CN110371106B - Steering stability method based on four-wheel independent drive electric automobile - Google Patents
Steering stability method based on four-wheel independent drive electric automobile Download PDFInfo
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- B—PERFORMING OPERATIONS; TRANSPORTING
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Abstract
The invention discloses a steering stability method based on a four-wheel independent drive electric automobile, which comprises the following steps: determining a yaw moment control strategy and a driving force distribution controller, and distributing driving moment in real time according to the steering working condition of the automobile; and determining an EPS active control strategy, and adjusting the power-assisted torque according to a control mode when the automobile has an understeer or oversteer working condition. The method for stabilizing the steering of the four-wheel independent drive electric automobile reduces instability of differential power-assisted steering and EPS (electric power steering) to automobile steering by adopting a direct yaw moment control method, thereby ensuring stable driving of the automobile.
Description
Technical Field
The invention relates to the technical field of automobile operation stability simulation, in particular to a steering stability method based on a four-wheel independent drive electric automobile.
Background
The invention and the popularization of the electric automobile accord with the theme of peaceful development of the current human society and the theme of creating a resource-saving and environment-friendly society in the sustainable development strategy of China, and the four-wheel independent drive electric automobile technology has the advantages of independent controllability of four-wheel driving moment and flexible operation, so that the comprehensive performance and corresponding products are quite competitive in the field of electric automobiles. When the four-wheel independent drive electric automobile is at a low speed, the turning radius is smaller, and the four-wheel independent drive electric automobile is more flexible and portable; at high speed, the return to positive is good, and the road feel is certain. This is a basic requirement of electric vehicles for steering. However, a large number of simulation tests show that when the electric automobile is steered under the common control of the EPS system and the differential power steering, no matter the electric automobile is at high speed or low speed, compared with the automobile without any control, the electric automobile has smaller turning radius, proper excessive steering characteristic and more flexible control characteristic. Therefore, a stability control strategy is added, so that when the electric automobile is steered at low speed, the electric automobile keeps labor-saving, flexible and certain over-steering characteristics; and when the automobile is steered at a high speed, certain road feel and understeer characteristics are kept, so that the automobile has better stability when being steered, particularly when being steered at a high speed, and the danger is avoided.
When the four-wheel independent drive electric automobile is at a low speed, the turning radius is smaller, and the four-wheel independent drive electric automobile is more flexible and portable; at high speed, the return to positive is good, and the road feel is certain. This is a basic requirement of electric vehicles for steering. When the electric automobile is steered under the common control of the EPS system and the differential power steering, no matter the speed is high or low, compared with the automobile without any control, the electric automobile has smaller turning radius, proper excessive steering characteristic and more flexible control characteristic. Therefore, a stability control strategy is required to be added, so that the electric automobile can keep labor-saving, flexible and certain over-steering characteristics during low-speed steering; and when the automobile is steered at a high speed, certain road feel and understeer characteristics are kept, so that the automobile has better stability when being steered, particularly when being steered at a high speed, and the danger is avoided.
Disclosure of Invention
The invention provides a steering stability method based on a four-wheel independent drive electric automobile for solving the technical defects at present, and the instability of differential power-assisted steering and EPS (electric power steering) on automobile steering is reduced by adopting a direct yaw moment control method, so that the stable running of an automobile is ensured.
The technical scheme provided by the invention is as follows: a steering stability method based on a four-wheel independent drive electric automobile comprises the following steps:
determining a yaw moment control strategy and a driving force distribution controller, and distributing driving moment in real time according to the steering working condition of the automobile;
and determining an EPS active control strategy, and adjusting the power-assisted torque according to a control mode when the automobile has an understeer or oversteer working condition.
Preferably, the drive force distribution controller adopts PID control.
Preferably, the drive force distribution controller specifically includes:
the input is total driving moment and additional yaw moment, the difference value of reference yaw velocity and actually measured yaw velocity is used as output compensation yaw moment, and the compensation yaw moment is symmetrically distributed to the left driving motor and the right driving motor in the form of torque.
Preferably, the reference yaw rate is determined according to a linear two-degree-of-freedom monorail model.
Preferably, the linear two-degree-of-freedom monorail model satisfies the following conditions:
in the formula: beta is the centroid slip angle; r is a yaw angular velocity; cf、CrFront and rear wheel cornering stiffness, respectively; i isZThe moment of inertia of the automobile around the z axis; i isf、IrThe distances from the front shaft and the rear shaft to the center of mass respectively; deltafIs a front wheel corner; mtRestoring moment of tyre deformation, MzThe wheel aligning moment is V, and the vehicle running speed is V.
Preferably, the reference yaw rate (the angular velocity of the vehicle rotating about the vertical axis in the body coordinate system) is:
in the formula, τeIs a time constant, δswTo the steering wheel angle, rssIs the yaw-rate steady-state gain.
Preferably, the yaw-rate steady-state gain is determined by:
when the automobile turns at a constant speed
Then according to the linear two-degree-of-freedom single-rail model, the following can be obtained:
preferably, when the steering is under a condition of large lateral acceleration, generally referring to a lateral acceleration range in which the lateral acceleration is greater than 0.4g while ensuring normal driving, the reference yaw rate is satisfied:
where μ is a coefficient of adhesion between the tire and the ground, and g is a gravitational acceleration.
Preferably, the method further comprises the following steps:
when the additional yaw moment Δ M is 0, the driving force distribution satisfies, in terms of the average distribution:
in the formula, FL、FRThe left front wheel driving force and the right front wheel driving force are respectively; f. ofL、fRThe left and right rear wheel driving forces are respectively; fGeneral assemblyIs the total driving force of the vehicle; bf、brRespectively the wheelbases of the front wheel and the rear wheel.
Preferably, the specific adjusting method of the boosting torque comprises the following steps:
when the steering is understeer, judging that the control mode is a basic power-assisted control mode, and increasing power-assisted torque; if the control mode is judged to be the return control mode, the power-assisted torque is reduced;
when the steering is oversteered, judging that the control mode is the basic power-assisted control mode, and reducing the power-assisted torque; and if the control mode is judged to be the return control mode, the power-assisted torque is increased.
The invention has the following beneficial effects: the invention provides a steering stability method based on a four-wheel independent drive electric automobile, which adopts a direct yaw moment control method to reduce instability of differential power-assisted steering and EPS (electric power steering) on automobile steering, thereby ensuring stable driving of the automobile.
Drawings
FIG. 1 is a linear two-degree-of-freedom monorail model of the present invention.
Fig. 2 is a drive force distribution controller model of the invention.
FIG. 3 is a vehicle trajectory for a vehicle with steering stability control and a vehicle without stability control according to the present invention.
FIG. 4 is a plot of yaw rate versus time for a vehicle with steering stability control and a vehicle without stability control in accordance with the present invention.
FIG. 5 is a graph of lateral acceleration versus time for an automobile with steering stability control and an automobile without stability control in accordance with the present invention.
FIG. 6 is a running trace of an automobile with steering stability control and an automobile without stability control according to the present invention.
FIG. 7 is a plot of yaw rate versus time for a vehicle with steering stability control and a vehicle without stability control in accordance with the present invention.
FIG. 8 is a graph of lateral acceleration versus time for an automobile with steering stability control and an automobile without stability control in accordance with the present invention.
Detailed Description
The present invention is further described in detail below with reference to the attached drawings so that those skilled in the art can implement the invention by referring to the description text.
When the automobile turns to drive, the lateral force can enable the automobile to incline outwards. The direct yaw moment control method is adopted to reduce the instability of the differential power-assisted steering and EPS to the automobile steering, thereby ensuring the stable running of the automobile. Specifically, the difference between the reference yaw rate and the actually measured yaw rate is used as the output compensation yaw moment, and finally, the compensation yaw moment is symmetrically distributed to the left driving motor and the right driving motor in the form of torque.
Generally, the wheel angle is linearly related to the steering wheel angle input, and the reference yaw rate of the direct yaw moment control may be determined according to a linear two-degree-of-freedom single-rail (Bicycle) model. When the automobile is turned at a low speed, the curvature of the motion track of the automobile is as follows:
when steering at low speeds (1-2 gear driving, usually less than 30 km/h), the curvature of the trajectory of the vehicle is:
wherein R is a steering radius, V is a vehicle speed, and R is a yaw rate;
when the vehicle steering angle is small, there are:
in the formula, deltaACalled Ackerman's angle, where l is the distance between the front and rear axes, and the reference transverse angular velocity omegadThe following were used:
fig. 1 shows a linear two-degree-of-freedom monorail model. The model is a two-degree-of-freedom automobile model which is supported on the ground by a front tire and a rear tire with lateral elasticity and has lateral and yaw motions.
The two-degree-of-freedom vehicle motion state equation is established as follows:
in the formula: beta is the centroid slip angle; r is a yaw angular velocity; cf、CrFront and rear wheel cornering stiffness, respectively; i isZThe moment of inertia of the automobile around the z axis; i isf、IrThe distances from the front shaft and the rear shaft to the center of mass respectively; deltafIs a front wheel corner; mtRestoring moment of tyre deformation, MzThe wheel aligning moment is V, and the vehicle running speed is V.
When the automobile turns at a constant speed, the automobile is provided with
The process is carried out in the above formula,
obtaining a yaw rate steady-state gain:
then, the reference yaw rate (the angular velocity of the vehicle rotating about the vertical axis in the body coordinate system) is defined as follows:
in the formula, τeIs a time constant, δswIs the steering wheel angle. Further, considering that the tire cornering ability limit is exceeded in the case of a large lateral acceleration, the reference yaw rate is also constrained by the following equation:
wherein μ is a tire-to-ground adhesion coefficient.
The driving force distribution controller is designed for reasonably distributing the driving forces of four driving wheels of the four-wheel independent driving electric automobile. The input of the driving force distribution controller is total driving moment and additional yaw moment, and the driving moment of the four in-wheel motors is distributed according to the control target.
The direction of the value delta M of the additional yaw moment is specified to be positive when the electric automobile turns left, and the direction delta M of the additional yaw moment is specified to be negative when the electric automobile turns right; the steering angle δ of the steering wheel when the automobile turns left is positive, and the steering angle δ of the steering wheel when the automobile turns right is negative.
The driving force distribution control strategy is:
when Δ M is 0, the vehicle is in neutral steering, no additional control is required, and the driving force distribution is in accordance with the average distribution principle:
in the formula: fL、FRThe left front wheel driving force and the right front wheel driving force are respectively; f. ofL、fRThe left and right rear wheel driving forces are respectively; fGeneral assemblyIs the total driving force of the vehicle; bf、brRespectively the wheelbases of the front wheel and the rear wheel.
For example: when Δ M > 0; when δ > 0, the driving force distribution controller needs to increase the yaw moment by Δ M at the time of left turn if an understeer condition of the vehicle occurs.
The rest are shown in table 1:
TABLE 1 drive Torque distribution Table
The driving force distribution controller model is shown in fig. 2. M in the model1、M2The torque of the front wheel and the torque of the rear wheel are respectively expressed, input into an Embedded control strategy, and respectively output the torque of the left wheel 1, the torque of the left wheel 2, the torque of the right wheel 3 and the torque of the right wheel 4 to form differential assistance.
Traditional electric power assisted steering system has mainly solved the light nature of operation when car low-speed turns to and the road feel problem when turning to at a high speed, can not increase the stationarity that turns to whole car, consequently builds EPS active control strategy: combining the yaw moment control strategy, if the automobile has understeer working condition, the EPS system judges whether to perform basic power-assisted control or return control, and if the automobile is in a basic power-assisted control mode, the power-assisted motor of the EPS system increases power-assisted torque; if the control is the return control, the assist torque is reduced. Other cases are shown in the following table:
table 2 EPS active control allocation table
(1) Low speed type J test. Test conditions are as follows: the speed of the vehicle is 10km/h, the steering wheel angle is 45 degrees, and the road adhesion coefficient is 0.85. Fig. 3 to 5 show the vehicle running track, yaw rate and lateral acceleration, respectively.
It can be seen from fig. 3 that the vehicle with steering stability control has a smaller steering radius than the vehicle without stability control, the maximum steering radius differing by 4.2 m. Experimental results show that the electric automobile with steering stability control at low speed has smaller turning radius and has good control effect under the working condition of low-speed turning.
As can be seen from FIG. 4, the vehicle with steering stability control at low speed has a lower steady state yaw rate than the vehicle without stability control; the reaction time is shorter, which shows that the steering response is quick and timely; the overshoot is smaller, which indicates that the execution error is small; the control effect is better.
As can be seen in FIG. 5, the vehicle with steering stability control at low speed has a greater steady state lateral acceleration than the vehicle without stability control; the reaction time is shorter, which shows that the steering response is quick and timely; the overshoot is smaller, which indicates that the execution error is small; the control effect is better.
(2) High speed type J test. Test conditions are as follows: the vehicle speed is 80km/h, the steering wheel angle is 45 degrees, and the road adhesion coefficient is 0.85. Fig. 6-8 show the driving path, the yaw rate signal and the lateral acceleration signal of the vehicle, respectively.
As can be seen from fig. 5, the vehicle with steering stability control has a larger steering radius than the vehicle without stability control, with a difference of 24.6m between the maximum steering radii. Experimental results show that the electric automobile with steering stability control at high speed has larger turning radius, and the control under the high-speed turning working condition has understeer characteristic, so that the steering is more stable.
As can be seen from FIG. 6, the vehicle with steering stability control has a lower steady state yaw rate than the vehicle without stability control; the reaction time is shorter, which shows that the steering response is quick and timely; the overshoot is smaller, which indicates that the execution error is small; the control effect is better.
As can be seen in FIG. 7, the vehicle with steering stability control has a lower steady state lateral acceleration than the vehicle without stability control; the reaction time is shorter, which shows that the steering response is quick and timely; the overshoot is smaller, which indicates that the execution error is small; the control effect is better.
While embodiments of the invention have been described above, it is not limited to the applications set forth in the description and the embodiments, which are fully applicable in various fields of endeavor to which the invention pertains, and further modifications may readily be made by those skilled in the art, it being understood that the invention is not limited to the details shown and described herein without departing from the general concept defined by the appended claims and their equivalents.
Claims (4)
1. A steering stability method based on a four-wheel independent drive electric automobile is characterized by comprising the following steps:
determining a yaw moment control strategy and a driving force distribution controller, and distributing driving moment in real time according to the steering working condition of the automobile;
determining an EPS active control strategy, and adjusting the power-assisted torque according to a control mode when the automobile has an understeer or oversteer working condition;
the drive force distribution controller adopts PID control;
the drive force distribution controller specifically includes:
the input is total driving moment and additional yaw moment, the difference value of the reference yaw velocity and the actually measured yaw velocity is used as the output compensation yaw moment, and the compensation yaw moment is symmetrically distributed on the left driving motor and the right driving motor in a torque mode;
the reference yaw angular velocity is determined according to a linear two-degree-of-freedom single-track model;
the linear two-degree-of-freedom single-rail model meets the following requirements:
in the formula: beta is the centroid slip angle; r is a yaw angular velocity; cf、CrFront and rear wheel cornering stiffness, respectively; i isZThe moment of inertia of the automobile around the z axis; i isf、IrThe distances from the front shaft and the rear shaft to the center of mass respectively; deltafIs a front wheel corner; mtRestoring moment of tyre deformation, MzThe wheel aligning moment is adopted, and V is the running speed of the vehicle;
the calculation method of the reference yaw rate comprises the following steps:
in the formula, τeIs a time constant, δswTo the steering wheel angle, rssIs the yaw rate steady state gain;
the method for determining the steady-state gain of the yaw rate comprises the following steps:
when the automobile turns at a constant speed
Then according to the linear two-degree-of-freedom single-rail model, the following can be obtained:
2. the steering stability method based on four-wheel independent drive electric vehicle according to claim 1, wherein when the lateral acceleration of the steering is greater than 0.4g, the reference yaw rate is satisfied:
where μ is a coefficient of adhesion between the tire and the ground, and g is a gravitational acceleration.
3. The steering stability method based on a four-wheel independent drive electric vehicle according to claim 2, further comprising:
when the additional yaw moment Δ M is 0, the driving force distribution satisfies, in terms of the average distribution:
in the formula, FL、FRThe left front wheel driving force and the right front wheel driving force are respectively; f. ofL、fRThe left and right rear wheel driving forces are respectively; fGeneral assemblyIs the total driving force of the vehicle; bf、brAre respectively asFront wheel and rear wheel wheelbase.
4. The steering stability method based on the four-wheel independent drive electric vehicle as claimed in claim 1, wherein the specific adjusting method of the power-assisted torque is as follows:
when the steering is understeer, judging that the control mode is a basic power-assisted control mode, and increasing power-assisted torque; if the control mode is judged to be the return control mode, the power-assisted torque is reduced;
when the steering is oversteered, judging that the control mode is the basic power-assisted control mode, and reducing the power-assisted torque; and if the control mode is judged to be the return control mode, the power-assisted torque is increased.
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| CN113682309B (en) * | 2021-08-31 | 2024-03-26 | 中国第一汽车股份有限公司 | Yaw control method of timely four-wheel drive system, vehicle and storage medium |
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| CN105015363B (en) * | 2015-07-23 | 2017-03-29 | 江苏大学 | A kind of distributed driving automotive control system and method based on hierarchical coordinative |
| CN107813869A (en) * | 2017-09-20 | 2018-03-20 | 江苏理工学院 | A kind of compound EPS control systems and method for distributed-driving electric automobile |
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