Disc drive apparatus with servo tracks offset from data tracks

A disc drive apparatus with a tracking servomechanism, comprising at least one disc medium having data surfaces where servo signal and data signal are recorded; transducer means having a first head for reproducing the data signal from the data tracks in a playback mode, and a second head for recording the data signal on the data tracks in a recording mode; a rotary arm for supporting the transducer means at one end thereof; actuator motor means for driving the rotary arm in a manner to move the transducer means over the data tracks; servo signal processing means for receiving and processing the servo signal reproduced by the transducer means and outputting first and second servo control signals; and positioning control means for controlling the actuator motor means in the playback and recording modes respectively in accordance with the first and second servo control signals so that the first head of the transudcer means is positioned precisely on any selected one of the data tracks in the playback mode and also that the second head is positioned precisely on any selected one of the data tracks in the recording mode.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a tracking servomechanism for a disc drive 
apparatus having a playback head and a recording head attached 
individually to a rotary support arm. 
2. Description of the Prior Art 
The conventional head displacement means known heretofore for use in a disc 
drive apparatus are generally classified into a linear type which moves a 
support arm linearly to displace a head in the radial direction of a disc, 
and a rotary type which rotates a support arm to displace a head in the 
radial direction of a disc. To realize a compact and lightweight structure 
of the drive apparatus, the rotary type is considered to be preferable. 
Meanwhile, a magnetoresistive type head capable of exhibiting excellent 
playback sensitivity characteristics even in a narrow track width has been 
developed to meet the requirement of achieving a higher track density. 
However, the magnetoresistive type head has no recording function. 
Therefore, when such magnetoresistive type head is employed, it is 
necessary to provide a recording head in addition thereto. 
Under the condition mentioned, it is still possible to produce a high 
density recording/reproducing disc drive apparatus of a compact and 
lightweight structure by using a rotary support arm and attaching a 
playback head and a recording head individually to such support arm. 
FIG. 1 schematically illustrates the structure of a conventional disc drive 
apparatus equipped with a rotary actuator, in which a base end of a 
support arm 1 is held rotatably by a rotary shaft 2. This shaft 2 is 
rotated by a driving force of an unshown motor to control the position of 
the support arm 1. A slider 3 is attached to a distal end of the support 
arm 1, and a magnetoresistive type playback head Hp and a coil type 
recording head Hr are secured to the slider 3 as illustrated in FIG. 2. 
The playback head Hp and the recording head Hr are disposed on a center 
line Cl of the support arm 1 while being spaced apart by a predetermined 
distance from each other, in such a manner that the recording head Hr is 
set at a front position while the playback head Hp is set at a rear 
position. The playback head Hp and the recording head Hr are so arranged 
as to be both in an exact tracking or "just-tracking" state at an 
innermost servo signal track Ti on a disc 4. Therefore at any other track, 
the recording head Hr is placed in an off-tracking state when the playback 
head Hp is at a "just-tracking" position. 
In such known disc drive apparatus equipped with a rotary actuator as shown 
in FIG. 1, the angle formed by the slider 3 and the tangential direction C 
to each track is changed in accordance with the track position. FIG. 1 
represents an exemplary case where a directional coincidence is attained 
therebetween at the innermost track position Ti. The above angle gradually 
increases in accordance with the displacement of the slider 3 toward the 
periphery of the disc and finally reaches a maximum value .theta. at the 
outermost track position To. 
In an exemplary case of FIG. 2 where the recording head Hr and the playback 
head Hp are so disposed on the slider 3 that the respective centers 
thereof are positioned exactly at the center line C2 of the innermost 
track Ti, when the rotary arm is controlled for aligning the center of the 
playback head Hp with the center line of any other track, the center of 
the recording head Hr comes to be positioned at a point deviated from the 
center line C2 of the track. When a tracking servo control is executed 
while a tracking servo signal prerecorded on the disc is read out 
therefrom by the playback head Hp, the center of the playback head Hp can 
be exactly positioned with respect to the track center, but there exists a 
positional deviation between the center of the recording head Hr and any 
other track center except the innermost track. Consequently, if a 
recording mode is executed in a state where merely the playback head Hp 
alone is aligned with the track center line as shown in FIG. 3, a newly 
recorded data track is formed with a positional deviation from the 
preceding data track. And when the signal recorded in this manner is 
reproduced, there arises a problem that some noise or read error is 
induced since the playback head Hp is so controlled that the center is 
continuously positioned on the track center line C2 while the recording 
head Hr scans the off-track position of the data signal at any track other 
than the innermost track Ti. 
There is known a discrete track disc where servo tracks and data tracks are 
formed alternately in the circumferential direction on a magnetic disc, 
and both tracks are composed of a magnetic layer while any other region is 
composed of a nonmagnetic layer. In such discrete track disc, the width of 
the data track is smaller than the width of the recording head as shown in 
FIG. 4, so that a signal recording operation can be performed without 
causing any exudation or incomplete erasure in a recording mode. 
However, even by the use of such a discrete track disc, when the off-track 
deviation becomes so great that the end of the recording head Hr scans the 
data track, some exudation or incomplete erasure is caused in the 
recording mode to eventually lose the merit of using a discrete track 
disc. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a disc drive 
apparatus with a tracking servomechanism where, in a recording mode, the 
"just-tracking" position of a servo signal is scanned by a recording head 
in every track, hence preventing occurrence of any noise or read error 
that may otherwise be derived from a state where a data signal is recorded 
at the off-track position of the servo signal. 
Another object of the present invention is to provide a disc drive 
apparatus with a tracking servomechanism which enables a recording head to 
scan each data track in a "just-tracking" one state to consequently record 
data signal properly even by the use of a discrete track disc as a 
recording medium. 
According to one aspect of the present invention, there is provided a disc 
drive apparatus with a tracking servomechanism comprising at least one 
disc medium having data surfaces on which a plurality of concentric data 
tracks are formed, wherein a servo signal is recorded on at least one of 
the data surfaces as well as data signal; transducer means having a first 
head for reproducing the data signal from the data tracks in a playback 
mode, and a second head for recording the data signal on the data tracks 
in a recording mode; a rotary arm for supporting the transducer means at 
one end thereof while being held rotatably around an axis; actuator motor 
means for driving the rotary arm in a manner to move the transducer means 
over the data tracks; servo signal processing means for receiving and 
processing the servo signal reproduced by the transducer means and 
outputting a first servo control signal and a second servo control signal; 
and positioning control means for receiving the first and second servo 
control signals and controlling the actuator motor means in the playback 
and recording modes respectively in accordance with the first and second 
servo control signals so that the first head of the transducer means is 
positioned precisely on any selected one of the data tracks in the 
playback mode and also that the second head of the transducer means is 
positioned precisely on any selected one of the data tracks in the 
recording mode. 
According to another aspect of the present invention, there is provided a 
disc drive apparatus with a tracking servomechanism comprising at least 
one disc medium having data surfaces on which a plurality of concentric 
tracks are formed, each of the tracks being divided into plural data 
tracks and plural servo tracks, wherein a data signal is recorded and 
reproduced by transducer means on and from the data tracks, while a servo 
signal is prerecorded and reproduced by the transducer means on and from 
the servo tracks; the transducer means having a first head for reproducing 
the data signal from the data tracks in a playback mode, and a second head 
for recording the data signal on the data tracks in a recording mode; a 
rotary arm for supporting the transducer means at one end thereof while 
being held rotatably around an axis; actuator motor means for driving the 
rotary arm in a manner to move the transducer means over the data tracks; 
servo signal processing means for receiving and processing the servo 
signal reproduced from the servo tracks and outputting a servo control 
signal; and positioning control means for receiving the servo control 
signal and controlling the actuator motor means in the playback and 
recording modes in accordance with the servo control signal; wherein the 
center of the servo tracks deviates from the center of the data tracks by 
an amount relative to a track address of the corresponding track, and the 
second head of the transducer means is positioned precisely on any 
selected one of the data tracks while the first head of the transudcer 
means is positioned precisely on the corresponding one of the servo 
tracks. 
The above and other features and advantages of the present invention will 
become apparent from the following description which will be given with 
reference to the illustrative accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinafter a preferred embodiment of the present invention will be 
described with reference to the accompanying drawings. 
FIG. 5 illustrates a format on a disc employed in a disc drive apparatus to 
which the present invention is applied. This format is based on a sector 
servo system where a multiplicity of tracks T of mutually different radii 
are formed around the rotation center of the disc 4, and servo areas 5 are 
formed at equal intervals on each track T of the same radius. In the disc 
4 based on such sector servo system, one sector is constituted of the 
servo area 5 and a data area 6 posterior thereto, and several tens of 
sectors are existent in such track. 
FIG. 6 shows a servo pattern of the servo areas 5, and FIG. 7 graphically 
shows the off-tracking characteristics of a fine address servo. In FIG. 6, 
a sector head ST with a recorded sector start signal is formed in a head 
portion of the servo area 5, and address portions AD1 and AD2 with 
recorded track addresses are formed posterior to the sector head ST. 
Immediately anterior to the address portion AD1, there is formed a portion 
X where a burst signal representing an odd track is recorded; and 
immediately posterior to the address portion AD2, there is formed a 
portion Y where a burst signal representing an even track is recorded. The 
track address is detected by comparing the levels of X and Y with each 
other and adopting the address of the portion AD1 when X&gt;Y or the address 
of the portion AD2 when X&lt;Y. Meanwhile, when X=Y, it is recognized that 
the playback head Hp is positioned between the tracks. Posterior to the 
address portions are portions A, B, C where signals A, B, C are recorded 
respectively. The portions A and B are formed at different positions in 
the longitudinal direction of the track while being spaced apart slightly 
from the track center line C2 and are provided integrally with the 
portions A and B of the adjacent track. Meanwhile the portion C is formed 
continuously with the corresponding portion of the adjacent track so as to 
be picked up without fail by the playback head Hp regardless of the 
scanning course thereof. A calculation of (A-B)/C is executed and then a 
value FA proportional to the off-track deviation from the track center 
line C2 as shown in FIG. 7 is computed to detect a fine address FA. In 
FIG. 7, TP stands for a track pitch, and TW for a track width. The present 
address of the playback head Hp can be obtained by combining the 
aforementioned track address with the fine address thus detected. 
FIG. 8 is a timing chart of receiving track addresses in a track seek mode. 
In FIG. 8, track addresses outputted from the playback head Hp are 
received synchronously with sector servo pulses in the track seek mode, 
and the head displacement speed V can be obtained by executing a 
calculation of (.vertline.a-b.vertline.TP)/t.sub.o (where a and b 
represent addresses received successively). 
FIG. 9 is a circuit block diagram of a track service mechanism. In this 
diagram, an output signal of the playback head Hp is fed via an amplifier 
7 to an address sampling circuit 8, which then samples a track address and 
a fine address. The fine address is obtained from a calculation of (A-B)/C 
as described above, and the resultant value (C to .+-.1) is converted into 
a digital percent value, for example, by means of an A-D 
(analog-to-digital) converter. Both the track address and the fine address 
are fed to an adder 9, where the values are added to each other so that 
the present address is obtained. The present address is supplied to a 
subtracter 10 and a speed calculating circuit 11. The subtracter 10 is 
supplied also with a desired address, and an address difference obtained 
by subtracting the present address from the desired address is supplied to 
a root curve generator 12. This generator 12 has speed information which 
corresponds to the address difference and represents the speed profile 
characteristic shown graphically in FIG. 10. A desired speed based on such 
characteristic is supplied to a subtracter 13. The speed calculating 
circuit 11 receives the present address in synchronism with the sector 
servo pulse and executes a calculation of .vertline.a-.vertline.b 
TP/t.sub.o as described above to thereby compute a head displacement 
speed. The present speed thus obtained is supplied to the subtracter 13, 
which substracts the present speed from the desired speed to calculate the 
speed difference and then supplies the speed difference data to a gain 
control amplifier 14. 
Meanwhile, the track address from the address sampling circuit 8 and a mode 
signal indicating either a playback state or a recording state of the 
drive apparatus are inputted to a correction value generating circuit 15, 
which calculates the corrected fine address in accordance with the input 
track address. The calculation of the corrected fine address is executed 
in the following manner. In FIG. 11, l represents the distance between the 
playback head Hp and the recording head Hr, and .theta. represents the 
angle formed by the center line C2 of a corrected track and the head 
center line C3, wherein a shift length X of the recording head Hr in the 
radial direction of the disc with respect to the playback head Hp is 
obtained by calculating an equation X=l sin .theta.. A fine address 
corresponding to a position S which is displaced by such shift length X in 
the reverse direction is determined to be the corrected fine address. In a 
recording mode, the corrected fine address thus obtained is outputted to a 
subtracter 16; and in a playback mode, a zero value (a "just-tracking" 
position) is outputted continuously as the corrected fine address. The 
fine address (present fine address) from the address sampling circuit 8 is 
inputted to the subtracter 16, and the fine address difference obtained by 
subtracting the present fine address from the desired fine address is 
outputted to a PID (proportional-integral-derivative) circuit 17. Either 
the output of such PID circuit 17 or the output of the aforementioned gain 
control amplifier 14 is selected by a selector switch 18, and the selected 
output is fed via an amplifier 19 to a motor 20 provided for driving the 
support arm. The action of the selector switch 18 is controlled by a 
switching control circuit 21 which receives the track address difference 
information from the subtracter 10. Upon start of the track seek mode, a 
control signal is outputted for selectively turning the switch 18 to the 
side of the gain control amplifier 14. When the track address difference 
is reduced to be within .+-.1/2 TP (track pitch), another control signal 
is outputted for changing the switch 18 to the side of the PID circuit 17. 
Now the function of the above constitution will be described below. 
Upon start of the track seek mode during the operation in the recording 
mode, the selector switch 18 is changed to connect the gain control 
amplifier 14. The address sampling circuit 8 samples a track address and a 
fine address from the playback output picked up by the playback head Hp, 
whereby the present address is obtained as an output of the adder 9. In 
the root curve generator 12, the desired speed data based on the speed 
profile characteristic is calculated from the address difference data and 
then is outputted to the subtracter 13. Meanwhile in the speed calculating 
circuit 11, the present speed is calculated from the present address fed 
thereto sequentially and then is outputted also to the subtracter 13. A 
driving control signal proportional to the speed difference is outputted 
from the gain control amplifier 14 to thereby control the rotation speed 
of the support arm 1. This rotation speed corresponding to the 
displacement speed of the playback head Hp is linearly increased to be 
approximate to the initial speed in the speed profile characteristic curve 
as indicated by a one-dot chained line in FIG. 10. Upon arrival at the 
speed in the speed profile characteristic curve, the control action is so 
executed as to attain a coincidence between the desired speed and the 
present speed, whereby the rotation speed is reduced in conformity with 
the speed profile characteristic. Upon the track address difference has 
been decreased to a range of .+-.1/2 TP, the selector switch 18 is changed 
to the position of the PID circuit 17 by the control signal fed from the 
switching control circuit 21. The decrease of the track address difference 
to a range of .+-.1/2 TP signifies that the playback head Hp has arrived 
at the desired track, so that the address of the corrected track is 
inputted from the address sampling circuit 8 to the correction value 
generating circuit 15. Then this circuit 15 outputs a corrected fine 
address corresponding to the input track address, and the PID circuit 17 
outputs a driving control signal corresponding to the position difference, 
thereby controlling the rotational position of the support arm 1. As 
illustrated in FIG. 12(a), the center position of the playback head Hp is 
set properly at the corrected fine address, so that the recording head Hr 
comes to scan the "just-tracking" position exactly on the desired track. 
Therefore, in the data area 6, the data signal is recorded at the position 
coincident with the track center line C2 as illustrated in FIG. 12(b). 
In the playback mode, the correction value generating circuit 15 
continuously outputs a zero value as the corrected fine address, so that 
the playback head Hp can be kept at the just-track position on the track 
contact line C2 to consequently reproduce a proper playback signal. 
Although the embodiment mentioned is concerned with an exemplary case of 
applying the present invention to a sector servo system, it is to be 
understood that the present invention is applicable also to a different 
servo system for recording a servo signal in a deep layer of a disc. And 
the present invention is further applicable to another servo system which 
uses a reference track with calculation of linear interpolation as 
disclosed in U.S. Pat. No. 4,122,503. 
According to the present invention, as described hereinabove, a playback 
head and a recording head are secured individually to a rotary support arm 
in a disc drive apparatus, and such playback head and recording head are 
moved substantially in the radial direction of a disc by the rotation of 
the support arm, wherein, during a recording mode, a shift length of the 
recording head in the radial direction of the disc is calculated with 
respect to the playback head at a desired track position, and a control 
action is so executed as to cause off-tracking of the playback head by the 
calculated shift length in the reverse direction. Consequently, the 
recording head scans the center line of the servo signal track and writes 
a data signal at a position coincident with the center of the servo signal 
track. Thus, in the playback mode, there is attainable a remarkable effect 
of eliminating any noise or read error that may result from the off-track 
scanning. 
Hereinafter another preferred embodiment of the present invention employing 
a discrete track disc will be described with reference to the accompanying 
drawings. The disc used in this embodiment also has a format pattern of 
FIG. 5 with servo sectors and data sectors arrayed alternately. FIG. 13 
illustrates the detail of such format pattern, wherein each servo area 
E.sub.S includes a servo track T.sub.S, and each data area E.sub.D 
includes a data track T.sub.D. The servo tracks T.sub.S and the data 
tracks T.sub.D denoted with oblique lines are composed of a magnetic 
layer, while any other region is composed of a nonmagnetic layer. The 
servo track T.sub.S comprises a track address portion T.sub.A and three 
fine address portions A, B, C. A track address signal is recorded in the 
track address portion T.sub.S, while burst signals of one frequency are 
recorded in the fine address portions A, B, C. The off-track deviation 
from the center line C3 of the servo track T.sub.S is obtained by 
calculating an expression (A-B)/C in accordance with the burst signal 
outputs picked up, and when the width d3 of the fine address portions A 
and B is rendered equal to the width d2 of the playback head Hp, the servo 
characteristics can be enhanced to bring about a satisfactory result. The 
data track T.sub.D is a region for recording a data signal, and its width 
is determined to be smaller than the width d1 of the recording head Hr but 
greater than the width d2 of the playback head Hp. At the innermost track 
Ti, as shown in FIG. 13(a), the center line C3 of the servo track T.sub.S 
becomes coincident with the center line C2 of the data track T.sub.D. 
However, at any track other than the innermost track Ti, as shown in FIG. 
13(b), the center line C3 of the servo track T.sub.S is shifted from the 
center line C2 of the data track T.sub.D. Such shifted position is denoted 
by S in FIG. 14 and is set by calculating a shift length X of the 
recording head Hr in the radial direction of the disc with respect to the 
playback head Hp at each rotational position of the support arm 1 and then 
shifting the center line C3 by such calculated length X in the reverse 
direction. The shift length X can be obtained by calculating an equation 
X=l sin .theta., in which l denotes the distance between the playback head 
Hp and the recording head Hr, and .theta. denotes the skew angle formed by 
the track center line C2 and the center line C1 of the support arm 1. The 
shift length X becomes greater toward the outermost track. 
Now the function of the above constitution will be described below. 
When the playback head Hp scans the servo track T.sub.S in a playback mode, 
the playback head Hp picks up a track address signal and a burst signal. 
The present track address being scanned is identified from the track 
address signal thus picked up, while the off-track deviation is detected 
from the burst signal, whereby the playback head Hp is kept scanning the 
servo track T.sub.S in a just-tracking state. At the innermost track Ti, 
as shown in FIG. 13(a), the playback head Hp and the recording head Hr are 
not shifted in the radial direction of the disc, so that the center line 
C3 of the servo track T.sub.S and the center line C2 of the data track 
T.sub.D are positionally coincident with each other. Therefore, both the 
playback head Hp and the recording head Hr scan the data track Tp in a 
just-tracking state. Meanwhile at any track other than the innermost track 
Ti, the playback head Hp and the recording head Hr are shifted in the 
radial direction of the disc as shown in FIG. 13(b), but since the center 
line C3 of the servo track T.sub.S is shifted from the center line C2 of 
the data track T.sub.D correspondingly to the above shift length, the 
playback head Hp scans the off-track position while the recording head Hr 
scans the "just-tracking" position. Consequently, the recording head Hr 
keeps scanning the just-track position on the data track T.sub.D and 
therefore records data signal properly on the data track T.sub.D without 
any exudation or incomplete erasure in the recording mode. Meanwhile the 
playback head Hp scans the off-track position on the data track T.sub.D. 
However, since the playback head Hp is formed to be narrower than the data 
track width, the end of the playback head Hp never protrudes outside the 
data track T.sub.D to eventually induce none of impediments to the signal 
reproduction. 
As described hereinabove, in a magnetic disc apparatus where a playback 
head and a recording head are attached individually to a rotary support 
arm and a discrete track disc is employed as a recording medium, the head 
positioning system of the present invention calculates a shift length of 
the recording head in the radial direction of the disc with respect to the 
playback head at each rotational position of the support arm, and then 
shifts the center of the servo track in the reverse direction by the 
calculated shift length from the center of the data track. Therefore the 
recording head scans the data track in a "just-tracking" state to 
eventually ensure a proper data signal recording operation without causing 
any exudation or incomplete erasure in the recording mode. Meanwhile the 
playback head scans the data track with an off-track deviation. However, 
since the playback head is generally formed to be narrower than the data 
track, it is possible for the playback head to scan the data track without 
protrusion therefrom, hence attaining an advantageous effect that 
eliminates any impediment to the signal reproduction.