Liquid jet device and liquid jet driving method

Abstract: At a dot cycle (T3), a dummy burst signal as well as a jet burst signal (26) are supplied to a vibration exciter (9a) to drive it so that the vibration exciter (9a) can provide liquid (30) with a periodic radiation pressure. The dummy burst signal is so determined as to jet no droplet (31) with a smaller number of pulses and so on. That allows a stable motion of the meniscus of ink (30), and it is thereby possible to stably jet the droplets (31). With this control provided is a technique to improve a shape of an ink surface from which the droplets (31) are created and stably jet the droplets (31) when a liquid jet unit (3) for jetting the droplets (31) from a liquid surface (93) is driven.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a technique to drive a liquid jet unit for jetting droplets from a liquid surface, and more particularly to a technique to jet droplets by providing vibration to a liquid having a surface limitedly exposed by an opening. In particular the invention relates to a device according to the preamble of claim 1, which is known from US-A-5 798 779. Description of the Background Art There has been a technique of depositing ink onto printing paper in a form of droplet to draw images and characters, i.e., ink jet printing. In this description, for distinction, a term "jet" is used when a plurality of ink droplets are simultaneously generated and a term "discharge" is used when droplets are sequentially generated one by one. To faithfully reproduce tone of an image proposed is a technique of ultrasonically providing ink with vibration to obtain droplets from an ink surface. For example, JP-A-2-303849 discloses a technique of controlling the length of period for continuously providing ink with ultrasound to control the amount of ink to be discharged. JP-A-10-128968 discloses a technique of providing ink with ultrasonic burst including a certain number of pulses repeatedly a plurality of times to control the amount of ink to be jetted with the repeat number. In the technique of controlling the length of period for continuously providing ink with ultrasound to control the amount of ink to be discharged, however, in order to discharge a large amount of ink, it is necessary to provide ink with continuous ultrasonic supply for a long time. That causes a marked rise of the level of ink surface from the opening with a radiation pressure as discussed later, resulting in an unstable discharge of droplets. The technique of providing ink with ultrasonic burst repeatedly a plurality of times is superior in not providing continuous radiation pressure. Since the ink jet printing, however, requires depositing of ink on printing paper with different tones for dots, it is necessary to adjust the timing of sending the printing paper and that of disposing ink for every dot. The length of period while no continuous ultrasonic is supplied varies from dot to dot in the technique of providing ink with ultrasonic burst repeatedly a plurality of times as well as in the technique of controlling the length of period for continuously providing ink with ultrasound. Since the length of period while a radiation pressure is provided therefore varies from dot to dot, the rise of ink level from the opening varies from dot to dot. This variation causes an unstable jetting of droplets, and further causes deterioration in graininess in terms of printing quality and difficulty in controlling gradation. SUMMARY OF THE INVENTION The document US-A-5 798 779, forming the preamble of claim 1, discloses an ultrasonic printing method by which recording of a high resolution can be achieved. In the conventional method, some or all of a plurality of ultrasonic oscillators are selectively driven in such phases that a difference in phase at a predetermined point between a reference ultrasonic wave from one of the selected ultrasonic oscillators and another ultrasonic wave from any other one of the selected ultrasonic oscillators is equal to or less than one-fourth a wavelength of the ultrasonic waves in a transmission medium for the ultrasonic waves from the selected ultrasonic waves to the predetermined point. The ultrasonic printing method can be applied to various printing apparatus for which recording of a high resolution is required. The object underlying the present invention is to provide a liquid jet driving device as specified in the preamble of claim 1 using a technique for particularly stable jetting of droplets by improving the shape of the liquid surface from which the droplets are generated. This object is solved in an advantageous manner by a liquid jet driving device comprising the features of claim 1. Advantageous further developments of the device according to the invention are set forth in the subclaims. By means of the liquid jet driving device according to the invention it is possible to suppress the variation in shape of the liquid surface in the opening in order to ensure an easy control of a liquid jet. Since the second vibration exciter is supplied with the suppressing signal, the first vibration exciter can be designed most suitably to the first frequency. These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram showing a configuration of a liquid jet driving device which may be applied to the present invention; Fig. 2 is a cross section schematically showing a structure of an ink head; Figs. 3A to 3C are waveform views for description of a jet burst signal; Fig. 4 is a cross section schematically showing a concept of a sound pressure; Fig. 5 is a cross section schematically showing a concept of a radiation pressure; Figs. 6A to 6D are waveform views showing a relation among the jet burst signal, the sound pressure and the radiation pressure; Figs. 7 to 14 are cross sections schematically showing states of meniscus of ink in an opening; Figs. 15A and 15B are waveform views showing a relation between the jet burst signal and meniscus motion of ink; Figs. 16A and 16B are waveform views showing a liquid jet driving technique; Figs. 17A and 17B are waveform views showing a further liquid jet driving technique; Fig. 18 is a cross section schematically showing a structure in accordance with an embodiment of the present invention; Figs. 19A to 19C are waveform views showing a liquid jet driving technique in accordance with a further embodiment of the present invention; Figs. 20A to 20C are waveform views showing a liquid jet driving technique in accordance with a further embodiment of the present invention; Figs. 21A to 21C are waveform views showing a liquid jet driving technique in accordance with a further embodiment of the present invention; Figs. 22A and 22B are waveform views showing a liquid jet driving technique in accordance with a further embodiment of the present invention; Figs. 23 to 31 are cross sections showing a liquid jet driving technique in accordance with the present invention; Fig. 32 is a cross section schematically showing an exemplary structure of a device usable in the present invention; Figs. 33 to 35 are cross sections showing a further liquid jet driving technique; and Fig. 36 is a cross section showing a further liquid jet driving technique. DESCRIPTION OF THE PREFERRED EMBODIMENTS A. Background of The Invention Before a detailed discussion on preferred embodiments of the present invention, findings on a radiation pressure as the background of the present invention will be discussed. Fig. 1 is a block diagram showing a configuration of a liquid jet driving device which is also applied to the present invention. The liquid jet driving device comprises an input-amount conversion circuit 1 for converting an image signal 20 having information on, for example, a tone value of an image to be drawn, into a jet signal-21, a basic-signal generation circuit 2 for generating a basic signal 28 consisting of continuous pulses of frequency f0, a driving circuit 3 adopting the basic signal 28 for a predetermined period based on the jet signal 21 to generate a jet burst signal 26 consisting of continuous pulses of frequency f0 in number k and an ink head 9 including a vibration exciter 9a driven with the jet burst signal 26. The ink head 9 has ink 30 therein. The vibration exciter 9a is driven with the jet burst signal 26 to jet the ink 30 as a droplet 31 from the ink head 9. The input-amount conversion circuit 1 performs conversion in consideration of conversion of dynamic range (e.g., converting tone values of 256 levels into 32 bits) and a non-linear relation between the amount of ink to be jetted from the ink head 9 and the repeat number of jet burst signal 26. Further, the driving circuit 3 generates a suppressing signal 27 and the vibration exciter 9a is also driven with the suppressing signal 27. Fig. 2 is a cross section schematically showing a structure of the ink head 9. The ink head 9 comprises a body 94 having, for example, rotary paraboloid as an inner wall to store the ink 30, a nozzle plate 93 having an opening 95 to limitedly expose a surface of the ink 30 and being communicated with the body 94 through the opening 95 and the vibration exciter 9a for providing ink 30 with vibration. Without the nozzle plate 93, the opening 95 may be disposed in the body 94. The vibration exciter 9a has an ultrasonic vibration exciter, for example, a piezoelectric vibrator 92, and further a protection sheet 91 between the body 94 and the piezoelectric vibrator 92, for protecting the piezoelectric vibrator 92 so that it should not be wetted by the ink 30. (A-1) Description on Jet Burst Signal Figs. 3A to 3C are waveform views of the basic signal 28, the jet burst signal 26 and a dot recording signal, respectively, which show a relation among the three signals. The basic signal 28 has a cycle T0 = 1/f0, assuming herein that, for example, f0 = 10 MHz (T0 = 100 ns). Each jet burst signal 26 is generated by collecting k pulses of the basic signals 28. It this case, k = 6. The jet burst signal 26 is repeated a predetermined number of times every predetermined burst cycle T2. The jet burst signal 26 is repeated burst number dj of times at the j-th dot #j, depending on a value of the jet signal 21 corresponding to a tone value required of one dot. A period corresponding to one dot is defined as a dot cycle T3, and for example, it is shown that burst numbers d1 = 25, d2 = 5 and d3 = 17 at the first dot #1, the second dot #2 and the third dot #3, respectively. Since T3-dj·T2 varies from dot to dot, an unstable level of ink surface is caused as above discussed. (A-2) Description on Emission Pressure Figs. 4 and 5 are cross sections schematically showing concepts of a sound pressure Ps and a radiation pressure Pi. Though a discussion will be made taking a case where the piezoelectric vibrator 92 makes thickness longitudinal vibration as an example, these two pressures may be applied to the ink 30 even by other-mode vibration. When the piezoelectric vibrator 92 provides the ink 30 with vibration with a frequency of f0, the sound pressure Ps having a frequency f0 drives the surface of the ink 30 exposed in the opening 95 almost vertically in two direction, as shown in Fig. 4. Since a rim of the opening 95 serves as a fixed end of the surface of the ink 30, a first surface wave is created from the rim of the opening 95. The first surface wave jets the fine droplet 31 from its loop. The first surface wave goes towards the center of the opening 95 at a velocity Vc, and is attenuated to vanish immediately after vibration excitement of the piezoelectric vibrator 92 is completed. On the other hand, since the ink 30 has an interface with air near the opening 95 and the vibration of the ink 30 is entirely reflected on the interface, the radiation pressure Pi drives the surface of the ink 30 almost vertically as shown in Fig. 5 in one direction from the piezoelectric vibrator 92 to the opening 93. Since the radiation pressure Pi vanishes after vibration excitement of the piezoelectric vibrator 92 is completed, a second surface wave is created from the rim of the opening 95. The second surface wave goes towards the center of the opening 95 at a velocity Vr. Figs. 6A to 6D are waveform views showing a relation among the jet burst signal 26, the sound pressure Ps and the radiation pressure Pi. The pulse train included in the jet burst signal 26 has a cycle T0 = 1/f0 and each jet burst signal 26 consists of pulses in a burst in number k. The burst cycle T2 is set at not less than k·T0, and therefore there is a relation of T2-k·T0≧0 between adjacent jet bust signals 26. The piezoelectric vibrator 92 has a shape of a thin plate perpendicular to a direction from the vibration exciter 9a to the opening 95 as shown in Fig. 3, and receives the jet burst signal 26 to excite thickness longitudinal vibration almost in a form of sine wave. If it is now considered that the ink 30 has a viscosity low enough to ideally follow the vibration applied by the piezoelectric vibrator 92, assuming that the velocity of sound propagated in the ink 30 (sound velocity), the maximum value of the velocity of the ink moving by vibration excitement (maximum velocity) and the density of the ink 30 are c, u and ρ, respectively, the sound pressure Ps has a cycle T0 and shows a sine wave with an amplitude ρ cu. When a pressure in a case of no vibration excitement for the ink 30 is 0, the sound pressure Ps varies within a range of ± ρ cu and drives the ink 30 in the two directions as shown in Fig. 4. It is assumed herein that a positive sign is taken in a direction from the piezoelectric vibrator 92 to the opening 95. On the other hand, the vibration of the ink 30 is entirely reflected on the interface with air near the opening 95, and that causes a base radiation pressure Bi showing a sine wave with a cycle T0/2 and an amplitude ρ u 2 /2. The base radiation pressure Bi varies from 0 to ρ u 2 . The maximum velocity u of the ink 30 is lower than the sound velocity c, and therefore the amplitude of the base radiation pressure Bi produces little effect on the motion of the ink when the sound pressure Ps is applied to the ink 30.. The base radiation pressure Bi, however, has an average value ρ u 2 /2 in a period T1 while the jet burst signal 26 exists. Since the radiation pressure Pi, as a pulse having the average value, is applied in one direction, when the vibration excitement continues to perform on the ink 30 for a long time, the meniscus of the ink 30 in the opening 95 greatly rises. Further, when the sound pressure Ps is not applied between the adjacent jet burst signals 26, the base radiation pressure Bi also vanishes. Therefore, the radiation pressure Pi produces a considerable effect, i.e., the second surface wave on the ink 30 entirely at the dot cycle T3. Further, JP-A-9-57963 discloses an aspect where the piezoelectric vibrator is driven with a triangular waveform and surface waves going from the rim to the center of the opening interfere with one another to discharge a droplet and another aspect where a plurality of fine droplets are jetted from an end of the surface wave. In both aspects, however, the triangular wave has to make a great drive on the liquid and is not a technique in consideration of radiation pressure accompanying the jet burst signal like the present invention. (A-3) Correction of Burst Cycle In a state where the jet burst signal 26 is repeatedly supplied, i.e., the period dj· T2 in the dot cycle T3 of the j-th dot #j referring to Fig. 3C, the burst cycle T2 is corrected on the basis of the velocity Vr of the second surface wave, to keep the state of the meniscus of the ink 30 in a common shape at the point of time when the jet burst signal 26 is supplied. This correction of the burst cycle is adopted in the preferred embodiments discussed later as the present invention. Figs. 7 to 14 are cross sections schematically showing states of meniscus of the ink 30 in the opening 95 at time t = 0 µs, 0.6 µs, 1µs, 2µs, 4µs, 8µs, 10µs and 15.6 µs, respectively, assuming that the sound pressure Ps of f0 = 10 MHz and k = 6 begins to be supplied at the time t = 0 µs. At the point of time when the sound pressure Ps begins to be supplied, as shown in Fig. 7, the meniscus of the ink 30 has a small rise from the opening 95 due to surface tension. By applying the sound pressure Ps, the droplets 31 are jetted from near the rim of the opening 95 until k/f0 = 0.6 µs. Since the meniscus has a small rise from the opening 95 as shown in Fig. 7, the droplets 31 are jetted radiantly from the opening 95. With the sound pressure Ps applied, the radiation pressure Pi is applied in a period from t = 0 to t = 0.6 µs. Considering that the radiation pressure Pi has one-half wavelength of fi = f0/2k, the velocity Vr of the second surface wave is obtained as fi·(2π σ / ρ fi 2 ) 1/3 . Assuming that the opening 95 is a circle with a diameter D , a time required for a to-and-fro movement of the second surface wave in the opening 95 is obtained as 2D/Vr. For example, assuming that the surface tension σ of the ink 30 is 5 × 10 -2 N/m, the density ρ is 1 × 10 3 kg/m 3 and D = 50 µm, the required time can be obtained as 2D/Vr = 15.6 µs. Therefore, as shown in Figs. 9 to 13, the state of meniscus has a complicate shape until t = 15. 6 µs due to propagation of the second surface wave, and returns to a state almost like that of t = 0, as shown in Fig. 14, when t = 15.6 µS. Setting the burst cycle T2 at 2D/Vr, the state of meniscus at the point of time when the jet burst signal 26 is supplied can be kept equally. There may be a case, naturally, where it is found from an actual measurement that an optimum result can be obtained by setting the burst cycle T2 at a value slightly shifted from the time required for the to-and-fro movement of the second surface wave in the opening 95. It falls within correction of burst cycle of this description, however, to adjust the cycle of variation in level of ink 30 due to the second surface wave and the burst cycle T2 for keeping equally the shape of meniscus at the point of time when the jet burst signal 26 is supplied. (A-4) Problem caused by Non-Continuous Jet Burst Signal Figs. 15A and 15B are waveform views showing a relation between the jet burst signal 26 and meniscus motion of the ink 30 at the dot cycle T3. Fig. 15A shows a timing for supplying the jet burst signal 26 while Fig. 15B shows the center position in the surface of the ink 30 which is exposed in the opening 95. In Figs. 15A and 15B, horizontally-extended lines are base lines, indicating that the pressure is 0 in the figure on the jet burst signal 26 and the center position of the liquid surface in the state of Fig. 7 in the figure on the position of the liquid surface. In a period while the jet burst signal 26 is stopped (T3-dj·T2 in Fig. 3), since no radiation pressure Pi is applied, the surface of the ink 30 which is driven with the radiation pressure Pi applied in a period while the jet burst signal 26 is supplied (dj ·T2 in Fig. 3) makes free vibration in a mode specific to the opening 95. The free vibration has a wavelength about several times as long as the burst period T2. As discussed in (A-1), when the tone level required of dot is different, dj becomes different and T3-dj·T2 also becomes different, and hence the first jet burst signal 26 in the next dot cycle T3 is not necessarily given to the state of meniscus shown in Fig. 7 or 14. Therefore, with control over the free vibration specific to the opening 95 which may occur in the period while the jet burst signal 26 is stopped, the surface of the ink 30 is controlled so that the jet burst signal 26 may be given in an appropriate state of meniscus. B. SPECIFIC EMBODIMENTS Figs. 16A and 16B are waveform views showing a liquid jet driving technique. Fig. 16A shows a timing for supplying the jet burst signal 26 and a dummy burst signal which is adopted as the suppressing signal 27 while Fig. 16B shows the center position in the surface of the ink 30. The base lines of Figs. 16A and 16B show the same as those of Fig. 15A and 15B. The dummy burst signal refers to a burst signal with which the vibration exciter 9a is driven to provide the ink 30 with vibration not sufficient to jet out from the opening 95. The dummy burst signal is generated as a burst signal consisting of a pulse train of a frequency f0, of which pulse number k is smaller than that of the jet burst signal 26. For example, when the jet burst signal 26 has pulses in number k = 6, the dummy burst signal has pulses in number k = 4. Alternatively, the dummy burst signal is generated as a burst signal having pulses in the same number k and the same frequency f0 as the jet burst signal 26 and a smaller amplitude than the jet burst signal 26. This dummy burst signal drives the vibration exciter 9a in the period while the jet burst signal 26 is not applied in the dot cycle T3, i.e., at a period T3-dj·T2. Therefore, over almost entire dot cycle T3, the jet burst signal 26 or the dummy burst signal is applied to the vibration exciter 9a almost at the burst cycle T2. Since the dummy burst signal does not jet the ink 30 unlike the jet burst signal 26 as discussed above, the tone level of the dot is not deteriorated. Since the dummy burst signal also provide the ink 30 with the radiation pressure, however, it is possible to control the free vibration as shown in Fig. 15B that the ink 30 can make in the period T3-dj·T2 to be almost the same as that in the period dj·T2 while the jet burst signal 26 is applied. Since the dummy burst signal is applied repeatedly at the same cycle as the jet burst signal 26 is applied, especially, the motion of the ink 30 can be kept in an almost stationary state. The dummy burst signal can be generated easily in the driving circuit 3 shown in Fig. 1. The driving circuit 3 continuously generates dj jet burst signals 26 with respect to the j-th dot on the basis of the information from the jet signal 21. After that, the driving circuit 3 continuously generates the dummy burst signals until the end of the dot cycle T3 to give the signals to the vibration exciter 9a as the suppressing signal 27. As discussed earlier, since the dummy burst signal has a frequency f0, the dummy burst signal can be easily generated from the basic signal 28 by using different number k of pulses or different amplitude, like the jet burst signal 26. In the dot cycle T3, the dummy burst signal does not necessarily have to be supplied after the jet burst signal 26. For example, earlier in the dot cycle T3, some dummy burst signals are supplied to the vibration exciter 9a, then the jet burst signals 26 are supplied and thereafter the dummy burst signals are supplied until the end of the dot cycle T3. Figs. 17A and 17B are waveform views showing a further liquid jet driving technique. Fig. 17A shows a timing for supplying the jet burst signal 26 and the dummy burst signal while Fig. 17B shows the center position in the surface of the ink 30 which is exposed in the opening 95. The base lines of Figs. 17A and 17B show the same as those of Fig. 15A and 15B. The dummy burst signal is supplied at a cycle different from the burst cycle T2. Since the wavelength of the free vibration of the ink 30 in a mode specific to the opening 95 can be obtained to be several times as long as the burst cycle T2 through calculation or actual measurement, the dummy burst signal, being adjusted to the wavelength, is supplied for the vibration exciter 9a. In this case, it is desirable to supply the dummy burst signal when the meniscus is displaced closest to the piezoelectric vibrator 92. Supplying the dummy burst signal at this point of time, the piezoelectric vibrator 92 provides the ink 30 with the radiation pressure in a direction from the piezoelectric vibrator 92 to the opening 95, ensuring an easy control of the free vibration. Though there may be a case, naturally, where it is found from an actual measurement that an optimum result can be obtained by supplying the dummy burst signal at a timing slightly shifted from the time when the meniscus is displaced closest to the piezoelectric vibrator 92, it is also possible to supply the dummy burst signals intermittently in the direction to suppress the free vibration as discussed above. In the embodiments described above, the dummy burst signal is used to generate the radiation pressure Pi. Instead of using the radiation pressure Pi accompanying the sound pressure Ps, however, a pressure independent of the sound pressure Ps can be used and by applying the pressure to the ink 30, the free vibration in the surface of the ink 30 in the opening 95 can be suppressed. Fig. 18 is a cross section schematically showing a structure of the ink head 9 additionally provided with a structure to apply that pressure. The body 94 and the vibration exciter 9a of Fig. 2 are replaced by a body 97 and a vibration exciter 9b, respectively. The vibration exciter 9b comprises a pressure pulse generator 96 as well as the protection sheet 91 and the piezoelectric vibrator 92. The body 97 is opened on a side of the vibration exciter 9b more widely than the body 94. The ink 30 is given pressure by the pressure pulse generator 96 as well as the piezoelectric vibrator 92 with the protection sheet 91 interposed. Since the pressure pulse generator 96 has no need of generating a sound pressure having a frequency f0, the ink 30 may be given a pressure by generating bubbles with a heating device, instead of being given vibration by a piezoelectric device. This pressure may be applied, for example, for the same period as the radiation pressure, i.e., the period T1 while the jet burst signal 26 exists, or may be applied for a shorter period. Further, the pressure does not necessarily have the same magnitude as that of the radiation pressure Pi by the jet burst signal 26. In other words, this is advantageous in designing flexibility which is greater than a case of using the dummy burst signal as the suppressing signal 27. In the following discussion, a pressure pulse signal is adopted as the suppressing signal 27, and a positive pressure is applied to the ink 30 in a period while the pressure pulse signal is "H". This pressure pulse signal can be generated easily by using a well-known technique in the driving circuit 3 of Fig. 1. Figs. 19A to 19C are waveform views showing a liquid jet driving technique in accordance with a further embodiment of the present invention. Fig. 19A shows a timing for supplying the jet burst signal 26, Fig. 19B shows a timing for supplying the pressure pulse signal and Fig. 19C shows the center position in the surface of the ink 30 which is exposed in the opening 95. The base lines of Figs. 19A and 19B indicate that the pressure is 0 and that of Fig. 19C indicates the center position of the liquid surface in the state of Fig. 7. This pressure pulse signal can be generated easily by using a well-known technique in the driving circuit 3. In this embodiment the dummy burst signal is replaced by the pressure pulse signal, but this embodiment can produce the same effect. (C-2) The Fourth Preferred Embodiment Figs. 20A to 20C are waveform views showing a liquid jet driving technique in accordance with a further embodiment of the present invention. Fig. 20A shows a timing for supplying the jet burst signal 26, Fig. 20B shows a timing for supplying the pressure pulse signal and Fig. 20C shows the center position in the surface of the ink 30 which is exposed in the opening 95. The base lines of Figs. 20A to 20C show the same as those of Figs. 19A to 19C. This pressure pulse signal can be generated easily by using a well-known technique in the driving circuit 3. In this embodiment the dummy burst signal is replaced by the pressure pulse signal, but this embodiment can produce the same effect. (C-3) The Fifth Preferred Embodiment By additionally providing the pressure pulse generator 96 as shown in Fig.18, a negative pressure pulse can be also applied to the ink 30. Only at a desired timing, the pressure pulse signal is made "L". This can be achieved in a case of using a heating device as the pressure pulse generator 96, by stopping heating only at a desired timing, as well as the case of using the piezoelectric device. The pressure pulse signal can be generated easily by using a well-known technique in the driving circuit 3. Figs. 21A to 21C are waveform views showing a liquid jet driving technique in accordance with a further embodiment of the present invention. Fig. 21A shows a timing for supplying the jet burst signal 26, Fig. 21B shows a timing for supplying the pressure pulse signal and Fig. 21C shows the center position in the surface of the ink 30 which is exposed in the opening 95. The base lines of Figs. 21A to 21C show the same as those of Figs. 19A to 19C. For convenience, it is shown that the negative pressure pulse signal is supplied at the point of time when the pressure pulse signal becomes "L". In this embodiment, it is desirable to supply the negative pressure pulse signal when the meniscus is displaced farthest from the piezoelectric vibrator 92. Supplying the negative pressure pulse signal at this point of time, the piezoelectric vibrator 92 provides the ink 30 with a pressure in a direction from the opening 95 to the piezoelectric vibrator 92, ensuring an easy control of the free vibration. Though there may be a case, naturally, where it is found from an actual measurement that an optimum result can be obtained by supplying the negative pressure pulse signal at a timing slightly shifted from the time when the meniscus is displaced farthest from the piezoelectric vibrator 92, it falls within this embodiment to supply the negative pressure pulse signals intermittently in the direction to suppress the free vibration. Thus, this embodiment can produce the same effect as the previous embodiment Since the pressure pulse generator 96 applies a pressure to the ink 30 also when the droplets 31 are jetted on the basis of the jet burst signal 26, the meniscus at that time is likely to have a shape protruding outside from the opening 95 as compared with the state of Fig. 7. Since the meniscus has the same shape when the jet burst signal 26 is supplied in the dot cycles T3, however, it is advantageously possible to avoid different controls of jetting for dots. Further, the embodiments described above can be also achieved with the ink head 9 having the structure of Fig. 2 by driving the piezoelectric vibrator 92 with the pressure pulse signal, instead of using the ink head 9 having the structure of Fig. 18. Since the piezoelectric vibrator 92, however, is designed to provide the ink 30 with larger vibration with near the frequency f0 in order to effectively generate the sound pressure Ps, there may be a case where the piezoelectric vibrator 92 does not effectively work, for example, when the pressure pulse is applied in the period T1 = k/f0. In other words, in the case of applying the pressure pulse, providing the pressure pulse generator 96 additionally to the piezoelectric vibrator 92 driven with the jet burst signal 26 is better for easy designing. The ink head 9 having the structure of Fig. 18 can apply the negative pressure pulse at the same timing as supply of the jet burst signal 26. The pressure pulse signal can be generated easily by using a well-known technique in the driving circuit 3. Figs. 22A and 22B are waveform views showing a liquid jet driving technique in accordance with a further embodiment of the present invention. Fig. 22A shows a timing for supplying the jet burst signal 26 and Fig. 22B shows a timing for supplying the pressure pulse signal. The base lines of Figs. 22A and 22B show the same as those of Figs. 19A and 19B. For convenience, it is shown that the negative pressure pulse signal is applied at the point of time when the pressure pulse signal becomes "L". This embodiment can dilute the radiation pressure Pi based on supply of the jet burst signal 26 by applying the negative pressure pulse at the same timing and can thereby suppress generation of the second surface wave. Therefore, even without correction of the burst cycle T2 which can be applied to the present invention in (A-3), a considerable effect can be achieved in preventing variation of meniscus. Fig. 23 is a cross section schematically showing the state of meniscus of the ink 30 in a waiting state of the ink head. The "waiting state of the ink head" refers to a stationary state where only atmospheric pressure is applied to the ink 30 for a long time. In the waiting state of the ink head, the ink 30 is retreated towards the piezoelectric vibrator 92 while keeping wetting against the inner wall of the nozzle plate 93 with surface tension, and the meniscus is out of contact with the rim of the opening 95. Fig. 24 is a cross section schematically showing the state of meniscus of the ink 30 when the jet burst signal 26 is applied to the piezoelectric vibrator 92 immediately after the waiting state of the ink 30. Since the liquid surface is out of contact with the rim of the opening 95 serving as a fixed end in the state immediately before the ink is vibrated, the first surface wave becomes hard to create and it becomes difficult to jet the droplets 31. When the jet burst signal 26 is supplied for the piezoelectric vibrator 92 several times, the surface level of the ink 30 gradually rises and the ink 30 reaches the rim of the opening 95 to appropriately jet the droplets 31. In consideration of this phenomenon, at least when the jet burst signal 26 is applied from the waiting state of the ink head, it is desirable to provide the ink 30 with hydrostatic pressure, which overwhelms the surface tension to push the liquid level up. On the other hand, once the liquid level of the ink 30 rises, it takes considerable time to reach a stationary state because at least one jet burst signal 26, dummy burst signal or pressure pulse signal is applied to the vibration exciters 9a and 9b every dot cycle T3. Fig. 26 is a cross section schematically showing the state of meniscus of the ink 30 after the jet burst signal 26 is supplied for the piezoelectric vibrator 92 several times, where the hydrostatic pressure continues to be applied. As shown in this figure, when the hydrostatic pressure continues to be applied to the liquid surface which once reaches the rim of the opening 95, the shape of the liquid surface is deformed. Fig. 27 is a cross section schematically showing the state of meniscus of the ink 30 when the jet burst signal 26 is supplied for the piezoelectric vibrator 92 in the state of Fig. 26. In a state immediately before the ink 30 is vibrated, since the liquid level greatly rises from the opening 95 or the liquid spills over, it becomes difficult in some cases to control jetting of the droplets 31. Further, in some cases, a large droplet is discharged from the liquid surface of the ink 30, to deteriorate the tone of dot. Therefore, when the jet burst signal 26 is supplied from the waiting state of the ink head, it is desirable to provide the ink 30 with the hydrostatic pressure, to push the liquid level of the ink 30 up to the rim of the opening 95, and to reduce the hydrostatic pressure or provide a negative hydrostatic pressure after the jet burst signal 26 is supplied for the piezoelectric vibrator 92. Fig. 29 is a cross section schematically showing the state where a positive hydrostatic pressure Pp is applied in the waiting state of the ink head. Thus, by applying the positive hydrostatic pressure Pp, the liquid surface of the ink 30 reaches the rim of the opening 95. Fig. 30 is a cross section schematically showing the state where the jet burst signal 26 is supplied for the piezoelectric vibrator 92 and a negative hydrostatic pressure Pn is applied. As discussed above, instead of applying the negative hydrostatic pressure, there may be a case where the positive hydrostatic pressure Pp is reduced to the degree smaller than that in the waiting state of the ink head. That makes the meniscus stable and avoids jet failure of the droplet 31 and discharge of large droplet. Fig. 32 is a cross section schematically showing an exemplary structure to achieve this aim. The ink 30 is supplied from an ink tank 34 through an ink supply tube 33 to the inside of the ink head 9. Applying the positive hydrostatic pressure Pp as shown in Fig. 29 is achieved by moving the ink tank 34 in the upward direction P and applying the negative hydrostatic pressure Pn or reducing the positive hydrostatic pressure Pp as shown in Fig. 30 is achieved by moving the ink tank 34 in a downward direction N. The moving operation of the ink tank 34 is achieved by using a well-known technique. The embodiment described above proposes a control to avoid the state where the meniscus of the ink 30 is out of contact with the rim of the opening 95 while preventing the ink 30 from spilling over from the opening 95. It is possible, however, to control the hydrostatic pressure while keeping the liquid surface of the ink 30 in contact with the rim of the opening 95 also in the state where the ink head is driven at the dot cycle T3, instead of the waiting state of the ink head. Figs. 33 to 35 are cross sections schematically showing the state of meniscus of the ink 30 in the period while the jet burst signal 26 is supplied for the piezoelectric vibrator 92. The negative hydrostatic pressure Pn is higher in Fig. 34 than in Fig. 33, and higher in Fig. 35 than in Fig. 34. As the negative hydrostatic pressure Pn becomes higher, the liquid surface of the ink 30 moves in a direction from the opening 95 towards the piezoelectric vibrator 92. Since the first surface wave is created when the liquid surface of the ink 30 is in contact with the rim of the opening 95, the more converged droplets 31 can be jetted as the negative hydrostatic pressure becomes higher, and narrowing the width of jetting, the droplets 31 responding to more delicate image can be jetted. The control of the shape of the meniscus shown in Figs. 33 to 35 is achieved by controlling the ratio of the burst cycle T2 to the period k·T0 for supplying the jet burst signal 26 to change the length of period to apply the radiation pressure Pi to the ink 30. Specifically, if the period k·T0 for supplying the jet burst signal 26 is fixed, the burst cycle T2 is made longer to retreat the shape of meniscus towards the piezoelectric vibrator 92 and that makes the width of jetting narrower. Further, in order to keep the shape of meniscus stable, it is desirable to supply the piezoelectric vibrator 92 with the dummy burst signal and the pressure pulse signal as discussed in the first and third preferred embodiments. As discussed earlier, the piezoelectric vibrator 92 supplied with the jet burst signal 26 provides the ink 30 with the radiation pressure Pi as well as the sound pressure Ps. Therefore, adjusting the timing of applying the radiation pressure Pi, i.e., the burst cycle T2 to the cycle of free vibration specific to the opening 95 allows great displacement of the meniscus. Fig. 36 is a cross section schematically showing the motion of the meniscus in accordance with this preferred embodiment Although the second surface wave is created from the rim of the opening 95 by the radiation pressure Pi, since the jet burst signal 26 is supplied at the cycle of the free vibration specific to the opening 95, the meniscus is greatly displaced to jet the large droplets 31. Thus, in this embodiment, by controlling the burst cycle T2, the small droplets 31 are jetted from the first surface wave to obtain an excellent graininess when the image data has a great number of tone levels, and the large droplets 31 are jetted from the second surface wave to obtain a sharp image when the image data has a small number of tone levels (e.g., binary image such as character information). Depending on whether the number of tone levels is large or small, the suitable droplets 31 can be deposited on printed paper. In this embodiment, in order to keep the shape of meniscus stable, it is desirable to supply the piezoelectric vibrator 92 with the dummy burst signal as discussed above and the pressure pulse signal as discussed above. Though there may be a case, naturally, where it is found from an actual measurement that an optimum result can be obtained when there is a slight difference between the burst cycle T2 and the cycle of free vibration specific to the opening 95, it it is also possible to control the burst cycle T2 to excite the above free vibration by utilizing resonance.

Claims: A liquid jet driving device, comprising: a liquid jet unit (9) having a jet surface (93) provided with an opening (95) adapted to limitedly expose a surface of liquid (30) to be jetted out, and a vibration exciter (9a, 9b) provided on the opposite side to the jet surface (93) with the liquid (30) interposed therebetween for providing the liquid (30) with vibration; and a driving unit (3) adapted to provide the vibration exciter (9a, 9b) with a suppressing signal (27) and a jet burst signal (26) consisting of a pulse train of a first frequency to drive the vibration exciter (9a, 9b); wherein vibration provided for the liquid (30) by the vibration exciter (9a, 9b) which is driven with the jet burst signal (26) is sufficient for the liquid (30) to jet out from the jet surface (93), and vibration provided for the liquid (30) by the vibration exciter (9a, 9b) which is driven with the suppressing signal (27) is not sufficient for the liquid (30) to jet out from the jet surface (93), characterized in that the vibration exciter (9b) has a first vibration exciter (92) supplied with the jet burst signal and a second vibration exciter (96) supplied with the suppressing signal. The device according to claim 1, wherein the suppressing signal (27) is a suppressing burst signal consisting of a pulse train of the first frequency. The device according to claim 1, wherein the suppressing signal (27) has less pulses in number than the jet burst signal (26). The device according to claim 1, wherein the suppressing signal (27) is a suppressing burst signal consisting of a pulse train of the first frequency which has a smaller amplitude than that of the jet burst signal (26). The device according to claim 1, wherein the first vibration exciter (92) is driven with the jet burst signal to provide the liquid (30) with a radiation pressure, wherein the liquid (30) makes free vibrations caused by the radiation pressure in a mode specific to the shape and size of the opening (95), wherein the suppressing signal is supplied for the second vibration exciter (96) near at least one timing when the free vibration takes an amplitude of extreme value, and wherein the second vibration exciter (96) is driven with the suppressing signal to provide the liquid (30) with a pressure which works in a direction to suppress the free vibration. The device according to claim 1, wherein the jet burst signal (26) and the suppressing signal (27) are supplied for the vibration exciter (9a, 9b) repeatedly with a second frequency. The device according to claim 6, wherein the second frequency is a frequency with which the liquid (30) makes free vibrations in a mode specific to the shape of the opening (95). The device according to claim 1, wherein the jet burst signal (26) is supplied for the vibration exciter (9a, 9b) repeatedly during a period required for a to-and-fro movement of a surface wave in the opening (95), which is excited on the basis of a radiation pressure to push the liquid (30) out from the opening (95) in a period while the vibration exciter (9a, 9b) is driven.

Inventors: AIZAWA JYUNICHI, FUKUMOTO HIROSHI, NAKAGAWA HIROYUKI, OZAKI YASUHIKO, AIZAWA, JYUNICHI, FUKUMOTO, HIROSHI, NAKAGAWA, HIROYUKI, OZAKI, YASUHIKO
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 2000
Main classification: B41J2
IPC codes: B41J2