UAV PLATFORMS

The very small platforms, micro and mini aerial vehicles (Table 12.1) can fly for less than 1 hour at an altitude below 250 m. Micro platforms are considerably smaller than mini platforms (i.e. <5 kg versus 20-150 kg), but both have a similar flying range. An example of micro UAV is the Phantom 1 or 2, with MTOW below 1.3 kg and a very light payload capacity; Aibot-X6 is another micro UAV with 3.4 kg MTOV and maximum payload of 2 kg. Mini is the most abundant type of platform produced for civilian applications, doubling the number of micro- and medium-range UAV platforms (UVS, 2014). An example of mini UAV is the Camcopter, with an MTOV of 68 kg and maximum

TABLE 12.1

Characteristics of Non-Military Remotely Piloted Aircraft Systems (RPAS)

Source: Adapted from UVS international (2008).

MAV, micro air vehicles; VTOL, vertical take-off and landing; LASE, low altitude, short endurance; LALE, low altitude, long endurance; MALE, medium altitude, long endurance; HALE, high altitude, long endurance. ^a According to national legal restrictions.

payload capacity of 25 kg. On the other side of the scale, medium-altitude, long-endurance (MALE) platforms (e.g. Talarion, Predator) and high-altitude, long-endurance (HALE) platforms (e.g. Global Hawk, Euro Hawk) have a flying endurance of several days at an altitude of 20,000 m. The latter aerial platforms are comparable in size to manned aircraft.

Nanodrones are miniature UAVs able to carry small still and video cameras. These UAVs can fly in all directions and perform manoeuvres and mid-air stunts. For example, the palm-size Micro Drone 2 weighs 0.034 kg and has a flying range of 120 m and endurance of 6-8 minutes. Other small UAVs are now flown as tethered aerial vehicles to circumvent the risks associated with flying. The Pocket Flyer by CyPhy Works is a 0.080 kg tethered platform that can fly continuously for 2 hours or more, sending back high-quality HD video the entire time. With improved tether technology, all data, control, and endurance can be built into the tether, providing long endurance. Furthermore, ZANO operates on a virtual tether connected to a smart device, allowing simple gestures to control it.

Small powered UAV platforms based on airframes can be grouped into two categories: rotarywing and fixed-wing UAVs. Fixed-wing UAVs have a relatively simple structure, making them stable platforms that are relatively easy to control during autonomous flights. Their efficient aerodynamics enables longer flight duration and higher speeds, making them ideal for applications such as aerial survey (requiring the capture of georeferenced imagery over large areas). On the down side, fixed- wing UAVs need to fly forward continuously and need space to both turn and land. These platforms are also dependent on a launcher (person or mechanical) or a runway to facilitate take-off and landing, which can have implications on the type of payloads they carry.

Typical lightweight fixed-wing UAVs currently in the commercial arena have a flying wing design with wings spanning between 0.8 and 1.2 m, and a very small fin at both ends of the wing. In-house vehicles tend to have slightly longer wings to enable carrying the required heavier sensors (Petrie, 2013a, b). A second type of design is the conventional fuselage. The dimensions are around 1.2—1.4 m length for the fuselage and 1.6-2.8 m wing length (Figure 12.1). In the UK, there are around 20 companies operating commercial airborne imaging services using fixed-wing UAVs (Petrie, 2013a, b).

Rotary-wing aircraft (N-copter or N-rotor) have complex mechanics, which translates into lower speeds and shorter flight ranges. Among their main strengths, rotary-wing UAVs can fly vertically, take-off and land in a very small space, and can hover over a fixed position and at a given height. This makes rotary-wing UAVs well suited for applications that require manoeuvring in tight spaces and the ability to focus on a single target for extended periods (e.g. structural inspections). On the downside, rotary-wing UAVs can be less stable than fixed-wing counterparts and also more difficult to control during flight.

Single-rotor and coaxial rotor platforms (with two counter-rotating rotors on the same axis) are similar to conventional helicopters, with a single lifting rotor and two or more blades. These platforms maintain directional control by varying blade pitch via a servo-actuated mechanical linkage. Singlerotor and coaxial rotor UAVs are typically radio-controlled and powered by electric motors, although some of the heaviest examples use petrol engines. They require cyclic or collective pitch control.

Multi-copters have an even number of rotors and utilise differential thrust management of the independent motor units to provide lift and directional control. As a general rule, the more the rotors, the higher the payload they can take and are functional in strong wind conditions, as the redundant lift capacity provides for increased safety, and more control in the event of a rotor malfunction or failure. Some platforms now available are capable of autonomous flight, which significantly improves the capability to undertake repeat aerial video and photography.

Sensors On-Board UAV for Monitoring Oil and Gas Pipelines

The information reachable by the survey mission is determined by the type and quality of the sensors carried on-board the flying platform. Despite an increasing range of sensors available for small- scale UAVs, thanks to miniaturisation and advancements in battery technology, for some of the most adequate oil or gas leak detection techniques, (e.g. fluorescence) there is still no sensor adapted to

FIGURE 12.1 Examples of lightweight commercial UAVs. Fixed-wing flying-wing design: (a) Trimble Gatewing X100; (b) swinglet CAM; (c) smartone. Conventional fuselage design: (d) MAVinci Sirius; (e) Composites Pteryx; (f) CropCam. Single rotor designs: (g) AT-10 (Advanced UAV Technology); (h) Syma S107C. Multi-rotor designs: (i) Parrot AR Drone quadcopter; (j) Cinestar 8 octocopter.

UAV conditions (i.e. laser fluorosensor). Selecting a combination of platform and sensor to provide the necessary data in adequate conditions for monitoring and mapping oil and gas pipelines remains a challenge. The main sensor types with commercial adaptations to UAV mechanics that can be used for monitoring oil and gas pipelines are listed in Table 12.2.

The essential difference between active and passive sensors originates from the source of energy illuminating the target objects (passive sensors rely on the sun; active sensors emit some kind of radiation themselves). Therefore, missions with an active sensor require higher lifting and carrying capacity UAV platforms. Active sensors emit some kind of radiation measuring the fraction reflected by the target objects and the difference in time between emission and reception. Active sensors require power supplied by a source, adding weight to the aerial system which makes active equipment less versatile for use on UAVs than passive equipment. The capacity to perform a particular monitoring task and to work under certain environmental conditions (e.g. topography, weather) is sensor dependent (Table 12.2).

Some examples of commercial sensors from most of the techniques listed in Table 12.2 which are adapted to small UAVs can be found in Gomez and Green (2015, report) and Colomina and Molina (2014).

AUXILIARY EQUIPMENT

A series of systems and elements support the aircraft and sensors to make an UAV mission successful. The most relevant systems are those dedicated to position and navigation, to autonomous flight, and for communications. The need of elements for the launch, recovery and retrieval, and the mechanics and payloads are UAV and mission dependent.

TABLE 12.2

Selection of Sensors Suitable for Monitoring Oil and Gas Pipelines; Strengths and Weaknesses for the Purpose and Typical Performing Tasks

(Continued)

TABLE 12.2 (Continued)

Selection of Sensors Suitable for Monitoring Oil and Gas Pipelines; Strengths and Weaknesses for the Purpose and Typical Performing Tasks

Source: Gomez and Green (2015).

The position and navigation systems play a crucial role in UAV missions, controlling the location of the UAV at all times by remote operator, or by the autonomous pre-programmed flight plan. Currently available Global Navigation Satellite System (GNSS) equipment for small UAVs is lightweight, compact, and capable of receiving signal from multiple satellite systems (e.g. GPS, Galileo, BeiDou) providing high-accuracy location information (Colomina and Molina, 2014), especially when operated as differential GPS, and facilitates all UAV navigation. For remote control from the ground, air, or sea, where non-autonomous operations are necessary, radar or radio tracking solutions are required.

Autonomous capacity is useful to take-off and land, as well as to repeat the same area survey at regular intervals, flying along a path defined with n-waypoints pre-programmed by the UAV operator. Another desirable capacity of UAVs, to assure safety operations, is the ability to navigate amongst obstacles in the flight path and to sense and avoid other vehicles. Ideally control software (e.g. Mission Planner, АРМ Planner, Droid Planner) enables switching between pre-programmed autonomous flight and manual control.

Communication between small UAVs and the control system (CS) is usually through radio frequency, commonly in the 900 MHz, 1.2 GHz, 2.4 GHz, and 5.8 GHz bands. Uplink transmission (i.e. CS to aircraft) consists of a flight plan, real-time flight-control commands, control commands to the different payloads, and updated positional information. The downlink information (i.e. aircraft to CS) consists of the payload data (e.g. imagery), positional data, and aircraft housekeeping data (e.g. battery or fuel state). Two different frequencies are necessary to keep the transmission of command information and sensor-acquired data independently, avoiding interference.

Fixed-wing vehicles require additional equipment to assist with launch and recovery. Launch equipment is typically an acceleration ramp with a trolley. Recovery equipment can be a parachute installed within the aircraft, combined with a means to absorb the impact energy (e.g. airbag or an easily replaceable piece of material). To secure the sensor and control the pointing direction and orientation during the flight, solutions go from a simple bracket and rubber mounts in between the UAV and the sensor to elaborated 3D gimbals.