US20180114540A1 - Accurate repeatable runout compensation in disk drives during seeks - Google Patents
Accurate repeatable runout compensation in disk drives during seeks Download PDFInfo
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- US20180114540A1 US20180114540A1 US15/331,380 US201615331380A US2018114540A1 US 20180114540 A1 US20180114540 A1 US 20180114540A1 US 201615331380 A US201615331380 A US 201615331380A US 2018114540 A1 US2018114540 A1 US 2018114540A1
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- feed
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
- G11B5/48—Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed
- G11B5/58—Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed with provision for moving the head for the purpose of maintaining alignment of the head relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following
- G11B5/596—Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed with provision for moving the head for the purpose of maintaining alignment of the head relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following for track following on disks
- G11B5/59633—Servo formatting
- G11B5/59661—Spiral servo format
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B20/00—Signal processing not specific to the method of recording or reproducing; Circuits therefor
- G11B20/10—Digital recording or reproducing
- G11B20/10009—Improvement or modification of read or write signals
- G11B20/10481—Improvement or modification of read or write signals optimisation methods
- G11B20/10509—Improvement or modification of read or write signals optimisation methods iterative methods, e.g. trial-and-error, interval search, gradient descent or feedback loops
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B20/00—Signal processing not specific to the method of recording or reproducing; Circuits therefor
- G11B20/10—Digital recording or reproducing
- G11B20/18—Error detection or correction; Testing, e.g. of drop-outs
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
- G11B5/48—Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed
- G11B5/54—Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed with provision for moving the head into or out of its operative position or across tracks
- G11B5/55—Track change, selection or acquisition by displacement of the head
- G11B5/5521—Track change, selection or acquisition by displacement of the head across disk tracks
- G11B5/5526—Control therefor; circuits, track configurations or relative disposition of servo-information transducers and servo-information tracks for control thereof
- G11B5/553—Details
- G11B5/5547—"Seek" control and circuits therefor
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
- G11B5/48—Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed
- G11B5/58—Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed with provision for moving the head for the purpose of maintaining alignment of the head relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following
- G11B5/596—Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed with provision for moving the head for the purpose of maintaining alignment of the head relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following for track following on disks
- G11B5/59627—Aligning for runout, eccentricity or offset compensation
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
- G11B5/48—Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed
- G11B5/58—Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed with provision for moving the head for the purpose of maintaining alignment of the head relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following
- G11B5/596—Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed with provision for moving the head for the purpose of maintaining alignment of the head relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following for track following on disks
- G11B5/59633—Servo formatting
- G11B5/59644—Acquisition or selection of servo format from a system reference
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
- G11B5/48—Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed
- G11B5/58—Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed with provision for moving the head for the purpose of maintaining alignment of the head relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following
- G11B5/596—Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed with provision for moving the head for the purpose of maintaining alignment of the head relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following for track following on disks
- G11B5/59633—Servo formatting
- G11B5/59666—Self servo writing
Definitions
- Disk drives typically have disk synchronous repeatable runout (RRO), which is the offset between the ideal track location for a particular track and an actual track location, which is defined by servo burst information for the track formed on the disk.
- RRO disk synchronous repeatable runout
- To accurately position a read or write head in the presence of such runout typically requires special RRO compensation algorithms. These algorithms inject a suitable control signal into a head actuator, so that the read or write head follows the desired RRO path with good accuracy.
- RRO is primarily induced by cyclically repeating phenomenon, such as disk eccentricity, clamping distortions, harmonic vibrations within the drive, and the like, the largest components of RRO error are harmonic in nature. Consequently, RRO compensation algorithms are designed to address the largest components of RRO by compensating for RRO that corresponds to various harmonics of disk rotational frequency.
- RRO compensation algorithms minimize or eliminate RRO at a desired harmonic by determining the amplitude and phase of an appropriate sinusoidal compensation signal that is then injected into the head actuator.
- the sine and cosine components of the sinusoidal compensating signals may be used instead of amplitude and phase.
- RRO compensation algorithms are typically adaptive, in that they continuously adjust the gain/phase or sin/cos compensation coefficients while the disk drive actuator servo system positions the read head on a certain disk drive track.
- the adaptation is typically slow, taking as many as several hundred disk revolutions to converge, and therefore is unable to converge to correct coefficient values when the read head is moved radially across the disk surface.
- the servo system cannot accurately compensate for RRO variation as the read head is moved across the surface of the disk radially, such as during seeks.
- One or more embodiments described herein provide systems and methods for accurately compensating for repeatable runout (RRO) while moving a magnetic head radially across a disk surface.
- An iterative learning control algorithm is employed to determine appropriate feed-forward coefficients for an RRO compensation signal for each of a plurality of radial locations across the disk surface.
- the feed-forward coefficients are determined by performing multiple iterations of continuously moving the magnetic head across the disk surface along a target path while measuring a position error signal that indicates the radial error between the magnetic head and the target path. With each iteration, the iterative learning control algorithm computes new feed-forward coefficients for each of the plurality of radial locations, where the new feed-forward coefficients are selected to reduce the measured position error signal when employed to move the magnetic head along the target path.
- a method of controlling a read head in a magnetic storage device includes the steps of rotating the disk having a surface with servo wedges, moving the read head continuously across the disk from a first radial location to a second radial location while positioning the read head based on the servo wedges and a feed-forward correction signal and generating a position error signal (PES), and measuring the generated PES at multiple radial locations of the disk as the read head is moved continuously in a radial direction, wherein each measured PES is input to an adaptive learning algorithm that corrects feed-forward coefficients for the feed-forward correction signal.
- PES position error signal
- a data storage device comprises a first data storage disk having a first recording surface with servo wedges, a second data storage disk having a second recording surface, first and second read/write heads, and a controller.
- the first and second read/write heads are configured to move in unison in a radial direction relative to the first and second recording surfaces, respectively.
- the controller is configured to rotate the first and second data storage disks, move the read head continuously across the first data storage disk from a first radial location to a second radial location while positioning the read head based on the servo wedges and a feed-forward correction signal and generating a position error signal (PES), and measure the generated PES at multiple radial locations of the first data storage disk as the read head is moved continuously in the radial direction, wherein each measured PES is input to an adaptive learning algorithm that corrects feed-forward coefficients for the feed-forward correction signal.
- PES position error signal
- FIG. 1 is a schematic view of an exemplary hard disk drive, according to one embodiment.
- FIG. 2 schematically illustrates a partial side-view of the multiple storage disks and sliders of the hard disk drive of FIG. 1 , according to an embodiment.
- FIG. 3 illustrates a recording surface of a storage disk with servo wedges and concentric data storage tracks formed thereon, according to an embodiment.
- FIG. 4 illustrates a recording surface of the storage disk of FIG. 3 with a single reference spiral written thereon, according to one embodiment.
- FIG. 5 is a schematic illustration of a portion of the recording surface indicated in FIG. 4 as a reference spiral is being written thereon, according to an embodiment.
- FIG. 6 is a graph showing PES during a seek by a read/write head while the servo system of the hard drive of FIG. 1 attempts to move read/write head along a target path for the reference spiral.
- FIG. 7 is a simplified control system block diagram illustrating a control system, according to some embodiments.
- FIG. 8 sets forth a flowchart of method steps for controlling a read head in a magnetic storage device, according to an embodiment.
- FIG. 1 is a schematic view of an exemplary hard disk drive (HDD) 100 , according to one embodiment.
- HDD 100 includes multiple storage disks 110 (only one of which is visible in FIG. 1 ) that each include one or two recording surfaces 112 on which a plurality of concentric data storage tracks are disposed.
- Storage disks 110 are coupled to and rotated by a spindle motor 114 that is mounted on a base plate 116 .
- An actuator arm assembly 120 is also mounted on base plate 116 , and includes multiple sliders 121 (only one of which is visible in FIG.
- each flexure arm 122 with a magnetic read/write head 127 that reads data from and writes data to the data storage tracks of an associated recording surface 112 .
- Each flexure arm 122 is attached to an actuator arm 124 that rotates about a bearing assembly 126 .
- Voice coil motor 128 moves all of the multiple sliders 121 radially relative to a recording surface 112 of a storage disk 110 , thereby positioning read/write head 127 over a desired concentric data storage track.
- Spindle motor 114 , read/write head 127 , and voice coil motor 128 are coupled to electronic circuits 130 , which are mounted on a printed circuit board 132 .
- Electronic circuits 130 include a read channel 137 , a microprocessor-based controller 133 , random-access memory (RAM) 134 (which may be a dynamic RAM and is used as a data buffer) and/or a flash memory device 135 and a flash manager device 136 .
- read channel 137 and microprocessor-based controller 133 are included in a single chip, such as a system-on-chip 131 .
- HDD 100 may further include a motor-driver chip 125 that accepts commands from microprocessor-based controller 133 and drives both spindle motor 114 and voice coil motor 128 .
- Read/write channel 137 communicates with the read/write head 127 via a preamplifier (not shown) that may be mounted on a flex-cable that is itself mounted on either base plate 116 , actuator arm 120 , or both.
- HDD 100 also includes an inner diameter (ID) crash stop 129 and a load/unload ramp 123 .
- ID crash stop 129 is configured to restrict motion of actuator arm assembly 120 to preclude damage to read/write head 127 and/or storage disk 110 .
- Load/unload ramp 123 is typically disposed proximate the outer diameter (OD) of storage disk 110 and is configured to unload read/write head 127 from storage disk 110 .
- FIG. 2 schematically illustrates a partial side-view of the multiple storage disks 110 and sliders 121 of HDD 100 , according to an embodiment.
- HDD is configured with multiple storage disks 110 and multiple read/write heads 127 .
- HDD 100 includes a storage disk 210 with recording surfaces 211 and 212 , a storage disk 220 with recording surfaces 221 and 222 , and a storage disk 230 with recording surfaces 231 and 232 .
- HDD 100 further includes read/write heads 211 A, 212 A, 221 A, 222 A, 231 A, 232 A that are each associated with a particular recording surface of one of storage disks 210 , 220 , and 230 , i.e., recording surfaces 211 , 212 , 221 , 222 , 231 , and 232 , respectively.
- actuator arm assembly 120 moves in an arc between the ID and the OD of the storage disk 110 .
- Actuator arm assembly 120 accelerates in one angular direction when current is passed in one direction through the voice coil of voice coil motor 128 and accelerates in an opposite direction when the current is reversed, thereby allowing control of the position of actuator arm assembly 120 and the attached read/write head 127 with respect to the particular storage disk 110 .
- Voice coil motor 128 is coupled with a servo system that uses the positioning data read from servo wedges on storage disk 110 by read/write head 127 to determine the position of read/write head 127 over a specific data storage track. For example, the servo system positions read/write head 211 A over recording surface 211 based on positioning data read from recording surface 211 , and positions read/write head 212 A over recording surface 212 based on positioning data read from recording surface 212 .
- the servo system determines an appropriate current to drive through the voice coil of voice coil motor 128 , and drives said current using a current driver and associated circuitry.
- the appropriate current is determined based in part on a position feedback signal of the read/write head 127 , i.e., a position error signal (PES).
- PES position error signal
- the PES is typically generated by using servo patterns included in the servo wedges on the recording surface 112 as a reference.
- FIG. 3 One embodiment of a recording surface 112 is illustrated in FIG. 3 .
- FIG. 3 illustrates a recording surface 112 of a storage disk 110 with servo wedges 300 and concentric data storage tracks 320 formed thereon, according to an embodiment.
- Servo wedges 300 may be written on recording surface 112 by either a media writer or by HDD 100 itself via a self-servo-write (SSW) process.
- SSW self-servo-write
- Servo wedges 300 may be substantially radially aligned.
- servo wedges 300 may be somewhat curved.
- servo wedges 300 may be configured in a spiral pattern that mirrors the path that would be followed by read/write head 127 if read/write head 127 were to be moved across the stroke of actuator arm assembly 120 while storage disk 110 is not spinning.
- servo wedges 300 are depicted as substantially straight lines in FIG. 3 .
- Each servo wedge 300 includes a plurality of servo sectors 350 containing servo information that defines the radial position and track pitch, i.e., spacing, of data storage tracks 320 .
- Data storage tracks 320 for storing data are located in data sectors 325 , and are positionally defined by the servo information written in servo sectors 350 .
- Each servo sector 350 contains a reference signal that is read by read/write head 127 during read and write operations to position read/write head 127 above a desired data storage track 320 .
- the actual number of data storage tracks 320 and servo wedges 300 included on recording surface 112 is considerably larger than illustrated in FIG. 3 .
- recording surface 112 may include hundreds of thousands of concentric data storage tracks 320 and hundreds of servo wedges 300 .
- servo wedges 300 written on one recording surface 112 of HDD 100 enable writing of reference spirals on a different recording surface 112 .
- servo wedges 300 are written on recording surface 211 and are employed to write one or more reference spirals on another recording surface 112 of HDD 100 , such as recording surface 221 or 231 .
- the servo system of HDD 100 can precisely control the radial location of read/write head 211 A, so that read/write head 211 A follows a target path on recording surface 211 , such as the path of an ideally formed reference spiral on recording surface 221 .
- another read/write head of HDD 100 such as read/write head 221 A or read/write head 231 A, can write on another recording surface a reference spiral having substantially the same shape as the target path on recording surface 211 , as illustrated in FIG. 4 .
- FIG. 4 illustrates recording surface 221 of storage disk 220 with a single reference spiral 401 written thereon, according to one embodiment.
- reference spiral 401 can be formed on recording surface 221 by controlling the radial location of read/write head 211 A (and therefore also the radial location of read/write head 221 A).
- the servo system of HDD 100 moves read/write head 211 A and 221 A radially across recording surfaces 211 and 221 , respectively, based on timing and position information read from the servo wedges 300 on recording surface 211 , and on an RRO compensation signal.
- the motion of read/write head 221 A as reference spiral 401 is written on recording surface 220 is shown in FIG. 5 .
- FIG. 5 is a schematic illustration of a portion 500 of recording surface 221 indicated in FIG. 4 as reference spiral 401 is being written thereon, according to an embodiment.
- a horizontal displacement in FIG. 5 corresponds to a circumferential displacement of read/write head 221 A with respect to recording surface 221 , caused by rotation of storage disk 220 .
- a vertical displacement in FIG. 5 corresponds to a radial displacement of read/write head 221 A with respect to recording surface 221 , caused by rotation of actuator arm assembly 120 .
- reference spiral 401 extends diagonally across recording surface 221 .
- Reference spiral 401 in conjunction with other reference spirals on recording surface 221 , is configured to provide position and timing information that enable the internal servo system of HDD 100 to perform a SSW process, thereby writing servo wedges 300 on recording surface 221 .
- position of read/write head 221 A and a target path 502 that indicates an ideal location for reference spiral 401 for optimal operation of HDD 100 .
- the servo system of HDD 100 can write servo wedges 300 onto recording surface 221 with the necessary precision for proper operation of HDD 100 .
- reference spiral 401 falls outside of maximum acceptable offset 503 , servo wedges 300 may not be written accurately, and issues affecting operation of HDD, such as track squeeze, may result.
- maximum acceptable offset 503 is depicted as an offset distance of reference spiral 401 from target path 502 that is measured perpendicular to target path 502 .
- maximum acceptable offset 503 is a radial offset distance 504 of reference spiral 401 from target path 502 .
- maximum acceptable offset 503 is a circumferential offset distance of reference spiral 401 from target path 502 (not shown).
- Writing reference spiral 401 on recording surface 221 involves precisely controlling the radial position of read/write head 221 A as read/write head 221 A is moved continuously across the stroke of actuator arm assembly 120 , for example from ID to OD of recording disk 220 or vice versa. That is, read/write head 221 A moves continuously in the radial direction while a different read/write head of HDD 100 servos off the servo wedges 300 on a different recording surface of HDD 100 , such as recording surface 211 .
- conventional techniques have proven inadequate for providing sufficiently precise control when writing with read/write head 221 A, as illustrated in FIG. 6 .
- FIG. 6 is a graph 600 showing PES during a seek by read/write head 221 A while the servo system of HDD 100 attempts to move read/write head 221 A along target path 502 for reference spiral 401 .
- the servo samples 600 (x-axis) are taken as read/write head 221 A is moved across recording surface 221 A from the ID of storage disk 220 to the OD of storage disk 220 , where the PES is an offset distance between target path 502 and an actual path of read/write head 221 A.
- the PES shown in graph 600 may be a radial offset distance 504 between target path 502 and the actual path of read/write head 221 A, as measured via servo wedges 300 on recording surface 211 .
- the PES for read/write head 221 A reaches values as high as 40% of servo track width, which is highly undesirable for reference spiral 401 .
- a control signal for positioning read/write head 127 is based on a position feedback signal, i.e., the currently measured PES, and a feed-forward signal, i.e., a sinusoidal compensation signal generated by an RRO compensation algorithm.
- the sinusoidal compensation signal compensates for RRO as read/write head 127 servos on a particular data storage track 320 .
- a position feedback signal i.e., the currently measured PES
- a feed-forward signal i.e., a sinusoidal compensation signal generated by an RRO compensation algorithm.
- the sinusoidal compensation signal compensates for RRO as read/write head 127 servos on a particular data storage track 320 .
- an iterative learning control (ILC) algorithm is employed to determine appropriate feed-forward coefficients for an RRO compensation signal for each of a plurality of radial locations across the disk surface. More specifically, the feed-forward coefficients are for an RRO compensation signal that compensates for RRO as read/write head 127 is moved continuously across recording surface 112 in a radial direction to follow a target path.
- a repetitive control algorithm determines appropriate feed-forward coefficients for an RRO compensation signal for a single specific data storage track 320 . That is, according to conventional techniques, feed-forward coefficients for an RRO compensation signal are determined for controlling the radial position of read/write head 127 as read/write head 127 servos on a single data storage track 320 .
- FIG. 7 is a simplified control system block diagram illustrating a control system 700 , according to some embodiments.
- Control system 700 includes a target position generator 710 , a controller 720 , SIN/COS coefficient adapter 730 , an RRO ILC coefficient adapter 740 , a COS generator 751 , a SIN generator 752 , and an actuator 760 , as shown.
- Control system 700 further includes various summers, multipliers, and feedback and feed-forward signals that interact with the above elements of control system 700 as shown.
- Control system 700 as a whole, or each of the elements of control system 700 set forth above, may be implemented as any suitable processor or logic circuit, such as a central processing unit (CPU), an application-specific integrated circuit (ASIC), a field programmable gate array (FPGA), any other type of processing unit, or a combination of different processing units.
- control system 700 or each of the elements of control system 700 set forth above, may be any technically feasible hardware unit capable of processing input signals or other data and/or executing software applications to facilitate operation of HDD 100 as described herein.
- Target position generator 710 provides the current target (radial) position r for a particular read/write head 127 , and controller 720 receives an error signal, i.e., PES 701 , and generates an actuator input signal 702 for controlling the position of read/write head 127 .
- SIN/COS coefficient adapter 730 is configured to receive PES 701 and generate gain/phase or sin/cos compensation coefficients for an RRO compensation signal 703 that modifies actuator input signal 702 .
- SIN/COS coefficient adapter 730 is configured to implement a repetitive control algorithm that continuously adjusts the gain/phase or sin/cos coefficients for RRO compensation signal 703 while the servo system for HDD 100 follows a particular data storage track 320 .
- COS generator 751 generates a disk-synchronous sinusoidal output signal 753 , for example cos(t* 2 * ⁇ /T), at a desired harmonic of disk rotational frequency.
- Sinusoidal output signal 753 is multiplied by a cosine amplitude coefficient G COS provided by SIN/COS coefficient adapter 730 .
- SIN generator 752 generates a disk-synchronous sinusoidal output signal 754 , for example sin(t* 2 * ⁇ /T), at the desired harmonic of disk rotational frequency.
- Sinusoidal output signal 754 is multiplied by a sine amplitude coefficient G SIN provided by SIN/COS coefficient adapter 730 .
- Cosine amplitude coefficient G COS and sine amplitude coefficient G SIN are selected by SIN/COS coefficient adapter 730 such that RRO compensation signal 703 forces actuator 760 to follow the desired harmonic with high accuracy.
- Actuator 760 represents actuator arm assembly 120 and voice coil motor 128 of HDD 100 in FIG. 1 . As such, actuator 760 is configured to receive a modified input signal 704 to move read/write head 127 from a current radial location to a target radial position p.
- control system 700 includes a control system 799 that may be employed in a conventional HDD.
- control system 799 may include elements equivalent to target position generator 710 , controller 720 , SIN/COS coefficient adapter 730 , COS generator 751 , SIN generator 752 , actuator 760 , and summers and multipliers associated therewith.
- control system 700 employs control system 799 , i.e., SIN/COS coefficient adapter 730 , COS generator 751 , SIN generator 752 , and actuator 760 , to servo read/write head 127 over a particular data storage track 320 .
- Cosine amplitude coefficient G COS and sine amplitude coefficient G SIN modify actuator input signal 702 so that RRO associated with the particular data storage track 320 is reduced or eliminated.
- control system 700 includes additional functionality over control system 799 .
- RRO ILC coefficient adapter 740 is configured to generate amplitude coefficients for modifying disk-synchronous sinusoidal output signal 753 and disk-synchronous sinusoidal output signal 754 when control system 700 causes read/write head 127 to follow a target path that results in read/write head 127 moving continuously from a first radial position to a second radial position. That is, the amplitude coefficients generated by RRO ILC coefficient adapter 740 are employed when read/write head 127 is controlled to follow a target path that crosses multiple data storage tracks 320 , such as when a reference spiral is being written by another read/write head 127 of HDD 100 .
- RRO ILC coefficient adapter 740 modifies the amplitude of RRO compensation signal 703 by adding amplitude coefficient factors F COS and F SIN to the outputs of SIN/COS coefficient adapter 730 .
- RRO ILC coefficient adapter 740 modifies the amplitude of RRO compensation signal 703 by replacing the outputs of SIN/COS coefficient adapter 730 with amplitude coefficient factors F COS and F SIN .
- the particular seek operation is performed over multiple iterations.
- block RRO COS ILC 741 computes amplitude coefficient F COS , based on the actuator position signal 705
- block RRO SIN ILC 742 computes amplitude coefficient F SIN , based on actuator position signal 705 . Because amplitude coefficients F COS and F SIN are feed-forward coefficients, the newly computed values for amplitude coefficients F COS and F SIN are employed in the next iteration in which the particular seek operation is performed.
- the ILC algorithm employed in block RRO COS ILC 741 and block RRO SIN ILC 742 to compute amplitude coefficients F COS and F SIN may be any suitable adaptive algorithm known in the art.
- block RRO COS ILC 741 and block RRO SIN ILC 742 employ the following exemplary equations to compute new values for amplitude coefficients F COS and F SIN for each iteration of the seek operation performed:
- F cos ( p, k+ 1) F cos ( p, k )+ K*G cos ( p+p o , k )
- F sin ( p, k+ 1) F sin ( p, k )+ K*G sin ( p+p o , k )
- the path followed by read/write head 127 falls within a maximum acceptable offset. For example, after a number of iterations of the seek operation, a path followed by read/write head 127 while being controlled to follow target path 502 in FIG. 5 falls within maximum acceptable offset 503 . At such time, the iterative process is considered to have converged, and values of amplitude coefficients F COS and F SIN are frozen, and are no longer modified.
- target path 502 represents an ideal path for reference spiral 401 on recording surface 221
- read/write head 221 A can now accurately write reference spiral 401 using the frozen values of amplitude coefficients F COS and F SIN while performing the appropriate seek operation.
- amplitude coefficient F COS and amplitude coefficient F SIN are stored in firmware tables, such as coefficient table 743 , for various radial locations.
- a suitable table lookup algorithm finds the table entry belonging to the current radial actuator position and computes amplitude coefficient F COS and amplitude coefficient F SIN accordingly.
- the values in coefficient table 743 are iteratively adapted as the multiple seek operations across the stroke are performed.
- the values of G cos and G sin are collected, and a suitable adaptation algorithm adjusts the RRO ILC tables such that the variation of G cos and G sin are minimized during seeks.
- Entries for any suitable number of radial positions may be included in coefficient table 743 .
- coefficient table 743 may include a value for amplitude coefficient F COS and amplitude coefficient F SIN for as few as 10 radial positions or as many as 1000 or more radial locations.
- the computational and storage costs associated with storing a value for amplitude coefficient F COS and amplitude coefficient F SIN for each data storage track 320 outweighs the benefits in more accurately implemented radial seeks.
- interpolation or any other estimating technique may be applied to determine values for amplitude coefficient F COS and amplitude coefficient F SIN .
- control system 700 is configured to generate accurate values for amplitude coefficient F COS and amplitude coefficient F SIN , so that read/write head 127 can be moved accurately in a seek operation that closely follows a particular target path in the presence of significant RRO.
- control system 700 is configured to compensate for a RRO associated with a single harmonic of HDD 100 , such as the first harmonic of the rotation speed of storage disks 110 .
- control system 700 may be supplemented with an additional RRO compensation system—one for each additional harmonic to be addressed.
- Each such additional RRO compensation system may include, without limitation, a SIN/COS coefficient adapter, RRO ILC Coefficient, COS generator, and SIN generator, all configured for the harmonic being addressed.
- FIG. 8 sets forth a flowchart of method steps for controlling a read head in a magnetic storage device, according to an embodiment.
- the control algorithms for the method steps may reside in microprocessor-based controller 133 , or, in some embodiments, an external host device that is temporarily coupled to HDD 100 and used to facilitate the calibration of HDD 100 .
- microprocessor-based controller 133 is assumed to perform said control algorithms for the method steps, although other external control devices can potentially be used in such a role.
- method 800 begins at step 801 , when microprocessor-based controller 133 rotates storage disks 110 , including a disk with servo wedges 300 formed thereon and a disk with a recording surface that does not have servo wedges formed thereon.
- microprocessor-based controller 133 moves a read/write head 127 from a first radial location, such as the ID storage disks 110 , to a second radial location, such as the OD of storage disks 110 .
- a seek operation is performed across some or all of the stroke of actuator arm assembly 120 .
- read/write head 127 is moved continuously across the disk with servo wedges 300 formed thereon, and therefore positions read/write head 127 based on timing and position information provided by servo wedges 300 .
- the positioning of read/write head 127 is based on a feed-forward correction signal, such as RRO compensation signal 703 .
- feed-forward coefficients for the feed-forward correction signal are fetched from coefficient table 743 . While moving from the first radial location to the second radial location, read/write head 127 generates a PES based on the servo wedges 300 formed on the disk surface.
- microprocessor-based controller 133 measures the PES generated in step 802 .
- microprocessor-based controller 133 measures PES at multiple radial locations as read/write head 127 is moved continuously across the disk surface on which servo wedges 300 are formed.
- the number of radial locations may be on the order of 10, 100, 1000, or more, but generally does not correspond with the number of data storage tracks 300 formed on the disk surface.
- microprocessor-based controller 133 determines whether the PES measured at one or more of the multiple radial locations between the first radial location and the second radial location exceeds a predetermined threshold value. For example, the PES measured at a radial location may be equal to or greater than a value indicating that the location of read/write head 127 falls outside maximum acceptable offset 503 . In such a case, method 800 proceeds to step 805 . If no PES measured during step 803 exceeds the predetermined threshold value, method 800 proceeds to step 810 .
- microprocessor-based controller 133 or SIN/COS coefficient adapter 740 computes new feed-forward coefficients for the feed-forward correction signal employed in step 801 to help position read/write head 127 .
- the feed-forward correction signal employed in step 801 is RRO compensation signal 703
- RRO SIN/COS ILC adapter 740 computes new values for amplitude coefficient factors F COS and F SIN .
- microprocessor-based controller 133 or RRO SIN/COS ILC coefficient adapter 740 stores the new feed-forward coefficients computed in step 805 .
- the previous values stored in coefficient table 743 for amplitude coefficient factors F COS and F SIN are replaced with the updated values computed in step 805 .
- read/write head 127 is positioned more accurately along the target path for the seek operation.
- method 800 returns to step 802 for another iteration of the seek operation.
- step 810 which is performed responsive to microprocessor-based controller 133 determining that no PES measured during step 803 exceeds the predetermined threshold value, microprocessor-based controller 133 causes a reference spiral to be written on a different surface than the surface with servo wedges 300 .
- read/write head 127 is moved continuously across the disk surface on which servo wedges 300 are formed while positioning read/write head 127 based on the servo wedges and an updated feed-forward correction signal.
- the updated feed-forward correction signal such as RRO compensation signal 703 , is based on the most recently updated feed-forward coefficients computed by RRO SIN/COS ILC coefficient adapter 740 .
- microprocessor-based controller 133 causes another read/write head 127 to write a reference spiral on a different surface than the surface with servo wedges 300 .
- feed-forward coefficients for the seek operation can be computed by an ILC, so that the seek operation can be precisely controlled, for example while writing a reference spiral.
- embodiments described herein provide systems and methods for accurately compensating for repeatable runout (RRO) while moving a magnetic head across a disk surface.
- An iterative learning control algorithm is employed to determine appropriate feed-forward coefficients for an RRO compensation signal for each of a plurality of radial locations across the disk surface.
- the feed-forward coefficients are determined by performing multiple iterations of continuously moving the magnetic head across the disk surface along a target path while measuring a position error signal that indicates the radial error between the magnetic head and the target path. With each iteration, the iterative learning control algorithm computes new feed-forward coefficients for each of the plurality of radial locations, the new feed-forward coefficients being selected to reduce the measured position error signal when employed to move the magnetic head along the target path.
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Abstract
Description
- Disk drives typically have disk synchronous repeatable runout (RRO), which is the offset between the ideal track location for a particular track and an actual track location, which is defined by servo burst information for the track formed on the disk. To accurately position a read or write head in the presence of such runout typically requires special RRO compensation algorithms. These algorithms inject a suitable control signal into a head actuator, so that the read or write head follows the desired RRO path with good accuracy. Because RRO is primarily induced by cyclically repeating phenomenon, such as disk eccentricity, clamping distortions, harmonic vibrations within the drive, and the like, the largest components of RRO error are harmonic in nature. Consequently, RRO compensation algorithms are designed to address the largest components of RRO by compensating for RRO that corresponds to various harmonics of disk rotational frequency.
- Typically, RRO compensation algorithms minimize or eliminate RRO at a desired harmonic by determining the amplitude and phase of an appropriate sinusoidal compensation signal that is then injected into the head actuator. Alternatively, the sine and cosine components of the sinusoidal compensating signals may be used instead of amplitude and phase. In either case, such RRO compensation algorithms are typically adaptive, in that they continuously adjust the gain/phase or sin/cos compensation coefficients while the disk drive actuator servo system positions the read head on a certain disk drive track. However, the adaptation is typically slow, taking as many as several hundred disk revolutions to converge, and therefore is unable to converge to correct coefficient values when the read head is moved radially across the disk surface. As a result, the servo system cannot accurately compensate for RRO variation as the read head is moved across the surface of the disk radially, such as during seeks.
- During some operations, it is desirable to minimize the magnitude of RRO while the head actuator is moving radially, i.e., moving across the stroke rather than servoing over a particular track. One such example is when a disk drive writes reference spirals on a disk as part of a spiral-based self-servo writing process. For an error-free and robust self-servo writing process, the reference spirals used should be precisely written on the disk surface, so that the drive can write servo wedges onto a surface of the disk with the necessary precision for proper operation of the drive. However, because existing RRO compensation algorithms may not provide sufficient head positioning accuracy while moving the head actuator radially to write reference spirals, such reference spirals can include an unacceptable level of error, thereby affecting operation of the drive. Accordingly, there is a need in the art for a method of accurately compensating for RRO when actuating a read or write head radially across a disk surface.
- One or more embodiments described herein provide systems and methods for accurately compensating for repeatable runout (RRO) while moving a magnetic head radially across a disk surface. An iterative learning control algorithm is employed to determine appropriate feed-forward coefficients for an RRO compensation signal for each of a plurality of radial locations across the disk surface. The feed-forward coefficients are determined by performing multiple iterations of continuously moving the magnetic head across the disk surface along a target path while measuring a position error signal that indicates the radial error between the magnetic head and the target path. With each iteration, the iterative learning control algorithm computes new feed-forward coefficients for each of the plurality of radial locations, where the new feed-forward coefficients are selected to reduce the measured position error signal when employed to move the magnetic head along the target path.
- A method of controlling a read head in a magnetic storage device, according to an embodiment, includes the steps of rotating the disk having a surface with servo wedges, moving the read head continuously across the disk from a first radial location to a second radial location while positioning the read head based on the servo wedges and a feed-forward correction signal and generating a position error signal (PES), and measuring the generated PES at multiple radial locations of the disk as the read head is moved continuously in a radial direction, wherein each measured PES is input to an adaptive learning algorithm that corrects feed-forward coefficients for the feed-forward correction signal.
- A data storage device, according to another embodiment, comprises a first data storage disk having a first recording surface with servo wedges, a second data storage disk having a second recording surface, first and second read/write heads, and a controller. The first and second read/write heads are configured to move in unison in a radial direction relative to the first and second recording surfaces, respectively. The controller is configured to rotate the first and second data storage disks, move the read head continuously across the first data storage disk from a first radial location to a second radial location while positioning the read head based on the servo wedges and a feed-forward correction signal and generating a position error signal (PES), and measure the generated PES at multiple radial locations of the first data storage disk as the read head is moved continuously in the radial direction, wherein each measured PES is input to an adaptive learning algorithm that corrects feed-forward coefficients for the feed-forward correction signal.
- So that the manner in which the above recited features of embodiments of the invention can be understood in detail, a more particular description of embodiments of the invention, briefly summarized above, may be had by reference to the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
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FIG. 1 is a schematic view of an exemplary hard disk drive, according to one embodiment. -
FIG. 2 schematically illustrates a partial side-view of the multiple storage disks and sliders of the hard disk drive ofFIG. 1 , according to an embodiment. -
FIG. 3 illustrates a recording surface of a storage disk with servo wedges and concentric data storage tracks formed thereon, according to an embodiment. -
FIG. 4 illustrates a recording surface of the storage disk ofFIG. 3 with a single reference spiral written thereon, according to one embodiment. -
FIG. 5 is a schematic illustration of a portion of the recording surface indicated inFIG. 4 as a reference spiral is being written thereon, according to an embodiment. -
FIG. 6 is a graph showing PES during a seek by a read/write head while the servo system of the hard drive ofFIG. 1 attempts to move read/write head along a target path for the reference spiral. -
FIG. 7 is a simplified control system block diagram illustrating a control system, according to some embodiments. -
FIG. 8 sets forth a flowchart of method steps for controlling a read head in a magnetic storage device, according to an embodiment. - For clarity, identical reference numbers have been used, where applicable, to designate identical elements that are common between figures. It is contemplated that features of one embodiment may be incorporated in other embodiments without further recitation.
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FIG. 1 is a schematic view of an exemplary hard disk drive (HDD) 100, according to one embodiment. For clarity, HDD 100 is illustrated without a top cover. HDD 100 includes multiple storage disks 110 (only one of which is visible inFIG. 1 ) that each include one or tworecording surfaces 112 on which a plurality of concentric data storage tracks are disposed.Storage disks 110 are coupled to and rotated by aspindle motor 114 that is mounted on abase plate 116. Anactuator arm assembly 120 is also mounted onbase plate 116, and includes multiple sliders 121 (only one of which is visible inFIG. 1 ), each mounted on aflexure arm 122 with a magnetic read/writehead 127 that reads data from and writes data to the data storage tracks of an associatedrecording surface 112. Eachflexure arm 122 is attached to anactuator arm 124 that rotates about abearing assembly 126.Voice coil motor 128 moves all of themultiple sliders 121 radially relative to arecording surface 112 of astorage disk 110, thereby positioning read/writehead 127 over a desired concentric data storage track.Spindle motor 114, read/writehead 127, andvoice coil motor 128 are coupled toelectronic circuits 130, which are mounted on a printedcircuit board 132. -
Electronic circuits 130 include a readchannel 137, a microprocessor-basedcontroller 133, random-access memory (RAM) 134 (which may be a dynamic RAM and is used as a data buffer) and/or aflash memory device 135 and aflash manager device 136. In some embodiments, readchannel 137 and microprocessor-basedcontroller 133 are included in a single chip, such as a system-on-chip 131. In some embodiments, HDD 100 may further include a motor-driver chip 125 that accepts commands from microprocessor-basedcontroller 133 and drives bothspindle motor 114 andvoice coil motor 128. Read/writechannel 137 communicates with the read/writehead 127 via a preamplifier (not shown) that may be mounted on a flex-cable that is itself mounted on eitherbase plate 116,actuator arm 120, or both. - HDD 100 also includes an inner diameter (ID)
crash stop 129 and a load/unload ramp 123.ID crash stop 129 is configured to restrict motion ofactuator arm assembly 120 to preclude damage to read/writehead 127 and/orstorage disk 110. Load/unload ramp 123 is typically disposed proximate the outer diameter (OD) ofstorage disk 110 and is configured to unload read/writehead 127 fromstorage disk 110. -
FIG. 2 schematically illustrates a partial side-view of themultiple storage disks 110 andsliders 121 ofHDD 100, according to an embodiment. As shown, HDD is configured withmultiple storage disks 110 and multiple read/writeheads 127. Specifically, in the embodiment illustrated inFIG. 2 ,HDD 100 includes astorage disk 210 withrecording surfaces storage disk 220 withrecording surfaces recording surfaces HDD 100 further includes read/writeheads storage disks recording surfaces - When data are transferred to or from a
particular storage disk 110 ofHDD 100,actuator arm assembly 120 moves in an arc between the ID and the OD of thestorage disk 110.Actuator arm assembly 120 accelerates in one angular direction when current is passed in one direction through the voice coil ofvoice coil motor 128 and accelerates in an opposite direction when the current is reversed, thereby allowing control of the position ofactuator arm assembly 120 and the attached read/writehead 127 with respect to theparticular storage disk 110.Voice coil motor 128 is coupled with a servo system that uses the positioning data read from servo wedges onstorage disk 110 by read/writehead 127 to determine the position of read/writehead 127 over a specific data storage track. For example, the servo system positions read/writehead 211A overrecording surface 211 based on positioning data read fromrecording surface 211, and positions read/writehead 212A overrecording surface 212 based on positioning data read fromrecording surface 212. - In positioning a read/write
head 127 over arecording surface 112, the servo system determines an appropriate current to drive through the voice coil ofvoice coil motor 128, and drives said current using a current driver and associated circuitry. Typically, the appropriate current is determined based in part on a position feedback signal of the read/writehead 127, i.e., a position error signal (PES). The PES is typically generated by using servo patterns included in the servo wedges on therecording surface 112 as a reference. One embodiment of arecording surface 112 is illustrated inFIG. 3 . -
FIG. 3 illustrates arecording surface 112 of astorage disk 110 withservo wedges 300 and concentric data storage tracks 320 formed thereon, according to an embodiment.Servo wedges 300 may be written onrecording surface 112 by either a media writer or byHDD 100 itself via a self-servo-write (SSW) process.Servo wedges 300 may be substantially radially aligned. In practice,servo wedges 300 may be somewhat curved. For example,servo wedges 300 may be configured in a spiral pattern that mirrors the path that would be followed by read/write head 127 if read/write head 127 were to be moved across the stroke ofactuator arm assembly 120 whilestorage disk 110 is not spinning. Such a curved pattern advantageously results in the wedge-to-wedge timing being independent of the radial position of read/write head 127. For simplicity,servo wedges 300 are depicted as substantially straight lines inFIG. 3 . Eachservo wedge 300 includes a plurality ofservo sectors 350 containing servo information that defines the radial position and track pitch, i.e., spacing, of data storage tracks 320. - Data storage tracks 320 for storing data are located in
data sectors 325, and are positionally defined by the servo information written inservo sectors 350. Eachservo sector 350 contains a reference signal that is read by read/write head 127 during read and write operations to position read/write head 127 above a desireddata storage track 320. Typically, the actual number of data storage tracks 320 andservo wedges 300 included onrecording surface 112 is considerably larger than illustrated inFIG. 3 . For example,recording surface 112 may include hundreds of thousands of concentric data storage tracks 320 and hundreds ofservo wedges 300. - In some embodiments,
servo wedges 300 written on onerecording surface 112 ofHDD 100 enable writing of reference spirals on adifferent recording surface 112. For example, referring toFIG. 2 , in one such embodiment,servo wedges 300 are written onrecording surface 211 and are employed to write one or more reference spirals on anotherrecording surface 112 ofHDD 100, such asrecording surface servo wedges 300 onrecording surface 211, as well as an RRO compensation signal, the servo system ofHDD 100 can precisely control the radial location of read/write head 211A, so that read/write head 211A follows a target path onrecording surface 211, such as the path of an ideally formed reference spiral onrecording surface 221. Simultaneously, another read/write head ofHDD 100, such as read/write head 221A or read/write head 231A, can write on another recording surface a reference spiral having substantially the same shape as the target path onrecording surface 211, as illustrated inFIG. 4 . -
FIG. 4 illustratesrecording surface 221 ofstorage disk 220 with asingle reference spiral 401 written thereon, according to one embodiment. As noted above,reference spiral 401 can be formed onrecording surface 221 by controlling the radial location of read/write head 211A (and therefore also the radial location of read/write head 221A). Specifically, the servo system ofHDD 100 moves read/write head recording surfaces servo wedges 300 onrecording surface 211, and on an RRO compensation signal. The motion of read/write head 221A asreference spiral 401 is written onrecording surface 220 is shown inFIG. 5 . -
FIG. 5 is a schematic illustration of aportion 500 ofrecording surface 221 indicated inFIG. 4 asreference spiral 401 is being written thereon, according to an embodiment. A horizontal displacement inFIG. 5 corresponds to a circumferential displacement of read/write head 221A with respect torecording surface 221, caused by rotation ofstorage disk 220. A vertical displacement inFIG. 5 corresponds to a radial displacement of read/write head 221A with respect torecording surface 221, caused by rotation ofactuator arm assembly 120. - As shown, a portion 501 of
reference spiral 401 extends diagonally acrossrecording surface 221.Reference spiral 401, in conjunction with other reference spirals onrecording surface 221, is configured to provide position and timing information that enable the internal servo system ofHDD 100 to perform a SSW process, thereby writingservo wedges 300 onrecording surface 221. Also shown are the position of read/write head 221A and atarget path 502 that indicates an ideal location forreference spiral 401 for optimal operation ofHDD 100. When the actual location ofreference spiral 401 is within a maximum acceptable offset 503 oftarget path 502, the servo system ofHDD 100 can writeservo wedges 300 ontorecording surface 221 with the necessary precision for proper operation ofHDD 100. However, whenreference spiral 401, as written, falls outside of maximum acceptable offset 503,servo wedges 300 may not be written accurately, and issues affecting operation of HDD, such as track squeeze, may result. - In the embodiment illustrated in
FIG. 5 , maximum acceptable offset 503 is depicted as an offset distance ofreference spiral 401 fromtarget path 502 that is measured perpendicular to targetpath 502. In other embodiments, maximum acceptable offset 503 is a radial offsetdistance 504 ofreference spiral 401 fromtarget path 502. In yet other embodiments, maximum acceptable offset 503 is a circumferential offset distance ofreference spiral 401 from target path 502 (not shown). -
Writing reference spiral 401 onrecording surface 221 involves precisely controlling the radial position of read/write head 221A as read/write head 221A is moved continuously across the stroke ofactuator arm assembly 120, for example from ID to OD ofrecording disk 220 or vice versa. That is, read/write head 221A moves continuously in the radial direction while a different read/write head ofHDD 100 servos off theservo wedges 300 on a different recording surface ofHDD 100, such asrecording surface 211. However, conventional techniques have proven inadequate for providing sufficiently precise control when writing with read/write head 221A, as illustrated inFIG. 6 . -
FIG. 6 is agraph 600 showing PES during a seek by read/write head 221A while the servo system ofHDD 100 attempts to move read/write head 221A alongtarget path 502 forreference spiral 401. The servo samples 600 (x-axis) are taken as read/write head 221A is moved acrossrecording surface 221A from the ID ofstorage disk 220 to the OD ofstorage disk 220, where the PES is an offset distance betweentarget path 502 and an actual path of read/write head 221A. For example, the PES shown ingraph 600 may be a radial offsetdistance 504 betweentarget path 502 and the actual path of read/write head 221A, as measured viaservo wedges 300 onrecording surface 211. As shown, the PES for read/write head 221A reaches values as high as 40% of servo track width, which is highly undesirable forreference spiral 401. - In positioning a read/
write head 127 over a particulardata storage track 320 ofrecording surface 112, a control signal for positioning read/write head 127 is based on a position feedback signal, i.e., the currently measured PES, and a feed-forward signal, i.e., a sinusoidal compensation signal generated by an RRO compensation algorithm. The sinusoidal compensation signal compensates for RRO as read/write head 127 servos on a particulardata storage track 320. However, as shown inFIG. 6 , as read/write head 127 is moved radially acrossrecording surface 112 and crosses a plurality of data storage tracks 320, the servo system ofHDD 100 does not accurately compensate for RRO variation when employing the above-described sinusoidal compensation signals for the data storage tracks being crossed. Thus, when read/write head 127 continuously seeks acrossrecording surface 112, for example when writingreference spiral 401, PES can have an undesirable magnitude. - According to various embodiments, an iterative learning control (ILC) algorithm is employed to determine appropriate feed-forward coefficients for an RRO compensation signal for each of a plurality of radial locations across the disk surface. More specifically, the feed-forward coefficients are for an RRO compensation signal that compensates for RRO as read/
write head 127 is moved continuously acrossrecording surface 112 in a radial direction to follow a target path. By contrast, in conventional techniques, a repetitive control algorithm determines appropriate feed-forward coefficients for an RRO compensation signal for a single specificdata storage track 320. That is, according to conventional techniques, feed-forward coefficients for an RRO compensation signal are determined for controlling the radial position of read/write head 127 as read/write head 127 servos on a singledata storage track 320. -
FIG. 7 is a simplified control system block diagram illustrating acontrol system 700, according to some embodiments.Control system 700 includes atarget position generator 710, acontroller 720, SIN/COS coefficient adapter 730, an RRO ILC coefficient adapter 740, aCOS generator 751, aSIN generator 752, and anactuator 760, as shown.Control system 700 further includes various summers, multipliers, and feedback and feed-forward signals that interact with the above elements ofcontrol system 700 as shown.Control system 700 as a whole, or each of the elements ofcontrol system 700 set forth above, may be implemented as any suitable processor or logic circuit, such as a central processing unit (CPU), an application-specific integrated circuit (ASIC), a field programmable gate array (FPGA), any other type of processing unit, or a combination of different processing units. Thus,control system 700, or each of the elements ofcontrol system 700 set forth above, may be any technically feasible hardware unit capable of processing input signals or other data and/or executing software applications to facilitate operation ofHDD 100 as described herein. -
Target position generator 710 provides the current target (radial) position r for a particular read/write head 127, andcontroller 720 receives an error signal, i.e.,PES 701, and generates anactuator input signal 702 for controlling the position of read/write head 127. SIN/COS coefficient adapter 730 is configured to receivePES 701 and generate gain/phase or sin/cos compensation coefficients for anRRO compensation signal 703 that modifiesactuator input signal 702. Specifically, SIN/COS coefficient adapter 730 is configured to implement a repetitive control algorithm that continuously adjusts the gain/phase or sin/cos coefficients forRRO compensation signal 703 while the servo system forHDD 100 follows a particulardata storage track 320. -
COS generator 751 generates a disk-synchronoussinusoidal output signal 753, for example cos(t*2*π/T), at a desired harmonic of disk rotational frequency.Sinusoidal output signal 753 is multiplied by a cosine amplitude coefficient GCOS provided by SIN/COS coefficient adapter 730. Similarly,SIN generator 752 generates a disk-synchronoussinusoidal output signal 754, for example sin(t*2*π/T), at the desired harmonic of disk rotational frequency.Sinusoidal output signal 754 is multiplied by a sine amplitude coefficient GSIN provided by SIN/COS coefficient adapter 730. Cosine amplitude coefficient GCOS and sine amplitude coefficient GSIN are selected by SIN/COS coefficient adapter 730 such thatRRO compensation signal 703 forces actuator 760 to follow the desired harmonic with high accuracy.Actuator 760 representsactuator arm assembly 120 andvoice coil motor 128 ofHDD 100 inFIG. 1 . As such,actuator 760 is configured to receive a modifiedinput signal 704 to move read/write head 127 from a current radial location to a target radial position p. - It is noted that
control system 700 includes acontrol system 799 that may be employed in a conventional HDD. As shown,control system 799 may include elements equivalent to targetposition generator 710,controller 720, SIN/COS coefficient adapter 730,COS generator 751,SIN generator 752,actuator 760, and summers and multipliers associated therewith. Thus, in typical operation,control system 700 employscontrol system 799, i.e., SIN/COS coefficient adapter 730,COS generator 751,SIN generator 752, andactuator 760, to servo read/write head 127 over a particulardata storage track 320. Cosine amplitude coefficient GCOS and sine amplitude coefficient GSIN modifyactuator input signal 702 so that RRO associated with the particulardata storage track 320 is reduced or eliminated. - According to embodiments,
control system 700 includes additional functionality overcontrol system 799. Specifically, RRO ILC coefficient adapter 740 is configured to generate amplitude coefficients for modifying disk-synchronoussinusoidal output signal 753 and disk-synchronoussinusoidal output signal 754 whencontrol system 700 causes read/write head 127 to follow a target path that results in read/write head 127 moving continuously from a first radial position to a second radial position. That is, the amplitude coefficients generated by RRO ILC coefficient adapter 740 are employed when read/write head 127 is controlled to follow a target path that crosses multiple data storage tracks 320, such as when a reference spiral is being written by another read/write head 127 ofHDD 100. Thus, during a seek of read/write head 127 in which the radial position of read/write head 127 follows a specific target path throughout the seek, RRO ILC coefficient adapter 740 modifies the amplitude ofRRO compensation signal 703 by adding amplitude coefficient factors FCOS and FSIN to the outputs of SIN/COS coefficient adapter 730. Alternatively, RRO ILC coefficient adapter 740 modifies the amplitude ofRRO compensation signal 703 by replacing the outputs of SIN/COS coefficient adapter 730 with amplitude coefficient factors FCOS and FSIN. - In some embodiments, to generate accurate values for amplitude coefficient factors FCOS and FSIN for a particular seek operation, such as the writing of a
particular reference spiral 401, the particular seek operation is performed over multiple iterations. In such embodiments, during each iteration of the seek operation, blockRRO COS ILC 741 computes amplitude coefficient FCOS, based on theactuator position signal 705, and blockRRO SIN ILC 742 computes amplitude coefficient FSIN, based onactuator position signal 705. Because amplitude coefficients FCOS and FSIN are feed-forward coefficients, the newly computed values for amplitude coefficients FCOS and FSIN are employed in the next iteration in which the particular seek operation is performed. The ILC algorithm employed in blockRRO COS ILC 741 and blockRRO SIN ILC 742 to compute amplitude coefficients FCOS and FSIN may be any suitable adaptive algorithm known in the art. - In some embodiments, block
RRO COS ILC 741 and blockRRO SIN ILC 742 employ the following exemplary equations to compute new values for amplitude coefficients FCOS and FSIN for each iteration of the seek operation performed: -
F cos(p, k+1)=F cos(p, k)+K*G cos(p+p o , k) -
F sin(p, k+1)=F sin(p, k)+K*G sin(p+p o , k) - In the above equations, k denotes iteration number; p denotes actuator position; Fcos(p, k+1) denotes the table entry to generate Fcos(p) at position p during iteration k+1; Fcos(p, k) denotes the table entry to generate Fcos(p) at position p during iteration k; Fsin(p, k+1) denotes the table entry to generate Fsin(p) at position p during iteration k+1; Fsin(p, k) denotes the table entry to generate Fsin(p) at position p during iteration k; Gcos(p+po, k) denotes the value of Gcos at actuator position p+po during iteration k; Gsin(p+po, k) denotes the value of Gsin at actuator position p+po during iteration k; K denotes a suitable adaptation gain (this is typically a tunable adaptation parameter); and po denotes a suitable position offset (this is typically a tunable adaptation parameter).
- With each iteration of the seek operation, the values of cosine amplitude coefficient GCOS and sine amplitude coefficient GSIN are collected. A suitable adaptation algorithm is then employed in block
RRO COS ILC 741 and blockRRO SIN ILC 742 that adjusts the values of amplitude coefficients FCOS and FSIN such that the variation of cosine amplitude coefficient GCOS and sine amplitude coefficient GSIN are each minimized during subsequent iterations of the seek operation. Thus, amplitude coefficients FCOS and FSIN approach values at which little or no PES is measured at the desired harmonic of disk rotational frequency for which these amplitudes are selected. Consequently, during a later iteration of the seek operation, the path followed by read/write head 127 falls within a maximum acceptable offset. For example, after a number of iterations of the seek operation, a path followed by read/write head 127 while being controlled to followtarget path 502 inFIG. 5 falls within maximum acceptable offset 503. At such time, the iterative process is considered to have converged, and values of amplitude coefficients FCOS and FSIN are frozen, and are no longer modified. In embodiments in which targetpath 502 represents an ideal path forreference spiral 401 onrecording surface 221, read/write head 221A can now accurately writereference spiral 401 using the frozen values of amplitude coefficients FCOS and FSIN while performing the appropriate seek operation. - Generally, with each iteration of the seek operation,
PES 701 is reduced and, consequently, the values of disk-synchronoussinusoidal output signal 753 and disk-synchronoussinusoidal output signal 754 are also reduced or approach zero. Thus, in some embodiments, when the particular seek operation is performed using the frozen values of amplitude coefficients FCOS and FSIN, outputs from SIN/COS coefficient adapter 730 may be disabled. - In some embodiments, amplitude coefficient FCOS and amplitude coefficient FSIN are stored in firmware tables, such as coefficient table 743, for various radial locations. A suitable table lookup algorithm finds the table entry belonging to the current radial actuator position and computes amplitude coefficient FCOS and amplitude coefficient FSIN accordingly. Thus, in such embodiments, the values in coefficient table 743 are iteratively adapted as the multiple seek operations across the stroke are performed. During each adaptation seek operation the values of Gcos and Gsin, are collected, and a suitable adaptation algorithm adjusts the RRO ILC tables such that the variation of Gcos and Gsin are minimized during seeks.
- Entries for any suitable number of radial positions may be included in coefficient table 743. For example, coefficient table 743 may include a value for amplitude coefficient FCOS and amplitude coefficient FSIN for as few as 10 radial positions or as many as 1000 or more radial locations. Generally, the computational and storage costs associated with storing a value for amplitude coefficient FCOS and amplitude coefficient FSIN for each
data storage track 320 outweighs the benefits in more accurately implemented radial seeks. For radial locations for which there is no table entry, interpolation or any other estimating technique may be applied to determine values for amplitude coefficient FCOS and amplitude coefficient FSIN. - It is noted that
control system 700 is configured to generate accurate values for amplitude coefficient FCOS and amplitude coefficient FSIN, so that read/write head 127 can be moved accurately in a seek operation that closely follows a particular target path in the presence of significant RRO. However,control system 700, as shown, is configured to compensate for a RRO associated with a single harmonic ofHDD 100, such as the first harmonic of the rotation speed ofstorage disks 110. To compensate for RRO introduced by additional harmonics ofHDD 100,control system 700 may be supplemented with an additional RRO compensation system—one for each additional harmonic to be addressed. Each such additional RRO compensation system may include, without limitation, a SIN/COS coefficient adapter, RRO ILC Coefficient, COS generator, and SIN generator, all configured for the harmonic being addressed. -
FIG. 8 sets forth a flowchart of method steps for controlling a read head in a magnetic storage device, according to an embodiment. Although the method steps are described in conjunction withHDD 100 inFIGS. 1, 2, and 7 , persons skilled in the art will understand that the method steps may be performed with other types of systems. The control algorithms for the method steps may reside in microprocessor-basedcontroller 133, or, in some embodiments, an external host device that is temporarily coupled toHDD 100 and used to facilitate the calibration ofHDD 100. For clarity of description, microprocessor-basedcontroller 133 is assumed to perform said control algorithms for the method steps, although other external control devices can potentially be used in such a role. - As shown,
method 800 begins atstep 801, when microprocessor-basedcontroller 133 rotatesstorage disks 110, including a disk withservo wedges 300 formed thereon and a disk with a recording surface that does not have servo wedges formed thereon. - In
step 802, microprocessor-basedcontroller 133 moves a read/write head 127 from a first radial location, such as theID storage disks 110, to a second radial location, such as the OD ofstorage disks 110. Thus, a seek operation is performed across some or all of the stroke ofactuator arm assembly 120. Instep 802, read/write head 127 is moved continuously across the disk withservo wedges 300 formed thereon, and therefore positions read/write head 127 based on timing and position information provided byservo wedges 300. In addition, the positioning of read/write head 127 is based on a feed-forward correction signal, such asRRO compensation signal 703. The values of feed-forward coefficients for the feed-forward correction signal are fetched from coefficient table 743. While moving from the first radial location to the second radial location, read/write head 127 generates a PES based on theservo wedges 300 formed on the disk surface. - In
step 803, microprocessor-basedcontroller 133 measures the PES generated instep 802. Generally, microprocessor-basedcontroller 133 measures PES at multiple radial locations as read/write head 127 is moved continuously across the disk surface on whichservo wedges 300 are formed. The number of radial locations may be on the order of 10, 100, 1000, or more, but generally does not correspond with the number of data storage tracks 300 formed on the disk surface. - In
step 804, microprocessor-basedcontroller 133 determines whether the PES measured at one or more of the multiple radial locations between the first radial location and the second radial location exceeds a predetermined threshold value. For example, the PES measured at a radial location may be equal to or greater than a value indicating that the location of read/write head 127 falls outside maximum acceptable offset 503. In such a case,method 800 proceeds to step 805. If no PES measured duringstep 803 exceeds the predetermined threshold value,method 800 proceeds to step 810. - In
step 805, microprocessor-basedcontroller 133 or SIN/COS coefficient adapter 740 computes new feed-forward coefficients for the feed-forward correction signal employed instep 801 to help position read/write head 127. For example, when the feed-forward correction signal employed instep 801 isRRO compensation signal 703, RRO SIN/COS ILC adapter 740 computes new values for amplitude coefficient factors FCOS and FSIN. - In
step 806, microprocessor-basedcontroller 133 or RRO SIN/COS ILC coefficient adapter 740 stores the new feed-forward coefficients computed instep 805. For example, in some embodiments, the previous values stored in coefficient table 743 for amplitude coefficient factors FCOS and FSIN are replaced with the updated values computed instep 805. Thus, for a subsequent iteration of the seek operation performed instep 802, read/write head 127 is positioned more accurately along the target path for the seek operation. Upon completion ofstep 806,method 800 returns to step 802 for another iteration of the seek operation. - In
step 810, which is performed responsive to microprocessor-basedcontroller 133 determining that no PES measured duringstep 803 exceeds the predetermined threshold value, microprocessor-basedcontroller 133 causes a reference spiral to be written on a different surface than the surface withservo wedges 300. Specifically, read/write head 127 is moved continuously across the disk surface on whichservo wedges 300 are formed while positioning read/write head 127 based on the servo wedges and an updated feed-forward correction signal. The updated feed-forward correction signal, such asRRO compensation signal 703, is based on the most recently updated feed-forward coefficients computed by RRO SIN/COS ILC coefficient adapter 740. Further, while read/write head 127 moves continuously across the disk, microprocessor-basedcontroller 133 causes another read/write head 127 to write a reference spiral on a different surface than the surface withservo wedges 300. - Thus, by performing multiple iterations of a particular seek operation, feed-forward coefficients for the seek operation can be computed by an ILC, so that the seek operation can be precisely controlled, for example while writing a reference spiral.
- In sum, embodiments described herein provide systems and methods for accurately compensating for repeatable runout (RRO) while moving a magnetic head across a disk surface. An iterative learning control algorithm is employed to determine appropriate feed-forward coefficients for an RRO compensation signal for each of a plurality of radial locations across the disk surface. The feed-forward coefficients are determined by performing multiple iterations of continuously moving the magnetic head across the disk surface along a target path while measuring a position error signal that indicates the radial error between the magnetic head and the target path. With each iteration, the iterative learning control algorithm computes new feed-forward coefficients for each of the plurality of radial locations, the new feed-forward coefficients being selected to reduce the measured position error signal when employed to move the magnetic head along the target path.
- While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.
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US11862196B1 (en) * | 2022-08-01 | 2024-01-02 | Kabushiki Kaisha Toshiba | Split-actuator drive that coordinates fractional-wedge timing of aggressor and victim for effective victim feedforward |
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US8537486B2 (en) * | 2011-08-10 | 2013-09-17 | Western Digital Technologies, Inc. | Disk drive writing spiral tracks on a slave surface using repeatable runout compensation for a master surface |
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