Servo writing method, magnetic disk apparatus and head position control method

According to one embodiment, a servo writing method for a magnetic disk apparatus that includes a magnetic disk and a head is provided. The method rotates the magnetic disk, derives a path obtained by adding a second specifying value to a first specifying value, the first specifying value corresponding to a separation distance from a rotation center of the magnetic disk, the second specifying value periodically displacing the separation distance in a radial direction synchronously with a rotation angle of the magnetic disk, the second specifying value being a value in which the separation distances at a start point and an end point of one rotation of the magnetic disk coincide, and, while causing the head to follow the path, writes servo information onto the magnetic disk with the head.

FIELD

Embodiments described herein relate generally to a servo writing method, a magnetic disk apparatus and a head position control method.

BACKGROUND

In magnetic disk apparatuses such as hard disks, servo patterns are written as servo information indicating information about positions on a magnetic disk. In the magnetic disk apparatus, by reading servo information from these servo patterns, a magnetic head can be moved to a target track.

The process of writing servo patterns onto the magnetic disk is called servo writing and is generally executed in the production process for magnetic disk apparatuses. In the conventional art, servo patterns are written at predetermined intervals (track pitches) to form tracks shaped like concentric circles or a spiral-shaped track with respect to the rotation center of the magnetic disk.

However, the track pitch of servo patterns may vary due to the influence of disturbance experienced during servo writing. If variation of the track pitch is large, servo gain varies, so that accuracy in detecting the head position, that is, accuracy in positioning the head decreases, and thus a malfunction such as a read error is likely to occur. Accordingly, a technique for reducing the influence of variation in the track pitch is desired.

DETAILED DESCRIPTION

In general, according to one embodiment, a servo writing method for a magnetic disk apparatus which comprises a magnetic disk and a head is provided. The servo writing method rotates the magnetic disk. The method derives a path obtained by adding a second specifying value to a first specifying value, the first specifying value corresponding to a separation distance from a rotation center of the magnetic disk, the second specifying value periodically displacing the separation distance in a radial direction synchronously with a rotation angle of the magnetic disk, the second specifying value being a value in which the separation distances at a start point and an end point of one rotation of the magnetic disk coincide. And, while causing the head to follow the path, the method writes servo information onto the magnetic disk with the head.

The servo writing method, magnetic disk apparatus, and head position control method according to an embodiment will be described in detail below with reference to the accompanying drawings. The present invention is not limited to this embodiment.

FIG. 1is a block diagram showing an example of the hardware configuration which a magnetic disk apparatus100of the present embodiment has. A magnetic disk111is a disk-shaped recording medium, and one or a plurality of magnetic disks111are provided along an axial direction of the rotation axis. The magnetic disk111is rotated by a spindle motor (SPM)112with the rotation axis (rotation center RC) as the center at predetermined rotation speed. Note that the SPM112is driven by a motor driver121.

A magnetic head122is provided at tip portion of an actuator arm115. The magnetic head122is moved by a voice coil motor (VCM)116driven by the motor driver121in radial directions of the magnetic disk111. While the rotation of the magnetic disk111is stopped, the magnetic head122is evacuated onto a ramp123. Note that the number of magnetic heads122(actuator arms115) is not limited to any number, but that a corresponding number of magnetic heads to the number of magnetic disks111are provided.

The magnetic head122has a write head and a read head (not shown). The write head writes data onto the magnetic disk111. The read head reads data from the magnetic disk111.

A head amplifier124amplifies the signal read by the magnetic head122from the magnetic disk111to supply to a RDC (Read Write Channel)125. Further, the head amplifier124amplifies a signal supplied from the RDC125to write data onto the magnetic disk111and supplies to the magnetic head122.

The RDC125code modulates data to be written onto the magnetic disk111supplied from an HDC131described later to supply to the head amplifier124. Further, the RDC125code demodulates a signal supplied from the head amplifier124into digital data to output to the HDC131.

A CPU (Central Processing Unit)126is a processor of the magnetic disk apparatus100. An SRAM (Static Random Access Memory)127that is a for-operation memory, a flash ROM (Read Only Memory)128that is a nonvolatile memory, and a for-temporary-storage buffer RAM129are connected to the CPU126.

The flash ROM128stores various programs (firmware, etc.) related to the operation of the magnetic disk apparatus100. Further, the flash ROM128stores a variety of setting information related to the operation of the magnetic disk apparatus100. For example, the flash ROM128stores a table in which first specifying values Ur1are associated with second specifying values Ur2as setting information. Here, the first specifying value Ur1is data specifying the radial position of a track (a track number or cylinder number). The radial position is the distance measured in a radial direction from the rotation center RC of the magnetic disk111to the track. An equal number of first specifying values Ur1to the number of tracks or cylinders written on the magnetic disk111are prepared. A second specifying value Ur2is prepared for each first specifying value Ur1. An equal number of second specifying values Ur2to the number of tracks written on the magnetic disk111may be prepared, or a second specifying value Ur2may be prepared for each of the zones partitioned along the radius. Further, second specifying values Ur2may be prepared for each magnetic head122or cylinder, or may be common to, not depending on, the magnetic heads122or cylinders. The details of the second specifying value Ur2will be described later.

The HDC (Hard Disk Controller)131provides the CPU126with access to/from function units such as the RDC125. Further, the HDC131provides the CPU126with access to/from a host computer (host)40. For example, the HDC131performs control of transmission/reception of data to/from the host computer (host)40via an I/F bus and control of the buffer RAM129. The buffer RAM129is used as a cache for data which is to be transmitted to or has been received from the host40.

The CPU126controls the operation of the magnetic disk apparatus100comprehensively by cooperating with a program stored in the flash ROM128. For example, the magnetic disk apparatus100performs servo writing to write servo information onto the magnetic disk111under the control of the CPU126. The servo information includes a variety of information used to detect the position of the magnetic head122(hereinafter called a head position) in a radial direction over the magnetic disk111. The servo information includes, e.g., servo marks, cylinder numbers, track numbers, sector numbers, a burst signal, and so on.

Further, the magnetic disk apparatus100performs head-position control to position the magnetic head122in a target position based on servo information written on the magnetic disk111and traversing velocity VSdescribed later under the control of the CPU126.

In the conventional art, servo writing at a predetermined track interval (SvTP: Servo track pitch) is performed to form concentric circle-shaped tracks with respect to the rotation center RC of the magnetic disk111. However, the SvTP may vary due to the influence of disturbance experienced at the time of servo writing. As an example of this disturbance, low-frequency noise lower than the rotation frequency of the SPM112can be cited. If variation in the SvTP is large, servo gain varies (oscillates), so that accuracy in detecting the head position decreases, and thus a malfunction such as a read error is likely to occur.

Accordingly, the magnetic disk apparatus100of the present embodiment performs processing to reduce the influence of variation in the SvTP when performing servo writing and head-position control. Specifically, the magnetic disk apparatus100performs servo writing with following servo tracks formed by adding sinusoidal waves or triangular waves to concentric circle-shaped tracks with respect to the rotation center RC. Further, when performing writing or reading data or so on for the magnetic disk111, the magnetic disk apparatus100controls the head position to describe concentric circles with respect to the rotation center RC. The control system of the magnetic disk apparatus100will be described below.

First, a control system related to servo writing (a servo-write control unit20) that the magnetic disk apparatus100comprises will be described.FIG. 2is a block diagram showing an example configuration of the servo-write control unit20that the magnetic disk apparatus100comprises.

As shown inFIG. 2, the servo-write control unit20comprises a first adder21, a subtractor22, a controller23, a second adder24, and a positioning mechanism unit25. Note that part or all of the servo-write control unit20may be constituted by software, which is implemented by the CPU126and a program cooperating. Or part or all of the servo-write control unit20may be constituted by hardware, which is implemented by a dedicated processor. Or part or all of the servo-write control unit20may be implemented by other function units such as the motor driver121and the HDC131.

The CPU126sequentially reads the first specifying value Ur1and the second specifying value Ur2of each track from the table in the flash ROM128and inputs to the first adder21. The first specifying value Ur1specifies a fixed radial position on the magnetic disk111. The second specifying value Ur2specifies a varying value by which to displace the first specifying value Ur1continuously through one rotation along the circular track with the rotation center RC as the center and with the radial position specified by the first specifying value Ur1as the radius.

As an example of the second specifying value Ur2, a sinusoidal wave Ur2_sinexpressed by the following equation (1) can be used. Here, A is an amplitude; θ is the rotation angle of the SPM112; and θofsis an arbitrary offset from the rotation angle θ.
Ur2_sin=Asin(θ+θofs)  (1)

Or as another example of the second specifying value Ur2, a triangular wave Ur2_triexpressed by the following equation (2) can be used.

Here, the second specifying value Ur2is adjusted so that the radial positions at the start point and the end point of the circular track specified by the first specifying value Ur1coincide. More specifically, the period or wavelength of the sinusoidal wave Ur2_sinor the triangular wave Ur2_triis adjusted so that one rotation of the circular track specified by the first specifying value Ur1coincides with n periods thereof, where n is ½ or an integer of one or greater. The amplitude A is set at a value greater than the SvTP (e.g., 15 times the SvTP). Where the triangular wave Ur2_triis used, the upper limit of the order k, that is, the number of sinusoidal components in the composite is not limited to any number. For example, the second specifying value Ur2may be a low-order component of the triangular wave Ur2_tricomposed of the first order to the fifth order (k=1 to 5).

FIG. 3is a diagram showing examples of the second specifying value Ur2. The horizontal axis represents the rotation angle θ of the SPM112. The vertical axis represents the value (amplitude δ) of the second specifying value Ur2with the first specifying value Ur1as a reference value (0). The graph G1denotes the case where the second specifying value Ur2is zero, that is, the circular track specified by the first specifying value Ur1. The graph G2denotes the track in the case where the sinusoidal wave Ur2_sinis used as the second specifying value Ur2. The graph G3denotes the track in the case where the triangular wave Ur2_triis used as the second specifying value Ur2.

As shown inFIG. 3, the second specifying value Ur2displaces the radial position specified by the first specifying value Ur1periodically (continuously) synchronously with the rotation angle θ. Where a low-order component of the triangular wave Ur2_triis used as the second specifying value Ur2, the track is in a shape as denoted by the graph G4.

The track of the graph G4is in a shape asymptotic to the graph G2as compared with the graph G3. AlthoughFIG. 3shows an example where one rotation of the circular track specified by the first specifying value Ur1is one period of the second specifying value Ur2, the invention is not limited to this. For example, the period of the second specifying value Ur2may be set at ½ or an integer of one or greater as mentioned above.

In the present embodiment, the table in the flash ROM128holds one rotation worth of position correction information representing the above-described sinusoidal wave Ur2_sinor triangular wave Ur2_trias the second specifying value Ur2. For example, if a cylinder (track) of the magnetic disk111has 400 sectors, the second specifying value Ur2holds 400 of position correction information corresponding to the sectors for that cylinder.

Referring back toFIG. 2, the first adder21adds the second specifying value Ur2to the first specifying value Ur1. The first adder21outputs the result of adding the first specifying value Ur1and the second specifying value Ur2as a target position Urto the subtractor22. The subtractor22calculates the offset of an observed head position ymfrom the target position Uras a deviation e (=Ur−ym).

The controller23generates a manipulation amount Uf_FEbased on the deviation e. The second adder24adds an FF control amount Uf_FFto the manipulation amount Ur_FB. The second adder24outputs the result of adding the manipulation amount Uf_FBand the FF control amount Uf_FFas a manipulation amount Ufto the positioning mechanism unit25.

Here, the FF control amount Uf_FFis a feedback control amount (manipulation amount) for suppressing the influence of disturbance d1applied at the time of servo writing. The FF control amount Uf_FFcan be obtained through learning repetitive control or the like. In the present embodiment, for example, the CPU126determines the FF control amount Uf_FFto be inputted to the second adder24based on the detecting result of a sensor (not shown) that detects a signal correlated with disturbance d1. Then the second adder24adds the FF control amount Uf_FFto the manipulation amount Uf_FBto output the manipulation amount Ufwith which accuracy in following the second specifying value Ur2is heightened.

The positioning mechanism unit25drives the VCM116according to the manipulation amount Ufgiven from the second adder24. That is, the positioning mechanism unit25moves the actuator arm115in a radial direction of the magnetic disk111. Thus, the magnetic head122is positioned in an actual head position y.

The servo-write control unit20detects the head position of the magnetic head122based on servo information and the like written on the magnetic disk111under the control of the CPU126. Here, the head position actually detected is the observed head position ymthat is disturbance d1at servo writing added to the actual head position y. The method of detecting the observed head position ymat servo writing is a publicly known technique, and hence description thereof is omitted.

Then the servo-write control unit20, with the FF control amount Uf_FFand the observed head position ymbeing inputted thereto, repeats the operation of writing new servo tracks at predetermined track pitches (SvTPs). Thus, multiple tracks worth of servo information is written as servo tracks onto the magnetic disk111. The magnetic disk apparatus100may be configured such that the CPU126inputs the FF control amount Uf_FFand the observed head position ymor that another function unit (e.g., the HDC131) inputs them.

FIG. 4is a diagram for explaining servo tracks written on the magnetic disk111. The plane formed by an X axis and a Y axis corresponds to a disk surface of the magnetic disk111. Further, the point P1in the middle of this plane corresponds to the rotation center RC of the SPM112(the magnetic disk111). Yet further, angles 0, 90, 180, and 270 plotted on the X axis and Y axis correspond to the rotation angle θ of the SPM112. Note thatFIG. 4shows a servo track written for a certain first specifying value Ur1.

The servo track ST1denotes a servo pattern in the case where the second specifying value Ur2is zero, that is, where only the first specifying value Ur1is used. As shown inFIG. 4, the path formed by the servo track ST1is in a circular shape (concentric circle) with the point P1as the center. The servo track ST2denotes a path in the case where the second specifying value Ur2is the sinusoidal wave Ur2_sin. The servo track ST3denotes a path in the case where the second specifying value Ur2is the triangular wave Ur2_tri. Note thatFIG. 4shows examples where one period of the second specifying value Ur2is one rotation of the circular path specified by the first specifying value Ur1.

As shown inFIG. 4, the paths formed by the servo tracks ST2, ST3are shaped in closed curves obtained by displacing each radial position on the servo track ST1by the amplitude δ according to the rotation angle θ. For example, the path formed by the servo track ST2is in a circular shape with the point P2offset by the amplitude δ from the point P1at an angle of 90 degrees, where the amplitude δ is maximal (δ=A), as the center. The path formed by the servo track ST3is generally in a heart shape with the point P2as the center as in the servo track ST2and with its tip being at the position of an angle of 90 degrees, where the amplitude δ is maximal (δ=A).

As such, the servo-write control unit20performs servo writing based on the target position Urobtained by adding the second specifying value Ur2specifying the sinusoidal wave Ur2_sinor triangular wave Ur2_trito the first specifying value Ur1specifying one of concentric circle-shaped tracks. Thus, servo tracks written on the magnetic disk111form non-concentric circle-shaped paths, whose distances (radial positions) from the rotation center RC vary continuously through one rotation.

Next, the operation at the time of servo writing will be described with reference toFIGS. 5 and 6. The servo writing can be divided into a servo write process of writing servo information onto each cylinder (track) and a during-servo process that is performed while writing servo information. First, the servo write process will be described with reference toFIG. 5.

FIG. 5is a flow chart showing an example procedure of the servo write process. Although this process describes an example where NULL-type servo information (servo pattern) is written, the type of the servo pattern is not limited to this.

For the NULL-type servo pattern, a burst pattern formed of an N phase and a Q phase is used. As the N phase and Q phase, magnetization patterns can be placed in which the polarity is inverted alternately at intervals of 180 degrees (=1 cylinder) when going along a radial direction of the magnetic disk111. Further, the N phase and Q phase can be offset in phase from each other by 90 degrees (=0.5 cylinder) when going along a radial direction of the magnetic disk111. For example, the N phase can be placed such that the polarity is inverted at the boundaries between adjacent tracks, and the Q phase can be placed such that the polarity is inverted at the center of each track.

The CPU126sets the cylinder number n to an initial value of 1, thereby setting the cylinder that is a first write position (B11). Then the CPU126positions the head position over the cylinder corresponding to the cylinder number n (B12).

Then the CPU126cooperates with the RDC125and the like to write a servo pattern (servo track) onto the magnetic disk111(B13). Note that while writing a servo track, the servo-write control unit20performs the during-servo process (seeFIG. 6).

When finishing writing the servo pattern for the cylinder number n, the CPU126determines whether the cylinder number n has reached an end number (B14). If determining that the cylinder number n has not reached the end number (No at B14), the CPU126sets 0.5 (cylinder) added to the cylinder number n as a new cylinder number n (B15) and returns to B12. Note that the CPU126writes servo patterns whose phases are offset from each other by 90 degrees for a cylinder number whose first digit after decimal point is zero and a cylinder number whose first digit after decimal point is 5.

On the other hand, if determining that the cylinder number n has reached the end number at B14(Yes at B14), the CPU126finishes this process.

Next, the during-servo process will be described with reference toFIG. 6.FIG. 6is a flow chart showing an example procedure of the during-servo process. This process is executed while writing a servo pattern in the above-described servo write process.

First, the CPU126detects the observed head position ym[k] of the magnetic head122(B21). Then the CPU126inputs the first specifying value Ur1[k] and the second specifying value Ur2[m] to the first adder21(B22). Here, the first specifying value Ur1[k] is the first specifying value Ur1for sample number k that specifies the cylinder (track) detected at the observed head position ym[k]. The second specifying value Ur2[m] is data for the sector (of sector number m) to perform servo writing on from among second specifying values Ur2registered in association with the first specifying value Ur1[k]. For example, if the cylinder has 400 sectors, 400 second specifying values Ur2are sequentially inputted.

The first adder21adds the first specifying value Ur1[k] and the second specifying value Ur2[m] to calculate the target position Ur[k] (B23). Then the subtractor22takes the difference between the target position Ur[k] and the observed head position ym[k] to calculate the deviation e[k] (B24). Then the controller23and the second adder24generates the manipulation amount Uf[k] based on the deviation e[k] (B25).

Then the positioning mechanism unit25drives the VCM116according to the manipulation amount Uf[k], thereby moving the magnetic head122to the target position Ur[k] (B26). The observed head position ym[k] of the magnetic head122moved by driving the VCM116is inputted recursively to the subtractor22.

Next, a control system related to head-position control (a head position control unit30) that the magnetic disk apparatus100comprises will be described.FIG. 7is a block diagram showing an example configuration of the head position control unit30that the magnetic disk apparatus100comprises.

As shown inFIG. 7, the head position control unit30comprises a first subtractor31, a second subtractor32, a controller33, and a positioning mechanism unit34. Note that part or all of the head position control unit30may be constituted by software, which is implemented by the CPU126and a program cooperating. Or part or all of the head position control unit30may be constituted by hardware, which is implemented by a dedicated processor. Or part or all of the head position control unit30may be implemented by other function units such as the motor driver121and the HDC131.

The CPU126inputs the first specifying value Ur1for a track or sector on which to perform writing or reading data or so on and the second specifying value Ur2corresponding to that the first specifying value Ur1to the first subtractor31. The first subtractor31subtracts the second specifying value Ur2from the first specifying value Ur1to output the subtracting result as the target position Urto the second subtractor32. The second subtractor32calculates the difference between the target position Urand the observed head position ymas the deviation e (=Ur−ym).

The controller33generates the manipulation amount Ufto be given to the positioning mechanism unit34based on the deviation e. The positioning mechanism unit34drives the VCM116according to the manipulation amount Ufgiven from the controller33. Thus, the magnetic head122is positioned in the actual head position y.

The head position control unit30detects the head position of the magnetic head122based on servo information read by the magnetic head122, the traversing velocity VSdescribed later, and the like under the control of the CPU126. Here, the head position actually detected is the observed head position ymthat is a path L, RRO (Repeatable RunOut), and disturbance d2added to the actual head position y. The path L is the path formed by the servo track written on the magnetic disk111according to the second specifying value Ur2in the servo write process. The RRO is a component corresponding to disturbance d1in the servo writing. The disturbance d2is a disturbance component applied when controlling the head position.

The head position control unit30recursively performs operation of positioning the magnetic head122in the target position Urwhile detecting the observed head position ym. The magnetic disk apparatus100may be configured such that the CPU126detects (inputs) the observed head position ymor that another function unit (e.g., the HDC131) detects (inputs) it. The method of detecting the observed head position ymis a publicly known technique, and hence description thereof is omitted.

In the above-described configuration of the head position control unit30, the second subtractor32subtracts the observed head position ymfrom the target position Ur. By this subtraction, the component of the second specifying value Ur2in the target position Urcancels out the component of the path L (the second specifying value Ur2) written in the servo write process. Therefore, the deviation e calculated by the second subtractor32is one obtained by removing the component of the second specifying value Ur2from the target position Ur. Thus, the actual head position y of the magnetic head122does not follow servo tracks but forms concentric circle-shaped paths with respect to the rotation center RC. Hence, the magnetic head122is substantially fixed over the track corresponding to the first specifying value Ur1(the target position Ur), so that as the magnetic disk111rotates, the magnetic head122goes across servo tracks written on the magnetic disk111.

FIG. 8is a diagram showing an example relation between servo tracks and data tracks. InFIG. 8, the horizontal axis represents the rotation angle θ of the SPM112. The vertical axis represents the track number for servo tracks written on the magnetic disk111. Note that inFIG. 8the paths of servo tracks are indicated by straight lines.

A graph G5on the right ofFIG. 8represents the state of the SvTP between servo tracks. In the graph G5, deviation to the left means that the SvTP becomes larger. Also with the servo writing method of the present embodiment, since being affected by the aforementioned disturbance d1, the SvTP varies between servo tracks as shown by the graph G5.

For example, in the case where servo tracks are based on the sinusoidal wave Ur2_sin, the head position control unit30controls the head position to follow the path of a data track DT2to remove the component of this sinusoidal wave Ur2_sin. In the case where servo tracks are based on the triangular wave Ur2_tri, the head position control unit30controls the head position to follow the path of a data track DT3to remove the component of this triangular wave Ur2_tri. By this means, the paths of the data tracks DT2, DT3are circular (in a concentric-circle shape) with respect to the rotation center RC. The data track DT1is the path in the case of following the servo track.

As shown inFIG. 8, the velocity V of the magnetic head122relative to a servo track can be decomposed into two components, velocity Vθin a circumferential direction and velocity (traverse velocity) VSin a radial direction. That is, the magnetic head122crosses servo tracks at traverse velocity V. AlthoughFIG. 8illustrates the velocity V for the data track DT3, the same applies to the data track DT2.

The head position control unit30detects the observed head position ymfrom servo information obtained by the magnetic head122crossing servo tracks, the traverse velocity VS, and the like and recursively performs operation of positioning the magnetic head122in the target position Urbased on the observed head position ym. That is, the head position control unit30detects and adjusts the observed head position ymusing multiple servo tracks which the magnetic head122has crossed.

As such, the magnetic disk apparatus100of the present embodiment detects and adjusts the head position using multiple servo tracks, thereby being able to reduce the influence of variation in the SvTP occurring between the servo tracks by an averaging effect. Thus, the magnetic disk apparatus100of the present embodiment can suppress variation in servo gain and improve accuracy in detecting the head position.

It is understood that as the traverse velocity VSbecomes larger, an offset is more likely to occur in the observed head position ymdetected. Hence, in order to reduce an offset of the demodulated position, it is preferable to decrease the maximum of the traverse velocity VS.

FIG. 9is a diagram showing examples of the traverse velocity VS. The horizontal axis represents the rotation angle θ of the SPM112. The vertical axis represents the traverse velocity VS. The graph G6indicates the traverse velocity VSin the case where the second specifying value Ur2is zero, that is, the head follows a servo track. The graph G7indicates an example of the traverse velocity VSin the case where the sinusoidal wave Ur2_sinis used as the second specifying value Ur2. The graph G8indicates an example of the traverse velocity VSin the case where the triangular wave Ur2_triis used as the second specifying value Ur2. Note that the graphs G6to G8ofFIG. 9correspond to the graphs G1to G3illustrated inFIG. 3respectively.

As shown inFIG. 9, the graphs G7, G8have shapes corresponding to the waveforms of the sinusoidal wave Ur2_sinand the triangular wave Ur2_trirespectively. The graph G8based on the triangular wave Ur2_triis seen to be lower in the maximum of the traverse velocity VSwhen comparing the graphs G7, G8. Hence, by using the triangular wave Ur2_trito write a servo track, an offset of the demodulated position can be reduced as compared with writing a servo track based on the sinusoidal wave Ur2_sin.

The graph G9indicates an example of the traverse velocity VSin the case where a low-order component of the triangular wave Ur2_triis used as the second specifying value Ur2. Note that the graph G9corresponds to the graph G4illustrated inFIG. 3.

As shown inFIG. 9, the graph G9varies more smoothly than the graph G8and is closer in shape to the graph G7. It is understood that the maximum of the traverse velocity VSof the graph G9is lower than the maximum of the graph G7. Therefore, by using a low-order component of the triangular wave Ur2_trito write a servo track, the effect of reducing an offset of the demodulated position is reduced as compared with the case of using the triangular wave Ur2_tri, but is improved as compared with the case of using the sinusoidal wave Ur2_sin.

FIG. 10is a flow chart showing an example procedure of the head position control process executed by the head position control unit30shown inFIG. 7. This process is executed to perform reading or writing data or so on for a specific track or sector.

First, the CPU126detects the observed head position ym[k] of the magnetic head122(B31). Then the CPU126inputs the first specifying value Ur1[k] and the second specifying value Ur2[m] to the first subtractor31based on the observed head position ym[k] (B32). Here, the first specifying value Ur1[k] is the first specifying value Ur1for sample number k that specifies the cylinder (track) detected at the observed head position ym[k]. The second specifying value Ur2[m] is data for the sector (of sector number m) to perform servo writing on from among second specifying values Ur2registered in association with the first specifying value Ur1[k]. For example, if the cylinder has 400 sectors, 400 second specifying values Ur2are sequentially inputted.

Then the first subtractor31subtracts the second specifying value Ur2[m] from the first specifying value Ur1[k] to calculate the target position Ur[k] (B33). Then the second subtractor32takes the difference between the target position Ur[k] and the observed head position ym[k] to calculate the deviation e[k] (B34). Then the controller33generates the manipulation amount Uf[k] based on the deviation e[k] (B35).

Then the positioning mechanism unit34drives the VCM116according to the manipulation amount Uf[k], thereby moving the magnetic head122to the target position Ur[k] (B36). The observed head position ym[k] of the magnetic head122moved by driving the VCM116is inputted recursively to the second subtractor32.

FIG. 11is a diagram for explaining the effect of suppressing variation by the servo writing method and head position control method of the present embodiment. InFIG. 11, the horizontal axis represents the type of the second specifying value Ur2that the first subtractor31subtracts from the first specifying value Ur1. Here, “FOLLOW” corresponds to the case of causing the head position to follow a servo track servo-written based on the sinusoidal wave Ur2_sin. “SINE WAVE” corresponds to the case where the sinusoidal wave Ur2_sinis used as the second specifying value Ur2. “TRIANGULAR WAVE (1st to 5th ORDERS)” corresponds to the case where a low-order component of the triangular wave Ur2_triis used as the second specifying value Ur2. “TRIANGULAR WAVE” corresponds to the case where the triangular wave Ur2_triis used as the second specifying value Ur2.

The vertical axis ofFIG. 11represents the servo gain. The servo gains for the types were obtained through simulation under the following conditions.Interval between servo tracks (SvTP): 63.5 nmVariation in SvTP: a sinusoidal wave of from 0.5 to 1.5 timesPeriod of variation in SvTP: 10 SvTPsNumber of sectors included in a servo track: 400First specifying value Ur1: 20 mmAmplitude A of the second specifying value Ur2: 1 μm

As shown inFIG. 11, the range of variation in the servo gain when causing the head to follow a servo track is larger than in the case of using the sinusoidal wave Ur2_sinor the triangular wave Ur2_tri. As such, when the servo writing method and head position control method of the present embodiment are used, variation in the servo gain associated with variation in SvTP can be suppressed. Further, because the maximum of the traverse velocity VSis lower when using the triangular wave Ur2_trithan when using the sinusoidal wave Ur2_sin, variation in the servo gain associated with variation in SvTP can be more efficiently suppressed.

For example, in the above embodiment, the self-servo writing configuration where the magnetic disk apparatus100itself performs servo writing has been described, but the invention is not limited to this. Specifically, the magnetic disk apparatus100may be configured such that a magnetic disk111onto which servo tracks have been written by an external apparatus different from the magnetic disk apparatus100is incorporated therein. Where this configuration is adopted, the external apparatus may comprise the servo-write control unit20to perform servo writing according to the above servo writing method.

Although in the above embodiment the magnetic disk apparatus is configured such that the first specifying value Ur1and the second specifying value Ur2are stored in a table before servo writing, the invention is not limited to this. For example, it may be configured such that, while determining the first specifying value Ur1and the second specifying value Ur2, it performs servo writing and stores the first specifying value Ur1and the second specifying value Ur2into a table.

Further, although in the above embodiment the magnetic disk apparatus is configured such that the second specifying value Ur2corresponding to the first specifying value Ur1is stored in a table, the invention is not limited to this. For example, it may be configured such that parameters (e.g., amplitude, phase, etc.) involved in creating the second specifying value Ur2are stored in association with the first specifying value Ur1in the flash ROM128. In this case, the CPU126generates the second specifying value Ur2using parameters in association with the first specifying value Ur1and performs servo writing and head-position control based on the first specifying value Ur1and the second specifying value Ur2. Where this configuration is adopted, the amount of information to be stored can be reduced as compared with the configuration where the second specifying value Ur2is stored as it is.

Yet further, although in the above embodiment the magnetic disk apparatus is configured such that the first specifying value Ur1and the second specifying value Ur2are stored in the flash ROM128, not being limited to this, it may be configured such that they are stored in another storage medium.