Data storage device detecting free fall condition from disk speed variations

This application claims the benefit of U.S. Provisional Application No. 61/939,643, filed on Feb. 13, 2014, which is hereby incorporated by reference in its entirety.

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 6 0 - 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 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.

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.

FIG. 2B shows control circuitry configured to generate a timing signal by reading data from the disk according to an embodiment.

FIG. 2C is a flow diagram according to an embodiment wherein the timing signal is evaluated to detect a free fall condition.

FIG. 3A shows a disk drive according to an embodiment comprising a head actuated over a disk.

FIG. 3B shows control circuitry configured to generate a speed signal representing a rotation speed of the disk.

FIG. 3C is a flow diagram according to an embodiment wherein the speed signal is evaluated to detect a free fall condition.

FIG. 4 illustrates an embodiment wherein an absolute value, derivative, and integral of the timing signal and/or speed signal are evaluated to detect a free fall condition.

FIGS. 5A and 5B are a flow diagram according to an embodiment wherein a potential free fall condition is detected and then confirmed based on the absolute value, derivative, and integral of the timing signal and/or speed signal.

FIG. 2A shows a data storage device in the form of a disk drive according to an embodiment comprising a disk 16 comprising data, and a head 18 actuated over the disk 16 . The disk drive further comprises control circuitry 20 (including timing control circuitry shown in FIG. 2B ) configured to execute the flow diagram of FIG. 2C , wherein a timing signal is generated by reading the data from the disk (block 22 ), and a frequency generator is adjusted based on the timing signal to generate a disk locked clock substantially synchronized to a rotation speed of the disk (block 24 ). A free fall condition is detected based on the timing signal (block 26 ), and when the free fall condition is detected (block 28 ) the head is parked (block 30 ).

In the embodiment of FIG. 2A , a plurality of concentric servo tracks 32 are defined by embedded servo sectors 34 0 - 34 N , wherein concentric data tracks are defined relative to the servo tracks 32 at the same or different radial density. The control circuitry 20 processes a read signal 36 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 20 filters the PES using a suitable compensation filter to generate a control signal 38 applied to a voice coil motor (VCM) 40 which rotates an actuator arm 42 about a pivot in order to actuate the head 18 radially over the disk 16 in a direction that reduces the PES. The servo sectors 34 0 - 34 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.

The disk drive of FIG. 2A may be utilized in any suitable electronic device, such as a laptop computer, tablet computer, or a consumer electronic device such as a smartphone, music player, etc. The disk drive may also be used as a portable backup drive such as a Universal Serial Bus (USB) portable disk drive. The portable nature of electronic devices means the disk drive may be subjected to a significant shock due to a user dropping the electronic device. Accordingly, in one embodiment the control circuitry 20 is configured to detect a free fall condition so that the head 18 may be parked before the impact shock causes the head 18 to slap the disk surface which may corrupt previously recorded data and/or damage the head 18 . The head 18 may be parked in any suitable manner, such as by parking the head 18 on a ramp mounted at the outer periphery of the disk 16 or parking the head 18 on a landing zone on the disk 16 (e.g., near an inner diameter of the disk 16 ). In one embodiment, a timing signal generated by timing control circuitry is evaluated to detect a free fall condition, and in another embodiment a speed signal representing a rotation speed of the disk is evaluated to detect a free fall condition.

FIG. 2B shows timing control circuitry according to an embodiment configured to generate a disk locked clock 44 substantially synchronized to a rotation speed of the disk. In the embodiment of FIG. 2B , a timing plant 48 comprises a frequency generator 48 A for generating the disk locked clock 44 at a frequency based on a timing control signal 50 . The timing plant 48 further comprises a block 48 B for measuring a phase 52 of the disk locked clock 44 relative to a rotation speed of the disk 16 . In one embodiment, the phase of the disk locked clock 44 may be measured relative to the frequency and/or phase of data read from the disk 16 , such as data recorded in the servo sectors 34 0 - 34 N . For example, a frequency of sync marks in the servo sectors and/or a phase of a periodic pattern in the servo sectors (e.g., a preamble or a servo burst) may be processed to measure the phase of the disk locked clock. The measured phase 52 is subtracted from a target phase 54 to generate a timing error signal 56 . A suitable compensator 58 filters the timing error signal 56 to generate the timing control signal 50 applied to the frequency generator 48 A, thereby adjusting the disk locked clock 44 so as to be synchronized with the rotation speed of the disk 16 . In one embodiment a timing signal generated by the timing control circuitry is evaluated to detect a free fall condition, such as the timing error signal 56 and/or the timing control signal 50 .

FIG. 3A shows a disk drive according to an embodiment comprising a head 18 actuated over a disk 16 and control circuitry 60 (including speed control circuitry shown in FIG. 3B ) configured to execute the flow diagram of FIG. 3C wherein a speed signal is generated representing a rotation speed of the disk (block 62 ). An absolute value of the speed signal, a derivative of the speed signal, and an integral of the speed signal is computed (block 64 ), and a free fall condition is detected based on the absolute value of the speed signal, the derivative of the speed signal, and the integral of the speed signal (block 66 ). When the free fall condition is detected (block 68 ), the head is parked (block 70 ).

FIG. 3B shows speed control circuitry according to an embodiment configured to rotate the disk 16 at a target rotation speed. In the embodiment of FIG. 3B , a speed plant 72 comprises a spindle motor 72 A for rotating the disk 16 at a rotation speed based on a speed control signal 74 . The speed plant 72 further comprises a block 72 B for measuring a rotation speed of the disk 16 , such as by evaluating the back electromotive force (BEMF) of the spindle motor 72 A and/or evaluating a frequency of a disk locked clock synchronized to the frequency/phase of the servo sectors 34 0 - 34 N . The measured rotation speed 76 is subtracted from a target rotation speed 78 to generate a speed error signal 80 . A suitable compensator 82 filters the speed error signal 80 to generate the speed control signal 74 applied to the spindle motor 72 A, thereby adjusting the rotation speed of the disk 16 to match the target rotation speed 78 . In one embodiment a speed signal generated by the speed control circuitry is evaluated to detect a free fall condition, such as the speed error signal 80 and/or the speed control signal 74 .

FIG. 4 illustrates an embodiment wherein the timing signal of FIG. 2B and/or the speed signal of FIG. 3B are represented as a nominal value (e.g., zero) when the feedback loops converge to a steady state with the disk drive in a normal operating mode. When the disk drive is subject to a free fall condition, the timing/speed signal diverges from the nominal value shown in the example of FIG. 4 as transitioning to a negative value. In the example of FIG. 4 , the timing/speed signal is illustrated as a noiseless signal but in practice the signals may be obfuscated by noise. In one embodiment, the noise in the timing/speed signal may be attenuated using suitable filtering, such as averaging or other low pass filtering, in order to improve the accuracy of the free fall detection.

As illustrated in FIG. 4 , in one embodiment the control circuitry evaluates an absolute value, a derivative, and an integral of the timing/speed signal in order to detect a free fall condition. In one embodiment, a potential free fall condition is detected when the absolute value exceeds a threshold (ABS_Th shown in FIG. 4 ), wherein the control circuitry begins evaluating the derivative and the integral of the timing/speed signal in order to confirm the free fall condition before parking the head. In one embodiment, the free fall condition is confirmed based on the derivative and the integral so that the head is not parked unnecessarily which degrades the performance of the disk drive by interrupting the host commands.

An example of this embodiment is illustrated in the flow diagram of FIGS. 5A and 5B wherein when the absolute value (ABS) of the timing/speed signal exceeds a threshold Th1 (block 84 ) a potential free fall condition is detected (block 86 ). The control circuitry then begins computing an integral of the timing/speed signal (block 88 ) as illustrated in FIG. 4 . While computing the integral during a first interval, the control circuitry periodically computes a derivative of the timing/speed signal (block 90 ), such as computing the derivative each time the timing/speed signal is sampled. If during the first interval the absolute value falls below the threshold Th1 or the absolute value exceeds a threshold Th5 greater than Th1 and the derivative exceeds a threshold Th6, the control circuitry determines the potential free fall condition is a false free fall condition and the flow diagram resets to block 84 .

If after the first interval (block 94 ) the integral of the timing/speed signal is less than a threshold 4*Th2 and greater than the threshold Th2, the control circuitry executes the flow diagram of FIG. 5B to further confirm the potential free fall condition. The control circuitry continues to compute the integral of the timing/speed signal (block 100 ) and continues to compute the derivative of the timing/speed signal (block 102 ). At the same time, if the absolute value falls below the threshold Th1 or the absolute value exceeds threshold Th5 and the derivative exceeds threshold Th6, the control circuitry determines the potential free fall condition is a false free fall condition and the flow diagram resets to block 84 ( FIG. 5A ). If during this second interval the integral exceeds a threshold 8*Th4 (which is greater than threshold Th2), the control circuitry determines the potential free fall condition is a false free fall condition and the flow diagram resets to block 88 ( FIG. 5A ) where the integral is reset to zero so that a potential free fall condition is still evaluated.

If the absolute value exceeds threshold Th3 and the integral exceeds threshold Th4 at block 106 , the control circuitry confirms the free fall condition and parks the head (block 110 ). The control circuitry keeps the head parked for a delay to ensure the head is parked when the impact shock occurs (block 112 ). After the delay (and in one embodiment after detecting the impact shock), the control circuitry loads the head onto the disk and waits at block 114 for the speed/timing signal to converge to the steady state nominal value (e.g., zero in FIG. 4 ). The flow diagram is then reset to block 84 ( FIG. 5A ) thereby enabling the detection and reaction to future free fall conditions.

Any suitable values may be selected for the thresholds described in the flow diagram of FIGS. 5A and 5B . In one embodiment, the thresholds may be calibrated and then adapted over time based on the accuracy of the free fall detection algorithm. For example, the thresholds may be adjusted if a free fall condition is confirmed based on the timing/speed signal (thereby parking the head), but then determined to be a false free fall condition if an impact shock does not follow. Conversely, the thresholds may be adjusted if a free fall condition is not detected (or a potential free fall condition is determined to be a false free fall condition) yet an impact shock is detected. Accordingly, the thresholds may be adapted over time so as to optimize performance while minimizing the chance of damage to the disk and/or the head due to an impact shock.

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.

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.