An ultrasonic anemometer for lightweight fixed-wing UAV airspeed measurements

Abstract

This thesis explores the development of an ultrasonic airspeed sensor for lightweight fixed-wing drones. The ultrasonic airspeed sensor acts as an alternative to Pitotstatic systems, a differential pressure sensor used for obtaining airspeed data for aircraft. The design takes inspiration from commercially available ultrasonic anemometers and ultrasonic distance sensors. The prototype consists of two ultrasonic transceivers facing each other at a predefined distance. One ultrasonic transceiver will transmit a signal, while the other will receive it. These roles will change to obtain an upstream and downstream airspeed measurement. The system records the time of initial transmission to receiving the signal, and by knowing the distance between the two ultrasonic sensors, the speed of sound can be established. Many factors can influence the speed of sound, however the only factor that has a directional component is wind. Taking advantage of this, the influence of wind and all other atmospheric conditions on the speed of sound can be separated. The ultrasonic signal is processed by an analog signal conditioning circuit consisting of amplifiers, filters and a comparator. A received ultrasonic signal peak is detected by the transition of comparator states, which is used to determine the transmission time. Cross-correlating upwind and downwind signal phases will determine the indicated airspeed measurement, which requires further calibration to obtain calibrated airspeed measurements.