Torque distribution control method for optimizing energy consumption in three-motor four-wheel drive electric vehicles

Abstract

This invention discloses a torque distribution control method for optimizing energy consumption in a three-motor, four-wheel-drive electric vehicle, belonging to the field of electric vehicle torque distribution control technology. The purpose of this invention is to utilize a dynamic model of tire longitudinal slip ratio and wheel drive torque to obtain a predictive model for the MPC controller. Multiple cost functions and adjustment weights are designed to distribute the total drive torque to the three drive motors. Drive torque constraints are designed for the front axle motor and the two rear axle drive motors to ensure driving safety. Finally, a performance evaluation index function is designed to achieve a torque distribution control method for optimizing energy consumption in a three-motor, four-wheel-drive electric vehicle. To achieve better coordinated control of stability and economy, the invention designs cost functions that ensure each power source generates a reasonable yaw moment during torque distribution, ensuring stability while allowing the front axle motor and the two rear wheel hub motors to operate in their highest efficiency range. This maximizes vehicle energy efficiency without affecting handling stability.

Description

Technical Field

[0001] This invention belongs to the field of electric vehicle torque distribution control technology. Background Technology

[0002] To alleviate global energy shortages and environmental pollution, the development of more environmentally friendly and energy-efficient electric vehicles is accelerating. Compared to traditional vehicles, electric vehicles can improve the efficiency of the entire system through lightweight design and efficient electric drive system configurations. Distributed four-wheel drive electric vehicles, in particular, simplify the overall vehicle structure by omitting the transmission system, making the design more flexible and optimizing control operability. During vehicle operation, while meeting the constraints of overall vehicle power performance, torque distribution among the power sources can be more freely achieved, improving the vehicle's fuel economy. Currently, torque distribution control in distributed drive electric vehicles primarily focuses on four-motor all-wheel drive systems. However, four-motor all-wheel drive vehicles not only have high configuration costs, but the simultaneous operation of all four motors can easily lead to power redundancy, increasing energy loss and making it difficult to achieve ideal energy-saving effects.

[0003] The following problems still exist:

[0004] 1. Current research on the energy efficiency of three-motor four-wheel drive electric vehicles focuses on how to allocate the total demand torque to the front axle motor and the two rear wheel hub motors to reduce motor energy efficiency losses. It mainly revolves around global optimization and instantaneous optimization. Global optimization is mostly used in relatively fixed linear operating conditions. Based on the global optimization results, it provides a basis for determining the torque distribution ratio between each drive motor. Although this method has the best optimization effect, it has poor adaptability to operating conditions. On the other hand, real-time optimization based on minimizing system power loss can achieve the best instantaneous energy consumption, but it may cause sudden changes in the output drive torque of the power source, making the state changes of each motor not smooth enough, resulting in more energy loss and making it difficult to achieve the best effect.

[0005] 2. Current research on distributed drive stability control often begins by strictly ensuring vehicle stability through yaw moment control. This limits the torque distribution ratio between the left and right motors, without considering whether side torque distribution will cause energy loss in the motors and thus affect vehicle economy. Furthermore, because stability is strictly constrained, a dynamic balance between stability and economy cannot be achieved.

[0006] 3. When the vehicle is driving under dynamic conditions such as acceleration and deceleration, current research has not considered the problem of energy dissipation due to longitudinal slippage of the tires caused by the saturation of driving force due to changes in the load on the front and rear axles; at the same time, it has not considered the safety problem that may occur due to wheel lock-up caused by unreasonable torque distribution under this condition. Summary of the Invention

[0007] The purpose of this invention is to utilize the dynamic model of tire longitudinal slip ratio and wheel drive torque to obtain the prediction model of the MPC controller in this invention, design multiple cost functions and adjustment weights to distribute the total drive torque to the three drive motors, design drive torque constraints for the front axle motor and two rear axle drive motors to ensure driving safety, and finally design a torque distribution control method for optimizing the energy consumption of a three-motor four-wheel drive electric vehicle by implementing a performance evaluation index function.

[0008] The steps of this invention are:

[0009] S1. Using the dynamic model of tire slip ratio and wheel drive torque, a prediction model based on energy consumption optimization MPC controller is obtained. Its state variables consist of four wheel slip ratios, and the control quantity is the drive wheel torque.

[0010] ① Calculate the driver's total torque demand command based on the desired longitudinal vehicle speed. :

[0011] (1)

[0012] in, , For PI controller parameters, , These are the tracking reference longitudinal velocity and the vehicle longitudinal velocity, respectively.

[0013] ②The yaw moment required to stabilize the vehicle is obtained by tracking the expected values ​​of the vehicle's center of gravity sideslip angle and yaw rate. ;

[0014] S2. Obtain the predictive model of the energy-optimized MPC controller.

[0015] ①The steady-state tire slip ratio is defined as:

[0016] (2)

[0017] in, For tire speed, Let be the wheel slip ratio, where These represent the front left wheel, the front right wheel, the rear left wheel, and the rear right wheel, respectively. Effective tire radius;

[0018] Longitudinal force is positively correlated with tire slip ratio, and the linear expression of longitudinal force is:

[0019] (3)

[0020] In the formula , The first i The longitudinal and vertical forces of each tire It is the moment of inertia;

[0021] Pushing wheel slip ratio With wheel torque The dynamic model of the relationship is as follows:

[0022] (4)

[0023] Considering The above formula can be written as:

[0024] (5);

[0025] ② Controller Design: The state variables of the prediction model consist of the slip ratios of four wheels, and the control variable is the torque of the drive wheels. The prediction model is as follows:

[0026] (6)

[0027] in , The state vector of this system is defined as:

[0028] Control quantity Torque is output to the four drive wheels: ,in The front axle motor outputs drive torque. This is the distribution coefficient of the drive torque from the front axle motor to the left and right wheels.

[0029] The prediction model is discretized based on the Euler equation, let... Given the sampling time, the discrete prediction model is as follows:

[0030] (7)

[0031] in , ;

[0032] The total drive torque is distributed to each wheel, so that each wheel... Minimum, satisfying the total driving force The difference in driving force between the left and right sides satisfies the required yaw moment. Each wheel drive force With total drive torque ,as well as The relationship is as follows:

[0033] (8);

[0034] S3. Design four cost functions, as follows:

[0035] First objective function:

[0036] (9)

[0037] in, The track width is the distance between the left and right wheels. , These are the weighting coefficients;

[0038] Second objective function:

[0039] (10)

[0040] To minimize tire slip power loss, the objective function is designed as follows:

[0041] (11)

[0042] The third objective function:

[0043] (12)

[0044] in , Boundary constraint values ​​for longitudinal slip. , As a weighting factor;

[0045] The fourth objective function:

[0046] The motor efficiency is expressed as a function of torque, and a sixth-order polynomial is used to fit the motor map data. The sixth-order polynomial is written as:

[0047] (13)

[0048] in, The fitted motor efficiency. These are the fitting coefficients. This refers to the motor torque;

[0049] When the motor is in drive mode, the objective function is:

[0050] (14)

[0051] The objective function when the motor is in regenerative braking mode is:

[0052] (15)

[0053] Due to the limitations of the motors' own load, the upper and lower limits of the output drive torque of the three motors are constrained as follows:

[0054] (16)

[0055] The control constraints can then be obtained:

[0056] (17)

[0057] The objective function is obtained as follows:

[0058] (18)

[0059] S4. When distributing drive torque between the front and rear axles, the front axle drive torque should not exceed the rear axle drive torque. The controller constraint should be designed accordingly. ,Right now:

[0060] (19)

[0061] The single motor on the front axle provides power to both the left and right wheels of the front axle, constrains the control input of the front axle, and simultaneously outputs either driving force or braking force.

[0062] (20)

[0063] Preventing rear wheel lock-up through torque distribution, To prevent the vehicle from skidding and posing a threat to driver safety, the controller is constrained as follows:

[0064] (twenty one)

[0065] By constraining the braking forces of the front and rear axles between the ideal I-curve and the lower limit of the ECE regulations, stability during braking is ensured.

[0066] (twenty two)

[0067] Right now

[0068] (twenty three)

[0069] in, L This is the distance from the front axle to the rear axle. It is the height of the center of mass. G It is gravitational acceleration;

[0070] The constrained problem to be solved is as follows:

[0071] (twenty four)

[0072] By optimizing and solving the above objective function, the control quantity is obtained as the wheel drive torque;

[0073] S5. Performance index evaluation functions were designed from four aspects: handling stability, motor energy saving performance, tire slip energy loss and longitudinal vehicle speed tracking.

[0074] ① Handling stability:

[0075] (25)

[0076] ②Energy-saving performance of motors:

[0077] (26)

[0078] ③ Tire slippage energy loss:

[0079] (27)

[0080] ④ Longitudinal vehicle speed tracking performance:

[0081] (28).

[0082] The beneficial effects of this invention are:

[0083] 1. This invention designs an energy management strategy based on model predictive control. It uses a sixth-order polynomial to fit the motor energy efficiency map and takes into account the real-time feedback of motor drive state changes through rolling optimization. It solves the high-efficiency operating point of each motor online and allocates torque so that the front axle motor and the two rear wheel hub motors can operate in their optimal efficiency range as much as possible. This strategy is not limited by operating conditions, can minimize the energy loss of the motor, and takes into account both real-time performance and economy.

[0084] 2. This invention considers various dynamic working conditions. For the phenomenon of excessive wheel slippage caused by changes in the front and rear axle load transmission during vehicle turning and acceleration / deceleration, a wheel slippage power loss matrix and a penalty matrix are designed to suppress the energy loss caused by wheel slippage. At the same time, to avoid the safety hazards caused by changes in axle load during acceleration and braking, corresponding constraints are set to limit it.

[0085] 3. In order to achieve better coordinated control of stability and economy, this invention does not strictly statically distribute additional yaw torque in yaw stability control while meeting the total drive torque requirement. Instead, it designs a cost function to make each power source generate a reasonable yaw torque during torque distribution to ensure stability while making the front axle motor and the two rear wheel hub motors work in the high-efficiency range as much as possible. This maximizes the vehicle's energy-saving goal without affecting handling stability. Attached Figure Description

[0086] Figure 1 This is a flowchart of a torque distribution control method based on energy consumption optimization for a three-motor four-wheel drive electric vehicle, as described in this invention.

[0087] Figure 2 This is a map diagram of the PD18 motor described in this invention;

[0088] Figure 3 This is the Map 6th degree polynomial fitting graph of the motor described in this invention;

[0089] Figure 4 Flowchart of online optimization process after motor Map fitting;

[0090] Figure 5 ECE braking regulation scope diagram;

[0091] Figure 6 Simulation diagram of motor output torque, tire slip ratio, yaw rate and center of gravity sideslip angle under static distribution for double lane change conditions, road surface adhesion coefficient, and vehicle speed maintained at around 60km / h.

[0092] Figure 7 This is a simulation diagram showing the motor output torque, tire slip ratio, yaw rate, and center of gravity sideslip angle under the torque distribution control of a three-motor four-wheel drive electric vehicle with optimized energy consumption, under the condition of double lane change, road surface adhesion coefficient, and vehicle speed maintained at around 60km / h.

[0093] Figure 8 This is a simulation diagram showing the motor output torque, tire slip ratio, yaw rate, and center of gravity sideslip angle of a four-motor four-wheel drive electric vehicle under torque distribution control, with the road surface adhesion coefficient and the vehicle speed maintained at around 60km / h, under the condition of double lane change.

[0094] Figure 9 Simulation diagram of motor output torque, tire slip ratio, yaw rate and center of gravity sideslip angle under torque distribution control for energy consumption optimization of a three-motor four-wheel drive electric vehicle during double lane change operation, road adhesion coefficient, and acceleration at vehicle speed of 50km / h~70km / h.

[0095] Figure 10 This is a simulation diagram showing the motor output torque, tire slip ratio, yaw rate, and center of gravity sideslip angle under the torque distribution control of a three-motor four-wheel drive electric vehicle during a double lane change operation with a road adhesion coefficient and a vehicle speed of 70km / h to 50km / h deceleration. Detailed Implementation

[0096] This invention addresses energy management and control strategies for a three-motor, four-wheel-drive electric vehicle from an energy consumption optimization perspective. First, a predictive model for the MPC controller in this invention is derived using a dynamic model of tire longitudinal slip ratio and wheel drive torque. Second, to dynamically coordinate the stability and economy of the three-motor, four-wheel-drive vehicle and maximize the reduction of overall vehicle energy consumption, multiple cost functions and weight adjustments are used to distribute the total drive torque to the three drive motors. This ensures reasonable yaw torque between each drive wheel, guaranteeing vehicle handling stability, allowing each motor to operate in its high-efficiency range, and reducing tire slip power loss. Third, considering wheel lock-up due to changes in front and rear axle loads under dynamic conditions, drive torque constraints are designed for the front axle motor and the two rear axle drive motors to ensure driving safety. Finally, a performance evaluation index function is designed to verify the energy efficiency of the proposed torque distribution control method for optimizing energy consumption in a three-motor, four-wheel-drive electric vehicle.

[0097] A torque distribution control method for optimizing energy consumption in a three-motor four-wheel drive electric vehicle is provided, comprising the following steps:

[0098] Step 1: Using the simulation software CarSim, a three-motor four-wheel drive electric vehicle model is designed to provide real-time vehicle status information. To meet the driving force requirements under different driving conditions, the front axle motor is configured as a Continental centralized motor, and the two rear wheel hub motors are configured as PD18 motors. The driver's total torque demand command is calculated based on the desired longitudinal vehicle speed. The yaw moment required to stabilize the vehicle is calculated by tracking the expected values ​​of the vehicle's center of gravity sideslip angle and yaw rate. .

[0099] Step 2: Using the dynamic model of tire slip ratio and wheel drive torque, a prediction model based on energy consumption optimization MPC controller is obtained. Its state variables consist of four wheel slip ratios, and the control quantity is the drive wheel torque.

[0100] Step 3: In this invention, the total driving torque is distributed to the front axle motor and the two rear wheel hub motors to achieve the control objectives of improving the overall vehicle's energy efficiency and ensuring driving safety. Four cost functions are designed: First, a sixth-order polynomial is used to fit the motor energy efficiency map to solve for the optimal motor efficiency online, ensuring that each motor operates in the high-efficiency region in both driving and braking modes to reduce motor energy loss and improve braking energy recovery efficiency; second, a longitudinal slip power loss matrix is ​​introduced to reduce tire dynamic slip energy dissipation; third, a penalty matrix is ​​designed to limit slippage beyond the boundary, further suppressing excessive tire slippage; finally, it is ensured that the required yaw torque can be generated between the motors to control the vehicle's handling stability.

[0101] Step 4: Considering the possibility of wheel lock-up due to changes in front and rear axle loads during vehicle acceleration and deceleration, constraints were designed on the drive torque of the front axle centralized motor and the rear axle motor. Furthermore, during deceleration, based on ECE regulations and the I-curve braking strategy, the output braking force of the front and rear axles was limited to ensure driving safety.

[0102] Step 5: Performance evaluation functions were designed from four aspects: handling stability, motor energy-saving performance, tire slip energy loss, and longitudinal vehicle speed tracking. The performance of the proposed torque distribution control method for optimizing energy consumption of a three-motor four-wheel drive electric vehicle was tested, demonstrating the energy-saving performance of the control method studied in this invention.

[0103] This invention takes a three-motor distributed four-wheel drive electric vehicle consisting of a single motor on the front axle and two hub motors on the rear axle as the research object, and realizes a torque distribution method for optimizing the energy consumption of the three-motor electric vehicle.

[0104] To explain in detail the technical content, structural features, and objectives of this invention, the following is a comprehensive explanation of the invention in conjunction with the accompanying drawings:

[0105] The flow of the torque distribution control method for optimizing energy consumption in a three-motor four-wheel drive electric vehicle described in this invention is as follows: Figure 1 As shown, the required total traction torque and the yaw moment required by each drive wheel are obtained from the driver model and stability control. In the MPC energy consumption optimization controller, the inputs are the longitudinal slip ratios of the four tires and the output measurements of the controlled object, and the outputs are the traction forces of each motor. Utilizing the dynamic relationship between tire slip and wheel drive torque, the total drive torque is distributed to the three drive motors through multi-objective function optimization. The MPC controller module in this invention is built in MATLAB / Simulink. The controlled object is a three-motor four-wheel drive electric vehicle model constructed using CarSim.

[0106] The control objective of this invention is that, based on real-time feedback signals, the control system uses the total traction torque obtained from the driver model and stability control to calculate and distribute the total driving torque to the three drive motors, so as to generate a reasonable yaw moment between each drive wheel to ensure vehicle handling stability, reduce motor power loss, and suppress longitudinal tire slippage.

[0107] This invention provides a co-simulation model based on the above operating principles and processes, and its construction and operation process are as follows:

[0108] 1. Software Selection

[0109] The simulation models of the controller and the controlled object of this control system were built using MATLAB / Simulink and CarSim software, respectively, with MATLAB R2022a and CarSim 2019.1, and a simulation step size of 0.001s. CarSim is a simulation software specifically designed for vehicle dynamics. Its main role in this invention is to provide a high-fidelity vehicle dynamics model, replacing a real three-motor four-wheel drive electric vehicle as the object of control in the simulation experiment, and providing a simulation environment for low-adhesion limit conditions. MATLAB / Simulink is used to build the simulation model of the controller, that is, to complete the controller's calculations in the control system through Simulink programming.

[0110] 2. Co-simulation settings

[0111] To achieve co-simulation between MATLAB / Simulink and CarSim, first, set the CarSim working path to the specified Simulink Model. Then, add the configured vehicle model to Simulink in CarSim and run Simulink to achieve co-simulation and communication. If the model structure or parameter settings in CarSim are modified, it needs to be resent.

[0112] 3. Model building of a three-motor four-wheel drive electric vehicle in co-simulation software

[0113] The CarSim electric vehicle model mainly consists of systems such as body, drivetrain, steering system, braking system, tires, suspension, aerodynamics, and operating condition configuration. A four-wheel drive vehicle is selected, with wheel torque inputs of IMP_MYUSM_L1, IMP_MYUSM_L2, IMP_MYUSM_R1, and IMP_MYUSM_R2. The electric vehicle parameters are shown in Table 1.

[0114] Table 1 Electric Vehicle Parameter Table

[0115]

[0116] 4. This invention is based on the torque distribution control principle for energy consumption optimization in three-motor four-wheel drive electric vehicles.

[0117] The controlled object of the present invention is a three-motor four-wheel drive electric vehicle, and the control objective is to optimize motor working efficiency and tire slippage loss while ensuring driving safety.

[0118] The following describes the specific steps of the control method of the present invention:

[0119] Step 1: Using the simulation software CarSim, a three-motor, four-wheel drive electric vehicle model is designed. This model is used to simulate a real controlled object, primarily providing real-time information on various vehicle states and allowing changes in vehicle motion state to be made using motor torque as input. To meet the driving force requirements under different driving conditions, the front axle motor is configured as a Continental centralized motor, and the two rear wheel hub motors are configured as PD18 motors. To achieve the torque distribution and handling stability requirements of the three motors, the driver's total torque demand command needs to be calculated based on the desired longitudinal vehicle speed. Furthermore, by tracking the expected values ​​of the vehicle's center of gravity sideslip angle and yaw rate, the yaw moment required for stable vehicle driving is calculated. .

[0120] ① A three-motor four-wheel drive electric vehicle model is characterized by consisting of one front axle drive motor and two rear wheel hub motors. Compared to traditional electric drive vehicles, the three-motor drive structure has the following advantages:

[0121] a. Compared with traditional single-motor all-wheel drive configurations

[0122] The three-motor configuration makes it easier to achieve independent control of each drive wheel, greatly simplifying the mechanical transmission system and thus reducing mechanical losses and overall vehicle weight.

[0123] b. Compared with four-wheel independent motor configuration

[0124] In a three-motor drive configuration, the hub motors located on the rear wheels inherit the advantages of ordinary wheeled distributed electric drive vehicles. The motors have accurate and rapid torque response and high control freedom. Furthermore, one front wheel hub motor is eliminated. The three-motor configuration is superior to the four-wheel independent motor configuration in terms of cost, space requirements, and control complexity.

[0125] In this invention, by configuring the front and rear axles with different motors, the driving needs under different working conditions can be better met. While meeting the total torque requirements of the vehicle, the loss caused by excess energy can be effectively reduced. Furthermore, by controlling the motor drive area, all three motors can operate at maximum efficiency.

[0126] ② Calculate the driver's total torque demand command based on the desired longitudinal vehicle speed. :

[0127] (1)

[0128] in, , For PI controller parameters, , These are the tracking reference longitudinal speed and the vehicle longitudinal speed, respectively.

[0129] ③ To meet the stability requirements, the yaw moment required for stable vehicle driving is calculated by tracking the expected values ​​of the vehicle's center of gravity sideslip angle and yaw rate. .

[0130] Step 2: Based on the steady-state slip ratio of the wheel, obtain the dynamic model of the wheel slip ratio and the wheel driving torque, and on this basis, obtain the prediction model based on the energy consumption optimization MPC controller.

[0131] ① To reduce tire slip energy loss, the steady-state tire slip ratio can be defined as:

[0132] (2)

[0133] in, For tire speed, Let be the wheel slip ratio, where These represent the front left wheel, the front right wheel, the rear left wheel, and the rear right wheel, respectively. Effective radius of the tire.

[0134] When the tire operates in the longitudinal linear region, the longitudinal force of the tire satisfies the small slip ratio model, where the longitudinal force is positively correlated with the tire slip ratio.

[0135] Therefore, the linear approximation of the longitudinal force can be expressed as:

[0136] (3)

[0137] In the formula , The first i The longitudinal and vertical forces of each tire Let be the moment of inertia.

[0138] The wheel slip ratio can be derived from the above equation. With wheel torque The dynamic model of the relationship is as follows:

[0139] (4)

[0140] Considering The above formula can be written as

[0141] (5).

[0142] ② Controller Design: The state variables of the prediction model consist of the slip ratios of four wheels, and the control variable is the torque of the drive wheels. The prediction model is as follows:

[0143] (6)

[0144] in , .

[0145] The state vector of this system can be defined as: Control quantity Torque is output to the four drive wheels: ,in The front axle motor outputs drive torque. This is the distribution coefficient of the driving torque from the front axle motor to the left and right wheels.

[0146] The above prediction model is discretized based on the Euler equation, and the sampling time is set to obtain the discretized prediction model as follows:

[0147] (7)

[0148] in , .

[0149] In this invention, the distribution of four-wheel drive force is explained as follows. On low-traction surfaces, when the wheel slip ratio... As the torque increases, the driving force of the wheels decreases due to saturation. To avoid this decrease, the torque of each wheel... It needs to be small enough to prevent saturation.

[0150] Therefore, in this invention, the total driving torque is distributed to each wheel so that each wheel... The minimum value satisfies the total driving force. The difference in driving force between the left and right sides satisfies the required yaw moment. Each wheel drive force With total drive torque ,as well as The relationship is as follows:

[0151] (8).

[0152] Step 3: In this invention, the total driving torque is distributed to the front axle motor and the two rear wheel hub motors to achieve the control objective of dynamically coordinating the stability and economy of the three-motor four-wheel drive vehicle and reducing the overall energy consumption of the vehicle. Four cost functions are designed, which are explained in detail below.

[0153] The first objective function is designed to distribute the total required traction torque to the drive motors and to generate yaw moments in each drive wheel to ensure handling stability:

[0154] (9)

[0155] in, The track width is the distance between the left and right wheels. , These are the weighting coefficients.

[0156] The second objective function aims to reduce tire dynamic slip energy dissipation by introducing a longitudinal slip power loss matrix:

[0157] (10).

[0158] To minimize tire slip power loss, the objective function is designed as follows:

[0159] (11).

[0160] The third objective function is to design a penalty matrix to address excessive tire slippage, thereby suppressing longitudinal tire slippage while achieving better stability performance.

[0161] (12)

[0162] in , Boundary constraint values ​​for longitudinal slip. , This is the weighting factor.

[0163] The fourth objective function aims to reduce motor energy efficiency losses. Based on the efficiency maps of the front and rear axle motors, a sixth-order polynomial fitting is performed. While meeting the total driving force requirements, online optimization based on MPC rolling optimization is used to find the motor's efficient operating point. Under different operating conditions, a reasonable torque distribution scheme is employed to ensure that the front axle motor and the two rear wheel hub motors operate in their high-efficiency regions as much as possible, thereby reducing motor energy loss.

[0164] When the motor is in drive mode, taking the PD18 motor map as an example, as follows: Figure 2 As shown, once the motor speed and torque are calculated, the corresponding efficiency value can be obtained from the motor efficiency graph. Therefore, motor efficiency can be described as a function of the motor's current speed and torque, but it is difficult to construct a mathematical function to fit the surface of the motor efficiency graph. In this invention, at each instant, the speed is assumed to be constant, such as... Figure 2 As shown by the dashed line, the motor efficiency can be expressed as a function of torque, thus representing the motor efficiency. This allows for subsequent online optimization of the motor's energy consumption. The solution flowchart is shown below. Figure 4 As shown, the process first determines the motor's drive mode, then determines the fitting function based on the motor's current speed range, and finally solves for the optimal efficiency point of this function.

[0165] To express motor efficiency as a function of torque, this invention uses a sixth-order polynomial to fit the motor map data. The sixth-order polynomial can be written as:

[0166] (13)

[0167] in, The fitted motor efficiency. These are the fitting coefficients. This represents the motor torque. The motor efficiency fitting curve is shown below. Figure 3 As shown.

[0168] When the motor is in drive mode, the drive torque of the three motors is rationally allocated with the goal of maximizing the overall drive efficiency of the power system. The objective function at this time is:

[0169] (14).

[0170] When the motor is in regenerative braking mode, the objective function is to recover as much braking energy as possible while meeting braking stability requirements.

[0171] (15).

[0172] Due to the limitations of the motors' own load, the upper and lower limits of the output drive torque of the three motors are constrained as follows:

[0173] (16).

[0174] The control constraints can then be obtained:

[0175] (17).

[0176] In summary, the objective function is as follows:

[0177] (18).

[0178] Step 4: Considering the potential wheel lock-up caused by changes in front and rear axle loads during vehicle acceleration and deceleration, design controller constraints and limit the output braking force of the front and rear axles during deceleration, based on ECE regulations and I-curve braking strategies.

[0179] Because the axle load shifts rearward during vehicle acceleration, increasing the vertical load on the rear axle and the available road adhesion for the rear wheels, the drive torque distribution between the front and rear axles should ensure that the front axle drive torque does not exceed the rear axle drive torque. This requires designing controller constraints. ,Right now:

[0180] (19).

[0181] The single motor on the front axle provides power to both the left and right wheels of the front axle, constrains the control input of the front axle, and simultaneously outputs either driving force or braking force.

[0182] (20).

[0183] The distribution of braking force in a car needs to be optimized for energy consumption while ensuring vehicle stability. Studies have shown that if the front wheels lock up during braking, the vehicle will be unable to steer; if the rear wheels lock up, the car may skid or roll over, both of which are dangerous.

[0184] The desired braking force distribution is to minimize the occurrence of wheel lock-up. If lock-up does occur, torque distribution should be used to prevent rear wheel lock-up. To prevent the vehicle from skidding and posing a threat to driver safety, the controller is constrained as follows:

[0185] (twenty one).

[0186] During deceleration, braking strategies based on ECE regulations and I-curves, such as Figure 5 As shown, the braking forces of the front and rear axles are constrained between the ideal I-curve and the lower limit of the ECE regulations to ensure stability during braking:

[0187] (twenty two)

[0188] Right now

[0189] (twenty three)

[0190] in, L This is the distance from the front axle to the rear axle. It is the height of the center of mass. G It is gravitational acceleration.

[0191] In summary, the constrained problems that need to be solved in this invention are as follows:

[0192] (twenty four)

[0193] By optimizing and solving the above objective function, the control quantity is obtained as the wheel drive torque.

[0194] Step 5: Design a performance evaluation index function and test the performance of the proposed torque distribution control method for optimizing energy consumption in a three-motor four-wheel drive electric vehicle; this demonstrates the energy-saving performance of the control method studied in this invention.

[0195] The performance evaluation function was designed from four aspects: handling stability, motor energy saving performance, tire slip energy loss, and longitudinal vehicle speed tracking, as shown below:

[0196] ① Handling stability:

[0197] (25).

[0198] Motor energy-saving performance:

[0199] (26).

[0200] Tire slip energy loss:

[0201] (27).

[0202] Longitudinal vehicle speed tracking performance:

[0203] (28).

[0204] The effectiveness of the torque distribution control method of the present invention is verified through simulation experiments of the following embodiments:

[0205] To verify the performance of the torque distribution control method described in this invention, simulation experiments were designed in the CarSim and MATLAB / Simulink co-simulation environment. The simulation test condition was set as a double lane change condition, with a road adhesion coefficient and vehicle speed maintained around 60 km / h. The sampling time was set to 0.001 s, and the prediction time domain was used. Under this condition, static distribution (such as...) was tested. Figure 6 As shown), the torque distribution of a four-motor, four-wheel-drive electric vehicle (as shown) Figure 8 As shown), and the torque distribution control method for energy consumption optimization of a three-motor four-wheel drive electric vehicle proposed in this invention (such as...). Figure 7 As shown in the figure), simulation verification was carried out, and the energy-saving and stability evaluation results of the three schemes are shown in Table 2.

[0206] Table 2 Performance Evaluation Table under Double Lane Change Speed ​​of 60km / h

[0207]

[0208] from Figure 6 , Figure 7 and Figure 8 Furthermore, as can be seen from Table 2, compared with systems without optimization objectives and constraints and systems driven by four motors and four wheels, the torque distribution control method for energy consumption optimization of three-motor four-wheel drive electric vehicles proposed in this invention significantly improves the overall efficiency of the motors and fully suppresses wheel slippage losses without sacrificing handling stability, thus achieving better coordinated control of stability and economy.

[0209] To further verify the effect of the torque distribution control method described in this invention under acceleration and deceleration conditions, a simulation experiment was designed in the CarSim and MATLAB / Simulink co-simulation environment. The simulation test condition was set as a double lane change condition, and the road adhesion coefficient was set to... Simulations were conducted to verify the performance of the vehicle during acceleration from 50 km / h to 70 km / h and deceleration from 70 km / h to 50 km / h. The simulation results (such as...) were used to verify the results. Figure 9 , Figure 10 As shown in the figure), and the performance evaluation results in Table 3, it is fully demonstrated that the torque distribution control method for energy consumption optimization of the three-motor four-wheel drive electric vehicle proposed in this invention has strong adaptability under acceleration and deceleration dynamic conditions, and achieves a significant reduction in the overall energy consumption of the vehicle.

[0210] Table 3 Performance Evaluation Table for Double Lane Change Vehicles under Acceleration Conditions of 50km / h~70km / h and Deceleration Conditions of 70km / h~50km / h

[0211] .

Claims

1. A torque distribution control method for optimizing energy consumption in a three-motor four-wheel drive electric vehicle, characterized in that: The steps are as follows: S1. Using the dynamic model of tire slip ratio and wheel drive torque, a prediction model based on energy consumption optimization MPC controller is obtained. Its state variables consist of four wheel slip ratios, and the control quantity is the drive wheel torque. ① Calculate the driver's total torque demand command based on the desired longitudinal vehicle speed. ; ②The yaw moment required to stabilize the vehicle is obtained by tracking the expected values ​​of the vehicle's center of gravity sideslip angle and yaw rate. ; S2. Obtain the predictive model of the energy-optimized MPC controller. Controller Design: The state variables of the prediction model consist of the slip ratios of four wheels, and the control variable is the torque of the drive wheels. The prediction model is as follows: （6） in , , The effective radius of the tire. The vehicle's longitudinal velocity is given; the state vector is defined as: Control quantity Torque is output to the four drive wheels: ,in The front axle motor outputs drive torque. This is the distribution coefficient of the drive torque from the front axle motor to the left and right wheels. Wheel moment of inertia; For tire longitudinal stiffness; Represents wheels slip ratio; subscript These represent the front left wheel, front right wheel, rear left wheel, and rear right wheel, respectively. and These are the drive torque output by the motor on the left rear axle and the drive torque output by the motor on the right rear axle, respectively. S3. Design four cost functions, each including the first objective function. The second objective function The third objective function and the first objective function The first objective function is designed to distribute the total required traction torque to the drive motor and generate yaw moment in each drive wheel to ensure handling stability. The second objective function is to reduce the energy dissipation of tire dynamic slip. The third objective function is to design a penalty matrix for excessive tire slip, suppressing longitudinal tire slip while obtaining better stability. The fourth objective function is to reduce the energy efficiency loss of the motor. Based on the efficiency maps of the front axle motor and the rear axle motor, a 6th-order polynomial fitting is performed. Under the condition of meeting the total driving force requirement, the efficient operating point of the motor is solved online based on MPC rolling optimization. The objective function is obtained as follows: （18） S4. When distributing the driving torque between the front and rear axles, the driving torque of the front axle should not be greater than that of the rear axle. Design controller constraints, optimize and solve the above objective function, and obtain the control quantity as the wheel driving torque. S5. Performance evaluation functions were designed from four aspects: handling stability, motor energy saving performance, tire slip energy loss, and longitudinal vehicle speed tracking.

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