Challenges and dawn of mass production and commercial use of control-by-wire steering

Steer-by-wire can be divided into broad and narrow senses. In a broad sense, steer-by-wire refers to a steering system that controls the steering function by electrical signals, and does not necessarily cancel the mechanical connection between the steering wheel and the steering wheel; in a narrow sense, steer-by-wire refers to a steering system that controls the steering function by electrical signals, and cancels the steering wheel and steering wheel. A mechanical connection between steering wheels in a steering system. In a narrow sense, the steering-by-wire system can be divided into those with a steering wheel and those without a steering wheel. If a steering wheel is provided, the steering wheel generally does not rotate with the wheels - such as Nexteer's silent steering-by-wire steering system.

In the context of this article, unless otherwise specified, steer-by-wire refers specifically to steer-by-wire in a narrow sense.

Currently commonly used in passenger cars is Electric Power Steering (EPS), which is a type of "generalized" steer-by-wire, and its penetration rate has exceeded 90%. As shown in the figure above, EPS means that after the driver turns the steering wheel, the torque sensor records the steering torque and transmits it to the electronic control unit. The electronic control unit calculates the steering assist and controls the operation of the servo motor to achieve power assistance.

Different from the electric power steering system, the steering by wire system (SBW) uses a steering motor controlled by an electronic control unit to drive the wheels to rotate, and there is no mechanical connection between the wheels and the steering wheel.

In the wire-controlled steering system, the wheel steering actuator adds a wire-controlled interface. The steering torque is completely output by the lower steering actuator. The direction and size of the output force of the lower steering actuator and the speed of rotation all depend on the signal of the control algorithm. . This means that the steering system can be completely taken over by the control algorithm without relying on manual turning of the steering wheel. This is a prerequisite for the realization of autonomous driving functions.

In fact, steer-by-wire is not a new concept. As early as the 1950s, TRW and Kasselmann proposed the concept of steer-by-wire, but at that time it was just a concept.

In 1990, the German Mercedes-Benz Company began research on steer-by-wire, and in 1996 applied the developed steer-by-wire system to the F200 concept car. Since then, many research institutions, OEMs, and parts manufacturers, such as the National University of Japan, BMW, and JTEKT, have conducted research on steer-by-wire and have successively launched some representative products.

In 1999, BMW launched the concept car BMW Z22. After applying steering-by-wire technology, this car can achieve a variable transmission ratio, and the maximum angle that the steering wheel needs to turn is reduced to 160 degrees, which greatly reduces the difficulty of the driver's operation during emergency steering.

In 2016, a prototype vehicle displayed after the merger of ZF and TRW was equipped with ZF TRW's front and rear axle steering system, which can achieve pure electric control. In 2018, Bosch Huayu's steering-by-wire product, which was developed in partnership with Audi, was mounted on the Audi A3 and toured the country.

However, to date, steer-by-wire products have not been widely used in passenger cars. In 2013, Infiniti's Q50 was launched. This car was equipped with steering-by-wire with mechanical redundancy. However, Infiniti recalled the Q50 in 2014 due to safety issues with the steering-by-wire system. Since then, there has been no mass production of cars on the market that are truly equipped with steer-by-wire.

Until April 2022, Toyota's bZ4X will be launched. The overseas version uses JTEKT's wire-steering system One Motion Grip. On this product, there is no mechanical connection between the steering wheel and the front wheels. Instead, there are wires and electronic systems. The steering wheel changes the steering ratio and force feedback according to the car's driving speed and wheel steering angle.

But it is difficult to say that the launch of bZ4X represents the successful mass production application of steer-by-wire in passenger cars.

In June 2022, Toyota implemented a recall of bZ4X in overseas markets due to wheel hub problems. At this time, most bZ4X had not yet been officially delivered. In October, Toyota recalled bZ4X again globally, and the reason was still the wheel hub problem. In the Chinese market, due to regulatory issues, bZ4X still uses a steering system with mechanical connection. Therefore, it still cannot be said that steer-by-wire has been successfully applied in mass production in passenger cars.

Some investors expressed that they are not optimistic about the implementation of steer-by-wire in the short term. The reason is that the current cost-effectiveness of steer-by-wire is too low and the cost is high, but it cannot bring significant improvement to the driver's experience. However, in October this year, ZF signed an agreement with NIO to jointly develop steering-by-wire products. Nexteer announced in its 2022 interim report that it had won a 2 billion yuan order for a steering-by-wire system from an OEM. It can be seen that even if the previous cases are not successful, there are still manufacturers that insist on research and development in this field.

1. The value of developing steer-by-wire systems

So, why do you want to study steer-by-wire? What kind of experience improvements can steering-by-wire bring to users?

1.1 Part of the chassis wire control

Some investors mentioned that in the era of intelligence, the electronic and electrical architecture of automobiles is developing from distributed to domain centralized and then to central integrated. In this context, steering is part of the chassis. With the decoupling of the upper and lower bodies and the realization of high-level autonomous driving, control-by-wire is an inevitable trend.

Algorithms can achieve faster response times and higher accuracy than humans. The lane emergency obstacle avoidance functions of many passenger cars today rely on the rapid response and precise control of algorithms. With the implementation of high-level autonomous driving, the use of algorithms to control steering will become a necessity.

Gao Feng, director of the Chassis Institute of Tsinghua University, believes that "braking, steering, and suspension wire control are all part of the centralized control of the chassis domain. Braking will develop towards IPB+RBU (<=L3) and EMB (L4&L5). Steering It will develop from mechanical coupling (<=L2) to no mechanical coupling (L3&L4) and chassis backup terminal (L5) without steering wheel. "

1.2 The body design is more flexible

Steer-by-wire eliminates the mechanical connection between the steering wheel and the steering wheel, bringing more flexibility to the body design. On the one hand, the elimination of the mechanical connection can save space in the driving position and the driver's legs can be more stretched; on the other hand, it also decouples the steering wheel and steering wheel because the steering wheel and steering wheel do not have to rotate at the same time.

After decoupling, the steering wheel can be retracted and used to implement other functions. If L4 level autonomous driving is realized in the future, in addition to controlling vehicle driving, the steering wheel can also support entertainment functions. You can imagine a scenario where the game interface is projected onto the HUD, the steering wheel acts as a handle, and the steering wheel is used to control the props in the game. In addition, you can also make some music-related rhythms on the steering wheel to give users a more realistic experience.

In addition, if >=L3 level of autonomous driving is achieved, when the driver does not need to take over, the steering wheel will always follow the wheels due to the mechanical connection, and the driver's experience will be relatively poor.

1.3 Transmission ratio is more flexible

When a car turns, the ratio of the angle of the steering wheel to the angle of the wheels is called the transmission ratio. In a traditional steering system, the steering wheel and wheel actuators are mechanically connected, and the transmission ratio is fixed. However, once the mechanical link is removed, the transmission ratio is variable.

Why do you need a variable transmission ratio? The main reason is that when driving at low speed, adjusting the transmission ratio can make the steering operation more effortless, and when driving at high speed, adjusting the transmission ratio can avoid accidentally deviating from the lane.

When driving at low speed, turning the steering wheel will be relatively difficult. When it is necessary to turn around, if the transmission ratio is not adjusted, the steering wheel needs to turn a larger angle to drive the wheels to turn the corresponding angle. If the transmission ratio is reduced when driving at low speed, the steering wheel only needs to turn a smaller angle to drive the wheels to turn the angle required for a U-turn, thus achieving a convenient U-turn.

When driving at high speed, the steering wheel is easy to rotate, and the driver's inadvertent movements may cause the steering wheel to rotate at a considerable angle. If the wheels follow the steering wheel and rotate at a corresponding angle, the vehicle may deviate from the lane and cause danger. Then, increasing the transmission ratio when driving at high speed can better avoid risks caused by driver's misoperation.

In addition, some customized designs can be made according to the scene. Luo Yongchang, general manager of Jiyu Technology, mentioned, "Although EPS can also change the steering ratio by adjusting the gear size during design, it cannot be changed once it is designed; in contrast, steering by wire is more flexible and has The car can also be customized according to the scene.”