A Four-Wheel Steering Control Method for Distributed Drive Electric Vehicles

Abstract

The present invention discloses a four-wheel steering control method for a distributed drive electric vehicle, which relates to the technical field of vehicle control. The front wheel steering angle δ is output by a driver model f , the throttle pedal opening α p , and the throttle pedal opening change rate Δα p . The ideal center-of-mass side slip angle β d and the ideal yaw angular velocity γ d are obtained through the two-degree-of-freedom dynamics equation of the front-wheel-steering vehicle. Then, based on the characteristics of the distributed drive torque vector control method and the rear-wheel active steering control method, and in combination with the actual center-of-mass side slip angle β and the actual yaw angular velocity γ of the vehicle, the vehicle target motion state tracking under different working conditions is carried out, and the additional yaw moment ΔM z and / or the rear wheel steering angle δ r are obtained. Finally, the coordinated control of the vehicle is realized through the four-wheel steering distributed drive vehicle model. While considering the control effect, the present invention takes into account the driving habits and driving needs of the driver to the greatest extent, not only improving the overall performance of the vehicle control system, but also ensuring the sensitivity, maneuverability and stability of the vehicle.

Description

Technical Field

[0001] The present invention relates to the technical field of vehicle control, and particularly relates to a four-wheel steering control method for a distributed drive electric vehicle. Background Art

[0002] In recent years, with the rapid development of automotive electronic technology, four-wheel steering technology and distributed drive control technology have become research hotspots in the current automotive control field. The four-wheel steering system technology can achieve active rear-wheel steering, enabling the vehicle to maintain a small sideslip angle of the center of mass during cornering, effectively improving the response ability of the steering system, and enhancing the driving stability of the vehicle. The distributed drive control technology, on the other hand, can control the traction force of different wheels by relying on torque vector distribution technology, generate an additional yaw moment, change the yaw motion of the vehicle, and obtain better cornering performance. However, when the four-wheel steering technology and the distributed drive control technology face the same control object, they will form a complex non-linear coupling system, greatly increasing the design difficulty of the control algorithm. Therefore, a simple and effective control method is needed. Summary of the Invention

[0003] The present invention provides a four-wheel steering control method for a distributed drive electric vehicle, and its main purpose is to solve the problems existing in the prior art.

[0004] The present invention adopts the following technical solutions:

[0005] A four-wheel steering control method for a distributed drive electric vehicle includes the following steps:

[0006] (1) The front-wheel steering angle δ f is output by the driver model, and the ideal sideslip angle β d of the center of mass and the ideal yaw angular velocity γ d are obtained based on the two-degree-of-freedom dynamics equation of the front-wheel steering vehicle;

[0007] (2) The driving conditions of the vehicle are divided into high-speed conditions, medium-speed conditions, and low-speed conditions. Combining the characteristics of the distributed drive torque vector control method and the rear-wheel active steering control method, a segmented fusion control strategy based on different condition requirements is formulated to obtain the additional yaw moment ΔM z and / or the rear-wheel steering angle δ r ; in this step, in the high-speed and low-speed conditions, the feedforward control in the rear-wheel active steering control method adopts a control method in which the front and rear wheel steering angles are proportional. The proportional coefficient k of the feedforward rear-wheel steering angle to the front-wheel steering angle is set, and the proportional coefficient k of the front and rear wheel steering angles is corrected based on different condition requirements:

[0008]

[0009] Wherein, m is the vehicle mass, a is the distance from the center of mass to the front axle of the chassis, b is the distance from the center of mass to the rear axle of the vehicle, u is the longitudinal vehicle speed at the center of mass, k1 and k2 are the equivalent cornering stiffnesses of the front and rear axles of the vehicle, L is the wheelbase of the vehicle, and u1 and u2 are the vehicle speed segmentation thresholds;

[0010] (3) The front wheel steering angle δ is output by the driver model f , the throttle pedal opening α p and the throttle pedal opening change rate Δα p . Combining the additional yaw moment ΔM z and / or the rear wheel steering angle δ r obtained by the segmented fusion control strategy, the cooperative control of the vehicle is realized through the four-wheel steering distributed drive vehicle model.

[0011] Further, in step (2), the segmented fusion control strategy includes:

[0012] a. Under high-speed conditions, with the ideal center of mass side slip angle β d and the ideal yaw angular velocity γ d as the control objectives, a cooperative control strategy that combines the distributed drive torque vector control method and the rear wheel active steering control method is adopted, and at the same time, the actual center of mass side slip angle β and the actual yaw angular velocity γ of the vehicle are combined to track the target motion state of the vehicle under high-speed conditions, so as to obtain the additional yaw moment ΔM z and the rear wheel steering angle δ r ;

[0013] b. Under medium-speed conditions, with the ideal yaw angular velocity γ d as the control objective, a single control strategy of the distributed drive torque vector control method is adopted, and at the same time, the actual yaw angular velocity γ of the vehicle is combined to track the target motion state of the vehicle under medium-speed conditions, so as to obtain the additional yaw moment ΔM z ;

[0014] c. Under low-speed conditions, with the ideal yaw angular velocity γ d as the control objective, a cooperative control strategy that combines the distributed drive torque vector control method and the rear wheel active steering control method is adopted, and at the same time, the actual yaw angular velocity γ of the vehicle is combined to track the target motion state of the vehicle under low-speed conditions, so as to obtain the additional yaw moment ΔM z and the rear wheel steering angle δ r .

[0015] Further, in step (2), the rear wheel active steering control method adopted under high-speed conditions is a closed-loop control method that combines feedforward and feedback; the rear wheel active steering control method adopted under low-speed conditions is a feedforward adaptive control method.

[0016] Furthermore, in step (2), first, based on the two-degree-of-freedom dynamic equation of a steady-state turning front-wheel steering vehicle, the calculation formula for the front and rear wheel steering angle ratio coefficient k during vehicle steady-state turning is as follows:

[0017]

[0018] Then, based on different working condition requirements, the front and rear wheel steering angle ratio coefficient k is corrected, and the steady-state yaw rate gain of the four-wheel steering vehicle to the front wheel input is obtained as:

[0019]

[0020] where K is the stability factor, with the unit of s 2 / m 2 , which is an important parameter characterizing the vehicle's steady-state response.

[0021] Furthermore, in step (1), the two-degree-of-freedom dynamic equation of the front-wheel steering vehicle is:

[0022]

[0023] where β is the vehicle's center-of-mass sideslip angle, γ is the vehicle's yaw rate, δ f is the front wheel steering angle, and I Z is the vehicle's yaw moment of inertia.

[0024] Furthermore, taking the center-of-mass sideslip angle of the vehicle in the non-sideslip state as the ideal value, that is, the ideal center-of-mass sideslip angle β d is 0.

[0025] Furthermore, taking the yaw rate of the vehicle under the steady-state turning condition as the ideal value and correcting this ideal value based on the influence of the road surface adhesion coefficient, the ideal yaw rate γ d The calculation formula is:

[0026]

[0027] where the stability factor , L is the vehicle wheelbase, μ is the road surface adhesion coefficient, g is the acceleration due to gravity, and 0.85 is the set safety factor.

[0028] Furthermore, under high-speed conditions, the feedback control in the rear-wheel active steering control uses sliding mode control based on the ideal value of the center-of-mass sideslip angle. According to the sliding mode variable structure control principle and combined with the two-degree-of-freedom dynamic equation of the front-wheel steering vehicle, the rear wheel steering angle control amount is obtained as:

[0029]

[0030] where s is the sliding mode surface and ε is the approaching speed, is the exponential approach term, and Δ is the boundary layer thickness of the saturation function.

[0031] Further, in step (2), the distributed drive torque vector control adopts an upper and lower hierarchical control method. The upper layer is the motion tracking layer, which obtains the additional yaw moment ΔM z and the longitudinal driving force T; the lower layer is the actuator control layer, which combines the output upper layer additional yaw moment ΔM z and the longitudinal driving torque T to distribute the torque to the left and right drive wheels, thereby realizing the yaw control of the vehicle.

[0032] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0033] A four-wheel steering control method for a distributed drive electric vehicle provided by the present invention formulates a segmented fusion control strategy based on different working condition requirements by combining the characteristics of the distributed drive torque vector control method and the rear-wheel active steering control method. The control method provided by the present invention takes into account the driving habits and driving needs of the driver to the greatest extent while considering the control effect, not only improving the overall performance of the vehicle control system, but also ensuring the sensitivity, maneuverability and stability of the vehicle. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 is a schematic diagram of a two-degree-of-freedom vehicle model in the present invention.

[0035] Figure 2 is a schematic diagram of the k-u relationship curve in the present invention.

[0036] Figure 3 is a schematic diagram of the yaw rate gain curves of front-wheel steering and four-wheel steering in the present invention.

[0037] Figure 4 is a schematic diagram of the corrected k-u relationship curve in the present invention.

[0038] Figure 5 is a schematic diagram of the corrected yaw rate gain curves of front-wheel steering and four-wheel steering in the present invention.

[0039] Figure 6 is a schematic diagram of the principle block diagram of the segmented fusion control strategy in the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0040] The following describes the specific embodiments of the present invention with reference to the drawings. To fully understand the present invention, many details are described below, but for those skilled in the art, the present invention can be implemented without these details.

[0041] The present invention is described by taking a distributed drive electric vehicle with front-wheel driver-operated steering and rear-wheel by-wire active steering as an example. The specific implementation steps are as follows:

[0042] 1. Ideal reference value calculation based on the vehicle two-degree-of-freedom model

[0043] Refer to Figure 1 , based on the vehicle two-degree-of-freedom model, the vehicle lateral and yaw motion equations can be obtained:

[0044] (1)

[0045] The lateral forces on the front and rear axles can be linearly expressed as:

[0046] (2)

[0047] The vehicle center-of-mass sideslip angle and the front and rear wheel sideslip angles are:

[0048] (3)

[0049] Integrating equations (1) to (3) gives the two-degree-of-freedom dynamics equation of the front-wheel-steering vehicle:

[0050] (4)

[0051] In equations (1) to (4):

[0052] F Y,FWS ——The lateral reaction force of the ground on the front-wheel-steering vehicle;

[0053] M Z,FWS ——The ground yaw moment on the front-wheel-steering vehicle;

[0054] F Y1 、F Y2 ——The lateral reaction forces of the ground on the front and rear wheels of the vehicle;

[0055] δ f ——The front-wheel angle;

[0056] m——The mass of the vehicle;

[0057] u——The longitudinal vehicle speed at the center of mass;

[0058] v——The lateral vehicle speed at the center of mass;

[0059] γ——The vehicle yaw angular velocity;

[0060] I Z ——The yaw moment of inertia of the vehicle;

[0061] k1, k2——The equivalent cornering stiffnesses of the front and rear axles of the vehicle;

[0062] α1, α2——The tire sideslip angles of the front and rear wheels;

[0063] β——Yaw angle of vehicle mass center;

[0064] a——Distance from the front axle center of the chassis to the mass center;

[0065] b——Distance from the rear axle center of the vehicle to the mass center.

[0066] The ideal reference values of a four-wheel steering distributed drive vehicle include the ideal yaw angle β of the vehicle mass center d and the ideal yaw rate γ d . Therefore, the front wheel steering angle δ can be output by the driver model f , and based on the two-degree-of-freedom dynamic equation of the front-wheel steering vehicle in Equation (4), the ideal yaw angle β of the vehicle mass center d and the ideal yaw rate γ d can be further obtained, as follows:

[0067] (1) Ideal yaw angle of vehicle mass center

[0068] The yaw angle of the vehicle mass center reflects the body attitude and trajectory deviation during vehicle driving. In the present invention, the yaw angle of the vehicle mass center in the non-side-slip state of the vehicle is used as the ideal value, that is, the ideal yaw angle β of the vehicle mass center d is 0, so as to ensure that the vehicle has a good driving trajectory.

[0069] (2) Ideal yaw rate

[0070] The yaw rate can directly reflect the steering control performance of the vehicle. Ignoring the transient response of the vehicle yaw rate to the front wheel steering angle δ f , the yaw rate under the steady-state steering condition of the vehicle is used as the ideal value. At this time, , , substituting into the two-degree-of-freedom dynamic equation of the front-wheel steering vehicle in Equation (4) to obtain the ideal yaw rate γ d is:

[0071] (5)

[0072] In Equation (5):

[0073] K——Stability factor, ; L is the wheelbase of the vehicle.

[0074] In addition, considering the limit force condition of the tire and the influence of the road surface adhesion coefficient on the vehicle, the ideal yaw rate is also subject to the following constraints:

[0075] (6)

[0076] In Equation (6):

[0077] μ——Road surface adhesion coefficient;

[0078] g——is the gravitational acceleration;

[0079] Among them, 0.85 is the set safety factor.

[0080] Therefore, considering the road surface adhesion coefficient, the ideal yaw rate is:

[0081] (7)

[0082] Then the ideal reference value of the four-wheel steering distributed drive vehicle is:

[0083] (8)

[0084] 2. Distributed drive torque vector control method

[0085] The distributed drive torque vector control method can rely on torque vector distribution technology to control the traction force of different wheels, generate additional yaw moment, change the yaw motion of the vehicle, and obtain better cornering ability. Specifically, the distributed drive torque vector control method of the present invention adopts upper and lower hierarchical control. The upper layer of the hierarchical control is the motion tracking layer, which specifically adopts a control method that combines feedforward control based on the ideal understeer degree and feedback control based on the ideal yaw rate γ d to output the additional yaw moment ΔM z and the longitudinal driving force T; the lower layer of the hierarchical control is the actuator control layer, which distributes the torque to the left and right drive wheels according to the requirements of different working conditions, combined with the output additional yaw moment ΔM z and the longitudinal driving torque T, so as to realize the yaw control of the vehicle.

[0086] 3. Design of the rear-wheel active steering control law based on the rear-wheel active steering control method

[0087] The rear-wheel active steering control enables the vehicle to maintain a small sideslip angle of the center of mass during cornering by directly controlling the rear-wheel steering angle δ r , thereby effectively improving the response ability of the steering system and enhancing the driving stability of the vehicle. According to different driving conditions or driving requirements, the rear-wheel active steering control method can adopt a closed-loop control method that combines feedforward and feedback, or can also adopt a feedforward adaptive control method alone. The following will introduce the feedforward control and feedback control of the rear-wheel active steering control method in detail:

[0088] (1) Feedforward control of the rear-wheel active steering control method

[0089] The feedforward control of the rear-wheel active steering control method adopts a control method in which the front and rear wheel steering angles are proportional. Let the proportional coefficient k of the rear-wheel steering angle to the front-wheel steering angle in the feedforward control be:

[0090] (9)

[0091] In Equation (9):

[0092] δ r —— Rear wheel steering angle;

[0093] δ f —— Front wheel steering angle;

[0094] k —— Proportionality coefficient of front and rear wheel steering angles.

[0095] When the vehicle is in steady-state steering, the yaw rate γ of the vehicle is a constant value. At this time, the yaw acceleration , and the lateral acceleration of the vehicle . Substituting the above conditions into the two-degree-of-freedom dynamic equation of the front-wheel steering vehicle in Equation (4), the proportionality coefficient k of the front and rear wheel steering angles when the sideslip angle β = 0 during the steady-state steering of the vehicle can be obtained as:

[0096] (10)

[0097] In Equation (10):

[0098] L —— Wheelbase of the vehicle.

[0099] Referring to Figure 2 , the proportionality coefficient k changes with the vehicle speed u. When the vehicle is steering at a low speed, k is negative. At this time, the front and rear wheels rotate in the opposite direction, thereby reducing the turning radius of the vehicle and improving its maneuverability. When the vehicle is steering at a high speed, k is positive. At this time, the front and rear wheels rotate in the same direction, thereby reducing the sideslip angle of the vehicle's center of mass and improving the vehicle's handling stability and safety.

[0100] When the vehicle enters the steady state, the yaw rate γ of the vehicle and the front wheel steering angle δ f are constant values, that is, at this time , . Combining the proportionality coefficient k of the front and rear wheel steering angles, the steady-state yaw rate gain of the four-wheel steering vehicle in response to the front wheel input can be obtained. Among them, the steady-state yaw rate gain is represented by the symbol , and is specifically as follows:

[0101] (11)

[0102] When k = 0, the steady-state yaw rate gain of the front-wheel steering vehicle can be obtained:

[0103] (12)

[0104] In Equation (12):

[0105] K —— Stability factor, with the unit of s 2 / m 2, is an important parameter characterizing the steady-state response of a vehicle.

[0106] Combined with vehicle parameters, the steady-state yaw rate gain curves of four-wheel steering vehicles and front-wheel steering can be calculated through Equations (11) and (12).

[0107] Refer to Figure 3 , compared with front-wheel steering vehicles, four-wheel steering vehicles with proportional feedforward control of front and rear wheel angles have a greater steady-state yaw rate gain during low-speed steering, indicating that when turning the same curve, four-wheel steering vehicles need to turn the steering wheel less than front-wheel steering vehicles, that is, four-wheel steering vehicles are more maneuverable and have better mobility; at high speeds, the steady-state yaw rate gain of four-wheel steering vehicles is smaller than that of front-wheel steering, indicating that when passing the same curve, four-wheel steering vehicles need to turn the steering wheel more than front-wheel steering.

[0108] Although it improves the active safety of the vehicle to a certain extent, the reduction in the steady-state yaw rate gain of four-wheel steering vehicles at high speeds compared to front-wheel steering is relatively large, resulting in a certain degree of understeer, thus bringing certain discomfort and burden to drivers accustomed to driving traditional front-wheel steering vehicles. At the same time, when the absolute value of the front and rear wheel angle ratio coefficient k is too large under low-speed conditions, it may cause the vehicle to fishtail and scrape, seriously affecting the driving habits of drivers. In view of this situation, the present invention proposes an improved calculation formula for the front and rear wheel angle ratio coefficient k based on vehicle speed segmentation, as follows:

[0109] (13)

[0110] In Equation (13):

[0111] u1, u2 - Vehicle speed segmentation thresholds, which can be determined through calculation and calibration.

[0112] Refer to Figure 4 and Figure 5 , based on the corrected front and rear wheel angle ratio coefficient k, not only can the steering flexibility at low speeds be improved, avoiding scraping caused by excessive rear wheel angles, but also the understeer degree at medium and high speeds can be weakened, improving the maneuverability, while taking into account the driving habits and driving needs of drivers to the greatest extent.

[0113] (2) Feedback control of the rear-wheel active steering control method

[0114] The feedback control of the rear-wheel active steering control method designs the control law based on the ideal sideslip angle of the center of mass, and combines the sliding mode variable structure control principle to set the sliding mode surface s as:

[0115] (14)

[0116] Taking the derivative gives:

[0117] (15)

[0118] Combined with the two-degree-of-freedom dynamic equation of the front-wheel steering vehicle in Equation (4), it can be obtained that:

[0119] (16)

[0120] Adopting the exponential reaching law, which has the characteristics of fast reaching speed and small speed near the switching surface, the functions of fast reaching and jitter weakening can be realized by adjusting parameters, that is:

[0121] (17)

[0122] In Equation (17):

[0123] ε —— represents the approaching speed;

[0124] —— represents the exponential approaching term.

[0125] By combining Equation (16) and Equation (17), the feedback control rear-wheel steering angle can be obtained:

[0126] (18)

[0127] In addition, to eliminate the jitter problem of the sliding mode controller, the saturation function sat(s / Δ) is used to replace the sign function sgn(s). That is:

[0128] (19)

[0129] In Equation (19), Δ is the boundary layer thickness of the saturation function. If Δ is too large, it will lead to instability; if Δ is too small, jitter will occur.

[0130] Finally, the rear-wheel steering angle control quantity is obtained as:

[0131] (20)

[0132] The finally actually output rear-wheel steering angle is the sum of the outputs of the feedforward control and the feedback control.

[0133] 4. Cooperative control of distributed drive torque vector control and rear-wheel active steering

[0134] Both the distributed drive torque vector control method (TVC) and the rear-wheel active steering control method (ARS) can change the dynamic response performance of the vehicle. Among them, the distributed drive torque vector control adjusts the vehicle attitude by adding a yaw moment ΔM z and the rear-wheel active steering control changes the dynamic response of the vehicle by directly controlling the rear-wheel steering angle δ r to change the dynamic response of the vehicle.

[0135] Referring to Figure 6 , the driving conditions of the vehicle are divided into high-speed conditions, medium-speed conditions, and low-speed conditions in the present invention. Combining the characteristics of the distributed drive torque vector control method and the rear-wheel active steering control method, that is, the distributed drive torque vector control uses the ideal yaw rate γ d as the closed-loop control, while the rear-wheel active steering control uses the ideal center-of-mass side slip angle β d as the closed-loop control. The former focuses on handling performance, and the latter focuses on stability performance. Thus, a segmented fusion control strategy based on different working condition requirements is further formulated:

[0136] (1) High-speed condition

[0137] The high-speed condition has a greater demand for vehicle stability. Therefore, taking the ideal center-of-mass side slip angle β d and the ideal yaw rate γ d as the control objectives, a cooperative control strategy that combines the distributed drive torque vector control method and the rear-wheel active steering control method is adopted. At the same time, the actual center-of-mass side slip angle β and the actual yaw rate γ of the vehicle are combined to track the target motion state of the vehicle under high-speed conditions, so as to obtain the additional yaw moment ΔM z and the rear-wheel steering angle δ r . Among them, the adopted rear-wheel active steering control method is a closed-loop control method that combines feedforward and feedback, and the feedback control only intervenes when the actual center-of-mass side slip angle of the vehicle exceeds a certain threshold β0, so as to ensure the stability of the vehicle and take into account the driving habits of the driver at the same time.

[0138] (2) Medium-speed condition

[0139] The demand for vehicle stability in the medium-speed condition is smaller than the demand for vehicle steering maneuverability. Therefore, taking the ideal yaw rate γ d as the control objective, a single control strategy of the distributed drive torque vector control method is adopted. At the same time, the actual yaw rate γ of the vehicle is combined to track the target motion state of the vehicle under medium-speed conditions, so as to obtain the additional yaw moment ΔM z . Then the rear-wheel cancels the active steering control function and uses a mechanical locking mechanism for automatic locking.

[0140] (3) Low-speed condition

[0141] The demand for vehicle stability in the low-speed condition is small, but the demand for vehicle steering maneuverability is large. Therefore, taking the ideal yaw rate γ d as the control objective, a cooperative control strategy that combines the distributed drive torque vector control method and the rear-wheel active steering control method is adopted. At the same time, the actual yaw rate γ of the vehicle is combined to track the target motion state of the vehicle under low-speed conditions, so as to obtain the additional yaw moment ΔM zand the rear wheel steering angle δ r . Among them, the adopted rear wheel active steering control method is a feedforward adaptive control method.

[0142] (4) Coordinated control output under various working conditions

[0143] Refer to Figure 6 , T fl , T fr , T rl , T rr are the torques of the drive motors of each wheel respectively, T is the total required torque, α p , Δα p are the throttle pedal opening and the change rate of the throttle pedal opening respectively, a y is the lateral acceleration, ω i is the wheel speed of each wheel, i = fl, fr, rl, rr. The front wheel steering angle δ f , the throttle pedal opening α p and the change rate of the throttle pedal opening Δα p are output by the driver model, and the ideal center of mass side slip angle β d and the ideal yaw rate γ d are obtained through the two-degree-of-freedom dynamics equation of the front wheel steering vehicle. Then, based on the characteristics of the distributed drive torque vector control method and the rear wheel active steering control method, combined with the actual center of mass side slip angle β and the actual yaw rate γ of the vehicle, the target motion state of the vehicle under different working conditions is tracked, and the control command of the rear wheel steering angle δ r and the additional yaw moment ΔM z are output. The additional yaw moment ΔM z is then used by the torque distributor to obtain the drive torques T fl , T fr , T rl , T rr at each wheel end, thereby realizing the coordinated control of the vehicle.

[0144] The present invention is described with a distributed drive electric vehicle as an example. Among them, the rear wheel active steering control technology is also applicable to non-distributed drive vehicles, and the integrated control strategy can be applied to the stability control of distributed drive vehicles.

[0145] The above is only the specific implementation manner of the present invention, but the design concept of the present invention is not limited thereto. Any non-substantive modification made to the present invention using this concept shall fall within the scope of infringement of the protection scope of the present invention.

Claims

1. A four-wheel steering control method for a distributed drive electric vehicle, characterized in that: It includes the following steps: (1) The front wheel steering angle δ is output by the driver model f , and the ideal center of mass side slip angle β d and the ideal yaw rate γ d are obtained based on the two-degree-of-freedom dynamics equation of the front-wheel-steering vehicle; (2) Divide the driving conditions of the vehicle into high-speed conditions, medium-speed conditions, and low-speed conditions. Combining the characteristics of the distributed drive torque vector control method and the rear-wheel active steering control method, formulate a segmented fusion control strategy based on the requirements of different conditions, so as to obtain the additional yaw moment ΔM z and / or the rear-wheel steering angle δ r ; In step (2), under high-speed conditions and low-speed conditions, the feedforward control in the rear-wheel active steering control method adopts a control method in which the front and rear wheel steering angles are proportional. Set the proportional coefficient of the feedforward rear-wheel steering angle to the front-wheel steering angle as k, and correct this proportional coefficient k based on the requirements of different conditions: Where, m is the vehicle mass, a is the distance from the center of mass to the front axle of the chassis, b is the distance from the center of mass to the rear axle of the vehicle, u is the longitudinal vehicle speed at the center of mass, k1 and k2 are the equivalent cornering stiffnesses of the front and rear axles of the vehicle, L is the wheelbase of the vehicle, and u1 and u2 are the vehicle speed segmentation thresholds; (3)Output the front wheel steering angle δ f and the throttle pedal opening α p and the throttle pedal opening change rate Δα p , combine the additional yaw moment ΔM z and / or the rear wheel steering angle δ r obtained by the segmented fusion control strategy, and realize the cooperative control of the vehicle through the four-wheel steering distributed drive vehicle model.

2. The four-wheel steering control method for a distributed drive electric vehicle according to claim 1, characterized in that: In step (2), the segmented fusion control strategy includes: a. Under high-speed conditions, with the ideal sideslip angle β of the center of mass d and the ideal yaw rate γ d as the control objectives, a coordinated control strategy that combines the distributed drive torque vector control method and the rear-wheel active steering control method is adopted. At the same time, the actual sideslip angle β and the actual yaw rate γ of the vehicle are combined to track the target motion state of the vehicle under high-speed conditions, so as to obtain the additional yaw moment ΔM z and the rear-wheel steering angle δ r ; b. Under medium-speed conditions, with the ideal yaw rate γ d as the control target, a single control strategy using the distributed drive torque vector control method is adopted, and at the same time, the actual yaw rate γ of the vehicle is combined to track the target motion state of the vehicle under medium-speed conditions, so as to obtain the additional yaw moment ΔM z ; c. Under low-speed conditions, with the ideal yaw rate γ d as the control target, a cooperative control strategy that combines the distributed drive torque vector control method and the rear-wheel active steering control method is adopted. At the same time, the actual yaw rate γ of the vehicle is combined to track the target motion state of the vehicle under low-speed conditions, so as to obtain the additional yaw moment ΔM z and the rear-wheel steering angle δ r .

3. The four-wheel steering control method for a distributed drive electric vehicle according to claim 2, wherein: In step (2), the rear-wheel active steering control method adopted under high-speed conditions is a closed-loop control method combining feedforward and feedback; the rear-wheel active steering control method adopted under low-speed conditions is a feedforward adaptive control method.

4. The four-wheel steering control method for a distributed drive electric vehicle according to claim 1, characterized in that: In step (2), first, based on the two-degree-of-freedom dynamics equation of the front-wheel steering vehicle during steady-state steering, the calculation formula for the proportional coefficient k during steady-state steering of the vehicle is obtained as: Then, based on the requirements of different working conditions, the proportional coefficient k is corrected, and the steady-state yaw rate gain of the four-wheel steering vehicle to the front-wheel input is obtained as: where K is the stability factor with the unit of s 2 / m 2 , which is an important parameter characterizing the steady-state response of the vehicle.

5. A four-wheel steering control method for a distributed drive electric vehicle according to claim 1, characterized in that: In step (1), the two-degree-of-freedom dynamics equation of the front-wheel steering vehicle is: Among them, β is the sideslip angle of the vehicle's center of mass, γ is the yaw angular velocity of the vehicle, and δ f is the front wheel steering angle, and I Z is the yaw moment of inertia of the vehicle.

6. The four-wheel steering control method for a distributed drive electric vehicle according to claim 5, wherein: Taking the sideslip angle of the vehicle's center of mass in the non-sideslip state as the ideal value, that is, the ideal sideslip angle of the center of mass β d is 0.

7. A four-wheel steering control method for a distributed drive electric vehicle according to claim 5, characterized in that: Taking the yaw rate under the steady-state steering condition of the vehicle as the ideal value, and correcting this ideal value based on the influence of the road surface adhesion coefficient, the ideal yaw rate γ is obtained accordingly. d The calculation formula is as follows: Among them, the stability factor , μ is the road surface adhesion coefficient, g is the acceleration due to gravity, and 0.85 is the set safety factor.

8. The four-wheel steering control method for a distributed drive electric vehicle according to claim 3, wherein: Under high-speed conditions, the feedback control in the rear-wheel active steering control adopts sliding mode control based on the ideal value of the sideslip angle at the center of mass. According to the sliding mode variable structure control principle, combined with the two-degree-of-freedom dynamics equation of the front-wheel steering vehicle, the rear-wheel steering angle control amount is obtained as: Where, s is the sliding mode surface, ε is the approaching speed, is the exponential approaching term, and Δ is the saturation function boundary layer thickness.

9. The four-wheel steering control method for a distributed drive electric vehicle according to claim 1, characterized in that: In step (2), the distributed drive torque vector control adopts an up-and-down hierarchical control method. The upper layer is the motion tracking layer, which obtains the additional yaw moment ΔM z and the longitudinal driving force T; the lower layer is the actuator control layer, which combines the output upper-layer additional yaw moment ΔM z and the longitudinal driving torque T to distribute the torque to the left and right drive wheels, thereby realizing the yaw control of the vehicle.

Images