Unmanned Systems Technology 027 l Hummingbird XRP l Gimbals l UAVs insight l AUVSI report part 2 l O’Neill Power Systems NorEaster l Kratos Defense ATMA l Performance Monitoring l Kongsberg Maritime Sounder

37 For a 40 kg fixed-wing UAV with a two-stroke engine, say, which is to be regularly flown in all weather conditions, operators will probably want at least a four-axis turret with internal and external gimbals for pan and tilt. Dedicating one axis to roll is particularly useful if the UAV will need compensation for buffeting by wind. This can also be accomplished digitally, essentially by post-processing the imagery to maintain a single angle in the roll axis, and cutting off unwanted parts of the picture. With well-written image processing software, a three-axis gimbal could be just as stable than a four-axis gimbal, if not more so. Digital stabilisation can be vital, as adding gimbals generally means a larger overall footprint or less free volume for sensors. The quality of digital image stabilisation is directly linked to the processing power available. In the past few years, powerful computing boards such as the Nvidia TX2 have become available that are sufficiently small and energy-efficient to be integrated into gimbal systems weighing less than 500 g. Such processors can run highly demanding applications – 3D image stabilisation, for example – or machine learning-based detection and tracking software, which could help security operators track the licence plates of suspicious vehicles, say, or carry out commercial autonomous inspections for signs of wear in bridges, pipelines or other critical infrastructure. Having more processing power also opens up the possibility of greater digital zoom, which may be preferable to greater mechanical zoom as that requires larger, heavier cameras with lenses of greater ranges of focal length. In addition to increasing the gimbal’s power draw though, this would also make the gimbal more challenging to balance, owing to how the lenses shift back and forth while zooming in and out, altering the centre of gravity (CoG). Previous attempts at such enhancement (and advanced tracking) capabilities have been made by installing FPGAs inside the housings. However, the hardware for that remains relatively expensive and complicated to develop software for, particularly when compared to the newest embedded computer processors. Selecting (and thoroughly testing for) the right IMUs is critical for true gyro stabilisation. Inertial measurement technology has continued to be miniaturised and made more cost- effective over the past few years, with ongoing algorithmic reductions in gyro drift in terms of bias instability and angular random walk. Reducing these errors is critical to ensuring a gimbal’s stability and accuracy, particularly as gimbals are increasingly expected to track moving targets. Also, an absolute minimum of gyro misalignment – in which the MEMS gyros are not perfectly oriented at 90 º to each other – is needed to maintain an accurate flow of inertial measurements and thus keep gimbals pointed in the right direction. For gimbals weighing more than 10 kg, fibre-optic gyros (FOGs) are still widely integrated for their higher accuracy than their MEMS equivalents. The development of FOGs using photonic ICs will provide greater performance still, while significantly lowering the costs associated with such systems, although their size and weight will still prohibit their use in miniature gimbals. Motor control Until a few years ago it was common for gimbal manufacturers to use open-loop control systems in their motors. That meant the motor was given a control signal and the required power to achieve the position commanded, and it was then assumed that the control action had been carried out as required. This method is not energy-efficient though, as it typically uses more current than is truly necessary for the Gimbals | Focus For gimbals that weigh more than 10 kg, fibre-optic gyros are still widely integrated for their higher accuracy than MEMS versions Unmanned Systems Technology | August/September 2019 Improvements in a gimbal’s mechanical and digital stabilisation contribute hugely to the level of mission-critical detail captured in images and videos, as seen in this comparison (Courtesy of Controp Precision)