The head / disk tester of the invention has a housing 43 that houses spindle 44 that rotatingly supports a magnetic disk 31. The housing supports a moveable
carriage 30 that, in turn, supports a magnetic head 32. Positioning means 39 and 41 are used to move the
carriage and the magnetic head across the magnetic disk. These positioning means include
stepper motors that realize coarse positioning of the magnetic head, and a piezo
actuator 37 that is used for fine positioning. Linear encoders 40 and 42 located at both sides of the
carriage provide feedback information to a closed-loop
positioning system that controls the piezo
actuator. A set of special signals ("
servo bursts") pre-written at a given track of the magnetic disk is used as an additional source of feedback information for the same closed-loop
positioning system. This
positioning system includes a
servo analyzer 45 that reads and processes
servo burst signals from the magnetic disk, and a position controller 49 that controls the piezo
actuator. The controller contains two control loops: a proportional-integral-derivative (PID) loop and an additional servo burst loop. During any movement of the magnetic head to a prescribed command position, the servo burst feedback is turned off, and feedback from linear encoders is used by the PID loop to move the head. When the magnetic head reaches the prescribed command position, servo burst loop is turned on; it changes the command position of the PID loop in a way to keep the ratio of pre-written burst signals constant. As a result, the position of the magnetic head with respect to the data track remains the same for as long as necessary, even in unstable temperature conditions.