Read and Write Heads
A hard disk drive usually has one read/write head for each platter surface (meaning that each platter has two sets of read/write headsone for the top side and one for the bottom side). These heads are connected, or ganged, on a single movement mechanism. The heads, therefore, move across the platters in unison.
Mechanically, read/write heads are simple. Each head is on an actuator arm that is spring-loaded to force the head into contact with a platter. Few people realize that each platter actually is “squeezed” by the heads above and below it. If you could open a drive safely and lift the top head with your finger, the head would snap back down into the platter when you released it. If you could pull down on one of the heads below a platter, the spring tension would cause it to snap back up into the platter when you released it. Figure below shows a typical hard disk head-actuator assembly from a voice coil drive.
Read/write heads and rotary voice coil actuator assembly.
When the drive is at rest, the heads are forced into direct contact with the platters by spring tension, but when the drive is spinning at full speed, air pressure develops below the heads and lifts them off the surface of the platter. On a drive spinning at full speed, the distance between the heads and the platter can be anywhere from 0.5 to 5 µ-inch or more in a modern drive.
In the early 1960s, hard disk drive recording heads operated at floating heights as large as 200300 µ-inch; today’s drive heads are designed to float as low as 10nm (nanometers) or 0.4 µ-inch above the surface of the disk. To support higher densities in future drives, the physical separation between the head and disk is expected to drop even further, such that on some drives there will even be contact with the platter surface. New media and head designs will be required to make full or partial contact recording possible.
To ensure the cleanliness of the interior of the drive, the HDA is assembled in a class-100 or better clean room. This specification means that a cubic foot of air cannot contain more than 100 particles that measure up to 0.5 microns (19.7 µ-inch). A single person breathing while standing motionless spews out 500 such particles in a single minute! These rooms contain special air-filtration systems that continuously evacuate and refresh the air. A drive’s HDA should not be opened unless it is inside such a room.
Although maintaining a clean-room environment might seem to be expensive, many companies manufacture tabletop or bench-size clean rooms that sell for only a few thousand dollars. Some of these devices operate like a glove box: The operator first inserts the drive and any tools required and then closes the box and turns on the filtration system. Inside the box, a clean-room environment is maintained, and a technician can use the built-in gloves to work on the drive.
In other clean-room variations, the operator stands at a bench where a forced-air curtain maintains a clean environment on the bench top. The technician can walk in and out of the clean-room field by walking through the air curtain. This air curtain is very similar to the curtain of air used in some stores and warehouses to prevent heat from escaping in the winter while leaving a passage wide open. Because the clean environment is expensive to produce, few companies, except those that manufacture the drives, are properly equipped to service hard disk drives.
As disk drive technology has evolved, so has the design of the read/write head. The earliest heads were simple iron cores with coil windings (electromagnets). By today’s standards, the original head designs were enormous in physical size and operated at very low recording densities. Over the years, head designs have evolved from the first simple ferrite core designs into the magneto-resistive and giant magneto-resistive types available today.
Head Actuator Mechanisms
Possibly more important than the heads themselves is the mechanical system that moves them: the head actuator. This mechanism moves the heads across the disk and positions them accurately above the desired cylinder.
The voice coil actuators used in hard disk drives made today use a feedback signal from the drive to accurately determine the head positions and adjust them, if necessary. This arrangement provides significantly greater performance, accuracy, and reliability than traditional stepper motor actuator designs.
A voice coil actuator works by pure electromagnetic force. The construction of the mechanism is similar to that of a typical audio speaker, from which the term voice coil is derived. An audio speaker uses a stationary magnet surrounded by a voice coil, which is connected to the speaker’s paper cone. Energizing the coil causes it to move relative to the stationary magnet, which produces sound from the cone. In a typical hard disk drive’s voice coil system, the electromagnetic coil is attached to the end of the head rack and placed near a stationary magnet. No physical contact occurs between the coil and the magnet; instead, the coil moves by pure magnetic force. As the electromagnetic coils are energized, they attract or repulse the stationary magnet and move the head rack. Systems such as these are extremely quick and efficient and usually much quieter than systems driven by stepper motors.
Voice coil actuators use a guidance mechanism called a servo to tell the actuator where the heads are in relation to the cylinders and to place the heads accurately at the desired positions. This positioning system often is called a closed-loop feedback mechanism. It works by sending the index (or servo) signal to the positioning electronics, which return a feedback signal that is used to position the heads accurately. The system also is called servo controlled, which refers to the index or servo information that is used to dictate or control head-positioning accuracy.
A voice coil actuator with servo control is not affected by temperature changes, as a stepper motor is. When temperature changes cause the disk platters to expand or contract, the voice coil system compensates automatically because it never positions the heads in predetermined track positions. Rather, the voice coil system searches for the specific track, guided by the prewritten servo information, and then positions the head rack precisely above the desired track, wherever it happens to be. Because of the continuous feedback of servo information, the heads adjust to the current position of the track at all times. For example, as a drive warms up and the platters expand, the servo information enables the heads to “follow” the track. As a result, a voice coil actuator is sometimes called a track-following system.
Automatic Head Parking
When you power off a hard disk drive using a CSS (contact start stop) design, the spring tension in each head arm pulls the heads into contact with the platters. The drive is designed to sustain thousands of takeoffs and landings, but it is wise to ensure that the landings occur at a spot on the platter that contains no data. Older drives required manual head parking; you had to run a program that positioned the drive heads to a landing zone, usually the innermost cylinder, before turning the system off. Modern drives automatically park the heads, so park programs are no longer necessary.
Some amount of abrasion occurs during the landing and takeoff process, removing just a “micro puff” from the magnetic mediumbut if the drive is jarred during the landing or takeoff process, real damage can occur. Newer drives that use load/unload designs incorporate a ramp positioned outside the outer surface of the platters to prevent any contact between the heads and platters, even if the drive is powered off. Load/unload drives automatically park the heads on the ramp when the drive is powered off.
One benefit of using a voice coil actuator is automatic head parking. In a drive that has a voice coil actuator, the heads are positioned and held by magnetic force. When the power to the drive is removed, the magnetic field that holds the heads stationary over a particular cylinder dissipates, enabling the head rack to skitter across the drive surface and potentially cause damage. In the voice coil design, the head rack is attached to a weak spring at one end and a head stop at the other end. When the system is powered on, the spring is overcome by the magnetic force of the positioner. When the drive is powered off, however, the spring gently drags the head rack to a park-and-lock position before the drive slows down and the heads land. On some drives, you could actually hear the “ting…ting…ting…ting” sound as the heads literally bounce-park themselves, driven by this spring.
On a drive with a voice coil actuator, you activate the parking mechanism by turning off the computer; you do not need to run a program to park or retract the heads. In the event of a power outage, the heads park themselves automatically. (The drives unpark automatically when the system is powered on.)
Air Filters
Nearly all hard disk drives have two air filters. One is called the recirculating filter, and the other is called either a barometric or breather filter. These filters are permanently sealed inside the drive and are designed never to be changed for the life of the drive, unlike many older mainframe hard disks that had changeable filters.
Although it is vented, a hard disk does not actively circulate air from inside to outside the HDA or vice versa. The recirculating filter permanently installed inside the HDA is designed to filter only the small particles scraped off the platters during head takeoffs and landings (and possibly any other small particles dislodged inside the drive). Because hard disk drives are permanently sealed and do not circulate outside air, they can run in extremely dirty environments
Air circulation in a hard disk.
The HDA in a hard disk drive is sealed but not airtight. The HDA is vented through a barometric or breather filter element that enables pressure equalization (breathing) between the inside and outside of the drive. For this reason, most hard disks are rated by the drive’s manufacturer to run in a specific range of altitudes, usually from 1,000 feet below to 10,000 feet above sea level. In fact, some hard disks are not rated to exceed 7,000 feet while operating because the air pressure would be too low inside the drive to float the heads properly. As the environmental air pressure changes, air bleeds into or out of the drive, so internal and external pressures are identical. Although air does bleed through a vent, contamination usually is not a concern because the barometric filter on this vent is designed to filter out all particles larger than 0.3 microns (about 12 µ-inch) to meet the specifications for cleanliness inside the drive. You can see the vent holes on most drives, which are covered internally by this breather filter. Some drives use even finer-grade filter elements to keep out even smaller particles.
I conducted a seminar in Hawaii several years ago, and several of the students were from one of the astronomical observatories atop Mauna Kea. They indicated that virtually all the hard disk drives they had tried to use at the observatory site had failed very quickly, if they worked at all. This was no surprise because the observatories are at the 13,796-foot peak of the mountain, and at that altitude, even people don’t function very well! At the time, they had to resort to solid-state (RAM) disks, tape drives, or even floppy disk drives as their primary storage medium. Since then, IBM’s Adstar division (which produces all IBM hard disks) has introduced a line of rugged 3.5-inch drives that are hermetically sealed (airtight), although they do have air inside the HDA. Because they carry their own internal air under pressure, these drives can operate at any altitude and can also withstand extremes of shock and temperature. The drives are designed for military and industrial applications, such as systems used aboard aircraft and in extremely harsh environments. They are, of course, more expensive than typical hard disks that operate under ambient air pressure.
Continue to The Components of Hard Disk Drive 4
PEOPLE FIND THIS PAGE BY THIS WORDS:
voice coil actuator suppliers disk drive; breath filter in hdd; voice coil motor hard disk drive; The typical hard disk drive has one read/write head for each; surface cleanliness of read-write heads; Moves the read/write heads across the disk surfaces in unison; hard drive voice coil; hard drive componnent breath filter; hard disk drive component clean; hard disk components magents to move the read write heads; what are some typical hard drive components;
