# Patent

## Title
Data storage device optimizing spindle motor power when transitioning into a power failure mode

## Abstract

A data storage device is disclosed comprising a head actuated over a disk, and a spindle motor configured to rotate the disk, wherein the spindle motor comprises a plurality of windings. The windings of the spindle motor are commutated based on a commutation sequence while applying a driving voltage to each winding, wherein the driving voltage comprises an operating polarity during normal operation. When a supply voltage falls below a threshold while the spindle motor is rotating the disk, the spindle motor is configured into a power generator by at least reversing the polarity of the driving voltage for an interval and then reversing the polarity of the driving voltage back to the operating polarity.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a divisional of U.S. patent application Ser. No. 14/582,068, filed on Dec. 23, 2014, 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.
The disk 2 is typically rotated by a spindle motor at a high speed so that an air bearing forms between the head and the disk surface. A commutation controller applies a driving signal to the windings of the spindle motor using a particular commutation sequence in order to generate a rotating magnetic field that causes the spindle motor to rotate. Prior art disk drives have typically controlled the commutation of the windings by measuring a zero-crossing frequency of a back electromotive force (BEMF) voltage generated by the windings of the spindle motor. Prior art disk drives may also utilize the BEMF voltage generated by the spindle motor as a power source during power failure to assist with power down operations, such as unloading the head onto a ramp. For example, a known technique to generate power from the spindle motor during a power failure is to short the windings to build current due do the inductance, and then release the short so that the fly-back current boosts the supply voltage. However, this technique may not optimize the power provided by the spindle motor, particularly when transitioning the spindle motor from normal operation into a power generator when a power failure is detected.



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 comprising a head actuated over a disk, and control circuitry powered by a supply voltage and configured to commutate windings of a spindle motor.
FIG. 2B shows an embodiment wherein when the supply voltage falls below a threshold due to a power failure, the spindle motor is configured into a power generator by at least reducing an amplitude of a driving voltage applied to the windings to substantially zero and then incrementally increasing the amplitude of the driving voltage by at least two steps toward an operating amplitude.
FIG. 3A shows an embodiment wherein the amplitude of the driving voltage is step incremented so as to increase the power generated by the spindle motor earlier during the power failure procedure.
FIG. 3B shows different waveforms representing the supply voltage decreasing due to a power failure and then boosted by the power generated by the spindle motor, wherein in one embodiment the step increments for the driving voltage as shown in FIG. 3A are selected to prevent the supply voltage from falling below a preamp fault threshold.
FIGS. 4A-4E show signal waveforms illustrating an embodiment for step increasing the amplitude of the windings driving voltages during the power failure procedure.
FIG. 5A shows an embodiment wherein when the supply voltage falls below a threshold due to a power failure, the spindle motor is configured into a power generator by at least reversing the polarity of the driving voltage for an interval, reversing the polarity of the driving voltage back to an operating polarity, and then step increasing the amplitude of the driving voltage.
FIG. 5B shows an embodiment wherein after reversing the polarity of the driving voltage back to the operating polarity, the amplitude of the driving voltage is configured to the average of the step increments shown in FIG. 5A.



DETAILED DESCRIPTION
FIG. 2A shows a data storage device in the form of a disk drive comprising a disk 16, a spindle motor 18 configured to rotate the disk 16, wherein the spindle motor comprises a plurality of windings, and a head 20 actuated over the disk 16. The disk drive further comprises control circuitry 22 powered by a supply voltage 24, wherein the control circuitry 22 is configured to commutate the windings based on a commutation sequence while applying a driving voltage to each winding, wherein the driving voltage comprises an operating amplitude during normal operation. When the supply voltage falls below a threshold, the control circuitry 22 configures the spindle motor 18 into a power generator by at least reducing the amplitude of the driving voltage to substantially zero and then incrementally increasing the amplitude of the driving voltage by at least two steps toward the operating amplitude an example of which is illustrated in FIG. 2B.
In one embodiment shown in FIG. 2A, a back electromotive force (BEMF) voltage 26 generated by the windings of the spindle motor 18 may be processed in order to drive the commutation sequence of a commutation controller 28. The control circuitry 22 may process a BEMF signal 30 which may be a square wave representing the BEMF zero-crossings as detected by a BEMF detector 32. The commutation controller 28 may generate a control signal 34 which configures the BEMF detector 32 to detect the zero-crossing of the BEMF voltage generated by each winding as the disk rotates. The commutation controller 28 also generates a control signal 36 applied to commutation logic 38. In the embodiment of FIG. 2A, the commutation logic 38 is configured by the control signal 36 to control the state of switches 40 in order to drive the windings with driving voltages +V and −V. The commutation logic 38 may operate in any suitable manner, such as by driving the switches 40 as linear amplifiers that apply continuous-time sinusoidal voltages to the windings. In another embodiment, the commutation logic 38 may drive the switches 40 using pulse wide modulation (PWM), such as using square wave PWM, trapezoidal PWM, or sinusoidal PWM. Regardless as to how the windings are driven, the commutation controller 28 generates the control signal 36 so that the windings are commutated at the correct periods, thereby generating the desired rotating magnetic field that causes the spindle motor to rotate. In one embodiment, the control circuitry 22 may generate a control signal 42 that controls the effective amplitude of the driving voltages (continuous or PWM), thereby controlling the speed of the spindle motor 18.
If a power failure occurs while the disk 16 is spinning, there is residual kinetic energy as the disk 16 continues to rotate the spindle motor 18, and therefore the spindle motor 18 can be converted into a power generator used to power the control circuitry 22 while executing power down operations, such as completing a current write operation to the disk 16 before unloading the head 20 onto a ramp. FIG. 3A shows a prior art technique for generating power from the spindle motor 18 by shorting the windings for an interval to build current due do the inductance, and then releasing the short so that the fly-back current boosts the supply voltage 24. For example, in one embodiment when a power failure occurs the negative supply voltage −V in FIG. 2A may be shorted to ground, and the windings of the spindle motor 18 may be shorted to ground by turning on the bottom switches 40. After the shorting interval, the upper switches 40 may be turned on to connect the windings to the supply voltage 24 which essentially drives the windings with a driving voltage at an amplitude proximate the BEMF voltage as illustrated in FIG. 3A.
The prior art technique of shorting/unshorting the windings may not provide sufficient power at the beginning of the power failure event, which may cause the supply voltage to dip below a preamp fault threshold and interrupt a current write operation since the preamp circuit is used to perform the write (e.g., by generating a write current applied to a write element). Accordingly, in one embodiment after shorting the windings to ground for an interval, the amplitude of the driving voltage may be incrementally increased by at least two steps toward the operating amplitude (i.e., toward the amplitude of the BEMF voltage). In one embodiment, the step increment and/or the interval of each step may be configured so as to increase the power generated by the spindle motor earlier during the power failure procedure in order to prevent the supply voltage from falling below the preamp fault threshold. FIG. 3B shows a number of waveforms representing the supply voltage during a power failure for different step increments and/or intervals. For example, certain step increments and/or intervals may still cause the supply voltage to fall below the preamp fault threshold. Accordingly, in one embodiment the step increments and/or intervals may be optimized to provide sufficient power at the beginning of a power failure event, thereby preventing the supply voltage (e.g., waveform 44 in FIG. 3B) from falling below the preamp fault threshold.
FIG. 4A shows a signal waveform representing the driving voltage for the windings according to an embodiment starting at substantially zero, and then incrementally increased by three steps (at each dot) over time. FIG. 4B illustrates the signal waveforms representing the power generated by each winding over time as well as the total power generated by the spindle motor 18. FIGS. 4C-4E show signal waveforms representing the current, driving voltage, and BEMF voltage of each winding (each phase) relative to time as the driving voltage is incrementally increased by the three steps. In this embodiment, the driving voltage applied to each winding comprises a sinusoidal voltage, and the driving voltage applied to each winding comprises a phase corresponding to a phase of the BEMF voltage across each winding. That is, the phase of the BEMF voltage and corresponding amplitude will vary depending on the rotation angle of the spindle motor 18 when a power failure occurs. The current flowing through each winding will also have a specific phase and amplitude when a power failure occurs. As illustrated in FIGS. 4C-4E, reducing the amplitude of the driving voltage to substantially zero and then incrementally increasing the amplitude of the driving voltage causes the current in each winding to reverse polarity, thereby converting the spindle motor 18 into a power generator. FIGS. 4C-4E also illustrate an embodiment wherein the driving voltage is incrementally increased toward the amplitude of the BEMF voltage, and in one embodiment the amplitude of the driving voltage eventually exceeds the BEMF voltage at the last incremental step which, in one embodiment, optimizes the power generation at the beginning of the power failure event.
The amplitude of the driving voltage applied to each winding may be increased incrementally in any suitable manner. In one embodiment, the control circuitry 22 may control the switches 40 as linear amplifiers in order to step increase the amplitude of the driving voltages. In another embodiment, the control circuitry 22 may drive the switches 40 using pulse width modulation (PWM) in order to control an average amplitude for the driving voltages. In addition, the control circuitry 22 may incrementally increase the amplitude of the driving voltage at any suitable step interval.
In one embodiment, the disk 16 as shown in FIG. 2A comprises a plurality of servo sectors 460-46N that define a plurality of servo tracks 48. The control circuitry 22 may be configured in one embodiment to increase the amplitude of the driving voltage when the head reaches a first servo sector by a first step increment and then step increase the amplitude of the driving voltage when the head reaches a second servo sector following the first servo sector by a second step increment. In one embodiment, the incremental steps shown in FIG. 2B for increasing the amplitude of the driving voltage are configured so as to increase the power generated by the spindle motor at the earlier servo wedges so as to prevent the supply voltage 24 from falling below the preamp fault threshold. The power generated by the spindle motor may be represented as:




  
    
      
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where k represents the servo sector index starting from k=1 as the first servo sector at the beginning of a power failure event, N represents a maximum number of servo sectors per data sector, p represents the phase of the winding (A,B,C), vD,P[k] represents the driving voltage for each winding, ip[k] represents the current for each winding, and w[k] represents a weighting factor. In one embodiment, the amplitude of the driving voltage vD,P[k] may be configured for each servo sector so as to maximize the above equation given a predetermined weighting w[k] assigned to each servo sector. In one embodiment, the weighting assigned to each servo sector may be represented as:

w[k]=α−k  Eq. (2)

In this manner the above Eq. (2) assigns a higher weighting to the earlier servo sectors so that the above Eq. (1) generates more power earlier during the power failure procedure to help provide enough power to complete a current write operation before the supply voltage 24 falls below the preamp fault threshold. For example in the embodiment of FIG. 3B, as α increases in Eq. (2) the supply voltage waveform may trend toward waveform 44.

FIG. 5A shows an embodiment wherein when the supply voltage 24 falls below a threshold, the control circuitry 22 may configure the spindle motor 18 into a power generator by at least reversing the polarity of the driving voltage for an interval and then reversing the polarity of the driving voltage back to an operating polarity. This embodiment may cause the current in the windings to reverse polarity faster so that the spindle motor 18 may generate more power earlier during the power failure procedure, which may help prevent the supply voltage from falling below the preamp fault as described above with reference to FIG. 3B. Any suitable technique may be employed to reverse the polarity of the driving voltage, such as by generating a negative supply voltage using the positive supply voltage 24, or by configuring the windings of the spindle motor 18 so as to generate reverse polarity driving voltages across each winding (i.e., use the BEMF voltage of the spindle motor 18 to generate a negative supply voltage).
In the embodiment of FIG. 5A, after reversing the polarity of the driving voltage for each winding, the control circuitry 22 may incrementally increase the amplitude of the driving voltage by at least two steps as described above. That is, the embodiment of FIG. 5A may combine the technique of reversing the polarity of the driving voltages with the technique of step increasing the driving voltages in order to increase the power generated early in the power failure procedure. In another embodiment shown in FIG. 5B, after reversing the polarity of the driving voltages back to the operating polarity, the amplitude of the driving voltages may be incremented by one step and then remain substantially constant in order to reduce the complexity of the power generating algorithm, thereby reserving processor bandwidth for other operations. In one embodiment, the constant amplitude for the driving voltage shown in FIG. 5B may be selected as the average value of the stepped voltages shown in FIG. 5A. That is, the optimal stepped driving voltage may be determined, for example, by optimizing the power generated based on the above Eq. (1), and then the constant amplitude of the driving voltage shown in FIG. 5B configured as the average of the optimal stepped driving voltage.
In one embodiment, the control circuitry 22 may short to ground the center tap 50 of the windings in the spindle motor 18 together with the opposite ends of the windings during the shorting interval. In this embodiment, shorting the center tap 50 of the windings may cause the current in the windings to reverse polarity faster so that the spindle motor 18 may generate more power earlier during the power failure procedure to help ensure a write operation may be completed before unloading the head onto a ramp.
In an embodiment described above, the control circuitry 22 may incrementally increase the amplitude of the driving voltages for each winding at each servo sector (i.e., as part of the servo sector interrupt service routine). In another embodiment, the control circuitry 22 may comprise dedicated hardware (e.g., registers and timers) that may be configured so as to automatically reverse the polarity of the driving voltages and/or step increase the amplitude of the driving voltages at any suitable interval independent of the servo sample rate when a power failure is detected.
In the embodiment described above with reference to FIG. 3B, the step increments and/or intervals for the driving voltage may be optimized to provide sufficient power at the beginning of a power failure event, thereby preventing the supply voltage (e.g., waveform 44 in FIG. 3B) from falling below a preamp fault threshold. In other embodiments, the step increments and/or intervals of the driving voltage may be optimized so as to remain above a different threshold during a power failure, such as a reset threshold that may cause the disk drive to automatically unload the head onto a ramp.
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.

## Claims

1. A data storage device comprising:
a disk;
a spindle motor configured to rotate the disk, wherein the spindle motor comprises a plurality of windings;
a head actuated over the disk; and
control circuitry powered by a supply voltage, the control circuitry configured to:
commutate the windings based on a commutation sequence while applying a driving voltage to each winding, wherein the driving voltage comprises an operating polarity during normal operation; and
when the supply voltage falls below a threshold while the spindle motor is rotating the disk, configure the spindle motor into a power generator by at least reversing the polarity of the driving voltage for an interval and then reversing the polarity of the driving voltage back to the operating polarity.

2. The data storage device as recited in claim 1, wherein after reversing the polarity of the driving voltage back to the operating polarity the control circuitry is further configured to incrementally step increase an amplitude of the driving voltage by at least one step.

3. The data storage device as recited in claim 1, wherein reversing the polarity of the driving voltage back to the operating polarity comprises configuring an amplitude of the driving voltage to substantially equal a back electromotive force (BEMF) voltage across each winding.

4. A method of operating a data storage device, the method comprising:
commutating windings of a spindle motor configured to rotate a disk based on a commutation sequence while applying a driving voltage to each winding, wherein the driving voltage comprises an operating polarity during normal operation; and
when a supply voltage falls below a threshold while the spindle motor is rotating the disk, configuring the spindle motor into a power generator by at least reversing the polarity of the driving voltage for an interval and then reversing the polarity of the driving voltage back to the operating polarity.

5. The method as recited in claim 4, wherein after reversing the polarity of the driving voltage back to the operating polarity the method further comprises incrementally step increasing an amplitude of the driving voltage by at least one step.

6. The method as recited in claim 4, wherein reversing the polarity of the driving voltage back to the operating polarity comprises configuring an amplitude of the driving voltage to substantially equal a back electromotive force (BEMF) voltage across each winding.

7. A controller for use in a data storage device comprising a disk, a spindle motor configured to rotate the disk, wherein the spindle motor comprises a plurality of windings, and a head actuated over the disk,
the controller comprising control circuitry powered by a supply voltage, the control circuitry configured to:
commutate the windings based on a commutation sequence while applying a driving voltage to each winding, wherein the driving voltage comprises an operating polarity during normal operation; and
when the supply voltage falls below a threshold while the spindle motor is rotating the disk, configure the spindle motor into a power generator by at least reversing the polarity of the driving voltage for an interval and then reversing the polarity of the driving voltage back to the operating polarity.

8. The controller as recited in claim 7, wherein after reversing the polarity of the driving voltage back to the operating polarity the control circuitry is further configured to incrementally step increase an amplitude of the driving voltage by at least one step.

9. The controller as recited in claim 7, wherein reversing the polarity of the driving voltage back to the operating polarity comprises configuring an amplitude of the driving voltage to substantially equal a back electromotive force (BEMF) voltage across each winding.