Abstract:
A tracking servo control system of an optical disk system produces a tracking control output signal for control of a tracking actuator which is adjusted to enable accurate positioning of a focussed light beam spot of the optical disk system at the center of a selected groove of a disk, for a variety of optical disks having various groove shapes. The output signal of a light receiver responsive to reflected image of the light beam spot focussed on the disk is processed to produce a tracking error signal:Y and a sum signal:X which are processed by a correction circuit for calculating the value: ##EQU1## where Z is a correction factor, the value of which is adjusted, to normalize the calculated value:E, in accordance with the peak detected value of E.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical disk system. More particularly, it relates to an apparatus for adjusting a tracking signal used for a tracking servo control to ensure stable operation of the tracking servo control for a variety of different optical disks. 
     2. Description of the Related Art 
     Both focus servo control and a tracking servo control must be carried out in an optical disk system. The focus servo control adjusts the position of an object lens to focus a beam emitted from a laser diode onto a track of an optical disk, so that any beam displacement at the track caused by an eccentricity in the motion of the optical disk and/or any deformation of the optical disk can be compensated. The tracking servo control adjusts the position of the read/write head to ensure that the head follows the track, to obtain a fine, or precise control of the position of the head. 
     Various different optical disks are used in an optical disk system and the tracking servo control is effected by using a light reflected from a groove formed in the optical disk. However, this reflected light is varied in response to the change of an exposure light used for data read and data write operations, dispersion of the groove shape due to differences between manufacturing lots and differences in manufacturing techniques, and differences in a groove shape, for example, a difference between a U-shaped groove and a V-shaped groove. In a prior art tracking servo control system, compensation for a differing reflection light due to differences between a write data exposure light and a read data exposure light is known, since the tracking servo control system receives a write mode signal and a read mode signal. However, in the prior art tracking servo control system, compensation for the dispersion of the groove shape is not yet practiced, and as a result, a fine or precise position control can not be fully carried out. In addition, compensation for differences in the groove shape is not practiced, and as a result, the optical disk system of the prior art can not use different types of optical disks having various groove shapes. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an optical disks system which adjust for dispersion of a groove shape. 
     Another object of the present invention is to provide an optical disk system usable for a variety of different optical disks having various groove shapes. 
     According to the present invention, there is provided an optical disk system including an optical disk provided with a plurality of grooves for a data read and write operations, and an optical head having an object lens, a light source, a light receiver, an optical system provided among the light source, the object lens, and the light receiver for directing a light emitted from the light source onto the object lens and for introducing a light from the groove to the light receiver, a focus actuator for moving the object lens to focus the light from the object lens on the groove, and a track actuator for moving the object lens to position the light from the object lens at the center of the groove. The optical disk system also includes a focus servo portion for driving the focus actuator, and a tracking servo portion for driving the tracking actuator, and having a circuit for receiving light from the light receiver and generating a tracking error signal:Y on the basis of the received light, a circuit for receiving the light from the light receiver and generating a sum signal:X of the received light, a correction circuit for calculating a value: E=(Y.Z)/X, where Z is a correction factor, and a circuit for phase-adjusting the calculated value. The optical disk system further includes a circuit for detecting a peak of the value calculated at the correction circuit, and a unit for controlling the focus servo portion and the tracking servo portion, and for adjusting a tracking signal. The control unit gives the correction factor:Z to normalize the calculated value:E, so that the tracking servo control is carried out by using the normalized tracking error signal:E. 
     The adjustment of the correction factor:Z for normalizing the value:E calculated at the correction circuit may be carried out by successively increasing or decreasing the correction factor until the peak of the calculated value:E lies within a predetermined range. Alternatively, the adjustment of the correction factor:Z for normalizing the value:E calculated at the correction circuit may be carried out by calculating Z=W/Ws, where W is a read of the peak of the calculated value:E, and Ws is a predetermined normalized amplitude. 
     The adjustment of the tracking signal may be carried out just after the energization of the focus servo control and prior to the energization of the tracking servo control. Alternatively, the adjustment of the tracking signal may be carried out periodically in a normal operation. 
     The tracking servo portion may include a digital-to-analog converter receiving the digital correction factor from the control unit and converting it to an analog correction factor to be supplied to the correction circuit. The tracking error signal generation circuit, the sum reflection light signal generation circuit and the correction circuit may be formed by analog circuits. The peak detection circuit may include an analog peak detection circuit receiving the output from the correction circuit and holding a maximum value, and an analog-to-digital converter converting the analog maximum value held at the peak detection circuit to a digital value and supplying the same to the control unit. The maximum value held at the peak detection circuit can be reset by the control unit. 
     The correction circuit may include two FET&#39;s having the same drain-source resistance and gates which are commonly connected, a first terminal of the first FET receiving the track error signal:Y, a first terminal of the second FET receiving the sum light signal:X, a second operational amplifier connected to a second terminal of the second FET at an inverted input terminal, another non-inverted input terminal thereof being grounded, an output thereof being connected to the common FET gate connected point, a resistor receiving the correction factor:Z at a terminal thereof and another terminal being connected to the connected point of the second terminal of the second FET and the input terminal of the second operational amplifier, a first operational amplifier having the same operational characteristic as the second operational amplifier and connected to another terminal of the first FET at an inverted input terminal, another non-inverted input terminal being grounded, and a feedback resistor connected between the inverted input terminal and an output of the first operational amplifier, and having the same resistance as the resistor. 
     The light receiver may comprise a four-division light receiver. Or, the light receiver may comprise a two-division light receiver. 
     The optical disk may be provided with a plurality of reflection type semi V-shaped grooves, or, with a plurality of transparent type semi V-shaped grooves. Alternatively, the optical disk may be provided with a plurality of reflection type U-shaped grooves, or with a plurality of transparent type U-shaped grooves. 
     The control unit may comprise a microprocessor unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and features of the present invention will be described in more detail with reference to the accompanying drawings, in which: 
     FIG. 1 is a block diagram of an optical disk drive system to which the present invention is applied; 
     FIG. 2a and 2b are graphs explaining the beam focus in FIG. 1; 
     FIGS. 3a to 3c are views explaining a detection of a light reflected from a groove of an optical disk in connection with FIGS. 2b; 
     FIG. 4 is a graph explaining the generation of a tracking error signal in FIG. 1; 
     FIG. 5 is a circuit diagram of a prior art tracking servo control system;. 
     FIGS. 6a and 6b are a sectional view of a V-shaped groove and a graph of a tracking error signal generated by a light reflected from the V-shaped groove; 
     FIGS. 7a and 7b are a sectional view of a U-shaped groove and a graph of a tracking error signal generated by a light from the U-shaped groove; 
     FIG. 8 is a graph representing a relationship between a groove .edge slope and a tracking error signal; 
     FIG. 9 is a circuit diagram of an embodiment of a tracking servo control system in accordance with the present invention; 
     FIG. 10 is, a specific circuit diagram of an adjustment circuit shown in FIG. 9; 
     FIG. 11 is a flow chart explaining the operation of the tracking servo control system shown in FIG. 9; and 
     FIGS. 12a to 12c are graphs explaining the operation of the tracking servo control system shown in FIG. 9. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Before describing the preferred embodiment of the present invention, an optical disk drive system and a prior art tracking servo control system incorporated therein will be described to provide an easy understanding of an optical disk control and to distinguish the differences between the prior art tracking servo control system and the embodiment of the present invention. 
     Referring to FIG. 1, an optical disk drive system includes an optical disk 1 rotated by a motor 1a, an optical head 2, a tracking servo portion 3&#39;, and a focusing servo portion 4. The optical head 2 includes an object lens 20, a tracking actuator 21 having a drive coil, a focus actuator 22 having a drive coil, an optical system 23, a light source 24 having a semiconductor laser diode, a light receiver 25, and a four-division light receiver 26. 
     First, in a seek control mode, the optical head 2 is moved to a desired track on the optical disk 1. Second, in a fine position control mode, the optical head 2 is finely positioned at the desired track. During the fine position control mode, reading of data from or writing of data onto the desired track can be carried out. To carry out the data read or the data write, a light emitted from the light source 24 in the optical head 2 is led to the object lens 20 through the optical system 23 and focused on a groove of the track in the optical disk 1. A beam spot focused by the object lens 20 and incident on the groove is reflected, and the reflected beam spot is received at the optical system 23 through the object lens 20. The beam spot received at the optical system 23 is supplied to the light receiver 25 to generate a reproduction signal RF. The reproduction signal is used for a data read and level control operations. Also, the beam spot received at the optical system 23 is supplied to the four-division light receiver 26 which output four electrical signals corresponding to the received light signals and which are used for generating a tracking error signal TES. The generation of the tracking error signal TES will be described later. 
     The optical disk 1 is provided with a large amount of grooves, forming tracks or pits, in a radial direction. A pitch between adjacent grooves is usually 1.6 μm. As the pitch is very small, a small eccentricity in the rotation of the disk may result in a large track displacement. In addition, the optical disk 1 is not perfectly even in the plane thereof, and this results in an undulating motion of the optical disk 1, which further displace the focus of the beam spot incident at the groove. Note, the beam spot must be focused on the groove within a range of 1 μm. 
     The focus servo portion 4 receives the tracking error signal TES from the four-division light receiver 26 and adjusts the focus position of the object lens 20 by moving the object lens 20 in a vertical direction, by means of the focus actuator 22 connected to the object lens 22. The tracking servo portion 3&#39; also receives the tracking error signal TES from the four-division light receiver 26 and adjusts the track position of the object lens 20 by moving the object lens 20 in a horizontal direction, through the tracking actuator 21, in response to the tracking error signal TES. 
     The operation of the tracking servo control will now be described in detail. The tracking servo control uses the diffraction of the beam spot BS by the groove 10 of the optical disk 1, as shown in FIG. 2a. Namely, the reflection light quantity distribution at the four-division light receiver 26 is varied in response to the position in the groove 10 of the beam spot BS, and thus a change of the light diffraction therefrom occurs as shown in FIG. 2b. The tracking error signal TES is obtained by using the change of the light diffraction. The four-division light receiver 26 consists of four light sensors a, b, c, and d, and outputs four electrical signals La, Lb, Lc, and Ld corresponding to the received light signals, respectively. The tracking error signal TES is defined by the following formula: 
     
         TES=(La+Ld)-(Lb+Lc)                                        (1) 
    
     When the center of the beam spot BS is at a position P 1  shown in FIG. 2b, two sensors a and d receive the reflected light as shown in FIG. 3a. As a result, the tracking error signal TES is a positive maximum value as shown in FIG. 4, in accordance with the formula (1). When the center of the beam spot BS is at a position P 0  as shown in FIG. 2b, the tracking error signal TES is 0 (zero) as shown in FIG. 4. Conversely, when the center of the beam spot BS is at a position P 2  as shown in FIG. 2b, the tracking error signal TES is a negative minimum value as shown in FIG. 4. Accordingly, by using the tracking error signal TES, the tracking servo portion 3&#39; drives the object lens 20 by using the tracking actuator 21 to place the center of the beam spot BS at the center of the groove 10 of the optical disk 1. 
     Referring to FIG. 5, the prior art tracking servo portion 3&#39; will be described. The track servo portion 3&#39; includes input resistors R1 to R8, bias resistors Ra and Rb, an operational amplifier 30 functioning as a subtractor, an operational amplifier 31 functioning as an adder, feedback resistors R11 and R12, a divider 32, a phase compensator 33, and a power amplifier 34. The operational amplifier 30 receives the four light sensed signals La, Lb, Lc, and Ld from the four-division light receiver 26 and outputs the tracking error signal TES, shown as output signal Y in FIG. 5 in accordance with the formula (1). The operational amplifier 31 receives the four light sensed signals La, Lb, Lc, and Ld and outputs a sum of reflected light signals in accordance with the following formula: 
     
         X=La+Lb+Lc+Ld                                              (2) 
    
     The divider 32 divides the tracking error signal Y by the sum reflection light signal X and outputs an automatic gain controlled value E. Namely, the automatic gain control or the adjustment of the amplitude of the track error signal TES (or Y), enables an even control of the difference in intensity of the exposed beam spot when the data read and the data write operations are carried out and a variation in the reflection of the beam spot exists. The phase compensator 33 compensates for the phase of the automatic gain controlled signal E&#39;. The power amplifier 34 amplifies the compensated and automatic gain controlled signal to an appropriate level for driving the tracking actuator 21, and the object lens 20 is moved to position the center of the beam spot BS therefrom at the center of the groove 10. 
     The optical disks used in the optical disk system are different: Namely, grooves in one optical disk type have a semi V-shape with a slope θ (degree) as shown in FIG. 6a, and grooves in another optical disk type have a U-shape with a smooth slope as shown in FIG. 7a. FIG. 6b shows a wave form of a tracking error signal TES when the optical disk having the groove 10a as shown in FIG. 6a is used. FIG. 7b shows a wave form of a tracking error signal TES when the optical disk having the groove 10b as shown in FIG. 7a is used. As can be seen by comparing FIGS. 6b and 7b, the amplitudes Wa and Wb and the respective frequencies are different. In addition, the amplitudes are varied by the respective slopes of the semi V-shaped groove or the U-shaped groove. FIG. 8 shows a change of amplitude caused by a change of a groove edge slope. The ordinate in FIG. 8 shows a normalized amplitude of a tracking error signal. Furthermore, the amplitude is varied in response to the depth and the width of a groove. Optical disks are subjected to a dispersion of the groove shape due to a difference in manufacture, and/or variations in production conditions. This dispersion varies the amplitude and the frequency of the tracking error signal. 
     An optical disk system should be able to handle a variety of different optical disks. However, the sum of the reflected light obtained at the operational amplifier 31 shown in FIG. 5 does not show such differences. Accordingly, the tracking servo control system can not adjust for the difference in amplitude of a tracking error signal, in spite of the execution of the automatic gain control. As a result, the servo gain may be varied and an out of servo scope and/or an abnormal oscillation of an optical head may occur. 
     Referring to FIG. 9, an embodiment of the tracking servo control system of the invention will now be described. 
     The tracking servo control system comprises a microprocessor unit (MPU) 6 such as an INTEL-8031, a tracking servo portion 3, a peak detection portion 5, and as in FIG. 1, the four-division light receiver 26, and the tracking actuator 21. The MPU 6 is used for focus servo control, tracking servo control, and read/write operations. The tracking servo portion 3 is provided with a digital-to-analog converter (DAC) 36 in addition to the tracking servo portion 3&#39; shown in FIG. 5. The divider 32 shown in FIG. 3 is replaced by a correction circuit 35. Other circuits of the track servo portion 3 shown in FIG. 9 are the same as those of the tracking servo portion 3&#39; shown in FIG. 5. The peak detection portion 5 comprises a peak detector 50 and an analog-to-digital converter (ADC) 51. The peak detector 50 detects the peak of the output E of the correction circuit 35, and the ADC 51 converts the peak analog value from the ADC 51 into a peak digital value. 
     The correction circuit 35 calculates an amplitude normalized and corrected tracking error signal E in accordance with the following formula: ##EQU2## where, 
     X denotes a sum of all reflected light obtained at the operational amplifier 31 in accordance with the formula (2), 
     Y denotes a tracking error signal TES, obtained at the operational amplifier 30 in accordance with the formula (1), and 
     Z denotes a correction factor supplied from the MPU 6 through the DAC 36. 
     FIG. 10 shows a specific circuit diagram of the correction circuit 35. The correction circuit 35 includes input resistors 351, 361, and 369, bias resistors 352 and 362, FET&#39;s 354 and 364, bypass resistors 353 and 363, series resistors 357 and 367, operational amplifiers 355 and 365, and a diode 366. The correction circuit 35 uses the characteristic that a resistance R DS  between a drain and a source of an FET is varied in response to the applied gate voltage. The operational amplifier 365 operates to maintain the applied input voltage which is applied to the inverted input terminal of the amplifier 365 at zero volt by negative feedback of the output therefrom to a gate of the FET 364. The FET&#39;s 354 and 364 are identical, and thus both FET&#39;s have the same source-drain resistance R DS  when the same gate voltage is supplied thereto. When an input Z of -1 V is supplied to the resistor 369 having a resistance R1 and an input X of +1 V is supplied to the drain of the FET 364 through the resistor 361, the operational amplifier 365 controls the gate of the FET 364 to maintain the input voltage applied to the inverted input terminal thereof at zero volt; namely, operational amplifier 365 functions to make the drain-source resistance R DS  FET 364 equal to the resistance R1 of the resistor 369. When an input Y of +1 V is supplied to the drain of the FET 354 and the resistance R2 of the resistor 356 is equal to the resistance R1, the drain-source resistance R DS  of the FET 354 becomes equal to the resistance R2. As the drain-source resistance R DS  of the FET 354 is equal to the resistance R2, the gain of the operational amplifier 355 becomes -1, thereby to produce an output E of -1 V. When the input X of +0.5 V is supplied to the FET 364, the drain-source resistance R DS  of each of the FET&#39;s 354 and 364 becomes one-half of the resistance R1 or R2, and as a result, the gain of the operational amplifier 355 becomes -2 thereby to produce an output E of -2 V. When the input X of +1 V, the input Z of -0.5 V and the input Y of +1 V are supplied, the drain-source resistances R DS  of each are double the resistance R1 or R2. Accordingly, the gain of the operational amplified 355 becomes 1/2 thereby to produce an output E of -0.5 V. 
     The circuit parameters of the correction circuit 35 are as follows: 
     FETs 354 and 364: 2N4393 
     Diode 366: IS1588 
     Operational amplifiers 356 and 365: TL082 (Dual) 
     Input voltage of Y: -2 to +2 V 
     Input voltage of X: 0 to +2 V 
     Input voltage of Z: -10 to 0 V 
     Resistance R1 and R2: 68 kΩ 
     Resistance of resistors 351 and 361: 910 Ω 
     Resistance of resistors 352 and 362: 100 Ω 
     Resistance of resistors 353, 363, 357, and 367: 600 kΩ 
     Referring to FIG. 11, the operation of the tracking servo control system shown in FIG. 9 will be described. The tracking signal adjustment is carried out upon every start up or restart of the optical disk system or upon every change of an optical disk, and prior to the start of a normal tracking servo control operation. 
     When the power is made ON or a restart is initiated, the MPU 6 starts operation. 
     Steps 001 and 002 (S001 and S002 in FIG. 11) 
     When a new optical disk is loaded, the MPU 6 sets a busy flag to inhibit the issue of a command from a host controller (not shown). 
     Steps 003 and 004 (S003 and S004) 
     The MPU 6 energizes the laser diode in the light source 24 shown in FIG. 1, thereby to emit light from the light source 24 (S003). The MPU 6 also drives the focus servo control system to focus the beam spot on a track on the optical disk by moving the object lens 20 by means of the focus actuator 22 shown in FIG. 1 (S004). The four-division light receiver 26 receives the reflected light beam. 
     Step 005 (S005) 
     The MPU 6 initializes a correction factor Zd. An initial value of the correction factor Zd may be zero. 
     Steps 006 to 008 (S006 to S008) 
     The MPU 6 outputs the correction factor Zd in digital form to the DAC 36 (S006). The DAC 36 outputs an analog correction factor Z corresponding to the digital correction factor Zd to the correction circuit 35. The MPU 6 reads the peak of E (i.e., the automatic gain controlled and corrected value E=Y·Z/X,  where Y.Z is the corrected version of (i.e., Y.Z=CTES) and thus corresponds to the tracking error signal Y (i.e., Y=TES) through the peak detection circuit 50 and the ADC 51 (S007). After the peak read operations, the MPU 6 outputs a reset pulse RESET to the peak detection circuit 50 to clear the held peak value. 
     FIGS. 12a and 12b show tracking error signals TES1 (Y) and TES2 (Y) of different groove shapes. In FIGS. 12a and 12b, intersections of the waveforms and the horizontal axis indicate centers of the grooves. Frequencies of the tracking error signals may be approximately 1 ms, and accordingly, the above peak read and reset can be carried out within 1 ms. 
     The tracking error signal TES obtained at the operational amplifier 30 is varied in response to a variation of the shape of the grooves, as shown in FIGS. 12a and 12b. The variation of the tracking error signal TES is corrected at the correction circuit 35 which produces the corrected tracking error signal CTES having a same amplitude Ws regardless of a variation in the amplitudes, as shown by W 1  in FIG. 12a and W 2  in FIG. 12b, by suitably adjusting the correction factor Zd. 
     The MPU 6 checks whether or not the read peak has reached or exceeded a predetermined peak to peak voltage aV P-P  , for example aV P-P  =1.0 V (S008). 
     Steps 008 to 010 (S008 to S010) 
     When the read peak has not reached or exceeded the predetermined peak to peak voltage aV P-P , the MPU 6 increases the correction factor Zd by one (S009). The MPU 6 changes the control to step 006 (S006) and the operation of steps 006 to 009 (S006 to S010) is continued until the read peak reaches or exceeds the predetermined peak to peak voltage aV P-P , unless the correction factor Zd exceeds a maximum value Zmax (S010). If the correction factor Zd exceeds the maximum value Zmax, the MPU 6 terminates the tracking signal adjustment. 
     Steps 008, 011 and 012 (S008, S011 and S012) 
     When the read peak reaches or exceeds the predetermined peak to peak voltage aV.sub. AV P-P , the MPU 6 holds the correction factor Zd, and as a result, the DAC 36 continues supplying the analog output Z, corresponding to the held correction factor Zd, to the correction circuit 35. The amplitude normalized and corrected circuit 35 then calculates the correction tracking error signal E in accordance with the equation (3) by using the correction factor Z from the DAC 36. The MPU 6 drives the tracking servo control system (S011). Thereafter, the MPU 6 resets the busy flag to permit issuance of a command from the host controller (S012), and the normal operation of the optical disk system can be carried out. 
     When the MPU 6 receives a seek command from the host controller, first a seek control is carried out, and then the tracking servo control is carried by using the normalized value E of the correction tracking error signal CTES. The amplitude of the correction tracking error signal is normalized regardless of the shape of the grooves of the optical disk, and accordingly, the tracking servo control can be achieved safely and accurately regardless of the type of optical disk. 
     In FIG. 11, the initial value of the Zd can be set to a predetermined value greater than zero to speed-up the track signal adjustment. 
     Also, the initial value of the Zd can be set to a predetermined value equal to a previously obtained correction factor. Also, the correction factor Zd can be increased or decreased by one in response to the value of the read peak. 
     When the final correction amplitude Ws shown in FIG. 12c is constant and is already known, the correction factor Zd can be formed by the following formula: ##EQU3## where, W denotes the value of the read peak. 
     The above tracking signal adjustment can be carried out not only at the start-up time, the restart time, and the loading of the optical disk, as described above, but also periodically and/or upon a change from a read to a write operation and vice versa. 
     In the above embodiment, a reflection type optical disk as shown in FIGS. 6a and 7a has been discussed, but the present invention can be also applied to a transparent type optical disk by installing a light receiving system above the optical disk 1 shown in FIG. 1. 
     The four-division light receiver 26 as shown in FIGS. 3a to 3c is described. However, the present invention can be achieved by using any other light receiver, for generating source signals used for a tracking error signal TES and a sum of the source signals, such as a two-division light receiver (not shown). 
     Also, the functions of the light receiver 25 for generating the RF signal and the light receiver 26 as shown in FIG. 1 can be in a single light receiver used in common for both functions. 
     In FIG. 1, the track actuator 21 can move both the object lens 20 and the optical system 23, instead of just providing the horizontal movement of the object lens 20. 
     Many widely different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not restricted to the specific embodiments described above, except as defined in the appended claims.