Track following servosystem for data storage apparatus

A data storage apparatus, such as an accessing head magnetic disk file, includes at least one data disk surface and a servo disk surface. In operation, a continuous position signal having high frequency components is derived from the servo surface, which has a quadature type servo signal prerecorded thereon. A circuit means modifies the derived position signal so that a substantially linear signal representing the displacement of the servo head from the servo track is obtained. A second position signal having a low frequency component is obtained from servo sector information registered on the data disk surface and together with the first position signal forms a hybrid position signal. This hybrid signal is used to control the movement of the data heads relative to the data tracks to ensure optimum transducing operation.

CROSS-REFERENCE TO RELATED APPLICATION 
In U.S. Pat. No. 4,072,990, entitled "Servo Positioning System for Data 
Storage Apparatus," filed July 19, 1976 in behalf of W. J. P. Case et al., 
data storage apparatus is described and claimed comprising a stack of 
recording disks mounted for rotation on a drive spindle, a plurality of 
recording and playback data heads each associated with a corresponding one 
of a plurality of data surfaces on the disks for performing transducing 
operations on data tracks thereon, each data track consisting of data 
sectors for recording and/or playback of data thereon by a data head 
alternating with servo sectors containing prerecorded data track position 
information for that track readable by the same data head, a servo head 
ganged for movement with the data heads and associated with servo tracks 
prerecorded on a servo surface on one of the disks, the servo tracks being 
distinct from the servo sectors on the data tracks and providing 
continuous data track position information readable by the servo head, and 
servo control circuits operable during track access operations in response 
to position information signals from the servo surface alone to control 
movement of the data heads across tracks and operable during track 
following operations in response to a hybrid signal composed of signals 
derived from the servo sectors on the data surface including the track 
being followed and high frequency components of position information 
signals derived from the servo surface. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to data storage apparatus, and in particular, to a 
track following servosystem useful in disk files. 
2. Description of the Prior Art 
Although the system described in U.S. Pat. No. 4,072,990 represents a 
marked improvement over previous systems, it has a disadvantage caused by 
the cyclic nature and hence the non-linearity of the position information 
signals, or position error signals (PES), used to control the servo. The 
problem exists as a result of disturbances which affect the mechanical 
stability of the apparatus and can lead to the servo head being 
considerably displaced from its true on-track position over the associated 
servo track, despite the data head being accurately positioned over the 
data track being followed. 
Under these circumstances, it is possible for the servo head to be so far 
off track that the position information or position error signals being 
supplied by the servo head are from the non-linear part of the signal. 
That is, they do not exhibit a linear relationship with respect to 
displacement from the servo on-track position. Accordingly, since the high 
frequency components of this signal are required to form the hybrid servo 
signal used during track following operations, the gain of the servo 
circuits at high frequencies varies with changing displacement from the 
servo on-track position. Since these servo head displacements are caused 
largely by external disturbances such as temperature variations or shocks 
and vibrations, as well as internal influencing factors such as 
eccentricity and tilt of disks on the disk spindle, they are largely 
uncontrollable. The continuous and unpredictable changing in gain of the 
servo circuits can lead to loop instability and/or increased head settle 
time, both of which are undesirable features. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide an improved track following 
servosystem. According to the invention, data storage apparatus comprises 
a stack of recording disks mounted for rotation on a drive spindle; a 
plurality of recording and playback heads each associated with a 
corresponding one of a plurality of data surfaces on the disks for 
performing transducing operations in data tracks thereon; each data track 
consisting of data sectors for recording and/or playback of data thereon 
by a data head alternating with servo sectors containing prerecorded data 
track position information for that track readable by the same data head; 
a servo head ganged for movement with the data heads associated with servo 
means providing continuous cyclic data track position information having 
substantially linear portions, each portion indicating by its magnitude 
and polarity the degree and direction of offset of the servo head from the 
end of an increment of movement equal to the distance between successive 
data tracks; modifying means for receiving said continuous data track 
position information signals supplied thereto and operable to supply 
modified position information signals at its output which are 
substantially linear with respect to displacement from one to another 
servo track over their entire range; and servo control circuits operable 
during track access operations in response to position information signals 
from the servo surface to control movement of the data heads across tracks 
and operable during track following operations in response to a hybrid 
signal composed of signals derived from the servo sectors on the data 
surface, including the track being followed, and high frequency components 
of modified signals supplied at the output to the modifying means.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1, a stack of magnetic recording disks 1 are mounted for rotation 
on a central spindle 2. Prerecorded servo tracks 3 are provided over one 
surface of one of the disks and are read by an associated servo head 4. 
Since this surface contains only servo information, it is referred to a a 
dedicated servo surface. The dedicated servo surface contains continuous 
information regarding the position of data tracks on the remaining surface 
of the disks which are accessed by a number of data recording the playback 
heads 5, one being provided for each of the remaining disk surfaces. The 
data heads 5 and the servo head 4 are all ganged together for movement 
across the disk surfaces by actuator mechanism 6. 
During track access operations, the continuous position error signals 
derived by the servo head 4 are passed through preamplifier 9 and AGC 
amplifier 10 to servo control circuit 11 which also receives the address 
and polarity of the selected data track for each track access operation 
from an external control system 12. From this information, the servo 
circuits produce the necessary drive currents for the actuator mechanism 6 
to coarsely position the data heads over the desired track. 
During track following operations, the fine position error signals required 
to maintain the data head accurately on track are derived from servo 
information prerecorded in sectors around the data track itself. Data and 
servo information read by a selected data head 5 is passed through 
preamplifier 7 and AGC amplifier 8 to servo control circuit 11. Here the 
dc and low frequency components of the sectored servo information from the 
data head is combined in the servo control circuits 11 with high frequency 
information derived from the dedicated servo surface. The resulting hybrid 
signal of wide bandwidth is then used to produce suitable drive currents 
to control the actuator 6 in closed loop mode to hold the data head track 
following the selected data track. Details of construction and operation 
of the circuits shown in FIG. 1 are fully described in U.S. Pat. No. 
4,072,990. 
The position error signals derived by the servo head 4 reading the 
prerecorded servo information 3 on the dedicated servo surface are 
illustrated in FIG. 2. Two types of servo tracks recorded on the disk give 
rise to these two position information signals which are called normal 
waveform N (FIG. 2b) and quadrature or displaced waveform Q (FIG. 2a). The 
production of these signals is described in copending application Ser. No. 
681,656, entitled "Positioning System for Data Storage Apparatus and 
Record Medium for Use Therewith,"filed Apr. 29, 1976, which is referred to 
in U.S. Pat. No. 4,072,990. As the servo head accesses the disk, the 
normal quadrature signals change in a cyclic manner passing through the 
waveform centerline, which may be zero volts, for example, as the servo 
head crosses the associated normal or quadrature servo tracks on the disk 
and reaches a maximum voltage midway between tracks. The zero crossings, 
or on-track positions 0, 1, 2, . . . , of the normal signal (waveform N) 
are prerecorded to coincide with corresponding data head on-track 
positions. However, disturbances to the apparatus often upset the 
mechanical stability and prevent the servo head-on track position from 
coinciding with the data on-track position. 
The nature of the particular servo patterns recorded on the dedicated disk 
surface is such that the resulting position information signals derived 
from a single track are substantially linear over a range of .+-.1/4 of a 
track width about the on-track position. Such a position is shown, for 
example, as P1 in FIG. 2b. Thus, provided the displacement of the servo 
head during data track following operations does not exceed .+-.1/4 of a 
track width the voltage generated per unit displacement is constant and 
the gain of the servo control circuit is also constant at the desired 
value. If, however, the displacement is such that the servo head becomes 
offset from its on-track position P1 by more than .+-.1/4 of a track 
width, so that during track following operations it lies at position P2, 
for example, then changes in position error voltage will no longer be 
linear with respect to changes of position about P2 and the servo position 
loop performance will be degraded. 
This problem is solved in a manner now to be described and involves using 
the quadrature position error signal not normally used during track 
following operation to effectively extend the linear region of the normal 
error signal beyond the .+-.1/4 track range thereby maintaining the ac 
gain of the servo circuits constant over the entire displacement range 
which may occur during track following operations. 
The particular servo pattern employed has the advantage that the linear 
portion of the quadrature waveform Q commences when the linear portion of 
the normal waveform N ends and vice versa. Thus, by applying a dc offset 
to the quadrature position error signal with or without inversion of the 
signal, the linear region of the normal signal can be extended, as shown 
for example, by the dotted waveform Q' in FIG. 2. Now, with the head 
displaced to position P2 its position error voltage P2' is still linear 
with respect to head displacement and the ac gain of the circuit is 
constant. 
The end of the linear region of the normal error signal is readily 
determined by detecting when the magnitude of the normal error signal N 
exceeds that of the quadrature error signal Q. When this is the case, the 
quadrature error signal plus a dc offset is used to effectively extend the 
linear region of the normal error signal N. The polarity of the required 
quadrature and offset voltage is determined by the polarity of the normal 
error signal to be extended and the direction of displacement either left 
or right from the on-track position. Thus, referring to FIG. 2 which shows 
position P2 as the on-track position for an even-numbered track 2, the 
following truth table gives the four possible conditions required to 
extend the linear region of the normal signal. The required polarity of 
quadrature and dc offset is given in each case together with the logic 
levels needed to control switching from normal linear region to extended 
linear region. 
______________________________________ 
Logic Levels 
Track Direction Of 
Signal N&gt;O Q-N Q+N 
Polarity 
Displacement 
Required N&gt;O Q-N Q+N 
______________________________________ 
Odd Left Q + Offset 
1 0 1 
Odd Right -Q - Offset 
0 1 0 
Even Left Q - Offset 
0 1 0 
Even Right -Q + Offset 
1 0 1 
______________________________________ 
Thus, in the event of a left hand displacement of a head in excess of a 
quarter of a track from track following an odd-numbered track the signal 
required to extend the normal linear region is the quadrature signal Q 
plus a dc offset of appropriate magnitude. Inspection of FIG. 2 shows that 
the magnitude of the dc offset required is twice the voltage V (FIG. 2) of 
the quadrature and normal signals when they are of equal magnitude. For a 
right hand displacement in excess of a quarter of a track from an odd 
polarity track, the signal required is the inverted quadrature signal, 
quadrature signal -Q, plus a negative dc offset. Similarly, a left hand 
displacement from an even-numbered track requires the quadrature signal Q 
plus a negative dc offset and a right hand displacement requires the 
inverted quadrature signal -Q plus a positive dc offset. 
The logic levels used to indicate head displacements in excess of one 
quarter of a track in either direction from the on-track positions of odd 
or even-numbered tracks are identified in the truth table and the 
waveforms showing the logic levels derived from the position error signals 
Q and N are shown and identified in FIG. 2c-f. The generation of these 
logic levels from the position error signals is achieved by conventional 
comparison circuits. 
By exclusively OR-ing the (Q-N) signal with (Q+N) a signal A having up 
levels corresponding to the nonlinear portions of the normal error signal 
is produced. Also, the required polarity of the quadrature signal is given 
by exclusively OR-ing the (N&gt;O) signal with the polarity signal from the 
control system which is at its up level for track accesses to 
even-numbered tracks and at its down-level for accesses to odd-numbered 
tracks. The EVEN signal as it is called is quite conventionally supplied 
from the control system 12 at the start of an access operation, as 
previously mentioned. 
A circuit for switching from the normal linear region to extended linear 
region when displacement in excess of the normal linear region occurs is 
shown in FIG. 3. The circuit consists of a standard three-channel 
operational amplifier 13, the appropriate channels being selected by a 
positive level on one of three corresponding channel select lines 14, 15 
and 16. The normal position error signal N is applied to input terminal 
17, and thence to the positive input of channel 1 of the amplifier 13. The 
quadrature position error signal Q is applied at input 18, and then to the 
positive input of channel 2 through resistor R1 and to the negative input 
of channel 3 through an identical resistor R2. The offset voltage V, equal 
to twice the crossover voltage of the quadrature and normal signals, is 
applied as a positive voltage to input 19 if the track being followed is 
an odd-numbered track or as a negative voltage of the same magnitude if an 
even-numbered track is being followed. The polarity of the offset voltage 
is switched under control of the EVEN signal from the control circuit 12. 
The offset voltage is added to the signals applied to the positive input 
of channel 2 and the negative input of channel 3. 
During operation, the normal error signal N, supplied to input 17, is 
transmitted to the output 20 by logic selection of channel 1 during the 
linear region of the normal signal, that is, during the down-level of 
signal A (FIG. 2f). At all other times, the quadrature signal supplied to 
input 18 is transmitted to output 20 with positive or negative dc offset 
voltage V by logic selection of channel 2, or inverted and transmitted to 
output 20 again with positive or negative offset voltage V added by logic 
selection of channel 3. 
It will be observed that the polarity of the slope of the signal at 20 is 
different for ODD and EVEN tracks for motion in a given direction. 
Accordingly, the signal must be inverted before it can be used to control 
the driver to track follow over alternate tracks. This is achieved by 
switching the signal appropriately under control of the EVEN signal. 
Logic circuits consisting of exclusive OR gates 21 and 22, AND gates 23 and 
24, and inverters 25 and 26 supplied with the appropriate logic signal 
levels perform the channel selection. 
Accordingly, the nonlinear region of the normal signal is provided by 
applying the (Q-N)&gt;0 signal to input 27 and the (Q+N)&gt;0 to input 28 of 
exclusive OR 21, the output of which is at an up-level at all times except 
during the linear region of the normal signal N. This signal, after 
inversion by inverter 25, is used therefore to select channel 1 during the 
linear region of the normal signal and to enable AND gates 23 and 24 
during the nonlinear regions. Further, the track polarity signal (EVEN) 
from the control system is applied to input 29 and the signal (N&gt;O) to 
input 30 of exclusive OR 22 to select the appropriate channel 2 or 3 
through AND gates 23 and 24. Thus, for an even-track with a left-hand 
displacement, (N&gt;0) will be negative and the output from exclusive OR 22 
will be at an up-level selecting channel 2 via AND gate 23. The output on 
line 20 will therefore be the linear region of the quadrature signal with 
a negative dc offset as shown in the truth table, supra. The other 
possible conditions can be easily verified with reference to the truth 
table, and the waveforms shown in FIG. 2. 
In order to avoid discontinuities in the extended linear region signal at 
the point where the quadrature puls offset is switched in, it is necessary 
that the offset voltage is accurately equal to twice the voltage of the 
normal and quadrature signals crossover voltage. The offset voltage can of 
course be produced to any desired accuracy but the magnitude of the 
crossover voltage is not constant, being subject to change in response to 
indeterminate circuit tolerances. Thus, the circuit shown in FIG. 3 
suffers from the disadvantage that discontinuities can occur during 
crossover from linear normal to extended linear quadrature. It is 
therefore necessary to incorporate a feedback loop in the circuit which 
controls the servo signal gain control so as to normalize the crossover 
voltage to half the dc offset. 
The next embodiment to be described, takes advantage of the ac coupling of 
the dedicated error signal inherent in the hybrid circuit described in 
copending application Ser. No. 706,313 to provide a solution to this 
problem. 
FIG. 4 shows a circuit for generating the hybrid servo signal used for 
track following with the extended linear region feature. The selection of 
the polarity of the quadrature signal Q applied to terminal 31 is made in 
this case by a two channel operational amplifier 32 which acts as a 
switchable inverter. The quadrature signal is passed unchanged through 
channel 1 selected by the up-level of the (N&gt;0) signal applied to terminal 
33 after inversion by inverter 34 and inverted via channel 2 selected 
directly by the up-level of the (N&gt;0) signal. The normal position error 
signal N is supplied to input terminal 35 unchanged when the seek is to an 
EVEN track, but inverted when it is to an ODD track. Inversion may be 
achieved, for example, by an inverting amplifier switched under control of 
the EVEN signal. The data head position error signals from the sectored 
servo information associated with the track being followed are supplied to 
input terminal 36. Capacitors C1 and resistors R1 connected as shown serve 
as high pass filters for the quadrature and normal signals from the 
dedicated surface and as a low pass filter for the sectored servo 
information from the data surface. A hybrid quadrature position error 
signal is thereby produced at node Q and a hybrid normal position error 
signal is produced at node N. To incorporate the extended linear region 
function, it is necessary to switch between these two signals without 
introducing a discontinuity. This is achieved by means of the switches S1 
and S2. As shown in the Figure, the circuit is shown in the normal linear 
region mode with S1 closed and S2 open. During this time the hybrid normal 
signal is supplied directly to the positive input of the amplifier 39 and 
appears as a version N' on the output line 40. By virtue of the feedback 
path around amplifier 39, this buffered hybrid position error signal N' 
appearing during the normal linear region forces the voltage of the Q node 
which is connected to the negative input of operational amplifier 39 via 
closed switch S1 to be equal to the voltage of the N node. Now, when the 
normal position error signal reaches the limit of its linear regions, S1 
opens and S2 closes under control of logic signal applied to input 42. 
Under these circumstances, the hybrid quadrature signal which is applied 
to the positive input of amplifier 41 appears as a buffered version Q' 
from its output. By virtue of the feedback path around amplifier 41, the 
buffered signal Q' now drives the N node which is connected to the 
negative input of operational amplifier 41 via closed switch S2 to acquire 
the voltage of the Q node. Since the voltage at Q was previously forced to 
be equal to the voltage at N, the dc offset of required magnitude is 
automatically provided with no discontinuity at the changeover point. The 
extended linear region hybrid error signal now appears at both the N and Q 
nodes. The logic signal for closing S1 and for opening S2 is the linear 
region signal (-A) appearing from inverter 25 in FIG. 3 which is applied 
to input 43. The connection is shown in dotted outline in FIG. 3. The 
output at 40 has the same polarity of slope at both ODD and EVEN tracks 
for motion in the same direction and additional switching is not required. 
Finally, it should be noted that the operation of the circuit shown in 
FIG. 4 using the two channel amplifier 32 together with the two additional 
amplifiers 39 and 41 fulfill the conditions set out in the truth table 
above as well as providing the required frequency mix of the hybrid 
position error signal. 
A problem well known in multiple switching circuits such as that shown in 
FIG. 4 is an unwanted buildup of the charge on the capacitors as a result 
of multiple switching and accumulation of small offset error voltages 
existing in the amplifiers and switches. As will be apparent to those 
familiar with such circuits, this problem can readily be resolved in a 
conventional manner by applying hysteresis to the logic comparators which 
produce the switching signal on line 42 for switches S1 and S2. 
In the embodiments described to illustrate the present invention, the 
continuous position information is derived from servo tracks prerecorded 
on a dedicated servo surface of the stack of disks. It should be 
understood that the invention is equally applicable where the data head 
position information is derived from other means, for example, an optical 
or inductive transducer cooperating with independent devices containing 
data track position information.