Miniaturising hard disk drive motors
08 May 2018

Hard disks (HDDs) are used in personal computers, Blu-ray recorders, servers, and many other kinds of information-processing equipment. The hard disk drive motor, which is the heart of the hard disks, is a high-tech product for which the production facilities must be on the same level as those for semiconductors.

A typical HDD has two electric motors; a spindle motor that spins the disks and an actuator (motor) that positions the read/write head assembly across the spinning disks. The disk motor has an external rotor attached to the disks; the stator windings are fixed in place. Opposite the actuator at the end of the head support arm is the read-write head; thin printed-circuit cables connect the read-write heads to amplifier electronics mounted at the pivot of the actuator. The head support arm is very light, but also stiff; in modern drives, acceleration at the head reaches 550g.

This spindle motor carries the storage disks, which it rotates around its own axis. The reading and writing heads moving over the disks can then magnetise and demagnetise the magnetic layers, thus writing or reading the digital information at extremely high speed and density on the disks in concentric circles.

Given the memory capacity of modern hard disks, the requirements placed upon the spindle motor are considerable. Since the heads move over the disks at distances in the nanometer range, spindle motors have to be manufactured and packed in cleanrooms, because even the smallest particle of dust would mean the immediate destruction of the hard disk. This is apart from the requirements on the mechanical precision of the components, their reliability and working life.

A particular technical challenge is the design of the bearing systems, which some years ago were changed from ball bearings to fluid dynamic bearing systems. This is due to the necessary improvement of concentric, quiet running properties. Every spindle motor has an application-specific hydrodynamic bearing system to ensure the best possible quality and reliability.

Key parameters which must be optimised in the motor include rotation accuracy, noise level, shock resistance, capacity, and low profile.

The actuator is a permanent magnet and moving coil motor that swings the heads to the desired position. A metal plate supports a squat neodymium-iron-boron (NIB) high-flux magnet. Beneath this plate is the moving coil, often referred to as the voice coil by analogy to the coil in loudspeakers, which is attached to the actuator hub, and beneath that is a second NIB magnet, mounted on the bottom plate of the motor (though some drives have only one magnet).

The voice coil itself is shaped rather like an arrowhead and made of doubly-coated copper magnet wire.

The HDD’s electronics control the movement of the actuator and the rotation of the disk and perform read and write operations on demand from the disk controller. Feedback of the drive electronics is accomplished by means of special segments of the disk dedicated to servo feedback. These are either complete concentric circles (in the case of dedicated servo technology), or segments interspersed with real data (in the case of embedded servo technology). The servo feedback optimises the signal to noise ratio by adjusting the voice-coil of the actuated arm. Modern disk firmware is capable of scheduling reads and writes efficiently on the platter surfaces and remapping sectors of the media which have failed.

Thinner notebooks

Thinner notebook PCs require the use of thinner components, including motors.  Currently, 7mm thick HDDs are the standard for notebook PCs. The thickness of the spindle motor built into the HDD is 70 to 75% of the thickness of the disk drive, meaning that the motors of 7mm HDDs must be approximately 5mm thick. 7mm thick HDDs have been mass-produced since 2010. Nidec, which is a leading supplier of motors for HDDs, also possesses the capabilities to produce HDDs as thin as 5mm.

The biggest barrier to successful thin motor design has been the assignment of sufficient space for magnetic circuits and bearings to support the rotor. If the magnetic circuit size is simply reduced, the desired magnetic force cannot be obtained. Also, the bearings around a shaft several millimetres in length must be rigid, so that one or two 2.5in disks can be rotated at a constant speed of 5,400 or 7,200rpm. In other words, reduced size is not enough by itself. Performance must be maintained or even enhanced at the same time.

Motors for 9.5mm thick HDDs (that were the standard before the advent of the 7mm HDD) initially used an internal stopper to keep the shaft in place. However, Nidec has drastically changed the structure of its motors and relocated the stopper outside the shaft, instead of developing a thinner internal stopper. The objective is to create a design that can be used for several generations in a world where the speed of evolution is extremely fast.

Helium-filled disk drives

Although HDD areal density has been increasing in the past due to refined magnetic heads and innovative recording methods, internal components of hard disk drives are usually exposed to the same air that surrounds the HDD itself. This stands in the way of further progress. The air resistance experienced by the disks as the HDD spindle motor rotates at speeds of several thousand or even tens of thousands of revolutions per minute leads to vibration, which in turn negatively affects the read/write precision of the HDD.

In addition to improving read/write precision, reducing the vibration of the disks by decreasing the drag would also allow them to be made even thinner, increasing the number of disks that can be fitted inside a single HDD. As a bonus, the electricity consumed by the spindle motor as it rotates the discs would decrease as well.

Nidec has set out to test hard disk drives filled with helium instead of air to reduce drag. Helium-filled hard disks are a result of numerous small but difficult steps.

The low density of helium – one-seventh that of air — results in considerably less drag, but a new problem arises as helium atoms, due to their small size, tend to leak through adhesive substances or cavities in the aluminium die-cast base plates. Ensuring that the helium stays sealed inside the HDDs for five years — the design lifetime of a hard disk drive — proved to be a particularly difficult challenge. If HDD base plates were scaled up to the size of 25m long swimming pools, any cavities would have to be thinner than strands of hair.

To ensure that the helium does not leak through the 1-2mm thin base plate, computer-aided engineering (CAE) analysis of the flow of the molten aluminium during casting is used to optimise every parameter of the casting process, including the shape of each part of the metal moulds and the temperature control.

Furthermore, in addition to using vacuum casting, Nidec has developed and adopted other technologies. These enable mass production of base plates, with very few defects, that can be sealed for five years without leaking any helium.

The adhesive used to seal the HDDs is another important factor in preventing helium leakage. After making sure that the adhesive is applied thickly, the individual components are joined together under very high pressure to maximise resistance to leakage. An adhesive with a low outgassing rate is chosen to avoid contamination caused by gases other than helium. Furthermore, the HDD components go through repeated baking procedures (removing gas in advance) during every step of the manufacturing process to minimise outgassing from the adhesive and resin.

For the fluid dynamic bearings, Nidec has also worked with an oil manufacturer to jointly develop a specialized oil with a low rate of evaporation and high resistance to external moisture.

High capacity low power-consumption helium-filled HDDs for servers in data centres are now commonplace. HDD manufacturers are pushing the limits with next-generation recording methods, such as heat-assisted recording and multilayer magnetic recording, and HDD capacity is expected to keep increasing in the future.

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