Ring Laser Gyros (RLGs) are a form of optical rotation sensor and unlike the preceding mechanical rotor systems, contain no moving parts, in their simplest form. Within the sensor, containing a machined quartz block, two laser beams are formed, one moving clockwise and the other anti-clockwise around an enclosed polygonal optical path loop of three, four or more sides with mirrors at the vertices. These laser beams interfere with each other, creating a standing wave(s) diffraction pattern observed by a photo-detector located at one of the vertex mirrors. This is a little like dropping two stones into a still pond, where the waves from each stone meet and form a pattern of waves with even higher peaks and lower troughs where they cancel out.
If the device is rotated, one beam experiences a shift up in frequency, whilst the other experiences a shift down, causing the interference pattern to move. As the device rotates the vertex photo-detector counts the fringes and hence measures the rotation of the sensor. This relativistic phenomenon, is known as the Sagnac effect after G. Sagnac (Frenchman) demonstrated and recognised the condition whilst conducting experiments to detect “the effect of the relative motion of the ether” (1913). Related experiments were also conducted by F. Harress in 1911 but his results were misdiagnosed at the time and attributed to “unexpected bias”.
Typically, a RLG consists of a triangular block of low-expansion quartz. The laser cavity is machined into the glass and filled with He & Ne creating an HeNe laser. A high voltage is applied across areas of this cavity to create the lasing action. At two points of the triangle, very high quality mirrors are placed and at the third vertice the beams are combined in a prism to produce the interference pattern which is detected by a photodiode array. Typically RLG size is around 8cm on a side.
The sensitivity of a Ring Laser Gyro is proportional to the area enclosed by the laser beams and the scale factor of the instrument depends on the ratio of the enclosed area to the path length. RLGs can be extremely accurate devices and can measure a range of rotations from as low as 0.01 deg/hr to more than 360 deg/s. This gives them an enormous dynamic range, of as much as 109. They have excellent scale-factor stability and linearity over this range.
Gyros performance is typically quantified in terms of bias stability and random walk. RLG can have bias levels of 0.01 deg/hr and random walks of 0.005 deg/rt (hr).
To ensure good sensor performance and bias stability the devices must be built in a high standard cleanroom, since any contaminants in the laser cavity will degrade performance. They must be machined from glass blocks with very low coefficient of thermal expansion to ensure that performance is maintained over a wide thermal range. The use of thermal shielding is essential for deeper oilfield applications.
RLGs suffer from a problem known as ‘lock-in’ where back scatter from the laser beams at a mirror causes the interference fringes to ‘lock’ together, giving the sensor a dead band, with no output at very low rotation rates. To minimise lock-in, extremely high quality mirrors are used. Also, typically the sensors are mechanically ‘dithered’, that is, vibrated rapidly and precisely through the dead band. It is small remaining periods in the dead band which causes the random walk performance of the sensor to deteriorate.
Ring laser gyros are very commonly used in inertial navigation systems in both civil and military aircraft, rocket launchers, tanks, artillery and high accuracy attitude systems, such as those used for geophysical surveys from the air.
RLG’s have only been utilised in one borehole survey system within the oil industry to-date. The RIGS Tool was developed by Sundstrand for Eastman Whipstock (Later to become part of Eastman Christiansen, Eastman Teleco and then Baker Hughes INTEQ). The Inertial Measurement Unit for a second generation RIGS was manufactured by Honeywell. The Tool was 5 ¼” in diameter, 14 ft long in standard configuration with a temperature rating of 100°C. A thermal shield allowed RIGS to survey to TD in wells with bottom hole temperatures of up to 150°C. The RIGS Tool demonstrated consistent lateral accuracy performance of 1-2 /1000 MD (2 sigma) at all attitudes. However, the tool was only ever operated within the North Sea Region. It was in commercial service from 1990 to 2006.
The main drawback of currently available RLGs is their relatively large size, which limits their use to >5” dia. Sonde Tools operating within >7” casings. The sensors commercially available have a temperature limitation of around 90 deg C. RLG technology is also currently covered by international arms trafficking laws and associated import and export restrictions which severely restricts product placement, R&M and Tool Utilization for any potential global operation by a service company.
Feedback
Post your comment on this topic.