82 Focus | IMUs, gyros and accelerometers rates are limited by the inherent bandwidth of the MEMS structures, optimising downstream signal-processing components such as analogue-to-digital converters and processors can go a long way towards achieving the 2000 Hz update speeds that some integrators are now requesting. Evolving FOG architectures The core internal parts of a FOG consist of an optical fibre that has been turned in a coil many times over, a high-performance light source for beaming down the length of that fibre in opposite directions, and a sensor for detecting the two light beams and any delay between them – which will be induced by and hence proportional to any angular rate imparted on the FOG, both from the host vehicle and the movement of the Earth. FOGs provide higher performance than MEMS gyros, being larger sensing systems at their core, and solid-state, counting photos moving out of the fibre to sense changes in inertia rather than requiring electromechanical elements to do this. Leading FOG manufacturers will build on the inherently low-bias nature of FOG designs by incorporating advantageous software features, such as the ability to reject common-mode interference and thus enhance bias repeatability. FOG companies are also converging around industry standard IMU form factors of below 10 cm diameter and height (and thus less than half a litre in volume). Within those bounds, such companies seek to engineer the highest-grade IMU they can, and they are gradually vertically integrating their design and production significantly more than MEMS firms tend to. To push the performance capabilities of today’s FOGs, manufacturers are choosing increasingly thin fibre designs and thinner forms of acrylic polymer coatings (relative to those used five to six years ago) to enable greater lengths of fibre to be wound more tightly into coils of smaller footprints than in past generations without any loss of linear polarisation. A longer coil means a longer distance for light to travel and hence greater sensitivity to delays indicative of rotational motions. Considerable knowhow also goes into ensuring the fibre and its coating can collectively maintain thermal stability across different environments and vibrations, or that effective compensatory measures can be embedded with the software in the device; both MEMS and FOG IMUs being expected to function with minimal error between -40 and 80 C. Some particularly high-end FOG IMUs are integrating their gyros in a pyramidal shape, rather than the traditional cylindrical or orthogonal arrangement. While the latter inevitably induces some differences in height within a housing, the former ensures all have the same height, and the same geometric orientation and offset relative to the X-, Y- and Z-axes. This approach was inspired by space applications, and while it is not as volume-efficient as the more industrystandard cylindrical or cubic FOG February/March 2024 | Uncrewed Systems Technology High-quality calibration and testing systems, such as this two-axis motion simulator, are critical for ensuring IMUs conform to standards (Image courtesy of Silicon Sensing)
RkJQdWJsaXNoZXIy MjI2Mzk4