Steer-by-wire control method with compatible braking function and redundant braking device

Abstract

This invention relates to the field of steer-by-wire technology, and more particularly to a steer-by-wire control method and redundant braking device compatible with braking function. The control method includes the following steps: S1: Obtain vehicle current operating status information and braking demand information. S2: If the longitudinal braking force provided by the failed wheel does not meet the braking expectation. S3: Obtain the lateral force of the failed wheel based on the current wheel load, the previous moment's wheel angle of the failed wheel, and the tire slip angle information. S4: Solve for the minimum wheel angle of the failed wheel. S5: Construct an optimization objective function and set constraints to calculate the target wheel angles. S6: Send the target wheel angles to the steering actuators of each wheel to achieve toe-in braking control under braking failure conditions. The advantage of this invention is that it fully utilizes the four-wheel independent steering and large-angle steering capabilities of the corner module vehicle, reconstructs the vehicle's braking capability through steering control, and improves the system redundancy utilization rate.

Description

Technical Field

[0001] This invention relates to the field of steer-by-wire technology, and in particular to a steer-by-wire control method and redundant braking device that are compatible with braking function. Background Technology

[0002] With the development of vehicle electrification, intelligentization, and drive-by-wire technology, traditional vehicle chassis are evolving from a mechanically centralized architecture towards electrification, distributed design, modularization, and high integration. Against this backdrop, modular vehicles are gradually becoming one of the important development directions for next-generation intelligent electric vehicle chassis.

[0003] Meanwhile, brake-by-wire technology is gradually replacing traditional hydraulic braking systems. Among them, the electro-mechanical brake (EMB) system uses a motor, reduction mechanism, and clamping actuator to directly drive the brake to generate braking force, eliminating traditional hydraulic lines, master cylinders, and vacuum boosters. It has advantages such as structural decoupling, fast response speed, high control precision, and easy independent wheel-end control.

[0004] However, compared to traditional hydraulic braking systems, EMB systems are more dependent on power supplies, sensors, communication networks, and electronic controllers. Their actuators, motors, reduction gears, and clamping mechanisms may all experience performance degradation or partial failure, leading to insufficient actual braking force, decreased vehicle deceleration, and increased braking distance. Particularly serious is the possibility of significant additional yaw moment in EMB failure conditions. This is due to inconsistent remaining braking force between the left and right wheels, unbalanced braking force distribution between the front and rear axles, asymmetrical tire force, or differences in adhesion utilization. This additional yaw moment can induce a yaw rate deviating from the driver's expectations, resulting in significant vehicle drift, lateral deviation, sideslip, or even fishtailing during braking. Especially under high-speed braking, low-adhesion road surface braking, and steering-braking coupling conditions, this yaw response caused by uneven braking is further amplified, seriously threatening the vehicle's directional control and braking safety. The severity of this threat often far exceeds a simple increase in braking distance.

[0005] Existing technologies mainly employ the following solutions to address EMB failures:

[0006] Redundant braking system: Improves system reliability by adding a backup brake steering actuator, but significantly increases system complexity and cost;

[0007] Braking force distribution optimization: The braking force is redistributed among the remaining brakes, but its compensation capability is limited in the case of asymmetric failure or severe failure, and it is difficult to effectively suppress additional yaw moment.

[0008] Differential brake assist: It uses the difference in braking force between the left and right wheels to generate yaw moment to correct the vehicle's direction. However, this solution comes at the cost of further reducing longitudinal braking capacity and is not effective on low-traction surfaces. It may even induce new instability.

[0009] However, none of the aforementioned existing technical solutions fully utilize the four-wheel independent steering capability of corner module vehicles. When the EMB experiences asymmetric or severe failure, the existing solutions either fail to effectively compensate for the sudden additional yaw moment or excessively sacrifice longitudinal braking capability, making it difficult to maintain vehicle directional stability while ensuring braking performance.

[0010] Therefore, how to utilize the four-wheel independent steering capability of corner module vehicles to actively and accurately compensate for the additional yaw moment caused by the imbalance of braking force under EMB failure conditions, so as to obtain the greatest improvement in directional stability with the least longitudinal braking loss, is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0011] Based on this, the present invention aims to provide a steer-by-wire control method and a redundant braking device compatible with braking functions. Under conditions where electromechanical braking (EMB) completely fails, this method fully utilizes the structural advantages of independent steering at large steering angles in modular vehicles. By controlling the wheel angle, it induces a force component in the tires that is beneficial for deceleration, thereby creating a toe-in braking effect. This effect improves the vehicle's equivalent braking strength and suppresses directional deviation during braking. By reconstructing the vehicle's longitudinal deceleration capability through steering control, while simultaneously considering lateral stability and yaw stability, the invention ensures that the vehicle maintains good braking safety distance and directional stability even when braking capacity is insufficient.

[0012] To achieve the above objectives, the technical solution of this invention is a steer-by-wire control method compatible with braking function, applied to a modular vehicle with four-wheel independent drive and four-wheel independent steering capabilities. When the vehicle's electromechanical braking system fails, the method reconstructs the vehicle's braking force by coordinating and controlling the rotation angles of the four wheels and utilizing the direction of wheel forces. The method includes the following steps:

[0013] S1: Obtain the vehicle's current operating status information and braking demand information, and determine whether the vehicle's electromechanical braking system is in a fault state;

[0014] S2: When the vehicle's electromechanical braking system is in a faulty state, identify the failed wheel and the longitudinal braking force that the failed wheel can provide; if the longitudinal braking force that the failed wheel can provide does not meet the driver's braking expectations, then activate the steering system redundant braking control.

[0015] S3: After the redundant braking control of the steering system is activated, the lateral force of the failed wheel is calculated based on the current wheel load of the failed wheel identified in step S2, the turning angle of the failed wheel at the previous moment, and the tire slip angle.

[0016] S4: Based on the force decomposition relationship between the wheel coordinate system and the vehicle coordinate system, calculate the longitudinal equivalent braking force and lateral force components that each failed wheel can provide at the current turning angle, and then solve for the minimum wheel turning angle of the failed wheel to ensure that the toe-in braking can generate additional deceleration capability.

[0017] S5: Based on the minimum wheel angle obtained in step S4, with the control objectives of tracking the desired longitudinal deceleration, suppressing the vehicle's lateral acceleration, suppressing the yaw moment, and reducing the change in wheel angle, an optimization objective function is constructed and constraints are set. Under the premise of satisfying the minimum wheel angle constraint, the target angles of the left front wheel, right front wheel, left rear wheel, and right rear wheel are calculated through an optimization solution algorithm.

[0018] S6: The obtained target steering angle is sent to each wheel steering actuator to realize toe-in braking control under braking failure conditions.

[0019] Furthermore, in step S1, the current operating status information of the vehicle includes at least: the vehicle's longitudinal speed, lateral speed, the working status of the electromechanical braking system, longitudinal acceleration, lateral acceleration, yaw rate, wheel load of each wheel, wheel angle of each wheel, remaining available electric motor power of each wheel, tire slip angle, and tire-road adhesion coefficient.

[0020] Furthermore, in step S2, when the electromechanical braking system of at least one wheel fails, toe-in braking control is performed on each failed wheel and its opposite wheel.

[0021] Furthermore, toe-in braking refers to adjusting the left and right wheels into a V-shape, so that one end of the wheel points towards the longitudinal centerline of the vehicle, in order to generate a longitudinal force component pointing towards the rear of the vehicle.

[0022] Furthermore, in step S3, the tire lateral stiffness of the failed wheel is calculated based on the current wheel load, the wheel rotation angle at the previous moment, and the tire slip angle. The lateral force of the failed wheel is then calculated using the tire lateral stiffness and the slip angle. Alternatively, the lateral force of the failed wheel can be calculated directly based on a preset nonlinear tire model. The preset nonlinear tire model is the Pacejka model.

[0023] Furthermore, in step S4, the equivalent longitudinal braking force of the vehicle body satisfies:

[0024] ;

[0025] in, This indicates the equivalent longitudinal braking force of the vehicle body. This indicates the longitudinal equivalent braking force of the failed wheel. The lateral force represents the failed wheel, and δ represents the wheel's rotation angle;

[0026] Assume the maximum braking force provided by the motor after EMB failure is To enable the steering system to provide additional braking effect, the following must be met: > ;

[0027] Under critical conditions, let = = A linear model of lateral force is adopted. ,get, Then the minimum wheel turning angle Let be the smallest positive real root of the following equation;

[0028] in, This indicates the lateral stiffness of the tire.

[0029] Furthermore, in step S5, the optimization solution model is constructed based on the relationship between the vehicle's longitudinal dynamics, lateral dynamics, and yaw dynamics;

[0030] The objective function is expressed as:

[0031] ;

[0032] Where J represents the optimization objective function, Indicates the desired longitudinal acceleration. This represents the actual longitudinal acceleration. This represents the desired lateral acceleration of the vehicle. This represents the actual lateral acceleration of the vehicle. This represents the resultant torque about the z-axis generated by the desired longitudinal force. This represents the resultant torque about the z-axis generated by the actual longitudinal force. , , , These represent the changes in wheel angle for the left front wheel, right front wheel, left rear wheel, and right rear wheel, respectively. to These represent the weighting coefficients for longitudinal acceleration, lateral acceleration, and yaw rate, respectively. to These represent the penalty coefficients for the changes in the rotation angle of the four wheels.

[0033] Furthermore, in step S5, the constraints include: wheel angle amplitude constraint, wheel angle change rate constraint, tire adhesion constraint, vehicle stability constraint, and steering actuator capability constraint; under the constraints, the optimal solution that minimizes the objective function J is obtained, thereby obtaining the target angle of each wheel; the vehicle stability constraint is used to limit the vehicle yaw rate deviation and lateral offset trend under braking failure conditions, and is dynamically adjusted in combination with road adhesion conditions, initial vehicle speed, fault degree, and driver braking requirements.

[0034] Furthermore, in step S5, the optimal solution that minimizes the objective function J is obtained through the optimization algorithm, and the tire slip angle increment of each wheel is obtained. The tire slip angle increment of each wheel is added to the corresponding wheel rotation angle at the previous moment to obtain the target rotation angle of the left front wheel, right front wheel, left rear wheel and right rear wheel at the current moment.

[0035] A redundant braking device for implementing the above-mentioned compatible braking function in a steer-by-wire control method includes:

[0036] The status acquisition module is used to acquire the vehicle's current operating status information and braking demand information, and to determine whether the vehicle's electromechanical braking system is in a fault state.

[0037] The failure detection module is used to identify the failed wheel and the longitudinal braking force that the failed wheel can provide when the vehicle's electromechanical braking system is in a faulty state; if the longitudinal braking force that the failed wheel can provide does not meet the braking expectation, the redundant braking control of the steering system is activated.

[0038] The lateral force calculation module is used to calculate the lateral force of the failed wheel based on the current wheel load, the current steering angle, and the tire slip angle information of the identified failed wheel after the redundant braking control of the steering system is enabled.

[0039] The cornering solution module is used to calculate the longitudinal equivalent braking force and lateral force components that each failed wheel can provide at the current cornering angle based on the force decomposition relationship between the wheel coordinate system and the vehicle coordinate system, and then solve for the minimum cornering angle of each wheel that enables the failed wheel to generate the required additional deceleration capability through toe-in braking.

[0040] The optimization control module is used to construct an optimization objective function and set constraints with the control objectives of tracking the driver's desired longitudinal deceleration, minimizing the absolute values ​​of the vehicle's lateral acceleration and yaw moment, and reducing the change in wheel steering angle. The optimization solution algorithm is used to calculate the target steering angles of the left front wheel, right front wheel, left rear wheel, and right rear wheel.

[0041] The execution control module is used to send the obtained target steering angle to each wheel steering actuator to realize toe-in braking control under braking failure conditions.

[0042] Compared with existing technologies, inventions and creations can achieve the following beneficial effects:

[0043] (1) In the case of failure of four-wheel EMB, the present invention is no longer limited to simple compensation of the remaining brakes, but makes full use of the independent steering and large-angle steering capabilities of the four wheels of the corner module vehicle, and makes full use of the remaining chassis steering actuator capabilities, and reconstructs the braking capability of the whole vehicle through steering control to improve the system redundancy utilization rate.

[0044] (2) By using the toe-in braking method, the present invention can generate additional braking force by converting the tire force direction when the braking capacity is severely insufficient or even when conventional braking fails completely. Under the premise of prioritizing directional stability, it can effectively shorten the braking distance and improve braking safety.

[0045] (3) While improving braking strength, the present invention incorporates vehicle lateral acceleration and yaw moment into the control target, which can effectively suppress vehicle deviation, sideslip and tail-swing, and improve vehicle directional stability and driving safety.

[0046] (4) The present invention uses an optimized solution method to allocate the target steering angle of the four wheels and allows the weight coefficient to be adaptively adjusted according to the working conditions and fault degree, which can better take into account the driver's braking intention tracking, vehicle stability and steering actuator smoothness.

[0047] (5) This invention is applicable to the safety control scenario of corner module vehicles after braking failure, and has strong engineering application value and promotion significance. Attached Figure Description

[0048] The accompanying drawings, which form part of the invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0049] Fig. 1 This is a flowchart of a steer-by-wire control method with compatible braking function provided according to an embodiment of the present invention;

[0050] Fig. 2 This is a schematic diagram of the force analysis of a single wheel according to an embodiment of the present invention;

[0051] Fig. 3 This is a schematic diagram of the toe-in braking principle provided in an embodiment of the present invention. Detailed Implementation

[0052] To make the purpose, technical solution, and advantages of the invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and do not constitute a limitation on the invention.

[0053] It should be noted that, where there is no conflict, the embodiments and features in the embodiments of the invention can be combined with each other.

[0054] In the description of an invention, it should be understood that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature specified with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of an invention, unless otherwise stated, "a plurality of" means two or more.

[0055] In the description of the invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in the invention based on the specific circumstances.

[0056] The invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0057] like Figs. 1-3 As shown, this embodiment of the invention provides a steer-by-wire control method compatible with braking function, applied to a corner module vehicle with four-wheel independent drive and four-wheel independent steering capabilities. The corner module includes wheels and a steering assembly, with the steering assembly driving the wheels to rotate. The steering assembly includes a steering motor and a reducer. The wheels include tires and hub motors. When the vehicle's electromechanical braking system fails, the vehicle's braking force is reconstructed by coordinating the rotation angles of the four wheels and utilizing the tire force direction, including the following steps:

[0058] S1: Obtain information on the current operating status of the vehicle and the driver's braking needs, and determine whether the vehicle's electromechanical braking system is in a faulty state.

[0059] The current operating status information of the vehicle includes at least the following: longitudinal speed, lateral speed, operating status of the electromechanical braking system, longitudinal acceleration, lateral acceleration, yaw rate, wheel load of each wheel, wheel angle of each wheel, remaining available electric motor power of each wheel, tire slip angle, and tire-road adhesion coefficient.

[0060] Accurately identify EMB failure states and activate redundant steering braking only when a fault occurs to avoid interfering with normal braking and driving; collect comprehensive information to provide a data foundation for subsequent stable, accurate, and safe control; pre-judge operating conditions before activating redundant control to avoid unnecessary optimization calculations and improve the real-time response of the system.

[0061] S2: When the electromechanical braking system is in a faulty state, identify the failed wheel and its remaining longitudinal braking force. If the longitudinal braking force provided by the failed wheel can meet the driver's braking expectations, maintain the current braking state; if not, further determine whether the remaining available longitudinal braking force of each wheel can meet the driver's braking expectations. If the longitudinal braking force meets the expectations, the driver's braking needs are met entirely through longitudinal braking force; if the longitudinal braking force still does not meet the expectations, activate the steering system redundant braking control.

[0062] During redundant braking control of the steering system, when the electromechanical braking system of only one wheel fails, toe-in braking control is applied to the failed wheel and its opposite wheel. Toe-in braking refers to adjusting the left and right wheels into a V-shape, with one end of the wheel pointing towards the longitudinal centerline of the vehicle, to generate a longitudinal force component pointing towards the rear of the vehicle. This arrangement causes the tires to generate a rearward force component, thereby providing additional longitudinal braking force, improving the vehicle's equivalent braking deceleration, and suppressing brake pull.

[0063] When there are multiple failed wheels, toe-in braking control is applied to all failed wheels and their corresponding opposite wheels, and the target steering angle of each axle is optimized in a coordinated manner according to the yaw moment requirements of the whole vehicle.

[0064] This step employs a tiered braking strategy: prioritizing the use of remaining longitudinal braking force, and activating steering braking when only the remaining longitudinal braking force is insufficient to optimize the allocation of braking resources; in the event of a single wheel failure, toe-in control is applied only to the failed wheel and its opposite wheel, resulting in fewer adjustment variables, smoother steering, and effective suppression of braking deviation; in the event of multiple wheel failures, toe-in braking control is applied to each failed wheel and its opposite wheel, and the steering angles of each wheel are coordinated to suppress additional yaw moments and prevent vehicle sideslip or fishtailing; this strategy employs differentiated control for different failure modes, resulting in low computational burden and high real-time performance.

[0065] S3: After the redundant braking control of the steering system is enabled, calculate the lateral force of the failed wheel based on the current wheel load, the steering angle of the failed wheel at the previous moment, and the tire slip angle information of the failed wheel identified in step S2.

[0066] Specifically, based on the current wheel load, the wheel rotation angle and tire slip angle of the failed wheel at the previous moment, the tire slip stiffness of the failed wheel is calculated, and the lateral force of the failed wheel is calculated using the tire slip stiffness and slip angle; or, the lateral force of the failed wheel is directly calculated based on a preset nonlinear tire model; wherein, the preset tire model is the Pacejka model.

[0067] The lateral force of the failed wheel is calculated based on the tire lateral stiffness and lateral angle, according to the following formula:

[0068] ;

[0069] ;

[0070] in, This represents the lateral force of the failed wheel. This represents the actual lateral force on the wheel. Indicates tire lateral stiffness. This represents the difference in tire slip angle. Indicates the wheel's turning angle. Indicates yaw rate. , These represent the vehicle's longitudinal speed and lateral speed, respectively. L represents the straight-line distance from the vehicle's center of gravity to the front axle, and B represents the track width.

[0071] The lateral forces of each failed wheel are calculated based on a pre-defined tire model, satisfying the following formula:

[0072] ;

[0073] Where D represents the peak factor, C represents the shape factor, and G represents the stiffness factor. The value represents the slip angle, and E represents the curvature factor. These parameters are pre-calibrated based on tire characteristics and road surface adhesion coefficient.

[0074] It should be noted that under small sideslip angle conditions, the linear sideslip stiffness method can meet the control accuracy requirements; under large sideslip angle conditions or when high-precision control is required, the Pacejka formula can be used. The two methods can be flexibly selected according to the actual operating conditions and vehicle configuration.

[0075] A dual-model adaptive strategy is adopted: a linear model is used for small sideslip angles to ensure calculation speed, while a Pacejka model is used for large sideslip angles to ensure calculation accuracy; the lateral force is estimated in real time based on the current wheel load, the previous steering angle, and the tire sideslip angle, which has strong robustness; it effectively compensates for the nonlinear characteristics of tire force and avoids control inaccuracy under large sideslip angle conditions; no additional sensors are required, resulting in low cost and easy mass production.

[0076] S4: Based on the force decomposition relationship between the wheel coordinate system and the vehicle coordinate system, calculate the longitudinal equivalent braking force and lateral force components that each failed wheel can provide at the current turning angle, and then solve for the minimum wheel turning angle of the failed wheel to ensure that the toe-in braking can generate additional deceleration capability.

[0077] like Fig. 2 As shown, for the wheel corner region of the corner module, after EMB failure, a single corner region can provide longitudinal force along the vehicle body direction. The formula is as follows:

[0078] The equivalent longitudinal braking force of the vehicle body meets the following requirements:

[0079] ;

[0080] in, This indicates the equivalent longitudinal braking force of the vehicle body. This indicates the longitudinal equivalent braking force of the failed wheel. The lateral force represents the failed wheel, and δ represents the wheel's rotation angle;

[0081] When the wheels are not turning, the braking force along the direction of the vehicle body is When the wheel rotates through an angle δ, the lateral force An additional component force is generated longitudinally on the vehicle body. To enable the steering system to provide additional braking effect, the longitudinal braking force after steering must be greater than the original braking force, that is: > ;

[0082] Adopting a linear model of lateral forces Under critical conditions, let = = Substituting into the above equation, we get:

[0083] ;

[0084] ;

[0085] The minimum wheel rotation angle is then the smallest positive real root of the above equation.

[0086] This step establishes a mathematical mapping relationship between wheel force and longitudinal braking force of the vehicle body; the principle is clear and easy to quantify. This is achieved by solving for the minimum effective turning angle. This avoids insufficient braking gain due to excessively small steering angles, prevents ineffective adjustment of the steering actuator, and ensures that the toe-in braking can produce an additional longitudinal deceleration effect. The entire mathematical derivation process is logically rigorous, clearly structured, and possesses good verifiability and engineering feasibility.

[0087] S5: Based on the minimum wheel angle obtained in step S4, with the control objectives of tracking the driver's desired longitudinal deceleration, controlling the vehicle's lateral acceleration, controlling the yaw moment, and reducing the change in wheel angle, an optimization objective function is constructed and constraints are set. Under the premise of satisfying the minimum wheel angle constraint, the target angles of the left front wheel, right front wheel, left rear wheel, and right rear wheel are calculated through an optimized solution algorithm.

[0088] It should be noted that the minimum wheel angle in step S4 refers to the lower limit of the angle required to generate the additional deceleration capability of toe-in braking. The target angle in step S5 refers to the final execution angle of each wheel determined by the optimization algorithm under the premise of satisfying the minimum wheel angle constraint.

[0089] Specifically, based on the longitudinal braking force that each failed wheel can provide in step S3 and the minimum turning angle of each failed wheel obtained in step S4, a four-wheel turning angle optimization solution model is established with the goal of tracking the driver's desired longitudinal deceleration, desired lateral acceleration and desired yaw moment. Under the constraints of braking strength, lateral stability and yaw stability, the target turning angle of each wheel is solved.

[0090] like Fig. 2 As shown, based on vehicle dynamics, the following longitudinal, lateral, and yaw dynamic equations are established. Longitudinal dynamic equation:

[0091] ;

[0092] Lateral dynamic equations:

[0093] ;

[0094] The equation of yaw dynamics:

[0095] ;

[0096] Where m represents the total vehicle mass; , These represent the vehicle's longitudinal acceleration and lateral acceleration, respectively. This represents the moment of inertia of the entire vehicle in the yaw direction about its center of mass; Indicates yaw acceleration; , These are the longitudinal force and lateral force of each wheel, respectively; Let i be the rotation angle of each wheel (i = fl, fr, rl, rr); , These are the distances from the center of mass to the front and rear axles, respectively.

[0097] The lateral force is calculated using the difference in sideslip angle compared to the previous moment, as shown in the following formula:

[0098] , (i=fl, fr, rl, rr);

[0099] ;

[0100] ;

[0101] ;

[0102]

[0103] With the control objectives of tracking the driver's desired longitudinal deceleration, suppressing vehicle lateral acceleration and yaw moment, and reducing wheel angle changes, the following optimization objective function is constructed: ;

[0104] in, Indicates the desired longitudinal acceleration. This represents the actual longitudinal acceleration. This represents the desired lateral acceleration of the vehicle. This represents the actual lateral acceleration of the vehicle. This represents the resultant torque about the z-axis generated by the desired longitudinal force. This represents the resultant torque about the z-axis generated by the actual longitudinal force. , , , These represent the changes in wheel angle for the left front wheel, right front wheel, left rear wheel, and right rear wheel, respectively. to These represent the weighting coefficients for longitudinal acceleration, lateral acceleration, and yaw rate, respectively. to These represent the penalty coefficients for the changes in the rotation angle of the four wheels. to The system adaptively adjusts based on vehicle operating conditions, braking errors, stability risk levels, and fault severity to balance braking strength, directional stability, and steering smoothness under different operating conditions.

[0105] For example, when the braking error is large (the actual deceleration is much smaller than the expected deceleration), the longitudinal acceleration weighting coefficient should be increased. When the vehicle shows a significant tendency to veer off course, increase the lateral acceleration weighting coefficient. and yaw rate weighting coefficient When the fault is severe, the penalty coefficient for angle change should be appropriately reduced. to This allows for a quicker steering response. Wheel angle and to The range of values ​​for is shown in Table 1.

[0106]

[0107] To ensure that the optimized target steering angle meets the vehicle dynamics constraints and the steering actuator physical constraints, the following constraints are set:

[0108] Wheel angle amplitude constraint: Ensure that the target angle does not exceed the mechanical limit of the steering actuator.

[0109] Wheel angle change rate constraint: ensures the continuity and smoothness of the steering process.

[0110] Tire adhesion constraint: Prevents tire forces from exceeding the road adhesion limit.

[0111] Vehicle stability constraints: Limit the vehicle's lateral and yaw responses under braking failure conditions.

[0112] Steering actuator capability constraint: Ensure that the obtained target steering angle is within the actual output range of the steering actuator.

[0113] The constraints include wheel angle amplitude, wheel angle change rate, tire adhesion, vehicle stability, and steering actuator capability. Under these constraints, the optimal solution that minimizes the objective function J is obtained, and the target angle of each wheel is then derived.

[0114] Vehicle stability constraints are used to limit the yaw rate deviation and lateral drift trend of the vehicle under braking failure conditions, and are dynamically adjusted in conjunction with road adhesion conditions, initial vehicle speed, fault severity, and driver braking demand. For example, when road adhesion conditions are poor, vehicle speed is high, and driver braking demand is large, k3 can be appropriately decreased and k1 increased to reduce the stress in the objective function. This measure ensures the driver's braking intensity. When the driver's braking demand is low and the yaw demand is high, k1 can be decreased and k3 increased to ensure that the desired response is followed.

[0115] The optimal solution that minimizes the objective function J is obtained by optimizing the solution algorithm. The tire slip angle increment of each wheel is obtained. The tire slip angle increment of each wheel is added to the corresponding wheel rotation angle at the previous moment to obtain the target rotation angle of the left front wheel, right front wheel, left rear wheel and right rear wheel at the current moment.

[0116] Toe-in braking refers to adjusting the left and right wheels into a V-shape, so that one end of the wheel points towards the longitudinal centerline of the vehicle, in order to generate a longitudinal force component pointing towards the rear of the vehicle.

[0117] This step simultaneously achieves four control objectives: ① tracking the driver's desired longitudinal deceleration; ② suppressing lateral acceleration to prevent braking deviation; ③ suppressing yaw rate to prevent skidding; and ④ reducing wheel angle variation to ensure smooth and comfortable steering. A weighted adaptive strategy is employed, dynamically adjusting weight coefficients based on vehicle operating conditions to balance braking intensity and directional stability under different conditions. Five types of constraints are set to protect the steering actuator and vehicle safety, including: wheel angle amplitude constraints, angle change rate constraints, tire adhesion constraints, vehicle stability constraints, and steering actuator capability constraints. The optimized steering angle is smooth and safe, not exceeding the mechanical limits of the steering actuator and the physical limits of the vehicle.

[0118] S6: The obtained target steering angle is sent to each wheel steering actuator to realize toe-in braking control under braking failure conditions.

[0119] The target rotation angle obtained in step S5 , , , The steering actuators at each wheel receive the target steering angle command via a vehicle communication network (such as CAN bus or in-vehicle Ethernet). Each steering actuator then drives the corresponding wheel to rotate to the target angle, thus achieving toe-in braking control under braking failure conditions.

[0120] Through the above control, in the case of complete failure of the electromechanical braking system, the vehicle can use its four-wheel independent steering capability to generate additional longitudinal braking force, while suppressing deviation and yaw during the braking process, thereby effectively shortening the braking distance and maintaining the vehicle's directional stability.

[0121] The direct-drive steering actuator offers fast response and precise control; it reconstructs the vehicle's braking force using four-wheel independent steering, providing effective deceleration even when the EMB fails completely; it achieves coordinated control of deceleration, yaw stability, and anti-deviation, significantly improving vehicle braking safety; it is particularly suitable for corner module vehicles, fully utilizing their chassis redundancy.

[0122] In summary, the steer-by-wire control method with compatible braking function of the present invention has the following advantages:

[0123] (1) In the case of failure of four-wheel EMB, the present invention is no longer limited to simple compensation of the remaining brakes, but makes full use of the independent steering and large-angle steering capabilities of the four wheels of the corner module vehicle, and makes full use of the remaining chassis steering actuator capabilities, and reconstructs the braking capability of the whole vehicle through steering control to improve the system redundancy utilization rate.

[0124] (2) By using the toe-in braking method, the present invention can generate additional braking force by converting the direction of tire force when the braking capacity is severely insufficient or even when conventional braking fails completely, thereby effectively shortening the braking distance and improving braking safety.

[0125] (3) While improving braking strength, the present invention incorporates vehicle lateral acceleration and yaw moment into the control target, which can effectively suppress vehicle deviation, sideslip and tail-swing, and improve vehicle directional stability and driving safety.

[0126] (4) The present invention uses an optimized solution method to allocate the target steering angle of the four wheels and allows the weight coefficient to be adaptively adjusted according to the working conditions and fault degree, which can better take into account the driver's braking intention tracking, vehicle stability and steering actuator smoothness.

[0127] (5) This invention is applicable to the safety control scenario of corner module vehicles after braking failure, and has strong engineering application value and promotion significance.

[0128] A redundant braking device for implementing the above-mentioned compatible braking function in a steer-by-wire control method includes:

[0129] The status acquisition module is used to acquire the vehicle's current operating status information and braking demand information, and to determine whether the vehicle's electromechanical braking system is in a fault state.

[0130] The failure detection module is used to identify the failed wheel and the longitudinal braking force that the failed wheel can provide when the vehicle's electromechanical braking system is in a faulty state; if the longitudinal braking force that the failed wheel can provide does not meet the driver's braking expectations, the redundant braking control of the steering system is activated.

[0131] The lateral force calculation module is used to calculate the lateral force of the failed wheel based on the current wheel load, the current steering angle, and the tire slip angle information of the identified failed wheel after the redundant braking control of the steering system is enabled.

[0132] The cornering solution module is used to calculate the longitudinal equivalent braking force and lateral force components that each failed wheel can provide at the current cornering angle based on the force decomposition relationship between the wheel coordinate system and the vehicle coordinate system, and then solve for the minimum cornering angle of each wheel that enables the failed wheel to generate the required additional deceleration capability through toe-in braking.

[0133] The optimization control module is used to construct an optimization objective function and set constraints with the control objectives of tracking the driver's desired longitudinal deceleration, minimizing the absolute values ​​of the vehicle's lateral acceleration and yaw moment, and reducing the change in wheel steering angle. The optimization solution algorithm is used to calculate the target steering angles of the left front wheel, right front wheel, left rear wheel, and right rear wheel.

[0134] The execution control module is used to send the obtained target steering angle to each wheel steering actuator to realize toe-in braking control under braking failure conditions.

[0135] The specific embodiments described above do not constitute a limitation on the scope of protection of this invention. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A steer-by-wire control method compatible with braking function, applied to a modular vehicle with four-wheel independent drive and four-wheel independent steering capabilities, characterized in that, When the vehicle's electromechanical braking system fails, the vehicle's braking force is reconstructed by coordinating and controlling the rotation angles of the four wheels and utilizing the direction of wheel forces. This includes the following steps: S1: Obtain the vehicle's current operating status information and braking demand information, and determine whether the vehicle's electromechanical braking system is in a fault state; S2: When the vehicle's electromechanical braking system is in a faulty state, identify the failed wheel and the longitudinal braking force that the failed wheel can provide; If the longitudinal braking force provided by the failed wheel does not meet the driver's braking expectations, then redundant braking control of the steering system is activated. S3: After the redundant braking control of the steering system is enabled, the lateral force of the failed wheel is calculated based on the current wheel load of the failed wheel identified in step S2, the turning angle of the failed wheel at the previous moment, and the tire slip angle information. S4: Based on the force decomposition relationship between the wheel coordinate system and the vehicle coordinate system, calculate the longitudinal equivalent braking force and lateral force components that each failed wheel can provide at the current turning angle, and then solve for the minimum wheel turning angle of the failed wheel to ensure that the toe-in braking can generate additional deceleration capability. S5: Based on the minimum wheel angle obtained in step S4, with the control objectives of tracking the desired longitudinal deceleration, suppressing the vehicle's lateral acceleration, suppressing the yaw moment, and reducing the change in wheel angle, an optimization objective function is constructed and constraints are set. Under the premise of satisfying the minimum wheel angle constraint, the target angles of the left front wheel, right front wheel, left rear wheel, and right rear wheel are calculated through an optimization solution algorithm. S6: The obtained target steering angle is sent to each wheel steering actuator to realize toe-in braking control under braking failure conditions.

2. The steer-by-wire control method with compatible braking function according to claim 1, characterized in that, In step S1, the current operating status information of the vehicle includes at least: vehicle longitudinal speed, lateral speed, working status of the electromechanical braking system, longitudinal acceleration, lateral acceleration, yaw rate, wheel load of each wheel, wheel angle of each wheel, remaining available electric motor power of each wheel, tire slip angle and tire-road adhesion coefficient.

3. The steer-by-wire control method with compatible braking function according to claim 1, characterized in that, In step S2, when the electromechanical braking system of at least one wheel fails, toe-in braking control is performed on each failed wheel and its opposite wheel.

4. The steer-by-wire control method with compatible braking function according to claim 3, characterized in that, The aforementioned toe-in braking refers to adjusting the left and right wheels into a V-shape, so that one end of the wheel points towards the longitudinal centerline of the vehicle, thereby generating a longitudinal force component pointing towards the rear of the vehicle.

5. The steer-by-wire control method with compatible braking function according to claim 1, characterized in that, In step S3, the tire lateral stiffness of the failed wheel is calculated based on the current wheel load, the wheel rotation angle and the tire slip angle information of the failed wheel at the previous moment, and the lateral force of the failed wheel is calculated using the tire lateral stiffness and the slip angle. Alternatively, the lateral force of the failed wheel can be directly calculated based on a preset nonlinear tire model; wherein the preset nonlinear tire model is the Pacejka model.

6. The steer-by-wire control method with compatible braking function according to claim 1, characterized in that, In step S4, the equivalent longitudinal braking force of the vehicle body satisfies: ； in, This indicates the equivalent longitudinal braking force of the vehicle body. This indicates the longitudinal equivalent braking force of the failed wheel. The lateral force represents the failed wheel, and δ represents the wheel's rotation angle; Assume the maximum braking force provided by the motor after EMB failure is To enable the steering system to provide additional braking effect, the following must be met: > ; Under critical conditions, let = = A linear model of lateral force is adopted. ,get, Then the minimum wheel turning angle Let be the smallest positive real root of the following equation; in, This indicates the lateral stiffness of the tire.

7. The steer-by-wire control method with compatible braking function according to claim 1, characterized in that, In step S5, the optimization solution model is constructed based on the relationship between the vehicle's longitudinal dynamics, lateral dynamics, and yaw dynamics. The optimization objective function is expressed as: ； Where J represents the optimization objective function, Indicates the desired longitudinal acceleration. This represents the actual longitudinal acceleration. This represents the desired lateral acceleration of the vehicle. This represents the actual lateral acceleration of the vehicle. This represents the resultant torque about the z-axis generated by the desired longitudinal force. This represents the resultant torque about the z-axis generated by the actual longitudinal force. , , , These represent the changes in wheel angle for the left front wheel, right front wheel, left rear wheel, and right rear wheel, respectively. to These represent the weighting coefficients for longitudinal acceleration, lateral acceleration, and yaw rate, respectively. to These represent the penalty coefficients for the changes in the rotation angle of the four wheels.

8. The steer-by-wire control method with compatible braking function according to claim 7, characterized in that, In step S5, the constraints include: wheel angle amplitude constraint, wheel angle change rate constraint, tire adhesion constraint, vehicle stability constraint, and steering actuator capability constraint; under the constraints, the optimal solution that minimizes the objective function J is obtained, and then the target angle of each wheel is obtained. The vehicle stability constraint is used to limit the vehicle's yaw rate deviation and lateral offset trend under braking failure conditions, and is dynamically adjusted in combination with road adhesion conditions, initial vehicle speed, fault degree and driver braking needs.

9. The steer-by-wire control method with compatible braking function according to claim 7, characterized in that, In step S5, the optimal solution that minimizes the objective function J is obtained through the optimization algorithm, and the tire slip angle increment of each wheel is obtained. The tire slip angle increment of each wheel is added to the corresponding wheel rotation angle at the previous moment to obtain the target rotation angle of the left front wheel, right front wheel, left rear wheel and right rear wheel at the current moment.

10. A redundant braking device for implementing the steer-by-wire control method with compatible braking function as described in any one of claims 1 to 9, characterized in that, include: The status acquisition module is used to acquire the vehicle's current operating status information and braking demand information, and to determine whether the vehicle's electromechanical braking system is in a fault state. The failure identification module is used to identify the failed wheel and the longitudinal braking force that the failed wheel can provide when the vehicle's electromechanical braking system is in a faulty state. If the longitudinal braking force provided by the failed wheel does not meet the braking expectation, then the redundant braking control of the steering system is activated. The lateral force calculation module is used to calculate the lateral force of the failed wheel based on the current wheel load, the current steering angle, and the tire slip angle information of the identified failed wheel after the redundant braking control of the steering system is enabled. The cornering solution module is used to calculate the longitudinal equivalent braking force and lateral force components that each failed wheel can provide at the current cornering angle based on the force decomposition relationship between the wheel coordinate system and the vehicle coordinate system, and then solve the minimum cornering angle of each wheel that enables the failed wheel to generate the required additional deceleration capability through toe-in braking. The optimization control module is used to construct an optimization objective function and set constraints with the control objectives of tracking the driver's desired longitudinal deceleration, minimizing the absolute values ​​of the vehicle's lateral acceleration and yaw moment, and reducing the change in wheel steering angle. The optimization solution algorithm is used to calculate the target steering angles of the left front wheel, right front wheel, left rear wheel, and right rear wheel. The execution control module is used to send the obtained target steering angle to each wheel steering actuator to realize toe-in braking control under braking failure conditions.

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