Hardware Reference
In-Depth Information
Figure 2.3: Rotary VCM actuator.
2.2 The Actuator
TheVCMisthetorqueproducingcomponentoftheheadpositioningservo-
mechanism. When current is passed through the coil of VCM suspended in the
magnetic field produced by permanent magnets, a force (torque) is generated.
The force (torque), proportional to coil current, can be controlled by changing
the amplitude and polarity of the current. There are two types of VCM actu-
ator - (i) Linear VCM and (ii) Rotary VCM. These are shown in Figure 2.3.
In the first of these types, the coil is wound around a central yoke placed
between two permanent magnets. The coil, when energized, is free to move
forward and backward. As a result, the actuator arm attached to the coil
structure moves in and out of the yoke. The VCM is fixed rigidly to the
base plate outside the area of the disk, and the movement of the arm takes
place along a radius of the disk (Figure 2.4). With this arrangement, the
orientation of the slider with respect to track remains the same at all radial
position of the slider. In a rotary VCM, the actuator arm is pivoted at a
point between the coil structure and the suspension arm. The coil is attached
using epoxy glue to one end of the arm and the suspensions carrying sliders
to the other end. The pivot point is nearer to the coil which is suspended
in the magnetic field of permanent magnets. Force is generated whenever the
coil is energized by allowing current to pass through it. This force makes the
coil move in a way that generates torque around the pivot point and causes
thesliderstomoveonanarc.Theanglebetweentheslider'sorientationand
the track underneath varies as a function of the radial position of the track
(Figure 2.5). The orientation of the head gap with respect to the track affects
the amplitude of the readback signal. However, this effect is insignificant and
all HDDs available in the market today use rotary actuators. Another effect of
this variation in orientation of the slider is the changes in micro-jog distance
