Magnetic disc, especially a flexible magnetic disc, for track adjustment and amplitude control

The invention relates to a magnetic disc for track adjustment and amplitude control having at least one control track with magnetic control signals, this track being arranged in the central region of the recording area, and the control signals consisting of two different signals. The invention also relates to the geometric arrangement of the control signals in the track, their number, their amplitude, their frequency and their duration.

This invention relates to a magnetic disc, especially a flexible magnetic 
disc, for track adjustment and amplitude control (hereinafter referred to 
as "adjustment and control disc") having at least one control track with 
magnetic control signals. 
With a magnetic disc, the writing or reading of data is effected through 
the movement, by means of a drive unit, of the disc relative to a magnetic 
head. 
The quality of the writing or reading operation is determined by the exact 
positioning of the magnetic head above the tracks of the recording medium 
as they move relative to one another. Especially in the case of floppy 
discs and their associated write and read apparatus it is important that 
the magnetic head and/or the head adjustment unit in particular should be 
able to be displaced relative to the recording medium. This operation is 
referred to as "track adjustment". So-called CE discs bearing control 
signals are used for this purpose in the case of rigid magnetic discs. 
Such control signals consist of magnetic flux changes numbering 
approximately 26,000 which extend alternately on either side of the center 
line of the track. With this digital information track adjustment can be 
effected, but the physical properties of the disc and/or the functioning 
of the drive unit cannot be checked. 
A further CE disc is known which has two signals displaced relative to the 
center line of the track, which signals when read sequentially, show that 
the head is adjusted correctly relative to the track when the said two 
signals cannot be distinguished on an oscillograph. A control method of 
this kind requires special read and decoding means, as well as a special 
video terminal. 
The object of the present invention is to provide a magnetic adjustment and 
control disc which is more versatile than prior art CE discs and can be 
used without any additional equipment. 
This object is achieved with a magnetic adjustment and control disc, 
especially a flexible magnetic adjustment and control disc, having at 
least one track with magnetic control signals, wherein the said track is 
arranged in the central region of the recording area and the control 
signals consist of orientation signals and adjustment signals, the 
adjustment signals extending alternately on either side of the centerline 
of the track, and the orientation signals having a different amplitude 
than the adjustment signals and being arranged so as to split up the 
adjustment signals into a plurality of groups. 
According to the invention, therefore, two signals which can be clearly 
distinguished from each other on a measuring instrument are used as 
control signals. Although it is very advantageous to employ only two 
control signals, i.e. orientation and adjustment signals, more than two 
control signals may be used. 
With the adjustment and control disc according to the invention, the whole 
length of the control track can be displayed on an oscillograph, as a 
result of which any eccentricity of the disc on the drive can be 
ascertained, and the disc can be optimally read despite this eccentricity. 
In an advantageous embodiment of the invention, the orientation and 
adjustment signals are arranged symmetrically with respect to the central 
hole in the disc. In this way, a reference centricity is obtained. 
In a further advantageous embodiment of the invention, the number of signal 
groups on the disc may vary between 4 and 36; preferably 6 or 12 groups 
are chosen, as a result of which the individual signals can be displayed 
on the screen of any commercially available oscilloscope. 
In yet another advantageous embodiment according to the invention, the 
amplitudes of the orientation signals are at least 10% larger than the 
amplitudes of the adjustment signals. As a result, the oscilloscope can be 
reliably triggered by the orientation signals. 
The orientation signals may conveniently have frequencies different to the 
frequency of the adjustment signals, which makes it easier to distinguish 
the signals from one another. 
Advantageously, the adjustment signals have the lower frequency of the two 
recording frequencies of the data system, so that the maximum signal 
amplitude can be measured by the read head. 
In a further embodiment of the invention, the orientation signals consist 
of three signals, namely a central signal having the lower frequency of 
the two recording frequencies, and two adjacent signals having the higher 
frequency of the two recording frequencies. 
As a result, the orientation signals are even easier to distinguish from 
the adjustment signals, and the frequency of the adjacent signals, 
equaling the higher of the two recording frequencies, can be used for 
adjusting the azimuth angle of the read head and for checking the 
amplifier. 
To sum up, then, the advantages of the adjustment and control disc of the 
invention are as follows: 
Track adjustment and a check on the track adjustment are possible 
therewith. It can be used to ascertain any disc eccentricity which occurs 
during loading, any disc deformation produced, for example, by climatic 
conditions, and any out-of-true running of the disc-bearing spindle. 
Moreover, it can be employed to check the functioning of the magnetic head 
and to adjust the azimuth angle of the head without there being any need 
for more than an oscilloscope such as in usually employed for servicing. 
Finally, control signals of approximately the same length may be used, the 
length being preferably 1 = arc 6.6.degree. .

The adjustment and control system 19 consists essentialy of a drive shaft 
16, a magnetic adjustment and control disc 15, a head positioning device 
11 with spindle 12 and magnetic head 13, and an oscilloscope 10 which is 
connected up to the magnetic head 13 via the device 11. The magnetic head 
13 can be moved on the spindle 12 in the directions indicated by the 
double arrow. The numeral 14 denotes a circular track in the central 
region of the recording area of the disc 15. In the case of a floppy disc 
this may for example be track no. 36 if the total number of tracks is 76. 
Arrow a indicates the direction of rotation of the drive shaft 16. The 
upper end 17 of the drive shaft, which protrudes through the disc 15, is 
slightly conical in order to facilitate mounting of the disc 15 thereon. 
FIG. 2 shows one possible arrangement of the control signals which are 
symmetrical with respect to the center line 18 of the abovementioned track 
no. 36. The reference symbol b indicates the width of the track. The 
control signals are composed of a multitude of magnetic flux changes. A 
total of six groups of signals -- I to VI -- can be seen in FIG. 2, each 
of these groups having a length c. Each group consists of three 
orientation signals 1 to 3 which are arranged symmetrically on the center 
line 18 of the track, and of six adjustment signals 4 to 9 which extend 
alternately on either side of the center line 18. The amplitudes of the 
orientation and adjustment signals should be different, so that the 
signals can be readily distinguished from one another. This can be 
achieved, for example, if orientation signals 1 to 3 consist of 
alternating signals of different frequency, e.g. of 2f, 1f, 2f. Here, the 
frequencies 2f and 1f are the recording frequencies of the data system. In 
the case of the floppy disc system, these frequencies are 500 kHz and 250 
kHz at 360 rpm. FIG. 3 shows an oscillogram of the signals between the 
dashed lines 20 and 21 (cf. FIG. 2) which include orientation signals 1 to 
3 and adjustment signals 4 to 9 of group II, as well as orientation 
signals 1 to 3 of group IV. As indicated in the drawing, the read voltage 
U, scanned by the magnetic head 13, is plotted on the vertical axis as a 
function of time t (horizontal axis). 
The orientation signals 1 to 3 are distinguishable as read voltages U2fc, 
U1fc and U2fc, the voltage U1fc being displayed with a larger amplitude 
than U2fc. In the illustrated embodiment, the amplitudes of read voltages 
U1Fi and U1fo are the same. Since the amplitudes of the read voltages 
produced by the magnetic head 13 depend upon the position of the head 
relative to the control track 14, the fact that the read voltages U1fi and 
U1fo have the same amplitude shows that 
a. the position of the head relative to the tracks is correct, 
b. the adjustment and control disc is situated absolutely centrically on 
the drive shaft, 
c. the hole in the disc (floppy disc) is circular and the disc itself is 
not deformed, 
d. the drive shaft is running true, and 
e. the magnetic head is functioning properly. 
FIGS. 4 and 5 show the read voltages U2fc, U1fc, U1fi and U1fo which 
correspond to the voltages shown in FIG. 3 only as far as their 
designation and their geometric position are concerned. Viewed from the 
left, burst 4 of FIG. 4, i.e. the read voltage U1fi, has an amplitude 
larger than the following burst 5, i.e. the read voltage U1fo. This 
amplitude difference (U1fi-U1fo) (x), which is positive, shows that a 
sector of the disc 15 is displayed towards the axis of the drive shaft. If 
the amplitude difference (U1fi-U1fo) is negative (y), as in FIG. 5, this 
means that a sector of the disc is displaced away from the axis of the 
drive shaft. The following equation can be used to determine the track 
displacement .DELTA.s from the desired position: 
##EQU1## 
wherein the track displacement .DELTA.s is expressed in .mu. and the track 
width s of the read head is likewise expressed in .mu.. By adjusting the 
position of the head on the spindle 12 it is possible to compensate for 
eccentricity of the disc and/or out-of-true running of the shaft. Such 
compensation is advantageously accomplished by adjusting the difference in 
amplitude of the read voltages of signal group n (n being one of the 
groups I to VI) and the difference in amplitude of the read voltages of 
the group n+3 so that they are equal, by moving the magnetic head. In this 
way, radial eccentricity can at least be reduced to a minimum. In this 
example of compensating for eccentricity, the number of signal groups on a 
track has been assumed to be six. For a different number N of signal 
groups, in the case of track displacement in the sector bearing the signal 
group n the amplitude differences of group n and n + (N/2) should be 
equalized. 
The frequencies 1fand 2f offer themselves for this purpose, but any 
frequencies suitable for other disc speeds may be employed. The frequency 
2f of the read voltages U2fc may be used advantageously for adjusting the 
azimuth angle of the magnetic head. 
We shall not describe the fundamental procedure for adjusting the head 
relative to the track for a floppy disc unit: 
After the disc 15 has been placed over the conical end 17 of the drive 
shaft 16 while the drive is running, the head is brought into read 
position. The oscilloscope is then connected to the read unit of the head 
positioning device 11. The oscilloscope is for example set to 20 
millisecond scale units and triggered by the orientation burst 2. When the 
magnetic head is located above track no. 36 the six signal groups 
illustrated in FIG. 3, 4 or 5 with their specific amplitudes are displayed 
on the screen. The oscilloscope is then adjusted to display the maximum 
amplitude of U1fc which should correspond to about 60 scale units on the 
screen. 
The head 13 is then suitably displaced on the spindle 12 until the track 
deviation of the n.sup.th group corresponds to that of the (n+3).sup.th 
group. Ideally, all signals 4 to 9 should have the same amplitude. 
However, since some eccentricity is usually present, this is hardly ever 
the case in practice. Finally, the magnetic head is locked in position. 
Head position accuracy can be checked as follows: 
The amplitude difference x or y (FIGS. 4 and 5) is determined and a 
quantity Z is ascertained: 
##EQU2## 
Z being the actual position of the head relative to the center line of the 
track on such an adjustment and control disc, and h indicating the maximum 
amplitude of the orientation signals. Idealy, Z = O if x = y. In practice 
it suffices if x + y .ltoreq. 3.3 scale units, so that Z is 20.mu. or 
less. 
In the embodiment described here, only two kinds of signals have been 
employed, i.e. the orientation signals and the adjustment signals, the 
orientation signals being identical and the adjustment signals being 
identical. However, it is also possible, for example, for the adjustment 
signals to differ slightly from one another in amplitude and/or frequency 
while possessing common characteristic features which make them clearly 
distinguishable from the orientation signals. Moreover, the orientation 
signals may also differ slightly from one another if it is ensured that 
they can be clearly and readily distinguished from the adjustment signals. 
Other embodiments of the invention which also come within the scope of 
protection of the claims are conceivable. Furthermore, it is obvious to 
those skilled in the art that the control signals are recorded on the 
adjustment and control disc by means of simple equipment, for example a 
floppy disc drive, and that other suitable measuring devices may be 
employed instead of the abovementioned measuring instruments.