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Inertial Reference System

Handout

Page 1

1. Laser Gyro

General

In a laser gyro two beams of light are generated, each traveling around the cavity (in this case a triangle) in opposite directions.

How it Works

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For 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.

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No 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 2011

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Since 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.

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Although the frequency Is determined by the gas that is "lasing", 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.

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The 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.

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Laser 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 "tuned cavity" 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.

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Inertial Reference System Detecting the Difference

Handout

Page 2

The 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 Pattern

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Anode 3000 Volt

In the center of the Cervit block is a device called a dither motor. The motor, which vibrates at 319 Hz, eliminates "laser lock," a hangup that sometimes occurs in the dead band around the zero-rate point.

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Photo Cells (2)

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Corner Prisma

Partially Transparent Mirror

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Light Beams

Piezoelectric Dither Motor

Anode 3000 Volt

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Figure 1 - Laser Gyro Module

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No 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 2011

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Gas Discharge Area Cathode

Cervit Block

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Servoed Mirror

Inertial Reference System

Handout

Page 3

Not Immune to Drift

Accuracy 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 Operation

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Interogator

The airplane's present position is calculated by adding the distance flown to the starting position.

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Start Position

The 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)

Acceleration

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Interogator

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As 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.

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Figure 2 - Accelerometer

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No 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 2011

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Velocity (GND Speed)

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Distance Flown

Actual Position

Inertial Reference System

Handout

Page 4

2. System Alignment

General

Measurement Process

Inertial System Alignment Process - Heading Determination

Roll Earth Rotation 15°/h

Pitch

Roll Gyro = 0°/h Pitch Gyro = 15°/h Heading = 270°

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Figure 3a

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Earth Rotation 15°/h

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© StrainTech EDV Support CH 8192 Glattfelden

No 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 2011

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Pitch

Roll

Figure 3b

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Roll Gyro = 15°/h Pitch Gyro = 0°/h Heading = 360°

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Inside 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 rotation

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During alignment, the inertial system determines local vertical, true north, aircraft latitude and aircraft heading.

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Inertial Reference System

Handout

Page 5

Roll Gyro = Pitch Gyro Heading = 315° Roll Pitch

Inertial System Alignment Process - Latitude Determination

Roll Gyro = 0°/h Pitch Gyro = 15°/h Latitude = 0° Roll Pitch

Roll Roll Gyro = Pitch Gyro = Latitude = 0°/h 0°/h 90°

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Figure 4a

Pitch

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Figure 4b

Earth Rotation 15°/h

Earth Rotation 15°/h

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© StrainTech EDV Support CH 8192 Glattfelden

No 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 2011

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Figure 3c

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The 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 "Off - Axis". In reality it is difficult for an Inertial system to align at high latitudes because all headings are direction South!

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Earth Rotation 15°/h

In 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 indicated

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Inertial Reference System

Handout

Page 6

Roll Gyro = 0°/h Pitch Gyro = 0 > < 15°/h Latitude = 45° Earth Rotation 15°/h

Roll

Pitch

Figure 4c

Inertial System Alignment Process - Heading & Latitude Determination

Roll Gyro = Rate due to Heading & Latitude Pitch Gyro = Rate due to Heading & Latitude Earth Rotation 15°/h

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Roll

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Figure 5

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© StrainTech EDV Support CH 8192 Glattfelden

No 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 2011

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In an actual example, these earth rate measurements, when combined, allow the Inertial systems computer to determine latitude and heading during alignment.

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Pitch

In 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.

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Inertial Reference System

Handout

Page 7

It cannot sense rotation in the two other directions!

Yaw Gyro

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Roll Gyro

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© StrainTech EDV Support CH 8192 Glattfelden

No 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 2011

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Yaw Gyro (HDG)

Figure 6 ­ Gyro Installation

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In 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!

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Pitch Gyro

A laser gyro can sense rotation only around its input axis.

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Roll Gyro

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Inertial Reference System

Handout

Page 8

During 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 Operation

Distance traveled

Present Position Departure Alignment

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Accelerations IRU COMPUTER Present Position

Initial Position

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Figure 7 ­ Navigation Computation

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Distance traveled is determined by measuring linear acceleration (from the accelerometers) and integrating the result twice to obtain velocity and distance.

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During 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.

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Operators Entry

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Inertial Reference System

Handout

Page 9

ANTI SKID AUTO BRAKE SYSTEM

Ground Speed Acceleration

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IRU

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Attitude Rates Inertial Altitude Altitude Acceleration Inertial Vertical Speed

Attitude, HDG Track Wind Drift 3- Axis Velocity Inertial Vertical Speed

Attitude

Position

Positions 3-Axis Position Acceleration MHDG THDG Attitude

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Yaw Rate Ground Speed Lateral Acceleration Roll Angle

EFIS, HSI VSI RDMI

WEATHER RADAR

FMS

GPWS

Position MHDG

ADAS

Figure 8 ­ General IRS / FMS Interface

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© StrainTech EDV Support CH 8192 Glattfelden

No 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 2011

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LAT /LONG Wind Drift Angle Flight Path Angle Inertial Vertical Speed

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AIR DATA COMPUTER

Altitude Rate Altitude TAS

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Initial Position THDG Track Angle Present Position Wind Speed & Direction Ground Speed

IRS MODE PANEL

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4. IRS Aircraft Interface

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FLIGHT CONTROL COMPUTER

3-Axis Rates & Accelerations Velocity, Attitude Track Angle, HDG, Discretes

YAW DAMPER

THRUST MANAGEMENT COMPUTER

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