Fibre optic gyroscope

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The interference on a Sagnac interferometer is proportional to the enclosed area. A looped fibre-optic coil multiplies the effective area by the number of loops.

A fibre optic gyroscope (FOG) senses changes in orientation using the Sagnac effect, thus performing the function of a mechanical gyroscope. However its principle of operation is instead based on the interference of light which has passed through a coil of optical fibre, which can be as long as 5 km.

Operation[edit]

Two beams from a laser are injected into the same fibre but in opposite directions. Due to the Sagnac effect, the beam travelling against the rotation experiences a slightly shorter path delay than the other beam. The resulting differential phase shift is measured through interferometry, thus translating one component of the angular velocity into a shift of the interference pattern which is measured photometrically.

Beam splitting optics launches light from a laser diode into two waves propagating in the clockwise and anticlockwise directions through a coil consisting of many turns of optical fibre. The strength of the Sagnac effect is dependent on the effective area of the closed optical path: this is not simply the geometric area of the loop but is enhanced by the number of turns in the coil. The FOG was first proposed by Vali and Shorthill[1] in 1976. Development of both the passive interferometer type of FOG, or IFOG, and a newer concept, the passive ring resonator FOG, or RFOG, is proceeding in many companies and establishments worldwide.[2]

Advantages[edit]

A FOG provides extremely precise rotational rate information, in part because of its lack of cross-axis sensitivity to vibration, acceleration, and shock. Unlike the classic spinning-mass gyroscope or resonant/mechanical gyroscopes, the FOG has no moving parts and doesn't rely on inertial resistance to movement. Hence, the FOG is an excellent alternative to a mechanical gyroscope. Because of their intrinsic reliability, FOGs are used for high performance space applications and military inertial navigation systems. [3]

The FOG typically shows a higher resolution than a ring laser gyroscope.

FOGs are implemented in both open-loop and closed-loop configurations.

Disadvantages[edit]

FOGs require initial calibration (determining which indication corresponds to zero angular velocity).

Some FOG designs are somewhat sensitive to vibrations[4]. However, when coupled with multiple-axis FOG and accelerometers, the impact is mitigated, making FOG systems suitable for high shock environments, including gun pointing systems for 105mm and 155mm howitzers.

Applications[edit]

  1. FOGs are used in Fibre optic gyrocompasses.[5]
  2. FOGs are used in the inertial navigation systems of some guided missiles.[6]
  3. FOGs can be a navigation aid in remotely operated vehicles and autonomous underwater vehicles.[7][8]
  4. FOGs are used in surveying.

See also[edit]

References[edit]

  1. ^ V.Vali, R.W. Shorthill, ‘Fiber Ring Interferometer, Appl Optics’, 15 (1976)
  2. ^ Hervé Lefèvre, "The Fiber-Optic Gyroscope", 1993, ARTECH HOUSE, INC. ISBN 0-89006-537-3
  3. ^ IXBlue website: Space Applications
  4. ^ https://www.osapublishing.org/ao/abstract.cfm?uri=ao-56-13-3848
  5. ^ Seamanship Techniques:Shipboard and Marine Operations, D. J. House, Butterworth-Heinemann, 2004, p. 341
  6. ^ https://cablegatesearch.wikileaks.org/cable.php?id=10STATE9939
  7. ^ http://www.oceantools.co.uk/oceanfog-3k-subsea-fibre-optic-gyro/p1
  8. ^ IXBlue website: Subsea Navigation and Positioning

Sources[edit]

  • Anthony Lawrence, Modern Inertial Technology: Navigation, Guidance, and Control, Springer, Chapters 11 and 12 (pages 169–207), 1998. ISBN 0-387-98507-7.
  • G.A. Pavlath, "Fiber-optic gyroscopes", IEEE Lasers and Electro-Optics Society (LEOS) Annual Meeting, 1994. LEOS '94 Conference Proceedings, Volume 2, pages 237–238. 31 Oct–3 Nov 1994.
  • R.P.G. Collinson, Introduction to Avionics Systems, 2003 Kluwer Academic Publishers, Boston. ISBN 1-4020-7278-3.
  • José Miguel López-Higuer, Handbook of Fibre Optic Sensing Technology, 2000, John Wiley & Sons Ltd.
  • Hervé Lefèvre, The Fiber-Optic Gyroscope, 1993, Artech House. ISBN 0-89006-537-3.