Dynamic magnetic information storage or retrieval – Record transport with head stationary during transducing – Disk record
Reexamination Certificate
2000-01-13
2004-10-26
Miller, Brian E. (Department: 2652)
Dynamic magnetic information storage or retrieval
Record transport with head stationary during transducing
Disk record
C310S090000, C384S536000
Reexamination Certificate
active
06809898
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for damping rocking mode vibrations in rotating devices. In particular, the present invention relates to a rocking mode vibration damper for computer disk drives.
BACKGROUND OF THE INVENTION
Disk drive memory systems are used in personal computer applications to store digital information on magnetic disks. In a typical disk drive assembly, information is stored on the disks in concentric tracks divided into sectors. The disks themselves are mounted on a hub, which rotates relative to the disk drive enclosure. Information is accessed by means of read/write heads mounted on pivoting arms that move radially over the surface of the disks. This radial movement of the transducer heads allows different tracks to be accessed. The disks are rotated by an electric motor to allow the read/write head to access different sectors on the disks. The motor is typically mounted within or beneath the hub, and usually rotates the disks at from 1,200 to 12,000 revolutions per minute.
Vibrations in a disk drive occur as a result of several influences. Defects in the ball bearings interconnecting the rotating hub to the disk drive enclosure are a first cause of vibrations. Ball bearings are not perfectly spherical and generally they contain some defects such as flat spots, crevices, or the like. Similarly, ball bearing races may also contain defects. Consequently, the movement of the hub as the bearing passes each defect produces an excitation which generates vibration in the hub. Because there will be several bearing defect frequencies associated with each hub speed, a multitude of ball bearing excitation frequencies can result, producing vibrations in the disk drive assembly. Environmental vibrations or shocks are another cause of vibration in computer disk drives. Sources of environmentally-induced vibrations include physical jarring of the disk drive installed in a computer, or any movement of the computer enclosing the disk drive. Deformations of the disk drive enclosure or base plate, known as vertical diaphragm vibrations, are yet another cause of vibrations in computer disk drives.
Three modes of vibration may occur in a spinning disk and hub assembly. The first mode of vibration for the disk and hub is in a radial direction relative to the axis of rotation. The second mode of vibration for the disk and hub is in an axial direction relative to the axis of rotation. The third mode of vibration is a rocking displacement of the disk and hub relative to the axis of rotation. When these vibrations are of sufficient amplitude, they can cause servo system errors and transducer/track misregistrations, thereby decreasing or inhibiting drive performance.
Every rotating disk drive inherently has an upper and lower rocking mode vibration. This rocking mode vibration is the result of the manufacturing tolerances of the bearing itself and the structural stiffness of the spindle motor and the disk drive housing or enclosure. Thus, any disk drive hub will exhibit specific upper and lower rocking resonances, the frequency of which will change depending upon the number of disks supported by the spindle, as well as the rotational speed of the hub. The rocking resonance can be affected by defects in the disk drive assembly bearings. In particular, the frequency of the vibrations caused by the disk drive assembly bearings can resonate with the natural rocking mode frequency of the disk drive assembly and result in high amplitude vibrations. These high amplitude vibrations can cause data misregistration errors as the storage disks rock beneath the read/write head. In addition, such nonrepetitive runout, if it occurs during servo or data write operations, can later reduce the track following capability of the disk drive during read operations. This in turn can lead to read/write head servo system failure or an increase in acoustical noise. Therefore, reductions in rocking mode vibrations and/or bearing defect vibrations can reduce disk drive errors, can lead to longer read/write head servo system life, and can reduce the level of noise produced by the disk drive. Also, the amplitude of these vibrations is directly associated with drive performance. In drives where the amplitude is kept relatively small, data can be stored in higher densities than in drives where such vibrations have a relatively large amplitude.
The frequencies at which the rocking mode and bearing defect vibrations interact will vary, depending on the particular set of bearings used in the drive and the geometry of the drive itself. The number and thickness of storage media disks carried by a disk drive also affects the frequencies at which rocking mode vibrations occur. For example, where a disk drive structure is used to carry from one to four storage media disks and is used in a first family of drives operated at a first speed and a second family of drives operated at a second speed, there are 16 upper and lower dynamic rocking mode frequencies. In addition, there are two distinct sets of bearing defect frequencies. Therefore, it is virtually impossible to avoid all harmful frequency interactions using a single disk drive design for a family of multiple disk disk drives. As a result, disk drive manufacturers are compelled to use a wide variety of spindle motor designs to accommodate the various disk drive platforms and performance standards that comprise their product lines while simultaneously attempting to minimize the effects of vibration.
The structure of the disk drive also has a significant effect on the amplitude of vibrations resulting from rocking mode and vertical diaphragm mode resonant frequency vibrations. Undamped structures tend to exhibit vibrations having a higher amplitude at their resonant frequencies than do equivalent structures that contain damping means. Consequently, for a given vibrational input (e.g. from spindle bearing defects) an undamped and rigid disk drive assembly will produce larger amplitude vibrations in the disk storage media at resonant frequencies than will equivalent disk drives having damping means. Conversely, disk drive assemblies having damping typically exhibit resonant frequencies of a lower amplitude. Because of the relatively low amplitude of the vibrations in a damped drive, data storage densities may be increased compared to undamped drives.
The natural resonance of the disk drive assembly during normal operation is also a major source of acoustical noise. The reduction of acoustical noise is an increasingly important consideration in the design of computer disk drives. In particular, where the computer disk drive is for use in computers placed in work spaces, it is desirable that the disk drive have a very low acoustical noise signature. By reducing the amplitude of the resonant frequencies in a disk drive assembly, damping lowers the acoustical noise signature of the assembly.
Efforts to dampen vibrations in disk drive assemblies have included the insertion of damping material between the spindle and the drive enclosure. An example of such an approach is found in U.S. Pat. No. 5,483,397 to Gifford et al. However, this approach cannot be applied to disk drive designs having rotating shafts. Other designs have attempted to reduce vibrations by mounting the stator of the disk drive spindle motor to the base through a damping material. This approach is taken in U.S. Pat. No. 5,365,388 to Maughan et al. Although such a design is useful at damping vibrations occurring in the stator, it is incapable of damping rocking mode vibrations set up in the rotating spindle and hub. Also, such a design requires more space than an otherwise identical design not having damping material. This is a disadvantage because disk drives increasingly must provide greater storage capacity and smaller form factors.
In light of the above-described problems, including those of misregistration errors and acoustical noise, it would be useful to find a way to avoid or dampen vibrations generated due to natural rocking resonances and bearing defect
Maxtor Corporation
Miller Brian E.
Sheridan & Ross P.C.
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