Data storage device comprising slew rate anti-windup compensation for microactuator

Description

BACKGROUND

Data storage devices such as disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and servo sectors. The servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the actuator arm as it seeks from track to track.

FIG. 1 shows a prior art disk format 2 as comprising a number of servo tracks 4 defined by servo sectors 6₀-6_N recorded around the circumference of each servo track. Each servo sector 6_i comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a servo track address, used to position the head over a target data track during a seek operation. Each servo sector 6_i further comprises groups of servo bursts 14 (e.g., N and Q servo bursts), which are recorded with a predetermined phase relative to one another and relative to the servo track centerlines. The phase based servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations. A position error signal (PES) is generated by reading the servo data 12 and servo bursts 14, wherein the PES represents a measured position of the head relative to a centerline of a target servo track. A servo controller processes the PES to generate a control signal applied to a head actuator (e.g., a voice coil motor) in order to actuate the head radially over the disk in a direction that reduces the PES.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art disk format comprising a plurality of servo tracks defined by servo sectors.

FIG. 2A shows a data storage device in the form of a disk drive according to an embodiment comprising a head actuated over a disk by a microactuator.

FIG. 2B is a flow diagram according to an embodiment wherein an input to a compensator is adjusted based on a saturated slew rate signal.

FIG. 3A shows control circuitry according to an embodiment wherein a microactuator control signal is differentiated to generate a slew rate signal, the slew rate signal is saturated, and the input to the compensator adjusted based on a difference between the slew rate signal and the saturated slew rate signal.

FIG. 3B shows control circuitry according to an embodiment wherein the input to the compensator is also adjusted based on a saturated amplitude signal.

FIG. 4 shows a reduced complexity embodiment that is equivalent to the embodiment of FIG. 3B.

DETAILED DESCRIPTION

FIG. 2A shows a data storage device in the form of a disk drive according to an embodiment comprising a disk 16, a head 18, and a microactuator 20 configured to actuate the head 18 over the disk 16. The disk drive further comprises control circuitry 22 configured to execute the flow diagram of FIG. 2B. An input signal to a compensator is generated (block 24), wherein the compensator is configured to generate a microactuator control signal. A slew rate signal representing a slew rate of the microactuator control signal is generated (block 26) and applied to a first saturator configured to generate a saturated slew rate signal (block 28). The input signal is adjusted based on the slew rate saturation signal (block 30), and the microactuator is controlled based on the microactuator control signal (block 32).

In the embodiment of FIG. 2A, a plurality of concentric servo tracks 34 are defined by embedded servo sectors 36₀-36_N, wherein concentric data tracks may be defined relative to the servo tracks 34 at the same or different radial density. The control circuitry 22 processes a read signal 38 emanating from the head 18 to demodulate the servo sectors and generate a position error signal (PES) representing an error between the actual position of the head and a target position relative to a target track. The control circuitry 22 filters the PES using a suitable compensator to generate a control signal 40 applied to a voice coil motor (VCM) 42 which rotates an actuator arm 44 about a pivot in order to actuate the head 18 radially over the disk 16 in a direction that reduces the PES. The control circuitry 22 also generates a control signal 46 applied to a microactuator 20 in order to actuate the head 18 over the disk 16 in fine movements. Any suitable microactuator may be employed, such as a piezoelectric actuator. In addition, the microactuator may actuate the head 18 over the disk 16 in any suitable manner, such as by actuating a suspension relative to the actuator arm, or actuating a slider relative to the suspension. The servo sectors 36₀-36_N may comprise any suitable head position information, such as a track address for coarse positioning and servo bursts for fine positioning. The servo bursts may comprise any suitable pattern, such as an amplitude based servo pattern or a phase based servo pattern.

In one embodiment, the slew rate of the control signal 46 applied to the microactuator 20 in FIG. 2A is limited (using a saturator) in order to reduce excitation of mechanical resonances when the disk drive is subjected to a disturbance, such as an external acoustic vibration, or a physical shock to the disk drive. However, saturating the slew rate of the control signal 46 for an extended period can lead to instability of the closed-loop servo control system. Accordingly, in one embodiment the control circuitry 22 that implements the servo control system achieves stability during slew rate saturation by adjusting the input to the compensator rather than modifying the compensator itself which can be overly complex.

FIG. 3A shows an example embodiment of control circuitry 22 that implements a servo control system for a VCM 42 and a microactuator 20. The combined response of the VCM 42 and microactuator 20 is measured by reading the servo sectors 36₀-36_N to generate a measured position 48 of the head 18. The measured position 48 is subtracted from a target position 50 to generate a position error signal (PES) 52. When the slew rate of the microactuator control signal 46 is unsaturated, the PES 52 is input into a microactuator compensator 54 unadjusted, wherein the micoractuator compensator 54 generates a microactuator control signal 56. The microactuator control signal 56 is differentiated 58 to generate a slew rate signal 60 representing the slew rate of the microactuator control signal 56. The slew rate signal 60 is applied to a slew rate saturator 62 configured to generate a saturated slew rate signal 64. The saturated slew rate signal 64 is integrated 65 to generate the control signal 46 applied to the microactuator 20. The effect on the position of the head 18 by the microactuator 20 due to the control signal 46 is estimated by processing the control signal 46 with a model 66 of the microactuator 20. The estimated change in position 68 is combined with the PES 52 to generate an adjusted PES 70 with the effect of the microactuator 20 canceled. The adjusted PES 70 is processed by a VCM compensator 72 which generates the VCM control signal 40 applied to the VCM 42.

When the slew rate signal 60 exceeds the limits of the slew rate saturator 62, a difference signal 74 is generated representing a difference between the slew rate signal 60 and the saturated slew rate signal 64. The difference signal 74 is applied to the model of the microactuator and the resulting response integrated 76 to generate a corresponding position signal 78 that is subtracted from the PES 52 in order to adjust the input to the microactautor compensator 54. Adjusting the input to the microactuator compensator 54 in this manner helps maintain stability of the closed-loop servo control system without requiring a more sophisticated anti-windup compensator.

In one embodiment, the control signal 46 applied to the microactuator 20 is also amplitude constrained (using a saturator) so as to prevent overdriving and damaging the microactuator 20. FIG. 3B shows control circuitry according to this embodiment wherein the amplitude signal 80 generated by the integrator 65 is input into an amplitude saturator 82. When the amplitude signal 80 exceeds the limits of the amplitude saturator 82, a difference signal 84 is generated representing a difference between the amplitude signal 80 and the saturated amplitude signal 46. The difference signal 84 is applied to the model of the microactuator 66 to generate a corresponding position signal 86 which is added to the position signal 78 to generate the position signal 88 used to adjust the input of the microactuator compensator 54. The overall system dynamic equations of the servo control system shown in FIG. 3B may be represented as:
u_c(k)=C_M(z)y_lin(k)
v(k)=u_c(k)−u_c(k−1)
v_m(k)=Sat_v(v(k))
u(k)=u(k−1)+v_m(k)
u_m(k)=Sat_u(u(k))
y_lin(k)=y_r(k)−y_dv(k)−y_du(k)
FIG. 4 shows a transformation of the above equations in order to implement an equivalent servo control system having reduced cost and complexity.

Any suitable saturation levels may be employed in the slew rate saturator 62 and the amplitude saturator 82 of FIG. 3B. In one embodiment, the saturations levels may be selected by optimizing a subset of disk drives, and then loading the optimized saturation levels into each production disk drive. In another embodiment, the saturation levels may be optimized (or further tuned) within each production disk drive by the control circuitry 22 executing any suitable optimization algorithm.

Any suitable control circuitry may be employed to implement the flow diagrams in the above embodiments, such as any suitable integrated circuit or circuits. For example, the control circuitry may be implemented within a read channel integrated circuit, or in a component separate from the read channel, such as a disk controller, or certain operations described above may be performed by a read channel and others by a disk controller. In one embodiment, the read channel and disk controller are implemented as separate integrated circuits, and in an alternative embodiment they are fabricated into a single integrated circuit or system on a chip (SOC). In addition, the control circuitry may include a suitable preamp circuit implemented as a separate integrated circuit, integrated into the read channel or disk controller circuit, or integrated into a SOC.

In one embodiment, the control circuitry comprises a microprocessor executing instructions, the instructions being operable to cause the microprocessor to perform the flow diagrams described herein. The instructions may be stored in any computer-readable medium. In one embodiment, they may be stored on a non-volatile semiconductor memory external to the microprocessor, or integrated with the microprocessor in a SOC. In another embodiment, the instructions are stored on the disk and read into a volatile semiconductor memory when the disk drive is powered on. In yet another embodiment, the control circuitry comprises suitable logic circuitry, such as state machine circuitry.

In various embodiments, a disk drive may include a magnetic disk drive, an optical disk drive, etc. In addition, while the above examples concern a disk drive, the various embodiments are not limited to a disk drive and can be applied to other data storage devices and systems, such as magnetic tape drives, solid state drives, hybrid drives, etc. In addition, some embodiments may include electronic devices such as computing devices, data server devices, media content storage devices, etc. that comprise the storage media and/or control circuitry as described above.

The various features and processes described above may be used independently of one another, or may be combined in various ways. All possible combinations and subcombinations are intended to fall within the scope of this disclosure. In addition, certain method, event or process blocks may be omitted in some implementations. The methods and processes described herein are also not limited to any particular sequence, and the blocks or states relating thereto can be performed in other sequences that are appropriate. For example, described tasks or events may be performed in an order other than that specifically disclosed, or multiple may be combined in a single block or state. The example tasks or events may be performed in serial, in parallel, or in some other manner. Tasks or events may be added to or removed from the disclosed example embodiments. The example systems and components described herein may be configured differently than described. For example, elements may be added to, removed from, or rearranged compared to the disclosed example embodiments.

While certain example embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions disclosed herein. Thus, nothing in the foregoing description is intended to imply that any particular feature, characteristic, step, module, or block is necessary or indispensable. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the embodiments disclosed herein.