## 1 Introduction  
The suspension system is one of the most important components of a transport vehicle, which can improve the ride comfort, running stability, and reduce the impact force caused by an uneven road. Therefore, it is very important to select the suspension design scheme and suspension parameters in a reasonable way. The large impact force on the vehicle load-bearing platform will aggravate the up and down vibration of the tire, affect the adhesion between the wheel and the ground, and affect the braking force, which will cause a great impact on the stability of the vehicle [1; 2; 3]. The hydraulic suspension system has great advantages, one of the main functions is to reduce the impact on the vehicle load-bearing platform. The hydraulic suspension system has been used for a long time, and it can not only improve the ride comfort of the vehicle, but also realize raising and lowering of the body. The transportation requirements are ride comfort, stability, and safety but because of the poor compressibility of hydraulic oil, the rigidity of the hydraulic suspension is substantial and the absorption of the impact force is poor. Through simulation analysis, [PERSON], [PERSON] improved the vibration reduction of a vehicle and optimized the parameters of the hydro-pneumatic suspension [4]. [PERSON], [PERSON] realized vibration control through a synovial control strategy [5]. [PERSON]. compared different gas models to calculate the spring force of a hydro-pneumatic suspension and found that the hydro-pneumatic suspension model on considering energy conversion was more accurate [6]. [PERSON] improved the ride comfort of a vehicle by changing the damping of the suspension under different working conditions [7]. Through simulation, [PERSON] obtained the best parameters for accumulator pressure for a heavy trailer suspension [8]. [PERSON] obtained a calculation method for the suspension accumulator according to the limiting working condition [9]. [PERSON] studied the compliance of self-adaption suspension group systems in self-propelled transporters [10]. [PERSON] proposed an observer-based active fault-tolerant controller for a vehicle suspension system [11]. [PERSON] analyzed the effects of an uneven road on a multi-axle vehicle with hydraulic suspension and found that increasing the axle number of the multi-axle vehicle can effectively suppress the dynamic load on the frame and suspension [12].  
The heavy hydraulic transport vehicle studied is shown in Figure 1. It is a special transport vehicle, not a conventional road vehicle. It is mainly used for transportation in construction sites.  
Based on the previous work, an improved method for a hydraulic suspension system is proposed in this paper. First, the structure of the suspension system was analyzed and expressed by a mathematical model. The main function of the accumulators in a hydraulic suspension system of a transport vehicle is to absorb the impact pressure, so the diaphragm type accumulator was selected. The hydraulic suspension system with accumulators was designed, then the mathematical model of the accumulator and the dynamic model of the vehicle established. Finally, the combined simulation was realized by ADAMS and AMESim, and the optimal parameters of the accumulator obtained. The experiment was compared with the simulation, which verified the correctness of the simulation and design.