Servo pattern for dedicated surface track following servo systems

A disk for use in a dedicated surface track following servo system has a plurality of radially spaced circumferential tracks on which synchronizing, position, and track region identifying transition pairs are recorded together with guard transition pairs between the synchronizing and position transitions pairs and between the position and track region identifying transition pairs. The guard and position transition pairs each bridge two adjacent tracks and are separated radially and circumferentially from track to track in repetitive staircase fashion.

This invention relates to servo patterns for reducing errors in dedicated 
surface track following servo systems; more particularly, it relates to 
servo patterns including synchronizing, position, and track region 
identifying information; and specifically it relates to such servo 
patterns including guard information between synchronizing and repetitive 
position information and between repetitive position and track region 
identifying information which serve to reduce amplitude abberations 
causing position errors due to proximity of synchronizing and track region 
identifying information to position information. 
BACKGROUND OF THE INVENTION 
It is known in the prior art to utilize a storage surface within a disk 
drive device to form a position transducing arrangement for purposes of 
positioning the data storage heads. The most common method is to provide a 
dedicated storage surface whereon a servo pattern is magnetically 
prerecorded. A dedicated magnetic playback or servo head reads the servo 
pattern and the signal obtained is demodulated to produce a position 
signal which is applied to a servo system to control an actuator that 
positions the heads. The servo system is referred to as a track following 
servo because it essentially follows a track defined by the pre-recorded 
servo pattern as it positions the servo head on the basis of the 
demodulated position signal. 
In order to provide for a high data storage density in disk drive type 
devices, the distance between adjacent data tracks must be made very 
small. This requires that the total positioning error be correspondingly 
small, such as tens of microniches or less. Therefore, it becomes 
necessary to reduce the causes of position errors. Since there are usually 
a large number of items that contribute to position errors and since some 
of them cannot be easily reduced, it is desirable to reduce as many errors 
as possible to the extent possible. Among the causes of positioning error 
is the servo pattern itself. 
Position errors caused by the servo pattern itself can be reduced as shown 
in U.S. Pat. Nos. 3,534,344 and 3,691,543. In U.S. Pat. No. 3,534,344 
position information is conveyed by transitions of opposite polarity at 
circumferentially spaced positions in adjacent servo tracks. In U.S. Pat. 
No. 3,691,543 position information is conveyed by transitions of like 
polarity at circumferentially spaced positions in adjacent tracks. 
To improve the signal noise ratio, particularly as servo track widths were 
narrowed to increase the number of tracks per inch, the prior art employed 
servo heads essentially twice the width of a servo track. However, the use 
of the servo heads twice the width of the servo track in combination with 
transitions of like polarity in adjacent servo tracks as in the 
forementioned U.S. Pat. No. 3,691,543 presented problems where it was 
necessary to include in the servo pattern dedicated synchronizing pulses 
for purposes of synchronizing the demodulation process, and track region 
identifying information and index information. 
As the output signal if the servo head is influenced by magnetic 
transitions proximate one another, peak shifting and reductions in 
amplitude of the output signal become more pronounced as the transitions 
are brought or crowded closer together to improve sampling rates. Where 
the servo pattern is uniform, as when it consists of only position 
information, the derived position information at the output of the servo 
head would not have any periodic amplitude abberations due to unequal 
pulse crowding effects. However, with the introduction of synchronizing 
and track region identifying information, the pattern is no longer 
homogenous and the derived position information suffers periodic amplitude 
abberations due to the pulse crowding influence of the synchronizing and 
track region identifying information on adjacent position information. 
These abberations contribute to position errors. 
In accordance with the invention, there are included in a servo pattern 
having synchronizing and track region identifying information, guard 
transitions between the sychronizing and repetitive position information 
and between the repetitive position information and track region 
identifying information which provides pattern uniformity with respect to 
the position information in that the pulse crowding effect of the guard 
transitions on position information is the same as the pulse crowding 
effect between position information, thereby avoiding periodic amplitude 
abberations in derived position information. 
An object of the invention is to provide, in a servo pattern which includes 
synchronizing, position and track region identifying information, means 
for avoiding periodic amplitude abberations in the derived position 
information. 
Another object of the invention is in the provision of guard transitions 
between position information and synchronizing and track region 
identification included in a servo pattern.

Referring now to the drawing wherein like reference numerals designate like 
or corresponding parts throughout the several views and wherein a 
preferred embodiment is disclosed, there is shown in FIG. 1 a portion of a 
dedicated surface 2 of a disk generally designated by reference numeral 2 
having, with reference to the center of the disk to which direction arrow 
A points, a plurality of radially spaced circumferential servo tracks, of 
which only eight, 3.sub.1 -3.sub.8 are depicted having abutting 
circumferential boundries 4. The portion of disk 2 shown includes only a 
single servo cell 5 of a series of servo cells. Recorded on the surface 1 
within a servo cell 5 is a servo pattern comprising closely spaced 
magnetic transitions 6 and 7 constituting a transition pair generally 
designated by reference numeral 8 which are spaced in a circumferential 
direction by a distance which is twice the distance between transitions of 
a pair 8, as indicated with reference to scale 9, which is not part of the 
pattern, having equidistant marks. It is to be here understood that the 
relative spacing between various transitions 6 and 7 of a pair 8 and 
between pairs 8 is for purpose of illustration and that actual 
implementation could utilize different relative spacing. 
The servo pattern is read by rotating the disk 2 in the direction of arrow 
B relative to a servo head 10 having read a width essentially twice the 
width of servo track 3. The resulting signal at the output of the head 10 
is demodulated such that two position signals having full quadrature 
relationship are derived for application to a servo head positioning servo 
loop including a head actuator to correct position error as will be 
understood in the art. 
The servo pattern recorded in the surface 1 of the disk 2 in each servo 
cell 5 comprises, in circumferential turn, a transition pair 8S in each 
track 3 dedicated to provide synchronizing information, transition pairs 
8G in selected tracks to provide leading guard information, as will 
hereinafter appear, and a plurality of servo subcells 12A-12F each 
embracing four transitions sites and each containing an equal number of 
transition pairs 8P arranged to provide full quadrature position 
information. Following subcell 12F the pattern includes additional 
trailing guard transition pairs 8G in selected tracks, and a transition 
pair 8I in each track 3 to provide track region identifying information. 
The transition pair 8I is selectively omitted (not shown) in certain 
regions of the pattern to convey region identifying information or index 
information, the latter a once or twice per revolution event. 
As shown in FIG. 1, it is seen that guard transition pairs 8G and the 
position transition pairs 8P each radially extend over two tracks 2, and 
are separated in the radial direction by two tracks. Also, 
circumferentially adjacent transitions 8 in the direction of disk rotation 
B overlap one track. The result is that the guard position transition 
pairs 8G and 8P are radially and circumferentially spaced from track to 
track in repeating staircase fashion. 
It will be appreciated that due to the interposition of the leading guard 
transition pairs 8G between the sync transition pairs 8S and the position 
transition pairs 8P in subcell 12A, the pulse crowding or amplitude 
reducing effect of the guard transition pair will be equivilent to the 
pulse crowding effect of transition pairs in abutting subcells 12, and 
that due to the interposition of the trailing guard transition 8G, the 
pulse crowding effect on the position transition pairs 8P in abutting 
subcell 12F is also the same as that between transition pairs in abutting 
subcells 12. Thus, the interposition of the leading and trailing guard 
transition pairs 8G guards the position transition pairs 8P from and 
significantly reduces the influence of the synchronizing and track region 
indentifying transition pairs 8S and 8I, respectively, on the position 
information which would cause periodic amplitude abberations and result in 
position errors. As the leading and trailing guard transitions 8G are not 
demodulated, the result of the use of guard transitions 8G is that 
magnetic uniformity of the pattern is not disrupted and position 
information having a homogenous uniform amplitude characteristic is 
provided. 
As will be appreciated to those skilled in the art, the synchronizing 
information is employed with a phase locked loop to time the demodulation 
process. 
While the transition pair 8I in one servo cell 5 would, to provide maximum 
pattern density, be spaced from the transition pair 8S in the next servo 
cell (not shown) by the same distance utilized between other transition 
pairs 8 in the pattern, other spacing between transition pairs 8I and the 
transition pair 8S in the next servo cell could be utilized. If the 
spacing between transition pairs 8I and transition pairs is in the next 
servo cell 5 is made relatively large, then this imparts a unique property 
to the pair 8S which aids in its detection. For the situation where a 
relatively large space between transition pairs 8I and following pairs 8S 
is utilized, then the leading guard transition pair and the trailing guard 
transition pair reduce the effect that such a large distance would 
otherwise have in disrupting the magnetic uniformity of the position 
transition pairs 8P. 
With reference to FIG. 2, the servo pattern described with reference to 
FIG. 1 is shown on the lower surface 1 of a disk 2 whose upper surface 13 
contains data tracks 14 on which magnetic data may be recorded or read by 
a data head 15. As shown in FIG. 2, the boundries 4 of the servo tracks 
are opposite the centerline of the data tracks 14. The invention has 
utility not only in the magnetic recording disk drives as shown in FIG. 2, 
but also in magneto-optical and electro-optical disk drives wherein a 
dedicated servo surface pattern is utilized.