Any body spinning about a movable axis can be considered to be a gyroscope, however the first practical application of the gyroscope was not until 1744, when Serson persuaded the British Admiralty to test at sea a spinning rotor which would indicate a stable horizontal reference for ships. The rotor was supported on a pivot so as to be free from disturbances caused by a ship ? forerunner of the gyroscope horizon used in modern aircraft.

In fact the most remarkable advances in the uses of gyroscopes has been in the air. The are used essentially for navigation and control in an accurate vertical reference. In artificial horizons and autopilots a gyroscope with a vertical axis (like an upright top) is used. As the aircraft climbs or rolls the angle is assumed relative to the unchanging gyro-axis provides a reading which can be used, either directly by the pilot or as a part of an automatic control system.

During the past 20 years these airborne applications have culminated in complex ?Inertial Navigation Systems?, at the heart of which are miniature precision gyroscopes capable of detecting turns of small fractions of a degree per hour ? much smaller than the slow rotations of the earth.

The system can operate as self-contained units, recording the precise position and orientation of the aircraft independent of any external source or reference, over flights of many thousands of miles.

The uses of gyroscopes as turn-and-bank indicators, aircraft horizons, and automatic pilots in aircraft and missiles are well known.

The latest in gyroscopic technology involves the use of laser, the laser gyro is an ideal candidate for many navigation, guidance, and attitude-reference applications.

Use of the laser gyro as an inertial component offers distinct advantages including;

Instantaneous operation

Input axis precisely defined by the lasing plane

Inherent digital output

Freedom from gravitational and accelerational errors, both angular and linear

The latest in laser gyroscopes have demonstrated performance accuracies to better then 0.01 degrees/hour.

The invention the laser makes it possible to construct a devise which can measure angular rotation by using light. The gas laser (usually helium-neon) has been commonly used because it can be operated continuously, has desirable gas properties and requires very little power.

The acronym ?laser? stands for ?Light Amplification by Stimulated Emission of Radiation?. To obtain ?lasing? action, two conditions must be satisfied. First, a gain (or amplification) medium to overcome losses must be present. This gain medium consists of a mixture of helium and neon gases at a very low pressure, which are excited with an electrical discharge.

A voltage is applied across the metallic anode and cathode. This voltage ionizes the gas, producing a glow discharge. Excited helium atoms collide with neon atoms, transferring energy to the neon atoms and raising them temporarily to higher energy levels.

A few neon atoms spontaneously emit a photon (or packet of light) as they fall back to a normal energy level. These photons strike other excited neon atoms, ?stimulating? the release of new photons, which, in turn, do the same. A cascade of photons is produced in all directions. The resulting distribution of neon atoms, more in the upper state than the lower state, provides a net gain, which can be set greater than the cavity loss.

The second condition required to obtain lasing is a resonant cavity. If mirrors are placed at each end of the tube, the photons travelling parallel to the tube will be reflected back and forth many times. The mirrors become the ends of a resonant cavity; they produce the positive feedback necessary to sustain oscillation. The length of the cavity is many thousands of times the optical wavelength. Therefore, the cavity is resonant at a large number of frequencies, but most of these frequencies do not receive sufficient gain to oscillate.

When any mode (or resonant frequency) receives gain greater than its losses, the radiation intensity builds up to a steady-state value and is emitted through the mirrors which may allow about 0.2 percent transmission. The emission is the laser radiation, which is essentially a single frequency. This resonant cavity provides a phase boundary condition in which the laser radiation is required to have zero phase shift per pass or trip around the cavity. This condition is most important to the gyro performance.

An operating gas discharge is used as a gyro by adding a third mirror and orientating the mirrors so that the radiation is reflected at 60 degrees. The radiation, instead of passing back and forth, now travels around an enclosed area in both directions simultaneously. A small portion of the light escapes through each mirror. Both clockwise and counterclockwise beams occupy the same space at the same time and have the same frequency when no input rate is present.

If the base in which the gas discharge and the mirrors are mounted is rotated, one beam will have a greater apparent distance to travel in making one revolution, and the other beam will have a shorter distance.

During this rotation, one beam has a somewhat different frequency that the other. That frequency difference is directly proportional to the rate with which the base is rotated. More specifically,

(area enclosed) (rotation rate)

D f = K --------------------------------------------- cps

(optical path length) (wavelength)

This triangular arrangement is often called a ring laser because the light is travelling around an enclosed area. The term laser gyro is more specific (a ring laser may have additional applications besides sensing rates) and is equally correct since the word gyro is from the Greek ?gyros? meaning ?ring?. The output is the integral of the input rate.

The fundamental limitations to laser gyro performance are;

Lock-in effects (Limits measurement of low input rates)

In summary, the design of the Laser Gyro overcomes the fundamental limitations to performance by incorporating;

Oscillating bias (dither) to minimise the effect of lock-in.

1. When was the first practical use of the gyroscope?

It was in 1744, when Serson persuaded the British Admiralty to test at sea a spinning rotor, which would indicate a stable horizontal reference for ships.

2. What are the common uses of a gyroscope?

The most remarkable advances in the uses of gyroscopes have been in the air. They are used primarily for navigation and control in an accurate vertical reference. The uses of gyroscopes as turn and bank indicators, aircraft horizons, and automatic pilots in aircraft and missiles are well known.

3. What are the advantages of the laser-gyro?

Use of the laser gyro as an inertial component offers distinct advantages including;

• Instantaneous operation
• Input axis precisely defined by the lasing plane
• Inherent digital output
• Freedom from gravitational and accelerational errors, both angular and linear
• Extreme bandwidth, limited at high rates only by the state of the art in electronics, not by fundamental limitations of the laser
• Simple, rugged, low cost design

1. How great an accuracy can the laser gyro demonstrate?

The latest gyroscopes have demonstrated performance accuracy to be better than 0.01 degrees per hour.

2. What are the limitations of the laser-gyro?

The fundamental limitations to laser gyro performance are;

• Lock-in effects (Limits measurement of low input rates)
• Variation of scale factor with gain and mode position (The angle represented by each output count of the laser gyro, varies with the laser gain and the mode position control0
• Variation of null with mode position and discharge current (All laser gyros exhibit a null shift which is a function of cavity length. Automatic length stabilization is incorporated to eliminate this source of error)

1. How are these limitations overcome?

• Oscillating bias (dither) to minimize the effect of lock-in.
• Active cavity length control to minimize scale factor and null variation due the changes in mode position.
• Discharge current control to minimize scale factor variation due to changes in gain.
• Differential current control to minimize null variation due to differential changes in current.

1. What is the common medium used to minimize losses in the transfer of energy?

A gain (amplification) medium to overcome losses must be present. This gain medium consists of a mixture of helium and neon gases at a very low pressure, which are excited with an electrical discharge.

2. How many mirrors does the ring laser-gyro incorporate and at what angle are they positioned?

There are three mirrors altogether, with two mirrors are placed at each end of the tube and become the ends of a resonant cavity. Whilst the third mirror is added as gyro by using an operating gas discharge. The mirrors are orientated so that the radiation is reflected at 60 degrees. Hence the radiation now travels around an enclosed area in both directions simultaneously.

3. Diagrammatically represent the ring laser-gyro configuration.

 

 

 

4. What are the two conditions that must be satisfied to obtain a "lasing" action?

There are two conditions must be satisfied. First, a gain (or amplification) medium to overcome losses must be present. This gain medium consists of a mixture of helium and neon gases at a very low pressure, which are excited with an electrical discharge. The second condition required to obtain lasing gas a resonant cavity. Here the mirrors become the ends of a resonant cavity, which are placed at each end of the tube. They produce the positive feedback necessary to sustain oscillation.

5. Mention the latest application in gyroscopic technology?

The latest in gyroscopic technology involves the use of laser. The laser gyro is an ideal candidate for many navigation, guidance and altitude-reference applications.

6. What is the progress of the gyroscope technology during the last twenty years?

During the last twenty years the uses of gyroscopes in airborne applications have culminated in complex "inertial navigation system", at the heart of which are miniature precision gyroscopes capable of detecting turns of small fractions of degree per hour ? much smaller than the slow rotations of the earth.

7. How is the gyroscope used as part of an automatic control system?

In artificial horizons and autopilots a gyroscope with a vertical axis (like an upright top) is used. As the aircraft climbs or rolls, the angle is assumed relative to the unchanging gyro-axis, which provides a reading, which can be used as a part of an automatic control system.

8. Why do we use gas laser as the common medium in the latest gyroscopic technology?

The invention of the laser technology makes it possible to construct a devise, which can measure angular rotation by using light. The gas laser (usually helium-neon) has been commonly used because it can be operated continuously, has desirable gas properties, and requires very little power.

9. What is the most important condition to the gyro performance required to achieve?