Tracking control apparatus

Abstract: A tracking control apparatus comprises an A/D converter (20), an edge detection circuit (25), a latch circuit (22) and a ROM (23) to change a tracking error signal (a) having a sinusoidal characteristic to a relative position of a light beam and a track on a disk to have a linear characteristic to the relative position. Stable tracking control free from off-tracking due to disturbance is attained even when amplitudes of the tracking error signal (a) and a tracking sum signal are not equal.

Description: BACKGROUND OF THE INVENTION An optical disk or a magneto-optical disk which uses an optical or magneto-optical signal recording system has recently been developed, and an optical recording and reproducing apparatus for recording and reproducing information to and from such a disk has also been developed. In such an apparatus, a tracking actuator is driven in accordance with a polarity and a level of a tracking error signal produced by an optical pickup to attain tracking control so that a track center is always followed without regard to an eccentricity of the disk. On the other hand, in order to improve a data transfer speed from the disk to a processor, the rotation speed of the disk is increased and a relative speed between a light beam and the track has been increased in spite of the reduction of the eccentricity of the disk. If the relative speed is lower than a pull-in speed determined by a tracking control band, the light beam is pulled into a target track but if it is higher than the pull-in speed, the pull-in is unstable and the light beam may be pulled into a different track. As a result, in an access operation which requires a jump over tracks, the light beam may be pulled into a track which is far distant from the target track and reaccess operation is needed. Thus, the access time is long. JP-A-63.181179 and GB-A-2 200 227 disclose a solution to the above problems. An arithmetic operation is carried out by dividing the tracking error signal (a) by the tracking sum signal, and the results for different input values are stored in a look-up table. In the disclosed method, the tracking is properly attained when amplitudes of a tracking error signal and a tracking sum signal are equal, but the waveform amplitudes are not equal in a disk in which track groove depth varies. Thus, the disclosed method, using the arithmetic operation mentioned, is not applicable to such a disk. In the disclosed method, an allowable range for the pull-in to the target track is |X| < P/2 when P is a track pitch and X is a deviation of the light beam in a radial direction, measured from the target track. Thus, the pull-in to the target track is not attained when |X| > P/2. (|X| represents the absolute value of X). SUMMARY OF THE INVENTION It is an object of the present invention to provide a tracking control apparatus which attains stable pull-in to a target track even if waveform amplitudes of a tracking error signal and a tracking sum signal are not equal or even if a light beam is positioned in an area of |X| > P/2, and which attain an access operation with a short access time. Instead of dividing the tracking error signal by the tracking sum signal, an equation is solved using level and amplitude of the tracking error signal. The object of the invention is achieved in a tracking control apparatus including an information recording and reproducing disk medium having tracks for recording and reproducing information by a light beam, by providing: signal generation means for generating a signal proportional to a displacement between the light beam and a target track, from a tracking error signal of sinusoidal waveform indicating a positioning error between the light beam and the target track and an on-track signal with two values indicating whether the light beam is on the target track or off the target track, in which the signal output from said signal generation means is a control signal used in tracking control means such that the light beam traces the target track and is moved transversely to the tracks of said information recording and reproducing medium, wherein said signal generation means includes: analog/digital conversion means for digitizing the tracking error signal indicating the positioning error between the light beam and the target track; amplitude detection means for detecting the amplitude of the tracking error signal indicating the positioning error between the light beam and the target track; memory means for receiving an output signal from said analog/digital conversion means, an output signal from said amplitude detection means, and the on-track signal; the memory means storing the result of the formula V'x = P 2π sin -1 Va A in which Va is the level and A the amplitude of the tracking error signal, P is the pitch and V'x is a correcting signal that corresponds to the particular values of the input signals, and outputting the result V'x corresponding to the input signals, digital/analog conversion means for receiving said output from said memory means and converting it into an analog signal (i) used in the tracking control. The result of the calculation of equation (1) is stored in a ROM. This facilitates substantially the calculation procedures and the further processing. The invention is, therefore, operative if the amplitudes of the tracking error signal and of the tracking sum signal are different, as is the case with continuous groove servo systems. It can be of advantage if the signal generation device comprises a counter device for counting up and down under control of the outputs of the tracking error detection device and the on-track detection device: a data processing device for processing the output of the memory device in accordance with the output of the counter device; and a digital/analog conversion device for converting the output of the data processing device to an analog signal. The data processing device detects the displacement between the light beam and the target track and the direction thereof by the count of the counter means and supplies the output of the memory device. when the light beam is on the target track and supplies the output of the counter device, when the light beam is off the target track. to the digital/analog conversion device through the data processing device. The counter device may include an external input setting device for externally setting a count. The present invention can change the tracking error signal having the sinusoidal characteristic to the relative position of the light beam and the track, to have the linear characteristic to the relative position by the signal generation device even when the waveform amplitudes of the tracking error signal and the tracking sum signal are not equal and the amplitude ratio is not constant. Further, the displacement from the target track and the direction thereof are detected by the counter device and the tracking error signal is corrected in accordance with the displacement and the direction. As result, the signal level is zero only at the target track and there is only one pull-in stable point. Accordingly, a force toward the target track acts on the actuator which drives the light beam and the light beam is positively pulled into the target. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a block diagram of a first embodiment of the present invention, Fig. 2 shows waveforms in the first embodiment, Fig. 3 shows a block diagram of a second embodiment of the present invention, Fig. 4 shows waveforms in the second embodiment, Fig. 5 shows a block diagram of a third embodiment of the present invention, Fig. 6 shows waveforms in the third embodiment, Fig. 7 illustrates a track jump operation in the third embodiment, Fig. 8 shows a block diagram of a fourth embodiment of the present invention, and Fig. 9 shows waveforms in the fourth embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In Fig. 1, a tracking error signal (a) is applied to an A/D converter 20 and converted to digital signals, that is, a polarity signal (b) representing a polarity of the tracking error signal (a) and a signal (j) representing an amplitude. Since an output of the A/D converter 20 is usually in an offset binary form, bits "1" and "0" of the signal (j) representing the amplitude are inverted depending on the polarity of the tracking error signal (a). For example, assuming that the amplitude signal (j) at +1V of the tracking error signal (a) is (0100010), the amplitude signal (j) at -1V of the tracking error signal (a) is (1011101). An absolute value circuit 21 checks the polarity signal (b) and if the tracking error signal (a) is negative, the bits "1" and "0" of the amplitude signal (j) are inverted and a signal (j') representing an absolute value of the amplitude is supplied to a latch circuit 22. On the other hand, an edge detector 25 detects a leading edge and a trailing edge of an on-track signal (e) which is derived by reshaping a tracking sum signal (d) shown in Fig. 2 to produce an edge detection signal (k). Since a level of the on-track signal (e) changes when the amplitude of the tracking error signal (a) is maximum, an amplitude A of the tracking error signal (a) is detected by latching the output (j') of the absolute value circuit 21 by the latch circuit 22 of Fig. 1 at the timing of the edge detection signal (k). An operation result of a formula (1) for the amplitude A of the tracking error signal (a) and a level va of the tracking error signal (a) when the on-track signal (e) is High, that is, where |X| < (P/4) is stored in a ROM 23, and a result of correction as shown by broken lines and arrow in (i) of Fig. 2 when the on-track signal is Low, that is, where (P/4) ≤ |X| < (P/2) is stored in the ROM 23. V'x = P 2π sin -1 Va A The data stored in the ROM 23 is supplied to a D/A converter 24 in accordance with the output (b) of the A/D converter 20, the amplitude signal (j), the output of the latch circuit 22 and the on-track signal (e). Thus, the D/A converter 24 produces a signal (i) which has a linear characteristic to a relative position of a light beam and a track. A second embodiment of the present invention is shown in Fig. 3, and waveforms of the embodiment of Fig. 3 is shown in Fig. 4. The like elements in Figs. 3 and 4 to those in Figs. 1 and 2 are designated by the like numerals or characters, and the explanation thereof is omitted. In the present embodiment, the amplitude of the tracking error signal (a) is asymmetric for positive and negative as a tracking error signal (a) shown in Fig. 4. The amplitude A of the tracking error signal (a) is detected separately for the positive and negative error signals. When the error signal (a) is positive, a positive amplitude A+ is used, and when the error signal (a) is negative, a negative amplitude A- is used. In Fig. 3, the output (k) of the edge detection circuit 25 is ANDed with the polarity signal (b) by the AND gate 31, which produces a signal (r) representing a positive maximum position of the tracking error signal (a). The latch circuit 22a latches the output (j') of the absolute value circuit 21 by the output (r) of the AND gate 31 to detect the positive amplitude A+ of the tracking error signal (a). On the other hand, the polarity signal (b) is applied to an AND gate 32 through an invertor 33 to AND the polarity signal (b) and the output (k) of the edge detector 25 so that the AND gate 32 produces a signal (s) representing a negative maximum position of the tracking error signal (a). The latch circuit 22b latches the output (j') of the absolute value circuit 21 by the output (s) of the AND gate 32 to detect the negative amplitude A- of the tracking error signal (a). The outputs of the latch circuits 22a and 22b are supplied to a multiplexor 34 which selectively outputs a signal representing the positive amplitude A+ or the negative amplitude A- of the tracking error signal in accordance with the polarity signal (b). A result of operation of the formula (1) for the amplitude A of the tracking error signal (a) and the level Va of the tracking error signal (a) when the tracking signal (e) is High, that is, where |X| < (P/4) is stored in the ROM 23, and a result of correction shown by broken lines and arrows in (i') of Fig. 2 when the on-track signal is Low, that is, where (P/4) ≤ |X| < (P/2) is stored in the ROM 23. The data stored in the ROM 23 is supplied to the D/A converter 24 in accordance with the output (b) of the A/D converter 20, the amplitude signal (j), the output of the multiplexor 34 and the on-track signal (e). The D/A converter 24 produces the signal (i) having a substantially linear characteristic to the relative position of the light beam and the track. In the present embodiment, since the amplitudes of the tracking error signal (a) are asymmetric for the positive and the negative and they are not sinusoidal, the signal (i) produced by the D/A converter 24 slightly deviates from the linear characteristic to the relative position of the light beam and the track but a signal having symmetric positive and negative levels is produced. Fig. 5 shows a third embodiment of the present invention, and Fig. 6 shows waveforms of the embodiment of Fig. 5. In Figs. 5 and 6, the like elements to those shown in Figs. 1 and 2 are designated by the like numerals or letters and the explanation thereof is omitted. In the present embodiment, the polarity signal (b) and the on-track signal (e) are applied to an up/down counter 27, which counts down when the light beams moves inwardly of the disk and the on-track signal (e) is Low and the polarity signal (b) changes from Low to High, and counts up when the light beams moves outwardly of the disk and the on-track signal (e) is Low and the polarity signal (b) changes from High to Low. A signal (g) for opening and closing a tracking control loop is applied to a reset terminal of the up/down counter 27 so that it is reset when the tracking control loop is open and the reset is released when the tracking control loop is closed. Accordingly, in a random access operation, the reset is released when the light beam reaches the target track and the tracking control loop is closed. The output of the up/down counter 27 is applied to a data processing circuit 26, which produces the output of the ROM 23 as it is when the content of the counter 27 is zero, produces an output indicating a minimum level, for example, (0, ----, 0) when the content of the counter 27 is negative, and produces an output indicating a maximum level, for example, (1, ----, 1) when the content of the counter is positive. The output of the data processing circuit 26 is converted to an analog signal by the D/A converter 24. As shown in Fig. 6(ℓ), the signal polarity does not change when |X| = (P/2) and the signal level is zero only at the target track. In the present embodiment, an external input device 100 may be connected to the up/down counter 27 to externally set the count so that the signal level is zero only at the target track. In this manner, the light beam may be moved to the target track by a so-called track jump. The operation of the track jump is explained with reference to Fig. 7. When the target track is reproduced, the signal level is zero only at the target track as a signal (ℓ) shown in Fig. 7. Thus, the target track is a sole stable point. The output of the up/down counter 27 is now zero. If the content of the counter 27 is set to "2", for example, by a signal (m) from the input device 100, the content of the counter is zero (the stable point) at two tracks ahead as a signal (ℓ') shown by solid lines in Fig. 7 and the light beam moves to that position. In this manner, the track jump is performed. The ±n track jump may be attained by setting the content of the counter to +n in accordance with the jump direction and the number of tracks to jump. Fig. 8 shows a fourth embodiment of the present invention, and Fig. 9 shows waveforms of the embodiment of Fig. 8. In Figs. 8 and 9, the like elements to those shown in Figs. 5 and 6 are designated by the like numerals or letters and the explanation thereof is omitted. In an optical disk, address information of each track is performed by pits in a track groove. In the address information recorded area, signal waveforms are distored as shown by the tracking error signal (a) and the tracking sum signal (d) of Fig. 9. As a result, the polarity signal (b) or the on-track signal (e) may be erroneously produced and the number of tracks to the target track and the amplitude A of the tracking error signal (a) may be misdetected. In the present embodiment, such misdetection is prevented by using a signal (q) which indicates the address information recorded area. In Fig. 8, the signal (q) which indicates the address information recorded area of the track is supplied to latch circuits 28 and 29 and an AND gate 30. The edge detection signal (k) produced by the edge detection circuit 25 is ANDed with the signal (q) by the AND gate 30 and an output signal (k') thereof is applied to the latch circuit 22, which latches the output (j') of the absolute value circuit 21 by the output (k') of the AND gate 30 to detect the amplitude A of the tracking error signal (a). The latch circuit 28 latches the on-track signal (e) while the signal (q) is Low. The latch circuit 29 latches the polarity signal (b) while the signal (q) is Low. The up/down counter 27 counts down when the light beam moves inwardly of the disk and the output (e') of the latch circuit 28 is Low and the output (b') of the latch circuit 29 changes from Low to High, and counts up when the light beam moves outwardly of the disk and the output (e') of the latch circuit 28 is Low and the output (b') of the latch circuit 29 changes from High to Low. The output of the latch circuit 28 is also applied to the ROM 23, and when the output (e') of the latch circuit 28 is Low, the corrected data as shown by broken line of (i') of Fig. 9 is supplied to the data processing circuit 26. The output of the up/down counter 27 is also applied to the data processing circuit 26, which produces the output of the ROM 23 as it is when the content of the counter is zero, produces a signal indicating a minimum level when the content of the counter is negative, and produces a signal indicating a maximum level when the content of the counter is positive. The output of the data processing circuit 26 is converted to an analog signal by the D/A converter, which produces a signal (ℓ) whose signal level is zero only at the target track. In the present embodiment, since the edge detection signal (k) generated by the address information recorded area is masked by the signal (q), the misdetection of the amplitude A of the tracking error signal (a) is prevented. Further, since the signals (b') and (e') which hold the immediately preceding level of the address information recorded area are applied to the up/down counter 27 as the polarity signal (b) and the on-track signal (e), respectively, there is no risk of misoperation by a pulse generated by the address information recorded area. In the present embodiment, the ±n track jump is attained by setting the content of the up/down counter 27 to ±n in accordance with the direction of jump and the number of tracks to jump, as it is in the embodiment of Fig. 3. In the above embodiment, if only the positive or negative edge of the tracking error signal (a) is to be detected, the absolute value circuit 21 is not necessary. While the latch circuit 22 latches the absolute value of the amplitude A of the tracking error signal (a) in the above embodiment, it may latch the polarity of the tracking error signal (a), that is, the polarity signal (b) as well and supply them to the ROM 23. In this case, the absolute value circuit 21 is not necessary, but the ROM 23 must store the data including the polarity of the amplitude A and it requires double capacity. Alternatively, only the data relating to the positive edge of the tracking error signal (a) may be stored in the ROM 23 and the output of the ROM 23 may be inverted in accordance with the polarity signal (b) to get the data of the negative edge. In this case, the capacity of the ROM 23 may be reduced to one half. Further, instead of detecting the amplitude at every edge of the on-track signal (e), it may be detected only the start time or at the track jump, or data of a plurality of detections may be averaged. When the amplitude (A) of the tracking error signal (a) is always kept constant by an automatic gain control (AGC) circuit (not shown), it is not necessary to latch the amplitude A but only the outputs (b) and (j) of the A/D converter 20 and the on-track signal (e) may be supplied to the ROM 23. In this case, the absolute value circuit 21 and the latch circuit 22 are not necessary and the capacity of the ROM 23 may be small. If the amplitude A of the tracking error signal (a) is detected smaller than an actual value due to the shift of timing of the on-track signal (e) or the offset of the tracking error signal (a), it may happen that (Va/A) > 1. In this case, there is no solution to the formula (1). Since the condition of (Va/A) > 1 takes place when the amplitude of the tracking error signal (a) is maximum, that is, in the vicinity of |X| = (P/4), the value at |X| = (P/4) may be stored in the ROM 23 and it may be read out when (Va/A) > 1. Various other modifications may be made. In essence, the tracking error signal having the sinusoidal characteristic to the relative position of the light beam and the track is changed to have the linear characteristic to the relative position by using the amplitude A of the tracking error signal (a) and the tracking error signal (a) in accordance with the formula (1). The present invention offers the following effects. (1) Since the tracking error signal having the sinusoidal characteristic to the relative position of the light beam and the track is changed to have the linear characteristic to the relative position by using only the tracking error signal, the present application is applicable even if the amplitudes of the tracking error signal and the tracking sum signal are not equal (A ≠ D) and the ratio of amplitudes (A/D) is not constant. (2) The sole pull-in stable point is attained by assuring that the polarity of the signal does not change at |X| = (P/2) and the signal level is zero only at the target track so that a force toward the target track is always applied to the actuator and the light beam is positively pulled into the target track. Since the stable point is only at the target track, the light beam is pulled back into the target track even if it is significantly shifted from the target track by disturbance. (3) The amplitude of the signal produced by the D/A converter does not depend on the amplitude of the tracking error signal but is always constant. The effect of the automatic gain control circuit is also expected.

Claims: A tracking control apparatus including an information recording and reproducing disk medium having tracks for recording and reproducing information by a light beam, comprising: signal generation means for generating a signal proportional to a displacement between the light beam and a target track, from a tracking error signal (a) of sinusoidal waveform indicating a positioning error between the light beam and the target track and an on-track signal (e) with two values indicating whether the light beam is on the target track or off the target track, in which the signal output from said signal generation means is a control signal (i, 1) used in tracking control means such that the light beam traces the target track and is moved transversely to the tracks of said information recording and reproducing medium, wherein said signal generation means includes: analog/digital conversion means (20) for digitizing the signal (a) indicating the positioning error between the light beam and the target track: amplitude detection means (21, 22, 25) for detecting the amplitude of the digitized signal (a) indicating the positioning error between the light beam and the target track; memory means (23) for receiving an output signal (b, j) from said analog/digital conversion means, an output signal from said amplitude detection means, and the on-track signal (e); the memory means holding the stored result of the formula V'x = P 2π sin -1 Va A in which Va is the level and A the amplitude of the signal (a), P is the pitch and V'x is a correcting signal that corresponds to the particular values of input signals (b), (j) and (e), and outputting result V'x corresponding to input signals b, j, & e; digital/analog conversion means (24) for receiving said output V'x from said memory means and converting it into an analog signal (i) used in the tracking control. A tracking control apparatus according to claim 1, comprising counter means (27) for counting up and down by the signal (a) indicating the positioning error between the light beam and the target track and the signal (e) indicating whether the light beam is on the target track or off the target track; data processing means (26) connected to said memory means, for processing an output from said memory means in response to an output from said counter means (27); and digital/analog conversion means (24) connected to said data processing means, for converting an output from said data processing means into an analog signal, whereby said data processing means detects the displacement and a direction between the light beam and the target track in response to a counting value of said counter means and supplying the output from said memory means when the light beam is on the target track, and supplying the output from said counter means when the light beam is off the target track, to said digital/analog conversion means. A tracking control apparatus according to claim 2, wherein said counter means includes external input setting means (100) for externally setting a counting value.

Inventors: FUKUI YUKIO, SUZUKI MOTOYUKI, SUZUKI YOSHIO, FUKUI, YUKIO, SUZUKI, MOTOYUKI, SUZUKI, YOSHIO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1992
Main classification: G11B7
IPC codes: G11B7, G11B21