Tribology International 33 (2000) 367–372www.elsevier.com/locate/triboint

Load/unload measurements using laser doppler vibrometry andacoustic emission

Stefan Weissner, Frank E. Talke*

CMRR, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0401, USA

Abstract

The dynamics of the load/unload process are studied using a so-called ‘periscope approach’ which allows us to follow the slidermotion during load/unload (L/UL) with the beam of a Laser Doppler Vibrometer (LDV). LDV signals and acoustic emission signalsare obtained for three different slider airbearing designs and for load/unload conditions with different vertical velocities and spindlespeeds. The load process is investigated statistically using the acoustic emission signal in order to determine the effect of verticalload speed and spindle speed on the probability of contacts between slider and disk.

The results indicate that small vertical load speeds decrease the number of head/disk contacts, and that slider designs with acavity centered close to the trailing edge enable a smooth unloading process. 2000 Elsevier Science Ltd. All rights reserved.

Keywords:Load/unload (L/UL); Laser Doppler Vibrometer (LDV); Head/disk interface; Periscope

1. Introduction

Dynamic load/unload (L/UL) has recently been intro-duced in 2.50 drives for laptop computers, and is nowalso being introduced in desktop drives. The mainadvantage of load/unload over contact start/stop is thatstiction during take-off and landing is essentially pre-vented. Furthermore, the head/disk interface in conven-tional start-stop hard drives can be damaged during ashock excitation (head-slap). In this situation, the sliderseparates from the disk due to a sudden high accelerationand then ‘slaps’ back on the disk, resulting in a largeimpact force. In load/unload drives the slider is parkedon a ramp when the drive is not operational, i.e., theslider is removed from the disk. Thus, even for highaccelerations due to shock excitation the slider is pre-vented from touching the disk.

In order to develop disk drives where contactsbetween the head and the disk during load/unload do notcause damage or are completely eliminated, optimizationof the load/unload process is necessary. For example, thepull-off force that is exerted on a subambient pressure

* Corresponding author. Tel.:+1 (858) 534-3646; fax:+1 (858)534-2720.

E-mail address:ftalke@ucsd.edu (F.E. Talke).

0301-679X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved.PII: S0301-679X(00 )00054-2

slider during the unload process can cause a significantflexure/gimbal deformation, followed by a head-slap.This pull-off force should be reduced in order to enablea smooth unloading of the slider.

A number of numerical investigations describing theload/unload process has been published in recent years.Hu et al. [1] used a ‘de-gramming’ rate to model theunload process for two different subambient pressure sli-ders. ‘De-gramming’ means that the suspension load islinearly reduced to zero to model the sliding of the sus-pension tip on the ramp. The suspension was modelledwith three de-coupled degrees of freedom. Furthermore,Hu et al. [1] captured the unloading process with a high-speed video camera.

Peng [2] used a numerical model that included thecalculation of the ramp reaction force. He investigatedboth the loading and unloading process for a subambientpressure slider and for a positive pressure tri-pad slider.Zeng et al. [3] introduced a suspension model with fourdegrees of freedom, and used finite element analysis andexperiments to calculate the suspension stiffness matrix.Their results showed that a positive pitch static attitude(PSA) improves the loading and unloading behaviour.

Experimental investigations of the load/unload pro-cess have also been performed by Jeong and Bogy [4],Levi and Talke [5], Fu and Bogy [6–8], and Suk andGillis [9].

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In Ref. 5, a periscope approach was used to followthe slider motion during load/unload with an LDV. Theresults indicated that contacts between the slider and diskoccurred predominantly for high vertical load speeds. InRef. 8, a magnetic read-back signal decrease wasobserved caused by slider-disk contact during loading.Zeng et al. [10] investigated the unload process for twotypes of 50% subambient pressure sliders using forceand LDV measurements. In order to keep the LDV laserspot on the slider, the ramp was moved under the slider.They observed large dimple separation caused by thepull-off force during unloading. Zeng et al. [11]presented an optimized airbearing surface design thatreduces the pull-off force. In this design, the center ofthe cavity was moved towards the slider trailing edge.

In this investigation we study the load/unload processexperimentally using the so-called ‘periscope’ approachof Ref. 5. This approach consists of deflecting the beamof a Laser Doppler vibrometer with two mirrors, one ofwhich is positioned over the slider and one over the axisof rotation of the swing arm actuator. With this setupthe beam of the LDV can follow the slider motion duringthe load/unload process.

The load/unload characteristics of three different pico-slider airbearing designs were investigated using acous-tic emission (AE) sensors and a high bandwidth Laser-Doppler Vibrometer (LDV). In particular, the effect ofspindle speed and vertical speed on the load/unload pro-cess was investigated. In addition, the loading processwas investigated statistically since the same loading con-ditions were not always found to result in contactbetween slider and disk.

2. Experimental setup

The experimental setup shown schematically in Fig.1 is similar to the one used by Levi and Talke [5]. Aperiscope is mounted on the actuator arm of a commer-cially available Contact Start-Stop (CSS) tester. Theperiscope holds one mirror mounted at a 45° angle cent-ered in the axis of rotation. This mirror deflects the verti-

Fig. 1. Experimental setup.

cal laser beam in the horizontal direction. A second 45°mirror is mounted directly over the slider to deflect thebeam onto the gimbal/slider assembly. The laser beamis reflected and returns the same way back to the LDV.Close to the suspension an acoustic emission sensor ismounted on the actuator arm. The periscope is adjustablein the length direction, and the mirrors can be adjustedfor alignment of the laser beam.

The LDV velocity signal and the AE signal are low-pass filtered and digitized using a 16-bit A/D board witha sampling frequency of up to 8 MHz. The AE signalis preamplified by 40 dB. Data acquisition is triggeredusing either the AE signal or the displacement signalfrom the LDV. The displacement signal is captured withan oscilloscope in the DC coupled mode.

Two LDV velocity decoders were available withbandwidths of 1.5 MHz and 10 MHz, respectively. Thus,the LDV has a large enough bandwidth to detect airbear-ing and even slider body vibrations. Slider bodyvibrations are considered to be a good indication forslider/disk contacts. The AE sensor can be used fordetecting vibrations up to 1.8 MHz, although the fre-quency response of the sensor is not flat.

Three different slider types were used in the experi-ments. A schematic picture of all three airbearing con-tours is shown in Fig. 2. We observe that the main differ-ence in the three airbearing designs is the position of thecavity. Types I and II have a cavity that is further for-ward than that of type III. Furthermore, the depth of thecavities are different: 930 nm for type I, 3.37µm fortype II, and 1.07µm for type III.

It should be noted that the laser beam could not beplaced directly on the back of the slider during ourexperiments since all investigated slider types are com-pletely covered by the suspension/gimbal assembly. Thelaser spot was directed at the gimbal/flexure of all slid-ers. Slider type I is attached to the gimbal over the wholesurface (no dimple), so that LDV measurements for thisslider type should be very close to the actual slidermotion.

Fig. 2. Investigated slider types.

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Fig. 3. Load profile for slider type I.

3. Experimental results and discussion

The load/unload profile for all slider designs was cap-tured using the LDV displacement output. The signalwas digitized with a low sampling rate (50 kHz). In Fig.3 the displacement during loading for a slider of type Iand three different vertical loading speeds is shown as afunction of time. We observe that the total displacementduring a load cycle is about 300µm, and that no dis-continuities in the LDV signal are present during a loadcycle. We also observe the increasing slope of the loadcurve as the vertical loading speed increases from 4mm/s to 26 mm/s.

In Fig. 4 the displacement profile of the unload pro-cess is shown for the same slider as in Fig. 3. Weobserve that a strong discontinuity exists at the begin-ning of the unload process, marked by ‘A’, and that wellpronounced oscillations in the displacement signal per-sist for approximately 50 ms. The strong discontinuity

Fig. 4. Unload profile for slider type I.

at point ‘A’ signals the collapse of the airbearing duringthe unload process, resulting in a sudden upward motionof the slider.

A comparison of the load and unload signals from theacoustic emission sensor is shown in Fig. 5 for the sliderof type I. Similar to the LDV results, the AE sensordoes not show any activity during loading but exhibitsa distinct burst at a time of approximately 40 ms duringthe unload process. The amplitudes of the AE signal dur-ing unloading are similar for the three vertical unloadspeeds investigated, but the time duration of the AE out-burst is longest for the highest unload speed of 26 mm/s(Fig. 5, lower trace).

In Figs. 6 and 7 the AE signal during loading andunloading is shown for sliders of type II and III, respect-ively. The results for slider type II are very similar tothose for sliders of type I. In particular, strong acoustic

Fig. 5. AE signal for loading (upper traces) and unloading (lowertraces) of slider type I.

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Fig. 6. AE signal for loading (upper traces) and unloading (lowertraces) of slider type II.

emission activity is observed for slider type II duringunloading for all three unload speeds investigated. Theslider of type III on the other hand, shows no acousticemission activity during unloading, indicating that theunload process for this slider is smoother and moredesirable than that for either slider type I or II. It isapparent that this difference in the unload behaviour isrelated to the different airbearing designs of the threeslider types. For type III sliders the cavity is further backthan for types I and II, resulting in a smoother unloadingprocess for these sliders. This result is in agreement withZeng and Bogy [11], who reported, based on numericalsimulation results, that moving the cavity center furtherto the trailing edge improves the unload behaviour.

From the AE signals shown in Figs. 5–7 we observethat the AE signal is small during loading for all threeslider types while the acoustic emission signal is muchhigher for sliders of type I and II during unloading. In

Fig. 7. AE signal for loading (upper traces) and unloading (lowertraces) of slider type II.

Figs. 5–7 they-axis scale for the AE during loading andunloading is the same to show this difference.Occasional small AE bursts can be detected during load-ing of all slider types. These bursts are too small to benoticeable in Figs. 5–7 and appear intermittently evenfor the same loading conditions. If we choose a lowthreshold for detecting these signals, we can derive stat-istics of contact occurrence versus vertical load speed.

Fig. 8 shows, as a function of the vertical load speed,the percentage number of contacts for 500 load cyclesfor a slider of type I. The AE signal was high-pass fil-tered at 200 kHz and the occurrence of an AE burstabove a threshold of 75 mV was considered to be indica-tive of contact between the slider and the disk. Weobserve that the frequency of contacts increases substan-tially with increasing vertical speed. In particular, at aload speed of 20 mm/s, all load cycles resulted in con-tact, while at a load speed of 5 mm/s contacts are essen-

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Fig. 8. Frequency of contacts during loading vs. vertical speed (slidertype I).

tially absent. This result is similar to the result obtainedby Levi and Talke [5] for much larger slider types.

Fig. 9 shows the percentage of contact events as afunction of disk speed. We observe that the number ofcontacts increases slowly with disk speed keeping thevertical load speed at 13 mm/s.

In order to obtain further details of the unload process,we have investigated the energy released during a headslap that was observed for sliders of type I and II. Fig.10 shows the squared and integrated AE voltage as afunction of disk speed. Ten experiments were averaged,and the points show the mean value. We observe a strongdecrease in the AE energy as the disk speed wasincreased above 1500 rpm. Zeng and Bogy [3] calculatedthe pull-off force as a function of disk rotational speedand obtained a similar trend. It is likely that the decreasein acoustic emission energy is caused by a reduced pull-off force, resulting in a less severe head-slap and asmoother unload process. Numerical simulations [3]show that the slider pitch angle increases with higherdisk velocities, causing a reduction of the pull-off force.

In order to detect slider airbearing and slider body

Fig. 9. Frequency of contacts during loading vs. disk speed (slidertype I).

Fig. 10. AE energy during unloading vs. disk speed (slider type I).

vibrations, the velocity output from the LDV was used.If the slider flies at a constant disk radius and the diskspeed is decreased, slider/disk interactions will increasein frequency and severity. This severe contact can bedetected using the LDV and the periscope setup. Fig. 11shows the frequency spectrum in the airbearing fre-

Fig. 11. Airbearing frequencies (top) and slider body frequencies(bottom) measured with the LDV.

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quency range (top) and in the slider body frequencyrange (bottom) for slider type I. Typical airbearing fre-quencies for pico-sliders were calculated numerically byZeng and Bogy [12] and were found to be in the rangefrom 90 kHz to 162 kHz. We observe strong frequenciesat 137.5 kHz, 164.5 kHz, and 170 kHz for a disk velocityof 3.5 m/s. These frequencies are seemingly related toairbearing frequencies. The large magnitude of these fre-quencies is related to the small flying height of the slider(,10 nm) at a disk velocity of 3.5 m/s.

From Fig. 11 we also observe a slider body frequencyat 1.6 MHz. This frequency, which corresponds to thefirst bending frequency calculated using FEM modalanalysis, is similar for all pico-sliders and is a good indi-cation of slider/disk contacts. A torsional slider bodymode is not present in Fig. 11 since the laser spot waspositioned on the center line of the slider.

4. Conclusions

A periscope approach was implemented to measurethe dynamics of three different pico-slider types duringthe load/unload process with a Laser-Dopplervibrometer. Results for unloading of subambient press-ure sliders show that head-slap occurs caused by a sud-den collapse of the (negative) airbearing. This head-slapis more severe for low disk velocities. Furthermore,slider airbearing designs with the center of the cavitycloser to the slider trailing edge can be unloaded moreeasily than slider airbearing designs with a cavity centercloser to the leading edge. This is in agreement withnumerical results obtained by Zeng and Bogy [11].

Our experiments showed that in order to avoid fre-quent contacts during loading, a small vertical speedshould be chosen. The effect of disk speed was foundto be small compared to the effect of vertical load speed.

Slider airbearing vibrations and slider body vibrationswere observed using the periscope for small disk velo-cities and small slider flying heights. However, a similarfrequency analysis during the load/unload process didnot show any physically relevant frequencies.

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