6 E. Bonisoli et al. 1.3 Product Development The geometrical parameters resulting from the optimization process represent the reference configuration for the product development phase. The aim is to convert a virtual prototype in a real device ready for the medium-large scale production, fully functional and ready to use when placed in training shoes. In this context, a consistent number of prototypes have been made to experimentally validate the design process and prove the project feasibility. 1.3.1 Virtual Prototype Starting from the reference configuration, a virtual prototype has been developed in according with the manufacturing requirements. The device consists of three plastic components, the case, the cap and the backcap, holding the cylindrical magnet and protecting the coils and the electronic interface. In order to optimize the available space, the elastic characteristic is obtained by means of two conic springs. As the step-counter will be insert in the sole of training shoes, a very important aspect is the design of the housing that must be sufficiently resistant to support the external load during the running and walking activity but, at the same time, not too thick in order to do not subtract space to the functional components. Therefore, 3D modelling, FEM analysis and the selection of the most suitable materials were key aspects in the product development of this device. The geometries that characterize the structural elements have been studied to minimize the internal stresses but always thinking to the ease of mounting. Figure 1.7 shows the section view of the device and an example of FEM analysis result. Device behavior has been simulated in almost ideal condition neglecting friction. Figure 1.8 shows the simulated electrical performance in terms of average harvested power over 22 s of activity with the first step after 2.5 s, voltage trend and energy recovery with respect to the energy requirement of the tenth step. As it can be observed, the energy harvested during the step in this condition is fully sufficient to support the electric interface requirements. 1.3.2 3D Printed Prototypes The first testing phase has been conducted on the 3D printed prototypes shown in Fig. 1.9. Tests have been performed reproducing on a shaker the same vibrational input used for the design process. Experimental evidences revealed that the springs, even if made of ductile iron, due to the strength of the magnetic induction of the floating element, shift on the lateral side of the magnet, precluded the right working of the device. To maintain the spring coaxial to the magnet, two plastic centering rings have been pasted on its flat surfaces. This allows the right linear oscillation of the floating elements but considerably reduces its available stroke resulting in a loss of performance of about 20 % when running as demonstrate by Fig. 1.10. Due to the smaller oscillation caused by the walking input, in this operative condition performance is the same as before; in fact, the stroke reduction do not affect magnet displacement when walking while strongly influence the behavior when running causing violent bumps that dissipate lot of energy. Real performance is much lower than simulated in almost ideal condition due to friction between floating element and guide, materials efficiency, components production uncertainty, assembling imperfection and all the inconvenient that differs the real from the ideal world. In order to overcome the loss due to the stroke reduction caused by the centering rings, ad hoc magnets, characterized by the same overall dimensions of the previous but presenting two centering holes on the flat surfaces, have been used. Exploiting larger stroke balances the smaller flux due to the smaller magnet volume. In addition, little adjustments of the guide component allowed reducing friction extending the magnet oscillation duration and increasing case resistance to the soldering high temperature. 1.3.3 Molded Prototypes A consistent number of molded prototypes have been produced through a semi-industrialized manufacturing process, see Fig. 1.11. Plots in Fig. 1.12 demonstrate the convenience of adopting ad hoc magnets and the effectiveness of the adjustment performed on the main plastic component. No significant differences of the device performance is registered under walking
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