Patent Publication Number: US-6213580-B1

Title: Apparatus and method for automatically aligning print heads

Description:
FIELD OF INVENTION 
     This invention relates generally to an apparatus and method for automatically aligning one or more print heads in an ink jet printing system and, more specifically, to an apparatus and method that automatically positions multiple stationary print heads with respect to three axes of movement. 
     BACKGROUND OF THE INVENTION 
     Ink jet printing involves ejecting ink droplets from orifices in a print head onto a receiving substrate to form an image. The image is made up of a grid-like pattern of potential drop locations, commonly referred to as pixels. The resolution of the image is expressed by the number of ink drops or dots per inch (dpi), with common resolutions being 300 dpi and 600 dpi. 
     Ink-jet printing systems commonly utilize either direct printing or offset printing architecture. In a typical direct printing system, ink is ejected from jets in the print head directly onto the final receiving substrate. In an offset printing system, the print head jets the ink onto an intermediate transfer surface, such as a liquid layer on a drum. The final receiving substrate is then brought into contact with the intermediate transfer surface and the ink image is transferred and fused or fixed to the substrate. 
     In many direct and offset printing systems, the print head and the final receiving substrate or the intermediate transfer surface move relative to one another in two dimensions as the print head jets are fired. Typically, the print head is translated along an X-axis in a direction perpendicular to media travel (Y-axis). The final receiving substrate/intermediate transfer surface is moved past the print head along the Y-axis. In this manner, the print head “scans” over the medium/substrate and forms a dot-matrix image by selectively depositing ink drops at specific pixel locations. To increase image density and allow for greater speeds, multiple print heads may be utilized. 
     Image resolution, print quality and speed are among the most important considerations in designing a printing system. Where greater speeds are paramount, it is known to utilize one or more stationary print heads to eliminate the necessity of scanning across the transfer surface or media. Multiple stationary print heads increase speeds while also allowing for greater image density and increased image width. 
     One challenge with a multiple print head architecture, whether scanning or stationary, is to maintain proper alignment among the print heads. If one print head is misaligned relative to the other print heads in the array, printing artifacts such as banding and misregistration can occur. Additionally, whenever a print head is installed in the print head array, it must be precisely aligned with the other print heads. 
     Alignment among multiple print heads may be expressed as the position of one print head relative to another print head within a coordinate system of multiple axes. For purposes of discussion, the X-axis will refer to a direction perpendicular to the media/intermediate transfer surface travel direction past a print head, the Y-axis will refer to a direction parallel to the media travel direction and the Z-axis will refer to a direction perpendicular to the X-Y axis plane. It will be appreciated that in this three dimensional coordinate system, a print head has six degrees of freedom of movement—three degrees of freedom of translation along the X, Y and Z axes, and three degrees of freedom of rotation about the three axes. 
     For optimal placement of ink drops on the receiving substrate, each print head in a multiple print head system should be aligned with the other print heads with respect to all six degrees of freedom of movement. It will be noted, however, that the printed image is a two-dimensional pattern of pixels arranged in the X-Y plane on the receiving substrate. Thus, the alignment of the print heads with respect to their position along the X and Y-axes and their angular rotation or roll about the Z-axis, also referred to as θ, will have the most impact on print quality and printing artifacts. 
     Prior art multiple print head systems have disclosed alignment mechanisms that utilize operator input to perform print head alignment along two axes. For example, in U.S. Pat. No. 5,428,375 to Simon et al. (the &#39;375 patent), each print head is supported by a platform that carries X and Y translation actuators. The X translation actuator moves the platform along a fixed lead screw in an X-axis direction. The Y translation actuator drives a plunger back and forth to move the platform in a Y-axis direction. An operator examines output from the printer for visual artifacts and manually adjusts the X and Y actuators to reposition the print heads. This mechanism does not allow for adjustment of individual print head “roll” or θ correction. 
     U.S. Pat. No. 5,241,325 to Nguyen (the &#39;325 patent) discloses a scanning or “swath type” printer that includes a mechanism for aligning two print cartridges with respect to a single axis of movement. One print cartridge is mounted in a fixed-position retaining shoe and the other print cartridge is mounted in a pivoting retaining shoe. Both retaining shoes are mounted on a carriage that scans across the media in an X-axis direction. 
     The print cartridges print test lines and an optical scanner measures the distance between test line segments. Horizontal or X-axis misalignment between the two print cartridges is addressed by adjusting the timing of the ink jet nozzle firing as the cartridges scan across the media. Vertical or Y-axis misalignment is addressed by nozzle selection and by mechanically adjusting the angular position about the X-axis of the adjustable retaining shoe relative to the fixed-position retaining shoe. 
     The mechanical adjustment is performed by advancing the print cartridges along the X-axis until a cam lever on the carriage engages an actuator arm. Movement of the cam lever rotates a position adjustment cam that bears against a cam follower flange on the adjustable retaining shoe. This rotates the adjustable retaining shoe and associated print cartridge about the X-axis while the fixed-position shoe and cartridge remain stationary. 
     One drawback to the adjustment mechanism in the &#39;325 patent is that it is limited to scanning or “swath type” printing systems, as movement of the print cartridges in the X-axis direction is required to actuate the mechanism. This mechanism is also limited to rotational adjustments about the X-axis. Additionally, like the mechanism in the &#39;375 patent, the mechanism in the &#39;325 patent does not allow for adjustment of print head “roll” or θ correction. 
     The present invention addresses the drawbacks of the prior art by providing an apparatus and method for automatically adjusting the relative position of multiple print heads with respect to three axes of movement, including rotational or θ adjustment about the Z-axis. The present invention also provides a method for automatically adjusting the position of a single print head with respect to its angular rotation about the Z-axis. 
     SUMMARY OF THE INVENTION 
     It is an aspect of the present invention to provide a method and apparatus for automatically aligning individual print heads within an array of print heads with respect to three axes of movement. 
     It is another aspect of the present invention that the method and apparatus may be utilized with direct and indirect or offset printing architectures. 
     It is another aspect of the present invention that the method and apparatus may be implemented in printing systems using scanning and fixed-position print heads. 
     It is a feature of the present invention that the method and apparatus allow an operator to replace individual print heads in an array of print heads without manually adjusting the alignment of the print heads. 
     It is another feature of the present invention that the method aligns multiple print heads with respect to a reference print head in the array. 
     It is yet another feature of the present invention that the method and apparatus may be utilized with any number of print heads in an array. 
     It is an advantage of the present invention that the method is a closed-loop electromechanical system that requires no input or intervention by an operator. 
     It is another advantage of the present invention that the method and apparatus align multiple print heads along an X-axis and Y-axis and rotationally about a Z-axis to correct print quality defects such as banding and misregistration. 
     It is yet another advantage of the present invention that the method and apparatus provide for rotational alignment about a Z-axis for all print heads in the array, including the reference print head. 
     To achieve the foregoing and other aspects, features and advantages, and in accordance with the purposes of the present Invention as described herein, an adjustable print head module mounting and related method for automatically aligning multiple print head modules with respect to three axes of movement are provided. The mounting includes first and second means for positioning the print head module. The first means for positioning translates the print head module in an X-axis direction, while the second means for positioning translates the print head module in a Y-axis direction and rotates the print head module about a Z-axis. The related method includes the steps of printing a test image, analyzing the test image to determine print head module adjustments and aligning the multiple print head modules linearly with respect to the X- and Y-axes and rotationally with respect to the Z-axis. 
     Still other aspects of the present invention will become apparent to those skilled in this art from the following description, wherein there is shown and described a preferred embodiment of this invention by way of illustration of one of the modes best suited to carry out the invention. As it will be realized, the invention is capable of other different embodiments and its details are capable of modifications in various, obvious aspects all without departing from the invention. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive. And now for a brief description of the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic illustration of a multiple print head offset ink jet printing apparatus that utilizes the apparatus and method of the present invention. 
     FIG. 2 is an enlarged elevational view of a print head module face plate having four arrays of ink jet nozzles for ejecting drops of ink. 
     FIG. 3 is a greatly enlarged illustration showing the spacing between two horizontally adjacent nozzles and two vertically adjacent nozzles on the face plate. 
     FIG. 4 is an elevational view of four face plates that are positioned to eject drops of ink that interleave with one another to form a solid fill image. 
     FIG. 5 is a schematic representation of a portion of a horizontal line printed by face plates  4  and  2  in FIG.  4 . 
     FIG. 6 is a schematic representation of a portion of a horizontal line comprised of Interleaved printed pixels from face plates  1 ,  2 ,  3  and  4  of FIG. 4, and a test pattern that includes printed pixels from each of the four face plates. 
     FIG. 6 a  is a schematic representation of an angled column of printed pixels from the test pattern of FIG. 6, with one of the printed pixels displaced from its properly aligned position. 
     FIG. 7 is a simplified block diagram showing the flow of data and information from an optical sensor to an adjustable print head module. 
     FIG. 8 is a front elevational view of an adjustable mounting for a print head module. 
     FIG. 9 is a bottom elevational view of the adjustable mounting for a print head module of FIG.  8 . 
     FIG. 10 is a right side elevational view of the adjustable mounting for a print head module of FIG.  8 . 
    
    
     Reference will now be made in detail to the present preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a schematic illustration of a multiple print head, offset or indirect ink jet printing apparatus  10  that utilizes the apparatus and method of the present invention. An example of an offset ink jet printer architecture is disclosed in U.S. Pat. No. 5,389,958 (the &#39;958 patent) entitled IMAGING PROCESS and assigned to the assignee of the present application. The &#39;958 patent is hereby specifically incorporated by reference in pertinent part. 
     The following description of a preferred embodiment of the present invention refers to its use in a multiple print head, offset printing apparatus. It will be appreciated, however, that the apparatus and method of the present invention may be used with various other ink-jet printing apparatus that utilize different architectures, such as direct printing in which ink is jetted directly onto a receiving medium. Accordingly, the following description will be regarded as merely illustrative of one embodiment of the present invention. 
     With continued reference to FIG. 1, the imaging apparatus  10  utilizes an offset printing process to place a plurality of ink drops in imagewise fashion on a final receiving substrate. In the preferred embodiment, the apparatus  10  includes  16  print head modules  12 A- 12 N,  12 P and  12 Q positioned around a support surface or drum  14 . The print head modules  12 A- 12 N,  12 P and  12 Q jet drops of ink in a molten or liquid state onto an intermediate transfer surface (not shown) on the drum  14 . The intermediate transfer surface is preferably a liquid layer that is applied to the drum  14  by contacting the drum with an applicator assembly  16 . Suitable liquids that may be used as the intermediate transfer surface include water, fluorinated oils, glycol, surfactants, mineral oil, silicone oil, functional oils and combinations thereof. The preferred liquid is amino silicone oil. 
     The applicator assembly  16  includes a reservoir  18 , a wicking pad  20  for applying the liquid and a metering blade  22  for consistently metering the liquid on the surface of the drum  14 . Wicking pad  20  is preferably formed from any appropriate nonwoven synthetic textile with a relatively smooth surface. A preferred configuration can employ the smooth wicking pad  20  mounted atop a porous supporting material, such as a polyester felt. Both materials are available from BMP Corporation as BMP products NR 90 and PE 1100-UL, respectively. 
     The support surface may take the form of a drum  14  as shown in FIG. 1, or alternatively may be a belt, web, platen, or other suitable design. The support surface  14  may be formed from any appropriate material, such as metals including, but not limited to, aluminum, nickel or iron phosphate, elastomers, including but not limited to, fluoroelastomers, per fluoroelastomers, silicone rubber and polybutadiene, plastics, including but not limited to, polytetrafluoroethylene loaded with polyphenylene sulfide, thermoplastics such as polyethylene, nylon, and FEP thermosets such as acetals or ceramics. The preferred material is anodized aluminum. 
     Liquid or molten ink is ejected from the print head modules  12 A- 12 N,  12 P and  12 Q onto the intermediate transfer surface on the drum  14  to form an ink image thereon. In the preferred embodiment, the ink utilized in the printer  10  is initially in solid form and is then changed to a molten state by the application of heat energy. The Intermediate transfer surface/drum  14  is maintained at a preselected temperature by a drum heater  28 . On the intermediate transfer surface/drum  14  the ink cools and partially solidifies to a malleable state. The media  11  is fed through a preheater  30  and into a transfix nip  32  formed between the drum  14  and a transfer roller  34 . The media  11  is shown as a continuous roll, but may also be individual sheets of media. As the media  11  passes through the nip  32 , it is pressed against the deposited ink image to transfer and fix (transfix) the ink image to the media. Additional processing of the ink image on the media  11  may be accomplished by a pair of post-processing rollers  36 ,  38  downstream from the transfix nip  32 . Preferably, all of the steps of depositing the ink image, heating the drum  14 , preheating the media  11 , applying the intermediate transfer surface to the drum  14 , transfixing the ink image to the media, and post-processing the ink image on the media are performed simultaneously or in parallel to maximize printing speed. 
     With reference now to FIG. 2, each print head module  12 A- 12 N,  12 P and  12 Q includes a face plate containing a plurality of nozzles  42  through which the liquid ink drops are ejected. The face plate  4  in FIG. 2 corresponds to the print head module  12 I in FIG.  1 . The following discussion of face plate  4  applies equally to the face plates on each of the other print head modules. In the preferred embodiment, face plate  4  includes four arrays  44 A- 44 D of nozzles  42 . Array  44 A is 12 nozzles across by 10 nozzles high, while arrays  44 B- 44 D are each 11 nozzles across by 10 nozzles high. This configuration yields a total of 450 nozzles  42  on the face plate  4 . 
     As explained in more detail below, in the preferred embodiment the nozzles  42  are spaced apart vertically and horizontally by a distance of about 20 pixels, and each pixel has an approximate diameter or width of {fraction (1/300)} inch (0.085 mm). The terms “horizontal” and “vertical” are used only in a general sense to indicate directions of reference, and should not be interpreted to refer to orthogonal directions. From the above description of the dimensions of the nozzle arrays  44 A  44 D, it will be appreciated that the face plate  4  can support  3  inch wide printing ((45 horizontal nozzles)×({fraction (1/15)} inch between nozzles)=3 inches). 
     FIG. 3 is a greatly enlarged illustration of horizontally adjacent nozzles  42 ′ and  42 ′″ and vertically adjacent nozzles  42 ′ and  42 ″. It will be appreciated that the relative placement of nozzles  42 ′,  42 ″ and  42 ′″ is representative of the relative placement of any vertically or horizontally adjacent nozzles  42  on the face plate  4 . As shown in FIG. 3, the horizontal centerline-to-centerline distance  20 H between horizontally adjacent nozzles  42 ′ and  42 ′″ is 20 pixels. As discussed above, a pixel represents a single dot location within an image. The size or dimensions of a pixel will vary depending on the resolution of the image. The preferred embodiment described herein refers to printing at 300 dpi (118 dots per cm.), or 300 pixels per inch. Thus, each pixel will have an approximate diameter or width of {fraction (1/300)} inch (0.085 mm.), and the above-referenced horizontal distance  20 H of 20 pixels is equal to {fraction (1/15)} inch. 
     With continued reference to FIG. 3, the vertical centerline-to-centerline distance  20 V between vertically adjacent nozzles  42 ′ and  42 ″ is 20 pixels, or {fraction (1/15)} inch. As shown in FIGS. 2 and 3, the vertical rows of nozzles  42  are angled slightly. Preferably, the horizontal centerline-to-centerline distance  2 H between vertically adjacent nozzles  42  is 2 pixels, or {fraction (1/150)} inch. Alternatively expressed, vertically adjacent nozzles are offset by 2 pixels, or {fraction (1/150)} inch. 
     With reference now to FIGS. 1 and 2, as the drum  14  moves past the face plate  4  of print head module  12 I, the nozzles  42  are selectively fired to place ink drops on the intermediate transfer surface on the drum. Given that vertically adjacent nozzles are horizontally offset by 2 pixels, a horizontal line printed by face plate  4  would have one pixel gaps between each printed pixel. Thus, to enable the printer  10  to print solid fill images, a second face plate  2  corresponding to print head module  12 K is horizontally aligned to interleave with face plate  4  (See FIG.  4 ). 
     More specifically, with reference to FIGS. 4 and 5, the nozzles in face plates  4  and  2  are horizontally offset by one pixel such that the one pixel gaps between vertically adjacent nozzles in face plate  4  are filled by the nozzles in face plate  2 . FIG. 5 illustrates a portion of a horizontal line printed by face plates  4  and  2 . Pixel  42 ′p is printed by nozzle  42 ′ of face plate  4 , pixel  43 ′p is printed by nozzle  43 ′ of face plate  2 , pixel  42 ″p is printed by nozzle  42 ″ of face plate  4 , pixel  43 ″p is printed by nozzle  43 ″ of face plate  2 , and so forth. 
     As explained above, in the preferred embodiment each print head module/face plate is capable of 3 inch wide printing. A pair of horizontally aligned face plates, such as face plates  4  and  2 , supports 3 inch wide printing at 300 dpi. With reference to FIG. 4, to enable the printer  10  to print 6 inch wide solid fill images, a second pair of horizontally aligned face plates  3  and  1 , corresponding to print head modules  12 J and  12 L, respectively, are interleaved with face plates  4 ,  2 . Preferably, the bottom four nozzles in the far right vertical row of face plates  3  and  1  interleave with the top four nozzles in the far left vertical row of face plates  4  and  2 , respectively. 
     With reference now to FIGS. 1 and 4, in the preferred embodiment the printer  10  utilizes four colors of ink, cyan, magenta, yellow and black, for full color printing. Two interleaved pairs of print modules/face plates, such as face plates  4 ,  3 ,  2  and  1 , are dedicated to each of the four colors. Thus, the printer  10  includes four sets of two interleaved pairs of print modules/face plates for a total of  16  print modules/face plates. The four sets of interleaved print modules/face plates are aligned horizontally to print full color, 6 inch wide images. It will be appreciated that any number of print head modules/face plates may be interleaved to allow for greater image widths. For example, four pairs of print head modules/face plates may be interleaved for each color to support 12 inch wide printing. 
     As discussed above, it is important to maintain proper alignment among the multiple print head modules to insure proper image quality. If one print head module is misaligned relative to another print head module, printing artifacts such as banding and misregistration can occur. Additionally, if a print head module is removed and reinstalled or replaced, the newly installed print head module must be aligned with the other print head modules, either manually by the operator or automatically. Accordingly, in an important aspect of the present invention, a method and apparatus for automatically aligning multiple print head modules will now be described. 
     The method of the present invention for automatically aligning multiple print head modules is based on the general concept of printing and analyzing a test pattern to determine whether the print head modules require repositioning. In an important and novel aspect of the present invention, the present method automatically aligns the print head modules with respect to three axes of movement. Additionally, as explained in more detail below, the method utilizes a single means for positioning a print head module to align the module with respect to two different axes of movement. 
     The printing of the test pattern will first be described. With reference to FIGS. 4 and 6 and as described above, printed pixels from face plates  1 ,  2 ,  3  and  4  may be interleaved to form a solid fill horizontal line. A greatly enlarged portion  102  of such a line is illustrated in FIG.  6 . Each circle in line portion  102  represents one printed pixel, and the number inside the circle corresponds to the face plate that jetted that printed pixel. To better illustrate the interleaving of printed pixels, the array  100  of printed pixels shows a vertically staggered breakdown of line portion  102 , with the pixels from face plates  1  and  2  shown above the pixels from face plates  3  and  4 . Additionally, groupings  101  in array  100  and  103  in line portion  102  contain printed pixels from each of the four face plates  1 ,  2 ,  3  and  4 . These groupings of printed pixels represent the interleaved portion or “seam” in a solid fill horizontal line that is printed using nozzles from all four face plates  1 ,  2 ,  3  and  4 . 
     With continued reference to FIG. 6, the test pattern  105  utilized by the method of the present invention is illustrated below line portion  102 . The test pattern  105  includes printed pixels from each of the four face plates  1 ,  2 ,  3  and  4 . With reference to FIG. 1, a test pattern  105  (not shown) is printed on the intermediate transfer surface on the drum  14  by print head modules  12 I- 12 L. As the drum rotates in the direction of action arrow D, the printed test pattern  105  Is advanced past an optical sensor  110 . An example of a suitable optical sensor is a contact image sensor from Dyna Image Corp., model number DL107-34AM. 
     With reference now to FIG. 7, the optical sensor  110  directs light from a light source  112  onto the drum  14  to illuminate the test pattern  105 . The light scattered from the test pattern  105  is received by a charge coupled device (CCD)  114  within the sensor  110  and focused onto a silicon sensor array (not shown). Data from the sensor array represents the positions of the printed pixels within the test pattern  105 . As described in more detail below, this data is then analyzed to determine whether one or more of the print head modules  12 I- 12 L requires repositioning. 
     With continued reference to FIG. 7, in the preferred embodiment data from the CCD  114  is transferred serially to an analog-to-digital converter (A/D)  116 . A suitable A/D converter is available from Harris-Hill Co, model number TDC1175-30. The A/D  116  transforms the voltage signal coming from the CCD  114  into 8 bit binary samples. These samples are then transferred into a FIFO memory  118  before being sent to the controller  120  for processing. A suitable FIFO memory is model number AM7202 available from AMD, Inc. The preferred controller is an i486 controller available from Intel. The FIFO memory  118  decouples the scanning rate of the sensor  110  from the speed that the controller  120  can accept and process the data. Additionally, a complex programmable logic device (CPLD)  122 , such as model number ispLSI2032 available from Lattice Semiconductor, generates control and timing signals for the sensor  110 , the A/D converter  116 , the FIFO memory  118  and the controller  120 . 
     Upon determining that a selected print head module requires repositioning, the controller  120  sends position information to a driver  124 . A suitable driver is the Mini SSC manufactured by Scott Edwards Electronics, model number 27912. The driver  124  transforms the position information into control signals that are used to reposition a print head mounting  150  that supports the selected print head module. The print head mounting is described in more detail below. 
     Returning to FIGS. 4 and 6, movement of the print head modules/face plates  1 - 4  and the positions of the printed pixels in the test pattern  105  will be discussed relative to an X-Y-Z coordinate system. The X-axis refers to a direction perpendicular to the drum travel direction T past a print head module, the Y-axis refers to a direction parallel to the drum travel direction T and the Z-axis refers to a direction perpendicular to the X-Y plane. With respect to the illustrations in FIGS. 4 and 6, the X-axis corresponds to a horizontal axis, the Y-axis corresponds to a vertical axis and the Z-axis corresponds to an axis coming out of the paper toward the reader. 
     It will be appreciated that in this three dimensional coordinate system, a print head has six degrees of freedom of movement - three degrees of freedom of translation along the X, Y and Z axes, and three degrees of freedom of rotation about the three axes. In an important aspect of the present invention, the print head modules/face plates are aligned relative to one another with respect to their position along the X- and Y-axes and individually aligned with respect to their angular rotation or roll about the Z-axis. 
     An example of analyzing the test pattern  105  to determine whether a selected print head module requires repositioning with respect to the X- and Y-axes will now be described. A reference print head module is first selected. In an important aspect of the present invention, the reference print head module is maintained in a fixed position while the other non-reference print head modules are aligned with respect to the reference print head module. In a separate step discussed below, the angular rotation about the Z-axis of each of the print head modules, including the reference print head module, is analyzed and corrected when appropriate. 
     For purposes of this example, print head module  12 L in FIG. 1, corresponding to face plate  1  in FIG. 4, is selected as the reference print head module. With reference to FIG. 6, printed pixels from face plate  1  are indicated by circles enclosing the number  1 . To determine whether one or more of the other three non-reference print head modules  12 K,  12 J and  12 I, corresponding to face plates  2 ,  3  and  4 , respectively, require repositioning along the X- and/or Y-axis, the positions of printed pixels from these other three print head modules are analyzed with respect to printed pixels from the reference print head module  12 L in test pattern  105 . 
     The test pattern  105  in FIG. 6 illustrates generally the output of four print head modules that are properly aligned relative to one another. It will be appreciated that in angled columns  210 ,  130  and  140 , the printed pixels lie on an imaginary line (not shown) extending between the printed pixels ejected from the reference print head module  12 L/face plate  1 . In FIG. 6 a,  one angled column  210  of printed pixels from the test pattern  105  is shown with the printed pixel  214  from print head module  12 K/face plate  2  displaced from its properly aligned position  214 ′. To align print head module  12 K with the reference print head module  12 L, first and second distances along the X- and Y-axes, respectively, between the actual position of printed pixel  214  and its properly aligned position  214 ′ on the imaginary line are calculated. With the first distance along the X-axis calculated, a first means for positioning in the print head mounting  150 , described in more detail below, translates the print head module  12 K along the X-axis by the calculated distance. Similarly, a second means for positioning in the print head mounting  150  translates the print head module  12 K along the Y-axis by the second calculated distance. In this manner, the selected print head module  12 K is aligned with the reference print head module  12 L. 
     In a situation where the printed pixel  214  is located along the imaginary line extending between the reference printed pixels  212 ,  126 , the method determines whether the printed pixel  214  is equidistant from adjacent printed pixels  128 ,  129  along the imaginary line. If the printed pixel  214  is not equidistant from the adjacent pixels  128 ,  129 , a third distance along the imaginary line is calculated between the printed pixel  214  and the properly aligned position  214 ′ along the imaginary line. 
     The same analyses are performed on angled columns  130  and  140  for the printed pixel from face plate  2 . The results from the three angled columns  210 ,  130 , and  140  are averaged to obtain an average deviation of the print head module  12 K/face plate  2  from its properly aligned position with respect to the reference print head module  12 L. The first and second means for positioning in the print head mounting  150  then translate the selected print head module  12 K along the X- and Y-axes to align it with the reference print head module  12 L. 
     It should be noted that in angled column  140  the printed pixel  126 ′ is shown in dotted outline to indicate that this is not an actual printed pixel in the test pattern  105 . Printed pixel  126 ′ is a theoretical projection of where a printed pixel from the reference print head module  12 L/face plate  1  would be located in column  140 . This projection of printed pixel  126 ′ allows angled column  140  to be completed and utilized to align the non-reference print head modules. 
     The above steps are performed to align the other two non-reference print head modules  12 I and  12 J with the reference print head module  12 L. Upon aligning these other two non-reference print head modules, the four print head modules are now properly aligned relative to one another. 
     The process of aligning each of the print head modules, including the reference print head module, with respect to its angular rotation about the Z-axis will now be described. To perform this alignment, a horizontal row of printed pixels from a single print head module is analyzed. With continued reference to FIG. 6, horizontal row  115  consists of five printed pixels from print head module  12 L/face plate  1 . These five printed pixels are analyzed to determine if they are equidistant along the X-axis. If they are not, the method calculates an amount and a direction of rotation of print head module  12 L about the Z-axis that will cause the print head module  12 L to eject ink drops that are equidistant along the X-axis. The same procedure is utilized to analyze horizontal rows  125 ,  135  and  145  and align print head modules  12 J,  12 K and  12 I, respectively, with respect to their angular rotation about the Z-axis. It will be appreciated that this method of aligning a single print head with respect to its angular position about the Z-axis is equally applicable to single print head printing systems. 
     With reference to FIG. 1, while the above-described steps have been described with respect to aligning four print head modules corresponding to a single color, the method of the present invention may also be utilized to align all of the print head modules  12 A- 12 N,  12 P and  12 Q to insure that all four colors are properly registered. For example, a first test pattern may be printed utilizing one print head module from each of the four groupings of four print head modules. Once these four print head modules are aligned, four more test patterns are printed, one for each color grouping of print head modules. The print head module in each grouping that was aligned with the first test pattern is designated the reference print head module, and the other three print head modules in each grouping are aligned with respect to the reference print head module as described above. 
     Print Head Module Mounting 
     With reference now to FIGS. 8 and 9, a mounting  150  for supporting and aligning a print head module  12  with respect to three axes of movement will now be described. The mounting  150  includes a base  160  and at least one flexure extending from the base for supporting the print head module  12 . In the preferred embodiment, three parallel adjustable support members  170 ,  180  and  190  extend from the base to support the print head module  12  (see also FIG.  10 ). Each support member  170 ,  180  and  190  is pivotally coupled at each end to the base  160  and to a flange extending from the print head module  12 . Advantageously, this allows the print head module to be positioned with respect to three degrees of freedom of movement, translation along the X- and Y-axes and rotation about the Z-axis, while also preventing significant movement in the other three degrees of freedom of movement. 
     Each support member  170 ,  180  and  190  includes a threaded connector  172 ,  182 ,  192 , respectively. As shown in FIG. 9, arms  174 ,  176  extend from threaded connector  172 . A first plug  175  is affixed to the end of arm  174  and a second plug  177  is affixed to the end of arm  176 . The first plug  175  is pivotally coupled to a shoulder  162  in the base  160 . The second plug  177  is pivotally coupled to a flange  200  extending from the print head module  12 . 
     With reference now to FIG. 10, arms  184 ,  186  extend from threaded connector  182 . A first plug  185  is affixed to the end of arm  184  and a second plug  187  is affixed to the end of arm  186 . The first plug  185  is pivotally coupled to a shoulder (not shown) in the base  160 . The second plug  187  is pivotally coupled to a flange  202  extending from the print head module  12 . 
     With reference now to FIG. 9, arms  194 ,  196  extend from threaded connector  192 . A first plug  195  is affixed to the end of arm  194  and a second plug  197  is affixed to the end of arm  196 . The first plug  195  is pivotally coupled to a shoulder  164  in the base  160 . The second plug  197  is pivotally coupled to a flange  204  extending from the print head module  12 . 
     It will be appreciated that other flexures or supporting means may be utilized to support the print head module, such as one or more springs, solid posts, cables, and the like. 
     In an important aspect of the present invention, the mounting includes a first means for positioning the print head module along a first axis of movement and a second means for positioning the print head module along a second axis of movement and about a third axis of movement. In the preferred embodiment shown in FIG. 9, the first means for positioning comprises a first camming surface  220  that engages a first control surface  222 . The first control surface  222  is positioned at the end of a lateral extension  224  that extends from a flange  226 . The flange  226  extends from a rear face  13  of the print head module  12 . 
     As best seen in FIG. 9, the first camming surface  220  is a sloping end portion of a rotatable cam  230 . The rotatable cam  230  is connected by shaft  232  to a servo motor  240  for rotating the first camming surface  220 . In this manner, when the servo motor  240  is actuated to rotate the first camming surface  220 , the first control surface  222  and connected print head module  12  are translated in an X-axis direction. 
     In an important aspect of the present invention, the second means for positioning the print head module moves the print head module with respect to two different axes of movement—translation along the Y-axis and rotation about the Z-axis. With reference to FIGS. 8 and 9, in the preferred embodiment the second means for positioning comprises a second camming surface  250  that engages a second control surface  252  on flange  202 , and a third camming surface  260  that engages a third control surface  254  on flange  204 . The second camming surface  250  is the periphery of a cylinder  251 , and the third camming surface  260  is the periphery of a cylinder  261 . 
     Both cylinders  251  and  261  are mounted for eccentric rotation by servo motors  270  and  280 , respectively. With reference to FIG. 8, simultaneous rotation of second camming surface  250  and third camming surface  260  causes the print head module  12  to move in a Y-axis direction. Alternatively, rotation of second camming surface  250  while maintaining third camming surface  260  stationary, or rotation of third camming surface  260  while maintaining second camming surface  250  stationary, results in rotating the print head module  12  about the Z-axis. Advantageously, the two camming surfaces  250 ,  260  and their associated servo motors  270 ,  280  allow for alignment of the print head module with respect to two different axes of movement. 
     With reference now to FIG. 9, a coil spring extends upwardly from the base  160  to the rear face  13  of the print head module  12 . The spring  290  is preferably in tension, such that it urges the first control surface  222  against the first camming surface, the second control surface  252  against the second camming surface  250  and the third control surface  254  against the third camming surface  260 . Advantageously, this insures that movement of any of the camming surfaces results in the desired movement of the print head module  12 . 
     It will be appreciated that other means for positioning the print head module may be utilized to practice the present invention, such as various combinations of stepper motors, d.c. motors and piezoelectric actuators with lead screws, levers and cams. 
     While the invention has been described above with references to specific embodiments thereof, it is apparent that many changes, modifications and variations in the materials, arrangements of parts and steps can be made without departing from the inventive concept disclosed herein. Accordingly, the spirit and broad scope of the appended claims is intended to embrace all such changes, modifications and variations that may occur to one of skill in the art upon a reading of the disclosure. All patent applications, patents and other publications cited herein are incorporated by reference in their entirety.