Resistive load There is an optimal value for the resistive load in terms of the power production. At this optimal value however the velocity will be minimal. Since the nonlinearity is more significant when the amplitude of motion is large, at optimal resistance where is the amplitude of motion is minimal, the nonlinear effects become less dominant. This phenomenon is illustrated in Fig. 5. (a) (b) Fig. 5: Relation between the resistive load across the piezoelectric patch and a) Tip velocity/base acceleration frequency response function and b) harvested power/ base acceleration2 frequency response function. The colors represent different base acceleration: blue: 0.15, green 0.3, red 0.74, cyan 1.5, magenta 3 m.s-2. Mono-stable Hybrid Harvester This section explores the behavior of hybrid mono-stable nonlinear harvester. The main difference between this section and previous section is the inclusion of the electromagnetic harvesting in experimental study. magnet distance Fig. 6 illustrates that the natural frequency of the harvester increases with magnet distance. The hardening nonlinearity however decreases with the magnet gap. The damping in the structure decreases with the magnet distance due to presence of eddy currents. Decreasing the distance therefore increases the bandwidth at the cost of reduction in motion amplitude. The decrease in the velocity magnifies in piezoelectric and electromagnetic power curves. The electromagnetic power is more sensitive to amplitude of oscillation than the piezoelectric power. As illustrated in Fig. 2 the coils are on the sides of the tip magnet. If the range of motion of the tip mass is below a certain limit, the magnet would not pass over the coils and therefore there would be a significant loss in electromagnetic power generation. 4 5 6 7 8 10 1 10 2 10 3 0 0.2 0.4 0.6 0.8 Frequency (Hz) Tip velocity FRF, =31mm R1 k 1/S 2 4 5 6 7 8 10 1 10 2 10 3 0 0.2 0.4 0.6 0.8 Frequency (Hz) Power FRF =31mm R1 k Watt / g 2 469
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