Inertial Instruments

8/7/2019 Inertial Instruments

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INERTIAL INSTRUMENTS: WHERE TO NOW?Neil M. Barbour, John M. Elwell, Roy H. Setterlund

The Charles Stark Draper Laboratory, Inc.Cambridge, Massachusetts 02139Abstract

Many types of inertial instruments have been invented inthe past, are currently being invented, and will continue to beinvented as the market for guidance, navigation, and controlcontinues to expand. Some of the inertial instruments havefound a niche in current applications, while some did notprogress much beyond the laboratory/prototype stage. Thiswill be true of future developments also. This paper begins bydescribing gyroscope and accelerometer technologies thatdominate the current market (e.g., strategic, aviation, space,tactical) and explains, in terms of performance and technology,why they have been successful. It is clear from this sectionthat electromechanical and ring laser technologies control thecurrent market. However, since accuracy requirements can beattained by existing technologies, the competition to insert thenew technologies into the current applications is driven by thedesire for low life-cycle cost, small size, and low productioncost. Thus, the success of future instruments will be driven bytechnologies that enable lower cost, highly reliableinstruments. This paper then describes what technologies arecurrently displacing the existing instruments and whattechnologies are expected to dominate in the future. Also, thepotential new applications and markets that will open upbecause of the batch processing and low cost of solid-state/microfabricated instruments are described. It becomesapparent that electromechanical instruments will be rapidlydisplaced over the next 20 years, surviving only where uniqueperformance cannot be matched, and that because of the fiercecompetition in the solid-state arena, the key to success willdepend on system architecture and market timeliness.

1. IntroductionElectromechanical inertial instruments have dominatedguidance, navigation, and control applications since the dawn

of inertial sensing. In recent years, new technologies haveenabled other kinds of sensors that are challenging thisdominance, and, in the case of the ring laser gyroscope, havealready replaced electromechanical instruments in certainapplications. New technologies continue to be developed tomeet future market needs. However, since accuracyrequirements can be atta ined by existing technology, and sincethe new and emerging technologies offer little in anyperformance improvement, the decision to insert or developthem will depend on low life-cycle cost, small size, and lowproduction cost. While several of the new technologies aredescribed herein, it is expected that only two or three willimpact the market in a significant way.

This paper attempts to present the sta tus of inertial sensortechnology in today's applications, and to explain why certainsensors have found a niche. This is then followed by aprediction of where inertial instrument technology is leading,and in what applications the future sensor technologies willfind a niche.

2. Current Inertial Sensor Technoloav Au~lic ati onsFigures 1 and 2 depict a perspective of current gyroscopeand accelerometer technologies, respec tively, and their

applications as related to ppm of scale factor (i.e., how well the

gyroscope or accelerometer reproduces the sensed rate oracceleration) and kg or deg/h of inherent bias stability (i.e.,the error independen t of inertial rate or acceleration). Whilethese performance factors are not the only ones that influencesensor selection, they are useful for comparison purposes.Referring to Figures 1 and 2, it is immediately apparent thatelectromechanical instruments, in the form of spinning massand force rebalance instruments, dominate today'sapplications. These are mature, proven technology concepts.

The spinning mass gyroscope first found a home around1920 in the single-degree-of-freedom rate gyro used as a basicturn indicator for instrument flying. After continuousevolution and improvement it was later used to provide leadangle data for aircraft fire control sights, and later still foraircraft and missile flight control systems. The basicconfiguration of a rate gyro is a ball bearing rotor housed in agimbal whose gyroscopic precession in response to an angularrate is restrained by a mechanical spring, making it relativelyinexpensive, very rugged, and reliable. Rate gyros dominatethe 10 deg/h and above applications such as flight control,stability augmentation, autopilots, etc.

With the need for better performance, such as aircraftnavigation, it was natural to improve on the rate gyro. Whenit was identified that the rate gyro's performance was limitedby its spring (i.e., the very mechanism that allows it tofinc tion also limits its -capability), the performancerequirements were mastered by the development of the single-degree-of-freedom, rate-int egrating gyroscope. Theintegrating gyro is basically a rate gyro in which the primaryrestraining torque on the gyro gimbal is a damping reactionwith a servo loop to maintain the gimbal at null. Theperfection of the integrating gyro is largely due to Dr. CharlesStark Draper, who isolated the gyro gimbal by means offlotation fluid and low reaction torque electromagneticsuspensions, pickoffs, and torquers. The floated integratinggyro went from revolutionizing aircraft navigation in the 50s

enabling strategic missile pidance, autonomous submarinenavigation, and space flight in the 60s, 70s and 80s.The gas bearing was a significant part of the floated gyroevolution, leading to better stability, and a self-aligning

capability for strategic missiles, a capability that no otherinstrument to date provides. Another benefit of the gasbearing is the reduction of the angle noise of the floatedinstruments, so that it is used in satellite navigation andcontrol; its most recent application is in the Hubble telescope.

Floated integrating gyros have relatively high cost, arelabor intensive, and have long warm-up (reaction) times.Clearly, if a suitable alternate technology could be found, itwould take advantage of these perceived problems; and this isexactly what happened.

The free rotor gyroscope, which is basically a ball bearingrotated spinning mass that is unrestrained about thegyroscope precession axes, was another early development.The use of one or two gimbals allows these instruments to beused as directional gyros for directional references and cockpit


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1 nautical mile earth rate

BlAS STABILITYFigure 1. Current gyro technology applications.

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BlAS STABILITY (kg)Figure 2 Current accelerometer technology applications.

displays such as the gyrocompass, artificial horizon, etc.These are very low-accuracy instruments, but have maintainedtheir role in the market place.Another early instrument that offered potential advantages

over the floated gyro was the two-degree-of-freedom, gas-filmsupported, free rotor gyroscope, but it was not until the mid-50s that this instrument became viable when the rotor timeconstant problem was solved. Rockwell's gas film free rotorgyroscope has been used for aircraft navigation andMinuteman boost guidance. This type of instrument has fastreaction times and results in lower cost because of i ts twodegrees of freedom, but it cannot match the best performingfloated gyros.

The free rotor gyro can be regarded as a precursor to thetwo-degree-of-freedom electrostatic gyro (ESG). In the early

50s, ESG development was begun and was carried on throughthe 60s and 70s. The ESG only became viable when machiningtechniques became available to generate the very precisefinishes and geometry required. The ESG was developed byHoneywell and Rockwell for inertial navigation and was usedfor aircraft and land navigation and surveying. Because of itsexceptionally high stability, enabling long periods betweencalibration or update, ESGs have been used for attacksubmarine navigation (ESGN) and the Trident submarine on-board monitor (ESGM). The ESG has much lower drift thanthe best floated gyros and is small; however, its applicationsare limited to relatively benign environments since it has low gcapability. ESGs are being replaced by lower cost technologiesthat are better suited for strapdown applications. For superaccuracy, approximately three orders of magnitude betterthan floated instruments, as required for relativisticmeasurements, Stanford University's Gravity Probe B (to be


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launched in the mid 90s) ESG is operated in a cryogenicenvironment.

In the early 60s, the dynamically-tuned free rotorgyroscope (DTG) was invented. The DTG is a two-degree-of-freedom instrument whose rotor is suspended by a universalhinge of zero stiffness at the tuned speed and rotated by a ballbearing. Because of their relatively low cost, fast reaction time,small size and ruggedness, DTGs have taken the market fromother mechanical instruments in most areas whereperformance is comparable. DTGs have been a phenomenalsuccess in strapdown and gimbaled systems, and have foundapplications in aircraft and missile navigation, stellar-aidedstrategic missile boost and reentry, north seekers, and AHARS.DTGs have demonstrated exceptionally low angle noise andare used in satellite navigation and control, and are also beingmade very small for tactical missile seeker heads. There is aworldwide manufacturing capability for DTGs.

At the same time that the DTG was being invented, theprinciple of detecting rotation by the Sagnac effect was firstdemonstrated (1963)in a ring laser gyroscope (RLG). The RLGoperates by setting up clockwise and counterclockwiseresonant light beams reflected around a closed cavity bymirrors and detecting phase shifts between these beams due toa rotation. The laser is inside the cavity, which contains thelasing medium; hence, the RLG is termed an active device.Development of RLG technology into a usable instrumentrequired significant time and investment to overcome lock-inproblems and to perfect the mirrors, and it was not until theearly 80s that an aircraft navigation-grade RLG becameavailable. Thereafter, RLGs advanced rapidly into the world'scivil aircraft (757, 767, Airbus) and military helicopters,displacing DTGs and integrating mechanical gyros. The RLGhas stellar-aided strategic missile boost grade performance,while the smallest versions are suitable for tactical missileapplications. RLG technology is still advancing, but is at thepractical limit for today's technology. There are many RLGmanufacturers worldwide.

The perseverance applied to RLG technology stemmedprimarily from the desire for strapdown operation, whichbecame more and more practical-as digital computing instrapdown algorithms grew in the 70s and beyond.Mechanical instruments can operate in the strapdown mode,but the RLG is an excellent strapdown device because of goodscale-factor (SF) linearity and SF stability in the tens of partsper billion compared with tens of parts per million formechanical sensors, and almost negligible g sensitivity. TheRLG has other attractive features such as digital output, veryfast reaction times, excellent dormancy characteristics, lowercost, and the absence of moving parts. The need to dither toavoid lock-in problems did introduce a mechanical componentto the RLG, but this can be avoided without mechanical meansas in Litton's ZLG (zero-lock gyro).2.2 Accelerometers

Referring to Figure 2, it is apparent that technologicallymature electromechanical accelerometers dominate today'smarket. The majority of these are the restrained mass or forcerebalance types, in which a proof mass is supported in a planeperpendicular to the input (sense) axis by a flexure, torsionbar, or pivot and jewel. The motion of this proof mass underchanges of acceleration is detected by a pickoff. A rebalanceforce may be generated through a servo feedback loop torestore the proof mass to its null position. Closed-loop, forcerebalance operation provides significant performanceadvantages such as linearity, wide dynamic range, reduced

cross-axis sensitivity, and good scale-factor stability. Open-loop operation is currently used primarily for environmentalmeasurements, switching, and vibration sensing, where highperformance is not an issue. This will change with certaintypes of new open-loop resonator accelerometers.The force rebalance type of accelerometer has beensuccessful not only because it is relatively small, simple, very

rugged, and reliable, but also because it can be designed tomeet different performance and application requirements bycareful selection of the flexure and mass configuration,electromagnetic pickoffs and forcers, servo electronics, fluidand damping, and materials. There are many manufacturersof force rebalance accelerometers such as Bell's Model XI,Litton's A-4, Kearfott's Mod VII, Sundstrand's Q-flex, andSystron Donner's 4841. Force rebalance accelerometers canoperate in strapdown or gimbaled modes. The output needsto be digitized.

The most accurate of the force rebalance accelerometers isthe Draper-designed and Honeywell-built Pulsed IntegratingPendulous Accelerometer (PIPA), which is a floated, single-degree-of-freedom, essentially zero restraint device, which isdigitally torque pulsed to maintain the float (or pendulum) atnull. The PIPA is used in submarine-launched strategicmissiles. PIPA accuracy is limited by nonlinear errorsproportional to the float velocity during pulse torquing.

The highest performance accelerometer available is thePendulous Integrating Gyro Accelerometer (PIGA), which isused for strategic missile guidance. The PIG part of the PIGAis identical to the floated single-degree-of-freedom, integratinggyro with the addition of a pendulous mass located on thespin axis. The PIG is mounted on a self-contained, servo-driven member (SDM) that rotates the PIG to produce agyroscopic torque to balance the pendulous torque. The SDMis equipped with a resolver or encoder to measure SDMrotation angle, which is proportional to the time integral of theinput acceleration. The PIGA is a very stable, linear device,with very high resolution over a wide dynamic range, and isthe only accelerometer to date capable of meeting strategicmissile thrust axis requirements. Its major advantage overforce rebalance accelerometers is the lower requirement onmeasuring SDM rotation angle and velocity versus torquepulse quantization, plus the error cancellation effect fromthe SDM rotation. PIGAs are relatively complex andperceived to have high life-cycle costs due to the three rotatingmechanisms (gas bearing, SDM ball bearing, and s lip ring).The Draper-designed PIGA is manufactured by Honeywelland Litton (U.S.A.); Sagem (France) and Russia alsomanufacture PIGAs.

Another type of accelerometer is the resonator or open-loop type such as the vibrating string accelerometer, whichwas used in early ICBM guidance and for local lunar gravitysurveying on the lunar rover. This device had low shocktolerance.

Angular accelerometers were initially used in the 50s fordynamic compensat ion of ac servomechanisms. The basicconfiguration is a fluid-filled ring with a vane extending intoit. Under rotational motion of the ring, the vane is restrainedby a torquer, whose current indicates the angulardisplacement. Devices, such as those built by Systron Donner,are used in applications requiring high bandwidths (2000 Hz),small magnitude stabilization, or jitter compensation.However, angular displacement sensors are not as accurate asfloated gyros or DTGs below about 20 Hz, but the high cost ofthose gyros restricts their use.


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For the purpose of reducing cost and size, multisensorsthat perform the combined function of measuring angular rateand linear acceleration were developed. The multisensor isideal for applications where redundant accelerationinformation is required.In the 70s, Draper developed a navigation-grade floatedmultisensor by adding a pendulous mass along the output axisof a floated single-degree-of-freedom gyro. This allowed two

axes of acceleration sensing and one axis for rate sensing. TheLitton multisensor incorporated two tuned rotors mounted onopposite ends of a ball bearing spin shaft; one of the rotors actsas a DTG, while the other acts as a two-degree-of-freedomaccelerometer due to an added mass imbalance.The ability to reduce size and cost significantly is achievedby sensing the Coriolis acceleration force on a rotating body inthe presence of an angular rate. Rockwell Collins and GECMarconi have developed multisensors consisting of two pairsof piezoelectric ceramic sensing elements, with each pairattached orthogonally on a rotating drive structure, whichprovide rate and acceleration inputs on two orthogonal axes.They are being used in tactical missiles. Sundstrand has

developed a multisensor using two accelerometers to producean angular velocity for Coriolis force sensing. Thisconfiguration provides one acceleration and .one gyro axis.Several other companies in the U.S. and Japan are developingmultisensors.

3. Where To Now?3.1 Inertial Instrument Technoloeies for the Future

Major changes are currently underway in thosetechnologies associated with inertial sensors such asgyroscopes and accelerometers. These changes are likely toresult in the proliferation of inertial sensors into a wide varietyof new military and commercial applications. This is expectedto occur, and is in many respects already underway, becauseinertial sensors are beginning to adopt many of the fabricationtechniques that have been developed by the solid-stateelectronics industry over the last decade. Inertial sensors arebeing fabricated in silicon, quartz, and with electro-opticmaterials, such as lithium niobate, by employing the low-labor-intensive batch processing techniques that weredeveloped earlier for solid-state electronics. It is predictedthat the utilization of these techniques will result in very lowassociated cost, high reliability, small size, and light weight forinertial sensors and for the higher level systems into whichthey are integrated. Some inertial sensors have already beenfabricated with dimensions so small that they are barelyvisible to the naked eye. It is not expected that there will beany performance improvement with these new sensors and, infact, the performance is often considerably less thanconventional inertial instruments provide. Nevertheless, thelargest majority of applications exist at these lowerperformance levels, especially in the commercial area.

Fiber-optic Gyros (RFOG. IFOG, and CFOQSagnac effect rotation rate sensors result from the counterpropagation of beams of light in a waveguide with optical

reciprocity between its clockwise and counterclockwisepropagation paths. Rotation normal to the waveguide planeupsets this symmetry, which is then photoelectronicallydetected and processed to provide an output of rate. The FOGis implemented using an integrated optics chip constructed in

lithium niobate, and fiber-optic sensing coil, diode lightsource, and photodetectors. This configuration is expected tobe supplemented eventually by quantum well technology,such as gallium arsenide, which will then allow integration ofmost of t he above components into a single substrate,increasing reliability, and reducing costs even further. FOGsensors have no gas or mirrors and do not exhibit lock-in atlow rate .

There are two fundamentally different implementations ofa FOG sensor; one is a resonant optical structure or RFOG, andthe other is an interferometric optical structure or IFOG. Sincethe IFOG sensor has been in active development longer thanthe RFOG, it is closer to a flight-testable prototype. Figure 3indicates the fundamental differences between the RFOG andIFOG.

KILOMETERS OF FIBERCOUNTERPROPAG ATING BEAMSMIXED ON CO MMON PHOTODETECTOR (PD)

RFOG

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COUNTERPROPAGATING BEAMS PROCES SEDONSEPARATE PHOTO DETECTORS (PD)

Fig. 3. FOG differences.The RFOG utilizes a short closed loop of fiber as anextremely high finesse resonant optical waveguide. Ideally,with no rotation, each of the two counterpropagating lightbeams remain in the fiber at resonance, and no light leaves theloop. Rotation shifts the beams from their resonancefrequency, and light then leaves the loop. An optical couplertogether with an integrated optics chip, directs this escapinglight for each beam direction onto separate photodetectors.

With these beams Doppler shifted off resonance in oppositefrequency directions-,-their corresponding photodeiectordifferentials provide a measure of the rotation rate. Because ofthe resonantoptical structure, the RFOG must be driven by avery narrow line laser diode. The RFOG is expected to havesize and radiation hardness advantages over the IFOG due toless fiber and cost advantages over the RLG; performancebetter than 1deg/h is expected.

In contrast to the RFOG, the IFOG uses a longer, multiple-turn, fiber coil with a nonresonant, very broad opticalstructure. As a result, a broadband diode is used as the lightsource. A single IFOG photodetector measures theinterference between the two counterpropagating light beams,each of which are driven from a common source. Withidentical optical path lengths (i.e., no rotation), thisphotodetector provides a measure of the correlation of thelight source after passage through the fiber coil. With rotation,the counter-rotating path propagation times change


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differentially, resulting in an optical phase shift between thesetwo beams, which is then read at the photodetector.The performance of EOGs has not been quite as good as

the RLGs to date for comparable size and weight, but that maybe only the result of the relative immaturity of the technology.However, it has a potential for extremely long MTBF (>150,000h), since it is truly a solid-state device (versus having a gas-filled cavity as in a RLG). The device may be open or closedloop (for higher performance versions). "Reentry-only" gradegyros (-1 deg /h) are approximately 3 inches in diameter;stellar-aided-boost quality gyros are about 7 inches indiameter . Only the 3-inch class models are out of thelaboratory stage of development at present.

By cryogenically cooling a fiber-optic gyro, certain types oflosses are practically eliminated, thereby enabling higherperformance in a very small instrument. Laser diode anddetector performance are also enhanced. A liability, of course,is the necessity of providing a cryogenic coolant source foroperation. It may be possible to incorporate a set of gyros on acooled focal plane (e.g., for an IR sensor) to provide asynergistic thermal design for such a sensor package. Becausethese would be very small instruments, the CFOGperformance may be too crude for most currently envisionedintegrated guidance missions.

Also under development is an integrated optics gyroscopein which the fiber is replaced with a waveguide on theintegrated optics; performance better than 1 deg/h isanticipated.

me vh so n !unction Gvro ( TTQSome theoretical work has been done at NADC an dTemple University on applying some properties of "high-temperature" superconducting materials to inertial sensing.This particular gyro concept utilizes the phase coherence ofthe superconducting electrons and a Josephson Junction tomeasure rotation around a sensitive axis. This is achieved by

producing a persistent current in a ring of superconductingmaterial. Again, the Sagnac effect is used as in RLGs, i.e., theelectrons (rather than light) experience a phase shift if arotation is applied to the ring. As with the RLG, the sensitivityof the instrument is a function of the area enclosed by the ringand the phase sensitivity of the electrons (versus light).Because electrons have a much higher phase sensitivity (-100)than photons, the JJG is theoretically capable of being twoorders of magnitude smaller (in volume) than an equivalentRLG. Again, this instrument is only in the theoretical stage ofdevelopment.

Hemisvherical Resonating Gvro (HRG)Under development by Delco for many years, the HRG

resembles a quartz "wine glass" shape. By exciting the wineglass into resonance, a standing wave pattern is produced onthe instrument. Pickoffs sense the position of this wavepattern, which rotates around the device if an inertial angularrate is applied about the instrument's sensitive axis. The sizeof the instrument increases if high performance is desired .Small, thimble size, instruments perform in the l-deg/h range.Considerably larger instruments would be required to supporta stellar-aided boost mission. The instrument is still in thedevelopment stage and has been funded (on and off) by boththe Navy and Air Force. Currently, new electronics are beingredesigned a nd tested to improve instrument performance.The HRG has been incorporated into Delco's Carousel INS forLufthansa.

This type of gyro is designed as an electronically-driventuning fork, often fabricated out of a single piece of quartz orsilicon. Such gyros operate in accordance with the dynamictheory that when an angle rate is applied to a translatingbody, a Coriolis force is generated. When this angle rate isapplied to the axis of an oscillating tuning fork, its tinesreceive a Coriolis force, which then produces torsional forcesabout the sensors axis. These forces are proportional to theapplied angle rate, which then can be measured capacitively(silicon) or piezoelectrically (quartz); the output needs to bedemodulated for digital processing. Systron Donner's QRS(Quartz Rate Sensor) has been incorporated into tacticalmissile guidance. Others, including Draper, are developingtuning fork gyros; Murata (Japan) uses an equilateral trianglefree-free-bar as a sensitive element.

Silicon micromechanical instruments can be made by bulkmicromachining (chemical etching) single crystal silicon or bysurface micromach ining layers of polysilicon. Manymanufacturers are developing gyros and accelerometers usingthis technology. Their extremely small size combined with thestrength of silicon makes them ideal for very high gapplications.

Several manufacturer's accelerometers incorporatepiezoresistive bridges such as those used in earlymicromechanical pressure gauges. More accurateaccelerometers are the force rebalance type that use closed-loop capacitive sensing and electrostatic forcing. Draper'smicromechanical accelerometer (Figure 4) is a typicalexample, where the accelerometer is a monolithic siliconstructure (i.e., no assembly of component parts) consisting of atorsional pendulum with capacitive readout and electrostatictorquer. This device is about 300 x 600 pm in size. Thependulum is supported by a pair of flexure pivots, and thereadout and torquing electrodes are built into the devicebeneath the tilt plate. The output of the angle sensor isintegrated and then used to drive the torquer to maintain thetilt plate in a fixed angular position. The torque required tomaintain this balance is proportional to the input acceleration;performance around 250 pg and 250 ppm SF have beenachieved.

Fig. 4. Silicon force rebalance accelerometer.


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Micromechanical accelerometers can also be fabricatedusing wafer bonding sandwich construction techniques, as inLitton's silicon accelerometer developed for AHRS and tacticalmissile guidance. Kearfott and Sundstrand are developingmicromechanical VBAs. Analog Devices has an interdigitatedpolysilicon capacitive accelerometer fabricated with an on-chip BiMOS process to include a precision voltage reference,local oscillators, amplifiers, demodulators, force rebalanceloop and self-test functions. Complete integration of sensorand electronics will become common in all futuremicromechanical instruments.

Micromechanical gyroscopes are primarily Coriolis forcesensors. Draper is developing two such devices. The first is adouble gimbal structure (Figure 5) with a vertical gold massmounted on the inner gimbal. The inner gimbal can bethought of as the gyro and the outer gimbal as the motor.Each gimbal is attached to the other through a set oforthogonal flexure pivots. These pivots are relatively weak intorsion, but strong in all other directions. When the outergimbal vibrates through a small angle, the inner gimbalbecomes sensitive to angular rates about the axis normal to theplane of gimbals. In the presence of an angular rate about theaxis normal to the plane of the gimbals, the inner gimbalresponds by vibrating at the outer gimbal vibration frequencyat an amplitude proportional to the input angular rate. Thisgyro is the companion to the Draper accelerometer, and couldultimately be processed on the same chip. Performance of 300deg/h has been achieved.

F= zmw

Gimballedgyro

IVE

In planegyroFig. 5. Operation of gimbaled and tuning fork gyro.

The second is an in-plane gyro design (Figure 5). Thecombs are excited so that electrostatic forces are generated thatdo not depend on the lateral position of the masses. Theflexures are sized to ensure that the tuning fork antiparallelmode is excited and that the translational modes areattenuated. The comb drives large-amplitude vibrations inopposite directions by a self-excited oscillator loop so thatlinear acceleration is rejected. Angular rate in the plane of thesubstrate lifts one mass up and the other down throughCoriolis acceleration, thus, the sense and input axis areidentical. Capacitors below the proof mass are used forsensing.

Development of micromechanical gyros having spinningwheels or configured as an HRG have been reported.Northrop is using a dual micromechanical accelerometerconfiguration as a gyro.pesonatin~Beam Accelerome taThe resonating beam accelerometer (also referred to as thevibrating beam accelerometer (VBA)) is similar in principle tothe vibrating string accelerometer concept that was developed

many years ago. This form of accelerometer is essentially anopen loop device, that is, the proof of mass is not rebalanced toits center position during the application of specific force. Foraccuracy, it relies on the scale-factor stability inherent in thematerial properties of the proof mass supports.

A schematic of the resonating accelerometer concept isshown in Figure 6 . These accelerometers can be constructedusing several different fabrication techniques. One method isto etch the entire device (proof mass, resonating tine, andsupport structure) from a single piece of quartz . Using suchtechniques can result in low-cost, highly reliableaccelerometers with performance capabilities of better than100pg. Constructing this accelerometer from a single piece ofquartz results in high thermal stability, along with dynamicranges approaching those obtainable in the timekeepingindustry. The principle of operation is similar to that of aviolin. When the violin string is tightened, its frequency ofoperation goes up. Similarly when the accelerometer proofmass is loaded, one tine is put into tension and the other intocompression. These tines are continually electrostaticallyexcited at frequencies in the hundreds of kilohertz range whenunloaded. As a result, when "g" loaded, one tine frequencyincreases while the other tine frequency decreases. Thisdifference in frequency is a measu re of the device'sacceleration.

'a' -ACCELERATION

Fig. 6. QRA schematic.Kearfott and Sundstrand have developed laboratoryprototypes aimed at strategic missile guidance, the drivingforce being that they are solid-state devices and therefore holdpotential for good MTBF. Sundstrand, Systron Donner,Draper Lab, and others have produced navigation- andtactical-grade quartz VBAs.Microwave Resonant Accelerometer (MRA)This is also a solid-state device with a strategicmissile performance goal. It is also an alternative to the


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high-performance VBA, and is approximately the same size asa mechanical instrument of comparable performance. Itoperates by maintaining the electromagnetic field in twoopposing cavities (separated by a proof mass) in resonance byinjecting microwaves into the cavities. The size (or volume) ofeach cavity dictates the resonant frequency. Underacceleration, the centered proof mass deflects, therebychanging the size of both cavities thus producing an u p an ddow n shift in the resonant frequencies. This frequencydifference is detected and related to the input specific force.The potential problems of radiation sensitivity and inversiontransients are being addressed early in the instrument'sdevelopment program. A schematic of MRA operation isshown in Figure 7.

ING CAVrrY SENSO WOPWSPROVIDECOMMOKMODE ERROR

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Fig. 7. Microwave resonator accelerometer block diagram.

Other devices that are still very immature are beingstudied, such as the Surface Acoustical Wave (SAW) device,Laser accelerometer, Tunneling accelerometer, and NuclearMagnetic Resonance (NMR) gyros. One other interestingcandidate that may have a potential for high performance isbased on using optical fibers in a mercury-filled, small (1 in3)three-axis instrument to interferometrically measureacceleration-induced pressure gradients in the fluid. This"FLUACC" concept is being studied at Draper.3.2 Future Technolovy Apvlications

The utilization of solid-state inertial sensors like thosedescribed above have potentially significant cost, size, andweight advantages over conventional instruments, which willresult in a rethinking of the options for which such devices canbe used in systems. While there are many such applications ofthe conventional military type, there are also many newerapplications that will emerge with the low cost and very smallsize inherent in such sensors, particularly at the lowerperformance end of the spectrum.

Predictions for the future path of technology, howeverinappropriate they may appear to be, can and have beenmade. In nearly every case, when these newer solid-stateinertial technologies have been evaluated against today'stechnology, given compatible technical requirements, this newclass of solid state inertial sensors becomes the winner . Thebasis for this technological selection is almost always lowercost. A vision of how the inertial instrument field might beexpected to change over the next 20 years is shown in Figures8 and 9 for the gyro and accelerometer, respectively. It isapparent that electromechanical instruments will becomerapidly displaced.

At the high-performance end however, fielded strategicmissile applications wi!l still be dominated by mechanicalfloated gyros for self-alignment, PIGAs, and force rebalance

accelerometers for cross-axis measurements. No new designefforts on these instruments are expected, and future activitywill be devoted to supporting longevity. New initiatives willinvolve solid-state strapdown, stellar or satellite-aidedmissions for which the development of solid-state thrust-axisaccelerometers (MRA and VBA) will be paramount, withIFOGs or RLGS for the gyro. It is quite likely that resonantaccelerometers will take over the cross-axis and reentrymeasurement functions. A solid-state thrust-axis accelerom-eter, such as the MRA or VBA, could meet the small sizerequired for advanced gradiometry borehole applications. Forthe esoteric relativistic measurements in space, only thecryogenic ESG will satisfy the requirements; super low noise(-1 nrad) floated gyros may be required for stabilization insome satellite applications.

The middle-performance ground is expected to becomedominated by optical gyros, primarily fiber optic, which willprocess their rate-induced Sagnac frequency shifts in lithiumniobate or gallium arsenide. The ring laser gyro is an excellentinstrument, but its intrinsic manufacturing is heavilydominated by precision machining processes and alignmentrequirements, which force its costs to remain relatively high.However, one particular area where the ring laser gyro isexpected to retain its superiority is in the area-of scale-factor.The laser gyro has its optical path in a gas, whereas the fiber-optic gyro has its path in glass, making the FOGfundamentally much more susceptible to environmentaleffects. For comparable performance applications, theselection between the FOG and the RLG will very likelydepend on the supplier of the system as well as the specificmission requirements. The use of GPS will greatly ease therequirements on inertial navigation systems. IFOGs are slatedfor the Boeing 777 INS.

The military and commercial space community tend to bevery conservative, so that the shift to new technology will takemuch longer and require significant evaluation. Thus, DTGsand floated gyros will continue to dominate the satellitemarket for the next 10 to 20 years. However, because of theirlow power and low cost, IFOGs and resonant accelerometerswill eventually captu re this market. Mechanical controlmoment gyros and ball bearing spin momentum and reactionwheels will continue to be used for attitude control.

Land navigation will be dominated by low-cost, rapid-reaction, thermally-modelable gyros, such as the RLG andFOG, and resonant accelerometers.

The need for a counterforce (such as Naval Theater MissileDefense) to the proliferation of tactical ballistic missiles, willinvolve gun-launched missile systems, and self-guidingKinetic Energy Weapons (KEW). These applications requirevery rapid reaction and possibly very high-g capabilityinstruments.

The low-performance end of the application spectrum willbe dominated by micromechanical inertial sensors. Thesecould be, for example, gyros and accelerometers or IMUsphotolithographically constructed in silicon or quartz andsubsequently etched in very large numbers as a single batch.It is expected that the military market will push thedevelopme nt of these sensors for applications such ascompetent and smart munitions, aircraft and missileautopilots, short time-of-flight tactical missile guidance, firecontrol systems, radar antenna motion compensation, smartskins utilizing embedded inertial sensors, multiple intelligentsmall projectiles such as flechettes or even "bullets," andwafer-scale GPS/inertial integrated systems.


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QUARTZ SILICON

Figure 8. Future gyro technology applications.

1,000--EI100->tdm

ACCELEROMETER

-. . BIAS STABILITY ( W g )

Figure 9. Future accelerometer technology applications.

These instruments will continue to evolve into the middle-performance ground. The commercial market formicromechanical inertial sensors is orders of magnitude largerthan any contemplated military market. The application ofmicromechanical gyro technology to the automobile industryis one case. For example, a true skid detector requires ameasure of inertial rate in order to operate successfully.Products designed for this industry must be inexpensive andreliable, both characteristics of solid-state technology. Many

other micromechanical inertial sensor applications exist forautomobiles such as airbags, braking, leveling, andaugmentation to GPS navigation systems. Additionalcommercial applications can be found in products such ascamcorders, factory automation, general aviation, medicalelectronics, and perhaps one of the largest areas of all,children's toys and games. If the cost can be brought downlow enough, one could expect to see an IMU in every home.


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Langford, R. C., "Unconventional Inertial Sensors,"Astronautics A da , Vol. 12, No. 4,1966.Ragan, R. R. et al., "Inertial Technology for the Future," IEEETransactions on Aerospace and Electronics Systems, Vol. AES-20,No. 4, July 1984.Savage, P. G., "Strapdown Sensors," NATOIAGARD LectureSeries 95,1978.Norling, B., "Accelerometers: Current and EmergingTechnology," Inter natio nal Association of Geodesy Symposia, 107,1991.Smith, R. B. and J. R. Weyraugh, "Gyroscopes: Current andEmerging Technologies," International Association of GeodesySymposia, 107,1991.

R. G. Brown Associates Inc., Inertial System Market Forecast1991-2005,Fourth Edition, 1991.Musoff, H. et al., Spacecraft Attit ude Determination and ControllerT e c h n o l o g y , CSDL-R-2252, The Charles Stark DraperLaboratory, Inc., Cambridge, MA 02139, June 1990."World Markets for Fiber-optic Sensors: New Methods ofSensing and Synergies with other Advanced ApplicationsDrive Growth," MIRC, 1990.Weinberg, M. S., "Emerging Technologies in Microguidanceand Control," Presented at NASA Microtechnologies andApplications to Space Systems Workshop, May 1992.Yun, W. and R. T. Howe, "Silicon MicrofabricatedAccelerometers: A Perspective on Recent Developments,"Sensors Expo Proceedings, 1991.

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