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

38 Focus | Gimbals required movement, to compensate for any error. This excess current also means the motors have to be unnecessarily large, at the expense of free space inside or around the gimbal. Most gimbal manufacturers have therefore switched to closed-loop control. Here, the motor controller can test if the rotation mechanism has performed as commanded. If not, the current can be altered to compensate for the error, increasing gimbal precision and energy efficiency. A position sensor, such as a Hall effect sensor or potentiometer, can be installed for this purpose, with its data feed being written into the gimbal’s control algorithm to measure the motion and compensate accurately. Closed-loop control also helps to compensate for changing loads, such as how zoom lenses dynamically alter the centre of gravity. Adding a sensor to the motor adds significant mechanical and algorithmic complexity though, particularly if slip rings are installed to provide 360 º continuous turn in any of the axes – which is why closed-loop control has not been universally adopted. New technologies are being developed and adopted to provide accurate and efficient gimbal control. For example, field-oriented control-based motor controllers are much more complex than those using trapezoidal control algorithms, but they provide faster speeds and better accuracy. Also, brushless motors are widely replacing gear motors, as they use power more efficiently, providing more stable and precise performance, as well as causing far less backlash and other problems associated with gear motors. Further into the future, improvements in the miniaturisation of axial flux (‘pancake’) BLDC motors will help integrate gimbals within much flatter spaces than has previously been possible. Ongoing development of edge drive motors could also greatly reduce the packaging footprint of gimbals. These operate in a similar way to rim-driven UUV thruster motors, using a thin fixed stator forming the outer circle of a given gimbal and a ring of permanent magnets forming the inner rotor. In addition to the space saved by their thin mechanical design, the wide diameters of such motors generally mean they naturally generate high torque, potentially reducing the power needed to control their gimbals. Their larger surface areas also help them cool faster than conventional motors, which improves their energy efficiency and lifetime. CAN bus could also come to replace PWM in gimbal motors, to reduce the internal density of wires as well as increasing comms bandwidths for sending and receiving position sensor measurements and other data. Structures Gimbal manufacturers can access a variety of software to design and analyse new models before moving on to production. CAD programs such as SolidWorks and Siemens NX can enable the analysis of a gimbal’s inertia, CoG, natural frequency and other parameters that the structure must account for. All high-end gimbals are enclosed systems, to protect expensive camera equipment from dust and moisture, and to minimise atmospherically driven vibration and motion that would interfere with stabilisation. A wide range of materials and manufacturing approaches are available for use in these protective housings, each of which provides different mechanical stiffnesses, tolerances and weights. Additive manufacturing (AM) has proven useful for quickly producing, testing and iterating new gimbal designs. After a CAD model of it has been examined in simulations, additively manufacturing a physical model – which can now be done using metals and alloys, rather than just plastics – can enable its approximate tolerances, inertia and balance to be tested. Such tests can reveal fundamental structural flaws in the design of a new gimbal, before any investment in tooling or machine programming is made for full- scale production. Furthermore, the ability to reduce manual input can be useful in some parts of gimbals. For example, the interface sections for mounting and connecting gimbals to their UAVs can contain a lot of tiny metal elements that have to be assembled manually in complex, time- consuming ways. August/September 2019 | Unmanned Systems Technology Advances in motor control technology (and the motors themselves) can enable greater accuracy and the ability to retract gimbals into their UAV’s hulls during flight or hard landings (Courtesy of Threod Systems)