Abstract:
An engraver having an engraving signal generating system for engraving a workpiece. The engraver includes a setup circuit enabling direct dimensional control of the cavities engraved by the engraver. Controls are provided for setting at least one of a plurality of parameters, such as a black cell width. These parameters are fed to the setup circuit which translates them into multiplication factors for an AC signal and a videosignal. The multiplied signals are combined with an offset signal to produce an engraving signal.

Description:
RELATED APPLICATION 
     This application is a continuation of Ser. No. 08/837,987 filed Apr. 15, 1997, now U.S. Pat. No. 5,808,749, which is a continuation of Ser. No. 08/476,657 filed Jun. 7, 1995, now U.S. Pat. No. 5,621,533, which is a continuation of Ser. No. 08/022,127 filed Feb. 25, 1993, now U.S. Pat. No. 5,424,845. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to engraving heads of the general type disclosed in Buechler U.S. Pat. No. 4,450,486. Such engraving heads comprise 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 a maximum arc of approximately 0.25 deg. at a frequency in the neighborhood of about 3,000 to 5,000 Hz. 
     A guide shoe is mounted on the engraving head in a precisely known position relative to the oscillating stylus. The engraving head is supported for tilting movement by a set of leaf springs secured to a rearwardly projecting bar. A DC motor rotates the bar so as to bring the guide shoe into contact with a printing cylinder to be engraved. When the guide shoe is in contact with the printing cylinder, the stylus oscillates from a position just barely touching the printing cylinder to a retracted position about 100 microns distant from the surface of the cylinder. 
     Once the guide shoe is in contact against the printing cylinder 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 entire printing surface of the printing cylinder. 
     In engraving systems of the type taught by Buechler, it is necessary for the machine operator to perform a tedious trial and error setup procedure at one end of the printing cylinder prior to commencement of engraving. This procedure involves adjustment of the gain on amplifiers for the sine wave driving signal and the video signal so as to produce “black” printing cells of a desired depth together with connecting channels of another desired depth and clean non-engraved white cells. Each change of one of the control variables interacts with the others, and therefore the setup becomes an iterative process. 
     It is therefore seen that a need has existed for an engraving system which may be quickly and easily set up to engrave cells of precisely controlled dimensions in the surface of a gravure printing cylinder. 
     SUMMARY OF THE INVENTION 
     The present invention provides an engraving apparatus and method wherein a plurality of parameter signals are supplied to a setup circuit for computing engraving parameters to control the engraving response of the engraving stylus to an input video signal. In a preferred embodiment an input AC signal and an input video signal are applied to a multiplication circuit wherein they are multiplied by multiplication factors which are generated by a setup circuit. The output signals from the multiplication circuits are combined with a white offset signal to produce an engraving signal for driving the engraving stylus to engrave a series of cells of the desired geometry. 
     The setup circuit is provided with input signals which indicate a desired black cell width, a desired channel width, a desired highlight cell width and the video voltage level at which a highlight cell of the desired width is to be engraved. Further, in accordance with the present invention the setup circuit may be provided with an input signal which adjusts the multiplication factor as appropriate for the geometry of the particular stylus tip which happens to be in use. This input signal corresponds to the cutting depth/width ratio, which in turn depends upon the tip angle of the stylus. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration, partly in perspective, of a programmable engraving system according to the present invention. 
     FIG. 2 is a schematic illustration of a series of cells engraved in a printing cylinder. 
     FIG. 3 is a schematic illustration of AC and video signals for controlling an engraving stylus and the engraving movement which results therefrom. 
     FIG. 4 is a block diagram of a stylus setup circuit. 
     FIG. 5 is a graphical plot of the maximum cell depths resulting from video input signals ranging from 0 to 10 volts. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1 there is illustrated a gravure printing cylinder  10  mounted for rotation by a drive motor  12  and engraving by an engraving stylus  20 . During the engraving operation, stylus  20  moves engravingly toward and away from printing cylinder  10  to produce a series of cells arranged along a track  30 . A lead screw motor  14  causes axial movement of stylus  20  so as to produce a track  30 . If lead screw motor  14  moves continuously, then track  30  will have a helical configuration. Intermittent movement of motor  14  produces a series of spaced circular tracks  30 . 
     Stylus  20  is driven into engraving contact with print cylinder  10  by an electromagnetic driver  16  operating in response to a drive control signal on line  60 . Electromagnetic driver  16  may be configured as generally disclosed in Buechler U.S. Pat. No. 4,450,486. 
     The signal on line  60  has an AC component, a video component and an offset component appropriate for producing an engraving action as hereinafter described. The AC component is derived from an AC input signal which is applied to a multiplier  36  and multiplied by a multiplication factor Ka obtained on a line  71  from a setup circuit  34 . The AC product signal from multiplier  36  is applied to a summing junction  40  where it is combined with a signal from another summing junction  26 . Summing junction  26  combines an offset signal WD from setup circuit  34  with an output signal from a second multiplier  38 . The function of multiplier  38  is to multiply an input video signal by a factor Kd generated by setup circuit  34  on line  73 . 
     The engraving operation may be controlled by an operator or by a programmed microprocessor. For setup purposes the system may be provided with a control panel  32  equipped with 8 controls  41 - 48 . Controls  41 - 46  respectively signal a desired black width, BW, a desired channel width, CW, a desired highlight width, HW, a stylus constant, Ks, a highlight voltage Vh, and a shoe offset S. These six signals are applied to a stylus setup circuit  34 . Control  47  provides for adjustment of a lead screw control circuit  24 , and control  48  provides for adjustment of a rotational speed control circuit  22  connected to drive motor  12 . 
     As hereinafter discussed in more detail, the AC component of the signal on line  60  causes stylus  20  to oscillate in a sinusoidal manner relative to printing cylinder  10  with a wavelength dependent upon the surface speed of the cylinder. The rotational speed of drive motor  12  must be adjusted so as to produce an engraving track  30  having an odd number of half wavelengths during a full engraving rotation. The lead screw control circuit  24  must be adjusted so as to cause lead screw motor  14  to advance stylus  20  an axial distance equal to one-half of a black cell width plus one-half of a connecting channel width, plus one separating wall width during each complete rotation of the printing cylinder  10 . 
     The geometrical configurations of typical black cells, connecting channels for black cells, highlight cells and separating walls are illustrated in FIG.  2 . That figure depicts a series of wide, deep black cells  70  and a series of shallower and narrower highlight cells  76 . The illustrated cells comprise portions of three side-by-side engraving tracks  30 . Black cells  70  have a maximum width BW. The control signal for the stylus is adjusted so as to produce connecting channels  72  between successively engraved black cells  70 . Channels  72  have a width CW, while highlight cells  76  have a width HW. The scalloped edges of the cells  70  result from the vertically oscillating cutting action of stylus  20  during rotational movement of printing cylinder  10  thereunder. As further illustrated in FIG. 2, a series of successively engraved black cells  70  may be separated by a wall  74  from a series of successively engraved cells  70  (also illustrated as being black cells) in an adjacent engraving track  30 . 
     A series of cells configured as illustrated in FIG. 2 will print a graphic pattern defining a diagonally extending screen. The tangent of the screen angle is the ratio of one-half the wavelength of the stylus cutting motion to the distance between adjacent engraving tracks  30 . The cutting wavelength is a function of the surface speed of the printing cylinder  10  and the oscillation frequency of stylus  20 . Thus the screen angle may be adjusted by adjusting the rotational speed of drive motor  12 , while holding the oscillation frequency constant but such adjustment must be made in incremental steps so as to maintain an odd number of half wavelengths around the circumference of the printing cylinder. 
     The driving signals for stylus  20  and the resulting vertical movement of the stylus are illustrated in FIG.  3 . The driving signal is obtained by adding an AC signal  80  to a video signal  82 . The illustrated video signal  82  has, by way of example, a white video level  86 , a black video level  88  and a highlight video level  90 . The video signal and the AC signal are combined with an offset such that the stylus is raised out of contact with the cylinder surface during the entire time that video signal  82  has a white level  86 . The minimum white elevation is WD. 
     When video signal  82  goes from a white level to a black level, stylus  20  moves into engraving contact with the cylinder as shown by stylus position line  84 . In this condition the stylus oscillates between a minimum depth CD and a maximum depth BD. When stylus  20  is at the depth CD, it engraves a connecting channel  72 . When video signal  82  shifts to a highlight level as indicated by the reference numeral  90 , stylus  20  oscillates between a position out of engraving contact with cylinder  10  to an engraving position having a maximum depth HD. AC signal  80 , video signal  82  and a white offset signal are produced by setup circuit  34 . 
     In general the depth of stylus  20  at any instant in time is given by the equation: 
     
       
           D ( t )= Ka*A *(sin(χ* t )−1)− WD+Kd*V ( t ) 
       
     
     where: 
     Ka=gain factor of the AC amplifier 
     A=maximum value of the AC reference signal 
     χ=frequency of AC reference signal 
     t=time 
     WD=white offset 
     Kd=gain factor of video amplifier 
     V(t)=video voltage at input (function of time) 
     The maximum black depth occurs when sin(χ* t)=1 and v(t)=Vmax. Therefore the black depth is given by: 
     
       
           BD=Kd*V max− WD   (1) 
       
     
     The channel depth CD occurs when sin(χ* t)=0 and v(t)=Vmax. Therefore the channel depth is given by: 
     
       
           CD=Ka*A−WD+Kd*V max  (2) 
       
     
     The highlight depth HD occurs when sin(χ* t)=1 and v(t)=highlight voltage Vh. Therefore: 
     
       
           HD=Kd*Vh−WD   (3) 
       
     
     When an engraving system is being set up, the values of BD, CD, and HD (or the functionally interchangeable cell width dimensions BW, CW and HW) are specified. Vh is also specified, and Vmax and A are known. Thus the problem becomes one of finding the values of the engraving parameters Kd, Ka and WD which will produce the specified values of BD, CD, and HD at the specified Vh. Setup circuit  34  accepts values for the six known setup parameters and performs a simultaneous solution of equations (1)−(3) to determine the three unknown engraving parameters. 
     The solution of the above equations may be carried out in many different ways, using either analog or digital devices. For example, a simply implemented digital procedure may use the following equivalent set of equations: 
     
       
           Kd =( BD−HD )/( V max− Vh )  (4) 
       
     
     
       
           WD=Kd*V max −ED   (5) 
       
     
     
       
           Ka =( CD+WD−Kd*V max) /A   (6) 
       
     
     Equations (4)-(6) may be solved in sequence. Thus the value of Kd obtained from the solution of Equation (4) may be used in the solutions of Equations (5) and (6), and the value of WD obtained from Equation (5) may be used in Equation (6). 
     If it is desired to use operator inputs BW, CW and HW from potentiometer devices, as illustrated in FIG. 1, then an analog to digital conversion is performed, and the resulting digital quantities are multiplied by a stylus constant Ks for conversion to equivalent values of BD, CD and HD. For a typical stylus-having a tip shape as illustrated in FIG. 1, Ks is given by the equation: 
     
       
           Ks =1/(2*tan(tip/2)) 
       
     
     where tip is the angle of the stylus tip. BD may also be derived from the screen angle, if desired. 
     In the event that there is a small error in the positioning of the shoe against printing cylinder  10 , then the setup parameter S may be supplied to setup circuit  34 . If this signal is generated, it is treated as an offset and is simply added to BD, CD and HD prior to performing the above outlined solution. 
     An equivalent analog solution is illustrated in block diagram form in FIG.  4 . In the illustrated arrangement some of the functions of control panel  32  are incorporated into setup circuit  34 . In particular, setup circuit  34  includes the controls  41 ,  42 ,  43  and  46  which are potentiometers having dial markings for manual setting of Black Width (BW), Channel Width (CW), Highlight Width (HW) and shoe offset (S) respectively. Signals indicating the stylus constant Ks and a highlight voltage Vh are received on input lines  54  and  55 . The BW, CW and HW inputs are multiplied by the stylus constant Ks to generate signals corresponding to the Black Depth, BD, Channel Depth, CD, and Highlight Depth, HD. 
     For the specific embodiment of the invention illustrated in FIG. 4, the video signal varies over a range from 0 to 10 volts as the graphic content in a scanned original document ranges from white to black. Highlight voltage Kh represents the video voltage which is to be assigned to a highlight cell of the width indicated by HW. The value of Kh is subtracted from 10 volts at a summing junction  104  to produce a dividend signal for a divider circuit  112 . Meanwhile summing junction  102  generates a divisor signal which is equal to BD—HD. BD may be optionally adjusted as described below. 
     The divisor signal is used by divider circuit  112  to produce a quotient on line  113  which is the cotangent of the slope angle for an engraving response line  250  as shown in the graph of FIG.  5 . Engraving response line  250  illustrates the full range of maximum cell depths to be engraved in response to video signals ranging from 0 to 10 volts. 
     Setup circuit  34  produces multiplier signals Kd and Ka on lines  73 ,  71  and an appropriate white offset signal WD on line  72  for causing engraving stylus  20  to engrave in a manner which will produce an engraving response line  250  having the desired slope and position. The production of signals Kd, Ka and WD involves applying the quotient on line  113  to multiplier  110  and multiplying it by Kh. The resulting product is added to HD at summing junction  107  and multiplied by 2 to produce a highlight offset HH on line  107 . 
     HH is multiplied by 0.5 and combined with BD at summing junction  108  to produce Kd. HH is also applied to summing junction  118  to contribute to the production of the white offset WD. Summing junction  118  also receives a shoe bias term from potentiometer  46 , an adjusted channel depth from summing junction  498  and the adjusted black depth from summing junction  496 . Ka is produced at summing junction  106  by subtracting the adjusted black depth from the adjusted channel depth. 
     It has been found that copper gravure printing cylinders respond in a linear manner to cutting by a diamond stylus only to a certain depth for a given cell geometry. A linear response is one in which a factor of x increase in head current results in a factor of x increase in stylus depth into the copper through the specified range of stylus movement. 
     In those cases where the non-linearity is objectionable, an optional linearizer  200  may be inserted into setup circuit  34 . Linearizer  200  corrects both the full cell width BW and the channel width CW. For this purpose linearizer  200  uses screen angle SA, horizontal resolution Ks, BW and CW as inputs. These parameters fully define the cell geometry and also are sufficient for a calculation of cylinder surface speed. It has also been found that engraving non-linearities for a copper cylinder are dependant upon the ratio of CW to BW. 
     In a typical embodiment, as illustrated in FIG. 4, the screen angle may be input to linearizer  200  as an analog signal. The horizontal resolution may be input manually in digital form at block  406  through use of pushbuttons configured to generate a BCD code indicating the number of lines per inch. Block  406  may include an EPROM for converting such a BCD code to binary form for transmission to another EPROM  408  and an EPROM  410 . The screen angle SA may be converted to a digital format by an A/D converter  402 . The digitized screen angle is also applied to EPROM  408  and EPROM  410 . 
     The stylus factor Ks is applied to an analog-to-digital converter  416  and digitized for transmission to an EPROM  412 . EPROM  412  converts the digitized stylus factor into a stylus angle for transmission to EPROM  408  and EPROM  410 . BD and CD are applied to A/D converter  414  which calculates the ratio CD/BD. This ratio (which is equal to CW/BW) is transmitted in digital form to EPROM  408  and EPROM  410 . 
     EPROM  408  calculates a correction for the black cell depth BD in digital form. The correction to BD is converted into analog form by digital-to-analog converter  418  and added to the raw value of BD at summing junction  496 . Similarly, EPROM  410  computes a correction to the channel depth CD which is converted to analog format by digital-to-analog converter  420  and added to the raw value of CD at summing junction  498 . 
     The necessary corrections for BD and CD are established empirically and stored in EPROM  408  and  410  in tabulated form. The corrections in each EPROM are tabulated as functions of CW/BW, stylus geometry, screen angle and horizontal resolution. Correction tables may be established by disconnecting D/A converters  418  and  420  from summing junctions  496 , 498  respectively and engraving a printing cylinder at different screen angles over a range of horizontal resolutions, a range of stylus angles and a range of values for CW/BW. At each setting analog signals are added to summing junctions  496  and  498  and are adjusted until the black width produced by the engraving stylus equals the input black width and the channel width equals the input channel width. The values of these signals represents the engraving error at the indicated cell geometry for the given stylus angles. These values are tabulated and stored in EPROM  408  and  410  for automatically linearizing the response of setup circuit  34  to the normal setup parameters. Consequently, a response as illustrated in FIG. 5 may be obtained. 
     Referring again to FIG. 5, the maximum cell depth is seen to be directly proportional to the video input signal. As illustrated in the figure, a maximum  10  volt video input signal produces the maximum cell depth BD required for engraving a black cell. For the illustrated example, setup circuit  34  has been given a highlight width HW=0.25 * BW. Hence the highlight depth HD is 25% of BD. The Figure also reflects a setting of 3 volts for Kh. Under those conditions a video signal having an amplitude equal to 30% of a “black” video signal produces a cut having a depth which is only 25% of the black cell depth. As a result the maximum cell depth goes to zero for a video input of about 0.7 volts. For video signals smaller than that amount, the cutting stylus remains out of contact with the printing cylinder. For a “white” video input the stylus is retracted from the engraving cylinder by a minimum distance WD, which is the white offset. 
     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 claims.