harddisk by lanyuehua

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									     Ruggedized Disk Drives for Commercial Airborne Computer Systems

                                         Robert G. Kaseta
                                    Vice President, Engineering
                                       Miltope Corporation



       The use of computer systems aboard commercial aircraft is rapidly growing as the

markets associated with in-flight entertainment and cabin management systems emerge.

Airborne environments include humidity, temperature, shock, vibration and altitude extremes

which significantly exceed the specifications of commercial disk drives. However, the need for

hard disk drives capable of performing reliably in airborne environments is critical in order to

overcome the extremely high cost associated with solid state memory devices. As usage

increases, the industry is realizing that the rigid disk drive is one of the more environmentally

sensitive commercial off-the-shelf (COTS) components used in these computer systems. The

following is a discussion of specific disk drive design parameters and their associated

environmental sensitivities as well as the solutions required to provide a ruggedized disk drive

that will perform reliably in airborne environments.



Commercial Disk Design Parameters and Sensitivities

       Commercial disk drive design involves a broad array of electromechanical technologies

and related components. The magnetic recording and retrieval of data is dependent on the

operation of many complex subsystems within the disk drive. The major subsystems within the

disk drive are:

       Flying head/magnetic media system

       Servo positioning voice coil actuator


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       Read/Write channel electronics

       Spindle and Spindle motor

       Servo control channel electronics

       Contamination control system

       Interface electronics

       Several of the subsystems within a disk drive are particularly sensitive to environmental

influences such as temperature, humidity, altitude, shock and vibration. Commercial disk drives

are designed to appropriate margins for an environment which is relatively benign compared to

the environment found when providing extended operation aboard commercial aircraft. These

include office environments or portable applications (transportable or “laptop” computer

systems) which are not intended to survive extremes in environmental parameters.



Head/Media Interface and Magnetic Recording

       “Winchester” is the technology of choice for today’s high capacity random access storage

requirements. Winchester provides the optimum tradeoff of cost per megabyte, speed, power and

storage capacity per cubic inch. It is a technology that is rapidly advancing at 60% - 100% per

year in areal density keeping pace with the growing appetite for storage in all computer markets.

Winchester technology employs a magnetic head that flies several micro inches above a disk

platter (media) on an air bearing generated by the spinning motion of the platter and the contour

of the head slider. Most Winchester technology disk drives employ contact start/stop, meaning

the head is in contact with the surface of the media when at rest and is in sliding contact with the

media when starting and stopping the spindle. The head flies only when the spindle speed is

above several hundred RPM. Because the head starts and stops in contact with the surface of the


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media, both the head surface and the media surface must be extremely smooth. The head surface

and media surface must be characterized during their manufacture to ensure that asperities do not

exist on either surface that is close to the flying height of the head. Head flying height is

proportional to the rotational surface speed of the disk media and is also a function of the air

temperature and atmospheric pressure (air density). Both ambient temperature and altitude

variations therefore affect the flying height of the head above the media surface. If a head flies

below its design limit for prolonged periods with the platters at high rotational speed, the head

may come into contact with asperities causing flying instability and creating the potential of head

crashes or excessive head/media wear. The following analogy may help put the challenge of

reliably flying a Winchester head in perspective. If the size of the flying head slider were

equated to the wingspan of a Boeing 747, the 747 would be flying less than one inch above the

ground at 600 miles per hour. The extreme smoothness required of the head and media surfaces

creates another environmental exposure. Excessive stiction, or breakaway friction, is a potential

problem with contact start/stop Winchester technology and is aggravated by excessive humidity.

Exclusion of air between the two smooth surfaces of the head and media when the spindle is at

rest may create a breakaway friction level that may be too high for the starting torque of the

spindle motor to overcome. If the spindle motor does overcome the excessive stiction, the head

may go through some unpredictable motions when it breaks away creating the potential of media

surface damage. The surfaces of the head and media are carefully optimized to reduce the

dangerous effects of excessive stiction including the use of special lubricants on the media

surface. High humidity conditions seriously increase the potential of excessive stiction between

the head/media interface and actual condensation will cause even more dire effects. Humidity

and condensation have proven to be a significant cause of failure of commercial disk drives used


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in an airborne environment. Numerous failed disk drives have been carefully disassembled in a

clean room environment, and analyses of the head and media components have confirmed the

causes of the failures. Aircraft are frequently parked for extended periods of time with

environmental control systems turned off, allowing equipment to experience the local external

climate. Condensation is prone to occur when environmental control systems are initially

activated.

        The magnetic parameters of both the head and media are sensitive to thermal effects.

Airborne environments typically exceed commercial disk drive specifications by 15° to 20°C at

both the high temperature and low temperature extremes. While the consequences of thermal

extremes are not usually as catastrophic as humidity, reliable operation of the disk drive may be

affected. This typically first manifests itself in high data error rates.

        Excessive shock may also affect the head/media interface when the head is at rest and

when it is flying. Although the mass of the flying head continues to be reduced and the durability

of the media surface continues to be improved, head slap caused by excessive shock can still

damage the head and media surfaces leading to head crashes and catastrophic failures.



Servo Positioning System

        The task of the servo positioning system is to provide a means for the recording heads to

be accurately positioned over any track on a disk platter so that data may be recorded or

retrieved. The heads are positioned by a rotary voice coil actuator which is comprised of a high

energy magnet and counteracting electromagnetic coil whose control current is derived from a

closed servo loop. The voice coil is attached to one end of a bearing mounted positioning arm.

Position feedback information is derived from information embedded on each track which is read


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by the data head and processed by the servo control channel to provide a position control current

to the voice coil of the actuator. The magnetic circuit of the voice coil actuator has a finite

stiffness or force capable of holding the heads accurately positioned over the appropriate track.

The servo system also has a finite response time to react to feedback information which informs

the system that the heads are moving off track. If external influences such as shock or vibration

act to misposition the data heads, then recording or readback data errors will occur. The high

track densities of today’s disk drives impose formidable requirements on the servo positioning

system. A positioning error on the order of tens of micro inches is sufficient to cause errors.



Bearings and Electronics

       The lubricated bearings contained in the disk spindle and voice coil actuator and the read

channel and servo channel electronics are all thermally sensitive components. Cold temperature

increases the viscosity of the lubricants used in the bearing systems. This effect has been known

to create functional problems. In some cases, the available starting torque of the spindle motor is

not sufficient to overcome the increased drag created by the now higher viscosity (cold)

lubricant. Changes in the voice coil actuator bearing lubricant viscosity may cause instability in

the servo positioning loop. These conditions are created aboard aircraft when parked with the

environmental control systems deactivated. These effects, while not permanent, prevent proper

operation when systems are first turned on after extended cold periods. Extreme high

temperature also has a negative effect on bearing life and head media reliability due to bearing

lubricant outgassing. While this phenomenon is present to some extent during operation in

normal environments, the effect is greatly accelerated when standard commercial temperature

ranges are exceeded.


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       The electronic systems within a disk drive, particularly the analog Read/Write channel

and servo channel, have also been known to malfunction due to parametric drift when operated at

temperatures beyond standard commercial disk drive specifications.



Solutions

       The solution to overcoming the problems previously discussed is threefold:

        1. Initial COTS disk drive evaluation and selection

        2. Sealed disk cartridge design

        3. Controlled production and test process including 100% environmental stress

            screening (ESS)



Evaluation and Selection

       The first critical element in achieving a successful ruggedized disk drive is the selection

and verification of a COTS disk drive. There is a large variation in the environmental

performance of COTS disk drives from vendor to vendor and from one model to the next. In a

ruggedized application, the COTS disk drive is being asked to perform beyond its commercial

design limits. A rugged disk product supplier must understand and prove that a product has

design margin beyond the commercial specifications. The evaluation must be thorough and

include an assessment of all technology areas which might be affected by extended

environmental operation. A significant sample size must be evaluated and virtually all vendors’

products must be evaluated to select not just one that works, but the one with the greatest design

margin. This process must be a continual one because of two driving forces. The first is the

short product life cycles in the commercial disk drive market. It is unusual for a product to have


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a life cycle longer than 6 months before it’s obsolete. The second driving force is the customer’s

desire for greater and greater storage capacity. As mentioned previously, the evaluation process

must include a significant sample size and evaluations for high and low temperature operation

(including cold start capability), thermal gradient performance, shock performance, and vibration

performance.

Sealed Disk Cartridge Design

       The second critical element in a ruggedized disk drive solution is to employ a sealed disk

cartridge design. The sealed disk cartridge design provides a sure solution to the problems

attendant with high humidity and condensation environments. This design approach protects

against external humidity/condensation conditions and allows purging and filling with a dry inert

gas to further protect the hard drive. Seal design must be validated to ensure that the integrity of

the seal is maintained across the entire temperature and pressure range encountered in the

airborne environment. While some tout Winchester technology as “sealed,” it is sealed from a

contamination perspective, but not sealed from a humidity perspective. Air is allowed to

exchange through a breather filter to accommodate pressure changes due to thermal and

atmospheric pressure variations. A sealed disk cartridge technology also provides protection

from the negative effects on head flying height caused by altitude/pressure variations. A sealed

cartridge design further provides the benefit of making the disk drive easily removable while

protecting it after removal.

Production and Test Process

       The third critical element in a rugged disk drive solution is a controlled production and

test process. A key element of this process is to perform a 100% environmental stress screening.

After a thorough evaluation and selection process to choose a COTS vendor and specific disk


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drive, the job is only partially complete. After qualification, there is also a significant exposure

to Engineering changes by the COTS vendor. These changes may be driven by cost, component

supplier changes or by design issues. Although these changes may not affect the success of the

product in a non-ruggedized environment, they could likely affect the performance of the product

in extended environmental applications. To protect against these issues, an environmental stress

screening test must be performed on 100% of products to be produced. The environmental stress

screening must be a comprehensive test process that is structured to identify design variations

that would prevent the product from performing at environmental extremes or that would lead to

reduced reliability. This testing must properly stress the appropriate drive parameters and must

ensure that the testing is non-destructive in nature. The testing must include extensive data

transfers and exercising the head actuator mechanism while using appropriate data patterns and

varying the ambient temperature. Performance must also be evaluated while operating through

rapid temperature transients and after allowing the product to soak for long periods at

temperature extremes. A significant volume of production product tested through a properly

constructed ESS screening criteria will be expected to exhibit a percentage of failure. It is rare to

find a COTS product that will exhibit a 100% yield in ESS testing throughout its product life

cycle. A typical scenario is to experience a low yield early in the product life cycle with

improving yield through ESS testing as the product matures. To accomplish this extensive

testing in an economic fashion requires that a major up-front investment be made in automated

test hardware and software. Although initial investment is necessary, automating the test

process minimizes the recurring costs associated with the required testing while ensuring no

compromises are incurred in the reliability and performance of the end product shipped to the

customer.


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         In summary, it is possible to provide a reliable rugged disk drive solution for airborne

environments if the limitations of the underlying technologies are well understood. A thorough

understanding of these limitations allows the selection of the COTS product with the greatest

design margins for environmental extremes. Employing sealed cartridge technology provides the

protection required by all rigid disk drives against humidity and altitude effects. Finally, an

appropriate environmental stress screening program performed on a 100% basis protects against

design and manufacturing variations affecting reliability and performance over the product life

cycle.




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