Hard Disk Storage - Upgrading and Repairing PCs - page 590

Itworksbysendingtheindex(orservo)signaltothepositioningelectronics,whichreturn
a feedback signal that positions the heads accurately. The system also is called servo con-
trolled , which refers to the index or servo information that accuracy dictates or controls
head-positioning.
Avoicecoilactuatorwithservocontrolisnotaffectedbytemperaturechangesasastepper
motor is. When temperature changes cause the disk platters to expand or contract, the
voice coil system compensates automatically because it never positions the heads in pre-
determined track positions. Rather, the voice coil system searches for the specific track,
guided by the prewritten servo information, and then positions the head rack precisely
above the desired track, wherever it happens to be. Because of the continuous feedback
of servo information, the heads adjust to the current 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
9.11 ) . The coil moves in and out on a track surrounded by the stationary magnets. The
primaryadvantageofthelineardesignisthatiteliminatestheheadazimuthvariationsthat
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 9.11 A linear voice coil actuator.