Dynamic magnetic information storage or retrieval – Automatic control of a recorder mechanism – Controlling the head
Reexamination Certificate
2001-09-28
2004-05-18
Hudspeth, David (Department: 2651)
Dynamic magnetic information storage or retrieval
Automatic control of a recorder mechanism
Controlling the head
C360S078070
Reexamination Certificate
active
06738220
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to computer data storage devices and, in particular, relates to a hard disk drive having an actuator controller that accounts for the bias torque on the actuator.
2. Description of the Related Art
Hard disk drive storage devices are an important component in virtually all computer systems. In particular, hard disk drives provide computer systems with the ability to store and retrieve data in a non-volatile manner such that the data is maintained even if power is removed from the device. The popularity of these devices is based on their ability to quickly store and retrieve large quantities of digital information at low cost. However, because the computer industry continually strives to provide computer systems with increased performance, there exists a need for improved disk drives having increased data access speeds.
The typical hard disk drive comprises one or more pivotally mounted disks having a magnetic recording layer disposed thereon and a plurality of magnetic transducer elements for affecting and sensing the magnetization states of the recording layer. The recording layer comprises a large number of relatively small domains disposed thereon that can be independently magnetized according to a localized applied magnetic field and that can be maintained in the magnetized state when the external field is removed. The domains are grouped into concentric circular tracks each having a unique radius on the disk and data is written to or read from each track by positioning the transducer over the disk at the corresponding radius while the disk is rotated at a fixed angular speed.
To position the transducer with respect to the disk, the typical hard disk drive further comprises a head stack assembly (HSA) that includes a transducer, a pivotally mounted actuator arm for supporting the transducer, a voice coil motor (VCM) for exerting a torque onto the actuator arm, and a servo-controller for controlling the VCM. The VCM comprises a coil of conducting wire wound into a plurality of loops and a permanent magnet disposed adjacent the coil. The servo-controller initiates movement of the actuator arm by directing a control current to flow through the coil which results in the permanent magnet applying a force onto the coil which is then transferred to the actuator arm in the form of a torque. Because the direction of the torque is dictated by the direction of control current flow, the servo-controller is able to reposition the transducer by first directing the control current through the coil so as to angularly accelerate the actuator arm in a first direction and then reversing the control current so as to angularly decelerate the actuator arm.
The HSA further comprises a pivot bearing and a thin flexible cable know as “flex cable”. The pivot bearing provides the pivotal support of the actuator arm while allowing precise rotational motion. The flex cable provides an interconnect between the transducer and the disk control circuitry. The flex cable is attached to the actuator arm, and is allowed to flex and unflex as the actuator arm moves back and forth.
The time required to reposition the transducer in the foregoing manner is known as the “seek time” of the drive and is an important performance factor that affects the throughput of the drive. For example, a drive having a short seek time will be able to access a requested track of data more quickly than a drive having a longer seek time. According the state of the art, the seek time required to reposition the transducer across a distance of 0.8-0.85 cm is typically in the range of 5-10 ms.
In a typical seek operation, the transducer accelerates, coasts, and decelerates according to the predetermined control of the current applied to the VCM. The transducer, through a feedback control, typically requires some settling time to settle on the proper target track. Once the transducer is on the proper track, a track following bias current is provided to the VCM in order to maintain tracking. The bias current is necessary to counteract a bias torque that is continuously exerted on the actuator arm.
The bias torque comprises any torque acting on the actuator arm, other than the torque due to the control current in the VCM (control torque). A torque on the actuator arm from the spring property of the flex cable is one potential source of such bias torque. For example, the flex cable may be stretched when the actuator arm moves in one direction, and squeezed when the actuator arm moves in the other direction thereby affecting the control torque on the actuator arm. Friction is another factor that contributes to the bias torque on the actuator arm.
Typically, the magnitude of the bias torque is much smaller than that of the control torque. As such, the bias torque is not significant when the control torque is being applied to the actuator arm for gross movements. During the settling phase and track following, however, small amounts of currents are applied by the VCM in a prescribed manner to make the transducer settle on the target track and maintain the track following thereafter. In these realms of operation, the effects of bias torque can become significant, especially in higher density drives.
In practice the bias torque is usually not characterized analytically, but measured. The feedback control system measures the current required by the VCM to counter the bias torque on the actuator arm so as to maintain the transducer at a given track. A plurality of such measurements are made at various locations on the disk, and the resulting curve yields the bias torque as a function of track location.
The bias torque curve as a function of track location is not unique. That is, bias torque measured when the transducer is moving from inner diameter side to the outer diameter side yields a first curve. Bias torque measured when the motion is in the opposite direction yields a second curve that typically does not retrace the first curve. When a full range of motion is made from the inner most track to the outer most track, and then back to the inner most track, the resulting bias torque measurements yield two distinct sets of curves, with the two curves joined at the inner most and the outer most points. This behavior resulting in the loop shape is known as hysteresis. Hysteresis is a phenomenon where a measured quantity depends on the direction of a process.
Hysteresis in general is caused by a “memory” of a system that makes certain aspects of the system to lag behind. In the case of the actuator arm, the memory comes from the mechanical memories of either the flex cable or the pivot bearing or both. That is, a bias torque depends on the recent history of the flex cable and/or the pivot bearing. If the actuator arm comes to a stop at a given track, it will feel a certain bias torque based on the memory of the previous movements, the memory including the direction of travel.
Hysteresis in bias torque is further complicated by its dependence on the size of a seek loop. A full seek loop is when the transducer goes from the inner most track to the outer most track, and returns to the inner most track. A smaller seek loop comprising a small length seek and its reverse, yields a smaller hysteresis loop that is nested within the larger full seek loop. Progressively smaller seek loops yield smaller hysteresis loops that are nested within the larger loops.
To achieve optimal seek performance, the controller must account for the bias torque and adjust the control current to the VCM. If the proper adjustment is not made, the transducer can overshoot or undershoot the target track sufficiently to cause a delay in settling, thus increasing the seek time. Thus, there is a need to adjust the control current to the VCM according to the bias torque acting on the actuator arm.
One possible solution is to consider only the average bias torque. Averaging of the bias torque hysteresis loop will yield a single bias torque curve as a function of track location (nominal bias curve). An X-
Knobbe Martens Olson & Bear
Shara, Esq. Milad G.
Western Digital Technologies Inc.
LandOfFree
Servo settling by utilization of a bias torque offset value... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Servo settling by utilization of a bias torque offset value..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Servo settling by utilization of a bias torque offset value... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3268989