Dynamic magnetic information storage or retrieval – Automatic control of a recorder mechanism – Controlling the head
Reexamination Certificate
1998-09-14
2001-04-17
Faber, Alan T. (Department: 2753)
Dynamic magnetic information storage or retrieval
Automatic control of a recorder mechanism
Controlling the head
C360S078090, C318S560000
Reexamination Certificate
active
06219196
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the suppression of mechanical resonance, and more particularly to a mechanical resonance suppression method using a notch filter.
DESCRIPTION OF RELATED ART
In hard disk drive head positioning, since the mechanical resonance of an actuator has a large amplitude of peak that falls in a frequency range of several kHz, which is higher than a servo frequency bandwidth, suppression of it should be provided by a notch filter, otherwise a stable servo loop can not be obtained. The notch filter has a fixed central frequency and a digital or analog filter which has a constant Q is normally used. The resonance frequency of the actuator can be selected as the central frequency of a notch filter, but if a small Q is selected and then the suppression of the resonance is obtained in a large range of frequencies, and there is an increase of the phase delay in a servo frequency bandwidth which is lower than the central frequency of the notch filter. Since this reduces the margin for the phase of the servo loop and degrades the control performance it is preferable that a notch filter has as large Q as is possible and that the central frequency consistently corresponds to the resonant frequency. However, since the resonant frequency of each actuator differs because of manufacturing variances and also because of temperature, the Q of the notch filter is set to a small value and the design of an actuator includes a margin that enables it to suppress the resonance even when the resonant frequency is slightly shifted. When designing an actuator, therefore, consideration must be given to reducing the incidence of manufacturing variances and the effect of temperature changes, and further, to ensuring that the amplitude of the resonance is small and the frequency is high. Consideration must also be given to the fact that when the number of disks in a hard disk drive is increased, the mass of the actuator is likewise increased, and accordingly, the resonant frequency is lowered; and also to the fact that when the number of revolutions of the disks is increased, the servo bandwidth is raised. These are problems that adversely affect the operation of a notch filter.
Techniques have been developed whereby the matching of the central frequency of a notch filter with the resonant frequency of an actuator is performed during the manufacturing process. Two methods are available: one employs a head position error signal output immediately after the seeking operation is completed; the other employs a transfer function of a open loop of the head positioning.
The first method involving the employment of a head position error signal output immediately after the seeking process is completed is disclosed in Japanese Unexamined Patent Publication No. Hei 1-235082. This invention relates to a method for adjusting a notch filter in an optical disk drive, and for obtaining a resonant frequency and a resonance value of a mechanism by sweeping the gain of a servo and the central frequency of a notch filter to calculate a point at which a head position error signal has a minimum value. Many problems arise when this method is actually employed. Specifically, the mechanism does not always have a single resonance, and the presence of multiple resonances adversely affects a position error signal. As a result, a plurality of locations at which the position error signal reaches a local minimum appear, as shown in FIG.
12
. The locations the position error signal actually reaches the minimum varies in accordance with the temperature of the device and the friction produced by the movable section of the mechanism. In addition, in many cases, an apparently minimum value obtained using the secondary function shown in
FIG. 2
of the above publication is not provided, the area in which the minimum value occurs is flattened, and an error of the minimum value causes a large frequency error. In other words, it is very difficult to detect a reliable resonant frequency. Further, since the gain of a control loop is changed in order to measure the resonant frequency, the control system tends to be unstable, and mechanical components, such as an actuator, may be damaged.
Several methods for employing the transfer function of an open loop of the head positioning have been developed. The invention described in Japanese Unexamined Patent Publication No. Hei 5-109217 relates to a method for adjusting a notch filter in a magnetic disk drive. A signal having a specific frequency is input into a VCM, the transfer function of the actuator is measured by sweeping that frequency, and the central frequency of the notch filter is set at the peak of the transfer function. However, many problems will arise when this method is actually employed. Specifically, a mechanical system does not always have a single resonance, and there may be a plurality of resonances occurring at very close frequencies, such as those shown as P
1
and P
2
in FIG.
13
. When the notch filter is adjusted to one of the peaks in accordance with the transfer function, the suppression of the other peak is not possible. In paragraph [0017] in this publication, the statement that “only one frequency point is assumed to reduce the gain. If the filter constant must be adjusted at a plurality of frequency points, the measured frequency range need only be changed and the same process repeated” means that another filter can control P
3
in
FIG. 13
, and it implies that when there are a plurality of peaks, such as P
1
and P
2
, in the same measurement range, they can not be controlled by a single notch filter.
The method for employing the transfer function of the actuator is also disclosed in Japanese Unexamined Patent Publication No. Hei 5-313751. This invention is very similar to that in Japanese Unexamined Patent Publication No. Hei 5-109217. As cited in the claims, “a maximum waveform value, at which the obtained gain change characteristic is shifted from a predetermined characteristic, a frequency of the maximum waveform value and the width of the waveform are detected, and a frequency characteristic that cancels the shifted waveform is calculated to reset the constant of the notch filter,” the peak of the transfer function is detected and is employed as the central frequency of the notch filter, and the width of the resonance peak is employed as its Q. The existence of a plurality of peaks can be handled as described in paragraph [0029] in this publication, where it says that “while there are many cases, such one where more than one peak exists, generally F
1
and F
2
of the notch filter are calculated and a damping depth G is determined to enclose gain peaks that fall within the range extending from F
1
to F
2
.” However, since the precise detection of F
1
and F
2
is difficult because the actual frequency characteristic does not have a smooth shape, as shown in FIG.
3
(B) of this publication, the accuracy of the obtained Q of the notch filter is low. Further, since the central frequency of the notch filter is set to the maximum peak frequency and the other peaks are handled by increasing the width of the notch filter, the Q of the notch filter is so set that it is lower than necessary, with the result that the phase margin of the servo loop is reduced and the control characteristic is deteriorated.
Although the above publications teach the adjustment of the notch filter not only during the manufacturing process but also during normal operation, this is not a desireable approach. If the same adjustment method as used in the manufacturing process is also employed during the normal operation of the apparatus, much time is required for the adjustment and this may serve to interrupt the normal operation function.
SUMMARY OF THE INVENTION
It is one object of the present invention to realize a stable feedback control by selecting an optimal notch filter, even when in an apparatus (e.g., a hard disk drive), which employs a positioning apparatus such as an actuator for positioning a control object, and mechanic
Kagami Naoyuki
Semba Tetsuo
Tokizono Akira
Faber Alan T.
International Business Machines - Corporation
Krall Noreen A.
Raissinia Abdy
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