The present invention relates to an information storage apparatus such as a magnetic disk apparatus, and more particularly to a magnetic head positioning control system suitable for improving positioning precision.
A conventional head positioning control system for a magnetic disk apparatus controls the position of a magnetic head by reading, with the magnetic head, servo signals written in advance on the surface of a magnetic disk. Various methods are known for writing a servo signal on the surface of a magnetic disk. Of these methods, a sector servo system is mainly used because of a recent improvement in the surface record density of a magnetic disk apparatus, which system intermittently writes servo signals over the whole surface of a magnetic disk.
In addition to servo signals written on the surface of a magnetic disk, many proposals have been made to provide sensors and detect more state quantities with the sensors in order to improve controllability. Of these, a first method is to mount an accelerometer on an apparatus housing, a cover, a circuit board or the like and detect, with the accelerometer, vibrations and impacts externally applied to the apparatus in order to intercept the data record operation or detect a repulsion force relative to a seek drive force in order to feed it back to a control system. A second method is to mount a sensor on a control object such as an actuator, a carriage and a magnetic head to detect a state quantity of the control object in order to feed it back. As the latter method, the following methods have been disclosed.
JP-A-60-136972 discloses a feedback control method of feeding back a state detected with an accelerometer mounted on a magnetic head slider. JP-A-63-42073 discloses a method of feeding back a state detected with an accelerometer mounted on a carriage in order to eliminate mechanism resonance of a pivot part of a rotary carriage. JP-A-2-226560 discloses a method of controlling rigidity of an arm part by providing a sensor and actuator for detecting bending vibration of the arm part. JP-A-3-76064 and JP-A-3-192585 disclose a method of improving a loop gain in the sector servo system by mounting an accelerometer near to or on a carriage or a magnetic head, or making a sampling period of a servo loop shorter than a sector servo period by picking up a signal of the accelerometer at a frequency higher than the sampling frequency of the sector servo system and synthesizing it with a servo signal to use a synthesized signal as a feedback position signal.
Increasing a storage capacity of a magnetic disk apparatus and making the apparatus compact requires improvements in record density and data surface efficiency. Improving on the record density requires to narrow a track width. In order narrowing the track width, it is important to maintain a sufficient recording/reproducing characteristic even if a magnetic head width is narrow and to improve a positioning precision. In order to improve a positioning precision, it is necessary to provide a sufficiently wide servo bandwidth of a control system. However, with a present sector servo system, servo signals are written in a partial area of the data area. Therefore, if servo signals are written so that they can be sampled at sufficiently high speed, an efficiency of the data surface is considerably lowered and so a sufficiently wide servo bandwidth cannot be maintained generally.
It is an object of the present invention to provide a high precision control system without degrading the data efficiency of a magnetic disk surface. It is a specific object to provide a servo control algorithm using an accelerometer.
According to the invention, an accelerometer is mounted on a magnetic head or a carriage to detect an acceleration of a magnetic head along a magnetic head positioning direction and constitute the following control system.
First, the control system has an AD converter for sampling an output signal of the accelerometer at a period shorter than a period of detecting servo signals, and a microprocessor having a compensator for feeding back a signal detected with the accelerometer to improve the dynamic characteristic of a carriage and an actuator.
Second, the microprocessor constitutes: a model control system including a control object model, a first compensator and a feedback loop for controlling the control object model; and a model following control system including a second compensator for comparing a displacement and an acceleration of the control object model with a servo signal and a signal detected with the accelerator, respectively, and making error signals zero and a loop for inputting an addition signal of an input signal to the control object model and an output of the second compensator to the actuator as its drive signal.
Third, the control system has means for calculating a speed signal from the signal detected with the accelerometer and the servo signal.
Fourth, the control system has a seek servo mode for moving the magnetic head to a target track, a following servo mode for positioning the magnetic head at the center of the track, means for setting an initial value to an internal variable of a compensator in the following servo mode when the seek servo mode is switched to the following servo mode, and means for calculating the initial value from the servo signal and the signal detected with the accelerometer when the mode is switched.
Fifth, the control system has a seek servo mode for moving the magnetic head to a target track, a following servo mode for positioning the magnetic head at the center of the track, and means for supplying an impulse train obtained from the servo signal and the signal detected with the accelerometer when the seek servo mode is switched to the following servo mode.
Sixth, the control system has means for calculating an acceleration of the magnetic head from the servo signal and the signal detected with the accelerometer, means for calculating a variation per one turn of a track from the signal detected with the magnetic head accelerometer means for storing the variation, and means for adding the variation to the positioning control loop.
Seventh, the control system has means for measuring the transfer characteristic from the DA converter to the accelerometer and means for controlling a compensator of the positioning control system in accordance with the measured transfer characteristic.
The above-described means may be provided separately or in combinations.
According to the present invention, a signal from the accelerometer mounted on a magnetic head as the control object or on the carriage supporting the magnetic head is detected at an optional sampling period. This sampling period is shorter than the sampling period of the servo signal, and two state quantities including the signal from the accelerometer and the servo signal are input to the microprocessor at different sampling periods. By using these input signals, the microprocessor realizes the above-described means for solving the problems by performing arithmetic operations to be detailed in the following embodiments.
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