Braking control method and control unit for commercial electric vehicle

Abstract

Disclosed is a brake control scheme for a commercial electric vehicle, in which a brake pedal opening critical value is determined at least based on a current vehicle weight and a road adhesion coefficient during vehicle driving, the brake pedal opening critical value defining a brake energy recovery exit condition during vehicle braking; in the vehicle braking process, the current brake pedal opening degree is detected; when the current brake pedal opening degree is smaller than the brake pedal opening degree critical value, brake energy recovery is executed and maintained on a motor driving wheel of the vehicle; and when the current opening degree of the brake pedal is larger than or equal to the opening degree critical value of the brake pedal, braking energy recovery executed on the motor driving wheel is quitted, and meanwhile the braking force of a vehicle mechanical braking element on the motor driving wheel is increased so as to make up for braking force deficiency on the motor driving wheel caused by quitting of the braking energy recovery.

Description

Technical Field

[0001] The present application relates to a braking control scheme for commercial electric vehicles, in which braking energy recovery is controlled based on the starting conditions of the anti-lock braking system (referred to as the ABS system). Background Art

[0002] An electric vehicle, that is, a vehicle driven by an electric motor, can use the drive motor as a generator during braking to recover the vehicle's motion energy for charging the power battery, achieving the purpose of energy conservation and range extension. The vehicle ABS system can automatically adjust the wheel braking force during braking to prevent the wheels from locking, avoid vehicle out of control, and obtain the best braking effect. During braking, compared with avoiding vehicle out of control, the priority of braking energy recovery is lower. Therefore, when the braking torque of the wheel approaches the critical point of wheel locking, the vehicle needs to promptly exit the braking energy recovery mode to ensure the normal operation of the ABS system.

[0003] Patent document CN109808502A proposes an energy feedback exit control method applicable to pure electric vehicles. Among them, taking the motor speed and wheel speed as parameters, it is judged whether the current state meets the braking energy feedback exit condition, and in specific cases, the energy feedback exit time is delayed to avoid the sudden disappearance of the energy recovery braking torque causing the vehicle to lunge forward. However, the control method in this document may not be able to promptly exit the braking energy feedback in some extreme cases, and the braking energy feedback will interfere with the operation of the ABS system. In addition, for commercial vehicles, the vehicle weight varies greatly with the load state, and the maximum braking torque on each driving wheel varies with the vehicle weight. The control method in this document does not consider the influence brought by the vehicle weight change. Summary of the Invention

[0004] The purpose of the present application is to provide a braking control scheme applicable to commercial electric vehicles, which can accurately confirm the starting conditions of the ABS system and exit the braking energy recovery mode before the ABS system starts.

[0005] To this end, in one aspect of the present application, a braking control scheme for commercial electric vehicles is provided, in which during vehicle driving, at least based on the current vehicle weight and road surface adhesion coefficient, the critical value of the braking pedal opening is determined, and the critical value of the braking pedal opening defines the braking energy recovery exit condition during vehicle braking; during vehicle braking, the current braking pedal opening is detected; when the current braking pedal opening is less than the critical value of the braking pedal opening, braking energy recovery is executed and maintained on the motor drive wheels of the vehicle; when the current braking pedal opening is greater than or equal to the critical value of the braking pedal opening, the braking energy recovery executed on the motor drive wheels is exited, and at the same time, the braking force of the vehicle mechanical braking element on the motor drive wheels is increased to make up for the lack of braking force on the motor drive wheels caused by the exit of the braking energy recovery.

[0006] According to the present application, an expression reflecting the relationship between the critical value of the brake pedal opening, the vehicle weight, and the road surface adhesion coefficient is established for commercial vehicles. During the braking process, when the current brake pedal opening reaches the critical value of the brake pedal opening, the brake energy recovery is exited, and the mechanical braking system completely takes over the wheel braking, increasing the mechanical braking torque to compensate for the change in the braking torque caused by the brake recovery torque, ensuring that the braking torque does not mutate and ensuring that the ABS system operates normally without being interfered by the brake energy recovery. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The foregoing and other aspects of the present application will be more fully understood and appreciated from the following detailed description taken in conjunction with the accompanying drawings, in which:

[0008] Figure 1 is a schematic block diagram of the vehicle components involved in the braking control scheme of the present application;

[0009] Figure 2 is an exemplary flowchart of the braking control scheme of the present application. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0010] The present application generally relates to a control scheme during the braking process of a commercial electric vehicle. Commercial vehicles, including commercial buses, commercial trucks, etc., are clearly defined in the industry. An electric vehicle refers to a vehicle in which some or all of the wheels are driven by electric motors. The vehicle components involved in the control scheme of the present application are schematically shown in Figure 1 as follows.

[0011] In Figure 1 all four wheels 1 of the commercial electric vehicle are shown as drive wheels, and each wheel 1 is driven by a separate electric motor 2. It should be noted that the control scheme of the present application is also applicable to the following situations: two front wheels 1 are driven by a single electric motor 2, and two rear wheels 1 are driven by a single electric motor 2; only two front wheels 1 are drive wheels, each equipped with a corresponding electric motor 2, or sharing a single electric motor 2; only two rear wheels 1 are drive wheels, each equipped with a corresponding electric motor 2, or sharing a single electric motor 2.

[0012] The commercial electric vehicle is equipped with a power battery 3 for supplying power to the electric motor 2. The power battery 3 is equipped with a battery management system 4.

[0013] The vehicle is equipped with a mechanical (hydraulic) braking system, which generally includes mechanical braking elements 5 provided on each wheel 1, a brake master cylinder 6 connected to each mechanical braking element 5 through a brake pipeline (possibly equipped with a brake booster), and the operation of the brake master cylinder 6 is controlled by a mechanical brake system controller (not shown) of the vehicle based on the opening (stroke) of the vehicle brake pedal.

[0014] The vehicle is also equipped with an ABS, which is used to prevent the wheels from being locked during braking under certain vehicle driving conditions, so as to prevent the vehicle from skidding or leaving the driving road surface due to wheel locking during braking. During braking, the ABS system can early identify the locking tendency of one or more wheels, and no longer apply braking force or reduce the braking force to the wheels with locking tendency, so as to achieve rapid braking on the premise of ensuring vehicle driving safety.

[0015] The ABS system has an ABS controller 7, which intervenes in the operation of the mechanical braking system when the wheels show a locking tendency during vehicle braking and controls the mechanical braking element 5 on the driving wheel 1.

[0016] The control scheme of the present application is implemented in the control unit 10. The control unit 10 is set to be communicatively connected to the motor 2, the battery management system 4, the ABS controller 7, the brake pedal opening sensor (or analog sensor), etc. The control unit 10 can be set separately or integrated into the vehicle controller.

[0017] The following describes the control scheme executed by the control unit 10.

[0018] The control scheme of the present application is based on the following concept: during braking, based on factors such as vehicle weight and road surface adhesion coefficient, determine the critical value of the brake pedal opening corresponding to the maximum possibility of vehicle locking; if the current brake pedal opening is greater than or equal to the critical value of the brake pedal opening, then exit the regenerative braking executed on each motor-driven wheel, and at the same time increase the mechanical braking force applied by the mechanical braking element 5 on each motor-driven wheel to make up for the braking force loss caused by the exit of regenerative braking; the ABS system is ready to intervene in braking at any time.

[0019] According to the above concept, the control scheme of the present application can be constructed in various specific ways.

[0020] According to one embodiment, first, calculate the locking critical torque of each wheel according to the vehicle weight and road surface adhesion coefficient. Here, each wheel mentioned includes all the wheels of the vehicle, including motor-driven wheels and non-driven wheels.

[0021] The vehicle weight can be measured by an independent weight sensor. For example, various force or pressure sensors are provided at different positions in the vehicle. When the vehicle is static or driving at a constant speed, the vertical (up and down) forces on different parts of the vehicle obtained through these sensors can be used to calculate the current vehicle weight. Further, the vehicle usually has an empty vehicle weight (net weight) marked at the factory. These sensors have initial measurement values corresponding to the empty vehicle state of the vehicle. When the vehicle weight changes, the current vehicle weight can be calculated based on the changes in the measurement values of these sensors.

[0022] Alternatively, vehicle longitudinal dynamics can also be utilized to calculate the current vehicle weight by means of the overall vehicle driving torque during vehicle acceleration and the sensed values of the longitudinal acceleration sensors in the vehicle.

[0023] The road adhesion coefficient is the ratio between the adhesion force between the wheel and the road surface and the normal pressure of the wheel on the road surface, and can be roughly regarded as the static friction coefficient between the tire and the road surface. The larger the road adhesion coefficient, the greater the available adhesion force, and the less likely the vehicle is to skid. The road adhesion coefficient is mainly determined by factors such as the road material, road surface condition, tire structure, tread pattern, material, and the speed of vehicle movement. Generally speaking, the adhesion coefficient of a dry and good asphalt or concrete road surface is the largest, reaching 0.7 - 0.8. While the adhesion coefficient of an ice and snow road surface is the smallest and it is most likely to skid. The adhesion coefficients of various road surface conditions can be stored, and the current road surface condition is determined through the road surface image captured by the vehicle image sensor or the tire sound recorded by the acoustic sensor, and then the corresponding road adhesion coefficient is retrieved based on the determined current road surface condition.

[0024] According to a simplified method, the vehicle weight can be distributed to each wheel based on the position of each wheel, and thus the normal pressure of each wheel on the road surface can be obtained. Based on the road adhesion coefficient and the normal pressure of each wheel, the maximum static friction force between each wheel and the road surface (i.e., the friction force when the wheel transitions from rolling friction to sliding friction with the ground) can be calculated, and based on the maximum static friction force and the wheel radius, the critical locking torque of each wheel can be calculated.

[0025] Optionally, according to a more accurate method, in order to improve the calculation accuracy of the critical locking torque of each wheel, factors such as the vehicle center of gravity position and road gradient can be taken into account in the calculation.

[0026] The vehicle center of gravity position, especially the center of gravity height, can be determined by the sensed values of various displacement or acceleration sensors in the vehicle, such as suspension height sensors, suspension stroke sensors, suspension angle sensors, body tilt sensors, body height sensors, body level sensors, etc.

[0027] Alternatively, the center of gravity position, especially the center of gravity height, can also be estimated through parameters such as the current vehicle weight. For example, based on the current vehicle weight and the approximate distribution of the load inside the vehicle (such as obtained by various force or pressure sensors, roughly determined using the images of the people or goods inside the vehicle, etc.), the center of gravity position (especially the center of gravity height) can be determined. The center of gravity position (especially the center of gravity height) determined in this way is actually an estimated value, but for the applications in this application, the accuracy is sufficient.

[0028] The road gradient can be measured by an independent gradient sensor, or it can also be calculated through the sensed values of the longitudinal acceleration sensor when the vehicle is traveling at a constant speed.

[0029] If the vehicle starts on a slope, accelerates, and then changes to a constant speed, the current vehicle weight sum can be decoupled and calculated based on the changes in the vehicle drive torque and longitudinal acceleration using the vehicle dynamics equations (especially the longitudinal dynamics equations).

[0030] When the vehicle is on a horizontal road surface, the longitudinal (front - rear direction) and lateral (left - right direction) positions of the vehicle's center of gravity affect the vehicle weight distributed to each wheel. When the vehicle is on an inclined road surface, the vertical (up - down direction) position of the vehicle also affects the vehicle weight distributed to each wheel. Therefore, based on the vehicle weight, the vehicle's center of gravity position, and the road slope, the vehicle weight distributed to each wheel, that is, the normal pressure of each wheel on the road surface, can be determined more accurately, and thus the locking critical torque of each wheel can be calculated more accurately.

[0031] The locking critical torque of each wheel is the maximum braking torque that the mechanical braking element can apply to the wheel without causing wheel lock.

[0032] After calculating the locking critical torque of each wheel, the minimum locking critical torque among all the wheels' locking critical torques can be determined. Or, considering that the braking energy recovery is only performed on the motor - driven wheels, only the locking critical torques of all the motor - driven wheels can be compared to find the minimum locking critical torque among them.

[0033] Regardless of which method is used to determine the minimum locking critical torque, next, the corresponding requested braking torque limit is determined based on this minimum locking critical torque. The requested braking torque limit is less than or equal to the minimum locking critical torque. For example, it can be the minimum locking critical torque multiplied by a coefficient less than or equal to 1. For example, the coefficient is 0.8 - 1, preferably 0.8 - 0.95.

[0034] Then, the corresponding critical value of the brake pedal opening is determined based on the requested braking torque limit. The critical value of the brake pedal opening defines the condition for exiting the braking energy recovery. Since the relationship between the brake pedal opening in the vehicle and the requested braking torque of each wheel is pre - stored or known, the corresponding critical value of the brake pedal opening can be found based on the requested braking torque limit.

[0035] In summary, the critical value of the brake pedal opening can be determined based on the current vehicle weight and road surface adhesion coefficient (parameters such as the vehicle's center of gravity position and road slope can be added to improve the calculation accuracy). In practice, an approximate formula or a look-up table can be used to express the relationship between the critical value of the brake pedal opening and the current vehicle weight and road surface adhesion coefficient (which may also include factors such as the vehicle's center of gravity position and road slope). When the vehicle weight, road surface adhesion coefficient (and possibly parameters such as the vehicle's center of gravity position and road slope) are obtained in real time, the critical value of the current brake pedal opening can be directly determined using the relationship expression between these parameters and the critical value of the brake pedal opening.

[0036] During the vehicle driving process, the control unit 10 has determined the current critical value of the brake pedal opening in the above manner and is implementing the detection of the brake pedal operation. When the driver steps on the brake pedal to apply braking, if the control unit 10 detects that the brake pedal opening reaches or even exceeds the critical value of the brake pedal opening, it is confirmed that the possibility of ABS intervention is large enough, and it is necessary to execute the exit from the regenerative braking energy recovery mode, and at the same time increase the braking torque applied by the mechanical braking elements to each driving wheel to supplement the lack of braking torque caused by the exit of the regenerative braking energy recovery. Thus, it is ensured that the regenerative braking energy recovery mode is completely exited before the ABS intervenes in braking, and there will be no situation of sudden change in braking torque.

[0037] According to a feasible implementation manner of the present application, a braking control method for a commercial electric vehicle is schematically shown Figure 2 in. This control method can be executed by the aforementioned control unit 10.

[0038] In this control method, in step S1, default values for initialization are set when the vehicle starts, where the vehicle weight is initialized to the unloaded vehicle weight and the road surface adhesion coefficient is initialized to a common value for a dry paved road surface.

[0039] Next, in step S2, during the vehicle acceleration phase, the vehicle weight and slope are estimated based on the sensed values of the sensors, or based on the vehicle's total drive torque and vehicle acceleration; the road surface adhesion coefficient is estimated based on the vehicle's image sensor, sound sensor or other methods. In step S2, the vehicle's center of gravity position, road slope, etc. may also be measured or estimated.

[0040] Next, in step S3, the critical value of the brake pedal opening as the condition for exiting the regenerative braking energy recovery is calculated based on the vehicle weight, road surface adhesion coefficient (possibly taking into account the vehicle's center of gravity position and road slope).

[0041] Next, in step S4, based on the signal that the brake pedal is depressed, the current brake pedal opening is obtained in real time.

[0042] Next, in step S5, it is determined whether the current brake pedal opening is greater than or equal to the critical value of the brake pedal opening; if the determination result is "no", step S6 is executed; if the determination result is "yes", step S7 is executed.

[0043] In step S6, the braking energy recovery is maintained.

[0044] In step S7, the braking energy recovery is exited, and the braking force of the mechanical braking element is increased.

[0045] Those skilled in the art can make various adaptive modifications to the various details described above for the control unit 10 and the control method under the principle of this application.

[0046] Generally speaking, the braking control method for a commercial electric vehicle of this application includes the following steps:

[0047] During vehicle driving, determine the critical value of the brake pedal opening at least based on the current vehicle weight and the road surface adhesion coefficient, and the critical value of the brake pedal opening defines the braking energy recovery exit condition during vehicle braking;

[0048] During vehicle braking, detect the current brake pedal opening;

[0049] When the current brake pedal opening is less than the critical value of the brake pedal opening, perform and maintain braking energy recovery on the motor drive wheels of the vehicle; when the current brake pedal opening is greater than or equal to the critical value of the brake pedal opening, exit the braking energy recovery performed on the motor drive wheels, and at the same time increase the braking force of the vehicle's mechanical braking element on the motor drive wheels to make up for the lack of braking force on the motor drive wheels caused by the exit of the braking energy recovery.

[0050] In the braking control method, the critical value of the brake pedal opening can be determined in the following manner:

[0051] Determine the locking critical torque of each wheel at least based on the current vehicle weight and the road surface adhesion coefficient;

[0052] Determine the requested braking torque limit value based on the locking critical torque of each wheel;

[0053] Determine the corresponding critical value of the brake pedal opening based on the requested braking torque limit value.

[0054] In the braking control method, the requested braking torque limit value can be determined based on the minimum locking critical torque among the locking critical torques of each wheel; it can also be determined based on the minimum locking critical torque among the locking critical torques of each motor drive wheel.

[0055] The requested braking torque limit value can be the minimum locking critical torque multiplied by a coefficient less than or equal to 1.

[0056] In the braking control method, the anti-lock critical torque of each driving wheel can be determined based on the current vehicle weight, the vehicle center of gravity, the road surface adhesion coefficient, and the road gradient.

[0057] In the braking control method, the current vehicle weight and the road gradient can be detected by using sensors in the vehicle; alternatively, the current vehicle weight and the road gradient can be decoupled and calculated based on the overall vehicle driving torque and acceleration when the vehicle is accelerating.

[0058] In the braking control method, the road surface adhesion coefficient can be estimated based on the sensing information of an image sensor or an acoustic sensor of the vehicle.

[0059] In the braking control method, the position of the vehicle center of gravity can be detected by using sensors in the vehicle; alternatively, the position of the vehicle center of gravity, especially the height of the center of gravity, can be estimated based at least on the current vehicle weight.

[0060] The present application also relates to a computer program product which contains executable instructions that can implement the above control method when executed by a processor.

[0061] According to the present application, for a commercial vehicle, an expression reflecting the relationship between the critical value of the brake pedal opening, the current vehicle weight, and the road surface adhesion coefficient is established. During the vehicle braking process, when the current brake pedal opening reaches the critical value of the brake pedal opening, the brake energy recovery is exited, and the mechanical braking system completely takes over the wheel braking, increasing the mechanical braking torque to compensate for the change in the braking torque caused by the brake recovery torque, ensuring that the braking torque does not change suddenly and ensuring that the ABS system works normally without being interfered by the brake energy recovery. Since the vehicle weight and the position of the center of gravity of a commercial vehicle often change greatly during actual use, the braking control solution of the present application can provide a more accurate brake energy recovery mechanism.

[0062] Although the present application is described here with reference to specific embodiments, the scope of the present application is not limited to the shown details. Various modifications can be made to these details without departing from the basic principles of the present application.

Claims

1. A braking control method for commercial electric vehicles, comprising the following steps: During vehicle driving, determine a critical value of the brake pedal opening at least based on the current vehicle weight and the road surface adhesion coefficient, and the critical value of the brake pedal opening defines the braking energy recovery exit condition during vehicle braking; During vehicle braking, detect the current brake pedal opening; When the current brake pedal opening is less than the critical value of the brake pedal opening, perform and maintain braking energy recovery on the motor drive wheels of the vehicle; when the current brake pedal opening is greater than or equal to the critical value of the brake pedal opening, exit the braking energy recovery performed on the motor drive wheels, and at the same time increase the braking force of the vehicle mechanical braking element on the motor drive wheels to make up for the lack of braking force on the motor drive wheels caused by the exit of braking energy recovery.

2. The braking control method according to claim 1, wherein The critical value of the brake pedal opening is determined in the following manner: Determine the critical locking torque of each wheel at least based on the current vehicle weight and the road surface adhesion coefficient; Determine the requested braking torque limit based on the critical locking torque of each wheel; Determine the corresponding critical value of the brake pedal opening based on the requested braking torque limit.

3. The braking control method according to claim 2, wherein, Determine the requested braking torque limit based on the minimum critical locking torque among the critical locking torques of each wheel; Optionally, determine the requested braking torque limit based on the minimum critical locking torque among the critical locking torques of each motor drive wheel.

4. The braking control method according to claim 3, wherein, The requested braking torque limit is the minimum critical locking torque multiplied by a coefficient less than or equal to 1.

5. The braking control method according to claim 2, wherein, Determine the critical locking torque of each drive wheel based on the current vehicle weight, the vehicle center of gravity, the road surface adhesion coefficient, and the road gradient.

6. The braking control method according to claim 5, wherein, Use the sensors in the vehicle to detect the current vehicle weight and the road gradient; Alternatively, decouple and calculate the current vehicle weight and the road gradient based on the vehicle's overall drive torque and acceleration during vehicle acceleration.

7. The braking control method according to claim 5, wherein, Estimate the road surface adhesion coefficient based on the sensing information of the vehicle's image sensor or sound sensor.

8. The braking control method according to claim 5, wherein, Use the sensors in the vehicle to detect the position of the vehicle center of gravity; Alternatively, estimate the position of the vehicle center of gravity, especially the center of gravity height, at least based on the current vehicle weight.

9. A braking control unit for commercial electric vehicles, configured to execute the braking control method according to any one of claims 1-8.

10. A computer program product, which contains executable instructions, and when the instructions are executed by a processor, can implement the braking control method according to any one of claims 1-8.

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