The electromagnetic load In principle, the electromagnetic load affects the power harvesting similar to the piezoelectric load. However the optimal electromagnetic load is in order of Ohms and for the examined device is less than the resistance of the wires. As illustrated in Fig. 9, the optimal electromagnetic load is less than the smallest shunt resistance and the power decreases with the electromagnetic resistance. The tip velocity and the piezoelectric power are almost insensitive to the electromagnetic shunt resistance. The situation has been predicated by modeling performed in [18]. (a) (b) (c) Fig. 9: Relation between the electromagnetic load and a) tip velocity transfer function, b) piezoelectric power transfer function, c) electromagnetic power transfer function. Bi-stable Harvester When the distance between the base and the tip magnets is less that 27mm the zero deflection equilibrium of the beam is not stable. There are two equilibriums on the sides which are stable. The motion of the harvester can be one of three forms: small oscillations about any of the stable equilibriums, chaotic motion, or large limit cycle oscillations circling both stable equilibriums. In the following we experimentally examine the conditions that give rise to any of the possible motion patters. 4 5 6 7 8 9 0 0.05 0.1 0.15 0.2 a b =1.47, R 1 =100k, =37(mm), legend: R 2 () Frequency (Hz) Tip vel/ Base accel (1/S) 2.2 4.7 6.8 10 4 5 6 7 8 9 0 0.2 0.4 0.6 0.8 1 1.2 x 10 -3 a b =1.47, R 1 =100k, =37(mm), legend: R 2 () Frequency (Hz) PZT power/ Base accel 2 (Kg.S) 2.2 4.7 6.8 10 4 5 6 7 8 9 0 0.5 1 1.5 2 2.5 3 3.5 x 10 -5 a b =1.47, R 1 =100k, =37(mm), legend: R 2 () Frequency (Hz) E.M. power/ Base accel 2 (Kg.S) 2.2 4.7 6.8 10 472
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