Compared with passenger cars, heavy vehicles have larger loads, longer bodies, more steering modes, and are more likely to cause improper steering angle relationships among the wheels, resulting in uncoordinated steering. This uncoordinated steering makes the tyres roll and drag, which results in abnormal lateral and longitudinal slips, changing the lateral and longitudinal mechanical characteristics of tyres and leading to low path tracking accuracy and poor steering performance of vehicles. Therefore, a tyre model containing lateral and longitudinal mechanical characteristics under heavy load is the key to solving the problem of uncoordinated steering in heavy vehicles. Firstly, with the physical tyre model, the lateral and longitudinal slip is described as carcass deflections. With the carcass deflections and contact pressure distribution of heavy load, the tyre forces under uncoordinated steering conditions are derived. Then, the tyre imprint parameters are obtained with a test bench, and a two-dimensional contact pressure distribution model is established to reproduce the contact pressure distribution of tyre under heavy loads and dynamic slips. The comparison results with Magic Formula (MF) and Trucksim are generally consistent, proving the effectiveness of the model. Finally, a lateral and longitudinal tyre force distribution control of vehicles is carried out based on the established model. The results of the Trucksim-Simulink co-simulation show that the path tracking accuracy of vehicles is significantly improved, which demonstrates that the tyre model incorporating lateral and longitudinal mechanical characteristics under heavy loads can provide a theoretical basis for solving the problem of uncoordinated steering in heavy vehicles.

Traditional multi-axle steering vehicle often adopts linear state feedback control, which is difficult to ensure high-precision trajectory tracking under all working conditions. Moreover, due to the failure to consider the optimal distribution of tire lateral and longitudinal forces, tires are prone to problems such as uneven load rates and wear. In this paper, considering the over-redundant and nonlinear characteristics of three-axle vehicle, a hierarchical coordinated control strategy is proposed. In the upper layer, the trajectory tracking controller is designed based on the robust nonlinear sliding mode control theory. In the middle layer, based on the optimization conditions of tire load rate and dissipative energy, the lateral and longitudinal forces optimal distribution controller is constructed under the constraint of friction circle. In the lower layer, the lateral and longitudinal forces are finally converted into tire angles and torques with the tire inverse model. The results show that the hierarchical coordinated control strategy can ensure that the multi-axle vehicle can achieve high-precision trajectory tracking under all-terrain load conditions, and the load rate and wear of each tire are relatively uniform. The coordinated control strategy proposed in this paper considers the influence of nonlinear characteristic of vehicle and tire lateral and longitudinal forces distribution on steering coordination, which can provide an important theoretical basis for the further improvement of steering coordination of multi-axle vehicle.

Heavy multi-axle vehicles with long bodies, large loads, and many steering axles are prone to one stuck axle with its steering system failed, which leads to a sharp drop in vehicle safety. Modulating the steering angles of the remaining non-faulty axles for compensation control can significantly improve the safety of the vehicle. Therefore, this paper proposes a method based on multi-axle steering compensation, which solves the large trajectory error, and instability of heavy multi-axle vehicles caused by the failure of one-axle steering system. Firstly, based on the Lyapunov method, and nonlinear model, the critical steering angle of the faulty axle leading to vehicle instability under the failure of one-axle steering system is clarified, which provides a quantitative index for vehicle stability evaluation. Then, a two-level controller is designed to maintain stability and reduce trajectory error of the faulty vehicle. The upper dual-input dual-output (DIDO) sliding mode controller (SMC) compensates for the faulty vehicle’s lateral force and yaw moment. And the lower controller distributes the non-faulty axles’ steering angles through a strategy considering tire workload and slip energy dissipation. Finally, a seven-axle vehicle model in the Trucksim and a Trucksim-Simulink co-simulation are used to verify the effectiveness of the proposed method. The results illustrate that the proposed method can maintain vehicle stability and reduce the lateral trajectory error by about 22%–91% in the failure of one-axle steering system. It proves that this method can provide a new scheme for active safety control of heavy multi-axle vehicles.

Electro-hydraulic servo steering system (EHSSS) is a key technology for heavy vehicles. The traditional EHSSS has high control accuracy but low energy efficiency. Therefore, this paper proposes a novel EHSSS based on independent metering system, which can combine high steering accuracy with high energy efficiency. Firstly, two servo-proportional valves are used to reduce the throttling loss at the meter-out orifice. The servo motor pump is used to keep the pump supply pressure at a low value. Then, to ensure high steering accuracy, a position-velocity-pressure combined control strategy is proposed for easy engineering practice. And a smooth switching strategy is proposed to achieve smooth mode switching at high frequency. Finally, the effectiveness of the proposed method is verified by experiments. The experimental comparison results show that the proposed method can achieve the same accuracy as the valve-controlled steering system with less energy, and can realize smooth mode switching.