Dynamic magnetic information storage or retrieval – Automatic control of a recorder mechanism – Controlling the head
Reexamination Certificate
1998-08-24
2001-03-20
Young, W. R. (Department: 2651)
Dynamic magnetic information storage or retrieval
Automatic control of a recorder mechanism
Controlling the head
C318S561000
Reexamination Certificate
active
06204988
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to magnetic disk drives and, more particularly, to a disk drive with a servo control system that self-characterizes its own frequency response and, if warranted, adapts itself to increase its operating stability.
BACKGROUND OF THE RELATED ART
It is well known that determining the frequency response of a control system can help identify problems which may arise during system operation. In the context of magnetic disk drives, however, the frequency response of the servo control system has generally been measured by external test equipment that costs thousands of dollars. The tests, moreover, must occur in a clean room environment because it requires invasive access to the drive's interior. Finally, it may take several minutes to several hours to complete the frequency characterization of a single drive using such external test equipment. Because of these limitations, the drive industry generally tests the frequency response of only a limited number of drives during product development.
During manufacture, each production drive is simply assumed to have a frequency response that is sufficiently “close” to the nominal disk drive(s) tested during development to make the drive operable. There is no mechanism to validate the frequency response on a drive by drive basis. There is conventionally no mechanism to vary the frequency response to salvage a marginal drive individually.
Unfortunately, the frequency response of each production drive may vary from the nominal response of the development drives due to an unpredictable combination of characteristics including the electromagnetic response of each individual read transducer, each transducer's location on the head stack, mechanical misalignments, and electronic component variations. Moreover, circumstances which cause deviations from the nominal frequency response are also likely to alter the magnitude and location of resonances and, thereby, reduce the servo system's gain margin, phase margin, or both, to the detriment of the servo system's operating stability.
The drive industry is highly competitive such that there is a clear need to reduce the percentage of customer returns and obtain a higher overall yield of disk drives in the mass production environment.
SUMMARY OF THE INVENTION
In a first aspect, the invention is implemented in a disk drive comprising a sampled servo feedback control system having a sampling frequency and including (1) a plant which has a nominal plant frequency response and (2) a servo controller which comprises (i) a compensator with a nominal compensator frequency response and (ii) a gain element with a nominal open-loop gain, the compensator and gain element being designed to define the overall sampled servo feedback control system with a desired open-loop bandwidth frequency, a desired gain margin, and a desired phase margin which collectively provide for stable closed-loop operation given the nominal plant frequency response, and may be regarded as a method of adaptively self-modifying the servo controller to compensate for deviations from the nominal plant frequency response owing to individual plant variations, the method comprising the steps of: autonomously determining a phase crossover frequency f
PC
by (1) measuring the sampled servo feedback control system's open-loop phase response to sinusoidal inputs at a plurality of discrete frequencies and (2) determining which of the plurality of discrete frequencies is associated with an open-loop phase response that is substantially equal or closest to −180 degrees; autonomously determining a measured gain margin by measuring the sampled servo feedback control system's open-loop magnitude response to a sinusoidal input at the phase cross-over frequency f
PC
; and adjusting the open-loop gain of the servo controller's gain element relative to the nominal open-loop gain if the measured gain margin is different than the desired gain margin to provide an adjusted gain margin that is substantially equal to the desired gain margin.
In a second aspect, the invention is implemented in a disk drive comprising a sampled servo feedback control system having a sampling frequency and including (1) a plant which has a nominal plant frequency response and (2) a servo controller which comprises (i) a compensator with a nominal compensator frequency response and (ii) a gain element with a nominal open-loop gain, the compensator and gain element being designed to define the overall sampled servo feedback control system with a desired open-loop bandwidth frequency, a desired gain margin, and a desired phase margin which collectively provide for a desired closed-loop operation in terms of stability and operating performance given the nominal plant frequency response, and may be regarded as a method of adaptively self-modifying the servo controller to compensate for deviations from the nominal plant frequency response owing to individual plant variations, the method comprising the steps of: autonomously determining a phase crossover frequency f
PC
by (1) measuring the sampled servo feedback control system's open-loop phase response to sinusoidal inputs at a plurality of discrete frequencies and (2) determining which of the plurality of discrete frequencies is associated with an open-loop phase response that is substantially equal or closest to −180 degrees; autonomously determining a measured gain margin by measuring the sampled servo feedback control system's open-loop magnitude response to a sinusoidal input at the phase cross-over frequency f
PC
; adjusting the open-loop gain of the servo controller's gain element relative to the nominal open-loop gain if the measured gain margin is different than the desired gain margin to provide an adjusted gain margin that is substantially equal or closest to the desired gain margin; and adjusting the compensator frequency response relative to the nominal compensator frequency response to compensate for the phase margin being moved relative to the desired phase margin by the step of adjusting the open loop gain and to provide an adjusted phase margin that is substantially equal to the desired phase margin.
In a third aspect, the invention is implemented in a disk drive having a sampled servo system having a sampling rate and a nominal bandwidth, wherein the sampled servo system comprises a plant and a servo controller that controls the plant using a compensator and a gain element with a nominal open loop gain, and the invention may be regarded as a method for adaptively modifying the servo controller to compensate for plant variations which are incompatible with the nominal gain and bandwidth comprising the steps of: implementing a self-generated bode plot to determine a gain margin and a phase margin, and if the gain margin is not greater than a predetermined minimum: adjusting the open loop gain of the servo controller to provide a gain margin which is greater than the predetermined minimum at a bandwidth which is different than the nominal bandwidth; and adjusting the compensator to provide a phase margin which is greater than a predetermined minimum.
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patent: 6072654 (2000-06-01), Eddy
Codilian Raffi
Sheh Edgar De-Jia
Yu Jie
Habermehl James L
Shara Milad G
Western Digital Corporation
Young W. R.
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