Dynamic magnetic information storage or retrieval – Automatic control of a recorder mechanism
Reexamination Certificate
2000-11-30
2004-05-04
Hudspeth, David (Department: 2651)
Dynamic magnetic information storage or retrieval
Automatic control of a recorder mechanism
C360S075000
Reexamination Certificate
active
06731450
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 method of monitoring the temperature of an actuator coil of a hard disk drive by sampling the electrical properties of the coil.
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 adjacent 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 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 time required to reposition the transducer in the foregoing manner is known as the “seek time” of the drive and is a critical performance factor that limits the throughput of the drive. For example, a drive having a short average seek time will generally be able to access a requested track of data more quickly than a drive having a longer average seek time. According to the state of the art, for a transducer having a linear acceleration greater than 500 meters/s
2
or 50 g's, the seek time required to reposition the transducer across a distance of 2.5 cm is typically in the range of 20-30 ms. Consequently, to provide such large acceleration, a relatively large (0.5 to
1
A) current is often required to flow through the coil.
Unfortunately, when large amounts of current are directed through the coil, the rate of heat gain caused by the finite resistance of the windings of the coil may exceed the rate of heat loss to the environment. Thus, if left unchecked for an extended period of time, a rapid succession of seek operations may excessively raise the temperature of the coil such that the drive will no longer be operable. For example, when subjected to an instantaneous or average current that is beyond the VCM's design limitations, the coil may generate excessive heat and rupture, or the coil overmold material may delaminate from the actuator assembly, lose its rigidity and/or outgas particulates into the disk drive enclosure, with deleterious results. Such outgassing from the coil overmold, coil insulators and/or from other materials applied to the coil wires (such as wire lubricants, for example) may occur even at relatively low temperatures (85° C., for example). Thus, there is a need to monitor the temperature of the VCM coil so as to reduce the likelihood that the VCM will become damaged from such overheating.
One possible solution to the problem of excessive coil temperature is to blindly limit the VCM control current, i.e. without sensing or estimating the coil temperature, so as to be absolutely sure that the temperature of the coil is less than a threshold value. For example, following a first seek operation, a subsequent seek could be delayed so as to be sure that heat added to the coil during the first seek operation is substantially dissipated to the environment before the subsequent seek occurs. Alternatively, the resistive heat gain in the coil could be reduced by reducing the commanded current through the coil. However, because of the difficulty in estimating how well the environment can remove heat from the coil, the foregoing methods of blindly limiting the coil current will likely require overly conservative limitations. Thus, while possibly preventing the coil from overheating, the foregoing solution will likely result in unacceptably slow drive performance.
Another solution is proposed in U.S. Pat. No. 5,594,603 to Mori et al. and assigned to Fujitsu Limited, Japan. In this patent, the current applied to the coil is used to approximate the coil temperature. This method attempts to mathematically model the thermal behavior of the coil by inter-relating a group of factors that includes the VCM control current, the heat naturally radiated by the coil, the ambient temperature, and the thermal capacity of the coil. However, such modeling, although providing an indication of the present VCM temperature, may not accurately provide a calculated temperature value that reliably matches the present and actual temperature of the VCM.
Indeed, a number of factors may skew the results obtained from such mathematical models. For example, the present temperature of the drive or the resistance of the VCM coil may not remain constant and may thus result in changing VCM control current magnitudes. As the VCM control current is used as the basis for the temperature calculations, the VCM may not be driven (i.e., supplied with VCM control current) in an optimal manner and the actuator may not sweep as rapidly across the disk as it might otherwise have, thereby needlessly limiting the overall performance of the drive. Alternatively, should the mathematical model prove to be an inaccurate predictor of actual VCM temperature, in certain situations, excessive VCM control currents may be generated, potentially causing damage to the VCM and to the drive. Over many iterations, recursively-applied mathematical models may cause an initial and relatively small error to grow to such a degree that the model no longer accurately reflects present operating conditions. Reliance upon such an inexact mathematical model in modulating the VCM control current may understandably result in less than optimal drive performance characteristics.
Another proposed solution is proposed in U.S. Pat. No. 5,128,813 to Lee (hereafter the '813 patent) and assigned to Quantum Corporation. In this patent, a discrete temperature-sensing element is used to dynamically sense the VCM temperature during the operation of the drive. This patent discloses that the thermistor is mounted for thermal conduction directly to the head and disk assembly. While the temperature sensing element may, in fact, provide a direct measurement of the temperature of the VCM (in contrast to the Mori et al. patent above, for example), this method requires mounting a high precision thermistor to the drive and requires that approp
Bennett George J.
Codilian Raffi
Hudspeth David
Knobbe Marten Olson & Bear
Mobarhan, Esq. Ramin
Olson Jason
Shara, Esq. Milad G.
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