Early write enable with off-track write protection

In a disk drive system, a servo controller is operative to perform a process of inhibiting write operations for writing data to tracks of a disk during a head settling period following a track seek operation. During each of a first plurality of sampling intervals transpiring during a first time period, the servo controller determines a present position value indicative of the position of the head during the present sampling interval, and also determines a predicted position value indicative of the position of the head during a subsequent sampling interval. Also during each of the first plurality of sampling intervals, the servo controller determines: whether the present position value is within a first error margin from the center of a target track; and whether the predicted position value is within the first error margin. If the present position value and the predicted position value are both within the first error margin, the servo controller enables write operations. During each of a second plurality of the sampling intervals transpiring during a second time period, the servo controller determines a predicted position value indicative of the position of the head during a subsequent sampling interval. Also during each of the second plurality of sampling intervals, the servo controller determines whether the predicted position value is within a second error margin from the center of the target track. If the predicted position value is not within the second error margin, the servo controller inhibits write operations.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to track seeking operations in a disk drive.
 More particularly, the present invention relates to a method of
 determining when to inhibit and when to enable write operations during a
 head settling period following an actuator seek operation in a disk drive.
 2. Description of the Prior Art
 Disk drive systems typically include: a disk having a thin magnetic coating
 upon which user data and position information is stored in the form of
 flux transitions disposed in a series of concentric tracks; a spindle
 assembly, having a spindle motor and an associated driver circuit, for
 supporting and rotating the disk in response to a spindle command signal;
 a read/write head for detecting the flux transitions in the magnetic
 material as the disk is rotated relative to the head, and for generating
 an analog read-back signal carrying data and position information in
 response thereto; a head-arm assembly for supporting and moving the head
 radially over the surface of the disk; and an actuator assembly, usually
 comprising a voice coil motor and an associated driver circuit, for
 driving the head-arm assembly in response to an actuator command signal in
 order to position the head relative to the tracks of the disk. Modern
 drive systems also include a servo system for controlling the position of
 the read/write head relative to the tracks of the disk. Components of the
 servo system typically include: a position error channel for receiving the
 read-back signal provided by the head via an arm electronics unit, and
 operative to demodulate and decode the read-back signal to generate a
 position error sensing (PES) signal; and a servo controller unit
 responsive to the PES signal, and operative to provide the actuator
 command signal and the spindle command signal to the actuator assembly and
 spindle assembly respectively.
 In digital servo control systems, position information is sampled at
 discrete times. Modern servo controllers, which may be implemented by a
 microprocessor or a digital signal processor, use state estimators to
 determine position, velocity, and acceleration parameters of the head as
 the head is moved radially over the disk by the actuator and head/arm
 assembly.
 A dedicated servo method is commonly used with multiple disk systems in
 which a servo head of a single dedicated servo disk surface controls
 movement of corresponding data read/write heads of a multiple platter disk
 drive. The entire surface of one side of the dedicated servo disk is
 pre-recorded with servo track information. The position of the servo head
 relative to the dedicated disk surface is used to indicate the position of
 the multiple data read/write heads relative to their corresponding disk
 surfaces. In a sector servo system, the tracks of the disk surface are
 divided into radial sectors having a short servo track information area
 followed by a data area. The servo track information area typically
 includes a sector marker, track identification data, and a servo burst
 pattern. The sector marker indicates that servo information immediately
 follows in the track. In both the dedicated servo and sector servo types
 of systems, a PES signal is used to generate a corrective input signal
 that is applied to the read/write head positioning servo.
 Servo track information usually includes: a synchronization field, such as
 for automatic gain control (AGC) or similar signal detecting purposes; a
 track identification field (TID field) typically comprising a digitally
 encoded gray code; a PES pattern field generally containing a servo
 pattern burst; and a customer data identification field generally
 including an identification synchronization pattern, identification data,
 and customer data.
 The PES field provides for generation of the PES signal as the head reads
 the PES field. The PES signal typically consists of three or four
 staggered bursts of transitions, (e.g., A, B, C and D bursts). The PES
 signal, which is proportional to the relative difference of the positions
 of the center of the servo head and the nearest track center, is a
 corrective signal providing an indication of which direction the head
 should be moved to during either track seeking or track following
 operations. The measured amplitude of the bursts indicates whether or not
 the head is in position.
 A servo system operates in several modes generally including a track
 following mode and a seek mode. In the track following mode, the servo
 controller maintains the head in a path over the centerline of a selected
 track to facilitate accurate reading and recording of data in the track.
 In the seek mode, the servo controller is directed to place the head on a
 target track different from the present track. A seek operation includes
 an acceleration sequence, and a deceleration sequence followed by a head
 settling period. As further explained below, during the head settling
 period, the head may overshoot the center of the target track and then
 oscillate about the center of the track before settling.
 The effect of servo surface irregularities or defects in the PES signal and
 hence on servo system performance can be severe. Large track
 misregistration contributions, excessive noise leading to unreliable
 operation, and non-optimal seek performance are the most significant
 effects. In order to improve linearity, and reduce sensitivity to disk
 surface effects, most position channels employ a quadrature technique, and
 some use a servo head twice as wide as the desired data track spacing. The
 essence of a quadrature system is the two-position error signals, often
 called normal and quadrature, are demodulated. The signals are derived
 from two sets of patterns which, when demodulated, produce position error
 signals that are in space (X direction) quadrature to each other. Having
 two signals allows use of only the most linear part of each. The normal
 and quadrature position error signals are quadrature signals because they
 are cyclic and out of phase by ninety degrees (one quarter phase).
 Track misregistration has two related aspects. Write-to-read track
 misregistration is the misregistration, for whatever cause, between the
 centerline of a recorded track and that of the read-back head following a
 seek to a desired record. Write-to-write track misregistration is the
 misregistration between a recorded track and an adjacent track, resulting
 in track encroachment or track-to-track squeeze depending on the direction
 of the misregistration on the adjacent recorded track. Among the physical
 factors that can result in track misregistration of both types are thermal
 track shift, incomplete head settling following a track seek, apparent
 run-out of the head/track combination due to spindle bearing and arm
 vibration of frequencies outside the capabilities of the servo system, and
 errors in the servo position detection circuits. The present application
 is principally concerned with reduction of track misregistration caused by
 incomplete head settling following a track seek.
 FIG. 1 shows a graph at 10 illustrating sampled values 12 of an exemplary
 position error signal 14 plotted against a time axis 16 during sampling
 intervals, having a period T, concurrent with a head settling period
 following a track seek operation of a typical prior art servo system. The
 sampled values of the PES signal are typically binary values including X
 bits. The depicted graph also includes: a center value 18 indicative of
 the center of the target track to which the head is settling; a positive
 error limit 20; and a negative error limit 22. The positive and negative
 error limits, forming an error margin, are used in a prior art process of
 inhibiting write operations during track seeking. In accordance with
 typical prior art method, write operations are inhibited during track
 seeking and track settling until a specified number, X, of consecutive
 sampled PES values 12 of the position error signal 14 are determined to be
 within the specified error margin 20, 22 from the center value 18
 indicating that the read/write head is within a specified distance from
 the center of the target track. Prior art methods typically require X=4 or
 5 consecutive sampled PES values to be within the specified error margins
 before enabling write operations.
 In the depicted example, a first sampled PES value 12, sampled at time
 t.sub.0, is greater than the positive error limit 20. However, the next
 fifteen PES values, sampled at times t.sub.1 -t.sub.15, are within the
 error margin 20, 22. Therefore, assuming that the servo system enables
 write operations after X=4 consecutive sampled PES values are within the
 specified error margin, the prior art servo system enables write
 operations at time t.sub.4 for the depicted PES signal 14. However, in the
 depicted example, it is actually safe to begin write operations at t.sub.1
 because the next 14 sampled PES values, sampled at time t.sub.1 -t.sub.14,
 are all within the specified error margin. Assuming a sampling period of
 100 .mu.sec, the delay incurred by the prior art servo system in enabling
 write operations would be equal to the time elapsed between time t.sub.1
 and time t.sub.4 which is approximately equal to 400 .mu.sec.
 An additional problem in prior art disk drives is that during the settling
 period, the head may settle to positions within the error margin for a
 period and then abruptly move to a position outside of the error margin.
 In the depicted example, the PES value 12 sampled at time t.sub.16 is
 outside of the negative error limit 22. However, in accordance with the
 prior art method of inhibiting write operations, the write operations
 enabled at time t.sub.4 are still enabled at time t.sub.16, and therefore
 a higher TMR occurs if the head writes at time t.sub.16 because the head
 is beyond the desired limit.
 What is needed is a process of enabling write operations during a head
 settling period following an actuator seek operation in a disk drive, the
 process providing minimal delay in enabling write operations while
 protecting against track misregistration due to incomplete head settling.
 SUMMARY OF THE INVENTION
 It is an object of the present invention to provide a process of enabling
 write operations during a head settling period following an actuator seek
 operation in a disk drive, the process providing minimal delay in enabling
 write operations while protecting against track misregistration due to
 incomplete head settling.
 Briefly, a presently preferred embodiment of the present invention includes
 a disk drive system having: a disk having position information stored in a
 plurality of tracks formed on the disk; an actuator assembly responsive to
 an actuator command signal and operative to position a read/write head
 relative to the tracks of the disk, the head being operative to read the
 position information in order to generate a position signal; a position
 channel unit responsive to the head signal and operative to generate a
 position error signal; a servo controller responsive to the position error
 signal and operative to determine position values indicative of the
 position of the head during each of a plurality of sampling intervals, and
 also being operative to provide and update the actuator command signal
 during the sampling intervals for controlling servo operations including a
 track seek for positioning the head relative to a target track.
 The controller means is further operative to perform a process of
 inhibiting write operations for writing data to the tracks of the disk
 during a head settling period following a track seek. During each of a
 first plurality of the sampling intervals transpiring during a first time
 period, the servo controller determines a present position value
 indicative of the position of the head relative to the center of the
 target track during the present sampling interval, and also determines a
 predicted position value indicative of the position of the head relative
 to the center of the target track during a subsequent sampling interval.
 Also during each of the first plurality of sampling intervals, the servo
 controller determines: (1) whether or not the present position value is
 within a first error margin from a center value indicative of the center
 of the target track; and (2) whether or not the predicted position value
 is within the first error margin from the center value. If the present
 position value and the predicted position value are both within the first
 error margin from the center value, the servo controller enables write
 operations.
 During each of a second plurality of the sampling intervals transpiring
 during a second time period, the servo controller determines a predicted
 position value indicative of the position of the head relative to the
 center of the target track during a subsequent sampling interval. Also
 during each of the second plurality of sampling intervals, the servo
 controller determines whether or not the predicted position value is
 within a second error margin from the center value. If the predicted
 position value is not within the second error margin from the center of
 the target track, the servo controller inhibits write operations.
 An important advantage of the present invention is that write operations
 following a track seek operations are enabled with minimal delay following
 the head settling period while protecting against track misregistration
 due to incomplete head settling.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Referring now to the drawing, FIG. 2 shows a block diagram at 30
 schematically showing a disk drive suitable for practicing the present
 invention. The disk drive 30 includes: a disk 32 having a thin magnetic
 coating formed thereon for storing user data and position information on a
 plurality or concentric tracks 33, the disk 32 being rotatably mounted to
 a spindle assembly 34 having a spindle motor and a spindle driver circuit,
 and also having an input 36 for receiving a spindle command signal used to
 control the rotational velocity of the disk. The disk drive 30 also
 includes one or more transducers or read heads 38 for reading magnetic
 transitions of the disk 32; a head/arm assembly 40 for supporting the read
 heads 38 in close proximity to the rotating disk surface, and having an
 output 42 providing a read-back signal carrying user data and position
 information provided by the head as the head reads the disk; and an
 actuator assembly 44, having an actuator driver circuit and an actuator
 motor for driving the arm assembly to control the position of the
 read/write head 38 in response to an actuator command signal received via
 an input 46. The arm assembly 40 may be a linear or rotary type assembly,
 and the actuator assembly 44 preferably comprises a voice coil motor.
 The disk drive 30 further includes: a servo controller 50 having an output
 52 providing a spindle command signal to input 36 of the spindle assembly
 34, an output 54 providing an actuator command signal to input 46 of the
 actuator assembly 44, and an input 56; an arm electronics circuit 58
 having an input 60 for receiving the read-back signal carrying user data
 and position information from the head 38 via output 42 of the arm
 assembly, and an output 64 providing a pre-amplified read-back signal; a
 channel unit 66, including a data channel and a position channel, and
 having an input 68 for receiving the pre-amplified read-back signal from
 output 64 of the arm electronics unit, and an output 70 for providing a
 position error sensing signal (PES signal) carrying position information
 to input 56 of the servo controller 50 via a node 72; and an interface
 controller 74 having a port 76 connected to output 70 of the channel unit
 66.
 The channel unit 66 is preferably of the PRML type and includes an
 automatic gain control circuit, a variable frequency oscillator circuit,
 and sync-byte detection circuitry. Descriptions of PRML channels are
 provided in commonly assigned U.S. Pat. Nos. 5,220,466 and 5,255,131.
 In the preferred embodiment, the servo controller 50 is a digital
 controller which includes means for processing the PES signal to determine
 head position parameters indicative of the position, velocity, and
 acceleration of the head for each sampling interval. Also, the servo
 controller 50 includes means for predicting head position parameters for
 subsequent sampling periods. A description of a method of predicting head
 position parameters for subsequent sampling intervals in a digital servo
 control system is provided in commonly assigned U.S. Pat. No. 4,679,103
 which is incorporated herein by reference. Also, a description of a disk
 file digital servo control system with coil current modeling is provided
 in commonly assigned U.S. Pat. No. 4,914,644 which is incorporated herein
 by reference.
 In varying embodiments, the controller includes either a microprocessor or
 a digital signal processor and may further include support logic such as
 counters, an interrupt controller, a direct memory access controller, a
 serial interface controller, and other components generally known to
 assist microprocessor control functions. The controller is normally
 associated with a predetermined amount of read only type memory for
 storing a control program, RAM, and a reference clock. The controller
 directly oversees operation of the interface controller 74, the channel
 unit 66, and the actuator and spindle control circuitry.
 The controller 50 controls the precise positioning of the head 38 during
 execution of track seek and track following operations by regulating the
 voltage of the actuator command signal provided to the actuator assembly
 44. The controller 50 implements closed loop servo control which utilizes
 the feedback of position information carried by the PES signal received
 from the disk via the channel unit 66 to find and maintain a position over
 a target track of the disk 32. It will be apparent to those skilled in the
 art of disk drive technology that the position information may be located
 on a single dedicated disk surface (i.e., a dedicated servo) or embedded
 on data tracks between portions of user data (embedded servo).
 FIG. 3 shows a graph at 100 illustrating sampled PES values 102 of an
 exemplary PES signal 104 plotted against a time axis 106 at sampling
 intervals, having a period T, during a head settling period following a
 track seek operation of the servo system 30 (FIG. 2) of the present
 invention. The depicted graph also includes: a center value 108 indicative
 of the center of a target track to which the head 38 (FIG. 2) is settling
 following a seek operation; a first positive error limit 110; a first
 negative error limit 112; a second positive error limit 114; and a second
 negative error limit 116. The first positive error limit 110 and first
 negative error limit 112 form a first error margin 110, 112. The second
 positive error limit 114 and second negative error limit 116 form a second
 error margin 114, 116. The first and second error margins are used in a
 process of inhibiting write operations according to the present invention
 as described below with respect to flow diagrams illustrated in FIGS. 4
 and 5.
 FIG. 4 shows a flow chart at 130 illustrating a first sequence of the
 process of inhibiting write operations according to the present invention.
 The depicted process begins with step 132 in which the servo controller 50
 (FIG. 2) determines a present position value indicative of the position of
 the head 38 (FIG. 2) relative to the center of a target track during a
 sampling interval. For example, at time t.sub.1 (FIG. 3), the servo
 controller determines the present PES value 102 corresponding to time
 t.sub.1. In step 143, the servo controller determines a predicted position
 value indicative of the position of the head relative to the center of the
 target track during a subsequent sampling interval. The subsequent
 sampling interval may be a next sampling interval, or may be an integer
 number N sampling intervals following the present sampling interval. For
 example, at time t.sub.1 (FIG. 3), the servo controller determines a
 predicted PES value 102 corresponding to the next sampling interval
 beginning at time t.sub.2.
 The process proceeds from 134 to 136 at which the servo controller
 determines whether the present position value determined in step 132 is
 within the first error margin 110, 112 (FIG. 3) relative to the center
 value 108 (FIG. 3). If it is determined at 136 that the present position
 value is within the first error margin, the depicted process proceeds to
 138. For example, at time t.sub.2 (FIG. 3), the servo controller
 determines that the present PES value corresponding with time t.sub.2 is
 within the first error margin 110, 112 relative to the center value 108
 (FIG. 3).
 From 136, the depicted process proceeds to 138 at which the servo
 controller determines whether the predicted position value determined in
 step 134 is within the first error margin 110, 112 (FIG. 3) relative to
 the center value 108 (FIG. 3). If it is determined at 136 and 138 that
 both the present position value and predicted position value are within
 the first error margin from the center value, the depicted process
 proceeds to step 142 in which the servo controller enables a write
 operation. For example, at time t.sub.2 (FIG. 3), the servo controller
 determines that the present PES value corresponding with time t.sub.2 is
 within the first error margin 110, 112 relative to the center value 108
 (FIG. 3), and that the predicted PES value associated with the next
 sampling interval beginning at time t.sub.3 is also within the first error
 margin. Therefore, write operations are enabled at time t.sub.2 (FIG. 3)
 in accordance with the process of the present invention.
 If it is determined at 136 that the present position value is not within
 the first error margin from the center value, the depicted process
 proceeds to step 140 in which the servo controller inhibits writing
 operations in order to avoid track misregistration due to incomplete head
 settling. For example, at time t.sub.1 (FIG. 3), the servo controller
 determines that the present PES value associated with time t.sub.1 is not
 within the first error margin from the center value. Also, if it is
 determined at 138 that the predicted position value is not within the
 first error margin from the center value, the depicted process proceeds to
 step 140 as described above. For example, at time t.sub.0 (FIG. 3), the
 servo controller predicts that the next PES value 102 corresponding with
 the next sampling interval beginning at time t.sub.1 (FIG. 3) is not
 within the first error margin 110, 112. After executing step 142, the
 depicted process proceeds to "A" (to FIG. 5).
 FIG. 5 shows a flow chart at 150 illustrating a second sequence of the
 process of inhibiting write operations according to the present invention.
 The depicted process steps proceed from "A" to step 152 in which the servo
 controller 50 (FIG. 2) determines a present position value indicative of
 the position of the head 38 (FIG. 2) relative to the center of a target
 track during a present sampling interval. In step 154, the servo
 controller determines a predicted position value indicative of the
 position of the head relative to the center of the target track during a
 subsequent sampling interval. The subsequent sampling interval may be a
 next sampling interval, or may be an integer number N sampling intervals
 following the present sampling interval.
 It is then determined at 156 whether or not the predicted position value
 determined in step 154 is within the second error margin 114, 116 (FIG. 3)
 from the center value 108 (FIG. 3). If it is determined at 156 that the
 predicted position value determined in step 154 is within the second error
 margin from the center value, the depicted process proceeds to step 160 in
 which the servo controller enables writing operations, and then proceeds
 back to step 152.
 If it is determined at 156 that the predicted position value is not within
 the second error margin from the center value, the depicted process
 proceeds to step 158 in which the servo controller inhibits writing
 operations, after which the process proceeds back to "A" (to FIG. 4). For
 example, the sampled PES values 102 sampled at times t.sub.3 -t.sub.9
 (FIG. 3) are all within the second error margin 114, 116. However, the
 sampled PES value sampled at time t.sub.10 (FIG. 3) is not within the
 second error margin 114, 116. Therefore, at time t.sub.9, the servo
 controller 50 (FIG. 2) determines that the predicted position value for
 the next sample interval beginning at time t.sub.10 is not within the
 second error margin, and therefore writing operations are inhibited.
 The use of predictive analysis provided in the process of the present
 invention allows for minimal delay in the enabling of write operations
 following a head settling period of a track seek operation while
 protecting against track misregistration due to incomplete head settling.
 The predictive analysis also provides for timely inhibiting of write
 operations upon prediction of a head position which is outside the
 specified margin of error.