The hard disk's actuator--or more correctly, its actuator assembly, comprising the
actuator, head arms, head sliders and read/write heads--is one of the most important
performance-limiting components in the hard disk. It's also one of the least-discussed and
least-understood; I discuss it in detail in this operation
section.
The primary impact that the actuator assembly has is on positioning performance. Since
random accesses require the heads to move over the surface of the disk to the correct
location, and the actuator controls this process, the actuator assembly is the primary
influence on the drive's seek time. Seek time in turn
is the largest component of access time. In order to
improve performance, manufacturers are constantly striving to reduce seek times by
improving the speed of the actuator assembly. The first step taken was to move to voice-coil actuators from stepper-motor designs.
Today, improvement is evolutionary, done by increasing the strength of the actuator
itself, reducing the weight and size of the actuator arms and sliders, and tweaking other
design parameters.
The characteristics of the actuator also have an impact, albeit relatively minor, on transfer
performance. The reason is that the actuator has an impact on cylinder switch time, which is a component of sustained transfer rate. Again, the impact is relatively
small compared to the impact of the other factors that influence STR.
Next: Spindle Motor Speed and Power
