CA1082282A - Method and apparatus for harmonic ink jet drop generation - Google Patents

Abstract

METHOD AND APPARATUS FOR HARMONIC INK JET DROP GENERATION
Abstract Modulation of a continuous fluid jet stream by an electrically responsive transducer driven by a sine wave and a second harmonic of the sine wave of selected amplitude and phase to cause the stream to break into a stream of uniform satellite-free drops.

Description

Back~round~of the Invention The-need for high quality printers with changeable formats has been evidénced in recent years.~ Developments ~have proceeded with respect to ink jet technologies to - ~ ~ answer this needr A technology that has been the subject of ~considerable development effort is the electrostatic pres-sure system. The major types of electrostatic pressure systems are the deflected type such as taught by Sweet, U.S.
20 Patent 3,596,275, and the binary type such as taught by Sweet et al, U.S. Patent 3,373,437.
Both types of systems project;one or more streams o~
ink which are perturbated to break into streams of drops.
In the de~lected type of sy tem, each drop in a single stream of ink drops is selectively charged at drop breakoff ~ ~ and passed through a uniform deflection field to impact -; various locations on a recording medium in accordance with the quantum of the-charge. Thus, by applying suitable ~ charging signals to the drops, a visible human-readable printed record may be formed on the recording medium. The binary type of system employs a pluralit~ of jets ;

1 in one or more rows, selectively charc~inq dro~s at dro~ brea'f~off

2 with a single charge amplitude to be deflected by a constant

3 field to an ink drop catcher. The uncharqed drops continue

4 along the original jet stream paths to impact a recordina medium. A visible human-readable printed record may thus be 6 formed by leaving uncharged those drops re~uired for printinq 7 during relative head to record medium motion~
8 Both types of electrostatic pressure ink jet systems 7 9 are highly dependent upon insuring that the drops receive only the intended charge so that the drops are then deflected a 11 predetermined lateral distance. The drop formation by pertur-12 bating the jet streams generally results in the formation of 13 both individual drops and at least temporary satellite drops 14 which subsequently merge with one of the adjacent drops. The perturbation may be by varying the pressure of the ink prior 16 to ejection from a nozzle, by vibrating the nozzler or by other 17 suitable means. The perturbation introduces varicosities into 18 the stream filament which gradually increase to form drops and 19 to pinch off the ink stream between drops. The pinchinq off is called drop breakoff.
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21 It has been observed that the satellites are formed 22 from a li~uid ligament portion of the ~et stream interconnecting 23 two large varicosities in the stream which become drops.
24 The ink employed in electrostatic ink jet is electrically conductive. Drops are charged by exposing the drop stream to 26 a selected charge signal at an adjacent charae electrode at the 27 point of drop breakoff. The ink is grounded inside the head, but 28 the pinched portion of the stream becomes, in effect, an open 29 circuit as breakoff occurs such that the ink beyond the break-off point retains a charge.

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1 Any ligament which becomes a satellite may first break off at the front and temporarily remain a part of the stream ; or may first break off at the rear and temporarily be a part of the drop it follows.
In the situations where breakoff occurs at rear of the ligament first, the satellite will be charged by the same signal as the drop it follows, whether the charge signal is then switched prior to or subsequent to the breakoff at the front of the ligament. If the satellite subsequently merges with the preceding drop (forward merge), the resultant drop will be properly charged. However, if the satellite merges with the succeeding drop (rearward merge), a portion of the charge intended for the preceding drop is transferred by the satellite to the succeeding drop.
In the situations where breakoff first occurs at the front of the ligament, the relative timing of rear breakoff and charge voltage switching becomes important. If the charge signal is switched prior to rear breakoff of the ligament, the satellite will be charged differently than the preceding drop. If the satellite subsequently forward merges, charge transfer will occur. Charge transfer will not occur if the satellite rearward merges. If the charge signal is switched subsequent to rear breakoff of the liga-ment, the satellite will then be charged the same as the preceding drop. Thus, charge transfer will not occur upon a forward merge of the satellite, and charge transfer will occur upon a rearward merge of the satellite.
The aforementioned U.S. Patent 3,928,855 broadly describes satellite control using asymmetric perturbation.
The satellite control results in drop generation without satellite generation or with forward or rearward merge of . ~, ~, , ~ - . , ~- , ' Z~82 1 satellites. However, the embodiment of the aforementioned U.S. Patent 3,928,855 concerns magnetic ink jet which employs magnetic deflection of ferrofluid drops. Hence, the drops are not charged and charge transfer is not a concern. The aforementioned U.S. Patent therefore describes no method or apparatus specifically arranged for preventing charge transfer.
Satellite control has also been a subject of Keur et al, U.S. Patent 3,683,396, which claims a reduction of a satel-lite problem by designing the nozzle structure to have a mechanical resonance at the drop generation frequency. This however specifically assumes that a ligament first breaking off at the rear will forward merge and vice versa, a condi-tion that is not assured.
Stauffer, U.S. Patent 3,334,351, also references satellite control thorugh the use of two separate vibration means acting in different directions to thereby impart a rolling motion to the ink drops to induce a merging of any satellites. The problem of possible charge transfer through merger of satellites and drops is not considered, however.
The result of charge transfer is that the drops receive or lose unintended charges and are therefore impro-perly deflected by the constant deflection field. For example, in the binary type of system, an "uncharged" drop receiving some charge by charge transfer may be deflected slightly by the deflection field and impact the recording medium at an unintended location. A "charged" drop losing some charge may be insufficiently deflected and impact a gutter or drop catcher at an unintended point and splatter ~
or bounce off and impact the recording medium. The deflec- -tion error may involve 25% of the total deflection distance.

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1 Summary of the Invention 2 It is therefore an object of -the present invention to 3 provide an improved method and apparatus for control of satellites 4 in a fluid jet stream.
In accordance with the present invention, there is provided for a fluid jet head including an inlet for pressurized 7 fluid, a manifold cavity communicating with the inlet, and at 8 least one nozzle orifice communicating with the manifold cavity 9 for projecting a fluid stream therefrom, the improvement of an electromechanical transducer for perturbatinq the fluid stream 11 to break the fluid stream into a stream of drops and an electric 12 signal generation means for supplying to the transaucer a funda-13 mental sine wave at the drop repetition rate and a secona harmonic 14 of the fundamental at the selected amplitude and phase with respect to the fundamental that causes ligaments forminq satel-16 lite drops to first break from the fluid stream at the end 17 opposite the drop with which the satellite merqes. Thus, the 18 amplitude and phase are selected which cause the liqament 19 forming a forward merging satellite to break irst at the end closest the fluid stream, and vice versa.
21 An advantage of the invention is that the control over 22 charge transfer is particularly useful in multi~orifice heaas 23 which have slightly differing charges is maintained larqely 24 independently of fine mechanical and fluid characteristic considerations.
26 , The foregoing and other objects, features a~d advantages 27 of the invention will be ap~arent from the following more 28 particular description of preferred embodiments of the invention, 29 as illustrated in the accompanying drawinqs.

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: ' , ' ~' ' ' :` .' ~ '' '' ' . ' 1 Brief Descri~tion of the Drawinqs 2 FIGURE 1 is a frontal and partiall~ schematic view of 3 an exemplary electrostatic ink jet head and electrode assembly 4 for implementing the present invention;
FIGURE 2 is a cross section view through the electro-6 static ink jet asse~ly of Figure 1;
7 FIGURE 3 is another cross section view through the 8 electrostatic ink jet assembly of Figure 1 with a partial.
¦ 9 schematic showing;
FIGURE 4 is an isometric view of the nozzle plate of 11 the electrostatic ink jet assembly of Fiqure 1;
12 FIGURE 5 is an isometric view of the charqe plate of 13 the electrostatic ink~jet assembly of Figure 1, 14 FIGURE 6 is a schematic circuit diaaram of an electric signal generator in accordance with the present invention-16 FIGURE 7 shows multiple fluid jet drop streams generated 17 with only the fundamental frequencyJ
lR FIGURE 8 shows multiple fluid jet drop streams generated 13 in accordance with the present invention;
FIGURE 9 is a cross-section view of a sinqle nozzle 21 ink jet head for implementing the present invention 22 FIGURE 10 shows single fluid jet drop streams generated 23 with only the fundamental frequency;
24 FIGURE 11 shows single fluid jet drop streams generated ~ 25 in accordance with the present invention;
26 FIGURE 12 is a schematic circuit diaaram of an alternative 27 electric signal generator in accordance with the present invention:
2~ FIGURE 13 is a detailed circuit diaqram of the attenuators 29 of Figure 12; and .. . . . : . .

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~ GURE 14 is a detailed circuit diaqram of the inteqrators 2 of Figure 12;
3 Description of the Preferred ~mbodiments _ _ _ _ _ _ ... .. .. . . ..
4 Referring to Figures 1 throuqh 3, an ink jet head assembly is shown for an eleckrostatic pressure ink jet s~stem of the 6 binary type. The head assembly includes a cavity plate 10 having 7 a manifold cavity 11 formed therein. Mounted wi~hin the manifold 8 are a piezoelectric crystal 12 and a nozzle plate 13. Referrinq 9 additionally to Figure 4, the nozzle plate includes two rows 14 ànd 15 of closely spaced ink jet orifices. The piezoelectric 11 crystal 12 is mounted on a mounting plate 16, which is clamped 12 to the cavity plate 10. A sealing ring 17 is mounted in slot 13 18 to provide a fluid seal of manifold 11 between cavity plate 14 10 and piezoelectric crystal 12. A charqe plate 20 is mounted on cavity plate 10 and is provided with two rows of charge 16 electrodes 21 and 22, each charge electrode being aliqned with 17 a corresponding orifice of nozzle plate 13. The charge plate 18 is illustrated in detail in Figure 5. Also aligned wi-th orifices 19 14 and 15 and charge electrodes 21 and ~2 r are oPenings 23 and 24 in cavity plate 10. These openings allow passaqe of ink jet 21 streams from the orifice plate, ~s shown in Figure 5 r the charge 22 plate 20 is also provided with a series of conductive lands 25, 23 each of which individually connects a charge electrode to a sepa-24 rate data source.
Referring to Figure 3, pressurized ink from fluid 26 supply system 26 is supplied via line 27 to connector 28 for 27 transmission through passage 29 in mounting block 16 and cavity 28 plate 10 to the interior of cavity 11. Passage 29 is sealed 29 at the juncture between mounting block 16 and cavity plate 10 ~o by means of O-ring 33.

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~822~2 1 The pressurized ink in manifold 11 is then ejected 2 through orifices 14 and 15 of nozzle plate 13. The piezoelectric 3 crystal driver 12 is then perturbated as will be explained to vary the internal volume of cavity ll. This perturbates the ink pressure, causing the ink jet streams emanating from ori~ices 14 and 15 to break into streams of uniformly sized drops. The 7 ink emanates from orifices 14 and 15 in the form of filaments 8 passing through openings 23 and 24 with the perturbations increas-g ing as the distance from orifice plate 13 increases. At some point within the charge electrodes 21 or 22, the drops break off 11 from the filaments. The ink in cavity 11 is grounded through 12 cavity plate lO to grounding terminal 34. Selected ones of the 13 charge electrodes 21 or 22 may be given a voltaqe signal, which 14 induces a corresponding signal in the filament within the corre-sponding charge electrode. At the moment of drop breakoff, the 16 conductive path between the drop or drop-and-liqament and the 17 grounded stream is broken. The drop or drop-and-liqament thus 18 retains a charge corresponding to the applied volta~e of the l9 charge electrode.
Uncharged drops proceed alona paths 30 and 31 to impact 21 a recording medium 32. A high voltaqe deflection plate 35 is 22 positioned intermediate the two drop flow paths 30 and 31.
23 Grounded deflection electrodes 37 and 38 are positioned respec-24 tively on the opposite sides of drop paths 31 and 32 from hiqh voltage deflection electrode 35. Deflection electrodes 37, 38 26 curve away from the drop paths and terminate in openin~s 41 and27 42 which communicate with cavities 43 and 44. The cavities 28 further communicate with tubes 45 and 46 which are connected to 29 a vacuum source 50 by, respectively, lines 52 and 53.

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~D8Z'~2 1 Electrostatic fields established between electrode 35 2 and electrodes 37 and 38 thus cause charged drops to be deflected 3 from normal uncharged drop paths 30 and 31 to contact, respectively, 4 electrodes 37 and 38. Electrodes 37 and 38 therefore also serve as gutters to intercept the drops which are deflected and not used 6 for recording pur~oses. The intercepted drops flow to the ends 7 of their respective electrodes and are drawn through the respec-8 tive opening 41 or 42 into cavity 43 or 44 by the vacuum source 9 50~ Accumulated ink is drawn from cavity 43 or 44 through the respective tube 45, 46 to the vacuum source 50. The ink may then 11 be recycled for subsequent recording use. In the example shown, 12 high voltage electrode 35 is hollow, havinq each side covered by 13 a fine mist screen 63. Thus, any ink mist is drawn throuqh the 14 screen and through a hollowed out screw 68 containing a passage 70 sealed from the atmosphere by O-ring seal 72. Passage 70 16 also communicates with tube 71 which is connected to vacuum 17 source 50 by line 73. Any ink mist is thus drawn therethrough 18 into the vacuum system 50.
19 Proper electrical considerations require that high voltage electrode 35 be insulated from the grounded electrodes and 21 support structure by means of insulators. This is accomplishea 22 b~ forming mounting blocks 78 and 79 from an insulating material 23 and by connecting vacuum line 73 to the vacuum system 50 separately 24 from lines 52 and 53 to prevent a short circuit through the con-ductive ink. Further, charge electrode plate 20 is made of an 26 insulating material, such as plastic, to prevent conduction be-27 tween various ones of the lands 25 or to prevent conduction be-28 tween various ones of the charge electrodes 21 and 22. The lands 29 may further be covered by an insulative and protective coating.

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1 Referrinq now in more detail to the ink iet perturhation, 2 piezoelectric driver 12 is held between cavity plate 10 and 3 mounting block 16 by the pressure of sealinq rinq 17. A cavity 4 80 is formed in mountinq block 16 behind the crvstal driver. A
wire 81 is connected between drive input 82 and the rear face of 6 the piezoelectric crystal driver 12. The front face of the crystal 7 driver is in contact with the electrically conductive ink in 8 cavity 11. The ink thus forms a conductive path between the face 9 of the crystal driver and mountinq block 10, which is qrounded at terminal 34. Thus, any electrical perturbation a~pearinq at input 11 82 is applied between the rear face of the Piezoelectric crystal 12 driver 12 and the grounded front face thereof.
13 As previously discussed, perturhation of the ink, such 14 as pressure perturbation due to the chan~e in volume of ca~7itY 11 by e~pansion and contraction of piezoelectric crystal driver 12, 1~ introduces varicosities into the ink jet stream which ~radually 17 increase to form drops and to pinch off the ink stream hetween 18 drops. The pinching off is called drop break-off.
19 Referrinq to Fiqure 7, a pluralitv of ink jet streams are shown emanating from a multi-orifice head. In the example shown, 21 the ink in the head is pressurized at approximately 22.5 psi and 22 issued from orifices of approximately 1 s~. mil area with an 23 80Khz perturbation rate. Each of the fluid filaments comprises 24 a series of ever larqer varicisities 90 and 91 connected by ligaments 92. When a satellite 93 is formed, the liqament pinches 26 or breaks off generally first at one end and then at the other 27 end. As discussed extensively ahove, the particular se~uence of 28 front and rear breakoff and switchinq of the charae siqnal deter-29 mines whether the satellite 93 has the same charae as the preced-ing drop 94 or whether it has the same charge as the succeedinq ~8'~2~Z
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l drop 91. Whether or not charqe transfer oCCUr.C, then de~ends upon 2 whether the satellite 93 forward meraes with drop 9~ or whether 3 it rearward merqes with droP 91.
4 In the example of Fiqure 7, the timina of the front and rear breakoff of liqament 96 from drops 97 an~ 9R to form 6 satellite 99 is considerahly different from the timina o the 7 front and rear breakoff of liqaments g2 from droPs 90 and 91 to 8 form satellites 9~. The timinq of the charae sianals is the 9 same for each stream, howe~rer. Thus, some satellites mav have the same charge as the succeedinq drop. Charqe transfer may ll therefore occur in some, but not all, of the streams if all 12 the satellites rearward merqe or if all the satellites forward 13 mer~e.
14 Char~e transfer causes droPs to be incorrectly charqed and incorrectly deflected by the constant electrostatic deflec-16 tion field.
17 Referrinq to Fiqure 2, drops intende~ to be uncharaed 18 which acquire some charqe throuqh the effect of charae transfer l9 will not proceed alonq ~aths 3~ or 31 directlv to the recor~ina medium 32, but will be deflecte~ outwardlY t~A7ard the aroun~e~
21 deflection plates 37 and 38. By beinq displaced in o~osite 22 directions to incorrectly imPact the recordina medium 32, the 23 drops adversely effect print quality. Charqed drops which lose 24 a portion of their charqe throuqh charqe transfer but ac~uire a correspondinq amount of uncharqed fluid from the merqina satellite.
26 retain the proper mass but have an im~roper charae. Such drops 27 are deflected by the constant deflection fiel~ towards the grounded 28 deflection plates 37 and 38 by a smaller amount than oriainallv 29 intended. Rather than beinq properly intercepted by electrodes 37 and 38 which serve as qutter~s, the droPs may splatter or bounce ~O~ B~:

1 off to partially or whol.ly impact recordinq me-li.um 32 to ~urther 2 have a deleterious effect upon the print quality.
3 Mer~er of ligaments 92 or 96 without charae tran.sfer is 4 accomplished by ap~lyina the fundamental and a small amount o:F
second harmonic thereof as the drive siqnal to input 82 of the 6 perturbation means. A preferred emhodiment of circuitrv for ac-7 complishin~ this function is illustrated in Fiqure 6. An oscil-8 lator 110 o~erates at 32 times the ~undamental fre~uenc~ of lf, 9 which may comprise 100KHz. The oscillator out~ut is su~plied to the input of counter 111 and to AN~ inverter 112. Counter 11 111 is a four-position recyclable, binarv counter. The outPuts 12 from the four staqes are supplied on cable 114 to excluslve OR
13 115. The output of the counter on cable 114 thus continuallY
14 cyclically counts from 0 to 15, recvclinq at the 2f rate. ~ire 118 is connected to the fourth staae of the counter 111. The 16 fourth stage goes on at the count of 8 and qoes off when the 17 counter recycles to zero. Thus, the counter produces a s~uaxe 18 wave of frequency 2f on line 118, which is su~lie~ to attenuator 19 120.
Attenuator 120 is preset to control the am~litude level 21 of the 2f square wave. The attenuate~ s~uare wave is su~lied 22 on line 121 to filter 122. The filter is extremely narrow 23 and centered on the second harmonic fre~uency so a.s to shape 24 the square wave from attenuator 120 into a ~ine wave~ The second harmonic sine wave is su~plied on line 124 to analoa 26 adder or summer 125.
27 ~xclusive OR circuitr~ 115 com~rises four se~arate 28 exclusive OR circuits, each of which is connected to one of 29 the wires in cable 114 and a corresPondin~ one of the wire.s of cable 127. Circuit 128 comprises four 1 or 0 preset input.s ' . , : . ' .

-;22~2 , 1 to be supplied on four lines 127. These pre.set inPuts rePresent a binary count whose variance Erom the binary count of 8 is the ; 3 phase variance between the lf freauency and the 2f sianal on 4 line 118. Thus, each binary count difference rePresents 1/16th of a 180 phase shift, with preset inputs less than 8 represent~
~ 6 ing a negative shift of the 2f freauency with respect to the lf : 7 frequency, and preset inputs qreater than 8 rePreSentina a po.si-8 tive phase shift of the 2f fre~uency with resPect to lf. Thus, 9 when counter 111 reaches the count represented ~Y the preset inPut.s of circuitry 128, exclusive OR circuits 115 all pro~7i~e positi~Te 11 inputs on cable 130 to and inverter 112~ Simultaneously, oscillator 12 110 provides an input siqnal at input 131 of AN~ inverter 112.
13 AND inverter 112 therefore provides neqative goina outPUt pulse on 14 line 133 to flip-flop 134 for the half-cycle duration of oscillator 110. At the next cycle of oscillator 110, counter 111 moves to the 16 next binary state to terminate the output of at least one of the 17 exclusive OR's of circuitry 115. Thus, n-, further output is 18 supplied from A~D inverter 112 until counter 111 has made another 19 complete cycle to the count of preset inPuts 128~ Flip-flop 134 changes state in resPonse to each input pulse aPPearina on line 21 133, thereby underqoin~ a complete cvcle for every t~7O cycles 22 of counter 111. The flip-flop therefore serves as a fre~uencv 23 divider to produce a fre~uencv one-half that o the 2f fre~uency 24 on line 118~ Flip-flop 134 may be initially set in a 180 Pha.se ~ 25 shift from that of the 2f freauency by the original initialization 26 setting of the flip-floP.
27 This lf frequency output from flip-flop 134 is supPlied 28 on line 138 to attenuator 140. Attenuator 140 sets the level 29 of the lf square wave output of flip-floP 134 at the desired level, ~hich is substantially greater than the 2f output of 2~

1 attenuator 120. The output of attenuator 140 sup~lled on line 2 141 to filter 142. Filter 142 also has very narrow freauency 3 pass characteristics to convert the sauare wave siqnal on line 4 141 to a sine wave on line 144. Analoq adder 125 therefore combines the sine waves appearinq on lines 144 and 124 to produce 6 the combined fundamental and second harmonic comhined drive sianal 7 on line 150. The combined drive siqnal on line 150 is then 8 supplied to drive input 82 in Fiqure 2 to o~erate the piezoelect-9 ric crystal driver 12 in accordance with the su~plied sianals.
Alternatively, analog adder 125 may also include an am~lifier so 11 as to provide an input siqnal to terminal 82 which is pro~erlv 12 matched to the specific driver.
13 An example of multiple ink jet streams driven both with 14 a fundamental and a second harmonic com~risinq an amplitude of 1 volt as compared to 8~ volts for the fundamental siqnal, and at 16 a phase shift of 158 with respect to the fundamental are shown 17 in Figure 8. When controlled in this manner, each liaament 160 18 first breaks off from the succeedina drop 161 to thereby as~ume 19 the same charqe as the precedinq drop 162, and then forward merqes with the same preceding drop.
21 Therefore, whether or not a momentary satellite is formed, 22 the ligament 160 forward meraes with the same dro~ with wwhich it 23 was associated ~Ihen the charqe was acquired at charqe electxodes 24 21 or 22 in Fiqures 1 and 2. As the result of use of the subject inventionj no charqe transfer cccurs.
26 The invention is equally aavantaaeous in sinale nozzle 27 heads as it is in multi-orifice heads of Fiaures 1-5. Sinqle 28 nozzle heads are most often used in deflected tvpe of electro-29 static pressure fluid jet systems, such as tauqht by Sweet, above. The adverse effects of charqe transfer may be even more ~0~ 28~

important in this type of system which often finally controls 2 the charging of individual droPs by separatinq dro~s that im~act 3 adjacent spots by only three volts at the charae electro~e while 4 charging drops over a range greater than 250 volts. In such systems, drops having no charqe are quttered whereas the variahly 6 charged drops are swept in a sinqle column to form various part.s 7 of the printed characters within the column. Thus, charqe trans-8 fer from an uncharged to charqed dro~ or vice versa can very g seriously affect placement of the drop on the recor~ina medium.
An example of a sinqle nozzle ink jet head is illustrated 11 in Figure 9. The head includes a head body 170 with a cavity 171 12 formed therein. A jewel nozzle 172 havina an orifice 173 is 13 cemeted to the face of the head body 170 overlayina and sealina 14 cavity 171~ A piezoelectric driver 174 is secured aqàinst mounting block 175 and within the cavity. An O-rinq 17~ provides a fluid 16 seal of the cavity between the piezoelectric crystal driver 174 17 and head body 170. Pressurized fluid is supplied from a pressure 18 source to input 177 and transmitted by passaae 178 into cavity 19 171, for e~ection through orifice 173 of nozz~e 172. Pin 179 provides a means for electrically connectinq the rear of piezo-21 electric crystal driver 174 to the piezoelectric drive line 15n 22 of the driving circuit. The other si~e of the piezoelectric 23 crystal driver is grounded through the ink in cavity 171 and 24 head body 170 to a groundinq point, such as post 180.
Therefore, application of the drive siqnal from the 26 circuitry of Figure 6 to pin 179 perturbates piezoelectric crystal 27 driver 174, 204, which further perturbates the pressure of the 28 fluid in cavity 171 to produce ink jet streams such as shown 29 in Figure 11.

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1 As previously discussed, perturhation of the ink, such 2 as pressure perturbation due to the chance in volume of cavity 171 3 by expansion and contraction of piezoelectric crystal driver 172, 4 introduces varicosities into the ink jet stream which ~radually increase to form drops and to pinch off the ink stream between 6 drops. The pinching off is called drop breakoff.
7 Referring to Fiaure 10, the fluid filament perturbated by only the fundamental comprises a series of even larger vari-9 cosities 190 and 191 connected by ligaments 192. The portion of a ligament connectinq it to the adjacent drops ultimately 11 pinches off generally first at one end and then at the other end 12 to form a satellite drop 194. One of the charqe electro~es may 13 expose the drop stream to a selected charae sianal at the instant 14 of drop breakoff. The ink is grounded by h~ad hody 170 ! but the 1~ pinched portion of the stream brakes the conductive path as 16 breakoff occurs s~lch that the ink beyond the breakoff point re-17 tains a charge. Thus, liqament 192 first breaks off from the 18 preceding drop 193 and secondly from the succeeding drop 191.
19 Thus, satellite 194 and drop 193 may both carry a charqe based on the same charge signal, dependinq uPon the timinq of the charqe 21 signal. I~ satellite 194 merqes with drop 143 prior to enterinq 22 the deflection field formed by deflection plates, no charqe trans-23 fer occurs and the resultant merged drop is pro~erly deflected.
24 Ligament 197 also breaks off first with respect to the precedinq drop 198 and secondly from the succeedina drop 199 to 26 form satellite 200. The satellite 200 and the succeedina drop 27 198 may thus be similarly charqed, aqain dependina uPon the 28 timing of the charge siqnal. The satellite, however, does not 29 rearwardly merge into the succeedinq drop, but rather merqes with preceding drop 201. The charqe, or lack of charqe, intended S~974044 -16~

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1 for succeedin~ drop 198 is therefore -transferred to precedi.n~ drop 2 201. Both drops 201 and 198 will thérefore be incorrectly char~ed 3 and incorrectly deflected by the constant electrostatic deflec-4 tion field.
Figure 11 illustra-tes examples of ink jet streams 6 corresponding to those of Fiqure 10, but driven both with a 7 fundamental and a second harmonic comprising an amplitude 3 per-8 cent that of the fundamental signal and at a phase shift of 180 9 with respect to the fundamental. When controlled in this manner, each ligament 210-212 first breaks off from the succeedinq drop 11 220-221 to thereby assume the same charqe as the precedin~ drop 12 230-232, and then forward merqes with the same precedinq drop.
13 Therefore, each momentary satellite forward merqes with 14 the same drop with which it was associated when the charae was acquired. As the result of use of the subject invention, no 16 charge transfer occurs.
17 Another way of statinq the situation is that the dro~
1~ and the ligament which merges with the drop break off from the 19 stream filament as a unit.
An alternative circuit to that shown in Fiqure 6 is 21 illustrated in Figure 12. The circuit of Figure 12 differs from 22 that of Figure 6 primarily by beinq adjustable to allow tests to 23 be run on a specific head to determine the optimum amplitudes of 24 the fundamental and second harmonics and the optimum relative phase. A substantial portion of the circuitry in Figure 12 is, 26 however, identical to that in Fiqure 6 and therefore retains the 27 same number as in Fiqure 6. Referrinq specifically to the dif-28 ferences, preset inputs 128 of Figure 6 are rePlaced by switches 29 251-254 in Figure 12. These switches comprise the various binary counts and are individually settable in the closed position to SA974~44 -17-~8~Z,~2 1 connect with ground terminal 255 or -to the open position, repre-2 senting respectively a binary 1 or a 0. Any oE 16 phase anqles 3 may therefore be set by the binary number represented by the 4 settings of the switches 251-254. The outputs of the switches are supplied on wires representinq cable 127 and supplied to 6 exclusive OR circuit 115. Operation of the oscillator 110, 7 counter 111, exclusive OR 150 and inverter 112, and fli~-flop 8 135 are precisely the same as discussed with respect to Fiqure 6.
9 In Figure 12, lines 118 and 138 are sup~lied to attenuators 258 and 259 respectively. Other inputs to the attenuators are 11 supplied by switches 260 and 261 respectively. The attenuators 12 158 and 159 are identical and are illustrated in detail in Fiqure 13 13. The attenuators include a potentiometer 262 for individually 14 adjusting the attenuation of the input signal at input 118 or 138.
Switch 260 or 261 may be selectively closed to kill the output 16 signal entirely. The output siqnal is supplied on lines 264 or 17 265 to differential amplifier 267 for transmission on lines 268 18 to integrator 269. These circuits together convert the current 19 input on lines 264, 265 to a voltaqe si~nal and convert the re-sultant signal to a combined and summed set of sine waves at the 21 fundamental and second harmonic freauencies to be suP~lied on 22 drive output 150. Differential amplifier 267 and inteqrator 23 269 are illustrated in specific detail in Fiqure 14.
24 In tests conducted with piezoelectric drivers and various types of fluid jet heads, charge transfer and satellite generation 26 have been controlled through application of a second harmonic 27 frequency of amplitudes ranginq between 1% and 10% of the funda-28 mental frequency and at various phase anqles ranging from 90 to 29 180 with respect to the fundamental. The variations in phase ~ZZ~3Z

1 angle are attributed to the differing characteristics of the 2 specific heads.
3 The ranges in phase and amplitude are largely the result 4 of various head designs and resonances, with each head apparently having an experimentally obtained optimum. In some heads the 6 invention also works where a rearward merger takes place. In 7 such cases, the optimum point is in the situation where the 8 ligament and succeeding drop break off from the filament as a g unit. This means that the succeeding drop breaks from the front of the following ligament and either the leadinq liqament does 11 not form a temporary satellite, or the leading liqament breaks 12 from the succeeding drop only after that drop has broken from 13 the stream filament.
14 While the invention has been particularly shown and described with reference to preferred embodiments thereof, it 16 will be understood by those skilled in the art that the fore-17 going and other changes in form and details may be made therein 18 without departing from the spirit and scope of the invention.
19 What is claimed is:

Claims (10)

1. In a fluid jet head having means for supplying pressurized fluid to at least one nozzle orifice for pro-jecting a fluid stream filament therefrom, the improvement comprising:
perturbation means for perturbating said fluid streams with a fundamental sine wave and a second harmonic of the fundamental at a selected amplitude and phase with respect to the fundamental which causes each ligament and the perturbation with which said ligament merges to break off from said fluid stream filament as a unit; for thereby forming a stream of uniform drops having the drop repetition rate of said fundamental.

2. The fluid jet head of Claim 1 wherein said per-turbation means comprises:
an electromechanical transducer means for perturbating said fluid stream; and an electric signal generation means for supplying to said transducer means said fundamental and said second harmonic.

3. The fluid jet head of Claim 2 wherein:
said fluid is electrically conductive; and charging means is additionally provided for selectively charging said fluid upon breakoff from said fluid stream filament.

4. The fluid jet head of Claim 3 having a plurality of orifices for projecting a plurality of fluid streams; and wherein said electromechanical transducer means is arranged to perturbate said plurality of fluid streams; and said electric signal generation means is arranged for supplying to said transducer means said fundamental sine wave and said second harmonic at a selected amplitude and phase with respect to the fundamental which causes liga-ments between perturbations of each of said fluid streams to first break off from said stream at the end opposite the perturbation with which the ligament merges.

5. The fluid jet head of Claim 4 wherein:
said electric signal generation means is further arranged to supply said second harmonic at a selected amplitude of less than 10% and selected phase within the range of 90% to 180% with respect to said fundamental.

6. The fluid jet head of Claim 5 wherein:
said electromechanical transducer means is piezo-electric crystal driver.

7. In a fluid jet system of the type for supplying an electrically conductive fluid under pressure to at least one nozzle orifice to eject a stream of fluid from each said orifice, the method for causing each said ejected stream to break into a serial stream of selectively charged drops, comprising the steps of:
perturbating said fluid with a fundamental sine wave and a second harmonic of the fundamental at a selected amplitude and phase with respect to the fundamental which causes each ligament and the perturbation with which said ligament merges to break off from said fluid stream filament as a unit; and selectively charging said fluid upon breakoff from said fluid stream.

8. The method of Claim 7 wherein:
said perturbating step additionally comprises pertur-bating the pressure of said fluid by means of an electro-mechanical transducer.

9. The method of Claim 8 wherein:
said perturbating step additionally comprises pertur-bating with said second harmonic at a selected amplitude of less than 10% and selected phase within the range of 90° to 180° with respect to said fundamental.

10. In a fluid jet assembly having an inlet for pressurized electrically conductive fluid, a manifold cavity communicating with said inlet, at least one nozzle orifice communicating with said cavity for projecting a fluid stream therefrom, and an electromechanical transducer for perturbating the interior volume of said cavity to cause a periodic pressure perturbation of each said fluid stream, the improvement comprising:
an electric signal generator for supplying to said transducer a fundamental sine wave and a second harmonic of the fundamental at a selected amplitude and phase with respect to the fundamental which causes each ligament between said stream perturbations and the perturbation with which said ligament merges to break off from said fluid stream filament as a unit;
a charging electrode for each said stream for selec-tively charging said fluid upon breakoff from said fluid stream filament; and electrostatic deflection means for deflecting said selectively charged drops.