Differential brake driving steering control method for electric automobile driven by hub motor

Abstract

The invention belongs to the technical field of automobile control, and particularly relates to a hub motor driven electric automobile differential brake drive steering control method, which comprises the following steps: acquiring vehicle state parameters in real time by using a vehicle state acquisition module, establishing a vehicle model representing differential brake drive steering, and designing a first-order dynamic sliding mode controller to complete steering control; whether the vehicle is unstable during steering and the steering form of the vehicle are judged, compensation yaw moment is generated through a fast terminal fuzzy sliding mode controller, active intervention is conducted on the vehicle, and the vehicle body posture is adjusted to complete neutral steering; after torque distribution, the hub motor on the side providing braking force conducts regenerative braking, and the energy utilization rate is increased. Regenerative braking is carried out while steering control is carried out, so that the vehicle can smoothly complete steering, the safety and the driving stability of the whole vehicle are improved, meanwhile, energy consumption can be reduced, and the endurance mileage of the vehicle is increased.

Description

Technical Field

[0001] The invention belongs to the technical field of automobile control, and in particular relates to a differential braking driving steering control method for an electric vehicle driven by a wheel hub motor. Background Art

[0002] In-wheel motor-driven electric vehicles are directly driven by in-wheel motors installed in the four wheels. A complex transmission system is no longer required. The wheel motion states are independent of each other, the torque is independently controllable, and the motor has a fast response speed. It is the best platform for realizing vehicle differential braking drive steering. When the steering motor in the vehicle's wire control system fails and cannot complete the steering smoothly, or when understeering or oversteering occurs during steering, the in-wheel motors on both sides can be controlled separately to provide the vehicle with differential braking drive torque, so that the front wheel angle of the vehicle turns to the corresponding angle to complete neutral steering, which is differential braking drive steering DBDS (Differential Brake Drive Steering).

[0003] Traditional car steering mainly relies on the left and right tie rods connected to the front wheel steering knuckle to pull the front wheel around the steering kingpin to complete the steering, and the vehicle will generate corresponding yaw angular velocity and center of mass sideslip angle. However, for vehicles using DBDS, by separately controlling the inner and outer wheel hub motors for braking and driving to generate differential braking drive torque, the vehicle can smoothly complete the steering under the action of torque, improving vehicle stability and driving safety. Among them, the inner wheel hub motor that provides braking torque for vehicle steering performs regenerative braking during braking, recovers braking energy, and improves energy utilization.

[0004] At present, the research on differential steering of electric vehicles driven by wheel hub motors mainly focuses on the differential drive steering control method, and lacks the research on differential brake drive steering control method. Tian Jie, Wang Qun and others from Nanjing Forestry University proposed a differential drive steering and stability control method, which enables the steering wheel of a nonlinear vehicle relying on differential drive steering to track the ideal center of mass sideslip angle and ideal yaw rate of the reference model well under different road conditions within the allowable turning angle range. However, when the vehicle is in extreme motion, the lateral force of the tire is easy to reach saturation, and it is difficult to change the understeering trend by increasing the driving torque difference to increase the front wheel turning angle, and the control effect is not ideal. The differential steering method proposed by Liu Jie and others from Beijing New Energy Automobile Co., Ltd. describes in detail the distribution method of braking torque and driving torque, but does not give a specific control method for yaw torque, as well as a control strategy when the vehicle is unstable.

[0005] Therefore, those skilled in the art urgently need to propose a wheel hub motor driven electric vehicle differential braking drive steering control method, so that the vehicle relying on differential braking drive steering can smoothly complete the steering, so as to better track the ideal yaw angular velocity and the ideal center of mass sideslip angle, improve the stability of the vehicle, and ensure driving safety. Summary of the invention

[0006] The purpose of the present invention is to solve the above-mentioned technical problems and proposes a differential braking drive steering control method for an electric vehicle driven by a hub motor, so that the vehicle can well track the ideal center of mass sideslip angle and ideal yaw angular velocity of a reference model on roads with different adhesion coefficients, and successfully complete steering. When the vehicle loses stability and understeers or oversteers, the vehicle body posture is adjusted by compensating for the yaw torque to complete neutral steering, and at the same time, energy is recovered to reduce energy consumption and improve vehicle cruising range.

[0007] In order to achieve the above-mentioned invention object, the technical solution proposed in this application is:

[0008] A method for controlling the differential braking drive steering of an electric vehicle driven by a wheel hub motor comprises the following steps:

[0009] 1) The vehicle state acquisition module collects the vehicle's steering wheel angle signal, vehicle speed signal, vehicle yaw rate r, vehicle center of mass sideslip angle β, road adhesion coefficient μ, and power battery state of charge SOC value, and transmits the data to the vehicle controller. The vehicle speed and steering wheel angle signals are obtained through the vehicle speed sensor and the steering wheel angle sensor.

[0010] 2) The vehicle controller determines whether the vehicle is turning based on the steering wheel signal. If it is determined not to turn, the current operation is maintained and returns to step 1).

[0011] If the vehicle is determined to be turning, the steering wheel angle signal and vehicle speed are input into the two-degree-of-freedom reference model to calculate the ideal yaw rate r d and the ideal center of mass sideslip angle β d .

[0012] Specifically, the motion equation of the two-degree-of-freedom reference model in step 2) is:

[0013]

[0014] Where: m is the vehicle mass, v x represents the longitudinal velocity at the center of mass of the vehicle, k f , k r Represents the tire cornering stiffness of the front and rear wheels of the vehicle, l f , l r Represents the distance from the center of mass of the vehicle to the front and rear axles, I z is the vehicle's moment of inertia, β is the vehicle's sideslip angle, is the sideslip velocity of the center of mass, r is the yaw velocity of the vehicle, is the vehicle's yaw acceleration, and δ is the vehicle's front wheel turning angle.

[0015] 3) Establish a vehicle differential braking drive model and convert the ideal yaw rate r d The difference between the vehicle's yaw rate r, the vehicle's center of mass sideslip angle β and the ideal center of mass sideslip angle β d The difference is input into the first-order dynamic sliding mode controller to obtain the yaw torque ΔM1 required for the vehicle to perform differential braking drive steering.

[0016] Specifically, the motion state equation of the vehicle differential braking drive model is established as:

[0017]

[0018] Where: R is the effective radius of the wheel, F xfl 、F xfr Respectively represent the longitudinal ground force on the left front wheel and the right front wheel, F yfl 、F yfr 、F yrl 、F yrr Respectively represent the ground lateral force on the left front wheel, right front wheel, left rear wheel, and right rear wheel, l s It is half of the left and right wheel tracks.

[0019] The side slip angles of coaxial tires are the same, and the side slip angle calculation formula for the front and rear tires is:

[0020]

[0021] Where: α f , α r They represent the front axle tire slip angle and the rear axle tire slip angle respectively.

[0022] The lateral ground force on the front and rear axle tires is:

[0023]

[0024] Since the front wheel turning angle is small, sinδ≈0,cosδ≈1.

[0025] Simplify the motion state equation of the vehicle differential braking drive model:

[0026]

[0027] The vehicle body steering dynamics equation is:

[0028]

[0029] in:

[0030] Simplifying, we get:

[0031]

[0032] Where: is the angular acceleration of the vehicle's front wheel, is the angular velocity of the front wheel, l is half of the tire trail, J eff , b eff Represent the effective moment of inertia and damping of the steering system, τ a is the tire aligning torque, τ f is the friction torque of the steering system, r σ The kingpin offset.

[0033] The final motion equation of the vehicle differential braking drive model is:

[0034]

[0035] Following the above scheme, the ideal yaw rate r d The difference between the vehicle's yaw rate r, the vehicle's center of mass sideslip angle β and the ideal center of mass sideslip angle β d The difference is input into the first-order dynamic sliding mode controller, where ΔM2=0.

[0036] The sliding surface of the first-order dynamic sliding mode controller is designed as: s = ε(rr d )+η(β-β d ), ε, η are adjustment parameters, which are determined by the yaw moment.

[0037] Power reaching law: k>0,0<α<1.

[0038] Derivative of the sliding surface:

[0039] Taking s as the system state, redesign the sliding surface for s:

[0040] After derivation, we get:

[0041] Let it be equal to the power reaching law:

[0042] in:

[0043] make

[0044] but:

[0045] So we get:

[0046] Then the output ΔM of the sliding mode controller is:

[0047] 4) The vehicle dynamics characteristics are used to establish the vehicle driving safety boundary to determine whether the vehicle is unstable. When the vehicle is unstable, the vehicle yaw rate r is used to determine the actual steering form of the vehicle. Then the ideal yaw rate is used as the control target, and the error and error change rate between the ideal yaw rate and the actual vehicle yaw rate are input into the fast terminal fuzzy sliding mode controller to output the additional yaw torque ΔM2 required to compensate for the vehicle's neutral steering.

[0048] In this step, the vehicle driving safety boundary is used to judge the vehicle stability. This is because the vehicle safety boundary judgment method is relatively conservative and meets the national automobile safety stability standards. There are many studies on the method of judging vehicle stability, which is not the innovation of the present invention. This application uses the yaw angular velocity and the sideslip angle of the center of mass as the horizontal and vertical coordinates, and obtains the coordinate calculation formula of the boundary in the vehicle driving safety boundary:

[0049] The coordinates of point A are determined by the steady-state yaw rate limit value and the rear axle slip angle saturation value:

[0050] Point A:

[0051] Saturated front wheel angle:

[0052] Where: s is the critical value of the front wheel turning angle, α fs , α rs Represent the saturation value of the sideslip angle of the front and rear axles respectively. The yaw rate and the sideslip angle of the center of mass determined by the critical value of the front wheel angle are at point B:

[0053] Point B:

[0054] Rotate the steering wheel in the opposite direction to obtain points C and D that are symmetrical to points A and B.

[0055] Since the lateral force of the front wheels in the upper right and lower left corners has little effect on the vehicle stability, they are regarded as non-safe areas and the boundaries of this area are reset to AEFCGHA.

[0056]

[0057] Where: e , f The principle of selecting the value of is that when the vehicle dynamic response becomes worse, it indicates that the selected initial safety margin is too large, and the values ​​of the two should be reduced. G and H are the symmetrical points of E and F.

[0058] Vehicle steering form judgment: Use the front and rear wheel slip angles and yaw angular velocity to describe the steady-state response.

[0059] According to the formula: Where L is the vehicle wheelbase, α f , α r They represent the front axle tire slip angle and the rear axle tire slip angle respectively.

[0060] get:

[0061] Furthermore: the yaw angular velocity is taken as the control target, and the fast terminal fuzzy sliding mode control algorithm is used to calculate the compensation yaw torque.

[0062] The sliding surface of the controller is designed as:

[0063] Where: The specific value is to be determined by experimental testing based on the applied vehicle parameters.

[0064] The purpose of stability judgment is to control the differential braking drive yaw torque through the vehicle state. In order to simplify the calculation and reduce the calculation amount of the controller, the equivalent control design sliding mode control law is adopted.

[0065] make According to the ideal two-degree-of-freedom reference model, we can get

[0066]

[0067] Let U eq =ΔM'2, combined with the two-degree-of-freedom reference model, The calculation formula is:

[0068]

[0069] By setting the values ​​of α, ρ, and λ, the system can reach equilibrium in a finite time. When it is on the sliding surface:

[0070]

[0071] The output of the fast terminal sliding mode controller can be obtained:

[0072]

[0073] Further settings are: suppressing chattering and switching control fuzziness.

[0074] Switching function U s Using the constant velocity approach law:

[0075] U s=-asgn(s)-bs, a,b>0

[0076] In order to suppress chattering, the switching control is fuzzy, and the fuzzy rules are:

[0077]

[0078] S represents the sliding surface function, φ represents the output of fuzzy control, and the fuzzy sets PB, PS, ZE, NS, and NB represent positive large, positive small, zero, negative small, and negative large, respectively.

[0079] The fuzzy input variable is the fast terminal sliding mode function S. The trapmf and trimf functions are used to design the fuzzy domain, and the distribution intervals are NB[-1,-1,-0.8,-0.5], NS[-0.8,-0.4,0], ZE[-0.4,0,0.4], PS[0,0.4,0.8], and PB[0.5,0.8,1,1].

[0080] The fuzzy output variable is φ, and the trapmf membership function is used to design the fuzzy domain. The distribution intervals are NB[-1,-1,-0.7,-0.5], NS[-0.7,-0.4,-0.2,-0.1], ZE[-0.2,-0.1,01,0.2], PS[0.1,0.2,0.4,0.7], and PB[0.5,0.7,1,1].

[0081] Then the control law is: U = U eq +φU s =ΔM2.

[0082] When φ=1, it is the traditional equivalent sliding mode control, and the control law is the same as the conventional switching function. When φ≠1, it is the control law that fuzzifies the switching function. The fuzzy control rule is used to adjust the change of φ to suppress vibration.

[0083] 5) Sum the yaw moments ΔM1 and ΔM2, and distribute the longitudinal force and yaw moment generated by the front and rear axles according to the front and rear axle load ratio of the vehicle;

[0084] In this step, the yaw moment is summed according to the following formula:

[0085]

[0086] Furthermore, the braking force / driving force allocated to each wheel is firstly made to meet the longitudinal force and yaw moment requirements of the vehicle. In addition, in order to prevent the single-axle wheel load from being too high and exceeding the limit value, the longitudinal force and yaw moment generated by the front and rear axles are allocated according to the front and rear axle load ratio, that is:

[0087]

[0088] Constraints:

[0089] in

[0090] Where: F zf 、F zr Represent the vertical loads on the front and rear axles, respectively, l d is the left and right wheelbase, a xd It is the sum of the longitudinal acceleration of the vehicle required by the driver and the longitudinal acceleration of the vehicle required to overcome various driving resistances.

[0091] The longitudinal force of each tire is calculated as:

[0092]

[0093] The yaw moment distributed between the front and rear axles is:

[0094]

[0095] Where: ΔM f , ΔM r Represents the yaw moment values ​​distributed to the front axle and rear axle respectively.

[0096] 6) According to the braking torque allocated by the hub motor, the control strategy of the four hub motors on the inner and outer sides of the front and rear axles is formulated, and the four hub motors are controlled to respond respectively.

[0097] The control strategy includes:

[0098] 61) The braking torque allocated by the hub motor is divided into the following levels:

[0099]

[0100] Where: T b The braking torque allocated to the inner wheel hub motor, T rb The maximum regenerative braking torque that the wheel hub motor can provide;

[0101] 62) The inner wheel hub motor provides braking torque, and the outer wheel hub motor provides driving torque, so as to keep the front wheel angle always within the allowable range of the steering wheel angle, and the left and right front wheel angles are consistent;

[0102] a. If the braking torque allocated by the wheel hub motor is the first level, determine whether the state of charge of the vehicle's power battery is less than 90% at this time. If so, the inner wheel hub motor performs regenerative braking to provide the required braking torque. If not, the inner wheel hub motor is still used for braking, but energy recovery is no longer performed, and the outer wheel hub motor provides the driving torque;

[0103] b. If the braking torque allocated by the wheel hub motor is the second level, determine whether the state of charge of the vehicle's power battery is less than 90% at this time. If so, the inner wheel hub motor performs regenerative braking and provides the maximum regenerative braking torque. If not, the inner wheel hub motor still provides the maximum braking torque, but no longer performs energy recovery. The remaining braking torque is provided by mechanical braking, and the outer wheel hub motor provides the driving torque;

[0104] c. If the braking torque allocated by the wheel hub motor is the third level, the inner side uses mechanical braking to provide braking torque, and the outer wheel hub motor provides driving torque.

[0105] The working principle of the present invention is as follows: the present invention fully utilizes the advantages of independent controllability of wheel hub motors to control four wheel hub motors and perform differential braking drive control on the vehicle; utilizes the safe driving boundary to determine the boundary of the stability domain, utilizes the generated compensatory yaw torque to actively intervene in the vehicle steering, and returns the vehicle to a stable state; while steering, the single-side wheel hub motor adopts regenerative braking to recover energy and improve energy utilization.

[0106] Compared with the prior art, the present invention has the following beneficial effects:

[0107] The present invention can enable a vehicle that relies on differential braking to drive steering to complete steering smoothly, can well track the ideal yaw angular velocity and center of mass sideslip angle, improve the stability of the entire vehicle, and ensure driving safety. At the same time, the present invention can also reduce energy consumption and increase the vehicle's cruising range. BRIEF DESCRIPTION OF THE DRAWINGS

[0108] Figure 1 Schematic diagram of the vehicle structure of the present invention in an embodiment.

[0109] Figure 2 Schematic diagram of the steering control strategy of the present invention in an embodiment.

[0110] Figure 3 4 is a flow chart of the regenerative braking control of the present invention in an embodiment.

[0111] Figure 4 Schematic diagram of the vehicle motion coordinate system established by the present invention in the embodiment.

[0112] Figure 5 A phase plane diagram for determining the safe driving boundary proposed by the present invention in an embodiment.

[0113] Figure 6 3 is a diagram showing the simulation results of the vehicle center of mass sideslip angle under double lane change conditions in the embodiment of the present invention.

[0114] Figure 7 4 is a diagram showing the simulation results of the vehicle yaw rate under double lane change conditions in the embodiment of the present invention.

[0115] Figure 8 This is a photo of a laboratory wheel hub motor driven electric vehicle prototype in the embodiment. DETAILED DESCRIPTION

[0116] The technology of the present invention is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0117] Example

[0118] This embodiment proposes a method for controlling the steering of a differential braking drive of an electric vehicle driven by a wheel hub motor. Figure 1 As shown, the vehicle status acquisition module acquires the corresponding status parameters of the vehicle and transmits the data to the vehicle controller. After calculation, the vehicle controller transmits the control signal to the wheel hub motor controller, and the motor controller controls the wheel hub motor to respond to the signal. Figure 2 As shown, the control method specifically includes the following steps:

[0119] 1) The vehicle state acquisition module collects the vehicle's steering wheel angle signal, vehicle speed signal, vehicle yaw rate r, vehicle center of mass sideslip angle β, road adhesion coefficient μ, and power battery state of charge SOC value, and transmits the data to the vehicle controller. The vehicle speed and steering wheel angle signals are obtained through the vehicle speed sensor and the steering wheel angle sensor.

[0120] 2) The vehicle controller determines whether the vehicle is turning based on the steering wheel signal. If it is determined not to turn, the current operation is maintained and returns to step 1).

[0121] If the vehicle is determined to be turning, the steering wheel angle signal and vehicle speed are input into the two-degree-of-freedom reference model to calculate the ideal yaw rate r d and the ideal center of mass sideslip angle β d .

[0122] Specifically, the motion equation of the two-degree-of-freedom reference model in step 2) is:

[0123]

[0124] Where: m is the vehicle mass, v x represents the longitudinal velocity at the center of mass of the vehicle, k f , k r Represents the tire cornering stiffness of the front and rear wheels of the vehicle, l f , l r Represents the distance from the center of mass of the vehicle to the front and rear axles, I z is the vehicle's moment of inertia, β is the vehicle's sideslip angle, is the sideslip velocity of the center of mass, r is the yaw velocity of the vehicle, is the vehicle's yaw acceleration, and δ is the vehicle's front wheel turning angle.

[0125] 3) Establish a vehicle differential braking drive model and convert the ideal yaw rate r d The difference between the vehicle's yaw rate r, the vehicle's center of mass sideslip angle β and the ideal center of mass sideslip angle β d The difference is input into the first-order dynamic sliding mode controller to obtain the yaw torque ΔM1 required for the vehicle to perform differential braking drive steering.

[0126] Specifically, Figure 4 As shown in the figure, the center of mass of the vehicle is taken as the origin of the vehicle motion coordinate system, the vertical, pitch and roll motions of the vehicle are ignored, the influence of the suspension on the body motion is ignored, the side slip angles of the coaxial tires are assumed to be the same, the mechanical properties of the hub motors are the same, and the slope resistance and tire rolling resistance are ignored. The motion state equation of the vehicle differential braking drive model is:

[0127]

[0128] Where: R is the effective radius of the wheel, F xfl 、F xfr Respectively represent the longitudinal ground force on the left front wheel and the right front wheel, F yfl 、F yfr 、F yrl 、F yrr Respectively represent the ground lateral force on the left front wheel, right front wheel, left rear wheel, and right rear wheel, l s It is half of the left and right wheel tracks.

[0129] The side slip angles of coaxial tires are the same, and the side slip angle calculation formula for the front and rear tires is:

[0130]

[0131] Where: α f , α r They represent the front axle tire slip angle and the rear axle tire slip angle respectively.

[0132] The lateral ground force on the front and rear axle tires is:

[0133]

[0134] Since the front wheel turning angle is small, sinδ≈0,cosδ≈1.

[0135] Simplify the motion state equation of the vehicle differential braking drive model:

[0136]

[0137] The vehicle body steering dynamics equation is:

[0138]

[0139] in:

[0140] Simplifying, we get:

[0141]

[0142] Where: is the angular acceleration of the vehicle's front wheel, is the angular velocity of the front wheel, l is half of the tire trail, J eff , b eff Represent the effective moment of inertia and damping of the steering system, τ a is the tire aligning torque, τ f is the friction torque of the steering system, r σ The kingpin offset.

[0143] The final motion equation of the vehicle differential braking drive model is:

[0144]

[0145] Following the above scheme, the ideal yaw rate r d The difference between the vehicle's yaw rate r, the vehicle's center of mass sideslip angle β and the ideal center of mass sideslip angle β d The difference is input into the first-order dynamic sliding mode controller, where ΔM2=0.

[0146] The sliding surface of the first-order dynamic sliding mode controller is designed as: s = ε(rr d )+η(β-β d ), ε, η are adjustment parameters, which are determined by the yaw moment.

[0147] Power reaching law: k>0,0<α<1.

[0148] Derivative of the sliding surface:

[0149] Taking s as the system state, redesign the sliding surface for s:

[0150] After derivation, we get:

[0151] Let it be equal to the power reaching law:

[0152] in:

[0153] make

[0154] but:

[0155] So we get:

[0156] Then the output ΔM of the sliding mode controller is:

[0157] Since the sliding mode controller will inevitably experience chattering, and it is largely caused by the discontinuous sliding mode control law. The discontinuous terms of the control input are placed in the first-order or even higher-order derivatives of the control input. Although the control input derivative is discontinuous, after integration, the control input becomes a continuous function, which can effectively suppress chattering. To simplify the calculation, the discontinuous terms are placed in the first-order derivative of the control input. Compared with the method of suppressing chattering by using saturation function and sigmoid function instead of sign function, this method has a better effect in suppressing chattering and will not reduce the robustness of the system.

[0158] 4) The vehicle dynamics characteristics are used to establish the vehicle driving safety boundary to determine whether the vehicle is unstable. When the vehicle is unstable, the vehicle yaw rate r is used to determine the actual steering form of the vehicle. Then the ideal yaw rate is used as the control target, and the error and error change rate between the ideal yaw rate and the actual vehicle yaw rate are input into the fast terminal fuzzy sliding mode controller to output the additional yaw torque ΔM2 required to compensate for the vehicle's neutral steering.

[0159] In this step, the vehicle driving safety boundary is used to judge the vehicle stability. This is because the vehicle safety boundary judgment method is relatively conservative and meets the national automobile safety and stability standards. Due to the uncertainty factors such as road complexity, vehicle force uncertainty and driver control characteristics during vehicle driving, which will affect the control of vehicle stability, it is more appropriate to use a conservative boundary determination method. Figure 5 As shown in the figure, the yaw rate and the sideslip angle of the center of mass are used as the horizontal and vertical coordinates, and the coordinate calculation formula of the boundary in the vehicle driving safety boundary is obtained as follows:

[0160] The coordinates of point A are determined by the steady-state yaw rate limit value and the rear axle slip angle saturation value:

[0161] Point A:

[0162] Saturated front wheel angle:

[0163] Where: s is the critical value of the front wheel turning angle, α fs , α rs Represent the saturation value of the sideslip angle of the front and rear axles respectively. The yaw rate and the sideslip angle of the center of mass determined by the critical value of the front wheel angle are at point B:

[0164] Point B:

[0165] Rotate the steering wheel in the opposite direction to obtain points C and D that are symmetrical to points A and B.

[0166] Since the lateral force of the front wheels in the upper right and lower left corners has little effect on the vehicle stability, they are regarded as non-safe areas and the boundaries of this area are reset to AEFCGHA.

[0167]

[0168] Where: e , f The principle of selecting the value of is that when the vehicle dynamic response becomes worse, it indicates that the selected initial safety margin is too large, and the values ​​of the two should be reduced. G and H are the symmetrical points of E and F.

[0169] Vehicle steering form judgment: Use the front and rear wheel slip angles and yaw angular velocity to describe the steady-state response.

[0170] According to the formula: Where L is the vehicle wheelbase, α f , α r They represent the front axle tire slip angle and the rear axle tire slip angle respectively.

[0171] get:

[0172] Furthermore: the yaw angular velocity is taken as the control target, and the fast terminal fuzzy sliding mode control algorithm is used to calculate the compensation yaw torque.

[0173] The sliding surface of the controller is designed as:

[0174] Where: The specific value is to be determined by experimental testing based on the applied vehicle parameters.

[0175] The purpose of stability judgment is to control the differential braking drive yaw torque through the vehicle state. In order to simplify the calculation and reduce the calculation amount of the controller, the equivalent control design sliding mode control law is adopted.

[0176] make According to the ideal two-degree-of-freedom reference model, we can get

[0177]

[0178] Let U eq =ΔM'2, combined with the two-degree-of-freedom reference model, The calculation formula is:

[0179]

[0180] By setting the values ​​of α, ρ, and λ, the system can reach equilibrium in a finite time. When it is on the sliding surface:

[0181]

[0182] The output of the fast terminal sliding mode controller can be obtained:

[0183]

[0184] Further settings are: suppressing chattering and switching control fuzziness.

[0185] Switching function U s Using the constant velocity approach law:

[0186] U s =-asgn(s)-bs, a,b>0

[0187] In order to suppress chattering, the switching control is fuzzy, and the fuzzy rules are:

[0188]

[0189] S represents the sliding surface function, φ represents the output of fuzzy control, and the fuzzy sets PB, PS, ZE, NS, and NB represent positive large, positive small, zero, negative small, and negative large, respectively.

[0190] The fuzzy input variable is the fast terminal sliding mode function S. The trapmf and trimf functions are used to design the fuzzy domain, and the distribution intervals are NB[-1,-1,-0.8,-0.5], NS[-0.8,-0.4,0], ZE[-0.4,0,0.4], PS[0,0.4,0.8], and PB[0.5,0.8,1,1].

[0191] The fuzzy output variable is φ, and the trapmf membership function is used to design the fuzzy domain. The distribution intervals are NB[-1,-1,-0.7,-0.5], NS[-0.7,-0.4,-0.2,-0.1], ZE[-0.2,-0.1,01,0.2], PS[0.1,0.2,0.4,0.7], and PB[0.5,0.7,1,1].

[0192] Then the control law is: U = U eq +φU s =ΔM2.

[0193] When φ=1, it is the traditional equivalent sliding mode control, and the control law is the same as the conventional switching function. When φ≠1, it is the control law that fuzzifies the switching function. The fuzzy control rule is used to adjust the change of φ to suppress vibration.

[0194] 5) Sum the yaw moments ΔM1 and ΔM2, and distribute the longitudinal force and yaw moment generated by the front and rear axles according to the front and rear axle load ratio of the vehicle;

[0195] In this step, the yaw moment is summed according to the following formula:

[0196]

[0197] Furthermore, the braking force / driving force allocated to each wheel is firstly made to meet the longitudinal force and yaw moment requirements of the vehicle. In addition, in order to prevent the single-axle wheel load from being too high and exceeding the limit value, the longitudinal force and yaw moment generated by the front and rear axles are allocated according to the front and rear axle load ratio, that is:

[0198]

[0199] Constraints:

[0200] in

[0201] Where: F zf 、F zr Represent the vertical loads on the front and rear axles, respectively, l d is the left and right wheelbase, a xd It is the sum of the longitudinal acceleration of the vehicle required by the driver and the longitudinal acceleration of the vehicle required to overcome various driving resistances.

[0202] The longitudinal force of each tire is calculated as:

[0203]

[0204] The yaw moment distributed between the front and rear axles is:

[0205]

[0206] Where: ΔM f , ΔM r Represents the yaw moment values ​​distributed to the front axle and rear axle respectively.

[0207] 6) According to the braking torque allocated by the hub motor, the control strategy of the four hub motors on the inner and outer sides of the front and rear axles is formulated, and the four hub motors are controlled to respond respectively.

[0208] like Figure 3 As shown, the control strategy includes:

[0209] 61) The braking torque allocated by the hub motor is divided into the following levels:

[0210]

[0211] Where: Tb The braking torque allocated to the inner wheel hub motor, T rb The maximum regenerative braking torque that the wheel hub motor can provide;

[0212] 62) The inner wheel hub motor provides braking torque, and the outer wheel hub motor provides driving torque, so as to keep the front wheel angle always within the allowable range of the steering wheel angle, and the left and right front wheel angles are consistent;

[0213] a. If the braking torque allocated by the wheel hub motor is the first level, determine whether the state of charge of the vehicle's power battery is less than 90% at this time. If so, the inner wheel hub motor performs regenerative braking to provide the required braking torque. If not, the inner wheel hub motor is still used for braking, but energy recovery is no longer performed, and the outer wheel hub motor provides the driving torque;

[0214] b. If the braking torque allocated by the wheel hub motor is the second level, determine whether the state of charge of the vehicle's power battery is less than 90% at this time. If so, the inner wheel hub motor performs regenerative braking and provides the maximum regenerative braking torque. If not, the inner wheel hub motor still provides the maximum braking torque, but no longer performs energy recovery. The remaining braking torque is provided by mechanical braking, and the outer wheel hub motor provides the driving torque;

[0215] c. If the braking torque allocated by the wheel hub motor is the third level, the inner side uses mechanical braking to provide braking torque, and the outer wheel hub motor provides driving torque.

[0216] Stability and track keeping are the main goals of vehicle dynamics control. The yaw rate characterizes the stability of the vehicle, and the sideslip angle characterizes the track keeping characteristics of the vehicle. The two are interrelated and influence each other, so they are usually used to characterize the stability of the vehicle. Figure 6 and Figure 7 The following are the simulation results of the vehicle's center of mass slip angle and yaw rate. During the simulation, the vehicle speed was set to 60 km / h, double lane change conditions, and the road adhesion coefficient was 0.85. From the simulation results, it can be seen that the vehicle can track the ideal center of mass slip angle and yaw rate very well.

[0217] Figure 8 This is a photo of a laboratory wheel hub motor driven electric vehicle prototype. This example uses this vehicle as the research object. The wheel hub motor driven electric vehicle is powered by two front and rear lithium-ion battery modules, and the vehicle is driven by the wheel hub motor in each wheel. The wheel hub motor uses a permanent magnet brushless DC motor.

[0218] The working principle of the present invention is as follows: the present invention fully utilizes the advantages of independent controllability of wheel hub motors to control four wheel hub motors and perform differential braking drive control on the vehicle; utilizes the safe driving boundary to determine the boundary of the stability domain, utilizes the generated compensatory yaw torque to actively intervene in the vehicle steering, and returns the vehicle to a stable state; while steering, the single-side wheel hub motor adopts regenerative braking to recover energy and improve energy utilization.

[0219] The above content describes the basic principles, methods and main features of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments. The above embodiments and the specification only describe the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention. The scope of protection claimed by the present invention is defined by the attached claims and their equivalents.

Claims

1. A method for controlling the differential braking drive steering of an electric vehicle driven by a wheel hub motor, characterized in that: The steps include: S1. When the vehicle is turning, the ideal yaw rate r of the vehicle is calculated based on the vehicle's steering wheel angle signal, vehicle speed and two-degree-of-freedom reference model. d and the ideal center of mass sideslip angle β d ; S2, establishing a vehicle differential braking drive model, and calculating the yaw moment ΔM1 required for the vehicle to perform differential braking drive steering; S3: Determine whether the vehicle is unstable. If so, use the vehicle yaw rate r to determine the actual steering form of the vehicle, and then use the ideal yaw rate r d As the control target, the fast terminal fuzzy sliding mode controller is used to calculate and output the additional yaw moment ΔM2 required to compensate for the vehicle's neutral steering; S4, summing the yaw moments ΔM1 and ΔM2, and distributing the longitudinal force and yaw moment generated by the front and rear axles according to the front and rear axle load ratio of the vehicle; S5. According to the braking torque allocated by the wheel hub motor, a control strategy for the four wheel hub motors on the inner and outer sides of the front and rear axles is formulated, and the four wheel hub motors are controlled to respond respectively.

2. The method for controlling the differential braking drive steering of an electric vehicle driven by a hub motor according to claim 1, characterized in that: The motion equation of the two-degree-of-freedom reference model in step S1 is: Where: m is the vehicle mass, v x represents the longitudinal velocity at the center of mass of the vehicle, k f , k r Represents the tire cornering stiffness of the front and rear wheels of the vehicle, l f , l r Represents the distance from the center of mass of the vehicle to the front and rear axles, I z is the vehicle's moment of inertia, β is the vehicle's sideslip angle, is the sideslip velocity of the center of mass, r is the yaw velocity of the vehicle, is the vehicle's yaw acceleration, and δ is the vehicle's front wheel turning angle.

3. The method for controlling the differential braking drive steering of an electric vehicle driven by a hub motor according to claim 2, characterized in that: The motion equation of the vehicle differential braking drive model in step S2 is: Where: is the angular acceleration of the vehicle's front wheel, is the angular velocity of the front wheel, l is half of the tire trail, J eff 、b eff Represent the effective moment of inertia and damping of the steering system, τ a is the tire self-aligning torque, τ f is the friction torque of the steering system, r σ The kingpin offset.

4. The method for controlling the differential braking drive steering of an electric vehicle driven by a hub motor according to claim 3, characterized in that: In step S2, the ideal yaw rate r d The difference between the vehicle's yaw rate r, the vehicle's center of mass sideslip angle β and the ideal center of mass sideslip angle β d The difference is input into the first-order dynamic sliding mode controller to obtain the yaw torque ΔM1 required for the vehicle to perform differential braking drive steering.

5. The method for controlling the differential braking drive steering of an electric vehicle driven by a hub motor according to claim 3, characterized in that: In step S3, the actual steering form of the vehicle is described by using the front and rear axle tire side slip angles and the vehicle yaw rate to describe the steady-state response, according to the formula: get: Among them, α f , α r They represent the front axle tire slip angle and the rear axle tire slip angle respectively, and L is the vehicle wheelbase.

6. The method for controlling the differential braking drive steering of an electric vehicle driven by a hub motor according to claim 5, characterized in that: In step S3, the fast terminal fuzzy sliding mode controller is to convert the ideal yaw angular velocity r d The error and error change rate of the actual vehicle yaw rate r are used as input variables, and fuzzy control is used to control the switching item to suppress the chattering phenomenon. The output is: ΔM2=U eq +φU s ; Among them, U eq is the fast terminal sliding mode control output variable, φ is the fuzzy control output variable, U s is the switching function.

7. The method for controlling the differential braking drive steering of an electric vehicle driven by an in-wheel motor according to claim 1, characterized in that: In step S5, the control strategy includes: S51, classify the braking torque allocated by the wheel hub motor into the following levels: Where: T b The braking torque allocated to the inner wheel hub motor, T rb The maximum regenerative braking torque that the wheel hub motor can provide; S52, providing a braking torque through the inner wheel hub motor and providing a driving torque through the outer wheel hub motor, so as to keep the front wheel angle always within the allowable range of the steering wheel angle and keep the left and right front wheel angles consistent; a. If the braking torque allocated by the wheel hub motor is the first level, determine whether the state of charge of the vehicle's power battery is less than 90% at this time. If so, the inner wheel hub motor performs regenerative braking to provide the required braking torque. If not, the inner wheel hub motor is still used for braking, but energy recovery is no longer performed, and the outer wheel hub motor provides the driving torque; b. If the braking torque allocated by the wheel hub motor is the second level, determine whether the state of charge of the vehicle's power battery is less than 90% at this time. If so, the inner wheel hub motor performs regenerative braking and provides the maximum regenerative braking torque. If not, the inner wheel hub motor still provides the maximum braking torque, but no longer performs energy recovery. The remaining braking torque is provided by mechanical braking, and the outer wheel hub motor provides the driving torque; c. If the braking torque allocated by the wheel hub motor is the third level, the inner side uses mechanical braking to provide braking torque, and the outer wheel hub motor provides driving torque.

8. The method for controlling the differential braking drive steering of an electric vehicle driven by an in-wheel motor according to any one of claims 1 to 7, characterized in that: The vehicle speed and steering wheel angle signals are acquired through a vehicle speed sensor and a steering wheel angle sensor.

Images