Hardware Reference
In-Depth Information
The actuator arms are made of solid steel or aluminum of signi
fi
cant thick-
ness not suitable for holding the read-write heads over the disk surface. An
extended arm, known as the suspension, carries the head slider. The suspen-
sion is made of thin sheet of stainless steel and is attached to the actuator arm.
In linear actuator, the suspension arm and the movement of the actuator arm
are collinear. The
fi
rst generation rotary actuators had their suspension arms
turned sideways to mimic the motion of linear actuator. Though the actuator
arm moves within a restricted angle (about 30
◦
), the motion of the suspension
arm follows a nearly straight line along the radius of the disk. In modern days,
the actuator arm and suspension of the rotary actuator are collinear making
the movement of the slider follow an arc and not a straight line. Each head
slider is attached to the tip of a suspension. Usually, there are as many sliders
in an HDD as the number of disk surface. All suspension arms are attached to
a single piece of actuator. Even if it is intended to move only one head from
an initial position at radius r
i
to a
fi
nal position at radius r
f
,allheadsare
moved together.
The slider
fl
oats over the spinning disk which has a certain degree of rough-
ness relative to the
fl
y height. The suspension provides a force on the slider
in the direction into the disk to counteract the upward aerodynamic forces of
the air bearing surface. This force must act precisely in the proper location;
otherwise a twisting force acting on the slider will cause one of its corners to be
too close to the disk and the other too far from the disk surface. In addition to
providing this downward force, the suspension must allow the slider to rotate
in the pitch and roll directions so that it can stay close to the surface despite
the presence of asperities on the disk surface. Any torque applied by the sus-
pension on the slider negatively affects the
fl
ying characteristics of the slider.
A good suspension gimbal design has very low rotational stiffness so that the
magnitude of the torque caused by slight deviation of static attitude from the
nominal is minimized. This asks for compliance in roll and pitch directions
of the suspension, but the servomechanism asks for high stiffness in the other
direction so that it can be swang back and forth rapidly during tracking and
seeking without producing excessive vibration. It should be pointed out here
that the HDD servomechanism uses error signal sensed by the read head at-
tached to one end of the actuator/suspension arm and the input torque for
positioning the head is generated by the VCM on the other end of the actu-
ator. The servo controller cannot accurately control the position of the head
if the structure connecting the sensing point and the actuating point is exces-
sively
fl
exible. The stiffer this intervening structure is, the more rapidly and
precisely the servo controller can position the head to compensate for off track
disturbances, i.e., more servo bandwidth.
In order to address this issue of competing requirements, low stiffness for
the gimbal and high stiffness for tracking, suspension arms are usually made
of two separate components - the flexure and the load beam. The slider is
adhesively bonded to the
fl
exure made of thin material and designed to give
