# A Review of Recent Advances in Roll Stability Control in On-Road and Off-Road Vehicles
## 5 Roll Stability Control in Off-Road Vehicles
### Active Roll Stability Control Methods  
#### 5.3.1 Active Leveling Control  
In operations on slopes and complex terrains, agricultural machinery faces significant challenges to roll stability and operational safety, due to dynamic parameter variations and posture instability [124; 125; 126]. [PERSON] et al. [127] conducted three-dimensional dynamic simulations to analyze the effects of ground slope and obstacle shape on tractor rollover and rearward tipping. The study concluded that slope is the primary factor causing rollover, with angles of up to 50\({}^{\circ}\), while obstacles with a sine shape increase the rollover risk and rectangular obstacles tend to cause rearward tipping. These findings offer valuable insights for tractor safety design. To improve the stability of agricultural machinery in complex terrains, researchers have developed several active roll stability control methods, utilizing multi-sensor fusion, advanced control algorithms, and coordinated actuator actions. For active leveling control of tractors, as shown in Figure 7a, [PERSON] et al. [128] designed an active posture control system for four-wheel-drive tractors operating in hilly terrain. The system adjusts the tractor's roll angle by linking the front and rear axles, and incorporates a slope stability model based on ground support forces. Simulations demonstrated that the system significantly improves lateral rollover stability, offering a practical solution for hilly-area operations. Further enhancing this, [PERSON] et al. [129] proposed a fuzzy switching-gain sliding mode control system for leveling, using a four-point leveling mechanism. Co-simulation results and test bench validations showed that the system can dynamically level the tractor within \(\pm\)2\({}^{\circ}\) on hilly terrain, restoring it to a 0\({}^{\circ}\) roll angle, thus providing a high-precision solution for mountain tractor body leveling. In addition to tractors, active leveling control for other agricultural machinery has garnered significant attention. As shown in Figure 7b, [PERSON] et al. [13] designed an omnidirectional leveling system to address vehicle body tilting and safety risks during operations on hilly terrain. The system, based on a \"three-layer framework\" articulated structure, optimizes key parameters using the NSGA-II algorithm. Both simulation and experimental results show that it effectively reduces the maximum tilt angle and leveling time, meeting operational requirements for hilly areas. As shown in Figure 7c, [PERSON] et al. [12] enhanced this system by incorporating a sliding mode synchronization control method based on a disturbance observer, improving the leveling accuracy and response speed. Experimental results indicated a 35.5% reduction in leveling time and hydraulic cylinder synchronization error within \(\pm\)6 \(\times\) 10\({}^{-4}\) m, enhancing safety in hilly terrains. As shown in Figure 7d, [PERSON] et al. [130] developed an adaptive leveling system for tracked agricultural machinery, addressing body tilting in complex terrains. The system enables three-dimensional adjustments (height, lateral, and longitudinal) using RecurDyn multi-posture simulation and a hydraulic drive model. Experimental results showed a leveling accuracy of \(\pm\)0.4\({}^{\circ}\), offering an effective solution for tracked machinery. Additionally, [PERSON] et al. [66] proposed an adaptive anti-rollover control method for articulated vehicles, using a dynamic stability index and predictive model for material weight, significantly improving anti-rollover performance. Combine harvesters frequently operate on uneven and sloped agricultural terrains, where maintaining lateral stability is critical to both operational safety and harvesting performance. To address this, modern harvesters are equipped with active leveling systems that automatically adjust the orientation of the working units--such as threshing and cleaning assemblies--to remain horizontal when the machine is traversing inclines.