What's the difference between the drive-by-wire chassis and the traditional chassis?

The drive-by-wire chassis is a mechanical structure part that cancels the hard connection of steering, braking and throttle system on the basis of the traditional chassis. Instead, the driver's operation is converted into electrical signals through sensors to realize vehicle control.

At present, the chassis by wire mainly includes steering by wire, braking by wire, driving by wire, suspension by wire and gear shifting by wire, which has the advantages of fast response, high controllability and low maintenance cost. Now, all major automobile companies are developing chassis by wire control technology.

First, the working principle

The control function of the chassis mainly includes five parts, namely: steering, braking, shifting, throttle and suspension. Among them, the most critical is the control of brake, throttle and steering. Let's take these three as examples to see how the traditional automobile chassis works.

1. Throttle: In a traditional fuel vehicle, the throttle pedal is connected with a pull rod (some of which are cables), and the opening and closing degree of the engine throttle is controlled to control the air intake of the cylinder, and then the fuel injection is controlled to drive the vehicle.

2. Steering: A steering lever is connected below the steering wheel to drive the steering arm to change the direction of wheel rotation. Steering has probably gone through several stages: mechanical steering, hydraulic power steering (HPS), electro-hydraulic power steering system (EHPS) and electric power steering system (EPS).

Mechanical steering is to change direction purely by the force between machines. On the basis of mechanical steering, HPS adds a hydraulic pump system to help the steering wheel by controlling oil pressure. EHPS is an electronic control unit added to HPS, which is used to control oil pressure, thus providing assistance for vehicle steering. ESP energizes the steering actuator on the basis of the previous one to make the steering lighter.

3. Braking: It has probably gone through several stages: mechanical braking, pressure braking and hydraulic braking by wire. The technical route is similar to the steering system, from the initial pure mechanical braking to power-assisted braking, and then to electric drive.

The traditional chassis has complex structure, heavy weight, large volume, high cost and difficult maintenance. For smart cars, the sensitivity of mechanical parts is not enough to meet the low delay response requirements of intelligent driving.

At this time, the advantages of the chassis by wire are highlighted. Because "wire" brings "electricity", as the name implies, the wire-controlled chassis can also be understood as "electric control chassis", that is, the functions originally realized by machinery are all controlled by "electricity" on the wire-controlled chassis.

Second, the difference between wire control and traditional chassis

1, the structure of the drive-by-wire chassis is simpler.

Compared with the traditional chassis, the drive-by-wire chassis eliminates most of the traditional mechanical, hydraulic and pneumatic connecting parts, which makes the chassis structure more concise and further reduces the production cost of the chassis. The chassis by wire can not only greatly reduce the weight of the car body and the volume of each component, but also reduce the vibration of the car body to a certain extent and achieve better silence.

2. The chassis controlled by wire has faster response speed.

The hydraulic mechanism of the brake-by-wire system was removed, and the sensor was installed through wires. The braking operation was directly transmitted to the brake through electrical signals, and its response speed was 0.35 seconds faster than that of traditional hydraulic braking. In extreme cases, this 0.35 second may play a crucial role.

3. The development prospect of the drive-by-wire chassis is good.

The reason why car companies want to develop wire-controlled chassis is mainly for the future automatic driving layout. Today's drive-by-wire chassis is actually the main carrier of automatic driving, with better controllability and faster response speed, which is beneficial for the automatic driving system to control the vehicle in time, so as to realize the higher-order automatic driving function as soon as possible.

4. The requirements for functional safety are higher.

At present, there are still insurmountable technical difficulties in the drive-by-wire chassis, and the drive-by-wire chassis technology often means high intelligence and electrification. If the vehicle electrical system fails, it is easy to get out of control without the traditional mechanical structure, so it requires higher functional safety.

As the main carrier of intelligent driving, the automobile drive-by-wire chassis technology will create a new future, and the future high-order automatic driving will be realized based on the drive-by-wire chassis. At present, the brake-by-wire and suspension-by-wire have opened the road of rapid penetration, and the steering-by-wire is about to land.

Third, the key technology of chassis by wire control

1, fault diagnosis and fault-tolerant control

As a complex advanced electronic system, the chassis by wire has not reached the same reliability as the mechanical or hydraulic components, and its failure mode is different from the traditional system. Therefore, how to effectively diagnose the fault in the new fault mode, and ensure that the system has fault-tolerant function, and can ensure the basic functions of steering and braking when some electronic components or software fail, is a problem that must be solved to realize the comprehensive application of chassis by wire.

The by-wire control system should be able to find the system fault in time, determine the fault source, and make corresponding fault-tolerant control actions. Fault-tolerant control means that when some components fail or fail, their functions in the system can be replaced by other components of the system, so that the system can continue to maintain the specified performance or not lose its basic functions, and further realize the optimal performance of the failed system.

There are two main design methods of fault-tolerant control: hardware redundancy method and analytical redundancy method. Hardware redundancy method is to provide backup for important components or components prone to failure, and analytical redundancy method is mainly to improve the redundancy of the whole system by designing the software of the controller.

In the online control system, compared with ECU, sensors and actuators are more prone to failure, so there are redundant backups between many sensors and actuators. However, although the reliability of ECU is relatively high, once the ECU fails, the consequences will be more serious. Because the sensor and actuator fail, the system may keep part of the work, and once the ECU fails, the system will be completely paralyzed and lose all functions.

However, hardware redundancy has the problem of high cost, which is also a major bottleneck in the development of current wire control technology. Considering the cost, it is an important development direction to make more use of analytical redundancy to improve fault tolerance.

2. Information acquisition and transmission

The premise of realizing chassis steering and braking by wire control is that the sensor continuously feeds back the driver's instruction information and vehicle state information to the controller in time, and then the controller controls the actuator according to the control strategy.

The precision and resolution of the sensor directly affect the precision and performance of the control system, so developing a sensor with low cost, good reliability, high precision and small size is also one of the key technologies to develop the by-wire control system.

Bus technology plays a decisive role in information transmission. The wide application of by-wire technology means the transformation of automobile from mechanical system to electronic system. Wire-by-wire technology requires fast data transmission, good time characteristics and high reliability.

The traditional CAN bus can't provide the required fault-tolerant function and bandwidth for the wire-controlled system: on the one hand, because the wire-controlled system requires high real-time and reliability, time-triggered communication protocol must be adopted; On the other hand, the by-wire system requires the communication network protocol to have fault-tolerant function, which means that even if different parts of the system fail, the system can continue to run as designed.

TTP/C and FlexRay protocols both contain fault-tolerant synchronous clocks, and the bus monitor is used to protect the communication channel from the influence of wrong nodes, which is the first choice for pure wired control systems.

3. Motor and its controller

The information is transmitted to the controller through the bus, and then the controller drives various motors to implement the work. The performance of the control motor and controller greatly affects the overall performance of the by-wire control system. The motor of the by-wire control system mainly takes the position, speed or torque as the control target, and the power varies from a dozen W to several thousand W. For low-power motors,

Stepper motors or DC motors can be used, such as throttle opening control motors and oil pump motors. In terms of high-power motors, permanent magnet synchronous motors have been widely used, such as EMB's steer-by-wire motor and brake motor.

Pure by-wire control system needs to consume more electric energy because multiple motors work at the same time. Therefore, it is necessary to improve the motor power density, controller power density, system efficiency and other indicators, and expand the range of high-efficiency areas. This can not only reduce the load of motor controller and system power supply, but also improve the redundancy of design. It is also of great significance to save energy and improve the dynamic performance of the by-wire control system.

In addition, the reliability, safety and electromagnetic compatibility of motor and controller are also important prerequisites for the safety of vehicle integrated control.

4. Power supply

The performance of power supply is also very important to ensure the stable operation of the whole line control system.

The actuators of the by-wire control system are mainly high-power motors and servo motors, which consume extremely high power compared with the traditional actuators. For example, the power range of a single steering motor is 550~800W, while the power of motor disc brake can reach1000 W. ..

If we continue to maintain the traditional 14V power supply system, we must increase the current to obtain higher power. However, excessive current will bring security risks to the whole system, and the heat energy consumption on the automobile circuit will also increase greatly, so the automobile power supply system must meet the demand for more electricity by increasing the voltage.