Hardware Reference
In-Depth Information
Figure 2.4: Movement of suspension arm for linear VCM.
as a function of track, which is explained next.
We can use the diagram in Figure 2.6 for a better visualization of the
problem of micro-jog distance. It was mentioned earlier that two different
heads fabricated on a single slider are used for reading and writing of data -
bits are written with the help of a thin film inductive (TFI) head while the
read head is a magnetoresistive (MR) sensor. As a result, there always exists
a physical gap between the read sensor and the write head. The read head
is also used to sense the servo patterns from the servo sectors which is used
to derive the position feedback signal. The read head is used as the position
sensor. During the operation of data reading, it is the read head whose position
is regulated by the servomechanism, making the sensor and point of control
collocated. On the contrary, during write operation, the point of control is
the write head but position feedback comes from the read head. If the gap
between the read head and the write head is known then that information
can be taken into consideration as offset while regulating the position of the
write head. This offset is known as the micro-jog distance. When a rotary
actuator is used, the micro-jog distance is different for different radial position
of the slider, which is illustrated in Figure 2.6. This figure shows the head
slider positioned by a rotary actuator over two different tracks, Trk m and
Trk n . Since the slider moves on an arc, the micro-jog distances for these two
locations (d m and d n )aredifferent; d m <d n for the illustration shown in
Figure 2.6. In this case, the head positioning servomechanism must use track-
dependent micro-jog distance to compensate for the offset. Many factors such
as head geometry, distance between actuator pivot and spindle-center affect
the micro-jog distance and its variation as a function of radius. The micro-
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