--Collect the "Automotive Driving Automation Classification" (GB/T 40429-2021)--
As we all know, the realization of automatic driving is inseparable from the synergy of the three major systems of perception, decision-making and control, of which motion control relies on the chassis technology of wire control. As an important part of the steer-by-wire chassis, steer-by-wire has become a familiar term.
So, what is the principle of steer-by-wire technology? What are the advantages? What is the current application? This article will carry out a detailed interpretation and demystify the steering-by-wire.
Figure 1 Steer-by-wire
Steer-by-wire vs conventional steering
The automotive steering system determines the lateral movement of the car, and the traditional steering system is a mechanical system: the driver manipulates the steering wheel, through the steering wheel and tie rod, to transmit the steering intention to the steering wheel, thus achieving steering movement.
The early mechanical hydraulic power steering system, the current popular electro-hydraulic power steering system and the electric power steering system are all based on mechanical components of the steering system. These mechanical systems have evolved to optimize the force transfer characteristics of the steering system, provide assistance for steering control, and improve the vehicle's handling stability and compliance. However, due to their mechanical structure, they cannot change the angular transmission characteristics of the steering system, that is, the steering characteristics of the car, so it is difficult to achieve the active control required for automatic driving.
Steering-By-Wire eliminates the mechanical connection between the steering wheel and the steering wheel, completely frees itself from the limitations of mechanical firmware and makes the steering entirely by electrical energy. The steer-by-wire system not only has all the advantages of the traditional mechanical steering system, but also can realize the optimization of the angle transfer characteristics that are difficult for the mechanical system to do. In the online steering control system, the driver's steering action becomes an electrical signal through the sensor, and the signal is analyzed and processed and transmitted directly to the actuator through the wire. Since it is not limited by the mechanical structure, it can achieve theoretical arbitrary steering intentions, so the steer-by-wire system is called the most advanced steering system at present.
Table 1 Summary of steering systems
The structure of the steer-by-wire system
The steer-by-wire steering system is mainly composed of steering wheel module, main controller, execution module, fault handling system, power supply and other parts. Among them, the steering wheel module, the main controller, and the execution module are the three main parts of the steer-by-wire steering, and the other modules belong to the auxiliary part.
Fig. 2 Structure of steer-by-wire system
The steering wheel module is an input module for steering intent, including steering wheel, angle sensor, torque sensor, positive torque motor and related accessories.
By measuring the angle and torque of the steering wheel, the steering wheel module converts the driver's steering intention into a digital signal and transmits it to the main controller; at the same time, the steering wheel module receives the torque signal feedback from the main controller, generates the steering wheel's positive torque, and provides the driver with a corresponding sense of road.
The main controller, the ECU, is the core of the steer-by-wire, equivalent to the brain, which determines the control effect of the steer-by-wire. Its main role is to analyze and process various signals, determine the steering intention and the state of motion of the car, and output the corresponding control instructions.
On the one hand, the main controller analyzes and processes the collected signals, sends instructions to the steering execution motor and the positive torque motor, and ensures that the two motors work together, so as to realize the simulation of the vehicle's steering motion and road sense. On the other hand, the main controller maintains real-time monitoring of the driver's operation and the status of the vehicle, enabling intelligent control.
When the system detects that the steering intention is unreasonable, the system instructions are wrong or the car is in an unstable state, the main controller can shield the wrong instructions in time and automatically control the vehicle in a reasonable way, so that the car can return to a stable state as soon as possible. In addition, when the steer-by-wire system fails, the main controller can take timely measures to remedy the situation and ensure the safety and stability of the driving.
The steering actuator module is to implement and execute the driver's steering intent, which consists of a steering actuator motor, a steering motor controller, a wheel steering component, and a wheel angle sensor. The execution module accepts the instructions of the main controller and controls the rotation of the steering wheel through the steering motor and its controller to achieve steering.
In addition, the wheel corner sensor synchronously feeds the measured wheel position signal back to the main controller for computational analysis and closed-loop control.
The fault handling system is also an important module of the steer-by-wire system, which contains a series of procedures for monitoring and countermeasures. When the steer-by-wire system fails, the fault handling system takes corresponding handling measures in accordance with the set program to avoid or mitigate the harm caused by the fault and ensure the safety of the car to the greatest extent.
In addition, the power supply, as a power supply facility, is also an indispensable part of the steer-by-wire system.
The principle of steering in-wire
Based on the structure of the steer-by-wire system, we can easily analyze how steer-by-wire works.
When the steering wheel is rotated, the steering wheel angle sensor and torque sensor respectively convert the measured torque and torque information into an electrical signal and transmit it to the main controller; at the same time, the main controller receives the wheel movement status signal collected by the corresponding sensor, such as vehicle speed, longitudinal acceleration, pendulum angle speed, etc. Based on the above signals, the main controller processes the steering wheel angle and torque signals, and sends control instructions to the steering motor to achieve reasonable steering.
On the other hand, the main controller receives the wheel information collected by the wheel corner sensor, combines the state information of the vehicle, and sends the corresponding torque command to the positive torque motor. The positive moment motor simulates the information of road surface feedback, thus providing the driver with a real-time sense of the road.
When the main controller is faulty or faulty, the fault handling module will make corresponding treatment according to the form and level of the fault to ensure that the driver can find the fault and keep driving safely.
Figure 3 How steer-by-wire works
Advantages and technical difficulties of steer-by-wire
From the structural analysis of the steer-by-wire system, it can be seen that the mechanical connection between the steering wheel and the steering wheel no longer exists, and instead the signal connection between the wire and cable is replaced. This steering system, which is based on pure electrical signal control, has a range of advantages that traditional mechanical steering systems do not have.
First, it's safer
Since the mechanical structure such as the steering column is eliminated, the damage of the steering column to the driver in a collision accident can be completely avoided.
Second, more comfortable
The intelligent ECU can judge in real time whether the driver's operation is reasonable according to the driving state of the car, and make corresponding adjustments. When the car is in a non-stable working condition, the steer-by-wire system can automatically control the stability of the car. For the people in the car, it will feel more stable and comfortable.
While eliminating the mechanical connection, the driver's legroom is increased. At the same time, the horizontal and vertical direction of the ground is not smooth, will not be directly transmitted to the driver's hands, road sense information is generated by the positive torque motor simulation, will filter useless information, only to the driver to provide useful information, thereby improving the driving comfort.
In addition, the steer-by-wire system has an important feature: the gear ratio is variable and can be set arbitrarily. As a result, the car can be driven according to the most ideal steering characteristics. Moreover, the steer-by-wire system can compensate for the parameters that change with the speed of the vehicle, so that the steering characteristics of the car no longer change with the change of speed, thereby reducing the burden on the driver.
Third, more economical
The traditional steering system has a large number of mechanical structures, and the elimination of these mechanical structures can significantly reduce the weight of the car and therefore reduce fuel consumption or power consumption. At the same time, the elimination of these mechanical connections also reduces the cost of producing parts for the car.
Fourth, smarter
Steer-by-wire is part of the vehicle, and the steer-by-wire controller can exchange and share data with other controllers of the car. As a result, the steering controller can obtain the overall motion state of the car and comprehensively improve the driving stability of the vehicle through algorithm optimization.
However, we must also be aware that there are still technical difficulties in steering-by-wire.
First, steer-by-wire needs to be reliable and robust enough. Since the mechanical structure is eliminated and all control is carried out by electrical signals, it is necessary to ensure that complex program operations are not wrong, otherwise it will lead to serious driving accidents.
The current practice is to improve the performance of the computing program on the one hand, and on the other hand, to provide redundant backups, so that when
When the primary system fails, there is still another redundant system that can work and keep driving normally. However, there are certain difficulties in the design, layout, and interaction between the two systems.
Secondly, the steer-by-wire system needs to simulate the road sense of the road in real time so that the driver can drive reasonably. This also puts forward higher requirements for the performance of the calculation program. Of course, if it is L5 level autonomous driving, completely cancel the steering wheel, then there is no problem, but the implementation of L5 is still far away.
In addition, the steer-by-wire system has high power requirements for the steering motor, and correspondingly, the cost of the steering motor will also increase.
The current state of application of steer-by-wire
Due to the unique advantages of the steer-by-wire system and the demand under the wave of autonomous driving, the steer-by-wire is gradually becoming popular.
The earliest production steer-by-wire system dates back to 2015, and the Infiniti Q50 is equipped with an active steer-by-wire system. The steering system of the Q50 is in normal working condition, there is no mechanical connection between the steering wheel and the steering gear, and the control and road sense are simulated completely by relying on electrical signals. When the system fails, through the clutch, the steer-by-wire system is turned into a mechanical steering system, which is the redundancy for the steer-by-wire system.
Figure 4 Infiniti Q50 steer-by-wire system
Unfortunately, the subsequent Q50 switch-by-wire version suffered a large-scale recall, and it seems that the reliability of the switch-by-wire did not meet the requirements of large-scale mass production at that time.
At present, a number of manufacturers have launched their own concept models of steer-by-wire, such as the traditional giant Bosch, etc., but there is no mass production of full-by-wire steering products that can support high-level automatic driving functions.
It is worth noting that in addition to the traditional Tier 1 such as Bosch and Continental, domestic independent brands have also begun to occupy the share of wire control. In 2021, Jidu, Weilai and Geely became the joint lead units for the development and standardization of steer-by-wire technology, and will lead the formulation of relevant national standards for steer-by-wire; Great Wall Motors also released its steer-by-wire technology that supports L4 level automatic driving, claiming that mass production will be achieved in 2023.
Although there are still technical difficulties, from the current technology iteration speed and industry trends, we believe that the complete wire control to mass production landing is not too far away.
Reprinted from Yanzhi Intelligent Car, the views in the text are only for sharing and exchange, and do not represent the position of this public account, such as copyright and other issues, please inform, we will deal with it in time.
-- END --