Micro actuator DC gain calibration scheme for HDD dual-stage actuator systems

A servo system for a hard disk drive comprising a first actuator, a second actuator, a head, and a disk on which is formed a plurality of tracks containing servo data. The servo system comprises a first stimulus, a second stimulus, and a calibration system. The first stimulus causes the first actuator to move the head to at least two calibration tracks on the disk. The second stimulus causes the second actuator to move the head relative to the at least two calibration tracks. The calibration system generates a calibration factor based on the second calibration signal and the movement of the second actuator relative to each of the at least two calibration tracks.

FIELD OF THE INVENTION

The present invention relates to digital storage devices and, more particularly, to systems and methods of calibrating the gain of the second stage of dual-stage actuator systems for hard disk drives.

BACKGROUND OF THE INVENTION

A disk drive is a digital data storage device that stores information on concentric tracks on a storage disk. The storage disk is coated on one or both of its primary surfaces with a magnetic material that is capable of changing its magnetic orientation in response to an applied magnetic field. During operation of a disk drive, the disk is rotated about a central axis at a constant rate. To read data from or write data to the disk, a magnetic transducer (or head) is positioned adjacent to a desired track of the disk while the disk is spinning.

Writing is performed by delivering a polarity-switching write current signal to the magnetic transducer while the transducer is positioned adjacent to the desired track. The write signal creates a variable magnetic field at a gap portion of the magnetic transducer that induces magnetically polarized transitions on the desired track. The magnetically polarized transitions are representative of the data being stored.

Reading is performed by sensing the magnetically polarized transitions on a track with the magnetic transducer. As the disk spins adjacent to the transducer, the magnetically polarized transitions on the track induce a varying magnetic field into the transducer. The transducer converts the varying magnetic field into a read signal that is delivered to a preamplifier and then to a read channel for appropriate processing. The read channel converts the read signal into a digital signal that is processed and then provided by a controller to a host computer system.

When data is to be written to or read from the disk, the transducer must be moved radially relative to the disk. In a seek mode, the transducer is moved radially inwardly or outwardly to arrange the transducer above a desired track. In an on-track mode, the transducer reads data from or writes data to the desired track. The tracks are typically not completely circular. Accordingly, in the on-track mode the transducer must be moved radially inwardly and outwardly to ensure that the transducer is in a proper position relative to the desired track. The movement of the transducer in on-track mode is referred to as track following.

Modern hard disk drives employ a dual-actuator system for moving the transducer radially relative to the disk. A first stage of a dual-actuator system is optimized for moving the transducer relatively large distances. A second stage of a dual-actuator system is optimized for moving the transducer relatively small distances. The present invention relates to hard disk drives having dual-stage actuator systems.

FIGS. 1 and 2depict a mechanical portion of an example disk drive10. The disk drive10further comprises control electronics typically including a preamplifier, a read/write channel, a servo control unit, a random access memory (RAM), and read only memory (ROM), spindle motor, and dual-stage driving electronics. The electronic portion is or may be conventional and will not be described herein beyond what is necessary for a complete understanding of the present invention.

FIGS. 1 and 2show that the mechanical portion of the disk drive10includes a disk12that is rotated by a spin motor14. The spin motor14is mounted to a base plate16. The disk drive10includes at least one and typically a plurality of disks12, each with one or two recording surfaces. During use, the disk12is rotated about a spindle axis A. The term “cylinder” is often used to refer to the tracks on each of the recording surfaces that are located at the same radial distance from the spindle axis.

The disk drive10further comprises what is commonly referred to as a head18. The head18comprises or supports the magnetic read/write transducer described above; the head18will be referred to herein as the component of the disk drive10that reads data from and writes data to the disk12.

FIGS. 1 and 2further illustrate a positioning system20of the disk drive10. The positioning system20comprises a bearing assembly22that supports at least one actuator arm assembly24. The actuator arm assembly24supports the head18adjacent to one recording surface26of one of the disks12. Typically, the bearing assembly22will support one actuator arm assembly24and associated head18adjacent to each of the recording surfaces26of each of the disks12. The actuator arm assemblies24allow each head18to be moved as necessary to seek to a desired track27in seek mode and then follow the desired track27in track following mode.

Typically, the actuator arm assemblies24are fixed relative to each other; the positioning system20thus moves at least a portion of the actuator arm assemblies24together. In this case, all of the actuator arm assemblies24will be located adjacent to the same track on each of the recording surfaces, or, stated alternatively, at the same cylinder.

The positioning system20depicted inFIGS. 1 and 2is a dual-stage system. Accordingly, each actuator arm assembly24comprises a first actuator structure30and a second actuator structure32. For ease of illustration,FIGS. 1 and 2depict the first and second actuator structures30and32as comprising first and second elongate actuator arms34and36, respectively, and the actuator structures30and32may be implemented as shown inFIGS. 1 and 2.

The actuator structures30and32may, however, be implemented using other structures or combinations of structures. For example, the first actuator structure30may comprise an elongate arm that rotates about a first axis B, while the second actuator structure32may comprise a suspension assembly rigidly connected to a distal end of the first actuator. In this case, the first actuator is able to rotate about an actuator axis, while the head18would be suspended from the second actuator for linear movement along the disk radius relative to the position of the first actuator. The actuator structures30and32may thus take any number of physical forms, and the scope of the present invention should not be limited to the exemplary actuator structures30and32depicted inFIGS. 1 and 2.

Conventionally, the bearing assembly22is also considered part of the first actuator structure30. In particular, the bearing assembly22supports a proximal end40of the first actuator arm34for rotation about a first axis B, while a distal end42of the first actuator arm34supports a proximal end44of the second actuator arm36for rotation about a second axis C. In this case, the head18is supported on a distal end46of the second actuator arm36.

FIG. 2also illustrates that the exemplary actuator structures30and32of the positioning system20form part of a first actuator50and a second actuator52. For the purposes of the following discussion, the first actuator50is identified as a voice coil motor (VCM) and the second actuator52is identified as a piezoelectric transducer (PZT). However, the actuators50and52may be formed by any device capable of movement in response to an electrical control signal as will be described below.

In particular, based on a first actuator control signal, the first actuator50moves the first actuator arm34to change an angular position of the head18relative to the first axis B. The second actuator52is supported by the distal end42of the first actuator structure30to rotate the head18about the second axis C based on a second actuator control signal. InFIG. 2, an angular position of the first actuator arm34is represented by reference character D, while an angular position of the second actuator arm36is represented by reference character E.

A range of movement “S” associated with the second actuator structure32is defined by the stroke “s+” and “s−” in either direction relative to a neutral position D defined by the first actuator arm34. The term “actual displacement” (ds inFIG. 2) refers to the angular difference at any point in time of the head18relative to the neutral position as defined by the position D of the first actuator structure30. When the head18is in the neutral position, the actual displacement of the second actuator arm36is zero.

FIG. 2further identifies arbitrary first and second tracks TAand TBon the disk12. The actuator arm assembly24is shown in an initial position by solid lines and in a target position by broken lines; the first track TAwill thus be referred to as the “initial track” and the second track TBwill be referred to as the “target track”. It should be understood that the terms “initial track” and “target track” are relative to the position of the head18before and after a seek operation. Any track27on the disk12may be considered the initial track or the target track depending upon the state of the disk drive10before and after a particular seek operation.

FIG. 3contains a block diagram of a servo system60incorporating a conventional two-stage actuator system. The servo system60will typically be embodied as a software program running on a digital signal processor, but one of ordinary skill in the art will recognize that control systems such as the servo system60described herein could be implemented in hardware.

The servo system60comprises a first stage62and a second stage64. As described above, the disk12defines a plurality of tracks27in the form of generally concentric circles centered about a spindle axis C. The first stage62controls the VCM50and the second stage64controls the PZT52to support the head18adjacent to a desired one of the tracks27. The first and second actuator control signals are generated as part of this larger servo system60.

More specifically, an input signal “R” is combined with a position error signal “PES” by a first summer70. The second stage position signal Y2is indicative of an actual position of the actuator52of the second stage64, and a second stage position estimate signal “Y2est” is indicative of an estimated position of the actuator52of the second stage64. The second summer72combines the second stage position estimate signal “Y2est” and the output of the first summer70. A first stage position signal “Y1” is indicative of the actual position of the first actuator50of the first stage62. A third summer74combines the first and second stage position signals “Y1” and “Y2”. System disturbances “d” are represented as an input to the third summer74. The position error signal “PES” thus represents the combination of the first and second position signals “Y1” and “Y2” with any system disturbances “d”.

The source of the input signal “R” and the first and second stage position signals “Y1” and “Y2” is or may be conventional and will be described herein only to the extent necessary for a complete understanding of the present invention. As will be described in further detail below, each of the tracks T contains data sectors containing stored data and servo sectors containing servo data. The servo data identifies each individual track T to assist in seek operations and is also configured to allow adjustment of the radial position of the head18during track following. As is conventional, a servo demodulation unit generates the position error signal “PES” and the first and second stage position signals “Y1” and “Y2” based on the servo data read from the disk12. The input signal “R” is generated by a host computer or is simply zero during track following.

Referring now back to the servo system60, the overall bandwidth of the system60is determined by the second stage64. The gain variation of the second stage64thus directly affects the bandwidth and stability margins of the entire system60. The need thus exists to calibrate the gain of the second stage64to improve drive performance (consistent system bandwidth) and reliability (consistent stability margins, accurate screening during the self-test).

However, PES non-linearities can adversely affect conventional methods of calibrating the gain of the second stage64. In particular, a conventional method of measuring the gain of the second stage64is to operate the first stage62to perform a track follow operation on a particular track while applying a calibration signal of known amplitude the second stage64. The calibration signal is predetermined to cause movement of the second actuator arm36relative to the position D of the first actuator arm34equal to approximately one-half the width of the track. By monitoring the servo data read by the head18while the calibration signal is applied to the second stage64, the actual displacement of the second actuator arm36relative to the position D can be measured.

The relationship between the calibration signal, the PES signal, and the actual displacement of the second actuator structure32during calibration thus allows a calibration factor to be generated for a particular positioning system. The PES signal is thus an important factor when calibrating the second stage64, and PES non-linearities adversely affect the ability of the positioning system to generate an accurate calibration factor.

The inventors have recognized that one important cause of PES non-linearities is the configuration of the servo sectors formed on the recording surfaces. In particular, referring now toFIG. 4, depicted therein is a somewhat simplified schematic representation of three adjacent tracks TX, TY, and TZ. The track TXis the outermost track on the recording surface in the example depicted inFIG. 4. The track TYis radially adjacent to the track TXbut is located inwardly of the track TX, while the track TZis radially adjacent to the track TYbut is located inwardly thereof.

As shown inFIG. 4, a plurality of data sectors120is associated with each of the tracks TX, TY, and TZ, and between each of the data sectors120is a servo sector122. As generally discussed above, the servo sectors122contain the servo data that allows the position of the head18to be determined for seek operations and accurate track following.

More specifically, each of the example servo sectors122comprises a plurality of A bursts130, B bursts132, C bursts134, and D bursts136. The bursts130-136are formed in radial sequences140,142,144, and146, with one such radial sequence140-146of each of the servo bursts130-136associated with each of the tracks T. Each of the radial sequences140-146of servo bursts130-136may, however, be associated with more than one of the tracks T.

An A/B burst seam150is defined between each sequence140of A bursts130and the adjacent sequence142of B bursts132. The A/B burst seams150extend along the tracks T and, ideally, define the centers of the tracks T. A C/D burst seam152is similarly defined between each sequence144of C bursts134and the adjacent sequence146of D bursts136. The C/D burst seams152are typically offset from the A/B burst seams. The burst seams150and152ideally extend in a direction parallel to the tracks TX, TY, and TZ.

In addition, burst transitions are formed between the leading and trailing edges of circumferentially adjacent bursts. The example inFIG. 4includes an A/B burst transition154formed between the trailing edge of each A burst130and the leading edge of the B burst132adjacent thereto. Similarly, a B/C burst transition156is formed between the trailing edge of each B burst132and the leading edge of each C burst134adjacent thereto. In the example shown inFIG. 4, a C/D burst transition158is also formed between the trailing edge of each C burst134and the leading edge of each D burst136adjacent thereto. The burst transitions154-158ideally extend in a direction perpendicular to the tracks TX, TY, and TZ.

FIG. 4is only one example of a configuration of a servo sector. Servo sectors may contain a lesser or greater number of servo bursts than the four servo bursts depicted inFIG. 4. In addition, the servo bursts may be formed in different patterns. However, burst seams that extend parallel to the track direction and burst transitions that extend perpendicular to the track direction will typically be defined between adjacent servo bursts.

It should be noted thatFIG. 4is highly idealized in that the various bursts are perfectly aligned with each of the burst seams150and152and burst transitions154-158. In practice, servo bursts are often not perfectly aligned with the burst seams150and152and/or the burst transitions154-158. For example,FIG. 5depicts a situation in which the trailing edges of the A bursts130associated with the track TXoverlap the leading edges of the B bursts132associated with that track TYat the A/B burst transitions154. The trailing edges of the A bursts associated with the track TX, on the other hand, are spaced from the leading edges of the B bursts associated with that track TZat the A/B burst transitions154.

AlthoughFIG. 5depicts a simplified example in which problems occur only at the A/B burst transitions154, similar problems may occur at the B/C burst transitions156, at the C/D burst transitions158, at the A/B burst seams150, and at the C/D burst seams152. Burst misalignment such as is depicted inFIG. 5will be referred to herein as burst alignment anomalies.

As is well-known in the art, the servo bursts are formed by specialized factory formatting equipment during what is referred to as a low-level disk format process. In particular, before the low-level disk format process is performed, the disk is blank and contains no information of any kind. To allow the hard disk drive to be used, the factory formatting equipment initially writes servo bursts to the blank disk surface.

As one example of the low-level format process, the outermost radial sequence of servo bursts associated with the outermost track (e.g., sequence140of D bursts136associated with the first track TXinFIG. 4) may first be written to the disk. If the low-level format process starts at the outermost edge of the recording surface, the servo bursts located radially inwardly of the outermost servo bursts (e.g., sequence142of A bursts130associated with the track TXinFIG. 4) are next written to the disk in a second radial sequence. This process is repeated by forming successive radial sequences of servo bursts, while moving towards the spindle, until all of the radial sequences of servo bursts have been written.

Burst alignment anomalies may be, and often are, created during the low-level disk format process. The inventors have further recognized that, because of the sequential nature in which servo bursts are written during the low-level disk format process, burst alignment anomalies tend to be consistent along burst seams and/or burst transitions between adjacent radial burst sequences and tend to vary from one burst seam and/or burst transition to another radially-spaced burst seam and/or transition.

In particular, referring for a moment back toFIG. 5, in that example, the spacing between the trailing edges of the A bursts130and the leading edges of the B bursts132associated with the first track TXis consistent along the track TX. Similarly, the spacing between the trailing edges of the A bursts130and the leading edges of the B bursts132associated with the second track TYis consistent along that track TY. However, a comparison of the relative locations of the A bursts and B bursts of the first and second tracks TXand TYillustrates that the burst alignment anomalies associated with these tracks TXand TYdiffer.

In the context of calibrating the gain of the second stage64, the inventors have further recognized that, because burst alignment anomalies tend to be consistent along burst seams and/or at burst transitions at adjacent radial burst sequences, the non-linearities in the PES will be consistent for a particular track. Accordingly, if the second stage is calibrated while following a particular track, the consistency of the burst alignment anomalies in that particular track creates a consistent, unknown non-linearity in the PES that cannot be eliminated by averaging, prediction, or other post-processing of the PES signal.

A need thus exists for improved positioning systems and methods for a dual-stage actuator of a disk drive and, in particular, for improved calibration systems and methods for the second stage of such positioning systems and methods.

SUMMARY OF THE INVENTION

The present invention may be embodied as a servo system for a hard disk drive comprising a first actuator, a second actuator, a head, and a disk on which is formed a plurality of tracks containing servo data. The servo system comprises a first stimulus, a second stimulus, and a calibration system. The first stimulus causes the first actuator to move the head to at least two calibration tracks on the disk. The second stimulus causes the second actuator to move the head relative to the at least two calibration tracks. The calibration system generates a calibration factor based on the second calibration signal and the movement of the second actuator relative to each of the at least two calibration tracks.

The present invention may also be implemented as a method of is calibrating the second stage of a dual-stage actuator. In particular, in a hard disk drive comprising a first actuator, a second actuator, a head, and a disk on which is formed a plurality of tracks containing servo data, the present invention may be embodied as method of calibrating a gain of the second actuator comprising the following steps. A first stimulus is applied to the first actuator to move the head to at least two calibration tracks on the disk. A second stimulus is applied to the second actuator to move the head relative to the at least two calibration tracks. A calibration factor is generated based on the second calibration signal and the movement of the second actuator relative to each of the at least two calibration tracks.

DETAILED DESCRIPTION

Referring now toFIGS. 6 and 7of the drawing, depicted therein is a servo system220constructed in accordance with, and embodying, the principles of the present invention. The exemplary servo system220operates in a calibration mode220aas shown inFIG. 6and in an operational (seek and read/write) mode220bas shown inFIG. 7. The servo system220will typically be embodied as a software program running on a digital signal processor, but one of ordinary skill in the art will recognize that the servo system220described herein could be implemented in hardware.

In both the calibration mode220aand the operating mode220b, the servo system220comprises the first and second actuator structures30and32and first and second actuators50and52described above. As will be described in detail below, the servo system220further generates the first and second actuator control signals that cause the actuators50and52to move the actuator structures30and32as generally described above.

Referring for a moment more specifically toFIG. 6, when in the calibration mode220a, the servo system220is configured to comprise first and second stages230and232, a calibration system234, and first and second summers236and238. The example first stage230comprises a first controller240and the VCM50. Optionally, a third summer242may be arranged between the first summer236and the controller240and an Adaptive Runout Compensation (ARC) system244(see U.S. patent application Ser. No. 10/318,316 filed Dec. 11, 2002 entitled “Method and Apparatus for Determining Embedded Runout Correction Values Using Feedback,” which is incorporated herein by reference) applying an ARC signal to the third summer242. In the calibration mode220a, the example second stage232comprises the PZT52.

The servo system220further comprises first and second signal sources250and252when operating in the calibration mode220a. The first signal source250generates a first calibration signal “r1”, and the second signal source252generates a second calibration signal “r2”. The purpose and characteristics of the first and second calibration signals “r1” and “r2” will be described in further detail below.

During operation in the calibration mode220a, first calibration source250applies the first calibration signal “r1” to the first summer236. The output of the first summer236is applied to the first controller240of the first stage230. The output of the first controller240is applied to the VCM50. A signal “Y1” indicative of a location of the first actuator50is applied to the second summer238. The output of the second summer238is the PES signal, which is applied to the first summer236to form a closed loop254. If used, the third summer242allows the parameters of the loop254to be altered based on the ARC signal generated by the ARC system244, which is or may be conventional.

The second signal source252applies the second calibration signal “r2” to the calibration system234and to the PZT52. A signal “Y2” indicative of a location of the second actuator52is also applied to the second summer238. The PES signal is applied to the calibration system234. When the servo system220is in the calibration mode220a, the second stage232is operated in an open loop mode. Based on the second calibration signal “r2” and the PES signal, the calibration system234generates a calibration factor for use by the servo system220in the operating mode220b, as will be described in further detail below.

The first calibration signal “r1” is a position reference stimulus that controls the VCM50through the first stage230. The first reference signal “r1” is predetermined to cause the VCM50to move the head18across a plurality of tracks T when the servo system220is in the calibration mode220a. The second calibration signal “r2” is a generally conventional calibration stimulus that causes the head18to move within the width of the track passing under the head18at any given point in time.

When the servo system220operates in the calibration mode220a, the head18will, at different points in time, be located over a plurality of tracks under the influence of the first calibration signal “r1” and located at different positions within any given one of this plurality of tracks under the influence of the second calibration signal “r2”. Accordingly, with the servo system220operating under the influence of both the first calibration signal “r1” and the second calibration signal “r2”, the PES signal is generated based on servo bursts associated with two or more radially spaced tracks.

Referring for a moment toFIGS. 8 and 9, one simplified example of the operation of the servo system220in the calibration mode220awill be described. In particular,FIGS. 8 and 9illustrate the effects of an example of the servo system220operating under the influence of both the first calibration signal “r1” and the second calibration signal “r2”.

Referring initially toFIG. 8of the drawing, depicted therein are broken lines schematically representing the boundaries between radially adjacent calibration tracks. The term “calibration tracks” is not used herein to suggest that the tracks depicted inFIG. 8are any different than any of the other tracks T on the disk surface26. Instead, the term “calibration tracks” simply refers to the fact that the gain calibration systems and methods described herein only requires data to be read from only a few of the tracks T.

The spaces between these broken lines represent the calibration tracks, and the calibration tracks are labeled T+5, T+4, T+3, T+2, T+1, TC, T−1, T−2, T−3, T−4, and T−5. The track TCis the center track among the illustrated tracks, and the positive numbers indicate tracks spaced radially outwardly from the center track TC, while the negative numbers indicate tracks spaced radially inwardly from the center track TC. The exact location of the center track TCon the recording surface26is not critical, but the location of the center track TCis ideally a location where PES non-linearities are expected to be low.

For illustrative purposes, boxes labeled “1” through “24” inFIG. 8indicate locations on the recording surface26at which calibration measurements are taken. These locations are further associated with different ranges of time during the overall calibration process. The overall calibration process is, however, continuously performed while the system220is in the calibration mode220aas suggested by the curve “r1”. The boxes “1” through “24” thus illustrate periodic snapshots of the calibration process and are not intended to imply that a system constructed in accordance with the principles of the present invention requires the use of a sequence of discrete, physically spaced calibration locations.

FIG. 8illustrates that the calibration measurements are taken across multiple tracks, with the pattern and frequency of movement of the head18as determined by the first calibration signal “r1”. The example first calibration signal “r1” represented inFIG. 8causes measurements to be taken at eleven different tracks. However, the number of tracks at which measurements are taken should be within a first range of between 5 and 15, a second preferred range of substantially between 3 and 20, and in any event should be within a third predetermined range of at least 2 tracks.

Referring now toFIG. 9of the drawing, depicted therein is a detail of the measurement process identified as box “10” inFIG. 8. In particular,FIG. 9illustrates movement of the head18caused by the second calibration signal “r2”. At the time of the measurement process “10”, the head18is located adjacent to the center track T−1, soFIG. 9schematically represents, between two broken lines, the center track of T−1. The boxes inFIG. 9are labeled “A” through “G” and illustrate locations of the head18within the track T−1at locations where the PES signal is sampled during the portion of the calibration process represented by box “10” inFIG. 8. The boxes “A” through “G” are associated with different points in time during a particular calibration measurement. Again, the calibration process is or may be continuous, but the boxes “A” through “G” represent sampling of PES at a discrete points in time during a single calibration measurement. A stroke value D10represents the distance traversed by the head under the influence of the second calibration signal “r2” during the portion of the measurement process represented by box “10” inFIG. 9.

Stroke values are similarly calculated throughout the calibration process through the tracks at which calibration measurements are taken during the calibration process. As is generally known in the art, the calibration factor may be generated by measuring the stroke value and correlating the stroke value with the magnitude of the second calibration signal “r2”.

In a typical implementation of the servo system220, the system220may sample the PES several times to obtain each calibration measurement, with the overall calibration process requiring thousands or tens of thousands of calibration measurements. The exact number of calibration measurements and/or samples taken is a tradeoff between accuracy and the time that the servo system220is operating in the calibration mode220a.

The example calibration system234generates the calibration factor by averaging the results of a predetermined number of calibration measurements. As an alternative, the number of calibration measurements and/or samples required may be determined by comparing the results of the successive calibration measurements and ending the measurement process when this comparison indicates that the change in the calculated calibration factor falls below an acceptable level.

Using the servo system220in the calibration mode220aas described above thus minimizes the effects of non-linearities arising from burst alignment anomalies on the gain measurements because the PES is not generated based on servo bursts associated with a single track.

Referring now toFIGS. 10 and 11, depicted therein is a comparison of gain measured at multiple points along a single track and gain measurements measured across a plurality of tracks according to the principles of the present invention. The plots ofFIGS. 10 and 11were taken under the same operating conditions.

FIG. 10contains a plot of gain values calculated for a number of different cylinders, where the gain values are calculated by performing measurements on only one track.FIG. 10illustrates that the gain values vary by as much 10%. The gain values may vary by as much as 25-30% under more extreme situations, such as when the read head is narrower or when localized squeeze is present.

FIG. 11, on the other hand, plots gain values calculated for a number of different cylinders using the techniques of the present invention. As shown inFIG. 11, the gain values differ by only approximately 1%. Even under more extreme conditions, the difference in gain values is still expected to be significantly less than the differences in gain values illustrated inFIG. 10.

The exact parameters of the first calibration signal “r1” and the second calibration signal “r2” are not critical to the principles of the present invention. The example calibration signals “r1” and “r2” are periodic signals. Further, for ease of implementation, the example calibration signals “r1” and “r2” are sinusoidal signals.

The exact frequencies of the calibration signals “r1” and “r2” are also not critical to the principles of the present invention; however, a frequency of the example second calibration signal “r2” should be different from that of the first calibration signal “r1” to allow the effects of the second calibration signal “r2” on the servo system220to be distinguished from the effects of the first calibration signal “r1”.

In particular, if periodic signals are used as the first and second calibration signals “r1” and “r2”, the frequency of the second calibration signal “r2” should be higher than that of the first calibration signal “r1”. With the frequency of “r2” higher than the frequency of “r2”, measurements (such as depicted at “A” through “G” inFIG. 9) may to be taken at each of the measurement locations (such as depicted at “1” through “24” inFIG. 8).

A ratio of the example second calibration signal “r2” to that of the first calibration signal “r1” is approximately 21:1 in the example servo system220. However, the ratio of the second signal “r2” to the first signal “r1” may be within a first predetermined range of from approximately 10:1 to approximately 100:1 or a second predetermined range of from at least 5:1 to approximately 1,000:1, but in any event should be within a third predetermined range of at least 2:1.

As a matter of convenience, the example first calibration signal “r1” is formed by a “1F” signal already available to the servo system220. The “1F” signal is a multiple of the power supply frequency and is a factor of the rotational speed of the spindle. In the example of a 7200 RPM spindle speed, the calibration signal “r1” is a sinusoidal signal having a frequency of 120 Hz. The example second calibration signal “r2” is also a sinusoidal signal, but has a frequency of approximately 2500 Hz, which, again as a matter of convenience, is close to the targeted cross-over frequency of the demodulator conventionally used to obtain the PES signal.

When the servo system220operates in the calibration mode, the head position “Y” is a function of the first and second stages230and232as stimulated by the calibration signals “r1” and “r2” and may be represented by the following formula:

Y=r⁢⁢1·Cv⁢Pv1+Cv⁢Pv︸A+r⁢⁢2·Pμ1+Cv⁢Pv︸B(1)
where CVrepresents the first controller240, PVrepresents the VCM50, and Pμrepresents the PZT52.

The term “A” in formula (1) represents a component of the head position “Y” associated with the first calibration signal “r1”, while the term “B” in formula (1) represents a component of the head position “Y” associated with the second calibration signal “r2”. Typically, the “A” component is a relatively low frequency component having relatively large amplitude, while the “B” component is a relatively high frequency component.

The following formulas represent the motion Y1of the VCM50, the motion Y2of the PZT (microactuator)52, and the position error signal perr.

The component “A” of the formula (4) represents the effects of the first calibration signal “r1” on the position error signal perr, while the component “B” of the formula (4) represents the effects of the second calibration signal “r2” on the position error signal perr.

In one form of the invention, the calibration system234generates the calibration factor by first mathematically removing the component “A” from the position error signal perr. Once the component “A” has been removed from the position error signal perr, the calibration system234generates the calibration factor by performing a Discrete Fourier Transform (DFT) on the component “B” of the position error signal perr at the frequency of the second calibration signal “r2” and dividing a result of the DFT by the magnitude of the second calibration signal “r2”. The calibration factor so generated may be used to calibrate the PZT (microactuator) gain as will be described in further detail below.

In an alternate form of the invention, the optional ARC system244may be activated while the servo system220is in the calibration mode220a. In this case, the ARC system compensates for the effects of the first calibration signal “r1” and eliminates the component “A” of the position error signal formula (4). With the component “A” of formula (4) eliminated by the ARC system244, the calibration system234generates the calibration factor by performing a DFT on the position error signal perr at the frequency of the second calibration signal “r2” and dividing by the magnitude of the second calibration signal “r2”. Again, the calibration factor may be used to calibrate the PZT (microactuator) gain.

Referring now back toFIG. 7, when in the operating mode220b, the servo system220is configured to comprise first and second stages260and262and first, second, and third summers264,266, and268. In this operating mode220b, the first stage260comprises a first controller270and the VCM50, while the second stage262comprises a second controller280, an estimator282, a calibrate block284, and the PZT52. Optionally, an ARC block290and third summer292are arranged to modify an input to the first controller270with an ARC signal.

When in the operating mode220bdepicted inFIG. 7, the servo system220allows the hard disk drive10to perform seek and track following (read/write) processes. The basic operation of the servo system220in the operating mode220bis or may be conventional and will not be described herein in detail beyond what is required for a complete understanding of the present invention.

When the servo system220is in the operating mode220b, the calibrate block284is arranged to alter the parameters of the system220based on the calibration factor. For example, the calibrate block284may multiply the output of the second controller280by a constant (e.g., “1”) when no calibration is required, and this constant is modified (e.g., “1.5” or “0.75”) based on the calibration factor as required. When the output of the second controller280is modified using the gain correction factor, the gain of the second stage262is calibrated, and the bandwidth and stability of the servo system220are significantly improved. However, the parameters of the servo system220may be altered in ways other than that depicted inFIG. 7to calibrate the second stage262of the servo system220operating in the operational mode220b.

The scope of the present invention should be determined with respect to the following claims and not the foregoing detailed description.