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Head Sliders, Arms and Actuator
When the hard disk platters are accessed for read and write operations using the read/write heads mounted on the top and bottom surfaces of each platter it is obviously, the read/write heads do not just float in space. They must be held in an exact position relative to the surfaces they are reading and also, they must be moved from track to track to allow access to the entire surface of the disk.
The heads are mounted onto a structure that facilitates this process which is often called the head assembly or actuator assembly or the head-actuator assembly. It is comprised of several different parts. The heads themselves are mounted on head sliders. The sliders are suspended over the surface of the disk at the ends of the head arms. The head arms are all mechanically fused into a single structure that is moved around the surface of the disk by the actuator.
Head Sliders
Each hard disk head is mounted to a special device called a head slider or just slider for short. The function of the slider is to physically support the head and hold it in the correct position relative to the platter as the head floats over its surface. Hard disk read/write heads are too small to be used without attaching them to a larger unit.
Sliders are given a special shape to allow them to ride precisely over the platter. As hard disk read/write heads have been shrinking in size, so have the sliders that carry them. The main advantage of using small sliders is that it reduces the weight that must be yanked around the surface of the platters, improving both positioning speed and accuracy. Smaller sliders also have less surface area to potentially contact the surface of the disk. Each slider is mounted onto a head arm to allow it to be moved over the surface of the platter to which it is mated.
Head Arms
The head arms are thin pieces of metal, usually triangular in shape onto which the head sliders carrying the read/write heads are mounted. There is one arm per read/write head, and all of them are lined up and mounted to the head actuator to form a single unit.
This means that when the actuator moves, all of the heads move together in a synchronized fashion. The arms themselves are made of a lightweight, thin material, to allow them to be moved rapidly from the inner to outer parts of the drive. Newer designs have replaced solid arms with structural shapes in order to reduce weight and improve performance.
Newer drives achieve faster seek times in part by using faster and smarter actuators and lighter, more rigid head arms, allowing the time to switch between tracks to be reduced. A recent trend in the hard disk industry has been the reduction in the number of platters in various drive families. Even some flagship drives in various families now only have three or even two platters, where four or five was commonplace a year or so ago.
One reason for this trend is that having a large number of head arms makes it difficult to make the drive with high enough precision to permit very fast positioning on random seeks. This is due to increased weight in the actuator assembly from the extra arms, and also problems aligning all the heads.
Head Actuator
The actuator is a very important part of the hard disk, because changing from track to track is the only operation on the hard disk that requires active movement. Changing heads is an electronic function, and changing sectors involves waiting for the right sector number to spin around and come under the head. Changing tracks means the heads must be shifted, and so making sure this movement can be done quickly and accurately is of paramount importance.
The actuator is the device used to position the head arms to different tracks on the surface of the platter to different cylinders, since all head arms are moved as a synchronous unit, so each arm moves to the same track number of its respective surface. Head actuators come in two general varieties:
- Stepper Motors
- Voice Coils
The main difference between the two designs is that the stepper motor is an absolute positioning system, while the voice coil is a relative positioning system.
All modern hard disks use voice coil actuators. The voice coil actuator is not only far more adaptable and insensitive to thermal issues. It is much faster and more reliable than a stepper motor. The positioning of actuator is dynamic and is based on feedback from examining the actual position of the tracks. This closed-loop feedback system is also sometimes called a servo motor or servo positioning system and is commonly used in thousands of different applications where precise positioning is important.
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