Ink jet recording apparatus having deflection means for deflecting droplets of ink emitted through a nozzle

A nozzle of an ink jet head is formed such that a droplet of ink is emitted in a direction nonparallel to a virtual plane formed by a scanning direction X and an electric field direction Z. A voltage is applied between a nozzle plate and a facing electrode for creating an electric field. First and second ink droplets are emitted from the same nozzle during one print cycle. The first ink droplet is emitted in an uncharged state in order not to be deflected by the electric field. On the other hand, the second ink droplet is emitted in a charged state in order to be deflected by the electric field. Because of the difference in deflection amount, the landing positions of the first and second ink droplets are varied, whereby dot density can be improved to be twice as great as nozzle density.

FIELD OF THE INVENTION
 The present invention relates to an ink jet recording apparatus. More
 particularly, the invention relates to an ink jet recording apparatus
 having a deflection means for deflecting droplets of ink emitted through a
 nozzle.
 BACKGROUND OF THE INVENTION
 Personal computer printers and the like employ ink jet recording
 technology. Ink jet recording apparatus have several advantages, such as
 easy handling, superior print performance, and low cost. Because of these
 benefits, ink jet recording apparatus have now become widespread. There
 exist various types of ink jet recording apparatus and they are classified
 according to how ink droplet is emitted. For example, there is an ink jet
 recording apparatus of the thermal type in which thermal energy is
 utilized to create a bubble in ink, and a droplet of the ink is emitted by
 a pressure wave caused by the created bubble. There is another ink jet
 recording apparatus of the electrostatic type in which an ink droplet is
 sucked and emitted by electrostatic force. There is still another ink jet
 recording apparatus of the piezoelectric type in which an ink droplet is
 ejected by means of an oscillator such as a piezoelectric element. In
 addition to these ink jet recording types, Japanese Unexamined Patent
 Gazette No. H05-278212 further discloses an ink jet recording apparatus
 which is a combination of the piezoelectric type and the electrostatic
 type.
 Whichever of the foregoing ink jet recording types is employed, ink
 droplets are emitted from many nozzles formed in an ink jet head in an ink
 jet recording apparatus. These emitted ink droplets land on a sheet of
 recording paper to form an ink dot. Then, by properly arranging many dots,
 printing of characters or images is performed on the recording paper
 sheet.
 However, in a typical ink jet recording apparatus, the number of ink
 droplets each nozzle emits during one print cycle is limited to one. This
 means that dot density (i.e., the number of dots per unit area) depends
 upon nozzle density (i.e., the number of nozzles formed in an ink jet head
 per unit area). Therefore, in order to provide an improved dot density,
 the nozzle density must be improved.
 However, with conventional ink jet head structures, it is difficult to
 provide improved nozzle densities for cost reasons. Accordingly, it has
 been considered that rapid improvement in dot density is difficult to
 achieve.
 Further, another problem arises. That is to say, the landing positions of
 ink droplets emitted through each nozzle align in line in a scanning
 direction. This results in the occurrence of a so-called white stripe due
 to the deviation of landing positions in a direction normal to the
 scanning direction, and due to the variation in ink-droplet emission
 volume among nozzles. Such white striping causes the quality of printing
 of characters and images to deteriorate.
 Bearing in mind the above-described problems with the prior art techniques,
 the present invention was made. Accordingly, an object of the invention is
 to provide improvement in the quality of printing of characters or images
 by heightening the dot density or by reducing the occurrence of white
 striping.
 SUMMARY OF THE INVENTION
 According to the present invention, the landing positions of droplets of
 ink emitted from nozzles are properly altered in a direction normal to a
 scanning direction.
 More specifically, an ink jet recording apparatus in accordance with the
 present invention comprises (a) an ink jet head having a nozzle through
 which a droplet of ink is emitted, (b) a relative movement means for
 causing the ink jet head and a recording medium placed face to face with
 the nozzle to relatively move in a scanning direction, with a specified
 clearance maintained between the nozzle and the recording medium, (c) a
 facing electrode disposed face to face with the nozzle in such a way as to
 put the recording medium between the nozzle and the facing electrode, and
 (d) a voltage applying means for electrifying an ink droplet emitted from
 the nozzle and for applying a voltage between the nozzle and the facing
 electrode thereby to create an electric field, wherein the nozzle is
 formed in such a way as to emit an ink droplet in a direction nonparallel
 with a virtual plane formed by the scanning direction and the direction of
 the electric field, and wherein a landing position varying means is
 provided which is capable of freely varying the landing position of an ink
 droplet emitted from the nozzle in a direction normal to the scanning
 direction on the recording medium.
 As a result of such arrangement, (i) the relative movement means relatively
 moves the ink jet head and the recording medium in a scanning direction,
 (ii) the ink jet head emits an ink droplet for each given print cycle, and
 (iii) characters, images, or the like are recorded on the recording
 medium. An ink droplet is emitted in a direction nonparallel with a
 virtual plane formed by the scanning direction and the direction of the
 created electric field. When the voltage applying means is actuated, the
 ink droplet is electrified, so that its flying direction is deflected by
 an electric field created between the nozzle and the facing electrode, and
 then the landing position of the ink droplet is altered in a direction
 normal to the scanning direction by the landing position varying means.
 Accordingly, it is possible to set the landing position of ink droplets
 which are emitted from the same nozzle at a plurality of landing points
 according to whether an ink droplet is charged or uncharged or by making a
 change in ink droplet charge amount. This makes it possible to heighten
 the dot density above the nozzle density as well as to suppress the
 occurrence of white striping.
 An arrangement, as shown in, for example, FIG. 6, may be made, wherein the
 ink jet head has a nozzle row of a plurality of nozzles arranged in a
 direction normal to the scanning direction and the ink jet head is
 constructed such that each of the plurality of nozzles is capable of
 emitting two or more ink droplets during a print cycle of a given interval
 of time and wherein the landing position varying means is constructed such
 that the two or more ink droplets emitted from each of the plurality of
 nozzles during the print cycle land on different landing positions along a
 direction normal to the scanning direction on the recording medium.
 As a result of such arrangement, during one print cycle two or more ink
 droplets are emitted from each of the nozzles of the ink jet head. Each
 ink droplet lands on different landing positions along a direction normal
 to a principal scanning direction on the recording medium, as a result of
 which the number of dots thus created exceeds the number of nozzles.
 Accordingly, the dot density is rapidly improved.
 An arrangement, as shown in, for example, FIG. 9, may be made, wherein the
 landing position varying means is constructed such that the voltage
 applying means applies a plurality of voltages of different levels during
 one print cycle.
 As described above, the voltage applying means applies, during one print
 cycle, a plurality of voltages of different levels. As a result of such
 voltage application, the landing point of each ink droplet is altered
 depending on the voltage level. To sum up, if a plurality of voltages of
 different levels are applied during one print cycle, this makes it
 possible to alter the landing point of each ink droplet according to the
 voltage level, which facilitates altering the landing points of ink
 droplets.
 An arrangement, as shown in, for example, FIG. 9, may be made, in which
 voltages which are applied by the voltage applying means are set in such a
 way as to vary at a period of 1/n of the print cycle, where the number n
 is a natural number equal to or greater than two.
 Accordingly, the voltage applying means varies a voltage which is applied
 at a period of 1/n of the print cycle, as a result of which the landing
 point of each ink droplet is varied at a period of 1/n in synchronization
 with the print period.
 In the way described above, if a voltage which is applied is varied at a
 period of 1/n of the print cycle, then the landing point of each ink
 droplet can be varied at a period of 1/n in synchronization with the print
 period.
 An arrangement, as shown in, for example, FIG. 9, may be made, in which a
 voltage which is applied by the voltage applying means during a print
 cycle is set in such a manner as to gradually increase in voltage level.
 As described above, the voltage applying means applies a voltage in such a
 manner that the applied voltage can gradually be increased in voltage
 level. Because of such arrangement, ink droplets are emitted in decreasing
 order of deflection amount, as a result of which the landing position
 deviation of the ink droplets accompanied with relative movement between
 the ink jet head and the recording medium is reduced.
 An arrangement, as shown in, for example, in FIG. 13 or FIG. 15, may be
 made, wherein the plurality of voltages which are applied by the voltage
 applying means during one print cycle include first and second voltages
 which differ in voltage level each other and wherein at least one of the
 first and second voltages is set in such a way to vary over a plurality of
 print cycles.
 As described above, the voltage applying means varies at least one of the
 first and second voltages which are applied during one print cycle over a
 plurality of print cycles. Accordingly, the ink droplet landing point is
 varied over a plurality of print cycles correspondingly to the first
 voltage or the second voltage, whichever is varied. As a result, the
 occurrence of white striping is suppressed.
 An arrangement, as shown in, for example, FIG. 3, may be made, wherein (a)
 the ink jet head has a plurality of nozzle rows arranged parallel with one
 another at given intervals of L in the scanning direction, (b) the
 relative movement means is constructed such that the ink jet head and the
 recording medium are relatively moved at a given relative velocity v, and
 (c) at least one of the first and second voltages which are applied by the
 voltage applying means is set in such a way as to vary at a period of
 T1=v/L.times.n, where the number n is a natural number.
 As a result of such arrangement, ink droplets emitted from the plurality of
 nozzle rows are varied synchronously with one another. As a result, for
 example, when ink droplets of a plurality of colors are emitted through a
 plurality of nozzle rows, color deviation due to the overlapping of ink
 droplets is prevented.
 An arrangement, as shown in, for example, FIG. 11, may be made, wherein the
 plurality of voltages which are applied by the voltage applying means
 comprise voltages of the same polarity.
 As described above, making utilization of a plurality of voltages of the
 same polarity, the voltage applying means performs variation in applied
 voltage. As a result, alteration of the landing point is carried out not
 by voltage ON/OFF control but by voltage variation control. For the case
 of voltages of different polarities, there may occur an undesirable
 situation in which ink droplets are not accelerated but decelerated by an
 electrostatic field. Such a situation can be prevented.
 An arrangement, as shown in, for example, FIG. 4, may be made, wherein (a)
 the ink jet head has an pressure chamber in communication with the nozzle
 for containing therein ink and pressurizing means for applying a pressure
 to the ink held in the pressure chamber so that the ink is emitted in the
 form of an ink droplet through the nozzle and (b) the landing position
 varying means is implemented by pressure varying means capable of varying
 the amount of pressure of the pressuring means so that the nozzle is able
 to freely emit a plurality of ink droplets of different emission
 velocities during one print cycle.
 Accordingly, the pressurizing means applies a pressure to ink held in the
 pressure chamber, and by virtue of such an applied pressure a droplet of
 the ink is emitted from the nozzle. The landing position varying means
 varies the pressure amount of the pressurizing means during one print
 cycle. As a result, during one print cycle a plurality of ink droplets of
 different emission velocities are emitted. Since an ink droplet of a high
 emission velocity has a shorter landing time, the amount of deflection
 thereof caused by an electric field is small. On the other hand, since an
 ink droplet of a low emission velocity has a longer landing time, the
 amount of deflection thereof caused by an electric field is great. As a
 result, there are made changes in landing position according to the
 difference in emission velocity between ink droplets. This facilitates
 changing the landing positions of ink droplets.
 An arrangement, as shown in, for example, FIGS. 18(a) and 18(b), may be
 made, wherein the pressure varying means is constructed such that, during
 one print cycle, a plurality of ink droplets are emitted from the same
 nozzle at the same emission volume and at different emission velocities.
 Accordingly, by virtue of the pressuring varying means, during one print
 cycle, a plurality of ink droplets of the same emission amount as well as
 of different emission velocities are emitted. The ink droplets thus
 emitted are identical in emission amount with one another, therefore
 forming their respective dots of the same diameter on the recording
 medium. On the other hand, these ink droplets differ in emission velocity
 from one another, so that the landing positions thereof become altered.
 Accordingly, it is possible to form on the recording medium dots of the
 same diameter while altering the landing position of an ink droplet.
 An arrangement may be made, wherein (a) the pressure varying means is
 constructed such that a plurality of ink droplets emitted from the same
 nozzle during one print cycle form at least first and second dots of the
 same diameter on the recording medium, (b) the first dot is formed of two
 or more ink droplets, while the second dot is formed of a single ink
 droplet, and (c) each of the ink droplets together forming the first dot
 each is emitted at a lower emission velocity in comparison with the
 droplet forming the second dot.
 As describe above, by virtue of the pressure varying means, a plurality of
 ink droplets are emitted during one print cycle, wherein a first dot is
 formed of two or more ink droplets, while a second dot is formed of a
 single ink droplet. The first dot is formed of two or more ink droplets.
 Therefore, when forming first and second dots of the same diameter, there
 is no need to emit ink droplets for forming these two dots at the same
 emission velocity. In other words each of ink droplets forming a first dot
 and an ink droplet forming a second dot can be emitted at different
 emission velocities, which therefore facilitates forming both the first
 and second dots.
 An arrangement may be made, wherein the pressurizing means is constructed
 such that the pressure amount varies at a period of 1/n of a print cycle,
 where the number n is a natural number equal to or greater than two.
 Accordingly, by virtue of the pressurizing means, the pressure amount is
 made to vary at a period of 1/n of a print cycle. As a result, the landing
 positions of ink droplets are altered at a period of 1/n in
 synchronization with the print cycle. Accordingly, it becomes possible to
 alter the landing positions of ink droplets at a period of 1/n in
 synchronization with the print cycle.
 An arrangement may be made, wherein the pressurizing means is constructed
 such that the pressure amount gradually increases during each print cycle.
 As described above, the pressurizing means increases the amount of pressure
 little by little. Accordingly, ink droplets will be emitted in decreasing
 order of deflection amount, as a result of which the landing position
 deviation of the ink droplets accompanied with relative movement between
 the ink jet head and the recording medium is reduced.
 An arrangement may be made, wherein (a) the pressuring means includes an
 oscillating plate which forms at least one of walls of the pressure
 chamber and a piezoelectric element for displacing the oscillating plate
 upon application of a voltage to the piezoelectric element, and (b) the
 pressure varying means is constructed such that the amount of pressure by
 the oscillating plate is varied by varying the waveform of a voltage which
 is applied to the piezoelectric element.
 Accordingly, application of a voltage to the piezoelectric element causes
 the oscillating plate to make a displacement, so that ink in the pressure
 chamber is placed under pressure. The pressure varying means changes the
 waveform of a voltage to be applied to the piezoelectric element, thereby
 to vary the amount of pressure by the oscillating plate. As a result,
 there is made a change in the pressure of the ink in the pressure chamber,
 so that droplets of the ink are emitted at different emission velocities.
 An arrangement, as shown in, for example, FIGS. 20(a) and 20(b), may be
 made, wherein the landing position varying means is formed by a charge
 amount varying means for varying the amount of ink droplet electrification
 so as to freely emit a plurality of ink droplets of different charge
 amounts during one print cycle.
 As described above, by virtue of the charge amount varying means, a
 plurality of ink droplets which are emitted during one print cycle are
 electrified so as to have different charge amounts. An ink droplet having
 a large charge amount is large in the amount of deflection by an electric
 field. On the other hand, an ink droplet having a small charge amount is
 small in the amount of deflection by an electric field. Accordingly, the
 landing positions of ink droplets are altered according to the difference
 in the amount of charge.
 An arrangement may be made, wherein the charge amount varying means is
 constructed such that the amount of charge which is given during each
 print cycle gradually increases.
 As described above, by virtue of the charge amount varying means, the
 amount of charge which is given during each print cycle is increased
 gradually. As a result of such an arrangement, ink droplets will be
 emitted in decreasing order of deflection amount, as a result of which the
 landing position deviation of the ink droplets accompanied with relative
 movement between the ink jet head and the recording medium is reduced.
 An arrangement, as shown in, for example, FIGS. 21(a) and 21(b), may be
 made, wherein the nozzle of the ink jet head is formed such that an ink
 droplet is emitted in a direction nonparallel with a virtual plane
 perpendicular to the scanning direction.
 Accordingly, ink droplets are emitted not only in a direction nonparallel
 with a virtual plane formed by the scanning direction and the direction of
 an electric field, but also in a direction nonparallel with a virtual
 plane perpendicular to the scanning direction. Although an ink droplet low
 in emission velocity has a longer landing time in comparison with an ink
 droplet high in emission velocity, and tends to undergo a greater landing
 point deviation accompanied with relative movement between the ink jet
 head and the recording medium, such an ink droplet undergoes a greater
 deviation by an electric field in comparison with an ink droplet large in
 size. Therefore, landing point deviation in the scanning direction due to
 the difference in emission velocity will be suppressed.
 An arrangement, as shown in, for example, FIG. 23, may be made, wherein (a)
 the ink jet head has a nozzle row of a plurality of nozzles arranged at a
 given pitch of P in a direction normal to the scanning direction and the
 ink jet head is constructed such that each of the plurality of nozzles is
 able to emit n ink droplets during a print cycle of a given interval of
 time, where the number n is a natural number equal to or greater than two
 and (b) the landing position varying means is constructed such that
 landing positions of n ink droplets emitted through each of the plurality
 of nozzles during the print cycle are deviated by P/n in a direction
 normal to said scanning direction.
 Accordingly, it is arranged such that n ink droplets emitted during one
 print cycle land on their respective positions deviated from one another
 by an amount of P/n in a direction normal to the scanning direction. As a
 result, a row of dots arranged at equal intervals is formed on the
 recording medium.
 An arrangement, as shown in, for example, FIG. 24, may be made, wherein (a)
 the ink jet head has a nozzle row of a plurality of nozzles arranged at a
 given pitch of P in a direction normal to the scanning direction and the
 ink jet head is constructed such that each of the plurality of nozzles is
 able to emit n ink droplets during a print cycle of a given interval of
 time where the number n is a natural number equal to or greater than two,
 and (b) the landing position varying means is constructed such that
 landing positions of n ink droplets emitted through each of the plurality
 of nozzles during the print cycle are deviated by P.times.m+P/n in a
 direction normal to the scanning direction, where the number m is a
 natural number.
 Accordingly, it is arranged such that n ink droplets emitted during one
 print cycle land on their respective positions deviated from one another
 by an amount of P.times.m+P/n. As a result, a dot formed without landing
 position alteration and a dot formed with landing position alternation are
 not arranged next to each other, therefore suppressing the occurrence of
 white striping.
 An arrangement, as shown in, for example, FIG. 25, may be made, wherein (a)
 the ink jet head has a nozzle row of a plurality of nozzles arranged at a
 given pitch of P in a direction normal to the scanning direction, and the
 ink jet head is constructed such that each of the plurality of nozzles is
 able to emit two ink droplets during a print cycle of a given interval of
 time, (b) the landing position varying means is constructed such that the
 landing positions of two ink droplets which are emitted from each of the
 plurality of nozzles can be varied among first to third landing points
 deviated each other by P/2 in a direction normal to the scanning direction
 on the recording medium, and in a first print cycle, two ink droplets
 which are emitted from each of the plurality of nozzles land on the first
 and second landing points respectively, while in a second print cycle
 posterior to the first print cycle, two ink droplets which are emitted
 from each of the plurality of nozzles land on the second and third landing
 points respectively, and (c) the first and second print cycles are set so
 as to be repeated in an alternating manner.
 Accordingly, in the first print cycle ink droplets land on the first and
 second landing points, respectively. On the other hand, in the following
 second print cycle ink droplets land on the second and third landing
 points, respectively. As a result of such arrangement, ink droplets will
 not land on the same landing point over a plurality of print cycles,
 thereby suppressing the occurrence of white striping.
 An arrangement, as shown in, for example, FIG. 26, may be made, wherein (a)
 the ink jet head has a nozzle row of a plurality of nozzles arranged at a
 given pitch of P in a direction normal to the scanning direction, and the
 ink jet head is constructed such that each of the plurality of nozzles can
 emit two ink droplets during a print cycle of a given interval of time,
 (b) the landing position varying means is constructed such that the
 landing positions of two ink droplets emitted from each of the plurality
 of nozzles can be varied among a first landing point on the recording
 medium, a second landing point deviating from the first landing point by
 an amount of 0.5 P in a direction normal to the scanning direction, and a
 third landing point deviating from the first landing point by an amount of
 1.5 P in the direction normal to the scanning direction, and in a first
 print cycle, two ink droplets emitted from each of the plurality of
 nozzles land on the first and second landing points respectively, while in
 a second print cycle posterior to the first print cycle, two ink droplets
 emitted from each of the plurality of nozzles land on the second and third
 landing points respectively, and (c) the first and second print cycles are
 set so as to be repeated in an alternating manner.
 Accordingly, in the first print cycle ink droplets land on the first and
 second landing points, respectively. On the other hand, in the following
 second print cycle ink droplets land on the second and third landing
 points, respectively. As a result of such arrangement, ink droplets will
 not land on the same landing point over a plurality of print cycles,
 thereby suppressing the occurrence of white striping.
 An arrangement, as shown in, for example, FIGS. 29(a) and 29(b), may be
 made, wherein (a) the ink jet head includes at least first and second
 nozzle rows, each of the first and second nozzle rows comprising a
 plurality of nozzles arranged at a given pitch P in a direction normal to
 the scanning direction, and the ink jet head is constructed such that each
 nozzle can freely emit at least two ink droplets during a print cycle of a
 given interval of time, (b) the first nozzle row includes a first nozzle
 for forming at least first and second dots on the recording medium, (c)
 the second nozzle row includes a second nozzle adjacent to said first
 nozzle for forming at least third and fourth dots on the recording medium,
 and (d) the second dot is set to lie between the third dot and the fourth
 dot, while the third dot is set to lie between the first dot and the
 second dot.
 As a result of such arrangement, the second dot is placed between the third
 and fourth dots, while the third dot is placed between the first and
 second dots. Accordingly, ink droplets emitted from the same nozzle will
 not land on adjoining landing points. As a result, there is provided
 improvement in the dot density because of the provision of a plurality of
 nozzle rows and the occurrence of white striping will be suppressed.
 An arrangement, as shown, for example, FIGS. 30(a) and 30(b), may be made,
 wherein the landing position varying means is constructed such that, in
 order to form a dot which is elongated in a direction normal to the
 scanning direction on said recording medium, a plurality of ink droplets
 which are emitted from each nozzle during one print cycle land on the
 recording medium in an overlapping manner while being deviated in the
 direction normal to the scanning direction.
 An arrangement, as shown in, for example, FIGS. 31(a) and 31(b), may be
 made, wherein (a) the ink jet head is constructed such that each of the
 nozzles can emit first and second ink droplet groups during one print
 cycle, each of the first and second ink droplet groups being formed of two
 or more ink droplets, (b) it is set such that the ink droplets of the
 first ink droplet group each land on the recording medium in an
 overlapping manner while being deviated in the scanning direction, to form
 on the recording medium a first dot which is elongated in the scanning
 direction, and (c) the ink droplets of the second ink droplet group each
 are emitted after the emission of the first ink droplet group in such a
 way as to differ in landing point from the first ink droplet group, and
 land on the recording medium in an overlapping manner while being deviated
 in said scanning direction, thereby to form a second dot which is
 elongated in the scanning direction at a position located at a given
 interval apart from the first dot in a direction normal to the scanning
 direction.
 An arrangement, as shown in, for example, FIG. 31, may be made, wherein (a)
 the ink jet head is constructed such that each of the nozzles can emit
 first and second ink droplet groups during one print cycle, each of the
 first and second ink droplet groups being formed of two or more ink
 droplets, (b) it is set such that the ink droplets of the first ink
 droplet group each land on the recording medium in an overlapping manner
 while being deviated in the scanning direction, to form on the recording
 medium a first dot which is elongated in the scanning direction, and (c)
 the ink droplets of the second ink droplet group each are emitted after
 the emission of the first ink droplet group in such a way as to differ in
 landing point from the first ink droplet group, and land on the recording
 medium in an overlapping manner while being deviated in said scanning
 direction, thereby to form a second dot which is elongated in the scanning
 direction at a position located a given interval apart from the first dot
 in a direction normal to the scanning direction.
 As a result of such arrangement, first and second dots which are elongated
 in the scanning direction are formed at given intervals in a direction
 normal to the scanning direction, thereby making it possible to provide
 multi-gradation recording by overlapping ink droplets. Further, the second
 ink droplet group is emitted after the first ink droplet group, therefore
 reducing the length of each dot in scanning direction.
 An arrangement, as shown in, for example, FIG. 27, may be made, wherein (a)
 the ink jet head includes a nozzle row of a plurality of nozzles arranged
 in a direction normal to the scanning direction, (b) the ink jet head is
 constructed such that each of the plurality of nozzles can freely emit a
 single ink droplet during a print cycle of a given interval of time, and
 (c) the landing position varying means is constructed such that the
 landing position of each of the ink droplets with respect to the recording
 medium is varied in a direction normal to the scanning direction for each
 print cycle.
 As a result of such arrangement, each nozzle emits one ink droplet during
 one print cycle. The landing position of each ink droplet is varied for
 each print cycle in a direction normal to the scanning direction,
 therefore suppressing the occurrence of a white stripe.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Preferred embodiments of the present invention will be described below by
 making reference to the attached drawings.
 Embodiment 1
 Structure of the Ink Jet Recording Apparatus
 FIG. 1 is a diagram illustrating the structure of an ink jet recording
 apparatus formed according to a first embodiment of the invention. As
 shown in FIG. 1, a carriage 2 is constructed such that the carriage 2 is
 reciprocated by a drive motor (not shown) while being guided by a carriage
 shaft 3. An ink jet head 1 is mounted on the carriage 2, so that the ink
 jet head 1 travels in a principal scanning direction X of FIG. 1 together
 with the carriage 2. The principal scanning direction X corresponds to
 what is defined as a scanning direction in the present invention. Both the
 carriage 2 and the carriage shaft 3 are disposed on a major surface side
 of a recording paper sheet 7 (defined as a recording medium in the present
 invention), and correspond to what is defined as a relative movement means
 in the invention. Disposed on the opposite surface side of the recording
 paper sheet 7 is a facing electrode 4 of metal. It is set such that the
 facing electrode 4 and the ink jet head 1 are spaced apart from each other
 by a distance of about 1 mm. The side of the ink jet head 1 is grounded,
 and a voltage of -2 kV is applied by a power supply 5 between the facing
 electrode 4 and the ink jet head 1. This power supply 5 corresponds to
 what is defined as a voltage applying means in the present invention.
 Reference numeral 6 indicates a paper feeding roller. The paper feeding
 roller 6 conveys the recording paper sheet 7 in a secondary scanning
 direction normal to the principal scanning direction, i.e., in a direction
 Y of FIG. 1 perpendicular to the carriage shaft 3.
 FIG. 2 is a top view illustrating a part of a head surface of the ink jet
 head 1. The ink jet head 1 is provided with four heads for a total of 4
 kinds of colors, namely a yellow head, a magenta head, a cyan head, and a
 black head. The respective heads emit ink of the respective colors.
 Referring to FIG. 2, there is shown a partial top view of a head that
 emits ink of one of the four colors. Each head has 300 nozzles 9 arranged
 at a given pitch of P (P=84.6 .mu.m) in the secondary scanning direction,
 and the head density is set to a value of 300 dpi. As shown in model form
 in FIG. 3, the yellow head (Y), the magenta head (M), the cyan head (C),
 and the black head (Bk) are arranged in that order in the principal
 scanning direction. A structure may be employed in which each of the Y, M,
 C, and Bk heads is made up of two rows of nozzles arranged at a given
 pitch of 169.3 .mu.m corresponding to a head density of 150 dpi.
 Compartmented and formed within the ink jet head 1 are pressure chambers 12
 for the respective nozzles 9. Each pressure chamber 12 is a long groove
 extending in the principal scanning direction and is arranged parallel
 with its adjoining pressure chamber 12. Each nozzle 9 is formed at the
 right-hand side edge of its associated pressure chamber 12. Compartmented
 and formed on the left-hand side of the pressure chambers 12 in the head 1
 is an ink supplying chamber 11 extending in the secondary scanning
 direction. Ink supplying passages 13 are formed between the ink supplying
 chamber 11 and each of the pressure chambers 12. The ink supplying chamber
 11 communicates with the pressure chambers 12 through the ink supplying
 passages 13.
 As shown in FIG. 4, the ink jet head 1 is constructed by sequential
 lamination of a nozzle plate 14 in which a nozzle orifice 10 is formed, a
 dividing wall 15 for compartment-forming the pressure chamber 12 and the
 ink supplying passage 13, and an actuator 17. Whereas the nozzle plate 14
 is formed of a stainless plate having a thickness of 20 .mu.m, the
 dividing wall 15 is formed of a lamination of stainless plates having a
 thickness of 280 .mu.m. As shown in FIG. 5 in an exaggeration manner, the
 actuator 17 is constructed by sequential lamination of an oscillating
 plate 18, a piezoelectric element 19, and an individual electrode 20. The
 oscillating plate 18 is formed of 2-.mu.m chrome (Cr) and functions also
 as a common electrode for applying a voltage to the piezoelectric element
 19 with the individual electrode 20. The piezoelectric element 19 is
 formed of 3-.mu.m PZT (lead zirconate titanate). The individual electrode
 20 is formed of 0.1-.mu.m platinum (Pt). Water ink is held within each of
 the pressure chambers 12.
 One of the features of the present invention is that each nozzle is opened
 in a direction non-parallel with a virtual plane formed by the principal
 scanning direction (X) and the direction of an electric field (Z). In the
 first embodiment, particularly, each nozzle 9 is opened along the
 secondary scanning direction Y in order that two droplets of ink emitted
 from the same nozzle during one print cycle can land at adjoining position
 in the second scanning direction Y. More specifically, as shown in FIG. 6,
 each nozzle is formed such that within a virtual plane normal to the
 principal scanning direction, an angle .alpha., which is formed by a
 perpendicular line extending from the nozzle orifice 10 down to the
 recording paper sheet 7 (i.e., a line in parallel with the electric field
 direction Z) and a direction in which the nozzle orifice 10 opens, is 10
 degrees. It is needless to say that the angle a is not limited to the
 foregoing value (i.e., 10 degrees). The angle .alpha. can be set to
 various values on the basis of the specification of the ink jet head 1.
 For example, a setting of the angle .alpha. can be performed on the basis
 of the result of experiments or simulation, as shown in FIG. 7.
 As shown in FIG. 8, a drive circuit 32 of the ink jet head 1 includes a
 control section 21 formed by a CPU, a ROM 22 for the storage of process
 routines or the like for use in various data processing, a RAM 23 for, for
 example, the storage of various data, a motor controlling circuit 24 for
 driving a conveying motor 26 of the paper feeding roller 6 and a carriage
 motor 28, a data receiving circuit 29 for receiving print data, and a
 pulse signal generating circuit 30. A driver 25 is disposed between the
 motor controlling circuit 24 and the conveying motor 26. A driver 27 is
 disposed between the motor controlling circuit 24 and the carriage motor
 28.
 The pulse signal generating circuit 30 is a circuit for generating a
 voltage that is applied for periodically displacing the piezoelectric
 element 19. The individual electrodes 20 are connected via switch circuits
 31 to the pulse signal generating circuit 30. Disposed between each switch
 circuit 31 and each individual electrode 20 are amplifying circuits not
 shown. The switch circuit 31 is a circuit for selectively outputting a
 pulse signal generated by the pulse signal generating circuit 30 to each
 individual electrode 20.
 The pulse signal generating circuit 30 is constructed so as to generate a
 pulse signal at a frequency of 20 kHz, in other words, the pulse signal
 generating circuit 30 generates a pulse signal for every 50 .mu.s. Such
 arrangement enables each of the nozzles 9 to emit two ink droplets during
 a print cycle of 100 .mu.s. That is to say, the instant drive circuit 32
 is constructed such that ink dots which are arranged next to each other in
 the principal scanning direction are formed for every 100 .mu.s.
 Operation of the Ink Jet Recording Apparatus
 Referring to FIG. 1, the entire operation of the ink jet recording
 apparatus will be described below. The recording paper sheet 7 is first
 conveyed by the paper feeding roller 6 to a desired position. Thereafter,
 each of the nozzles 9 of the ink jet head 1 emits ink droplets while the
 carriage 2 is conveyed by a drive motor (not shown) from a position X1 to
 a position X2 along the principal scanning direction. As a result, a
 recording image for one scanning operation of the ink jet head 1 is
 recorded on the recording paper sheet 7. Subsequently, the recording paper
 sheet 7 is conveyed by the paper feeding roller 6 for a desired amount
 while the carriage 2 is returned from the position X2 to the position X1.
 Thereafter, each of the nozzles 9 is caused to emit ink droplets while the
 carriage 2 is again shifted from the position X1 to the position X2. As a
 result, a recording image for another one scanning operation is recorded
 on the recording paper sheet 7. Such an operation is repeatedly carried
 out for the realization of forming an image on the recording paper sheet
 7.
 Operation of Emitting Ink Droplets
 Next, the operation of emitting droplets of ink from the nozzle 9 will be
 described. Upon application of a voltage to the piezoelectric element 19,
 the piezoelectric element 19, together with the oscillating plate 18,
 bends in a direction in which the pressure chamber 12 decreases in volume.
 This accordingly results in placing an ink in the pressure chamber 12
 under higher pressure, thereby causing the ink to fly in the form of a
 droplet from the nozzle 9 towards the recording paper sheet 7.
 At this time, when no voltage is applied between the nozzle plate 14 and
 the facing electrode 4, the nozzle 9 emits an ink droplet which is not
 electrified (i.e., an uncharged ink droplet). Such an uncharged ink
 droplet flies along the opening direction of the nozzle orifice 10 as
 indicated by a solid line of FIG. 6, and lands on a position on the
 recording paper sheet 7 which is on the extension of the nozzle orifice
 10.
 On the other hand, application of a voltage between the nozzle plate 14 and
 the facing electrode 4 induces a positive charge to an ink within the
 nozzle 9, whereby an ink droplet positively electrified (i.e., a charged
 ink droplet) is ejected from the nozzle 9. Additionally, an electric field
 is created between the nozzle plate 14 and the facing electrode 4, because
 of which the charged ink droplet will undergo a deflection by the created
 electric field as indicated by a broken line of FIG. 6, resulting in
 landing at a position different from the one where the aforesaid uncharged
 ink droplet landed.
 Since an uncharged ink droplet is not accelerated by an electric field, it
 will fly at its initial velocity all the way. On the other hand, a charged
 ink droplet is accelerated by an electric field and therefore, its mean
 flying velocity becomes greater than the initial velocity. Accordingly,
 charged ink droplets have a shorter landing time (i.e., a period of time
 taken from emission to landing) in comparison with uncharged ones.
 Therefore, in the present embodiment, ink droplet emission is carried out
 such that uncharged ink droplets and charged ink droplets are emitted in
 that order. In other words, during a single print cycle in which first and
 second ink droplets are emitted, an uncharged ink droplet is emitted as
 the first ink droplet and a charged ink droplet is emitted as the second
 ink droplet. Such arrangement diminishes positional deviation in the
 principal scanning direction occurring between the first and second ink
 droplets due to the movement of the carriage 2.
 The above will be described using an concrete example in which (i) the ink
 droplet initial velocity is 5 m/s, (ii) the voltage between the nozzle
 plate 14 and the facing electrode 4 is 2 kV, (iii) the clearance between
 the nozzle plate 14 and the facing electrode 4 is 1 mm, and the travelling
 speed of the carriage 2 is 416 mm/s. In this case, the charged ink droplet
 has a landing time of 152.2 .mu.s, while the uncharged ink droplet has a
 landing time of 203.1 .mu.s. Therefore, the difference in landing time
 between these charged and uncharged ink droplets is 203.1-152.2=50.8
 .mu.s. Accordingly, if an uncharged ink droplet is emitted after a charged
 ink droplet, then the amount of landing position deviation
 (misregistration) is (50.08+50.0) .mu.s.times.416 m/s=42 .mu.m. On the
 other hand, if a charged ink droplet is emitted after an uncharged ink
 droplet, then the amount of landing position deviation is (50.08-50.0)
 .mu.s.times.416 m/s=0.3 .mu.m. To sum up, the arrangement that emission of
 charged ink droplets follows that of uncharged ink droplets will diminish
 the degree of landing position deviation in the principal scanning
 direction.
 Deflection Control
 As shown in FIG. 9, the power supply 5 performs ON/OFF control for each
 print cycle of T. In the present embodiment, the power supply 5 is set
 such that switching between the state of ON and the state of OFF occurs in
 that order, for causing an uncharged ink droplet and a charged ink droplet
 to be emitted in that order.
 As described above, each of the nozzles 9 emits a first and a second ink
 droplet during one print cycle. More specifically, the first ink droplet
 is emitted when the power supply 5 is in the state of OFF. On the other
 hand, the second ink droplet is emitted when the power supply 5 is in the
 state of ON. The second ink droplet is emitted after an elapse of T/2 from
 the time the first ink droplet was emitted. In other words, each ink
 droplet is emitted for every T/2 cycle. Here, since the print cycle is set
 to a value of 50 .mu.s, each ink droplet is emitted for every 25 .mu.s.
 The emission velocity, at which each ink droplet is emitted, is set such
 that the landing positions of the first and second ink droplets are
 arranged side by side at intervals of 1/2 of the nozzle pitch P in the
 secondary scanning direction. Here, since the nozzles are formed at a
 pitch of 84.6 .mu.m (density: 300 dpi), the landing positions of ink
 droplets are set to be arranged side by side at 42.3-.mu.m intervals. In
 the present embodiment, the emission velocity of each of the first and
 second ink droplets is set to a value of 5 m/s in order that the first and
 second ink droplets can be arranged at intervals of the aforesaid value.
 The emission velocity can be set easily by controlling, for example, a
 voltage that is applied to the piezoelectric element 19.
 Accordingly, in the first embodiment, first and second ink dots D1 and D2
 each are formed on the recording paper sheet 7 at a density of 600 dpi, as
 shown in FIG. 10. That is to say, a second dot D2 formed by landing of a
 second ink droplet (indicated by broken-line circle) is sandwiched between
 each first dot D1 formed by landing of a first ink droplet (indicated by
 solid-line circle), thereby making it possible to provide an improved dot
 density twice the conventional dot density. As a result, despite the
 nozzle density of 300 dpi, the dot density is 600 dpi.
 Effects of the First Embodiment
 As described above, in accordance with the present embodiment, it is
 designed such that each of the nozzles 9 emits two ink droplets, i.e., a
 first and a second ink droplet, during one print cycle. In addition, one
 of the first and second ink droplets is subjected to deflection, which
 makes it possible to cause the first and second ink droplets to land on
 the recording paper sheet 7 in a side by side fashion in the secondary
 scanning direction. As a result of such arrangement, it becomes possible
 to provide an improved dot density greater than the nozzle density.
 Accordingly, it becomes possible to provide a rapidly improved dot density
 thereby making it possible to perform high-quality recording.
 Additionally, ink droplet deflection can be accomplished easily by
 electrifying ink droplets and by creating an electric field between the
 nozzle plate 14 and the facing electrode 4, therefore making it possible
 to easily and inexpensively provide a means for the deflection of ink
 droplets. Further, it is possible to easily control an ink-droplet
 deflection amount by controlling the voltage between the nozzle plate 14
 and the facing electrode 4.
 It is arranged such that a charged ink droplet is emitted after emission of
 an uncharged ink droplet, which makes it possible to make the landing time
 of the ink droplet emitted in the second place shorter than that of the
 ink droplet emitted in the first place. Because of this, it becomes
 possible to diminish the landing position deviation (misregistration) in
 the principal scanning direction of these ink droplets accompanied with
 the movement of the carriage 2.
 Variation Example 1
 In the first embodiment describe above, it is arranged such that two
 different ink droplets, of which one is a charged ink droplet and the
 other is an uncharged ink droplet, are emitted during one print cycle.
 However, the number of ink droplets to be emitted during one print cycle
 is not limited to two (2). Three or more ink droplets may be emitted
 during one print cycle, in which case a plurality of charged ink droplets
 are to be emitted during one print cycle and the charge amount of each of
 the charged ink droplets or the applied voltage is set to a plurality of
 levels. Because of such arrangement, these charged ink droplets differ in
 the amount of deflection from each other, so that the amount of landing
 point deviation of each of the charged ink droplets is varied.
 Accordingly, it becomes possible to increase the dot density more than
 three times as large as the nozzle density.
 Variation Example 2
 Further, in the first embodiment, the power supply 5 is on/off controlled
 such that the landing positions of the first and second ink droplets are
 deviated. Such control for the deviation of the landing positions of the
 first and second ink droplets should not be considered restrictive. For
 example, as shown in FIG. 11, there may be made a change in ink-droplet
 landing position by the use of voltage switching control for alternately
 switching between a first plus voltage V1 and a second plus voltage V2
 greater than the first plus voltage V1. In such case, both the first and
 second ink droplets are subject to deflection; however, the landing
 positions of the first and second ink droplets are deviated by a given
 pitch in the secondary scanning direction because the second ink droplet
 undergoes a greater deflection than the first ink droplet. The ratio of
 the first plus voltage V1 to the second plus voltage V2 can be set to any
 values as long as the landing positions of the first and second ink
 droplets are deviated by a given amount. It is preferably desired that the
 ratio is 1:5 or above.
 Embodiment 2
 With a view to preventing the occurrence of so-called white striping, in a
 second embodiment of the invention there is made modification to the
 on/off control of the power supply 5 of the first embodiment.
 Ink jet recording apparatus have some problems. For example, some of plural
 nozzles fail to emit an ink droplet of a predetermined diameter and the
 emission direction of ink droplets is deviated because the nozzle orifice
 10 becomes clogged or because the piezoelectric element 19 deteriorates in
 quality. In such cases, as shown in FIG. 12, there is produced an
 unintentional spacing between dots. These spacings, when arranged in
 series in the principal scanning direction, result in the occurrence of a
 white stripe 33. If a great number of such white stripes 33 occur, this
 causes the quality of printing characters or images to deteriorate.
 Particularly, in the present ink jet recording apparatus, it is arranged
 such that two ink droplets are emitted from the same nozzle, which means
 that there is produced a series of white stripes 33 in the secondary
 scanning direction. Therefore, how to cope with the occurrence of white
 striping becomes important.
 Accordingly, a measurement is taken in the second embodiment. More
 specifically, a voltage, which is applied between the nozzle plate 14 and
 the facing electrode 4, is controlled in such a way as to periodically
 vary over a plurality of print cycles (over eight print cycles in the
 present embodiment), as shown in FIG. 13. If the traveling velocity of the
 carriage 2 is v and the interval between nozzle rows of different colors
 is L (see FIG. 3), the voltage is made to vary at a cycle of
 T1=L/v.times.N, the number N being a natural number.
 Here, the voltage is made to vary, so that the amount of landing position
 deviation by voltage variation is 1/4 of a length (i.e., 10.6 .mu.m)
 corresponding to a density of 600 dpi. More specifically, it is required
 that the applied voltage be varied by 384 V for achieving a landing
 position deviation of 10.6 .mu.m, as shown in FIG. 14. Therefore, it is
 arranged in the present embodiment such that the applied voltage is made
 to periodically vary in a range of 2 kV.+-.192 V.
 Accordingly, in the second embodiment, the landing position of charged ink
 droplets varies with the variation in applied voltage, which makes it
 possible to reduce the occurrence of the white stripe 33. The quality of
 recording characters, images, or the like becomes stable.
 Additionally, since the voltage varies at a cycle of v/L.times.N, the
 landing positions of ink droplets of colors of yellow (Y), magenta (M),
 cyan (C), and black (B) will vary in synchronization with one another.
 Accordingly, although the landing positions of ink droplets of these four
 colors vary, the relative positional relationship between dots of
 different colors remains unchanged. This positively ensures that color
 deviation caused by an overlapping of unintentional dots is prevented.
 The landing position of ink droplets can arbitrarily be adjusted by
 performing control of the applied voltage. For example, based on the
 relationship of FIG. 14, adjustment of the applied voltage can be made
 such that the landing position deviation amounts to a predetermined value.
 Variation Example 1
 As shown in FIG. 15, voltage variation control for the prevention of the
 occurrence of white striping is, needless to say, applicable to the second
 variation example of the first embodiment. In the second variation example
 of the first embodiment, the landing positions of ink droplets are varied
 using the first and second plus voltages. As a result of such arrangement,
 it becomes possible to apply voltage variation control to both of the
 first and second plus voltages. However, as shown in FIG. 15, it is
 preferred to apply such voltage variation control to the first plus
 voltage relatively lower in voltage value than the second plus voltage.
 The reason is that since a charged ink droplet (to which a smaller voltage
 is applied) is much affected by voltage variation in comparison with a
 charged ink droplet (to which is a greater voltage is applied), the amount
 of voltage variation necessary for securing a predetermined amount of
 deviation can be reduced, as shown in FIG. 16.
 For example, assume now that the amount of variation in the ink-droplet
 landing position is, as shown in FIG. 16, 10.6 .mu.m. As described in the
 second embodiment, when the second plus voltage (the greater one) is
 subjected to variation, a variation range of 384 V is required. On the
 other hand, when the first plus voltage (the smaller one) is subjected to
 variation, a variation range of 241 V suffices. Accordingly, voltage
 variation control can be carried out in a simple and easy manner.
 Voltage variation control can, needless to say, be applied to both of the
 first and second plus voltages.
 Embodiment 3
 In a third embodiment of the invention, while the voltage applied between
 the nozzle plate 14 and the individual electrode 20 is maintained uniform,
 the nozzle 9 emits first and second ink droplets at different emission
 velocities.
 An ink jet recording apparatus of the present embodiment is identical in
 structure with that of the first embodiment, and the description thereof
 is omitted here.
 Control of the Emission Velocity of Ink Droplet
 The controlling of the emission velocity of ink droplets can be carried out
 by adjustment of the displacement velocity of the oscillating plate 18.
 Adjustment of the displacement velocity of the oscillating plate 18 is
 performed by adjustment of the waveform of a voltage which is applied to
 the piezoelectric element 19.
 For example, as is illustrated in FIG. 17, by adjusting the inclination
 angle of a rising part of a pulse voltage waveform applied to the
 piezoelectric element 19, it becomes possible to emit two ink droplets at
 the same emission volume, but at different emission velocities.
 Relationship Between Velocity and Deflection
 When emission velocity is great as shown in FIG. 18A, landing time is
 relatively short. Therefore, the degree of acceleration by an electric
 field is small and the component of velocity in the secondary scanning
 direction is great. Accordingly, the resulting landing position is much
 deviated from a point directly under the nozzle orifice 10. On the other
 hand, when emission velocity is small as shown in FIG. 18B, landing time
 is relatively long. As a result, the degree of acceleration by an electric
 field is great and the component of velocity in the secondary scanning
 direction becomes relatively small. Accordingly, the amount of landing
 position deviation from a point directly under the nozzle orifice 10
 becomes relatively small. A first ink droplet (which is emitted at a lower
 emission velocity) and a second ink droplet (which is emitted at a higher
 emission velocity) are emitted during one print cycle, thereby making it
 possible to form two dots in the secondary scanning direction during a
 single print cycle.
 Deflection Control
 In accordance with the third embodiment, during one print cycle of T, a
 first ink droplet of a lower emission velocity is emitted and thereafter,
 a second ink droplet of a high emission velocity is emitted. Each ink
 droplet is emitted at intervals of T/2, as in the first embodiment.
 Effects of the Third Embodiment
 As a result of arrangement described above, the third embodiment also
 provides the same effects that the first embodiment does. Additionally, it
 is designed such that a second ink droplet of a higher emission velocity
 is emitted after a first ink droplet of a lower emission velocity, which
 makes it possible to diminish landing position deviation in the principal
 scanning direction accompanied with the movement of the carriage 2.
 Variation Example
 The number of ink droplets which are to be emitted during one print cycle
 is not limited to two (2). An arrangement may be employed in which three
 or more ink droplets of different emission velocities are emitted during
 one print cycle.
 In order to prevent the occurrence of white striping, it may be arranged
 such that the emission velocity of at least one of the first and second
 ink droplets is varied over a plurality of print cycles.
 Embodiment 4
 In a fourth embodiment of the invention, first, second, and third ink
 droplets are emitted during one print cycle, wherein the landing positions
 of the first and second ink droplets become collected at the same point
 thereby to form a first dot, while the third ink droplet forms a second
 dot.
 In accordance with the foregoing third embodiment, two ink droplets are
 emitted at the same emission volume, but at different emission velocities.
 Generally, it is easier to emit two ink droplets at different emission
 volumes and at different emission velocities than emitting two ink
 droplets at the same emission volume, but at different emission
 velocities. Therefore, in the fourth embodiment the following control is
 performed in order to facilitate the controlling of pulse voltages.
 That is, in accordance with the fourth embodiment, two pulse waveforms
 almost similar to each other in shape are employed, as shown in FIG. 19.
 More specifically, first and second ink droplets are emitted in succession
 at an emission volume of 7.5 pl and at an emission velocity of 4.3 m/s.
 Thereafter, after an elapse of T/2, a third ink droplet is emitted at an
 emission volume of 15 pl and at an emission velocity of 10 m/s.
 As a result of such arrangement, the first and second ink droplets land on
 the same position on the recording paper sheet 7 in an overlapping fashion
 to form a single dot (i.e., a first dot). On the other hand, the third ink
 droplet independently forms a second dot. Here, the third ink droplet
 undergoes a less deflection than the first and second ink droplets because
 of its higher emission velocity. Accordingly, the first dot and the second
 are arranged side by side along the secondary scanning direction,
 therefore providing an improved dot density.
 As described above, in accordance with the fourth embodiment, there is no
 need to emit ink droplets, which are to be emitted at different emission
 volumes, at the same emission velocity, thereby making it possible to
 easily and correctly generate a voltage pulse which is applied to the
 piezoelectric element 19.
 Embodiment 5
 In a fifth embodiment of the invention, first and second ink droplets are
 emitted through the nozzle 9, having different charge amounts.
 An ink jet recording apparatus of the present embodiment is identical in
 structure with that of the first embodiment, and the description thereof
 will be omitted accordingly.
 Control of the Ink Droplet Charge Amount
 The charge amount of ink droplet can be controlled by adjustment of the
 displacement velocity of the oscillating plate 18. The displacement
 velocity of the oscillating plate 18 is adjusted by adjustment of the
 waveform of a voltage which is applied to the piezoelectric element 19.
 For example, as is illustrated in FIG. 17, by adjusting the inclination
 angle of a rising part of a pulse voltage waveform applied to the
 piezoelectric element 19, it becomes possible to emit two ink droplets at
 the same emission volume, but at different charge amounts.
 Relationship Between Charge and Deflection
 As shown in FIG. 20A, when charge amount is small (i.e., when charge
 density is small), the amount of landing position deviation from a point
 directly under the nozzle 9 is relatively great because the degree of
 acceleration by an electric field is small. On the other hand, as shown in
 FIG. 20B, when charge amount is great (i.e., when charge density is
 great), the landing-position deviation amount is relatively small because
 the degree of acceleration by an electric field is great. Therefore, a
 first ink droplet of a smaller charge amount and a second ink droplet of a
 greater charge amount are emitted during one print cycle, thereby making
 it possible to form two dots in the secondary scanning direction during
 one print cycle.
 Deflection Control
 In accordance with the fifth embodiment, during one print cycle of T, the
 first ink droplet small in the charge amount is first emitted and
 thereafter, the second ink droplet great in the charge amount is emitted.
 Each ink droplet is emitted at intervals of T/2, as in the first
 embodiment.
 Effects of the Fifth Embodiment
 As a result of arrangement described above, the fifth embodiment also
 provides the same effects that the first embodiment does. Additionally, it
 is designed such that after the first ink droplet of a smaller charge
 amount is emitted, the second ink droplet of a larger charge amount is
 emitted, which makes it possible to diminish the amount of landing
 position deviation in the principal scanning direction accompanied with
 the movement of the carriage 2.
 Variation Example
 Also, the number of ink droplets which are to be emitted during one print
 cycle is not limited to two (2) in the present embodiment. An arrangement
 may be employed in which three or more ink droplets of different charge
 amounts are emitted during one print cycle.
 In order to prevent the occurrence of white striping, it may be arranged
 such that the charge amount of at least one of the first and second ink
 droplets is varied over a plurality of print cycles.
 Embodiment 6
 In a sixth embodiment of the invention, the opening direction of the nozzle
 9 is in nonparallel with a virtual plane formed by a scanning direction
 and an electric field direction, as in the first to fifth embodiments. In
 addition, in the sixth embodiment, the nozzle opening direction is also in
 nonparallel with a virtual plane perpendicular to the scanning direction.
 Generally, as the ink droplet diameter increases, the emission velocity
 likewise increases, in other words, as the ink droplet diameter decreases,
 the emission velocity likewise decreases. For this reason, with the
 movement of the carriage 2, a landing position deviation in the principal
 scanning direction due to the emission velocity, occurs.
 To cope with the above problem, the nozzle opening is inclined towards the
 direction in which the carriage 2 advances during print operation by a
 predetermined angle .beta. from a virtual plane PL perpendicular to the
 scanning direction. Note that the predetermined angle .beta. is set to a
 value of 12 degrees here.
 Although the emission velocity (v1) of a larger ink droplet is higher than
 the emission velocity (v2) of a smaller ink droplet, its charge amount is
 greater than that of the smaller ink droplet, so that the acceleration
 component (w1) thereof is greater than the acceleration component (w2) of
 the smaller ink droplet. Accordingly, landing position deviation due to
 the difference in emission velocity will be offset by landing position
 deviation based on the difference in deflection amount by an electric
 field, as a result of which the landing positions of both the ink droplets
 are almost the same.
 In accordance with the sixth embodiment, it becomes possible to have a
 larger-diameter ink droplet and a smaller-diameter ink droplet land at
 almost the same point, so that deviation of the landing position due to
 the movement of the carriage 2 can be prevented.
 Embodiment 7
 In a seventh embodiment of the invention according to the foregoing first
 to sixth embodiments, there is made a change in the pattern of forming a
 dot onto the recording paper sheet 7.
 For the sake of providing a simplified description, nozzles 9a, 9b, and so
 on and corresponding dots D11, D12, D21, D22, and so on are represented,
 as shown in FIG. 22, by symbols such as .circle-solid., .box-solid.,
 .diamond-solid., .star-solid., .tangle-soliddn., and .tangle-solidup.. As
 shown in FIG. 23, in each of the foregoing embodiments it is designed such
 that first and second ink droplets emitted from each nozzle form two dots
 arranged next to each other in the secondary scanning direction. For
 example, a first ink droplet emitted through the first nozzle 9a to form
 the first dot D11 and a second ink droplet emitted through the first
 nozzle 9a to form the second dot D12 are arranged next to each other in
 the secondary scanning direction. A row of .circle-solid. dots formed of
 these first and second dots D11 and D12 (rows of .circle-solid. dots
 vertically arranged in FIG. 23) are arranged side by side in the secondary
 scanning direction (i.e., in the lateral direction in FIG. 23).
 On the other hand, in the seventh embodiment, it is designed such that the
 first dot D11 formed by a first ink droplet emitted from the first nozzle
 9a and the second dot D12 formed by a second ink droplet emitted from the
 first nozzle 9a are not arranged next to each other, and that the third
 dot D21, which is formed by a first ink droplet emitted from the second
 nozzle 9b, locates between the first and second dots D11 and D12.
 As described above, in accordance with the seventh embodiment, the first
 and second ink droplets emitted from the same nozzle are not formed side
 by side in the secondary scanning direction, therefore making it possible
 to effectively suppress the occurrence of white striping. For example, if
 ink droplets are emitted through the second nozzle 9b, having a diameter
 smaller than a predetermined diameter, then the third and fourth dots D21
 and D22 formed by the ink droplets thus emitted are scattered without
 being arranged next to each other. The occurrence of a white stripe is
 suppressed accordingly.
 Variation Example 1
 The amount of landing position variation of two ink droplets emitted from
 each nozzle can be set to any values as long as these two ink droplets are
 not arranged next to each other in the secondary scanning direction. Such
 a variation amount is not limited to the above-described embodiment.
 Additionally, the number of ink droplets which are emitted from each
 nozzle is not limited to two (2). The number may be set to three (3) or
 more. For example, if each nozzle emits n ink droplets during one print
 cycle where the number n is a natural number equal to or greater than two
 (2), each ink droplet may be deviated by P+P/n where P is the nozzle
 pitch. Further, each droplet may be deviated by m.times.P+P/n, where the
 number m is a natural number equal to or greater than two (2).
 Variation Example 2
 As shown in FIG. 25, it may be designed such that the first dot D11 formed
 by a first ink droplet and the second dot D12 formed by a second ink
 droplet are arranged next to each other in the secondary scanning
 direction, and that these two dots D11 and D12 snake-zigzag in the
 principal scanning direction. Such a dot pattern will be formed, for
 example, in the following way. In other words, the ink jet head 1 is
 constructed in such a way as to enable each nozzle to emit three kinds of
 ink droplets having different landing positions, wherein in a specific
 print cycle ink droplets are made to land on first and second landing
 positions, in a subsequent print cycle ink droplets are made to land on
 the second and third landing positions, and in the next print cycle ink
 droplets are made to land again on the first and second landing positions.
 These print cycles are repeated a plurality of times, as a result of which
 the aforesaid dot pattern will be formed easily.
 Variation Example 3
 As shown in FIG. 26, it may be designed such that in a specific print cycle
 the first and second dots D11 and D12 are arranged next to each other in
 the secondary scanning direction, while on the other hand in another print
 cycle the first and second dots D11 and D12 are not arranged next to each
 other. Such a dot pattern can be formed in the same way as the second
 variation example.
 Embodiment 8
 In an eighth embodiment of the invention, landing-position variation
 control is employed not for the improvement in dot density but for
 preventing the occurrence of white striping.
 As shown in FIG. 27, in the eighth embodiment, each nozzle emits a single
 ink droplet during one print cycle and ink droplet landing position is
 varied periodically. Here, ink droplet landing position is varied for
 every two print cycles.
 In accordance with the eighth embodiment, ink droplets emitted from the
 same nozzle are not arranged next to each other, thereby making it
 possible to suppress the occurrence of white striping.
 Embodiment 9
 In a ninth embodiment of the invention, there is made a change to the ink
 jet head 1 and a nozzle arrangement structure is employed in which nozzles
 are formed in a zigzag pattern as shown in a model form by FIG. 28A.
 Referring to FIG. 28A, an ink jet head formed in accordance with the ninth
 embodiment is provided with first and second nozzle rows N1 and N2, each
 of the nozzle rows N1 and N2 comprising a plurality of nozzles arranged at
 a predetermined pitch P in the secondary scanning direction. The pitch
 (P') between nozzles of the first nozzle row N1 and their corresponding
 adjoining nozzle of the second nozzle row N2 is set to a value of P/2.
 Accordingly, the present ink jet head has an increased nozzle density
 twice that of the ink jet head 1 of the first embodiment. Each of the
 nozzles is constructed in such a way so as to emit two ink droplets during
 one print cycle.
 As shown in FIG. 28B, if the amount of variation in the landing position of
 ink droplets emitted from each nozzle is set to a value of P/4, then the
 dot density is increased to be fourfold. In this case, high-density
 recording becomes possible with a less deflection amount.
 Accordingly, much higher-density recording can be accomplished by (a)
 providing n rows of nozzles, (b) setting the inter-nozzle pitch P' between
 nozzles of one of the n nozzle rows and their corresponding adjoining
 nozzles of another of the n nozzle rows to a value of P/n, (c) causing
 each nozzle to emit m ink droplets during one print cycle, and (d) setting
 the amount of landing position deviation of each of the ink droplets to a
 value of P/(m.times.n), where both the numbers m and n are natural numbers
 not less than two (2).
 Variation Example
 As shown in a model form by FIG. 29A, the pitch P', i.e., the pitch between
 nozzles of the first nozzle row N1 and their adjoining nozzles of the
 second nozzle row N2 may be set to a value of P/4 (P'=P/4), thereby
 forming a special zigzag pattern. Also in this case, it is designed such
 that each nozzle emits two droplets during one print cycle, and the amount
 of landing position variation of the ink droplets is set to a value of
 P/2.
 As a result, as shown in FIG. 29B, the ink droplets are arranged at a pitch
 of P/4. Similar to the ninth embodiment, it becomes possible to provide
 high print quality of characters or images. Further, in the present
 variation example, since the two dots D11 and D12 formed of two ink
 droplets emitted from each nozzle are not arranged next to each other in
 the secondary scanning direction, thereby suppressing the occurrence of
 white striping. That is to say, in accordance with the present variation
 example, not only high-density recording is accomplished, but also the
 occurrence of white striping can be suppressed.
 Further, much higher-density recording can be accomplished while
 suppressing the occurrence of white striping by (a) providing n rows of
 nozzles, (b) setting the inter-nozzle pitch between each nozzle of one of
 the n nozzle rows and its adjoining nozzle of another of the n nozzle
 rows, i.e., the pitch P', to a value of P/(2n), (c) causing each nozzle to
 emit m ink droplets during one print cycle, and (d) setting the amount of
 landing position deviation of each of the ink droplets to a value of P/n,
 where both the numbers m and n are natural numbers not less than two (2).
 Embodiment 10
 In a tenth embodiment of the invention, it is designed such that a
 plurality of ink droplets land on a recording paper sheet while deviating
 them in the secondary scanning direction, to form a dot D1 of an elliptic
 shape which is elongated in the secondary scanning direction (hereinafter
 called the elliptic dot).
 As shown in FIG. 30, in the tenth embodiment, two ink droplets (d1 and d2)
 are emitted during one print cycle. It is set such that the landing
 positions of the first and second ink droplets d1 and d2 are deviated, so
 that these two ink droplets are overlapped in part with each other.
 As a result of such arrangement, it becomes possible to reduce the gap
 defined between elliptic dots arranged next to each other in the secondary
 scanning direction, thereby reducing the occurrence of white striping.
 Particularly, if elliptic dots arranged next to each other in the
 secondary scanning direction are contacted together, this prevents a white
 stripe from occurring. This makes it possible to provide higher print
 quality.
 Embodiment 11
 In an eleventh embodiment of the invention, elliptic dots D1 and D2 are
 formed which are elongated in the principal scanning direction.
 As shown in FIG. 31, in the eleventh embodiment, first to eighth ink
 droplets d1-d8 are sequentially emitted during one print cycle. Each ink
 droplet is emitted at predetermined intervals according to the movement in
 the principal scanning direction so that the landing positions of the ink
 droplets are deviated by a predetermined distance in the principal
 scanning direction. More specifically, the first to fourth ink droplets
 d1-d4 (which makes up a first ink droplet group) each are deflected,
 together forming an elliptic dot D1 which is elongated in the principal
 scanning direction. On the other hand, the fifth to eighth ink droplets
 (which makes up a second ink droplet group) are not deflected, together
 forming an elliptic dot D2 which is located side by side with the elliptic
 dot D1 in the secondary scanning direction.
 Accordingly, in accordance with the eleventh embodiment, it becomes
 possible not only to provide high-density recording but to also provide
 so-called overlap recording by causing ink droplets to land on the
 recording paper sheet in an overlapping manner. As a result,
 multi-gradation recording can be done.
 Further, an arrangement may be employed in which the first ink droplet
 group forming the first dot D1 and the second ink droplet group forming
 the second dot D2 are alternately emitted. However, as in the present
 embodiment, it is arranged such that the ink droplets d5-d8 together
 forming the second dot D2 are emitted after all the ink droplets d1-d4
 together forming the first dot D1 are emitted, which arrangement makes it
 possible to reduce the length of the dots D1 and D2 in the principal
 scanning direction.
 Embodiment 12
 As shown in FIG. 32, a twelfth embodiment of the invention is directed to a
 recording apparatus having a so-called full-line head. More specifically,
 an ink jet head 1a of the present embodiment is formed in such a way as to
 laterally run across the recording paper sheet 7. Nozzles 9 of the ink jet
 head 1a are arranged in such a way as to extend in a lateral direction
 relative to the recording paper sheet 7 (i.e., a direction X shown in FIG.
 32).
 At the time of recording on the recording paper sheet 7, the paper feeding
 roller 6 conveys the recording paper sheet 7 in a scanning direction
 (i.e., a direction Y shown in FIG. 32), and a plurality of dots are formed
 on the recording paper sheet 7 by the ink jet head 1a. A recording method,
 i.e., a method of forming dots on the recording paper sheet 7, can be
 performed in the same way as in the first to eleventh embodiments. The
 present embodiment makes it possible to perform printing of characters or
 images all over the recording paper sheet 7 with a single scanning
 operation.
 Incidentally, prior art full-line heads have the great difficulty in
 providing high-density recording and preventing the occurrence of white
 striping. The reason is as follows. A full line head completes a recording
 on the recording paper sheet 7 by a single scanning operation. Repeating
 such a scanning operation a plurality of times results in increasing not
 only the size of apparatus but also the cost, which is unpractical.
 Accordingly, by implementing the ink jet head 1a in the form of a
 full-line head, it becomes possible to provide high-density recording. In
 addition, the effect of preventing the occurrence of white striping in the
 present invention will be exhibited more significantly.
 Other Embodiments
 It is to be noted that the range of application of the present invention is
 not limited to so-called piezoelectric type recording apparatus typical
 examples of which are disclosed in the aforesaid first to twelfth
 embodiments. The present invention is, needless to say, applicable to
 bubble type recording apparatus in which droplets of ink are emitted by
 bubbles created by rapid application of heat to the ink.
 It will be appreciated by those of ordinary skill in the art that the
 invention can be embodied in other specific forms without departing from
 the spirit or essential character thereof.
 The presently disclosed embodiments are therefore considered in all
 respects to be illustrative and not restrictive. The scope of the
 invention is indicated by the appended claims rather than the foregoing
 description, and all changes which come within the meaning and range of
 equivalence thereof are intended to be embraced therein.