Hardware Reference
In-Depth Information
Servo Mechanisms
Three servo mechanism designs have been used to control voice coil positioners over the
years:
Thethreedesignsareslightlydifferent,buttheyaccomplishthesamebasictask:Theyen-
able 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 repres-
ented 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 po-
sition 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 in-
terior of the HDA, it requires a clean-room environment.
A servowriter is an expensive piece of machinery, costing thousands of dollars, and often
mustbecustom-madeforaparticularmakeormodelofdrive.Somedrive-repaircompan-
ies 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 re-
written.
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 format-
