Note: Descriptions are shown in the official language in which they were submitted.
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'' .
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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
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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
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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
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_ 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.
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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
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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
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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
98
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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~
- 20 -
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
- 23 -
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,
,
- 25 -
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~
- 26 -
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
, . ~
- 27 -
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~
- 29
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
- 30 -
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
- 31 -
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
~
- 32 -
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~
- 33 -
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
- 34 -
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~
- 35 -
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
- 39 -
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.
