Fibre optic gyroscope
Encyclopedia
A fibre optic gyroscope (FOG) senses changes in orientation, 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.
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.
The development of diode (semiconductor) lasers
and low-loss single-mode optical fibre in the early 1970s for the telecommunications industry enabled Sagnac effect fibre optic gyros to be developed as practical devices. 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 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 .
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
, the FOG has absolutely no moving parts and doesn't rely on inertial resistance to movement. Hence this is perhaps the most reliable alternative technology to the mechanical gyroscope. Because of their intrinsic reliability, FOG are used for high performance space applications.
The FOG typically shows a higher resolution than a ring laser gyroscope
but suffered from greater drift and worse scale factor performance until the end of the 1990s.
FOGs are used in surveying , stabilization and inertial navigation system
s.
FOGs are implemented in both open-loop
and closed-loop configurations.
Gyroscope
A gyroscope is a device for measuring or maintaining orientation, based on the principles of angular momentum. In essence, a mechanical gyroscope is a spinning wheel or disk whose axle is free to take any orientation...
. However its principle of operation is instead based on the interference of light which has passed through a coil of optical fibre
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
which can be as long as 5 km.
Two beams from a laser are injected into the same fibre but in opposite directions. Due to the Sagnac effect
Sagnac effect
The Sagnac effect , named after French physicist Georges Sagnac, is a phenomenon encountered in interferometry that is elicited by rotation. The Sagnac effect manifests itself in a setup called ring interferometry. A beam of light is split and the two beams are made to follow a trajectory in...
, the beam travelling against the rotation experiences a slightly shorter path delay than the other beam. The resulting differential phase shift
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
is measured through interferometry, thus translating one component of the angular velocity
Angular velocity
In physics, the angular velocity is a vector quantity which specifies the angular speed of an object and the axis about which the object is rotating. The SI unit of angular velocity is radians per second, although it may be measured in other units such as degrees per second, revolutions per...
into a shift of the interference pattern which is measured photometrically.
Photodiode
A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation.The common, traditional solar cell used to generateelectric solar power is a large area photodiode....
The development of diode (semiconductor) lasers
Laser diode
The laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. The most common type of laser diode is formed from a p-n junction and powered by injected electric current...
and low-loss single-mode optical fibre in the early 1970s for the telecommunications industry enabled Sagnac effect fibre optic gyros to be developed as practical devices. 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
Sagnac effect
The Sagnac effect , named after French physicist Georges Sagnac, is a phenomenon encountered in interferometry that is elicited by rotation. The Sagnac effect manifests itself in a setup called ring interferometry. A beam of light is split and the two beams are made to follow a trajectory in...
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 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 .
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
Gyroscope
A gyroscope is a device for measuring or maintaining orientation, based on the principles of angular momentum. In essence, a mechanical gyroscope is a spinning wheel or disk whose axle is free to take any orientation...
, the FOG has absolutely no moving parts and doesn't rely on inertial resistance to movement. Hence this is perhaps the most reliable alternative technology to the mechanical gyroscope. Because of their intrinsic reliability, FOG are used for high performance space applications.
The FOG typically shows a higher resolution than a ring laser gyroscope
Ring laser gyroscope
A ring laser gyroscope consists of a ring laser having two counter-propagating modes over the same path in order to detect rotation. It operates on the principle of the Sagnac effect which shifts the nulls of the internal standing wave pattern in response to angular rotation...
but suffered from greater drift and worse scale factor performance until the end of the 1990s.
FOGs are used in surveying , stabilization and inertial navigation system
Inertial navigation system
An inertial navigation system is a navigation aid that uses a computer, motion sensors and rotation sensors to continuously calculate via dead reckoning the position, orientation, and velocity of a moving object without the need for external references...
s.
FOGs are implemented in both open-loop
Open-loop
Open-loop may refer to:*Open-loop controller of a dynamical system*Open-loop model in game theory*Open loop rhetorical device...
and closed-loop configurations.
Applications
- FOGs are used in the inertial navigation systems of many guided missiles.
- FOGs can be a navigation aid in ROVs
See also
- Ring laser gyroscopeRing laser gyroscopeA ring laser gyroscope consists of a ring laser having two counter-propagating modes over the same path in order to detect rotation. It operates on the principle of the Sagnac effect which shifts the nulls of the internal standing wave pattern in response to angular rotation...
- GyroscopeGyroscopeA gyroscope is a device for measuring or maintaining orientation, based on the principles of angular momentum. In essence, a mechanical gyroscope is a spinning wheel or disk whose axle is free to take any orientation...
- Vibrating structure gyroscopeVibrating structure gyroscopeA vibrating structure gyroscope is a type ofgyroscope that functions much like the halteresof an insect.The underlying physical principle is that a vibratingobject tends to continue vibrating in the same planeas its support rotates...
- Quantum gyroscopeQuantum gyroscopeA quantum gyroscope is a very sensitive device to measure angular rotation based on quantum mechanical principles. The first of these has been built by Richard Packard and his colleagues at the University of California, Berkeley...
- Inertial measurement unitInertial measurement unitAn inertial measurement unit, or IMU, is an electronic device that measures and reports on a craft's velocity, orientation, and gravitational forces, using a combination of accelerometers and gyroscopes. IMUs are typically used to maneuver aircraft, including UAVs, among many others, and...