Abstract:
In a method for determining the axial position of an engraving element in an engraving machine used for engraving impression cylinders, the engraving machine has a fixed toothed comb in which a significant tooth flank of each tooth presents an axial distance which is a multiple of the tooth spacing of the toothed comb to a reference point. A photoelectric barrier which serves as axial reference mark is positioned on the engraving element and the toothed comb serves as black-out element for said photo-electric barrier. The engraving element is displaced by means of the photoelectric barrier from a momentary position to the nearest relevant tooth flank of the toothed comb. The approximate distance between the reference mark and the reference point is measured by means of a distance meter. Next, the approximate distance is compared with the multiple of the tooth spacing of the toothed comb and the exact distance between the reference mark of the engraving element and the reference point is determined from this comparison.

Description:
BACKGROUND OF THE INVENTION 
     The invention is in the field of electronic reproduction technology and is directed to a method and to an apparatus for the determination of the axial position of an engraving element in an electronic engraving machine for engraving print cylinders for rotogravure, and is also directed to an engraving machine with such an apparatus. 
     When engraving print cylinders in an electronic engraving machine, an engraving element, which comprises, for example, an electromechanical engraving element with an engraving stylus as cutting tool, moves in the axial direction along a rotating print cylinder. The engraving stylus controlled by an engraving control signal cuts a sequence of cups of different depth arranged in an engraving raster into the generated surface of the print cylinder. The engraving control signal is formed by superimposition of an image signal, which represents the gradations to be engraved between “light” (white) and “dark” (black), with a periodic raster signal. Whereas the periodic raster signal effects a vibrating lifting motion of the engraving stylus for producing the engraving raster, the image signal values determined the depths of the cups engraved into the generated surface of the print cylinder and, thus, the engraved gradations. 
     In order to axially position the engraving element before the engraving and move it along the print cylinder in the axial direction during the engraving, the engraving element is driven by a spindle drive that is often designed as a stepping motor drive. The stepping motor is driven by a motor clock sequence, each clock thereof corresponding to a traversed, axial path increment of the engraving element. By counting the clocks of the motor clock sequence with a position counter, thus, the respective axial position of the engraving element can be identified, the engraving element can be displaced onto a defined axial position by counting a predetermined number of clocks. 
     Before the start of engraving, the position counter of the stepping motor drive must be reset and the respective, axial actual position of the engraving element upon reset—called the zero position—must be identified, so that the engraving element, proceeding from the identified zero position, can be subsequently shifted to a desired, axial rated position. 
     U.S. Pat. No. 5,492,057 already discloses a method for determining the axial position of engraving elements in an electronic engraving machine with the assistance of sensors. 
     U.S. Pat. No. 5,074,690 likewise already discloses that a toothed comb be employed as an absolute value sensor for determining the zero position in a matrix printer. 
     The traditional method for determining the zero position of an engraving element occurs either with a first light barrier that recognizes a shadowing element called zero flag that is attached to the engraving element or with an absolute value sensor. Particularly given simultaneous engraving with a plurality of engraving elements, the traditional method has the disadvantage that a plurality of zero flags corresponding in number to the plurality of engraving elements must first be optimally precisely adjusted in order to achieve an adequate precision in the determination of the zero position. Over and above this, the employment of a plurality of absolute value sensors is expensive. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to improve a method and an apparatus for determining the axial position of at least one engraving element in an electronic engraving machine for engraving print cylinders for rotogravure as well as an electronic engraving machine having such an apparatus such that a high precision in the position detection is achieved with relatively little expanse and an automatic implementation becomes possible. 
     According to the present method and apparatus of the present invention for determining an axial position of at least one engraving element in an electronic engraving machine for engraving a print cylinder, an engraving element engraves a series of cups arranged in an engraving raster into a print cylinder, engraved depths of the cups determining gradations to be engraved between light and dark. For planar engraving of the cups, with the engraving element executing a feed motion along the print cylinder that is directed in an axial direction of the print cylinder. A momentary axial position of the engraving element relative to the print cylinder is determined before the engraving. A stationary toothed comb is provided directed in an axial direction of the print cylinder whereby one of tooth faces of each and every tooth represents an axial distance from an axial reference point as a multiple of a toothed division of the toothed comb. The engraving element has an axial reference mark. The engraving element together with its reference mark is displaced from its momentary position onto a closest, relevant toothed face of the toothed comb into the axial position of the engraving element to be determined. An approximate distance of the reference mark from the reference point is measured. The measured approximate distance is compared to the multiple of the toothed division of the toothed comb. An exact distance of the position of the engraving element from the reference point is determined from the comparison. 
    
    
     The invention is explained in greater detail below on the basis of FIGS. 1 through 4. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block circuit diagram of an engraving machine for print cylinders; 
     FIG. 2 is a side view of an apparatus for determining the axial position of an engraving element; 
     FIG. 3 is a first exemplary embodiment of an apparatus for determining the axial position of an engraving element, shown in a front view; and 
     FIG. 4 is a second exemplary embodiment of an apparatus for determining the axial position of an engraving element, shown in a front view. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a schematic block circuit diagram of an engraving machine for engraving print cylinders for rotogravure. For example, the engraving machine is a HelioKlischograph® of Hell Gravure Systems GmbH, Kiel, DE. 
     A print cylinder  1  is rotationally driven by a cylinder drive  2 . 
     Particularly in packaging rotogravure, the engraving on the print cylinder  1  occurs with a single engraving element  3  that, for example, is designed as an electromagnetic engraving element with an engraving stylus  4  as cutting tool. 
     In the illustrated exemplary embodiment, the engraving element  3  with its engraving support is located on an engraving carriage  5  on which the engraving element  3  can be manually displaced on its engraving support in the axial direction of the print cylinder  1  and locked. Via a spindle  6 , the engraving carriage  5  is driven in the axial direction of the print cylinder  1  by an engraving carriage drive  7  in order to position the engraving carriage  5  with the engraving element  3  and move it along the print cylinder  1  during the engraving. 
     The engraving carriage drive  7  is designed, for example, as a stepping motor drive. The stepping motor is driven by a motor clock sequence, each clock thereof corresponding to a traversed, axial path increment of the engraving element. By counting the clocks of the motor clock sequence with a position counter, thus, the respective axial position of the engraving element can be identified or, the engraving element can be displaced onto a defined axial position by counting a predetermined plurality of clocks. 
     The engraving stylus  4  of the engraving element  3  cuts a series of cups arranged in a print raster into the generated surface of the rotating print cylinder engraving line by engraving line while the engraving carriage  5  with the engraving element  3  moves along the print cylinder  1  in feed direction. Alternatively, the engraving element  3  can also be coupled to the rotating spindle  6  with a spindle nut. In this case, the common engraving carriage  5  is eliminated. 
     The engraving stylus  4  of the engraving element  3  is controlled by an engraving control signal GS. The engraving control signal GS is formed in an engraving amplifier  8  from the superimposition of a periodic raster signal R on a line  9  with an image signal B that represents the gradations between “light” (white) and “dark” (black) of the cups to be engraved. Whereas the periodic raster signal (R) effects a vibrating lifting motion of the engraving stylus  4  for producing the engraving raster, the image signal values B—in conformity with the gradations to be engraved—determine the respective geometrical dimensions such as penetration depth, transverse diagonal and longitudinal diagonal of the cups engraved into the generated surface of the print cylinder  1 . 
     The analog image signal B is acquired in a D/A converter  10  from engraving data (GD) that are stored in an engraving data memory  1  and read out therefrom engraving line by engraving line and supplied to the D/A converter  10 . An engraving datum of at least one byte that, among other things, contains the gradation between “light” and “dark” to be engraved as engraving information is thereby allocated to each engraving location for a cup on the print cylinder  1 . 
     A controller  12  generates the raster signal (R) on the line ( 9 ), a read clock sequence (T) on a line  13  for reading the engraving data (GD) out from the engraving data memory  11  and a feed command S 1  on a line  14  to the engraving carriage drive  7  for controlling the step-by-step advance of the engraving carriage  5 . 
     Before the start of engraving, the position counter (not shown) in the engraving carriage drive must be reset, and the respective axial zero position of the engraving element  3  following the reset must be identified in order, for example, to position the engraving element  3  to the desired, axial start of engraving point proceeding from the identified zero position. 
     For determining the axial position of the engraving element  3 , for example the zero position, the engraving machine inventively comprises an apparatus  15  that is composed of a light barrier  16  as a reference mark attached to the engraving element or the engraving support and of a stationary toothed comb  17  as a shadowing element for the light barrier  16 . 
     For example, the light barrier  16  is designed as bifurcated light barrier and comprises a light source  18  and a light detector  19  lying thereopposite, between which the stationary toothed comb  17  is located. The optical axis of the light source  18  and of the light detector  19  lies in a plane that is perpendicular to the feed direction of the engraving carriage  5  and proceeds through the engraving stylus  4 . For determining the zero position, the engraving element  3  supplies corresponding signals via a multiple line  20  to the controller  12 . The toothed comb  17  secured to the base of the engraving machine is directed in the feed direction of the engraving carriage  5  and has its longitudinal extent extending at least over the maximum axial displacement range of the engraving element  3  along the print cylinder  1 . For example, the toothed comb  17  is designed as a milled metal rail. 
     FIG. 2 shows a side view of the apparatus  15  for determining the axial position of the engraving element  3 . What are shown are the stationary toothed comb  17  in crossection and the engraving element  3  with the bifurcated light barrier  16  comprising the light source  18  and the light detector  19 . 
     FIG. 3 shows a front view of the apparatus  15  for determining the axial position of the engraving element  3 . What are shown are the stationary toothed comb  17  with teeth  21  and tooth gashes  22  in its longitudinal expanse and the engraving element  3  with the light barrier  16  overlapping the toothed comb  17 . Together with the engraving carriage  5 , the engraving element  3  moves in the direction of the longitudinal extent of the toothed comb  17  by means of the spindle  6  and the engraving carriage drive  7 . One of the tooth faces  23  of each and every tooth  21 , for example each negative tooth face  23 , represents the zero position of the engraving element  3  or, respectively, the distance D of the zero position from an axial reference point  24  at the edge of the displacement range as a multiple of the tooth division of the toothed comb  17 , whereby the tooth division corresponds to the distance of two relevant tooth faces  23  from one another. 
     Given displacement of the engraving element  3  from a momentary position along the toothed comb  17  with the engraving carriage drive  7 , the light barrier  16  “seeks” the closest, relevant tooth face  23  as a brightness transition between a tooth  21  and a tooth gash  22 , and the engraving element  3  supplies a stop signal to the controller  12  via the multiple line  20 . As a result of the stop signal, the engraving element  3  is arrested in the zero position at the corresponding face  23  of the toothed comb  17 . Since which relevant tooth face  23  the engraving element  3  has stopped at is not known, the actual distance D of the zero position of the engraving element  3  from the reference point  224  is not yet known. Given an assumed tooth division of 10 cm, for example, the actual distance D can amount to D=10 cm or a multiple thereof, i.e. 20 cm, 30 cm, 40 cm, etc. 
     FIG. 3 shows a first exemplary embodiment of the apparatus  15  wherein the apparatus  15  inventively comprises a distance measuring means  25  that initially measures the approximate distance D′ of the zero position of the engraving element  3  from the reference point  24 . For example, the distance measuring unit  25  is an ultrasound sensor. The zero position of the engraving element  3  is marked, for example, by a reflector  26  for the ultrasound that is attached to the engraving element  3 . The distance measurement can ensue in an axial measuring channel attached to the engraving machine. 
     The approximate measured result is forwarded via the multiple line  20  to the controller  12  wherein the exact distance D between the zero position and the reference point  24  is determined on the basis of the approximate distance D′. When, for example, the approximate distance amounts to D′≈22 cm, the exact, actual distance is D=20 cm, whereby the precision of the actual distance is dependent on the manufacturing precision of the toothed comb  17 . 
     For magazine rotogravure, a plurality of engraving lanes of predetermined lane widths lying side-by-side in axial direction of the print cylinder  1  are engraved with a respectively allocated engraving element  3 . In this case, the engraving elements  3  are positioned and arrested such on the engraving carriage  5  such that they are spaced from one another in conformity with the predetermined lane widths. Alternatively, the engraving elements  3  can also be individually coupled to the rotating spindle  6  with spindle nuts. In this case, the common engraving carriage  5  is again eliminated. 
     In this case, the approximate distances D′ of the individual engraving elements  3  are preferably successively measured, whereby the reflectors  26  required for the measurement are successively pivoted into the measuring channel, for example with electromagnets. 
     FIG. 4 shows a second exemplary embodiment of the apparatus ( 15 ), whereby an identifier in the form of a binary code ( 27 ) of respectively n bits is allocated to each relevant tooth face ( 23 ) of the toothed comb ( 17 ). The actual relevant tooth face at which an engraving element ( 3 ) is stopped in its zero position can be determined on the basis of the “read” binary code ( 27 ),and, thus, the actual distance D between the zero position and the reference point ( 24 ) can be directly identified. 
     In the illustrated exemplary embodiment, the binary codes ( 27 ) are black and white marks located on a steel band ( 28 ) whose longitudinal extent extends in axial direction and that is stationarily secured to the base of the engraving machine. For reading the binary code ( 27 ) on the steel band, the engraving element ( 3 ) (not shown) comprises a sensor ( 29 ) having a plurality of sensor elements corresponding in number to the plurality of bits, said sensor elements interpreting the light reflected by the binary code ( 27 ). 
     The binary code ( 27 ) that has been read is communicated via the multiple line ( 20 ) from the engraving element ( 3 ) to the controller ( 12 ) in FIG. 1 in which the actual distance D between the zero position of the engraving element ( 3 ) and the reference point ( 24 ) is determined on the basis of the binary code ( 27 ) that has been read, that tooth ( 21 ) of the toothed comb ( 17 ) allocated to the binary codes ( 27 ) and the tooth division. 
     2 x  axial positions can be discriminated with an x-bit binary coding.  32  axial positions can be identified with the 5-bit binary coding shown in FIG.  4 . 
     When the zero positions of a plurality of engraving elements ( 3 ) are to be identified, each of the engraving elements ( 3 ) comprises a sensor ( 29 ), and the binary code ( 27 ) read by each sensor ( 29 ) is transmitted via the multiple line ( 20 ) to the controller ( 12 ) in FIG.  1  and is correspondingly interpreted there. 
     Although various minor changes and modifications might be proposed by those skilled in the art, it will be understood that my wish is to include within the claims of the patent warranted hereon all such changes and modifications as reasonably come within my contribution to the art.