Dual sector servo system for disk file with separate read and write heads

A method and servo system is provided to compensate for misalignment of separate read and write heads when reading and writing data on a magnetic disk. Two sets of servo information are generated for each data track on the disk. These sets are radially displaced with respect to each other by a distance equal to the respective misalignments between the read and write heads at each respective data track. One set is generated by the write head. The other set is generated by the write head while the read head is positioned using said one set as a position reference. Thereafter, to write data on a selected track, the read head is first aligned with said one set of servo information and then data is written on said track with the write head. To read data on a selected track, the read head is aligned to said other set of servo information and then the data is read by said read head.

This invention relates to a servo method and means for compensating for 
misalignment of separate read and write heads with respect to a plurality 
of concentric data tracks in a magnetic disk file, and more particularly 
to such a method and means wherein separate read and write heads in a dual 
head structure are independently positioned on a selectable disk track for 
reading and writing by use of separate sets of servo information. 
BACKGROUND OF THE INVENTION 
When using separate heads for reading and writing, such as 
magneto-resistive (MR) heads for reading and inductive heads for writing, 
it is difficult to position alternately the read head to the track during 
reading, and the write head to the track during writing. This is not a 
serious problem when the misalignment between the read head and the write 
head is much smaller than the overall track misregistration and the track 
pitch of the system. However, in high track density applications or in 
systems with rotary actuators, where the read to write misalignment is no 
longer insignificant when compared to the track pitch, one is faced with 
two choices: (1) increase the degree of write-wide and read-narrow, i.e., 
(a) increase the write track width which results in lower track density, 
or (b) reduce read track width which results in lower signal-to-noise 
ratio--either of which results in performance degradation; or (2) devise a 
servo scheme to position the write head on the track for writing and the 
read head on the track for reading. 
One arrangement heretofore proposed to independently position the read and 
the write heads to the track is described in the October 1978 issue of the 
IBM Technical Disclosure Bulletin at page 2005 and depicted in FIGS. 1A 
and 1B. When positioning the head for writing, the write head is used to 
read the sector servo information, as in FIG. 1A. When positioning the 
head for reading, the read head is used to read the sector servo 
information, as in FIG. 1B. 
Another arrangement proposed to solve this misalignment problem is 
described in the June 1974 issue of the IBM Technical Disclosure Bulletin, 
at page 217. It requires reading a calibration pattern with both the read 
head and write head, and using that calibration pattern to measure the 
misalignment between the read and write heads. The misalignment is then 
stored and used by the head positioning servo system to offset the head 
position during read to compensate for the misregistration between the 
read and write elements. This approach requires the write head to read 
servo type information only during the calibration phase, which can be 
done in the factory, but requires a write head designed to read the 
calibration pattern and electronic circuitry that will permit reading with 
the write head. 
In both approaches, the write head not only has to write data but also has 
to read the servo information. This requirement put significant 
limitations on applicability of the scheme, especially in high track 
density applications with MR-read/inductive-write heads. First, inductive 
heads have lower signal-to-noise ratio; and although this problem can be 
remedied to some degree with more turns in the coil, it adds process 
complexity and cost to head fabrication. Second, an inductive head 
optimized for writing will most likely have difficulty reading data at 
data frequency; and although servo information can be written at lower 
data rate, it adds to the real estate taken up for servo. Third, this 
arrangement requires one additional read channel for the write head. 
Finally, and also most important, thin film inductive heads with very 
narrow pole widths are likely to have domain problems for reading 
purposes, rendering them unusable for high track density applications. 
There is a need for a method and means for compensating for misalignment of 
separate but linked read and write heads when reading and writing data on 
a magnetic disk whether such misalignment varies due to the arcuate 
direction of access with a rotary actuator or is of essentially constant 
magnitude due to variations in the relative positions at which the heads 
are mounted during manufacture of a radial linear actuator. 
SUMMARY OF THE INVENTION 
Toward this end and according to the invention, the following method and 
means is provided to compensate for misalignment of separate but linked 
read and write heads when reading and writing data on a magnetic disk. 
Two sets of servo information are generated for each data track on the 
disk. These sets are radially displaced with respect to each other by a 
distance equal to the respective misalignments between the read and write 
heads at each respective track. One set is generated by the write head. 
The other set is generated by the write head while the read head is 
positioned using said one set as a position reference. Thereafter, to 
write data on a selected track, the read head is first aligned with said 
one set of servo information and then data is written on said track with 
the write head. To read data on a selected track, the read head is aligned 
to said other set of servo information and then the data is read by said 
read head.

DESCRIPTION OF PRIOR ART 
As illustrated in FIG. 1A during writing of data on a selected data track 
of a magnetic disk (not shown), write head 10 is aligned with and reads 
the servo information 11 on the track; and read head 12 is misaligned a 
distance d from the write head and servo information. As illustrated in 
FIG. 1B, during reading of data, read head 12 is aligned with and reads 
the servo information 11, while the write head 10 is now misaligned the 
distance d from the read head and servo information. 
DESCRIPTION OF PREFERRED EMBODIMENT 
As illustrated in FIG. 2A, a write head 20 and a read head 21 travel in the 
direction of arrow 22 relative to a plurality of concentric data tracks on 
a magnetic disk. While write head 20 and read head 21 are traveling in the 
direction of arrow 22, the write head is used to write and generate a 
first set 23 of sector servo information for each data track. Read head 21 
will be misaligned the distance d from the write head and set 23 at this 
time. 
A second set 24 of sector servo information is now generated, as 
illustrated in FIG. 2B, for each data track by aligning read head 21 using 
the servo information set 23 and concurrently writing the second servo 
information set 24 with write head 20. 
The steps just described generate, for all data tracks, two sets of servo 
information unique to each data track. Note that if the heads are mounted 
on a rotary actuator, the misalignment distance d will vary from track to 
track because of the non-radial (i.e., arcuate) path traversed by the 
actuator and hence the heads during track access. 
As illustrated in FIG. 3, data is written at a selected data track by 
aligning read head 21 to the first set 23 of servo information and then 
writing data on that track with write head 20. Since the data thus written 
is aligned with the second set 24, misalignment d of the heads will be 
automatically compensated for when the data is subsequently read in the 
following manner. 
As illustrated in FIG. 4, data is read at a selected track by aligning read 
head 21 to the second set 24 of servo information and then reading the 
data with the read head. At this time, write head 20 will be misaligned 
distance d from read head 21 and second set 24 of servo information. 
Thus, according to the invention, and as best shown in FIGS. 3 and 4, these 
two sets 23, 24 of servo information are provided at separate 
circumferential positions on the magnetic disk. One radially extending set 
(24) is in line with each data track and the other set (23) is offset in a 
generally radial direction from said one set by the distance d that will 
be of constant magnitude if the disk is accessed with a radial linear 
actuator or will vary from track to track if the disk is accessed with a 
rotary actuator. 
FIG. 5 depicts a sector servo control apparatus for implementing the 
invention. This apparatus comprises a dual element head structure 30 
comprising a read head 31 and a write head 32. As illustrated, head 
structure 30 is mounted on a rotary actuator 33 that moves the head 
structure 30 in an arcuate path to a selected track on magnetic or optical 
disk 29. An external position reference sensor 34 provides position 
information to servo control electronic circuitry 35 for actuator 33 which 
controls position of head structure 30 using a voice coil motor 36. Write 
circuits 37 provides signals to the write head 32 to write and to thereby 
generate the first set 23 of servo information on the disk. Next, read 
head electronic circuitry 38 processes the signals from read head 31 to 
provide position information to actuator servo control circuitry 35. This 
is used to position read head 31 over the first set 23 of servo 
information while writing and hence generating the second set 24 of servo 
information. The foregoing operation is controlled by timing and control 
electronic circuitry 39. Amplifiers 40, 41 amplify the write signal and 
read signal, respectively. 
As earlier noted, the distance d will vary track to track if the heads are 
mounted on a rotary actuator. However, the invention is also of value when 
the heads are mounted on a radial linear actuator, especially in high 
track density applications. In this latter case, variations in distance d 
can occur from actuator to actuator due to the relative positioning of the 
heads during the manufacturing process; however, the misalignment distance 
d will be substantially constant for any given radial linear actuator. 
The invention has been successfully implemented using a magneto-resistive 
read head and an inductive write head mounted on a rotary actuator. The 
invention can also be implemented with any sector servo arrangement, 
provided two sets of servo information are generated--one for writing and 
one for reading. 
Although the servo arrangement herein disclosed requires additional storage 
capacity to provide the two sets of servo information, it desirably 
permits achievement of higher track densities by eliminating design 
concerns over misalignment of read and write heads. 
While the invention has been shown and described with respect to a 
preferred embodiment thereof, it will be understood by those skilled in 
the art that changes in form and detail may be made without departing from 
the scope and teaching of the invention. Accordingly, the method and means 
herein disclosed are to be considered merely as illustrative, and the 
invention is to be limited only as specified in the claims.