An electro-hydraulic composite braking method and system considering stability

Abstract

The present invention proposes an electro-hydraulic composite braking method and system that takes stability into consideration, and belongs to the technical field of vehicle composite braking. When the vehicle is in a stable area, a composite braking strategy for the stable area is executed: the boundaries of the total braking torque distribution of the front axle wheels are determined; the regenerative braking torque is reserved according to the braking intensity and the braking torque required by each wheel; the front and rear axle braking torques and the regenerative braking torque of each wheel are distributed according to the boundaries with the highest recovery efficiency as the goal; when the vehicle exceeds the stable area, a composite braking strategy for the unstable area is executed: the total braking torque of each wheel is first distributed; the regenerative braking torque is reserved according to the braking intensity and the braking torque required by each wheel; and the regenerative braking torque of each wheel is distributed. The present invention coordinates hydraulic braking and electric motor braking while taking vehicle stability into consideration, so that the vehicle achieves a balance between efficient energy recovery and excellent braking performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of vehicle compound braking, and is based on an electro-hydraulic compound braking vehicle, specifically an electro-hydraulic compound braking method and system taking stability into consideration. Background Art

[0002] Electro-hydraulic hybrid braking is an advanced braking technology that combines electric motor regenerative braking with hydraulic braking. During braking, the hydraulic braking system exhibits a hysteresis effect. To address this, the electric motor's braking force is retained to compensate for the hysteresis. However, this compromises the efficiency of brake energy recovery. When the vehicle is in an unstable area, brake energy recovery efficiency is also difficult to guarantee. Summary of the Invention

[0003] The present invention addresses the deficiencies in the prior art and provides an electro-hydraulic composite braking method and system that takes vehicle stability into consideration. By coordinating hydraulic braking and electric motor braking, the vehicle achieves a balance between efficient energy recovery and excellent braking performance.

[0004] To achieve the above object, the present invention adopts the following technical solutions:

[0005] In a first aspect, the present invention provides an electro-hydraulic composite braking method taking stability into consideration, comprising:

[0006] Collect vehicle data and judge the vehicle's stability through the ω-β phase plane, where ω is the yaw rate and β is the sideslip angle of the center of mass;

[0007] When the vehicle is in the stable zone, the stable zone composite braking strategy is implemented, including: determining the boundaries for the total braking torque distribution of the front axle wheels; reserving regenerative braking torque based on the braking intensity and the braking torque required by each wheel; and distributing the front and rear axle braking torque and the regenerative braking torque of each wheel based on the boundaries with the goal of maximizing recovery efficiency.

[0008] When the vehicle exceeds the stable area, the unstable area composite braking strategy is implemented, including: firstly distributing the total braking torque to each wheel; reserving the regenerative braking torque according to the braking intensity and the braking torque required by each wheel; and then distributing the regenerative braking torque to each wheel;

[0009] After determining the hydraulic braking torque and regenerative braking torque of each wheel, the EHB and wheel hub motor are controlled to generate corresponding braking torque.

[0010] Optionally, in the stable area compound braking strategy and the unstable area compound braking strategy, if the braking intensity, vehicle speed, and battery status do not meet the requirements of regenerative braking, regenerative braking does not participate in braking, and the braking torque is entirely provided by EHB.

[0011] Optionally, the stable region of the ω-β phase plane is divided using a double straight line method, and the stable region formula is expressed as:

[0012] |ω+B1β|≤ρB2;

[0013] Where B1 and B2 are boundary parameters, ρ is the stable region reservation coefficient, and ρ<1;

[0014] Let B = |ω+B1β|, when B≤ρB2, the vehicle is in the stable region; when B>ρB2, the vehicle is out of the stable region.

[0015] Optionally, in the stable region compound braking strategy, the boundaries of the total braking torque distribution of the front axle wheels are determined according to the equal desired braking force line, the ECE regulation curve, the I curve and the f-line group, specifically:

[0016] The equal expected braking force line satisfies:

[0017] F xb =z d mg=F xb1 +F xb2 ;

[0018] Where, F xb is the total braking force, F xb1 is the front axle braking force, F xb2 is the rear axle braking force, z d is the expected braking intensity, m is the vehicle mass, and g is the acceleration due to gravity;

[0019] Curve I is the ideal braking force distribution curve, and the braking forces on the front and rear axles satisfy the following relationship:

[0020]

[0021] Where z is the braking strength, a is the distance from the center of mass to the front axle, b is the distance from the center of mass to the rear axle, L is the wheelbase, and h is the height of the center of mass.

[0022] The f line group is a set of lines formed by the change of the front and rear wheel braking force with the adhesion coefficient when the front wheel is locked first. The expression is as follows:

[0023]

[0024] Where, is the road adhesion coefficient;

[0025] The relationship between the front and rear axle braking forces that meets the ECE regulatory curve is as follows:

[0026]

[0027] The horizontal and vertical axes are F xb1 and F xb2 Draw the ECE regulation curve, I curve, f line of the current road adhesion coefficient and the equal expected braking force line in the coordinate system to obtain the front axle braking force F xb1 The boundary of (F Dxb1 ,F Exb1 ), and then the left front wheel braking torque T 11 and the right front wheel braking torque T 12 The boundary is R W is the effective radius of the wheel.

[0028] Optionally, in the stable region composite braking strategy, regenerative braking torque is reserved according to the braking intensity and the braking torque required by each wheel, specifically:

[0029] Define the motor motive force reserve constraint coefficient q:

[0030]

[0031] Where z is the vehicle braking strength, z m The maximum braking strength that the hub motor can produce;

[0032] Define the contribution of each wheel to the braking intensity c ij for:

[0033]

[0034] Where, T ij is the braking torque of each wheel, and the subscripts ij = 11, 12, 21, and 22 represent the left front wheel, right front wheel, left rear wheel, and right rear wheel, respectively;

[0035] The braking intensity is weighted by the contribution degree to obtain the reserved constraint coefficient q of the motor force of each wheel ij :

[0036]

[0037] Therefore, the regenerative braking torque T of each wheel provided by the motor is mij ≤q ij T max , T max The maximum braking torque that the motor can generate.

[0038] Optionally, in the stable region composite braking strategy, the front and rear axle braking torques and the regenerative braking torques of each wheel are distributed as follows:

[0039] When distributing the regenerative braking torque of the wheels, the objective function J is defined as:

[0040] minJ=-(η 11 +η 12 +η 21 +η 22 )

[0041] η ij =f(T mij ,n ij );

[0042] Where η ij is the energy recovery efficiency of each hub motor, which is obtained by checking the map of the hub motor. f represents the mapping function, and n ij is the speed of each wheel;

[0043] Use particle swarm algorithm to search for the optimal solution, set particle swarm X i =(T 11 ,T m11 ,T m12 ,T m21 ,T m22 ), fitness function Fitness(x) = J, and the boundary of the particle swarm is obtained according to the boundary of the total braking torque of the front axle wheels:

[0044]

[0045] Where, the left front wheel braking torque T 11 and the right front wheel braking torque T 12 The boundary is Front axle braking force F xb1 The boundary of (F Dxb1 ,F Exb1 ), R W is the effective radius of the wheel;

[0046] When the number of iterations m is reached pso When the fitness value of multiple iterations is less than the preset value, the braking torque T of each wheel is output. ij and regenerative braking torque T mij , then the hydraulic braking torque of each wheel T hij =T ij -T mij .

[0047] Optionally, in the unstable area composite braking strategy, a hierarchical control strategy is adopted to distribute the total braking torque of each wheel. The input of the upper controller is the difference between the vehicle's yaw rate and the reference yaw rate, and the difference between the center of mass sideslip angle and the reference center of mass sideslip angle, and the output is an additional yaw moment; the lower controller distributes the braking torque according to the longitudinal required braking force and the additional yaw moment.

[0048] Optionally, the lower controller distributes the braking torque according to the required longitudinal braking force and the additional yaw moment, specifically as follows:

[0049] Considering only the longitudinal force of the tire, the objective function J is set as:

[0050]

[0051] Where, F xij is the braking force of each wheel, F zij is the vertical force acting on each wheel, is the road adhesion coefficient, and the subscripts ij = 11, 12, 21, and 22 represent the left front wheel, right front wheel, left rear wheel, and right rear wheel, respectively;

[0052] The following constraints are met when distributing the braking force:

[0053]

[0054] Where, F x is the total longitudinal force of the vehicle, z d is the expected braking strength, m is the vehicle mass, g is the acceleration of gravity, D is the wheel track, T Max is the maximum braking torque that the brake unit can generate, R W is the effective radius of the wheel;

[0055] Combine the objective function and the constraints to obtain the braking force F of each wheel xij , and then calculate the braking torque T of each wheel ij =F xij R W .

[0056] Optionally, in the unstable region composite braking strategy, the regenerative braking torque of each wheel is distributed as follows:

[0057] According to the braking intensity and the braking torque T of each wheel ij Get the reserved constraint coefficient q of each wheel motor motive force ij , search for the regenerative braking torque of each wheel with the highest recovery efficiency through the particle swarm algorithm; select the particle swarm X i =(T m11 ,T m12 ,T m21 ,T m22 ), fitness function Fitness(x)=-(η 11 +η 12 +η 21 +η 22 ), T mij is the regenerative braking torque of each wheel, η ijis the energy recovery efficiency of each hub motor, and the boundary of the particle swarm is:

[0058]

[0059] When the number of iterations m is reached pso When the fitness value of multiple iterations is less than the preset value, the regenerative braking torque T of each wheel is output. mij , then the hydraulic braking torque of each wheel T hij =T ij -T mij .

[0060] In a second aspect, the present invention provides an electro-hydraulic composite braking system taking stability into consideration, comprising: a vehicle data acquisition unit, a vehicle stability monitoring unit, a yaw moment control unit, a front axle braking torque distribution boundary acquisition unit, a regenerative braking torque reservation unit, a regenerative braking torque distribution unit, and a vehicle execution unit;

[0061] The vehicle data collection unit collects vehicle data;

[0062] The vehicle stability monitoring unit determines the stability of the vehicle through the ω-β phase plane, where ω is the yaw rate and β is the sideslip angle of the center of mass;

[0063] When the vehicle is in a stable region, the front axle braking torque distribution boundary acquisition unit determines a boundary for distributing the total braking torque to the front axle wheels; the regenerative braking torque reservation unit reserves the regenerative braking torque based on the braking intensity and the required braking torque of each wheel; and the regenerative braking torque distribution unit distributes the front and rear axle braking torques and the regenerative braking torque of each wheel based on the boundary with the goal of achieving the highest recovery efficiency.

[0064] When the vehicle exceeds the stable area, the yaw moment control unit first distributes the total braking torque to each wheel, the regenerative braking torque reservation unit reserves the regenerative braking torque according to the braking intensity and the braking torque required by each wheel, and the regenerative braking torque distribution unit then distributes the regenerative braking torque to each wheel;

[0065] After determining the hydraulic braking torque and regenerative braking torque of each wheel, the vehicle execution unit controls the EHB and wheel hub motor to generate corresponding braking torque.

[0066] The beneficial effects of the present invention are:

[0067] The present invention takes into account the situation where the vehicle is in an unstable area when distributing the braking force. The total braking torque of each wheel is distributed through the yaw moment control unit, and the regenerative braking torque of each wheel is distributed through the regenerative braking torque distribution unit. This not only improves the safety of the vehicle, but also ensures the efficiency of braking energy recovery in the unstable area.

[0068] When the vehicle is in a stable area, the present invention dynamically distributes the braking force of the front and rear axle wheels based on conditions such as the braking force demand, the ECE regulatory curve, the I curve and the f-line group, thereby expanding the range of distributable regenerative braking torque and thereby improving the efficiency of braking energy recovery.

[0069] When reserving the motor braking force, the present invention proposes a motor braking force reservation constraint coefficient for each wheel based on the contribution of each wheel braking torque to the vehicle braking strength, which can more effectively compensate for the hysteresis effect of the hydraulic system and enable the actual braking force to track the expected braking force more quickly.

[0070] The present invention adopts a particle swarm optimization algorithm to distribute the regenerative braking torque, and has the advantages of fast convergence speed, strong robustness, good dynamic adaptability, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0071] Figure 1 This is a flow chart of an electro-hydraulic composite braking method taking stability into consideration.

[0072] Figure 2 Schematic diagram of the ECE regulation curve, I curve, f-line of the current road adhesion coefficient, and the expected braking force line. DETAILED DESCRIPTION

[0073] The present invention will now be described in further detail with reference to the accompanying drawings.

[0074] Example 1

[0075] This embodiment proposes an electro-hydraulic composite braking system that takes stability into consideration, including a vehicle data acquisition unit, a vehicle stability monitoring unit, a road surface recognition unit, a braking intention recognition unit, a yaw moment control unit, a front axle braking torque distribution boundary acquisition unit, a regenerative braking torque reservation unit, a regenerative braking torque distribution unit, and a vehicle execution unit.

[0076] Example 2

[0077] Based on the first embodiment, this embodiment proposes an electro-hydraulic composite braking method considering stability, such as Figure 1 The specific steps are as follows:

[0078] Step 1: The vehicle data acquisition unit collects the data required by this method, such as brake pedal travel, steering wheel angle, vehicle yaw rate, center of mass sideslip angle, longitudinal speed, lateral speed, wheel speed, etc., and sends the collected data to the corresponding data processing unit for calculation and processing.

[0079] Step 2: The vehicle stability monitoring unit judges the stability through the ω-β phase plane and classifies the compound braking strategy according to the stability, which is divided into a stable area compound braking strategy and an unstable area compound braking strategy.

[0080] Step 3: Identify the braking intention based on fuzzy control, take the brake pedal stroke and brake pedal operation speed as the input of the braking intention identification unit, and take the expected braking intensity z as the input of the braking intention identification unit. d As the output of the braking intention recognition unit.

[0081] Step 4: The road surface recognition unit processes the information collected by the vehicle data acquisition unit to obtain the current road surface adhesion coefficient

[0082] Step 5.1: When in the stable area, the front axle braking torque distribution boundary acquisition unit determines the boundaries of the total braking torque distribution of the front axle wheels based on the braking force demand (equal expected braking force line), ECE regulatory curve, I curve and f-line group; the regenerative braking torque reservation unit reserves the regenerative braking torque based on the braking intensity and the required braking torque of each wheel; the regenerative braking torque distribution unit dynamically distributes the front and rear axle braking torques and the regenerative braking torque of each wheel based on the above boundaries with the highest recovery efficiency as the goal.

[0083] If the braking intensity, vehicle speed and battery status do not meet the requirements of regenerative braking, regenerative braking will not be involved in braking, and the braking torque will be provided entirely by EHB.

[0084] Step 5.2: When the vehicle exceeds the stable area, the yaw moment control unit first distributes the total braking torque to each wheel; the regenerative braking torque reservation unit reserves the regenerative braking torque based on the braking intensity and the braking torque required by each wheel; and the regenerative braking torque distribution unit then distributes the regenerative braking torque to each wheel to improve the efficiency of energy recovery.

[0085] If the braking intensity, vehicle speed and battery status do not meet the requirements of regenerative braking, regenerative braking will not be involved in braking, and the braking torque will be provided entirely by EHB.

[0086] Step 6: After determining the magnitude of the hydraulic braking torque and regenerative braking torque of each wheel, the vehicle execution unit controls the EHB and the hub motor to generate the corresponding braking torque.

[0087] Furthermore, the total desired braking torque, the hydraulic braking torque, and the regenerative braking torque are described.

[0088] Total expected braking torque:

[0089] T d =z d ikB W =TH +T E

[0090] Where m is the mass of the vehicle, g is the acceleration due to gravity, and R W is the effective radius of the wheel, T H is the total hydraulic braking torque, T E is the total regenerative braking torque.

[0091] Braking force of each wheel T ij :

[0092] T ij =F xij R W =T mij +T hij

[0093] Where, F xij is the wheel braking force, T mij is the motor torque, T hij is the hydraulic braking torque. (Subscript i = 1, 2 represents front and rear, subscript j = 1, 2 represents left and right, 11 represents the left front wheel, 12 represents the right front wheel, 21 represents the left rear wheel, and 22 represents the right rear wheel).

[0094] Regenerative braking torque T mij Provided by the hub motor, the hydraulic braking torque T hij Provided by EHB. Whether regenerative braking is involved in braking depends on factors such as battery SOC value, vehicle speed v, and braking intensity z.

[0095] When the braking intensity z≥0.7, it is emergency braking, which requires strong braking force. The motor braking force cannot meet the instantaneous deceleration requirements, and only EHB braking is used, and the motor does not provide braking torque; when the braking intensity z<0.7, the motor participates in braking. The braking intensity constraint coefficient k1 is:

[0096]

[0097] When the SOC is too high, continuing to charge the battery will affect the battery life, and the motor torque is usually not used. The SOC constraint coefficient k2 is:

[0098]

[0099] Where Q SOC is the SOC value.

[0100] When the vehicle speed is low, the power generation generated by the motor driving force is very small. Considering the power generation efficiency, the vehicle speed constraint coefficient k3 is:

[0101]

[0102] In summary, the braking torque T of each wheel ij =k1k2k3T mij +T hij .

[0103] Furthermore, the I curve, f line group, ECE regulation curve, and equal expected braking force line are described in detail.

[0104] The equal expected braking force line satisfies:

[0105] F xb =z d mg=F xb1 +F xb2

[0106] Where, F xb is the total braking force, F xb1 is the front axle braking force, F xb2 is the rear axle braking force, z d is the expected braking intensity.

[0107] Curve I is the ideal braking force distribution curve, which can ensure that the front and rear wheels are locked at the same time and maximize the use of the road adhesion coefficient. The ideal braking force distribution of the front and rear axle braking forces should meet the following relationship:

[0108]

[0109] Where z is the braking strength, a is the distance from the center of mass to the front axle, b is the distance from the center of mass to the rear axle, L is the wheelbase, and h is the height of the center of mass.

[0110] The f line group is a set of lines formed by the change of the front and rear wheel braking force with the adhesion coefficient when the front wheel is locked first. The expression is as follows:

[0111]

[0112] Where, is the road adhesion coefficient.

[0113] ECE regulations set specific requirements for the distribution of braking forces between the front and rear axles to ensure the braking performance and safety of the vehicle under various road conditions. The relationship between the front and rear axle braking forces that meets the regulations is as follows:

[0114]

[0115] Next, step 2 is described in detail.

[0116] The vehicle stability monitoring unit uses the ω-β phase plane to determine stability. The stable region of the ω-β phase plane is divided using the double straight line method, and the stable region formula is expressed as:

[0117] |ω+B1β|≤ρB2

[0118] Where coefficients B1 and B2 are boundary parameters, ρ is the stable region reserve coefficient, and ρ<1. ω is the yaw rate, and β is the sideslip angle of the center of mass.

[0119] Let B = |ω+B1β|. When B≤ρB2, the vehicle is considered to be in the stable area and the stable area composite braking strategy is adopted; when B>ρB2, the vehicle is considered to be in the unstable area and the unstable area composite braking strategy is adopted.

[0120] Next, step 3 is described in detail.

[0121] Braking intention recognition uses a triangular function to fuzzify the input variables. The fuzzy subsets for brake pedal travel are (L, M, H), brake pedal speed are (L, M, H), and braking intensity are (L, M, H). The defuzzification process uses a weighted average method to calculate the exact braking intensity. The specific fuzzy rules are shown in Table 1.

[0122] Table 1 Fuzzy rules

[0123]

[0124] Next, step 5.1 is described in detail.

[0125] The specific contents of the stable area compound braking strategy are as follows:

[0126] When the braking intensity z<0.7 and k1k2k3=1, the braking forces of the front and rear axles are dynamically distributed on the equal braking deceleration line, and the braking forces at the left and right ends of the front and rear axles are distributed equally.

[0127] F xb =z d mg=F xb1 +F xb2

[0128] F xb1 =F x11 +F x12

[0129] F xb2 =F x21 +F x22

[0130] F x11 =F x12

[0131] F x21 =F x22

[0132] Where, Fx11 is the longitudinal force of the left front wheel, F x12 is the longitudinal force of the right front wheel, F x21 is the longitudinal force of the left rear wheel, F x22 is the longitudinal force on the right rear wheel.

[0133] When the braking intensity z ≥ 0.7, the vehicle enters the emergency braking state. To maximize the road adhesion coefficient, the front and rear axle braking torques are distributed according to the I curve. At this time, the motor brake is disengaged, and the vehicle's braking force is provided solely by the EHB. When k1k2k3 = 0, the braking torques of each wheel are also distributed according to the following relationship:

[0134]

[0135] Furthermore, the acquisition of the total braking torque distribution boundary of the front and rear axle wheels is described in detail.

[0136] Draw the ECE regulation curve, I curve, f line of the current road adhesion coefficient, and the expected braking force line. Figure 2 shown.

[0137] The intersection of the equal expected braking force line and the lower part of the OABC area is E, and it is assumed that xb1 The intersection points of the axis, the ECE regulation curve, and the f-line are E1, E2, and E3 respectively. The intersection point of the constant expected braking force line and the I curve is D.

[0138] To meet ECE regulations and prevent wheel lock, the front and rear axle braking forces must be distributed within the OABC zone. Furthermore, to achieve the desired braking force, the front and rear axle braking forces must adhere to the desired braking force line. This means that the front and rear braking forces must be distributed along line segment DE.

[0139] By combining the expected braking force line and the line segment OABC, the coordinates of points D and E can be obtained. The horizontal coordinates of points D and E are recorded as F Dxb1 ,F Exb1 .

[0140] Then the front axle braking force F xb1 The boundary of (F Dxb1 ,F Exb1 ), the corresponding rear axle braking force F xb2 =z d mg-F xb1 .

[0141] The braking torque of each wheel is:

[0142]

[0143] Where, T 11 is the left front wheel braking torque, T 12 is the right front wheel braking torque, T21 is the left rear wheel braking torque, T 22 is the braking torque of the right rear wheel.

[0144] T 11 The boundary is

[0145] Furthermore, the regenerative braking torque reserve unit is described in detail.

[0146] During braking, the hydraulic braking system has a hysteresis effect. When the deceleration changes instantaneously, the total braking torque can still follow the target value, but there will still be a certain difference. The fast response and high precision characteristics of the motor braking system can be used to compensate for the hysteresis effect of hydraulic braking and ensure that the actual braking force quickly matches the target value. To achieve this goal, the motor's braking torque must be reserved. As the braking intensity increases, the reserved motor braking torque should also increase. Define the motor braking torque reserve constraint coefficient q:

[0147]

[0148] Where z m The maximum braking strength that the hub motor can produce.

[0149] Considering the braking torque T of each wheel of the vehicle ij The distribution is not the same, the braking torque T of each wheel ij The contribution to the vehicle's braking strength z is different. When reserving the electric braking force, the contribution should be taken into account. The higher the contribution to the braking strength, the more sufficient the electric braking force of the wheel should be reserved. Define the contribution of each wheel c ij for:

[0150]

[0151] The braking intensity is weighted by the contribution degree to obtain the reserved constraint coefficient q of the motor force of each wheel ij :

[0152]

[0153] In summary, the regenerative braking torque T provided by the motor mij ≤q ij T max , T max The maximum braking torque that the motor can generate.

[0154] Furthermore, the regenerative braking torque distribution unit is described in detail.

[0155] When distributing the regenerative braking torque to the wheels, the primary goal is to maximize the total braking energy recovery efficiency. The objective function can be defined as:

[0156] minJ=-(η 11 +η 12 +η 21 +η 22 )

[0157] η ij =f(T mij ,n ij )

[0158] Where η ij is the energy recovery efficiency of each hub motor, which can be obtained by looking up the hub motor map. ij =f(T mij ,n ij ), n ij is the speed of each wheel.

[0159] This embodiment uses a particle swarm algorithm to search for the optimal solution. The specific steps of the particle swarm algorithm are as follows.

[0160] In the D-dimensional target search space, there are N particles forming a community, where the i-th particle is represented as a D-dimensional vector:

[0161] X i =(x i1 ,x i2 ,…,x iD )i=1,2,…,N

[0162] The flight speed of the i-th particle is also a D-dimensional vector. The speed represents the direction and distance the particle moves in the next iteration, which is recorded as:

[0163] V i =(v i1 ,v i2 ,…,v iD )i=1,2,…,N

[0164] Set the fitness function Fitness(x) = J, take the optimization target as the fitness function, the smaller the function value, the higher the fitness. The fitness value of the optimal position searched by the i-th particle is f i,pb , the fitness value of the optimal position searched by the group is f gb .

[0165] The optimal position searched by the i-th particle is also the individual optimal solution, which is recorded as:

[0166] p i,pb =(p i1 ,p i2 ,…,p iD )

[0167] The optimal position searched by the group is also the group optimal solution, which is recorded as:

[0168] p gb =(p 1,gb ,p 2,gb ,…,p D,gb )

[0169] When the i-th particle in the t-th generation evolves to the t+1-th generation, it is updated according to the following formula:

[0170] V i (t+1)=wV i (t)+c1r1(t)[p i,pb (t)-X i (t)]+c2r2(t)[p gb (t)-X i (t)]

[0171] X i (t+1)=X i (t)+V i (t+1)

[0172] Where w is the inertia weight, c1 is the self-learning factor, c2 is the group learning factor, and r1(t) and r2(t) are random.

[0173] Particle Swarm X i =(T 11 ,T m11 ,T m12 ,T m21 ,T m22 ), initialize particle parameters, including particle swarm size N, particle swarm position Xi, particle swarm velocity V i , number of iterations m pso , inertia weight w, self and group learning factors c1, c2, etc.

[0174] According to the front axle braking torque distribution boundary acquisition unit and the regenerative braking torque reservation unit, the boundary of the particle swarm can be obtained as follows:

[0175]

[0176] When the number of iterations m is reached pso When the adaptation value of multiple iterations is less than the preset value, the optimal solution is output. That is, the braking torque T of each wheel that makes the total recovery efficiency the highest is output. ij and the regenerative braking torque T of each wheel mij , hydraulic braking torque of each wheel T hij =T ij -T mij , the vehicle execution unit controls the EHB and wheel hub motor to provide corresponding braking torque.

[0177] Furthermore, step 5.2 is described in detail.

[0178] When the vehicle exceeds the stable area, the yaw moment control unit first distributes the total braking torque to each wheel to ensure vehicle safety, and then distributes the regenerative braking torque.

[0179] In this embodiment, the yaw moment control unit adopts a hierarchical control strategy. The upper-level controller is based on model predictive control. The input is the difference between the vehicle's yaw angular velocity and center of mass sideslip angle and the reference yaw angular velocity and center of mass sideslip angle. The output is an additional yaw moment. The lower-level controller distributes the braking torque according to the required longitudinal braking force and the required yaw moment.

[0180] Upper controller

[0181] Establish a two-degree-of-freedom model of the vehicle:

[0182]

[0183] Where, v x is the longitudinal velocity of the vehicle, v y is the lateral speed of the car; k1 and k2 are the front and rear wheel cornering stiffnesses respectively; δ f represents the front wheel turning angle; ω is the vehicle's yaw rate, β is the center of mass side slip angle, I z Represents the vehicle's moment of inertia around the Z axis, M Z To correct the yaw moment.

[0184] Selecting the state variables as the sideslip angle β and the yaw rate ω, the state space equation is as follows:

[0185]

[0186] State variable x = [βω] T , output y = [βω] T , control quantity u=M Z , front wheel turning angle δ f As a known quantity, it can be used as system disturbance and written as the following state equation:

[0187]

[0188] Where,

[0189]

[0190] Discretize the above continuous system, and the sampling time t s , the discretized state space expression is:

[0191]

[0192] In order to reduce or eliminate the static error, an integral term is introduced and the state space equation is rewritten into the following incremental form:

[0193]

[0194] Among them, Δx(k)=x(k)-x(k-1), Δu(k)=u(k)-u(k-1), ΔΨ(k)=Ψ(k)-Ψ(k-1).

[0195] Assume that the prediction time domain is P, the control time domain is m, and the control time domain does not exceed the prediction time domain. Outside the control time domain, Δu(k+i)=0(i≥m). Assuming that the external input interference remains unchanged within the prediction time domain P, that is, ΔΨ(k+i)=0(i=1,2,…,P-1), the P-step prediction output vector and the m-step control input vector can be defined as follows:

[0196] Y P (k+1|k)=[y(k+1|k) y(k+2|k)…y(k+P|k)] T

[0197] ΔU(k+1|k)=[Δu(k+1) Δu(k+2)…Δu(k+m-1)] T

[0198] Where y(k+i|k) is the output of the system at time k+i predicted at time k, and Δu(k+i) is the change in system control at time k+i.

[0199] According to the incremental state space equation, the P-step prediction output Y can be derived P The expression for (k+1|k):

[0200] Y P (k+1|k)=S x Δx(k)+Ν y y(k)+S u ΔU(k)+S Ψ ΔΨ(k)

[0201] Where:

[0202]

[0203] Where I is the unit vector, S x ,Ν y ,S u ,S Ψ Y P The coefficient matrix of (k+1|k).

[0204] Define the reference sequence as follows:

[0205] R(k+1)=[r(k+1) r(k+2)…r(k+P)] T

[0206] Where r(k+i) represents the reference value at time k+i, r(k+i)=[β r ω r ] T , β r is the reference center of mass sideslip angle, ω r is the reference yaw rate.

[0207] During steady-state driving, the vehicle's center of mass lateral slip angular velocity is Yaw angular acceleration Substituting into the above state space equation we get:

[0208]

[0209] Where, is the stability coefficient, β d is the ideal center of mass sideslip angle, ω d is the ideal yaw rate.

[0210] The reference center of mass slip angle is set to 0 to ensure the stability of the vehicle at high speed. Considering that the road adhesion condition will impose certain constraints on the yaw rate, the reference center of mass slip angle and reference yaw rate used for optimization can be expressed as:

[0211]

[0212] In order to achieve the ideal control effect, the actual yaw rate and sideslip angle should be made to coincide with the corresponding ideal values ​​as quickly as possible. At the same time, the control amplitude of the vehicle yaw moment should be taken into account. The objective function is selected as follows:

[0213]

[0214] Among them, Γ y,i is the weight matrix of the output error predicted in the i-th step, Γ u,i is the weight matrix of the control increment predicted at step i, Γ y,i ,Γ u,i are all diagonal matrices.

[0215] Set the constraints as follows:

[0216]

[0217] Among them, the subscripts max and min correspond to the maximum and minimum values ​​respectively.

[0218] Rewrite the objective function into matrix form:

[0219] J(x(k),ΔU(k),m,P)=||Γ y (Y P (k+1|k)-R(k+1))|| 2 +||Γ u ΔU(k)|| 2

[0220] Among them, Γ y =diag(Γ y,1 ,Γ y,2 ,…,Γ y,P ), Γ u =diag(Γ u,1 ,Γ u,2 ,…,Γ u,P ).

[0221] The optimal control sequence ΔU can be obtained * (k) is:

[0222]

[0223] E P (k+1|k)=R(k+1)-S x Δx(k)-Ν y y(k)-S Ψ ΔΨ(k)

[0224] Take ΔU * The first item in the (k) sequence gives the current control input, i.e. u(k) = u(k-1) + Δu(k), which is the current required yaw moment value M. Z .

[0225] Lower-level controller

[0226] Tire utilization can characterize the stability margin of the tire and can be selected as the optimization target. Since the lateral force of the tire is uncontrollable, only the longitudinal force of the tire is considered, and the objective function is:

[0227]

[0228] Where, F zij is the vertical force acting on each wheel.

[0229] The following constraints must be met when distributing the braking force:

[0230]

[0231] Where, F x is the total longitudinal force of the vehicle, D is the wheel track, T Max It is the maximum braking torque that the brake unit can generate.

[0232] Combining the objective function and the constraints, the braking force F required for each wheel can be obtained. xij , and then calculate the braking torque T required for each wheel ij , T ij =F xij R W .

[0233] If k1k2k3=0, the regenerative braking condition is not met and the braking torque is provided entirely by EHB, i.e. T ij =T hij If k1k2k3=1, the regenerative braking condition is met, and the distribution strategy of the regenerative braking torque is as follows:

[0234] The regenerative braking torque reserve unit is based on the braking intensity and the braking torque of each wheel T ij Get the reserved constraint coefficient q of each wheel motor motive force ij .

[0235] Furthermore, the regenerative braking torque distribution process is described in detail.

[0236] Through the particle swarm algorithm, we search for the regenerative braking torque of each wheel with the highest recovery efficiency. i =(T m11 ,T m12 ,T m21 ,T m22 ), fitness function Fitness(x)=J=-(η 11 +η 12 +η 21 +η 22 ), initialize particle parameters, including particle swarm size N, particle swarm position X i , particle swarm velocity V i , number of iterations m pso , inertia weight w, self and group learning factors c1, c2.

[0237] According to the regenerative braking torque reserved unit, the boundary of the particle swarm can be obtained as follows:

[0238]

[0239] When the number of iterations m is reached pso When the adaptation value of multiple iterations is less than the preset value, the optimal solution is output. That is, the regenerative braking torque T of each wheel with the highest efficiency is output. mij , hydraulic braking torque of each wheel T hij =T ij -T mij , the vehicle execution unit controls the EHB and wheel hub motor to provide corresponding braking torque.

[0240] The above are merely preferred embodiments of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions based on the principles of the present invention are within the scope of protection of the present invention. It should be noted that for those skilled in the art, various improvements and modifications that do not depart from the principles of the present invention should be considered within the scope of protection of the present invention.

Claims

1. An electro-hydraulic composite braking method considering stability, characterized in that: include: Collect vehicle data and judge the vehicle's stability through the ω-β phase plane, where ω is the yaw rate and β is the sideslip angle of the center of mass; When the vehicle is in the stable region, a stable region composite braking strategy is executed, including: determining the boundaries for the total braking torque distribution of the front axle wheels; reserving regenerative braking torque based on the braking intensity and the required braking torque of each wheel; and distributing the front and rear axle braking torques and the regenerative braking torque of each wheel based on the boundaries with the goal of maximizing recovery efficiency. In the stable region composite braking strategy, the boundaries for the total braking torque distribution of the front axle wheels are determined based on the equal desired braking force line, the ECE regulation curve, the I curve, and the f-line group, specifically: The equal expected braking force line satisfies: F xb =z d mg=F xb1 +F xb2 ; Where, F xb is the total braking force, F xb1 is the front axle braking force, F xb2 is the rear axle braking force, z d is the expected braking intensity, m is the vehicle mass, and g is the acceleration due to gravity; Curve I is the ideal braking force distribution curve, and the braking forces on the front and rear axles satisfy the following relationship: Where z is the braking strength, a is the distance from the center of mass to the front axle, b is the distance from the center of mass to the rear axle, L is the wheelbase, and h is the height of the center of mass. The f line group is a set of lines formed by the change of the front and rear wheel braking force with the adhesion coefficient when the front wheel is locked first. The expression is as follows: Where, is the road adhesion coefficient; The relationship between the front and rear axle braking forces that meets the ECE regulatory curve is as follows: The horizontal and vertical axes are F xb1 and F xb2 Draw the ECE regulation curve, I curve, f line of the current road adhesion coefficient and the equal expected braking force line in the coordinate system to obtain the front axle braking force F xb1 The boundary of (F Dxb1 ,F Exb1 ), and then the left front wheel braking torque T 11 and the right front wheel braking torque T 12 The boundary is R W is the effective radius of the wheel; When the vehicle exceeds the stable area, the unstable area composite braking strategy is implemented, including: firstly distributing the total braking torque to each wheel; reserving the regenerative braking torque according to the braking intensity and the braking torque required by each wheel; and then distributing the regenerative braking torque to each wheel; After determining the hydraulic braking torque and regenerative braking torque of each wheel, the EHB and wheel hub motor are controlled to generate corresponding braking torque.

2. The electro-hydraulic composite braking method considering stability according to claim 1, characterized in that: In the stable area compound braking strategy and the unstable area compound braking strategy, if the braking intensity, vehicle speed, and battery status do not meet the requirements of regenerative braking, regenerative braking will not participate in braking, and the braking torque will be provided entirely by the EHB.

3. The electro-hydraulic composite braking method considering stability according to claim 1, characterized in that: The stable region of the ω-β phase plane is divided using the double straight line method, and the stable region formula is expressed as: |ω+B1β|≤ρB2; Where B1 and B2 are boundary parameters, ρ is the stable region reservation coefficient, and ρ < 1; Let B = |ω + B1β|, when B ≤ ρB2, the vehicle is in the stable region; when B > ρB2, the vehicle is out of the stable region.

4. The electro-hydraulic composite braking method considering stability according to claim 1, characterized in that: In the stable region composite braking strategy, the regenerative braking torque is reserved according to the braking intensity and the braking torque required by each wheel, specifically: Define the motor motive force reserve constraint coefficient q: Where z is the vehicle braking strength, z m The maximum braking strength that the hub motor can produce; Define the contribution of each wheel to the braking intensity c ij for: Where, T ij is the braking torque of each wheel, and the subscripts ij = 11, 12, 21, and 22 represent the left front wheel, right front wheel, left rear wheel, and right rear wheel, respectively; The braking intensity is weighted by the contribution degree to obtain the reserved constraint coefficient q of the motor force of each wheel ij : Therefore, the regenerative braking torque T of each wheel provided by the motor is mij ≤q ij T max , T max The maximum braking torque that the motor can generate.

5. The electro-hydraulic composite braking method considering stability according to claim 4, characterized in that: In the stable region composite braking strategy, the front and rear axle braking torques and the regenerative braking torques of each wheel are distributed as follows: When distributing the regenerative braking torque of the wheels, the objective function J is defined as: Where η ij is the energy recovery efficiency of each hub motor, which is obtained by checking the map of the hub motor. f represents the mapping function, and n ij is the speed of each wheel; Use particle swarm algorithm to search for the optimal solution, set particle swarm X i =(T 11 ,T m11 ,T m12 ,T m21 ,T m22 ), fitness function Fitness(x) = J, and the boundary of the particle swarm is obtained according to the boundary of the total braking torque of the front axle wheels: Where, the left front wheel braking torque T 11 and the right front wheel braking torque T 12 The boundary is Front axle braking force F xb1 The boundary of (F Dxb1 ,F Exb1 ), R W is the effective radius of the wheel; When the number of iterations m is reached pso When the fitness value of multiple iterations is less than the preset value, the braking torque T of each wheel is output. ij and regenerative braking torque T mij , then the hydraulic braking torque of each wheel T hij =T ij -T mij .

6. The electro-hydraulic composite braking method considering stability according to claim 1, characterized in that: In the unstable region composite braking strategy, a hierarchical control strategy is employed to distribute the total braking torque to each wheel. The upper-layer controller receives as input the difference between the vehicle's yaw velocity and a reference yaw velocity, as well as the difference between the center of mass slip angle and a reference center of mass slip angle, and outputs an additional yaw moment. The lower-layer controller distributes the braking torque based on the required longitudinal braking force and the additional yaw moment.

7. The electro-hydraulic composite braking method considering stability according to claim 6, characterized in that: The lower controller distributes the braking torque according to the required longitudinal braking force and the additional yaw moment, specifically: Considering only the longitudinal force of the tire, the objective function J is set as: Where, F xij is the braking force of each wheel, F zij is the vertical force acting on each wheel, is the road adhesion coefficient, and the subscripts ij = 11, 12, 21, and 22 represent the left front wheel, right front wheel, left rear wheel, and right rear wheel, respectively; The following constraints are met when distributing the braking force: Where, F x is the total longitudinal force of the vehicle, z d is the expected braking strength, m is the vehicle mass, g is the acceleration of gravity, D is the wheel track, T Max is the maximum braking torque that the brake unit can generate, R W is the effective radius of the wheel; Combine the objective function and the constraints to obtain the braking force F of each wheel xij , and then calculate the braking torque T of each wheel ij =F xij R W .

8. The electro-hydraulic composite braking method considering stability according to claim 7, characterized in that: In the unstable region composite braking strategy, the regenerative braking torque of each wheel is distributed as follows: According to the braking intensity and the braking torque T of each wheel ij Get the reserved constraint coefficient q of each wheel motor motive force ij , search for the regenerative braking torque of each wheel with the highest recovery efficiency through the particle swarm algorithm; select the particle swarm X i =(T m11 ,T m12 ,T m21 ,T m22 ), fitness function Fitness(x)=-(η 11 +η 12 +η 21 +η 22 ), T mij is the regenerative braking torque of each wheel, η ij is the energy recovery efficiency of each hub motor, and the boundary of the particle swarm is: When the number of iterations m is reached pso When the fitness value of multiple iterations is less than the preset value, the regenerative braking torque T of each wheel is output. mij , then the hydraulic braking torque of each wheel T hij =T ij -T mij .

9. An electro-hydraulic composite braking system considering stability, used for implementing the electro-hydraulic composite braking method considering stability as claimed in any one of claims 1 to 8, characterized in that: include: Vehicle data acquisition unit, vehicle stability monitoring unit, yaw moment control unit, front axle braking torque distribution boundary acquisition unit, regenerative braking torque reservation unit, regenerative braking torque distribution unit and vehicle execution unit; The vehicle data collection unit collects vehicle data; The vehicle stability monitoring unit determines the stability of the vehicle through the ω-β phase plane, where ω is the yaw rate and β is the sideslip angle of the center of mass; When the vehicle is in a stable region, the front axle braking torque distribution boundary acquisition unit determines a boundary for distributing the total braking torque to the front axle wheels; the regenerative braking torque reservation unit reserves the regenerative braking torque based on the braking intensity and the required braking torque of each wheel; and the regenerative braking torque distribution unit distributes the front and rear axle braking torques and the regenerative braking torque of each wheel based on the boundary with the goal of achieving the highest recovery efficiency. When the vehicle exceeds the stable area, the yaw moment control unit first distributes the total braking torque to each wheel, the regenerative braking torque reservation unit reserves the regenerative braking torque according to the braking intensity and the braking torque required by each wheel, and the regenerative braking torque distribution unit then distributes the regenerative braking torque to each wheel; After determining the hydraulic braking torque and regenerative braking torque of each wheel, the vehicle execution unit controls the EHB and wheel hub motor to generate corresponding braking torque.

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