Multi-operating-condition compound braking control method for electric vehicles based on stability

Abstract

The present invention discloses a multi-operating-condition composite braking control method for electric vehicles based on stability. Based on information such as the driver's required braking intensity, battery state of charge, and motor speed, an optimization objective function is introduced to determine the distribution ratio of electric and hydraulic braking between the front and rear axles. The vehicle's yaw angular velocity is used to determine whether the vehicle is braking on an oncoming road. If so, the ABS is switched to low-select control, while the motor provides braking torque to maintain vehicle stability. Otherwise, the ABS is switched to single-wheel control mode. Normal brake pressure control and brake disc speed control are used to suppress brake pad stick-slip vibration during friction braking at a constant speed downhill. Using this composite braking method to brake an electric vehicle can achieve tracking of the driver's braking force requirements at a relatively low cost. It also balances vehicle braking stability and energy recovery during emergency braking on an oncoming road. It also suppresses stick-slip vibration during constant speed downhill braking, reducing brake pad wear.

Description

Technical Field

[0001] The present invention relates to the technical field of automobile braking, and in particular to a multi-operating-condition composite braking control method for an electric vehicle based on stability. Background Art

[0002] While electric vehicles can easily implement brake energy recovery, motor regenerative braking cannot independently meet the braking stability and performance requirements of electric vehicles under different braking conditions due to the influence of vehicle speed, motor power, battery state of charge, and electric braking force. To achieve brake energy recovery while meeting the braking stability requirements of the electric vehicle and the braking performance requirements of the driver, existing electric vehicles often use a hybrid braking system that combines an electric motor braking system with a hydraulic braking system.

[0003] Based on the configuration between the electric motor force and the hydraulic brake force, the existing electric vehicle compound braking system is generally divided into a parallel compound braking system and a series compound braking system.

[0004] A parallel compound braking system directly superimposes electric braking on traditional friction braking. Both friction and electric braking are linearly proportional to the brake pedal's opening angle, with no coordination between them. Consequently, the total braking force output exceeds the driver's desired braking force. This braking force control method can affect vehicle braking stability and reduce braking feel, but it is easy to implement and has low development costs.

[0005] A tandem compound braking system coordinates friction and electric braking, typically aiming to maximize regenerative braking energy recovery efficiency or optimize braking stability. Its total braking force follows the driver's desired braking force, resulting in good braking stability. However, this system requires electronic control of the hydraulic braking force, which results in high development costs.

[0006] Based on the braking conditions, the compound braking of electric vehicles can be divided into the compound braking control method for conventional braking conditions and the compound braking control method for emergency braking conditions.

[0007] The design focus of the composite braking control method for conventional braking conditions is the method of distributing the braking force between the front and rear axles. Under normal traffic conditions, 95% of drivers generally do not exceed a braking deceleration of 3.5 m / s on dry roads. 2 Therefore, the compound braking control method for conventional braking conditions is extremely important.

[0008] The design of a composite braking control method for emergency braking conditions focuses on anti-lock braking. If the front wheels lock before the rear wheels, the vehicle loses steering ability; if the rear wheels lock before the front wheels, it can easily cause the vehicle to spin out, an unstable condition. Traditional vehicles use ABS control systems to address these conditions. During emergency braking on a split road, ABS systems often employ low-select control, controlling the tire slip rate based on the road surface with the lower adhesion coefficient. This method can effectively bring the vehicle to a safe stop, but the disadvantage is a longer braking distance.

[0009] When a vehicle travels downhill at a constant speed, the brake pads of traditional hydraulic brakes experience stick-slip vibrations due to the intermittent changes in dynamic and static friction, as well as the negative correlation of the friction coefficient with relative speed. This in turn generates low-frequency noise, exacerbating brake pad wear and shortening its service life. Existing literature often passively controls stick-slip vibrations by placing damping devices. However, due to the limited space within the vehicle's braking system, this approach is impractical and has high development costs. Summary of the Invention

[0010] Based on the technical problems existing in the background technology, the present invention proposes a multi-condition composite braking control method for electric vehicles based on stability, which can track the driver's braking force requirements at a lower cost; take into account the stability and energy recovery of vehicle braking under emergency braking on oncoming roads; and suppress stick-slip vibration and reduce brake pad wear during uniform downhill descent.

[0011] The present invention proposes a multi-operating-condition composite braking control method for electric vehicles based on stability, and the method steps are as follows:

[0012] S1: When braking on a uniform road surface, the distribution ratio of the electro-hydraulic composite braking force between the front and rear axles is determined based on the vehicle's current state information;

[0013] S2: When emergency braking is performed on an oncoming road, the electro-hydraulic braking force distribution ratio of the four wheels is determined based on the vehicle's current state information;

[0014] S3: When braking at a constant speed downhill, the electro-hydraulic braking force distribution ratio of the four wheels is determined based on the vehicle's current state information; the electro-hydraulic braking force is adjusted secondary based on the brake pad speed information to suppress the stick-slip vibration phenomenon of the brake pad.

[0015] Preferably, the method steps for braking on a uniform road surface are as follows:

[0016] S11: Based on ECE regulations, the rated power of the motor, and the charging power, establish the constraints on the motor's power and the utilized adhesion coefficient at the current moment, as follows:

[0017]

[0018]

[0019] In formula (1), Utilize the adhesion coefficient for the front axle, is the rear axle adhesion coefficient, z is the braking intensity; in formula (2), T n is the maximum driving torque of the motor, T e_max is the maximum motor torque, n is the motor speed, n r is the rated speed of the motor, P e is the rated power of the motor; in formula (3), SOC is the battery state of charge, T cha is the maximum charging torque of the motor; F in formula (4) R is the maximum braking force of the motor, r w is the effective radius of the tire.

[0020] S12: establishing an optimization objective function with the vehicle's utilized adhesion coefficient and the motor's braking force as optimization variables and minimizing the difference between the utilized adhesion coefficient and the driver's required braking intensity as the optimization goal;

[0021] S13: Calculate the motor motive force using a quadratic programming optimization algorithm based on the optimization objective function and constraints;

[0022] The optimization objective function and constraints are organized into the following standard form:

[0023]

[0024] In formula (10), the coefficient matrix H = A1 T WA1, coefficient matrix Control variables F R1 is the front axle electric motor power, F R2 is the rear axle motor force. When the braking intensity z<0.3, the coefficient matrix Coefficient matrix When the braking intensity z≥0.3, the coefficient matrix Coefficient matrix in,

[0025] M2=(1 1),

[0026] The quadratic programming optimization algorithm is used to solve formula (10) and the motor force F is obtained. R1 、F R2 .

[0027] S14: Calculate the front axle hydraulic braking force and the rear axle hydraulic braking force respectively according to the driver's required braking force and the electric motor braking force.

[0028] F f =β(Gz-F R1 -F R2 ) (11)

[0029] F r =(1-β)(Gz-F R1 -F R2 ) (12)

[0030] In formulas (11) and (12), F f is the front axle hydraulic braking force, F r is the rear axle hydraulic braking force, vehicle load G=wG full +(1-w)G e , G full is the vehicle's fully loaded weight, G e is the vehicle's unladen weight, w is the load coefficient, and β is the hydraulic braking force distribution coefficient of the front and rear axles.

[0031] Preferably, the optimization objective function established in S12 is as follows:

[0032]

[0033] In formula (5), w is the load factor, The adhesion coefficient is used for the fully loaded front axle. The rear axle is fully loaded and utilizes the adhesion coefficient. is the front axle unloaded adhesion coefficient, is the rear axle unloaded adhesion coefficient, F R1 is the front axle electric motor power, F R2 Powering the rear axle electric motor.

[0034] The utilization adhesion coefficient under compound braking conditions is:

[0035]

[0036] In formulas (6)-(9), L is the vehicle wheelbase, a is the front axle wheelbase, b is the rear axle wheelbase, and h is g is the centroid height.

[0037] Preferably, the method steps for emergency braking on an open road are as follows:

[0038] S21: Establish a vehicle dynamics model with the vehicle's yaw rate and lateral velocity as state variables and the yaw torque as the control variable;

[0039] S22: The vehicle yaw rate r and lateral velocity v in the vehicle dynamics model y As the state variable x k , yaw torque M zAs the control variable u k , establish the optimization objective function of the DLQR control algorithm, and use the vehicle dynamics model as the constraint condition of the optimization objective function, as follows:

[0040]

[0041] In formula (16), x k is the state variable, x N is the state variable information at the termination time, u k is the control variable, Q, R are weight matrices, A k is the state transfer matrix, B k is the control matrix, and the superscript T of the matrix represents the transposed matrix.

[0042]

[0043] To solve the control variable u k , establish the iterative formula as follows:

[0044] P(k)=Q+A T [I+P k+1 BR -1 B T ] -1 P k+1 A (17)

[0045] In formula (17), I is the identity matrix. When |P(k)-P(k-1)| < ε, the iteration ends. Substitute the matrix P into the following formula to calculate the control variable u:

[0046] k=(R+B T PB) -1 B T PA (18)

[0047] u=-kx (19)

[0048] The vehicle's yaw torque is controlled by applying braking force through the wheel hub motor.

[0049] S23: According to the changes in the vehicle's yaw rate and longitudinal speed, the tire slip rate is controlled to achieve emergency braking of the vehicle.

[0050] When the vehicle's yaw rate exceeds 0.01 rad / s, the ABS system switches to low-select control. This involves comparing the slip changes of the wheels on both sides of the vehicle and selecting the wheel with the faster slip increase as the control target. By controlling the hydraulic braking force, the slip rates of both wheels are simultaneously controlled. When the vehicle's yaw rate is less than 0.01 rad / s, single-wheel control is employed, controlling the slip rates of each wheel individually to fully utilize the adhesion coefficient of each wheel. Finally, when the vehicle's longitudinal speed is less than 5 km / h, the parking brake is applied, locking the tires with hydraulic braking force to stop the vehicle.

[0051] Preferably, the vehicle dynamics model established in S21 is as follows:

[0052]

[0053]

[0054] In formulas (13)-(15), F y1 is the lateral force on the front wheel, F y2 is the lateral force on the rear wheel, C f is the total cornering stiffness of the front wheel, C r is the total lateral stiffness of the rear wheel, a is the front axle wheelbase, b is the rear axle wheelbase, m is the vehicle mass, v x is the longitudinal velocity, I z is the vehicle's yaw moment of inertia, v y is the lateral velocity, r is the yaw angular velocity, M z is the yaw torque, and δ is the front wheel angle.

[0055] Preferably, the method steps for braking at a constant speed downhill are as follows:

[0056] S31: According to ECE regulations, the rated power of the motor and the charging power, the constraints of the motor force and the utilized adhesion coefficient at the current moment are established, which are the same as formulas (1)-(4);

[0057] S32: establishing an optimization objective function with the vehicle's utilization adhesion coefficient and the motor force as optimization variables and minimizing the difference between the utilization adhesion coefficient, the sum of the driver's required braking intensity, and the downward component of gravity along the slope as the optimization goal;

[0058] S33: Calculate the motor motive force using a quadratic programming optimization algorithm based on the optimization objective function and constraints;

[0059] The optimization objective function and constraints are organized into the following standard form:

[0060]

[0061] In formula (25), the coefficient matrix H = A1T WA1, coefficient matrix Optimization variables F R1 is the front axle electric motor power, F R2 is the rear axle motor force. When z<0.3, the coefficient matrix Coefficient matrix When z ≥ 0.3, the coefficient matrix coefficient in,

[0062]

[0063]

[0064] M2=(1 1),

[0065] The quadratic programming optimization algorithm is used to solve formula (25) and the motor force F is obtained. R1 、F R2 .

[0066] S34: Calculate the front axle hydraulic braking force and the rear axle hydraulic braking force respectively according to the driver's required braking force and the electric motor braking force;

[0067] F f =β[G(z+sinθ)-F R1 -F R2 ] (26)

[0068] F r =(1-β)[G(z+sinθ)-F R1 -F R2 ] (27)

[0069] In formulas (26) and (27), F f is the initial hydraulic braking force of the front axle, F r is the initial hydraulic braking force of the rear axle, vehicle load G=wG full +(1-w)G e , G full is the vehicle's fully loaded weight, G e is the vehicle's unladen weight, w is the load factor, z is the driver's required braking intensity, β is the front and rear axle hydraulic braking force distribution coefficient, F R1 is the front axle electric motor power, F R2 Powering the rear axle electric motor.

[0070] S35: Establish a three-degree-of-freedom model of the brake system disc-block;

[0071] S36: Calculate the correction amount of the hydraulic braking force and the motor speed.

[0072] From an energy perspective, this control method considers the rotating brake disc as an energy source and the brake pad as an oscillator. Friction exerts work on the brake pad. When the work is greater than 0, the brake pad's amplitude gradually increases. When the work remains constant, the brake pad exhibits a stable periodic oscillation. When the work is less than 0, the brake pad's amplitude gradually decreases. Therefore, the energy formula is established:

[0073]

[0074] The brake pad is in the sliding stage, that is, the relative speed v rel <0, the speed changes sinusoidally, so Friction force F f Affected by the normal force and relative velocity, F f =μ(v rel )·F N , during the braking process, E>0, the brake pad performs a stable periodic stick-slip vibration. In order to make the input energy E<0, the brake pressure correction formula is The motor speed correction formula is: Substituting into formula (30), we get:

[0075]

[0076] In formula (33), F0 is the initial braking pressure, is the fluctuation pressure amplitude, is the phase difference between the brake pressure change and the brake pad speed change, is the brake pad speed amplitude, T is the stick-slip vibration period, k is the brake disc speed adjustment coefficient, v0 is the brake disc initial speed, μ s is the static friction coefficient, δ is the model parameter, v rel is the relative speed, ω is the vibration angular velocity of the brake pad, and t is the time. Let E<0, when When k>1, the energy will reach the minimum value. rel <0, so Substituting the above values ​​into the brake pressure correction formula, we get:

[0077]

[0078] In formula (34), F N is the normal force, F0 is the initial hydraulic braking force, is the amplitude of the fluctuating hydraulic braking force, is the brake pad velocity amplitude, is the brake pad speed, ω is the brake pad vibration angular velocity. Measured by the speed sensor, F0 is calculated by S34, the front wheel rear wheel

[0079] The brake pressure is received by the controller through the brake pad speed sensor signal, which controls the hydraulic valve to adjust the brake pressure; the brake disc speed is directly controlled by the wheel hub motor.

[0080] Preferably, the optimization objective function established in S32 is as follows:

[0081]

[0082] In formula (20), θ is the slope angle, w is the load factor, The adhesion coefficient is used for the fully loaded front axle. The rear axle is fully loaded and utilizes the adhesion coefficient. is the front axle unloaded adhesion coefficient, is the rear axle unloaded adhesion coefficient, z is the braking intensity, F R1 is the front axle electric motor power, F R2 Powering the rear axle electric motor.

[0083] During the downhill process, the vehicle is subjected to the gravity component along the slope direction, so the adhesion coefficient formula is corrected:

[0084]

[0085] In formulas (21)-(24), L is the vehicle wheelbase, a is the front axle wheelbase, b is the rear axle wheelbase, and h is g is the height of the center of mass, G e is the vehicle's unloaded weight, G full is the vehicle's fully loaded weight, F R1 is the front axle electric motor power, F R2 Powering the rear axle electric motor.

[0086] When the braking intensity z is set to 0, it is a constant speed downhill. In the control of stick-slip vibration, it is necessary to set the utilization coefficient of the motor force, F R1 =c1·max(F R1 ), F R2 =c2·max(F R2 ), c1 and c2 are limiting coefficients.

[0087] Preferably, the brake system disc-block three-degree-of-freedom model established in S35 is as follows:

[0088]

[0089] In formula (28), m1, m2, m3 are the mass of the brake disc assembly, the mass of the brake assembly, and the mass of the brake pad assembly, respectively; k1, k2, k3 are the stiffness of each assembly, respectively; d1, d2 are the damping of the brake disc assembly and the damping of the brake assembly, respectively; x1, x2, x3 are the displacement of the brake disc, the displacement of the brake, and the displacement of the brake pad, respectively; They are the vibration speed of the brake disc, the vibration speed of the brake, and the vibration speed of the brake pad. They are the brake disc vibration acceleration, brake vibration acceleration, and brake pad vibration acceleration, respectively. f For friction.

[0090] The friction model is:

[0091]

[0092] In formula (29), μ s ,μ d are the static friction coefficient and the kinetic friction coefficient, F N is the normal force, δ is the model parameter, v rel is the relative velocity, in the three-degree-of-freedom model is the vibration speed of the brake pad, is the vibration speed of the brake disc, v b is the linear velocity of the brake disc. rel When it is close to zero, that is, when static friction occurs, the friction force at the model is a set-value mapping interval, and the value is determined according to the resistance encountered by the brake pad during vibration, which is more in line with the actual situation.

[0093] Beneficial technical effects of the present invention:

[0094] (1) The present invention provides an electro-hydraulic composite braking force distribution method for braking on a uniform road surface. The method uses the vehicle's utilized adhesion coefficient and the motor's braking force as optimization variables, and minimizes the difference between the utilized adhesion coefficient and the driver's required braking intensity as the optimization goal. An optimization objective function is established. Therefore, the driver's required braking intensity can be accurately tracked. The front and rear axle hydraulic braking force distribution adopts a traditional fixed-proportional distribution, with the motor's rated power and the power battery's charging power as constraints for the objective function. This fully utilizes the motor's braking capacity and reduces the development cost of the electro-hydraulic composite brake.

[0095] (2) The present invention's electro-hydraulic hybrid brake distribution method for emergency braking on open roads uses the vehicle's lateral velocity and yaw rate as control targets. The DLQR algorithm calculates the yaw torque required to maintain the control targets at zero. The motor applies the yaw torque to the vehicle, coordinating with the vehicle's ABS control to achieve emergency braking on open roads. Compared to conventional ABS low-select control modes, this method shortens the stopping distance for emergency braking.

[0096] (3) The electro-hydraulic composite brake distribution method of the present invention in uniform downhill braking minimizes the work done by friction on the brake pad by actively correcting the hydraulic brake pressure and motor speed, thereby significantly suppressing the stick-slip vibration phenomenon of the brake pad. BRIEF DESCRIPTION OF THE DRAWINGS

[0097] Figure 1 This is a schematic structural diagram of the composite braking system for an electric vehicle proposed by the present invention;

[0098] Figure 2 This is a schematic diagram of the control of the brake pad stick-slip vibration system proposed by the present invention;

[0099] Figure 3 This is a schematic diagram of the dynamic model of the electric vehicle proposed in the present invention during emergency braking on an oncoming road;

[0100] Figure 4 This is a schematic diagram of the peak braking torque-speed curve of the hub motor proposed in the present invention;

[0101] Figure 5 This is a schematic diagram of the charging power-SOC curve of the hub motor proposed in the present invention;

[0102] Figure 6 Time domain diagrams of (a) vehicle lateral velocity, (b) yaw rate, (c) yaw torque, and (d) longitudinal velocity according to Example 1 of the present invention;

[0103] Figure 7 Brake pad displacement-velocity phase diagrams of Example 2 proposed in the present invention under (a) no control, (b) brake disc speed control, (c) normal force control, and (d) combined control;

[0104] Figure 8 Time-domain diagrams of brake pad velocity in Example 2 of the present invention (a) without control, (b) with brake disc speed control, (c) with normal force control, and (d) with combined control;

[0105] Figure 9 These are frequency domain diagrams of the brake pad displacements of Example 2 proposed in the present invention under (a) no control, (b) brake disc speed control, (c) normal force control, and (d) combined control. DETAILED DESCRIPTION

[0106] The present invention will be further explained below with reference to specific embodiments.

[0107] Reference Figure 1The electric vehicle of an embodiment of the present invention is a four-wheel drive vehicle driven by in-wheel motors. In the figure, dashed lines represent electrical connections, and solid lines represent mechanical connections. The vehicle is equipped with a driving speed sensor, a vehicle yaw angular velocity sensor, a vehicle lateral velocity sensor, a proportional valve, four brake pad angular velocity sensors, four in-wheel motors M1, M2, M3, and M4, four brake hydraulic control valves H1, H2, H3, and H4, and four ABS control valves ABS1, ABS2, ABS3, and ABS4. A motor controller is connected to the in-wheel motors to control electric braking under different operating conditions. The motor controller is electrically connected to the power battery; during electric braking, the motor charges the power battery. The proportional valve is used to distribute the hydraulic braking force between the front and rear axles. The ABS control valve receives a slip ratio signal calculated from wheel speed information to control the wheel slip ratio. An angular velocity sensor is installed on the brake pad in the braking system. The angular velocity sensor transmits the detected speed information to the hydraulic control valve and motor controller. The hydraulic control valve adjusts the brake pressure according to a control method, and the motor controller adjusts the brake disc speed according to a control method.

[0108] Example 1

[0109] Reference Figure 1 The vehicle of this embodiment is a 4×4 in-wheel motor driven passenger car, with the following parameters: wheelbase L = 2.6m, front axle wheelbase a = 1.04m, rear axle wheelbase b = 1.56m, center of mass height h g =0.54m, vehicle unloaded weight G e =12111.82N, vehicle fully loaded G full =16111.82N, load factor w = 0.5, front and rear axle braking force distribution coefficient β = 0.7, tire effective radius r w =0.298m. Specific parameters of hub motor: rated speed n r =800rpm, maximum torque T e_max =400Nm, rated power P e =33kW, peak power P e_max =55kW.

[0110] The front half of the vehicle of this embodiment performs braking on a uniform road surface, and the rear half performs emergency braking on a split road surface.

[0111] The steps of the compound braking control method for the front half of the vehicle on a uniform road surface are as follows:

[0112] (1) Establishing constraints on motor motive force and utilizing adhesion coefficient

[0113] The utilization of the adhesion coefficient and the motor power are respectively subject to the constraints of ECE regulations and motor status information. The rated power constraint of the motor is as follows: Figure 4 As shown, the constraints of charging power are as follows Figure 5 Shown, including:

[0114]

[0115] In formulas (1)-(4), Utilize the coefficient of adhesion for the front axle; is the rear axle adhesion coefficient; z is the braking intensity; n is the motor speed; SOC is the battery state of charge; T n is the maximum driving torque of the motor; T cha is the maximum charging torque of the motor; F in formula (4) R The maximum braking force of the motor.

[0116] (2) Establishing the optimization objective function

[0117] The optimization objective function uses the vehicle's utilized adhesion coefficient and the motor's braking force as optimization variables, and minimizes the difference between the utilized adhesion coefficient and the driver's required braking intensity as the optimization goal. The optimization objective function is as follows:

[0118]

[0119] In formula (5), The adhesion coefficient is used for the fully loaded front axle. The rear axle is fully loaded and utilizes the adhesion coefficient. is the front axle unloaded adhesion coefficient, is the rear axle unloaded adhesion coefficient, z i is the braking strength, F R1 is the front axle electric motor power, F R2 Powering the rear axle electric motor.

[0120] The utilization adhesion coefficient under compound braking conditions is:

[0121]

[0122]

[0123] (3) Calculation of motor dynamics

[0124] The optimization objective function and constraints are organized into the following standard form:

[0125]

[0126] In formula (10), the coefficient matrix H = A1 T WA1, coefficient matrix Optimization variables F R1 is the front axle electric motor power, F R2 is the rear axle motor force. When z<0.3, the coefficient matrix Coefficient matrix When z ≥ 0.3, the coefficient matrix Coefficient matrix in,

[0127]

[0128]

[0129] M2=(1 1),

[0130] The motor force F is obtained by using the quadratic programming optimization algorithm to calculate formula (10): R1 、F R2 .

[0131] (4) Calculation of hydraulic braking force

[0132] The front axle hydraulic braking force and the rear axle hydraulic braking force are calculated respectively according to the driver's required braking force and the electric motor braking force.

[0133] F R1 =0.7×(Gz-F R1 -F R2 ) (11)

[0134] F R2 =(1-0.7)×(Gz-F R1 -F R2 ) (12)

[0135] In formulas (11) and (12), vehicle load G = 0.5 × 16111.82 + (1 - 0.5) × 12111.82.

[0136] Setting different braking intensities, battery SOC, and motor speeds, solving equations (10), (11), and (12) yields the electric braking forces and hydraulic braking forces of the front and rear axles, as shown in Table 1.

[0137] Table 1 Solution results

[0138]

[0139] Comparing the required braking intensity with the actual front and rear axle adhesion coefficients proves that the composite braking control method of the present invention can accurately track the driver's braking demand.

[0140] In the second half, the vehicle enters the split road and performs emergency braking. The adhesion coefficient of the left side of the split road is 0.2, and the adhesion coefficient of the right side of the split road is 0.5. The specific steps are as follows:

[0141] (1) Establish a two-degree-of-freedom vehicle dynamics model. The model diagram is as follows Figure 3 As shown:

[0142]

[0143] In formulas (13)-(15), F y1 is the lateral force on the front wheel, F y2 is the lateral force on the rear wheel, v x is the longitudinal velocity, v y is the lateral velocity, r is the yaw angular velocity, M z is the yaw torque.

[0144] (2) Calculation of yaw torque

[0145] The vehicle yaw rate r and lateral velocity v in the vehicle dynamics model are y As the state variable x k , yaw torque M z As the control variable u k , establish the optimization objective function of the DLQR control algorithm, and use formula (13) as the constraint condition of the optimization objective function.

[0146]

[0147] In formula (16), x k is the state variable, u k is the control variable, Q, R are weight matrices, A k is the state transfer matrix, B k is the control matrix.

[0148]

[0149] R=1000;

[0150] To solve the control variable u k , establish the iterative formula as follows:

[0151] P(k)=Q+A T [I+P k+1 BR -1 B T ] -1 P k+1 A (17)

[0152] Set the initial value of P to Q. When |P(k)-P(k-1)|<ε, the iteration ends. Substitute the matrix P into the following formula to calculate the control variable u:

[0153] k=(R+B T PB) -1 B T PA (18)

[0154] u=-kx (19)

[0155] The calculated yaw torque is given by Figure 6 (c) Finally, the yaw torque of the vehicle is controlled by applying braking force through the wheel hub motor.

[0156] (3) Control tire slip rate

[0157] Based on the changes in the vehicle's yaw rate and longitudinal speed, different tire slip control methods are selected to achieve emergency braking. Slip control is achieved by the vehicle's hydraulic ABS device.

[0158] When the vehicle's yaw rate exceeds 0.01 rad / s, the ABS system switches to low-select control. This compares the slip changes of the wheels on both sides of the vehicle and selects the wheel with the faster slip increase. By controlling the hydraulic braking force, the slip rates of both wheels are controlled synchronously. When the vehicle's yaw rate is less than 0.01 rad / s, single-wheel control is adopted, which controls the slip rate of each wheel individually to fully utilize the adhesion coefficient of each wheel. Finally, when the vehicle's longitudinal speed is less than 5 km / h, the parking brake is applied, using the hydraulic braking force to lock the tires to stop the vehicle.

[0159] The final result can be found in Figure 6 , Figure 6 (a), Figure 6 (b) shows that the vehicle lateral velocity is stable in the range of (-0.025, 0.025) m / s, and the vehicle yaw rate is stable in the range of (-0.05, 0.05) rad / s. Figure 6 (d) shows that during emergency braking on the oncoming road, the vehicle's speed drops from an initial speed of 65 km / h to 0 after about 8 seconds.

[0160] Example 2

[0161] See also Figure 1 The vehicle in the embodiment is a 4×4 wheel hub motor driven passenger car, with specific parameters: wheelbase L = 2.6m, front axle wheelbase a = 1.04m, rear axle wheelbase b = 1.56m, center of mass height h g =0.54m, vehicle unloaded weight G e =12111.82N, vehicle fully loaded weight G full=16111.82N, load factor w = 0.5, braking intensity z = 0, front and rear axle braking force distribution coefficient β = 0.7, tire effective radius r w =0.298m. Specific parameters of hub motor: rated speed n r =800rpm, maximum torque T e_max =400Nm, rated power P e =33kW, peak power P e_max =55kW. Specific parameters of the brake system: brake pad static friction coefficient μ s =0.5, dynamic friction coefficient μ d =0.3, friction model coefficient δ=3, brake disc assembly mass m1=10kg, brake assembly mass m2=6kg, brake pad assembly mass m3=0.1kg, brake disc assembly stiffness k1=1e06N / m, brake assembly stiffness k2=1e06N / m, brake pad stiffness k3=1e04N / m, brake disc assembly damping d1=1N·s / m, brake assembly damping d2=1N·s / m.

[0162] This embodiment determines the distribution ratio of electro-hydraulic composite braking force between the front and rear axles based on the current vehicle state information when driving downhill at a constant speed; and adjusts the electro-hydraulic braking force secondary to the brake pad angular velocity information. The specific steps are as follows:

[0163] (1) Establishing constraints on motor motive force and utilizing adhesion coefficient

[0164] Same as step (1) in conventional braking on a uniform road surface in Example 1.

[0165] (2) Establishing the optimization objective function

[0166]

[0167] In formula (20), θ is the slope angle, The adhesion coefficient is used for the fully loaded front axle. The rear axle is fully loaded and utilizes the adhesion coefficient. is the front axle unloaded adhesion coefficient, is the rear axle unloaded adhesion coefficient, F R1 is the front axle electric motor power, F R2 Powering the rear axle electric motor.

[0168] The utilization adhesion coefficient under compound braking conditions is:

[0169]

[0170] In the control of stick-slip vibration, it is necessary to set the utilization coefficient of the motor force, F R1 =c1·max(FR1 ), F R2 =c2·max(F R2 ), c1 and c2 are limiting coefficients, which are taken as 0.8 in this embodiment.

[0171] (3) Calculation of motor dynamics

[0172] The objective function (20) and the constraints (1), (2), (3), (4) are organized into the following standard form:

[0173]

[0174] In formula (25), the coefficient matrix H = A1 T WA1, coefficient matrix Optimization variables F R1 is the front axle electric motor power, F R2 is the rear axle motor force. When z<0.3, the coefficient matrix Coefficient matrix When z ≥ 0.3, the coefficient matrix Coefficient matrix in,

[0175]

[0176] M2=(1 1),

[0177]

[0178] (4) Calculation of hydraulic braking force

[0179] The hydraulic braking force is calculated based on the driver's required braking force and the motor braking force calculated in step (3). The specific formula is as follows:

[0180] F f =0.7[G(z+sinθ)-F R1 -F R2 ] (26)

[0181] F r =(1-0.7)[G(z+sinθ)-F R1 -F R2 ] (27)

[0182] In formulas (26) and (27), vehicle load G = 0.5 × 16111.82 + (1 - 0.5) × 12111.82.

[0183] (5) Establish a three-degree-of-freedom model of the brake system disc-block

[0184] according to Figure 2 The three-degree-of-freedom model shown establishes the dynamic model as follows:

[0185]

[0186] In formula (28), x1, x2, and x3 are the brake disc displacement, brake displacement, and brake pad displacement, respectively. They are the vibration speed of the brake disc, the vibration speed of the brake, and the vibration speed of the brake pad. They are the brake disc vibration acceleration, brake vibration acceleration, and brake pad vibration acceleration, respectively. f For friction.

[0187] The friction model used is:

[0188]

[0189] In formula (29), v rel is the relative velocity, in the three-degree-of-freedom model When v rel When it is close to zero, that is, when static friction occurs, the friction force of the model is a set-value mapping interval, and the value is determined according to the resistance encountered by the brake pad during vibration, which is consistent with the actual situation.

[0190] The stability of the system can be determined according to the Lyapunov exponent. The calculation formula of the Lyapunov exponent of a discrete system is as follows:

[0191]

[0192] In formula (30), λ is the Lyapunov exponent, m is the eigenvalue of the variational equation of the dynamic system, and n is the dimension of the dynamic equation.

[0193] The variational equation of the dynamic system is:

[0194]

[0195] is the Jacobian matrix of the dynamic equation f:

[0196] When v rel ≠0,

[0197] When v rel =0,

[0198] Solving equations (28) and (31) simultaneously, we can obtain the solution of the variational equation φ T (x0, t0), T is usually 10 times the system motion period, by solving φ TSubstituting the eigenvalues ​​of (x0, t0) into Equation (35), we obtain six Lyapunov exponents. The solution is shown in Table 2.

[0199] Table 2 Iterative solution results

[0200]

[0201] The system has a Lyapunov exponent of 0.0092 and the rest are negative, so the system has a stable limit cycle.

[0202] (6) Calculate the correction amount of hydraulic braking force and motor speed

[0203] From an energy perspective, this control method considers the rotating brake disc as an energy source and the brake pad as an oscillator. Friction exerts work on the brake pad. When the work is greater than 0, the brake pad's amplitude gradually increases. When the work remains constant, the brake pad exhibits a stable periodic oscillation. When the work is less than 0, the brake pad's amplitude gradually decreases. Therefore, the energy formula is established:

[0204]

[0205] In order to make the input energy E<0, the brake pressure correction formula is: The motor speed correction formula is: Substituting into formula (32), we get:

[0206]

[0207] In formula (33), F0 is the initial braking pressure, is the fluctuation pressure amplitude, is the phase difference between the brake pressure change and the brake pad speed change, is the brake pad speed amplitude, T is the stick-slip vibration period, k is the brake disc speed adjustment coefficient, v0 is the brake disc initial speed, μ s is the static friction coefficient, δ is the model parameter, v rel is the relative speed, ω is the vibration angular velocity of the brake pad, and t is the time. Let E<0, when When k>1, the energy will reach the minimum value. rel <0, so Substituting the above values ​​into the brake pressure correction formula, we get:

[0208]

[0209] The speed state of the brake pad Measured by the speed sensor, the front wheel rear wheel

[0210] The calculated normal force F N Substituting into formula (29), we can get the friction force F f , F f Substitute into equation (28) to solve the differential equation, and the final result is shown in Figure 7-9 .

[0211] In order to demonstrate the beneficial technical effects of the present invention, the results of no control, speed control, normal braking force control, and speed-normal braking force combined control are respectively displayed for comparison.

[0212] Figure 2 This is a schematic diagram of the brake pad stick-slip vibration system control. The speed sensor transmits the brake pad speed information to the hydraulic system and motor respectively. The hydraulic valve controls the pressure of the wheel cylinder based on the speed information, and the motor adjusts the brake disc speed based on the speed information.

[0213] Figure 7 (a) is the displacement-velocity phase diagram of the brake pad when there is no control. Figure 7 (b) is the displacement-velocity phase diagram of the brake pad when the brake disc speed is controlled. Figure 7 (c) is the displacement-velocity phase diagram of the brake pad under normal force control. Figure 7 (d) is the velocity-displacement phase diagram of the brake pad during combined control;

[0214] Figure 8 (a) is the time domain diagram of the brake pad speed when there is no control. Figure 8 (b) is the time domain diagram of the brake pad speed when the brake disc speed is controlled. Figure 8 (c) is the time domain diagram of the brake pad velocity under normal force control. Figure 8 (d) is the time domain diagram of the brake pad velocity during joint control;

[0215] Figure 9 (a) is the displacement frequency domain diagram of the brake pad when there is no control. Figure 9 (b) is the frequency domain diagram of the brake pad displacement when the brake disc speed is controlled. Figure 9 (c) is the displacement frequency domain diagram of the brake pad under normal force control. Figure 9 (d) is the displacement frequency domain diagram of the brake pad during joint control;

[0216] from Figure 7 (a) The displacement of the brake pad is (2×10 -3 ,5.5×10 -3 ) Figure 9 (a) shows that the peaks are distributed at equal intervals, which indicates that the brake pad is vibrating periodically in the uncontrolled state. Figure 7 (b) The displacement of the brake pad is (2×10 -3 ,5.5×10 -3 ) range, with Figure 7(a) No significant changes compared to the control group; Figure 8 (b) shows that the speed adjustment range is small due to the limitation of the maximum torque value of the motor; Figure 9 (b) The frequency peaks are distributed at equal intervals, indicating that the brake pad still vibrates periodically under speed control. Figure 7 (c) The displacement of the brake pad is (3.5×10 -3 ,5.5×10 -3 ) range, compared with Figure 7 (a), Figure 7 (c) The amplitude of the brake pad is significantly reduced; Figure 8 (c) Displays the normal pressure F N During the brake pad sliding phase, the speed of the brake pad is adjusted and the two have a phase difference of π; Figure 9 (c) shows that the frequency peaks are distributed at equal intervals, so the brake pad vibrates periodically under normal force control, but the amplitude is reduced. Figure 7 (d) The displacement of the brake pad is Figure 7 (c) significantly reduced; Figure 8 (c) Displays the normal pressure F N During the brake pad sliding phase, the brake disc speed is adjusted in the positive direction according to the brake pad speed and the two have a phase difference of π. Figure 9 In (d), there is only an obvious peak at the zero point, which proves that the movement of the brake pad eventually converges to the equilibrium point.

Claims

1. A multi-operating-condition composite braking control method for electric vehicles based on stability, characterized in that: The steps are as follows: S1: When braking on a uniform road surface, the distribution ratio of the electro-hydraulic composite braking force between the front and rear axles is determined based on the vehicle's current state information; S2: When emergency braking is performed on an oncoming road, the electro-hydraulic braking force distribution ratio of the four wheels is determined based on the vehicle's current state information; S3: When braking at a constant speed downhill, the four-wheel electro-hydraulic braking force distribution ratio is determined based on the vehicle's current state information. Based on the brake pad speed information, the electro-hydraulic braking force is adjusted secondary to suppress the brake pad's stick-slip vibration. The steps for braking at a constant speed downhill are as follows: S31: Establish constraints on the motor's power and the utilized adhesion coefficient at the current moment based on ECE regulations, the motor's rated power, and the charging power; S32: establishing an optimization objective function with the vehicle's utilization adhesion coefficient and the motor force as optimization variables and minimizing the difference between the utilization adhesion coefficient, the sum of the driver's required braking intensity, and the downward component of gravity along the slope as the optimization goal; S33: Calculate the motor motive force using a quadratic programming optimization algorithm based on the optimization objective function and constraints; S34: Calculate the front axle hydraulic braking force and the rear axle hydraulic braking force respectively according to the driver's required braking force and the electric motor braking force; S35: Establish a three-degree-of-freedom model of the brake system disc-block; The three-degree-of-freedom model of the brake system disc-block is as follows: （28） In formula (28), They are the mass of the brake disc assembly, the mass of the brake assembly and the mass of the brake pad assembly, are the stiffness of each assembly, They are brake disc assembly damping and brake assembly damping respectively; They are brake disc displacement, brake displacement, and brake pad displacement, respectively. They are the vibration speed of the brake disc, the vibration speed of the brake, and the vibration speed of the brake pad. They are brake disc vibration acceleration, brake vibration acceleration, and brake pad vibration acceleration. is the friction force; S36: Calculate the correction amount of the hydraulic braking force and the motor speed.

2. The multi-operating-condition compound braking control method for an electric vehicle based on stability according to claim 1, characterized in that: The steps for braking on a smooth road are as follows: S11: Establish constraints on the motor's power and the utilized adhesion coefficient at the current moment based on ECE regulations, the motor's rated power, and the charging power. S12: establishing an optimization objective function with the vehicle's utilized adhesion coefficient and the motor's braking force as optimization variables and minimizing the difference between the utilized adhesion coefficient and the driver's required braking intensity as the optimization goal; S13: Calculate the motor motive force using a quadratic programming optimization algorithm based on the optimization objective function and constraints; S14: Calculate the front axle hydraulic braking force and the rear axle hydraulic braking force respectively according to the driver's required braking force and the electric motor braking force.

3. The multi-operating-condition compound braking control method for an electric vehicle based on stability according to claim 1, characterized in that: The optimization objective function established in S12 is as follows: （5） In formula (5), w is the load factor, The adhesion coefficient is used for the fully loaded front axle. The rear axle is fully loaded and utilizes the adhesion coefficient. is the front axle unloaded adhesion coefficient, is the rear axle unloaded adhesion coefficient, is the braking strength, Powering the front axle electric motor, Powering the rear axle electric motor.

4. The multi-operating-condition compound braking control method for an electric vehicle based on stability according to claim 1, characterized in that: The steps for emergency braking on an oncoming road are as follows: S21: Establish a vehicle dynamics model with the vehicle's yaw rate and lateral velocity as state variables and the yaw torque as the control variable; S22: The vehicle yaw rate in the vehicle dynamics model and lateral speed As a state variable , yaw torque As a control variable , establish the optimization objective function of the dlqr control algorithm; S23: According to the changes in the vehicle's yaw rate and longitudinal speed, the tire slip rate is controlled to achieve emergency braking of the vehicle.

5. The multi-operating-condition compound braking control method for an electric vehicle based on stability according to claim 4, characterized in that: The vehicle dynamics model established in S21 is as follows: （13） （14） （15） In formulas (13)-(15), a is the front axle wheelbase, b is the rear axle wheelbase, is the lateral force on the front wheel, is the lateral force on the rear wheel, is the total cornering stiffness of the front wheel, is the total cornering stiffness of the rear wheel, is the front axle wheelbase, is the rear axle wheelbase, is the vehicle mass, is the longitudinal velocity, is the vehicle's yaw moment of inertia, is the lateral velocity, is the yaw angular velocity, is the yaw torque, is the front wheel turning angle.

6. The multi-operating-condition compound braking control method for an electric vehicle based on stability according to claim 1, characterized in that: The optimization objective function established in S32 is as follows: （20） In formula (20) is the slope angle, w is the load factor, The adhesion coefficient is used for the fully loaded front axle. The rear axle is fully loaded and utilizes the adhesion coefficient. is the front axle unloaded adhesion coefficient, is the rear axle unloaded adhesion coefficient, is the braking strength, Powering the front axle electric motor, Powering the rear axle electric motor.

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