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70 flow from the tank to the engine (and back, in the case of a return system). Traditional fuel flow meters have been based on the concept of measuring the speed of an impeller driven by the flow of fuel, but the latest development is to use ultrasonic measurement technology instead. Ultrasonic transducers emit and receive high-frequency sound waves, turning electrical energy into sound and vice versa. Ultrasonic flow measurement is based on the so-called transit time of flight measurement principle, whereby opposite sending and receiving transducers transmit and receive signals through the flow. The outgoing signal moves with the flow while the returning signal moves against it, and thus travels slower, so the flow rate can be calculated from the time difference. This principle is used by ultrasonic fuel flow meters recently developed for Formula One and Le Mans racing. In those applications accuracy is vital to ensure no competitor gains an unfair advantage, while size and weight are also of paramount importance, along with the ability to withstand an extremely hostile vehicle environment. These solid-state devices, in removing mechanical moving parts from the flow path, ensure minimal pressure drop across the sensor, providing true flow rate data with little impact on the flow itself. Also, they eliminate mechanical damping and enable measurement of flows pulsating at high frequency. As an injector closes, it creates a momentary back-pressure throughout the fuel system: measured in microseconds, the flow reversal will not be detected by an impeller-type device, whereas this type of sensor can read it. The accuracy of these devices is said to rival that of industry-standard Coriolis fuel flow meters, as used in dyno cells, which are far too cumbersome for in-vehicle use. Solid-state sensors Given that reliability, space and weight are crucial factors for UAVs, it is no surprise that the industry is transitioning towards solid-state sensors. Most designers and system developers like the idea of no moving parts and hence no mechanical wear, relying on electronics rather than physical moving parts. The advantages of solid state are readily apparent – reliability; long, maintenance-free service life; space and weight saving; and programmability. A solid-state sensor can also be programmed to measure just the distance the design requires rather than a predetermined stroke. With a mechanical system it may be necessary to use a sensor measuring 100 mm for example but only, say, 72 mm needs to be measured; in older technologies this leads to lower resolution and lower accuracy. Solid-state sensors however can be programmed to get the maximum resolution out of the 72 mm required, which means that often a smaller and lighter sensor can be designed, saving weight. The downside of solid-state sensors is that they are still a lot more expensive than mechanical sensors. Designers still baulk at having to pay $80 for a solid- state potentiometer when they have traditionally paid only about $2. However, with market estimates predicting that the number of UAVs October/November 2016 | Unmanned Systems Technology The design of flow meters is evolving to allow autonomous system designers more flexibility (Courtesy of Sentronics) hydraulic actuator, and the movement of a piston can be measured without the need to physically touch it. That opens up a lot of opportunities for designers who may want to measure the stroke of a piston inside a hydraulic actuator for an aileron or gearbox, or some other component that is critical, but where they cannot physically put a sensor inside the moving part because of lack of space, for example. However, by bolting a sensor to the outside that is able to detect through the wall of the housing and the piston – and measure it very accurately – has clear advantages. Fuel consumption measurement Measurement of fuel consumption is particularly crucial for UAV operators so they can work out how long they can continue to fly. One method of doing this is to measure the fuel injector opening time as set by the injector drivers within the ECU, although that has to be referenced to fuel pressure, engine speed and the prevailing mapping. On top of that the injector dynamics have to be understood, starting with the calibration of each injector with reference even to battery condition. A more elaborate alternative is to use a fuel flow meter to measure the physical