GB1589000A - Track follower system for positioning magnetic heads - Google Patents

Description

(54) TRACK FOLLOWER SYSTEM FOR POSITIONING MAGNETIC HEADS (71) We, BASF AKTIENGESELLSCHAFT, a German Joint Stock Company of 6700 Ludwigshafen, Federal Republic of Germany, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a track follower system for positioning the data head over a preselected concentric track of a magnetic recording disc, signals being derived, via a servo head, from a servo track lying in a plane beneath said preselected track, which signals, after evaluation in an electronic circuit arrangement, are used to control a positioning mechanism which keeps the data head on the preselected track.

Track follower systems are known for disc memories in which a plurality of recording discs with magnetizable coatings are assembled into a magnetic disc pack. Associated with every disc surface is a write/read head, all of the heads being mechanically linked to a common support. The recorded data are stored in a large number of concentric tracks in the magnetizable coatings of the magnetic discs. In the case of the earliest known disc storage systems, the magnetic heads were positioned as accurately as possible above one another, and the entire assembly of magnetic heads was moved to a preselected track by mechanical and pneumatic means. In these systems the necessary positioning information was provided by the magnetic and optical scanning of bars or glass rods. Due to the mechanical tolerances and thermal expansion of metal parts, all that could be obtained with recording systems of this type was a track density of not much more than 4 tracks per mm.

A further increase in the storage capacity on a given recording area by increasing track density was achieved by recording servo signals on at least one surface of a recording disc in a magnetic storage unit which were then scanned by means of servo heads and used to position the data write/read heads. German Published Application DAS 10 74 887 discloses such a system in which servo signals are recorded in pairs of circular servo tracks on the surface of one magnetic disc in a disc pack. When the servo head is located between a pair of servo tracks, the two control signals of the adjacent servo tracks balance each other out. As soon as the head moves away from its median position between the two tracks, a control signal is generated in an electronic processing unit which returns the head to its original position.

Owing to the rigid mechanical linking of the servo head to the other data write/read heads, the center of all data tracks must therefore lie exactly between two servo tracks.

These servo signals are also employed for clocking the data write and read operations. For this reason, the frequencies of the data signals have a predetermined relationship to those of the servo signals. However, a disadvantage of this system is that, owing to the use of separate tracks for servo signals, part of the surface available for recording data is lost. For example, disc memories are known in which one servo disc surface is required for 19 data disc surfaces.

This ratio becomes less and less favorable as the number of discs in a disc pack decreases. In the case of the frequently used single-disc cassettes in small computer systems 50% of the available recording surface would be taken up by servo signals if such a system were employed.

To overcome this disadvantage, recording discs have been disclosed (U.S. Patent 3,219,353 and German Laid-Open Application DOS 25 12 456) which consist of two different, superposed magnetic layers, the lower layer being recorded with servo information which can neither be erased nor over-written when the upper layer is subsequently written and read. It is thus possible to write servo information and data above one another, and hence to optimally utilize the surface area of the disc for recording data. Another type of coating which serves a similar purpose is also known. Such a magnetic coating contains two oxides of widely different coercitivity, as a result of which it is possible to indelibly write servo information for positioning the data head, using a special magnetic head.

Recording discs of this kind are scanned in the usual manner by a magnetic head which has only one gap and thus scans servo signals added to data signals. In view of the effect these signals have upon one another, it is very difficult to completely separate the servo signals from the data by means of electric filters. In hitherto known and utilized servo systems, the frequency of the servo signals is lower than the data signals frequencies by a factor of 2 to 10, because it is from the latter signals that the clock signals for writing and the synchronizing pulses for reading the data are derived. Since high demands are made on the tolerance of the time sequence of synchronizing pulses in the coding procedures in use, it is necessary to use a very narrow half-pulse width for the servo signals.

Besides, the servo signals are characterized by a high harmonic content, so that, due to the fixed relationship between data and servo signals frequencies, the harmonics of the servo signals coincide with the fundamental wave and the harmonics of the data signals on which they become superimposed. This, in conjunction with pattern-dependent phase displacement, leads to phase errors which, in the case of digital recording, manifest themselves in the form of frequent reading errors. Moreover, due to certain data patterns, for example as a result of the multiple repetition of letters or figures, subharmonic signals are produced which add themselves to the signals of the servo recording, and this produces a rogue control signal which pulls the data head away from the correct track position. This results in positioning of the data head over the wrong track or at least in positioning errors which cause the data head to extend over part of the adjacent track, which again leads to phase errors, In another servo system, a separation between the magnetic effects of data information and of servo information is achieved by means of a magnetic head comprising two portions, one which determines the rate at which the magnetic flux varies and the other which determines the absolute flux magnitude (German Laid-Open Application DOS 21 01 906). The disadvantage of this system is. however, that the flux-sensing portion of the magnetic head which reads the servo information is influenced by any stray magnetic field and hence requires appropriate screening which is expensive and, in the case of flying heads, is difficult to carry out.

An aim of the present invention is therefore to provide a track following system for positioning magnetic heads in the case of rotating magnetic recording discs with concentric recording tracks, by means of servo signals located within the magnetizable recording layer, but a plane below the plane for recording data, the recording and/or reproduction of the servo and/or data signals being effected by means of a magnetic-head system, which track follower system does not suffer from the abovementioned disadvantages.

This aim is achieved according to the invention by providing one magnetic head for the data signals and another head for the servo signals, the gaps in said heads being arranged at an angle a to one another such that. for a given recording wavelength X and a given track width W, the value of log A is at least 1.5, A being determined by the equation (I) sin 7r x W x tan a A X (I) x 7r x W x tan a Further details of the invention are disclosed by way of example, in the following description of the embodiments of the track follower system illustrated in the accompanying drawings, in which Fig. l is a schematic view of a memory processing unit and of recording tracks on a magnetic recording disc, Figs. 2a and 2b shown cross sections of recording media used in the system of the invention, Figs. 3a and 3b show arrangements of separate magnetic heads for data and servo information on coinciding tracks, Figs. 4a and 4h show arrangements of separate magnetic heads for data and servo information on non-coinciding tracks.

Fig. 5 shows the damping of the fundamcntal waves of the servo signals. in dependence on the radius of the recording medium according to Example l, and Fig. 6 shows the damping of the fundamental waves of servo signals, in dependence on the radius of the recording medium according to Example 2.

Figure 1 shows, in diagrammatic representation, the layout of a memory processing unit comprising a drive unit with at least one rotating magnetic recording disc 1 and a magnetic head 2 which is movable rectilinearly across the rotating recording medium, by means of an actuator 3 controlled by a servo circuit 4, for reading and writing the information, contained within concentric tracks 6, 6', 6", via a data write/read unit 5.

In Fig. 2a a recording medium suitable for the track follower system of the invention is shown in cross-section. At least one side of the rigid base disc 20 is provided with the recording planes 21, 22 for the servo information and the data information respectively. In one embodiment of the recording medium, a dual-coercitivity recording layer is employed.

Here, the upper layer 22 consists of a low-coercitivity magnetic material for storing data information, while the lower layer 21 consists of high-coercitivity magnetic material for storing the servo information. A recording medium of this type has been disclosed, for example, in U.S. Patent 3,219,353.

Another embodiment, disclosed in German Laid-Open Application DOS 25 12 456, according to Figure 2b, enables servo and data signals to be recorded in a similar fashion; in this case the layers 21 and 22, while having the same or nearly the same coercivity, are separated by means of a third non-magnetic intermediate layer 23 of sufficient thickness.

Once the servo information has been written by means of suitable devices in the part of the recording layer earmarked for this purpose, it can no longer be affected or altered by the data recording or erasing operations. However, whenever data and servo information provided for controlling the track position of the scanning devices have to be read at the same time, the problems discussed above in connection with the state of the art arise.

These disadvantages are, however, overcome by the system of the invention. Figures 3a and b and 4a and b illustrate possible embodiments for the arrangement of the magnetic heads which record/reproduce data and/or servo information.

In Figures 3a and 3b, the two magnetic heads, one denoted by the reference numeral 31 for the data information and the other denoted 33 for the servo information, are arranged one behind the other in the recording direction A. In this arrangement, both the data and the servo information are located within the same track position in the recording layer, but are vertically separated from one another. In Figure 3a, the two magnetic heads are isolated magnetically from one another by means of a screening element 35 known per se. As the various tracks are addressed, the dual-head assembly moves along the positioning line P in a radial direction across the concentric tracks of the rotating recording disc. The two gaps 32 and 34 are at an angle to one another which is predetermined according to the invention. A particularly advantageous arrangement is shown in Figure 3b: the two magnetic heads 31,33 are mounted on a common support 36 at equal distances d from the positioning line.

The track follower system of the invention for positioning magnetic heads is not limited to use with servo and data information stored in coinciding tracks. Figures 4a and 4b show other head arrangements which enable the data track and the servo track to be arranged at a distance n from one another. In Figure 4a, the two magnetic heads 31 and 33 are separated from one another by the data/servo track spacing n in the positioning direction P. All other details are the same as in Figure 3a. According to Figure 4b, the two magnetic heads, mounted on a common support 46, are separated from one another by a distance corresponding to a power of n It is essential in the head arrangements shown here or capable of being derived therefrom that the gaps of the magnetic heads should be at an angle to one another.

From analyses of recording and playback operations it has long been known that, when reproducing recordings, certain frequencies can be attenuated by inclining the magnetic head This This effect, known as azimuth attenuation, can be determined by the following equation (II): sin IT x W x tan B Azimuth attenuation (db)=20 x log ~~~~~~~~~~~~~~~~ ~ A - (II),(it), rr x W x tan wherein W is the track width of the recording, the angle between the head gap and the direction of recording, and X the recorded wavelength.

With the track-follower system of the invention, which is characterized by the use of two heads for recording and reproducing the data and servo information respectively and the arrangement of the two gaps at an angle a to one another, the number of tracks and hence the storage capacity can be increased in a simple manner and at the same time the mutual interference of data and servo signals can be eliminated.

By virtue of the fact that the servo information is a mixture of frequencies consisting of fundamentals and harmonics, the angle formed by the data head gap and tbe direction in which the servo information is recorded is fixed, using equation (it), so that the fundamental wave of the servo information falls in the first infinity digit of the attenuation characteristic, the first harmonic thus falls in the second infinity digit. the second harmonic in the third infinity digit, etc. Consequently. frequency components of the servo information are no longer contained in the reproduced data information or they are so attenuated that they no longer have any influence on the data signals. Since the attenuation characteristic described here is reversible, this also prevents a frequency component of the data information from being identical with the fundamental wave of the servo information and from interfering with the filtered fundamental wave of the servo information which is used as control signal.

In the course of the development of the system of the invention, it has been found advantageous for the attenuation of the signals in the other magnetic head to be at least 30 db.

This requirement facilitates the setting of the angle formed by the gaps. In an advantageous embodiment of the system of the invention, the gaps of the two magnetic heads are at an angle of 90" to one another, the angle formed by each gap and the positioning line of the dual-head assembly being greater than 0 and smaller than 90 , and preferably 45".

In a particularly advantageous embodiment of the system of the invention, the two magnetic heads are arranged as shown in Figure 3b. The recording of information on rotating magnetic discs is effected by means of a magnetic head moving stepwise across the disc surface, thus producing concentric tracks in the recording layer. At a constant speed of rotation of the recording medium and a constant recording frequency, the wavelength decreases as the head moves toward the center of the disc, as a result of the decreasing radius.

The change in wavelength X would, according to equation (I), necessitate - in the case of a constant track width - a radius - dependent alteration of the angle a formed by the two gaps, in order to ensure mutual attenuation of the signals to the required degree. However, if the gaps are arranged at a distance d from each side of the positioning line coincident with the radius, as shown in Figure 3b, the distance d may be so chosen that the angle between the recording direction A and each of the gap of the servo head and the gap of the data head varies in dependence on the radius in the same way as the wavelength of the recording.

The invention is further illustrated by the following Examples.

EXAMPLE 1 A rigid base disc 14" in diameter is first provided with a metallic magnetic coating, obtained by electrodeposition and consisting of a cobalt/nickel alloy, then with a nonmagnetic oxide layer and finally with a magnetic oxide upper layer. The servo information is written with a servo head, the middle of which points radially toward the axis of rotation of the recording disc, the head gap forming an angle of about 45" with the radius. The gap width of this special servo recording head is 7.5,am which enables it to write the lower servo layer.

The disc rotates at a speed of 3,600 rpm. After the recording operation during which servo information is written, which is standardized in the case of a 12-disc pack, the information in the upper layer is erased, so that only the servo information in the lower layer is left. Using a second head, i.e. a data read head, the middle of which also points radially toward the axis of rotation, the head gap also forming an angle of about 45" with the radius, but pointing in the opposite direction to the servo head gap. an attempt is made to read the previously written servo information. Since the gaps of the data head and the servo head form a total angle of 90 , the data head can no longer read the servo signals.

The attenuation characteristic is ascertained with the aid of a computer program. An angle adjustment tolerance of + 3 is assumed. The track width of each head is 1 two clam, the fundamental wave of the servo pattern is 400 kHz, and the harmonics thereof a multiple of this figure. Figure 5 shows the attenuation characteristic D of the fundamental wave as a function of the recording track S. The outer track Sa has a radius of 163.7 mm while the inner track Si has a radius of 108.0 mm. Attenuation is always higher than 30 db.

EXAMPLE 2 A floppy disc is produced which has a diameter of approx. 192 mm and a metallic sub-layer, a non-magnetic intermediate layer and an oxide upper layer. Alternating tracks are recorded with servo signals of two different frequencies (80 kHz and 90 kHz respectively). The speed of the disc is 360 rpm. The middle of the servo head is moved along a positioning line which is parallel to the positioning line along which the middle of the data head is moved. The distance between the two parallel positioning lines is 10.86 mm, i.e. each is at a distance of 5.43 mm from the radius. Each of the head gaps forms a fixed angle of 45" with the head positioning line. The total angle formed by the recordings of the heads on the outer track Sa, which has a radius of 91.75 mm, is therefore 23.7 . The amount by which this angle varies from the outer to the inner track Si - the radius of the latter being 51.54 mm - is 11.5 . Each of the heads employed has a track width of 350 ,um.

With these parameters. the attenuation characteristic of Figure 6 was obtained, in dependence on the radius.

WHAT WE CLAIM IS: 1. A track follower system for positioning magnetic heads in the case of rotating magnetic recording discs having concentric recording tracks. by means of servo signals which are located within the magnetizable recording layer in a plane below the plane intended for the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. infinity digit, etc. Consequently. frequency components of the servo information are no longer contained in the reproduced data information or they are so attenuated that they no longer have any influence on the data signals. Since the attenuation characteristic described here is reversible, this also prevents a frequency component of the data information from being identical with the fundamental wave of the servo information and from interfering with the filtered fundamental wave of the servo information which is used as control signal. In the course of the development of the system of the invention, it has been found advantageous for the attenuation of the signals in the other magnetic head to be at least 30 db. This requirement facilitates the setting of the angle formed by the gaps. In an advantageous embodiment of the system of the invention, the gaps of the two magnetic heads are at an angle of 90" to one another, the angle formed by each gap and the positioning line of the dual-head assembly being greater than 0 and smaller than 90 , and preferably 45". In a particularly advantageous embodiment of the system of the invention, the two magnetic heads are arranged as shown in Figure 3b. The recording of information on rotating magnetic discs is effected by means of a magnetic head moving stepwise across the disc surface, thus producing concentric tracks in the recording layer. At a constant speed of rotation of the recording medium and a constant recording frequency, the wavelength decreases as the head moves toward the center of the disc, as a result of the decreasing radius. The change in wavelength X would, according to equation (I), necessitate - in the case of a constant track width - a radius - dependent alteration of the angle a formed by the two gaps, in order to ensure mutual attenuation of the signals to the required degree. However, if the gaps are arranged at a distance d from each side of the positioning line coincident with the radius, as shown in Figure 3b, the distance d may be so chosen that the angle between the recording direction A and each of the gap of the servo head and the gap of the data head varies in dependence on the radius in the same way as the wavelength of the recording. The invention is further illustrated by the following Examples. EXAMPLE 1 A rigid base disc 14" in diameter is first provided with a metallic magnetic coating, obtained by electrodeposition and consisting of a cobalt/nickel alloy, then with a nonmagnetic oxide layer and finally with a magnetic oxide upper layer. The servo information is written with a servo head, the middle of which points radially toward the axis of rotation of the recording disc, the head gap forming an angle of about 45" with the radius. The gap width of this special servo recording head is 7.5,am which enables it to write the lower servo layer. The disc rotates at a speed of 3,600 rpm. After the recording operation during which servo information is written, which is standardized in the case of a 12-disc pack, the information in the upper layer is erased, so that only the servo information in the lower layer is left. Using a second head, i.e. a data read head, the middle of which also points radially toward the axis of rotation, the head gap also forming an angle of about 45" with the radius, but pointing in the opposite direction to the servo head gap. an attempt is made to read the previously written servo information. Since the gaps of the data head and the servo head form a total angle of 90 , the data head can no longer read the servo signals. The attenuation characteristic is ascertained with the aid of a computer program. An angle adjustment tolerance of + 3 is assumed. The track width of each head is 1 two clam, the fundamental wave of the servo pattern is 400 kHz, and the harmonics thereof a multiple of this figure. Figure 5 shows the attenuation characteristic D of the fundamental wave as a function of the recording track S. The outer track Sa has a radius of 163.7 mm while the inner track Si has a radius of 108.0 mm. Attenuation is always higher than 30 db. EXAMPLE 2 A floppy disc is produced which has a diameter of approx. 192 mm and a metallic sub-layer, a non-magnetic intermediate layer and an oxide upper layer. Alternating tracks are recorded with servo signals of two different frequencies (80 kHz and 90 kHz respectively). The speed of the disc is 360 rpm. The middle of the servo head is moved along a positioning line which is parallel to the positioning line along which the middle of the data head is moved. The distance between the two parallel positioning lines is 10.86 mm, i.e. each is at a distance of 5.43 mm from the radius. Each of the head gaps forms a fixed angle of 45" with the head positioning line. The total angle formed by the recordings of the heads on the outer track Sa, which has a radius of 91.75 mm, is therefore 23.7 . The amount by which this angle varies from the outer to the inner track Si - the radius of the latter being 51.54 mm - is 11.5 . Each of the heads employed has a track width of 350 ,um. With these parameters. the attenuation characteristic of Figure 6 was obtained, in dependence on the radius. WHAT WE CLAIM IS:

1. A track follower system for positioning magnetic heads in the case of rotating magnetic recording discs having concentric recording tracks. by means of servo signals which are located within the magnetizable recording layer in a plane below the plane intended for the

storage of data, the recording and/or reproduction of the servo signals and/or the data signals being effected by means of a magnetic-head system, wherein a separate magnetic head is provided for the data signals and for the servo signals, the gaps in said heads being arranged at an angle a to one another such that, for a given recording wavelength A and a given track width W, the value of log A is at least 1.5, A being determined by the equation (I) sin # x W x tan α A =# (I) # x W x tan α x

2. A track follower system according to claim 1, wherein the angle a is 90 , and neither gap is aligned with the positioning line of the magnetic heads on the magnetic disc.

3. A track following system according to claim 1 or claim 2, wherein the angle a is 90" and each gap forms an angle of 45 with the positioning line of the magnetic heads on the magnetic disc.

4. A track follower system for positioning magnetic heads in the case of rotating magnetic recording discs having concentric recording tracks, by means of servo signals which are located within the magnetizable recording layer in plane below the plane intended for the storage of data, the recording and/or reproduction of the servo signals and/or the data signals being effected by means of a magnetic-head system, wherein a separate magnetic head is provided for the data signals as well as for the servo signals, both heads having a common support and the gaps in said heads being arranged at an angle a to one another such that, for a given recording wavelength X and a given track width W, the value of log A is at least 1.5, A being determined by the equation (I) sin IT x W x tan a A = # (I) # x W x tan α #

5. A track follower system according to claim 4, wherein the two magnetic heads are mounted adjacent to one another on a common support at equal distances from the radial positioning line, this distance being so chosen that the angle formed between each head gap and the recording direction varies, in dependence on the radius, proportionally to the radius-dependent wavelength shift.

6. A track follower system substantially as hereinbefore described in either of the foregoing Examples.

7. A track following system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.