Hardware Reference
In-Depth Information
The disadvantage of a rotary system is that as the heads move from the outer to the inner cylinders,
they rotate slightly with respect to the tangent of the cylinders. This rotation results in an azimuth error
and is one reason the area of the platter in which the cylinders are located is somewhat limited. By
limiting the total motion of the actuator, the azimuth error is contained to within reasonable
specifications. Virtually all voice coil drives today use rotary actuator systems.
Servo Mechanisms
Three servo mechanism designs have been used to control voice coil positioners over the years:
• Wedge servo
• Embedded servo
• Dedicated servo
The three designs are slightly different, but they accomplish the same basic task: They enable the head
positioner to adjust continuously so it is precisely positioned above a given cylinder on the disk. The
main difference between these servo designs is where the gray code information is actually written on
the drive.
All servo mechanisms rely on special information that is written to the disk when it is manufactured.
This information is usually in the form of a special code called a gray code —a special binary
notational system in which any two adjacent numbers are represented by a code that differs in only
one bit place or column position. This system enables the head to easily read the information and
quickly determine its precise position.
At the time of manufacture, a special machine called a servowriter writes the servo gray code on the
disk. The servowriter is basically a jig that mechanically moves the heads to a given reference
position and then writes the servo information at that position. Many servowriters are themselves
guided by a laser-beam reference that determines its own position by calculating distances in
wavelengths of light. Because the servowriter must be capable of moving the heads mechanically, the
process requires either that the lid of the drive be removed or that access be available through special
access ports in the HDA. After the servowriting is complete, these ports are usually covered with
sealing tape. You often see these tape-covered holes on the HDA, usually accompanied by warnings
that you will void the warranty if you remove the tape. Because servowriting exposes the interior of
the HDA, it requires a clean-room environment.
A servowriter is an expensive piece of machinery, costing thousands of dollars, and often must be
custom-made for a particular make or model of drive. Some drive-repair companies have
servowriting capability, which means they can rewrite the servo information on a drive if it becomes
damaged. If a servowriter is not available, a drive with servo code damage must be sent back to the
drive manufacturer for the servo information to be rewritten.
Fortunately, damaging the servo information through the disk read and write processes is impossible.
Drives are designed so the heads can't overwrite the servo information, even during a low-level
format. One myth that has been circulating (especially with respect to ATA drives) is that you can
damage the servo information by improper low-level formatting. This is not true. An improper low-
level format can compromise the performance of the drive, but the servo information is protected and
can't be overwritten. Even so, the servo information on some drives can be damaged by a strong
adjacent magnetic field or by jarring the drive while it is writing, which causes the heads to move off
track.
 
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