Hardware Reference
In-Depth Information
actuator as the suspension in this case is moved side-to-side unlike the linear
actuator whose movement is along the length of the load beam. Modes other
than the torsion and sway modes have small amplitude or lie in frequencies
very far away.
It is hard to control precisely the position of the head using an actuator
with lightly damped resonant modes, which are subject to variation from drive
to drive and in a single drive over time. As a result, these resonances limit
achievable bandwidth of the servomechanism. Servo engineers try to eliminate
the effects of these resonances by using notch filters in series with the amplifier
driving the VCM. But it is not very effective to use analog notch filters when
the resonance frequencies are not fixed. Application of digital filtersissuitable
for adaptation to such variations, but implementation of digital notch filter is
restricted by relatively low sampling frequency in hard disk drives. This led to
the design of multi-rate notch filter for HDD servomechanism. Design of notch
filter as well as multi-rate compensator are explained in details in chapter 3.
An alternative solution to the problems of resonance is to use other materials
that make stiff and yet light weight suspension arms.
The relatively heavier part of the actuator to which all the suspension arms
are attached is known as the E-block. If it is rigid then the dynamics of one
suspension is not coupled to that of another. In such case, each suspension
resonates by itself and does not interfere with other arms. Frequency and
damping of the resonant mode of one suspension arm are slightly different
from the resonant frequency and damping of another arm due to manufacturing
tolerances. If the E-block is flexible, resonant modes of different suspensions
interact with one another degenerating into two new modes - an in-phase mode
at lower frequency and an out-of-phase mode at higher frequency. The in-phase
mode further limits the bandwidth of the servo system.
2.5 Noise and Disturbances
The disturbances affecting the performance of HDD head positioning servo-
mechanism are contributed by some internal components of the drive as well
as sources external to the HDD. The external disturbances are typically in
the form of shock and vibration that come from the environment. For ex-
ample, a moving vehicle or a machine running in a factory or an accidental
hit by the user of desktop computer or carrying a laptop computer causes the
servo loop to be subjected under external forces. Besides these external distur-
bances, drive components and their interconnections also give rise to several
disturbing forces acting on the control loop. Even the interaction of the aero-
dynamic forces with different structures inside the drive enclosure affects the
performance of the servomechanism.
