Unmanned Systems Technology 036

40 Focus | IMUs, gyros and accelerometers components in a conventional FOG, this monolithic design greatly reduces system vulnerabilities and losses, be they from shock, vibration, heat or other factors that would damage the joints and splices of various parts. That improves the reliability, lifespan and consistency of performance in this new type of FOG. Manufacturing a PIC starts with a wafer substrate assembly, typically silicon, followed by heteroepitaxy to grow the necessary features and components onto the substrate. After that, the interfaces are defined to core-out the die, and individual PICs are separated and then tested (although the production process is inspected throughout to ensure design goals are being met). After passing inspections, the PICs are moved into the manufacturing line to be fitted with the remaining FOG components. For example, a fibre optic sensing coil is still installed separate to the PIC but as indicated, using the latter component means 80% of the conventional FOG design now has little to no impact on unit costs or hand work. Critical to this stage are close alignments between the PICs, fibre coils, modulator assemblies and housings, to minimise losses, optimise polarisation- holding and generally maximise resistance to shock, vibration and temperature. It is worth noting that by standardising the bulk of FOG componentry in this way, considerable modularity is enabled. That means new fibre and housing systems could be quickly installed and tested with the PIC, with minimal costs or r&d. As new forms of fibre coil are developed, particularly those with smaller February/March 2021 | Unmanned Systems Technology Fibre-optic gyro developers continue to experiment with different forms of fibre to improve the size and weight of their products (Courtesy of iXblue) Incorporating photonic integrated chips is enabling FOG-based systems to be produced in quantities (and at costs) competitive with MEMS devices while improving lifespans (Courtesy of KVH) diameters and less weight, so PIC FOGs will be well-poised to integrate such coils, enabling faster SWaP-C advancements of this technology than was possible before. Future IMU technologies In addition to advances in software and processing capabilities, there are a few projects in development that stand to open new possibilities and fields in inertial sensing products. For instance, products that integrate IMUs with cameras are gaining in popularity as a means of achieving accurate navigation, through optical flow measurements of far-off objects and geography, without relying on GNSS signals. Also in development are navigation arrays that incorporate large numbers of IMUs; as discussed, some IMU PCBs are incorporating higher numbers of processors to enhance bandwidth and shorten latencies. As MEMS accelerometers and gyros become smaller as well, this policy can be extended to these components to aggregate their bias stability, range and other critical parameters. For example, by incorporating Products that integrate IMUs with cameras are becoming more popular as a way of navigating accurately without GNSS