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16.15.2. Ring Laser Gyro. Accuracy and dependability of first generation systems have greatly
improved with the introduction of the ring laser gyro (RLG) INS. The RLG INS replaces the three
pendulous mass accelerometers with three RLG accelerometers. Technically the RLG is not a gyroscope
since it has no moving parts, but it gives the same information as a gyro. A RLG is made from a single
block of glass with three holes drilled through the glass to form a triangular path. Two of the openings
are plugged with mirrors and the triangular tube is filled with helium neon or other lazing gas. When the
gas is charged, the lazing gas produces two counter-rotating laser beams that are reflected around the
path by the mirrors. Both laser beams emerge through the third hole in the glass and are superimposed
upon each other to produce an interference pattern. As the RLG moves, one beam has a longer path to
travel; the other a shorter path. This causes changes in the interference pattern, which are detected by
photocells. The angular rate and direction of motion are computed as accelerations.
16.15.3. Acoustic Gyros. Another recent development is the inertial sensor based on vibrating quartz
crystal technology. Like the RLG, these are not true gyros. Acoustic gyros are manufactured from a
single piece of microminiature quartz rate sensor. Angular accelerations affect the patterns produced by
a vibrating tuning fork and result in torque on the fork proportional to the angular acceleration. These
gyros appeared in inertial units in the late 1990s.
16.16. Stable Platform. Stable platforms have been used for years in bombing and fire control systems.
Autopilots and attitude indicators use gyrostabilized platforms. Inertial navigation simply requires a
stable platform with higher specifications of accuracy. A gyro-stabilized platform on which
accelerometers are mounted is called a stable element. It is isolated from the aircraft's angular motions
by three concentric gimbals. The stable element is the mounting for the linear accelerometers,
gyroscopes, and other supporting equipment. The supporting equipment includes torque motors, servo
motors, pickoffs, amplifiers, and wiring. The effectiveness of the stable platform is determined by all
parts of the platform, not just the accelerometers and gyros.
16.16.1. The linear accelerometers measure acceleration in all directions and the gyros control the
orientation of the platform. The platform must contain at least two gyros with two degrees of freedom. A
simple diagram of a two-degrees-of-freedom gyro mounted on a single-axis platform is shown in Figure
16.2. If one-degree-of-freedom rate gyros are used, three units are needed, each gyro having its own
independent feedback and control loop. The original gimbaled gyro was not very accurate by today's
standards, producing sizeable amounts of gyroscopic precession. Recent developments such as the airbearing
gyro and the electronically suspended gyro have only 1/10,000,000 the friction of a standard
gyro and negligible real precession. Today's gyros have real precession rates of less than 360° in 30
years.
AFPAM11-216 1 MARCH 2001 333
Figure 16.2. Stable Platform.
16.16.2. The desired property of a gyro that we want to capitalize on is its stability in space. A spinning
gyro tends to remain in its original position. A free spinning gyro aligned in space tends to remain
pointed in the same direction unless a force acts on it. On a stable platform, any displacement of the
stable element from its frame of reference is sensed by the electrical pickoffs in the gyroscopes. These
electrical signals are amplified and used to drive the platform gimbals to realign the stable element in the
original position. More advanced INS have a four-gimbal platform in a three-axis configuration (Figure
16.3).
Figure 16.3. Gimbal Platform.
16.16.3. The four-gimbal mounting provides a full 360° freedom of rotation about the stable element,
thus allowing it to remain level with respect to local gravity and to remain oriented to true north. This is
north as established by the gyros and accelerometers, regardless of the in-flight attitude of the aircraft.
The azimuth, pitch, and outer roll gimbals have a 360° freedom of rotation about their own individual
axis. The fourth or inner roll gimbal has stops limiting its rotation about its axis. This gimbal is provided
334 AFPAM11-216 1 MARCH 2001
to prevent gimbal lock, which is a condition that causes the stable element to tumble. Gimbal lock can
occur during flight maneuvers, such as a loop, when two of the gimbal axes become aligned parallel to
each other, causing the stable element to lose one of its degrees of freedom.
16.17. Measuring Horizontal Acceleration. The key to a successful inertial system is absolute
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