Patent Publication Number: US-2020294541-A1

Title: Magnetic disk device and head seek control method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2019-048723, filed Mar. 15, 2019, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a magnetic disk device and a head seek control method. 
     BACKGROUND 
     Magnetic disk devices change a current or a voltage applied to a voice coil motor (VCM) when a head is accelerated during head seek. Magnetic disk devices can apply a larger current to a VCM when a head is decelerated during head seek as compared to a case where a head is accelerated by a counter electromotive force or the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating an example of a configuration of a magnetic disk device according to an embodiment. 
         FIG. 2  is a plan view pictorially illustrating an example of a position of a head with respect to a disk. 
         FIG. 3  is a diagram illustrating an example of a target velocity curve according to an embodiment. 
         FIG. 4  is an enlarged view illustrating an example of a settling mode switching distance of the target velocity curve illustrated in  FIG. 3 . 
         FIG. 5  is a diagram illustrating an example of a change in head acceleration with respect to a remaining distance to the target radial position corresponding to the target velocity curve illustrated in  FIG. 4 . 
         FIG. 6  is a diagram illustrating an example of a current flowing to a VCM when a seek operation of a head according to an embodiment is performed. 
         FIG. 7  is a diagram illustrating an example of a head velocity when the seek operation illustrated in  FIG. 6  is performed. 
         FIG. 8  is a diagram illustrating an example of a position of a head when the seek operation illustrated in  FIG. 6  is performed. 
         FIG. 9  is a block diagram illustrating an example of a control system of head seek processing according to an embodiment. 
         FIG. 10  is a flowchart illustrating an example of each sample processing during acceleration according to an embodiment. 
         FIG. 11  is a flowchart illustrating an example of processing of updating a mode switching condition according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, a magnetic disk device comprises: a disk; a head that writes data to the disk and reads data from the disk; an actuator comprising a voice coil motor that moves the head over the disk; and a controller that increases a second current value applied to the voice coil motor during deceleration of the head in a seek according to a first current value that is maximum value capable of being applied to the voice coil motor during acceleration of the head in the seek on the disk. 
     Hereinafter, embodiments will be described with reference to the drawings. It should be noted that the drawings are merely examples and do not limit the scope of the invention. 
     Embodiment 
       FIG. 1  is a schematic diagram illustrating an example of a configuration of a magnetic disk device  1  according to an embodiment. 
     The magnetic disk device  1  includes a housing HS, a head disk assembly (HDA)  10 , a driver IC  20 , a head amplifier integrated circuit (hereinafter referred to as a head amplifier IC or a preamplifier)  30 , a volatile memory  70 , a buffer memory (buffer)  80 , a nonvolatile memory  90 , and a system controller  130  that is a 1-chip integrated circuit. In addition, the magnetic disk device  1  is connected to a host system (hereinafter simply referred to as a host)  100 .  FIG. 1  pictorially illustrates a cross section of the HDA  10 . 
     The HDA  10  includes a magnetic disk (hereinafter referred to as a disk) DK, a spindle motor (hereinafter referred to as SPM)  13  that rotates the disk DK around a spindle  12 , an arm AM that carries a head HD, and a voice coil motor (hereinafter referred to as a VCM)  14 . The SPM  13  and the VCM  14  are secured to the housing HS. The disk DK is attached to the spindle  12  and is rotated by the driving of the SPM  13 . The head HD faces the disk DK. The arm AM and the VCM  14  constitute an actuator AC. The actuator AC rotates around the rotation axis to position the head HD attached to the tip of the arm AM at a particular position of the disk DK. Two or more disks DK and two or more heads HD may be provided. For example, at least two disks DK and at least two heads HD are provided. 
       FIG. 2  is a plan view pictorially illustrating an example of the position of the head HD with respect to the disk DK. In the radial direction of the disk DK, the direction on the spindle  12  side is referred to as inward direction (inward), and the direction opposite to the inward direction is referred to as an outward direction (outward). The direction orthogonal to the radial direction of the disk DK is referred to as a circumferential direction.  FIG. 2  illustrates the rotational direction of the disk DK in the circumferential direction. It should be noted that the rotational direction may be opposite to the direction illustrated in  FIG. 2 . 
     In the disk DK, a user data area UA available to a user, and a system area SA to which information necessary for system management (hereinafter also referred to as system information) is written are allocated to the area to which the data can be written. Hereinafter, a particular position of the disk DK in the radial direction may be referred to as a radial position, and a particular position of the disk DK in the circumferential direction may be referred to as a circumferential position. The radial position corresponds to, for example, a track, and the circumferential position corresponds to, for example, a sector. The radial position and the circumferential position may be collectively referred to simply as a position. 
     The head HD includes a slider as a main body, and includes a write head WH and a read head RH mounted on the slider so as to face the disk DK. The write head WH writes data to the disk DK. The read head RH reads data recorded on the track of the disk DK. As illustrated in  FIG. 2 , for example, the head HD slides in the horizontal plane of the disk DK during seek as the actuator AC rotationally drives around a bearing BR. Hereinafter, the position (for example, the radial position) of the head HD in the disk DK may be simply referred to as a head position. 
     The driver IC  20  drives the SPM  13  and the VCM  14  according to the control of the system controller  130  (specifically, the MPU  50  described later). The driver IC  20  includes an SPM control unit  21  and a VCM control unit  22 . The SPM control unit  21  controls the rotation of the SPM  13 . The VCM control unit  22  controls the VCM  14  by controlling the supplied current. It should be noted that part of the configuration of the driver IC  20  (for example, the SPM control unit  21  and the VCM control unit  22 ) may be included in the system controller  130 . 
     The head amplifier IC (preamplifier)  30  amplifies a signal read from the disk DK and outputs the amplified signal to the system controller  130  (specifically, a read/write (R/W) channel  40  described later). In addition, the head amplifier IC  30  outputs, to the head HD, a write current corresponding to a signal output from the R/W channel  40 . The head amplifier IC  30  includes a write signal controller  31  and a read signal detection unit  32 . The write signal controller  31  controls a write current to be output to the head HD according to the control of the system controller  130  (specifically, the MPU  60  described later). The read signal detection unit  32  detects a signal written by the write head or a signal read by the read head. It should be noted that part of the configuration of the head amplifier IC  30  (for example, the write signal controller  31  and the read signal detection unit  32 ) may be included in the system controller  130 . 
     The volatile memory  70  is a semiconductor memory in which stored data is lost when power supply is cut off. The volatile memory  70  stores data and the like necessary for processing in each unit of the magnetic disk device  1 . The volatile memory  70  is, for example, a dynamic random access memory (DRAM) or a synchronous dynamic random access memory (SDRAM). 
     The buffer memory  80  is a semiconductor memory that temporarily records data and the like transmitted and received between the magnetic disk device  1  and the host  100 . It should be noted that the buffer memory  80  may be integrated with the volatile memory  70 . The buffer memory  80  is, for example, a DRAM, a static random access memory (SRAM), an SDRAM, a ferroelectric random access memory (FeRAM), and a magnetoresistive random access memory (MRAM). 
     The nonvolatile memory  90  is a semiconductor memory that records stored data even when power supply is cut off. The nonvolatile memory  90  is, for example, a NOR type or NAND type flash read only memory (FROM). 
     The system controller (controller)  130  is realized by, for example, using a large scale integrated circuit (LSI) called a system-on-a-chip (SoC) in which a plurality of elements are integrated on a single chip. The system controller  130  includes a read/write (R/W) channel  40 , a hard disk controller (HDC)  50 , and a microprocessor (MPU)  60 . The system controller  130  is electrically connected to the driver IC  20 , the head amplifier IC  30 , the volatile memory  70 , the buffer memory  80 , the nonvolatile memory  90 , and the host system  100 . It should be noted that the system controller  130  may include the SPM control unit  21 , the VCM control unit  22 , the write signal controller  31 , and the read signal detection unit  32 . In addition, the system controller  130  may include the driver IC  20  and the head amplifier IC  30 . 
     The R/W channel  40  performs signal processing of read data transferred from the disk DK to the host  100  and write data transferred from the host  100  in response to an instruction from the MPU  60  described later. The R/W channel  40  has a circuit or function that measures the signal quality of the read data. The R/W channel  40  is electrically connected to, for example, the head amplifier IC  30 , the HDC  60 , and the MPU  60 . 
     The HDC  50  controls data transfer between the host  100  and the R/W channel  40  in response to an instruction from the MPU  60  described later. The HDC  50  is electrically connected to, for example, the R/W channel  40 , the MPU  60 , the volatile memory  70 , the buffer memory  80 , and the nonvolatile memory  90 . 
     The MPU  60  is a main controller that controls each unit of the magnetic disk device  1  in response to an instruction from the host  100  or the like. The MPU  60  controls the actuator AC through the driver IC  20  and performs servo control for positioning the head HD. The MPU  60  controls the write operation of data to the disk DK and selects the storage destination of the write data. In addition, the MPU  60  controls the operation of reading data from the disk DK and controls the processing of read data. The MPU  60  is connected to each unit of the magnetic disk device  1 . The MPU  60  is electrically connected to, for example, the driver IC  20 , the R/W channel  40 , and the HDC  50 . 
     The MPU  60  includes a seek controller  61 , a limit current calculation unit  62 , and a velocity controller  63 . The MPU  60  performs the processing of these units, such as the seek controller  61 , the limit current calculation unit  62 , and the velocity controller  63 , on firmware. It should be noted that the MPU  60  may include these units as circuits. Part of the configuration of the MPU  60  may be included in the HDC  50 . For example, the seek controller  61 , the limit current calculation unit  62 , and the velocity controller  63  may be provided in the HDC  50 . In addition, the MPU  60  may include the configuration or function of the HDC  50 . 
     The seek controller  61  controls the seek of the head HD from a particular radial position of the disk DK (hereafter also referred to as a start position) to a target radial position of the disk DK (hereafter referred to as a target position or a target radial position) in response to an instruction from the host  100  or the like. The seek controller  61  controls the velocity of the head HD (hereinafter also referred to as a head velocity) while seeking the head HD from the start position to the target position. During seek, the seek controller  61  controls the head velocity from the start position, completes the control of the head velocity near the target position, and switches the control of the head velocity to the settling of the head. For example, during seek, the seek controller  61  accelerates the head HD in a section in a particular radial direction (hereinafter also referred to as an acceleration section) or in a particular period (hereinafter also referred to as an acceleration period), moves the head HD at a constant velocity in a particular section (hereinafter also referred to as a constant velocity section) or in a period (hereinafter also referred to as a constant velocity period) subsequent to the acceleration section, decelerates the head HD in a particular section (hereinafter also referred to as a deceleration section) or in a period (hereinafter also referred to as a deceleration period) subsequent to the constant velocity section, and positions the head HD at the target position by settling in a particular section (hereinafter also referred to as a settling section) or in a period (hereinafter also referred to as a settling period) subsequent to the deceleration section. In other words, the seek controller  61  switches the control mode of the head velocity during seek. The seek controller  61  switches an acceleration mode for accelerating the head HD, a constant velocity mode for moving the head HD at a constant velocity, a deceleration mode for decelerating the head HD, and a settling mode for settling the head HD during seek. In addition, a state in which the acceleration mode is being performed may be referred to as an acceleration timing, a state in which the constant velocity mode is being performed may be referred to as a constant velocity timing, a state in which the deceleration mode is being performed may be referred to as a deceleration timing, and a state in which the settling mode is being performed may be referred to as a settling timing. It should be noted that, during seek, the seek controller  61  may accelerate the head HD in the acceleration section or the acceleration period, decelerate the head HD in the deceleration section subsequent to the acceleration section or the deceleration period subsequent to the acceleration period, and position the head HD at the target position by settling in the settling section subsequent to the deceleration section or the settling period subsequent to the deceleration period. 
     The limit current calculation unit (limit voltage calculation unit)  62  calculates a current value (voltage value) to be applied to the VCM  14 . Hereinafter, the “current value or voltage value to be applied to the VCM  14 ” may be simply referred to as a “current value or voltage value”. The limit current calculation unit (limit voltage calculation unit)  62  detects a current value (hereinafter referred to as a saturation current value) or a voltage value (hereinafter referred to as a saturation voltage value) in a case of saturation acceleration on the head HD, and calculates (or estimates) a current value of the limit where a current or voltage does not saturate (hereinafter referred to as a limit current value) or a voltage value of the limit where a current or voltage does not saturate (hereinafter referred to as a limit voltage value) based on the saturation current value or the saturation voltage value. The limit current value (or the limit voltage value) is a value smaller than the saturation current value (or the saturation voltage value) and close to the saturation current value (the saturation voltage value). During acceleration, the limit current calculation unit  62  performs saturation acceleration on the head HD in a particular section (hereinafter also referred to as a saturation acceleration section) or in a particular period (hereinafter also referred to as a saturation acceleration period), detects (or estimates) a saturation current value, and calculates a limit current value based on the saturation current value. It should be noted that the limit current calculation unit  62  can estimate a resistance of the VCM  14  (hereinafter also referred to as a VCM resistance or a VCM resistance estimation value) based on the saturation current value and the head velocity when the saturation current value is applied to the VCM  14 . The limit current calculation unit  62  can estimate a coil temperature of the VCM  14  (hereinafter also referred to simply as a coil temperature) based on the VCM resistance. In addition, the limit current calculation unit  62  may detect the current value or the voltage value. 
     The velocity controller  63  controls the head velocity through the VCM  14 . In other words, the velocity controller  63  controls the head velocity by the current value (or the voltage value) applied to the VCM  14 . It should be noted that the velocity controller  63  may control the head velocity by controlling the current value (or the voltage value) applied to the VCM  14  according to the VCM resistance or the coil temperature. The velocity controller  63  controls the head velocity according to a set velocity condition of the head HD, for example, a change in target head velocity with respect to the remaining distance to the target radial position (hereinafter referred to as a target velocity curve). Hereinafter, the “target head velocity” may be referred to as a “target velocity”. The velocity controller  63  controls the head velocity according to the target velocity curve (hereinafter also referred to as a nominal velocity curve) set based on a design nominal value, for example, a current value (hereinafter referred to as a nominal current value) or a voltage value (hereinafter referred to as a nominal voltage) set by design, an environmental temperature (hereinafter referred to as a nominal environmental temperature) set by design, and a coil temperature (hereinafter referred to as a nominal coil temperature) set by design, and the like. 
     The velocity controller  63  changes the velocity condition during deceleration, and controls the head velocity according to the changed velocity condition during deceleration. The velocity controller  63  increases the head velocity during deceleration or the current value (or the voltage value) during deceleration as compared to the head velocity during deceleration or the current value (or the voltage value) during deceleration, which is set by the design nominal value at the time of decelerating, for example, the nominal current value (or the nominal voltage value) during deceleration, the nominal environment temperature during deceleration, and the nominal coil temperature during deceleration, based on the limit current value (or the limit voltage value) calculated during acceleration. In other words, the velocity controller  63  increases the head velocity during deceleration or the current value (or the voltage value) during deceleration as compared to the head velocity during deceleration or the current value (or the voltage value) during deceleration, which is set by the design nominal value during decelerating, based on the saturation current value (or the saturation voltage value) detected during acceleration. The velocity controller  63  changes a target velocity curve (hereinafter referred to as a target deceleration curve), which is a reference during deceleration which is set based on the design nominal value during deceleration, to a target deceleration curve (hereinafter referred to as a limit deceleration curve) when the limit current value is applied to the VCM  14 , based on the limit current value (or the limit voltage value). When changing from the target deceleration curve to the limit deceleration curve, the velocity controller  63  corrects the limit deceleration curve such that the state when switching from the deceleration mode to the settling mode in the limit deceleration curve matches the state before the change. For example, when changing from the target deceleration curve to the limit deceleration curve, the velocity controller  63  corrects the limit deceleration curve, such that the remaining distance to the target radial position (hereinafter referred to as the settling mode switching distance) for switching from the deceleration mode to the settling mode in the limit deceleration curve matches the settling mode switching distance in the target deceleration curve, and the acceleration of the head HD of the limit deceleration curve at the settling mode switching distance (hereinafter also referred to as head acceleration) matches the head acceleration of the target deceleration curve at the settling mode switching distance, and controls the head velocity according to the corrected limit deceleration curve. Hereinafter, the “head acceleration of the target deceleration curve at the settling mode switching distance” may be referred to as “settling mode switching acceleration”. The settling mode switching distance is a distance that corresponds to the distance of the boundary between the decelerating section and the settling section and corresponds to the end of the decelerating section or the decelerating period. It should be noted that the velocity controller  63  may change the target deceleration curve set based on the design nominal value during deceleration to the target deceleration curve (hereinafter also referred to as the saturation deceleration curve) when the saturation current value is applied to the VCM  14 , based on the saturation current value, may correct the limit deceleration curve such that the settling mode switching state is matched as in the limit deceleration curve, and may control the head velocity according to the corrected saturation deceleration curve. 
     For example, the velocity controller  63  calculates the limit deceleration curve by multiplying the target deceleration curve by the coefficient x calculated based on a margin (hereinafter referred to simply as a current margin or a voltage margin) of the current value (or the voltage value) applied to the VCM  14  during acceleration, which is calculated according to the limit current value. For example, the velocity controller  63  calculates the coefficient x (=√(limit current value/nominal current value)) corresponding to the square root of the ratio of the limit current value to the nominal current value that is the reference from the current margin, and calculates the limit deceleration curve by multiplying the target deceleration curve by the coefficient x. The nominal current value is, for example, smaller than the limit current value. It should be noted that the velocity controller  63  may calculate the coefficient x (=√(limit voltage value/nominal voltage value during acceleration)) corresponding to the square root of the ratio of the limit voltage value to the nominal voltage value during acceleration from the voltage margin, and calculate the limit deceleration curve by multiplying the target deceleration curve by the coefficient x. The nominal voltage value is, for example, smaller than the limit voltage value. The target deceleration curve represents the change in the target velocity with respect to the remaining distance. Therefore, when this is multiplied by the coefficient x, the change in the remaining distance per time and the change in the target velocity per time are x times, respectively. The acceleration, that is, the change in velocity per time, is approximately x{circumflex over ( )}2 times. The velocity controller  63  detects a head velocity (hereinafter referred to as a corresponding velocity) and a head position (hereinafter referred to as a corresponding position) in the limit deceleration curve corresponding to the settling mode switching acceleration (the slope of the target deceleration curve at the settling mode switching distance), and calculates a target deceleration curve (hereinafter referred to as a corrected deceleration curve) in which the limit deceleration curve is corrected, based on a head velocity (hereinafter also referred to as a settling mode switching velocity) corresponding to the settling mode switching distance in the target deceleration curve and a difference value (hereinafter referred to as a velocity correction value) of the corresponding velocity, and a difference value (hereinafter referred to as a distance correction value) between the settling mode switching distance and the corresponding distance. The velocity controller  63  controls the head velocity according to the corrected deceleration curve during deceleration. 
     For example, the velocity controller  63  calculates the corrected deceleration curve by Equation (1) below. 
         Vref _ cr=f ( p +( p 2− p 1))× x +( v 2− v 1)  (1)
 
     Vref_cr is the corrected deceleration curve, x is the coefficient for calculating the limit deceleration curve by multiplying the target deceleration curve, f(p) is the target deceleration curve, p 1  is the settling mode switching distance, p 2  is the corresponding distance corresponding to the slope f′(p 2 )×x of the limit deceleration curve that is the same as the slope (settling mode switching acceleration) f′(p 1 ) of the target deceleration curve at p 1 , that is, f′(p 2 )×x=f′(p 1 ), v 1  is the value of the target deceleration curve f(p 1 ) when p=p 1 , that is, the settling mode switching velocity, and v 2  is the value of the limit deceleration curve f(p 2 )×x when p=p 2 , that is, the corresponding velocity. In addition, f′(p) is a derivative of f(p). In other words, f′(p) corresponds to the first derivative of f(p). 
       FIG. 3  is a diagram illustrating an example of the target velocity curve according to an embodiment. In  FIG. 3 , a horizontal axis indicates a remaining distance, and a vertical axis represents a head velocity. In the horizontal axis of  FIG. 3 , a value increases in the direction of the positive value as it proceeds in the direction of the positive arrow from the origin (=0), and a value decreases in the direction of the negative value as it proceeds in the direction of the negative arrow from the origin (=0). In the horizontal axis of  FIG. 3 , the origin (0) corresponds to the target position in the seek. In the vertical axis of  FIG. 3 , a value increases in the direction of the positive value as it proceeds in the direction of the positive arrow from the origin (=0), and a value decreases in the direction of the negative value as it proceeds in the direction of the negative arrow from the origin (=0).  FIG. 3  illustrates a target deceleration curve L 1 , a limit deceleration curve L 2 , and a corrected deceleration curve L 3 . 
     In the example illustrated in  FIG. 3 , the velocity controller  63  calculates the limit deceleration curve L 2  by multiplying the target deceleration curve L 1  by the coefficient x, and corrects the limit deceleration curve L 2  to the corrected deceleration curve L 3 . The velocity controller  63  controls the velocity of the head HD based on the remaining distance according to the corrected deceleration curve L 3  during deceleration. 
       FIG. 4  is an enlarged view illustrating an example of the settling mode switching position D 1  of the target velocity curve illustrated in  FIG. 3 , and  FIG. 5  is a diagram illustrating an example of the change in head acceleration with respect to the remaining distance corresponding to the target velocity curve illustrated in  FIG. 4 .  FIG. 5  illustrates changes AL 1 , AL 2 , and AL 3  in head acceleration with respect to the remaining distance. The change AL 1  in head acceleration illustrated in  FIG. 5  corresponds to the target deceleration curve L 1  illustrated in  FIG. 4 , the change AL 2  in head acceleration illustrated in  FIG. 5  corresponds to the limit deceleration curve L 2  illustrated in  FIG. 4 , and the change AL 3  in head acceleration illustrated in  FIG. 5  corresponds to the corrected deceleration curve L 3  illustrated in  FIG. 4 . 
     In  FIGS. 4 and 5 , a horizontal axis represents a remaining distance. In the horizontal axes of  FIGS. 4 and 5 , a value increases in the direction of the positive value as it proceeds in the direction of the positive arrow from the origin (=0), and it decreases in the direction of the negative value as it proceeds in the direction of the negative arrow from the origin (=0). In the horizontal axes of  FIGS. 4 and 5 , the origin (0) corresponds to the target position in the seek. In the horizontal axes of  FIGS. 4 and 5 , the remaining distance D 1  corresponds to the settling mode switching position p 1  described above, and the remaining distance D 2  corresponds to a corresponding position corresponding to the slope (acceleration) of the limit deceleration curve that is the same as the slope (settling mode switching acceleration) of the target deceleration curve L 1  at the settling mode switching position p 1  described above. In addition, in the horizontal axis of  FIG. 4 , a remaining distance D 3  is a particular remaining distance farther from the target position than the remaining distance D 1  and D 2 . 
     In  FIG. 4 , a vertical axis represents a head velocity. In the vertical axis of  FIG. 4 , a value increases in the direction of the positive value as it proceeds in the direction of the positive arrow from the origin (=0), and a value decreases in the direction of the negative value as it proceeds in the direction of the negative arrow from the origin (=0). The vertical axis of  FIG. 4  represents a head velocity (settling mode switching velocity) VEL 1  of the target deceleration curve L 1  corresponding to the remaining distance (settling mode switching position) D 1 , a head velocity (corresponding velocity) VEL 2  of the limit deceleration curve L 2  corresponding to the remaining distance (corresponding position) D 2 , a head velocity VEL 3  of the target deceleration curve L 1  corresponding to the remaining distance D 3 , and a head velocity VEL 4  of the corrected deceleration curve L 3  corresponding to the remaining distance D 3 . 
     In  FIG. 5 , a vertical axis represents a head acceleration. The value on the vertical axis of  FIG. 5  corresponds to the first derivative of the value on the vertical axis of  FIG. 4 . In the vertical axis of  FIG. 5 , a value increases in the direction of the positive value as it proceeds in the direction of the positive arrow from the origin (=0), and a value decreases in the direction of the negative value as it proceeds in the direction of the negative arrow from the origin (=0). The vertical axis in  FIG. 5  represents a head acceleration (settling mode switching acceleration) ACC 1  of a change AL 1  in head acceleration corresponding to the remaining distance (settling mode switching position) D 1 , a head acceleration ACC 2  of a change AL 2  in head acceleration corresponding to the remaining distance D 2 , and a head acceleration ACC 3  of a change AL 3  in head acceleration corresponding to the remaining distance D 1 . In  FIG. 5 , the head acceleration (settling mode switching acceleration) ACC 1  and the head acceleration ACC 3  match each other. 
     For example, the velocity controller  63  calculates the coefficient x corresponding to the square root of the ratio of the nominal current value during acceleration to the limit current value based on the current margin during acceleration calculated according to the limit current value, and calculates the limit deceleration curve L 2  by multiplying the target deceleration curve L 1  by the coefficient x as illustrated in  FIG. 4 . The velocity controller  63  detects the corresponding velocity VEL 2  of the limit deceleration curve L 2  corresponding to the head acceleration ACC 2  and the corresponding position D 2  corresponding to the head acceleration ACC 2  in the change AL 2  in head acceleration that is the same as the settling mode switching acceleration ACC 1  of the settling mode switching position D 1  in the target deceleration curve L 1 . The velocity controller  63  corrects the limit deceleration curve L 2  to the corrected deceleration curve L 3  based on the position correction value DD corresponding to the difference value between the settling mode switching position D 1  and the corresponding position D 2  and the velocity correction value DVL corresponding to the difference value between the settling mode switching velocity VEL 1  and the head velocity VEL 2 . In other words, based on the position correction value DD and the velocity correction value DVL, the velocity controller  63  calculates the corrected deceleration curve L 3  by moving the limit velocity curve L 2  such that the corresponding velocity VEL 2  at the corresponding position D 2  of the limit velocity curve L 2  matches the settling mode switching velocity VEL 1  at the settling mode switching position D 1  of the target deceleration curve L 1 . At this time, the head acceleration ACC 3  of the change AL 3  in the head acceleration corresponding to the settling mode switching position D 1  matches the settling mode switching acceleration ACC 1  corresponding to the settling mode switching position D 1 . For example, the corrected deceleration curve L 3  is expressed as follows from Equation (1) described above. 
         L 3= f ( p +( D 2− D 1))× x +( VEL 2− VEL 1)  (2)
 
     The velocity controller  63  controls the head velocity according to the corrected deceleration curve L 3  during deceleration. 
     In the example illustrated in  FIG. 4 , the head velocity VEL 4  at the remaining distance D 3  of the corrected deceleration curve L 3  is larger than the head velocity VEL 3  at the remaining distance D 3  of the target deceleration curve L 1 . In addition, as described above, the corrected deceleration curve L 3  and the target deceleration curve L 1  match each other at the settling mode switching position D 1 . By controlling the head velocity according to the corrected deceleration curve L 3  as illustrated in  FIG. 4 , it is possible to maximize the value of the current applied to the VCM  14 , and it is possible to improve the head velocity during deceleration and stabilize the settling. For example, when the head velocity is controlled according to the target deceleration curve L 1 , the velocity controller  63  can apply, to the VCM  14 , the current value up to about 70% of the nominal current value during deceleration, but when the head velocity is controlled according to the corrected deceleration curve L 3 , the velocity controller  63  can apply, to the VCM  14 , the current value up to about 90% of the nominal current value during deceleration. 
       FIG. 6  is a diagram illustrating an example of the current flowing to the VCM when the seek operation according to the present embodiment is performed.  FIG. 7  is a diagram illustrating an example of the head velocity when the seek operation according to the present embodiment is performed.  FIG. 8  is a diagram illustrating an example of the head position when the seek operation according to the present embodiment is performed. In  FIGS. 6 to 8 , a horizontal axis represents a time. In the horizontal axes of  FIGS. 6 to 8 , the time progresses as it progresses in the direction of the tip of the arrow. In  FIG. 6 , a vertical axis represents the current applied to the VCM  14  during the seek of the head  15  (hereinafter also referred to as a seek current). In the vertical axis of  FIG. 6 , the seek current increases in the direction of the positive value as it proceeds in the direction of the positive arrow from the origin (=0), and the seek current decreases in the direction of the negative value as it proceeds in the direction of the negative arrow from the origin (=0).  FIG. 6  illustrates the change CCL in seek current when deceleration control is performed based on the corrected deceleration curve, for example, the corrected deceleration curve L 3  of  FIGS. 3 and 4 , and the change TCL in seek current when deceleration control is performed based on the target deceleration curve, for example, the target deceleration curve L 1  of  FIGS. 3 and 4 . In  FIG. 7 , a vertical axis represents a head velocity between seeks of the head  15 . In the vertical axis of  FIG. 7 , the head velocity increases in the direction of the positive value as it proceeds in the direction of the positive arrow from the origin (=0), and the head velocity decreases in the direction of the negative value as it proceeds in the direction of the negative arrow from the origin (=0).  FIG. 7  illustrates the change CVL in head velocity when deceleration control is performed based on the corrected deceleration curve, for example, the corrected deceleration curve L 3  of  FIGS. 3 and 4 , and the change TVL in head velocity when deceleration control is performed based on the target deceleration curve, for example, the target deceleration curve L 1  of  FIGS. 3 and 4 . In  FIG. 8 , a vertical axis represents a head position between seeks of the head  15 . In the vertical axis of  FIG. 8 , the head position increases in the direction of the positive value as it proceeds in the direction of the positive arrow from the origin (=0), and the head position decreases in the direction of the negative value as it proceeds in the direction of the negative arrow from the origin (=0).  FIG. 8  illustrates the change CPL in head position when deceleration control is performed based on the corrected deceleration curve, for example, the corrected deceleration curve L 3  of  FIGS. 3 and 4 , and the change TPL in head position when deceleration control is performed based on the target deceleration curve, for example, the target deceleration curve L 1  of  FIGS. 3 and 4 . In the vertical axis of  FIG. 8 , the origin (=0) corresponds to the target position. 
     As illustrated in  FIG. 6 , the seek current during deceleration of the change CCL in the seek current is increased as compared to the seek current during deceleration which corresponds to the tip side of the arrow of the horizontal axis in the change TCL in seek current. In other words, when the deceleration control is performed based on the corrected deceleration curve, the seek current during deceleration is increased as compared to the case where the deceleration control is performed based on the target deceleration curve. 
     As illustrated in  FIG. 7 , when the deceleration control is performed based on the corrected deceleration curve, the change in head velocity during deceleration of the head velocity change CVL becomes steeper than the change in head velocity during deceleration of the head velocity change TVL, as compared to the case where the deceleration control is performed based on the target deceleration curve. In other words, when the deceleration control is performed based on the corrected deceleration curve, the change in head velocity becomes steeper as compared to the case where the deceleration control is performed based on the target deceleration curve. 
     As illustrated in  FIG. 8 , the head position during deceleration of the change CPL in head position reaches the target position more quickly than the seek current during deceleration of the change TPL in head position. In other words, the seek time is shortened by performing the deceleration control based on the corrected deceleration curve, as compared to the case where the deceleration control is performed based on the target deceleration curve. 
       FIG. 9  is a block diagram illustrating an example of a control system SY of the seek processing of the head HD according to the present embodiment. 
     The magnetic disk device  1  includes a seek control system SY that performs seek processing of the head HD. The seek control system SY includes a target velocity generator S 1 , a position feedback (FB) controller S 2 , a velocity feedback (FB) controller S 3 , a mode switch S 4 , an acceleration controller S 5 , a VCM S 6 , a state estimator S 7 , a limit current estimation unit S 8 , and operating units C 1 , C 2 , and C 3 . 
     The target velocity generator S 1  generates a target velocity of the head HD. The position feedback controller S 2  performs feedback control associated with the position (for example, radial position) of the head HD. The velocity feedback controller S 3  performs feedback control associated with the velocity of the head HD. The mode switch S 4  switches the seek mode of the head HD, for example, an acceleration mode for accelerating the head HD, a constant velocity mode for moving the head HD at a constant velocity, a deceleration mode for decelerating the head HD, and a settling mode for performing settling, and the like. The acceleration controller S 5  controls the acceleration of the head HD. The VCM S 6  corresponds to the VCM  14  described above. The state estimator S 7  is a state observer and has a plant model or an internal state variable. The state estimator S 7  estimates the position (for example, radial position) of the next head HD. The limit current estimation unit S 8  estimates (or calculates) the limit current value based on the saturation limit current and the like. 
     In the seek control system SY, the target position and the estimated velocity are input to the operating unit C 1 . The operating unit C 1  inputs an output based on the target position and the estimated velocity to the target velocity generator S 1 , the position feedback controller S 2 , and the mode switch S 4 . The target velocity generator S 1  inputs, to the operating unit C 2 , an output (target velocity) based on the target position, the estimated velocity, and the output (limit current value) from the limit current estimation unit. The operating unit C 2  inputs an output from the target velocity generator S 1  and an output based on the estimated position to the velocity feedback controller S 3  and the mode switch S 4 . The position feedback controller S 2  inputs an output based on the target position to the mode switch S 4 . The velocity feedback controller S 3  inputs an output based on the output from the operating unit C 2  to the mode switch S 4 . In addition, in the seek control system SY, an elapsed time from the start of the seek of the head HD (hereinafter simply referred to as elapsed time) is input to the mode switch S 4  and the acceleration controller S 5 . The acceleration controller S 5  inputs an output based on the elapsed time and the output from the limit current estimation unit S 8  to the mode switch S 4 . Based on the target position, the elapsed time, the output from the target velocity generator S 1 , the output from the position feedback controller S 2 , the output from the velocity feedback controller S 3 , and the output from the acceleration controller S 5 , the mode switch S 4  performs mode switching, for example, switching of the acceleration mode, the constant velocity mode, the deceleration mode, and the settling mode, and inputs a control signal for performing the switched mode to the VCM S 6 , the state estimator S 7 , and the limit current estimation unit S 8 . The VCM S 6  is driven based on the control signal from the mode switch S 4 , moves the head HD to the observation position, and inputs an output (observation position) based on the driving amount to the operating unit C 3 . The operating unit C 3  inputs an output based on the output (observation position) from the VCM S 6  and the output (estimated position) from the state estimator S 7  to the state estimator S 7 . The state estimator S 7  calculates the estimated acceleration, the estimated velocity, and the estimated position based on the control signal from the mode switch S 4  and the output from the operating unit C 3 , inputs the estimated position to the operating unit C 2 , the operating unit C 3 , and the position feedback controller S 2 , inputs the estimated velocity to the operating unit C 1 , and inputs the estimated acceleration to the limit current estimation unit S 8 . The limit current estimation unit S 8  estimates the limit current value based on the control signal from the mode switch S 4  and the estimated acceleration from the state estimator S 7 , and inputs the limit current value to the target velocity generator S 1  and the acceleration controller S 5 . 
       FIG. 10  is a flowchart illustrating an example of each sample processing during acceleration according to the present embodiment. 
     The MPU  60  calculates the head position (B 1001 ), estimates the head velocity and the head acceleration (B 1002 ), and detects the saturation acceleration section (B 1003 ). The MPU  60  determines a current instruction value to be applied to the VCM  14  so as to perform saturation acceleration (B 1004 ), and applies the determined current instruction value to the VCM  14  (B 1005 ). The MPU  60  estimates the next sample state (B 1006 ), updates the mode switching condition, for example, the settling mode switching condition (B 1007 ), determines whether mode switching is to be performed (B 1008 ), and ends the processing. 
       FIG. 11  is a flowchart illustrating an example of the processing of updating the mode switching condition according to the present embodiment.  FIG. 11  corresponds to the processing of updating the mode switching condition of B 1007  of  FIG. 10 . 
     The MPU  60  updates an equivalent current force constant and a VCM resistance estimation value (B 1101 ), and determines whether an equivalent current force constant and a VCM resistance estimation value of a previous sample change (B 1102 ). When it is determined that the change has not occurred (No in B 1102 ), the MPU  60  ends the processing. When it is determined that the change has occurred (Yes in B 1102 ), the MPU  60  calculates the coefficient x (B 1103 ). The MPU  60  calculates the corresponding position p 2  that satisfies f′(p 2 )×x=f′(p 1 ) (B 1104 ). The MPU  60 , for example, calculates the limit deceleration curve by multiplying the target deceleration curve Vref by the coefficient x, and detects the settling mode switching position p 1  and the settling mode switching velocity v 1  corresponding to the settling mode switching position p 1 . The MPU  60  calculates the corresponding velocity v 2 =f(p 2 ) based on the corresponding position p 2 . The MPU  60  corrects the target deceleration curve Vref to the corrected deceleration curve based on the coefficient x, the settling mode switching position p 1 , the settling mode switching velocity v 1 , the corresponding position p 2 , and the corresponding velocity v 2  (B 1105 ), updates the settling mode switching condition based on the corrected deceleration curve (B 1106 ), and ends the processing. 
     According to the present embodiment, during acceleration, the magnetic disk device  1  performs saturation acceleration on the head HD, detects the saturation current value, and calculates the limit current value based on the saturation current value. The magnetic disk device  1  calculates the target deceleration curve based on the design nominal value during deceleration, and calculates the limit deceleration curve by multiplying the target deceleration curve by the coefficient x calculated based on the current margin during acceleration calculated according to the limit current value. When changing from the target deceleration curve to the limit deceleration curve, the magnetic disk device  1  detects the corresponding position and the corresponding velocity of the limit deceleration curve corresponding to the settling switching acceleration, and corrects the limit deceleration curve to the corrected deceleration curve based on the position correction value and the velocity correction value. When the head velocity is decelerated according to the corrected deceleration curve to perform settling during deceleration, the magnetic disk device  1  can increase the current value applied to the VCM  14  during deceleration, and can improve the head velocity during deceleration and stabilize settling. Therefore, the magnetic disk device  1  can shorten the seek time. Therefore, the magnetic disk device  1  can improve the access performance. 
     While certain 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. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.