Method and arrangement for positioning a magnetic head to various tracks of a magnetic tape

For exact positioning of a magnetic head to various tracks of a magnetic tape by use of a stepping motor, a plurality of motor control steps required in order to position the magnetic head to the various tracks proceeding from a reference position are identified. For the compensation of manufacturing tolerances, individual positional values are identified for every magnetic tape recorder. These positional values indicate a precise position of the magnetic head on the center of the respective track. The positional values are stored in a read-only memory, or are formed by addition of stored standard values and correction values stored in a read-only memory.

BACKGROUND OF THE INVENTION 
The invention relates to a method for positioning a magnetic head to 
various tracks of a magnetic tape in a magnetic tape recorder wherein the 
magnetic head is positioned to the various tracks perpendicular to a 
moving direction of the magnetic tape by use of a positioning device 
provided with an electric motor. 
German OS No. 32 44 149, corresponding to U.S. Pat. No. 4,609,959, 
incorporated herein by reference, discloses an arrangement for positioning 
a magnetic head to various tracks of a magnetic tape wherein the magnetic 
tape is accommodated in a cassette which is inserted into the magnetic 
tape recorder in a longitudinal direction. The magnetic tape is moved in a 
longitudinal direction by use of a tape drive capstan, and the recording 
occurs on a plurality of parallel tracks by use of a magnetic head. The 
magnetic head is provided, for example, with two write/read heads arranged 
above one another and the recording occurs, for example, in four parallel 
tracks. The magnetic head is positioned to the various tracks by use of a 
positioning device. The positioning device contains a stepping motor at 
which a worm gearing is situated, this worm gearing cooperating with a 
screw gearing via which a magnetic head carrier accepting the magnetic 
head is shifted perpendicular to the moving direction of the magnetic 
tape. 
Although precise component parts are used for the positioning device, a 
deviation of a write/read head of up to .+-.40 .mu.m from the precise 
center of a track is possible when the magnetic head is positioned at the 
outer tracks. In magnetic tape recorders having a low number of tracks, 
this deviation does not present any difficulties in the recording or 
playback of signals. When, however, the number of tracks on the same 
magnetic tape is increased to, for example, 8 or even 20, such tolerances 
of .+-.40 .mu.m can no longer be accepted. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to specify a method and an 
arrangement for positioning a magnetic head to various tracks of a 
magnetic head given a situation in which a precise positioning of the 
write/read heads to the centers of the tracks is possible, even given a 
high number of tracks on the magnetic tape. 
Given the method of the type initially cited, this object is achieved in 
accordance with the invention by identifying a substantially exact desired 
position of the write/read head of the magnetic head at a center of the 
various tracks on the magnetic tape during a measuring procedure by a 
positioning to at least one track relative to the reference position. 
Positional values for the electric motor are allocated to substantial 
exact positions. During normal operation of the magnetic tape recorder, 
control signals are generated for a motor controller connected to the 
motor by a comparison of actual positions of the write/read head to the 
substantially exact desired positions so as to drive the electric motor of 
the positioning device. 
The method of the present invention has the advantage that, even in 
magnetic tape recorders wherein a high number of tracks is employed, the 
component parts of the magnetic tape recorders for the lower track density 
can be utilized without higher demands made of the tolerances, since the 
compensation of the tolerances does not occur by means of mechanical 
techniques but by means of electronic techniques. No added costs 
whatsoever are thus required for the mechanical structure in order to be 
able to undertake precise positioning to a plurality of tracks. 
The position values identified during the measuring procedure are 
preferably stored in an erasable and re-writable read-only memory. 
It is also possible to form the positional values from standard values 
which are identical for a plurality of magnetic tape recorders, and from 
correction values individually identified for every magnetic tape recorder 
during the measuring procedure. 
For example, a center track can be provided as a reference position and the 
positional values of the positions of the write/read heads from the 
centers of the respective outer tracks are identified during the measuring 
procedure. An outer track can also be employed as the reference position 
and the positional values in the direction toward the other outer track 
are identified during the measuring procedure. Furthermore, an edge of the 
magnetic tape can also be employed as a reference position. 
When the positioning device contains a stepping motor, the positional 
values respectively indicate a prescribed number of steps which correspond 
to the position. When the positional values are formed from the standard 
values and the correction values, these steps are respectively subtracted 
or added from the number of normal steps in order to precisely position 
the write/read head to the center of the track. 
It is also conceivable to employ a dc motor for the positioning device, 
this dc motor being equipped with a timing disk and allocated 
opto-electronic sensing elements. In this case, the positional values or 
the correction values indicate respective prescribed pluralities of clock 
pulses or the number of clock pulses to be subtracted or added from the 
respective number of normal clock pulses. 
The positioning devices also frequently have a different amount of play, 
dependent on the direction in which the positioning event occurs. Thus, 
for example, the number of steps for positioning a center track can differ 
dependent on the direction in which positioning to this center track is 
carried out, i.e. from which outer track positioning in the direction of 
this center track is undertaken. For the compensation of this play, the 
positional values or the correction values for the various tracks can be 
of various sizes, dependent on the direction from which positioning to 
this track is undertaken. 
The identification of the positional values or of the correction values can 
occur in that the write/read head is precisely positioned to the center of 
every track during a measuring procedure, and the positional values or the 
correction values are thereby identified. It is also possible, for 
example, proceeding from the center reference position, to identify the 
positional values or the correction values at the two outer tracks, and to 
identify the positional values or the corrections for the intervening 
tracks by means of interpolation. It can also be sufficient to 
respectively combine the correction values of a group of mutually 
adjoining tracks and to respectively employ the correction value allocated 
to this group when positioning to an arbitrary track of this group. 
An advantageous arrangement for the implementation of the method of the 
invention is that a control unit is provided which generates the control 
signals for a motor control of the electric motor of the positioning 
device. It generates these control signals from the difference between a 
value allocated to the actual position of the magnetic head and a rated or 
desired positional value identified during the measuring procedure. The 
control unit can contain an erasable and re-writable read-only memory in 
which the positional values individually identified for every magnetic 
tape recorder are stored after the measuring procedure and which, during 
positioning to a track, emits the positional values to a comparator which 
compares these positional values to values for the respected actual 
position of the magnetic head which are stored in a memory. The control 
unit can also contain a read-only memory in which the standard values 
allocated to all tracks are stored. It can also contain a further memory 
in which the correction values are stored, and an adder which adds the 
correction values to the standard values when a specific track is 
selected. The correction values can have positive or negative operational 
signs, so that an addition or a subtraction occurs in the adder. 
The control unit can also comprise a microcomputer which generates the 
control signals for the positioning device by use of the positional values 
or of the correction values. 
The memory for the correction values can also be fashioned as a switch unit 
having a plurality of switches which, for example, emit a correction value 
which occurs given positioning to an outer track. The correction values of 
the inner tracks can then be concluded from this correction value by 
interpolation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The arrangement shown in FIG. 1 shows a portion of a magnetic tape recorder 
means which is designed as a cassette magnetic tape recorder. Such a 
cassette magnetic tape recorder can be derived, for example, from German 
OS No. 32 44 165 or from German OS No. 32 44 149, both incorporated herein 
by reference. Situated in a housing 1 of the magnetic tape cassette 
recorder are a drive arrangement (not shown) for the magnetic tape 3 and a 
positioning device for a magnetic head 4 in order to position write/read 
heads of the magnetic head 4 to various tracks of the magnetic tape 3. A 
magnetic tape cassette 2 (shown with broken lines) comprising the magnetic 
tape 3 is insertable into the magnetic tape recorder. During operation of 
the magnetic tape recorder, the magnetic head 4 engages into an opening of 
the cassette 2. The magnetic head 4 is secured to a head carrier 5 by use 
of two bearings 6 and 7, and is displaceable along an axis or shaft 8 in a 
transverse direction to the magnetic tape 3. 
The displacement of the magnetic head 4 occurs by use of an electric motor 
9 preferably fashioned as a stepping motor, but which can also be 
fashioned as a DC motor. A worm 10, which is in functional communication 
with a worm wheel 11, is secured to the shaft of the motor 9. In the 
region of the worm wheel 11, the shaft 8 comprises an outside thread 12 
which is in functional communication with an inside thread of the worm 
wheel 11. When the worm 10 turns, the worm wheel 11 is driven, so that 
this moves in an axial direction of the shaft 8. A compression spring 13 
presses the head carrier 5 against the worm wheel 11 so that this follows 
the movement of the worm wheel 11 in an axial direction and thus positions 
the magnetic head 4 to various tracks of the magnetic tape 3. The head 
carrier 5 is prevented from turning with the rotation of the worm wheel 11 
during the movement of the head carrier 5 in an axial direction of the 
shaft 8. During insertion or removal of the cassette 2, the magnetic head 
4 together with the head carrier 5, however, is pivotable out of the 
working position into an idle position. The motor 9 receives control 
signals from a motor controller 15, these control signals being fashioned, 
for example, as stepping pulses, when the motor 9 is a stepping motor. The 
motor controller 15 is driven by a control unit 16 which receives the 
control signals when a write/read head in the magnetic head 4 is 
positioned to a specific track of the magnetic tape 3. It receives these 
control signals in order to proceed from an actual position, for example 
from a specific track, to a rated or desired position, for example to 
another track. The function of such a control unit 16 is well known for a 
normal positioning and may be derived, for example, from U.S. Pat. No. 
4,313,141, incorporated herein by reference. 
The control unit 16 is informed of a specific track by a central control 
unit of the magnetic tape recorder or by a microcomputer which are 
schematically shown as a desired track emitter 17. The actual position in 
which the magnetic head 4 is respectively situated is stored in a memory 
18. 
At the beginning of operation of the magnetic tape recorder, the magnetic 
head 4 is brought into a reference position which is determined by the 
reference position emitter 25. This reference position, for example, is an 
outer edge of the magnetic tape 3 which is identified by a method as 
disclosed in German OS No. 31 12 886, corresponding to U.S. Pat. No. 
4,476,503, incorporated herein by reference. The reference position can 
also be a first, for example, outer or center track of the magnetic tape 3 
or, as disclosed by U.S. Pat. No. 4,313,141, can be a defined position of 
the positioning device which is recognized by means of a light barrier. 
This reference position is then the actual position and is stored in the 
memory 18. 
The control unit 16 contains an allocator 19 fashioned as a read-only 
memory which converts the track number output by the desired track emitter 
17 into a rated or desired position. Both the desired position output by 
the allocator 19 as well as the actual position output by the memory 18 
are supplied to a comparator 20 which uses the difference between the 
desired and actual positions in order to identify the spacing, both with 
respect to amount as well as with respect to direction, and initiates a 
pulse generator 20 to emit pulses for the stepping motor 9 to the motor 
controller 15 until the desired position and the actual position coincide. 
The magnetic head 4 is thus positioned to the desired track upon 
employment of the motor 9. At the same time, the pulses are supplied to 
the memory 18 so that this contains the new actual position. For this 
purpose, the memory 18 can contain an adder which respectively subtracts 
the corresponding numbers of pulses from the actual position, or adds them 
thereto. It can also be designed as a counter whose counter reading 
indicates the respective actual position and which is respectively 
incremented or deincremented by the pulses emitted by the pulse generator 
21. 
As already mentioned, the motor 9 can also be designed as a dc motor. In 
this case, a timing disk can be situated on the shaft of the dc motor for 
precise positioning, a plurality of slots in the timing disk being sensed 
by opto-electronic elements. In this case, the pulse generator 21 emits 
signals to the motor controller 15 until a prescribed number of clock 
pulses has been sensed, this prescribed number of clock pulses 
corresponding to the difference between the rated position and the actual 
position. 
When the recording of data on the magnetic tape 3 occurs in four parallel 
tracks, and the magnetic head 4 contains two write/read heads which are 
respectively positionable to two of the tracks, the corresponding desired 
positions are permanently stored in the allocator 19 as identical standard 
values N for a plurality of magnetic tape recorders, and the positioning 
occurs without difficulties. When, however, recording on the magnetic tape 
occurs in a great number of tracks, for example 20 tracks, it can occur 
that the same desired positions can no longer be stored for all magnetic 
tape recorders of a series, since the write/read heads are no longer 
precisely positioned to the centers of the various tracks as a consequence 
of mechanical tolerances in the positioner device. 
The illustration in FIG. 2 shows a short section of the magnetic tape 3 on 
which the data are recorded in a plurality n of tracks. A center track n/2 
can be employed, for example, as a reference position B1 from which 
positioning to other tracks is undertaken. When from this reference 
position B1 positioning is undertaken, for example, to the tracks n or 0, 
deviations d1 through d4 which no longer coincide with the provided 
centers of the tracks can occur as a consequence of mechanical tolerances 
in the positioning device. Deviations d5 and d6 can occur in a 
corresponding fashion when, proceeding from an outer track 0 as a 
reference position B2, or from the edge of the magnetic tape 3 as a 
reference position B3, positioning is undertaken to the other outside 
track n. These deviations d5 and d6 can, for example, be greater than the 
deviations d1 through d4 since the path which the magnetic head 4 
traverses is greater in these cases than given employment of the reference 
position B1. 
In the illustration in FIG. 3, the deviations in the positioning to the 
tracks 0 through n are shown in the form of a diagram, whereby it is 
assumed that the outer track 0 is selected as a reference position. An 
analogous case applies given employment of a center track as reference 
position B1, or of the edge of the magnetic tape 3 given the reference 
position B3. During a measuring procedure after mounting of the 
positioning device and of the magnetic head 4, either the precise 
positional values P necessary for positioning to the centers of the 
desired tracks, and which are to be stored in the allocator 19, or 
deviations from the centers of the tracks identified during positioning to 
the standard values N stored in the allocator 19, are identified in the 
form of correction values d, these indicating the exact positions or the 
deviations by a respective plurality of steps, given employment of a 
stepping motor, or by a respective plurality of clock pulses given the 
employment of a dc motor with a timing disk. Given positioning to the 
track n, these correction values correspond to the deviations d5 or d6. In 
order to proceed from the track 0 to track n, n.times.m steps in 
accordance with the content of the allocator 19 are required as standard 
value Nn, these being indicated in the ordinate direction when a stepping 
motor is employed as the electric motor 9. In a corresponding fashion, n/2 
.times.m steps are required as a standard value N in order to proceed to a 
center track n/2. Even more or fewer steps can be required for precise 
positioning to the center of this track, these being defined, together 
with the standard value N, by the positional value P or by the respective 
correction value d for the track. The positional values P, or the 
correction values d, are identified and stored for every magnetic tape 
recorder after assembly of the positioning device. The corresponding 
positional value or correction value d can therefore be identified for 
every track. Proceeding from the respective reference position, it is also 
possible when positioning to a remote track, for example the outer track, 
to identify the overall value and to identify the values at the other 
tracks by interpolation. It is also possible to respectively combine a 
group of mutually adjoining tracks and to provide them with a correction 
value d. 
The deviation can amount to .+-.40 .mu.m when positioning from an outer 
track to the other outer track, or from the edge of the magnetic tape 3 to 
the outer track at the other edge of the magnetic tape. This deviation can 
correspond to .+-.8 steps of the stepping motor 9. The correction value d 
can then be specified by a four bit word, whereby the most significant bit 
indicates the operational sign and the remaining three bits indicate the 
number of steps. The respective correction value d for every track or 
every group of tracks can then be determined by interpolation, for example 
linear interpolation. When positioning to a prescribed track, the 
allocator 19 then does not specify the normal number of steps, but rather 
the number of steps corrected by the correction value. 
The allocator 19 shown in FIG. 1 can be constructed as what is referred to 
as a EEPROM (electrically erasable and programmable read-only memory), and 
the positional values P for every track are stored in it after the 
measuring procedure. 
The allocator 19 shown in FIG. 4 contains, on the one hand, a memory 22 for 
the standard values N of the plurality of steps for positioning to the 
various tracks. A memory 23 for the correction values d is additionally 
provided. When positioning to a specific track which is specified by the 
desired track emitter 17, the standard value N and the correction value d 
are read out of the memory 22 or the memory 23, and are added in an adder 
24 and are output to the comparator 20 as a desired position. As already 
mentioned, the correction values d can be provided in the memory 23 for 
every track. The same correction value d can also be present for one 
respective group of tracks. Furthermore, the correction values d can 
differ, depending upon the direction in which positioning to a specific 
track is undertaken in order to compensate a play in the positioning 
device which is dependent on rotational sense. 
The control unit 16' shown in FIG. 5 contains a microcomputer 26 to which 
the desired track emitter 17 can supply the respective desired track 
number, and to which the signals allocated to the reference position can 
also be supplied by the reference position emitter 25. The control unit 
16' contains a memory for the positional values P which is fashioned as an 
electrically erasable and programmable read-only memory, or contains the 
two memories 22 and 23 for the standard values N or the correction values 
d. The microcomputer 26 uses the positional values P or the normal values 
N and the correction values d to identify the respective desired rated 
position, and emits the corresponding control signals to the motor 
controller 15. 
Given the control unit 16" shown in FIG. 6, this likewise comprises a 
microcomputer 26. The switch memory 23' for the correction values d is 
constructed as a switch unit which comprises a plurality of switches, for 
example four switches. During a measuring procedure after assembly of the 
positioning device, the deviation d5 or d6 when positioning to the other 
outer track proceeding, for example, from an outer track or the edge of 
the magnetic tape 3, is identified and the corresponding number of steps 
of the stepping motor 9 is input by three switches in binary coded 
fashion. A further switch indicates the operational sign of the deviation. 
When, as already set forth above, the deviation can comprise up to .+-.8 
steps, this deviation can be set in binary coded form by the three 
switches, and the fourth switch indicates the operational sign. One step 
corresponds to a deviation of .+-.5 .mu.m. By means of linear 
interpolation, the microcomputer 26 then identifies the deviation for the 
respective track, or for a group of tracks, in order to be able to 
undertake positioning to the center of the respective track with optimum 
precision. 
Given employment of a read-only memory for the positional values P, the 
overall number of steps which corresponds to the sum of the standard value 
N and the correction value d, is identified for every track during the 
measuring procedure, and is stored in the read-only memory, which then 
represents the allocator 19. In this case, an addition or subtraction of 
the correction values d is not necessary since the correction values d are 
already taken into consideration in the measuring procedure. 
Although various minor changes and modifications might be proposed by those 
skilled in the art, it will be understood that I wish to include within 
the claims of the patent warranted hereon all such changes and 
modifications as reasonably come within my contribution to the art.