1. Field of the Invention
The present invention relates to a position sensitivity adjusting method which is used for a disk apparatus and a disk apparatus using the same for positioning its heads in accordance with a servo signal on a servo surface of a disk medium and adjusts the adjustment value of an AGC (Automatic Gain Control) amplifier for acquiring the servo signal.
2. Description of the Related Art
In magnetic disk apparatuses, the track density of magnetic disks is increased to increase the memory capacity. While magnetic disks have a high track density, it is still necessary to precisely position a head to a target track. This positioning is accomplished by a servo control system.
According to the servo control system, a magnetic disk is provided with a servo surface where a servo signal is written. In accordance with this servo signal from the servo surface, the on-track control of a data head on a data surface is performed.
Even with the use of such a servo control system using the servo surface, when the ambient temperature varies, the degree of expansion/contraction differs from one magnetic disk to another. Even when servo control is executed, therefore, the data head is likely to come off the target track. To prevent this thermal off-tracking, the off-track (offset) amount of the data head is measured every given time. At the time of servo control, the offset amount is added to the control amount to execute the on-track control of the data head. This processing is called offset compensation.
FIG. 10 is a structural diagram of prior art, and FIG. 11 is an explanatory diagram for a servo signal in the prior art.
As shown in FIG. 10, two magnetic disks 90 are spun by a spindle motor 91. Of the four sides of the two magnetic disks 90, the three sides are used as data surfaces 90-1 and one side as a servo surface 90-2. A servo signal is written on the servo surface 90-2. Provided at the data surfaces 90-1 are magnetic heads (data heads) 92-1 which read and write data from and on the respective data surfaces 90-1. Provided at the servo surface 90-2 is a magnetic head (servo head) 92-2 which reads data from the servo surface 90-2.
A rotary actuator (voice coil motor) 93 moves those magnetic heads 92-1 and 92-2 in the radial direction of the magnetic disks 90 and positions the magnetic heads. An AGC (Automatic Gain Control) amplifier 94 performs automatic gain control of signals read by the servo head 92-2 to control the signal levels to a constant level.
A demodulator 95 demodulates the output of the AGC amplifier 94 to two position signals POSN and POSQ having a phase difference of 90 degrees. As shown in FIG. 11, the two position signals POSN and POSQ are sinusoidal waves having a phase difference of 90 degrees.
A controller 96 is constituted of a processor. An adder 97 adds a control amount from the controller 96 to a control amount from the demodulator 95 to control the reference value of the AGC amplifier 94.
The positioning operation of this magnetic disk apparatus is performed as follows. When receiving a seek command from a high-rank apparatus, the controller 96 executes coarse control. That is, the controller 96 generates a speed curve according to the number of tracks to the target track. Then, the controller 96 calculates the actual speed from the position signals POSN and POSQ. The controller 96 obtains a speed error or a speed difference between the target speed on the speed curve and the actual speed. Based on this speed error, the controller 96 controls the rotary actuator 93.
The controller 96 detects the head position from the position signals POSN and POSQ. When detecting that the head has reached near the target position, the controller 96 switches the control to fine control. Then, the controller 96 generates a fine control signal Fine POS as shown in FIG. 11 from the position signals POSN and POSQ. In accordance with the fine control signal Fine POS and the offset amount, the controller 96 controls the rotary actuator 93.
AS shown in FIG. 11, one period of the position signals POSN and POSQ consists of four tracks T1 to T4. The fine control signal Fine POS is generated by switching the position signals POSN and POSQ from one to the other at each boundary of tracks.
There is a difference between the levels of the signals at the inner and outer tracks of each magnetic disk 90. In this positioning control, the AGC amplifier 94 controls to make the difference at constant value. Each position signal indicates the head position, so that this position signal may lead to a variation in the position detection sensitivity. In this respect, the controller 96 adjusts the control amount of the AGC amplifier 94 to keep the sensitivity of the position signal to a predetermined level.
Conventionally, seeking over four tracks (one period of the position signal) is performed to acquire the position signals POSN and POSQ as shown in FIG. 11. The adjustment value of the AGC amplifier 94 is controlled in such a manner that voltage values at the intersections of the position signals POSN and POSQ become set values V1 and V2. This measurement is carried out at the outermost position and the innermost position of a magnetic disk. The adjustment values for the other tracks are obtained by interpolation of those measured values.
FIG. 12 is an explanatory diagram showing a boundary between tracks, and FIG. 13 is a diagram showing a waveform at the track boundary.
To increase the memory capacity of the magnetic disk apparatus, the track density should be improved. Improving the track density reduces the intervals between tracks. When the offset compensation of the data head is performed, therefore, the servo head 90-2 may be positioned at the boundary between servo tracks T1 and T2 as shown in FIG. 12.
Since this track boundary is where the two position signals POSN and POSQ are switched from one to the other, switching between the position signals POSN and POSQ occurs frequently, as shown in FIG. 12.
As shown in FIG. 13, therefore, the position signals POSN and POSQ fluctuate to thereby vibrate the head. This leads to a positioning error.