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`Inertial Reference SystemHandoutPage 11. Laser GyroGeneralIn a laser gyro two beams of light are generated, each traveling around the cavity (in this case a triangle) in opposite directions.How it WorksStr aFor those who might find it hard to understand that the laser gyro turning about its axis shortens the path length for one beam and lengthens it for the other, here is another way to explain the phenomenon: Consider a particle of light, a photon, just leaving the cathode and traveling toward the mirror on the right hand corner (see figure 1). If the gyro turns clockwise on its axis, the mirror would move closer to the photon that was on its way toward it. Hence the photon's path length is shortened in the distance from cathode to mirror, and in the entire distance around the triangular race. Remember the photon is traveling in inertial space; it is not fixed to the gyro. Thus, this one photon and the millions of its traveling companions move around the circuit in a shorter time and in doing so they compress the waves in the laser beam and raise its frequency (number of cycles in a given time). Of course the opposite happens to the photon traveling clockwise from the emitter, because when the gyro turned clockwise about its axis, its mirrors, moved away from the clockwise-traveling photon, forcing it to travel further to reach the mirror and to complete the circuit.© StrainTech EDV Support CH - 8192 GlattfeldenNo part of this paper may be used or reproduced in any form or by any means, or stored in a databse or retrievial sytem, without prior written permission of the authors! Edition: January 2011inTechESince both contra-rotating beams travel at the same constant speed (speed of light), it takes each the same exact time to complete its circuit. However, if the gyro were rotated on its axis, the path length of one beam would be shortened, while that for the other would be lengthened. Since, as explained, the laser beam adjusts its wavelength for the length of the path, the beam that traveled the shorter distance would rise in frequency (wavelength decreases), while the beam that traveled the longer distance to complete the circuit would encounter a frequency decrease. This frequency difference between the two beams is directly proportional to the angular rate of turn about the gyro's axis. Simply stated, that is the principle of the laser gyro. Thus, frequency difference becomes a measure of rotation rate. If the gyro doesn't move about its axis, both frequencies remain equal (since the path lengths of both beams are equal) and the angular rate is zero.DV SAlthough the frequency Is determined by the gas that is &quot;lasing&quot;, it can be varied somewhat by changing the path length over which the waves have to travel. For a given path length there are an integral number of waves (cycles that occur over the complete path). If the path length is altered, the waves will be either compressed or expanded, but there always will be an integral number of cycles that occur over the complete path. If the waves are compressed more cycles occur per unit time, hence frequency increases. If expanded the opposite is true.uppoThe laser beams, even though in the light spectrum, have coherent wave-like properties, undulating between zero and peak sine-wave fashion. The light is said to be a pure frequency. In the Honeywell helium-neon laser gyro, as defined by its wavelength (the reciprocal of frequency), it is 6,328 Angstroms.rt GLaser gyros are not gyros at all, but sensors of angular rate of rotation about a single axis. As exemplified in the Honeywell design, they are made of a triangular block of temperature-stable glass weighing a little more than two pounds. Very small tunnels are precisely drilled parallel to the perimeter of the triangle and reflecting mirrors are placed in each corner. A small charge of heliumneon gas is inserted and sealed into an aperture in the glass at the base of the triangle. When high voltage is run between the anodes and cathode the gas is ionized, and in the energy exchange process many of the atoms of the gas are transformed into light in the orange-pink part of the visible spectrum. This action is abetted by the &quot;tuned cavity&quot; effect of the physical dimensions of the light path inside the glass block. The randomly moving particles resonate at a single frequency resulting in a thin, high energy beam of coherent light traveling through the triangle of tunnels. The mirrors serve as both reflectors and optical filters, reflecting the light frequency for which they were designed and absorbing all others.lattfeldenInertial Reference System Detecting the DifferenceHandoutPage 2The difference in frequency in the laser gyro is measured by an optical detector that counts the fringes of the fringe pattern generated by the interference of the two light waves. Since the fringes are seen as pulses by the photocell, the detected frequency difference appears at the output of the detector in digital form, ready for immediate processing by the system's associated digital electronics. Note that there are two photocells. (see figure 1) The function of one is to tell the direction in which the fringes are moving, which is an indication of whether the gyro is rotating to the left or right. As indicated in the more detailed diagram, the three corner mirrors are not identical. One is servoed so that it can make micro-adjustments to keep the physical path always the same. Another (the one at the apex of the triangle in the diagram) permits a small amount of light to pass through so as to impinge on the photocell detectors. The prism, as can be seen, flips one beam around causing it to meet the interfere with the beam aimed directly at the photocells. The interfering beams alternately cancel and reinforce each other, thus generating the fringe pattern. The block of glass used for the Honeywell laser gyro is made from Cervit, a special glass, the physical dimensions of which remain constant over a wide temperature range (specified as -65° to +180° F). To start the lasing action 3,000 volts are applied across the anodes to the cathode. Although one can't see the laser beams in the laser gyro, a plasma is formed between the cathode and the two anodes that glows an orange pink that is in the same part of the visible spectrum as the 6,328 Angstrom beams. This plasma can be seen.Fringe PatternDV SupAnode 3000 VoltIn the center of the Cervit block is a device called a dither motor. The motor, which vibrates at 319 Hz, eliminates &quot;laser lock,&quot; a hangup that sometimes occurs in the dead band around the zero-rate point.port GlatPhoto Cells (2)hEtfeCorner PrismaPartially Transparent MirrorinTecLight BeamsPiezoelectric Dither MotorAnode 3000 VoltStr aFigure 1 - Laser Gyro Module© StrainTech EDV Support CH  8192 GlattfeldenNo part of this paper may be used or reproduced in any form or by any means, or stored in a databse or retrievial sytem, without prior written permission of the authors! Edition: January 2011ldeGas Discharge Area CathodeCervit BlocknServoed MirrorInertial Reference SystemHandoutPage 3Not Immune to DriftAccuracy of a laser gyro is influenced by the length of its optical path. The longer the path, the higher the accuracy. The relationship is not linear. For example, a small increase in path length makes for a larger increase in accuracy. Honeywell said it chose the triangular path because this geometry gave it the highest accuracy for the smallest overall area. For installation in the IRS, the laser gyros are each inserted in a triangular aluminum can. The can has an external pin connector to mate it with the system. The assembly weight is about 2.3 lb.Accelerometer OperationupInterogatorThe airplane's present position is calculated by adding the distance flown to the starting position.poStart PositionThe sensing device for the IRU is an accelerometer. The accelerometer is shown as a weight, centered in a case by two springs. As the airplane accelerates, the weight is displaced from center, causing an electrical pickoff signal to be generated. This signal is amplified and applied as feedback to re-center the weight. The amount of signal required to keep the weight centered is there fore proportional to acceleration. This re-centering operation allows the accelerometer to sense over a wide range and also be able to sense very small changes in acceleration. The re-centering signal is integrated once to give velocity and integrated a second time to give distance.Re-Centering (Feedback)AccelerationDV Srt GInterogatorlatAs with spinning wheel gyros. the major source of error in a laser gyro is random drift. While in spinning wheel gyros the root cause is imperfect bearings and mass imbalances, in the laser types it is noise, due almost exclusively to imperfect mirrors including mirror coatings.hEAmplifierecFigure 2 - AccelerometerStr a© StrainTech EDV Support CH  8192 GlattfeldenNo part of this paper may be used or reproduced in any form or by any means, or stored in a databse or retrievial sytem, without prior written permission of the authors! Edition: January 2011inTtfeVelocity (GND Speed)ldenDistance FlownActual PositionInertial Reference SystemHandoutPage 42. System AlignmentGeneralMeasurement ProcessInertial System Alignment Process - Heading DeterminationRoll Earth Rotation 15°/hPitchRoll Gyro = 0°/h Pitch Gyro = 15°/h Heading = 270°DV SFigure 3aupEarth Rotation 15°/hecStr a© StrainTech EDV Support CH  8192 GlattfeldenNo part of this paper may be used or reproduced in any form or by any means, or stored in a databse or retrievial sytem, without prior written permission of the authors! Edition: January 2011inThEPitchRollFigure 3bpoRoll Gyro = 15°/h Pitch Gyro = 0°/h Heading = 360°rt GlatInside the Inertial system, the three laser gyros and three accelerometers sense any aircraft movement. Since the aircraft is stationary during the alignment, the movement is due to earth's rotation. The Inertial computer measures this motion and can then calculate latitude, true north and heading.The earth rotates at approx. 15°/h. The Roll gyro senses this total rate, and the Pitch gyro senses no rotation. The IRU computer uses this information to determine that the aircraft is pointing North or South depending on polarity of the sensed rotation.In this orientation the Pitch gyro senses 15°/h and the Roll gyro reads zero. The IRU computer interprets this is an aircraft pointing West or East depending on polarity of rotationtfeDuring alignment, the inertial system determines local vertical, true north, aircraft latitude and aircraft heading.ldenInertial Reference SystemHandoutPage 5Roll Gyro = Pitch Gyro Heading = 315° Roll PitchInertial System Alignment Process - Latitude DeterminationRoll Gyro = 0°/h Pitch Gyro = 15°/h Latitude = 0° Roll PitchRoll Roll Gyro = Pitch Gyro = Latitude = 0°/h 0°/h 90°hEDV SFigure 4aPitchupFigure 4bEarth Rotation 15°/hEarth Rotation 15°/hStr a© StrainTech EDV Support CH  8192 GlattfeldenNo part of this paper may be used or reproduced in any form or by any means, or stored in a databse or retrievial sytem, without prior written permission of the authors! Edition: January 2011inTecport GFigure 3clatThe earth rate is also used to determine latitude. There, since the Pitch gyro is in the correct orientation, it senses the full 15°/h earth rate. The Roll gyro senses nothing Here on the North Pole, neither gyro would sense rotation. both are &quot;Off - Axis&quot;. In reality it is difficult for an Inertial system to align at high latitudes because all headings are direction South!tfeEarth Rotation 15°/hIn any other orientation, each gyro senses a portion of the maximum 15°/h earth rotation rate. By determining how much is seen by each gyro, the IRU computer can calculate the aircraft heading. In this case, with both gyro rates equal, a heading of 315° is indicatedldenInertial Reference SystemHandoutPage 6Roll Gyro = 0°/h Pitch Gyro = 0 &gt; &lt; 15°/h Latitude = 45° Earth Rotation 15°/hRollPitchFigure 4cInertial System Alignment Process - Heading &amp; Latitude DeterminationRoll Gyro = Rate due to Heading &amp; Latitude Pitch Gyro = Rate due to Heading &amp; Latitude Earth Rotation 15°/hpoRollDV SupFigure 5Str a© StrainTech EDV Support CH  8192 GlattfeldenNo part of this paper may be used or reproduced in any form or by any means, or stored in a databse or retrievial sytem, without prior written permission of the authors! Edition: January 2011inTechEIn an actual example, these earth rate measurements, when combined, allow the Inertial systems computer to determine latitude and heading during alignment.rt GlatPitchIn this third, more realistic example, the Pitch gyro is between the positions shown in figure 4a and figure 4b. It therefore would sense a portion of the earth rate. From this information the latitude can be determined. Also, because the Pitch gyro has less rotation signal to work with, it takes longer for the measurement process.tfeldenInertial Reference SystemHandoutPage 7It cannot sense rotation in the two other directions!Yaw GyroupRoll GyroinTecStr a© StrainTech EDV Support CH  8192 GlattfeldenNo part of this paper may be used or reproduced in any form or by any means, or stored in a databse or retrievial sytem, without prior written permission of the authors! Edition: January 2011hEDV SYaw Gyro (HDG)Figure 6 ­ Gyro InstallationpoIn each Inertial system there are three laser gyros, mounted at 90° angles from each other so as to measure rotation in the Roll, Pitch and Yaw axes. The Roll and Pitch gyros are used to determine heading and latitude!rt GlattfePitch GyroA laser gyro can sense rotation only around its input axis.ldeRoll GyronInertial Reference SystemHandoutPage 8During alignment, the operator enters latitude and longitude coordinates. These entries are compared to the stored and calculated values. If all numbers match, the alignment is completed and the system is ready to navigate.3. Navigation OperationDistance traveledPresent Position Departure AlignmentDV SupAccelerations IRU COMPUTER Present PositionInitial PositionStr a© StrainTech EDV Support CH  8192 GlattfeldenNo part of this paper may be used or reproduced in any form or by any means, or stored in a databse or retrievial sytem, without prior written permission of the authors! Edition: January 2011inTechEFigure 7 ­ Navigation Computationport GDistance traveled is determined by measuring linear acceleration (from the accelerometers) and integrating the result twice to obtain velocity and distance.latDuring initialization. the latitude and longitude of the starting point are entered into the inertial, reference unit computer. Present position at all future times is determined by adding the distance traveled onto the coordinates of the initial starting position.tfeldeOperators EntrynInertial Reference SystemHandoutPage 9ANTI SKID AUTO BRAKE SYSTEMGround Speed Accelerationrt GIRUlatAttitude Rates Inertial Altitude Altitude Acceleration Inertial Vertical SpeedAttitude, HDG Track Wind Drift 3- Axis Velocity Inertial Vertical SpeedAttitudePositionPositions 3-Axis Position Acceleration MHDG THDG AttitudetfeYaw Rate Ground Speed Lateral Acceleration Roll AngleEFIS, HSI VSI RDMIWEATHER RADARFMSGPWSPosition MHDGADASFigure 8 ­ General IRS / FMS InterfaceStr a© StrainTech EDV Support CH  8192 GlattfeldenNo part of this paper may be used or reproduced in any form or by any means, or stored in a databse or retrievial sytem, without prior written permission of the authors! Edition: Jauary 2011inTechEDV SLAT /LONG Wind Drift Angle Flight Path Angle Inertial Vertical SpeedupAIR DATA COMPUTERAltitude Rate Altitude TASpoInitial Position THDG Track Angle Present Position Wind Speed &amp; Direction Ground SpeedIRS MODE PANELlde4. IRS Aircraft InterfacenFLIGHT CONTROL COMPUTER3-Axis Rates &amp; Accelerations Velocity, Attitude Track Angle, HDG, DiscretesYAW DAMPERTHRUST MANAGEMENT COMPUTER`

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