Servo control appartaus and method using absolute value input signals

Abstract

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.

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.  
 
     
     
       2. The control system of  claim 1 , wherein the determining logic compares an absolute value of the first predicted value with the second value of the observable variable and compares an absolute value of the second predicted value with the second value of the observable variable to determine which of the first and second predicted values of the observable variable corresponds to the second value of the observable variable. 
     
     
       3. The control system of  claim 1 , wherein the control system maintains the controlled variable near a nominal value and the nominal value is between the first and second possible values of the controlled variable. 
     
     
       4. The control system of  claim 3 , wherein the control system operates in accordance with discrete time intervals and wherein the first and second estimators receive the first and second possible values of the controlled variable only after the control system determines that the controlled variable will take values on above and below the nominal 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.  
 
     
     
       6. The control system of  claim 5 , wherein the determining logic compares an absolute value of the first predicted value with the second value of the observable variable and compares an absolute value of the second predicted value with the second value of the observable variable to determine which of the first and second predicted values of the observable variable corresponds to the second value of the observable variable. 
     
     
       7. The control system of  claim 6 , wherein the control system maintains the controlled variable near a nominal value and the nominal value is between the first and second possible values of the controlled variable. 
     
     
       8. The control system of  claim 7 , wherein the control system operates in accordance with discrete time intervals and wherein the first and second estimators receive the first and second possible values of the controlled variable only after the control system determines that the controlled variable will take values on above and below the nominal value. 
     
     
       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 X f  to produce a function f(y) j ;  
 a map responsive to f(y) j  to produce a positive value y j   +  and a negative value y j   − ;  
 a first estimator responsive to y j   +  provide an estimated value ŷ 1j ;  
 a second estimator responsive to y j   −  to provide an estimated value ŷ 2j ; and  
 nonlinear logic responsive to ŷ 1j  and ŷ 2j  to generate an estimate ŷ k  of the displacement.  
 
     
     
       10. A servo control system in accordance with  claim 9 , further comprising a second sampling circuit for sampling a value of a servo mechanism for providing a scalar position error output y at a sampling frequency of X s  to produce a value y   k   , and wherein the first and second estimators respond to the value y   k    as an input thereto when produced by the second sampling circuit. 
     
     
       11. A servo control system in accordance with  claim 10 , wherein the sampling frequency X f  is substantially greater than the sampling frequency X s . 
     
     
       12. A servo control system in accordance with  claim 10 , wherein the servo mechanism comprises a magnetic disk drive having a magnetic head laterally positionable relative to a plurality of concentric tracks on a rotatable disk, the disk having sectors of servo signal interspersed with recorded data and the magnetic head providing a signal from which is derived the error signal f(y). 
     
     
       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.  
 
     
     
       14. A method in accordance with  claim 13 , wherein the step of using nonlinear logic includes the step of assuming that the sign of an absolute value has not changed since the last sampling thereof if the value of the absolute value is larger than a predetermined level. 
     
     
       15. A method in accordance with  claim 13 , wherein the step of using nonlinear logic includes the step of assuming that the sign of an absolute value has not changed since the last sampling thereof if the value of the absolute value is less than or equal to a predetermined level and the next sampling predicted value of the generated estimates has the same sign as the generated estimate of the previous step. 
     
     
       16. A method in accordance with  claim 13 , wherein the step of using nonlinear logic includes the step of determining if the value of an absolute value is less than or equal to a predetermined level when the next sampling predicted value of the value estimate has a sign opposite the sign of the value estimate of the previous sampling, and if so, then estimating the sign of the value by the steps of comparing the absolute value of the positive and negative estimates with the absolute value of the corresponding positive and negative signals values, and assuming that the sign of the absolute value is the sign of the positive and negative estimates whose output magnitude deviates the least from the input magnitude. 
     
     
       17. A method in accordance with  claim 16 , wherein the servo control system includes a magnetic head and means for positioning the magnetic head relative to a track of given width and the predetermined level comprises a predetermined fraction of the given width of the track. 
     
     
       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.  
 
     
     
       19. A servo system in accordance with  claim 18 , wherein the rate of occurrence of the bursts of servo signals at the magnetic head defines a first sampling rate for the first set of position error signals, and the means responsive to the passage of the data at the magnetic head generates a second set of error signals at a second sampling rate which is substantially greater than the first sampling rate. 
     
     
       20. A servo system in accordance with  claim 19 , wherein the means for applying the position error signals from the first and second sets applies the signals at a correction rate which is greater than the first sampling rate and less than the second sampling rate. 
     
     
       21. A servo system in accordance with  claim 20 , wherein the first sampling rate is at 15 kHz, the second sampling rate is at 240 kHz, and the correction rate is at 60 kHz., 
     
     
       22. A servo system in accordance with  claim 18 , wherein each pair of possible position error signal values includes a positive value and a negative value and the processing of the pair of possible position error signal values includes choosing one of the pair of possible position error signal values that appears to best estimate the position error. 
     
     
       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.  
 
     
     
       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.     
     
     
       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.     
     
     
       26. The control system of  claim 25 , wherein said comparison circuit is configured to compare said first predicted value to said second state value and compare said second predicted value to said second state value. 
     
     
       27. The control system of  claim 26 , further comprising control logic configured to make adjustments to said control system based on results of comparing said first predicted value to said second state value and comparing said second predicted value to said second state value. 
     
     
       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. 
     
     
       29. The control system of  claim 25 , wherein said first possible value and said second possible value each have an associated sign and magnitude, and wherein said first possible value and said second possible value have different signs. 
     
     
       30. The control system of  claim 25 , wherein said first state value and said second state value correspond to an observable variable, and wherein the control system maintains a controlled variable corresponding to the observable variable near a nominal value, and wherein the nominal value is between said first possible value and said second possible value. 
     
     
       31. The control system of  claim 30 , wherein said control system operates in accordance with discrete time intervals and wherein the first and second estimators receive the first and second possible values only after the control system determines that the controlled variable will take on values above and below the nominal value. 
     
     
       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.     
     
     
       33. The method of  claim 32 , wherein said identifying includes comparing said first predicted value to said second state value and comparing said second predicted value to said second state value. 
     
     
       34. The method of  claim 32 , wherein said first possible value and said second possible value each have an associated sign and magnitude, and wherein said first possible value and said second possible value have different signs. 
     
     
       35. The method of  claim 32 , further comprising adjusting said system based at least in part on results of said identifying. 
     
     
       36. The method of  claim 32 , further comprising generating said first possible value and said second possible value from said first state value. 
     
     
       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.       
     
     
       38. The disk drive of  claim 37 , further comprising a read/write head configured to access one or more storage locations on said disk, wherein said control logic is configured to control said read/write head. 
     
     
       39. The disk drive of  claim 38 , wherein said comparison circuit is configured to compare said first predicted value to said second state value and compare said second predicted value to said second state value. 
     
     
       40. The disk drive of  claim 39 , wherein said control logic is configured to adjust positioning of said read/write head based on results of comparing said first predicted value to said second state value and comprising said second predicted value to said second state value. 
     
     
       41. The disk drive of  claim 37 , 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. 
     
     
       42. The disk drive of  claim 37 , wherein said first possible value and said second possible value each have an associated sign and magnitude, and wherein said first possible value and said second possible value have different signs. 
     
     
       43. The disk drive of  claim 37 , wherein said first value and said second state value correspond to an observable variable, and wherein the disk drive maintains a controlled variable corresponding to the observable variable near a nominal value, and wherein the nominal value is between said first possible value and said second possible value. 
     
     
       44. The disk drive of  claim 43 , wherein said control logic operates in accordance with discrete time intervals and wherein the first and second estimators receive the first and second possible values only after the control logic determines that the controlled variable will take on values above and below the nominal value. 
     
     
       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.     
     
     
       46. The integrated circuit of  claim 45 , wherein said comparison circuit is configured to compare said first predicted value to said second value and compare said second predicted value to said second state value. 
     
     
       47. The integrated circuit of  claim 46 , wherein said integrated circuit is configured to generate said control signals based on results of comparing said first predicted value to said second state value and comparing said second predicted value to said second state value. 
     
     
       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. 
     
     
       49. The integrated circuit of  claim 45 , wherein said first possible value and said second possible value each have an associated sign and magnitude, and wherein said first possible value and said second possible value have different signs. 
     
     
       50. The integrated circuit of  claim 45 , wherein said first state value and said second state value correspond to an observable variable, and wherein the integrated circuit maintains a controlled variable corresponding to the observable variable near a nominal value, and wherein the nominal value is between said first possible value and said second possible value. 
     
     
       51. The integrated circuit of  claim 50 , wherein said integrated circuit operates in accordance with discrete time intervals and wherein the first and second estimators receive the first and second possible values only after the integrated circuit determines that the controlled variable will take on values above and below the nominal value. 
     
     
       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.       
     
     
       53. The computer system of  claim 52 , further comprising a read/write head, wherein said control logic is configured to control said read/write head, and wherein said one or more disks include magnetic disks. 
     
     
       54. The computer system of  claim 53 , wherein said comparison circuit is configured to compare said first predicted value to said second state value and compare said second predicted value to said second state value. 
     
     
       55. The computer system of  claim 54 , wherein said control logic is configured to adjust positioning of said read/write head based on results of comparing said first predicted value to said second state value and comparing said second predicted value to said second state value. 
     
     
       56. The computer system of  claim 53 , 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. 
     
     
       57. The computer system of  claim 53 , wherein said first possible value and said second possible value each have an associated sign and magnitude, and wherein said first possible value and said second possible value have different signs. 
     
     
       58. The computer system of  claim 53 , wherein said first state value and said second state value correspond to an observable variable, and wherein the computer system maintains a controlled variable corresponding to the observable variable near a nominal value, and wherein the nominal value is between said first possible value and said second possible value. 
     
     
       59. The computer system of  claim 58 , wherein said control logic operates in accordance with discrete time intervals and wherein the first and second estimators receive the first and second possible values only after the control logic determines that the controlled variable will take on values above and below the nominal value.