METHOD AND APPARATUS FOR ON LINE PHASING OF MULTIPLE NOZZLE INK JET PRINTERS

Abstract: A multiple nozzle continuous ink jet printer has a common nozzle block (10) with an array of nozzles (1, 2, 3, 4, 5, 6, 7) having respective charge electrodes (12) operated by respective drop charging circuits (45, 46, 47, 48, 49, 50, 51). A pilot nozzle (34) is provided in operative proximity to the nozzles (1 to 7) to monitor and detect changes in phase due to changes in ink parameters such as temperature, viscosity and pressure. A phase selection means (42) is arranged to alter the clock phase (33) applied to the pilot nozzle drop charging circuit (44) responsive to a sensor means (38, 40) thereby maintaining optimal charging by the pilot charge electrode (36). During system initialisation the correct break-off phase relationship is established for each charge electrode (12). During printing the phase selection logic (25, 26, 27, 28, 29, 30, 31) for the printing array (1 to 7) is altered based on the alteration of the clock phase to the pilot nozzle (34).

Description: METHOD AND APPARATUS FOR ON LINE PHASING OF MULTIPLE NOZZLE INK JET PRINTERS This invention relates to continous ink jet printing with a plurality of ink jets. Such ink jets are typically produced by an array of nozzles. In order to operate the nozzles, it is necessary to phase the jet streams correctly before printing commences. This is accomplished in a set-up or phasing mode of operation prior to the initiation of the printing operation. Phasing is done by time shifting the charge pulse supplied to the charging electrode associated with each. ink jet and by measuring the charge induced on the ink drops. This data is then used by a micro-processor based controller to optimize drop charging. Due to changes in operating temperature, ink viscosity, pressure, as well as other environmental factors, it has been necessary not only to phase the printing array before printing begins, but periodically to stop printing to rephase the array. Because such printers are typically used in high output environments, where it is necessary to maximize printing, this interruption for phasing purposes is undesirable. More specifically, such printers are often used to mark products on assembly lines and the like with date codes, product codes and similar information. The use of these printers on high speed assembly lines can be tolerated only if the printers are both reliable and capable of providing high quality marking over extended periods of operation under changing environmental conditions. _ 1 _ SUBSTITUTESHEET It is accordingly an object of the present invention to provide a method and apparatus to achieve on line phasing of a multiple nozzle ink jet printer, thereby avoiding the need for periodic interruption of the printing process for rephasing the jets. It is known for an ink jet printer to have a plurality of print nozzles for emitting ink droplets, charge means for electrically charging selected ink droplets using selected phases of a multi-phase clock, and droplet deflection means for deflecting charged droplets. Accordingto one aspect of the invention, an ink printer has a pilot nozzle disposed in operative proximity to the print nozzles, a charge means is arranged for electrically charging selected pilot nozzle droplets using a selected phase of the multi-phase clock, a sensor means is arranged to monitor the charges on the pilot nozzle droplets, a phase selection means is provided for altering the pilot nozzle clock phase responsive to the sensor means to maintain optimal charging of the pilot nozzle droplets, and a means is provided for coupling the phase selection means to the means for electrically charging the print nozzle droplets to alter the print nozzle clock phases as a function of pilot nozzle clock phase changes, whereby variation in print nozzle operation due to temperature, ink pressure or ink viscosity changes will be automatically compensated without interrupting printing. The - 2 - SUBSTITUTESHEET sensor means preferably includes means for detecting the charge induced on the pilot nozzle droplets. The means for detecting the charge preferably includes a charge sensor. It is also known for apparatus, for controlling the clock phasing of charging circuits for a multiple nozzle ink jet printer, to have each nozzle charging circuit using preselected phases of a multi-phase clock. According to another aspect of the invention apparatus, for controlling the clock phasing of charging circuits for a multiple nozzle ink jet printer, has a pilot nozzle disposed in operative proximity to the print nozzles, a charging circuit provided for charging selected droplets emitted from the pilot nozzle using a preselected phase of the multi-phase clock, a sensor means arranged to monitor the charges on the pilot nozzle droplets, a phase selection means provided for altering the pilot nozzle clock phase responsive to the sensor means to maintain optimal charging of the pilot nozzle droplets, and a means provided for coupling the charging circuits for the print nozzles to the phase selection means to alter the print nozzle clock phases as a function of pilot nozzle clock phase changes whereby environmental changes due to variation in operating temperature, ink pressure or ink viscosity will be compensated for without interrupting printing. - 3 - SUBSTITUTESHEET It is also known for a method of controlling the clock phasing of charging circuits for a multiple nozzle ink jet printer to have each nozzle charging circuit arranged to use a preselected phase of a multi-phase clock. According to a further aspect of the invention a method of controlling the clock phasing of charging circuits for a multiple nozzle ink printer includes emitting ink droplets from a pilot nozzle disposed in operative proximity to print nozzles, charging selected droplets emitted from the pilot nozzle using a preselected phase of the multi-phase clock, monitoring the charges on the pilot nozzle droplets, altering the clock phase to maintain optimal charging of the pilot nozzle droplets, and altering the clock phases of the print nozzle charging circuits as a function of pilot nozzle clock phase changes to compensate for environmental changes due to variation in operating temperature, ink pressure or ink viscosity without interrupting printing. The method preferably includes monitoring the pilot nozzle droplet charges by sensing the magnitude of the charges. The method may include altering the pilot nozzle clock phase by comparing at least the two adjacent clock phases with the current clock phase to determine which produces the best charge on said pilot nozzle droplets, and by selecting the clock phase which produces the best charge. - 4 - SUBSTITUTESHEET The invention accordingly enables a printing device to monitor phasing continuously during the printing operation and to adjust phase to maintain optimal printing without interrupting printing. By providing a separate pilot nozzle, the present invention enables changes in phase due to environmental factors to be monitored continuously, thereby permitting automatic correction as changes in phase occur. The present invention provides for continuous monitoring of the phase relationship between the drop charging signal and the break-off of drops from an ink stream in an ink jet printing system. It also provides for automatic correction of that relationship while printing. As a result, phasing can be accomplished without user intervention and continuous printing is permitted since phasing is accomplished without interruption. In addition, reduced system cost may be realized because less precision will be required to maintain ink pressure, temperature, viscosity and nozzle excitation voltages constant since variations therein can be tolerated and accounted for by the continuous monitoring of the printer phasing. The present invention teaches the use of a pilot nozzle, spaced slightly from the printing array of a typical nozzle block. A separate charge electrode is provided for the pilot nozzle, along with a separate catcher and means for sensing the phase of the pilot ink stream separately from the streams which form the printing array. Circuit means are provided for - 5 - SUBSTITUTESHEET detecting the phase of the pilot stream and for altering phase selection logic for the printing array based on changes detected in the pilot stream. During system initialization, the correct drop break-off phase relationship for each jet in the printing array is established. The correct phase relationship for the pilot jet is also established at this time. During printing, the pilot jet is separately and continuously monitored. Its phase is continuously adjusted for optimum charging. The phase control for the jets in the printing array are linked to the pilot jet phase control circuits such that a change in the pilot jet phase is matched in magnitude and direction by a change in phase for the printing jets. This is successful to optimize printing because those factors which affect the phase of the pilot jet, i.e., variation in ink pressure, temperature, viscosity and nozzle excitation level, are common to the printing array and the pilot jet. Accordingly, these factors affect both the printing array and the pilot jet equally. Because the pilot jet used for phase monitoring is never used for printing, phase sensing and correction is permitted to occur continuously rather than at periodic intervals during which the printer must be taken off line. The invention will now be described, by way of example only, with reference to the accompanying drawings, in which:- - 6 - SUBSTITUTESHEET Figure 1 is a schematic drawing illustrating part of a printer employing a seven-high nozzle array with a separate pilot nozzle and, due to its size, has been split into two sheets designated Figure 1A and Figure IB; Figure 2 is the initialization routine flow diagram; Figure 3 is the phase test routine flow diagram, and Figure 4 is the run-time phasing flow diagram. Figure 1 diagrammatically illustrates the application of the invention to an ink jet printer having seven nozzles 1, 2, 3, 4, 5, 6, 7 arranged seven high in an ink jet nozzle block 10. The invention may, however, use a greater or lesser number of jet nozzles. In Figure 1 various functions are illustrated, for the sake of convenience, using electro¬ mechanical switches. However, it should be noted that these mechanical switches would be replaced, in practice, by electronic switching circuits under control of the printer's micro-processor. A practical system might have one hundred or more printing nozzles, as opposed to the seven illustrated here for clarity. In such a case, it will be desirable to group the nozzles for phasing purposes rather than to phase each printing nozzle individually as described hereafter. Additionally, the circuit illustrated in Figure 1, shows the - 7 - SUBSTITUTESHEET print cycle being divided into four overlapping clock phases. It will be recognized by those skilled in this art that a larger number of clock phases may be employed to maintain the phase closer to the theoretically perfect phase for each ink jet. From Figure IB it will be noted that the seven printing nozzles 1 to 7 emit respective jets of ink under pressure and these are jointly indicated by the bracket 16. As is well known in this art, these ink streams are broken up, by applying stimulation energy to the nozzles 1 to 7, into discrete ink drops which can be electrically charged by a charge electrode 12. The drops next pass a high voltage deflection electrode 14 causing those which were charged to be deflected from their initial flight path onto an unshown substrate or surface to be marked. Drops which are not charged by one of the charge electrodes 12, pass to an unshown catcher and are returned to the ink system for reuse. Alternatively, the charged drops can pass to the catcher while the uncharged drops are used for printing. For initial phasing purposes, the ink jets 16, are phased using a common charge sensor 18, the output of which is provided (see Figure 1A) to a phase sensing circuit 20 (contact or capacitively coupled sensors are acceptable for this purpose) . The phase sensing circuit 20 is connected through switches 22 and 24 to a separate phase selection logic circuits 25 to 31 each associated with one of the seven ink - 8 - SUBSTITUTESHEET jet nozzles 1 to 7 as illustrated in Figures 1A and IB. Each of the phase selection logic circuits 25 to 31 is connected, as shown, to inputs from a multi-phase clock generator 33 which provides four phases, each phase being separated by approximately ninety degrees. During set up of the printer, each of the phase selection circuits 25 to 31 is employed to select the clock phase, for the corresponding nozzle 1, 2, 3, 4, 5, 6 or 7, which produces the best printing results. Thereafter, the circuits 25 to 31 are not used. Instead, phase control is provided as a function of a pilot nozzle 34 and its separate phase sensing circuit 40 and phase selection logic 42. In addition to the seven printing nozzles 1 to 7, the present invention provides the separate pilot nozzle 34 which has a charge electrode 36 and a charge sensor 38, separate from the common charge sensor 18. It should particularly be noted that, since the pilot nozzle 34 is employed solely for phase control, there is no need for it to be in operative relationship with the deflection electrode 14. It is only necessary that the pilot nozzle be mounted in operative proximity to the seven printing nozzles. As illustrated, the pilot nozzle 34 is mounted on the nozzle block 10 with the printing nozzles 1 to 7 so that it is subject to the same environmental conditions, such as temperature variation and vibration, and is supplied with the same ink and stimulating energy as the printing nozzles 1 to 7. - 9 - SUBSTITUTESHEET The output of the pilot charge sensor 38 is provided to a phase sensing circuit 40, which provides an input to a phase selection logic 42, dedicated to the pilot nozzle 34. As with the printing nozzles 1 to 7, the phase selection logic 42, controls a drop charging circuit 44, which functions to control operation of the specific charge electrode to which it is dedicated, in this case, the pilot electrode 36. Likewise, the printing nozzle charge electrodes 12 are controlled by drop charging circuits 45 to 51 as shown in Figures 1A and IB. The phase selection logic simply chooses the clock phase to which the drop charging voltage will be synchronized. A digital multiplexer, such as that identified by the common manufacturers designation 74-139, is controlled by the printer's controller to select one of the clock phases. The output of the multiplexer is then used to gate the charging voltage on and off. During the setup or calibration mode, a calibration pulse is provided along line 41 to switch 43 to permit sequential calibration of each of the seven printing nozzles 1 to 7 in a manner which will be explained later. This calibration pulse is applied to the drop charging circuits 45 to 51 by the line 41 through respective switches which are ganged as indicated as by the dashed line 52 and are shown in the calibrate position. In their alternate position, these ganged switches connect the drop charging circuits 45 to 51 to the correct one of the seven drop printing signals (as indicated by the - 10 - SUBSTITUTESHEET bracket 54) which are supplied from the printer's controller in a manner well known in the art. When the system is first powered up, it will enter the calibration or initialization mode. After an appropriate time has elapsed and the system has warmed up, the printer's controller will begin applying the calibration pulses along the line 41, and through switch 43 to nozzle 1, and via a connection 60 to the pilot nozzle 34. It should be noted that the switch 43 only connects the line 41 temporarily to the drop charging circuit 51 for the first nozzle 1 whilst the drop charging circuit 44 for the pilot nozzle 34 is always connected to the line 41 by the connection 60. All four phases of the clock 33 will be tested and the best one selected to maximize the induced charge or drops from the first nozzle 1. While there are many methods for phasing, most are reasonably compatible with the present invention. In that connection, reference is made to U.S. Patent No. 3,750,119 to Naylor as an example of phasing techniques suitable for use in the present invention. As known in the art, the correct phase for a nozzle is a function of the break-off point of the drops from the ink stream within the associated charge electrode 12. Thus, in any array of nozzles, there will be nozzle to nozzle variation in the dimensions and other physical characteristics of the nozzle causing small, but meaningful variation in the break- off point of drops. Unless the charge is applied at the right - 11 - SUBSTITUTESHEET time, much of it will be lost. Hence the need for phasing each of the printing nozzles before commencing operation. After the first nozzle is phased, the selector switch 43 is moved clockwise to the next position to begin phasing of nozzle 2. This procedure is repeated until all seven nozzles have been initially phased. When all of the printing nozzles 1 to 7 have been tested and their respective charging circuits 45 to 51 have been set to the best phase, the pilot phase selection logic 42 is rechecked to determine if the phase has drifted during the phase selection process for the printing nozzles. The pilot nozzle 34 is monitored continuously throughout the phase setting mode. If there has been a change in the phase of the pilot nozzle 34 during this period, due possibly to system warm-up or interaction between the nozzle streams, some of the initial phase settings for the nozzles 1 to 7 may be wrong. Therefore, the entire procedure is repeated until there is no phase change occurring during the set-up of the print nozzles 1 to 7. At this point, it may be safely assumed that the system is stable and that only gradual changes, due to temperature, pressure, viscosity or stimulation voltage variation, need be expected. The system is then ready to enter the printing mode. Figures 2 and 3 illustrate the logic flow of the computer control during the initialization process. More specifically, Figure 2 phases the pilot nozzle 34 and print nozzles 1 to 7 - 12 - SUBSTITUTE SHEET by repeatedly calling the phase test routine of Figure 3 until it is satisfied that the phasing is correct and the printer is ready to enter the print mode (Figure 4) . These flow diagrams are self-explanatory in view of the operating description provided herein and will not be further described. To enter the printing mode, the ganged switches 52 are moved from the CALIBRATE position illustrated in Figures 1A and IB to the alternate position, as indicated by the arrow PRINT in Figure IB, in which the drop charging circuits 45 to 51 are connected to the drop printing signals 54 from the ■ controller. At this time, each of the seven nozzles 1 to 7 has its phase set and locked on one of the four phases of the clock 33. This has been accomplished by the phase selection logic 25 to 31 which receive inputs from the common charge sensor 18 and determine which of the four clock phases produce the best charge for a particular nozzle 1 to 7. In addition to moving the ganged switches 52 to the print mode, it is also necessary to disconnect the common charge sensor 18 and to couple the phase selection logic 25 to 31 to the output of the phase sensing circuit 40 which monitors only the charge sensor 38 for the pilot electrode 34. This is accomplished by shifting the position of the ganged switches, indicated by dashed line 62, from the CALIBRATE position to the PRINT position. This switching permits all of the printing nozzles 1 to 7 to be controlled by the sensor circuitry for the pilot nozzle 34. - 13 - SUBSTITUTESHEET In the print mode, the phase selection logic 25 to 31 for the printing jets 16 is controlled solely by the phase sensing circuit 40 associated with the pilot jet 34. In a stable, properly operating system, phase changes are gradual. Accordingly, the pilot jet circuit need test only three phases: the phase currently used and the two adjacent phases, one earlier and one later. Should the earlier phase provide a better signal, the phase sensing circuit 40 will signal the phase selection logic 25 to 31 and 42 to select an earlier phase as the new phase for each of the printing nozzles 1 to 7 and the pilot nozzle 34. Thus, all the printing nozzles 1 to 7 operate in a master-slave relation with the pilot nozzle 34. Because the phase selection can be accomplished without sending calibration pulses to any of the printing jets 16, it is not necessary to interrupt printing in order to adjust phasing. Similarly, if a later clock phase produces a better signal for the pilot jet 34, all of the printing jets 1 to 7 will similarly be switched to a later phase of the clock 33. In this manner, continuous phase monitoring is accomplished and phase adjustments made on the fly without any interruption in the printing operation of the device. The phasing mode need only be employed once per day at the start up of the printer. Thereafter drift due to changes in operating conditions will be automatically accounted for by virtue of the continuous monitoring of the pilot nozzle 34. The program flow diagram for the monitoring process is shown in Figure 4. - 14 - SUBSTITUTESHEET As indicated earlier, the electro-mechanical switches 22, 24, 43, 52 and 62 would be replaced, in a preferred embodiment, with electronically controlled switches under control of the system micro-processor. As also indicated, a greater or lesser number of printing nozzles can be used, and nozzles can be grouped together for phasing rather than individually if desired. Also, a greater or lesser number of clock phases can be employed as desired for print optimization. - 15 - SUBSTITUTE SHEET

Claims: WHAT IS CLAIMED: 1. An ink jet printer having a plurality of print nozzles (1, 2, 3, 4, 5, 6, 7) for emitting ink droplets, charge means (12) for electrically charging selected ink droplets using selected phases of a multi-phase clock (33), and droplet deflection means (14) for deflecting charged droplets, characterised in that a pilot nozzle (34) is disposed in operative proximity to the print nozzles (1 to 7), a charge means (36) is arranged for electrically charging selected pilot nozzle droplets using a selected phase of the multi-phase clock (33), a sensor means (38, 40) is arranged to monitor the charges on the pilot nozzle droplets, a phase selection means (42) is provided for altering the pilot nozzle clock phase (44) responsive to the sensor means (38, 40) to maintain optimal charging of the pilot nozzle droplets, and a means (62) is provided for coupling the phase selection means (42) to the means (45, 46, 47, 48, 49, 50, 51) for electrically charging the print nozzle droplets to alter the print nozzle clock phases as a function of pilot nozzle clock phase changes, whereby variation in print nozzle operation due to temperature, ink pressure or ink viscosity changes will be automatically compensated without interrupting printing. 2. An ink jet printer, as in Claim 1, characterised in that the sensor means (38, 40) includes means (38) for • detecting the charge induced on the pilot nozzle droplets. - 16 - SUBSTITUTE SHEET 3. An ink jet printer, as in Claim 2, characterised in that the means (38) for detecting the charge includes a charge sensor. 4. Apparatus for controlling the clock phasing of charging circuits (45, 46, 47, 48, 49, 50, 51) for a multiple nozzle (1, 2, 3, 4, 5, 6, 7) ink jet printer, in which each nozzle charging circuit (45 to 51) uses preselected phases of a multi-phase clock (33), characterised in that a pilot nozzle (34) is disposed in operative proximity to the print nozzles (1 to 7), a charging circuit (36, 44) is provided for charging selected droplets emitted from the pilot nozzle (34) using a preselected phase of the multi-phase clock (33), a sensor means (38, 40) is arranged to monitor the charges on the pilot nozzle droplets, a phase selection means (42) is provided for altering the pilot nozzle clock phase (44) responsive to the sensor means (38, 40) to maintain optimal charging of the pilot nozzle droplets, and a means (62) is provided for coupling the charging circuits (45 to 51) for the print nozzles (1 to 7) to the phase selection means (42) to alter the print nozzle clock phases as a function of pilot nozzle clock phase changes whereby environmental changes due to variation in operating temperature, ink pressure or ink viscosity will be compensated for without interrupting printing. - 17 - SUBSTITUTESHEET 5. Apparatus, as in Claim 4, characterised in that the sensor means (38, 40) includes means (38) for detecting the charge induced on the pilot nozzle droplets. 6. Apparatus, as in Claim 5, characterised in that the means (38) for detecting the charge includes a charge sensor. 7. A method of controlling the clock phasing of charging circuits for a multiple nozzle ink jet printer in which each nozzle charging circuit (45, 46, 47, 48, 49, 50, 51) uses a preselected phase of a multi-phase clock (33), characterised by emitting ink droplets from a pilot nozzle (34) disposed in operative proximity to print nozzles (1, 2, 3, 4, 5, 6, 7), charging selected droplets emitted from the pilot nozzle using a preselected phase of the multi-phase clock, monitoring the charges on the pilot nozzle droplets, altering the clock phase to maintain optimal charging of the pilot nozzle droplets, and altering the clock phases of the print nozzle charging circuits (45 to 51) as a function of pilot nozzle clock phase changes to compensate for environmental changes due to variation in operating temperature, ink pressure or ink viscosity without interrupting printing. 8. A method, as in Claim 7, characterised in that the pilot nozzle droplet charges are monitored by sensing the magnitude of the charges. - 18 - SUBSTITUTESHEET 9. A method, as in Claim 7, characterised in that the pilot nozzle clock phase is altered by comparing at least the two adjacent clock phases with the current clock phase to determine which produces the best charge on said pilot nozzle droplets, and by selecting the clock phase which produces the best charge. - 19 - SUBSTITUTE SHEET

Inventors: EMERSON JOHN KEIGHTLY, EMERSON, JOHN, KEIGHTLY
Applicants: GEN ELECTRIC CO PLC, THE GENERAL ELECTRIC COMPANY PLC
Application year: 1993
Main classification: B41J2
IPC codes: B41J2