Measuring UAV Propeller RPM with FMCW Radar: Validation with Calibrated Accelerometers

The use of Unmanned Aerial Vehicles (UAVs) in non-military applications has become more widespread in recent years. Safety concerns during their operation have also increased. For this reason, developing detection techniques targeting such devices is an area of scientific interest. The detection of drones can be achieved through the use of various sensors, including optical (video, or Light Detection And Ranging, or electromagnetic sensors. By resorting to the latter family of sensing technologies, different types of information about the UAV can be collected, which motivates the focus of this work on Radar sensors. Machine Learning and Deep Learning approaches may allow to classify the type of UAV, but more quantitative figures can be obtained as well, from the Radar signals. Among them, the rotational speed of a UAV propeller was already quantified by a Frequency Modulated Continuous Wave Radar, but the technique used can be investigated more in-depth to better understand the interaction with chassis vibrations, to evaluate the accuracy of the obtained values. To this aim, a series of tests are carried out on a mockup quadcopter. The results output by the Radar, are compared to the values provided by a calibrated accelerometer, showing that the mean vibration frequency is exactly measured, while a difference in the order of tens of micrometers is found on the mean vibration displacement. These outcomes prove that the vibration detected by the Radar is actually relatable to the rotational speed of the UAV propeller.