Detecting head/disk contact using timing jitter

A disk drive that includes a head and a disk. The disk drive also includes a circuit that can detect head/disk contact from a jitter determined from a read signal provided by the head. The jitter may correspond to the time interval between two detected sync marks. Alternatively, the jitter may correspond to a change in frequency of a read clock generated from the read signal. These approaches allow for detection of head movement in a down track direction. Down track is a direction that is essentially parallel with the longitudinal axis of the head.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to detection of contact between a head and a disk of a hard disk drive.

2. Background Information

Hard disk drives contain a plurality of magnetic heads that are coupled to rotating disks. The heads write and read information by magnetizing and sensing the magnetic fields of the disk surfaces. Each head is attached to a flexure arm to create a subassembly commonly referred to as a head gimbal assembly (“HGA”). The HGA's are suspended from an actuator arm. The actuator arm has a voice coil motor that can move the heads across the surfaces of the disks.

The disks are rotated by a spindle motor of the drive. Rotation of the disks creates an air flow within the disk drive. Each head has an air bearing surface that cooperates with the air flow to create an air bearing between the head and the adjacent disk surface. The air bearing eliminates or minimizes the mechanical wear between the head and the disk. The height of the air bearing is commonly referred to as the flying height of the head.

The magnetic field detected by the head is inversely proportional to the flying height of the head. Likewise, the strength of the magnetic field written onto the disk is also inversely proportional to the fly height. A larger fly height will produce a weaker magnetic field on the disk.

Due to various reasons the fly height of the heads may vary during operation of the drive. Such a variation in fly height may result in poorly written data on the disk. Weakly written data may create errors during a read routine. It would be desirable to monitor the fly height of the heads. It would also be desirable to provide such a monitoring function without significantly changing the components, cost, or operation of the drive.

There have been developed heads which include a heater coil. Current is provided to the heater coil to generate heat and thermally expand the head to move the read and write elements closer to the disk. These types of heads are sometimes referred to as fly on demand (“FOD”) heads. The flying height of FOD heads can be varied by changing the amount of power provided to the heater coil.

It is desirable to create a flying height that is nearly zero. To achieve a minimum flying height it is desirable to know the point of contact between the head and the disk. Contact between the head and the disk can cause vibration and associated head movement. The vibration movement can be in a variety of directions. For example, the head can move perpendicular to and from the disk. This head movement will vary the amplitude of the read signal. There have been schemes developed to determine head contact by analyzing the amplitude of the read signal to detect mechanical vibration. The head contact may also cause cross-track vibration that varies the position error signal (“PES”) used in the servo routine of the drive. There have also been schemes to determine head contact by analyzing the PES of the servo. Head assemblies that quickly damp perpendicular or cross-track vibration limit the amount of signal data that can be analyzed to determine head contact.

BRIEF SUMMARY OF THE INVENTION

A hard disk drive that includes a head that is coupled to a disk and provides a read signal. The disk drive further includes a circuit that detects a contact between the head and the disk from a jitter associated with the read signal.

DETAILED DESCRIPTION

Disclosed is a disk drive that includes a head and a disk. The disk drive also includes a circuit that can detect head/disk contact from a jitter determined from a read signal provided by the head. The jitter may correspond to the time interval between two detected sync marks. Alternatively, the jitter may correspond to a change in frequency of a read clock generated from the read signal. These approaches allow for detection of head movement in a down track direction. Down track is a direction that is essentially parallel with the longitudinal axis of the head. Such a scheme is particularly desirable for head gimbal assemblies that quickly damp vibration in the perpendicular and cross-track directions.

Referring to the drawings more particularly by reference numbers,FIG. 1shows an embodiment of a hard disk drive10of the present invention. The disk drive10may include one or more magnetic disks12that are rotated by a spindle motor14. The spindle motor14may be mounted to a base plate16. The disk drive10may further have a cover18that encloses the disks12.

The disk drive10may include a plurality of heads20located adjacent to the disks12. As shown inFIG. 2the heads20may have separate write22and read elements24. The write element22magnetizes the disk12to write data. The read element24senses the magnetic fields of the disks12to read data. By way of example, the read element24may be constructed from a magneto-resistive material that has a resistance which varies linearly with changes in magnetic flux. The head20may be a perpendicular recording head. The head20may also include a heater element (not shown). Such heads are commonly referred to as fly on demand (“FOD”) heads.

Referring toFIG. 1, each head20may be gimbal mounted to a flexure arm26as part of a head gimbal assembly (HGA). The flexure arms26are attached to an actuator arm28that is pivotally mounted to the base plate16by a bearing assembly30. A voice coil32is attached to the actuator arm28. The voice coil32is coupled to a magnet assembly34to create a voice coil motor (VCM)36. Providing a current to the voice coil32will create a torque that swings the actuator arm28and moves the heads20across the disks12.

The hard disk drive10may include a printed circuit board assembly38that includes a plurality of integrated circuits40coupled to a printed circuit board42. The printed circuit board40is coupled to the voice coil32, heads20and spindle motor14by wires (not shown).

FIG. 3shows an electrical circuit50for reading and writing data onto the disks12. The circuit50may include a pre-amplifier circuit52that is coupled to the heads20. The pre-amplifier circuit52has a read data channel54and a write data channel56that are connected to a read/write channel circuit58. The pre-amplifier52also has a read/write enable gate60connected to a controller64. Data can be written onto the disks12, or read from the disks12by enabling the read/write enable gate60.

The read/write channel circuit62is connected to a controller64through read and write channels66and68, respectively, and read and write gates70and72, respectively. The read gate70is enabled when data is to be read from the disks12. The write gate72is to be enabled when writing data to the disks12. The controller64may be a digital signal processor that operates in accordance with a software routine, including a routine(s) to write and read data from the disks12. The read/write channel circuit62and controller64may also be connected to a motor control circuit74which controls the voice coil motor36and spindle motor14of the disk drive10. The controller64may be connected to a non-volatile memory device76. By way of example, the device76may be a read only memory (“ROM”). The non-volatile memory76may contain the instructions to operate the controller and disk drive. Alternatively, the controller may have embedded firmware to operate the drive.

FIG. 4is a schematic of a circuit100that can detect contact between a head and a disk. The circuit100includes a read channel102and a timing measurement circuit104. The disks are typically organized to have a plurality of concentric tracks. Each track is divided into a plurality of sectors. Each sector has a servo field and one or more data fields. There is typically a sync mark associated with each servo field. Likewise, there is typically a sync mark associated with each data field. The sync marks are used to synchronize reading of the servo and data fields.

The read channel102can detect two different sync marks. The detection of the sync marks is provided to the measurement circuit which determines a time interval between the detection of the marks. A jitter calculation block106determines the deviation between the measured time interval and a stored time interval value. The stored time interval value is the time interval that should occur between detection of two marks if there is no head/contact. Head contact with a disk surface will reduce the head speed and increase the time interval between detected marks. It is advantageous to detect sync marks for the servo fields if head contact were to occur during a write operation because such marks exits before writing. Conversely, because there are typically more data sync marks such marks can be used to provide more sensitive to high frequency jitter.

A contact detection block108compares the computed jitter with a threshold value. If the jitter exceeds the threshold then a head contact signal is generated by detection block108.

FIG. 5is a schematic of an alternate embodiment of the circuit100′. The read channel102′ generates a read clock from the read signal. The read clock is generated with a phase lock loop that phase locks the clock signal to a frequency. The jitter calculation block104′ determines a jitter of the read clock frequency. The jitter can be the difference in clock frequency between the computed frequency and a stored frequency. The jitter can also be a rate change in frequency of the read clock. The stored frequency is a frequency of the clock signal when there is no head/disk contact.

The contact detection block108′ compares the computed jitter with a threshold value. The detection block108′ can output a head/contact detection signal when the computed jitter exceeds a threshold value.

The function blocks102,102′,104,104′,106,108and108′ can all be performed by the controller64of the disk drive.