1. Field of the Invention
The present application relates to disk drive devices in general, and in particular to an architecture for low-profile rigid disk drive devices.
2. Description of the Related Art
Rigid (hard) disk drive devices are well-known. Rigid disk drives are commonly comprised of one or more rigid information bearing disks fixedly mounted to a spindle motor, an actuator having a plurality of arms upon which are placed read/write heads for reading from or writing to the rigid disks, and associated control circuitry. The term "head-disk assembly," or HDA, as used below refers to the spindle motor, rigid disk combination, along with the read/write heads disposed between and adjacent the rigid disks. The term "low-profile" refers to the thickness of the complete disk drive package measured perpendicular to the diameter of the rigid disks.
The current trend in rigid disk drive design technology is toward devices having a low-profile package and containing disks having increasingly smaller outer diameters. This trend is fueled by the demand for smaller palmtop or laptop computers. As each generation of successively smaller disk drives is produced, the design architectures employed in preceding generations are modified to resolve design features which impede miniaturization.
Rigid disk drive devices currently under development contain rigid magnetic disks having outer diameters of 1.8 inches or less. Rigid disks of this size are designed with center holes, used to mount the disks to the spindle, which are as small as 10 mm in order to maximize the amount of disk surface area used for information storage. As a result, information storage is performed at increasingly smaller inner diameters of the disk. Most disk drive devices are operated at constant angular velocity, typically 3600 RPM. Consequently, the rate of change of magnetic flux caused by the magnetized areas on the magnetic disk decreases at the inner information tracks, thereby reducing the read signal. However, noise pickup from the spindle motor occurs undiminished, and may increase, as the head moves inward toward the center of the disk. Therefore, the increased noise acts to reduce a ratio between the read signal and noise even further, such that the signal to noise ratio reaches unacceptable levels.
An in-hub motor architecture may be used in low-profile disk drive devices incorporating rigid disks having an outer diameter of 2.5 inches or larger (FIG. 10). The in-hub motor architecture locates the spindle motor 1010 inside an inner diameter of the rigid disks 1020 between the spindle bearings 1025. The spindle motor 1010 is sized to fit within the hole defined in the rigid disks 1020, the size of the hole typically being fixed by standard industry practice.
An advantage of the in-hub motor architecture is that the overall thickness of the HDA can be reduced because the motor 1010 is disposed between the rigid disks 1020, as opposed to below the rigid disks. However, the spindle motor 1020 cannot be reduced to fit within the 10 mm hole of a 1.8 inch (or less) rigid disk. First, the hole diameter provides insufficient space for a rotor and stator. Additionally, reduction of the diameter of the in-hub motor is limited by the dimensions of the spindle bearings. The spindle bearings required to stabilize the rotational run-out of the recording disks have an irreducible height, even when used as a cartridge containing a pair of pre-loaded bearings. Moreover, reducing the size of the bearings results in a motor that is too fragile for use in portable equipment. In addition, reduction of the motor diameter reduces the motor volume, thereby producing a motor with insufficient torque to operate the disk drive. Therefore, the in-hub architecture is inadequate for disk drive devices incorporating disks having an outer diameter of 1.8 inches or less.
A second prior art architecture incorporates a low-profile brushless DC motor 710 below a disk 720, as shown in FIG. 7. A pair of read/write heads 730 and 731 are disposed on arms 748 of an actuator 745 (shown in FIG. 8) which positions the read/write heads adjacent an upper surface 722 and lower surface 721 of the disk 720.
The motor 710 comprises a housing 711 in which is disposed a stator comprised of magnetic laminations 713 and coil windings 714. A rotor 715 is press fitted to a motor shaft 716 which is rotatably disposed in a central portion of the motor 710 by means of bearings 760 on an outer portion of the rotor are disposed rotor magnets 718. A clamp 719 holds the disk 720 to a mounting surface 727, both the clamp 719 and mounting surface 727 being carefully machined to provide a uniform clamping force on the disk 720. A rotor/stator gap 745 is located between the outer portion of the rotor and the housing 711. Disposed on an outer portion of the housing 711 is a ferrite shield 740. The ferrite shield 740 acts to protect the head 730 from magnetic flux produced by the motor 710. A space t separates the lower surface 721 of the disk 720 and an upper surface of the ferrite shield. The space t, typically 1.5 mm, is the necessary distance between the ferrite shield 740 and the disk 720 to allow access of the head 730.
The benefit of the low-profile architecture is that small diameter disks may be used without sacrificing motor size. However, when the read/write head 730 is disposed near the rotor/stator gap 745 between the spindle motor 710 and the lower data surface 721, the read/write head 730 experiences severe electromagnetic interference from the commutating pulses emanating from the motor 710. The interference results because of a space separating the stationary ferrite shield 740 from the rotating rotor 715. The small space between the ferrite shield 740 and the rotor 715 allows magnetic flux to create interference in the vicinity of the rotor/stator gap 745. This interference has been found to cause the playback signal to contain errors, rendering invalid retrieval of stored information or servo signals, at times making these signals impossible to recover.
One solution to this problem has been to restrict the access of the read/write head 730 to storage tracks of the disk 720 which are fully protected by the ferrite shield 740. This solution obviously reduces the amount of disk area available for information storage by restricting the accessed disk area to the more outer diameters of the lower surface 721. In addition, present disk drive devices commonly fix all read/write heads to a common actuator mechanism. Therefore, restricting the access of the read/write head 730 to a sweep angle .theta., as shown in FIG. 8, also restricts the access of the read/write 731. This results not only in sacrificing the inner diameter storage area on the lower surface 721, but also an equal loss of storage area on the upper surface 722. This loss also occurs on any additional disks disposed above disk 720 on a multi-disk prior art HDA.
Another solution is to limit the outer diameter of the motor 710, thereby effectively moving the rotor/stator gap 745 toward the central portion of the motor 710. This solution allows access to storage tracks located at a smaller diameter of the disk 720 than with a larger diameter motor. However, motor power is a function of motor volume. Therefore, a reduction in motor diameter requires that the thickness of the motor 710, and hence the thickness of the entire HDA, be increased in order to produce the same amount of power. In addition, since signal level decreases as the read/write head reads tracks located closer to the rotor shaft, even if the rotor/starter gap 745 is moved inward, the signal to noise ratio decreases as inner tracks are accessed. Thus, there is a conflict between reducing motor diameter and reducing motor thickness.
In balancing the conflict between the motor diameter and motor thickness, a motor having a diameter of approximately 22 mm and a thickness of approximately 10 mm is suitable for disk drive devices incorporating the low-profile motor described above and rigid disks having a diameter of 1.8 inches. Including the space t below the disk 720 and an additional space t above the upper surface 722, and including a disk thickness of 0.6 mm, the minimum thickness of the HDA shown in FIG. 7, as measured from the bottom surface of the housing 711 to the top surface to the rotor shaft 716, is approximately 13.6 mm. In practice an additional space is necessary above the heat 731.
FIGS. 9A and 9B illustrate two- and three-disk embodiments of HDAs incorporating the prior art low-profile architecture. The spacing between adjacent disks 720 is approximately two times the space t, or 3.0 mm, to provide space for the two read/write heads 730 and 731 disposed between the adjacent disks. Thus, each additional disk 720 increases the thickness of the prior art HDA by 3.6 mm (two spaces t and 0.6 mm for the thickness of the additional disk). Therefore, the thickness of the two-disk, four-head HDA of FIG. 9A is approximately 17.2 mm, and the thickness of the three-disk, six-head HDA of FIG. 9B is approximately 20.8 mm. Ferrite shields 910 are illustrated in both FIGS. 9A and 9B.
It is an objective of the present invention to provide a design architecture for reduced-diameter, low-profile disk drive devices which overcomes the limitations. of the prior art design architectures.
It is further an objective of the present invention to provide a design architecture which reduces the thickness of a disk drive device while substantially maintaining or increasing its information storage capacity.
It is further an objective of the present invention to provide a design architecture which increases the amount of usable storage area on magnetic disks, while decreasing the amount of signal error caused by the proximity of the magnetic head to the spindle motor.