Disk drive device having reduced thickness

A disk drive system in accordance with the present invention comprises a surface opposing type spindle motor which in turn comprises a spindle which is rotatably supported through a bearing by a chassis and to which is to be mounted a disk, a rotor yoke securely attached to the spindle, a driving magnet assembly, a printed-circuit board upon which are mounted a plurality of driving coils and a stator yoke disposed in opposing relationship with the driving magnet through the driving coils. A chassis has an opening in which is housed a head and the stator yoke is formed with a first recess in a region corresponding to a space in which the head is displaced. The bearing is a radial bearing having a diameter greater than that of the spindle. In order to avoid the interference between the driving coils and the first recess, the driving coils are disposed in an opening formed through the printed-circuit board or the space defined at the position corresponding to the recess. Furthermore, in order to balance the attractive forces of the driving magnet assembly or to cancel the cogging torque, the stator has a second recess at a position in opposing relationship with the first recess across the bearing.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a disk drive device for rotating a 
disk-shaped recording medium such as floppy disks. 
2. Description of the Prior Art 
FIG. 1 is a sectional view illustrating a spindle motor portion which is 
one of the major component parts of a conventional floppy disk drive (to 
be referred as "FDD" in this specification hereinafter). Reference numeral 
1 respesents a chassis or a main body base made by the aluminum die 
casting process or the press; 2, a disk the surface of which the data is 
written and read out, and 23, 23', bearings for supporting a spindle 25 
which supports and rotates the disk 2. 
The disk 2 is cramped on the spindle 25 by a center cone 40. 41 indicates a 
bearing; 42, a center shaft which is securely carried by a pressure plate 
43 by a retaining ring 44. 45 is a spring for transmitting the clamping 
force from the pressure plate 43 to the center cone through the bearing 
41. 26 is a rotor or which driving magnets 7 are securely attached. 
8 is a printed circuit board for a motor for rotating a disk. 29 are 
driving coils arranged in the circumferential direction and in 
equidistantly spaced apart and coplanar copartner relationship with each 
other, and the rotating force produced by using the electromagnetic 
conversion force produced by a conventional current switching method is 
imparted to the driving magnetic. 10 is a stator yoke and establishes the 
closed magnetic circuit together with the driving magnets 7. 11 is a 
magnetic head for writing and reading the data on and off from the surface 
of the disk 2 which is securely connected to a head carriage (not shown) 
so that the magnetic head can move in the directions indicated by the 
double pointed arrow F. 
22 is a setscrew for securely attaching the rotor 26 to the spindle 25. 13 
is a housing for securely holding the bearings 23 and 23' which is 
securely mounted on the chassis 1. 
The conventional FDD of the type described above has the following 
problems. 
(1) In the bearing assembly, the spindle 25, the bearings 23 and 23' and 
the rotor 26 are superposed so that the reduction in thickness of the 
bearing assembly is limited. 
(2) The thickness of the magnetic head 11 is added to that of the motor so 
that the reduction in overall thickness is also limited. 
SUMMARY OF THE INVENTION 
The primary object of the present invention is therefore to provide a disk 
drive device which can be made compact in size by absorbing the overall 
thickness of the motor and the magnetic head by suitable design and 
construction of another parts. 
Another object of the present invention is to provide a disk drive device 
in which the balance of attractive forces of the driving magnets can be 
improved and which can be made thinner by suitable design and 
construction. 
A further object of the present invention is to provide a disk drive system 
in which torgue ripple due to occurrence of cogging torgue can be 
suppressed to a minimum so that precision of rotation can be improved and 
vibration sound can be reduced. 
In the firest aspect of the present invention, a disk drive device of the 
type comprises: 
a spindle rotatably supported by a chassis on which is to be mounted a 
disk; 
a head disposed on the chassis so as to be movable in the radial direction 
of the disk; 
a rotor yoke securely mounted on the spindle and securely carrying a 
disk-shaped driving magnet assembly; 
a printed-circuit board disposed in opposing relationship with the driving 
magnet assembly and provided with a plurality of driving coils; and a 
stator yoke mounted on the chassis in opposing relationship with the 
driving magnet assembly through the plurality of driving coils, wherein 
the chassis has an opening for housing therein the head; 
the stator yoke has a recess in a portion corressponding to a region in 
which the head is displaced; 
the printed circuit board has an opening in opposing relationship with the 
recess; and 
at least one driving coil is disposed in the opening. 
Here, the bearing may be a radial bearing whose inner diameter is greater 
than that of the spindle and the spindle is fitted into an opening defined 
by the inner ring of the radial bearing. 
The recess may be a rectangular groove formed by the drawing operation and 
having the same width throughout the whole length of the groove. 
In the second aspect of the present invention, a disk drive device 
disposing a surface opposing type spindle motor comprises: 
a spindle rotatably supported through a radial bearing by a chassis on 
which is to be mounted a disk; 
a rotor yoke securely attached to the spindle; 
a driving magnet assembly; 
a plurality of driving coils disposed in opposing relationship with driving 
magnet assembly; and a stator yoke, being in opposing relationship with 
the driving magnet assembly, sandwiching the plurality of the driving 
coils therebetween; wherein the improvement comprises the bearing being a 
radial bearing having an inner diameter greater than the diameter of the 
spindle and the spindle being fitted into the hole defined by the inner 
ring of the radial bearing. 
Here, the following relationship may be satisfied, 
EQU D.sub.r .gtoreq.D 
Where 
D is an inner diameter of the disk and 
D.sub.r is a diameter of a raceway surface of an inner ring of the radial 
bearing. 
The following relationship may be satisfied, D.sub.B &lt; a diameter of an 
innermost track of the disk, where D.sub.B is a diameter of the outer ring 
of the radial bearing. 
The following relationship may be satisfied, 
EQU H.ltoreq.6 mm 
where H is a height from the center of a bearing ball of the radial bearing 
to a disk mounting surface of the spindle. 
A spindle which is made of a plastic mixed with at least a metal selected 
from the group consisting of aluminum, an aluminum alloy, stainless steel, 
brass, zinc and a zinc alloy and filter and which has a low coefficient of 
thermal expansion, may be securely fitted into an inner hole defined by an 
inner ring of the radial bearing. 
In the third aspect of the present invention, a disk drive device of the 
type having a spindle rotatably supported by a chassis on which is to be 
mounted a disk, a head disposed on the chassis so as to be movable in the 
radial direction of the disk, a rotary yoke securely mounted on the 
spindle and securely carrying a circular driving magnet assembly, a 
plurality of driving coils disposed in opposing relationship with the 
circular driving magnet assembly, and a stator yoke disposed on the 
chassis in opposing relationship with the circular driving magnet assembly 
through the plurality of driving coils; wherein the improvement comprises: 
the chassis having an opening in which the head is housed; 
the stator yoke having a recess in a portion corresponding to a region in 
which the head is displaced; 
satisfying the condition P=4n+2, where P being the number of magnetized 
poles of the circular driving magnet assembly and 3n (n: plus integers) 
being the number of the driving magnetic coils; and 
the plurality of driving coils being arranged in the form of a circle 
leaving a space corresponding to the recess of the stator yoke. 
Here, the bearing may be a radial bearing whose inner diameter is greater 
than that of the spindle and the spindle is fitted into an opening defined 
by the inner ring of the radial bearing. 
The recess may be a rectangular groove formed by the drawing operation by 
the mechanical press and having the same width throughout the whole length 
of the groove. 
In the fourth aspect of the present invention, a disk drive device of the 
type having a spindle which is rotatably supported through a bearing on a 
chassis and over which is mounted a disk; 
a head disposed on the chassis so as to be movable in the radial direction 
of the disk; 
a rotor yoke which is securely attached to the spindle and to which is 
attached a circular driving magnet ring; 
a plaurality of driving coils disposed in opposing relationship with the 
circular driving magnet assembly; and a stator yoke disposed on the 
chassis in opposing relationship with the circular driving magnet assembly 
through the plurality of driving coils; wherein the improvement comprises: 
the chassis having an opening in which the head is housed; 
the stator yoke having a recess in a portion corresponding to a region in 
which the head is displaced; and means, disposed at a position in opposing 
relationship with the recess across the bearing, for balancing attractive 
forces of the circular driving magnet assembly acting on the stator yoke. 
Here, the balancing means may comprise another yoke member disposed within 
spaces of the driving coils. 
The balancing means may be a second recess formed by the drawing operation. 
The second recess may be extended into an inside space of one of the 
plurality of driving coils. 
In the fifth aspect of the present invention, a disk drive device disposing 
a surface opposing type spindle motor comprises: 
a spindle rotatably supported through a radial bearing on a chassis and 
upon which is placed a disk; 
a rotor yoke securely attached to the spindle; 
a driving magnet assembly; 
a plurality of driving coils disposed in opposing relationship with the 
driving magnet assembly; and 
a stator yoke disposed in opposing relationship with the driving magnet 
assembly with the plurality of driving coils sandwiched therebetween; 
wherein the improvement comprises: 
the bearing being a radial bearing having an inner diameter greater than 
the diameter of the spindle and the spindle being fitted into the hole 
defined by the inner ring of the radial bearing. 
In the sixth aspect of the present invention, a disk drive device of the 
type having a spindle rotatably supported by a chassis on which is to be 
mounted a disk, a head disposed on the chassis so as to be movable in the 
radial direction of the disk, a rotary yoke securely mounted on the 
spindle and securely carrying a circular driving magnet assembly, a 
plurality of driving coils disposed in opposing relationship with the 
circular driving magnet assembly, and a stator yoke disposed on the 
chassis in opposing relationship with the circular driving magnet assembly 
through the plurality of driving coils; wherein the improvement comprises: 
the chassis having an opening in which the head is housed; and 
the stator yoke having a first recess formed in a portion corresponding to 
a region in which the head is displaced and a second recess formed as as 
to cancel the cogging torque produced by the first recess. 
Here, the driving magnet assembly may be equiangularly divided by a 
predetermined angle .theta..sub.o into an even number of alternately 
magnetized poles defined by driving by .theta. are positively and 
negatively; 
the first and second recesses of the stator yoke are substantially in the 
form of a rectangle and are extend in different radial directions, 
respectively, of the stator; and 
the side edges of the recesses extending outwardly of the stator yoke are 
not in parallel with boundary lines of the poles of the driving magnet 
assembly, when the side edges and the boundary lines become in opposing 
relationship, by viewing from a point of a line perpendicular to the 
surface of the stator yoke. 
An even number of poles defined by equiangularly dividing the circular 
driving magnet assembly by .theta. may be alternately positively and 
negatively magnetized; 
the first and second recesses of the stator yoke are substantially in the 
form of a rectangle and are exended in the different radial directions, 
respectively, of the stator; and 
an angle between the first and second recesses is substantially equal to 
(n+.alpha.) times the predetermined angle .theta..sub.o, wherein n is 0 or 
an integer, and 
.alpha. is 1/4, 1/2 or 3/4. 
The angle between the first and second recesses may be 180 degrees plus 1/4 
or 1/2 or 3/4 of the angle .theta. or is 180 degrees minus 1/4 or 1/2 or 
3/4 of the angle .theta..sub.o. 
The plurality of driving coils may consist of a predetermined number of 
first driving coils and a predetermined number of second driving coils 
which are different in size from the first driving coils, each pair of the 
first and second coils being in the same phase; and 
the first and second driving coils are so interconnected that output torque 
in each phase becomes equal. 
The plurality of driving coils may be arranged in the form of a circle 
leaving spaces so as not to interfere with the first and second recesses 
of the stator yoke, whereby the first and second recesses are positioned 
within the spaces, respectively. 
The above and other objects, effects, features and advantages of the 
present invention will become more apparent from the following description 
of embodiments thereof taken in conjunction with the accompanying drawings 
.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring first to FIGS. 2 and 3, an embodiment of the present invention 
will be described. 
Reference numeral 2 indicates a disk 2 or a recording medium and a magnetic 
head 11 carried by a head carriage (not shown) makes sliding contact with 
the surface of the disk 2 to write and read the data onto and out of the 
surface thereof. 
A chassis 1 or a main body base of an FDD is made by aluminum die casting 
or a suitable press machine. A large-diameter radial bearing 3 is securely 
attached to the chassis 1 by a retaining ring 4 and an elongated opening 
1a is formed through the chassis 1 so that a space for permitting the 
displacement of the magnetic head 11 is obtained. A spindle 5 is securely 
fitted into the inner ring or race of a bearing 3. The disk 2 is securely 
clamped over the surface of the spindle 5 by the center cone 40 which 
constitutes a clamping means shown in FIG. 1. 
A disk-shaped rotor yoke (to be referred to as "a rotor" in this 
specification hereinafter) 6 is securely connected to the lower surface of 
the spindle 5 by one or more setscrews 12. Driving magnets 7 are securely 
mounted on the upper surface of the rotor 6. As shown in FIG. 5, the 
driving magnets 7 are arranged in the form of a circular ring, defining a 
hole 7a coaxially of the magnets 7. Even numbers of regions obtained by 
equiangularly dividing the magnet ring 7 in the form of a 360-degree ring 
are alternately polarized by different magnetic poles. Because of the 
magnetic forces of the driving magnets 7, upward forces are imparted to 
the rotor 6 so that the bearing 3 is subjected to the previous 
pressurization. 
A printed-circuit board 8 has control circuits for controlling the rotation 
of the disk drive motor and is formed with a hole 8b the center portion of 
the printed-circuit board 8 through which is extended the bearing 3. The 
printed-circuit board 8 is fixed to the chassis 1 in parallel and opposing 
relationship with the driving magnet ring 7. And they are spaced apart 
from each other by a predetermined distance. Driving coils 9 are securely 
mounted on the printed-circuit board 8 as will be described in detail 
hereinafter. A stator yoke (to be referred to as a stator hereinafter) 10 
is disposed above the driving coils 9. More specifically, the stator 10 is 
securely attached to the chassis 1 in parallel with the driving magnet 
ring 7 so that the driving coils 9 are sandwiched between them. The stator 
10 and the rotor 6 construct the magnetic circuit of the driving magnets 
7. 
The spindle motor with the above-described construction is a DC brushless 
three-phase Hall motor in which the driving magnets and the driving coils 
are in opposing relationship in a horizontal plane. 
As shown in FIG. 3, the stator 10 is in the form of a rectangle or a disk 
and is formed at the center portin thereof a hole 10b through which is 
extended the bearing 3. In addition, it is formed with a rectangular 
recess 10a extended in the radial direction by the reduction method so as 
to permit the displacement of the magnetic head 11 in the directions 
indicated by the double pointed arrow F, shown in FIG. 2. That is, the 
magnetic head 11 is displaced in the recess 10a in the directions 
indicated by the double pointed arrow F; that is in the radial direction 
of the disk 2. 
But there is no space to dispose the driving coil 9 at the recess 10 
between the printed-circuit board 8 and the stator 10 so that according to 
the first embodiment of the present invention, the driving coils 9 are 
arranged as shown in FIG. 3. The printed-circuit board 8 is formed with an 
opening 8a corresponding to the recess 10a and one of the driving coils 9 
is disposed as indicated by 9a, which are securely bonded by an adhesive 
or the like to the printed-circuit board 8 or the stator 10. 
In FIG. 3, the driving coils 9 (or 9a) are shown (two coils are not shown 
for the sake of simplicity of the figure) as a three-phase drive with six 
driving coils, but the three-phase drive with nine driving coils may be 
also used in the present invention. 
But according to the present invention, the disposition of the driving coil 
in the region of the recess 10a may be omitted as shown in FIGS. 4-6. That 
is, the driving magnet assembly 7 consists of 14 magnetic poles (4 
poles.times.3+2 poles as shown in FIG. 5 and 9 driving coils (3.times.3 
phases) as shown in FIG. 6. In this case, in order to avoid the 
disposition of the driving coils 9 in the region of the recess 10a of the 
stator 10, an arcuate space corresponding to that in which two magnetic 
poles could be located is defined as indicated by 15. Therefore, the 
recess 10a of the stator 10 is positioned in the space 15. In the latter 
case, it is not needed to form an opening in the printed-circuit board 8. 
Thus, by switching energization and de-energization of the driving coils in 
a conventional three phase full-wave driving method, the rotor 6 is 
rotated by the magnetic force between the driving coils 9 and the driving 
magnets 7, whereby the spindle 5 and hence the disk are rotated. The 
magnetic head 11 makes sliding contact with the surface of the disk to 
write and read the data onto and off from the surface of the disk 2. 
When the spindle motor assembly has the construction described above, it 
becomes possible to use part of the space in which are disposed driving 
coils 9 as the passage in which is displaced the magnetic head 11. In 
addition, the spindle is fitted into the large-diameter bearing 3. Thus, 
the disk drive device can be made thin in size. 
Next referring to FIGS. 7-9, a second embodiment of the present invention 
will be described in detail below. 
In addition to the disk drive motor assembly described above in the first 
embodiment, the second embodiment has a means for balancing the attractive 
forces of the driving magnets 7 with respect to the stator 10. When the 
balance of the attractive forces is distorted, the rotor 6 is inclined in 
the direction in which the attractive force is greater so that both the 
parallelism between the rotor 6 and the spindle 5 and the precision of 
rotation are degraded. As a result, the sliding contact of the magnetic 
head 11 and the surface of the disk 2 is adversely affected so that in the 
case of writing and reading the data, errors result. The balance between 
the attractive forces acting on the rotor 6 is measured at both ends of 
the rotor in opposing relationship through the bearing 3 and it is 
preferable that the results of the measurements are that a degree of 
unbalance is no in excess of 20%. 
Still referring to FIG. 7, a yoke 31 made of a magnetic material is 
disposed on the printed-circuit board 8 at least one portion of the spaces 
within a plurality of driving coils 9 which opposes to the recess 10a of 
the stator 10 across the bearing 3, thereby balancing the attractive 
forces of the driving magnets 7. 
In stead of the provision of the yoke 31, as shown in FIGS. 8 and 9, the 
stator 10 may be subjected to the reduction press to form second recesses 
32 as in the case of the recess 10 which are extended into the inner sides 
of the driving coils 9. 
Next referring to FIGS. 10-14, an embodiment of the present invention 
applied to a 5.25" FDD will be described below. 
As shown in FIGS. 10-11, the inner diameter of the inner ring 3.sub.i of a 
large-diameter radial bearing 3 is greater than that of the disk 2 and the 
spindle 5 having the diameter substantially equal to that of the inner 
diameter D of the disk 2 is fitted into the inner ring 3.sub.i. 
With this construction, the attractive forces of the driving magnets 7 are 
determined to become considerably greater than the clamping force of the 
center cone 40 (for instance, attractive forces-clamping force&gt;300 g). The 
attractive forces impart the upward pressure to the bearing 3 so that the 
stabilization can be attained. 
Recesses 10c and 10d are formed on the opposite side of the recess 10a of 
the stator 10 across the bearing 3 so that the attractive forces are 
balanced and the cogging due to non-uniformity of torque can be decreased. 
The recesses 10c and 10d will be described in more detail in the following 
embodiments. 
The driving coils 9 are arranged in a manner substantially similar to that 
described above with reference to FIG. 14 and the recess 10a of the stator 
10 is inserted into the space 15 and recesses 10c and 10d, into the space 
16. As shown in FIG. 13, the magnet assembly 7 consists of 12 alternately 
magnetized poles. In general the conventional three-phase full-wave drive 
includes 9 driving coils, but according to the present embodiment, 6 
driving coils are used as shown in FIG. 14. When each phase is uniformly 
eliminated, the smoothness of torque is not lost. 
In the case of a 5.25" FDD, the diameter D.sub.B of the outer ring 3.sub.o 
of the bearing 3 is preferably smaller than the radius of 34.13 .sub.mm of 
the 79-th track of one surface of the disk which is the innermost position 
of the magnetic head 11. Therefore when D.sub.B &gt;68.sub.mm, the 
interference between the bearing 3 and the magnetic head 11 can be 
prevented and they can be arranged in series in coplanar relationship with 
each other. 
When the diameter Dr of the raceway surface of the inner ring 3.sub.i of 
the radial bearing 3 is greater than the inner diameter D 
(.phi.28.57.+-.0.025.sub.mm) of the disk 2, the diameter Dr of the raceway 
surface of the inner ring 3 becomes almost greater than the diameter of 
the points on which forces act so that when even one bearing is used, the 
stabilized precision of rotation can be ensured. 
Furthermore, the lower the height H from the center of bearing balls 3b to 
the disk 2, the smaller the surface vibration of the spindle assembly and 
the vibration of the spindle become and the thinner the disk drive 
becomes. 
The present invention has taken the thickness of the motor assembly and the 
size of the center cone 40 required for clamping the disk 2 into 
consideration. The result is that H&lt;6.sub.mm is considered to be 
effective. 
In addition when the inner diameter of the radial bearing.gtoreq.the inner 
diameter of a recording medium or disk 2, a spindle made of a material 
such as aluminum, an aluminum alloy, stainless steel, brass, zinc, a zinc 
alloy or the like and a filler and having a low coefficient of thermal 
expansion can be fitted into the inner ring. Thus, the standard bearings 
can be used and the thickness of the spindle and the thickness of the 
bearing are not cumulated in the vertical direction, whereby the FDD can 
be made thinner. 
Referring next to FIG. 15, an embodiment of the present invention in which 
the inner ring itself of the bearing is used as spindle will be described. 
In FIG. 15, the inner diameter 50a of the inner ring of a large-diameter 
radial bearing 50 is made substantially equal to the inner diameter of the 
disk 2. In addition, the inner ring is formed with a tapered portion 50b 
in order to guide the disk 2. Therefore the inner ring has the function of 
the spindle such as centering of the disk 2 and rotating the same. The 
periphery of the center hole of the rotor 51 is bent by the press forming 
process as indicated by 51a and securely fitted into the inner hole 50a of 
the bearing 50. 
So far in the above-described embodiments of the present invention, the 
motor has been described as the surface opposing type DC brushless motor, 
but it is apparent that the belt drive can be used. In the latter case, 
the rotor 6 or 51 is replaced by a pulley. Even in this case, the 
stabilized rotation can be ensured because the inner ring is pressed in 
one direction under the force of the clamping force of the center cone 40 
and the diameter of the raceway surface of the inner ring of the bearing 
is greater than the inner diameter of the recording medium or the disk. 
Next an embodiment in which cogging torque resulting from the recess 10a of 
the embodiments described above with reference to FIGS. 2-9 can be 
cancelled will be described. Same reference numerals are used to designate 
similar parts in the following figures and the figures used to explain the 
above described embodiments so that no further explanation of the parts 
already made will not be repeated in this specification. However, in the 
fifth embodiment, in order to distinguish between the driving coils and 
their phases, reference numerals 91U, 91V, 91W, 92U, 92V and 92W are used 
in to indicate the driving coils. 
First referring to FIG. 16, the construction of the spindle motor assembly 
of the disk drive device will be described. Parts which have not shown in 
FIG. 4 and are component parts of the spindle motor assembly are Hall 
elements 13a and 13b for detecting the phase of rotation of the driving 
magnets 7. Furthermore, a head carriage 14 for mounting thereon the 
magnetic head 11 and displacing the same is shown. As described above, in 
order to secure the space in which the head carriage 14 can be displaced, 
the chassis 1 is formed with the elongated rectangular opening 1a. The 
difference in construction of the fifth embodiment shown in FIG. 16 and 
the embodiment described above with reference to FIGS. 4-6 is as follows. 
(1) the shape of the stator 10, 
(2) the number of magnetized poles of the driving magnet assembly 7 and 
(3) the number, arrangement and size of the driving coils. 
These differences will be described in more detail below. 
First referring to FIGS. 16-18, the shape of the stator 10 will be 
described. The stator 10 is in the form of a disk. In order to secure the 
space in which the magnetic head can be displaced in the radial direction 
of the disk 2, the rectangular recess 10a extended in the radial direction 
of the stator 10 is formed by the drawing operation by the machine press 
as in the case of the above-described embodiments, but according to the 
fifth embodiment, in addition to the recess 10a, the stator 10 is formed 
with a second recess 10b in order to cancel the clogging torque resulting 
from the recess 10a. The rectangular recess 10b is extended in the radial 
direction different from that of the recess 10a, but it is similar in 
cross section to the recess 10a; that is, both have the same trapezoidal 
cross section. Furthermore, both the recesses 10a and 10b have the same 
width L and depth D. 
The positional relationship between the recesses 10a and 10b is determined 
by the angle .phi. between the recesses 10a and 10b as shown in FIGS. 16 
and 17. In this case, the angle .phi. is determined as follows. That is, 
the whole circle of 360 degrees of the driving magnet assembly is 
equiangularly divided by a predetermined angle .theta..sub.o into an even 
number of alternately magnetized poles. The angle .phi. is then determined 
depending upon the center angle .theta. of each magnetized pole. When the 
number of poles is represented by P, the center angle of each magnetized 
pole is obtained by 2.pi./P (rad). The angle .phi. between the recesses 
10a and 10b is obtained by the following equation: 
EQU .phi..apprxeq.(2.pi./P)(n+.alpha.)rad 
where 
n is 0 or any other integer, and 
.alpha. is 1/4, 1/2 or 3/4 as an optimum value selected in response to the 
dependability of the cogging torque on the rotational speed. However, in 
order to balance among the attractive forces of the driving magnets 7 as 
better as possible, it is preferable that the angle .phi. is as closer as 
possible to 180 degrees. 
The number of poles of the driving magnet assembly 7 is not limited, but 
the magnitude of torque obtained and the width of the recess 10a for 
permitting the displacement of the magnetic head 11 are taken into 
consideration so that in the fifth embodiment the number is selected 12 as 
shown in FIGS. 16 and 19. Furthermore the period of the cogging torque is 
selected by selecting the value of .alpha.=1/2 by taking the 
above-mentioned angle .theta..sub.o into consideration. In view of the 
above, the angle .phi. between recesses 10a and 10b in this embodiment is 
determined as follows: 
EQU .phi..apprxeq.(2.pi./12)(5+1/2)=165 degrees. 
Next the number and arrangement of the driving coils will be described. 
Since the driving magnetic assembly 7 consists of 12 poles, in the case of 
the three-phase Hall motor, as many as nine driving coils can be used in 
priniciple, but in the fifth embodiment, the recesses 10a and 10b are 
provided so that as shown in FIG. 19, the space 15 corresponding to one 
driving coil and the space 16 corresponding to two driving coils are 
defined in order to prevent the interference between the recesses 10a and 
10b of the stator 10 and the driving coils. As a result, the whole number 
of six driving coils 91 and 92 are arranged in the form of two arcs which 
have the same center of a circle. In this case, it is seen that the spaces 
15 and 16 are in opposing relationship with each other and the recesses 
10a and 10b are positioned in the spaces 15 and 16, respectively. 
Furthermore, as shown in FIG. 19, 6 driving coils are divided into two 
types depending on the size and the effective length for producing torque. 
the driving coils 91U, 91V and 91W have a longer length capable of 
producing torque greater than that produced by the other type driving 
coils 92U, 92V and 92W. In order to dispose on the printed-circuit board 8 
the Hall elements 17a and 17b for detecting the phase of rotation of the 
driving magnet assembly 7 radially inwardly of the driving coils, the 
driving coils 92W and 92U have the shorter effective length. The driving 
coil 92V is substantially similar in size to the driving coils 92U and 92W 
so that the output torque obtained from the phase U,V and W becomes equal. 
The output torque in the U phase is the sum of torque produced by the 
coils 91U and 92U; the output torque by the V phase, the sum of the torque 
produced by the coils 91V and 92V; and the output torque by the W phase, 
the sum of the torque produced by the coils 91W and 92W. 
According to the construction of the spindle motor of the fifth embodiment 
described above, the cogging torque produced by the recess 10a of the 
stator 10 can be cancelled by the cogging torque produced by the recess 
10b as will be described in detail below with reference to FIGS. 20 and 
21. 
First, the FIG. 20 illustrates the positional relationship between the 
poles of the driving magnet assembly 7 and the recesses 10a and 10b of the 
stator 10. In FIGS. 16 and 17, the recesses 10a and 10b are shown as being 
angularly spaced about 165 degrees. This fact is same with the fact that 
they are angularly spaced apart from each other by .pi./12 (rad)=15 
degrees from the standpoint of the phase of rotation of the driving magnet 
assembly 7. 
When the driving magnet 7 rotates in the direction indicated by the arrow 
in FIG. 20, because of the existence of the recess 10a, the driving magnet 
assembly 7 is subjected to the cogging torque which varies in a manner 
substantially similar to the sine wave as indicated by the solid line a as 
shown in FIG. 21, in which the positive (+) torque acts in the direction 
of the rotation while the negative (-) torque acts in the direction 
opposite to the direction of rotation. The phase of rotation shown in FIG. 
20 is that at the point Q in FIG. 21. 
On the other hand, because of the presence of the recess 10b, the driving 
magnet assembly 7 receives the cogging torque which varies in a manner 
substantially similar to the sine wave as indicated by the broken line b. 
Both the cogging torque is naturally same in frequency and strength. From 
the standpoint of the phase of rotation of the driving magnet assembly 7, 
the recesses 10a and 10b are angularly spaced apart from each other by 15 
degrees, which is one half of the center angle 30 degrees of one pole of 
the driving magnet assembly 7 so that the phase of the cogging torque b is 
deviated from the phase of the cogging torque a by one half of the period 
of the torque variation. As is apparent from FIG. 21, the cogging torque a 
is cancelled by the cogging torque b. 
Next referring to FIGS. 24A, 24B and FIG. 25, operation according to the 
shape of the recess 10a of the stator 10 will be described and this 
description also applies to the recess 10b in common. FIG. 24A shows the 
present invention: that is, the stator 10 with the substantially 
rectangular recessed 10a and the driving magnet assembly 7 while FIG. 24B 
shows a comparative example; that is, a stator 10' having a fan-shaped 
recess 20a whose central angle is substantially equal to that of each pole 
of said driving magnet assembly. The direction and strength of the torque 
are determined by the positional relationship between the side edges 101, 
102, 201 and 202 of the recesses 10a and 20a extended in the outward 
direction of the disks 10 and 10', respectively, and the boundary lines 71 
and 72 of each pole of the driving magnet assembly 7. 
According to the fifth embodiment of the present invention, when the stator 
10 is viewed from a position on the line perpendicular to the surface of 
the stator 10 and when the side edges 101 and 102 of the recess 10a are in 
opposing relationship with the boundary lines 71 and 72 of the pole of the 
driving magnet assembly 7, they are not parallel with each other. As a 
result, the direction of the torque inside of the circle is opposite to 
the direction of the torque outside of the circle as shown by the arrows 
so that the cogging torque is cancelled by each other. 
On the other hand, in the case of the comparative example, as shown in FIG. 
24B, the side edges 201 and 202 of the recess 20a are substantially in 
parallel with the boundary lines 71 and 72 of the pole of the magnetic 
drive system 7. as a result, the directions of the torque inside and 
outside of the circle are same so that the torque strength is increased. 
FIG. 25 illustrates angle dependence characteristics of the cogging torque 
of the angle of rotation of the driving magnet assembly 7 of the present 
embodiment shown in FIG. 24A and the comparative example showing FIG. 24B, 
respectively. It is seen that the stator with the recess 10a in accordance 
with the present invention has the smaller cogging torque which is 
substantially similar the sine wave so that the angle dependence 
characteristics thereof makes it easy to cancel the cogging torque. 
As described above, the cogging torque is determined by the positional 
relationship between the side edges of the recess 10a and the boundary 
lines of each pole of the driving magnet assembly 7 so that when the width 
L of the recess 10a shown in FIG. 24A is selected properly, it becomes 
possible to obtain the cogging torque whose amplitude of oscillation is 
less and which is substantially similar in shape to a sine wave. The 
recess 10a is substantially in the form of a rectangle but the shape of 
the recess 10a may be an inverted shape of each pole of the driving magnet 
assembly 7. 
As described above, in the case of the spindle motor of the fifth 
embodiment of the present invention, the cogging torque can be cancelled 
or made negligible so that the torque ripples are decreased, the precision 
of rotation can be considerably improved and the starting torque can be 
increased. 
Furthermore, the angle .phi. between the recesses 10a and 10b of the stator 
10 is selected substantially equal or closer to 180 degrees so that the 
attractive forces of the driving magnet assembly 7 can be substantially 
balanced so that a couple of forces acting on the center cone 40 and the 
bearing 3 can be decreased. As a result, the clamping of the disk can be 
ensured. In addition, the pressure previously applied to the bearing 3 can 
be also balanced so that the degradation of the bearing 3 can be 
prevented. 
Moreover, the effective length of the driving coil is lengthened as long as 
practical and the two types of the driving coils different in size are so 
combined that the degree of torque at each phase can be made equal, 
whereby the output torque can be increased. 
As described above, the construction according to the fith embodiment of 
the present invention leaves some unbalanced attractive forces of the 
driving magnet assembly 7. In order to solve this problem, it is 
preferable to use the stator shown in FIGS. 12, 22 or 23. 
In the sixth embodiment, the provision of the recess 10a of the stator 10 
for securing the space in which the magnetic head can be displaced is 
substantially similar to those shown in the above described embodiments, 
but two more recesses 10c and 10d are defined in order to cancel the 
cogging torque. They are substantially in the form of an elongated 
rectangle extended in the radial direction and are same in depth with the 
recess 10a, but their width is one half L/2. The angle .phi. between the 
recesses 10a and 10b and the angle .phi.' between the recesses 10a and 10d 
are substantially equal to 165 degrees. 
With this construction, when the attractive force of the recess 10a due to 
the driving magnet assembly 7 is represented by Fa, the attractive forces 
Fc and Fd of the recesses 10c and 10d are expressed by the following 
equation: 
EQU Fc=Fd=(1/2)Fa. 
It follows therefore that the moment Fm of the attractive force of the 
driving magnet assembly 7 acting in the direction perpendicular to the 
stator 10 as the center point Q as a pivot point can be expressed by the 
following equation: 
EQU Fm=Fa-(FC+Fd)=0 
As a result, the attractive forces are balanced. 
As described above, according to the sixth embodiment of the present 
invention, the attractive force between the driving magnet assembly 7 and 
the stator 10 can be balanced so that the inclination of the spindle 5 and 
the bearing 3 due to the unbalanced attrative forces can be prevented. 
Next from the standpoint of the cogging torque, when the angle of rotation 
of the driving magnet assembly 7 is represented by .theta. (rad), the 
cogging torque Ta of the recess is expressed by the following equation: 
EQU Ta=.tau..sub.0 sin (12.theta.) 
where .tau..sub.0 is amplitude of cogging torque. The cogging torque Tc and 
Td of the recessed 10c and 10d, respectively, are represented by 
EQU Tc=(.tau..sub.0 /2) sin {12(.theta.+2.pi./12.times.5.5)}=-(.tau..sub.0 /2) 
sin (12.theta.), and 
EQU Td=(.tau..sub.0 /2 sin {12(.theta.-2.pi./12.times.5.5)}=-(.tau..sub.0 /2) 
sin (12.theta.) 
It follows therefore that the whole cogging torque Ttotal is expressed by 
##EQU1## 
Therefore the cogging torque can be cancelled. 
It is to be understood that the present invention is no limited to FDDs of 
the type described above and that it may be equally applied to other disk 
drive systems. 
The invention has been described in detail with respect to preferred 
embodiments, and it will now be apparent from the foregoing to those 
skilled in the art that changes and modifications may be made without 
departing from the invention in its broader aspects, and it is the 
invention, therefore, in the appended claims to cover all such changes and 
modifications as fall within the true spirit of the invention.