Distributed steer-by-wire vehicle fault-tolerant control method

Abstract

The invention provides a distributed steer-by-wire vehicle fault-tolerant control method. The method comprises the steps that a distributed steer-by-wire vehicle dynamics model, a two-degree-of-freedom reference model and a steering motor failure model are established; in the design of the upper fault-tolerant controller, a self-adaptive terminal sliding mode control method is adopted, the method has the characteristics of high robustness and self-adaptive parameter change, and the robustness of the system can be considered while the target tracking performance is ensured. In the design of a lower-layer torque and rotation angle optimal distribution reconstruction controller, a failure factor is introduced, two more objective functions are established on the basis of minimizing the tire load rate, and errors between longitudinal and lateral forces and expected target values and errors between yawing moment and additional yawing moment are reduced while the optimized objective functions are ensured to have solutions. According to the method, the uncertainty of the system during the steering failure is fully considered, and the robustness of the system can be considered while the target tracking performance is ensured, so that the control stability of the distributed steer-by-wire vehicle during the steering failure is improved.

Description

Technical Field

[0001] The present invention relates to the field of automobile assisted driving, and in particular to a fault-tolerant control method for a distributed steer-by-wire vehicle. Background Art

[0002] The distributed steer-by-wire system has two sets of steering actuators on the front and rear axles (front and rear axle type) or four sets of steering actuators on four wheels (four-wheel distributed). Compared with the front-wheel steer-by-wire system, the distributed steer-by-wire system is free from the mechanical connection constraints between the left and right wheels, and can provide a wider range of lateral control force. The wheel angle is no longer limited by the trapezoidal mechanical structure, and the steering is more flexible, which improves the vehicle's maneuverability and flexibility. It exhibits more precise, agile and stable steering performance during the steering process, and can complete special motion conditions such as low-speed steering, on-the-spot steering, oblique driving and translation.

[0003] Distributed steer-by-wire systems have multiple actuators. As the number of actuators increases, the probability of actuator failure also increases. In addition, distributed steer-by-wire systems are susceptible to harsh environments, which may damage their structures. Therefore, it is urgent to introduce fault-tolerant control in distributed steer-by-wire systems to reduce the impact of actuator failures on the handling stability control of distributed steer-by-wire vehicles. Distributed steer-by-wire vehicle handling stability control currently uses control methods such as ordinary sliding mode control, PID control, model predictive control, and neural network control. Although ordinary sliding mode control has the advantages of simple implementation and strong robustness, it has the defect of jitter problem and is not suitable for application in distributed steer-by-wire vehicle stability control. Although PID control has the characteristics of simple design and flexible structure, it is mostly used in single-input and single-output systems. Model predictive control linearizes the vehicle model, which can reduce the amount of calculation in the control process and improve the real-time performance of the control, but it also reduces the control accuracy. Its calculation burden will increase with the complexity of the system dynamics. Neural network control can approximate external disturbances and modeling errors, and has strong robustness and fault tolerance, but it has disadvantages such as slow learning speed and long calculation time, and it is difficult to prove the convergence and stability of the learning process in an analytical way.

[0004] At present, most of the torque and steering angle distribution reconstruction controllers of distributed steer-by-wire systems only use minimizing the tire load rate as the optimization goal. Under complex working conditions, it is easy to have no solution, so repeated solutions are needed, resulting in long optimization solution time and slow tire force distribution response. Summary of the invention

[0005] The purpose of the present invention is to propose a fault-tolerant control method for a distributed steer-by-wire vehicle; the uncertainty of the system when the steering fails is fully considered, and the robustness of the system can be taken into account while ensuring the target tracking performance, so as to improve the handling stability of the distributed steer-by-wire vehicle when the steering fails.

[0006] To achieve the above object, the technical solution of the present invention is: a distributed steer-by-wire vehicle fault-tolerant control method, comprising the following steps:

[0007] S1. Establish a distributed steer-by-wire vehicle dynamics model;

[0008] S2, establishing a two-degree-of-freedom vehicle reference model;

[0009] S3, determining a desired longitudinal speed and an ideal lateral vehicle speed;

[0010] S4. Establishing a failure model of the steering motor of a distributed steer-by-wire vehicle;

[0011] S5. Establishing an upper adaptive terminal sliding mode fault-tolerant controller for a distributed steer-by-wire vehicle;

[0012] S6. Establish a distributed steer-by-wire vehicle lower-layer torque and steering angle optimization distribution reconstruction controller.

[0013] Preferably, the establishment of a distributed steer-by-wire vehicle dynamics model is specifically as follows:

[0014] Longitudinal dynamic equation:

[0015]

[0016] Lateral dynamics equation:

[0017]

[0018] Yaw dynamics model:

[0019]

[0020] Among them are:

[0021]

[0022] Where m is the mass of the vehicle; v x and v y are the components of the vehicle's center of mass velocity on the x-axis and y-axis respectively; a x is the longitudinal acceleration; a y is the lateral acceleration; ω r is the yaw rate of the vehicle; β is the sideslip angle of the center of mass of the vehicle; I z is the yaw moment of inertia of the vehicle around the z-axis; l f , l r are the distances from the center of mass of the vehicle to the front and rear axles respectively; b is half of the wheelbase; ij = fl, fr, rl, rr represent the left front wheel, right front wheel, left rear wheel and right rear wheel of the vehicle respectively; F tyij 、Ftxij are tire lateral force and tire longitudinal force respectively; F xij 、F yij F txij 、F tyij The longitudinal force and lateral force of each wheel decomposed and synthesized in the vehicle coordinate system; F x 、F y F txij 、F tyij The total longitudinal force and total lateral force decomposed and synthesized in the vehicle coordinate system; M z is the yaw moment of the vehicle; ij is the wheel angle;

[0023] Build the wheel dynamics model:

[0024]

[0025] Where, J is the moment of inertia; ω ij is the angular velocity of each wheel; T ij is the driving torque; F zij is the vertical load; f ij is the rolling resistance coefficient; r ij is the rolling radius;

[0026] According to the moment balance relationship, the vertical load on each wheel is calculated as follows:

[0027]

[0028] In the formula, g is the acceleration due to gravity; h is the height of the center of mass from the ground;

[0029] Building the Magic Formula Tire Model:

[0030] Y(x)=Dsin{Carctan[Bx-E(Bx-arctan(Bx))]} (7)

[0031] Where Y(x) is the lateral force F of the tire tyij Or the longitudinal force F txij ; x is the tire slip angle α ij Or tire slip rate λ ij ; B, C, D, and E are fitting parameters in the tire characteristic curve, which are stiffness factor, curve shape factor, curve peak factor, and curve curvature factor, respectively.

[0032] Preferably, the tire slip rate of each wheel is calculated as:

[0033]

[0034] In the formula, u ij is the wheel center speed of each wheel; ωij is the angular velocity of each wheel; r is the rolling radius of the wheel;

[0035] The wheel center speed of each wheel can be expressed as:

[0036]

[0037] Preferably, the slip angles of the four wheels are calculated as:

[0038]

[0039] Preferably, the establishment of a two-degree-of-freedom vehicle reference model is specifically as follows:

[0040]

[0041] In the formula, k f is the front axle equivalent cornering stiffness of the reference model; k r is the equivalent cornering stiffness of the rear axle of the reference model; δ is the front wheel turning angle of the reference model;

[0042] The ideal yaw rate of a car in steady-state driving is as follows:

[0043]

[0044] In the formula, ω rd is the ideal yaw rate of the car when it is driving in a steady state, and K is the stability factor:

[0045]

[0046] The expected yaw rate must satisfy the following equation:

[0047]

[0048] Where μ represents the road adhesion coefficient. The ideal yaw rate of the reference model based on equations (12) and (14) can be expressed as:

[0049]

[0050] Preferably, the desired longitudinal velocity is expressed as:

[0051]

[0052] In the formula, v xd is the expected longitudinal velocity; v x0 is the longitudinal speed at the initial moment; a xd is the target longitudinal acceleration.

[0053] Preferably, the ideal lateral vehicle speed is expressed as:

[0054] vyd =v x tanβ d (17)

[0055] In the formula, v yd is the ideal lateral speed, β d is the ideal center of mass sideslip angle.

[0056] Preferably, the establishment of a distributed steer-by-wire vehicle steering motor failure model is specifically as follows:

[0057] The output torque expression of the steering motor fault is:

[0058] u λ =ζu d +Δu (18)

[0059] Where ζ∈[0,1] is the steering motor failure factor; u λ is the fault output torque of the steering motor; u d is the expected output torque of the steering motor; Δu is other faults of the steering motor;

[0060] When ζ=0 and Δu=0, the steering motor fails completely, the brake mechanism in the steering system takes effect, and the wheel maintains the wheel angle when the steering motor fails completely, and fault-tolerant control is performed by adding yaw torque.

[0061] Preferably, the establishment of the upper layer adaptive terminal sliding mode fault-tolerant controller in S5 comprises the following steps:

[0062] S5.1. Modify the two-degree-of-freedom vehicle reference model to a four-wheel independent steering two-degree-of-freedom vehicle model and add an additional yaw moment ΔM:

[0063]

[0064] S5.2. The tracking error of the additional yaw moment adaptive terminal sliding mode controller is established as:

[0065] e r =ω r -ω rd (20)

[0066] S5.3. Establish the sliding mode surface of the additional yaw moment adaptive terminal sliding mode controller:

[0067]

[0068] Where k is an adaptive parameter, c>0, q and p are positive odd numbers, and the expression of k is:

[0069]

[0070] Wherein, σ>0,ξ>0,0<ε<1; w is any real number; e is the natural logarithm;

[0071] S5.4, using the exponential reaching law to switch the additional yaw moment adaptive terminal sliding mode controller:

[0072]

[0073] Where η>0,κ>0

[0074] S5.5. Combining equations (19) to (23), the final control law of the adaptive terminal sliding mode controller is obtained:

[0075]

[0076] Where ΔM is the additional yaw moment, and the positive and negative represent the steering direction of the vehicle, positive for left turn and negative for right turn; S5.6, establish the tracking error of the longitudinal speed adaptive terminal sliding mode controller of the distributed wire-controlled steering vehicle:

[0077] e x =v x -v xd (25)

[0078] S5.7. Establish the sliding mode surface of the distributed steer-by-wire vehicle longitudinal speed adaptive terminal sliding mode controller:

[0079]

[0080] In the formula, c x is a controller parameter with a value greater than zero, q x 、p x is a positive odd number, k x The expression is:

[0081]

[0082] In the formula, σ x >0,ξ x >0,0<ε x <1; w x is any real number; e is the natural logarithm;

[0083] S5.8. Use the exponential reaching law to switch the longitudinal vehicle speed adaptive terminal sliding mode controller:

[0084]

[0085] Where η x , κ x are controller parameters with values ​​greater than zero;

[0086] S5.9, combining equation (1), equation (25)-equation (28), the final control law of the longitudinal vehicle speed adaptive terminal sliding mode controller is obtained:

[0087]

[0088] In the formula, F xd is the expected total longitudinal force;

[0089] S5.10. Establish the tracking error of the distributed steer-by-wire vehicle lateral speed adaptive terminal sliding mode controller:

[0090] e y =v y -v yd (30)

[0091] S5.11. Establish the sliding mode surface of the distributed lateral speed adaptive terminal sliding mode controller of the wire-controlled vehicle:

[0092]

[0093] In the formula, c y is a controller parameter with a value greater than zero, q y 、p y is a positive odd number, k y The expression is:

[0094]

[0095] In the formula, σ y >0,ξ y >0,0<ε y <1; w y is any real number; e is the natural logarithm;

[0096] S5.12, use the exponential reaching law to switch the lateral vehicle speed adaptive terminal sliding mode controller:

[0097]

[0098] Where η y , κ y are controller parameters with values ​​greater than zero;

[0099] S5.13, combining equation (2), equation (30)-equation (33), the final control law of the lateral vehicle speed adaptive terminal sliding mode controller is obtained:

[0100]

[0101] In the formula, F yd is the expected total lateral force.

[0102] Preferably, the distributed steer-by-wire vehicle lower layer torque and steering angle optimization allocation reconstruction controller of step S6 is established by comprising the following steps:

[0103] S6.1. Establish the optimization objective function of the torque optimization distribution reconstruction controller:

[0104]

[0105] In the formula, F txdij To optimize the distribution of longitudinal forces on the tire; F tydij To optimize the distribution of tire lateral forces; F zij is the tire vertical load; μ represents the road adhesion coefficient; τ ij is the weight coefficient of each wheel, which is used to adjust the proportion of each tire force in the entire optimization objective function;

[0106] S6.2. Establish a second optimization objective function to prevent the first objective function optimization solver from having no solution under complex working conditions and to reduce the error between the longitudinal and lateral forces and the expected target values; the second optimization objective function is as follows:

[0107] min J2=min(η1(F x -F xd ) 2 +η2(F y -F yd ) 2 ) (36)

[0108] Where, η1 and η2 are the longitudinal and lateral force error weight coefficients;

[0109] S6.3. Establish a third optimization objective function to ensure that the optimization objective function has a solution and reduce the error of the yaw moment and the additional yaw moment. The third optimization objective function is as follows:

[0110] minJ3=min(η1(F x -F xd ) 2 +η2(F y -F yd ) 2 +η3(ΔM-M z ) 2 ) (37)

[0111] Where η3 is the yaw moment error weight coefficient;

[0112] S6.4. Establish the overall optimization model of torque and angle optimization distribution reconstruction controller:

[0113] When the steering motor of a wheel fails completely, the output torque of the steering motor is zero, and the lateral force is no longer allocated to the wheel. The optimization allocation controller will optimize the allocation among the remaining steering wheels. The lateral force to be allocated to the faulty wheel is:

[0114]

[0115] In the formula, failure time ζ ij =0, otherwise ζ ij =1;

[0116] Since the faulty wheel cannot provide the required lateral force, the failure factor is added to the constraint condition to obtain the overall optimization model:

[0117]

[0118] Where, T ijmax is the maximum output torque of the hub motor;

[0119] S6.5. Establish an inverse model of tire cornering characteristics:

[0120]

[0121] In the formula, C α is the tire cornering stiffness; p is a constant;

[0122] Combining equations (35)-(40) and (10), the expected lateral forces of the four wheels are obtained, and the expected wheel turning angle δ is obtained by using the inverse model of the tire side slip characteristics. dij .

[0123] Compared with the prior art, the present invention has the following beneficial effects:

[0124] In the design of the upper-layer fault-tolerant controller, the present invention adopts an adaptive terminal sliding mode control method. The adaptive terminal sliding mode control has strong robustness, and the adaptive parameters can be adaptively changed according to the changes of the sliding mode surface and the system state. While reducing the sliding mode control chattering, the convergence speed of the sliding mode is improved, and the convergence time is reduced. It is possible to take into account the robustness of the system while ensuring the target tracking performance; in the design of the lower-layer torque and angle optimization allocation reconstruction controller, three objective functions are established. On the basis of minimizing the tire load rate, two more objective functions are established to reduce the errors between the longitudinal and lateral forces and the expected target values ​​and the errors between the yaw moment and the additional yaw moment, thereby ensuring that the optimization objective function has a solution to improve the handling stability of the distributed wire-controlled steering when the steering fails. BRIEF DESCRIPTION OF THE DRAWINGS

[0125] Figure 1 A diagram of the distributed steer-by-wire vehicle dynamics model of the present invention;

[0126] Figure 2 This is a fault-tolerant control flow chart of a distributed steer-by-wire vehicle according to the present invention. DETAILED DESCRIPTION

[0127] The following is combined with Figure 1-2 , the technical solution of the present invention is specifically described.

[0128] like Figure 1 As shown, a distributed steer-by-wire vehicle fault-tolerant control method comprises the following steps:

[0129] S1. Establish a distributed steer-by-wire vehicle dynamics model:

[0130] Longitudinal dynamic equation:

[0131]

[0132] Lateral dynamics equation:

[0133]

[0134] Yaw dynamics model:

[0135]

[0136] Among them are:

[0137]

[0138] Where m is the vehicle mass; v x and v y are the components of the vehicle's center of mass velocity on the x-axis and y-axis respectively; a x is the longitudinal acceleration; a y is the lateral acceleration; ω r is the yaw rate of the vehicle; β is the sideslip angle of the center of mass of the vehicle; I z is the yaw moment of inertia of the vehicle around the z-axis; l f , l r are the distances from the center of mass of the vehicle to the front and rear axles respectively; b is half of the wheelbase; ij = fl, fr, rl, rr represent the left front wheel, right front wheel, left rear wheel and right rear wheel of the vehicle respectively; F tyij 、F txij are tire lateral force and tire longitudinal force respectively; F xij 、F yij F txij 、F tyij The longitudinal force and lateral force of each wheel decomposed and synthesized in the vehicle coordinate system; F x 、F y F txij 、Ftyij The total longitudinal force and total lateral force decomposed and synthesized in the vehicle coordinate system; M z is the yaw moment of the vehicle; ij is the wheel angle.

[0139] Build the wheel dynamics model:

[0140]

[0141] Where, J is the moment of inertia; ω ij is the angular velocity of each wheel; T ij is the driving torque; F zij is the vertical load; f ij is the rolling resistance coefficient; r ij is the rolling radius.

[0142] Considering the vertical load transfer of each wheel caused by the longitudinal and lateral movement of the vehicle during driving, according to the moment balance relationship, the vertical load calculation formula of each wheel is as follows:

[0143]

[0144] Where g is the acceleration due to gravity and h is the height of the center of mass from the ground.

[0145] Building the "Magic Formula" tire model:

[0146] Y(x)=Dsin{Carctan[Bx-E(Bx-arctan(Bx))]} (7)

[0147] Where Y(x) is the lateral force F of the tire tyij Or the longitudinal force F txij ; x is the tire slip angle α ij Or tire slip rate λ ij ; B, C, D, and E are fitting parameters in the tire characteristic curve, which are stiffness factor, curve shape factor, curve peak factor, and curve curvature factor, respectively.

[0148] The tire slip rate of each wheel is calculated as:

[0149]

[0150] In the formula, u ij is the wheel center speed of each wheel; ω ij is the angular velocity of each wheel; r is the rolling radius of the wheel.

[0151] The wheel center speed of each wheel can be expressed as:

[0152]

[0153] The slip angles of the four wheels are:

[0154]

[0155] S2. Establish a two-degree-of-freedom vehicle reference model:

[0156]

[0157] In the formula, k f is the front axle equivalent cornering stiffness of the reference model; k r is the equivalent cornering stiffness of the rear axle of the reference model; δ is the front wheel steering angle of the reference model.

[0158] The ideal yaw rate of a car in steady-state driving is as follows:

[0159]

[0160] In the formula, ω rd is the ideal yaw rate of the car when it is driving in a steady state, and K is the stability factor, which can be expressed as:

[0161]

[0162] Considering that the vehicle is affected by the adhesion coefficient between the tires and the ground when driving, the desired yaw rate must satisfy the following formula:

[0163]

[0164] The ideal yaw rate of the reference model based on equations (12) and (14) can be expressed as:

[0165]

[0166] S3, determining a desired longitudinal speed and an ideal lateral vehicle speed;

[0167] The longitudinal velocity of the vehicle is adjusted by controlling the longitudinal acceleration. The desired longitudinal velocity can be expressed as:

[0168]

[0169] In the formula, v xd is the expected longitudinal velocity; v x0 is the longitudinal speed at the initial moment; a xd is the target longitudinal acceleration.

[0170] Let the ideal center of mass sideslip angle be zero, and the ideal lateral speed can be obtained as:

[0171] v yd =v x tanβ d (17)

[0172] In the formula, v yd is the ideal lateral speed, β d is the ideal center of mass sideslip angle.

[0173] S4. Establishment of a failure model for the steering motor of a distributed steer-by-wire vehicle:

[0174] The output torque expression of the steering motor fault is:

[0175] u λ =ζu d +Δu (18)

[0176] Where ζ∈[0,1] is the steering motor failure factor; u λ is the fault output torque of the steering motor; u d is the expected output torque of the steering motor; Δu is other faults of the steering motor.

[0177] Only the complete failure of the steering motor with ζ=0 and Δu=0 is considered. When the steering motor fails completely, the brake mechanism in the steering system takes effect to keep the wheel at the wheel angle when the steering motor fails completely. When the steering motor fails, fault-tolerant control is performed by adding yaw torque.

[0178] S5. Establish an upper-layer adaptive terminal sliding mode fault-tolerant controller for a distributed steer-by-wire vehicle.

[0179] S6. Establish a distributed steer-by-wire vehicle lower-layer torque and steering angle optimization distribution reconstruction controller.

[0180] like Figure 2 As shown, the establishment of the upper layer adaptive terminal sliding mode fault-tolerant controller in S5 includes the following steps:

[0181] S5.1. Modify the two-degree-of-freedom vehicle reference model to a four-wheel independent steering two-degree-of-freedom vehicle model and add an additional yaw moment ΔM:

[0182]

[0183] S5.2. The tracking error of the additional yaw moment adaptive terminal sliding mode controller is established as:

[0184] e r =ω r -ω rd (20)

[0185] S5.3. Establish the sliding mode surface of the additional yaw moment adaptive terminal sliding mode controller:

[0186]

[0187] Where k is an adaptive parameter, c>0, q and p are positive odd numbers, and the expression of k is:

[0188]

[0189] In the formula, σ>0,ξ>0,0<ε<1; w is an arbitrary real number; e is the natural logarithm.

[0190] S5.4, using the exponential reaching law to switch the additional yaw moment adaptive terminal sliding mode controller:

[0191]

[0192] Where η>0,κ>0

[0193] S5.5. Combining equations (19) to (23), the final control law of the adaptive terminal sliding mode controller can be obtained:

[0194]

[0195] Where ΔM is the additional yaw moment, and its positive and negative values ​​represent the turning direction of the vehicle, with a positive value for left turn and a negative value for right turn.

[0196] S5.6. Establish the tracking error of the longitudinal speed adaptive terminal sliding mode controller of the distributed steer-by-wire vehicle:

[0197] e x =v x -v xd (25)

[0198] S5.7. Establish the sliding mode surface of the distributed steer-by-wire vehicle longitudinal speed adaptive terminal sliding mode controller:

[0199]

[0200] In the formula, c x is a controller parameter whose value is greater than zero, q x 、p x is a positive odd number, k x The expression is:

[0201]

[0202] In the formula, σ x >0,ξ x >0,0<ε x <1; w x is any real number; e is the natural logarithm.

[0203] S5.8. Use the exponential reaching law to switch the longitudinal vehicle speed adaptive terminal sliding mode controller:

[0204]

[0205] Where η x , κ x are controller parameters, and their values ​​are all greater than zero.

[0206] S5.9, Combining equation (1), equation (25)-equation (28), the final control law of the longitudinal vehicle speed adaptive terminal sliding mode controller can be obtained:

[0207] In the formula, F xd is the expected total longitudinal force.

[0208] S5.10. Establish the tracking error of the distributed steer-by-wire vehicle lateral speed adaptive terminal sliding mode controller:

[0209] e y =v y -v yd (30)

[0210] S5.11. Establish the sliding mode surface of the distributed lateral speed adaptive terminal sliding mode controller of the wire-controlled vehicle:

[0211]

[0212] In the formula, c y is a controller parameter whose value is greater than zero, q y 、p y is a positive odd number, k y The expression is:

[0213]

[0214] In the formula, σ y >0,ξ y >0,0<ε y <1; w y is any real number; e is the natural logarithm.

[0215] S5.12, using the exponential reaching law to switch the lateral vehicle speed adaptive terminal sliding mode controller:

[0216]

[0217] Where η y , κ y are controller parameters, and their values ​​are all greater than zero.

[0218] S5.13, Combining equation (2), equation (30)-equation (33), the final control law of the lateral vehicle speed adaptive terminal sliding mode controller can be obtained:

[0219]

[0220] In the formula, F yd is the expected total lateral force.

[0221] like Figure 2 As shown, the distributed steer-by-wire vehicle lower layer torque and steering angle optimization distribution reconstruction controller in S6 is established including the following steps:

[0222] S6.1. Establish the optimization objective function of the torque optimization distribution reconstruction controller:

[0223]

[0224] In the formula, F txdij To optimize the distribution of longitudinal forces on the tire; F tydij To optimize the distribution of tire lateral forces; F zij is the tire vertical load; μ represents the road adhesion coefficient; τ ij is the weight coefficient of each wheel, which is used to adjust the proportion of each tire force in the entire optimization objective function.

[0225] S6.2. In order to prevent the first objective function optimization solver from having no solution in complex working conditions and to minimize the error between the longitudinal and lateral forces and the expected target values, a second optimization objective function is established:

[0226] min J2=min(η1(F x -F xd ) 2 +η2(F y -F yd ) 2 ) (36)

[0227] Where η1 and η2 are the longitudinal and lateral force error weight coefficients.

[0228] S6.3. In order to further ensure that the optimization objective function has a solution and reduce the error of the yaw moment and the additional yaw moment, the third optimization objective function is established:

[0229] minJ3=min(η1(F x -F xd ) 2 +η2(F y -F yd ) 2 +η3(ΔM-M z ) 2 ) (37)

[0230] Where η3 is the yaw moment error weight coefficient.

[0231] S6.4. Establish the overall optimization model of torque and angle optimization distribution reconstruction controller:

[0232] When the steering motor of a wheel fails completely, the output torque of the steering motor is zero, and the lateral force is no longer allocated to the wheel. The optimization allocation controller will optimize the allocation among the remaining steering wheels. The lateral force to be allocated to the faulty wheel is:

[0233]

[0234] In the formula, failure time ζ ij =0, otherwise ζ ij =1.

[0235] Since the faulty wheel cannot provide the required lateral force, the failure factor is added to the constraint condition to obtain the overall optimization model:

[0236]

[0237] Where, T ijmax is the maximum output torque of the hub motor.

[0238] S6.5. Establish an inverse model of tire cornering characteristics:

[0239]

[0240] In the formula, C α is the tire cornering stiffness; p is a constant.

[0241] Combining equations (35)-(40) and (10), the expected lateral forces of the four wheels can be obtained, and the expected wheel turning angle δ can be obtained by using the inverse model of tire cornering characteristics. dij .

[0242] It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as those generally understood by those skilled in the art in the art to which the present invention belongs. It should also be understood that terms such as those defined in common dictionaries should be understood to have meanings consistent with the meanings in the context of the prior art, and will not be interpreted with idealized or overly formal meanings unless defined as herein.

[0243] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A distributed steer-by-wire vehicle fault-tolerant control method, characterized in that: The following steps are involved: S1. Establish a distributed steer-by-wire vehicle dynamics model; S2, establishing a two-degree-of-freedom vehicle reference model; S3, determining a desired longitudinal speed and an ideal lateral vehicle speed; S4. Establishing a failure model of the steering motor of a distributed steer-by-wire vehicle; S5. Establishing an upper adaptive terminal sliding mode fault-tolerant controller for a distributed steer-by-wire vehicle; S6. Establish a distributed steer-by-wire vehicle lower-layer torque and steering angle optimization distribution reconstruction controller.

2. The fault-tolerant control method for a distributed steer-by-wire vehicle according to claim 1, characterized in that: The distributed steer-by-wire vehicle dynamics model is established as follows: Longitudinal dynamic equation: Lateral dynamics equation: Yaw dynamics model: Among them are: Where m is the mass of the vehicle; v x and v y are the components of the vehicle's center of mass velocity on the x-axis and y-axis respectively; a x is the longitudinal acceleration; a y is the lateral acceleration; ω r is the yaw rate of the vehicle; β is the sideslip angle of the center of mass of the vehicle; I z is the yaw moment of inertia of the vehicle around the z-axis; l f , l r are the distances from the center of mass of the vehicle to the front and rear axles respectively; b is half of the wheelbase; ij = fl, fr, rl, rr represent the left front wheel, right front wheel, left rear wheel and right rear wheel of the vehicle respectively; F tyij 、F txij are tire lateral force and tire longitudinal force respectively; F xij 、F yij F txij 、F tyij The longitudinal force and lateral force of each wheel decomposed and synthesized in the vehicle coordinate system; F x 、F y F txij 、F tyij The total longitudinal force and total lateral force decomposed and synthesized in the vehicle coordinate system; M z is the yaw moment of the vehicle; ij is the wheel angle; Build the wheel dynamics model: Where, J is the moment of inertia; ω ij is the angular velocity of each wheel; T ij is the driving torque; F zij is the vertical load; f ij is the rolling resistance coefficient; r ij is the rolling radius; According to the moment balance relationship, the vertical load on each wheel is calculated as follows: In the formula, g is the acceleration due to gravity; h is the height of the center of mass from the ground; Building the Magic Formula Tire Model: Y(x)=D sin{C arctan[Bx-E(Bx-arctan(Bx))]} (7) Where Y(x) is the lateral force F of the tire tyij Or the longitudinal force F txij ; x is the tire slip angle α ij Or tire slip rate λ ij ; B, C, D, and E are fitting parameters in the tire characteristic curve, which are stiffness factor, curve shape factor, curve peak factor, and curve curvature factor, respectively.

3. The fault-tolerant control method for a distributed steer-by-wire vehicle according to claim 2, characterized in that: The tire slip rate of each wheel is calculated as: In the formula, u ij is the wheel center speed of each wheel; ω ij is the angular velocity of each wheel; r is the rolling radius of the wheel; The wheel center speed of each wheel can be expressed as:

4. The fault-tolerant control method for a distributed steer-by-wire vehicle according to claim 2, characterized in that: The slip angles of the four wheels are calculated as:

5. The fault-tolerant control method for a distributed steer-by-wire vehicle according to claim 1, characterized in that: The establishment of the two-degree-of-freedom vehicle reference model is specifically as follows: In the formula, k f is the front axle equivalent cornering stiffness of the reference model; k r is the equivalent cornering stiffness of the rear axle of the reference model; δ is the front wheel turning angle of the reference model; The ideal yaw rate of a car in steady-state driving is as follows: In the formula, ω rd is the ideal yaw rate of the car when it is driving in a steady state, and K is the stability factor: The expected yaw rate must satisfy the following equation: Where μ represents the road adhesion coefficient. The ideal yaw rate of the reference model based on equations (12) and (14) can be expressed as:

6. The fault-tolerant control method for a distributed steer-by-wire vehicle according to claim 1, characterized in that: The desired longitudinal velocity is expressed as: In the formula, v xd is the expected longitudinal velocity; v x0 is the longitudinal speed at the initial moment; a xd is the target longitudinal acceleration.

7. The fault-tolerant control method for a distributed steer-by-wire vehicle according to claim 1, characterized in that: The ideal lateral vehicle speed is expressed as: v yd =v x ·tanβ d (17) In the formula, v yd is the ideal lateral speed, β d is the ideal center of mass sideslip angle.

8. The fault-tolerant control method for a distributed steer-by-wire vehicle according to claim 1, characterized in that: The establishment of the steering motor failure model of the distributed steer-by-wire vehicle is as follows: The output torque expression of the steering motor fault is: you λ =ζu d +Δu (18) Where ζ∈[0,1] is the steering motor failure factor; u λ is the fault output torque of the steering motor; u d is the expected output torque of the steering motor; Δu is other faults of the steering motor; When ζ=0 and Δu=0, the steering motor fails completely, the brake mechanism in the steering system takes effect, and the wheel maintains the wheel angle when the steering motor fails completely, and fault-tolerant control is performed by adding yaw torque.

9. The fault-tolerant control method for a distributed steer-by-wire vehicle according to claim 1, characterized in that: The establishment of the upper layer adaptive terminal sliding mode fault-tolerant controller in S5 includes the following steps: S5.

1. Modify the two-degree-of-freedom vehicle reference model to a four-wheel independent steering two-degree-of-freedom vehicle model and add an additional yaw moment ΔM: S5.

2. The tracking error of the additional yaw moment adaptive terminal sliding mode controller is established as: e r =ω r -oh rd (20) S5.

3. Establish the sliding mode surface of the additional yaw moment adaptive terminal sliding mode controller: Where k is an adaptive parameter, c>0, q and p are positive odd numbers, and the expression of k is: Wherein, σ>0,ξ>0,0<ε<1; w is any real number; e is the natural logarithm; S5.4, using the exponential reaching law to switch the additional yaw moment adaptive terminal sliding mode controller: Where η>0,κ>0 S5.

5. Combining equations (19) to (23), the final control law of the adaptive terminal sliding mode controller is obtained: Where ΔM is the additional yaw moment, and the positive and negative values ​​represent the turning direction of the vehicle, with a positive value for left turn and a negative value for right turn; S5.

6. Establish the tracking error of the longitudinal speed adaptive terminal sliding mode controller of the distributed steer-by-wire vehicle: yes x =v x -v xd (25) S5.

7. Establish the sliding mode surface of the distributed steer-by-wire vehicle longitudinal speed adaptive terminal sliding mode controller: In the formula, c x is a controller parameter with a value greater than zero, q x 、p x is a positive odd number, k x The expression is: In the formula, σ x >0,ξ x >0,0<ε x <1; w x is any real number; e is the natural logarithm; S5.

8. Use the exponential reaching law to switch the longitudinal vehicle speed adaptive terminal sliding mode controller: Where η x , κ x are controller parameters with values ​​greater than zero; S5.9, combining equation (1), equation (25)-equation (28), the final control law of the longitudinal vehicle speed adaptive terminal sliding mode controller is obtained: In the formula, F xd is the expected total longitudinal force; S5.

10. Establish the tracking error of the distributed steer-by-wire vehicle lateral speed adaptive terminal sliding mode controller: yes y =v y -v yd (30) S5.

11. Establish the sliding mode surface of the distributed lateral speed adaptive terminal sliding mode controller of the wire-controlled vehicle: In the formula, c y is a controller parameter with a value greater than zero, q y 、p y is a positive odd number, k y The expression is: In the formula, σ y >0,ξ y >0,0<ε y <1; w y is any real number; e is the natural logarithm; S5.12, use the exponential reaching law to switch the lateral vehicle speed adaptive terminal sliding mode controller: Where η y , κ y are controller parameters with values ​​greater than zero; S5.13, combining equation (2), equation (30)-equation (33), the final control law of the lateral vehicle speed adaptive terminal sliding mode controller is obtained: In the formula, F yd is the expected total lateral force.

10. The fault-tolerant control method for a distributed steer-by-wire vehicle according to claim 1, characterized in that: The distributed steer-by-wire vehicle lower layer torque and steering angle optimization distribution reconstruction controller of step S6 is established by the following steps: S6.

1. Establish the optimization objective function of the torque optimization distribution reconstruction controller: In the formula, F txdij To optimize the distribution of longitudinal forces on the tire; F tydij To optimize the distribution of tire lateral forces; F zij is the tire vertical load; μ represents the road adhesion coefficient; τ ij is the weight coefficient of each wheel, which is used to adjust the proportion of each tire force in the entire optimization objective function; S6.

2. Establish a second optimization objective function to prevent the first objective function optimization solver from having no solution in complex working conditions and to reduce the error between the longitudinal and lateral forces and the expected target values. The second optimization objective function is as follows: min J2=min(η1(F x -F xd ) 2 +η2(F y -F yd ) 2 ) (36) Where, η1 and η2 are the longitudinal and lateral force error weight coefficients; S6.

3. Establish a third optimization objective function to ensure that the optimization objective function has a solution and reduce the error of the yaw moment and the additional yaw moment. The third optimization objective function is as follows: min J3=min(η1(F x -F xd ) 2 +η2(F y -F yd ) 2 +η3(ΔM-M z ) 2 ) (37) Where η3 is the yaw moment error weight coefficient; S6.

4. Establish the overall optimization model of torque and angle optimization distribution reconstruction controller: When the steering motor of a wheel fails completely, the output torque of the steering motor is zero, and the lateral force is no longer allocated to the wheel. The optimization allocation controller will optimize the allocation among the remaining steering wheels. The lateral force to be allocated to the faulty wheel is: In the formula, failure time ζ ij =0, otherwise ζ ij =1; Since the faulty wheel cannot provide the required lateral force, the failure factor is added to the constraint condition to obtain the overall optimization model: Where, T ijmax is the maximum output torque of the hub motor; S6.

5. Establish an inverse model of tire cornering characteristics: In the formula, C α is the tire cornering stiffness; p is a constant; Combining equations (35)-(40) and (10), the expected lateral forces of the four wheels are obtained, and the expected wheel turning angle δ is obtained by using the inverse model of the tire cornering characteristics. dij .

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