Timing-Based Track-Following Servo For Linear Tape ... - IEEE Xplore

IEEE TRANSACTIONS ON MAGNETICS, VOL 34, NO 4, JULY 1998

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- ~ o ~ ~ o w Servo i n g for Linear Tape Systems R. C. Barrett, E. H. Klaassen, and T. R. Albrecht IBM Almaden Research Center, 650 Harry Road, San Jose, CA 95 120

G. A. Jaquette IBM Storage Systems Division, 9000 S.Rita Road, Tucson, AZ 85744 I

J. H. Eaton IBM Storage Systems Division, 5600 Cottle Road, San Jose, CA 95 193

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Abstract -- Timing-Based Servo (TBS) is a unique servo technology developed specifically for linear tape drives. In TBS

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the pattern. Position signals a r e nearly perfectly linear over multiple track widths, and highly immune to errors caused by head wear, head instability, debris, and media defects. Multitrack TBS servo patterns a r e written in a single pass using a novel multigap horizontal thin film servo writing head. T h e design of the pattern and its dimensions a r e optimized to provide sampling rate, noise level, and e r r o r rate suitable for the intended application. An all-digital TBS servo channel provides a speed-invariant position signal. Pattern recognition algorithms detect servo signal errors, providing a highly robust servo signal. Test results show approximately 1 p m linearity and 0.3 p m noise level over a width of 400 p m width with 18 kHz sampling rate. The TBS pattern allows flexibility for encoding additional information without affecting the position signal. By shifting transitions as little as 0.1 p m from their nominal pattern positions, a low e r r o r rate serial bitstream can be encoded in the servo track. This technique allows tape longitudinal position to be encoded with a resolution of about 2 mm, allowing efficient and precise tape transport control based on the servo signal alone.

p=\(X)

Index Term position encoding.

--

Servo, servowriting, tape, timing-based,

I. INTRODUCTION

Areal densities in linear tape storage systems have arrived at the point where further progress requires implementation of track-following servo to allow narrower data tracks. Tape products without servo now operate in vicinity of 500 tracks per inch (TPI), about the same track pitch at which disk storage systems adopted track-following servo several years ago. While servo technology originally developed for disk systems may be applied to tape systems, the unique technical challenges of the latter favor an altemate approach. Among these challenges are unreliable spatial response of servo heads, media dropouts, and extendability and interchange issues due to removability. Timing-Based Servo (TBS) is a novel system for addressing these challenges. Conventional servosystems generally fall into a category referred to as boundaT systems, in which the servo track or

_ _ _-

-~ -

4 - q(Y)

“X” Reeion

R,(X) = Amplitude (X)

(a)

(a) Boundary servo pattem using a wide servo head and (b) continously variable servo pattem using a narrow read head. Fig. 1.

pattern is laterally divided into two or more regions, separated by linear boundaries as shown in Fig. l(a). The distinct regions have different properties which can be detected by the head; for example, the regions may be recorded at different frequencies or phases, or they may contain bursts occurring at distinct times. The head element straddles a boundary between regions, and the ratio of the amplitude of the response of the head to each region provides the position signal upon which the track-following servo operates. Some specific examples of boundary servo patterns are the Imation QICTMpattern shown in Fig. 2(a) and the IBM 3590 pattem shown in Fig. 2(b). The QICTMpattern is written with a dual-element head; the first element writes a wide single frequency track, and the second element selectively erases sections of this track, creating boundaries for the servo system to follow. The position signal is the ratio of the amplitudes of the two portions (‘1’ and ‘2’ in Fig. 2) of the servo cycle, read back with a wide servo read head. The IBM 3590 pattern is also recorded with a dual-element servo write head which records a half-track at one frequency, with a second half-track below the first, consisting of bursts of a second frequency. The position signal is a function of the amplitudes during the two portions of the servo cycle. Boundary-type servoes in tape systems are particularly susceptible to position signal errors. To provide sufficient

0018-9464/98$10.00 0 1998 IEEE

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Freq B

Freq A 1

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2

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1

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B K Fig. 2 (a) QICTUservo pattern and (b) IBM 3590 servo track.

lateral dynamic range, head elements tend to be approximately as wide as a full track width (typically 30-80 p m at present). Head instabilities, head wear, localized debris on the head or tape, and media defects all contribute to temporary or long-term shifts in the spatial response of the servo head to the recorded servo pattern. 11. TIMING-BASED SERVOPATTERNS

TBS systems use a continuously variable servo pattern (shown conceptually in Fig. I(b)), which does not require a wide servo read head for dynamic range. Each of the sources of position error cited above scales to some degree with the width of the servo read head. The maximum error is less than the width of the servo read head; hence errors tend to be smaller when narrower servo read heads are used. It is interesting to note that even in the case of boundary-type servo patterns, wide heads provide inferior position signal quality: Only the diflerence in amplitude response to each region provides useful signal for determining position; wider heads do not increase this difference, but merely increase the amplitude of media noise in the system. This effect is quite different than for the case of reading recorded data, which shows improved signal-to-noise with wider heads. Since dedicated servo tracks are generally favored in tape systems, there is freedom to optimize the servo head element width for best servo performance. A simple example of a continuously variable TBS pattern is shown in Fig. 3. This pattern is similar to versions of TBS proposed in the past [l]. The pattern consists of stripes (regions of one polarity) recorded at two distinct azimuthal orientations. Speed-invariant position information is given by the ratio of the duration of variable-length A intervals to fixed length B intervals. The position signal derived in this manner is inherently highly linear. Dynamic range is limited by the width of the pattern, which may be multiple data tracks wide. The servo head is much narrower than the pattern, its width being limited only by detector noise and media jitter at very narrow track widths. Detectable synchronization features (empty gaps in the case of the pattern shown) are provided periodically in the pattern to allow the servo decoder to lock to the pattern and measure the length of the necessary intervals.

Head Position P =

M

Time (A) . Time (B)

Fig. 3 . A simple example of a TBS pattern. By dividing the timing between servo marks as shown, a speed-independent position signal is obtained.

To exploit fully the advantages of TBS, a more complex pattern such as the “diamond” pattern shown in Fig. 4 may be used. This pattern provides a higher density of transitions per unit length and maximum timing differentiation for a given azimuth and pattern width, which improves signal-to-noise. The increased pattern complexity also lends itself to improved error checking, discussed below. This pattern is decoded by taking the ratio of the sums of four A and four B intervals. Periodic bursts of five are present for synchronization. TBS systems have advantages for interchange, removability, and extendability. Because of the good linearity and reproducibility of the position signal over a 15

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Fig. 4 (a) A new TBS servo pattern allows improved error checking and a higher number of transitions per unit length. (b) Bursts of four and five marks allow for the generation of a speed-independent position signal.

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wide position range, the system may operate at arbitrary positions. Calibrations and offsets may be implemented with high precision. A single TBS track pattern may serve multiple generations of products operating at a variety of track pitches; data track positions are independent of the servo pattern. For further comparison of conventional and TBS systems see [ 2 ] .

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111. SERVOWRITING HEAD

Writing a TBS pattern with its stripes at two distinct azimuthal angles requires a novel servowriting head design. Fig. 5 shows side and top views of the head. It has two consecutive gaps per track, each of which writes a distinct shape on the media. Since the trailing gap must not overwrite patterns written by the leading gap, short duration unipolar current pulses are applied to the head. Each pulse “prints” an image of both gaps on the tape. Since the write current is not continuous, the media must be erased prior to servowriting. The process for producing heads is shown in Fig. 6. The gaps are formed photolithographically in a plated horizontal film of NiFe. The mechanical module is machined from a prelaminated ferriteiglass structure, and the two-turn coil is wound by hand through slots in the module. The gap structure design shown in Fig. 5 results in regions of disparate height on the tape bearing surface. During operation, the tape may sag into depressions on the tape bearing surface, resulting in debris collection and head wear due to increased tape contact pressure at step edges. The ideal tape bearing surface would be perfectly smooth. A “shunted” gap layout shown in Fig. 7(b) nearly achieves this ideal. Conventional wisdom in head design calls for large gaps between the magnetic poles except for the narrow write gaps where the write field is to be generated. All other bridges between the poles are typically filled with nonmagnetic material. However, in the shunted head design, the poles are bridged with a continuous film of NiFe, leaving only the 2 pm wide write gaps open. At first glance, this structure appears to shunt the magnetic flux through the continuous NiFe film, leaving little flux in the gaps. With

Open Slot

Femte

Fig. 6 . Servo write-head fabrication process flow. (a) A ferrite-glass-ferrite sandwich is made using glass bonding techniques. (b) After deposition of a plating seed layer, a photoresist mask defines the head gaps. A 2-3 pm NiFe layer is plated on the seed layer. (c) After removal of the photoresist, several laminations of a NiFeNiFeN film are sputtered on the tape bearing surface to improve wear characteristics of the head.

sufficiently high drive current, however, the entire NiFe film saturates, resulting in sufficient field in the write gaps. The continuous NiFe film has surprisingly little effect on the magnetic efficiency of the head. Fig. 8 shows a comparison of the readback amplitude for various write currents applied to shunted and nonshunted heads while writing 1600 Oe metal particle tape. Shunted heads require one special design consideration: The ends of the write gaps do not have a well-defined cutoff. Fig.9 shows flares which have been incorporated into the ends of the gaps to ‘reduce the write field at a well defined position, producing a written pattern of precisely controlled lateral width. The gaps of the shunted head are not filled, and may accumulate debris (including magnetic media debris) during operation. However, this has not been shown to affect head performance. The relatively wide gap produced by the horizontal head process produces a deep write field (on the order of 1 pm), which minimizes the effects of spacing loss due to debris or other factors during writing.

Glass (a)

Side View:

(b)

NiFe

Top View:

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Fig. 5. (a) Side view of servo writing head with dimensions shown (not to scale). The two turns of wire carry unipolar 2 A write current pulses. (b) Top view of servo writing head, showing a magnified view of the gaps. This example shows an unshunted head.

(a)

(b)

Fig. 7. (a) The head as shown in Fig. 5(b) has too much surface topology, but a shunted head (b) eliminates the depressions around the write gaps.

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Fig. 8. The readback signals from a 1600 Oe metal particle tape written by a shunted and unshunted head are very similar in amplitude.

Fig. 10 shows two photomicrographs of a Bitter pattern of a section of metal particle tape written with the TBS servo pattern. The servo track is 408 pm wide, and the spacing between each 4-5 burst pair is 221 pm. The spacing between the marks in each pattern is 5 pm. IV. PATTERNOPTIMIZATION There are a number of performance parameters which must be considered in designing a TBS pattern. The pattern width must be somewhat greater than the desired servo range per servo head element. The sampling rate is limited by the length and arrangement of intervals in the pattern. Using the naming conventions shown in Fig. 1 1, the position signal formula can be written as

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Fig. 10. Photomicrographsof a Bitter pattern showing a TBS servo track.

where ai and b, are time intervals measured, and B is the written length of bi. The variation of P with respect to jitter in the transitions U , vi, and wi is

Flare

If ui, vi, and wjhave a normal distribution with variance then the standard deviation of P is

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-Q+~ --tan0 / Q ~2N

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(3)

where o is the standard deviation in the measurement of the position of a single transition, N is the number of A or B intervals measured per sample, and Q is given by Fig. 9. Flares added to the ends of the write gaps provide a well-defined cutoff for the write field. This figure is not to scale.

I= ai

Q=-. i i

bi

(4)

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noise adds to the physical measurement noise in an uncorrelated fashion. For our system, the clock frequency is 20 MHz and the tape velocity is 2 m/s. This leads to a CJq of 0.03 pm for the measurement of each transition position and a resulting standard deviation in the position signal of 0.08 pm at the sampling rate of 18 kHz.

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V. ERRORDETECTION

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HEAD PATH

Fig. 11. Cross-track positions U, v, and w and the measured time intervals a and b

The noise in the measurement of a single transition, 6, depends in part on the pulse width associated with each transition. The spreading of the pulses with increasing azimuth can be described by the following approximation: CJ = e(d+ wtano)

(5)

where w is the width of the servo read head, d is the detection pulse width of a perpendicular transition, and E is the fractional precision with which the peak of the transition may be detected (govemed by the pulse resolution, media noise, and peak detector noise). For our system, using a 12.5 pm wide read head with d measured to be 1.3 pm, the coefficient E was found to have a value of 0.03. If the head is too wide for a given azimuth and stripe width, the head may span the entire stripe width, responding to transitions of opposite polarity at the leading and trailing edges of the stripe. Beyond this angle O,, given by

where s is the stripe width in the direction of tape motion, the resolution deteriorates rapidly; this regime should be avoided when designing the system. The clock speed of the decoder contributes additional noise to the system due to the asynchronous nature of the decoding process. The standard deviation in the measurement of the position of a single transition due to this quantization error is given by

Pattern recognition may be used to detect errors during decoding of TBS signals. In conventional systems, error detection is accomplished mainly by monitoring the amplitude of servo signals relative to a target amplitude range. However, significant position errors may occur before the error threshold is reached. While amplitude monitoring may provide useful information in predicting the occurrence of errors in TBS systems, pattern recognition provides a more powerful technique. The signals for the diamond TBS pattern comprise bursts of four and five pulses separated by variable length intervals. Counting the pulses in bursts and checking against the known 4-4-5-5 pattern has been found experimentally to detect more than 99% of all servo errors. Fig. 12 shows an example of errors detected and corrected using this type of algorithm. The correction algorithm used in this demonstration is simply to replace erroneous samples with the previous good sample. More complex pattem recognition schemes could be used to achieve better error detection capability. VI. ENCODING A SERIAL BIT STREAM IN THE TBS PATTERN

In order to provide a means for efficient and precise tape transport control, a serial bitstream may be encoded directly into the tape servopattern. This encoding can be achieved without affecting the position error signal. Fig. 13 shows that the second and third servo mark of the groups of four, as well as the second and fourth mark in the group of five, can be shifted towards or away from each other. The value of the position signal is not affected, since the sum of the A intervals and the sum of the B intervals remain unchanged. In the I

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f

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Tv rJq = 2J3

(7)

where T is the clock period, and v is the tape velocity. This measurement error can be substituted for CT in equation (3) to determine the contribution to the position signal noise. This

Fig. 12. Examples of an uncorrected (top) and corrected (bottom) servo position error signal.

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“Zero”

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R3

R4

Fig. 13. Information is embedded in the servo pattern by shifting some of the pulses towards or away from the center of each of the bursts.

particular case depicted in Fig. 13, all the A intervals are unchanged by the shift. Interval B, becomes shorter by twice the shift distance, but B2 and B3 each become longer by a single shift distance; hence the sum of B intervals remains the same as for the unshifted case. A digital “zero” or “one” can be stored by moving the appropriate servo marks outward or inward, respectively. During readback of the servo signal, the digital bitstream is decoded using the algorithm shown in Fig. 14. The pulses shown represent the output of the peak detector when reading the servo pattern. In the five burst, two accumulators measure the total of the various P- and R-intervals as indicated in the figure. After the pulse from the fifth closing chevron mark, a comparison of these two sums yields the encoded bit. A similar procedure is employed for the four burst. The comparison, however, is now between the sum of the P intervals and half the sum of the R intervals, since the nominal P intervals are half as long in this case. With this pulse shifting scheme, longitudinal tape position can be embedded into the servo pattern. A coding scheme using 20-bit serial words can be used that allows for error detection and correction. In this fashion, one word can be written every 2.2 mm of tape length, providing high resolution position information for tape transport control using the servo signal alone. Experiments were conducted to determine the amount of shift necessary for low error-rate readback of the encoded information. It was found that 100 nm pulse shifts yielded an error rate of 5x 1O-7. This was the smallest shift increment of interest, since it represents a single clock period advance or delay with a 2 m/s tape speed for the 20 MHz clock speed used in the servo system. Shifts of 200 to 800 nm lowered the error rate to less than These error rates were measured as the number of incorrectly decoded bits per good servo bursts. Shifts larger than 800 nm began to cause intersymbol distortion, which had a negative impact on the position signal and the servo error rate.

Fig. 14. The data embedded in the servo pattern is decoded by comparing the sums of different intervals between pulses.

VII. OUTLOOK A TBS system has been implemented in the IBM 3570 Magstar MP series of tape storage products. Table I shows the specifications and results measured in the laboratory with a system similar to that used in products. Because of its unique advantages in linear tape systems for position signal linearity, reliability, and error detection capability, TBS may become a widely used technology.

TABLE I TIMING BASEDSERVO SYSTEM PARAMETERS

DESIGN PARAMETERS: tape speed data track width servo sampling rate decoder clock speed closed loop servo bandwidth POSITION ERROR CONTRIBUTORS: pattern distortions (write head) write velocity error (kO.l%) decoder clock speed electronics and media noise SERVOERROR RATES(DETECTED): single channel dual channel, uncorrelated

2 m/sec 48 pm 18 kHz 20 MHz 500 Hz

f l pm 0.2 pm f 0.08 pm @ 18 kHz, k0.013 pm @ 500 Hz

*

f0.3pm@18kHz, f 0.05 pm @ 500 Hz 2 x 105(1 every2.8sec @ 18 kHz) 4 x 1O-Io (1 every 39 hrs @ 18 kHz)

REFERENCES [I] See, for example, U.S. Patent # 3,686,649 by Michael Behr. [2] T.R. Albrecht, R.C. Barrett, and J.H. Eaton, “Time-based, Track-following Servo for Linear Tape Drives,” Data Storage, Oct. 1997 (in press).