(3)
A second form of apparent precession is due to vehicle motion over the curved surface of the earth. Consider the gyro to be in an airplane flying north along a meridian from the equator to the pole. If the gyro spin axis were oriented horizontal and parallel with the meridian when at the equator, the gyro spin axis (being space stabilized) would appear to rotate to a vertical orientation relative to earth as the airplane flies north. Consequently, in order to use the gyro as an earth-reference device in a moving airplane, it must be torqued to compensate for north-south airplane motion over the earth's surface in addition to earth rate.
(4)
Similarly, when flying east or west and starting with the gyro spin axis oriented in the horizontal and aligned east-west, the spin axis will appear to tilt due to travel east or west. This tilt is in addition to that caused by earth rate. Therefore, a gyro precessing torque is also necessary to maintain the earth reference orientation during east-west travel.
(5)
The total gyro torquing signals necessary to slave a moving gyro to an earth-related coordinate system consist of an earth rate component, a vehicle velocity component, and a gyro drift rate compensation component.
D. Inertial Platform
EFFECTIVITY
AIRPLANES WITH DELCO INS CONFIG 1 03 A Page 14 Apr 25/8634-41-00
(1)
An airplane moving in three dimensions over the earth's surface has 6 degrees of freedom, three translational (north-south, east-west, and up-down) and three rotational (roll, pitch, and azimuth). Actual airplane motion is a changing combination of all 6 degrees of motion. Sensing of this motion is accomplished by a gyro stabilized frame of reference known as an inertial platform. The platform consists of a tilt table suspended in a servo-driven gimbal system. The gimbal system isolates the acceleration sensing devices from rotational motion of the airplane and maintains the platform parallel to the earth's surface regardless of airplane motion or earth rotation. The gimbal system effectively decouples the inertial platform from the airframe and provides all-attitude freedom of motion.
(2)
Platform stabilization is achieved by use of three single-degree of freedom gyros mounted on the platform in the al1-attitude gimbal system. The gyros are mounted with their sense (input) axes mutually perpendicular. Any minute tilt or rotation of the platform produces an input torque about the sense axis of one or more of the gyros. This causes induced gyroscopic precession which causes the gyro gimbal to rotate about the gyro output axis and generate an output signal from the gyro pickoff which is representative of the angular displacement of the gimbal. This signal is amplified and used to drive the platform gimbal torquer through a corresponding angle of rotation to maintain the gyro output at a null, and thus maintain correct platform orientation.
(3)
Stabilization of the inertial platform allows a fixed coordinate system to be established within which airplane accelerations can be measured. This is accomplished by two accelerometers, mounted on the stable platform with their sense axes mutually perpendicular and aligned with reference axes on the platform. The accelerometers sense airplane translational motion and are maintained parallel to the earth's surface. A third accelerometer is mounted on the platform to sense vertical acceleration.
E. Computer
(1) The inertial navigation problem uses continuously changing navigation parameters and system error correction factors to obtain a high degree of accuracy in the attitude and heading computations. For this reason a computer is used as the information processing center for the inertial navigation system. The computer:
(a)
solves mechanization equations associated with great circle navigation
(b)
supplies control information to the inertial platform to keep it earth referenced
(c)
supplies digital information to various INS and interface components in order to display system status and navigation parameters.
(d)
monitors its own operation and that of other INS components
(e)
supplies guidance signals to the autopilot to steer the airplane along a great circle path
(f)
solves mechanization equations associated with calibration and updating performance parameters of the INS components
EFFECTIVITY
AIRPLANES WITH DELCO INS CONFIG 1 03 A Page 15 Apr 25/8634-41-00
IN DUAL DME UPDATING, THE INS USES SLANT RANGE INPUTS FROM TWO DME STATIONS IN THE PROCESS OF UPDATING THE PRESENT POSITION OF THE AIRPLANE. POINT A REPRESENTS THE ACTUAL PRESENT POSITION OF THE AIRPLANE AND POINT I REPRESENTS THE PRESENT POSITION AS CONTAINED IN THE INS SYSTEM. THE INS KNOWS THE POSITION COORDINATES OF POINT I AND BOTH DME STATIONS.
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本文鏈接地址：747飛機維護手冊AMM CHAPTER 34 - NAVIGATION 第34章導航2(84)
