Unmanned Systems Technology 014 | Quantum Tron | Radio links and telemetry | Unmanned Aerial Vehicles | Protonex fuel cell | Ancillary systems | AUVSI 2017 Show report

64 rotor (gerotor) and internally generated rotor (IGR) pumps, which operate on the same principles but with detail differences. The two types are positive- displacement pumps with two intermeshing, rotating elements in a common casing, one revolving inside the other on a parallel rather than concentric axis. The inner rotor is closer to one side of the outer rotor than the other. This divides the space between the two rotors into low-pressure and high- pressure areas through which the fuel must pass. Naturally, the inlet is near the area of lowest pressure and largest volume, the reverse applying to the outlet. In the gerotor type, semicircular lobes in the outer rotor fit into concave spaces between the ‘teeth’ of the inner, which resembles a sprocket and presents a surface with a continuously changing curvature, the two elements forming expanding and contracting pockets of fuel as they rotate. Ports in the adjacent housings provide the inlet and outlet timing to seal off the high- and low- pressure areas from one another. The IGR does the same thing, but the outer element has a continuously changing radius of curvature and the inner carries cylindrical rolls. IGRs are more expensive than gerotors but are better at pumping low-viscosity fuels such as gasoline. Gerotor pumps are also commonly used in lubrication systems. Fuel flow measurements Measuring fuel flow in real time is vital to calculating a UAV’s remaining endurance and range at any point in the mission. UAVs require accurate measurement of small volumes of fuel, and rely extensively on what are called positive-displacement meters. In these devices, flowing fuel moves mechanical components such as pistons, gears or turbines that turn a shaft equipped with a means of measuring its rotation rate. Piston meters are candidates for the most accurate way to measure the flow of low-viscosity fluids, with some achieving ±0.2% of the reading and wide flow ranges of 200:1 or more. In one of the latest examples for UAV use, four pistons reciprocate in cylinders arranged around a central crankshaft in an arrangement reminiscent of a radial aero engine. Two fluid ports in the cylinder wall, a port at the top of the cylinder and a grooved piston enable the fuel to be measured and pumped out by each piston in turn, with the valve action for each cylinder carried out by the adjacent piston. Hall effect sensors in a transmitter detect the position of a magnet spun by the crankshaft, with a claimed angular resolution of 0.36°. Changes in the magnet’s position are tracked by a microprocessor, whose output is proportional to the fuel flow rate and updated once every millisecond. By contrast, turbine flow meters use a freely suspended turbine or rotor spun by the fuel flow through the meter’s body. According to one supplier, the turbine’s rotational speed is a true representation of the volumetric flow rate. As it spins, the turbine generates a train of electrical pulses that are sensed by an external pick-up. The pulse rate can be converted into an analogue current or voltage, or displayed as a flow rate in standard units. One emerging new technology is the use of non-intrusive ultrasound technology. Ultrasonic transducers emit and receive high-frequency sound waves (the ultrasonic range is about 18,000 Hz). Ultrasonic flow measurement is based on the so-called transit time principle whereby a pair of sending and receiving transducers transmit and receive signals through the flow. The outgoing signal moves with the flow while the returning one moves against it and thus inherently travels slower. The flow rate can be calculated from the time difference. Ultrasonic fuel flow sensors are solid- state devices that can handle multiple fuels. They use a pair of piezo-ceramic transducers, the faces of which are wetted by the flow, the flow having been carefully conditioned within a short measurement tube. The removal of mechanical moving parts from the flow path ensures minimal pressure drop across the sensor, providing true flow rate data with little impact on the flow itself. Fuel tanks There is also a connection between UAVs and motorsport in fuel tanks, as both challenge suppliers with complex shapes, composite structures and June/July 2017 | Unmanned Systems Technology Focus | UAV engine ancillary systems This piston flow meter is claimed to be the most accurate type for use with low- viscosity fluids, some achieving a ±0.2% error (Courtesy of Max Machinery)