APPARATUS FOR MONITORING AND ADJUSTING LIQUID JETS IN INK JET PRINTERS

Abstract: The production of uniform droplets (5) in ink jet printers is effected by introducing a periodic variscosity into the liquid stream (3) which leaves each jet body (2) of the printer. Effective printing with these droplets (5) requires the correct synchronism between breakoff of droplets from the liquid stream and the application of a charge to the droplets, via a charging electrode (4). To enable compensation to be made to ensure this synchronism remains correct, despite variations in the properties of the printing liquid, droplets from one printing jet (monitor jet) are monitored. Whenever the monitor jet observation shows that the synchronism has varied, a correction signal (ADV, RTD) is generated to cause the application of variscosity, or the application of charge, to be altered to compensate for the drift in synchronism. A preferred monitor jet construction includes a collector (6) for charged droplets, which receives droplets (5) from the monitor jet and periodically discharges collected liquid (7). A sense amplifier (11) is responsive to the net charge on the collector (6), and the signal from the sense amplifier (11) is used to generate the correction signal.

Description: TITLE: "APPARATUS FOR MONITORING AND ADJUSTING LIQUID JETS IN INK JET PRINTERS" 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 a 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 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 inherently unstable and tend to collapse at random intervals, forming droplets of uneven size. Uniform dropsize, however, is required for uniform image reproduction on the recording medium and a number of methods have been used to improve the uniformity of droplet size. 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. Other methods that have been used to produce uniform droplets include squeezing or periodically constricting 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 method used is to modulate the stream velocity and the 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 uniform 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, including the amplitude of the initiating perturbation and properties of the liquid; in particular, the surface energy, the 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 this time lapse is mechanical change in the total jet assembly. Mechanical change occurs as a result of variations of 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 coefficient of expansion. In this type of ink jet printer the formed droplets are 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 surface is positioned generally orthogonal to the droplet stream and further downstream from the deflection field with the result that each droplet strikes the recording surface and forms a small spot thereon. 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. In most jet printers of this kind, a collector is placed between the deflection field and the recording 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. However, if the charging signal is in transition from one charging voltage to another at the time of 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 assign the exact charge on a droplet by the charging means at the time of separation, it is necessary to determine the proper instant of droplet separation in relation to the charging signal. 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 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 &Λ_ 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 to 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 shifted to correct for this. Unfotunately, such a detection system causes problems due to ink buildup on the receiving element which decreases the sensitivity of the systems. 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 of 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 synchronisation 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 developed which may be used to control an electric or electromechanical drop forming means. The specification of US Patent No 3,750,191, to Naylor, entitled "Synchronisation of Multiple Ink Jets", describes a method in which a plurality of ink jet printing heads are monitored and controlled to obtain sequential synchronisation of drops propelled from the heads. 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 (a) the need for sequential corrective action on each jet, (b) the time devoted to servicing of a separate test mode, and (c) the need for a separate sensor for each jet. It is an objective of one aspect of the present invention to provide a method and apparatus which overcomes these drawbacks of the prior art techniques and ensures that all the printing jets (or a predetermined number of the printing jets) have their jets in synchronism with each other and in fixed phase relationship with a monitor jet. The basis of the present invention is the discovery, from diligent observation of the breakoff instant of droplets from an ink jet fluid filament, that drift of the breakoff instant relative to the periodic perturbing signal applied to the piezo-electric deforming transducer is dependent mainly on changes in fluid properties; and furthermore, that the breakoff instants of a number of such fluid filaments issuing from identical ink jet heads communicating with a 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 apply an appropriate correction to the monitor jet and also to a number of the printing jets (normally all the printing jets) of the ink jet printer. It is an object of a second aspect of the present invention to provide improved droplet monitoring equipment which can be used in the arrangement of the first aspect of this invention, and which is simple to fabricate, more economical to construct than other types of droplet monitor, and which can be seen by a user of an ink jet printer to be functioning. This last point is important, for prior art droplet monitors such as those using charge sensors, acoustic arrangements or photo-detectors - provide no visual indication that they are functioning correctly, and this is regarded by operators of ink jet printers as a serious drawback, creating uncertainty in the minds of such operators. To achieve this objective ' of the first aspect of the present invention, a monitor jet for the ink jet printer is mounted near to the printing jets (in fact, it may be one of the printing jets) and is supplied with printing fluid from the same source as the (or the other) printing jets. If the monitor jet is separate from the printing jets, its construction is similar to that of the main printing jets, so that its droplet forming characteristics and performance generally are the same as the main printing jets. Thus the droplet stream from the monitor jet experiences the same variations in fluid properties as the printing jets and consequently suffers from the same unpredictability of 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 interval for proper operation of the ink jet printer, a signal is generated which is used by a feedback servo loop to correct the monitor jet phase synchronism. Since an identical drift in the breakoff instant exists in all the jets in this multiple jet system, the signal developed to correct the monitor jet phase synchronism is also used to correct all (or a predetermined number of) the jets of the printer in a parallel fashion without the need for individual droplet detection or interruption of the printing operation of the printing jets. According to the first aspect of the present invention, there is provided a method of monitoring and correcting the phase relationship between the instant of droplet formation and the application of a charge to a droplet in an ink jet printer having (i) a plurality of substantially identical jet bodies with respective orifices, each adapted to supply a stream of liquid, and including means 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 (ii) a charging electrode associated with each jet body, for inducing a charge on droplets produced from the respective stream of liquid, said method comprising the steps of a) observing the droplets generated by a monitor jet in the jet printer; b) generating a signal whenever said observation indicates that the time interval between the introduction of variscosity to the liquid stream from said monitor jet and the application of a charging voltage to the charge electrode of the monitor jet departs from the value of this time interval for proper operation of the monitor jet; c) applying said signal to a servo loop to vary said time interval to reduce the departure thereof from the value for proper operation of the monitor jet; and d) applying said signal to at least one other printing jet of the jet printer, to effect the same variation of said interval, respectively, in each said other printing jet to which said signal is applied. - The application of the signal to the servo loop of the monitor jet and to the other printing jets may be to adjust the application of the periodic signal which causes the onset of variscosity, or it may be to adjust the application of the charging signal. Whichever approach is used, the phase synchronism between the signals for introducing variscosity and applying charge is varied. The adjustment will normally be by an amount of up to a single cycle of the periodic signal which causes the onset of variscosity. The first aspect of this invention also encompasses apparatus for performing this method, as recited in the claims of this specification. According to the second aspect of the present invention, there is provided a droplet monitor for use in an ink jet printer having a plurality of substantially identical jet bodies, each with a respective charging electrode, said droplet monitor comprising a) a monitor jet body having a construction substantially the same as each jet body of said plurality of jet bodies, with an associated charging electrode; b) collection means adapted to receive the droplets generated by said monitor jet body and adapted to discharge therefrom liquid collected by said collection means; c) means, in electrical isolation from said collection means, for receiving liquid discharged from said collection means; d) means for applying a voltage signal to the charging electrode associated with said monitor jet body, said voltage signal varying between a positive voltage value and an equal negative voltage value; e) sensing means adapted to sense the net charge of the liquid collected by said collection means; and f) logic means, responsive to said sensing means, for generating a signal indicative of the qualitative change required to ensure the correct phase relationship between the instant of droplet formation and the application of said voltage signal to the charging electrode associated with said monitor jet body. Preferably the signal from the logic means is used to increase or reduce, by an amount of up to a single cycle of either the periodic signal which causes the onset of variscosity to the liquid stream of the monitor jet, or the varying voltage signal, the time between a predetermined instant in the application of the periodic signal causing onset of variscosity and the application of the voltage signal to the charging electrode, in both the monitor jet body and in at least one of the plurality of jet bodies of the jet printer. Embodiments of both aspects of the present invention, and modificatons and variations ^ that may be incorporated therein, 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 second aspect of the present invention, together with charging electrode, charge sensing tube and scavenging fluid collector. Figure 2 is a functional diagram showing the inter-relationship of the monitor jet and phase control system. Figure 3 illustrates waveforms generated by the control system of the first aspect 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 form of apparatus that may be used to achieve the proper phase relationship between the printing jets and the monitor jet. Figure 7 is another form of the apparatus shown in Figure 6, with individual sensors and controls for printing jet phase synchronisation. Figure 8 is a schematic diagram of the apparatus of Figure 7, in which the sensor and feedback 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 which communicates with a stable pressurised ink supply source (not shown). 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 14. The droplets 5 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 ink supply source via tube 10. By maintaining a sufficient distance between sensor tube 6 and scavenging collector 9, complete electrical isolation of sensor tube 6 is achieved relative to the bulk ink supply. This complete electrical isolation from the bulk fluid supply allows sensor tube 6 to function as a highly reliable and accurate charge sensing device. Instead of a tubular collector 6 with an aperture at its lower, closed end, any other suitable collection device (such as an inclined plate with a channel formed therein) may be used to receive and collect the droplets 5, then periodically discharge the collected liquid. A charge electrode driver 12 applies a time periodic alternating voltage 15 (which may have a square wave form) to the charge electrode 4. Signal 15 alternates from a positive voltage to a complementary negative voltage, relative to ink supply zero potential, at the same frequency as the periodic transducer drive signal 14. At the instant of separation from the fluid stream 3, droplets 5 acquire a charge induced by charge electrode 4. The induced charge is opposite in sign to the voltage applied to electrode 4. This induced charge is collected on tube 6 and is detected by a sense amplifier 11, which produces a signal indicating whether the breakoff instant occurs on the negative level, the positive level, or on one of the transition slopes of signal 15. In the arrangement shown in Figure 2, the sense amplifier 11 produces a signal which is interpreted by a phase control logic unit 20. Logic unit 20 is adapted to 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 signal 64F (comprising a series of clock pulses 25) 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. 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 and thence to charge electrode 4 to induce charges on droplets formed from the stream 3 flowing from nozzle 16. In Figure 3, the ideal breakoff instant relative to 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 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 15. 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. 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 causes the phase control logic unit 20 to assert signal ADV, which instructs the ' variable phase shifter 18 to advance the phase of the transducer modulating sinewave 14 by one step on each pulse of the second clock 26. 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 phase negative. The effect of a single instance of this procedure is shown in Figure 4. Solid lines 30 and 31 represent, respectively, the transducer modulating signal and a schematic representation of the droplet breakoff. Dotted lines 32, 33 indicate the response to a single 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, neither will be set true. In this last case, no adjustment to the variable phase shifter is made on the occurrence of the sample clock pulse as the breakoff 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 Drop. The duration of STROD is exactly one drop period and its timing is 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. 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 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 25. 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 and RTD. Truth table 5b illustrates, in numeric tabular representation, the effect on the periodic output of phase shifter 18. The value of the contents of three 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+1 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, , » the digital value will be n, n+1, or n+2 depending on whether the 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 this adjustment will result in accurate synchronism of the breakoff instant of the monitor jet with the reference signals. Thus these reference signals (that is, the transducer modulating signal 14 and the reference signals STROD and STROD/4) , can be the means for supplying the required primary signals to the printing jets of a 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 breakoff 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 coupling coefficient of the piezo-electric deforming 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 in Figure 6. In this apparatus, a monitor jet assembly 40 produces three output signals, namely, transducer modulating signal 14, signal 23 (STROD) and signal 24 (STROD/4). The signals 23 and 24 are properly phased with respect to the break off instant of the monitor jet. By adjustment of a respective potentiometer 37, the strength of signal 14 applied to 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. 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 (27). 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 35. 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 35 to pass ramp signal 41 to electrodes 39, then the resulting gated signal appearing on electrodes 39 is 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. 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 easi »ly 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 ant other device (one example being a phase change circuit) to bring about a modification of the breakoff instant of the droplet relative to the reference signal. Signals STROD and STROD/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 40 produces and supplies a transducer modulating signal 14, signal 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 deflection field (not shown). 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 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, 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 used 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 unit 44. When this adjustment has been performed according to the earlier teaching of this invention, the sensor 56 is then multiplexed to sample position 57 and the control output of feed back controller 59 is multiplexed to apply correction to breakoff control 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.

Claims: CLAIMS 1. A method of monitoring and correcting the phase relationship between the instant of droplet formation and the application of a charge to a droplet in an ink jet printer having (i) a plurality of substantially identical jet bodies with respective orifices, each adapted to supply a stream of liquid, and including means 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 (ii) a charging electrode associated with each jet body, for inducing a charge on droplets produced from the respective stream of liquid, said method characterised by the steps of a) observing the droplets (5) generated by a monitor jet (2) in the jet printer; b) generating a correction signal (ADV, RTD) whenever said observation indicates that the time interval between the introduction of variscosity to the liquid stream (3) from said monitor jet (2) and the application of a charging voltage to the charge electrode (4) of the monitor jet departs from the value of this time interval for proper operation of the monitor jet; c) applying said correction signal to a servo loop to vary said time interval to reduce the departure thereof from the value for proper operation of the monitor jet; and d) applying said correction signal to at least one other printing jet of the jet printer, to effect the same variation of said interval, respectively, in each said other printing jet to which said signal is applied. 2. A method as defined in claim 1, in which said correction signal is applied to said servo loop and to each said other printing jet to adjust the application of a periodic signal (14) to said means for applying variscosity, said periodic signal (14) causing onset of variscosity in the respective streams of liquid. 3. A method as defined in claim 2, in which the adjustment of said periodic signal (14) is by an amount of up to a single cycle of said periodic signal. 4. A method as defined in claim 1, in which said correction signal is applied to means for applying a voltage signal (15) to the charging electrode (4) of said monitor jet and to the means for applying a voltage signal to the respective charging electrode of each said other printing jet, to adjust the application of the voltage signal (15) to the respective charging electrodes. 5. A method as defined in claim 4, in which the voltage signal (15) applied to the charging electrode (4) of the monitor jet is a periodic voltage signal and the adjustment effected by said correction signal is by an amount of up to one single cycle of said periodic voltage signal. 6. A method as defined in any preceding claim, in which the adjustment is effected in steps upon the occurrence of signal pulses from a clock (21). 7. A method as defined in claim 6, in which said correction signal is generated upon the occurrence of a pulse from a second clock (26). 8. Apparatus for monitoring and correcting the phase relationship between the instant of droplet formation and the application of a charge to a droplet in an ink jet printer having (i) a plurality of substantially identical jet bodies with respective orifices, each adapted to supply a stream of liquid, and including means 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 (ii) a charging electrode associated with each jet body, for inducing a charge on droplets produced from the respective stream of liquid, said apparatus characterised by: a) means for observing the droplets (5) generated by a monitor jet in the jet printer; b) means for generating a correction signal (ADV, RTD) whenever said observation indicates that the time interval between the introduction of variscosity to the liquid stream (3) from said monitor jet and the application of a charging voltage to the charge electrode (4) of the monitor jet departs from the value of this time interval for proper operation of the monitor jet; c) means for applying said correction signal to a servo loop to vary said time interval to reduce the departure thereof from the value for proper operation of the monitor jet; and d) means for applying said correction signal to at least one other printing jet of the jet printer, to effect the same variation of said interval, respectively, in each said other printing jet to which said signal is applied. A droplet monitor for use in an ink jet printer having a plurality of substantially identical jet bodies, each with a respective charging electrode, said droplet monitor characterised by: a) a monitor jet body (2) having a construction substantially the same as each jet body of said plurality of jet bodies, with an associated charging electrode (4); b) collection means (6) adapted to receive the droplets (5) generated by said monitor jet body and adapted to discharge therefrom liquid (7) collected by said collection means; c) means (9), . in electrical isolation from said collection means, for receiving liquid (8) discharged from said collection means (6); d) means (12) for applying a voltage signal (15) to the charging electrode (4) associated with said monitor jet body, said voltage signal varying between a positive voltage value and an 5 equal negative voltage value; e) sensing means (11) adapted to sense the net charge of the liquid (7) collected by said collection means (6); and f) logic means (20), responsive to said sensing 10 means (11), for generating a signal (ADV, RTD) indicative of the qualitative change required to ensure the correct phase relationship between the instant of droplet formation and the application of said voltage signal (15) to 15 the charging electrode (4) associated with said monitor jet body. 10. A droplet monitor as defined in claim 9, in which said monitor jet is one of the printing jets of the jet printer. 2011. A droplet monitor as defined in claim 9 or claim 10, in which the signal (ADV, RTD) from the logic means (20) is used to increase or reduce the time between a predetermined instant in the application of a periodic signal (14) causing onset 25 of variscosity to the liquid and the application of a voltage signal (15) to the charging electrode (14) of the monitor jet. 12. Apparatus as defined in claim 8, in which said means for observing the droplets generated by the monitor jet and means for generating a correction signal, in combination, comprise a droplet monitor as defined in claim 9, claim 10 or claim 11. 13. Apparatus as defined in claim 12, in which a) each said means for applying a periodic variscosity includes an electromechanical transducer which is actuated by a respective transducer driver (13); b) each said transducer driver (13) is responsive to a signal from a clock (21); c) a phase shifter (18) is included between said clock (21) and the transducer driver (13); and d) said signal from the logic means (20) is applied to said phase shifter (18) to cause said increase or reduction of said time. 14. Apparatus as defined in claim 13, in which said signal from said clock (21) is also applied to said means (19) for applying a voltage signal to the charging electrode (4) associated with said monitor jet, whereby the signals (14) applied to said electromechanical transducer and the voltage signal (15) applied to the charging electrode (4) of said monitor jet have the same frequency. 15. Apparatus as defined in claim 14, in which said means for applying a voltage signal to the charging electrode associated with said monitor jet includes signal generating means (19) for generating said voltage signal (15), said signal generating means being responsive to signals from said clock (21). 16. Apparatus as defined in claim 15, including a 5 charge electrode driver (12) which receives signals from said signal generating means (19) and applies said alternating voltage signal (15) to the charging electrode (4) associated with said monitor jet. 1017. Apparatus as defined in claim 16, in which said signal generating means (19) also generates at least one reference signal. 18. Apparatus as defined in any one of claims 12 to 17, in which the alternating voltage signal (15) 15 applied to ' the charging electrode (4) of said additional jet body is a square wave. 19. Apparatus as defined in any one of claims 12 to 18, including a second clock (26), operating asynchronously to other waveforms in said 20 apparatus; said second clock having an output comprising a series of pulses, said second clock output being connected to said logic means (20), said logic means being adapted to generate said signal indicative of the qualitative change 25 required to ensure said correct phase relationship in response to each pulse of said output from said second clock (26). 20. Apparatus as defined in claim 17, or in claim 18 when appended to claim 17, or in claim 19 when appended to claim 17, including adjustment means to adjust the instant of formation of droplets from the liquid streams of said plurality of jet bodies to be in synchronism with each other and in a fixed phase relationship to the instant of formation of droplets (5) from the liquid stream (3) of said monitor jet, said adjustment means comprising: a) a plurality of sensor units (53, 54, 55) associated with respective jet bodies (47, ' 48, 49) of said plurality of jet bodies; b) a plurality of feedback control units (50, 51, 52) each responsive to the output of a respective one of said sensor units; c) a plurality of droplet breakoff control units (44, 45, 46), each responsive to a signal from a respective one of said feedback control units to vary the breakoff instant of the droplets from an associated one of said plurality of jet bodies; and d) means, including a switch (35), for applying a ramp voltage signal (41) to the charging electrodes (39) of said plurality of jet , bodies, said switch being controlled by said reference signal or one of said reference signals. 21. Apparatus as defined in claim 20, in which said plurality of feedback control units comprises a single control unit (59) adapted to be responsive to each sensor unit (56, 57, 58) in turn to apply a signal to the respective droplet breakoff control unit. 22. A method of monitoring and correcting the phase relationship between the instant of droplet formation and the application of a charge to a droplet in an ink jet printer, substantially as hereinbefore " described with reference to the accompanying drawings. 23. Apparatus for monitoring and correcting the phase relationship between the instant of droplet formation and the application of a charge to a droplet in an ink jet printer, substantially as hereinbefore described with reference to the accompanying drawings. 24. A droplet monitor, substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

Inventors: WILLS LESLIE JAMES, WILLS, LESLIE, JAMES
Applicants: COMMW SCIENT IND RES ORG, COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH OR
Application year: 1985
Main classification: B41J3
IPC codes: B41J2