Emergency head unload system for magnetic disk drive

摘要

Circuit means for use in a moving head type magnetic disk drive for unloading the heads under emergency conditions, such as power loss, to avoid loss of data and head or disk damage. The circuit means include a dynamic brake circuit which responds to an emergency condition to electrically short a positioner coil to thereby reduce the head support structure radial velocity to substantially zero. After a short delay, a velocity control circuit is activated, first in a low current mode to move the support structure at a controlled velocity toward the disk outer edge and then in a high current mode to enable the support structure to climb an unload ramp to move the heads axially away from the disk surface. The dynamic brake and velocity control circuits are powered by a capacitor which is charged during normal operation.