Dynamic magnetic information storage or retrieval – Head mounting – For shifting head between tracks
Reexamination Certificate
1999-05-17
2001-10-16
Klimowicz, William (Department: 2652)
Dynamic magnetic information storage or retrieval
Head mounting
For shifting head between tracks
Reexamination Certificate
active
06304421
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to voice coil motors for disc drives. More particularly, the present invention relates to a voice coil motor plate having a non-uniform cross section for increasing the magnetic flux capacity of the plate without increasing the size of the disc drive.
BACKGROUND OF THE INVENTION
Disc drives employ a variety of means for moving a head/arm assembly across a magnetic medium. Demands for increased performance and capacity have lead to the nearly exclusive use of rotary voice coil motor (“VCM”) actuators as the motive force for moving the head/arm assembly.
A rotary VCM typically includes a coil of wire positioned between opposing fixed magnetic structures. The fixed magnetic structures include one or more magnets connected to plates (also referred to as “poles” by those skilled in the art) which are fabricated from magnetically permeable material such as steel. The wire coil is shaped to include two opposing radial arms (
FIG. 1
) which allow direct current to pass in opposite directions through the radial arms. A magnetic field generated by the direct current passing through each radial arm interacts with the permanent magnetic field to apply a motive force to the radial arm. However, movement of the coil between the magnetic structures is limited so that each opposing arm experiences an opposite magnetic flux (FIG.
2
). The current flowing in opposite directions through the two arms cooperates with the opposite magnetic flux experienced by each radial arm to ensure that the motive force applied to the two radial arms is cumulative.
Sophisticated control logic applies a precise amount and polarity of direct current to the windings within the coil to controllably move the coil within the fixed magnetic field. The coil and the head/arm assembly of the disc drive are attached on opposite sides of a pivot shaft (
FIG. 1
) to move the head/arm assembly across the magnetic medium in response to movement of the coil. The speed with which the coil moves between the fixed magnetic structures of the VCM, and thus the speed or “access time” of the disc drive, depends on the torque capability of the VCM. This torque capability depends in turn on a number of factors including the size and strength of the permanent magnet, the number of windings contained within the coil, the amount of power applied to the coil and the size of the plates above and below the coil, among others.
FIG. 2
illustrates a schematic of a prior art VCM comprising a top plate
20
, a magnet
22
, a coil
24
(shown in section) and a bottom plate
26
. The magnet
22
is divided along a centerline
27
between North and South poles
28
and
30
, respectively, and flux lines
32
illustrate the magnetic circuit between the poles of the magnet
22
and the plates
20
and
26
. An air gap
34
between the magnet
22
and the bottom plate
26
allows for movement of the coil
24
within the fixed magnetic field. The total height dimension or vertical space allotted for the VCM (i.e., the distance between the top of the top plate
20
and the bottom of the bottom plate
26
) is commonly referred to as the “z-height” of the VCM.
The desire to increase the access time for disc drives conflicts with a further desire to reduce the size of such drives. Indeed, relatively small hard drives (e.g., drives approximately 1 cm high) are highly desirable for use with notebook or smaller-sized computers. Because the VCM typically fits within an outer casing of the disc drive, the z-height for the VCM of a small disc drive will be less than 1 cm. Thus, a number of compromises are typically necessary in the design of a VCM. For example, in order to increase the size (and thus the power) of both the magnet
22
and the coil
24
, the thickness of the top and bottom plates
20
and
26
may be reduced to the point where magnetic flux leaks outside of the plates
20
and
26
, as designated by the arrows
36
in FIG.
2
.
Flux leakage
36
occurs when the flux density within the plates
20
and
26
exceeds the maximum flux density for the particular material (e.g., a flux density of 18,000 Gauss for steel). Flux leakage
36
is highly undesirable since the leakage of the magnetic flux outside of the closed VCM circuit reduces the power of the VCM (thereby increasing the access time) while simultaneously interfering with the electronic circuitry and the magnetic medium within the disc drive. Because the magnetic flux within a VCM depends on the strength of the magnet
22
, the flux density within the plates
20
and
26
increases as the thickness of the plates decreases. Thus,
FIG. 2
illustrates the case where the thickness of the plates
20
and
26
is too small to handle the flux within the VCM.
The flux leakage can further be illustrated by plotting the flux density over the length or angle of the plates
20
and
26
. Due to the rotary nature of the VCM, both the plates
20
,
26
and the magnet
22
are curved to follow the arcuate path of the coil
24
.
FIG. 3
illustrates a prior art top plate
20
and magnet
22
and further defines an angle &THgr; determined by the arc of the magnet
22
.
FIG. 4
illustrates the linear increase of the flux density within the plate
20
from a point adjacent each end of the plate
20
to the middle of the plate (i.e., from ±&THgr;/2 to a 0° angle as shown in FIGS.
3
and
4
).
FIG. 4
further illustrates that the peak flux at the middle of the plate
20
exceeds the maximum flux density that can be accommodated by the steel plate
20
as designated by the horizontal line
38
. Thus,
FIG. 4
graphically illustrates the flux leakage
36
shown schematically in FIG.
2
.
One method of reducing flux leakage is to simply increase the cross-sectional area of the plates
20
and
26
. Due to the cramped conditions within the disc drive and other constraints on the shape of the plates, the cross-sectional area of the plates
20
and
26
is typically increased by increasing the height of the plates. For example, the height of the plates
20
and
26
in
FIG. 2
could be increased until the peak flux density in
FIG. 4
was lower than the maximum flux density of the steel plates
20
and
26
(i.e., below the dashed line
38
). However, as noted above, all the components of a VCM must fit within the maximum z-height allotted within the disc drive. Therefore, increasing the height of the plates
20
and
26
to reduce flux leakage necessitates a corresponding reduction in the size of the magnet
22
, the coil
24
, or both, with a resulting reduction in the power of the VCM. This tradeoff between the size of the different components within a VCM requires a careful optimization process to maximize the power of the VCM for a given z-height.
FIGS. 5 and 6
illustrate one prior art method of increasing VCM power by increasing the available z-height of the VCM. In essence, an opening
40
is formed in a top cover
42
of a prior art disc drive to allow the entire top plate
20
to protrude upward through the opening
40
. The z-height of the VCM is thus increased by the thickness of the top cover
20
, and a label or other adhesive covering (not shown) is then placed over the opening
40
to maintain the airtight seal within the disc drive. Although the top cover
42
is only approximately 0.5 millimeters thick, this small thickness can account for approximately a 7% increase in the effective z-height for small disc drives such as a 9.5 millimeter drive which has a nominal VCM z-height of 7.2 millimeters.
Because the opening
40
must be sufficiently large to accommodate the entire top plate
20
, the size of the opening
40
makes it difficult to maintain an airtight seal within the disc drive. First, due to the tight fit of the top plate
20
within the disc drive, at least one edge
44
of the opening
40
is formed immediately adjacent an edge
46
of the top cover
42
. The small surface area along the edge
46
reduces the structural integrity of the top cover
42
and can lead to deformation of the top cover
42
, particularly in light o
Klimowicz William
Merchant & Gould P,C,
Phillips John B.
Seagate Technology LLC
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