IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-20, NO. 5 , SEPTEMBER 1984 921

DYNAMIC RESPONSE OF A WINCHESTER-TYPE SLIDER HEASURED BY LASER DOPPLER INTERFEROMETRY

D.K. Miu , G. Bouchsrd*, D.B. Bogy , and F.E. Talke**

ABSTRACT

Optical in terferometr ic techniques have been used extensively in the past to measure the timeaveraged flying height of self-act ing, gas- lubricated s l ider bear ings commonly found i n magnetic recording disk files. Recently, t h e i r a p p l i c a t i o n h a s b e e n e x t e n d e d t o d y n a m i c a l measurements[ 1,2]. Previously used optical techniques have an inherent disadvantage i n that they require a special ly made glass disk, which is different from the usual oxide coated aluninum disks. Also, they are applicable only to s t e a d y s t a t e measurements. This paper presents a l a s e r Doppler technfque capable of measuring the dynamic response of a s l ider in an unmodified magnetic disk f i le.

INTRODUCTIO'IP

Since the introduction of the I B M Ramac d i s k f i l e i n 1957, there has been a steady increase in performance of magnetic recording mass storage devices[3]. Much of this progress i s associated with the improvement of the mechanical design of the self-acting, gas-lubricated slider bearings. The s l ider to media clearance, or :flying height", of recent commercial products has been reduced to a mere 0.3 p, canpared to the 20.p of earlier designs. Over the years , many t h e o r e t i c a l and experimental: investigations relevant to sl$der bearing research have been published. Various experimental techniques have been introduced i n order to investigate the performance of new bearing designs and to verify numerical resu l t s from proposed theoretical m o d e l s .

A review paper was wr i t ten by Lin in 1973[4] , which sumnarized the s ta te of the art in f lying height measurement techniques. Among those described were capacitive methods such as that documented by Brunner e t a1.[5], and opt ical

I- loser

beam Mikr I

differential amplifier

Fig.1 Optical and e lectr ical component8 of the LDV.

* Department of Mechanical Engineering, University of

** IBM Corporation, San Jose Research Iaboratories, California, Berkeley.

California.

methods such as that presented by Lin and Sullivan[6]. The former technique measures the clearance variation between a s l ider and the media by measuring the corresponding changes i n capacitance of the lubricated film; it offers reasonable frequency response but requires modification of the sl ider bearing i n order to house a metallic electrode. The l a t t e r technique ut i l izes the pr inciple of optical interference and permits the measurement of the bearing separation by set t ing up a c lass ica l two-beam interferometer between the s l ider and the disk. This technique allows one to measure the t ime averaged f lying heights but requires one of the bearing sur faces to be t ransparent , i.e., e i t h e r a g l a s s d i s k o r g lass s l ider must be used.

Due to the small physical size of the recent sl ider designs, capacit ive techniques, which are capable of measuring dynamic motions, are now d i f f i cu l t t o implement. White light interferometry, on the other hand, continues to be an important too l in studying the steady state f lying height of s l ider bearings.

More recent e f for t s inc lude those of F le i scher and L i n [ l ] , and Nigam[Z]. I n [ l ] a n i n f r a r e d l a s e r interferometer was used to measure both the timeaveraged and the dynamic variations in flying height by continuously moni tor ing the in tens i ty var ia t ions of t h e f i r s t o r d e r interference fringe. Such a technique has inherent noise problems and requires extreme care i n c a l i b r a t i o n and optical alignment. Nigam reported an al ternat ive technique i n which t h e s l i d e r is assumed t o v i b r a t e a t a c e r t a i n fundamental frequency and the amount of v i b r a t i o n i s obtained by s a l v i n g a p a i r of transcendental equations involving the amplitudes of the measured signals. However, th i s method is not capable of measuring actual transient s l ider motions such as those due to an impulse disturbance on the suspension arm or an asper i ty on t h e moving d isk surface. Moreover, a l l previous interferanetric techniques require an op t i ca l ly f l a t g l a s s d i sk , which can be quite different mechanically fran the oxide coated aluninum disks cammonly used i n present day magnetic disk f i les.

This paper describes a novel laser Doppler technique capable of direct ly measuring t ransient motions of the s l ider in an unmodified magnetic disk file. Results w i l l be presented to i l l u s t r a t e t he dynamic response of the s l ider due to a a n a l l mechanical defect on the disk surface.

EXPERIMENTAL APPARATUS

experimental apparatus incl@es 1) a Laser Doppler Vibrometer (LDV) for measuring normal motions of a r e f l e c t i v e s u r f a c e , 2) a high speed A/D conver te r and a laboratory minicomputer for data acquisition and analyses, and 3) a cormnercial ly avai lable 5 1/4 in. magnetic disk f i l e .

L ight re f lec ted o f f a moving surface i s frequency shifted by an amount proportional to the velocity of the motion. Such a phenomenon i s a particular case of the well- known Doppler effect and i s u t i l i z e d i n t h e LDV. Fig. 1 i l l u s t r a t e s the optical and e lec t r ica l components of such an instrument.

Unlike conventional interferometric techniques, where the limitation is the wavelength of light, the resolution and bandwidth of the LDV depend en t i re ly on the capability of the FM demodulator to derive the velocity canponent from the measured signal. Figure 2 i l lust rates the usable range of our LDV instrunentation.

The o p t i c a l s e c t i o n of our LDV was modified from a c-ercially available unit made by DISA Electronics. A similiar LDV was used in Wlezien, Miu, and Kibens[7] to character ize the surface contour of a r o t a t i n g f l e x i b l e d isk , and i t s performance was compared with that of a capacitance probe.

0018-9464/84/0900-927$01.0001984 IEEE

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VELOCITY (m/s)

AMPLITUDE (ml

Fig.2 Usable range of the LDV.

RESULTS A m DISCUSSION

In o r d e r t o e x c i t e t h e s l i d e r and to s tudy i t s dynamical behavior, a small physical defect was a r t i f i c i a l l y introduced on the surface of the disk. This defect is merely an impact-induced depression of substantial depth relative t o t h e f l y i n g h e i g h t o f t h e s l i d e r ; i t measures approximately 1.2 mn long and 0.3 mn wide with i t s length along the radius of the disk. As the depression t rave ls past the bearing region, it creates a very localized low pressure region which causes the slider to deviate frau its steady state flying position. The defect i s located near the 5 an radius where the surface velocity is 19 d s for a rotational speed of 3600 rpm. The e f f e c t of this depression on the s l ider motion is observed when the R / W head is located on the following tracks : track n m k r 58 to 76 (depression under the right r a i l of the slider), 83 t o 92 (center ra i l ) , and 99 to 117 ( l e f t r a i l ) .

The i n i t i a l measurements are concerned with the general runout of the d i sk and the corresponding motion of t h e s l i d e r . The l a s e r beam is f i r s t focused on the d i sk a t a track affected by the defect and the velocity signal due to the disk profile is recorded. Then, the sl ider is positioned an the same track and its ver t ical veloci ty i s recorded a s well. Figure 3 presents these results together with the cor- responding displacements obtained by integration. The sam- pling rate i s 50 kHz, so that 2000 data points represent a t o t a l of 40 ms, which is s l i g h t l y more than two revolutions of the disk. In Fig. 3(A), we observe thzit the disk velocity signal has three large spikes d& to the depression (one per revolution), while Fig. 3(C) shows that the slider signal has smaller spikes a t the same locations corresponding to i t s dynamical response to the excitation. The disk velocity a l s o e x h i b i t s some high frequency components evidently related to the disk surface roughness. Moreover, one can observe that the displacements of the disk and the s l ider are nearly identical. %e slider follows the disk runout v e r y c l o s e l y , so t h a t any difference between t h e two displacements w i l l be r e l a t i v e l y small. The peak-trrpeak displacenent of the disk is found to be less than 20 p but this depends strongly on the way the disk is m o ~ t e d on the spindle.

The transient motion of the sl ider i s next investigated in more detai l . This is accmplished by using a higher sam- p l i n g r a t e t o r e c o r d a smal l por t ion of the p rev ious ly displayed signals concerned with the excitation. Figure 4 presen t s t he ve loc i ty s igna l at the r ea r cen te r of the s l ider as it f l i e s over the defect, together with its FFT and the resulting displacement. The sampling rate is 2 MHz. T& spectrm shws a clear separation between low frequency components associated with the disk runout and higher frequency terms related to the transient response. This is

\ E E O

W i

z -20 -

-1oL 0 10 20 30 4 0 m s

Fig.3 General runout: disk velocity (A), disk displacement (B), sl ider veloci ty (C), and slider displacement (D).

-1oL

-. 2 L

@ n 0 .25 . 5 .75 I m s

( C ) n

Fig.4 Transient motion of the slider: velocity (A), displacement (B) , and veloci ty spectm (c) .

a lso observed on the displacement &ere a high frequency motion i s superposed on a low frequency one. In the following dynamical study, the signal is therefore high-pass f i l t e r e d a t 1.5 kHz to eliminate the low-frequency disk runout components.

A more complete description of the slider motion can be achieved by recording the vertical displacements a t its four corners. Assunling that these displacements remain small canpared to the overa l l dimensions of the s l i d e r (5.4 mn length by 3.6 mn width), the pitch angle, roll angle, and ve r t i ca l mo t ion a t t he geomet r i c cen te r are e a s i l y calculated. Data is obtained with the s l i d e r positioned on track 108, 80 that the perturbation occurs on the l e f t rail. The corner motions appear i n Fig. 5. me calculated center displacements, pitch motions, and r o l l motiolis are shown in Fig. 6. The sampling rate in these curves is 1 MHz.

Note that the peak displacements observed l o c a l l y a t t he co rne r s a r e much g rea t e r t han t he ca l cu la t ed cen te r motions (200 nm v s 70 nm). These r e s u l t s emphasize the localized character of the excitation. Since the width of the defect is very small compared to that of the s l ider , it bare ly genera tes enough force t o p e r t u r b t h e s l i d e r ver t ical ly: hence the small center displacements. On the other hand, precisely because of its small size, the defect generates a substantial pitch and r o l l motion as it t rave ls across the bearing region: hence the large corner d i s p l a c e m e n t s . One c a n a l s o o b s e r v e t h e d a m p i n g characteristics of the transient signals. In particular, a l l disturbances are damped out after 1 ms, which corresponds to about f o q times the length of the slider.

Finally, we examine the frequency content of the front and rear s l ider motions. Figures 7(A) and (B) are obtained by perfomiw a FE’T on data similar to that presented i n Figs. 5(B) and 5(D) respectively, but at a lower sampling

Fig. 5

i l g -. 25

Y - 2 5 r W ( C )

Transient motion of the s l ider : ver t ical displacements of the 4 corners at track 108. As viewed from the top: front right (A) , front l e f t (B) , rear right (C), and rear l e f t (D).

. l -

E 1

cn 0

L i o +i

-. 1 -

- 1 a -60

Fig.6 Calculated vertical center motion (A), pitch (B), qnd r o l l (C) of the slider at track 108.

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rate (2 MHz) for better frequency resolution. The s l ider has a steady state pitch angle with larger spacing in front so that the stiffness of the air-bearing at the trail ing end is higher than that of the leading end. Therefore, one w u l d expect that when excited, the.rear part of the s l ider wil l vibrate a t a s l i g h t l y higher frequency. Indeed, fran Fig. 7, one can observe that the predominant frequency of the front motion is a t 8 kHz while that o f the rear motion is around 11 kHz. Also, one can observe that many other resonances e x i s t , which a re r e l a t ed t o t he coup l ing of t he va r ious s l ider mot ions and to the na tura l f requencies o f the suspension a m These resul ts qual i ta t ively eorrelate with those published by On0 [8], who used capacitance probes t o measure the dynamic response of a s l ider due to ve r t i ca l disk motions. The resonance frequencies found here, +w=ver, a r e somewhat higher, probably because of the different physical characteristics of the slider bearings used.

0 5 10 15 ZOkHz

Fig.7 Yelocity spectra of the front (A), and rear (B) s l ider motions.

conccLusIoI A technique uti l izing the Doppler effect has been

applied here to measure the dynamic response of a s l ider bearing i n an unmodified magnetic disk f i l e . Un l ike previously used interferanetric techniques, this schene does not require a transparent disk or slider and is capable of detect iq t ransient s l ider motions in an actual d isk f i le . In order to demonstrate the feasibi l i ty of this instrunent, a comnercially available “Winchester“ disk drive with a small mechanical defect on the disk surface was used. The response of the s l ider t o t h i s a r t i f i c i a l p r e s s u r e disturbance was measured, and the daninant frequencies and the dampirq: characteristics of t h e s l i d e r / d i s k i n t e r f a c e y r e examined. A strong coupling between the various slider motions i s observed, id ica t ing the need for a nonlinear dynamical analysis i f one attempts t o model the head/disk interface analytically.

ACKWOVLEDGEHEEBTS

This research was sponsored by the Center for Magnetic Recording Research d e r Contract UCB ENG-5730. One of us, DKM, a lso acknowledges the support fran the Hertz Foundation.

REFERENCES

Fleischer, J.M., and Lin, C., IBM J. R 6 D, 18, 1974, 529 Nigam, 11, Trans. ASME, Ser. F, 104, 1982, 60 Harker, J.M., et al . , IBM J. R & D, 25-5, 1981, 677 Lin, C., IEEE Trans. Magn., 9-4, 1973, 673 Brunner, RK., e t al., IBM J. R & D, 3-3, 1959, 260 Lin, C,, and Sullivan, RF., I B M J. R & D, 16, 1972, 269 Wlezien, R, et al., J. SPIE, July 1984 Gno, It, et al., Bull. JSME, 22-173, 1979, 1672