## 3 The Switch Control Strategy of the Hydro-Pneumatic Suspension  
### Switch Control Strategy  
Based on the conflicting relationship between them, it is difficult to optimize both aspects of ride comfort and handling stability of the rescue vehicle based on the control of the hydro-pneumatic suspension. However, when the vehicle is in different driving conditions or driving on different road surfaces, level of magnitude of the two aspects can be sorted. For instance, when the road surface is relatively flat and the driving state of the vehicle is unlikely to cause the vehicle to be dangerous, the ride comfort is the main goal pursued by the suspension system in this case. On the contrary, when the vehicle is driving on an uneven road or steering at a high speed, the possibility of rolling of the vehicle is relatively large at this time, and the handling stability of the vehicle is the major demand of  
Figure 4: Simulated displacement curves of the left and right tires.  
the vehicle. As a result, it should be possible to analyze the weight of the vehicle's ride comfort and the handling stability under different conditions, so as to achieve the corresponding switch according to different requirements of working conditions. For the above-mentioned idea, the switch control strategy is presented in this paper, and the schematic diagram of the switch control strategy of the hydro-pneumatic suspension is shown in Figure 5.  
where \(\delta_{f}\) is the steering angle of the wheel, and \(a_{y}\) is the lateral acceleration of the spring mass, \(a_{y}=v_{x}(\dot{\beta}+\gamma)\).  
For the switch control strategy, an indicator is needed to determine whether and when the switch occurs. In order to evaluate the possibility of the vehicle's danger, LTR is selected as the performance index to judge whether the switch action between the ride comfort and handling stability controllers is triggered, and LTR is defined as follows.  
\[LTR=\frac{2m_{s}}{mT}\bigg{(}(h_{r}+h\cos\varphi)\frac{a_{y}}{g}+h\sin\varphi \bigg{)} \tag{7}\]  
Figure 5: Sliding mode switch control strategy of adjustable hydro-pneumatic suspension.  
When the LTR is equal to 0, the vehicle has a stable roll dynamic, and the risk becomes higher with the increase of LTR [25]. The threshold of LTR is defined as Q, then the switch control strategy can be illustrated in Equation (8). In order to ensure sufficient time for the controller to react before the rollover danger occurs, Q is set to 0.8 in this paper. On this occasion, the controller has enough time to lower the LTR to avoid the rollover before obtaining high values of LTR.  
\[controller=\left\{\begin{array}{l}ride\,comfort\,controller,LTR\leq Q\\ handing\,stability\,controller,LTR>Q\end{array}\right. \tag{8}\]  
In order to easily understand the proposed control strategy, the control block diagram of the switch control strategy of the hydro-pneumatic suspension is shown in Figure 6. As shown in Figure 6, when the value of LTR is lower than Q, it indicates that the vehicle is not at risk of rollover at this time. As a result, the major task of the controller is to optimize the ride comfort of the suspension system, then the ride comfort SMC (RCSMC) is active. The handling stability SMC (HDSMC) is not activated until the LTR reaches the threshold Q.