In most disk drives, a representative example of which is the hard disk drive, a magnetic head (hereinafter referred as “head”) records and reproduce data on the from a disk that is a magnetic recording medium. In the disk drive, the disk is secured to the shaft of a spindle motor and can be rotated by the spindle motor (SPM).
In the disk drive of such a structure, the disk may undergo a phenomenon called disk runout, because of, for example, the error in positioning the spindle motor. If the disk runout occurs, the servo track (servo cylinder) will deviate from the rotational orbit around the rotational center of the disk (i.e., rotational center of the SPM). (Thus, so-called “servo track runout occurs.) The disk runout results in dynamic offset (DO), changing the read/write (R/W) offset as the disk rotates once.
In the disk drive, the head has a read head and a write head, which are spaced apart from each other. Since the read head and the write head are separated from each other, an offset (positional displacement) of a specific value exists between the track loci of the read head and write head in the radial direction of the disk. This offset shall hereinafter be referred to as “R/W offset.”
The disk drive has a servo control function of performing write dynamic offset control (WDOC) for controlling the dynamic offset. The write dynamic offset control adjusts the R/W offset value at the time of writing data to the disk. In order to perform the WDOC appropriately, the value of disk runout (hereinafter referred to, when necessary, as “runout value” or “runout data”) must be measured with high precision and the data representing this value must be stored in a memory or the disk. Methods of measuring the runout value have hitherto been proposed (see, for example, Jpn. Pat. Appln. KOKAI Publications Nos. 9-128915 and 9-330571).
In the method disclosed in the prior-art documents specified above, a servo signal is read and the runout is measured while the head remains physically stopped and stored in a memory in the form of a runout correction table. The controller uses the runout value in the table, correcting the position of the head. The head can thereby write data to a data track truly circular around the rotational center of the disk. The methods disclosed in the above-identified documents are designed to generate and store rotational locus servo data that controls the head, causing the same to move along its rotational locus, relative to the disk that keep rotating. The controller controls the head in accordance with the rotational locus servo data.
The method disclosed in the above-identified documents corrects the runout, by using the runout data obtained by measuring the runout in an inner-periphery push scheme or a servo-free scheme. The head is thereby controlled to move along a truly circular locus, with respect to the disk.
In the inner-periphery push scheme, the actuator (i.e., carriage holding the head) is pushed to the stopper provided at the inner periphery of the disk. In this state, the head reads the servo data (position data), thereby measuring the disk runout. However, the method is disadvantageous in that fine dust particles are made (this event is known as “contamination”) as the actuator is pushed onto the stopper every time the drive is activated. The dust particles may adversely influence the disk drive. In view of this, the method needs improvement to be utilized in practice.
In the servo-free scheme, the current of a specific value is supplied to the voice coil motor (VCM) of the actuator, by means of feed-forward control. At this point, the servo data (position data) is read, thereby calculating the runout value. This method, however, can hardly achieve stable measuring of the runout if disturbance occurs, because the head position deviates in the radial direction of the disk.