(1) Field of The Invention
The present invention relates to a servo circuit for controlling the positioning of an object under servo control (hereinafter referred to as a "servo object") to a target position by speed control and then position control.
(2) Description of the Related Art
Wide use is being made of servo positioning circuits for the positioning of magnetic heads on tracks of magnetic disk devices. In such servo positioning circuits, there is a desire for a technique enabling stable positioning even at high speeds.
That is, in the speed control of conventional servo circuits, at the stage of acceleration control at the start of the speed control, due to the overly large difference between the real speed and the target speed of the voice coil motor, linear control of the control current flowing through the voice coil motor is not possible and there results a state of voltage control such that closed loop control by the servo circuit cannot be performed for that period.
If closed loop control cannot be performed, then the control current for the acceleration control flows uncontrolled. Therefore, the rise of the current is large, resulting in oscillation of the voice coil motor. At the stage of acceleration control, parasitic oscillation easily occurs and further the parastic oscillation cannot be suppressed, so turns into residual oscillation. This has the problem of reducing the seek speed and hindering high speed seek operations.
In particular, along with the increasingly higher speeds of seek operations as in recent years, the seek speed has been shortened to a value close to the cycle of the residual oscillation, making the problem of easy occurrence of residual oscillation much worse.