Dynamic magnetic information storage or retrieval – Automatic control of a recorder mechanism – Controlling the head
Reexamination Certificate
2000-09-06
2004-08-03
Hudspeth, David (Department: 2697)
Dynamic magnetic information storage or retrieval
Automatic control of a recorder mechanism
Controlling the head
C360S075000
Reexamination Certificate
active
06771455
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-251927, filed Sep. 6, 1999, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
This invention relates to a control system and a control method for moving a magnetic head (to be referred to as “head” hereinafter) to a target of a head position above a disk medium (to be referred to as “disk” hereinafter) to which the head should be moved and relating to said control system and said control method for positioning the head at this target of the head position, and in particular relates to a control system and a control method for digitally comparing the head position with a predetermined value and positioning the head above a desired position on the disk due to a feedback control of a controlled object based on the comparison.
The control system for positioning the head above a desired position is mounted in a hard disk drive (to be referred to as “HDD” hereinafter). HDD is a storage device in which information is written on the disk surface of a storage medium, and/or information is read from the disk surface by the head. According to the invented control system, the head is moved to a position above the target track on the disk surface (seek control). The control system also enables the precise positioning of a head within a track range (track following control). The track is part of the disk, where information has been recorded or information can be recorded.
As a control system for positioning the HDD head above a desired position, the sector servo method is known which divides a disk surface into many servo areas and records servo data there. The servo data is a magnetic record of data for positioning a head.
In general, in the sector servo method, the servo data recorded on the disk is read out with a specific sampling interval. The sampling interval is a time interval with which the head reads servo data from the disk. Based on the servo data, an operating signal with which the head will be controlled is provided by a digital circuit. Then an electric current calculated from the operating signal drives an actuator holding the head.
The actuator has a resonance frequency because it is composed of a physical substance. When the actuator is driven, a mechanical resonance phenomenon can take place in the actuator. On the other hand, it is known that the sector servo method relying on digital circuits has difficulty to control frequencies that are higher than half the sampling frequency (referred to as Nyquist Frequency hereinafter). In HDDs, the resonance frequency of the actuator is higher than the Nyquist Frequency in many cases. For example, the Nyquist Frequency is 2-3 kHz in 2.5″ HDDs. Most of the resonance frequencies of actuators are 4 kHz or higher. In 2.5″ HDDs, therefore, the actuator suffers from resonance affected by the Nyquist Frequency made by the sector servo method relying on digital circuits.
Such a mechanical resonance phenomenon causes an unstable control of actuators and leads to a failure in positioning a head accurately above a desired position. Also during seek control, while the head moves over tracks, vibration of the actuator causes noise (seek noise).
In the head positioning system of prior art, analog-based low-pass filters and notch filters may be added to a driving circuit that provides signals for controlling actuators in order to reduce the influence of the above-mentioned mechanical resonance phenomenon.
An analog-based driving circuit, however, requires many components in its circuit, compared with a digital-based driving circuit. Many components require many steps for their mounting on a circuit board. Increase in the number of components and manufacturing processes leads to a higher manufacturing cost of the driving circuit. As a result, the analog-based driving circuit cost more than those based on the digital circuit. The analog-based driving circuit, therefore, becomes more costly than the digital-based driving circuit.
Many components requires a large circuit board for their mounting. In a large circuit board, the driving circuit becomes large as well. Thus analog-based driving circuits become larger than digital-based driving circuits.
Further, analog-based driving circuits have larger fluctuation in the properties of components, compared with digital-based circuits. Fluctuation in the component properties leads to failures in realizing desired operation. A circuit having such fluctuation in component properties cannot be shipped as a commercial product. The manufacturing yield of analog-based driving circuits becomes thereby lower than that of digital-based driving circuits.
In addition, since analog-based driving circuits are more affected by the operating environment than digital-based driving circuits, their application range and condition are restricted. Analog-based driving circuits are thus used in limited operation environments and conditions. In particular, 2.5″ HDDs are often used in operating environments in which temperature and humidity fluctuate significantly. Then the above restriction in terms of operating environments and conditions can be a serious problem for 2.5″ HDDs.
Compared with digital-based driving circuits, analog-based driving circuits are difficult to change circuit properties as desired. If the circuit properties are hard to change, it also becomes hard to change the actuator drive mode, and then separate circuits will be necessary in many cases. Then the choice of actuator drive modes is accordingly limited in analog-based driving circuits. For example, in head control systems for using analog-based driving circuits, separate circuits will be necessary for the seek control and the track following control for precision positioning of the head, if a change for operating one selected from the seek and track following controls is needed to the circuit.
As described above, since analog-based driving circuits have many problems, other methods are necessary for a solution to those problems.
In order to raise the Nyquist Frequency, the number of servo areas should be increased to raise the sampling frequency. An increase in the number of servo areas, however, leads to a longer time for the head to consume reading servo data. Then the efficiency in formatting the disk becomes accordingly low. Further, in order to increase the Nyquist frequency, the rotating speed of the disk is required to be large. A large rotating speed, however, leads to a decrease in accuracy for writing data on and reading data from the disk. For a large Nyquist Frequency, a higher performance is required in the CPU but such CPUs are expensive. Then the actuator driving circuit becomes costly.
BRIEF SUMMARY OF THE INVENTION
The primary object of the present invention is to provide a control system and a control method for positioning the head above a desired position on the disk in the disk storage device where factors causing mechanical resonance are effectively restricted and a system stability is secured by installing a notch filter that can work on frequencies higher than the Nyquist Frequency of the digital servo system and then reduce gain in arbitrary frequencies.
Another object of the present invention is to reduce gain in arbitrary frequencies that is determined by the gain and phase characteristics of each signal line by providing such an output that is equal to more than one output of operating signals provided during one sampling interval.
Still another object of the present invention is to reduce fluctuation in characteristics of the circuit for driving the actuator and enable to set the circuit characteristics as desired.
Yet another object of the present invention is to reduce gain in arbitrary frequencies without increasing the number of servo areas per track (namely, without shortening the sampling interval).
In addition, another object of the present invention is to reduce seek noise by reducing the mecha
Colon Rocio
Hudspeth David
Kabushiki Kaisha Toshiba
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