阅读说明：本技术 一种基于线控制动的能量回收控制方法 (Energy recovery control method based on brake-by-wire ) 是由 常九健 张煜帆 于 2021-09-17 设计创作，主要内容包括：本发明涉及一种基于线控制动的能量回收控制方法,包括：获取分布式电驱动整车实时的数据和参数；建立EMB电子机械制动器系统模型；判断是否启动制动能量回收；获取制动强度Z；根据制动强度Z,依据设定的前后轴制动力曲线得到此时前轴制动力矩T-(bf)和后轴制动力矩T-(br)；将当前制动踏板信号Brk、实时纵向车速V以及电池的SOC值作为输入,利用模糊控制得到电机制动力矩占比系数K；得到每个轴上的制动能量回馈力矩,电机根据整车控制器VCU给定的制动能量回收力矩进行制动。本发明通过实时条件计算制动强度,分配每个轴上的制动力大小,然后通过模糊控制计算的到每个轴上再生制动力的占比,从而得到回收的能量。(The invention relates to an energy recovery control method based on brake-by-wire, which comprises the following steps: acquiring real-time data and parameters of a distributed electric drive whole vehicle; establishing an EMB electronic mechanical brake system model; judging whether to start braking energy recovery; obtaining braking strength Z; according to the braking intensity Z, obtaining the front axle braking torque T at the moment according to the set front and rear axle braking force curve bf And rear axle braking torque T br (ii) a Taking a current brake pedal signal Brk, a real-time longitudinal vehicle speed V and a battery SOC value as input, and obtaining a motor braking torque ratio coefficient K by utilizing fuzzy control; and obtaining braking energy feedback torque on each shaft, and braking the motor according to the braking energy recovery torque given by the VCU of the vehicle control unit. The invention calculates the braking strength through real-time conditions, distributes the braking force on each shaft, and then obtains the recovered energy through the proportion of the regenerative braking force on each shaft calculated through fuzzy control.)

1. An energy recovery control method based on brake-by-wire is characterized in that: the method comprises the following steps in sequence:

(1) acquiring real-time data and parameters of the distributed electric drive whole vehicle: acquiring real-time longitudinal speed V, current brake pedal signal Brk and SOC value of a battery of the distributed electric drive whole vehicle under different working conditions; acquiring distributed electric drive vehicle parameters including a distance a from a center of mass to a front shaft, a distance b from the center of mass to a rear shaft, a height hg of the center of mass, a mass m of the vehicle, a wheel base L, an effective friction radius r of a brake disc, a braking area A of a brake piston of a brake, a friction coefficient mu of the brake, a braking efficiency eta of the brake and a specific factor c of the brake;

(2) establishing an EMB electronic mechanical brake system model;

(3) judging whether to start braking energy recovery or not according to the current brake pedal signal Brk, the real-time longitudinal vehicle speed V and the SOC value of the battery;

(4) obtaining braking intensity Z according to current vehicle parameters and current brake pedal signals Brk; according to the braking intensity Z, obtaining the front axle braking torque T at the moment according to the set front and rear axle braking force curve_bfAnd rear axle braking torque T_br；

(5) Taking a current brake pedal signal Brk, a real-time longitudinal vehicle speed V and a battery SOC value as input, and obtaining a motor braking torque ratio coefficient K by utilizing fuzzy control; and obtaining braking energy feedback torque on each shaft, and braking the motor according to the braking energy recovery torque given by the VCU of the vehicle control unit.

2. The brake-by-wire based energy recovery control method according to claim 1, characterized in that: the step (2) specifically comprises the following steps:

(2a) establishing a mathematical model of the brushless direct current motor:

T_m(t)＝C_m·i(t)

E＝C_E·N

wherein U is the armature voltage; i (t) is the armature current; l is_mIs the armature inductance; r_mIs the overall loop resistance; e is armature back EMF; tm (t) is the torque produced by the motor; cm is the torque coefficient; c_EIs the motor induced electromotive force coefficient; n is the rotor speed;

(2b) establishing a mathematical model of the planetary gear reduction mechanism:

T_x＝T_a·i_x·η_x

wherein, T_xIs the output torque of the planet carrier; t is_a＝T_mIs the input torque of the sun gear; i.e. i_xIs the transmission ratio; eta_xIs the transmission efficiency of the planetary gear;

(2c) establishing a mathematical model of the ball screw pair:

T_g＝F·P_h/(2π)

T_g＝T_x·η_g

wherein, T_gIs the driving torque of the ball screw; f is the screw rod thrust; p_hIs the lead of the lead screw; eta_gIs the transmission efficiency of the ball screw;

(2d) establishing an EMB electromechanical brake system model:

F＝2π·C_m·i(t)·i_x·η_x·η_g/P_h。

3. the brake-by-wire based energy recovery control method according to claim 1, characterized in that: the step (3) specifically comprises the following steps: judging whether to start braking energy recovery according to the current brake pedal signal Brk, the real-time longitudinal speed V and the SOC value of the battery: when the SOC of the battery is greater than 90%, closing braking energy recovery; when the real-time longitudinal speed V is less than 5km/h, closing the braking energy recovery; and if the current brake pedal has signal input, the real-time longitudinal speed V is greater than 5km/h, and the SOC value of the battery is not greater than 90%, starting to recover the braking energy.

4. The brake-by-wire based energy recovery control method according to claim 1, characterized in that: the step (4) specifically comprises the following steps: according to AVL Cruise theory handbook, the following formula between the braking torque and the braking pressure is found:

T＝2pAημrc

wherein T is the braking torque required by the current automobile; p is the brake pressure converted into the current required by the mechanical brake of the automobile; a is the braking area of the brake piston of the brake; eta is the brake efficiency of the brake; mu is the friction coefficient of the brake; r is the effective friction radius of the brake disc; c is a brake specific factor;

according to the formula of brake strength:

in the formula, m is the mass of the whole vehicle; r is the radius of the wheel, and the braking strength Z is obtained;

according to the braking strength Z, a front axle braking force Fx1 and a rear axle braking force Fx2 are obtained by combining a designed front and rear axle braking force curve;

the point A is an intersection point of an ECE regulation curve and an abscissa axis, corresponds to the first braking strength Z1, and corresponds to the first front axle braking force Fx 1A; the curve I is an ideal braking force distribution curve;

when the brake intensity Z is less than the first brake intensity Z1, front and rear axle brake forces are distributed according to an OA line segment, namely all the brake forces are provided by the front axle, and the rear axle does not participate in braking;

the point B corresponds to the second braking intensity Z2, and is a coordinate point position obtained when the braking intensity Z is equal to 0.2, that is, Z2 is 0.2; the corresponding second front axle braking force is Fx2A, the second rear axle braking force is Fx2B, and when the braking strength Z is between Z1 and Z2, the braking forces of the front axle and the rear axle are distributed according to an AB line;

the point C is a coordinate point position obtained by increasing the front axle braking force while keeping the rear axle braking force constant when the braking strength Z is equal to 0.4, that is, when the braking strength Z3 is 0.4, and the point C corresponds to the third braking strength Z3;

the corresponding third front axle braking force is Fx1C, the third rear axle braking force is Fx2C, and when the braking strength is between Z2 and Z3, the second rear axle braking force is Fx2B and is kept unchanged, and the front axle braking force is increased, namely a BC section;

the point D is a coordinate point position obtained when the braking intensity Z is equal to 0.6, that is, Z4 is 0.6, and corresponds to the fourth braking intensity Z4; the fourth front axle braking force is Fx1D, the fourth rear axle braking force is Fx2D, and when the braking strength is between Z3 and Z4, the front and rear axle braking forces are distributed according to a CD line;

the braking intensity corresponding to the point E is Z5, the point E is a coordinate point position obtained when the braking intensity Z is equal to 0.75, Z5 is 0.75, and emergency braking is performed after the point E is larger than the point E;

when the braking strength is between Z4 and Z5, the braking force of the front and rear axles is distributed according to the DE line;

when the braking strength is greater than Z5, the braking force of the front and rear shafts is distributed according to the curve I, and the motor exits the braking link; the distribution of the front and rear axle braking forces must be between the ECE regulation curve and the I curve;

knowing the braking intensity Z, the allocation strategy is as follows:

when Z < Z1, the braking force distribution line segment for the front and rear axles is OA:

g is gravity, Fx1 is front axle braking force, Fx2 is rear axle braking force;

when Z1< Z2, the front-rear axis braking force distribution line segment is AB:

when Z2< Z3, the front-rear axis braking force distribution line segment is BC:

when Z3< Z4, the front-rear axis braking force distribution line segment is CD:

when Z4< Z5, the front-rear axis braking force distribution line segment is DE:

when Z is more than Z5, the front and rear axle braking force distribution line segment is an I curve;

in the formula: k_AB、K_CD、K_DEThe slopes of the line segments AB, CD and DE respectively;

the Fx1 is the front axle braking force, the Fx2 is the rear axle braking force, and the front axle braking torque T is obtained_bfAnd rear axle braking torque T_br。

5. The brake-by-wire based energy recovery control method according to claim 1, characterized in that: the step (5) specifically comprises the following steps: the braking energy feedback torque on each shaft is obtained by using the following formula, and the motor performs braking according to the braking energy recovery torque given by the VCU of the vehicle controller:

T_regn＝T_brk*K

wherein, T_regnIs the regenerative braking torque; t is_brkIs the total braking torque; k is the motor braking torque ratio coefficient.

Technical Field

The invention relates to the technical field of energy recovery of electric automobiles, in particular to an energy recovery control method based on brake-by-wire.

Background

The pure electric vehicles account for more and more of the global quantity of the vehicles, and the positions of the pure electric vehicles are more and more important. The pure electric vehicle has the characteristics of zero emission, zero pollution, high efficiency and energy conservation, so that the pure electric vehicle becomes the development trend of the vehicle. The energy saving of the electric automobile is partially realized by converting braking force, and the braking energy is converted into electric energy to be stored in a battery by recovering the braking energy.

However, if the conventional hydraulic brake is used to cooperate with the motor to recover the braking energy, great difficulty may be caused because the braking force cannot be precisely controlled. If the brake-by-wire is used, the dynamic accurate control of the braking force can be realized. On the basis of brake-by-wire, the accurate control of the braking force can be realized. In order to improve the efficiency of braking energy recovery, an efficient braking force distribution and control system is required to reasonably distribute regenerative braking of the motor and mechanical friction braking.

The research on the mutual combination of an electronic mechanical brake system, a hub motor, a storage battery and the like in China rarely improves the recovery efficiency of automobile brake energy, and most of the research is carried out on the motor, the power supply and the like independently.

Disclosure of Invention

The invention aims to provide an energy recovery control method based on brake-by-wire, which realizes the recovery of more brake energy by real-time distribution control of braking force on the premise of ensuring the braking stability.

In order to achieve the purpose, the invention adopts the following technical scheme: an energy recovery control method based on brake-by-wire, the method comprising the following sequential steps:

(1) acquiring real-time data and parameters of the distributed electric drive whole vehicle: acquiring real-time longitudinal speed V, current brake pedal signal Brk and SOC value of a battery of the distributed electric drive whole vehicle under different working conditions; acquiring distributed electric drive vehicle parameters including a distance a from a center of mass to a front shaft, a distance b from the center of mass to a rear shaft, a height hg of the center of mass, a mass m of the vehicle, a wheel base L, an effective friction radius r of a brake disc, a braking area A of a brake piston of a brake, a friction coefficient mu of the brake, a braking efficiency eta of the brake and a specific factor c of the brake;

(2) establishing an EMB electronic mechanical brake system model;

(3) judging whether to start braking energy recovery or not according to the current brake pedal signal Brk, the real-time longitudinal vehicle speed V and the SOC value of the battery;

(4) according to the currentAcquiring braking intensity Z according to vehicle parameters and a current brake pedal signal Brk; according to the braking intensity Z, obtaining the front axle braking torque T at the moment according to the set front and rear axle braking force curve_bfAnd rear axle braking torque T_br；

(5) Taking a current brake pedal signal Brk, a real-time longitudinal vehicle speed V and a battery SOC value as input, and obtaining a motor braking torque ratio coefficient K by utilizing fuzzy control; and obtaining braking energy feedback torque on each shaft, and braking the motor according to the braking energy recovery torque given by the VCU of the vehicle control unit.

The step (2) specifically comprises the following steps:

(2a) establishing a mathematical model of the brushless direct current motor:

T_m(t)＝C_m·i(t)

E＝C_E·N

wherein U is the armature voltage; i (t) is the armature current; l is_mIs the armature inductance; r_mIs the overall loop resistance; e is armature back EMF; tm (t) is the torque produced by the motor; cm is the torque coefficient; c_EIs the motor induced electromotive force coefficient; n is the rotor speed;

(2b) establishing a mathematical model of the planetary gear reduction mechanism:

T_x＝T_a·i_x·η_x

wherein, T_xIs the output torque of the planet carrier; t is_a＝T_mIs the input torque of the sun gear; i.e. i_xIs the transmission ratio; eta_xIs the transmission efficiency of the planetary gear;

(2c) establishing a mathematical model of the ball screw pair:

T_g＝F·P_h/(2π)

T_g＝T_x·η_g

wherein, T_gIs the driving torque of the ball screw; f is the screw rod thrust; p_hIs the lead of the lead screw; eta_gIs the transmission efficiency of the ball screw;

(2d) establishing an EMB electromechanical brake system model:

F＝2π·C_m·i(t)·i_x·η_x·η_g/P_h。

the step (3) specifically comprises the following steps: judging whether to start braking energy recovery according to the current brake pedal signal Brk, the real-time longitudinal speed V and the SOC value of the battery: when the SOC of the battery is greater than 90%, closing braking energy recovery; when the real-time longitudinal speed V is less than 5km/h, closing the braking energy recovery; and if the current brake pedal has signal input, the real-time longitudinal speed V is greater than 5km/h, and the SOC value of the battery is not greater than 90%, starting to recover the braking energy.

The step (4) specifically comprises the following steps: according to AVL Cruise theory handbook, the following formula between the braking torque and the braking pressure is found:

T＝2pAημrc

wherein T is the braking torque required by the current automobile; p is the brake pressure converted into the current required by the mechanical brake of the automobile; a is the braking area of the brake piston of the brake; eta is the brake efficiency of the brake; mu is the friction coefficient of the brake; r is the effective friction radius of the brake disc; c is a brake specific factor;

according to the formula of brake strength:

in the formula, m is the mass of the whole vehicle; r is the radius of the wheel, and the braking strength Z is obtained;

according to the braking strength Z, a front axle braking force Fx1 and a rear axle braking force Fx2 are obtained by combining a designed front and rear axle braking force curve;

the point A is an intersection point of an ECE regulation curve and an abscissa axis, corresponds to the first braking strength Z1, and corresponds to the first front axle braking force Fx 1A; the curve I is an ideal braking force distribution curve;

when the brake intensity Z is less than the first brake intensity Z1, front and rear axle brake forces are distributed according to an OA line segment, namely all the brake forces are provided by the front axle, and the rear axle does not participate in braking;

the point B corresponds to the second braking intensity Z2, and is a coordinate point position obtained when the braking intensity Z is equal to 0.2, that is, Z2 is 0.2; the corresponding second front axle braking force is Fx2A, the second rear axle braking force is Fx2B, and when the braking strength Z is between Z1 and Z2, the braking forces of the front axle and the rear axle are distributed according to an AB line;

the point C is a coordinate point position obtained by increasing the front axle braking force while keeping the rear axle braking force constant when the braking strength Z is equal to 0.4, that is, when the braking strength Z3 is 0.4, and the point C corresponds to the third braking strength Z3;

the corresponding third front axle braking force is Fx1C, the third rear axle braking force is Fx2C, and when the braking strength is between Z2 and Z3, the second rear axle braking force is Fx2B and is kept unchanged, and the front axle braking force is increased, namely a BC section;

the point D is a coordinate point position obtained when the braking intensity Z is equal to 0.6, that is, Z4 is 0.6, and corresponds to the fourth braking intensity Z4; the fourth front axle braking force is Fx1D, the fourth rear axle braking force is Fx2D, and when the braking strength is between Z3 and Z4, the front and rear axle braking forces are distributed according to a CD line;

the braking intensity corresponding to the point E is Z5, the point E is a coordinate point position obtained when the braking intensity Z is equal to 0.75, Z5 is 0.75, and emergency braking is performed after the point E is larger than the point E;

when the braking strength is between Z4 and Z5, the braking force of the front and rear axles is distributed according to the DE line;

when the braking strength is greater than Z5, the braking force of the front and rear shafts is distributed according to the curve I, and the motor exits the braking link; the distribution of the front and rear axle braking forces must be between the ECE regulation curve and the I curve;

knowing the braking intensity Z, the allocation strategy is as follows:

when Z < Z1, the braking force distribution line segment for the front and rear axles is OA:

g is gravity, Fx1 is front axle braking force, Fx2 is rear axle braking force;

when Z1< Z2, the front-rear axis braking force distribution line segment is AB:

when Z2< Z3, the front-rear axis braking force distribution line segment is BC:

when Z3< Z4, the front-rear axis braking force distribution line segment is CD:

when Z4< Z5, the front-rear axis braking force distribution line segment is DE:

when Z is more than Z5, the front and rear axle braking force distribution line segment is an I curve;

in the formula: k_AB、K_CD、K_DEThe slopes of the line segments AB, CD and DE respectively;

obtaining front axle braking force Fx1 and rear axle braking force Fx2, namely obtaining front axle braking torque T_bfAnd rear axle braking torque T_br。

The step (5) specifically comprises the following steps: the braking energy feedback torque on each shaft is obtained by using the following formula, and the motor performs braking according to the braking energy recovery torque given by the VCU of the vehicle controller:

T_regn＝T_brk*K

wherein, T_regnIs the regenerative braking torque; t is_brkIs the total braking torque; k is the motor braking torque ratio coefficient.

According to the technical scheme, the beneficial effects of the invention are as follows: firstly, the invention relates to a distributed type electric drive energy recovery control strategy based on brake-by-wire, which calculates the brake intensity through real-time conditions, distributes the brake force on each shaft, and then calculates the proportion of the regenerative brake force on each shaft through fuzzy control so as to obtain the recovered energy; secondly, compared with the prior art, the invention abandons the hydraulic brake of the traditional automobile, adopts the brake-by-wire, and utilizes the brake force distribution curve obtained by design to distribute the brake force, thereby further improving the recovery efficiency of energy and ensuring that the brake force distribution is easier to realize; thirdly, the control strategy can be built by combining the distribution of the braking force and the fuzzy control, so that the accuracy of the braking force is improved and the driving range is improved on the premise of ensuring the stability and the safety of the braking.

Drawings

FIG. 1 is a flow chart of a method of the present invention;

FIG. 2 is a front and rear axle braking force distribution diagram;

FIG. 3 is a diagram of SOC membership function;

FIG. 4 is a graph of vehicle speed membership function;

FIG. 5 is a graph of a brake pedal displacement membership function;

FIG. 6 is a motor brake ratio K membership function diagram;

FIG. 7 is a comparative plot of battery pack SOC under the NEDC cycle conditions;

FIG. 8 is a graph comparing energy recovered by a battery pack under the NEDC cycle conditions;

FIG. 9 is a comparative diagram of SOC of a battery under CLTC-P cycle conditions;

FIG. 10 is a graph comparing the energy recovered from a battery under CLTC-P cycling conditions.

Detailed Description

As shown in fig. 1, an energy recovery control method based on brake-by-wire includes the following sequential steps:

(1) acquiring real-time data and parameters of the distributed electric drive whole vehicle: acquiring real-time longitudinal speed V, current brake pedal signal Brk and SOC value of a battery of the distributed electric drive whole vehicle under different working conditions; acquiring distributed electric drive vehicle parameters including a distance a from a center of mass to a front shaft, a distance b from the center of mass to a rear shaft, a height hg of the center of mass, a mass m of the vehicle, a wheel base L, an effective friction radius r of a brake disc, a braking area A of a brake piston of a brake, a friction coefficient mu of the brake, a braking efficiency eta of the brake and a specific factor c of the brake;

(2) establishing an EMB electronic mechanical brake system model;

(3) judging whether to start braking energy recovery or not according to the current brake pedal signal Brk, the real-time longitudinal vehicle speed V and the SOC value of the battery;

(4) obtaining braking intensity Z according to current vehicle parameters and current brake pedal signals Brk; according to the braking intensity Z, obtaining the front axle braking torque T at the moment according to the set front and rear axle braking force curve_bfAnd rear axle braking torque T_br；

(5) Taking a current brake pedal signal Brk, a real-time longitudinal vehicle speed V and a battery SOC value as input, and obtaining a motor braking torque ratio coefficient K by utilizing fuzzy control; and obtaining braking energy feedback torque on each shaft, and braking the motor according to the braking energy recovery torque given by the VCU of the vehicle control unit.

The step (2) specifically comprises the following steps:

(2a) establishing a mathematical model of the brushless direct current motor:

T_m(t)＝C_m·i(t)

E＝C_E·N

wherein U is the armature voltage; i (t) is the armature current; l is_mIs the armature inductance; r_mIs the overall loop resistance; e is armature back EMF; tm (t) is the torque produced by the motor; cm is the torque coefficient; c_EIs the motor induced electromotive force coefficient; n is the rotor speed;

(2b) establishing a mathematical model of the planetary gear reduction mechanism:

T_x＝T_a·i_x·η_x

wherein, T_xIs the output torque of the planet carrier; t is_a＝T_mIs the input torque of the sun gear; i.e. i_xIs the transmission ratio; eta_xIs the transmission efficiency of the planetary gear;

(2c) establishing a mathematical model of the ball screw pair:

T_g＝F·P_h/(2π)

T_g＝T_x·η_g

wherein, T_gIs the driving torque of the ball screw; f is the screw rod thrust; p_hIs the lead of the lead screw; eta_gIs the transmission efficiency of the ball screw;

(2d) establishing an EMB electromechanical brake system model:

F＝2π·C_m·i(t)·i_x·η_x·η_g/P_h。

the step (3) specifically comprises the following steps: judging whether to start braking energy recovery according to the current brake pedal signal Brk, the real-time longitudinal speed V and the SOC value of the battery: when the SOC of the battery is greater than 90%, closing braking energy recovery; when the real-time longitudinal speed V is less than 5km/h, closing the braking energy recovery; and if the current brake pedal has signal input, the real-time longitudinal speed V is greater than 5km/h, and the SOC value of the battery is not greater than 90%, starting to recover the braking energy.

The step (4) specifically comprises the following steps: according to AVL Cruise theory handbook, the following formula between the braking torque and the braking pressure is found:

T＝2pAημrc

wherein T is the braking torque required by the current automobile; p is the brake pressure converted into the current required by the mechanical brake of the automobile; a is the braking area of the brake piston of the brake; eta is the brake efficiency of the brake; mu is the friction coefficient of the brake; r is the effective friction radius of the brake; c is a brake specific factor;

according to the formula of brake strength:

in the formula, m is the mass of the whole vehicle; r is the radius of the wheel, and the braking strength Z is obtained;

according to the braking strength Z, a front axle braking force Fx1 and a rear axle braking force Fx2 are obtained by combining a designed front and rear axle braking force curve;

as shown in fig. 2, point a is an intersection point of an ECE law curve and an abscissa axis, and corresponds to the braking strength Z1, and the corresponding first front axle braking force is Fx 1A; the curve I is an ideal braking force distribution curve;

when the brake intensity is smaller than Z1, front and rear axle brake forces are distributed according to an OA line segment, namely all the brake forces are provided by the front axle, and the rear axle does not participate in braking;

the point A is an intersection point of an ECE regulation curve and an abscissa axis, corresponds to the first braking strength Z1, and corresponds to the first front axle braking force Fx 1A; the curve I is an ideal braking force distribution curve;

when the brake intensity Z is less than the first brake intensity Z1, front and rear axle brake forces are distributed according to an OA line segment, namely all the brake forces are provided by the front axle, and the rear axle does not participate in braking;

the point B corresponds to the second braking intensity Z2, and is a coordinate point position obtained when the braking intensity Z is equal to 0.2, that is, Z2 is 0.2; the corresponding second front axle braking force is Fx2A, the second rear axle braking force is Fx2B, and when the braking strength Z is between Z1 and Z2, the braking forces of the front axle and the rear axle are distributed according to an AB line;

the point C is a coordinate point position obtained by increasing the front axle braking force while keeping the rear axle braking force constant when the braking strength Z is equal to 0.4, that is, when the braking strength Z3 is 0.4, and the point C corresponds to the third braking strength Z3;

the corresponding third front axle braking force is Fx1C, the third rear axle braking force is Fx2C, and when the braking strength is between Z2 and Z3, the second rear axle braking force is Fx2B and is kept unchanged, and the front axle braking force is increased, namely a BC section;

the point D is a coordinate point position obtained when the braking intensity Z is equal to 0.6, that is, Z4 is 0.6, and corresponds to the fourth braking intensity Z4; the fourth front axle braking force is Fx1D, the fourth rear axle braking force is Fx2D, and when the braking strength is between Z3 and Z4, the front and rear axle braking forces are distributed according to a CD line;

the braking intensity corresponding to the point E is Z5, the point E is a coordinate point position obtained when the braking intensity Z is equal to 0.75, Z5 is 0.75, and emergency braking is performed after the point E is larger than the point E;

when the braking strength is between Z4 and Z5, the braking force of the front and rear axles is distributed according to the DE line;

when the braking strength is greater than Z5, the braking force of the front and rear shafts is distributed according to the curve I, and the motor exits the braking link; the distribution of the front and rear axle braking forces must be between the ECE regulation curve and the I curve;

knowing the braking intensity Z, the allocation strategy is as follows:

when Z < Z1, the braking force distribution line segment for the front and rear axles is OA:

g is gravity, Fx1 is front axle braking force, Fx2 is rear axle braking force;

when Z1< Z2, the front-rear axis braking force distribution line segment is AB:

when Z2< Z3, the front-rear axis braking force distribution line segment is BC:

when Z3< Z4, the front-rear axis braking force distribution line segment is CD:

when Z4< Z5, the front-rear axis braking force distribution line segment is DE:

when Z is more than Z5, the front and rear axle braking force distribution line segment is an I curve;

in the formula: k_AB、K_CD、K_DEThe slopes of the line segments AB, CD and DE respectively;

obtaining front axle braking force Fx1 and rear axle braking force Fx2, namely obtaining front axle braking torque T_bfAnd rear axle braking torque T_br。

The step (5) specifically comprises the following steps: the braking energy feedback torque on each shaft is obtained by using the following formula, and the motor performs braking according to the braking energy recovery torque given by the VCU of the vehicle controller:

T_regn＝T_brk*K

wherein, T_regnIs the regenerative braking torque; t is_brkIs the total braking torque; k is the motor braking torque ratio coefficient.

After the moment distribution of the front and rear shafts is finished, the moment on each shaft is distributed. Using a fuzzy control method, inputting fuzzy control: the input comprises a real-time longitudinal vehicle speed V; acquiring a brake pedal signal Brk; acquiring the SOC value of the battery;

(1) the battery SOC is set as follows:

when a battery of an electric automobile is charged and discharged, the SOC needs to be monitored in real time, and the SOC value is damaged when being too high or too low, so that the braking energy can be recovered only when the SOC value is within a certain range in consideration of the safety of the battery, and the SOC values are set to three different levels, { high (G), medium (Z) and low (D) }, as shown in fig. 3;

(2) the vehicle speed V is set as follows:

when the vehicle speed is lower than a certain value, the regenerative braking is closed, and when the vehicle speed is higher than the value, the regenerative braking ratio is properly increased. The vehicle speed is divided into 4 different levels, { high (G), medium (Z), low (D), very low (HD) }, as shown in fig. 4;

(3) the brake pedal stroke Brk is set as follows:

the brake pedal is the braking demand of the driver, and acts to limit the output torque according to the demand of the driver. The brake pedal travel is divided into three different levels, { high (G), medium (Z), low (D) }, as shown in fig. 5;

(4) the output motor braking duty ratio K is set as follows:

the braking force proportion k of the regenerative braking is set in the range of [0,1 ]. The fuzzy subsets are set to 5, { very High (HG), high (G), medium (Z), low (D), very low (HD) }, as shown in fig. 6;

formulating fuzzy rules as shown in table 1; and obtaining a motor braking torque ratio coefficient K. When the braking torque provided by the motor is insufficient, the electronic mechanical braking system provides residual mechanical braking force for supplement.

TABLE 1

In order to better verify and explain the technical effects adopted in the invention, the embodiment selects a braking energy recovery strategy with a fixed proportion to perform a comparison test with the method of the invention, and compares the experimental results by a scientific and rigorous means to verify the authenticity of the method of the invention.

The following are comparisons under the NEDC condition and the CLTC-P condition, respectively.

First, the NEDC operation is shown in fig. 7 and 8.

Table 1: NEDC operating mode

	SOC	Amount of reduction of SOC	Amount of energy recovery
				Control strategy of the invention	85％—82.0407％	2.9593％	792.453KJ
Fixed ratio control strategy	85％—81.4037％	3.5963％	463.56KJ

Next, the CLTC-P operating condition is shown in fig. 9 and 10.

Table 2: CLTC-P regime

	SOC	Amount of reduction of SOC	Amount of energy recovery
				Control strategy of the invention	85％—80.4734％	4.5266％	1193.59KJ
Fixed ratio control strategy	85％—80.1305％	4.8695％	738.622KJ

Therefore, in the two typical working conditions, compared with the existing fixed proportion recovery control strategy, the recovery efficiency is effectively improved, and the consumption of battery energy is reduced.

In summary, the invention provides a distributed electric drive energy recovery control strategy based on brake-by-wire, which calculates the brake strength through real-time conditions, distributes the brake force on each shaft, and then calculates the proportion of the regenerative brake force on each shaft through fuzzy control, thereby obtaining the recovered energy; compared with the prior art, the invention abandons the hydraulic brake of the traditional automobile, adopts the brake-by-wire, and utilizes the brake force distribution curve obtained by design to distribute the brake force, thereby further improving the recovery efficiency of energy and ensuring that the brake force distribution is easier to realize; according to the invention, a control strategy can be established by combining the distribution and fuzzy control of the braking force, so that the effects of improving the accuracy of the braking force and improving the driving range on the premise of ensuring the stability and safety of braking are realized.