Dynamic magnetic information storage or retrieval – Automatic control of a recorder mechanism – Controlling the head
Reexamination Certificate
2000-06-30
2003-10-21
Sniezek, Andrew L. (Department: 2651)
Dynamic magnetic information storage or retrieval
Automatic control of a recorder mechanism
Controlling the head
C360S078080, C360S078060, C360S078090
Reexamination Certificate
active
06636377
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to magnetic disk drives and, more particularly, to a method of tuning feed-forward control in a disk drive.
BACKGROUND OF THE RELATED ART
Magnetic disk drives generally read and write data on the surface of a rotating magnetic disk with a transducer that is located at the far end of a moveable actuator. A servo control system uses servo control information recorded amongst the data, or on a separate disk, to controllably drive a voice coil motor (VCM) that moves the transducer from track to track (“seeking”) and to hold the transducer at a desired position (“track following”). A detailed discussion of servo control systems is unnecessary because such systems are well known as set forth, for example, in patent application Ser. No. 09/138,841 that was filed on Aug. 8, 1998, now U.S. Pat. No. 6,204,988 entitled “DISK DRIVE CAPABLE OF AUTONOMOUSLY EVALUATING AND ADAPTING THE FREQUENCY RESPONSE OF ITS SERVO CONTROL SYSTEM,” and is commonly owned by the assignee of this application.
Most disk drives have previously been used for storing conventional data files of the type that are associated with personal computers. In such applications, data integrity is paramount relative to other considerations such as seek times and the reduction of acoustic noise. It presently appears, however, that disk drives are likely to become popular for recording and replaying audiovisual data—e.g. a drive based recording device that replaces video cassette recorders (VCRs). A drive-based recording device of this nature will benefit from using a disk drive with faster seek times because it will spend less time moving its actuator where it needs to be and have more time to record or recover information in any given unit of time. The drive-based recording device, therefore, may be able to record and/or playback more audiovisual data streams that otherwise possible. At the same time, the drive-based recording device is likely to be located adjacent to a television or be in some other location where acoustic noise issues arise. Accordingly, it is equally important for the disk drive to implement its seeks in as quiet a manner as possible.
The drive industry has progressed through several stages of development as related to seeks. Of particular relevance here, is the prior use of a so-called “bang-bang” seek profile wherein the transducer is rapidly accelerated at the start of a seek and then rapidly decelerated at the end of a seek using saturated levels of command effort (current or voltage). A bang-bang seek profile moves the transducer to a target position in as rapid a manner as possible. On the other hand, because the bang-bang acceleration profile is a square wave, it is relatively loud and it contains many high frequency components that may detrimentally excite a mechanical resonance that causes the transducer to take longer to settle into the target position. It has previously been determined that a quieter, faster settling seek is possible by “shaping” the transducer's acceleration profile so that it does not appear like a square wave, but rather approximates a single frequency sine wave. The result is a shaped seek profile that is “close” to a bang-bang square wave but is quieter and does not contain the high frequency components that may excite the drive's resonant frequencies.
Modem disk drives generally use a sampled servo control system that only periodically receives position information (e.g. once per servo sector) and shortly thereafter outputs a command effort based on a deviation between the indicated position and the target position. The servo control system in such a drive implements a shaped seek profile as a feed-forward profile using a feed-forward control path as a feed-forward profile using as discussed,for example, in co-pending patent application Ser. No. 09/538,931 pending that was filed on Mar. 31, 2000, entitled “DISK DRIVE WITH FEED-FORWARD CONTROL PATH THAT RECEIVES A REFERENCE POSITION SIGNAL TO APPLY A FEED-FORWARD COMMAND EFFORT AT A RATE GREATER THAN A SERVO SAMPLING RATE,” (the “Multi-rate Feed-Forward Application”), which application is commonly owned by the assignee of this application and hereby incorporated by reference in its entirety.
Modeling errors in the feed-forward control path, however, may cause the servo to inaccurately follow the intended feed-forward profile. In the Multi-rate Feed-Forward Application, the feed-forward control path models the VCM and overall plant as a rigid body that responds equally to all input frequencies in terms of magnitude and phase, i.e. it implements a simple double derivative of the PES reference signal to form a sinusoidal seek profile without regard to the frequency-dependent variance of the VCM. Obviously, in such case, the effectiveness of the feed-forward control path will diminish to the extent that the actual VCM response G(z) differs from the modeled VCM response G
0
(z).
The most prevalent modeling errors are gain errors due to variation in the values of the motor torque constant (K
T
) and the motor winding resistance (R
W
) due to changes in temperature, calibration errors, track pitch errors, and other factors. These modeling errors may cause the actuator to either overshoot or undershoot the target position at the end of the shaped seek, and thereby increase the required settling time for seeks of all lengths.
One could simply rely on the feedback loop that contains the feedback control path to repeatedly, reactively reign in any deviation caused by the feed-forward control path's modeling error in a sample-by-sample fashion (as in the above-referenced Multi-rate Feed-forward application), but system performance would be improved if the effect of the modeling error could be eliminated or reduced.
One approach to addressing the deviation between actual and modeled frequency response is the inclusion of an adaptive control path that characterizes the difference and dynamically accounts for it during each seek. One such system monitors the error signal e during the first several samples of a seek in order to characterize any modeling error between the modeling VCM response G
0
(z) and the actual VCM response G(z) and thereafter outputs an adaptive command effort during subsequent samples to compensate for such modeling error and thereby tends to make the error signal e approach zero at the end of the seek or, equally speaking, to make the transducer
80
arrive at the target position at the end of the seek notwithstanding the modeling error. This approach was discussed in detail in co-pending patent application Ser. No. 09/539,349 pending that was filed on Mar. 31, 2000, entitled “DISK DRIVE WITH ADAPTIVE CONTROL PATH THAT PRODUCES AN ADAPTIVE CONTROL EFFORT BASED ON A CHARACTERIZED DEVIATION BETWEEN ACTUAL AND MO[D]ELED PLANT RESPONSE,” (the “Adaptive Control Path Application”), which application is commonly owned by the assignee of this application and hereby incorporated by reference in its entirety.
The Multi-rate Feed-Forward Application and the Adaptive Control Path Application represent significant advances. Neither, however, does anything to eliminate or reduce the modeling error between the modeled VCM response G
0
(z) and the actual VCM response G(z) in advance of actually starting a seek. That modeling error is embodied in the disk drive's feed-forward control, and, more particular, in the relationship between the position reference signal and the feed-forward control signal that were cooperatively derived on the basis of the modeled VCM response G
0
(z).
There remains a need, therefore, for a method of tuning feed-forward control in a disk drive.
SUMMARY OF THE INVENTION
The invention may be regarded as a method of tuning a servo control system in a disk drive having a disk with servo control information recorded thereon, a transducer for reading the servo control information, a servo control system that produces a control signal by combining (1) a feed-forward command effort signal u
ffwd
and (2) a feedback comm
Joan David B.
Yu Jie
Myers Dawes Andras & Sherman
Shara, Esq. Milad G.
Sniezek Andrew L.
Western Digital Technologies Inc.
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