Dynamic magnetic information storage or retrieval – Automatic control of a recorder mechanism – Controlling the head
Reexamination Certificate
2001-06-05
2004-03-09
Hudspeth, David (Department: 2651)
Dynamic magnetic information storage or retrieval
Automatic control of a recorder mechanism
Controlling the head
Reexamination Certificate
active
06704158
ABSTRACT:
TECHNICAL FIELD
The present invention relates to disk drive actuator arms used with read/write heads in computer disk drives.
BACKGROUND ART
A computer disk drive stores and retrieves data by positioning a magnetic read/write head over a rotating magnetic data storage disk. The head, or heads, which are typically arranged in stacks, read from or write data to concentric data tracks defined on surface of the disks which are also typically arranged in stacks. The heads are included in structures called “sliders” into which the read/write sensors and transducers are imbedded during fabrication. The goal in recent years is to increase the amount of data that can be stored on each hard disk. If data tracks can be made narrower, more tracks will fit on a disk surface, and more data can be stored on a given disk. The width of the tracks depends on the width of the read/write head used, and in recent years, track widths have decreased as the size of read/write heads have become progressively smaller. This decrease in track width has allowed for dramatic increases in the recording density and data storage of disks.
Hard disk drives are typically formed with an actuator arm having a gimbal assembly with the read/write head attached. The actuator arm is positioned by using a voice coil motor (VCM), and as the disks rotate, the VCM pivots the actuator arm, moving the heads over the disk surfaces. By this rotating motion, the actuator operates to position the head above the disk to read or write data on a desired track. As storage capacity of disks increases, and tracks get narrower, the task of positioning the read/write head becomes increasingly difficult, coupled with the fact that another goal is to decrease “seek time” or the amount of time that it takes for a read/write head to locate and position itself over a desired track.
There have been several approaches to the problems of increasing the accuracy and speed of positioning the read/write heads. A current system for positioning a read/write head over a data track employs a single stage actuator. The single stage actuator includes an actuator assembly, which pivots the actuator arm, enabling the head to read data from a particular data track. The actuator assembly typically uses the VCM alone to position the head. Disk drives with single stage actuators typically achieve a memory storage density of tens of thousands of data tracks per inch (TPI), but this density can not exceed the limit of a VCM's precision.
In an effort to achieve finer control over the head positioning, a secondary actuator has sometimes been used with a two-stage actuator arm. A Head Stack Assembly (HSA) of the actuator arm is used as the two-stage actuator arm, and generally has a fixed portion and a movable portion, within the overall arm. The read/write head is attached to the end of the movable portion, and micro-actuators are connected between the two portions. When activated, the micro-actuator provides finer positioning adjustments to the coarser positioning provided by the VCM. These micro-actuators have used piezoelectric materials, which vary their length or shape when a voltage is applied to them. Some prior actuators have had a hinge portion connecting the fixed and movable portions, and others have had the two portions completely separated from each other with only the micro-actuators connecting them. However, this type of actuator can have problems with out-of plane movement, as the slider is caused to roll slightly. This type of motion can potentially risk damage to the disk surface, or detrimental change in fly height (spacing of head to disk). In addition to this potential out-of-plane motion, these actuators can possibly excite the load beam vibratory modes. Such excitation would limit the ability of the servo control system to achieve high bandwidth.
The data tracks are ideally symmetrical and uniform in curvature, but in practice, irregularities occur. These irregularities make it necessary for the actuators to make small adjustments in position in order for the head to remain centered on the tracks. Disks typically include servo information, which is read along with the other stored data. This servo information is sent to the control system, which then generates control signals, which help to steer the head back on track. For example, as the head encounters an irregularity in the track and begins to deviate from the track, the servo information signal can communicate this change to the control system, which then may activate a voltage to one of the micro-actuator motors to steer the head back in the direction of the track. The speed with which these irregularities can be sensed and corrected is an important factor in the proper operation of the disk drive. The number of these minute corrections, which can be achieved each second is referred to as “bandwidth”, and is measured in cycles per second. The greater the bandwidth, the greater the reliable operating speed of the disk drive can be, and ultimately, this allows greater storage capacity of the system.
Servo control systems generally operate better if the servo sensor and the actuator are near each other, rather than being separated in distance. The positioning of the sensor, in this case the read/write head, and the actuator in the same location is known as co-location or “collocation” as the term has evolved in the industry, and collocation generally results in improved bandwidth, and faster response time.
Potential difficulties exist with collocation configurations. The actuators are desired to be as small as possible so that they do not adversely affect the performance of the drive by adding too much additional mass or bulk to the actuator arm. The face of the slider, which faces the disk is configured as an Air Bearing Surface (ABS) which has a distributed load over its face from the air pressure generated between the ABS and the spinning disk surface. This is balanced by a spring force generated through the load beam, a force, which can be as low as 30 mN (3 grams-force) or less. The performance of the actuator to move the slider as it makes its many micro-corrections of position could be adversely affected by the friction inherent in the system. The smaller the friction force to be overcome, the smaller the actuator can be, with accompanying advantages of reduced mass and volume, and thus improved performance for the disk drive, and increased practical storage capacity for the system.
FIG. 2
shows an actuator arm
2
which has an arm beam
6
, which is configured in two parts which include a stationary part
12
and a movable part
14
which are connected by a narrow hinge portion
16
. Two piezoelectric actuators
18
are connected between the two parts
12
,
14
, and act as a push-pull mechanism to direct the movable part
14
. A slider
20
including a read/write head
22
is shown at the end of the movable part
14
of the actuator arm
2
. A Voice Coil Motor (VCM)
9
acts as a primary actuator
7
and provides coarse positioning of the overall arm, and the piezoelectric actuators act as secondary actuators
8
.
This is an example of a secondary actuator
8
in which the sensor, the read/write head
22
is separated from the actuator mechanisms
18
, and thus the bandwidth is typically in the range of 1.5-3.0 kHz, which is improved compared to a bandwidth of 800-1 kHz typical for an actuator without secondary actuation, but less than that expected from a collocated actuator. In contrast, it is estimated that the bandwidth of collocated actuators is typically greater than 5 kHz.
Collocated actuators can have many configurations, but generally they include a Head Gimbal Assembly (HGA) having a load beam, and a flexure, also known as a gimbal, which is a thin springy member which functions as a leaf spring, as well as a slider containing the read/write head. Typically, actuators in the form of strips or tiny bars have been used, which have two ends, which will be referred to as the head end and the foot. In this configuration, the foot end is attached to the flexure, whi
Fanslau Edmund B.
Hawwa Muhammad A.
Young Kenneth F.
Stetina Brunda Garred & Brucker
Tzeng Fred F.
Western Digital (Fremont) Inc.
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