Handbook for Sound Engineers

1086 Chapter 28

maintains alignment of the coil as the head moves in a
straight line along a radius of the disk.
A rotary actuator is a pivoting device that moves the
head in an arc across the surface of the disk. The arc
causes the head to depart from absolute tangency to the
disk, but the error is relatively small since only 30% of
the disk’s radius contains data. The actuating force is
once again generated by a coil and magnet, but in this
case the components are curved to match the pivoting
motion. The support bearings and structure of the rotary
actuator are simpler and less expensive than the linear
actuator’s sled.
The actuators are part of a closed loop control
system that moves the head to the appropriate radius on
the disk. The desired address comes from a translator
that converts a data address into a physical radius. The
feedback to the control system is the actual data that is
being read from the disk. Older systems used one
surface of one disk in the stack of disks for nothing but
positioning information. These dedicated servo drives
contained prerecorded positioning information defining
the radius and the angle of rotation. The actuator would
move to match the desired radius to the data being read
from the servo platter.
Newer drives save the expense of a servo platter by
using information written on the normal data surfaces.
These embedded servo schemes have short blocks of
address information scattered around each circular track
at regular intervals. These systems can also sense
changes in the readout pattern when the head begins to
move off the centerline of the data track.
Embedded servos have allowed designers to greatly
increase the track density on the surface of the disk.
Any small changes due to temperature and wear are
actively sensed at the exact point where the data is
being written and read, not at a remote location on
another disk. Our example drive uses an embedded
servo to pack 40,000 tracks per inch of radius.
The current trend is to utilize digital signal processor
(DSP) chips for the tracking servo and spindle motor
control. Other electronics tasks include a buffered
digital data interface with the host computer, error
detection and correction and encode/decode of the data
to optimize the read/write process.
All of the mechanisms and servos in a disk drive
would be useless if any dirt gets into the system. When
a head is flying at a spacing of less than 1μinch, even a
particle of cigarette smoke can cause a catastrophic
collision that might destroy the head and/or disk. To
avoid contamination problems, the entire head and disk
assembly are enclosed in a clean environment. Any air
entering the sealed head/disk assembly for cooling or

atmospheric pressure balancing passes through a filter

that traps all dirt.

28.5.8 Hard Disk Electronics

The hard disk drive also features very dense circuit

packaging. A typical drive has less than 20 in^2 of circuit

board with just a few highly integrated chips. The block

diagram of Fig. 28-55 shows the basic functions that are

squeezed into this small space.

The spindle motor controller provides the servo loop

that turns the disk at a constant speed. The controller

also provides the acceleration and deceleration profiles

during startup and shutdown.

The actuator servo controls the linear or rotary voice

coil motor that positions the head actuator at the proper

radius of the disk. This servo must provide rapid seeks

to the desired data and track any eccentricities or other

disturbances that might cause a tracking error. The

current trend is to program custom DSP chips to serve

as digital servos for both the actuator servo and spindle

motor controllers.

The data path circuitry provides many of the inter-

leaving, error correction, and modulation code functions

described in conjunction with the digital tape recorder

above. In addition, the interface provides data format-

Figure 28-55. Block diagram of a hard disk drive.

Host computer

interface

To computer

Data buffer

Error correction

Data

encode/decode

Read and write

electronics

Read/write

heads

Supervisor

Spindle motor

controller

Spindle motor

Head position

controller

Head actuator