Drone-based single-shot grating projection for 3D measurement of surface anomalies

• Applicant: Professor Dr.-Ing. Peter Lehmann
• Subject Area: Measurement Systems
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 541032875

Drone-based measurement systems are currently primarily being used for long distance inspection at locations, which are hard to reach. In this context, typically visual optical inspection tasks or thermographic measurements are carried out. The proposed project aims at mesoscale optical 3D measurements with comparatively small fields of view and high lateral resolution. Applications are related to the detection and analysis of surface anomalies with lateral extent in the millimeter or centimeter range, which occur, for example, on rotor blades of wind turbines due to delamination buckling or mechanical distortions resulting from damage caused by hail. A quantitative assessment of such damages is necessary to decide upon further actions, which are necessary to guarantee the proper function of the turbine. Today, typically pure visual inspections are carried out in this context via wire rope techniques. In our project we work on drone based mesoscale 3D measurements. Preliminary investigations show that single-shot fringe projection is an adequate and promising measurement method for these applications. The method projects a static fringe pattern onto the surface area under investigation and records a single camera image under a well-known triangulation angle. Afterwards, the digital fringe pattern will be analyzed with respect to local phase anomalies. From these phase anomalies surface height anomalies will be obtained. Compared to rather complex 3D objects, which are typically measured via fringe projection techniques, the surface height changes are quite moderate for the objects studied here. Consequently, no phase ambiguities are to be expected and the surface topography can be reconstructed based on a single fringe pattern. Apart from a robust height discrimination, special challenges in context with drone-based measurements are related to securing the correct working distance and position of the measuring system with respect to the measured surface. Thus, the methodological approach we follow in the project analyses the images taken by the drone’s camera, determines the distance from the surface under investigation by an ultrasonic distance sensor using phase-runtime analysis, and uses a pulsed illumination and image acquisition triggered by the distance sensor. This measurement strategy will be elaborated in detail as a part of the project. In order to assess the achieved or achievable measuring uncertainty, comparative measurements will be conducted in the laboratory. In addition, an uncertainty analysis of the camera-based phase measurement will be carried out, combining Monte-Carlo simulations and statistical uncertainty calculations based on GUM.

• DFG Programme: Priority Programmes
• Subproject of: SPP 2433: Metrology on flying platforms