Abstract:
This invention relates to an improved wide format color digital print engine, and in particular to a drum-based large format digital ink jet print engine having a vertically articulated carriage portion which carries both the print heads and an imaging array, adjustable saber angles for print heads, a novel service station, and an extremely robust weight-bearing carriage architecture. In particular, the present invention provides a highly manufacturable print engine benefiting from a discrete few controlled mounting locations so that accurate mounting of the cooperating sub-assemblies allow a very stable carriage assembly for driving said carriage laterally with respect to a printing medium coupled to the exterior of a drum.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuing application (continuation-in-part) of Ser. No. 08/711,796 filed Sep. 10, 1996 now U.S. Pat. No. 5,871,292 and entitled “Cooperating Mechanical Sub-Assemblies for a Drum-Based Wide Format Digital Color Print Engine.” 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the field of wide format color digital printing systems, and in particular to an improved drum-based ink jet printer having a novel service station for precisely cleaning and storing a plurality of marking elements associated with ink emitting print heads. The present invention finds use in the digital printing and imaging industry where successful repeatable printing requires precise placement of dots on print substrates including paper, vinyl, film, and other print substrates of varying thickness that typically include emulsion coatings thereon to promote consistent interaction with printing media marking materials such as ink, toner, or various printing compounds. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an improved apparatus for supporting, driving, maintaining, and operating print heads coupled to a carriage across a printing media so that ink droplets may be placed thereon to reproduce text and images with precision. Prior art mechanisms for driving a print carriage typically utilize a belt and pulley actuated mechanism or helical gear attached to the print carriage. These prior art carriage mounting and drive systems are known to cause errors in printed output. 
     Prior art print head rail members for large format print engines further encounter limitations due simply to the length and mass of a typical rail, carriage drive means, and carriage assembly as well as mechanical control difficulties related to precisely controlling all the specifications and tolerances during manufacturing and installation. A phenomena described as “tolerance stacking” contributes a significant component of error in an assembly process wherein at least two precision machining events occur at differing times on the same assembly or same time for different assemblies. In relation to a carriage assembly for a large format print engine, such tolerance stacking occurs at a number of discrete points of manufacture. For example, a carriage typically precisely supports at least one, and oftentimes several, print heads, a portion of the circuitry for such print heads, and all intervening means for driving the carriage assembly upon a guideway or track in a highly controlled manner (e.g., belts, pulleys, gears, encoders, motors, etc.). Thus, machining portions of subassemblies to arrive at an exact location of the print head(s) relative to: other print head(s), the printing medium, and the rail assembly can all contribute positional error relative to a design criteria possessing rigorous tolerance specifications. Prior art carriages typically couple to opposing sides of a single rail and often include a driving force to both impart axial motion and to constrain movement of the carriage. The inventors believe that using an attachment point for driving a carriage causes errors in the locations of ink droplets on the paper as described hereinbelow. 
     Additional problems with prior ink jet head configurations involve the mounting of the print head for accurate placement and movement across the printed image. The rail structure for the print head must adequately support the print head not only over the entire printed image, but also for any cleaning, maintenance, and temporary storage of the print heads. It is common to provide a zone, away from the printing medium within which to “park” the print head to perform auxiliary “service” functions, this zone is commonly known as a service station. These auxiliary functions may include manipulating the carriage, certain calibration functions, cleaning and capping of the print heads. To accommodate the park zone, the support system, or rails, must support the head over a distance greater than the width of the printing medium. For example, printers handing printing medium about 36 inches wide may have rails about 48 inches long. 
     Accurate placement and movement of the print head becomes more and more difficult as the length of the print scan (i.e., the width of the image) increases. Most prior ink jet printers over about 17 inches wide employ either a two-rail structure, or a single-rail and outrigger structure, for head carriage X-directional travel. Both of these techniques provide two separate and independently adjustable support points for the carriage. Multiple support systems were used on wide format printers because it was believed that a single rail could not provide adequate support and stability for the print head over a large distance. Multiple support systems were utilized to provide a wider support base for the print head and carriage to lessen the effect of any stability problems, as well as to provide additional strength to lessen rail flexing problems. Vibration problems may occur if the print head undergoes movement with respect to the rail structure. The print head may slightly rotate or shake about an axis parallel to the rails, causing the print head placement with regard to the paper surface to be inaccurate. Alternatively, the print head may slightly rotate or shake from side to side on the rails, perhaps due to the direction of print head travel. 
     Dual support systems have not proven to be altogether feasible for graphics quality large format printing because it is generally difficult to maintain parallelism of the supports across the entire width of the large format media and oftentimes either more or less than three discrete points constrain the carriage. More particularly, each support introduces positional error, resulting in non-parallel guide paths for the carriage. Further, prior art two-rail systems employ a pair of circular rails, with the print head mounted on a carriage which is in turn mounted on the rails. The carriage is generally supported by circular sets of ball bearings wrapped around each of the circular rails. Non-parallelism of the rails introduces vibration through the ball bearings to the carriage, often causing instantaneous horizontal velocity errors. If the supports are not parallel, the rollers on the carriage will bind or have excess freedom at particular locations along the rails, and cause further stability and vibration problems. If bending of the rails occurs and the railings are not maintained completely straight, errors occur in positioning the print head. Additional problems occur due to the space that the rails take up, interfering with the transfer of electronics and ink from the printer housing to the print head. It will be appreciated that these problems are magnified as the length of the rail or rails becomes greater, as in large-format printing. Accordingly, a print head configuration is desired which will avoid these various problems. 
     Furthermore, such prior art carriage and drive systems typically are not designed for in-field replacement with minimum personnel and requiring a minimum amount of service time. In fact, due to the obvious competing design objectives of mechanical positional accuracy and field replacement convenience, the inventors are aware of only one other such rail system offering similar design benefits as the present invention. The inventors refer to U.S. Pat. No. 5,592,202, issued Jan. 7, 1997 and titled “Ink Jet Printer Rail Assembly” which application is commonly assigned with the present invention to ColorSpan Corporation (formerly known as LaserMaster Corporation) of Eden Prairie, Minn. 55344. In the application cited, a single rail pivotably attaches to at least one end of a print engine chassis so that the carriage riding thereon may be removed for field service and replacement. The benefits of such a system relate to diminished down-time, reduced required service, and efficient repairs, thereby reducing the overall cost of ownership involved in operating one or more large format digital print engines. 
     Prior art digital printing systems typically operate by moving a print medium transverse to an articulated imaging print head. The print head frequently includes a plurality of discrete imaging elements suitably arranged in a pattern wherein one or more linearly aligned sets of ink emitting nozzles, or jets, disposed perpendicular to the direction of movement of the printing substrate for providing maximum coverage. The net result is that discrete dots are placed on precise locations on the printing media so that a pleasing visual image is rendered upon the printing media. However, as the number of print heads (and therefore jets) increases registration among the different print heads become more complex at the same that it becomes more crucial to the quality of the printed output formed by the printer. A picture element or pixel generally refers to a coverage area on the print media (or a sensing element in an imaging array) defined by resolution in a “vertical” direction relative to a “horizontal” direction. These pixels must be controlled very carefully to impart desired quality of the image, and the physical and chemical interaction between the marking material and the printing media and the environmental conditions under which the marking material is deposited upon the printing media all contribute to the quality of the actual image rendered. In fact, due largely to media characteristics, the inventors prefer to utilize a single pass printing mode wherein all the image pixels are formed in a single pass of the carriage assembly over the printing substrate to avoid such problems. 
     Most digital print engines that typically use one or more of a subset of the four subtractive primary colors: cyan, magenta, yellow, and black (“CMYK”) and rely upon color blending of these four ink colors to achieve accurate representations of desired color(s). Upon combining ink colors at a given pixel that a particular color combination can be formed by having multiple ink colors at a particular pixel location, either in a dot-on-dot or a dot-next-to-dot configuration. In sum, digital printing processes involve precisely placing a number of tiny dots onto particular locations on a printing medium. Any number of these small dots, when viewed some distance away from a printing medium such as film or paper, are perceived as a continuous-tone visual image and means for registering the dots is crucial, although only x-y registration was heretofore available. Thus, it can be appreciated that even slight variance in the actual positional location of the ink dots can significantly effect the overall visual impression created by the printed image. In one subset of digital printing technology, aqueous ink is expelled from a plurality of ink jet nozzles to form dots on the printing media via thermal, piezoelectric, acoustic, and other technologies used for reliably ejecting a tiny droplet of ink onto a printing substrate. This is known as “ink jet” printing and its popularity and the innovation related thereto have greatly increased the accuracy and therefore the photorealistic quality of the images printed, while at the same time attempting to lower the costs of ownership as larger and larger wide format print engines are commercially introduced. While the types and numbers of inks, and ink jet cartridges, usable with such printers have increased thereby increasing the complexity of controlling interaction among the inks, cartridges, and printing medium, reduced costs of ownership and ease of serviceability continue to drive a large amount of innovation in this field of endeavor. Thus, a continued need exists in the art for low cost and at the same time technically advanced, highly accurate means of performing wide format color digital ink jet printing. 
     SUMMARY OF THE INVENTION 
     The reader is encouraged to cross reference and review the present document with a number of [now allowed] U.S. patent applications, each filed on Sep. 10, 1996, each commonly assigned, and the contents of each are hereby incorporated by reference in their entirety herein; specifically, U.S. patent application Ser. No. 08/711,796 entitled “Cooperating Mechanical Sub-Assemblies for a Drum-Based Wide Format Digital Color Print Engine;” U.S. patent application Ser. No. 08/709,803 “Calibration and Registration Compensation Method for Manufacturing a Drum-based Print Engine;” U.S. patent application Ser. No. 08/709,804 “Method for Manufacturability of a Low Cost Printing Drum;” U.S. patent application Ser. No. 08/709,803 “Method and Apparatus for Compensating for Faulty Ink Emitting Elements in a Digital Output Device;” and U.S. patent application Ser. No. 08/711,815 “Method of Selecting an Ink Set of an Ink Jet Printer.” Furthermore, applicant herein incorporates by reference U.S. patent application Ser. No. 08/922,297 “Method and Apparatus for Registration and Color Fidelity Control in a Multihead Digital Color Print Engine” which was filed on Sep. 3, 1997; U.S. Pat. No. 5,369,429 titled “Continuous Ink Refill System for Disposable Ink Jet Cartridges Having a Predetermined Ink Capacity;” and U.S. Pat. No. 5,469,201 directed to a guideway for a continuous ink refill system, all three of which are also commonly owned by the present assignee. 
     The present invention relates to a low cost, large format print engine featuring field replaceable subassemblies such as: a carriage having a vertically-articulated articulated portion (which retains a plurality of print heads and an imaging sensor array) and carriage drive means preferably comprising a combination rail and mechanically isolated lead screw assembly, a motor to both drive a printing drum and provide reliable means for monitoring and controlling drum rotation, two electronics subassemblies—a first for operating the printing system software (disposed in an electronics bay), and a second for handling all print related image data management and printing operations (disposed on the carriage assembly), and a service station subassembly for cleaning and maintaining a plurality of ink jet cartridges operating in said large format ink jet print engine. The print engine described with reference to preferred embodiments herein solves prior art difficulties and offers a new series of low-cost accurate wide format print engines. The print engine of the present invention contains a CD-ROM reader, hard drive, and Intel Pentium Processor chip and related circuitry so that relatively complex large format digital color computerized printers as described and enabled herein may be reliably and simply fabricated, operated, and serviced and thereby producible in high volume at reduced cost making ownership of such machines less expensive overall. Some of the key advantage of a print engine taught herein include; a two stage carriage assembly having a vertically articulated portion housing ink jet print heads and preferably an optical imaging array; an extremely resilient three-point (planar) stance where the carriage assembly rides on the printer rail structure; a mechanically-isolated attachment to a lead screw drive means; and an improved service station assembly. 
     The improved drum-based printer of the present invention again utilizes virtually no critical parallel alignments; in that the inventors have again implemented a pair of complementary alignment plates that are virtually impervious to errors during manufacture and which posses extremely robust behavior in almost every orientation. In fact the two alignment plates of the present invention must practically be physically bent, or grossly out of relatively parallel alignment before any of the sub-assemblies of the present invention are affected. Thus, the alignment plates handle a great deal of chassis movement without deleterious effects on the head height relative to the print medium. 
     The present invention may be performed with most types of large format digital print head technologies since the characteristics of the print head are more or less independent of the manner in which a carriage for conveying a print head or heads across a print media to create a large format digital image. Thus, the present invention encompasses a carriage assembly which supports the print heads in close proximity to a print medium via a vertically articulated portion of the carriage assembly and couples to a helical lead screw member at a mechanically isolated engagement point to reliably control the transverse motion of the carriage relative to the printing medium. 
     In a preferred embodiment, the carriage contacts three support points, thereby forming an extremely stable planar foundation from which deflection is unlikely and is not supported but is rather only driven via engagement at a single drive location by a novel, spring-biased helical lead screw for reliably propelling the carriage with a minimum of resistance. In this embodiment, the drive location is preferably spring-biased to one side of the grooves that form the driving portion of said lead screw and provided with a single-degree of freedom spring-biased attachment sub-assembly. This attachment subassembly imparts an axial force that absorbs a surprisingly large amount of non-axial vibration and movement, which are potential sources of tracking error for the entire carriage assembly particularly when the lead screw reverses rotation. Furthermore, the three-point stance and minimal rolling resistance absorbs an unprecedented amount of force without deflection. Thus, heavy pre-printing, printing, or post-printing assemblies and associated electronics can be easily accommodated with this design. Also, the service station assembly has an independent suspension for all “stations” and uses the print heads themselves for guidance accuracy during service station operation. Finally, the vertically articulated print heads can adjust to accommodate myriad printing substrates of vastly differing thickness. 
     The following drawings are representative only and as such should be viewed as illustrative and not limiting to any particular embodiment of the present invention or of any relative scale of the features depicted therein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view of a supporting and drive assembly for a prior art drum and rails assembly. 
     FIG. 2 is a perspective view of an embodiment of a carriage assembly for releasably retaining eight ink jet cartridges in close proximity to a printing medium attached to a rotating drum member and using the rail apparatus of FIG.  1 . 
     FIG. 3 is a perspective view of an embodiment of a prior art carriage assembly for releasably retaining eight ink jet cartridges in close proximity to a printing medium attached to a rotating drum member. 
     FIG. 4 is a perspective view of an embodiment of a carriage assembly made pursuant to the present invention for releasably retaining eight ink jet cartridges on an articulated saddle portion of said carriage in close proximity to a printing medium attached to a rotating drum member and depicting the location of an imaging array within said articulated saddle portion. 
     FIG. 5 is a perspective view of the carriage assembly depicted in FIG. 4 taken from a lower perspective to illustrate the lower portion of said carriage and the location of said imaging array. 
     FIG. 6 is a plan view of the lower portion of the carriage assembly of FIG.  4  and FIG.  5  and illustrating the close spacing between eight print heads and an imaging array practiced in accordance with the present invention. 
     FIGS. 7 is an elevational side view in cross-section of a lateral side of an embodiment of the carriage assembly depicted in FIG.  5  and FIG. 6, showing details of the coupling points for the carriage to a slide rail and a driven lead screw, a stepper motor for vertically articulating the saddle portion of said carriage and the apertures for receiving flex cables to electrically connect to print heads disposed in print head sockets. 
     FIG. 8 is an elevational side view of a lateral side of an embodiment of the carriage assembly depicted in FIG.  5  and FIG. 6, showing details of the coupling points for the carriage to a slide rail and a driven lead screw, line traces representing electrical and electronic communication between printer circuitry, head board circuitry, and four print heads disposed in print head sockets along one side of said carriage assembly. 
     FIG. 9 is a perspective view of a service station assembly taught herein depicting a slideable service station assembly and associated service function locations thereon. 
     FIG. 10A and 10B depict plan views of the service station plates with the wiping and spitting sub-stations clearly illustrated, as well as the guideposts for ensuring alignment of the cartridges and said sub-stations. 
     FIG. 11 is a perspective view of a ducted plenum assembly which is used to convey a source of vacuum through a valved control device and to an accumulator vessel that is usable with the present invention. 
     FIG.  12 A and FIG. 12B depict an exploded view and an assembled view, respectively, of a biased attachment nut for mechanically isolating a driving force provided by a lead screw. 
     FIG. 13 is a copy of a printed set of calibration patterns useful in practicing certain aspects of the present invention related to use of an imaging array to determine head height and to render cartridge de-skewing information. 
     FIG. 14A and 14B depict perspective views of the enclosure of the print engine of the present invention in a closed and open configuration, respectively. 
     FIG. 15 is a graph of data used to correct skewing of a cartridge with respect to a portion of data captured by an imaging array showing rotational offset for a select subset of printed pixels. 
     FIG. 16 is a graph depicting a worst case scenario of positional error related to a lead screw having a 0.5 pitch per revolution. 
     FIG. 17 is a graph depicting a worst case scenario of positional error related to a lead screw having a 0.1 pitch per revolution. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An apparatus for reliably and precisely propelling a print carriage  32  along print rails  22 , 24  proximate a rotating drum  26  thus forming a low cost high precision printer  40  containing many field replaceable sub-assemblies is disclosed and taught herein. Referring to FIG. 1 depicting key elements of a prior art rail and carriage drive assembly useful with the present invention as typically disposed within suitable print engine chassis housing  1  two alignment support plates  20  oriented at either end of rail assembly  10  and mechanically coupled to only the components whose alignment relative to one another is critical to print operations; namely, bar rail  22 , shaft rail  24 , and drum  26  (FIG.  1 ). Lead drive screw  28  engages linear actuator  30  which is preferably an anti-backlash lead screw nut  31  biased in the axial direction parallel to said lead screw  28  and attached to intermediate assembly nut  31  (FIG.  12 A and FIG. 12B) and with one or more sources of bias (e.g., springs, magnets, electronic circuits, switches, resin-based straps, etc.) connected to a lower portion of a carriage assembly  32 , to provide a single degree of freedom mechanism, so that as lead screw  28  rotates, the linear actuator  30  propels along the axis of lead screw  28  in proportion to the rate of rotation of lead screw  28  with tolerance for tracking error of said lead screw  28  relative to carriage assembly  32 . Linear actuator  30  preferably self lubricates and requires no maintenance whatsoever during its intended service life. Lead screw  28  preferably comprises a two start, or dual thread path, lead screw, pitched relative to vertical such that the linear actuator  30  travels less than one half inch (0.5) in an axial direction for every complete rotation of the lead screw  28  and preferably approximately one tenth of one inch (0.1) to significantly decrease transmission of drive train errors to the ultimate location of the ink droplets on the printing substrate (see FIG.  16  and FIG. 17 which depict positional error between 0.5 and 0.1 pitch lead screw). Lead screw  28  furthermore is sized to appropriately handle the drive load of the carriage assembly  32  (shown in FIGS. 4-9) such that in one embodiment the diameter of lead screw  28  is three quarters of an inch (¾″). The reason for this choice of diameter relates to the fact that encountering a natural frequency during driving of a lead screw will cause many vibration errors detrimental to quality digital printing. The nut assembly  31  has an upwardly extending dowel-like pin  33  for engaging a corresponding not shown aperture formed on a lower surface of the carriage  32  and a rotational axis orthogonal to said pin  33 , which is loosely constrained with two pins to ease assembly, removal, and reconnection of the carriage  32  to a first two start connector  30  via said nut assembly  31  and ultimately lead screw  28 . 
     Bar rail  22  preferably mechanically couples at a distal end of a carriage assembly  32 , where a set of opposing low friction rollers (not shown) contacts opposing sides of the polished exterior surface of bar rail  22 . In addition, a spring provides a biasing force to the rollers to the sides of bar rail  22  so when an upward force is received from service station capping (see below) the carriage assembly does not fail to engage the rail. The carriage assembly  32  similarly contacts two locations of the shaft rail  24  so that a stable three-point support orientation which retains the carriage assembly  32  in a stable, but not overly-constrained, secure manner. Lead screw  28  is preferably disposed at a center of resistance of the carriage assembly with respect to the aggregate components of drag induced by the sliding action of the carriage  32  at bar rail  22  and shaft rail  24  thus promoting an evenly distributed propelling force that inherently reduces jitter and other oscillation of the carriage during printing operations. 
     Carriage assembly  32  mechanically couples to shaft rail  24  around a majority portion the circumference of cylindrical shaft rail  24  (as shown with arrow denoted with reference numeral  25 ) such that the carriage assembly  32  is disengageable from bar rail  22  by loosening three screws that connect nut assembly  31  to actuator  30  and sliding the peg  33  out of the aperture formed on the lower portion of the carriage  32 . To remove the carriage assembly  32  from shaft rail  24  by loosening the bearing that forms a partial radius connector and sliding it axially until it is clear of the carriage assembly  32 . Then an operator may simply lift the carriage assembly  32  from its supported position from the rail(s)  22 , 24 . 
     At each end of lead screw  28  where it connects to ancillary supports  42 , 43  ballast  34  can be added to counteract the naturally sagging the occurs in a length of periodically supported rail, such as used in the present invention. Thus, even though the shaft rail  24  is supported upon the two alignment support plates  20  and at approximately two additional locations upon adjustable bracket supports  36 , the ballast  34  operates to eliminate sagging (during initial fabrication only ) and maintain shaft rail  22  straight and parallel to bar rail  24 , which is supported along a majority of its length directly upon a support plate rigidly fastened to the rear of the enclosure  40 . Lead screw  28  furthermore preferably is spring biased at one end (either  48  or  58 ) to help ensure uniform and reproducible motion of actuator  30  for every repetition of axial movement of actuator  30 , and therefore, to a large degree, carriage assembly  32 . 
     A drive mechanism for lead screw  28  consists of a low torque motor, which must ultimately move the carriage assembly  32  which itself requires a minimum of approximately 8 in/ounces of torque to move the carriage assembly, driving a timing belt and pulley  44  coupled to a spring tensioned timing belt  45  further coupled to gear  46 , spaced apart from pulley  44 , which is in turn coupled to an end  48  of lead screw  28 . Low torque motor  42  is electrically coupled to suitable motor control circuitry to drive the pulleys  44 , 46  and therefore lead screw  28 , precisely during printing operations and periodic service station visits (as described elsewhere herein). In addition, a similar low-torque motor can be used to drive the drum  26  into stable rotation. In a preferred embodiment, said low torque motor  42  comprises an analog stepper motor driven by two analog voltage inputs to achieve rotation and wherein an optical encoder is provided that generates a count value as the lead screw  28  rotates. This rotational counter is correlated to the axial slewing distance of the carriage assembly  32 , and at least one calibration value of optical encoder counts correlated to a known slewing distance is stored in flash memory in the associated circuitry of the engine  40  as described hereinbelow 
     Another aspect of the present invention is that positional error or other defect in the spacing, sizing, or pitch of the lead screw  28  can be accurately eliminated during a calibration sequence following assembly of a print engine. The sequence involves printing two dots at a desire spacing, then measuring the spacing with an object standard, comparing the measurement to the desired spacing (i.e., maximum print swath end points) and then providing a calibration input signal to the rotary motor  42  driving the lead screw  28 . Thereafter, the rotary motor  42  utilizes an initial nominal counter calibration input signal value (stored in flash during initial assembly) which drives the carriage  32  a known distance (e.g., 36″). 
     Following initial fabrication of the engine  40 , and subsequently after major service to drive components of said engine  40 , a calibration sequence is run as follows: A first and a second discrete printed mark is printed with a command for a desired separation between said marks (using the value stored in the flash memory). The separation between the first and the second mark is measured using highly accurate measuring instrument capable of resolution on the order of the print head resolution. If the desired and the actual separations differ, the value stored in the flash memory is replaced with a new counter value corresponding to the exact distance. This method reduces error for a typical low cost lead screw as preferably used herein from approximately 20 mils per yard (0.020″/36″) to less than two mils (&lt;0.002″). This calibration sequence can be implemented following replacement or repair of any sub-assembly involved in translating the carriage assembly  32  laterally during printing and servicing operations. 
     A service station assembly  200  as disclosed and enabled herein operates to clean and maintain the print heads intermittently during sustained print operations and during temporary storage, for example, overnight. Notably the service station subassembly  207  also mechanically releasably couples directly to alignment supports  20  so that the relative parallel alignment among the drum surface axis (and therefore surface) and the service station assembly  200  (shown in detail in FIG. 9) remains intact. The carriage assembly  32  meets lead screw  28  at interface  29  (which receives actuator  30  and biased nut assembly  31 ) and the shaft rail  24  meets the carriage at the location denoted by the reference numeral  25  (arrow  25 ). This design provides for robust weight-bearing performance of the carriage assembly  32  since it can accommodate approximately 200 pounds of downward force without significant deflection, even through the design is essentially a cantilever. Thus, the carriage assembly  32  can accommodate and carry a variety of printing implements of varying size and weight. For example, a source of ultra-violet (UV) or infrared (IR) radiation *or other sources of post-print processing energy), significantly all electronics for the printer, or even an extended ink supply could be carried on the carriage assembly  32  of the present invention without degrading performance of said carriage. Of course, a suitably rated motor  42  for propelling the carriage assembly  32  would be required to practice this form of the invention. 
     In yet another preferred embodiment, an imaging array assembly  150  with LED&#39;s of selected values disposed around the periphery of the array  150 . The array  150  may be a linear or two-dimensional array and is preferably disposed on a lower portion of said carriage assembly  32  proximate a printing substrate attached to the drum  26 . The imaging array  150  is electrically coupled as described in detail in co-pending U.S. patent application Ser. No. 08/922,297 (filed Sep. 3, 1997); owned by common assignee ColorSpan Corporation (formerly known as LaserMaster Corporation) of Eden Prairie, Minn., and entitled “Method and Apparatus for Registration and Color Fidelity Control in a Multihead Digital Color Print Engine.” The contents of said patent application are hereby incorporated in their entirety herein as if fully set forth herein. While this reference reveals in detail the operational considerations for an imaging array attached to a roll-fed large format carriage printer, all said teaching applies to the instant drumbased print engine. For present purposes the imaging array is preferably positioned between a preferred v-shaped plurality of print head sockets  58  at either or an adjustable height relative to said ink emitting surface, or orifice plate. The imaging array  150  is used to detect marks of calibration patterns  220  (see portions  222 , 224 , 226 , of FIG. 13) and possesses a native pixel size and forms a corresponding bitmap image when scanned across, or disposed opposite, marks formed by ink droplets from the print heads  50 . Assuming a nominal diameter of said marks, a correlation between the number of imaging array pixels that comprise a discrete mark and head height can be determined. In addition, due to the fact that the imaging array is fixed with respect to the carriage  32  and rails  22 , 24  of the printer  40  (in the x-y plane) the bitmap image captured can be accurately analyzed to determine the degree of skew, or the saber angle, between a given print head  50  and one or more other print heads with reference to a parallel reference (using the fixed relationship of imaging array  150  the carriage and rails of the engine  40 . The bitmap is analyzed and a correlation located with respect to an ideal printout. Then each dot of the pattern is analyzed to determine rotational and axial errors from the ideal. When the axial offset is versus rotational offset produces a slope (that cannot be attributed to a consistent skew of the imaging array itself) a best fit routine will produce the skew of the cartridges. Then a correlation between a pitch of a threaded set screw disposed against an angled surface (and biased with a spring) will urge each cartridge to pivot to a desired de-skewed position. The imaging array  150  typically is first tested to confirm it is operating within parameters with a simple white-point/black-point pattern  222  signal comparison (to confirm signal strength of the array). The imaging array is preferably surrounded by sources of visual radiation (LED&#39;s, optical fibers, lasers, operating in the visual spectrum) to maximize the signal returning therefrom. Then, after allowing time for ink to dry, and setting a drum brake (not shown) by switching a solenoid switch (not shown), one or more searching techniques can be used so that the array “intercepts” one of a plurality of calibration patterns  220  (see FIG. 13 printed on a selected printing substrate and either provides feedback on its signal output or passes said signal output to comparator circuitry for analysis. Herein, scaling marks  226  are inspected (following confirmation of signal strength) to accurately gauge the distance separating the focal plane of the imaging array  150  and the printing substrate this distance can be determined in a variety of ways (e.g., focus imaging array on the periphery of scaling pattern marks and determine head height by applying a ratio of the number of imaging pixels of the array  150  to each dot-shaped mark on the printing media. Then, given a known linear separation between the plane of the ink emitting surfaces of the print heads and the focal plane of the imaging array the height of the ink emitting surface over the printing media can be ascertained. The testing and calibration pattern depicted in FIG. 13 are created by energizing the individual ink emitting nozzles, or jets, while the carriage assembly  32  remains motionless and the drum  26  rotates (four turns for the patterns depicted). Then, a solenoid switch (not shown) is activated to release a drum brake (not shown) that forcefully engages a lateral non-printing surface of said drum element so that the carriage assembly  32  may slew back and forth to register and calculate values of print head skew, or saber angle, as more fully described hereinbelow (by inspecting pattern  226 ). This Figure also depicts the limited axial separation, or footprint, of a preferred print head orientation for the present invention. 
     Another embodiment of said head height adjustment technique useful for simply avoiding detrimental “head strikes”(i.e., contact between the ink emitting surfaces and the printing substrate) as opposed to optimizing said head height parameter as in the foregoing discussion, can be implemented as follows: a first head height separation setting is input to the printer  40  so that a nominal head height, or home position is achieved (either independently or in reference to a published manufacturers media thickness measurement) and single pass, or slewing operation, of the carriage assembly  32  occurs. While the carriage assembly  32  slews laterally, instantaneous axial location information (from a linear optical encoder as is known in the art) and instantaneous drum-rotation velocity information is correlated from a rotary encoder (not shown) optically coupled to a low torque motor  42  driving a belt  45  to gear  46  to rotate the drum  26  so that each head strike event can be recorded as a momentary discontinuity or alteration of the control loop signal. Then, as head strikes are detected, the head height spacing, or gap, is adjusted upward until no further head strike events are recorded. After the carriage assembly  32  has traversed the entire printed area of the printing substrate it is returned to its original position in preparation of subsequent printing passes over the same printing media. Note that creases, adhesive tape, wrinkles, etc. unique to each piece of printing media may require a head strike clearing scan of the carriage assembly  32  prior to performing every print. The repeatable “home” head height position is preferably preset when the printer  40  is manufactured so that printer initialization and calibration can proceed accurately during the life of the printer. This home position is set as follows. A stepper motor  101  attached to the upper “saddle” portion  107  of a vertically articulated part of the carriage assembly  32 . The motor  101  drives a threaded shaft  104  coupled to a threaded nut  102  mounted to the lower half of said articulated saddle  107 . A series of hard stops  105  are provided at locations of maximum desired upward travel of said saddle  107 . Thus, the home position is found by activating the stepper motor  101  until hard stops  105  retards the upward progress of the saddle portion  107 , then preferably approximately sixty (60) mils of travel downward for the saddle portion  107  is completed, then a final upward movement of about twenty (20) 20 mils is applied—to remove all backlash from the system—and the “home” position is found. This home position is set at a relatively safe separation that will provide high quality printing in the event that one or more of the head height adjustment components later becomes inoperative. While not depicted herein a similar articulated motion (but this time a pivoting motion not a linear translation as with the stepper  101 ) of a lateral portion of the carriage can be used to correct any yaw, or sideways angle of the carriage. 
     With respect to the service station assembly  200 , as can be seen in FIGS. 9,  10 , and  11 , each ink emitting surface of each cartridge  50  has a corresponding service location (provided with independent suspension for each cartridge) of three stations of said service station assembly; namely wiping, spitting, and capping sub-stations. In a preferred embodiment, the capping station is coupled to a source of relative vacuum  211  (preferably Hewlett-Packard Company part #C3195-60059 primer assembly with a 3.5 p.s.i. maximum) operating to provide approximately 1-2 p.s.i. which fluidly couples to an accumulator vessel  210  wherein excess ink material can be eliminated by evacuation, venting, boiling, or drying. This process allows a large volume of ink to be removed including material that themselves plug an ink emitting orifice, or can later form “soft plugs” if left unattended, as is known in the art. In a preferred embodiment, The accumulator vessel  210  is used intermediate of said source of vacuum  211  and said print head cap mounts or mounting bosses  205  and said accumulator vessel  210  can be outfitted with a drain member  212  to protect the source of vacuum pressure and a float valve  208  (having output signal means for providing an output warning signal  208 ′—optionally to remove display apparatus  151 ) so that an electric signal may be initiated when the vessel  210  is nearing capacity. In the preferred embodiment having eight print heads  50  the source of vacuum  211  is provided with a plenum assembly  225  that directs the source of vacuum  211  from its original four ducts advantageously to all eight cap stations or mounting bosses  205  for each of the eight print heads  50 . The plenum assembly  225  preferably includes at least one length of flexible tubing  250  that is capable of fluidly coupling at least one mounting boss  205  to accumulator vessel  210 . 
     Referring to FIG. 3, depicting a portion of a prior art carriage assembly  32 , including print heads  50  and ink tubing receiving grooves  52  formed into carriage top cap  54 , and a lateral end cap  56  sealing the end of carriage assembly  32 . As can be seen in FIG. 13, a perspective view calibration patterns  220  shows that no two print heads  50  operate to print a common print swath (testing and calibration patterns  220  from said print heads  50 ). Thus, in a preferred eight print head embodiment of the carriage assembly  32  the print heads  50  disposed on a common side of the carriage  32  are minimally spaced apart from each adjacent print head  50  axially and in the lateral direction to minimize the footprint of the carriage assembly  32 . Furthermore, as can be seen in FIG. 5, the print heads  50  are disposed spaced from one another circumferentially, so that each print head  50  maintains a common distance from the exterior surface of drum  26  which is of course cylindrical even when the head height is adjusted. In a preferred embodiment, the entire articulated saddle portion  107  of the carriage assembly  32  pivotably attaches to an threaded attachment point where a threaded bore  102  meets a threaded stepper axle  104  driven by a stepper motor having a native axial resolution of approximately 0.5 mils. This attachment point is furthermore preferably an electrical stepper motor  101  operated from a remote touch pad, or console  41  on a surface of the printer  40  for ease of control by an operator to elevate and lower the saddle portion  107  of the carriage assembly  32 , and thus the ink emitting jets, approximately ¼″ (to desired or optimum spacing above a selected printing substrate). The articulated attachment point could comprise a variety of devices, including a manually operated set screw, a ratchet assembly, or a bushing etc. and should have a home position or set point easily repeated after long use. 
     In a further preferred embodiment, another articulated attachment point is advantageously employed at one or more cartridge mounting sockets  58  so that one or more of said cartridges (or print heads)  50  can be skewed relative to one or more of the other of said plurality of print heads  50  to enable certain specialized high speed print modes (typically having high slew-rates for carriage assembly  32 ) and to improve registration among two or more of said print heads that inadvertently contain a saber angle between the marks they produce on a printing substrate. In one embodiment, only those print heads containing ink particularly perceptible to a human viewer contain such an articulate attachment point, in another embodiment, more than one such articulated attachment point is provided so that a particular print head, or a bank or more than one print heads can be appropriately disposed relative to one or more of said plurality of print heads  50 . In addition, more than one axis of rotation may be enabled with more than one set screw adjustment per cartridge so that three dimensional skewing, or alignment can be achieved between cartridges. 
     Referring to FIG. 5, with respect to the cartridge socket mounting surface  58 , the print heads  50  are disengageably mounted onto their respective mounting location with a simple and effective three point retaining coupling. Three point retaining coupling consists of two foot members for receiving the corresponding two feet of a family of thermal ink jet cartridges manufactured by Hewlett-Packard, and known as model 51645 series, or family, or cartridges, and brethren using similar external bosses and mounting feet. The model 51645 print heads have 300 ink emitting nozzles per cartridge but only 224 are preferably used in conjunction with the present invention due largely to the difficulty of getting all the nozzles formed on the periphery of the orifice plate of said 51645 cartridges to reliably emit ink when energized. The inventors have discovered that such an engagement mechanism provides a stable and repeatable means of attachment without further adjustment or manual tightening by an end user. Furthermore, the inventors have further discovered that such a design offers economy of space since nothing protrudes beyond the outer surface of the cartridge  50  itself while mounted for printing operations. For reference, many prior art cartridge attachment means rely upon relatively more complicated and less robust designs to typically mount cartridge  50  so that an array comprised of a plurality of ink jetting nozzles (not shown) associated therewith are oriented parallel to the axial movement of a reciprocating print carriage operating in a drum-based large format digital print engine. 
     Referring to FIG. 7, which is also an elevational view in partial cross section along lines  5 — 5  of FIG. 4, of the full length of carriage assembly  32  apertures  70  formed in first carriage assembly structural member  76 , admit electrically conducting flex circuits (not shown) to each of the print heads  50  at a first end and a second end terminates at circuit board  300  (not shown) disposed intermediate the two carriage assembly structural members forming articulated saddle portion  107  of carriage  32 , and line  306  electrically couples to remove circuitry. 
     Referring to FIG. 8, the coupling of carriage assembly  32  to heads  50  is illustrated in an elevational view of an embodiment of the rails and connection pins to drive carriage assembly  32  axially along the shaft rail  24 . 
     A spring loaded cartridge maintenance location, service station assembly  200 , is disposed at one end of the rail operates to perform wiping, spitting, and capping functions to the ink emitting nozzles of the cartridges  50  so they perform within specification. The service station assembly  200  was designed with a minimum of moving parts, and is actuated by passively receiving a boss-shaped datum  59  located at a proximal end of the carriage assembly  32  with a tab  61 ′ disposed adjacent to block-shaped member  61  so that the datum  59  is retained therebetween, thereby further promoting field service efficiency and precision alignment with a minimum of critical alignments. With reference to FIG. 9, depicting a perspective view of an embodiment of the service station assembly  200  having three main functional areas: spitting area  203 , wiping area  201 , and capping area  205  having a radius similar to the radius for the cartridges  50  with respect to spatial orientation. Each mounting boss  205  is provided with a spring-biased base section  223  for promoting a firm seal around the periphery of each group of nozzles of each cartridge  50 . In operation, a lower rib portion of cartridge  50  (proximate the jets) meets guide posts  218  which acts as extremely accurate guidance vehicles for alignment of the cartridges  50  during service station operation. The guide posts  218  ensure an initial proper axial alignment of the wiper elements  201  and spitting aperture  203  with respect to the cartridges on the carriage assembly  32 . The frame member  220  thus mechanically articulates on a parallel axis to the axis of lead screw  28  so that after momentarily aligning an ink receiving vessel with the ink emitting nozzles of each row of print cartridges  50 , a short segment of double-finned wiper armature  201  clears ink from the surface of the nozzles. To maintain the spatial orientation of the left and right array plates  231 ,  233  with a minimum of additional components a strap member  206  couples to both plates  231 ,  233  and to the upper portion of the service station assembly  200  and springs underneath (not shown) provide a degree of freedom. Thus, the guide posts  218  of the plates  231 ,  233  provide affirmative contact with a portion of the print heads themselves to ensure accurate alignment during wiping and spitting operations. Note that the carriage assembly  32  need not reverse direction so that the second set of cartridges  50  receive the same treatment as the first set of cartridges  50 . Then, the carriage  32  preferably translates the service station sub-assembly  207  laterally on pins  215  which engage rising surface  216  to promote sealing connection between mounting bosses  205  and cartridges  50  (with approximately eight pounds of force—thereby necessitating the dual, spring biased rollers at the distal end of carriage  32  earlier noted). A source of vacuum  211  is ported via plenum assembly  225  to a lower port formed below the mounting bosses  205  and said vacuum is applied for a fraction of a second which is long enough for ink to be removed from the ink emitting nozzles. This ink material is preferably routed to an accumulator vessel  210  outfitted with a float valve  208  which provides a signal (on line  208 ′) to processing electronics, and optionally to a monitor means  151 , when the vessel is nearing capacity. In addition a drain member  212  can be coupled to the vessel  210  so that the source of vacuum  211  is protected from over pressure conditions. Preferably the drain member  212  is a check valve  300 . Following the evacuation of ink from the ink emitting nozzles, the print heads  50  continue to engage the mounting bosses  205  of said service station assembly  200  where they remain prepared until a next required printing operation or for manual replacement with a different set of marking materials by the operator. In designing the service station assembly  200  of the present invention, the inventors consulted and considered, among other things, capping station pressures, evaporation rates, and other criteria for designing the capping station, including a psychometric chart (used in evaporation calculations), and tolerance stack-up allocation for components. 
     In one embodiment of the present service station assembly  200 , a tab  63  formed on a lower portion of carriage  32  is oriented so that said tab  63  trips a split sensor  65  as the carriage prepares to enter the service station zone (thereby indicating the location of the carriage assembly). This split sensor  65  provides the location to carriage control circuitry so that the drive screw  28  may operate at its maximum torque setting to drive the service station into sealing contact with the boots. Also, to aid the low torque motor  42  in efficiently performing service station functions, the vertically articulated saddle portion  107  of the carriage assembly  32  which retains the print heads  50  can be raised slightly to thereby reduce the force needed to cap the nozzles of print heads  50 . As can be appreciated, the print engine of the present invention operates independently of absolute carriage and print head position sensing or calculation. As a result this split sensor  65  provides a location signal of adequate resolution for the accurate control of the carriage during the initial phase of a service station visit. 
     A preferred method of manufacture of a drum member  26  usable with the present print engine  40  is detailed in the applications referred to above. The reader is encourage to review the disclosure therein for a fuller understanding of the design considerations and criteria for the present drum member. A current embodiment of a drum subassembly  26  useable with the present invention basically consists of a resin-based, or plastic, sleeve bonded to two endcaps with total indicated run-out (“TIR”) of not more than 0.005″ over the entire surface of the drum, and a static balance of not greater than 0.8 inch lbs, and a diameter of the drum or 15.900″+/−0.005″ and a width of 36.25″+0.040″−0.000″ and a torque rating during printing operations one (1) inch ounce maximum. 
     Rotary encoder assembly used herein preferably employs a one thousand (1000) counts per revolution optical chopper—a part customized for the print engine described herein although quite similar in some respects to Hewlett-Packard 6000 Series three channel optical encoder reader. Since these types of encoders are well known in the art, no further discussion of the encoder follows. 
     The drum  26  is driven about its axis of rotation by a pulley subassembly coupled to a simple low torque electric motor residing in an enclosed space so that only a drive wheel protrudes into a printing bay of the engine  40 . The belt driven between the motor and a driving surface on an end of the drum preferably is a timing-type belt with geared teeth to reliably engage the drive wheel of the motor. In order to further control tolerance stacking and creep, a variety of drum drive belt stretch test data was captured so that a predictable amount of wear and stretching of the drive belt can be tolerated by the printing engine. Since the rotation of the drum and the drive screw are independently operated, and the precise control and stability of both directly impact the accuracy of the final printed output, very little stretch can be tolerated, and therefore the belts have a recommended replacement frequency that ensures the desired quality of printing. 
     The drum member is preferably statically and dynamically balanced. First of all known static balancing techniques ensure that the drum is balanced at rest. Then the drum is rotated to a stable rotational velocity and torque measured at various periods of time. As is known in the art a plug of material is typically added so one or both lateral sides of the rotary drum member so that undesired vibration during rotation is eliminated. 
     While the present invention has been described with reference to certain embodiments, it is clear to one of skill in the art that various other embodiments and changes of form shall are covered by the spirit of the invention and as such shall be considered part of the invention as particularly claimed in the following claims, which alone define the metes and bounds of the invention herein disclosed, enabled, and taught. 
     For example, the extremely robust three point stand of the print head of the present invention can be adapted to carry a large variety of additional equipment, including bulk ink reservoirs associated with extended ink supply systems referenced above as well as fans, heaters, and other means of drying the printed materials. 
     The selection of materials for the components described herein can be varied, but preferably the vertically articulated saddle portion  107  of said carriage assembly  32  is preferably formed of aluminum, the drum of PVC, and the chassis for the printer of either metal or resin. The useful range of the head height adjustment described herein has been estimated at approximately one-quarter inch (0.25″). The output from the imaging array  150  can be ported to a display means  151  located on the printer itself, to a remote monitor, both, or neither, since the image capture does not generally require human interpretation. The manner of dot detection used with said imaging array can be based on the periphery of a dot, the calculated “center” of a dot, an arbitrary weight associated with each pixel element of the imaging array, or other methods as known in the imaging art. The degree of print head skew with respect to a given print head can be determined with reference to a variety of sources, including a logical co-axial vector parallel to the direction of print head slew, or simply from the relative reference frame of the field of view of the imaging array  150  since said field of view (and therefore “skew” of said imaging array) is relative for all print heads inspected by the same imaging array. The useful range of skewing of a print head has been determined to be about sixty mils (0.060″) at a point remote from a pivot location of said model 51645 print head, although the location of the set screw and the type and size of print head will dictate other possible combinations that can be reliably correlated to a skewing angle of said print head for compensation purposes. The values of head height, encoder counts per unit of carriage slew, home head height, skewing angles of the print heads and other operating values of the printer are preferably stored in a flash-type memory so that they can be reliably retrieved even after long periods of printer inactivity.