## 4 Experimental Results
### Pump-Level Test Results
Therefore, the maximum magnitude of asymmetric pitch control command should be limited, as indicated in Figure 8. Endurance tests of electric oil pumps should be conducted after applying the asymmetric pitch control. In addition, as the proposed method is based on instantaneous changes in the current command of the electric oil pump, the control performance directly affects the noise characteristics of the system. The gains and execution period for the current controller should be carefully selected, and the real-time property should be secured without delay. To analyze the effects of these pulsation shape changes, the spectrum analysis was conducted as follows. Figure 11 presents the results of the frequency spectrum analysis  
Figure 10: Pressure fluctuation according to the control methods. Figure 9: Hydraulic performances according to the control methods. of measured noise and vibration when the pump is operating at 1500 rpm. In the case of normal control, the highest peaks of the frequency band are 550 Hz and 825 Hz, which are harmonic components of 275 Hz. The 275 Hz is the combination frequency of the rotational speed (25 Hz) and the number of teeth (11). When the asymmetric pitch control is applied, it can be confirmed that the noise level of the frequencies of 550 Hz and 825 Hz is reduced. In contrast, it was confirmed that the frequency components of 525 Hz, 575 Hz, 800 Hz, and 825 Hz, which are distributed at intervals of 25 Hz based on 550 Hz and 825 Hz, tend to increase symmetrically. This is because the size of the asymmetric pitch control command corresponding to each tooth was set to one rotation cycle, as in Equation (4), and it can be confirmed that this result has the same tendency as the results of a previous study that mechanically applied the asymmetric pitch as in Equation (2) [5,6]. According to the vibration measurement results in Figure 11b, it can be confirmed that the vibration is symmetrically distributed based on 550 Hz, similarly to the noise measurement results. As summarized in Table 3, when the asymmetric pitch control is applied, the maximum peak value of the noise is reduced by 4.5 dB(A), but the overall size of the entire noise is maintained at the same level. This shows that when applying the asymmetric pitch, rather than absolutely reducing the noise, it plays a role in dispersing the frequency components and changing them into a form of noise that is less irritating to the driver's ears. Figure 12 shows the results of the frequency spectrum analysis of noise when driving at 3000 rpm (50 Hz). Compared to the 1500 rpm driving condition, the reduction in peak noise at 1100 Hz order frequencies is only 3 dB(A) when applying the asymmetric pitch control, but the tendency to be symmetrically distributed at 50 Hz rotation frequency  
Figure 11: Frequency spectrum according to the control methods (1500 rpm): (**a**) noise; (**b**) vibration. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Category**} & \multicolumn{2}{c}{**Noise [dB(A)]**} & \multicolumn{2}{c}{**Vibration [dB]**} \\ \cline{2-5}  & **Overall** & **Max Peak** & **Overall** & **Max Peak** \\ \hline Normal Control & 58.2 & 52.0 & 134.0 & 118.1 \\ Asymmetric Pitch Control & 58.0 & 47.5 & 133.5 & 113.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Noise and vibration measurement results at 1500 rpm. intervals is the same. As the magnitude of the asymmetric pitch control command value was set to 20% of the rated current, it can be seen that the effect of applying the asymmetric pitch control decreases as the load current increases. Figure 13 shows the noise color map according to the control methods at 1500 rpm and 3000 rpm.