PC Hardware

Actuator Arms

The goal of a hard disk drive is to quickly and directly access data stored on a flat surface. To do this, two different motions are required. As the disk spins, the R/W heads move across the platter perpendicularly to the motion of the disk. The R/W heads are mounted on the ends of the actuator arms (much like the arm of an old record player). A critical element in hard disk drive design is the speed and accuracy of these actuator arms.

Early hard disk drives used a stepper motor to move the actuator arms in fixed increments or steps. This early technology had several limitations:

  • The interface between the stepper motor and actuator arm required that slippage be kept to a minimum. The greater the slippage, the greater the error.
  • Time and physical deterioration of the components caused the positioning of the arms to become less precise. This deterioration eventually caused data transfer errors.
  • Heat affected the operation of the stepper motor negatively. The contraction and expansion of the components caused positioning accuracy errors. (Components expand as they get warmer and contract as they cool. Even though these changes are very small, they make it difficult to access data, written while the hard drive is cold, after the disk has warmed up.)
  • The R/W heads need to be "parked" when not in use. Parking moves the heads to an area of the disk that does not contain data. Leaving the heads on an area with data can cause that data to be corrupted. Old hard disk drives had to be parked with a command. Most drives today automatically park the heads during spin-down.
Older hard disk drives require that the heads be parked before moving the computer. It is recommended that you use the appropriate command to park the heads. The actual command can vary depending on the drive manufacturer, but you can try typing PARK at an MS-DOS prompt.

Hard disk drives with stepping motor actuator arms have been replaced by drives that employ a linear motor to move the actuator arms. These linear voice coil motors use the same type of voice coil found in an audio loudspeaker, hence the name. This principle uses a permanent magnet and a coil on the actuator arm. By passing electrical current through the coil, it generates a magnetic field that moves the actuator arm into the proper position.

Voice coil hard disk drives offer several advantages:

  • The lack of mechanical interface between the motor and the actuator arm provides consistent positioning accuracy.
  • When the drive is shut down (the power is removed from the coil), the actuator arm, which is spring-loaded, moves back to its initial position, thus eliminating the need to park the head. In a sense, these drives are self-parking.

There is a drawback: because a voice coil motor can't accurately predict the movement of the heads across the disk, one side of one platter is used for navigational purposes and so is unavailable for data storage. The voice coil moves the R/W head into an approximate position. Then the R/W heads on the reserved platter use the "map" to determine the head's true position and make any necessary adjustments. This is why hard drive specifications list an odd number of heads.

Head to Disk Interference

Head to Disk Interference (HDI) is a fancy term for head crash. These terms describe the contact that sometimes occurs between the fragile surface of the disk and the R/W head. This contact can cause considerable damage to both the R/W head and the disk. Never move a hard disk drive until it is completely stopped; the momentum of the drive can cause a crash if it is moved or dropped during operation.

Picking up a disconnected hard disk drive that is still spinning is not a good idea either. The rotation force of the platters can wrench it out of your hands, and the drive is not likely to survive the trip to the floor.