A 10,000-word article explains the current status and trends of the development of commercial vehicle electronic steering systems

[Abstract] The electronic control of commercial vehicle steering systems is the only way to achieve energy saving and intelligence of commercial vehicles. For this reason, commercial vehicle steering systems have developed from traditional hydraulic power steering systems to electronic steering systems represented by electro-hydraulic coupling power steering systems. This paper reviews the electronic steering technology of commercial vehicles, sorts out the research on commercial vehicle electronic steering technology from three aspects: typical configuration of electronic steering systems, intelligent steering control and functional safety design, and summarizes the key research areas and future development directions of electronic steering technology. The summary found that: (1) In terms of system configuration, the electronic steering system of commercial vehicles is mainly based on electro-hydraulic coupling steering assist technology, focusing on realizing the intelligent steering function of commercial vehicles while taking into account energy saving needs; the electric power steering system of commercial vehicles is an ideal electronic steering technology solution in the future. (2) In terms of intelligent control, in order to adapt to the development of commercial vehicle intelligence, steering execution control focuses on solving the nonlinear and time-lag problems caused by the hydraulic system and the lateral dynamics control problem during trajectory following; the assisted driving function focuses on overcoming the discontinuous steering torque caused by the nonlinearity of the hydraulic system and the allocation of human-machine control rights; the automatic driving function focuses on improving safety and fuel economy. (3) In terms of system functional safety design, the commercial vehicle electronic steering system focuses on designing fault diagnosis and fault-tolerant control strategies that meet functional safety standards.

Preface

In recent years, with the continuous development of automobile technology, power electronics technology, control theory, communication and network technology, smart cars have become the strategic direction of the development of the global automobile industry. Under the social background of "carbon neutrality", smart transportation, and smart highways, the "Smart Automobile Innovation and Development Strategy" jointly issued by the National Development and Reform Commission and 11 other departments clearly proposed to strengthen the research and development of smart automobile technology and industrial layout, and promote the industrialization process of smart cars [1]. At the same time, the State Council also issued the "14th Five-Year Plan Comprehensive Work Plan for Energy Conservation and Emission Reduction", which proposed the goal of vigorously developing smart transportation and solidly promoting energy conservation and emission reduction [2]. Under this general background, the development of commercial vehicles will focus on their intelligence and energy saving; the steering system of commercial vehicles, as the main actuator of the lateral movement of the whole vehicle, is an important carrier of the lateral intelligence of the vehicle. To achieve this goal, it must be based on its high degree of electronic control.

In the past few decades, commercial vehicles have widely adopted hydraulic power steering (HPS) systems. This system has strong power, simple structure and low cost. However, the system power characteristics are fixed at the factory, which makes it difficult to balance the lightness of the vehicle when turning at low speed (or in place) and the stability of the vehicle when turning at high speed. In addition, in terms of energy consumption, HPS will consume about 3% of the fuel of the commercial vehicle [3], and this figure can reach up to 8% at high speed [4]. The HPS steering oil pump is driven by the engine, but new energy commercial vehicles may not have an engine, such as pure electric and fuel cell buses. Therefore, an electric pump is used instead of the engine to supply energy to the hydraulic system, that is, an electro-hydraulic power steering (EHPS) system [5] is used. This system can control the hydraulic power and has achieved significant results in enhancing road feel and reducing energy consumption [6-7]. However, as commercial vehicles have an increasingly urgent demand for steering functions such as intelligent driving, EHPS has been unable to meet the demand due to its inability to actively intervene in steering control. Therefore, a large amount of innovative research work has been carried out at home and abroad on commercial vehicle electronic steering systems that can implement active steering intervention control. Different system configurations and rich intelligent steering control strategies have been proposed, which has greatly promoted the intelligent development of commercial vehicles.

This paper will review the recent research results of domestic and foreign scholars in the configuration of commercial vehicle electronic steering systems, intelligent steering control technology (mainly including steering execution control strategy, lateral dynamics control and intelligent driving control), functional safety design and fault-tolerant control strategy. On this basis, it will further summarize the main challenges faced by commercial vehicle electronic steering systems and future development directions.

1 Commercial vehicle electronic steering system configuration

With the continuous development of commercial vehicles in terms of electrification, intelligence and networking, commercial vehicle steering systems must provide more abundant steering functions, such as intelligent driving. The configuration of electronic steering systems that can provide active steering intervention, namely intelligent steering function, is the most basic work. This section will sort out the mainstream configurations of commercial vehicle electronic steering systems.

1.1 Electro-hydraulic coupling power steering (EHCPS)

The EHCPS system is essentially an EPS (electric power steering) motor system integrated into the steering input of the HPS system. The system is mainly composed of a sensor unit, a motor, a control unit, and a worm gear reducer. The system can not only provide a wider design range for steering road feel by realizing the combined assistance of motor power and hydraulic power through steering motor control, but also realize active steering intervention through motor control to meet the needs of intelligent steering. At present, intelligent commercial vehicles equipped with EHCPS systems have been used in closed scenes such as structural highways, mining areas, and ports, significantly reducing operating costs, improving operating efficiency and safety [8].

Different configurations are produced according to the different locations of the motor system integrated into the EHCPS. The ReAX-ColumnMounted system designed by ZF [9] integrates the motor system into the steering column, as shown in Figure 1. This structure is similar to the C-EPS configuration of passenger cars. It achieves the control of the steering input torque with relatively small changes to the steering system. However, the torque input capacity of this system is limited and it is mainly suitable for small and medium-sized commercial vehicles.

The ReAX Gear Mounted system designed by ZF [10] and the ServoTwin system designed by Bosch [11] (see Figure 2) integrate the motor system into the input shaft of the steering gear, similar to the P-EPS configuration of passenger cars.

In terms of intelligence, the EHCPS system can provide greater freedom for steering feel design through the design of motor control algorithms to improve the driver's steering feel, and can control the motor to actively intervene in steering to achieve intelligent steering function. Therefore, it not only overcomes the problems inherent in the HPS system of commercial vehicles, such as heavy steering, insensitivity to the middle position, and poor returnability [12], but can also perform steering actions independently of the driver's intentions, providing a foundation for the intelligent steering of commercial vehicles.

In terms of energy saving, with the assistance of the superimposed electric power steering, the rated flow rate of the hydraulic power steering can be appropriately reduced to reduce the overflow loss of the oil pump assembly, the throttling loss of the rotary valve and the along-the-line loss of the pipeline. The increase in the rotary valve input torque caused by the reduction in flow will be compensated by the electric power steering. Therefore, compared with the HPS system, the EHCPS system can ensure the lightness of steering while also having the advantage of low energy consumption. Therefore, it has been well applied in traditional internal combustion engine commercial vehicles.

In order to further reduce the steering energy consumption, EHPS and EHCPS can be combined. By reasonably allocating the proportion of electric power assistance and hydraulic power assistance under different working conditions, the steering system can not only achieve active steering but also significantly reduce the system energy consumption. The basic configuration of the composite steering system (EHPS-EHCPS system) composed of EHPS and EHCPS is shown in Figure 3. In this composite configuration, the electric oil pump serves as the power source of the hydraulic subsystem, replacing the engine-driven steering oil pump. This part of the structure is similar to the EHPS system [13].

References [14] and [15] tested the energy consumption of the EHPS-EHCPS system under multiple working conditions. The results showed that the system could save 50% energy and improve the fuel economy of the vehicle by 1% compared with the HPS system. Reference [16] improved the hydraulic circuit of the EHPS by using accumulators and solenoid valves, so that the hydraulic power assist was changed from flow control to pressure control, thereby realizing two modes of full electric power assist and electro-hydraulic coupling power assist, further reducing energy consumption. Its energy consumption was reduced by 83% compared with the EHPS system. In addition, optimizing the key parameters in the EHPS-EHCPS system can effectively improve system performance and reduce system energy consumption. References [17] to [20] optimized the key design parameters of the EHPS-EHCPS system based on the multi-objective particle swarm optimization method, improved the steering power assist performance, and reduced the steering energy consumption. The specific optimization methods, optimization objectives, and optimization parameters involved in the above references are shown in Table 1.