GB2279491A

GB2279491A - A magnetic disk drive

Info

Publication number: GB2279491A
Application number: GB9411354A
Authority: GB
Inventors: Koji Kurachi; Toyomi Ohsawa; Takao Matsui; Yuji Kigami
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1993-06-25
Filing date: 1994-06-07
Publication date: 1995-01-04
Anticipated expiration: 2014-06-07
Also published as: CN1101160A; JPH0737216A; TW247958B; KR950001603A; GB9411354D0; GB2279491B

Abstract

The object is to improve the read error rate for identification (ID) information In a sector servo type magnetic disk drive with a head comprising separate read and write elements. A pair of ID fields 21a and 21b are set up in each sector In each track of a magnetic disk for recording ID Information. The pair of ID fields are separated from each other along the track and radially displaced with respect to each other by a distance equal to an offset between the read element 24 and the write element 23 associated with the track. In a data write operation for a trace the magnetic head is positioned so as to read the ID information in the ID field 21b with the read element 23. In a read operation for the track, the head is positioned so as to read the ID information in the ID field 21a with the read element 23. <IMAGE>

Description

MAGNETIC DISK DRIVE The present invention relates to a magnetic disk drive with a magnetic head comprising separate read and write elements.

When a magnetic disk drive employs a sector servo system, each of a plurality of tracks on the recording surface of a disk is segmented into a plurality of sectors, each of which includes a servo field followed by an ID field and data fields. In reading or writing data from or to a data field in a desired sector, it is necessary for the magnetic disk to read ID information from ID fields of sequential sectors. Recently, a dual element head comprising separate read and write elements has been widely used as the magnetic head.

Effective offset between the read and the write elements should be taken into consideration for the dual element head. When the head is positioned to a track, the read and the write elements are displaced laterally with respect to the longitudinal direction of the track or in the radial direction of the disk. Such offset results from manufacturing tolerance or skew of the head to the track. Due to the presence of the offset, if in writing data the head is positioned to align the center line of the write element to that of the track (or the data field), the center line of the read element is displaced from that of the track.

Also, if in reading data the head is positioned to align the center line of the read element to that of the track, the center line of the write element is displaced from that of the track. It is important that the read element can properly read the ID information in either situation.

A typical format of each sector according to the prior art is as shown in Figure 3, in which one ID field 31 is set to be symmetric with respect to the center line 35 of the track predetermined by servo information recorded in a servo field 30. If the offset value between a read element 34 and a write element 33 is represented as d, in this example, the center line 37 of a data field 32 is displaced by a distance of d/2 to the right from the center line 35 of a track and an ID field.

In writing data, the head is positioned to align the center of the write element 33 to the center line 37 of. the data field 32 as shown in Figure 3 (A), while in reading the data, the head is positioned to align the center of the read element 34 to the center line 37 as shown in Figure 3 (B). In either case, because the read element 34 is positioned entirely in the range of the ID field 31, the ID information can be read with a somewhat improved read error rate.

However, as seen from Figure 3, in both read and write operations, the read element 34 is positioned at its center to be on the line 36 or 37 laterally displaced by a distance of d/2 from the center line 35 of the ID field, so that it is unavoidable that the read error rate for the ID information is higher than that for data. In addition, because data is usually attached with an error correction code, the read error rate for data is significantly improved. Thus, the ID information can be a determining factor in overall error.

The present invention thus provides a technique for improving the read error rate for ID information. More specifically, the present invention provides a magnetic disk drive provided with a pair of ID fields which are selectively used in the data read and write operations so as to improve the read error rate for ID information. Furthermore the present invention provides a method for setting up a pair of ID fields for each of a plurality of sectors in the magnetic head of a sector servo type magnetic disk drive so as to improve the read error rate for ID information, as well as a method for setting up and utilizing such ID fields.

In accordance with the present invention, there is now provided a magnetic disk drive with a magnetic head comprising separate read and write elements, characterized by: a magnetic disk having a plurality of tracks each of which is segmented into a plurality of sectors, each sector in each track having a pair of identification fields which are separated from each other along the track and radially displaced with respect to each other by a distance equal to an offset between the read and write elements associated with the track, the identification field having identification information for the sector recorded in it; and means for positioning the magnetic head so as to read one of the pair of identification fields by the read element in a data write operation for any of the tracks and for positioning the magnetic head so as to read the other of the pair of identification fields by the read element in a data read operation for the track.

In a preferred embodiment of the present invention there is provided a sector servo type magnetic disk drive with a magnetic disk comprising separate read and write elements. A pair of identification fields are set up in each sector in each track of the magnetic disk for recording ID information on that sector. The pair of identification fields are separated from each other along the track and radially displaced with respect to each other by a distance equal to the offset between the read and write elements. The magnetic head is positioned so that one of the pair of ID fields is read by the read element in a data write operation for a track, and positioned so that the other ID field is read by the read element in a data read operation for the track.

A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic view of the arrangement of a sector servo type magnetic disk drive implementing the present invention; Figure 2 is a diagram showing various positional relationships between a sector with a pair of ID fields according to the present invention, and a read element and a write element; Figure 3 is a diagram showing various positional relationships between a sector with one ID field according to the prior art, and a read element and a write element; and Figure 4 is a graph showing the relationship between off-track distance for a read element and read error rate.

As schematically shown in Figure 1, a sector servo type magnetic disk drive for implementing the present invention comprises a magnetic disk 1, a magnetic head 2, a rotary actuator 3, a preamplifier 4, a read/write channel 5, a controller 6, and a servo control and drive circuit 7. The head 2 is a dual element head comprising separate read and write elements. The disk 1 has a plurality of tracks segmented into a plurality of sectors. The controller 6 drives the actuator 3 through the servo control and drive circuit 7, thereby making it possible to position the head 2 to a desired track. The controller 6 may include a hard disk controller, a microprocessor and a storage device.

Next, Figure 2 shows various positional relationships between sectors formatted according to the present invention and the head 2.

However, only the read element 24 and the write element 23 are shown for the head 2, while a slider supporting both elements is omitted. A specific example of a head 2 is one using a magneto-resistance effect (MR) type transducer as the read element 24 and using a thin film induction (TFI) type transducer as the write element 23. As seen from this figure, a pair of ID fields, that is, a read ID field 21a and a write ID field 21b, are set up between a servo field 20 and a data field 22. The arrangement of the read ID field 21a and the write ID field 21b as shown is only an example, and the order of the two ID fields may of course be reversed. The ID fields 21a and 21b are separated from each other along the track, and radially displaced (lateral direction in Figure 2) with respect to each other by a distance equal to an offset value S representing the radial offset between the read element 24 and the write element 23 relating to this track. A suitable procedure for setting such ID fields 21a and 21b for each of a plurality of sectors in a track is as follows.

(I) First, an offset value is prepared which represents a radial offset between the read and the write elements when positioning the head 2 for each of a plurality of tracks. When the rotary actuator 3 (Figure 1) is used, the radial offset between the read and the write elements differs for each track because the skew of the head 2 for a track differs from one track to another. Accordingly, it is necessary to prepare a different offset value for each track.

Such an offset value may be found, for example, through calculation that takes into consideration displacement between the two elements due to manufacturing tolerance or the skew of the head 2 for each track.

(II) S is taken as the offset value prepared in relation to a selected track, and T is taken as a value predetermined with consideration of the radial width of a servo field in that track. Under the control of the controller 6 of Figure 1, as shown in Figure 2, the head 2 is positioned se as to position the center of the read element 24 on a line 26 representing the first radial position which is displaced by a distance T to the left from the center line 25 of the track determined by the servo information in the servo field 20. Then, with suitable timing, the read ID field 21a is set up by writing respective identification information in each of a plurality of sectors in that track with the write element 23.

Although T = S/2 in this example, it is not limited to such a value but may be any other value. Because the offset value is S, the center of the write element 23 exists on a line 27 representing the second radial position which is displaced by a distance of T = S/2 to the right from the center line 25 of the track. Therefore, the line 27 becomes the center line of the read ID field 21a, and consequently the center line of the data field 22.

(III) Under the control of the controller 6, as shown in Figure 2 (B), the head 2 is positioned so as to position the center of the read element on a line 26 representing the second radial position which is displaced by a distance of T + S to the left from the center line 26 of the track. Then, with suitable timing, the write ID field 21b is set up by writing respective identification information in each of a plurality of sectors in that track with the write element 23. In this case, because the center of the write element 23 exists on the line 26, the line 26 becomes the center line of the write ID field.

(IV) The operations (II) and (III) are repeated for all of a plurality of tracks. The sequence of (II) and (III) is of course switched to first set up the write ID field 21b, and then set up the read ID field 21a.

The read ID field 21a and the write ID field 21b thus set up are selectively used in writing or reading data. First, in a data write operation for a track, as shown in Figure 2 (C), the head 2 is positioned so as to position the center of the read element 24 on the center line 27 of the write ID field 21b, and the ID information is read from this write ID field 21b by the read element 24. Then, data sent through the read/write channel 5 and the preamplifier 4 of Figure 1 is written in the data field 22 by the write element 23. On the other hand, in a data read operation for that track, as shown in Figure 2 (D), the head 2 is positioned so as to position the center of the read element 24 on the center line of the read ID field 21a and the data field 22, and by the read element 24, the ID information is read from the read ID field 21a and data from the data field 22.

Next, the read error rate is considered. Typically, the read error rate is represented in the form of 10In. This means that one bit error occurs with every IOn bits read. In Figure 4, a substantially V-shaped curve E schematically shows the relationship between the off-track distance of the read element and the read error rate when a read element width corresponding to about 5000 TPI is used. However, for the error rate, only its logarithmic value or -n is shown. According to the present invention, in either a data write or read operation, the center of the read element 24 is positioned on the center line of either one of the two ID fields 21a and 21b so that the off-track distance is near zero, and therefore the logarithmic value of the read error rate is maintained at the low value of -10 at the minimum value of the curve E.

On the other hand, in the case of the prior art described with reference to Figure 3, because the read element 34 is in the off-track state where its center is always displaced by a distance of d/2 to the left or right from the center line of the ID field 31, the read error rate becomes much higher (worse). As an example, to satisfy a requirement where less than one ID read error is allowed in a data transfer of 1010 bits in a case where the ID field 31 has a length of 7 bytes (56 bits) and the ID field 32 has a length of 512 bytes (4096 bits), the following relational expression is established if the read error rate required for the ID is 10-X: 1 - (1 - 10-X)56 = (101 /4096)-1 From this relational expression, -X = -8.14 is obtained. In Figure 4, the error rate for this value corresponds to the off-track distance of about 0.5 pm. In the prior art, the requirement to make the abovementioned d/2 about 0.5 pm is quite severe. It is very difficult to further lower the error rate by further shortening this distance. If the track density is further increased or the dimensions of the disk are made larger, such a requirement becomes so severe that the prior art cannot satisfy it any more. Nevertheless, the present invention can sufficiently satisfy it.

The present thus invention allows improvement of the read error rate for ID information.

Claims

1. A magnetic disk drive with a magnetic head comprising separate read and write elements, characterized by: a magnetic disk having a plurality of tracks each of which is segmented into a plurality of sectors, each sector in each track having a pair of identification fields which are separated from each other along the track and radially displaced with respect to each other by a distance equal to an offset between the read and write elements associated with the track, the identification field having identification information for the sector recorded in it; and means for positioning the magnetic head so as to read one of the pair of identification fields by the read element in a data write operation for any of the tracks and for positioning the magnetic head so as to read the other of the pair of identification fields by the read element in a data read operation for the track.

2. A magnetic disk drive as claimed in Claim 1, wherein each sector has data fields, the center line of the other of the pair of information fields being at substantially the same radial position as the center line of the data field.

3. A magnetic disk drive as claimed in Claim 1 or 2, wherein the magnetic head positioning means has a rotary actuator, and the radial offset differs from one track to another.

4. A magnetic disk drive as claimed in any one of Claims 1 to 3, wherein the read element is a magneto-resistance effect type transducer, and the write element is a thin film induction type transducer.

5. In a magnetic disk drive having a magnetic head comprising separate read and write elements and a magnetic disk having a plurality of tracks each of which is segmented into a plurality of sectors, a method for setting up identification fields in each of the plurality of sectors, comprising the steps of: calculating an offset value for each of the plurality of tracks, the offset representing a radial offset between the read and write elements when the magnetic head is positioned on each track, and in each of a pair of fields which are separated from each other in each sector in each track along each track and radially displaced with respect to each other by the offset value calculated with respect to each track, writing identification information associated with the sector.

6. A method as claimed in Claim 5 comprising the steps of: (a) positioning the read element on one of first and second radial positions which are displaced from the center line of a selected track by distances T and T + S, respectively, in the same direction, and writing respective identification information by the write element into each of a plurality of sectors in the track to set up a first identification field, the S being an offset value prepared for the selected track, the T being a value which is predetermined in view of the width of the servo field in the track, and the center line of the track being determined by the servo information of the servo field; (b) positioning the read element on the other of the first and second radial positions and writing by the write element into each of the plurality of sectors in the track the respective identification information to set up a second identification field; and (c) repeating the steps (a) and (b) until the set up of the first and second fields is completed for all of the plurality of tracks.

7. A method as claimed in Claim 6, wherein the magnetic disk drive is of the type driving the magnetic head with a rotary actuator, a different offset value being calculated for each track.

8. A method as claimed in Claim 6 or Claim 7 comprising: in a data write operation for a track, positioning the magnetic head so as to position the center of the read element on the center line of either one of the first and second identification fields to read the identification information from the one identification field, and in a data read operation, positioning the magnetic head so as to position the read element on the other one of the first and second identification fields to read the identification information from the other identification field.