Abstract:
A system and method relates to an ink volume determination for determining the amount of ink to be applied by the printer. The ink volume determination involves generating a composite cylinder layout of at least one image for the engraved cylinder, generating a set of data corresponding to the composite cylinder layout and then using the set of data to determine the volume of ink. The printer is then filled with a volume of ink related to this calculated ink volume during a printing process. The method of determining ink volume may be used independently of the printing process, for example, in order to control or manage the amount of ink used in the printing press.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a method of predicting ink consumption in a gravure printing process. Such a process may use an electro-mechanically engraved gravure printing cylinder; for example, a gravure printing cylinder which has been engraved in accordance with the method disclosed in copending application, Ser. No. 08/022,127, filed Feb. 25, 1993. Such printing cylinders are engraved by an engraving head comprising a diamond stylus carried by a holder mounted on an arm projecting from a torsionally oscillated shaft. A sine wave driving signal is applied to a pair of opposed electromagnets to rotate the shaft through an arc of approximately 0.25° at a frequency in the neighborhood of about 3,000 to 5,000 Hz. 
     A video signal is added to the sine wave driving signal for urging the oscillating stylus into contact with the printing cylinder thereby engraving a series of controlled depth cells in the surface thereof. The printing cylinder rotates in synchronism with the oscillating movement of the stylus while a lead screw arrangement produces axial movement of the engraving head so that the engraving head comes into engraving contact with the printing surface of the printing cylinder. The system has setup controls for quickly and easily setting up the engraving head to engrave cells of precisely controlled dimensions in the surface of a gravure printing cylinder. 
     When such a printing cylinder is used in a gravure printing process, ink will be applied in an amount which is related to the total volume of all of the cells which have been so engraved. This is likewise true for gravure printing processes using printing cylinders which have been engraved by other engraving techniques. Regardless of the particular engraving technique which is used, it has been common to engrave connecting channels between cells having a depth which is greater than some predetermined amount. This has complicated the task of predicting the amount of ink which will be required for a particular printing job. Heretofore, ink volume estimation has required a tedious trial and error process and has been subject to error. This has made it necessary to stock excess amounts of ink in order to avoid shortages. 
     SUMMARY OF THE INVENTION 
     It is, therefore, a primary object of this invention to provide an ink management system which permits the efficient management of ink. 
     In one aspect, this invention comprises a method for determining a volume of ink for an engraved cylinder, said method comprising the steps of: (a) generating a composite cylinder layout of at least one image for said engraved cylinder; (b) generating a set of data corresponding to said composite cylinder layout; and (c) using said set of data to determine said volume of ink. 
     An object of this invention is to provide a system or method for determining a volume of ink for an engraved cylinder. 
     Another object of this invention is to provide a system or method of determining said ink volume in response to at least one input parameter, such as cell width, cell wall size, channel width, engraving width, taper requirements, circumferential linearization, balance correction, edge enhancement level, screen and screen angle. 
     Still another object is to provide a system or method for determining the amount of ink consumed by an engraved cylinder during printing. 
     These advantages and others may be more readily understood in connection with the following specification, claims and drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a general schematic view of the system and method according to an embodiment of this invention; 
     FIG. 2 is a fragmentary view of a cylinder surface showing a plurality of cells, cell walls and highlight cells; 
     FIG. 3 is a schematic illustration of AC and DC signals for controlling an engraving stylus on an engraving head of an engraver and the engraving movement which results therefrom; 
     FIG. 4 is a fragmentary view of the cylinder showing an engraving stylus and associated angle cut into the cylinder; 
     FIG. 5 is a graph showing the relationship between a voltage supplied to the engraving head and the cell width; 
     FIG. 6 is a fragmentary view showing a cell wall width; 
     FIG. 7 is a graph showing the relationship between the length and the depth of a cell with a channel; and 
     FIG. 8 is a graph showing the relationship between the length and the depth of a cell without a channel. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A method of predicting ink consumption in accordance with the present invention utilizes a series of steps as illustrated generally in FIG. 1. The object is to print N copies of an original image or of a composite image comprised of multiple images. The data defining an original or composite image may be generated and downloaded from a computer or it could be scanned, for example, from a graphic master or other medium capable of being scanned. 
     The method of the invention begins by inputting the image data as indicated by block 24 of FIG. 1. The image data could be a group of files representing multiple images, each obtained from a different source. Alternatively, the images could be a single file of scanned or computer generated image data. 
     After the files of image data have been read, a composite cylinder layout (block 25) is composed. This cylinder layout identifies the portion of each image which is to be engraved on the cylinder surface and specifies the exact geometric placement of that portion of each image. In order to compose the composite cylinder layout, one or more of a plurality of engraving parameters (not shown) may be input into the computer. For example, the parameters of engraving width, taper requirements, circumferential linearization, balance correction, edge enhancement level, screen and screen angle, as well as others, may be input into the computer. These parameters affect the size and placement of engraved cells on the cylinder. For example, taper and circumferential linearization adjust engraved cells to eliminate visual discontinuities caused by spiral engraving. The edge enhancement level parameters provide a method to improve contrast at line or image edges. The screen and screen angle generally describe cell population and cell shape. 
     The computer comprises means for considering each of the above parameters, as well as others, and for adjusting the densities of certain cells accordingly. 
     After the cylinder layout and engraving parameters are specified, cell shape parameters are input (block 26) which complete the definition of an engrave job. A histogram representing the image densities of each of the pixels may then be generated for the engrave job. Density values for a conventional electronic engraving machine are generally proportional to the voltages supplied to the engraving head. As discussed in detail below, an electronic engraving machine is driven by a video signal and an AC signal. The video signal is generally adjusted so as to be proportional to a desired printing density. The density values used to compile the histogram are used for the engraving operation, as well as used to predict ink volume. 
     In block 28, the computer prepares a table of data representing a histogram of density values associated with the composite cylinder layout. Preferably the densities are digitized and set to one or another of a predetermined number of discrete values. A vector of length 1025 has been found to be convenient for this purpose. Each time the examination indicates a particular density value, the appropriate vector position is adjusted. This process continues until a histogram or table of densities is generated for the entire cylinder. 
     After the density table has been generated, the computer begins reading the tabulated density values (block 29) for calculation of associated cell volumes. Calculations are performed at blocks 31 and 32 to determine the volumes of each of the different cell sizes corresponding to the different density levels. Each computed cell volume is multiplied by the number of occurrences of that cell volume to obtain a cell volume subtotal (block 33). The subtotals are accumulated (for example, at block 34) in order to read the total volume of all engraved cells. 
     The cell volume calculations use the setup parameters generated at block 26 to define the cell shape and geometry. These same parameters are used for controlling the engraving process (blocks 40-47) substantially as shown and described in Ser. No. 08/022,127 which is assigned to the same assignee as the present invention and which is herein incorporated by reference and made a part hereof. In short, a highlight voltage and cell width, a shadow voltage and cell width and a stylus angle are selected and input by the operator. The voltage and cell width corresponding to a shadow cell and a highlight cell may define a linear or non-linear function. In the embodiment being described, the voltage and cell width define a generally linear function, as shown in FIG. 5. Thus, given the voltage, for example, of a shadow cell, the computer determines the width of that shadow cell. 
     A series of engraved shadow cells 70 and highlight cells 76 may be engraved on the surface of a cylinder 10 as generally illustrated in FIG. 2. Shadow cells 70 may be connected by channels 72, the width of which may be adjusted by adjusting the video signal used for driving the engraving tool. If the shadow cells are not connected by a channel, the distance between cells in the direction of engraving is the vertical cell spacing 71, as shown in FIG. 2. 
     Referring now to FIG. 3, an engraving tool 20 oscillates into cutting contact with cylinder 10 under control of a driving signal which is the sum of a video signal 82 and an AC signal 80. Video signal 82 may have a white level value 86 such that the tip of the engraving tool never gets closer to cylinder 10 than a predetermined white depth WD. When the video signal 82 drops to the value 88, the engraver engraves full depth shadow cells having a maximum depth BD. The tool then engraves a contour 84 having a minimum depth CD which is the channel depth. When the video signal shifts upwardly to a value 90, the engraver engraves highlight cells having a maximum depth HD. Reference may be made to application Ser. No. 08/022,127 for equations which relate white depth and shadow depth voltages to a desired channel depth in a desired highlight depth. 
     The preferred embodiment of this invention utilizes a minimum diagonal wall size 49 (FIG. 6) as a setup parameter. The minimum diagonal wall size 49 is the perpendicular distance between tangent lines to adjacent cell walls. 
     The cell shape description mentioned above may be fine tuned if desired. For example, the channel width associated with a cell shape description may be entered in which case the computer recalculates or adjusts the minimum diagonal wall size 49. It is to be noted that a channel width of zero, indicating no channel, may be entered into the computer. In this event, a vertical spacing between cells may be entered into the computer, and again, the computer will recalculate or adjust the minimum diagonal wall size. Therefore, it is significant to note that the computer comprises means for tuning the cell shape description to accommodate various inputs and parameters which may affect cell shape, geometry, and volume. 
     All input parameters and fine tuning inputs may be shown on a monitor (not shown) which is operatively coupled to the computer. 
     The volumes of the individual cell types are calculated by a process indicated by decision point 30 and blocks 31 and 32. The process involves calculating the cross-sectional area of the cell as a function of cell location (e.g. position along the cell) and then integrating the area along the length of the cell in the direction of the engraving track. The integration may be carried out in closed form (as defined by the equations below) or performed numerically. If the integration is carried out numerically, then a check is made immediately following each pass through the integration loop to determine whether integration of the cell has been completed. 
     In the preferred embodiment, the volume for a cell is determined using one or the other of equations (1) and (2) below. It is to be noted that the cell volume will differ for a cell with a channel as opposed to a cell without a channel. 
     The volume of a cell with a channel is given by the equation: ##EQU1## θ is the stylus tip angle. s is screen in lines/micron. 
     b is the length of the side of a normal cell in microns. ##EQU2## P is the period of the sine wave mentioned earlier herein. φ is the screen angle. 
     D 0  =depth of channel in microns. 
     D 1  =total depth the stylus travels into copper. 
     For ease of illustration, FIG. 7 graphically illustrates the variables P, D 0 , and D 1  for a cell with a channel. 
     The volume of a cell without a channel is given by the equation: ##EQU3## b, s, θ and P are as defined above. ##EQU4## L is the cell length in direction of cutting. D 1  is the depth of the cell. 
     D 0  is the amplitude of the sine wave (to be derived from user inputs) minus the depth of the cell. 
     For ease of illustration, FIG. 8 graphically illustrates the variables P, L, D 0  and D 1  for a cell without a channel. 
     After integration of the first cell size has been completed, the process proceeds to select the next cell size and repeats the integration process. After completion of each volume computation, a check is made (Point 35) to determine whether the volumes of all cell sizes have been determined. If so, then the process proceeds to block 36 for a calculation of the volume of ink required for a single impression. Here, the total computed cell volume is multiplied by a release factor R. The release factor accounts for factors, such as the absorption properties of the printing substrate, the viscosity of the ink, speed of the press and the like. This ink volume is multiplied by the number of impressions N (block 37) to obtain the required volume of ink for an entire press run. This completes the prediction of ink consumption and usage at block 38. In the embodiment being described, the ink volume may then be used to provide an estimate of the amount of ink to fill an ink well of the printer (block 39). 
     If the integration is carried out numerically, then it is most convenient to perform the integration over a one-half wavelength distance and thereafter double the result. The numerical integration proceeds by moving from station-to-station along a profile of FIG. 3 and calculating the cross-sectional area at each station. This area is multiplied by the incremental distance between computing positions to obtain an incremental volume. 
     A typical stylus 20 for use in the practice of this invention is illustrated in phantom outline in FIGS. 3 and 4. The tip of stylus 20 has two bevelled faces which produce a tip angle θ, which may be about 120°. The stylus cuts a corresponding angular channel in the surface. It will be appreciated that FIG. 4 is a view taken perpendicular to the view of FIG. 3. Thus, the walls have a sinusoidal profile when viewed in a direction perpendicular to the engraving direction and conform to the shape of the engraving tip when viewed in a direction parallel to the direction of engraving. 
     Each of the depressions illustrated in FIG. 3 represents an engraved printing cell. Thus the Figure depicts three deep printing cells interconnected by two channels and two shallower printing cells which are not connected to any other cell. The volume of any printing cell may be computed by calculating the cross-sectional area as viewed in FIG. 4 and integrating that area over a one wavelength distance in the direction of engraving (e.g. parallel to engraving tracks 30). In the special case where stylus 20 has a tip configuration as illustrated in FIGS. 3 and 4, the cross-sectional area of the cut is given by the expression: 
     
         S=d.sup.2 * tan (θ/2) 
    
     The wavelength distance is given by the period of the sine wave as defined above. At each computing interval, it is necessary to check the sign of d to assure that it has a positive value. Whenever d is found to have a negative value, the computer forces it to a value of zero. 
     Advantageously, this invention provides an ink management system and printing method for precisely determining the amount of ink required by a print cylinder, such as a gravure cylinder having a plurality of cells. 
     It is to be noted that the video data generated at block 28 may be applied to an engraving controller (not shown) for generation of an engraving signal at block 40. This engraving signal is used to position an engraving stylus, as described in detail below. The engraving stylus engraves a cell (block 44) and continues engraving cells until the last cell has been engraved (decision point 46). 
     While the method herein described, and the form of apparatus for carrying this method into effect, constitute preferred embodiments of this invention, it is to be understood that the invention is not limited to this precise method and form of apparatus, and that changes may be made in either without departing from the scope of the invention, which is defined in the appended.