Method and apparatus for ramp loading in a hard disk drive using pulse width modulation

This invention relates to improvements in methods and circuits for controlling a velocity of a read/write head of a disk drive, and more particularly, to obtaining precise control of the movement of a voice coil motor by measuring a back electromotive force (BEMF) voltage generated in the voice coil. A method and apparatus to control application of a drive current to a voice coil in a hard disk drive system modulates a periodic waveform between either a reference level and a signal associated with a stored back electromotive force (BEMF) in the voice coil. The drive current is applied to the voice coil during periods of time when the periodic waveform is modulated by the signal associated with the stored BEMF. The amplitude and duration of the drive current correspondingly decreases as the BEMF in the voice coil decreases. Thus, the voice coil is controlled based on its BEMF and is provided with decreasing power to move a head mechanism to or from a ramp assembly.