CA1196098A

CA1196098A - Optical memory apparatus

Info

Publication number: CA1196098A
Application number: CA000416313A
Authority: CA
Inventors: Takeshi Maeda; Masahiro Takasago; Tokuya Kaneda; Yasumitsu Mizoguchi; Motoo Uno; Kouji Muraoka
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1981-04-02
Filing date: 1982-11-25
Publication date: 1985-10-29

Abstract

Abstract:
In an optical memory apparatus predetermined information is optically recorded on a recording medium along guide grooves previously formed and can then be played back. The invention is concerned with improvements in apparatus for accurately positioning the light beam at a desired target guide groove. Using a tracking signal and a signal indicating the quantity of reflected light at the time at which a projected light beam passes across the guide grooves, the directions in which the light beam passes across the guide grooves and the number of guide grooves to be passed are detected. Seek control is performed by adding and subtracting the numbers of the guide grooves to be passed, depending upon the directions of passage, and fine positioning is thereafter performed using this tracking signal. The invention provides high precision of accessing in a system that can have a track density one or two orders of magnitude higher than a conventional magnetic disc.

Description

Optical me _ The present invention relates to an optical memory apparatus which records and plays back information optically. ~ore particularly, it relates to an access device for positioning a light beam at a desired yuide groove.
An optical memory apparatus called an "optical disc" has been proposed wherein a rotary disc, in which a predetermined substrate has been vapor-coated with an information recording medium (e.g., a metal film), is irradiated with a laser beam which is focused to a spot of approximately 1 ~m in diameter.
The irradiation power of the beam is modulated whereby thermally to form recesses (called "pitsi') in the recording medium and thus record information. At playback, a feeble laser beam is condensed and projected onto the recording medium and the information is read by utilizing variations in the quantity of reElected light from the pits. Such a proposal was made in "Electronics", Nov. 23, 1978, p. ~5, "Ten Billion Bits Fit onto Two Sides of 12-inch disc".
2Q According to the invention there is provided an optical memory apparatus comprising: a recording medium on which predetermined information is optically recorded along guide grooves previously formed and from which said information is read out: projecting means projecting a laser beam on the ~5 recording medium; light reception means receiving reflected light from the recording medium; means generating a tracking .~i'' .
. ~

- 1a -error signal on the basis of an output from the light reception means; characterized by a reflected light signal generator generating a signal corresponding to the total quantity of light reflected from the recording medium from an information signal containing bits obtained from the light reception means during the passage of at least one light spot across the guide grooves; and edge signal generator for generating two signals corresponding to the direction of passage each time the light beam passes through the guide grooves, on the bais of the total quantity of light and the tracking error signal; a difference detecting circuit detect-ing a difference between the guide groove where the light beam exists and a target guide groove on the basis of the output from said edge signal generator fro gennerating a first control signal for controlling the position of the light beam incorrespondance with said difference; and light beam position-control meand for bringing the light beam near to the target guide groove in reponse to the first control signal To enable the background of the oinvention to be described with the aid of diagrams,the figures of the accompanying drawings will first be listed.
Figure 1 is a schematic arrangement view of an optical memory apparatus;
Figure 2 is a partially enlarged sectional view of a disc;
Figure 3 is a diagram for explaning the relationships between light spot traces and eccentricity;
Figures 4(a), 4(b) and 4(c) are diagrams for explaining methods of detecting signals a-t the time of passage through a -track;
E`igure 5 is a waveform diagram for explaining a position detecting method;
Figure 6 is a circuit block diagram for explaining positional detection;
Figure 7 is a circuit block diagram for explaining speed detection;
Figure 8 is a waveform diagram for explaining the timing of the positional control;
Figures 9 and 10 are circuit block diagrams for explaining the positional control;
Figure 11 is a block diagram showing an embodiment of the present invention;
Figure 12 is a block diagram showing another embodi-ment of the present inventioni Figures 13 and 14 are diagrams for explaining the operation of the embodiment in Figure 12;
Figure 15 is an arrangement diagram of a simulator circuit for use in the embodiment of Figure 12;
Figure 16 is a diagram for explaining a mirror deflection detecting method for use in the present invention Figure 17 is a block diagram showing still another embodiment of the present invention;
Figure 18 is a block diagram showing a further embodiment of the presen-t invention;
Figure 19 is a time chart showing the signal waveforms of various par-ts in the embodiment of Figure 18;
Figure 20 is an explanatory diagram showing the trace of a light spot;
Figure 21 ls a block di.agram showiny a further embodiment of the present invention;
Figures 22(a) and 22(b) are views showing the construc-tion of a two-dimensional actuator; and Figures 23(a) arld 23(b), Figures 24(a) and 24(b), Figures 25(a) and 25(b) and Figures 26(a) and 26(b) are . .
., diagrams for explaining methods oE detectin(~ a tracking error signal and a total reflected-light quantity signal for use in the present invention.
A typical example of an arrangement of the memory apparatus is shown in Figure 1. A disc 3 (partly broken away in the figure) having a diameter of approximately 30 cm is rotated in the direction of the arrow about a shaft 4 by a motor 5. An optical head 2, consisting of a laser source and an optical system, is carried on a swing arm actuator 1 of the type used with magnetic discs and is driven in the radial direction of the disc 3.
The disc 3 has its surface covered with a transparent protective film 6 of glass or the like, a]so shown partly broken away.
Methods of recording and playing back information in this arrangement will now be described with reference to Figure 2 in which the illustrated part A of the disc 3 is shown on an enlarged scale.
On a substrate 11 of glass or plas-tic, guide grooves (called "tracks") 13 of a concave sectional shape and having a predetermined width and depth are formed by the use of an ultraviolet-setting resin 14 or the like. Further, a metal film 10 is evaporated onto the resin 14, whereupon the protective film 6 is deposited~ When recording, the focused spot of light from -the optical head 2 is guided along the guide grooves 13, i.e., the light spot tracks the guide grooves 13, and the metal film 10 is dissolved by the ligh-t spot to form the pits 12. For playback, a light spot is similarly projected onto the guide grooves 13, and the quantity of the resulting reflected light is read.
Signals for controlling the light spot are detected from the quantity of the reflected light. These signals are, in the main, a focal deviation detecting signal for detecting the focal deviation ascribable to vertical oscillations of the disc, and a tracking deviation detecting signal for detecting any deviation between the center of the light spo-t and the center line of the guide groove. All such signals use the quan-tity of the reflected light from the metal film, ~l~96~

namely, the area other than the pits.
Assuming the pi-tches of the guide grooves to be 1.6 ~m, one side of the disc having a diameter of 300 mm is provided with about 50,000 guide grooves, and the data to be received per guide groove become about 4,000 bytes.
In each guide groove, a plurality of sectors for indicating the limits of the data are provided in advance in the rotational direction of the disc. When recording external information at any desired position or playing back the recorded information, an access operation of looking for one guide groove in the surface of the disc and finding one sector of this guide groove is required. That is, a so-called "seek" operation is necessary, moving the light spot to a selected guide groove where the desired information is recorded or is to be recorded, and the so-called tracking operation of maintaining the light spot on the center line of the guide groove with the minimum deviation throughout the period of time during which the information is being read or recorded.
Magnetic disc apparatus has heretofore required such an access operation. Since, however, the track pitches of magnetic disc are about 150 ~m to 30 ~m and are one to two orders greater than the pitches of the guide grooves of the optical disc, positioning based on the same type of access operation as with a magnetic disc cannot be applied to an optical disc. More specifically, when a magnetic head is brought to a desired track by an actuator (for example, a voice coil type of linear motor), a steady state error (an offset Erom the targe-t position) of about 5 - 10 ~m develops, differing depending upon the construction and performance of the actuator. It is caused by friction. Moreover, in the transient state of the positional control, an overshoot can take place with respect to the target position, amounting to about S ~m. In this manner, with the access means employed with a magnetic disc, the stop precision is very low, e.g., about 10 llm. Since, as described before, the pitches of the guide grooves of an optical disc are presently about 1.6 ~m at the least, positioning on an op-tical disc is difficult using the type of access method employed with magnetic discs.
Besides, in cases where the actuator undergoes maximum acceleration and maximum deceleration, there is a risk that the actuator itself will vibrate by an amoun-t of the order of 1 ~m. This leads to the problem that information from the disc cannot be read out during the seek operation.
Furthermore, unlike the magnetic disc, the optical disc does not include a disc and a servo head for positioning.
This leads to the problem that the exact position of the optical head cannot be detected.
An object of the present invention is to solve these problems and provide an optical memory apparatus that can achieve positioning of high precision.
In order to achieve high-precision positioning, first of all, the exact position of the optical head during an access operation must be detected. To -this end, it is considered to use a tracking deviation signal (tracking signal) at the time at which a light spot passes across a guide groove (track). However, when executing the seek control and the follow-up con-trol using this signal, the following disadvantage is involved. At the start and end of the seek control, the moving speed of the optical head becomes very low. When this speed has become lower than the speed caused by the eccentricity of the tracks, a miscount occurs in counting the number of tracks each time one track is passed and the exact position cannot be detected.
More specifically, referring to Figure 3, the trace 40 of the light spot corresponds to a case where the light spot has passed across a group of eccentric tracks at the maximum speed of the eccentricity. Individual solid lines represent the variations-with-time of the positions of the tracks in the radial direction of the disc. In this case, the count value of the passage through the tracks and the number of the tracks having been passed are in agreement.
In contrast, the trace 41 of -the light spo-t corresponds to a case where the light spot has passed through the group of tracks at a speed lower than the maximum speed of the ~9~

eccentricity. In this case, tlle count value of the passage through the tracks does not agree with the number of the tracks actually passed; the former is larger.
In the present invention, therefore, the direction in which the light spot passes across the guide grooves (tracks) and the number of the guide grooves that have been passed are detected by the use of a signal indicating the total quantity of reflected light during passage of the light spot across the guide grooves and the tracking deviation signal (tracking signal). Addition or subtraction of the guide grooves passed is executed in accordance with the direction of the passage whereby to detect the exact position of the optical head, that is, the light spot.
Alternatively, -the optical head can be provided with a scale, by means of which the position of the optical head during the access operation is exactly detected without detecting the passage of the light spot across the guide groove.
More specifically, the invention provides in an optical memory apparatus having a recording medium on which predetermined information is optically recorded along guide grooves previously formed and from which it can be played back, projection means for projecting a laser beam onto the recording medium, light reception means for receiving reflected light from the recording medium, light recep-tion means for receiving the reflected light from the first-mentioned light reception means, first generation means for generating a tracking signal for causing the laser beam to track the guide groove on -the basis of an output from the second-mentioned ligh-t reception means, and second generation means for generating a signal indicative of a quantity oE
the reflected Light on the basis of the second-mentioned light reception means; the improvement comprising first means for generating a signal corresponding to a direction of passage each time the light beam passes across a yuide groove on the basis of outputs Erom said first and second generation means, second means Eor detecting a difference bet:ween the locat:ion oE the guide groove where the light 3~ 3~

beam exists and a target quide yroove on the basis of the output of said Eirst means and for generating a first control sigrlal for controlling the position of the light beam i.n correspondence with such difference, third means for generating a second control signal for positioning -the light beam to the target guide groove on -the basis of the tracking signall and light beam position-control means for bringing the light beam near to the target guide groove in response to the first control signal and for positioning the light beam to a center line of the target groove in response to the second control signal.
The invention also provides in an optical memory apparatus having a recording medium on which predetermined information is optically recorded along guide grooves previously formed and from which it can be played back, projection means for projecting a laser beam onto the record-ing medium, light reception means for receiving reflected light from the recording medium, light reception means for receiving the reflected light from the first-mentioned light reception means, and first generation means for generating a tracking signal for causing the laser beam to track the guide groove on the basis of an output from the second-mentioned light reception means; the improvement comprising an external scale, first means for generating an output at each pitch of said scale on the basis of a position signal of said scale, second means for detecting a number of pitches of said scale corresponding to a difference between the guide groove where the light beam exists and a target guide groove on the basis of the output of said first means and for generating a first control signal for controlling a position of the light beam in correspondence with the number of pitches, third means for generating a second control signal for positioning the light beam to the target guide groove on the basis of the tracking signal, and light beam position-colltrol means for bringing the light beam near to the target guide groove in response -to the first control signal and .or positloning the light beam to a center line oE the target groove in response to the second control slgnal.

The present invention can include a first actuator that has a movable ranye extending over the full radius of the disc, and a second actuator that has a minute variable range and which exhibits high responsiveness. The position-ing precision unattainable with only one actuator is thus realized by interlocking the two actuators. However, the controlling of the two actuators becomes a problem. This problem can be solved in such a way that the movement of the second actuator for high-precision positioning is detected, the first actuator for approximate positioning over the full radius of the disc being driven in interlocking rela-tionship with the detec-ted movement.
There will first be described a method of exactly detecting the position of an optical head from a signal indicating the total quantity of reflected light and a tracking signal. Figures 4(a), 4(b) and 4(c) are explanatory diagrams of methods of producing signals ind:icative ~f the direction in which the optical head passes along a track, and the passage along the track, in order to detect exactly the position of the optical head from the disc. Referring to Figure 4(a), light rays emergent from the light source of the optical head are condensed by an objective (not shown), to form a spot 50 on a metal film 10 through a substrate 11 of the disc and a UV resin 14 forming guide grooves (tracks) 13. At this time, assuming that the N.A. (numerical aperture) of the objective is 0.50 and tha-t the wavelength of the light source is 830 nm, a spot size (a diameter at which an intensity of l/e2 is established) becomes about 1.6 ~m. It is assumed that the pitch of the guide grooves on the disc is 1.6 ~m. Then, as the spot moves radially of the disc, as shown by the arrow, a tracking signal 52 expressing the deviation between the center line oE the guide groove and the center of the spot varies as shown in Figure 4(b). ~eyarding the production of this signal, there is a method employing two spots as disclosed in Japanese Laid-open Patent Application No. 49-50954, a method oE spot wobble as disclosed in Japanese Laid open Patent Application No. 49-94304, a method of track wobble - ~L19~a3~3 _ 9 _ as disclosed in Japanese Laid-open Patent Application No. 50-68413 and a method employinc3 di:Efracted light as disclosed in Japanese Laid-open Patent Application No. ~9-60702. In addition, when the spot moves in the direction of the arrow, the total quantity of reflected light from the disc varies as shown in Figure 4(c). The total quantity of reflected light becomes smallest at the center line of the guide groove and largest at the middle line be-tween adjacent guide grooves. The signal 51, which is obtained by detecting the total quantity of reflected light and converting it into an electric signal by means of a photodetector, is so related to the tracking signal 52 as to be equal in periodicity and shifted by 90 in phase. The tracking signal 52 becomes null at the center line of the guide groove/ and its sign differs depending upon whether the light spot lies on the right or the left side of the guide groove (corres-ponding to the outer peripheral side or the inner peripheral side of the disc). By utilizing this feature, the direction in which the guide groove is being passed can be determined.
The "total quantity of reflected light" mentioned here denotes the total quantity of light that has passed and arrived through the aperture of a lens having a certain specified numerical aperture, when the reflected light from the disc has been condensed by the lens. This light quantity is used for detecting the information signal recorded on the disc. This information signal is obtained by the light beam arriving through the lens aperture is condensed onto the light receiving face of a single photodetector and is converted into a photocurrent, the light beam being projected on a group of photodetectors having a plurality of light receiving faces, photocurrents from -the respective photo-detectors being summed, or the photocurrents being converted into voltages, which are added up. The resulting signal can be used as the signal 51 of the total quantity of reflected light.
There wi.ll now be explained a method of executing exact posit.ional detection by the use of the tracking signal 52 and the total reflected-light quantity signal 51.

~9~S~638 Figure 5(a) shows the A.C. cornponent of the total reflected-light quantity signal. Figure 5(b) shows the tracking signal. In this embodiment, letting plus (+) denote a situation where the spot lies on the inner side of the disc and minus (-) denote a situation where it lies on the outer side of the disc, the variations of the two signals versus a time axis are illustrated for a case where the light spot has moved from the outer side toward the inner side of the disc, has stopped halfway and then moved in the opposite direction. Figure 5(c) shows a track signal 90 indicating places a-t which guide grooves exist. This signal exploits the fact that the total quantity of reflected light decreases in places where guide grooves exist. The signal of the total quantity of reflected light is compared with a lS voltage El. When the former is smaller than the latter, this corresponds to the state of logic level "0". When variation of the signal 90 is observed on the time axis, each falling edge of the waveform corresponds substantially to an edge of a guide groove at which the spot begins to traverse such guide groove. Therefore, pulses 92 of small time width in Figure 5(e) are prepared from these falling edges. On the other hand, in order to know the direction in which the spot passes through the guide groove, a signal 91 (Figure 5(d)), called the "track sign signal", is prepared by comparing the tracking signal 52 with the zero level. The direction of passage through the guide groove can be determined by comparing the track sign signal 91 with the timing of the -track passage edge signal 92. Accordingly, when it is desired to know the number of guide grooves through which the light spot has passed when moving from the outer side towards the inner side, the number of pulses of a track passage edge signal 53 (Figure 5(g)) at the time when the tracking signal 52 becomes minus, that is, at which the track sign signal 91 has a low level, can be counted. The same applies to movement in the opposite direction.
An exa~ple of a practicable circuit for realizing the operations described above is shown in Figure 6. The total reflected-light quantity signal 51 is applied to the (+) terminal of a comparator 93a, and the voltage El is applied to the (-) terminal thereof, whereb~ -to make a comparison between the -total reflected-light quantity signal 51 and the voltage El. An output logic level becomes "1"
when the level of the signal 51 is greater than E1, and it becomes "0" in the reverse case. The output signal 90 ~s applied to a monostable multivibrator 94 to form pulses of fixed width from the falling edges of -the signal 90.
This output signal 92 is applied to one terminal of each of AND circuits 95a and 95b for making logical products. The other terminals of the AND circuits 95b and 95a are respectively supplied with the sign signal 91 obtained by applying the tracking signal 52 to a comparator 93b, and a signal obtained by inverting the sign signal 91 by means of an inverter circuit 96. The respective AND circuits 95b and 95a deliver a pulse signal 54 (Figure 5(f)) each time the light spot passes through a track from the inside to the outside, and a pulse signal 53 each time the light spot passes through a track from the outside to the inside. From these signals it is possible to know the number of tracks remaining to the target track being accessed, which number is required for the speed control.
In the circuit of Figure 6, an access sign signal 56 indicating the direction of access is caused to correspond to, for example, the logic level "0" when accessing the target track from the outside to the inside. A logic circuit consisting of logic elements 97, 98, 99, 100, 101, 102, and 103 selects the plus direction pulse 54 and applies it to the Up terminal of a counter 104 and the minus direction edge signal 53 and applies it to the Down terminal of the counter 10~. At the start of accessing, the counter 104 is loaded with the absolute value 55 of the distance to the target guide groove. When the light spot has started moving from outside to inside, the minus direction pulse signal 53 appears and decreases the content of the counter 104 each time the light spot traverses one guide groove. On the other hand, when the light spot comes back halfway for any reason and traverses one guide groove moving from the inside towards the outside, the plus direction pulse signal 5~
appears and increases the content of the coun-ter lO~. The counter 104 thus delivers the exact absolute value 57 of the remaining number of guide grooves to be -traversed in the access operation. When the content of the counter 104 becomes zero, a pulse A indicating this fact is provided from its BR terminal, indica-ting that the light spot has reached the edge of the target guide groove.
Using -the plus and minus direction pulse signals 54 and 53, the absolute value of the speed of the access operation (this value being required for the speed control) is known. By way of example, in the circuit of Figure 7, the minus direction pulse signal 53 is applied to a frequency-to-voltage converter 105, while the plus direction pulse signal 54 is applied to a frequency-to-voltage converter 106.
Letting p denote the pitch of the guide grooves and _ the absolute value of the speed of passage across the guide grooves, the frequency f of a train of pulses each of which appears at the edge of a guide groove during passage across such guide groove is given by the following expression:
E = v/p By knowing this frequency, the absolute value of the speed at which the light spot passes across the guide grooves is known. The direction of passage is known from the existence of the signal 53 or 54. The circuit of Figure 7 is a practice example for performing these operations. The output of the frequency-to-voltage converter (hereinbelow, termed "F/V converter") 105 or 106 is such that, corresponding to the sign of passage across each track, the speed of passage is converted into an analog value in the form of a voltage, this form being convenient for the later speed comparison. The ou-tputs of the F/V converters 105 and 106 have their difference taken by a differential amplifier 107, the output of which is applied to both the input of an inverter circuit 103 and a switching circuit 109. The out-put of the inverter circuit 108 is applied to a switching circuit 110, which is controlled by the inverted signal of ,. ~

the control 56 for controlLing the swltching circuit 109.
The outpu-ts of -the switching circuits 109 and 110 are combined to form a signal lll indicative of the absolute value of the speed. More specifically, as regards the sign signal 56, access from the inside towards the outside now corresponds to logic level "1"~ Thus, when accessing from the inside to the outside, the F/V conversion output of the signal 54 gives the differential output a + sign.
Since the access sign signal is "1", the switching circuit 109 turns ON~ and this appears as the absolute value signal 111 of the speed. Conversely, when accessing from the Ollt-side to the inside, the access sign signal 5~ is "0" in terms of the logic level. The switching circuit 110 is thus turned ON by the output of an inverter 112 for inverting the sign signal 56. The F/V conversion output of the minus direction signal 53 gives the output of the differential amplifier 107 a - sign, but the latter is rendered + by the inverter circuit 108. This appears as the absolute value signal 111 of the speed.
The procedure for preparing a timing signal for changing-over speed control to positional control will now be described with reference to Figure 8. The servo system of the positional control is usually designed on the assumption of linear operation. This flows from an easy analysis and a simple circuit arrangement. As shown in Figure 4(b), however, the tracking error signal 52 varies sinusoidally as a function of the track position and exhibits a nonlinear characteristic as a control input. In such a system, the timing at which operation of the servo system is started becomes an importan-t factor for stable operation of the system.
Referring to Figure 8(a), when the spot has -traversed the disc from the inside to the outside to come close to the N-th target guide groove, the tracking error signal 52 varies as illustrated. When the tracking error signal is expressed as a sinusoidal wave whose origin is a target point 115 (the zero point of the tracking error signal), the timing of the st~rt of the positional control for executing stable operation extends, according to experiments, between the peak points ~36~

of the + and - signs closest to the target point (within + ll/2 in terms of the phase of the sinusoidal wave). More suitable is a linear region whose point of syr~etry is the origin. In addition, the servo systern needs to be operated a-t an edge, before passage through the zero point of the target guide groove. With this taken in-to consideration, when the spot approaches the target guide groove from the inside of the disc, the positional servo system can be turned ON after the spot has passed through the zero point of the guide groove directly preceding -the target guide groove and has passed through the next plus peak point.
Conversely, when the spot approaches the target guide groove from the outside, the positional servo system is turned ON after the spot has passed through the zero point of the guide groove directly preceding the target guide groove and has passed through the next minus peak point.
Circuits for realizing the above are shown in Figures 9 and 10. Figure 8(a), (b), (c) and (g) respec-tively show the tracking error signal 52 at the time when -the spot has approached the target guide groove from the inside of the disc, a signal 113 indicating linear regions, a signal B
indicating the turn-ON of the positional servo system, and a signal 144 indicating arrival at the target guide groove.
Referring to Figure 9, the tracking error signal 52 is applied to the -~ terminal of a comparator 117, the -terminal of which is supplied with a voltage E2. As indicated in Figure 8(a) the voltage E2 is set at a positive level substantially linear to the target point llS of the tracking error signal 52. The outpu-t of the comparator 117 is applied to one input terminal of an AN~ circuit 120, the other input terminal of which is supplied with the access polarity signal 56. The tracking error signal 52 is also applied to the - -terminal of a comparator 118, the terminal oE which is supplied with a voltage E3. As indicated ln Figure 8(a), the voltage E3 is set at a negative level substantially linear to the target point 115 oE the tracking error signal 52. The output of the comparator 118 is applied to one input of an AND circuit 121 .. .
., the other input of which is supplied with a signal obtained by inverting the access sign signal 56 by means of an inverter 119. The outputs of the A~D circuits 121 and 120 are applied to an OR circuit 122 which takes the loyical sum thereof.
S Thus, the output 113 of the OR circuit 122 becomes the wave-form shown in Figure 8(b), when the access sign signal 56 is '11", and the waveform shown in Figure 8(e), when the access sign signal 56 is "0". In both cases, the falling edge of a pulsative signal represents the end of the linear region whose center is the target point. In order to achieve positional control of the liyh~ spot to the target point 115 of the target guide groove, the linear region of the target guide groove needs to be known. Therefore, the BR output A
(the signal provided when the counter content 57 has become zero) of the counter 104, which has been explained with reference to Figure 6, is used. As explained with reference to Figure 5(f) and (g), the pulses 54 and 53 indicative of the passage across the guide grooves develop at those edges of the guide grooves to-b~-passed which appear in time precedence. Accordingly, the rising edges of the pulses correspond substantially to the peak points of the tracking error signal. Supposing that the signal A is a pulse signal which rises when the content of the counter 104 has ~ecome 2ero, it is applied to a flip-flop 128 to prepare a signal 114 that rises with the rise of the signal A. The signal 114 is applied to one input of an AND circuit 123, and the signal 113 is applied to the other input thereof, whereby the linear region of the target guide groove is selected by the signal 114. The output of the AND circuit 123 is applied to a trailing edge reaction -type (master-slave type) flip-flop 124, to generate a positional control start signal B
which rises at the trailing edge of the applied input.
The signal B can also be formed by the circuit shown in Figure 10. The tracking error signal S2 is applied to a switching circuit 125, while it is applied -to and inverted by an inverting amplifier 116, the inverted signal entering a switching circuit 126. The switching circuit 125 is controlled by the access sign signal 56, and the switching circuit 126 is controlled by a signal obtained by inverting the access siyn signa]. 56 by means of an inverter 119. The outputs of the switching circuits 125 and 126 are combined and then applied to the -~ terminal of a comparator 127. The voltage E2 is applied to -the - -terminal of -the comparator 127. Thus, the signal 113 is generated in which the falling edge of the comparator output represents the end of the linear region having the target point as its center. The subsequent processing is the same as the operations in Figure 9. In this case, the positive peak level and the negative peak level of the tracking error signal 52 must be substantially equal. The circuit of Figure 10 corresponds to a case of analogously processing the first half of the circuit of Figure 9.
A general system for performing an access operation in accordance with the present invention as thus far stated, will now be described with reference to Figure 11. In this embodiment, the seek control and the follow-up control are carried out by a single actuator, the optical head on the actuator being made light in weight and small in size. By way of example, the optical head can be placed on a swing arm as explained with reference to Figure 1.
The quantity of reflected light detected by the optical head 2 is subjected to photoelectric conversion by a photodetector (not shown), and the resulting electric signal is applied to a tracking error signal generator 201 and a total reflected-light quantity signal genera-tor 200.
The method of preparing the tracking error signal will not be described in detail. The tracking error signal 52 is obtained from the generator 201, while the -total reflected-light quantity signal 51 is obtained from the generator 200.
The signals 52 and 51 are applied to an edge signal generator 202, which generates the plus directiGn signal 54 and the minus direction signal 53. The arrangement of the generator 202 has been described in detail in conjunction with Figure 6. The signals 54 and 53 are applied to both a dif~erential counter circui-t 203 for calculating the distance to the target guide groove and a velocity detecting 6~

circuit ~04, whlch respectively deliver the absolute value signal 57 of the distance to the targe-t guide groove and the absolu-te va]ue signal 111 oE the velocity. Regardlng these, the arrangement and operations of the differential counter circuit 203 have been described in detail with reference to Figure 6, while the arrangement and operations of the velocity detecting circuit 20~1 have been described in detail with reference to Figure 7.
The absolute value signal 57 of the distance to the target guide groove is applied to a target velocity curve generator 205 which delivers the optimum velocity in accordance with -the distance to the target guide groove.
Ordinarily, the recommended optimum velocity is proportional to the square root of the distance. Here, since the output of the counter 104 is ln digital form, a table of square roots is-stored in a ROM and a target velocity signal 206 is delivered digitally in accordance with the absolute value signal 57. The target velocity signal 206 is applied to a D/A (digital-to-analog) converter 207 to be converted into an analog quantity that is applied to one input of a differential amplifier 208. The other input thereof receives the absolute velocity value signal 111 from the velocity detecting circuit 204, and a difference is taken.
The output of the differential amplifier 208 is applied to a sign inversion circuit 209. Since this output is an absolute value, the sign inversion circuit 209 performs the operation of ascribing a sign to the velocity difference in correspondence with the logic level of the access sign signal 56. Accordingly, the output of this circuit becomes the difference between the target velocity and the actual velocity and having the appropriate sign. This difference enters a seek control/positional control switching circuit 210, which is controlled by the -timing signal B for the start of the positional control. More specifically, when 3~ the timing signal B is LOW, seek control is established, and the signal of the velocity difference appears at the output oE the switchirlg circuit 210 and drives the swing arm 1 through a swing arm driver circuit 251. When seelc ~3~

control has ended with the light spot arriving at the target guide groove, -the timiny signal B becomes HIGH to switch to positional control. Regarding the Elow oE slgnals for positional control, the tracking error signal 52 is applied to a switching circuit 211, which is coupled to a phase compensator 212 under the control of the timing signal B or when this timing signal is "high". The output of the phase compensator 212 is applied to an adder 213 along with a jump signal D to be described later, and the applied signals are added therein, the sum being applied to the switching circuit 210. In this manner, positional control is started by the timing signal B, and the light spot can be stably introduced to the target guide groove.
The tracking error signal 52, along with the access sign signal 56 and the signal A, is also applied to a timing signal yenerator 214 for producing the timing signal B.
The arrangement and operation of the circuit 214 haS been described in detail with reference to Figures 9 and 10.
The target guide groove is trac~ed by the above operations, and the address information stored in the guide groove is read out. Reading means therefor is omitted from the present description. The read-out information is transmitted to a controller (not shown), to decide whether or not the particular guide groove is the target guide groove.
This controller is a control unit that controls the whole optical disc apparatus. It usually gives instructions or commands to the driving unit which includes the driving mechanism and the driving circuitry required for reading and writing data, and it controls the driving unit in order to read or write data. The address of the desired guide groove is received in an access operation from a computer connected with the controller. Such address is compared with the address of a guide groove read out currently, the absolute value and sign of the difference between the current and desired guide grooves being calculated, and the result transmitted to the driving unit. The driving unit executes the seek control and the positional control by itse]E, and begins to reacl data from the target guide groove ~ 19 -or a guide groove close thereto. The controller tllen decodes the data to know the address of t:he gu1de groove currently read out and to judge the subsequent access procedure. ~y way of example, when the read-out guide groove is the target guide groove, the controller transmits as a jump number signal 58, a signal indicative of one guide groove, and a jump sign signal indicative of the direction of jump from the outside to the inside of the disc, on the assumption that guide grooves on the disc are recorded spirally outwardly. The jump number signal 58 is applied to a jump counter 215 which transmits the sign signal to a jump signal generator 216, and generates pulses for starting the jump by the number of guide grooves to be jumped, at specified time intervals. Upon receiving the pulses, the jump signal generator 216 produces the driving slgnal D for executing the jump in accordance with the jump sign signal. Details of such a jump operation are contained in 'Philips Technical Review', Vol. 33, p. 178, and are omitted here.
Accordingly, in order steadily to read out the target guide groove upon reaching it, the jump number signal 58 is transmitted from the controller in a manner including a signal indicative of one guide groove to be jumped and a jump sign signal indicative of the direction of the jump from the outside to the inside, each time the disc executes one revolution. In a case where, when the address content recorded in the guide groove to which the light spot has been positionally controlled at the end of the access is read out, this guide groove di~fers from the target guide groove, the light spot is moved to the target guide groove by performing a jump, subject to the con-dition that the difference between the guide groove currently read out and the target guide groove is smaller than a certain set number (for example, 6~ or 128). At this time, the controller transmits the jump number signal 58 which contains the number o~ guide grooves to the JI~

target guide groove and the direction o~ the needed jump.
If the difference between the current guide groove and the target guide groove is greater than the set value, an access operation including the speed control will be started. ThiS operation is the repetition of the access procedure already explained.
As described above, according to the present embodiment, whether the light spot passes across the guide grooves on the disc moving inwardly or outwardly is known from the total reflected-light quantity signal and the tracking error signal. The position of the optical head can thus be exactly detected ~7ithout errors attributed to any eccentricity~ mechanical vibrationsl etc. The afore-mentioned signals at the passage of the light spot across the guide grooves are also utilized for the velocity detection, whereby the relative velocity between the light spot and the guide grooves can be exactly detected.
Further, according to the present embodiment, even when a swing arm is used as the actuator, a positioning operation is possible from rough positioning over the full radius of the disc to fine positioning of about 0.1 llm.
Another embodiment of the present invention will now be described with reference to Figure 12. In the fore-going embodient of Figure 11, both the rough and the fine positioning are executed by a single actuator. With some actuators, however, the frequency characteristic of dis-placement versus driving current becomes problematical, and the cutoff frequency cannot be made high when using a servo system for positional control. It is accordingly desirable to use a first actuator for rough positioning and a second actuator that can move in only a minute range, but has sufficiently good frequency responsiveness to make the cutoff frequency high even when a servo system is used. In this case, interlocking of the two actuators during an access operation becomes a problem. The present embodiment aE~ords an expedient Eor solving this problem.

3~3 A linear motor 314 is an example of a ~irst actuator for the rough positioning. ~owever, the present invention applies also to other actuators. A galvano-mirror 308 or a pivot mirror is employed as the second actuator o~ high responsiveness for follow-up control, within a minute range. The disc 3 is rotating in a predetermined direction about the axis 4. An optical head 2' is placed on a bed 315 movable on a base 309 on rollers 310. The movable bed 315 is coupled to a coil 311 through a supporting mechanism 313, and it is driven by the electrotnagnetic forces of a magnet 312 when current flows through the coil 311. The optical head 2' includes therein an objective 306 which serves to form the light spot on the disc, the galvano mirror 308 which serves as means for deflecting the light spot onto the surface of the disc, a photodetector 307 which receives reflected light from the disc surface, a light source, an optical system which conducts a light beam from the light source to the objective, and an optical system which conducts the reflected light to the photodetector. Since details of the light source and the optical systems are unnecessary for explaining the present invention, they are not illustrated.
The processes by which the total reflected-light quantity signal 51 and the tracking signal 52 are produced from the output of the photodetector 307 and by which the signals 53 and 54 indicating the directions of passage through guide grooves are produced, and the process by which the velocity control is achieved using these signals, have been described in detail above. The same blocks are therefore merely indicated and are not explained again.
The block 214 for producing the timing signal B of the positional control from the tracking error signal 52, and the portion for e~fecting the jump function are not explained, either, because they are the same as before.
Only the procedure ~or positional control will be ~ r~ ~

explained. The switching circuit 211 is turned "on"
by the timing signal B of the positional control, and it leads the tracking error signal 52 to the phase compensator 212 so as to subject the signal to phase compensation for enhancing the stability and fol]ow-up performance of the control system. After the compensated signal has been added to the jump signal ~ by the adder circuit 213, it becomes a mirror driving signal E. The mirror driving signal E drives the galvano-mirror 308 through a driver circuit 305 to cause the light spot to track a guide groove. At this stage there is no target signal for the position at which the first actuator is positioned, and hence a positioning signal needs to be prepared.
When the linear motor moves radially on the disc surface under the condition that the light spot is fixed at the center of the field of view of the objective, the tracking error signal 52 varies with the magnitude of movement, as illustrated in Figure 13. It is also possible to use this tracking signal 52 as the positioning signal Eor the linear motor. Since, however, the linear range of this signal is only the extent of the width of the guide groove, the control will be impossible unless the follow-up precision ~ falls within this range. With conventional linear motors, the follow-up precision amounts to 2 - 3 ~m and even up to about 10 ~m when it is large.
However, the pitch p of the guide grooves in the optical disc is abou-t 1.6 ~m in order to record information at high density, and the width ~ becomes about 0.8 - 0.6 ~m.
It is accordingly impossible to perform positioning control of the linear motor by using the tracking error signal 52 with the center line 405 of the guide groove as a target.
It is therefore necessary to prepare a signal that expresses the deviation between the target guide groove being tracked and the linear motor and whose linear region is wider than the linear region of the tracking error signal, and to achieve positioning of the linear motor using this signal. As such a signal, there is the trace of the light spot followed up by the galvano-mirror. More specifically, each circular region indicated by a dotted line in Figure 14 is the field of view 402 of the objec-tive, and a trace 403 is the trace versus the time t of the guide groove being tracked by the galvano--mirror. The guide groove being followed within the lens view field 402 varies sinusoidally versus the time due to an eccentricity, as illustrated in the figure. Since the objective is fixed to the movable bed of the linear motor, the center 404 of the lens view field 402 moves unitarily with the linear motor. The neutral point of the galvano-mirror (determined when the mirror has been mechanically set on the movable bed of the linear motor) is uniquely determined owing to the spring supporting mechanism when the driving signal E
is null. Usually, adjustments are so made that when the galvano-mirror lies at the neutral point, the light spot is situated at the center 404 of the objective view field 402. The reason for such adjustment is that the residual aberration Oe the lens i9 least at the center of the lens view field. ~etween the amount of movement of the light spot within the lens view field and the angle of rotation of the galvano-mirror, there is a certain linear relation-ship that is determined by an optical arrangement relation and the focal distance of the objective. Accordingly, the deviation from the center of the lens view field to the guide groove ~eing tracked by the light spot can be known erom the rotational angle of the galvano-mirror.
The rotational angle of the galvano-mirror can be known from the driving signal E. Although the rotational angle oE the galvano-mirror has a characteristic dieeering in dependence upon the frequency components Oe the driving signal E (that is, a frequency response), this character-istic is already known. Even when the center o~ the lens - ~4 --view ~ield is heLd in agreement with the center line ~OS
of the guide groove in Figure 13 and the linear motor is stopped, the galvano-mirror is driven and the light spot is moved from one end to the other end of the view field of the objective, the tracking error signal 52 being detected similarly to the foregoing explanation. The galvano-mirror driving signal E at this time becomes null at the center line 40S of the target groove, and it has a minus sign at one end of the lens view field and a plus sign at the other end. It falls into a linear relation to the light spot within the lens view field, and its linear region extends over the whole view field of the objective.
In the arrangement of Figure 12, the driving signal E is applied to a circuit 300 for simulating the frequency characteristic of the galvano-mirror, to form a signal F
representing the deviation of the light spot from the center of the lens view field. The signal F is passed through a phase compensator 301 via a switching circuit 316, which is turned "on" by the timing signal B of the positional control, so as to drive the linear motor. The position of the linear motor is thus so controlled that the light spot can come to the center of the lens view field. Herein, the deviation signal F of the light spot from the center of the lens view field has a wide linear region, which is at least lO0~ m or so. The signal thus has no problem even when the follow-up precision of the linear motor amounts to 2 - 3 ~m.
In Figure 14, each circular region 406 in solid line is the field of view of the objective after the above operations have been performed.
More specifically, since the light spot tracks the guide groove trace ~03, positioning o~ the linear motor is performed upon detecting the deviation between the light spot and the center 407 o~ the lens view field. The center of the lens view Eield, indicated by each white circle 407, , unitary with the linear motor, follows up the light spot, but it deviates by the component of the aforementioned positioning error ~ (explained as the ~ollow-up precision).
While the positioning error differs, depending upon the characteristics of the positioning servo system of the linear motor, Figure 14 illustrates the case of a system that has a servo band capable of following up a large eccentric component. Enhancing the band of the linear motor positioning servo system in the present embodiment has another effect. That is, since the linear motor is controlled so that the light spot may always come to the vicinity of the center of the view field of the objective, a region of little residual aberration becomes usable. As a result, the spot size ~the diameter at which the light intensity distribution becomes 1/e2 of the maximum value) of the light spot becomes the smallest. Thus, in a play-back operation, the amplitude of a playback signal from a recorded pit becomes large, while in a recording operation, the emission power of the light source required for forming a pit of prescribed diameter can be low. Conversely, when the spot size required for the optical disc apparatus is determined at a certain value, the aberration can be reduced at only the center of the field of view, and, hence, the objective becomes smaller in relation to the number of constituent lenses and becomes lighter in weight, smaller in size and lower in cost, as compared with the objective needed in the method in which the spot is moved within the objective view field by the yalvano-mirror.
A practicable example of the circuit 300, by which the deviation signal F of the light spot from the center of the lens view field is produced from the driving signal E, is shown in Figure 15. The driving signal E enters a buffer ampllfier 302 and is delivered to an electric circuit h`aving a characteristic s;milar to the fre~uency characteristic of the galvano-mirror. Since the driving 9~

voltage (or current) -versus- deflection angle character-istic oE a conventional galvano-mirror exhibits the characteristic of a second order, low-pass ~ilter, this embodiment uses a second-order, low-pass active filter which consists of capacitors Cl, C2, resistors Rl, R2 and a buffer amplifier 304. The output of this filter accord-ingly represents the deflection of the ga]vano-mirror.
The deflection of the galvano-mirror and the movement of the spot on the lens view field are usually in a linear relationship. Therefore, the deviation signal F of the light spot from the lens view field is obtained by compen-sating the sensitivity ~as to the deflection angle and the spot movement value) through a linear amplifier 303.
For detecting the deviation signal of the light spot from the lens view field, there is a method of detecting the deflection angle of the mirror directly, otherwise than electrical simulation from the mirror driving signal E. Figure 16 shows a practicable example of such a method. A light beam 328 emergent from a light source (not shown) enters a mirror 320 along an optical axis 329, and is reflected at ~5 to have its optical path curved toward an objective (not shown). A permanent magnet 321 is mounted on the rear surface of the mirror 320, and an electromagnetic force is generated by current flowing through a coil 322 surrounding the magnet so that the mirror is rotated about a bearing 331. The bearing 331 is fixed to a part 332 of the optical head by a supporting rod 330. I'he bearing 331 is formed of a rubber material which is flexible. This structure is a kind of pivot 3Q mirror. In order to detect the deflection angle, a light beam from a light emitting diode 326 is condensed on the reflective surface of the mirror 320 by a lens 327, and the resulting reflected light beam is received by two photodetectors 323 and 324. The optical axis 329 of the light beam reflected by the mirror 320 is aligned to agree 33~1 , . ~

with the optical axis of the objective when the driving voltage is null. Thereafter, the light beam from the light emitting diode is adjusted to be equally received by the photodetectors 323 and 32~. Then, when the outputs of the photodetectors 323 and 32~ are applied to a differential amplifier 325 to measure the difference between them, the output F' becomes a signal indicative of the deflection angle of the mirror. Directly detecting the deflection angle of the mirror in this manner makes it possible to know the movement of the mirror due to any mechanical oscillationO It is therefore effective in a case where the mirror located on the linear motor might oscillate when the linear motor performs an acceleration or attenuation of the maximum number G during rough positioning. That is, the movemen~ of the mirror is detected, the mirror is positioned to the first set point and the optical axis of the objective can be prevented from fluctuatingO Accord-ingly, the signal F' can be used for the foregoing oper-ation when rough positioning is being executed by velocity control of the linear motor, and it can be used as the deviation signal from the center of the lens view field when the mirror 320 is deflected to minutely position the light spot. An embodiment in which the above signal F', obtained by directly detecting the mirror deflection angle, is used for accessing, is shown in Figure 17. The velocity control, employing the linear motor, is the same as already described, and portions represented by the same blocks are common and will not be explained again~ Since the timing signal B of the positional control is "low" in the velocity control, a switching circuit 333 allows the mirror deflection signal F' to pass therethrough during this period only, and it prevents the deflection mirror 320 from oscillating mechanically, to keep it at the set point. When the timiny signal B of the positional control has become "high", the tracking error signal 52 is allowed .~ .

-- 2~ -to pass, and tracking of the liyht spot by the mirror is performed. ~n the other hand, the mirror deflection signal F' is applied to the switching circuit 316 through an amplifier 334 for sensitivity compensation. Only when the timing signal B of the positional control is "high", the signal passes through the switching circuit and drives the linear motor to achieve control, so that the guide groove being tracked by the mirror can lie at the center of the 1ens view field.
As described above, by combining the first actuator, which can conduct the rough positioning but is inferior in the follow-up precision, with the second actuator which has only the minute movable range but is of high response rate and can be made good in follow-up precision, the present embodiment enables realisation of accessing that has a high response rate and high ~ollow-up precision.
Referring to Figure 18, there will now be explained an embodiment wherein an optical head is provided with a scale, by means of which the accurate positioning of an actuator is detected to perform rough positioning, where-upon fine positioning is performed by the use of a tracking signal.
In this embodiment, the seek control and the follow-up control are executed by a single actuator, and a swing arm is used as in the embodiment of Figure 11.
This embodiment is characterized in that the scale for detecting the rotational angle of the swing arm is disposed outside. A scale utilizing moire or a magnetic scale can be used, by way of example. Here, the use of a moire scale will be exemplified.
As shown in Figure 18, the moire scale 240 is mounted on the driving part of the swing arm 1, and a positional signal 242 based on the moire (hereinbelow, termed "moire signal") (Figure l9(b)) is transmitted from a moire detector circuit 241. The pitch of a moire scale ~3~

that can be presently fabricated is very rough in compar-ison with the pitch of the guide grooves. For example, it is about 10 ~m at the minimum and is typically about 50 ~Im.
There will now be described the process of access from the guide groove currently being read by the optical head, to a desired target guide groove. The difference between the current guide groove and the target guide groove is calculated by a superior control ~nit (not shown), to obtain a signal 243 which indicates the number of moire pitches corresponding to the guide groove difer-ence and also the direction of th~e difference. This signal 243 is introduced into the present apparatus by a latch circuit 244, and is set in a down counter circuit 245.
The other input of the down counter 245 is supplied with a moire pulse 246 to be described later, and the set value of the down co~nter 245 is subtracted by this pulse. To prepare the moire pulse 246 the moire signal 242 is applied to a moire pulse generator 247, which generates one pulse for each moire pitch.
The output of the down counter 245 represents the number of remaining moire pitches up to a moire position near the target guide groove, and it is applied to a velocity curve generator 248/ which generates the optimum velocity signal for the seek control. This optimum vel-ocity signal is applied as one input of a differential amplifier 249. The other input of the differential ampli-fier 249 is supplied with an actual velocity signal. While there are various expedients for detecting the actual velocity, the present embodiment detects the actual velocity by applying the moire pulses 246 to an F/V
(frequency-to~voltage) converter 250. As another exped-ient, there is a method in which the current driving the swing arm is integrated.
The diferential amplifier 249 compares the optimum velocity signal and the actual velocity signal, and pro-vides the difference. This output is applied to a first switching circuit 210 for switching the seek control and the follow-up control. It effects the switching in accor-dance with a logic sequenee to be stated later, so as to drive the swing arm through a power amplifier 251.
The moire signal 2~2 is further applied to a second switching circuit 253. It is seleet:ed in accordance with a logic sequenee to be stated later, and is applied to ]0 an adder 213 through a phase compensator 254. The other input of the adder 213 is supplied with a jump signal D
(Figure l9(e)) for jumping guide grooves one by one.
The output of the adder 213 is applied as the other input of the first switching circuit 210. In the case of follow-up control, it is selected and drives the swing arm through the power amplifier 251. When follow-up control has been performed, the output of the second switching eireuit 253 serves also as a signal indieative of the follow-up precision. Therefore, it is applied to a sequence circult 256 whieh generates the timing of the logic sequence.
The other input of the seeond switching eircuit 253 is supplied with the tracking signal (Figure l9(d)) which expresses the deviation bet~een the center line of the guide groove where the optical head is detected, and the central position of the light spot. This signal is used when one guide groove is selectively followed up by the second switching circuit 253. Since the means for detect-ing the traeking signal from the refleeted light of the disc has been described in detail in the foregoing embodiment, it is omitted from this figure.
In the present embodiment, in order to execute minute positioning up to the target guide groove by a jump operation, the number of a guide groove nearby is once read after the rough positioning based on the moire, the differ-ence up to the target guide groove is calculated by the superior control unit, and a signal 5~ indicative of the difference and the direction is set in a jump counter 215. The jump counter 215 generates pulses of a certain specified period by the number of guide grooves to be jumped. A jump signal generator 216 is started by these pulses and its output D is applied to the adder 213 as described before.
The operation of the present e~bodiment will now be described with reference to the time chart of Figure 19 and the trace diagram of the light spot in Figure 20. When the number o~ moire pitches corresponding to the differ-ence between the target guide groove and the current guide groove has been set in the latch circuit 244 as the signal 243, the swing arm starts moving along the optimum velocity curve to the zero point xO of the moire signal closest to the target guide groove at time to. Meantime, the moire pulse generator 247 detects the moire signal 242 and applies the moire pulses 246 to the down counter 245.
When the set value has become null, the down counter 245 generates a signal A' which makes known the approach to the moire closest to the target guide groove, and applies it to the sequence circuit 256.
In the sequence circuit 256, a switching signal B' (Figure 19(a~) for executing positioning based on the moire signal is produced from the moire signal 242 entered through the second switching circuit 253 and the afore-mentioned signal A'. This switching signal B' is applied to the first switching circuit 210. In consideration of the stable pull-in of the servo system, the timing of the switching signal B' is most suitably the time at which the swing arm has moved into a part where the moire signal is linear with respect to the zero point thereof. With this timing, the trace of the light spot shown in Figure 20 is introduced into the guide groove corresponding to the target value xO of the moire. In the figure, the half ~

cycle of the moire pitch is denoted by ~, and the eccentric value of the disc is denoted by ~. In order to know if the moire positioning has arrived within a certain precision, it may be detected that the voltage level of the moire signal 242 falls within certain set values in the sequence circuit 256. After a period of time t2, the switching signal B tFigure l9(e)) for executing positioning based on the tracking signal 5~ is generated and is applied to the second switching circuit 253.
The light spot is then introduced into a guide groove 71 at that moment and reacls out the address inform-ation recorded in that guide groove. By reading out this address information, the superior control unit calculates the number of remaining guide grooves and the direction of movement and sets the values in the jump counter 215. The peri.od of time required therefor is t3. Subsequently, the optical head comes to reach the target guide groove 72 while jumping the guide grooves one by one in accordance with the jump signal D (Figure l9(c)) delivered from the jump signal generator 216. In order to correct any error in the jump operation, it is desirable to so perform such operation that when a plurality of guide grooves has been jumped, the address information of the guide groove cur-rently reached is read out and again acknowledged.
The em~odiment has been described as pulling the light spot into the zero point of the moire signal. How-ever, when the final velocity at the time at which the content of the down counter 245 has become null under the seek control is small, or when the magnitude of eccen-tricity is small and the velocity based on the eccen-tricity of the guide grooves is small, it becomes possible to position the swing arm within the detection range of the tracking signal. It is therefore possible to perform the follow-up control using only the tracking signal.
In this case, the seting time t2 is dispensed with.

3~

The present embodiment has referred to the employ-ment of the swing arm. Therefore, when a moire scale of equal pitches is attached to the coil part oF the swing arm, an error develops between a positicn on the disc surface and the count number of moire pitches. In order to adsorb the error, the pitches of the moire scale can be changed or the error can be compensated during calculation in the superior control unit.
This problem is not involved in an ac-tuator, such as the linear motor, that drives the optical head recti-linearly. The present embodiment is also applicable to a structure in which the optical head is placed on the actuator, such as the linear motor, performing a recti-linear motion.
In this manner, according to the present embodiment, the position of the optical head during the access oper-ation can be reliably known by the external scale without reading out the guide groove formed on the disc, and seek and follow-up control can be reliably achieved.
Figure 21 shows another embodiment of the present invention. It can achieve positioning of the center line of the guide groove with high precision, likewise to the embodiment of Figure 12. It is so constructed that a second actuator having a small follow-up range is mounted on the optical head 2', and that the optical head 2' is carried on a first actuator which is not good in stop precision but which can move over the full radius of the disc, whereby positioning with high precision can be achieved over the whole surface of the disc. In the present embodiment, similarly to the embodiment of Fiyure 12, the linear motor 314 is used as the first actuator, and the galvano-mirror 308 i5 used as the second actuator.
The moire scale 240 is disposed on a linear carriage 315, as shown in the figure. The arrangement for performing seek and posit;onal control of the linear motor 314 using the moire scale 240 and the access procedure, are the same as in the case of Figure 18. Points of difference will be explained below.
After the linear carriage 315 has been positioned with the moire signal, it is acknowledged that the light spot has fallen within a certain precision (5 - 10 ~m), and the switching signal B for bringing the tracking servo system into the operating state is applied to the switching circuit 211~ In addition, the switching circuit 211 is supplied with the tracking signal 52 prepared from the output signal of the photodetector 307. In accordance with the switching signal B, the tracking signal 52 is passed through the compensator 212 via the switching circuit 211 and is applied to one input of the adder 213. The other input of the adder 213 is supplied with the jump signal D.
The output E of the adder 213 is applied to the mirror driver 305, the mechanical output of which deflects the mirror 308 to perform the tracking.
When using an external scale as in the present embodiment, it is possible to perform positioning of still higher precision and to shorten the access time by employ-ing the two actuators.
As the second actuator for tracking the limited region in the embodiment of Figure 12, Figure 17 or Figure 21, there is a two-dimensional actuator which moves the objective in parallel with the optical axis, to effect focusing, and which is moved perpendicularly to the optical axis and in the radial direction of the disc, to effect tracking. An example of such a two-dimensional actuator is shown in Figure 22. It is a mechanism that moves an objective 3~0 in parallel with an optical axis 342 for the purpose of focusing, and that moves it perpendicularly to the optical axis for the purpose of tracking. Figure 22(a) is a top plan view, while 22(b) is a side elev-ational view. I'he optical axis 342 is curved by a mirror ~t~

343, and agrees with the optical axis of the objective 3~0. The objective 340 is supported by a metallic spiral ring spring 341. A frame member 361 for keeping the outer peripheral part of the spring is coupled to a supporting portion for driving the track direction. A coil 344 is wound on the lower part of a frame member 362 coupled to the inner peripheral part of the spring, and the objective 340 is driven in parallel with the optical axis by current caused to flow through the coil 344 electromagnetically by a magnetic circuit consisting of a permanent magnet 345, a center pole 3~6 and a yoke. The mirror 343 is coupled on the center pole 346. On the other hand, as seen from Figure 22(a), a coil 348 is wound on the end of the sup-porting portion 347, and the objective is driven in the radial direction of the tracks by a magnetic circuit consisting of a permanent magnet 349, a center pole 350 and a yoke. A bearing 351 is coupled to the frame member 361 for keeping the ring spring 341~ A shaft 352 lies in contact with the bearing 351, and the shaft 352 is mounted on a bed 353 for supporting it and is fixed to a base. In the radial direction of the tracks, accordingly, the objec-tive in parallel with the optical axis and the mirror 342 are driven unitarily. In the above structure, when positioning the tracks by means of a linear motor, the center of the field of view of the lens conforms with the movement of the two-dimensional actuator in the track direction, and, hence, the positional deviation of the mechanism unitarily supporting the objective 3~0 and the mirror 3~2 can be known.
A permanent magnet 35~ is mounted on the bearing 351, and Hall elements 355 and 356 are mounted on the base to which the shaft supporting portion 353 is fixed. When the outputs of the two elements are applied to a differ-ential amplifier 357, the output of this differential amplifier indicates the deviation between the geometrical center of the two Hall elements 355 and 356 and the 3~

- 36 ~

permanent magnet 354. When the permanent magnet 354 is arranged on the extension of a perpendicular drawn from the optical axis of the objective 340 down to the slide shaft 352, the optical axis of the objective and the positions of the tracks correspond at 1 to 1 in the tracking of the tracks by the two-dimensional actuator, and, hence, the output of the differential amplifier 357 represents the deviation between the track and the geometrical center of the ~all elements 355 and 356 set in the linear motor.
~ccordingly, the output of the differential amplifier 357 is used as the signal F' for positiona:L control of the linear motor.
Since, in the above, the method of detecting the information signal and the method of detecting the tracking error signal have not been described in detail, these methods will now be outlined with reference to Figures 23, 24 and 25. Figure 23 is a diagram of a tracking signal detecting system which utilizes diffracted light, and 23(a) shows a simplified arrangement of an optical system. Light rays from a light source 504 ~for example, a semiconductor laser) are converted into a collimated beam by a coupling lens 503. This beam is passed through a polarizing beam splitter 502 and a quarter-wavelength plate 501, and is converged by an objective 500 onto the disc 3 which rotates about the axis 4. The resulting reflected light passes through the objective 500 again and has its polarization plane rotated by 90 with respect to that of the incident light by means of the quarter-wavelength plate. The beam has its optical path curved toward a converging lens 505 by the polarizing beanl splitter 502, and is condensed toward a convergence point 506 by the converging lens 505.
photodetector 507 is arranged between the converging lens 505 and the convergence point 506. Figure 23(b) explains the structure of the photodetector 507 and means for detecting the tracking signal 52 and the information signal - 37 ~

512. The photodetector 507 is constructed of bisected photodetectors 50B and 509. The tracking signal 52 is obtained in such a way that outputs from these photo-detectors have their difference taken by a differential amplifier 510, while the information signal 512 is obtained in such a way that the sum of the outputs of the photodetectors 508 and 509 is taken by an adder 5110 Figure 24 is a diagram of a tracking signal detect-ing system which employs two spots. In Figure 24(a) the difference from Figure 23(a) is that a diffraction grating 51~ is arranged behind the coupling lens 503 to split the collimated beam into three. Three spots are thus formed on the surface of the disc. One of them is arranged in the middle of the track, and the remaining two spots are arranged symmetrically slightly shifted from the middle of the track. When the photodetector 513 is arranged on the convergence point 506 of the converging lens, three spots enclosed with a dotted line are formed thereon as shown in Figure 24(b). The photodetector 513 is constructed of three independent photodetectors correspondlng to the three spots. An output from the middle photodetector passes thro~gh a buffer amplifier 515 and becomes the information signal 512, while outp~ts from the remaining two photodetectors enter the differential amplifier 510 and generate the tracking signal 52.
Figure 25 is a diagram of a wobbling or pre-wobbling tracking signal detection system. Figure 25(a) shows a photodetector 516 placed at the convergence point 506 of the converging lens 505, with an optical system similar to that of Fiyure 23. In Figure 25(b) the photo~
detector 516 is a photodetector having a single light rece~ving portion, a single light spot (hatched area) being formed on the face of the photodetector. When the output of the photodetector 516 is amplified by a buffer amplifier 517, it becomes the information signal 512.

- 38 ~

This signal is passed through an envelope detection circuit 519 to eliminate the influence of a recorded data signal, and is passed through a band-pass filter 520, the center frequency of which is the frequency of wobbling or prewob-bling, so as to extract the wobbling or prewobbling com-ponent. It is then applied to a synchronous detection circuit 521. This circuit 521 is supplied with the signal 522 of the wobbling frequency having a reference phase, and it performs synchronous detection to provide the tracking signal 52. The signal 522 with the reference phase is produced from the information signal 512 in the case of prewobbling, and is produced from a signal for driving the optical head or the deflector in the track direction, in case of wobbling.
Means for detecting the total reflected-light quant-ity signal 51 from the information signal 512 will now be explained. In a case where an information bit 12 is non-existent in a guide groove 13, the information signal 512 is equal to ~he total reflected-light quantity signal 51.
In contrast, when an information bit 12 exists, the inform-ation signal 512 varies as shown in Figure 26, in corres-pondence with Figure 4(c). The portion enclosed between the solid and dotted lines denotes the modulation of the reflected light quantity by the information bit. As shown in Figure 26(a), this signal 512 is applied to an envelope detection circuit 524 through a buffer amplifier 523 and is delivered through a buffer amplifier 525. The total reflected-light quantity signal 51 is then obtained. The values of a capacitor C and a resistor R, which determine the time constant of the envelope detection circuit 524, are selected so that this time constant becomes suffic-iently smaller than the lowest repetition frequency due to the information bit in the information signal 512 and sufficiently greater than the highest repetition frequency of the total reflected-light quantity signal 51 at passage across the guide groove. As regards the tracking signal 52, means similar to the above is applicable in order to eliminate the influence of the information bit.
There has thus been provided an optical disc that is one or two orders higher in track density than the conventional magnetic disc, has a high precision of about 0.1 ~m to a target guide groove and an access time equiv~
alent to that of the conventional magnetic disc, even in the presence of eccentricity of the guide groove.

Claims

Claims:

1. An optical memory apparatus comprising:
a recording medium on which predetermined information is optically recorded along guide grooves previously formed and from which said information is read out;
projecting means projecting a laser beam on the recording medium;
light reception means receiving reflected light from the recording medium;
means generating a tracking error signal on the basis of an output from the light reception means;
characterized by a reflected light signal generator generating a signal corresponding to the total quantity of light reflected from the recording medium from an information signal containing bits obtained from the light reception means during the passage of at least one light spot across the guide grooves;
an edge signal generator for generating two signals corresponding to the direction of passage each time the light beam passes through the guide groove, on the basis of the total quantity of light and the tracking error signal;
a difference detecting circuit detecting a difference between the guide groove where the light beam exists and a target guide groove on the basis of the output from said edge signal generator for generating a first control signal for controlling the position of the light beam in correspondence with said difference; and light beam position-control means for bringing the light beam near to the target guide groove in response to the first control signal.

2. An optical memory apparatus according to claim 1, characterized in that the reflected light signal generator comprises an envelope detection circuit.

3. An optical memory apparatus according to claim 1, characterized in that the envelope circuit comprises a capacitor C and a resistor R, which determine a time constant of the envelope detection circuit, the values of the capacitor C and resistor R are so selected that the time constant becomes smaller than a lowest repetition frequency due to the information bit in the information signal and becomes greater than the highest repetition frequency of the total reflected-light quantity signal at the passage through the guide groove.

4. An optical memory apparatus according to claim 1, characterized in that said edge signal generator comprises a first circuit for detecting the moment of the passage of the light beam through the guide groove on the basis of the total quantity of light signal to produce a first output signal, a second circuit for detecting a sign of the output from said tracking deviation signal corresponding to that moment, to produce a second output signal, and a logic circuit for receiving said first and second output signals from said first and second circuits and for supplying the first output signal of said first circuit to said difference detecting circuit as separate third and fourth signals in accordance with the second output signal of said second circuit.

5. An optical memory apparatus according to claim 4 characterized in that said first circuit comprises a comparator for transforming the total quantity of light signal into pulses, and a mono-stable circuit generating a pulse output at a negative edge of the pulse output of the comparator, the pulse output being the first output signal;
the second circuit transforms the tracking error signal into pulses, and the logic circuit supplies the first output signal from said mono-stable circuit to the difference detecting circuit, separately, in accordance with a level of the second output signal.

6. An optical memory apparatus according to claim 1, characterized in that said means generating the tracking error signal comprise an envelope detector which eliminates the influence of information-pits located on the recording medium.

7. An optical memory apparatus according to claim 1, characterized by a circuit for generating a second control signal for positioning the light beam to a center line of the target groove on the basis of the tracking error signal in response to the second control signal and a switch circuit for switching the first control signal into the second control signal prior to the light beam position-control means.

8. An optical memory apparatus according to claim 7, characterized in that the light beam position-control means include a single actuator on which said projection means are placed.

9. An optical memory apparatus according to claim 7, characterized in that the light beam position-control means include a first actuator on which the projection means are placed, and a second actuator which is disposed in the projection means, the first actuator responding to the first control signal, while the second actuator responds to the second control signal.

10. An optical memory apparatus according to claim 9, characterized in that the first actuator is a linear motor and the second actuator is a deflection mirror.

11. An optical memory apparatus according to claim 7, further comprising a jump circuit generating a jump signal for moving the light beam to the target guide groove, subject to the condition that the difference between the guide groove currently read out and the target groove is smaller than a certain set number.

12. An optical memory apparatus comprising:
a recording medium on which predetermined information is optically recorded along a track having a fluctuation value, said track being one of a plurality of tracks, and from which said information is played back, projection means for projecting a laser beam on the recording medium as a light spot, light reception means for receiving reflected light from the recording medium, first means for generating a tracking signal for causing the laser beam to move along the track on the basis of an output from said light reception means, a first actuator for moving said projection means radially with respect to said recording medium, a second actuator having a higher response rate than said first actuator for moving the light spot radially on the recording medium to follow a desired track, on the basis of the tracking signal characterized by:
detecting means for detecting the movement of said second actuator at least during the following of the desired track;
wherein said first actuator is actuated such that the movement value of the light spot can be smaller than the fluctuation value of said track and follows the desired track in response to the output of the detecting means, in cooperation with the second actuator.

13. An optical memory apparatus according to claim 12, wherein said detecting means comprises a means for detecting electrically the movement of said second actuator during the following of the desired track on basis of the tracking signal.

14. An optical memory apparatus according to claim 13, wherein said means for detecting electrically has a two-dimensional low pass filter.

15. An optical memory apparatus according to claim 13, further comprising a velocity control means for generating a velocity signal to control said first actuator, so that said light spot is moved from the track where said light spot exists currently near to the desired track, subject to the condition that difference between the current track and the desired track is larger than a certain set number, and switching means for switching said velocity signal to the output of said detecting means for supplying said first actuator.

16. An optical memory apparatus according to claim 12, wherein said detecting means comprises means for detecting optically the movement of the second actuator, while said second actuator is in operation.

17. An optical memory apparatus according to claim 16, wherein said means for detecting optically comprises projection means for projecting a laser beam on a movable portion of said second actuator, light reception means for receiving reflected light from said portion and detecting means for detecting the movement of said portion.

18. An optical memory apparatus according to claim 17, wherein said second actuator comprises a deflecting mirror for deflecting the light beam.

19. An optical memory apparatus according to claim 12, wherein said detecting means comprises means for detecting magnetically the movement of said second actuator.

20. An optical memory apparatus according to claim 19, wherein said second actuator is an actuator which moves an objective lens, having a field of view, perpendicular to an optical axis and in the radial direction of said recording medium for the purpose of following and said means for detecting magnetically comprises means for detecting the deviation between the optical axis and the field of view of said objective lens.

21. An optical memory apparatus according to claim 15, comprising a velocity control means for generating the velocity signal to drive said first actuator which moves the laser spot from the current track to the desired track subject to the condition that the difference between the track currently read out and the desired track is larger than a certain set number;
first switching means for switching said velocity signal to the output of said detecting means and for supplying said first actuator and;

second switching means for switching the output of said detecting means to the tracking signal and for supplying said second actuator.

22. An optical memory apparatus according to claim 12, further comprising jumping means for generating a jump sign signal for controlling said second actuator and performing a jump by which the laser beam is moved to the desired track subject to the condition that the difference between the track currently read out and the desired track is smaller than a certain set number.

23. An optical memory apparatus according to claim 22, comprising adder means for adding said jump sign signal to the first control signal.