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Head Actuator Mechanisms

Possibly more important than the heads themselves is the mechanical system that moves them: the head actuator. This mechanism moves the heads across the disk and positions them accurately above the desired cylinder. Many variations on head actuator mechanisms are in use, but all of them can be categorized as being one of two basic types:

  Stepper motor actuators
  Voice-coil actuators

The use of one or the other type of positioner has profound effects on a drive's performance and reliability. The effect is not limited to speed; it also includes accuracy, sensitivity to temperature, position, vibration, and overall reliability. To put it bluntly, a drive equipped with a stepper motor actuator is much less reliable (by a large factor) than a drive equipped with a voice-coil actuator.

The head actuator is the single most important specification in the drive. The type of head actuator mechanism in a drive tells you a great deal about the drive's performance and reliability characteristics. Table 1.2 shows the two types of hard disk drive head actuators and the affected performance parameters.

Table 1.2  Characteristics of Stepper Motor Versus Voice-coil Drives.

Characteristic Stepper Motor Voice Coil

Relative access speed Slow Fast
Temperature sensitive Yes (very) No
Positionally sensitive Yes No
Automatic head parking Not usually Yes
Preventive maintenance Periodic format None required
Relative reliability Poor Excellent

Generally, a stepper motor drive has a slow average access rating, is temperature-sensitive during read and write operations, is sensitive to physical orientation during read and write operations, does not automatically park its heads above a save zone during power-down, and usually requires annual or biannual reformats to realign the sector data with the sector header information due to mistracking. Overall, stepper motor drives are vastly inferior to drives that use voice-coil actuators.

Some stepper motor drives feature automatic head parking at power-down. If you have a newer stepper motor drive, refer to the drive’s technical reference manual to determine whether your drive has this feature. Sometimes, you hear a noise after power-down, but that can be deceptive. Some drives use a solenoid-activated spindle brake, which makes a noise as the drive is powered down and does not involve head parking.

Many of the older, less expensive, low-capacity hard disks use a stepper motor system. Most hard disks with capacities of more than 40MB have voice-coil actuators, as do all drives with capacities of more than 100MB, which means all drives being manufactured today.

Voice Coil

A voice-coil actuator is found in all higher-quality hard disk drives, including most drives with capacities greater than 40MB and virtually all drives with capacities exceeding 80MB. Unlike the blind stepper motor positioning system, a voice-coil actuator uses a feedback signal from the drive to accurately determine the head positions and to adjust them, if necessary. This system allows for significantly greater performance, accuracy, and reliability than traditional stepper motor actuators offer.

A voice-coil actuator works by pure electromagnetic force. The construction of this 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 connected to the speaker’s paper cone. Energizing the coil causes the coil to move relative to the stationary magnet, which produces sound from the speaker cone. In a typical hard disk voice-coil system, the electromagnetic coil is attached to the end of the head rack and placed near a stationary magnet. No contact is made 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. Such systems are extremely quick and 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, much like the slide of a trombone. 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, servo-controlled mechanism. Closed loop indicates that the index (or servo) signal is sent to the positioning electronics in a closed-loop system. This loop sometimes is called a feedback loop because the feedback from this information is used to position the heads accurately. Servo controlled refers to this index or the servo information that is used to dictate or control head-positioning accuracy.

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