The Hemispherical Resonator Gyroscope, also called wine-glass gyroscope or mushroom gyro, is a compact, low noise, high performance angular rate or rotation sensor. An HRG is made using a thin solid-state hemispherical shell, anchored by a thick stem. This shell is driven to a flexural resonance by electrostatic forces generated by electrodes which are deposited directly onto separate fused-quartz structures that surround the shell. The gyroscopic effect is obtained from the inertial property of the flexural standing waves. Although the HRG is a mechanical system, it has no moving parts, and can be very compact.
Operation
The HRG makes use of a small thin solid-state hemispherical shell, anchored by a thick stem. This shell is driven to a flexural resonance by dedicated electrostatic forces generated by electrodes which are deposited directly onto separate fused quartz structures that surround the shell. For a single-piece design made from high-purity fused quartz, it is possible to reach a Q-factor of over 30-50 million in vacuum, thus the corresponding random walks are extremely low. The Q-factor is limited by the coating and by fixture losses. Such resonators have to be fine-tuned by ion-beam micro-erosion of the glass or by laser ablation in order to be perfectly dynamically balanced. When coated, tuned and assembled within the housing, the Q-factor remains over 10 million. In application to the HRG shell, Coriolis forces cause a precession of vibration patterns around the axis of rotation. It causes a slow precession of a standing wave around this axis, with an angular rate that differs from input one. This is the wave inertia effect, discovered in 1890 by British scientist George Hartley Bryan. Therefore, when subject to rotation around the shell symmetry axis, the standing wave does not rotate exactly with the shell, but the difference between both rotations is nevertheless perfectly proportional to the input rotation. The device is then able to sense rotation. The electronics which sense the standing waves are also able to drive them. Therefore, the gyros can operate in either a “whole angle mode” that sense the standing waves' position or a “force rebalance mode” that holds the standing wave in a fixed orientation with respect to the gyro. Originally used in space applications, HRG is now used in advanced Inertial navigation system, in Attitude and Heading Reference System and gyrocompass.
Advantages
The HRG is extremely reliable because of its very simple hardware. It has no moving parts; its core is made of a monolithic part which includes the hemispherical shell and its stem. They demonstrated outstanding reliability since their initial use in 1996 on the NEAR_Shoemaker spacecraft. The HRG is highly accurate and is not sensitive to external environmental perturbations. The resonating shell weighs only a few grams and it is perfectly balanced which makes it insensitive to vibrations, accelerations and shocks. The HRG exhibits superior SWAP characteristics compared to other gyroscope technologies. The HRG generates neither acoustic nor radiated noise because the resonating shell is perfectly balanced and operates under vacuum. The material of the resonator, the fused quartz, is naturally radiation hard in any space environment. This confers intrinsic immunity to deleterious space radiation effects to the HRG resonator. Thanks to the extremely high Q-factor of the resonating shell, the HRG has an ultra-low low angular random walk and extremely low power dissipation. The HRG, unlike optical gyros, has inertial memory: if the power is lost for a short period of time, the sensitive element integrates the input motion so that when the power returns, the HRG signals the angle turned in the interval of power loss.
Disadvantages
The HRG is a very high-tech device which requires sophisticated manufacturing tools and process. The control electronics required to sense and drive the standing waves, is somewhat sophisticated. This high level of sophistication strongly limits the dissemination of this technology and only few companies were able to develop it. Up to now, only three companies are manufacturing HRG in series: Northrop Grumman Corporation, Safran and Raytheon Anschutz. Classical HRG is relatively expensive due to the cost of the precision ground and polished hollow quartz hemispheres. This manufacturing cost restricts its use to high added value applications such as satellite and spacecraft. Nevertheless manufacturing costs can be dramatically reduced by design changes and engineering controls. Rather than depositing electrodes on an internal hemisphere that must perfectly match the shape of the outer resonating hemisphere, electrodes are deposited on a flat plate that matches the equatorial plan of the resonating hemisphere. In such configuration, HRG becomes very cost effective and is well suitable for high grade but cost sensitive applications.
