Toward Navigation Grade Acousto-Optic Gyroscope

2020-03-12T17:52:10Z (GMT) by Ashraf Mahmoud
Accurate control of navigation is critical for many existing and emerging applications such as military missions, self-driving cars, pico-satellites and drones in high G-shock events and high-vibration environment. Precise and accurate navigation is made possible by the use of GPS. But what if GPS service is not available at all? This could happen due to several reasons related to the nature of the GPS communication as an RF signal. In those situations, the denial of GPS drives navigation systems to rely on the other systems.
Inertial Navigation Systems (INS), which rely on using inertial sensors such as gyroscopes and accelerometers, are a potential approach to achieve navigation in GPS-denied environments. Commercial MEMS INS have a small form factor and power consumption suitable for the emerging applications while they suffers from noise and drift. Tactical and navigation grade optical INS show better performance but at the expense of size and power. This leads to a technological challenge and opens research opportunities targeting the decrease of noise and increase of stability of inertial sensors at low power consumption and small form factor.
In this dissertation, we focus on one type of MEMS inertial sensors, namely a gyroscope, to enable positioning, navigation, and timing (PNT) in GPS-denied environments. We invented and demonstrated a new type of gyroscopes called the acousto-optic gyroscope (AOG). The AOG is based on the idea of the surface acoustic wave gyroscope (SAWG) but uses a photonic read-out to decrease input referred noise and increase signal stability.
The AOG offers well established performance in terms of high G-shock survivability, low g-sensitivity, and wide operating bandwidth. The AOG experiences challenges in achieving high sensitivity due to the relatively small Coriolis force. In the present work, the sensitivity of AOG and long-term stability are investigated. Finite element and analytical methods are used to optimize the design of the AOG. These investigations resulted in improvements in AOG sensitivity by acting on the Coriolis mass and the quality factor of SAW resonators. Similarly, a fabrication process was developed to ensure that large masses could be synthesized and device non-idealities reduced.
The results of these investigation lead to the 1st functional prototype of both SAWGs and AOGs, which exhibited low noise to signal ratio expressed as angle random walk (ARW) as low as 0.25 Β°/βˆšβ„Žπ‘Ÿ are demonstrated.
Long-term stability was also studied using a monolithically integrated micro-oven, which helped maintaining the temperature and operating frequency variations within Β±10πœ‡πΎ andΒ±0.2 π‘π‘π‘š , respectively. This study eliminated the impact of temperature and stress induced drifts, showing that these devices can attain record breaking stability up to 300 sec. SAW resonators were also tested at CMU and ARL surviving high G-shock of more than 50,000 Gs.