Note: Descriptions are shown in the official language in which they were submitted.
?101520253035CA 02264038 1999-02-26WO 98/08687 PCT/US97/14685lSPECIFICATIONTITLE OF THE INVENTIONINKJET PRINT HEAD FOR PRODUCING VARIABLEVOLUME DROPLETS OF INKCROSS-REFERENCE TO RELATED APPLICATIONSThis application is a continuation-in-part of coâpendingapplication Serial No. 08/703,974, filed August 27, 1996.This application is hereby incorporated by reference in itsentirety.Background of the Invention1. Field of the InventionThe present invention pertains to the field of inkjetprinters, and more specifically, to dropâon-demand piezo-electric inkjet printers.2. Description of Related ArtDrop-on-demand inkjet printers having piezoelectriccomponents are well known in the art. In general, piezo-electric drop-on-demand inkjet printers are constructed witha piezoelectric transducer component which reacts to theapplication of an electrical signal with a mechanicalmovement or distortion, such that a drop of ink is expelledfrom a print head ink channel or cavity that is in mechanicalcommunication with the transducer component. Prior attemptsto expel variable volume drops of ink from known inkjet printhead apparatuses have employed the use of an array of printhead channels, the outputs of which are selectively combinedto generate a larger, variable volume drop of ink. However,this method of printing typically requires a bulky print headapparatus since a g?urality of separate ink channels isrequired to generate a single ink drop. Other attempts togenerate variable volume ink drops have focussed on methodsto change the amplitude or shape of an electrical drivesignal which is applied to the inkjet printer transducers;however, while these methods produce variable volume inkdrops, they also generally resulted in inkjet systems where.i...i_.,..,,_T____________,,im,. .. ........... F1541?l0l5202530CA 02264038 1999-02-26W0 98l08687 PCT/U S97/ 146852the drop velocities of the expelled ink drops are notconsistent from one ink drop to the next. This results inpotential printing problems because variations in expelleddrop velocities result in drop placement errors on the printmedium, degrading output print quality.Therefore, there is a need for an inkjet print head whichcan be operated to expel variable volume ink drops from asingle ink channel, but which can also be operated such thatthe variable volume ink drops are expelled at substantiallythe same drop velocity.Summary of the InventionA drop on demand inkjet print head apparatus accordingto the present invention comprises a piezoelectric inkjetprint head having a transducer mechanically coupled to anink channel, wherein the electrical actuation of the inkchannel transducer results in the expulsion of a drop ofink from an ink channel orifice. The volume of the ex~pelled drop of ink can be selectively varied by controllingthe number of electrical signal pulses utilized to drivethe print head transducer. Generally, the more pulsesemployed to expel a single drop of ink, the greater thevolume of the expelled ink drop.One aspect of the present invention relates to themodification of the amplitude of successive electricaldrive signal to compensate for the tendency of the printhead to expel ink drops at increasing drop speeds whenusing multiple pulses to expel a single drop of ink. Forthis aspect of the present invention, the drive signalcomprises a "burst" series of electrical pulses havingamplitudes which decrease as a function of an increase inthe number of pulses within each burst series. One embodi-ment comprises decreasing amplitudes for the pulses withina burst series, such that different pulses within a single?1015202530CA 02264038 1999-02-26WO 98/08687 PCT/US97/ 146853burst series have different amplitudes. Another embodimentcomprises a burst series in which the amplitude of eachpulse within the burst series is set at a uniform ampli-tude, but the overall amplitude of the burst series de-creases in proportion to the number of pulses in that burstseries.Another aspect of the present invention relates toformation of relatively large ink drops. Relatively largeink drops are created by successively reducing the ampli-tude of the initial signal pulses within a burst series andthen successively increasing the amplitude of the lateroccurring signal pulses.Another aspect of the present invention relates to theoperation of the inkjet printer apparatus at approximatelythe resonant frequency of the ink channel. The preferredfrequency time factor (i.e., period) of an applied electri-cal drive signal is near or at 4L/C, where L equals thelength of the ink channel, and C equals the speed of soundof ink contained in the inkâfilled ink channel. If abipolar sinusoidal waveform is employed as the electricaldrive signal, then the period of each positive or negativecomponent of the drive signal waveform is preferably set at2L/C.Another aspect of the present invention relates to theuse of a switch array comprising an array of unipolarswitches to control the application of electrical drivesignals to an array of ink channel transducers. In thisembodiment of the present invention, the electrical drivesignals have distinct positive and negative components, andthe positive component by itself is not enough to expel adrop of ink from the print head ink channel. However, thecombined energy of both the positive and negative compo-nents is sufficient to expel an ink drop. Thus, an arrayof unipolar switches can be used to selectively block only?1015202530CA 02264038 1999-02-26W0 98/08687 PCT/US97/146854the negative component of the drive signal of selected inkchannels to effectively control the firing of certainchannels within an array of print head channels. Thepolarities of the electrical drive signals described inconnection with this aspect of the present invention can bereversed to the same effect.Yet another aspect of the present invention relates toan apparatus and method to generate "cancellation pulses."Typically, firing a given ink channel to expel a drop ofink results in the presence of residual pressure waveswhich reflect within the ink channel, even after the dropof ink has been fully expelled. These residual pressurewaves may interfere, constructively or destructively, withthe firing of the next drop of ink from that same inkchannel, e.g., by influencing the drop velocity of the nextink drop. in the present invention, cancellation pulsesare generated at the appropriate amplitude and phase tocreate pressures waves to cancel and counter the effects ofthe residual pressure waves within the ink channels.These and other aspects of the present invention aredescribed more fully in following specification and illus-trated in the accompanying drawing figures.Brief Description of the DrawingsFig. 1 depicts a crossâsectional side view of a singlechannel of an inkjet print head.Fig. 2 is a crossâsectional side view of an inkjetprint head for a single ink channel according to a pre-ferred embodiment of the present invention.Fig. 3 is a partial perspective view of the inkjetprint head of Fig. 2.Fig. 4 is a diagram of an embodiment of a sinusoidalmulti-pulse drive signal according to the present inven-tion.?1015202530W0 98l08687CA 02264038 1999-02-26PCT/US97/ 146855Figs. 5A-E depict the expulsion of a multiâpulse inkdrop corresponding to the sinusoidal waveform of Fig. 4.Figs. 6-8 depict alternate waveform shapes useful inthe present invention.Fig. 9 is a plot illustrating the change in ink dropvolume relative to a change in the number of pulses perburst series for an embodiment of the present invention.Fig. 10 is a plot illustrating the change in ink dropspeed relative to a change in the number of pulses perburst series for an embodiment of the present invention.Fig. 11 is a plot illustrating the change in ink dropspeed relative to a change in the amplitude of a firedburst series for an embodiment of the present invention.Fig. 12 depicts a progression of sinusoidal burstseries signals with varying burst series amplitudes accord-ing to an embodiment of the present invention.Fig. 13 depicts a progression of sinusoidal burstseries signals having varying pulse amplitudes within theburst series according to an embodiment of the presentinvention.Fig. 14 depicts a functional block diagram of a signalgenerator according to an embodiment of the present inven~tion.Figs. 15 and 16 depict sinusoidal burst series drivesignals according to alternate embodiments of the presentinvention.Fig. 17 depicts an alternate embodiment of a waveformgenerator in which cancellation pulses are generated.Fig. 18 depicts a sinusoidal burst series signal withvarying burst series amplitudes according to an embodimentof the present invention.Figs. 19 and 20 depict sinusoidal burst series drivesignals with varying burst series amplitudes according toalternate embodiments of the present invention.?1015202530CA 02264038 1999-02-26W0 98l08687 PCT/US97I146856Fig. 21 depicts a sinusoidal burst series signal withvarying burst series amplitudes according to an embodimentof the present invention.Figs. 22 and 23 depict sinusoidal burst series drivesignals with varying burst series amplitudes according toalternate embodiments of the present invention.DESCRIPTION OF THE PREFERRED EMBODIMENTSFig. 1 is diagrammatic representation of the principalcomponents of a single channel of a drop-onâdemand piezo-electric inkjet print head structure 20 according to thepresent invention. In the embodiment of Fig. 1, print headstructure 20 comprises an ink channel 16 which is suppliedwith ink from an ink reservoir 10. A nozzle plate 14comprising an orifice 12 is disposed along one end of theink channel 16. A transducer 4 is in mechanical communica-tion with the ink channel 16, and may define part of theinner wall or inner surface area of the ink channel 16.The transducer 4 is typically formed of a piezoelectricmaterial, such as PZT, which responds to the application ofan electrical signal with a mechanical distortion of thetransducer material. This mechanical distortion causes achange in the positioning and/or dimensions of the trans-ducer material, thereby resulting in a change of the totalvolume of the ink channel 16. In operation, the applica-tion of a voltage potential across transducer 4 createsvolume changes within the ink channel 16 which, in turn,causes the expulsion of ink drops through orifice 12 innozzle plate 14. A signal generator 6 is employed togenerate electrical drive signals to excite the transducermaterial via two or more electrodes 8. A typical inkjetprint head having multiple ink channels may comprise anarray of print heads structures 20.?1015202530CA 02264038 1999-02-26WO 98/08687 PCT/US97/146857The structure of the preferred piezoelectric inkjetprint head apparatus useful in conjunction with the presentinvention is described in more detail in copending U.S.application serial no. (N/A), Lyon & Lyon Docket No.220/202, entitled "Inkjet Print Head Apparatus", which isbeing filed concurrently with the present application, andthe details of which are hereby incorporated by referenceas if fully set forth herein. The following detaileddescription with reference to Figs. 2 and 3, sets forth themajor features of the inkjet print head apparatus describedin that copending application. However, the present inven-tion is capable of operation with a broad range of otherpiezoelectric inkjet print head structures, and thus, theparticular inkjet print head described in connection withFigs. 2 and 3 is presented for illustrative purposes only,and is not intended to be limiting in any way.Fig. 2 is a crossâsectiona1 side view of a singlechannel of piezoelectric inkjet print head structure 20constructed in accordance with one embodiment of the pres-ent invention. Print head structure 20 comprises an inkchannel 29 which is supplied with ink from an ink reservoir10 through an ink passageway 47 in rear cover plate 48. Inoperation, print head structure 20 expels ink from inkchannel 29 though an orifice 38 in nozzle plate 33. Refer-ring to Fig. 3, more detail can be seen of the preferredstructure of print head 20. Print head transducer 2 com-prises a first wall portion 32, a second wall portion 34,and a base portion 36. The upper surfaces of the first andsecond wall portions 32 and 34 define a first face 7 of theprint head transducer 2, and the lower surface of the baseportion 36 defines a second, opposite face 9 of the printhead transducer 2. Ink channel 29 is defined on threesides by the inner surface of the base portion 36 and theinner wall surfaces of the wall portions 32 and 34, and is?1015202530CA 02264038 1999-02-26WO 98/08687 PCT/US97/ 146858an elongated channel cut into the piezoelectric material ofthe print head transducer 2, leaving a lengthwise openingalong the first face 7 of the print head transducer 2. Afirst metallization layer substantially coats the innersurfaces of ink channel 29 and is also deposited along thefirst face 7 of the print head transducer 2. This firstmetallization layer forms a common electrode 24 for theprint head structure 20, and is preferably connected toground. An ink channel cover 31 is bonded over the firstface 7 of the print head transducer 2, closing off thelengthwise opening in the ink channel 29. A secondmetallization layer coats the outer surfaces of the baseportion 36, and also extends approximately halfway up eachof the outer surfaces of the first and second wall portions32 and 34. This second metallization layer forms theaddressable electrode 22. The poling direction (i.e., theoverall polarization direction) of the piezoelectric mate-rial forming print head transducer 2 preferably lies sub-stantially in the direction shown by arrow 30 in Fig. 3.As disclosed in more detail in copending application serialno. (N/A), Lyon & Lyon Docket No. 220/202, this polingdirection provides for the inkjet print head structure 20to be actuated in both the normal mode and shear mode uponthe generation of a voltage difference between the firstand second metallization layers (e.g., when an electricaldrive signal is applied to the addressable electrode 22).The piezoelectric inkjet print head 20 operates by theapplication of an electrical drive signal from a signalgenerator 6 to the piezoelectric material of print headtransducer 2. The application of this electrical drivesignal produces a dimensional and/or positional distortionof the piezoelectric material of the print head transducer2, resulting in a change in the interior volume of the inkchannel 29. This change of volume within the ink channel?1015202530CA 02264038 1999-02-26WO 98/08687 PCT/US97/14685929 generates an acoustic pressure wave within the inkchannel 29, and the movement of the pressure wave withinthe ink channel 29 provides energy to expel ink from theink channel 29 onto a print medium via orifice 38. Ofparticular importance to the operation of the inkjet printhead 20, and to the creation of these acoustic pressurewaves within the ink channel 29, are the particular parame-ters of the electrical drive signal applied to the piezo-electric material of the print head 20. For example,manipulating the parameters of an applied electrical drivesignal (e.g., the amplitude, frequency, and/or shape of theapplied electrical waveform) significantly affects thecharacteristics of the acoustic pressure wave(s) actingwithin the ink channel 29, which in turn affects the size,volume, shape, speed, and/or quality of the ink drop ex-pelled from the print head 20.The present specification describes and teaches the useof an electrical drive signal having a numericallyselectable series of individual pulses to generate a vari-able volume ink drop from an inkjet print head. This canbe best described by way of example, as shown in Figs. 4and 5A-E. In Fig. 4, there is shown an example of a elec-trical drive signal having one or more individual signalpulses which may be applied to the electrodes of an inkjetprint head transducer according to the present invention,and this type of electrical signal is hereafter referred toas a "burst series". The particular burst series shown inFig. 4 is a sinusoidal waveform comprising a series of fourindividual sine-wave electrical signal pulses 72, 74, 76,and 78 ("bursts"). Each of the individual signal pulses orbursts 72, 74, 76, and 78 comprises both the negative andpositive components of a sinusoidal signal. To betterexplain the operation of the burst series of Fig. 4, theeffects of the individual signal pulses 72, 74, 76, and 78?1015202530CA 02264038 1999-02-26WO 98/08687 PCT/U S97/ 1468510upon an inkjet print head will now be explained in detail.In the first individual signal pulse 72 shown in Fig.4, the negative portion 72a of the sinusoidal waveformbegins the process of expelling a droplet of ink from aninkjet print head. The negative portion 72a of the firstsignal pulse 72, when applied to the addressable electrode22 of the preferred print head 20 (Fig 3), moves the baseportion 36 and the wall portions 32 and 34 of the printhead transducer 2 outwardly to expand the volume of the inkchannel 29, creating an underpressure in the ink channel29, which generates an acoustic pressure wave that rever-berates and reflects within the ink channel 29. The fol-lowing positive portion 72b of the electrical waveform ofsignal pulse 72 deflects the base portion 36 and wallportions 32 and 34 of the print head transducer 2 in theopposite direction (inwardly into the ink channel 29),reducing the volume of the ink channel 29, which generatesanother pressure wave within the ink channel 29. Thecharacteristics of the waveform of signal pulse 72 arepreferably selected and timed such that the energy andmovement of the pressure wave(s) from the negative portion72a of the signal pulse 72 are substantially synchronizedwith the energy and movement of the pressure wave(s) creat-ed by the positive portion 72b of signal pulse 72, suchthat the substantially combined energy of both the negativeand positive portions of signal pulse 72 unite to expel amicroâdroplet 60 of ink from the orifice 38 of print head20.Fig. 5A corresponds to a view of orifice 38 at timeperiod T1 of Fig. 4, and shows the expulsion of a singleink microâdroplet 60 from orifice 38 upon the applicationof a first electrical signal pulse 72. During time periodT2 (Fig. 4), a second signal pulse 74 is applied to theaddressable electrode 22 of print head 20, and as shown in?1015202530CA 02264038 1999-02-26W0 98l08687 PCT/US97/ 1468511Fig. 5B, a second ink microâdroplet 62 is expelled follow-ing the application of signal pulse 74. As illustrated inFig. 5B, because signal pulse 74 is applied immediatelyfollowing the application of signal pulse 72, the subse-quently expelled ink microâdroplet 62 is typically stillattached to the preceding microâdroplet 60 by a thin seg-ment of ink. Fig. 5C depicts the orifice 38 at time periodT3, after the application of a third electrical signalpulse 76. As is also shown in Fig. 5C, a third ink micro-droplet 64 is expelled as a result of the third signalpulse 76, and this third microâdroplet 64 may also beconnected to the preceding micro-droplet 62 by a thinsegment of ink. By this time period, the surface tensionof the ink may have already begun the process of drawingthe first two micro-droplets 60 and 62 together into asingle macro-droplet of ink. At time period T4, Fig. 5Dshows the result of the application of a fourth electricalsignal pulse 78, which causes the expulsion of a fourthmicroâdroplet 66 from the orifice 38. As before, micro-droplet 68 may still be connected to the preceding micro-droplet by a thin segment of ink. By this time period, thefirst three micro-droplets may have already begun coalesc-ing into a single macro-droplet of ink. Fig. 5E depictsthe expelled ink macro-droplet 70 at a later period intime, when the surface tension of the ink has pulled allthe separate ink micro-droplets 60, 62, 64, and 66 for thisparticular burst series together, so that the micro-drop-lets have merged inâflight into a generally spherical inkmacro-droplet 70 prior to impact upon a print medium. Themicro-droplets may also merge at time of impact upon theprint medium.In general, by repeatedly applying these electricalsignal pulses in a "burst series" of such signal pulses, anink drop can be expelled having a crossâsectional diameter?1015202530CA 02264038 1999-02-26WO 98/08687 PCT/US97/1468512larger than the diameter of the print head orifice 38 fromwhich the ink drop is expelled. The size of the ink macro-droplets is dependent upon the number of electrical signalpulses in a burst series, with the size of the macroâdrop-let expelled generally increasing as the number of pulsesin a burst series increases. Although the example of Figs.4 and 5A-E show a burst series consisting of only fourindividual signal pulses, the number of pulses within aburst series can be significantly higher. The principallimitation as to the number of signal pulses which can bewithin a single burst series is that, at some point intime, the series of pulses should be discontinued to allowthe next burst series to begin firing, in order to maintainthe proper spacing between the ink drops to be printed uponthe print medium. Fig. 9 graphically displays the resultsof experiments conducted with a preferred inkjet print head20, showing the increase in volume of an expelled ink dropas the number of electrical signal pulses per burst seriesincreases. The test data shown in Fig. 9 are for thepurposes of illustration only, and the actual volume in-creases may differ if an inkjet printer having other con-figurations and/or dimensions is employed, or if differentparameters for the electrical drive signal are applied tothe inkjet print head.The present invention is preferably practiced in aresonant mode of operation, wherein the frequency of thepulses of an electrical drive signal is near or at theresonant frequency of the print head ink channel 29.Employing a resonant frequency time component (i.e., aperiod corresponding to the resonant frequency) for theelectrical drive signal allows the inherent resonance ofthe ink channel 29 to assist in the expulsion of ink fromthe print head, since residual energy from pressure wavesgenerated in the ink channel 29 by earlier signal pulses?1015202530CA 02264038 1999-02-26WO 98108687 PCT/U S97/ 1468513will combine with the energy of pressure waves from one ormore later signal pulse to expel ink drops. In the pre-ferred embodiment of the present invention, the expelledink drops are produced by generating an energizing electri-cal drive signal having major Fourier components near theink channel's resonant frequency, where the ink channelresonant frequency is preferably calculated to include theeffect of having ink contained within the ink channel. Ifa sinusoidal electrical drive signal is employed, such asthe waveform depicted in Fig. 4, then the period of eachsignal pulse is preferably approximately 4L/C; thus, thewidth for each of the positive or negative components foreach signal pulse (i.e., oneâhalf period of each signalpulse) is approximately 2L/C, where L equals the length ofthe ink channel and C equals the velocity of sound of inkcontained in the ink channel.Although the preferred embodiment employs a period of4L/C, other frequency time components for the signal pulsesin each burst series are expressly contemplated within thescope of the present invention, and the actual frequency tobe employed is dependent upon the particular applicationfor which the invention is utilized, and upon the specificsystem and drive signal parameters to be used. For exam-ple, drive signals may be utilized having frequencies nearthe resonant frequency of the ink channel, e.g., having aperiod within 0â10% of 4L/C. Alternative embodimentswithin the scope of the present invention may employ drivesignals having a period which is also near the resonantfrequency of the ink channel, but which varies by more than10% from the resonant frequency. In addition, the frequen-cy of each burst series may be selectively varied basedupon the number of pulses within the burst series. Thus,an alternate embodiment comprises the use of a frequencytime component which is less than or is at 4L/C for a burst?10152025'30CA 02264038 1999-02-26WO 98/08687 PCT/US97/ 1468514series having a greater number bursts, but which has aperiod that increases up to or is greater than 4L/C for aburst series having a lesser number of bursts, with asingle-pulse burst series having the largest frequency timecomponent.One effect of applying multiple pulses to eject a dropof ink is that the velocity of the expelled ink drops tendsto increase as the number of pulses within a burst seriesis increased. Fig. 10 graphically depicts the experimentalresults with a presently preferred inkjet print head struc-ture 20, where it has been confirmed that in a piezoelec-tric resonating inkjet printer, the drop speed of an inkdrop formed with fewer pulses is more likely to be slowerthan the drop speed of an ink drop formed with a greaternumber of pulses. In part, this results from a residualbui1dâup of energy in the ink channel 29 from the multiplenumber of signal pulses in a burst series. As discussedabove, each individual signal pulse generates pressurewaves in the ink channel which act to expel a microâdropletof ink from the ink channel. However, residual energy fromeach induced pressure wave may remain in the ink channel toadd to the pressure wave induced by the next applied burstpulse, which then produces a succeeding ink microâdropletthat is somewhat faster and larger than the preceding inkmicroâdroplet. Essentially, the sizes of the followingsub-droplets are increased as a result of the echo effectof the resonance left over in the channel from the previoussignal pulses. In addition, the first microâdroplet thatis ejected typically has a slower overall velocity than theink microâdroplets that follow. This is partially becausethe energy in the first sub-droplet is partially dissipatedin breaking the surface tension of the ink meniscus at theorifice of the ink channel 29. Thus, the net velocity of acombined macroâdroplet will be greater for an applied drive?1015202530CA 02264038 1999-02-26WO 98/08687 PCTIUS97/1468515signal having more bursts than an applied signal havingless bursts.Visible printing imperfections may occur if the printhead expels drops of ink at inconsistent drop speeds, sincethe inkjet print head typically traverses across a printmedium at a substantially constant speed. This contributesto a degradation in the quality of the resulting print,since the ink drops may not uniformly line up or be uni-formly spaced on the print medium if the print head ismoving at a constant speed while the ink drops are beingexpelled at different speeds. Therefore, although the useof multiple signal pulses within a burst series allows theexpulsion of a variable volume ink drop from a print head,the use of multiple pulses may also create variations inthe drop speed, which affects the final quality of an imageprinted onto a print medium.The present invention overcomes this problem, by pro-viding a method and apparatus for varying the volume of anexpelled ink drop, while simultaneously maintaining acontrolled, constant drop speed for the expelled ink drop.This is accomplished by using the principle that decreasingthe amplitude of the electrical signal applied to a trans-ducer of a print head to expel a drop of ink results in adecrease of the drop velocity of the expelled ink drop. Byvarying the amplitude of the electrical drive signal, inconjunction with the use of an electrical drive signalhaving multipleâpulses per burst series, the volume of theexpelled ink drop can be increased while maintaining asubstantially constant drop speed. For the preferredembodiment of the present invention, when utilizing theinkjet print head 20 of Figs. 2 and 3, substantially con-stant drop speeds for expelled variableâsize ink drop isproduced by varying the amplitude of signal pulses inaccordance with the parameters shown in Fig. 11. For?1015202530CA 02264038 1999-02-26WO 98/08687 PCT/US97/1468516example, as indicated in Fig. 11, the amplitude of a four-pulse burst series used to expel an ink drop should be setat approximately 80% of the amplitude of a singleâpulse inkdrop in order to maintain a constant drop speed. The plotdepicted in Fig. 11 is for the purpose of illustration onlyas it was derived particularly to be used in conjunctionwith the above-described preferred inkjet print head 20,and is not intended to be limiting in any way, since ampli-tude compensation levels will necessarily vary dependingupon the particular application in which the present inven-tion is utilized, and may also vary depending upon manyother conditions, some of which may include, for example,the actual dimensions and structure of the inkjet printhead employed, the material used to construct the printhead, the shape of the print head, the type, frequency, andamplitudes of the electrical input waveform employed, andthe characteristics of the ink employed.Referring to Fig. 12, shown is an illustrative embodi-ment of electrical signal waveforms which can be employedto generate variable volume ink drops having substantiallyconstant drop speed. This figure shows varying amplitudeburst series signals which may be used to maintain theconstant drop speed of an expelled ink drop, wherein theamplitude of each individual pulse within a particularburst series is the same, but the overall amplitude of theburst series varies depending upon the number of pulseswithin each burst series. As shown in Fig. 12, the ampli-tude of a singleâpulse burst series waveform 100 is at aheight of A1, which produces an ink drop at drop velocityV. To maintain a substantially constant drop speed, theamplitude for each pulse in a twoâpulse burst series 102 isset at amplitude A2, which is preferably smaller thanamplitude Al, so that the twoâpulse burst series 102 expelsan ink drop at substantially the same velocity V as the?1015202530CA 02264038 1999-02-26WO 98/08687 PCT/US97/ 1468517single-pulse burst series 100. The amplitude A3 for three-pulse burst series 104 is smaller than the amplitude A2 forthe two-pulse burst series 102, and the amplitude A4 for afourâpulse burst series 106 is even smaller than the ampli~tude A3 for the threeâpulse burst series 104. The ampli-tudes shown in Fig. 12 are for the purposes of illustrationonly, to allow a pictorial representation of the types ofamplitude changes which may be utilized in the presentinvention. To maintain substantially constant drop speed,the preferred embodiment employs the amplitude changesshown in Fig. 11, as applied to the preferred inkjet printhead 20 described further in connection with Figs. 2 and 3.Thus, as indicated in Fig. 11, in the preferred embodiment,the amplitude of a fourâpulse burst series is set at ap-proximately 80% the amplitude of a single-pulse burstseries to maintain the substantially constant drop speed ofthe expelled ink drops.Referring to Fig. 13, an alternate embodiment of thepresent invention is shown which can be utilized to gener-ate variable volume ink drops having a substantially con-stant drop velocity. In contrast to the embodiment of Fig.12, the embodiment shown in Fig. 13 makes use of signalpulses having varying amplitudes within a single burstseries to maintain the substantially constant drop velocityof expelled ink drops. As shown in Fig. 13, the amplitudeof a single-pulse burst series 110 is at a height of B1,which produces an ink drop at drop velocity V. In thisembodiment, to cause a two-pulse burst series to expel adrop of ink at substantially the same drop velocity V as asingle-pulse burst series, a two-pulse burst series 112preferably has a first pulse 117 which is also set at anamplitude of B1, but the second pulse 118 of burst series112 is preferably set at an amplitude of B2, with amplitudeB2 being smaller than amplitude B1. It has been experimen-?1015202530CA 02264038 1999-02-26wo 98/08687 PCTIUS97/1468518tally confirmed that in the present invention, the properselection of decreasing amplitudes for the successivepulses of a multi-pulse burst series will compensate forthe tendency of an expelled ink drop to increase its dropvelocity when the number of pulses is increased. Thus, fora three-pulse burst series 114 in this embodiment, thefirst pulse 119 will again be set at an amplitude B1, andthe successive two pulses 120 and 121 will be set at ampli-tudes of B2 and B3, respectively, with the amplitude of thesuccessive pulses decreasing as necessary to maintain anexpelled ink drop at the consistent drop velocity V. For afourâpulse burst series, or even more pulses per burstseries, the same decreasing amplitude pattern is repeatedto generate a multi-pulse ink drop with substantially thesame drop velocity V. The amplitudes shown in Fig. 13 arefor the purposes of illustration, to allow a pictorialrepresentation of the types of amplitude changes which arepreferably made to utilize this aspect of the presentinvention.Another embodiment comprises burst series waveforms inwhich one or more of the individual signal pulses have thesame amplitude, but other signal pulses within that sameburst series have different amplitudes to maintain constantdrop velocities of the expelled ink drop. This methodemploys burst series waveforms which are a variation ofthose shown in Figs. 12 and 13, and can also be utilized toestablish constant ink drop velocities.While the above described burst series waveforms cangenerate variable volume ink drops having substantiallyconstant drop speed, when producing larger ink drops, forexample, ink drops created by more than four pulses in aburst series, these burst series waveforms can provideunsatisfactory print quality. The reason for this is thatafter approximately the third pulse in a burst series, the?1015202530CA 02264038 1999-02-26W0 98l08687 PCT/U S97/ 1468519resonance energy built in the print head ink channel 29peaks at its maximum level. Once the resonance energy inprint head ink channel 29 peaks, the velocity of the ex-pelled ink microâdroplet will also peak. This can causethe laterâexpelled ink micro-droplets to fail to combinewith the larger ink macro-droplet created by the earlierpulses of the burst series. Furthermore, the earlierexpelled ink micro-droplets have traveled a substantialdistance to the print medium before the later expelled inkmicro-droplets have even been expelled. When this happens,the trailing ink micro-droplets might not strike the printmedium at the same location as the larger ink macro-droplet(which is comprised of the earlier expelled ink micro-droplets). Instead, the later expelled ink micro-dropletscould strike the print medium at a location adjacent towhere the larger ink macro-droplet struck the medium. Thisis highly undesirable, as it can lead to poor print quali-ty.It is therefore desirable to vary the amplitude of thepulses in the burst series so as to maintain the same timeof flight to the print medium of the aggregate drop, i.e.,the macroâdroplet, for all ink drops, regardless of howmany micro-droplets comprise the macroâdroplet. Thus, in apreferred embodiment, the amplitude of the later pulses ina burst series is increased in order to increase the reso-nance energy in the print head ink channel 29. The in-creased resonance energy in the ink channel 29 will in-crease the velocity of the later expelled ink micro-drop-lets, thereby allowing those later expelled ink drops toeither remain attached to the larger ink macro-dropletformed by the previously expelled micro-droplets or catchup to the earlier expelled ink drops. If the later ex-pelled ink micro-droplets cannot catch up to the ink macro-droplet created by the earlier expelled ink micro-droplets,?1015202530CA 02264038 1999-02-26W0 93/03587 PCT/U S97/ 1468520the increased velocity should allow them to impact theprint medium at substantially the same location as the inkmacro~droplet created by the earlier expelled microâdrop-lets. In preferred embodiments of the present invention,the amplitude of the signal pulses are progressively re-duced until reaching a predetermined signal pulse in aburst series. Beginning with the predetermined signalpulse, the amplitude of the signal pulses of the burstseries is then progressively increased until the burstseries is completed.In the presently preferred embodiment, the predeter-mined number of signal pulses is four. Thus, in the pres-ently preferred embodiment, the amplitude of the firstthree signal pulses is progressively reduced. Then, begin-ning with the fourth signal pulse, the amplitude of thesignal pulses is progressively increased. An exemplarysevenâpulse burst series 180 of the presently preferredembodiment is shown in Fig. 18. In the embodiment of Fig.18, the amplitude of the earlier occurring signal pulses inthe burst series 180 is progressively reduced to maintainthe substantially constant drop velocity of the initiallyexpelled ink drops. The amplitude of the first pulse 182of the sevenâpulse burst series 180 has a height of E1,which produces an ink droplet at drop velocity V. Thesecond pulse 184 of burst series 180 is preferably set atan amplitude of E2, with amplitude E2 being smaller thanamplitude E1. The third pulse 186 of burst series 180 ispreferably set at an amplitude of E3, with amplitude E3being smaller than amplitude E2.Beginning at the fourth pulse 188 of burst series 180,the amplitude is gradually increased to increase the veloc-ity of the later expelled ink microâdroplets. Thus, theamplitude of the fourth pulse 188 of burst series 180 ispreferably set at an amplitude of E4, with amplitude E4?1015202530CA 02264038 1999-02-26WO 98/08687 PCT/US97/1468521being larger than amplitude E3. The amplitude of the fifthpulse 190 of burst series 180 is preferably set to ampli-tude E5, with amplitude E5 being larger than amplitude E4.The sixth pulse 192 of burst series 180 is preferably setat an amplitude of E6, with amplitude E6 being larger thanamplitude E5. Finally, the seventh pulse 194 of burstseries 180 is preferably set at an amplitude of E7, withamplitude E7 being larger than amplitude E6.The amplitudes shown in Fig. 18 are for the purposes ofillustration only to allow a pictorial representation ofthe types of amplitude changes which are preferably made toutilize this aspect of the present invention. Moreover,the number of signal pulses that are decreased in amplitudeprior to increasing is dependent upon several factors,including the type and viscosity of ink used, environmentalfactors such as temperature and humidity, and the type ofprint head used. As discussed the presently preferredpredetermined number of signal pulses is four. However,using the teachings of this invention, one could vary thenumber signal pulses having progressively decreasing ampli-tudes to ensure high quality printing.Another preferred method of varying the amplitude ofthe pulses in the burst series to maintain the same time offlight to the print medium of larger aggregate drops, i.e.,the macroâdroplets, for all ink drops is shown in Fig. 21.In this preferred embodiment, the amplitude of all thepulses in a burst series is progressively increasedthroughout the burst series. This is done to increase theresonance energy in the print head ink channel 29. Just asin the embodiment of Fig. 18, this method allows the laterexpelled ink drops to either remain attached to the largerink macro-droplet formed by the previously expelled micro-droplets or catch up to the earlier expelled ink drops. Ifthe later expelled ink microâdroplets cannot catch up to?1015202530CA 02264038 1999-02-26wo 98/08687 PCT/US97/1468522the ink macro-droplet created by the earlier expelled inkmicroâdroplets, the increased velocity should allow them toimpact the print medium at substantially the same locationas the ink macro-droplet created by the earlier expelledmicro-droplets.In the embodiment of Fig. 21, the amplitude of thefirst pulse 202 of the sevenâpulse burst series 180 has aheight of H1, which produces an ink droplet at drop veloci-ty V. The second pulse 204 of burst series 200 is prefera-bly set at an amplitude of H2, with amplitude H2 beinglarger than amplitude H1. This produces an ink droplet ata drop velocity slightly higher than V. The third pulse206 of burst series 200 is preferably set at an amplitudeof H3, with amplitude H3 being larger than amplitude H2.This produces an ink droplet at a drop velocity slightlyhigher than that produced by second pulse 204. The fourthpulse 208 of burst series 200 is preferably set at anamplitude of H4, with amplitude H4 being larger than ampliâtude H3. This produces an ink droplet at a drop velocityslightly higher than that produced by third pulse 206.Finally, the fifth pulse 210 of burst series 200 is prefer-ably set at an amplitude of H5, with amplitude H5 beinglarger than amplitude H4. This produces an ink droplet ata drop velocity slightly higher than that produced byfourth pulse 208.The amplitudes shown in Fig. 21 are for the purposes ofillustration only to allow a pictorial representation ofthe types of amplitude changes which are preferably made toutilize this aspect of the present invention. Moreover,the number of signal pulses that are decreased in amplitudeprior to increasing is dependent upon several factors,including the type and viscosity of ink used, environmentalfactors such as temperature and humidity, and the type ofprint head used. As discussed the presently preferred?1015202530CA 02264038 1999-02-26WO 98/08687 PCT/US97/ 1468523predetermined number of signal pulses is four. However,using the teachings of this invention, one could vary thenumber signal pulses having progressively decreasing ampli-tudes to ensure high quality printing. Further, dependingupon these factors, it is possible that no pulses withdecreased amplitude will be necessary.The specific amplitude levels described and depicted inconnection with Figs. 11, 12, 13, 18 and 21 are not intend-ed to be limiting in any way as to the scope of the presentinvention, since it is contemplated that this invention maybe employed with other types of piezoelectric inkjet printheads or other types of electrical drive signals, and suchother applications may employ the same inventive principlestaught herein to utilize amplitude levels that are differ-ent from that described and shown in connection with Figs.11-13, 18 and 21. For example, some of the factors whichmay affect the use and/or selection of amplitude changes ofthe signal pulses or burst series in accordance with thepresent invention include the viscosity and properties ofthe particular ink type employed, the physical dimensionsof the ink channel, the specific transducer material em-ployed, the configuration of the transducer material, theshape and frequency of the electrical drive signal, and thesize of the ink channel orifice.A decrease in the amplitude of an applied electricalsignal, while resulting in decreased drop speed, may alsodecrease the drop volume of an expelled ink drop. However,the reduction in volume of an ink drop resulting from adecrease in signal pulse amplitude is typically less thanthe volume increase achieved by increasing the number ofpulses in a multipleâpulse burst series waveform to createan increase in the volume of the expelled ink drop.Although the above embodiments depict the utilizationof a sinusoidal electrical input waveform, other waveform?1015202530WO 98/08687CA 02264038 1999-02-26PCTIU S97! 146852 4shapes may also be employed within the scope of the presentinvention. For example, a trapezoidal waveform, as shownin Fig. 6, can also be employed in the present invention.Other waveforms, such as half-sinusoidal pattern, or asquare wave (Fig. 7), or a triangular wave pattern (Fig. 8)may also be employed in the present invention. Of course,the choice of waveform shapes may change depending upon theparticular configuration of the inkjet printer apparatusemployed with the present invention or the application forwhich it is used. For these other waveform shapes, theperiod of the signal pulses may also be near or at theresonant mode for operation of the inkjet print head. Forexample, for the square wave shown in Fig. 7, the period ofeach signal pulse is preferably 4L/C.Fig. 14 is a functional block diagram showing theprincipal components of a preferred signal generator 6useful in the present invention. Signal generator 6 gener-ates and transmits the electrical drive signal which drivesthe transducer material 141 in the inkjet print head. Theoperational sequence of signal generator 6 begins with theapplication of a waveform control signal 130 to a burstseries waveform generator 134 from an outside signal source132, such as a print head controller 132 Waveform controlsignals 130 may also be sent from an external encoder ormicroprocessor, which outputs control signals linked to themotion of the print head, so that the expelled ink dropsare ejected with optimal timing to impact the print mediumat the correct position. The waveform generator 134 pro-duces burst series waveform 136, comprising one or morepulses per burst series, which is applied to an amplifier138, which increases the amplitude of the burst serieswaveform 136 to an appropriate voltage level to drive thetransducer 141 in the print head. The amplified burstwaveform 139 from the amplifier 138 is connected to a?1015202530CA 02264038 1999-02-26W0 98l08687 PCT/US97Il468525switch array 140, a series of digitally controlled switch-es, which selectively controls which individual channels ofthe array of print head channels will be permitted toreceive the actuating amplified burst waveform 139. Theamplified burst waveform is then applied to selected chan-nels of the print head transducer 141.The preferred burst series waveform generator 134comprises a lookup table controller 150 which directs theoperation of a lookup table 152. Lookup table controller150 receives waveform control signals 130 from an outsidesignal source 132 which provides control signals pertainingto the timing and waveform parameters of the burst serieswaveform to be generated by the signal generator 6. Someof the parameters which may be specified by the waveformcontrol signal 130 include the frequency of the waveform,the number of pulses within a burst series, and the shapeof the waveform.Lookup table 152 is programmed with a table ofwaveformâdata points which define electrical burst serieswaveforms that can be utilized to drive the print headtransducers 141. In the preferred embodiment, the burstseries waveform is defined by a series of (X, y) coordinatepoints, where the xâcoordinate represents a point in timeon a burst series waveform plot, and the y-coordinaterepresents the voltage or amplitude of the waveform at thatparticular xâcoordinate. Each individual waveformâdatapoint in the lookup table 152 corresponds to an (x, y)coordinate location on a desired waveform. For each burstseries, the lookup table controller 150 outputs a stream oftimeâfactor coordinates (xâcoordinates) to the lookup table152, and the lookup table 152 in turn outputs a series ofamplitude values (yâcoordinates) corresponding to each X-coordinate. The lookup table controller 150 contains acounter which increments through the lookup table's input?1015202530WO 98/08687CA 02264038 1999-02-26PCT/U S97/ 146852 6range at a rate determined by the specific contents of thewaveform control signal 130. The waveformâdata points fromthe lookup table 152 are applied in sequence to a digi-tal/analog converter ("DAG") 154, which outputs an analogburst series waveform 136 that corresponds to the series of(X, y) coordinate points, and which is a low power versionof the signal that will be applied to transducer 141 of theprint head. Other DACs may also receive control inputssignals which partially determine several parameters of theburst series waveform 136. For example, a separate DAC maybe employed to control the overall amplitude of the outputburst series waveform.The function of the switch array 140 is to selectivelyallow the amplified burst series waveform to fire onlycertain ink channels of the-print head 141. In the pre-ferred embodiment, switch array 140 generally comprises anarray of switches, e.g., field effect transistors, whichare controlled to selectively allow the amplified burstwaveform(s) to pass only to certain ink channels of theprint head 141. The preferred embodiment employs an opto-isolator 144 to apply switch control signals 146 from theprint head controller 132 to the switch array 142. Theprint head controller 132 provides switch control signals146 that control whether a given channel of the print headis or is not printing at a given point in time as the printhead 141 is moved across the print medium.In one embodiment of the present invention, switcharray 140 comprises an array of biâdirectional switches,with each individual ink channel within the print headhaving a corresponding switch within the switch array 140.This type of switch array allows the inkjet print head tooperate with the waveforms characterized by Figs. 12, 13,18 and 21, wherein the transmission of both the positiveand negative half portions of each signal pulse to theâ ââ"ââ"âTâ"ââ*'-'~"-~"'- - -'- ~- «~â~â~a-~------â«âââââ-â.âââ...__..._.. ...___,_.,?1015202530CA 02264038 1999-02-26WO 98/08687 PCT/U S97! 1468527array of inkjet channel transducers can be selectivelycontrolled by the switch array 140, thereby controllingwhich individual ink channel is allowed to fire a drop ofink. However, bi-directional switches typically cost morethan equivalent unipolar switches, and the greater manufac-turing cost resulting from using biâdirectional switchesincreases proportionally when a print head contains agreater number of ink channels that needs to be controlled.Thus, an alternate embodiment of the present inventioncomprises a switch array 140 which employs an array ofunipolar power switches, where only the positive componentor only the negative component of each pulse of a burstseries waveform need to be controlled to operate an inkjetprint head. In this alternate embodiment when applying abipolar pulse waveform, the application of a first halfpulse waveform to the transducer, which can be either thepositive or negative component, generates a pressure waveinside the ink channel, which reflects and reverberateswithin the inner walls of the ink channel. The applicationof the second half pulse waveform, which is opposite inpolarity to the first half pulse waveform oneâhalf periodlater, creates another pressure wave which is timed tocoincide with the reflected first pressure wave such thatthe combined energy of both pressure waves unite to expel adroplet of ink from the ink channel orifice. Because theenergy of the two half pulse waveforms are combined toproduce the desired expulsion of ink, the second half pulsewaveform need not necessarily be as large as the firstwaveform to generate the desired droplet size. In fact,each half pulse waveform may be small enough in amplitudesuch that it will not, by itself, expel a drop of ink, butthe combined energy of both halfâpulse waveforms will beenough to expel a drop of ink. Thus, in this embodiment,the firing of a particular ink channel can be effectively?1015202530CA 02264038 1999-02-26wo 98/08687 PCT/US97/1468528controlled by selectively blocking the application of one-half of each pulse waveform to the transducer, while theother half pulse waveform of opposite polarity is allowedto be transmitted to each desired ink channel. Therefore,this embodiment allows the utilization of an array ofunipolar switches in switch array 140, instead of a morecostly array of bipolar switches.The array of unipolar switches can be employed with thealternate electrical waveforms shown in Figs. 15, 19 and22. Figs. 15, 19 and 22 correspond to a modification ofthe signal waveforms shown in Figs. 13, 18 and 22, whereinthe amplitude of the positive portion of each pulse in theburst series is set at approximately oneâhalf of the ampli-tude of the immediately preceding negative portion. Inthis embodiment, the positive portion of each pulsewaveform of the burst series will always be applied to eachink channel of the print head; however, the positive por-tion is not enough, by itself, to generate the necessaryenergy in the ink channels to expel a drop of ink. Thus,the selective transmission of the negative portions of thepulse waveforms of the burst series will allow the firingof the particular ink channels to which both the positiveand negative portions of the waveforms are applied. Theamplitude of the positive portion of each pulse waveform ispreferably set high enough such that a meaningful pressurewave contribution can be made within the ink channelstoward the expulsion of ink drops from the ink channels,but the amplitude is set low enough that spurious or unin-tended expulsion of ink is prevented from occurring whenonly the positive portions of the pulse waveforms aretransmitted to the ink channels.The preferred signal generator described in more detailabove in conjunction with Fig. 14 can be easily modified toimplement this aspect of the present invention. The modi?1015202530CA 02264038 1999-02-26WO 98108687 PCTIUS97/ 1468529fied switch array 140 consists of an array of unipolarswitches which can selectively switch the application ofthe negative portions of the amplified pulse waveforms 139to the print head transducers 141, but would universallyallow the transmission of all of the positive portions ofthe pulse waveform. The lookup table 152 is programmedwith waveform-data coordinate points corresponding to avarying waveform, such as illustrated in Fig. 15, whereinthe amplitude of the positive portion of each output pulseis set at approximately oneâhalf the amplitude of thepreceding negative portion, and this output is convertedinto the desired analog signal by D/A converter 154.Alternatively, the lookup table 152 outputs a digitalsequence corresponding to a typical sinusoidal-likewaveform wherein the positive and negative portions are atthe same amplitude, but the D/A converter 154 is controlledto output the positive portions at oneâhalf the amplitudeof the negative portions.This embodiment of the present invention can be alter-natively implemented with the described polarities re-versed, wherein the negative portion of each pulse waveformis universally allowed to pass to the print head transduc-er, but the positive portions are selectively switched tofire only selected ink channels. In addition, the ampli-tude levels shown in Figs. 15, 19 and 22 are illustrativeonly, and numerous other variations upon this theme areexpressly within the scope of the present invention. Forexample, Figs. 16, 20 and 23 depict alternative signalwaveforms, where the positive portions of the pulsewaveforms for each burst series are at a constant amplitudeD5 (Fig. 16), G8 (Fig. 20), or J6 (Fig. 23) and are univer-sally applied to the ink channel transducers, but thenegative portions are selectively switched to fire onlyspecific ink channels. In this embodiment, only the change?1015202530CA 02264038 1999-02-26WO 98/08687 PCT/US97/ 1468530in amplitude of the negative portions of the pulsewaveforms is employed to maintain constant drop speed ofthe expelled ink drop.Referring now to Fig. 17, another aspect of the presentinvention comprises the generation of "cancellation pulses"within the print head ink channels. One effect of operat-ing a piezoelectrically actuated print head is that pres-sure waves induced in the ink channel may create residualpressure waves which reverberate within the ink channel,even after an ink drop is fired from the channel. Residualpressure waves within the ink channel may be even morepronounced when the print head functions in a resonancemode. Because the residual pressure waves may interfere,either constructively or destructively, with the preciseoperation of the print head for the immediately succeedingfiring of the ink channel, it is typically necessary towait a period of time after each droplet is ejected from anink channel before the next firing of the ink channel toallow the residual energy from the previous pressure wavesto dissipate. Thus, it is desirable to be able to generatecancellation pulses which can be used to substantiallycancel the effects of the residual pressure wave energythat is present in the ink channels in order to permit moreimmediate ejection of successive ink droplets.The present invention comprises an apparatus and methodfor generating such cancellation pulses. Utilizing cancel-lation pulses results in improved operation of an inkjetprinter, since the firing of a given ink channel will notgenerate residual pressure waves which interfere with asucceeding firing of that same ink channel. In addition,because a given ink channel does not have to be signifi-cantly rested between firings to permit residual pressurewaves to dissipate, the number of pulses within a particu-lar burst series can be extended as long as possible before?1015202530CA 02264038 1999-02-26wo 98/08687 PCTIUS97/1468531the firing of the next burst series.A cancellation pulse generated in accordance with thepresent invention, when applied to a print head transducer,generates counterâpressure waves which substantially cancelthe effects of residual pressure waves present in the inkchannels. To be effective, a cancellation pulse mustproduce a counter-pressure wave which is substantially atthe same absolute energy level as the existing residualpressure wave in the ink channel, but which is out of phasewhen compared to the phase of the existing residual pres-sure wave. This counter-pressure wave is preferablygenerated by a calculated pulse burst of the same generaltype and shape as the signals pulses which are used to firethe ink channels, but the cancellation pulse is of a lesseramplitude and out of phase when compared to the priorpulses which created the residual pressure waves.Fig. 17 shows an embodiment of a waveform generator 160which can be utilized to practice the cancellation pulseaspect of the present invention. The waveform generator160 contains a lookup table controller 162 which transmitscontrol signals to both a burst series lookup table 164 anda cancellation pulse lookup table 166. Burst series lookuptable 164, like the lookup table 152 of Fig. 14, is prefer-ably programmed with a table of waveform coordinate pointsthat define one or more burst series signal waveforms whichcan be utilized to drive the print head transducers 141 tocontrollably eject droplets of ink from the print head.Cancellation pulse lookup table 166 is preferably pro-grammed with a table of waveform coordinate points whichdefine a set of one or more cancellation pulse waveformsoptimized for canceling residual pressure waves createdfrom the waveforms programmed into the burst series lookuptable 164.?1015202530CA 02264038 1999-02-26WO 98/08687PCT/US97/ 146853 2In the preferred embodiment, lookup table controller162 comprises a counter which increments and outputs a setof time factor coordinates (x-coordinates) to the burstseries lookup table 164. In response to the signal appliedfrom the lookup table controller 162, the burst serieslookup table 164 transmits a series of voltage amplitudecoordinates (yâcoordinates), corresponding to each timefactor X-coordinate, which together define a desired burstseries waveform. This output is applied to a digital-analog converter (âDAC") 168 to generate an analog burstseries waveform. The firing of a given burst serieswaveform sets and starts a timer 167, which is employed totrack the total elapsed time since the firing of the burstseries waveform. After a designated period of time, thetimer 167 triggers the lookup table controller 162, whichincrements and outputs a set of cancellation pulse timefactor coordinates (xâcoordinates) to the cancellationpulse lookup table 166. The cancellation pulse lookuptable 166 produces a series of voltage amplitudes (yâcoorâdinates), which correspond to each of the xâcoordinates,which together define a waveform which is appropriate togenerate counter-pressure waves to cancel the residualpressure waves in the ink channel created by the mostrecent burst series waveform. The output of the cancella-tion pulse lookup table 166 is then applied to the DAC 168to produce an analog cancellation pulse signal waveform.An alternate embodiment comprises the triggering of thecancellation pulse waveform immediately after the firing ofthe burst series waveform. In this embodiment, a timer 167is not required, as the lookup table controller 162 isprogrammed to immediately begin outputting a series ofcontrol signals to the cancellation pulse lookup table 166immediately following the transmission of control signalsto the burst series lookup table 164. An alternate embodi-?1015202530CA 02264038 1999-02-26W0 93/03687 PCTIUS97/1468533ment triggers the cancellation pulse after a fixed timedelay following the end of the previous burst serieswaveform. Yet another embodiment employs different cancel-lation pulse waveform shapes depending upon the number ofindividual pulses in the main burst series waveform.The cancellation pulse coordinate values within thecancellation pulse lookup table 166 can be generated in aplurality of ways. For example, the lookup table valuescan be derived empirically, by repeatedly firing a progres-sion of burst series pulses and corresponding "test" can-cellation pulses through the transducer of a given inkchannel. By incrementally adjusting the amplitude andphase parameters of the sample cancellation pulses whichare fired, a set of appropriate cancellation pulse parame~ters can be plotted which correspond to the preprogrammedset of burst series pulses which are used to drive theinkjet print head. The preferred values of the cancella-tion pulses generally correspond to experimental firingswherein the burst series pulses can be fired at a highereffective repetition rate of firing without interferencefrom prior firings. In general, the higher the effectivefiring repetition rate when using a particular cancellationpulse, then the more effective that cancellation pulse willbe to cancel residual pressure waves in the ink channels.Alternatively, the lookup table values can be obtainedby simulating the pressure wave effects resulting from thefiring of a given burst series in an ink channel, andmathematically calculating the parameters of a cancellationpulse signal to generate appropriate counterâpressurewaves. The simulation determines the amplitude and timingof a pulse which minimizes or cancels the residual pressurewaves at a certain time period after the firing of a previ-ous burst series. The character of the cancellation pulserequired is related both to the amplitude and phase of the.. .m.._.. ...TL___.__________,______________,___ â?1015202530WO 98108687CA 02264038 1999-02-26PCT/U S97/ 146853 4pressure waves present in the ink channel and to the par-ticular geometry and characteristics of the ink channel.The cancellation pulse for the preferred embodiment ispreferably either a half pulse, or one full pulse, of asinusoidal wave displaced in time from the type of pulse ina burst series used to fire an ink drop. Various simula-tions may be performed to determine the optimal character-istics and parameters of the desired cancellation pulses.The effectiveness of these cancellation pulses can beexperimentally confirmed, preferably in conjunction withthe repetition rate method described above.This aspect of the present invention is particularlyadvantageous when operated in conjunction with the multi-pulse aspect of the present invention, since by appropriatecancellation pulses, the pulses within an individual burstseries can thus be fired until very shortly before thefiring of the next burst series, without leaving residualpressure waves to interfere with the next firing. However,this aspect of the present invention can also be employedfor numerous other applications also, including singlepulse applications for example, when there is a need tofire many pulses near each other in time, without having toworry about interfering with the next firing of the inkchannel.While embodiments, applications and advantages of theinvention have been shown and described with sufficientclarity to enable one skilled in the art to make and usethe invention, it will be equally apparent to those skilledin the art that many more embodiments, applications andadvantages are possible without deviating from the inven-tive concepts disclosed anddescribed herein. The invention therefore should only berestricted in accordance with the spirit of the claimsappended hereto and to their equivalents, and is not to be?CA 02264038 1999-02-26WO 98/08687 PCT/US97Il46853 Srestricted by the description of the preferred embodiments,the specification or the drawings. .__w_______â______.«_._.-_M.._ 0 _.,_T_._â__
