Evaluating dual stage actuator response in a disk drive by adding sinusoid to control signal

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 5 0 - 5 N recorded around the circumference of each servo track. Each servo sector 5 i 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 5 i 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.

As the density of the data tracks increases, a microactuator may be employed in combination with the VCM to improve the tracking performance of the servo system. Any suitable microactuator may be employed such as a suitable piezoelectric (PZT) actuator. It may be desirable to evaluate the performance of the dual stage actuator (VCM+microactuator), such as evaluating the error rejection curve (ERC), open loop response, and/or closed loop response.

FIG. 1 shows a prior art disk format comprising a plurality of servo 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 by a dual stage actuator (DSA).

FIG. 2B shows a DSA servo loop comprising a voice coil motor (VCM) servo loop and a microactuator servo loop according to an embodiment of the present invention.

FIG. 2C is a flow diagram according to an embodiment of the present invention wherein a closed loop response of the VCM servo loop and microactuator servo loop are computed as B/A.

FIG. 3A shows a prior art VCM driver for applying a VCM control current to the VCM through a resistor.

FIG. 3B shows an embodiment of the present invention wherein to measure the closed loop response of the VCM servo loop a sinusoidal current is added to the VCM control current, and the response of the VCM control current is measured using a current sensor.

FIG. 4A shows a prior art microactuator driver for applying a microactuator control voltage to the microactuator through a differential amplifier.

FIG. 4B shows an embodiment of the present invention wherein to measure the closed loop response of the microactuator servo loop a sinusoidal voltage is added to the microactuator control voltage, and the response of the microactuator control voltage is measured.

FIG. 5 shows an embodiment of the present invention wherein after measuring the closed loop response of the VCM and microactuator servo loops, other performance metrics may be computed, such as an error rejection curve (ERC), open loop response, and/or closed loop response of the overall DSA servo loop.

FIGS. 2A-2C illustrate a method of evaluating a dual stage actuator (DSA) servo loop in a disk drive. The disk drive of FIG. 2A comprises a head 16 , a disk surface 18 , and a dual stage actuator (DSA) servo loop shown in FIG. 2B comprising a voice coil motor (VCM) servo loop and a microactuator servo loop operable to actuate the head over the disk surface. The disk drive of FIG. 2A further comprises control circuitry 20 that in one embodiment executes the flow diagram of FIG. 2C wherein a first sinusoidal signal A 1 is added to a VCM control signal B 1 generated by the VCM servo loop (step 22 ). A response of the VCM control signal B 1 to the first sinusoidal signal A 1 is measured (step 24 ), and a closed loop response of the VCM servo loop is computed in response to A 1 and B 1 (step 26 ). A second sinusoidal signal A 2 is added to a microactuator control signal B 2 generated by the microactuator servo loop (step 28 ), and a response of the microactuator control signal B 2 to the second sinusoidal signal A 2 is measured (step 30 ). A closed loop response of the microactuator servo loop is computed in response to A 2 and B 2 (step 32 ).

In the embodiment of FIG. 2A , the disk 18 comprises embedded servo sectors 34 0 - 34 N that define a plurality of servo tracks 36 . The control circuitry 20 processes a read signal 38 emanating from the head 16 to demodulate the servo sectors 34 0 - 34 N and generate a position error signal (PES) 40 shown in FIG. 2B representing an error between the actual position of the head 42 and a target position 44 relative to a target track. The control circuitry 20 filters the PES 40 using a suitable VCM compensator 46 to generate a VCM control signal 48 applied to a VCM 50 which rotates an actuator arm 52 ( FIG. 2A ) about a pivot in order to actuate the head 16 radially over the disk in coarse movements so as to reduce the PES 40 . The control circuitry 20 also filters the PES 40 using a suitable microactuator compensator 54 ( FIG. 2B ) to generate a microactuator control signal 56 applied to a microactuator 58 which actuates the head radially over the disk in fine movements so as to further reduce the PES 40 . The servo sectors 34 0 - 34 N 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 (e.g., a quadrature servo pattern), or a suitable phase-based servo pattern.

Any suitable microactuator 58 may be employed in the embodiments of the present invention, such as a piezoelectric (PZT) microactuator. In the embodiment shown in FIG. 2A , the microactuator 58 rotates a suspension relative to the actuator arm 52 in order to actuate the head radially over the disk. However other microactuator configurations may be employed, such as a microactuator that perturbs a gimbal assembly that connects the head to the suspension.

In the embodiment of FIG. 2B , a first sinusoidal signal A 1 60 is added to the VCM control signal B 1 48 , and the effect of the first sinusoidal signal A 1 60 on the VCM control signal B 1 48 is measured in order to compute the closed loop response of the VCM servo loop as the ratio B 1 /A 1 . A second sinusoidal signal A 2 62 is added to the microactuator control signal B 2 56 , and the effect of the second sinusoidal signal A 2 62 on the microactuator control signal B 2 56 is measured in order to compute the closed loop response of the microactuator servo loop as the ratio B 2 /A 2 .

FIG. 3A shows a prior art VCM driver comprising a digital-to-analog converter (DAC) 64 for generating a VCM control current B 1 48 which is applied to the voice coil of the VCM 50 through a resistor R. FIG. 3B shows a VCM driver according to an embodiment of the present invention wherein a current sensor 66 is connected in series with the VCM control current B 1 48 and a sinusoidal current A 1 60 is added to the VCM control current B 1 48 . The sinusoidal current A 1 60 may be generated using any suitable signal generator. As described in greater detail below, the current sensor 66 and signal generator for generating the sinusoidal current A 1 60 may be implemented as part of the control circuitry 20 in the disk drive of FIG. 2A , or they may be external components used to evaluate a printed circuit board (PCB) of a production disk drive.

FIG. 4A shows a prior art microactuator driver comprising a differential amplifier 68 for amplifying a difference between a microactuator control voltage B 2 56 generated by a DAC 70 and a reference voltage 72 . The microactuator control voltage B 2 56 is connected to a first input of the differential amplifier 68 through a resistor R 2 , and the reference voltage 72 is connected to a second input of the differential amplifier 68 through a resistor R 3 . FIG. 4B shows a microactuator driver according to an embodiment of the present invention wherein a second end of a resistor R 5 is connected to the second input of the differential amplifier 68 , and a sinusoidal voltage A 2 62 is connected to a first end of the resistor R 5 . The sinusoidal voltage A 2 62 may be generated using any suitable signal generator. As described in greater detail below, the voltage detector for measuring the microactuator control voltage B 2 56 and the signal generator for generating the sinusoidal voltage A 2 62 may be implemented as part of the control circuitry 20 in the disk drive of FIG. 2A , or they may be external components used to evaluate a PCB of a production disk drive.

In an alternative embodiment, instead of connecting the second end of the resistor R 5 to the second input of the differential amplifier as shown in FIG. 4B , the second end of the resistor R 5 may be connected to the first input of the differential amplifier 68 . This embodiment reverses the sign of the computed closed loop response B 2 /A 2 of the microactuator servo loop.

In one embodiment, the reference voltage 72 of FIG. 4B corresponds to a DC offset of the differential amplifier 68 , and the sinusoidal voltage A 2 62 is generated with a corresponding DC offset (DC coupling). In one embodiment, the resistor R 5 is selected to match the resistor R 3 and the sinusoidal voltage A 2 62 is generated with a DC offset that matches the reference voltage 72 (this embodiment assumes R 4 /R 3 =R 1 /R 2 ). In the embodiment described above where the second end of the resistor R 5 is instead connected to the first input of the differential amplifier, resistor R 5 is selected to match resistor R 2 .

In one embodiment, to measure the closed loop response of the servo loops the sinusoidal signals A 1 and A 2 are swept (separately) over a range of frequencies (with optionally different amplitudes for A 1 and A 2 ) and the effect on the control signals is measured at each frequency. After computing the closed loop response of the servo loops, other performance metrics may be computed, such as an error rejection curve (ERC), open loop response, and/or closed loop response of the overall DSA servo loop as illustrated in the equations of FIG. 5 .

In one embodiment, the control circuitry 20 within the disk drive of FIG. 2A generates the sinusoidal signals and measures the response of the control signals in order to compute the closed responses of the servo loops. In another embodiment, the PCB of a production disk drive may be evaluated by adding external sinusoidal signals to the control signals and measuring the response of the control signals using suitable probing instruments. For example, in the embodiment of FIG. 3B the sinusoidal current A 1 60 may be added to the VCM control current B 1 48 by soldering a wire to the appropriate trace of the PCB, and connecting the wire to an external signal generator. An external current sensor 66 may be used to sense the VCM control current B 1 48 by cutting the appropriate trace of the PCB at two points and wiring the external current sensor 66 in series with the trace. In the embodiment of FIG. 4B , the sinusoidal voltage A 2 62 may be added to the microactuator control voltage B 2 56 by soldering a second end of the resistor R 5 to the second input of the differential amplifier 68 and coupling an external signal generator to the first end of resistor R 5 . The response of the control voltage B 2 56 may be measured by soldering a wire to the appropriate trace of the PCB, and connecting the wire to an external voltage detector.

In the embodiment where the PCB of a production disk drive is evaluated, the disk drive may be issued a command that causes the disk drive to maintain the head over a target track while evaluating the DSA servo loop. For example, a host connected to the disk drive may command the disk drive to read the same LBA multiple times so that the disk drive attempts to read the same track multiple times while the DSA servo loop is evaluated.

The flow diagrams in the embodiments of the present invention may be implemented using any suitable computer system. In one embodiment, the computer system comprises a microprocessor for executing instructions stored on any suitable computer readable medium (e.g., optical or magnetic disk, semiconductor memory, etc.). The computer system may accumulate the performance data associated with the DSA (the response of the control signals B to the sinusoidal signals A), and then compute the closed loop response of the servo loops as B/A as well as the error rejection curve, open loop response, and closed loop response of the DSA servo loop as described above.

In an alternative embodiment, any suitable disk drive 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.