An electro-hydraulic braking force distribution method for pure electric vehicles

Abstract

A method for electro-hydraulic brake force distribution of a pure electric vehicle, including establishing a kinematic model of the sprung mass of the vehicle; discretizing the kinematic model of the sprung mass to obtain a vehicle state prediction model; establishing an objective function of the brake force distribution controller, setting constraint conditions for the objective function of the distribution controller, and calculating the target motor braking torque and the target hydraulic braking torque according to the vehicle state prediction model and the objective function of the distribution controller. This method can balance braking safety and energy recovery efficiency and reasonably distribute the electro-hydraulic brake force.

Description

Technical Field

[0001] The present invention relates to the technical field of braking energy recovery for pure electric vehicles, and particularly to a method for electro-hydraulic braking force distribution for pure electric vehicles, which is specifically applicable to the reasonable distribution of electro-hydraulic braking force for pure electric vehicles. Background Art

[0002] Under the background of the increasing shortage of global non-renewable resources, the braking energy recovery technology for electric vehicles has attracted more and more attention, and the use of this technology can effectively improve the cruising range of vehicles. As the core content of the braking energy recovery technology, the electro-hydraulic braking force distribution directly affects the energy-saving effect and safety of the braking energy recovery technology. Therefore, it is necessary to propose a method for electro-hydraulic braking force distribution for pure electric vehicles to reasonably distribute the electro-hydraulic braking force and maximize the recovered energy on the premise of meeting braking safety.

[0003] At present, the electro-hydraulic braking force distribution methods can be divided into two categories, namely the superposition type distribution method and the coordinated type distribution method. The superposition type is also called the parallel type or the independent type distribution method. In this control method, the mechanical braking system and the motor energy recovery function work independently, and the two braking force sources cannot be uniformly controlled. The coordinated type distribution method is also called the series type distribution method. In this type of distribution method, the electric braking force and the mechanical braking force can be uniformly coordinated and controlled, but how to coordinate energy recovery and braking safety is still a major problem. Summary of the Invention

[0004] The purpose of the present invention is to overcome the problem in the prior art that it is difficult to coordinate the maximum recovery of energy and the braking safety performance, and provides a method for electro-hydraulic braking force distribution for pure electric vehicles that can balance comfort, maximize the recovered energy, and take into account safety at the same time.

[0005] To achieve the above purpose, the technical solution of the present invention is:

[0006] A method for electro-hydraulic braking force distribution for pure electric vehicles, the distribution method includes the following steps:

[0007] Step 1: Establish a kinematic model of the sprung mass of the vehicle;

[0008] Step 2: Discretize the kinematic model of the sprung mass to obtain a vehicle state prediction model;

[0009] Step 3: Establish an objective function of the braking force distribution controller, set the constraint conditions of the objective function, and calculate the target electric motor braking torque and the target hydraulic braking torque according to the vehicle state prediction model and the objective function.

[0010] In the first step mentioned above, establishing the kinematic model of the sprung mass of the vehicle includes: establishing the kinematic equation of the sprung mass and establishing the kinematic equations of the front and rear wheels.

[0011] In the first step mentioned above, the kinematic equation of the sprung mass specifically includes:

[0012]

[0013]

[0014] F b1 = K D sin(K c tan -1 {K B s1 - K E [K B s1 - tan -1 (K B s1)]})F z1 (3);

[0015] F b2 = K D sin(K C tan -1 {K B s2 - K E [K B s2 - tan -1 (K B s2)]})F z2 (4);

[0016]

[0017]

[0018]

[0019]

[0020] In Equations (1)-(8), m sm is the total sprung mass, is the derivative of the vehicle speed v with respect to time, F b1 is the ground braking force of the front wheel, F b2 is the ground braking force of the rear wheel, F a is the air resistance, C d is the air resistance coefficient, A F is the frontal area, ρ a is the air density, K B 、K C 、K D 、K EThey are all coefficients of the Pacejka model. s1 is the slip ratio of the front wheel, F z1 is the vertical load of the front wheel, s2 is the slip ratio of the rear wheel, F z2 is the vertical load of the rear wheel, ω1 is the rotational speed of the front wheel, R t is the wheel rolling radius, ω2 is the rotational speed of the rear wheel, g is the acceleration due to gravity, l is the wheelbase, a is the horizontal distance from the center of mass of the sprung mass to the front axle, b is the horizontal distance from the center of mass of the sprung mass to the rear axle, h g is the height of the center of mass.

[0021] In the first step, the motion equations of the front and rear wheels specifically include:

[0022]

[0023]

[0024] T h1 =(T driver -T m )β (11);

[0025]

[0026]

[0027] ω m =i t ω1 (14);

[0028] In equations (9)-(14), is the derivative of the front wheel rotational speed ω1 with respect to time, is the derivative of the rear wheel rotational speed ω2 with respect to time, J w1 is the moment of inertia of the front wheel, i t is the total reduction ratio, T h1 is the braking torque of the front wheel brake, J w2 is the moment of inertia of the rear wheel, T h2 is the braking torque of the rear wheel brake, J m is the moment of inertia of the motor, T m is the motor torque, T driver is the driver's required braking torque, β is the brake force distribution coefficient, R t is the wheel rolling radius, F b1 is the ground braking force of the front wheel, F b2 is the ground braking force of the rear wheel, I m is the current output by the motor, Loss(T m , ω m ) is the power loss of the motor, V acc is the voltage of the power battery, ω m is the motor speed.

[0029] Step 2, discretizing the sprung mass kinematic model to obtain a vehicle state prediction model, specifically includes:

[0030] Discretize the sprung mass motion equation and the front and rear wheel motion equations to obtain a vehicle state prediction model. The sampling period of the vehicle state prediction model is Δt. The vehicle state prediction model is as follows:

[0031]

[0032]

[0033] F b1 (k) = K D sin(K C tan -1 {K B s1(k) - K E [K B s1(k) - tan -1 (K B s1(k))]})F z1 (k) (17);

[0034] F b2 (k) = K D sin(K C tan -1 {K B s2(k) - K E [K B s2(k) - tan -1 (K B s2(k))]})F z2 (k) (18);

[0035]

[0036]

[0037]

[0038]

[0039]

[0040]

[0041] T h1 (k) = (T driver (k) - T m (k))β (25);

[0042]

[0043]

[0044] In equations (15)-(27), v(k) is the vehicle speed at time k, v(k+Δt) is the vehicle speed at time k+Δt, v(k-Δt) is the vehicle speed at time k-Δt, F b1 (k) is the ground braking force of the front wheels at time k, F b2 (k) is the ground braking force of the rear wheels at time k, F a (k) is the air resistance at time k, s1(k) is the slip ratio of the front wheels at time k, s2(k) is the slip ratio of the rear wheels at time k, F z1 (k) is the vertical load of the front wheels at time k, F z2 (k) is the vertical load of the rear wheels at time k, ω1(k) is the rotational speed of the front wheels at time k; ω2(k) is the rotational speed of the rear wheels at time k, ω1(k+Δt) is the rotational speed of the front wheels at time k+Δt, ω2(k+Δt) is the rotational speed of the rear wheels at time k+Δt, T m (k) is the motor torque at time k, ω m (k) is the motor speed at time k, T h1 (k) is the braking torque of the front-wheel brake at time k, T h2 (k) is the braking torque of the rear-wheel brake at time k, T driver (k) is the braking torque demanded by the driver at time k, I m (k) is the motor current at time k, Loss(T m (k), ω m (k)) is the power loss of the motor at time k.

[0045] In the third step described above, the objective function J(k) of the braking force distribution controller is as follows:

[0046]

[0047] In the formula, w u is the comfort weight; w s is the safety weight; w P is the economic weight; h c is the prediction time domain length; u(k+i|k) is the controller input at time k+i; u(k+i-Δt|k) is the controller input at time k+i-Δt; s1(k+i|k) is the slip ratio of the front wheels at time k+i; P m (k+i|k) is the motor recovery power at time k+i;

[0048] The controller input is the motor torque.

[0049] The recovered power P of the motor m is: P m = T m ω m - Loss(T m , ω m );

[0050] Where: T m is the motor torque, ω m is the motor speed, Loss(T m (k), ω m (k)) is the power loss of the motor.

[0051] In step three, the constraint conditions of the objective function include:

[0052] T m_min (ω m ) ≤ T m ≤ 0;

[0053] s1 ≤ s max ;

[0054] I m ≤ I max ;

[0055] Where, T m is the motor torque, s1 is the slip ratio of the front wheel, I m is the current output by the motor. The T m_min (ω m ) is the external characteristic curve of the motor in the fourth quadrant; s max is a constant, which is the slip ratio limit value of the wheel, and I max is the charging current limit value of the battery.

[0056] In the said step three, calculating and outputting the target motor braking torque and the target hydraulic braking torque according to the vehicle state prediction model and the objective function of the distribution controller specifically includes:

[0057] Calculating the values of the state variables within the prediction time domain according to the vehicle state prediction model and the values of the current state variables, substituting the values of the state variables within the prediction time domain into the objective function of the distribution controller for calculation and solution to obtain the motor torque that minimizes the objective function value. The obtained motor torque is the target motor braking torque T1;

[0058] Calculating the target front-wheel brake braking torque T2 and the target rear-wheel brake braking torque T3 according to the target motor braking torque T1, the current driver demand braking torque T driver , and the brake force distribution coefficient β:

[0059] T2 = (T driver - T1)β;

[0060]

[0061] The target front-wheel brake braking torque T2 and the target rear-wheel brake braking torque T3 are the target hydraulic braking torques.

[0062] The state variables include: vehicle speed v, front-wheel speed ω1, rear-wheel speed ω2, motor torque T m , front-wheel brake braking torque T h1 , rear-wheel brake braking torque T h2 , driver demand braking torque T driver , motor speed ω m , motor output current I m .

[0063] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0064] 1. In the electro-hydraulic braking force distribution method for a pure electric vehicle of the present invention, a sprung mass motion model is first established, and then the sprung mass motion model is discretized to obtain a vehicle state prediction model. Based on the current vehicle speed, front-wheel speed, rear-wheel speed, motor torque, front-wheel brake braking torque, rear-wheel brake braking torque, driver demand braking torque, motor speed, motor output current and other state variables and the vehicle state prediction model, the values of the state variables within the prediction time domain can be calculated, and the objective function is solved through the values of the state variables within the prediction time domain to obtain the target motor torque and the target hydraulic braking torque, so as to reasonably distribute the electro-hydraulic braking force. Therefore, in this design, the values of the state variables within the prediction time domain are calculated through the vehicle state prediction model, and then the target motor torque and the target hydraulic braking torque are calculated by combining the values of the state variables within the prediction time domain and the objective function, and the electro-hydraulic braking force can be reasonably distributed.

[0065] 2. In the electro-hydraulic braking force distribution method of a pure electric vehicle according to the present invention, in order to recover braking energy as much as possible, it is necessary to increase the braking energy recovery power as much as possible on the premise of ensuring that the tire slip ratio is within the safe range. During the entire braking process, the braking torque is provided jointly by the motor and the hydraulic braking system. The motor torque can be changed rapidly by adjusting the current, but the change rate of the hydraulic braking torque is relatively slow. Therefore, during the braking process, the change in the motor torque should be as small as possible to ensure the smoothness of the deceleration during braking. According to the above optimization objectives of reducing the change in the motor torque, ensuring that the tire slip ratio is within the safe range, and increasing the braking energy recovery power, the objective function of the braking force distribution controller is established. By solving the objective function of the distribution controller and the values of the state variables within the prediction time domain, the target motor torque and the target hydraulic braking torque are obtained. The motor controller and the braking controller respectively control the motor and the hydraulic braking system to respond according to the target motor torque and the target hydraulic braking torque, and a greater energy recovery power can be achieved on the premise of ensuring smooth deceleration. Therefore, in this design, the objective function of the distribution controller is established by comprehensively considering braking comfort, safety, and economy, and then the driver's required braking torque is reasonably distributed through the objective function of the distribution controller, which can achieve the effects of smooth deceleration and increased braking energy recovery power.

[0066] 3. In the electro-hydraulic braking force distribution method of a pure electric vehicle according to the present invention, the constraint conditions of the objective function of the distribution controller are set, and the motor torque during braking is limited by the external characteristic curve of the motor in the fourth quadrant. At the same time, the slip ratio of the front wheels and the current output by the motor during braking are limited, effectively ensuring the safety of braking. Therefore, in this design, the safety of braking is effectively ensured by setting the constraint conditions of the objective function. Description of the Drawings

[0067] Figure 1 is the flowchart of the present invention. Detailed Embodiment

[0068] The present invention will be further described in detail below with reference to the description of the drawings and the detailed embodiment.

[0069] See Figure 1 , an electro-hydraulic braking force distribution method for a pure electric vehicle, the distribution method includes the following steps:

[0070] Step 1: Establish a kinematic model of the sprung mass of the vehicle;

[0071] The sprung mass is the mass of the vehicle supported by the suspension, that is, all components above the suspension are collectively called the sprung mass, and the sprung mass includes the frame, the body, the engine, and the transmission system.

[0072] Step 2: Discretize the kinematic model of the sprung mass to obtain a vehicle state prediction model;

[0073] Step 3: Establish the objective function of the braking force distribution controller, set the constraint conditions of the objective function of the distribution controller, and calculate and output the target motor braking torque and the target hydraulic braking torque according to the vehicle state prediction model and the objective function of the distribution controller.

[0074] The establishment of the kinematic model of the vehicle's sprung mass in Step 1 includes: establishing the motion equation of the sprung mass and the motion equations of the front and rear wheels.

[0075] In Step 1, the motion equation of the sprung mass specifically includes:

[0076]

[0077]

[0078] F b1 =K D sin(K C tan -1 {K B s1-K E [K B s1-tan -1 (K B s1)]})F z1 (3);

[0079] F b2 =K D sin(K C tan -1 {K B s2-K E [K B s2-tan -1 (K B s2)]})F z2 (4);

[0080]

[0081]

[0082]

[0083]

[0084] In equations (1)-(8), m sm is the total sprung mass, that is, the total mass of the sprung part, is the derivative of the vehicle speed v with respect to time, F b1 is the ground braking force of the front wheels, Fb2 is the ground braking force of the rear wheel. The ground braking force is the tangential force exerted by the ground on the wheel caused by the braking torque, F a is the air resistance, C d is the air resistance coefficient, A F is the frontal area of the vehicle, ρ a is the air density, K B 、K C 、K D 、K E are all coefficients of the Pacejka model. Among the coefficients of the Pacejka model, K B is the stiffness factor, K C is the shape factor, K D is the peak factor, K E is the curvature factor coefficient, s1 is the slip ratio of the front wheel, F z1 is the vertical load of the front wheel, s2 is the slip ratio of the rear wheel, F z2 is the vertical load of the rear wheel, ω1 is the rotational speed of the front wheel, R t is the wheel rolling radius, ω2 is the rotational speed of the rear wheel, g is the acceleration due to gravity, l is the wheelbase, that is, the distance between the center of the front axle and the center of the rear axle of the vehicle, a is the horizontal distance from the center of mass of the sprung mass to the front axle, b is the horizontal distance from the center of mass of the sprung mass to the rear axle, h g is the height of the center of mass.

[0085] In the first step, establishing the motion equations of the front and rear wheels specifically includes:

[0086] Since the wheel rotational speeds can be obtained from the motion equations of the front and rear wheels, the motion equations of the front and rear wheels of a front-wheel drive vehicle are:

[0087]

[0088]

[0089] T h1 =(T driver -T m )β (11);

[0090]

[0091] In equations (9)-(12), is the derivative of the front wheel rotational speed ω1 with respect to time, is the derivative of the rear wheel rotational speed ω2 with respect to time, J w1 is the moment of inertia of the front wheel, i t is the total reduction ratio, T h1 is the braking torque of the front wheel brake, J w2 is the moment of inertia of the rear wheel, T h2is the braking torque of the rear-wheel brake, J m is the moment of inertia of the motor, T m is the motor torque, T driver is the braking torque demanded by the driver, β is the braking force distribution coefficient of the brake, R t is the wheel rolling radius, F b1 is the ground braking force of the front wheel, F b2 is the ground braking force of the rear wheel;

[0092] At a given rotational speed and torque, the current I output by the motor m is:

[0093]

[0094] Since the front wheel is rigidly connected to the drive motor through the transmission system, the motor speed ω can be known from the front wheel speed m is:

[0095] ω m =i t ω1; (14);

[0096] In Equation (13), Loss(T m , ω m ) is the power loss of the motor at a rotational speed of ω m and a torque of T m . By testing the loss power of the motor at different rotational speeds and torques on the test bench, the power loss of the motor corresponding to different rotational speeds and torques can be obtained. V acc is the voltage of the power battery. The voltage of the power battery is related to the current and SOC of the battery. However, for each control cycle, the change in the battery voltage is small. Therefore, the battery voltage is regarded as a constant.

[0097] Step 2: Discretize the sprung mass kinematic model to obtain the vehicle state prediction model, which specifically includes:

[0098] Discretize the motion equations of the sprung mass and the front and rear wheels to obtain the vehicle state prediction model. The sampling period of the vehicle state prediction model is Δt. The vehicle state prediction model is as follows:

[0099]

[0100]

[0101] F b1 (k)=K D sin(K C tan -1 {K B s1(k)-K E [KB s1(k) - tan -1 (K B s1(k))]})F z1 (k) (17);

[0102] F b2 (k) = K D sin(K C tan -1 {K B s2(k) - K E [K B s2(k) - tan -1 (K B s2(k))]})F z2 (k) (18);

[0103]

[0104]

[0105]

[0106]

[0107]

[0108]

[0109] T h1 (k) = (T driver (k) - T m (k))β (25);

[0110]

[0111]

[0112] In formulas (14) - (26), v(k) is the vehicle speed at time k; v(k + Δt) is the vehicle speed at time k + Δt; F b1 (k) is the ground braking force of the front wheels at time k; F b2 (k) is the ground braking force of the rear wheels at time k; F a (k) is the air resistance at time k; s i (k) is the slip ratio of the wheels at time k; F z1 (k) is the vertical load of the front wheels at time k, F z2 (k) is the vertical load of the rear wheels at time k; v(k - Δt) is the vehicle speed at time k - Δt; ω1(k) is the rotational speed of the front wheels at time k; ω2(k) is the rotational speed of the rear wheels at time k; ω1(k + Δt) is the rotational speed of the front wheels at time k + Δt; Tm (k) is the motor torque at time k; T h1 (k) is the braking torque of the front-wheel brake at time k; ω2(k+Δt) is the rear-wheel speed at time k+Δt; T h2 (k) is the braking torque of the rear-wheel brake at time k; T driver (k) is the driver's required braking torque at time k. During the calculation of the optimized target motor braking torque, the driver's required braking torque is a constant; I m (k) is the motor current at time k; ω m (k) is the motor speed at time k; Loss(T m (k), ω m (k)) is the power loss of the motor at time k.

[0113] In engineering practice, since the controller continuously monitors the state variables online, when the braking process just starts, the controller inputs the state variables measured at the previous moment at time k as the state variables at time (k-1) into the model.

[0114] To recover braking energy as much as possible, it is necessary to increase the braking energy recovery power as much as possible on the premise of ensuring that the tire slip rate is within the safe range. During the entire braking process, the braking torque is provided jointly by the motor and the hydraulic braking system. The motor torque can be changed rapidly by adjusting the current, but the change rate of the hydraulic braking torque is slow. Therefore, during the braking process, the change of the motor torque should be as small as possible to ensure the smoothness of the deceleration during braking. According to the above optimization objectives, the objective function J(k) of the braking force distribution controller is established:

[0115]

[0116] In the formula, w u is the comfort weight; w s is the safety weight; w P is the economic weight; h c is the prediction time domain length; u(k+i|k) is the controller input at time k+i; u(k+i-Δt|k) is the controller input at time k+i-Δt; s1(k+i|k) is the front-wheel slip rate at time k+i; P m (k+i|k) is the motor recovery power at time k+i;

[0117] The controller input u(k) is the motor torque T m (k), that is, the controller input at time k+i is the torque of the motor at time k+i, and the controller input at time k+i-1 is the torque of the motor at time k+i-1.

[0118] The motor recovery power P mis: P m = T m ω m - Loss(T m , ω m );

[0119] In the formula: T m is the motor torque, ω m is the motor speed, Loss(T m (k), ω m (k)) is the power loss of the motor.

[0120] In the third step, the constraint conditions of the objective function include:

[0121] T m_min (ω m ) ≤ T m ≤ 0;

[0122] s1 ≤ s max ;

[0123] I m ≤ I max ;

[0124] In the formula, T m is the motor torque, s1 is the slip ratio of the front wheel, I m is the current of the motor, and the T m_min (ω m ) is the external characteristic curve of the motor in the fourth quadrant; s max is a constant, which is the slip ratio limit value of the wheel, and I max is the charging current limit value of the battery.

[0125] In the third step, calculating and outputting the target motor braking torque and the target hydraulic braking torque according to the vehicle state prediction model and the objective function of the distribution controller specifically includes:

[0126] Calculating the values of the state variables within the prediction time domain according to the vehicle state prediction model and the values of the current state variables, substituting the values of the state variables within the prediction time domain into the objective function of the distribution controller for calculation and solution to obtain the motor torque that minimizes the objective function value, and the obtained motor torque is the target motor braking torque T1;

[0127] Calculating the target front wheel brake braking torque T2 and the target rear wheel brake braking torque T3 according to the target motor braking torque T1, the current driver demand braking torque T driver , and the brake force distribution coefficient β:

[0128] T2 = (T driver - T1)β;

[0129]

[0130] The target front-wheel brake braking torque T2 and the target rear-wheel brake braking torque T3 are the target hydraulic braking torques.

[0131] The state variables include: vehicle speed v, front-wheel rotational speed ω1, rear-wheel rotational speed ω2, motor torque T m , front-wheel brake braking torque T h1 , rear-wheel brake braking torque T h2 , driver demand braking torque T driver , motor rotational speed ω m , motor output current I m .

[0132] The principle of the present invention is described as follows:

[0133] When the electric vehicle brakes, the vehicle control unit receives the motor state information and battery state information transmitted by the motor control unit and the battery control unit, and calculates the target motor torque and the target hydraulic braking torque through the objective function and its constraint conditions, with the braking comfort, safety, and economy as the optimization objectives, and sends the target motor torque to the motor control unit and the target hydraulic braking torque to the brake control unit. The motor control unit and the brake control unit respectively control the motor and the hydraulic braking system to respond to the control target.

[0134] Embodiment 1:

[0135] A method for electro-hydraulic braking force distribution of a pure electric vehicle, the distribution method comprising the following steps:

[0136] Step 1, establish a kinematic model of the sprung mass of the vehicle;

[0137] Step 2, discretize the kinematic model of the sprung mass to obtain a vehicle state prediction model;

[0138] Step 3, establish an objective function of the braking force distribution controller, set the constraint conditions of the objective function of the distribution controller, and calculate the target electric motor braking torque and the target hydraulic braking torque according to the vehicle state prediction model and the objective function of the distribution controller.

[0139] In the above step 1, establishing a kinematic model of the sprung mass of the vehicle includes: establishing a motion equation of the sprung mass and establishing front and rear wheel motion equations. The motion equation of the sprung mass specifically includes:

[0140]

[0141]

[0142] F b1 = K Dsin(K C tan -1 {K B s1-K E [K B s1-tan -1 (K B s1)]})F z1 (3);

[0143] F b2 =K D sin(K C tan -1 {K B s2-K E [K B s2-tan -1 (K B s2)]})F z 2 (4);

[0144]

[0145]

[0146]

[0147]

[0148] In equations (1)-(8), m sm is the total sprung mass, is the derivative of vehicle speed v with respect to time, F b1 is the front wheel ground braking force, F b2 is the rear wheel ground braking force, F a is the air resistance, C d is the air resistance coefficient, A F is the frontal area, ρ a is the air density, K B , K C , K D , K E are all coefficients of the Pacejka model, s1 is the slip ratio of the front wheel, F z1 is the vertical load of the front wheel, s2 is the slip ratio of the rear wheel, F z2 is the vertical load of the rear wheel, ω1 is the front wheel speed, R t is the wheel rolling radius, ω2 is the rear wheel speed, g is the acceleration due to gravity, l is the wheelbase, a is the horizontal distance from the center of mass of the sprung mass to the front axle, b is the horizontal distance from the center of mass of the sprung mass to the rear axle, h g is the height of the center of mass.

[0149] The specific front and rear wheel motion equations include:

[0150]

[0151]

[0152] T h1 = (T driver - T m )β (11);

[0153]

[0154]

[0155] ω m = i t ω1 (14);

[0156] In equations (9)-(14), is the derivative of the front-wheel rotational speed ω1 with respect to time, is the derivative of the rear-wheel rotational speed ω2 with respect to time, J w1 is the moment of inertia of the front wheel, i t is the total reduction ratio, T h1 is the braking torque of the front-wheel brake, J w2 is the moment of inertia of the rear wheel, T h2 is the braking torque of the rear-wheel brake, J m is the moment of inertia of the motor, T m is the motor torque, T driver is the driver's required braking torque, β is the brake force distribution coefficient, R t is the wheel rolling radius, F b1 is the ground braking force of the front wheel, F b2 is the ground braking force of the rear wheel, I m is the current output by the motor, Loss(T m , ω m ) is the power loss of the motor, V acc is the voltage of the power battery, ω m is the motor speed.

[0157] In step two, the sprung mass kinematic model is discretized to obtain a vehicle state prediction model, which specifically includes:

[0158] The equations of motion of the sprung mass, the front and rear wheels are discretized to obtain a vehicle state prediction model. The sampling period of the vehicle state prediction model is Δt, and the vehicle state prediction model is as follows:

[0159]

[0160]

[0161] F b1 (k) = K D sin(K C tan -1 {K B s1(k) - K E [K B s1(k) - tan -1 (K B s1(k))]}F z1 (k) (17);

[0162] F b2 (k) = K D sin(K C tan -1 {K B s2(k) - K E [K B s2(k) - tan -1 (K B s2(k))]}F z2 (k) (18);

[0163]

[0164]

[0165]

[0166]

[0167]

[0168]

[0169] T h1 (k) = (T driver (k) - T m (k))β (25);

[0170]

[0171]

[0172] In equations (15) - (27), v(k) is the vehicle speed at time k, v(k + Δt) is the vehicle speed at time k + Δt, v(k - Δt) is the vehicle speed at time k - Δt, F b1 (k) is the ground braking force of the front wheels at time k, F b2 (k) is the ground braking force of the rear wheels at time k, F a(k) is the air resistance at time k, s1(k) is the slip ratio of the front wheels at time k, s2(k) is the slip ratio of the rear wheels at time k, F z1 (k) is the vertical load of the front wheels at time k, F z2 (k) is the vertical load of the rear wheels at time k, ω1(k) is the rotational speed of the front wheels at time k; ω2(k) is the rotational speed of the rear wheels at time k, ω1(k+Δt) is the rotational speed of the front wheels at time k+Δt, ω2(k+Δt) is the rotational speed of the rear wheels at time k+Δt, T m (k) is the motor torque at time k, ω m (k) is the motor speed at time k, T h1 (k) is the braking torque of the front-wheel brake at time k, T h2 (k) is the braking torque of the rear-wheel brake at time k, T driver (k) is the braking torque demanded by the driver at time k, I m (k) is the motor current at time k, Loss(T m (k), ω m (k)) is the power loss of the motor at time k.

[0173] In the third step described above, the objective function J(k) of the braking force distribution controller is:

[0174]

[0175] In the formula, w u is the comfort weight; w s is the safety weight; w P is the economic weight; h c is the prediction horizon length; u(k+i|k) is the controller input at time k+i; u(k+i-Δt|k) is the controller input at time k+i-Δt; s1(k+i|k) is the slip ratio of the front wheels at time k+i; P m (k+i|k) is the motor recovery power at time k+i;

[0176] The controller input is the motor torque.

[0177] The motor recovery power P m is: P m =T m ω m -Loss(T m , ω m );

[0178] In the formula: T m is the motor torque, ω m is the motor speed, Loss(T m (k), ω m (k)) is the power loss of the motor.

[0179] In step 3, the constraint conditions of the objective function include:

[0180] T m_min (ω m ) ≤ T m ≤ 0;

[0181] s1 ≤ s max ;

[0182] I m ≤ I max ;

[0183] Wherein, T m is the motor torque, s1 is the slip ratio of the front wheel, I m is the current output by the motor, and the T m_min (ω m ) is the external characteristic curve of the motor in the fourth quadrant; s max is a constant, which is the slip ratio limit value of the wheel, and I max is the charging current limit value of the battery.

[0184] Embodiment 2:

[0185] Embodiment 2 is basically the same as Embodiment 1, and the difference is that:

[0186] In step 3, calculating and outputting the target motor braking torque and the target hydraulic braking torque according to the vehicle state prediction model and the objective function of the distribution controller specifically includes:

[0187] According to the vehicle state prediction model and the values of the current state variables, the values of the state variables within the prediction time domain can be calculated, and the values of the state variables within the prediction time domain are substituted into the objective function of the distribution controller for calculation and solution to obtain the motor torque that minimizes the objective function value, and the obtained motor torque is the target motor braking torque;

[0188] According to the target motor braking torque, the driver's required braking torque T driver , and the brake force distribution coefficient β of the brake, the target hydraulic braking torque is calculated:

[0189] Target front-wheel brake braking torque = (T driver - target motor braking torque) * β;

[0190] Target rear-wheel brake braking torque = target front-wheel brake braking torque * (1 - β) / β;

[0191] The state variables include: vehicle speed v, front-wheel speed ω1, rear-wheel speed ω2, motor torque T m , front-wheel brake braking torque T h1, Rear wheel brake braking torque T h2 , Driver demand braking torque T driver , Motor speed ω m , Current I output by the motor m .

[0192] The above is only the preferred embodiment of the present invention, and the protection scope of the present invention is not limited to the above embodiment. Any equivalent modification or change made by those of ordinary skill in the art according to the disclosure of the present invention shall be included in the protection scope recorded in the claims.

Claims

1. A method for electro-hydraulic braking force distribution of a pure electric vehicle, characterized in that: The distribution method includes the following steps: Step 1: Establish a kinematic model of the sprung mass of the vehicle; Step 2: Discretize the kinematic model of the sprung mass to obtain a vehicle state prediction model; Step 3: Establish an objective function of the braking force distribution controller, set the constraint conditions of the objective function, and calculate the target motor braking torque and the target hydraulic braking torque according to the vehicle state prediction model and the objective function; In the step 3, the objective function J(k) of the braking force distribution controller is: where, w u is the comfort weight; w s is the safety weight; w P is the economy weight; h c is the prediction horizon length; u(k+i|k) is the controller input at time k+i; u(k+i-Δt|k) is the controller input at time k+i-Δt; s1(k+i|k) is the front wheel slip ratio at time k+i; P m (k+i|k) is the motor regenerative power at time k+i; the controller input is the motor torque; In the step 3, calculating and outputting the target motor braking torque and the target hydraulic braking torque according to the vehicle state prediction model and the objective function of the distribution controller specifically includes: Calculating the values of the state variables within the prediction time domain according to the vehicle state prediction model and the values of the current state variables, substituting the values of the state variables within the prediction time domain into the objective function of the distribution controller for calculation and solution to obtain the motor torque that minimizes the objective function value, and the obtained motor torque is the target motor braking torque T1; According to the target motor braking torque T1 and the current driver demand braking torque T driver and the brake force distribution coefficient β of the brake, the target front-wheel brake braking torque T2 and the target rear-wheel brake braking torque T3 are calculated as follows: T2 = (T driver - T1)β; The target front-wheel brake braking torque T2 and the target rear-wheel brake braking torque T3 are the target hydraulic braking torques.

2. The method for electro-hydraulic braking force distribution of a pure electric vehicle according to claim 1, characterized in that: In the step 1, establishing a kinematic model of the sprung mass of the vehicle includes: establishing a sprung mass motion equation and establishing front and rear wheel motion equations.

3. The method for electro-hydraulic braking force distribution of a pure electric vehicle according to claim 2, characterized in that: In the step 1, the sprung mass motion equation specifically includes: F b1 = K D sin(K C tan -1 {K B s1 - K E [K B s1 - tan -1 (K B s1)]})F z1 (3); F b2 = K D sin(K C tan -1 {K B s2 - K E [K B s2 - tan -1 (K B s2)]})F z2 (4); In formulas (1)-(8), m sm is the total sprung mass, is the derivative of vehicle speed v with respect to time, F b1 is the front wheel ground braking force, F b2 is the rear wheel ground braking force, F a is the air resistance, C d is the air resistance coefficient, A F is the frontal area, ρ a is the air density, K B 、K C 、K D 、K E are all coefficients of the Pacejka model, s1 is the slip ratio of the front wheel, F z1 is the vertical load of the front wheel, s2 is the slip ratio of the rear wheel, F z2 is the vertical load of the rear wheel, ω1 is the front wheel speed, R t is the wheel rolling radius, ω2 is the rear wheel speed, g is the acceleration due to gravity, l is the wheelbase, a is the horizontal distance from the center of mass of the sprung mass to the front axle, b is the horizontal distance from the center of mass of the sprung mass to the rear axle, h g is the height of the center of mass.

4. The method for electro-hydraulic braking force distribution of a pure electric vehicle according to claim 3, characterized in that: In the step 1, the front and rear wheel motion equations specifically include: ω m = i t ω1 (14); In formulas (9)-(14), is the derivative of the front wheel speed ω1 with respect to time, is the derivative of the rear wheel speed ω2 with respect to time, J w1 is the moment of inertia of the front wheel, i t is the total reduction ratio, T h1 is the braking torque of the front wheel brake, J w2 is the moment of inertia of the rear wheel, T h2 is the braking torque of the rear wheel brake, J m is the moment of inertia of the motor, T m is the motor torque, T dziver is the braking torque demanded by the driver, β is the braking force distribution coefficient of the brake, R t is the wheel rolling radius, F b1 is the ground braking force of the front wheel, F b2 is the ground braking force of the rear wheel, I m is the current output by the motor, Loss(T m , ω m ) is the power loss of the motor, V acc is the voltage of the power battery, ω m is the motor speed.

5. The method for electro-hydraulic braking force distribution of a pure electric vehicle according to claim 4, characterized in that: In the step 2, discretizing the kinematic model of the sprung mass to obtain a vehicle state prediction model specifically includes: Discretizing the sprung mass motion equation and the front and rear wheel motion equations to obtain a vehicle state prediction model, the sampling period of the vehicle state prediction model is Δt, and the vehicle state prediction model is as follows: F b1 (k) = K D sin(K C tan -1 {K B s1(k) - K E [K B s1(k) - tan -1 (K B s1(k))]})F z1 (k) (17); F b2 (k) = K D sin(K C tan -1 {K B s2(k) - K E [K B s2(k) - tan -1 (K B s2(k))]})F z2 (k) (18); T h1 (k) = (T driver (k) - T m (k))β (25); In Equations (15)-(27), v(k) is the vehicle speed at time k, v(k+Δt) is the vehicle speed at time k+Δt, v(k-Δt) is the vehicle speed at time k-Δt, F b1 (k) is the front-wheel ground braking force at time k, F b2 (k) is the rear-wheel ground braking force at time k, F a (k) is the air resistance at time k, s1(k) is the slip ratio of the front wheel at time k, s2(k) is the slip ratio of the rear wheel at time k, F z1 (k) is the vertical load of the front wheel at time k, F z2 (k) is the vertical load of the rear wheel at time k, ω1(k) is the front-wheel rotational speed at time k; ω2(k) is the rear-wheel rotational speed at time k, ω1(k+Δt) is the front-wheel rotational speed at time k+Δt, ω2(k+Δt) is the rear-wheel rotational speed at time k+Δt, T m (k) is the motor torque at time k, ω m (k) is the motor rotational speed at time k, T h1 (k) is the braking torque of the front-wheel brake at time k, T h2 (k) is the braking torque of the rear-wheel brake at time k, T driver (k) is the driver's required braking torque at time k, I m (k) is the motor current at time k, Loss(T m (k), ω m (k)) is the power loss of the motor at time k.

6. The method for electro-hydraulic braking force distribution of a pure electric vehicle according to claim 1, characterized in that: The recovered power P of the motor m is: P m = T m ω m - Loss(T m , ω m ); Where: T m is the motor torque, ω m is the motor speed, Loss(T m (k), ω m (k)) is the power loss of the motor.

7. The method for electro-hydraulic braking force distribution of a pure electric vehicle according to claim 6, characterized in that: In the step 3, the constraint conditions of the objective function include: T m_min (ω m ) ≤ T m ≤ 0; s1≤s max ; I m ≤I max ; Where, T m is the motor torque, s1 is the slip ratio of the front wheel, I m is the current output by the motor, and the T m_min (ω m ) is the external characteristic curve of the motor in the fourth quadrant; s max is a constant, which is the slip ratio limit value of the wheel, and I max is the charging current limit value of the battery.

8. The method for electro-hydraulic braking force distribution of a pure electric vehicle according to claim 1, characterized in that: The state variables include: vehicle speed v, front wheel speed ω1, rear wheel speed ω2, motor torque T m , front wheel brake braking torque T h1 , rear wheel brake braking torque T h2 , driver demand braking torque T drier , motor speed ω m , motor output current I m .

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