Abstract:
In a disk drive having a dual stage actuator, a method to improve head switching time by aligning a target head with a target track before a head switch occurs by using a second stage actuator attached to the target head to align the target head with the new track. The radial offset required to align the target head with the target track is either calibrated at startup or is measured dynamically during normal disk operations. A dual stage actuator with either a milli-actuator second stage or a micro-actuator second stage, positions a read/write head over a disk surface. The disk drive has a radial offset table for storage of radial offsets for head switches.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method in a disk drive to adjust the radial offset between heads of a dual stage actuator, where the dual stage actuator uses either a milli-actuator or a micro-actuator, to position a read/write head during a head switch. 
     BACKGROUND OF THE INVENTION 
     A huge competitive market exists for disk drives that store massive amounts of data in computers. This competition requires disk drive manufacturers to provide ever-increasing data storage capacity and higher performance in their products. One way to increase capacity is to provide more data tracks on a given disk surface, which generally requires that the tracks be more densely packed since the form factor of the disk is limited by compatibility standards. In current disk drives, track density or pitch of 10,000 tracks per inch is becoming available. 
     As tracks are placed closer together, the problem of maintaining a read/write transducer in position over the track becomes more difficult. The industry presently prefers a rotary type actuator, which employs a voice coil motor to pivotally swing an array of vertically stacked arms carrying read/write transducers over tracks disposed on surfaces of a corresponding stack of disks. A sampled servo system reads servo wedges interspersed at equal intervals around the data tracks to maintain the position of a transducer over a track. At some point in the march toward higher track densities, these conventional actuators and servo systems may be unable to provide the bandwidth and precision control required to keep the head positioned over a target track without encroaching adjacent tracks. 
     Dual stage actuators have been proposed as a solution to the bandwidth and precision control problem. The dual stage actuator generally provides two pivot points and two motors with a second stage motor and pivot point being smaller and positioned closer to the transducer, therefore providing for higher control bandwidth and precision. Typically the span or range of motion of the second stage actuator is limited to a few tracks or even to the width of a single track or less. 
     Dual stage actuators may be broadly characterized as milli-actuators or micro-actuators. In a milli-actuator design, the second stage actuator moves a suspension arm that suspends the read/write head. The milli-actuator can adjust the radial offset of the read/write head by moving the suspension arm, and thus the read/write head, in relation to the first stage pivot point. 
     In the micro-actuator design, the second stage, mounted on the suspension arm, moves the read/write head directly. The micro-actuator can adjust the position of the read/write head by moving the read/write head in relation to the suspension. 
     The performance of disk drives employing dual stage actuators, as in conventional disk drives, is and will be impacted by the time required to position a transducer over a target data track to perform a read or write operation, characterized as access time. One major component of access time is the time required to move or swing the actuator from a present track to a target track, known as seek time. Another significant component of access time is the time required to perform a head switch, which entails selecting a target transducer head on a different one of the vertically stacked arms of the actuator and establishing the precise position of that head over the present track. The head switch occurs frequently during read or write operations of sequential data where data is conventionally recorded on a stack of respective tracks at the same radial location on each disk surface, known as a cylinder. This recording technique has been historically employed to minimize the delay in continuing sequential data transfer because a head switch can be accomplished in less time than a seek. 
     Although the stacked arms and transducer heads in a disk drive are closely aligned, there are sufficient variations between the disks, arms and heads to require that the target head be re-positioned when a head switch is performed, even though the previous head was perfectly aligned over a corresponding track. This is especially true with more densely spaced tracks and the problem may even be exacerbated with dual stage actuators. The instantaneous difference in position between heads on the stacked arms owing to these variations may be termed radial offset. As a consequence, there is a continuing need to improve head switch times and provide higher performance disk drives. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to minimize the time required to perform a head switch from a current head to a target head. The invention takes advantage of the fact that a dual stage actuator provides an individual actuator for each head and therefore advantageously aligns a target head with a target track with the target head second stage actuator before a head switch occurs. The radial offset required to align the target head with the target track may be either calibrated at startup during a calibration period or may preferably be measured dynamically during normal disk operations. The invention can be applied to dual stage actuators using a milli-actuator structure or a micro-actuator structure. 
     In one aspect, the invention provides a method to measure the radial offset of a read/write head for head switching operations in a disk drive. The disk drive has a plurality of disk surfaces, a head actuator having a first stage actuator means and a plurality of head arms rotatable in unison about a pivot by the first stage actuator means. Each head arm has a head for reading and writing on a respective disk surface. Each head is independently movable by a respective second stage actuator means. The disk drive has a servo controller and a servo read channel for reading servo signals from a head and positioning the first and second stage actuator means. The method comprises the steps of: track following at a selected cylinder location with a first head with the servo controller to establish a first position error signal; switching to a target head; reading a second position error signal with the servo controller at the selected cylinder location with the target head; computing a measure of the radial offset between the first head and the target head based on a difference between the first position error signal and the second position error signal; and writing the radial offset into a radial offset table. 
     Preferably the radial offset measurement is performed over a predetermined number of head switches wherein the radial offset is averaged over the predetermined number of head switches. The radial offset averaging may be performed during a calibration period or may be performed during normal disk operations. 
     The invention also provides a method to adjust the radial offset of a head for performing a head switch from a first head to a target head over a target track. The method is employed in a disk drive having a plurality of disk surfaces, a head actuator having a first stage actuator means and a plurality of head arms rotatable in unison about a pivot by the first stage actuator means. Each head arm has a head for reading and writing on a respective disk surface and each head is independently movable by a respective second stage actuator means. The disk drive further comprises a radial offset table for storing mutual radial offsets between heads, and a servo controller for reading servo signals from a head for positioning the first and second stage actuator means. The method of the invention reads a radial offset for a target head from the radial offset table and calculates an offset compensation signal for the target head second stage actuator for positioning the target head based on the radial offset. The method of the invention then applies the offset compensation signal to the target head second stage actuator before the head switch occurs from the first head to the target head. 
     In an alternate embodiment, the disk drive has a plurality of servo read channels and the method includes the steps of track following on a first head using a first servo read channel for performing data read or write operations and, while continuing to track follow on the first head, determining a target head for a head switch and reading a position error signal from the target head using a second servo read channel. The invention calculates a compensation signal for the target head and applies the compensation signal to the target head second stage actuator means to correct the position of the target head. After completing the data read or write operation with the first head the method performs a head switch from the first head to the target head. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a disk drive of the invention employing dual stage actuators. 
     FIG. 2 shows a schematic diagram of two stacked dual stage actuator arms deployed over two stacked magnetic disks which are shown offset for clarity and showing the radial offset between heads. 
     FIG. 3 shows a milli-actuator and a servo control system of the invention. 
     FIG. 4 shows a micro-actuator and a servo control system of the invention. 
     FIG. 5 shows the method of the invention to measure the radial offset of a read/write head and store the radial offset in a radial offset table for use with a dual stage actuator. 
     FIG. 6 shows the method of the invention to adjust the radial offset of a target head prior to a head switch. 
     FIG. 7 shows the disk drive of the invention having a dual channel servo system. 
     FIG. 8 shows the method of the invention with a dual channel servo system to adjust the radial offset of a target head by track following on the target head. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Refer now to FIG. 1 which shows a disk drive  10  with a dual stage actuator head stack assembly  18  controlled by a servo controller  37 . The dual stage actuator head stack assembly  18  positions the multiple read/write heads  30 A and  30 B over a set of disks  101 A and  101 B. The disk drive  10  reads and writes data on the set of disks  101 A and  1 O 1 B via the heads  30 A,  30 B. 
     The dual stage actuator head stack assembly  18  has a first stage actuator  19  and multiple second stage actuators  44 A and  44 B. The first stage actuator  19  and multiple second stage actuators together cooperate to move the multiple read/write heads  30 A and  30 B. The first stage actuator  19  comprises a voice coil motor, VCM  20  and multiple arm/suspension assemblies  52 A and  52 B. The multiple ann/suspension assemblies  52 A and  52 B are simultaneously rotated about pivot  53  by VCM  20 . Arm/suspension assemblies  52 A and  52 B support two second stage actuators  44 A and  44 B, respectively. Two embodiments of a second stage actuator are provided and are described in more detail below. The read/write heads  30 A and  30 B are precisely moved by attachment to the second stage actuators  44 A and  44 B. 
     A head disk assembly  15  comprises the set of disks  101 A and  101 B , a spindle  102  and a spindle motor  107 . The set of disks  101 A and  101 B are mounted on spindle  102  in a well known manner. Spindle motor  107  drives the spindle  102  at a constant speed also in a well known manner. Each one of the set of disks  101 A and  101 B further has multiple servo wedges for use with the servo controller  37 . The servo controller  37  controls the position of the multiple read/write heads  30 A and  30 B. Each servo wedge further comprises multiple servo tracks created and configured in a well-known manner. 
     The disk drive  10  controls the operation of the head stack assembly  18  with the servo controller  37 . The servo controller  37  comprises a microprocessor  12 , a servo read channel  25 , a second stage driver  36 , a digital to analog converter, DAC  22 , and a current driver  23 . DAC  22 , converts digital control signals, from microprocessor  12  via line  11 , to analog control signals that are further provided to the current driver  23 . The VCM  20  is connected to the current driver  23  by line  35 . The VCM  20  swings arm/suspension assemblies  52 A,  52 B and attached second stage actuators  44 A,  44 B in response to commands from the microprocessor  12 . The second stage actuators  44 A and  44 B provide a limited range of motion for the multiple read/write heads  30 A and  30 B over the set of disks  101 A and  101 B respectively. Microprocessor  12  provides commands to second stage driver array  36  to provide signals to move second stage actuators  44 A, via control lines  56 A and  56 B respectively. The microprocessor  12  further comprises memory  38  for storage of data such as parameter tables used to control the second stage actuators  44 A and  44 B. Preamplifier  31  selectively processes signals from heads  30 A,  30 B via line  33  to provide a position error signal to the servo channel  25  for calculation of head position. 
     In summary, for seek operations, Microprocessor  12  rotates head stack assembly  18  to position the heads over a target cylinder by controlling first stage actuator  19 . Assuming for example that the targeted track is on disk  101 A, head  30 A is selected to provide position signals to servo channel  25  and Microprocessor  12 . When head  30 A is over the target track, Microprocessor keeps the head on track precisely in a closed loop servo process by controlling second stage actuator  44 A and if necessary, first stage actuator  19  responsive to position error signals from head  30 A and servo channel  25 . Similarly, if the target track is on disk  101 B, head  30 B provides position error signals and is maintained on track by controlling second stage actuator  44 B. 
     Refer now to FIG. 2 which shows a schematic representation of two disk/actuator combinations illustrating the radial offset between heads  3 A and  3 B. The first disk  101 A has a track  103 A with servo information written on it in a conventional manner. Similarly a second disk  101 B has a track  103 B also with servo information written on it in a conventional manner. The set of disks  101 A and  101 B are rotating on a common spindle axis  105 . The disks are shown offset for clarity of presentation. Arm/suspension assemblies  52 A and  52 B are connected to first stage actuator  19  (shown in FIG. 1) and are pivoted in unison about a common axis  106 . Second stage actuator  44 A is pivoted about axis  48 A and is supported by arm/suspension assembly  52 A. Similarly second stage actuator  44 B pivots about axis  48 B and is supported by arm suspension assembly  52 B. As indicated in FIG. 2, the second stage actuators  44 A and  44 B are able to independently (of each other) pivot about respective axes  48 A,  48 B. Due to variations in manufacturing and assembly of head stack assembly  18  and its components, an offset shown as  47  can exist between heads. In the example shown, when head  30 A is centered on track  103 A, head  30 B is radially displaced from track  103 B, creating offset  47 . In a reversed situation, when head  30 B is centered on track  103 B, head  30 A may be offset from track  103 A. 
     The radial offset between heads, such as that indicated by  47  in FIG. 2, can detrimentally impact head switching time if the head switch is performed conventionally, i.e. serially, by first switching to the target head, waiting for a servo wedge, reading the servo information, and then applying a correction to center the head. Frequently multiple servo sectors must be read and multiple corrections applied to insure that the new target head is centered on the track. Instead, the invention provides for correcting the offset prior to the head switch so that the new target head is in position for a read or write operation in a more timely manner. 
     FIG. 5 shows the method of the invention to measure the radial offset of a read/write head and store radial offset values in a radial offset table for use with a dual stage actuator. The method starts at step  240  by closed loop track following using a first head to establish a position error signal which is nominally zero. A head switch to a second head is performed in step  242 . The position error signal is then read from the second head in step  244  in an open loop process. In step  246 , the measurement of the radial offset between the first and second heads is calculated based on the difference in position error signals between the first and second heads. The new radial offset value, for the switch from the first head to the second head, is written to a radial offset table in step  248 . 
     In a preferred embodiment of the invention, the radial offset is averaged over multiple head switches. This averaging of the radial offset value accounts for normal statistical variations versus a single radial offset measurement. The average value may be accumulated during a specific period of time such as power-up of the disk drive or, preferably during drive operation, a running average may be accumulated over a predetermined number of recent head switches. These statistical variations may be caused by noise in the read channel, environmental factors such as temperature and vibration, and runout. Radial offset values may be taken for multiple head switches from the same two heads, for example, from head  30 A to head  30 B. Since the method of the invention tracks changes in radial offset during operation, a separate calibration procedure is not needed. The invention constantly adjusts for variations in radial offset. 
     A table of radial offset entries for each head is stored in memory. One example table is shown in Table A. Table A shows a table of offsets from a current head. In this example a head switch from may be made to five other heads. 
     
       
         
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE A 
               
             
             
               
                   
               
               
                 CURRENT 
                   
               
               
                 HEAD 
                 TARGET HEAD → 
               
             
          
           
               
                 ↓ 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
               
               
                   
               
               
                 1 
                 0 
                 OFFSET 1-2 
                 OFFSET 1-3 
                 OFFSET 1-4 
                 OFFSET 1-5 
                 OFFSET 1-6 
               
               
                 2 
                 OFFSET 2-1 
                 0 
                 OFFSET 2-3 
                 OFFSET 2-4 
                 OFFSET 2-5 
                 OFFSET 2-6 
               
               
                 3 
                 OFFSET 3-1 
                 OFFSET 3-2 
                 0 
                 OFFSET 3-4 
                 OFFSET 3-5 
                 OFFSET 3-6 
               
               
                 4 
                 OFFSET 4-1 
                 OFFSET 4-2 
                 OFFSET 4-3 
                 0 
                 OFFSET 4-5 
                 OFFSET 4-6 
               
               
                 5 
                 OFFSET 5-1 
                 OFFSET 5-2 
                 OFFSET 5-3 
                 OFFSET 5-4 
                 0 
                 OFFSET 5-6 
               
               
                 6 
                 OFFSET 6-1 
                 OFFSET 6-2 
                 OFFSET 6-3 
                 OFFSET 6-4 
                 OFFSET 6-5 
                 0 
               
               
                   
               
             
          
         
       
     
     In one embodiment, coefficients proportional to the force necessary to move the micro-actuator may be stored in the table. The table is used during normal disk operations to move the target head close to its target track. 
     Although Table A shows entries for all combinations of current head to target head offsets, the table may be minimized to reduce storage requirements. For example, by simply providing offset entries referenced to a single head such as head  1 , only 5 entries would be required. In this embodiment, the offset between any two heads would be determined by calculating the difference in offset relative to the reference head. 
     FIG. 6 shows the method of the invention to adjust the radial offset of a read/write head prior to a head switch with a dual stage actuator. The head switch is to be made from a current head to a target head over a target track. The process in step  230  provides track following on a current head during normal operations. In step  234 , an anticipated target head is specified. While track following on the current head is maintained at step  232 , the radial offset for the switch from a current head to the target head is read from the radial offset table in step  238 . In step  242 , a DC Bias is calculated for the target head&#39;s second stage actuator from the value read from the radial offset table. In step  246 , the DC bias is applied to the target head&#39;s second stage actuator. The DC Bias translates to the force necessary to move the head enough to compensate for the radial offset. At step  236  operations are completed on the current track and at step  240 , a head switch is performed to the target head which has been pre-positioned to be closer to target track center. 
     FIG. 3 shows one example embodiment of the dual stage actuator as a milli-actuator. The milli-actuator comprises a first stage actuator  180  having a body portion  184 . A voice coil motor  182  connected to the body  184  rotates the first stage actuator  180  around a pivot  185 . An arm  186  is cantilevered from the body  184 . The second stage actuator  190  of the dual stage actuator comprises a milli-actuator motor  191 , a suspension  194 , and a pivot  192 . The second stage actuator  190  is connected to a suspension arm  186 . The milli-actuator motor  190  moves the suspension  194  about the pivot  192 . A read/write head  196  is supported by the suspension  194 . The read/write head  196  reads a servo signal from the disk  188  to a preamplifier  172 . The disk  188  rotates about spindle  198 . The preamplifier  172  provides a signal to a demodulator  170  that provides a position error signal to a controller  176 . The controller  176  controls a voice coil motor driver  178  that coarsely controls the radial position of the actuator through voice coil motor  182 . The controller  176  also provides a signal to a second stage driver  174  that finely positions the suspension  194  and head  196  over a target track by controlling the milli-actuator motor  191 . 
     FIG. 4 shows an alternative example embodiment of the dual stage actuator as a micro-actuator. The micro-actuator comprises driven first stage actuator  210  comprising a voice coil motor  212  which rotates the entire mechanism about a pivot  215 . A body  214  holds suspension arm  216 . The second stage  225  of the dual stage actuator comprises an arm  218 , and a micro-actuator  220 . The arm  218  is attached to the suspension arm 216 . The arm  218  holds the micro-actuator  220 , which may be a piezo-electric positioner such as a piezo-electric motor, to fine position the read/write head  222  across a disk  226 . Those skilled in the art will appreciate that other types of fine positioning mechanisms, such as voice coil based mechanisms, may be used in the micro-actuator without deviating from the spirit and scope of the invention. The disk  226  rotates on spindle  224 . The read/write head  222  provides a signal to the preamplifier  202 . The preamplifier  202  provides a signal to a demodulator  200  that demodulates the amplified signal from preamplifier  202 . The demodulator  200  provides a position error signal to a controller  206 . The controller  206  drives driver  204  which controls the micro-actuator  220 . The controller  206  also provides a signal to a voice coil motor driver  208  to control the position of the suspension arm  216  about pivot bearing  215 . In operation the controller, during a head switch, positions the read/write head  222  over a track and the micro-actuator adjusts for radial offset. 
     The methods of the invention are useful for both single channel and dual channel servo control systems. 
     Refer now to FIG. 7 which shows a disk drive  110  with a dual stage actuator head stack assembly  118  controlled by a dual channel servo controller  137 . The dual stage actuator head stack assembly  118  positions the multiple read/write heads  130 A and  130 B over a set of disks  101 A and  101 B. The set of disks  1101 A and  1101 B are part of a head disk assembly  115 . 
     The dual stage actuator head stack assembly  118  has a first stage actuator  119  and multiple second stage actuators  144 A and  144 B. The first stage actuator  119  and second stage actuators  144 A, 144 B cooperate to move the read/write heads  130 A and  130 B over tracks on disks  1101 A,  1101 B. The first stage actuator  119  comprises a voice coil motor, VCM  120  which swings multiple arm/suspension assemblies  152 A and  152 B about a pivot  153 . The arm/suspension assemblies  152 A and  152 B support two second stage actuators  144 A and  144 B, respectively. Two embodiments of a second stage actuator are provided as described in more detail above. The multiple read/write heads  130 A and  130 B are attached to the second stage actuators  144 A and  144 B respectively. 
     The head disk assembly  115  comprises the set of disks  1101 A and  1101 B, a spindle  1102  and a spindle motor  1107 . The set of disks  1101 A and  1101 B are mounted on spindle  1102  in a well known manner. Spindle motor  1107  drives the spindle  1102  at a constant speed also in a well known manner. Each one of the set of disks  1101 A and  1101 B further have multiple servo wedges for use with the dual channel servo controller  137 . Each servo wedge further comprises multiple servo tracks created and configured in a well-known manner. 
     The disk drive  110  controls the operation of the multiple arm dual stage actuators with the dual channel servo controller  137 . The dual channel servo controller  137  comprises a microprocessor  112 , a first servo read channel  125 A, a second servo read channel  125 B, a second stage driver array  136 , a digital to analog converter, DAC  122 , and a current driver  123 . DAC  122  converts digital control signals, from microprocessor  112  via line  111 , to analog control signals that are further provided to the current driver  123 . The VCM  120  is connected to the current driver  123  by line  135 . The VCM  120  employs a voice coil  126  that varies a magnetic field in the proximity of a permanent magnet  132 . The magnetic field is varied by changing the VCM coil  126  current. Microprocessor  112 , implementing a sampled servo control program, controls the current to the VCM  120  through DAC  122  and current driver  123 . The VCM  120  simultaneously rotates arm/suspension assemblies  152 A,  152 B about pivot  153  in response to commands from the microprocessor  112 . Second stage actuators  144 A and  1441 B are also moved by VCM  120  due to their attachment to arm/suspension assemblies  152 A,  152 B. The second stage actuators  144 A and  144 B provide a limited range of movement of each of the read/write heads  130 A and  130 B to provide fine positioning over tracks on the set of disks  1101 A and  1101 B respectively. The second stage actuators  144 A,  144 B are driven by control signals from second stage driver array  136  on lines  156 A,  156 B respectively. Microprocessor  112  send commands to second stage driver array  136  to selectively move second stage actuators  144 A, 144 B. The microprocessor  112  further comprises memory  138  for storage of data such as parameter tables used to control second stage actuators  144 A and  144 B. Preamplifier  131 A conditions position signals from the head  130 A and preamplifier  131 B conditions position signals from the head  130 B. Preamplifier  131 A provides a position error signal to a servo read channel  125 A. Preamplifier  131 B provides a position error signal to a servo read channel  125 B. 
     Multiple second stage actuators  144 A and  144 B are coarsely positioned by first stage actuator  119 . In an exemplary embodiment, for read/write operations on disk  1101 A, read/write head  130 A is selected to provide a position error signal to microprocessor  112  through preamplifier  131 A and servo channel  125 A. The position error signal from head  130 A then provides for closed loop control of first stage actuator  119  and second stage actuator  144 A. When a head switch is anticipated to continue read/write operations on disk  1101 B on the current cylinder, read/write head  130 B is selected to provide a position error signal to microprocessor  112  through preamplifier  133 B and servo read channel  125 B. While track following on disk  1101 A, a limited range of motion is available for head  130 B using second stage actuator  144 B. Second stage driver array  136  provides current to drive each one of second stage actuators  144 A and  144 B via lines  156 A and  156 B respectively. Microprocessor  112  loads registers in second stage driver array  36  to independently control the actuators  144 A and  144 B. In this manner, when a head switch is performed from head  130 A to  130 B, head  130 B will be pre-positioned on track center and a read or write operation can resume without delays required to re-position or settle the target head. 
     FIG. 8 shows the method of the invention to provide a limited track following capability on a target head while track following on a current head when a second servo read channel is available. The method begins operation at  332  by track following on a current track with a PES read from the currently selected head using the first servo read channel. A targeted head for a next head switch is determined at  334 . While continuing to track follow and perform operations on the current head in step  333 , a PES is read from the target head on the second servo read channel at  336 . A compensation is calculated at  338  for the target head actuator and at  340  the compensation is applied to the target head second stage actuator. When operations on the current track are completed at  335 , the method performs a head switch to the target head at  337 . Beneficially the target head is pre-positioned at track center by performing track following on the target head in parallel with track following on the Rent head.