Hardware Reference
In-Depth Information
The track-following capabilities of a servo-controlled voice coil actuator eliminate the positioning
errors that occur over time with stepper motor drives. Voice coil drives are not affected by
conditions such as thermal expansion and contraction of the platters. In fact, many voice coil drives
today perform a special thermal-recalibration procedure at predetermined intervals while they run.
This procedure usually involves seeking the heads from cylinder 0 to some other cylinder one time
for every head on the drive. As this sequence occurs, the control circuitry in the drive monitors how
much the track positions have moved since the last time the sequence was performed, and a thermal-
recalibration adjustment is calculated and stored in the drive's memory. This information is then used
every time the drive positions the heads to ensure the most accurate positioning possible.
At one time, most drives had to perform the thermal-recalibration sequence every 5 minutes for the
first 30 minutes that the drive was powered on and then once every 25 minutes after that. With some
drives, this thermal-recalibration sequence was noticeable because the drive essentially stopped
what it was doing, and you heard rapid ticking for a second or so. This was often misinterpreted as
the drive having a problem reading data and having to reread it, but this was not true.
As multimedia applications grew in popularity, thermal recalibration became a problem with some
manufacturers' drives. The thermal-recalibration sequence sometimes interrupted the transfer of a
large data file, such as an audio or a video file, which resulted in audio or video playback jitter.
Consequently, some companies released special A/V (audio/visual) drives to hide the thermal-
recalibration sequences so they never interrupt a file transfer. Virtually all of today's ATA drives are
A/V capable, which means the thermal-recalibration sequences do not interrupt a data transfer. A/V-
capable ATA drives are also used in set-top boxes that are utilized for digital recording, such as the
popular TiVo device.
While we are on the subject of automatic drive functions, most of the drives that perform thermal-
recalibration sequences also automatically perform a function called a
disk sweep
. Also called
wear
leveling
by some manufacturers, this procedure is an automatic head seek that occurs after the drive
has been idle for a period of time. The disk-sweep function moves the heads to a cylinder in the outer
portion of the platters, which is where the head float-height is highest (because the head-to-platter
velocity is highest). Then, if the drive continues to remain idle for another period, the heads move to
another cylinder in this area, and the process continues indefinitely as long as the drive is powered
on.
The disk-sweep function is designed to prevent the head from remaining stationary above one
cylinder in the drive for too long, where friction between the head and platter eventually would dig a
trench in the medium. Although the heads are not in direct contact with the medium, they are so close
that the constant air pressure from the head floating above a single cylinder could cause friction and
excessive wear.
Figure 9.13
shows both a wedge and an embedded servo.
