Drone design for scientific research

Materials World magazine
,
30 May 2019

Carbon-fibre and 3D printing are improving the quality of surveillance through the use of drones at Liverpool John Moores University.

Drones offer a unique opportunity to study the environment and are used across multiple disciplines from agriculture to search and rescue. The Faculty of Science at Liverpool John Moores University, UK, specialises in the use of drones for research in wildlife conservation and environmental monitoring. The efficacy of drones for this work is directly related to their payload carrying capability and flight endurance, both of which are inversely proportional to this. Carbon-fibre components are key to reducing airframe weight without adversely affecting structural stiffness. The material is used for simple shapes such as plates and tubes, as complex shapes are hard to make due to the parts’ small sizes, but is increasingly possible with the likes of 3D printing technology. Machined aluminium parts are used for complex shapes. The greatest opportunities for further weight saving comes from the use of advanced 3D-printed parts, such as those reinforced with carbon-fibre. Their use is currently limited to ancillary elements such as payload mounts. However, as their long-term properties, including damage tolerance and fatigue life, are still unknown, they are unsuitable for major structural components.

Multirotor drones

Multirotor drones are limited to relatively short flights due to their poor aerodynamic efficiency. To offset this, every effort possible is made to minimise the weight of airframe components to enable the use of heavier, higher capacity batteries. Unfortunately, this is complicated by the requirement for the airframe to be incredibly rigid so as to ensure the rotors remain in the same relative position and orientation under all flight load conditions.

Early multirotors used aluminium tubing for the arms but this limited their length due to weight and stiffness constraints. Shorter arms force the use of smaller rotors, which compounds their poor aerodynamic efficiency leading to even shorter flight times and limited payload capacity. Modern multirotors make extensive use of carbon-fibre components, such as roll-wrapped tube or rigid sheet, joined together by aluminium parts where needed. This use of carbon-fibre allows much longer arms and larger rotors without a significant increase in weight or loss of rigidity, leading to increased flight time and payload capacity. When comparing a new and old system, despite being 75% larger, the newer airframe is only 45% heavier due to the extensive use of carbon-fibre. The increase in efficiency gained by using larger rotors means the modern system can fly a 1kg payload for up to 30 minutes, compared to eight for the old system.

Fixed-wing drones

The poor efficiency of multirotor drones stems from their use of high-speed rotors to generate lift. This causes a lot of turbulence and noise, all of which represents lost energy. In contrast, fixed-wing drones are just like regular aeroplanes and use high-efficiency wings to produce their lift. An added benefit to fixed-wing drones is they only require one motor to propel themselves forwards thus reducing weight. A typical fixed-wing drone, as shown above, is the same physical size as the large multirotor pictured, but has an airframe weight of less than 30% and could carry a similar payload for up to 90 minutes.

An additional constraint to the design of fixed-wing drones is that their outer shape is critical for aerodynamic performance. Lifting surfaces such as wings must have a carefully designed aerofoil shape for correct performance. Traditional methods of manufacturing these components follow a similar process to full-sized aircraft, by first constructing an internal structure to support the outer surface and then applying a skin to form its outer shape. For drones, lightweight woods such as balsa are used extensively and carbon-fibre rods add rigidity. These built up wings are lightweight, owing to the fact the internal structure is largely hollow. However, they are complex and time-consuming to produce and can be difficult to repair in the event of an accident.

An alternative means of producing lifting surfaces is to use expanded polypylene (EPP) foam shaped by a moulding or CNC hot-wire cutter. These EPP structures are very lightweight but exhibit poor rigidity in their own right, therefore they are typically reinforced with carbon-fibre tube and strips. Both built-up and EPP surfaces may be reinforced with additional outer skin of either plastic, glass-fibre or carbon-fibre.

Planning for problems

The drawback of using aluminium metal is its high oxygen sensitivity. Cadmium (Cd) and bismuth (Bi) have a relatively high vapour pressure, which means the metal may also evaporate during the growth process, rather than have a catalytic effect on the growth. In that case, even a few atoms can compromise the performance of the device. Gallium (Ga) is also unfavourable because it has a very low melting and eutectic point (30°C). It melts and tends to migrate on the sample’s surface, making the size and position control of the catalyst very difficult. Silver (Ag) has a high eutectic temperature (845°C), therefore it is not compatible with temperature-sensitive substrates and low-temperature processes. Hence, tin (Sn), platinum (Pt), nickel (Ni), palladium (Pd) and titanium (Ti) could be possible options of catalyst material for silicon nanostructure growth via a VLS mechanism if high self-doping is to be avoided. But the eutectic temperature for Pt, Pd and Ti with silicon (Si) is higher than 750°C. Ti has been used as a template material to grow the silicon nanowires – Ti-coated substrate was annealed at 920°C and then wires were grown at 670°C.

Payloads

For the majority of our projects, the drone is just a tool to allow us to fly a particular sensor over an area of interest. For example, many of our projects revolve around the use of thermal infrared (TIR) cameras to detect the heat signatures of particular things, such as animals, people or underground fires. In the past it was common to simply attach the payload sensor directly to the airframe and this method is still used for many fixed wing systems as their inflight motion is usually fairly gentle. Multirotors, however, often manoeuvre much more aggressively and therefore a directly attached camera would experience significant blurring in its images. To prevent this, multirotor sensors are usually attached by means of a gimbal stabilising mechanism.

Gimbal stabilisation is achieved by attaching the camera to the drone via at least two orthogonal motors, to provide isolation from its longitudinal and lateral rotations. A third motor may optionally be used to provide isolation in direction as well. These motors are controlled automatically to keep the camera in a fixed orientation as the drone rotates, removing any image blurring which may otherwise have been introduced. It is essential that the centre of mass of the camera is in the centre of the gimbal in order to function correctly. One method of ensuring this is to produce a gimbal mechanism with a large amount of adjustability to allow the camera position to be optimised. This approach is common, but leads to a larger and heavier gimbal. Alternatively, a customised gimbal can be produced for each camera, which is our favoured approach.

Advanced 3d printing

To produce custom gimbals we use the Markforged Mark Two – an advanced 3D printer capable of producing high-quality fibre-reinforced parts. The printer uses dual nozzles to function as a regular nylon printer and also  lay down a continuous thread of fibre reinforcement at specified layers in the part. A number of different reinforcement fibres are available including carbon-fibre and Kevlar. This method produces custom gimbal parts for each of our cameras, which are incredibly lightweight and much smaller than a regular 3D-printed part.

In addition to custom gimbals, the faculty is experimenting with fibre-reinforced 3D-printed parts for other airframe components, such as Kevlar reinforced undercarriage mounts.

One challenge which is limiting the team’s use of 3D-printed parts for more structurally critical airframe components is the uncertainty in their fatigue life, particularly under exposure to persistent vibration. As more experience is gained in less critical areas, we will no doubt expand their use.


Owen McAree is Senior Research Officer at the faculty of science, Liverpool John Moores University.