77 solutions, companies are enhancing their manufacturing facilities to optimise production output and costs, and devising new means of leveraging external aiding sources to go beyond the traditional INS for tomorrow’s uncrewed vehicle. Size, weight, power Some readers will doubtless be familiar with the ecosystem of IMU types. For those who are not, there are three broad categories, across which size, weight and power (SWaP), and performance quality are all positively correlated. Performance encompasses varying facets of accuracy, precision, measurement range and resolution, and resilience against drift, as well as other forms of error. At the lower end of the three categories are microelectromechanical system (MEMS) IMUs. Fibre-optic gyro (FOG) IMUs are typically associated with higher performance and SWaP-C (cost), with ring laser gyro (RLG) IMUs the highest across all categories. MEMS are generally the most prolific among uncrewed systems, followed by FOGs. RLGs are the rarest, traditionally only seen in very long-range, long endurance UUVs and some spacecraft, and they are seldom built or marketed explicitly for uncrewed systems engineers. An oft-repeated catchphrase describing the future of US defence strategy is “small, cheap, smart and many”, which one might apply not only to the uncrewed swarms that defence futurists around the world envision, but also to MEMS devices, which continue to strike the lowest SWaP-C figures and the highest production outputs via their sensing instruments. Among the latest MEMS IMUs available for uncrewed systems are devices weighing no more than a few grams, with some rare high-end products lighter than 1 g, only a few millimetres tall, and 15-20 mm wide and long. Making an IMU smaller and less expensive typically means reducing its sensitivity, and hence its sensing performance, but such devices are integrating gyroscopes capable of in-run bias stabilities of less than 1.5°/hr and random walk values of 0.16°/√hr, along with accelerometers whose in-run bias stabilities are below 19 μg with random walks of 0.2 m/s/√hr. IMUs, gyros and accelerometers | Focus Uncrewed Systems Technology | February/March 2024 Improvements in SWaP accuracy and stability over time without GNSS dependency are critical requirements for modern IMUs across the uncrewed world (Image courtesy of Advanced Navigation) High-end MEMS components and highdensity PCB designs are key to packing MEMS IMUs inside tighter enclosures (Image courtesy of Silicon Sensing) Inertial systems, by contrast, cannot be congested or frustrated in the way satellite signals can. They remain a critical hallmark of modern navigation
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