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

Abstract

The invention relates to a method for braking a vehicle (10) having an electric drive motor (14), in which a target value for a braking torque at the electric drive motor (14) is specified in a first braking phase, a current speed of the vehicle (10) is detected as an actual speed, and if the actual speed reaches a threshold value (v0) for the vehicle speed, the target value (v0) of the braking torque at the electric drive motor (14) is determined. In the first braking phase, the presetting of the target value of the braking torque is ended, and in the second braking phase, a speed target value trajectory (202) is specified, the speed target value trajectory starting from a threshold value (v0) of the vehicle speed up to zero speed, and the threshold value (v0) of the vehicle speed is determined from the current deceleration and the impact value of the vehicle (10).

Description

Technical Field

[0001] The invention relates to a method for braking a vehicle having an electric drive motor, 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 the vehicle, such as wheel brakes or electric motors which are operated like generators so as to induce a braking magnetic field in the coils of the electric motor or which are energized in a targeted manner so that the electric motor is loaded with a torque which decelerates the vehicle.

[0003] The regulation of electric motors in vehicles is usually based on a target torque as a command variable. Since electric motors can provide both positive and negative torques independently of the direction of rotation, the problem in the low-speed range is to accurately provide a specific position or speed of the electric motor. 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. If, for example, it is desired to brake the vehicle to a standstill with the aid of the electric motor, it must be ensured that, after the standstill, no reverse travel occurs immediately due to the corresponding torque of the electric motor.

[0004] The detection of a vehicle stop is usually based on an analysis of the vehicle speed, which is in turn derived from the wheel speed. For this purpose, incremental transmitters are usually used, so that the vehicle speed signal is only available as a discontinuous function. Due to measurement errors and the fact that there are delays in signal processing, it is not suitable to use the wheel speed as an input variable for regulating the vehicle speed in the low speed range, especially at speeds close to zero.

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

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

[0007] In detail, the present invention relates to a method for braking a vehicle having an electric drive motor, wherein in a first braking phase, a target value for a braking torque at the electric drive motor is predetermined, wherein a current speed of the vehicle is detected as an actual speed, wherein if the actual speed reaches a threshold value for the vehicle speed, the presetting of the target value for the braking torque is terminated, and in a second braking phase, a speed target value trajectory is predetermined, wherein the speed target value trajectory extends from a threshold value for the vehicle speed to zero speed, and wherein the threshold value for the vehicle speed is determined from a current deceleration of the vehicle and an impact value.

[0008] Within the scope of the present invention, a possibility for improving the braking, in particular the stopping, of a vehicle having an electric drive motor is presented, in which a speed target value trajectory is determined in a special manner.

[0009] It has been shown that a relatively high control quality must be achieved for a comfortable stopping operation. The overshoot at the end of the stopping process is particularly critical, which can accelerate the vehicle in the opposite direction of the desired direction of travel and can lead to vehicle behavior that is unbelievable to the driver. Events that occur during the stopping process, such as bumps or obstacles (kerbs), are also problematic. These events appear as disturbances to the controller, which tries to compensate for them, which can also be unbelievable to the driver.

[0010] The invention is therefore particularly directed to determining a velocity target value trajectory within the framework of a stopping process. The main goal is maximum driving comfort and avoiding unbelievable vehicle behavior. The jerk, as the rate of change of the acceleration over time, is decisive for the comfort of the driving maneuver. The smaller the jerk, the more comfortable the maneuver is perceived by the driver. A constant jerk results in a linear change in the acceleration. In contrast, a linear acceleration curve produces the smallest jerk for a given change in the acceleration within a fixed time.

[0011] In one embodiment, the threshold value of the vehicle speed is determined as a quotient of the square of the current deceleration of the vehicle and twice the impact value. In this variant, only a few parameters are introduced into the determination, so that this can be performed very easily.

[0012] In one embodiment, the impact value is a predetermined impact value. The impact value can be predetermined by the manufacturer, for example, based on comfort aspects. The impact value can also be predetermined based on vehicle specifications or drive specifications, for example, the minimum necessary impact that can be provided by the vehicle.

[0013] In one embodiment, the speed target value trajectory includes a linear decrease in the deceleration starting from the current deceleration at the beginning of the second braking phase to a zero value at zero speed. This results in the smallest possible jerk and thus the greatest possible comfort.

[0014] If the actual speed at a certain point in time is lower than the target speed predefined by the speed target value trajectory, in one embodiment the target speed is reduced to the actual speed and the speed target value trajectory is continued starting from the speed target value corresponding to the actual speed. Thus, efficient disturbance compensation can be achieved.

[0015] In one embodiment, the deceleration target value trajectory is determined from the speed target value trajectory, and the braking torque precontrol value trajectory is calculated and predefined as a function of the deceleration target value trajectory. With such a precontrol, the speed regulator only needs to intervene as little as possible in the form of regulation, since the actuating variable, here the braking torque, is essentially already predefined by the precontrol value.

[0016] In one embodiment, the braking torque precontrol value trajectory is determined as a function of the target value of the braking torque at the end of the first braking phase, the current deceleration at the beginning of the second braking phase, and the standstill torque required to maintain zero speed of the vehicle at the end of the second braking phase. In this variant, only a few parameters are introduced into the determination, so that this can be carried out very easily, wherein in particular the standstill torque is also already taken into account and therefore does not need to be calibrated again later.

[0017] In one embodiment of the preceding claim, the braking torque precontrol value trajectory extends linearly from the target value of the braking torque at the end of the first braking phase to the static torque required to maintain the zero speed of the vehicle. With this variant, a linear deceleration reduction in the speed target value trajectory can be achieved very easily.

[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] The implementation in the form of a computer program or computer program product with a program code for executing all method steps according to the method of the present invention is also advantageous, because this causes particularly low costs, especially if the controller to be implemented is also used for other purposes and therefore originally exists. Finally, a machine-readable storage medium is provided, which has a computer program as described above stored thereon. Suitable storage media or data carriers for providing computer programs are especially magnetic, optical and electrical memories, such as hard disks, flash memory, EEPROM, DVD, etc. It is also feasible for the program to be downloaded through a computer network (the Internet, intranet, etc.). Such downloading can be realized here either wired or wired or wirelessly (e.g., by WLAN networks, 3G-, 4G-, 5G- or 6G-connections, etc.). BRIEF DESCRIPTION OF THE DRAWINGS

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

[0021] The present invention is schematically shown in the drawings based on exemplary embodiments and is described below with reference to the drawings.

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

[0023] Figure 2 The diagram shows the course of travelled route, vehicle speed, acceleration and impact during an exemplary stopping process according to one embodiment of the present invention.

[0024] Figure 3 The diagram shows the course of the driving route, the vehicle speed, the acceleration and the torque during an exemplary stopping process according to one embodiment of the present invention. DETAILED DESCRIPTION

[0025] Figure 1 A schematic diagram of a vehicle 10 is shown, which is configured to carry out an exemplary embodiment of the method according to the invention. The vehicle 10 comprises a control unit 12, which can in particular be a control unit of a power electronics device. The vehicle 10 further comprises an electric drive motor 14, which is configured to drive at least one wheel 11 of the vehicle 10. A sensor device 15 is arranged at the electric drive motor 14 in such a way that the sensor device 15 detects the angular position and / or the rotational speed of the electric drive motor 14 and transmits it to the control unit 12. The control unit 12 is connected to the electric drive motor 14 via a signal line, so that the electric drive motor 14 can be regulated and / or controlled by the control unit 12.

[0026] If the driver wants to stop the vehicle, for example, the driver usually actuates the brake for this purpose. In the framework of an embodiment, it is provided that a braking process is performed using the drive motor 14, as this point is described below in a continuous and unified manner with reference to the accompanying drawings as an example.

[0027] exist Figure 2 , an exemplary stopping process is shown in the form of a diagram, wherein the travel distance or position 201, the vehicle speed 202, the acceleration / deceleration 203 and the impact 204 are plotted against time. It can be seen that in the first braking phase until the time t0, braking is performed with a constant deceleration, which is a0, for example, and is achieved by presetting a target value for the braking torque at the electric drive motor.

[0028] At time t0, a speed v0 is reached as a threshold value for the vehicle speed, which represents a triggering event for the transition to the second braking phase. At this time, a speed target value trajectory is determined, as explained further below, and the drive motor 14 is braked to a standstill according to the provisions of this speed target value trajectory 202. In particular, this results in a linear decrease of the deceleration 203 to a zero value of zero speed at time t1.

[0029] A method for calculating a speed target value trajectory according to one specific embodiment of the present invention is described below.

[0030] The starting speed is calculated based on the vehicle acceleration, and the speed regulation starts at this starting speed. Correspondingly, the initial condition (t=t0) is:

[0031] Initial acceleration

[0032] v(t0) initial velocity

[0033] At the end of the trajectory (t=t1), the vehicle must stop. Correspondingly, the end condition is:

[0034]

[0035] v(t1)=0

[0036] Based on the boundary conditions, the time course of the velocity trajectory can be calculated for the linear decrease in acceleration.

[0037] Integral of acceleration:

[0038] a(t)=k·t+a0 (1)

[0039] Caused by speed

[0040]

[0041] At the end of the trajectory, not only the velocity but also the acceleration reaches the value 0: t=T: v=0, a=0.

[0042] Therefore, equation (1) can be transformed into

[0043] 0=k·T+a0 (3)

[0044] Equation (2) is transformed into

[0045]

[0046] By combining (3) and (4), the slope k (impact) can be eliminated, which results in the duration of the trajectory until the standstill is reached:

[0047] It is now possible to specify equations for acceleration v, velocity a and position x during the stopping process (assuming x(t=0)=0)

[0048]

[0049] The impact k defines whether the stop feels hard or soft to the driver. It is therefore possible to define k as a calibration parameter and to calculate the entry point of the speed control sequence accordingly. In this way, the second braking phase can be started at the point at which an acceleration ramp with a slope k, starting from the current acceleration a0, brings the stop exactly.

[0050] Assuming k is defined for a given vehicle, the initial velocity can be calculated by eliminating the duration T from the equation

[0051]

[0052] Correspondingly, when the actual speed reaches the threshold value v0, v≤v0, the second braking phase begins.

[0053] External forces can cause the vehicle to brake faster than desired, for example because of driving over bumps or curb edges. In this case, it is advantageous to readjust the trajectory, which leads to a shorter braking distance overall. Without such a readjustment, the control of the speed reduction would be suppressed by the acceleration in order to reach the target value of the original trajectory. This should be avoided.

[0054] Therefore, in one embodiment of the present invention, an interference correction is provided, as is exemplified below with reference to Figure 3 As described, wherein according to one embodiment of the present invention, the curves of acceleration a, vehicle speed v, travel distance or position x and torque M during an exemplary stopping process are shown. The curve 301 without interference is represented by a dashed line, and the curve 302 with interference and correction is represented by a solid line. The interference time point is represented by line 303. The earlier stopping time point achieved by interference and readjustment is represented by line 304.

[0055] The corresponding processing method is based on the following assumptions:

[0056] If the actual speed at a specific time 303 is less than the speed target value generated by the speed target value trajectory at this time, a disturbance, such as a bump, is detected. In this case, the new initial value existing after the disturbance is used. to reinitialize the velocity target trajectory. However, the acceleration slope k is maintained.

[0057] New initial speed = is equal to the speed measured after the disturbance. However, it can be assumed—for example due to value differences—that the acceleration signal measured after the disturbance is affected by noise and / or delayed by filters and is therefore not available for setting a new

[0058] The new initial acceleration is obtained from equation (9):

[0059]

[0060] Therefore, the reinitialization corresponds to a time step on the original speed target value trajectory, that is, if the actual speed is lower than the target speed predetermined by the speed target value trajectory at a certain point in time, the target speed is reduced to the actual speed and the speed target value trajectory is continued starting from the speed target value corresponding to the actual speed.

[0061] The disturbance correction also functions properly in the case of multiple disturbances within the same speed control sequence or even in the case of “continuous” disturbances, ie when the actual speed is permanently below the calculated speed.

[0062] In one embodiment of the present invention, the presetting of the braking torque is set in a pre-control manner, such as in Figure 3 Although the drive motor is in speed control in the second braking phase (for example, a speed target value is predetermined by a higher-level controller in the vehicle, such as a so-called VCU (English: Vehicle Control Unit)), the result can be improved by additionally providing a pre-control torque.

[0063] This is based on the following assumptions:

[0064] The pre-control torque Mplt should be:

[0065] At the beginning of the speed control sequence (that is, at the end of the first braking phase): Target value of the braking torque at the end of the first braking phase (M 0,EM ).

[0066] At the end of the speed control sequence (ie at the end of the second braking phase or at standstill): equal to the static torque M SS,EM , which is required, for example, to compensate for downhill running forces. By subtracting the derivative of the speed from the measured longitudinal acceleration of the inertial sensor, the static torque M can be estimated in particular. SS,EM .

[0067] During speed regulation: proportional to the desired acceleration.

[0068] Therefore, in one design, the pre-control torque of the drive motor is:

[0069]

[0070] The equation is also valid when correcting for interference. Just note that a0 and M 0,EM It is the initial value at the beginning of the entire speed control sequence and is not allowed to be reinitialized in the event of turbulence.

Claims

1. A method for braking a vehicle (10) having an electric drive motor (14), in, In a first braking phase, a target value for the braking torque at the electric drive motor (14) is predefined, wherein the current speed of the vehicle (10) is detected as the actual speed, If the actual speed reaches a threshold value (v0) for the vehicle speed, the presetting of the target value for the braking torque is terminated, and a speed target value trajectory is predefined in the second braking phase (202), The speed target value trajectory extends from a threshold value (v0) of the vehicle speed to zero speed, and The threshold value (v0) of the vehicle speed is determined from the current deceleration of the vehicle (10) and the impact value.

2. The method according to claim 1, wherein: The threshold value (v0) of the vehicle speed is determined as a quotient formed by the square of the current deceleration of the vehicle (10) and twice the impact value.

3. The method according to claim 1 or 2, wherein: The impact value is a minimum impact greater than zero that can be provided by the vehicle or a predetermined impact value.

4. A method according to any one of the preceding claims, wherein: The speed target value trajectory (202) includes a linear decrease of the deceleration from the current deceleration at the beginning of the second braking phase to a zero value at zero speed.

5. A method according to any one of the preceding claims, wherein: If at a certain point in time the actual speed falls below a target speed predefined by the speed target value trajectory (202), the target speed is reduced to the actual speed and the speed target value trajectory (202) is continued starting from a speed target value corresponding to the actual speed.

6. A method according to any one of the preceding claims, wherein: A deceleration target value trajectory is determined from the speed target value trajectory (202), wherein a braking torque precontrol value trajectory is calculated and predetermined based on the deceleration target value trajectory.

7. A method according to the preceding claim, wherein: The target value of the braking torque at the end of the first braking phase, the current deceleration at the beginning of the second braking phase, and the static torque (M) required to maintain the zero speed of the vehicle (10) at the end of the second braking phase are calculated. SS ) to determine the braking torque pre-control value trajectory.

8. A method according to the preceding claim, wherein: The braking torque precontrol value trajectory extends linearly from the target value of the braking torque at the end of the first braking phase to the static torque (M) required for maintaining the zero speed of the vehicle (10). SS ). 9 . A computer unit which is designed to carry out all method steps of the method according to claim 1 . 10 . A computer program that causes a computing unit to execute all method steps of the method as claimed in claim 1 when the method is executed on the computing unit.

11. A machine-readable storage medium having stored thereon the computer program according to claim 10.

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