United States Patent
`
`[19]
`
`Ayata et al.
`
`[11] Patent Number:
`
`4,463,359
`
`[45] Date of Patent:
`
`Jul. 31, 1984
`
`[54] DROPLET GENERATING METHOD AND
`APPARATUS THEREOF
`
`[75]
`
`Inventors:. Naoki~Ayata;.~Yoshiaki Shirato, both
`of Yokohama; ’ Yasushi Takatori,
`Sagamihara; Mitsuaki Seki,
`Machida, all of Japan
`
`[73] Assignee:
`
`Canon Kabushiki Kaisha, Tokyo,
`Japan
`
`[21] Appl. No.: 133,327
`
`[22] Filed:
`
`Mar. 24, 1980
`
`Foreign Application Priority Data
`[30]
`Apr. 2, 1979 [JP]
`Japan ............
`.................. .. 54-39467
`Apr. 2, 1979 [JP]
`Japan
`54-39468
`Apr. 2, 1979 [JP]
`Japan
`54-39469
`Apr. 2, 1979 [JP]
`Japan
`54-39470
`Apr. 2, 1979 [JP]
`Japan
`54-39471
`Apr. 2, 1979 [JP]
`Japan
`54-39472
`Apr. 11, 1979 [JP]
`Japan
`54-43849
`Feb. 22, 1980 [JP]
`Japan
`.. 55-21348
`
`.. GOID 15/13
`Int. CL3 ..................................... ..
`[51]
`[52] U.S.Cl.
`................................. 346/1.1; 346/140R
`[58] Field of Search ............ .. 346/140 R, 75, 140 PD,
`346/1.1; 355/73; 358/297-302, 285, 296, 293
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`346/75 X
`......... ..
`2/1971 Taylor et al.
`3,560,641
`355/73 X
`2/1973 Knechtel et al.
`3,716,294
`346/140 R
`3,988,745 10/1976 Sultan ........... ..
`.. 346/140 PD
`4,074,284
`2/1978 Dexter et a1.
`..... ..
`.. 346/ 140 PD
`4,095,237
`6/1978 Amberntsson el al.
`4,126,868 11/1978 Kirner ........................ .. 346/140 PD
`
`1/1981 Kobayashi et al.
`4,243,994
`2/1981 Hara et al.
`4,251,824
`4,296,421 10/1981 Hara et al.
`
`346/140 R
`346/140 R
`................. .. 346/140 PD
`
`FOREIGN PATENT DOCUMENTS
`
`2253125
`2450463
`2549758
`1566988
`
`355/73
`5/1974 Fed. Rep. of Germany
`346/140 R
`5/1976 Fed. Rep. of Germany
`5/1977 Fed. Rep. of Germany .... .. 346/140
`PD
`'
`5/1980 United Kingdom .......... .. 346/ 140 R
`
`Primary Exam1'ner—Haro1d I. Pitts
`Assistant Examiner—W. J. Brody
`Attorney, Agent, or F1’rm——-Fitzpatrick, Cella, Harper &
`Scinto
`
`[57]
`
`ABSTRACT
`
`This invention provides a method of and apparatus for
`emitting a droplet from an orifice by generating a bub-
`ble in a small liquid chamber in response to a droplet
`generating instruction. The method and apparatus are
`particularly characterized having gradual contraction
`of the bubble to prevent excessive recession of the ori-
`fice in the liquid chamber which recession would hinder
`subsequent droplet generation.
`For achieving the above-mentioned functions the drop-
`let generating apparatus, or the droplet emitting head, is
`provided with a structure which allows effective gener-
`ation and annihilation of the bubble. The present inven-
`tion also provides a novel apparatus having emission
`heads in a staggered block arrangement for improving
`alignment density of the devices and elements and time-
`division drive circuitry.
`
`51 Claims, 69 Drawing Figures
`
`'
`
`¢
`. . . . .
`02'.°.°.'o'c?v!¢I*Iv$I?Z¢.‘I
`
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`U.S. Patent
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`Jul. 31, 1984
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`Sheet 1 of 37
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`4,463,359
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`U.S. Patent
`
`Jul. 31, 1984
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`Jul. 31, 1984
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`Sheet 3 of 37
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`Jul. 31, 1984
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`U.S. Patent
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`Jul. 31, 1984
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`Jul. 31, 1984
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`Jul. 31, 1984
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`4,463,359
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`Jul. 31, 1984
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`Jul. 31, 1984
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`U.S. Patent
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`Jul. 31, 1984
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`Jul. 31, 1984
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`Sheet 16 of 37
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`Jul. 31, 1984
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`U.S. Patent
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`Ju1.31, 1984
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`Jul. 31, 1984
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`U.S. Patent
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`Jul. 31, 1984
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`Sheet 26 of 37
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`U.S. Patent
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`Jul. 31, 1984
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`Jul. 31, 1984
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`Jul. 31, 1984
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`Jul. 31, 1984
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`Jul. 31, 1984
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`Jul. 31, 1984
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`

`
`1
`
`DROPLET GENERATING METHOD AND
`APPARATUS THEREOF
`
`4,463,359
`
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`The present invention relates to a droplet generating
`method and apparatus therefor, and more particularly
`to a method and apparatus therefor for emitting a drop-
`let from an orifice communication with a small liquid
`chamber containing liquid therein. More specifically,
`the present invention providesa method for repeated
`high-speed generation of the droplets and an apparatus
`for allowing arcuate generation of droplets of a uniform
`diameter.
`2. Description of the Prior Art
`In the related field there are already known ink jet
`recording apparatus for example of drop-on-demand
`type, which are recently attracting particular attention
`because of their negligibly low noise and absence of 20
`unnecessary ink being deposited at the recording. Such
`recording is considered particularly useful in the ability
`of recording on plain paper without a particular fixing
`treatment. In the field of such drop-on-demand type ink
`jet recording there have been proposed various appara-
`tus some of which are already in commercial use, while
`others are still in the courseof development.
`,
`In summary, the ink jet recording of the drop-on-
`demand type performs recording by emitting a droplet
`of recording liquid, called ink, from a small orifice in
`response to an instruction signal _and depositing the
`droplet onto a recording material. In the known meth-
`ods, the droplet generation is achieved for example by
`the use of a piezoelectric element. <
`The present invention relates to a novel drop-on-
`demand type ink jet recording method which is differ-
`ent from the conventional method utilizing the piezo-
`electric element and which effects the droplet emission
`from a small orifice by applying a drive signal to the
`liquid introduced to a small liquid chamber thereby
`causing bubble formation in the liquid. Also in a related
`field U.S. Pat. No. 3,878,519 discloses another appara-
`tus, which while not requiring pressurizing and deflect-
`ing means, does provide droplets of insufficient even-
`ness because of a weak droplet forming force and fur-
`ther requires a liquid recovery mechanism for unused
`droplets, and thus cannot be compactized.
`SUMMARY OF THE INVENTION
`
`A principal object of the present invention is to pro-
`vide a solution to technical problems which have not
`been resolved in this technical field by the conventional
`technology.
`_
`Another object of the present invention is to provide
`apparatus well adapted for example for high-speed.
`droplet emission without the trouble of the defect or
`lack of droplet emission.
`Still another object of the present invention is to
`provide apparatus having a very short induction period
`before reaching a stable droplet emission state and
`achieving a gradual retraction of the liquid meniscus
`after the droplet emission.
`Still another object of the present invention is to
`provide a droplet emitting apparatus allowing easy
`maintenance.
`The droplet emitting apparatus embodying the pres-
`ent invention provides stability in size of emitted drop-
`lets, a stable emission period, a high emission frequency,
`
`2
`a significant compactization of the droplet emission
`head because of a much simpler structure and ease of
`fine mechanical working thereof, and further provides
`for the use of a multiple-nozzle head indispensable for
`example to a high-speed recording apparatus owing to
`such a simple structure and easy mechanical working.
`In addition the apparatus permits an arbitrary array
`structure of the emitting orifices in the designing of a’
`multi-nozzle emission head, thus permitting the use of a
`block structure in the emission head suitable for mass
`production.
`'
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIGS. 1, 2 and 3 are explanatory schematic views
`showing the working principle of the present invention;
`FIG. 4 is a waveform chart showing various drive
`signals;
`FIG. 5 is a view showing schematically the structure
`of the droplet emission head;
`FIGS. 6A and 6B are a partial perspective view and
`a cross-sectional view of the head, respectively;
`FIG. 7 is a cross-sectional view of another embodi-
`ment of the invention;
`FIG. 8 is a schematic circuit diagram showing an
`example of the drive circuit;
`FIGS. 9 and 10 are waveform charts showing the
`drive signals;
`.
`FIG. 11 is a schematic view of color recording;
`FIGS. 12A, 12B and 12C are views showing the
`examples of the head structure;
`FIG. 13 is a block diagram showing an example of the
`drive circuit;
`_
`FIG. 14 is a perspective view showing another exam-
`ple of the head structure;
`FIG. 15 is a perspective view of the emission head of
`a cassette structure;
`FIG. 16 is a cross-sectional view of the ink supply
`section;
`,
`FIG. 17 is a perspective view of an example of the full
`multiple head;
`FIG. 18, is an enlarged front view of a part of the
`head;
`FIG. 19 is a perspective view showing another exam-
`ple of the cassette structure;
`FIG. 20 is a perspective view showing a full multiple
`head obtained therefrom;
`FIG. 21 is a circuit diagram showing an example of
`the drive circuit;
`FIG. 22 is a waveform chart showing the drive sig-
`nals;
`FIGS. 23A and 23B, when combined as shown in
`FIG. 23, are a circuit diagram showing another example
`of the drive circuit;
`FIG. 24 is a waveform chart showing the drive sig-
`nals therefor;
`FIGS. 25, 26 and 27 are circuit diagrams showing
`other examples of the drive circuit;
`FIG. 28 is a perspective view showing still another
`example of the head;
`FIG. 29 is a cross-sectional view of the head;
`FIGS. 30 and 31 are cross-sectional views showing
`2
`still other examples of the head;
`FIG. 32 is a perspective view showing still another
`example of the head;
`FIG. 33 is an enlarged front view showing the head
`and the relation thereof to the reading sensor;
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`_FIG. 34 is a perspective view showing an example of
`the original reading unit;
`FIG. 35 is a schematic block diagram showing an
`example of the control-unit for the entire apparatus;
`FIGS. 36 and 37 are waveform charts showing the 5
`drive signals;
`FIG. 38 is a partial view showing the configuration of V
`the memory;
`FIG. 39 is a chart showing the locations of the mem-
`ory contents at the information, reading;
`FIGS. 40A and 40B, when combined as shown in
`FIG. 40, are a flow chart showing the operations
`thereof;
`FIG. 41 is a View showing an example of the struc-
`ture of the reading and recording heads;
`FIG. 42 is a circuit diagram showing an example of
`the drive circuit for the recording head;
`._FIG. 43 is a schematic block diagram thereof;
`FIG. 44 is a waveform chart showing the operations
`thereof;
`FIG. 45 is a circuit diagram showing another example
`of the drive circuit;
`FIG. 46 is a schematic block diagram thereof;
`FIG. 47 is a view showing another example of the
`head structure;
`FIG. 48 is a view showing still another example of the
`head structure;
`FIGS. 49X and 49Y are cross-sectional views thereof;
`FIGS. 50 and 51 are schematic views of still other
`examples;
`FIG. 52 is a perspective View of still another example
`of the heat-generating unit of the head;
`FIG. 53 is a schematic view of an example of the
`apparatus;
`FIG. 54 is a perspective view of the drum unit;
`FIG. 55 is a cross-sectional view of an example
`thereof;
`'
`’
`FIG. 56 is a schematic view showing the principle for
`correcting the head position;
`FIG. 57 is a cross-sectional view of an example of the
`apparatus;
`g
`FIG. 58 is a perspective view of an example of the
`vertical adjust means;
`FIG. 59 is a perspective view of an example of the
`mechanism for changing the ink emitting direction;
`FIG. 60 is a cross-sectional view useful in understand-
`ing the principle of bubble elimination; and _
`,
`FIG. 61 is a cross-sectional view of an example of the
`ink chamber.
`
`4
`and the liquid present in the chamber W1 is displaced
`rapidly in the direction of the orifice OF and in the
`opposite direction by an amount equal to the volume
`increase of the formed bubble B. Consequently a part of
`the liquid present in the portion 1 of the chamber W is
`emitted from the orifice OF. The emitted liquid consti-
`tutes a liquid column, of which thefront end accumu-
`lates the kinetic energy supplied thereto until ‘the
`growth thereof is terminated. In case the bubble B col-
`lides with the ceiling of the liquid chamber W1, the
`colliding force is diverted in the longitudinal direction
`to enhance the droplet propelling force.
`Upon termination of the drive signal supplied to‘ the
`heating element H1, the temperature thereof gradually
`lowers so that the bubble B initiates volume contraction’
`'after a slight delay in time. Along with the volume
`contraction, the liquid is replenished into portion A]
`from the direction of the orifice OF and from the oppo-
`site direction. In this manner the liquid present in the
`vicinity of the orifice OF is retracted to the chamber
`W1, so that the kinetic energy of the front end portion
`of the liquid column is directed opposite to that of the
`portion of the‘ liquid column close to the orifice OF.
`Thus the front end portion becomes separated from the
`liquid column to constitute a minute droplet ID which -
`flies toward a member PP and is deposited on a deter—'»
`mined position thereon. The bubble B on the heating
`element H1 gradually disappears by heat dissipation.
`The gradual annihilation of the bubble B causes a slow
`retraction of the meniscus IM while maintaining a stable
`surface thereof, so that it is possible to resolve the prob-
`lems attended to subsequent droplet emission resulting
`from excessive meniscus retraction caused by air intro-
`duction from the destructed meniscus. The position of
`the bubble formation should be selected suitably since
`the bubble B itself may also be emitted from the orifice
`OF to destroy the droplet ID if the position is exces-
`sively olose to the orifice IF while a bubble generated‘
`an excessive distance from the orifice IF may be unable
`to cause the droplet emission. The aforementioned
`gradual contraction of the bubble B is caused by the
`heat dissipation of the heating element (since the trailing
`down time of a thermal signal is longer than the leading
`rise time thereof), bubble or liquid, condensation to
`liquid, capillary liquid supply or the combinations
`thereof.
`The dimension of the droplet ID emitted from the
`orifice OF is dependent on the parameters of the appa-
`ratus such as the quantity of energy appplied, width A1
`of the portion subjected to the energy application, inter-
`nal diameter d of the liquid chamber W, distance 1 from
`the orifice OF to the heating element H1 etc., and the
`physical properties of the liquid IK such as specific
`heat, thermal conductivity, thermal expansion coeffici-
`ent, viscosity, etc. Also the aforementioned heating
`element may be replaced by instantaneous irradiation
`with a laser pulse LZP which ‘similarly causes rapid
`generation and gradual annihilation of the bubble B to
`emit a droplet. In such a case the element H1 in the
`portion Al may be utilized, if desirable, as a reflector or
`a heat accumulator for improving the heat generation
`by the laser pulse LZP. Furthermore the liquid IK is not
`necessarily limited to a recording liquid but also in-
`cludes other liquids such as water, solution of chemicals
`or fertilizers etc.
`FIG. 2 schematically shows the procedure of liquid
`emission in the steps ‘of ti] to t9, wherein there are
`shown the liquid chamber W, heating element H1 and
`
`'
`
`45
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`The present invention will now be explained in detail
`on the embodiments thereof shown in the attached
`drawings.
`.
`Reference is now made to FIG. 1 showing, in a sche-
`matic view, the principle of droplet emission by the
`droplet emission head of the present invention.
`A liquid chamber W constituting the emission head is
`supplied with liquid IK. Upon receipt of a drive signal,
`a heating element H1 having a width Al and located at
`a distance 1 from an orifice OF initiates the temperature
`rise. When heating element H1 reaches a temperature
`above the evaporating point of the liquid contained in '
`the chamber‘ W1, a bubble B is formed on heating ele-
`mentH1. With the rise of temperature thereof, bubble B ’
`rapidly increases the volume thereof. As the result the
`pressure in the liquid chamber W1 rapidly increases,
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`orifice OF, and the liquid IK is supplied by capillary
`action from a direction P. FIG. 3A shows an example of
`the drive pulse, wherein times t0—t9 respectively corre-
`spond to those in FIG. 2. FIGS. 3B and 3C respectively
`show the temperature change of the heating element H1
`and the volumic change of the bubble B. The time t0
`represents the state prior to the droplet emission, and a
`drive pulse E is supplied to the heating element H1 at a
`time tp between t0 and t1. As illustrated, the heating
`element H1 initiates the temperature rise simultaneously
`with the application of the drive pulse E. At time t1 the
`heating element reaches a temperature exceeding the
`vaporizing point of the liquid IK whereby bubbles B
`appear and the meniscus IM becomes expanded from
`the orifice corresponding to the displacement of the
`liquid IK by bubbles B. At time t2 the bubbles B are
`further developed to form a more protruding meniscus
`IM. The meniscus IM becomes further expanded at time
`t3 when the drive pulse E is terminated as shown in
`FIG. 3A and the heating element H1 assumes a highest
`temperature as shown in FIG. 3B. At time t4, although
`the temperature of the heating element H1 is already
`descending as shown in FIG. 3B, the bubble B reaches
`a largest volume as shown in FIG. 3C so that the menis-
`cus IM is even larger. At time t5 the bubble B starts to
`contract so that thelliquid IK is retracted into the liquid
`chamber W from the protruding portion corresponding
`to the volumic contraction of the bubble B, whereby the
`meniscus IM develops a constricted portion Q. At time
`t6, the droplet ID is separated from the meniscus IM’ 30
`due to further advanced contraction of the bubble B. At
`time t7 the liquid droplet ID is completely emitted,
`while the meniscus IM’ further approaches the face of
`the orifice OF due to the continued contraction of the
`bubble B. At time t8 where the bubble B is close to 35
`annihiliation, the meniscus IM is further retracted to a
`position inside the orifice OF. At time t9 the liquid IK
`is replenished to assume the original state t0.
`As will be apparent from the foregoing, the form of
`the drive signal supplied to the heating element H1 is an 40
`important factor for achieving stable emission of the
`liquid IK. Also important for droplet separation is the
`contraction of the bubble which can however be easily
`controlled by the form of the drive signal. Furthermore
`it is possible to increase the droplet emitting frequency
`by the form of the drive signal.
`FIG. 4 shows various examples of the drive signal
`and the corresponding temperature changes in the heat-
`ing element H1 and the volumic changes of the bubble
`B.
`
`These drive signal pulses are capable of satisfactory
`droplet emission. The waveform (a) is particularly ad-
`vantageous in not requiring a special provision in‘ the
`drive circuitry for the high resistor in the CR discharge
`circuit in case piezoelectric drive is not required. The 55
`waveform (b) performs a pre-heating before the start of
`the pulse thereby reducing the pulse width for droplet
`emission. The waveform achieves a rapid bubble devel-
`opment and is effective for improving the emission
`speed and the emission frequency. Also the pre-heating,
`conducted only at the droplet emission, prevents exces-
`sive heating of the liquid. The waveform (c) performs a
`post-heating subsequent
`to the drive pulse thereby
`achieving a further gradual retraction of the meniscus
`after the droplet is separated. The waveform is effective
`in avoiding the air introduction into the liquid chamber
`after the droplet emission, thus ensuring smooth emis-
`sion for the subsequent droplets. Also in this case the
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`post-heating is effected only at the recording so that the
`bubble is completely annihilated to ensure the subse-
`quent droplet emission. The waveform (d) involves a
`gradual heat dissipation for realizing a smooth droplet
`separation and preventing excessive meniscus retraction
`thereafter, and is effective in achieving a gradual menis-
`cus retraction without losing the droplet speed. Also
`the waveform (e) is an effective drive signal obtained
`from the combination of the waveforms (b) and (d).
`In either case it is possible to realize very gradual heat
`dissipation of the heating element and bubble contrac-
`tion merely by controlling the drive signal and without
`the use of, for example, an external high resistance. This
`prevents the trouble of lack of droplet emission upon
`receipt of the subsequent emission instruction pulse,
`which results from a rapid meniscus retraction induced
`by air introduction from the orifice. The preferred rela-
`tion of the generation and contraction of the bubble
`with the drive signal is determined by the pulse width S
`and pulse height L. The waveform (a) is preferred in
`consideration of the function of the large-scale inte-
`grated circuits, and particularly preferred in the case
`where a laser pulse is used. Also in the case of a solid-
`state laser it is quite easy to control the intensity of the
`laser pulse and to obtain waveforms similar to those of
`(b) to (e), thus achieving control over the heat genera-
`tion or the bubble behavior by the laser light. Conse-
`quently the term “heating element” to be used hereinaf-
`ter shall include other heat generating means such as a
`laser beam or infrared beam.
`FIG. 5 shows an example of the head structure in a
`schematic exploded perspective view, in which a sub-
`strate SS1 is provided on the surface thereof with heat-
`ing elements H1—H7, a common electrode D1 and se-
`lecting electrode 11-17. Heating elements H1—H7 are of
`the same area and the same resistance and are positioned
`respectively corresponding to the liquid chambers. A
`plate GL1-is provided with a liquid supply inlet IS,
`small grooves M1—M7 constituting the liquid chambers
`and a common groove MD for supplying the liquid to
`the liquid chambers. Grooved plate GL1 is further
`provided, if necessary, with an orifice plate (not shown)
`at the droplet emitting side. Grooved plate GL1 is com-
`posed of a glass plate which is subjected to an etching
`process for forming common groove MD and plural
`grooves Ml—M7, which are subsequently combined
`with the substrate SS1 to constitute a plurality of liquid
`chambers. Consequently grooves M1—M7 are so ad-
`hered as to respectively correspond to the heating ele-
`ments. The heating elements H1—H7 selectively effect
`heat generation of an energy level corresponding to the
`input signal level. It is also possible to use substrate SS1
`as a simple liquid support, instead of mounting the heat-
`ing elements thereon, in which case a solid-state laser,
`for example, a semiconductor laser head LZH slidably
`mounted on a carriage guide CG, is intermittently or
`continuously displaced to selectively irradiate those
`grooves with laser pulses LZP of a determined length
`through the plate GL1. Otherwise plural laser heads
`LZH may be fixedly provided in plate of the heating
`elements. FIGS. 6A and 6B show, in a partial view, the
`details of the substrate SS1 having the heating elements
`H1—H7 thereon. A substrate AM for example composed
`of aluminum is provided thereon with a heat accumulat-
`ing layer SO (several micrometers), a heat-generating
`resistance layer H composed of Zrl§2 (800 A) and an
`aluminum electrode layer AL (5000 A) which are selec-
`tively etched to form heating elements H1, H2, H3 etc.
`
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`each of 60 pm in width and 75 um in length, the com-
`mon electrode D1 and selecting electrodes 11, 12, 13 etc.
`As shown in FIG. 6B the heating elements and the
`electrode layer AL are overlaid by a Si02 protective
`layer K (1 pm).
`FIG. 7 shows, in a cross-sectional view, another em-
`bodiment of the droplet emitting heat basically similar
`to the foregoing and provided with plural heating ele-
`ments HI, HII, HIII, etc. for controlling the tonal rendi-
`tion. As illustrated therein the substrate SS1 provided
`with heating elements HI, F111 and H111 is placed on a
`metal heat sink HS and is covered with the grooved
`plate GL1 as explained in the foregoing to constitute a
`liquid chamber S at
`the junction therebetween.
`Grooved plate GL1 is provided with a liquid supply
`inlet IS and a stopper FF with an O-ring OR for bubble
`removal at the liquid filling and for nozzle cleaning.
`Supply inlet IS is provided with a filter FL for remov-
`ing minute dusts, a filter holder block FH for supporting
`the filter, a pipe holding rubber piece for supporting a
`pipe IP for external ink supply, and a rubber piece
`holder RH for supporting the rubber piece. At the front
`end of the liquid chamber W there is provided an orifice
`plate OP for obtaining a droplet of a desired shape, the
`orifice plate‘ being however dispensed with in case the
`liquid chamber W itself is structured to constitute the
`orifice as shown in FIGS. 5 and 6.
`As exaggeratedly illustrated in FIG. 7, the liquid
`chamber W is provided, along the longitudinal direc-
`tion thereof, with plural heating elements I-II, H11 and
`H111 which are selectively energized to cause a state
`change in the adjacent liquid, involving the generation
`and annihilation of bubbles as explained in the forego-
`ing, which are schematically illustrated as a single bub-
`ble B. The volume change induced by the generation of 35
`bubble B in the liquid chamber causes the emission,
`from the orifice plate OP, of a droplet IDI,' IDII or
`IDIII different in dimension due to a tonal rendition
`explained in the following. Namely if the heating ele-
`ments HI, H11 and H111 are formed with different thick-
`ness or length to have different resistances,
`it
`is ren-
`dered possible to generate a bubble corresponding to
`the applied energy and to vary the volume of the drop-
`let according to the energy, thus obtaining droplets of
`different sizes. Also a similar effect can be obtained by 45
`selecting plural heating elements simultaneously or~in
`succession.
`FIG. 8 is a block diagram showing control circuitry
`for selectively driving five heating elements as shown in
`FIG. 7. Input analog signals supplied from an input
`terminal 20 are introduced, through buffer circuits 211
`to 215, to comparators 221-215. The comparator 221 is
`designed to release an output signal in response to a
`lowest
`input
`signal
`level, and other comparators
`222-225 are designed to respond to successively high
`input signal levels.
`The output signals from comparators 221-225 are
`respectively supplied to AND gates 261-264 of a gate
`circuit 26, whereby only one gate corresponding to the
`input signal level is enabled. A drive circuit 27 is acti-
`vated by the output signal from the comparator 221 to
`an output signal of a determined pulse width and pulse
`amplitude to AND gates 281-285, of which onlyvone is
`opened by the output signal selected in the gate circuit '
`26 to transmit the output signal from the drive circuit27
`to one of the output terminals 291-295. Assuming that
`the terminal 291 is connected to a heating element of the
`highest resistance while the terminal 295 is connected to
`
`8
`a heating element of the lowest resistance, the former or
`the latter is energized respectively corresponding to a
`low-level input or a high-level input. In case the input
`signals are level-indicating digital signals, those com-
`parators can be dispensed with and the input signals
`directly select the gate circuits and selectively drive the
`corresponding heating elements. Also different resis-
`tances of the heating elements may be achieved by using
`different materials instead of using different dimensions.
`In contrast to the foregoing embodiment in which the
`diameter of the emitted droplet is modified by control-
`ling the heating energy through the selection of plural
`heating elements of differen