SYSTEM FOR SENSING THE ROTATIONAL POSITION OF A ROTATING DISK HAVING COARSE SEEK TRACKS

Abstract: Sector marks are selectively placed in each coarse servo track (M-15) of rotating disk (10). These concentric coarse seek tracks (11-15) pre-written on the disk (10), are used by a servo system for the positioning of the read/write mechanism. Coarse track read means simultaneously sense at least two adjacent coarse seek tracks (54-55). Detection means (60) sense the relative position of these coarse seek tracks (54-55) within the sensing range of the coarse track read means, and this position information is used by the servo system to selectively control the radial position of the coarse track read means with respect to the rotating disk (10). The sector marks written in the coarse seek tracks (11-15) are likewise detected, and a signal is produced indicating when each sector begins and ends. An oscillator (69) is locked to this sector signal such that a fixed number of cycles of the oscillator signal corresponds to the passage of one sector of the disk under the coarse track read means. A counter (70) is then used to count the appropriate number of cycles. An index mark (30), representing a zero degree reference point, is also placed in the coarse seek track (54). When sensed, the index mark (30) resets the counter (70). The count contained in the counter thus always indicates the rotational position (sector location) of the disk (10).

Description: SΪSTEM FOR SENSING THE ROTATIONAL POSITION OF A ROTATING DISK HAVING COARSE SEEK TRACKS BACKGROUND 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 system 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 OMPI V . 1PO of the disk so that data contained in a specified sector of the disk can be readily located. 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 under 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 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 marks are encoded in the coarse seek track with a technique that requires no additional disk surface space beyond that already used by the coarse seek tracks. Further, the same read head may 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 rotates 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 underneath 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, is also sensed fay 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 rotational position (sector location) of the disk, regardless of whether the data read/write heads are moved from one track to another. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, advantages, and features of the present invention will be more apparent from the following more particular description thereof, presented in connection with the following drawings, wherein: FIG. 1 • is a simplified top plan view of an information disk showing some of the coarse seek tracks located thereon; FIG. 2 is a diagramatic representation illustrating the system of the present invention wherein sector marks are selectively placed in the coarse seek tracks of FIG. 1; and FIG. 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 THE INVENTION The' following is a description of the best presently contemplated 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. FIG. 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 FIG. 1 as concentric circles. (While only five coarse seek tracks are shown in FIG. 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 application Serial No. 438,133, filed 11/1/82, "Fine and Coarse Servo System For Access and Tracking On An Optical Disk," assigned to the same assignee as is this application. The coarse seek tracks 11-15 are selectively spaced about the center 16 of the disk. Data is written on and read from a multiplicity of data tracks, not shown in the figure, located in the region or band 19 between the coarse seek tracks. Numerous fine servo systems could be employed in order to access a given data track once the region or band 19 has been accessed 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. 438,133i which application is incorporated by reference herein. This preferred servo system is designed for use with an optical disk, and so the description that follows will likewise be explained 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 systems. 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 19 , 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% off 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 * fCFR 3 OMPI ^ ., Wϊi' ° N ΎKF 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 sector, and would still provide a continuous clock source for other purposes. Regardless 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 disk 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' mounte 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 OMPI 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 therewith 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. 438,133• The outputs of the two band pass filters 65, 66 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 OMPI ^NATlQ^ / applied to the 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 connected to a counter 70 and to an index decoder 71- The output of the OR circuit 68 is also applied to the index decoder 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 departing from the spirit and scope of the present invention. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein. OMPI <£Ri\ TlO

Claims: CLAIMS What is claimed is: 1. 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 with respect to a surface of said rotating disk for simultaneously sensing information written on at least two adjacent of said coarse seek tracks, a system for sensing the rotational position of said disk comprising: sector marks selectively 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 relationship with respect to the time it takes one of said equal sectors to pass by said coarse head means; v 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 "ξfiHE OMPI 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 rotational position count in said counting means. 2. The system as defined in claim 1 wherein each of said sectors of said disk have a gap therebetween. 3. The system as defined in claim 2 wherein said sector marks comprise a first marking scheme placed in said coarse seek track for indicating the presence of said sector, and a second marking scheme placed in said coarse seek track for indicating the gap between adjacent sectors. 4. The system as defined in claim 3 wherein said first marking scheme comprises a continuous signal written on said coarse seek track, and wherein said second marking scheme comprises the absence of said continuous signal. 5. The system as defined in claim 4 wherein said first marking scheme comprises a clock signal having a first phase associated therewith and wherein said second marking scheme comprises said clock signal having a second phase associated therewith. 6. The system as defined in claim 2 wherein said oscillator means comprises an oscillator locked to said output signal of said detection means. OMPI ^RNATIO∑S 7. The system as defined in claim 6 wherein one cycle of said oscillator corresponds to the movement of one sector and one sector gap past said coarse head means. 8. The system as defined in claim 7 wherein said reset means comprises: an index mark placed on each coarse seek track of said disk at a zero degree reference location; means for sensing that passage of said index mark past said coarse head means; and means responsive to the sensing of said index mark for resetting said counting means. 9. The system as defined in claim 1 wherein said disk storage system is an optical disk storage system. 10. 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 rotating it at a known rotational speed; (d) simultaneously sensing with said read/write head the 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 said disk, and whereby the count held in said counter circuit represents an accurate measure of the rotational position of said disk regardless of the radial movement of said read/write head.

Inventors: DEAN ROBERT E, DEAN, ROBERT, E.
Applicants: STORAGE TECHNOLOGY PARTNERS, STORAGE TECHNOLOGY PARTNERS II
Application year: 1984
Main classification: G11B27
IPC codes: G11B7, G11B21, G11B27