Patent Publication Number: US-8982501-B1

Title: Data storage device compensating for repeatable disturbance when commutating a spindle motor

Description:
BACKGROUND 
     Data storage devices such as disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and servo sectors. The servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the actuator arm as it seeks from track to track. 
       FIG. 1  shows a prior art disk format  2  as comprising a number of servo tracks  4  defined by servo sectors  6   0 - 6   N  recorded around the circumference of each servo track. Each servo sector  6 , comprises a preamble  8  for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark  10  for storing a special pattern used to symbol synchronize to a servo data field  12 . The servo data field  12  stores coarse head positioning information, such as a servo track address, used to position the head over a target data track during a seek operation. Each servo sector  6 , further comprises groups of servo bursts  14  (e.g., N and Q servo bursts), which are recorded with a predetermined phase relative to one another and relative to the servo track centerlines. The phase based servo bursts  14  provide fine head position information used for centerline tracking while accessing a data track during write/read operations. A position error signal (PES) is generated by reading the servo bursts  14 , wherein the PES represents a measured position of the head relative to a centerline of a target servo track. A servo controller processes the PES to generate a control signal applied to a head actuator (e.g., a voice coil motor) in order to actuate the head radially over the disk in a direction that reduces the PES. 
     The disk  2  is typically rotated by a spindle motor at a high speed so that an air bearing forms between the head and the disk surface. A commutation controller applies a driving signal to the windings of the spindle motor using a particular commutation sequence in order to generate a rotating magnetic field that causes the spindle motor to rotate. Prior art disk drives have typically controlled the commutation of the windings by measuring a zero-crossing frequency of a back electromotive force (BEMF) voltage generated by the windings of the spindle motor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a prior art disk format comprising a plurality of servo tracks defined by servo sectors. 
         FIG. 2  shows a data storage device in the form of a disk drive according to an embodiment comprising a head actuated over a disk, and control circuitry comprising a commutation controller configured to commutate windings of a spindle motor based on compensation values. 
         FIG. 3  shows control circuitry according to an embodiment wherein the commutation controller commutates the windings of the spindle motor based on back electromotive force (BEMF) voltage zero crossings of the spindle motor. 
         FIG. 4  shows control circuitry according to an embodiment wherein the commutation controller commutates the windings of the spindle motor based on the servo sectors. 
         FIG. 5  shows control circuitry according to an alternative embodiment wherein the commutation controller commutates the windings of the spindle motor based on the servo sectors. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  shows a data storage device in the form of a disk drive according to an embodiment comprising a head  16  actuated over a disk  18 , and a spindle motor  20  configured to rotate the disk  18 , wherein the spindle motor  20  comprises a plurality of windings. The disk drive further comprises control circuitry  22  including a commutation controller  24  configured to commutate the windings based on a commutation sequence. The control circuitry is configured to generate compensation values  26  based on:
 
 A 1 sin ω1 t+B 1 cos ω1 t  
 
where A1 and B1 are adaptable coefficients and ω1 represents a first frequency based on a first mechanical parameter of the spindle motor  20 . The compensation values  26  are used to drive the commutation sequence of the commutation controller  24 .
 
     In one embodiment shown in  FIG. 2 , a back electromotive force (BEMF) voltage  27  generated by the windings of the spindle motor  20  may be processed in order to drive the commutation sequence of the commutation controller  24 . In another embodiment, the commutation sequence may be driven based on timing data recorded on the disk  18 , such as servo sectors  28   0 - 28   N  that define servo tracks  30 . Regardless as to how the commutation sequence is driven, in one embodiment the compensation values  26  are used to drive the commutation sequence in order to compensate for a mechanical parameter of the spindle motor. For example, in one embodiment the pole pairs of the spindle motor may exhibit an asymmetrical alignment due to manufacturing tolerances such that the optimal time to commutate the windings may be based on a fundamental frequency (the rotation frequency of the spindle motor) plus an additional frequency based on a mechanical parameter of the spindle motor. In one embodiment, the mechanical parameter comprises a number of pole pairs of the spindle motor, and in another embodiment, the mechanical parameter comprises a number of slots of the spindle motor. 
     In one embodiment, while the disk is being spun up and/or when synchronizing to the timing data on the disk  18  is lost, the control circuitry  22  may process a BEMF signal  32  which may be a square wave representing the BEMF zero-crossings as detected by a BEMF detector  34 . The commutation controller  24  may generate a control signal  36  which configures the BEMF detector  34  to detect the zero-crossing of the BEMF voltage generated by each winding as the disk rotates. The commutation controller  24  also generates a control signal  38  applied to commutation logic  40 . In the embodiment of  FIG. 2 , the commutation logic  40  is configured by the control signal  38  to control the state of switches  42  in order to drive the windings with driving voltages +V and −V. The commutation logic  40  may operate in any suitable manner, such as by driving the switches  42  as linear amplifiers that apply continuous-time sinusoidal voltages to the windings. In another embodiment, the commutation logic  40  may drive the switches  42  using pulse wide modulation (PWM), such as using square wave PWM, trapezoidal PWM, or sinusoidal PWM. Regardless as to how the windings are driven, the commutation controller  24  generates the control signal  38  so that the windings are commutated at the correct periods, thereby generating the desired rotating magnetic field that causes the spindle motor to rotate. In one embodiment, the control circuitry  22  may generate a control signal  44  that controls the effective amplitude of the driving voltages (continuous or PWM), thereby controlling the speed of the spindle motor. 
     In one embodiment, the commutation controller  24  may disable the driving voltage applied to the winding that the BEMF detector  34  is evaluating for a zero-crossing during a zero-crossing window. However, disabling the driving voltage typically induces current transients in the windings of the spindle motor, which can result in acoustic noise, torque/speed jitter, and disk vibration. Accordingly, in one embodiment after the disk  18  has spun up to a target operating speed and the control circuitry  22  has synchronized to the timing data on the disk  18 , the control circuitry  22  switches from using the BEMF signal  28  to using the timing data on the disk  18  to drive the commutation sequence. If synchronization to the timing data is lost for any reason, the control circuitry  22  may switch back to using the BEMF signal  28  to drive the commutation sequence. 
     In one embodiment, the control circuitry  22  processes a read signal  46  emanating from the head  16  to demodulate the servo sectors  28   0 - 28   N  and generate a position error signal (PES) representing an error between the actual position of the head and a target position relative to a target track. The control circuitry  22  filters the PES using a suitable compensation filter to generate a control signal applied to a voice coil motor (VCM)  48  which rotates an actuator arm  50  about a pivot in order to actuate the head  16  radially over the disk  18  in a direction that reduces the PES. The servo sectors  28   0 - 28   N  may comprise any suitable head position information, such as a track address for coarse positioning and servo bursts for fine positioning. The servo bursts may comprise any suitable pattern, such as an amplitude based servo pattern or a phase based servo pattern. 
     The compensation values  26  may be used to drive the commutation sequence of the commutation controller  24  in any suitable manner.  FIG. 3  shows an embodiment wherein the BEMF signal  32  (representing zero-crossings in the BEMF voltage of the spindle motor) may be converted at block  52  into a commutation phase  54  of the spindle motor, wherein the commutation phase  54  initializes a counter  56 , and the output of the counter  56  represents the current commutation state in the commutation sequence of the commutation controller  24 . The BEMF signal  32  is also converted into a rotational velocity  60  of the spindle motor at block  58  (i.e., the rotation velocity  60  represents the fundamental rotation frequency of the spindle motor). The compensation values  26  are added to the rotational velocity  60  to generate a control signal  62  applied to an oscillator  64 , where the output frequency  66  of the oscillator  64  is used to clock the counter  56 . Accordingly, the counter  56  is driven at a frequency based on the fundamental rotation frequency of the spindle motor plus an additional frequency based on the compensation values  26  (i.e., to compensate for a mechanical parameter of the spindle motor). In one embodiment, the counter  56  is a modulo-N counter (where N+1 represents the number of states of the commutation sequence) so that the counter  56  continuously cycles from 0 to N as the commutation sequence is repeated. 
     In one embodiment, the control circuitry may be further configured to generate the compensation values  26  based on:
 
 A 2 sin ω2 t+B 2 cos ω2 t  
 
where A2 and B2 are adaptable coefficients and ω2 represents a second frequency based on a second mechanical parameter of the spindle motor. For example, the compensation values  26  may be generated by adding the compensation values generated based on two sinusoids, each compensating for a different mechanical parameter of the spindle motor (e.g., number of pole pairs and number of slots). Other embodiments may generate the compensation values using any suitable number of sinusoids in order to compensate for any suitable number of mechanical parameters of the spindle motor.
 
     In the above-described embodiments, the compensation values  26  may be generated as a continuous time sinusoid(s) (i.e., a continuous time base t). For example, in one embodiment the compensation values  26  may be generated using one or more oscillators that generate a continuous time sinusoidal signal and used, for example, to adjust the frequency of an oscillator (e.g., oscillator  64  in  FIG. 3 ). In other embodiments, the compensation values  26  may be generated as a discrete time sinusoid(s) (i.e., a discrete-time base k). For example, in one embodiment the compensation values  26  may be generated as discrete values of a sinusoid and used, for example, to adjust the frequency of an oscillator (e.g., oscillator  64  in  FIG. 3 ). 
     In one embodiment, the control circuitry  22  is further configured to adapt the coefficients A1 and B1 based on: 
     
       
         
           
             
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                 ⁢ 
                 
                     
                 
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                   1 
                   k 
                 
               
               - 
               
                 γ 
                 · 
                 
                     
                 
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                   tErr 
                   k 
                 
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                   sin 
                   ⁡ 
                   
                     ( 
                     
                       
                         ω 
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               B   ⁢           ⁢     1     k   +   1         =       B   ⁢           ⁢     1   k       -     γ   ·           ⁢     tErr   k     ·     cos   ⁡     (       ω   ⁢           ⁢   1   ⁢     t   k       -   φ1     )                 
where γ is a learning coefficient, and tErr k  is a timing error based on a measured rotation period of the spindle motor. For example, in one embodiment the timing error tErr k  is generated based on:
 
 t Err k =period k −(period k-1   +ff   k-1 )
 
wherein period k  represents a measured period (based on a fixed clock) between a previous zero crossing and a current zero crossing in the BEMF voltage  27 , and ff k-1  represents a timing compensation value applied to the previously measured period k-1  due to the compensation values  26 . Accordingly, in one embodiment the control circuitry  22  adapts the coefficients A1 and B1 of the compensation values  26  until the timing error tErr k  converges to substantially zero, thereby compensating for the mechanical parameter of the spindle motor.
 
     In one embodiment, the control circuitry may adapt the coefficients A1 and B1 at a frequency based on the BEMF voltage  27  zero crossings of the spindle motor. For example, the discrete-time base k in the above adaptation equation may be incremented at each BEMF voltage  27  zero crossing (or corresponding timing data read from the disk). In one embodiment, the control circuitry may generate the compensation values  26  more frequently than the coefficients A1 and B1 are adapted. For example, the control circuitry may generate the compensation values  26  as a continuous time sinusoid, or as a discrete-time sinusoid having a sampling period less than the sampling period used to adapt the coefficients A1 and B1. In the embodiments described below, the control circuitry may also generate the compensation values  26  more frequently than a closed-loop control signal used to drive the commutation sequence is updated (e.g., based on a measured velocity of the disk). For example, the measured velocity of the disk may be sampled at each BEMF voltage  27  zero crossing (or corresponding timing data read from the disk), whereas the compensation values  26  may be generated as a continuous time sinusoid, or as a discrete-time sinusoid having a lesser sampling period. Generating the compensation values  26  at a higher resolution than the coefficients A1 and B1 are adapted and/or at a higher resolution than the velocity measurements are taken during closed loop control of the commutation sequence may better compensate for the mechanical parameter of the spindle motor. 
       FIG. 4  shows control circuitry according to an embodiment wherein servo data  68  read from the disk  18  is processed at block  70  to detect a wedge number representing the current servo sector. Block  72  converts the detected wedge number into a phase of the commutation controller  24  which is loaded into a counter  56  over line  74 . The servo data  68  is also processed at block  76  to synchronize a disk locked clock to a rotation frequency of the disk  18  using any suitable technique (e.g., using a phase-locked loop). The disk locked clock is converted at block  78  into a measured rotational velocity  80  of the disk  18 . The compensation values  26  are added to the rotational velocity  80  to generate a control signal  82  applied to an oscillator  64 , where the output frequency  66  of the oscillator  64  is used to clock the counter  56 . Accordingly, the counter  56  is driven at a frequency based on the fundamental rotation frequency of the spindle motor plus an additional frequency based on the compensation values  26  (i.e., to compensate for a mechanical parameter of the spindle motor). 
     In one embodiment, each time a new servo sector is detected (wedge number is detected at block  70 ), a first conversion counter  84  is incremented and a second conversion counter  86  is incremented. When the second conversion counter  86  reaches a predetermined threshold, the disk locked clock generated at block  76  is converted at block  78  into the measured rotation velocity  80  of the disk, thereby updating the control signal applied to the oscillator  64 . Accordingly in this embodiment, the rotational velocity of the disk is measured periodically as determined by the period of the second conversion counter  86 . 
     When the first conversion counter  84  reaches a predetermined threshold, the wedge number detected at block  70  is converted into a phase at block  72  which is used to re-initialize the phase of the commutation controller  24  by loading the phase value into the counter  56  via the control line  88 . Accordingly in this embodiment, the phase of the commutation controller is periodically re-initialized as determined by the period of the first conversion counter  84  in order to periodically compensate for a cumulative phase error. In one embodiment, the first and second conversion counters  84  and  86  may operate based on different periods such that the detected wedge number may be converted into the phase more or less frequently than the disk locked clock is converted into the measured rotational velocity of the disk. 
       FIG. 5  shows control circuitry according to an embodiment wherein servo data  68  read from the disk  18  is processed at block  70  to detect a wedge number representing the current servo sector. Block  72  converts the detected wedge number into a detected phase  90  of the commutation controller  24  which is compared to the actual phase  92  of the commutation controller  24  (at the output of counter  56 ) to generate a phase error  94 . The phase error  94  is filtered using a suitable compensator  96  (e.g., proportional-integral-derivative (PID) control). The compensation values  26  are added to the filtered phase error  98  to generate a control signal  100  applied to an oscillator  64 , where the output frequency  66  of the oscillator  64  is used to clock the counter  56 . Accordingly, the counter  56  is driven at a frequency based on the fundamental rotation frequency of the spindle motor plus an additional frequency based on the compensation values  26  (i.e., to compensate for a mechanical parameter of the spindle motor). 
     In one embodiment, the timing error tErr k  in the above equation used to adapt the coefficients A1 and B1 may be based on the commutation phase error  94  shown in  FIG. 5 . That is, in one embodiment the coefficients A1 and B1 may be adapted in a manner that drives the commutation phase error  94  toward zero, thereby compensating for the mechanical parameter of the spindle motor when driving the commutation sequence. 
     Any suitable control circuitry may be employed to implement the flow diagrams in the above embodiments, such as any suitable integrated circuit or circuits. For example, the control circuitry may be implemented within a read channel integrated circuit, or in a component separate from the read channel, such as a disk controller, or certain operations described above may be performed by a read channel and others by a disk controller. In one embodiment, the read channel and disk controller are implemented as separate integrated circuits, and in an alternative embodiment they are fabricated into a single integrated circuit or system on a chip (SOC). In addition, the control circuitry may include a suitable preamp circuit implemented as a separate integrated circuit, integrated into the read channel or disk controller circuit, or integrated into a SOC. 
     In one embodiment, the control circuitry comprises a microprocessor executing instructions, the instructions being operable to cause the microprocessor to perform the flow diagrams described herein. The instructions may be stored in any computer-readable medium. In one embodiment, they may be stored on a non-volatile semiconductor memory external to the microprocessor, or integrated with the microprocessor in a SOC. In another embodiment, the instructions are stored on the disk and read into a volatile semiconductor memory when the disk drive is powered on. In yet another embodiment, the control circuitry comprises suitable logic circuitry, such as state machine circuitry. 
     In various embodiments, a disk drive may include a magnetic disk drive, an optical disk drive, etc. In addition, while the above examples concern a disk drive, the various embodiments are not limited to a disk drive and can be applied to other data storage devices and systems, such as magnetic tape drives, solid state drives, hybrid drives, etc. In addition, some embodiments may include electronic devices such as computing devices, data server devices, media content storage devices, etc. that comprise the storage media and/or control circuitry as described above. 
     The various features and processes described above may be used independently of one another, or may be combined in various ways. All possible combinations and subcombinations are intended to fall within the scope of this disclosure. In addition, certain method, event or process blocks may be omitted in some implementations. The methods and processes described herein are also not limited to any particular sequence, and the blocks or states relating thereto can be performed in other sequences that are appropriate. For example, described tasks or events may be performed in an order other than that specifically disclosed, or multiple may be combined in a single block or state. The example tasks or events may be performed in serial, in parallel, or in some other manner. Tasks or events may be added to or removed from the disclosed example embodiments. The example systems and components described herein may be configured differently than described. For example, elements may be added to, removed from, or rearranged compared to the disclosed example embodiments. 
     While certain example embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions disclosed herein. Thus, nothing in the foregoing description is intended to imply that any particular feature, characteristic, step, module, or block is necessary or indispensable. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the embodiments disclosed herein.