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[1] The inertial reference system (IRS) is one part of the Flight Management System (FMS). It is responsible for providing baseline navigation information, vertical speed, and airplane altitude information. The IRS consists of two inertial reference unit (IRU), a mode select unit (MSU) , an inertial system display unit (ISDU) and two digital-to-analog adapter (DAA). Latitude and longitude are entered into the system at the starting point during alignment. After alignment, airplane movement is sensed by inertial laser gyros and accelerometers. The movement is computed by the IRS, and provided as output data to other airplane systems. Output data includes ground speed, drift angle, present position, attitude, heading, and angular information .
Fig. 32-1 inertial reference system
[2] The IRS provides heading and attitude reference information by utilizing computations based on accelerometer and laser gyro sensed signals. Three accelerometers and three laser gyros are used. The accelerometers are positioned in the inertial reference units so that they are oriented along the x, y and z axes of the airplane. This orientation allows the IRU to sense accelerations along each of the three axes. The three laser gyros are positioned to sense pitch, roll and yaw rotation around the x, y , and z axes. These accelerometers and laser gyros are in the strap-down configuration. That is , they are oriented with respect to each of the aircraft's three axes and are not allowed to move when the aircraft rotate about or accelerates along any of these axes. 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.
Fig. 32-2 IRS general theory
[3] The first requirement which must be met for proper IRS operation is alignment, IRS alignment basically consists of determination of local vertical and true north. Both accelerometer and laser gyro inputs are used for alignment. The alignment computations use the basic premise that the only accelerations during alignment are caused by earth's gravity. The only motion during alignment is caused by the earth's rotation. Acceleration because of gravity are always perpendicular to the earth's surface, see figure 32-3. Three accelerometers are used to sense the gravity components of the airplane. The three signals are manipulated within the computer to give us local verticle. Three laser gyros are used to sense the earth's angular rate components on the airplan's lateral, longitudinal and vertical axes. The IRU computer uses the direction of angular rate to determine the direction of true north .The present position ( latitude/longitude) must be entered during the 10 minutes alignment period. When the present position has been entered into the IRU, it is compared to the last known stored present position. If the entered present position is within 1 degree of the stored present position, the IRU will enter the navigation mode. 中國(guó)航空網(wǎng) m.k6050.com 航空翻譯 www.aviation.cn 本文鏈接地址：第31課 大氣數(shù)據(jù)系統(tǒng) LESSON 31 AIR DATA SYSTEM(6)
