between them. Spindle of a typical HDD used for desktop application supports
4 disks. However, actual number of disks depends on the desired capacity of
the drive and the capacity of individual disks. Several years ago, a single disk
could record approximately 10 GB of data, and 4 such disks were used in a
40 GB drive. Technology involved in producing head and media has improved
tremendously over last decade. Servomechanism and channel electronics were
also improved. Together they made it possible to record more than 60 GB on
a single surface of a 3.5 inch disk. An HDD of 60 GB capacity doesn't even
need to use both surfaces of the disk.
Read-Write Head Positioning Actuator
Movement of the read-write head between different radii of the disk surface is
effectuated by an actuator. The actuator used in RAMAC supported a single
pair of head, and moved radially to access different tracks on a surface and ver-
tically to reach different surfaces. It used cables and pulleys. Such mechanism
was replaced by hydraulic actuators in the series of drives (IBM 2314) intro-
duced in the late sixties and early seventies. Some of the early generations
of HDD used stepper motors as actuator. These actuators, controlled open
loop, performs well within the speci fi cations when track density is low, i.e., the
space between two adjacent tracks is quite large. With increasing density of
data tracks, open loop control failed to perform and closed loop control using
VCM replaced the stepper motors. The fi rst linear VCM actuator was devel-
oped by IBM in 1965, followed by the rotary VCM actuator . The VCM is
a moving coil type actuator, in which a coil is held suspended in the magnetic
fi eld produced by pairs of permanent magnets fi xed to the casing of the HDD.
The suspended coil is free to move within a restricted area. When a current is
passed through the coil, it moves (the motion is governed by Faraday's Law).
The actuator arm is glued to this coil; a movement of the coil makes the actu-
ator move. In a linear VCM, the actuator arm moves in and out of the yoke
holding the permanent magnets. On the contrary, the coil of the rotary VCM
moves sidewise and, with the arm pivoted, the tip of the actuator arm moves
on an arc (Figure 1.5).
Figure 1.5: Linear (left) and Rotary (right) voice coil motor actuator