Hardware Reference
In-Depth Information
position of the track at all times. For example, as a drive warms up and the platters expand, the servo
information enables the heads to “follow” the track. As a result, a voice coil actuator is sometimes
called a track-following system.
The two main types of voice-coil positioner mechanisms are
• Linear voice-coil actuators
• Rotary voice-coil actuators
The two types differ only in the physical arrangement of the magnets and coils.
A linear actuator moves the heads in and out over the platters in a straight line (see
Figure 9.12
)
. The
coil moves in and out on a track surrounded by the stationary magnets. The primary advantage of the
linear design is that it eliminates the head azimuth variations that occur with rotary positioning
systems. (
Azimuth
refers to the angular measurement of the head position relative to the tangent of a
given cylinder.) A linear actuator does not rotate the head as it moves from one cylinder to another,
thus eliminating this problem.
Figure 9.12. A linear voice coil actuator.
Although the linear actuator seems to be a good design, it has one fatal flaw: The devices are much
too heavy. As drive performance has increased, the desire for lightweight actuator mechanisms has
become important.
Rotary actuators also use stationary magnets and a movable coil, but the coil is attached to the end of
an actuator arm. As the coil moves relative to the stationary magnet, it swings the head arms in and
out over the surface of the disk. The primary advantage of this mechanism is its light weight, which
means the heads can accelerate and decelerate quickly, resulting in fast average seek times. (Refer to
Figure 9.10
, which shows a rotary voice coil actuator.)