U.S. patent application number 13/844643 was filed with the patent office on 2014-09-18 for in situ sensor based contact detection.
The applicant listed for this patent is SEAGATE TECHNOLOGY LLC. Invention is credited to CheeWee Cheng, Andy Chou, SweeChuan Samuel Gan, Subhash Guddati, Stefan Ionescu, ShengYuan Lin, Richard Martin, Scott Ryun, Jesse Speckhard.
Application Number | 20140268406 13/844643 |
Document ID | / |
Family ID | 51493416 |
Filed Date | 2014-09-18 |
United States Patent
Application |
20140268406 |
Kind Code |
A1 |
Cheng; CheeWee ; et
al. |
September 18, 2014 |
In Situ Sensor Based Contact Detection
Abstract
Apparatus and method for positional sensing and control. In
accordance with some embodiments, a transducer is positioned
adjacent a recording medium. The transducer includes a write
element, a read element, a heater and a thermally responsive
sensor. Power is applied to the heater to establish a selected fly
height of the transducer relative to the medium. A contact event
between the transducer and the medium is detected responsive to an
accumulated plural count of pulses in a bias signal obtained from
the thermally responsive sensor.
Inventors: |
Cheng; CheeWee; (Singapore,
SG) ; Guddati; Subhash; (Singapore, SG) ; Lin;
ShengYuan; (Singapore, SG) ; Chou; Andy;
(Singapore, SG) ; Gan; SweeChuan Samuel;
(Singapore, SG) ; Ionescu; Stefan; (Burnsville,
MN) ; Ryun; Scott; (Victoria, MN) ; Martin;
Richard; (Longmont, CO) ; Speckhard; Jesse;
(Douglas, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SEAGATE TECHNOLOGY LLC |
Cupertino |
CA |
US |
|
|
Family ID: |
51493416 |
Appl. No.: |
13/844643 |
Filed: |
March 15, 2013 |
Current U.S.
Class: |
360/75 |
Current CPC
Class: |
G11B 5/607 20130101;
G11B 5/6076 20130101 |
Class at
Publication: |
360/75 |
International
Class: |
G11B 5/60 20060101
G11B005/60 |
Claims
1. A method comprising: positioning a transducer with respect to a
recording medium, the transducer comprising a write element, a read
element, a heater and a thermally responsive sensor; applying power
to the heater to establish a selected fly height of the transducer
relative to the medium; and detecting a contact event between the
transducer and the medium responsive to an accumulated plural count
of pulses in a bias signal obtained from the thermally responsive
sensor.
2. The method of claim 1, wherein the power applied to the heater
is at a power level selected responsive to the thermally responsive
sensor.
3. The method of claim 1, further comprising performing a write
operation to write data by applying a write signal to the write
element, wherein the contact event is detected during the
application of the write signal.
4. The method of claim 1, further comprising performing a read
operation to read data by applying a read bias signal to the read
element, wherein the contact event is detected during the
application of the read bias signal.
5. The method of claim 1, wherein the heater is a first heater
operative during a write operation during a writing of data, the
transducer further comprises a separate, second heater operative
during a read operation during a reading of data, and the power
applied during the applying step is to a selected one of the first
or second heaters selected responsive to the thermally responsive
sensor.
6. The method of claim 5, wherein the power applied to the first
heater during the write operation is a first power level, wherein
the power applied to the second heater during the read operation is
a different, second power level, and wherein the first and second
power levels are respectively selected responsive to the thermally
responsive sensor.
7. The method of claim 1, wherein the detected contact event is
used to select a power level applied to the heater during the
applying step.
8. The method of claim 1, wherein one or both of the heater and the
thermally responsive sensor are combined with one or both of the
write element or the read element.
9. The method of claim 1, wherein the detecting step comprises:
writing data to the medium using the write element coincident with
monitoring the bias signal obtained from the thermally responsive
sensor; accumulating a first total number of pulses in the bias
signal during said monitoring; counting a total number of servo
wedges on the medium during the writing; and determining the
accumulated plural count of pulses responsive to a difference
between the first total number of pulses and the total number of
counted servo wedges.
10. The method of claim 1, wherein the positioning step further
comprises coupling a preamplifier/driver circuit to the transducer
to supply write signals to the write element, to supply read bias
signals to the read element, to supply heater signals to adjust the
fly height of the transducer, and to supply a bias current through
the thermally responsive sensor to generate the bias signal
therefrom.
11. An apparatus comprising: a recording medium; a transducer
adjacent the recording medium comprising a write element, a read
element, a heater and a thermally responsive sensor; and a control
circuit coupled to the transducer and adapted to apply power to the
heater to establish a selected fly height of the transducer
relative to the medium, and to detect a contact event between the
transducer and the medium responsive to an accumulated plural count
of pulses in a bias signal obtained from the thermally responsive
sensor.
12. The apparatus of claim 11, wherein the control circuit
generates and stores in a memory a power level value responsive to
the thermally responsive sensor, the control circuit further
operative to apply the power level value to a driver circuit which
in turn applies the power to the heater in an amount associated
with a magnitude of the power level value.
13. The apparatus of claim 11, wherein the heater is a first heater
operative during a write operation during a writing of data to the
medium using the write element, the transducer further comprises a
separate, second heater operative during a read operation during a
reading of data from the medium using the read element, and the
control circuit applies a first power level to the first heater
during the write operation and applies a different, second power
level to the second heater during the read operation, wherein the
control circuit further operates to select the respective first and
second power levels responsive to the thermally responsive
sensor.
14. The apparatus of claim 11, wherein the control circuit
comprises a comparator adapted to compare the bias signal from the
thermally responsive sensor to a selected threshold value to
identify a first count of potential contact events between the
transducer and the medium, the selected threshold value determined
responsive to the thermally responsive sensor.
15. The apparatus of claim 14, wherein the control circuit further
comprises a fault register which accumulates a total accumulated
count of servo wedges on the medium, and an analysis circuit which
subtracts the total accumulated count of servo wedges from the
first count of potential contact events from the comparator to
determine the accumulated plural count of pulses in the bias signal
obtained from the thermally responsive sensor.
16. The apparatus of claim 11, wherein the detected contact event
is used by the control circuit to subsequently select and store in
a memory a power level for the heater during a subsequent read or
write operation.
17. The apparatus of claim 11, wherein the control circuit is
further adapted to: write data to the medium using the write
element while monitoring the bias signal obtained from the
thermally responsive sensor; accumulate a first total number of
pulses in the bias signal during the monitoring; count a total
number of servo wedges on the medium during the writing; and
determine the accumulated plural count of pulses responsive to a
difference between the first total number of pulses and the total
number of counted servo wedges.
18. The apparatus of claim 11, wherein the thermally responsive
sensor comprises more than one sensor, each sensor placed in a
location to collect information regarding the contact event.
19. An apparatus comprising: a recording medium adapted for
rotation about a central axis, the recording medium storing a
plurality of spaced apart servo wedges that define concentric
tracks adapted to store data in data sectors between adjacent pairs
of the servo wedges; a transducer adjacent the recording medium
comprising a write element adapted to store data to the data
sectors, a read element adapted to read back the data stored to the
data sectors and to read back servo data from the servo wedges, a
write heater adapted to lower a fly height distance between the
write element and the medium responsive to an applied write heater
value, a read heater adapted to lower a fly height distance between
the read element and the medium responsive to an applied read
heater value, and at least one thermally responsive sensor; and a
preamplifier/driver circuit adapted to, during a write operation,
apply write signals to the write element to write data to a
selected track, apply the write heater value to the write heater to
establish a first fly height distance for the write element, apply
a bias current to the thermally responsive sensor, and to detect a
contact event during the write operation responsive to an
accumulated plural count of pulses in a bias signal obtained from
the applied bias current to the thermally responsive sensor.
20. The apparatus of claim 19, wherein the preamplifier/driver
circuit further generates and stores the write heater value and the
read heater value in a memory responsive to at least one detected
pulse from the thermally responsive sensor.
Description
SUMMARY
[0001] Various embodiments of the present disclosure are generally
directed to positional sensing and control.
[0002] In accordance with some embodiments, a transducer is
positioned adjacent a recording medium. The transducer includes a
write element, a read element, a heater and a thermally responsive
sensor. Power is applied to the heater to establish a selected fly
height of the transducer relative to the medium. A contact event
between the transducer and the medium is detected responsive to an
accumulated plural count of pulses in a bias signal obtained from
the thermally responsive sensor.
[0003] These and other features and aspects which characterize
various embodiments of the present disclosure can be understood in
view of the following detailed discussion and the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] FIG. 1A is a functional representation of a data storage
device.
[0005] FIG. 1B is another functional representation of the device
of FIG. 1A.
[0006] FIG. 2 is a functional representation of another data
storage device.
[0007] FIG. 3 shows an arrangement of the data transducer of FIG. 2
in accordance with some embodiments.
[0008] FIG. 4 depicts the storage medium of FIG. 2 in accordance
with some embodiments.
[0009] FIG. 5 is a comparator circuit of the preamp of FIG. 3.
[0010] FIG. 6 depicts a heater power control circuit of the preamp
of FIG. 3.
[0011] FIG. 7 is a noise floor calibration routine.
[0012] FIG. 8 is a contact detection routine.
[0013] FIG. 9 depicts spectral response of the thermally responsive
sensor(s) of FIG. 3.
[0014] FIG. 10 shows bias response characteristics of the thermally
responsive sensor(s).
[0015] FIG. 11 provides an avalanche response in accumulated counts
from the thermally responsive sensor(s) based on actual (qualified)
contact events.
[0016] FIG. 12 graphically represents bias signals obtained from
the thermally responsive sensor(s) during respective read and write
operations.
DETAILED DESCRIPTION
[0017] The present disclosure generally relates to positional
control systems, such as the type used to control a fly height of a
data transducer adjacent a rotatable data storage medium.
[0018] Some data storage devices use a transducer to write data to
and read data from a rotatable data storage medium. The transducer
may be hydrodynamically supported adjacent the rotating data medium
by fluidic currents that interact with a fluidic (air) bearing
surface. A fly height adjustment mechanism can be used to adjust
the fly height of the transducer to an appropriate level, and a
contact detection system can be used to detect contact events
between the transducer and the medium surface.
[0019] A calibration routine can be carried out to select
appropriate fly heights during read and write operations to allow
the transducer to fly in close, stable proximity to the medium
surface while substantially avoiding contact events. Such
calibration routines can rely on a variety of sensor inputs such as
read sensor data from a read element of the transducer used to read
data from the medium, motor control inputs from a voice coil motor
(VCM) used to position the transducer, etc. Signals from the read
element can include position error signals (PES), VCM signals, etc.
These approaches generally rely on horizontal displacement
(off-track detection) to identify a contact event.
[0020] While operable, one limitation with such approaches is the
reliance on the detection of horizontal off-track displacement to
sense a change in vertical displacement of the transducer relative
to the medium surface. Contact occurs when the vertical
displacement (fly height) essentially becomes zero. Once contact is
made, however, the transducer may be displaced laterally
(horizontally), exhibiting off-track error that can be sensed from
measured positional error or changes in readback signal amplitude.
Relying on horizontal displacement measurements is reactive since
the displacement can generally be measured only once contact has
taken place.
[0021] Another limitation with such current generation off-track
situ detectors is that significant contact with the medium may be
required before contact can be detected. Detectors may exhibit
different responses at different skew angles and/or radial
locations on the medium, as well as different responses based on a
number of operational parameters including temperature, write
quality, read quality, servo errors, offsets, actuator tolerances,
and so on. Extended contact situations can increase burnishing of a
medium surface and other deleterious effects.
[0022] As continued increases in areal data storage densities drive
higher track densities, the individual tracks become smaller, which
decreases servo margin (e.g., servo signal to noise ratio SNR).
Using servo based algorithms to compute the degree of off-track
becomes increasingly less precise for lower levels of servo
SNR.
[0023] Accordingly, various embodiments of the present disclosure
are generally directed to an in situ vertical displacement
detection and control system. As explained below, in some
embodiments a transducer is adapted to fly in non-contacting
relation to a rotating data storage medium. The transducer includes
a write element, a read element, a heater unit and a thermally
responsive sensor. In some embodiments, the heater unit and/or the
thermally responsive sensor can be included in or combined with the
write element or the read element.
[0024] A control circuit is adapted to provide respective signals
to each of the transducer elements. These signals may include a
write signal to the write element to write data to the medium
during a write operation, a read bias signal to the read element to
read back data stored to the medium during a read operation, a
heater signal to the heater to adjust a fly height of the
transducer, and a thermal bias signal to the thermally responsive
sensor to detect a contact event between the transducer and the
medium.
[0025] The control circuit is adapted to perform a noise floor
calibration routine to establish appropriate signal detection
thresholds during read and write operations, followed by a
detection routine to establish suitable fly heights for the
transducer during such read and write operations. Thereafter, the
control circuit can monitor the system for the occurrence of
contact events and take corrective actions to reduce the impact on
system performance.
[0026] In some embodiments, the control circuit forms a portion of
a preamplifier/driver (preamp) circuit of a data storage device.
The preamp performs in situ calibration and detection at
appropriate times during the operational life of the device. The
thermally responsive sensor may be a high temperature coefficient
of resistance (TCR) element that translates small temperature
changes to large electrical signal changes. The preamp biases the
thermally responsive sensor, amplifies the output signal from the
sensor, and processes the amplified signal.
[0027] Some embodiments derive the final fly height values for the
transducer based on three main variables: the filter bandwidth
necessary to capture a contact signal from the output of the
sensor, the bias level applied to the sensor to obtain optimal SNR
response, and the detection threshold level suitable to reliably
detect a contact event. The first two variables can be empirically
determined. The third can be established by the noise floor
calibration routine to be discussed in detail below.
[0028] FIG. 1A is a functional block diagram of a data system 100
in accordance with some embodiments. The data system 100 includes a
control circuit 102, and a transducer 104 adjacent a data storage
medium 106. The transducer 104 incorporates a slider (not
separately shown) with hydrodynamic features such as an air bearing
surface (ABS) to facilitate stable support of the transducer 104
above and in close proximity to the storage medium 106 during
rotation of the medium.
[0029] The control circuit 102 interfaces with various operational
elements of the transducer 106. These elements include a write
element 108, a read element 110, and optionally a separate heater
112 and thermally responsive sensor 114.
[0030] The write element 108 is used to magnetically write data to
data tracks defined on the medium surface. The write element may
employ perpendicular magnetic recording and heat assisted magnetic
recording (HAMR) techniques. The read element 110 is used to sense
the previously written magnetic data, and may utilize a
magneto-resistive (MR) sensor or similar design. The heater 112
generally constitutes a thermally responsive material that
mechanically expands due to the application of power (e.g.,
current) in relation to a coefficient of thermal expansion of the
material. The thermal expansion of the heater 112 brings the write
and read elements 108, 110 closer to the medium surface. Separate
write and read heaters may be used as desired. In some embodiments
the write element 108 may serve as heater 112.
[0031] The thermally responsive sensor 114 comprises a resistive
material that operates as a highly sensitive thermal transducer.
The control circuit 102 applies a sense bias current through the
sensor 114 and pulses are induced in the bias current responsive to
changes in thermal state. Multiple sensors can be used, including
sensors arranged at different corners or other locations on the
slider. Individual bias currents may be supplied to each of the
thermally responsive sensors 114. In some embodiments the read
element 110 may also serve as thermally responsive sensor 114.
[0032] FIG. 1B is another representation of the device 100 of FIG.
1A. A combined writer/heater (W/H) is depicted at 116, and a
combined reader/sensor (R/S) is depicted at 118.
[0033] FIG. 2 depicts another data system 120 similar to the system
100 of FIGS. 1A-1B. The data system 120 of FIG. 2 is characterized
as a hard disc drive (HDD) data storage system, although such is
merely exemplary and not limiting. The system 120 includes a
controller 122 that provides top level control for the device. A
read/write (R/W) channel 125 includes a write channel portion
operable to encode input write data from the host to provide a
serialized data stream to a preamplifier/driver (preamp) 128.
[0034] The preamp 128 provides a sequence of write currents to a
transducer (head) 130 to write data to a magnetic data recording
medium 132 during a write operation. During a read operation, the
preamp 120 obtains readback signals from the transducer 130,
conditions and amplifies the readback signals, and provides the
same to a read channel portion of the R/W channel 125. The read
channel portion applies signal processing techniques to recover the
originally stored data, which may be stored in a buffer of an
interface (I/F) circuit (not shown) pending subsequent transfer to
the host.
[0035] During both read and write operations, specially configured
servo positioning data on the medium 132 may be recovered by the
transducer 130 and supplied to a servo control circuit (not
separately shown. A fault register (FR) 138 is provided in the
preamp 128 which can be used during calibration processing as
discussed below.
[0036] FIG. 3 depicts the transducer 130 of FIG. 2 in accordance
with some embodiments. A slider 140 supports a write element 142, a
read element 144, a write heater 146, a read heater 148, and four
thermal sensors 150 denoted as sensors S1-S4. The preamp 128 (FIG.
2) supplies appropriate control signals to each of these elements
during operation. The thermal signals from the sensors 150 can be
separately analyzed or combined into a composite signal, as
desired. It will be appreciated that the relative placement and
locations of the various elements can vary as desired. The sensors
150 can be nominally identical or individually tailored for
different lateral locations on the slider 140. Sensors S1-S4 can be
placed in locations where information regarding contact events may
be collected. The total number of sensors 150 can include two,
three, five or any amount of sensors that meet the needs of the
system 100.
[0037] FIG. 4 shows the storage medium 132 of FIG. 2 in accordance
with some embodiments. The medium 132 may be characterized as a
perpendicular magnetic recording disc, heat assisted recording
disc, or bit patterned disc, although such is merely for purposes
of illustration and is not limiting.
[0038] The data tracks on the medium 132 are grouped together into
a number of concentric zones 154. The zones are denoted herein as
Zones 1-4 (Z1-Z4). Separate write and read fly height calibration
values can be determined for each zone. Any number of different
zones can be defined based on the desired resolution including
zones of different sizes, locations and radial widths.
[0039] The servo data used by the servo circuit 134 in FIG. 2 can
be arranged as a plurality of servo wedges, two of which are
represented at 156. The servo wedges are arrayed in spaced apart
fashion around the circumference of the medium 132 much like spokes
of a wheel. While only two servo wedges 156 are depicted in FIG. 4,
it will be appreciated that several hundred such wedges (or more)
may be provisioned around the medium. User data are written to
sectors along the tracks in the areas between adjacent pairs of the
servo wedges 156. The servo wedges 156 provide the requisite servo
positioning control data to the servo circuit 134 (FIG. 2) to
facilitate precise positioning of the respective read and write
elements 142, 144 during read and write operations as well as
during calibration processing.
[0040] FIG. 5 represents a calibration processing circuit 160 which
can be incorporated into the preamp 128. A comparator 162 receives
an input signal on path 164 from one or more of the thermally
responsive sensors 150 (S1-S4) of FIG. 3. Upstream signal
amplification, filtering and other processing may be applied by the
preamp 128 to the input signal on path 164. A detection threshold T
is supplied as a second input to the comparator 162 via path 166.
Potential contact events are output as pulses on output path 168
responsive to a comparison of the inputs on paths 164 and 166.
[0041] The fault register 138 (see FIG. 2) of the preamp 128
receives a data input on path 170 to enable the register to
accumulate a count of servo wedges over each disc revolution. Any
number of suitable signals can be provided, such as a write gate
signal or other signal. A servo wedge count is output on path 172.
An analysis block 174 subtracts the servo wedge count from the
potential contact events to output a detected contact event count
on path 176. This allows contact events that arise other than from
the passage of a servo wedge to be identified.
[0042] FIG. 6 provides a heater control circuit 180 of the preamp
128 in accordance with some embodiments. The heater control circuit
180 includes a digital to analog converter 182 and a driver 184.
Generally, an input heater power level H, expressed in digital form
as a multi-bit representation over a selected range, is converted
to a corresponding analog input to the driver 184, which in turn
supplies power (such as in the form of applied voltage and/or
current) to the associated heater (e.g., the write heater 146 or
the read heater 148 of FIG. 3).
[0043] A separate driver can be provided for each heater, or the
circuit 180 can be multiplexed using suitable switching circuitry
(not separately shown) to apply the appropriate heater signals to
the respective heater elements. The various parameters used by the
system, such as the threshold detection values, the heater values,
etc., may be stored in a local memory 186 incorporated into or
accessible by the preamp 128. Control functions discussed herein
can be carried out by a local preamp controller 188 or by another
control circuit, such as the controllers 102, 122 in FIGS. 1-2.
[0044] FIG. 7 illustrates an adaptive noise floor calibration
routine 200. The routine represents processing carried out by
and/or under the direction of the preamp 128 or other control
circuit at suitable times, such as during idle periods, extended
initialization periods, etc. The routine may be executed on a
periodically scheduled basis, as well as responsive to a high
number of detected contact events.
[0045] Generally, the routine 200 determines appropriate detection
threshold levels T (path 166, FIG. 5) to be supplied to the output
signal(s) from one or more of the thermal sensors 150. Both read
and write thresholds can be determined for use during respective
read and write operations.
[0046] During the routine 200, the transducer 130 is moved to a
test track and the routine establishes a predetermined non-contact
fly-height level. The thermal sensor(s) are initialized with an
initial threshold and other parameters, and the preamplifier fault
register (FR block 138 in FIG. 2) is cleared. Separate write and
read operations are carried out to arrive at final threshold values
which are then saved for future reference. It will be noted that
the thresholds generally represent system noise thresholds and
constitute a sum of the amplified sensor noise and electronic noise
input to the comparator 162.
[0047] The routine 200 of FIG. 7 includes moving the associated
transducer 130 to a test location, step 202. For purposes of
illustration, it will be contemplated that the first test location
is located within Zone 1 in FIG. 4. A write calibration sequence is
performed first, followed by a read calibration sequence. The write
heater 146 is activated at step 204 to establish a predetermined
fly height known to be in non-contacting relation to the recording
surface. This can be carried out by applying an input digital
heater value HW to the DAC 182 in FIG. 6. An initial write
detection threshold TW is set at step 206 and applied to the
comparator 162 (FIG. 5).
[0048] Test data are written to the test track at step 208 using
the write element 142 (FIG. 3) over one or more consecutive
rotations of the medium. During the writing of data, the respective
outputs of the comparator 162 and the preamp fault register (FR)
138 are monitored, step 210. Decision step 212 determines whether
any actual contact events were detected. If not, the write
detection threshold TW is decremented at step 214 and the process
is repeated.
[0049] At some point the write detection threshold TW will have
been decreased sufficiently to allow at least one contact event to
the detected by the analysis block 174. It will be appreciated that
an actual contact event may or may not have actually occurred;
rather, the output of the block 174 indicates the write detection
threshold TW is now at a level sufficient to detect noise in the
system. Accordingly, the flow passes from decision step 212 to step
216 where the TW is incremented by a backoff value to provide a
final write threshold value TWF which is stored at step 216.
[0050] The foregoing processing is repeated to establish a final
read threshold value TRF. The read heater 148 (FIG. 3) is activated
to establish a non-contact fly height. It will be appreciated that
different fly heights may be achieved based on the fact that the
write element 142 is not active during the read operation sequence,
and therefore less heating will be applied to the transducer 130.
The read processing is similar to the write processing except that
the previously written track is now read by the read element.
[0051] Once final TWF and TRF values are stored for the selected
location (e.g. Zone 1), decision step 218 determines whether
corresponding threshold values should be determined for additional
zones. If so, the foregoing process is repeated until all of the
desired write and read threshold values are obtained for the
selected transducer 130. Decision step 220 determines whether
additional transducers should be evaluated, and if so,
corresponding threshold values are obtained for each transducer on
a zone-by-zone basis. Once all threshold values for the system have
been obtained, the routine ends at step 222.
[0052] The final write and read threshold values can be expressed
as follows:
TWF=TWC+B1
TRF=TRC+B2 (1)
[0053] Where TWF is the final write threshold, TRF is the final
read threshold, TWC is the write threshold that was the first to
exhibit a detected contact event during write processing, TRC is
the read threshold that was the first to exhibit a detected contact
event during read processing, and B1 and B2 are backoff values. B1
may be set equal to B2, or these may be different values.
[0054] The backoff values B1 and B2 are used because the sensor(s)
150 tend to produce increased sensor noise responsive to increases
in temperature. During contact detection, the heater power
increases and causes the sensor temperature to increase. The back
off value(s) may be determined empirically by evaluating a
population of nominally identical devices. A goal is to select
appropriate backoff value(s) that are sufficiently high enough to
avoid false triggers caused by system noise while being
sufficiently low enough to ensure actual contact event declarations
do not come too late to avoid inaccuracy or burnishing.
[0055] As noted above, in many cases it is expected that the final
write threshold TWF will be greater than the final read threshold
TRF due to the combination of the preamp behavior due to write and
read mode switching during write processing as the sensor reacts to
temperature differences as the transducer switches between writing
(over data sectors) and reading (over servo wedges). Since during
read processing the read element is maintained continuously on over
both the data sectors and servo wedges, in some cases the servo
counts may not trigger and so the write fault register may not be
needed to determine the baseline read noise level.
[0056] Once the final write and read threshold values TWF and TRF
are determined, the processing continues in FIG. 8 which provides a
contact detection routine 230. The routine 230 is also carried out
by and/or under the direction of the preamp 128 or other control
circuit to establish appropriate write and read heater values using
the detection thresholds from FIG. 7. As before, write processing
is carried out first, followed by read processing using one or more
test tracks.
[0057] Generally, the routine 230 operates to start at a
non-contact fly height and perform write operations while
monitoring for detected contact events. Write heater power is
successively incremented until a final write heater power level HWF
is selected and saved. Read processing is carried out in a similar
manner to select and save a final read heater power level HRF. The
final heater power levels HWF and HRF are thereafter used during
normal read and write operations.
[0058] During the write processing, the number of detected contact
events from the analysis circuit 174 (FIG. 5) is accumulated. If
the accumulated count does not exceed a predetermined count
threshold TC, the transducer is deemed to not have contacted the
medium and the heater power level HW is incremented. The
predetermined count threshold is the minimum number of total fault
counts at the end of the write operation that is deemed necessary
to declare contact. This limit may be determined by the minimum
number of contact faults per revolution multiplied by the total
number of revolutions in the write operation.
[0059] If the parameters are selected properly, a single fault
count may be sufficient to identify an actual contact event. In
practice, however, the operational environment of an in situ
application within a drive or other device can be relatively noisy,
leading to the use of multiple counts in order to declare an actual
contact event. Post processing steps such as moving averages of the
contact count can be applied to declare an actual contact event and
filter out spurious signals while reliably providing early
detection of actual contacts.
[0060] With specific reference to FIG. 8, the system is initialized
at step 232, which includes moving the transducer 130 to a selected
test track or other location, such as in a selected zone (e.g.,
Zone 1). Other parametric initializations can take place at this
time.
[0061] An initial write heater value HW is applied at step 234. It
is contemplated that the HW value may be initially relatively low
to ensure non-contact during initial stages of the write
processing. The detection threshold from FIG. 7 is recalled from
memory and applied to the comparator 162 (FIG. 5) at step 236. For
this first pass through the routine 230, write processing will be
applied so that the final write fault threshold TWF is initially
used. During subsequent read processing, the read fault threshold
TRF will be used.
[0062] Test data are next written to the test track at step 238
over one or more revolutions of the medium. During this writing,
the comparator 162 and the fault register (FR) 138 are monitored to
establish an accumulated contact count, step 240.
[0063] Decision step 242 determines whether the total number of
accumulated contacts equals or exceeds the threshold count TC. If
not, the heater power HW is increased by a suitable increment at
step 244, thereby bringing the transducer 130 closer to the medium
132, and the foregoing steps are repeated.
[0064] Once the accumulated count reaches or exceeds the threshold
TC, a contact event is identified and the process continues to
decision step 246, which determines whether the identified contact
event has been qualified. The contact event can be qualified in a
variety of ways such as using a time-based rolling average or other
statistical means to verify that an actual contact event was
detected. For example, a number of detected counts in a relatively
localized area as compared to spurious single contact points that
are widely distributed around the circumference of the track may be
indicative of an actual contact event. Similarly, a localized
off-track deflection coincident with or immediately following the
contact events may be indicative of an actual contact event.
[0065] If the contact is not qualified, the test is repeated as
shown by step 248 using the same parameters to determine if the
contact event can be repeated. If the contact event is qualified
and determined to have occurred with sufficient confidence, the
routine passes to step 250 where a final write heater value HWF is
selected and stored. The final write heater value may be derated
from the last heater value used during the last pass through the
routine.
[0066] The foregoing steps are then repeated to determine a final
read heater value HRF, which is also selected and stored at step
250, after which the process ends at step 252.
[0067] It will be appreciated that the routine 230 of FIG. 8
advantageously identifies final write and read heater power levels
for subsequent use during normal operation. Relevant portions of
the routine 230 can be executed by the preamp during normal read
and write operations, respectively, to accumulate counts and
identify, as desired, actual contact events.
[0068] In the event a contact event is detected during subsequent
normal operation, a variety of actions can be taken by the device
120 including a repeating of the associated write or read
operation, the application of a write/read verify to ensure the
data are correctly written, application of higher levels of
on-the-fly error detection/correction to recovered read data to
ensure proper readback, on-the-fly adjustments to the applicable
write and/or read heater power levels, and so on. Should a
statistically significant number of qualified contact events be
detected, the system may elect to proceed with a new calibration
sequence to obtain updated heater power levels.
[0069] FIG. 9 presents a graphical representation of first and
second signal spectrum waveforms 260, 262 plotted against a
frequency x-axis 264 and an amplitude y-axis 266. The waveform 260
represents the signal spectrum from a selected thermally responsive
sensor 150 at two different relative fly heights above the
associated medium surface.
[0070] Waveform 260 is a first lower fly height in which the slider
140 is in close contact proximity, and waveform 262 is a second
higher fly height in which the slider 140 is maintained in
non-contacting relation to the medium. Peak 268 indicates a
localized increase in thermal energy, and this localized peak can
be used as part of the detection methodology discussed above.
Suitable operational bandwidth and gain settings for the preamp 128
can be derived from such empirical data.
[0071] FIG. 10 depicts bias design point curves 270, 272 plotted
against a bias x-axis 274 and amplitude (SNR) y-axis 276. The bias
represents the power biasing, such as in the form of applied
voltage and/or current, that is supplied to the respective sensors
150 during the foregoing detection processing. Suitable bias levels
in substantially linear regions can be selected.
[0072] FIG. 11 depicts an exemplary accumulated count curve 280
applied against a position (tracks) x-axis 282 and accumulated
count y-axis 284. The curve represents exemplary types of response
for both read and write processing during the routine of FIG. 8. As
can be seen, an avalanche type response can be observed resulting
from actual contact events between the transducer 130 and the
medium 132. Suitable TC thresholds can be selected accordingly,
such as denoted at 286.
[0073] FIG. 12 shows representative read and write bias (response)
curves 290, 292 plotted against a time x-axis 294 and signal
amplitude y-axis 296. The read response curve 290 represents the
input response from a selected thermally responsive sensor 150
during read processing, and the write response curve 292 represents
the corresponding input response from the sensor 150 during write
processing. A write gate signal 298 denotes the periodic occurrence
of the servo wedges 156 (FIG. 3) and the associated drops in write
response from the sensor 150. This demonstrates the efficacy of
counting and subtracting out the servo wedge count from the fault
register 138 in the preamp 128 to obtain a more accurate assessment
of peaks in the write response curve 292.
[0074] From the foregoing it will be understood that the various
embodiments disclosed herein can provide a number of benefits. The
in situ contact detection scheme advantageously detects actual
vertical contact between the transducers and the media directly,
rather than merely relying on horizontal (e.g., off-track)
after-contact displacement. Consistent and repeatable measurements
can be obtained across all media/transducer/radius combinations.
The methodology is readily adaptable for field use in both setting
suitable read and write heater power levels, and also in
subsequently detecting actual contact events.
[0075] In some embodiments, the methodology reduces the myriad
variables affecting fly height into the three basic
parameters--gain/amplitude levels, bias levels and threshold
detection levels--and these three parameters can be updated as
required during subsequent field operation. Reduced slider/media
contact can reduce burnishing, lubrication disturbance and other
effects, thereby increasing system reliability. It has been found
in some cases that the disclosed methodology can provide a
significantly reduced overall calibration time to arrive at
appropriate heater levels and an integrated contact detection
mechanism that continuously verifies and, as necessary, adjusts
these levels.
[0076] The in situ vertical displacement contact detection scheme
(VIS Det) disclosed herein has been found to provide better and
more accurate contact detection results than those that can be
obtained from current generation off-track detection (OT Det)
methodologies. In one example, both earlier contact and reduced
variation results were obtained as compared to a current generation
detection system, as set forth in Table 1.
TABLE-US-00001 TABLE 1 Read Processing Write Processing OT Det VIS
Det Delta OT Det VIS Det Delta Mean 103.2 91.5 -11.7 59.3 55.8 -3.5
Sigma 15.8 15.4 -0.4 11.8 10.4 -1.4
[0077] The values in Table 1 are in terms of digital input of
heater power when contact was detected. It can be seen that both
the average power of an actual contact event (mean) and the
variation (sigma) are more sensitive and repeatable with the system
disclosed herein (VIS Det) as compared to current generation
off-track (horizontal displacement) configurations (OT Det).
[0078] It is to be understood that even though numerous
characteristics and advantages of various embodiments of the
present disclosure have been set forth in the foregoing
description, together with details of the structure and function of
various embodiments, this detailed description is illustrative
only, and changes may be made in detail, especially in matters of
structure and arrangements of parts within the principles of the
present invention to the full extent indicated by the broad general
meaning of the terms in which the appended claims are
expressed.
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