effectively use energy required when a pulse voltage rises, there is a method known in which a time period ranging from the trailing edge of a pulse voltage to the leading edge of the next pulse voltage is set to ½ of a natural vibration period of a nozzle portion. However, according to this method, vibration of the next ink-jetting is overlapped with vibration generated when a piezoelectric element returns to a stable position after a vibration of ink-jetting is stopped. This method does not provide a counter-measure executed during the continuous vibration at a high printing frequency. 
     Additionally, as shown in Japanese Published Unexamined Patent Application No. Sho 61-120764, a method is known in which a drive signal for a piezoelectric element is controlled with reference to a dot interval in such a manner that the volume of droplets of ink remains constant regardless of the dot interval. However, this method is not able to prevent fluctuation of the volume of ink droplets of a second and subsequent continuous dots. 
     SUMMARY OF THE INVENTION 
     The present invention solves the above-mentioned problems, and provides an ink ejecting method and apparatus in which a printing frequency, used when continuous dots are printed, is set to a predetermined value so that stable ink-jetting is possible during continuous vibration, fluctuation of jetting speeds and volumes of ink droplets of a second dot, and subsequent dots are prevented and excellent ink-jet printing quality is provided. 
     In order to attain the above-described objects, according to a first aspect of the present invention, there is provided an ink ejecting method in which a pressure wave is generated within an ink chamber by applying a jet pulse signal to an actuator which changes a capacity of the ink chamber containing a quantity of ink to apply a pressure to the ink thereby jetting droplets of ink from a nozzle. This ink ejecting method uses a printing frequency such that volumes of ink droplets of a second dot and subsequent dots become substantially equal to a volume of the ink droplet of the first dot when the jet pulse signal is applied to the actuator in accordance with a printing command for a plurality of consecutive dots. According to this method, fluctuation of the volume of droplets of ink required when droplets of a plurality of dots are continuously ink-jetted is prevented, thereby making it possible to realize high frequency printing. 
     Also, according to a second aspect of the present invention in an ink ejecting method, the jet pulse signal applied to the actuator in accordance with the printing command for the plurality of consecutive dots has a frequency that is equal to a reciprocal of a value approximately equal to a quantity time T, in which a pressure wave propagates in one direction within the ink chamber, multiplied by a multiplier that is an integer plus 0.5. According to this method, setting the jet pulse signal frequency equal to a reciprocal of the product of the quantity time T and an odd integer decreases the speeds and volumes of droplets of ink of a second dot and subsequent dots. Alternatively, setting the frequency equal to a reciprocal of the product of the quantity time T and an even integer increases the speeds and volumes of droplets of ink of a second dot and subsequent dots. However, setting the jet pulse signal frequency equal to a reciprocal of the product of the quantity time T and an integer plus 0.5 maintains the speeds and volumes of droplets of ink of a second dot and subsequent dots at substantially constant values. 
     Also, according to a third aspect of the present invention, there is provided an ink ejecting apparatus which is comprised of an ink chamber that contains a quantity of ink, an actuator that changes a capacity of the ink chamber, a driving power source that applies an electrical signal to the actuator, and a controller. The controller controls a volume capacity of the ink chamber with selective application of a jet pulse signal to the actuator from the driving power source to generate a pressure wave within the ink chamber and application of a pressure to a quantity of ink contained in the ink chamber by decreasing the volume capacity from an increased state to a natural state after a time that is an integer multiple of T elapsed to jet droplets of ink. The controller controls the driving power source to apply a jet pulse signal with a frequency that is the reciprocal of the approximate product of the quantity T and an integer plus 0.5 to the actuator in accordance with a printing command of a plurality of consecutive dots. As a result of the this arrangement, the volume and print speed associated with a second dot and subsequent dots is substantially maintained. 
     According to the present invention, if the jet pulse signal frequency for printing a plurality of consecutive dots is set in such a manner that ink droplet volumes of the second dot and subsequent dots are equal to that of the first dot, then even when dots are printed at a high frequency, stable ink jetting is possible during continuous vibration so that the ink jetting speeds and ink droplet volumes are maintained. In particular, the jet pulse signal frequency is set equal to the reciprocal of the approximate product of the quantity time T and an integer plus 0.5, whereby the speeds and volumes of the ink droplets used when dots are continuously printed are maintained provide high quality printing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the present invention will be described in detail with reference to the following figures wherein: 
     FIGS. 1 a diagram showing a driving waveform of an ink droplet jetting apparatus according to an embodiment of the present invention; 
     FIG. 2A is a graph showing measured data of ink droplet speeds obtained when an ink droplet jetting frequency is varied; 
     FIG. 2B is a graph showing measured data of ink droplet speeds of first to fifth dots obtained when the apparatus is driven at a variety of periods; 
     FIG. 3A is a graph showing measured data of ink droplet volumes obtained when an ink droplet jetting frequency is varied; 
     FIG. 3B is a graph showing measured data of ink droplet volumes of first to fifth dots obtained when the apparatus is driven at a variety of periods; 
     FIG. 4 is a diagram showing a driving circuit of an ink droplet jetting apparatus; 
     FIG. 5 is a diagram showing a storage area of a ROM of a controller of the ink droplet jetting apparatus; 
     FIGS. 6A,  6 B,  6 C are diagrams showing the manner in which ink droplets are jetted from a nozzle when the ink droplet jetting apparatus is driven at a variety of printing frequencies; 
     FIG. 7 is a diagram used to explain the manner in which a pressure within a pressure chamber is changed when a jetting pulse is applied thereto; 
     FIG. 8A is a longitudinal sectional view of an ink jet portion of a recording head, and FIG. 8B is a cross-sectional view of the longitudinal section view illustrated in FIG. 8A viewed from the line of sight identified by  8 B— 8 B; and 
     FIG. 9 is a longitudinal sectional view showing an operation of an ink jet unit of a recording head. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention will hereinafter be described with reference to the drawings. An exemplary arrangement of a mechanical portion of the apparatus for jetting droplets of ink according to this embodiment is illustrated in FIGS. 8A and 8B, and therefore need not be described. 
     Exemplary sizes of a present ink droplet jetting apparatus  600  will now be described. A length L of an ink chamber  613  may be, for example, 15 mm. A size of a nozzle  618  is such that a diameter of an ink drop jetting side is, for example, 40 μm. A diameter of an ink chamber  613  side is 72 μm and a length is 100 μm for example. A viscosity of ink, as used in the experiments may be about 2 mPa·s at 25° C. and its surface tension may be 30 mN/m. Thus, a ratio of the length, L, to the speed of sound, a, in the ink contained in this ink chamber  613  is for example 15 μsec. The ratio of the length, L (in meters), to the speed of sound, a (in meters per second), is equal to the quantity of time, T, required for a sound wave to traverse the length of the ink chamber  613 . The quantity T can be considered a period for a sound wave to propagate the length of the ink chamber  613 . The quantity of time T is essentially a period of a signal with pulses traversing the length of the ink chamber  613  individually, with no more than one pulse traversing the length of the ink chamber at any time. 
     FIG. 1 shows a waveform of a driving voltage applied to an electrode  619  disposed within the ink chamber  613  according to an embodiment of the present invention. An illustrated driving waveform  10  is a jet pulse signal A that is used to jet droplets of ink when one dot is printed. A peak voltage value of the driving waveform is 20 (v), for example. 
     A pulse width of the jet pulse signal A is the quantity of time T, or an odd-multiple of the time T. The period of the jet pulse signal A is approximately (N+0.5)T where N is an integer. Time period T is the time necessary for a pressure wave to travel a length of the ink chamber in one-direction. The period of the jet pulse signal A required when subsequent dots are printed continuously becomes 100 μsec when the frequency of the driving waveform is set to 10 kHz because frequency is the reciprocal of period. 
     When jet pulse signal A is applied in accordance with a printing command of a plurality of continuous dots, a printing frequency is used such that volumes of droplets of ink of a second dot and subsequent dots become approximately equal to that of the first dot. More specifically, as is clear from ink droplet measured data shown in FIGS. 2 and 3, which will be described below, the frequency of the jet pulse signal A is set approximately equal to the reciprocal of the product of the period T multiplied by the sum of an integer and 0.5. 
     FIG. 2A shows ink droplet speeds measured when the ink droplet jet frequency was varied, and FIG. 2B shows ink droplet speeds of the first five dots obtained when the ink droplet jet apparatus is driven at a variety of different frequencies corresponding to periods 6.0T through 10.0T. FIG. 3A shows ink droplet volumes obtained when the ink droplet jet frequency was changed, and FIG. 3B shows ink droplet volumes of the first five dots obtained when the ink droplet jet apparatus is driven at a variety of frequencies corresponding to periods 6.0T to 10.0T. In FIG. 2A, the solid line indicates the results from plotting measured data obtained when the ink droplet speed for the second dot is measured at a variety of driving waveform frequencies. A dashed line indicates the results from plotting measured data obtained when the third dot is measured at a variety of driving waveform frequencies. A dot-and-dash line represents ink droplet speeds and volumes of the first dot regardless of driving waveform frequency. As illustrated in FIG. 2A, the ink droplet speed of the first dot is maintained at approximately 7 m/s regardless of the driving waveform frequency. Similarly, as illustrated in FIG.  3 A,the volume of the ink droplets for the first dot remain constant at approximately 40 pl (picoliter). 
     As shown in FIGS. 2A and 3A, the ink droplet speeds and volumes for the second and third dots are increased when the period of the driving waveform is even-numbered multiples of the period T, for example, 6T, 8T, 10T. The ink droplet speeds and volumes for the second and third dots are decreased when the period of the driving waveform is odd-numbered multiples of the period T, for example, 7T, 9T. When the driving waveform period is equal to 6T, 90 μsec when T equals 15 μsec, the associated driving waveform frequency is approximately 11 kHz. In FIGS. 2A and 2B, the periods of the areas, shown by circles, in which the characteristic curves for the second and third dots cross the dot-and-dash line, which represents the value of the first dots, are located at approximately 6.5T, 7.5T, 8.5T, 9.5T. Therefore, the ink droplet volumes and speeds are approximately the same for the first, second and third dots at the frequencies within these circular areas mathematically represented as the product of the quantity time T and the sum of integers plus 0.5. Accordingly, by selecting these periods, it is possible to make the ink droplet speeds and the volumes of the second and third dots equal to those of the first dots. This will be understood from the graphs of FIGS. 2B and 3B. Therefore, by manipulating the period of the drive waveform equal droplet volume and speed is provided. This is performed by manipulating the drive waveform frequency because frequency is the reciprocal of the period. 
     A controller for realizing the aforementioned driving waveform  10  according to a preferred embodiment will be described with reference to FIGS. 4 and 5. A controller  625 , shown in FIG. 4, comprises a charging circuit  182 , a discharging circuit  184  and a pulse control circuit  186 . A piezoelectric material of an actuator wall  603  and electrodes  619 ,  621  are equivalently expressed by capacitor  191 . Reference numerals  191 A and  191 B denote terminals of the capacitor. 
     Input pulse signals are input into terminals  181  and  183 . These input pulse signals are used to set voltages supplied to the electrode  619  within the ink chamber  613  to E (v) and 0 (v), respectively. The charging circuit  182  comprises resistors R 101 , R 102 , R 103 , R 104 , R 105  and transistors TR 101 , TR 102 . 
     When an ON signal (+5 v) is input to the input terminal  181 , the transistor TR 101  is controlled through the resistor R 101  so that a current flows from a positive power supply  187  through the resistor R 103  to the transistor TR 101  along the collector to the emitter direction. Therefore, divided voltages of the voltage applied to the resistors R 104  and R 105  connected to the positive power supply  187  are raised and a current that flows in the base of the transistor TR 102  increases, thereby controlling the emitter-collector path of the transistor TR 102 . A voltage 20(v) from the positive power source  187  is applied through the collector and the emitter of the transistor TR 102  and the resistor R 120  to the capacitor  191  at the terminal  191 A. 
     The discharging circuit  184  will be described next. The discharging circuit  184  comprises resistors R 106 , R 107  and a transistor TR 103 . When an ON signal (+5 v) is input to the input terminal  183 , the transistor TR 103  is controlled through the resistor R 106 , thereby resulting in the terminal  191 A on the side of the resistor R 120  of the capacitor  191  being connected to the ground through the resistor R 120 . Therefore, electric charges applied to the actuator wall  603  of the ink chamber  613 , shown in FIGS. 8 and 9, are discharged. 
     The pulse control circuit  186  generates pulse signals that are input to the input terminal  181  of the charging circuit  182  and the input terminal  183  of the discharging circuit  184 . The pulse control circuit  186  is provided with a CPU  110  for performing a variety of computations. To the CPU  110 , there are connected a RAM  112  for memorizing printing data and a variety of data and a ROM  114  for memorizing sequence data in which on/off signals are generated in accordance with a control program and a timing of the pulse control circuit  186 . The ROM  114  includes, as shown in FIG. 5, an ink droplet jet control program area  114 A and a driving waveform data storage area  114 B. The sequence data of the driving waveform  10  is stored in the driving waveform data storage area  114 B. 
     Further, the CPU  110  is connected to an I/O bus  116  for exchanging a variety of data, and a printing data receiving circuit  118  and pulse generators  120  and  122  are connected to the I/O bus  116 . An output from the pulse generator  120  is connected to the input terminal  181  of the charging circuit  182 , and an output from the pulse generator  122  is connected to the input terminal  183  of the discharging circuit  184 . 
     The CPU  110  controls the pulse generators  120  and  122  in accordance with the sequence data memorized in the driving waveform data storage area  114 B. Therefore, by memorizing various kinds of patterns of the above-mentioned timing in the driving waveform data storage area  114 B within the ROM  114  in advance, it is possible to supply the drive pulse of the driving waveform  10  shown in FIG. 1 to the actuator wall  603 . 
     The quantity of each of the pulse generators  120 ,  122 , charging circuit  182  and discharging circuit  184  are equal to the number of nozzles in an apparatus. Therefore, while this embodiment typically describes the manner in which one nozzle is controlled, other nozzles are controlled similarly as described above. 
     FIGS. 6A,  6 B and  6 C illustrate variations of droplets of ink jetted from the nozzle depending upon the printing frequency. FIG. 6A illustrates how the sizes of droplets of ink jetted from the nozzle when droplets of ink of continuous dots (here, one(1) to five(5) dots) are jetted at a period (integer +0.5) times the period T. FIG. 6B illustrates how the droplets of ink are jetted from the nozzle when the period is an even-number multiple of the time T. FIG. 6C illustrates how droplets of ink are jetted from the nozzle when the period is an odd-number multiple of the time T. In FIG. 6A, the speeds and volumes of the ink droplet  14  of the continuous dots are not changed at all based on the dot being formed. In FIG. 6B, as a result of increasing the period to an even multiple of T, the speed and the volume of the second ink droplet  16  are increased relative to the first ink droplet  15 , as indicated by a change in droplet size and the larger number of drops produced for the fifth dot(5) in relation to the first dot(1). In FIG. 6C, as a result of increasing the period to an odd multiple of T, the speed and the volume of the second ink droplet  18  are decreased relative to the first ink droplet  17  of the continuous dots. 
     FIG. 7 is a diagram used to explain the manner in which the pressure within the ink chamber  613 , referred to as a pressure chamber, changes when a jetted pulse is applied to the ink droplet jetting apparatus  600 . Reference numerals 1T to 10T denote time transitions. At the leading edge time 0 of the jetted pulse, the capacity of the pressure chamber increases to generate a negative-pressure pressure wave. At a trailing edge timing point of the jetted pulse obtained after the time 1T, the capacity of the pressure chamber is decreased to the natural state resulting in a positive-pressure pressure wave. The positive pressure induced by the positive-pressure pressure wave becomes negative pressure induced by the negative-pressure pressure wave during a time period of 2T. The phase of the pressure will hereinafter be inverted at every time T and attenuated. 
     Since the pressure changes as a result of the jet pulse, as described above, if the ink droplet jet apparatus is continuously driven at a period that is an even multiple of the period T, then the speeds and volumes of the droplets for the second and third dots increase. If the ink droplet jet apparatus is continuously driven at a period that is an odd multiple of the period T, then the speeds and volumes of the droplets second and third dots decrease. Therefore, if the ink droplet jet apparatus is driven at an approximately intermediate period between the even and odd multiples of the period T, it is possible to suppress the speed and volume of the ink droplet from being fluctuated. 
     While the embodiment has been described so far, the present invention is not limited thereto. For example, while there is illustrated only the driving signal having one jet pulse signal A as the main driving signal as described above, the present invention is not limited thereto, and a main driving signal may comprise two jet pulses, for example. 
     Also, the ink droplet jet apparatus  600  is not limited to the arrangement of the above-mentioned embodiment, and it is possible to use such an ink droplet jet apparatus in which a polarization direction of a piezoelectric material is reversed. 
     While the air chambers  615  are provided on both sides of the ink chamber  613 , as described above, air chambers need not be provided, and ink chambers may be located adjoining to each other. Further, while the actuator may be of a shearing mode type, the present invention is not limited thereto, and an actuator may be of such a type that piezoelectric materials are laminated and a pressure wave is generated by a deformation of its laminated direction. Also, the material is not limited to the piezoelectric material; rather, any material and structure that generate a pressure wave in an ink chamber may be used.