US20170092310A1

US20170092310A1 - Data storage device concurrently controlling and sensing a secondary actuator for actuating a head over a disk

Info

Publication number: US20170092310A1
Application number: US15/246,332
Authority: US
Inventors: Timothy A. Ferris; Jaesoo Byoun
Original assignee: Western Digital Technologies Inc
Current assignee: Western Digital Technologies Inc
Priority date: 2015-09-25
Filing date: 2016-08-24
Publication date: 2017-03-30
Also published as: US9437231B1

Abstract

A data storage device is disclosed comprising a voice coil motor (VCM) and a secondary actuator configured to actuate a head over a disk. A control signal is applied to the secondary actuator while processing a sensor signal generated by the secondary actuator. A vibration signal is generated based on the sensor signal, wherein the vibration signal has a cut-off frequency between ten percent and ninety percent of a bandwidth of a control loop for controlling the secondary actuator.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 14/865,858 (Atty. Docket No. T8190), filed on Sep. 25, 2015, entitled “DATA STORAGE DEVICE CONCURRENTLY CONTROLLING AND SENSING A SECONDARY ACTUATOR FOR ACTUATING A HEAD OVER A DISK,” which is hereby incorporated by reference in its entirety

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 60-6N recorded around the circumference of each servo track. Each servo sector 6i 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 6i 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 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 voice coil motor (VCM) and a secondary actuator.

FIG. 2B is a flow diagram according to an embodiment wherein a control signal is applied to the secondary actuator while processing a sensor signal generated by the secondary actuator, wherein a high-pass vibration signal is generated based on the sensor signal.

FIG. 2C shows an embodiment wherein the secondary actuator control loop has a high-pass response, and the vibration signal has a cut-off frequency above a cut-off frequency of the high-pass response of the secondary actuator.

FIG. 3 shows control circuitry according to an embodiment wherein the vibration signal is generated based on a difference between the sensor signal and an estimated capacitive voltage of the secondary actuator.

FIG. 4 shows control circuitry according to an embodiment wherein a gain of a sensor capacitor is adapted based on the sensor signal and the estimated capacitive voltage of the secondary actuator.

FIG. 5 shows an embodiment wherein the vibration signal has a cut-off frequency higher than a cut-off frequency of the VCM control loop.

DETAILED DESCRIPTION

FIG. 2A shows a data storage device in the form of a disk drive according to an embodiment comprising a voice coil motor (VCM) 16 and a secondary actuator 18 configured to actuate a head 20 over a disk 22. The disk drive further comprises control circuitry 24 configured to execute the flow diagram of FIG. 2B, wherein a control signal is applied to the secondary actuator while processing a sensor signal generated by the secondary actuator (block 26). A vibration signal is generated based on the sensor signal, wherein the vibration signal is a high-pass signal (block 28).
In the embodiment of FIG. 2A, the disk 22 comprises a plurality of servo sectors 300-30N that define a plurality of servo tracks 32, wherein data tracks are defined relative to the servo tracks at the same or different radial density. The control circuitry 24 processes a read signal 34 emanating from the head 20 to demodulate the servo sectors 300-30N 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 24 filters the PES using a suitable compensation filter to generate a control signal 36 applied to the voice coil motor (VCM) 16 which rotates an actuator arm 38 about a pivot in order to actuate the head 20 radially over the disk 22 in a direction that reduces the PES. The control circuitry 24 also generates a control signal 40 applied to the secondary actuator 18 in order to actuate the head 20 over the disk 22 in fine movements. The servo sectors 300-30N 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.
The secondary actuator 18 may comprise any suitable elements for actuating the head 20 over the disk 22, such as one or more piezoelectric elements. Further, the secondary actuator 18 may actuate the head 20 in any suitable manner, wherein in the example of FIG. 2A, the secondary actuator 18 actuates a suspension 41 about the distal end of the actuator arm 38. In other embodiments, the secondary actuator 18 may actuate the head 20 about the distal end of the suspension 41. In yet other embodiments, the secondary actuator may comprise multiple actuators, such as a milliactuator configured to actuate the suspension 41 about the actuator arm 38, and a microactuator configured to actuate the head 20 about the suspension 41.
In one embodiment, the secondary actuator 18 may operate as a sensor for sensing vibrations affecting the disk drive. That is, a vibration may cause a rotational displacement of the actuator arm 38 which may induce an electrical response (sensor signal) in the secondary actuator 18. In one embodiment, the sensor signal may manifest on the same electrical lead used to apply the control signal 40 to the secondary actuator 18, and in other embodiments, there may be a dedicated lead coupled to the secondary actuator 18 for conducting the sensor signal. In one embodiment, the sensor signal may be processed to generate a vibration signal representing a vibration affecting the disk drive (magnitude and/or phase). The vibration signal may be used for any suitable purpose, such as for aborting a write operation to prevent an off-track write, or for generating a feed-forward control signal that compensates for the vibration in the servo control loop.
FIG. 2C shows an embodiment wherein the vibration signal 42 generated based on the sensor signal 40 emanating from the secondary actuator 18 is a high pass signal meaning that the vibration signal 42 is responsive to higher frequency vibrations affecting the disk drive (above a cut-off frequency) with essentially no response at DC. In one embodiment, the vibration signal has a cut-off frequency between ten percent and ninety percent of a bandwidth of the control loop for controlling the secondary actuator 18. In another embodiment, the control loop for controlling the secondary actuator 18 has a high-pass response 44 such as shown in FIG. 2C, and the high-pass vibration signal has a cut-off frequency above a cut-off frequency of the high-pass response of the secondary actuator control loop. In yet another embodiment shown in FIG. 5, the high-pass vibration signal 42 has a cut-off frequency higher than a cut-off frequency of a response of the VCM control loop 46. In one embodiment, generating the high-pass vibration signal 42 above the response of the VCM control loop 46 helps attenuate cross-talk interference from the VCM control loop when using the vibration signal as feed-forward compensation for the secondary actuator control loop.
FIG. 3 shows control circuitry according to an embodiment comprising a read/write channel 48 configured to process the read signal 34 emanating from the head 20 when reading the servo sectors. The read/write channel 48 demodulates the read signal 34 into a measured position 50 of the head 20 over the disk 22. The measured position 50 is subtracted from a reference position 52 to generate a position error signal (PES) 54. A VCM compensator 56 processes the PES 54 to generate the control signal 36 applied to the VCM 16, and a secondary actuator compensator 58 processes the PES 54 to generate the control signal 40 applied to the secondary actuator 18. In the embodiment of FIG. 3, the secondary actuator compensator 58 generates a digital control signal 60 that is adjusted at adder 62 by a feed-forward compensation signal 64. The resulting digital control signal 66 is converted into an analog control signal 40 by a digital-to-analog converter (DAC) 68. The analog control signal 40 is processed at block 70 to estimate a capacitive voltage 72 of the secondary actuator 18, and at block 74, a vibration signal 76 is generated based on the estimated capacitive voltage 72 and the analog control signal 40. Block 78 processes the vibration signal 76 to generate the feed-forward compensation signal 64, wherein block 78 may implement any suitable conversion algorithm to convert the vibration signal 76 (an acceleration signal) into a feed-forward control signal 64. In the embodiment of FIG. 3, the feed-forward control signal 64 compensates for the vibration by essentially anticipating the effect of the vibration on the PES 54 and controlling the position of the head 20 so as to follow the vibration.
Any suitable control circuitry may be employed to implement blocks 70 and 74 in FIG. 3. FIG. 4 shows control circuitry according to an embodiment comprising a sensor capacitor 80 comprising a capacitance C′ and a gain K that effectively estimate the capacitance C within the secondary actuator 18. The control circuitry of FIG. 4 further comprises a suitable current mirror F that generates a sensor current 82 proportional to a current applied to the secondary actuator 18 due to the control signal 40. An estimated capacitive voltage 72 of the secondary actuator 18 is generated by applying the sensor current 82 to the sensor capacitor 80, and the vibration signal 76 is generated at adder 84 based on a difference between the sensor signal 40 and the estimated capacitive voltage 72. This embodiment effectively cancels the voltage component in the sensor signal 40 due to the capacitance C of the secondary actuator 18 so that the vibration signal 76 represents mainly the voltage component 88 generated by the secondary actuator 18 due to the effect of the vibration on the disk drive.
In the embodiment of FIG. 4, the control circuitry adapts the gain K of the sensor capacitor 80 based on the sensor signal 40 and the estimated capacitive voltage 72. In one embodiment, a difference signal 86 is generated at adder 89 based on a difference between the absolute value (block 90A) of the control signal 40 and the absolute value (block 90B) of the estimated capacitive voltage 72. In the embodiment of FIG. 4, the control circuitry comprises a proportional-integral-derivative (PID) compensator 92 that low pass filters the difference signal 86 to generate a low-pass signal 94, and adapts the gain K of the sensor capacitor 80 based on the low-pass signal 94. In this manner, the gain K of the sensor capacitor 80 is adapted substantially based on the control signal generated by the secondary actuator compensator 58 rather than on the sensor signal 40 due to the response of the secondary actuator 18 to vibrations. In one embodiment, the gain K is adapted until the low-pass signal 94 is substantially zero wherein the capacitance of the sensor capacitor 80 will substantially match the capacitance C of the secondary actuator 18.
In one embodiment, the ratio of the current mirror F and the gain K are selected to enable the capacitance C′ of the sensor capacitor 80 to be significantly less than the capacitance C of the secondary actuator 18 (e.g., two times less). In this manner, the capacitor C′ in the sensor capacitor 80 may be fabricated as part of an integrated circuit rather than implemented as a more expensive external capacitor. For example, if the capacitor C′ is fabricated to be approximately two times smaller than the capacitor C of the secondary actuator 18, the current mirror F may be fabricated with an approximately unitary ratio and the gain K adapted to approximately two. In other embodiments, the ratio of the current mirror F and/or the gain K may be selected so that the capacitor C′ of the sensor capacitor 80 may be larger than the capacitor C of the secondary actuator 18.
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 other embodiments, the control circuitry may be implemented within a suitable preamp circuit, within a power large scale integrated (PLSI) circuit, or within a stand-alone integrated circuit.
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 some embodiments, the control circuitry may comprise suitable conversion circuitry so that at least some of the operations are implemented in the digital domain, and in other embodiments at least some of the operations are implemented in the analog domain.
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.

Claims

What is claimed is:

1. A method of operating a data storage device, the method comprising:

actuating a head over a disk using a voice coil motor (VCM) and a secondary actuator;

applying a control signal to the secondary actuator and concurrently process a sensor signal generated by the secondary actuator; and

generating a vibration signal based on the sensor signal and a sensor capacitor, wherein a capacitance of the sensor capacitor is at least two times less than a capacitance of the secondary actuator.

2. The method as recited in claim 1, wherein the vibration signal has a cut-off frequency between ten percent and ninety percent of a bandwidth of a control loop for controlling the secondary actuator.

3. The method as recited in claim 1, wherein:

a control loop for controlling the secondary actuator has a high-pass response; and

the vibration signal has a cut-off frequency above a cut-off frequency of the high-pass response of the control loop for the secondary actuator.

4. The method as recited in claim 1, wherein the vibration signal has a cut-off frequency higher than a cut-off frequency of a response of a control loop for controlling the VCM.

5. The method as recited in claim 1, further comprising:

generating a sensor current proportional to a current applied to the secondary actuator due to the control signal;

estimating a capacitive voltage of the secondary actuator based on the sensor current; and

generating the vibration signal based on a difference between the sensor signal and the estimated capacitive voltage.

6. The method as recited in claim 5, further comprising estimating the capacitive voltage of the secondary actuator by applying the sensor current to the sensor capacitor.

7. The method as recited in claim 5, further comprising adapting a gain of the sensor capacitor based on the sensor signal and the estimated capacitive voltage.

8. The method as recited in claim 7, further comprising:

low pass filtering a difference between the sensor signal and the estimated capacitive voltage to generate a low-pass signal; and

adapting the gain of the sensor capacitor based on the low-pass signal.

9. The method as recited in claim 1, further comprising generating a feed-forward compensation signal applied to the secondary actuator based on the vibration signal.

10. Control circuitry configured to control a voice coil motor (VCM) and a secondary actuator to actuate a head over a disk, the control circuitry configured to:

apply a control signal to the secondary actuator and concurrently process a sensor signal generated by the secondary actuator; and

generate a vibration signal based on the sensor signal and a sensor capacitor, wherein a capacitance of the sensor capacitor is at least two times less than a capacitance of the secondary actuator.

11. The control circuitry as recited in claim 10, wherein the vibration signal has a cut-off frequency between ten percent and ninety percent of a bandwidth of a control loop for controlling the secondary actuator.

12. The control circuitry as recited in claim 10, wherein:

the vibration signal has a cut-off frequency above a cut-off frequency of the high-pass response of the secondary actuator.

13. The control circuitry as recited in claim 10, wherein the vibration signal has a cut-off frequency higher than a cut-off frequency of a response of a control loop for controlling the VCM.

14. The control circuitry as recited in claim 10, further configured to:

generate a sensor current proportional to a current applied to the secondary actuator due to the control signal;

estimate a capacitive voltage of the secondary actuator based on the sensor current; and

generate the vibration signal based on a difference between the sensor signal and the estimated capacitive voltage.

15. The control circuitry as recited in claim 14, further configured to estimate the capacitive voltage of the secondary actuator by applying the sensor current to the sensor capacitor that is proportional to a capacitance of the secondary actuator.

16. The control circuitry as recited in claim 14, further configured to adapt a gain of the sensor capacitor based on the sensor signal and the estimated capacitive voltage.

17. The control circuitry as recited in claim 16, further configured to:

low pass filter a difference between the sensor signal and the estimated capacitive voltage to generate a low-pass signal; and

adapt the gain of the sensor capacitor based on the low-pass signal.

18. The control circuitry as recited in claim 10, further configured to generate a feed-forward compensation signal applied to the secondary actuator based on the vibration signal.

19. A data storage device comprising:

a disk;

a head;

a voice coil motor (VCM) and a secondary actuator configured to actuate the head over the disk; and

control circuitry configured to:

generate a vibration signal based on the sensor signal, wherein the vibration signal has a cut-off frequency between ten percent and ninety percent of a bandwidth of a control loop for controlling the secondary actuator.

20. The data storage device as recited in claim 19, wherein:

the control loop for controlling the secondary actuator has a high-pass response; and

21. The data storage device as recited in claim 19, wherein the vibration signal has a cut-off frequency higher than a cut-off frequency of a response of a control loop for controlling the VCM.

22. The data storage device as recited in claim 19, wherein the control circuitry is further configured to:

23. The data storage device as recited in claim 22, wherein the control circuitry is further configured to estimate the capacitive voltage of the secondary actuator by applying the sensor current to a sensor capacitor that is proportional to a capacitance of the secondary actuator.

24. The data storage device as recited in claim 23, wherein the control circuitry is further configured to adapt a gain of the sensor capacitor based on the sensor signal and the estimated capacitive voltage.

25. The data storage device as recited in claim 24, wherein the control circuitry is further configured to:

adapt the gain of the sensor capacitor based on the low-pass signal.

26. The data storage device as recited in claim 23, wherein a capacitance of the sensor capacitor is at least two times less than the capacitance of the secondary actuator.

27. The data storage device as recited in claim 19, wherein the control circuitry is further configured to generate a feed-forward compensation signal applied to the secondary actuator based on the vibration signal.