Page | 55
quadrature effect is present in the majority of CVG designs and necessitates quadrature suppression control loop electronics, signal processing and compensation. Two CVG systems show potential [Tuning Fork Gyro (TFG) and the Hemispherical Resonator Gyro (HRG)] and these sensor developments are continuously kept under observation by the industry. 9.10.1 Micro electro mechanical System (MEMS) – Tuning Fork Gyro A typical MEMS-TFG incorporates a single or dual (contra – mass-balanced) tuning fork (Proof Mass) arrangement, integrated on a silicon chip. Capacitive measurements are made between the fork tines (Proof Mass) as they contort relative to device rotation (Coriolis Effect). Current fabrication and manufacturing techniques enables the production of very small sensor devices with proof mass size typically only 1-5 mm. However, even the best commercially available MEMS gyro struggles to reach a bias performance of 1 deg/hr, with units in associated defence developments approaching 0.3 deg/hr. Oilfield applications requires the sensor have a performance capability within the range 0.1- 0.01 deg/hr. MEMS Gyros have to-date found a wide market in low performance applications. They are used in cars, smartphones, gaming systems etc. In the navigation world they are used to assist GPS acquisition in artillery shells, but have not yet been used for any form of unaided inertial navigation. The small size and mass of current sensors results in lower performance and resolution with poorer signal to noise characteristics. Their size does however lend the device to thermal shield encapsulation providing the necessary thermal operating stability. Improved performance is expected to follow as the industry succeeds in developing techniques and production methods to produce thicker and larger component parts including the proof mass. This should result in improved stability and signal to noise characteristics and hence improved performance. These developments will hopefully lead to low volume, high accuracy, high value applications and the Holy Grail for down- hole surveying in small, rugged, robust, vibration insensitive, accurate inertial navigation systems which can operate throughout all phases of the drilling process. 9.10.2 Hemispherical Resonator Gyro (HRG) The HRG is often referred to as the Wineglass gyro as the CVG properties for a wineglass were first discovered and noted by Bryan in 1891. Due to Coriolis Forces, a vibration standing wave pattern induced on a hemispherical, cylindrical, or similarly shaped resonating cavity, rotates relative to the gyro case by a fraction of the rotation angle experienced about the angle of symmetry. The wave rotation scale factor is a function of resonator geometry but for h m sph s ≈ 0.3. High quality Hemispherical Resonators are commonly machined from quartz, due to its excellent mechanical properties and it is the dimensional accuracy of the precision ground and polished unit which determines its accuracy. Temperature effects remain critical in this respect producing quadrature non-uniform mass distribution. The system is robust, with almost no moving parts and can be very accurate, under strictly controlled temperature environments. Litton, now part of Northrop Grumman, produces a unit with < 0.01°/hr performance which is used by the military and for space flight in which it has recorded millions of operating hours without failure. This system has potential for oilfield use but is expensive, requires significant temperature stabilization which results in size implications. The system is also covered by the international arms trafficking laws with all the implications and restrictions applicable as noted above for RLG.
CONTENTS
Made with FlippingBook flipbook maker