# Patent

## Title
Disk drive detecting when head is on ramp

## Abstract

A disk drive is disclosed comprising a head actuated over a disk surface, the head comprising a temperature sensitive (TS) fly height sensor. The disk drive further comprises a ramp for parking the head, and control circuitry configured to process a fly height signal generated by the TS fly height sensor, wherein the fly height signal represents a fly height of the head over the disk, and detect whether the head is on the ramp in response to the fly height signal.

## Background
CROSS REFERENCE TO RELATED APPLICATION(S)
This application is a continuation-in-part of application Ser. No. 13/237,727 filed on Sep. 20, 2011, which is hereby incorporated by reference in its entirety. This application claims priority to provisional U.S. Patent Application Ser. No. 61/880,058, filed on Sep. 19, 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 servo controller to control the velocity of the actuator arm as it seeks from track to track.
FIG. 1 shows a prior art disk format 1 as comprising a number of servo tracks 3 defined by servo sectors 5O-5N recorded around the circumference of each servo track. Each servo sector 5i comprises a preamble 7 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 9 for storing a special pattern used to symbol synchronize to a servo data field 11. The servo data field 11 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 5i further comprises groups of servo bursts 13, which are recorded with precise intervals and offsets relative to the track centerlines. The servo bursts 13 provide fine head position information used for centerline tracking while accessing a data track during write/read operations.
When the disk drive is idle (not servicing host commands), the head is typically parked on a ramp located near an outer edge of the disk. The control firmware typically maintains the state of the head (parked or un-parked) so that the disk drive knows whether the head should be unloaded from the ramp before executing a new host command. However if the control firmware malfunctions when determining the state of the head, the disk drive may fail when attempting to execute a new host command.
It is also typically desirable to detect when the head first contacts the ramp during an unload operation to determine the maximum stroke of the actuator arm. For example, the location of the ramp may be determined prior to self-servo writing a disk so that the servo tracks may be written from the inner diameter of the disk to the edge of the ramp (or very near thereto), thereby maximizing the usable disk space.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a prior art disk format comprising a plurality of tracks defined by embedded servo sectors.
FIG. 2A shows a disk drive according to an embodiment of the present invention comprising a head actuated over a disk surface and a ramp for parking the head.
FIG. 2B shows a head according to an embodiment of the present invention comprising a fly height actuator for controlling a fly height of the head.
FIG. 2C is a flow diagram according to an embodiment of the present invention wherein after decreasing the fly height of the head whether the head is on the ramp is detected by evaluating a fly height signal.
FIG. 3A is a flow diagram according to an embodiment of the present invention wherein the head is detected as on the ramp when the fly height signal exceeds a threshold.
FIG. 3B shows an embodiment of the present invention wherein when seeking the head toward the ramp the fly height is deceased and the head contacting the ramp is detected when the fly height signal exceeds a threshold.
FIG. 4 is a flow diagram according to an embodiment of the present invention wherein a ramp detection fly height threshold is calibrated by performing a touchdown calibration.
FIG. 5 shows control circuitry according to an embodiment of the present invention for detecting a head touchdown event.
FIG. 6A shows an embodiment of the present invention wherein the disk drive comprises a plurality of heads actuated over respective disk surfaces.
FIG. 6B is a flow diagram according to an embodiment of the present invention wherein a touchdown calibration is executed for each head, and then the head having the maximum touchdown control signal is selected to execute the ramp procedure.
FIG. 7 is a flow diagram according to an embodiment where the head comprises a temperature sensitive (TS) fly height sensor for generating a fly height signal processed to detect whether the head is on the ramp.
FIG. 8 shows control circuitry according to an embodiment wherein a bias current is applied to the TS fly height sensor such that the fly height signal comprises the resulting voltage across the TS fly height sensor.
FIG. 9 is a flow diagram according to an embodiment wherein the fly height actuator is controlled to decrease the fly height of the head and heat the TS fly height sensor in order to detect when the head contacts the ramp when seeking toward the ramp.



DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
FIG. 2A shows a disk drive according to an embodiment of the present invention comprising a head 14 actuated over a disk surface 16, the head 14 comprising a fly height actuator 18 (FIG. 2B) for controlling a fly height of the head 14. The disk drive further comprises a ramp 20 for parking the head 14, and control circuitry 22 operable to execute a ramp procedure shown in FIG. 2C, wherein a control signal is applied to the fly height actuator to decrease the fly height and a corresponding fly height signal is received (block 24). Whether the head is on the ramp is detected in response to the fly height signal (block 26).
In the embodiment of FIG. 2A, the disk surface 16 comprises embedded servo sectors 280-28N that define a plurality of servo tracks 30. The control circuitry 22 processes a read signal 32 emanating from the head 14 to demodulate the servo sectors 280-28N 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 22 filters the PES using a suitable compensation filter to generate a control signal 34 applied to a voice coil motor (VCM) 36 which rotates an actuator arm 38 about a pivot in order to actuate the head 14 radially over the disk in a direction that reduces the PES. The servo sectors 280-28N may comprise any suitable 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 the amplitude-based servo pattern shown in FIG. 1, or a suitable phase-based servo pattern. In an embodiment described below, during the ramp procedure the control circuitry 22 seeks the head 14 toward the ramp 20 by processing the servo sectors 280-28N, and decreases the fly height of the head as the head approaches the ramp 20. The location of the ramp 20 is then detected when the head 14 contacts the ramp 20 and the fly height signal exceeds a threshold.
In another embodiment, the control circuitry 22 executes the ramp procedure of FIG. 2C to determine whether the head is parked on the ramp 20 or flying over the disk surface 16. For example, due to a programming error the control circuitry 22 may lose track of the location of the head 14. Therefore, the control circuitry 22 decreases the fly height of the head 14 and evaluates the fly height signal to determine whether the head 14 is parked on the ramp 20. If the head 14 is parked on the ramp 20, the fly height signal will not change as the fly height decreases because the head 14 is not over the disk surface 16. If the head 14 is over the disk surface 16, the fly height signal will change as the fly height decreases.
In an alternative embodiment, the control circuitry 22 executes the ramp procedure of FIG. 2C to determine the location of the ramp 20 relative to the disk surface 16. For example, in one embodiment the disk surface 16 may be servo written with the embedded servo sectors 280-28N prior to installing the disk into the disk drive. After installing the disk into the disk drive, the ramp 20 is installed with the edge extending out over the disk surface 16 by an unknown amount. The control circuitry 22 executes the ramp procedure to detect the location of the ramp 20 relative to the servo tracks 30 so that the data tracks can be defined up to the edge of the ramp in order to maximize the recording area. In an alternative embodiment, the embedded servo sectors 280-28N may be self written by the control circuitry 22 after installing the disk into the disk drive. Prior to servo writing the servo sectors 280-28N, the control circuitry 22 executes the ramp procedure to detect the location of the ramp 20 relative to the disk surface 16. In this manner, the servo sectors 280-28N can be written to define the servo tracks 30 up to the edge of the ramp in order to maximize the recording area.
FIG. 3A is a flow diagram according to an embodiment of the present invention wherein the ramp procedure is executed to detect the location of the ramp 20 relative to the disk surface 16. The fly height of the head is increased (block 40) prior to seeking the head toward the outer diameter of the disk to avoid head disk contact during the seek operation. The seek operation is started (block 42), and when the head approaches the ramp (block 44), the fly height of the head is decreased (block 46) and the fly height signal is evaluated (block 48). When the fly height signal exceeds a threshold (block 50), the head contacting the ramp is detected (block 52). That is, as the head contacts the ramp and begins lifting away from the disk surface, the fly height signal will increase and eventually exceed a threshold as illustrated in FIG. 3B. In one embodiment, the fly height threshold is selected to account for noise in the fly height signal that may cause a false detection of the ramp.
In one embodiment, when decreasing the fly height to detect the edge of the ramp the fly height may be decreased to a level lower than a fly height used to execute one of a normal write and read operation. During normal write and read operations, the fly height may be higher to protect the head from contacting asperities on the disk, whereas during the ramp procedure the fly height may be lower in order to detect the edge of the ramp as soon as possible as the head seeks toward the ramp. Damage due to contacting asperities during the ramp procedure is less of a concern since the ramp procedure is executed infrequently (e.g., one time to detect the edge of the ramp).
In one embodiment, the embedded servo sectors may be processed in order to seek the head toward the ramp during the ramp procedure as described above. In an alternative embodiment, the disk surface 16 may be written with seed servo tracks (e.g., spiral tracks) that are processed in order to self-servo write the embedded servo sectors 280-28N. In this embodiment, the seed servo tracks may be processed in order to seek the head toward the ramp during the ramp procedure. In yet another embodiment, seeking the head toward the ramp may be carried out without reading any position information from the disk. In this embodiment, the head may be moved toward the ramp using an open loop control system, or using a closed loop control system wherein the back electromotive force (BEMF) voltage generated by the VCM may be processed to estimated the velocity of the head. For example, the head may be positioned at the inner diameter of the disk (using the inner diameter crash stop) and then moved toward the ramp while adjusting the velocity to substantially match a velocity profile. In one embodiment, when the head contacts the edge of the ramp the rotational angle of the disk is saved based on a clock locked to the rotation frequency of the disk. That is, the rotation angle of the disk when the head contacts the ramp represents the location of the ramp relative to the disk surface.
Any suitable technique may be employed to generate the fly height signal in various embodiments of the present invention. In one embodiment, the head 14 may comprise a suitable fly height sensor 54 (FIG. 2B), such as a suitable capacitive sensor or semiconductor sensor such as a tunneling sensor. In another embodiment, the head 14 may comprise a read element 56, such as a magnetoresistive (MR) read element, for reading data from the disk and that may also be used as a fly height sensor for generating the fly height signal. In the embodiment of FIG. 3B, the fly height signal is directly proportional to the fly height of the head, whereas in other embodiments the fly height signal may be inversely proportional to the fly height of the head (i.e., the fly height signal may increase as the fly height decreases). In this embodiment, the fly height signal may be inverted prior to comparing it to a threshold (block 50 of FIG. 3A), or the comparison may be inverted (i.e., detect the ramp when the fly height signal falls below a threshold).
FIG. 4 shows a flow diagram according to an embodiment of the present invention for calibrating the fly height threshold used to detect whether the head is on the ramp. The head is positioned near the outer diameter of the disk (block 58) and then a touchdown procedure is executed wherein the fly height of the head is decreased (block 60) until the head touching down on the disk is detected (block 62). The fly height of the head is then increased from the touchdown level by a predetermined margin (block 64). The fly height signal is then evaluated in order to calibrate the fly height threshold used to detect whether the head is on the ramp. For example, a statistical analysis of the fly height signal (e.g., mean, variance, etc.) may be computed to establish the fly height threshold.
FIG. 5 shows control circuitry for detecting the head touchdown according to an embodiment of the present invention. A touchdown sensor generates a touchdown signal 68 that is highpass filtered 70 (or alternatively bandpass filtered), and the filtered signal 72 compared 74 to a first threshold Th1. When the filtered signal 72 exceeds the threshold Th1 a counter 76 is incremented, and when the output 78 of the counter 76 exceeds a second threshold Th2 at comparator 80, a touchdown event is detected. The counter 76 is reset periodically such that the touchdown event is detected when the filtered touchdown signal 72 exceeds the first threshold Th1 for a corresponding percentage of a detection window.
In one embodiment, the touchdown detection circuitry of FIG. 5 is also used to detect when the head is on the ramp. That is, the touchdown signal 82 output by the comparator 80 represents the fly height signal at block 26 of FIG. 2C for detecting whether the head is on the ramp. After decreasing the fly height of the head while seeking the head toward the ramp, the ramp is detected when the touchdown signal 82 toggles from high to low as illustrated in FIG. 5. Alternatively, when detecting whether the head is parked on the ramp or over the disk, the fly height is decreased and the touchdown signal 82 evaluated (where a high state indicates the head is over the disk). When calibrating the fly height threshold, either the first threshold Th1 or the second threshold Th2 or both thresholds of FIG. 5 may be adjusted at block 66 of FIG. 4.
FIG. 6A shows an embodiment of the present invention wherein the disk drive comprises a plurality of disk surfaces (e.g., disk surface 16A) and a head actuated over each disk surface (e.g., head 14A). In one embodiment, the ramp procedure of FIG. 2C may be executed for each head in order to detect the ramp location relative to each disk surface. In another embodiment, the ramp procedure is executed for only one of the heads, wherein FIG. 6B is a flow diagram according to an embodiment of the present invention for selecting the head to execute the ramp procedure.
The heads are positioned near the outer diameter of the disk (block 84), and then the touchdown calibration is executed for each head. A first head is selected (block 86) and its fly height initialized (block 88). The fly height is then decreased (block 90) by increasing the fly height control signal applied to the fly height actuator until the touchdown event is detected (block 92). The touchdown value for the fly height control signal that caused the touchdown event is saved (block 94) and the process is repeated for the next head (block 96) until the touchdown value for the fly height control signal has been determined for each head. The head having the maximum touchdown value is then selected (block 98) for executing the ramp procedure (block 100).
In one embodiment, the fly height sensor 54 in FIG. 2B comprises a suitable temperature sensitive (TS) fly height sensor. Any suitable TS fly height sensor may be employed in the embodiments, such as a magnetoresistive (MR) element or a semiconductor metal (e.g., Ni80Fe20) having a resistance that varies with temperature (thermistor). In embodiments employing a thermistor, the TS fly height sensor may comprise a positive or negative temperature coefficient meaning the resistance may increase or decrease with a rising temperature. In one embodiment, the TS fly height sensor may be the read element 56 in the head 14, and in another embodiment the TS fly height sensor may be a dedicated fly height sensor 54 separate from the read element 56 as shown in FIG. 2B. In one embodiment, the TS fly height sensor may be used to adjust the fly height of the head 14 for normal access operations (e.g., using touchdown detection and/or feedback control and/or feed-forward control), and in another embodiment the TS fly height sensor may be dedicated to detecting whether the head 14 is on the ramp 20 (i.e., the TS fly height sensor may not necessarily be used for fly height control). FIG. 7 shows a flow diagram according to an embodiment, wherein the TS fly height sensor generates a fly height signal (block 102), and the control circuitry 22 detects whether the head is on the ramp 20 based on the fly height signal (block 104).
In one embodiment, when the head 14 is over the disk surface 16 as shown in FIG. 2A, the disk surface 16 acts as a heat sink which cools the TS fly height sensor. When the head 14 contacts the ramp 20 and lifts away from the disk surface 16, the heat sinking effect of the disk surface 16 dissipates causing an increased heating of the TS fly height sensor, and a corresponding deviation in the fly height signal. In one embodiment, the TS fly height sensor is very sensitive to the heating effect when the head 14 lifts away from the disk surface 16 and onto the ramp 20, and therefore the resulting deviation in the fly height signal provides a very precise indication of when the head 14 contacts the ramp 20.
FIG. 8 shows control circuitry according to an embodiment wherein block 106 applies a bias current (iBIAS) to a TS fly height sensor 108 such that the fly height signal comprises a sensor voltage (vBIAS) across the TS fly height sensor 108. A digital-to-analog converter (DAC) 110 generates an offset voltage vTh that is subtracted from the sensor voltage (vBIAS) at subtractor 112 to generate a compensated voltage. A low pass filter (LPF) 113 filters the compensated voltage, and block 114 converts the resulting differential end voltage to a single end voltage representing the fly height signal (FHS) 116. In one embodiment, a gain, DC offset, and/or bandwidth of the LPF 113 may be configured to improve the accuracy of the FHS 116.
FIG. 9 is a flow diagram according to an embodiment for detecting when the head 14 contacts the ramp 20 by evaluating the fly height signal generated by the TS fly height sensor. The bias current 106 in FIG. 8 is initialized to a high value (block 118), and then the bias current is decreased (block 122) until the sensor voltage (vBIAS) in FIG. 8 falls below a target level (block 120). The LPF 113 in FIG. 8 is configured (block 124), such as by configuring a gain, DC offset, and/or bandwidth. A power applied to the fly height actuator 18 (FIG. 2B) is increased to heat the TS fly height sensor (block 126). In one embodiment, heating the TS fly height sensor shifts the sensor to a more linear operating range, thereby increasing the sensitivity of the sensor. However, in one embodiment heating the TS fly height sensor increases the resistance and therefore increases the sensor voltage (vBIAS). In one embodiment, the offset voltage vTh in FIG. 8 is adjusted to compensate for the increase in the sensor voltage (vBIAS). The offset voltage vTh is first set to zero (block 128), and then the offset voltage vTh is increased (block 132) until the sensor voltage falls below a target level (block 130). The head is then moved from a reference position over the disk surface (e.g., from an inner diameter) toward the ramp (block 134). At some point during the move, such as during the beginning of the move or as the head approaches the ramp, a ramp threshold is set (block 136), for example, by adding a six-sigma offset to the average of the fly height signal (FHS) 116 in FIG. 8. As the head approaches the ramp, the FHS 116 is monitored, and when the FHS exceeds the ramp threshold (block 138), the head contacting the ramp is detected (block 140).
Any suitable control circuitry may be employed to implement the flow diagrams in the embodiments of the present invention, 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 steps 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 an SOC.
In one embodiment, the control circuitry comprises a microprocessor executing instructions, the instructions being operable to cause the microprocessor to perform the steps of 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 embodiments disclosed herein.

## Claims

1. A disk drive comprising:
a disk surface;
a head actuated over the disk surface, the head comprising a temperature sensitive (TS) fly height sensor;
a ramp for parking the head; and
control circuitry configured to:
process a fly height signal generated by the TS fly height sensor, wherein the fly height signal represents a fly height of the head over the disk surface; and
detect whether the head is on the ramp in response to the fly height signal.

2. The disk drive as recited in claim 1, wherein the temperature sensitive fly height sensor comprises a thermistor.

3. The disk drive as recited in claim 2, wherein the thermistor comprises one of a positive temperature coefficient and a negative temperature coefficient.

4. The disk drive as recited in claim 1, wherein the head further comprises a fly height actuator and the control circuitry is further configured to:
control the fly height actuator to heat the TS fly height sensor; and
seek the head toward an outer diameter of the disk surface and detect when the head contacts the ramp in response to the fly height signal.

5. The disk drive as recited in claim 4, wherein the control circuitry is further configured to apply a bias current to the TS fly height sensor such that the fly height signal comprises a sensor voltage across the TS fly height sensor.

6. The disk drive as recited in claim 5, wherein the control circuitry is further configured to adjust the bias current until the sensor voltage reaches a target level while the head is over the disk surface.

7. The disk drive as recited in claim 6, wherein the control circuitry is further configured to:
configure a threshold based on the fly height signal while the head is over the disk surface; and
detect when the head contacts the ramp based on when the fly height signal exceeds the threshold.

8. The disk drive as recited in claim 6, wherein the control circuitry is further configured to:
adjust the bias current until the sensor voltage reaches a target level while the head is over the disk surface prior to controlling the fly height actuator to decrease the fly height of the head and heat the TS fly height sensor; and
after controlling the fly height actuator to decrease the fly height of the head and heat the TS fly height sensor, subtract an offset voltage from the sensor voltage to generate a compensated voltage, wherein the fly height signal comprises the compensated voltage.

9. The disk drive as recited in claim 2, wherein when the head contacts the ramp resulting in an increase in the fly height, a temperature of the TS fly height sensor is increased.

10. A method of operating a disk drive comprising:
processing a fly height signal generated by a temperature sensitive (TS) fly height sensor, wherein the fly height signal represents a fly height of a head over a disk surface; and
detecting whether the head is on a ramp in response to the fly height signal.

11. The method as recited in claim 10, wherein the temperature sensitive fly height sensor comprises a thermistor.

12. The method as recited in claim 11, wherein the thermistor comprises one of a positive temperature coefficient and a negative temperature coefficient.

13. The method as recited in claim 10, wherein the head further comprises a fly height actuator and the method further comprises:
controlling the fly height actuator to heat the TS fly height sensor; and
seeking the head toward an outer diameter of the disk surface and detect when the head contacts the ramp in response to the fly height signal.

14. The method as recited in claim 13, further comprising applying a bias current to the TS fly height sensor such that the fly height signal comprises a sensor voltage across the TS fly height sensor.

15. The method as recited in claim 14, further comprising adjusting the bias current until the sensor voltage reaches a target level while the head is over the disk surface.

16. The method as recited in claim 15, further comprising:
configuring a threshold based on the fly height signal while the head is over the disk surface; and
detecting when the head contacts the ramp based on when the fly height signal exceeds the threshold.

17. The method as recited in claim 15, further comprising:
adjusting the bias current until the sensor voltage reaches a target level while the head is over the disk surface prior to controlling the fly height actuator to decrease the fly height of the head and heat the TS fly height sensor; and
after controlling the fly height actuator to decrease the fly height of the head and heat the TS fly height sensor, subtracting an offset voltage from the sensor voltage to generate a compensated voltage, wherein the fly height signal comprises the compensated voltage.

18. The method as recited in claim 11, wherein when the head contacts the ramp resulting in an increase in the fly height, a temperature of the TS fly height sensor is increased.