## 3 Numerical Simulation of Drop Process of Single DOF System
### Rotor Dynamic Behavior under Failure of Upper Unit  
The upper unit circuit is cut off, and the rotor dynamic behavior is tested as shown in Figure 20.  
According to Figure 20, when the electromagnetic system of the upper unit fails, the variation trend of rotor rubbing displacement obtained by experiment is basically consistent with the theoretical simulation. The theoretical value of rotor displacement ranges from \(-12.4\) um to \(-29.1\) um. The rubbing displacement is \(16.7\) um and the rubbing time is \(0.07\) s. The experimental value ranged from \(-16.6\) um to \(-45.2\) um, the rubbing displacement is \(28.6\) um, and the rubbing time is \(4.21\) s. The experimental rubbing displacement is \(11.9\) um larger than that the theoretical value. The rubbing time is \(4.14\) s longer than that the theoretical value. This phenomenon is due to unilateral electromagnetic control adopted in the test stand and the phenomenon of oil leakage in MLDSB. It will lead to the speed of rubbing is smaller and the response time is longer.  
When the upper unit fails, the pressure variation trend of the upper/lower oil obtained by experiment is basically consistent with the theoretical simulation. The theoretical value of upper pocket pressure \(p_{r,1}\) decreases from \(0.060\) MPa to \(0.035\) MPa. The experimental value decreases from \(0.070\) MPa to \(0.041\) MPa. The theoretical value of lower pocket pressure \(p_{r,2}\) increases from \(0.161\) MPa to \(0.419\) MPa. The experimental value increases from \(0.137\) MPa to \(0.189\) MPa. Because the final balance position of the rotor is lower than the initial position, the lower pocket pressure changes more than that of the upper pocket pressure.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Symbol** & **Variable** & **Size** \\ \hline \(i_{0}\) & Biasing circuit & \(0.8\) A \\ \(p_{s}\) & Supply pressure & \(0.04\) MPa \\ \(\mu\) & Dynamic viscosity & \(0.041\) Pa\(\cdot\)s \\ \(m\) & Mass of rotor & \(4\) kg \\ \(\mu_{0}\) & Permeability of air & \(4\pi\times 10^{-7}\) H/m \\ \(h_{0}\) & Oil film thickness & \(70\) μm \\ \(B\) & Flow coefficient & \(0.71\) \\ \(S\) & Area of pole & \(1080\) mm\({}^{2}\) \\ \(N\) & Turns per coil & \(50\) \\ \(Q\) & Load & \(0\) N \\ \(\beta\) & Throttling ratio & \(2\) \\ \(l\) & Plating thickness & \(30\)μm \\ \hline \hline \end{tabular}
\end{table}
Table 2: Initial test parameters of MLDSB.  
In order to improve the stability of the electronic control system, the unilateral control strategy is adopted to the control voltage of the electromagnetic system on the test stand. The control voltage is only input to one of the upper or lower units, and the other side is only input to the bias voltage or no voltage input. Therefore, when the displacement sensor detects the gradual decline of the rotor, the upper unit continuously increases the regulation voltage to increase the electromagnetic attraction to prevent the rotor rubbing trend. As the electromagnetic coil circuit of the upper unit is manually disconnected, the electromagnetic attraction is zero, and the rotor keeps falling until the control voltage of the upper unit reaches the limit value of 4 V as shown in Figure 22e. Then the rotor continues to move down until the hydrostatic system balances the rotor and reaches the new balanced state again.