CN110371106A - A kind of steering stability method based on four motorized wheels electric car - Google Patents

Abstract

The invention discloses a kind of steering stability methods based on four motorized wheels electric car, comprising: determines yaw moment control plan and driving force dispensing controller, according to motor turning operating condition, distributes driving moment in real time；It determines EPS active control strategies, when understeer or oversteering operating condition occurs in automobile, booster torquemoment is adjusted according to control model.A kind of steering stability method based on four motorized wheels electric car is provided, differential power-assisted steering and EPS are reduced to motor turning bring unstability using the method for direct yaw moment control, to guarantee that vehicle steadily travels.

Description

A Steering Stability Method Based on Four-Wheel Independent Drive Electric Vehicle

technical field

The invention relates to the technical field of vehicle handling stability simulation, in particular to a steering stability method based on four-wheel independently driven electric vehicles.

Background technique

Energy shortage and environmental pollution are putting more and more pressure on human beings. The invention and popularization of electric vehicles are in line with the theme of peace and development of human society today, and also in line with the goal of creating a resource-saving and environment-friendly society in my country's sustainable development strategy. The four-wheel independent drive electric vehicle technology has the advantages of individually controllable four-wheel drive torque and flexible handling, making its comprehensive performance and corresponding products quite competitive in the field of electric vehicles. The four-wheel independent drive electric vehicle has a smaller turning radius, more flexible and lighter at low speeds; at high speeds, it has good return to center and has a certain sense of the road. This is the basic requirement for steering in electric vehicles. However, through a large number of simulation tests, it can be seen that when the electric vehicle under the joint control of the EPS system and the differential power steering is turned, no matter at high speed or low speed, it has a smaller turning radius than the vehicle without any control. Excessive steering characteristics, more flexible handling characteristics. Therefore, it is necessary to add a stability control strategy to make the electric vehicle maintain labor-saving, flexible and certain oversteer characteristics when steering at low speeds; Especially when turning at high speed, it has better stability and avoids danger.

The four-wheel independent drive electric vehicle has a smaller turning radius, more flexible and lighter at low speeds; at high speeds, it has good return to center and has a certain sense of the road. This is the basic requirement for steering in electric vehicles. When the electric vehicle under the common control of the EPS system and the differential power steering is turned, no matter at high speed or low speed, compared with the car without any control, it has a smaller turning radius, appropriate excessive steering characteristics, and more flexible. Manipulation characteristics. Therefore, it is necessary to add a stability control strategy to enable the electric vehicle to maintain labor-saving, flexible and certain oversteer characteristics when steering at low speeds; Especially when turning at high speed, it has better stability and avoids danger.

Contents of the invention

In order to solve the shortcomings of the current technology, the present invention provides a steering stability method based on four-wheel independent drive electric vehicles, and adopts the method of direct yaw moment control to reduce the inconvenience caused by differential power steering and EPS to vehicle steering. Stability, so as to ensure the stable driving of the car.

The technical solution provided by the present invention is: a steering stability method based on four-wheel independently driven electric vehicles, comprising:

Determine the yaw moment control strategy and the driving force distribution controller, and distribute the driving torque in real time according to the steering conditions of the vehicle;

Determine the EPS active control strategy, and adjust the power assist torque according to the control mode when the car appears understeer or oversteer.

Preferably, the driving force distribution controller adopts PID control.

Preferably, the driving force distribution controller specifically includes:

The input is the total drive torque and the additional yaw moment, and the difference between the reference yaw rate and the actual measured yaw rate is used as the output compensation yaw moment, which is symmetrically distributed to the left and right drive motors in the form of torque.

Preferably, the reference yaw rate is determined according to a linear two-degree-of-freedom single-track model.

Preferably, the linear two-degree-of-freedom monorail model satisfies:

In the formula: β is the side slip angle of the center of mass; _r is the yaw rate; C _f , C _r are the cornering stiffnesses of the front and rear wheels respectively; I _Z is the moment of inertia of the vehicle around the z-axis _; , the distance from the rear axle to the center of mass; δ _f is the front wheel rotation angle; M _t is the restoring moment when the tire is deformed, M _z is the wheel returning torque, and V is the vehicle speed.

Preferably, the reference yaw rate (the angular velocity at which the vehicle rotates around the vertical axis in the body coordinate system) is:

In the formula, τ _e is the time constant, δ _sw is the steering wheel angle, and r _ss is the steady-state gain of the yaw rate.

Preferably, the method for determining the steady-state gain of the yaw rate is:

When the car turns at a constant speed, it satisfies According to the linear two-degree-of-freedom single-track model, we can get:

Preferably, when the steering is at a relatively large lateral acceleration, it generally refers to the range of lateral acceleration greater than 0.4g while ensuring normal driving, and the reference yaw rate needs to meet:

In the formula, μ is the coefficient of adhesion between the tire and the ground, and g is the acceleration due to gravity.

Preferably, it also includes:

When the additional yaw moment ΔM=0, the distribution of driving force satisfies according to the average distribution:

In the formula, F _L , F _R are the driving forces of the left and right front wheels respectively; f _L , f _R are the driving forces of the left and right rear wheels respectively; F _always is the total driving force of the vehicle; b _f , b _r are respectively Front and rear wheelbase.

Preferably, the specific adjustment method of the assisting torque is:

When the steering is understeer, if the control mode is judged to be the basic booster control mode, the booster torque will be increased; if the control mode is judged to be the return control mode, the booster torque will be decreased;

When oversteering, it is judged that the control mode is the basic power assist control mode, then the power assist torque is reduced; if it is judged that the control mode is the normalization control mode, the power assist torque is increased.

Beneficial effects of the present invention: the present invention provides a steering stability method based on four-wheel independent drive electric vehicles, which uses direct yaw moment control to reduce the instability caused by differential power steering and EPS to vehicle steering performance, so as to ensure the stable driving of the car.

Description of drawings

Fig. 1 is the linear two-degree-of-freedom monorail model of the present invention.

Fig. 2 is the driving force distribution controller model of the present invention.

Fig. 3 is the automobile vehicle trajectory of the present invention with steering stability control and without stability control.

4 is a graph of yaw rate versus time for an automobile with steering stability control and an automobile without stability control according to the present invention.

Figure 5 is a graph of lateral acceleration versus time for an automobile with steering stability control and an automobile without stability control of the present invention.

Fig. 6 is the running track of the automobile with steering stability control and the automobile without stability control according to the present invention.

FIG. 7 is a graph of yaw rate versus time for an automobile with steering stability control and an automobile without stability control according to the present invention.

Figure 8 is a graph of lateral acceleration versus time for an automobile with steering stability control and an automobile without stability control of the present invention.

Detailed ways

The present invention will be further described in detail below in conjunction with the accompanying drawings, so that those skilled in the art can implement it with reference to the description.

As the car turns, lateral forces cause the car to tilt outward. The method of direct yaw moment control is used to reduce the instability caused by differential power steering and EPS to the steering of the car, so as to ensure the stable driving of the car. Specifically, the difference between the reference yaw rate and the actually measured yaw rate is used as the output compensation yaw moment, which is finally symmetrically distributed to the left and right drive motors in the form of torque.

Generally speaking, the wheel rotation angle has a linear relationship with the steering wheel angle input, and the reference yaw rate velocity for direct yaw moment control can be determined according to a linear two-degree-of-freedom monorail (Bicycle) model. When turning at low speed, the curvature of the vehicle trajectory is:

When turning at a low speed (1-2 gears, usually below 30 kilometers per hour), the curvature of the car's trajectory is:

In the formula, R is the turning radius, V is the vehicle speed, and r is the yaw rate;

When the vehicle steering angle is small, there are:

In the formula, δ _A is called the Ackermann angle, l is the distance between the front and rear axles, and the above two formulas are combined to obtain, and the reference lateral angular velocity ω _d is as follows:

As shown in Figure 1, it is a linear two-degree-of-freedom single-track model. The model is a two-degree-of-freedom car model supported on the ground by two laterally elastic tires at the front and rear, and has lateral and yaw motions.

The motion state equation of the two-degree-of-freedom vehicle is established as follows:

In the formula: β is the side slip angle of the center of mass; _r is the yaw rate; C _f , C _r are the cornering stiffnesses of the front and rear wheels respectively; I _Z is the moment of inertia of the vehicle around the z-axis _; , the distance from the rear axle to the center of mass; δ _f is the front wheel rotation angle; M _t is the restoring moment when the tire is deformed, M _z is the wheel returning torque, and V is the vehicle speed.

When the car turns at a constant speed, there is into the above formula,

Obtain the steady-state gain of the yaw rate:

Then, the reference yaw rate (the angular velocity at which the vehicle rotates around the vertical axis in the body coordinate system) is defined as follows:

In the formula, τ _e is the time constant, and δ _sw is the steering wheel angle. In addition, considering that the limit of tire turning ability will be exceeded in the case of large lateral acceleration, the reference yaw rate is also constrained by the following formula:

In the formula, μ is the adhesion coefficient between the tire and the ground.

In order to reasonably distribute the driving force of the four driving wheels of a four-wheel independent drive electric vehicle, a driving force distribution controller is designed. The input of the driving force distribution controller is the total driving torque and the additional yaw moment, and the driving torque of the four hub motors is distributed according to the control target.

It is stipulated that the direction of the value ΔM of the additional yaw moment when the electric vehicle turns left is positive, and that of ΔM when it turns right is negative; the direction of the steering wheel angle δ when the car turns left is positive, and the direction of the steering wheel angle δ when the car turns right is negative .

The driving force distribution control strategy is:

When ΔM=0, the vehicle is neutral steering, no need to add control, and the driving force distribution follows the principle of equal distribution:

In the formula: F _L , F _R are the driving forces of the left and right front wheels respectively; f _L , f _R are the driving forces of the left and right rear wheels respectively; F _always is the total driving force of the vehicle; b _f , b _r are respectively Front and rear wheelbase.

For example: when ΔM>0; δ>0, when turning left, if the vehicle appears understeer, the driving force distribution controller needs to increase the yaw moment of ΔM.

The rest of the situation is shown in Table 1:

Table 1 Driving torque distribution table

The driving force distribution controller model is shown in Figure 2. In this model, M ₁ and M ₂ represent the torques of the front wheels and rear wheels respectively, which are input into the Embedded control strategy, and output the left wheel 1 torque, left wheel 2 torque, right wheel 3 torque, and right wheel 4 torque to form the difference power assist.

The traditional electric power steering system mainly solves the problem of easy operation when the car turns at low speed and the road feeling when turning at high speed, and cannot increase the stability of the overall car steering. Therefore, an EPS active control strategy is built: combined with the above-mentioned Yaw torque control strategy, if the car has an understeer condition, the EPS system judges whether to perform the basic power assist control or the centering control. If it is the basic power assisting control mode, the EPS system assists the motor to increase the power assist torque; then reduce the assist torque. Other situations are shown in the table below:

Table 2 EPS Active Control Allocation Table

(1) Low-speed J-type test. Test conditions: vehicle speed 10km/h, steering wheel angle 45 degrees, road surface adhesion coefficient 0.85. Figures 3-5 show the vehicle trajectory, yaw rate and lateral acceleration respectively.

It can be seen from Figure 3 that the car with steering stability control has a smaller turning radius than the car without stability control, and the maximum turning radius difference is 4.2m. The experimental results show that the electric vehicle with steering stability control at low speed has a smaller turning radius, and the control effect is good in low-speed cornering conditions.

It can be seen from Figure 4 that the car with steering stability control at low speed has a smaller steady-state yaw rate than the car without stability control; the reaction time is shorter, indicating that the steering response is fast and timely; the overshoot is smaller, indicating that The execution error is small; the control effect is better.

It can be seen from Figure 5 that the vehicle with steering stability control at low speed has greater steady-state lateral acceleration than the vehicle without stability control; the reaction time is shorter, indicating that the steering response is quick and timely; the overshoot is smaller , indicating that the execution error is small; the control effect is better.

(2) High-speed J-type test. Test conditions: vehicle speed 80km/h, steering wheel angle 45 degrees, road surface adhesion coefficient 0.85. As shown in Figure 6-8, the vehicle's driving trajectory, yaw rate signal and lateral acceleration signal are respectively.

It can be seen from Figure 5 that the car with steering stability control has a larger turning radius than the car without stability control, and the maximum turning radius difference is 24.6m. The experimental results show that the electric vehicle with steering stability control at high speed has a larger turning radius, and the control has understeer characteristics under high-speed cornering conditions, and the steering is more stable.

It can be seen from Figure 6 that the car with steering stability control has a smaller steady-state yaw rate than the car without stability control; the reaction time is shorter, indicating that the steering response is fast and timely; the overshoot is smaller, indicating that the implementation error Small; better control effect.

It can be seen from Figure 7 that the vehicle with steering stability control has smaller steady-state lateral acceleration than the vehicle without stability control; the reaction time is shorter, indicating that the steering response is fast and timely; the overshoot is smaller, indicating that The execution error is small; the control effect is better.

Although the embodiment of the present invention has been disclosed as above, it is not limited to the use listed in the specification and implementation, it can be applied to various fields suitable for the present invention, and it can be easily understood by those skilled in the art Therefore, the invention is not limited to the specific details and examples shown and described herein without departing from the general concept defined by the claims and their equivalents.

Claims (10)

1. a kind of steering stability method based on four motorized wheels electric car characterized by comprising

It determines yaw moment control plan and driving force dispensing controller, according to motor turning operating condition, distributes driving moment in real time；

It determines EPS active control strategies, when understeer or oversteering operating condition occurs in automobile, is helped according to control model adjusting Power torque.

2. the steering stability method according to claim 1 based on four motorized wheels electric car, which is characterized in that

The driving force dispensing controller takes PID control.

3. the steering stability method according to claim 2 based on four motorized wheels electric car, which is characterized in that The driving force dispensing controller specifically includes:

Input is total driving moment and additional yaw moment, by the difference of reference yaw velocity and actually measured yaw velocity It is worth the compensation yaw moment as output, and is symmetrically distributed in left and right driving motor in the form of torque.

4. the steering stability method according to claim 3 based on four motorized wheels electric car, which is characterized in that

It is described to be determined with reference to yaw velocity according to linear two degrees of freedom single track model.

5. the steering stability method according to claim 4 based on four motorized wheels electric car, which is characterized in that The linear two degrees of freedom single track model meets:

In formula: β is side slip angle；R is yaw velocity；C_f、C_rRespectively front and rear wheel cornering stiffness；I_ZIt is automobile around z-axis Rotary inertia；I_f、I_rRespectively distance of the axle to mass center；δ_fFor front wheel angle；M_tRestoring moment when squeegee action, M_zFor wheel aligning torque, V is Vehicle Speed.

6. the steering stability method according to claim 5 based on four motorized wheels electric car, which is characterized in that The calculation method with reference to yaw velocity are as follows:

In formula, τ_eFor time constant, δ_swFor steering wheel angle, r_ssFor yaw velocity steady-state gain.

7. the steering stability method according to claim 6 based on four motorized wheels electric car, which is characterized in that The determination method of yaw velocity steady-state gain are as follows:

Automobile meets when at the uniform velocity turning toIt is then available according to linear two degrees of freedom single track model:

8. the steering stability method according to claim 7 based on four motorized wheels electric car, which is characterized in that When the side acceleration of steering is greater than 0.4g, need to meet with reference to yaw velocity:

In formula, μ is tire and ground attaching coefficient, and g is acceleration of gravity.

9. the steering stability method according to claim 8 based on four motorized wheels electric car, which is characterized in that Further include:

As additional yaw moment Δ M=0, driving force distribution meets according to mean allocation:

In formula, F_L、F_RRespectively left and right side front wheel drive force；f_L、f_RRespectively left and right side rear wheel drive force；F_AlwaysIt is always driven for vehicle Power；b_f、b_rRespectively front-wheel, hind axle away from.

10. the steering stability method according to claim 1 based on four motorized wheels electric car, feature exist In the specific adjusting method of booster torquemoment are as follows:

When understeer, judge that control model for basic Power assisted control mode, then increases booster torquemoment；Judge that control model is Rotary transform tensor mode, then reduce booster torquemoment；

When oversteering, judge that control model for basic Power assisted control mode, then reduces booster torquemoment；Judge that control model is Rotary transform tensor mode, then increase booster torquemoment.