1 As the main mode of transportation for passengers on roads, the passenger car has received more attention from the industry for its safe operation. Due to the large difference in axle load between the no-load and the full-load, the passenger car is prone to skidding on the slippery road (under low adhesion coefficient) on the road surface, which leads to the deterioration of the vehicle's power and Slippery.

In order to avoid the deterioration of the vehicle's power and the occurrence of traffic accidents due to side slippage, Yutong Group has a drive anti-skid system on its large passenger car products ZK6120HB and ZK6120R41. ASR is a kind of active safety device. It uses the mathematical algorithm and control logic to make the driving wheel of the vehicle get the best longitudinal driving force under harsh road or complex road conditions according to the dynamic changes of the driving condition of the vehicle. It can be used in the driving process. Especially in starting, accelerating, cornering and other processes to prevent the excessive slip of the drive wheels, so that the car in the driving process to maintain directional stability and steering ability and improve the acceleration performance. The anti-slip system ASR is an extension of the ABS anti-lock brake system technology and is developed on the basis of the increasingly mature ABS technology. In essence, it is the development and application of the basic ideas of ABS in the field of drive. It is therefore also called the traction control system TCS.

2 working principle

2.1 The theoretical basis of driving anti-skid control

When the car is driving on the road, the driving force depends on the engine output torque, but the driving conditions are limited by the conditions of the ground attachment. That is: F z F t F (1) where: F z is the vehicle's driving resistance; F t is the vehicle's driving force; F is the road adhesion.

The adhesion provided by the pavement is related to the adhesion coefficient. On weakly adhering roads, there are two situations in which the wheel's movement on the road is rolling and sliding. In the driving anti-skid system, the wheel slip rate is used to represent the slippage in the longitudinal movement of the wheel. The proportion of the total.

=r - vr 100% (2) where: r is the free rolling radius of the wheel; it is the rotational angular velocity of the wheel; v is the longitudinal velocity of the wheel center. The relationship between the adhesion coefficient and the slip rate is shown as 1. An optimal slip ratio corresponds to the largest adhesion coefficient, but as the slip ratio increases, the lateral adhesion coefficient decreases rapidly. Therefore, considering the utilization ratio of the vertical adhesion coefficient and the lateral adhesion coefficient, the slip rate of the wheel should generally be controlled within the illustrated range, which is approximately between 0. 08 0. 15 .

2. 2 control principle of the anti-skid system

During the start-up or driving of the vehicle, the sensor transmits the change of the slip rate of each wheel () to the electronic control unit. The electronic control unit changes the traction force distributed to each wheel through the control path of ASR to make the vehicle obtain a higher value. Acceleration ability and stable steering ability.

In the starting stage of the vehicle, the driving anti-skid control should be to improve the car's starting acceleration performance as the main control target; to make full use of the adhesion of each driving wheel to obtain the maximum traction force for the control principle, because at low speeds, even if the various driving wheels are The generated traction force has a large degree of imbalance and does not have much impact on the stability of the car's driving direction. Therefore, the control of the slip ratio of each driving wheel at this stage should be carried out according to the principle of independence.

In the case of linear acceleration at low speed and low adhering speed, if the adhesion is low, the wheel braking force should be adjusted to quickly control the slippage of the drive wheel so as to fully utilize the ground adhesion and exert the vehicle acceleration performance as the main control target; Low adhesion and separation of the road, the independent control of the wheel slip rate, in the use of vehicle acceleration performance, while ensuring the stability of the vehicle.

When driving at medium speed, the control of anti-slip rotation should be to ensure the stability of the car's driving direction as the main control target, but it should also take into account the acceleration performance of the car. At this time, each driving wheel can be controlled together with the principle of low selection, so that the slip ratio of the driving wheels with poor attachment conditions is within the range of the lateral and longitudinal adhesion coefficients, so as to ensure that the driving wheels generate the same traction force, and Make each drive wheel have a strong anti-sliding ability, so that the car to obtain a better direction of stability. However, when the braking intervention is used to control, the brake intervention time must be adjusted to prevent the brake from overheating and excessive wear due to prolonged braking torque.

In the process of high-speed driving, the control of anti-skid rotation should be to ensure the stability of the car's driving direction as the only control target. In the process of driving anti-skid rotation, the traction generated by each driving wheel should always be consistent. In order to prevent overheating and excessive wear of the brakes, the slip ratio of the drive wheels should not be controlled by the brake intervention method at this stage, but the drive acting on the drive wheels should be adjusted by reducing the output torque of the engine and the gear ratio of the transmission. Torque, the slip ratio of the drive wheel is controlled within the range of the lateral adhesion coefficient, to ensure that the car has a strong anti-sliding ability. Especially when driving on a straight road on a separated road, due to the difference in adhesion coefficient, the difference in the ground driving force of the left and right driving wheels will produce a tendency to rotate the vehicle around the center of mass, which is very dangerous at high speeds, so the The drive wheel is controlled according to the principle of low selection, to prevent the driving stability of the vehicle from being affected by the ground driving force provided by the two sides of the ground on the drive wheel.

3 ASR Control Routes

The control of the drive wheel slip rate is achieved by an electronic control device that drives the anti-skid system through various ways to adjust the forces and moments acting on the drive wheels. In a condition where the condition of the tire and the road surface is constant, the adhesion force F between the wheel and the road surface depends on the wheel load FG and the adhesion coefficient therebetween. The driving torque MT acting on the driving wheels originates from the engine and is transformed and distributed by the power train. While the driving torque acts on the driving wheels, there is also a braking torque MZ applied to the driving wheels, so the force of the driving wheels is as follows 2 shows.

The force analysis of the driving wheel can obtain the following mechanical relationships: MT - MZ - FT r = I gd dt MT = M eigio MZ = PFTF = FG (3) where: MT is the driving torque acting on the driving wheel; MZ is The braking torque acting on the driving wheel; FT is the driving force acting on the driving wheel; FG is the load of the driving wheel; The torque distribution coefficient of the differential; r is the rolling radius of the driving wheel; Ig is the driving wheel And its associated part of the moment of inertia; d / dt is the angular acceleration of the drive wheel; the efficiency factor of the brake; P is the brake pressure to drive the wheel brake; for the adhesion coefficient of the drive wheel.

When ignoring the moment of inertia of the wheel, the driving torque must be smaller than the torque produced by the adhesion force, ie, the MTF, so that the driving wheel does not slip. Therefore: M eigio - PFG r (4) where: M e is the output torque of the engine; ig is the transmission ratio of the transmission; io is the transmission ratio of the main reducer.

It can be seen that increasing the adhesion of the driving wheels is a necessary condition for improving the acceleration performance and the maximum speed of the car. The purpose of increasing the adhesion can be achieved by increasing the load and the adhesion coefficient of the driving wheels. Controlling the driving wheel slip rate can keep the adhesion coefficient between the wheel and the road surface near the peak value, and controlling the slip ratio of the driving wheel can be realized by adjusting the driving torque acting on the driving wheel. Therefore, the non-slip of the driven wheels can be satisfied by adjusting the engine output torque Me, the gear ratio ig of the transmission, the torque distribution coefficient of the differential, the load FG of the driving wheels, and the braking pressure P of the driving wheel brakes. condition.

3.1 Adjusting Engine Output Torque

When the driving torque is too large, the driving wheels are liable to slip. At this time, if the output torque of the engine can be adjusted adaptively and the driving torque acting on the driving wheels can be reduced, the slip ratio of the driving wheels can be realized. control. The method for adjusting the output torque of the engine mainly includes: adjusting the throttle opening degree of the engine to achieve the reduction of the intake air amount; adjusting the fuel injection amount of the engine supply system; and adjusting the ignition advance angle or the injection advance angle of the engine.

3. 2 change the gear ratio of the transmission

Changing the transmission ratio ig of the transmission can change the driving torque transmitted to the driving wheels and reduce the degree of slippage of the driving wheels, thereby realizing the driving anti-skid control.

3. 3 controllable slip differential

A variable ratio distribution of the drive torque can be achieved with a controllable slip differential. Ordinary conical planetary gear differentials distribute torque in equal amounts. When the adhesion between the drive wheels and the ground is different, the ground adhesion cannot be fully utilized. Electronic control of high-friction differentials allows the differential to achieve unequal torque distribution when traveling at low speeds, and equalizes differentials at high speeds. This will be able to make full use of the ground adhesion in different situations to reduce the phenomenon of slipping.

3. 4 adjust the load distribution of the drive wheels

The use of an electrically controlled suspension enables the adjustment of the load distribution of the drive wheels. When the vehicle is equipped with an electronically-controlled suspension, the vehicle load can be adjusted between the wheels by adjusting the suspension. When the attachment conditions of the driving wheels are not uniform, the load can be distributed more heavily by the active adjustment capability of the suspension. On the driving wheels with better conditions, the total adhesion force of each driving wheel should be as large as possible, which is beneficial to increase the driving torque and improve the starting acceleration performance of the vehicle; also, the active adjustment of the suspension can be used to distribute the load more. On the driving wheels with poor attachment conditions, reducing the difference in the adhesion force of each driving wheel is beneficial to the balancing of the driving torque between the driving wheels and the directional stability during driving.

3. 5 adjust the brake pressure of the driving wheel brake

Increasing the brake pressure P of the driving wheel brakes is a brake intervention on the driving wheels. By applying a braking torque to the driving wheels that have slipped, the driving torque of the driving wheels is reduced, so that the driving wheel slip can be controlled. The purpose of the transfer. This kind of control method responds quickly and is the most rapid control method to prevent slip, and it can realize independent control of each wheel for systems with independent control channels. The effect is particularly significant when the adhesion coefficient separates the road surface; but it is easy to cause brakes. The fever can not be used for a long time.

4 ASR control algorithm

The commonly used control algorithms for driving anti-slip control system can be divided into three categories, including classical control methods, modern control methods and intelligent control methods. Among them, logic threshold control and PID control belong to the category of classical control theory, and optimal control and sliding mode variable structure. Control belongs to the category of modern control theory. Fuzzy control and neural network control belong to the category of intelligent control theory.

4.1 Logical Threshold Control Algorithm

The logic gate limit control, also called two-position control, is a classic logic threshold-based control method. It takes the slip rate and acceleration of the wheel or the wheel speed difference between the two wheels as the control threshold, and controls the supercharging, decompression, and holding pressure so that the wheel slip rate fluctuates at the point of optimal slip rate. If the car is running, if a certain parameter of the wheel exceeds the set positive threshold, the control system will generate an action to adjust the output torque of the engine or apply a braking force to reduce it to reach the negative threshold value. The system generates actions and increases driving force, so that the cycle achieves control of the slip rate. However, due to the large inertia of the driven wheels, the wheel speed difference between the two sides of the drive wheel often exceeds the set threshold value, but the acceleration of the wheel slip is not large, and the wheel is still in a stable working state. If the control pressure is applied according to the wheel speed difference threshold, excessive pressure tends to occur and the wheel is locked.

4.2 PID Control Algorithm

PID control system consists of PID controller and controlled object, as shown in 3.

The PID controller controls the proportion of the deviation between the given value r(t) and the actual output value y(t), proportion, and derivative (d if ferential) by a linear combination. Control. Its control law is: u (t) = kpc (t) + 1 T t 0 c (t) dt + TD dc (t) dt (5) Therefore, its transfer function is: G (s) = U (s) E (s) = kp (1 + 1 T 1 s + TD s) (6) where: kp is the proportional coefficient; T 1 is the integral time constant; TD is the differential time constant.

Simply put, the role of each PID controller calibration section is as follows.

(1) Proportional link: proportionally reflects the deviation signal of the control system. Once the deviation is generated, the controller immediately produces a control function to reduce the deviation; (2) Integral link: It is mainly used to eliminate the static error and improve the system without any difference. ;

(3) Derivative link: The variation trend (rate of change) of response deviation, and can introduce an effective early correction signal in the system before the deviation signal becomes too large, thereby speeding up the system's movement speed and reducing the adjustment time.

The PID control method does not need to understand the mathematical model of the controlled object, and controls the controller parameter value setting based on experience. Relative to the threshold control, the PID control method is more stable. However, the PID control method has a major drawback, that is, it is too sensitive to changes in the controlled object's parameters, and has poor adaptability to working conditions. Specifically, its parameters must change with changes in the road conditions. Control parameters for low adhesion coefficient pavements cannot be used for high adhesion coefficient road surfaces. To be applied, its parameters must be set automatically with the road conditions, but general PID control cannot be achieved, which limits its popularization and application.

4. 3 optimal control algorithm

Optimal control means that under a given mathematical model and initial conditions, an objective function of the characterization process is selected to determine an optimal control function so that the given system can reach the termination state from the initial state, and the performance index is minimized. value. Optimal control excels at handling multiple input-multiple output systems and complex systems with high control accuracy requirements. The most commonly used in the passenger car driving force anti-skid system is the linear quadratic performance index to determine the optimal control rate: J = 1 2 X (tf)S x(tf) = 1 2 tft 0 dt where: S is nn semi-positive definite The real matrix is ​​called terminal circle matrix; Q is nn positive positive definite or positive definite real matrix, which is called state weight matrix; R is mn positive definite real matrix, which is called control weight matrix.

Then, based on this performance index, a dynamic model of the engine's external output torque is established, and the torque output from the crankshaft of the engine is transmitted to the differential case via the clutch, transmission, output shaft, and main reducer, and the torque transmission of this transmission process is performed. And the equation of motion, then coupled with the dynamic equation of the semi-axle to the drive wheel torque transmission, under certain constraints, the optimal value or optimal area for the slip ratio control can be obtained. However, optimal control is a control method based on model and represented in the time domain with the state space. It is based on an accurate mathematical model. The accuracy of the system mathematical model has an important influence on its control effect. The mathematical model of the vehicle system in the control algorithm is a mathematical model based on the simplification of the vehicle's power system. It is easy to cause the control effect to be unsatisfactory. Therefore, the use of optimal control in driving the anti-skid system is still relatively small.

4. 4 sliding mode variable structure control algorithm

Sliding mode variable structure control is actually a control strategy in variable structure control and is a special type of nonlinear control method. Sliding mode variable structure control is discontinuous control. According to the system's current state, deviation and its derivative value, in different control areas, the size and symbol of the control quantity are switched in the ideal switching mode, so that the system can switch back and forth in the vicinity of the switching line. Movement until the movement of the system state becomes a slip along the switching line, that is, a small high-frequency up and down movement (called a sliding mode movement) along a prescribed state trajectory, and therefore, the power of the controller and the controlled object system can be made Academic changes, parameter changes, and external disturbances have all and strong insensitivity, that is, they have good robustness (Ro bustness).

The slip ratio of the drive wheel is related to the conditions of the road surface, the running state of the vehicle, the driving form of the vehicle, and the driver's operation. These factors have obvious time-varying, nonlinear and uncertainties, which require that the control theory used should be robust.

Academician Guo Konghui of Jilin University studied the application of sliding mode variable structure control in driving anti-slip system in the 1990s. He used the method of adjusting the throttle opening of the engine to control the slip rate, and performed simulation calculation. The ability of the sliding mode variable structure control algorithm to drive the anti-skid system to improve vehicle drivability and steering stability is described.

4. 5 fuzzy control algorithm

The fuzzy control method is to convert the precise digital quantity into the membership function of the fuzzy set. Then fuzzy logic inference is made according to the fuzzy rules of the control quantity to obtain a fuzzy output membership function. Finally, the membership function obtained by inference is used in different ways. Find a representative accurate value as a control quantity and add it to the actuator to implement the control. Fuzzy control is easy to implement control of time-varying nonlinear objects, and does not need to know the mathematical model of the controlled object; it has strong robustness to the change of the controlled object's characteristic parameters, and has strong inhibition to the interference of the controlled object. ability.

The main purpose of automobile anti-skid control is to control the slip ratio near the optimal working slip rate. The best slip rate is represented by 0. The actual slip rate is expressed by E = - 0, which is the deviation value according to the deviation. The value E and the rate of change in deviation C(E) are the overall control of the drive torque MT. Since both E and C(E) are accurate calculations, blurring is required. A fuzzy controller for establishing the slip rate is used. The fuzzy controller automatically adjusts the driving torque according to the change of the slip rate, so that the actual slip rate approaches the ideal value of the slip rate.

4. 6 neural network control algorithm

By establishing a BP neural network controller to control the accelerator opening and the brake wheel pressure respectively, an ideal control effect is obtained.

5 Conclusion

Through the analysis of the working principle of ASR, it can be concluded that although the control object of several control algorithms is the wheel slip rate, the process of improving the slip situation is different because of different control algorithms and actuators. In the actual bus ASR, it should be based on the actual needs of the situation, select the appropriate control algorithm or several control algorithms combined with each other in order to optimize the control effect of driving the anti-skid.

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