Data storage device balancing and maximizing quality metric when configuring arial density of each disk surface

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, 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.

BRIEF DESCRIPTION OF THE DRAWINGS

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

FIGS. 2A and 2B show a data storage device in the form of a disk drive comprising a plurality of disk surfaces and a head actuated over each disk surface.

FIG. 2C is a flow diagram according to an embodiment wherein a quality metric is measured at a plurality of discrete areal densities for each disk surface, and then based on a target capacity an areal density is selected for each disk surface such that the combined areal densities satisfies the target capacity and such that the quality metrics across the disk surfaces are substantially equal and substantially maximized.

FIG. 3 shows an embodiment wherein the quality metric measured for each areal density comprises an off-track read capability (OTRC).

FIG. 4 shows an example of a quality metric measured at a plurality of discrete areal densities for a disk surface.

FIG. 5 shows an example of an areal density selected for two disk surfaces such that the corresponding quality metrics are substantially equal and substantially maximized.

FIG. 6 is a flow diagram according to an embodiment wherein if after selecting the areal densities for the disk surfaces the balanced quality metric is less than a threshold, the target capacity is reduced and the process of selecting the areal density for each disk surface is repeated.

DETAILED DESCRIPTION

FIGS. 2A and 2B show a data storage device in the form of a disk drive according to an embodiment comprising a plurality of disk surfaces 16₀-16₃, and a head 18, actuated over each disk surface 16_i. The data storage device further comprises control circuitry 20 configured to execute the flow diagram of FIG. 2C, wherein for each disk surface, a quality metric is measured at a plurality of discrete areal densities including a first areal density comprising a first radial density and a first linear density, and a second areal density comprising a second radial density different from the first radial density and a second linear density different from the first linear density (block 22). Based on a target capacity, an areal density is selected for each disk surface such that the combined areal densities satisfies the target capacity and such that the quality metrics across the disk surfaces are substantially equal and substantially maximized (block 24).

In the embodiment of FIG. 2B, the disk drive comprises two disks with a head actuated over each of the top and bottom disk surfaces. However, other embodiments may comprise a single disk or more than two disks. In addition, other embodiments may not comprise a head actuated over each disk surface, for example, the top surface of the top disk and the bottom surface of the bottom disk may remain unused. In the embodiments described herein, there are at least two disk surfaces with at least one head actuated over each of the two disk surfaces.

In the embodiment of FIG. 2A, a plurality of concentric servo tracks 26 are defined by embedded servo sectors 28₀-28_N, wherein concentric data tracks are defined relative to the servo tracks at the same or different radial density. The control circuitry 20 processes a read signal 30 emanating from the head 18₀ 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 32 applied to a voice coil motor (VCM) 34 which rotates an actuator arm 36 about a pivot in order to actuate the head 18₀ radially over the disk surface 16₀ in a direction that reduces the PES. The servo sectors 28₀-28_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.

Any suitable quality metric may be measured for each disk surface at each areal density. FIG. 3 illustrates an example embodiment wherein the quality metric comprises an off-track read capability (OTRC) wherein for a given areal density the ability to read a data track is measured at each of a plurality of off-track offsets. A corresponding OTRC metric is generated for each off-track offset, such as a bit error rate. As the off-track offset increases, the bit error rate increases, wherein the OTRC is defined as the width of the off-track offset relative to a threshold level (Th) for the OTRC metric as illustrated in FIG. 3. A bit error rate metric results in a “bathtub” curve for the OTRC metric with a minimum at zero off-track offset as shown in FIG. 3; however, other OTRC metrics may exhibit an inverted bathtub curve having a maximum at zero off-track offset and an OTRC defined relative to when the OTRC metric falls below a threshold. Any suitable OTRC metric may be evaluated to measure the OTRC, such as the log-likelihood ratios of an iterative correction code or an iterative sequence detector, branch metrics of a Viterbi sequence detector, mean squared error between expected and actual read signal sample values, gain control and/or timing recovery errors, read signal amplitude, etc.

Examples of other quality metrics that may be measured for each disk surface at each areal density include an error rate (e.g., bit error rate) or a squeeze margin defined as an amount of tolerable track squeeze before reaching a suitable metric. In one embodiment, multiple quality metrics may be measured for each disk surface at each areal density and the resulting quality metrics processed in any suitable manner, such considering the maximum, minimum, or computing an average of the quality metrics.

FIG. 4 illustrates an embodiment wherein a quality metric is measured for a plurality of discrete areal densities (represented by block dots). Each measurement represents a specific radial and linear density, wherein in the embodiment of FIG. 4, the quality metric is measured for a plurality of different radial densities at one of a plurality of linear densities. The resulting measurements are then curve fitted to a polynomial using any suitable technique so that the areal density may be determined for any desired radial density at the corresponding linear density. This illustrated in FIG. 4 where the quality metric is measured for four different linear densities (by varying the radial density) and the resulting measurements curve fitted to four curves each represented by coefficients of a polynomial. FIG. 4 also illustrates that for a given areal density, there is a linear density (and corresponding radial density) that maximizes the quality metric. For example, at the areal density 38A the quality metric is maximum at linear density LD_2, and at the areal density 38B the quality metric is maximum at linear density LD_4. Accordingly, in one embodiment the areal density space (represented by the possible radial and linear densities) versus the quality metric is searched in order to select an areal density for each disk surface that achieves a target capacity as well as balances (makes substantially equal) and maximizes the quality metrics so that each disk surface provides a similar recording quality.

An example of this embodiment is illustrated in FIG. 5 which shows the areal density curves generated for a first and second disk surface. By searching these areal density spaces using any suitable technique (e.g., interpolation and extrapolation using curve fitted polynomials), it is possible to select an areal density (radial and linear density) for each disk surface that achieves a target capacity as well as balances (makes substantially equal) and maximizes the quality metrics so that each disk surface provides a similar recording quality. That is, there is a point in each areal density space where the combined areal densities will substantially equal a target capacity as well as result in substantially equal quality metrics that are maximized. By measuring discrete points in the areal density space, a near optimal areal density may be selected for each disk surface by interpolating/extrapolating the discrete points (e.g., using polynomials), or by rounding to the nearest discrete point.

In one embodiment, searching for the optimal points of areal density for each disk surface may involve setting a very high quality metric and determining whether the target capacity may be achieved by selecting corresponding points in the areal density space of each disk surface. If not, the quality metric may be reduced and the process repeated until the target capacity is achieved. The example of FIG. 5 which shows the balancing/maximizing of the quality metric for two disk surfaces may be extended to any suitable number of disk surfaces. In another embodiment, each disk surface may be divided into a number of zones, where each zone comprises a number of consecutive servo tracks. The above process may then be repeated for each zone of each disk surface in order to optimize the areal density for each zone (i.e., select an optimal radial and linear density for each zone).

FIG. 6 is a flow diagram according to an embodiment which extends on the flow diagram of FIG. 2C, wherein a target capacity for the data storage device is determined (block 40), for example, as part of a manufacturing procedure. After measuring the plurality of discrete areal densities for each disk surface (block 22) and selecting the areal densities to balance/maximize the quality metrics across the disk surfaces (block 24), the resulting quality metric is evaluated at block 42. If the maximum quality metric for the selected areal densities corresponds to an insufficient recording quality (e.g., if the maximum quality metric is less than a threshold in the example of FIG. 6), the target capacity for the disk drive may be reduced and the process repeated. That is, the data storage device may be “waterfalled” into a lower capacity product that may be marketed and sold as such.

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.