Magnetic head positioning servo arrangement for a system for recording/reproducing information on a magnetic recording medium, particularly on a flexible magnetic recording medium

A magnetic head positioning servo arrangement for a system for recording or reproducing information on a flexible magnetic recording medium, the position of the head being adjustable, relative to the magnetic tracks of the recording medium, by means of an actuator, and each magnetic track being provided with at least one reference signal which, when sensed, provides information about the momentary position of the head and, in the case of mistracking, generates in a comparator circuit a control signal for the actuator which moves the magnetic head back over the center of the track, the reference signal consisting of at least one pair of pulse sequences which are recorded asymmetrically with respect to the center of the track and which, when sensed, produce increasing or decreasing signal pulses.

The present invention relates to a magnetic head positioning servo 
arrangement for a system for recording/reproducing information on a 
magnetic recording medium, particularly on a flexible magnetic recording 
medium, the position of the head being adjusted, relative to the magnetic 
tracks of the recording medium, by means of an actuator, and at least one 
reference signal being recorded within or beneath each magnetic track, 
which signal, when sensed, provides information about the momentary 
position of the head, this information being compared, in a comparator 
circuit, with information about the correct position of the head above the 
track center, and if there is a difference, a control signal is generated 
for the actuator which moves the magnetic head back to its correct 
position, and a magnetic recording medium for effecting head position 
control. 
German Laid-Open Application DOS 2,439,546 discloses a servo arrangement 
for positioning the magnetic head in a data storage system in which servo 
information is recorded adjacent to or in the data track and is read by 
means of part of the data head or of a separate servo head and used for 
controlling the head position during read/write operations. 
It is also known that in a magnetic disk storage system, in which the 
recording surface of the magnetic disk is divided into sectors, separate 
servo signals for each track may be stored in each sector and used for 
adjusting the radial position of the magnetic head. This servo information 
is recorded after the magnetic disk has been manufactured and normally 
remains stored unchanged for the life of the disk. In particular, the 
known servo signals (German Laid-Open Application DOS 2,619,601) in the 
sectors provide information about the excentricity and changes in shape of 
the magnetic disk. The servo recordings known hitherto are either too 
complicated or technically unsatisfactory. 
German Published Application DAS 1,424,516 generally describes transducer 
positioning control signals which, like the data signals, are recorded 
diagonally to the track and are sensed with the same magnetic head. The 
signals are recorded diagonally for the purpose of eliminating guard bands 
between adjacent tracks, i.e. the tracks lie immediately next to one 
another, alternate tracks being recorded diagonally in opposite 
directions. However, it is disadvantageous that, in practice, the servo 
signals do not differ substantially from the data signals. 
It is an object of the present invention to shape the servo signals in such 
a manner that simple detection of a mistracking condition and immediate 
and accurate head readjustment is achieved. 
According to the invention, this object is achieved by a magnetic head 
positioning servo arrangement for a system for recording/reproducing 
information on a magnetic recording medium, particularly a flexible 
magnetic recording medium, the position of the head being adjustable, 
relative to the magnetic tracks of the recording medium, by means of an 
actuator, and at least one reference signal being recorded within or 
beneath each magnetic track, which signal, when sensed, provides 
information about the momentary position of the head, this information 
being compared, in a comparator circuit, with information about the 
correct position of the head above the track center, and, if there is a 
difference, a control signal is generated for the actuator which moves the 
magnetic head back to its correct position, wherein the reference signal 
consists of at least one pair of pulse sequences which are recorded 
asymmetrically with respect to the center of the track and which, when 
sensed, produce increasing or decreasing signal pulses, the individual 
pulses of the sensed signal being detected and compared by means of a 
comparator circuit, and a control signal for the actuator being generated 
in the event of a difference in signals. 
Such a reference signal, the shape of which is allocated to the sensing 
location, can be detected reliably and without difficulty and can be 
converted in a simple manner into the control signal and thus makes 
possible immediate head readjustment. 
In an advantageous embodiment, the waveform of each pulse sequence of the 
reference signal is triangular, this being a signal shape which can be 
readily detected. 
In a further embodiment of the invention, the pair of pulse sequences 
consists of a pulse pattern on the track, this pattern having a marker 
zone, at least part of which extends diagonally across the track. As a 
result, the reference signals can be produced in a very simple manner. 
On the one hand, such a marker zone, which may for example be triangular or 
strip-shaped, can be produced by partially erasing the pulse pattern. On 
the other hand, the marker zone can also be formed by a pulse pattern 
between neighboring areas which are erased diagonally to the track. Such 
marker zones can also be produced by overwriting a pulse pattern or by 
removing correspondingly shaped areas of the magnetic layer. 
In a further advantageous embodiment of the invention, the first pulse 
sequence is recorded on one half of the track and the second pulse 
sequence on the other half, the second pulse sequence being arranged 
behind the first pulse sequence in the direction of recording and both 
pulse sequences having the same waveform. 
Advantageously, the comparator circuit is a summing circuit in which the 
pulses of the first sequence are stored and the pulses of the second 
sequence are removed. In practice, the summing circuit can be in the form 
of a counter which is incremented by the pulses of the first sequence and 
decremented step by step by the pulses of the second sequence. 
It is advantageous, with a view to achieving as faultless a signal reading 
operation as possible, to provide several pairs of pulse sequences on each 
magnetic track. If an optimum number of reference signals is used, for 
example on each track of a flexible recording disk, it is possible to 
effect continuous automatic control of the material-dependent changes in 
the shape of the disk by using an automatic head positioning device so 
that both write and read operations can be carried out without being 
affected by these changes. 
The invention also relates to a magnetic recording medium, particularly a 
flexible magnetic recording medium, for effecting head position control, 
which consists of a base provided with a magnetic layer and possessing, in 
this layer, signals which are to be or are recorded on a plurality of 
tracks, wherein said magnetic track comprises at least one reference 
signal having at least one area, at least part of which extends diagonally 
across the track, and which is magnetically different from the areas 
adjoining it. 
As explained above, the areas of the reference signal can be of any 
suitable shape; for example, magnetic erasure of a strip-shaped or 
triangular region is considered to be a very favorable method of producing 
these areas. 
In order to cover, for the purpose of head position control, as much of the 
total area of the recording medium as possible, it is advantageous if a 
plurality of reference signals are provided on each track. 
With the abovementioned use of a plurality of such reference signals on one 
track--whether it be a longitudinal track on a magnetic tape or a circular 
track on a rigid or flexible magnetic disk--a considerable increase in the 
storage capacity of the recording media can be achieved since the 
available recording surfaces can be utilized almost completely and the 
error rate due to poor head positioning can be kept very low.

A data recording/reproducing system, in which the head positioning servo 
arrangement according to the invention and the magnetic recording medium 
according to the invention can be used, is described below, by way of 
example, with reference to a floppy disk drive. 
According to FIG. 1, the read channel of such a disk drive consists of the 
following stages: 
Read head 21, read amplifier 22, noise suppression filter 23, 
differentiator 24, zero detector 25, pulse shaper 26 and output terminal 
26. The read signal induced in the read head winding is thus amplified, 
low-frequency and high-frequency noise signals are removed by limiting the 
frequency range, and the user information signal, the information being 
contained in the position of the pulse spikes, is then differentiated so 
that the position of the pulse spikes is converted to zero crossovers. In 
the zero detector the differentiated signal is converted into a 
rectangular signal which is then converted into a sequence of narrow read 
pulses which are available at the output 20. During the generation of the 
rectangular signal, a minimum signal amplitude is set in order to obtain a 
read signal which is free of noise components. Thereafter, the signal 
amplitude is limited at approximately 1/10-1/3 of the maximum read 
amplitude in such a manner that only read pulses of higher amplitude are 
detected. In order to obtain such a minimum read pulse amplitude, a 
minimum read track width b (see FIG. 3a) is established. If this track 
width b is not achieved, the read pulse sequence at the output 20 no 
longer agrees with the recorded signal sequence. 
A flexible magnetic disk (such as a Flexy Disk.RTM. (trademark of BASF 
Aktiengesellschaft, 6700 Ludwigshafen, Germany) has data tracks with 
standardized formatting. A data track 27 has 26 sectors, each with a 
sector identification signal (ID) consisting of 13 bytes, an 
identification gap (IDG) consisting of 11 bytes and the data block (DB) 
consisting of 137 bytes and the data gap (DG) of 27 bytes. These format 
blocks are as shown in FIG. 2. 
According to the present invention, in the data gap DG a reference signal 
for head position control is provided which does not in any way interfere 
with the data format. Initialization of a data track 27 can for example be 
carried out as follows. Triggered by a track start signal ST (FIG. 4a) 
which can be generated by an index mark or a fault on the magnetic disk, 
80.times.A bytes, 30.times.S bytes and 78.times.B bytes are recorded 
continuously as sector identification signal with the read/write gap of, 
for example, a recording/playback head in FIG. 5. A, B and S bytes are 
here easily distinguishable signals, each of which has a predetermined 
pattern. After the last B bytes, A bytes are recorded until the next track 
start signal is detected. In the read operation, during the next 
revolution the recorded data patterns are detected and checked by means of 
the read channel of FIG. 1. As each signal transition from B to A bytes is 
detected, a delay generator is started for a period T1. After the time T1 
has elapsed, the erase head as shown in FIG. 5 is supported with an erase 
current for the erasing period T3. Due to the distance l 5 between the 
read gap 28 and the inclined erase gap 29 (FIG. 5), a time delay T2 arises 
which, in conjunction with T1, controls the timing of the erase current in 
such a manner that it becomes effective only in the region of the S bytes. 
The S byte areas are partially erased by the inclined erase gap, as shown 
in FIG. 2a. With the period T2 predetermined by the distance between the 
head gaps 28 and 29, the period T1 is so adjusted that the length l 1 
(FIG. 2a) has the optimum length for the particular format used (in the 
present embodiment, 1 1=from 0 to 2 bytes). l 1 designates the distance 
between the sector reference line (SR), running approximately through the 
center of the data gap DG, and the beginning of the erased diagonal strip 
SS which represents an important characteristic of the servo signal 
according to the invention. The period T1 may also be easily varied from 
track to track so that the length l 1 can be maintained for all 
tracks--even with circular tracks of different lengths. 
The erasing time T3 is advantageously such that an erased length l 2 (FIG. 
2a) of approximately from 2 to 16 bytes is obtained. As determined by the 
sensitivity of the read circuit (FIG. 1), after this erasure the following 
information is still readable from the 30.times.S byte fields: 
a number of Z1.times.S bytes from length l 1 and the still readable part of 
l 2 
a number of Z2.times.S bytes from length l 4 and the still readable part of 
l 3. 
As already noted above, the limit of readability can be adjusted to 
approximately from 1/10 to 1/3 of the normal amplitude of the data 
signals. 
In FIG. 4e the information contents of a track obtained after diagonal 
erasure are represented. 
During the next revolution of the magnetic disk the number (Z1) of the 
readable S bytes up to the diagonal strip SS is determined and then 
compared with the number of S bytes (Z2). If the head is in its nominal 
position, i.e. over the center of the track, agreement of the numbers 
Z1=Z2 is established so that the write operation can be started and the 
sector, consisting of sector identification signal ID, identification gap 
IDG and data block DB, is continuously initialized. After initialization 
of the sector, the write operation is stopped so that a few A bytes 
(approximately 9 bytes) are left until the subsequent S byte field is 
recognized. This procedure is repeated for every sector until all sectors 
are initialized (see FIGS. 4f and 3). The bytes written are selected under 
the aspect of distinguishability so that flux changes in the magnetic 
coating, which differ in space and time, are allocated to the A, B and S 
bytes. 
The procedure for using a recording medium containing servo information as 
described above is as follows: 
In FIG. 3a, a track 27 containing servo information as shown in FIG. 2a is 
represented. To the right of this FIG. 3a, FIG. 3b shows ten different 
positions of a read head cap 28. The minimum read track width of a signal, 
which can still be recognized as a decodable data signal, is b. In the 
embodiment described here, the total track width is 5b. If the track 27 is 
read with a read circuit according to FIG. 1 in an arbitrary head position 
(from 1 to 10 in FIG. 3b), a number of M1 bytes must be identified from 
the group of S1 bytes and a number of N1 bytes must be identified from the 
group of S2 bytes. If the read track is identical to the written track 
(head gap position 6) in FIG. 3b, M1=M=N1=N. M and N are not necessarily 
identical to Z1 and Z2 as shown in FIG. 4 if different drives are used, 
since normally each amplifier chain has a different sensitivity, so that 
it must be considered to be an advantage that the sensitivity of the 
amplifier does not affect the head position control signal. 
If the S1, S2 signal patterns in FIG. 3 are read with a read head position 
corresponding to head position 4 in FIG. 3b, only a number of M1 bytes of 
the S1 signal pattern are recognized, and from the S2 signal pattern only 
a number of N1=N bytes are recognized. From the difference F=M1-N1 of the 
bytes read, a line K according to FIG. 3c can be determined, each of the 
possible read head positions (KP) (from 1 to 10) in FIG. 3b being 
associated with S2 bytes. Head deviation from the center line of the 
written track, the position of the latter being fixed by the 
initialization, is proportional to the difference F=M1-N1 (in this 
embodiment, head position 4, FIG. 3c), and the direction of the deviation 
is defined by the sign of the difference. If the M1 and N1 byte numbers 
are fed to an electronic counter circuit, so that the M1 bytes increment 
the counter and the N1 bytes decrement the counter step by step, the 
result is a number of F=M1-N1 bytes which represents the head deviation 
and which can now be utilized by means of known circuits for correcting 
the position of the read head. 
Such a circuit is shown diagrammatically in FIG. 5. In this circuit, the 
read head 21 is followed by the pulseshaping stage 26, a decoder (DEC) 29 
for the S bytes and a counter circuit, symbolized by a flip-flop 30, which 
should have a negative and positive output. In a conventional manner, a 
direct-current servo motor (MOT) 33 is controlled via D/A convertors, 
which are not shown, and subsequent separate amplifiers 31 and 32, this 
servo motor, in turn, controlling a positioning device (POS) 34 for the 
read head 1, for example within 3 msec. The position of the read head 21 
is adjusted by means of suitable devices, for example directly by means of 
the head positioner of the drive used. 
The decoder DEC 29 recognizes the A, B or S byte types of information and 
detects the data transitions. There is no response if a first S signal 
pattern is followed by an A or B signal pattern since, for example, faulty 
information may be present. If a first S signal pattern is followed by a 
second S pattern, the S signals are added, for example by means of the 
counter C, up to a predetermined pause or until the signal readability 
limit is reached again, and after the pause, or when the signal 
readability limit has been reached again, the counter is decremented step 
by step by the subsequent S signals, i.e. the subsequent S signals (Z2) 
are substracted. 
The steps for effecting servo initialization of a magnetic recording medium 
and the control of the position of the read head, including the 
compensation procedure for a possible head alignment error, have been 
described above. The essential component of the servo information is an 
area SS or SD, at least part of which extends diagonally across the track 
and the magnetization of which differs from that of the neighboring areas, 
different electric pulse patterns thus being produced when these areas are 
sensed. 
The strip SS or the triangle SD can be a magnetically erased area within a 
predetermined fixed pulse pattern or a written area within an erased area. 
The strip SS can also be an area which has been overwritten with a signal, 
or area SS or SD can be an area on the magnetic recording medium, a 
correspondingly shaped portion of whose magnetic layer has been removed. 
If the servo information is to be initialized by the user, a magnetically 
erased strip or, for example, a triangular area which can differ from 
device to device is to be preferred to a permanent marking; otherwise, it 
is preferable for the servo information to be initialized by the 
manufacturer. 
The strip SS or the inclined line of triangular area SD can run, for 
example, at an angle of approximately 20.degree. to 70.degree. to the 
transverse axis of the track. 
The edges of the strip SS may be straight, as shown in FIGS. 2a and 3a, or 
step-shaped (FIG. 2b), the latter shape being produced by step-by-step 
erasure with the read/write gap at head gap positions which differ from 
track section to track section (as shown in FIG. 3b). For example, this 
read/write head displacement relative to the track can be achieved by 
means of a head stepping device, the individual steps of which are 
fractions of the track width. 
A further type of servo information according to the invention is shown in 
FIG. 7a. Here, the magnetized areas R and P are arranged in staggered 
relationship on either side of the center of the track, being merely 
separated by a narrow erased guard band n1. Such a magnetic signal 
produces a read signal as shown in FIG. 7b, the amplitudes of each signal 
area R and P decreasing continuously from their maxima to zero. Such a 
decrease in amplitude can be produced during initialization of a recording 
medium by appropriately changing the amplitude of the write current, by 
effecting erasure at an angle as described above, or by switching the 
write current on and off at irregular intervals to give different 
distances between flux changes. The areas Q and U are advantageously 
erased magnetically. 
The servo signals shown in FIG. 7 can be produced in the following manner, 
explained with reference to a magnetic track on a flexible magnetic disk. 
1. Erase disk. Set head to nominal track position. 
2. Use write head 2 to write A bytes in nominal track position, then write 
5 B bytes at a distance equal to r+p+n1+n2. 
3. Set position of upper track half (approximately 180 .mu.m displacement 
from the center line in the case of flexible magnetic disks). After 
reading 1 A byte, write the servo signal pattern in area R with decreasing 
amplitudes, using one of the processes specified; do not write in zone Q. 
4. Set position of lower track half (approximately 180 .mu.m displacement 
from the center line in the case of flexible magnetic disks). After 
reading 1 A byte, write the servo signal pattern after a delay 
time--corresponding to the distance r--in the area P with decreasing 
amplitudes, using one of the processes specified. 
5. Set write head and tunnel erase head to nominal track position. Tunnel 
erase the entire track, thus producing the track width according to FIG. 
7a. 
6. With the head in its nominal track position, read servo signal patterns 
R and P, counting detectable numbers of pulses from R and P; if a 
predetermined difference between the numbers in exceeded, correct by new 
initialization. 
The further measures, i.e. recording of data signals, reading and 
evaluation of servo signals and readjustment of the head position, can be 
carried out as described above. 
With the present head positioning servo arrangement it is possible to 
detect head deviation from the ideal position any number of times within a 
magnetic track, and to correct the head position. In the case of flexible 
magnetic disks, i.e. floppy disks or FlexyDisks.RTM., it is possible to 
achieve accurate head tracking up to an excentricity of 50 .mu.m during 
one revolution of the disk, the head position being readjusted 26 times 
per track. The necessary initialization of disks or magnetic tape can be 
carried out either by the manufacturer or later by the user. Apart from 
reducing the error rate in data storage using flexible recording media, it 
is possible to at least triple the track density of, for example, 
FlexyDisks.RTM..