Hardware Reference
In-Depth Information
Figure 2.2: Closed Loop Head Positioning Servomechanism.
erence marks that define the center of the track; these reference marks are
created on the disk at the time of servo track writing. These references are
special magnetic patterns written in designated areas on the disk surface known
as servo sectors. The servo sectors are created at the time of manufacturing
and are never overwritten or erased. The closed loop servomechanism uses
the feedback signal generated by decoding the information written in these
sectors. The servo sectors and the demodulation of the written information
are explained later in section 2.3. The servo information and user data are
multiplexed in space around the track. When the disks spin, this spatial mul-
tiplexing becomes temporal multiplexing. The feedback signal constitutes of
both digitally coded track number and Position Error Sensing (PES) signal.
The PES signal is proportional to the radial distance between the track-center
created during servo track writing and the actual position of the read head.
The composite feedback signal is used by the control algorithm which is im-
plemented on a digital processor. There are separate heads for reading and
writing, but the two heads are fabricated on a single slider. Since the sus-
pension arms accessing different surfaces are mounted on the same actuator,
all heads are moved simultaneously even though the tracks and sectors are
accessed one surface at a time. The suspension provides a preload to press the
slider down towards the surface of the disk. Care must be taken while design-
ing the suspension so that the effects of different torsion and sway modes of
the drive mechanics on the servo loop are minimized.
The head positioning servomechanism moves the read/write head as fast
as possible from one track to another when asked by the host system (Track
Seek). Once the head reaches the target track, it is regulated precisely over
the track so that the PES is minimized (Track Following). Smooth settling,
i.e., transition between the track seek and the track following modes without
any jerk is another important feature expected in HDD servomechanism.
This process is carried out by another high precision mechatronics system, Servo Track
Writer, which is discussed in chapter 5.
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