Data storage device calibrating fly height actuator based on read mode touchdown resistance of touchdown sensor

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 embedded servo sectors. The embedded servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo controller to control the velocity of the actuator arm as it seeks from track to track.

Data is typically written to the disk by modulating a write current in an inductive coil to record magnetic transitions onto the disk surface in a process referred to as saturation recording. During readback, the magnetic transitions are sensed by a read element (e.g., a magnetoresistive element) and the resulting read signal demodulated by a suitable read channel. Heat assisted magnetic recording (HAMR) is a recent development that improves the quality of written data by heating the disk surface with a laser during write operations in order to decrease the coercivity of the magnetic medium, thereby enabling the magnetic field generated by the write coil to more readily magnetize the disk surface.

Since the quality of the write/read signal depends on the fly height of the head, conventional heads may comprise an actuator for controlling the fly height. Any suitable dynamic fly height (DFH) actuator may be employed, such as a heater which controls fly height through thermal expansion, or a piezoelectric (PZT) actuator. It is desirable to determine the appropriate DFH setting (e.g., appropriate current applied to a heater) that achieves the target fly height for the head.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A, shows a data storage device in the form of a disk drive according to an embodiment comprising a head actuated vertically over a disk by a fly height actuator (FHA).

FIG. 1B shows an embodiment wherein the head comprises a write element (WE), a read element (RE), the FHA, a touchdown sensor (TDS), and a laser (LS).

FIG. 1C is a flow diagram according to an embodiment wherein a write touchdown FHA setting that will cause the head to contact the disk while in a write mode is estimated based on a touchdown read mode resistance and a non-touchdown write mode resistance of the touchdown sensor.

FIG. 1D shows an embodiment wherein a plurality of touchdown read mode resistances of the touchdown sensor are measured while adjusting the FHA setting until the head contacts the disk, and then the write touchdown FHA setting is estimated based on the touchdown read mode resistance of the touchdown sensor.

FIG. 2 shows an embodiment wherein the write touchdown FHA setting is estimated by extrapolating a plurality of non-touchdown write mode resistances toward the touchdown read mode resistance of the touchdown sensor.

FIG. 3 shows an embodiment wherein the write touchdown FHA setting is estimated based on a delta from a read touchdown FHA setting, wherein the delta is based on the touchdown read mode resistance and the non-touchdown write mode resistance of the touchdown sensor.

FIG. 4 shows an embodiment wherein the write touchdown FHA setting is estimated based on a plurality of non-touchdown read mode resistances of the touchdown sensor and a plurality of non-touchdown write mode resistances of the touchdown sensor.

FIG. 5A shows an embodiment wherein the write touchdown FHA setting is estimated based on a shift between the non-touchdown read mode resistances of the touchdown sensor and the non-touchdown write mode resistances of the touchdown sensor.

FIG. 5B shows an embodiment wherein the write touchdown FHA setting is estimated based on a shift between a non-touchdown read mode resistance of the touchdown sensor and a non-touchdown write mode resistance of the touchdown sensor.

FIG. 6 shows an embodiment wherein the write touchdown FHA setting is estimated based on a slope of a line passing through a touchdown write mode resistance and a touchdown read mode resistance of the touchdown sensor.

DETAILED DESCRIPTION

FIG. 1A shows a data storage device in the form of a disk drive according to an embodiment comprising a disk 2, a head 4 (FIG. 1B) comprising a touchdown sensor 6, and a fly height actuator (FHA) 8 configured to actuate the head 4 vertically over the disk 2 based on an FHA setting 10. The data storage device further comprises control circuitry 12 configured to execute the flow diagram of FIG. 1C. The data storage device is configured into a read mode (block 14), and while in the read mode, the FHA setting is adjusted until the head contacts the disk and a touchdown read mode resistance of the touchdown sensor is measured (block 16). The data storage device is configured into a write mode and the FHA setting is adjusted so the head is not contacting the disk (block 18). While in the write mode, a non-touchdown write mode resistance of the touchdown sensor is measured (block 20), and a write touchdown FHA setting that will cause the head to contact the disk while in the write mode is estimated based on the touchdown read mode resistance and the non-touchdown write mode resistance (block 22).

Any suitable head 4 may be employed in the embodiments, wherein in the embodiment of FIG. 1B the head 4 comprises a suitable write element 30 (e.g., an inductive coil), a suitable read element 32 (e.g., a magnetoresistive element), and a suitable fly height actuator (FHA) 8 configured to actuate the head 4 vertically over the disk 2. Any suitable FHA 8 may be employed, such as a heater that actuates through thermal expansion, or a piezoelectric actuator that actuates through mechanical deflection. The head 4 in the embodiment of FIG. 1B also comprises a suitable laser 34 (e.g., a laser diode) configured to heat the surface of the disk during write operations for HAMR disk drives.

In one embodiment the FHA setting 10 is configured so that the head 4 maintains a target fly height during write and read operations. This helps ensure the data is recorded on the disk reliably (e.g., by generating the appropriate magnetic write field), as well as helps ensure the data is recovered reliably (e.g., by providing an adequate signal-to-noise ratio (SNR) in the read signal). In one embodiment, the FHA setting corresponding to the target fly height during read operations is determined by finding the FHA setting that causes the head to touchdown onto the disk, and then backing off the touchdown FHA setting by a delta. However, in one embodiment employing the touchdown technique to find a write mode touchdown FHA setting may be undesirable since inducing a touchdown may degrade or damage one or more of the write components. For example, applying a write current to a write coil may cause the write coil to protrude toward the disk due to thermal expansion. Similarly, applying a write current to a laser for HAMR disk drives may cause a near field transducer (NFT) to protrude toward the disk due to thermal expansion. This thermal protrusion may result in a single point contact by the write element during touchdown which may degrade or damage the write component, and therefore in one embodiment the write mode touchdown FHA setting is estimated without actually performing a write mode touchdown procedure (or by performing a small number of write mode touchdown procedures).

Any suitable touchdown sensor 6 may be employed in the embodiments, such as a suitable temperature sensitive sensor (e.g., a suitable magnetoresistive element having a thermal coefficient of resistance (TCR)). As the fly height of the head 4 changes, there is a corresponding change in resistance of the touchdown sensor 6 due to a corresponding change in temperature. This change in resistance may be monitored in order to measure the fly height of the head as well as detect when the head has contacted the disk during a touchdown event.

FIG. 1D illustrates an example embodiment wherein the resistance of the touchdown sensor is measured while adjusting the FHA setting until the head contacts the disk and a touchdown read mode resistance 24 of the touchdown sensor is measured. While in the write mode, a non-touchdown write mode resistance 26 of the touchdown sensor is measured, and a write touchdown FHA setting 28 that will cause the head to contact the disk while in the write mode is estimated based on the touchdown read mode resistance 24 and the non-touchdown write mode resistance 26.

Any suitable technique may be employed to estimate the write touchdown FHA setting 28 that will cause the head to contact the disk based on the touchdown read mode resistance 24 and the non-touchdown write mode resistance 26. FIG. 2 illustrates an embodiment wherein a plurality of non-touchdown read mode resistances (represented by “x”s) are measured while adjusting the FHA setting until the head contacts the disk. Any suitable technique may be employed to detect when the head contacts the disk, such as by processing a touchdown signal generated by the touchdown sensor using any suitable statistical analysis. After determining the touchdown read mode resistance 24, a plurality of non-touchdown write mode resistances (represented as “o”s) are measured while increasing the FHA setting up until the non-touchdown write mode resistance 26B nears the touchdown read mode resistance 24. The write touchdown FHA setting 28 is then estimated by extrapolating the non-touchdown write mode resistances toward the touchdown read mode resistance as illustrated in FIG. 2. The write touchdown FHA setting 28 is less than the read touchdown FHA setting 36 in FIG. 2 due to the protrusion of a write component (e.g., NFT) during the write mode. This embodiment estimates the touchdown FHA setting 28 through extrapolation so as to avoid head/disk contact that would otherwise occur if the write mode FHA setting were increased until touchdown.

FIG. 3 illustrates an alternative embodiment for estimating the write touchdown FHA setting 28 based on the touchdown read mode resistance 24. While in the read mode, a non-touchdown read mode resistance 38 of the touchdown sensor is measured for a plurality of the FHA settings. A first linear fit of the plurality of non-touchdown read mode resistances 38 is generated and a delta (d) is generated representing a difference between the read mode FHA setting 36 that causes the head to contact the disk and a FHA setting 40 corresponding to a first intercept 42 of the first linear fit with the touchdown read mode resistance 24 of the touchdown sensor. While in the write mode, the FHA setting is adjusted and a non-touchdown write mode resistance 44 of the touchdown sensor is measured for a plurality of the FHA settings. A second linear fit of the plurality of non-touchdown write mode resistances 44 is generated, and the write touchdown FHA setting is estimated based on the delta (d) and a second intercept 46 of the second linear fit with the touchdown read mode resistance 24 of the touchdown sensor. In other words, the non-touchdown write mode resistances 44 are extrapolated through a linear curve fit to find the intercept 46 of the line with the touchdown read mode resistance 24 and the corresponding FHA setting 48. The write touchdown FHA setting 28 is then estimated by adding the delta (d) to the FHA setting 48, wherein the delta (d) corresponds to the difference between the read mode FHA setting 40 and the read mode FHA setting 36.

FIG. 4 illustrates another embodiment for estimating the write touchdown FHA setting 28 based on the touchdown read mode resistance 24. In this embodiment, a FHA setting delta (s) is measured between the plurality of non-touchdown read mode resistances 38 of the touchdown sensor and the plurality of non-touchdown write mode resistances 44 of the touchdown sensor. In this embodiment, the FHA setting delta (s) represents the shift of the curve representing the read mode resistance of the touchdown sensor and the curve representing the write mode resistance. The FHA setting delta (s) or shift between the two curves may be measured in any suitable manner. In one embodiment, the FHA setting delta (s) may be estimated by minimizing the equation:
|w(k)−f(x*+s)|²
where w(k) represents the measured non-touchdown write mode resistances 44, f(x) represents a curve fitted function of the measured non-touchdown read mode resistances 38, and x* represents the FHA settings used to measure w(k). The above equation reaches a minimum when the partial derivative with respect to the FHA setting delta (s) reaches zero. For example, if f(x) is represented using a third-order polynomial, then the partial derivative of f(x) is a second-order polynomial and the partial derivative of the above equation is a fifth-order polynomial. Once the FHA setting delta (s) is estimated, in one embodiment the write touchdown FHA setting 28 is estimated by subtracting the FHA setting delta (s) from the read touchdown FHA setting 36.

FIG. 5A illustrates a simplified embodiment for estimating the FHA setting delta (s) described above with reference to FIG. 4. In the embodiment of FIG. 5A, a non-touchdown write mode resistance (A) and a non-touchdown read mode resistance (B) is measured at various FHA settings. A slope of the line representing either or both of the non-touchdown resistances is also estimated. For each set of non-touchdown resistances (e.g., A1 and B1), a corresponding shift value C between the two lines is computed as shown in FIG. 5A. The FHA setting delta (s) may then be estimated as the mean of the computed shift values (e.g., mean of C1 to C4). In an alternative embodiment shown in FIG. 5B, a single shift value C1 may be computed based on a single FHA setting and corresponding non-touchdown resistances A1 and B1. This latter embodiment reduces the processing time to estimate the write touchdown FHA setting 28 since it requires fewer non-touchdown resistance measurements.

In one embodiment, the write touchdown FHA setting 28 may be estimated for a plurality of different write mode settings (e.g., different write current applied to a write coil and/or different write current applied to a laser). For example, a different write mode setting may be employed under different environmental conditions (temperature, altitude, etc.) and/or a different write mode setting may be employed at different radiation locations across the disk. Accordingly, one or more of the techniques described above may be used to estimate the write touchdown FHA setting 28 across various possible write mode settings. In one embodiment, only a subset of the write mode settings are evaluated to estimate a corresponding plurality of write touchdown FHA settings 28 which may then be interpolated and/or extrapolated so as to fill in gaps between the unmeasured write mode settings. During normal operation, when the control circuitry 12 adjusts the write mode setting, the control circuitry 12 makes a corresponding adjustment to the write mode FHA setting so as to maintain a target fly height for the head during write operations.

In the above embodiments, it may be assumed the touchdown resistance of the touchdown sensor will not change between the read mode and the write mode. That is, it may be assumed the dashed line representing the touchdown resistance in FIG. 1D may be horizontal (slope of zero) such that the touchdown write mode resistance will be substantially the same as the touchdown read mode resistance 24. In another embodiment illustrated in FIG. 6, the touchdown resistance of the touchdown sensor may vary as the write mode setting is adjusted (e.g., as the write current of the write coil and/or the write current of the laser is adjusted) resulting in a non-zero linear slope of the touchdown resistance. In one embodiment, the slope of this line shown in FIG. 6 may be estimated by measuring a touchdown write mode resistance 50 relative to the touchdown read mode resistance 24. The touchdown write mode resistance 50 may be measured by configuring a target write mode setting and then increasing the FHA setting until touchdown is detected. Since the write mode touchdown is performed a single time (or a few times), the degradation to the write components is minimized. Once the slope of the touchdown resistance has been estimated, it may be used to modify the above embodiments to more accurately estimate the write touchdown FHA setting 28 across multiple write mode settings while avoiding an actual touchdown measurement.

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.