Bias compensation method and appropriate bias force profile generation method, recording media and hard disk drive

Provided is a method of compensating for a bias for efficient servo control of a hard disk drive, and optimally compensating for bias force based on the elapsed usage time of a hard disk drive, and a bias force profile generation method, recording media, and a hard disk drive appropriate to the bias compensation method. The bias compensation method for compensating for a bias force applied to a magnetic head of a hard disk drive includes: measuring the bias force at the current position of a head; selecting a bias force profile having a bias force similar to the bias force measured at the current position of the head, from among a plurality of bias force profiles that are obtained by measuring bias forces of a disc from based on the elapsed usage time of the hard disk drive; and compensating for the bias force according to the selected bias force profile. According to the bias force compensation method, a plurality of bias force profiles based on the elapsed usage time of the hard disk drive are generated, and by selecting an optimum bias force profile from among the stored profiles, bias compensation can be performed accurately and quickly.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Korean Patent Application No. 10-2005-0045302, filed on May 28, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of compensating for a bias for efficient servo control of a hard disk drive, and, more particularly, to a bias compensation method of optimally compensating for a bias force with respect to elapsed usage time of a hard disk drive, and a bias force profile generation method, recording media, and a hard disk drive which is appropriate to the bias compensation method.

2. Description of the Related Art

Generally, a hard disk drive is an apparatus for magnetically storing data on a rotating magnetic disk. Because of its large capacity and high speed, it is widely used as an auxiliary memory device of a computer system.

In the hard disk drive, data is stored in tracks formed as concentric circles on a magnetic disk. These tracks are accessed by a magnetic head that reads data from and writes data to the magnetic disk. Moving the magnetic head over the magnetic disk to one of the tracks is referred to as track seek.

A disturbance acting on the magnetic head during the track seek of the hard disk drive is referred to as a bias. In an HDD, major factors which affect the bias force include a pressure difference occurring when an air flow generated by the rotation of the disk pushes sides of the head, a tension of a flexible cable connected to the head, the characteristics of a bearing axis of a voice coil motor, etc. Accordingly, this bias force varies with respect to the position, the movement distance and the movement direction of the magnetic head. More specifically, the initial driving power of the voice coil varies with respect to the position and movement direction of the magnetic head and the initial movement speed of the magnetic head varies with respect to the movement distance in a seek operation. Accordingly, bias should be compensated with respect to the position, the movement distance, and the movement direction of the magnetic head. This bias force compensation method is disclosed in Japanese Patent Application Laid-Open Nos. Hei 6-215508, Hei 4-339371, and Hei 6-20408, etc.

According to the conventional bias force compensation method, a surface of a disk is divided into a plurality of areas ranging from its inner circumference to its outer circumference, and the bias force in each area is measured and stored in a bias compensation table. Then, the bias compensation table is referred to for bias compensation on a track seek operation.

However, a bias force can vary with respect to user environments. Accordingly, in actual practice, it is widely adopted that if the difference between the measured bias force according to the positions of the head and the bias force of the corresponding head position stored in the bias force table is considerably large, then the bias forces in that vicinity of the corresponding head position or in all areas in the bias force table are updated.

However, if only the bias forces in that vicinity are updated, since the remaining areas keep the previous bias forces without change, a deviation between bias forces in adjacent areas becomes larger.

Meanwhile, if the bias forces of all areas are updated, it is difficult to calculate a new adequate bias force for each area, and the calculation takes [too] much time. Further, assuming that the bias forces change non-linearly, because it needs to adapt a proportional constant with respect to the position of the head on the disk, the calculation takes quite a lot of time.

SUMMARY OF THE INVENTION

An aspect of the present invention is to provide a bias compensation method for optimally compensating for a bias force with respect to an elapsed usage time of a hard disk drive.

Another aspect is to provide a bias force profile generation method for a hard disk drive appropriate to the bias compensation method.

A further aspect is to provide computer readable recording media storing programs to execute the above methods.

An additional aspect is to provide a hard disk drive appropriate to the bias force compensation method.

According to an aspect of the present invention, there is provided a bias compensation method for compensating for a bias force applied to a magnetic head of a hard disk drive, the method including: measuring the bias force at the current position of a head; selecting a bias force profile having a bias force most similar to the bias force measured at the current position of the head, from among a plurality of bias force profiles that are obtained by measuring bias forces of a disc from its inner circumference to its outer circumference and generated with respect to the elapsed usage time of the hard disk drive; and compensating for the bias force according to the selected bias force profile.

According to another aspect of the present invention, there is provided a bias force profile generation method for generating a bias force profile for compensating for a bias force applied to a head, the method including: generating a bias force profile by measuring a bias force with respect to the position of a head of a hard disk drive by continuously moving the head from the inner circumference to the outer circumference of a disk; generating n bias force profiles by repeatedly performing the bias force profile generation n times at predetermined time intervals with respect to the elapsed usage time of the hard disk drive; and storing the n bias force profiles in a recording area that is accessed by the hard disk drive.

According to still another aspect of the present invention, there is provided a computer readable recording medium having embodied thereon a computer program for executing a bias compensation method for compensating for a bias force applied to a magnetic head of a hard disk drive, wherein the bias compensation method includes: measuring the bias force at the current position of a head; selecting a bias force profile having a bias force most similar to the bias force measured at the current position of the head, from among a plurality of bias force profiles that are obtained by measuring bias forces of a disc from its inner circumference to its outer circumference and generated with respect to the elapsed usage time of the hard disk drive; and compensating for a bias force according to the selected bias force profile.

According to yet still another aspect of the present invention, there is provided a computer readable recording medium having embodied thereon a computer program for executing a bias force profile generation method for generating a bias force profile for compensating for a bias force applied to a head, wherein the bias force profile generation method includes: generating a bias force profile by measuring a bias force with respect to the position of a head of a hard disk drive by continuously moving the head from the inner circumference to the outer circumference of a disk; generating n bias force profiles by repeatedly performing the bias force profile generation n times at predetermined time intervals with respect to the elapsed usage time of the hard disk drive; and storing the n bias force profiles in a recording area that is accessed by the hard disk drive.

According to a further aspect of the present invention, there is provided a hard disk drive including: a disk which stores information; a spindle motor which rotates the disk; a head which writes information to and reads information from the disk; a voice coil motor which moves the head; a memory which stores a bias force profile for compensating for a bias force applied to the head; and a controller which measures a bias force in a track following operation, selects a bias force profile having a bias force most similar to the bias force measured at the current position of the head, from among a plurality of bias force profiles that are obtained by measuring bias forces of a disc from its inner circumference to its outer circumference and generated with respect to the elapsed usage time of the hard disk drive, updates the memory with the selected bias force profile, and compensates for a bias force according to the bias force profile stored in the memory in a track seek mode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1illustrates bias force profiles generated by a bias force profile generation method according to an embodiment of the present invention;

Referring toFIG. 1, the horizontal axis indicates the distance from the outer circumference to the inner circumference of the disk, and the vertical axis indicates the bias force. Thus a bias force profile shows the changes of the bias force with respect to the position of the head on the disk.

According to a bias force profile generation method of the present invention, a plurality of bias force profiles are made from the time when the hard disk drive begins to be used, that is, when data begins to be written or read as a track seek or track following operation is performed.

Each bias force profile has bias forces measured while moving the head (continuously) from the inner circumference to the outer circumference of the disk. A bias force is represented by a DC component of a driving current applied to a voice coil motor when the head follows a track.

The bias force profiles shown inFIG. 1are prepared in the manufacturing process of the hard disk drive, preferably in a burn-in test process from the inner diameter (ID) to the outer diameter (OD). The bias force profiles are stored in a system area (maintenance area) of the disk, and referred to when compensation of a bias force is needed for example, in a track seek operation.

FIG. 1shows 5 bias force profiles. Referring toFIG. 1, it can be seen that the bias force converges on a predetermined value as time elapses after the hard disk drive begins to be used. For example, from the bottom to the top on the left-hand side ofFIG. 1♦, □, Δ, x, and * are shown, and these marks indicate values measured at the initial use time (0) of the hard disk drive and at 2 minutes, 4 minutes, 6 minutes and 8 minutes after the initial use time respectively. As can be seen, the bias force converges on *, that shows the best value[s] as time elapsed.

Accordingly, it can be learned that the bias force can be compensated for by selecting one of the bias force profiles shown inFIG. 1by referring to the elapsed usage time after the hard disk drive began to be used.

However, in this bias force compensation method the elapsed time after the hard disk drive began to be used must be known at the time of bias compensation.

In order to detect this elapsed time, it is possible to use a timer, but this is not so efficient. This is because it is difficult to accurately calculate variables such as a pause time, the operation conditions of the hard disk drive, etc, which should be considered in determining the elapsed time.

In the present invention, a bias force profile to be used for bias force compensation is selected by a more practical way. More specifically, in the present invention, a bias force profile having a similar bias force to the actual bias force measured is selected.

This method is more effective when respective bias forces of bias profiles with respect to a head position are sufficiently separated from each other.

This method enables an optimum compensation of bias force not only with respect to the elapsed usage time of the hard disk drive but also with respect to other variables for example, operation temperature, humidity, atmospheric pressure, etc.

According to experiment results, if there are about 5 bias force profiles measured at intervals of 2 minutes, then the bias force can be substantially compensated for.

FIG. 2is a flowchart of the operations performed by a bias force compensation method according to an embodiment of the present invention.

It is assumed that bias force profiles for bias force compensating as shown inFIG. 1are prepared beforehand. The bias force profiles for bias force compensating as shown inFIG. 1are stored in a system area of a hard disk drive or a non-volatile memory.

First, if power is provided to a hard disk drive, a bias force profile to be used for bias force compensating in the initial stage of usage is selected, and stored in a bias force compensation table in operation S202.

If power is provided to the hard disk drive, a controller (not shown) reads a bias force profile corresponding to initial use from among the prepared bias force profiles, for example, that marked with ♦ inFIG. 1, and stores it in preparation for bias force compensation afterward.

Track seek is performed in operation S204. The track seek operation is performed together with a bias force compensation, and the bias force compensation is performed by using the bias force profile stored in the bias force compensation table.

The bias force is measured in operation S206. The bias force is measured when the track seek operation is finished and a track following operation is performed. The bias force is detected with the DC component of a voice coil motor driving current.

An optimum bias force profile, which has a bias force equal to that measured, with respect to the position of the head on the disk, is selected in operation S208.

It is assumed that A, B, C, and D are respectively the measured bias force, a bias force of the bias force profile being used for a bias force compensation (the current bias force profile), a bias force of a bias force profile at a time before the current bias force profile, and a bias force of a bias force profile at a time after the current bias force profile. Here, the bias force profiles before and after the current profile can be easily understood by considering that in the bias force profile generation method according to the present invention, bias force profiles are generated at predetermined time interval from the time when the hard disk drive begins to be used. For example, if the current bias force profile is that marked with □ ofFIG. 1, the bias force profiles before and after the current profile are those marked with ♦ and Δ, respectively.

A bias force profile to use is selected as follows.

If |A−B|<|A−C|& |A−B|<|A−D|, then maintain the current bias force profile.

If |A−C|<|A−B|& |A−C|<|A−D|, then replace the current bias force profile with the bias force profile at a time after the current bias force profile.

If |A−D|<|A−C|& |A−D|<|A−B|, then replace the current bias force profile with the bias force profile at a time prior to the current bias force profile.

That is, by comparing the differences between the measured bias force and the bias forces of the current, before and after bias force profiles, the bias force profile having a bias force (most) similar or closest to the measured bias force is selected.

The selected bias force profile, the bias compensation table is updated in operation S210. The controller selects an optimum bias force profile having a bias force identical to the bias force measured, respect to the position of the head on the disk, from among the bias force profiles recorded in the system area of the disk, and updates the bias compensation table by the selected profile.

As the result, the bias compensation table has a bias force profile to be referred to in the next track seek operation.

In the bias force compensation method according to the present invention, measuring and updating of the bias force can be performed after a track seek is performed for more than a predetermined distance.

In the present invention, when a track seek operation is performed, bias compensation by referring to a bias force profile stored in the bias compensation table is performed, and in a track following operation after the track seek operation, an actual bias force is measured and the bias force profile to be used in next bias force compensation is updated.

Though selection of a bias force profile before track seek may be good, this is not desirable for maximizing the operation speed of the hard disk drive.

In the present invention, measuring of the bias force may be performed periodically at predetermined time intervals during a track following operation.

FIG. 3illustrates the structure of a hard disk drive10. The drive10includes at least one disk12rotated by a spindle motor14. The drive10also includes a head16positioned close to the surface18of the disk12.

The head16can read information from or write information to the rotating disk12by sensing a magnetic field of the disk12or magnetizing a part of the disk12. Typically, the head16is coupled with one of the surfaces18of disk12. Though a single head16is shown, this should be understood as including a recording head for magnetizing the disk12and a separate read head for sensing the magnetic field of the disk12. The read head is made with a magneto-resistive (MR) element.

The head16can be integrated into a slider20. The slider20has a structure for generating an air bearing between the head16and the surface of the disk12. The slider20is coupled with a head gimbal assembly22. The head gimbal assembly is attached to an actuator arm24having a voice coil26. The voice coil26is located close to a magnetic assembly28defining a voice coil motor (VCM)30. A current provided to the voice coil26generates torque that rotates the actuator arm24about the bearing assembly32. The rotation of the actuator arm24moves the head16across the surface18of the disk12.

Typically, information is stored in circular tracks34of the disk12. Each track34generally includes a plurality of sectors. Each sector includes a data field and an identification field. The identification field is comprised of a gray code for identifying a sector and a track (cylinder). The head16moves across the surface18of the disk12in order to read information from or to write information to another track. An operation of moving the head across to another track is generally referred to a track seek routine.

FIG. 5illustrates the circuit structure of the apparatus shown inFIG. 4. As shown inFIG. 5, the hard disk drive includes a disk12, a magnetic head16, a pre-amp210, a write/read channel220, a buffer230, a controller240, a ROM250A, a RAM250B, a host interface260, and a voice coil motor (VCM) driving unit270.

The ROM250A stores a variety of commands and data to be used by the controller240in order to execute a software routine. One such software routine is a seek control routine for moving the head16from one track to another track while compensating for a bias force by referring to a bias force profile selected from among a plurality of bias force profiles generated considering the elapsed usage time of the hard disk drive. Also, as an embodiment, the ROM250A stores equations for generating acceleration, velocity, and position trajectories of a sine waveform for track seek. Furthermore, the ROM250A can also store a program for measuring the bias force when a track following operation is performed; selecting a bias force profile to be referred to when a track seek operation is performed, by comparing the measured bias force with a plurality of bias force profiles recorded in the system area of the disk; and storing the selected bias force profile in a bias compensation table for bias force compensation.

The RAM250B stores information required for driving the hard disk drive, which is read from the ROM250A or the disk12when the drive begins to operate. In particular, the RAM250B stores bias force profiles to be referred to for bias force compensation when a track seek operation is performed in the form of a bias compensation table.

The controller240analyzes a command received from a host device (not shown) through the host interface260, and performs control of the hard disk drive40corresponding to the analysis result. The controller240provides a control signal to the VCM driving circuit270in order to control the movement of the VCM and the magnetic head16.

First, the normal operation of the hard disk drive will be explained.

In a data read mode, the hard disk drive first amplifies in the pre-amp210an electric signal sensed by the read head the magnetic head16from the disk12. Then, in the write/read channel220, a gain is controlled by an automatic gain control circuit (not shown), and the signal amplified in the pre-amp210is amplified to a predetermined level. The analog signal amplified to the predetermined level by the automatic gain control circuit is coded into a digital signal that can be read by the host device (not shown); converted into stream data; temporarily stored in the buffer230; and then transmitted to the host device through the host interface260.

Next, in a data write mode, the hard disk drive receives input data from the host device through the host interface260; temporarily stores the data in the buffer230; converts the data into a binary data stream appropriate to the write channel, by sequentially outputting the data stored in the buffer230; and then sends a write current amplified by the pre-amp210to the disk12through the write head of the magnetic head16.

Then, the track seek method performed in the controller240will be explained in detail.

In the track seek operation, the controller240calculates a seek time with respect to a seek length, generates acceleration, velocity, and position trajectories based on the calculated seek time, and executes a process of controlling the driving current of the voice coil motor. At this time, the controller240compensates for the driving current of the voice coil motor by referring to the bias force profile stored in the RAM250B.

FIG. 5illustrates a typical track seek servo control system implemented by a hardware and a software executed by the controller240shown inFIG. 4.

A seek trajectory generator60calculates the position xd(n), velocity vd(n), and design acceleration ad(n) of the head16from the sine wave acceleration trajectory and the velocity and position trajectories obtained by integrating the acceleration trajectory whenever the head16reads a gray code of the track34.

The state estimator62performs a process for estimating a state variable value of the head motion including the actual position and actual velocity information of the head, by using a state equation from a position error signal and control signal uk that are being input.

A first adder64subtracts an actual position value xr(n) from a position value xd(n). A position control gain compensator66generates a position compensation value obtained by multiplying the difference of the position value and the actual position value calculated by the first adder64, by the position gain (kp) for position compensation.

Next, a second adder68adds a design velocity value vd(n) to the position compensation value generated by the position control gain compensator66, and then subtracts an actual velocity value vr(n).

Then, a velocity control gain compensator70generates a velocity compensation value obtained by multiplying the value calculated by the second adder68by a velocity gain (kv) for velocity compensation.

Next, a third adder72generates a seek driving current control value uk(n) by adding the velocity compensation value and the design acceleration value.

A bias force compensator74generates the seek driving current control value obtained by compensating for the bias force in the seek driving current control value uk(n) with respect to the position of the head16by referring to the bias force profile, and applies this to the VCM driver76. Then, the VCM driver provides a current corresponding to the seek driving current control value in which the bias force is compensated for, to the voice coil of the head disk assembly10to rotate the voice coil motor and move the head16.

The acceleration, velocity, position trajectories and current trajectory according to a seek servo algorithm of a sine waveform can be obtained from the following equation 1:

Here, TSKdenotes a seek time, IMdenotes a maximum current provided to the voice coil, and KA denotes an acceleration constant.

In relation to a given seek length (XSK), a time (t) is identical to the seek time (TSK) and their relationship is described by the following equation 2:

From the equation 2, a seek time (TSK) required for a given seek length (XSK) can be obtained by using the following equation 3:

According to equation 3, it can be seen that a calculated seek trajectory of a sine waveform restricted by the current (IM) applied to the voice coil motor is generated.

However, since the motion of the head16is actually disturbed by bias force acting on the head16, it is necessary to compensate for the disturbance.

The controller240compensates for the current (IM) applied to the voice coil motor by referring to the bias force profile stored in the RAM250B.

For this, as described above with reference to the flowchart ofFIG. 2, when power is applied to the hard disk drive, the controller240selects the bias force profile for an initial use of the hard disk drive, for example, that marked with ● inFIG. 1, from among a plurality of bias force profiles in the system area of the disk, and stores it in the RAM250B.

After that time, the controller240detects the bias force at the current position on the disk using the DC component of the current (IM) applied to the voice coil motor when a track following operation is performed. Once the bias force is detected, the controller240compares the measured bias force with the bias forces of the plurality of bias force profiles recorded in the system area of the disk, selects an optimum bias force profile, and replaces the bias force profile stored in the RAM250B with the selected bias force profile.

If a track seek operation begins, the controller240compensates for the bias force by referring to the bias force profile stored in the RAM250B.

In the bias force compensation method according to the present invention, bias force profiles are generated by referring to the elapsed usage time of the hard disk drive, but when these are used for actual compensation, one of them is selected by referring to a measured bias force. Accordingly, the present invention is robust against the influences of operation temperature, humidity, and atmospheric pressure, as well as the elapsed usage time of the hard disk drive.

For example, after a usage time of 2 minutes, if the operation temperature changes, the bias force profile indicated by ▪ may not be appropriate. In the present invention, a bias force profile is not selected according to the elapsed usage time. Instead, according to the measured bias force, an optimum bias force profile is selected from among a plurality of bias force profiles prepared in advance and compensation is performed using the selected profile. Accordingly, comprehensive compensation can be performed in consideration of temperature, humidity, atmospheric pressure and the like.

According to the bias force compensation method of the present invention as described above, a plurality of bias force profiles based on the elapsed usage time of the hard disk drive are generated, and by selecting an optimum bias force profile from among the profiles in order to compensate for a bias force, bias compensation can be performed accurately and quickly.