Method and apparatus for driving ink jet recording head

Abstract: A method is described for driving an ink jet recording head comprising the steps of: retreating a vibrating plate to a predetermined position from a nozzle opening at such a speed as to allow a meniscus at the nozzle opening to be jetted from the nozzle opening while applying a drive voltage to a piezoelectric vibrating element; holding the vibrating plate at the position; and advancing the vibrating plate toward the nozzle opening when the meniscus has returned to a position 1/3 or more of the farthest retreat position thereof. As a result, a pressure chamber is contracted to thereby apply pressure to ink when an inertial stream of the ink becomes stable and it heads toward the nozzle opening, producing an ink droplet to be jetted at a predetermined speed irrespective of the position of the meniscus. Furthermore, an appartus for driving an ink jet recording head comprising a nozzle plate (3), a vibrating plate (8), piezoelectric vibrating elements (1) and means for driving said piezoelectric vibrating elements (1) is described.

Description: The invention relates to a method and an apparatus of driving an ink jet recording head. As disclosed in Japanese Patent Publication No. Hei 2-24218, ink jet recording heads having disk-shaped piezoelectric vibrating substrates secured to a resilient plate forming pressure chambers has heretofore been used for recording apparatuses. In the ink jet recording heads of this type, displacement of each piezoelectric vibrating element is so small that a large effective area must be provided, which in turn ensures a relatively large area. These ink jet recording heads are of such a structure that the pressure chambers are located distant from the nozzle openings and communicate with the nozzle openings through flow paths. This not only makes the recording heads large in structure, but also entails the complicated operation of adjusting the fluid resistances of the respective ink flow paths so as to be consistent. To overcome the above problems, proposed in, e.g., the specification of U.S. Patent No. 4,697,193, is an ink jet recording head that produces ink droplets by arranging rod-like piezoelectric vibrating elements and abutting these elements against the resilient plate that forms the pressure chambers to allow the piezoelectric vibrating elements to vibrate vertically. Since the piezoelectric vibrating elements can be disposed so as to confront the nozzle openings in this type of ink jet recording head, the flow paths for connecting the pressure chambers to the nozzle openings can be dispensed with. In addition, the piezoelectric vibrating elements can be prepared in lamination form, which contributes not only to decreasing the drive voltage, but also to improving the print speed owing to the fact that the eigenfrequency of the piezoelectric vibrating element is comparatively large and that this permits high-speed driving of the head. The ink jet recording head utilizing vertical vibration is driven by a so-called "pull-and-strike" method, in which a drive voltage is applied to the piezoelectric vibrating elements to contract them before forming dots and the drive voltage is then discharged to expand the piezoelectric vibrating elements so that ink droplets are produced. Such pull-and-strike method not only allows elastic energy prestored in the piezoelectric vibrating elements and the vibrating plate to be utilized, but also ensures that ink will be introduced to the pressure chambers. On the other hand, if the frequency of repeating the operation of the piezoelectric vibrating elements is increased to improve the print speed, the meniscus varies from one position to another each time an ink droplet is formed, thus varying the size and speed of the ink droplets and making the print quality inconsistent. To avoid these problems, a driving technique in which the piezoelectric vibrating element contracting speed is set to a smallest possible value so as to minimize the varying distance of the meniscus; the piezoelectric vibrating elements are held as contracted for a predetermined time period until the meniscus returns to the original position and stays stationary; and the piezoelectric vibrating elements are expanded by applying a second drive voltage. Document US-A-4 383 264 discloses a demand drop forming device with interacting transducer and orifice combination in which a controlled quantity of ink is formed and expelled from a chamber and through a nozzle in response to a signal that is applied to a deforming element. Document US-A-4 509 059 discloses a method of operating an ink jet in which a voltage drives the jet ink transducer so as to project a droplet of ink from the orifice of an ink chamber on demand. After the chamber of the ink jet has been filled, an abrupt change in drive voltage level disturbs the meniscus of ink in the orifice of the jet to move the meniscus away from the chamber to a predetermined position and velocity, at which point the drive voltage supplied to the ink jet transducer is substantially changed to eject the droplet from the orifice. Document EP-A-0 159 188 discloses a method for operating an ink jet device to obtain high resolution printing in which the volume of the ink chamber of an ink jet device is rapidly expanded to pull an ink meniscus into the chamber from the ink jet orifice, forming a cusp shaped disturbance on the meniscus. This causes a small droplet of ink to break away from the meniscus and be ejected through the orifice. This drive technique contributes to ensuring a stable print quality, but imposes a new problem that the high-speed response of each piezoelectric vibrating element cannot be fully utilized. The invention has been made in view of the above circumstances. Accordingly, an object of the invention is to provide a method of driving an ink jet recording head utilizing the vertical vibration mode that allows high-speed drive while maintaining high print quality. Another object of the invention is to provide an apparatus on which to achieve the above method. These objects are solved by the method of driving an ink jet recording head according to independent claim 1 and the apparatus for driving an ink jet recording head according to independent claim 2. Further advantageous features, aspects and details of the method and the apparatus are evident from the dependent claims, the description and the drawings. The invention provides a method according to claim 1 and an apparatus according to claim 2 of driving an ink jet recording head that ejects an ink droplet from a nozzle opening by displacing a resilient plate forming a pressure chamber with a rod-like piezoelectric vibrating element and compressing the pressure chamber by such displacement. The invention provides a method of driving an ink jet recording head in which pressure chambers are formed by disposing a vibrating plate so as to communicate with nozzle openings and in which ends of piezoelectric vibrating elements of a vertical vibration mode are fixed on the vibrating plate. The method involves the steps of: retreating the vibrating plate from the nozzle opening to a predetermined position at such a speed as to allow a meniscus at a nozzle opening to be jetted from the nozzle opening by applying a drive voltage to a piezoelectric vibrating element; holding the vibrating plate at the position; and advancing the vibrating plate to the nozzle opening when the meniscus has returned by 1/3 or more of the retreat distance. The meniscus retreats from the nozzle opening as the piezoelectric vibrating element contracts. The meniscus temporarily stops retreating before and after the piezoelectric vibrating element stops contracting, but at a next instance, the meniscus forms a stream toward the nozzle opening. As the direction of inertial streams of the ink is switched and at the time the meniscus has returned to a position about 1/3 of the original position, the piezoelectric vibrating element expands. As a result, the inertial streams and the pressure produced by the expansion of the piezoelectric vibrating element are applied to the ink adjacent to the nozzle opening, allowing the ink to splash out efficiently from the nozzle opening. When the meniscus returns to a position more than 1/3 of the original position, an ink droplet splashes out at a certain speed independently of the position of the meniscus, ensuring that the printed dots will have stable form and size. Figure 1 is a sectional perspective view showing an embodiment of an ink jet recording head according to the invention; Figure 2 is a sectional view of the ink jet recording head shown in Figure 1; Figure 3 is a diagram showing an arrangement of ink flow paths and nozzle openings of the ink jet recording head shown in Figure 1; Figure 4 is a diagram showing motion of piezoelectric vibrating elements in the ink jet recording head shown in Figure 1; Figures 5(I) through 5(IV) are diagrams illustrative of the operation of the ink jet recording head shown in Figure 1; Figure 6 is a circuit diagram showing an embodiment of a circuit for driving the ink jet recording head shown in Figure 1; Figures 7(I) through 7(IV) are diagrams illustrative of a drive signal applied to the drive circuit, a voltage at the terminal of a piezoelectric vibrating element, a charging current, and motion of a meniscus; Figure 8 is a diagram showing a relationship among the contraction, the holding, and the expansion process of the piezoelectric vibrating element; Figure 9 is a diagram illustrative of the contraction, the holding, and the expansion process of a piezoelectric vibrating element that contracts by discharging; Figure 10 is a diagram showing an example of a model used to simulate the behavior of ink within a pressure chamber by a drive method of the invention; Figures 11(I) through 11(XXI) are diagrams chronologically showing the behavior of the meniscus when the piezoelectric vibrating element is contracted and left as contracted; Figures 12(I) through 12(X) are diagrams chronologically showing the behavior of the meniscus when the piezoelectric vibrating element is contracted and left as contracted for 2 µsec and then expanded; Figures 13(I) through 13(X) are diagrams chronologically showing the behavior of the meniscus when the piezoelectric vibrating element is contracted and left as contracted for 4 µsec and then expanded; Figures 14(I) through 14(X) are diagram chronologically showing the behavior of the meniscus when the piezoelectric vibrating element is contracted and left as contracted for 6 µsec and then expanded; Figures 15(I) through 15(X) are diagrams chronologically showing the behavior of the meniscus when the piezoelectric vibrating element is contracted and left as contracted for 8 µsec and then expanded; Figures 16(I) through 16(X) are diagrams chronologically showing the behavior of the meniscus when the piezoelectric vibrating element is contracted and left as contracted for 10 µsec and then expanded; Figures 17(I) through 17(X) are diagrams chronologically showing the behavior of the meniscus when the piezoelectric vibrating element is contracted and left as contracted for 12 µsec and then expanded; Figures 18(I) through 18(X) are diagrams chronologically showing the behavior of the meniscus when the piezoelectric vibrating element is contracted and left as contracted for 14 µsec and then expanded; Figures 19(I) through 19(X) are diagrams showing the profile of an ink droplet at each of the above-mentioned holding times; Figure 20 is a diagram showing a relationship among the holding time, the ink jetting speed, and the ink jetting amount; Figure 21 is a diagram showing a relationship between the drive frequency and the ink jetting amount; Figure 22 is a diagram showing a relationship between the drive frequency and the ink jetting speed; Figure 23 is a diagram showing a relationship between the time constant and the damped vibration amplitude of a drive voltage when the piezoelectric vibrating element is expanded; and Figure 24 is a circuit diagram showing a circuit for driving a conventional ink jet recording head. The invention will be described in detail with reference to embodiments shown in the accompanying drawings. Figures 1 and 2 show an embodiment of an ink jet recording head utilizing a vertical vibration mode to which a drive apparatus of the invention will be applied. Reference numeral 1 designates a piezoelectric vibrating element that vibrates in the vertical vibration mode. The piezoelectric vibrating element is formed by sandwiching drive electrodes and a piezoelectric vibrating material while connecting the drive electrodes that interpose the piezoelectric vibrating material in parallel. The advantage of this structure is that all the piezoelectric vibrating layers can be driven by a single voltage; ink droplets can be formed at a drive voltage of about 30 volts. One end of the piezoelectric vibrating element 1 is fixed on a base 9 by an insulating adhesive 7 whose elastic coefficient is relatively large, whereas the other end thereof carries a vibrating plate 8 which is made of a highly elastic material and which forms a pressure chamber 10 through a pressure transmitting member 2. A nozzle plate 3 having nozzle openings 4 is fixed on the base 9 through spacers 11 so as to provide a certain gap with respect to the vibrating plate 8. A space formed between the vibrating plate 8 and the nozzle plate 3 provides the pressure chamber 10, so that ink is supplied from a not shown ink tank through an ink supply path 5 that is a recessed portion arranged in the base 9. The nozzle openings 4 are arranged in a plurality of rows so as to be staggered in an auxiliary scanning direction as shown in Figure 3. This arrangement permits printing with dots as dense as possible. With reference to Fig. 5 (I) in the thus structured ink jet recording head, a meniscus 15 is positioned almost on the surface of the nozzle opening 4 when no drive voltage is applied to the piezoelectric vibrating element 1. When a first drive voltage is applied to the piezoelectric vibrating element 1 so that the element 1 will contract (in a direction indicated by reference character "a" in Figure 4), the vibrating plate 8 that is fixed on the end of the piezoelectric vibrating element 1 retreats from the nozzle opening 4 while being elastically deformed relative to the nozzle plate 3. As a result, the pressure chamber 10 expands and this causes the ink to flow in a direction indicated by reference character "A" in Figure 5(II). This in turn causes the meniscus 15 to retreat toward the piezoelectric vibrating element 1. At the same time, the ink flows into the pressure chamber 10 from the ink supply flow path 5 while ushered by the stream of ink as indicated by reference character "B" (Figure 5(II)). When the piezoelectric vibrating element 1 is maintained as contracted, streams C of the ink toward the nozzle opening are produced in the pressure chamber 10 by inertia (Figure 5(III)), so that the meniscus 15 starts advancing toward the nozzle opening 4. When a drive voltage is applied to the piezoelectric vibrating element 1 so that the element 1 will expand (in a direction indicated by reference character "b" in Figure 4) while the meniscus is advancing, the vibrating plate 8 is biased against the piezoelectric vibrating element 1 to be elastically deformed toward the nozzle opening to thereby contract the pressure chamber 10. A pressure D thereby produced is added to the inertial streams C (Figure 5(III)) that have been produced in the previous process to thereby splash out the ink in the pressure chamber 10 as an ink droplet 16 from the nozzle opening 4 (Figure 5(IV)). While the pressure produced by the expansion of the piezoelectric vibrating element 1 produces a stream E toward the ink supply flow path 5 from the pressure chamber 10, such pressure is checked by the inertial stream B produced in the previous process to some degree. This contributes to reducing pressure drop in the pressure chamber 10 to some degree (Figure 5(IV)). Figure 6 shows an embodiment of an apparatus for driving the above-mentioned ink jet recording head. A first switching circuit 20 includes three transistors 21, 22, 23 and turns on when a H-level voltage is applied to an input terminal 24. A second switching circuit 25 includes transistors 26, 27 and is connected to the input terminal 24 through an inverter 28 so that the circuit 25 turns on when a L-level voltage is applied to the input terminal 24. A capacitor 29 forms a time constant circuit. This capacitor 29 is charged by a power supply voltage VH through a resistor 30 when the first switching circuit 20 turns on and discharges through a resistor 31 when the second switching circuit 25 turns on. A current buffer 32 includes transistors 33, 34 and supplies a voltage proportional to the terminal voltage of the capacitor 29 to the piezoelectric vibrating element 1. In the thus configured circuit, when an input signal such as shown in Figure 7(I) is applied to the input terminal 24 and when the signal goes high, the transistor 22 turns on, which in turn causes a drive voltage VP to rise to the voltage VH. Since the transistor 22 and the capacitor 29, as well as the current buffer 32 connected thereto form a Miller integrator, the waveform of the drive voltage VP applied to the piezoelectric vibrating element 1 depicts a rising straight line with a certain gradient as shown in Figure 7(II). When the drive voltage VP equals the power supply voltage VH, the voltage stops rising and the drive voltage VP is held at the power supply voltage VH level for a predetermined time thereafter. When the input signal goes low, the capacitor 29 discharges through the resistor 31, so that the drive voltage VP of the piezoelectric vibrating element 1 depicts a falling line that is substantially symmetrical with respect to the rising portion of the waveform thereof. By the way, a rising time τ1 of the drive voltage V P and a falling time τ2 thereof are determined by the capacitor 29 and the resistors 30, 31, which constitute a circuit. Assuming that the capacitance of the capacitor 29 is C, the resistance of the resistor 30 is R1, the resistance of the resistor 31 is R2, the base-emitter voltages of the transistors 21,27 are Vbe1, Vbe2, then τ1 and τ2 are expressed as τ1 = C × R1 × VH / VBE1 τ2 = C × R2 × VH / VBE2 As a result, a current IP flowing through the piezoelectric vibrating element 1 is expressed as IP = Cp × VH / τ1 = Cp × VBE1 / (C × R1) IP = Cp × VH / τ2 = Cp × VBE2 / (C × R2) As is apparent from the current IP, this is a constant current circuit designed to cause a certain current to flow through the piezoelectric vibrating element 1. In contrast thereto, there is a conventional constant voltage circuit that is often used as a circuit for driving a piezoelectric element such as shown in Figure 24. To drive a piezoelectric element with this constant voltage circuit, a current IP' flowing through the piezoelectric element and a voltage VP' applied to the piezoelectric element vary with time. That is, assuming that the resistance of a resistor 61 is R1' and the resistance of a resistor 62 is R2', the current IP' and the voltage VP' are expressed as IP' = (t) = VH / R1' × exp {-t / (Cp × R1')} VP' = (t) = VH × [1 - exp {-t / (Cp × R1')}] The time required for the drive voltage VP' to reach the power supply voltage VH is very long in this circuit. However, a time period t0, which is an interval in which the drive voltage reaches a voltage VP'(t) that is 0.9 times the power supply voltage VH can be expressed as t0 = 2.3 × Cp × R1' assuming that the voltage that can ensure the deformation of the piezoelectric element is the voltage VP'(t) that is 0.9 times the power supply voltage. If R1' is set so that the voltage VP'(t) rises within the time period t0 that is equal to the rising time τ1, the maximum of IP', that is IP'(0) is expressed as IP'(0) = VH / R1' = VH × 2.3 × Cp / τ1 = 2.3 × IP This indicates that the constant voltage circuit requires the maximum current 2.3 times that of the constant current circuit in order to rise the voltage within the same time period. The above also applies to the falling time. As described above, the use of the constant current circuit proposed in this embodiment allows the maximum current flowing through the circuit to be reduced even if the charging time and the discharging time are shortened compared with the conventional drive circuit. As a result, the drive circuit can be fabricated with small components and economically. Figures 7(I) through 7(IV) are diagrams showing the operation of the above-mentioned drive circuit in waveform. Upon input of a signal such as shown in Figure 7(I) from a host machine, the first switching circuit 20 turns on as the signal goes high at time T1 to charge the capacitor 29 through the resistor 30. This charging current varies with a time constant τ1, and a voltage that increases at a certain rate is applied to the piezoelectric vibrating element 1 by the Miller integrating function. Accordingly, the piezoelectric vibrating element 1 contracts at a constant speed, not only causing the vibrating plate 8 to retreat at a constant speed to thereby expand the pressure chamber 10, but also causing the meniscus to retreat as time elapses (Figure 7(IV)). When the terminal voltage VP of the piezoelectric vibrating element 1 has reached the power supply voltage VH at time T2, the voltage stops rising, causing the expansion of the pressure chamber 10 to stop as well. With the pressure chamber 10 maintained as expanded for a predetermined time (holding time T) and when the meniscus returns to a predetermined position that will be described later, the input signal goes low at time T3. As a result, the second switching circuit 25 turns on to cause the capacitor 29 to discharge through the resistor 31 with a time constant τ2. Accordingly, the terminal voltage VP of the piezoelectric vibrating element 1 decreases at a certain rate, causing the piezoelectric vibrating element 1 to expand at a constant speed which in turn causes the pressure chamber 10 to contract at a certain rate. The example in which the piezoelectric vibrating element contracts by charging and expands by discharging has been described in this embodiment. It goes without saying that if a piezoelectric vibrating element that contracts by discharging and expands by charging is used, the similar operation can be achieved by generating a drive voltage whose rising time constant is τ1, whose holding time is T, and whose falling time constant is τ2 with the 0-volt level as a symmetrical line while using a drive signal that first goes high and then goes low as shown in Figure 9. By the way, the behavior of the ink in the drive method of the invention will be described in detail by simulation. That is, the drive method involves the steps of: first contracting the piezoelectric vibrating element 1 by applying the first drive voltage to the ink jet recording head to thereby retreat the meniscus formed on the nozzle opening from the surface of the nozzle opening; and then expanding the piezoelectric vibrating element 1 at a proper time when the meniscus starts advancing. The result of a simulation in which a recording head whose nozzle opening diameter is 40 µm and whose gap g between the nozzle plate 3 and the vibrating plate 8 in the stop condition is 80 µm is operated using an ink whose viscosity is 10 mPs using an acoustic model shown in Figure 10 will be presented. This model is an acoustic concentrated constant circuit that models respective sections of the ink jet head as concentrated constants. A concentrated constant circuit 40 for the nozzle section connects a pressure 44 of the meniscus 15 and an acoustic inertance 45 and a resistance 46 of a nozzle in series. A concentrated constant circuit 41 includes the piezoelectric vibrating element 1 and the vibrating plate 8 as single vibrating components, respectively, and connects a pressure producing source 48, a compliance 49, a resistance 50, and an acoustic inertance 57 in series. A concentrated constant circuit 42 representing the pressure chamber 10 connects a resistance 52 and an acoustic inertance 53 in series. A concentrated constant circuit 43 representing deformation of the entire part of the pressure chamber 10 is expressed by a compliance 55. A concentrated constant circuit for the entire part of the ink jet head connects the concentrated constant circuits 40, 41, 42, and 43 in parallel. By the way, the acoustic inertance 45 and the resistance 46 at the time when the meniscus 15 fills up the nozzle opening 4 are greater than those at the time when the meniscus 15 is in retreat. For this reason, in this simulation, the pressure, the acoustic inertance 45, and the resistance 46 constituting the concentrated constant circuit 40 for the nozzle section are treated as a nonlinear element that changes in accordance with the amount of retreat of the meniscus 15. In this simulation, the pressure plate 8 removes the ink at a volume velocity Uv when the pressure producing source 48 is driven. As a result, volume velocities Un, Uc, Us of the ink at the respective sections varying in a nonsteady-state manner can be obtained. Figures 11 through 18 show a simulation of splashing ink droplets while giving the calculated volume velocity Un of the nozzle section as a boundary condition for a general-purpose differential calculus that is capable of handling the behavior of a free surface. The eigenfrequency of the vibrating system including the acoustic inertances 45, 53 and the compliance 55 is termed as a resonance frequency of the pressure chamber. Figures 11(I) through 11(XXI) show a free behavior of the meniscus adjacent to the nozzle opening when the first drive voltage changing at a certain gradient is applied to the piezoelectric vibrating element for 10 µsec and held thereafter. The meniscus continues to retreat from the time at which the drive voltage has been applied (Figure 11(I)) to the time that is 10 µsec thereafter (Figure 11(VI)). Then, the meniscus is directed toward the nozzle opening. Upon elapse of 22 µsec from the application of the drive voltage (Figure 11(XII)), the ink droplet projects from the nozzle opening 4, so that the ink droplet can be developed into such a liquid column as to define a final form thereof (Figure 11(XXI)). Figures 12 through 18 respectively show, in a sum, the flow of the ink adjacent to the nozzle opening in the case where the meniscus in various positions contracts the pressure chamber by the application of the second drive voltage. The selected time constants for the process of both contracting and expanding the piezoelectric vibrating element 1 are 10 µsec, respectively. The various positions of the meniscus are achieved by changing the time between the end of contraction and the start of expansion of the piezoelectric vibrating element, i.e., the above-mentioned holding time. Figure 12 shows flow of the ink when the pressure chamber is contracted upon elapse of 12 µsec from the application of the first drive voltage, i.e., with the holding time being 2 µsec. The pressure chamber is compressed as the meniscus stops retreating and starts an initial phase of advance thereafter while helped by the inertial stream. As a result, the ink splashes out in columnar form, i.e., with the diameter at the end being substantially the same as that at the middle. Figure 13 shows flow of the ink when the pressure chamber is contracted upon elapse of 14 µsec from the application of the first drive voltage, i.e., with the holding time being 4 µsec. This is a flow of the ink when the vibrating plate is pushed out into the nozzle opening side as the meniscus makes a small advance toward the nozzle opening after the retreat phase thereof. The ink jetted out of the nozzle opening similarly becomes columnar. Figure 14 shows flow of the ink when the pressure chamber is contracted upon elapse of 16 µsec from the application of the first drive voltage, i.e., with the holding time being 6 µsec. This is a flow of the ink when the piezoelectric vibrating element is expanded as the meniscus makes a small advance toward the nozzle opening after the retreat phase thereof and returns to a position about 1/3 of the farthest retreat position. Since a part of the front end of an ink droplet jetted out of the nozzle opening is constricted and transformed into a spherical form in the course of splashing, an ideally formed dot can be formed on a recording sheet. Figures 15 through 18 respectively show flows of the ink when the time elapsing from the application of the second voltage is set to 18 µsec (the holding time is 8 µsec), 20 µsec (the holding time is 10 µsec), 22 µsec (the holding time is 12 µsec), and 24 µsec (the holding time is 14 µsec). It has been verified that the end of the ink droplet jetted out from the nozzle opening becomes spherical when the piezoelectric vibrating element is expanded at the respective times. Figure 19 shows various profiles of an ink droplet photographed by a high-speed camera in function of the holding time. The profiles of the ink droplet were photographed under the following steps using the recording head which is the model of the above-mentioned simulation. The photographs were taken by the steps of: jetting an ink droplet from the nozzle opening when the first drive voltage was applied for 10 µsec at a certain rate, i.e., at such a speed as to allow an ink droplet to splash in columnar form from the nozzle opening helped by the inertial stream if the meniscus was left as it was, so that the piezoelectric vibrating element was contracted at a certain rate; and leaving the piezoelectric vibrating element as contracted for an arbitrary time thereafter; and applying a drive voltage that changes at a certain rate for 10 µsec, so that the piezoelectric vibrating element was contracted. It is verified that the profiles with the holding time being 6 µsec or more (Figure 19(III)) are such that the front end of an ink droplet jetted from the nozzle is constricted into a spherical form, whereas the profiles with the holding time being 4 µsec or less (Figures 19(II), 19(I)) are all columnar. The amount of ink jetted from the nozzle opening every holding time and the ink jetting speed are as shown in Figure 20. The amount of ink jetted (the solid line in Figure 20) exhibits a slight increase with increasing holding time. The ink jetting speed drops drastically with increasing holding time up to 6 µsec, or a time at which the meniscus returns to a position about 1/3 of the farthest retreat position, while maintaining a constant value of about 12.5 m/sec with a holding time longer than 6 µsec. By the way, it is known that when the ink jetting speed is high, the ink droplet jetted from the nozzle becomes columnar, whereas that when the ink jetting speed is low, the ink droplet becomes spherical. It is also known that when the front end of a jetted ink droplet is spherical, the dot formed on a recording sheet is substantially circular. It has been verified from the above that an ideal dot, i.e., a circular dot can be printed with the above-mentioned model if the pressure chamber is contracted after the holding time of 6 µsec, i.e., after the meniscus has returned to a position 1/3 of the farthest retreat position. The above fact has been further checked by observing various behaviors of the meniscus while changing parameters such as nozzle opening size, ink viscosity, and the gap between the nozzle plate and the vibrating plate. As a result, it is verified that the front end of an ink droplet splashing from the nozzle opening becomes spherical irrespective of parameters as long as the pressure chamber is contracted at a time when the meniscus has returned to a position 1/3 of the farthest retreat position although the time in which the meniscus returns to such position 1/3 of the farthest retreat position varies. The above means that the ink droplet can splash in spherical form irrespective of the position of the meniscus when the inertial stream produced as the vibrating plate is retreated expands the pressure chamber at such a high speed as to allow the ink droplet to be jetted from the nozzle opening and when the pressure chamber is contracted after the meniscus has returned to a position 1/3 of the farthest retreat position. This means, in terms of the ink jet recording head drive frequency, that the ink jetting amount and speed become constant between the-drive frequencies from 1 kHz to about 10 kHz as indicated by the solid lines in Figures 21, 22. This further means not only that high quality printing can be maintained at high speeds, but also that a certain print quality can be ensured for any dot forming mode, whether dots are formed contiguously, alternately, or every two dots. In contrast thereto, the conventional drive method (the dotted lines in Figures 21, 22) exhibits a decrease in the ink jetting amount around a drive frequency of 3 kHz and a sudden rise of the ink jetting speed, which, as a result, causes the print quality thereof to be negatively affected by the drive frequency. By the way, it is verified that when the vertical frequency mode is employed, the piezoelectric vibrating element is more difficult to be affected by components such as the vibrating plate and the ink compared with a piezoelectric vibrating element in the flexural vibration frequency mode and that the piezoelectric vibrating element vibrates only at the resonance frequency of the piezoelectric element alone. On the other hand, the resonance frequency of the pressure chamber is determined by the acoustic inertances 45, 53, and the compliance 55. The resonance frequency fc of the pressure chamber is expressed as: f = 1 2π Mn + Ms Cc × Mn × Ms where Mn is the acoustic inertance 45, Ms is the acoustic inertance 53, and Cc is the compliance. It has been verified that when the pressure chamber is vibrated freely, the pressure chamber vibrates at the resonance frequency fc. During the expansion and contraction of the pressure chamber, the residual vibration of the pressure chamber can be minimized after the expansion of the pressure chamber as well as the formation of the ink droplet by increasing the piezoelectric vibrating element displacing speed 0.9 times the characteristic vibrational period of the pressure chamber, i.e., by increasing the rising time and falling time of a voltage applied to contract and expand the piezoelectric vibrating element 0.9 times or more the characteristic vibrational period of the pressure chamber. Also, to suppress the residual vibration of the piezoelectric vibrating element, the piezoelectric vibrating element displacing speed may be made equal to the characteristic vibrational period of the piezoelectric vibrating element. That is, as shown in Figure 23, by setting the time constant of the discharge waveform at the time the voltage is being discharged to the range of from 90 to 120% of the characteristic vibrational period of the piezoelectric vibrating element itself, the residual vibration of the piezoelectric vibrating element itself after the displacement thereof as well as the residual vibration of the vibrating plate coupled thereto can be suppressed within about 10% of the amplitude at the time they are driven. As described above, the residual vibration of the pressure chamber as well as the residual vibration of the piezoelectric vibrating element can be suppressed by setting the piezoelectric vibrating element displacing speed, i.e., the rising and/or falling time of a voltage to arbitrary values, the voltage being applied to the piezoelectric vibrating element. However, it is apparent that timings matching other conditions such as the dimension, material, etc. of the piezoelectric vibrating element or the pressure chamber forming components must be selected. That is, the following settings are preferable from the relationship between the characteristic vibrational period Ta of the piezoelectric vibrating element and the characteristic vibrational period Tc (= 1/fc) of the pressure chamber. If Ta < Tc, then τ1, τ2 ≧ 0.9 × Tc. If Ta ≧ Tc, then 0.9 × Ta ≦ τ1, τ2 ≦ 1.2 × Ta. By setting the rising time and/or falling time of the voltage to be applied to the piezoelectric vibrating element in the above manner, dots can be formed immediately without providing a residual vibration attenuation wait time after the jetting of the ink which greatly affects particularly the frequency for repetitively driving the recording head, or the repetitive frequency, hence further improving the repetitive frequency. While the case where the invention is applied to the ink jet recording head with the ink flow paths formed only on one side of each pressure chamber has been described in the above embodiment, it is apparent that the same effect can be obtained by applying the invention to an ink jet recording head having ink flow paths on both sides of each pressure chamber. Also, the same effect can be obtained by applying the invention to a so-called face ejected ink jet recording head in which the vibrating plate is formed so as to confront the nozzle openings as well as to, e.g., a so-called edge ejected ink jet recording head in which an ink droplet is jetted parallelly to a vibrating element. As described on the foregoing pages, the invention is characterized as involving: the first step of retreating the vibrating plate to a predetermined position from the nozzle opening at such a speed as to allow the meniscus at the nozzle opening to be jetted from the nozzle opening while applying a drive voltage to the piezoelectric vibrating element; the second step of holding the vibrating plate at the position; and the third step of advancing the vibrating plate toward the nozzle opening when the meniscus has returned to a position 1/3 or more of the farthest retreat position thereof. As a result, not only the ink stream that is produced adjacent to the nozzle opening when the piezoelectric vibrating element is contracted can be utilized positively, but also the pressure chamber can be contracted in a zone in which the ink jetting speed does not depend on the position of the meniscus, thereby allowing the size and splashing speed of an ink droplet to be maintained substantially constant in a wide range of drive frequencies. Thus, a high-seed recording apparatus can be achieved. Further, since the rising and falling time for the charging or discharging at the time of jetting an ink droplet are set so as to match the vibrational periods of the pressure chamber and the piezoelectric vibrating element, the amplitude of the residual vibration of the vibration system including the piezoelectric vibrating elements after the jetting of an ink droplet can be reduced, thereby contributing to improving the repetitive drive frequency.

Claims: A method of driving an ink jet recording head, the ink jet recording head including a nozzle plate (3) forming nozzle openings (4) therein, a vibrating plate (8) which opposes said nozzle plate (3) to form a pressure chamber (10) communicating with said nozzle openings (4) in cooperation with said nozzle plate (3), piezoelectric vibrating elements (1), each having one end opposing each of said nozzle openings (4) respectively and being fixed to said vibrating plate (8), said piezoelectric vibrating elements (1) essentially vibrating in the direction of ink jetting, said method comprising the steps of: retreating said vibrating plate from said nozzle opening by applying a drive voltage to said piezoelectric vibrating element; wherein said retreating step retreating said vibrating plate from said nozzle opening to a predetermined position at such speed as to allow a meniscus of said nozzle opening to be moved away from said nozzle opening to a predetermined retreat distance by applying a drive voltage to said piezoelectric vibrating element; holding said vibrating plate at said predetermined position; and advancing said vibrating plate toward said nozzle opening when said meniscus has returned by 1/3 or more of said retreat distance, such that an ink drop splashes from said recording head irrespective of the further position of the meniscus. An apparatus for driving an ink jet recording head, the ink jet recording head including a nozzle plate (3) forming nozzle openings (4) therein, a vibrating plate (8) which opposes said nozzle plate (3) to form a pressure chamber (10) communicating with said nozzle openings (4) in cooperation with said nozzle plate (3), piezoelectric vibrating elements (1), each having one end opposing each of said nozzle openings (4) respectively and being fixed to said vibrating plate (8), said piezoelectric vibrating elements (1) essentially vibrating in the direction of ink jetting, wherein a drive circuit for supplying a first signal to each said piezoelectric vibrating element (1) for retreating said vibrating plate from said nozzle opening to a predetermined position and having a magnitude that is sufficient to contract said piezoelectric element (1) at a speed that allows a meniscus (15) formed at said nozzle opening to be withdrawn from said nozzle opening (4) to a predetermined retreat distance; said drive circuit also for supplying a second signal to said piezoelectric element (1) to maintain said piezoelectric element in the contracted condition for holding said vibrating plate at said predetermined position for an interval T≧Tin, where T is a second signal holding time and Tin is the time from which said first signal reaches a steady-state, corresponding to said vibrating plate reaching said predetermined position, to the time at which said meniscus has returned to 1/3 of said retreat distance; and said drive circuit also for supplying a third signal to said piezoelectric element (1) that is sufficient to expand said piezoelectric element to jet an ink droplet from said nozzles irrespective of the further position of said meniscus. An apparatus according to claim 2, wherein a time constant of said first and/or said third drive signal is set to a value 0.9 times a characteristic vibrational period of said pressure chamber (10). An apparatus according to claim 2, wherein a time constant of said third drive signal is set to a value 0.9 to 1.2 times a characteristic vibrational period of said pressure chamber (10).

Inventors: ABE TOMOAKI, HOSONO SATORU, KATAKURA TAKAHIRO, KITAHARA TSUYOSHI, YONEKUBO SHUJI, ABE, TOMOAKI, HOSONO, SATORU, KATAKURA, TAKAHIRO, KITAHARA,TSUYOSHI, YONEKUBO, SHUJI, ABE, TOMOAKI SEIKO EPSON CORPORATION, HOSONO, SATORU SEIKO EPSON CORPORATION, KATAKURA, TAKAHIRO SEIKO EPSON CORPORATION, KITAHARA,TSUYOSHI SEIKO EPSON CORPORATION, YONEKUBO, SHUJI SEIKO EPSON CORPORATION
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1992
Main classification: B41J2
IPC codes: B41J2