Abstract:
Both an inkjet printer and method are provided for contone inkjet printing on a receiver. The inkjet printer includes a printhead having at least one nozzle for ejecting a stream of ink droplets, a droplet deflector for generating a flow of gas that impinges on the stream of ejected droplets to deflect the trajectories of the droplets, and a controller for varying the velocity of the gas flow in order to vary the degree of trajectory deflection so the droplets intended to print on a particular pixel in the receiver land on top of one another without elongation despite relative movement between the printhead and the receiver. The printer provides improved image quality and productivity while reducing image artifacts.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    Reference is made to U.S. application Ser. No. 09/750,946, entitled Printhead Having Gas Flow Ink Droplet Separation And Method Of Diverging Ink Droplets, filed in the name of Jeanmaire and Chwalek on Dec. 28, 2000; U.S. application Ser. No. 09/751,232, entitled A Continuous Ink-Jet Printing Method And Apparatus, filed in the names of Jeanmaire and Chwalek on Dec. 28, 2000; U.S. application Ser. No. 09/751,563, entitled Ink Jet Apparatus Having Amplified Asymmetric Heating Drop Deflection, filed in the names of Chwalek, Delametter and Jeanmaire on Dec. 28, 2000; and U.S. application Ser. No. 09/777,426, entitled Continuous Inkjet Printhead and Method of Translating Ink Drops, filed in the names of Hawkins and Jeanmaire on Feb. 6, 2001. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention generally relates to inkjet printing, and is specifically concerned with an apparatus and method for continuously displacing the trajectories of droplets ejected from an inkjet printhead toward a relatively moving receiver so that droplets intended for a particular location on the receiver land on top of one another.  
         BACKGROUND OF THE INVENTION  
         [0003]    There are two types of inkjet printers, including drop-on-demand printers in which the printhead nozzles eject droplets only when it is desired to print ink onto a receiver, and continuous inkjet printers in which the printhead nozzles eject droplets continuously, the droplets not desired to be printed being captured by a gutter. Both methods are currently practiced.  
           [0004]    In drop-on-demand printers, the printhead  1  typically includes a linear row of nozzles  3  which is scanned across a stationary receiver  5  in a fast scan direction  7  as shown in PRIOR ART FIG. 1 a . Commercially available desktop printers, for example those made by Epson, operate in this manner. After each fast scan the printhead moves in a slow scan direction  9  relative to the receiver, the slow scan direction being orthogonal to the fast scan direction. Typically, the receiver is moved in the slow scan direction  9  rather than the printhead to effect the relative movement, and another row of printing is completed as is indicated in phantom.  
           [0005]    In continuous inkjet printers, the receiver is often moved in the fast scan direction rather than the printhead due to the size and complexity of the printhead. In many cases, the printhead is pagewide and extends across the entire width of the paper to obviate the need for a second scanning movement. The fast scan motion of the printhead relative to the receiver is typically parallel to the length of the printhead.  
           [0006]    Drop-on-demand and continuous inkjet printers print droplets on a regularly spaced grid of printing locations or pixels on a receiver, typically at a density of from a few hundred to more than two thousand pixels per inch. Both types of inkjet printers may operate in either a binary (black and white) mode of printing or a contone (also referred to as grayscale) mode of printing. In the binary mode, either a single droplet of a fixed size is printed at each pixel or no droplet is printed. In the contone mode of printing, the amount of ink printed onto a given pixel can be varied over a range of sizes or levels; for example, 10 or more levels. One method to vary the amount of ink printed at each pixel is in contone printing to eject droplets of differing size. However, such an approach is well known in the art to be difficult if substantial variations in droplet size are required, which is usually the case in contone printing. Another method is to print more than one droplet of a fixed size at a given pixel at different times. For example, a second droplet may be printed on a subsequent fast scan pass. This method greatly slows the printing process, especially if substantial variations in the amount of ink per pixel are required. A third more widely practiced method is to eject all of the droplets required at a given pixel during a single scan pass print in rapid sequence so that the droplets print at substantially the same time. In some cases this has been achieved by arranging for each group of sequentially ejected droplets to combine together before landing on the receiver. However, droplets which combine before landing on the receiver may not land at exactly the desired position, since they have been ejected over a range of times. Also the combined droplet may not be spherical when it lands, resulting in image artifacts. In other printers, a group of droplets is sequentially ejected so that the droplets land on the same pixel on the receiver. However, if the receiver is moving quickly relative to the printhead (as desired to achieve high productivity) the droplets landing in a group may be printed as an elongated group that is smeared on the pixel in the direction of receiver motion. Such an elongation within the printed pixel also produces image artifacts and lowers image quality.  
           [0007]    To overcome these problems, U.S. Pat. No. 6,089,692, issued to Anagnostopoulos on Aug. 8, 1997, discloses a contone printing method wherein the motion of the receiver is modulated with respect to the printhead by rapidly starting and stopping the receiver in the fast scan direction. This method advantageously allows sequential droplets ejected in a group to be printed at an identical location, thus avoiding pixel smearing. Preferably, the printhead ejects a sequence of equally sized droplets that do not combine before landing on the receiver. During printing of a group of droplets, the receiver motion with respect to the printhead is effectively stopped, and the receiver is moved before the next droplet or group of droplets is printed. Unfortunately, this method requires expensive and precise mechanical controls and hence adds to the cost of the printer and additionally may reduce printer speed due to the time required to accelerate and decelerate heavy components. It is, of course, possible to accelerate the printhead relative to the receiver. But if this is attempted, the printhead may perform poorly due to fluid acceleration and consequent pressure differentials in the ink along the length of the printhead. This is particularly true for pagewide printheads because of the long fluid channels that are distributed over the entire length of the printhead, especially if the displacement occurs rapidly.  
           [0008]    Clearly, there is a need for an improved method for contone printing in which a printhead ejects groups of identically sized droplets that land at a single location on the receiver in order to achieve high image quality at no expense to productivity. It would be desirable if such a method could be achieved without the need for expensive and precise mechanical controls that modulate relative movement between the printhead and receiver. Ideally, such a method should be applicable to both drop-on-demand and continuous stream printers. In the case of continuous stream printers, such a method should be achieved without the need for adding any new and expensive droplet steering mechanisms to the printer.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention includes both an apparatus and method for contone inkjet printing using printheads which eject groups of identically sized ink droplets intended to be printed together at a single printing location or pixel. In accordance with the present invention, droplets in such a group land at a single location on the receiver despite the fact that the receiver moves uniformly with respect to the printhead. The trajectories of droplets ejected sequentially in the group are continuously altered so that droplets ejected later in time travel further in the direction of motion of the receiver than do droplets ejected earlier in time. Such trajectory alteration is accomplished by means of the same droplet deflector that is used to separate printing from non-printing droplets. The droplet deflector generates a flow of gas that impinges on the droplet stream comprised of larger and smaller droplets to deflect the larger droplets away from a gutter that captures and recycles the smaller droplets. A controller varies the speed of the deflecting gas flow to further deflect the trajectories of the larger droplets intended for printing so that the droplets intended for a particular pixel land on top of one another despite continuous relative movement between the printhead and the receiver. The apparatus and method are useful in reducing image artifacts and improving image quality and productivity.  
           [0010]    While the preferred application of the invention is in a continuous stream inkjet printer, the invention may also be used in a drop-on-demand type inkjet printer.  
           [0011]    The droplet deflector includes a tube having an outlet for directing a gas flow into trajectory-altering impingement with the droplets. In one embodiment of the invention, the controller includes a gas flow restrictor for varying the gas flow velocity exiting the tube outlet by variably restricting the gas flow through the tube. The gas flow restrictor may take the form of an expandable bladder disposed within the tube interior. Alternatively, the gas flow restrictor may include a plurality of movable cantilevers, which are either electrostatically or thermally controlled via bimetallic elements that are mounted around the inner surface of the tube. In still another embodiment, the gas flow restrictor may include a plurality of movable vanes disposed within the tube, which restrict more or less of the gas flow in the same manner as venetian blinds.  
           [0012]    In still other embodiments of the invention, the controller may include a pressure pulse generator for varying the gas flow velocity in the deflector tube. The pressure pulse generator may include a speaker-like diaphragm in communication with the tube that is connected to an armature which rapidly moved by a piezoelectric transducer. In still another embodiment, the pressure pulse generator may include a diffuser disposed within the tube in combination with a vibrational mechanism that variably vibrates the tube and diffuser toward and away from the droplet stream to create pressure waves within the tube.  
           [0013]    In still another group of embodiments, the controller may include an oscillating mechanism for variably oscillating the outlet of the tube with respect to the droplet stream. The direction of the oscillations may be perpendicular to a longitudinal axis of the tube. Alternatively, the oscillations may be in a pivotal direction around a point on the longitudinal axis of the tube.  
           [0014]    In all cases, the controller varies the degree of trajectory deflection for the droplets in the stream such that droplets intended for printing on a selected pixel on the receiver are deposited substantially on top of one another despite relative movement between the printhead and the receiver. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 a  shows a prior art method of printing with mechanical translation of an inkjet printhead scanned over a receiver.  
         [0016]    [0016]FIGS. 1 b  and  1   c  show partial cross-sectional view of an inkjet printer in accordance with the present invention having a droplet deflector that employs a flow of air from an air tube located above a row of nozzles to deflect ink droplets.  
         [0017]    [0017]FIG. 1 d  is a side view of the air tube and printhead nozzles of FIGS. 1 b  and  1   c  along the line  1   d - 1   d.    
         [0018]    [0018]FIGS. 2 a - 2   b  are top views of the air tube of FIGS. 1 b  and  1   c  located above a row of nozzles in a continuous inkjet printhead wherein the airstream flows at different velocities to deflect ejected droplets a greater or lesser amount of flow of an airstream through the air tube.  
         [0019]    [0019]FIGS. 2 c ,  2   d , and  2   e  depict side views of the air tube in FIGS. 2 a  and  2   b , and the effect the different airstream velocities have on the printed droplet.  
         [0020]    [0020]FIGS. 3 a  and  3   b  show a side cross-sectional view of an air tube having an airflow restricter at the end of the air tube in a contracted (FIG. 3 a ) and an extended (FIG. 3 b ) position.  
         [0021]    [0021]FIG. 3 c  shows a top view of the location of droplets printed on a receiver from a fast and a slow airstream corresponding to the contracted and expanded restricter of FIGS. 3 a  and  3   b  respectively.  
         [0022]    [0022]FIGS. 3 d  and  3   e  show a side cross-sectional view of an air tube having an airflow restricter centrally located in the air tube in a contracted (FIG. 3 d ) and an extended (FIG. 3 e ) position.  
         [0023]    [0023]FIG. 3 f  shows a top view of the location of droplets printed on a receiver from a fast and a slow airstream corresponding to the contracted and expanded restricter of FIGS. 3 d  and  3   e , respectively.  
         [0024]    [0024]FIGS. 3 g  and  3   h  show a side cross-sectional view of an air tube having a rectangular and tapered channel, respectively, at the end of the air tube.  
         [0025]    [0025]FIG. 3 i  shows a top view of the location of droplets printed on a receiver from a fast and a slow airstream corresponding to the rectangular and tapered channels of FIGS. 3 g  and  3   h , respectively.  
         [0026]    [0026]FIGS. 4 a  and  4   b  show a side cross-sectional view of an air tube having a contracted and expanded upper and lower control surface, respectively.  
         [0027]    [0027]FIG. 4 c  shows a top view of the location of droplets printed on a receiver from a fast and a slow airstream corresponding to the contracted and extended upper and lower control surfaces of FIGS. 4 a  and  4   b , respectively.  
         [0028]    [0028]FIG. 4 d  shows a three dimensional view of a control surface having cantilevers in a state corresponding to an extended control surface.  
         [0029]    [0029]FIG. 4 e  shows a top view of an airstream including a first and second set of guide vanes for altering the direction of the airstream, both guides being horizontal.  
         [0030]    [0030]FIG. 4 f  shows a side cross-sectional view of an air tube of an airstream deflector having a first and second set of guide vanes for controlling airflow, the second guide vanes being angled.  
         [0031]    [0031]FIG. 4 g  shows a top view of the location of droplets printed on a receiver corresponding to the horizontal and angled second guide vanes of FIGS. 4 e  and  4   f , respectively.  
         [0032]    [0032]FIG. 4 h  shows a side cross-sectional view of an air tube of an airstream deflector having a transducer and plate located centrally.  
         [0033]    [0033]FIG. 4 i  shows a side cross-sectional view of an air tube of an airstream deflector having a diffuser located centrally. The air tube and diffuser are mechanically displaced periodically in the direction of diffuser motion.  
         [0034]    [0034]FIG. 5 a  shows a side cross-sectional view of an air tube of an airstream deflector vertically spaced from the membrane in which the printhead nozzles are defined.  
         [0035]    [0035]FIG. 5 b  shows a side cross-sectional view of an air tube of an airstream deflector with a reduced vertical spacing from the membrane.  
         [0036]    [0036]FIG. 5 c  shows a top view of the location of droplets printed on a receiver corresponding to the vertical spacing and the reduced vertical spacing of FIGS. 5 a  and  5   b , respectively.  
         [0037]    [0037]FIG. 6 shows a side cross-sectional view of an air tube of an airstream deflector for two positions of the air tube, a upwardly angled air tube and a lower angled air tube, and a top view of the location of droplets printed on a receiver corresponding to the two angled air tube positions.  
         [0038]    [0038]FIGS. 7 a - 7   d  show the trajectories of four ink droplets sequentially ejected from a printhead and landing at a common location on a moving receiver. FIG. 7 a  illustrates the average airflow velocities experienced by each drop.  
         [0039]    [0039]FIGS. 8 a - 8   d  show schematically four examples of the printed drop displacement (vertical axis) as a function of time (horizontal axis) for corresponding plots of airstream velocity (vertical axis) as a function of time (horizontal axis) for an airstream deflector. In each case, the periodic dependence of airflow on time is of the same duration as the time required for an ejected droplet to traverse the airstream. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0040]    [0040]FIGS. 1 b  and  1   c  schematically illustrate a continuous stream inkjet printer  10  in accordance with the present invention, the printer  10  having a printhead  12 , a receiver  14 , and a droplet deflector  15  that utilizes an airflow to deflect differently sized ink droplets. Ink droplets  16  are ejected from a nozzle  18 , the nozzle  18  typically having been formed in a membrane  20  overlying an ink cavity  22 . The ejected droplets  16  are selected to have at least one of two sizes, a large size  26  and a small size  28 . Such selective sizing of the ink droplets may be accomplished by means of small annular heating elements  30  that circumscribe each of the nozzles  18 . Electrical power is conducted in pulses to each of the heating elements  30  as droplets are ejected therefrom. Depending upon the frequency of the pulses, the surface tension of the ink is affected such that small droplets  26  are generated during higher frequencies, while larger droplets  28  are generated during lower frequencies. An airstream  34  flows across the trajectory followed by the ejected droplets  16 , and a gutter  36  is provided to capture the large size droplets  26  that impinge on the end of the gutter  14 . The airstream  34  is shown in FIG. 1 c , extending from the open end of the air tube  32 . Printed droplets, which are of the small size  28 , experience a greater deflection angle when passing through the airstream  34  than do guttered droplets, which are of the large size  26 . As is shown in FIG. 1 d , the opening  33  of the air tube  32  is somewhat elongated in shape and positioned over and to the side of the nozzles  18  of the printhead  12 , each of which is ejecting a combination of small and large droplets in accordance with the frequency of pulses received by their respective heating elements  30 . In the subsequent discussions, only the trajectory of the printed droplets is considered.  
         [0041]    In order to illustrate the principal of operation of the invention and its embodiments, FIGS. 2 a  and  2   b  show top views of the air tube  32  of the inkjet printer  10  and the droplets printed on a receiver which results from simultaneously ejecting small droplets from each nozzle. The location of the edge of the gutter is shown as a phantom line in FIGS. 2 a - 6 . The phantom line is a useful reference point in indicating the displacement of the printed droplets  38  in the fast scan direction due to passage though the airstream  34 . The velocity of airflow  34  in the air tube  32  is about the same as the airstream velocity outside the tube and near its outlet  33 .  
         [0042]    The airflow in the air tube  32  in FIG. 2 a  is shown as having a higher airflow velocity, in comparison with the velocity shown in FIG. 2 b , where the airflow is shown as having a lower airflow velocity. As shown by the difference in the distance of the printed droplets from the gutter location, the lower airflow velocity reduces the displacement experienced by the droplets while traversing the airstream; in other words, the deflection angle in FIG. 1 c  has been reduced. As will be described later, although the change in displacement of the printed droplets  38  in FIGS. 2 a  and  2   b  has been described as a case in which airflow  34  in the air tube  32  and hence the airflow velocity is constant in time, the same result holds on average if the airflow velocity is changing at any point of the droplet trajectory in the airflow. The displacement of droplets is approximately proportional the average airflow velocity experienced by the droplet during passage through the airflow  34 . It should be noted that while reference is frequently made herein to a change in airflow velocity, such velocity changes are made over a baseline velocity which is the minimum necessary for the airflow  34  to deflect the small droplets  20  beyond the capturing edge of the gutter  36 .  
         [0043]    [0043]FIGS. 2 c - 2   d  show a cross-section of the air tubes and airstreams  34  of FIGS. 2 a  and  2   b , respectively. The airstream  34  extends near the end of the air tubes  32  vertically from the bottom to top of the air tubes. FIG. 2 e  is a schematic representation of the displacements of printed droplets  38   c, d  with respect to the gutter position (dotted line) corresponding to the airstream velocities of FIGS. 2 c  and  2   d , respectively. The format of FIGS. 2 c - 2   d  is used subsequently in describing preferred embodiments of the apparatus of the present invention.  
         [0044]    [0044]FIGS. 3 a - 3   c  show a cross-section of an air tube  32  having an airflow restrictor  40  at the open end of the air tube  32 . The airflow restrictor  40  comprises a moveable solid or solid surface which can be extended into the air tube  32  to partially block airflow  34  in the tube  32 . For example, an airflow restricter  40  may be an expandable elastic membrane  42  which can be extended into the air tube by inflating the cavity between the membrane  42  and the top of the inner wall of the air tube  32 . FIG. 3 b  shows the airflow restricter in the contracted state, in which case the airflow velocity is high. FIGS. 3 c  shows the airflow restricter in the extended state, in which case the airflow velocity is lowered. FIG. 3 c  is a schematic representation of the displacements of printed droplets  38   a, b  with respect to the gutter position (dotted line) corresponding to the airstream velocities of FIGS. 3 a  and  3   b , respectively. The format of FIGS.  2   c - 2   d  has been used in FIGS. 3 a - 3   c  in describing these preferred embodiments of the apparatus employed to alter the displacement of printed droplets.  
         [0045]    [0045]FIGS. 3 d - 3   e  show a cross-section of an air tube  32  having an airflow restrictor  44  centrally located in the air tube  32 . A central location is advantageous in that the effects of small geometrical imperfections in the airflow restricter  44  are averaged out to an appreciable extent by the time the flowing air reaches the open end of the air tube  32 . Again, an airflow restricter comprises a moveable solid or solid surface  46 , which can be extended into the air tube  32  to partially block airflow  34  in the air tube  32 , as in the previous embodiment. FIG. 3 d  shows the airflow restrictor  44  in the contracted state, in which case the airflow velocity is high. FIG. 3 e  shows the airflow restricter in the extended state, in which case the airflow velocity is lowered. FIG. 3 f  is a schematic representation of the displacements of printed droplets  38   d, e  with respect to the gutter position (dotted line) corresponding to the airstream velocities of FIG. 3 d  and  3   e , respectively. Again, the format of FIGS. 2 c - 2   d  has been used in FIGS. 3 d - 3   f  in describing this embodiment of the apparatus.  
         [0046]    [0046]FIGS. 3 g - 3   h  show a cross-section of an air tube  32  having a tapered end portion  48  at the end of the air tube. A central location of such a tapered portion  48  in the air tube  32  could also be used advantageously for the same reasons cited in the previous embodiment. The tapered portion  48  could be provided by mechanical alteration of the top and bottom portions of the air tube  32 , for example by hinging the top and bottom sections. FIG. 3 g  shows the air tube  32  having a rectangular cross-section, in which case the airflow velocity is high. FIG. 3 h  shows the air tube  32  having a tapered end portion  48 , in which case the airflow velocity is lowered. FIG. 3 i  is a schematic representation of the displacements of printed droplets  38   g, h  with respect to the gutter position (dotted line) corresponding to the airstream velocities of FIG. 3 g  and  3   h , respectively. Again, the format of FIGS. 2 c - 2   d  has been used in FIGS. 3 g - 3   I  in describing this embodiment.  
         [0047]    [0047]FIGS. 4 a - 4   b  are a cross-sectional view of an air tube  32  having an airflow control surface centrally located in the air tube  32 . An airflow control surface is known to the art of microstructure fabrication as a solid surface having moveable cantilevers  50  which may be extended upwards to partially redirect airflow. Typically, the cantilevers  50  are conductive and are fabricated in an extended state. Their motion is controlled by application of a voltage to the cantilevers  50  by control means (not shown) which results in their motion due to electrostatic attraction. Typical cantilever dimensions are in the range of 1 to 100 microns in width and 10-1000 microns in length. Also known to the art of control surfaces are bimetallic actuators, in which the cantilevers  50  are formed by stacking two materials (insulated one from another if both are metallic) having different thermal expansion coefficients and passing a current through one to heat the structure thereby causing a curling motion. FIG. 4 a  shows the cantilevers  50  in a contracted state, in which case the airflow velocity is high. FIG. 4 b  shows the cantilevers  50  in an extended state, in which case the airflow velocity is lowered. FIG. 4 c  is a schematic representation of the displacements of printed droplets  38   a, b  with respect to the gutter position (dotted line) corresponding to the airstream velocities of FIG. 3 j  and  3   k , respectively. Again, the format of FIGS. 2 c - 2   d  has been used in FIGS. 4 a - 4   b  in describing this embodiment. The cantilevers  50  are shown in FIG. 4 d  as rectangular, but their shape is not required to be rectangular so long as the individual cantilevers  50  can be controlled.  
         [0048]    [0048]FIGS. 4 e - 4   f  represent a side cross-section of an air tube  32  having two sets of airflow control vanes  52 ,  54  located in the air tube  30 , one near the air tube end (fixed airflow control vane) and the other centrally located (adjustable airflow control vanes  54 ). Such an airflow control vane can be constructed from a freestanding thin film, which may be tilted away from the direction of airflow  34  in a manner similar to a venetian blind. FIG. 43 shows both sets of vanes  52 ,  54  oriented parallel to the airflow, in which case the airflow  34  velocity is high. FIG. 4 f  shows the central airflow control vanes  52  to be angled, so that the airpath is now perturbed. FIG. 4 g  is a schematic representation of the displacements of printed droplets  38   e, f , with respect to the gutter position (dotted line) corresponding to the airstream velocities of FIGS. 4 e  and  4   f , respectively. Again, the format of FIGS. 2 c - 2   d  has been used in FIGS. 4 e - 4   f  in describing this embodiment. The perturbed airflow  34  reduces the airstream velocity and hence reduces the distance by which the printed droplets  38   e, f  are swept while traversing the airstream.  
         [0049]    In yet another preferred embodiment, shown in FIG. 4 h , the air tube contains a pressure pulse generator  56 , for example a piezo transducer  58 , capable of changing its vertical dimension in the presence of an applied electric voltage. The piezo transducer  58  is mounted on the top of the air tube  32 , with a diaphragm  59  attached to the bottom of the transducer  58  via an armature  60  so that vertical motion “d” of the diaphragm displaces a significant mass of air and creates a compressive wave  62 . Preferably, the diaphragm  59  of the transducer  58  extends entirely across the air tube  32  as viewed from the top, and preferably the maximum extent of motion “d” of the diaphragm  59  of the transducer  58  is several percent of the height of the air tube  32 , that is from 10 to 1000 microns. As is well known in acoustic technology, when a voltage is applied to the piezo transducer  58 , armature  60  moves the diaphragm  59  downward in response, creating a pressure pulse in the flow of air through the air tube  32 . This results in a forward pressure wave  62  which travels rapidly to the end of the tube  32 . This pressure wave  62  is used in accordance with the present invention to modulate the airstream velocity and thereby the droplet trajectories. For example, an oscillatory motion of the diaphragm at moderate acoustic frequencies, such as frequencies of from 1 to 50 kHz, will result in periodic pressure waves in the tube  32  and hence in periodic changes in the velocity of the airstream  34 ,  34 ′ (shown in phantom) and thus in the trajectory of droplets. Although not shown in FIG. 4 h , it is advantageous to minimize the airspace above the diaphragm by filling this region with a closed cell elastic foam extending to the top side of the air tube  32 , so that motion of the diaphragm does not cause airflow perturbations above the plate. Changes in the locations of printed droplets  38 ,  38 ′ resulting from such a pressure pulse generator  56  in the air tube  32  are shown with respect to the gutter position in FIG. 4 a.    
         [0050]    [0050]FIG. 4 i  shows yet another embodiment employing a pressure pulse generator via the air tube  32 . Here an airflow diffuser mounted centrally in the air tube  32  and rigidly attached to the air tube walls. The diffuser  64  has a large surface area of contact with all air flowing through the air tube  32  and there is no region of air in the diffuser that is far from a diffuser wall. Such a diffuser  64  can be a bundle of straight, thin-wall tubes aligned along the airflow direction occupying the entire cross-section of the air tube. In such case, the dimensions of the thin-wall tubes are preferably in the range of from 10 to 100 microns in diameter and 1 mm to 1 cm in length. Alternatively, the diffuser  64  can be made by sintering together solid spheres, as is well known in the field of chemical engineering. In this case, the diffuser  64  may comprise spheres of a diameter of from 10 to 100 microns and occupying the entire air tube cross-section over a length of from 1 mm to 1 cm. The diffuser is tightly coupled to the air in the air tube by virtue of its geometry, so that when the diffuser  64  is moved by a mechanical oscillator  66 , for example, rapidly back and fourth in the direction of airflow, pressure waves  62  in the airstream are induced. Such mechanical motion is easily accomplished by moving the air tube  32  itself periodically along its axis, resulting in a forward pressure wave  62  which travels rapidly to the end of the tube  32 . This pressure wave  62  is used in accordance with the present invention to modulate the airstream velocity and thereby the droplet trajectories. For example, an oscillatory motion of the air tube  62  along its length (indicated by the dotted arrow in FIG. 4 b ) at moderate acoustic frequencies, for example frequencies of from 1 to 50 kHz, will result in periodic pressure fluctuations in the tube and hence in periodic changes in the velocity of the airstream  34 ,  34 ′ and thus in the trajectory of droplets traversing the airstream. Changes in the locations of printed droplets  38 ,  38 ′ resulting from an oscillating air tube  32  are shown with respect to the gutter position in FIG. 4 i.    
         [0051]    [0051]FIGS. 5 a - 5   b  show yet another embodiment which achieves the objective of altering the trajectories of droplets ejected from the nozzle  8  from a printhead  12 . In this embodiment, the vertical spacing shown in FIG. 5 a  from the bottom of the air tube to the top of the printhead membrane is periodically changed between an increased D 1  and a reduced D 2  spacing by oscillating the air tube  32  via mechanical oscillator  68 . When the spacing is increased to D 1 , the effect of the airstream  34  on the trajectories of the printed droplets is larger than for the reduced spacing, because the velocities of ejected droplets decrease as the droplets travel further from the printhead  12  and thus the time the droplets spend traversing the airstream  34  increases. As in the previous embodiments, such an oscillatory vertical motion of the air tube  34  at moderate acoustic frequencies, for example frequencies of from 1 to 50 kHz, will result in periodic changes in displacement of printed droplets  38   a ,  38   b  as shown in FIG. 5 c.    
         [0052]    [0052]FIG. 6 shows a related embodiment which achieves the objective of altering the trajectories of ejected droplets by periodically varying the angle of the air tube  34  from a upper inclination to a lower inclination via a mechanical oscillator  70 . When the angle is being increased to the upper inclination, the effect of the airstream  34  on the trajectories of the printed droplets is larger than for the reduced spacing, because the airstream  34  is tracking the ejected droplets, which thus spend more time in the airstream  34 . As in the previous embodiments, such an oscillatory angular motion of the air tube  32  at moderate acoustic frequencies, for example frequencies of from 1 to 50 kHz, will result in periodic changes in the displacement of printed droplets  38 ,  38 ′.  
         [0053]    [0053]FIGS. 7 a - 7   e  show schematically how the present invention adjusts the trajectories of a group of ejected droplets to print them at a common location on a moving receiver  14 . In FIG. 7 a , a first printed droplet A has already landed on the receiver  14 , which is moving left. At an earlier time when the first droplet was ejected, the velocity of the airstream was set to a low value and was additionally caused to gradually increase at a rate whose value will be discussed shortly. Thus the average velocity of the airstream experienced by the first droplet during the time it traverses the airstream is somewhere between the value of the airstream velocity when it was ejected and the value of the airstream velocity when it lands on the receiver. In FIG. 7 a , a second, third, and fourth droplets, following trajectories B, C, and D, are also shown along with arrows representing the average velocity of the airstream experienced by each droplet. Because the airstream velocity is still increasing during ejection of droplets along trajectories B, C, and D, the average velocity horizontal (i.e., velocity in the directions of the airstream) experienced by subsequent droplet increases. Here, average means a time average of the horizontal velocity from the time of droplet ejection to the time the droplet lands on the receiver.  
         [0054]    [0054]FIG. 7 b  shows schematically the trajectories of the group of droplets at a time slightly later than FIG. 7 a . Because the average airstream velocity experienced by the second droplet along trajectory B was greater than that experienced by the first droplet, the second droplet lands on the receiver at a position further left with respect to the nozzle than did the first droplet. However, because the receiver  14  has moved a distance also during the time between the landing of the first and second droplets, the second droplet lands directly on the first. This is in fact the criterion for determining the needed rate of increase in airstream velocity.  
         [0055]    [0055]FIG. 7 c  shows schematically the trajectories of the group of droplets at a time slightly later than FIG. 7 b . Because the average airstream velocity experienced by the third droplet along trajectory C was greater than that experienced by the second droplet, the third droplet lands on the receiver  14  at a position when further left with respect to the nozzle than did the second droplet. Again, because the receiver  14  has moved a distance also during the time between the landing of the second and third droplets, the third droplet lands directly on the first two droplets.  
         [0056]    [0056]FIG. 7 d  shows schematically the trajectories of the group of droplets at a time slightly later than FIG. 7 c . Again, because the average airstream velocity experienced by the fourth droplet along trajectory D was greater than that experienced by the third droplet, and because the receiver  14  has moved a distance also during the time between the landing of the third and fourth droplets, the fourth droplet lands directly on the first three droplets. At this time, the airstream velocity is reduced to its lowest value, i.e., the value it had at the time of ejection of the first droplet, and the process is repeated with another group of droplets.  
         [0057]    [0057]FIGS. 8 a - 8   d  illustrate, in graphical form, variations in the displacement of printed drops in response to four different types of time dependent variations of the velocity of the airstream. The different time dependencies of the airstream velocity, all useful in the practice of the current invention, are shown in FIGS. 8 a - 8   d . In these cases, the airstream velocity is varied periodically in time with a period which is chosen, for simplicity of illustration, to be approximately equal to the time required for an ejected drop to traverse the airstream. In each case, only a single period of the variation in airstream velocity is graphed, the repetitions being thereafter identical. The airstream velocities are indicated by heavy dashes in FIGS. 8 a - 8   d  and the printed drop displacements are indicated by light dashes. In all cases, the airstream velocity (vertical axis) is plotted as a function of time (horizontal axis). The left end of time axis in each figure is defined as time t=0 and the right end corresponds to one period of the variation in airstream velocity.  
         [0058]    In each case, only variations of the airstream velocity are show, although generally, in accordance with the present invention, these variations may be superposed on a constant airstream velocity, chosen so that printed drops are deflected sufficiently to miss the gutter. Typically, the magnitude of the time dependent portion of the airstream velocity is a fraction of the magnitude of the constant portion of the airstream velocity, for example one tenth to nine tenths the constant portion. However, this range should not be construed as limiting. In fact, because the time dependent portion of the airstream velocity itself can sufficiently deflect the printed drops so as to miss the gutter, the present invention can be practiced even in the absence of a time independent portion of the airstream velocity.  
         [0059]    The time dependent portion of the airstream velocity results in a variation of drop displacement relative to any fixed reference position on the printhead itself, for example the position of the edge of the gutter. The amount of drop displacement in each of the cases of FIGS. 8 a - 8   d  varies depending on the time of drop ejection relative to t=0. Thus the printed drop displacement, relative to the edge of the gutter, is plotted on the vertical axis as a function of the delay time between the ejection of the drop and the start (t=0) of the periodic variation of the airstream velocity. In this sense, the time axis has a different interpretation for the airstream velocity versus the printed drop displacement. For the printed drop displacement, the left end of the time axis corresponds to the case that the drop is ejected into the airstream at t=0, at which time, in these illustrative examples the velocity of the airstream is beginning to increase; whereas the middle of the time axis corresponds to the case that the drop is ejected into the airstream at a time halfway through the periodic variation of the airstream velocity, etc.  
         [0060]    In all cases the velocities and displacements are scaled to the value of their maximum excursions, for example the peak height of the plotted velocities in each of FIGS. 8 a - 8   d  represents 100% of its maximum time variation. These curves have been modeled assuming that the force in the direction of the airstream on drops traversing the airstream is at any moment proportional to the airstream velocity at the location of the drop and that the drop velocities in the direction of the airstream are small compared to the drop velocities perpendicular to the airstream.  
         [0061]    In FIG. 8 a , the airstream velocity is modulated in time in a sinusoidal manner, about an average value represented by the central horizontal line in the graph. In this case, the resulting dependence of the printed drop displacement (light dashed line) on the delay time between the ejection of the drop and the start (t=0) of the periodic variation of the airstream velocity is also a sinusoidal function having a delayed phase, that is , a cosine function. In this case, the drops are maximally displaced when launched at the time the time dependent portion of the airstream velocity is rising at its maximum rate.  
         [0062]    In FIG. 8 b , the airstream velocity is modulated in time in square wave manner, about an average value represented by the central horizontal line in the graph. In this case, the resulting dependence of the printed drop displacement on the delay time between the ejection of the drop and the start (t=0) of the periodic variation of the airstream velocity is a triangular function, as shown by the light dashed line.  
         [0063]    In FIG. 8 c , the airstream velocity is shown modulated in time in a triangular manner, about an average value represented by the central horizontal line in the graph. In this case, the resulting dependence of the printed drop displacement on the delay time between the ejection of the drop and the start (t=0) of the periodic variation of the airstream velocity is maximal when the ejected drop is launched midway during the rise of the airstream velocity.  
         [0064]    In FIG. 8 d , the airstream velocity is modulated in time in an asymmetric manner. The central horizontal line in the graph is the mid point of the modulation extrema. In this case, the resulting dependence of the printed drop displacement is a distorted triangular function, again as shown by the light dashed line.  
         [0065]    While all waveforms are in principal useful in controlling the landing locations of drops passing through the airstream, in practice modulation of the airstream velocity in a asymmetric manner is preferred in order to provide a sustained and linear increase in the displacements of subsequently ejected drops, which ensures the possibility of all drops landing in a common location on a uniformly moving receiver. The maximum amplitude of the modulation of the airstream velocity is chosen so that the change in displacement of subsequent drops matches the distance moved by the receiver over the time interval between subsequently ejected drops. Many other functional forms of the time dependent velocity component of the airstream velocity may be usefully employed, including cases in which groups of drops desired to be printed in identical positions are ejected over a time which is only a fraction of the repetition time of the airstream velocity variations, in order that more than one such group of drops can be ejected during the repetition time.  
         [0066]    The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. In particular, while the droplet deflector is preferably of the airstream type, any type of droplet deflector is within the scope of the invention, which is limited only by the claims appended hereto and equivalents thereto.  
       PARTS LIST  
       [0067]    1. Printhead  
         [0068]    3. Row of nozzles  
         [0069]    5. Receiver  
         [0070]    7. Fast scan direction  
         [0071]    9. Slow scan direction  
         [0072]    10. Inkjet printer  
         [0073]    12. Printhead  
         [0074]    14. Receiver  
         [0075]    15. Droplet deflector  
         [0076]    16. Ink droplets  
         [0077]    18. Nozzle  
         [0078]    20. Membrane  
         [0079]    22. Ink cavity  
         [0080]    26. Large size droplets  
         [0081]    28. Small size droplets  
         [0082]    30. Heating element  
         [0083]    32. Air tube  
         [0084]    33. Outlet  
         [0085]    34. Airflow  
         [0086]    36. Gutter  
         [0087]    38. Printed droplets  
         [0088]    40. Restrictor  
         [0089]    42. Membrane  
         [0090]    44. Restrictor  
         [0091]    46. Membrane  
         [0092]    48. Tapered end portion  
         [0093]    50. Moveable cantilevers  
         [0094]    52. Movable vanes  
         [0095]    54. Fixed vanes  
         [0096]    56. Pressures pulse generator  
         [0097]    58. Piezo transducer  
         [0098]    59. Diaphragm  
         [0099]    60. Armature  
         [0100]    62. Compressive wave  
         [0101]    64. Diffuser  
         [0102]    66. Mechanical oscillator  
         [0103]    68. Mechanical oscillator  
         [0104]    70. Mechanical oscillator