Hardware Reference
In-Depth Information
Figure 9.13. A wedge and an embedded servo.
Wedge Servo
Early servo-controlled drives used a technique called a
wedge servo
. In these drives, the gray-code
guidance information is contained in a “wedge” slice of the drive in each cylinder immediately
preceding the index mark. The index mark indicates the beginning of each track, so the wedge-servo
information was written in the PRE-INDEX GAP, which is at the end of each track. This area is
provided for speed tolerance and normally is not used by the controller.
Some controllers had to be notified that the drive was using a wedge servo so they could shorten the
sector timing to allow for the wedge-servo area. If they were not correctly configured, these
controllers would not work properly with the drive.
Another problem was that the servo information appears only one time every revolution, which means
that the drive often needed several revolutions before it could accurately determine and adjust the
head position. Because of these problems, the wedge servo never was a popular design; it no longer
is used in drives.
Embedded Servo
An embedded servo is an enhancement of the wedge servo. Instead of placing the servo code before
the beginning of each cylinder, an embedded servo design writes the servo information before the
start of each sector. This arrangement enables the positioner circuits to receive feedback many times
in a single revolution, making the head positioning much faster and more precise. Another advantage
is that every track on the drive has its own positioning information, so each head can quickly and
efficiently adjust position to compensate for any changes in the platter or head dimensions, especially
for changes due to thermal expansion or physical stress.
Most drives today use an embedded servo to control the positioning system. As in the wedge servo
design, the embedded servo information is protected by the drive circuits, and any write operations
are blocked whenever the heads are above the servo information. Thus, it is impossible to overwrite
the servo information with a low-level format, as some people incorrectly believe.
Dedicated Servo
