Hardware Reference
In-Depth Information
Stepper Motor Actuators
A stepper motor is an electrical motor that can “step,” or move, from position to position, with
mechanical detents or click-stop positions. Stepper motors can't position themselves between step
positions; they can stop only at the predetermined detent positions. The motors are small (between 1
inch and 3 inches) and 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-induced errors.
Floppy disk drives position their heads by using a stepper motor actuator. Stepper motor actuators
were commonly used on hard drives made during the 1980s and early 1990s with capacities of
100MB or less. All HDDs being manufactured today use voice coil actuators because stepper motors
can't achieve the degree of accuracy necessary.
Voice Coil Actuators
The voice coil actuators used in virtually all HDDs made today—unlike stepper motor actuators—
use a feedback signal from the drive to accurately determine the head positions and adjust them, if
necessary. This arrangement provides significantly greater performance, accuracy, and reliability than
traditional stepper motor actuator designs.
A voice coil actuator works by pure electromagnetic force. The construction of the mechanism is
similar to that of a typical audio speaker, from which the term
voice coil
is derived. 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 the stationary magnet, which produces sound from the
cone. In a typical HDD's voice coil 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 attract or repulse the stationary magnet and move the head rack. Systems like these are extremely
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 where the heads are in relation to the
cylinders and to place the heads accurately at the desired positions. This positioning system often is
called a
closed loop feedback mechanism
. It works by sending the index (or servo) signal to the
positioning electronics, which return a feedback signal that positions the heads accurately. The system
also is called
servo controlled
, which refers to the index or servo information that accuracy dictates
or controls head-positioning.
A voice coil actuator with servo control is not affected by temperature changes as a stepper motor is.
When temperature changes cause the disk platters to expand or contract, the voice coil system
compensates automatically because it never positions the heads in predetermined track positions.
Rather, the voice coil system searches for the specific track, guided by the prewritten servo
information, and then positions the head rack precisely above the desired track, wherever it happens
to be. Because of the continuous feedback of servo information, the heads adjust to the current
