U.S. patent application number 09/401688 was filed with the patent office on 2002-05-09 for system and method for measuring absolute transducer-medium clearance using a thermal response of an mr transducer.
Invention is credited to SMITH, GORDON JAMES.
Application Number | 20020054446 09/401688 |
Document ID | / |
Family ID | 23588798 |
Filed Date | 2002-05-09 |
United States Patent
Application |
20020054446 |
Kind Code |
A1 |
SMITH, GORDON JAMES |
May 9, 2002 |
SYSTEM AND METHOD FOR MEASURING ABSOLUTE TRANSDUCER-MEDIUM
CLEARANCE USING A THERMAL RESPONSE OF AN MR TRANSDUCER
Abstract
A method and apparatus for measuring absolute clearance between
a magnetoresistive (MR) transducer and a moving medium utilizes a
transduced signal that varies as a function of transducer-medium
clearance. The medium may be devoid of, or include, magnetic
information. The velocity of the medium is reduced relative to the
MR transducer, and a rate of change of the signal is monitored.
Using data associated with the rate of change of the signal during
spindown, absolute clearance between the MR transducer and the
medium is computed for a nominal medium-transducer velocity, such
as a full operational velocity. Computing absolute
transducer-medium clearance involves determining a transition
velocity at which the rate of change of the signal exceeds a
pre-established threshold. Computing absolute clearance may further
involve associating the transition velocity with an absolute
clearance value obtained using a clearance profile associated with
the MR transducer. The clearance profile may be representative of a
relationship between landing or take-off velocity of the MR
transducer and associated transducer-medium clearance. The
transition velocity is used to compute the absolute clearance
between the MR transducer and the medium for a nominal
medium-transducer velocity. The transition velocity represents a
medium-transducer velocity at which the
Inventors: |
SMITH, GORDON JAMES;
(ROCHESTER, MN) |
Correspondence
Address: |
MARK A. HOLLINGSWORTH
ALTERA LAW GROUP LLC
6500 CITY WEST PARKWAY
SUITE 100
MINNEAPOLIS
MN
55344
US
|
Family ID: |
23588798 |
Appl. No.: |
09/401688 |
Filed: |
September 23, 1999 |
Current U.S.
Class: |
360/73.03 ;
G9B/5.227; G9B/5.23 |
Current CPC
Class: |
G11B 5/6005 20130101;
G11B 5/59683 20130101 |
Class at
Publication: |
360/73.03 |
International
Class: |
G11B 015/46 |
Claims
What we claim is:
1. A method of measuring absolute clearance between a
magnetoresistive (MR) transducer comprising an MR element and a
medium using a thermal response of the MR element, the medium
moving relative to the MR transducer, the method comprising:
producing, using the MR element, a signal that varies as a function
of clearance between the MR element and the medium, the signal
comprising a thermal component representing a thermal response of
the MR element; reducing a velocity of the medium relative to the
MR transducer; monitoring, while reducing the medium-transducer
velocity, a rate of change of the signal; and computing, for a
nominal medium-transducer velocity, absolute clearance between the
MR transducer and the medium using the rate of change of the
signal.
2. The method of claim 1, wherein computing absolute clearance
between the MR transducer and the medium further comprises
determining a transition velocity at which the rate of change of
the signal exceeds a pre-established threshold and using the
transition velocity to compute the absolute clearance between the
MR transducer and the medium for the nominal medium-transducer
velocity.
3. The method of claim 2, wherein monitoring the rate of change of
the signal comprises performing a plurality of rate of change
computations using the signal to establish the pre-established
threshold.
4. The method of claim 2, wherein the transition velocity is
representative of a medium-transducer velocity at which the rate of
change of the signal exceeds the pre-established threshold by about
10 percent or more.
5. The method of claim 2, wherein the transition velocity is
representative of a transition from a first thermal transport
mechanism associated with the MR transducer to a second thermal
transport mechanism associated with the MR transducer.
6. The method of claim 2, wherein the transition velocity is
representative of a medium-transducer velocity at which appreciable
contact occurs between the transducer and the medium.
7. The method of claim 1, wherein computing absolute clearance
between the MR transducer and the medium further comprises
associating a transition velocity at which the rate of change of
the signal exceeds a pre-established threshold with an absolute
clearance value obtained using a clearance profile associated with
the MR transducer.
8. The method of claim 7, wherein the clearance profile is
representative of a relationship between landing or takeoff
velocity of the MR transducer and associated transducer-medium
clearance.
9. The method of claim 1, wherein the signal is representative of a
resistance or voltage of the MR element that varies as a function
of clearance between the MR element and the medium.
10. The method of claim 1, further comprising biasing the MR
element using a constant current.
11. The method of claim 1, further comprising adjusting the
medium-transducer velocity or a characteristic of a slider that
supports the MR transducer to maintain a desired clearance between
the MR transducer and the medium.
12. The method of claim 1, further comprising: determining a
transition velocity at which the rate of change of the signal
exceeds a pre-established threshold; and determining a state of
lubricity between a slider that supports the MR transducer and the
medium using the rate of change of the signal for medium-transducer
velocities lower than the transition velocity.
13. The method of claim 1, further comprising: determining a
transition velocity at which the rate of change of the signal
exceeds a pre-established threshold; and characterizing a surface
profile of a slider that supports the MR transducer using the rate
of change of the signal for medium-transducer velocities lower than
the transition velocity.
14. An apparatus for measuring absolute clearance between a slider
and a medium moving relative to the slider, the apparatus
comprising: a magnetoresistive (MR) transducer comprising an MR
element and supported by the slider, the MR transducer producing a
signal that varies as a function of clearance between the MR
element and the medium, the signal comprising a thermal component
representing a thermal response of the MR element; a controller
that controls a velocity of the medium relative to the slider, the
controller reducing the velocity of the medium relative to the
slider; and a processor, coupled to the controller and MR
transducer, that computes a rate of change of the signal during
reduction of the medium-slider velocity and determines a
medium-slider threshold velocity at which the rate of change of the
signal exceeds a pre-established threshold, the processor
determining, for a nominal medium-slider velocity, absolute
clearance between the slider and the medium using the threshold
velocity and a clearance profile associated with the slider.
15. The apparatus of claim 14, wherein the processor performs a
plurality of rate of change computations using the signal to
establish the pre-established threshold.
16. The apparatus of claim 14, wherein the transition velocity is
representative of a medium-slider velocity at which the rate of
change of the signal exceeds the pre-established threshold by about
10 percent or more.
17. The apparatus of claim 14, wherein the transition velocity is
representative of a medium-slider velocity at which appreciable
contact occurs between the slider and the medium.
18. The apparatus of claim 14, wherein the clearance profile is
representative of a relationship between landing or takeoff
velocity of the slider and associated slider-medium clearance.
19. The apparatus of claim 14, wherein the signal is representative
of a resistance or voltage of the MR element that varies as a
function of clearance between the slider and the medium.
20. The apparatus of claim 14, further comprising a programmable
filter, coupled to the processor, that filters the rate of signal
change computations produced by the processor.
21. The apparatus of claim 20, wherein the programmable filter
comprises a finite impulse response (FIR) filter.
22. The apparatus of claim 20, wherein the programmable filter is
programmed to perform a least-squares linear fit using the rate of
signal change computations produced by the processor.
23. The apparatus of claim 14, wherein the processor cooperates
with the controller to adjust the medium-slider velocity or a
characteristic of the slider to maintain a desired clearance
between the slider and the medium.
24. The apparatus of claim 14, wherein the processor determines a
state of lubricity between the slider and the medium using the rate
of change of the signal for medium-slider velocities lower than the
transition velocity.
25. The apparatus of claim 14, wherein the processor characterizes
a surface profile of the slider using the rate of change of the
signal for medium-slider velocities lower than the transition
velocity.
26. A data storage system, comprising: a data storage disk mounted
to a motor; at least one magnetoresistive (MR) transducer
comprising an MR element, the MR transducer supported by a slider
and producing a signal that varies as a function of clearance
between the slider and the disk, the signal comprising a thermal
component representing a thermal response of the MR element; a
movable support structure, the support structure supporting the
slider and moving slider across the disk; a controller, coupled to
the motor, that controls a velocity of the disk relative to the
slider, the controller reducing the velocity of the disk relative
to the slider; and a processor, coupled to the controller and MR
transducer, that computes a rate of change of the signal during
reduction of the disk-slider velocity and determines a disk-slider
transition velocity at which the rate of change of the signal
exceeds a pre-established threshold, the processor determining, for
a nominal disk-slider velocity, absolute clearance between the
slider and the disk using the transition velocity and a clearance
profile associated with the slider.
27. The system of claim 26, wherein the processor performs a
plurality of rate of change computations using the signal to
establish the pre-established threshold.
28. The system of claim 26, wherein the transition velocity is
representative of a disk-slider velocity at which the rate of
change of the signal exceeds the pre-established threshold by about
10 percent or more.
29. The system of claim 26, wherein the transition velocity is
representative of a disk-slider velocity at which appreciable
contact occurs between the slider and the disk.
30. The system of claim 26, wherein the clearance profile is
representative of a relationship between landing or takeoff
velocity of the slider and associated slider-disk clearance.
31. The system of claim 26, wherein the signal is representative of
a resistance or voltage of the MR element that varies as a function
of clearance between the slider and the disk.
32. The system of claim 26, further comprising a programmable
filter, coupled to the processor, that filters the rate of signal
change computations produced by the processor.
33. The system of claim 26, wherein the programmable filter is
programmed to perform a least-squares linear fit using the rate of
signal change computations produced by the processor.
34. The system of claim 26, wherein the processor cooperates with
the controller to adjust the disk-slider velocity or a
characteristic of the slider to maintain a desired clearance
between the slider and the disk.
35. The system of claim 26, wherein the processor determines a
state of lubricity between the slider and the disk using the rate
of change of the signal for disk-slider velocities lower than the
transition velocity.
36. The system of claim 26, wherein the processor characterizes a
surface profile of the slider using the rate of change of the
signal for disk-slider velocities lower than the transition
velocity.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to data storage
systems and, more particularly, to a system and method for
measuring absolute clearance between a magnetoresistive transducer
and a medium using a thermal response of the MR transducer.
BACKGROUND OF THE INVENTION
[0002] In many applications, it is desirable to ascertain the
clearance or spacing between an MR transducer and a data storage
disk. By way of example, an unexpected change in head-disk
clearance of a particular head is generally indicative of a problem
with the head or head assembly. An appreciable decrease in
head-disk clearance by one head of a disk drive system relative to
other heads may be indicative of a suspect head.
[0003] One known method for determining head-disk clearance is
referred to as a Harmonic Ratio Flyheight (HRF) clearance test. A
HRF testing approach typically requires employment of a dedicated
tester which may take several minutes to complete HRF testing of a
disk drive. A HRF testing approach, as well as other known
flyheight evaluation techniques, require that dedicated tracks of
magnetic information be available over which each of the heads must
pass with relative high precision in order to obtain an accurate
head flying height measurement. Accordingly, such known flyheight
measuring approaches require the presence of a previously recorded
magnetic pattern on the disk.
[0004] Moreover, present HRF measurement techniques are very
sensitive to off-track deviations. By way of example, approximately
one-half of the time required to perform a HRF measurement during
manufacturing involves accurately locating the centerline of the
dedicated magnetic test track. Further, HRF testing data becomes
highly inaccurate when the low pole frequency of the arm
electronics (AE) is near HRF readback frequencies. In such cases,
additional attenuation may cause HRF clearance measurement errors
in excess of 50 nanometers (nm) for some disk drive systems.
Although such attenuation and may be compensated for if the AE low
frequency pole can be accurately estimated, known approaches for
accurately estimating the AE low frequency pole are problematic for
a variety of reasons.
[0005] Other known head flyheight evaluation techniques involve the
use of the thermal response of an MR head. Reference is made to
co-owned U.S. Pat. No. 5,751,510 which discloses techniques
concerning the identification, processing, and uses of the thermal
signal component of a readback signal, including head flyheight
evaluation. U.S. Pat. No. 5,751,510 is hereby incorporated herein
by reference in its entirety.
[0006] The thermal response of an MR head is particularly useful
when evaluating disk topography variations. Techniques that exploit
the thermal response of the MR head for purposes of determining
head flyheight generally require that the disk be operated at a
fixed speed, and generally rely on various calibration or
normalization methods so that the thermal response of the MR head
is useable. Although such techniques provide for an accurate
representation of relative head-disk spacing, such known methods
that utilize the thermal response of an MR head are generally not
capable of providing an estimate as to absolute head-disk
clearance.
[0007] There exists a need in the data storage system manufacturing
community for an apparatus and method for measuring absolute
head-disk clearance at the time of manufacturing and, importantly,
during subsequent use in the field. There exists a further need for
an apparatus and method for detecting absolute head-disk clearance
without the need for dedicated test tracks. There exists yet a
further need to provide such an apparatus and method which is
suitable for incorporation into existing data storage systems, as
well as into new system designs, and one that operates fully
autonomously in-situ a data storage system. The present invention
is directed to these and other needs.
SUMMARY OF THE INVENTION
[0008] The present invention is directed to an apparatus and method
for measuring absolute clearance between a magnetoresistive
transducer and a medium which moves relative to the MR transducer.
The medium may be devoid of magnetic information or may include
magnetic information. In accordance with the principles of the
present invention, a signal is produced using an MR element of the
MR transducer, such that the signal varies as a function of
clearance between the MR element and the medium. The signal
comprises a thermal component representative of a thermal response
of the MR element.
[0009] The velocity of the medium is reduced relative to the MR
transducer. While reducing the medium-transducer velocity, the rate
of change of the signal produced by the MR transducer is monitored.
Using data associated with the rate of change of the signal during
spindown, absolute clearance between the MR transducer and the
medium is computed for a nominal medium-transducer velocity, such
as a full operational velocity.
[0010] Computing the absolute clearance between the MR transducer
and the medium may further involve determining a transition
velocity at which the rate of change of the signal exceeds a
pre-established threshold. The transition velocity is used to
compute the absolute clearance between the MR transducer and the
medium for a nominal medium-transducer velocity. In one embodiment,
for example, a transition velocity represents a medium-transducer
velocity at which the rate of change of the signal exceeds the
pre-established threshold by about 10% or more.
[0011] The transition velocity is representative of a
medium-transducer velocity at which the heat transfer behavior of
the MR transducer assembly transitions from a first thermal
transport mechanism to a second thermal transport mechanism. The
transition velocity coincides with a medium-transducer velocity at
which appreciable contact occurs between the transducer and the
medium.
[0012] Computing absolute clearance between the MR transducer and
medium may further involve associating a transition velocity, at
which the rate of change of the signal exceeds the pre-established
threshold, with an absolute clearance value obtained using a
clearance profile associated with the MR transducer. For example,
the clearance profile may be representative of a relationship
between landing or take-off velocity of the MR transducer and
associated transducer-medium clearance.
[0013] Monitoring the rate of change of the signal produced by the
MR element involves performing a plurality of rate of change or
slope computations using the signal to establish the
pre-established threshold. The signal produced by the MR element
may be representative of a resistance of the MR element that varies
as a function of transducer-medium clearance. The signal may
alternatively be representative of a voltage across the MR element
that varies as a function of transducer-medium clearance. During
the absolute transducer-medium clearance measurement procedure, the
MR element may be biased using a constant current.
[0014] According to another embodiment of the present invention,
the thermal spindown technique for measuring absolute
transducer-medium clearance may also be employed to evaluate
lubricity between a slider that supports the MR transducer and the
medium. According to this embodiment, the transition velocity at
which the rate of change of the signal produced by the MR
transducer exceeds a pre-established threshold is determined. The
state of lubricity between a slider that supports the MR transducer
and the medium may then be determined using the rate of change of
the signal for medium-transducer velocities lower than the
transition velocity.
[0015] The rate of change signal samples acquired for
medium-transducer velocities lower than the transition velocity may
be characterized in terms of a curve having a given slope. The
slope of the curve is related to the state of the temperature
profile of slider/MR transducer. If the slope of such a curve is
appreciably greater than the slope of previously computed curves
for the same slider assembly, the resulting increase in
slider/medium frictional heating may be due to insufficient
provision of a lubricant between the slider and medium.
[0016] According to a further embodiment of the present invention,
a surface profile or crown of a slider may be characterized be
evaluating the slope of a curve developed using rate of change
signal samples acquired for medium-transducer velocities lower than
the transition velocity. A transition velocity at which the rate of
change of the signal exceeds a pre-established threshold is
determined. Since the slope of this curve may also be viewed as a
function of the configuration of the slider crown, a surface
profile of the slider may be characterized by evaluating changes in
the slope of the curve relative to previously computed curves for
the same slider.
[0017] In accordance with yet another embodiment, information
acquired during the thermal spindown procedure may be used to
adjust the medium-transducer velocity or a characteristic of a
particular slider in order to maintain a desired clearance between
the slider and the medium. Slider pre-load or pitch angle, for
example, may be adjusted, alone or in combination with a
medium-transducer velocity, to maintain a given slider at a desired
flyheight relative to the medium.
[0018] An apparatus for measuring absolute clearance between a
slider and a medium moving relative to the slider according to the
principles of the present invention includes an MR transducer
comprising an MR element which is supported by the slider. The MR
transducer produces a signal that varies as a function of clearance
between the MR element and the medium. The signal comprises a
thermal component representing a thermal response of the MR
element. The apparatus includes a controller that controls a
velocity of the medium relative to the slider, and, in particular,
reduces the velocity of the medium relative to the slider during
the absolute clearance measurement procedure.
[0019] A processor, which is coupled to the controller and MR
transducer, computes a rate of change of the signal during
reduction of the medium-slider velocity and determines a
medium-slider threshold velocity at which the rate of change of the
signal exceeds a pre-established threshold. The processor
determines, for a nominal medium-slider velocity, absolute
clearance between the slider and the medium using the threshold
velocity and a clearance profile associated with the slider.
[0020] The processor performs a number of rate of change
computations using the signal to establish the pre-established
threshold. The transition velocity is representative of a
medium-slider velocity at which the rate of change of the signal
exceeds the pre-established threshold by about 10 percent or more.
An apparatus for measuring absolute clearance between a slider and
a medium moving relative to the slider according to the principles
of the present invention may be incorporated in a data storage
system, such as a direct access storage device (DASD), or a test
apparatus.
[0021] The apparatus may further include a programmable filter,
which is coupled to the processor, that filters the rate of signal
change computations produced by the processor. In one embodiment,
the programmable filter comprises a finite impulse response (FIR)
filter. The programmable filter may be programmed to perform a
least-squares linear fit or other type of curve fitting technique
using the rate of signal change computations produced by the
processor.
[0022] The processor may also determine a state of lubricity
between the slider and the medium using rate of signal change data
acquired for medium-slider velocities lower than the transition
velocity. The processor may further characterize a surface profile
of the slider using rate of signal change data for medium-slider
velocities lower than the transition velocity. In another
embodiment, the processor cooperates with the controller to adjust
the medium-slider velocity or a characteristic of the slider to
maintain a desired clearance between the slider and the medium.
[0023] The above summary of the present invention is not intended
to describe each embodiment or every implementation of the present
invention. Advantages and attainments, together with a more
complete understanding of the invention, will become apparent and
appreciated by referring to the following detailed description and
claims taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a top perspective view of a disk drive system with
its upper housing cover removed;
[0025] FIG. 2 is a side plan view of a disk drive system comprising
a plurality of data storage disks;
[0026] FIG. 3 is a graph of normalized MR element voltage plotted
as a function of disk speed (RPM), the slope of the resulting curve
varying significantly between two dominant regimes representative
of two different thermal transport mechanisms associated with an MR
element assembly;
[0027] FIG. 4 is a plot of normalized MR element voltage as a
function of disk speed (RPM) showing a comparison of theoretical
and actual data obtained while performing an absolute head-disk
clearance procedure according to the principles of the present
invention;
[0028] FIG. 5 is a plot of disk landing speed (RPM) as a function
of full speed head-disk clearance which is developed for a
particular head of a given disk drive system;
[0029] FIG. 6 is a flow diagram of various process steps associated
with measuring absolute head-disk clearance in accordance with the
principles of the present invention;
[0030] FIG. 7 shows thermal spindown curves for seven different
sliders of a particular disk drive system obtained in accordance
with the principles of the present invention; and
[0031] FIG. 8 is a plot of the data shown in FIG. 6 subject to
finite impulse response (FIR) filtering.
[0032] While the invention is amenable to various modifications and
alternative forms, specifics thereof have been shown by way of
example in the drawings and will be described in detail
hereinbelow. It is to be understood, however, that the intention is
not to limit the invention to the particular embodiments described.
On the contrary, the invention is intended to cover all
modifications, equivalents, and alternatives falling within the
scope of the invention as defined by the appended claims.
DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS
[0033] In the following description of the illustrated embodiments,
references are made to the accompanying drawings which form a part
hereof, and in which is shown by way of illustration, various
embodiments in which the invention may be practiced. It is to be
understood that other embodiments may be utilized, and structural
and functional changes may be made without departing from the scope
of the present invention.
[0034] An absolute head-disk clearance measurement methodology of
the present invention may be implemented to estimate the absolute
clearance between an MR element and a medium which is moving
relative to the MR element. Various embodiments of the present
invention contemplate use of the disclosed methodology in an
apparatus that employs a slider, upon which an MR element is
supported, and a rotatable medium, such as a disk substrate.
[0035] A significant advantage of the present invention concerns
the ability to perform absolute head-medium clearance measurements
for disks that are devoid of a magnetic coating or magnetic
information. As such, absolute head-medium clearance measurements
according to the principles of the present invention may be
performed using disk blanks, such as glass, ceramic or aluminum
polished or unpolished disk substrates, for example.
[0036] The following system description contemplates employment of
an absolute head-medium clearance measurement methodology according
to the present invention in the context of a disk drive system that
includes one or more magnetizable disks. It is to be understood
that the systems and methods described herein are provided for
illustrative purposes only, and are not to be regarded as limiting
embodiments.
[0037] Referring to the drawings, and more particularly to FIGS. 1
and 2, there is illustrated a data storage system 20 within which
an absolute head-disk clearance measurement methodology of the
present invention may be implemented. The disk drive system 20, as
is best shown in FIG. 2, typically includes one or more rigid data
storage disks 24 which are stacked coaxially in a tandem spaced
relationship, and rotate about a spindle motor 26 at a relatively
high rate of rotation.
[0038] As is depicted in FIG. 1, each disk 24 is typically
magnetically formatted to include a plurality of spaced concentric
tracks 50. One or more of the disks 24 may alternatively be
magnetically formatted to include a spiraled track configuration,
or a combination of concentric and spiraled track configurations.
Digital information is typically stored in the form of magnetic
transitions along the tracks 50. The tracks 50 are generally
divided into a number of sectors 52, with each sector 52 comprising
a number of information fields, including fields for storing data,
and sector identification and synchronization information, for
example.
[0039] Writing data to a magnetic data storage disk 24 generally
involves passing a current through the write element of the
transducer assembly 27 to produce magnetic lines of flux which
magnetize a specific location of the disk surface 24. Reading data
from a specified disk location is typically accomplished by a read
element of the transducer assembly 27 sensing the magnetic field or
flux lines emanating from the magnetized locations of the disk
surface 24. As the read element passes over the rotating disk
surface 24, the interaction between the read element and the
magnetized locations on the disk surface 24 results in the
production of electrical signals, commonly referred to as readback
signals, in the read element.
[0040] An actuator 30 typically includes a number of interleaved
actuator arms 28 with each arm having one or more transducer 27 and
slider assemblies 35 mounted to a load beam 25 for transferring
information to and from the data storage disks 24. The slider 35 is
typically designed as an aerodynamic lifting body that lifts the
transducer 27 off the surface of the disk 24 as the rate of spindle
motor rotation increases and causes the transducer 27 to hover
above the disk 24 on an airbearing produced by high speed rotation
of the disk 24. The distance between the slider 35 and the disk
surface 24, which is typically on the order of 30-100 nanometers
(nm), is commonly referred to as head-disk clearance or
spacing.
[0041] The actuator 30 is typically mounted to a stationary
actuator shaft 32 and rotates on the shaft 32 to move the actuator
arms 28 into and out of the stack of data storage disks 24. A coil
assembly 36, mounted to a coil frame 34 of the actuator 30,
generally rotates within a gap 44 defined between the upper and
lower magnet assemblies 40 and 42 of a permanent magnet structure
38 causing the actuator arms 28, in turn, to sweep over the surface
of the data storage disks 24. The spindle motor 26 typically
comprises a DC motor energized by a power supply 46 and adapted for
rotating the data storage disks 24. The coil assembly 36 and the
upper and lower magnet assemblies 40 and 42 of the permanent magnet
structure 38 operate in cooperation as an actuator voice coil motor
39 responsive to control signals produced by a servo processor 56.
The servo processor 56 controls the direction and magnitude of
control current supplied to the voice coil motor 39. The actuator
voice coil motor 39 produces a torquing force on the actuator coil
frame 34 when control currents of varying direction and magnitude
flow in the coil assembly 36 in the presence of a magnetic field
produced by the permanent magnet structure 38. The torquing forces
imparted on the actuator coil frame 34 cause corresponding
rotational movement of the actuator arms 28 in directions dependent
on the polarity of the control currents flowing in the coil
assembly 36.
[0042] The data storage system 20 shown in FIG. 1 preferably
employs a closed-loop servo control system for positioning the
read/write transducers 27 to specified storage locations on the
data storage disk 24. During normal data storage system operation,
a servo transducer, generally mounted proximate the read/write
transducers, or, alternatively, incorporated as the read element of
the transducer assembly 27, is typically employed to read
information for the purpose of following a specified track (i.e.,
track following) and locating (i.e., seeking) specified track and
data sector locations on the disk surface 24.
[0043] In accordance with one servo technique, embedded servo
pattern information is written to the disk 24 along segments
extending in a direction generally outward from the center of the
disk 24. The embedded servo patterns are thus formed between the
data storing sectors of each track 50. It is noted that a servo
sector typically contains a pattern of data, often termed a servo
burst pattern, used to maintain optimum alignment of the read/write
transducers 27 over the centerline of a track 50 when transferring
data to and from specified data sectors on the track 50. The servo
information may also include sector and track identification codes
which are used to identify the location of the transducer assembly
27.
[0044] The servo processor 56, which cooperates with read channel
electronics 57, regulates the actuator voice coil motor 39 to move
the actuator arms 28 and transducers 27 to prescribed track 50 and
sector 52 locations when reading and writing data to and from the
disks 24. The servo processor 56 is loosely coupled to a disk drive
controller 58. The disk drive controller 58 typically includes
control circuitry and software that coordinate the transfer of data
to and from the data storage disks 24. Although the servo processor
56 and disk drive controller 58 are depicted as two separate
devices in FIG. 1, it is understood that the functionality of the
servo processor 56 and disk drive controller 58 may be embodied in
a single multi-purpose processor, which typically results in a
reduced component cost.
[0045] A system and method in accordance with the principles of the
present invention generally provide for measuring an absolute
head-disk clearance using the thermal response of an MR transducer.
A system and method according to the present invention is well
suited for implementation in-situ a disk drive system of the type
described above with reference to FIGS. 1 and 2, it being
understood that the invention is not limited to use in a disk drive
system nor to use in the particular operating environment described
in the above discussion.
[0046] An absolute head-disk clearance measurement methodology
consistent with the principles of the present invention exploits a
phenomenon observed by the inventor by which the rate of change of
the thermal response of an MR head changes significantly as the MR
transducer transitions from flying above a medium to a state of
contacting the medium. In particular, and with reference to FIG. 3,
the temperature, and therefore resistance and voltage, of an MR
transducer changes significantly as the transducer transitions from
a fully flying state to a contacting state relative to a medium.
This change in MR transducer signal output can be seen in FIG. 3 as
a change in the slope of the curve 61 as the curve 61 passes from
regime-1 to regime-2.
[0047] It is noted that the data plotted in FIG. 3 was obtained
using a particular head of a test disk drive system. The data was
collected by first placing the arm electronics (AE) in an MR
resistance measurement mode and then monitoring the MR resistance
by measuring the MR voltage at constant MR bias current while the
disk velocity was reduced and the heads were held at the inner
diameter landing zone radius.
[0048] The two regimes, regime-1 and regime-2, represent different
thermal transport mechanisms associated with the slider supporting
the MR transducer as the slider transitions from a non-contacting
flying state to a contacting state. As can be seen in FIG. 3, the
slope of curve 61 in regime-1 is nearly constant, which will later
be shown to be proportional to the resistance of the MR element and
other factors that are essentially constant. In regime-1, as will
later be shown, the absolute spacing of the MR transducer relative
to the medium surface is not a governing factor. In regime-2, in
contrast to regime-1 wherein the slider is flying above the disk,
the slider is sliding on the disk surface.
[0049] In an embodiment in which the slider exhibits a positive
surface profile or crown, contact between the slider and the disk
surface induces a forward pitching motion of the slider, due to its
positive crown, which greatly reduces the rate at which heat is
transferred from the MR element. This pitching motion, coupled with
the decrease in disk speed, creates a much steeper slope of curve
61 in regime-2 as compared to that in regime-1.
[0050] An absolute head-disk clearance methodology of the present
invention monitors the slope or rate of change of curve 61 and
detects appreciable changes in the slope of curve 61. Detection of
an appreciable increase in the slope of curve 61 may be used to
determine the speed at which a particular head is no longer flying
above the disk surface without making appreciable contact with the
disk surface.
[0051] As was stated hereinabove, the two regimes, regime-1 and
regime-2, depicted in FIG. 3 represent two different slider/MR
element thermal transport mechanisms which will now be discussed in
greater detail. With respect to the thermal transport mechanism
associated with regime-1, it is assumed that a slider is flying
above the disk surface and that any sporadic intermittent contact
between the slider and disk surface does not alter this overall
assumption.
[0052] Assuming a laminar thermal boundary layer between the moving
disk and relatively stationary slider, a one-dimensional heat
transfer relationship may be developed as follows: 1 k 2 T y 2 = -
( u y ) 2 [ 1 ]
[0053] where, k and .mu. represent the thermal conductivity and
absolute viscosity of air, respectively, T represents temperature,
u represents the velocity of the disk, and y represents the
vertical spacing distance between the slider and the disk
surface.
[0054] Although Equation [1] may be used to derive the thermal
gradient in the air film, what is of particular interest is the
temperature of the MR element as a function of disk velocity, u.
Using a well-known axiom in boundary layer theory, it is assumed
that, at the boundary between a solid body and a fluid, the
transfer of heat is due solely to conduction.
[0055] In view of this axiom, the Nusselt Number, Nu, may be used
and defined as follows: 2 Nu = ql k ( Tw - T .infin. ) = ql k ( T
MR - T DISK ) [ 2 ]
[0056] where, q represents the heat dissipation in the direction of
the disk from the MR element, l represents a characteristic length,
such as the width of the MR strip, k represents the thermal
conductivity of air, T.sub.MR represents the temperature of the MR
element, T.sub.DISK represents the temperature of the disk, Tw
represents wall temperature, and T.sub..infin.represents a fixed
reference temperature.
[0057] When considering the boundary layers, a relationship between
the Nusselt Number, Nu, and the Reynolds Number, Re, may be used,
as is characterized in the following equation: 3 Nu = Re .times. f
3 ( x l , P ) [ 3 ]
[0058] where, P represents local pressure, l represents a
characteristic length, such as the width of the MR stripe, and x
represents a portion of the characteristic length, l. It is noted
that for purposes of the instant analysis, the function f.sub.3 is
of no interest.
[0059] The Reynolds Number, Re, may be defined as follows: 4 Re = U
.infin. l v = U DISK l v [ 4 ]
[0060] where, U.sub.DISK and v represent the disk velocity and
kinematic viscosity of air, respectively. Combining Equation [3]
and Equation [4] above gives the needed relationship between MR
element and disk temperatures, as is characterized in the following
equation:
T.sub.MR-T.sub.DISK=q.LAMBDA.(U.sub.DISK).sup.-0.5 [5]
[0061] where, 5 = vl k 2 [ 6 ]
[0062] and q represents the heat dissipation of the MR element.
[0063] Equation [5] above may be restated in a more useful form by
noting that the MR element heat dissipation, q, may be expressed in
terms of the MR resistance, R, bias current, i.sub.b, and the
fraction, F, of energy emanating into the air (that is, the
fraction of energy not emanating to the shields). Given the above,
Equation [5] above may be rewritten as follows: 6 T MR - T DISK = i
b 2 RF ( U DISK ) - 0.5 [ 7 ]
[0064] Data produced using Equation [7] above is plotted in FIG. 4,
along with the same data points obtained from actual data plotted
in FIG. 3. FIG. 4 demonstrates good correlation between theoretical
and actual measurements with respect to regime-1.
[0065] Using Equation [7] above, an equation which characterizes
the MR element voltage may be obtained. MR element voltage may be
obtained by multiplying the temperature coefficient of resistance
of the MR element, .alpha., by the MR current, i.sub.b, and
resistance, R, as is characterized by the following equation:
E.sub.MR=.LAMBDA.F.alpha.R.sup.2i.sub.b.sup.3(U.sub.DISK).sup.-0.5
[8]
[0066] Equation [8] above demonstrates that head-disk spacing is
not a significant factor with respect to regime-1. The slope of the
thermal spindown curve in regime-1 is driven primarily by the MR
resistance, R, and current, i.sub.b. This observation has been
verified experimentally by the inventor.
[0067] FIG. 4 also demonstrates a significant departure between
theory and experiment when the speed of the disk, measured in RPM,
is small compared to nominal disk speed. The departure or error is
due to a new thermal regime, namely, regime-2, which is best shown
in FIG. 3.
[0068] In regime-2, which occurs when the disk velocity is low
enough to cause the slider to touch and slide along the disk
surface, the dominant heat transfer mechanism is conduction between
the slider/MR element and the surrounding air. Unlike regime-1,
however, a simple parallel surface geometry between the slider and
disk surface cannot be assumed. Rather, in regime-2, pitching or
rocking motion of the slider due to contact with the disk surface
must be considered.
[0069] When a slider with a positive surface profile or crown
touches the surface of a moving disk, a pitching motion is induced
which causes the slider to pitch forward. This forward pitching
motion increases the distance between the MR element and the disk
surface. It has been observed experimentally that there is a
one-to-one correspondence between the time when the pitching motion
begins and the time when the thermal spindown enters regime-2.
[0070] As the slider settles onto the disk surface as disk velocity
gradually decreases, the forward pitching motion increases and the
rate at which heat leaves the MR element into the surrounding air
decreases, owing to the lower air velocity adjacent to the MR
element. A quasi-static dynamic equation that characterizes this
phenomenon is given below as:
d=b(P.sub.1-KU.sub.DISK)C.sub.fa/K.sub..theta. [9]
[0071] where, d represents the MR element-to-disk spacing, a
represents the distance between the mean plane of the airbearing
surface in the center of slider pitch rotation, b represents the
distance between the mean plane of the airbearing surface and the
location of the MR element, P.sub.1 represents the pre-load force
on the slider, KU.sub.DISK represents the force created by the
airbearing which is in the opposite direction of P.sub.1, C.sub.f
represents the dynamic coefficient of friction between the slider
and disk surface, and K.sub..theta. represents slider pitch
stiffness.
[0072] By taking the first derivative of Equation [9] above with
respect to U.sub.DISK, it can be seen that the rate of increase in
MR element-to-disk spacing, d, should be constant in regime-2.
Since the rate of heat transfer is linear with respect to changes
in MR element-to-disk spacing, as is indicated by Fourier's law of
heat conduction, it can be seen that spacing, and not disk
velocity, is the likely dominant mechanism impacting changes in MR
element temperature, and, thus, MR element voltage.
[0073] Having established a theoretical and corresponding
experimental foundation for the above-described thermal spindown
phenomenon, procedures which exploit this thermal spindown
phenomenon may be developed for performing absolute head-disk
clearance measurements in accordance with the principles of the
present invention. It is important to appreciate that there is no
inherent calibration necessary when employing a thermal spindown
approach of the present invention, as would otherwise be required
if a magnetic spindown technique according to a prior art approach
were to be employed.
[0074] Instead, many of the significant thermal spindown factors
associated with an absolute head-disk clearance measurement
procedure according to the present invention do not impact actual
head-disk clearance. For this reason, absolute head-disk clearance
measurements may be based on a known relationship between the
landing velocity and absolute flying height of a slider, and on the
disk velocity at which a transition from regime-1 to regime-2
slider behavior is detected.
[0075] Referring now to FIG. 6, there is illustrated various
process steps associated with an absolute head-disk clearance
measurement methodology in accordance with the principles of the
present invention. Initially, a slider is positioned 100 at a
desired radial location above the disk surface. As was discussed
previously, the disk may, but need not, include a magnetic coating,
as the magnetic readback signal component is not needed to measure
absolute head-disk clearance according to the present
invention.
[0076] It is assumed that the slider is initially flying at a
desired location above the disk at a given speed so that no
appreciable head-disk contact occurs, such as at a full or nominal
disk speed. The desired disk location may be a landing zone
provided at an inner diameter or outer diameter disk location, or
other location on the disk. As was discussed previously, it is not
necessary to position the slider over a dedicated track or location
of the disk, as is required when conventional head flyheight
measurement approaches are employed.
[0077] After positioning 100 the slider at the desired disk
location, the speed of the disk is reduced 102 While the disk speed
is being reduced, the voltage of the MR element is monitored 104.
It is understood that resistance, voltage, current, temperature, or
other parameter of the MR element which varies as a function of
head-disk spacing may be monitored, but that voltage in the instant
embodiment is a more convenient parameter to use.
[0078] Monitoring 104 the MR element voltage during disk spindown
typically involves computing the rate of change of the MR element
voltage during disk spindown, which corresponds to computing the
slope of curve 61 shown in FIG. 3, for example. The computed rate
of change of MR element voltage is compared 106 to a
pre-established threshold, such as a maximum MR element voltage
rate of change threshold. If the pre-established threshold is not
exceeded, disk spindown and monitoring of MR element voltage change
continues.
[0079] The maximum MR element voltage rate of change threshold may
be computed in several ways. In accordance with one embodiment, the
rate of change of MR element voltage, or other parameter of the MR
element that varies as a function of head-disk spacing, is computed
dynamically on a repeated basis during disk spindown 102.
[0080] A statistical evaluation of a number of MR element voltage
samples is performed to determine whether the rate of change of the
MR element voltage indicates a transition from regime-1 slider
behavior to regime-2 slider behavior. An appreciable change in MR
element voltage associated with a most recent MR element voltage
sample relative to previous MR element voltage samples typically
indicates the beginning of regime-2 slider behavior and, thus,
appreciable contact 108 between the slider and disk surface.
[0081] By way of example, a series of MR element voltage rate of
change (i.e., slope) computations made during initial disk spindown
102 will typically indicate that the MR element voltage changes at
a constant rate relative to the rate of disk speed reduction. This
relationship between MR element voltage rate of change and disk
velocity change is characteristic of regime-1 slider dynamics, as
is indicated by the constant slope of curve 61 in FIG. 3. As disk
spindown 102 continues, MR element voltage slope computations are
repeatedly compared against previously computed voltage slope
values to determine whether a new data point deviates significantly
from those generated by previous computations.
[0082] For example, an appreciable change of a given MR element
voltage data point may represent a 10% or greater deviation from
slope computations associated with previously acquired MR element
voltage data. A significant deviation in MR element voltage or rate
of voltage change indicates a transition from regime-1 to regime-2
slider dynamics and, consequently, detection 108 of head-disk
contact.
[0083] At the time the transition from regime-1 to regime-2 is
detected, which represents appreciable contact 108 between the
slider and disk surface, the disk speed is stored and used to
calculate 110 absolute head-disk clearance. In particular, the disk
velocity at which an appreciable increase in the rate of change of
MR element voltage is detected is used to determine the absolute
head-disk clearance for the particular head at full or nominal
speed. A head-disk clearance profile developed specifically for
each head is used to relate a regime-1 regime-2 transition velocity
to an absolute head-disk spacing at a full or nominal disk
velocity.
[0084] The clearance profile illustrated in FIG. 5, for example,
graphically depicts a relationship between nominal slider flying
height at full disk speed and slider landing velocity for the type
of slider being used in a particular disk drive family. This
general relationship between full speed slider flyheight and
landing velocity is well-understood, and can be obtained in several
ways, such as through use of airbearing modeling, empirically
through use of known HRF techniques or through use of acoustic
emission transducers, for example. In the case of the relationship
depicted in FIG. 5, the landing velocity data was determined by use
of known HRF methods.
[0085] A clearance profile which can be used to calculate full
speed absolute head-disk clearance may be estimated on the basis of
landing velocity or, alternatively, on the basis of takeoff
velocity by using a simple linear relationship, as is indicated by
the straight line in FIG. 5. It is understood, however, that actual
landing zone roughness will influence the landing velocity. Since
disk surface roughness is typically dominated by laser bumps with
good height control, the landing velocity is dominated by the
actual head flying height. In future disk drive systems, very
smooth disks will likely be used which will have even tighter disk
roughness distributions.
[0086] For purposes of illustration, and with reference to FIGS. 3
and 6, it may be assumed that a disk is rotated at a nominal speed
(e.g., full speed) of 7,200 RPM. MR element voltage change
computations are made while the speed of the disk is reduced from
the nominal disk speed of 7,200 RPM. As is shown in FIG. 3, the
slope of curve 61 changes appreciably at about 3,200 RPM.
Thirty-two hundred RPM, in this illustrative example, represents
the speed at which the slider subjected to the absolute head-disk
clearance measurement procedure transitions from regime-1 to
regime-2 behavior.
[0087] The disk velocity value of 3,200 RPM is then applied to the
clearance profile shown in FIG. 5. As can be seen in FIG. 5, a disk
landing speed of 3,200 RPM corresponds to a full speed absolute
head-disk clearance of 5 nm. In other words, the particular head
under evaluation will be flying at approximately 5 nm above the
disk surface at a nominal disk speed of 7,200 RPM during normal
operation. In this case, a full speed flyheight of 5 nm may be
indicative of a suspect head, especially if this and other heads
for a given disk drive system are designed to have a clearance of
approximately 20-40 nm under nominal conditions.
[0088] It is noted that the ambient pressure within the disk drive
system has a significant influence on head flyheight. In cases of
reduced pressure, such as when operating the disk drive system at
high altitudes, a comparison between the full speed flyheight of a
given head relative to that of other heads may be used as a basis
for detecting a suspect head.
[0089] By way of further illustration, and assuming that a given
slider experiences a transition between regime-1 and regime-2 at
approximately 2,400 RPM, the head-disk clearance associated with
this head at full speed (i.e., 7,200 RPM) is approximately 12 nm,
which is obtained directly from the data plotted in FIG. 5. As a
further example, a particular head which experiences an appreciable
change in MR element voltage at 1,800 RPM will realize a nominal
flyheight of approximately 20 nm at full disk speed.
[0090] FIG. 7 shows a number of thermal spindown curves for seven
different sliders of a particular disk drive system. One slider,
which is characterized by curve 63, appears to be in near contact
with the disk surface at full/nominal disk speed (i.e., 7,200 RPM
in this illustrative example). This conclusion is based on the
observation that thermal regime-2 behavior is detected at about
4,500 RPM for this particular slider.
[0091] Based on the linear fit shown in FIG. 5, this low-flying
head has an estimated clearance of less than 1 nm. In this
illustrative example, it can be appreciated that detecting this
low-flying condition is extremely important to the long-term
reliability of the subject disk drive system.
[0092] In order to automate the process of identifying thermal
regime-2 slider behavior in the acquired thermal spindown data, a
simple MR element voltage threshold may be established based on the
slope of the thermal voltage signal, as previously discussed. Use
of the thermal voltage slope approach advantageously enhances
detection of head-disk contact, since the slope in regime-2 is many
times greater than the slope associated with the thermal voltage in
regime-1, as can clearly be seen in FIG. 3.
[0093] A three-point finite impulse response (FIR) filter may be
used to enhance identification of the starting point of regime-2 in
the acquired thermal spindown data. By way of example, a
three-point FIR filter programmed to operate as a least-squares
linear slope predictor may have coefficients of [+0.5, 0,
-0.5].
[0094] Taking the same data shown in FIG. 7 and applying this data
to the FIR filter having coefficients of [+0.5, 0, -0.5] results in
the production of the data plotted in FIG. 8. A clear
stratification between sliders flying with sufficient head-disk
clearance and those flying with insufficient head-disk clearance is
evident in the filtered data plotted in FIG. 8. An appropriately
programmed digital filter, such as the above-described programmed
FIR filter, may be used to pre-process thermal spindown data before
a threshold is applied.
[0095] By setting a threshold of 0.1 on the vertical axis of FIG.
8, for example, head-disk clearances can be estimated. The
particular threshold used is based primarily on the type of slider
employed and the MR bias current. It can be seen, for example, that
each of the curves in FIG. 8 represents a unique function of disk
speed (RPM), and that all of the curves are monotonically
increasing within the range of about 0.075 to about 0.12. A
threshold may be established to fall within this range (e.g., 0.1)
for purposes of accurately estimating head-disk clearance and
reliably detecting low flying heads.
[0096] In accordance with another embodiment, the thermal spindown
technique discussed hereinabove may be used to characterize the
slider crown. Referring again to FIG. 3, the slope of curve 61 in
thermal regime-2 may be viewed as being dependent on the profile of
the slider crown. If the slider is completely flat or has a
negative crown, it is much less likely that any slider pitching
motion will occur in regime-2, and thus the curve 61 will be much
flatter. The curve 61 associated with such a slider will still have
some slope, however, because of frictional heating as a result of
sliding.
[0097] Sliders which have positive crowns are associated with
curves 61 having steeper slopes in thermal regime-2. As such, the
slope of curve 61 may be used to characterize the crown of a given
slider, such as in terms of profile or change in profile over
time.
[0098] In accordance with yet another embodiment, the lubricity of
the head-disk interface (HDI) may be evaluated using the
above-described thermal spindown technique. For a given slider with
a certain crown, any significant increase in the slope of curve 61
in regime-2 can result from the lack of a sufficient amount of
lubricant which, of course, results in increased frictional heating
of the slider/MR transducer during head sliding. Thus, the slope of
curve 61 in regime-2 may be monitored for all heads, and this data
may be used as a measure of the lubricity of the head-disk
interface. The slope of curve 61 may be estimated, such as by use
of a linear fit approach, and then compared to the estimated slope
over time.
[0099] The slope of curve 61 may also be measured at different disk
radii. The radial distribution of a lubricant is due, in part, to
centrifugal and other forces that tend to make the lubricant spin
off toward the outer diameter of the disk. This movement of the
lubricant may take several months, and must eventually be
replenished. An example of an in-situ lubricant replenishment
system is disclosed in commonly assigned U.S. Pat. No. 5,305,301,
which is hereby incorporated herein by reference in its
entirety.
[0100] The servo feedback for the system disclosed in U.S. Pat. No.
5,305,301 is ideally head-disk friction, which can be estimated
using the slope of curve 61 in regime-2. Of course, the point of
transition from regime-1 to regime-2 may also change as a function
of lubricity. This transition point may be estimated by using an
estimate of the y-intercept derived from the linear fit mentioned
previously.
[0101] In accordance with another embodiment, the information
obtained from a thermal spindown according to the principles of the
present invention may be used to adjust disk speed or some other
controlling means, such as a slider pre-load or pitch angle, in
order to keep the heads flying at some desired flying height. For
near-contact recording, for example, it is desirable to keep the
sliders of a disk drive system as close to the disk surface as
possible, without causing the sliders to slide on the disk. Since
the y-intercept discussed in the previous paragraph identifies this
flying-to-sliding transition, the sliders may be set to remain in
this regime regardless of altitude, temperature, or age of the disk
drive system.
[0102] An absolute head-disk clearance methodology according to the
principles of the present invention employs thermal spindown data
as a basis for estimating absolute head-disk clearance. The thermal
spindown approach of the present invention advantageously avoids
many of the significant problems associated with magnetic clearance
methods, such as known HRF techniques.
[0103] In particular, the thermal spindown methodology of the
present invention obviates the need to perform time consuming track
alignment. Also, a thermal spindown approach of the present
invention is not affected by the low pole of the arm electronics
(e.g., pre-amplification circuitry). The thermal spindown
techniques of the present invention may be performed in-situ a disk
drive by locating the sliders at a known location, such as against
a crash stop, switching the arm electronics into MR resistance
measurement mode, monitoring the MR element voltage during a disk
spindown procedure, and calculating the clearance based on the
expected landing velocity versus clearance profile for the
particular slider used in the disk drive under evaluation.
[0104] The foregoing description of the various embodiments of the
invention has been presented for the purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise form disclosed. Many modifications and
variations are possible in light of the above teaching. It is
intended that the scope of the invention be limited not by this
detailed description, but rather by the claims appended hereto.
* * * * *