Fig. 2 (b) shows the main phases of the process: 1. the tire is not touching the ground, so the scavenger is subjected to radial (centripetal) acceleration, therefore a centrifugal force acts on the device, 2. the tire is arriving on the ground and is deforming, so the scavenger is subjected to an acceleration peak, 3. the tire is on the ground (contact patch, footprint) and it is moving with a straight motion, the radial acceleration quickly drops to zero, 4. the tire leaves the ground contact and the scavenger is again subjected to another acceleration peak. Mechanical model An accurate model of the energy scavenger requires the study of different phenomena, in particular: • magnetic circuit and interaction with magnet movement, taking into account nonlinearities of materials, • interaction with external electric load and definition of a scavenger equivalent circuit, • dynamic simulation of the floating magnet as a response from external acceleration, • evaluation of the wasted energy due to dry friction and pneumatic forces arising from mass motion. The design phase of the energy scavenger requires the definition of a simulation tool that is able to couple the different physical phenomena involved. The complete block diagram is shown in Fig. 3. The block-oriented sub-structuring technique has the following properties [4]: • starting from analytical or experimental component behavior, to predict system performance in time domain, for several operating conditions; • to underline correspondences between model blocks and physical system, input/output relations between components and their interactions on system behavior; • to allow a high level of flexibility in pre-development and development phases, through the use of a multi-sharing component library and the adoption of a user-friendly graphical interface; • to develop and compare performance and limitations of different electric/electronic or mechanical layouts, in particular focusing on power absorption and efficiency of the whole system. Fig. 3 Block diagram of the energy scavenger simulator Each substructure is connected to the others through a direct link (e.g., in Fig. 4 the coil and the preload magnet interact on the floating magnet with forces) and a feedback link (the evaluated forces are dependent on the state of the floating magnet) that produces the corresponding reaction to the acting state. 341
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