Disk drive generating feed-forward fly height control based on temperature sensitive fly height sensor

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to provisional U.S. Patent Application Ser. No. 61/807,603, filed on Apr. 2, 2013, which is hereby incorporated by reference in its entirety.

BACKGROUND

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 VCM servo controller to control the actuator arm as it seeks from track to track.

An air bearing forms between the head and the disk due to the disk rotating at high speeds. Since the quality of the write/read signal depends on the fly height of the head, conventional heads (e.g., a magnetoresistive heads) may comprise an actuator for controlling the fly height. Any suitable fly height actuator may be employed, such as a heater which controls fly height through thermal expansion, or a piezoelectric (PZT) actuator. A dynamic fly height (DFH) servo controller may measure the fly height of the head and adjust the fly height actuator to maintain a target fly height during write/read operations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a disk drive according to an embodiment comprising a head actuated over a disk.

FIG. 1B shows a head according to an embodiment comprising a fly height actuator (FHA) and a temperature sensitive fly height sensor (TSS).

FIG. 1C is a flow diagram according to an embodiment wherein a conversion function is configured based on an FHA actuation curve, wherein the conversion function is operable to convert a TSS fly height signal (FHS) in first units (e.g., volts) into a corresponding fly height (FH) of the head in second units (e.g., nm).

FIG. 1D shows a closed loop feedback system including a feed-forward controller for controlling the fly height of the head using the TSS according to an embodiment.

FIGS. 2A-2C show a comparison of the disk topography measurements between HMS 2T/6T, SVGA, and TSS fly height measurements at different radiuses (i.e., OD MD and ID).

FIG. 3 shows an example FHA actuation curve generated during a touchdown calibration procedure according to an embodiment.

FIG. 4 illustrates an embodiment wherein a gain of the feed-forward controller may be varied so as to vary the amplitude of a feed-forward (FF) control.

FIG. 5 illustrates an embodiment wherein the resolution of DAC settings representing the FF control may be increased by dithering between the settings.

FIG. 6 is a flow diagram according to an embodiment wherein the FF control is phase shifted by one or more servo sectors, and the FHA control signal is generated from the phase shifted FF control in order to compensate for a transient response (time constant) of the FHA.

FIG. 7 is a flow diagram according to an embodiment wherein the FF control waveforms may be calibrated across the radius of the disk at specific environmental conditions (e.g., at a specific ambient temperature).

DETAILED DESCRIPTION

FIG. 1A shows a disk drive according to an embodiment comprising a head 2 actuated over a disk 4, wherein the head 2 comprises a fly height actuator (FHA) 6 shown in FIG. 1B operable to actuate the head 2 vertically over the disk 4. The head 2 further comprises a temperature sensitive fly height sensor (TSS) 8 operable to generate a fly height signal (FHS) 10 representing a fly height of the head 2 over the disk 4. The disk drive further comprises control circuitry 12 operable to execute the flow diagram of FIG. 1C, wherein a FHA actuation curve for the FHA 6 is measured (block 14). A conversion function 16 (FIG. 1D) is configured based on the FHA actuation curve (block 18), wherein the conversion function 16 is operable to convert the FHS 10 in first units (e.g., volts) into a corresponding fly height (FH) 11 of the head 2 in second units (e.g., nm).

In the embodiment of FIGS. 1B and 1D, a feed-forward controller 20 generates feed-forward (FF) control based on the FHS 10 and the conversion function 16 (block 24), wherein the FF control is used to generate an FHA control signal 26 so that the head follows a topography variation of the disk 4. In other embodiments, the conversion function 16 of FIG. 1D may be used to convert the FHS 10 into a corresponding FH 11 for other reasons, such as to evaluate a waviness of the disk 4 as part of a manufacturing screening process.

In the embodiment of FIG. 1B, the FHA 6 may comprise any suitable actuator, such as a heater that actuates through thermal expansion, or a piezoelectric actuator that actuates through mechanical deflection. Also in the embodiment of FIG. 1B, the head 2 comprises a suitable write element 28, such as an inductive coil, and a suitable read element 30, such as a magnetoresistive element. In one embodiment, the control circuitry 12 calibrates a DC control signal applied to the FHA 6 in order to maintain the head 2 at an average fly height during write/read operations (where a different target fly height may be used for write and read operations). The topography of the disk 4 may vary for any number of reasons, including a warpage of the disk 4 caused by an uneven clamping force when clamping the disk 4 to a spindle motor that rotates the disk 4. In one embodiment, the DC control signal applied to the FHA 6 is modulated by an AC control signal so that the head follows the topography of the disk 4 during write/read operations.

In one embodiment, the AC control signal is generated as a feed-forward (FF) control signal used to generate the FHA control signal 26 (FIG. 1D). An error signal 32 is generated as a difference between a target fly height 34 and the measured FH 11 of the head 2. A switch 36 is closed in order to measure the FH variations due to the variation in the disk topography as reflected by the error signal 32. After measuring the FH variations, the feed-forward controller 20 generates FF control that will cause the head to follow the measured topography of the disk 4, thereby driving the error signal 32 toward zero. In one embodiment, the FF control is added to a DC control signal to generate the FHA control signal 26 applied to the FHA 6. During normal write/read operations, the switch 36 may be opened, or the switch 36 may remain closed in order to further adapt the FF control in response to the error signal 32.

The ability of the feed-forward controller 20 to cause the head 2 to follow the topography of the disk 4 depends, at least partly, on the accuracy of the FH 11 measurement. A known technique for measuring the FH 11 is a harmonic ratio technique (Wallace spacing equation) that measures an absolute head-media spacing (HMS) according to the ratio of the amplitude of the read signal at two different harmonics while reading a periodic pattern from the disk 4. This harmonic ratio can be generated by reading a periodic pattern at two different frequencies (e.g., a 2T and 6T pattern) and therefore may be referred to as an HMS 2T/6T technique. Another known technique for measuring the FH 11 is to evaluate the control signal applied to a variable gain amplifier (VGA) which attempts to maintain the amplitude of the read signal at a target amplitude when reading a periodic pattern from the disk, for example, a preamble in a servo sector. This technique may therefore be referred to as a servo VGA or SVGA technique. As describe below, these known techniques have drawbacks that may be overcome by using a temperature sensitive fly height sensor, such as a suitable magnetoresistive element having a thermal coefficient of resistance (TCR). In one embodiment, the temperature sensitive fly height sensor (TSS 8) may also be used as a touchdown sensor (TDS) for detecting when the head 2 contacts the disk 4 during a fly height calibration procedure.

FIGS. 2A-2C show a comparison of the disk topography measurements between HMS 2T/6T, SVGA, and TSS at different radiuses (i.e., OD MD and ID). At the inner diameter (ID) of the disk 4, all the measured topographies using the three different methods are correlated. At the outer diameter (OD) of the disk 2, the topography from SVGA clearly show three low points, which is due to magnetic property variations causing read back signal amplitude variations (e.g., during the sputtering process disk clamp holders affect magnetic layers especially at OD). Topographies measured based on HMS 2T/6T at OD show clamp sputter shadow effect to some extent. TSS 8 is a thermal transducer so that it is not influenced by this effect; hence it may measure good topography at any radius. In addition, the TSS 8 may be significantly wider than the read element 30, thereby spanning multiple tracks of the disk 2 as compared to the single track of the read element 30. The wider coverage of the TSS 8 may improve the topography measurement, for example, by increasing the signal-to-noise ratio (SNR) as compared to the single track magnetic read back methods.

The TSS 8 may transduce the fly height of the head into an FHS 10 which may be, for example, a voltage or a current that varies due to a thermally induced change in resistance of the TSS 8. In one embodiment, the FHS 10 generated by the TSS 8 is converted into a corresponding FH 11 by configuring a conversion function 16. The conversion function 16 may, for example, convert the FHS 10 in the form of a voltage signal into an absolute fly height measurement (e.g., nm). In one embodiment, the conversion function 16 is configured based on a FHA actuation curve that may be generated, for example, when performing a touchdown calibration for the head 2 in order to determine the DC level for the FHA control signal 26.

FIG. 3 illustrates an example FHA actuation curve generated during a touchdown calibration procedure by increasing the DC level of the FHA control signal 26, such as in step increments, and taking a corresponding FH measurement at each increment. In one embodiment, the FH measurement may be generated using the above described harmonic ratio technique which generates an absolute FH measurement representing the head-media spacing. The FH measurement may be generated while reading a periodic pattern in a servo sector, such as a preamble and/or burst pattern, wherein the measurement taken at each servo sector are averaged over a disk revolution to generate a DC FH measurement. The DC level of the FHA control signal 26 is increased and a new FH measurement taken until a touchdown is detected indicating the head 2 has contacted the disk 4. In one embodiment, the FHA actuation curve may be generated while servoing the head over an ID data track where the magnetic property variations for the read element 30 may be minimal, thereby increasing the accuracy of the FH measurements.

An operating DC level for the FHA control signal 26 may be generated by backing off from the touchdown level by an offset as illustrated in FIG. 3, wherein the backoff setting may correspond to a target (T) fly height. In one embodiment, during the touchdown calibration procedure the FHS 10 generated by the TSS 8 is saved for at least a subset of the DC level increments of the FHA control signal 26. For example, ten FHS 10 measurements may be taken proximate the target fly height, wherein the ten FHS 10 measurements and corresponding FH measurements may be fitted to a curve representing the conversion function 16. The resulting curve may be linear such that the conversion function 16 comprises a first order function, or the resulting curve may be more complex such that the conversion function 16 comprises a higher order function (e.g., quadratic). In addition, the FHA actuation curve shown in FIG. 3 is linear, whereas in other embodiments the FHA actuation curve may be more complex (e.g., quadratic).

After configuring the conversion function 16, the FHS 10 may be evaluated at each servo sector around a revolution of the disk. The FHS 10 is converted into a FH 11 by the conversion function 16 representing the topography of the disk. The FH 11 variation from the target (T) fly height represents the error signal 32 in terms of an absolute fly height measurement. This error signal 32 may be converted into units of the FHA control signal 26 using the FHA actuation curve such as shown in FIG. 3. The waveform of the error signal 32 representing the topography of the disk may be inverted in order to generate the FF control that causes the head 2 to follow the topography, thereby driving the error signal 32 toward zero.

In one embodiment, after generating the FF control by inverting the waveform of the measured error signal 32, there may be a residual amplitude in the error signal 32 due to an error in the FH 11 measurement. For example, there may be an error in the conversion function 16 causing an amplitude error in the FHS 10 to FH 11 conversion. FIG. 4 illustrates an embodiment wherein a gain of the feed-forward controller 20 may be varied so as to vary the amplitude of the FF control. That is, the waveform amplitude representing the inverted error signal 32 is varied and the resulting residual amplitude of the error signal 32 is measured. The gain of the feed-forward controller 20 that minimizes the amplitude of the residual error signal 32 is selected for normal operations.

In the example of FIG. 4, the feed-forward controller 20 generates the FF control in units of a digital-to-analog converter (DAC). The gain of the feed-forward controller 20 is varied by varying the peak-to-peak DAC setting representing the peak-to-peak amplitude of the inverted error signal 32 waveform. In the example of FIG. 4, the peak-to-peak DAC setting of plus/minus two values (FF DAC−2) minimizes the amplitude of the residual error signal 32 waveform. Referring again to FIG. 3, the DC level of the FHA control signal 26 corresponding to the target (T) fly height is modulated by plus/minus two DAC settings corresponding to the peak-to-peak amplitude of the inverted error signal 32 waveform of FIG. 4.

In one embodiment, the resolution of the DAC settings representing the FF control may be increased by dithering between the settings. An example of this embodiment is shown in FIG. 5 which corresponds to the peak-to-peak DAC setting of plus/minus two values (FF DAC−2) that minimizes the amplitude of the residual error signal 32 waveform of FIG. 4. FIG. 5 shows the original FH measurements at each servo sector together with the dithered DAC values. When the FH measurement corresponds to an intermediate DAC setting (e.g., between 0 and 1), the DAC is dithered between the two settings in order to generate an averaged FHA control signal 26 that corresponds to the intermediate FH measurement. In other embodiments, a higher resolution DAC may be employed to increase the resolution of the FHA control signal 26 without needing to dither the DAC.

FIG. 6 is a flow diagram according to an embodiment which extends on the flow diagram of FIG. 1C, wherein the FF control is phase shifted by one or more servo sectors (block 38), and the FHA control signal 26 generated from the phase shifted FF control (block 40) in order to compensate for a transient response (time constant) of the FHA 6. In one embodiment, the gain of the FF control is adjusted until the error signal 32 is minimized as described above, and then the FF control is phase shifted by one or more servo sectors. In one embodiment, the FF control may be phase shifted incrementally (e.g., by one servo sector) until the error signal 32 reaches a minimum.

In one embodiment, the conversion function 16 of FIG. 1D is configured at block 18 of FIG. 1C based on the FHA actuation curve (e.g., FIG. 3) generated while servoing the head 2 over an ID data track. The conversion function 16 may then used to generate the FF control at different radial locations across the disk 4 to account for topography variations that may occur across the disk radius. The different FF control waveforms may be stored in memory, and during normal operation the FF control may be read from the memory based on the radial location of the head 2. In another embodiment, the saved FF control waveforms may be interpolated in order to generate a FF control waveform corresponding to an intermediate radial location between two of the measured radial locations. In yet another embodiment, the FF control waveforms measured at the different radial locations may be curve fitted to a function, and during normal operation the FF control may be generated based on the function with the radial location of the head as the input to the function.

In one embodiment, the response of the TSS 8 may vary based on an environmental condition of the disk drive, such as the ambient temperature. Accordingly, in one embodiment the conversion function 16 of FIG. 1D may be calibrated over a range of different environmental conditions (e.g., different ambient temperatures) by at least partially regenerating the FHA actuation curve for a plurality of different environmental conditions. In another embodiment, the conversion function 16 may be recalibrated while the disk drive is deployed in the field when a change in an environmental condition is detected. In either embodiment, the FF control waveforms may be calibrated across the radius of the disk at specific environmental conditions (e.g., at a specific ambient temperature). This embodiment is illustrated in the flow diagram of FIG. 7, wherein when a change in an environmental condition is detected while the disk drive is deployed in the field (block 42), the FF control waveforms corresponding to the different radial locations may be adjusted based on the corresponding adjustment to the conversion function 16 (block 44). For example, if the adjustment to the conversion function 16 is a change in gain, there may be a corresponding change in the amplitude of the saved (or generated) FF control waveforms to compensate for the change in the environmental condition.

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.

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 inventions disclosed herein.