Disk drive having reduced magnetic flux leakage

In a voice coil motor (VCM) of a disk drive, a coined feature formed on a ferromagnetic plate reduces magnetic flux leakage from a disk drive without increasing the weight of the drive or adding complexity to the manufacturing process. The coined feature is shaped and positioned to alter the path of magnetic flux lines produced by the VCM in a way that significantly reduces the flux leakage from the VCM. The coined feature may be formed without adding material to the ferromagnetic plate by the same stamping process used to fabricate the plate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention relate generally to disk drives and, more particularly, to disk drives having reduced magnetic flux leakage.

2. Description of the Related Art

Disk drives, and especially disk drives for mobile applications, are tightly constrained to simultaneously achieve low cost, low weight, and low power consumption. As higher performance, e.g., shorter seek time, and greater data capacity are demanded for such drives, satisfying cost, weight, and power requirements becomes increasingly difficult. This is particularly true for the voice coil motor (VCM) used in disk drives to position the read/write head over a desired data storage track.

When shorter seek times are desired, or the inertia of the head stack assembly is increased to enable higher drive capacity, the VCM must have higher torque. Because increased power consumption is generally not acceptable, higher torque is typically achieved by increasing the magnetic flux density in the VCM. Magnetic flux density can be increased by using a higher energy magnet material and/or by increasing the volume of the magnets. In either case, without other design changes to the VCM, the increased magnetic flux in the VCM produces a higher level of magnetic flux leakage outside the disk drive, which may be unacceptable.

Increased flux leakage from the VCM and, consequently, from the disk drive, is typically controlled with a larger return path or by creating additional return paths for the magnetic flux. Steel plates generally form the return paths for magnetic flux in a disk drive VCM, and increasing the width and/or thickness of such plates in high flux density areas can control flux leakage when higher energy or higher volume magnets are used. However, the addition of material to high flux density areas in the steel plates in a VCM results in undesirable VCM weight increases and will usually increase cost of the VCM. To offset such weight increases, weight is removed in other areas, adding complexity to the stamping process that is typically employed for fabricating the steel plates. In addition, removal of material from other areas of the steel plates creates design compromises that may reduce design margin, increase cost, and limit performance of the disk drive.

In light of the above, there is a need in the art for a means to reduce magnetic flux leakage from a disk drive without increasing weight or adding complexity to the manufacturing process.

SUMMARY OF THE INVENTION

One or more embodiments of the invention provide a means for reducing magnetic flux leakage from a disk drive without increasing the weight of the drive or adding complexity to the manufacturing process. The embodiments contemplate a coined feature or bump formed in at least one ferromagnetic plate in a voice coil motor (VCM) of a disk drive, where the ferromagnetic plate provides magnetic flux return paths for the magnet or magnets of the VCM. The coined feature is shaped and positioned to alter the path of magnetic flux lines produced by the VCM in a way that significantly reduces the flux leakage from the VCM. The coined feature may be formed without adding material to the ferromagnetic plate by the same stamping process used to fabricate the plate.

A disk drive according to an embodiment of the invention includes at least one storage disk, at least one actuator arm assembly including a transducer element using which data is read from and written to the storage disk, and a voice coil motor for positioning the actuator arm assembly, wherein the voice coil motor includes at least one ferromagnetic plate on which at least one magnet is mounted and the at least one ferromagnetic plate has a coined section that: (i) has substantially the same thickness as other parts of the ferromagnetic plate, (ii) projects away from the at least one magnet, and (iii) is positioned near a neutral zone of the at least one magnet. The neutral zone is the transition between regions of different magnetic polarity.

A voice coil motor for a disk drive according to an embodiment of the invention includes a first ferromagnetic plate on which a magnet is mounted, and a second ferromagnetic plate on which a magnet is mounted, wherein the first ferromagnetic plate has a stamped section that: (i) has substantially the same thickness as other parts of the first ferromagnetic plate, (ii) projects away from the magnets, and (iii) is positioned near a neutral zone of the magnets.

A voice coil motor for a disk drive according to another embodiment of the invention includes at least one magnet mounted between a pair of ferromagnetic plates, one of the ferromagnetic plates having a stamped section formed therein proximate the neutral zone of the at least one magnet, wherein the stamped section is configured to project away from the at least one magnet and add minimal reluctance to the magnetic flux flow.

DETAILED DESCRIPTION

Embodiments of the invention provide a means for reducing magnetic flux leakage from a disk drive without increasing the weight of the drive or adding complexity to the manufacturing process. The embodiments contemplate a coined feature or bump formed in a ferromagnetic plate in a voice coil motor (VCM) of a disk drive, where the ferromagnetic plate provides magnetic flux return paths for the magnet or magnets of the VCM. The coined feature is shaped and positioned to alter the path of magnetic flux lines produced by the VCM in a way that significantly reduces the flux leakage from the VCM, while having a minimal effect on VCM performance.

The coined feature is disposed in a region of the ferromagnetic plate that is saturated with magnetic flux and/or is closer to saturation with respect to other regions of the plate. In addition, the coined feature projects from the plane of the ferromagnetic plate away from the magnets of the VCM, and includes at least one stamped edge that alters the path of magnetic flux lines produced by the VCM in a way that significantly reduces the flux leakage from the VCM. In one embodiment, the stamped edge is substantially perpendicular to the plane of the ferromagnetic plate and substantially parallel to the primary lines of flow of magnetic flux in a high flux density region of the plate. When so oriented, the stamped edge or edges of the coined feature discourage magnetic flux lines from bunching or concentrating in the high flux density region of the ferromagnetic plate, thereby distributing the flux lines more evenly throughout the plate. In other embodiments, the coined feature may have other shapes (e.g., very sharp transitions) that add little or no reluctance in the direction of the primary lines of flow of magnetic flux and provide substantially more reluctance in the direction perpendicular to the primary lines of flow of magnetic flux. The coined feature may be formed without adding material to the ferromagnetic plate and by the same stamping process used to fabricate the plate. Consequently, the coined feature as described herein adds no weight to the disk drive or complexity to the manufacturing process, while significantly reducing magnetic flux leakage from the disk drive.

FIG. 1is a plan view of a disk drive100according to an embodiment of the invention. For clarity, disk drive100is illustrated without a top cover. Disk drive100includes a base104, an actuator arm assembly116and a magnetic disk108. For clarity of description, disk drive100is illustrated with a single storage disk108and actuator arm assembly116. Disk drive100may, however, also include multiple storage disks108and multiple actuator arm assemblies116. Storage disk108is interconnected to base104by a spindle motor (not shown) mounted within or beneath a hub112, such that storage disk108can be rotated relative to base104. Actuator arm assembly116is interconnected to base104by a bearing120and includes a transducer element124constructed on actuator arm assembly116as shown. Transducer element124includes a read element and a write element and transfers data to and from a surface of storage disk108. A VCM128pivots the actuator arm assembly116about bearing120to radially position transducer element124with respect to storage disk108. By changing the radial position of the transducer element124with respect to storage disk108, transducer element124accesses different tracks132on storage disk108. VCM128is operated by a controller136that is, in turn, operatively coupled to a host computer (not shown). A channel140processes information read from storage disk108by transducer element124, and a servo control system144controls the position of transducer element124with respect to the particular track132being followed.

FIG. 2is a partial plan view of VCM128, which includes an upper ferromagnetic plate201, a curved upper magnet202, a lower ferromagnetic plate203(shown inFIGS. 3A,3B), and a curved lower magnet204(also shown inFIGS. 3A,3B), according to an embodiment of the invention. Curved upper magnet202is mounted on the lower, or hidden, surface of upper ferromagnetic plate201, and curved lower magnet204is mounted on the upper surface of lower ferromagnetic plate203. Curved upper magnet202is magnetized to have two sections of opposite polarity, denoted N and S, the transition between the sections being the neutral zone212. Alternately, magnet202can be made of two separate pieces positioned with the polarities opposite. Curved lower magnet204is similarly configured, with two sections of opposite polarity and a neutral zone214, which is illustrated inFIGS. 3A and 3B.

FIGS. 3A and 3Bare partial cross-sectional views of two types of VCM design in which one or more embodiments of the invention can be implemented.FIG. 2fairly represents the partial plan views of both types of VCM design.

FIG. 3Ais a partial cross-sectional view of a first type of VCM128taken at section line A-A inFIG. 2, illustrating the relative positions of upper ferromagnetic plate201, curved upper magnet202, lower ferromagnetic plate203, and curved lower magnet204. With the magnetic poles oriented as shown inFIG. 3A, magnetic flux lines301,302flow in a loop, with the highest flux density usually being in the area of neutral zone212and/or neutral zone214. An electrical coil (not shown for clarity) is disposed in the air gap303and is used to generate torque for VCM128. Upper ferromagnetic plate201and lower ferromagnetic plate203are comprised of a ferromagnetic material, such as low carbon steel. Because ferromagnetic materials have several orders of magnitude greater permeability than the surrounding air, the majority of magnetic flux lines flow through the material of upper ferromagnetic plate201and lower ferromagnetic plate203. However, as the material of upper ferromagnetic plate201and lower ferromagnetic plate203approach saturation, for example, when a VCM design is modified to include a higher energy magnet, magnetic flux is conducted less efficiently, resulting in greater magnetic flux leakage from VCM128. According to an embodiment, a coined feature250, as shown inFIG. 2, is disposed on the top surface of upper ferromagnetic plate201and/or the bottom surface of lower ferromagnetic plate203to alter the magnetic flux flow in upper ferromagnetic plate201, and is described in greater detail below.

FIG. 3Bis a partial cross-sectional view of a second type of VCM128taken at section line A-A inFIG. 2. The configuration and relative positions of upper ferromagnetic plate201, curved upper magnet202, lower ferromagnetic plate203, and curved lower magnet204are substantially similar to those illustrated inFIG. 3A, except that upper ferromagnetic plate201and lower ferromagnetic plate203are joined by vertical legs310. Vertical legs310are also comprised of ferromagnetic material and are often incorporated as features of one or both of the ferromagnetic plates201and203. They are used to create multiple flux loops320in VCM128as shown. Multiple flux loops320reduce the total magnetic flux through the neutral zone, which in turn reduces the flux density of upper ferromagnetic plate201and lower ferromagnetic plate203. One of skill in the art will appreciate that even when configured with vertical legs310, upper ferromagnetic plate201and lower ferromagnetic plate203may develop high flux density regions and produce unacceptable levels of magnetic flux leakage unless modified with coined feature250.

Referring back toFIG. 2, coined feature250is disposed in a region of upper ferromagnetic plate201that is saturated with magnetic flux and/or is closer to saturation with respect to other regions of the plate. One of skill in the art will appreciate that such regions are frequently located adjacent neutral zone212and offset toward the inner radius221of curved upper magnet202. This is because the lines of magnetic flux follow the path of least reluctance, which for curved upper magnet202is primarily through upper ferromagnetic plate201, rather than through the surrounding air. And since the path of least reluctance also tends towards the shortest path, the flux density in ferromagnetic plate201adjacent neutral zone212is not uniform across the cross-section of upper ferromagnetic plate201, and is instead concentrated at a region closer to inner radius221of curved upper magnet202than to outer radius222. One of skill in the art, using conventional methods of modeling and direct measurement, can readily determine an optimal location for coined feature250on upper ferromagnetic plate201.

Coined feature250projects from the plane of upper ferromagnetic plate201away from the magnets of the VCM, and includes at least one stamped edge251that is shaped to provide substantially more reluctance to magnetic flux flow in direction402compared to the direction of the primary lines of flow of magnetic flux. In one embodiment, stamped edge251has walls that are substantially perpendicular to the plane of the ferromagnetic plate and extend substantially parallel to the primary lines of flow of magnetic flux. For example, coined feature250is configured with a rectangular footprint, as depicted inFIG. 2, and consequently has two substantially parallel stamped edges,251and252. Stamped edges251,252alter magnetic flux flow in a way that reduces the flux density of upper ferromagnetic plate201in the region proximate coined feature250. Namely, by increasing reluctance in upper ferromagnetic plate201to any components of magnetic flux perpendicular to the main flow, i.e., components of magnetic flux parallel to neutral zone212, the magnetic flux lines contained in upper ferromagnetic plate201are discouraged from concentrating asymmetrically therein and are more evenly distributed throughout upper ferromagnetic plate201. In this way, the flux density of “hot spots” in upper ferromagnetic plate201is significantly reduced. Optimal configurations of coined feature250are described below in conjunction withFIGS. 4,5, and6A-C.

Because the embodiments of coined feature250described herein may be formed in upper ferromagnetic plate201as part of a conventional stamping process used to fabricate upper ferromagnetic plate201, the complexity and cost of the stamping process is not affected. Because no material is added to upper ferromagnetic plate201, the weight of VCM128is not increased and performance thereof is not adversely affected by the addition of coined feature250.

Depending on the specific configuration of disk drive100, high flux density areas in upper ferromagnetic plate201, lower ferromagnetic plate203, or both may produce unacceptable levels of magnetic flux leakage. In one embodiment of the invention, coined feature250may be disposed on lower ferromagnetic plate203instead of upper ferromagnetic plate201. In yet another embodiment, both lower ferromagnetic plate203and upper ferromagnetic plate201may have coined feature250disposed thereon to alter magnetic flux flow in a way that reduces flux density. In yet another embodiment, multiple coined features250may be formed in upper ferromagnetic plate201, lower ferromagnetic plate203, or both. In such an embodiment, the coined features may be formed proximate each other, in which case the stamped edges of each may be oriented substantially parallel to each other. Alternatively, the multiple coined features250may be formed in substantially distal regions of a given ferromagnetic plate, in which case the orientation of each stamped feature may vary.

One of skill in the art will appreciate that the above-described embodiments of the invention may be useful for other types of VCM design than those illustrated inFIGS. 3A,3B, such as a single magnet VCM design.FIG. 3Cis a partial cross-sectional view of a single magnet VCM350that may benefit from one or more coined features250. As shown, single magnet VCM350includes upper ferromagnetic plate201, lower ferromagnetic plate203, and a single magnet351. In one embodiment, a coined feature250is located on the ferromagnetic plate that does not have a magnet mounted thereto, i.e., upper ferromagnetic plate201, and is positioned to reduce the flux density of hot spots in upper ferromagnetic plate201. For example, coined feature250may be located opposite the neutral zone314of single magnet351, as shown.

As noted above in conjunction withFIG. 2, embodiments of the invention contemplate that coined feature250may be configured to provide increased reluctance in a ferromagnetic plate of VCM128to any components of magnetic flux perpendicular to the main flow without adding significant reluctance to components of magnetic flux parallel to the main flow.FIG. 4is a partial plan view of a ferromagnetic plate400having a coined feature250that meets these requirements, according to embodiments of the invention. Coined feature250may have a substantially rectangular footprint with two substantially parallel stamped edges,251and252and two ends261,262, as shown. Stamped edges251,252are illustrated inFIG. 5and different embodiments of ends261,262are illustrated inFIGS. 6A-C.

FIG. 5is a partial cross-sectional view of coined feature250taken at section line B-B inFIG. 4illustrating one configuration of stamped edges251,252. As noted above in conjunction withFIG. 2, when coined feature250is disposed proximate neutral zone212or214, stamped edges251,252may be oriented substantially parallel to the primary flow lines of magnetic flux and perpendicular to neutral zone212or214. In addition, stamped edges251,252are substantially perpendicular to the plane of the ferromagnetic plate, i.e., surface401, to more effectively add reluctance to magnetic flux that is parallel to arrow402. Modeling by the inventor indicates that the optimum depth D of coined feature250is generally less than half of thickness T of the ferromagnetic plate. When depth D is significantly thicker than half of thickness T, magnetic flux leakage in the high flux density region surrounding coined feature250has been shown to increase.

FIGS. 6A-Care partial cross-sectional views of alternative configurations of a coined feature implemented on a ferromagnetic plate of a VCM. InFIG. 6A, coined feature250is a smoothly curving arch in cross section. InFIG. 6B, coined feature250has a single point in cross section, formed from a single stamped bend. InFIG. 6C, coined feature250is formed from two parallel bends256,257, and is substantially trapezoidal in cross-section. AlthoughFIGS. 6A-Cshow sharp corners, there will typically be a radius at one or more of the transitions. Modeling has indicated that the embodiments of coined feature250illustrated inFIGS. 6A-Care effective at reducing magnetic flux leakage from VCM128. For a particular VCM with a ferromagnetic plate 1.2 mm thick, the embodiment of coined feature250illustrated inFIG. 6Ahaving bump displacement271of 0.5 mm is predicted to reduce peak magnetic flux leakage from VCM128from about 380 gauss to about 290 gauss. For the same plate thickness and bump displacement, the embodiment of coined feature250illustrated inFIG. 6Bis predicted to reduce peak magnetic flux leakage from VCM128from about 380 gauss to about 300 gauss. For the same plate thickness and bump displacement, the embodiment of coined feature250illustrated inFIG. 6Cis predicted to reduce peak magnetic flux leakage from VCM128from about 380 gauss to about 270 gauss.

Other configurations of coined feature250are also contemplated; however the effect of coined feature250on magnetic flux leakage is enhanced when ends261,262of coined feature250do not have the “step-like” vertical offset characteristic of stamped edges251,252, as illustrated inFIG. 5. Instead, it is preferred that ends261,262smoothly transition into and out of the plane formed by surface401, where surface401is the surface of ferromagnetic plate400in contact with a magnet, e.g., curved upper magnet202or curved lower magnet204. In this way, little or no additional reluctance to magnetic flux parallel to the primary lines of flow of magnetic flux is introduced by coined feature250. Because no material is added to the ferromagnetic plate during the stamping process used to fabricate coined feature250, the thickness255of coined feature250is substantially equal to thickness T of the ferromagnetic plate into which coined feature250is formed.