Hysteresis compensation in a disc drive

Systems and methods for compensating for hysteresis in a disc drive are described. In one embodiment, a method may use an inverse hysteresis model to linearize effects of hysteresis of a microactuator in the disc drive. The hysteresis model may be a Coleman-Hodgdon hysteresis model. The hysteresis of the microactuator may be characterized, and the inverse hysteresis model may be based at least in part on the characterization. The inverse hysteresis model may be used to implement a digital filter. The digital filter may be employed in series with the microactuator to linearize the effects of hysteresis.

SUMMARY

The present disclosure is directed to methods and systems for compensating for hysteresis in a disc drive. In some embodiments, the present systems and methods may linearize the effects of hysteresis of a microactuator of the disc drive.

A storage device for hysteresis compensation is described. In one embodiment, the storage device may include a data storage medium, at least one microactuator configured to move at least one read/write head relative to the data storage medium, and a hysteresis compensator to compensate for hysteresis of the at least one microactuator using a digital filter based at least in part on an inverse hysteresis model, wherein the inverse hysteresis model is determined by the hysteresis compensator based at least in part on the hysteresis of the at least one microactuator.

In some embodiments, the hysteresis compensator may use a Coleman-Hodgdon hysteresis model. In some embodiments, the digital filter may be in series with the at least one microactuator. In some embodiments, the hysteresis compensator may characterize the hysteresis of the at least one microactuator and determine parameters for the inverse model of the hysteresis based at least in part on the characterized hysteresis of the at least one microactuator. In some cases, the storage device may further include a servo controller associated with the at least one microactuator. The servo controller may be set to a single-stage mode for the hysteresis compensator to characterize the hysteresis of the at least one microactuator. In some configurations, the hysteresis compensator may characterize the hysteresis of the at least one microactuator while the servo controller drives at least two heads associated with the at least one microactuator out of phase using a direct current (DC) voltage. The servo controller may vary the DC voltage over an entire driving range of the at least one microactuator for the characterizing.

An apparatus for hysteresis compensation is also described. In one embodiment, the apparatus may include at least one microactuator configured to move at least one read/write head relative to a data storage medium, and a hysteresis compensator to compensate for hysteresis of the at least one microactuator using an inverse hysteresis model.

A method for hysteresis compensation is also described. In one embodiment, the method may include obtaining a characterization of hysteresis of a microactuator in a disc drive, determining parameters for an inverse model of the hysteresis according to the characterization, and compensating for the hysteresis of the microactuator using the inverse hysteresis model.

DETAILED DESCRIPTION

The following relates generally to hysteresis compensation in a disc drive. A disc drive typically includes one or more microactuators for driving read/write heads relative to a storage medium. The microactuator(s) may exhibit hysteresis. For example, a piezoelectric transducer (PZT) microactuator, which may be employed to obtain precise tracking, may suffer from significant hysteresis. Servo performance of the disc drive may be degraded by the non-linear behavior of the microactuator caused by the hysteresis if not addressed. For example, such hysteresis may result in a non-linear gain in the servo loop, may cause distortion of feedforward signals through the associated non-linear transfer function, and may be a source of error when a DC head skew table is updated.

In one embodiment, an efficient method to implement a digital filter is disclosed. The digital filter may be placed in series with the microactuator (e.g., the PZT element thereof) to linearize the effects of the hysteresis of the microactuator. As used herein, the term “linearize” is intended to mean “make more linear.” Thus, it should be understood that linearizing may not result in a complete linearization of the hysteresis effects, but at least results in a more linear performance of the microactuator as compared to that achieved without hysteresis compensation.

The filter described herein may be implemented using a Coleman-Hodgdon hysteresis model. Coleman-Hodgdon models have been used to model and simulate hysteresis. However, such models have not been used to compensate for hysteresis as described herein, namely using an inverse of the model to linearize a system.

In the case of a PZT microactuator, the hysteresis of the microactuator is characterized, such as described further below, and parameters of the Coleman-Hodgdon hysteresis model are determined based on the characterization of the hysteresis (e.g., to fit the actual hysteresis curve that characterizes the hysteresis of the microactuator). Once the parameters are determined, the inverse of the Coleman-Hodgdon hysteresis model may be implemented as a digital filter for the microactuator.

The present disclosure also describes a method to characterize the hysteresis of the microactuator. In one embodiment, the characterization process involves switching to a single-stage mode, for example, by setting a servo controller associated with the microactuator to the single-stage mode. Normally, tracking control in a disc drive employing a microactuator operates in a dual-stage mode in which the microactuator and a second actuator (for moving a slider, described further below) are regulated by the servo controller simultaneously. In the single-stage mode, the servo controller regulates only the second actuator (e.g., through a voice coil motor (VCM)) with the microactuator being open loop. This allows different signals to be applied to the microactuator for characterizing the hysteresis. In one embodiment, a direct current (DC) voltage is applied to drive a pair of read/write heads out of phase and a change in DC skew is measured. The DC voltage is ramped up and down over an entire driving range of the microactuator (e.g., the PZT element) with measurements being made to generate a hysteresis curve for the microactuator. As discussed above and further herein, the parameters of the hysteresis model being employed (e.g., Coleman-Hodgdon) are determined so that the model fits the inverse of the generated hysteresis curve.

FIG. 1is a block diagram illustrating one embodiment of a data storage system100(e.g., a disc drive system) in which the present systems and methods may be implemented. The data storage system100includes media106, such as a plurality of discs107, which are mounted on a spindle motor140by a clamp108. Each surface of the media106has an associated slider110, which carries a read/write head111for communication with the media surface. Sliders110are supported by suspensions and track accessing arms of an actuator mechanism116. For example, the actuator mechanism116can be of the type known as a rotary moving coil actuator and includes a voice coil motor (VCM)118. The VCM118rotates actuator mechanism116about a pivot shaft to position sliders110over a desired data track along an arcuate path between an inner diameter (ID) and an outer diameter (OD) of respective discs7. The VCM118is driven by electronic circuitry based on signals generated by the read/write heads111and a host computer150. Although not shown inFIG. 1(but illustrated inFIG. 3), each of the sliders110may be implemented with a microactuator that pivots to move the associated read/write head111along a second arcuate path.

As previously discussed, media106can includes a plurality of discs107. Each disc107has a plurality of substantially concentric circular tracks. Each track is subdivided into a plurality of storage segments. As defined herein, a storage segment is the basic unit of data storage in media106. Each storage segment is identified and located at various positions on media106. In the disc-type media example, storage segments or data sectors are “pie-shaped” angular sections of a track that are bounded on two sides by radii of the disk and on the other side by the perimeter of the circle that defines the track. Each track has related logical block addressing (LBA). LBA includes a cylinder address, head address and sector address. A cylinder identifies a set of specific tracks on the disk surface to each disc107which lie at equal radii and are generally simultaneously accessible by the collection of read/write heads111. The head address identifies which head can read the data and therefore identifies which disk from the plurality of discs107the data is located. As mentioned above, each track within a cylinder is further divided into sectors for storing data and servo information. The data sector is identified by an associated sector address.

The data storage system100includes a system processor136, which is used for controlling certain operations of data storage system100in a known manner. The various operations of data storage system100are controlled by system processor136(e.g., storage controller) with the use of programming and/or instructions stored in a memory137. The data storage system100also includes a servo controller138, which generates control signals applied to the VCM118and spindle motor140(as well as the microcontroller, not shown). The system processor136instructs the servo controller138to seek read/write head111to desired tracks. The servo controller138is also responsive to servo data, such as servo burst information recorded on disc107.

The data storage system100further includes a preamplifier (preamp)142for generating a write signal applied to a particular read/write head111during a write operation, and for amplifying a read signal emanating from a particular read/write head111during a read operation. A read/write channel144receives data from the system processor136during a write operation, and provides encoded write data to the preamplifier142. During a read operation, the read/write channel146processes a read signal generated by the preamplifier142in order to detect and decode data recorded on the discs107. The decoded data is provided to the system processor136and ultimately through an interface148to a host computer150.

In some configurations, the data storage system100may include a hysteresis compensator, such as a hysteresis linearization module130. In one example, the data storage system100may be a component of a host (e.g., operating system, host hardware system, etc.). The hysteresis linearization module130may compensate for hysteresis of the microactuator (not shown), for example, by implementing a digital filter in series with the microactuator.

As described herein, the characterization module205is configured to characterize the hysteresis of the microactuator. Although various details for characterizing the hysteresis of the microactuator are described, it should be understood that the hysteresis of the microactuator may be determined or otherwise obtained in any suitable manner. As described further below, the characterization module205determines an estimate of the hysteresis that can be used for modeling the hysteresis.

As discussed above, one model for hysteresis is a Coleman-Hodgdon (C-H) model. The C-H hysteresis model is particularly suitable for modeling the hysteresis of a PZT microactuator as described herein. However, it should be understood that any other hysteresis model may be employed for the described systems and methods, for example, depending on the characteristics of the particular microactuator.

From known literature, the C-H hysteresis model may be expressed in terms of a difference equation:
y(t+1)−y(t)=−p1y(t)|u(t+1)−u(t)|
+p2h[−u(t)]u(t)|u(t+1)−u(t)|
+p3h[u(t)]u(t)|u(t+1)−u(t)|
+p4h[DR−u(t)]u(t)|u(t+1)−u(t)|
+p5h[u(t)−DL]u(t)|u(t+1)−u(t)−p6ξ1(t)|u(t+1)−u(t)|
−p7ξ2t|u(t+1)−ut|+p8[u(t+1)−u(t)]
+p9h[u(t)−DR]h[DL−u(t)][u(t+1)−u(t)],

and p1through p9are model coefficients

The C-H hysteresis model as expressed above is a non-linear equation that includes fifteen (15) constant parameters, nine (9) of which are independent (parameter dependence being omitted for brevity).

The parameters of the C-H hysteresis model can be estimated by forming a data vector Φ(t) and a parameter vector Θ as follows:
Φ(t)=[−y(t)|u(t+1)−u(t)|,h[−u(t)]u(t)|u(t+1)−u(t)|,
h[u(t)]u(t)|u(t+1)−u(t)|,h[DR−u(t)]u(t)|u(t+1)−u(t)|,
h[u(t)−DL]u(t)|u(t+1)−u(t)|,−ξ1(t)|u(t+1)−u(t)|,
−ξ2(t)|u(t+1)−u(t)|,[u(t+1)−u(t)],
h[u(t)−DR]h[DL−u(t)][u(t+1)−u(t)]]T
θ=[p1,p2, . . . ,p9]T

From the data vector Φ(t) and a parameter vector Θ, a pseudo linear form of the difference equation of the C-H hysteresis model can be obtained:
y(t+1)−y(t)=ΦT(t)θ

As expressed in the pseudo linear form, the parameters of the C-H hysteresis model can be estimated, for example, using a least-mean-square-error (LMSE) estimation technique. Because the model is non-linear, iterations may be used to refine and update dependent non-linear parameters that appear in the data vector Φ(t).

As discussed above, the parameters of the C-H hysteresis model are determined based at least in part on the characterization of the microactuator hysteresis provided by the characterization module205. More specifically, the parameters of the C-H hysteresis model are determined to fit the inverse of the hysteresis characterization.

Either the characterization module205or the hysteresis compensator module210can obtain the inverse of the hysteresis characterization. For example, where the obtained characterization of the hysteresis for the microactuator is expressed as y=f(u), the inverse can be obtained by swapping vectors y and u to obtain the inverse expressed as u=g(y). This approach avoids an inefficient mathematical computation to arrive at the inverse u=f−1(y). However, it should be understood that obtaining the inverse u=f−1(y) mathematically and then determining parameters for a hysteresis model according to the obtained inverse is possible.

The parameters of the C-H hysteresis model then can be determined, for example, by the hysteresis compensator module215. For example, the LMSE estimation technique described above may be employed (with the vectors y and u swapped) to obtain the parameters, and thus obtain the inverse model for compensation.

The hysteresis compensator module215then uses the inverse model to linearize the hysteresis of the microactuator. For example, the hysteresis compensator module215can implement a digital filter based on the inverse model, which is placed in series with the microactuator (e.g., between the microactuator and the servo controller). The digital filter can be implemented using the difference equation with u(t) being the pre-compensation control input from the servo controller and y(t) being the compensated output fed to the microactuator.

FIG. 3shows an example of hysteresis associated with a microactuator as characterized in accordance with various aspects of this disclosure. For the sake of clarity, a depiction of subassemblies300that may be employed in the data storage system100shown inFIG. 1. Each subassembly300includes a slider310, which carries a read/write head311for communication with the media surface, represented by tracks307. The slider110is supported by suspensions and track accessing arms of an actuator mechanism316. As shown, the slider110has two portions connected by a microactuator360that is configured to move the portion carrying the read/write head311relative to the other portion. Both portions of the slider310are moved by the actuator mechanism316.

A characterization process may be implemented using the subassemblies300to determine a hysteresis curve365associated with the microactuator360. The characterization process corresponds to the method for characterizing the hysteresis of the microactuator described above. The process/method may be performed by the characterization module205ofFIG. 2, for example, in conjunction with various components of the data storage system100ofFIG. 1.

As illustrated inFIG. 3, the characterization process involves switching to a single-stage mode so that the actuator mechanism316is regulated by the servo controller (e.g.,138inFIG. 1) and the microactuators360are open loop, allowing different signals to be applied to the microactuators360for characterizing the hysteresis. Once in the single-stage mode, a head switch is performed to determine (e.g., calibrate) a baseline DC skew. Then, a direct current (DC) voltage is applied to the microactuators360to drive the respective read/write heads311out of phase, and a change in DC skew is measured. The applied DC voltage is ramped up and down over an entire driving range of the microactuators360with measurements being made to generate the hysteresis curve365for the microactuators360. As shown inFIG. 3, the uncompensated hysteresis of the microactuators360is significantly non-linear, exhibiting a 4.7 micro inch (On) displacement D between the read/write heads311at a zero volt input.

FIG. 4is a flow chart illustrating an example of a method400for hysteresis compensation in a disc drive, in accordance with various aspects of the present disclosure. One or more aspects of the method400may be implemented in conjunction with the data storage system100ofFIG. 1, the hysteresis linearization module130-aofFIG. 2, and/or the subassemblies300ofFIG. 3. In some examples, a storage device may execute one or more sets of codes to control the functional elements of the storage device to perform one or more of the functions described below. Additionally or alternatively, the storage device may perform one or more of the functions described below using special-purpose hardware.

At block405, the method400may include characterizing the hysteresis of the microactuator(s). Characterizing the hysteresis at block405may be performed in any suitable manner, such as described above with reference toFIG. 3. At block410, the method400may include determining a hysteresis model for modeling the hysteresis of the microactuator(s). As illustrated by the dotted line of block410, such operation(s) may be optional, for example, when the hysteresis model is predetermined for the microactuator(s) needing hysteresis compensation. In some cases, however, the method400may include the operation(s) at block410to be able to perform hysteresis compensation for multiple different types of microactuators. For example, the operation(s) at block410may involve determining the type of microactuator(s) to be compensated, and selecting a hysteresis model from a plurality of available hysteresis models based at least in part on the type of microactuator(s).

Next at block415, the method400may include determining parameters for an inverse hysteresis model according to the characterized hysteresis of the microactuator(s). As discussed above, the operation(s) at block415may involve performing iterative estimations of the parameters to fit the hysteresis model to the characterized hysteresis (e.g., hysteresis curve of the microactuator(s)). Then at block420, the method400may include implementing a hysteresis compensator, such as a digital filter placed in series with the microactuator(s), based on the inverse hysteresis model.

The operation(s) at block405-420may be performed using the hysteresis linearization module130described with reference toFIGS. 1 and 2and/or another module. Thus, the method400may provide for hysteresis compensation in a disc drive that employs microactuators for precise tracking. It should be noted that the method400is just one implementation and that the operations of the method400may be rearranged, omitted, and/or otherwise modified such that other implementations are possible and contemplated.

FIG. 5is a flow chart illustrating an example of another method500for hysteresis compensation in a disc drive, in accordance with various aspects of the present disclosure. More specifically, the method500may be employed to characterize the hysteresis of one or more microactuators in a disc drive. One or more aspects of the method500may be implemented in conjunction with the data storage system100ofFIG. 1, the hysteresis linearization module130-aofFIG. 2, and/or the subassemblies300ofFIG. 3. In some examples, a storage device may execute one or more sets of codes to control the functional elements of the storage device to perform one or more of the functions described below. Additionally or alternatively, the storage device may perform one or more of the functions described below using special-purpose hardware.

At block505, the method500may include setting a servo controller associated with the microcontroller(s) to a single-stage mode. At block510, the method500may include determining or otherwise obtaining a baseline direct current (DC) skew for a pair of heads associated with the microactuator(s). At block515, the method500may include applying a DC voltage to the microactuator(s) to drive the heads out of phase. At block520, the method500may include measuring a change in DC skew between the heads with the applied voltage.

Next at block525, the method500may include determining whether the entire driving range has been covered for the microactuator(s) by the applied DC voltage. For the first instance of applying a voltage at block515, the entire range is not covered. Thus, the method500may continue to block530, at which the method500may include adjusting the voltage. The method500then returns to block515, at which the adjusted voltage is applied.

The operations at blocks515-525are repeated using the adjusted voltage from block530for each iteration. The operation(s) at block530may be performed so that the voltage is ramped progressively up and down the entire driving range of the microactuator(s). Thus, after a certain number of iterations, the determination at block525will be that the entire driving range of the microactuator(s) has been covered. In such case, the method500jumps to block535, at which the servo controller is returned to a dual-stage mode (e.g., for performing normal read/write operations). Having covered the entire driving range of the microactuator(s), the method500should result in sufficient measurements to establish a hysteresis curve for the microactuator(s).

The operations at blocks505-535may be performed using the hysteresis linearization module130described with reference toFIGS. 1 and 2and/or another module. Thus, the method500may provide a characterization of hysteresis of a microactuator(s) for implementing hysteresis compensation as described herein. It should be noted that the method500is just one implementation and that the operations of the method500may be rearranged, omitted, and/or otherwise modified such that other implementations are possible and contemplated. For example, depending on a size of the voltage adjustment at block530, the method500may continue through two or more iterations of the entire driving range of the microactuator(s) to obtain sufficient measurements to characterize the hysteresis. In such case, the determination at block525may be modified, for example, to a determination as to whether sufficient measurements have been made, or whether a desired number of iterations over the driving range have been completed.

In some examples, aspects from the methods400and500may be combined and/or separated. It should be noted that the methods400and500are just example implementations, and that the operations of the methods400and500may be rearranged or otherwise modified such that other implementations are possible.

The various illustrative blocks and components described in connection with this disclosure may be implemented or performed with a general-purpose processor, a digital signal processor (DSP), an ASIC, an FPGA or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, and/or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, multiple microprocessors, one or more microprocessors in conjunction with a DSP core, and/or any other such configuration.

In addition, any disclosure of components contained within other components or separate from other components should be considered exemplary because multiple other architectures may potentially be implemented to achieve the same functionality, including incorporating all, most, and/or some elements as part of one or more unitary structures and/or separate structures.

This disclosure may specifically apply to security system applications. This disclosure may specifically apply to storage system applications. In some embodiments, the concepts, the technical descriptions, the features, the methods, the ideas, and/or the descriptions may specifically apply to storage and/or data security system applications. Distinct advantages of such systems for these specific applications are apparent from this disclosure.

The process parameters, actions, and steps described and/or illustrated in this disclosure are given by way of example only and can be varied as desired. For example, while the steps illustrated and/or described may be shown or discussed in a particular order, these steps do not necessarily need to be performed in the order illustrated or discussed. The various exemplary methods described and/or illustrated here may also omit one or more of the steps described or illustrated here or include additional steps in addition to those disclosed.

Furthermore, while various embodiments have been described and/or illustrated here in the context of fully functional computing systems, one or more of these exemplary embodiments may be distributed as a program product in a variety of forms, regardless of the particular type of computer-readable media used to actually carry out the distribution. The embodiments disclosed herein may also be implemented using software modules that perform certain tasks. These software modules may include script, batch, or other executable files that may be stored on a computer-readable storage medium or in a computing system. In some embodiments, these software modules may permit and/or instruct a computing system to perform one or more of the exemplary embodiments disclosed here.

This description, for purposes of explanation, has been described with reference to specific embodiments. The illustrative discussions above, however, are not intended to be exhaustive or limit the present systems and methods to the precise forms discussed. Many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to explain the principles of the present systems and methods and their practical applications, to enable others skilled in the art to utilize the present systems, apparatus, and methods and various embodiments with various modifications as may be suited to the particular use contemplated.