1.  Field of the Invention
The present invention relates to a disk device and a disk device calibrating method for performing stable high-speed movement by a feedforward servo control for movement of a head to a target track. 
Recently, advances have been made in making the capacity a disk device larger and increasing the speed of the disk device, and making of a servo technique for moving a head at a higher speed is required.  However, in a servo technique which moves a head, only by feedback control, delay of a driving signal is large so that the delay of the driving signal cannot be ignored with respect to a seeking time, and thereby, high-speed movement cannot be achieved. ( In the feedback control, the head is driven using a feedback error between target information for moving the head to a target position and demodulated information which is obtained as a result of demodulating feedback information from the head.)  Therefore, a feedforward control has been used in general, in which a current waveform which moves the head is previously stored as feedforward information, which is added to the feedback error, and the head is controlled thereby. 
2.  Description of the Related Art
With reference the following figures, a disk device in the related art will now be described. 
FIG.  1 shows an example of an arrangement of the disk device in the related art, in which feedforward servo control is performed. 
As shown in FIG.  1, the disk device includes servo control circuit 21, VCM (Voice Control Motor) 22, a power amplifier 23 and a position demodulating circuit 24.  The servo control circuit 21 performs servo control for moving a head 33 to a target track on a disk 25.  The VCM 22 moves the head 33 to a target position.  The power amplifier 23 drives the VCM 22 using a driving current which is output from the servo control circuit 21.  The position demodulating circuit 24 converts position information which is provided from the head 33 in a feedback manner into a digital signal. 
Further, the servo control circuit 21 includes a servo control device 26, a target information table 27, a demodulated information table 28, a feedforward table 29, a detection control device 30, a comparator 31 and an adder 32.  The servo control circuit 21 has a function of performing the servo control for desired movement of the head 33.  The servo control device 26 controls a series of sequences of the servo control for moving the head 33 to a target track on the disk 25.  The target information table 27 stores target information.  The demodulated information table 28 stores demodulated information.  The feedforward table 29 stores feedforward information which is common for all the disk devices.  The detection control device 30 acts as a detection control unit.  The comparator 31 compares the target information and the demodulated information with one another and sends a feedback error to the detection control device 30.  The adder 32 adds the feedforward information and the feedback error to one another and drives the power amplifier 23. 
In the above-described arrangement, when a user of the disk device performs an operation for reading data stored in the disk 25, the servo control device 26, which receives a reading instruction from the outside or externally, performs the servo control for movement of the head 33.  This operation will be described with reference to an operation flowchart shown in FIG.  2. 
The VCM 22 moves the head 33 to a measurement starting track which corresponds to an initial value of position measurement in accordance with the instruction of the servo control device 26 (in a step S101). 
The servo control device 26 reads a parameter for moving the head 33, for example, such as that shown in FIG.  3A, from the feedforward table 29, and supplies a current to the power amplifier 23, which current corresponds to the parameter.  The power amplifier 23, having the current supplied thereto, drives the VCM 22.  As a result of being driven by the VCM 22, the head 33 starts seeking for the specified number of tracks (in a step S102).  The above-mentioned parameter of the feedforward table 29 is previously set. 
The head 33 sends position information from seek start (in a step S102) to seek end (in a step S104) in analog values, such as that shown in FIG.  3B, to the position demodulating circuit 24.  The position demodulating circuit 24, receiving the position information, converts the analog signal to a digital signal at each sampling position, and sends demodulated position information, for example, such as that shown in FIG.  8C, to the demodulated information table 28.  The information demodulated table 28 stores the information (in a step S103).  The head 33 finishes the seeking (in the step S104), and enters an on-track condition at the target position (in a step S105). 
The comparator 31 obtains the difference between the demodulated position information stored in the demodulated information table 28 and the target position information previously stored in the target information table 27, for example, such as that shown in FIG.  3D. Then, the comparator 31 sends the feedback error, for example, such as that shown in FIG.  8E, to the detection control device 30.  The detection control device 30, receiving the feedback error, determines, based on a predetermined determination reference range, whether the feedback error is within the range (in the step S106). 
When the feedback error is within the predetermined range (Yes in the step S106), the detection control device 30 informs the servo control device 26 that the head 33 has moved to the target position.  Thus, the user of the disk device can cause the disk device to read the data recorded on the disk 25. 
When the feedback error is out of the predetermined range (No in the step S106), the detection control device 30 informs the servo control device 26 that the head 33 has not moved to the target position.  The servo control device 26, receiving the information, instructs the VCM 22 to move the head 33 to the measurement starting track which corresponds to the initial value of the position measurement (in a step S107).  Then, the servo control device 26 again reads, from the feedforward table 29, the parameter for moving the head 33, and supplies, to the adder 32, the current value corresponding to the parameter.  The adder 32 adds the feedback error to the current value, and supplies the addition result to the power amplifier 23.  The power amplifier 23, having the current supplied thereto, drives the VCM 22, and the head 33 again starts seeking as a result of being driven by the VCM 22. 
The servo control circuit 21 repeats the operation from the step S101 to the step S107 until the feedback error becomes within the predetermined range. 
If the disk device is ideal, the head 33 can move to the target track only with the feedforward information. 
However, in an actual operation, because the performance of each component varies among disk devices, it is not possible to provide an accurate movement of the head 33 in a condition where the feedforward information is fixed among all the disk devices.  In the servo control, the feedback error between the target position information and the demodulated position information is added to the feedforward information, and head movement is performed also using the feedback control.  Thereby, delay occurs due to the feedback loop, and high-speed seeking cannot be achieved. 
Further, as the track density increases, the determination reference range for determining whether the head has reached a target accurately becomes narrower.  However, in the related art, the feedforward information in a stage of simulation or debug of the disk device is commonly stored in a ROM or the like for all the disk devices.  Therefore, a position error, occurring when the head has moved only in accordance with the feedforward information, cannot be corrected by using the feedback control.  Thus, it is difficult to perform stable servo control.  Further, due to the same reason, a margin for externally applied shake or shock, or variation of performances of components among disk devices is reduced.