METHOD AND APPARATUS FOR OPTIMIZING PHASE AND IMPROVING RESOLUTION IN INK JET PRINTERS

Abstract: An ink jet printer and method of use of an ink jet printer incorporating the present invention is a continuous stream printer using pressure to eject ink from a capillary along a predetermined path. A piezo-electric crystal applies cyclical lateral vibration to the ejection end of the capillary to aid in obtaining greater control over the actual position of the droplet formation point and the time of droplet formation. A charging tunnel, positioned along the path and encompassing the point at which the stream breaks up into droplets, selectively charges the droplets. A deflection unit positioned further downstream deflects droplets having a particular charge, while permitting other droplets to pass to a recording medium. The invention further includes apparatus for maintaining a synchronism and phase relationship between the voltage applied to the charging tunnel and the cyclical vibrations of the piezo-electric vibrator. This permits the charging of a single droplet. This cooperation with the droplet formation point control provided by the vibrator, thus provides accurate control of the amount of ink deposited in a particular area of the recording surface and the placement of droplets on the recording surface.

Description: METHOD AND APPARATUS FOR OPTIMIZING PHASE AND IMPROVING RESOLUTION IN INK JET PRINTERS BACKGROUND OF THE INVENTION Field of the Invention This invention relates to an apparatus and a method for use in ink jet printing and more particularly for controlling formation and charging of the droplets in a continuous stream deflectin type ink jet printer to promote improved image accuracy and quality. Background of the Invention The field of ink jet printing has witnessed rapid advances in technology in the past. These advances, however, have not resulted in a completely satisfactory means for controlling the placement of droplets on the recording surface. Thus, there has been an inability to produce high resolution images on a consistent basis with a minimum of manual intervention. In continuous stream type ink jet printers of the type disclosed and claimed in U.S. Patent No. 4,639,736 to Jochimsen, an ink stream is pressure ejected from a capillary along a preselected path toward a recording surface. The stramof ink spontaneously forms into droplets at a so-called "droplet-formation" point. Portions of the droplet stream are charged to a first potential, while the other portions of the stream have a second potential. The droplets are then separated by an electric field placed in the path, so that the droplets having one potential are deflected from the path and away from the recording surface and droplets having the other potential pass to the recording surface. In the past, control of the droplet placement and charge passed to the droplets has orginarily required manual intervention and adjustment to correct problems in this regard. It has been known to use an oscillating device to gain some control over the droplet size and formation point, as discussed in U.S. Patent No. 3,916,421 to Hertz. The devices disclosed in U.S. Patent Nos. 4,639,736 and 3,916,421, while having improved image resoltuion over other ink jet printers in the prior art, are still subject to limitations in image resolution relating to the control of the placement of droplet on the recording surface. Uncontrolled variations in droplet placement affect both the quality of the images and the time required to obtain the images, since manual intervention is required to correct variations in placement. SUMMARY OF THE INVENTION It is an object of the present invention to provide an ink jet printer which produces images on a recording medium having a high degree of accurate placement of droplets on the medium. It is a further object of the present invention to provide an ink jet printer which reproduces images accurately with minimal manual intervention. It is a further object of this invention to provide a method for producing images having an accurate placement of droplets on a recording surface. An ink jet printer incorporating my invention is a continuous stram printer using pressure to eject ink from a capillary along a predetermined path. A piezo-electric crystal applies cyclical lateral vibration to the ejection end of the capillary to aid in obtaining greater control over the actual position of the droplet formation point. A charging tunnel, positioned along the path and encompassing the point at which the stream breaks up into droplets, selectively charges the droplets. A deflection unit positined further downstream deflects droplets having a particular charge, while permitting other droplets to pass to a recording medium. The invent in further includes apparatus for maintaining a synchronism between the voltage applied to the charging tunnel and the cyclical vibrations of the piezo-electric vibrator. This permits the charging of a single droplet. This cooperation with the roplet formation point control provided by the vibrator, thus provides accurate control of the amount of ink deposited in a particular area of the recording surface and the pal cement of droplets on the recording surface. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagrammatic representation of an ink jet printer embodying the present inventin. Fig. 2 is a diagram of the control circuit of the printer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to Fig. 1, an ink jet printer embodying the present invention includes an ink jet 1 having a capillary 3, contained in a housing 9. A pump 19 pumps electrically conductive ink 4 (e.g. having a bulk resistivity of approximately 200ohm-cm or less) from reservoir 13 to the ink jet 1. At the rearward end of the jet the ink 4 contacts a conductive charge adaptor 5 which is virtually grounded, as described below. The ink is expelled from the capillary 3 through the forward end thereof, which serves as a nozzle 15. The resulting ink stream 17 breaks up into droplets a droplet formation point 18. A piezo-electric vibrator 7 laterally vibrates the nozzle 15 to provide a uniform breaking of the stram into droplets at the droplet formation point. A charging mechanism, in this instance a charging tunnel 21, which surrounds the ink stram 17 as it passes through the tunnel 21, is positioned to encompass the droplet formation point 18. The electrical potentil of the tunnel 21 is selectively alternated between a first level and second level. The forward end of the ink stream 17 takes potential with a polarity opposite to that of the tunnel 21. tHis corresponds to the induction of a charge at the end of the stream. More specifically, each ink droplet, as it breaks off, is charged by the electrical potential existing between the tunnel 21 and the end of the ink stream 17. Any changes in the potential of the tunnel 21 after a particular droplet has separated from the stream will not affect the charge of the droplet. It is the controlled application of this charge to individual droplets to which this invention is primarily directed. The succession of droplets next passes into a deflection unit 23, where an electric field is applied between an upper deflection electrode 25 and a lower waste disposal or gutter electroe 27. The upper electrode has an electrical potential of the same polarity as that of the charge of the droplets that are to be deflected and the gutter electrode 27 has a ground potential. These droplets are thus directed into the gutter electrode 27 by the electric field between electrodes 25 and 27 by the electrid field between electrodes 25 and 27, while the other droplets, only slightly charged or uncharges, pass to a recording medium 41 which is mounted on a drum 43. The drum is rotated with respect to the ink jet 1 by a driver unit and the housing 9 of the ink jet 1 is mounted on a base 50 which is moved parallel to the drum ais. thus, the uncharged droplets fromthe ink stram 17 cause, in essence, raster type lines to be formed on the medium. In this case a space 47 of the platform is not covered by the medium 41. With reference to Fig. 2, the charge adaptor 5 is connected to a virtual ground at the inverting input terminal of an operational amplifier 149 a current-tovoltage convertor 150. This induction of a charge at the end of the ink stream 17 (Fig. 1) results in a current to or from ground in the ink stream and the output voltage of the converter 105 corresponds to this current. This output is passed to an analog-to-digital convertor 105 to a digital processing unit 107. The processing unit provides an output to a digital-to-analog convertor 109 to control the phase delay that a phase control unit 111 imparts to the output of a crystal driving oscillator 113. The crystal oscillator 113 controls the vibration of the vibrator 7. Its phase-adjusted output also times the voltage applied to the charging tunnel 21. This is, the output of the phase-control unit 111 is applied to a data control unit 115 to clock the input data for the printer to a voltage source 117. The source 117, thus, applies the requisite potentials to the charging tunnel 21 synchronously with the vibrations of the vibrator 7, but with a phse difference that provides a control of the charge applied to indiviudual droplets. More specifically, in this particular embodiment during each revolution of the drum 43, when a longitudinal strip not covered by the recording medium 41 is oppoiste the ink jet 1, a signal from unit 53 (see Fig. 1) is sent tothe processing unit 107. The processing unit thereupon enters a phase adjustment routine. First, it applies a signal to the voltage source 117 to apply an offset voltage to the tunnel 21 so tht all droplets are deflected to the gutter electrode 27. Then it applies a control signal to the data control unit 115 which generates and outputs a pseudo data signal to the source 117. At the same time the processing unit 107 "dithers" the phase control unit to cyclically increase and decrease the phase delay imparted to the signal applied to the tunnel 21. As it does so, the processing unit 107 monitors the output of the analog-to-digital convertor 105 and records the digitized output of the current-to-voltage converter 150. The processor 107 analytically determines a phase setting which is based primarily on that setting corresponding to a minimum current in the ink stream 17. This setting allows the charging of a single droplet for a unitary pulse applied to the tunnel 21 and is applied to the phase control unit 111 when the recording medium 41 is again opposite jet 1. I am currently using a signal source of between 7501200kHz, whichpreferably is between 900-1000kHz, to drive the crystal so that a full cycle of the crystal's movement is approximately 1 microsecond. Also, the voltage source 117, which I use during printing causes the charge tunnel 21 to be at a high or gound potential to permit the ink droplets to be selectively charged or uncharged. However, it may be found in certain systems and under certain conditions that it is preferably to slightly charge or bias the droplets for printing and fully charge the droplets to be deflected. This merely requires adjusting the ink nozzle position 15 and the DC bias voltage applied to the tunnel 21 by the voltage source 117 accordingly. I have also found that with the deflection electrode at 1000v to 2000v and the gutter electrode at ground that the droplets to be deflected are charged by the tunnel 21 having a high potential of between -25v to -100v with approximately -50v being preferred. The reference offset voltage, which is applied to the charging tunnel 21 by the processing unit 107 during the phase adjustmen routine that I have usedr is approximately 50v. In this embodiment, as previously described, the crystal oscillator 113 provides both the signal to drive the crystal and the informational signal to the phase control unit 111 regarding the state of the crystal 7. The crystal 7 cyclically vibrates the ink jet so that the ink stream as it is ejected is slightly agitated so that the stream tends to break up in droplets at substantially the same distance from the nozzle 15 (i.e., an approximately constant time interval betwen the formation of each droplet) so that a droplet breaks off from the stream at a rate approixmately equal to the clock cycle of the crystal oscillator 113. The duration of the pulses which are directed by the voltge source to the charging tunnel are periods which are multiples of the clock cycle of the crystal oscillator 113 and phse adjusted, as previously described. Thus, to charge a single drop. the voltage source 117 applies a voltage to the charge tunnel 21 for a phase adjusted cycle of the crystal oscillation unit 113 (e.g., in my system, approximately 1 microsecond). As explained, the desired result is to only charge the potential of the tunnel 21 at a controlled time between the separation of a droplet from the stream and the separation of the next droplet from the stream. It will be appreciated that minor variations in the system will occur over time. Such variations will cause deleterious effects on printing, so I hve provided means for controlling the pulsing of the charging tunnel to optimize the charging of the droplets relative to the state of the signal from the crystal oscillator 113 to the crystal unit 7. Thus, substantial control over the charging and placement of the droplets cab be achieved. In the actual system I have used and described, I have developed an apparatus and technique for recalibrating the ink jet on a per revolution of the medium basis to recalibrate the system. A voltage is measured, in this instance, by measuring the current at the charge adapter 5 through the current-to-voltage convertor 150. This occurs during the period that the space 47 is exposed to the ink jet and when the test pattern signal is generated by the data control unit 115. It will be understood that this measurement can also occur during the exposure of a margin on the medium to the ink jet. This measured voltage is then used to recalibrate the system. In the embodiment shown in Fig. 1, the ink jet printer is one having a unitary jet of the type depicted and discussed in U.S. Patents Nos. 4,639,736 and 4,668,959, which are assigned to the same assignee as this invention. It will be apparent and is contemplated that this invention can be easily incorporated into multijet ink jet printers. Also, while not all of the various improvements shown and discussed in these Patents are disclosed herein, a person skilled in the art will recognize the ease with which they may be incorporated herein without departure from the nature and the spirit of the present invention. Likewise, it will be apparent to those skilled in the art that the present invention has broad applications fro all ink jet printers using the continuous stream deflection type technology. This is true, whether the deflected ink is deflected to a gutter or is deflected for printing. It will be recognized in one case, assuming the other variables can be controlled, that within the spirit and scope of this invention one can interconnect the piezoelectric crystal or other cyclically vibrating device with the high voltage driver so that the charge tunnel, or other apparatus for charging the droplets breaking off from the stream, is charged to the desired potential at the appropriate time. Although this device has been described with respect to an ink jet printer, it also has applications to other fields where a high degree of control over the quantity of a liquid to be delivered to a substrate is required. For instance, certain medicines now in manufacture and use require delivery of small an precise dosages. This invention permits users to deliver to a substrate a precise amount of such liquids. The device would consist of the jet, with a pump and reservoir containing the drug as previously described, and the charging and deflection unit. A substrate would be positioned along the predetermined path. The machine would be calibrated in the normal manner described, and the precise dose required would then be sent to the substrate. What is claimed as new and desired to be secured by Letters Patent of the United States is:

Claims: CLAMS 1. A device comprising a capillary through which an electrically conductive liquid is ejected in a stream along a predetermined path toward a substrate, a means for selectively charging droplets of the liquid in the ink stream, a deflection unit for directing certain droplets away from the predetermined path whereby only selected droplets reach the substrate, means for detecting the level of charge imparted to the droplets, and means for determining from the detected level of charge imparted to the droplets the appropriate timing for selectively alternating said means for selectively charging. 2. An ink jet printer comprising a capillary, a pressure generating unit for pressure ejection of a conductive ink stream from said capillary, a charging electrode for selectively varying the potential of selected portions of said stream, a deflection unit for selectively deflecting portions of said stream according to the potential of said portions of said stream whereby predetermined portions of said stream impact a recording surface and the other portions are directed to a waste collector, a voltage source which varies the potential of the charging electrode, and means for controlling the timing of pulses from the voltage source, whereby the potential of the charging electrode is varied only during the time interval between the breaking off of one droplet and the breaking off of the next droplet. 3. An ink jet printer according to claim 2 further comprising a vibrator for cyclically vibrating the ejection end of the capillary and a driving unit which controls the cycle rate of the vibrator wherein said voltage source varies the pulses as function of the vibrator. 4. An ink jet printer according to claim 3 further comprising a sampling device wherein the potential of droplets forming at the droplet formation point can be analyzed to determine an appropriate phase shift in the pulses from the voltage source relative to the driving unit. 5. A device according to claim 1 further comprising means for vibrating the ejection end of the capillary in a cyclical fashion wherein the timing for selectively alternating said charging electrode is varied as a function of the vibration of said capillary. 6. A device according to claim 5 wherein the charging means further includes a voltage source which generates voltage pulses to selectively alternate the potential of the charge electrode and the duration of the pulses to the charge electrode are phase adjusted multiples of the clock cycle of the vibrating means. 7. An ink jet printer according to claim 4 wherein said means for charging the droplets includes a charging electrode which is selectively alternated between a first and second potential to generate an electric field through which the ink stream passes and said charging electrode is positioned along said path at an area including the area where said ink stream breaks up into droplets. 8. An ink jet printer according to claim 2 further comprising sampling means for generating a test pattern and for monitoring the charge applied to the droplets during said test pattern and means for setting an appropriate timing signal for varying the potential applied to said charging electrode. 9. An ink jet printer according to claim 8 further comprising a vibrator for vibrating the nozzle end of a capillary, a clock unit controlling the cycle of the vibrato, a phase control unit for receiving the timing signal from said sampling means to vary the phase of the vibrator potential relative to said crystal rate. 10. A method for improved printing in an ink jet printed comprising the steps of: ejecting ink from a capillary in a continuous stream; selectively charging portions of said ink so that certain droplets have a higher charge relative to the other droplets; deflecting ink droplets having one of said charges, thereby permitting predetermined droplets to pass to a recording surface; controlling the timing of the charging of said droplets relative to the formation of droplets from the ink stream; recalibrating the timing of the charging of said droplets relative to the formation of said droplet during non-printing periods in a printing scan; generating a test routine during such non-printing periods; and determining an appropriate timing for the step of charging of said droplets, whereby droplets tend to be either at a first or second charge level without a substantial amount of droplets having a charge which is between the first and second charge level. 11. The method of claim 10 further comprising the steps of cyclically vibrating the nozzle end of the capillary; and controlling the rate of the cyclical vibration wherein said charging step includes passing said droplets through an electrode at the droplet formation point which is alternated between a first and second potential whereby the droplets tend to have the first or second charge level imparted to them according to the state of the electrode at the point when the ink droplet separates from the ink stream, and said step of controlling said timing of the charging includes varying the potential applied to the electrode in a phase relative to the cyclical vibration of the nozzle.

Inventors: FARGO FOSTER M, FARGO, FOSTER, M.
Applicants: IRIS GRAPHICS INC, IRIS GRAPHICS, INC.
Application year: 1988
Main classification: B41J3
IPC codes: B41J2