Invasion of the hard disk drives. Joe Desposito.
If Simon and Garfunkel were to sing about hard disks, they might use the lyric "Where have you gone, five megabytes?" Though this was a popular storage capacity a few years ago, 5Mb hard disk drives have all but vanished from the marketplace. Ironically, though storage capacity has increased, price has not--it has decreased dramatically. Nowadays a 10Mb drive can be purchased by mail order for under $700, complete with controller and software. We are certainly getting more bytes for fewer bucks. But what other changes are occurring in hard disks?
Like most products associated with electronics, hard disks are shrinking in size. Half-height 5.25" drives are common, while drives with 3.5" platters are gaining in popularity. Speed is a critical issue, so some hard drives are significantly faster than others. Another issue is the disk medium itself. Should it be oxide coated, plated, or something else? Should it be fixed inside the drive or removable?
Since hard disk drives are sealed in an enclosure, it is difficult to ascertain the features of a particular drive. Let's examine current hard disk technology to gain a better understanding of what goes on inside this popular storage device.
The Media Stores the Message
The term "hard disk" derives from the rigid platter that resides inside the drive. A typical hard disk drive may contain one or more of these platters. The bare disk, or substrate, is made of aluminum, which is then covered with a metallic substance.
Oxide coated disks have been an industry staple since 1955, when they were used in the first disk drives ever constructed. The magnetic layer is created by coating the aluminum substrate with a paste containing gammaferric oxide particles. This paste is then cured and polished. Next a protective lubricant is added, and the layer is burnished smooth. Each particle is the magnetic layer can be polarized by the read/write head to represent data.
Another category of media that has been around for some time is metallic thin-film plated disks. This kind of disk is manufactured by immersing aluminum substrates in a series of chemical plating baths, coating the substrate with layers of metallic film. The last layer is a very thin (three micro-inch) cobalt alloy in which the magnetic transitions are stored.
A newer kind of medium is metallic thin-film sputtered disks. These are manufactured by coating the aluminum substrate with a nickel phosphorus layer, which is smoothed and polished. Then a continuous vacuum deposition process called sputtering is begun. In this process, magnetic layers as thin as two microinches are deposited on the disk similar to the way wafers are coated with thin metallic films in the semiconductor industry. The sputtering technique is then used to lay down a diamond hard one-microinch protective carbon overcoat.
So why the discussion of media? Why should you care if your hard drive has oxide coated, plated, or sputtered disks? The answer is the diference between a 5Mb and a 20Mb drive--storage capacity.
In a hard disk drive the read/write head(s) doesn't touch the disk, but flies over the surface as close to it as possible. If the head were to come in contact with the surface of a disk while it was spinning, a head "crash" would result and data would be lost.
As the flying height of the head decreases, the storage capacity of the disk increases. But since we are talking about flying heights in the microinch range, the smoothness of the surface of the disk also becomes critically important.
Though the surfaces of oxide-coated and plated disks appear smooth to the naked eye, under an electron microscope hills and valleys appear. Here sputtered disks have an edge. The disk surface of a sputtered disk is so smooth that heads can fly reliably just eight microinches above the surface.
Another factor influencing capacity is the thinness of the magnetic layer placed on the disk. Both metallic plated and sputtered disks have magnetic layers as thin as two microinches; oxide-coated disks, on the other hand, have magnetic layers that are typically 30 microinches thick.
Another advantage of plated and sputtered media is that the intensity of the magnetic field at any given point is almost twice that of oxide-coated media. This provides the higher signal amplitudes needed for good signal-to-noise performance, which in turn increase the capacity of the disk.
One of the manufacturers of plated media, the Tandon Corporation, has come up with a new technique, which consists of depositing a protective overcoat layer on top of electrolytically plated cobalt layers with the sputtering process. The sputtered overcoat of carbon provides strength, smoothness, and integrity to the plated disks.
Why, you might ask, is oxide-coated material used at all if plated and sputtered disks seem to be so superior? One reason is that the oxide-coated disk has been old reliable for many years. Another is the cost factor. Oxide-coated disks are much easier to mass produce and thus can be sold much more cheaply. Which type of media do you think resides in a $699 10Mb drive?
The Head Bone Connects to the Arm Bone
Disks store your data, but it is the read/write heads that transfer the data from the disk to the drive electronics. And it is the arms that hold the heads as they move from track to track.
The conventional head in a hard drive is a ferrite head. However, a more advanced head technology is a tiny electromagnet called a thin-film head. Thin-film heads can read and write information in denser patterns on a disk, which means that more information can be put on a disk. Besides higher recording densities, thin-film heads tend to be more reliable and lighter than ferrite heads. But cost again becomes an issue. The reason ferrite heads are still used in hard drives is that the cost of manufacturing thin-film heads is higher.
There are two kinds of arm assemblies that hold the heads. One is a Winchester, and the other is a Whitney. Whitney is the name commonly associated with second generation Winchester technology. As can be seen in Figure 1, the Whitney arm is much more streamlined than the Winchester. The Whitney arm improves read/write signal reliability by improving flying height stability and tracking accuracy, and reducing interfering signals from adjacent tracks (thus yielding a higher signal-to-noise ratio). Ferrite heads are normally used with the Winchester arm, while thin-film heads are used with the Whitney arm.
What is Your Position(er)
One of the hard disk specs often bandied about is average access time. This is the mean time needed to reposition and read/write head from one data record to another and includes average latency, or the rotational delay of the spinning disk. One drive might have an average access time of 85 milliseconds (ms), while another has a time of 38 ms or less. Why the difference?
The difference lies in the type of positioning mechanism used in the drive. A low-cost, low-capacity drive usually uses an open-loop stepper motor to move the heads from track to track. Once a command to seek a particular track is given, the stepper motor shaft clicks off the desired number of steps. This system relies on the mechanical accuracy of the motor to bring the heads to the correct location. And since this is a purely mechanical system, parts wear out over time and cause failures and longer access times.
A more accurate (and more expensive) way to position heads is with a closed loop, rotary voice coil positioner. (You may recall that a voice coil is also used to move the cone of a speaker; it is a hollow cylinder around which coils of wire are wrapped.) A closed-loop servo system employs electrical feedback to find a desired location on the disk. The information needed by the positioning mechanism to determine its distance from a specified location is contained either on a dedicated servo surface (DSS), which is one of the hard disk surfaces, or embedded in all the record gaps on every track.
Some of Tandon's new disk drives use a pseudo closed-loop head positioning system. In this method, servo positioning information is embedded in the microscopic index wedge located on each data track. The drive checks its location during each disk revolution and corrects its position if necessary. This positioner is more accurate than an open-loop system, yet less expensive than a closed-loop servo system. However, it does not produce faster access times.
Small But Rugged
Though 5.25" hard disk drives have become a standard for microcomputer systems, 3.5" hard drives are waiting in the wings. The smaller drives are a natural for desktop computers with small footprints and for portable models. And drives not only get smaller but they get thinner, too. Half-height 5.25" drives are already available. It seems only a matter of time before one-third height 5.25" drives and half- and one-third height 3.5" drives become available.
Both 5.25" and 3.5" drives are more rugged these days, too. It used to be that hard drives were very susceptible to head crashes if the drive was moved. Now there are drives that automatically park and lock the heads in a dedicated landing zone upon power down. Other drives use a mechanism that lifts the arm up and away from the disk any time power is shut off.
And Away We Go
One of the drawbacks of using a hard disk drive is that the disk cannot be removed and transported. In fact, Winchester technology is based on the condition that disks and heads reside in a protected, sealed environment. However, some companies have tried to circumvent this limitation by manufacturing drives with removable hard disks. The problem is that inserting or removing the cartridge exposes both the media and the drive to contamination and possible damage.
Though these companies have a good idea--removable media--reliability has been suspect. But now drive manufacturers are trying to lick the reliability problem by incorporating fully retracting heads into their drives. Like a frightened turtle, the drive pulls its head into its body (and away from the disk) whenever it senses a loss of power. The heads are re-extended only after the disk has come up to speed and the cartridge compartment has been completely filtered of contaminants.
Reliability problems will no doubt continue to plague new efforts in the removable media area. But the success of companies such as Iomega, whose Bernoulli Box disk drive looks like a floppy system but acts like a removable hard drive, indicate that interest is strong in the microcomputer community.
Putting It All Together
If 5Mb hard disk drives are already gone, what is the future of 10Mb drives? It seems that the microcomputer world is caught up in the larger (storage capacity), faster, smaller (physical size) madness when it comes to hard disks. There are many factors that enable one hard disk to outperform another. And as performance continues to improve, it will be imperative for users to find out what is inside the drives that are so carefully protected from the outside world.