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1. General

A. The inertial reference system (IRS) provides inertial navigation data to user systems. It uses a ring laser gyro instead of the conventional rate gyro to sense angular rate about the roll, pitch and yaw axes. The system is termed strapdown since its sensors are, in effect, directly mounted to the airframe.

B. The inertial reference system (IRS) includes two inertial reference units (IRU), one inertial system display unit (ISDU), one mode select unit (MSU), one master caution unit (MCU), two digital/analog adapters (DAA) and two radio digital distance magnetic indicators (RDDMI). The IRS provides the inertial navigation data and the inertial flight control data to other systems.

C. The main function of each IRU is to sense and compute linear accelerations and angular turning rates about the airplane's pitch, roll, and yaw axes. This data is used for pitch and roll displays and navigational computations.

D. Each IRU contains three laser gyros and three accelerometers. These sense angular rates and linear accelerations, respectively. The sensed data is resolved to local vertical coordinates and combined with air data inputs to compute the following:

(1) position (latitude, longitude)

(2) attitude (pitch, roll, yaw)

(3) true and magnetic heading

(4) windspeed and direction

(5) velocity

(6) accelerations

(7) angular rate data

(8) altitude

E. The IRS outputs are displayed on the flight instrument system displays. They are also displayed on the flight management computer system control display unit (FMCS-CDU) (Ref 34-62-01). Preselected parameters are also displayed on the inertial system display unit (ISDU).

2. IRS Theory of Operation

A. General

(1) The IRS provides basic heading and attitude reference accomplished through computations based on accelerometer and laser gyro sensed signals. Three accelerometers and three laser gyros are used. The accelerometers and laser gyros are of the strap-down type and are positioned in the inertial reference units so that they are oriented along each of the three axis of the airplane. This orientation allows the IRU to sense accelerations along and rotation about each of the three axis. Computer manipulation of the signals from all six sensors provide the basic heading and attitude reference signals along with present position, accelerations, ground speed, drift angle and attitude rate information. The first requirement which must be met for proper IRS operation is alignment. IRS alignment basically consists of determination of local vertical and initial heading.

B. Alignment

(1) IRS alignment consists of determining local vertical and initial heading. Both accelerometer and laser gyro inputs are used for alignment. The alignment computations use the basic premise that the only accelerations during alignment are due to the earth's gravity; the only motion during alignment is due to the earth's rotation. Accelerations due to gravity are always perpendicular to the earth's surface and thus define the local vertical. This local vertical is used to erect the attitude data so that it is accurately referenced to vertical. Initially, only a coarse vertical is established. Once vertical is established, the laser gyro sensed earth rate components are used to establish the heading of the airplane. As the alignment continues, both the vertical reference and the heading determinations are fine tuned for maximum accuracy.

(2) The orientation of the vertical axis of the attitude reference relative to the earth's surface is based on airplane position input to the IRU. The initial position entry can be made at any time during the alignment period. Earth rate sensing by the laser gyros allows the IRU to determine initial latitude. This gyro determined latitude is compared to the crew entered latitude. Crew entered longitude is compared to the last stored longitude. These comparisons must be favorable to complete the alignment period. During the alignment period all outputs of the IRU, except for present position, are set to NCD (No Computed Data). The minimum duration of the align mode is 10 minutes.

C. Navigate Mode

(1) In the navigate mode the IRUs provide outputs for attitude, heading, present position, accelerations, track angle, drift angle, ground speed, and wind data. These outputs are all derived from gyro and accelerometer strap down sensor data. The initial attitude, heading and velocity signals are modified by inputs from the sensors to establish real time present parameters through integration and computer calculations. Additional calculations by the computer establish such parameters as present position, ground speed and drift angle. Inputs from the air data computers are used for inertially smoothed altitude and altitude rate (baro altitude) and wind speed/direction (true airspeed).

D. Accelerometer

(1) Each IRU contains three accelerometers, one for each of the three axes: longitudinal, lateral and vertical. Acceleration along the input axis moves the proof mass. Capacitive pickoff converts the position change into an electrical error signal to the servo amplifier. The servo amplifier nulls out the error signal by returning the proof mass to the zero-position using the torquer coil. The current in the torquer coil needed to null the error signal is the analog output signal representing acceleration.




(2) The analog output signal is integrated once to give velocity and integrated a second time to give distance.

(3) A temperature sensor is provided for each axis (X, Y, Z) to improve accelerometer accuracy. Each sensor provides a signal proportional to temperature. This signal is used by the IRU for compensation and correction of sensor data.



E. Laser gyro

(1) The ring laser gyro uses laser light to measure angular rotation. Each gyro is a triangular-shaped, helium-neon laser that produces two light beams, one traveling in the clockwise direction and one in the counterclockwise direction. Production of the light beams, or lasing, occurs in the gas discharge region by ionizing the low pressure mixture of helium-neon gas with high voltage to produce a glow discharge. Light produced from the lasing is reflected around the triangle by mirrors at each corner of the triangle to produce the clockwise and counterclockwise light beams.

(2) The path length around the cavity is carefully monitored and adjusted so that it is an integral multiple of the peak power laser wavelength.

(3) When the laser gyro is at rest, the frequencies of the two opposite travelling laser beams are equal. When the laser gyro is rotated about an axis perpendicular to the lasing plane, a frequency difference between the two laser beams results. The frequency difference is created because the speed of light is constant. One laser beam will thus have a greater apparent distance to travel than the other laser beam in completing one pass around the cavity.

(4) A small amount of light from the two laser beams passes through one of the mirrors (less than 0.2%). The beams are combined by optical frequencies to produce a beat frequency. This takes the form of a fringe (interference) pattern. This beat frequency of light is analogous to two different audio frequencies which combine to produce a third difference frequency.

(5) When the laser beam frequencies differ, a fringe pattern of alternate dark and light stripes is created. Photodiodes sense the fringe pattern rate and direction of movement. The frequency and relative phase of the two diode outputs indicate magnitude and the direction of the gyro's rotation.

(6) At low rotation rates, the small frequency difference between the laser beams leads to beam coupling. This locks the frequencies together at a single false value. To compensate for this effect a peizoelectric dither motor is used to vibrate the laser block through the lock-in region. Dither vibration has a net zero average. It produces no net inertial rotation. The dither motor vibration can be felt on the IRU case and produces an audible hum.

3. IRS Mode Select Unit (MSU)

A. The mode select unit (MSU) provides mode selection for two IRU's through two mode select switches and monitors the operation of the IRU's through two sets of alert lights mounted on the unit.

B. The MSU is mounted on the aft overhead panel in the flight control cabin. The MSU is mounted with four DZUS fasteners and is interfaced with airplane wiring through two rear mounted connectors.

C. The MSU contains two rotary mode select switches, one for the left (L) IRU and one for the right (R) IRU. The positions are OFF, ALIGN, NAV and ATT.

D. When the mode selector switches are in OFF, power is removed from the IRUs.

E. When ALIGN is selected, power is applied to the IRU's and the IRU's normally progress through an alignment period of approximately 10 minutes before the navigational mode is armed. When the switches are maintained in ALIGN, however, the IRU's remain in the align mode. Normal alignment requires the entry of present position into the IRU's.

(1) The NAV position of the switches enables the navigational mode, provided the alignment period is completed in the IRU's. The NAV position may be selected directly from OFF. The NAV mode is the normal operating mode for the IRS. In this mode, the IRS performs inertial navigation functions and outputs normal IRS data to be displayed or used by other systems.

(2) The ATT position of the switches places the IRU's into a reversionary mode of operation. The ATT (attitude) mode is used when failure or total power loss (AC and DC power) is detected in the NAV mode. In this mode, only attitude and heading data is output to the user systems.

F. Each mode select switch has detented positions. The detented positions prevent accidental movement of the switch. In a detented position, the switch must first be pulled out of the detent before selecting another position to prevent damage to the switch. In the NAV position, the switch must first be pulled out of detent before selecting the ATT position. In the ATT position, the switch must first be pulled out of detent before selecting the NAV position. In the ALIGN position, the switch must first be pulled out of detent before selecting the OFF position.

G. Two sets of four annunciators are installed on the MSU, one set for each IRU. Each set of annunciators consists of an ALIGN annunciator (white when lit) and three warning annunciators (amber when lit). The three warning annunciators are ON DC, DC FAIL, and FAULT. DC power for lamp illumination is derived from the IRU or from the master dim test circuit.

(1) The ALIGN light illuminates steady during alignment or flashes when an abnormal alignment is sensed.

(2) The ON DC lights illuminate when 115 volt ac power is removed from the IRU's and the IRU's operate on 28 volt dc. The 28 volt dc continues to power the left IRU but after 5 minutes, power is removed from the right IRU.

(3) The DC FAIL lights illuminate when the airplane battery power is insufficient to maintain IRU operation.

(4) The FAULT Lights illuminate when an abnormal condition exists in the IRUs.

H. The lights may be tested by pressing each light switch assembly, by activating the bright-dim-test circuit or by initiating an IRU test.

4. Inertial System Display Unit (ISDU)

A.The inertial system display unit (ISDU) provides pilot interface with the IRUs. The ISDU allows entry of initialization data for the IRU's. The display of track angle, ground speed, present position, wind direction and speed, heading and system status is available. The ISDU is located on the aft overhead panel, P5. The unit weighs four pounds (1.8 kgms) and is interfaced with airplane wiring through two rear mounted connectors.

C. The ISDU front panel contains two sets of, seven segmented displays. Mounted on the left side of the front panel is a two-position system display (SYS DSPL) switch and a five position display select (DSPL SEL) switch. On the right side of the panel are a set of twelve keys called the keyboard.

(1) The SYS DSPL switch is a two position switch for selecting either the left or the right IRU. The ISDU can only initialize or receive data from the IRU which has been selected on the SYS DSPL switch.

(2) The DSPL SEL switch selects the type of data to be displayed on the ISDU numeric displays. The IRU, as selected on the SYS DSPL switch, supplies the data. The five switch positions and the data displayed for each position is as follows:

(a) TEST - The test position provides a remote test signal to the selected IRU. When held in the TEST position, fixed outputs from the IRU selected by the SYS DSPL switch are displayed on the ISDU and the associated pilot's instruments. The test switch is inhibited during the navigational mode when the airplane ground speed is greater than twenty knots and during the attitude mode. During the first 2 seconds, all display segments and lighted pushbuttons, except the lightplate lamps, illuminate simultaneously and the highlighter bars on the ENT and CLR keys illuminate. Then, for 8 seconds fault messages are generated which are followed by test value displays.

(b) TK/GS (Track Angle/Ground Speed) - True track angle from 0 to 359.9 degrees is displayed in the left display with a resolution of 0.1 degree. Ground speed from 0 to 2000 knots is displayed in the right display with a resolution of 1 knot.

(c) PPOS (Present Position) - Latitude from 90S to 90N is displayed in the left display and longitude from 180E to 180W is displayed in the right display. Resolution is 0.1 minute. The display is used when inserting present position during initialization of the two IRUs or when monitoring present position from an IRU during flight.

(d) WIND (Wind speed and Direction) - Wind speed from 0 to 256 knots is displayed in the right display with a resolution of 1 knot. Wind direction from 0 to 359 degrees is displayed in the left display with a resolution of 1 degree. When the airplane is on the ground, the wind speed display will read 100 knots.

(e) HDG/STS (heading/Status) - True heading from 0 to 359.9 degrees is displayed in the left display with a resolution of 0.1 degree. The ALIGN STATUS appears in the left side of the right display and is a countdown of the last 7 minutes of the ALIGN cycle. Malfunction codes (M/C) appear in the right side of the right display.

(3) BRT - A brightness control knob is concentric with the DSPL SEL switch and is a potentiometer to control brightness of the displays.

(4) There are two numeric displays on the ISDU. When the ISDU is receiving IRU data, the DSPL SEL switch determines the data on the display. When the ISDU keyboard is used to initialize an IRU, the data punched in at the keyboard is shown on the two displays. For invalid data from an IRU, both displays are blanked. A brightness control for the displays is located concentric within the DSPL SEL switch.

(5) The keyboard consists of twelve lighted keys. To change the numeric display from the IRU receive mode to a keyboard display mode, one of the following keys must first be pressed, N(2), W(4), H(5), E(6), or S(8). Any other initial key closure is ignored.

(a) N(2), W(4), E(6), or S(8) when pressed the first time cause a N or W to appear in the left display or an E or S to appear in the right display. They represent north, west, east, and south and are used to initialize the IRU when inserting longitude and latitude. Numbered data can now be inserted which will be appropriately displayed on the numeric display (in ALIGN Mode only).

(b) The H(5) key when initially pressed first, also changes the ISDU display to the keyboard display mode. This key is used in the ATT mode to enter the magnetic heading.

(c) Keys with letters on them as well as numbers, represent the letter value when they are the initially pressed key in a program sequence.

(d) When the N, S, E, W or H key is pressed, the ENT highlighter bar illuminates and stays lit while digits are keyed in. When the ENT key is pressed, the ENT highlighter bar extinguishes and a reasonableness check is done on the data. When reasonable, the data is transmitted to the IRU and the display shows the selected parameters.

(e) If the data is unreasonable, the CLR highlighter bar illuminates and the display retains the unreasonable entry. Pressing the CLR key causes the CLR highlighter bar to extinguish and the ISDU to display the selected parameters.

D. The ISDU is powered from both IRU's with 28 volt dc. The two power sources are connected in parallel to the filter which supplies dc for the operation of the unit. A single 28 volt dc source is sufficient to power the unit.

E. Both IRU's supply ARINC 429 format into the ISDU. The system select switch selects one of the busses and supplies it to the processor which continually updates the stored information from the selected bus. In addition, the processor senses pilot inputs during alignment or during the attitude mode. When supplied with information through the keys from the pilot, the processor functions as a transmitter, supplying ARINC 429 information to the IRU's. When the ENT or CLR keys are pressed, the processor functions as a receiver.

F. The processor consists of a clock, synchronized by the selected IRU, a 32 bit register, which converts the ARINC 429 32 bit word to parallel 8 bit information, the data processor which controls unit operation and a bit sensor. The data processor senses discrete inputs from the display selector and controls the display accordingly.

G. The display circuit consists of a display logic, drivers, decoders and 13 display units. The logic circuit senses display selector position, decoder input and dimming control and accordingly controls the light segmented display.

5. Inertial Reference Unit (IRU)

A. The inertial reference unit (IRU) provides attitude, acceleration, angular rates, velocity, true and magnetic headings, positional data, absolute altitude and wind data signals. The signals are developed from a set of three laser gyros and three accelerometers mounted to airplane reference. The signals are provided to other systems including the flight management computer system, the digital flight control system, the electronic flight instruments system, the autothrottle, the VHF navigation system and the pilots' instruments.

B. The IRU has two power sources, one a 115 volt ac source and one a 28 volt dc source. Either source is sufficient for operation but both are required for initial startup.

C. Inside the IRU, the power control circuit automatically switches to a backup source (hot battery bus) when the normal 115 volt ac source is not available. During startup, the control circuit verifies that the battery is connected by switching off the 115 volt ac input. Inputs from the mode select switch controls the power supply switching circuits. The output of the power supply provides control voltage for the IRU and the ISDU and high voltage excitation to the three laser gyros.

D. All IRU input data is routed into the computation circuit. The air data source supplies the ARINC 429 bus input containing air data. Initialization data originates from either the flight management computer or the ISDU. The IRU program pins tell the computation circuits which way the IRU is facing in the airplane.

E. Software does all computations, including compensations, navigational calculations, and coordinate transformations. Gyro and accelerometer outputs are compensated for sensor bias, scale factor, misalignment, and thermal changes. The compensated signals are used in computing airplane pitch, roll and heading relative to the local navigation coordinates. Airplane attitude is used to resolve acceleration into components relative to the local navigation coordinate system. Barometric altitude from an air data computer is used to compensate the vertical speed computations.

F. Three identical ARINC 429 data buses (high speed) provide digital data to flight management, autoflight, and flight instrument systems. A status discrete is applied to the MSU.

G. AIRPLANES WITH IRU -109 AND PREVIOUS;Built-in test equipment (BITE) circuits isolate faults to the LRU level. Faults are indicated by the yellow fault ball on the front of the IRU, the amber FAULT light on the MSU, and a status bit on the data buses to the user systems. The user systems may record an inflight fault based upon the fault from the IRU. In addition, status words are stored in a non-volatile memory in the IRU for at least the previous nine flights. Contents of the non-volatile memory can be extracted on the ground for maintenance.

H. AIRPLANES WITH IRU -110 AND SUBSEQUENT; Built-in test equipment (BITE) circuits isolate faults to the LRU level. Faults are indicated by the amber FAULT light on the MSU, and a status bit on the data buses to the user systems. The user systems may record an inflight fault based upon the fault from the IRU. In addition, status words are stored in a non-volatile memory in the IRU for at least the previous nine flights. Contents of the non-volatile memory can be extracted on the ground for maintenance.

I. Parameters continuously monitored by the BITE include the laser gyros, accelerometer and power supply outputs, computer and memory operation, and temperature. When the IRU is initially turned on, the battery power is tested. The ON DC indication displays during the battery check.

L. The manual test is initiated by pressing the test switch on the IRU or by holding the display selector in TEST position. The test is also performed as part of the IRS BITE from the FMCS CDU. The test can be done in ALIGN mode, or in NAV mode when ground speed is less than 20 knots. It is inhibited in ATT mode.



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