# Study on Coupled Vertical Vehicle-Bridge Dynamic Performance of Medium and Low-Speed Maglev Train
## 4 Test Results and Analysis
### Parametric Analysis
#### 4.2.1 Vehicle Load  
Effects of vehicle load on the vertical acceleration of suspension bogie are shown in Figure 8. It can be seen from the time domain diagram that the period with obvious acceleration amplitude (such as multiple peaks) represented the levitation (or landing) of the suspension bogie. Since several suspension bogies work orderly, there were multiple peaks in the diagram. The peak with maximum amplitude was generated by the second suspension bogie (vertical acceleration test point), and the relatively smooth area during 40-90 s was the response of complete suspension. It can be seen that the maximum vertical accelerations under overload and no-load conditions were 15.529 and 8.279 m/s2, respectively. Accordingly, the maximum vertical acceleration of the suspension bogie increased by 7.25 m/s2 when the load was increased by 6 tons. Since the levitation force could not adapt to the change of load, vertical acceleration was generated. However, the load change in unit time was aggravated when the load increased, and the adaptability of suspension force to load change was further weakened, it led to the vertical acceleration and airgap change. The vertical acceleration of the suspension bogie increased when overloaded, which confirmed the simulation results in Figure 3c. In addition, in Figure 8b, the vibration acceleration within 40 s to 100 s was larger than that when overload. It can be regarded that the instantaneous point dropping or rail smashing occurred during static levitation, which causes track excitation, and it was transmitted to suspension bogie by the levitation force. Figure 8: _Cont._  
Figure 7: Sensors in the field test. According to the train-bridge coupling state, the whole static levitation process is divided into three stages: floating, levitation, and landing. In order to accurately analyze the spectrum relationship, only the suspension stage was analyzed. It can be seen from Figure 8 that the load had almost no effect on the vibration spectrum. The first-order vibration frequency was 5.18 Hz when overloaded and 4.97 Hz with no load. The frequency values of the two conditions were similar. In Figure 9, from the curve change trend within 20-40 s, it can be seen that the response speed of overload was much slower than that of the no-load condition, which was consistent with the law shown in Figure 3a. The no-load state was finally stable near the ideal gap (8.5 mm), while the overload state had a steady-state error of about 1 mm. It indicates that the real-time and accuracy of levitation control were reduced, and the response speed of the airgap was slowed down due to the increase in load. In order to overcome the adverse effects of load, the reliability and robustness of the suspension control algorithm should be further improved. As shown in Figure 10, the vertical vibration of bridge under different loads was analyzed in the time and frequency domains. The maximum vertical acceleration of the bridge was 1.177 m/s\({}^{2}\) under overload conditions and 0.776 m/s\({}^{2}\) under no-load conditions. It indicates that the vibration acceleration amplitude of the bridge increased by 0.401 m/s\({}^{2}\) when the load was increased by 6 tons. Therefore, the influence of load on the vertical acceleration of the suspension bogie was more significant than that of the bridge, which indicates the rigidity of the suspension bogie structure was smaller than that of the bridge. According to the frequency domain diagram, it can be seen that the energy near 70 Hz was  
Figure 8: Effects of vehicle load on the vertical acceleration of suspension bogie. Figure 9: Effect of vehicle load on the airgap. high, which represents the forced vibration of the bridge caused by track excitation, and the energy in the overloaded condition was higher than that of the no-load condition. It could be judged that the increase in load made the levitation force increase, and the external excitation of the track was further enhanced. In addition, the first natural frequency of the bridge was about 7.1 Hz.