25 A Concept for the Estimation of Displacement Fields in Flexible Wind Turbine Structures 253 With a TRAC value of 0.9998, the two estimated displacements in the tower structure, q Tw y (t) and q Tw z (t), show a very good correlation with the reference curves, see Fig. 25.4a. In addition, relative errors of less than 3% confirm that even by applying only one single IMU, the displacement field can be accurately recovered within the tower structure. Regarding the blade structure in Fig. 25.4b, a TRAC value of 0.9988 indicates a very good correlation as well, but the relative errors of up to 35% reveal larger magnitude differences between estimated and reference displacements. The latter effect can be ascribed to an insufficient representation of the actual ODS by the selected static modes only and needs to be investigated carefully. Nonetheless, the sole number of load cycles is retained correctly, compare the detailed view of Fig. 25.4c, which is important for a subsequent fatigue analysis. 25.7 Conclusion and Outlook As a central part of model-based fatigue monitoring in structurally loaded mechanical systems, the displacement fields at critical spots need to be recovered accurately on the basis of a limited number of measurements. For that purpose, the present contribution proposes a sequential framework to apply the well-known concept of Modal Decomposition, inherently restricted to linear models, to nonlinear wind turbine systems. The high quality of this approach is demonstrated by a numerical multibody simulation of a small-scale wind turbine test rig based on virtual IMU measurements in combination with static mode shapes. Within the next project phases, the proposed framework will be implemented on a detailed flexible multibody model of a full wind turbine to account for more realistic operating conditions. Furthermore, the appropriate definition of a modal basis for the MDE concept will be carefully analyzed to ensure accurate state estimations within the entire operating range. Up until now, perfect knowledge of the required state measurements, independent of a specific sensor concept, has been assumed. Future investigations will aim at utilizing Intertial Measurement Units for data acquisition on the real turbine. Acknowledgments This research is part of the DynAWind2 project funded by the German Federal Ministry for Economic Affairs and Energy under grant number 0325228E/F/G. References 1. Luthe, J., Schulze, A., Rachholz, R., Zierath, J., Woernle, C.: State observation in beam-like structures under unknown excitation. In: Kecskeméthy, A., Geu Flores, F. (eds.) 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