AU594031B2 - Monitor jet control for ink jet printer - Google Patents

Description

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5 940MP3 1 COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952-1962 "a m u0iWwt "Qnius ft srmenowro' mode "un~m Itto 49 isd Om~w c~~t ot ptritkg Application Number: Lodged: Complete Specificati COMPLETE SPEC IFI CATION (Original FOR OFFICE USE: Cl ass Int. Class K9' 44 0 4., 44 44

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Priority: Related Art: 4 4 4 44 44 4 4 44 4 444 9, 4 4 .44 t 9 Name of Applicant: Address of Applicant Actual Inventor(s): COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH ORGANIZATION Limestone Avenue Campbell, in the Australian capital Territory, Commronwealth of Australia LESLIE JAMES WILLS Address for Service: DAVIES COLLISON, Patent Attorneys, AMP Building, Hobart Place, Canberra, ACT 2601 Complete Specification for the invention entitlIed: "APPARATUS FOR MONITORING AND ADJUSTING LIQUID JETS IN INK JET PRINTElRS" The following statement is a full description of this including the best aiethod of performiing it known to us -1Iinvention, L1 IA FIELD OF THE INVENTION This invention relates to the control of multi-jet ink jet printers of the high pressure synchronous drop type. In particular it concerns the maintenance of the proper phase relationship between the charging voltage and the droplet breakoff instant of the drops in e multi-jet printer.

BACKGROUND TO THE INVENTION AND PRIOR ART In ink jet printers, various devices have been used to improve the positional accuracy of recorded marks made by the impact of droplets on a recording medium.

In the case of jet printers of the charge amplitude 15 controlling variety, devices have been developed to maintain the proper phase relationship between the instant of droplet formation and the application of a charging potential to the charge electrode of the printer.

Streams of liquid are propelled through respective orifices by the static pressure applied to a contained fluid. These streams or filaments of liquid are ti a inherently unstable and tend to collapse at random i intervals, forming droplets of uneven size. Uniform *t" however, is required for uniform image 1 reproduction on the recording medium and a number of methods have been used to improve the uniformity of droplet size.

1 2 In one of these methods, uniform droplets are formed from the liquid stream by vibrating the stream issuing orifice at the resonant frequency of the orifice assembly using a piezo-electric deforming transducer to which is applied an alternating electric field. The amplitude of the initial perturbation on the fluid stream is determined by the strength of this electric field. In this method, droplet formation follows the introduction of a regular variscosity into the liquid filament by the regular vibration of the orifice.

Ceher methods that have been used to produce uniform aa*. droplets include squeezing or periodically constricting o the orifice so that uniform drop formation takes place a short distance from the nozzle of the droplet generating head of the jet printer. Variscosities have also been introduced into the fluid stream, to aid uniform drop formation, using the heat from a modulated light source to introduce a periodic temperature profile in the fluid stream. However, the most common 20 method used is to modulate the stream velocity and the a.

pressure within the chamber preceding the stream orifice using an electromechanical transducer which couples energy directly into the fluid.

*In all of these methods of uniform droplet formation, the position at which the stream breaks off into Suniform drops is a distance away from the orifice aperture. Thus there is a time lapse between the introduction of the regular variscosity and the instant the droplet separates from the stream. The duration of this time lapse is determined by several factors, *4 -3including the amplitude of the initiating perturbation and properties of the liquid; in particular, the surface energy, tihe viscosity and the specific gravity of the liquid. The variation of these properties in response to temperature changes, evaporation of liquid and other adventitious occurrences causes this time lapse to vary with time. Known methods exist to maintain a constant amplitude of the initiating perturbation. Hence a major factor in the unpredictability of this time lapse is the temporal change that occurs in these fluid properties.

Another factor that affects the variation of thise- time lapse is mechanical change in the total jet assembly.

Mechanical change occurs as a result of variations of 15 the characteristicS of the piezo-electric deforming transducer due to ageing, stress relaxation of the jet assembly and its mounting structure, and changes of the orifice size due to wear and/or the temperature 4 coefficient of expansion.

4t 20 In this type of ink jet printer the formed droplets are i selectively and variably charged by a charge field from a charge electrode and are subsequently deflected along a desired trajectory downstream by an electric field established by known means. A suitable recording 25 surface is positioned generally orthogonal to the t droplet stream and further downstream from the deflection field with the result that each droplet w i strikes the recording surface and forms a small spot thereon.

i 4 A charging electrode may comprise any suitable electrically conducting surface in close proximity to the unbroken stream (for example, a tube which surrounds the fluid or a pair of parallel plates positioned with the fluid filament between them).

The size of the charge on a drop depends on maintaining the proper phase relationship between the applied charging voltage and the droplet breakoff instant. When the droplet is formed during the transition from one charging voltage to another, charge size cannot be predicted and consequently droplets are misplaced on the printing surface.

A.r In most jet printers of this kind, a collector is placed between the deflection field and the recording 15 surface to intercept the undeflected stream of drops while droplets charged by the charging means are deflected by the deflection field to impact on the recording surface at a predetermined position.

S However, if the charging signal is in transition from "20 one charging voltage to another at the time of S. separation of the droplet from the fluid filament, then the charge induced on the droplet will be some function of the initial value, the transition slope and the final value of the charging signal. Thus, in order to 0 25 assign the exact charge on a droplet by the charging .j means at the time of separation, it is necessary to determine the proper instant of droplet separation in relation to the charging signal.

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*i In other words, it is necessary to precisely 'synchronise the droplet separation instant with the application of the charging signal at a time when the signal transition will have no influence on the value of charge imposed.

Since the time varying quantities affecting the time lapse between the introduction of the variscosity by the deforming transducer and the droplet breakoff instant vary only slowly, a phase synchronism also exists between the charging signal and the droplet formation transducer drive signal. To compensate for temporal changes in the fluid properties which affect the droplet breakoff instant, the phase relationship between these two signals must be continually corrected 15 to maintain the synchronism between breakoff instant and correct application of the charging signal.

Various techniques have been proposed for continuously maintaining this relationship. For example, in the specification of US Patent No 3,465,350 to Keur et al entitled "Ink Drop Writing Apparatus", there is described an arrangement for detecting whether ink droplets are properly charged which involves placing an ink drop detector at the location tt which they should be deflected. If the ink drop detector does not detect ink droplets, the phase of the instant of formation of the ink drops relative to the charging signal is sh3fted to correct for this. E, such a detection system causes problems due to ink buildup on the receiving element which decreases the sensitivity of the systems.

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"-t -6 The specification of US Patent No 3,769,630, to Hill et al, discloses a failure detection mode which makes use of a second collector (called a gutter) that receives drops when properly phased. Such a detection system, however, prohibits use of printing jets in a continuous printing operation due to print interruption for test mode servicing.

The specification o'2 US Patent No 3,769,632, to Julisburger et al, describes a system involving a special test cycle phase, separate from the print cycle, in which synchronizsation measurements are made.

Adjustments of the phase of the charging circuitry are made in the subsequent test cycles.

*in the specification of US Patent No 3,836,912, to Ghougasian et al, entitled "Drop charge Sensing Apparatus for an Ink Jet Printing System", there is disclosed an inductive charge sensing device 'which detects charges impressed on droplets passing adjacent to it but not impinging on it. A signal is devaloped S 20 whiob may be used to control an electric or electromechanical drop forming means.

The specification of US Patent No 3p7501191# to Naylor, entitled "Synchronization of Multiple Ink Jots"# 25describes a method in which a plurality of ink jet 2printing heads are monitored tnd controlled to obtain sequential synahronisati'i of dropa propelled from the heads.

f X 7 When ink jet printers are to be used for uninterrupted printing processes, such as the printing of continuous lengths of textile webs, a requirement for a period of time to be devoted, periodically, to the sampling of the droplets of the printing jets to test for phase synchronism is clearly contradictory to the continuous printing requirements.

DISCLOSURE OF THE PRESENT INVENTION The phase synchronisation methods outlined above have several drawbacks. The most serious drawbacks are the need 'dor sequential corrective action on each jet, the time devoted to servicing of a separate test mode, and the need for a separate sensor for each jet.

1 0 15 It is an objective of the present invention to provide a a method and apparatus which overcomes these drawbacks S- of the prior art techniques and ensures that all the printing jets (or a predetermined nuioetr of the printing jets) have their droplet formation and S. e 20 application of a rharge in synchronism, and in fixed phase relationsL with a monitor jet.

The basis of the present invention is the discovery, *from diligent observation of the breakoff instant of S droplets from an ink jet fluid filament, that drift of 25 the beakoff instant relative to the periodic perturbing signal applied to the pieao-olctrie deforming transducer is dependent mainly on changes in fluid properties; and furthermore, that the breakoff i: instants of a number of such fluid filaments issuing 30 from identical k heads ommu with a from identical ink jet heads coimmunicating with a 8 common ink supply reservoir tend generally to follow the same drift pattern. Thus, by monitoring the performance of one jet (which may be one of the printing jets but which is preferably a separate jet), it is possible to derive a correction signal from that "monitor jet" and use that correction signal to vary (apply an appropriate correction to) the monitor jet and also a number of the printing jets (normally all the printing jets) of the ink jet printer.

Thus to achieve the objective of the present invention, a monitor jet for the ink jet printer is mounted near to the printing jets and is supplied with printing fluid from the same source as the printing jets. The monitor jet is constructed to be the same as the printing jets of the jet printer, so that its droplet 1forming characteristics and performance generally are identical to the printing jets. Thus the droplet stream from the monitor jet experiences the same *variations in fluid properties as the printing jets and 20 consequently suffers from the same unpredictability of a time interval between the introduction of variscosity to the stream and the droplet breakoff. Whenever the monitor jet 'construction provides an indication that this time interval has varied from the required ,25 interval for proper operation of the ink jet printer, a sign&a is generated which is used to vary, and to correct, the monitor jet phase synchronism. Since an S* identical drift in the breakoff instant exists in all Sthe jets in this multiple jet system, the signal developed to correct the monitor jet phase synchronism S. is also used to correct all (or a predetermined number of) the jets of the printer in a parallel fashion

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To be more specific, according to the present invention, there is provided a method of maintaining a required phase relationship between the instant of droplet formation and the instant of application of a charge to a droplet in a liquid jet printer having a plurality of substantially identical jet bodies with respective orifices, said jet bodies being adapted to supply respective streams of liquid through said orifices; (ii) respective electromechanical transducer means associated with the jet bodies, said transducer means being activated in asynchronism by respective transducer drivers, for applying a periodic variscosity to their associated streams of liquid to cause said a associated streams to break up into droplets of uniform size; and (iii) a respective charging electrode associated with each jet body, to which voltage signals 20 are applied to induce charges on droplets generated S from its associated stream of liquid when a respective charge electrode driver gate is activated; said method comprising the steps of: establishing monitor jet on or within said jet v 25 printer, said monitor jet having a monitor jet body S which is constructed to be the same as said jet "o bodies of the jet printer and including a monitor 4 jet electromechanical transducer for applying a periodic variscosity to the stream of liquid produced by the monitor jet body, said monitor jet transducer being driven by a monitor jet driver, said monitor jet driver being activated by a

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06I 4 n r i* a *\M-k r 7 10 periodic signal from an activation circuit incorporating a signal advance/retard facility, a monitor jet charging electrode being mounted for inducing a charge on droplets of the monitor jet; applying a square wave alternating voltage signal to the monitor jet charging electrode, said square wave signal having a frequency equal to that of the periodic signal from the activation circuit; sensing the charge of a plurality of droplets of the monitor jet charged by the monitor jet charging electrode; whenever the sensed charge of the monitor jet droplets exceeds a predetermined positive or negative value, generating a correction signal which is applied to said activation circuit to advance or retard said periodic signal and thus vary the time of application of the periodic signal to the monitor jet driver relative to the time of application of the square wave signal to the monitor jet charging electrode; and applying a signal in phase synchronism with the square wave signal to the charge electrode driver gate of at least one of the jet bodies of the jet printer and applying the signal from the activation circuit to the transducer driver of at least said one of the jet bodies of the jet printer.

Also according to the present invention, there is provided apparatus for maintaining a required phase relationship between the instant of droplet formation 30 and the instant of application of a charge to a droplet in a liquid jet printer having a plurality of substantially idi~ntical jet bodies with respective Ce..

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M^f ^3 A.iMgB, 11 orifices, each jet body being adapted to supply a stream of liquid through its orifice; (ii) respective electromechanical transducer means associated with each jet body, each transducer means being activated by a respective transducer driver, for applying a periodic variscosity to its associated stream of liquid to cause said associated stream to break up into droplets of uniform size, and (iii) a respective charging electrode associated with each jet body, to which voltage signals are applied to induce ,,harges on droplets generated from its associated stream of liquid when its associated charge electrode driver gate is activated; said apparatus comprising: a monitor jet body mounted on or within said jet printer, said monitor jet body having the same construction as the jet bodies of the jet printer and including a monitor jet charging electrode for *inducing a charge on droplets of a stream of 04 droplets produced by the monitor jet body by the 20 action of a monitor jet electromechanical *transducer driven by a monitor jet driver connected for activation thereof to the output of an 0"0 0 activation circuit adapted to produce a periodic voltage signal at said output, said activation circuit including a signal advance/retard facility, a. i a the output of said activation circuit being 4. connected to the transducer driver of at least one I *of the jet bodies of the jet printer; vo,?age signal generating means connected to the S* 30 monitor jet charging electrode, said voltage signal I generating means being adapted to produce a square wave voltage signal having a frequency equal to the periodic signal at the output of said activation 1L ~RRn~ 12 circuit; said voltage signal generating means being synchronised with the charge electrode driver gate of said at least one jet body of the jet printer; sensing means for sensing the charge of a plurality of droplets of said monitor jet; and correction signal generating means responsive to an output signal from said sensing means to generate a correction signal if the sensed charge of a plurality of monitor jet droplets exceeds a predetermined positive or negative value, said correction signal being applied to said activation circuit to advance or retard the periodic signal and thus vary the time of application of said periodic signal to the monitor jet driver relative to the time of application of the square wave signal to the monitor jet charging electrode.

A preferred embodiment of the present invention will now be described with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a generalised view (partly schematic, partly perspective, and partly magnified) of a monitor jet assembly constructed in accordance with the present invention.

Figure 2 is a functional diagram showing the inter-relationship of the monitor jet and phase control system.

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I 'V t' 7 -1 13 Figure 3 illustrates waveforms generated by the control system of the present invention, relative to the ideal droplet breakoff instant.

Figure 4 is a schematic diagram showing the effect of an incremental phase change in the transducer modulating signal on the droplet break off instant.

Figure 5 is a diagram in three parts illustrating the change in a portion of a single waveform when it is adjusted in accordance with the first aspect of the present invention.

Figure 6 is a schematic diagram showing one way in which the apparatus of' the present invention may be incorporated into a jet printer.

Figure 7 illustrates another way in which the apparatus 15 of the present invention may be incorporated into a jet printer.

Figure 8 is a schematic diagram of the apparatus of Figure 7, in which the sensor and control elements are multiplexed to service a number of jets in a time-position serial mode.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS In the apparatus shown in Figure 1, ink or dye solution is propelled through a jet nozzle 16 in a fine stream 3 from an ink jet body 2 connected to a supply tube 1 25 which communicates with a stable pressurised ink supply source (not shown).

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14 A transducer driver 13 applies a time periodic alternating voltage 14 to electrically deformable transducers within the ink jet body 2. The liquid stream 3 issues from the nozzle 16 with a regular periodic variscosity which causes the stream to break up into droplets 5 within a tubular charge electrode 4.

The droplets 5 are formed at the same frequency as that of the transducer drive signal i4. The droplets impinge downstream on the inclined interior surface of a collection tube 6, and flow evenly down this surface to join a small volume of liquid 7 contained within tube 6 by surface tension. Liquid from the collected volume 7 periodically drips from an aperture in the lower end of tube 6 into a scavenging system collector 9 to be returned to the ihk supply source via tube 10. By maintaining a ,ufficient distance between the sensor tube 6 and the scavenging collector 9, complete electrical isolation of sensor tube 6 is S. achieved relative to the bulk ink supply. This complete S' 20 electrical isolation from the bulk fluid supply allows rc 0A. the sensor tube 6 to function as a highly reliable and S#*4 accurate charge sensing device.

so# A Instead of a tubular collector 6 with an aperture at its lower, closed end, any other suitable collection SS 25 device (such as an inclined plate with a channel formed S a therein) may be used to receive and collect the a. droplets 5, then periodically discharge the collected 0, 4 liquid. i rJIII A charge electrode driver 12 applies a square wave voltage 15 to the charge electrode 4. The signal r alternates from a positive voltage to a complementary 0

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At the instant of separation from the fluid stream 3, droplets 5 acquire a charge induced by the charge electrode 4. The induced charge is opposite in sign to the voltage applied to the electrode 4. This induced charge is collected on the tube 6 and is detected by a sense amplifier 11, which produces a signal which is indicative of the charge on the collected droplets.

Thus the signal generated by the sense amplifier 11 is indicative of whether the breakoff instant occurs at a point such that when a droplet is influenced by the charge electrode 4, the signal 15 is at its negative level, the positive level, or at one of its transition slopes.

Referring now to Figure 2, the sense amplifier 11 produces a signal which is interpreted by a phase control logic unit 20. This logic unit 20 is adapted to 20 produce two signals, designated ADV and RTD. Signals ADV and RTD are used to instruct a variable (incremental) phase shifter 18 to alter its output signal. Variable phase shifter 18 produces a 64-step approximate sine-wave in response to an input 25 signal 64F (comprising a series of clock pulses from a clock 21. A drop synchroniser 19 (simultaneously) receives signal 64F from clock 21 and produces signals 23, 24 and 22 (which are designated STROD, STROD/4 and SIGNAL PHASE respectively), each at one-sixtyfourth the frequency of signal 64F.

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-7 i I ii: -i 16 In a prototype of the illustrated embodiment, SIGNAL PHASE was a square wave which alternated in voltage between +12V and -12V at the same frequency as the signal from the transducer driver. Any one of several alternative circuits may be used to generate this square wave signal; an integrated circuit chip type 1488 available as a standard item from the Signetics Company Bipolar Division has been found to work in a satisfactory manner. SIGNAL PHASE is applied to the charge electrode driver 12 to control the temporal application of charge to the electrode 4. The charge on the electrode 4 induces charges on droplets formed from the stream 3 flowing from nozzle 16.

In Figure 3, the ideal breakoff instant relative to 15 SIGNAL PHASE is shown together with its relationship to the above-mentioned signals 64F, STROD and STROD/4.

If the breakoff instant occurs when SIGNAL PHASE is at a positive level, sense amplifier 11 detects a negative charge on drops 5, causing the phase control logic ar a* W4 to a a 0 a so# a a a a *a a.

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k 41 17 unit 20 to assert signal RTD which instructs the incremental phase shifter 18 to retard the phase of the transducer modulating sine wave 14 by one step on each clock pulse 25 of clock 21 relative to signal A second clock 26 operates asynchronously to other waveforms (in the prototype embodiment, this clock 26 produced a signal which oscillated at 3 cycles per second). Consequently, for each pulse of the second clock 26, the actual breakoff"point retards by one step towards the ideal breakoff point whenever it occurs with signal phase positive.

0#00 "However, if the actual droplet breakoff occurs when SIGNAL PHASE is at a negative level, then the sense amplifier 11 detects a positive charge on drops 5 and 15 causes the phase control logic unit 20 to assert signal ADV, which instructs the variable phase S. shifter 18 to advance the phase of the transducer modulating sinewave 14 by one step on each pulse of the second clock 26.

*4 S4 20 Consequently, for each pulse of sample clock 26, the actual breakoff point advances by one step towards the ideal breakoff point whenever it occurs with signal S" phase negative.

The effect of a single instance of this procedure is f, 0 5 shown in Figure 4. Solid lines 30 and 31 represent,r respectively, the transducer modulating signal and a schematic representation of the droplet breakoff.

Dotted lines 32, 33 indicate the response to a single 1 -V *11 18 cycle assertion of the signal ADV. Both signal 32 and droplet breakoff point 34 have advanced towards the ideal breakoff point shown in Figure 3.

If the breakoff instant occurs near or on the transition from negative to positive of the signal phase, sense amplifier 11 detects an indeterminate charge on drops 5. In this situation, the phase control logic unit 20 assumes one of three possible states. ADV will be set true if the indeterminate charge on drops 5 is above a threshold positive level, RTD will be set true if the indeterminate charge is less than the negative threshold level, and whenever the charge level is between these two thresholds, oo neither will be set true. In this last case, no 15 adjustment to the variable phase shifter is made on the occurrence of the sample clock pulse as the breakoff ii* point is now in the correct timing relationship with the reference signal SIGNAL PHASE and thus resides at its ideal position at the ideal breakoff point.

The signal 23 (also shown as signal STROD in the drawings) produced by drop synchroniser 19 is a signal *in phase with SIGNAL PHASE (also shown as signal 22).

STROD is a mnemonic for STRObe Dropg The duration of STROD is exactly one drop period and its timing is 25 centred on the ideal breakoff point of the monitor jet.

Signal 24 (also shown as signal STROD/4), which is one quarter the duration of the STROD signals, is also timed in this relationship. 4 1 I 19 Signals 23 and 24 are only output by the synchroniser 19 after the assertion of the input signal DROD (signal 27, the terminology being a mnemonic from the words DROp Demand), and are used as the reference signals to the printing jets or as the primary datum focus.

Both the variable phase shifter 18 and the drop synchroniser 19 are digital read only memory elements (ROM) which contain digital values representing signals 14, 15, 23 and 24 shown in Figures 2 and 3.

Signal 14 is an approximate sine wave of period the same as that of signal 64F derived by means of a digital to analog converter and amplifier transducer driver 13 from the digital values stored in variable 15 phase shifter 18. Signals 15, 23 and 24 are binary digital signals, again having the same period as signal 64F generated in synchronism with the input pulses Truth table 5a of Figure 5 illustrates the instantaneous response at the output of variable phase shifter 18 to the application of the input signals ADV t* and R2D. Truth table 5b illustrates, in numeric tabular S' representation, the effect on the periodic output of phase shifter 18. The value of the contents of three S* I successive locations which represent the value of successive steps of the approximate sinewave 14 is shown in column 1. These values are n-1, n and n+1.

Columns 2, 3 and 4 show the next step value at any point in response to the directive states shown in table 5a. At any step n of the approximate sinewave 14, the next natural sequence step would be to n+l as indicated in row n column 1. if, however, the input signal ADV is asserted, the next step would have the value of location n+2, thus advancing the sinewave by one step on its natural sequence. If the input signal RTD is asserted, the next step would remain at the value of location n, thus effectively retarding the sinewave by one step from its natural sequence.

In this way the sinewave is advanced or retarded as shown in Figure 5c. At time t~nl the digital value will be n, n+11 or n+2 depending on whether the o directive state is retard, no change, or advance.

The effect of this action causes a change in the phase relationship between the transducer modulating signal 14 and the digital reference signals 15, 23 ':*and 24.

By continual performance of this adjustment to the transducer modulating signal and the relevant reference signals of the monitor, it will now be clear to one skilled in this art that repeated performance of thitb adjustment will result in accurate synchronism of 'a the breakoff instant of the monitor jet with the ***tooreference signals.

Thus these reference signals (that is, the transducer modulating signal 14 and the reference signals STJROD and STROD/4), can be the means for supplying 'the required primary signals to the printing jets of ti -21 multiple jet printer. The signals 23 and 24 are used as print command signals to a charge electrode driver gate and signal 14 is applied to the droplet forming means, namely the liquid jet modulator. These signals will be so adjusted by the monitor jet control means that the source of drift in the droplet breokoff instant of the printing jets, which is the same as the source of drift in the monitor jet, will be automatically and continuously corrected.

In addition to the temporal changes in droplet formation which have been described above, there exist also individual differences between ink jet modulators due to manufacturing tolerances between components.

Two parameters contribute the major part of this variance, namely, jet orifice size variation and variation in the coup.,,ng coefricient of the *piezo-clectric deformirng transducer used to impart the ~initial perturbation onto the ink jet stream.

Differences in these components at the time of manufacture will translate into differences in the break off distance of the modulated jets.

**,one form of apparatus that may be used to correct these individual differences between jet modulators is shown A* in Figure 6. In this apparatus# a monitor jet assembly 40 produces three output signals# namely, transducer modulating signal 14, signail 23 (STROD) and signal 24 (STROD/4). The signals 23 and 24 are properly phased with respect to the ):reak off instant of the monitor jet. By adjustment of a respective potentiometer 37, the strength of signal 14 applied to r 0 22 each jet modulator can be increased or decreased. This has the effect of varying the amplitude of the mechanical deformation of the respective piezo-electric transducer and directly controls the time lapse to the break off instant.

9999 9 9 9.99 0999 9 99 999, 9s 9990 It 9 09 99 9 4D a A method by which the break off instants of the printing jets can be made to occur at the same instant as each other, and at the same instant as (or in fixed phase relationship to) the monitor jet, and hence be in proper phase relationship with the printing signal STROD, using the equipment illustrated in Figure 6, will now be described.

Signals STROD (23) and STROD/4 (24) are both output during normal printing operation on demand from input signal DROD Either full width printing signal STROD (23) or quarter width test mode printing signal STROD/4 (24) may be selectively applied by means of a switch 36 to enable charge electrode driver switch Switch 35 enables ramp signal 41 to be applied to charge electrodes 39 through a current limiting protection resistor 42.

When switch 36 enables STROD/4 (24) to enable switch to pass ramp signal 41 to electrodes 39, then the resulting gated signal appearing on electrodes 39 is 25 present for only one quarter of a droplet cycle time.

This gated signal is represented in timing diagram form in Figure 6 as signal 43.

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*4 0 00 4 *4 0 *4 *0 *04 I' 04 I4 As is well known to those skilled in this art, the application of such, a narrow pulse to a droplet charging means such as electrode 39 will succeed in properly charging droplets only if the droplet formation instant occurs when the signal is present on the electrodes 39. This can be made to occur by adjustment of potentiometer 37. The observation or monitoring of the efficacy of the adjustment can be done in several ways. For example, the printed output of an operating ink jet printer may be observed whilst adjusting potentiometer 37 and whenever the raster printed output on the printed surface appears reasonably regular to the observer, the breakoff instant will be in proper phase relationship with signal STROD (23) for all normal printing operations.

A very close timing relationship with the breakoff instant of the monitor jet will also exist and hence automatic corrections to the breakoff instant of the monitor jet assembly by the control means in Figure 2 will also be a correction to the printing jets since the same transducer modulating signal from amplifier 13 is used. Thus setting of the breakoff instant of each of the printing jets to the same instant as a monitor jet assembly can be made easily using this apparatus.

Variations or modifications of the illustrated apparatus and techniques described above are possible.

For example, potentiometer 37 may be replaced by any other device which can be used to attenuate an electrical signal. Also, the same effect can be achieved by using any other device (one example being a

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.d's M I I I I -1 11 .1 A V I, 1 11 I I 1 1-1. "Wr 24 phase change circuit) to bring about a modification of the breakoff instant of the droplet relative to the reference signal. Signals STROD and STOD/4 may be substituted by other forms of signal which display the same intent, and other methods for checking for the breakoff instant of the printing drops relative to the breakoff instant of the monitor jet may be used, without departing from the present inventive concept.

The apparatus described above can readily be implemented in a form which does not require the operator to observe the printing operation. Such apparatus is now described with reference to Figure 7.

In the apparatus of Figure 7, a monitor jet assembly produces and supplies a transducer modulating S" 15 signal 14, si.ynal STROD (23) and signal STROD/4 (24).

Sensor units 53, 54 and 55 are used to determine if the breakoff instants are properly co-ordinated with the breakoff instant of the monitor jet. This sensor unit may be a charge sensing device for detecting charge on the droplets or it may be an optical device such as a silicon photo-detector or an array of such silicon or similar photo-detectors arranged to determine if the drops are properly charged by observing the resultant trajectory pattern after the droplets have traversed a 25 deflection field (not shown).

t, The outputs of sensor units 53, 54 and 55 act as directive inputs to respective feedback control units 50, 51 and 52, which control breakoff control units 44, 45 and 46. Each breakoff control unit may be *4; M sie*''s<s h s~iio .^~iliii-ijM 25 any one of a number of known devices for varying the breakoff instant of a single jet. It may be a signal attenuator such as potentiometer 37 of Figure 6; it may be a phase change element such as integer 18 of Figure 2; or it may be any other of the previously disclosed devices generally used for this purpose.

An arrangement to reduce the amount of hardware required to realise this invention on a jet printer having a large number of individual jets is shown in Figure 8. In this arrangement, the transducer modulating signal is applied to the piezo-electric deforming transducers of jet bodies 47, 48 and 49 after passing through breakoff control units 44, 45 and 46, respectively. A feedback control unit 59, which may be a micro computer or other specialised hardware, is used to apply the correction to breakoff control units 44, s* 45 and 46 as a result of observations made by sensor element 56.

In the configuration shown in Figure 8, the feedback controller is being ured in a time-position serial multiplexed mode. Sensor 56 is sampling a response from the droplets generated using jet body 47 and applying the corrective result to breakoff control S| ,unit 44. When this adjustment has been performed according to the earlier teaching of this invention, Sthe sensor 56 is then multiplexed to sample position 57 a* and the control output of feed back controller 59 is multiplexed to apply correction to breakoff control -if- .s i.t a o is W S, v f 26 unit 45. This action is repeated until all the jet bodies of the array have received break off phase synchronisation servicing.

Apparatus to perform this procedure could consist of an optical sensor such as a television camera, to observe the printing of the jet streams in turn, and a mechanically actuated tool to engage a series of potentiometers in place of units 44, 45 and 46.

INDUSTRIAL APPLICABILITY The present invention has been developed for use in jet printers which are used to print patterns on textiles.

However, the invention is applicable to any type of jet printer.

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Claims (6)

1. A method of maintaining a required phase relationship between the instant of droplet formation and the instant of application of a charge to a droplet in a liquid jet printer having a plurality of substantially identical jet bodies with respective orifices, said jet bodies being adapted to supply respective streams of liquid through said orifices; (ii) respective electromechanical transducer means associated with the jet bodies, said transducer means being activated in asynchronism by respective transducer drivers, for applying a periodic variscosity to their associated streams of liquid to cause said associated streams to break up into droplets of uniform size; and (iii) a respective charging electrode associated with each jet body, to which voltage signals are applied to induce charges on droplets generated from its associated stream of liquid when a respective charge electrode driver gate is activated; said method comprising the steps of: establishing a monitor jet on or within said jet printer, said monitor jet having a monitor jet body which is constructed to be the same as said jet bodies of the jet printer and including a monitor jet electromechanical transducer for applying a periodic variscosity to the stream of liquid produced by the monitor jet body, said monitor jet transducer being driven by a monitor jet driver, said monitor jet driver being activated by a periodic signal 11$ 0 0 *9 1 :1 r C1 fl~.lirl l 'J 28 from an activation circuit incorporating a signal advance/retard facility, a monitor jet charging electrode being mounted for inducing a charge on droplets of the monitor jet; applying a square wave alternating voltage signal to the monitor jet charging electrode, said square wave signal having a frequency equal to that of the periodic signal from the activation circuit; sensing the charge of a plurality of droplets of the monitor jet charged by the monitor jet charging electrode; *b whenever the sensed charge of the monitor jet droplets exceeds a predetermined positive or negative value, generating a correction signal which is applied to said activation circuit to 0 advance or retard said periodic signal and thus vary the time of application of the periodic signal to the monitor jet driver relative to the time of application of the square wave signal to the monitor jet charging electrode; oand applying a signal in phase synchronism with the square wave signal to the charge electrode driver gate of at least one of the jet bodies S of the jet printer and applying the signal from the activation circuit to the transducer driver of at least said one of the jet bodies of the jet printer. ,1

2. A method as defined in claim 1, in which the frequency of said periodic signal is a function of the frequency of signals generated by a first I v A 29 clock, and said correction signal is generated by a logic circuit, said logic circuit being responsive to a signal indicative of said sensed charge of a plurality of monitor jet droplets and (ii) to signals from a second clock,,

3. A method as defined in claim 1 or claim 2, in which said sensing of the charge of a plurality of monitor jet droplets is effected by collecting said monitor jet droplets in a tubular container and periodicially discharging collected liquid from said tubular container, and sensing the charge on the tubular container with a sense amplifier. *i.4

4. A method as defined in claim 1, substantially as hereinbefore described with reference to the o f accompanying drawings. Apparatus for maintaining a required phase relationship between the instant of droplet formation and the instant of application of a 1 charge to a droplet in a liquid jet printer having a plurality of substantially identical jet bodies with respective orifices, each jet body being adapted to supply a stream of liquid through its orifice; (ii) respective electromechanical transducer means associated with each jet body, L each transducer means being activated by a respective transducer driver, for applying a periodic variscosity to its associated stream of 1 liquid to cause said associated stream to break up into droplets of uniform size, and (iii) a respective charging electrode associated with e/ach 0 ii4 I C 30 jet body, to which voltage signals are applied to induce charges on droplets generated from its associated stream of liquid when its associated charge electrode driver gate is activated; said apparatus comprising: a monitor jet body mounted on or within said jet printer, said monitor jet body having the same construction as the jet bodies of the jet printer and including a monitor jet charging electrode for inducing a charge on droplets of a stream of droplets produced by the monitor jet body by the action of a monitor jet electromechanical transducer driven by a ,r monitor jet driver connected for activation *l thereof to the output of an activatin circuit adapted to produce a periodic voltage signal at said output, said activation circuit including 4 a signal advance/retard facility, the output of said activation circuit being connected to the transducer driver of at least one of the jet bodies of the jet printer; voltage signal generating neans connected to the monitor jet charging electrode, said 0 voltage signal generating means being adapted to produce a square wave voltage signal having I, a frequency equal to the periodic signal at the S V output of said activation circuit; said voltage 0 0 4 signal generating means being synchro-ised with ,the charge electrode driver <'ate of said at least one jet body of the jet printer; sensing means for sensing the charge of a plurality of droplets of said monitor jet; and 40 i, L AI r I I s I. 1^ 5) -31 correction signal generating means responsive to an output signal from said sensing means to generate a correction signal if the sensed charge of a plurality of monitor jet droplets exceeds a predetermined positive or negative value, said correction signal being applied to said activation circuit to advance or retard the periodic signal and thus vary the time of application of said periodic signal to the monitor jet driver relative to the time of application of the square wave signal to the monitor jet charging electrode. Apparatus as defined in claim 5, in which said sensing means comprises a tubular container mounted to receive droplets of said monitor jet and an associated sensing amplifier which generates said toot means output signal in response to the irecharge on said tubular container, said tubular container being adapted to periodically discharge liquid from it.

7. Apparatus as defined in claim 5 or claim 6, including a first clock producing signals from which the frequency of said periodic signal is derived, and a second clock producing signals which are input to sai.d signal correction generating me~ans.

8. Apparatus as defined in claim 5, substantially as h~treinbef ore described with reference to the accompanying drawings. DATED this fourth day of December 1989 COMMONWEALTH SCItNTIPIC AND XNDUST1ntAL RlESEARCH ORGANISATION 0 by its Patent Attorneys DAVXBS &COLLISON