Vibration Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 120

The sawtooth waveform observed in Figure 4.10 is the rectification of the
AC voltage pulses of the piezoelectric push-button igniter. After the AC-DC
rectification, the unregulated DC voltage is filtered by a parallel connected
capacitor of 2.2 F added to provide some smoothing effects on the DC voltage

as well as to store the harvested energy. The waveforms of the capacitor and
the output of the linear regulator are shown in Figure 4.11 where Channel 1
is the DC voltage across the capacitor and Channel 2 is the regulated voltage
output from the linear regulator voltage.
It can be seen from Figure 4.11 that the maximum voltage accumulated
across the capacitor is 7.84 V, and the capacitor is discharged completely in
300 ms. Using the formula

1
2 CV cap

E cap = (4.8)

the amount of electrical energy that is harvested after the piezoelectric push
button is depressed is 67.61 J. As for the linear regulator, the output DC

voltage is regulated at 3.3 V, and it remains constant for around 50 ms before
it starts to drop to zero with respect to the capacitor discharge rate. Hence,
the challenge is that within the 50 ms of regulated DC 3.3 V, the 12-bit ad-
dress/data information has to be transmitted from the HT-12E encoder via
the radio transmitter. As mentioned, the harvested energy occurs only for a
short pulse through actuation of the piezoelectric pulse generator; therefore,
in evaluating the performance of our energy harvesting circuit design, the
voltage waveforms of the discharging capacitor together with the DC linear

CH1 = 2V
DC

CH3 = 2 V
DC

50 ms/div
(50 ms/div)
NORM:20 kS/s

100:1 1:1

CH1: Capacitor Voltage

CH2: Linear
Regulator Voltage

=Trace1= Max 7.84 OV

FIGURE 4.11
Voltage waveforms of a capacitor and output of a linear regulator.