80 J. Maierhofer et al. double-notch type flexure metal piece impact excitation system in deflected shape test structure holder part equilibrium electromagnet Driver cam Stopper Main body Hammer X 0 d 0 x(t) y(t) x(t) y(t) k c g 0 Fig. 9.3 Further principles of automated modal hammers. (a) Pendulum hammer [6]. (b) Beam deflection hammer [3] 9.3 AMimpact All available automated impact hammers have one drawback: They are very high priced and very big. Therefore, they are quite cumbersome to mount in a way to excite the structure as desired. This becomes particularly obvious when working on structures on a whole car. The challenge was to set up a simple mechatronic system to fulfill the need for a handy and yet repeatable way of impacting light structures. After some calculations and tests the AMimpact was developed. 9.3.1 Description of the System The whole system of the AMimpact is packed into a light 3D printed housing, which can be mounted on any available tripod. In order to use it, only a power source and a PC connected to the USB port are required. With a little software the user can set up all the possible parameters, which are explained later, and can even save them as presets. In Fig. 9.4a the finished system is shown, whereas Fig. 9.4b depicts the principle scheme of the AMimpact. The core functionality comes from a magnetic linear actuator, which works on the principle of reluctance forces. As current runs through the coils, the bolt receives a forward driving force. On the front side of the bolt, a common piezo force sensor (Head of Model 086E80, PCB) is mounted. A spring is attached to the backside of the bolt and connected with the housing to reset the position of the bolt. The key idea is to switch off the current just the moment before the impact, so that the bolt flies to the structure ballistically. Due to the repulsion, double impacts are nearly impossible. Fig. 9.4 The automated modal hammer of the Chair of Applied Mechanics, called AMimpact. (a) Picture of AMimpact. (b) Diagram of system
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