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Inertial Navigation System: Difference between revisions
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However, they do have their disadvantages. One is reliability -- the spinning gyros and gimbals all move, so you have wear and tear and they can fail or lose their accuracy. However, a more prominant issue is with "gimbal lock."[http://history.nasa.gov/alsj/gimbals.html] This is when two of those three gimbals align. Since they both function about one axis, and the other only does one axis too, you only have two axes -- any rotation about the last axis cannot occur, so the platform is swung and misaligned with rotation about that axis. There are two main solutions: navigate around it or a fourth gimbal. On Apollo 11 there were only three gimbals, so they planned their maneuvers around gimbal lock. Their computers told them where not to go to keep two gimbals from aligning. The second, but more complex, solution is a fourth gimbal. This gimbal is motorized to keep it always oriented away from the other gimbals, so you keep three independent axes at all times. However, this is mechanically more complex. The fourth-gimbal system is used more in more recent gimballed inertial navigation systems. | However, they do have their disadvantages. One is reliability -- the spinning gyros and gimbals all move, so you have wear and tear and they can fail or lose their accuracy. However, a more prominant issue is with "gimbal lock."[http://history.nasa.gov/alsj/gimbals.html] This is when two of those three gimbals align. Since they both function about one axis, and the other only does one axis too, you only have two axes -- any rotation about the last axis cannot occur, so the platform is swung and misaligned with rotation about that axis. There are two main solutions: navigate around it or a fourth gimbal. On Apollo 11 there were only three gimbals, so they planned their maneuvers around gimbal lock. Their computers told them where not to go to keep two gimbals from aligning. The second, but more complex, solution is a fourth gimbal. This gimbal is motorized to keep it always oriented away from the other gimbals, so you keep three independent axes at all times. However, this is mechanically more complex. The fourth-gimbal system is used more in more recent gimballed inertial navigation systems. | ||
===Strapdown=== | ===Strapdown=== | ||
Strapdown systems have all their sensors mounted on a platform that changes orientation like the plane. Instead of mechanical gyros to hold it level, it has three more accurate gyros that sense the orientation of the system. Additionally, it has the same three acceleration sensors. | |||
Whereas the gimballed system just senses the orientation of the platform to get the aircraft's attitude, the strapdown systems have three gyroscopes that sense the rate of roll, pitch, and yaw. It integrates them to get the orientation, then calculates the acceleration in each of the same axes as the gimballed system. | |||
Due to the sensing of the rate of rotation, rather than just holding a platform level, very accurate and sensitive gyroscopes are needed. There are several sensitive gyroscope types available, but the most commonly used on is the ring laser gyro. This basically is a circular path (actually, usually triangular) that laser light travels around. This light goes both ways. When it is rotated, one direction appears to go faster than the other -- and when they get around and meet, they interfere. This creates a pattern that is picked up by sensors. These gyroscopes are so accurate the alignment for strapdown systems consists of finding true north by sensing the earth's rotation. | |||
Strapdown systems have fewer moving parts, so they are more reliable and simpler than other systems. However, they do need more accurate gyroscopes and better computers, so they are a more recent development. | |||
==References== | ==References== | ||
'''''Page under construction''''' | '''''Page under construction''''' | ||