Dynamic magnetic information storage or retrieval – General processing of a digital signal – Data clocking
Reexamination Certificate
2001-03-02
2004-01-06
Holder, Regina N. (Department: 2651)
Dynamic magnetic information storage or retrieval
General processing of a digital signal
Data clocking
C360S075000, C360S077080
Reexamination Certificate
active
06674593
ABSTRACT:
The present invention relates to a method and apparatus for writing a servo track on a storage medium.
Information for systems such as data processing systems is typically stored on storage media. Particular use is made of storage disks such as magnetic disks, opto-magnetic disks, and the like. One type of magnetic disk arrangement is a so-called “head disk assembly” which is intended normally to be permanently fixed in a data processing system; the head disk assembly includes the magnetic disk medium itself and the associated read and write head or heads which write data to the disk and read data from the disk. Another type of magnetic disk is of a type known as “removable media” which normally consists of a magnetic disk medium in a protective plastics case which can be used to transfer data between data processing systems by physical transfer of the disk itself from one machine to another.
In a known method of manufacturing storage media such as hard disk drives, a head disk assembly consisting of the head(s), the disk or disks, the motor and arm electronics, is mounted in a mastering station known as a servo-writer. The servo-writer writes a pattern of magnetic information (the “servo track pattern”) onto the disk. The servo track pattern becomes the master reference which is used by the disk drive during normal operation in order to locate the tracks and sectors on the disk for data storage and retrieval.
In order to be able to write the servo tracks onto the correct positions on the disk during manufacture of the disk, in one commonly used process, a (usually temporary) clock track is written onto the disk to serve as a timing reference during writing of the servo pattern. Conventionally, a separate clock head is used to write the clock track onto the disk and to read the clock track from the disk so that the servo tracks can be phase aligned with respect to each other. However, the use of a dedicated clock track writing head is an expensive addition to the manufacturing process and further requires that the servo tracks be written in a clean room because the clock track writing head has to be inserted into the open (unsealed) head disk assembly. Moreover, in practice the clock heads have to be replaced on a daily basis because of damage which occurs during use.
U.S. Pat. No. 5,485,322 discloses a method and system for writing a clock track on a storage medium using an internal recording head of a hard disk drive. A timing pattern is generated on the storage medium with the internal recording head and a radial positioning value used in radially positioning the internal recording head is determined. The servo pattern is written at the locations determined by the generated timing pattern and radial positioning value. In effect, in this prior art system, the clock pattern is written in an iterative manner across the disk. However, there is a problem in that this method can introduce phase and frequency differences in a clock pattern between respective tracks on the disk. As a disk will often require many thousands of clock tracks across the disk, even very small phase and frequency errors in the clock track can cumulatively become very large.
U.S. Pat. No. 5,448,429 discloses another example of a system for writing clock tracks across a disk in which a written clock signal is read to provide a reference for the writing of the clock signal to a subsequent track.
U.S. Pat. No. 5,668,679 discloses another example of a method of self-servowriting a disk drive in which a clock track is written followed by spiral tracks across the disk. The clock track and the spiral tracks have missing pulses or bits and are used to locate the arm of the disk drive when the servo information is written.
In our WO-A-98/31015, the entire disclosure of which is incorporated herein by reference, there is disclosed a method and apparatus for writing clock data to a storage medium, such as a disk. The storage medium has tracks on which data can be stored. Clock data is written to a present track on the storage medium. The written clock data is read from that track. Clock data for a subsequent track on the storage medium is generated from the clock data read from said present track. The phase of the generated clock data for the subsequent track is compared with a reference timing signal and the phase of the generated clock data for the subsequent track is adjusted in accordance with said comparison. The phase adjusted clock data for the subsequent track is then written to said subsequent track. In this method and apparatus, it is again not necessary to provide a dedicated clock read/write head. As in other proposals of this type, the servo information is written to the disk using the clock track data to position the servo information very accurately. To save time, it is preferred that the servo information in a track be written alternately with the clock data in that track, i.e. in any track, a portion of clock data is written, that clock data being phase aligned with the clock data in the previous track, and then a burst of servo data is written, again appropriately phase aligned with the previous track, followed by the next portion of clock data, and so on.
However, there are a number of problems associated with using an analogue phase locked loop in a servo track writer. For example, analogue phase locked loops are extremely noise sensitive and the quality of the frequency and phase lock is difficult to determine exactly. If there is a defect on the disk where the clock track is written, analogue phase locked loops respond unfavourably to the disturbance. If the analogue signal detected through the clock head is disturbed by noise, then there is again an unfavourable response in the phase locked loop circuit. Conventional analogue phase locked loops are sometimes used in a servo track writer during the actual writing of the servo tracks in a mode in which they are not phase locked, but remain locked at the last detected frequency and phase, but this means that inevitably the loop is locked at that last frequency and phase. The analogue phase locked loop is also prone to drift and other inaccuracies in this hold mode. Also, analogue phase locked loop circuits induce errors not only into the phase but also the frequency when used in this locked mode because the last detected frequency is in principle not known. This means that analogue phase locked loops are difficult to use in a system in which one clock track is generated from a previous clock track. Finally, analogue phase locked loops require tuning to different operating frequencies due to the stability and control issues of such devices.
A further problem with the prior art is that the only connection between the two states, frequency and phase, of the servo tracks to be written and the frequency and phase of the clock track with which phase alignment is to be achieved is by means of the analogue phase locked loop, but phase and frequency are inextricably linked in an analogue phase locked loop. This makes it difficult to achieve phase alignment and frequency coherence, especially at high frequencies.
According to a first aspect of the present invention, there is provided a method of writing a servo track to a storage medium having at least one clock track thereon which is used to control the phase of the servo track written to the storage medium, the method comprising the steps of:
rotating a storage medium at a rotational frequency which is obtained from a reference frequency;
obtaining a servo pattern signal having a pattern frequency from said reference frequency;
reading a clock track from said storage medium to obtain a clock signal having a clock frequency;
adjusting the phase of the servo pattern signal relative to the clock signal so that the servo pattern signal is in phase with the clock signal; and,
writing a servo track in accordance with the servo pattern signal to the storage medium.
The motor frequency and the pattern frequency are both obtained from the same reference frequency. This greatly facilitates the achievement of frequ
Jolly Paul H. R.
Levin Paul A.
Holder Regina N.
Pillsbury & Winthrop LLP
Xyratex Technology Limited
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