On the other hand, voice coil actuators are fast, not affected by temperature changes, and
extremely reliable. The type of actuator used in a disk drive tells you a lot about the
drive’s overall performance and reliability.Stepper Motor Actuators
Asteppermotoris an electrical motor that moves in a series of steps. The motor cannot stop be-
tween steps and must advance from one step to the next to operate. On a disk drive that uses
a stepper motor actuator to move the read/write heads, the stepper motor is located outside
of the HDA and connects to the head arm gang through a sealed hole in the HDA case.
The actual mechanism that connects the stepper motor to the heads is either a flexible
steel band that is wrapped around the motor’s spindle or through a rack-and-pinion gearing
arrangement. Each step of the motor typically represents one track on the disk surface. So,
in order to move 30 tracks, in or out, the stepper motor must move to the 31 step in the
appropriate direction. A big problem with this approach is that over the life of the disk
drive, the heads, head arms, and other mechanical parts of the disk drive may drift
slightly from their original positions. A disk drive using a stepper motor actuator posi-
tions the heads over the disk with ablind location systembecause the heads are at the
mercy of the stepper motor to place them over the correct part of the disk.Voice Coil Actuators
Avoice coil actuatoris used in many disk drives with capacities above 40MB, nearly all
disk drives with 80MB of capacity or more, and virtually all high-end hard disk drives. It
gets its name from the construction of its core mechanism, which is very similar to that
used in the voice coil of a typical audio speaker. Audio speakers use a large magnet
enclosed by a voice coil that is directly connected to the paper speaker cone. By energizing
the coil, it interacts with the magnet to produce sound from the speaker cone.
On a hard disk drive, the electromagnetic coil is placed on the end of the head gang,
and then positioned near a stationary magnet. The coil and the magnet never touch, but
as the coil is energized with positive or negative polarity, the head gang is attracted to or
pushed away from the stationary magnet. Because there are no real moving parts in the
actuator itself, voice coil actuators are fast and very quiet.
Unlike the stepper motor actuator, a voice coil system does not have predetermined
stepstousetopositiontheheads.Instead,itusesaguidancesystemthatisabletopositiona
head above a particular track on the disk. This system, called aservo, tells the actuator
exactly where the heads are in relation to the tracks and cylinders on the disk and when the
heads are over the target locations. Unlike the blind system used by the stepper motor,
voicecoilactuatorsreceivefeedbacksignalsfromtheharddiskdrivefindheadpositions.
Nearly all voice coil servo systems use arotary voice coil actuator. At one time, another
type was used, thelinear voice coil actuator, but it was too heavy and slow for today’s
faster,higher-densitydiskdrives.Rotaryactuatorsattachtheircoiltotheendofanactuator
arm that is mounted like a pivot. As the stationary magnet moves the coil, the head arm
swings in and out moving the heads over the surface of the disk. The advantage of this
system is that it is lightweight and very fast. The disadvantage of this system is that(^180) PC Hardware: A Beginner’s Guide