Predicting Full-Field Strain on a Wind Turbine for Arbitrary Excitation Using Displacements of Optical Targets Measured with Photogrammetry

Abstract

Wind turbine blades and other structures are often subjected to dynamic loading that may not be predicted or measurable at critical locations of interest. Therefore, a non-contacting measurement technique that can provide information throughout an entire structure with the absence of instrumented sensors is desirable. Such an approach is particularly beneficial and relevant to operating rotor or wind turbine blades. In this paper, a three-bladed wind turbine placed in a semi-built-in boundary condition was subjected to a variety of different loadings. The turbine was excited using a sinusoidal excitation, a pluck test, arbitrary impacts on three blades, and random force excitations with a mechanical shaker. The response of the structure to these excitations at optical targets mounted to the blades was measured using three-dimensional point tracking. The limited set of measured displacement at the optical targets was expanded using a modal expansion algorithm. The expanded displacement was used in conjunction with a finite element model of the turbine to extract dynamic strain throughout the entire structure. The results from the technique were compared to instrumented strain gages and are shown be in close agreement. The predicted strain using the proposed approach is not limited to the locations of the optical targets or where the cameras have line of sight. This new technique may enable a new structural health-monitoring approach that has the ability to interrogate an entire structure, inside and outer surface.