CN118342985A

CN118342985A - Braking control method and device for electric automobile and storage medium

Info

Publication number: CN118342985A
Application number: CN202410503836.7A
Authority: CN
Inventors: 王念; 赵春来; 张泽阳; 吴婷; 吴昊
Original assignee: Dongfeng Motor Group Co Ltd
Current assignee: Dongfeng Motor Group Co Ltd
Priority date: 2024-04-25
Filing date: 2024-04-25
Publication date: 2024-07-16

Abstract

The invention provides a braking control method of an electric automobile, which is applied to a vehicle control unit, wherein the front axle of the electric automobile adopts electrohydraulic braking, and the rear axle adopts electromechanical braking, and the method comprises the following steps: acquiring displacement information of a brake pedal of the electric automobile, the mass and the current longitudinal speed of the electric automobile and the current road gradient; determining the final required torque of each of four hubs of the electric automobile based on displacement information, mass, gradient and longitudinal speed; and distributing each final required torque based on a preset torque mixing rule to obtain the respective mixed torques of four hubs of the electric automobile, wherein the mixed torques comprise the output torque of a friction brake and the output torque of a hub motor. The invention provides an enhanced function such as hybrid torque, so that the vehicle can keep a stable motion state under the condition of gradient, and provides a continuous control strategy of hybrid torque which is not available in the prior art, and the stability and the driving comfort of the vehicle are obviously improved.

Description

Braking control method and device for electric automobile and storage medium

Technical Field

The invention relates to the technical field of direct yaw moment control, in particular to a braking control method and device of an electric automobile, a storage medium and the electric automobile.

Background

With the continuous development of electric automobile (ELECTRIC VEHICLES, EVS) technology, vehicle chassis systems are also continuously innovating. The novel electromechanical chassis system integrates a brake-by-wire system and a rear axle hub driving system, integrates vehicle chassis control, and provides higher-level safety functions and performance characteristics. Based on the traditional vehicle chassis system, the new technologies provide more control and optimization options for the electric vehicle. The novel electromechanical chassis system not only provides common safety functions (such as an anti-lock braking system (ABS)), but also can integrate advanced functions (such as torque mixing), and is expected to improve the overall safety of the automobile. Meanwhile, by applying the continuous control strategy, the stability and the control performance of the vehicle are improved, and the driving experience and the dynamic response of the vehicle are enhanced.

Electric vehicles, which typically employ individually controlled In-Wheel Motors (IWMs), are receiving increasing attention In the automotive industry and research area for their high performance, design flexibility, and meeting traffic electrification and automation requirements. One of the most attractive but also challenging features of electric vehicles with hub drive is the hybrid operational possibilities of friction braking systems and regenerative braking systems (represented by IWMs operating in generator mode), which can essentially improve energy efficiency, fail-safe, and motion control performance in critical driving situations. To achieve this hybrid operation, the friction brake system preferably employs a decoupling architecture, i.e. the connection between the driver and the brake calipers is achieved by means of a brake pedal simulator, and the clamping force of the calipers can be controlled separately in a precisely controlled manner. This decoupling is an inherent feature of most Brake-by-Wire (BBW). The brake-by-wire system may be implemented using electro-hydraulic, electro-mechanical, or very few magnetorheological actuators, as well as hybrid actuators. Electro-hydraulic braking (Electro-hydraulicBrakes, EHB) and Electro-mechanical braking (Electro-MECHANICAL BRAKES, EMB) employ different methods to control the clamping force that creates the frictional contact. However, there is still no widely accepted method in this regard. One of the common problems with EHB operation is associated with poor alignment between brake pressure control and antilock brake system control, which may result in undesirable pressure peaks and oscillations. Therefore, the EHB control loop possibly comprises a related attenuation mechanism, an automobile brake slip rate control method based on an integrated electrohydraulic brake system is designed in related technology, slip rate calculation, slip rate control and motor pressure building cylinder target pressure calculation are carried out through motion state information of a vehicle to determine the automobile brake slip rate, then the automobile brake slip rate is regulated by a position-pressure double-loop switching controller, a speed loop, a weak magnetic controller, a current loop and a voltage restraint device in sequence, and finally a permanent magnet synchronous motor is driven to operate through three-phase current, so that a complete closed-loop control strategy is formed. The integrated drive-by-wire hydraulic braking system is effectively helped to overcome the friction of a transmission mechanism, the nonlinear characteristics of a hydraulic system and the like, the rapid and accurate control of hydraulic pressure is realized, the driving safety and the driving comfort of a vehicle are improved, and an execution foundation is laid for realizing high-level automatic driving. Brake-by-wire designs, however, require attention not only to the proper clamping force control, but also to the brake mix procedure to explore the possibility of efficient and reliable combined operation of the electric motor and friction brakes.

Accordingly, the braking method of the electric vehicle in the related art has a problem in that there is no hybrid braking scheme controlled by employing a combined operation of the electric motor and the friction braking.

Disclosure of Invention

The invention provides a brake control method of an electric automobile, which solves the problem that a hybrid brake scheme adopting combined operation control of an electric motor and friction brake is not adopted in the prior art, determines brake torque distributed to each hub by combining brake pedal displacement information output by a driver, the state of a vehicle and the current road condition, provides hybrid torque of combined operation of the electric motor and the friction brake on each hub, integrates vehicle chassis control, provides an enhancement function such as hybrid torque, and provides a continuous control strategy of the hybrid torque which is not available in the prior art because the determination of the brake torque considers the current road gradient to enable the vehicle to keep a stable motion state under the condition of gradient, and has obvious improvement in the aspects of vehicle stability and driving comfort.

Other features and advantages of the application will be apparent from the following detailed description, or may be learned by the practice of the application.

In a first aspect, the present invention provides a brake control method for an electric vehicle, applied to a vehicle control unit, where a front axle and a rear axle of the electric vehicle respectively use electro-hydraulic braking and electromechanical braking, the method including:

acquiring displacement information of a brake pedal of the electric automobile, the mass and the current longitudinal speed of the electric automobile and the current road gradient;

Determining respective final required torques of four hubs of the electric vehicle based on the displacement information, the mass, the gradient, and the longitudinal speed;

and distributing each final required torque based on a preset torque mixing rule to obtain the respective mixed torques of four hubs of the electric automobile, wherein the mixed torques comprise the output torque of a friction brake and the output torque of a hub motor.

Optionally, according to the braking control method of an electric automobile provided by the present invention, the final required torques are distributed based on a preset torque mixing rule to obtain respective mixed torques of four hubs of the electric automobile, where the mixed torques include a friction brake output torque and a hub motor output torque, and the method specifically includes:

If the final required torque of any hub is larger than the maximum achievable electric torque of the corresponding motor, determining corresponding mixed torque based on a preset torque mixing algorithm, wherein the mixed torque comprises friction brake output torque and hub motor output torque;

And if the final required torque of any hub is smaller than the maximum achievable electric torque of the corresponding motor, determining that the output torque of the hub motor of the corresponding hub is the final required torque, and determining that the output torque of the corresponding friction brake is zero.

Optionally, according to the braking control method of an electric automobile provided by the present invention, the determining the final required torque of each of the four hubs of the electric automobile based on the displacement information, the mass, the gradient and the longitudinal speed specifically includes:

determining initial required torque of each of four hubs of the electric vehicle based on the displacement information, the mass, the gradient, and the longitudinal speed;

and adjusting each initial braking torque based on a preset tire sliding control algorithm to obtain the final required torque of each of the four hubs of the electric automobile.

Optionally, according to the braking control method of an electric automobile provided by the present invention, the determining initial required torques of four hubs of the electric automobile based on the displacement information, the mass, the gradient and the longitudinal speed specifically includes:

Determining a total braking torque based on the displacement information, the mass, the grade, and the longitudinal speed;

And determining initial required torques of four hubs of the electric automobile respectively through a preset torque distribution algorithm based on the total braking torque.

Optionally, according to the braking control method of an electric automobile provided by the invention, the determining the total braking torque based on the displacement information, the mass, the gradient and the longitudinal speed specifically includes:

determining a driver requested braking torque based on the displacement information and the mass;

determining a grade-compensating braking torque based on the mass, the grade, and the longitudinal speed;

a total braking torque is determined based on the driver requested braking torque and the grade compensating braking torque.

Optionally, according to the method for controlling braking of an electric automobile provided by the present invention, the determining, based on the total braking torque, initial required torques of four hubs of the electric automobile respectively through a preset torque distribution algorithm specifically includes:

determining the vertical loads of four hubs of the electric automobile respectively based on the total braking torque;

Based on each vertical load, determining the initial required torque of each of four hubs of the electric automobile.

Optionally, according to the braking control method of the electric automobile provided by the invention, the preset tire slip control algorithm is a tire slip control algorithm based on proportional integral control of anti-integral saturation or a tire slip control algorithm based on integral slip mode control.

In a second aspect, the present invention provides a brake control device for an electric vehicle, in which a front axle and a rear axle of the electric vehicle are respectively braked by an electro-hydraulic brake and an electro-mechanical brake, the device comprising:

An acquisition unit for acquiring displacement information of a brake pedal of the electric vehicle, a mass and a current longitudinal speed of the electric vehicle, and a current road gradient;

A determining unit configured to determine final required torques of four hubs of the electric vehicle, respectively, based on the displacement information, the mass, the gradient, and the longitudinal speed;

and the mixing unit is used for distributing each final required torque based on a preset torque mixing rule to obtain the respective mixed torques of the four hubs of the electric automobile, wherein the mixed torques comprise the output torque of the friction brake and the output torque of the hub motor.

In a third aspect, the present invention provides a computer-readable storage medium having stored therein at least one program code loaded and executed by a processor to implement operations performed by the brake control method of an electric vehicle according to the first aspect.

In a fourth aspect, the present invention provides an electric vehicle, which includes one or more processors and one or more memories, where at least one program code is stored in the one or more memories, and the at least one program code is loaded and executed by the one or more processors to implement the operations performed by the braking control method of an electric vehicle according to the first aspect.

According to the braking control method and device for the electric automobile, the storage medium and the electric automobile, the braking torque distributed to each hub is determined by combining the brake pedal displacement information output by a driver, the state of the automobile and the current road condition, the mixed torque of the electric motor and the friction braking combined operation on each hub is provided, the control of the chassis of the automobile is integrated, the enhancement function such as the mixed torque is provided, the current road gradient is considered in the determination of the braking torque, the automobile keeps a stable motion state under the condition of gradient, a continuous control strategy of the mixed torque which is not available in the prior art is provided, and the stability and the driving comfort of the automobile are improved obviously.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the application as claimed.

Drawings

In order to more clearly illustrate the invention or the technical solutions of the prior art, the following description will briefly explain the drawings used in the embodiments or the description of the prior art, and it is obvious that the drawings in the following description are some embodiments of the invention, and other drawings can be obtained according to the drawings without inventive effort for a person skilled in the art.

Fig. 1 is a flowchart of a brake control method of an electric vehicle of the present invention;

FIG. 2 is a diagram of the overall controller control flow provided by the present invention;

Fig. 3 is a diagram of an electric automobile brake control structure of the hub type driving and full decoupling line control braking system provided by the invention;

fig. 4 is a schematic structural diagram of a brake control device of an electric automobile according to the present invention;

fig. 5 is a schematic diagram of a physical structure of an electric vehicle according to the present invention.

Detailed Description

For the purpose of making the objects, technical solutions and advantages of the present invention more apparent, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings, and it is apparent that the described embodiments are some embodiments of the present invention, not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.

The following describes a brake control method, apparatus and storage device of an electric vehicle according to the present invention with reference to fig. 1 to 5.

Fig. 1 is a flowchart of a brake control method of an electric vehicle according to the present invention. As shown in fig. 1, the method of the present invention is applied to a vehicle control unit (VCU, vehicle Control Unit) in which the front axle and the rear axle of an electric vehicle are braked electro-hydraulically and electro-mechanically, respectively, the method comprising the steps of:

Step 110, obtaining displacement information of a brake pedal of the electric automobile, mass and current longitudinal speed of the electric automobile, and current road gradient.

Since the vehicle is driven by a person, the initiation signal is generated by the brake pedal application force F _ped and the steering wheel torque T _SW, which results in pedal displacement s _ped and angular displacement (i.e., steering wheel angle delta _SW). The two signals also show parameters fed back by the driver, which provide the driver with information such as road irregularities or friction condition changes.

Therefore, under the condition of manual control braking, firstly, the displacement information of the brake pedal output by the driver is required to be acquired, so as to judge whether the current intention of the driver is sudden braking, slow braking, spot braking and the like, namely, the pedal displacement information output by the driver is converted into corresponding braking torque based on a braking controller; secondly, the mass and current longitudinal speed of the electric vehicle and the current road gradient parameters are also considered, because if the electric vehicle is in a road condition of an ascending or descending slope, when the driver controls to execute the braking torque output, the road gradient needs to be compensated for so that the electric vehicle needs additional climbing torque or braking torque, for example, if the electric vehicle is in a descending slope section, an additional braking torque is added on the basis of the braking torque converted based on the brake pedal information of the driver, and the additional braking torque is used for balancing the trend of the electric vehicle undershoot formed by the whole gravity of the electric vehicle along a component force of the ascending slope caused by the compensating slope.

It should be noted here that, the displacement information of the brake pedal of the electric vehicle, the current longitudinal speed and the current road gradient are determined based on real-time acquisition of the vehicle-mounted sensor, and the mass of the electric vehicle defaults to the factory mass, which is preset, and the mass error of the vehicle-mounted mass is negligible.

Step 120, determining the final required torque of each of the four hubs of the electric vehicle based on the displacement information, the mass, the gradient, and the longitudinal speed.

Specifically, the total braking torque may be determined based on the displacement information, the mass information, the gradient and the longitudinal speed, because the total braking torque includes both the driver's output braking intention (representing the displacement information of the brake pedal) and the road gradient, the vehicle mass and the additional braking torque due to the vehicle longitudinal mass, i.e. braking on a slope compared to a flat road, in addition to considering the driver's requested braking torque due to the driver's output brake pedal information, the tendency to balance the "undershoot" or "slip" due to the force component of the compensating gravity down the slope is also required. And then, distributing the total braking torque to four hubs of the electric automobile based on a preset torque distribution algorithm to obtain the final required torque of the four hubs of the electric automobile respectively.

And 130, distributing each final required torque based on a preset torque mixing rule to obtain the respective mixed torques of four hubs of the electric automobile, wherein the mixed torques comprise the output torque of a friction brake and the output torque of a hub motor.

In particular, electric vehicles employ a hub drive system that can be used to slow down when the in-wheel motor is operating in generator mode, which can recover kinetic energy for battery recharging, for example, to enhance endurance. When each hub executes the output of the final required torque, the output of the braking torque can be performed through the hub motor, and the mixed output of the braking torque can also be performed through the hub motor and the friction brake (namely, the brake pad), namely, for the final required torque of any hub, one part of the final required torque is output through the motor corresponding to any hub, and the rest of the final required torque is output through the friction brake corresponding to any hub. In the torque mixing rule, the output of the final required torque by using the hub motor is preferentially considered, because the electric automobile is characterized in that reverse current is generated to charge the battery during the braking of the hub motor, the endurance mileage can be enhanced, and the final required torque is output by considering the mode of combining the braking of the hub motor and the braking of the braking block, so that the torque mixing rule can be a rule of preferentially selecting the hub motor for output, and the mixing distribution algorithm between the specific hub motor and the braking block is not particularly limited.

According to the braking control method of the electric automobile, provided by the invention, the braking torque distributed to each hub is determined by combining the braking information output by a driver, the state of the automobile and the current road condition, and then the mixed torque of the combined operation of the electric motor and the friction braking on each hub is provided, so that the control of the chassis of the automobile is integrated, the enhancement function such as the mixed torque is provided, the automobile can keep a stable motion state under the condition of gradient, and the continuous control strategy of the mixed torque is obviously improved in the aspects of the stability and the driving comfort of the automobile.

Based on the above embodiment, in the method, the final required torque is distributed based on a preset torque mixing rule to obtain respective mixed torques of four hubs of the electric vehicle, where the mixed torques include a friction brake output torque and a hub motor output torque, and the method specifically includes:

Specifically, the preset torque mixing rule preferably uses the hub motor to output the final required torque, and only when the required motor torque T _WSC (i.e., the final required torque) is higher than the maximum achievable electric torque T _{max_EM}, the manner of mixing the friction brake (i.e., the brake pad) and the hub motor is adopted, that is, the friction brake output braking torque (T _{re_FB}) and the motor output braking torque (T _{re_EM}) work in parallel to increase the hub torque (T _w) to the total required amount (T _WSC). Thus, a blend factor, α, is defined as the value at each wheel angle, is introduced herein. If 0< α ^ij <1, the in-wheel motor brake and the friction brake are operated in parallel, if α ^ij =1, the in-wheel motor brake and the friction brake are operated in series, wherein α ^ij represents a mixing factor of the ijth wheel, i=1 represents a front wheel, i=2 represents a rear wheel, j=1 represents a left wheel, and j=2 represents a right wheel. The related description formula of the preset torque mixing algorithm is as follows:

Wherein, For () to be a saturated function,Wherein T _{max_EM} ^ij is the upper domain limit of the saturation function,Wherein 0 is the lower limit of the definition domain of the saturation function;

According to the above description formula of the torque mixing algorithm, the friction brake (i.e., brake pad) output braking torque (T _{re_FB}) and the motor output braking torque (T _{re_EM}) of each hub can be calculated.

It should be further noted that some limiting parameters may be added to the motor torque T _WSC (i.e., the final required torque) requested by any of the hub wheels ij in the above-mentioned preset torque mixing algorithm to limit the braking torque requirement, where specific limiting parameters and limiting modes are as follows:

1) Battery state X: in particular, the charge current I _B and the cell voltage are used to predict the state of charge of the battery.

2) Vehicle speed v _x: at higher speeds, the maximum electric torque is reduced due to power limitations, and at low speeds only friction torque is used, since no energy recovery is performed at this time.

3) Motor/battery temperature (θ _EM,θ_B): it may be desirable to reduce the motor torque request to avoid overheating of the components.

The mixing factor alpha is adjusted according to all constraints. When the motor is saturated, the friction brake is activated independently, regardless of the phase selected; to smoothly transition between the friction brake and the motor, a first order filter with a 5Hz cut-off frequency would modify the mixing factor and then send it to the controller.

Based on any one of the above embodiments, in the method, the determining the final required torque of each of the four hubs of the electric vehicle based on the displacement information, the mass, the gradient, and the longitudinal speed specifically includes:

Specifically, the torque output by the engine is distributed to each wheel according to the current running state of the electric automobile and the road surface condition, namely, the torque distribution proportion of the four wheels of the automobile is determined from the running information of the electric automobile and the input of a brake pedal by a driver. The total braking torque is distributed to each hub by adopting a preset torque distribution algorithm, so that the braking torque required to be output by each hub is obtained, namely, the initial required torque of each of the four hubs is obtained, and the braking torque is output by each of the four tires, so that the instability of the vehicle caused by the braking of the independent front wheels or the braking of the independent rear wheels can be avoided; the preset torque distribution algorithm is not specifically limited herein.

In addition, there are further provided safety control, that is, a preset tire slip control algorithm for preventing wheel slip and anti-lock braking, which controls the tire slip rate within a proper range to ensure that the wheels do not slip and lock up, and there are various preset tire slip control algorithms, which may be a tire slip control algorithm based on proportional integral control against integral saturation or a tire slip control algorithm based on integral slip mode control, etc., without being particularly limited thereto.

Based on any one of the above embodiments, in the method, the determining initial required torque of each of the four hubs of the electric vehicle based on the displacement information, the mass, the gradient, and the longitudinal speed specifically includes:

Specifically, when a situation that the driver needs to decelerate occurs, the driver may step on a brake pedal, the basic brake controller may convert the pedal displacement into a driver requested brake torque that needs to be executed by the electric vehicle, and a preset mapping relationship exists between the driver requested brake torque and the pedal displacement, for example, the larger the pedal displacement is, the larger the brake torque is, the larger the change of the pedal displacement in a certain time is, the more urgent the driver brakes, and the larger the brake torque is, where the construction method of the preset mapping relationship is not specifically limited.

In addition to the driver requested braking torque determined by the driver output brake pedal displacement information, which is required to be executed by the electric vehicle, there is an additional braking torque determined based on the mass of the electric vehicle, the current road gradient and the current longitudinal speed, which is used to compensate for the driver requested braking torque, resulting in a total braking torque required by the electric vehicle.

And distributing the torque output by the engine to each wheel according to the current running state of the electric automobile and the road surface condition, namely determining the torque distribution proportion of four wheels of the vehicle from the running information of the electric automobile and the input of a brake pedal by a driver. And a preset torque distribution algorithm is adopted to distribute the total braking torque to each wheel hub, so that the braking torque required to be output by each wheel hub is obtained, namely, the initial required torque of each wheel hub is respectively output by four wheels, and the braking torque is output by each tire, so that the instability of the vehicle caused by the braking of a single front wheel or the braking of a single rear wheel can be avoided. The preset torque distribution algorithm is not particularly limited, and various ones are available.

Based on any of the above embodiments, in the method, the determining the total braking torque based on the displacement information, the mass, the gradient, and the longitudinal speed specifically includes:

Specifically, the foundation brake controller is used to generate a driver requested brake torque, and when the driver depresses the brake pedal to output brake pedal displacement information, the foundation brake controller can convert the brake pedal displacement information into a corresponding driver requested brake torque T _re, specifically, the driver requested brake torque T _re is calculated by the following formula:

T_re＝m_va_refr_w

Where a _ref describes a reference deceleration related to brake pedal effort characteristics, r _w is tire radius, and m _v is vehicle mass.

In general, these characteristics are highly dependent on the parameters of the hydraulic assembly, in particular the pipe diameter and length. Due to the BBW system involved, the brake pedal appears to be artificially generated by a brake pedal simulator. In the event of a system failure (e.g., a power interruption), all valves are opened, thereby establishing a direct hydraulic linkage between the master cylinder and the front actuator. The brake pedal simulator stops operating and the brake pedal feel is as if the hydraulic system were not supported by the brake booster.

In order to maintain a smooth motion of the vehicle in a sloped condition, additional braking torque is also required to be compensated for in order to balance the tendency of gravity to "slip" or "undershoot" during both uphill and downhill conditions of the vehicle, caused by the component of gravity in parallel to the slope. Accordingly, embodiments of the present invention also incorporate vehicle mass and road grade observers to determine additional braking torque. Since vehicle mass is directly related to the calculation of torque demand and road grade creates additional drag, the equation for the balance of longitudinal forces using Newton's second law is as follows:

Where m _v is the total mass of the vehicle, v _x is the longitudinal speed, v _x is the derivative of the longitudinal speed, F _d and F _air are the driving force and air resistance, respectively, g is the gravitational acceleration, Is the road grade angle, T _d is the torque to drive the tires (i.e., the extra braking torque), r _w is the wheel radius, c _w is the air resistance coefficient, ρ is the air density, a _v is the effective frontal area of the vehicle, v _air is the air speed, and F _slope is the grade resistance.

Depending on the inertial measurement unit with which the vehicle is equipped, the longitudinal acceleration (a _x) and the gravity (g) can be measured directly on a flat road. However, when the vehicle is traveling on a grade, the signal of a _x may be superimposed by the gravitational component. Thus, the actual acceleration will be higher or lower than the measured value. The previous formula can be modified as follows:

By the above method, the additional braking torque T _d (i.e., the gradient compensation braking torque) and the driver-requested braking torque T _re can be determined, and then the total braking torque T _total is determined by the following formula:

T_total＝T_re+T_d。

Based on any one of the foregoing embodiments, in the method, the determining, based on the total braking torque, initial required torques of four hubs of the electric vehicle by a preset torque distribution algorithm specifically includes:

Specifically, after the total torque demand is generated, it is transmitted through the transmission to be distributed to the individual wheels. The torque distribution ratio of the four wheels of the vehicle is determined according to the current running state and the road surface condition, namely, the vehicle running information and the input of the driver to the control system. The following two sets of formulas give the ideal torque distribution, and assuming that two axles are utilized simultaneously, the vertical load F _zf,F_zr applied to the front and rear wheels, and the braking force F _xf,F_xr applied to the front and rear wheels are expressed as follows:

W＝T_total×r_w

wherein r _w is the tire radius, T _total is the total braking torque, L is the wheelbase of the vehicle, a, b are the distances between the rear axle and the front axle in the x-axis direction of the vehicle and the center of gravity respectively, H represents the height of the center of mass of the electric automobile, mu is the friction coefficient of the contact point between the front axle tire and the road surface, and W represents the load of the whole vehicle; further, the friction coefficient mu should satisfy the following formula to avoid early locking of the rear wheels,

Wherein a _x is longitudinal acceleration.

Based on any one of the above embodiments, in the method, the preset tire slip control algorithm is a tire slip control algorithm based on proportional integral control against integral saturation or a tire slip control algorithm based on integral slip mode control.

Specifically, the tire slip describes the deviation of the wheel speed ω ^ij at the tire-to-road contact point relative to the vehicle speed, specifically expressed by the following formula:

Wherein, Representing the difference between the vehicle speed of the ij-th wheel and the speed of the particular wheel, v _x is the vehicle speed, v _w is the speed of the particular wheel, ω ^ij is the angular speed of the ij-th wheel, and r _d ^ij is the dynamic tire radius of the loaded wheel after one complete revolution of the ij-th wheel.

The tire slip control system is very important for driving safety, and it can suppress excessive slip (lambda < 0) and rotation (lambda > 0) of the wheel during ABS or running traction control. For this reason, whenever the slip exceeds a fixed threshold (referred to as a reference slip lambda _ref ^ij), the controller is activated and remains active until stopped when the driver depresses the brake or accelerator pedal. In order to reduce the slip again below the threshold value, the required torque is corrected by means of a so-called reactive component (T _rea ^ij). The preset tire slip control algorithm is defined as a tire slip control algorithm based on proportional integral control against integral saturation or a tire slip control algorithm based on integral slip mode control in the embodiment of the present invention, and the two tire slip control algorithms are specifically described below.

1. Proportional-integral control against integral saturation

Proportional-integral control is very easy to implement. Typically, an additional differentiating section is included to suppress any rise in the output signal. In the present design, the derivative portion is omitted because it would make the controller more sensitive to noise. The feedback saturation portion implemented compensates for the integral component when the control error is saturated, otherwise it is inactive. The error signal lambda _e, the difference between the actual slip lambda and the reference slip lambda _ref, is defined by the following formula.

The error is an input to the tire slip mode controller, where the proportional-integral control law is as follows:

Where K _P is the proportional gain, τ _i is the time constant of integration, τ _a is the time constant of the anti-integral saturation mechanism part, sat represents the saturation function, and u _PI is the output variable of the proportional-integral controller (i.e., the final output torque of each hub). The active state is a state in which the tire has slipped, and the inactive state is a stationary state of the automobile.

2. Integral sliding mode control

The integrated sliding mode control (ISM) has better performance than the proposed proportional-integral control because the ISM combines the advantages of the integrated part to smoothly track the reference slip and the fast switching control of the sliding mode method. Thus, the jitter effect can be reduced in a more accurate error compensation manner. The control law of ISM control is shown in the following formula,

u_ISM＝u_c+u_d

Wherein u _ISM is composed of two parts, namely a continuous part u _c and a discontinuous part u _d, u _ISM is taken as the output quantity of the controller and is the final output torque of each hub, wherein the continuous part is equal to PI control in the proportional-integral control algorithm for resisting integral saturation, and the discontinuous part can be described by the following formula.

u_d＝-K_ISMsign(s)

Where s is the slip plane, K _ISM integrates the slip gain, sign represents the sign function, and is represented by the slip variable s ₀＝λ_e and an integral partAnd (3) determining:

wherein the input matrix B is derived from the representation of the wheel slip quarter dynamics vehicle model:

Where J _w is the inertia of the wheel, F _x is the longitudinal tire force, and T _w is the torque applied by the controller output u _ISM.

To avoid control degradation caused by estimation uncertainty, system uncertainty is introduced The former formula becomes:

To reduce jitter, the discontinuous portion is low pass filtered as expressed by the following formula:

Where τ _f is the time constant of the first order filter.

Based on any one of the above embodiments, the present invention further provides a method for controlling braking of an electric vehicle with a hub-type driving and full decoupling brake-by-wire system, and fig. 2 is a control flow chart of an overall controller provided by the present invention, as shown in fig. 2, and specific implementation steps are as follows:

1) And (5) a mechanical and electrical integrated system description. And researching and analyzing the structure of the hybrid power line control braking system. The driver's brake process sensor operation, the target vehicle component composition, and the associated parameter settings of the brake-by-wire system are determined.

Specifically, fig. 3 is a schematic diagram of a brake control structure of an electric vehicle with a hub-type driving and full-decoupling brake-by-wire system, as shown in fig. 3, where the main purpose of the control system is to design a brake control arrangement, and provide a safety system (such as wheel slip control) and an enhancement function (such as torque mixing), so that the vehicle can maintain a stable running state under a gradient condition.

The braking demand of the driver is measured by a travel sensor. To ensure fail-safe, the system employs two redundant travel sensors. By means of the pressure sensor, the Vehicle Control Unit (VCU) can rationalise the travel and pressure, improving redundancy and reliability of the system. Within the VCU, the braking demand is converted to an equivalent torque demand and distributed to the corners of the vehicle according to an in-built electronic brake force distribution (EBD) definition. Thus, K _f and K _r are introduced as factors of torque to pressure and torque to force transfer for generating the input signals of EHB and EMB. These signals are transmitted by a Brake Control Unit (BCU). The relationship between the two factors is shown as follows:

Wherein R _disc represents the effective brake disc radius; a _pist represents a brake caliper piston cross section; mu _br represents the coefficient of friction between the brake disc and the brake pad. Wherein K _f =0.02 bar/Nm and K _r = 6.963N/Nm.

Such hybrid brake-by-wire systems initially have a hybrid function, but without any switching valves. In addition to the decoupled braking system, the target vehicle is equipped with a hub drive at the rear axle, the equipment used being designed on the basis of a liquid-cooled synchronous machine with a maximum power of 110kW and a maximum torque of 1500Nm (boost mode). Furthermore, the mechatronic design is used for various vehicle applications, such as passenger cars, SUVs, pick-ups or minivans. The knuckle adapter is suitable for most vehicles on the market and is convenient to implement. It also includes a friction brake device with an Electronic Parking Brake (EPB), an electromechanical sliding caliper that is mounted Brembo in the described design. In contrast to conventional electric drive systems, which are directly connected to the wheels, there are many advantages in terms of weight, packaging and efficiency.

2) And (5) designing an observer. There are some parameters in the target vehicle that cannot be directly quantified, which are very important for vehicle dynamics control, and the estimation of different signal parameters is required.

Many sensors in modern vehicles are able to measure different signals, but also some parameters cannot be directly quantified or can only be measured with high technical and cost effort. Since some of these non-measurable parameters are important for vehicle dynamics control, it is necessary to estimate these parameters.

Accordingly, embodiments of the present invention also incorporate vehicle mass and road grade observers to determine additional braking torque. Since vehicle mass is directly related to the calculation of torque demand and road grade creates additional drag, the equation for the balance of longitudinal forces using Newton's second law is as follows:

3) And (5) designing a controller. The combined and integrated controller is designed to provide an anti-lock braking system and enhance the functions of regenerative braking and the like through the hub motor.

In order to combine the advantages and functions of the driveline and the brake system, a suitable control strategy is required. Therefore, the technology adopts an integrated controller, can provide common safety functions such as ABS and the like, and realizes functional enhancement such as regenerative braking and the like through IWM.

3.1A foundation brake controller.

The basic brake controller is used for generating a driver request brake torque, when the driver presses a brake pedal to output brake pedal displacement information, the basic brake controller can convert the brake pedal displacement information into a corresponding driver request brake torque T _re, and specifically, the driver request brake torque T _re is calculated by the following formula:

T_re＝m_va_refr_w

T_total＝T_re+T_d。

After the total torque demand is generated, it is transmitted through the transmission to be distributed to the individual wheels. The torque distribution ratio of the four wheels of the vehicle is determined according to the current running state and the road surface condition, namely, the vehicle running information and the input of the driver to the control system. The following two sets of formulas give the ideal torque distribution, and assuming that two axles are utilized simultaneously, the vertical load F _zf,F_zr applied to the front and rear wheels, and the braking force F _xf,F_xr applied to the front and rear wheels are expressed as follows:

W＝T_total×r_w

Wherein a _x is longitudinal acceleration.

3.2 Torque hybrid controller).

The target vehicle employs a hub drive system that may be used to slow down when the electric machine is operating in generator mode. This approach may recover kinetic energy for battery recharging, for example, to enhance endurance mileage.

The preset torque mixing rule preferably adopts an in-wheel motor to output the final required torque, and only when the required motor torque T _WSC (i.e. the final required torque) is higher than the maximum achievable electric torque T _{max_EM}, a mode of mixing and outputting a friction brake (i.e. a brake pad) and an in-wheel motor is adopted, namely the friction brake output braking torque (T _{re_FB}) and the motor output braking torque (T _{re_EM}) work in parallel to increase the in-wheel torque (T _w) to the total required amount (T _WSC). Thus, a blend factor, α, is defined as the value at each wheel angle, is introduced herein. If 0< α ^ij <1, the in-wheel motor brake and the friction brake are operated in parallel, if α ^ij =1, the in-wheel motor brake and the friction brake are operated in series, wherein α ^ij represents a mixing factor of the ijth wheel, i=1 represents a front wheel, i=2 represents a rear wheel, j=1 represents a left wheel, and j=2 represents a right wheel. The related description formula of the preset torque mixing algorithm is as follows:

a) Battery state X: in particular, the charge current I _B and the cell voltage are used to predict the state of charge of the battery.

B) Vehicle speed v _x: at higher speeds, the maximum electric torque is reduced due to power limitations, and at low speeds only friction torque is used, since no energy recovery is performed at this time.

C) Motor/battery temperature (θ _EM,θ_B): it may be desirable to reduce the motor torque request to avoid overheating of the components.

4) And establishing proportional-integral control and integral sliding mode control for resisting integral saturation, improving a traditional proportional-integral control strategy, and constructing a cost function and an optimization method.

4.1 Proportional-integral control against integral saturation

4.2 Integral sliding mode control

u_ISM＝u_c+u_d

u_d＝-K_ISMsign(s)

Where τ _f is the time constant of the first order filter.

5) Brake control systems, including safety systems (hub slip control, etc.) and enhancement functions (torque mixing, etc.), are implemented and tested to evaluate performance of the continuous control strategy.

The complete flow of the implementation and test integrated control system is as follows:

And carrying out environment creation and experimental verification on a vehicle dynamics model of the target movement type multifunctional vehicle through CarMaker. The normalized integral of the absolute value of the control action (IACA) is determined by the following formula:

Wherein, T _WSC represents the control force of the hub type driving motor and the wire control system at each angle, T ₀ is the test starting time, T _N is the test ending time, and the wear of the actuator can be evaluated through IACA parameters.

First, by testing with a high and low coefficient of friction μ, a continuous control strategy is achieved that reduces braking distance compared to closing hub slip control. Secondly, testing the continuous method can improve the driving comfort level under the condition that emergency braking is not needed and testing at different pedal application speeds, and the variation of the braking distance and the yaw rate is less than 1%. Accordingly, on the underground surface with the uneven road surface friction coefficient mu, namely mu _split, the continuous control method and the rule-based control method are tested, so that the comfort is improved, the longitudinal jolt is obviously reduced, and the rule-based control method has a larger jolt problem, so that the running experience is poor. Finally, in combination with the torque hybrid controller, setting the mixing factor α=1 on roadways with different friction coefficients, the vehicle shows a significant difference in stability compared to the case of unmixed torque, especially a significant reduction in the control yaw rate in both PI and ISM.

The following describes an embodiment of the apparatus of the present application, which can be used to perform the brake control method of the electric vehicle in the above embodiment of the present application. For details not disclosed in the embodiment of the device of the present application, please refer to the embodiment of the braking control method of the electric vehicle.

Based on any of the above embodiments, fig. 4 is a schematic structural diagram of a brake control device of an electric vehicle according to the present invention. As shown in fig. 4, the apparatus includes an acquisition unit 410, a determination unit 420, and a mixing unit 430, wherein,

The acquiring unit 410 is configured to acquire displacement information of a brake pedal of the electric vehicle, a mass and a current longitudinal speed of the electric vehicle, and a current road gradient;

The determining unit 420 is configured to determine final required torques of four hubs of the electric vehicle, respectively, based on the displacement information, the mass, the gradient, and the longitudinal speed;

The mixing unit 430 is configured to distribute each of the final required torques based on a preset torque mixing rule, so as to obtain respective mixed torques of four hubs of the electric vehicle, where the mixed torques include a friction brake output torque and a hub motor output torque.

The device provided by the embodiment of the invention integrates the control of the chassis of the vehicle by combining the brake information output by the driver, the state of the vehicle and the current road condition to determine the brake torque distributed to each hub and then providing the mixed torque of the combined operation of the electric motor and the friction brake on each hub, thereby providing the enhancement function such as the mixed torque, ensuring that the vehicle keeps in a stable motion state under the condition of gradient, and obviously improving the continuous control strategy of the mixed torque in the aspects of vehicle stability and driving comfort.

Based on any of the foregoing embodiments, in the apparatus, the mixing unit is specifically configured to:

Based on any one of the foregoing embodiments, in the apparatus, the determining unit is specifically configured to:

Based on any one of the above embodiments, in the device, the determining initial required torque of each of the four hubs of the electric vehicle based on the displacement information, the mass, the gradient, and the longitudinal speed specifically includes:

Based on any of the above embodiments, in the apparatus, the determining the total braking torque based on the displacement information, the mass, the gradient, and the longitudinal speed specifically includes:

Based on any one of the foregoing embodiments, in the device, the determining, based on the total braking torque, initial required torques of four hubs of the electric vehicle according to a preset torque distribution algorithm specifically includes:

Based on any one of the above embodiments, in the apparatus, the preset tire slip control algorithm is a tire slip control algorithm based on proportional integral control against integral saturation or a tire slip control algorithm based on integral slip mode control.

Fig. 5 illustrates a schematic physical structure of an electric vehicle, and as shown in fig. 5, the electric vehicle may include: processor 510, communication interface (Communications Interface) 520, memory 530, and communication bus 540, wherein processor 510, communication interface 520, memory 530 complete communication with each other through communication bus 540. Processor 510 may invoke logic instructions in memory 530 to perform a method of braking control of an electric vehicle, the method comprising: acquiring displacement information of a brake pedal of the electric automobile, the mass and the current longitudinal speed of the electric automobile and the current road gradient; determining respective final required torques of four hubs of the electric vehicle based on the displacement information, the mass, the gradient, and the longitudinal speed; and distributing each final required torque based on a preset torque mixing rule to obtain the respective mixed torques of four hubs of the electric automobile, wherein the mixed torques comprise the output torque of a friction brake and the output torque of a hub motor.

Further, the logic instructions in the memory 530 described above may be implemented in the form of software functional units and may be stored in a computer-readable storage medium when sold or used as a stand-alone product. Based on this understanding, the technical solution of the present invention may be embodied essentially or in a part contributing to the prior art or in a part of the technical solution, in the form of a software product stored in a storage medium, comprising several instructions for causing a computer device (which may be a personal computer, a server, a network device, etc.) to perform all or part of the steps of the method according to the embodiments of the present invention. And the aforementioned storage medium includes: a usb disk, a removable hard disk, a Read-only memory (ROM), a random access memory (RAM, randomAccessMemory), a magnetic disk, or an optical disk, or other various media capable of storing program codes.

In another aspect, the present invention also provides a computer program product, where the computer program product includes a computer program, where the computer program can be stored on a non-transitory computer readable storage medium, and when the computer program is executed by a processor, the computer can execute a braking control method of an electric automobile provided by the above methods, and the method includes: acquiring displacement information of a brake pedal of the electric automobile, the mass and the current longitudinal speed of the electric automobile and the current road gradient; determining respective final required torques of four hubs of the electric vehicle based on the displacement information, the mass, the gradient, and the longitudinal speed; and distributing each final required torque based on a preset torque mixing rule to obtain the respective mixed torques of four hubs of the electric automobile, wherein the mixed torques comprise the output torque of a friction brake and the output torque of a hub motor.

In still another aspect, the present invention also provides a non-transitory computer readable storage medium having stored thereon a computer program which, when executed by a processor, is implemented to perform the method for controlling braking of an electric vehicle provided by the above methods, the method comprising: acquiring displacement information of a brake pedal of the electric automobile, the mass and the current longitudinal speed of the electric automobile and the current road gradient; determining respective final required torques of four hubs of the electric vehicle based on the displacement information, the mass, the gradient, and the longitudinal speed; and distributing each final required torque based on a preset torque mixing rule to obtain the respective mixed torques of four hubs of the electric automobile, wherein the mixed torques comprise the output torque of a friction brake and the output torque of a hub motor.

The apparatus embodiments described above are merely illustrative, wherein the elements illustrated as separate elements may or may not be physically separate, and the elements shown as elements may or may not be physical elements, may be located in one place, or may be distributed over a plurality of network elements. Some or all of the modules may be selected according to actual needs to achieve the purpose of the solution of this embodiment. Those of ordinary skill in the art will understand and implement the present invention without undue burden.

From the above description of the embodiments, it will be apparent to those skilled in the art that the embodiments may be implemented by means of software plus necessary general hardware platforms, or of course may be implemented by means of hardware. Based on this understanding, the foregoing technical solution may be embodied essentially or in a part contributing to the prior art in the form of a software product, which may be stored in a computer readable storage medium, such as ROM/RAM, a magnetic disk, an optical disk, etc., including several instructions for causing a computer device (which may be a personal computer, a server, or a network device, etc.) to execute the method described in the respective embodiments or some parts of the embodiments.

Finally, it should be noted that: the above embodiments are only for illustrating the technical solution of the present invention, and are not limiting; although the invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical scheme described in the foregoing embodiments can be modified or some technical features thereof can be replaced by equivalents; such modifications and substitutions do not depart from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. The brake control method of the electric automobile is characterized by being applied to a vehicle control unit, wherein the front axle of the electric automobile adopts electrohydraulic braking, and the rear axle of the electric automobile adopts electromechanical braking, and the method comprises the following steps:

2. The method for controlling braking of an electric vehicle according to claim 1, wherein the final required torques are distributed based on a preset torque mixing rule to obtain respective mixed torques of four hubs of the electric vehicle, and the mixed torques include a friction brake output torque and a hub motor output torque, and specifically include:

3. The brake control method of an electric vehicle according to claim 1 or 2, characterized in that the determination of the respective final required torques of the four hubs of the electric vehicle based on the displacement information, the mass, the gradient and the longitudinal speed, specifically comprises:

4. The brake control method of an electric vehicle according to claim 3, wherein the determining initial required torque of each of four hubs of the electric vehicle based on the displacement information, the mass, the gradient, and the longitudinal speed specifically includes:

5. The brake control method of an electric vehicle according to claim 4, characterized in that the determining of the total brake torque based on the displacement information, the mass, the gradient, and the longitudinal speed, specifically includes:

6. The method for controlling braking of an electric vehicle according to claim 5, wherein determining initial required torques of four hubs of the electric vehicle respectively by a preset torque distribution algorithm based on the total braking torque specifically comprises:

7. The brake control method of an electric vehicle according to claim 3, wherein the preset tire slip control algorithm is a proportional-integral control based tire slip control algorithm against integral saturation or an integral slip mode control based tire slip control algorithm.

8. The utility model provides a braking control device of electric automobile, its characterized in that, electric liquid braking and electromechanical braking are adopted respectively to electric automobile's front axle and rear axle, the device includes:

9. A computer-readable storage medium, wherein at least one program code is stored in the computer-readable storage medium, the at least one program code being loaded and executed by a processor to implement the operations performed by the brake control method of an electric vehicle according to any one of claims 1 to 7.

10. An electric vehicle, characterized in that it comprises one or more processors and one or more memories in which at least one program code is stored, loaded and executed by the one or more processors to implement the operations performed by the braking control method of an electric vehicle according to any of claims 1 to 7.