Apparatus and method for automatically aligning print heads

An apparatus and related method for automatically aligning one or more print head modules in an ink jet printing system are provided. A mounting supports and aligns a print head module with respect to three axes of movement. The mounting includes rotatable cams that contact control surfaces connected to the print head module to move the print head module in a desired direction. The related method automatically positions multiple stationary print heads with respect to three axes of movement, including rotational adjustment about a Z-axis. The method also automatically adjusts the position of a single print head with respect to its angular rotation about the Z-axis.

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 .theta., 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 '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 .theta. correction.
 U.S. Pat. No. 5,241,325 to Nguyen (the '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 '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 '375 patent, the
 mechanism in the '325 patent does not allow for adjustment of print head
 "roll" or .theta. 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 .theta. 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.

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 '958 patent)
 entitled IMAGING PROCESS and assigned to the assignee of the present
 application. The '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 12A-12N, 12P and 12Q
 positioned around a support surface or drum 14. The print head modules
 12A-12N, 12P and 12Q 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 12A-12N, 12P
 and 12Q 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 12A-12N, 12P and 12Q
 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 12I 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
 44A-44D of nozzles 42. Array 44A is 12 nozzles across by 10 nozzles high,
 while arrays 44B-44D 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 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 44A 44D, it will be appreciated
 that the face plate 4 can support 3 inch wide printing ((45 horizontal
 nozzles).times.(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 20H 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 1/300 inch (0.085 mm.), and the
 above-referenced horizontal distance 20H of 20 pixels is equal to 1/15
 inch.
 With continued reference to FIG. 3, the vertical centerline-to-centerline
 distance 20V between vertically adjacent nozzles 42' and 42" is 20 pixels,
 or 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 2H between vertically adjacent nozzles 42 is 2 pixels, or 1/150
 inch. Alternatively expressed, vertically adjacent nozzles are offset by 2
 pixels, or 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 12I, 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 12K 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 12J and 12L,
 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 12I-12L. 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 12I-12L 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 12L 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
 12K, 12J and 12I, 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 12L 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 12L/face plate 1. In
 FIG. 6a, one angled column 210 of printed pixels from the test pattern 105
 is shown with the printed pixel 214 from print head module 12K/face plate
 2 displaced from its properly aligned position 214'. To align print head
 module 12K with the reference print head module 12L, 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 12K 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 12K along the Y-axis by the second calculated distance.
 In this manner, the selected print head module 12K is aligned with the
 reference print head module 12L.
 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 12K/face plate 2 from its properly aligned position with
 respect to the reference print head module 12L. The first and second means
 for positioning in the print head mounting 150 then translate the selected
 print head module 12K along the X- and Y-axes to align it with the
 reference print head module 12L.
 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
 12L/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 12I and 12J with the reference print head module 12L. 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 12L/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 12L about the Z-axis that will cause the print head
 module 12L 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 12J, 12K and 12I, 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 12A-12N, 12P and 12Q 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.