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

Abstract

The present invention discloses an electro-hydraulic composite steering control method and system for an electric wheel vehicle under extreme working conditions, comprising: S1: determining whether the vehicle is in an unstable state; S2: selecting a vehicle drive control mode; S3: performing electro-hydraulic composite coordinated distribution on the vehicle's four wheels; S4: obtaining the optimal motor drive force and hydraulic braking force; and S5: selecting a corresponding electro-hydraulic composite coordinated control mode based on the vehicle's current steering working condition and the optimal motor drive force and hydraulic braking force obtained in S4. By proposing coordinated control of hydraulic braking and motor drive under extreme working conditions, different wheels are controlled separately, compensating for each other, and accurately and timely controlling the vehicle body stability under extreme working conditions, achieving a combination of different control modes and maximizing electro-hydraulic coupled optimized stability control. The vehicle's working condition is determined based on yaw angular velocity, steering wheel angle, and steering wheel angular velocity, and coupling optimization of the electro-hydraulic composite ESP is achieved by regulating hydraulic braking and motor drive.

Description

Technical Field

[0001] The present invention relates to an electro-hydraulic composite steering control method and system for an electric wheel vehicle under extreme working conditions, belonging to the technical field of electric wheel vehicles. Background Art

[0002] As an emerging mode of transportation, electric vehicles (EVs) utilize electric motors to drive their wheels, offering higher energy efficiency and significant environmental advantages compared to traditional internal combustion engine vehicles. Utilizing electric motors instead of traditional internal combustion engines not only improves vehicle acceleration but also reduces exhaust emissions and noise pollution. However, in EV control systems, effectively integrating electric motor control with hydraulic braking systems to enhance vehicle maneuverability and safety remains a key research topic, particularly under highly dynamic conditions.

[0003] In electric vehicles, the Electronic Stability Program (ESP) plays a key role. Electro-hydraulic ESP is an advanced electronic stability control system that combines electric motor control with hydraulic braking to achieve more precise vehicle dynamics control. By monitoring vehicle dynamic parameters such as yaw rate, lateral acceleration, and speed in real time, the system precisely adjusts braking force and torque distribution as needed, thereby enhancing vehicle handling and stability.

[0004] However, under certain extreme operating conditions, such as sharp turns at high speeds and emergency avoidance on slippery roads, the electric vehicle's motor control characteristics, while responsive and precise, are insufficient in handling the additional yaw torque required. This is because the electric motor's braking or driving capabilities cannot meet the requirements under all operating conditions. While the hydraulic braking system's control characteristics can output a large braking torque at all vehicle speeds, its response speed is slow and the braking torque control accuracy is poor. In this case, a single control strategy cannot meet actual needs and cannot complement the electro-hydraulic characteristics to simultaneously meet the requirements of electro-hydraulic hybrid ESP coordinated control and electro-hydraulic decoupling regulation. Summary of the Invention

[0005] Purpose of the invention: In response to the deficiencies in the prior art, the present invention provides an electro-hydraulic composite steering control method and system for electric-wheel vehicles under extreme working conditions. The method utilizes the rapid response characteristics of motor braking and is based on the complementary electro-hydraulic characteristics in electric-wheel vehicles to achieve more accurate and efficient distribution of braking and driving forces, thereby realizing electro-hydraulic coupling optimization during electro-hydraulic composite braking.

[0006] Technical solution: A method for controlling electro-hydraulic composite steering of an electric wheel vehicle under extreme working conditions, comprising the following steps:

[0007] 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.

[0008] 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;

[0009] 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.

[0010] S4: Optimize the distribution result of S3 to obtain the optimal motor driving force and hydraulic braking force;

[0011] 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.

[0012] Preferably, the calculation process of the center of mass sideslip angle deviation is specifically as follows:

[0013] Get the expected center of mass sideslip angle β d :

[0014] 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 Get the desired center of mass sideslip angle calculation:

[0015]

[0016] Among them, V e is the vehicle speed; θ is the steering wheel angle; m is the vehicle mass; 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 center of mass to the front axle; l r is the distance from the center of mass to the rear axle;

[0017] According to the steering angle of the car and the current speed V e Get the actual center of mass sideslip angle β D :

[0018]

[0019] Among them, β D is the actual center of mass sideslip angle, V Y is the lateral velocity at the vehicle's center of mass, V X is the longitudinal velocity at the vehicle's center of mass;

[0020] Get the center of mass sideslip angle deviation Δβ:

[0021] Δβ=β d -β D .

[0022] Preferably, the calculation process of the yaw rate deviation is specifically as follows:

[0023] Get the desired yaw rate:

[0024] 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 The desired yaw rate is expressed as:

[0025]

[0026] Wherein, L is the wheelbase, is the stability coefficient, which reflects the steady-state response of the car; K f , 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 center of mass to the front axle; l r is the distance from the center of mass to the rear axle;

[0027] Get the yaw rate deviation Δr:

[0028] Δr=r d -r D

[0029] Among them, r D is the actual yaw angular velocity.

[0030] Preferably, the vehicle speed V e The specific calculation process is:

[0031] When the car is in a turning state, calculate the linear speed of the four wheels:

[0032]

[0033] where ω fl 、ω fr 、ω rl 、ω rr represents the angular velocity of the four wheels; R is the wheel radius;

[0034] Vehicle speed indication:

[0035]

[0036] Preferably, the step S2 further includes obtaining the total additional yaw moment and total longitudinal force of the vehicle, specifically:

[0037] Get the total additional yaw moment:

[0038] The sliding surface s is defined by the yaw rate and the center of mass roll angle:

[0039] s=ε(r p -r d )+ξ(β D -β d )

[0040] Among them, r D refers to the actual yaw angular velocity; β D is the actual center of mass roll angle; ε and ξ are distribution weight coefficients and ξ + ε = 1; the additional yaw moment is obtained by differentiation and combined with the vehicle motion equation:

[0041]

[0042] Where (-asgns-bs) is the set exponential convergence law, which is used to ensure that the system's moving point approaches the sliding plane; a>0, b>0; s is the defined sliding surface; 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 center of mass to the front axle; l r is the distance from the center of mass to the rear axle; A is the stability increase value, and its expression is as follows:

[0043]

[0044] Where, I is the moment of inertia; 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 center of mass to the front axle; l r 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 desired center of mass sideslip angle.

[0045] Obtain the total longitudinal force ∑F(t):

[0046] Taking the target speed and reference speed as input, the PID method is used to implement the control law as follows:

[0047]

[0048] Among them: K p is the proportional coefficient; K t is the integral coefficient; K D is the differential coefficient.

[0049] Preferably, the pure motor driving force and the four-wheel additional yaw moment distribution values ​​are obtained in S2, specifically:

[0050] Get pure motor driving force:

[0051] 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:

[0052]

[0053] Where: h g is the height of the vehicle's center of mass; a x is the longitudinal acceleration; ∑F x Represents the sum of all driving forces; ∑M Z 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:

[0054] I z Expressed as the moment of inertia around the Z axis; F mfl Left front wheel motor driving force; F mfr Right front wheel motor driving force; F mrl Left rear wheel motor driving force; F mrr Right rear wheel motor driving force;

[0055] Get the additional yaw torque distribution value of the four wheels driven by pure electric motor:

[0056] M zn =F n ·jl n

[0057] M zn They are respectively represented as the additional yaw torque of the four wheel hub motors; F n Respectively expressed as F mfl 、F mfr 、F mfr 、Fmrr ;jl n Expressed as the distance from the four wheel hub motor action points to the center of mass of the vehicle.

[0058] Preferably, the S3 is specifically:

[0059] Get the four-wheel additional yaw moment distribution value during pure hydraulic braking:

[0060]

[0061] in:

[0062]

[0063] Where ΔF x , ΔF y is the change in wheel longitudinal / lateral force; l x 、l y Respectively represent the straight-line distance between the longitudinal / lateral force and the center of mass; d represents the wheelbase; a and b represent the vertical distance between the longitudinal / lateral force and the center of mass; ΔF in ΔM x ·l x +ΔF y ·l y Indicates the outer front wheel of the electric wheel car brake, Indicates the rear wheel inside the electric wheel car brake;

[0064] Get the braking force during pure hydraulic braking:

[0065] 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:

[0066]

[0067] Among them, F braken Indicates the braking force of the four wheels; ΔM n represents the additional yaw moment of the four wheels; R is the wheel radius; p is the distance from the point of application of the wheel braking force to the center of mass of the vehicle;

[0068] Electro-hydraulic coordinated distribution of the four wheels of the vehicle:

[0069] 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:

[0070]

[0071] Among them: F hfl Left front wheel hydraulic braking force; Fhfr Right front wheel hydraulic braking force; F hrl Left rear wheel hydraulic braking force; F hrr Right rear wheel hydraulic braking force; F mfl Left front wheel motor driving force; F mfr Right front wheel motor driving force; F mrl Left rear wheel motor driving force; F mrr Right rear wheel motor driving force; Wheel steering angle; d wheelbase;

[0072] 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:

[0073]

[0074] Among them, F xi 、F yi is the longitudinal force and lateral force of each wheel, μ i is the road adhesion coefficient of the corresponding wheel, F zi is the vertical load of each wheel;

[0075] The torque that the motor can provide is constrained by the motor's external characteristics:

[0076]

[0077] Where: T imax (v) is the peak torque of the motor; F mrr Right front wheel motor driving force; F mrl Left rear wheel motor driving force; F mrr Right rear wheel motor driving force;

[0078] The longitudinal force is constrained by the road adhesion condition and the vertical load as follows:

[0079] -μF zi ≤F xi ≤μF zi ,i=fl,fr,rl,rr。

[0080] Preferably, the S4 is specifically:

[0081] Based on the allocation results in S3, perform quadratic programming to optimize the allocation method:

[0082] According to the above optimization objectives and constraints, the standard form of the quadratic programming method is sorted out as

[0083]

[0084] constraint:

[0085] in,

[0086] Among them, F zi Represents the braking and driving forces of the four wheels in the numerical direction; diag represents a diagonal matrix;

[0087] Where u=[F fl F fr F rl F rr ] T , G is a matrix: d represents the wheelbase;

[0088] The above calculations can be used to determine the optimal motor driving force and hydraulic braking force in u.

[0089] Preferably, the electro-hydraulic coordinated control mode in S5 is specifically:

[0090] 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.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] 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;

[0097] Mode 8: When the yaw rate deviation is positive, the steering wheel angular velocity is positive, and the steering wheel angle is zero, the vehicle is in a left understeer condition. The left rear wheel is hydraulically braked, while the motor increases the right front wheel drive torque.

[0098] 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.

[0099] 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.

[0100] A system for implementing an electro-hydraulic composite steering control method for an electric wheel vehicle under extreme working conditions includes a wheel hub motor, a wheel speed sensor, a pressure sensor, a hydraulic control unit, a motor control unit, a vehicle control unit (ECU), a steering wheel angle sensor, a lateral acceleration sensor, a yaw rate sensor, and a brake master cylinder. The wheel hub motor, wheel speed sensor, and pressure sensor are respectively mounted on the wheel hub; the hydraulic control unit is connected to the brake master cylinder; the motor control unit is connected to the wheel hub motor; and the wheel speed sensor, pressure sensor, steering wheel angle sensor, lateral acceleration sensor, yaw rate sensor, and brake master cylinder are respectively signal-connected to the vehicle control unit (ECU).

[0101] Beneficial effects: The present invention proposes coordinated control of hydraulic braking and motor drive under extreme working conditions, controls different wheels separately, compensates for each other, and accurately and timely controls the stability of the vehicle body under extreme working conditions, realizes the combination of different control modes, and maximizes the stability control of electro-hydraulic coupling optimization; at the same time, the working condition of the vehicle is judged according to the yaw angular velocity, steering wheel angle and steering wheel angular velocity, and then the coupling optimization of electro-hydraulic composite ESP is realized by regulating hydraulic braking and motor drive. BRIEF DESCRIPTION OF THE DRAWINGS

[0102] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0103] Figure 1 is a flow chart of the method of the present invention;

[0104] Figure 2 This is the wheel driving / braking force distribution diagram when understeering left;

[0105] Figure 3 It is the wheel driving / braking force distribution diagram when turning left oversteering;

[0106] Figure 4 This is the wheel driving / braking force distribution diagram when understeering right;

[0107] Figure 5 It is the wheel driving / braking force distribution diagram when turning right oversteering;

[0108] Figure 6 Schematic diagram of the system of the present invention. DETAILED DESCRIPTION

[0109] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0110] In the description of the present invention, it should be understood that the terms "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating directions or positional relationships, are based on the directions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and therefore should not be understood as limiting the present invention.

[0111] In the present invention, unless otherwise expressly specified or limited, a first feature being "above" or "below" a second feature may include the first and second features being in direct contact, or may include the first and second features being in contact not directly but through another feature between them. Furthermore, a first feature being "above," "above," and "above" a second feature may include the first feature being directly above or obliquely above the second feature, or may simply mean that the first feature is higher in level than the second feature. A first feature being "below," "below," and "below" a second feature may include the first feature being directly below or obliquely below the second feature, or may simply mean that the first feature is lower in level than the second feature.

[0112] like Figure 1 As shown, a method for controlling electro-hydraulic composite steering of an electric wheel vehicle under extreme working conditions includes the following steps:

[0113] 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.

[0114] In this embodiment, the maximum center of mass sideslip angle threshold specified for a vehicle is mainly determined by the vehicle model. For an ordinary family sedan, the maximum center of mass sideslip angle threshold is generally 3°-5°; for a sports utility vehicle (SUV), due to its taller body and relatively higher center of gravity, the maximum center of mass sideslip angle threshold is 2°-4°; for a light truck, the maximum center of mass sideslip angle threshold is 2°-3°; for a large passenger vehicle, the maximum center of mass sideslip angle threshold is 2°-3°; and for an off-road vehicle, the maximum center of mass sideslip angle threshold can reach 10°-15°.

[0115] The maximum yaw rate threshold required to maintain vehicle stability is between ±3° / s and ±5° / s for passenger cars and between ±2° / s and ±4° / s for commercial vehicles.

[0116] The calculation process of the center of mass sideslip angle deviation is specifically as follows:

[0117] Get the expected center of mass sideslip angle β d :

[0118] 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 Get the desired center of mass sideslip angle calculation:

[0119]

[0120] Among them, V e is the vehicle speed; θ is the steering wheel angle; m is the vehicle mass; 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 center of mass to the front axle; l r is the distance from the center of mass to the rear axle;

[0121] According to the steering angle of the car and the current speed V e Get the actual center of mass sideslip angle β D :

[0122]

[0123] Among them, βD is the actual center of mass sideslip angle, V Y is the lateral velocity at the vehicle's center of mass, V X is the longitudinal velocity at the vehicle's center of mass;

[0124] Get the center of mass sideslip angle deviation Δβ:

[0125] Δβ=β d -β D

[0126] The calculation process of the yaw rate deviation is specifically as follows:

[0127] Get the desired yaw rate:

[0128] 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 The desired yaw rate is expressed as:

[0129]

[0130] Wherein, L is the wheelbase, is the stability coefficient, which reflects the steady-state response of the car; 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 center of mass to the front axle; l r is the distance from the center of mass to the rear axle;

[0131] Get the yaw rate deviation Δr:

[0132] Δr=r d -r D

[0133] Among them, r D is the actual yaw rate, which is directly obtained from the yaw rate sensor.

[0134] The vehicle speed V e The specific calculation process is:

[0135] When the car is in a turning state, calculate the linear speed of the four wheels:

[0136]

[0137] where ω fl 、ω fr 、ω rl 、ω rr represents the angular velocity of the four wheels; R is the wheel radius;

[0138] Vehicle speed indication:

[0139]

[0140] 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;

[0141] The step S2 also includes obtaining the total additional yaw moment and total longitudinal force of the vehicle, specifically:

[0142] Get the total additional yaw moment:

[0143] Because the center of mass slip angle and yaw rate are coupled, these two variables need to be controlled jointly. Controlling only the yaw rate will result in an excessively large slip angle, while controlling only the slip angle will deviate from the desired yaw rate. Therefore, the sliding surface s is defined jointly by the yaw rate and the center of mass roll angle:

[0144] s=ε(r D -r d )+ξ(β D -β d )

[0145] Among them, r D refers to the actual yaw angular velocity; β D is the actual center of mass roll angle; ε and ξ are distribution weight coefficients and ξ + ε = 1; the additional yaw moment is obtained by differentiation and combined with the vehicle motion equation:

[0146]

[0147] Where (-asgns-bs) is the set exponential convergence law, which is used to ensure that the system's moving point approaches the sliding plane; a>0, b>0; s is the defined sliding surface; 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 center of mass to the front axle; l r is the distance from the center of mass to the rear axle; A is the stability increase value, and its expression is as follows:

[0148]

[0149] Where I is the moment of inertia; 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 center of mass to the front axle; lr 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 desired center of mass sideslip angle.

[0150] Obtain the total longitudinal force ∑F(t):

[0151] Taking the target speed and reference speed as input, the PID method is used to implement the control law as follows:

[0152]

[0153] Among them: K p is the proportional coefficient; K t is the integral coefficient; K D is the differential coefficient.

[0154] The pure motor driving force and the four-wheel additional yaw moment distribution values ​​are obtained in S2, specifically:

[0155] In this embodiment, when the car turns left, the front and rear wheels on the left are the inner wheels, and the front and rear wheels on the right are the outer wheels. When the car turns right, the front and rear wheels on the left are the outer wheels, and the front and rear wheels on the right are the inner wheels.

[0156] The equilibrium equation of force and torque is expressed as:

[0157]

[0158] Where, F Y1 is the front wheel cornering force, F Y2 is the rear wheel cornering force; m is the vehicle mass; a y is the lateral acceleration of the vehicle's center of mass; I z is the moment of inertia of the car about its center of mass; is the yaw angular acceleration; l f is the distance from the center of mass to the front axle; l r is the distance from the center of mass to the rear axle.

[0159] The drive motors in electric vehicles generate a yaw moment by varying the drive torques of the motors on both sides, creating a torque difference. For example, when an electric vehicle experiences understeer, the following example illustrates this: as the cornering force on the front wheels decreases, the slip angles of the two front wheels decrease; as the cornering force on the rear wheels increases, the slip angles of the two rear wheels increase, causing the vehicle to understeer. For example, if the vehicle is turning right and experiencing understeer instability, increasing the drive torque on the left rear wheel and decreasing the drive torque on the right rear wheel will generate an opposite yaw moment. Similarly, driving the left front wheel can alleviate understeer. Similarly, to alleviate oversteer, the drive motor can drive both the inner front and inner rear wheels.

[0160] The longitudinal and lateral forces generated by driving the outer front wheel both generate a yaw moment that helps the vehicle turn; the longitudinal and lateral forces generated by driving the inner rear wheel both generate a yaw moment that inhibits the vehicle from turning. Therefore, when the electric motor of an electric vehicle is driving, the drive utilization rate of the outer front wheel and the inner rear wheel is significantly higher than that of the other two wheels.

[0161] The advantages of the motor-driven torque response are used to compensate for the slow response, long delay and inability to accurately control the braking force and torque of the hydraulic brake. The hydraulic brake can output a large braking torque at any vehicle speed to compensate for the shortcomings of the motor's small output torque and even smaller output torque at high speed.

[0162] Get pure motor driving force:

[0163] 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:

[0164]

[0165] Where: h g is the height of the vehicle's center of mass; a x is the longitudinal acceleration; ∑F x Represents the sum of all driving forces; ∑M Z 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:

[0166] I z Expressed as the moment of inertia around the Z axis; F mfl Left front wheel motor driving force; F mfr Right front wheel motor driving force; F mrl Left rear wheel motor driving force; F mrr Right rear wheel motor driving force;

[0167] The value obtained by the above formula needs to be compared with the actual set peak driving force of the hub motor. When the value obtained by the above formula is less than the actual set peak driving force of the hub motor, the vehicle is driven only by the pure electric motor, and there is a difference with the motor driving force under the actual operation of the vehicle. Based on this difference, the motor on the corresponding side is driven by the pure electric motor. Otherwise, the electro-hydraulic composite coordinated control is entered.

[0168] When the vehicle is confirmed to enter the extreme operating condition, the value obtained by the above formula is used as one of the constraints of the electro-hydraulic composite coordinated control formula of the electric wheel vehicle and is solved jointly.

[0169] Get the additional yaw torque distribution value of the four wheels driven by pure electric motor:

[0170] M zn =F n ·jl n

[0171] M zn They are respectively represented as the additional yaw torque of the four wheel hub motors; F n Respectively expressed as F mfl 、F mfr 、F mfr 、F mrr ;jl n Expressed as the distance from the four wheel hub motor action points to the center of mass of the vehicle.

[0172] 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.

[0173] The equilibrium equation of force and torque is expressed as:

[0174]

[0175] Where, F Y1 is the front wheel cornering force, F Y2 is the rear wheel cornering force; m is the vehicle mass; a y is the lateral acceleration of the vehicle's center of mass; I z is the moment of inertia of the car about its center of mass; is the yaw angular acceleration; l f is the distance from the center of mass to the front axle; l r is the distance from the center of mass to the rear axle.

[0176] When the cornering force on the front wheels decreases, the corresponding slip angles of the front wheels decrease; when the cornering force on the rear wheels increases, the corresponding slip angles of the rear wheels increase, causing the vehicle to understeer. If the longitudinal force on the outer rear wheel increases and the force on the inner rear wheel decreases, or if the longitudinal force on the outer front wheel increases and the force on the inner front wheel decreases, both of these forces act in opposite directions on the vehicle, mitigating the understeer effect.

[0177] Therefore, when the vehicle is understeering, the hydraulic pressure can brake the inside front and rear wheels. Similarly, when the vehicle is oversteering, the hydraulic pressure can brake the outside front and rear wheels.

[0178] Hydraulic braking of the same-side wheels can achieve the same control effect, but electro-hydraulic coordinated control primarily addresses cornering situations. Tire lateral and longitudinal forces are also coupled, and the yaw moments generated by the same-side wheels vary in magnitude. For example, when turning left, the longitudinal and lateral forces generated by braking the right front wheel both generate a yaw moment that inhibits steering, while the longitudinal and lateral forces generated by braking the left rear wheel both generate a yaw moment that facilitates steering. Therefore, in electro-hydraulic coordinated control, braking the outer front wheel can mitigate oversteer, while braking the inner rear wheel can better mitigate understeer.

[0179] According to the different steering conditions faced by hydraulic braking and motor drive, the method of obtaining electro-hydraulic decoupling is as follows:

[0180] Control methods for oversteering

[0181] When the vehicle is oversteering, the in-wheel motor of the inner rear wheel is driven first because the drive motor has the advantage of fast and precise torque response. At the same time, the in-wheel motor of the outer wheel is reduced, generating an additional yaw moment in the opposite direction of the yaw motion of the electric wheel vehicle. The outer front wheel is hydraulically braked, also generating a yaw moment in the opposite direction.

[0182] Control method for understeering

[0183] When the vehicle is understeering, the drive motor's fast and precise torque response makes it possible to increase the driving force of the outer front wheel's hub motor first. Simultaneously, the driving force of the inner wheel's hub motor is reduced, generating an additional yaw moment in the opposite direction of the vehicle's yaw motion. Hydraulic braking then applies to the inner rear wheel, also generating a yaw moment in the opposite direction.

[0184] Get the four-wheel additional yaw moment distribution value during pure hydraulic braking:

[0185]

[0186] in:

[0187]

[0188] Where ΔF x , ΔF y is the change in wheel longitudinal / lateral force; l x 、l y Respectively represent the straight-line distance between the longitudinal / lateral force and the center of mass; d represents the wheelbase; a and b represent the vertical distance between the longitudinal / lateral force and the center of mass; ΔF in ΔM x ·l x +ΔF y ·l yIndicates the outer front wheel of the electric wheel car brake, Indicates the rear wheel inside the electric wheel car brake;

[0189] Get the braking force during pure hydraulic braking:

[0190] 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:

[0191]

[0192] Among them, F braken Indicates the braking force of the four wheels; ΔM n represents the additional yaw moment of the four wheels; R is the wheel radius; p is the distance from the point of application of the wheel braking force to the center of mass of the vehicle;

[0193] Electro-hydraulic coordinated distribution of the four wheels of the vehicle:

[0194] 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:

[0195]

[0196] Among them: F hfl Left front wheel hydraulic braking force; F hfr Right front wheel hydraulic braking force; F hrl Left rear wheel hydraulic braking force; F hrr Right rear wheel hydraulic braking force; F mfl Left front wheel motor driving force; F mfr Right front wheel motor driving force; F mrl Left rear wheel motor driving force; F mrr Right rear wheel motor driving force; Wheel steering angle; d wheelbase;

[0197] 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:

[0198]

[0199] Among them, F xi 、F yi is the longitudinal force and lateral force of each wheel, μ i is the road adhesion coefficient of the corresponding wheel, F zi is the vertical load of each wheel;

[0200] The torque that the motor can provide is constrained by the motor's external characteristics:

[0201]

[0202] Where: T imax (v) is the peak torque of the motor; F mfr Right front wheel motor driving force; F mrl Left rear wheel motor driving force; F mrr Right rear wheel motor driving force;

[0203] The longitudinal force is constrained by the road adhesion condition and the vertical load as follows:

[0204] -μF zi ≤F xi ≤μF zi ,i=fl,fr,rl,rr。

[0205] S4: Optimize the distribution result of S3 to obtain the optimal motor driving force and hydraulic braking force;

[0206] Based on the allocation results in S3, perform quadratic programming to optimize the allocation method:

[0207] According to the above optimization objectives and constraints, the standard form of the quadratic programming method is sorted out as

[0208]

[0209] constraint:

[0210] in,

[0211] Among them, F zi Represents the braking and driving forces of the four wheels in the numerical direction; diag represents a diagonal matrix;

[0212] Where u=[F fl F fr F rl F rr ] T , G is a matrix: d represents the wheelbase;

[0213] The above calculations can be used to determine the optimal motor driving force and hydraulic braking force in u.

[0214] Because when a car needs to perform active safety control, it is first necessary to determine whether the car is in an unstable state. At the same time, the motor drive is the main active safety control in the entire process, and hydraulic braking is the compensation for the motor drive. Therefore, in order to determine whether electro-hydraulic composite coordinated control is required, the parameters and settings of the hub motor of the electric wheel car are first selected. For example, this embodiment takes a brushless DC motor as an example, and specifies the peak torque, rated speed, inductor winding internal resistance, motor torque coefficient, and motor magnetic constant information according to actual conditions.

[0215] The calculated total longitudinal force and additional yaw moment, as well as the calculated driving force of the pure electric motor drive, are compared with the driving force provided by the peak torque of the selected hub motor. If the driving force of the pure electric motor drive is less than the peak driving force of the selected hub motor, then only the hub motor will be used for control; if the driving force of the pure electric motor drive is greater than the peak driving force of the selected hub motor, then the electro-hydraulic composite coordinated control will be used.

[0216] S5: Based on the vehicle's current steering condition and the optimal motor driving force and hydraulic braking force obtained in S4, the corresponding electro-hydraulic composite coordinated control mode is selected:

[0217] 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.

[0218] 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.

[0219] 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.

[0220] 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.

[0221] 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.

[0222] 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.

[0223] 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;

[0224] Mode 8: When the yaw rate deviation is positive, the steering wheel angular velocity is positive, and the steering wheel angle is zero, the vehicle is in a left understeer condition. The left rear wheel is hydraulically braked, while the motor increases the right front wheel drive torque.

[0225] 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.

[0226] 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.

[0227] Since there may be a delay in the driver's response, the angular velocity of the steering wheel angle is included in the judgment to avoid violating the driver's intention.

[0228] When driver response lag occurs, the yaw rate deviation has the same sign as the steering wheel angle but different sign from its angular velocity, which means that the actual dynamic response of the vehicle is inconsistent with the driver's expected operation.

[0229] The angular velocity of the steering wheel angle is added for joint judgment, mainly based on the following principles:

[0230] Improved Judgment Accuracy: Relying solely on yaw rate deviation and steering wheel angle cannot fully and accurately understand the vehicle's dynamic state and the driver's intentions. The angular velocity of the steering wheel angle provides additional information about the rate of change of the driver's input, allowing for a more accurate assessment of whether the vehicle's behavior meets the driver's expectations.

[0231] Reduce misjudgment: Helps distinguish whether the driver is intentionally slow in operation or not in time due to delayed reaction.

[0232] Therefore, when the yaw rate deviation is positive, the steering wheel angle is positive, and the steering wheel angular velocity is negative; when the yaw rate deviation is negative, the steering wheel angle is negative, and the steering wheel angular velocity is positive; under these two working conditions, electro-hydraulic composite ESP control is not performed.

[0233] Incorporating the angular velocity of the steering wheel angle into the joint judgment can provide a more comprehensive and accurate understanding of the vehicle's dynamics and the driver's intentions, thereby better coordinating the vehicle control system with the driver's operations and improving driving safety and comfort.

[0234] Modes 1 and 8 described above are described as the electric wheel vehicle being in an understeering and unstable state of turning left. Figure 2 Modes 2, 3, and 9 can be described as an oversteering instability state when the electric wheel vehicle is turning right, and the following steps are performed: Figure 5 Modes 4, 5, and 6 can be described as an oversteering instability state when the electric wheel vehicle is in a left turn, and the following steps are performed: Figure 3 Modes 7 and 10 can be described as the understeering instability state of the electric wheel vehicle turning right, as shown in FIG. Figure 4 Assign operation shown.

[0235] The pure motor drive control in the present invention is achieved by combining the pure motor drive force and yaw torque and the pure motor drive control mode. The pure motor drive control mode includes:

[0236] Mode 1: When the car is in a left understeer state, the wheel hub motor increases the driving torque of the outer wheel.

[0237] Mode 2: When the car is in the process of oversteering to the left, the in-wheel motor increases the driving torque of the inner wheel.

[0238] Mode 3: When the vehicle is in the right understeer state, the wheel hub motor increases the driving torque of the outer wheel.

[0239] Mode 4: When the car is in the state of oversteering to the right, the hub motor increases the driving torque of the inner wheel.

[0240] like Figure 6 As shown, a system for realizing an electro-hydraulic composite steering control method for an electric wheel vehicle under extreme working conditions includes 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 ECU6, a steering wheel angle sensor 7, a lateral acceleration sensor 8, a yaw rate sensor 9, and a brake master cylinder 10. The wheel hub motor 1, wheel speed sensor 2, and 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, pressure sensor 3, steering wheel angle sensor 7, lateral acceleration sensor 8, yaw rate sensor 9, and brake master cylinder 10 are respectively connected to the vehicle control unit ECU6 for signal signals.

[0241] When the car enters a steering state and requires electro-hydraulic coordinated control, the driver will manipulate the steering wheel according to the steering situation. However, there may also be problems of driver manipulation errors and driver response delays. Therefore, the following three quantities are analyzed.

[0242] When the car enters extreme working conditions, the yaw angular velocity sensor 9 and the steering wheel angle sensor 7 transmit signals to the vehicle control unit ECU6. The vehicle control unit ECU6 will compare and correspond the measured values ​​with the 10 modes in the electro-hydraulic composite control, determine the working conditions and status of the car, and match them in real time to complete the arrangement of hydraulic braking and motor drive.

[0243] At the same time, the hydraulic brake is set to brake the tires specifically, and the motor drive generates a yaw torque that mainly changes the driving torque of the hub motor 1 to form a torque difference. Therefore, when the motor drives a certain wheel, the opposite side motor can also be braked appropriately accordingly.

[0244] By obtaining the tires and values ​​corresponding to the braking force and driving force, the hydraulic control unit 4 transmits the braking signal to the hydraulic actuator. At this time, the pressure sensor 3 increases, maintains and reduces the pressure of the hydraulic system of the wheel. The braking process is that when the hydraulic system brakes and increases pressure, the brake master cylinder 10 will provide pressure. At the same time, the brake fluid will flow from the brake master cylinder 10 to the hydraulic cylinder and then to the control valve, thereby generating a series of pressures to push the piston to move, causing the pressure plate to displace, completing the entire hydraulic braking process.

[0245] The motor control unit 5 transmits the received drive signal to the hub motor 1 on each wheel in the form of an electrical signal. The motor control unit 5 determines whether the pure motor driving force is greater than the maximum driving force that the hub motor 1 can provide. If it is less, then only motor drive control is required. If the required yaw torque is greater than the maximum driving force that the drive motor can provide, then electro-hydraulic composite coordinated control is performed, and the distribution can be carried out according to the obtained distribution results and the corresponding control mode.

[0246] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. Reference can be made to the common and similar parts between the various embodiments. For the devices disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple, and the relevant parts can be referred to the method description.

[0247] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for controlling electric-hydraulic composite steering of an electric wheel vehicle under extreme working conditions, characterized in that: The following steps are involved: 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 electric wheel vehicles under extreme working conditions according to claim 1 is characterized in that: 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 electric wheel vehicles under extreme working conditions according to claim 1 is characterized in that: The calculation process of the yaw rate deviation is specifically 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 an electric wheel vehicle 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 electric wheel vehicles under extreme working conditions according to claim 4 is characterized in that: The pure motor driving force and the four-wheel additional yaw moment distribution values ​​are obtained in S2, 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 electric wheel vehicles under extreme working conditions according to claim 5 is characterized in that: The 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 electric wheel vehicles under extreme working conditions according to claim 6, characterized in that: The 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 electric wheel vehicles under extreme working conditions according to claim 1 is characterized in that: The electro-hydraulic coordinated control mode in S5 is specifically as follows: 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, the vehicle is in a left understeer condition. The left rear wheel is hydraulically braked, while 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 implementing the electro-hydraulic composite steering control method for electric wheel vehicles under extreme working conditions as described in any one of claims 1 to 8, characterized in that: The invention 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 angular velocity sensor (9), and a brake master cylinder (10). 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 angular velocity sensor (9), and the brake master cylinder (10) are respectively signal-connected to the vehicle control unit ECU (6).

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