Note: Descriptions are shown in the official language in which they were submitted.
7~
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 rrlagnetic 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 thereonto.
Numerous approaches have been proposed in the art for providing
the desired access and tracking capability. Prior art approaches
relating to access and tracking systems are discussed in Patent
Cooperation Treaty (PCT) International Application No. WO/84/01849,
published 10 May 1984 by the World Intellectual Property
Organization (WIPO). While a discussion of such prior-art
approaches provides interesting background information, applicant
does 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
3~37~
that can be used by the appropriate servo system to guide the
positioning of the read/write head to a desired radial position wi-th
respect to the disk9 and to maintain this desired position once
reached. In the above cited application, a detector array is
disclosed for this purpose. According to the teachings therein, a
narrow strip of radiant energy incident to the 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. By 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 from 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 readtwrite head relative to a given coarse track. This error
signal is used, in turn, by a coarse position servo system to place
the read/write head at a desired location so as to provide the
requisite access and tracking capability.
While- the detector array disclosed in the above-cited
application adequately performs its intended function, and
represents the best mode of carrying out the invention disclosed
therein at the time the invention was made, such a detector array is
not without its drawbacks. An array is by definition a collectior,
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 an~ther. rrhese 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
equivalent9 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.
As a still further disadvantage, the amplitude of the error
signal generated in arrays of the type disclosed in the above-cited
application 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 of 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 disk, which reflected radiant energy may vary a
great deal in intensity.
Also, detector arrays of the type disclosed in the
above-described application must be realized from somewhat complex
circuits, employing a large number of discrete components. Such
complex circuits are expensive (in terms of both time and money) to
build and maintain.
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~ hat is needed, therefore, is a simple, less-expensive detection
system that provides a continuous linear output signal that
indicates the position of a narrow strip of radiant energy incident
thereto, and that is insensi-tive 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 of an
optical disk data storage system that generates a position error
signal having an amplitude that is linearly proportional to the
location of a strip 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 for use with a
coarse 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 optics to the linear detector
system of the present invention.
A stil1 further object of the present invention is to provide
such a linear detector system wherein the amplitude of the position
error signal is substantially independent of the intensity of the
incident radiant energy falling thereon.
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 dependant upon the use of clock
signals, or equivalent, in order to gain access to and process the
position information sensed by said detector system.
A still further object of the present invention is to provide
such a linear detector system that is simple and inexpensive to
build, yet that prnvides repeatable, reliable performance.
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 systemO
The optical disk storage system includes means for rotating an
optical disk and means for controllably 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
radiant energy through the read/write head to the surface of the
rotating disk. This radiant energy strikes an area large enough on
the surface of the disk to insure that at least a sector of one
coarse servo track is always illuminated. Reflected radiant energy
from the surface of the disk therefore includes the location of the
coarse track sector within the illuminated area. This reflected
energy is directed back through the read/write head to the linear
detection system of the present invention.
The linear detection system generates an error signal having an
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amplitude that is linearly proportional to the distance at which the
narrow strip of radiant energy (reflected from the coarse track on
the surface of the dislc) falls on a collection surface of a detector
used within said system as measured relative to a fixed reference
point on said collection surface. The amplitude of the error signal
is substantially independent of the intensity of the radiant
energy. Two reference signals are derived from circuitry associated
with the collection surface. 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.
The position error signal is used by the coarse servo
positioning systern as a feedback signal to control the radial
position of the read/write head with respect to said disk. In a
seek or access mode, the read/write head will be moved radially with
respect to said disk until the read/write head is above or near a
desired coarse servo track. ~hile 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
7~a
is assumed during whicll the read/write head is held ;n a fixed position
relative to the desired coarse servo track by monitoring the cmlplitude of the
position error signal
Thus in accordance with one broad aspect of the inventionJ 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 pre-written on a rotating disk said linear de-tector
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 ploportional 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 dis-
tance that said light beam is from a second end of said collection surface;
summation means for adding said first and second signals and yroducing a sum
signal therefrom having an amplitude equal to the sum of the amylitude 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 firs-t and second signals;
and dividing means for dividing said difference signal by said sum signal and
producil~g an output signal therefrom said output signal having a signal
amplitude that is proportional to the linear position of said light beam on
said collector surface as measured relative to one of said ends thereof
In accordance with another broad aspect of the inventioll there is
provided a method for generating a lincar position error signal for use in an
optical disk storage system that indicates the linear position of a narrow
strip of radiant energ~ incident to a collection surface of a linear detector
said strip of radiant energy corresponding to reflec-ted energy from a segment
- 1-
- 7a -
of a coarse data band writtell of a rotating disk used witllin said storage system,
said position being measured rela-tive to a kllol~n reference point on said
collection surface, said method comprising thc steps of:
(a) generati]lg a first re-Eerellcc signal having an amplitude proportional
to the intensi-ty of said strip of radiant energy and linearly proportiollal to
the location that said incidellt strip of radiant energy falls UpOIl said collec-
tion surface as measured relative to a first reference pOillt thereo-l;
~ b) gellerating a second referellce signal having an amplitude propor-
tional to the intellsity of said strip of radiallt energy and linearly propor-
tional to the location that said strip of radiant energy falls UpOII said
collection surface as measured relative to a second reference point thereon;
(c) summing the amplitude of said first and second reference signals
to produce a sum signal;
(d) subtracting the amplitude of said first and second reference
signals to produce a difference signal; and
(e) dividing said difference signal by said sum signal to produce said
linear position error signal, said position error signal havillg an amplitude
linearly proportional to the distclllce that said beann of light falls upon said
collection surface as measuredrelative to one of said first or second reference
points, and said position signal amplitude being substantially independent of
the intensity of said beam of light.
In accordancewith another broad aspect of the inventioll there is
provided apparatus for producillg a linear position error signal for use in a
servo control system of an optical disk storage system, said linear position
error signa] being used to controllably 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:
7~3
- 7b -
first means for generating a laser beam; second means for directillg said laser
beam through said read/wri-te head to said ro-tating disk, said laser beam fall-
ing upon a surface of sai.d rotating disk with a spot si~e sufficiently large
to illuminate at least a segment o-~ one of -thc concelltric coarse servo tracks
located on said dis~, each o-f said coarse servo tracks on said disk bcing COIl-
figured so as to changc the reflectivity characteristics of said disk wherever
said coarse servo tracks are placed; third means for direc-ting -those portiolls
of said laser beam reflected from said disk through said read/write head to a
collection surface of a stati.onarily moullted linear detector, said lincar
detector comprising: first signal generating means for generating a fi.rst
reference signal having an amplitude proportional to the intensity of the laser
beam energy incident to said collection surface, and linearly proportional to
the location at whi.ch 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 generating means for generating a secolld reference
signal having an amplitude proportional to the intensity of the laser beam
energy incident to said collection surface, and linearly proportional to the
location at which 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 the
a~nplitude of said first and second reference signals to produce a difference
signal; and sixth means for dividing said difference signal by said sum signal
to produce said linear position error signal, said linear position error signal
having an amplitude that is linearly proportional to the distance that said
laser beam energy falls upon said collection surface as measured relative to
one of said first or second collection surface reference points, and said
linear position error signal amplitude being sllbstantially independent
of the intensity of said laser beam energy at said collection surface.
~RIFF DESCRIPTI _ OF THE DRAWINGS
The above and other objects, features, and advantages of
the present invention will be more apparent from the following more
particular description thereof, presented in conj~mction wi.th the
following drawings whereill:
Fi.gure 1 is a block diagram of a coarse/fine servo system
used in an optical disk data storage system, and illustrates the
environment in which the present invention is designed to be used;
Figure 2 schematically shows the principle elements of Fig-
ure l;
Figure 3 is a side view of an optical disk drive and
schematically S]IOWS the rel.ationship between the optical disk~ fixed
and moving optics packages, and a linear actuator for controllably
positioning the read/write head;
Figure ~ is a block diagram of the coarse track detection
system of the present invention; and
Figure 5 shows the waveform of the output signal from the
detection system shown in Figure ~ during radial movement of the read/
write head across the disk.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is best understood by reference to
the accompanying drawings wherein like numerals will be used to des-
cribe like elements or parts throughout.
7~9
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 pre-recorded 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.
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 a 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
7~
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 illurninator 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 fine access/tracking servo system 25d.
The servo system for access to and tracking of the coarse servo
tracks includes a coarse illuminator 30 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 lla 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
lla of the disk surface. It is noted that the illuminated portion
lla of the disk surface spans at least the distance between two
servo tracks, an~ thereby always illuminates at least one servo
track. As shown in FIG. 29 light is reflected From the portion lla
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 it
will be described more fully below.
The output of the coarse detector and processing circuitry 31 is
a coarse track position error signal, which signal has an amplitude
proportional to the location at which the reflected strip of light
~ t7~
--10--
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 wnite 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 through the data read system 25c.
The read or write beams 52' from the illuminator 25a 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 galvenometer controlled mirror(s), is controlled by the
fine access/tracking servo system 25d.
In order to allow the servo tracks to be easily discriminated
from the data tracks, the servo tracks are preferrably 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 (represented schematically
in FIG. 2 by the carriage optics 23). 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
7~
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/wri-te relationship
with any of the tracks on the aisk 11. The carriage actuator 24 may
be realized with a linear motor, such as a voice coil motor9 that
includes a stationary magnet 41 and a moveable coil 49. The optical
path for either the read or write light beam(s) to the surface of
the disk 11 includes an objective lens 50, mirror 42, 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
fixed optic plate 48 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 a strip of light, or
narrow strip of radiant energy, representing that segment of the
coarse track illuminated in the area lla (FIG. 2), is directed to
the coarse detector 31.
Referring next to only FIG. 2, it is seen that the coarse
detector 31 comprises a detector 61 having a radiant energy
collection surface 62 upon which the strip of light 63, reflected
from the appropriate coarse servo track, is projected. 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 66. The position error signal, the output from
the signal processing circuitry 64, is directed to the coarse
access/tracking servo system 34 over signal line 67.
~ '7~
Referring next to FIG. 45 there is shown a more detailed block
diagram of the coarse detector 31 of the present inventionO As
explained in the preceding paragraph, the detector 61 includes a
collection surface 62 upon which a strip of radiant energy 63 from a
coarse track is projected. The collection surface 62 has a known
length L associated therewith. (This collection surface also has a
width associated therewith~ but the width is not an important
consideration for purposes of the present invention.) The strip of
radiant energy 63 reflected from the coarse track has a width w
associated therewith. This width w is, of course, related to the
actual width of the coarse servo tracks 16-19 (FIG. 2) that are
pre-written on the disk 11. In practice, the width w is small in
comparison to the length L.
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 d
between a first end of the collection surface 62 and the location
where the strip 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 L - d between a second end of the collection surface 62 and
the point where the strip 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 voltage output signal from the
transimpedance amplifier 70 is then subtracted from the output
voltage signal from the transimpedance amplifier 71 in a difference
7~
-13-
amplifier 72. Similarly, the voltage output signal from the
transimpedance amplifier 70 is summed with the voltage output signal
from the transimpedance ampliFier 71 in a summing circuit 73. The
outputs of the difference amplifier 72 and sumrlling amplifier 73 are
then coupled to a divider circuit 74 in such a manner so as -to cause
the output of the difference amplifier 72 to be divided by the
output of the summing amplifier 73. The output signal from the
divider circuit 74 is the desired position error signal.
An analysis of the configuration shown in FIG. 4 reveals that
the position error signal will have an amplitude proportional to the
distance d (or the distance L - d), but independent of the intensity
of the radiant energy falling upon the collection surface 62.
Hence, the desired characteristics (proportional to distance but not
to intensity) have been realized.
~ eferring to FIG. 5, there is shown a waveform representing the
shape of the position error signal as the read/write head is
radially moved past several coarse tracks. At one extreme, as the
strip of radiant energy corresponding to the coarse servo track
first falls upon one end of the collection surface 62, the error
signal assumes a large value, such as -A (FIG. 5). For example, in
the preferred embodiment, a large negative amplitude corresponds to
the strip of light 63 falling upon the bottom of the collection
surface 62 as shown in FIG. 4~ As the read/write head moves,
thereby causing the strip of radiant energy to move towards the top
of the collection surface 62, the amplitude of the position error
signal linearly increases from the negative amplitude -A to the
positive amplitude A. When the strip of light 63 is falling upon a
center point of the collection surface 62 (which center point is
indicated by a dashed line C in FIG. 4)~ then the amplitude of the
s~
~14-
position error signal will be zero. For the actual situation shown
in FIG. 4, tile strip of radiant energy 63 is falling on the
co11ection surface 62 a distance d from the top end thereof. This
distance d is roughly one half of the distance to the center line
C. Therefore, the position error signal would assume a value of
approximately A/2. Hence, the position error signal provides a
continuous signal whose amplitude is proportionai to the location of
the strip of radiant energy 63 on the surface 62 of the detector 61.
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
applicant for this function. This device has an active area
(collection surface 62) of 0.115 square centimeters. The dimension
L shown in FIG. 4 is roughly 0.21 inches (0.53 cm.), while the
dimension w shown in FIG. 4, the width of the strip of radiant
energy, is typically 0.085 inches (0.033 cm.) in the preferred
embodiment.
Any suitable transimpedance amplifier, available from numerous
IC 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,
the difference and summing amplifiers 72 and 73 may be realized
using commercially available integrated circuit operational
amplifiers, such as the LF353 manufactured by National
3~3
~15-
Semiconductor. The divider circuit 74 may be reali~ed with an AD535
Divider, manufactured by Analog Devices.
While a partlcular embodiment of the invention has been shown
and 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.
