HDD actuator is a device which moves head arm assembly. Its task is, not only to start and secure continuous movement of hard drive head assembly, but also to fully control it. Securing and control of this movement is of crucial importance for performing elementary function of hard drives.
Surface area of HDD platters is divided into large (and increasing) number of sectors for data storage. Actuator brings head arm and its sliders to the right spot over HDD platters, from where data should be read/or where new data should be written. At the same time, electronic controller manage HDD motor operation. Platters are set in position so that firstly actuator, and then sliders, are able to perform their task.
If you consider platters RPM (up to 15000, usually 5400/7200) and the number of sectors which is "packed" on the platters of modern hard drives, it's clear how complex is task of HDD actuators in locating exact sector almost instantaneously.
Servomechanism (left) and Stepper motor (right)
Previously, role of the HDD actuator was performed by stepper motor - electric motors which converts digital electric impulses into predefined angular displacement. This principle of head arm positioning was good enough in the times when density of sectors on HDD platters was significantly lower.
Constant demands for HDD capacity increase (and thus higher density of sectors on the platters) reduced usability of stepper motors, as they became unable to accurately position head arm sliders. In addition, stepper motors are known for their overheating issue - which led to thermal expansions of platters and increased possibility of contact between heads and platters surfaces - aka head crash.
In today's era, stepper motors are no longer used as HDD actuators. They are replaced by so called - servomechanism. Head arm movement is secured and controlled by changing amplitude and polarity of current through the voice coil in the field of permanent magnet.
Voice coil is integral part of head assembly and it is located on its backside. Thus, its movement in the field between two permanent magnets brings entire head assembly into motion.
Magnetic field is secured by using pair of very strong neodymium magnets. These magnets are strong enough to attract a mass which is up to 1000 times greater than their own. Direction of this magnetic force inside hard drives is strictly vertical, otherwise it could cause damage to the data on the platters.