Settle control systems and methods for the second stage of a dual stage actuator

An actuator control system and method for a hard disk drive comprising first and second actuators that support a head relative to a disk is disclosed. The actuator control system comprises a first stage, a second stage, a memory device, and a maximum stroke calculator. The first stage displaces the first actuator relative to the disk. The second stage displaces the second actuator relative to the first actuator. The memory device stores at least one data value indicative of a predicted response of the second stage. The maximum stroke calculator generates a predicted maximum stroke based on the at least one data value stored by the memory device. The second stage is deactivated when the disk drive initiates a seek operation. The second stage is reactivated based on a comparison of the predicted maximum stroke with a position of the second actuator.

FIELD OF THE INVENTION

The present invention relates to digital storage devices and, more particularly, to 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 above (or below) 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 above (or below) 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 below (or above) 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 to a desired track. In a seek mode, the transducer is moved radially inwardly or outwardly to arrange the transducer above the 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 may 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.

FIG. 1depicts a disk drive10comprising 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 VCM controller driving electronics. The preamplifier, read/write channel, servo control unit, RAM, and ROM are or may be conventional and will not be described herein beyond what is necessary for a complete understanding of the present invention.

FIG. 1shows that the disk drive10includes a disk12, a spin motor14, and a base plate16. The disk12is rotated by a spin motor14, and 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 shown inFIG. 2.

The disk drive10further comprises what is commonly referred to as a head18. The head18comprises or supports the magnetic read/write transducer described above and will thus 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 track28in seek mode and then follow the desired track28in track following mode.

The exemplary positioning system20depicted inFIGS. 1 and 2is a dual-stage system. Accordingly, each actuator arm assembly24comprises a first actuator30and a second actuator32. The principles of the present invention are currently of primary importance when applied to the second actuator of a dual-stage actuator system, and that application of the present invention will be described herein.

For ease of illustration,FIGS. 1 and 2depict the first and second actuators30and32as comprising elongate arms34and36, respectively, and the actuators30and32may be implemented as shown inFIGS. 2 and 3. Conventionally, the bearing assembly22is also considered part of the first actuator30.

As perhaps best shown inFIG. 2, the bearing assembly22supports a proximal end40of the arm34of the first actuator30for rotation about a first axis B, while a distal end42of the first actuator arm34supports a proximal end44of an arm36of the second actuator32for rotation about a second axis C. In this case, the head18is supported on a distal end46of the second actuator arm36.

The actuators30and32may, however, be implemented using other structures or combinations of structures. For example, the first actuator30may comprise an elongate arm that rotates about a first axis B, while the second actuator32may 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 actuators30and32may thus take any number of physical forms, and the scope of the present invention should not be limited to the exemplary actuators30and32depicted inFIGS. 2 and 3and described herein.

FIG. 2also illustrates that the exemplary actuators30and32of the positioning system20further comprise a first electromechanical transducer50and a second electromechanical transducer52. In response to a first control signal, the first transducer50moves the first actuator arm34to change an angular position of the head18relative to the first axis B. The second transducer52is supported by the distal end42of the first actuator30to rotate the head18about the second axis C in response to a second control signal. The first transducer50may be a voice coil motor (VCM), while the second transducer52may be a piezo-electric transducer (PZT), but other types of transducers may be used as the first and second transducers50and52.

InFIG. 2, an angular position of the first actuator arm34relative to the first axis B is represented by reference character D, while an angular position of the second actuator arm36relative to the second axis C is represented by reference character E. When the head18is above the neutral position D (on-track mode), the displacement of the second actuator arm36is zero.

FIG. 2also shows that a range of movement utotal is associated with the second actuator36relative to a neutral position D defined by the first actuator arm34. The stroke of the actuator arm36in either direction from the neutral position will be referred to herein as umax. The terms u+max and u−max used inFIG. 2indicate the direction of the stroke with respect to the neutral position D.

An actual position uA of the second stage actuator32corresponds to the angular position of the second actuator32relative to the neutral position D at any point in time. An initial offset uO of the second stage actuator32is the actual position signal uA of the second stage actuator32at the time a seek operation is initiated. The terms uO+ and uO− will be used to identify not only the magnitude but also the direction of the initial offset.

FIG. 2further identifies arbitrary first and second tracks28aand28bon the disk12. The actuator arm assembly24is shown in an initial position by solid lines and in a target position by broken lines; the first track28awill thus be referred to as the “initial track” and the second track28bwill 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 track28on 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 other forms such as using discrete hardware components.

The servo system60comprises a first stage servo62and a second stage servo64. As described above, the disk12defines a plurality of tracks28in the form of generally concentric circles centered about a spindle axis C. The first stage servo62controls the first transducer50and the second stage servo64controls the second transducer52to support the head18adjacent to a desired one of the tracks28. 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 signal uA of the transducer52of the second stage servo64, and a second stage position estimate signal Y2estis indicative of an estimated position of the transducer52of the second stage servo64. The second summer72combines the second stage position estimate signal Y2estand the output of the first summer70. A first stage position signal Y1is indicative of the actual position signal uA of the first transducer50of the first stage servo62. A third summer74combines the first and second stage position signals Y1and 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 Y1and Y2with any system disturbances d.

The sources of the input signal R and the first and second stage position signals Y1and Y2are or may be conventional and will be described herein only to the extent necessary for a complete understanding of the present invention. Briefly, each of the tracks28includes data sectors having stored data and servo sectors having servo data. The servo data identifies each individual track28to assist in seek operations and is also configured to allow adjustment of the radial position of the head18during track following. A servo demodulation unit generates the position error signal PES and the first and second stage position signals Y1and Y2based 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.

The seek operation may be divided into a seek phase and a settle phase. During the seek phase, the servo system60displaces the head18most of the distance from the initial track28ato the target track28b.

During the settle phase, the second stage servo64is conventionally deactivated, and the head18repeatedly crosses over the target track28bas the relatively low bandwidth first stage servo62of the servo system60attempts to lock onto the desired track28b. The second stage servo64is typically deactivated while the disk drive10performs a seek operation because the stroke S of the second stage servo64is too limited to have any significant effect during the seek phase of a seek operation.

Once the seek portion of the seek operation is completed, however, the relatively high bandwidth second stage servo64would ideally be activated to speed up the settle phase of the seek operation.

The fundamental problem with using the second stage servo64to assist during the settle phase of the seek operation is when to deactivate and reactivate the second stage actuator. The conventional method is to set the actuator64to the neutral position at the beginning of the seek operation, deactivate the actuator64, and then reactivate the actuator64when the position error signal PES equals zero.

The conventional method of improving settle times using the second stage servo64creates several problems. First, the first stage servo62takes too long to lock onto the desired track to place the system10in track following mode. The second stage servo64would ideally be activated earlier in the seek operation to assist the actuator system60in finding the desired track. Second, the second stage servo64may be significantly offset from the neutral position (out of stroke) when it is determined that the system is on-track. In this case, the second stage servo64may saturate and will thus be unable to cancel out displacement of the first stage servo62, which may not yet have settled. As a result of this post-settle saturation, a bump in the position error signal PES may occur.

The need thus exists for improved systems and methods of controlling the second stage of a dual stage actuator during the settle phase of a seek operation.

SUMMARY OF THE INVENTION

The present invention may be embodied as an actuator control system for a hard disk drive comprising first and second actuators that support a head relative to a disk. The actuator control system comprises a first stage, a second stage, a memory device, and a maximum stroke calculator. The first stage displaces the first actuator relative to the disk. The second stage displaces the second actuator relative to the first actuator. The memory device stores at least one data value indicative of a predicted response of the second stage. The maximum stroke calculator generates a predicted maximum stroke based on the at least one data value stored by the memory device. The second stage is deactivated when the disk drive initiates a seek operation. The second stage is reactivated based on a comparison of the predicted maximum stroke with a position of the second actuator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now toFIG. 4of the drawing, depicted therein is an actuator control system120constructed in accordance with, and embodying, the principles of the present invention. The actuator control system120comprises a servo system122and a settle control system124. The actuator control system120forms part of a two-stage actuator system similar to the two-stage actuator system formed by the servo system60described above.

The servo system122will 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 system122described herein could be implemented in hardware. The example servo system122comprises a first stage130, a second stage132, and first, second, and third summers134,136, and138. The first stage130comprises a first control circuit140and the first transducer50described above. The second stage132comprises a second control circuit150, an estimator circuit152, and the second transducer52described above.

As with the conventional servo system60described above, an input signal R is combined with a position error signal PES by the first summer134. The first control circuit140generates a first control signal VVCMfor operating the first transducer50based on the output of the first summer134and a second stage position estimate signal Y2—est. A first stage position signal Y1is indicative of the actual position of first transducer50of the first stage130.

When activated, the second control circuit150generates a second control signal VPZTthat controls the second transducer52based on the output of the first summer134. A second stage position signal Y2is indicative of an actual position signal uA of the second transducer50of the second stage132. The estimator circuit152generates the second stage position estimate signal Y2—est based on the second control signal VPZT. The second stage position estimate signal Y2—est is indicative of an estimated position of the second transducer52of the second stage132.

The third summer138combines the first and second stage position signals Y1and Y2. System disturbances d are represented as an additional input to the third summer138. The position error signal PES thus represents the combination of the first and second position signals Y1and Y2with any system disturbances d.

The sources of the input signal R and the first and second stage position signals Y1and Y2are or may also be conventional and will be described herein only to the extent necessary for a complete understanding of the present invention. Each of the tracks28contains data sectors containing stored data and servo sectors containing servo data. The servo data identifies each individual track28to assist in seek operations and is also configured to allow adjustment of the radial position of the head18during track following. A servo demodulation unit generates the position error signal PES and the first and second stage position signals Y1and Y2based on the servo data read from the disk12.

Given the foregoing, one of ordinary skill in the art will recognize that the first and second stages130and132form first and second servo loops.

The settle control system124of the actuator control system120activates or deactivates the second stage132based on pre-calculated data tables and operating conditions before and during the seek operation. More specifically, the settle control system124generates at least one predicted maximum stroke S(PES,PES—v)max based on the position error signal PES, an estimate of a radial velocity PES—v of the head18during the seek operation, and data tables indicative of overshoot of the head18relative to the desired track under expected operating conditions. The predicted maximum stroke S(PES,PES—v)max represents an estimate of the predicted maximum stroke required of the second stage132to compensate for the overshoot of the head18under a given set of conditions.

In use, the settle control system124deactivates the second stage132at the start of the seek operation. During the seek operation, the settle control system124compares the at least one predicted maximum stroke value S(PES,PES—v)max with the second stage position estimate signal Y2—est. Based on this comparison, the settle control system124determines whether the second stage132has sufficient stroke to compensate for the expected overshoot. When the comparison indicates that the second stage132has sufficient stroke to effectively assist with the settle phase of the seek operation, the second stage132is reactivated.

The details of construction and operation of several example implementations the settle control system124of the actuator control system120will now be described.

Referring initially toFIG. 4, that figure shows that the settle control system124comprises a maximum stroke calculator160, a memory device162, and a velocity estimator164. The memory device162stores at least one data value or data table from which the normalized predicted maximum stroke Snorm as shown inFIG. 4may be generated. The velocity estimator164generates a velocity estimate signal PES—v. The maximum stroke calculator160combines the normalized predicted maximum stroke Snorm with the velocity estimate signal PES—v and the position error signal PES to obtain the predicted maximum stroke Smax.

FIG. 4and the notation used therein is generalized to apply to any one of a number of settle algorithms that may be implemented by the maximum stroke calculator160. Several examples of settle algorithms will be described in further detail below.

In the following discussion, the term S(0,1) will be used to refer to the velocity component of the normalized predicted maximum stroke Snorm. Similarly, the term S(1,0) will be used to refer to the position component of the normalized predicted maximum stroke Snorm. In addition, the term S(0,PES—v) may be used to refer to the velocity component of the stroke signal Smax, while the term S(PES,0) may be used to refer to the position component of the predicted maximum stroke Smax. The predicted maximum stroke Smax may also be referred to in the following discussion as S(PES,PES—v)max.

Because of the nature of the pre-calculated position and velocity response curves, the predicted maximum stroke Smax may be represented as a positive value and a negative value. The notations S+max or S+(PES,PES—V)max will be used to identify the positive value, and the notations S−max or S−(PES,PES—V)max will be used to identify the negative value.

The settle control system124further comprises a comparator170. The comparator170deactivates or reactivates the second stage132based on a comparison of the predicted maximum stroke S(PES,PES—v)max with the second stage actuator actual displacement value uA. The second stage actuator actual displacement value uA is calculated based on the second stage position estimate signal Y2—est.

Referring now toFIGS. 5 and 6, depicted therein are plots of normalized velocity and position responses for a particular implementation of the servo system122. In particular,FIG. 5contains a trace representing a velocity response signal S(0,1) associated with the servo system122, whileFIG. 6contains a trace representing a position response signal S(1,0) associated with the servo system122.

The example signals S(0,1) and S(1,0) represent predicted or pre-calculated overshoot of the servo system122resulting from different initial velocity and position states PES—v and PES of the servo system122prior to initiation of a seek operation. The signals S(0,1) and S(1,0) represent typical servo loop overshoot in that they each contain an initial peak in one direction and a second peak in the opposite direction before they substantially stabilize at or near zero. The signals S(0,1) and S(1,0) are non-directional in that they are valid in either radial direction of the head18relative to the disk12.

The example signals S(0,1) and S(1,0) are generated based on a simulation of the servo system122. The signals S(0,1) and S(1,0) may also be generated by actual testing on a design of servo system122and/or as part of a calibration process conducted on a particular physical device implementing the servo system122.

In the following discussion, the maximum value of the initial peak of the signal S(0,1) is referred to as S+(0,1)max, and the maximum value of the second peak of the signal S(0,1) is referred to as S−(0,1)max. Similarly, the maximum value of the initial peak of the trace S(1,0) is referred to as S+(1,0)max, and the maximum value of the second peak of the trace S(1,0) is referred to as S(1,0)max.

The example signals S(0,1) and S(1,0) are normalized such that the peak with the greatest magnitude equals 1.00 on the ordinate axis. In the example signals S(0,1) and S(1,0), the initial peaks are larger than the second peaks. Accordingly, S+(0,1)max and S+(1,0)max are both set equal to 1.00, while S−(0,1)max is approximately 0.55 and S−(1,0)max is approximately 0.30. The signals S(0,1) and S(1,0) are both examples provided for illustrative purposes only, and other servo systems may have different normalization schemes, curves, and/or maximum values.

As will be described in further detail below, the memory device162shown inFIG. 4stores at least one value representing or associated with the velocity and position traces S(0,1) and S(1,0) shown inFIGS. 5 and 6. The normalized predicted maximum stroke Snorm thus comprises at least one value generated from the data stored in the memory device162based on one or both of the normalized signals S(0,1) and S(1,0). The example maximum stroke calculator160thus also generates the predicted maximum stroke Smax based on the normalized predicted maximum stroke Snorm, the velocity estimate signal PES—v, and the position error signal PES.

The maximum stroke calculator160generates the predicted maximum stroke S(PES,PES—v)max using any one of a number of settle algorithms. Three possible settle algorithms for computing the predicted maximum stroke S(PES,PES—v)max will now be discussed.

The first settle algorithm calculates S(PES,PES—v)max based on the absolute value of the sum of the maximums of the signals S(0,1) and S(1,0) in positive and negative directions. In particular, the memory device162stores the values of S+(0,1)max, S+(1,0)max, S−(0,1)max, and S−(1,0)max. The velocity estimate signal PES—v is multiplied by the values of S+(0,1)max and S−(0,1)max to obtain S+(0,PES—v)max and S−(0,PES—v)max, respectively. Similarly, the position error signal PES is multiplied by the values of S+(1,0)max and S−(1,0)max to obtain S+(PES,0)max and S−(PES,0)max, respectively.

The maximum stroke calculator160thus calculates the predicted maximum stroke values S+(PES,PES—v)max and S−(PES,PES—v)max according to the following equations E1-1 and E1-2:
S+(PES,PES—v)max=|S(PES,0)max|+|S(0,PES—v)max|  (E1-1)
S−(PES,PES—v)max=|S(PES,0)max|+|S(0,PES—v)max|  (E1-2)

The comparator170turns on the second stage132according to the following conditional statement CS1:
if |S+(PES,PES—v)max+uO+|<umax, and
if |S−(PES,PES—v)max+uO−|<umax, then  (CS1)
activate the second stage132.

The steps associated with equations E1-1 and E1-2 and conditional statement CS1 are repeated from the time the seek operation is initiated and the second stage actuator is deactivated until the second stage132is reactivated.

This first settle algorithm will improve the settle phase of a seek operation. Further, because it is generated only based on the values S(PES,0)max and S(0,PES—v)max, the first settle algorithm is simple to implement.

However, the first settle algorithm may be too conservative in some cases because it assumes a worst case scenario that is unlikely to occur during actual operation of a disk drive employing the servo system122. In particular, a comparison of the example traces S(0,1) and S(1,0) inFIGS. 5 and 6shows that the maximum S(1,0)max of the position trace S(1,0) occurs earlier in time than the maximum S(0,1)max of the velocity trace S(0,1). Because the peaks of the traces S(1,0) and S(0,1) do not occur at the same time, S(PES,PES—v)max calculated as described in formula (1) above will likely be significantly greater than the overshoot possible given the parameters of the servo system122. The result of combining the peaks of the traces S(1,0) and S(0,1) is a possibly significant delay in the reactivation of the second stage132.

A second settle algorithm calculates S(PES,PES—v)max based on knowledge of the transfer function of the servo system122. In particular, the transfer function from initial PES to PES is Gy—v0(s), and the transfer function from initial velocity to PES is Gy—y0(s). It has been proven that Gy—v0(s)=s Gy—v0(s). Therefore, the combined response due to initial velocity and position may be predicted as the vector sum of two vectors with a 90 degree phase difference between them.

Accordingly, the memory device162stores the values of S+(0,1)max, S+(1,0)max, S−(0,1)max, and S−(1,0)max, and the second settle algorithm calculates S(PES,PES—v)max according to the following equation E2-1 and E2-2:
S+(PES,PES—v)max=sqrt(S+(PES,0)max^2+S+(0,PES—v)max^2)  (E2-1)
S−(PES,PES—v)max=sqrt(S−(PES,0)max^2+S−(0,PES—v)max^2)  (E2-2)

The comparator170then turns on the second stage132according to the following conditional statement CS2:
if |S+(PES,PES—v)max+uO+|<umax, and
if |S−(PES,PES—v)max+uO−|<umax, then  (CS2)activate the second stage132.

The steps associated with equations E2-1 and E2-3 and conditional statement CS2 are repeated from the time the seek operation is initiated and the second stage actuator is deactivated until the second stage132is reactivated.

This second settle algorithm is, like the first settle algorithm, based only on S(PES,0)max and S(0,PES—v)max and is thus simple to implement. The second settle algorithm is also less conservative than the first settle algorithm.

One drawback of the second settle algorithm is that it is based on the assumption that the closed loop response can be approximated as a sine wave signal. In addition, the assumption that a 90 degree phase difference exists is only valid for steady state conditions. If the closed loop dynamics are not dominated by a pair of poles and/or the poles are well damped, the prediction of overshoot obtained from equations 2-1 and 2-2 may not be accurate.

A third settle algorithm that may be used by the system of the present invention employs the entire traces of the position response signal S(1,0) and velocity response signal S(0,1) shown inFIGS. 5 and 6.

In general, the third settle algorithm comprises the following steps. First, the combined response of various combinations of possible position error signals PES and velocity error signals PES—V are generated based on the position response S(1,0) and velocity response S(0,1) signals. Second, the peak values of each combination are stored in the memory device162. Third, the peak values associated with a given initial position error signal PES and initial velocity estimate signal PES—v are read and compared with the predicted maximum stroke of the second stage132. Based on this comparison, the second stage132is either left deactivated or reactivated to assist with the settle phase of the seek operation.

More specifically, the third settle algorithm operates in a pre-seek mode and in a settle mode. The pre-seek computation mode may be summarized by the following equations E3-1, E3-2, and E3-3:
S(1,βi)=S(1,0)+βi*S(0,1);  (E3-1)
Si+max=max(S(1,βi)); and  (E3-2)
Si−max=min(S(1,βi)); where  (E3-3)=
β=PES—v/PES;
i=1:N;
N=table length;
1≧βi≧−1; and
2*N=memory space.

N values of Si+max and Si−max for i=1: N are thus stored in the memory spaced defined by the memory device162. The pre-seek computation process may be performed any time prior to the seek operation and is typically performed off-line. The value βifunctions as an index that allows the settle control system124to access the appropriate values Si+max and Si−max during the seek operation.

In particular, when the seek process is initiated, the third settle algorithm operates according to the following equations E3-4 and E3-5:
S+(PES,PES—v)max=PES*S+max(1,βi),  (E3-4)
S−(PES,PES—v)max=PES*S−max(1,βi), where  (E3-5)β is rounded to the nearest βi.

The following table is an example that may be used by the third settle algorithm to generate the valuesS+(PES,PES—v)max and S−(PES,PES—v)max.

In the foregoing example table, the odd values v1–v41 represent the positive maximum values of a curve resulting from combination of the position and velocity responses, while the even values v2–v42 represent negative maximum values of a curve resulting from the combination of the position and velocity responses. In the example table, only one index β is employed to access the position and response values because the elements on the same diagonal line (e.g., S(0.1,0.1), S(0.2, 0.2) . . . S(α, α) . . . (S(1.0, 1.0)) may be consolidated into the single table element αS(1, 0.5).

In the example table set forth above, if β=0.92, that number is rounded to the closest βi, or in this case β20. The values v39 and v40 associated with β20are thus substituted for the expressions S+max(1,β) and S−max(1,β) in equations E3-4 and E3-5.

In any event, once the appropriate values are obtained, the comparator170turns on the second stage132according to the following conditional statement (CS3):
if |S+(PES,PES—v)max+uO+|<umax, and
if |S−(PES,PES—v)max+uO−|<umax, then  (CS3)activate the second stage132.

The conditional statement CS3 represents a comparison of predicted maximum stroke required with available stroke. The steps associated with equations E34 and E3-5 and conditional statement CS3 are repeated from the time the seek operation is initiated and the second stage actuator is deactivated until the second stage132is reactivated.

An example of the combined response used by the third settle algorithm is shown inFIG. 7.FIG. 7illustrates that the normalized maximums S+max and S−max of the combined response can be less than one. The third settle algorithm is more accurate and less conservative than the first and second settle algorithms discussed above and, thus, reactivates the second stage132sooner than either of the first and second settle algorithms.

As briefly described above, the settle algorithms described herein are examples only, and other settle algorithms may be used within the scope of the present invention. For example, for a particular servo system, the designer may decide that one or the other of the velocity and position responses is dominant and may generate Smax based only on the dominant response.

Alternatively, the designer may alter one of the settle algorithms above. If the value Smax is calculated using the first settle algorithm, Smax may be multiplied by a factor of less than one to obtain a value that is less conservative.

In the settle algorithms described herein, the initial offset uO of the second stage actuator is taken into account by the conditional statements CS1, CS2, or CS3 implemented by the comparator170. In any one of the settle algorithms, the second stage132may be reset at the time the seek operation is initiated. If the second stage132is reset, the initial offset uO will be zero. Abruptly resetting the second stage132to zero can, in some cases, introduce instabilities into the servo system122.

Alternatively, instead of simply resetting the second stage132, the initial offset uO may be gradually reduced to zero during the seek operation. Gradually reducing the initial offset uO to zero eliminates the contribution of the initial offset uO to the actual position signal uA of the second stage132while reducing or eliminating system instabilities caused by abruptly resetting the second stage132.

If the second stage132is not reset at the beginning of the seek operation, the actual position signal uA of the second stage actuator comprises a component contributed by the initial offset uO. The term discharge portion uD will be used herein to refer to the component of the actual position signal uA contributed by the initial offset uO at any point during the seek operation.

Referring toFIG. 8, illustrated therein is an actuator control system220constructed in accordance with, and embodying, the principles of another embodiment of the present invention. The actuator control system220is designed to gradually reduce the discharge portion uD of the actual offset uA to zero.

In particular, the actuator control system220comprises a servo system222, a settle control system224, and a discharge profile system226. The actuator control system220forms part of a two-stage actuator system similar to the two-stage actuator system formed by the servo system60described above. The discharge profile system226eliminates, according to a predetermined discharge profile, the discharge portion uD of the actual position signal uA associated with the second stage actuator232.

The servo system222comprises a first stage230, a second stage232, and first, second, and third summers234,236, and238. The first stage230comprises a first control circuit240and the first transducer50described above. The second stage232comprises a second control circuit250, an estimator circuit252, and the second transducer52described above. The settle control system224comprises a maximum stroke calculator260, a memory device262, and a velocity estimator264. One of ordinary skill in the art will recognize that the first and second stages230and232form first and second servo loops.

The servo system222and settle control system224operate in the same basic manner as the servo system122and settle control system124described above. The details of construction and operation of the servo system222and the settle control system224thus will not be repeated below. The signal names and description used in connection withFIGS. 3 and 4will also be used in connection withFIG. 8.

The discharge profile system226modifies the output of the second control circuit250during the seek operation based on a predetermined discharge profile. The discharge profile gradually reduces the output of the second control circuit250until the discharge portion uD of the initial offset uO to the actual position signal uA of the second stage actuator232is gradually eliminated.

In particular, the example discharge profile system226comprises a discharge profile generator280, a multiplier282, and a summer284. The output of the discharge profile generator280is a discharge signal that represents the discharge profile. The discharge signal is multiplied by the control signal VPZTapplied to the second transducer52and combined with the output of the control circuit250. So connected, the discharge profile system226gradually reduces the discharge portion uD of the actual position signal uA during the seek operation.

FIG. 9illustrates a combined response that results under one example set of initial conditions when a discharge profile system226is used to gradually reset the initial offset uO. Co-pending U.S. patent application Ser. No. 10/366,544, which is incorporated herein by reference, discloses in detail at least one example of a discharge profile system that may be used as the discharge profile system226.

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