A servo control apparatus and method controls systems at least partially on the basis of an observable variable that has an absolute value functional relationship with the controlled variable and does not change sign for positive and negative variations from a nominal value. When applied to the positional control of an object, the control system observes a value of a position error signal and maps that signal to two different potentially correct displacement values. Two estimators within the control system are initiated, one using the positive displacement and the other using the negative displacement, and the two estimators each predict the future position of the object and the corresponding position error signal for each estimated position. A new position error signal is detected and compared to the two estimated position error signals. After sufficient system evolution, the control system can select one or the other of the estimators as being correct and the associated displacement is identified as correct and is used for future positioning applications, preferably until the sign of the displacement of the head again becomes ambiguous. The control system can be used in combination with other control mechanisms including those using complimentary control information that provides more complete positioning information. The control method, system and apparatus find particularly advantageous application in magnetic storage hard disk drive systems.
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13. A method of estimating the value of an error signal in a servo control system, comprising the steps of:
determining an error rate of the servo control system;
sampling the error rate at a sampling frequency;
using a nonlinear map to determine scalar position error values corresponding to each sampled error rate;
generating positive and negative estimates of the absolute values of the scalar position error values; and
using nonlinear logic to generate an estimate of an actual error signal based on the positive and negative estimates.
9. A servo control system comprising the combination of:
a signal source providing an error rate function f(y) corresponding to a displacement y, the displacement y characteristic of a system to be controlled
a sampling circuit for sampling the value of f(y) at a sampling frequency of Xf to produce a function f(y)j;
a map responsive to f(y)j to produce a positive value yj+ and a negative value yj−;
a first estimator responsive to yj+ provide an estimated value ŷ1j;
a second estimator responsive to yj− to provide an estimated value ŷ2j; and
nonlinear logic responsive to ŷ1j and ŷ2j to generate an estimate ŷk of the displacement.
0. 32. A method for controlling a system, comprising:
determining a first state value indicative of a current state of said system;
estimating a first predicted value predicative of a future state of said system based at least in part upon a first possible value corresponding to said first state value;
estimating a second predicated value predictive of a future state of said system based at least in part upon a second possible value corresponding to said first state value;
determining a second state value corresponding to a subsequent state of said system;
identifying whether said first predicted value or said second predicted value more accurately corresponds to said second state value.
0. 25. A control system, comprising:
a first circuit configured to determine a first state value indicative of a current state of said control system, said first circuit further configured to determine a second state value indicative of a subsequent state of said control system;
a first estimator configured to estimate a first predicted value predicative of a future state of said control system, wherein said first predicted value is based at least in part upon a first possible value corresponding to said first state value;
a second estimator configured to estimate a second predicted value predictive of a future state of said control system, wherein said second predicted value is based at least in part upon a second possible value corresponding to said first state value;
a comparison circuit configured to determine whether said first predicted value or said second predicted value more accurately corresponds to said second state value.
0. 37. A disk drive, comprising:
a disk configured to store data;
control logic configured to control said disk drive, said control logic including:
a first circuit configured to determine a first state value indicative of a current state of said disk drive, said first circuit further configured to determine a second state value indicative of a subsequent state of said disk drive;
a first estimator configured to estimate a first predicted value predictive of a future state of said disk drive, wherein said first predicted value is based at least in part upon a first possible value corresponding to said first state value;
a second estimator configured to estimate a second predicted value predictive of a future state of said disk drive, wherein said second predicted value is based at least in part upon a second possible value corresponding to said first state value;
a comparison circuit configured to determine whether said first predicted value or said second predicted value more accurately corresponds to said second state value.
0. 45. An integrated circuit, comprising:
a first circuit configured to determine a first state value indicative of a current state of a storage device, said first circuit further configured to determine a second state value indicative of a subsequent state of said storage device;
a first estimator configured to estimate a first predicted value predicative of a future state of said storage device, wherein said first predicted value is based at least in part upon a first possible value corresponding to said first state value;
a second estimator configured to estimate a second predicted value predictive of a future state of said storage device, wherein said second predicted value is based at least in part upon a second possible value corresponding to said first state value;
a comparison circuit configured to determine whether said first predicted value or said second predicted value more accurately corresponds to said second state value;
wherein said integrated circuit is configured to provide control signals to said storage device.
23. A servo system for positioning a magnetic head relative to a track which is movable relative to the head, the track having a succession of burst of servo signals therealong and data signals between the bursts of servo signals, comprising the combination of:
means responsive to the passage of the bursts of servo signals at the magnetic head for generating a first set of error signals;
means responsive to the passage of the data signals at the magnetic head for generating a second set of error signals; and
means responsive to the first and second sets of error signals for applying the position error signals from the first and second sets to correct the position of the magnetic head relative to the track,
wherein the means responsive to the passage of the data signals at the magnetic head generates each of the second set of error signals by generating a possible error signal during each of a succession of samplings of the data track and observing any changes in sign and absolute value of the possible error signal during the succession of samplings.
18. A servo system for positioning a magnetic head relative to a track while is movable relative to the head, the track having a succession of bursts of servo signals therealong and data signals between the bursts of servo signals, comprising the combination of:
means responsive to the passage of the bursts of servo signals at the magnetic head for generating a first set of error signals;
means responsive to the passage of the data signals at the magnetic head for generating a second set of error signals; and
means responsive to the first and second sets of error signals for apply the position error signals from the first and second sets to correct the position of the magnetic head relative to the track,
wherein the means responsive to the passage of the data signals at the magnetic head generates at least some of the second set of error signals by producing a pair of possible position error signal values in response to each sampling of the data track and processing the pair of possible position error signals values to choose one that best estimates position error of the magnetic head relative to the track.
0. 52. A computer system, comprising:
a storage sub-system including one or more disks configured to store data;
control logic configured to control said storage sub-system, said control logic including:
a first circuit configured to determine a first state value indicative of a current state of said storage sub-system, said first circuit further configured to determine a second state value indicative of a subsequent state of said storage sub-system;
a first estimator configured to estimate a first predicated value predictive of a future state of said storage sub-system, wherein said first predicted value is based at least in part upon a first possible value corresponding to said first state value;
a second estimator configured to estimate a second predicted value predicative of a future state of said storage sub-system, wherein said second predicted value is based at least in part upon a second possible value corresponding to said first state value;
a comparison circuit configured to determine whether said first predicted value or said second predicted value more accurately corresponds to said second state value.
5. A control system for a controlled variable representative of a system, wherein the system is characterized by an observable variable and wherein at least two values of the controlled variable correspond to a single value of the observable variable, the control system comprising:
a signal source providing first and second values of the observable variable, the second value of the observable variable subsequent in time to the first value of the observable variable;
mapping logic, the mapping logic receiving the first value of the observable variable and outputting a first and a second possible value of the controlled variable;
a first estimator capable of estimating a future state of the system to be controlled, the first estimator receiving the first possible value of the controlled variable and producing a first predicted value of the observable variable responsive to the first possible value of the controlled variable;
a second estimator capable of estimating a future state of the system to be controlled, the second estimator receiving the second possible value of the controlled variable and producing a second predicted value of the observable variable responsive to the second possible value of the controlled variable; and
determining logic determining which of the first and second estimators more accurately predicts the second value of the observable variable.
1. A control system for use in adjusting a controlled variable representative of a system to be controlled, wherein the controlled variable is represented within an observable variable and at least two values of the controlled variable correspond to a single value of the observable variable, the control system comprising:
a signal source providing first and second values of the observable variable, the second value of the observable variable subsequent in time to the first value of the observable variable;
mapping logic, the mapping logic receiving the first value of the observable variable and outputting a first and a second possible value of the controlled variable;
a first estimator capable of estimating a future state of the system to be controlled, the first estimator taking as an input the first possible value of the controlled variable and producing a first output variable representative of a first predicted value of the observable variable responsive to the first possible value of the controlled variable;
a second estimator capable of estimating a future state of the system to be controlled, the second estimator taking as an input the second possible value of the controlled variable and producing a second output variable representative of a second predicted value of the observable variable responsive to the second possible value of the controlled variable; and
determining logic determining which of the first and second predicted values of the observable variable more accurately corresponds to the second value of the observable variable.
0. 24. A control system for use in adjusting a controlled variable representative of a system to be controlled, wherein the controlled variable is represented within an observable variable and at least two values of the controlled variable correspond to a single value of the observable variable, the control system comprising:
mapping logic, the mapping logic receiving a first value of the observable variable and outputting a first and a second possible value of the controlled variable;
first logic configured for estimating, the first logic capable of estimating a future state of the system to be controlled, the first estimator taking as an input the first possible value of the controlled variable and producing a first output variable representative of a first predicted value of the observable variable responsive to the first possible value of the controlled variable;
second logic configured for estimating, the second logic capable of estimating a future state of the system to be controlled, the second estimator taking as an input the second possible value of the controlled variable and producing a second output variable representative of a second predicted value of the observable variable responsive to the second positive value of the controlled variable; and
determining logic configured to determine which of the first and second predicted values of the observable variable more accurately corresponds to a second value of the observable variable, wherein the second value of the observable variable is subsequent in time to the first value of the observable variable.
2. The control system of
3. The control system of
4. The control system of
6. The control system of
7. The control system of
8. The control system of
10. A servo control system in accordance with
11. A servo control system in accordance with
12. A servo control system in accordance with
14. A method in accordance with
15. A method in accordance with
16. A method in accordance with
17. A method in accordance with
19. A servo system in accordance with
20. A servo system in accordance with
21. A servo system in accordance with
22. A servo system in accordance with
0. 26. The control system of
0. 27. The control system of
0. 28. The control system of 25, further comprising mapping logic configured to generate said first possible value and said second possible value based at least in part on said first state value.
0. 29. The control system of
0. 30. The control system of
0. 31. The control system of
0. 33. The method of
0. 34. The method of
0. 35. The method of
0. 36. The method of
0. 38. The disk drive of
0. 39. The disk drive of
0. 40. The disk drive of
0. 41. The disk drive of
0. 42. The disk drive of
0. 43. The disk drive of
0. 44. The disk drive of
0. 46. The integrated circuit of
0. 47. The integrated circuit of
0. 48. The integrated circuit of 45, further comprising mapping logic configured to generate said first possible value and said second possible value based at least in part on said first state value.
0. 49. The integrated circuit of
0. 50. The integrated circuit of
0. 51. The integrated circuit of
0. 53. The computer system of
0. 54. The computer system of
0. 55. The computer system of
0. 56. The computer system of
0. 57. The computer system of
0. 58. The computer system of
0. 59. The computer system of
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1. Field of the Invention
The present invention relates to control systems and methods and more particularly to control systems and methods useful where a system characteristic observed for control purposes can take values of the same sign and magnitude for both positive and negative variations of a variable that is adjusted to control the system. While aspects of this invention are believed to relate very generally to many different control systems and methods, aspects of the invention find their most immediate application to the positional control of detectors used in data acquisition systems. Specific embodiments of the invention are useful in servo mechanical control systems such as high density magnetic disk drives from which data are read by precisely positioning a magnetic read element adjacent to a set of predefined data storage locations.
2. Discussion of the Related Art
The related art is illustrated with reference to several simple control systems. A common task assigned to control systems is maintaining the relative position of one object with respect to another object, where both objects may be moving in an unpredictable manner. One useful system to consider is an optical disk player, schematically illustrated in
It is not typically practical to measure the distance between the lens 3 and the data storage system, so disk players indirectly observe this distance. For example, light reflected from the data storage surface may pass through a beam splitter 4, be collected and refocused by lens 5 and directed to an optical detector 6. The optical detector 6 is divided into four quadrants, as illustrated in FIG. 2. The system is designed so that, when the distance between the lens 3 and the data storage surface 2 is equal to the nominal “in focus” distance, the light incident on the detector 6 is in focus and has an intensity distribution that varies symmetrically on the surface of the detector. Such a symmetric, “in focus” state is illustrated in FIG. 3. Each quadrant of the detector provides a separate output voltage VA, VB, VC, VD, so the symmetric state of
Asymmetry is introduced into the laser light used to read the data storage surface 2 so that too short of a separation between the lens 3 and the data storage surface 2 produces an asymmetric, out of focus pattern on the detector 6 like that illustrated in FIG. 2. This asymmetry is characterized by high intensity light on quadrants B and C of detector 6 and low intensity light on quadrants A and D. Too long a separation between the lens 3 and the data storage surface 2 produces the out of focus pattern shown in
In the
While it is possible to control the system of
Variations of the system of
Systems like that indicated in
Dither has a variety of drawbacks. It is complex, requiring introduction of a detectable amount of motion between objects. Moreover, dither is itself a noise source, and so is generally undesirable. Consequently, it is more common to design a system to have an observable variable that provides both a magnitude and direction or sign for a correction than it is to use a system like that illustrated in
The above example relates to the control of the optics of an optical disk player and illustrates the difficulty of trying to control such a system using an absolute value observable variable. In this sense, an absolute value observable variable is one that varies with the variable to be controlled, but only in magnitude as illustrated by the graph of
Magnetic disk drive data storage systems provide high volume, long term data storage that is comparatively fast and relatively inexpensive, at least as measured on a per-bit basis. For example, magnetic disk storage is faster than present optical storage options and is comparatively less expensive than present flash memory based storage devices. Industry today relies on magnetic disk drives for long term data storage in various types of computer systems and in certain consumer electronics applications such as video recording and playback, and both types of uses continue to grow. Research into magnetic storage disk systems continues and the performance of such systems is expected to continue to improve.
Rotational storage devices and in particular disk drives store data on one or more faces of a rotating media, often referred to as platters or disks. In the case of a conventional hard disk drive, data are stored by generating a magnetic modulation within a magnetic material coated on a data storage surface of the disk. Data are read back by subsequently detecting this modulation with a read head. Typically data are written to a disk using a write element and data are read from the disk using a read element, where both the write and read elements are provided as physically distinct elements on a single head. By recording data in the form of magnetic signals on the rotating disk, data can both be stored and subsequently recovered after even long periods of time. Data may be organized into a plurality of radially displaced, tangentially extending tracks, with the data stored on the tracks generally organized into a plurality of data blocks. To read or write data from or to a particular data block, the typical disk drive positions the read and write head over the track containing the target data block in what is known as a seek operation. The read and write head of the disk drive then reads or writes the data on the storage surface, as desired. Data read and write operations, seek operations and other operations such as using Grey codes to identify track positions are described in U.S. Pat. Nos. 5,523,902, 5,796, 543, and 5,847,894, each of which is hereby incorporated by reference.
The read and write head 22 is a small assembly provided on the end of an arm or transducer assembly 24 that moves the head 22 over the storage surface 10. The transducer assembly may move the head 22 by rotation, by translation or by a combination of rotations and translations. For example, many present drives provide larger movements by rotating the transducer assembly about a pivot on the end of the transducer assembly opposite that of the head 22. Additional adjustments may be accomplished using fine translations, which might be accomplished, for example, using piezo-electric elements. In general, the mechanical rotational and translational movements of the head 22 are preferably accomplished under servo control using, for example, voice coil motors or other compact, fast response systems. The read and write head 22 of the transducer assembly is typically not rigidly attached to the transducer assembly. Rather, the read and write head is preferably mounted on a slider coupled to the transducer assembly through a flexible assembly. Typically the slider is designed to “fly” on an air bearing over the data storage surface created between the shaped undercarriage of the slider and the disk.
A read and write head 22 associated with a storage surface is precisely positioned with respect to data storage locations along a track through the use of servo control mechanisms within the disk drive that operate in conjunction with positional servo information stored on the storage surface of the disk drive. Various servo schemes have been used historically for magnetic storage disk drives, with the industry presently preferring the use of servo information included on each data storage surface on the disks within the disk drive. In reliably performing a track seeking operation, the disk drive uses the read element 28 of the head to detect servo position information that is used by control circuitry to position the transducer assembly and the head over the target track. The servo position information identifies the position of each track and provides at least a relative identification for each of the tracks on the disk drive.
Positional control or servo information most often is stored within radially extending sector servo wedges, described in greater detail in the above-referenced patents, precisely placed on the disk's data storage surface during the original manufacture of the disk storage device. The positional and other servo information may be written with a servo writer like that described in U.S. Pat. No. 4,920,442 or in accordance with the methods described therein. Servo writers are used in a factory initialization process to write positional and other servo information on the storage surfaces of the disks, along with other information to prepare the storage surface for use. The servo writer, typically using precise positional information provided by a laser positioning mechanism, most often places servo information on each track along predefined radial spokes, defining the beginning of each sector on the disk.
Regardless of whether zones are differentiated on the data storage surface, the servo wedges may includes a significant amount of information useful for positioning the head and for reading and writing data to the disk. An illustration of the information that may be included in the servo wedge is provided, for example, in previously incorporated by reference U.S. Pat. No. 5,796,543 and is reproduced in
The servo preamble 42 provides information used to adjust the read channel electronics for reading and processing the positional servo information. The servo position portion 44 of the servo wedge provides the actual position data to be read by the read element 28 and used for positioning the head 22. The illustrated servo preamble 42 begins with a pre-burst gap 48 in which no transitions are recorded followed by an automatic gain control (AGC) field 50 that might include a regular pattern of transitions (e.g., a position 3T pattern followed by a negative 3T pattern) used to adjust the gain of the read channel electronics. The servo preamble next includes a sync pattern 52 for setting the clock in the read channel electronics when reading the servo positional information, which may be followed by a servo address mark 54 that indicates to the read channel electronics that the subsequent information will be servo positional information, as opposed to data. Next the servo preamble 42 may include an index field 56 that provides positional information within the track, i.e., whether the servo wedge is that designated as the first servo wedge on the track.
After the servo preamble 42 is the servo position information 44, including coarse position information 58 and fine position information 60-66. The coarse position information 58 may, for example, comprise Grey codes the numerically designate each of the tracks on the storage surface. Generally, a gap separates the coarse position information 58 and the finer track positioning information provided by servo bursts 60-66. The checkerboard pattern 60-66 of offset servo bursts A, B, C, D of recorded information are written to have precise and desired positions with respect to the centerlines of different tracks within a predetermined grouping of tracks. This allows the read element to generate a control signal related to the linear offset with respect to a desired position relative to a track, such as the track centerline, which control signal can be used to adjust the position of the head with respect to the track.
The illustrated checkerboard pattern consisting of the A, B, C, D servo bursts is formed by a servo writer using multiple write and erase passes during manufacture so that each of the servo wedges includes the illustrated pattern of four rectangular servo bursts repeated at desired radial and tangential positions. The servo bursts A, B, C, D might internally consist, for example, of a repeating 3T pattern, with the servo bursts surrounded by regions without recorded transitions. In the normal operation of the disk drive, the boundaries of the servo busts are detected in track seek and track following operations to periodically generate a position error signal (PES) that can be used to adjust the position of a head with respect to a data track. In between the servo bursts, multiple (typically 3-5) data blocks are stored along the track. The servo control mechanism works in cooperation with the buried servo information to place the head accurately at a desired position with respect to the track as the servo burst passes beneath the head. No additional positioning information is available until the next servo wedge passes by the head. Accordingly, the servo control mechanism attempts to hold the head in a fixed position with respect to the track position identified by the most recent servo burst. It is possible for the head or the disk to move due to mechanical impacts, vibrations, thermal variances or other disturbances in the system before reading the next servo burst.
In addition to the track identification information within the servo wedge, storage surfaces are sometimes provided with additional information to indicate when the desired track and sector has been located in a seek operation. An ID header block may optionally be provided between the servo burst and the first data block of a sector. The ID header primarily includes identification for the track and the following sector. Aspects of the use of synchronization patterns and headers are described, for example, in U.S. Pat. Nos. 5,541,783 and 5,796,534, which patents are hereby incorporated by reference. While header information may be provided at the start of sectors in many systems, other techniques for identifying tracks that do not use headers are known. For example, ID and header information can be included within the servo bursts as described in the article by Finch, et al., “Headerless Disk Formatting: Making Room for More Data,” Data Storage (April 1997), pp. 51-54, or servo information can cross-reference information stored in a corresponding table in memory as described in the IBM Storage publication by Hetzler, “No-ID Sector Format,” dated Jan. 8, 1996.
Following a servo wedge 40 (and ID header block when used), multiple blocks of data (typically 3-5) can be stored along a track, as shown in FIG. 10. Each block of data 70, 72, 74 includes a data synchronization pattern 76, 78, 80 positioned adjacent the data storage region of the block. Typically, a data block 70, 72, 74 is followed by an ECC block 82, 84 that stores error identifying and correcting codes for the preceding data block. The data storage region of each data block is typically of sufficient size to store data signals to represent 512 bytes of data.
The data synchronization pattern includes synchronization information that can be extracted to establish a sampling frequency and phase for recovering data from a data storage region. The conventional synchronization pattern 76, 78, 80 is written by the write element of a head in an operation in which the associated data block 70, 72, 74 is written. The clock rate used to write the synchronization pattern is also used to write the subsequent data blocks. During a subsequent read operation, the read element of a head passes over the synchronization pattern and detects a pattern of transitions (e.g., a 2T or 3T pattern) from which a clock is derived for reading the subsequent data blocks. Conventionally it is preferred that the synchronization pattern be substantially uniform in the radial direction, varying only in the tangential direction for a read element positioned in a desired manner with respect to the track. Typically, the disk control logic and the actual rotational speed of the disk determine the data rate written for the data synchronization pattern and the data storage region that follows. Accordingly, the actual data rate can vary from block to block and sector to sector and, consequently, the amount of space occupied by the stored data can change. To accommodate these changes, there is typically a gap (an interblcok gap) or data pad 86, 88 following each data block to insure that there is sufficient physical separation between successive blocks along a track to allow data blocks to be written without overwriting a subsequent block header or trailing servo burst.
There is a tension in designing data storage systems between increasing the track density, which typically requires denser servo wedge patterns, and loss of storage area due to the provision of increased densities servo information. It is desirable to provide additional servo information without reducing the area on the data storage surface devoted to the actual storage of data. In other words, it is desirable to increase the storage density without increasing the overhead necessary for accurately storing and retrieving information.
In contrast to the well-written track 90, the badly-written track 94 is subject to poor positional control and mechanical disturbance. The path of the badly-written track varies within the track pitch in an irregular manner. It is possible for such an irregular write path to be sufficiently misaligned that the read width is partially off of the written area, as indicated at 94. Such a misalignment reduces the quality of the read out data and can lead to read errors. To avoid this problem, it is conventional to increase the size of the write width relative to the read width. A different error, indicated at 96 in
Each of the potential errors illustrated in
The preceding discussion has set forth aspects of the servo and other informational structures provided on magnetic disk drives and how this information is used. With this background established, the discussion now returns to control systems for magnetic storage disk drives and how different observable variables are used in such systems. Commonly, the position error signals (PES) that are observed for controlling the position of the read and write head with respect to the track position are derived dedicated servo patterns. In particular, fine positional control is effected by deriving PES from the A, B, C, D servo burst patterns illustrated in
U.S. Pat. No. 5,233,487 to Christensen, et al., relates to magnetic disk drives and the control of head position with respect to a track using an absolute value observable variable for the control system. The Christensen patent describes a disk drive that periodically interrupts data reading and writing operations to perform a calibration of the optimal head offset with respect to a data track for data writing operations. The calibration proceeds by translating a read head across the track through a range of intentional mislaingments with respect to the track. Data are read in the deliberately misaligned positions and the errors that occur in reading data from the track are obtained as a function of misaligned position. Error rates are derived by detecting blocks of data and then applying the standard data block error correction using the ECC circuitry of the disk drive with reference to the ECC data encoded with the data stored on the disk. The disk drive detects at what track position, and hence what level of intentional misalignment, that the observed error rate exceeds certain target error rates having a known association with known track positions. This allows the characteristics of the operating disk drive to be compared to known standards.
It should be noted that the target error rates (indicated in FIG. 2 of the Christensen patent) are absolute value functions. As such, the target error rates that are derived in the Christensen patent's system do not indicate the direction in which the head is misaligned with respect to the data track. The sign of the misalignment is known by the control system because of the intentional positioning of the read head, much in the same way that dither can be used to generate a known direction for a correction within an absolute value observable control system.
The track positions detected using the Chritensen patent's method can be used to calculate a track center position, recalibrate the relative motion achieved by application of a controlled displacement voltage, and establish an optimal offset for reading data from and writing data to the track. In this way the Christensen patent performs a servo control function using information other than the servo wedge and servo burst information illustrated in
The Christensen patent performs servo functions using an observable variable, a bit error rate derived from ECC operations on data blocks, that provides only absolute value information relating to the positional error. On the other hand, practice of the Christensen patent's method requires use of a head to track displacement that shares many problems with dithering and so cannot be used as data are gathered. It is desirable to provide a method and system capable of using absolute value information to perform a control function.
It is an object of the present invention to provide a method for controlling at least one aspect of a system on the basis of an observable variable that either increases for both positive and negative variations about a nominal or target value or decreases for both positive and negative variations about the nominal or target value. Such an observable variable has the property that a value of the observable variable maps to two or more values of a variable representing the controlled aspect of the system. Such an observable variable is identified here as an absolute value variable and the system an absolute value system.
Aspects of the present invention provide a servo control apparatus and method that can be used to control an absolute value system on the basis of an absolute value observable variable. An observed value of such an “absolute value” variable could correspond to either a positive or negative displacement of the controlled variable from its nominal value. The possibility of mapping the observed value to two different displacements makes it difficult to control the system. This is because the value of the observed variable does not indicate the direction in which the controlled variable should be adjusted to move the controlled variable closer to its nominal value. Aspects of the present invention allow control of systems at least partially using an observable variable that has absolute value characteristics with respect to the controlled variable.
A preferred aspect of a system in accordance with the invention observers a value of the observable variable and the system maps that value to the two possible displacements corresponding to the observed value. Two estimators are prepared, with each estimator designed to predict the present or future state of the system based on a set of input values including the displacement. The first estimator takes one of the possible displacements as an input. The second estimator takes the other of the possible displacements as an input. Both of the estimators are used to predict values of the observed variable after the system has evolved through a time interval. The absolute values of the observable variables predicted by the estimators are compared to one or more newly observed values of the observable variable to determine which estimator embodies the proper assumption as to the sign or direction of the initial displacement. Control functions are preferably performed using the sign and displacement identified in this operation as being correct.
Most preferably, the incorrect estimator is reset to the state of the correct estimator after the sign or direction of the displacement has been predicted.
When applied to a position control system for an object, a preferred embodiment of the control system observes a value of a position error signal and maps that signal to two potentially correct displacement values. Two estimators within the control system are initiated, one using one of the potentially correct displacements and the other using the other displacement, and the estimators predict the future object position and the corresponding position error signal for each estimate future position. A new absolute value position error signal is detected and compared to the absolute value of the two estimated position error signals. After sufficient system evolution, the control system can select one or the other of the estimators as being correct. The correct displacement, including its sign, is identified as correct and is used for future positioning operations, preferably until the sign of the displacement of the object again becomes ambiguous.
The control method, system and apparatus find particularly advantageous application in disk drive servo control systems where the object subject to positional control is the head of the disk drive. Most preferably the absolute value observable that is used in controlling the system is a signal derived from the data or data signals read from the storage surface of the disk within the hard disk drive. For example, the absolute value observable might represent error information characteristic of the data or data signals retrieved from the disk. In the alternative, the observable position error signal might be derived in accordance with a constraint characteristic of the data or data signals retrieved from the disk.
An aspect of the present invention relates to a control system for use in adjusting a controlled variable representative of a system to be controlled, where the controlled variable is represented within an observable variable and at least two values of the controlled variable correspond to a single value of the observable variable. The control system comprises a signal source providing first and second values of the observable variable, the second value of the observable variable subsequent in time to the first value of the observable variable. Mapping logic receives the first value of the observable variable and outputs a first and a second possible value of the controlled variable. Two estimators are provided. A first estimator is capable of estimating a future state of the system to be controlled, the first estimator taking as an input the first possible value of the controlled variable and producing a first output variable representative of a first predicted value of the observable variable responsive to the first possible value of the controlled variable. A second estimator is capable of estimating a future state of the system to be controlled, the second estimator taking as an input the second possible value of the controlled variable and producing a second output variable representative of a second predicted value of the observable variable responsive to the second possible value of the controlled variable. Determining logic determines which of the first and second predicted values of the observable variable more accurately corresponds to the second value of the observable variable.
Another aspect of the invention provides a servo control system including a signal source providing an error rate function f(y) corresponding to a displacement y, the displacement y characteristic of a system to be controlled. A sampling circuit samples the value of f(y) at a sampling frequency of Xf to produce a function f(y)j. A map responsive to f(y)j to produce a positive value yj+ and a negative value yj−. A first estimator responsive to yj+ to provide an estimated value ŷ1j. A second estimator responsive to yj− to provide an estimated value ŷ2j. Nonlinear logic responsive to ŷ1j and ŷ2j generates an estimate ŷk of the displacement.
A still further aspect of the invention provides a method of estimating the value of an error signal in a servo control system. The method includes determining an error rate of the servo control system and sampling the error rate at a sampling frequency. A nonlinear map is used to determine scalar position error values corresponding to each sampled error rate. Positive and negative estimates are generated of the absolute values of the scalar position error values. Nonlinear logic generates an estimate of an actual error signal based on the positive and negative estimates.
A servo system for positions a magnetic head relative to a track which is movable relative to the head, the track having a succession of bursts of servo signals therealong and data signals between the bursts of servo signals. The servo system includes means responsive to the passage of the bursts of servo signals at the magnetic head for generating a first set of error signals. Means responsive to the passage of the data signals at the magnetic head for generate a second set of error signals. Means responsive to the first and second sets of error signals apply the position error signals from the first and second sets to correct the position of the magnetic head relative to the track.
These and other aspects of the present invention, along with various attendant benefits related to use and practice of these aspects, may be better understood with reference to the drawings, which form a part of the disclosure.
The present invention provides a control method, system and apparatus capable of controlling an absolute value system using an absolute value observable as an input variable. As used here, an absolute value observable variable is one in which the variable either increases or decreases for both positive and negative variations about a nominal or target value. A sufficiently large value of the absolute value observable variable indicates a displacement to be corrected, but could represent a positive displacement or a negative displacement. Control functions based on the observable variable preferably account for the lack of sign or direction indication for the correction to be made.
A very brief introduction is useful here to provide a working framework for illustration of aspects of the present invention.
Control systems incorporate models, whether implicit or explicit, of how the system being controlled reacts to input data. For example, most control systems for magnetic storage disk drives include an explicit model of the disk drive that predicts how the position of a read head changes in response to application of a certain voltage to an actuator. The disk drive control system determines how to position the head of the disk drive by predicting, i.e., calculating using a model of the disk drive and actuator response, how much voltage should be applied to the actuator to move the head to its desired position. The model uses the state of the disk drive such as the present position of the head as well as aspects of the history of the disk drive and receives input data about the changes to be made to the disk drive. From this information, the model within he control system of the disk drive predicts system behavior.
This discussion references estimators that can predict or estimate the future or present state of a system on the basis of a model of present or future system behavior. It is convenient to view an estimator as estimating or predicting a state of a system on the basis of a model of the system, the history of the system and the inputs to the system, but variations from this framework are possibe. What are referred to in this discussion are, in the art, more precisely known as state estimators in that the estimator predicts various characteristics of a system that, taken together, accurately describe a system. Those of skill in the art of control theory will recognize that the term “observer” is sometimes used interchangeably with the term estimator. The following describes control logic that uses two slightly different estimators. For ease of understanding, it can be assumed that each estimator uses the same model of the system, the same input data representative of observed system behavior and, for the most part, the same prior history of the system. The two estimators each include slightly different presumptions about what has happened and what is happening in the system under control. By comparing the estimates of system behavior represented by these two estimators to the actually observed behavior of the system, the control logic can determine which of the two estimators included the more accurate description of the system. The control system can then adjust system characteristics on the basis of the estimated sign and displacement information.
Aspects of the invention are now illustrated through an example in which the variable to be controlled is the displacement of an object from its nominal target position and the observable variable represents a position error signal. Displacements can be positive or negative with respect to the nominal target position and the position error signal has the same sign for positive and negative displacements. A sufficiently large observed value of the position error signal indicates a displacement to be corrected, but could represent a positive displacement or a negative displacement. In other words, the position error signal has absolute value characteristics as a function of displacement. The position error signal may bear an asymmetric, nonlinear relationship to the displacement or may have a simpler relationship. Thus, any observed position error signal corresponds to two different possible displacements where the two displacements have different signs and might have different magnitudes. Systems with even more complicated mappings are possible and can be processed in a similar manner, typically by using additional estimators.
In accordance with aspects of the present invention, this exemplary system can be controlled on the basis of the observed position error signal using knowledge of the mapping between the position error signal and the displacement, in conjunction with a model of the system. The position error signal is observed at a first time interval. The first position error signal might correspond to a positive displacement +D1 or a negative displacement −D2 of the object with respect to its nominal position, so there is an
y=CX (1)
where X is the state vector, y is the scalar output and u is the scalar input. The matrices A, B, C each has appropriate dimensions with constant coefficients. This model is an approximation of a more complex model of the servo dynamics than is preferred in some instances and is discussed below. A simple model that captures the dominant characteristics of the dynamics is preferred in other instances because a simple model leads to a simpler and computationally less demanding algorithm and control design.
The output y of the servomechanism 140 is an input to a nonlinear map 142 of y. Here, y is the controlled variable displacement, which is not directly observable and the nonlinear map produces the position error signal, which is observable. An example of the map 142 is shown in FIG. 18 and corresponds generally to the error information graphs illustrated in
Block 144 in the arrangement of
sec. The output of the S/H device 144 is
f(y)j=f(jT240 )
which is a sequence of steps with values f(y)j at times t=jT240, j=1,2, . . . In other words, f(y)j=f(y(t)) for jT240≦t<(j+1)T240.
A block 146 opposite the block 144 is a sample and hold (S/H) device that samples and holds the value of y at a sampling frequency of Xs kHz. In the present example, Xs=15 kHz. This means that low frequency, linear servo information component of y is available every
sec. The output of S/H is a sequence of steps with values y
The sample and hold device 144 is coupled to an inverse map 148. The inverse map 148 provides the inverse of the function provided by the nonlinear map 142 and the inverse of the function plotted in FIG. 18. Whereas the nonlinear map 142 provides the error rate f(y) for the scalar value y, the inverse map 148 provides the scalar displacement values yj+ and yj− in response to the error rate f(y)j. More particularly, the inverse map 148 uses the sampled values of f(y)j and the nonlinear mapping to obtain the corresponding sets of possible values of y at time intervals t=jT240. As shown in
yj−=y−(jT240), yj+=y+(jT240).
An output of the inverse map 148, and the output of the sample and hold device 146 are coupled to a first estimator 150 (Estimator 1). As a practical matter, the inverse map 148 and the estimators are implemented as software, although aspects could be implemented in hardware. Estimator 1 is described by the following equations in which the letters A, B, C indicate discrete forms of the matrices discussed above:
Where
Ideally,
Note that if for each j, yj is positive which means yj+ is the value of y that generated f(y)j then Estimator 1 assures that ŷj→yj as j→∞.
Estimator 2, which is block 152 in
where
Estimator 2 assumes that the values of y in the inverse mapping of (y) is negative and uses yj− as the possible correct value of y at t=jT240, with the exception of instants of time where j=16
The inputs to the nonlinear logic block 154 are ŷ1j=ŷ1 (jT240) ŷ2j=ŷ2(jT240),y
In other words, the estimate ŷk=ŷ(kT60) is the operation at the time instant (k=4
The estimated value ŷk of y at t=kT60 is used to design a controller to operate at 60 kHz frequency. The design of an appropriate servo system is conventional and might, for example, be designed in accordance with the general teachings of chapter 14 of Franklin, et al., Digital Control of Dynamic Systems 649-687 (1998), which chapter describes the so-called Workman model and is hereby incorporated by reference. The block diagram of the controller together with the blocks for generating ŷk are shown in FIG. 20. The illustrated control system, including the estimators, nonlinear logic, and controller, is preferably implemented using software, for example within a digital signal processor as is presently conventional in the art.
The arraignments of
sec. The controller 160 is described by the equations
{circumflex over (x)}k+1=A60{circumflex over (x)}k+B60uk+G60[C60 {circumflex over (x)}k−ŷk]
where
The arrangement of
uk=−K60{circumflex over (x)}k
where
In the simulations illustrated in the graphs of
where Y(s), U(s) denote the Laplace transforms of the head position and actuator input, respectively. The parameters of the model are listed in Table 1.
TABLE 1
System Parameters
Parameter
Description
Value
Units
m
Moving mass of actuator
0.2
Kilograms
Kt
Force constant
20
Newton/
amp
kv = kt/m
Normalization constant
200
Ky
Position measurement gain
10000
Volts/M
Kf
Viscous friction constant
2.51
N-sec/M
ω1
Resonance: VCM isolator
2π70
Rad/sec
ω2
Resonance: head suspension
2π2200
Rad/sec
ω3
Resonance: actuator arm carrier
2π4000
Rad/sec
ω4
Resonance: coil structure
2π9000
Rad/sec
B1
First resonance coupling
−0.00575
—
B2
First resonance coupling
0.0000115
sec−1
B3
Second resonance coupling
0.0230
—
B4
Second resonance coupling
0
sec−1
B5
Third resonance coupling
0.8185
—
B6
Third resonance coupling
0
sec−1
B7
Fourth resonance coupling
0.1642
—
B8
Fourth resonance coupling
0.0273
sec−1
ζ1
First resonance damping
0.05
—
ζ2
Second resonance damping
0.005
—
ζ3
Third resonance damping
0.05
—
ζ4
Fourth resonance damping
0.005
—
A careful consideration of the transfer function model given by equation (2) set forth above servo mechanism 140 of
and therefore the hard disk drive dynamics can be approximated by the following transfer function model
which represents the reduced-order dynamics to be used for control design and is also described by equation (2) of the servo mechanism 140. The servo controller is designed based on the reduced-order model but is simulated and analyzed using the full-order system.
As discussed above, the preferred source for position error signals (PES) for positional control of a head with respect to a track in a disk drive is the data within the track itself. More particularly, the preferred PES signals are derived from information about errors characteristic of the data or the data signals that are stored on the disk. Another related, preferred source of PES is derived from constraint information characteristic of the data or data signals in conjunction with the data or data signals themselves. The derivation of this sort of error and constraint information is described in the above-referenced and incorporated Despain application.
The read channel illustrated in
Referring to
The signal output by the variable gain amplifier is analog and has peak voltages more precisely in the desired range. The variable gain amplifier is coupled to an analog equalizer 210. Analog equalizer 210 adjusts the signal level across a range of received frequencies to remove some of the frequency dependent effects of the read head and other parts of the read channel circuitry. After equalization, the signal is provided to the analog to digital converter 212, which operates in accordance with a clock signal provided by the clock recovery circuit 214 to filter the input analog signals. As illustrated in
The output of the analog to digital converter 212 is output to a second equalizer 216 that is used to further equalize the signal. In the illustrated circuit, the second equalization circuit is provided in the form a finite impulse response (FIR) filter. The equalization provided by FIR filter 216 is adjusted according to the detected signal levels to accommodate the data received by the circuit. This adaptive equalization process is accomplished by adjusting the weighting of the FIR filter using information provided by a slicer 218. The slicer 218 generates estimated sample values by comparing individual sample values to expected sample values. For example, the slicer might compare each sample received to values set for logic signals of +1, 0 and −1 and assign to each sample the logic signal that comes closest to matching its value. These estimates are used for adjusting the gain, for adjusting the clock recovery circuitry, and for adjusting the equalization process.
As part of the functions of the slicer 218, the slicer generates a measure of channel quality by comparing each of the values received by the slicer to the value that the slicer assigned to that sample. For example, a difference measurement can be calculated for each sample. A first sample provided to the slicer has a voltage VS. The slicer determines that the voltage VS is closest in value to the voltage V1 that is expected for samples that correspond to the logic value +1. That sample is preliminarily assigned the logic value +1 and the slicer can then generate a error measure for that sample equal to VS−V1. Alternatively, the error measure for that sample might correspond to the absolute value of this difference or the square of this difference. Various statistical analyses might be performed on the collected error measures generated by sequences of samples. These collected error measures conventionally provide the mean squared error (MSN) and the channel quality measure (CQM) within the chip.
The error measure output from the slicer is typically accumulated on the read channel chip to evaluate the performance of the chip or the storage system. In accordance with the present invention, however, the error measure generated by the slicer 218 is provided to one of the output pins or terminals for the chip. This error signal has absolute value qualities and is generated at a rate that can be as high as the data rate. As such, the signal is particularly useful in practicing aspects of the present invention.
Referring again to
The decoder 220 outputs the sequence of best fit data points 224 as the data output from the read channel circuitry. The decoder 220 also generates an error measure for each sample that relates to the difference between the Viterbi decoder assigned value (224) and the actually received value (222). This difference 226 is identified in the graph as gray arrows associated with each of the samples. In normal usage, this error measure 226 might be accumulated within the read channel chip as a measure of system performance or channel quality. Conventionally this error measure is not available while data are being transmitted off the read channel chip.
Preferred embodiments of the
The output error measure signal is available at terminals 230, 232 at rates as high as the data rate and has absolute value characteristics. In fact, the error measure 226 (
It should be appreciated that the error measure provided as an output from the read channel chip need not be available continuously because there may be reasons that the terminal is switched to other uses through multiplexing. Appropriate error measure signals are available from the preferred read channel chip at a higher data rate than conventional servo signals and are available while the data are being read and as data are being output from the chip. In addition, it is preferred that the read channel chip continue to process and output conventional linear servo information in the conventional manner so that it can be available for use in the manner described above where linear servo information is used in combination with absolute value data. Of course, if all servo operations are performed without the linear servo information, there would be no need to provide for processing linear servo information within the read channel chip.
Although the present invention has been described in detail with reference only to the presently preferred embodiments, those of ordinary skill in the art will appreciate that various modifications can be made without departing from the invention. For example, while the disclosure has described servo control systems according to the invention in terms of hard disk drive (HDD) applications, systems according to the invention are applicable to other servo control applications as well. Moreover, the present invention has been described in terms of a hard disk drive in which both of linear servo and absolute value information are used in following a track. This need not be true. For example almost all operations can be performed without using the linear servo operations. It is furthermore conceivable that no conventional servo information would be stored on at least parts of a disk within a disk drive practicing aspects of the present invention. Accordingly, the invention is defined by the following claims and is not to be limited to the particular preferred embodiments described here.
Despain, Alvin M., Ioannou, Petros A., Kosmatopoulos, Elias B.
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