Method and system to control torque distribution

Abstract

The disclosure relates to a method to control torque distribution among a plurality of electric machines connected to at least one front wheel and at least one rear wheel of a vehicle during operation, comprising: acquiring the total torque requested; obtaining the most energy efficient torque distribution mode by using a loss model or loss map; evaluating the actual driving situation; determining if a mode switch is allowed depending on the actual driving situation; switching the torque distribution mode, if allowed; and preventing a mode switch, if not allowed.

Claims

1. A computer-implemented method to control torque distribution among a plurality of electric machines connected to at least one front wheel or at least one rear wheel of a vehicle during operation, the computer-implemented method comprising: acquiring, by a system operatively coupled to a processor, a total torque requested; obtaining, by the system, a most energy efficient torque distribution mode by using a loss model or loss map; evaluating, by the system, an actual driving situation; determining, by the system, if a mode switch is allowed depending on the actual driving situation; switching, by the system, the torque distribution mode, if allowed; and preventing, by the system, a mode switch, if not allowed.

2. The computer-implemented method according to claim 1, wherein the mode switch is not allowed, if the evaluation of the actual driving situation results in the mode switch not complying with safety or comfort requirements based on a determination that at least one of the following conditions is met: a lateral acceleration of the vehicle exceeds a predefined threshold; a friction force between at least one wheel of the vehicle or the ground falls below a predefined threshold; a time passed since a previous distribution mode switch is shorter than a predefined minimum time; or an understeer gradient of the vehicle is outside normal bounds.

3. The computer-implemented method according to claim 2, wherein the thresholds are determined at forehand such that values measured while driving can be compared with the respective values of the at forehand determined thresholds.

4. The computer-implemented method according to claim 1, wherein the acquired total torque is distributed: only to the electric machines connected to front wheels of the vehicle, defining a first torque distribution mode; only to the electric machines connected to rear wheels of the vehicle, defining a second torque distribution mode; or to the electric machines connected to front wheels and to rear wheels of the vehicle, defining a third torque distribution mode.

5. The computer-implemented method according to claim 4, wherein each electric machine connected to a wheel is directly controllable through torque requests or is disconnectable from the respective wheel.

6. The computer-implemented method according to claim 4, further comprising: decoupling, by the system, the electric machines to which no torque is distributed.

7. The computer-implemented method according to claim 4, wherein during propulsion, torque is distributed to the electric machines of an axle with the lowest normal load, and wherein during braking, torque is distributed to the electric machines of an axle with the highest normal load.

8. The computer-implemented method according to claim 1, wherein the loss model or loss map comprises electrical losses as well as tire losses.

9. The computer-implemented method according to claim 8, wherein the electrical losses are measured experimentally and tire losses are modelled.

10. The computer-implemented method according to claim 1, wherein the loss model or loss map comprises battery losses, resistive losses in cables or aerodynamic losses by air drag.

11. The computer-implemented method according to claim 1, wherein the loss model or loss map is pre-generated and implemented in the vehicle.

12. The computer-implemented method according to claim 1, wherein the loss model or loss map comprises evaluation of effects of temperature or tire pressure on power loss.

13. The computer-implemented method according to claim 1, wherein for each drive mode of the vehicle, at least one of eco driving, dynamic driving, a corresponding loss model or loss map is used.

14. The computer-implemented method according to claim 1, wherein at least one loss model or loss map is used that has been pre-generated by using a brute force method.

15. The computer-implemented method according to claim 1, wherein acquiring the total torque requested further comprises calculating the total torque requested using: $T_{m,\mathrm{req}}=\frac{F_{x,\mathrm{req}}*R_1}{n},$ wherein “T.sub.m,req” is the requested motor torque, “F.sub.x,req” is the longitudinal force request, “R.sub.1” is the average loaded radius for all wheels and “n” is the gear ratio.

16. A system to control torque distribution among a plurality of electric machines connected to at least one front wheel and at least one rear wheel of a vehicle, the system comprising: a processor that executes computer executable components stored in at least one memory; a torque allocation module that acquires the total torque requested and obtains the most energy efficient torque distribution mode on basis of a loss model or loss map; and a vehicle motion state estimator that evaluates the actual driving situation.

17. The system according to claim 16, wherein the torque allocation module has at least one port that receives data from the vehicle motion state estimator or a sensor.

18. The system according to claim 17, wherein the data received via the port relates to the actual driving situation and comprises information about motor speed, wheel speed, wheel slip, side-slip angle, road friction, or lateral acceleration.

19. The system according to claim 16, wherein the torque allocation module has at least one port that activates at least one actuator.

20. A computer-readable recording medium storing a program that causes a processor to execute a computer-implemented method comprising: acquiring a total torque requested; obtaining a most energy efficient torque distribution mode by using a loss model or loss map; evaluating an actual driving situation; determining if a mode switch is allowed depending on the actual driving situation; switching the torque distribution mode, if allowed; and preventing a mode switch, if not allowed.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0068] These and other aspects of the present disclosure will become apparent from the embodiments described hereinafter.

[0069] FIG. 1 shows a first block diagram in order to explain the proposed method,

[0070] FIG. 2 shows a second block diagram in order to explain the proposed system,

[0071] FIG. 3 shows a diagram in which the x-axis represents the torque and the y-axis represents the power loss, and

[0072] FIG. 4 shows an example of a loss map in which the x-axis represents the vehicle speed and the y-axis represents the torque requested.

DESCRIPTION OF EMBODIMENTS

[0073] FIG. 1 illustrates the main steps of the method proposed. At first, in step 100, the total torque requested by a driver or a machine is calculated. A computer or processor integrated in a torque allocation module may execute the calculation. If a driver is steering the vehicle, the torque requested by the driver may be represented by the position of an accelerator or brake pedal. In this case, before step 100 is executed, the accelerator or brake pedal position has to be translated into a longitudinal force request, e.g. by means of a driver interpreter. The driver interpreter transmits this information to the torque allocation for acquiring the total torque requested.

[0074] In step 200, the torque allocation module may determine the most energy efficient torque distribution mode by using a loss model or loss map that has been implemented in the torque allocation module. Thus, the loss model or loss map has been pre-generated, e.g. in form of a look-up table. An example of a loss map is shown in FIG. 4 and will be explained further below. In one embodiment, a loss map is used that considers not only electrical losses (effected by the electric machines, inverters, and/or battery), but also tire losses. The consideration of tire losses improves the preciseness of the method proposed compared to methods known from the background art. Both, electrical losses and tire losses have the biggest impact on the total loss. Further losses, such as aerodynamic losses due to air drag, may be neglected.

[0075] The loss map defines thresholds suggesting a mode switch when a threshold is passed. The mode switch may take place between three different torque distribution modes, which are torque distribution [0076] only to front wheels, [0077] only to rear wheels or [0078] equally distributed between front and rear wheels.
Optimal torque distribution serves to reduce power consumption, which can be directly read off the loss map.

[0079] Because a mode switch has an effect on the driving dynamics, in step 300 of the method proposed, before executing a mode switch, the actual driving situation is evaluated. If this evaluation comes to the result, that a mode switch would affect the driving dynamics such that the occupant's safety or comfort is negatively influenced, the mode switch is not allowed. If the evaluation comes to the result, that safety and comfort are not negatively influenced, the mode switch is allowed.

[0080] Accordingly, in step 400 of the method proposed, after the mode switch has been allowed, the mode switch is taking place, i.e. the torque distribution to the individual electric machines of the vehicle is changed.

[0081] FIG. 2 illustrates an embodiment of a system 10 for executing the method proposed. The system 10 comprises a vehicle motion state estimator 30 and a torque allocation module 20. The vehicle motion state estimator 3 provides the torque allocation module 20 with information, e.g. relating to side-slip angle, road friction, lateral acceleration. On basis of this information, the torque allocation module 20 evaluates the actual driving situation. The information/data is received by the torque allocation module 20 via port 21. Further information/data, e.g. relating to motor speed, wheel speed, wheel slip, may be provided by at least one sensor (not shown) and may be received by the torque allocation module 20 via port 22.

[0082] The torque allocation module 20, may execute steps 100 to 400 of the method proposed. Thus, at least one loss map is implemented in the torque allocation module 20. If the torque allocation module 20 suggests and allows a mode switch, a signal is transmitted via port 23 to at least one actuator 40, 41, 42, 43 in order to change torque distribution.

[0083] FIG. 3 illustrates power losses from a typical electric machine at three different speeds (see curves A. B and C). The power loss is mainly dependent on torque and angular speed of the electric machine. As seen from FIG. 3, when the speed increases the power loss also increases, not linear but exponentially. Assumed, that there are several electric machines available for control. Further assumed, that a request of a total propulsive torque can freely be distributed among the electric machine. Due to the non-linearity, the total loss from the electric machines is dependent on which distribution is used.

[0084] FIG. 4 illustrates a loss map for use in the method proposed. At first, desired torque distribution modes are defined. e.g. propulsive torque only at the front axle (mode 1) and equal torque distribution between front and rear axle (mode 2). When the complete loss model is known, an offline optimization can be made to determine a distribution strategy depending on angular motor speed and total propulsive torque requested. In FIG. 4, mode 1 is represented by the dark grey color, mode 2 by the light grey color.

[0085] One interpretation of FIG. 4 is, that mode 2 should only be used for high total torque request, e.g. during high acceleration and/or uphill driving. The switch torque from mode 1 to mode 2 changes with motor speed, being lower at low velocities and higher at high velocities. The speed axis (x-axis) can be interpreted as vehicle speed and the torque axis (y-axis) can be interpreted as requested longitudinal force, depending on the selected transmission ratio.

[0086] When a loss map according to FIG. 4 is implemented in the vehicle, the torque distribution can be done dependent on actual speed and requested torque. When passing through different regions on the loss map, a change from one mode to another occurs. i.e. a mode switch.