DISK HEAD POSITIONING

8/31/2002

Disk Head positioning. Historically, two methods have been used to position disk read/write heads to a specific cylinder/track. Stepper motor positioning is used in most floppy drives and was used in early hard drives. In the 1980s, voice coil positioning replaced stepper motor positioning in hard drives.

Stepper motors are motors that are cleverly wired so that applying power to them causes the motor to start to turn, but the rotor magnets almost immediately encounter a oppositely polarized magnetic field that stops the rotor after a small "step" Each pulse of power to a stepper motor moves it one step. Given a few gears and appropriate linkages, a set of pulses to a stepper motor can translate into repeatable, determinate, positioning of disk read/write heads. Other than noise, the principle difficulties with stepper motor positioning is that the speed to move large distances is rather low as each discrete step must be completed before the next can be started. There are also limits to how accurately heads can be positioned because the positioning is done without reference to the data on the disk. There is no simple way to trim the positioning to allow for thermal expansion of the disk platters or other phenomenon that might be encountered with recording large numbers of narrow tracks.

Voice coil positioners replace the stepper motor with a positioning coil similar to the voice coil in a speaker. The positioning of the read/write heads is a function of the voltage applied to the voice coil. Voice coil positioning allows the heads to be slewed across the disk at higher speeds than stepper motors can achieve. It also allows positioning to be fine tuned. positioning is done relative to track number information ("servos") recorded on the disk surface. Earlier disk drives used a separate servo surface. Modern drives have "embedded servos" with track number information interspersed with recorded data.

Return To Index Copyright 1994-2002 by Donald Kenney.