Actuator In Hard Disk

What is Actuator in Hard Disk?

An actuator in a hard disk refers to the electronic device intended to move the head arm of the drive. It is controlled by a motor. In simple terms, it signifies the specific part of the hard drive connecting the read-write arm.

Technically, the actuator is a part of the actuator assembly, which typically signifies both the actuators as well as the head assemblies.

Understanding Actuator

What is Actuator in Hard Disk

Hard drives are mechanical devices containing spinning platters to store data on them magnetically. These splatters are divided into different sectors, called data sectors.

The read and write head of the hard drive needs to reach the exact location on these data sectors for reading a particular data or writing new data on them, and it is the actuator that helps it to do so. The actuator glides the head arm correctly onto the desired area.

The head arm is connected to the axis point by this actuator, which controls its positioning when the computer needs to access specific data and sends a signal to the actuator to move the head arm to the correct position.

The arm does not move across the entire surface of the hard drive platter. In fact, it moves only within a small range which is permitted by the axis point and its movement is controlled by the actuator.

As for the spaces out of the reach of the head arm, it is the spinning platters that help in this particular aspect.

Usually the actuators move very quickly over the platter and quite frequently. But there may be times when it will not be in use.

During those times, the actuator ensures that the read and write head rests on a specific space which does not contain any user data.


Typically, in a single-stage Hard Disk Drive, there is only one single VCM actuator to handle the positioning aspect of the read and write head.

On the other hand, in the dual-stage systems, there are two different types of motor actuators used as follows:

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The actuators are exceptionally accurate, and now they have become even more accurate with the developments in storage technology and in their design.

You will now find micro-actuators on the head assembly of the modern hard drives. These actuators offer even more head accuracy and data tracking.

This, in turn, helps in improving data density and the manufacturers can now design hard disks with larger capacities and lower seek times.

Ideally, an actuator is the part of the actuator assembly which is made up of different parts such as:

The heads are located at the tip of the actuator arm and the sliders are attached to the arms and other parts of the actuator.

Working Process

The working process of the actuator in the hard drive needs all components to work in perfect tandem to achieve the common goal of reading or writing data from or to the disk. The entire process involves all of the following:

So, considering all the steps as well as the Revolutions Per Minute (RPM) of the mechanical platters, which can be anywhere from 5400 to 15000, along with the large number of data sectors packed on these platters, the hard drive actuators surely have a tough task in hand, given the fact that they have to locate the exact data sector more or less instantly.

Failing Actuators

The actuator is one of the most vital components in a hard drive, and it is also one of the most vulnerable components in it to failure.

This is because these are mechanical components, and just like any other mechanical part, unfortunately, the actuators may fail eventually.

When it fails over time, the head assembly will become less than perfect. It may either stop functioning or may even remove the magnetic material on the platters physically by coming into direct contact with it. This will cause permanent data loss.

As said earlier, the actuators are terribly accurate, but they may show some specific signs of failure as follows:

There are several reasons for an actuator to fail and data tracking may not be done properly by it. Some of the most common reasons are:

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All these situations must be evaluated physically by the data recovery engineers to find out the type and degree of the failure. Based on that, you may have to either:

All these are very complex processes and need specialized tools and must be done by certified technicians.

Functions of an Actuator

The basic function of the actuator in a hard drive is to move the actuator arm in order to position the read and write head at the right place, based on the signals received from the computer.

Typically, when the platter in the hard drive spins, the position of the access arm is adjusted to move the read and write head along the axis point so that it can read, write or delete data, as the case may be.

The actuator of the HDD moves the head arm assembly but its function is not limited to only that. In addition to initiating and securing constant movement of the head assembly of the Hard Disk Drive, the job of the actuator also involves controlling it completely.

Typically, one of the crucial factors in the performance of the hard drives is to control and secure the movement of the head arm so that it is positioned at the exact location as desired to access the required data.

This is not an easy job because the relatively large surface area of the platters of the Hard Disk Drives is split into a large, as well as in an increasing number of sectors to store data in them.

Finding the right spot for the right data on this huge surface area on the HDD platters is not easy.

It needs to be precise and fast, and that is what the actuator of the hard drive does.

It slides the actuator arm containing the read and write head to the precise spot on the platters to read old data from there, write new data on that spot, or delete it from there.

The entire operation is controlled by an electronic controller all the time. This controller ensures that first the platters are set in position and then the actuator, and the sliders, in that specific order, can perform their respective functions.

What Type of Motor Control is the Actuator of the Hard Disk?

In the earlier designs of an actuator in a hard drive, there was a stepper motor used to control its movement. However, over time, with the advancement of technology, there is a more advanced servo motor used in the design now to move the actuator in the modern hard drive.

Ideally, the actuators in the older hard drives with a stepper motor could move the heads of the drives according to the stepper pulses and their response to these particular pulses.

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In these older models, the actuator of the hard drive could perform its role pretty well since the capacity of the drives as well as the density of the data sectors on the platter itself were pretty low.

So these stepper motors, which were basically electric motors, were efficient enough to convert digital electric pulses into calculated angular movements.

However, over time, the demands for the capacity of the Hard Disk Drives increased constantly.

This called for platters with a higher density of sectors. This, in turn, reduced the efficacy of the stepper motors since they were not able to position the actuator and the head arm sliders perfectly into the desired positions.

In addition to that, the stepper motors also encountered significant issues due to overheating, resulting in thermal expansions of the platters.

This, in turn, increased the prospects of contact between the platter surface and heads. This phenomenon is called head crash.

Therefore, the demand of the modern and challenging era is for more efficient HDD actuators, which is why the stepper motors were replaced by a process called the servomechanism.

In this specific process, the movement of the head arm is controlled and secured by altering the polarity and amplitude of the current passing through the voice coil within the magnetic field created by the permanent magnet.

In the modern hard drives, therefore, a servo motor guidance system is used which actually comes with a voice coil actuator.

This actuator comes with a Voice Coil Motor or VCM, which typically controls the movement of the coil toward a permanent magnet or away from it.

This movement is determined on the basis of the amount of current passing through it.

In these actuators, the voice coil is the most essential part of the head assembly.

It is usually located on the back side, which helps in bringing the complete head assembly into motion by moving the actuator in the magnetic field between the two permanent magnets.

The strength of the magnetic field is ensured by the pair of very powerful neodymium magnets. These specific types of magnets can attract a mass that is up to 1000 times greater than the mass of their own.

The direction of the magnetic force is essentially vertical inside the hard drives. This ensures that there are no damages caused to the data stored on the platters of the hard drive.


The actuator in the hard drive plays a very vital role in the proper functioning of it, and, in turn, the computer system as a whole.

It has a motor that controls the movement and placement of the actuator and operates hand in hand with all the other moving elements of the drive to execute its functions quickly and perfectly.