Patent Publication Number: US-6705696-B1

Title: Method of driving inkjet recording head and inkjet recording apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to a method of driving an inkjet recording head and an inkjet recording apparatus, and specifically, to a driving technique for driving an inkjet recording head for the recording of characters and images by the ejection of minute ink droplets from an ink nozzle in an inkjet recording apparatus. 
     BACKGROUND ART 
     As a conventional inkjet recording system, a drop-on-demand type inkjet system is known whereby an electro-mechanical transducer such as a piezoelectric actuator is used to cause a pressure wave (acoustic wave) to be generated in a pressure chamber filled with a liquid ink, so that the pressure wave ejects an ink droplet from a nozzle coupled with the pressure chamber. Such inkjet recording method using the drop-on-demand type inkjet system is disclosed in Japanese Patent Publication No. Sho. 53-12138, for example. An example of the structure of the inkjet recording head of this type is shown in FIG.  22 . 
     Referring to FIG. 22, a pressure chamber  61  is connected with a nozzle  62  for the ejection of ink and an ink supply path  64  for guiding ink from an ink reservoir (not shown) through a common ink chamber  63 . A vibrating plate  65  is mounted on the bottom surface of the pressure chamber. 
     When an ink droplet is to be ejected, a piezoelectric actuator  66  mounted outside the pressure chamber  61  operates to displace the vibrating plate  65 , whereby the volume within the pressure chamber  61  is changed and thus a pressure wave is generated therein. This pressure wave causes a part of the ink filled in the pressure chamber  61  to be ejected through the nozzle  62  as a flying ink droplet  67 . The flying ink droplet lands on a recording medium such as a recording paper and forms a recorded dot thereon. Such formation of recorded dots are repeated on the basis of image data, thereby recording characters or images on the recording paper. 
     In order to achieve a high image quality in this type of inkjet recording head, it is necessary to minimize the diameter of the ejected ink droplet (droplet diameter). Specifically, in order to obtain a smooth image with little graininess, the recording dot (pixel) formed on the recording paper must be made as small as possible. For this reason, the diameter of the ink droplet ejected must be minimized in size. Generally, the graininess of the image decreases greatly as the dot diameter becomes 40 μm or less. As the dot diameter becomes 30 μm or less, it becomes so difficult to visually recognize the individual dots even in the highlight portion of the image that the image quality improves greatly. 
     The relationship between the ink droplet diameter and the dot diameter depends on the rate of flight of the ink droplet (droplet velocity), physical properties of the ink (viscosity, surface tension), the type of the recording paper, and so on. Normally, the dot diameter is about twice the size of the ink droplet diameter. Accordingly, in order to obtain a dot diameter of 30 μm or less, the ink droplet diameter must be set at 15 μm or less. In the present description, the diameter of the ink droplet (droplet diameter) refers to the diameter of a spherical droplet substituting the total amount of ink (including the satellites) ejected in a single act of ejection. 
     The most effective way of minimizing the ink droplet diameter is to reduce the nozzle diameter. Practically, however, the nozzle diameter cannot be reduced to less than about 25 μm, given technical difficulties in the manufacture and the fact that as the nozzle diameter is reduced, the nozzle tends to be clogged. Accordingly, it is impossible to obtain an ink diameter on the order of 15 μm solely by decreasing the nozzle diameter. To solve this problem, it is known to reduce the droplet diameter of the ejected ink droplet by way of the driving method employed, and some effective methods are proposed. 
     As one such example, Japanese Patent Laid-open Publication No. Sho. 55-17589 discloses a meniscus control technique whereby the pressure chamber is once expanded immediately before ejection, and then an ink droplet is ejected when the ink meniscus at the nozzle opening is drawn towards the pressure chamber. FIG. 23 shows an example of the driving waveform for driving the piezoelectric actuator using this technique. In the present description, the relationship between the driving voltage and the piezoelectric actuator operation is such that as the driving voltage increases, the volume of the pressure chamber decreases and, conversely, as the driving voltage decreases, the volume of the pressure chamber increases. Generally, the polarities are often reversed depending on the structure of the piezoelectric actuator and the direction of polarization of the piezoelectric element. 
     Referring to the driving waveform shown in FIG. 23, a voltage fall  71  from V 1  to zero volt expands the volume of the pressure chamber. A subsequent voltage rise  71  from zero volt to V 2  compresses the volume of the pressure chamber to thereby eject an ink droplet. The interval of each of the fall time t 1  and rise time t 2  is generally on the order of 2-10 μs, which is longer than an inherent period Ta of the conventional piezoelectric actuator. 
     FIGS.  25 ( a ) to ( d ) illustrate the movement of the ink meniscus at the nozzle opening portion upon application of the driving waveform of FIG.  23 . The ink meniscus has a flat upper portion during the initial state (FIG.  25 ( a )). As the pressure chamber is expanded immediately before the ejection, the top portion of the ink meniscus assumes a concave shape, as shown in FIG.  25 ( b ). As the pressure chamber is compressed by voltage rise  71  when there is such a concave ink meniscus, a thin liquid column  83  is formed in the center of the ink meniscus as shown in FIG.  25 ( c ). This is followed by the formation of an ink droplet  84  as the tip of the liquid column is separated (FIG.  25 ( d )). The ink droplet diameter is substantially equal to the thickness of the liquid column thus formed and is smaller than the nozzle diameter. Thus it is possible to eject an ink droplet with a smaller diameter than the nozzle diameter by using such driving method. 
     As described above, the meniscus control system enables the ejection of an ink droplet with a smaller diameter than the nozzle diameter. However, when such driving waveform as shown in FIG. 23 is used, the smallest diameter of the droplet that could actually be obtained was about 25 μm, which is still not good enough to satisfy the need for higher image quality. 
     FIG. 24 shows another driving waveform as a driving means for enabling the ejection of a smaller droplet. In this waveform shown in FIG. 24, a voltage fall  73  draws the ink meniscus immediately prior to the ejection. A subsequent voltage rise  74  compresses the volume of the pressure chamber and thereby causes a liquid column to be formed. A voltage fall  75  separates a droplet from the tip of the liquid column at an early period. A voltage rise  76  suppresses the reverberations of the pressure wave remaining after the ejection of the ink droplet. In other words, the driving waveform of FIG. 24 is based on the conventional meniscus control system in which a pressure wave control is incorporated for the early separation of the ink droplet and for the suppression of the reverberations. This arrangement allows an ink droplet with a droplet diameter on the order of 20 μm to be ejected in a stable manner. 
     However, it was still difficult to eject an ink droplet with an ink diameter of 20 μm or less easily even by using this improved driving waveform, and particularly an ink diameter of 15 μm or less was impossible. Thus, there was no driving method that could achieve the ink diameter of 15 μm or less, which was required for image quality reasons. One of the biggest reasons for this was that in the conventional inkjet recording head, the ink droplet ejection was carried out by the pressure wave that was governed by the acoustic capacity of the pressure chamber. This reason will be explained in detail below. 
     FIG. 26 shows the result of observation of velocity changes in the ink meniscus (particle velocity change) by a laser Doppler meter, the changes being caused when a driving waveform of FIG. 24 is applied to the piezoelectric actuator. As shown in the figure, the ink meniscus vibrates due to the pressure wave generated in the pressure chamber. In the example of FIG. 26, the inherent period Tc of the pressure wave is 13 μs, and pressure waves generated at the respective nodes of the driving waveform are superposed, resulting in a complex velocity change in the ink meniscus. 
     The volume of the ejected ink droplet can be thought of as substantially proportional to the product of a shaded area defined by the initial positive half-cycle of the pressure wave of FIG.  26  and the area of the nozzle opening. Namely, an estimate of the droplet diameter (drop volume) on the assumption that the ink is ejected from the nozzle with a positive rate (velocity in the direction out of the nozzle) and flies as an ink droplet corresponds well with an actually measured droplet diameter (drop volume). Although when the meniscus control system is used, a liquid column which is thinner than the nozzle diameter is formed and therefore the effective nozzle opening area decreases, the relationship where the ink droplet volume is substantially proportional to the shaded area of FIG. 26 is still valid. Accordingly, inorder to reduce the droplet diameter (drop volume), it is important to reduce the area of the above-mentioned shaded portion. 
     There are roughly two ways for the reduction of the shaded portion area. One sets the amplitude of the particle velocity small, as shown in FIG.  27 . The other sets the period of the particle velocity vibration short, as shown in FIG.  28 . The former method, by which the amplitude of the particle velocity is set small, is difficult to implement in actual applications. This is because the drop velocity is substantially proportional to the average particle velocity of the shaded portion, and so if the amplitude of the particle velocity is set small, the flying velocity (drop velocity) of the ink droplet drops significantly, which poses a problem in image recording. 
     Accordingly, in order to perform a minute-drop ejection, the inherent period of the pressure wave must be set very small as shown in FIG.  28 . Specifically, in order to eject an ink droplet with a droplet diameter of 15 μm at a drop velocity of 6 m/s, the inherent period of the pressure wave must be set on the order of 3 to 5 μs. 
     However, it was very difficult to set the inherent period of the pressure wave at such small values in the conventional inkjet recording head. This was because of the fact that in order to obtain the inherent period on the order of 3 to 5 μs, the volume of the pressure chamber must be set very small and at the same time the rigidity of the walls forming the pressure chamber must be very high, as will be described later. Those measures, however, are difficult to realize in the conventional head manufacturing method where the pressure chamber is constructed by stacking and bonding perforated board materials. 
     Even if the above-mentioned conditions are met, the reduction in the limit ejection frequency of the ink droplet cannot be avoided. Specifically, while it is necessary to set the volume of the pressure chamber small in order to shorten the inherent period of the pressure wave, a certain area must be secured for the actuator unit for the application of displacements by the piezoelectric actuator, which necessarily results in the pressure chamber having a flat shape. As a result, the flow-path resistance of the pressure chamber significantly increases, which in turn lengthens the refill time (the time for the returning of the ink meniscus after ejection), thereby making it difficult to repeat the ejection at a high frequency. 
     As explained above, the conventional inkjet recording head had the disadvantage that it is unable to eject an ink droplet with such a droplet diameter as required for the significant improvement of the image quality, namely a minute ink droplet with a droplet diameter on the order of 15 μm. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method of driving an inkjet recording head which is capable of ejecting an ink droplet with a droplet diameter of 15 μm or less without adversely affecting the ejection property in the high-frequency region and without requiring a specialized head manufacturing technology, and to provide an inkjet recording apparatus using such driving method. 
     Another object of the present invention is to enable both high-quality and high-speed recording by ensuring a wide range of droplet diameter modulation when performing a grayscale recording by modulating the droplet diameter of the ejected ink droplet in multiple levels. 
     In order to achieve those objects, the present invention is directed to a method of driving an inkjet recording head having a pressure chamber filled with a liquid ink, said pressure chamber including an ink supply port for supplying the liquid ink and an ink nozzle for ejecting said ink in the form of at least one ink droplet, and an electro-mechanical transducer disposed such that a pressure wave is generated in said pressure chamber by applying a driving voltage in order to eject the ink droplet via said ink nozzle, said transducer having an inherent vibration period Ta, said method characterized in that: 
     said driving voltage has a first driving voltage waveform, said first driving voltage waveform including consecutively a first waveform portion having a first time length t 1  for contracting a volume of said pressure chamber and a second waveform portion having a second time length t 2  for expanding the volume of said pressure chamber, said first and second time lengths t 1  and t 2  being set equal to or longer than the inherent vibration period Ta of said electro-mechanical transducer. 
     An inkjet recording apparatus according to the present invention includes: an inkjet recording head including a pressure chamber having an ink supply port for supplying a liquid ink and an ink nozzle for ejecting the ink as at least one ink droplet, the pressure chamber being filled with liquid ink, and an electro-mechanical transducer disposed such that the ink droplet is ejected from the ink nozzle by the generation of a pressure wave in the pressure chamber by application of a driving voltage, the transducer having an inherent vibrating period Ta; and 
     a driving waveform generating circuit for generating one or more driving waveforms for the driving voltage to be applied to the electro-mechanical transducer, wherein: 
     the driving waveform includes a first waveform portion having a first time length for the compression of the volume of the pressure chamber and a second waveform portion having a second time length for the expansion of the volume of the pressure chamber, the first and second time lengths being set equal to or longer than the inherent vibrating period Ta of the electro-mechanical transducer. 
     In accordance with the method of driving the inkjet recording head and the inkjet recording apparatus according to the invention, the electro-mechanical transducer element is actuated by a driving waveform having a rise time and a fall time which are shorter than the inherent vibrating period of the electro-mechanical transducer element, whereby a minute ink droplet having a diameter of 15 μm or less can be ejected from the ink nozzle and therefore the printing precision can be improved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an equivalent circuit diagram of the inkjet recording head in accordance with the principle of the present invention. 
     FIG. 2 shows a part of the equivalent circuit of FIG.  1 . 
     FIG. 3 shows another part of the equivalent circuit of FIG.  1 . 
     FIG. 4 shows another part of the equivalent circuit of FIG.  1 . 
     FIG. 5 is a chart showing the driving waveform of the driving voltage having a voltage rise portion for use in the invention. 
     FIG. 6 is a chart showing the driving waveform of the driving voltage having a voltage rise portion and a voltage fall portion for use in the invention. 
     FIG. 7 is another chart showing the driving waveform of the driving voltage having a voltage rise portion and a voltage fall portion for use in the invention. 
     FIG. 8 is a graph showing the drop velocity at the nozzle portion against time in the case of the driving waveform of FIG. 5 having a larger rise time than the inherent period of the pressure chamber. 
     FIG. 9 is a graph showing the drop velocity at the nozzle portion against time in the case of the driving waveform of FIG. 5 having a smaller rise time than the inherent period of the pressure chamber. 
     FIG. 10 is a graph showing the drop velocity at the nozzle portion against time in the case of the driving waveform of FIG.  6 . 
     FIG. 11 is a graph showing the drop velocity of the nozzle portion against time in the case of the driving waveform of FIG.  7 . 
     FIG. 12 is a block diagram of a configuration of the driving circuit for the piezoelectric actuator. 
     FIG. 13 is a block diagram of another configuration of the driving circuit for the piezoelectric actuator. 
     FIG. 14 is an exemplary chart of the driving waveform to be used in the method of driving the inkjet recording head according to an embodiment of the invention. 
     FIG. 15 is a graph showing the drop velocity at the nozzle portion actuated by the driving waveform of FIG.  14 . 
     FIG. 16 is a graph showing the relationship between the pressure rise time and the droplet diameter. 
     FIG. 17 is another exemplary chart of the driving waveform to be used in the driving method for the inkjet recording head according to an embodiment of the invention. 
     FIG. 18 is a graph showing the drop velocity along with time in the case of the driving voltage of FIG.  17 . 
     FIG. 19 is a chart showing a driving waveform to be used in the method of driving the inkjet recording head in another embodiment of the invention for producing a small ink droplet in accordance with another embodiment of the invention. 
     FIG. 20 is a chart showing another driving waveform for use in the method of driving the inkjet recording head in accordance with another embodiment of the invention for producing a middle-size ink droplet in accordance with another embodiment of the invention. 
     FIG. 21 is a chart showing a driving waveform for use in the method of driving the inkjet recording head in accordance with another embodiment of the invention for producing a large ink droplet in accordance with another embodiment of the invention. 
     FIG. 22 is a depiction of a Kaiser-type inkjet recording head. 
     FIG. 23 is another chart showing a driving waveform. 
     FIG. 24 is another chart showing a driving waveform for enabling the ejection of a smaller droplet. 
     FIGS.  25 ( a ) to ( d ) illustrate the movement of the ink meniscus at the nozzle opening portion upon the application of the driving waveform of FIG.  23 . 
     FIG. 26 shows the result of observation of velocity changes in the ink meniscus. 
     FIG. 27 is another graph showing the drop velocity at the nozzle portion against time. 
     FIG. 28 is another a graph showing the drop velocity at the nozzle portion against time. 
    
    
     PREFERRED EMBODIMENTS OF THE INVENTION 
     Before describing the preferred embodiments of the present invention, the principle of the present invention will be described based on the result of theoretical analysis of the inkjet recording head by referring to a lumped-constant circuit model. 
     FIG. 1 shows a circuit diagram of the general ink jet recording head as substituted by an equivalent electric circuit. In the figure, reference m designates an inertance [kg/m 4 ], reference r designates an acoustic resistance [Ns/m 5 ], reference c designates an acoustic capacitance [m 5 /N], and reference u designates a volume rate [m 3 /s], and reference φ designates a pressure [Pa]. Indexes [ 0 ], [ 1 ], [ 2 ], and [ 3 ] designate the actuator unit, pressure chamber, ink supply path, and nozzle, respectively. 
     In the conventional inkjet recording head, when a piezoelectric actuator that operates in a longitudinal vibration mode is used, the circuit of FIG. 1 can be thought of as consisting of the three circuits shown in FIGS. 2-4. FIG. 2 shows a circuit relating to the actuator unit having a piezoelectric actuator and a vibrating plate. Its inherent period Ta can be expressed by the equation: 
     
       
           Ta =2 π{square root over (m 0 c 0 )}.   
       
     
     The inherent period Ta of the circuit of FIG. 2 can be considered as an inherent period of a bar with a fixed-end and a free-end longitudinally vibrating, and can be approximately determined by the equation:          Ta   =     4      L            ρ   p       E   p             ,                   
     where L is the length of the piezoelectric actuator, ρ p  and Ep are density and coefficient of elasticity of the piezoelectric actuator material, respectively. Ta is on the order of 1-5 μs in the conventional inkjet recording head. 
     The partial circuit of FIG. 3 includes a pressure chamber referenced by an acoustic capacitance c 1  of the pressure chamber. The pressure wave generated by the inherent vibrating mode within the pressure chamber is defined by the circuit of FIG.  3 . Namely, in the conventional inkjet recording head, the ejection of the ink droplet is carried out by the pressure wave defined by this circuit. An inherent period Tc of the circuit of FIG. 3 is expressed by the equation:        Tc   =     2                 π                  m   2                     m   3           m   2     +     m   3         ·   c1       .                       
     Tc is on the order of 10-20 μs in the conventional inkjet recording head. 
     Acoustic capacitance c 1  of the pressure chamber is expressed by the equation:        c1   =       W   1       k   ·     K   1                         
     wherein W 1 [m 3 ] is the volume of the pressure chamber, κ[Pa] is the volume coefficient of elasticity of the ink, and K 1  is a constant dependent on the rigidity of the pressure chamber wall. 
     Thus, in order to decrease inherent period Tc, it is desirable to set volume W 1  of the pressure chamber smaller and set the rigidity of the pressure chamber wall higher (set K 1  larger). 
     The circuit of FIG. 4 is a circuit which is governed by acoustic capacitance c 3  by the surface tension of the ink meniscus and is related to the refill property. An inherent period Tm of the circuit of FIG. 4 is expressed by the following equation: 
     
       
           Tm= 2π{square root over (( m   2   +m   3 )· c   3 )}. 
       
     
     Tm is on the order of 20-50 μs in the conventional inkjet recording head. 
     In the circuits of FIGS. 2-4, the present invention utilizes the properties of the circuits of FIGS. 2 and 3. In particular, while the conventional inkjet recording head utilized the properties of the circuit of FIG. 3 for the ejection of the ink droplet, the present invention uses the inherent vibration of the actuator unit (piezoelectric actuator) per se for the ejection of the ink droplet. 
     FIG. 5 shows an example of a pressure (pressure wave) φ within the pressure chamber shown in FIG. 22 in proportion to the driving voltage. FIGS. 8 and 9 each show a drop velocity v 3  (particle velocity) at the opening of the nozzle related to the pressure wave of FIG.  5 . Particle velocity v 3  is equal to the quotient when a volume velocity u 3  is divided by the area of opening of the nozzle. 
     FIG. 8 shows the particle velocity in the inkjet recording head when rise time t 1  of pressure φ is set larger than inherent period Ta of the circuit, used in the method of driving the conventional inkjet recording head. Particle velocity v 3  vibrates at an inherent period Tc. Thus, particle velocity v 3  is defined only by the circuit of FIG. 3 in the conventional inkjet recording head. 
     FIG. 9 shows particle velocity v 3  of the inkjet recording head when rise time t 1  of pressure φ is set equal to or smaller than inherent period Ta of the actuator unit in accordance with the principle of the present invention. In this case, the inherent vibration of the actuator unit of FIG. 2 is excited, and, as a result, the vibration of particle velocity v 3  corresponds to the vibration of inherent period Tc superposed with the vibration of inherent period Ta. In other words, by setting the rise time of pressure φ equal to or smaller than inherent period Ta, the ink meniscus can be vibrated at the inherent period of the piezoelectric actuator per se. 
     Referring to FIG. 6, there is shown the case where the pressure wave generated in the pressure chamber is trapezoidal in shape. In the figure, rise time t 1  and fall time t 2  are both set equal to or smaller than inherent period Ta of the circuit, and the time difference (t 3 ) between the start time of voltage rise and the start time of voltage fall is set such that Ta/2≦t 3 ≦Ta. As the pressure wave of FIG. 6 is generated in the pressure chamber, particle velocity v 3  of the ink meniscus varies as shown in FIG.  10 . In this case, the piezoelectric actuator is sharply elongated by a voltage rise portion  141 A of FIG. 6, and a voltage fall  142 A for contracting the piezoelectric actuator is applied in synchronism with the timing of contraction of the elongated piezoelectric actuator by its own inherent vibration. As a result, the piezoelectric actuator sharply contracts and particle velocity v 3  returns to the position of FIG. 10 where v 3 −0 at a very quick timing. Thus, the area of the shaded portion corresponding to the initial positive half cycle of the particle vibration becomes smaller than the shaded portion of FIG. 9, so that there can be obtained a favorable condition for the ejection of a small drop. 
     When pressure φ is made trapezoidal in shape as shown in FIG. 6, the initial positive half cycle of the particle velocity of FIG. 10 will contain a plurality of ridges as shown in FIG.  10 . In this case, the area of the shaded portion may increase, i.e., the diameter of the ink droplet may increase, resulting in the creation of a satellite ink droplet and at the same time an unstable ejection may result. 
     To prevent such a scenario, it is preferable to have such a variation of pressure φ as shown in FIG.  7 . The pressure wave of FIG. 7 has its shaded portion formed by a single maximum point as shown in FIG. 11 by setting the amount of pressure drop  142 B greater than the amount of pressure rise  141 B. The single maximum point permits a reduction of the area of the shaded portion, thereby allowing for a stable ink ejection. 
     As explained above, the inherent period of the ink meniscus vibration can be greatly reduced by setting the rise/fall time of the driving waveform equal to or smaller than the inherent period Ta of the piezoelectric actuator and at the same time by setting time difference t 3  between the rise start and drop start time such that Ta/2≦t 3 ≦Ta. By so doing, the area of the shaded portion can be reduced as shown in FIGS. 10 and 11, whereby it becomes possible to eject a smaller drop than in accordance with the conventional driving methods. Further, by setting the voltage change amount of the drop portion larger than the voltage change amount of the rise portion, an even smaller ink droplet can be ejected. 
     In the following, the present invention will be described by way of preferred embodiments. The principle of the invention was applied to a sample of the inkjet recording head having the basic structure of FIG.  22 . 
     The sample of the inkjet recording head was produced by stacking and bonding a plurality of thin plates perforated by etching and the like. In the present embodiment, stainless plates with a thickness of 50-75 μm were bonded by means of an adhesive layer (about 20 μm in thickness) including a thermosetting resin. Its head has a plurality of pressure chambers  61  arranged in a direction perpendicular to the sheet of FIG.  22 . The pressure chambers  61  are connected by a common ink chamber  63 . The common ink chamber  63  is connected to an ink reservoir (not shown) and operates to guide ink to the respective pressure chambers  61 . 
     Each of the pressure chambers  61  is communicated to the common ink chamber  63  via an ink supply path  64 , and the pressure chamber  61  is filled with ink. Each of the pressure chambers  61  is also provided with a nozzle  62  for the ejection of ink. 
     In the present embodiment, the nozzle  62  and the ink supply path  64  have an identical shape, with an opening diameter of 30 μm, a hem diameter of 65 μm and a length of 75 μm, thus forming a tapered shape. The perforation was given by a press. 
     The bottom surface of the pressure chamber  61  has a vibrating plate  65 , and the volume of the pressure chamber can be increased or decreased by a piezoelectric actuator (piezoelectric vibrator)  66  as the electro-mechanical transducer mounted externally to the pressure chamber  61 . In the present embodiment, a nickel thin plate formed by electroforming is used for the vibrating plate  65 . 
     The piezoelectric actuator  66  was a stacked piezoelectric ceramics. The shape of the driving column for the application of displacements to the pressure chamber  61  is 1.1 mm in length (L), 1.8 mm in width (W) and 120 μm in depth (along the direction perpendicular to the sheet of FIG.  22 ). The piezoelectric material used had a density ρ p of 8.0×10 3  kg/m3, and a coefficient of elasticity Ep of 68 GPa. The measured inherent period Ta of the piezoelectric actuator per se was 1.6 μs. 
     As the volume of the pressure chamber  61  is varied by the piezoelectric actuator  66 , a pressure wave is generated in the pressure chamber  61 . The pressure wave moves the ink of the nozzle portion  62 , whereby an ink droplet  67  is formed. In the present invention, inherent period Tc of the head is 14 μs. 
     Next, the basic configuration of the driving circuit for driving the piezoelectric actuator will be described by referring to FIGS. 12 and 13. 
     FIG. 12 shows an example of the configuration of the driving circuit in the case where the diameter of the ejected ink droplet is fixed, i.e., there is no ink diameter modulation. The driving circuit shown in FIG. 12 includes a waveform generating circuit  121 , an amplifier circuit  122  and a switching circuit (transfer gate circuit)  123  for driving a piezoelectric actuator  124 . An driving waveform signal is generated and power-amplified, and then supplied to the piezoelectric actuator for driving the same, such that characters and images are printed on a sheet of recording paper. The waveform generating circuit  121  is composed of a digital-analog converter circuit and an integrating circuit. It analog-converts the driving waveform data and then integrates the data in order to generate a driving waveform signal. The amplifier circuit  122  voltage- and current-amplifies the driving waveform signal supplied from the waveform generating circuit  121  and outputs the signal as an amplified driving waveform signal. The switching circuit  123  controls the on-off of the ink droplet ejection by applying the driving waveform signal to the piezoelectric actuator  124  on the basis of a signal generated from the image data. 
     FIG. 13 shows an example of the configuration of the driving circuit in the case where the diameter of the ejected ink droplet is switched in multiple levels, i.e. an ink diameter modulation is carried out. The driving circuit of FIG. 13 includes three kinds of waveform generating circuits  131 ,  131 A and  131 B for modulating the droplet diameter in three levels (large, middle and small), respectively, and the individual waveforms are amplified by amplifier circuits  132 ,  132 A and  132 B, respectively. During recording, the driving waveform to be applied to the piezoelectric actuator  134  is switched by the switching circuit  133  based on the image data, such that an ink droplet of a desired diameter can be ejected. 
     It should be noted that the configuration of the driving circuit for driving the piezoelectric actuator is not limited to that of FIG. 12 or  13 , and other configurations may be used. 
     FIG. 14 shows an example of the driving waveform generated by the driving circuit of FIG. 19 for the formation of an ink droplet with a diameter of about 20 μm by using the inkjet recording apparatus according to the embodiment of the invention. The driving waveform has a rise time t 1  (0.5 μs) which is shorter than the inherent period Ta (1.6 μs), and a first rise portion  11  increasing from an initial voltage Vb (6 volts) to V 2  (20 volts) for contracting the pressure chamber. The waveform further includes a first drop portion  12  which starts a t 3  time after the start time of the first rise portion, has a fall time t 2  (0.5 μs) which is shorter than inherent period Ta, and drops from V 2  to zero volt. The drop portion  12  expands the pressure chamber. Furthermore, the waveform has a second rise portion  13  which starts a t 4  (14 μs) after the end of the drop portion  12  and has a rise time t 5  (30 μs) for returning from zero volt to initial voltage Vb. By this arrangement, t 3  satisfies Ta≦t 3 ≦3Ta. 
     FIG. 15 shows the result of observation of the movement of the ink meniscus by a laser Doppler meter when the driving waveform of FIG. 14 was applied. During the observation, in order to measure the movement of the ink meniscus accurately, the application voltage was set low at 1/15, and the results of FIG. 15 indicate values obtained by multiplying the measured particle velocity by a factor of 15, in light of the fact that particle velocity v 3  is proportional to the applied voltage. 
     In FIG. 15, the ink meniscus vibrates where the vibration of inherent period Ta and the vibration of inherent period Tc are superposed. Since the piezoelectric actuator is contracted at the timing of t 3 =1 μs, the initial maximum point returns to the point where v 3 =0 at a very early point of t=2 μs. Thus, the area of the shaded portion of the initial half cycle is very small, which is advantageous for the ejection of a minute drop. 
     When an ejection experiment was actually conducted by using the sample head with the driving waveform of FIG. 14, it was observed that an ink droplet with a diameter of 21 μm was ejected at a drop velocity of 5.5 m/s. When the experiment was conducted by using a driving waveform where t 1 =t 2 =t 3 =2 μs&gt;Ta, as in the conventional driving method, the minimum diameter of the minute drop that could be ejected was 28 μm. 
     FIG. 16 shows the results of observing changes in the droplet diameter as rise time t 1  was varied, where fall time t 2  was set such that t 2 =t 1 , and time t 3  was set such that t 3 =1 μs when t 1 ≦1 μs and t 3 ˜t 1  when t 1 &gt;1 μs. Applied voltages V 1  and V 2  were adjusted with respect to respective t 1  such that the drop velocity was 6 m/s. 
     In FIG. 16, there can be seen a sharp change in droplet diameter at around t 1 =Ta where there is obviously a change in the ejection mechanism. In other words, while in the region where t 1 &gt;Ta the ejection occurs due to the ink meniscus vibration with inherent period Tc, in the region where t 1 ≦Ta, the ejection occurs due to the ink meniscus vibration with inherent period Ta. As will be seen from FIG. 16, the droplet diameter can be greatly reduced by using the driving method according to the present invention as compared with the conventional one. 
     FIG. 17 shows an example of the driving waveform used for the ejection of a minute drop with a droplet diameter 15 μm or less in the above-mentioned inkjet recording head. The driving waveform of FIG. 17 includes a voltage fall  33  for meniscus control prior to a voltage rise  31 . Thus, the driving waveform of FIG. 17 uses a driving method combining the ejection mechanism based on the inherent vibration of the piezoelectric actuator per se with the meniscus control system. Accordingly, it is possible to eject an ink droplet with an even smaller droplet diameter than in the case of using the driving waveform of FIG.  14 . 
     The driving waveform of FIG. 17 includes a first drop portion  33  having a fall time (t 6 =3 μs) which is larger than inherent period Ta and smaller than inherent period Tc for dropping from an initial voltage Vb (40 volts) to V 3  (18 volts). The first drop portion  33  occurs a t 7  time (4 μs) earlier than a first voltage rise  31  which raises the voltage by V 1 . Such driving waveform makes it possible to combine the driving technique based on the inherent vibration of the piezoelectric actuator per se with the meniscus control technique. 
     Specifically, the first drop portion  33  has a fall time t 6  (3 μs) which is larger than inherent period Ta and smaller than inherent period Tc and expands the pressure chamber. The first rise portion  31  has a voltage rise V 1  for contracting the pressure chamber and has a shorter rise time t 1  (0.5 μs) than inherent period Ta. The second drop portion  32  starts a t 3  time (1 μs) after the start of the first rise portion  31 , has a fall time t 2  (0.5 μs) and expands the pressure chamber with a voltage change amount of V 2  (36 volts) to bring the voltage to zero. The second rise portion  34  restores the voltage from zero back to initial voltage Vb and has a rise time (30 μs). 
     FIG. 18 shows the results of observation of the movement of the ink meniscus by a laser Doppler meter when the driving waveform of FIG. 17 was applied. During the observation, the applied voltage was set low at 1/15, and the results of FIG. 18 indicate values obtained by multiplying the actually measured particle velocity by a factor of 15. 
     As shown in FIG. 18, when the driving waveform of FIG. 17 is applied to the piezoelectric actuator, initially a negative particle velocity is generated by the voltage fall  33 , by which the ink meniscus is drawn into the pressure chamber, resulting in an concave ink meniscus. When a first voltage change process  31  is applied, v 3 &gt;0 and the ink meniscus is displaced towards the outside of the nozzle. Since the previous ink meniscus was concave, a thin liquid column is formed at the center of the nozzle. According to the result of observation of the state of drop ejection (strobe observation), the thus formed liquid column had a thickness of about 15 μm (about one half the nozzle diameter). 
     After the formation of the liquid column, the second rise portion  32  is applied at a timing of Ta/2≦t 3 ≦Ta, whereby the pressure chamber quickly contracts and returns to v 3 =0 at a very early period. As a result, the area of the shaded portion of FIG. 18 becomes very small and results in a waveform which is advantageous to the ejection of a minute drop. 
     When an ejection experiment was conducted by using the driving waveform of FIG. 17, it was observed that an ink droplet with a droplet diameter of 14 μm was ejected at a drop velocity of 6 m/s. The reason that the droplet diameter decreased further than when the driving waveform of FIG. 14 was used is that the meniscus control technique was used in combination. That is, it can be assumed that the use of the meniscus control resulted in an effect equivalent to the reduction of the nozzle diameter. When the experiment was conducted by using, for comparison, a driving waveform where t 1 =t 2 =t 3 =2 μs&gt;Ta as in the conventional driving waveform, the minimum diameter of the minute drop that could be ejected was 21 μm. 
     The purpose of setting the driving waveform of FIG. 17 set such that Ta&lt;t 6 ≦Tc is to effect a stable ink meniscus shape control. If the setting is such that t 6 ≦Ta, there will occur the vibration of inherent period Ta even during the time interval of t&lt;t 6 +t 7 , causing such problems as a difficulty in accurately controlling the ink meniscus shape or an occurrence of unwanted ejection. Similarly, if the setting is such that t 6 &gt;Tc, the change of particle velocity v 3  during the time interval t≦t 6 +t 7  will be complicated, thereby also making it difficult to accurately control the ink meniscus shape. In particular, a large property variability tends to occur in the case of a multi-nozzle head. 
     Accordingly, it is desirable that time t 6  is within the range Ta&lt;t 6 ≦Tc, in which case there occurs no vibration of inherent period Ta during the time interval t≦t 6 +t 7 , thus making it possible to control the ink meniscus shape in a stable manner. However, in the case of a single-nozzle head or other heads where a high uniformity can be ensured between the nozzles, the waveform may be set such that t 6 &lt;ta or t 6 &gt;Tc. 
     FIGS. 19-21 show driving waveforms used for the modulation of the ejected ink droplet into three sizes of small, middle and large drop in the above-mentioned inkjet recording head. The small-drop waveform of FIG. 19 is identical in shape to the driving waveform of FIG.  17 . The middle- and large-drop waveforms shown in FIGS. 20 and 21, respectively, have a rise time (t 11 , t 12 ) set larger than inherent period Ta of the circuit and for use with a driving method which does not involve the excitation of the inherent vibration of the piezoelectric actuator. 
     The middle-drop driving waveform of FIG. 20 has a first drop portion  53 A having a fall time t 61  (3 μs) for the drop from the initial voltage to a voltage fall amount V 3 A, whereby the ink meniscus is made to assume a concave shape immediately before the ejection. After a first retaining time t 71  (4 μs), the pressure chamber is compressed by a voltage rise  51 A with a rise time t 11  (3 μs) which is larger than inherent period Ta, followed by a second retaining time 13 μs (t 31 −t 11 ) which is larger than inherent period Ta. Thereafter the waveform is returned back to initial voltage Vb (40V) by a second drop portion  52 A with a fall time t 21  (30 μs). 
     In the case of the large-drop driving waveform of FIG. 21, the pressure chamber is compressed by a voltage rise  51 B having a large rise time t 12  (10 μs) following the initial voltage, and then the voltage slowly returns back to initial voltage by way of a voltage fall  52 B having a fall time t 22  (30 μs), thereby expanding the volume of the pressure chamber. The driving waveform of FIG. 21 does not involve the drawing of the ink meniscus immediately prior to the ejection. 
     Referring to FIG. 13, the driving waveforms for the small-, middle- and large-drops, respectively, were generated by individual waveform generating circuits ( 131 ,  131 A,  131 B). By switching the waveforms to be applied to the piezoelectric actuator  134  based on the image data, a grayscale recording was performed. 
     With the use of the driving waveforms of FIGS. 19-21, it was possible to eject a small-drop with a droplet diameter 14 μm at a drop velocity 6 m/s, a middle-drop with a droplet diameter 28 μm at a drop velocity 6.2 m/s and a large-drop with a droplet diameter 41 μm at a drop velocity 7 m/s, in each case with a driving frequency of 10 kHz. Thus it was possible to realize a wider drop-diameter modulating range of 14-41 μm than in the prior art while maintaining a high driving frequency. 
     It should be noted that the driving waveforms for the large- and middle-drops are not limited to the waveforms illustrated in the above embodiments and may employ other waveform shapes. For example, in the case of the large-drop driving waveform as well, the ejection stability can be improved by incorporating a voltage change process for making the shape of the ink meniscus slightly concave immediately before the ejection. 
     Also, while in the above embodiments the number of levels of drop-diameter modulation was three consisting of large, middle and small, the number of the drop-diameter levels may be more or less than 3 and still the present invention can be implemented. 
     Further, as mentioned above, by using the ejection principle based on the inherent vibration of the piezoelectric actuator according to the present invention for the ejection of a minute drop in the inkjet recording head performing the drop-diameter modulation, and by using the pressure wave under the control of acoustic capacity c 1  of the pressure chamber for the ejection of a larger-diameter drop as according to the conventional inkjet recording head, a very wide drop-diameter modulation range can be obtained, thereby making it possible to realize both high-quality recording and high-speed recording at the same time. 
     Although the invention was described above by way of preferred embodiments, those embodiments should not be taken as limiting the present invention. For example, while in the above embodiments t 1 &lt;t 3  and there was the voltage retaining portion (flat portion) between the first and second voltage change processes, it may be that t 1 =t 3 , i.e., the driving waveform may have no constant voltage portion. 
     Furthermore, while the driving waveform in the embodiments did not involve a compulsory suppression of reverberations after the ink droplet ejection, such a reverberation suppressing process as shown in FIG. 24 may be incorporated. 
     In the above embodiments, inherent period Ta of the piezoelectric actuator per se (actuator unit) was set at 1.6 μs, but it may be set at other values. It is desirable, however, to set inherent period TA at 5 μs or less if a minute ink droplet with a droplet diameter on the order of 15 μm is to be ejected. 
     Furthermore, while bias voltage (initial voltage) Vb was set such that the application voltage to the piezoelectric actuator was positive at all times in the embodiments, bias voltage Vb may be set at other voltages, e.g., zero V, provided a negative voltage can be applied to the piezoelectric actuator without any problems. 
     While in the embodiments, the piezoelectric actuator included a longitudinal vibration-mode piezoelectric actuator with a piezoelectric constant d 33 , other types of actuators may be used, such as a longitudinal vibration-mode actuator with a piezoelectric constant d 31 . In the embodiments, the stacked-type piezoelectric actuator was used, but the same advantages can be obtained by using a single plate-type piezoelectric actuator. If inherent period Ta can be set small enough, it is also possible to use a deflection vibration-mode piezoelectric actuator. 
     While the embodiments employed such a Kaiser-type inkjet recording head as shown in FIG. 22, the present invention can be applied in other inkjet recording heads with different structures, such as a recording head having a groove provided in the piezoelectric actuator as the pressure chamber. Furthermore, the invention can be applied in such inkjet recording heads that employ other types of electro-mechanical transducers than the piezoelectric electric actuator, such as actuators utilizing electrostatic force or magnetic force. 
     Thus, in accordance with the method of driving the inkjet recording head and the inkjet recording apparatus using the method according to the present invention, it is possible to eject a micro drop with a droplet diameter on the order of 15 μm, so that the image quality can be greatly improved. 
     In accordance with a preferred embodiment of the invention, the ejection of such micro drop is possible without setting the volume (W 1 ) of the pressure chamber small, whereby the ejection can be made at a high driving frequency without causing an increase in the refill time. 
     In accordance with a further preferred embodiment of the invention, the ejection principle taking advantage of the inherent vibration of the piezoelectric actuator in accordance with the invention can be used in combination with the conventional ejection principle that takes advantage of the pressure wave governed by the acoustic capacitance (c 1 ) of the pressure chamber, so that there can be obtained a wide drop-diameter modulation range, making it possible to provide high-image quality and high-recording speed at the same time.