Centering device for the manufacture of a center hole in disks
BACKGROUND OF THE INVENTION 
     The invention relates to a centering device for the manufacture of a center hole in disks, particularly metallized video disks, with, for example, spiral information tracks and an information-free center. A measuring head is employed for the emission of a light beam onto the x-y disk plane and for the reception of the radiation picked up by a detector upon exploitation of the light diffraction at the information tracks, whereby the measuring head and the disk to be centered can be displaced relative to one another in parallel planes. 
     Optically scanned video disks which rotate with a speed of 1500 or, respectively, 1800 rpm exhibit a center hole into which a shaft for rotation is inserted during the play-back operation. If the center hole is not precisely situated in the center of the disk, then an eccentricity of the informational tracks with respect to the axis of rotation would be effected. Slight eccentricity amounts which are smaller than 50 μm can be compensated by the play-back device. Greater eccentricities can exceed the correction capability of the play-back device and, accordingly, can have a negative influence on the play-back. 
     Due to the high demands made with respect to a slight eccentricity, pressing the center hole in the same work step as the manufacture and duplication of the disks is not possible. The center hole, therefore, must be produced in an additional work step. To this end, a device for centering the disks with high precision is required. 
     A method and a device for localizing the center of a circular video disk is known from German AS No. 25 38 383, corresponding to U.S. Pat. No. 3,915,576, incorporated herein by reference. There, a light source for the emission of a light beam onto the surface of the disk to be centered and a light receiver for the acceptance of the light beam diffracted by the informational tracks on the disk are provided. The light receiver is disposed in the optical axis of the light source. The video disk is moved in the disc plane situated perpendicular to said optical axis until the light receiver indicates a diffraction maximum. The localization of the center can be determined from said criterion in such manner that the center of the informational tracks is precisely disposed on the optical axis at a diffraction maximum in the light receiver. 
     However, this known method fails when the disk to be centered exhibits edge defects, since the measurement of the diffraction maximum is falsified in this case due to the light beam striking the edge area of the disk. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to eliminate the above disadvantage and to specify a further solution for centering disks in which the diffraction grating property of the informational tracks is exploited. 
     This object is inventively achieved in that the light beam emitted by the measuring head is focused onto the surface of the disk in the perpendicular beam direction. The disk is rotatably seated in order to be able to execute a rotational movement in the x-y disc plane. The measuring head executes a relative motion in a radial direction with respect to the disk in such manner that the light beam is precisely situated on the boundary between the informational portion and the portion free of information during a full disk rotation (transition or boundary criterion). This criterion is evaluated for the identification of the center. 
     It is further advantageous that an electronic control circuit driven by the detector with a first control signal is provided. During a disk revolution, the control circuit emits a second control signal derived from the first control signal across a measuring transducer. The second control signal is emitted to a measuring head drive for moving said measuring head. After the disk revolution, the second control signal is evaluated across a measuring transducer for the follow-up of the translational motion of the disk in x-y disk plane. 
     The advantage achieved with the invention particularly consists of the high measuring precision for identifying the center, namely independently of imprecisions of the edge of the disk. It is thereby guaranteed that the center hole is always punched out precisely at the location provided for this purpose. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block diagram of a device for centering according to the invention; 
     FIG. 2 is a graph showing the time relationship of the radiation received by the detector given incidence of the light beam on the information-free part of the disk; 
     FIG. 3 is a graph showing the time relationship of the radiation received by the detector given incidence of the light beam on the boundary between the information part and the information-free part of the disk; 
     FIG. 4 is a graph showing the time relationship of the radiation received by the detector given incidence of the light beam on the information part of the disk; 
     FIG. 5 is a block diagram of a circuit for generating the control signal S2 in the measuring transducer MW; 
     FIG. 6 is a block diagram of the measuring transducer MU; 
     FIG. 7 is a graph showing the manner of functioning of the angle indicator or generator WG; 
     FIG. 8 is a block diagram of the control means KE; and 
     FIG. 9 is a detailed illustration of the measuring head drive MKA, of the rotational drive ROT-An, and of the drives X-An and Y-An. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The arrangement illustrated in FIG. 1 shows a light source LQ which emits a light beam, preferably a laser beam L. Via a deflection mirror US, the laser beam L arrives at the measuring head MK which exhibits a tilting mirror KS and a lens system in the optical path of the laser beam L. The lens system consists of a focusing lens FL in order to focus the laser beam L in a vertical beam direction onto the surface of the disk P1 to be centered, and further consists of a cylindrical lens ZL by means of which the laser beam L is focused in the shape of a line. The line focusing is executed in order to make the device insensitive to disruptions due to dust and surface damage on the disk. The tilting mirror KS, which is provided in order to facilitate the evaluation, oscillates with a relatively low frequency, so that the light spot is moved back and forth by approximately 200 to 300 μm in a radial direction on the disk P1. Furthermore, the measuring head MK has a detector which, for the purpose of optimum reception of the radiation, is disposed in the direction of a diffraction order of the information tracks functioning as a diffraction grid. 
     FIGS. 2, 3 and 4 show the time relationship of the radiation received by the detector D as a function of the point of incidence of the laser beam L on the disk P1 to be centered. 
     When picked up on the information-free part IFT, the laser beam L emitted by the measuring head MK is not diffracted, so that the detector D can likewise not receive any radiation in this case, and thus the control signal S1 for driving the control circuit ST exhibits the value 0 (FIG. 2). 
     In contrast thereto, the laser beam L is diffracted when it strikes the information part IT of a disk P1 provided with information tracks. In this case, the control signal S1 assumes a constant magnitude (FIG. 4). FIG. 3 shows the time relationship of the control signal S1 when the laser beam L is precisely situated on the boundary between the information part IT and the information-free part (IFT). 
     At the beginning of the measuring operation, the measuring head MK, and thus the laser beam L focused on a disk P1, proceeding from the information-free part IFT, is moved in a radial direction with respect to the disk P1 until the beginning of the information part IT. When the beginning of the information part IT has been found, then the motion of the measuring head MK is controlled in such manner that the focused laser beam L is situated precisely on the boundary between the information part IT and the information-free part IFT during a full disk revolution. An electronic control circuit ST driven by the detector D with the signal S1 is provided for control of the motion of the measuring head MK. The signal S1 at the input E1 of the electronic control circuit ST is converted into the signal S2 across the measuring transducer MW, said signal S2 arriving at the measuring head drive MKA via the output A1 of the electronic control circuit ST, so that the focused laser beam L is held at the boundary between the information part IT and the information-free part IFT during a full revolution. 
     FIG. 5 shows a detailed block diagram for generating the control signal S2 in the measuring transducer MW for motion of the measuring head MK. For reasons of greater clarity, the precise generation of the control signal S2 is not shown in FIG. 1. As can be derived from FIG. 5, the control signal S2 contains the setting pulse P and the direction signal R for the measuring head drive MKA equipped with a stepping motor with which an incremental feed is generated. In order to derive the directional signal R, the tilting mirror KS is driven via the amplifier VKS with a sine signal arising at the output of the filter F. The resonant frequency of the filter F is identical to the clock frequency f o  =500 Hz emitted by the oscillator Os. Via the optical path, the laser beam L diffracted at the surface of the disk P1 arrives at the detector D whose output signal S1 drives the comparator K via the amplifier V. The switching threshold of the comparator K is a function f of the maximum value of the control signal S1, whereby an identification of the boundary between the information-free part IFT and the information part IT which is independent of disruptive magnitudes such as fluctuations of intensity of the laser or contamination of the disk, is guaranteed. 
     The signal supplied by the comparator is connected through in proper phase as the directional signal R by means of the oscillator clock in the flip-flop FF delayed in the time-delay unit VZ1. 
     After the directional signal R has been connected through, the delayed setting pulse P is generated with a second time delay unit VZ2. 
     The control signals S2 emitted to the measuring head drive MKA during a full disk revolution are converted, after the disk revolution, into control commands in a measuring transducer MU for the follow-up of the translational motion of the disk P1 in the x-y disk plane which is necessary for the centering. Thereby, the measuring transducer MU is a unit consisting of two forward/backward counters VRZ1 or, respectively, VRZ2, as can be seen from FIG. 6. The control signal S2, with the respective directional signal R and the setting pulse P, arrives at the input side of both counters VRZ1 and VRZ2. The counter VRZ1 identifies the eccentricity components in the x-direction. The counter VRZ2, accordingly, identifies the eccentricity components in the y-direction. For this purpose, the two counters VRZ1 and VRZ2 are driven by the angle indicator or generator WG via the terminals E3, E4 as a function of the rotational angle β of the disk P1. 
     The manner of functioning of the angle indicator or generator WG is illustrated in FIG. 7. The identification of the eccentricity components in the x-direction ensues in the rotation angle range 0≦β≦π of the disk P1. The identification of the eccentricity components in the y-direction is offset by the angle π/2 and ensues in the rotational angle range π/2≦β≦(3/2)·π (shaded areas). Thus, the entire identification of the eccentricity values is terminated when the rotational angle β=(3/2)·π is reached. The actual centering operation, i.e., the follow-up of the translational motion of the disk P1 in the x-y disk plane then ensues in the rotational angle range (3/2)·π≦β≦2π. For this purpose, the signal x is emitted by the counter VRZ1 of the measuring transducer MU via the output A2 of the electronic control circuit ST to the drive x-An for motion of the disk in the x-direction and the signal y is emitted via the output A3 to the drive y-An for motion in the y-direction. 
     The rotary drive ROT-An for the disk P1 controlled by the motor M exhibits a switch S for setting the direction of rotation. In switch position a, the disk P1 executes the full revolution, for example, in a clockwise direction, required given the setting of the focused laser beam at the boundary between the information part IT and the information-free part IFT. In switch position b, the disk P1 is returned to its initial position in the reverse direction of rotation in order to check the centering. The signal S2 measured during said return is supplied to a control means KE which releases the punching-out of the central hole M given a position of the center Z within adjustable tolerance limits. 
     FIG. 8 shows the block diagram of the control means KE. The signal S2 measured during the return arrives at the forward/backward counter VRZ3, which identifies the motion of the measuring head MK in a radial direction Δr across the plate P1 which has already been centered. The counter reading Δr of the counter VRZ3 which corresponds to the motion of the measuring head MK is continuously compared to a prescribed maximum value Δrm by means of a comparator KO, the tolerance limits being determined by means of said maximum value Δrm. A memory MO follows the comparator KO. The memory MO is always set when, during the return of the disk P1, a deviation from the ideal orbit is identified which is greater than a permitted maximum. The signal in the memory MO arrives via the inverter I to the input side of the AND gate G at whose second input the changeover pulse of the switch S arriving via the input E2 always lies during the return. Thus, no signal arises at the output of the AND gate G, whereby the operation &#34;release punch-out&#34; FS is blocked. If, in contrast thereto, the deviation Δr determined during the return of the disk P1 is less than that which is maximally admissible, then the memory MO is not set. A signal which triggers the punch-out operation arises at the output of the control means KE across the inverter I and the AND gate G. The punching device itself is not illustrated in the drawing. 
     Circuit blocks MKA, ROT-AN, x-An and Y-An are illustrated in greater detail in FIG. 9. The disk P1 on a vacuum suction lining 1 is turned around the cutting tool 5 by means of the stepping motor 2, pinion 3 and the turntable 4. The vacuum suction lining 1 and the turn table 4 can be mutually displaced in the coordinate direction, but cannot be rotated with respect to one another. The measuring head MK is moved without play against a pressure spring 9 by means of the stepping motor 6, a claw coupling 11 and a threaded spindle 12. Motion in the other coordinate direction occurs in the same manner offset by 90° (not illustrated). 
     Although various minor modifications may be suggested by those versed in the art, it should be understood that we wish to embody within the scope of the patent warranted hereon, all such embodiments as reasonably and properly come within the scope of my contribution to the art.