On the feasibility of replacing base excitation by modal forces in qualification testing - A case study on the PPF satellite

The conventional dynamic qualification of space structures requires the availability of base-driven vibration test facilities. For large and unwieldy structures which cannot be tested by base excitation, an alternative approach, the modal force combination method, can be utilised. The method employs special multi-point forces to simulate the vibrations of a base excitation. The principles of the modal force combination method are explained and basic equations are derived. With the purpose of investigating the feasibility of the method, an analytical case study based on experimental modal data is performed on the PPF satellite. The derivation of the PPF simulation model from a comprehensive modal survey test is described. The structural responses to an exemplary base acceleration input are calculated and compared to the calculated responses of a modal force combination excitation. The results demonstrate the feasibility of the method. Due to the required input data, the modal force combination method can be excellently combined with a modal survey test. Thus, the combined performance of a modal survey test and modal force simulation test delivers both the basis for tile validation of the analytical model and the dynamic qualification of the structural design.