Hardware Reference
In-Depth Information
can be square, cylindrical, or flat. Stepper motors are outside the sealed HDA, although
the spindle of the motor penetrates the HDA through a sealed hole.
Stepper motor mechanisms are affected by a variety of problems, but the greatest one is
temperature. As the drive platters heat and cool, they expand and contract, and the tracks
on the platters move in relation to a predetermined track position. The stepper mechanism
can't move in increments of less than a single track to correct for these temperature-in-
duced errors. The drive positions the heads to a particular cylinder according to a prede-
termined number of steps from the stepper motor, with no room for nuance.
Floppy disk drives position their heads by using a stepper motor actuator. Stepper motor
actuatorswerecommonlyusedonharddrivesmadeduringthe1980sandearly1990swith
capacities of 100MB or less. The accuracy of the stepper mechanism is suited to a floppy
disk drive because the track densities usually are nowhere near those of a hard disk. The
track density of a 1.44MB floppy disk is 135 tracks per inch, whereas HDDs have densit-
iesofmorethan5,000tracksperinch.AllHDDsbeingmanufacturedtodayusevoicecoil
actuators because stepper motors can't achieve the degree of accuracy necessary.
Voice Coil Actuators
ThevoicecoilactuatorsusedinvirtuallyallHDDsmadetoday—unlikesteppermotorac-
tuators—use a feedback signal from the drive to accurately determine the head positions
and adjust them, if necessary. This arrangement provides significantly greater perform-
ance, accuracy, and reliability than traditional stepper motor actuator designs.
A voice coil actuator works by pure electromagnetic force. The construction of the mech-
anism is similar to that of a typical audio speaker, from which the term
voice coil
is de-
rived. An audio speaker uses a stationary magnet surrounded by a voice coil, which is
connected to the speaker's paper cone. Energizing the coil causes it to move relative to
thestationarymagnet,whichproducessoundfromthecone.InatypicalHDD'svoicecoil
system, the electromagnetic coil is attached to the end of the head rack and placed near a
stationary magnet. No physical contact occurs between the coil and the magnet; instead,
the coil moves by pure magnetic force. As the electromagnetic coils are energized, they
attractorrepulsethestationarymagnetandmovetheheadrack.Systemsliketheseareex-
tremely quick, efficient, and usually much quieter than systems driven by stepper motors.
Unlike a stepper motor, a voice coil actuator has no click-stops or detent positions; rather,
a special guidance system stops the head rack above a particular cylinder. Because it has
no detents, the voice coil actuator can slide the heads in and out smoothly to any position
desired. Voice coil actuators use a guidance mechanism called a
servo
to tell the actuator
wheretheheadsareinrelationtothecylindersandtoplacetheheadsaccurately atthede-
siredpositions.Thispositioningsystemofteniscalleda
closed loop feedback mechanism
.
