## 4 Measurements, Simulations and Discussion
Previous measurements have pointed out that, given controllable parameters T\({}_{\text{D}}\) and D, the programmed Arduino board is capable of self-adapting in order to generate the suitable PWM signals with respect to the AC input voltage frequency. Figure 13 shows the AC-DC converter's output voltage with a linear sweeping of the input voltage frequency from 10 up to 100 Hz, in a time-window of 15 s. The input voltage amplitude remains constant at 220 mV\({}_{\text{peak}}\). The controllable parameters have been fixed at D = 75% and T\({}_{\text{D}}\) = 1%. The colored rectangular boxes represent three different time spans that have been analyzed in the corresponding top panes (same axes colors), where the output voltage and PWM signals are shown. It is noticeable that the output voltage (second pane from the top) is continuously increasing over time, up to about 3 V when the input frequency approaches 100 Hz. Furthermore, the Arduino board correctly measures the input frequency and properly drives the switching signals to the MOSFET's gates in a self-adapting fashion. Figure 11: Mapping of the RMS power efficiencies with respect to Duty Cycle (D) and Time Delay (T\({}_{\text{D}}\)) at different input voltage frequencies (f\({}_{\text{in}}\)). The insets show the top view of each surface, while the red dots point out the maximums. This measurement is representative of a generic AC source behavior, which is typical of an EH device that exploits an engine vibration during its transient phases. With the aim to explore the operating conditions and behaviors, a randomic impulse-like voltage has been given as input to the AC-DC boost converter, thanks to the arbitrary waveform generator. The signal is designed as shown in Figure 14b, where two heartbeat-like pulses, with peaks equal to about 200 mV and 20 ms time period, are shifted by 20 ms. This signal is representative of a typical output voltage coming from an EH device based on magnetostrictive materials, installed under the asphalt on a vi  
Figure 12: Ripple Factor (RF) values with respect to Duty Cycle (D) and Time Delay (T\({}_{\text{D}}\)) at different input voltage frequencies (f\({}_{\text{in}}\)). The insets show the top view of each surfaces, while red dots identify the maximum values. Figure 8:0 Measured signals of a linear swept\(-\)frequency input voltage over 15 s time interval, at D = 75% and T\({}_{\text{D}}\) = 1%. Colored boxes represent three different time spans, where the input frequency is varying around about 25 Hz (_green_), 55 (_orange_) and 85 Hz (_purple_), respectively. Top panes show the output voltage and PWM signals for each analyzed time span. In Figure 13(a), a screenshot of the oscilloscope (model: LeCroy WaveRunner 6030A) showing the measurements of output and PWM voltages together with the random impulse-like input voltages, at D = 75% and T\({}_{\text{D}}\) = 1%, over a 1 M\(\Omega\) resistor is shown. The yellow line is the output voltage, the magenta line is the input voltage while the green and blue lines are PWM1 and PWM2 signals, respectively. The output voltage increases at each impulse because of the energy stored by the inductor and transferred to the capacitor. The considered load is representative of a possible configuration where a super-capacitor with very low equivalent series resistance (low ESR) is adopted to store electric energy. Figure 13(b) shows the output voltage when steady-state condition is reached (after about 90 s), with a periodic impulse-like input voltage. It is noticeable that in such working conditions, the output voltage reaches a maximum value of about 9.4 V.  
Figure 8:1 Oscilloscope screenshots of the measurements with impulse–like input voltage. Yellow line is the output (V\({}_{\text{out}}\)), magenta line is the input voltage (V\({}_{\text{in}}\)) while green and blue lines are PWM1 and PWM2 signals, respectively. Controllable parameters are D = 75% and T\({}_{\text{D}}\) = 1%, while R\({}_{\text{out}}\) = 1 M\(\Omega\).