Method, computing unit and computer program for braking a vehicle with two electric drive motor

Abstract

The invention relates to a method for braking a vehicle (10) having a first electric drive motor (14) and a second electric drive motor (16), in which, in a first braking phase, a first target value for a braking torque is specified for the first electric drive motor (14) and a second target value for a braking torque is specified for the second electric drive motor (16), a current speed of the vehicle (10) is detected as an actual speed, and when the actual speed reaches a first threshold value (v0) for the vehicle speed, a first target value for the braking torque is increased and a second target value for the braking torque is reduced at the same time in a redistribution phase, when the actual speed reaches a second threshold value (v1) for the vehicle speed, the specification of the first target value for the braking torque is ended, and a speed target value trajectory (202) is specified in the second braking phase, said speed target value trajectory extending from the second threshold value (v1) for the vehicle speed to a speed zero.

Description

Technical Field

[0001] The invention relates to a method for braking a vehicle having two electric drive motors, as well as to a computing unit and a computer program for carrying out the method. Background Art

[0002] Various technical devices are available for braking a vehicle, for example wheel brakes or electric motors, which are either operated as generators so that a braking magnetic field is induced in the coils of the electric motor, or which are energized in a targeted manner so that the electric motor is exerted with a torque that decelerates the vehicle.

[0003] The regulation of electric motors in vehicles is usually based on a target torque as a reference variable. Since electric motors can provide both positive and negative torques regardless of the direction of rotation, there is the problem of precisely setting a specific position or speed of the electric motor in the low speed range. It must be ensured that no torque is generated at any time that would accelerate the vehicle in the opposite direction of the desired direction of movement. For example, if the vehicle is to be braked to a standstill by means of an electric motor, it must be ensured that the vehicle does not immediately reverse after the standstill, which is introduced by a corresponding torque of the electric motor.

[0004] The stationary state of a vehicle is usually detected based on an evaluation of the vehicle speed, which in turn is derived from the wheel speed. For this purpose, an incremental encoder is usually used, so that the speed signal of the vehicle only appears as a discontinuous function. Due to this measurement inaccuracy and the fact that there are delays in signal processing, the use of the wheel speed as an input variable for regulating the speed of the vehicle in the low speed range, in particular at speeds close to zero, is not suitable.

[0005] For the purpose of improvement, DE 10 2019 205 180 A1 proposes a method for braking a vehicle comprising an electric drive motor, wherein the vehicle is brought to a standstill while using the electric drive motor for speed control. Summary of the invention

[0006] According to the invention, a method for braking a vehicle having two electric drive motors and a computing unit and a computer program for carrying out the method are proposed having the features of the independent patent claims. Advantageous embodiments are the subject matter of the dependent claims and the following description.

[0007] The invention is advantageously suitable for vehicles with multiple electric drive motors or multiple driven axles, for example so-called four-wheel drive (AWD, all-wheel drive), in particular vehicles that support purely electric driving and in which speed-controlled stopping / starting processes can be implemented. With the invention, in particular in vehicles with multiple electrically driven axles, a gentle, comfortable, jolt-free and noise-free stopping process without deceleration fluctuations can be ensured.

[0008] The invention provides for this by ensuring that the stopping process to a standstill is carried out exclusively by means of the electric drive machine, even if two or more electric drive motors are used in the vehicle for normal operation.

[0009] The present invention relates specifically to a method for braking a vehicle having a first electric drive motor and a second electric drive motor, wherein in a first braking phase, a first target value for a braking torque is predefined for the first electric drive motor and a second target value for a braking torque is predefined for the second electric drive motor, wherein the current speed of the vehicle is detected as the actual speed, wherein when the actual speed reaches a first threshold value for the vehicle speed, the first target value for the braking torque is increased and the second target value for the braking torque is simultaneously reduced in a redistribution phase, wherein when the actual speed reaches a second threshold value for the vehicle speed, the predefinition of the first target value for the braking torque is terminated and a speed target value trajectory is predefined in a second braking phase, wherein the speed target value trajectory extends from the second threshold value for the vehicle speed to zero speed.

[0010] In other words, when the vehicle is stopped, the entire braking torque is therefore first concentrated on the electric drive motor in the redistribution phase, which is then also used only for braking. In particular, the total braking torque is kept constant in the redistribution phase in order to avoid irritating the driver. The redistribution phase is hereby arranged in particular such that the redistribution phase ends in good time when the second braking phase begins with the predetermination of the speed target value trajectory.

[0011] It has been shown that the simultaneous use of two drive motors in speed-controlled operation, which is advantageous for the stopping process itself, is disadvantageous. Due to certain differences in the transmissions (reduction stages, differentials) and tires (different sizes, different degrees of wear) of the drive motors, different wheel speeds occur between the wheels driven by the drive motors (for example, front and rear wheels). Speed ​​control (or position control) of the two drive motors for stopping can therefore lead to tensions and noises in the chassis due to the different wheel speeds and can also lead to vibrations.

[0012] Within the scope of the invention, only one electric drive motor is now used for stopping, so that corresponding known methods as described in the introduction can in particular also be used for this purpose.

[0013] In one embodiment, during the redistribution phase, the first target value for the braking torque is increased to the sum of the first target value and the second target value at the beginning of the redistribution phase, and the second target value for the braking torque is reduced to zero. This is advantageous in order to switch one of the drive motors to inactivity. It can then be operated at idle speed.

[0014] In one embodiment, the first threshold value for the vehicle speed is determined by the time period that is at least required to increase the first target value for the braking torque to the sum of the first target value and the second target value and to reduce the second target value for the braking torque to zero at the beginning of the redistribution phase. In this embodiment, the shortest possible redistribution phase is assumed, wherein the first threshold value defines the beginning of the redistribution phase.

[0015] In one embodiment, the first threshold value for the vehicle speed is determined by the current deceleration of the vehicle. This is advantageous because the greater the current deceleration, the greater the first threshold value (i.e., the start speed of the redistribution), whereby the redistribution is terminated at the latest when the second threshold value (i.e., the start time of the stationary state control) is reached.

[0016] In one specific embodiment, the first threshold value for the vehicle speed is determined by the second threshold value for the vehicle speed. This is advantageous so that the redistribution ends at the latest when the second threshold value is reached, ie the start time of the stationary state control.

[0017] In one embodiment, the first threshold value for the vehicle speed is determined as the sum of the product of the second threshold value for the vehicle speed and the current deceleration of the vehicle and the time period required to increase the first target value for the braking torque to the sum of the first target value and the second target value at the beginning of the redistribution phase and to reduce the second target value for the braking torque to zero. With this solution, the redistribution phase is as short as possible and can be performed at the latest possible time before the second braking phase.

[0018] A computing unit according to the present invention, for example a control unit of a vehicle, is configured, in particular in terms of programming, to carry out the method according to the present invention.

[0019] It is also advantageous to implement the method according to the invention in the form of a computer program or computer program product with a program code for executing all method steps, because this leads to particularly low costs, in particular if the controller used for implementation is also used for other tasks and therefore already exists. Finally, a machine-readable storage medium is provided, with a computer program stored thereon as described above. Suitable storage media or data carriers for providing computer programs are in particular magnetic, optical and electrical memories, such as hard disks, flash memories, EEPROMs, DVDs, etc. The program can also be downloaded via a computer network (Internet, intranet, etc.). Such downloading can be done here either wired or wired or wirelessly (e.g., via a WLAN network, 3G, 4G, 5G or 6G connection, etc.).

[0020] Further advantages and improvements of the invention are apparent from the description and the accompanying drawings.

[0021] The invention is schematically illustrated in the drawings using exemplary embodiments and is described below with reference to the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 A schematic diagram of a vehicle set up to carry out an exemplary embodiment of the method according to the invention is shown;

[0023] Figure 2 The speed profile of the embodiment according to the invention during an exemplary stopping process is shown. DETAILED DESCRIPTION

[0024] Figure 1 The schematic diagram of a vehicle 10 which is set up for carrying out an exemplary embodiment of the method according to the invention is shown. The vehicle 10 comprises a control unit 12, which can in particular be a control unit of a power electronics system. The vehicle 10 also comprises a first electric drive motor 14 which is set up for driving at least one wheel 11 of the vehicle 10, for example a rear wheel or a rear axle. A sensor device 15 is arranged on the first electric drive motor 14 in such a way that the sensor device 15 detects the angular position and / or the rotational speed of the first electric drive motor 14 and transmits it to the control unit 12.

[0025] The vehicle 10 further comprises a second electric drive motor 16 which is designed to drive at least one (other) wheel, for example a front wheel or a front axle, of the vehicle 10. The sensor device 17 is arranged on the second electric drive motor 16 in such a way that the sensor device 17 detects the angular position and / or the rotational speed of the second electric drive motor 16 and transmits it to the control unit 12.

[0026] The control unit 12 is connected to the first electric drive motor 14 and the second electric drive motor 16 via a signal line, so that the electric drive motor 14 or 16 can be regulated and / or controlled by the control unit 12 .

[0027] For example, if the driver wants to bring the vehicle to a standstill, the driver usually actuates the brakes for this purpose. In the context of the embodiment, it is provided that the braking process is first carried out using the first drive motor 14 and the second drive motor 16 and finally only carried out using the first drive motor 14, as described below in a coherent and comprehensive manner by way of example with reference to the drawings.

[0028] Figure 2, wherein the vehicle speed v is plotted against the time t. In a first braking phase until the time t0, deceleration is performed using the first drive motor 14 and the second drive motor 16, for example by controlling the first drive motor 14 and the second drive motor 16 with a target value for a constant, in particular the same, braking torque. In this first braking phase, a first target value for the braking torque is therefore predefined for the first electric drive motor 14 and a second target value for the braking torque is predefined for the second electric drive motor 16.

[0029] At time t0 , a speed v0 is reached as a first threshold value for the vehicle speed, which represents a triggering event for the transition to the redistribution phase.

[0030] In the redistribution phase, the first target value for the braking torque is increased and the second target value for the braking torque is reduced at the same time. In particular, in the redistribution phase, the first target value for the braking torque is increased to the sum of the first target value and the second target value at the beginning of the redistribution phase, and the second target value for the braking torque is reduced to zero, and the sum of the first and second target values ​​remains constant. Thus, the braking process continues unchanged for the driver, but the braking torque is concentrated on the first drive motor. For example, braking can be performed with a constant deceleration, which is, for example, a.

[0031] At time t1, the speed v1 as the second threshold value for the vehicle speed is reached, which represents a triggering event for the transition to the second braking phase. At this time, the speed target value trajectory is determined (in particular as v(t)), and the first drive motor 14 is then braked to a standstill according to the speed target value trajectory. The second drive motor 16 is running at idle speed.

[0032] In particular, the speed target value trajectory may result in a decrease in the deceleration from the current value a to a value of zero at zero speed. The decrease in the deceleration may be linear, ie the slope is constant.

[0033] The method for calculating the redistribution phase or first threshold value v0 for the vehicle speed is described below.

[0034] The starting point can be the current deceleration, which is given by the current braking torque and therefore remains unchanged during the redistribution phase according to the above-described design, i.e., still prevails at the end of the redistribution phase. For this current deceleration or the braking torque associated therewith, a time period tRamp, i.e., the time period for redistribution, is known, which is required in order to increase the first target value for the braking torque to the sum of the first target value and the second target value and to reduce the second target value for the braking torque to zero at the beginning of the redistribution phase. For example, this time period tRamp can be determined in advance in a test bench measurement and stored in a characteristic field or the like.

[0035] Based on the time period tRamp and the current deceleration a, the speed drop during the reallocation can be determined as: tRamp*a, and if the reallocation starts at v at the current moment, the predicted speed at the end of the reallocation is: v-abs(tRamp*a).

[0036] For example, the second threshold value v1 for the vehicle speed is derived from the desire to brake from a speed v-abs(tRamp*a) and a deceleration a with a constant slope to a deceleration of zero and a speed of zero. The slope defines whether the stopping force felt by the driver is strong or weak. Thus, the entry point of the speed regulation sequence can be calculated. For example, a suitable slope value can also be selected based on comfort or safety considerations.

[0037] The first threshold value v0 for the vehicle speed is then obtained from the following formula: v0=v1+abs(tRamp*a).

Claims

1. A method for braking a vehicle (10) having a first electric drive motor (14) and a second electric drive motor (16), in, In a first braking phase, a first target value for a braking torque is predefined for the first electric drive motor (14), and a second target value for a braking torque is predefined for the second electric drive motor (16). wherein the current speed of the vehicle (10) is detected as the actual speed, wherein, when the actual speed reaches a first threshold value (v0) for the vehicle speed, in a redistribution phase the first target value for the braking torque is increased and at the same time the second target value for the braking torque is reduced, wherein, when the actual speed reaches a second threshold value (v1) for the vehicle speed, the predefinition of the first target value for the braking torque is terminated and a speed target value trajectory is predefined in a second braking phase (202), Therein, the speed target value trajectory extends from a second threshold value (v1) for the vehicle speed to a speed of zero.

2. The method according to claim 1, wherein: In the redistribution phase, the first target value for the braking torque is increased to the sum of the first target value and the second target value at the beginning of the redistribution phase, and the second target value for the braking torque is reduced to zero.

3. The method according to claim 2, wherein: The first threshold value (v0) for the vehicle speed is determined by the time period (tRamp) required to increase the first target value for the braking torque to the sum of the first target value and the second target value and to reduce the second target value for the braking torque to zero at the beginning of the redistribution phase.

4. A method according to any one of the preceding claims, wherein: A first threshold value (v0) for the vehicle speed is determined from the current deceleration of the vehicle (10).

5. A method according to any one of the preceding claims, wherein: The first threshold value (v0) for the vehicle speed is determined from the second threshold value (v1) for the vehicle speed.

6. The method according to the combination of claims 3 to 5, wherein: A first threshold value (v0) for the vehicle speed is determined as the sum of a second threshold value (v1) for the vehicle speed and the product of a current deceleration (a) of the vehicle (10) and a time period (tRamp) required to increase a first target value for the braking torque to the sum of the first target value and the second target value at the beginning of the redistribution phase and to reduce the second target value for the braking torque to zero.

7. A computing unit (12) which is designed to carry out all method steps of the method according to any of the preceding claims. 8 . A computer program which, when executed on a computing unit, causes the computing unit to carry out all method steps of the method according to claim 1 .

9. A machine-readable storage medium having stored thereon the computer program according to claim 8.

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