Disk drive adjusting rotation speed of disk to compensate for blind area when measuring frequency response of servo control system

A disk drive is disclosed comprising a head and a disk comprising a plurality of servo tracks, wherein each servo track comprises a plurality of servo sectors. The disk drive further comprises control circuitry comprising a servo control system operable to actuate the head over the disk in response to the servo sectors. The disk is rotated at a first speed and the servo sectors are read at a first servo sample frequency to generate first servo samples. The disk is rotated at a second speed different from the first speed and the servo sectors are read at a second servo sample frequency to generate second servo samples. The second servo samples are processed to measure a frequency response of the servo control system proximate the first servo sample frequency.

BACKGROUND

FIG. 1shows a prior art disk format2as comprising a number of servo tracks4defined by servo sectors60-6Nrecorded around the circumference of each servo track. Each servo sector6, comprises a preamble8for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark10for storing a special pattern used to symbol synchronize to a servo data field12. The servo data field12stores 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 sector6, further comprises groups of servo bursts14(e.g., N and Q servo bursts), which are recorded with a predetermined phase relative to one another and relative to the servo track centerlines. The phase based servo bursts14provide fine head position information used for centerline tracking while accessing a data track during write/read operations. A position error signal (PES) is generated by reading the servo bursts14, wherein the PES represents a measured position of the head relative to a centerline of a target servo track. A servo controller processes the PES to generate a control signal applied to a head actuator (e.g., a voice coil motor) in order to actuate the head radially over the disk in a direction that reduces the PES.

DETAILED DESCRIPTION

FIG. 2Ashows a disk drive according to an embodiment comprising a head16and a disk18comprising a plurality of servo tracks20, wherein each servo track comprises a plurality of servo sectors220-22N. The disk drive further comprises control circuitry24comprising a servo control system operable to actuate the head over the disk in response to the servo sectors220-22N, the control circuitry24operable to execute the flow diagram ofFIG. 2B. The disk is rotated at a first speed and the servo sectors are read at a first servo sample frequency to generate first servo samples (block26). The disk is rotated at a second speed different from the first speed and the servo sectors are read at a second servo sample frequency to generate second servo samples (block28). The second servo samples are processed to measure a frequency response of the servo control system proximate the first servo sample frequency (block30).

In the embodiment ofFIG. 2A, the control circuitry24processes a read signal32emanating from the head16to demodulate the servo sectors220-22Nand 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. In one embodiment, the target track comprises a target data track defined relative to the servo tracks20, wherein the data tracks may be recorded at the same or different radial density than the servo tracks20. The control circuitry24filters the PES using a suitable compensation filter to generate a control signal34applied to a voice coil motor (VCM)36which rotates an actuator arm38about a pivot in order to actuate the head16radially over the disk18in a direction that reduces the PES. The control circuitry24may also generate a control signal40applied to a microactuator42in order to actuate the head16over the disk18in fine movements. Any suitable microactuator42may be employed in the embodiments, such as a piezoelectric actuator. In addition, the microactuator42may actuate the head16over the disk18in any suitable manner, such as by actuating a suspension relative to the actuator arm, or actuating a slider relative to the suspension. The servo sectors220-22Nmay comprise any suitable head 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 an amplitude based servo pattern or a phase based servo pattern.

In one embodiment, it may be desirable to measure a frequency response of the servo control system for actuating the head16over the disk18in order, for example, to identify resonant frequencies of the servo system. In one embodiment, the servo control system may be modified based on the identified resonant frequencies, such as by adding and/or modifying notch filters that attenuate the frequency response at the resonant frequencies. In another embodiment, the identified resonant frequencies may be used to identify defective servo components, such as a defective VCM36or microactuator42, so that the disk drive may be discarded or reworked to replace the defective components.

In one embodiment, the technique for measuring the frequency response of the servo control system may exhibit a blind area proximate the servo sampling frequency (where the servo sampling frequency is based on the rotation speed of the disk). For example, when the disk18is rotated at a normal operating speed used to access the data tracks (write/read operations), the control circuitry24reads the servo sectors at a corresponding servo sample frequency to generate servo samples. The control circuitry24may then measure the frequency response of the servo control system based on the servo samples (e.g., the PES generated at each servo sector). An example frequency response measured for the servo control system is shown inFIG. 3, which includes a magnitude response and a phase response. When the disk is rotated at a first rotation speed (e.g., the operating rotation speed used to access the disk during normal operations), there may be a blind area in the measured frequency response (solid line response) shown inFIG. 3, where the blind area corresponds to a frequency range proximate the servo sampling frequency. Accordingly, in order to measure the frequency response across the blind area, in one embodiment the rotation speed of the disk is adjusted (increased or decreased) which effectively shifts the blind area in the measured frequency response. In one embodiment, the frequency response is measured over a desired range of frequencies outside the blind area while the disk is rotating at the first speed (e.g., operating speed), and then the frequency response is measured for the blind area after adjusting the rotation speed of the disk.

In one embodiment, the technique for measuring the frequency response of the servo control system may exhibit multiple blind areas, such as at an integer multiple of the servo sample frequency. Accordingly, in one embodiment after adjusting the rotation speed of the disk from the first speed to the second speed, the frequency response of the servo control system may be measured proximate an integer multiple of the first servo sample frequency.

In one embodiment, the degree to which the control circuitry24adjusts the rotation speed of the disk18depends on the width of the blind area shown inFIG. 3. That is, as the rotation speed of the disk18is increased or decreased, there is a corresponding change in the servo sample frequency, and a corresponding shift of the blind area in the frequency response of the servo control system. In one embodiment, the control circuitry is operable to adjust (increase or decrease) the rotation speed of the disk to the second speed so that:
Fs—new>Fs+BLIND_WIDTH or
Fs—new<Fs−BLIND_WIDTH
where Fs represents the first servo sample frequency when the disk is rotated at the first speed, and Fs_new represents the second servo sample frequency when the disk is rotated at the second speed. In one embodiment, the width of the blind area (BLIND_WIDTH) in the frequency response requires an adjustment to the rotation speed of the disk of not more than ten percent of the first speed.

Any suitable technique may be employed to measure the frequency response of the servo control system. In one embodiment, the control circuitry processes the second servo samples to measure the frequency response of the servo control system proximate the first servo sample frequency using a signal processing algorithm capable of measuring the frequency response of the servo control system at frequencies higher than half the second servo sample frequency. Such a signal processing algorithm may include an anti-aliasing multi-rate (Nx) bode algorithm which is understood with reference toFIGS. 4A and 4B.FIG. 4Arepresents the closed-loop sampled servo control system wherein Gp(jω) represents the plant under test (e.g., a compensator and actuator), r(t) represents a reference input, y(t) represents the sampled output (e.g., the PES measured at each servo sector), and Gh(jω) represents a zero order hold function. In one embodiment, the frequency response of the closed-loop servo control system shown inFIG. 4Ais measured at discrete frequencies (e.g., frequency ω0) by injecting a sinusoid having a frequency ω0as the reference input R(jω). The effective transfer function Heff(jω) may be derived as shown inFIG. 4Bwhere T represents the servo sample period. The term HΣ(jω) represents the discrete-time transfer function of the closed-loop system evaluated at z=ejωT, and ωsrepresents the servo sample frequency. Since the effective transfer function Heff(jω) does not exhibit aliasing (anti-aliasing) it may be measured at any frequencies, including frequencies beyond half the servo sample frequency (the Nyquist frequency). However, when using the above-described multi-rate (Nx) bode algorithm, the frequency response is undefined when the frequency of the reference input R(jω) is proximate an integer multiple of the servo sample frequency (kωs) resulting in blind areas proximate an integer multiple of the servo sample frequency such as illustrated inFIG. 3.

In one embodiment, the above-described multi-rate (Nx) bode algorithm (or another algorithm) may be used to measure the frequency response of the servo control system over a desired range of frequencies outside the blind areas while the disk is rotating at the first speed (e.g., the operating speed). The control circuitry24may then adjust the rotation speed of the disk to the second speed in order to measure the frequency response of the servo control system over the blind areas that were not measured while the disk was rotating at the first speed.

FIG. 5Ashows an embodiment wherein the control circuitry24is operable to adjust the rotation speed of the disk (increase the rotation speed) from the first speed to the second speed in a number of step increments. For example, the control circuitry24may adjust the rotation speed higher and then lower in at least five step increments. This gradual adjustment to the rotation speed may help avoid airflow dynamics that may perturb the head causing contact with the disk, and/or help avoid undesirable actuator mechanical modes which may corrupt the frequency response measurement.

FIG. 5Bis a flow diagram according to an embodiment wherein the disk is rotated (block44) at a first peed (e.g., the normal operating speed). The servo control system is then disabled (block46) and the rotation speed of the disk is increased in increments to a second speed (block48) as illustrated inFIG. 5A. The servo control system is enabled in order to synchronize to the servo sectors at the second servo sample frequency (block50). While the disk is rotating at the second speed, a frequency response of the servo control system is measured (e.g., using the above-described multi-rate (Nx) bode algorithm) proximate the first servo sample frequency (block52) based on the second servo samples taken at the second servo sample frequency. The servo control system is disabled (block54) and the rotation speed of the disk is decreased in increments to the first speed (block56). The servo control system is then enabled in order to synchronize to the servo sectors at the first servo sample frequency (block58). In the embodiment ofFIG. 5B, the first speed is the normal operating speed wherein a data track may be accessed during write/read operations (block60).