-~o~~owing Servo for Linear Tape Systems

Transcription

1 1872 EEE TRANSACTONS ON MAGNETCS, VOL 34, NO 4, JULY ~o~~owng Servo for Lnear Tape Systems R. C. Barrett, E. H. Klaassen, and T. R. Albrecht BM Almaden Research Center, 650 Harry Road, San Jose, CA G. A. Jaquette BM Storage Systems Dvson, 9000 S. Rta Road, Tucson, AZ J. H. Eaton BM Storage Systems Dvson, 5600 Cottle Road, San Jose, CA Abstract -- Tmng-Based Servo (TBS) s a unque servo,~ ~- technology developed specfcally for lnear tape drves. n TBS 4 X Reeon the pattern. Poston sgnals are nearly perfectly lnear over multple track wdths, and hghly mmune to errors caused by head wear, head nstablty, debrs, and meda defects. Multtrack TBS servo patterns are wrtten n a sngle pass usng a novel multgap horzontal thn flm servo wrtng head. The desgn of the pattern and ts dmensons are optmzed to provde samplng rate, nose level, and error rate sutable for the ntended applcaton. An all-dgtal TBS servo channel provdes a speed-nvarant poston sgnal. Pattern recognton algorthms detect servo sgnal errors, provdng a hghly robust servo sgnal. Test results show approxmately 1 pm lnearty and 0.3 pm nose level over a wdth of 400 pm wdth wth 18 khz samplng rate. The TBS pattern allows flexblty for encodng addtonal nformaton wthout affectng the poston sgnal. By shftng transtons as lttle as 0.1 pm from ther nomnal pattern postons, a low error rate seral btstream can be encoded n the servo track. Ths technque allows tape longtudnal poston to be encoded wth a resoluton of about 2 mm, allowng effcent and precse tape transport control based on the servo sgnal alone. ndex Term -- Servo, servowrtng, tape, tmng-based, poston encodng.. NTRODUCTON Areal denstes n lnear tape storage systems have arrved at the pont where further progress requres mplementaton of track-followng servo to allow narrower data tracks. Tape products wthout servo now operate n vcnty of 500 tracks per nch (TP), about the same track ptch at whch dsk storage systems adopted track-followng servo several years ago. Whle servo technology orgnally developed for dsk systems may be appled to tape systems, the unque techncal challenges of the latter favor an altemate approach. Among these challenges are unrelable spatal response of servo heads, meda dropouts, and extendablty and nterchange ssues due to removablty. Tmng-Based Servo (TBS) s a novel system for addressng these challenges. Conventonal servosystems generally fall nto a category referred to as boundat systems, n whch the servo track or p=\(x) - q(y) R,(X) = Ampltude (X) (a) Fg. 1. (a) Boundary servo pattem usng a wde servo head and (b) contnously varable servo pattem usng a narrow read head. pattern s laterally dvded nto two or more regons, separated by lnear boundares as shown n Fg. l(a). The dstnct regons have dfferent propertes whch can be detected by the head; for example, the regons may be recorded at dfferent frequences or phases, or they may contan bursts occurrng at dstnct tmes. The head element straddles a boundary between regons, and the rato of the ampltude of the response of the head to each regon provdes the poston sgnal upon whch the track-followng servo operates. Some specfc examples of boundary servo patterns are the maton QCTM pattern shown n Fg. 2(a) and the BM 3590 pattem shown n Fg. 2(b). The QCTM pattern s wrtten wth a dual-element head; the frst element wrtes a wde sngle frequency track, and the second element selectvely erases sectons of ths track, creatng boundares for the servo system to follow. The poston sgnal s the rato of the ampltudes of the two portons ( 1 and 2 n Fg. 2) of the servo cycle, read back wth a wde servo read head. The BM 3590 pattern s also recorded wth a dual-element servo wrte head whch records a half-track at one frequency, wth a second half-track below the frst, consstng of bursts of a second frequency. The poston sgnal s a functon of the ampltudes durng the two portons of the servo cycle. Boundary-type servoes n tape systems are partcularly susceptble to poston sgnal errors. To provde suffcent /98$ EEE

2 873 Freq A Freq B Servo Head D A K ) B K - M Tme (A) Head Poston P=. Tme (B) Fg. 2 (a) QCTU servo pattern and (b) BM 3590 servo track. lateral dynamc range, head elements tend to be approxmately as wde as a full track wdth (typcally pm at present). Head nstabltes, head wear, localzed debrs on the head or tape, and meda defects all contrbute to temporary or long-term shfts n the spatal response of the servo head to the recorded servo pattern. 11. TMNG-BASED SERVO PATTERNS TBS systems use a contnuously varable servo pattern (shown conceptually n Fg. (b)), whch does not requre a wde servo read head for dynamc range. Each of the sources of poston error cted above scales to some degree wth the wdth of the servo read head. The maxmum error s less than the wdth of the servo read head; hence errors tend to be smaller when narrower servo read heads are used. t s nterestng to note that even n the case of boundary-type servo patterns, wde heads provde nferor poston sgnal qualty: Only the dflerence n ampltude response to each regon provdes useful sgnal for determnng poston; wder heads do not ncrease ths dfference, but merely ncrease the ampltude of meda nose n the system. Ths effect s qute dfferent than for the case of readng recorded data, whch shows mproved sgnal-to-nose wth wder heads. Snce dedcated servo tracks are generally favored n tape systems, there s freedom to optmze the servo head element wdth for best servo performance. A smple example of a contnuously varable TBS pattern s shown n Fg. 3. Ths pattern s smlar to versons of TBS proposed n the past [l]. The pattern conssts of strpes (regons of one polarty) recorded at two dstnct azmuthal orentatons. Speed-nvarant poston nformaton s gven by the rato of the duraton of varable-length A ntervals to fxed length B ntervals. The poston sgnal derved n ths manner s nherently hghly lnear. Dynamc range s lmted by the wdth of the pattern, whch may be multple data tracks wde. The servo head s much narrower than the pattern, ts wdth beng lmted only by detector nose and meda jtter at very narrow track wdths. Detectable synchronzaton features (empty gaps n the case of the pattern shown) are provded perodcally n the pattern to allow the servo decoder to lock to the pattern and measure the length of the necessary ntervals. Fg. 3. A smple example of a TBS pattern. By dvdng the tmng between servo marks as shown, a speed-ndependent poston sgnal s obtaned. To explot fully the advantages of TBS, a more complex pattern such as the damond pattern shown n Fg. 4 may be used. Ths pattern provdes a hgher densty of transtons per unt length and maxmum tmng dfferentaton for a gven azmuth and pattern wdth, whch mproves sgnal-to-nose. The ncreased pattern complexty also lends tself to mproved error checkng, dscussed below. Ths pattern s decoded by takng the rato of the sums of four A and four B ntervals. Perodc bursts of fve are present for synchronzaton. TBS systems have advantages for nterchange, removablty, and extendablty. Because of the good lnearty and reproducblty of the poston sgnal over a $ r n 6 Servo Head ( b ).Ready \\\ 1111 \\\\\ \\\ Sample Sample Sample +Ready Ready Fg. 4 (a) A new TBS servo pattern allows mproved error checkng and a hgher number of transtons per unt length. (b) Bursts of four and fve marks allow for the generaton of a speed-ndependent poston sgnal. 4 t

3 1874 wde poston range, the system may operate at arbtrary postons. Calbratons and offsets may be mplemented wth hgh precson. A sngle TBS track pattern may serve multple generatons of products operatng at a varety of track ptches; data track postons are ndependent of the servo pattern. For further comparson of conventonal and TBS systems see [2] SERVOWRTNG HEAD Cylndrcal Dual Gaps Photoresst Plat,ng Mask NFe Plated 1 NFeN/FeN F1 oat Wrtng a TBS pattern wth ts strpes at two dstnct azmuthal angles requres a novel servowrtng head desgn. Fg. 5 shows sde and top vews of the head. t has two consecutve gaps per track, each of whch wrtes a dstnct shape on the meda. Snce the tralng gap must not overwrte patterns wrtten by the leadng gap, short duraton unpolar current pulses are appled to the head. Each pulse prnts an mage of both gaps on the tape. Snce the wrte current s not contnuous, the meda must be erased pror to servowrtng. The process for producng heads s shown n Fg. 6. The gaps are formed photolthographcally n a plated horzontal flm of NFe. The mechancal module s machned from a prelamnated ferrteglass structure, and the two-turn col s wound by hand through slots n the module. The gap structure desgn shown n Fg. 5 results n regons of dsparate heght on the tape bearng surface. Durng operaton, the tape may sag nto depressons on the tape bearng surface, resultng n debrs collecton and head wear due to ncreased tape contact pressure at step edges. The deal tape bearng surface would be perfectly smooth. A shunted gap layout shown n Fg. 7(b) nearly acheves ths deal. Conventonal wsdom n head desgn calls for large gaps between the magnetc poles except for the narrow wrte gaps where the wrte feld s to be generated. All other brdges between the poles are typcally flled wth nonmagnetc materal. However, n the shunted head desgn, the poles are brdged wth a contnuous flm of NFe, leavng only the 2 pm wde wrte gaps open. At frst glance, ths structure appears to shunt the magnetc flux through the contnuous NFe flm, leavng lttle flux n the gaps. Wth Open Slot Femte Fg. 6. Servo wrte-head fabrcaton process flow. (a) A ferrte-glass-ferrte sandwch s made usng glass bondng technques. (b) After deposton of a platng seed layer, a photoresst mask defnes the head gaps. A 2-3 pm NFe layer s plated on the seed layer. (c) After removal of the photoresst, several lamnatons of a NFeNFeN flm are sputtered on the tape bearng surface to mprove wear characterstcs of the head. suffcently hgh drve current, however, the entre NFe flm saturates, resultng n suffcent feld n the wrte gaps. The contnuous NFe flm has surprsngly lttle effect on the magnetc effcency of the head. Fg. 8 shows a comparson of the readback ampltude for varous wrte currents appled to shunted and nonshunted heads whle wrtng 1600 Oe metal partcle tape. Shunted heads requre one specal desgn consderaton: The ends of the wrte gaps do not have a well-defned cutoff. Fg.9 shows flares whch have been ncorporated nto the ends of the gaps to reduce the wrte feld at a well defned poston, producng a wrtten pattern of precsely controlled lateral wdth. The gaps of the shunted head are not flled, and may accumulate debrs (ncludng magnetc meda debrs) durng operaton. However, ths has not been shown to affect head performance. The relatvely wde gap produced by the horzontal head process produces a deep wrte feld (on the order of 1 pm), whch mnmzes the effects of spacng loss due to debrs or other factors durng wrtng. Glass NFe ( a ) Sde Vew: (b) Top Vew: 2T Gaps Gaps Shunted Fg. 5. (a) Sde vew of servo wrtng head wth dmensons shown (not to scale). The two turns of wre carry unpolar 2 A wrte current pulses. (b) Top vew of servo wrtng head, showng a magnfed vew of the gaps. Ths example shows an unshunted head. (a) (b) Fg. 7. (a) The head as shown n Fg. 5(b) has too much surface topology, but a shunted head (b) elmnates the depressons around the wrte gaps.

4 - 0.8,_- v 1875 Fg. 8. The readback sgnals from a 1600 Oe metal partcle tape wrtten by a shunted and unshunted head are very smlar n ampltude. Fg. 10 shows two photomcrographs of a Btter pattern of a secton of metal partcle tape wrtten wth the TBS servo pattern. The servo track s 408 pm wde, and the spacng between each 4-5 burst par s 221 pm. The spacng between the marks n each pattern s 5 pm. V. PATTERN OPTMZATON There are a number of performance parameters whch must be consdered n desgnng a TBS pattern. The pattern wdth must be somewhat greater than the desred servo range per servo head element. The samplng rate s lmted by the length and arrangement of ntervals n the pattern. Usng the namng conventons shown n Fg. 1 1, the poston sgnal formula can be wrtten as P= +C= +C 2tm8C, b 2 tan 8 x(w - u) Fg. 10. Photomcrographs of a Btter pattern showng a TBS servo track. where a and b, are tme ntervals measured, and B s the wrtten length of b. The varaton of P wth respect to jtter n the transtons U, v, and w s Flare 408 pn f u, v, and wj have a normal dstrbuton wth varance 02 then the standard devaton of P s /Q~-Q+~ G -- P- tan0 2N (3) where o s the standard devaton n the measurement of the poston of a sngle transton, N s the number of A or B ntervals measured per sample, and Q s gven by Fg. 9. Flares added to the ends of the wrte gaps provde a well-defned cutoff for the wrte feld. Ths fgure s not to scale. = a Q=-. b (4)

5 876 HEAD -.- PATH Fg. 11. Cross-track postons U, v, and w and the measured tme ntervals a and b The nose n the measurement of a sngle transton, 6, depends n part on the pulse wdth assocated wth each transton. The spreadng of the pulses wth ncreasng azmuth can be descrbed by the followng approxmaton: CJ = e(d+ wtano) (5) where w s the wdth of the servo read head, d s the detecton pulse wdth of a perpendcular transton, and E s the fractonal precson wth whch the peak of the transton may be detected (govemed by the pulse resoluton, meda nose, and peak detector nose). For our system, usng a 12.5 pm wde read head wth d measured to be 1.3 pm, the coeffcent E was found to have a value of f the head s too wde for a gven azmuth and strpe wdth, the head may span the entre strpe wdth, respondng to transtons of opposte polarty at the leadng and tralng edges of the strpe. Beyond ths angle O,, gven by where s s the strpe wdth n the drecton of tape moton, the resoluton deterorates rapdly; ths regme should be avoded when desgnng the system. The clock speed of the decoder contrbutes addtonal nose to the system due to the asynchronous nature of the decodng process. The standard devaton n the measurement of the poston of a sngle transton due to ths quantzaton error s gven by Tv rjq = - 2J3 where T s the clock perod, and v s the tape velocty. Ths measurement error can be substtuted for CT n equaton (3) to determne the contrbuton to the poston sgnal nose. Ths (7) nose adds to the physcal measurement nose n an uncorrelated fashon. For our system, the clock frequency s 20 MHz and the tape velocty s 2 m/s. Ths leads to a CJq of 0.03 pm for the measurement of each transton poston and a resultng standard devaton n the poston sgnal of 0.08 pm at the samplng rate of 18 khz. V. ERROR DETECTON Pattern recognton may be used to detect errors durng decodng of TBS sgnals. n conventonal systems, error detecton s accomplshed manly by montorng the ampltude of servo sgnals relatve to a target ampltude range. However, sgnfcant poston errors may occur before the error threshold s reached. Whle ampltude montorng may provde useful nformaton n predctng the occurrence of errors n TBS systems, pattern recognton provdes a more powerful technque. The sgnals for the damond TBS pattern comprse bursts of four and fve pulses separated by varable length ntervals. Countng the pulses n bursts and checkng aganst the known pattern has been found expermentally to detect more than 99% of all servo errors. Fg. 12 shows an example of errors detected and corrected usng ths type of algorthm. The correcton algorthm used n ths demonstraton s smply to replace erroneous samples wth the prevous good sample. More complex pattem recognton schemes could be used to acheve better error detecton capablty. V. ENCODNG A SERAL BT STREAM N THE TBS PATTERN n order to provde a means for effcent and precse tape transport control, a seral btstream may be encoded drectly nto the tape servopattern. Ths encodng can be acheved wthout affectng the poston error sgnal. Fg. 13 shows that the second and thrd servo mark of the groups of four, as well as the second and fourth mark n the group of fve, can be shfted towards or away from each other. The value of the poston sgnal s not affected, snce the sum of the A ntervals and the sum of the B ntervals reman unchanged. n the Uncorrected Corrected 1 1 f Fg. 12. Examples of an uncorrected (top) and corrected (bottom) servo poston error sgnal.

6 Data=(Rl+W+W+R4)>(Pl+P2) One Zero Fg. 13. nformaton s embedded n the servo pattern by shftng some of the pulses towards or away from the center of each of the bursts. partcular case depcted n Fg. 13, all the A ntervals are unchanged by the shft. nterval B, becomes shorter by twce the shft dstance, but B2 and B3 each become longer by a sngle shft dstance; hence the sum of B ntervals remans the same as for the unshfted case. A dgtal zero or one can be stored by movng the approprate servo marks outward or nward, respectvely. Durng readback of the servo sgnal, the dgtal btstream s decoded usng the algorthm shown n Fg. 14. The pulses shown represent the output of the peak detector when readng the servo pattern. n the fve burst, two accumulators measure the total of the varous P- and R-ntervals as ndcated n the fgure. After the pulse from the ffth closng chevron mark, a comparson of these two sums yelds the encoded bt. A smlar procedure s employed for the four burst. The comparson, however, s now between the sum of the P ntervals and half the sum of the R ntervals, snce the nomnal P ntervals are half as long n ths case. Wth ths pulse shftng scheme, longtudnal tape poston can be embedded nto the servo pattern. A codng scheme usng 20-bt seral words can be used that allows for error detecton and correcton. n ths fashon, one word can be wrtten every 2.2 mm of tape length, provdng hgh resoluton poston nformaton for tape transport control usng the servo sgnal alone. Experments were conducted to determne the amount of shft necessary for low error-rate readback of the encoded nformaton. t was found that 100 nm pulse shfts yelded an error rate of 5x 1 O-7. Ths was the smallest shft ncrement of nterest, snce t represents a sngle clock perod advance or delay wth a 2 m/s tape speed for the 20 MHz clock speed used n the servo system. Shfts of 200 to 800 nm lowered the error rate to less than These error rates were measured as the number of ncorrectly decoded bts per good servo bursts. Shfts larger than 800 nm began to cause ntersymbol dstorton, whch had a negatve mpact on the poston sgnal and the servo error rate. R R2 R3 R4 Fg. 14. The data embedded n the servo pattern s decoded by comparng the sums of dfferent ntervals between pulses. V. OUTLOOK A TBS system has been mplemented n the BM 3570 Magstar MP seres of tape storage products. Table shows the specfcatons and results measured n the laboratory wth a system smlar to that used n products. Because of ts unque advantages n lnear tape systems for poston sgnal lnearty, relablty, and error detecton capablty, TBS may become a wdely used technology. TABLE TMNG BASED SERVO SYSTEM PARAMETERS DESGN PARAMETERS: tape speed data track wdth servo samplng rate decoder clock speed closed loop servo bandwdth POSTON ERROR CONTRBUTORS: pattern dstortons (wrte head) wrte velocty error (ko.l%) decoder clock speed electroncs and meda nose SERVO ERRORATES (DETECTED): sngle channel dual channel, uncorrelated 2 m/sec 48 pm 18 khz 20 MHz 500 Hz REFERENCES fl pm * 0.2 pm f khz, k Hz f0.3pm@18khz, f Hz 2x 105(1 18 khz) 4 x 1O-o (1 every khz) [] See, for example, U.S. Patent # 3,686,649 by Mchael Behr. [2] T.R. Albrecht, R.C. Barrett, and J.H. Eaton, Tme-based, Track-followng Servo for Lnear Tape Drves, Data Storage, Oct (n press).