WO1999006992A9

WO1999006992A9 - Method and apparatus for increasing the sample rate of a disc drive with low overhead

Info

Publication number: WO1999006992A9
Application number: PCT/US1998/015482
Authority: WO
Inventors: Daniel R Zaharris
Original assignee: Seagate Technology; Daniel R Zaharris
Priority date: 1997-07-31
Filing date: 1998-07-27
Publication date: 1999-08-05
Also published as: WO1999006992A8; WO1999006992A1

Abstract

A disc drive is disclosed having disc files (20), a spindle motor (27), ganged read/write heads (23), an actuator in the form of a voice coil motor (VCM) (24), servo electronics (30), and a servo information sector (32) recorded on a magnetic recording data surface (26). The recorded servo information is a sequence of mini burst fields (110) interspersed between consolidated burst fields (100). The consolidated burst fields (100) include a preamble field (101), a servo address mark field (102), a digital Gray code field (103), and a series of position burst fields (104-107). The mini burst fields (110) include a preamble field (111), a sync mark field (112) half the length of the servo address mark field (102), and a series of position burst fields (113, 116) that are shorter than the position burst fields (104-107) of the consolidated burst field (100).

Description

METHOD AND APPARATUS FOR INCREASING THE SAMPLE RATE OF A DISC DRIVE WITH LOW OVERHEAD

RELATED APPLICATION

This application claims priority to co-pending Provisional Patent Application Serial No. 60/073,378 entitled: "METHOD TO INCREASE THE SAMPLE RATE OF A DISC DRIVE WITH LOW OVERHEAD" , filed 07/31/97 by the same applicant. TECHNICAL FIELD

The present invention relates to disc drives. More particularly, the present invention relates to servo patterns written on magnetic media used by the disc drive's servo positioning systems for accessing and following data tracks. Even more particularly, the present invention relates to servo patterns which are periodically sampled by the disc drive's positioning system to maintain the disc drive's read/write head on-track.

BACKGROUND ART In a disc drive using magnetic recording, data is recorded in concentric tracks on a plurality of surfaces. The recording heads are mechanically ganged together and arranged one per surface with all heads nominally positioned over the same radial track location. The heads are positioned in unison using a voice coil motor. Recorded data, consisting of a certain topology of magnetic transitions is recorded on the surface of the magnetic media. To follow the recorded data, servo positioning information is pre-recorded in a plurality of spaced servo records written on the magnetic media and is used by the disc drive for maintaining the read/write head assembly in a desired track position. The servo information is written continuously along all tracks of one surface (dedicated servo), or is written in small segments of each track on all surfaces (sectored). In either case, the servo information is read back and demodulated to obtain (1) a digital track number, and (2) an analog signal indicating positions from track center. The analog signal indicating position from track center is referred to as the position error signal. The read/write head's position relative to the disc is responsive to the position error signal. The response to maintain the read/write head on track center may be attributable to either a sampled servo system, or a non-sampled servo system sampling of the position error signal. The control signals are considered to be "overhead", while data stored on the discs are considered to be non-overhead.

With the advent of the laptop computer, disc drives have become smaller and now incorporate such features as a microactuator. In order to take advantage of the servo control loop bandwidth increase allowed by the utilization of a microactuator, a higher servo sample rate is required. Simply increasing the number of servo bursts is an obvious approach but this may not be the most efficient way to achieve a higher sample rate.

Thus, a need is seen to exist for a method and apparatus for increasing the sample rate of a disc drive servo without simply increasing the number of servo bursts in the pre-recorded pattern.

DISCLOSURE OF INVENTION In accordance with these and other objects and features of the present invention, there is provided a servo burst field having a mini-burst field interspersed between normal servo bursts. The mini-burst fields are smaller for lower overhead. The mini-burst field has no Gray Code field and has a sync mark field half the length of a typical servo address mark field. By interspersing the abbreviated mini-burst field between normal servo bursts, the result is to effectively double the sample rate without doubling the overhead.

The advantages of the present invention are the following: 1. the servo sample rate can be increased without a significant increase in overhead; 2. the same servo burst demodulator hardware can be used for the normal and mini-burst demodulation;

3. the timing information from the mini-burst sync mark could be used to increase the timing resolution of sector mark generation;

4. this can be applied to any position burst demodulator design; and 5. if the mini-burst field is small enough a PLO field following the burst may not be needed for split data.

Other features of the present invention are disclosed or apparent in the section entitled: "BEST MODE FOR CARRYING OUT THE INVENTION. "

BRIEF DESCRIPTION OF DRAWINGS

For fuller understanding of the present invention, reference is made to the accompanying drawing in the following detailed description of the Best Mode of Carrying Out the Present Invention. In the drawings:

Fig. I is a simplified block diagram of a sector servo in accordance with the invention.

Fig. 2 is a broken fragmentary view of the recording surface of a portion of a disc useful in a disc drive in accordance with the invention, and indicating a sector servo pattern thereon.

Fig. 3 is a schematic diagram illustrating the servo pattern provided in the servo information sector of a recording data surface.

Fig. 4 is a diagram of a consolidated burst field of the type shown in Figs., 2 and 3 illustrating the sequence of the component fields thereof.

Fig. , 5 is a diagram of a mini burst field in accordance with the invention illustrating the sequence of the component fields thereof. Fig. 6 is a diagram illustrating the sequence of the mini burst fields of Fig.

5 interspersed with the consolidated burst fields of Fig. 4.

Reference numbers refer to the same or equivalent parts of the present invention throughout the several features of the drawing.

BEST MODE FOR CARRYING OUT THE INVENTION

As hard disc drives, disc surfaces, servo control systems, VCM actuators, and read/write heads are all known in the art, in order to avoid confusion while enabling those skilled in the art to practice the claimed invention, this specification omits many details with respect to those items.

Referring now to Fig. 1, there is shown a simplified block diagram of a sector servo in accordance with the present invention. Disc files 20 are information storage devices which utilize one or more rotatable discs 21 , 22 with concentric data tracks containing information, one or more heads 23 for reading or writing data onto the various tracks, and an actuator or voice coil motor (VCM) 24 connected by a support arm assembly 25 to the head or heads 23 for moving one of the heads 23 to the desired track and maintaining it over the track centerline during read or write operations. Each disc 21, 22 has two data surfaces 26, and the discs 21, 22, being disposed on a spindle, are driven by a spindle motor 27. The read/write information is electrically coupled to and from the heads 23 by way of a read/write (R/W) channel 28. The R/W channel 28 may include amplifiers, filters and detectors.

The movement of the head 23 to a desired track is referred to as track accessing or "seeking", while the maintaining of the head 23 over the centerline of the desired track during a read or write operation is referred to as track "following" . The actuator or VCM 24 typically comprises a coil movable through the magnetic field of a permanent magnetic stator. The application of current to the VCM 24 causes the coil, and thus the attached head 23, to move radially. The acceleration of the coil is proportional to the applied current, so that ideally there is no current to the coil if the head 23 is perfectly stationary over a desired track.

In disc files which have a relatively high density of data tracks on the disc, it is necessary to incorporate a servo control system to maintain the head precisely over the centerline of the desired track during read or write operations. This is accomplished by utilizing prerecorded servo information either on a dedicated servo disc or on sectors angularly spaced and interspersed among the data on a data disc. The servo information sensed by the read/write head (or the dedicated servo head if a dedicated servo disc is used) is demodulated to generate a position error signal (PES) which is an indication of the position error of the head away from the nearest track centerline.

There are many ways of obtaining the PES signal well known to those skilled in the art. For purposes of illustration, the present invention is shown as being used with a disc drive digital sector servo employing a pattern of servo information prerecorded on sectors interspersed among data on data discs. It should be understood that this is by way of example only, and that the invention is not limited to use with this particular method of obtaining the PES signal. The prerecorded servo information is customarily written to the discs 21, 22 by a servowriter at the factory. The prerecorded servo information is read off the discs 21, 22 and is applied to the input of servo electronics 30 which drives the VCM 24.

Referring now to Fig. 2, there is shown a broken, fragmentary view of the recording data surface 26 of a portion of a disc 21, 22 that may be used in the present invention. The recording data surface 26 has a sector servo pattern thereon. The usable recording data surface 26 may be divided into a plurality of data storage sectors 31, and a plurality of servo information sectors 32. The servo information sectors 32 may be distributed around the discs 21, 22 between adjacent data storage sectors 31.

Referring now to Fig. 3, there is shown a schematic diagram illustrating the servo pattern provided in the servo information sector 32 of a recording data surface 26. The servo pattern is composed of four sections: an automatic gain control (AGC) preamble 34; a Servo Sync Field 35; a Digital Gray Code 36; and a Position Error Signal (PES) A/B burst portion 33. The AGC preamble 34 provides a full amplitude signal coherently written across tracks for providing automatic gain control of the readback signal. The Servo Sync Field 35 provides a unique frequency which enables the digital processing electronics to synchronize to the information in the digital Gray Code 36. The Servo Sync Field 35 may be as simple as a DC erase gap, as indicated in Fig. 3. The digital Gray Code 36 includes digital encoding of such information as inner and outer guardband track regions, absolute cylinder number, and index and sector mark. The PES A/B burst portion 33 provides a linear PES voltage which is a measure of the offtrack position of the head reading that voltage.

Every servo sample provides information identifying the track, along with an analog component that identifies when the head 23 is to the left or to the right of track center. This analog component is derived from the constant frequency PES A/B burst portion 33. When the constant frequency bursts of servo patterns A and B are read as the disc revolves under the head, the head reads a portion of A and a portion of B. Signals A and B are demodulated and an amplitude comparison is made. The amplitude of A relative to the amplitude of B represents off-track positioning in either a positive or a negative direction. As the head moves across the tracks, the difference between the amplitude of the A burst and the amplitude of the B burst produces a difference signal called the PES. When the PES is zero, the head is on track center.

The Gray Code has encoded in it the address of the track. Upon arrival of the head on the track, verification is received that it is on the desired track. Once positioned on the track, the A-B burst information is used for track following. It will be seen that instead of continuous PES information being generated from the disc surface 26, position samples are obtained once every servo information sector 32.

While a specific servo signal format for a disc has been shown, it will be understood that other well known varieties of servo signal format may also be used, if desired.

It should be understood that the foregoing servo signal format is equally applicable to a discrete or digital servo system as to a continuous or analog servo system.

It should be understood that the servo control signals are considered to be "overhead" while data signals are considered to be non-overhead.

Referring now to Fig. 4, there is shown a diagram illustrating the sequence of the fields making up a servo signal format such as that shown in Figs. 2 and 3. A consolidated burst field 100 is made up of a preamble field 101, a servo address mark field 102, a digital Gray code field 103, a position burst A field 104, a position burst B field 105, a position burst C field 106 and a position burst D field 107. The preamble field 101 is for AGC acquisition, the servo address mark field 102 is for timing information, the digital Gray code field 103 contains the servo track address, and the position burst fields 104-107 contain the fine radial track position information. Referring now to Fig. 5, there is shown a mini burst field 110 in accordance with the principles of the present invention. The mini burst field 110 is made up of a preamble field 111, a sync mark field 112, a position burst A field 113, a position burst B field 114, a position burst C field 115 and a position burst D field 116. The preamble field 111 is the same as the preamble field 101 of the consolidated burst field 100. The sync mark field 112 is half the length of the servo address mark field 102. The position burst fields 113-116 provide the same information as the position burst fields 104-107 of the consolidated burst field 100.

However, the position burst fields 113-116 of the mini burst field 110 are smaller than the other burst fields 104-107 so as to provide a lower overhead.

Referring now to Fig. 6, in accordance with the principles of the present invention the mini burst fields 110 are interspersed between the consolidated burst fields 100. Thus, there is a first consolidated burst field 100a, followed by a first mini burst field 110a, followed by a second consolidated burst field 100b, followed by a second mini burst field 110b, followed by a third consolidated burst field 100c. The arrangement of interspersed mini burst fields 110 as shown in Fig. 6 effectively doubles the sample rate without doubling the overhead. The mini burst field 110 is made as small as possible. This is done by eliminating the digital Gray code 103, using a sync mark field 112 instead of a servo address mark field 102, and by shortening the position burst fields 113-116. A short mini burst field 110 may eliminate the need for a PLO field for split data. Conventionally, a PLO field is used to identify split data. One advantage of the use of the interspersed mini burst field 110 is that the same servo burst demodulator hardware may be used to demodulate both the consolidated burst field 100 and the mini burst field 110. Also, timing information from the mini sync mark field 112 of the min burst field 110 may be used to increase the timing resolution of sector mark generation. It is contemplated that the principles of the present invention may be applied to any position burst modulator design.

The present invention has been particularly shown and described with respect to certain preferred embodiments of features thereof. However, it should be readily apparent to those of ordinary skill in the art that various changes and modifications in form and detail may be made without departing from the spirit and scope of the invention as set forth in the appended claims. The invention disclosed herein may be practiced without any element which is not specifically disclosed herein.

Claims

CLAIMSWhat is claimed is:

1. Apparatus for increasing disc drive sample rate without substantially increasing overhead, said apparatus comprising: means for providing disc drive servo information including first and second consolidated burst fields, each consolidated burst field including a preamble field, an address mark field having a preselected length, a digital Gray code field, and a first position burst field having a preselected length, and including a mini burst field 5 in between said first and second consolidated burst fields, said mini burst field including a preamble field, a sync mark field having a length substantially half the length of said address mark field, and a second position burst field having a length less than the length of said first position burst field; and a disc drive servo operatively coupled to said means for utilizing said disc 10 drive servo information.

2. An article of manufacture for increasing sample rate of a disc drive without substantially increasing overhead, said article of manufacture comprising: a magnetic disc for use in a servo system for a disc drive, said magnetic disc having servo information magnetically recorded thereon, said servo information 5 including: a plurality of mini burst fields interspersed between a plurality of consolidated burst fields; said consolidated burst fields including a preamble field, an address mark field having a preselected length, a digital Gray code field, and a first position burst 10. field having a preselected length; said mini burst fields including a preamble field, a sync mark field having a length substantially half the length of said address mark field, and a second position burst field having a length less than the length of said first position burst field.

3. A process for increasing disc drive sample rate without substantially increasing overhead, said process comprising the following steps: recording on a magnetic disc first and second consolidated burst fields, each consolidated burst field including a preamble field, an address mark field having a preselected length, a digital Gray code field, and a first position burst field having a preselected length; recording on said disc a mini burst field in between said first and second consolidated burst fields, said mini burst field including a preamble field, a sync mark field having a length substantially half the length of said address mark field, and a second position burst field having a length less than the length of said first position burst field.