Abstract:
A continuous stream ink jet printhead includes an ink droplet forming mechanism operable to selectively create a stream of ink droplets having a plurality of volumes and a droplet deflector having a gas source. The gas source is operable to interact with the stream of ink droplets thereby separating ink droplets having one of the plurality of volumes from ink droplets having another of the plurality of volumes. A sensor senses ambient pressure transients and is coupled to a controller which adjusts the gas flow, through a pressure compensation mechanism, to compensate for pressure transients.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application is related to application Ser. No. 09/750,946 filed on Dec. 28, 2000, the disclosure of which is incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates generally to the field of printing devices, and in particular to improving the quality of print yielded from continuous stream ink jet printers in which a liquid ink stream is broken into droplets, some of which are selectively deflected by a gas stream.  
         BACKGROUND OF THE INVENTION  
         [0003]    Traditionally, digitally controlled ink jet color printing is accomplished by one of two technologies. Both can utilize independent ink supplies for each of the colors of ink provided. Ink is fed through channels formed in the printhead and each channel includes a nozzle from which droplets of ink are selectively ejected and deposited upon a print medium, such as paper. Typically, each technology requires separate ink delivery systems for each ink color used in printing. Ordinarily, the three primary subtractive colors, i.e. cyan, yellow and magenta, are used because these colors can produce, in general, up to several million shades or color combinations.  
           [0004]    The first technology, commonly referred to as “drop on demand” (DOD) ink jet printing, provides ink droplets for impact upon a recording surface using a pressurization actuator, such as a thermal actuator, piezoelectric actuator, or the like. Selective activation of the actuator causes the formation and ejection of a flying ink droplet that crosses the space between the printhead and the print media and strikes the print media. The formation of printed images is achieved by controlling the individual formation of ink droplets, as is required to create the desired image. Typically, a slight negative pressure within each channel keeps the ink from inadvertently escaping through the nozzle, and also forms a slightly concave meniscus at the nozzle helping to keep the nozzle clean.  
           [0005]    With heat actuators, a heater, placed at a convenient location, heats the ink causing a quantity of ink to phase change into a gaseous steam bubble that raises the internal ink pressure sufficiently for an ink droplet to be expelled. With piezoelectric actuators, an electric field is applied to a piezoelectric material possessing properties that create a mechanical stress in the material causing an ink droplet to be expelled. Some naturally occurring materials possessing these characteristics are quartz and tourmaline. The most commonly produced piezoelectric ceramics are lead zirconate titanate, barium titanate, lead titanate, and lead metaniobate.  
           [0006]    The second technology, commonly referred to as “continuous stream” or “continuous” inkjet printing, uses a pressurized ink source which produces a continuous stream of ink droplets. Conventional continuous inkjet printers utilize electrostatic charging devices that are placed close to the point where a filament of working fluid breaks into individual ink droplets. The ink droplets are electrically charged and then directed to an appropriate location by deflection electrodes having a large potential difference. When printing is desired, the ink droplets are deflected into an ink capturing mechanism and either recycled or discarded. When printing is desired, the ink droplets are not deflected and allowed to strike a print media. Alternatively, deflected ink droplets may be allowed to strike the print media, while non-deflected ink droplets are collected in the ink capturing mechanism. Typically, continuous inkjet printing devices are faster than droplet on demand devices and can produce high quality printed images and graphics.  
           [0007]    U.S. Pat. No. 1,941,001, issued to Hansell, and U.S. Pat. No. 3,373,437 issued to Sweet et al., each disclose an array of continuous ink jet nozzles wherein ink droplets to be printed are selectively charged and deflected towards the recording medium. This technique is known as “binary deflection” continuous ink jet printing.  
           [0008]    Continuous ink jet printers that utilize electrostatic charging devices and deflector plates require many components and large spatial volumes in which to operate. This results in continuous inkjet printheads and printers that are complicated, have high energy requirements, are difficult to manufacture, and are difficult to control.  
           [0009]    U.S. Pat. No. 3,709,432, issued to Robertson, discloses a method and apparatus for stimulating a filament of ink to break up into uniformly spaced ink droplets through the use of transducers. The lengths of the filaments before they break up into ink droplets are regulated by controlling the stimulation energy supplied to the transducers, with high amplitude stimulation resulting in short filaments and low amplitudes resulting in long filaments. A flow of air is generated across the paths of the fluid at a point intermediate to the ends of the long and short filaments. The air flow affects the trajectories of the filaments before they break up into droplets more than it affects the trajectories of the ink droplets themselves. By controlling the lengths of the filaments, the trajectories of the ink droplets can be controlled, or switched from one path to another. As such, some ink droplets may be directed into a catcher while allowing other ink droplets to be applied to a print media.  
           [0010]    U.S. Pat. No. 4,190,844, issued to Taylor, on Feb. 26, 1980, discloses a continuous inkjet printer in which a printhead supplies a filament of working fluid that breaks into individual ink droplets. The ink droplets are then selectively deflected by a first pneumatic deflector, a second pneumatic deflector, or both. The first pneumatic deflector is an “on/off” or an “open/closed” type having a diaphram that either opens or closes a nozzle depending on one of two distinct electrical signals received from a central control unit. This determines whether the ink droplet is to be printed or non-printed. The second pneumatic deflector is a continuous type having a diaphram that varies the amount a nozzle is open depending on a varying electrical signal received the central control unit. This oscillates printed ink droplets so that characters may be printed one character at a time. If only the first pneumatic deflector is used, characters are created one line at a time.  
           [0011]    The use of an air flow to deflect droplets in a continuous inkjet printhead reduces the complexity of the printhead. However, such printheads are sensitive to environmental conditions and thus can produce inconsistent print quality.  
         SUMMARY OF THE INVENTION  
         [0012]    An object of the present invention is to improve the quality of print from of a continuous ink jet printhead. To achieve this and other objects, a first aspect of the invention is an apparatus for printing an image comprising an ink droplet forming mechanism configured to selectively create a stream of ink droplets having a plurality of volumes and traveling along a trajectory path. A droplet deflector is configured to generate a gas flow at an output thereof interacting with the stream of ink droplets thereby separating ink droplets having one of a plurality of volumes from ink droplets having another of a plurality of volumes. A pressure sensor is positioned proximate the output and configured to generate a pressure indication signal. A controller is coupled to the pressure sensor and configured to output a compensation signal based on the indication signal, and a pressure mechanism is operatively coupled to the controller to adjust the gas flow generated by the droplet deflector. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0013]    Other features and advantages of the present invention will become apparent from the following description of the preferred embodiment of the invention and the accompanying drawings, in which:  
         [0014]    [0014]FIG. 1 is a schematic view of a print mechanism in accordance with a preferred embodiment of the present invention;  
         [0015]    [0015]FIG. 2 is a graph of an example of heater activation frequency and the resulting ink droplets;  
         [0016]    [0016]FIG. 3 is a schematic side view of a print apparatus of the preferred embodiment illustrating the ink droplet trajectory;  
         [0017]    [0017]FIG. 4 is a partial sectional view of a gas plenum of the preferred embodiment;  
         [0018]    [0018]FIG. 5 is a partial sectional view of an alternative gas plenum; and  
         [0019]    [0019]FIG. 6 is a partial sectional view of an alternative pressure compensation mechanism.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    [0020]FIG. 1 illustrates a print head mechanism in accordance with a preferred embodiment of the invention. Mechanism  100  includes printhead  2 , at least one ink supply  20 , and controller  10 . Although mechanism  100  is illustrated schematically and not to scale for the sake of clarity, one of ordinary skill in the art will be able to readily determine the specific size and interconnections of the elements. Printhead  2  can be formed from a semiconductor material, such as silicon, using known semiconductor fabrication techniques, such as complementary metal oxide semiconductor (CMOS) fabrication techniques and micro electro mechanical structure (MEMS) fabrication techniques, or from any materials using any known or future fabrication techniques.  
         [0021]    Plural nozzles  5  are formed in printhead  2  to be in fluid communication with ink supply  20  through ink passages (not shown) also formed in printhead  2 . Each ink supply  20  may contain a different color ink for color printing. Any number of ink supplies  20  and corresponding nozzles  5  can be used in order to provide color printing using three or more ink colors. Additionally, black and white or single color printing may be accomplished using a single ink supply  20 .  
         [0022]    Heaters  4  are positioned on printhead  2  around a corresponding nozzle  5 . Although each heater  4  may be disposed radially away from an edge of a corresponding nozzle  5 , heaters  4  are preferably disposed close to an edge of a corresponding nozzle  5  in a concentric manner. In a preferred embodiment, heater  4  is formed in a substantially circular or ring shape. However, heater  4  may be formed in a partial ring, square, or any appropriate shape. Heater  4  can include an electric resistive heating element electrically connected to pad  6  via conductor  8  or any other type of heating element.  
         [0023]    Conductors  8  and pads  6  may be at least partially formed or positioned on printhead  2  and provide an electrical connection between controller  10  and heaters  4 . Alternatively, the electrical connection between controller  10  and heaters  4  may be accomplished in any known manner. Controller  10  may be a logic controller, programmable microprocessor, or the like, operable to control heaters  4  and other components of mechanism  100  as described below.  
         [0024]    [0024]FIG. 2 illustrates an example of the activation signal frequency provided by controller  10  to one of heaters  4 , plotted as signal amplitude versus time, and the resulting individual ink droplets  102  and  104 . A high frequency, e.g., a frequency resulting from time t 2  between pulses, of activation of heater  4  results in a small volume droplet  102  and a low frequency, e.g., a frequency resulting from time t 1  between pulses, of activation of heater  4  results in large volume droplets  104 . Activation of heaters  4  may be controlled independently based on the ink color required, movement of printhead  20  relative to a print media P and an image to be printed. A plurality of droplets may be created having a plurality of volumes, including a mid-range activation frequency of heater  4  resulting in a medium volume droplet. As such, reference below to large volume droplets  104  and small volume droplets  102  is for example purposes only and should not be interpreted as being limiting in any manner.  
         [0025]    [0025]FIG. 3 illustrates an ink jet print apparatus of the preferred embodiment. Large volume ink droplets  104  and small volume ink droplets  102  are ejected in a stream from printhead  2  along ejection path X. Droplet deflector system  40  applies a force to ink droplets  102  and  104  as the ink droplets travel along path X. The force interacts with ink droplets  102  and  104  along path X, causing the ink droplets  102  and  104  to be deflected. As ink droplets  102  and  104  have different volumes and masses, the force causes small droplets  102  to separate from large droplets  104  with small droplets  102  diverging from path X along deflection angle D. While large droplets  104  are only slightly affected by the force.  
         [0026]    Droplet deflector system  40  can include a pressurized gas source  42  that provides the force in the form of a gas flow. Gas source  42  can be a fan for moving ambient air or any other source of pressurized gas. Plenum  44  is coupled to gas source  42  to direct the flow of gas in a desired manner. An output end of plenum  44  is positioned proximate path X. Ink recovery conduit  30  is disposed substantially in opposition to plenum  44  to facilitate recovery of non-printed, i.e., deflected ink droplets for subsequent use. Of course, there can be a separate droplet deflection mechanism and ink recovery conduit for each ink color. However, only one of each of these elements is illustrated for simplicity.  
         [0027]    In operation, a print media P is transported in a direction transverse to path X in a known manner. Transport of print media P is coordinated with movement of printhead  2  using controller  10  in a known manner. Pressurized ink is ejected through nozzles  5  creating filaments of ink. Heaters  4  are selectively activated at various frequencies causing the filaments to break up into a streams of individual ink droplets  102  and  104  as described above.  
         [0028]    During printing, deflector system  40  is operated. As gas exiting the output of plenum  44  interacts with the stream of ink droplets, the individual ink droplets separate depending on each the volume and mass of each droplet. Accordingly, gas source  42  can be adjusted to permit large volume droplets  104  to strike print media P while small volume droplets  102  are deflected as they travel downward into recovery plenum  30 . Accordingly, heaters  4  can be controlled in a coordinated manner to cause ink of various colors to impinge on print media P to form a desired image. Alternatively, deflected droplets can impinge on media P and non-deflected droplets can be recovered.  
         [0029]    Large volume droplets  104  and small volume droplets  102  can be of any appropriate relative size. However, the droplet size is primarily determined by ink flow rate through nozzles  5  and the frequency at which heaters  4  are cycled. The flow rate is primarily determined by the geometric properties of nozzles  5  such as nozzle diameter and length, pressure applied to the ink, and the fluidic properties of the ink such as ink viscosity, density, and surface tension. As such, typical ink droplet sizes may range in site from 1 to 10,000 picoliters.  
         [0030]    Although a wide range of droplet sizes are possible, at typical ink flow rates, for a 12 micron diameter nozzle, large volume droplets  104  can be formed by cycling heaters  4  at a frequency of about 10 kHz producing droplets of about 60 microns in diameter and small volume droplets  102  can be formed by cycling heaters  4  at a frequency of about 150 kHz producing droplets that are about  25  microns in diameter. These droplets typically travel at an initial velocity of 10 m/s. Even with the above droplet velocity and sizes, a wide range of separation distances between large volume droplets  104  and small volume droplets  102  after deflection is possible, depending on the physical properties of the gas used, the velocity of the gas, and the distance over which the gas interacts with droplets  102  and  104 . For example, when using air as the gas, typical air velocities may range from, but are not limited to 100 to 1000 cm per sec while interaction distances may range from, but are not limited to, 0.1 to 10 mm. Gases, including air, nitrogen, etc., having different densities and viscosities can be used for deflection.  
         [0031]    It follows that, the separation amount is dependent on the ambient pressure because, assuming constant operation parameters of gas source  42 , the velocity of the gas ejected therefrom will vary with the ambient pressure. Accordingly, pressure transients, such as pressure changes caused by activation or termination of a cooling unit in a room containing the printing device, the opening of a door or a window, or any other change in ambient conditions, can cause poor performance of the printing apparatus. For example, a small ambient pressure transient may cause a droplet or portion of a droplet that is intended to go into the recovery conduit  30  to impinge upon the print media P. Accordingly, the preferred embodiment includes a mechanism for compensating for changes in ambient air pressure.  
         [0032]    As illustrated in FIGS. 1 and 3, pressure sensor  12  is disposed in print mechanism  120  proximate an output of plenum  44  but in a position in which it does not interfere with the flow of gas from plenum  44 . Controller  10  includes logic for receiving a pressure indication signal from sensor  12  and determining a compensation value based on the indication signal. For example, the logic can include a lookup table having corresponding compensation value for each indication signal value or for each range of such values. The indication signal can represent any type of pressure indication, such as, actual pressure, an absolute value of a change in pressure from ambient, or the like.  
         [0033]    Controller  10  can include any necessary logic in logic section  11  for determining ambient pressure, such as time based filters, averaging algorithms, or the like. Sensor  12  can comprise plural sensing elements and controller  10  can utilize or and function or the like between the sensing elements to avoid erroneous readings.  
         [0034]    Controller  10  can be coupled to a gas flow adjustment mechanism. For example, as illustrated in FIG. 4, the adjustment mechanism can comprise one or more baffles  46  disposed in plenum  44  to selectively restrict the flow of gas therethrough. Baffles  46  can be activated by actuators  48 , such as piezoelectric actuators, MEMs actuators, electromagnetic solenoids, or any other type of actuators. For example, baffles  46  can be moved from a retracted position, represented by the dashed lines, to an extended position, represented by the solid lines. Baffles  46  can be actuated independently or in concert with one another. Baffles  46  may be positioned at any appropriate position. It can be seen that, when baffles  46  are in the extended plenum  44  will be reduced.  
         [0035]    As illustrated in FIG. 5, the pressure compensation mechanism can be a device for selectively altering the size or shape of plenum  44  to restrict gas flow therethrough. For example, actuators  48  can be positioned between a rigid outer portion  50  and a flexible inner portion  52  of plenum  40  to press on inner portion  52  when actuated and thereby adjust the cross-sectional area of plenum  44 . When the cross sectional area is reduced, gas flow through plenum  44  is reduced. Once again, actuators  48  can be of any type, such as piezoelectric, MEMS, solenoids, or the like.  
         [0036]    [0036]FIG. 6 illustrates an alternative pressure compensation mechanism  60  in the form of an acoustic wave generator generating acoustic waves in a manner to interfere with gas flow from plenum  44 . Speaker  64  is coupled to wave generator  62  to selectively generate acoustic waves to oppose the gas flow out of the output of plenum  44  and thus selectively restrict the velocity of the gas flow. Wave generator  62  can be controlled by controller  10  in response to the indication signal to control the frequency and/or amplitude of the acoustic waves.  
         [0037]    The compensation values can be determined mathematically or through experimentation. Compensation values can be stored as a lookup table, a linear or non linear mathematical formula, or the like.  
         [0038]    Printhead  2  can be manufactured using known techniques, such as CMOS and MEMS techniques and can incorporate a heater, a piezoelectric actuator, a thermal actuator, etc. There can be any number of nozzles  5  and the separation between nozzles  5  can be adjusted in accordance with the particular application to avoid smearing and deliver the desired resolution.  
         [0039]    Droplet deflector system  40  can be of any configuration and can include any number of appropriate plenums, conduits, blowers, fans, etc. Additionally, droplet deflector system  40  can include a positive pressure source, a negative pressure source, or both, and can include any elements for creating a pressure gradient or gas flow. Recovery plenum  30  can be of any configuration for catching deflected droplets and can be ventilated if necessary. Gas source  42  can be any appropriate source, including a gas pressure vessel or generator, a fan, a turbine, a blower, or electrostatic air moving device.  
         [0040]    Any mechanism can be disposed in plenum  48  or at any other position to selectively adjust gas flow based on the sensing of pressure transients. For example, baffles orifices templates or the like can be used. The gas flow adjustment mechanism can be any internal or external mechanism for adjusting the gas flow. The baffles can be of any size, shape, or configuration.  
         [0041]    Print media P can be of any type and in any form. For example, the print media can be in the form of a web or a sheet. Additionally, print media P can be composed from a wide variety of materials including paper, vinyl, cloth, other large fibrous materials, etc. Any mechanism can be used for moving the printhead relative to the media, such as a conventional raster scan mechanism, etc.  
         [0042]    While the foregoing description includes many details and specificities, it is to be understood that these have been included for purposes of explanation only, and are not to be interpreted as limitations of the present invention. Many modifications to the embodiments described above can be made without departing from the spirit and scope of the invention, as by the following claims and their legal equivalents.  
       Parts List  
       [0043]    [0043] 2  Printhead  
         [0044]    [0044] 4  Heaters  
         [0045]    [0045] 5  Nozzels  
         [0046]    [0046] 6  Pad  
         [0047]    [0047] 8  Conductor  
         [0048]    [0048] 10  Controller  
         [0049]    [0049] 11  Logic Section  
         [0050]    [0050] 20  Ink Supply  
         [0051]    [0051] 30  Recovery Conduit  
         [0052]    [0052] 40  Deflector System  
         [0053]    [0053] 42  Gas Source  
         [0054]    [0054] 44  Plenum  
         [0055]    [0055] 46  Baffles  
         [0056]    [0056] 48  Actuators  
         [0057]    [0057] 50  Plenum Outer Portion  
         [0058]    [0058] 52  Plenum Inner Portion  
         [0059]    [0059] 60  Acoustic Wave Generator  
         [0060]    [0060] 64  Speaker  
         [0061]    [0061] 62  Wave Generator  
         [0062]    [0062] 100  Print Mechanism  
         [0063]    [0063] 102  Small Droplet  
         [0064]    [0064] 104  Large Droplet