Steering Control Method and Device for a Four-Wheel Independently Driven and Steered Electric Vehicle

Abstract

The present invention provides a steering control method and device for an electric vehicle with four-wheel independent drive and steering, belonging to the field of vehicle steering. When a fault detector detects a fault in the steering actuator, the fault state at this time is first quantified and transmitted to the adaptive fault-tolerant controller in real time. The fault-tolerant controller calculates the required additional yaw moment according to the residual state of the current steering actuator and the driving state of the vehicle, and then uses the differential drive of the in-wheel motors and the residual steering actuator to jointly complete the vehicle steering function. If the vehicle cannot be maintained in the stable region after the failure of the steering actuator, the steering is degraded and the vehicle is parked for processing. The present invention can utilize the differential drive of the four in-wheel motors and the residual steering actuator to complete vehicle steering for different failure types of the steer-by-wire system, while ensuring the vehicle has a steering function and reducing costs.

Description

Technical Field

[0001] The present invention belongs to the field of vehicle steering, and particularly relates to a steering control method and device for a four-wheel independently driven and steered electric vehicle. Background Art

[0002] A four-wheel independently driven and steered electric vehicle integrates drive, brake, steering and suspension, achieving a high degree of integration of the chassis system; it can travel in all directions, such as front-wheel steering, four-wheel steering, diagonal driving, in-situ steering, crab walking, etc.; this architecture also provides a good development platform for intelligent driving. Since the wheels are completely decoupled from each other and all four wheels are independently controllable, four steering motors are required to jointly complete the steering. The increase in steering actuators makes steering redundancy more important. When a fault occurs in the steering actuator mechanism, formulating a suitable fault-tolerant control strategy can greatly reduce the impact of the fault on the system and thus improve the stability of the system.

[0003] Existing redundancy technologies can be roughly divided into two categories. One is the redundancy technology of hardware backup, and the other is the algorithm fault-tolerant technology. Hardware backup provides backup for important components and components prone to failure, such as the structure of dual actuators or dual controllers. This method can effectively increase the safety of the vehicle but will increase the mass and cost of the system; algorithm fault tolerance mainly relies on the fault-tolerant algorithm of the controller to improve the redundancy of the entire system. When a fault occurs in the steering system, the remaining normal operating steering system devices are controlled, or the current work is completed using the remaining devices.

[0004] A wheel hub motor-driven electric vehicle directly installs the wheel hub motor inside or near the drive wheel, with the characteristics of simple structure, short transmission path, and high transmission efficiency. Since the drive motors can be controlled separately, additional yaw torque can be generated by outputting different driving torques on the left and right motors to enable the vehicle to complete steering. Summary of the Invention

[0005] The purpose of the present invention is to overcome the defects of the existing technology and provide a steering control method and device for a four-wheel independently driven and steered electric vehicle when a fault occurs in the steering actuator of the four-wheel independently driven and steered electric vehicle.

[0006] To achieve the above purpose, the present invention adopts the following technical solutions:

[0007] In the first aspect, the present invention provides a steering control method for a four-wheel independently driven and steered electric vehicle, including:

[0008] Input the desired steering wheel angle, obtain the desired wheel angle through the Ackermann steering relationship, and transmit it to the steering actuator for execution;

[0009] When the deviation between the actual wheel angle of the steering actuator and the desired wheel angle exceeds the upper bound of the stable error value, it is determined that the steering actuator has failed. Then, it is judged whether the vehicle is in a stable state according to the yaw rate of the vehicle: if so, steering redundancy control is adopted, and the residual functions of the normal steering actuator and the faulty steering actuator are used to steer the vehicle; if not, steering degradation processing is adopted, and differential braking is used to generate braking torques for the four wheels and the front wheel toe-in is increased to stop the vehicle.

[0010] When a failure occurs in the vehicle steering actuator, according to the failure type and degree of failure, the four wheel angles are output, and the residual functions of the normal steering actuator and the faulty steering actuator are used to complete the steering; if the steering cannot be completed, differential steering is used for auxiliary steering.

[0011] When dangerous situations such as vehicle instability occur, differential drive / braking is used to generate additional yaw moments to handle the vehicle stop while maintaining vehicle body stability.

[0012] Optionally, judging whether the vehicle is in a stable state according to the yaw rate of the vehicle specifically is:

[0013] When the deviation between the actual yaw rate and the desired yaw rate exceeds the upper bound of the stable error value, the vehicle is in an unstable state; otherwise, the vehicle is in a stable state.

[0014] Optionally, when the fault detector detects that a failure has occurred in the steering actuator, differential drive of the in-wheel motors and the residual steering actuator are used together to complete the steering redundancy control. The process of the steering redundancy control includes:

[0015] Identify the failure type of the steering actuator, set the fault factor input to the adaptive fault-tolerant controller according to the failure type, the adaptive fault-tolerant controller outputs the four wheel angles to the vehicle, and the vehicle feeds back the actual yaw rate ω to the sliding mode controller;

[0016] Output the desired yaw rate ω through the four-wheel independent steering two-degree-of-freedom vehicle model d to the sliding mode controller, and the sliding mode controller outputs the four wheel driving torques to the vehicle according to the input ω and ω d , in combination with the set constraint conditions and objective function.

[0017] Optionally, if the vehicle cannot remain within the stable region after the failure of the steering actuator, steering degradation and vehicle stop processing are performed. The process of the steering degradation processing includes:

[0018] The vehicle feeds back the actual yaw rate ω to the sliding mode controller, and at the same time outputs the desired yaw rate ω through the four-wheel independent steering two-degree-of-freedom vehicle model d to the sliding mode controller, and the sliding mode controller outputs the four wheel driving torques to the vehicle according to the input ω and ω d, combined with the set constraint conditions and objective function, outputs the driving torques of the four wheels to the vehicle;

[0019] The vehicle increases the front wheel toe-in by controlling the wheels to rotate towards the central axis direction and outputs the steering angles of the four wheels to the vehicle.

[0020] Optionally, during the steering redundancy control, the adaptive fault-tolerant controller outputs the steering angles of the four wheels to the vehicle, specifically:

[0021] Construct the state-space equation of the vehicle when the steering actuator fails:

[0022]

[0023] where the state variable x(t) = [β ω] T , β and ω are the actual sideslip angle of the center of mass and the yaw rate respectively, the superscript · represents differentiation, and the superscript T represents transpose; the input variable u f (t) = [δ fl δ fr δ rl δ rr , δ fl 、δ fr 、δ rl and δ rr are the steering angles of the left front wheel, right front wheel, left rear wheel and right rear wheel respectively; A represents the system matrix and B represents the input matrix;

[0024] Define the tracking error e as:

[0025]

[0026] where x d (t) = [β d ω d T , β d and ω d are the desired sideslip angle of the center of mass and the yaw rate respectively;

[0027] Calculate according to the state-space equation and the tracking error to get k is an arbitrary positive constant;

[0028] The adaptive fault-tolerant controller outputs the steering angles δ f 、δ fl 、δ fr 、δ rl and δ rr of the four wheels to the vehicle.

[0029] Optionally, during the steering redundancy control, the four-wheel independent steering two-degree-of-freedom vehicle model is:

[0030] ​

[0031] In the formula, β and ω are the actual sideslip angle of the center of mass and the yaw angular velocity respectively, and the superscript · represents differentiation; a and b are the distances from the center of mass to the front and rear axles respectively; m represents the mass of the vehicle; C f and C r are the cornering stiffnesses of the front and rear wheels respectively; u x is the longitudinal vehicle speed; δ fl , δ fr , δ rl and δ rr are the wheel angles of the left front wheel, right front wheel, left rear wheel and right rear wheel respectively; I z is the moment of inertia of the vehicle about the z-axis.

[0032] Optionally, during the steering redundancy control, the sliding mode controller outputs the driving torques of the four wheels to the vehicle, specifically:

[0033] The sliding mode controller selects the sliding mode surface according to the yaw angular velocity error e ω = ω d - ω, and calculates the resultant torque M d of the driving torques of the four wheels;

[0034] The dynamic model of the differential steering system is constructed as:

[0035]

[0036] In the formula, the subscript n = f, r; J f and J r are the effective moments of inertia of the front and rear differential steering systems respectively; b f and b r are the effective damping of the front and rear differential steering systems respectively; δ f and δ r are the wheel angles of the front and rear wheels respectively; ΔM′ f and ΔM r ′ are the differences in the steering torques about the kingpin of the front and rear wheels respectively; ΔM f and ΔM r are the additional yaw torques of the front and rear wheels respectively, ΔM f + ΔM r = M d ; T dfr and T drr are the driving torques of the right front wheel and right rear wheel respectively; T dfl and T drl are the driving torques of the left front wheel and left rear wheel respectively; τ af and τ ar are the total self-aligning torques of the front and rear differential steering systems respectively; C f and C rare the cornering stiffnesses of the front and rear wheels respectively; α f and α r are the cornering angles of the front and rear wheels respectively; l is half of the track width; l s is half of the tire trail; r σ is the kingpin offset; r is the wheel radius; The superscript · represents differentiation, and the superscript ·· represents second - order differentiation;

[0037] Under the dynamic model of the differential steering system, the constraint conditions are set as:

[0038]

[0039] In the formula, the subscript i = fl, fr, rl, rr, representing the left front wheel, right front wheel, left rear wheel and right rear wheel respectively; T max is the maximum torque output by the motor; F xi , F yi and F zi are the lateral force, longitudinal force and vertical force of the wheel respectively; μ i is the road friction coefficient of the wheel;

[0040] Taking the lowest tire load rate J as the objective function, the objective function is set as:

[0041]

[0042] According to the objective function and constraint conditions, the driving torques of the four wheels are obtained.

[0043] Optionally, the process of steering degradation treatment satisfies the following vehicle states:

[0044]

[0045] In the formula, I z is the moment of inertia of the vehicle about the Z - axis; ω is the actual yaw angular velocity; F xfl , F xfr , F xrl and F xrr are the lateral forces of the left front wheel, right front wheel, left rear wheel and right rear wheel respectively; F yfl , F yfr , F yrl and F yrr are the longitudinal forces of the left front wheel, right front wheel, left rear wheel and right rear wheel respectively; l is half of the track width; a and b are the distances from the center of mass to the front axle and rear axle respectively; φ and are the front and rear wheel steering angles respectively.

[0046] Optionally, during the process of steering degradation treatment, the sliding - mode controller outputs the driving torques of the four wheels to the vehicle, specifically:

[0047] Under vehicle conditions, the constraint conditions are set as follows:

[0048]

[0049] In the formula, F fl , F fr , F rl and F rr are the braking forces of the left front wheel, right front wheel, left rear wheel and right rear wheel respectively; F zfl , F zfr , F zrl and F zrr are the vertical forces of the left front wheel, right front wheel, left rear wheel and right rear wheel respectively; μ fl , μ fr , μ rl and μ rr are the road friction factors of the left front wheel, right front wheel, left rear wheel and right rear wheel respectively;

[0050] Taking the lowest tire load rate J as the objective function, the objective function is set as:

[0051]

[0052] In the formula, the subscript i = fl, fr, rl, rr;

[0053] According to the objective function and constraint conditions, the driving torques of the four wheels are obtained.

[0054] In a second aspect, the present invention provides an electronic device, including: a memory, a processor, and a computer program stored on the memory and executable on the processor. When the processor executes the computer program, the steering control method of the four-wheel independent drive and steer electric vehicle in the first aspect is implemented.

[0055] The beneficial effects of the present invention are: The present invention can formulate a suitable fault tolerance control strategy when the steering actuator fails, greatly reducing the impact of the fault on the system, and thus improving the stability of the system; The present invention can utilize the differential drive of the four-wheel hub motors and the residual steering actuator to complete vehicle steering for different failure types of the steer-by-wire system, reducing costs while ensuring that the vehicle has a steering function. Description of the Drawings

[0056] Figure 1 is the tire force analysis diagram of a four-wheel steering vehicle.

[0057] Figure 2 is the steering control flowchart of a four-wheel independent drive and steer electric vehicle.

[0058] Figure 3 is the steering control strategy diagram of a four-wheel independent drive and steer electric vehicle.

[0059] Figure 4 It is a schematic diagram of the dynamic model of the differential steering system. Specific implementation manner

[0060] Next, the technical solutions in the embodiments of the present application will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present application.

[0061] In one embodiment, the present invention proposes a steering control method for an electric vehicle with four-wheel independent drive and steering. The forces on the tires of a four-wheel steering vehicle are as Figure 1 shown. Since all four wheels are independently controllable, when a part of the steering actuator fails, the residual functions of the normal steering actuator and the faulty steering actuator are used together to complete the steering of the vehicle. At the same time, to ensure the stability of the vehicle, differential drive / braking is used to provide additional yaw force to ensure the stability of the vehicle.

[0062] When the vehicle controller detects that the body angle does not steer according to the steering wheel angle, it is determined that the steering system has a fault, and at this time, the steering fault tolerance control system intervenes. The vehicle controller obtains the desired wheel angle of the driver at this time according to the steering wheel angle of the driver, and obtains the steering wheel angle, longitudinal vehicle speed signal, yaw angular velocity, and sideslip angle of the center of mass of the running vehicle. At this time, it is judged whether the vehicle steering actuator is faulty. According to the fault tree, the actuator faults are divided into three categories: partial failure, communication interruption, and jamming fault, as shown in Table 1 specifically, where ε and λ are fault factors.

[0063] Table 1 Actuator fault types

[0064] Fault type ε λ Normal 1 0 Partial failure 0-1 0,1 Communication interruption 0 0,1 Jamming fault 0 0,1

[0065] It should be noted that the above faults are classified for a single steering actuator. In this embodiment, the vehicle is an electric vehicle with four-wheel independent drive and steering, and the vehicle has a total of four steering actuators, each of which is independently controllable. The purpose of this embodiment is to use the four independently controllable steering wheels of the vehicle to complete steering for different failure types and degrees of failure of the steering actuator.

[0066] Define the steering actuator fault input u f (t) as:

[0067] u f (t) = ε i u(t) + λ i u o (t) (1);

[0068] In the formula, u o (t) represents a jamming fault, u(t) represents a partial failure and communication interruption fault of the steering actuator, i = fl, fr, rl, rr represent the four wheels of the left front wheel, right front wheel, left rear wheel, and right rear wheel, and εi , λ i represents the fault factor corresponding to the wheel.

[0069] According to the four-wheel independent steering two-degree-of-freedom vehicle model:

[0070]

[0071] In the formula, β and ω are the actual sideslip angle of the center of mass and the yaw angular velocity respectively, and the superscript · represents differentiation; a and b are the distances from the center of mass to the front and rear axles respectively; m represents the mass of the vehicle; C f and C r are the cornering stiffnesses of the front and rear wheels respectively; u x is the longitudinal speed of the vehicle; δ fl , δ fr , δ rl and δ rr are the wheel angles of the left front wheel, right front wheel, left rear wheel and right rear wheel respectively; I z is the moment of inertia of the vehicle about the z-axis.

[0072] The state-space equation can be obtained through Equation (2) as follows:

[0073]

[0074] In the formula, the state variable x(t) = [β ω] T , the input variable u(t) = [δ fl δ fr δ rl δ rr , y(t) represents the output variable; A is the system matrix, B is the input matrix, and C is the output matrix.

[0075] The steering control process of the four-wheel independent drive and steering electric vehicle is as Figure 2 shown. When the vehicle is steering normally, after the driver inputs the desired steering wheel angle, the wheel angles of the four wheels can be obtained through the Ackermann steering relationship and transmitted to the four steering actuators. When the deviation between the actual angle executed by the vehicle steering actuator and the expected angle is too large, that is, when Equation (4) is not satisfied, it is considered that the steering actuator fails and the adaptive fault-tolerant controller intervenes. After the fault-tolerant controller intervenes, to ensure the stability of the vehicle, in this embodiment, the safety range of the vehicle is jointly determined according to the wheel angle and the yaw angular velocity:

[0076] |δ i - δ id | ≤ C1 (4);

[0077] |ω - ω d | ≤ C2 (5);

[0078] Among them, C1 and C2 are the upper bounds of the steady-state error values, and their specific values can be obtained through experiments for different vehicle models. When the inequality (5) is not satisfied, the vehicle will be in an unstable state. δ i and δ id represent the actual and desired wheel angles respectively, and ω and ω d represent the actual and desired yaw rates respectively. When both inequalities (4) and (5) are satisfied, it is determined that the steering actuator has no fault. At this time, take ε = 1 and λ = 0.

[0079] When the steering actuator is normal, the steering redundancy controller does not intervene, and only the steering actuator is used to complete vehicle steering.

[0080] Partial failure means that the steering actuator can perform steering at a certain angle but cannot reach the driver's expected angle. At this time, the remaining normal steering actuators, the residual functions of the faulty steering actuator, and differential steering are used to complete vehicle steering.

[0081] When there is a communication fault in the steering actuator, only differential steering is used to complete steering. When the steering actuator is stuck, if the inequality (5) is satisfied, the faulty motor is locked, and at the same time, the remaining normal steering actuators are used to complete vehicle steering. If the inequality (5) is not satisfied, steering degradation is performed, and differential braking is used to keep the vehicle stable and stop.

[0082] In summary, as Figure 3 shown, when the steering actuator fails, the steering redundancy controller outputs the wheel angles of the four wheels according to the failure type and degree of failure, and uses the normal steering actuator and the residual functions of the faulty steering actuator to complete steering. If steering cannot be completed, differential steering is used for auxiliary steering. When dangerous situations such as vehicle instability occur, differential drive / braking is used to generate an additional yaw moment to maintain the stability of the vehicle body while performing parking.

[0083] For different types of failure modes and degrees of failure of the steering system, the values of ε and λ are different. The following are the specific value-taking methods:

[0084] When a certain steering actuator does not satisfy the inequality (4) but the vehicle body state satisfies the inequality (5), it is determined that the steering actuator of the vehicle is partially failed at this time, and take δ i as the maximum angle that the wheel can reach at this time, and δ id as the wheel angle expected by the driver.

[0085] When the wheel angle does not respond to the steering wheel angle for two consecutive cycles, if the vehicle controller can receive the feedback signal sent back by the steering controller at this time, it is determined that the steering actuator is stuck. If the feedback signal sent by the steering actuator cannot be received, it is determined that the signal is interrupted.

[0086] When it is a jamming fault, at this time, ε of the actuator is 0, and the remaining normal steering actuators and differential steering are used to jointly complete vehicle steering.

[0087] When there is a communication interruption, at this time, the steering actuator cannot normally receive the instructions sent by the vehicle controller, stops controlling the steering motor, ε = 0, and four-wheel differential drive is used to complete vehicle steering.

[0088] It should be noted that in any fault state, when the vehicle body state does not satisfy Equation (5), that is, the vehicle has exceeded the stability boundary, λ = 1 is taken for steering degradation processing. At this time, differential braking is used to generate additional yaw moment and the vehicle is stably stopped by increasing the front wheel toe-in.

[0089] When the vehicle takes steering degradation processing, the front wheel toe-in is increased by controlling the wheels to rotate towards the central axis direction, that is, the left wheel rotates to the right and the right wheel rotates to the left.

[0090] Additional yaw angular velocity is generated by controlling the braking torques of the four wheels to balance the additional yaw angular velocity of the vehicle and make the vehicle stop stably. For the current vehicle state, there is:

[0091]

[0092] In the formula, I z is the moment of inertia of the vehicle about the Z axis; ω is the actual yaw angular velocity; F xfl , F xfr , F xrl and F xrr are the lateral forces of the left front wheel, right front wheel, left rear wheel and right rear wheel respectively; F yfl , F yfr , F yrl and F yrr are the longitudinal forces of the left front wheel, right front wheel, left rear wheel and right rear wheel respectively; l is half of the wheelbase; a and b are the distances from the center of mass to the front axle and rear axle respectively; φ and are the front and rear wheel steering angles respectively. At this time, the total braking force of the four wheels should be within the adhesion circle with μF z as the radius, and μF z represents the maximum braking force that the ground can provide:

[0093]

[0094] In the formula, F fl , F fr , F rl and F rr are the braking forces of the left front wheel, right front wheel, left rear wheel and right rear wheel respectively; F zfl , F zfr , F zrl and Fzrr The vertical forces of the left front wheel, right front wheel, left rear wheel, and right rear wheel respectively; μ fl , μ fr , μ rl and μ rr The road friction coefficients of the left front wheel, right front wheel, left rear wheel, and right rear wheel respectively.

[0095] Taking the minimum tire load rate as the objective function, the driving torques of the left and right wheels are obtained according to the constraint conditions. Objective function:

[0096]

[0097] Combining equations (6), (7), and (8), the driving torques of the four wheels can be obtained.

[0098] When a fault occurs in the vehicle steering system but the vehicle is still in the stable region, that is, equation (4) is not satisfied but equation (5) is satisfied. At this time, the residual functions of the normal steering actuator and the faulty steering actuator and the differential steering of the left and right hub motors are used together to enable the vehicle to accurately steer according to the driver's intention. At this time, the input of the four-wheel angles is u f (t) = [δ fl δ fr δ rl δ rr , that is, the input when the steering actuator fails. From equations (1) and (3), the state space equation at this time is:

[0099]

[0100] Define the tracking error e as:

[0101]

[0102] In the formula, x d (t) = [β d ω d T , β d and ω d are the desired sideslip angle of the center of mass and yaw rate respectively;

[0103] Design the input k is an arbitrary positive constant used to ensure the stability of the control system.

[0104] Select the Lyapunov function V1:

[0105]

[0106] Taking the derivative of V1 gives:

[0107] ​

[0108] It can be obtained that the designed steering redundancy controller is stable.

[0109] In order to enable the vehicle to complete steering according to the driver's intention even when the steering system fails to different degrees, in this embodiment, differential steering is used for auxiliary steering in the fault state.

[0110] First, the sliding mode controller is used to calculate the additional yaw moment required to track the reference model.

[0111] According to the yaw rate error e ω = ω d - ω, the sliding mode surface s is selected as:[[]]

[0112] s = ce ω (13);

[0113] In the formula, the value of c is a positive constant, which is the controller parameter that satisfies the Routh-Hurwitz stability criterion.

[0114] The exponential reaching law is selected to reduce the system jitter, that is:[[]]

[0115]

[0116] In the formula,[[]] and k ω are controller parameters, both greater than zero. When k ω is increased while is decreased, it can make the transition during the sliding mode switching smoother. From the characteristics of the sliding mode controller, it can be known that it can be obtained that:[[]]

[0117]

[0118] Therefore, the required resultant moment M d is:[[]]

[0119]

[0120] Since the differential torque has an approximately linear relationship with the wheel angle, in this embodiment, it is approximately considered that there is the following proportional relationship between the front and rear wheel angles and the differential torque of the front and rear axles:[[]]

[0121]

[0122] In the formula, ΔM f and ΔM r represent the differential torques required for the front and rear axles of the vehicle. When the front and rear wheel angles of the vehicle are in the same direction, k δ is positive, and vice versa, the value of k δ is negative. At this time, ΔM f + ΔM r = Md 。

[0123] Build a dynamic model of the differential steering system as shown in Figure 4 :

[0124]

[0125] In the formula, the subscript n = f, r; J f and J r are the effective moments of inertia of the front and rear differential steering systems respectively; b f and b r are the effective damping of the front and rear differential steering systems respectively; δ f and δ r are the steering angles of the front and rear wheels respectively; ΔM′ f and ΔM r ′ are the differences in the steering torques around the kingpin of the front and rear wheels respectively; ΔM f and ΔM r are the additional yaw torques of the front and rear wheels respectively, ΔM f +ΔM r = M d ; T dfr and T drr are the driving torques of the right front wheel and the right rear wheel respectively; T dfl and T drl are the driving torques of the left front wheel and the left rear wheel respectively; τ af and τ ar are the total self-aligning torques of the front and rear differential steering systems respectively; C f and C r are the cornering stiffnesses of the front and rear wheels respectively; α f and α r are the sideslip angles of the front and rear wheels respectively; l is half of the track width; l s is half of the tire trail; r σ is the kingpin offset; r is the wheel radius.

[0126] To achieve steering by using the residual steering actuator and the differential drive of the hub motor when the steering system fails and stable parking when the vehicle is out of control. The following constraint conditions are formulated:

[0127]

[0128] In the formula, T max is the maximum torque output by the motor.

[0129] According to equations (8), (16)-(19), the driving torques of the four wheels can be obtained, enabling the vehicle to complete steering according to the driver's intention.

[0130] In another embodiment, the present invention provides an electronic device, comprising: a memory, a processor, and a computer program stored on the memory and executable on the processor. When the processor executes the computer program, the steering control method of the four-wheel independently-driven and steered electric vehicle in the foregoing embodiment is implemented.

[0131] Those of ordinary skill in the art can realize that the units and algorithm steps of each example described in combination with the embodiments disclosed in the present application can be implemented by electronic hardware or a combination of computer software and electronic hardware. Whether these functions are executed in a hardware or software manner depends on the specific application and design constraints of the technical solution. Professional technicians can use different methods to implement the described functions for each specific application, but such implementation should not be considered to exceed the scope of the present application.

[0132] The above are only the preferred embodiments of the present invention, and the protection scope of the present invention is not limited to the above embodiments. All technical solutions falling within the concept of the present invention belong to the protection scope of the present invention. It should be noted that for those of ordinary skill in the art in the technical field, several improvements and refinements made without departing from the principle of the present invention should be regarded as within the protection scope of the present invention.

Claims

1. A steering control method for a four-wheel independently-driven and steered electric vehicle, characterized in that Including: Input the desired steering wheel angle, obtain the desired wheel angles through the Ackermann steering relationship, and transmit them to the steering actuator for execution; When the deviation between the actual wheel angle of the steering actuator and the desired wheel angle exceeds the upper bound of the stable error value, it is determined that the steering actuator has a fault. Then, judge whether the vehicle is in a stable state according to the yaw rate of the vehicle: If so, adopt steering redundancy control and use the residual functions of the normal steering actuator and the faulty steering actuator to steer the vehicle; If not, adopt steering degradation processing, use differential braking to generate braking torques for the four wheels and increase the front wheel toe-in to stop the vehicle; The process of the steering redundancy control includes: Identify the fault type of the steering actuator, set the fault factor input to the adaptive fault-tolerant controller according to the fault type, the adaptive fault-tolerant controller outputs the four wheel angles to the vehicle, and the vehicle feeds back the actual yaw rate ω to the sliding mode controller; Output the desired yaw rate ω through a four-wheel independent steering two-degree-of-freedom vehicle model d to the sliding mode controller, which, based on the input ω and ω d , combines the set constraint conditions and objective function and outputs the driving torques of the four wheels to the vehicle; During the process of the steering redundancy control, the adaptive fault-tolerant controller outputs the four wheel angles to the vehicle, specifically: Construct the state space equation of the vehicle when the steering actuator fails: Among them, the state variable \(x(t)=[\beta\ \omega]^T\), where \(\beta\) and \(\omega\) are the actual sideslip angle of the center of mass and the yaw angular velocity respectively, the superscript \(\cdot\) represents differentiation, and the superscript \(T\) represents transpose; the input variable \(u(t)=[\delta_1\ \delta_2\ \delta_3\ \delta_4]^T\), where \(\delta_1\), \(\delta_2\), \(\delta_3\), and \(\delta_4\) are the wheel angles of the left front wheel, right front wheel, left rear wheel, and right rear wheel respectively; \(A\) represents the system matrix, and \(B\) represents the input matrix. T , where \(\beta\) and \(\omega\) are the actual sideslip angle of the center of mass and the yaw angular velocity respectively, the superscript \(\cdot\) represents differentiation, and the superscript \(T\) represents transpose; the input variable \(u\) f (t)=[\delta fl \(\delta\) fr \(\delta\) rl \(\delta\) rr ], \(\delta\) fl 、\(\delta\) fr 、\(\delta\) rl and \(\delta\) rr are the wheel angles of the left front wheel, right front wheel, left rear wheel, and right rear wheel respectively; \(A\) represents the system matrix, and \(B\) represents the input matrix. Define the tracking error e as: where x d (t) = [β d ω d T , β d and ω d are the desired sideslip angle of the center of mass and the yaw angular velocity, respectively;​ Calculated based on the state space equation and the tracking error k is an arbitrary positive constant; The adaptive fault-tolerant controller outputs the steering angles δ f (t) of the four wheels to the vehicle according to u fl , δ fr , δ rl and δ rr ; The process of the steering degradation processing includes: The vehicle feeds back the actual yaw rate ω to the sliding mode controller, and at the same time outputs the desired yaw rate ω through the four-wheel independent steering two-degree-of-freedom vehicle model d to the sliding mode controller, which, based on the input ω and ω d , combines the set constraint conditions and objective function and outputs the driving torques of the four wheels to the vehicle; The vehicle rotates the wheels towards the central axis direction to increase the front wheel toe-in and outputs the four wheel angles to the vehicle; The process of the steering degradation processing satisfies the following vehicle state: where, I z is the moment of inertia of the vehicle about the Z-axis; ω is the actual yaw rate; F xfl , F xfr , F xrl and F xrr are the lateral forces of the left front wheel, right front wheel, left rear wheel and right rear wheel respectively; F yfl , F yfr , F yrl and F yrr are the longitudinal forces of the left front wheel, right front wheel, left rear wheel and right rear wheel respectively; l is half of the track width; a and b are the distances from the center of mass to the front axle and rear axle respectively; φ and are the front and rear wheel steering angles respectively; During the process of the steering degradation processing, the sliding mode controller outputs the four wheel driving torques to the vehicle, specifically: Under the vehicle state, set the constraint conditions as follows: Where, F fl , F fr , F rl and F rr are the braking forces of the left front wheel, right front wheel, left rear wheel and right rear wheel respectively; F zfl , F zfr , F zrl and F zrr are the vertical forces of the left front wheel, right front wheel, left rear wheel and right rear wheel respectively; μ fl , μ fr , μ rl and μ rr are the road friction coefficients of the left front wheel, right front wheel, left rear wheel and right rear wheel respectively; Take the lowest tire load rate J as the objective function, and set the objective function as: In the formula, the subscript i = fl, fr, rl, rr; According to the objective function and the constraint conditions, obtain the four wheel driving torques.

2. The steering control method of a four-wheel independently-driven and -steered electric vehicle according to claim 1, characterized in that: The specific method of judging whether the vehicle is in a stable state according to the yaw rate of the vehicle is: When the deviation between the actual yaw rate and the desired yaw rate exceeds the upper bound of the stable error value, the vehicle is in an unstable state; otherwise, the vehicle is in a stable state.

3. The steering control method of a four-wheel independently driven and steered electric vehicle according to claim 1, characterized in that: During the process of the steering redundancy control, the four-wheel independent steering two-degree-of-freedom vehicle model is: where β and ω are the actual sideslip angle of the center of mass and the yaw rate, respectively, and the superscript · represents differentiation; a and b are the distances from the center of mass to the front and rear axles, respectively; m represents the mass of the vehicle; C f and C r are the cornering stiffnesses of the front and rear wheels, respectively; u x is the longitudinal speed of the vehicle; δ fl , δ fr , δ rl and δ rr are the wheel angles of the left front wheel, right front wheel, left rear wheel, and right rear wheel, respectively; I z is the moment of inertia of the vehicle about the z-axis.

4. The steering control method for a four-wheel independently driven and steered electric vehicle according to claim 1, characterized in that: During the process of the steering redundancy control, the sliding mode controller outputs the four wheel driving torques to the vehicle, specifically: The sliding mode controller selects the sliding mode surface according to the yaw rate error e ω = ω d - ω, and calculates the resultant torque M of the driving torques of the four wheels d ; Construct the dynamic model of the differential steering system as: where the subscript n = f, r; J f and J r are the effective moments of inertia of the front and rear differential steering systems respectively; b f and b r are the effective damping of the front and rear differential steering systems respectively; δ f and δ r are the front and rear wheel angles respectively; ΔM′ f and ΔM r ′ are the differences in the steering torques about the kingpin of the front and rear wheels respectively; ΔM f and ΔM r are the additional yaw torques of the front and rear wheels respectively, ΔM f +ΔM r = M d ; T dfr and T drr are the driving torques of the right front wheel and the right rear wheel respectively; T dfl and T drl are the driving torques of the left front wheel and the left rear wheel respectively; τ af and τ ar are the total self-aligning torques of the front and rear differential steering systems respectively; C f and C r are the cornering stiffnesses of the front and rear wheels respectively; α f and α r are the sideslip angles of the front and rear wheels respectively; l is half of the track width; l s is half of the tire trail; r σ is the kingpin offset; r is the wheel radius; the superscript · represents differentiation, and the superscript ·· represents second - order differentiation; Under the dynamic model of the differential steering system, set the constraint conditions as: where the subscript i = fl, fr, rl, rr, representing the left front wheel, right front wheel, left rear wheel, and right rear wheel respectively; T max is the maximum torque output by the motor; F xi , F yi and F zi are the lateral force, longitudinal force, and vertical force of the wheel respectively; μ i is the road friction coefficient of the wheel; Take the lowest tire load rate J as the objective function, and set the objective function as: According to the objective function and the constraint conditions, obtain the driving torques of the four wheels.

5. An electronic device, characterized in that, Including: A memory, a processor, and a computer program stored on the memory and executable on the processor. When the processor executes the computer program, it implements the steering control method of the four-wheel independent drive and steering electric vehicle as described in any one of claims 1-4.

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