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High-Rate Skew Estimation for Tape Systems
7th IFAC Symposium on MechatronicUniversity, Systems UK September 5-8, 2016. Loughborough 7th IFAC Symposium on MechatronicUniversity, Systems UK September 5-8, 2016. Loughborough 7th IFAC Symposium on Mechatronic Systems online September 5-8, 2016. Loughborough Available University, UK at www.sciencedirect.com September 5-8, 2016. Loughborough University, UK
ScienceDirect IFAC-PapersOnLine 49-21 (2016) 007–012 High-Rate Skew Estimation for Tape Systems High-Rate Skew Estimation for Tape Systems High-Rate Skew Estimation High-Rate Skew Estimation for for Tape Tape Systems Systems
Giovanni Cherubini, Simeon Furrer, and Mark A. Lantz Giovanni Cherubini, Simeon Furrer, and Mark A. Lantz Giovanni Cherubini, Simeon Furrer, and Mark A. Lantz Giovanni Cherubini, Simeon Furrer, and Mark A. Lantz IBM Research – Zurich, 8803 Rüschlikon, Switzerland IBM Research – Zurich, 8803 Rüschlikon, Switzerland (e-mail:–{cbi,sfu,mla}@zurich.ibm.com) IBM Research Zurich, 8803 Rüschlikon, Switzerland (e-mail:–{cbi,sfu,mla}@zurich.ibm.com) IBM Research Zurich, 8803 Rüschlikon, Switzerland (e-mail: {cbi,sfu,mla}@zurich.ibm.com) (e-mail: {cbi,sfu,mla}@zurich.ibm.com) Abstract: In several recent magnetic tape drive designs, an actuator with two degrees of freedom has Abstract: In several recent magnetic tape drive designs,This an actuator with two degrees of freedom has been adopted jointrecent skew- magnetic and track-following enables the of flanges from guide Abstract: In for several tape drive control. designs, an actuator withremoval two degrees of freedom has been adopted for joint skewand track-following control. This enables the removal of flanges from guide Abstract: In tape several recent magnetic tape drive designs, an actuator with two degrees of freedom has rollers in the path, thus avoiding debris build-up, tape-edge damage, and high-frequency lateral tape been adopted for joint skew- and track-following control. This enables the removal of flanges from guide rollersadopted in the tape path,skewthus avoiding debris build-up, tape-edge damage, and high-frequency lateral tape been for joint and track-following This enables the removal of flanges from guide motion. compensation is needed to keep thecontrol. head perpendicular to and the direction of tapelateral motion to rollers inSkew the tape path, thus avoiding debris build-up, tape-edge damage, high-frequency tape motion.in Skew compensation is neededin tothe keep the head perpendicular to excursions the direction of tapelateral motion to rollers the tape path, thus avoiding debris build-up, tape-edge damage, and high-frequency tape enable read-while-write functionality presence of large lateral tape originating from the motion. Skew compensation is needed to keep the head perpendicular to the direction of tape motion to enable read-while-write functionality into the presence of large lateral tape originating from the motion. Skew compensation is needed keep the head perpendicular to excursions the direction of skew tape motion to removal of roller flanges. In this paper, we present a novel skew estimator that generates estimates enable read-while-write functionality in the presence of large lateral tape excursions originating from the removal of roller flanges. In this paper, we skew estimator that generates skew estimates enable read-while-write functionality the present presencea novel of large lateral tape excursions originating from the at a high rate for an extended range ofinallowable skew values. A skew estimate given by removal of roller flanges. In this paper, we presenttape-to-head a novel skew estimator that generates skewis estimates at a high rate for an extended range of allowable tape-to-head skew values. A skew estimate is given by removal of roller flanges. In this paper, we present a novel skew estimator that generates skew estimates the sum of a fractional and an integer part, extracted from information provided by two parallel at a high rate for an extended range of allowable tape-to-head skew values. A skew estimate is given by the sum of a fractional and an integer part, extracted from information provided by two parallel at a high rate for an extended range of allowable tape-to-head skew values. A skew estimate is given by synchronous channels on part, timing-based The fractional parttwo obtained the sum of aservo fractional andoperating an integer extractedservo frompatterns. information provided by parallel synchronous servo channels operating on timing-based servo patterns. The fractional part is obtained the sum of aservo fractional and an integer extracted from information provided by two parallel from the accurate measure of operating timing intervals between dibit correlation peaks, whereas the integer part is synchronous channels on part, timing-based servo patterns. The fractional part is obtained from thefrom accurate measure of timing information. intervals between dibit correlation peaks, whereas the integer part is synchronous servo channels operating on timing-based servo patterns. The fractional part is obtained derived longitudinal position The new method was implemented in an FPGA and from the accurate measure of timing intervals between dibit correlation peaks, whereas the integer part is derived from longitudinal position information. The new waspeaks, implemented in an FPGA and from theexperimentally accurate measure of timing intervals between dibit method correlation whereas the integer part is verified using a commercial tape drive. derived from longitudinal position information. The new method was implemented in an FPGA and verified experimentally using a commercial tape drive. derived from longitudinal position information. The new method was implemented in an FPGA and verified experimentally usingFederation a commercial tape drive. © 2016, IFAC (International of Automatic Control) Hosting by Elsevier Ltd. All rights reserved. Keywords: Skew estimation, control, tape systems. verified experimentally usingskew-following a commercial tape drive. Keywords: Skew estimation, skew-following control, tape systems. Keywords: Skew estimation, skew-following control, tape systems. Keywords: Skew estimation, skew-following control, tape systems. 1. INTRODUCTION 1. INTRODUCTION 1. INTRODUCTION In modern magnetic tape drives, the system reliability is 1. INTRODUCTION In modern magnetic tape drives, the system reliability is enhanced performing a read operation simultaneously with In modernbymagnetic tape drives, the system reliability is enhanced bymagnetic performing a read operation simultaneously with In modern tape drives, the system reliability is write operations to verify that the data has been correctly enhanced by performing a read operation simultaneously with write operations to verify that the datausing has been correctly enhanced by performing a read operation simultaneously with written. This verification is performed a second head write operations to verify isthat the datausing has been correctly written. Thisread verification performed a second head write operations toelements, verify that the isdata has been correctly module positioned adjacent to written. with This verification is which performed using a second head module with read elements, which is positioned adjacent to written. This verification is performed using a second head the head module writing the data. This second head module is module with read elements, which is positioned adjacent to the head module writing the data. This second head module is module with read elements, which is positioned adjacent to typically physically attached to the first head module, with the head module writing the data. This second head module is typically physically attached to the first head module, with the head module writing the data. This second head module is read physically elements ofattached the second aligned withwith the typically to themodule first head module, the read elements ofattached the module. second module aligned with the typically physically to theData firstishead module, with write elements of the first written as the tape the read elements of the second module aligned with the write elements the Data is then written the tape the read elements offirst the module. second module aligned with the is streamed overof module and readas and write elements ofthe thefirst firsthead module. Data is written asback the tape is streamed over the module and read and write elements ofwritten thefirst firsthead module. Data is then written asback the head tape verified as the data passes over the second is streamed over the first head module and then read back and verified as over the data passes the read second is streamed the first head and back and module. too written many raw module errors over are then detected in head this verified asIf the written data passes over the second head module. If too many raw errors are detected in head this verified as the written data can passes over the second verification step, the data be rewritten immediately module. If too many raw errors are detected in this verification step, the data can be rewritten immediately module. If too many raw errors are detected in without stopping processthis to verification step, the the tape. data For can this be verification rewritten immediately without stopping the tape. For this verification process to verification step, the data can be rewritten immediately function properly, the tape motion must be essentially without stopping thethetape. Formotion this verification process to function properly, tape must be essentially without stopping the tape. For this verification process to perpendicular to the the axis tape of themotion arrays ofmust writers readers function properly, be and essentially perpendicular to the such axis tape of the arrays ofmust writers and readers function properly, the motion beremain essentially in the two modules, that the read elements in the perpendicular to the axis of the arrays of writers and readers in the two modules, that thearrays read elements in the perpendicular the such axis of the of writersremain and readers shadows thetowrite elements. in the twoofmodules, such that the read elements remain in the shadows the writesuch elements. in the twoofmodules, that the read elements remain in the shadows of the write elements. shadows of the is writea elements. Tape skew measure of the deviation from Tape skew is ofa themeasure the perpendicularity angle ofof tapedeviation relative tofrom the Tape skew is a measure ofthethe deviation from perpendicularity of the angle of the tape relative tofrom the Tape skew is a measure of the deviation read/write head. In previous generations of tape drives, tape perpendicularity of the angle of the tape relative to the read/write head. In previous generations of tape drives, tape perpendicularity of the angle of the tape relative to the skew was constrained by flanged roller guides limittape read/write head. In previous generations of tapethat drives, skew was constrained by flanged roller guides that limit the read/write head. In previous generations of tape drives, tape lateral motion of the tape that causes skew. the skew was constrained by flanged rollertape guides thatWhen limit the lateral motion the tape that these causes skew. skew was constrained by flanged rollertape guides thatWhen limit the tape comes intoof high-frequency lateral motion of contact the tapewith that causesflanges, tape skew. When the tape comes intoof contact with high-frequency lateral motion thecan tape that these causes tape skew. the tape motion result that isflanges, difficult for When the tracktape comes into contact with these flanges, high-frequency lateral tapeactuator motion canfollow result that can isflanges, difficult for the tracktape comes into contact with and these high-frequency following to also cause tape-edge lateral tape motion can result that is difficult for the trackfollowing actuator to alsoflanges cause tape-edge lateral tape motion canfollow result and that can is difficult for the trackdamage. To alleviate been following actuator to these followproblems, and can the alsoflanges cause have tape-edge damage. To alleviate problems, the have been following actuator to these follow andofcan also causetape tape-edge removed from the guide rollers more recent drives. damage. To alleviate these problems, the flanges have been removed from the guide rollers of more recent tape drives. damage. To alleviate these problems, the flanges have been Unfortunately, removing the flanges fromrecent the guide rollers removed from the guide rollers of more tape drives. Unfortunately, removing the flanges from the guide rollers removed from the guide rollers of more recent tape drives. results in a significant in lateral tapeguide motion and Unfortunately, removingincrease the flanges from the rollers results in a significant in lateral tapeguide motion and Unfortunately, removingincrease the flanges from the rollers results in a significant increase in lateral tape motion and results in a significant increase in lateral tape motion and
hence in dynamic tape skew. This, in turn results in larger hence in dynamic tape skew. This, inread turn results inrelative larger and more frequent misalignment of the hence in dynamic tape skew. This, in turnelements results in larger and more frequent misalignment of the read elements relative hence in dynamic tape skew. This, in turn results in larger to written tracksmisalignment during read-while-write verification. To andthe more frequent of the read elements relative to the written tracks during read-while-write verification. To and more frequent misalignment of the read elements relative address this issue, flangeless tape drives use an actuator with to the written tracks during read-while-write verification. To address this issue, flangeless tape drives use an actuator with to the written tracks during read-while-write verification. To two degrees of freedom for joint skew- use and an track-following address this issue, flangeless tape drives actuator with two degrees ofskew freedom forcontroller joint skewand an track-following address this issue, flangeless tape drives use actuator with control. The servo ensures that the read two degrees of freedom for joint skew- and track-following control. The skew servo ensures that read two degrees freedom forcontroller joint skewand track-following elements are ofaligned with the freshly written tracksthe control. The skew servo controller ensures that theduring read elements are aligned with controller theand freshly written tracks control. The skew servo ensures that theduring read read-while-write verification hence is a key enabler for elements are aligned with the freshly written tracks during read-while-write verification hence issystems. a key enabler for elements are aligned with theand freshly written tracks during achieving the very high reliability of tape read-while-write verification and hence is a key enabler for achieving the veryverification high reliability tape issystems. read-while-write and of hence a key enabler for achieving the very high reliability of tape systems. achieving the very high reliability tape systems. 2. TAPE-TO-HEAD SKEW ANDof TRACK FOLLOWING 2. TAPE-TO-HEAD SKEW AND TRACK FOLLOWING 2. TAPE-TO-HEAD ANDmodules TRACK FOLLOWING Figure 1 illustrates SKEW the head a tape drive, 2. TAPE-TO-HEAD SKEW AND TRACK of FOLLOWING Figure 1 illustrates the head modules of read a tape including the servo readers and the data and drive, write Figure 1 the illustrates the headandmodules of read a tape drive, including servo readers the data and write Figure 1 illustrates the head modules of a tape elements, and skew readers θ between thedrive, tape including the aservo andthe thetape datahead readandand write elements, while and skew θ between andand the tape including the aservo readers andthe thetape datahead read write medium, the tape is being transported from a supply elements, and a skew θ between the tape head and the tape medium, while tape isa being transported supply elements, and a the skew θatbetween the tape headfrom and athe tape reel to a take-up reel nominal velocity. Servo bands, medium, while the tape is being transported from a supply reel to straddle a while take-up nominal velocity.from Servo bands, medium, thereel tapeatbands, isa being transported a supply which the data are prewritten on tape media reel to straddle a take-up a nominal velocity. on Servo bands, which the reel dataat are prewritten tape media reel to tape a take-up reel atbands, atonominal velocity. Servo bands, during manufacturing allow simultaneous reading of which straddle the data bands, are prewritten on tape media during tape manufacturing to allow simultaneous reading of which straddle the data bands, are prewritten on tape media servo signals from two adjacent servo bands. Each servo during tape manufacturing to allow simultaneous reading of servo signals from two adjacent servo bands. Each servo during tape manufacturing to allow simultaneous reading of band servo (TBS) et al. servo contains signals timing-based from two adjacent servoframes bands.(Barrett Each servo band contains timing-based servo (TBS) frames (Barrett et al. servo signals from two adjacent servo bands. Each servo (1998)), fromtiming-based which servoservo parameters, such (Barrett as the ettape band contains (TBS) frames al. (1998)), fromtiming-based which servoservo parameters, such (Barrett as the band contains frames ettape al. velocity, lateral head position, and(TBS) longitudinal position (1998)), from which servo parameters, such tape as the tape velocity, lateral head position, and servo longitudinal position (1998)), from which servo parameters, such tape asarethe tape (LPOS), are extracted. The TBS frames written velocity, lateral head position, and longitudinal tape position (LPOS), are extracted. Theangles TBS framesof are written velocity, head position, and servo longitudinal tape position with two lateral different azimuth and consist four servo (LPOS), are extracted. The TBS servo frames are written with two different azimuth angles andstripes consist of are fourwritten servo (LPOS), are extracted. The TBS servo frames bursts, i.e., two bursts with four servo followed by two with two different azimuth angles and consist of four servo bursts, i.e.,different two bursts with four servo stripes followed two with two azimuth angles and consist of consists fourbyservo bursts with five stripes. Therefore a servo frame of bursts, i.e., two bursts with four servo stripes followed by two bursts with fivebursts stripes. Therefore a [4 servo frame consists of bursts, i.e.,arranged two with four servo stripes followed by two 18 stripes in a sequence of 4 5 5] bursts. During bursts with five stripes. Therefore a servo frame consists of 18 stripes arranged in a sequence of [4 4 5 5] bursts. During bursts with five stripes. Therefore a servo frame consists of tape-drive operation, servo reader the magnetic 18 stripes arranged in aa sequence of [4 4reads 5 5] bursts. During tape-drive operation, a servo reader reads the magnetic 18 stripes arranged in a sequence of [4 4 5 5] bursts. During transitions associated awith the reader servo stripes, resulting in tape-drive operation, servo reads the magnetic transitions associated the reader servo stripes, resulting in tape-drive operation, awith servo reads the magnetic pulses usually called dibits (Furrer et al. (2012)). The tape transitions associated with the servo stripes, resulting in pulses usually called dibits (Furrer etare al. (2012)). The tape transitions associated with the servo stripes, resulting in velocity and the lateral head position estimated from the pulses usually called dibits (Furrer et al. (2012)). The tape velocity and thecalled lateral head position from the pulses usually dibits (Furrer etare al. estimated (2012)). The tape relative arrival time of dibits associated with servo stripes. velocity and the lateral head position are estimated from the relative arrival time of head dibits with stripes. velocity and the lateral are estimated from the TBS patterns also allow bit position ofassociated additional LPOSservo information relative arrival time of 1dibits associated with servo stripes. TBS patterns also allow bit ofassociated additional with LPOSservo information relative arrival time of 1dibits stripes. TBS patterns also allow 1 bit of additional LPOS information 7 TBS patterns also allow 1 bit of additional LPOS information
Copyright © 2016 IFAC 2405-8963 © IFAC (International Federation of Automatic Control) Copyright © 2016, 2016 IFAC 7 Hosting by Elsevier Ltd. All rights reserved. Peer review©under of International Federation of Automatic Copyright 2016 responsibility IFAC 7 Control. Copyright © 2016 IFAC 7 10.1016/j.ifacol.2016.10.503
2016 IFAC MECHATRONICS 8 GiovanniUK Cherubini et al. / IFAC-PapersOnLine 49-21 (2016) 007–012 September 5-8, 2016. Loughborough University,
⎛ v (τˆ2 − τˆ1 ) ⎞ Δx , ⎟≈ b ⎝ ⎠ b
to be encoded per frame using pulse-position modulation (PPM) on selected stripes, without affecting the generation of the transversal position estimates. In LTO-7 drives, the latest drive of the LTO* family, LPOS information is comprised within an LPOS word consisting of 36 servo frames. Each 36-bit LPOS word starts with a known 8-bit synchronization word, followed by 24 information bits. Sync words are periodically embedded into the LPOS data stream such that a sync word is always repeated after a distance of 36 servo frames. Detection of the servo patterns and extraction of the servo information parameters are achieved by a synchronous servo channel using a matched-filter interpolator/correlator, which turns out to be optimal in the presence of additive white Gaussian noise, and considerably increases both system robustness and measurement accuracy in the presence of media noise and other disturbances (Cherubini et al. (2015)).
θˆ = arctan⎜
where v is the tape velocity in the longitudinal direction, the function arctan(x) is approximated by its argument, assuming small values of x, and Δx = v (τˆ2 − τˆ1 ) corresponds to the distance travelled by the tape in the time interval. Position ref.
PES controller
Skew ref.
ch1
Two-degree of freedom r2 (t ) actuator
u SES (t )
-
τˆ1 (t n )
r1 (t )
u PES (t )
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θˆl
-
ˆy k
ˆy1 (t n )
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ˆy2 (t n )
τˆ1 (t n )
Skew estimator
τˆ 2 (t n )
Position estimator
ˆy2 (t n )
ˆy1 (t n )
Fig. 2. Skew- (SES = skew error signal) and track-following (PES = position error signal) control system. 3. HIGH-RATE SKEW ESTIMATION In the method described in Cherubini et al. (2015), there is an inherent ambiguity equivalent to the length 2d of a servo frame, e.g., 200 μm for the TBS servo format adopted for LTO-1 to 6 tape drives. 4
Channel 1
Correlation peak
Correlator output 1
3 2 1
Fig. 1. Illustration of head modules and tape-to-head skew.
0 -1
The block diagram of a skew- and track-following control system is shown in Fig. 2. Dual synchronous servo channels ch1 and ch2 receive servo signals r1(t) and r2(t), respectively, from the two servo readers reading servo patterns on adjacent servo bands. They provide estimates yˆ1 and yˆ 2 of the lateral head position, and estimates τˆ1 and τˆ2 of the times at which
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time [ms]
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peaks of the correlator output signals are observed at the servo channels, indicating the arrival times of the dibits in the servo bursts with high precision, as illustrated in Fig. 3. By defining b as the distance between servo readers in the same head module and assuming that the shift between servo frames in adjacent servo bands is zero, an estimate of the skew-error signal (SES) is given by
4
Channel 2
3
Correlator output 2
2 1 0 -1
*
Linear Tape-Open and LTO are trademarks of HP, IBM, and Quantum in the U.S. and other countries.
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IBM is a trademark of International Business Machines Corporation, registered in many jurisdictions worldwide.
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time [ms]
Fig. 3. Skew estimation using correlation signal peaks. 8
2016 IFAC MECHATRONICS September 5-8, 2016. Loughborough University, GiovanniUK Cherubini et al. / IFAC-PapersOnLine 49-21 (2016) 007–012
9
Start Compute skewInt using LPOS sync-flags
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Cnt2→7 yes
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frameFlag=1 intFlag=0 skewEst_1=skewEst Compute skewEst frameFlag=0 intFlag=0
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frameFlag=0 intFlag=1 no
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skewEst_1<-a & skewEst>a yes
skewEst_1>a & skewEst<-a
skewInt=skewInt-2d
yes
skewInt=skewInt+2d
Fig. 4. Flow chart of the skew estimation method. represent an issue. However, in future tape drive generations, reliable read-while-write operation in the presence of large offsets in the alignment of the head modules and trackfollowing disturbances due to compressional tape vibration modes will dictate implementations of the skew-following loop for which the reference skew for skew-following control may be in a wide range of values, well beyond the assumed limits of ±d. Active compensation for changes in tape width due to changes in the environmental conditions by skewing the head to change its effective span would also require the ability to skew-servo on arbitrary reference values. Furthermore, the method described in Cherubini et al. (2015) generates only one skew estimate per servo frame. The rate at which estimates are generated in the TBS technique increases linearly with tape velocity. Higher skew estimate generation rates are desirable to reduce estimation delays in the skew-
This ambiguity arises because the skew estimate is obtained as the difference of the arrival times of correlation peaks that correspond to dibits occupying the same position in the [4 4 5 5] sequences of dibits in the two adjacent servo bands, without distinguishing whether they belong to two servo frames encoding the same LPOS symbol, characterized by zero shift, or to different LPOS symbols, characterized by a shift equal to an integer multiple of 2d, as illustrated in Fig. 3. This leads to the drawback that a reliable skew estimate is limited to the interval [–d, d] and that controlling the skew to values of approx. ±d μm + K × 2d μm, where K is an integer, poses a serious challenge because the skew measured during skew following would toggle between values around +d and –d. If perfect alignment between servo readers and servo frames is required, i.e., if the head is to be kept perpendicular to the tape, the skew is controlled to around zero, and the ambiguity mentioned above does not 9
2016 IFAC MECHATRONICS 10 GiovanniUK Cherubini et al. / IFAC-PapersOnLine 49-21 (2016) 007–012 September 5-8, 2016. Loughborough University,
following control loop and hence to enable improved controller performance, especially at low tape velocities.
shows the estimation of the skew process, highlighting the fractional part of the estimate. Figure 5(b) illustrates the evolution of the relevant variables in the time interval around the crossing of the boundary corresponding to a skew equal to d. The variable estFlag, not shown in the flow chart of Fig. 4, indicates the instants at which skew estimates are computed. The method ensures that the skew is correctly estimated over a wide range of skew values, thus enabling skew-following control for any value of the skew reference, to compensate large offsets in the position of the head modules, to enable active tape dimensional stability compensation, and to mitigate disturbances in the trackfollowing loop arising from compressional tape vibration modes. The method can also be readily applied for skew compensation in legacy LTO drives, where the reference in the skew-following loop needs to take into account the presence of the shift in the servo pattern for servo band identification.
Our solution for high-rate skew estimation without ambiguity can be described with reference to the flow chart shown in Fig. 4. Let us consider the i-th skew estimate si, expressed as the sum of an integer and a fractional part, i.e.,
s = n × 2d + μ , i
i
i
where μi denotes the fractional part in the interval [–d, d], and ni denotes the number of integer servo-frame lengths. The first step in the recursive method illustrated in Fig. 4 consists of estimating the initial value of ni (variable skewInt) using the LPOS sync-word detection flags provided by the two servo channels. Upon detecting the 8-bit sync word that is part of every 36-bit LPOS word, each servo channel outputs an LPOS sync-word detection flag. The time difference between a pair of sync-word detection flags corresponding to the same 36-bit LPOS word output by the synchronous servo channels provides the information about the initial value of the tape-to-head skew, from which the initial value of ni is estimated.
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Then, at each clock interval kT, where T denotes the period of the clock provided to the servo channels, the system monitors the values achieved by the counters modulo-18 in servo channels 1 and 2, which advance by one whenever a new correlation peak is detected in a [4 4 5 5] sequence of dibits within a servo frame (variables Cnt1 and Cnt2, respectively). Selecting channel 2 as a reference, a new observation frame is started (variable frameFlag set to 1) whenever the value of Cnt2 increases from 6 to 7, indicating that the third dibit in the second burst of the [4 4 5 5] sequence of dibits has been detected on channel 2. During an observation frame, monitoring of the variables Cnt1 and Cnt2 continues to detect the increase of at least one of the two counter values from 15 to 16, indicating that the third dibit in the fourth burst of the [4 4 5 5] sequence of dibits has been detected. Detection of the first such transition of a counter value from 15 to 16 within a frame triggers the start of a measurement interval (variable intFlag is set to 1) and resets the observation frame flag (variable frameFlag is set to 0). The transition of the second counter value from 15 to 16 within the frame leads to the reset of both the observation frame and the measurement flags (both variables intFlag and frameFlag are set to 0), and to the computation of a new fractional skew estimate (variable skewEst). As special cases, if in the same clock interval Cnt2 increases from 6 to 7 and Cnt1 increases from 15 to 16, or both Cnt1 and Cnt2 increase from 15 to 16, a new fractional skew estimate (variable skewEst) is computed immediately without starting a new observation frame or measurement interval. If a variation Δ between the fractional parts of two consecutive skew estimates (skewEst_1 and skewEst) that is larger in absolute value than a threshold 2a is observed, with 2a ~ d/2, the integer part of the last skew estimate is augmented by a term equal to –2d × sign(Δ).
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(a)
(b) Fig. 5. Illustration of (a) skew estimation and (b) relevant variables. With this approach, up to four skew estimates can be generated per servo frame, considering one reference dibit per servo burst. This is achieved by interleaving the generation of the skew estimates using four reference counter values, i.e., one per servo burst, to determine the start of an observation frame, and selecting appropriate shifts of the values of the counters modulo-18 to set the observation and
Simulation results, which illustrate the new method applied to a drive reading a tape with an LTO-7 servo format, for a tape velocity of 1.8 m/s, a servo channel SNR of 24.7 dB, and open-loop skew estimation, are shown in Fig. 5. Figure 5(a) 10
2016 IFAC MECHATRONICS September 5-8, 2016. Loughborough University, GiovanniUK Cherubini et al. / IFAC-PapersOnLine 49-21 (2016) 007–012
the measurement flags. Therefore the method allows an increase of up to a factor of four in the rate of generation of skew estimates with respect to the state-of-the-art approach. Furthermore, averaging the arrival times of multiple dibits in a burst to reduce estimation noise may also be considered.
tracks the skew value through and beyond the ±d boundary. Although a sine wave was applied as skew actuator signal, the estimated skew does not exactly resemble a sine wave, but shows a compression with ringing at maximum amplitude. The latter is due to a mechanical limit in the achievable skew range, which is specific to the tape drive used. Figure 7 also shows a short period of invalid skew estimates (indicated by a zero skew value) at around 3.54 s, which is caused by a servo pattern defect in one of the servo bands that forces a re-acquisition in one of the servo channels.
4. EXPERIMENTAL RESULTS A hardware implementation of the high-rate skew estimation algorithm described in Section 3 was designed in VHDL. The implementation provides two skew estimates per servo frame, and interfaces to two high-performance servo channels described in Cherubini et al. (2015). These hardware units are connected to a NIOS II soft-core microprocessor to form a programmable System-on-Chip (SoC), implemented on a field programmable gate array (FPGA)-based prototyping platform, as shown in Fig. 6. In combination with an IBM TS1150 tape drive and a current driver, this experimental setup enables open-loop and closed-loop track-following and skew-following servo control experiments.
11
skew (legacy) skew (high−rate)
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Fig. 7. Skew estimated by the legacy skew estimator unit and the new high-rate skew estimator with increased dynamic range, measured in fractions of the servo frame length.
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Fig. 6. Experimental setup with IBM TS1150 tape drive and FPGA hardware prototyping platform.
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To test the functionality of the new high-skew-rate servo channel, we performed an experiment using a particulate tape medium sample formatted with an LTO-7 servo pattern in an IBM TS1150 tape drive. The drive was operated in closedloop track-following mode at a tape speed of 2.7 m/s, and a 10 Hz sinusoidal waveform was applied to the skew actuator in (skew-servo) open-loop mode. Figure 7 shows the output of both the legacy skew estimator detailed in Cherubini et al. (2015) and the new high-rate skew estimator discussed in this paper, both implemented in the FPGA platform. The estimated skew is plotted in fractions of the servo frame length 2d. As shown in Fig. 7, the positive skew exceeds 1/2 of the servo frame length 2d, which causes a wrap around (ambiguity) in the legacy skew estimate at approximately 3.49 s. In contrast, the new high-rate skew estimator properly
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0
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3 time (s)
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Fig. 8. Output of the high-rate skew estimator measured in micrometers. In a second experiment, we replaced the tape drive in Fig. 6 with a low-disturbance tape path with an IBM TS1140 electronics card, also used for the 85.9 Gb/in2 and 123 Gb/in2 recording areal density demonstrations described in Furrer et al. (2015a) and Lantz et al. (2015), and a particulate tape medium sample with an experimental TBS servo pattern with a 24-degree azimuth angle and a servo frame length of 2d = 11
2016 IFAC MECHATRONICS 12 GiovanniUK Cherubini et al. / IFAC-PapersOnLine 49-21 (2016) 007–012 September 5-8, 2016. Loughborough University,
Furrer, S., et al. (2015a), 85.9 Gb/in2 recording areal density on barium ferrite tape. IEEE Trans. Magn. 51(4), article 3100207.
96 µm, described in Furrer et al. (2015b). Figure 8 depicts the skew (in micrometers) estimated by the new high-rate skew estimator during a closed-loop track-following experiment at 2.1 m/s tape speed, in which the head skew was manually decreased by means of a micrometer stage. Towards the end of the capture, the skew exceeds two servo frame lengths, i.e. si > 2 × 2d, which corresponds to a skew estimate si whose integer part is equal to ni = –2.
Lantz, M. A., et al. (2015), 123 Gbit/in2 recording areal density on barium ferrite tape. IEEE Trans. Magn. 51(11), article 3101304. Furrer, S., Pantazi, A., Cherubini, G., and Lantz, M. A. (2015b), Resolution limits of timing-based servo schemes in magnetic tape drives. IEEE Trans. Magn. 51(11), article 3101504.
In future work, we plan to implement a skew-following controller on the microprocessor shown in Fig. 6 to close the skew servo loop. This will enable the closed-loop skewfollowing servo to i) exploit the increased update rate in the skew estimation and ii) perform skew following with a large nonzero skew reference. 5. CONCLUSIONS We demonstrated a novel skew estimator that generates skew estimates with a high rate over an extended range of allowable tape-to-head skew values. The increased rate of estimation will reduce delay in the skew servo control loop and is expected to lead to improved skew-following performance, particularly at low tape speeds. In addition, the extended estimation range will enable new functionality in future tape drives, such as active tape-dimensional stability compensation via head skewing to control the effective span of the array of read/write transducers in the head.
ACKNOWLEDGMENT The authors would like to thank A. Pantazi for her help with the experimental results, and E. Eleftheriou for his support of this work.
REFERENCES Barrett, R. C., Klaassen, E. H., Albrecht, T. R., Jaquette, G. A., and Eaton, J. H. (1998). Timing-based trackfollowing servo for linear tape systems. IEEE Trans. Magn. 34(4), pp. 1872-1877. Furrer, S., Jubert, P.-O., Cherubini, G., Cideciyan, R. D., and Lantz, M. A. (2012), Analytical expressions for the readback signal of timing-based servo schemes. IEEE Trans. Magn. 48(11), pp. 4578-4581. Cherubini, G., Eleftheriou, E., Jelitto, J., and Hutchins, R. (2007). Synchronous servo channel design for tape drive systems. In: Proc. 17th Annual ASME Information Storage and Processing Systems Conf. ISPS 2007, Santa Clara, CA, pp. 160-162. Cherubini, G., Furrer, S., and Jelitto, J. (2015). Highperformance servo channel for nanometer head positioning and longitudinal position symbol detection in tape systems. IEEE/ASME Trans. Mechatronics. DOI: 10.1109/TMECH.2015.2462718.
12
ScienceDirect IFAC-PapersOnLine 49-21 (2016) 007–012 High-Rate Skew Estimation for Tape Systems High-Rate Skew Estimation for Tape Systems High-Rate Skew Estimation High-Rate Skew Estimation for for Tape Tape Systems Systems
Giovanni Cherubini, Simeon Furrer, and Mark A. Lantz Giovanni Cherubini, Simeon Furrer, and Mark A. Lantz Giovanni Cherubini, Simeon Furrer, and Mark A. Lantz Giovanni Cherubini, Simeon Furrer, and Mark A. Lantz IBM Research – Zurich, 8803 Rüschlikon, Switzerland IBM Research – Zurich, 8803 Rüschlikon, Switzerland (e-mail:–{cbi,sfu,mla}@zurich.ibm.com) IBM Research Zurich, 8803 Rüschlikon, Switzerland (e-mail:–{cbi,sfu,mla}@zurich.ibm.com) IBM Research Zurich, 8803 Rüschlikon, Switzerland (e-mail: {cbi,sfu,mla}@zurich.ibm.com) (e-mail: {cbi,sfu,mla}@zurich.ibm.com) Abstract: In several recent magnetic tape drive designs, an actuator with two degrees of freedom has Abstract: In several recent magnetic tape drive designs,This an actuator with two degrees of freedom has been adopted jointrecent skew- magnetic and track-following enables the of flanges from guide Abstract: In for several tape drive control. designs, an actuator withremoval two degrees of freedom has been adopted for joint skewand track-following control. This enables the removal of flanges from guide Abstract: In tape several recent magnetic tape drive designs, an actuator with two degrees of freedom has rollers in the path, thus avoiding debris build-up, tape-edge damage, and high-frequency lateral tape been adopted for joint skew- and track-following control. This enables the removal of flanges from guide rollersadopted in the tape path,skewthus avoiding debris build-up, tape-edge damage, and high-frequency lateral tape been for joint and track-following This enables the removal of flanges from guide motion. compensation is needed to keep thecontrol. head perpendicular to and the direction of tapelateral motion to rollers inSkew the tape path, thus avoiding debris build-up, tape-edge damage, high-frequency tape motion.in Skew compensation is neededin tothe keep the head perpendicular to excursions the direction of tapelateral motion to rollers the tape path, thus avoiding debris build-up, tape-edge damage, and high-frequency tape enable read-while-write functionality presence of large lateral tape originating from the motion. Skew compensation is needed to keep the head perpendicular to the direction of tape motion to enable read-while-write functionality into the presence of large lateral tape originating from the motion. Skew compensation is needed keep the head perpendicular to excursions the direction of skew tape motion to removal of roller flanges. In this paper, we present a novel skew estimator that generates estimates enable read-while-write functionality in the presence of large lateral tape excursions originating from the removal of roller flanges. In this paper, we skew estimator that generates skew estimates enable read-while-write functionality the present presencea novel of large lateral tape excursions originating from the at a high rate for an extended range ofinallowable skew values. A skew estimate given by removal of roller flanges. In this paper, we presenttape-to-head a novel skew estimator that generates skewis estimates at a high rate for an extended range of allowable tape-to-head skew values. A skew estimate is given by removal of roller flanges. In this paper, we present a novel skew estimator that generates skew estimates the sum of a fractional and an integer part, extracted from information provided by two parallel at a high rate for an extended range of allowable tape-to-head skew values. A skew estimate is given by the sum of a fractional and an integer part, extracted from information provided by two parallel at a high rate for an extended range of allowable tape-to-head skew values. A skew estimate is given by synchronous channels on part, timing-based The fractional parttwo obtained the sum of aservo fractional andoperating an integer extractedservo frompatterns. information provided by parallel synchronous servo channels operating on timing-based servo patterns. The fractional part is obtained the sum of aservo fractional and an integer extracted from information provided by two parallel from the accurate measure of operating timing intervals between dibit correlation peaks, whereas the integer part is synchronous channels on part, timing-based servo patterns. The fractional part is obtained from thefrom accurate measure of timing information. intervals between dibit correlation peaks, whereas the integer part is synchronous servo channels operating on timing-based servo patterns. The fractional part is obtained derived longitudinal position The new method was implemented in an FPGA and from the accurate measure of timing intervals between dibit correlation peaks, whereas the integer part is derived from longitudinal position information. The new waspeaks, implemented in an FPGA and from theexperimentally accurate measure of timing intervals between dibit method correlation whereas the integer part is verified using a commercial tape drive. derived from longitudinal position information. The new method was implemented in an FPGA and verified experimentally using a commercial tape drive. derived from longitudinal position information. The new method was implemented in an FPGA and verified experimentally usingFederation a commercial tape drive. © 2016, IFAC (International of Automatic Control) Hosting by Elsevier Ltd. All rights reserved. Keywords: Skew estimation, control, tape systems. verified experimentally usingskew-following a commercial tape drive. Keywords: Skew estimation, skew-following control, tape systems. Keywords: Skew estimation, skew-following control, tape systems. Keywords: Skew estimation, skew-following control, tape systems. 1. INTRODUCTION 1. INTRODUCTION 1. INTRODUCTION In modern magnetic tape drives, the system reliability is 1. INTRODUCTION In modern magnetic tape drives, the system reliability is enhanced performing a read operation simultaneously with In modernbymagnetic tape drives, the system reliability is enhanced bymagnetic performing a read operation simultaneously with In modern tape drives, the system reliability is write operations to verify that the data has been correctly enhanced by performing a read operation simultaneously with write operations to verify that the datausing has been correctly enhanced by performing a read operation simultaneously with written. This verification is performed a second head write operations to verify isthat the datausing has been correctly written. Thisread verification performed a second head write operations toelements, verify that the isdata has been correctly module positioned adjacent to written. with This verification is which performed using a second head module with read elements, which is positioned adjacent to written. This verification is performed using a second head the head module writing the data. This second head module is module with read elements, which is positioned adjacent to the head module writing the data. This second head module is module with read elements, which is positioned adjacent to typically physically attached to the first head module, with the head module writing the data. This second head module is typically physically attached to the first head module, with the head module writing the data. This second head module is read physically elements ofattached the second aligned withwith the typically to themodule first head module, the read elements ofattached the module. second module aligned with the typically physically to theData firstishead module, with write elements of the first written as the tape the read elements of the second module aligned with the write elements the Data is then written the tape the read elements offirst the module. second module aligned with the is streamed overof module and readas and write elements ofthe thefirst firsthead module. Data is written asback the tape is streamed over the module and read and write elements ofwritten thefirst firsthead module. Data is then written asback the head tape verified as the data passes over the second is streamed over the first head module and then read back and verified as over the data passes the read second is streamed the first head and back and module. too written many raw module errors over are then detected in head this verified asIf the written data passes over the second head module. If too many raw errors are detected in head this verified as the written data can passes over the second verification step, the data be rewritten immediately module. If too many raw errors are detected in this verification step, the data can be rewritten immediately module. If too many raw errors are detected in without stopping processthis to verification step, the the tape. data For can this be verification rewritten immediately without stopping the tape. For this verification process to verification step, the data can be rewritten immediately function properly, the tape motion must be essentially without stopping thethetape. Formotion this verification process to function properly, tape must be essentially without stopping the tape. For this verification process to perpendicular to the the axis tape of themotion arrays ofmust writers readers function properly, be and essentially perpendicular to the such axis tape of the arrays ofmust writers and readers function properly, the motion beremain essentially in the two modules, that the read elements in the perpendicular to the axis of the arrays of writers and readers in the two modules, that thearrays read elements in the perpendicular the such axis of the of writersremain and readers shadows thetowrite elements. in the twoofmodules, such that the read elements remain in the shadows the writesuch elements. in the twoofmodules, that the read elements remain in the shadows of the write elements. shadows of the is writea elements. Tape skew measure of the deviation from Tape skew is ofa themeasure the perpendicularity angle ofof tapedeviation relative tofrom the Tape skew is a measure ofthethe deviation from perpendicularity of the angle of the tape relative tofrom the Tape skew is a measure of the deviation read/write head. In previous generations of tape drives, tape perpendicularity of the angle of the tape relative to the read/write head. In previous generations of tape drives, tape perpendicularity of the angle of the tape relative to the skew was constrained by flanged roller guides limittape read/write head. In previous generations of tapethat drives, skew was constrained by flanged roller guides that limit the read/write head. In previous generations of tape drives, tape lateral motion of the tape that causes skew. the skew was constrained by flanged rollertape guides thatWhen limit the lateral motion the tape that these causes skew. skew was constrained by flanged rollertape guides thatWhen limit the tape comes intoof high-frequency lateral motion of contact the tapewith that causesflanges, tape skew. When the tape comes intoof contact with high-frequency lateral motion thecan tape that these causes tape skew. the tape motion result that isflanges, difficult for When the tracktape comes into contact with these flanges, high-frequency lateral tapeactuator motion canfollow result that can isflanges, difficult for the tracktape comes into contact with and these high-frequency following to also cause tape-edge lateral tape motion can result that is difficult for the trackfollowing actuator to alsoflanges cause tape-edge lateral tape motion canfollow result and that can is difficult for the trackdamage. To alleviate been following actuator to these followproblems, and can the alsoflanges cause have tape-edge damage. To alleviate problems, the have been following actuator to these follow andofcan also causetape tape-edge removed from the guide rollers more recent drives. damage. To alleviate these problems, the flanges have been removed from the guide rollers of more recent tape drives. damage. To alleviate these problems, the flanges have been Unfortunately, removing the flanges fromrecent the guide rollers removed from the guide rollers of more tape drives. Unfortunately, removing the flanges from the guide rollers removed from the guide rollers of more recent tape drives. results in a significant in lateral tapeguide motion and Unfortunately, removingincrease the flanges from the rollers results in a significant in lateral tapeguide motion and Unfortunately, removingincrease the flanges from the rollers results in a significant increase in lateral tape motion and results in a significant increase in lateral tape motion and
hence in dynamic tape skew. This, in turn results in larger hence in dynamic tape skew. This, inread turn results inrelative larger and more frequent misalignment of the hence in dynamic tape skew. This, in turnelements results in larger and more frequent misalignment of the read elements relative hence in dynamic tape skew. This, in turn results in larger to written tracksmisalignment during read-while-write verification. To andthe more frequent of the read elements relative to the written tracks during read-while-write verification. To and more frequent misalignment of the read elements relative address this issue, flangeless tape drives use an actuator with to the written tracks during read-while-write verification. To address this issue, flangeless tape drives use an actuator with to the written tracks during read-while-write verification. To two degrees of freedom for joint skew- use and an track-following address this issue, flangeless tape drives actuator with two degrees ofskew freedom forcontroller joint skewand an track-following address this issue, flangeless tape drives use actuator with control. The servo ensures that the read two degrees of freedom for joint skew- and track-following control. The skew servo ensures that read two degrees freedom forcontroller joint skewand track-following elements are ofaligned with the freshly written tracksthe control. The skew servo controller ensures that theduring read elements are aligned with controller theand freshly written tracks control. The skew servo ensures that theduring read read-while-write verification hence is a key enabler for elements are aligned with the freshly written tracks during read-while-write verification hence issystems. a key enabler for elements are aligned with theand freshly written tracks during achieving the very high reliability of tape read-while-write verification and hence is a key enabler for achieving the veryverification high reliability tape issystems. read-while-write and of hence a key enabler for achieving the very high reliability of tape systems. achieving the very high reliability tape systems. 2. TAPE-TO-HEAD SKEW ANDof TRACK FOLLOWING 2. TAPE-TO-HEAD SKEW AND TRACK FOLLOWING 2. TAPE-TO-HEAD ANDmodules TRACK FOLLOWING Figure 1 illustrates SKEW the head a tape drive, 2. TAPE-TO-HEAD SKEW AND TRACK of FOLLOWING Figure 1 illustrates the head modules of read a tape including the servo readers and the data and drive, write Figure 1 the illustrates the headandmodules of read a tape drive, including servo readers the data and write Figure 1 illustrates the head modules of a tape elements, and skew readers θ between thedrive, tape including the aservo andthe thetape datahead readandand write elements, while and skew θ between andand the tape including the aservo readers andthe thetape datahead read write medium, the tape is being transported from a supply elements, and a skew θ between the tape head and the tape medium, while tape isa being transported supply elements, and a the skew θatbetween the tape headfrom and athe tape reel to a take-up reel nominal velocity. Servo bands, medium, while the tape is being transported from a supply reel to straddle a while take-up nominal velocity.from Servo bands, medium, thereel tapeatbands, isa being transported a supply which the data are prewritten on tape media reel to straddle a take-up a nominal velocity. on Servo bands, which the reel dataat are prewritten tape media reel to tape a take-up reel atbands, atonominal velocity. Servo bands, during manufacturing allow simultaneous reading of which straddle the data bands, are prewritten on tape media during tape manufacturing to allow simultaneous reading of which straddle the data bands, are prewritten on tape media servo signals from two adjacent servo bands. Each servo during tape manufacturing to allow simultaneous reading of servo signals from two adjacent servo bands. Each servo during tape manufacturing to allow simultaneous reading of band servo (TBS) et al. servo contains signals timing-based from two adjacent servoframes bands.(Barrett Each servo band contains timing-based servo (TBS) frames (Barrett et al. servo signals from two adjacent servo bands. Each servo (1998)), fromtiming-based which servoservo parameters, such (Barrett as the ettape band contains (TBS) frames al. (1998)), fromtiming-based which servoservo parameters, such (Barrett as the band contains frames ettape al. velocity, lateral head position, and(TBS) longitudinal position (1998)), from which servo parameters, such tape as the tape velocity, lateral head position, and servo longitudinal position (1998)), from which servo parameters, such tape asarethe tape (LPOS), are extracted. The TBS frames written velocity, lateral head position, and longitudinal tape position (LPOS), are extracted. Theangles TBS framesof are written velocity, head position, and servo longitudinal tape position with two lateral different azimuth and consist four servo (LPOS), are extracted. The TBS servo frames are written with two different azimuth angles andstripes consist of are fourwritten servo (LPOS), are extracted. The TBS servo frames bursts, i.e., two bursts with four servo followed by two with two different azimuth angles and consist of four servo bursts, i.e.,different two bursts with four servo stripes followed two with two azimuth angles and consist of consists fourbyservo bursts with five stripes. Therefore a servo frame of bursts, i.e., two bursts with four servo stripes followed by two bursts with fivebursts stripes. Therefore a [4 servo frame consists of bursts, i.e.,arranged two with four servo stripes followed by two 18 stripes in a sequence of 4 5 5] bursts. During bursts with five stripes. Therefore a servo frame consists of 18 stripes arranged in a sequence of [4 4 5 5] bursts. During bursts with five stripes. Therefore a servo frame consists of tape-drive operation, servo reader the magnetic 18 stripes arranged in aa sequence of [4 4reads 5 5] bursts. During tape-drive operation, a servo reader reads the magnetic 18 stripes arranged in a sequence of [4 4 5 5] bursts. During transitions associated awith the reader servo stripes, resulting in tape-drive operation, servo reads the magnetic transitions associated the reader servo stripes, resulting in tape-drive operation, awith servo reads the magnetic pulses usually called dibits (Furrer et al. (2012)). The tape transitions associated with the servo stripes, resulting in pulses usually called dibits (Furrer etare al. (2012)). The tape transitions associated with the servo stripes, resulting in velocity and the lateral head position estimated from the pulses usually called dibits (Furrer et al. (2012)). The tape velocity and thecalled lateral head position from the pulses usually dibits (Furrer etare al. estimated (2012)). The tape relative arrival time of dibits associated with servo stripes. velocity and the lateral head position are estimated from the relative arrival time of head dibits with stripes. velocity and the lateral are estimated from the TBS patterns also allow bit position ofassociated additional LPOSservo information relative arrival time of 1dibits associated with servo stripes. TBS patterns also allow bit ofassociated additional with LPOSservo information relative arrival time of 1dibits stripes. TBS patterns also allow 1 bit of additional LPOS information 7 TBS patterns also allow 1 bit of additional LPOS information
Copyright © 2016 IFAC 2405-8963 © IFAC (International Federation of Automatic Control) Copyright © 2016, 2016 IFAC 7 Hosting by Elsevier Ltd. All rights reserved. Peer review©under of International Federation of Automatic Copyright 2016 responsibility IFAC 7 Control. Copyright © 2016 IFAC 7 10.1016/j.ifacol.2016.10.503
2016 IFAC MECHATRONICS 8 GiovanniUK Cherubini et al. / IFAC-PapersOnLine 49-21 (2016) 007–012 September 5-8, 2016. Loughborough University,
⎛ v (τˆ2 − τˆ1 ) ⎞ Δx , ⎟≈ b ⎝ ⎠ b
to be encoded per frame using pulse-position modulation (PPM) on selected stripes, without affecting the generation of the transversal position estimates. In LTO-7 drives, the latest drive of the LTO* family, LPOS information is comprised within an LPOS word consisting of 36 servo frames. Each 36-bit LPOS word starts with a known 8-bit synchronization word, followed by 24 information bits. Sync words are periodically embedded into the LPOS data stream such that a sync word is always repeated after a distance of 36 servo frames. Detection of the servo patterns and extraction of the servo information parameters are achieved by a synchronous servo channel using a matched-filter interpolator/correlator, which turns out to be optimal in the presence of additive white Gaussian noise, and considerably increases both system robustness and measurement accuracy in the presence of media noise and other disturbances (Cherubini et al. (2015)).
θˆ = arctan⎜
where v is the tape velocity in the longitudinal direction, the function arctan(x) is approximated by its argument, assuming small values of x, and Δx = v (τˆ2 − τˆ1 ) corresponds to the distance travelled by the tape in the time interval. Position ref.
PES controller
Skew ref.
ch1
Two-degree of freedom r2 (t ) actuator
u SES (t )
-
τˆ1 (t n )
r1 (t )
u PES (t )
SES controller
θˆl
-
ˆy k
ˆy1 (t n )
τˆ 2 (t n )
ch2
ˆy2 (t n )
τˆ1 (t n )
Skew estimator
τˆ 2 (t n )
Position estimator
ˆy2 (t n )
ˆy1 (t n )
Fig. 2. Skew- (SES = skew error signal) and track-following (PES = position error signal) control system. 3. HIGH-RATE SKEW ESTIMATION In the method described in Cherubini et al. (2015), there is an inherent ambiguity equivalent to the length 2d of a servo frame, e.g., 200 μm for the TBS servo format adopted for LTO-1 to 6 tape drives. 4
Channel 1
Correlation peak
Correlator output 1
3 2 1
Fig. 1. Illustration of head modules and tape-to-head skew.
0 -1
The block diagram of a skew- and track-following control system is shown in Fig. 2. Dual synchronous servo channels ch1 and ch2 receive servo signals r1(t) and r2(t), respectively, from the two servo readers reading servo patterns on adjacent servo bands. They provide estimates yˆ1 and yˆ 2 of the lateral head position, and estimates τˆ1 and τˆ2 of the times at which
-2 0.5
0.51
0.52
0.53
time [ms]
0.54
0.55
0.54
0.55
τ 2 − τ1
peaks of the correlator output signals are observed at the servo channels, indicating the arrival times of the dibits in the servo bursts with high precision, as illustrated in Fig. 3. By defining b as the distance between servo readers in the same head module and assuming that the shift between servo frames in adjacent servo bands is zero, an estimate of the skew-error signal (SES) is given by
4
Channel 2
3
Correlator output 2
2 1 0 -1
*
Linear Tape-Open and LTO are trademarks of HP, IBM, and Quantum in the U.S. and other countries.
-2 0.5
IBM is a trademark of International Business Machines Corporation, registered in many jurisdictions worldwide.
0.51
0.52
0.53
time [ms]
Fig. 3. Skew estimation using correlation signal peaks. 8
2016 IFAC MECHATRONICS September 5-8, 2016. Loughborough University, GiovanniUK Cherubini et al. / IFAC-PapersOnLine 49-21 (2016) 007–012
9
Start Compute skewInt using LPOS sync-flags
Advance clock: k=k+1 no
Cnt2→7 yes
Cnt2→16 & Cnt1→16
no
yes
Cnt2→16 | Cnt1→16
yes
Cnt1→16
no
yes
no yes
frameFlag=1 intFlag=0 skewEst_1=skewEst Compute skewEst frameFlag=0 intFlag=0
intFlag=1 no
frameFlag=1
no
yes
frameFlag=0 intFlag=1 no
no
skewEst_1<-a & skewEst>a yes
skewEst_1>a & skewEst<-a
skewInt=skewInt-2d
yes
skewInt=skewInt+2d
Fig. 4. Flow chart of the skew estimation method. represent an issue. However, in future tape drive generations, reliable read-while-write operation in the presence of large offsets in the alignment of the head modules and trackfollowing disturbances due to compressional tape vibration modes will dictate implementations of the skew-following loop for which the reference skew for skew-following control may be in a wide range of values, well beyond the assumed limits of ±d. Active compensation for changes in tape width due to changes in the environmental conditions by skewing the head to change its effective span would also require the ability to skew-servo on arbitrary reference values. Furthermore, the method described in Cherubini et al. (2015) generates only one skew estimate per servo frame. The rate at which estimates are generated in the TBS technique increases linearly with tape velocity. Higher skew estimate generation rates are desirable to reduce estimation delays in the skew-
This ambiguity arises because the skew estimate is obtained as the difference of the arrival times of correlation peaks that correspond to dibits occupying the same position in the [4 4 5 5] sequences of dibits in the two adjacent servo bands, without distinguishing whether they belong to two servo frames encoding the same LPOS symbol, characterized by zero shift, or to different LPOS symbols, characterized by a shift equal to an integer multiple of 2d, as illustrated in Fig. 3. This leads to the drawback that a reliable skew estimate is limited to the interval [–d, d] and that controlling the skew to values of approx. ±d μm + K × 2d μm, where K is an integer, poses a serious challenge because the skew measured during skew following would toggle between values around +d and –d. If perfect alignment between servo readers and servo frames is required, i.e., if the head is to be kept perpendicular to the tape, the skew is controlled to around zero, and the ambiguity mentioned above does not 9
2016 IFAC MECHATRONICS 10 GiovanniUK Cherubini et al. / IFAC-PapersOnLine 49-21 (2016) 007–012 September 5-8, 2016. Loughborough University,
following control loop and hence to enable improved controller performance, especially at low tape velocities.
shows the estimation of the skew process, highlighting the fractional part of the estimate. Figure 5(b) illustrates the evolution of the relevant variables in the time interval around the crossing of the boundary corresponding to a skew equal to d. The variable estFlag, not shown in the flow chart of Fig. 4, indicates the instants at which skew estimates are computed. The method ensures that the skew is correctly estimated over a wide range of skew values, thus enabling skew-following control for any value of the skew reference, to compensate large offsets in the position of the head modules, to enable active tape dimensional stability compensation, and to mitigate disturbances in the trackfollowing loop arising from compressional tape vibration modes. The method can also be readily applied for skew compensation in legacy LTO drives, where the reference in the skew-following loop needs to take into account the presence of the shift in the servo pattern for servo band identification.
Our solution for high-rate skew estimation without ambiguity can be described with reference to the flow chart shown in Fig. 4. Let us consider the i-th skew estimate si, expressed as the sum of an integer and a fractional part, i.e.,
s = n × 2d + μ , i
i
i
where μi denotes the fractional part in the interval [–d, d], and ni denotes the number of integer servo-frame lengths. The first step in the recursive method illustrated in Fig. 4 consists of estimating the initial value of ni (variable skewInt) using the LPOS sync-word detection flags provided by the two servo channels. Upon detecting the 8-bit sync word that is part of every 36-bit LPOS word, each servo channel outputs an LPOS sync-word detection flag. The time difference between a pair of sync-word detection flags corresponding to the same 36-bit LPOS word output by the synchronous servo channels provides the information about the initial value of the tape-to-head skew, from which the initial value of ni is estimated.
0.5
Then, at each clock interval kT, where T denotes the period of the clock provided to the servo channels, the system monitors the values achieved by the counters modulo-18 in servo channels 1 and 2, which advance by one whenever a new correlation peak is detected in a [4 4 5 5] sequence of dibits within a servo frame (variables Cnt1 and Cnt2, respectively). Selecting channel 2 as a reference, a new observation frame is started (variable frameFlag set to 1) whenever the value of Cnt2 increases from 6 to 7, indicating that the third dibit in the second burst of the [4 4 5 5] sequence of dibits has been detected on channel 2. During an observation frame, monitoring of the variables Cnt1 and Cnt2 continues to detect the increase of at least one of the two counter values from 15 to 16, indicating that the third dibit in the fourth burst of the [4 4 5 5] sequence of dibits has been detected. Detection of the first such transition of a counter value from 15 to 16 within a frame triggers the start of a measurement interval (variable intFlag is set to 1) and resets the observation frame flag (variable frameFlag is set to 0). The transition of the second counter value from 15 to 16 within the frame leads to the reset of both the observation frame and the measurement flags (both variables intFlag and frameFlag are set to 0), and to the computation of a new fractional skew estimate (variable skewEst). As special cases, if in the same clock interval Cnt2 increases from 6 to 7 and Cnt1 increases from 15 to 16, or both Cnt1 and Cnt2 increase from 15 to 16, a new fractional skew estimate (variable skewEst) is computed immediately without starting a new observation frame or measurement interval. If a variation Δ between the fractional parts of two consecutive skew estimates (skewEst_1 and skewEst) that is larger in absolute value than a threshold 2a is observed, with 2a ~ d/2, the integer part of the last skew estimate is augmented by a term equal to –2d × sign(Δ).
- 0.5
(a)
(b) Fig. 5. Illustration of (a) skew estimation and (b) relevant variables. With this approach, up to four skew estimates can be generated per servo frame, considering one reference dibit per servo burst. This is achieved by interleaving the generation of the skew estimates using four reference counter values, i.e., one per servo burst, to determine the start of an observation frame, and selecting appropriate shifts of the values of the counters modulo-18 to set the observation and
Simulation results, which illustrate the new method applied to a drive reading a tape with an LTO-7 servo format, for a tape velocity of 1.8 m/s, a servo channel SNR of 24.7 dB, and open-loop skew estimation, are shown in Fig. 5. Figure 5(a) 10
2016 IFAC MECHATRONICS September 5-8, 2016. Loughborough University, GiovanniUK Cherubini et al. / IFAC-PapersOnLine 49-21 (2016) 007–012
the measurement flags. Therefore the method allows an increase of up to a factor of four in the rate of generation of skew estimates with respect to the state-of-the-art approach. Furthermore, averaging the arrival times of multiple dibits in a burst to reduce estimation noise may also be considered.
tracks the skew value through and beyond the ±d boundary. Although a sine wave was applied as skew actuator signal, the estimated skew does not exactly resemble a sine wave, but shows a compression with ringing at maximum amplitude. The latter is due to a mechanical limit in the achievable skew range, which is specific to the tape drive used. Figure 7 also shows a short period of invalid skew estimates (indicated by a zero skew value) at around 3.54 s, which is caused by a servo pattern defect in one of the servo bands that forces a re-acquisition in one of the servo channels.
4. EXPERIMENTAL RESULTS A hardware implementation of the high-rate skew estimation algorithm described in Section 3 was designed in VHDL. The implementation provides two skew estimates per servo frame, and interfaces to two high-performance servo channels described in Cherubini et al. (2015). These hardware units are connected to a NIOS II soft-core microprocessor to form a programmable System-on-Chip (SoC), implemented on a field programmable gate array (FPGA)-based prototyping platform, as shown in Fig. 6. In combination with an IBM TS1150 tape drive and a current driver, this experimental setup enables open-loop and closed-loop track-following and skew-following servo control experiments.
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Fig. 7. Skew estimated by the legacy skew estimator unit and the new high-rate skew estimator with increased dynamic range, measured in fractions of the servo frame length.
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Fig. 6. Experimental setup with IBM TS1150 tape drive and FPGA hardware prototyping platform.
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To test the functionality of the new high-skew-rate servo channel, we performed an experiment using a particulate tape medium sample formatted with an LTO-7 servo pattern in an IBM TS1150 tape drive. The drive was operated in closedloop track-following mode at a tape speed of 2.7 m/s, and a 10 Hz sinusoidal waveform was applied to the skew actuator in (skew-servo) open-loop mode. Figure 7 shows the output of both the legacy skew estimator detailed in Cherubini et al. (2015) and the new high-rate skew estimator discussed in this paper, both implemented in the FPGA platform. The estimated skew is plotted in fractions of the servo frame length 2d. As shown in Fig. 7, the positive skew exceeds 1/2 of the servo frame length 2d, which causes a wrap around (ambiguity) in the legacy skew estimate at approximately 3.49 s. In contrast, the new high-rate skew estimator properly
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Fig. 8. Output of the high-rate skew estimator measured in micrometers. In a second experiment, we replaced the tape drive in Fig. 6 with a low-disturbance tape path with an IBM TS1140 electronics card, also used for the 85.9 Gb/in2 and 123 Gb/in2 recording areal density demonstrations described in Furrer et al. (2015a) and Lantz et al. (2015), and a particulate tape medium sample with an experimental TBS servo pattern with a 24-degree azimuth angle and a servo frame length of 2d = 11
2016 IFAC MECHATRONICS 12 GiovanniUK Cherubini et al. / IFAC-PapersOnLine 49-21 (2016) 007–012 September 5-8, 2016. Loughborough University,
Furrer, S., et al. (2015a), 85.9 Gb/in2 recording areal density on barium ferrite tape. IEEE Trans. Magn. 51(4), article 3100207.
96 µm, described in Furrer et al. (2015b). Figure 8 depicts the skew (in micrometers) estimated by the new high-rate skew estimator during a closed-loop track-following experiment at 2.1 m/s tape speed, in which the head skew was manually decreased by means of a micrometer stage. Towards the end of the capture, the skew exceeds two servo frame lengths, i.e. si > 2 × 2d, which corresponds to a skew estimate si whose integer part is equal to ni = –2.
Lantz, M. A., et al. (2015), 123 Gbit/in2 recording areal density on barium ferrite tape. IEEE Trans. Magn. 51(11), article 3101304. Furrer, S., Pantazi, A., Cherubini, G., and Lantz, M. A. (2015b), Resolution limits of timing-based servo schemes in magnetic tape drives. IEEE Trans. Magn. 51(11), article 3101504.
In future work, we plan to implement a skew-following controller on the microprocessor shown in Fig. 6 to close the skew servo loop. This will enable the closed-loop skewfollowing servo to i) exploit the increased update rate in the skew estimation and ii) perform skew following with a large nonzero skew reference. 5. CONCLUSIONS We demonstrated a novel skew estimator that generates skew estimates with a high rate over an extended range of allowable tape-to-head skew values. The increased rate of estimation will reduce delay in the skew servo control loop and is expected to lead to improved skew-following performance, particularly at low tape speeds. In addition, the extended estimation range will enable new functionality in future tape drives, such as active tape-dimensional stability compensation via head skewing to control the effective span of the array of read/write transducers in the head.
ACKNOWLEDGMENT The authors would like to thank A. Pantazi for her help with the experimental results, and E. Eleftheriou for his support of this work.
REFERENCES Barrett, R. C., Klaassen, E. H., Albrecht, T. R., Jaquette, G. A., and Eaton, J. H. (1998). Timing-based trackfollowing servo for linear tape systems. IEEE Trans. Magn. 34(4), pp. 1872-1877. Furrer, S., Jubert, P.-O., Cherubini, G., Cideciyan, R. D., and Lantz, M. A. (2012), Analytical expressions for the readback signal of timing-based servo schemes. IEEE Trans. Magn. 48(11), pp. 4578-4581. Cherubini, G., Eleftheriou, E., Jelitto, J., and Hutchins, R. (2007). Synchronous servo channel design for tape drive systems. In: Proc. 17th Annual ASME Information Storage and Processing Systems Conf. ISPS 2007, Santa Clara, CA, pp. 160-162. Cherubini, G., Furrer, S., and Jelitto, J. (2015). Highperformance servo channel for nanometer head positioning and longitudinal position symbol detection in tape systems. IEEE/ASME Trans. Mechatronics. DOI: 10.1109/TMECH.2015.2462718.
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