9 Development of a Low Cost Automatic Modal Hammer for Applications in Substructuring 79 Electromagnet (a) (b) (c) Upper support Nut Fixinghole Elongated slot Bolt Bottom support Male-Female XLR Cable AC Power Cord Trigger input: Contact Switch Closure, Pulse Generator, PLC, etc. Connection to Data Acquisition System Fig. 9.1 Three different modal hammers with linear actuation. (a) Bridge hammer [8]. (b) Measuring a milling machine [9]. (c) AS1220 [1] Fig. 9.2 Two different commercial modal hammers with rotational actuation. (a) NV-Tech Design—SAM [5]. (b) Maul-Theet—vImpact20 [2] Beam-Bending Hammers The second type of hammer works in principle by bending a beam and using either the inertia force or the resetting spring force to generate an impact on the structure. The Scalable Automatic Modal Hammer (SAM) in Fig. 9.2a is produced by the company NV Tech Design GmbHand is specialized to characterize nonlinear structures. The device comes with software and claims very high reproducibility [5]. The working principle is to stop the elastic hammer shaft shortly before hitting the structure. Due to the inertia, the tip of the shaft bends through and touches the surfaces, i.e. the desired impact. The vImpact series fromMaul Theet work with the same principal and are also available in different sizes. They are not controlled via a PC but with an external controller which allows the different settings [2]. The third hammer is described in [3] and works in a slightly different manner. The beam gets deflected using an electro magnet. The impact is then generated by the pullback through the resetting spring force and the following motion because of the mass inertia. Rotating Hammer The last hammer presented here works passively. The tip is mounted on a pendulum which gets displaced and then released. Using gravity the pendulum accelerates and hits the structure. This is a very simple construction but is limited in the orientation of the working direction [6] (Fig. 9.3).
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