The active steering system can not only drive at high speeds on straight roads, but also show cat-like agility on sharp bends. This is the dream state of automotive engineers when designing steering systems. It was not until the launch of BMW's new 5 Series that this dream was truly realized. Since the invention of the automobile, the gearing used to drive steering has generally been fixed. In other words, whether it is slowly driving on narrow urban streets or driving on the highway, the steering angle ratio of the steering wheel to the front wheels remains unchanged. Therefore, this is also a more difficult choice faced by engineers: if direct steering is used, the driver does not need to turn the steering wheel significantly when passing sharp corners, but when driving at high speed, subtle movements of the steering wheel will affect driving stability; On the other hand, the more indirect the steering system, the higher the vehicle's driving stability on the highway, but the handling when cornering must be sacrificed. Therefore, traditional steering systems must make a trade-off between safety and comfort. After several years of research and development efforts, BMW engineers developed a revolutionary active steering system, which first appeared in the new BMW 5 Series. For the first time in automotive history, this system ensures ideal steering control at any speed while simultaneously taking into account maximum driving pleasure, agility and safety. The basic principle of this genius design is actually very simple. The steering wheel system is equipped with a gearbox that adjusts the steering transmission according to vehicle speed. This system consists of a fist-sized planetary gear and two input shafts. One of the input shafts is connected to the steering wheel, while the other is controlled by an electric motor through a helical gear. Dr. Philip K?hn, project manager in charge of BMW's active steering system, explained: "When the vehicle speed is low, the control motor and the steering column rotate in the same direction to increase the steering angle; and when driving at high speeds , controlling the motor to rotate in the opposite direction, thereby reducing the steering angle. "BMW's active steering system greatly enhances driving safety. The driver of the new 5 Series can still maintain an ideal sitting posture when continuously cornering, and almost does not need to move his hands. He can only complete the control through the multi-function switch and SMG shift handle within easy reach on the steering wheel. This design also provides convenience for parking. You can easily park the car into the parking space with just two turns of the steering wheel. The active steering system makes driving on the highway more relaxed. This is because the system can reduce steering sensitivity at high speeds, and steering wheel vibrations caused by external factors, such as driving on rough roads, have a smaller impact on directional stability. Dr. Kong even pointed out: "When turning at high speed, the steering becomes simpler and smoother." Even in the face of sudden steering movements, such as avoiding obstacles that suddenly appear in front, the system actions are still smooth and natural. In addition, with the original steering power servo system, the steering torque will provide more power in accordance with the vehicle speed to avoid losing control of the steering wheel. The traditional steering system uses a fixed transmission ratio of 18:1 regardless of the speed of the vehicle, which means that the steering wheel turns 18 degrees and the wheels turn 1 degree. The ratio of BMW's active steering system is within a certain range, from 10:1 at rest to 20:1 at high speed. That is to say, when the steering wheel turns half a turn (180 degrees), if the vehicle speed is low, the wheels will rotate 18 degrees, and if the vehicle speed is high, the wheels will only rotate less than 9 degrees. The control components of the active steering system are connected to the engine electronics, the Dynamic Stability Control (DSC) and two yaw rate sensors. Based on the information provided by these systems, it provides the most real-time and ideal steering angle at an average calculation speed of 100 times per second. By measuring the steering angle, the system can understand the driver's intentions. The Dynamic Stability Control system calculates vehicle speed based on the number of wheel rotations, while a yaw rate sensor monitors the stability of the vehicle's vertical axis at all times. The active steering system always knows whether the new 5 Series is driving on the ideal route or has a tendency to deviate from the route. All cars naturally oversteer when a particularly critical situation occurs, such as evading. Active Steering senses this from the start and adjusts the steering angle accordingly within milliseconds. In other words, the system can automatically reverse the steering system to balance the vehicle body without the driver's knowledge, thus improving driving safety.
And if the active steering system itself is not enough to keep the vehicle on a stable advanced route, the dynamic stability control system will intervene in time to reduce engine horsepower or apply brakes to individual wheels. What if the control software fails? BMW engineers have already anticipated this. In a pure steer-by-wire system, steering is controlled by electronic signals, and there is no direct mechanical connection between the steering wheel and the wheels. The new 5 Series equipped with an active steering system is different. Even if the system fails, it can still perform steering actions, but its steering angle cannot be increased or decreased. Serious steering errors caused by software obstacles are absolutely not allowed to occur. Dr. Philip Koon explained: "All information is analyzed in two computers in different ways. Only when the results of the two computers are the same are the instructions accepted. , if the results are contradictory, the system will shut itself down. "The clever use of mechanical and electronic devices complement each other, making BMW the first company to combine cornering agility and straight-line stability in one car.