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

The EHPS-EHCPS system configuration retains the advantages of both the EHPS system and the EHCPS system. It can realize active steering function through the coordinated control of the power motor and the hydraulic system, and can reduce energy consumption by adjusting the hydraulic power according to the driving conditions through the control of the electric pump, so that the two major problems of intelligence and energy saving faced by the commercial vehicle steering system have a unified solution. However, the increase in system complexity has brought about an increase in cost, especially for traditional internal combustion engine commercial vehicles, which require additional electric oil pump components. Therefore, it is often used in new energy commercial vehicles. This is because most new energy commercial vehicles already have the EHPS system foundation and only need to add the motor system in the EHCPS.

1.2 Commercial vehicle electric power steering (CV-EPS)

At present, EPS has been gradually popularized in passenger cars, but has not been widely used in commercial vehicles. This is because the EPS system limits the steering force output by the steering system and cannot adapt to the large steering force requirements of commercial vehicles. For this reason, an electric power steering system suitable for commercial vehicles, namely CV-EPS, must be designed. CV-EPS eliminates the hydraulic system and has a simple structure. It can effectively improve the accuracy and speed of system response. It uses the motor as the power source of the steering system, which can realize the intelligent steering function. The motor assists more directly and does not require the traditional hydraulic system to provide assistance. Therefore, compared with the HPS system, its energy efficiency is significantly improved.

For medium and small commercial vehicles, CV-EPS can be achieved by further improving the power of the steering motor and the maximum torque that the rack and pinion structure can withstand based on the EPS of passenger cars. For example, the HO EPS system proposed by Nexteer can output a maximum rack force of 24 kN [21]. The advantage of this solution is that it can directly transplant the mature structural solution of passenger cars and has low development difficulty. The disadvantage is that the rack and pinion structure has a strength limit and is currently difficult to apply to large commercial vehicles.

The CV-EPS of large commercial vehicles still needs to solve two problems: (1) The 24 V low-voltage power supply system currently used in commercial vehicles can only provide limited power and cannot cover all steering conditions of commercial vehicles; (2) The existing steering transmission mechanism cannot bear the maximum torque required for commercial vehicle steering.

Based on the significant advantages of CV-EPS and the necessity of its application, CV-EPS will definitely become the next-generation solution for commercial vehicle steering actuators. Existing studies have attempted to explore CV-EPS in order to solve the above two problems.

The CV-EPS system designed based on the 48 V on-board power system can effectively solve the problem of insufficient power. For example, ZF has developed a CV-EPS prototype ReAXEPS based on a 48 V system [22] (see Figure 4). Its structure includes an integrated controller, a reducer, a sensor, and a high-torque motor with a torque of up to 70 N·m, which can be installed on heavy-duty commercial vehicles.

In addition, with the gradual coverage of commercial vehicle electrification, the high-voltage power supply of the drive system can theoretically be directly used for the steering system, which can also solve the current problem of insufficient steering power in internal combustion engine commercial vehicles. However, due to the long-term close contact between the steering system and people, from a safety perspective, there are still few steering system applications using high-voltage motors.

In terms of transmission structure design, Tianjin Songzheng Company proposed a design scheme of planetary gears and roller screws [23] (see Figure 5). A set of planetary gears and roller screws with high load capacity and large reduction ratio is installed on the input shaft of the traditional recirculating ball steering gear. The reduction mechanism can carry a large torque, but the connection between the reduction mechanism and the steering gear is relatively weak and easy to fall off. Wenling Dongling Company proposed a design scheme of cycloid pinwheel and bevel gears [24] (see Figure 6). This mechanism has high transmission efficiency and can provide a large torque output, but it uses two sets of reduction mechanisms, cycloid pinwheel and bevel gear, with a complex mechanical structure and a large overall mechanism volume.

The commercialization of CV-EPS will still require a long period of research and verification. In the current CV-EPS solution, the 48 V system causes too much change to the vehicle structure and is difficult to implement. If the 24 V system is still used, a small-volume, high-power low-voltage motor must be designed. The existing reducer design has engineering problems such as large internal friction, insufficient connection strength, and unguaranteed service life, so its mechanical structure needs to be further optimized.

2 Intelligent Steering Control for Commercial Vehicles

As the market demand for intelligent commercial vehicles becomes increasingly urgent, the electronic steering system that cannot provide active steering function can no longer meet user needs, and the CV-EPS system cannot be put into practical use in the short term. Therefore, the aforementioned EHCPS configuration becomes the most feasible commercial vehicle intelligent steering system solution in the transition period before the CV-EPS technology matures. It is mainly manifested in that through the design of the power assist motor control strategy, it can not only realize active steering functions such as speed-dependent power assist, emergency steering, active return, and side wind compensation, but also provide the necessary control interface for the upper-level advanced driver assistance system [25].

The steering system is the main actuator for commercial vehicles to achieve intelligent control. In the typical vehicle chassis dynamics hierarchical cascade control architecture [26], the control block diagram is shown in Figure 7. The actuator controller receives the desired front wheel angle command sent by the upper-level lateral dynamics controller, generates the bottom-level control command, drives the steering actuator to track the desired angle, and adjusts the vehicle's lateral posture. Therefore, the control performance of the steering actuator is a key factor in measuring the intelligence of commercial vehicles.

Intelligent steering control for commercial vehicles based on EHCPS faces the following two problems: (1) The system is a typical complex electro-mechanical-hydraulic coupled system, which makes the design of precise servo control strategies for wheel angle and torque extremely challenging; (2) The human-machine collaboration in commercial vehicle assisted driving causes problems in the allocation of human-machine control rights, as well as energy saving and safety issues in autonomous driving.

According to the hierarchical cascade control architecture of vehicle dynamics, the steering control execution of commercial vehicles can be divided into two levels: the upper-level control strategy performs vehicle dynamics control to calculate the desired front wheel steering angle; the lower-level steering execution control strategy is used to track the upper-level desired front wheel steering angle. Compared with the EPS system of passenger cars, the lower-level controller of the EHCPS system of commercial vehicles must also consider the nonlinearity and high time lag caused by the hydraulic system in the compensation steering system [27]; the upper-level controller design must also consider the low threshold of vehicle rollover acceleration, the risk of vehicle rollover during steering, and the uncertainty of commercial vehicle parameters and the nonlinearity of lateral dynamic response caused by external uncertain interference [28].

The main control contents and common methods in intelligent steering control of commercial vehicles summarized according to the control characteristics of the EHCPS system of commercial vehicles are shown in Table 2. The following will focus on the contents in Table 2.

2.1 Steering actuator control

The intelligent steering system for commercial vehicles based on EHCPS is a typical complex system of mechanical-electrical-hydraulic coupling. When active steering intervenes, the steering motor generates a driving force that deforms the HPS torsion bar, and then the relative rotation of the rotary valve causes a pressure difference at both ends of the hydraulic cylinder piston to form a hydraulic booster. The motor driving force and the hydraulic driving force overcome the ground steering resistance torque of the wheel to achieve wheel deflection. Due to the existence of the torsion bar of the hydraulic booster system itself during active steering, deformation will occur during the process of transmitting torque, resulting in deviations in the estimated wheel angle. The deformation angle of the torsion bar of the EHCPS system must be compensated in the design of the lower-level actuator controller to achieve accurate tracking of the angle during steering execution.

Considering the strong correlation between the torsion bar deformation angle and the steering resistance torque, a sliding mode observer is designed in reference [29] to observe the steering resistance torque, thereby compensating the torsion bar deformation angle by calibrating the lookup table. References [30] and [31] obtain the steering resistance torque under the corresponding vehicle speed and steering wheel angle conditions through theoretical analysis of the steering resistance torque components. However, this method has the problem of poor robustness when the load mass of commercial vehicles varies over a large range. Reference [32] proposes a torsion bar deformation angle estimation strategy based on a fuzzy neural network identifier. This strategy uses a neural network to identify the nonlinear model of the EHCPS system to obtain an estimated value of the torsion bar deformation angle.