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12 Platform one Design engineers have long used gimbals, pods and pan-tilt systems as a way to protect payloads, but gimbal design requirements are being driven by advances in technology that demand greater precision and pointing stability. Gimbals rely on good sealing design and engineering to ensure they are able to point accurately. This precision accuracy reduces jitter and has a direct impact on performance as well as image quality. While gimbal seals help protect this sensitive equipment in harsh environments, they can reduce precision by adding friction to the system. Minimising friction in the seal design should therefore be a priority, to allow a free range of motion. Both static and dynamic friction impact the system. Typically, static friction (also referred to as stiction or breakout) is higher than dynamic friction. Every time a gimbal changes direction, it experiences a jump in friction caused by this static element, making precise adjustments more challenging. Thus effective gimbal seals not only have low dynamic friction but keep static friction jumps to a minimum as well. Gimbal seal performance can be optimised to reduce friction by taking into account key seal characteristics such as profile, material and energiser. Polymer- filled PTFE (whose static coefficient of friction is close to its dynamic coefficient of friction) is preferred because it offers low friction, low stick slip, wide temperature variation tolerance, and environmental and chemical resistance. However, because PTFE is susceptible to cold flow, a spring energiser is necessary to ensure proper sealing. A canted coil spring provides a relatively constant spring force that when paired A ‘U-cup’ profile is preferred for gimbal sealing with a flexible seal lip helps to address large gland height tolerances and wear issues. With a canted coil spring, spring forces can be controlled precisely, often within 10% of the nominal working deflection over a wide range. For gimbal sealing, a ‘U-cup’ seal profile is preferred because it can better handle wide temperature variations and large diameters with smaller cross-sections. This design also helps to minimise the effect of dimensional changes caused by changes in temperature. The key is to combine materials to achieve high lubricity with a geometry that dramatically reduces the amount of torque necessary to move a gimbal without compromising sealability. A spring-energised gimbal seal, shown above in cross-section, uses a unique geometry that optimises seal lip contact while keeping friction to a minimum. Calculated forces exerted by the canted coil spring energiser help to promote uniform wear and prolong service life. Bal Seal Engineering’s Karina Chavez and Brandon Grant contributed to this article. Gimbal precision issue is sealed Manufacturing February/March 2018 | Unmanned Systems Technology Dr Donough Wilson Dr Donough Wilson is innovation lead at VIVID/ futureVision, which specialises in game- changing thinking for defence, homeland security, and both manned and unmanned aviation innovations. He was first to propose the automatic tracking and satellite download of airliner black box data, technology which is now being adopted. His defence innovations include the automatic cockpit vision system that protects military aircrew from asymmetric high-energy laser attack. As a pilot, he has more than 3000 hours of flying experience in both military and civil environments, and is currently a flying instructor and a flight test examiner. Paul Weighell Paul has been involved with electronics, computer design and programming since 1966. He has worked in the real-time and failsafe data acquisition and automation industry using mainframes, minis, micros and cloud-based hardware on applications as diverse as defence, Siberian gas pipeline control, UK nuclear power, robotics, the Thames Barrier, Formula One and automated financial trading systems. Ian Williams-Wynn Ian has been involved with unmanned and autonomous systems for more than 20 years. He started his career in the military, working with early prototype unmanned systems and exploiting imagery from a range of unmanned systems from global suppliers. He has also been involved in ground-breaking research including novel power and propulsion systems, sensor technologies, communications, avionics and physical platforms. His experience covers a broad spectrum of domains from space, air, maritime and ground, and in both defence and civil applications including, more recently, connected autonomous cars. Unmanned Systems Technology’s consultants