Experimental Dynamic Load Simulation by means of Modal Force Combination.

Kurzfassung

The increase of dimensions and recent advances in lightweight design of space systems have led to the requirement of new structural dynamic qualification techniques. Dynamic qualification means experimental simulation of the structural dynamic environment in order to check the dynamic behaviour of spacecraft. Presently, structural dynamic qualification of space systems is accomplished by means of single-axis electrodynamic shaker systems. For the last ten years multi-axis hydraulic shaker systems have also been under investigation to supplement and possibly replace the current single-axis testing philosophy. Both methods apply base acceleration at the interface between the satellite and launcher. Thus, they require high forces in order to accelerate the total mass of the test structure. An alternative test philosophy was proposed and investigated in a study funded by ESA. The new concept called the Modal Force Combination (MFC) technique replaces the base acceleration by having a set of forces acting directly in a selected number of structural points. The forces compensate for the d'Alembert's inertia forces, which arise due to the transient multi-axial base acceleration. The transient forcing functions are calculated by means of modal deformations and related modal force distributions in order to get the same structural responses of the structure as would occur by base acceleration. The direct substitution of distributed forces by means of a set of single forces yielding the same responses is generally impossible. This would require too many exciters. However, the MFC method offers two possibilities to perform a simulation. The first version is based on the demand for the equality of the input energy expressed bymeans of the modes of vibration. The second version starts with the demand for the equality of the structural responses in the points of