Note: Descriptions are shown in the official language in which they were submitted.
AMPLITUDE MODULATED COARSE
POSITION ERROR SIGNAL. GENERATION
IN AN OPTICAL DISK STORAGE SYSTEM
EMPLOYING COARSE SERVO TRACKS
BACKGROUND OF THE INVENTION
This invention relates to optical disk data storage systems, and
more particularly to a system and method for generating a coarse
position error signal for use in a coarse servo system of an optical
disk data storage system.
Optical data storage systems that utilize a disk to optically
store information have been the object of extensive research. Like
their counterpart magnetic disk units, these optical disk storage
units must have a servo system which controls the positioning of a
read/write head to provide direct access to a given track of data
recorded on the rotating disk. Further, once a desired track has
been accessed, the servo system must cause the read/write head to
accurately follow this track while it is being read or when data is
initially written there onto.
Numerous approaches have been proposed in the art for providing
the desired access and tracking capability. While a discussion of
such prior-art approaches provides interesting background
information applicants do not believe that such a discussion is
necessary to teach and understand the operating principles of the
invention described herein. Accordingly no such background
discussion is repeated.
Whatever the type of access and tracking system employed some
sort of detection means must be used to generate an error signal
that can be used by the appropriate servo system to guide the
positioning of the read write head to a desired radial position with
respect to the disk, and to maintain this desired position once
reached. For example, a coarse/fine servo system may be used for
this purpose. In such a system, coarse servo tracks, selectively
placed on the disk, allow the coarse servo system to access and
track a relatively large band on the disk. The fine servo system is
then used to access and track a desired data track within the band.
With respect to the course servo system, a narrow strip of radiant
energy incident to a detector array can be sensed, and a signal
generated having an amplitude proportional to the location at which
the strip of radiant energy strikes the array. my selectively
placing spaced-apart coarse servo tracks on the disk, and then by
illuminating through the read/write head an area of the disk large
enough to always include a segment of one of these coarse servo
tracks, the reflected radiant energy prom the illuminated coarse
servo track becomes a narrow strip of radiant energy that may be
directed back through the read/write head to the surface of the
detector array. The signal generated by the array can then be used
as the needed error signal to indicate the location of the
read/write head relative to a given coarse track This error signal
is used, in turn, by a coarse positioning servo system to place the
read/write head at a desired location SO as to provide the requisite
coarse access and tracking capability.
The use of a detector array as described above is not without
its drawbacks. An array is by definition a collection of discrete
radiation-sensitive elements arranged in a systematic fashion. As
such, the output signal generated will have minor discontinuities
therein as the radiant energy moves from one element to another.
These discontinuities may impact the linearity of the signal thus
generated, and are therefore undesirable.
Further depending upon the size of the array and the number of
elements used therein, it may actually be necessary to store the
information sensed by each element and serially pass this
information out of the array through a single pin or terminal,
thereby minimizing the number of input/output pins associated with
the detector array. If such is the case, a clock signal, or
equivalent, must be used in order to clock the data out of the
device. This imposes a finite processing time during which the
sensed position data is serially passed out of the array,
reconfigured, and examined. This "processing time" may
disadvantageously limit the access speed associated with moving the
read/write head from one coarse track to another. Further, the
circuitry required to carry out -this signal processing is quite
complex and expensive to implement, and all the multiplexing
involved generates undesirable noise that may adversely impact the
data.
As a still further disadvantage, the amplitude of an error
signal generated in arrays of the type described above may not only
be a function of the sensed position of the radiant energy (as
desired), but it may also be a function of the intensity of the
radiant energy as it strikes the array surface. Thus in order to
preserve the integrity o-f the position error signal, the intensity
of the radiation incident to the detector must be held more or less
constant. Unfortunately, this is an extremely formidable task when
dealing with radiant energy that is reflected off of a rotating
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disk, which reflected radiant energy may vary a great deal in
intensity.
Moreover, more often than not, radiation incident to the array
may be reflected from more than just the desired coarse servo track,
e.g., from data tracks. Some means is needed therefore to identify
and distinguish the desired reflected radiation from any undesired
reflected radiation that may be present.
What is needed, therefore, is a detection system that provides a
continuous linear output signal that indicates the position of
radiant energy incident thereto, that is reflected from only a
coarse servo track, and not from adjacent data tracks, and that is
insensitive to variations in the intensity of the incident radiation.
SUMMARY OF_THE_INVENTION
It is an object of the present invention to provide a linear
detector system for use with a coarse positioning servo system ox an
optical disk data storage system that generates a position error
signal having an amplitude that is proportional to the location of a
strip or band of radiant energy incident thereto.
It is a further object of the present invention to provide such
a linear detector system that it is especially suited fox use with a
coarse positioning servo system employing concentric coarse servo
tracks on an optical disk, a reflected image of a segment of a
coarse servo track being directed through appropriate optical
elements to the linear detector system of the present invention.
A still further object of the present invention is to provide
such a linear detector system wherein the amplitude of the position
5 ~%~
error signal is substantially independent of the intensity of the
incident radiant energy falling thereon.
An additional object of the present invention is to provide such
a linear detector system that responds only to radiation reflected
from the coarse servo tracks, and that is insensitive to radiation
reflected from other than the coarse servo tracks.
Still another object of the present invention is to provide such
a linear detector system wherein the position error signal is
continuously generated, and is not dependent upon the use of clock
signals, or equivalent, in order to gain access to and process the
position information sensed by said detector system.
The above and other objects of the invention are realized by
employing a linear detector system, described more fully below, as
an element in a coarse servo positioning system of an optical disk
data storage system.
The optical disk storage system includes means for rotating an
optical disk and means for controllable positioning a read/write
head radially with respect to said disk, thereby allowing radiant
energy, typically laser energy, passing through said read/write head
to be directed to desired locations on the surface of the rotating
disk. Such radiant energy is used to selectively mark (write) the
disk with desired information, or to read (sense radiant energy
reflected from the previously-written marks) the information already
on the disk.
Included within the coarse servo positioning system are coarse
servo tracks, typically concentrically placed on the disk. As
described below, these coarse tracks are used as markers or sign
posts to guide the read/write head to a desired radial position with
respect to a given coarse track. Coarse illumination means direct
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radiant energy through the read/write head to the surface of the
rotating disk. This radiant energy strokes an area large enough on
the surface of the disk to ensure that at least a segment of one
coarse servo track is always illuminated. Reflected radiant energy
from the surface of the disk therefore includes the coarse track
segment within the illuminated area. This reflected energy is
directed back through the read/write head to the linear detection
system of the present invention.
Advantageously, the coarse servo track has a particular
reflectivity pattern associated therewith such that radiation
reflected therefrom can be uniquely distinguished from radiation
reflected from other portions of the area illuminated by the coarse
illumination means. Lyons, by using appropriate discrimination
means to identify the coarse servo track radiation, the linear
detection system of the present invention generates an error signal
having an amplitude that is linearly proportional to the distance at
which this coarse servo radiation falls on a collection surface of a
detector used within said system. This distance is measured
relative to a fixed reference point on the collection surface.
Advantageously, the amplitude of the error signal generated by the
linear detection system of the present invention is substantially
independent of the intensity of the radiant energy.
In the preferred embodiment the reflectivity pattern of the
coarse servo track is a repetitive on off (reflectivity
high/reflectivity low) sequence such that when the disk is rotated
at a constant speed, the reflected radiation from the coarse servo
track assumes a pulsed condition having a known frequency. Once
converted to a corresponding electrical signal, filtering techniques
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are used to distinguish this fixed-frequency radiation from other
radiation.
Two reference signals are derived from circuitry associated with
the collection surface of the detector. A first reference signal
has an amplitude proportional to the intensity of the radiant energy
and the location that said radiant energy falls on the collection
surface relative to a first reference point. A second reference
signal has an amplitude proportional to the intensity of the radiant
energy and the location that the radiant falls on the collection
surface relative to a second reference point. The sum and
difference of the amplitudes of these first and second reference
signals are derived to produce sum and difference signals,
respectively. The difference signal is then divided by the sum
signal to produce the desired error signal, which error signal has
an amplitude that is substantially independent of the intensity of
the radiant energy. Filtering, or other suitable discrimination
techniques, may be used at essentially any point prior to dividing
the difference signal by the sum signal as these reference signals
are processed in order to limit the sensitivity of the detection
system to only that radiation reflected from the coarse servo tracks.
The position error signal is used by the coarse servo
positioning system as a feedback signal to control the radial
position of the read/write head with respect to the disk In a seek
or access mode, the read/write head will be moved radially with
respect to the disk until the read/write head is above or near a
desired coarse servo track. While so moving, the position error
signal assumes a sawtooth waveform, each cycle of which corresponds
to the movement from one servo track to an adjacent servo track.
Once a desired coarse servo track has been reached, a tracking mode
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is assumed during which the read/write head is held in a fixed position relative
to the desired coarse servo track by monitoring the amplitude of the position
error signal.
Thus, in accordance with a broad aspect of the invention, there
is provided a linear detector for generating a position error signal for use
in a head positioning servo system of an optical disk storage system having
coarse servo tracks rewritten on a rotating disk, said coarse servo tracks
being adapted to modulate light energy reflected therefrom according to a pro-
scribed pattern, said linear detector comprising: signal generating means for
generating two signals in response to an incident light beam falling on a
collection surface of said generating means, a first of said signals having a
signal amplitude proportional to the distance of said light beam from a first
end of said collection surface, and the second of said signals having a signal
amplitude proportional to the distance that said light beam is from a second
end of said collection surface; sunmlation means for adding said first and second
signals and producing a sum signal therefrom having an amplitude equal to the
sum of the amplitude of said first and second signals; difference means for
subtracting said first and second signals and producing a difference signal
therefrom having an amplitude equal to -the difference between the amplitudes of
said first and second signals; signal processing means for demodulating said
sum and difference signals; and dividing means err dividing said demodulated
difference signal by said demodulated sum signal and producing an output signal
therefrom said output signal having a signal amplitude that is proportional
to the linear position of said light beam reflected from said coarse servo
track on said collector surface as measured relative to one of said ends there-
of.
In accordance with another broad aspect of the invention there is
- pa
provided a method for generating a linear position error signal for use in
an optical disk storage system that indicates the linear position of a eon-
trod of radiant energy incident to a collection surface of a linear detector,
said strip of radiant energy corresponding to reflected energy from a segment
of a coarse data track written of a rotating disk used within said storage
; system, said position being measured relative to a known reference point on
said collection surface, said method comprising the steps of:
(a) modulating radiant energy reflected from said coarse servo
track according to a known pattern;
(b) generating a first reference signal having an amplitude pro-
portion Al to the intensity of radiant energy and linearly proportional Jo the
location that the sauntered of said radiant energy falls upon said collection
surface as measured relative to a firs reference point thereon;
. (c) generating a second reference signal having an amplitude
I` proportional to the intensity of said radiant energy and linearly proportional
to the location that the sauntered of said radiant energy falls upon said
I,
I- collection surface as measured relative to a second reference point thereon;
(d) processing said eeriest and second reference signals so as to
effectively eliminate therefrom all signal components attributable to sources
other than the modulated radiant energy from said coarse servo crack;
(e) summing the amplitude of said first and second reference
signals to produce a sum signal;
(f) subtracting the amplitude of said first and second reference
signals to produce a difference signal; and
(g) dividing said difference signal by said sum signal -to produce
said linear position error signal, said position error signal having an amply-
tune linearly proportional to the distance that said beam of light falls upon
- 8b -
said collection surface as measured relative to one of said first or second
reference points, and said position signal amplitude being substantially in-
dependent of the intensity of said beam of light.
In accordance with another broad aspect of the invention there is
provided apparatus for producing a linear position error signal for use in
a servo control system of an optical disk storage system, said linear position
error signal being used to controllable position a read/write head of said
optical disk storage system with respect to one of a plurality of concentric
coarse servo tracks located on a rotating disk, said apparatus comprising:
first means for generating a laser beam; second means for directing said
laser beam through said read/write head to said rotating disk, said laser
beam falling upon a surface of said rotating disk with a spot size suffix
: ciently large to illuminate at least a segment of one of the concentric
coarse servo tracks located on said disk, each of said coarse servo tracks
on said disk being configured so as to modulate that portion of said laser
beam reflected from said coarse servo track with a fixed frequency as said
disk rotates at a constant velocity; third means for directing those port
lions of said laser beam reflected from said disk through said read/write
head to a collection surface of a stationarily mounted linear detector, said
linear detector comprising: first signal generating means for generating a
first reference signal having an amplitude proportional to the intensity of
the laser beam energy incident to said collection surface, and linearly pro-
portion Al to the location at which an energy sauntered of said reflected laser
beam energy strikes said collection surface as measured with respect to a
first reference point on said collection surface, and second signal general-
in means for generating a second reference signal having an amplitude
; proportional to the intensity of the laser beam energy incident to said
,
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Jo collection surface, and linearly proportional to the location at which the
energy sauntered of said reflected laser beam energy strikes said collection
surface as measured with respect to a second reference point on said collection
surface; fourth means for summing the amplitude of said first and second
, reference signals to produce a sum signal; fifth means for subtracting thei, amplitude of said first and second reference signals to produce a difference
signal; demodulation means for limiting said sum and difference signals to
only that portion of said laser beam reflected from said coarse servo tracks;
to`, and sixth means for dividing said difference signal by said sum signal to pro-
,~, 10 dupe said linear position error signal, said linear position error signal having
, an amplitude that is linearly proportional to the distance that said laserbeam energy falls upon said collection surface as measured relative to one of
said first or second collection surface reference points, and said linear post-
lion error signal amplitude being substantially independent of the intensity
of said laser beam energy at said collection surface.
In accordance with another broad aspect of the invention there is
provided a system for generating a position error signal for use in a head
5. positioning servo system of an optical disk storage system having a plurality
of data tracks interposed between coarse servo tracks on a rotating disk, said
20 system comprising means for placing a first reflectivity pattern into each of, said coarse servo tracks such that when a given coarse servo track is optically
I/ read by measuring the radiant energy reflected therefrom, said reflected radiant
energy periodically assumes maximum and minimum values having a known frequency
associated therewith wren said disk is rotated at a known rotational speed;
means for selectively controlling the reflectivity patterns associated with
each of said data tracks in accordance tooth a desired data pattern such that
when a given data tract is optically rear by measuring the radiant allergy
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a'
reflected therefrom" said reflected radiant energy assumes maximum and r,ninimur3l
values in accordance with said data pattern; means for simultaneously sensing
the radiant energy reflected from an illuminated area of said disk, said area
including a segment of at least one of said coarse servo tracks and segments
of a plurality of said data tracks, whereby said radiant energy reflected from
said area comprises a composite of the radiant energy reflected from segments
of at least one of said coarse servo tracks and a plurality of said data tracks;
radiation detection means for detecting the radiant energy reflected from
said illuminated area and for generating two signals in response thereto, a
first of said signals having a signal amplitude proportional to the distance
that a sauntered of said reflected radiant energy falls on a collection surface
of said radiation detection means relative to a first reference point on said
collection surface and the second of said signals having a signal amplitude
proportional to the distance that said sauntered of said reflected radiant
energy falls on said collection surface relative to a suckled reference point
on said collection surface; first signal processing means for respectively
processing said two signals and effectively eliminating therefrom all components
except those attributable to the reflected radiant energy reflected from said
coarse servo track at said known frequency; second signal processing means
for processing said two signals so as to generate therefrom said position error
signal, said position error signal having a signal amplitude that is proper-
tonal to the distance that the reflected energy from said coarse servo track
falls on said collection surface relative to a known reference point on said
collection surface.
In accordance with another broad aspect of the invention there is
provided apparatus for producing a linear position error signal for use in a
servo control system of an optical disk storage system, said linear position
. .
:.
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;
error signal being used to controllable position a read/write head of said
optical disk storage system with respect to one of a plurality of concentric
coarse servo tracks located on a rotating disk, said apparatus comprising:
first means for generating a laser beam; second means for directing said laser
beam through said read rite head to said rotating disk, said laser beam falling
UpOlI a surface of said rotating disk with a spot size sufficiently large to
illuminate at least a segment of one of the concentric coarse servo tracks
located on said disk, each of said coarse servo tracks on said disk being con-
figured so as to modulate that portion of said laser beam reflected from said
lo coarse servo track with a fixed frequency as said disk rotates at a constant
velocity; third means for directing those portions of said laser beam reflect-
Ed from said disk through said read/write head to a collection surface of a
` stationarily mounted linear detector; said linear detector comprising signal
generating means for generating at least one reference signal having an amply-
tune proportional to the intensity of the laser beam energy incident to said
collection surface, and linearly proportional to the location at which an
' energy sauntered of said reflected laser beam energy strikes said collection
surface as measured with respect to a reference point on said collection sun-
face; fourth means for limiting said reference signal to only that portion of
said laser beam reflected from said coarse servo tracks; and fifth means for
maintaining the intensity of said reflected laser beam as measured at said
collection surface at a substantially constant level, whereby said reference
signal assumes an amplitude that is substantially solely a function of the
location at which said energy sauntered strikes said collection surface.
BRIEF DESCRIPTION OF TIRE DRAWINGS
-
The above and other objects, features, and advantages of the
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9 ' .
I,
present invention will be more apparent from the following more part-
I, cuter description thereof, presented in conjunction with the follolling
I Drawings wherein:
Figure 1 is a block diagram of a coarse/fine servo system
used in an optical disk data storage system, and illustrates the environ-
mint in which the present invention is designed to be used;
; Figure 2 schematically shows the principle elements of
Jo Figure l;
,,
Figure 3 is a side view of an optical disk drive and schemata
icily shows the relationship between the optical disk fixed and moving
' optics packages, and a linear actuator for controllable positioning
; the read/write head;
j Figure 4 conceptually illustrates how the intensity of the
radiation reflected from the optical disk may vary as a function of disk
radial position between two coarse servo tracks;
Figure 5 is a block diagram of the coarse track detection
` system of the present invention;
Figure 6 is an expanded view of a segment of the optical disk
surface and conceptually illustrates the reflectivity-high/reflectivity-
low pattern placed in the coarse servo tracks;
Figure 7 is a timing diagram that illustrates the type of
radiation signal reflected from the coarse illuminated area ha of
1 I
g
the optical disk surface (FIG. 6) when the disk is rotated at a
constant angular velocity, both before and after filtering;
FIG. 8 is a block diagram of an alternative configuration of the
coarse track detection system of the present invention; and
FIG 9 is a schematic diagram of the configuration shown in FIG.
8.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is best understood by reference to the
accompanying drawings wherein like numerals will be used to describe
like elements or parts throughout.
FIG. 1 shows a block diagram of a coarse/fine servo system of a
type with which the present invention could be used. The various
optical paths associated with the system shown in FIG. 1 are
illustrated as bold lines, whereas electrical paths are indicated by
fine lines. Mechanical coupling, as occurs between a carriage
actuator 24 and the carriage optics 23, is indicated by a dashed
line.
Referring next to both FIG. 1 and FIG. 2, the optical disk
storage system can be explained. The system allows reading and
writing from and to the surface of a disk 11 having a rotational
axis 10 and a plurality of concentric data bands 12-14 (shown in
FIG. 2). Each of the data bands includes a plurality of data tracks
concentrically spaced about the rotational axis. The surface of the
disk 11 has prerecorded thereon, during manufacture, a plurality of
optically readable servo tracks 16-19~ concentrically and uniformly
spaced about the rotational axis of the disk and positioned between
the data bands.
.
--lo--
The disk 11 is rotated about its axis 10 by conventional means.
An optical read/write head depicted by the carriage optics block
23, is positioned adjacent to the surface of the disk 11. Carriage
actuator 24 selectively moves the read/write head along a radial
axis 20 (FIG. 2), thereby moving the carriage optics 23 in a radial
direction with respect to the disk 11 in order to access the data
bands thereon. Mechanical motion of the carriage optics 23 is
depicted in FIG. 2 as a dotted line 45, with motion being possible
in both directions as indicated by the double headed arrow 45'.
A fine read/write servo illuminator and detector 25 (FIG. 2)
projects read or write light beam(s) 52' to the surface of the disk
11 so as to access data tracks thereon. In order to access the disk
surface, this beam 52' is reflected by a fine tracking mirror 26,
passes through a beam combiner and separator 27, as well as through
the carriage optics 23. Included within the illuminator and
detector 25 is a read detector 25b (FIG. 1) that reads light which
has been reflected from the accessed recorded data track. This
reflected light passes through the carriage optics 23 and beam
combiner and separator 27 before reaching the read detector 25b.
The read detector converts this light to an equivalent electrical
signal(s). This read electrical signal is in turn, supplied to a
data read system 25c, and to a wine access/tracking servo system 25d.
The servo system for access to and tracking of the coarse servo
tracks includes a coarse illuminator I which projects light,
represented as dashed double-dot lines in FIG. 2, through a coarse
servo beam separator 36, a beam combiner and separator 27, and the
carriage optics 23 onto a relatively broad portion ha of the disk
surface (FIG. 2). An optical detector 31 detects reflected light,
represented as dashed single-dot lines in FIG. 2, from the portion
L
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" _ 1 1 _
ha of the disk surface. It is noted that the illuminated portion
ha of the disk surface spans at least the distance between two
coarse servo tracks, and thereby always illuminates at least one
coarse servo track. As shown in FIG. 2, light is reflected from the
portion ha of the disk 11 between servo tracks 16 and 18 with the
servo track 17 being projected onto coarse detector and processing
circuitry 31. It is this coarse detector and processing circuitry
31 that comprises the principle element of the present invention,
and is described more fully below.
The output of the coarse detector and processing circuitry 31 is
a coarse track position error signal (PUS), which signal has an
amplitude proportional to the location at which the reflected
radiation from the illuminated coarse servo track falls on the face
of the detector 31. This error signal from the detector 31 is
applied to a coarse access/tracking system 34. This system is
connected in a servo loop with the actuator 24, which actuator moves
the read/write head (represented schematically by the carriage
optics 23) into radial proximity of a selected servo track so that
the fine access and tracking system 25d can accurately position read
or write beams on a selected data track.
As indicated previously, light reflected from a single data
track on the disk is passed by means of the carriage optics 23, beam
separator 27, and tracking mirror 26, and is detected by read
detector 25b, the output of which is applied to the fine
access/tracking servo system 25d. The read or write beams 52' from
the illuminator aye are moved radially with respect to the optical
disk 11 by means of the tracking mirror 26, thereby providing for
fine selective control of the beam's radial position. The tracking
mirror 26, which may be a conventional galvanometers controlled
~%~ 8
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;`
,` -12-
'I
mirror(s), is controlled by the fine access/tracking servo system
Jo 25d.
In order to discriminate radiation reflected from servo tracks
-from that reflected from data tracks or other areas of the disk
surface, the servo tracks have an on/off
(reFlectivity-high/reflectivity-low) pattern placed therein that may
be conceptually thought of as a dashed line, as shown best in FIG.
6. This is explained more fully below. Further, the servo tracks
are preferably three to five times the width of the data tracks.
The servo tracks provide improved data track following capability by
providing coarse tracking control of the read/write head. The
coarse tracks are also used to permit rapid random access to a data
band, regardless of whether any data has been recorded in the fine
track area. (Note, a data band is that region of the disk surface
between servo tracks.) This provides the ability to skip to
randomly selected data bands for reading or writing. seeking to a
selected band may be accomplished by counting coarse tracks, in
conjunction with analog or digital servo techniques commonly used in
magnetic disk drives.
FIG. 3 is a side view that schematically shows the relationship
between the optical disk 11 and a moving optics package 40 that is
driven by the carriage actuator 24 into a read/write relationship
with any of the tracks on the disk 11. The carriage actuator 24 may
be realized with a linear motor, such as a voice coil motor, that
i includes a stationary magnet 41 and a movable coil 49. The optical
s ` path for either the read or write light team to the surface of
the disk 11 includes an objective lens SO, mirror aye telescope lens
43~ and mirror 44. Light is transmitted to and from the moving
optics package 40 through a suitable optics package 47 mounted to a
-13-
fixed optic plate I on which the remainder of the optics are
mounted. The details associated with this optics package are not
pertinent to the present invention. Any suitable technique could be
used within the optics package so long as the radiation reflected
from that segment of the coarse track illuminated in the area ha
(FIG. I) is directed to the coarse detector 31.
Referring next to only FIG. pa it is seen that the coarse
detector 31 comprises a detector 61 having a radiant energy
collection surface 62 upon which radiation reflected from the disk
surface area ha is projected. This radiation has a energy sauntered
or "center-of-mass" 63 associated therewith, which energy sauntered
represents that point at which a single ray of radiation, having an
intensity equivalent to all the radiation falling upon the surface
62, would fall on the surface 62. The detector 61~ as explained
more fully below, generates two separate output signals that are
directed to signal processing circuitry 64 over signal lines 65 and
The output from the signal processing circuitry 64, the PUS
signal, is directed to the coarse access/tracking servo system 34
over signal line 67.
FIG. 4 conceptually depicts the levels of radiation that would
be reflected from the surface of -the disk along a radial axis
thereof as a function of radial position. At a first coarse servo
track N, a large amount of radiation is reflected (assuming that the
writing of the coarse servo track creates a high-reflectivity
condition). In the data band between this first Crusoe servo track
N and an adjacent second coarse servo track Nil a varying amounts of
radiation are reflected depending upon the presence or absence of
data tracks and the type of data therein. This radiation is
typically much lower in intensity than the radiation associated with
, --14--
. `
the coarse servo tracks because the width of the data tracks is 3-5
times smaller than the width of the coarse servo tracks.
Nonetheless, the radiation reflected from the data tracks within the
data band can adversely impact the location of the energy sauntered
of all the radiation reflected from the illuminated area ha (FIGS.
2 6). Hence, in order to assure consistency in locating the
energy sauntered regardless of whether data tracks are present or not
within the data band, some means must be employed to identify only
that radiation reflected from the coarse servo track. A known
reflectivity pattern is placed in the coarse servo tracks for this
purpose so that the reflected radiant energy therefrom can be
distinguished from reflected radiant energy from the data tracks
which may or may not be present.
In the preferred embodiment, the reflectivity pattern selected
for the coarse servo tracks is a repetitive on/off scheme such that
the coarse servo track appears as a dashed line. This concept is
best illustrated by the coarse servo tracks 17 and 18 in FIG. 6. A
small segment of the coarse servo track aye is written, causing a
high reflectivity condition to exist. This high reflectivity
segment aye is followed by a segment 18b where no coarse servo track
is written, causing a low reflectivity condition to exist. (In the
preferred embodiment, the optical disk 11 exhibits low reflectivity
if not written upon, and high reflectivity if written upon. This
situation could, of course, be reversed without alienating the basic
operating principles of the present invention.) As the disk 11 is
rotated at a constant angular velocity, the coarse servo track
illuminated in the coarse illuminated area ha will alternately
reflect high and low amounts of radiation. By making the high
reflectivity segments of the coarse servo track equal in length, and
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by making the low reflectivity segments also equal in length, the
reflected radiation from the coarse servo track assumes a periodic
pulsed pattern having a known frequency. By selecting the
fundamental frequency of the reflected radiation From the coarse
servo track to be different from the primary frequency components
associated with data that is recorded on the data tracks, this
coarse track frequency can then be used as the mechanism for
distinguishing the radiation reflected from the coarse servo track
from that reflected from the data tracks.
The waveform A in FIG. 7 conceptually illustrates how the
reflected radiation from the illuminated area ha of the disk's
surface appears as a function of time, i.e., as different portions
of the disk 11 are rotated into and out of the illuminated area ha
at a constant velocity. The waveform B in FIG. 7 depicts how the
waveform A could be "cleaned up" using filtering or equivalent
techniques in order to pick out just those components of the
waveform A that are attributable to the fixed-~requency radiation
reflected from the coarse servo track.
Referring next to FIG. 5 there is shown a block diagram of the
coarse detector 31 of the present invention. As explained
previously, the detector 61 includes a collection surface 62 upon
which reflected radiation from the coarse illuminated area ha (FIGS
2 & 7) is projected. This collection surface 62 is schematically
depicted in FIG. 5 as a current generator because, as explained
below, it generates two currents, each having an amplitude
proportional to the intensity and location that the radiation falls
on the collection surface respective to known reference points
thereon. (The collection surface is typically a rectangle having
known dimensions associated therewith. In the preferred embodiment,
8~8
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the reference location points are the respective ends of the
collection surface.) As explained, radiation of varying levels
actually falls upon much of the collection surface due to the
radiation reflected from the data tracks. Louvre, all of this
radiation is equivalent to a single ray of radiation 63 falling upon
the collection surface at the "energy sauntered" location.
As explained, a first signal generated by the detector 61 is a
current signal having an amplitude proportional to the intensity of
the radiant energy falling upon the collection surface 62 and the
distance between a first end of the collection surface 62 and the
location where the sauntered of radiant energy 63 strikes the
collection surface 62. A second output signal from the detector 61
is likewise a current signal having an amplitude proportional to the
intensity of the radiant energy incident to the collection surface
62 and the distance between a second end of the collection surface
62 and the point where the sauntered of radiant energy 63 falls upon
the surface 62.
The processing circuitry 64 includes transimpedance amplifiers
70, 71 that respectively convert the current signals from the
detector 61 to voltage signals. The outputs of the transimpedance
amplifiers 70, 71 are then directed to respective band pass filters
72, 73, which band pass filters are designed to have a center
frequency equal to the fixed-frequency of the radiation reflected
from the coarse servo tracks. Thus, while the input signals to the
band pass filters 72, 73 may be a composite of all the radiation
striking the collection surface 62, such as illustrated in waveform
A of FIG. 7, the output signals from these band pass filters are
limited Jo only that radiation reflected from the coarse servo
tracks, such as illustrated in wave-form B of FIG. 7. A rectifier
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and envelope detector circuit 74, 75 is then employed to generate a
signal proportional to the amplitude of the signal outputted from
the respective band pass filter circuits 72, 73. The output signal
from the rectifier and envelope detector circuit 74 is then
subtracted from the output signal from the rectifier and envelope
detector circuit 75 in a difference amplifier 76. Similarly, the
output signal from the rectifier and envelope detector circuit 74 is
summed with the output signal from the rectifier and envelope
detector circuit 75 in a summing circuit 77. The outputs of the
difference amplifier 76 and summing amplifier 77 are then coupled to
a divider circuit 78 in such a manner so as to cause the output of
the difference amplifier 76 to be divided by the output of the
summing amplifier 77. The output signal from the divider circuit 78
is the desired position error signal, or PEST
An analysis of the configuration shown in FIG. 5 reveals that
the position error signal will have an amplitude proportional to the
distance from one of the ends of the collection surface 62 that the
sauntered of the radiant energy associated with the coarse servo
track falls upon said surface, but substantially independent of the
intensity of the radiant energy falling upon said surface 62. the
divider circuit normalizes any energy variations.) Hence the
desired characteristics (proportional to distance but not to
intensity) have been realized.
It should be noted that the processing circuit 64 ox FIG. 2
could be realized using alternate configurations from that shown in
FIG. 5. One such alternate configuration is discussed below in
connection with FIG. 8. Another alternate configuration would
involve a system that maintains the average intensity of the
reflected laser energy as sensed at the detector 61 at a
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substantially constant level. Such a system would typically include
feedback from the processing circuitry 64 to the coarse illuminator
30 (shown as a dotted line 31' in FIG. 2) in order to control the
intensity of the incident laser beam. A suitable tolerancir,g system
for controlling the laser beam intensity and disk reflectivity could
also be used.
Referring next to FIG. 8, there is shown a block diagram of an
alternative configuration from that shown in FIG. 5. In FIG. 8, the
order or sequence of processing the signals from the detector 61 has
been altered from the processing sequence associated with FIG. 5.
In FIG. 8, the use of the detector 61 and transimpedance amplifiers
70, 71 remains unchanged from FIG. 5. However, in FIG. 8 buffer
amplifiers 80, 81 are interposed between the transimpedance
amplifiers 70, 71 and a sum amplifier 83 and difference amplifier
82. Band pass filters 84, 85, followed by demodulation circuits 86,
871 and Lopez filter circuits 88, 89, are then employed to process
the outputs from the sum and difference amplifiers 82, 83,
respectively, prior to presenting these processed signals to the
divider circuit 78.
FIG. 9 is a schematic diagram of the configuration shown in the
block diagram of FIG. 8, not including the detector 61 and
transimpedance amplifiers 70, 71. The details associated with the
band pass filter 85~ demodulator 87, and low pass filter 89 are not
shown in FIG. 9 because they are either identical to or easily
derived from the circuits of the band pass filter 84, demodulator 86,
and low pass filter 88, respectively. Representative circuit
components for the schematic diagram of FIG. 9 are as indicated in
Table 1. The components specified in Table 1 assume a detector 61
is used as described below.
. lug
Table 1
Representative Component values for FIG. 9
Rl-lOK Cl-8pf CRl-IN4448
R2-30K C2-3pf Ul,U2,U3,U4-LF356A
R3-3.3K C3-18pf U5-NE592
R4-60K C~-120pf U6-LF357A
R5-lOK Caliph U7,U8,U9-LF353A
R6-320 C6-500pf U10-AD535
R7-100 C7-820pf
R8-lK Caliph
R9-35K C9-5pf
R10-l.lK C10-6800pf
Lowe C11-1800pf
The detector 61, including the collection surface 62, may be
realized using a commercially available component manufactured by
United Detector Technology, Inc., of Santa Monica, California. A
United Technology "LSC" position sensing detector is particularly
well suited for this use. Specifically, a United Detector
Technology part number PIN-LSC/5D has been successfully used by
applicants for this function. This device has an active area
(collection surface 62) of 0.115 square centimeters. The length of
the collection surface is roughly 0~21 inches (0.53 cm.).
Any suitable transimpedance amplifier, available from numerous
IT manufacturers, could be employed for the amplifiers 70 and 71.
In particular, an operational amplifier HA-5170 manufactured by
Harris Semiconductor could be used for this purpose. (As those
skilled in the art will recognize, any operational amplifier can be
configured to function as a transimpedance amplifier.) Similarly,
as described above in conjunction with FIG. 9, the difference and
'
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summing amplifiers 76 and 77 (or 82 and 83) may be realized using
commercially available integrated circuit operational amplifiers,
such as the LF353 manufactured by National Semiconductor. The
divider circuit 78 may be realized with an AUDI Divider,
manufactured by Analog Devices.
; While a particular embodiment of the invention has been shownand described, various modifications could be made thereto that are
within the true spirit and scope of the invention. The appended
; claims are, therefore, intended to cover all such modifications.
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