Head flight characterization using thermal asperity detection

Details Head flight characterization using thermal asperity detection Head flight characterization using thermal asperity detection

1999-09-14

2002-11-19

Hudspeth, David (Department: 2651)

Dynamic magnetic information storage or retrieval

Automatic control of a recorder mechanism

Controlling the head

C360S025000

Reexamination Certificate

active

06483657

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to the field of disc drive storage devices, and more particularly, but not by way of limitation, to improvements in the detection of head flight characteristics in a disc drive.
BACKGROUND OF THE INVENTION
Disc drives are commonly used as the primary data storage and retrieval devices in modem computer systems. In a typical disc drive, user data are magnetically stored on one or more discs that are rotated at a constant high speed and accessed by a rotary actuator assembly having a plurality of read/write heads that fly adjacent the surfaces of the discs.
When the disc drive is deactivated, the heads are typically moved from an aerodynamically supported position over data recording portions of the discs and brought to rest onto texturized landing zone portions of the discs. More particularly, the heads are said to “touchdown” onto the landing zones as the disc rotational velocity decreases to a level just insufficient to continue to aerodynamically support the heads. Once the heads are located over the landing zones, a latch secures the actuator assembly to prevent inadvertent movement of the heads out onto the data recording portions of the disc as a result of a mechanical shock to the deactivated disc drive.
Upon subsequent reinitialization of the drive, current is applied to a spindle motor to accelerate the discs to operational velocity and the heads “takeoff” from the landing zones when the velocity of the discs reach a sufficient takeoff velocity just sufficient to aerodynamically support the heads. The actuator assembly is thereafter unlatched and the heads are moved out over the data recording portions for normal disc drive operation.
It is desirable for a variety of reasons to determine with some precision the respective disc velocities at which the heads take off and touchdown. Heads are typically mounted on “slider” assemblies which provide the requisite aerodynamic features that enable the heads to fly above the disc surfaces; hence, head take off and touchdown information is useful in evaluating the performance of various alternative slider designs. Moreover, since disc drives typically use the spindle motors as generators when power is removed from the drives and use this derived power to quickly move the heads to a latched position over the landing zones before the discs come to rest, information regarding head take off and touchdown characteristics is invaluable in configuring the deinitialization operation of new drive designs. Another area where head flight characteristics are periodically measured is during extended reliability testing where a population of drives is operated over a long period of time to observe changes in operational performance.
One prior art approach to determining head take off and touchdown information involves the use of an acoustic emissions (AE) sensor which basically comprises a tiny microphone (transducer) that is bonded to the actuator assembly near the heads. Since a great deal of audible noise is generated as the heads drag along the discs (before take off), and this noise is removed when the heads subsequently separate from the discs, the AE sensor has been used to detect this change in acoustic output and correlate this change to the rotational velocity of the discs.
While operable, this and other similar prior art approaches have suffered from various drawbacks. Insertion of an AE sensor requires that the protective housing surrounding the discs and heads be opened, both allowing for the introduction of contaminants into the housing as well as altering the mechanical configuration of the disc drive. Moreover, the bonding of the extraneous sensor to the actuator assembly, usually carried out using an adhesive, provides additional risk of contamination to the disc drive, and adversely introducing uncertainties in the observed data.
Accordingly, there is a need for improvements in the art to enable disc drive manufacturers to evaluate head flight characteristics in a noninvasive and easily implementable manner. It is to such improvements that the present invention is directed.
SUMMARY OF THE INVENTION
The present invention is directed to a method for evaluating head flight characteristics in a disc drive.
In accordance with preferred embodiments, a methodology is first presented to identify a head touchdown velocity. A disc of a disc drive is first accelerated to an initial rotational velocity sufficient to aerodynamically support a head over the disc at a nominal flying height. Next, an appropriate read bias current of selected magnitude is applied to a read element of the head to generate a readback signal determined in relation to voltage drop across the read element. A thermal asperity threshold level is also selected and applied which the readback signal exceeds in response to contact between the head and the disc as the disc rotates.
Thereafter, the head touchdown velocity is determined as a velocity just insufficient to aerodynamically support the head by decelerating the disc to decrease the flying height of the head and detecting subsequent contact between the head and the disc when the readback signal exceeds the thermal asperity threshold level.
Further, a methodology is provided to determine a head take off velocity. First, rotation of the disc is initiated while the head remains in contact with the disc. Next, appropriate read bias currents and thermal asperity threshold levels are selected and applied. The head takeoff velocity is thereafter determined as a velocity just sufficient to aerodynamically support the head by detecting lift off of the head from the disc as the readback signal falls below the thermal asperity threshold level.
With knowledge of the head take off and touchdown velocities, the distances (in disc revolutions) that the head remains in contact with the disc before take off and after touchdown can be determined.
These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.


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