Hardware Reference
In-Depth Information
Chapter 2
Head Positioning
Servomechanism
When an HDD is powered up, the disks are spun to a precisely regulated speed
and the heads are allowed to move radially over the disk surfaces. Limited ver-
tical movement within a very small range, self-regulating by the formation of
an air bearing surface (ABS) between the head and slider is also allowed. Ac-
curate and precise control of radial position of the head slider is done by the
head positioning servomechanism.Thisservomechanismisafeedbacksystem
consisting of a sensing element that measures the displacement of the head, a
servo motor and actuator, an amplifier, and a controller controlling the move-
ment of the actuator. In the early generations of HDD, the controller used
to be implemented using analog electronics but all modern drives come with
digital controller. Nowadays almost all practical servomechanisms, HDD head
positioning servo being one of them, use ยต-controller or digital signal processor
(DSP) to implement the controller. In this chapter, functions and principle of
operations of different components of the HDD servomechanism are discussed,
sources of noise and disturbances are explained, and the basic guidelines for
design of controller are presented.
2.1 The Servo Loop
Binary bits are stored in an HDD by setting a small area of magnetic material
coated on the disk to one of two possible polarities. This tiny area, called a bit
cel l , consists of several grains of the magnetic material alloy. If all grains in a
bit cell are magnetized in the same polarity, it is said to be storing a binary
'0'. On the other hand, a bit cell where a transition of magnetization takes
place is considered as storing a binary '1'. The bit cells are created by the
write head while the disk spins causing the bits to be arranged in concentric
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