Disc drives of the type known as “Winchester” disc drives, or hard disc drives, are well known in the industry. Such disc drives magnetically record digital data on a plurality of circular, concentric data tracks on the surfaces of one or more rigid discs. The discs are typically mounted for rotation on the hub of a brushless DC spindle motor. In disc drives of the current generation, the spindle motor rotates the discs at speeds of up to 15,000 RPM.
Data are recorded to and retrieved from the discs by an array of vertically aligned read/write head assemblies, or heads, which are controllably moved from track to track by an actuator assembly. The read/write head assemblies typically consist of an electromagnetic transducer carried on an air bearing slider. This slider acts in a cooperative pneumatic relationship with a thin layer of air dragged along by the spinning discs to fly the head assembly in a closely spaced relationship to the disc surface. In order to maintain the proper flying relationship between the head assemblies and the discs, the head assemblies are attached to and supported by flexures attached to the actuator.
The actuator assembly used to move the heads from track to track has assumed many forms historically, with most disc drives of the current generation incorporating an actuator of the type referred to as a rotary voice coil actuator. A typical rotary voice coil actuator consists of a pivot shaft fixedly attached to the disc drive housing base member closely adjacent the outer diameter of the discs. The pivot shaft is mounted such that its central axis is normal to the plane of rotation of the discs. The actuator is mounted to the pivot shaft by precision ball bearing assemblies within a bearing housing. The actuator supports a flat coil which is suspended in the magnetic field of an array of permanent magnets, which are fixedly mounted to the disc drive housing base member. These magnets are typically mounted to pole pieces which are held in positions vertically spaced from another by spacers at each of their ends.
On the side of the actuator bearing housing opposite to the coil, the actuator assembly typically includes a plurality of vertically aligned, radially extending actuator head mounting arms, to which the head suspensions mentioned above are mounted. These actuator arms extend between the discs, where they support the head assemblies at their desired positions adjacent the disc surfaces. When controlled DC current is applied to the coil, a magnetic field is formed surrounding the coil which interacts with the magnetic field of the permanent magnets to rotate the actuator bearing housing, with the attached head suspensions and head assemblies, in accordance with the well-known Lorentz relationship. As the actuator bearing housing rotates, the heads are moved generally radially across the data tracks of the discs along an arcuate path.
As explained above, the actuator assembly typically includes an actuator body that pivots about a pivot mechanism disposed in a medial portion thereof. The function of the pivot mechanism is crucial in meeting performance requirements associated with the positioning of the actuator assembly. A typical pivot mechanism has two ball bearings with a stationary shaft attached to an inner race and a sleeve attached to an outer race. The sleeve is also secured within a bore in the actuator body. The stationary shaft typically is attached to the base deck and the top cover of the disc drive.
As disc drive consumers demand ever higher storage capacity and data access speeds, track densities have increased to the point at which a single 3.5 inch disc can store over 40 gigabytes of data. Track densities are projected to increase far beyond these numbers. Because tracks are increasingly smaller and closer together, it is more important than ever that the actuator and servo system be designed so as to minimize undesirable actuator movement caused by vibration and external disturbances.
Undesirable actuator movement is exacerbated by resonance within a vibrating actuator. All moving mechanical systems are characterized by natural resonance frequencies. When an actuator vibrates in a particular mode at a frequency equal to the resonant frequency of that mode, the vibrations intensify until the servo system can no longer effectively control actuator movement. It is therefore generally desirable that an actuator system be designed such that the resonant frequencies in each mode are as high as possible so as to prevent resonance within the actuator system.
An actuator system has four primary bending modes, each having a resonant frequency a designer must be concerned with. One such bending mode, conventionally known as a “first bending mode,” involves bending of the actuator arm out of the rotational plane of the actuator, where the bending takes place near the pivot cartridge. Another bending mode, conventionally known as a “second bending mode,” similarly involves bending out of the rotational plane of the actuator, but where the bending takes place further away from the pivot axis, near the flexure support end of the actuator arm. A third bending mode is the “first torsion mode,” in which the actuator arm twists about a longitudinal axis of the actuator arm, such that the plane of the actuator intersects but is no longer parallel to the rotational plane of the actuator. A fourth primary bending mode is the “first sway mode,” in which the actuator arm bends within the rotational plane of the actuator. As the servo system directs the actuator to move the head from track to track, the actuator will vibrate in these various modes. As long as the frequencies generated by the servo system remain below the various resonant frequencies of the actuator, the drive will continue to function properly. It should be clear that the speed at which the drive may operate is limited by the resonant frequencies of the actuator system. It is generally a goal of actuator design, therefore, to raise the natural resonant frequencies of the actuator system to allow for faster drive operation.
This has typically been accomplished by maximizing the stiffness of the actuator assembly. The conventional method for increasing actuator stiffness has been to (1) increase the thickness of the actuator, (2) increase the width of the actuator, or (3) manufacturing the actuator from a different material. There are several disadvantages associated with increasing stiffness in this way. First, increasing thickness or width increases the moment of inertia of the actuator. This decreases the speed at which the actuator may change direction, begin moving a head to a track, or to settle once the head has reached a desired track. Second, improvement of overall stiffness using this method is minimal, possibly even insignificant. And third, it is impossible to individually tune the various mode shape frequencies. For example, thickening the actuator would stiffen it against bending, torsion and sway, thereby proportionally increasing the frequencies of all four modes even if the resonant frequencies in some of these modes may be sufficiently high.
What the prior art has been lacking is an actuator design methodology which allows individual tuning of the various mode shapes, producing an actuator which is optimized to conform to servo bandwidth requirements.