Magnetic disk device and method

A magnetic disk device comprising:

• a magnetic disk;
• a magnetic head configured to write and read data to and from the magnetic disk;
• an actuator arm having the magnetic head at a distal end;
• a motor configured to move the actuator arm to move the magnetic head in a radial direction of the magnetic disk;
• a drive circuit configured to generate a first current that drives the motor by an amount according to an instruction value and supply the generated first current to the motor;
• a first sensor configured to detect a predetermined physical quantity;
• and
• a controller configured to:
• detect a position of the magnetic head;
• generate a first instruction value so as to reduce a deviation amount of the detected position of the magnetic head from a target position of the magnetic head;
• correct the first instruction value by a first feedforward control based on a first detection value obtained by the first sensor;
• input a second instruction value, which is a value obtained by correcting the first instruction value, to the drive circuit as the instruction value;
• and
• execute a first operation in accordance with occurrence of a set event, the first operation being an operation of acquiring a first waveform that is a waveform of the deviation amount, acquiring from the first waveform a first frequency band that is a frequency band whose amplitude is greater than a first threshold, and adjusting a coefficient of a transfer function of the first feedforward control so as to suppress the first detection value in a second frequency band different from the first frequency band and output a correction amount for the first instruction value based on the first detection value in the first frequency band.