Disk drive with a pivot embedded torque generating track...

Details Disk drive with a pivot embedded torque generating track... Disk drive with a pivot embedded torque generating track...

2001-06-11

2004-05-04

Hudspeth, David (Department: 2651)

Dynamic magnetic information storage or retrieval

Automatic control of a recorder mechanism

Controlling the head

Reexamination Certificate

active

06731454

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a disk drive, and more particularly to a disk drive having a pivot embedded torque generating track follow actuator.
2. Description of the Related Art
Growth in areal density (bits/sq. inch) of a hard disk drive (HDD) is achieved through an increase in track density and bit density metrics. Technical advancement in electromechanical components and servo system architecture facilitates the increase in track density. Indirectly, an increase in track density requires a commensurate increase in crossover frequency of the track following servo transfer function. A 3.5″ HDD for server class applications reached a track density of 30 kTPI (tracks per inch) in year 2000, and the growth is expected to continue into the next decade.
Actuator resonance modes have become fundamental limiters in achieving higher servo crossover frequency required for high TPI design.
The sector servo system of a 3.5″ server class HDD with a 1 kHz openloop crossover frequency has been able to meet 30 kTPI (tracks per inch) track-following accuracy requirements. However, the growth of track density to higher than 30 kTPI has emerged as a major challenge to the actuator and servo system design.
Further, mechanical system resonance is a key limiter to higher bandwidth control. Use of microelectromechanical (MEMs) devices has been studied to increase actuator response characteristics. A major innovation in the actuator system design to increase the servo crossover frequency is desirable, but the storage industry needs cost-effective innovations in servo system design. A drastic change in the actuator system design does not retain the time-proven simple actuator system concepts. Thus, an alternative servo-mechanics approach is required to meet the high track density challenges. However, prior to the present invention, such an alternative, optimized approach has not been presented or developed.
For example, turning to
FIGS. 1A-1B
, a conventional rotary actuator assembly
110
of a disk drive has a single voice coil motor (VCM)
120
. It produces a force about a pivoting point in order to generate a change in radial position of the read/write head.
FIG. 1A
shows the conventional rotary actuator assembly
110
found in a HDD. The actuator (and actuator arm
115
) is made to pivot (e.g., by a pivot bearing assembly
150
) about an axis when the VCM
120
is activated. As shown the actuator assembly
110
further includes a pivot assembly body
130
.
The pivot itself is composed of a pair of ball bearings
160
A,
160
B, as shown in
FIG. 1B
, which are assembled with an appropriate preload so that the pivoting function is made to be sufficiently free of rotational stiffness. The ball bearings
160
A,
160
B, along with an inner shaft
170
, are fitted inside of a bearing sleeve or housing
180
, with the pivot assembly body
130
being fitted over the pivot bearing assembly
150
. Thus, the shaft and ball bearings support the entire actuator assembly
110
. The linear radial stiffness of the bearings
160
A,
160
B is high enough to maintain the resonance of a rigid actuator to be around 10 kHz. In a “real world” application, the radial stiffness of the pivot-bearing contributes to general reduction of the free-body vibration of the actuator assembly
110
. Early recognition of pivot stiffness induced dynamics as a detractor and a solution to it can be found in commonly-assigned U.S. Pat. No. 5,267,110, incorporated herein by reference.
Recently several institutions have shown initiative in addressing the problem of finite radial stiffness (e.g., see K. Aruga, “High-speed orthogonal power effect actuator for recording at over 10,000 TPI, IEEE Transactions on Magnetics, Vol. 32, No. 3, May 1996).
Turning now to
FIGS. 2A-2B
, there are several actuator resonance modes associated with a 3.5″ form factor HDD.
FIG. 2A
shows a graph of magnitude with respect to frequency. That is, when a force (current) is applied to the actuator, the head is anticipated to move in a certain way (e.g., a certain frequency will result in the conventional actuator arm assembly).
The first important mode (e.g., resonance peak) that occurs around 7 kHz is understood to arise from bending of the actuator voice coil motor around its pivoting point. The coil bending resonance (CBR) is associated with a 180-degree phase change (e.g., see
FIG. 2B
which shows the phase as a function of frequency) and in certain configurations the magnitude/phase combination could produce an unstable condition of the track-follow servo. This bending mode characteristic also is sensitive to temperature, pivot parameters and other design parameters of a disk drive.
Conventional approaches of managing the presence of this mode have been to introduce a digital notch filter in series with the servo controller during a seek and track-follow mode. A notch filter reduces the negative effect of the peak gain that occurs due to the coil bending resonance (CBR). Because of the temperature-induced drift of the resonance frequency as well as the manufacturing variability encountered within a population of a product, the digital notch filters are designed to have wider than required attenuation bandwidth, thereby resulting in a corresponding phase loss in the crossover region of the servo loop. The loss of phase in turn limits the achievable crossover frequency of the track-follow servo system.
Another industry effort to tackle the CBR has been to include an active damping servo loop within the conventional positioning servo (e.g., see F. Huang, T. Semba, W. Imaino and F. Lee, “Active Damping in HDD Actuator,” Digests of APMRC2000,” ISBN 0-7803-6254-3, November 2000, page MB6-01). This method, which is theoretically equivalent to that of an optimized digital notch filter, has been implemented in some server class HDDs.
A passive method to enhance the CBR resonance through structural modification is proposed in J. Heath, “Boosting servo bandwidth,” Digests of APMRC2000,” ISBN 0-7803-6254-3, November 2000, page MP20-01. Briefly, suppressing the CBR by various methods has a time limited advantage, and it does not allow for progressive growth in servo crossover frequency required for next generation HDDs.
Thus, the impact of coil resonance in the track-follow servo transfer function must be minimized, and hence requires new innovations. The present actuator system with a single VCM is primarily optimized for seek operation. The track-follow performance is extracted from the same actuator structure as a secondary challenge. However, this constraint must be removed in order to achieve not only an optimum access but also a high track density settle-out and track follow performance. H. Yamura and K. Ono, “New H-infinity design for track-following,” Digests of APMRC2000,” ISBN 0-7803-6254-3, November 2000, page TA4-01 proposes a configuration in which the contribution of CBR is circumvented by a second actuator.
FIG. 3
shows a conventional disk torque generating actuator concept in which a generic torque producing VCM configuration for track-following operation is suggested (e.g., see the above-mentioned U.S. Pat. No. 5,267,110, incorporated herein by reference).
In
FIG. 3
, the torque generator
300
includes a main VCM
310
, a pivot
320
, a “mini-VCM”
325
, a load-beam
330
, and a head
340
which provides an input to a servo
350
. The servo
350
also receives an input from a rotation velocity sensor/servo
360
coupled to the main VCM
310
. The servo
350
provides outputs to the main VCM
310
and the mini-VCM
325
to move the head about the pivot.
It is noted that this system developed in that the previous conventional system employed only the main VCM. However, a problem arose in that, in applying a force to the arm (and thus the head) by the main VCM
310
(e.g., based on a signal from the servo), a clockwise torque should result, thereby moving the head in a clockwise direction.
However, because of the configuration of the previous conventional devic

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