Frank E Talke

Lin Zhou

Shinji Yonemura

CiteSeerX

ccur,
. In
pler
of a
valu-
ed.
alog
cited
Downloaded FromShinji Yonemura
Hitachi, Ltd., Japan,
Kanagawa, Odawara, Kozu, 2880, Japan
Lin Zhou
Frank E. Talke
University of California San Diego,
Center for Magnetic Recording Research
(CMRR),
9500 Gilman Drive,
La Jolla, CA 92093-0401, U.S.A.
An Investigation of Slider
Vibrations in Near Contact
Recording Using a Digital Laser
Doppler Vibrometer
At a flying height of 10 nanometers, contacts between slider and disk are likely to o
and control of contact-induced slider vibrations is an important design consideration
this study, slider vibrations during contact are investigated using a digital laser Dop
vibrometer (LDV). The noise level of the digital interferometer is compared with that
conventional analog LDV. In addition, acoustic emission (AE) sensors are used to e
ate the contact behavior of the slider. A comparison of AE and LDV data is perform
The results show that the noise level of the digital LDV is lower than that of the an
LDV, and that suspension sway mode vibrations and torsion mode vibrations are ex
during contact as a function of the skew angle.@DOI: 10.1115/1.1540124#e
e
n
s
a
a
D
u
u
i
n
t
p
t
lus
and
the
e-
t
the
tal
nt
og
the
ea-
t is
re-
utes
re-
e is
ith
am-
less
ical
is
for
digi-
ed
in-
dis-
er
ari-
theeIntroduction
The reduction of flying height is critical for increasing the r
cording density of hard disk drives. The flying height for comm
cially available disk drives with a recording density of 50 Gb/i2
is presently less than 20 nm. In order to achieve a recording d
sity of 100 Gb/in2, it is estimated that the flying height of a slide
would have to be about 6 to 7 nm@1#. To achieve flying heights in
the 6–7 nm range, it is necessary to decrease the roughne
both the slider and the disk. Since a decrease in the surface ro
ness of the slider/disk interface causes an increase in stiction,
important to investigate the dynamic characteristics of sliders
function of surface roughness of both the slider and the disk.
Laser Doppler vibrometry~LDV ! and acoustic emission~AE!
sensors have been used extensively to investigate the dyn
characteristics of sliders. Knigge and Talke@2–4# used joint time-
frequency analysis to investigate slider-disk contacts. They
served nonlinearities of air bearing frequencies using both L
and AE sensors. Kohira et al.@5# investigated slider vibrations
using laser Doppler vibrometry. They applied filtering techniqu
and showed that pitch angle, air bearing surface design, and l
cant thickness affect slider vibrations. Sheng et al.@6# studied
head-disk impact during unloading using laser Doppler vibro
etry and AE analysis. Their results showed that limiters and
loading conditions were important design parameters during
loading. Fu et al. @7# investigated head-disk contact durin
dynamic loading. They established a relationship between L
measured impact velocity and disk surface damage by estima
the impact stress between slider and disk. Zeng et al.@8# measured
flying height modulation of sliders at sub-10 nm flying heigh
using laser Doppler vibrometry. They found that the LDV veloc
output was more accurate than the displacement signal.
A conventional analog LDV uses the phenomenon of freque
shift to measure the velocity of a moving body. Integration of t
velocity signal yields the displacement. The integration opera
is limited by the sampling frequency of the displacement out
and often results in a drift of the displacement output signal. If
measurement position is stationary and the object is moving w
velocity vs , the relationship between the measured frequencyf m
and the source frequencyf s is:
Contributed by the Tribology Division for publication in the ASME JOURNAL OF
TRIBOLOGY. Manuscript received by the Tribology Division March 27, 2002; r
vised manuscript received August 6, 2002. Associate Editor: C.-P. R. Ku.Copyright © 2Journal of Tribology
: https://tribology.asmedigitalcollection.asme.org on 07/01/2019 Terms of Use: h-
r-
en-
r
s of
ugh-
it is
s a
mic
ob-
V
es
bri-
m-
n-
un-
g
DV
ting
ts
ty
cy
he
ion
ut
he
ith
f s
f m
516
vs
c
(1)
wherec is the phase velocity of the wave in the medium. The p
sign is used when the object is moving towards the observer
the minus sign is used when the object is moving away from
observer.
A digital LDV, on the other hand, measures not only the fr
quency shift but also demodulates continuously the phase shifDw
of the reflected laser light. The phase shift is proportional to
d placementDx of the observed object:
Dw5~4p/l!Dx (2)
wherel is the wavelength of the source laser light. The digi
LDV used in this study offers both velocity and displaceme
output at high resolution and accuracy.
Typically, analog LDV’s incorporate a large number of anal
mixers and other analog components, which contribute to
overall noise of the system. Furthermore, to obtain different m
surement ranges in an analog LDV, the interferometer outpu
mixed with specific oscillator frequencies. These oscillator f
quencies are generated using a Phase Locked Loop~PLL! circuit.
The PLL circuit generates so-called phase noise which contrib
to the 1/f noise. Since the number of analog components is
duced in a digital laser Doppler interferometer, much less nois
being generated. Although analog LDV’s can be equipped w
phase shift electronics to give displacement output at high s
pling frequencies, higher resolution of the measurement with
noise is to be expected for digital LDV’s@9#.
Table 1 shows the comparison of the specifications of a typ
digital LDV and a typical analog LDV. Real-time analog output
not available for the digital LDV because of the time demands
data processing. However, the data acquisition system of the
tal LDV yields the measured data as an ASCII file.
In this paper, a newly available commercial digital LDV is us
to measure slider vibrations for near contact conditions. Both
plane and out-of-plane vibrations are investigated using the
placement output of the digital LDV. The dependence of slid
vibrations on flying height and skew angle is studied and comp
son is made with AE data for the same conditions.
Experiments
1 Noise Level Comparison of Digital LDV and Analog
LDV. Table 1 shows the comparison of the specifications of
digital LDV ~Polytec VDD-660! and the analog LDV~Polytec
-
003 by ASME JULY 2003, Vol. 125 Õ 571
ttp://www.asme.org/about-asme/terms-of-use
i
d
l
p
t
s
i
a
e
e
isk
m
of
ata
tion
ura-
ents.
the
lider
t a
y 6
de
and
e 6
ngle
re-
ly
Downloaded FromOFV-3001 OFV-512 with Mistras2001 E1.71 data acquisitio
system! used in the present study. The data acquisition system
a fixed high-pass filter at 1 kHz. An acoustic emission~AE! sensor
was mounted on the slider holder. The same data acquisition
tem for the analog LDV was also used for AE analysis with
band-pass filter of 200–500 kHz to detect contacts between
slider and the disk. Real-time analog output is not available for
digital LDV because of the time demands for data process
However, the data acquisition system of the digital LDV yiel
the measured data as an ASCII file.
The background noise level of the analog and digital LDV us
in our experiments was first compared as a function of samp
frequency using a stationary hard disk surface. Since a high
filter is present at 1 kHz in the data acquisition system of
analog LDV, a high pass filter of 1 kHz was also incorporated
the digital LDV. During the experiments the LDV head was po
tioned about 300 mm from the disk surface. The displacem
result of the analog LDV is obtained by integration of the veloc
signal.
Figure 1 and Fig. 2 compare the noise level of the analog
the digital LDV at sampling frequencies (f s) of 200 and 256 kHz,
and 4 and 5.12 MHz, respectively. We observe that the noise l
of the digital LDV is lower than that of the analog LDV at bot
low and high sampling frequency.
2 Observation of Slider Vibrations Using the Digital LDV.
Figure 3 shows the experimental set-up used for the LDV m
surement. A commercially available CSS tester is used as a
stand. In-plane and out-of-plane slider motions were investiga
using the digital LDV. The air bearing surface design of the slid
used is shown in Fig. 4~a!. Flying height is controlled by changing
Table 1 Comparison between the digital and analog LDV
Digital LDV Analog LDV
Primary output displacement velocity
Real-time
analog output
not available available
Data acquisition VIBSOFT
~with ASCII
output!
User supplied
Fig. 1 Comparison of noise level of analog and digital LDV at
low sampling frequency: „a… analog LDV „f sÄ200 kHz…; and „b…
digital LDV „f sÄ256 kHz….572 Õ Vol. 125, JULY 2003
: https://tribology.asmedigitalcollection.asme.org on 07/01/2019 Terms of Use: hn
has
sys-
a
the
the
ng.
s
ed
ing
ass
he
in
i-
ent
ty
nd
vel
h
ea-
spin
ted
rs
the disk velocity. The relationship between flying height and d
velocity of the slider is shown in Fig. 4~b!. The experiments used
65 mm (2.59) carbon coated disks with a roughness of 0.8 n
(Ra). The disks were lubricated with PFPE~2 nm thickness!.
To observe slider in-plane vibrations, a sampling frequency
128 kHz was chosen with a total record length of 64 ms. The d
were averaged over 10 measurements. For out-of-plane vibra
experiments, measurements were taken at 5.12 MHz for a d
tion of 3.3 ms. The data were averaged over 50 measurem
The LDV laser beam was focused on the side of the slider for
in-plane measurements and on the inner trailing edge of the s
for the out-of-plane measurements~Fig. 3!.
Measurement Results of Slider Vibrations
Figure 5 shows the spectrum of the in-plane slider motion a
skew angle of 0 deg as a function of flying height~ !. As we can
see from Fig. 5, two well-defined peaks exist at approximatel
kHz and 11 kHz, corresponding to the torsion and sway mo
frequencies of the suspension. The amplitude of the torsion
sway modes increases with decreasing flying height. Figur
shows the spectrum of the out-of-plane vibrations at a skew a
of 0 degrees as a function of flying height. The air bearing f
quencyvp of the pitch mode at approximately 200 kHz is clear
Fig. 2 Comparison of noise level of analog and digital LDV at
high sampling frequency: „a… analog LDV „f sÄ4 MHz…; and „b…
digital LDV „f sÄ5.12 MHz….
Fig. 3 Schematic illustration of an experimental setupTransactions of the ASME
ttp://www.asme.org/about-asme/terms-of-use
t
e
y
k
a
e
a rob-
ht.
at
ns
the
af-
he
ith
po-
he
e vi-
kew
Downloaded Fromobservable at flying heights of 5 nm and 4 nm. Harmonics of
air bearing pitch frequency are observed at a flying height o
nm, but not at flying heights of 5 nm or higher.
Figure 7 and Fig. 8 show the spectrum of the in-plane a
out-of-plane slider motion, respectively, at a skew angle of
degrees. In Fig. 7, the peaks of the torsion mode~about 6 kHz!
and the sway mode~about 11 kHz! of the suspension are observe
at all flying heights investigated. Figure 8 shows the out-of-pla
frequency characteristics at a skew angle of 10 degrees. Th
sults are similar to the slider vibration characteristics of Fig.
High order harmonics of the pitch frequency are observed onl
a flying height of 4 nm.
Figure 9 shows the peak to peak value of the AE signal at s
angles of 0 and 10 degrees as a function of flying height. T
intensity of the AE signal increases dramatically when cont
occurs between head and disk. The intensity of the AE signal
degree skew angle is higher than that at 10 deg skew angle,
though the spectra of the LDV measurements of the out-of-pl
vibrations are almost the same for both cases~Fig. 6 and Fig. 8!.
The slider vibrations observed using either AE or LDV measu
ments increase substantially when the slider flies lower tha
‘‘critical flying height.’’ This critical flying height is generally
defined as the ‘‘avalanche flying height.’’ The ‘‘avalanche flyin
height’’ of the AE signal at a skew angle of 10 degree is lower~4
nm! than that at a skew angle of zero degrees~5 nm!.
Discussions
High frequency out-of-plane vibrations of a slider are likely
be caused by impacts between the slider and the disk. These
Fig. 4 Air bearing design and flying height of a slider: „a… air
bearing surface; and „b… flying height as a function of disk
speed.Journal of Tribology
: https://tribology.asmedigitalcollection.asme.org on 07/01/2019 Terms of Use: hhe
f 4
nd
10
d
ne
re-
6.
at
ew
he
act
t 0
ven
ne
re-
n a
g
to
fre-
quencies are the rigid body bending and torsion modes. The p
ability that impacts occur increases with decreasing flying heig
Figure 6 and Fig. 8 show significant out-of-plane vibrations
flying heights of 5 nm and 4 nm. The out-of-plane vibratio
observed were not a strong function of the skew angle. On
other hand, the in-plane vibrations of the slider are strongly
fected by the skew angle, as is shown in Figs. 5 and 7. T
in-plane vibrations result from friction forces which increase w
decreasing flying height. As the skew angle increases, the com
nent of the friction force perpendicular to the centerline of t
suspension increases. This causes an increase in the in-plan
brations. Figures 7 shows noticeable in-plane vibrations at a s
Fig. 5 Spectrum of slider in-plane vibrations as a function of
flying height at skew angle of 0 deg „0 dBÄ1 m…: „a… hÄ7 nm;
„b… hÄ6 nm; „c… hÄ5 nm; and „d… hÄ4 nm.JULY 2003, Vol. 125 Õ 573
ttp://www.asme.org/about-asme/terms-of-use
e
z
t
-
ng
ider
in-
AE
dif-
ew
ns
ere
Downloaded Fromangle of 10 degrees for flying heights of 6 nm and 7 nm, wher
slider in-plane vibrations are nearly absent at a skew angle of
degrees for flying heights of 6 nm and 7 nm~Fig. 5!. Thus, exci-
tation of torsion and sway modes of the suspension is a func
of the skew angle. On the other hand, we observe that the s
angle does not affect the out-of-plane vibrations of the slider.
In general, AE signals have much higher background no
level than LDV measurements, since AE sensors reacts to s
waves from all sources in the neighborhood of the head/disk
terface. This results in a lower sensitivity of AE data compared
LDV measurements. Figure 10 shows a comparison of the a
Fig. 6 Spectrum of slider out-of-plane vibrations as a function
of flying height at skew angle of 0 degree „0 dBÄ1 m…: „a… hÄ7
nm; „b… hÄ6 nm; „c… hÄ5 nm; and „d… hÄ4 nm.574 Õ Vol. 125, JULY 2003
: https://tribology.asmedigitalcollection.asme.org on 07/01/2019 Terms of Use: has
ero
tion
kew
ise
ress
in-
to
va-
lanche flying height~i.e., the flying height at which slider vibra
tions are detected! for a skew angle of 0 degree and 10 deg usi
three different methods of observation. At 0 deg skew angle, sl
vibrations are detected at the same flying height using LDV
plane measurements, LDV out-of-plane measurements, and
analysis. However, these methods detect slider vibrations at
ferent flying heights at the skew angle of 10 deg. At the sk
angle of 10 deg and a flying height of 5 nm, slider vibratio
could not be detected using the AE signal, although they w
detected using LDV measurements.
Fig. 7 Spectrum of slider in-plane vibrations as a function of
flying height at a skew angle of 10 degrees „0 dBÄ1 m…: „a… hÄ7
nm; „b… hÄ6 nm; „c… hÄ5 nm; and „d… hÄ4 nm.Transactions of the ASME
ttp://www.asme.org/about-asme/terms-of-use
t
s
n
s
cur
and
em-
ies
ra-
PI
ards
act
e-
-
ct
l.,
d-
ion
J.
of
r in
ic
ty
ing
D.
Downloaded FromSummary and Conclusions
Slider in-plane and out-of-plane vibrations were investiga
using a new commercially available digital LDV. The results
the LDV measurements were compared with acoustic emis
measurements. The experimental results show that
1. The digital LDV exhibits a lower noise level than conve
tionally available analog LDV’s. Thus, the digital LDV i
more suitable to high precision frequency measureme
than the analog LDV.
Fig. 8 Spectrum of slider out-of-plane vibrations as a function
of flying height at a skew angle of 10 degrees „0 dBÄ1 m…: „a…
hÄ7 nm; „b… hÄ6 nm; „c… hÄ5 nm; and „d… hÄ4 nm.Journal of Tribology
: https://tribology.asmedigitalcollection.asme.org on 07/01/2019 Terms of Use: hed
of
ion
-
nts
2. In-plane vibrations of the slider/suspension assembly oc
during contacts between slider and disk. The sway mode
the torsion mode vibrations of the slider/suspension ass
bly increase with increasing skew angle.
3. LDV measurements of in-plane and out-of-plane frequenc
showed a higher sensitivity towards detecting slider vib
tions than AE measurements.
Acknowledgments
We appreciate the cooperation of Mario Pineda of Polytec
Inc. with our studies.
References
@1# Menon, A. K., 1999, ‘‘Critical Requirements for 100 Gb/in2 Head/Disk Inter-
face,’’ ASME Proceedings of the Symposium on Interface Technology Tow
100 Gb/ in2, 9, ASME, New York, pp. 1–9.
@2# Knigge, B., Talke, F. E., Baumgart, P., and Harrison, J., ‘‘Slider Disk Cont
Dynamics of Glide Heads and Proximity Recording Sliders using Joint Tim
Frequency Analysis and Scalogram,’’ 1999,ASME Proceedings of the Sympo
sium on Interface Technology Towards 100Gbit/ in2, 9, ASME, New York,
pp. 23–30.
@3# Knigge, B., and Talke, F. E., 2001, ‘‘Slider Vibration Analysis at Conta
Using Time-Frequency Analysis and Wavelet Transforms,’’ ASME J. Tribo
123, pp. 548–554.
@4# Knigge, B., and Talke, F. E., 2001, ‘‘Nonlinear Dynamic Effects at the Hea
Disk Interface,’’ IEEE Trans. Magn.,37, pp. 900–905.
@5# Kohira, H., Tanaka, H., Matsumoto, M., and Talke, F. E., 2001, ‘‘Investigat
of Slider Vibrations Due to Contact with Smooth Disk Surface,’’ ASME
Tribol., 123, pp. 616–623.
@6# Sheng, G., Chen, Q., Hua, W., and Liu, B., 2001, ‘‘An Experimental Study
Dimple Separations and Head-Disk Impacts of Negative Pressure Slide
Unload Process,’’ IEEE Trans. Magn.,37~4!, pp. 1859–1862.
@7# Fu, T. C., and Bogy, D. B., 2000, ‘‘Analysis of Stresses Induced by Dynam
Load Head-Disk Contacts,’’ ASME J. Tribol.,122~1!, pp. 233–237.
@8# Zeng, Q. H., Thornton, B. H., Bogy, D. B., and Bhatia, C. S., 2001, ‘‘Flyabili
and Flying Height Modulation Measurement of Sliders with sub-10 nm Fly
Heights,’’ IEEE Trans. Magn.,37~2!, pp. 894–899.
@9# Pineda, M., 2001, personal communication, Polytec PI Inc., CMRR, UCS
Fig. 10 Avalanche flying height of slider vibration detected by
different observation methods
Fig. 9 AE signal intensity as a function of flying heightJULY 2003, Vol. 125 Õ 575
ttp://www.asme.org/about-asme/terms-of-use


An Investigation of Slider Vibrations in Near Contact Recording Using a Digital Laser Doppler Vibrometer

http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=5E23BE72AF2957F1BCEE59DD9D734CCF?doi=10.1.1.1046.2687&rep=rep1&type=pdf

An Investigation of Slider Vibrations in Near Contact Recording Using a Digital Laser Doppler Vibrometer

Abstract

Similar works

Full text

Available Versions

CiteSeerX