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export restrictions which severely restricts product placement, R&M and Tool Utilization for any potential global operation by a service company.
9.8 Fibre Optic Gyro
Fibre Optic Gyros (FOGs) consist of a coil of fibre optic cable in which two light beams travel through the entire cable length in opposite directions and are then combined. The development of low loss, single mode, optical fibre in the ly 1970’s, n bled Sagnac effect fibre optic gyros to be developed. The sensor operates on a similar principle to the RLG, where the interference pattern created from the counter-propagating light waves, after travelling through the fibre, is a measure of the angular rotation of the device. However, they differ in that an incoherent broadband light source is used. Fibre optic gyros tend to be packaged in cylindrical containers, for example 10cm diameter by 2.0cm deep. A sensor may contain as much as 5 kilometres of fibre. FOG sensitivity is a function of coil radius (enclosed area) and optical path length, so once again larger sensors tend to be more accurate sensors.
FOG performance in general, is similar to but not quite as good as RLG. Bias stabilities of 0.1 deg/hr or better and random walks of 0.005 deg/rt(hr) would be typical. RLG has inherently better scale factor stability.
Outside of the oilfield, FOG sensors tend to be used for similar applications to RLGs, but for those applications where environmental conditions and accuracy is less important and where cost is a factor. Although experimental devises have been developed, no FOG system has been commercially marketed within the oilfield. Once again sensor size and temperature concerns are limiting factors. In general FOG performance is more sensitive than RLGs to environmental conditions such as shock, vibration and temperature gradients.
Large diameter, high accuracy systems have been developed and are in use in space and submarine applications, where size restrictions are secondary.
9.9 Vibrating Structure Gyroscope Coriolis Effect
The Coriolis Effect is an inertial force first described by the 19th century French engineer-mathematician Gustave- Gaspard Coriolis in 1835. Coriolis showed that if the ordinary Newtonian laws of motion of bodies are to be used in a rotating frame of reference, an inertial force – acting to the right of the direction of body motion for counter clockwise rotation of the reference frame or to the left for clockwise rotation – must be included in the equations of motion. The effect of the Coriolis force is an apparent deflection of the path of an object that moves within a rotating coordinate system. The object does not actually deviate from its path, but it appears to do so because of the motion of the coordinate system. A simple demonstration example of the effect is a ball rolling across the surface of a rotating merry-go-round.
9.10 Coriolis Vibratory Gyros (CVG)
A Coriolis Vibratory Gyro (CVG) operates on the principal that a vibrating object (mass) tends to keep vibrating in the same plane as its support is rotated in space. This type of device is known as a Coriolis Vibratory Gyro where the plane of oscillation of a proof mass is rotated, the orthogonal response resulting from the Coriolis term in the equations of motion, is detected by a pickoff transducer. CVGs have been produced in various forms, including the original Foucault pendulum (1851), vibrating beams, tuning forks, vibrating plates and vibrating shells. In the Foucault pendulum (non-commercial), the swing path of the pendulum rotates a fraction of the E h’s rotation, dependent on the location latitude. Due to friction effects in the mounting fixture, some of the energy is transposed into quadrature effects, so that the pendular path becomes elliptical and theoretically, ultimately circular, negating the angular measuring capability of the system. This unwanted
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