CN118907073B

CN118907073B - An electro-hydraulic composite steering control method and system for electric wheel vehicles under extreme working conditions

Info

Publication number: CN118907073B
Application number: CN202411241866.1A
Authority: CN
Inventors: 张厚忠; 司卫健; 徐兴; 孙晓强; 王峰
Original assignee: Jiangsu University
Current assignee: Jiangsu University
Priority date: 2024-09-05
Filing date: 2024-09-05
Publication date: 2025-10-03
Anticipated expiration: 2044-09-05
Also published as: CN118907073A

Abstract

The invention discloses an electrohydraulic composite steering control method and system of an electric wheel automobile under a limiting working condition, wherein the method comprises the following steps of S1, judging whether the automobile is in a unstable state, S2, selecting an automobile driving control mode, S3, electrohydraulic composite coordination distribution of four wheels of the automobile, S4, obtaining optimal motor driving force and hydraulic braking force, S5, obtaining the optimal motor driving force and the optimal hydraulic braking force according to the current steering working condition of the automobile, and S4, and selecting a corresponding electrohydraulic composite coordination control mode. The method comprises the steps of providing coordinated control of hydraulic braking and motor driving under a limit working condition, respectively controlling different wheels, mutually compensating, accurately and timely controlling the stability of a vehicle body under the limit working condition, realizing combination of different control modes, maximally realizing stability control of electrohydraulic coupling optimization, judging the working condition of an automobile according to yaw rate, steering wheel angle and steering wheel angular velocity, and realizing coupling optimization of electrohydraulic compound ESP by regulating and controlling hydraulic braking and motor driving.

Description

Electric-hydraulic composite steering control method and system for electric-wheel automobile under limit working condition

Technical Field

The invention relates to an electro-hydraulic composite steering control method and system for an electric wheel automobile under a limiting working condition, and belongs to the technical field of electric wheel automobiles.

Background

An electric-wheel automobile is an emerging vehicle, which drives wheels through an electric motor, and has higher energy utilization efficiency and remarkable environmental protection advantages compared with a traditional internal combustion engine automobile. The electric wheel automobile is driven by using the motor to replace the traditional internal combustion engine, so that the acceleration performance of the automobile is improved, and the exhaust emission and noise pollution are reduced. However, in the control system of the electric wheel car, especially in the high dynamic working condition, how to effectively integrate the motor control and the hydraulic braking system to improve the operability and the safety of the car is still an important research topic.

In an electric-wheel vehicle, the electronic stability program ESP, electronic Stability Program plays a key role. The electrohydraulic compound ESP is a advanced electronic stability control system which combines the characteristics of motor control and hydraulic braking to realize more accurate dynamic control of the vehicle. The system accurately adjusts braking force, torque distribution and the like of the vehicle according to the needs by monitoring dynamic parameters of the vehicle, such as yaw rate, lateral acceleration, vehicle speed and the like in real time, so that the operability and stability of the vehicle are improved.

However, under certain limit conditions, such as abrupt steering during high-speed running, emergency avoidance on wet road surfaces, etc., the motor control characteristics of the electric-wheel vehicle, while responding quickly and with precise control, may be insufficient in coping with the additional yaw moment demand. This is because the braking or driving capability of the motor cannot meet the demands under all conditions. The control characteristic of the hydraulic braking system can output larger braking torque at any vehicle speed, but the response speed is slower, and the control accuracy of the braking torque is poorer. Under the condition, a single control strategy cannot meet actual demands, and electrohydraulic characteristics cannot be complemented, and simultaneously electrohydraulic compound ESP coordination control and electrohydraulic decoupling adjustment are met.

Disclosure of Invention

Aiming at the defects in the prior art, the invention provides the electrohydraulic composite steering control method and the electrohydraulic composite steering control system for the electric wheel automobile under the limit working condition, which utilize the characteristic of quick response of motor braking and are based on electrohydraulic characteristic complementation in the electric wheel automobile so as to realize more accurate and efficient braking force and driving force distribution and realize electrohydraulic coupling optimization during electrohydraulic composite braking.

The technical scheme is that the electro-hydraulic composite steering control method of the electric wheel automobile under the limit working condition comprises the following steps:

S1, judging whether the automobile is in an unstable state, namely calculating a centroid side deviation angle deviation through an expected centroid side deviation angle and an actual centroid side deviation angle, judging whether the difference is larger than a maximum centroid side deviation angle threshold value specified by the automobile, if so, judging the current state of the automobile through calculating a yaw rate deviation through an expected yaw rate and an actual yaw rate, if so, judging the current state of the automobile, if the yaw rate deviation is smaller than the maximum yaw rate threshold value for keeping the automobile stable, then the automobile is in a stable state, and if the yaw rate deviation is larger than the maximum yaw rate threshold value for keeping the automobile stable, then the automobile is in an unstable state;

S2, selecting an automobile driving control mode, namely judging whether the driving force of the pure motor is smaller than the peak driving force of the actually set hub motor or not by acquiring the driving force of the pure motor and acquiring four-wheel additional yaw moment distribution values under the driving control of the pure motor, and selecting the hub motor to perform the driving control of the pure motor if the driving force is smaller than the peak driving force of the actually set hub motor;

S3, electro-hydraulic compound coordination distribution, namely acquiring braking force and four-wheel additional yaw moment distribution values thereof during pure hydraulic braking, carrying out electro-hydraulic compound coordination distribution on four wheels of the automobile by combining the driving force of the pure motor and the four-wheel additional yaw moment distribution values thereof in S2, and entering S4 after the distribution is finished;

S4, optimizing the distribution result of the S3 to obtain optimal motor driving force and hydraulic braking force;

and S5, selecting a corresponding electrohydraulic composite coordination control mode according to the current steering working condition of the automobile and the optimal motor driving force and hydraulic braking force obtained in the step S4.

The preferred method comprises the following steps of:

obtaining a desired centroid slip angle beta _d:

designing the expected centroid slip angle as the centroid slip angle estimation method based on Kalman filtering, and when the vehicle is in steady-state steering, the inverse of the expected centroid slip angle is expected Obtaining a desired centroid slip angle calculation:

wherein V _e is the speed of the vehicle, θ is the steering wheel angle, m is the mass of the vehicle, K _r、K_f is the cornering stiffness of the rear and front wheels, θ is the steering wheel angle, l _f is the distance from the centroid to the front axle, and l _r is the distance from the centroid to the rear axle;

Acquiring an actual centroid slip angle beta _D according to the turning angle of the automobile steering and the current vehicle speed V _e:

Where β _D is the actual centroid slip angle, V _Y is the lateral speed at the vehicle centroid, and V _X is the longitudinal speed at the vehicle centroid;

Obtaining centroid slip angle deviation delta beta:

Δβ=β_d-β_D。

Preferably, the calculation process of the yaw rate deviation specifically includes:

acquiring a desired yaw rate:

the yaw rate is derived from the dynamic formula of the body movement, and the expected value is mainly the yaw rate expected when the vehicle enters steady-state steering, and the yaw rate acceleration at this time is calculated The desired yaw rate is found as:

Wherein L is the wheelbase, Is a stability coefficient which is used for reflecting the steady state response of the automobile, K _f、K_f is the cornering stiffness of the rear and front wheels, theta is the steering wheel corner, l _f is the distance from the mass center to the front axle, and l _r is the distance from the mass center to the rear axle;

Acquiring a yaw rate deviation deltar:

Δr=r_d-r_D

Where r _D is the actual yaw rate.

Preferably, the calculation process of the vehicle speed V _e specifically includes:

When the automobile is in a steering working condition, calculating the linear speeds of four wheels:

Wherein omega _fl、ω_fr、ω_rl、ω_rr represents the angular speeds of four wheels;

vehicle speed represents:

the step S2 further comprises the step of acquiring the total additional yaw moment and the total longitudinal force of the automobile, wherein the total additional yaw moment and the total longitudinal force are specifically as follows:

acquiring total additional yaw moment:

The slip plane s is defined by the yaw rate and centroid camber angle in combination:

s=ε(r_p-r_d)+ξ(β_D-β_d)

Wherein r _D refers to the actual yaw rate, beta _D refers to the actual centroid camber angle, epsilon and zeta are assigned weight coefficients and epsilon + epsilon=1, and the additional yaw moment is obtained by differentiation and combination with the whole vehicle motion equation:

Wherein (-asgns-bs) is a set exponential approach law, the approach law is used for ensuring that the approach motion of a system motion point tends to a sliding film plane, a is more than 0, b is more than 0, s is a defined sliding mode surface, K _r、K_f is the cornering stiffness of a rear wheel and a front wheel, θ is the steering wheel angle, l _f is the distance from a centroid to a front shaft, l _r is the distance from the centroid to the rear shaft, and A is a stability increment value, wherein the expression is as follows:

Wherein I is rotational inertia, K _r、K_f is cornering stiffness of a rear wheel and a front wheel, theta is steering wheel corner, l _f is distance from a centroid to a front shaft, and l _r is distance from the centroid to the rear shaft; is the desired yaw acceleration; Is a differential value of the actual centroid slip angle and the desired centroid slip angle.

Acquiring a total longitudinal force Σf (t):

The target vehicle speed and the reference vehicle speed are used as inputs, and a PID method is selected to realize the control rule:

Wherein K _p is a proportional coefficient, K _t is an integral coefficient, and K _D is a differential coefficient.

The preferred option, the driving force of the pure motor and the four-wheel additional yaw moment distribution value thereof are obtained in the step S2, specifically:

and obtaining the driving force of the pure electric motor:

for motor driving, torque differential distribution is performed to enable four hubs to generate ideal additional yaw moment, so that the running stability of the vehicle is maintained, and the driving force values of four wheels are as follows:

wherein h _g is the overall vehicle centroid height, a _x is the longitudinal acceleration, Σf _x represents the sum of all driving forces, Σm _Z represents the sum of all yaw moments, and B represents the variance obtained from the centroid slip angle of the extended kalman filter, expressed as:

I _z is expressed as moment of inertia about the Z-axis, F _mfl left front wheel motor drive, F _mfr right front wheel motor drive, F _mrl left rear wheel motor drive, F _mrr right rear wheel motor drive;

Acquiring a pure motor driving four-wheel additional yaw moment distribution value:

M_zn＝F_n·jl_n

M _zn is respectively represented as the additional yaw moment of the four in-wheel motors, and F _n is respectively represented as F _mfl、F_mfr、F_mfr、F_mrr;jl_n as the distance from the action point of the four in-wheel motors to the mass center of the automobile.

Preferably, the step S3 specifically includes:

acquiring four-wheel additional yaw moment distribution values during pure hydraulic braking:

Wherein:

Wherein DeltaF _x、ΔF_y is the variation value of the longitudinal/lateral force of the wheel, l _x、l_y is the linear distance between the longitudinal/lateral force and the centroid point, d is the wheel track, a and b are the distance between the longitudinal/lateral force and the centroid point in the vertical direction, deltaF _x·l_x+ΔF_y·l_y is the front wheel outside the braking of the electric wheel automobile in DeltaM, The brake inner rear wheel of the electric wheel automobile is shown;

acquiring braking force during pure hydraulic braking:

in the case of purely hydraulic braking, i.e. the additional yaw moment required for active safety control of the vehicle can be provided by hydraulic braking alone, the braking force calculation formula for each wheel required for an electric wheel vehicle is as follows:

Wherein F _braken represents the braking force of four wheels, deltaM _n represents the additional yaw moment of the four wheels, R is the radius of the wheels, and p is the distance from the application point of the braking force of the wheels to the mass center of the automobile;

electro-hydraulic compound coordination distribution is carried out on four wheels of the automobile:

And (3) adding constraint conditions to the pure electric drive in the S2 and the pure hydraulic drive in the S3, namely acquiring the relation between the yaw moment and the longitudinal force and the driving force and braking force of each wheel:

wherein, the hydraulic braking force of the left front wheel of F _hfl, the hydraulic braking force of the right front wheel of F _hfr, the hydraulic braking force of the left rear wheel of F _hrl, the hydraulic braking force of the right rear wheel of F _hrr, the motor driving force of the left front wheel of F _mfl, the motor driving force of the right front wheel of F _mfr, the motor driving force of the left rear wheel of F _mrl and the motor driving force of the right rear wheel of F _mrr; wheel steering angle, d wheel track;

According to the optimal allocation algorithm of the minimum tire utilization rate, the constraint condition, namely the stability objective function, is set as the sum of squares of the minimum utilization rates of four wheels of the vehicle:

Wherein F _xi、F_yi is the longitudinal force and the lateral force of each wheel, mu _i is the road adhesion coefficient of the corresponding wheel, and F _zi is the vertical load of each wheel;

the amount of torque that can be provided by a motor is constrained by the motor external characteristics:

Wherein T _imax (v) is motor peak torque, F _mrr right front wheel motor driving force, F _mrl left rear wheel motor driving force, F _mrr right rear wheel motor driving force;

the longitudinal force is constrained by road surface adhesion conditions and vertical loads:

-μF_zi≤F_xi≤μF_zi,i=fl,fr,rl,rr。

Preferably, the step S4 specifically includes:

And (3) carrying out a quadratic programming optimization allocation method based on the allocation result in the step (S3):

According to the optimization target and the constraint condition, the standard type of the quadratic programming method is arranged as

Constraint:

Wherein, the

Wherein F _zi represents the braking and driving forces of the four wheels in the numerical direction;

where u= [ F _flF_frF_rlF_rr]^T, G is the matrix: d represents the track width;

the optimal motor driving force and hydraulic braking force in u are calculated by the above.

The preference, the electrohydraulic composite coordination control mode in the S5 specifically comprises:

when the yaw rate deviation is positive, the steering wheel angular velocity is positive, the steering wheel angle is positive, the left-turning understeer working condition is met, the left rear wheel is hydraulically braked, and meanwhile, the motor increases the driving moment of the right front wheel;

mode 2, when the yaw rate deviation is positive, the steering wheel angular speed is positive and the steering wheel angle is negative, the left front wheel is hydraulically braked under the working condition of right turning oversteer, and meanwhile, the motor increases the driving moment of the right rear wheel;

Mode 3, when the yaw rate deviation is positive, the steering wheel angular speed is negative and the steering wheel angle is negative, the left front wheel is hydraulically braked under the working condition of right turning oversteer, and the motor increases the driving moment of the right rear wheel;

Mode 4, when the yaw rate deviation is negative, the steering wheel angular velocity is positive and the steering wheel angle is positive, the steering wheel is in a left turning oversteer working condition, the right front wheel is hydraulically braked, and meanwhile, the motor increases the driving moment of the left rear wheel;

when the yaw rate deviation is negative, the steering wheel angular velocity is positive and the steering wheel angle is zero, the left-turning oversteer working condition is adopted, the right front wheel is hydraulically braked, and meanwhile, the motor increases the driving moment of the left rear wheel;

Mode 6, when the yaw rate deviation is negative, the steering wheel angular velocity is negative and the steering wheel angle is positive, the left-turning oversteer working condition is adopted, the right front wheel is hydraulically braked, and meanwhile, the motor increases the driving moment of the left rear wheel;

mode 7, when the yaw rate deviation is negative, the steering wheel angular speed is negative and the steering wheel angle is negative, the steering wheel is in a right-turning understeer working condition, the right rear wheel is hydraulically braked, and meanwhile, the motor increases the driving moment of the left front wheel;

mode 8, when the yaw rate deviation is positive, the steering wheel angular speed is positive and the steering wheel angle is zero, the left-turn understeer working condition is adopted, the left rear wheel is hydraulically braked, and meanwhile, the motor increases the driving moment of the right front wheel;

Mode 9, when the yaw rate deviation is positive, the steering wheel angular velocity is negative and the steering wheel angle is zero, the steering wheel is in a right-turning oversteer condition, the left front wheel is hydraulically braked, and meanwhile, the motor increases the right rear wheel driving moment;

And 10, when the yaw rate deviation is negative, the steering wheel angular speed is negative and the steering wheel angle is zero, the steering wheel is in a right-turning understeer working condition, the right rear wheel is hydraulically braked, and meanwhile, the motor increases the driving moment of the left front wheel.

The system for realizing the electrohydraulic composite steering control method of the electric wheel automobile under the limit working condition comprises a hub motor, a wheel speed sensor, a pressure sensor, a hydraulic control unit, a motor control unit, a whole vehicle control unit ECU, a steering wheel corner sensor, a lateral acceleration sensor, a yaw rate sensor and a brake master cylinder, wherein the hub motor, the wheel speed sensor and the pressure sensor are respectively arranged on a hub, the hydraulic control unit is connected with the brake master cylinder, the motor control unit is connected with the hub motor, and the wheel speed sensor, the pressure sensor, the steering wheel corner sensor, the lateral acceleration sensor, the yaw rate sensor, the brake master cylinder and the whole vehicle control unit ECU are respectively in signal connection.

The invention has the advantages that through the coordinated control of hydraulic braking and motor driving under the limit working condition, different wheels are respectively controlled to compensate each other, the vehicle body stability under the limit working condition is accurately and timely controlled, the combination of different control modes is realized, the stability control of electrohydraulic coupling optimization is maximally realized, meanwhile, the working condition of an automobile is judged according to the yaw rate, the steering wheel angle and the steering wheel angular velocity, and the coupling optimization of the electrohydraulic compound ESP is realized by regulating and controlling the hydraulic braking and the motor driving.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings that are required to be used in the embodiments or the description of the prior art will be briefly described below, and it is obvious that the drawings in the following description are only embodiments of the present invention, and that other drawings can be obtained according to the provided drawings without inventive effort for a person skilled in the art.

FIG. 1 is a flow chart of the method of the present invention;

FIG. 2 is a wheel drive/brake force distribution diagram for a left-turn understeer;

FIG. 3 is a wheel drive/brake force distribution diagram for a left turn oversteer;

FIG. 4 is a graph of wheel drive/brake force distribution for right-turn understeer;

FIG. 5 is a wheel drive/brake force distribution diagram for right turn oversteer;

FIG. 6 is a schematic diagram of a system according to the present invention.

Detailed Description

The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.

In the description of the present invention, it should be understood that the terms "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. indicate orientations or positional relationships based on the drawings, are merely for convenience in describing the present invention and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a specific orientation, be configured and operated in a specific orientation, and thus should not be construed as limiting the present invention.

In the present invention, unless expressly stated or limited otherwise, a first feature "above" or "below" a second feature may include both the first and second features being in direct contact, as well as the first and second features not being in direct contact but being in contact with each other through additional features therebetween. Moreover, a first feature being "above," "over" and "on" a second feature includes the first feature being directly above and obliquely above the second feature, or simply indicating that the first feature is higher in level than the second feature. The first feature being "under", "below" and "beneath" the second feature includes the first feature being directly under and obliquely below the second feature, or simply means that the first feature is less level than the second feature.

As shown in fig. 1, the electro-hydraulic composite steering control method of the electric wheel automobile under the limit working condition comprises the following steps:

In the embodiment, the maximum centroid side deflection angle threshold value specified by the automobile is mainly determined by the automobile type, and for a common household car, the maximum centroid side deflection angle threshold value is usually 3-5 degrees, the maximum centroid side deflection angle threshold value of the sports utility car SUV is 2-4 degrees due to the fact that the car body is higher and the center of gravity is relatively higher, the centroid side deflection angle maximum threshold value of the pickup truck is 2-3 degrees, the maximum centroid side deflection angle threshold value of the large bus is 2-3 degrees, and the centroid side deflection angle maximum threshold value of the off-road car can reach 10-15 degrees;

The maximum yaw rate threshold for keeping the vehicle stable is between + -3 DEG/s and + -5 DEG/s for the passenger vehicle and between + -2 DEG/s and + -4 DEG/s for the commercial vehicle.

The calculation process of the centroid side deviation angle deviation specifically comprises the following steps:

obtaining a desired centroid slip angle beta _d:

Obtaining centroid slip angle deviation delta beta:

Δβ=β_d-β_D

the calculation process of the yaw rate deviation specifically comprises the following steps:

acquiring a desired yaw rate:

Wherein L is the wheelbase, Is a stability coefficient which is used for reflecting the steady state response of the automobile, K _r、K_f is the cornering stiffness of the rear and front wheels, theta is the steering wheel corner, l _f is the distance from the mass center to the front axle, and l _r is the distance from the mass center to the rear axle;

Acquiring a yaw rate deviation deltar:

Δr=r_d-r_D

where r _D is the actual yaw rate. Obtained directly by a yaw rate sensor.

The calculation process of the vehicle speed V _e specifically comprises the following steps:

vehicle speed represents:

The step S2 is also to acquire the total additional yaw moment and the total longitudinal force of the automobile, and specifically comprises the following steps:

acquiring total additional yaw moment:

since there is a coupling relationship between the centroid slip angle and the yaw rate, it is necessary to control these two variables in combination. Controlling only the yaw rate may cause the slip angle of the vehicle to be excessively large, and controlling only the slip angle may deviate from the desired yaw rate. Thus, the slip plane s is defined by the yaw rate and the centroid camber angle in combination:

s=ε(r_D-r_d)+ξ(β_D-β_d)

Wherein I is rotational inertia, K _r、K_f is cornering stiffness of rear and front wheels, theta is steering wheel corner, l _f is distance from mass center to front axle, and l _r is distance from mass center to rear axle; is the desired yaw acceleration; Is a differential value of the actual centroid slip angle and the desired centroid slip angle.

Acquiring a total longitudinal force Σf (t):

And S2, acquiring a pure motor driving force and four-wheel additional yaw moment distribution value thereof, wherein the method specifically comprises the following steps:

In this embodiment, when the vehicle turns left, the front and rear wheels on the left side are the inner side wheels, the front and rear wheels on the right side are the outer side wheels, and when the vehicle turns right, the front and rear wheels on the left side are the outer side wheels, and the front and rear wheels on the right side are the inner side wheels.

The equilibrium equation for force and moment is expressed as:

Wherein F _Y1 is front wheel side bias force, F _Y2 is rear wheel side bias force, m is automobile mass, a _y is automobile mass center lateral acceleration, and I _z is automobile moment of inertia around the mass center; Is yaw acceleration, l _f is the distance from the centroid to the front axis, and l _r is the distance from the centroid to the rear axis.

The electric wheel automobile driving motor generates yaw moment by changing driving moment of motors at two sides to form torque difference. In the same way, the understeer condition of the electric-wheel automobile is taken as an example, the side deflection force of the front wheels is reduced, the side deflection angles of the corresponding front wheels are reduced, the side deflection force of the rear wheels is increased, the side deflection angles of the corresponding rear wheels are increased, the understeer condition of the automobile is caused, the understeer condition of the automobile is assumed to occur when the automobile turns right, and if the driving moment of the left rear wheel is increased, the driving moment of the right rear wheel is reduced, so that an opposite yaw moment can be generated. The left front wheel can be driven by the electric wheel automobile in the same way, the understeer can be relieved, and the inner front wheel and the inner rear wheel can be driven by the oversteer relieving driving motor in the same way.

The longitudinal and lateral forces generated by driving the outer front wheels can generate a yaw moment favorable for steering the vehicle, and the longitudinal and lateral forces generated by driving the inner rear wheels can generate a yaw moment favorable for inhibiting the steering of the vehicle. Therefore, when the motor of the electric wheel automobile drives, the driving utilization rate of the outer front wheel and the inner rear wheel is obviously higher than that of the other two wheels.

The hydraulic braking system utilizes the advantages of quick and accurate motor driving torque response to overcome the defects of slow hydraulic braking response, long delay and incapability of accurately regulating and controlling braking torque, and utilizes hydraulic braking to output large braking torque at any vehicle speed to overcome the defects of smaller torque which can be output by a motor and even smaller torque which can be output at high speed.

And obtaining the driving force of the pure electric motor:

the value obtained by the above formula is required to be compared with the actual set peak driving force of the hub motor, when the value obtained by the above formula is smaller than the actual set peak driving force of the hub motor, the automobile is driven by the pure electric motor only, and forms a difference value with the driving force of the motor of the automobile under actual running, the pure electric motor driving control is carried out on the motor at the corresponding side according to the difference value, and otherwise, the electric-hydraulic compound coordination control is carried out.

When the automobile confirms to enter the limiting working condition, the value obtained through the formula is used as one of constraints of the electrohydraulic composite coordination control formula of the electric wheel automobile, and joint solution is carried out.

M_zn＝F_n·jl_n

The equilibrium equation for force and moment is expressed as:

The front wheel side deflection force is reduced, the side deflection angle of the corresponding front two wheels is reduced, the rear wheel side deflection force is increased, and the side deflection angle of the corresponding rear two wheels is increased, so that the vehicle is not steered enough. If the longitudinal force of the outer rear wheels is increased and the longitudinal force of the inner rear wheels is reduced, or the longitudinal force of the outer front wheels is increased and the longitudinal force of the inner front wheels is reduced, the two are equivalent to the opposite additional yaw moment on the automobile, and the understeer effect is relieved.

Thus, when the vehicle is under-steered, the hydraulic pressure can brake the inner front wheels and the inner rear wheels. Similarly, when the vehicle is oversteering, the hydraulic pressure may brake the outer front wheels and the outer rear wheels.

The same control effect can be achieved by hydraulically braking wheels on the same side, but the electrohydraulic compound coordination control is mainly used for researching the situation when an automobile turns, the lateral force of a tire and the longitudinal force have the same coupling relation, and the yaw moment generated by the wheels on the same side is divided into a large part and a small part. For example, when the vehicle turns left, both the longitudinal and lateral forces generated by braking the right front wheel can generate a yaw moment that inhibits the steering of the vehicle, and both the longitudinal and lateral forces generated by braking the left rear wheel can generate a yaw moment that facilitates the steering of the vehicle. Therefore, during electrohydraulic compound coordination control, the oversteer of the front wheel outside the brake can be improved, and the understeer of the rear wheel inside the brake can be better improved.

According to different steering working conditions faced by hydraulic braking and motor driving, the method for obtaining the electro-hydraulic decoupling comprises the following steps:

Control method for oversteer

When the automobile is in over-steering, the in-wheel motor of the inner rear wheel can be driven in advance because the driving motor has the advantage of quick and accurate torque response. The wheel hub motor of the outer wheel can be reduced to generate an additional yaw moment opposite to the yaw movement direction of the electric wheel automobile, and the hydraulic braking brakes the outer front wheel to generate a reverse yaw moment.

Control method for understeer

When the automobile is under-steered, the driving force of the outer front wheel hub motor can be increased first because the driving motor has the advantage of quick and accurate torque response. Meanwhile, the driving force of the hub motor of the inner wheel can be reduced, an additional yaw moment which is opposite to the yaw movement direction of the electric wheel automobile can be generated, and the inner rear wheel is braked by hydraulic braking, so that a reverse yaw moment is also generated.

Wherein:

acquiring braking force during pure hydraulic braking:

Wherein T _imax (v) is motor peak torque, F _mfr right front wheel motor driving force, F _mrl left rear wheel motor driving force, F _mrr right rear wheel motor driving force;

-μF_zi≤F_xi≤μF_zi,i=fl,fr,rl,rr。

Constraint:

Wherein, the

where u= [ F _flF_frF_rlF_rr]^T, G is the matrix: d represents the track width;

Because the automobile needs to be actively controlled, firstly, whether the automobile is in a instable state is judged, and meanwhile, the motor driving is mainly actively controlled in the whole process, and the hydraulic braking is compensation for the motor driving, so that in order to judge whether the electrohydraulic compound coordination control is needed, firstly, parameters and settings of the hub motor of the electric wheel automobile are selected, for example, the brushless direct current motor is taken as an example, and peak torque, rated rotation speed, internal resistance of an inductance winding, motor torque coefficient and motor magnetic flux constant information are regulated according to actual conditions.

The calculated total longitudinal force and the additional yaw moment and the calculated driving force of the electric-only motor are compared with the driving force provided by the peak torque of the selected wheel hub motor, if the driving force of the electric-only motor is smaller than the peak driving force of the selected wheel hub motor, the electric-hydraulic compound coordination control is performed if the driving force of the electric-only motor is larger than the peak driving force of the selected wheel hub motor.

S5, selecting a corresponding electrohydraulic composite coordination control mode according to the current steering working condition of the automobile and the optimal motor driving force and hydraulic braking force obtained in the S4:

Because the condition of response lag of the driver can occur, the angular speed of the steering wheel angle is taken into account for judging, so as to avoid the violation with the intention of the driver,

When a driver response lag occurs, the yaw rate deviation is the same as the steering wheel angle, but different from its angular velocity, meaning that the actual dynamic response of the vehicle is inconsistent with the operation expected by the driver.

The angular speed of the steering wheel is added for common judgment, and the method is mainly based on the following principles:

The judgment accuracy is improved, namely the dynamic state of the vehicle and the intention of a driver cannot be comprehensively and accurately known only by virtue of yaw rate deviation and steering wheel rotation angle. The angular velocity of the steering wheel angle can provide additional information about the rate of change of the driver's operation, thereby more accurately determining whether the behavior of the vehicle meets the driver's expectations.

Reducing false positives helps to distinguish whether the driver is intentionally slow operating or is operating untimely due to reaction delays.

Therefore, when the yaw rate deviation is positive, the steering wheel angle is positive and the steering wheel angular velocity is negative, the yaw rate deviation is negative, the steering wheel angle is negative and the steering wheel angular velocity is positive, and the electrohydraulic compound ESP control is not performed under the two working conditions.

The angular speed of the steering wheel corner is added for common judgment, so that the dynamics of the vehicle and the intention of a driver can be more comprehensively and accurately understood, the operation of a vehicle control system and the driver can be better coordinated, and the driving safety and the driving comfort are improved.

The described modes 1, 8 are expressed as the electric wheel car is in the left-turn understeer unstable state to perform the distribution operation as shown in fig. 2, the modes 2, 3, 9 may be expressed as the electric wheel car is in the right-turn oversteer unstable state to perform the distribution operation as shown in fig. 5, the modes 4, 5, 6 may be expressed as the electric wheel car is in the left-turn oversteer unstable state to perform the distribution operation as shown in fig. 3, and the modes 7, 10 may be expressed as the electric wheel car is in the right-turn understeer unstable state to perform the distribution operation as shown in fig. 4.

The pure electric motor driving control in the invention completes the pure electric motor driving control by combining the driving force and the yaw moment of the pure electric motor and the pure electric motor driving control mode, wherein the pure electric motor driving control mode comprises the following steps:

and in the mode 1, when the automobile is in left turning and understeer, the hub motor increases the driving moment of the outer side wheel.

And 2, when the automobile is in left-turn oversteer, the hub motor increases the driving moment of the inner side wheel.

And 3, when the automobile is in the right turning understeer, the hub motor increases the driving moment of the outer side wheel.

And 4, when the automobile is in right-turn oversteer, the hub motor increases the driving moment of the inner side wheel.

As shown in fig. 6, a system for realizing the electrohydraulic composite steering control method of the electric wheel automobile under the limit working condition comprises a hub motor 1, a wheel speed sensor 2, a pressure sensor 3, a hydraulic control unit 4, a motor control unit 5, a whole vehicle control unit ECU6, a steering wheel angle sensor 7, a lateral acceleration sensor 8, a yaw rate sensor 9 and a brake master cylinder 10, wherein the hub motor 1, the wheel speed sensor 2 and the pressure sensor 3 are respectively arranged on a hub, the hydraulic control unit 4 is connected with the brake master cylinder 10, the motor control unit 5 is connected with the hub motor 1, and the wheel speed sensor 2, the pressure sensor 3, the steering wheel angle sensor 7, the lateral acceleration sensor 8, the yaw rate sensor 9 and the brake master cylinder 10 are respectively in signal connection with the whole vehicle control unit ECU 6.

After the automobile enters a steering working condition and needs electrohydraulic compound coordination control, a driver can control the steering wheel according to the steering condition, but the problems of error control of the driver and corresponding retardation of the driver can also exist at the same time, so the following three quantities are analyzed.

When the automobile enters a limit working condition, after the yaw rate sensor 9 and the steering wheel angle sensor 7 transmit signals to the whole vehicle control unit ECU6, the whole vehicle control unit ECU6 compares and corresponds the measured numerical value with 10 modes in electrohydraulic compound control, and the working condition and the state of the automobile are judged to be matched in real time, so that the arrangement of hydraulic braking and motor driving is completed.

Meanwhile, the hydraulic braking is set to be braking specific to the tire, and the yaw moment generated by motor driving mainly changes the driving moment of the hub motor 1 to form a torque difference, so that when the motor drives a certain wheel, the motor on the opposite side can be correspondingly braked appropriately.

After the hydraulic control unit 4 transmits the braking signals to the hydraulic actuator through the obtained tires and the obtained values corresponding to the braking force and the driving force, the pressure sensor 3 pressurizes, maintains and decompresses the hydraulic system of the wheels at the moment, and the braking process is that when the hydraulic system brakes and pressurizes, the brake master cylinder 10 provides pressure, meanwhile, the brake fluid flows to the control valve after flowing to the hydraulic cylinder from the brake master cylinder 10 so as to generate a series of pressure to push the piston to move, so that the pressure disc generates displacement, and the whole hydraulic braking process is completed.

The motor control unit 5 transmits the received driving signals to the hub motors 1 on the wheels through the form of electric signals, the motor control unit 5 judges whether the driving force of the pure electric motor is larger than the maximum driving force which can be provided by the hub motors 1, if the driving force is smaller than the maximum driving force which can be provided by the hub motors, motor driving control is only needed, if the required yaw moment is larger than the maximum driving force which can be provided by the driving motors, electro-hydraulic compound coordination control is needed, and the distribution is needed through the obtained distribution result and the corresponding control mode.

In the present specification, each embodiment is described in a progressive manner, and each embodiment is mainly described in a different point from other embodiments, and identical and similar parts between the embodiments are all enough to refer to each other. For the device disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant points refer to the description of the method section.

The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for controlling electro-hydraulic composite steering of an electric wheel vehicle under extreme operating conditions, characterized by comprising the following steps:

S1: Determine whether the vehicle is in an unstable state: Calculate the center of mass sideslip angle deviation by using the expected center of mass sideslip angle and the actual center of mass sideslip angle, and determine whether the difference is greater than the maximum center of mass sideslip angle threshold specified for the vehicle. If so, the vehicle is in an unstable state. If less, calculate the yaw rate deviation by using the expected yaw rate and the actual yaw rate to determine the vehicle's current state. If the yaw rate deviation is less than the maximum yaw rate threshold required to maintain vehicle stability, the vehicle is in a stable state. If the yaw rate deviation is greater than the maximum yaw rate threshold required to maintain vehicle stability, the vehicle is in an unstable state.

S2: Select vehicle drive control mode: By obtaining the pure electric motor driving force and the four-wheel additional yaw torque distribution value under pure electric motor drive control, it is determined whether it is less than the actual set peak driving force of the hub motor. If it is less, the hub motor is selected for pure electric motor drive control; if it is greater, the electro-hydraulic composite coordinated control is selected and the process proceeds to S3;

The step S2 also includes obtaining the total additional yaw moment and total longitudinal force of the vehicle, specifically:

Get the total additional yaw moment:

The sliding surface is defined by the yaw rate and the center of mass roll angle :

;

in, is the actual yaw rate; It refers to the actual center of mass roll angle; 、 is the allocation weight coefficient and + =1;

is the desired sideslip angle of the center of mass; is the desired yaw rate; by differentiation and combining with the vehicle motion equation, the additional yaw moment is obtained:

+A;

in,( ) is the exponential reaching law, which is used to ensure that the moving point of the system approaches the sliding plane; a>0, b>0; s is the defined sliding surface; is the lateral stiffness of the rear and front wheels; is the steering wheel angle; is the distance from the center of mass to the front axle; is the distance from the center of mass to the rear axle; A is the stability increase value, and its expression is as follows:

;

Where I is the moment of inertia; is the lateral stiffness of the rear and front wheels; is the steering wheel angle; is the distance from the center of mass to the front axle; is the distance from the center of mass to the rear axle; is the desired yaw angular acceleration; is the differential value of the actual center of mass sideslip angle and the expected center of mass sideslip angle;

Get the total longitudinal force :

Taking the target speed and reference speed as input, the PID method is used to implement the control law as follows:

;

in: is the proportionality coefficient; is the integration coefficient; is the differential coefficient;

S3: Electro-hydraulic coordinated distribution: The braking force and the four-wheel additional yaw moment distribution values during pure hydraulic braking are obtained. Combined with the pure electric motor driving force and the four-wheel additional yaw moment distribution values in S2, electro-hydraulic coordinated distribution is performed on the four wheels of the vehicle. After the distribution is completed, the process enters S4.

S4: Optimize the distribution result of S3 to obtain the optimal motor driving force and hydraulic braking force;

S5: Select the corresponding electro-hydraulic coordinated control mode based on the current steering condition of the vehicle and the optimal motor driving force and hydraulic braking force obtained in S4.

2. The electro-hydraulic composite steering control method for an electric wheel vehicle under extreme working conditions according to claim 1, wherein the calculation process of the center of mass sideslip angle deviation is specifically as follows:

Get the expected center of mass sideslip angle :

The expected center of mass sideslip angle is designed as a center of mass sideslip angle estimation method based on Kalman filter. When the vehicle performs steady-state steering, the inverse of the expected center of mass sideslip angle is =0, and the expected center of mass sideslip angle is calculated:

;

in, is the vehicle speed; is the steering wheel angle; For car quality; is the lateral stiffness of the rear and front wheels; is the distance from the center of mass to the front axle; is the distance from the center of mass to the rear axle;

According to the car's steering angle and current speed Get the actual center of mass sideslip angle :

;

in, is the actual center of mass sideslip angle, is the lateral velocity at the vehicle's center of mass, is the longitudinal velocity at the vehicle's center of mass;

Get the center of mass sideslip angle deviation :

;

3. The electro-hydraulic composite steering control method for an electric wheel vehicle under extreme operating conditions according to claim 1, wherein the yaw rate deviation is calculated as follows:

Get the desired yaw rate:

The yaw rate is derived from the dynamic formula of the vehicle body motion. The so-called expected value mainly represents the expected yaw rate when the vehicle enters a steady-state turn. The yaw acceleration at this time =0, the desired yaw rate is expressed as:

;

Among them, L is the wheelbase, K= is the stability coefficient, which reflects the steady-state response of the vehicle; is the lateral stiffness of the rear and front wheels; is the steering wheel angle; is the distance from the center of mass to the front axle; is the distance from the center of mass to the rear axle;

Get yaw rate deviation :

;

in, is the actual yaw angular velocity.

4. The electro-hydraulic composite steering control method for electric wheel vehicles under extreme working conditions according to claim 2 or 3, characterized in that the vehicle speed The specific calculation process is:

When the car is in a turning state, calculate the linear speed of the four wheels:

;

in 、 represents the angular velocity of the four wheels; R is the wheel radius;

Vehicle speed indication:

;

5. The electro-hydraulic composite steering control method for an electric wheel vehicle under extreme working conditions according to claim 4, wherein the step S2 obtains the pure motor driving force and the four-wheel additional yaw moment distribution value, specifically:

Get pure motor driving force:

For motor drive, torque differential distribution is performed to generate an ideal additional yaw moment on the four wheel hubs, thereby maintaining the vehicle's driving stability. The driving force values of the four wheels are:

;

in: is the height of the vehicle's center of mass; is the longitudinal acceleration; Represents the sum of all driving forces; represents the sum of all yaw moments, and B represents the variance of the sideslip angle of the center of mass obtained by the extended Kalman filter, which is expressed as:

B= , Expressed as the moment of inertia around the Z axis; Left front wheel motor driving force; Right front wheel motor driving force; Left rear wheel motor driving force; Right rear wheel motor driving force;

Get the additional yaw torque distribution value of the four wheels driven by pure electric motor:

;

They are represented as the additional yaw torques of the four wheel hub motors; Respectively expressed as 、、、 ; Expressed as the distance from the four wheel hub motor action points to the center of mass of the vehicle.

6. The electro-hydraulic composite steering control method for an electric wheel vehicle under extreme working conditions according to claim 5, wherein S3 is specifically:

Get the four-wheel additional yaw moment distribution value during pure hydraulic braking:

;

in:

;

in, 、 is the change in wheel longitudinal/lateral force; Respectively represent the straight-line distance of the longitudinal/lateral force from the center of mass; Indicates wheelbase; a, b indicate the vertical distance of longitudinal/lateral force from the center of mass; middle Indicates the outer front wheel of the electric wheel car brake, Indicates the rear wheel inside the electric wheel car brake;

Get the braking force during pure hydraulic braking:

In pure hydraulic braking, that is, the additional yaw torque required for the vehicle's active safety control can be provided by hydraulic braking alone, the braking force required for each wheel of the electric wheel vehicle is calculated as follows:

;

in, Indicates the braking force of the four wheels; represents the additional yaw moment of the four wheels; is the wheel radius; is the distance from the point of application of the wheel braking force to the center of mass of the vehicle;

Electro-hydraulic coordinated distribution of the four wheels of the vehicle:

Combined with the pure electric drive in S2 and the pure hydraulic drive in S3, constraints are added to obtain the relationship between the yaw moment, longitudinal force, and the driving force and braking force of each wheel:

;

in: Left front wheel hydraulic brake force; Right front wheel hydraulic brake force; Left rear wheel hydraulic brake force; Right rear wheel hydraulic brake force; Left front wheel motor driving force; Right front wheel motor driving force; Left rear wheel motor driving force; Right rear wheel motor driving force; Wheel steering angle; wheelbase;

According to the optimization allocation algorithm of minimum tire utilization, its constraint condition, that is, the stability objective function, is set as the minimum sum of squares of the utilization of the four wheels of the vehicle:

= ;

in, 、 are the longitudinal and lateral forces of each wheel, is the road adhesion coefficient of the corresponding wheel, is the vertical load of each wheel;

The torque that the motor can provide is constrained by the motor's external characteristics:

;

in: is the peak torque of the motor; Right front wheel motor driving force; Left rear wheel motor driving force; Right rear wheel motor driving force;

The longitudinal force is constrained by the road adhesion condition and the vertical load as follows:

;

7. The electro-hydraulic composite steering control method for an electric wheel vehicle under extreme working conditions according to claim 6, wherein S4 is specifically:

Based on the allocation results in S3, perform quadratic programming to optimize the allocation method:

According to the above optimization objectives and constraints, the standard form of the quadratic programming method is sorted out as

;

constraint: ;

in, ;

in, Indicates the braking and driving forces of the four wheels in the numerical direction; represents a diagonal matrix;

in , G is a matrix: , d represents the wheelbase;

The above calculations show The optimal motor driving force and hydraulic braking force.

8. The electro-hydraulic composite steering control method for an electric wheel vehicle under extreme working conditions according to claim 1, wherein the electro-hydraulic composite coordinated control mode in S5 is specifically:

Mode 1: When the yaw rate deviation is positive, the steering wheel angular velocity is positive, and the steering wheel angle is positive, the vehicle is in a left understeer condition. The left rear wheel is hydraulically braked, and the motor increases the right front wheel drive torque.

Mode 2: When the yaw rate deviation is positive, the steering wheel angular velocity is positive, and the steering wheel angle is negative, indicating a right oversteer condition, the left front wheel is hydraulically braked while the motor increases the right rear wheel drive torque.

Mode 3: When the yaw rate deviation is positive, the steering wheel angular velocity is negative, and the steering wheel angle is negative, indicating a right oversteer condition, the left front wheel is hydraulically braked while the motor increases the right rear wheel drive torque.

Mode 4: When the yaw rate deviation is negative, the steering wheel angular velocity is positive, and the steering wheel angle is positive, indicating a left oversteer condition, the right front wheel is hydraulically braked, while the motor increases the left rear wheel drive torque.

Mode 5: When the yaw rate deviation is negative, the steering wheel angular velocity is positive, and the steering wheel angle is zero, indicating a left oversteer condition, the right front wheel is hydraulically braked, while the motor increases the left rear wheel drive torque.

Mode 6: When the yaw rate deviation is negative, the steering wheel angular velocity is negative, and the steering wheel angle is positive, indicating a left oversteer condition, the right front wheel is hydraulically braked, while the motor increases the left rear wheel drive torque.

Mode 7: When the yaw rate deviation is negative, the steering wheel angular velocity is negative, and the steering wheel angle is negative, it is in the right understeer condition, the right rear wheel is hydraulically braked, and the motor increases the driving torque of the left front wheel;

Mode 8: When the yaw rate deviation is positive, the steering wheel angular velocity is positive, and the steering wheel angle is zero, it is in the left understeer condition. The left rear wheel is hydraulically braked, and the motor increases the right front wheel drive torque.

Mode 9: When the yaw rate deviation is positive, the steering wheel angular velocity is negative, and the steering wheel angle is zero, it is in a right oversteer condition. The left front wheel is hydraulically braked, and the motor increases the driving torque of the right rear wheel.

Mode 10: When the yaw rate deviation is negative, the steering wheel angular velocity is negative, and the steering wheel angle is zero, it is in the right understeer condition, the right rear wheel is hydraulically braked, and the motor increases the driving torque of the left front wheel.

9. A system for realizing the electro-hydraulic composite steering control method of an electric wheel vehicle under extreme working conditions as described in any one of claims 1 to 8, characterized in that it comprises a wheel hub motor (1), a wheel speed sensor (2), a pressure sensor (3), a hydraulic control unit (4), a motor control unit (5), a vehicle control unit ECU (6), a steering wheel angle sensor (7), a lateral acceleration sensor (8), a yaw rate sensor (9), and a brake master cylinder (10), wherein the wheel hub motor (1), the wheel speed sensor (2), and the pressure sensor (3) are respectively mounted on the wheel hub, the hydraulic control unit (4) is connected to the brake master cylinder (10), the motor control unit (5) is connected to the wheel hub motor (1), and the wheel speed sensor (2), the pressure sensor (3), the steering wheel angle sensor (7), the lateral acceleration sensor (8), the yaw rate sensor (9), and the brake master cylinder (10) are respectively signal-connected to the vehicle control unit ECU (6).