78 Focus | IMUs, gyros and accelerometers Such systems can be integrated as subsystems into more complete solutions, such as the attitude and heading reference system (AHRS) or the INS with dynamic roll and pitch accurate to 0.04°, or dynamic heading output to 0.13°. Reaching these figures uses some proprietary IP, guarded by IMU companies, but much of it comes from carefully picking among MEMS suppliers to find the latest technology. Accelerometer and gyroscope fabricators typically take three to five years between step improvements in the SWaP, performance and production throughput of the vital core components they supply to IMU makers. Those components are predominantly silicon-based, although MEMS made from metals, silica (silicon dioxide, also known as quartz), ceramics and plastics are also commercially available. Regardless of material, the core principles remain the same: a microscopic structure is designed to move in response to acceleration (an accelerometer) or angular rate (a gyroscope), with that movement inducing a corresponding voltage that can be detected, measured and reprocessed into inertial – and hence movement – information. Ever smaller On top of this, manufacturers are leveraging high-density printed circuit board (PCB) designs to pack MEMS into smaller product enclosures, without leading to interferences or resonances inside those spaces that could degrade performance. Additionally, optimising these designs is still hugely dependent on real-world prototyping, testing and iteration; such are the myriad nuances of IMU performance that virtual simulations cannot yet capture. Although MEMS may be a byword for minimised-SWaP, fibre-optic inertial systems have also made huge strides over the past few years to create FOG-based IMUs that can be as light as 100-200 g. While this remains many times heavier than the lightest MEMS IMUs, it is multiple times lighter than the typical commercial FOG IMU offerings for uncrewed systems just a few years ago. This new wave of systems gets by on 1-2 W of power in standard operations, output readings with angular, random walk values of 0.03-0.05°/√hr and bias stabilities of about 1°/hr. Such devices perform with high immunity to magnetic interference by leveraging either internal or external magnetic shielding, with magnetic response values of 0.1-0.3°/h/Gauss. The ability of FOG IMU manufacturers to push SWaP limits without scuppering performance comes from a similar place to many other advancements being made in FOG-based systems (which we will expand upon), and that involves leveraging advances and experience in fibre optics from across industry to create ever-smaller sub-components. The principal bottleneck is the physical bend radius of the fibre itself – bend it tighter and you create a smaller circle, and hence a single-axis FOG with a smaller footprint, but bend it too tightly and you risk breaking the fibre. Among the smallest FOG IMUs we have seen in the uncrewed space, the fibre of choice is a polarisation-maintaining fibre (PMF) with a thickness of 40 µm. Linearly polarised light maintains its linear polarisation state during propagation and when exiting the fibre. The choice of PMF is key to highly precise interferometry; the process by which FOGs sense angular rate, while the thickness enables the fibre to be wound in circles tight enough to manufacture FOG IMUs with radii of 20-25 mm. As we will discuss, other FOG (and MEMS) companies are making similarly judicious choices for the growing variety of IMU architectures appearing across the market, with optimisations going beyond just SWaP. All in the timing For customers seeking IMUs for navigation applications, precise timing is a critically valued quality, often as important as in-run bias stability, as without proper time synchronisation it is impossible to correctly consolidate inertial data between different accelerometers February/March 2024 | Uncrewed Systems Technology Some fibre-optic inertial systems are coming in smaller and smaller architectures (Image courtesy of Fizoptika)
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