Feasibility of Drone-Based Modal Analysis Using ToF-Grayscale and Tracking Cameras

The application of modal analysis is a well-established procedure for detecting damage in civil structures. However, implementing monitoring systems on civil structures could involve a large number of sensors leading to expensive and cumbersome setups. On the other hand, unmanned aerial vehicles (UAVs) are already employed for monitoring purposes. Moreover, using UAVs inspections can be executed close to structures and using a single measurement node that can be moved from one measurement point to another. In this article, a vision-based measurement procedure composed of a system mounted on a prototype drone is presented in order to perform modal analysis of a structure. This measuring system is equipped with a grayscale camera, a time-of-flight (ToF) sensor, and a trajectory tracking camera. This composition is necessary to measure vibrations by tracking features on the structure (grayscale camera) to convert vibrations from pixels to millimeters (ToF sensor) and to track the trajectory of the drone (trajectory tracking camera). This latter sensor is used to reduce the effect of the movement of the drone in relation to the vibrations of the structure and to reconstruct the 3-D geometry of the structure. The grayscale camera on the drone is synchronized with another fixed grayscale camera, that frames a point on the structure, used as the reference point for modal analysis. This measurement procedure was tested on a mock-up of a structure a few meters in size, and the results are compared with those obtained using accelerometers. The outcome of these tests proves that it is possible to correctly measure resonance frequencies (with errors lower than 0.3%) and to reconstruct mode shapes with cameras assembled on a drone, as well as to reconstruct the 3-D geometry of the points measured.