## 6 Experimental Verification and Discussion  
A prototype of the proposed CC&OV-PT controlled two-stage WPT system was designed and verified, as shown in Figure 11. The experimental system consists of a Buck converter, Inverter, coupling inductors, coupling capacitors, Rectifier, Resistor and Radio frequency (RF) module. The technical parameters are the same as those listed in Table 2. The vertical distance between the primary-side coil and the secondary-side coil is 30 cm. The experimental waveforms of the primary side voltage under the PT control and CC&OV-PT control methods are shown in Figures 12 and 13, respectively. It can be seen from Figure 12 that, under the traditional PT control method, the output voltage, the inverter input voltage and the converter inductor current all have low-frequency oscillations with the frequency of about 120 Hz. In a low-frequency oscillation cycle, 108 pulses can be observed, including 33 high power pulses and 75 low power pulses, namely the pulse combination of one oscillation cycle is 33 \(P_{\text{H}}\)-75 \(P_{\text{L}}\). When the WPT system operates at a steady state, the oscillation ranges of output voltage \(v_{\text{o}}\), inverter input voltage \(v_{1}\) and the inductor current \(i_{\text{L}}\) are [8.8 V, 11.1 V], [8.3 V, 12.5 V] and [0 A, 2.1 A], respectively. These results show that the PT control method can result in unavoidable low-frequency oscillation in this two-stage WPT system. Figure 11: Prototype of the proposed CC&OV-PT controlled two-stage WPT system. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Working Conditions**} & \multirow{2}{*}{**Control Methods**} & \multicolumn{2}{c}{**Regulating Voltage**} \\  & & **Peak-to-Peak** & **Regulating Time/ms** \\  & & **Value/V** & \\ \hline \multirow{2}{*}{Output load changed} & PWM control & 2.5 & 37.5 \\  & Traditional PT control & 3.1 & 3 \\  & CC\&OV-PT control & 0.7 & 3 \\ \hline \multirow{2}{*}{Input voltage changed} & PWM control & 3 & 30 \\  & Traditional PT control & 3.6 & 3 \\  & CC\&OV-PT control & 0.09 & 4 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The regulation time and peak-to-peak values of system output voltage under different control methods and abrupt changes in different working conditions. As shown in Figure 13, when the CC&OV-PT control is applied, no low-frequency oscillation phenomena are observed in the waveforms of the output voltage \(v_{\rm o}\), the inverter input voltage \(v_{\rm 1}\), and the converter inductor current \(i_{\rm L}\). It can be seen from the waveforms of the inductor current and power pulses, only 11 power pulses are measured in an oscillation cycle, and the combination of the power pulses is \(1P_{\rm H}\)-\(5P_{\rm L}\)-\(1P_{\rm H}\)-\(4P_{\rm L}\). When the WPT system operates at steady state, the output voltage \(v_{\rm o}\) and inverter input voltage \(v_{\rm 1}\) are stabilized at 10 V and 7 V, respectively. Meanwhile, the variation range of inductor current \(i_{\rm L}\) is [1.1 A, 1.5 A]. These results show that the low-frequency oscillations have been eliminated by using the proposed CC&OV-PT control method. The experimental waveforms corresponding to CC&OV-PT control and PWM control are presented in Figure 14a,b, respectively, when the load reduces from 10 \(\Omega\) to 7 \(\Omega\). It can be seen that, for the CC&OV-PT controlled WPT system, the transient time is 5.6 ms and the overshoot of output voltage \(v_{\rm o}\) is 1.1 V. However, when the conventional PWM method is applied, the transient time is 100 ms, and the overshoot of output voltage \(v_{\rm o}\) is 1.3 V. Hence, the CC&OV-PT control is superior to the PWM control due to quicker transient response and smaller overshot of the output voltage. Figure 12: Experimental waveform under the traditional PT control. Figure 13: Experimental waveform under the proposed CC&OV-PT control.