Disc drive spindle motor having tuned stator with adhesive grooves

A disc drive spindle motor includes a central axis, a stationary member, a rotatable member which is rotatable with respect to the stationary member and a bearing interconnecting the rotatable member with the stationary member. A data storage disc is attached to the rotatable member. A stator is supported by the stationary member at an interface for rotating the rotatable member about the central axis with a driving force frequency. The stator has a resonant vibrational frequency. The interface has a plurality of recesses which are spaced axially from one another with respect to the central axis. An adhesive is applied within a selected set of the plurality of recesses and forms a bond between the stator and the stationary member. The set of recesses is selected such that the resonant vibrational frequency is different than the driving force frequency.

BACKGROUND OF THE INVENTION 
The present invention relates to disc drive spindle motors and, more 
particularly, to a spindle motor having a tuned stator resonant frequency. 
Disc drive data storage devices, known as "Winchester" type disc drives, 
are well known in the industry. In a Winchester disc drive, digital data 
is written to and read from a thin layer magnetizable material on the 
surface of a rotating disc. Write and read operations are performed 
through a transducer which is carried on a slider body. The slider and the 
transducer are sometimes collectively referred to as a head, and typically 
a single head is associated with each disc surface. The heads are 
selectively moved under control of electronic circuitry to any one of a 
plurality of circular, concentric data tracks on the disc surface by an 
actuator device. In the current generation of disc drive products, the 
most commonly used type of actuator is a rotary moving coil actuator. 
The discs are typically mounted in a "stack" on the hub structure of a 
brushless DC spindle motor. The rotational speed of the spindle motor is 
precisely controlled by motor drive circuitry which controls both the 
timing and the power of commutation pulses directed to the stator windings 
of the motor. Traditional spindle motor speeds have been in the range of 
3,600 RPM. Current technology has increased spindle motor speeds to 10,000 
RPM and above. 
Analysis of the various types of disc drives has brought to light several 
different modes of acoustic noise generation which are attributable to the 
spindle motor and its control logic. One mode of noise generation is 
sympathetic vibration of the disc drive housing in response to the 
rotating mass of the spindle motor. Another mode of acoustic noise 
generation is electromagnetic disturbances caused by the excitation of the 
stator mass by the application and removal of the commutation pulses that 
are used to drive the motor and control its speed. The commutation pulses 
are timed, polarization-selected DC current pulses which are directed to 
sequentially selected stator windings. The rapid rise and fall times of 
these pulses act as a striking force and set up sympathetic vibrations in 
the stator structure. Interaction of resonant vibrational frequencies of 
the stator and its support structure with the fundamental forcing 
frequencies of the commutation pulses and their harmonics is a well known 
contributor to disc drive acoustic noise, and especially pure tone 
vibrations. 
Several attempts have been made to tune the stator resonant frequency away 
from the fundamental forcing frequencies and their harmonics. For example, 
features have been machined into the spindle shaft of an in-hub spindle 
motor or into the stator mounting boss of an under-the-hub spindle motor. 
These "designed-in" machined features are effective at tuning the stator 
torsional resonant frequency, but cannot easily account for natural part 
variations associated with high volume manufacturing of spindle motor 
components such as shafts, stators and motor bases. 
Improved methods of tuning the stator resonant vibrational frequency in 
high volume manufacturing processes are desired. 
SUMMARY OF THE INVENTION 
One aspect of the present invention relates to a method of mounting a 
stator, which has a resonant vibrational frequency, to a stationary member 
in a disc drive spindle motor. The method includes: providing a plurality 
of recesses in an interface between the stator and the stationary member; 
selecting a set of the plurality of recesses as a function of the resonant 
vibrational frequency; and placing an adhesive in the selected set of 
recesses to form a bond between the stator and the stationary member. 
Another aspect of the present invention relates to a method of mounting the 
stator to the stationary member, which includes bonding the stator to the 
stationary member with a selected bond geometry such that the stator and 
the stationary member have a selected vibrational resonant frequency. 
Another aspect of the present invention relates to a disc drive spindle 
motor. The spindle motor includes a central axis, a stationary member, a 
rotatable member which is rotatable with respect to the e stationary 
member and a bearing interconnecting the rotatable member with the 
stationary member. A data storage disc is attached to the rotatable 
member. A stator is supported by the stationary member at an interface. 
The stator rotates the rotatable member about the central axis with a 
driving force frequency and has a resonant vibrational frequency. The 
interface has a plurality of recesses which are spaced axially from one 
another with respect to the central axis. An adhesive is applied within a 
selected set of the plurality of recesses and forms a bond between the 
stator and the stationary member. The set of recesses is selected such 
that the resonant vibrational frequency is different than the driving 
force frequency.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention is a disc drive data storage device having a spindle 
motor with selectively-filled adhesive grooves between the stator and its 
supporting member for tuning the torsional resonant frequency of the 
stator. FIG. 1 is a perspective view of a typical disc drive 10 in which 
the present invention is useful. Disc drive 10 includes a housing with a 
base 12 and a top cover (not shown) . Disc drive 10 further includes a 
disc pack 16, which is mounted on a spindle motor 17 by a disc clamp 18. 
Disc pack 16 includes a plurality of individual discs which are mounted 
for co-rotation about a central axis 20. Each disc surface has an 
associated disc head slider 22 which is mounted to disc drive 10 for 
communication with the disc surface. In the example shown in FIG. 1, 
sliders 22 are supported by suspensions 24 which are in turn attached to 
track accessing arms 26 of an actuator 28. The actuator 28 shown in FIG. 1 
is of the type known as a rotary moving coil actuator and includes a voice 
coil motor (VCM), shown generally at 30. Voice coil motor 30 rotates 
actuator 28 with its attached sliders 22 about a pivot shaft 32 to 
position sliders 22 over a desired data track along arcuate path 34. 
FIG. 2 is a sectional view of spindle motor 17. Spindle motor 17 includes 
shaft 40, hub 42, bearings 44 and 46, and stator 48. In the embodiment 
shown in FIG. 2, shaft 40 is a non-rotating shaft which is fixed with 
respect to base 12. Shaft 40 defines central axis 20. Hub 42 is 
interconnected with shaft 40 through bearings 44 and 46 for rotation about 
shaft 40. Bearings 44 and 46 are ball bearing assemblies having inner 
races which are attached to the outer diameter of shaft 40 and outer races 
which are attached to the inner diameter of hub 42. Hub 42 includes a 
lower disc carrier member 50 which extends from the outer diameter of the 
hub for supporting disc pack 16. Disc pack 16 includes a plurality of 
individual discs 52A-52K. Disc pack 16 is held on disc carrier member 50 
by disc clamp 18 (shown in FIG. 1). 
Hub 42 carries a permanent magnet 60 on its inner diameter, which acts as a 
rotor for spindle motor 17. Rotor magnet 60 is magnetized to form one or 
more magnetic poles. Rotor magnet 60 can be formed as a unitary, annular 
ring, or can be formed of a plurality of individual magnets which are 
spaced about the inner circumference of hub 42. Hub 42 includes a 
back-iron 42A, which is formed of a magnetic material, for rotor magnet 
60. 
Stator 48 is supported by stationary member 70 which is in the form of a 
cylindrical mounting boss which extends vertically from base 12 along 
shaft 40. In an alternative embodiment, stator 48 is supported directly by 
shaft 40. Stator 48 includes a stator lamination 72 and a stator winding 
74. Stator lamination 72 and stator winding 74 are spaced radially inward 
of rotor magnet 60 to allow rotor magnet 60 and hub 42 to rotate about 
central axis 20. Commutation pulses applied to stator winding 74 generate 
a rotating magnetic field which communicates with rotor magnet 60 and 
causes rotor magnet 60 and hub 42 to rotate about central axis 20 on 
bearings 44 and 46. The commutation pulses are timed, 
polarization-selected DC current pulses which are directed to sequentially 
selected stator windings to drive rotor magnet 60 and control its speed. 
Stator 48 has various modes of vibration, such as torsional, rocking, 
flapping, axial and radial modes of vibration. The frequency of the 
commutation pulses applied to stator windings 74 is an excitor of these 
modes of vibration, and in particular torsional vibration. Interaction of 
the resonant vibrational frequency of stator 48 with the commutation pulse 
frequencies and their harmonics, contributes to disc drive acoustic noise 
and pure tone vibrations. Acoustic noise and pure tone vibrations are 
reduced with the present invention by modifying interface 76 between 
stator 48 and stationary member 70 to adjust the stator's resonant 
vibrational frequency, such as the resonant torsional frequency, away from 
the fundamental forcing frequencies and their harmonics. 
Interface 76 is shown in greater detail in FIG. 3. Interface 76 is formed 
by inner diameter surface 80 of stator lamination 72 and outer diameter 
surface 82 of stationary member 70 and includes a plurality of 
circumferential grooves 84A-84D which are arranged perpendicular to 
central axis 20 and are spaced axially from one another along central axis 
20. In the embodiment shown in FIGS. 2 and 3, grooves 84A-84D are machined 
into outer diameter surface 82 of stationary member 70. In another 
embodiment, grooves 84A-84D are machined into inner diameter surface 80 of 
stator lamination 72. 
A selected set of the grooves 84A-74D, such as grooves 84A and 84C, are 
filled with an adhesive 86 to form a bond between stator 48 and stationary 
member 70. The remaining grooves 84B and 84D are left free of adhesive. 
For a particular one of the grooves 84A-84D to participate in the bond 
between stator 48 and stationary member 70, the groove must be filled with 
an adhesive. The grooves that are filled with an adhesive are selected as 
a function of the stator's resonant vibrational frequency, such as the 
torsional resonant frequency, to move the resonant frequency away from the 
driving force frequencies of the commutation pulses that are applied to 
the stator and their harmonics. Varying the selection of grooves varies 
the spacing and number of the bonds between stator 48 and stationary 
member 70, which varies the stiffness of the stationary member as a spring 
relative to the various modes of vibration. By varying the selection of 
grooves that are filled with adhesive, the stator's resonant vibrational 
frequency can be tuned away from the driving force frequencies and their 
harmonics. 
In a high volume manufacturing process, each disc drive is manufactured 
with the same grooves. Normal variations in the tolerances of individual 
parts within each disc drive may result in variations in the resonant 
vibrational frequency of each disc drive or each manufacturing lot of disc 
drives. These resonant vibrational frequencies can be tuned away from the 
driving force frequencies on a drive-by-drive basis or lot-by-lot basis by 
selecting the grooves to be filled with adhesive without changing the 
design or geometry of individual parts in the disc drive. 
FIG. 4 is a sectional view of a spindle motor 110 having a fixed-shaft, 
"in-hub" stator configuration. Spindle motor 110 includes fixed shaft 112, 
hub 114, bearings 116 and 118 and stator 120. Shaft 112 is supported by 
and extends from housing base 122 and defines a central axis 124. Hub 114 
is interconnected with shaft 112 through bearings 116 and 118. Hub 114 
includes disc carrier member 126 for supporting disc pack 128. Magnet 130 
is attached to the inner diameter of hub 114 and acts as a rotor for 
spindle motor 110. 
Stator 120 is attached to the outer diameter of shaft 112 and is positioned 
within an internal cavity of hub 114, between bearings 116 and 118. Shaft 
112 acts as a stationary member for supporting stator 120. Stator 120 
includes stator laminations 132 and stator windings 134. 
A plurality of grooves 140A, 140B, 140C and 14D are machined into the outer 
diameter surface of shaft 112, along the inner diameter of stator 
laminations 132. Grooves 140A-140D are selectively filled with adhesive 
150 as a function of the resonant vibrational frequency of stator 120 for 
forming a bond between shaft 112 and stator 120 and for tuning the 
resonant vibrational frequency away from the stator driving frequencies, 
according to the present invention. 
FIG. 5 is a sectional view of a disc drive spindle motor 200 having an 
in-hub, rotating shaft configuration, according to another alternative 
embodiment of the present invention. Spindle motor 200 includes rotating 
shaft 202, hub 204, stationary sleeve 206 and stator 208. Hub 204 is 
supported by rotating shaft 202 and supports a magnet 210 at an inner 
diameter 220. Stationary sleeve 206 is supported by base 216 and is 
interconnected with rotating shaft 202 through bearings 212 and 214. 
Stator 208 includes stator laminations 222 and stator windings 224. The 
inner diameter of stator laminations 222 is supported by the outer 
diameter of stationary sleeve 206. 
In the embodiment shown in FIG. 5, grooves 230A-230F are machined into the 
inner diameter surface of stator laminations 222 at the interface with 
stationary sleeve 206. Grooves 230A-230F are selectively filled with an 
adhesive 232 for adjusting the stator's resonant vibrational frequency to 
a selected resonant vibrational frequency, as discussed above. 
FIG. 6 is a flow chart of a method of mounting a stator to a stationary 
member in a disc drive spindle motor according to one embodiment of the 
present invention. At step 300, a plurality of recesses are formed in the 
interface between the stator and the stationary member. The recesses can 
be formed in the stator, the stationary member or both the stator and the 
stationary member. The recesses can have a variety of shapes, such as a 
plurality circumferential, annular grooves or spaced depressions which can 
be arranged in a variety of geometries. At step 302, a set of the 
plurality of recesses are selected as a function of the resonant 
vibrational frequency of the stator. At step 304, an adhesive is placed in 
the selected set of recesses to form a bond between the stator and the 
stationary member. 
FIG. 7 is a flow chart of a method of tuning the stator resonant 
vibrational frequency in a high volume manufacturing process. At step 400, 
a first of a plurality of spindle motors is assembled from a given lot of 
manufacturing parts. Each spindle motor has a stator which is supported by 
a stationary member at an interface having a plurality of adhesive 
recesses. The geometric pattern of the plurality of recesses is the same 
in each spindle motor. At step 402, an adhesive is placed in a selected 
first set of the plurality of recesses in the first spindle motor to form 
a bond between the stator and the stationary member. At step 404, the 
resonant vibrational frequency of the stator in the first spindle motor is 
measured. The resonant vibrational frequency can be measured by exciting 
the stator windings of the assembled motor with white noise or a 
sinusoidal sweep of commutation frequencies to induce vibration in the 
stator. The frequency response of the stator vibration can then measured 
over a broad range of excitation frequencies with accelerometers, 
laser-based sensors, or other non-contact displacement probes, for 
example. The resonant vibrational frequency of the stator is determined 
from the measured frequency response and compared with the expected 
driving force frequencies of the stator and their harmonics. 
At step 406, a second set of the plurality of recesses is selected as a 
function of the measured vibrational frequency. The second set of recesses 
is selected such that the resonant vibrational frequency is different than 
the driving force frequencies of the stator and their harmonics. The 
remainder of the spindle motors of the given manufacturing lot are then 
assembled, at step 408, with adhesive being placed in the second set of 
recesses. 
Although the present invention has been described with reference to 
preferred embodiments, workers skilled in the art will recognize that 
changes may be made in form and detail without departing from the spirit 
and scope of the invention. For example, the adhesive grooves can have 
various configurations. The spindle motor can have a fixed shaft or a 
rotating shaft. The stator can be positioned at various locations along 
the central axis, such as within the hub or below the hub. The stator can 
have a radial position which is either internal to the hub or external to 
the hub. The bearing between the stationary member and the hub be a ball 
bearing assembly or a hydrodynamic bearing assembly.