A method for controlling the braking lateral stability of a four-wheel independent drive vehicle

Abstract

The application discloses a kind of four-wheel independent drive vehicle brake lateral stability control method, comprising: step one, vehicle speed and steering wheel angle in actual driving are obtained by sensor, the expected yaw angular velocity and expected mass center side slip angle of vehicle are calculated;Step two, the difference between expected and actual mass center side slip angle and the difference between expected and actual yaw angular velocity are as input, the additional yaw moment needed is as output, according to the road adhesion coefficient selection direct yaw moment control strategy;Wherein, when road adhesion coefficient s≤0.6, PID control method is used;When road adhesion coefficient s>0.6, fuzzy control method is used;Step three, based on the additional yaw moment needed, the brake force needed by each wheel of four-wheel independent drive vehicle is calculated. In the process of turning braking, the lateral stability of vehicle can be improved by adjusting the brake force of each wheel and exerting yaw moment on vehicle.

Description

Technical Field

[0001] This invention relates to a method for controlling the lateral stability of a four-wheel independent drive vehicle during braking, belonging to the field of electric vehicle braking technology. Background Technology

[0002] During cornering and braking, four-wheel independent drive electric vehicles are affected by the tire lateral slip characteristics. When the vehicle reaches the tire adhesion limit, it will enter an extremely unstable state. At this time, the Direct Yaw Torque Control (DYC) system can actively adjust the vehicle posture by judging the driver's intention to avoid problems such as understeer or oversteer.

[0003] Fuzzy control is an intelligent reasoning control method based on fuzzy set theory, fuzzy logic reasoning, and fuzzy linguistic variables. The fuzzy control system mainly consists of variable definition, fuzzification, fuzzy reasoning, and declarative analysis. The fuzzy controller is based on experience gained from a large number of inputs and outputs. The main operation method is to fuzzify, fuzzy rule, and declaratively analyze the input variables, summarize the obtained data, and determine the appropriate output variables.

[0004] The direct yaw moment control provided by this invention calculates the additional yaw moment required by the vehicle through feedback adjustment, and then calculates the braking force required to be applied to each wheel, thereby adjusting the lateral stability of the four-wheel independent drive vehicle. Summary of the Invention

[0005] This invention designs and develops a method for controlling the lateral stability of a four-wheel independently driven vehicle during braking. By adjusting the braking force of each wheel during cornering, a yaw moment is applied to the vehicle, thereby improving the vehicle's lateral stability.

[0006] The technical solution provided by this invention is as follows:

[0007] A method for controlling the lateral stability of a four-wheel independent drive vehicle during braking includes:

[0008] Step 1: Obtain the vehicle speed and steering wheel angle during actual driving, and calculate the vehicle's expected yaw rate and expected sideslip angle.

[0009] Step 2: Obtain the road surface adhesion coefficient, the actual yaw rate of the vehicle, and the actual sideslip angle of the vehicle's center of gravity. Use the difference between the expected and actual sideslip angle and the difference between the expected and actual yaw rate as inputs, and the required additional yaw moment as output. Select the yaw moment control strategy based on the road surface adhesion coefficient.

[0010] When the road surface adhesion coefficient s≤0.6, the PID control method is adopted;

[0011] When the road surface adhesion coefficient s > 0.6, a fuzzy control method is adopted;

[0012] Step 3: Calculate the additional yaw moment required by the difference between the actual and desired sideslip angles of the vehicle's center of gravity, and the difference between the actual and desired yaw angular velocities of the vehicle.

[0013] Calculate the required braking force for each wheel of a four-wheel independent drive vehicle based on the additional yaw moment required;

[0014] The expression for calculating the braking force of each wheel of the four-wheel independently driven vehicle is as follows:

[0015]

[0016] In the formula, m is the total mass of the four-wheel independent drive vehicle, in kg; h g 1 is the distance between the vehicle's center of gravity and the ground, in meters; b is the horizontal distance between the rear axle and the center of gravity of a four-wheel independent drive vehicle, in meters; L is the wheelbase of a four-wheel independent drive vehicle, in meters; F ua The braking force provided by the front axle brake, F ub The braking force provided to the rear axle brake, measured in N.

[0017] Preferably,

[0018] The vehicle's desired yaw rate γ _NO The calculation formula is:

[0019]

[0020] In the formula, v x The actual speed of the vehicle during travel, in m / s; δ f The front wheel steering angle of a four-wheel independently driven vehicle is expressed in degrees (deg); i is the steering gear ratio; V ch This is the reference value for the longitudinal speed of the vehicle, in m / s;

[0021] The vehicle's center of gravity sideslip angle β _NO The calculation formula is:

[0022]

[0023] In the formula, a is the horizontal distance between the four-wheel independent drive vehicle and the center of gravity, in meters, and k2 is the lateral stiffness of the rear axle, in N / rad.

[0024] Preferably, the vehicle longitudinal speed reference value V ch The calculation formula is:

[0025]

[0026] In the formula, k1 is the lateral stiffness of the front axle of the vehicle, in N / rad.

[0027] Preferably, the wheelbase of the four-wheel independent drive vehicle is L = 2.776m, the horizontal distance between the four-wheel independent drive vehicle and the center of gravity is a = 1.11m, the horizontal distance between the rear axle of the four-wheel independent drive vehicle and the center of gravity is b = 1.666m, and the total mass of the four-wheel independent drive vehicle is m = 1370kg.

[0028] Preferably, in step two, the fuzzy control in the direct yaw moment control strategy includes:

[0029] The difference between the expected and actual centroid sideslip angle Δβ and the difference between the expected and actual yaw rate Δγ are respectively converted into quantization levels in the fuzzy universe of discourse;

[0030] The difference between the expected and actual centroid sideslip angle Δβ and the difference between the expected and actual yaw rate Δγ are input into the fuzzy control model. The difference between the expected and actual centroid sideslip angle Δβ is divided into 5 levels, with the fuzzy set being {NB, NS, Z, PB, PS}. The difference between the expected and actual yaw rate Δγ is also divided into 5 levels, with the fuzzy set being {NB, NS, Z, PB, PS}.

[0031] The output of the fuzzy control model is the additional yaw moment ΔM that needs to be added. The additional yaw moment ΔM is divided into 7 levels, and the fuzzy set is {NB, NM, NS, Z, PB, PM, PS}.

[0032] Both the input and output membership functions are triangular membership functions.

[0033] Preferably, in the fuzzy control, the universe of discourse for the difference between the expected and actual yaw angular velocities Δγ is {-1,1}, the universe of discourse for the difference between the expected and actual centroid sideslip angles Δβ is {-1,1}, and the universe of discourse for the additional yaw moment ΔM is {-1,1}.

[0034] Preferably,

[0035] The formula for calculating the difference between the expected and actual yaw rate of the four-wheel independent drive vehicle is as follows:

[0036] Δγ=γ _NO -γ;

[0037] The formula for calculating the difference between the expected and actual sideslip angle of the four-wheel independent drive vehicle is as follows:

[0038] Δβ=β _NO -β;

[0039] In the formula, γ is the actual yaw rate of the four-wheel independent drive vehicle, and β is the actual sideslip angle of the center of gravity of the four-wheel independent drive vehicle.

[0040] Preferably, the PID control is a continuous closed-loop control system, and the system state expression is:

[0041]

[0042] In the formula, e(t) represents the difference in yaw rate Δγ and the difference in sideslip angle Δβ at each step length, μ(t) represents the PID system output value at each step length, and K... p K is the proportionality coefficient. i K is the integral coefficient. d is the differential coefficient.

[0043] The beneficial effects of this invention are as follows: The lateral stability control method for four-wheel independent drive vehicles provided by this invention improves the lateral stability of the vehicle by adjusting the braking force of each wheel during the cornering braking process of a four-wheel independent drive electric vehicle, thereby avoiding problems such as understeer or oversteer. Attached Figure Description

[0044] Figure 1 This is a framework diagram of the fuzzy control system described in this invention.

[0045] Figure 2 Δγ and Δβ are the membership functions of the input variables Δγ and difference Δβ of the fuzzy controller described in this invention.

[0046] Figure 3 ΔM is the membership function of the output variable ΔM of the fuzzy controller described in this invention.

[0047] Figure 4 This is a framework diagram of the PID control system described in this invention.

[0048] Figure 5 This is a diagram showing the relationship between the applied braking force and the generated yaw moment of the wheel described in this invention.

[0049] Figure 6(a) is a schematic diagram comparing the yaw rate described in this invention.

[0050] Figure 6(b) is a schematic diagram comparing the centroid side deflection angle described in this invention.

[0051] Figure 6(c) shows the lateral acceleration curve described in this invention.

[0052] Figure 6(d) is a schematic diagram of the vehicle's driving trajectory according to the present invention. Detailed Implementation

[0053] The present invention will now be described in further detail with reference to the accompanying drawings, so that those skilled in the art can implement it based on the description.

[0054] like Figure 1As shown in Figure 6, this invention provides a method for controlling the lateral stability of a four-wheel independently driven vehicle during braking. During cornering braking, by adjusting the braking force of each wheel, a yaw moment is applied to the vehicle, thereby improving the vehicle's lateral stability. The method includes:

[0055] Step 1: Obtain the vehicle speed and steering wheel angle during actual driving using sensors, and calculate the vehicle's desired yaw rate and desired center of gravity deflection angle.

[0056] In this invention, as a preferred embodiment, the vehicle speed and steering wheel angle during actual driving are simulated using Carsim software data as input.

[0057] Vehicle desired yaw rate γ _NO The calculation formula is:

[0058]

[0059] In the formula, v x The actual speed of the vehicle during travel, expressed in m / s; δ f V is the front wheel steering angle of a four-wheel independent drive vehicle, measured in degrees; i is the steering gear ratio; V ch This is the reference value for the longitudinal speed of the vehicle, in m / s.

[0060] Vehicle center of gravity sideslip angle β _NO The calculation formula is:

[0061]

[0062] In the formula, a is the horizontal distance between the four-wheel independent drive vehicle and the center of gravity, in meters; k2 is the lateral stiffness of the rear axle, in N / deg.

[0063] Vehicle longitudinal speed reference value V ch The calculation formula is:

[0064]

[0065] In the formula, k1 is the lateral stiffness of the front axle of the vehicle, in N / deg.

[0066] In this invention, as a preferred embodiment, the wheelbase of the four-wheel independent drive vehicle is L = 2.776m, the horizontal distance between the four-wheel independent drive vehicle and the center of gravity is a = 1.11m, the horizontal distance between the rear axle of the four-wheel independent drive vehicle and the center of gravity is b = 1.666m, and the total mass of the four-wheel independent drive vehicle is m = 1370kg.

[0067] Step 2: Obtain the road surface adhesion coefficient, the actual yaw rate of the vehicle, and the actual sideslip angle of the vehicle's center of gravity. Use the difference between the expected and actual sideslip angle and the difference between the expected and actual yaw rate as inputs, and the required additional yaw moment as output. Select the yaw moment control strategy based on the road surface adhesion coefficient.

[0068] The formula for calculating the difference between the expected and actual yaw rate of a four-wheel independent drive vehicle is as follows:

[0069] Δγ=γ _NO -γ;

[0070] The formula for calculating the difference between the expected and actual sideslip angle of a four-wheel independently driven vehicle is as follows:

[0071] Δβ=β _NO -β;

[0072] In the formula, γ is the actual yaw rate of the four-wheel independent drive vehicle, and β is the actual sideslip angle of the center of gravity of the four-wheel independent drive vehicle.

[0073] When the road surface adhesion coefficient s≤0.6, the PID control method is adopted;

[0074] PID control is a continuous closed-loop control system, and the system state expression is:

[0075]

[0076] In the formula, e(t) represents the difference in yaw rate Δγ and the difference in sideslip angle Δβ at each step length, μ(t) represents the PID system output value at each step length, and K... p K is the proportionality coefficient. i K is the integral coefficient. d is the differential coefficient.

[0077] When the road surface adhesion coefficient s > 0.6, a fuzzy control method is adopted.

[0078] Fuzzy control includes:

[0079] The difference between the expected and actual centroid sideslip angle Δβ and the difference between the expected and actual yaw rate Δγ are respectively converted into quantization levels in the fuzzy universe of discourse;

[0080] The difference between the expected and actual centroid sideslip angle Δβ and the difference between the expected and actual yaw rate Δγ are input into the fuzzy control model. The difference between the expected and actual centroid sideslip angle Δβ is divided into 5 levels, with fuzzy sets {NB, NS, Z, PB, PS}. The difference between the expected and actual yaw rate Δγ is also divided into 5 levels, with fuzzy sets {NB, NS, Z, PB, PS}.

[0081] The output of the fuzzy control model is the additional yaw moment ΔM, which is divided into 7 levels and the fuzzy set is {NB, NM, NS, Z, PB, PM, PS}.

[0082] In this invention, as a preferred embodiment, both the input and output membership functions are triangular membership functions.

[0083] The universe of discourse for the difference between the expected and actual yaw angular velocities Δγ is {-1,1}, the universe of discourse for the difference between the expected and actual sideslip angles of the center of mass Δβ is {-1,1}, and the universe of discourse for the additional yaw moment ΔM is {-1,1}.

[0084] Step 3: Calculate the additional yaw moment required by the difference between the actual and desired sideslip angles of the vehicle's center of gravity, and the difference between the actual and desired yaw angular velocities of the vehicle.

[0085] The required braking force for each wheel of a four-wheel independent drive vehicle is calculated based on the additional yaw moment required.

[0086] The expression for calculating the braking force of each wheel in a four-wheel independently driven vehicle is as follows:

[0087]

[0088] In the formula, m is the total mass of the four-wheel independent drive vehicle, in kg; h g 1 is the distance between the vehicle's center of gravity and the ground, in meters; b is the horizontal distance between the rear axle and the center of gravity of a four-wheel independent drive vehicle, in meters; L is the wheelbase of a four-wheel independent drive vehicle, in meters; F ua The braking force provided by the front axle brake, F ub The braking force provided to the rear axle brake, measured in N.

[0089] The relationship between the additional yaw moment and the braking force of each wheel is as follows Figure 5 The braking force distribution rule curve is determined.

[0090] Yaw moment control is achieved through a yaw moment controller, which consists of a fuzzy controller and a PID controller.

[0091] like Figure 1 As shown, the fuzzy control system takes the difference between the expected and actual yaw rate and the difference between the center of gravity sideslip angle as inputs. After the fuzzy controller performs inference calculations, it outputs the target torque required by the vehicle. Then, based on the target torque, it calculates the braking force required by each wheel and distributes it to the motor control system.

[0092] The universe of discourse for the input variables of the fuzzy controller, the difference between the actual yaw rate and the expected value Δγ, and the difference between the actual sideslip angle and the expected value Δβ, is {-1, -0.5, 0, 0.5, 1}. The universe of discourse for the output variable yaw moment ΔM is {-1, -0.35, -0.75, 0, 0.35, 0.75, 1}. The fuzzy linguistic set for the input variables is: NB (negative large), NS (negative small), Z (zero), PB (positive small), PS (positive large); the fuzzy linguistic set for the output variables is: NB (negative large), NM (negative medium), NS (negative small), Z (zero), PB (positive small), PM (positive medium), PS (positive large). The membership graphs of both the input and output variables adopt a triangular function distribution, such as... Figure 2 and Figure 3 As shown in Table 1, the fuzzy control rules are as follows.

[0093] Table 1 Fuzzy Control Rules

[0094]

[0095] If the fuzzy levels corresponding to the difference in vehicle yaw rate Δγ and the difference in vehicle center of gravity sideslip angle Δβ are both "PB" or "PS", then the fuzzy level corresponding to the vehicle's additional yaw moment ΔM is "PS", meaning that the vehicle's output additional yaw moment is the largest and the direction is positive.

[0096] If the fuzzy levels corresponding to the difference in vehicle yaw rate Δγ and the difference in vehicle center of gravity sideslip angle Δβ are both "NB" or "NS", then the fuzzy level corresponding to the vehicle's additional yaw moment ΔM is "NS", meaning that the vehicle's output additional yaw moment is the minimum and its direction is negative.

[0097] If the vehicle's additional yaw moment ΔM output participation level is "NB, NM, or NS", then the direction of the output additional yaw moment is negative; if the vehicle's additional yaw moment ΔM output participation level is "PB, PM, or PS", then the output additional yaw moment is positive; if the vehicle's additional yaw moment ΔM output participation level is "NB or PB", then the output additional yaw moment is maximum; if the vehicle's additional yaw moment ΔM output participation level is "NM or PM", then the output additional yaw moment is moderate; if the vehicle's additional yaw moment ΔM output participation level is "NS or PB", then the output additional yaw moment is small; if the vehicle's additional yaw moment ΔM output participation level is "Z", then the output additional yaw moment is zero, and the vehicle does not need additional yaw moment.

[0098] like Figure 4As shown, the PID control system takes the difference between the yaw rate and the difference between the center of gravity sideslip angle as input. After calculation by the PID controller, it outputs the target torque required by the vehicle. Then, based on the target torque, it calculates the braking force required by each wheel and distributes it to the motor control system.

[0099] Let the direction of the car's left turn be the positive direction, and the counterclockwise yaw moment be the positive direction. Figure 5 As shown, when the braking force gradually increases, the yaw moment curves of the outer front wheel and the inner rear wheel fluctuate significantly. When the outer front wheel is subjected to braking force, the vehicle body will generate a clockwise yaw moment. When the inner rear wheel is subjected to braking force, the vehicle body will generate a counterclockwise yaw moment. When the outer rear wheel and the inner front wheel are subjected to the same braking force, they can also generate corresponding yaw moments, but the generated yaw moments are smaller than those of the outer front wheel and the inner rear wheel. Therefore, the outer front wheel and the inner rear wheel are used as the controlled wheels of DYC control. As long as these two wheels are controlled, the vehicle stability can be guaranteed.

[0100] When a vehicle has an excessive tendency to turn, the corresponding braking force is applied to the wheels other than the front wheels to generate a compensating yaw moment that is opposite to the direction of the vehicle's yaw rate. When a vehicle has an understeer tendency, the corresponding braking force is applied to the inner rear wheels to generate a compensating yaw moment that is in the same direction as the vehicle's yaw rate, thereby ensuring the stability of the vehicle when turning. The specific selection rules are shown in Table 2.

[0101] Table 2 Selection Rules for Controlled Wheels

[0102]

[0103] The lateral stability control method for four-wheel independent drive vehicles provided by this invention improves the lateral stability of the vehicle by adjusting the braking force of each wheel during the cornering braking process of a four-wheel independent drive electric vehicle, thereby avoiding problems such as understeer or oversteer.

[0104] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and illustrations shown and described herein.

Claims

1. A method for controlling the lateral stability of a four-wheel independent drive vehicle during braking, characterized in that, include: Step 1: Obtain the vehicle speed and steering wheel angle during actual driving, and calculate the vehicle's expected yaw rate and expected sideslip angle. Step 2: Obtain the road surface adhesion coefficient, the actual yaw rate of the vehicle, and the actual sideslip angle of the vehicle's center of gravity. Use the difference between the expected and actual sideslip angle and the difference between the expected and actual yaw rate as inputs, and the required additional yaw moment as output. Select a direct yaw moment control strategy based on the road surface adhesion coefficient. When the road surface adhesion coefficient s≤0.6, the PID control method is adopted; When the road surface adhesion coefficient s > 0.6, a fuzzy control method is adopted; Step 3: Calculate the additional yaw moment required by the difference between the actual and desired sideslip angles of the vehicle's center of gravity, and the difference between the actual and desired yaw angular velocities of the vehicle. Calculate the required braking force for each wheel of a four-wheel independent drive vehicle based on the additional yaw moment required; The expression for calculating the braking force of each wheel of the four-wheel independently driven vehicle is as follows: In the formula, m is the total mass of the four-wheel independent drive vehicle, in kg; h g 1 is the distance between the vehicle's center of gravity and the ground, in meters; b is the horizontal distance between the rear axle and the center of gravity of the four-wheel independent drive vehicle, in meters; L is the wheelbase of the four-wheel independent drive vehicle, in meters. F ua The braking force provided by the front axle brake, F ub The braking force provided to the rear axle brake, measured in N.

2. The method for controlling the lateral stability of a four-wheel independently driven vehicle under braking according to claim 1, characterized in that, The vehicle's desired yaw rate γ _NO The calculation formula is: In the formula, v x The actual speed of the vehicle during travel, in m / s; δ f The front wheel steering angle of a four-wheel independently driven vehicle, measured in degrees (deg). i is the steering system gear ratio; V ch This is the reference value for the longitudinal speed of the vehicle, in m / s; The vehicle's center of gravity sideslip angle β _NO The calculation formula is: In the formula, a is the horizontal distance between the four-wheel independent drive vehicle and the center of gravity, in meters, and k2 is the lateral stiffness of the rear axle, in N / rad.

3. The method for controlling the lateral stability of a four-wheel independently driven vehicle under braking according to claim 2, characterized in that, The vehicle longitudinal speed reference value V ch The calculation formula is: In the formula, k1 is the lateral stiffness of the front axle of the vehicle, in N / rad.

4. The method for controlling the lateral stability of a four-wheel independently driven vehicle under braking according to claim 3, characterized in that, The wheelbase of the four-wheel independent drive vehicle is L = 2.776m, the horizontal distance between the four-wheel independent drive vehicle and the center of gravity is a = 1.11m, the horizontal distance between the rear axle of the four-wheel independent drive vehicle and the center of gravity is b = 1.666m, and the total mass of the four-wheel independent drive vehicle is m = 1370kg.

5. The method for controlling the lateral stability of a four-wheel independently driven vehicle under braking according to claim 4, characterized in that, In step two, the fuzzy control in the direct yaw moment control strategy includes: The difference between the expected and actual centroid sideslip angle Δβ and the difference between the expected and actual yaw rate Δγ are respectively converted into quantization levels in the fuzzy universe of discourse; The difference between the expected and actual centroid sideslip angle Δβ and the difference between the expected and actual yaw rate Δγ are input into the fuzzy control model. The difference between the expected and actual centroid sideslip angle Δβ is divided into 5 levels, with the fuzzy set being {NB, NS, Z, PB, PS}. The difference between the expected and actual yaw rate Δγ is also divided into 5 levels, with the fuzzy set being {NB, NS, Z, PB, PS}. The output of the fuzzy control model is the additional yaw moment ΔM that needs to be added. The additional yaw moment ΔM is divided into 7 levels, and the fuzzy set is {NB, NM, NS, Z, PB, PM, PS}. Both the input and output membership functions are triangular membership functions.

6. The method for controlling the lateral stability of a four-wheel independently driven vehicle under braking according to claim 5, characterized in that, In the fuzzy control described above, the universe of discourse for the difference between the expected and actual yaw angular velocities Δγ is {-1,1}, the universe of discourse for the difference between the expected and actual centroid sideslip angles Δβ is {-1,1}, and the universe of discourse for the additional yaw moment ΔM is {-1,1}.

7. The method for controlling the lateral stability of a four-wheel independently driven vehicle under braking according to claim 6, characterized in that, The formula for calculating the difference between the expected and actual yaw rate of the four-wheel independent drive vehicle is as follows: Δγ=γ _NO -c; The formula for calculating the difference between the expected and actual sideslip angle of the four-wheel independent drive vehicle is as follows: Δβ=β _NO -b; In the formula, γ is the actual yaw rate of the four-wheel independent drive vehicle, and β is the actual sideslip angle of the center of gravity of the four-wheel independent drive vehicle.

8. The method for controlling the lateral stability of a four-wheel independently driven vehicle under braking according to claim 7, characterized in that, The PID control is a continuous closed-loop control system, and the system state expression is: In the formula, e(t) represents the difference in yaw rate Δγ and the difference in sideslip angle Δβ at each step length, μ(t) represents the PID system output value at each step length, and K... p K is the proportionality coefficient. i K is the integral coefficient. d is the differential coefficient.

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