Note: Descriptions are shown in the official language in which they were submitted.
Ti : SYSTEM FOR SENSING THE ROTATIONAL POSITION OF A ROTATINC
DISK HAVING COARSE SEEK TRACKS
Inventor: Robert E. Dean
BACKGRO~ND OF THE INVENTION
This invention relates to a system for sensing the rotational
position of a rotating disk. More particularly, the invention
relates to a systern and method for placing sector marks on a
rotating disk having coarse seek tracks in a manner that saves space
on the disk and that eliminates the need for a special sector mark
detector.
In a disk storage system, regardless of whether such a system is
a magnetic disk or an optical disk, data is written on or read from
the disk surface as the disk rotates past read/write heads (the term
"head" will be used in this disclosure to describe the device which
senses the data during a read operation or causes the data to be
written during a write operation). Data is typically recorded on
the disk in concentric rings called tracks. Each track is
advantageously divided into a number of segments called sectors.
Such sectors provide an identifiable area on the disk where data may
be stored and indexed for future retrieval. Before a read or write
operation can be initiated, the head must be positioned over the
desired track, and the disk must be rotated until the desired sector
of the selected track is positioned under the head.
The read/write heads are positioned at the correct track by
means of a servo system. There are many types of head positioning
servo systems known in the art, anyone of which could be used with
the present invention.
In addition to positioning the head radially with respect to the
disk, however, it is also necessary to sense the rotational position
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of the disk so that data contained in a specified sector of the disk
can be readily locatedO A variety of techniques have been used in
the prior art to provide such sector information. One technique
uses a separate sector track that has the sector information written
on it. Unfortunately, this method uses an entire track for the
sector information and also requires a completely separate head just
to read the sector marks.
Another method places sector information with the regular data
to be stored by the system. This approach divides the disk into
sectors of a fixed size and writes a sector number at the beginning
of each sector when the disk is formatted before data is written
thereon. This method has the advantage of allowing the same read
head to be used to read both the sector information and the data
that is later written in the sector; but it has the disadvantage of
using some of the sector space for the sector number (thereby
decreasing data storage capacity), and it requires the use of
special decoding circuitry to decode the sector number that is read,
as well as to compare it with the desired sector number.
Another sector locating system known in the art is to merely
find the desired track and wait for an index mark to pass the read
head. The index mark is a unique type of sector mark that is used
to mark the zero degree point of rotation. Typically each sector is
marked by the absence of data, i.e., a data gap, at a fixed
rotational position from the index mark. Thus, since there is a gap
between each sector where no information is written, this system
merely counts the gaps from the index mark as the disk rotates to
determine a desired sector location~ However, because it takes less
time for a sector to pass by the head than it takes for the head to
move from one track to another, the sector count is unfortunately
lost when the head is moved to a new track until such time as the
index mark again passes unAer the head. Hence, this approach slows
down the operation of the disk storage system since more than one
revolution may be required before the desired sector can be found.
There exists a need in the art, therefore, for an improved disk
sector-locating method and system whereby a desired sector location
of a given data track may be quickly identified without slowing down
the operation of the disk system (i.e., without having to wait for
the disk to rotate a full revolution); without decreasing the data
storage capacity of the disk (i.e., without taking up additional
disk surface area with pre-recorded sector information); and without
requiring special hardware, such as an additional read head, to
detect and/or separate the sector information from the other data
stored on the disk.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a system for
sensing the rotational position of a rotating disk that overcomes
the aforecited disadvantages associated with prior art rotational
position sensing systems. More particularly, it is an object of the
present invention to provide a system for recording sector
information on a rotating disk in a manner that does not use disk
space that could be used for storing data, and in a manner that does
not require special hardware to detect the sector information.
It is a further object of the present invention to provide such
an improved rotational position sensing system for use with a disk
employing coarse seek tracks, a coarse seek track being a servo
track that is used to radially position the head so as to have
access to a given band of the disk surface, each band containing a
relatively large number of data tracks.
The above and other objects, features, and advantages of the
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present invention are realized with a system wherein sector marks
are selectively recorded in the coarse seek tracks that are
pre-recorded on the disk, which coarse seek tracks are used by the
servo system to position the read/write heads radially with respect
to the disk. Advantageously, no additional disk space is required
to record the sector marks. Rather, the sector rnarks are encoded in
the coarse seek track with a technique that requires no additional
disk surface space beyond that already used by tha coarse seek
tracks. Further, the same read head rnay be used to read the sector
information as is used by the coarse servo system to find the coarse
seek track. Still further advantageously, a large part of the
circuitry used by the coarse positioning servo system may also be
used to detect the sector marks.
In a preferred embodiment, at least two coarse seek tracks will
lie under the coarse track read head. The sector marks included in
each coarse seek track divide the disk into equal sectors. As the
disk rotate~ the coarse track head means used to follow the coarse
seek track senses the sector marks and, in cooperation with
appropriate detection circuitry, generates an output signal
indicating when each sector begins and ends. Because the disk
rotates at a constant speed, and because each sector is equal in
size, this output signal assumes a fixed frequency, the period of
which corresponds to the time it takes one sector to pass un~erneath
the coarse track read head. Using phase lock loop techniques, an
oscillator is locked onto this output signal. A counter is then
used to count the cycles of the oscillator. An index mark,
representing a zero degree reference point for the rotating disk, ls
also sensed by the coarse track read head for each revolution of the
disk. This index mark, when sensed, is used to reset the counter.
Thus, the counter always contains a count therein indicative of the
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rotational position (sector location) of the disk, regardless of whether the
data read/write heads are moved from one track to another.
Thus, in accordance with one broad aspect of the invention,
there is provided, in a disk storage system having a rotating disk on which
data may be written and from which data may be read, said rotating disk having
a multiplicity of concentric coarse seek tracks thereon, and said disk storage
system including coarse head means positioned for select radial movement wi-th
respect to a surface of said rotating disk for simultaneously sensing informa-
tion written on at least two adjacent of said coarse seek -tracks, a system for
sensing the rotational position of said disk comprising: sector marks selec-
tively placed on each of said coarse seek tracks, said marks defining the
boundaries between a multiplicity of equal sectors on the surface of said disk;
detection means coupled to said coarse head means for detecting and signaling
the movement of said sector marks past said coarse head means; oscillator means
coupled to an output signal from said detection means for generating a periodic
signal synchronized with the movement of said sector marks past said coarse
head means, whereby the period of said periodic signal bears a fixed relation-
ship with respect to the time it takes one of said equal sectors to pass by
said coarse head means; counting means coupled to said periodic signal for
counting the number of sectors that pass by said coarse head means; and reset
means for resetting said counting means once for each revolution of said disk,
said reset occurring at a known rotational position of said disk relative to
said coarse head means; whereby said counting means provides an indication of
the rotational position of said disk as measured by the number of sectors that
have passed by said coarse head means relative to the known rotational position
of said disk; and further whereby said coarse head means may be selectively
moved radially with respect to said disk while still maintaining an accurate
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rotational position count in said couting means.
In accordance with another broad aspect of the invention there
is provided a method for sensing the rotational position of a rotating disk
used in an information storage system, said method comprising the steps of:
(a) prerecording concentric servo tracks on a surface of said
disk, said servo tracks being used by a servo system to selectively position
a read/write head radially with respect to said disk;
~ b) prerecording aligned sector marks into each of said servo
tracks that divide the surface of said disk into equal sectors;
~c) inserting said disk into said storage system and ro-tating
it at a known rotational speed;
~d) simultaneously sensing with said read/write head tlle sector
marks from at least two adjacent servo tracks;
(e) synchronizing an oscillator with the sector marks sensed
in step (c);
(f) counting the cycles of said oscillator in a counter circuit,
an integer number of said cycles corresponding to a one sector rotation of said
disk; and
(g) resetting said counter circuit at the same point in the
rotation of said disk, whereby said counter circuit is reset once for each
revolution of sai.d di.sk, and whereby the count held in said counter circuit
represents an accurate measure of the rotational position of said disk regard-
less of the radial movement of said read/write head.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, advantages, and features of the pre-
sent invention will be more apparent from the following more particular
description thereof, presented in connection with the following drawings,
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wherein:
Figure 1 is a simpli-fied top plan view of an informatioll disk
showing some oE the coarse seek tracks located thereon;
Figure 2 is a diagrammatic representation illustrating the system
of the present invelltiol- wherein sector marks are selectively placed in the
coarse seek tracks of Figure l; and
Figure 3 is a block diagram of the present invention, including
detection circuitry that could be used to detect and decode the sector marks
placed in the coarse seek tracks of an information disk in accordance with
the present invention.
DETAILED DESCRIPTION OF Tl-IE INVENTION
The following is a description of the best presently contem-
plated mode of carrying out the present invention. It is given only to
illustrate the objectives, features, and advantages of the invention and is
not to be taken in a limiting sense. The true scope of the invention can be
ascertained by referring to the appended claims.
Figure 1 is a simplified top plan view of an information disk
10 that, when inserted in an appropriate disk storage system, is rotated about
its center 16. On the disk 10 are a plurality of coarse seek tracks 11-15,
represented in Figure 1 as concentric circles. (While only five coarse seek
tracks are shown in Figure 1, it is to be understood that any number of such
tracks could be used.) The primary function of the coarse seek tracks 11-15
is to provide radial position information -to the servo system. A servo system
employing such coarse seek tracks is disclosed in Canadian application
Serial No. ~39,881, filed October 27, 1983, "Fine and Coarse Servo System For
Access and Tracking On An Optical Disk," assigned to the same assignee as is
t~t;jr
this ayplication.
The coarse seek tracks 11-15 are selectively spaced about
the center 16 o the disk. Data is written on and read from a multipli-
city of data tracks, not showll in the figure, located in the region or band
19 be-tween the coarse seek tracks. Nllmerous fine servo systems could be
employed in order to access a given data track once the region or band 19
has been accesse~ by a coarse servo system. Further, numerous coarse servo
systems could be employed. For the description that follows, however, the
preferred embodiment will be described in connection with the fine and
coarse servo system described in the aforecited patent application, Serial
No. 439,881. This preferred servo system is designed for use wi-th an
optical disk, and so the description that follows will likewise be explain-
ed in terms of an optical disk. However, it is to be understood that
the present invention could be used equally as well with magnetic or other
information disk storage system.
In the optical disk embodiment, a light source is used
to project an elongated spot 17 or 18 on the surface of the disk 10.
This elongated spot is reflected to a linear detector which can detect
the position of the light spot on one or more of the coarse seek tracks.
The length of the elongated spot of light is slightly longer than
two data bands l9, and therefore covers two or three
coarse seek tracks. Two different representations of the elongated
light spot are included in FIG. 1~ The light spot 17 is shown
impinging on three coarse seek bands, while the light spot 18 is
only impinging on two coarse seek bands. The spot 18 is the normal
position of the spot during a read/write operation.
FIG. 2 is a diagramatic representation illustrating how the
system of' the present invention divides two representative coarse
seek bands 54-55 into sectors. Lines 40-51 are shown emanating from
the center of the disk 16 to divide the coarse bands 54-55 into a
multiplicity of sectors. The lines 40-51 are shown only for
illustrative purposes and do not actually exist on the disk
surface. The two coarse seek tracks 54-55, and all the other coarse
seek tracks on the disk but not shown in the figure, are divided
into a multiplicity of sectors represented in the figure as 20-29.
Each sector spans the same number of degrees and all gaps between
sectors are equal. An index mark 30 is a unique sector mark
indicating the zero degree reference point of rotation.
The sectors are actually formed by writing the coarse seek track
an appropriate number of degrees, stopping or altering the writing
for the appropriate number of degrees to form a gap between the
sectors, and repeating this process until all sectors of the band
are written. For example, the signal used to write the coarse seek
track may be continuously on for the duration of the sector, and
modulated 50% on and 50% of`f in the gap between sectors. All coarse
seek bands are written by a special servo writing machine such that
the sector marks are precisely aligned. In the preferred
embodiment, for example, each sector may be .022 degrees in length,
with a gap between sectors of .022 degrees.
In an alternative embodiment, incorporating a clock in the servo
track, it may be advantageous to mark each sector by changing the
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phase of the clock an appropriate amount (such as 90 or 180 degrees)
in the gap between sectors, rather than physically leaving a gap in
the coarse servo track or altering the modulation scheme of the
signal used to write the coarse servo track. Such a phase change
could be easily detected to mark the beginning or end of each
sector9 and would still provide a continuous clock source for other
purposes.
~ egardless of the marking scheme used, however, it is important
to note that the sector information is contained within the servo
tracks, and as such, no additional aisk surface area is required to
include this sector information on the disk.
FIG. 3 is a simplified block diagram showing the principal
components of the present invention, including how the sector marks
may be detected. Light 59, reflected off the surface of the disk
10, impinges on a linear detector 60. The reflected light 59
(represented as a dashed-double dot line in FIG. 3) originates from
a light source 58, such as a laser, which light source directs
incident light 58' (represented as a dashed line in FIG. 3) to the
disk 10 so as to form an elongated spot 17 or 18 (FIG. 1) thereon.
The incident and reflected lights are directed to and from the disk
10 through a head assembly 57 mounted to a movable carriage 56.
Suitable optic elements (lenses, mirrors, etc.) are, of course, used
to direct the incident and reflected light along the desired paths.
The carriage 56 is radially positioned with regard to the disk 10
under control of the servo system.
The linear detector 60 has two outputs 61 and 62. These outputs
have a signal that is proportional to the amount and position of
light impinging on their respective half of the linear detector.
The reflected light 59 varies in intensity as it passes over the
sector marks in the coarse seek bands. This variation in intensity
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occurs at a position along the length of the light spot
corresponding to the relative position of the coarse seek tracks
within this elongated light spot. Thus, the output signals 61, 62
vary as a function of the relative position and content (off` or on)
of the reflected light 59 that strikes the detector 60. As such,
these signals will typically have a frequency associated thare~ith
that is a function of the rotational speed of the disk and the
sector gap and length, and an amplitude that is a function of the
relative position of the servo tracks within the elongated spot.
Low pass filters 63, 64 and band pass filters 65, 66 are used to
filter the outputs of the linear detector 60. These outputs will
vary in sinusoidal fashion as the disk rotates and as the sector
gaps are encountered. This sinusoidal waveform is supplied to each
of the inputs of a summing amplifier 67. The polarity associated
with the inputs to the amplifier 67 is such that the amplifier 67
subtracts the two signals and supplies the result to the servo
system. The servo system responds to this signal by moving the
light spot until it is centered on the desired coarse seek band.
When this occurs, the two outputs of the linear detector 60 will be
equal, the two inputs to the amplifier 67 will be equal, and the
signal to the servo controller will be zero. Further details
relative to the operation of the light spot, linear detector, and
servo system may be found in the previously referenced copending
patent application, Serial No. 439J881
The outputs of the two band pass filters 659 o6 are also
combined in a functional OR circuit 68. Since the reflected light
spot 59 is always over at least one coarse seek track, at least one
of the inputs to the OR circuit 68 will have a sine wave applied to
it corresponding to the frequency generated by the light spot as the
sector marks rotate by it. The output of the OR circuit 68 is
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applied to t~le input of a phase lock loop oscillator 69, which
oscillator will not pass the irregularity in the sine wave caused by
the index mark 30 (see FIG. 2). The output of the phase lock loop
oscillator is connectsd to a counter 70 and to an index decoder 71.
The output of the OR circuit 68 is also applied to the index ~ecoder
71.
The counter 70 is incremented by the output of the phase lock
loop oscillator 69. The frequency of the oscillator 69 is, in turn,
locked to the rate at which the sector marks (gaps) pass through the
light spot. This frequency may be directly related to the sector
count or an integer multiple of the sector count. In either event,
the counter is configured so as to maintain a running count of the
sector count as the disk rotates, even when the servo is moving the
light spot while seeking a different track.
The index decoder 71, using techniques that are well known to
those skilled in the art of logic design, compares the output
waveform of the OR circuit 68, which includes the irregularity of
the index mark, to the output of the phase lock loop oscillator,
which does not include the irregularity of the index mark. When the
index mark is detected, the index decoder 71 generates an output
signal which clears the counter 70, and the running count of the
sector number starts over. Thus, the count held in the counter 70
is always synchronized with the index mark, and the count thus
indicates the rotational position of the disk by indicating the
number of sector locations from the index mark that the disk has
rotated.
While the invention described herein has been described by means
of a specific embodiment and application thereof, numerous
modifications and variations could be made thereto by those skilled
in the art without dep&rting from the spirit and scope of the
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present invention. It is therefore to be understood that within the
scope of the appended claims, the invention may be practiced
otherwise than as speci~ically described herein.
