Development of a Low Cost Automatic Modal Hammer for Applications in Substructuring

One key goal of Dynamic Substructuring (DS) is the coupling of measured components and simulated ones. This can be done using a frequency-based formulation of the system dynamics. For the experimental determination of the components dynamics, good and reliable measurements are extremely important to achieve correct results. Usually the frequency response functions (FRF) are obtained using a modal hammer with a force sensor tip. Some of the problems that occur are that the excitation positions vary with every hit, that the angle is very hard to determine and furthermore that it is nearly impossible to bring the same energy into the system with every hit. This contribution gives a short motivation why the automatisation of modal analysis experiments could improve the method of experimental substructuring. At the Chair of Applied Mechanics at TU-Munich, we developed a low-cost automatic modal hammer which is presented here. The whole device is positioned in front of the structure with a stand, so there is no need for readjustment in order to perform multiple impacts on the structure. The energy of the impacts can be adjusted by tuning the parameter settings of the automatic hammer. The motion in the hammer is induced by an electromagnetic reluctance actuator. The principle is shown and a simple multi-body model set up in this paper. This model is used to tune parameters in a way to avoid double impacts and to predict the impact forces. The actuator is driven by some electronics with a microcontroller, whereby the acceleration time, voltage and the impulse series can be adjusted via PC. Consistence between the model and the real device are shown using a fully instrumented test rig. Furthermore, test series were carried out to prove repeatability. Finally, a demonstration application on an academic structure is shown. Here the differences between classical modal analysis using a hand hammer and using the new automatic hammer are evident. Generally, the FRFs using the automatic hammer are less noisy and the coherence function is better.