For mobile applications, the emphasis is on power and
environmental robustness, with slimness and low weight being
desirable. The favorite design is a ‘2.5-in. form factor’ with one or
two 65 mm diameter disks spinning at typically 5400 rpm. For
desktop applications, price per gigabyte rules and the favorite
design is the ‘3.5-in. form factor’ with up to five 95 mm disks
spinning at 7200 rpm. For ‘server’ or enterprise applications, the
emphasis is on performance (data-rate and accesses per second)
and on reliability, with power becoming an increasingly important
consideration. Here there is more variety of both form factor and
disk diameter. Spindle speeds are much higher at 10,000 or
15,000 rpm and the actuator access times are correspondingly
shorter.
The actuator must be very light (low inertia) in order to rapidly
move the heads from track to track radially across the disk
without consuming excessive power or generating too much heat.
However, it must also be very stiff so that a high-bandwidth
control loop can be closed for accurate track-following. Bandwidths
of several kHz are required to counteract disturbances
from turbulence inside the enclosure as well as from external
vibrations. These bandwidths are similar to those used to transmit
telephone conversations. To boost bandwidth further and to
progress to even higher track-densities, it will be necessary at
some point to include a secondary actuator mounted close to each
individual head. Some trial products have been made along these
lines but current HDD products have so far universally managed
to avoid the complication of a secondary actuator.