Patent Publication Number: US-9418689-B2

Title: Data storage device generating an operating seek time profile as a function of a base seek time profile

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 14/510,951 filed on Oct. 9, 2014, entitled “DATA STORAGE DEVICE DYNAMICALLY REDUCING COAST VELOCITY DURING SEEK TO REDUCE POWER CONSUMPTION,” to Mingji Lou et al., which is hereby incorporated by reference in its entirety. 
    
    
     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   i  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   i  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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a prior art disk format comprising a plurality of servo tracks defined by servo sectors. 
         FIG. 2A  shows a data storage device in the form of a disk drive according to an embodiment comprising a head actuated over a disk. 
         FIG. 2B  is a flow diagram according to an embodiment wherein an operating seek time profile is generated based on a base seek time profile. 
         FIG. 2C  shows example embodiments of operating seek time profiles generated based on a base seek time profile. 
         FIG. 2D  shows an embodiment wherein the operating seek time profile may be adjusted as a function of a runtime parameter, such as workload. 
         FIG. 3  shows an example seek profile including a coast velocity that is updated during the seek based on the measured velocity, the deceleration distance, and the deceleration time. 
         FIG. 4A  shows a function representing a deceleration distance relative to a coast velocity according to an embodiment. 
         FIG. 4B  shows a function representing a deceleration time relative to a coast velocity according to an embodiment. 
         FIG. 5  shows control circuitry according to an embodiment configured to generate an actuator control signal during the seek operation. 
         FIG. 6  shows control circuitry configured to generate the actuator control signal during the seek operation, including a specific embodiment for updating the coast velocity. 
         FIG. 7  shows an embodiment wherein the seek profile and corresponding coast velocity may vary relative to different operating seek time profiles such as shown in  FIG. 2C . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2A  shows a data storage device in the form of a disk drive according to an embodiment comprising a disk  16  comprising a plurality of tracks  18 , and an actuator  20  configured to actuate a head  22  over the disk  16 . The disk drive further comprises control circuitry  24  configured to execute the flow diagram of  FIG. 2B , wherein an operating seek time profile is generated as a function of a base seek time profile (block  26 ), wherein the seek time profiles define a seek time to seek the head radially over the disk by a seek length. A seek operation is executed (block  28 ) to seek the head by a target seek length by at least generating a target seek time based on the target seek length and the operating seek time profile (block  30 ), and seeking the head the target seek length based on the target seek time (block  32 ). 
     In the embodiment of  FIG. 2A , the disk  16  comprises a plurality of servo sectors  36   0 - 36   N  that define a plurality of servo tracks, wherein data tracks  18  are defined relative to the servo tracks at the same or different radial density. The control circuitry  24  processes a read signal  38  emanating from the head  22  to demodulate the servo sectors  36   0 - 36   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  24  filters the PES using a suitable compensation filter to generate a control signal  40  applied to a voice coil motor (VCM)  20  which rotates an actuator arm  42  about a pivot in order to actuate the head  22  radially over the disk  16  in a direction that reduces the PES. The servo sectors  36   0 - 36   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. 
       FIG. 2C  illustrates examples of different operating seek time profiles (B,C,D) that are generated based on a base seek time profile (A). Each operating seek time profile may correspond to a desired performance of the disk drive; that is, each operating seek time profile may change various performance metrics of the disk drive. For example, in one embodiment, the base seek time profile (A) may correspond to the shortest seek times and therefore increase throughput performance but also increase power consumption and/or acoustic noise. In one embodiment, the performance of the disk drive may be improved in terms of power consumption and/or acoustic noise by executing seek operations using an operating seek time profile generated by adding an offset to the base seek time profile (A) or by scaling the base seek time profile (A) by a scalar. Examples of these embodiments are illustrated in  FIG. 2C  by operating seek time profile (B) and operating seek time profile (C). Other operating seek time profiles may be generated based on the base seek time profile (A) to meet desired performance criteria, such as an operating seek time profile that achieves a desired average random seek time while limiting the maximum seek time, an example of which as illustrated by the operating seek time profile (D) in  FIG. 2C . In one embodiment, the operating seek time profile (D) may be generated as a complex function based on the seek lengths and corresponding seek times in the base seek time profile. 
     In one embodiment, the operating seek time profile may be adjusted as a function of a runtime parameter of the disk drive. For example, in one embodiment the operating seek time profile may be adjusted relative to a current workload of the disk drive, which may be measured in any suitable manner, such as based on a number and/or size of access commands queued in a command queue.  FIG. 2D  illustrates an example of this embodiment wherein the operating seek time profile (B) may be adjusted by adjusting a scalar for scaling the base seek time profile (A) relative to the workload of the disk drive. During periods of higher workloads, the scalar may be decreased so as to decrease the seek times and thereby prevent or reduce the bottleneck of throughput as seen from the host. When the workload decreases, the scalar may be increased to increase the seek times and thereby decrease power consumption and/or acoustic noise. 
     In one embodiment, the control circuitry  24  is configured to execute a seek operation based on a seek profile comprising an acceleration phase, a constant velocity phase, and a deceleration phase such as shown in  FIG. 3 . In prior art disk drives, the seek profile is configured prior to executing the seek, such as by configuring the seek profile based on a seek length and a latency available to perform the seek. However, executing a seek operation based on a preconfigured seek profile may not result in optimal seek performance in terms of power consumption, seek settling, bias learning, etc. Accordingly, in one embodiment the control circuitry  24  is configured to recursively adjust the coast velocity of the seek profile during the seek operation based on a measured velocity of the head during the seek, as well as a corresponding deceleration distance and deceleration time. Also in one embodiment, generating and/or adjusting an operating seek time profile based on a base seek time profile as described above is facilitated by configuring the seek profile on-the-fly during a seek operation based on the seek time derived from the operating seek time profile. 
       FIG. 3  illustrates an example of this embodiment wherein during the acceleration phase of a seek, the current velocity of the head is measured. A corresponding deceleration distance and deceleration time are generated based on the measured velocity of the head assuming the current measured velocity has converged to a corresponding coast velocity. A target coast velocity is then generated based on the updated deceleration distance and deceleration time. A control signal applied to the actuator is then generated based on a difference between the measured velocity and the target coast velocity. In one embodiment, the control signal increases the velocity of the head toward the target coast velocity. The above process for updating the coast velocity is repeated recursively until the measured velocity converges to the target coast velocity. 
       FIG. 4A  shows an example second order phase plane function according to an embodiment representing a relationship between the deceleration distance and the velocity of the head, and  FIG. 4B  shows a third order phase plane function according to an embodiment representing a relationship between the deceleration time and the velocity of the head. In one embodiment, these phase plane functions are used during the acceleration phase of a seek to generate the deceleration distance and deceleration time parameters that are input into the recursive algorithm for updating the coast velocity as described above with reference to  FIG. 3 . 
       FIG. 5  shows control circuitry according to an embodiment comprising a state estimator  44  configured to estimate the states  46  of the servo system, such as the current position and current velocity of the head during a seek operation. During the acceleration phase of the seek profile, block  48  recursively updates the coast velocity  50 , and block  52  generates the actuator control signal  40 A based on the updated coast velocity  50  and the estimated states  46 . When the head reaches the deceleration phase of the seek profile, block  54  generates the actuator control signal  40 B based on the estimated states  46  in order to decelerate the head toward the target track. Block  54  also generates the deceleration distance and deceleration time parameters  56  (e.g., based on  FIGS. 4A and 4B ) that are input into block  48  for updating the coast velocity during the acceleration phase of the seek profile. 
       FIG. 6  shows control circuitry according to an embodiment wherein the coast velocity  v (k)  50  is updated during the seek based on: 
                       v   _     ⁡     (   k   )       =       L   -     x   ⁡     (   k   )       -     DecelDistance   ⁡     [     v   ⁡     (   k   )       ]           T   -     (   k   )     -     DecelTime   ⁡     [     v   ⁡     (   k   )       ]                   Eq   .     (   1   )                 
where k represents a current time for the seek, L represents a seek length for the seek, x(k) represents a current position of the head at time k, v(k) represents the measured velocity of the head at time k, DecelDistance[v(k)] represents the deceleration distance for the seek based on the measured velocity, DecelTime[v(k)] represents the deceleration time for the seek based on the measured velocity, and T represents a seek time for the seek which, in one embodiment, is generated based on the operating seek time profile such as shown in  FIG. 2C . In other words, in this embodiment the above equation updates the coast velocity  v (k)  50  by dividing the current coast distance for the seek by the current coast time for the seek.
 
     In the embodiment of  FIG. 6 , block  52  generates the actuator control signal u(k)  40 A based on:
 
 u ( k )=min{ C _ slew·W ( k ), f (   v   ( k ), v ( k ))}  Eq.(2)
 
where W(k) represents a coast time for the seek based on the measured velocity, C_slew is a predetermined scalar, and f represents a suitable function of the updated coast velocity  v (k) and the measured velocity v(k), such as a proportional-integral-derivative (PID) filter that filters a difference between the coast velocity  v (k) and the measured velocity v(k) as shown in the example of  FIG. 6 . The first term in the above Eq.(2) ensures the actuator control signal u(k)  40 A decreases to zero prior to reaching the deceleration phase of the seek profile in the event the measured velocity of the head has not yet converge to the target coast velocity. In one embodiment, the coast time is generated based on:
 
                     W   ⁡     (   k   )       =         sx   ⁡     (   k   )       -     v   ⁡     (   k   )             sv   ⁡     (   k   )       +     a   ⁡     (   k   )                   Eq   .     (   3   )                 
where k represents a current time for the seek, x(k) represents a current position of the head at time k, v(k) represents the measured velocity of the head at time k, a(k) represents an acceleration of the head at time k, and s represents a slope of the phase plane function for computing the deceleration distance such as shown in  FIG. 4A .
 
     In one embodiment, the control circuitry shown in  FIG. 6  generates the actuator control signal u(k)  40 A at the beginning of the acceleration phase based on:
 
 u ( k )=min{ u _coast( k ), u _slewup}  Eq.(4)
 
where u_coast(k) is the actuator control generated based on the above Eq.(2) and u_slewup is computed as k·C where C is a constant. At the beginning of the seek, the first term in the above Eq.(4) is very large such that the actuator control signal u(k)  40 A is generated as the slew up constant u_slewup.
 
     In the example embodiment of  FIG. 3 , the measured velocity v(k) of the head eventually converges to the updated coast velocity  v (k) wherein the recursive update of the coast velocity  v (k) terminates and the coast phase of the seek profile is executed using the final value of the updated coast velocity  v (k). In one embodiment, the recursive update of the coast velocity  v (k) may terminate when the actuator control signal generated based on the above Eq.(2) falls below a threshold. In another embodiment, the measured velocity v(k) of the head may not converge to the updated coast velocity  v (k) in which case the right shaded area shown in  FIG. 3  representing the update of the coast velocity  v (k) may extend left all the way to the deceleration phase (left shaded area). In this embodiment, the actuator control signal u(k)  40 A is eventually generated based on the first term in the above Eq.(2). 
       FIG. 7  illustrates two different seek profiles generated during a seek operation according to an embodiment for seeking by a target seek length based on two different seek times each corresponding to a different operating seek time profile (e.g., as shown in  FIG. 2C ). For example, the seek profile  58 A may correspond to a seek time T generated by the operating seek time profile (A) in  FIG. 2C , and the seek profile  58 B may correspond to a seek time T generated by the operating seek time profile (B) in  FIG. 2C . Because the seek time T generated based on the operating seek time profile (A) is shorter than that generated by operating seek time profile (B), the seek profile  58 A in  FIG. 7  comprises a higher coast velocity and a shorter coast time as compared to the seek profile  58 B. 
     In one embodiment, configuring the seek profile on-the-fly based on a target seek time T during a seek operation facilitates the ability to generate any arbitrary operating seek time profile based on a base seek time profile such as shown in  FIG. 2C . In this manner, the operating seek time profile may be configured and/or adjusted to achieve any desired performance metric without needing to store a number of different operating seek time profiles and/or corresponding seek profiles. 
     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.