Figure 18.8 shows calculated and measured results of the open circuit voltage in two cases. When the magnet pair is set to have antiparallel magnetization, the energy harvester shows a high open circuit voltage with a peak value of 544 mV at a vibration frequency of 54 Hz and an acceleration amplitude of 0.57 x g. As expected, for the case when the two bias magnets are arranged with parallel magnetizations, the output voltage has a significantly lowered peak value of 8 mV at double the driving frequency (i. e. at 108 Hz). It is interesting to note that the mechanical vibration source is a sine wave signal, while the output voltage is not, but with narrow peaks with a full width at half maximum of 1 ms. This is related to the nonuniform magnetic field spatial distribution, leading to the approximate square wave time varying magnetic flux, shown in Fig. 18.9. The free end amplitude dependent flux is also plotted. It is clear that the flux in the beam is reversed
immediately while passing the nonstable equilibrium position in the middle of the hard magnet pair. The sharp drop and rise in flux is the reason for the large induced voltage.
Figure 18.10 shows the frequency response of the harvester. The maximum measured output power is 74 mW across a 1 £2 load and with a time average value 5 mW at an acceleration amplitude of 0.57 x g corresponding to a maximum power density of 1.07 mW/cm3 or 1.88 mW/(gcm3).
