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Triple-stage actuator: innovations in hard drives

Triple-Stage Actuator: Innovations in Hard Drives

A hard disk drive (HDD) is an extremely complex device, involving both hardware and software components. Composed of around 300 parts sourced from various suppliers around the world, its manufacture requires extremely high precision, with many of the design criteria having tolerances of less than 1 nm. In addition, it has more than a million lines of software code to manage communication with the host, mechanical operations, and data writing and reading.

The increase in the number of tracks per inch (TPI) and the growth in areal density, which indicate the amount of data that can be stored on a hard disk, promote greater capacities and are the result of constant innovation in HDD technology.

Mechanical innovations to increase TPI and areal density include the Triple Stage Actuator (TSA), present in Western Digital's DC HC550 18TB CMR and Ultrastar DC HC650 20TB SMR hard drives.

The actuator arm and its evolution

The image below shows a diagram of a hard disk actuator arm, illustrating the arc movement that the arm makes to position the read/write head over the disks.

Actuator arm of a hard disk

Actuator arm of a hard disk. Image credit: Disclosure

It is a single-stage actuator arm – the voice coil motor (VCM) takes care of all the movement through an electric current applied to the coil (number 30 in the figure), making it move on an axis (32 in the figure).

Arc movement

The actuator arm moves in an arc over the HDD platters to access different data tracks. This is necessary to position the read/write head at the exact point where the data is located.

Positioning 34, 35 and 36, in the image

34: It shows a more outer position in the arc of motion, where the arm accesses tracks on the outermost part of the platters, which generally offers faster transfer rates due to the higher data density at the edges of the disk, allowing more information to be read or written per revolution of the disk.

35: It indicates an intermediate position in the range of motion, where the arm would be accessing tracks located in a more median region of the disc.

36: Shows the position of the actuator arm over the inner edge of the HDD platters, representing a track closer to the center.

The movement described in numbers 34, 35, and 36 represents the variation in the actuator's position as it moves along the data tracks, adjusting as necessary for reading and writing. The movement of the arm is guided by precise commands, enabling access to different storage areas on the HDD.

But there is a problem of misalignment of the read/write head (in green and red) in relation to the data track (in orange), due to the arc movement of the actuator arm. This misalignment occurs because the arm follows a fixed arc, while the tracks are arranged in concentric circles on the HDD platters.

This effect is known as radial displacement error or off-track error, and it occurs because the head movement is not linear, but rather curved. This problem is more pronounced on the inner and outer tracks (such as the positions indicated by numbers 34 and 36 in the image), where the angle of inclination between the position of the read head and the track is more evident. The read head does not position itself correctly on the disc track.

Positioning accuracy is critical for modern hard disk drives, which are becoming increasingly faster, higher capacity, higher data density, and miniaturized. The need for higher bandwidth servo systems capable of quickly and accurately positioning the read/write head across a high track density is becoming increasingly urgent.

Actuator technology has evolved from single stage to dual stage to triple stage.

Two-stage actuator and three-stage actuator. Image credit: Disclosure

Two-stage actuator and three-stage actuator. Image credit: Disclosure

Each evolution increases the accuracy of read/write head positioning, while decreasing the time required to access and write data to media, as well as reducing vibration.

Dual-stage actuators use the milliactuator to move the loadbeam, which is the metal suspension at the end of the actuator arm. This is where the head gimbal assembly (HGA) is located, where the slider is installed, which "flies" approximately 3nm from the disk surface and is responsible for magnetic reading and writing.

Milli and micro piezoelectric actuators

Milli and micro piezoelectric actuators. Image credit: Disclosure

The miliactuator and microactuator each have a pair of piezoelectric elements (or "piezo elements") connected to different suspension components. The actuator makes a small rotation when a voltage is applied, causing one piezoelectric element to expand while the opposite piezo element contracts.

The miliactuator can rotate up to 200 nm, while the microactuator can rotate up to 100 nm, the latter acting on the HGA.

The three points of the actuators

The three points of the actuators. Image credit: Disclosure

TSA with three pivot points can increase granularity during tracking and trail finding.

This allows for more precise positioning of the head on the track.

During the search, the actuator arm moves from one track to another. As the arm approaches the target track, the milliactuator and microactuator are connected to align with the target track more quickly. Finer positioning control allows for faster trail access while reducing noise and vibration.

And here is the piezoelectric actuator that NGK mass-produces for hard disk drives.

Atuador piezoelétrico fabricado pela NGK Insulators

Piezoelectric actuator manufactured by NGK Insulators. Image credit: Disclosure

Pretty cool, right?

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