Dynamic magnetic information storage or retrieval – Automatic control of a recorder mechanism – Controlling the head
Reexamination Certificate
2001-10-09
2004-08-31
Hudspeth, David (Department: 2651)
Dynamic magnetic information storage or retrieval
Automatic control of a recorder mechanism
Controlling the head
Reexamination Certificate
active
06785080
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the filtering of control information in a disk drive servo control system. In particular, the present invention relates to a providing an oversampled digital filter for resonance compensation in connection with disk drive servo control systems.
BACKGROUND OF THE INVENTION
Computer disk drives store information on magnetic disks. Typically, the information is stored on each disk in concentric tracks that are divided into servo sectors and data sectors. Information is written to or read from a disk by a transducer head, mounted on an actuator arm. The actuator arm is capable of moving to position the transducer head radially over the disk. Accordingly, the movement of the actuator arm allows the transducer head to access different tracks. The disk is rotated by a spindle motor at a high speed, allowing the transducer head to access different sectors within each track on the disk.
The actuator arm is interconnected to an actuator, such as a voice coil motor (VCM) to move the actuator arm such that the transducer head can access different tracks. Operation of the actuator is controlled by a servo control system. The servo control system generally performs two distinct functions: seek control and track following. In general, the seek function is initiated when a host computer associated with the disk drive issues a command to read data from or write data to a target track on a disk. Once the transducer head has been moved sufficiently close to the target track by the seek function of the control system, the track following function of the control system is activated to center and maintain the transducer head on the target track until the desired data transfers are completed.
Typically, the transducer head will oscillate about the center line of the target track for a period of time following the transition of the servo control system from the seek mode to the track following mode. In addition, while in the track following mode, adjustments to the position of the transducer head with respect to the center line of the target track are often required. Such small adjustments are required to correct drift in the position of the transducer head relative to the target track. The precise control of the position of the transducer head relative to a target track has become increasingly important as data densities in disk drives have increased.
In a typical disk drive, a digital signal processor or microprocessor is used to implement the servo control system. The digital control output of the digital signal processor or microprocessor is converted to an analog control signal and applied to the actuator (e.g., the voice coil motor) to position the transducer head relative to the target track. The processor used to implement the servo control system is often used to also implement the read/write channel of the disk drive to reduce the cost of the disk drive. Therefore, as more processing resources are devoted to implementing the servo control system, fewer of those resources are available for read and write operations, reducing the performance of the disk drive.
The mechanical actuator assembly, including the actuator, the actuator arm, and the transducer head, has flexible resonance modes. The phase and amplitude of the flexible resonance modes are difficult to control in mechanical manufacturing processes. Therefore, notch filters have been applied around the resonant frequencies to attenuate excitation of the actuator assemblies at those frequencies. However, the resonant frequencies may be greater than the Nyquist frequency of the servo control system.
The Nyquist frequency is defined as one-half the sampling frequency of a digital system. Signals having frequencies above the Nyquist frequency result in aliasing. Aliasing causes higher frequency signals present in the system to be reflected back to a frequency below the Nyquist frequency. With respect to a servo control system, the number of servo sectors containing positioning information used to correct the position of the transducer head during track following operations, the rate of rotation of the magnetic disks, and the speed and computational power of the microprocessor or digital signal process used to implement the read/write channel and the servo control system of the disk drive, determine the sampling rate of the servo control system. In general, the number of servo sectors utilized in a disk drive, and therefore the sampling frequency of the servo control systems, is limited, because portions of the disk devoted to servo sectors reduces the area of the disk available for user data, while the computational power of the processor is limited due to cost considerations.
One approach that has been proposed to remove unwanted signals at frequencies greater than the Nyquist frequency of the system is to apply a notch filter about the frequency at which the higher frequency signal is mirrored due to aliasing. However, the placement of a notch filter in the vicinity of the cross-over frequency of the closed loop servo control system results in high phase losses. As a result, the control system can become unstable, resulting in poor tracking of the target track by the transducer head.
Another approach to filtering frequencies from the control signal that can cause undesirable resonance in the actuator assembly is to apply a symmetrical, double sampling rate notch filter to the system. According to this approach, for each instance of position information acquired from the servo sectors, the digital notch filter provides two position control outputs. The output from the digital notch filter is applied symmetrically in that the additional control signal is equally spaced between the preceding and succeeding first control signals generated in response to each set of position information. Although such systems are effective in filtering high frequencies, a relatively large proportion of digital signal processor or microprocessor resources are required. In particular, because of the symmetrical spacing of the oversampled control outputs, processing overhead is increased because two interrupts must be generated for each sampling period in order to produce the oversampled control output. Accordingly, the overall performance of the disk drive is degraded, as relatively fewer resources in the digital signal processor or microprocessor are available for other functions, such as read and write operations.
It would be advantageous to provide a disk drive that is capable of providing an oversampled disk drive servo control system filter that did not require a large amount of processor overhead to implement. Furthermore, it would be advantageous to provide a disk drive having a servo control system that was capable of providing an oversampled control output to an actuator that did not require the execution of more than one interrupt service routine during a sampling period. Furthermore, it would be advantageous to provide a disk drive servo control system that provided an oversampled control output, that had low processor overhead requirements, that was inexpensive to implement, and that was reliable in operation.
SUMMARY OF THE INVENTION
In accordance with the present invention, a method and an apparatus for providing a variable rate oversampling digital filter for resonance compensation in disk drive servo control systems are provided. The present invention generally allows frequencies corresponding to resonance modes in actuator arm assemblies of a disk drive to be filtered, even though those frequencies are in excess of the Nyquist frequency of the servo control system. In addition, such filtering is achieved with relatively little digital signal processor or microprocessor overhead.
In accordance with an embodiment of the present invention, position information received from a servo sector is applied to a proportional-integral-derivative (PID) controller and the output of the PID controller is then applied to an oversampling digital filter. The oversampling digital filter provides a plurality
Brunnett Don
Hu XiaoPing
Sun Yu
Hansra Tejpal S.
Hudspeth David
Maxtor Corporation
Tzeng Fred F.
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