Magnetic head pivotal support with compact drive means

A positioning device for objects of low mass, particularly magnetic heads in memory processing units having at least one pivotable head support and at least one pivotable support for at least one coil, which coil support is located in the air gap of a magnet assembly, the magnet assembly comprising pairs of flat permanent magnets whose air gaps are arranged next to each other and whose poles are so arranged that poles of unlike polarity are opposite each other, and the coil(s) being of elongate shape, the longitudinal conductors of which coil(s) are always located inside the said air gaps and the transverse conductors are always located outside said air gaps. The device of the invention can be used for all positioning operations requiring a very high degree of accuracy, in particular to position write/read devices of every kind and measuring and testing probes in control engineering and the laboratory.

The present invention relates to a device for positioning objects of low 
mass, particularly magnetic heads over preselected tracks on at least one 
magnetic disc which can be coupled to a drive in a memory processing unit 
in which there are provided at least one support for at least one magnetic 
head, which support is pivotable about an axis parallel to the axis of 
rotation of the magnetic disc, and at least one coil support for a drive 
means for pivoting the head support, and in which the said drive means 
consists of at least one flat coil which projects into at least one 
working air gap of a magnet assembly. 
A device of the above type is described for example in German Laid-Open 
Application DOS 26 23 572, in which a magnetic coil in the form of spiral 
conductors arranged on a flat support acts as drive means, and projects 
into the air gaps between the legs of two horseshoe magnets. In the case 
of this positioning device the coil is oval in shape, the effective 
portions of the windings being arranged parallel to imaginary straight 
lines passing through the axis of the head arm. 
This prior art device is disadvantageous because 
(a) only a small portion of the windings plays a part in effecting pivoting 
of the head support, 
(b) the head support and the support for the drive means are arranged in 
parallel planes which are far apart from one another, 
(c) it constitutes a single-head positioning device for a single-disc 
memory, 
(d) it has two magnet systems that have to be mounted separately, which 
makes it necessary for them to be aligned relative to one another on two 
supports that are at a large distance from one another, and 
(e) the magnet systems are not suitable for an extremely flat construction. 
Another positioning device is disclosed for example in German Published 
Application DAS 22 09 522, in which there is provided a common support for 
at least one magnetic head, the support being pivotable about an axis 
parallel to the axis of rotation of the magnetic disc, and for a drive for 
pivoting the support, the magnetic head being arranged at the fork-shaped 
end of the support and the said drive at the other free end of the 
support. 
However, this device has the disadvantage that the magnetic coil is a 
hollow rectangular coil of elongate shape which projects into the air gaps 
of an E-shaped stator, a magnet assembly of the short air gap type thus 
being formed. 
The prior art devices have the disadvantage that they are too large, 
particularly in the vertical direction, which makes their production as 
well as the assembly of the individual parts very expensive. 
It is therefore an object of the present invention to provide a positioning 
device which, as compared with the prior art devices, is cheaper to 
produce in large numbers, permits extremely short head positioning times 
and hence extremely short access times, exhibits a better vibrational 
behavior, and enables the number of heads and hence the number of discs 
and consequently the capacity of memory processing units to be readily 
increased. 
This object is achieved with a device for positioning objects of low mass, 
particularly magnetic heads, wherein the magnet assembly consists of at 
least two pairs of flat permanent magnets, the two working air gaps of 
which are arranged next to each other in the same plane, and the poles of 
which are so arranged that poles of unlike polarity are opposite each 
other, wherein each pair of adjacent poles of opposite polarity is 
magnetically connected together via a common flux-conducting member, and 
wherein the flat coil is of elongate shape, the longitudinal conductors of 
said coil being always located inside the working air gaps of the pairs of 
permanent magnets, and the transverse conductors being always located 
outside said air gaps. 
As a result of the flat shape of the coil, it is possible to optimally 
design the entire pivoting arm assembly and hence to keep the overall 
height of the positioning device low, while increasing the pivoting torque 
and consequently shortening the access times. With this compact and yet 
powerful construction it is also possible to combine one or more of such 
positioning devices and a corresponding number of magnetic discs in a 
housing to form a fixed head storage module. In addition, such a 
positioning device is suitable for positioning any kind of object of low 
mass, e.g. optical devices such as lenses and optical scanners. 
The term "positioning", as used herein, means not only the moving and 
aligning of a first member relative to a second member, but also the 
controlled movement of the two members relative fo one another, e.g. the 
oscillation thereof at any desired frequency. 
In a further advantageous embodiment of the invention the coil supports and 
head supports are so designed that they can be stacked above one another 
and arranged in a predetermined position relative to their axis of 
rotation. 
The positioning device can thus be modified with respect to the number of 
objects (e.g. magnetic heads) to be positioned and the number of members 
(e.g. magnetic discs) associated therewith and hence suitably adapted for 
a particular task. 
In another advantageous embodiment of the invention, sets of two pairs of 
permanent magnets are provided with flux-conducting members, the pairs of 
magnets and the flux-conducting members being arranged above one another. 
Arrangement of the magnets in this way enables a construction of very low 
overall height to be achieved. 
In yet another advantageous embodiment, each head support is provided with 
at least one coil support which is preferably detachably connected 
thereto. 
With this design of the positioning device it is possible to adjust the 
pivoting torque acting on the head support(s) as required by altering the 
number of coils. 
In an even further advantageous embodiment of the invention, there is 
provided on the axis an adjustable rotatable hub for carrying the coil and 
head supports, and the said supports are provided with apertures so that 
they can be mounted on the hub. 
In a preferred embodiment of the invention, the flat coil consists of a 
plurality of substantially rectangular conductors, the direction of 
current flow in the longitudinal conductor portions being so chosen that 
the resulting force vectors in the magnetic fields in the two working air 
gaps are in the same direction. 
Further advantages are achieved in the production of the device of the 
invention and in the further reduction of its overall height if the flat 
coil is applied to the support in the form of a printed circuit; 
preferably, such a coil is applied to each side of the support. This 
design enables the advantages of low overall height and at the same time 
adequate pivoting torque to be achieved particularly easily. 
If a coil is on each side of the support instead of being on one side only, 
each coil having the same number of conductors, twice the induced force 
can be obtained when the conductor cross section is the same and the air 
gap is the same. 
It is advantageous to rigidly connect the head supports to a tachometer 
consisting of a coil and a fixed magnet. 
In another advantageous embodiment of the device of the invention, the 
pairs of permanent magnets consist of small flat pieces of magnetic 
material having a high energy product and a low demagnetization factor. To 
achieve a particularly flat construction, the pieces of magnetic material 
are made much thinner than the flux-conducting members. 
In summary, the invention provides an inexpensive device which is 
distinguished particularly by its compact design and which can be modified 
as desired, i.e. increased or reduced in size, due to the fact that the 
coil and head supports can be stacked on top of one another. 
In yet another advantageous embodiment, a magnet having two air gaps is 
employed in which two coils designed according to the present invention 
are arranged. As a result, a very sturdy device from the dynamics and 
strength points of view is obtained which offers particular advantages 
with respect to its vibrational behavior.

The positioning device 40 consists essentially of an arm having two 
sections of different length. The long section is formed by the head 
support 7 and the magnetic heads 8 fastened thereto, and the short section 
by a flat coil support 2 and coil(s) 14 which may be of conventional 
design or, advantageously, in the form of substantially rectangular 
conductors produced for example by printed circuit techniques. 
The above-described flat coil forms, together with the magnetic device 16, 
advantageously a permanent magnet assembly, the heart of the drive. The 
arm is mounted for pivotal movement in the directions indicated by double 
arrow 25 about a fixed axis 15 which is fastened to chassis 10 by means of 
flange 13, it being possible for pivoting to be effected simultaneously in 
more than one horizontal plane, as shown in FIG. 1. Two pairs of 
anti-friction bearings 5, e.g. ball bearings, which are fastened with a 
tight fit on shaft 6, serve as bearings. The housing for the bearings 5 is 
formed by hub 4 which carries the superposed coil supports 2 and head 
supports 7. The coil supports 2 and head supports 7 provided with circular 
apertures (cf. FIG. 2) are supported by flange 13 via sleeve 17. The 
number of coil supports 2 and head supports 7 can therefore be increased 
for example by 2 or 3. It is also possible to employ a split hub provided 
with flanges which is rotatable about axis 15, the arms for the objects to 
be positioned being fastened to one side of the hub and the coil supports 
to the other. The axial position of the head supports 7 and hub 4 is 
adjusted and fixed at the top via a flanged bearing 18 whose collar is 
firmly pressed against the end of hub 4. As a result, the position of the 
magnetic heads 8 relative to the surfaces of the discs 9a, 9b and 9c is 
predetermined. The coil support 2 which in the case of printed circuits 
may consist of an appropriate plastics insulating material is detachably 
fastened, e.g. by means of screws (not shown), to the head supports 7 at 
points a and a'. The magnetic device 16 is fastened to the chassis 10 by 
means of, for example, screws represented by the dot-dash line 35 in FIG. 
3. Other screws (not shown) serve to join the parts of the magnetic device 
16 together. To achieve rapid and accurate positioning with the device of 
the invention, it is necessary to ascertain the speed at which the heads 8 
pivot. For this purpose there is provided a tachometer which may for 
example consist of a permanent magnet system 12, fastened to chassis 10, 
and a rectangular coil 11 which has a large number of windings and whose 
support member 19 is attached to the lower end of hub 4. The voltage 
generated in the rectangular coil 11 by movement thereof is a measure of 
the momentary velocity of the head support 7 and heads 8. Pivotal movement 
of the head support 7 is advantageously limited by an adjustable stop 20 
(FIG. 2). 
Further details of the positioning device will be apparent from the 
following explanations concerning its function: 
The magnetic heads 8 which are attached to the head supports 7 via spring 
members 21 should be able to be moved very quickly and precisely across 
the surfaces of magnetic discs 9a, 9b and 9c and positioned over 
preselected tracks by pivotal movements in horizontal planes. To this end, 
current is passed through the coils 14 of conventional design or formed by 
conductors 1, as a result of which a torque is applied to the coil 
supports, thus causing the head supports 7 to rotate about axis 15, so 
that the magnetic heads 8 move along an arcuate path. The direction in 
which the head supports 7 pivot is determined by the direction of current 
flow in the coils 14. Slowing down and stopping of the head supports 7 
before one of the heads 8 reaches the desired track can be controlled via 
the tachometer 11, 12 and hence very accurate positioning can be achieved. 
The control device required for this purpose is not described in further 
detail herein or shown in the drawings. The stop 20 determines the end 
position of head supports 7 when the heads 8 are pivoted over the tracks. 
Once the head 8 has been positioned over the desired track, it is kept on 
this track by applying a current to the coils 14, the direction in which 
they act being opposite to the direction of head deviation. Selection of 
the desired track and location of the head 8 thereon is achieved by a 
servo system (not shown) which is usually used in memory processing units. 
As shown in FIG. 3, the magnetic device 16 is so designed that the pairs of 
permanent magnets 22 and 23, poles of opposite polarity facing each other, 
are arranged adjacent to one another in planes parallel to the plane of 
the coil 14 at a distance 24 from one another. 
The coil(s) is (are) approximately rectangular in shape and, as shown in 
FIG. 2, is (are) in the form of rectangular conductor loops 1 which are 
electrically connected together in a manner which is not shown, to form 
one or more spirals. 
The width of the permanent magnets 22 and 23, i.e. of their poles, is 
advantageously such that the conductor portions 1a and 1b always stay 
within the working air gaps 26a and 26b of the magnetic device 16 when the 
head supports 7 pivot through their maximum range of travel, i.e. at no 
time do these conductor portions even partially project beyond the air 
gaps 26a and 26b. As shown more clearly in FIG. 3, in an embodiment having 
conductors 1 on both sides of the coil support 2 the magnetic induction in 
each air gap 26a and 26b becomes effective at the same time. Reference 
numerals 27 and 28 indicate the directions of magnetic flux in the gaps. 
The transverse conductor portions 29a and 29b which link the longitudinal 
portions 1a and 1b are always located outside the air gaps 26a and 26b 
(cf. FIG. 2). Owing to the fact that the directions of flux 27 and 28 are 
opposite one another in the air gaps 26a and 26b and the direction of 
current flow is at right angles to the direction of flux--depending on the 
arrangement of the coil windings--forces acting in the same direction 30 
are exerted on the two conductor portions located within the two air gaps 
(cf. FIG. 3). 
If a plurality of drive units, consisting essentially of coils and coil 
supports arranged above one another, are used according to the invention 
to increase torque, the drive units are advantageously so arranged that 
the opposing poles 31a and 31b of adjacent units are of like polarity to 
prevent an undesirable weakening of the magnetic flux density. The 
directions of flux in the two central flux-conducting members 32 are the 
same, as a result of which the magnetic flux density is not weakened. To 
achieve this, the current in the coils of the two units must flow in 
opposite directions and the current strength must be the same, as a result 
of which the forces exerted are the same and the directions in which they 
are exerted are also the same. It is of course also possible to arrange 
two or more drive units next to each other in the same plane. 
The upper and lower flux-conducting members bear the reference numeral 38. 
Between adjacent flux-conducting members 38 and 32 there is arranged a 
non-magnetic distance piece 39, which ensures that the lines of force 
close via the pair of permanent magnets 23. 
The coil supports 2 can be fastened to the head supports 7 by appropriate, 
advantageously removable, connecting means, e.g. screws; for example, two 
or more coil supports can be attached to a single head support 7. 
The coil supports 2 and head supports 7 are held in position relative to 
one another by means of a common centering member, e.g. hub 4 shown in 
FIG. 1, and a hole 33 in each head support 7 and a pin 34, threaded at 
each end, which fits snugly therein (cf. FIG. 2). The coil supports 2 are 
connected together at their outer ends, for example in pairs, 
advantageously via distance pieces 36, so that a strong assembly is 
achieved in conjunction with the mounting of the other ends of the coil 
supports on the hub 4, it thus being possible to adjust and maintain the 
twin air gaps with their extremely small dimensions. The height of the 
distance pieces depends of course on the width of the air gaps and the 
thickness of the flux-conducting members 32 situated between the air gaps. 
It is advantageous to use seven printed circuit conductors having a 
thickness of not more than 70 .mu.m and a width of about 1 mm each, the 
distance between two adjacent conductors being 0.5 mm, if a steady current 
of 4 amperes is to flow. 
At a mean magnetic induction in the air gap of about 3000 to 3500 gauss the 
track-to-track access time is of the order of 6 milliseconds in the case 
of printed circuit conductors having the above dimensions, the average 
access time being about 35 milliseconds and the access time from the outer 
track to the inner track and vice versa being 70 milliseconds. These 
results were obtained in tests using an experimental set-up as shown in 
FIG. 1. 
A sufficiently high magnetic flux density is achieved if magnetic material 
is employed which has a high energy product and a low demagnetization 
factor. Small flat pieces of permanent magnet material which have a high 
energy product (BH).sub.max and are highly insensitive to demagnetizing 
magnetic fields and are based on cobalt and rare earths are very suitable. 
However, other suitable magnetic materials may also be used. The small 
flat pieces of permanent magnet material are thin compared with the 
flux-conducting member, the thickness ratio being for example 1:8. 
Any suitable plastics material of low density and high strength, e.g. 
glass-fiber-reinforced epoxy resin, may be used as coil support material. 
To keep the head supports' mass down and to impart great rigidity to them, 
a magnesium alloy is used as the material of construction and a T-shaped 
cross section for instance may be employed. However, any other 
material/design combination which is suitable from the strength point of 
view may be used. 
FIG. 4 shows a flat single-disc memory 50 in diagrammatic representation. 
Housing 49 mounted on chassis 10 encloses a positioning device 40 
consisting of a magnet assembly 16, a coil support 2 and a fork-shaped 
head support 47 whose magnetic heads 48 scan the tracks on magnetic disc 
9. 
A disc-accommodating member 46 and drive shaft 45 mounted in bearings 44 
enable the disc 9 to be rotated. Coil support 2 and head support 47 are 
mounted for pivotal movement about axis 43. For the sake of simplification 
the drive motor and current leads have been omitted. 
X designates the overall height of the disc memory above the chassis. This 
height X is very small, being about 3 cm in practice. As a result, an 
extremely flat construction is obtained which opens up a wide variety of 
applications for disc memories, particularly in the office.