## 1 Introduction  
Differing from general vehicles operating on prepared roads, special vehicles equipped with tracked chassis are designed for negotiating harsh terrains. The irregular loads and impacts transmitted from such uneven terrains lead to maneuverability loss and ride performance degradation. Hence, the hydropneumatic suspension is widely used in off-road tracked vehicles, not only because of its good non-linear elastic stiffness and superior damping performance [1] but also for the high-power density with lightweight. With such good properties, the tracked vehicles can better handle the conflict between the ride comfort and handling stability than ordinary trailing arm suspension [2, 3]. The characteristics of hydropneumatic suspension have already attracted wide research attention in wheeled vehicles. [PERSON] et al. [4] studied the hydropneumatic suspension implemented on the front axle of an articulated vehicle by sensitivity analysis and optimization for the design parameters. The results suggested that the hydropneumatic suspension can help preserve directional stability and reduce driving vibration by nearly 30%. [PERSON] et al. [5] presented an analytical model of air spring suspensions and obtained its first-order Taylor series linearized model, both of which are validated with experimental measurements. [PERSON] et al. [6] discussed the hydropneumatic suspension characteristics of a \(6\times 6\) amphibious vehicle. A mathematical model of the whole vehicle, including thermal transfer of the actuator and the hydraulic circuit, has been realized and experimentally tuned. [PERSON] et al. [7] presented a highly non-linear hydropneumatic suspension with strong regressive spring stiffness for motorcycles. By connecting the hydropneumatic suspension and the spring in series, the grip can be increased while ensuring that the comfort level is basically unaltered. These publications based on wheeled vehicles provide effective theoretical methods and models for the research of hydropneumatic suspension for tracked vehicles. Current studies on the hydropneumatic suspension of tracked vehicles mainly focus on heavily armored vehicles. [PERSON] and [PERSON] [8] investigated the non-linear elastics and the damping characteristics of an in-arm hydropneumatic suspension unit (ISU). The results showed that the desirable vertical displacement could be achieved by adjusting both the orifice diameter and pre-displacement of disc springs. [PERSON] and [PERSON] [9] validated the mathematical model of a hydro-gas suspension with experimental data. This model was incorporated in an in-plane model and the influence of suspension parameters were studied for different sinusoidal inputs. Although different hydropneumatic suspension designs and configurations have been proposed, their ride performance analysis methods still rely on the traditional analysis methods served for wheeled vehicles. The systematic dynamics research for the ride performance analysis on various road surfaces of tracked vehicles started in the 1990s. [PERSON] [10; 11] and [PERSON] and [PERSON] [12] systematically derived tracked vehicle dynamics models which considered the mobility over soft terrains, ride dynamics over rough surfaces and maneuverability. They also studied the ride quality of tracked vehicle over deformable terrains. [PERSON] and [PERSON] et al. [13; 14] developed a ride dynamic simulation model of a typical tracked vehicle assuming constant vehicle speed and non-deformable terrain surface. The wheel/track-terrain interactions and dynamic track loads were also taken into consideration. [PERSON] et al. [15] conducted a theoretical investigation into the effect of suspension configurations on a tracked vehicle performance over bump terrains. The suspension configurations, especially the numbers and locations of suspension dampers, were found to have a significant effect on vehicle mobility.